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Catalyst recycling—A survey of recent progress and current status
Coordination Chemistry Reviews 349 (2017) 1–65
Contents lists available at ScienceDirect
Coordination Chemistry Reviews journal homepage: www.elsevier.com/locate/ccr
Review
Catalyst recycling—A survey of recent progress and current status Árpád Molnár a,⇑, Attila Papp b a b
Department of Organic Chemistry, University of Szeged, Szeged, Hungary Soneas Research Ltd, Hungary
a r t i c l e
i n f o
Article history: Received 22 June 2017 Accepted 14 August 2017
Keywords: Complex Heterogeneous catalyst Nanoparticle Recycling Ionic liquid Magnetic catalyst
a b s t r a c t This review gives a comprehensive treatment to show and illustrate current efforts in the field of recyclable catalysis. Results of recycling studies performed with a wide range of soluble homogeneous and immobilized complexes as well as heterogeneous catalysts developed in recent years have been collected and discussed. Among others, transformations including hydrogenation, reduction, oxidation, varied asymmetric syntheses and coupling reactions are covered. A thorough analysis of the available data and discussion of issues related to recyclability in general are also given. The review includes selected literature examples until the beginning of 2017. Ó 2017 Elsevier B.V. All rights reserved.
Contents 1. 2. 3.
4.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 About valid assessment of recycling ability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Catalyst recycling from homogeneous systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 3.1. Temperature-controlled phase-separable catalyst recycling. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 3.2. Aqueous biphasic catalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 3.3. Ionic liquid-based systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 3.4. Recycling of soluble catalysts used in monophasic reactions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 3.5. Stabilized soluble metal nanoparticles in reuse studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Recycling of solid catalysts applied in heterogeneous systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 4.1. Catalysts systems based on siliceous support materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 4.1.1. The use of supports with ordered structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 4.1.2. Catalysts with amorphous silica support . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Abbreviations: AAS, atomic absorption spectroscopy; AB, ammonia–borane (H3N:BH3); acac, acetylacetonate; BAIB, [bis(acetoxy)iodo]benzene; bdmi, 1-butyl-2,3dimethylimidazolium; BARF, tetrakis-[3,5-bis(trifluoromethyl)phenyl]borate; BINAM, 1,10 -binaphthyl-2,20 -diamine; BINAP, 2,20 -bis(diphenylphosphano)-1,10 -binaphthyl; bmim, 1-butyl-3-methylimidazolium; BOX, bisoxazoline; BWT, black wattle tannin; C10mim, 1-decyl-3-methylimidazolium; Cnh, carbon nanohorn; Cnt, carbon nanotube; DABCO, 1,4-diazabicyclo[2.2.2]octane; dansyl, [5-(dimethylamino)naphthalene-1-sulfonyl]; dba, dibenzylideneacetone; DMAc, N,N-dimethylacetamide; DMF, N,Ndimethylformamide; dmpz, 3,5-dimethylpyrazole; DA, dopamine; DPEN, 1,2-diphenyl-1,2-diaminoethane; ee, enantiomeric excess; emim, 1-ethyl-3-methylimidazolium; FESEM, field emission scanning electronic microscopy; FSG, fluorous silica gel; F-SPE, fluorous solid-phase extraction; GO, graphene oxide; rGO, reduced graphene oxide; graph, graphite; HBPE, hyperbranched polyethylene; HMS, hexagonal mesoporous silica; HPEI, hyperbranched polyethylenimine; HRTEM, high-resolution transmission electron microscopy; ICP-AES, inductively coupled plasma atomic emission spectroscopy; ICP-OES, inductively coupled plasma optical emission spectroscopy; IL, ionic liquid; l/br, linear to branched ratio; LC, laponite clay; MCF, mesocellular foam; MAB, methylamine–borane (MeH2N:BH3); mim, methylimidazolium; MTBE, methyl tert-butyl ether; MW, microwave heating; NHC, N-heterocyclic carbene; NMP, N-methyl-2-pyrrolidone; Oct, octyl; olac, oleic acid; omim, 1-methyl-3-octylimidazolium; PAA, polyacrylic acid; PE, polyethylene; PEI, polyethyleneimine; PEG, poly(ethylene glycol); PET, polyethylene terephthalate; PIB, polyisobutylene; pmim, 1-propyl-3-methylimidazolium; PMO, periodic mesoporous organosilica; poly, polymer; ppy, 2-phenylpyridine; PS, polystyrene; PVP, poly(4-vinylpyridine); PVPy, poly(N-vinyl-2-pyrrolidone); RCM, ring-closing metathesis; rGO, reduced graphene oxide; scCO2, supercritical scCO2; SFM, scanning force microscopy; TBAAc, tetrabutylammonium acetate; TBAB, tetrabutylammonium bromide; TBHP, tert-butyl hydroperoxide; TEOS, tetraethyl orthosilicate; TEMPO, 2,2,6,6-tetramethylpiperidine 1-oxyl; TfO, triflato (CF3SO3, trifluoromethylsulfonato); TGA, thermogravimetric analysis; TOF, turnover frequency; TON, turnover number; TPPTS, P(C6H4-mSO3Na)3; TRPTC, thermoregulated phase-transfer catalysis; TsDPEN, N-(p-toluenesulfonyl)-1,2-diphenylethylenediamine; VSM, vibrating sample magnetometer. ⇑ Corresponding author at: Dóm tér 8, H-6720 Szeged, Hungary. E-mail addresses: [email protected] (Á. Molnár), [email protected] (A. Papp). http://dx.doi.org/10.1016/j.ccr.2017.08.011 0010-8545/Ó 2017 Elsevier B.V. All rights reserved.
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4.1.3. Catalysts with other silicon-based supports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Use of oxide-based catalysts. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.1. Oxides as catalyst support . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.2. Oxides serving as catalytically active phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Polymer-supported catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.1. Catalysts based on synthetic polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.2. Catalyst supported on natural polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4. Varied carbon materials serving as catalyst support. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5. Magnetic and magnetically tagged catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.1. Recycling in hydrogenation and hydrogenolysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.2. Reuse studies in oxidations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.3. Recycling of magnetic catalysts in asymmetric syntheses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.4. C–C bond forming reactions induced by magnetic catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.5. Reuse in hydrogen generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.6. Recycling of magnetic catalysts in other transformations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.6. Catalyst based on MOF materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Additional examples. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Summary analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1. Successful examples of recycling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2. The advantage of recycling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3. The problem of catalyst deactivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4. What to do?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.
5. 6. 7.
1. Introduction ‘‘Green” has become a buzzword in recent decades for sustainability. Sustainable chemistry is of extremely high importance stimulating the development of clean processes and technologies. It includes waste reduction, minimization of materials and energy, renewability, introduction and use of environmentally benign reagents and efficient processes. In particular, recycling, that is the reuse of catalysts, the topic of this review, is an important issue when considering the limited availability and dwindling supply of expensive noble metals. A long catalyst lifetime and the ability to easily recycle the catalyst are highly desirable for industrial applications. That is, both environmental and economic considerations push for the development of processes that enable the separation and recovery as well as reuse of the catalyst. The general requirements of any catalyst are high activity, selectivity and stability for prolonged use. Furthermore, a catalytic process should have low environmental impact. Homogeneous (soluble) metal complexes are well-known for their high, in certain instances, extremely high activities and selectivities to induce varied transformations. No wonder that they are widely used in both academia and industry in synthetic organic chemistry and commercial chemical technologies. The downside, however, is often low stability and the difficulty in reutilization because of the complexity of their separation being time consuming and requiring high energy. These problems may be solved using ‘‘non-conventional” solvents (ionic liquids, water, fluorinated and supercritical solvents) and multiphase homogeneous catalysis [1]. Heterogenization that is immobilization of catalytically active species on suitable carriers may provide another solution to these needs [2]. Solid heterogeneous catalysts are easy to handle and can easily be separated by simple techniques (filtration, centrifugation, magnetic separation, etc.) allowing catalyst recycling. Immobilization, however, introduces additional synthetic steps in catalyst preparation. Furthermore, the activity and selectivity of immobilized catalysts are frequently lower compared to the corresponding soluble homogeneous complexes. One of the main reasons is the much lower number of available active sites, compared to the single-site nature of homogeneous complexes. Additionally, deactivation resulting from poisoning, metal leaching
27 29 29 30 31 31 37 39 42 42 44 46 47 50 51 52 55 58 59 59 59 59 60 60
and decomposition take place (see a detailed discussion in Section 7.3). A further major problem is environmental concerns arising from the use of costly and often poisonous materials applied in catalyst synthesis. To compile a general review of the field of catalyst recycling is an immense task. A review by Cole-Hamilton [3], as well as two excellent books edited by Cole-Hamilton and Tooze [4] and de Vos, Vankelecom and Jacobs [5] have been published. However, all three have focused only on soluble and immobilized homogeneous complexes. Additional related reviews covering even narrower fields are also available for interested readers [1,6]. Gladysz has discussed general problems of catalyst recycling in two relevant papers [7]. A comprehensive review about catalyst recycling comprising varied possibilities of catalyst recovery and reuse and, at the same time, covering diverse transformations has not been compiled before. This review is intended to be illustrative by presenting and discussing a selection of varied examples of the last 6–7 years with a handful of results from 2017. Nevertheless, it is a comprehensive treatment to show and illustrate current efforts in the development of the field. In addition to recent achievements, a few noteworthy examples from earlier studies will also be treated here as appropriate. A thorough analysis of the available data and discussion of issues related to recyclability in general are also included. The aim of this review is to show a wide spectrum of reactions and catalyst systems with the potential of catalyst recycling. Still, the strict selection rules applied in related previous reviews by the principal author are also applicable here [8]. Specifically, the general rule for data selection is that decrease in percentage activity (conversion or yield) cannot be higher than the number of cycles. Of studies with 5–9 runs only results without decreases in activity are treated. There are, however, a few notable exceptions (for example, novel catalysts and rarely studied reactions). A further precondition for selection, in all cases, is the availability of appropriate catalyst characterization information. Whereas consistently high yields in repeated runs cannot be taken as satisfactory evidence for high catalyst stability, they testify to the robust nature of a catalyst system, which is a necessary requirement for long-term uses (see also Section 2).
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Strictly speaking, studies about continuous flow systems do not belong to the topic of this review. Nevertheless, a handful of excellent examples with high catalyst stability affording high productivities (TON numbers) are included to demonstrate the merits of such approaches. Readers may find it useful to consult a recent review [9]. 2. About valid assessment of recycling ability Before undertaking the elaboration of the topic, it appears to be important to start with a short discussion of this closely related, important question of recycling chemistry. Characteristically, the consistently high yields of a catalyst in a few repeated runs are taken as evidence to prove its high recycling potential. In the great majority of published data, however, small but consistent drops of yields are experienced. Naturally, this is a clear sign of unsatisfactory catalyst stability in the long run. A number of examples exist when steadily and significantly decreasing tendencies in yields are erroneously characterized by the authors to state satisfactory catalytic activities. Kinetic data may provide important pieces of information about the nature of active species. As pointed out by Gladysz [10], an induction period and catalytic activity of the separated reaction mixture as well as induction periods in repeated runs of the recovered catalyst often with decreased activity are clear signs indicating that the ‘‘recycled” species is certainly not the active catalyst. Induction periods detected in successive catalytic cycles show the regeneration of the active catalyst in every new cycle that is the catalyst resting state is actually recycled. It is quite unfortunate that kinetic analysis has been rarely performed in recycling studies. An additional key problem is the efficacy of catalyst recovery, which is seldom determined. Considering practical applications, a high cumulative TON is more crucial than good recycling ability of a catalyst that is yields as a function of cycles. Furthermore, average TOF values giving the rate of product formation measured in each cycle are valid and meaningful data. Data sets collected by such careful and thorough studies will certainly allow the unequivocal assessment of catalyst systems with respect to recyclability, stability and deactivation if any. See also a related excellent discussion by Jones [11]. 3. Catalyst recycling from homogeneous systems A basic and important necessity of an efficient and recyclable catalytic process is the easy and, as far as possible, complete separation of the catalyst from the reaction system. Considering homogeneous complexes, either heterogenization or the use of two-phase systems affords a suitable solution. In the latter case, the catalyst is dissolved in one phase and the substrate/product in the other. Phase-separation at the end of the reaction allows reusing the catalyst-containing phase in the next run. The most frequently applied methods are organic/organic separation including fluorous biphasic chemistry, the use of aqueous biphasic catalysis and ionic liquid-based systems. These are discussed in Sections 3.1–3.3. Other specific options are recycling of soluble solid catalysts used in monophasic reactions (3.4) and the use of stabilized soluble metal nanoparticles (3.5). For readers interested in pursuing the subject further, a few additional sources may prove valuable to find important background information [12]. Additional readings are about practical aspects of these methods [13] including industrial applications [14].
3
[15]. A liquid–liquid system can be transformed into a single phase by changing the temperature of the reaction mixture. After reaction, the biphasic system is formed again by temperature change with one phase containing the product and the other the catalyst. Phase separation may also be achieved by perturbing the reaction mixture by the addition of water. Another possibility is to have a homogeneous monophasic reaction system, which upon cooling results in precipitation of the solid catalyst. Interested readers are advised to consult recent publications for further details [16]. As mentioned in Introduction, this review covers primarily results of recent years. However, this subsection cannot be treated without briefly mentioning the seminal work by Horváth and Rábai who developed the fluorous biphasic catalyst recovery concept [17] with the main motivation to carry out hydroformylation of high-molecular-weight alkenes. They reported the use of a rhodium catalyst prepared in situ from Rh(acac)(CO)2 and fluoroussoluble phosphane P[CH2CH2(CF2)5CF3]3. When applied in a 1:1 t oluene–perfluoromethylcyclohexane (C6F11CF3) solvent mixture in the hydroformylation of dec-1-ene, a homogeneous singlephase system is formed at reaction temperature (100 °C). There are two great advantages of this catalyst system: (i) it can be utilized for the hydroformylation of alkenes of both low and high molecular weights and (ii) the biphasic system formed at room temperature allows facile catalyst separation since the product phase is easily separated from the catalyst-containing fluorous phase, which is then recycled. Recent results with respect to the application of thermomorphic solvent systems are collected in Table 1. These afford catalysis to take place in a single phase followed by catalyst recovery via phase separation. Behr and co-workers have developed thermomorphic multicomponent solvent systems and performed varied transformations [6a,18]. Upon studying in detail the ternary model systems DMF–decane–dodec-1-ene and DMF–decane–dodecanal, hydroformylation of dodec-1-ene was carried out in a 1:1 DMF– decane mixture containing 16 wt% dodec-1-ene induced by the Rh(acac)(CO)2/Biphephos (1) (1:20) catalyst system [19]. The polar DMF phase separating at 10 °C was reused in an eight-run recycling study (entry 1). The original l/br (linear/branched) ratio of 98:2 decreased to 84:16 in run 5 but increased upon adding fresh ligand 1a. Cumulative losses of Rh and ligand in eight runs were, respectively, 40 and 229 ppm. Significantly lower levels of leaching were reported in a follow-up study when the catalyst system was reused in 30 runs (entry 2) [20]. The first efficient recycling of an iridium catalyst used in hydroformylation of oct-1-ene has recently been reported [21]. The reaction was induced by Ir(cod) (acac) in the presence of sulfonated triphenylphosphane ligands in the mixed polar solvent DMF and water under homogeneous conditions. Product separation was achieved by extraction with the nonpolar solvent 2,2,4-trimethylpentane. Recycling data using TPPMS (monosulfonylphenylphosphane) are collected in entry 3.
3.1. Temperature-controlled phase-separable catalyst recycling Catalyst recovery from thermoregulated or thermomorphic solvent systems is easily performed by a temperature change
Recent efforts by the Behr group with respect to hydroformylation to explore other possibilities in catalyst recycling including
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Table 1 Catalyst recycling in thermoregulated systems. Entry
Reaction
1
Catalysta
Conditions
Number of cycles
Yield (%)b
Ref.
Rh(acac)(CO)2 (0.1)
1a, 20 bar (CO/H2 = 1:1), DMF/decane (1:1), 100 °C, 2 h
8
74.7 (78–72)
[19]
Rh(acac)(CO)2 (0.1) Ir(cod)(acac) (0.8) PdII-2 (1)
1a, 20 bar (CO/H2 = 1:1), DMF/decane (1:1), 100 °C, 2 h 30 bar (CO/H2 = 2:1), TPPMS, DMF/H2O (39:1), 100 °C, 5 h DABCO, CuI, DMF/FC 77 (1:1), 135 °C, 4 h c
30
71 (79–54)
[20]
5
72 (67–75.54)
[21]
8 8
99.9 (100 ? 99) 98 (100–92)d
[24]
PdII-2 (1)
DABCO, CuI, DMF, 135 °C, 4 h
96 (100–74) 100e >99f
[24]
R= C10H21 2
R = C10H21
3
R = C6H13
4 O2N
Ph
I + Ph O2N
Et3N, heptane/DMF (1:1), 100 °C, 16 h
7
0.19% Pd -3HBPE (0.05) 4-poly (2)
14 8 10
(Boc)2O, heptane/EtOH (1:1), rt, 1 h
20
91
[27]
8
4-poly (5)
Heptane/EtOH (1:1), 80 °C, 24 h
20
91
[27]
9
4.26% PdII-5PIB (1)
Et3N, heptane/DMF (1:1), 75 °C, 46 h, under nitrogen
10
>97f
[28]
10
CrIII-6a*-PIB (1)
Heptane/EtOH (1:1), rt, 24 h, under nitrogen
5
85 (70–99)
[29]
CrIII-6b*-PIB (5)
Heptane/EtOH (1:1), rt, 12 h, under nitrogen
6
58.5 (42–95) 10–22g
[29]
12
CrIII-7-PE (5)
Toluene, 75 °C, 3 h, under nitrogen
6
96 (88–100)
[29]
13
RuII-8-PE (5)
Toluene, PEG, 70 °C, 8 min, under nitrogen
5
86
[30]
14
11.4% IrIII-9PIB (1)
Bu3N (2 equiv), Hantzsch ester (2 equiv), MeCN/heptane, 85 °C, 3 h
10
91 (86–96)
[31]
15
11.4% IrIII-9PIB (2.5)
Bu3N (10 equiv), HCCOH (10 equiv), MeCN/ heptane, 85 °C, 3 h
10
67.5 (60–76)
[31]
16
11.4% IrIII-9PIB (0.023)
iPr2NEt, MeCN/heptane, 90 °C, 30 min
11h
83:17–81:19i
[31]
5
II
6
OH
11
O + Me3SiN3
a b c d e f g h i
[25]
N3
Percentage data show loading of active species (wt%) when available; data in parentheses indicate mol% used. Average of yields. Data in parentheses indicate range of yields. The last run in 6 h. Without CuI; the last two runs in 8 h. Without CuI; the first two runs in 2 h. Conversion. Range of enantiomeric excess. Number of samplings. Range of Z/E ratios.
the use of solvent systems, organic solvent nanofiltration and varied technical solutions are useful further readings [22]. A review about nanofiltration is also advised [23]. Lu and co-workers applied the fluorous biphasic concept by conducting Sonogashira reactions under two conditions [24]. Catalyst PdII-2 with fluorous ponytails applied in the mixture of DMF and perfluorinated solvent FC 77 could be separated by cooling
the reaction mixture to 0 °C to have the fluorous phase with the dissolved catalyst (entry 4). When working in DMF alone, the catalyst precipitates upon cooling the reaction mixture and then it is separated by centrifugation. High yields decreased in the last three runs, when the reaction was performed in the presence of CuI in 14 cycles, whereas the reaction time was increased to maintain full conversion in the absence of CuI (entry 5).
Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65
5
monophasic reaction mixture by addition of 10 vol% of water and then reusing the heptane phase. Significantly increasing yields were observed in oxirane ring opening with 4-methoxythiophenol (entry 10) and Me3SiN3 with low enantioselectivity (entry 11). In contrast, thermomorphic phase separation could be used when ring opening of cyclohexene oxide was catalyzed with complex 7 bearing a polyethylene (PE) oligomer support. Applied in toluene at 75 °C, the solid catalyst precipitates from the cold reaction mixture. After separation by centrifugation, it works with increasing activity and leaches 0.3% of metal (entry 12). A hyperbranched polyethylene (HBPE) with disulfide functionalities as binding sites for PdII–diimine moiety 3 proved to be a highly stable catalyst (0.19% PdII-3-HBPE) in Heck coupling in heptane–DMF (entry 6) [25]. The reaction mixture separates at room temperature allowing the reuse of the catalyst-containing heptane phase. Pd leaching after the first three runs (3.88 ppm) becomes negligible in further reuses (0.7 and 0.6 ppm). High conversions of >99% were also measured in runs 1 and 10 in 4 h. Formation of small Pd clusters was detected in the first several cycles. Another recycling application of an oligomeric PE-bound catalyst is the use of immobilized Hoveyda–Grubbs second generation Ru complex in RCM [30]. The reactant and catalyst RuII-8-PE are dissolved in toluene containing PEG as a cosolvent. This reaction mixture gives a gel upon cooling. The product is dissolved with toluene and the gel is recycled by adding fresh substrate. An average yield of 86% was reported with 0.19% Rh leaching in run 4 (entry 13). A number of soluble polymer systems have been developed and applied in thermoresponsive separating mode by the Bergbreiter group [26]. Terpolymer 4 containing dodecyl, dansyl [5-(dimethyla mino)naphthalene-1-sulfonyl] and N-butyl-N-dansylaminomethyl groups attached to polystyrene was found to significantly increase phase-selective solubility in alkane. Accordingly, functionalized polymer 4 (catalyst 4-poly) with immobilized organocatalyst 4dimethylaminopyridine has been tested in acylation and transesterification in a heptane–ethanol mixture [27]. Reactions carried out at room temperature are monophasic but the addition of 5 vol% water perturbs the system and induces separation of the heptane phase containing the polymeric catalyst. High average yields were reported in 20-run studies (entries 7, 8) with 0.09% and 0.002% catalyst leaching, respectively. Polyisobutylene (PIB) is another soluble catalyst support used as the handle for immobilizing NHC (N-heterocyclic carbene) ligands. The formed catalyst 4.26% PdII-5-PIB was successfully applied in the Heck reaction in DMF–heptane exhibiting high and stable activities in 10 cycles (entry 9) [28]. After removing the DMF phase the remaining heptane solution of the catalyst was reused.
A homogeneous, soluble polyisobutylene-tagged iridium complex (11.4% IrIII-9/poly) has been employed in three studies in the acetonitrile–heptane thermomorphic solvent system [31]. It was used in the photocatalytic deiodination at elevated temperatures (entry 14) and in deiodination–cyclization (entry 15) exhibiting high stability upon repeating the reactions 10 times. The catalyst was recovered by cooling the reaction mixture and then reusing the acetonitrile phase containing the catalyst. In another application, photocatalytic E/Z alkene isomerization was performed with continuous recycling of the catalyst phase in a flow process in a 30-min experiment. The heptane-saturated solution of the substrate and the catalyst dissolved in heptane was pumped into a microreactor and irradiated. Stable Z/E ratios (entry 16) with the loss of 2.6% of Ir in the first mmol of converted substrate were measured. Ir loss leveled off well below 0.1% (2 ppm).
3.2. Aqueous biphasic catalysis The immobilized CrIII-salen catalysts 6⁄ anchored to PIB prepared by Bergbreiter et al. have also been used in heptane–ethanol [29] (here and in similar anchored catalysts, only a single moiety is shown). Again, the catalyst is separated by perturbing the
As already mentioned, the development of aqueous biphasic catalysis has been motivated by easy catalyst recycling. At an appropriately selected temperature the reaction mixture is
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Table 2 Catalyst recycling in aqueous biphasic solvent systems. Catalysta
Conditions
Number of cycles
Yield (%)b
Ref.
1
[RuCl2(PTA)4] (5)
H2O/tert-BuOH (1:1), 100 °C, 24 h
7
86.4 (69–93)c
[36]
2
[Ir(cod)Cl]2 (0.2)
(L)-cysteine, H2O/toluene (4:2), buffer (pH 10), 50 bar, 80 °C, 21 h
5
77.4 (67–82)
[37]
3
Rh(acac)(CO)2 (0.2)
10, H2O/toluene/(CH2OH)2 (8:4:1), buffer (pH 7.2), 50 bar, 80 °C, 24 h
5
79.6 (66–80)
[37]
4
Rh(acac)(CO)2 (0.2) [Rh(cod)2] [BF4] (0.5)
10, H2O/(CH2OH)2 (9:1), buffer (pH 7.2), 50 bar, 80 °C, 24 h 11, H2O/scCO2, 30 bar, 56 °C, 2 h
5
67.6 (61–72)
[37]
5
>94 (81 ? 99)c
[38]
[Rh(cod)2] [BARF] (0.5) [Rh(cod)2] [BARF] (0.5) AuIII-14 (2.5)
12*, H2O/scCO2, 30 bar, 56 °C, 2 h
5
[38]
13*, H2O/scCO2, 30 bar, 56 °C, 2 h
5
H2O/toluene (1:1), rt, 2 h
10
100c 39–43d 100c 97–99d 100
[39]
Rh(acac)(CO)2 (0.1)
Me(OCH2CH2)nPPh2, 40 bar, pentane, H2O, 90 °C, 4 h c
8
98.6 (99.9–97.3)
[40]
Ref.
Entry
Reaction
5
6 7 8
9
a b c d
[38]
Percentage data show loading of active species (wt%) when available; data in parentheses indicate mol% used. Average of yields. Data in parentheses indicate range of yields. Conversion. Range of enantiomeric excess.
Table 3 Catalyst recycling in IL-based multiphase solvent systems. Entry 1
a b c d e f g
Reaction
Catalysta
Conditions
Number of cycles
Yield (%)b
RhCl3 (0.1)
15, TPPTS, 50 atm (H2/CO = 1:1), 85 °C, 5 h
35
81 (34.9–96.6)c
2
Rh(acac)(CO)2 (0.1)
16, [bmim][Tf2N], Tf2NH, MeOH, 20 bar, 80 °C, 2h
17
3
[Rh(cod)2] [BF4] (1)
17*, [bmim][BF4], MeOH, 1.5 atm, 25 °C, 2.5 h
8
4
[Mo(O) (O2)2(H2O)n]
[omim][PF6], dmpz, H2O, 60 °C, 4 h
5
RuCl3 (0.1)
6
Pd(OAc)2 (0.3)
d
[43]
97 (98–92) 10–30e 88–51f 99.75 (100–98) 91–84g
[44]
10
95 (91–99)
[46]
35 atm, [Oct3NMe]Cl, H2O/isooctane (1:1), 150 °C, 16 h
6
93 (60–100)
[47]
18, DTBPMB, MeOH, 22 bar (CO/C2H4/ Ar = 2:2:1), 80 °C, 20 min
15
98.5 (100–97.5)
[48]
[45]
Percentage data show loading of active species (wt%) when available; data in parentheses indicate mol% used. Average of yields. Data in parentheses indicate range of yields. Run 5 = 96.5, run 16 = 96.6, run 23 = 95.4, run 32 = 84.7%. Conversions. Aldehyde selectivities. Dimethylacetal selectivities. Range of enantiomeric excess.
monophasic but it becomes biphasic at room temperature. Another possibility called thermoregulated (temperature-controlled) phase-transfer catalysis (TRPTC) was developed by Jin and coworkers [32,33]. In a biphasic aqueous–organic TRPTC system at high temperatures above the cloud point, the catalyst is transferred from the aqueous into the organic phase thus allowing a homogeneously catalyzed process to take place. At lower temperatures it returns into the aqueous phase and the latter can be recycled.
The method, for example, allows the hydroformylation of water-immiscible alkenes. A prime example is the Ruhrchemie–R hône-Poulenc technology for the hydroformylation of C2–C5 alkenes operating with a Rh complex with sulfonated triarylphosphane as ligand [34]. For further information about this and related reactions, interested readers are directed to book chapters [35]. [RuCl2(PTA)4] (PTA = 1,3,5-triaza-7-phosphaadamantane) was found by Frost and Lee to be effective in nitrile hydration in a
Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65
biphasic mixture of water and tert-butyl alcohol [36]. High but significantly changing conversions were observed (Table 2, entry 1) with increasing amounts of Ru leaching into the organic phase (run 1 = 2.9 ppm, run 2 = 24.5 ppm, run 7 = 77.2 ppm). Although the monophasic aqueous reaction was much faster and gave constant conversions (99%), yields were significantly lower (average yield = 53%, range of yields = 18–67%). Paganelli et al. successfully applied (L)-cysteine and (S)captopril (10) as thioligands and Rh(acac)(CO)2 and [Ir(cod)Cl]2 in selective hydrogenation of a,b-unsaturated carbonyl compounds [37]. In recycling studies, the Ir–(L)-cysteine system worked satisfactorily in the hydrogenation of 2-cyclohexen-1-one (entry 2). The Rh complex, in turn, performed better with ligand 10 in the selective transformation of trans-cinnamaldehyde either in water–toluene–ethylene glycol (entry 3) or in water–ethylene glycol (entry 4).
Leitner and co-workers used a so-called inverted supercritical CO2 (scCO2)–aqueous solvent system for Rh-catalyzed hydrogenations [38]. This allows the separation of the aqueous phase containing the products and the reuse of the catalyst dissolved in scCO2 without depressurizing the autoclave. Methyl 2acetamidoacrylate was tested to show increasing and then stable activities in a five-run study induced by catalyst [Rh(cod)2][BF4] and phosphane ligand 11 (entry 5). Catalyst [Rh(cod)2][BARF] {BARF = tetrakis-[3,5-bis(trifluoromethyl)phenyl]borate} in combination with the chiral ligand (R)-Cl-MeO-BIPHEP (12⁄) afforded full conversions but moderate enantioselectivities (entry 6). Neither Rh nor P leaching was observed. Finally, quantitative conversions and excellent enantioselectivities were measured in the presence of chiral ligand (R,S)-3-H2F6-Binaphos (13⁄) (entry 7). Loss of Rh and P in the first run is 1.4 ppm and 5.2 ppm, respectively; however, leaching was not measurable in further uses. These favorable characteristics are attributed to the use of the BARF ligand: it is not prone to hydrolysis, increases the solubility of the Rh complex in scCO2 and enhances catalyst stability.
The novel AuIII-NHC complex 14 existing as an equilibrium mixture [Eq. (1)] was shown to induce cyclization of c-alkynoic acids to enol-lactones in aqueous biphasic media [39]. Surprisingly, byproduct formation from alkyne hydration was not observed. Complete conversion and 100% selectivity were observed when used in 10 repeated reactions in the cycloisomerization of a 1,6diyne at room temperature when isomer 14b is the major catalytic species (entry 8).
7
Bönnemann and co-workers prepared a RhIII complex by reacting Rh(acac)(CO)2 and thermoregulated ligand Me(OCH2CH2)nPPh2 (molar mass = 918) and used it in hydroformylation in a TRPTC system [40]. It exhibited high activity in the biphasic hydroformylation of oct-1-ene, albeit with a gradual decrease in aldehyde yield in reuse (entry 9). This was attributed to the formation and subsequent agglomeration of colloidal Rh particles. 3.3. Ionic liquid-based systems Ionic liquids (ILs) used in catalysis are characteristically salts with a melting point typically <100 °C. It is a diverse family with rich possibilities for functionalization with a wide range of anion and cation combinations. This allows fine-tuning their physical and chemical properties. Catalysis in ionic liquids offers special advantages. Namely, reactions can be carried out either in monophasic systems or, more characteristically, in liquid–liquid biphasic conditions. Furthermore, ILs greatly simplify product isolation by simple decantation or extraction. In fact, the use of ILs combines catalysis taking place in the homogeneous phase with heterogeneous catalysis providing easy product isolation, catalyst recovery and reuse. In addition, they often increase catalyst stability and affect selectivity. For comprehensive treatments and references readers are advised to consult relevant literature affording additional useful information [41] including practical applications [42]. In Tables 3–5, examples for catalyst recycling of multiphase solvent systems based on ILs are collected. Guanidinium methanesulfonates with a polyether tag were used in Rh-catalyzed biphasic hydroformylation of higher olefins in the presence of TPPTS [tris(m-sulfonylphenyl)phosphane] [43]. At reaction temperature, it is a liquid–liquid biphasic system. Upon cooling, the IL phase solidifies allowing easy separation and catalyst recycling. When applied in a reuse study of the hydroformylation of oct-1-ene with RhCl3 and IL 15, it exhibited high levels of recyclability with small drops after run 29 (Table 3, entry 1). Rh (and P) leaching after runs 1, 7 and 16, respectively, are 0.06% (0.16%), 0.07% (0.11%) and 0.04% (0.13%).
Hydroformylation of oct-1-ene was studied by applying Rh (acac)(CO)2 and ligand 16 with a lysine tag in the mixed solvents [bmim][Tf2N] (bmim = 1-butyl-3-methylimidazolium) and methanol [44]. A recycling study of 17 runs was executed in the presence of Tf2NH, which protonates ligand and, therefore, decreases Rh leaching to 0.3–0.4%. In addition to isomeric aldehydes, large amounts of dimethyl acetals are formed (entry 2). Rh complexes [Rh(cod)2][BF4] and [Rh(cod)Cl]2 in combination with ligand 17⁄ were shown to provide high ee values in the asymmetric hydrogenation of methyl (Z)-2-acetamidocinnamate with the former showing higher stability [45]. The reaction was performed in [bmim][BF4]–methanol (entry 3) with Rh leaching of 0.1% in the first runs.
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Table 4 Catalyst recycling in hydroaminomethylation of alkenes.a.
a b c d e f g
Entry
Cycle
R
Rh (mol%)
Conv. (%)
Sel. (%)a
l/br
1b 2 3 4 5
1 2 3 4 5
C6H13 C8H17 C4H9 C10H21 C6H13
0.056 0.022 0.022 0.028 0.028
96.4 88.4 92.7 87.8 89.1
90.8 96.5 92.4 96.8 97.3
35.1 44.4 33.5 20.2 18.1
6c 7 8 9 10
1c 2d 3e 4e 5g
C6H13 C6H13 C6H13 C6H13 C6H13
0.13 0.13 0.13 0.13 0.13
94.3 94.1 90.1 91.2 89.2
94.9 93.4 87.7 78.9 77.3
28.1 44.8 68.5f 72.8f 78.4f
Selectivity of amine formation. 19/Rh = 4.5 in entries 1–5. 19/Rh = 4.0 in entries 6–10. 110 °C. 110 °C, 6 h. Calculation includes enamine intermediate. 110 °C, 4 h.
Table 5 Cycling studies with ILs in monophasic solvent systems. Catalysta
Conditions
Number of cycles
Yield (%)b
Ref.
1
20 (1)
(DHQ)2-PHAL, NMO, MeCN/H2O (4:1), rt, 0.5–2 h, under argon
5
97.2 (95–99) 70–67c
[50]
2
20 (1)
(DHQ)2-PHAL, NMO, MeCN/H2O (4:1), rt, 0.5–2 h, under argon
5
91.6 (85–95) 81–74c
[50]
3
21
205 °C, 8 h
7
[51]
4
[HO3S-C3mim] [HSO4] g
H2O, 170 °C, 5 h
6
100d 94.9 (95.3– 94.3)e 95.0 (95.7–94.3)f 51.5% (58–66)
[52]
5
[H3NCH2CH2NH3] [HSO4]2 (0.2)
70 °C, 2 h
7
96h
[53]
6
22 (1)
Oct2ImBr, DAB (2 mol%) 50 °C, 3 h
9
90 (89–92)
[54]
7
[{Rh(l-OSiMe3) (cod)}2] (0.001)
23, 120 °C, 1 h, under argon
10
96.5 (99–97)
[55]
8
PdII-24 (0.05)
[Bpy][BF4], Et3N, 100 °C, 1.5 h
5
91.5 (86–93)
[56]
Entry
a b c d e f g h
Reaction
Percentage data show loading of active species (wt%) when available; data in parentheses indicate mol% used. Average of yields. Data in parentheses indicate range of yields. Range of enantiomeric excess. Conversion. Yield of dibutyl terephthalate. Yield of ethylene glycol. 550 mg cellulose, 1 g IL. Major isomer = o,p0 (45% selectivity).
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Alkene epoxidation in an aqueous–IL biphasic system was developed by Montilla, Galindo and co-workers [46]. Catalyst [Mo(O)(O2)2(H2O)n] is applied in [omim][PF6] with aqueous H2O2 and 3,5-dimethylpyrazole (dmpz) an N-donor base additive, which inhibits hydrolysis and enhances activity. The results acquired in a 10-run study with dmpz added after each cycle show high, stable activity (entry 4). Upgrading biomass-derived levulinic acid to c-valerolactone via hydrogenation was accomplished in an aqueous–organic (isooctane)–IL {trioctlymethylammonium chloride, [Oct3NMe]Cl} multiphase system [47]. Quantitative conversion with full selectivity
could be achieved either with Ru/C or using RuCl3. Whereas the supported catalyst gave decreasing activities in recycling, RuCl3 after an initial yield of 60% afforded yields of 100% (entry 5). Since the catalyst is completely segregated into the IL phase, recycling was conducted by removing the lower product-containing aqueous phase and then adding the reactant dissolved in water. Brønsted acid ILs prepared by Riisager and coworkers have been applied as both promoters and reaction media in the methoxycarbonylation of ethylene to synthesize methyl propanoate [48]. Reactions were catalyzed by Pd(OAc)2 and phosphane ligand 1,2-bis(di-tert-butylphosphinomethyl)benzene (DTBPMB). The reaction system with the use of IL [1-(4-sulfonylbutyl)-3-methyli midazolium][pTsO] (18) reveals high durability in 15 cycles (entry 6). After runs 5, 10 and 15, the upper product layer was decanted, the solvent evaporated and the IL phase containing the catalyst was reused.
Table 6 Recycling of BOX complexes in organic transformations. Entry
Reaction
Catalysta II
*
Conditions
Number of cycles c
Yield (%)b
Ref. d
1
Cu -25 -CTC (10)
CH2Cl2, 50 °C, 44–19 h, under argon
10
97.4 (95–76) 92–64e
[58]
2
CuII-25*-CTC (10)
CH2Cl2, 20 °C, 6–24 hf
10
87 (92–76) 70–64g
[59]
3
CuII-26*-CTC (10)
CH2Cl2, 20 °C, 1.5–2 h,h under argon
14
84.3 (92–40)i 73–87j
[60]
CuII-26*-CTC (10)
CH2Cl2, 50 °C, 3–1.5 h, under argon
6k
[60]
5
Cu(OAc)2-27a* (5)
iPrOH, rt, 24 h, under argon
11
87.8 (84–92) 85–89g 87 (77–96)l 89–75g
6
Cu(OAc)2-27b* (5)
iPrOH, rt, 24 h, under argon
8
[62]
7
Cu(OAc)2-28* (5)
iPrOH, rt, 24 h, under argon
14
8
Cu(OAc)2-29* (5)
iPrOH, rt, 24 h, under argon
13
K2CO3, DMF/H2O (1:1), 70 °C, 1 h
12
82 (78–78)m 26–44g 70 (86–69)n 66–74g 63 (51–50)o 92–86g 96 (98.5–94)
R= Me 4
9
a b c d e f g h i j k l m n o
R=H
II
Pd -31 (0.4)
[61]
[62] [62] [64]
Percentage data show loading of active species (wt%) when available; data in parentheses indicate mol% used. Average of yields. Data in parentheses indicate range of yields. Runs 1, 2 = 44 h, run 3 = 30 h, run 4 = 20 h, runs 5–10 = 19 h. Run 7 = 96%. Range of enantiomeric excess of the major anti isomer. Run 1 = 12 h, runs 2–7 = 6 h; runs 8 (12 h) and 9 (24 h) were performed at 20 °C, run 10 = 6 h. Range of enantiomeric excess. Run 12 = 18 h, –10 °C, run 14 = 44 h, 30 °C. Yields changed from 73% to 92% and then dropped to 40% in run 14. Enantiomeric excess of the major endo isomer. Continued use of catalyst applied in entry 3. A yield of 83% in run 1 dropped to 77% in run 2 then increased to 89% (run 3), dropped again to 80% (run 5) and then increased to 90, 95 and 96% in the final runs. Values increased to 91% (run 4) then decreased. Values increased to 95% (run 2), decreased to 47% (run 8) then increased again to 89% (run 12). Values increased to 75% (run 7) then decreased.
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Vogt and co-workers conducted hydroaminomethylation of alkenes induced by [Rh(cod)2][BF4] in the presence of Sulfoxantphos ligand 19 and [pmim][BF4] (pmim = 1-propyl-3-methylimidazolium) under biphasic conditions [49]. Table 4 includes the results of two separate recycling studies. In entries 1–5, varied alkenes were reacted with changing metal loadings. The first three runs were performed by adding new alkene to the reaction mixture without removing the product. In contrast, the upper product layer was removed after every reaction in entries 6–10. In this case, because of shorter reactions and lower temperatures, the enamine intermediate could not be fully hydrogenated. Decreasing regioselectivities shown in the first five entries may be attributed to the formation of isomeric alkenes that remained in the IL phase. This is in accordance with the opposite trend in the second set of experiments allowing much slower isomerization of the alkene.
Additional examples of recycling studies performed in or induced by ILs carried out in monophasic reactions are collected in Tables 5 and 6. The third-generation dendritic osmium catalyst 20 was successfully applied in asymmetric dihydroxylations. Increasing, slightly fluctuating yields and high enantioselectivities induced by biscinchona alkaloid 1,4-bis(9-O-dihydroquininyl)phthalazine [(DHQ)2-PHAL] and N-methylmorpholine N-oxide (NMO) as reoxidant were reported in five-run studies (entries 1, 2) [50]. The residue after evaporation of the reaction mixture was dissolved in acetone and the catalyst and (DHQ)2-PHAL were precipitated by the addition of water.
depolymerization of cellulose to levulinic acid performed in a Teflon-lined SS autoclave [52]. Yields increased in recycling (entry 4). The aqueous solution of IL was extracted with MIBK, dried under vacuum, adjusted to 1 g (adding 5%) and then reused. A range of C1–C4 alkylammonium hydrosulfates were evaluated in the condensation of phenol with formaldehyde to form bisphenol isomers [53]. Of these acidic ILs, [H3NCH2CH2NH3][HSO4]2 afforded the best performance (entry 5).
Degradation (depolymerization) of natural rubber, a process of practical importance to produce high-value products, has been accomplished by applying Hoveyda–Grubbs second generation Ru complex 22 in IL Oct2ImBr in the presence of 1,4-diacetoxy-2butene (DAP) as chain transfer agent [54]. Recycling experiments showed that the catalytic system could be reused nine times in this metathetic degradation with stable activity (entry 6). Varied allyland vinyl-ammonium-based ILs were used by Maciejewski and coworkers in hydrosilylations catalyzed by Ru complexes [55]. The best result in the hydrosilylation of oct-1-ene with IL trimethylvinylammonium ethanesulfonate (23) is shown in entry 7.
The phosphane-ligated ionic palladium complex PdII-24, prepared by reacting the corresponding bisphosphanoimidazolium derivative with [Pd(MeCN)Cl2], was well-characterized by means of XRD and 31P NMR spectroscopy [56]. It works as a precatalyst in carbonylative Sonogashira coupling in IL [Bpy][BF4] (Bpy = N-butylpyridinium) with increasing yields in five runs (entry 8).
The Brønsted–Lewis acidic IL 21 was applied in the butanolysis of polyethylene terephthalate (PET) affording full conversion and high yields of dibutyl terephthalate and ethylene glycol (entry 3) [51]. [HO3S-C3mim][HSO4] [C3mim = 1-methyl-3-(3-sulfopropyl)i midazolium hydrogen sulfate] showed lower activity in 3.4. Recycling of soluble catalysts used in monophasic reactions
Scheme 1. Use of diazabisoxazolin and triazabisoxazolin CuII complexes in cyclopropanation.
Catalyst separation from monophasic systems, as illustrated in this section, can easily be made by precipitating the catalyst or removing the product by distillation and then reuse the catalyst. Extracting either the catalyst or the product can also be applied. An easy possibility when solid catalysts soluble at high temperatures precipitate at room temperature. However, some unique examples have also been collected here. BOX complexes, for example, can be precipitated at the end of reaction by forming charge transfer complexes, which are reused in subsequent runs. The use of novel self-assembled and self-supported coordination polymer materials links homogeneous and heterogeneous catalyses. When the reactant and reagent are added to the reaction mixture, the polymers break up and dissolve. Upon complete consumption of the educts the polymers reassemble and precipitate allowing
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Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65 Table 7 Nobel metal derivatives reused in varied organic transformations. Catalysta
Conditions
Number of cycles
Yield (%)b
Ref.
1
RhIII-32* (5)
H2O, 60 °C, 5–7 h
7
78 (58–88) 58–74c
[65]
2
RhIII-33 (0.5)
NaOH, H2O, 100 °C, 8 h
20
94 (98–83)d
[66]
3
Pt0-34a (0.5)
H2O, 30 °C, 6 h
8
>95e 55:45–60:40f
[67]
4
Pt0-34b (0.5)
H2O, 30 °C, 6 h
9
>95e 56:44–65:35f 76 (83–79)g
[67]
55.7 (71–56)h 2.3 104– 3.7 104 i 98.5 (100– 94.5)
[69]
Entry
a b c d e f g h i j k l m
Reaction
II
5
Pd -35 (1)
KOH, propan-2-ol/H2O (1:1), 70 °C, 1 h
10
6
RuII-36 (0.002)
70 atm (CO2/H2 = 1:1), DMF, 110 °C, 5–7 h
12
7
RuII-37 (0.1)
PPh3 (0.1 mol%), tBuOK (0.5 mol%), toluene/ tBuOH (9:1), rt, 24 h
25j
8
RhIII-38
30 bar (CO/H2 = 1:1), 50 °C, 120 h
40j
63 (68–60)e,k e
[68]
[70]
[71]
9
Rh(acac)(CO)2 (2 105)
1a, 20 bar (CO/H2 = 1:1), toluene, 120 °C, 72 h
5
100 93–90l
[72]
10
IrIII-39 (2 105)
20 bar He, 50 °C, 20 h
11
235 (208– 245)m
[73]
Percentage data show loading of active species (wt%) when available; data in parentheses indicate mol% used. Average of yields. Data in parentheses indicate range of yields. Enantiomeric excess of the trans isomer. Run 19 = 93%, run 20 = 83%. Conversion. Range of b-(E)/a ratios. Run 3 = 57%, run 7 = 88%, run 9 = 65%. Run 2 = 44%, run 3 = 67%, run 7 = 47%, run 9 = 63%. Range of TON values. Number of samplings. Run 1 = 28%, run 2 = 59%, run 3 = 68%. Selectivity for hydroformylation vs isomerization and hydrogenation. Pressure (bar).
Table 8 The reuse of metallopolymer NiII-40* in Michael additions.
a b c d e
Entry
R1
R2
R3
Ar
Number of cycles
Conv. (%)a
ee
1 2c 3 4 5 6d 7e
MeO
Me
H
Ph
EtO MeO EtO Me Me
Et Me Et Me Me
H H Me H H
Ph 4-BrC6H4 Ph Ph Ph
8 13 8 7 8 7 8
99 99 (98–95) 99 99 99 99 98.8 (99–97)
89.5 (92–76) 83 (85–76) 86 (92–61) 83.5 (93–56) 87.3 (92–69) 87.7 (91–75) 85 (91–58)
Average of conversions. Data in parentheses indicate range of conversions. Average of ee values. Data in parentheses indicate range of ee values. Room temperature. dr = 1.05:1. dr = 3.5:1.
b
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easy recovery and reuse. This dynamic self-supported catalysis has emerged as a new strategy for catalyst reuse. A recent review summarizes the advances of the field [57]. Results reported with the use of BOX complexes are collected in Table 6 and Scheme 1. Additional studies about the application of other catalysts in varied reactions are found in Tables 7–11. In an earlier study, Schulz and co-workers prepared and applied complex 25⁄ in Diels–Alder reactions [58]. Catalyst recovery was performed by adding 2,4,7-trinitrofluoren-9-one to the reaction mixture to form a brown charge transfer complex (CTC) precipitating with pentane. In further cycles, consequently, this charge transfer complex (25⁄-CTC) was reused. Yields are lower at the end of the recycling study because of shorter reactions (Table 6, entry 1;
only the major endo isomer is shown). Complex 25⁄-CTC was also used in ene reactions [59] with similar catalyst performance (entry 2). Note, however, that both reaction time and temperature were changed. Complex 26⁄ (more precisely, the corresponding CTC complex) was also reused in the Diels–Alder reaction of Nacyloxazolidinones with cyclopentadiene [60]. Yields with the methyl-substituted dienophile changed significantly in 14 cycles and dropped to 40% in the last run when the reaction was performed at 30 °C (entry 3). The catalyst was then further tested with the unsubstituted compound affording satisfactory stability and enantioselectivity (entry 4).
29⁄ exhibited similar behavior that is they afforded widely changing activities and enantioselectivities (entries 6–8) [62].
Cu(TfO)2 complexes of the other two diazabisoxazolin ligands [Cu(TfO)2-27c⁄ and Cu(TfO)2-27d⁄] and that of triazabisoxazolin 30⁄ were applied in cyclopropanation carried out in CH2Cl2 [63]. These work as discussed above with activities varying significantly, whereas changes in selectivities are minimal (Scheme 1). Pd-BOX complex PdII-31 applied in Suzuki reaction shows high activity and stability in coupling reactions performed in DMF–water solution (entry 9) [64]. Recycling is made by extracting the product with hexane and then charging the catalyst phase with substrates and base.
Table 9 Hydroformylation of oct-1-ene with Rh(acac)(CO)2 and ligand 41.
The García group has developed a number of multitopic oxazoline-based chiral ligands. These form coordination polymers of chain and layered structures in noncoordinating solvents when reacted with CuII salts. Bis(oxazoline) complex 27a⁄ formed with Cu(OAc)2 was applied in the asymmetric Henry reaction [61]. High activities changed during reuse (entry 5). Complexes 27b⁄, 28⁄ and
a
Cycle
Time (h)
Conversion (%)
l/br
Isomers (%)
Rh leach (%)
P leach (%)
1 2 3 4 5 6a 7 8
18 18 19 19 25 17 18 20
90 73 75 60 63 77 61 68
43.3 42.3 13.9 7.6 3.9 34.8 40.4 22.7
5.7 4.5 39.5 50.9 78.6 14.0 6.1 25.5
0.6 0.8 7.4 3.5 1.3 2.1 1.2
20 15 18 11 11
Ligand 41 is added.
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Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65 Table 10 Reuse studies induced with various other catalyst preparations. Entry
Reaction
1
Catalysta
Conditions
Number of cycles
Yield (%)b
Ref.
KI (1.88), K2CO3 (0.45)
25 bar, 120 °C, 1 h
12
84.5 (87–82.5)
[76]
MeOH, 60 °C, 30 min
12
86 (89–83.5)
[76]
2
3
4-poly (5)
heptane, 80 °C, 12 h
12
90
[27]
4
4-poly (5)
heptane, 90 °C, 12 h
8
87
[27]
5
42 (1.5)
30% H2O2, MeCN, 60 °C, 1 h
98 (99–97) 99.5 (100–98)
[77]
6
saccharin + DMAP
25 °C, 2 h
10 42a 42b 10
>99
[79]
7
saccharin + DMAP
25 °C, 2–4 h
10
>99
[79]
8
AgI (2.5), Cs2CO3
DMF, H2O, 25 °C, 16 h
CuI (0.1)
43 (1 mol%), H2O, 25 °C, 3 h
80 78 80 81 85 90 89 91
[80]
9
R = H, X = none 55 R = 4-F, X = none 24 R = MeO, X = none 26 R = H, X = (CH2)2 32c R = H, X = CH2O 30 R = 3-OH, X = none 21 R = 2-Cl, X = none 20c 10
[81]
10
CuII-DC/ Br-poly
[PEGMA500]/[EBPA]/[HCc]/[CQ]/[TEA] = 100/2/1/0.5/1, heptane/EtOH (4:3), white LED (9.6 W); 25 °C, 22 h
5
97 (96.5–97.4)d
[82]
11
CuBr2/45
[MMA]/[EBPA]/[CuBr2]/[PIL]/[AIBN] = 200:1:1:4:0.8; 70 °C, 6 h
10
62 (62.5–67.8)d,e 98 (100–95)f
[83]
R = Me, iPr
a b c d e f
Percentage data show loading of active species (wt%) when available; data in parentheses indicate mol% used. Average of yields. Data in parentheses indicate range of yields. 50 °C, 24 h. Conversion. Run 2 = 51.8%, run 5 = 69.1%, run 10 = 67.8%. Recycling efficiency: percentage of residual catalyst in solution.
Candeias, Afonso and co-workers have prepared and applied homochiral dirhodiumII complexes in asymmetric intramolecular C–H insertion of a-diazo acetamides in aqueous solution [65]. Water-soluble complex RhIII-32⁄ could be recycled after product extraction with diethyl ether. Increasing yields, changing stereoselectivities (cis/trans 1:0.3–1:0.6) and moderate enantioselectivities were measured (Table 7, entry 1). Binuclear complex RhIII-33 soluble in water is an active catalyst in the aerobic oxidation of alcohols [66]. In a recycling study, 1-(4-methoxyphenyl)ethanol was transformed to the corresponding ketone in aqueous solution. The catalyst exhibited high, slowly decreasing activities with a significant drop in the last run (entry 2).
Two water-soluble NHC Pt0 complexes (Pt0-34a, Pt0-34b) have been applied in hydrosilylation of alkynes in aqueous solution [67]. Recycling studies were performed until conversions of >95%
were achieved (entries 3, 4). Activities changing in a wide range were found with the Pd NHC complex Pd(bmim-y)Br2(pyridine) (PdII-35) (bmim-y = 1-butyl-3-methylimidazol-2-ylidene) applied in Suzuki coupling (entry 5) [68]. Pd particles formed under reaction conditions were shown by the Hg0 test not to be involved in the coupling process.
Ru pincer complex RuII-36 was used in N-formylation of amines with CO2 and H2 [69]. In the reaction of dimethylamine, at a catalyst loading of 9.3 105 mol%, a TON of 6 105 was achieved. Varied yields were found with dimethylammonium dimethylcarbamate to form dimethyl formamide (entry 6). Recycling was made by removing the product with vacuum distillation and recharging the residue. Another Ru catalyst, diphosphane/diamine complex
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Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65
catalyst. Under different conditions, a TON value of 5 106 was measured in a single run and the catalyst was active in continuous operation in about 3.5 months. The CO content of the product mixture is below the detection limit (less than 6 ppm), which makes this system suitable for delivering hydrogen to fuel cell applications.
Table 11 Recycling of catalyst 14.9% 46*-4G3 in Michael additions.
Cycle
R1
R2
Yield (%)
ee
1 2 3 4 5 6 7
Et Pent Pr Me Me Me Me
Ph Ph Ph Ph 4-MeOC6H4 2-furyl 4-BrC6H4
99 99 99 99 98 97 98
99 99 99 99 96 93 95
syn/anti ratios = 97:3–90:10.
RuII-37, was applied in the asymmetric hydrogenation of a-tetralone [70]. It was used in a small-scale continuous nanofiltration study, with co-feeding the solution of catalyst, substrate (S/C = 9400:1) and PPh3 as well as the base into the reactor filled with catalyst solution. At 18 h, when the conversion at the reactor outlet and that in the retentate decreased a few percents, the substrate feed was cut into half (entry 7). A cumulative TON of 4750 corresponding to a TON of 250 indicates that the catalyst was reused in 15 cycles. The system performance is the equivalent of 60 recycles.
(Diphenylphosphinoethyl)dimethylethoxysilane and silanolterminated polydimethyl-siloxane were used in the synthesis of low-molecular weight ligand 38. This was then mixed with Rh (acac)(CO)2 to produce homogeneous complex RhIII-38 with a P/Rh ratio of 3 after washing out nonbound Rh with toluene [71]. A continuous hydroformylation of oct-1-ene in toluene (30:70 v/v, feed rate = 13 mL h1) was performed to have a stabilized conversion of 60%, an aldehyde selectivity of 97%, a l/br ratio of 2.5 and a cumulative TON 1.29 104 (entry 8). The stabilized Rh loss in the permeate after 40 h on stream was 1–2 ppm with a cumulative loss of 11%. Controlled tandem hydroformylation and isomerization of fatty substrates including undec-10-enonitrile were induced by Rh(acac)(CO)2 and Ir(acac)(cod) with the former exhibiting much better stability [72]. Linear aldehydes were formed with high selectivity (l/br ratio = 99:1) and the catalyst showed high levels of recyclability (entry 9). Catalyst recycling was carried out reusing the residue after vacuum distillation. Both catalysts gave high cumulative TON values [Rh(acac)(CO)2 = 2.5 105 (5 runs), Ir(acac)(cod) = 1.1 105 (3 runs)]. The homogeneous Ir complex of 1,10-phenanthroline-4,7-diol (39) works as a highly efficient catalyst in the generation of hydrogen from formic acid under pressure [73]. Catalyst IrIII-39 delivered a mixture of H2 and CO2 affording a cumulative TON of 3.85 104 when reused in 11 runs (entry 10). It precipitates after reaction upon cooling to 20 °C that is it can be recycled as a heterogeneous
Chiral NiII-40⁄ is a dynamic, self-supported catalyst with Evans NiII complex existing as a polymer in its resting state [74]. When used in enantioselective Michael additions, it dissolves in the reaction medium and disaggregates to release the active complex bearing only a single diamine ligand. The catalyst is insensitive to air and moisture and displays high activity and enantioselectivity. It is also robust when reused up to 13 cycles (Table 8). It can be easily recovered by precipitating with the addition of diethyl ether.
Kluwer, Reek et al. applied a reversible catalyst adsorption strategy with non-covalent interactions. Hydroformylation of oct1-ene was carried out using the Rh(acac)(CO)2 complex and ligand 41 [75]. The catalyst system provided the corresponding aldehydes with high productivity (TON = 6 104, 5 104 mol% Rh) and a selectivity of 93.8% in a batch process. When performed in a flow reactor, the system after reaction was cooled to room temperature and the reaction mixture was pumped through a column filled with end-capped silica-alumina scavenging the RhIII complex and ligand through specific non-covalent adsorption mode. This is reversible and, therefore, when the column is flushed with THF in a reverse flow the catalyst desorbs and can be used again. Small Rh leaching and a significant loss of the ligand resulted in large drops in linear/branched ratios in an eight-run recycling study (Table 9). In run 5 the ratio is close to that of the ligand-free system. Addition of ligand in run 6 restores both activity and selectivity.
Cheng, Zhang and co-workers used the aqueous solution of a simple binary salt mixture in the synthesis of dimethyl carbonate [76]. First ethylene oxide was reacted with CO2 in the presence of KI and K2CO3 (Table 10, entry 1) and then the product mixture
Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65
was further treated with methanol to perform transesterification of intermediate ethylene carbonate (entry 2). The salt mixture was isolated and reused after evaporating the liquid phase. Functionalized polymer 4 (catalyst 4-poly) discussed earlier as a thermoresponsive catalyst applied in a heptane–ethanol mixture (see Section 3.1) has also been used in heptane to induce alcohol acetylation and cyclopropane ring opening with concomitant acetylation [27]. In these reactions products were extracted with acetonitrile and then the heptane phase with dissolved catalyst was reused in the next cycle. High average yields were reported (entries 3, 4) with metal leaching of 0.015 and 0.005%, respectively. Zhou, Kühn, and coworkers synthesized octamolibdate-based solids (42) and used them in alkene epoxidation as ‘‘reactioncontrolled phase-transfer catalysts” [77]. The term was introduced by Xi et al. [78]. The catalyst systems are soluble under reaction conditions, that is, they are transferred from the solid to the liquid phase and then self-separate at the end of reaction and can be reused after washing with water. The two catalysts showed similar performance in the epoxidation of cis-cyclooctene (entry 5).
The salt of saccharin and 4-N,N-dimethylaminopyridine (DMAP) as catalyst has been used in the acylation of alcohols with acid anhydrides [79]. The salt precipitates upon addition of hexane to the reaction mixture. Varied alcohols and phenols undergo clean transformation under base- and solvent-free conditions with repeated uses of the catalyst. The best examples are shown in entries 6 and 7. In an interesting study, Hong and coworkers have performed the carboxylation of terminal acetylenes with carbon dioxide exhaust gas [80]. CO2 generated by a burning candle was captured in ethanolamine and then reacted with AgI and Cs2CO3 in aqueous DMF. The reaction solution could be recycled with maintained activity. Selected examples are summarized in entry 8. Astruc and coworkers have fabricated amphiphilic dendrimer 43 bearing 27 triethylene glycol (TEG) termini and nine intradendritic triazole groups (G) [81]. The resulting catalyst after treatment with CuSO4 was used in Huisgen-type alkyneazide cycloaddition (CuAAC) ‘‘click” reaction in water induced by CuI ions formed under reaction conditions. The catalyst system is highly stable and efficient in 10 reuses without drop in activities (entry 9). The TEG termini make the dendrimer water-soluble allowing product extraction and reuse of the aqueous phase after drying. Moreover, a reaction performed with 1 ppm CuI gave a TON of 5.1 105 and a TOF of 2.12 104 h1 (a yield of 50% in 24 h).
A recycling study of an atom-transfer radical polymerization (ATRP) process induced by a CuII complex was accomplished by Zhang, Cheng and coworkers [82]. The hybrid catalyst complex (HCc) was fabricated by carrying out the polymerization of poly (ethylene glycol) methyl ether methacrylate (PEGMA500),
15
2-bromophenylacetate (EBPA) as ATRP initiator, N,Ndibutyldithiocarbamate copperII [Cu(DC)2], a random copolymer of octadecyl acrylate (OA) and MA-Ln [2-(bis(pyridin-2-ylmethyl) amino)ethyl acrylate] [POA-ran-P(MALn), 44] and (2,4,6trimethylbenzoyl) diphenylphosphane oxide (TPO). The resulting complex CuII-DC/Br-poly (HCc) formed upon irradiation with a mercury lamp for 10 h was dissolved with heptane and used to catalyze the polymerization of the above mixture. The involvement of copper catalyst DC–Cu–Br/CuBr/CuBr2 = 0.6:0.33:0.07, a ternary complex with POA-ran-P(MA-Ln) was speculated. HCc induced polymerization upon irradiation with visible light exhibiting high stability (entry 10). Adding water to the reaction mixture allowed easy product separation. In a subsequent study they developed a catalyst system and used it again in ATRP [83]. It is a thermoregulated random copolymer polyionic liquid (PIL) polyelectrolyte with side-chains bearing both PEG-based ILs and ATRP ligands (45). It was used with CuBr2 in the polymerization of methyl methacrylate (MMA) affording moderate, varying conversions and high catalyst recycling efficiency (entry 11). Catalyst separation and recycling, in this case, were achieved by decreasing polymerization temperature to room temperature. CuI and CuII complexes in a reversible, dynamic equilibrium play a pivotal role in the mechanism. A recent review addressing the progress in separation and recycling of transition metal catalysts in ATRP is highly advised to interested readers [84].
Proline-derived Jørgensen–Hayashi organocatalyst (S)-a,a-dip henylprolinol trimethylsilyl ether (46⁄) was grafted to azidoterminated phosphorus dendrimers and the resulting catalysts were applied in asymmetric Michael addition of aldehydes and nitroalkenes [85]. The third-generation dendrimer (14.9% 46⁄4G3) displayed high stability in seven consecutive reactions with varied educts (Table 11). The catalyst was recovered by precipitation with pentane after evaporating the solvent.
Bourne, George and co-workers reported a process combining a monophasic reaction with biphasic separation. The organic substrate is mixed with fluorous solvent 3-ethoxy-2-(trifluorome thyl)dodecafluorohexane (HFE) and a porphyrin-based catalyst modified with fluorous ponytail fluorous-47 [86]. The reaction studied was photo-oxidation with singlet oxygen of citronellol (48) to form the corresponding isomeric peroxides 49a and 49b in scCO2 [Eq. (2)]. In this process with continuous recycling of the fluorous phase containing the catalyst, conversions decreased slowly in a 20-h test reaction (Fig. 1) and a TON of 2.7 104 was calculated. This, however, is an apparent value since about 12% of the catalyst remained in the product phase for each cycle.
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Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65
70
Conversion (%)
60 50 40 30 20 10 0 1
3
5
7
9
11
13
15
17
19
21
23
25
Cycle number
yields also increased from 27 to 40% (3 h) and 60 to 66% (20 h). A combined Rh loss of 24 ppm was measured in six cycles. The Wang group tested Rh nanoparticles in a number of reactions. Hydrogen reduction of RhCl3 in the presence of PEG4000 gave nanoparticles of 1.7 ± 0.4 nm, which were applied in a toluene–heptane mixture at 120 °C in hydroformylation of oct-1ene [89]. The lower PEG phase containing Rh particles was reused in 21 runs giving high, randomly changing yields (entry 2). Particle size did not change, which was attributed by the authors to the pseudocrown ether interactions between PEG4000 and Rh particles. Stabilized Rh nanoparticles of 1.3 nm were also prepared in IL 50 [90]. This system reveals high stability giving full conversions in six runs with high aldehyde yields (entry 3). This is despite an increase in particle size to 2.4 nm and a Rh loss of 0.3%.
Fig. 1. Continuous recycling of the fluorous phase containing catalyst fluorous-47 in the singlet oxidation of citronellol (2 104 mol% catalyst, flow rate = 0.1 mL min1, full reaction time = 20 h; see Eq. (2). Reprinted (adapted) from Ref. [86], Copyright (2017), with permission from The Royal Society of Chemistry.
3.5. Stabilized soluble metal nanoparticles in reuse studies The primary motive driving the use of varied solvent systems is, naturally, efficient catalyst recovery and reuse. In most cases, the recycling performance of mixed biphasic thermoregulated solvents consisting of an organic solvent in combination with water, PEG, polymers or another organic solvent proved to be quite satisfactory. Catalyst reuse in other organic–aqueous or IL solvents was accomplished by recharging the reaction mixture with fresh educts after extracting or decanting the product. In the first part of this subsection, examples of recycling of organo-soluble nanoparticles are discussed. In Table 12, results with respect to hydroformylation and hydrogenations are collected. In these cases, particles applied in thermoregulated biphasic solvent systems transfer from one phase to the other at reaction temperature. After complete reaction, they return to the original phase, which is separated and then reused in the next cycle. Reactions performed in other solvent systems are listed in Table 13. A recent review about the use of homogeneous recyclable metal nanoparticles is advised [87]. Behr and co-workers studied the hydroformylation of dodec-1ene by using Rh nanoparticles of 3 nm prepared by hydrogenating Rh(acac)(CO)2 dissolved in DMF [88]. Reactions were carried out in a mixed DMF–decane solution finding slightly increasing conversions by recycling the DMF phase (Table 12, entry 1). Aldehyde
Rh nanoparticles (2.4 ± 0.3 nm) were also generated from RhCl3 treated under hydrogen pressure (40 bar, 70 °C, 2 h) in the presence of phosphane ligand Ph2P(CH2CH2O)16Me in a butanol–water mixture (1:1) [91]. This system worked efficiently in the hydrogenation of alkenes [92]. Product yields in the hydrogenation of cyclohexene were found to be 100% in a seven-run study (entry 4). Particle aggregation was not detected and only a slight increase in the mean diameter from 2.6 to 3.2 nm was measured. A Rh leaching of 1.1 wt% in run 1 decreased significantly in further cycles (cumulative Rh loss in six runs = 1.67 wt%). Additional successful applications of Rh and Pd metal nanoparticles in varied hydrogenations have also been reported. Decreasing yields were found in the selective hydrogenation of cyclooctadiene to cyclooctene in the presence of PEG-based IL 51 [93], which may be due to an increase in particle size from 1.7 ± 0.2 nm to 2.5 ± 0.4 nm (entry 5). Alkyne semihydrogenation was promoted with Ru particles prepared by hydrogen treatment of RuCl3 in PEG2000 [94]. Catalyst recycling was carried out in a toluene–heptane biphasic system (entry 6). A small increase in particle size from 1.6 ± 0.4 to 1.8 ± 0.4 nm was detected after 10 cycles and a significant metal leaching of 1.7 wt% per cycle were measured. Elemental mercury completely stopped the reaction testifying to the heterogeneous nature of the reaction.
Hou and co-workers fabricated Pd particles by reducing Pd (OAc)2 with NaBH4 in the presence of IL 52 in CH2Cl2 [95]. According to data acquired by NMR spectroscopy TPPTS anions surround Pd particles and IL cations form an outer layer thereby providing stabilization. After complete phase separation at 0 °C the IL phase is recycled. The stabilized nanoparticles gave 100% yield in the hydrogenation of styrene under mild conditions without particle aggregation (entry 7).
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Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65 Table 12 Recycling of soluble metal nanoparticles in thermoregulated systems. Entry
Reaction
1
Catalysta
Conditions
Number of cycles
Yield (%)b
Ref.
Rh0 (0.1)
30 bar, DMF/decane (1:1), 110 °C, 3 h or 20 h
6
62 (59–66)c,d 93 (92–95)c,e
[88]
Rh0 (0.1)
60 bar, PEG4000, toluene/heptane (3:1), 120 °C, 3h 50, 50 bar, cyclohexane, 120 °C, 4 h
21
93.5 (99–88)f
[89]
R= C10H21 2 3
a b c d e f g
R = C6H13
0
R = C6H13
Rh (0.25) 0
6
c
100 99.5 (100–99) 100
[90]
4
Rh (0.1)
Ph2P(CH2CH2O)16Me, 10 bar, butanol/H2O (1:1), 60 °C, 1 h
7
[92]
5
Rh0 (0.05)
51, 10 bar, toluene/heptane (4:1), 60 °C, 6 h
11
95 (99–92)c 84.5 (89–82)
[93]
6
Rh0 (0.1)
11
92 (94–91)
[94]
7
Pd0 (0.1)
20 bar, PEG2000, toluene/heptane (3:1), 100 °C, 1h 52, 1 atm, EtOAc, 45 °C, 3 h
10
100
[95]
8
3.47% Pd0-53PIB (1)
54-PIB, K2CO3, THF/DMF/Polywax (4:4:3), 90 °C, 12 h, under nitrogen
5
89
[96]
9
Au0-EOEOVE (2)
KOH, H2O, 27 °Cg
6
>99
[97]
Percentage data show metal loading (wt%) when available; data in parentheses indicate mol% used. Average of yields. Data in parentheses indicate range of yields. Conversions. Three reactions in 3 h. Three reactions in 20 h. Yields decreased from 99% to 88% in run seven and then increased to 94%. Reaction time not reported.
Table 13 Recycling soluble metal nanoparticles in other solvent systems. Entry
a b c d
Reaction
Catalysta 0
Conditions
Number of cycles
Yield (%)b
Ref.
1
Rh (1)
55a, 40 bar, [bdmi][Tf2N], 75 °C, 3 h
10
95.6 (72–100)c
[98]
2 3
Rh0 (1) 16.7% Pt0-56 (0.065)
55b, 40 bar, [bdmi][Tf2N], 75 °C, 3 h 1.1 bar, MeOH/heptane (1:5), 20 °C
10 14
90.1 (32–100)c 100c
[98] [99]
4
Pd0 (1)
57, 3 bar, glycerol, 100 °C, 3 h
10
94 (95–92)d
[100]
5
27.7% Pd0-(SL) (0.003)
K2CO3, H2O, 60 °C, 6 h
8
97 (98–96)
[101]
Percentage data show metal loading (wt%) when available; data in parentheses indicate mol% used. Average of yields. Data in parentheses indicate range of yields. Conversions. Run 4 = 96%, run 5 = 97%, run 9 = 91%, run 10 = 92%.
A polyisobutylene-bound ortho-metallated Pd catalyst system developed by Bergbreiter and co-workers (3.74% Pd0-53-PIB) was applied in heptane–DMF in Heck and Suzuki couplings [96]. Average yields of 89–96% in five runs were observed requiring, however, the increase in reaction times from 6 h to 12 h. The reason is a loss of about 7% Pd to the reaction mixture. This problem was addressed by performing the reaction with added polyethylene oligomer (Polywax) and PIB-bound phosphane 54 to afford high, stable yields (entry 8). In this case, the hot THF–DMF–Polywax monophasic mixture was perturbed by adding water. Cooling results in the separation of a Polywax phase containing both PIBbound components. A minimal Pd leaching of 31 ppm (0.12%) was measured. Pd particles of about 50 nm are formed during reaction and effectively recycled. Au nanoclusters were prepared in the presence of a star polymer consisting of 2-(2-ethoxy)ethoxyethyl vinyl ether (EOEOVE) in water and used in the aerobic oxidation
of benzyl alcohol with high efficiency and stability (entry 9) [97]. The catalyst was recovered by decantation at 60 °C.
Additional reactions, performed in other solvent systems including organic–aqueous or IL solvents are collected in Table 13. Catalyst reuse is accomplished by recharging the reaction mixture with fresh educts after extracting or decanting the product. A mixture of Rh(allyl)3 and phosphane-functionalized ILs was treated with hydrogen in [bdmi][Tf2N] (bdmi = 1-butyl-2,3dimethylimidazolium) to form stabilized Rh particles with mean particle diameters of 2.3 ± 0.5 nm (in 55a) and 1.7 ± 0.5 nm
Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65
(in 55b) [98]. The formed solutions were used in the hydrogenation of toluene to find induction periods in the first few cycles and increasing conversions (entries 1, 2). This is caused by incomplete reduction of the precursor allowing the formation of nanoparticles during reaction. In addition, a small decrease in particle size was also observed in the case of the catalyst made in IL 55a. Catalyst recycling was performed by extracting the product with pentane and drying the catalyst phase after each use.
Platinum particles of 3 nm were fabricated by reducing H2[PtCl6] with NaBH4 in aqueous solution in the presence of a polymer with dendritic core–multishell architectures [99]. It is composed of a hydrophilic core (hPG3000 = hyperbranched polyglycerol) and a hydrophilic outer shell of polyethylene glycol with an inner hydrophobic shell of alkyl chains in between connected through carboxamide bonds (56). Catalyst 16.7% Pt0-56 containing stabilized particles was used in the hydrogenation of methyl crotonate in a methanol–heptane two-phase system (entry 3) to have full conversions but significantly decreasing TOF values. Pt leaching into the product phase in the first, second and fifth run, respectively, was determined to be 0.22, 0.38 and 0.16 ppm. After removing the heptane phase, the methanol solution with the catalyst showing no sign of particle aggregation was recycled. A cumulative TON of 2.2 104 was measured in 14 cycles.
Gómez and coworkers have generated Pd nanoparticles by treating Pd(cod)Cl2 in glycerol in the presence of N-benzylated PTA-based ligand 57 (PTA = 1,3,5-triazaphosphaadamantane) as stabilizer under hydrogen (3 bar, 60 °C) [100]. The black colloidal solution was used directly in the hydrogenation of (E)-4phenylbut-3-en-2-one in 10 runs with randomly changing yields and reused after product extraction with CH2Cl2 (entry 4). Interestingly, the particle size of 3.2 ± 0.8 nm decreased to 1.7 ± 0.6 nm during reuse. Monodispersed Pd nanoparticles were generated by reducing PdCl2 with the use of the extract of herbal tea Stachys lavandulifolia (SL) serving as reductant, capping agent and stabilizer [101]. Water-soluble spherical Pd particles (5–7 nm) were used as a homogeneous catalyst [27.7% Pd0-(SL)] in aqueous solution for Suzuki coupling. The solid formed by adding ethyl acetate to the reaction mixture was separated by filtration and was used in subsequent runs in a recycling study (entry 5). Pd leaching was below the detection limit of 0.1 ppm. An amphiphilic hyperbranched copolymer bearing a hydrophilic hyperbranched polyethylenimine (HPEI) core and a hydrophobic shell formed by linear palmitamide (C16) (58a) were loaded with Au nanoparticles prepared by reducing HAuCl4 with NaBH4 (catalyst Au0-58a-HPEI) [102]. The polymer-stabilized organosoluble particles were applied in the reduction of 4-nitrophenol. Catalyst stabilities in recycling studies were characterized by measuring the time necessary to achieve full conversion. The best result is an average reaction time of 3 min in 17 runs (range of time = 2–7 min; Fig. 2, shaded bars). However, significantly longer times of 34 min and 60 min were required in the 18th and 19th
cycles. The average particle diameter increased from 3.2 nm to 15 nm during recycling. These studies were extended to the synthesis and use of catalysts with dendritic shells [103]. Another sample with particles of 3.0 ± 0.5 nm and a generation 2 dendritic shell (catalyst Au0-58b-HPEI/dendr) exhibited the best performance applied with a lower Au loading (Fig. 2, open bars). In the first nine runs full conversions were accomplished in 3–50 min. Again, significant further increases in reaction times were necessary in subsequent cycles (run 10 = 125 min, run 11 = 290 min). More importantly, however, an unprecedented cumulative TON of 2.3 104 could be achieved.
4. Recycling of solid catalysts applied in heterogeneous systems Heterogeneous catalysis has been used overwhelmingly in the chemical industry [104]. Suggested additional readings are general treatments [105], and reviews and books about other, more specific topics discussing the use of ILs [106], nanoparticles [107], functional resins [108], and supported organocatalysts [109]. A multi-author book about asymmetric reactions over varied support materials with chiral active species may be of special interest [110]. Readers’ attention is also directed to an early review [111] and a book about dendrimers [112], in particular, to chapters about homogeneous enantioselective (Chapter 9) and heterogeneous catalysts (Chapter 11). An important chapter summarizes
350
Time for full conversion (min)
18
300 250 200 150 100 50 0 1
3
5
7
9
11
13
15
17
19
Cycle numbers Fig. 2. Reduction of 4-nitrophenol induced by organo-soluble Au nanoparticle catalysts. Shaded bars: Au0-58a-HPEI (8.3 mol%), H2O/CH2Cl2 (10:2.5), 32 °C; open bars: Au0-58b-HPEI/dendr (0.21 mol%), H2O/CHCl3 (10:2), 32 °C. Reprinted (adapted) from Refs. [102,103], Copyright (2017), with permissions from Wiley and The Royal Society of Chemistry, respectively.
Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65
industrial applications [113]. Three books focusing on the fine chemical industry are also advised [114]. Varied possibilities are available to separate and then reuse solid catalysts from simple filtration through other methods requiring more sophisticated techniques such as ultracentrifugation and membrane separation. In a few rare cases—monolith, mesh, membrane, thin film samples, dendrimers—the catalyst can be very simply removed (see examples in the following subsections). Magnetic catalyst separation has recently become a popular and widely studied research area (Section 4.5). Note, that examples for catalyst recovery accomplished by precipitation including soluble polymers were treated separately (Sections 3.1 and 3.5). Organization in this part of the manuscript is made according to the type of support materials.
19
sample delivered diaryl sulfides in high yields by the coupling of halobenzenes and thiophenols with high activity and stability (entry 5) [121]. Similar performances were reported for catalysts with an immobilized Rh complex (RhIII-62a-MCM-41) used in the hydrothiolation of alkynes (entry 6) [122] and sample 2.98% RhIII-62bMCM-41 inducing the hydrophosphanylation of alkynes with Ph2P(O)H (entry 7) [123]. The latter reaction was also accomplished with 4% RhIII-62a-MCM-41 (entry 8) [124]. Similar results in a C–S coupling with a higher Rh loading are shown in entry 9 [125].
4.1. Catalysts systems based on siliceous support materials 4.1.1. The use of supports with ordered structure Porous solids with ordered structure—silica-based mesoporous materials (MCM, SBA, and others) synthesized using surfactantdirected self-assembly, zeolites (microporous crystalline aluminosilicates), and recent discoveries including periodic mesoporous organosilicas (PMOs)—have attracted great attention. One of their most useful application possibilities is catalysis. Important features are high surface area, uniform pore size distribution, easy functionalization and high stability to generate readily accessible, covalently bound active sites. Suggested readings provide rich information about ordered mesoporous materials [115] and industrial application of zeolites [116]. Results achieved with ordered mesoporous silica materials are collected in Tables 14–16, whereas Table 17 shows recycling studies with zeolites and related crystalline materials. The Veisi group fabricated Pd catalysts starting with aminopropyl-functionalized SBA-15 and studied them in Suzuki coupling. The best sample was synthesized by anchoring melamine–pyridine to the amino group followed by treatment with PdCl2 to get immobilized complex 3.1% PdII-59-SBA-15 [117]. The catalyst characterized by means of appropriate physical methods (UV–Vis, FT-IR, TEM, SEM, XRD, EDX, XPS and ICP-AES) was tested in Suzuiki coupling showing high stability in 8 runs (Table 14, entry 1). Pd leaching was not detected by either hot filtration tests or a poisoning experiment with mercaptopropyl-functionalized silica. Immobilized catalyst complex PdII-60-SiO2, synthesized with the use of azido-functionalized mesoporous silica followed by click reaction with ethynylpyridine and, finally, a treatment with PdCl2 [118], gave consistently high yields in a 10-run recycling study (entry 2).
Complexes anchored to MCM-41 developed by Cai et al. were successfully applied in C–C couplings. Sample PdII-61a-MCM-41 with a Pd loading of 2.87 wt% was efficiently recycled in the coupling of 4-chlorobenzoyl chloride with BiPh3 in N-methyl-2pyrrolidone (NMP) as solvent and Bu3N as the base (entry 3) [119]. Catalyst 2.86% CuI-61b-MCM-41, in contrast, has somewhat increased stability in homocoupling (entry 4) [120]. A NiII-MCM-41
In a recent contribution the Cai group reported the synthesis of an immobilized CuI bipyridine complex by reacting MCM-41 with bis-[3-(triethoxysilyl)propylaminomethyl]-2,20 -bipyridine and then with CuI (6.7% CuI-63-MCM-41) [126]. It was successfully applied in a less frequently studied coupling reaction of arylboronic acids with diaryl diselenides to form organic selenides with satisfactory stability in 10 runs (entry 10). Hot filtration test and ICP-AES analysis indicated the heterogeneous nature of the reaction. In a synthesis process aminopropylated MCM-41 was reacted with pyridine-2-carboxaldehyde and then the product was loaded with CuI [127]. The resulting catalyst bearing a Schiff base–pyridine bidentate CuI complex (2.6% CuI-64-MCM-41) exhibited similar features in Sonogashira reaction (entry 11). Related sample 2.86% CuI-61b-MCM-41 with an immobilized bidentate CuI complex exhibited satisfactory stability in Buchwald N-arylation (entry 12) [128].
Data for miscellaneous other reactions induced by catalysts with ordered mesoporous silica supports are found in Table 15 and shown in Fig. 3. Catalyst 4% Pd0-SBA-15 prepared by depositing Pd particles into the nanocages of aminopropylated SBA-16 was also applied in aerobic alcohol oxidations with an impressive stability in 12 runs (entry 1) [129]. The mixture of trioctylphosphane, octadecene and Pd(OAc)2 was heated at 260 °C for 30 min to generate Pd nanoparticles. These were immobilized by mixing with SBA-15 in toluene (rt, 24 h) and then calcination gave PdO/SBA-15 [130]. It exhibited high, stable activity in seven cycles (entry 2). No further reaction was detected in a hot filtration experiment. Karimi and co-workers have fabricated a periodic mesoporous organosilica (PMO) material by hydrolysis–polycondensation of 1, 3-bis(3-trimethoxysilylpropyl)imidazolium chloride (65) and tetramethoxysilane in the presence of Pluronic P123 as structure directing agent followed by adsorbing Pd(OAc)2 (1.06% PdII-65PMO) [131]. Decrease in BET surface area and pore volume testified to the incorporation of IL into the silica wall. Intensity loss of diffraction peaks indicated that Pd species were located in the
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Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65
Table 14 Reactions forming C–C and C–heteroatom bonds. Catalysta
Conditions
Number of cycles
Yield (%)b
Ref.
1
3.1% PdII-59-SBA-15 (0.3)
K2CO3, EtOH/H2O (2:1), 50 °C, 0.8 h, in air
8
98
[117]
2 3
PdII-60-SiO2 (2) 2.87% PdII-61a-MCM-41 (1.5)
K2CO3, EtOH, 80 °C, 6 h Bu3N, NMP, 80 °C, 3 hc
10 10
>99 93.5 (95–92)
[118] [119]
4
2.86% CuI-61b-MCM-41 (1)
piperidine, CH2Cl2, 25 °C, 4 h
10
94.5 (96–93)
[120]
5
NiII-MCM-41 (5)
NaOH, DMF, 70 °C, 1 h
5
94
[121]
6
4% RhIII-62a-MCM-41 (0.3)
EtOH, 40 °C, 24 h, under argon
10
94 (95–93)
[122]
7
2.98% RhIII-62b-MCM-41 (3)
toluene, 80 °C, 4 h, under argon
10
88.5 (90–87)
[123]
8 9 10
4% RhIII-62a-MCM-41 (2) 4% RhIII-62a-MCM-41 (5) 6.7% CuI-63-MCM-41 (5)
toluene, 70 °C, 3 h, under argon CH2Cl2, Et3N, 30 °C, 24 h, under argon DMSO/H2O (2:1), 110 °C, 14 h, in air
10 10 10
90 (91–89) 93 (95–92) 94.6 (96–93)
[124] [125] [126]
11
2.6% CuI-64-MCM-41 (5)
PPh3, K2CO3, DMF, 100 °C, 6 h, under argon
10
95 (96–94)
[127]
12
2.86% CuI-61b-MCM-41 (4)
K3PO4, toluene, 110 °C, 24 h
10
96.5 (98–95)
[128]
Entry
Reaction
(PhSe)2 + MeO
a b c
B(OH)2
SePh
MeO
Percentage data show metal loading (wt%) when available; data in parentheses indicate mol% used. Average of yields. Data in parentheses indicate range of yields. 1 h in run 1.
pores of PMO [132]. Increasing reaction time was needed to achieve close to complete conversions in the oxidation of 1phenylethanol with better than 99% selectivity (entry 3). The negative result of the hot filtration test and Pd content below the detection limit indicate that leached Pd is not involved in the reaction. Poisoning tests provided additional proof.
Jeong and co-workers immobilized human carbonic anhydrase (HCA) onto gold nanoparticles deposited either to aminopropylated or mercaptopropylated SBA-15 (H2N-SBA-15, HS-SBA-15). The thiol-functionalized sample exhibited somewhat better stability than the amino-functionalized catalyst in the hydrolysis of 4nitrophenyl acetate in 20 cycles [133] (entries 4 and 5; 4% and 7% loss of the original activity, respectively). In contrast, a significant loss of 21% was measured for the Au-free catalyst. The higher stability of the Au-loaded samples was attributed to the strong electrostatic interaction of the enzyme with Au particles. The catalysts were separated by centrifugation and were washed with the buffer before reuse. Much larger differences in stability were detected with the use of Pseudomonas fluorescens lipase immobilized on unmodified and aminopropylated SBA-15 [134]. Significant drops in activity are seen after the 10th run when PF/SBA-15 is reused in the solvent-
free esterification with ethanol of sunflower oil (biodiesel production; Fig. 3, open bars). PF/H2N-SBA-15, in turn, showed still satisfactory activity in the 20th cycle (shaded bars), albeit with a higher enzyme loading (502 ± 24 vs. 256 ± 15 mg g1 support). Interestingly, PF/SBA-15 proved to be more active in the hydrolysis of tributyltin. SBA-15 with the immobilized CoII Schiff base complex with low Co loading (1.8% CoII-66-SBA-15) developed by Luque and coworkers is a useful catalyst of high activities in organic transformations. High activity was reported in 12 runs when used in solvent-free Pechmann condensation of resorcinol and ethyl acetoacetate (entry 6) with small decreases in yields [135]. It showed lower stability in the formation of benzimidazoles through oxidative condensation (data not shown) [136].
The Cai group conducted tandem reactions to form 2-aminosubstituted benzo[d]thiazoles using catalyst 2.7% CuII-61c-MCM41 with immobilized CuSO4 and found satisfactory stability (entry 7) [137]. A 2% loss of Cu was measured by ICP analysis. In a recent
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Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65 Table 15 Use of ordered mesoporous silicas in other reactions. Catalysta
Conditions
Number of cycles
Yield (%)b
Ref.
1
4% Pd0-SBA-16 (1)
1 bar, K2CO3, H2O, 50 °C, 4 h
12
99 (100–99)
[129]
2
PdO/SBA-15 (0.66)
7
95
[130]
3
1.06% PdII-65-PMO (0.25)
K2CO3, toluene, 70 °C, 4 h, aerobic conditions K2CO3, trifluoromethyltoluene, 95 °C, 3–5.5 h
10
99
[132]
4
(HCA)Au0-HS-SBA-15
MeCN, Tris buffer (pH 6.4), 25 °C, 15 min
20
97 (100–96)c
[133]
5
(HCA)Au0-H2N-SBA15 1.8% CoII-66-SBA-15 (0.1)
MeCN, Tris buffer (pH 6.4), 25 °C, 15 min
20
96 (99–92)c
[133]
110 °C, 3 h
12
96.5 (98–92)
[135]
Entry
6
a b c d e f
Reaction
7
2.7% CuII-61c-MCM41 (0.25)
Et3N, DMSO, 80 °C, 8 h
10
93.5 (95–92)
[137]
8
3% CuI-67-MCM-41 (10)
Cs2CO3, DMF, 60 °C, 12 h, under argon
10
94 (95–93)
[138]
9
10% CuII-SBA-15 (8)
CH2Cl2, rt, 16 h
5
91.5 (85–97)
[139]
10
H5[PW10V2O40]/ MCM-48 (0.25)
100 °C, 1 h
10
69 (70–68)
[140]
11
HPW-mim/KCC-1
10 bar, 90 °C, 1.5 h
10
97.5 (98–97)
[141]
12
9.7% Pd0-H2N-PEI/ KCC-1 (5)
cyclohexane, 4 Å MS, 130 °C, 20 h
8
95.5 (94–97)
[142]
13
2.76% Au0-68*-SBA15 (2)
CD2Cl2, rt, 0.5 h
11
92.3 (94–90) 95–97d
[143]
14
2.7% RuII-69*-SiO2 (1)
H2O, 40 °C, 3 h
10
99 (>99–98)e 98–97d
[144]
15
4.7% Ru0-MCM-41
[dami][CF3(CF2)3SO3], 40 bar (CO2/ H2 = 1:1), H2O, 80 °C, 5 h
10
17,640 (17,760– 17,570)f
[145]
Percentage data show loading (wt%) of active species when available; data in parentheses indicate mol% used. Average of yields. Data in parentheses indicate range of yields. Relative activities. Range of enantiomeric excess. Conversion. TON values.
contribution they reported the fabrication of a tridentate CuI complex immobilized on MCM-41 (3% CuI-67-MCM-41) [138]. The synthesis included functionalizing the support with [3-(2-aminoethylamino)-propyl]trimethoxysilane followed by a reaction with pyridine-2-carbaldehyde and CuI. It is a leach-free, highly-active and stable catalyst in a cascade reaction to form quinazolines (entry 8). Cu(NO3)2 loaded onto SBA-15 in an aqueous solution at pH 10.5 gave CuII-SBA-15 with a copper loading of 10% [139]. It induced azide–alkyne cycloaddition affording 1,4-disubstituted triazoles in a regioselective manner with increasing activity upon recycling (entry 9).
Tayebee and Maleki loaded Keggin-type heteropoly acid H5[PW10V2O40] on MCM-48 and recycled the catalyst in the synthesis of 4-nitrophenyl-14H-dibenzo[a,j]xanthene. Constant yields
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Table 16 Recycling of catalysts RhIII-70 in the hydroformylation of oct-1-ene.
Entry
Run
Catalyst
Pressure (bar)
Conversion (%)
Linear
Cumulative TON
TOFa
1 2 3 4 5 6 7 8 9 10 11 12 13
1 2 3 4 5 6 7 8 9 10c 15 16 17
1.81% RhIII-70a-SBA-15
50 50 50 50 50 50 50 20 20 50 20 20 20
90.5 95.8 98.7 86.5 74.4 34.3b 92.6 94.2 91.6 86.9 90.0 78.8 93.4
90 91 88 88 88 88 91 92 94 91 85 76 54
671 1306 1932 2511 3030 3221 3885 4597 14,748 25,589 67,716 68,225 68,640
195
Rh(acac)(CO)2 Rh(acac)(CO)2
50 20
33.9 53.5
93b 95b
0.106% RhIII-70b-glass
50 50 50 50 20 20
79.4 84.0 85.1 44.2 95.0 94.3
74.2 82.6 87.1 92.3 91.3 91.2
14 15 16 17 18 19 20 21 a b c d
1 2d 3d 8d 9 10
95 126 96 306 995 836 226 67 119 120 149
519 1113 1789 4624 5318 5979
76 62 102 15 115
Values were measured at 20–40% conversion. Measured after 2 h. Reaction run in neat oct-1-ene. In toluene/isopropyl alcohol (12:1) solvent mixture.
Table 17 Studies using other ordered porous materials. Catalysta
Conditions
Number of cycles
Yield (%)b
Ref.
1
2.1% PdCl2(phen)/NaY (0.7)
Et2N, DMAc, 20 bar, 130 °C, 1 h
16
98.5 (97–99)
[148]
2
4.06% CoII-SAPO-5 (1.7)
CHP,c air bubbling, DMF, 90 °C, 5 h
8
82 (83–81)
[149]
3
Pd0-zeolite/poly (0.1)d
air, 250 °C
11
91 (92.2–89.7)
[150]
AO/peroxidase-MCF (1)
20
96 (100–95)
[151]
10
97.5 (99–96)
[152]
Entry
5
1.6% Ru -71-MCF (5)
H2O2, citrate buffer (pH 4), 30 °C, 1–10 min CH2Cl2, 50 °C
6
1.6% RuII-71-MCF (5)
CH2Cl2, 50 °C
10
94.5 (99–91)
[152]
7
1.6% RuII-71-MCF
CH2Cl2, 50 °C, 40 mine
10
97 (98–98)
[152]
4
a b c d e
Reaction
Decolorization of RBBR
II
Percentage data show loading (wt%) of active species when available; data in parentheses indicate mol% used. Average of yields. Data in parentheses indicate range of yields. Cumyl hydroperoxide (0.2 mM) as initiator. 0.1 g. 36 mol catalyst, 2.2 mM substrate/run, 0.15 M solution, flow rate: 5 mL min1.
in short reactions could be maintained in a solventless system (entry 10) [140]. A catalyst based on KCC-1, a new class of fibrous silica nanospheres, was fabricated and tested by Sadeghzadeh [141]. KCC-1 functionalized with chloropropyl moieties was reacted with 1-methylimidazole (mim) followed by ion-exchange with H3[PW12O40] to get catalyst HPW-mim/KCC-1. It showed high
activity and stability in the solventless synthesis of ethylene carbonate in 10 reuses (entry 11). A low level of agglomeration of catalyst particles and a loss of 3.6% HPW were detected in the recovered sample. In another study, KCC-1 was modified with 3-glycidoxypropyltrimethoxysilane and then reacted with polyethyleneimine forming a highly-branched network of amino
Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65
functionalities. In the final step, highly dispersed Pd nanoparticles (1.8–3.5 nm) were generated to give sample 9.7% Pd0-H2N-PEI/ KCC-1 [142]. Decarbonylation of 2-naphthaldehyde underwent smoothly with high product yields (entry 12). The Somorjai group has reported the one-step synthesis of immobilized gold complexes by reacting SBA-15 with molecular gold complexes [(phosphane)AuBF4] [143]. Reduction with ascorbic acid resulted in the formation of Au0 species located inside the channels. Regioselective asymmetric lactonization of allenoic acid was induced with catalyst 2.76% Au0-68⁄-SBA-15 synthesized with phosphane 68⁄ affording the highest activity and enantioselectivity. Lactonization could be repeated with similar efficiency in recycling (entry 13). The loss of the molecular complex (37%, measured by FT-IR) and 3.2% Au leaching (ICP-OES) were found after 11 cycles.
A unique material, a chrysanthemum-like mesoporous silica was functionalized with TsDPEN [N-(p-toluenesulfonyl)-1,2-diphe nylethylenediamine] and then treated with [RuCl2(C5Me5)] to generate chiral active sites (catalyst 2.7% RuII-69⁄-SiO2). It afforded high, stable activities and enantioselectivities in transfer hydrogenation of acetophenone (entry 14) [144]. Catalysts composed of silica or MCM-41 loaded with ruthenium were used in combination with ILs to test their performance in the selective hydrogenation of CO2 to formic acid [145]. 4.7% Ru0-MCM-41 and IL [dami] [CF3(CF2)3SO3] [dami = 1,3-di(N,N-dimethylaminoethyl)-2-methyli midazolium] gave the best results in terms of TON and TOF values achieving a cumulative TON of 1.77 105 in 10 cycles (entry 15). After separating the aqueous phase the IL phase containing the catalyst was reused.
Hydroformylation of oct-1-ene was studied by Reek, Marras and co-workers using Nixanthphos-based Rh–diphosphane complexes
Conversion (%)
100 80 60 40 20 0 1
3
5
7
9
11
Run
13
15
17
19
Fig. 3. Ethanolysis of sunflower oil. Open bars: PF/SBA-15, shaded bars: PF/H2NSBA-15. Ethanol/oil ratio: 8:1, 30 °C, 7 h. Reprinted (adapted) from Ref. [134], Copyright (2017), with permission from Wiley.
23
anchored onto solid supports (70; Rh/ligand ratio = 1/15). 1.81% RhIII-70a-SBA-15 was made by grafting the corresponding trimethoxysilyl derivative onto SBA-15 then capping and, finally, reacting the formed support with Rh(acac)(CO)2 [146]. The catalyst exhibited high stability and gave high conversions in 15 runs (Table 16). In runs 16 and 17 (entries 12, 13) linear to branched ratios (l/br) decreased significantly. A remarkable cumulative TON of 6.9 104 could be achieved in 17 cycles. Surprisingly, the supported catalyst exhibited higher activity than the homogeneous complex (compare TOF values of entries 1 and 9 to those in entries 14 and 15, respectively). This was attributed to the formation of an inactive dinuclear complex when the homogeneous catalyst was used. Unfortunately, a second batch of the same catalyst gave inferior results with a drop to 67% of the linear isomer and increased isomerization. In a follow-up study the complex was immobilized on microscope cover slips by a drop by drop dilution method that is, slowly adding the functionalized ligand and the capping agent into refluxing toluene (sample 0.106% RhIII-70b-glass, Rh/ ligand = 1/15) [147]. The method assures a more homogeneous distribution than by immersing the glass into ligand solution and then capping. This catalyst, however, gave only a cumulative TON of 5979 under identical conditions in 10 runs (entries 16–21).
A few examples of recycling studies with other ordered porous materials are collected in Table 17. Encaging the palladium complex of 1,10-phenanthroline [PdCl2(phen)] into the supercage of NaY zeolite gave a catalyst, which is highly active and stable in aminocarbonylation [2.1% PdCl2(phen)/NaY] [148]. After separation by centrifugation, reactions could be repeated with consistently high yields in 16 runs with a low level of Pd leaching of 0.08% (Table 17, entry 1). Pd ions were shown to undergo reduction to Pd0 without forming particles. The complex adsorbed on NaY, Pd/C and Pd/Y exhibited inferior characteristics. SAPO-5, a crystalline silicoaluminophosphate of high silica content prepared by the dry-gel conversion, was treated with Co(OAc)2 to form 4.06% CoII-SAPO-5 through ion-exchange with a Si/Al ratio of 0.6 [149]. High activities were recorded in the epoxidation of various alkenes under air with satisfactory stability in recycling in the reaction of isopropenylbenzene (entry 2). A hybrid composite material with Pd particles (45.3 ± 9.4 nm) and natural zeolite embedded on an electrospun polymer nanofiber membrane was developed and studied for water treatment [150]. Specifically, zeolite microparticles of this multifunctional membrane are capable of adsorbing ammonia and then Pd can induce recovery of N2 by oxidation. With about 5% catalyst loss in 11 uses, high degrees of N2 recovery with slow decreases were recorded (entry 3). A recombinant dye-decolorizing peroxidase was immobilized on a mesocellular silica foam (MCF) and used to decolorize Remazol Brilliant Blue R (RBBR) an anthraquinone dye [151]. The foam was prepared by polymerization of TEOS in the presence of 1,3,5trimethylbenzene to form a mesoporous silica material with large pores (25 nm) and a cage-like structure interconnected through windows of about 10 nm. The peroxidase produced from Aspergillus oryzae was immobilized at pH 5 (catalyst AO/peroxidaseMCF). Complete decolorization took place in the first four cycles
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in a 20-run study in about 1 min. Increasing time, however, was needed in further cycles with slow decreases in activities. The whole recycling experiment took 50 min to complete (entry 4). Another MCF-based catalyst material with an anchored Ru complex was applied in metathesis reactions [152]. Immobilization was performed by reacting MCF partially capped with trimethylsilane with a triazoles-modified ligand followed by treatment with second-generation Hoveyda–Grubbs RuII complex. Supported catalyst 1.6% RuII-71-MCF showed god stability in recycling in RCM (entries 5,6). Even better results were found with diethyl diallyl malonate when the sample was used in a circulating flow reactor (entry 7). Ru leaching in the first four and the following eight runs were 8.4 ppm and 2.9 ppm, respectively. Significant decreases in activity were experienced, however, when the reaction was performed in toluene attributed to higher solubility of ethylene in toluene resulting in slower ethylene removal from the system.
4.1.2. Catalysts with amorphous silica support Oxidative fluoroalkylation of aromatics has been achieved applying an electrogenerated NiIII bipyridyl complex stabilized by silica nanoparticles [153]. In a reuse study, 2-phenylpyridine and perfluoroheptanoic acid reacted in the presence of NiIII(2bpy)xSiO2 (2bpy = 2,20 -bipyridine) affording high, stable yields (Table 18, entry 1). A ‘‘high internal phase emulsion” (HIPE) open-cell hybrid matrix was fabricated by the hydrolysis of TEOS by Backov and co-workers. The monolith thus prepared was grafted with (3-gly cidyloxypropyl)trimethoxysilane (glymo) followed by immobilizing the lipase Candida rugosa [154]. 19 runs could be conducted in the esterification of oleic acid with butan-1-ol with full conversion with drops in activity in the final runs to 98% and then 57% (entry 2). This was ascribed to the partial collapse of the monolith and the loss of enzyme. The catalyst without the stabilizing glymo moiety could only be reused in three cycles with a slower kinetics. The next entry is about oxidative sulfur removal. IL [Oct3MeN]2[W2O11] was incorporated into mesoporous amorphous silica made by polymerizing TEOS in the presence of the IL as template followed by calcination at 450 °C [155]. Desulfurization of dibenzothiophene in octane occurred with high efficiency in recycling (entry 3). 2.5% Pd0-72-Mn3O4/SiO2 was prepared by treating porous hollow silica nanospheres with a Mn3O4 layer on the interior surface with a solution of Na2[PdCl4] at pH 2.4 [156]. The method allowed the generation of Pd particles (2.0 ± 0.4 nm) exclusively on the interior cavity surface with the concomitant partial dissolution of Mn3O4. The catalyst is highly efficient in the hydrolytic oxidation of hydrosilanes to silanols (5 104 mol% Pd, TON = 1.98 105) with sufficient stability in recycling (entry 4) and it outperforms commercial 5% Pd/C (range of yields = 95–33%). Werner and Kohrt treated aminopropyl-functionalized silica with iodoethanol and isolated silica-immobilized ammonium salt 73-SiO2 [157]. It retained high activity in the formation of cyclic carbonates in 13 runs (entry 5). The yield with an analogous sample with polystyrene support dropped to 60% in the eighth run.
Hasaninejad and co-workers performed multicomponent reactions to form 4H-benzo[b]pyrans induced by DABCO immobilized on silica gel (11.7% DABCO-74 a-SiO2) [158]. Increasing reaction time was required to sustain high yields in a 15-run recycling study (entry 6). Acid-treated silica reacted first with (3iodopropyl)trimethoxysilane and then acetylacetonate and, finally, treated with Pt(OAc)2 in acetonitrile gave a black solid with immobilized Pd nanoparticles of 6–12 nm (0.43% Pd0-75-SiO2) [159]. The air- and moisture-insensitive catalyst afforded a cumulative TON of 1.34 104 in the Suzuki coupling of iodobenzene and phenylboronic acid (entry 7).
The Marciniec group developed and tested silica-anchored Rh and Ru complexes. Rh siloxide surface complex 2.23% Rh0-76SiO2 was studied in alkene hydrosilylation exhibiting high activity and excellent recycling properties (entry 8) [160]. Rh siloxide phosphane complexes 77-SiO2 showed similar futures in 10-run recycling (entries 9–11) [161]. RCM of diethyl diallyl malonate was induced by a mixture of Ru carbene siloxide complexes 78 and 79 with varying activities (entry 12) [162]. After 15 runs, 95% of the catalyst could be recovered. In a hot filtration test the reaction mixture after removing the catalyst at 20% conversion showed no further reaction.
Helmchen and Malakar have recently reported the first example of the preparation and use of immobilized iridium catalysts in allylic amination [163]. These were prepared by loading cationic Ir(p-allyl) complexes on silica modified with p-ethylphenylsulfonic acid. The loading process was ion-exchange with the trimethylammonium salt. Thorough recycling studies were performed with catalyst IrIII-80a⁄-SiO2 (dbcot = dibenzooctadiene). Characteristic examples are collected in Table 19. The effect of solvents (entries 1–3), catalyst loading (compare entry 1 and 4), and the substituent (compare entry 1 and 5) were examined. The use of the enantiomer of catalyst IrIII-80a⁄-SiO2 was also tested (compare entries 1 and 6). In most cases, stable activities found in many repeated uses dropped significantly toward the end of the studies attributed to catalyst loss. The quantity of the iridium salt remaining on the catalyst in entries 1, 2 and 3 is, respectively, 0.13, 0.64,
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Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65 Table 18 Varied reactions studied with catalysts supported on amorphous silica. Catalysta
Conditions
Number of cycles
Yield (%)b
Ref.
1
NiIII(2bpy)x-SiO2 (1)
MeCN, rt, 24 h, under argon
5
85
[153]
2 3
CR/glymo-SiO2(HIPE) [Oct3MeN]2[W2O11]/SiO2 (5)c
heptane, 37 °C, 24 h 30% H2O2, octane, 60 °C, 1 h
21 8
98 (100–57) 99.5 (98–100)d
[154] [155]
4
2.5% Pd0-72-Mn3O4/SiO2 (1) 73-SiO2 (2)
H2O, acetone, rt, 30 min
10
97 (98–95)e
[156]
1 bar, 90 °C, 6 h
13
97.5 (98–97)
[157]
Entry
Reaction
5
6
11.7% DABCO-74a-SiO2 (6)
EtOH, rt, 35–55 min
15
93.2 (96–90)
[158]
7
0.43% Pd0-75-SiO2 (1)
NaHCO3, reflux, 6–7 min
6
2254 (2350–2050)f
[159]
8
2.23% Rh0-76-SiO2 (0.01)
100 °C, 1 h, under argon
95 (97–87) 89 (88–91)g
[160]
9
1.28% Rh0-77a-SiO2 (0.1)
toluene 100 °C, 1 h, under argon
99 99
[161]
2.78% Rh0-77b-SiO2 (0.1)
toluene 100 °C, 1 h, under argon toluene 100 °C, 1 h, under argon
R = Bu 15 R = 20 SiMe(OSiMe3)2 R = C14H29 10 R = 10 (3,4-diMeO)Bn 10
99
[161]
10
70 (65–72)h
[161]
15
75.5 (78–73)
[162]
10
a b c d e f g h
R = Ph
11
3.29% Rh0-77c-SiO2 (0.1)
12
1.5% Ru0-78 + 79-SiO2 (5)
CH2Cl2, 40 °C, 12 h, under argon
Percentage data show loading (wt%) of active species when available; data in parentheses indicate mol% used. Average of yields. Data in parentheses indicate range of yields. 5 mg of catalyst. Sulfur removal efficiency. Conversion. TON values. Run 15 = 83%. Run 5 = 84%.
and 1.83 mol%. Note, that although catalyst 80a⁄ was used in these studies, when phenyl-substituted allyl carbonate was reacted (entries 1–4 and 6) it is transformed to IrIII-80b⁄-SiO2, which is the resting state recovered and reused.
Immobilized ILs are useful catalyst materials applied frequently in recycling studies. Here, results with the use of ILs as the active material loaded onto amorphous silica supports are collected (Tables 20 and 21). Immobilization may be achieved through non-covalent interaction or covalent attachment (supported ionic liquid-like phase catalysis, SILC, SILP or SILPC) [164]. A combina-
tion of these two approaches is the confinement of an ionic liquid onto a support modified with covalently-bound IL. Readers are advised to consult recent books about detailed discussions [165]. Note, however, that studies with the use of ILs immobilized on magnetic supports are included in Section 4.5. Bis(imidazolium)-tagged TEMPO derivatives (81) noncovalently immobilized on various supports have been applied in the oxidation of varied alcohols with [bis(acetoxy)iodo]benzene (BAIB) [166]. Both catalysts applied with 82 gave quantitative yields in 10 runs in the oxidation of 1-phenylethanol with 10 mol% loading. Steady performances could also be achieved with 1 mol% catalyst in the oxidation of varied alcohols (Table 20).
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Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65
Table 19 Reuse of catalyst IrIII-80a*-SiO2 in allylic aminations.
a b c d e f g
Entry
R
Runs
Catalyst (mol%)
Solvent
t (min)
Conv. (%)
Yield (%)
bra
ee
1 2 3 4 5 6f
Ph Ph Ph Ph Me Ph
33 43 24 11 23 30
2.4 2.4 2.4 0.42 2.32 2.32
THF toluene hexane THF THF THF
10–200b 10–180c 20 1.33–24d 15–90e 10–30g
100–67 100–69 n.r. n.r. n.r. n.r.
87–57 88–59 87–78 89–70 91–63 89–61
97–89 96–90 95–89 95–85 97–93 98–90
98–92 93–91 90–83 95–96 91–93 99–98
Branched isomer selectivity relative to linear. 10 min in the first 38 runs. 10 min in the first 21 runs. Hour. 15 min in the first 20 runs. The enantiomer of IrIII-80a*-SiO2 was used. 10 min in the first 25 runs.
Table 20 Oxidation of alcohols induced by immobilized TEMPO derivatives 81. Cycle
Reaction
81a + 82-SiO2
81b + 82-SiO2
Time (h)
Yield (%)
Time (h)
Yield (%)
1
0.5
>95
0.5
>95
2
0.5
>95
0.5
>95
3
0.5
>95
>95
4
0.5
>95
>95
5
5
>95
5
>95
6
3
>95
3
>95
7
0.5
92
0.5
92
8
0.5
>95
0.5
>95
9
0.5
>95
0.5
>95
10
0.5
>95
0.5
>95
11
0.5
92
12
0.5
92
Reaction conditions: 1 mol% 81, 25 wt% loading relative to 82, CH2Cl2, BAIB (1.1 equiv), rt.
Additional results are collected in Table 21. Wei and co-workers developed silica-supported multilayer imidazole-based IL brushes (Im-83). Im-83a-SiO2 proved to be a highly stable catalyst in 10 cycles in a nucleophilic substitution (entry 1) [167]. The brush with the PF 6 anion (Im-83b) was loaded with PdCl2 and then Pd nanoparticles with uniform distribution (mean diameter = 0.95 nm) were generated by reduction with NaBH4 (catalyst 2% Pd0-Im-83b-SiO2) [168]. Full conversion could be maintained in a 15-run study in the solventless hydrogenation of nitrobenzene with a slight increase in reaction time to achieve full conversion (entry 2). Whereas the average particle size increased to 1.52 nm, no sign of particle aggregation was detected by TEM analysis.
Hagiwara used mercaptopropyl-functionalized silica gel (HSSiO2) to load 10 wt% [bmim][BF4] and Pd(OAc)2 [169]. Both this sample and a catalyst with supported [bmim][PF6] gave full conversions in the hydrogenation of cyclohexene in 10 uses (entries 3, 4). The hydrogenation catalyst [RhCl(PPh3)3] co-entrapped with IL [bmim][PF6] in silica by the sol–gel method [170] revealed high
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Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65 Table 21 Recycling of ILs immobilized on amorphous silica. Catalysta
Conditions
Number of cycles
Yield (%)b
Ref.
1
Im-83a-SiO2 (0.4)
H2O, 80 °C, 5 h
10
99.3 (99.8–98.8)
[167]
2
2% Pd0-Im-83b-SiO2 (0.029)
1 bar, 30 °C, 8.5–9 h
15
100c
[168]
3
5.43% Pd0-HS-SiO2/[bmim] [BF4] (5)
1 atm, EtOH, rt, 45 mind
10
100c
[169]
4
5.23% Pd0-HS-SiO2/[bmim] [PF6] (5) 0.06% [RhCl(PPh3)3]-SiO2/ [bmim][PF6] (0.001) RuII-84* + 85-SiO2/[bmim] [BF4] (0.1)
1 atm, EtOH, rt, 80 min
10
100c
[169]
27 bar, CH2Cl2, 100 °C, 30 min
10
98c
[170]
KOH, iPrOH, 30 bar, 20 °C, 6 h
5
100 79–75e
[171]
0.02% RhIII-86*-SiO2
[emim][NTf2], scCO2 flow, 120 bar, 40 °C, 81 h
33f
1.43 105 99–70e
Entry
5
6
7
a b c d e f g
Reaction
g
[172]
Percentage data show Pd loading (wt%) when available; data in parentheses indicate mol% of metal used. Average of yields. Data in parentheses indicate range of yields. Conversion. Average time to reach full conversion. Range of enantiomeric excess. See text. Cumulative TON.
stability in the hydrogenation of styrene in 10 runs (entry 5). A threecomponent catalyst composed of chiral Ru complex RuCl2(PPh3)2 (S,S-DPEN) (RuII-84⁄), IL 85 and [bmim][BF4] immobilized on silica could be repeatedly used with high efficiency in the asymmetric hydrogenation of acetophenone (entry 6) [171]. Catalyst samples prepared with the use of other supports (MCM-48, MCM-41 and SBA-15) were less stable with a large drop of yield in the fourth cycle. Since Ru loss was not detected, decreasing activities were attributed to pore blocking. Amorphous silica with larger pores, in turn, is not affected. Moreover, a higher TON of 1000 could be obtained in comparison to that of the homogeneous complex (200).
Asymmetric hydrogenation of dimethyl itaconate has been studied in a continuous flow system over a SILP catalyst with immobilized Schrock–Osborn-type chiral phosphane–phosphora midite 1-naphthyl-QUINAPHOS Rh complex (0.02% RhIII-86⁄-SiO2) in the presence of IL [emim][NTf2] and scCO2 [172]. In an 81-h continuous operation with 33 samplings, a very high cumulative TON value and slowly decreasing enantioselectivities were measured (entry 7).
4.1.3. Catalysts with other silicon-based supports The group of Béland, Pagliaro and coworkers has developed a series of sol–gel entrapped Pd catalysts, which are now commercially available. In the fabrication of a catalyst with Pd particles first a sol is prepared via an alcohol-free hydrolysis–polycondensa tion of MeSi(OEt)3 and Si(OEt)4 followed by adding K2[PdCl4] and then hydrogenation [173]. The siloxane matrix 87 in the resulting catalyst (0.32% Pd0-87-SiliaCat) encapsulates Pd particles of 3.2 nm. An excellent performance in a seven-run reuse study in Suzuki coupling was found with a low leaching of Pd (0.43 ppm; Table 22, entry 1). Another SiliaCat preparation with higher loading (0.67% Pd0-87-SiliaCat) with Pd nanoparticles (mean particle size = 5.7 nm) showed high activity and stability in the hydrogenation of methyl 4-nitrobenzoate at room temperature with a minimal leaching of Pd and Si (1–2 ppm) (entry 2) [174].
Ervithayasuporn et al. fabricated a catalyst by depositing palladium onto polyhedral oligomeric silsesquioxane (POSS) modified with pmim. Palladium ions undergo in situ reduction to form Pd particles (3.0–1.5 nm). Catalyst 10.7% Pd0-pmim-POSS allowed the execution of 10 cycles without any decrease in activity in the coupling of 4-bromoacetophenone and phenylboronic acid (entry 3) [175]. The Yang group has developed a novel method for catalyst recycling by working in a Pickering emulsion catalytic system. This is a particle-stabilized emulsion–organic biphasic system composed of Pd nanoparticles (2–4 nm), silica modified with pH-sensitive triamine (MeO)3Si(CH2)3(NHCH2CH2)2NH2 and hydrophobic octyltrimethoxysilane [176]. The key features are the strong adsorption of catalyst particles on the surface of emulsion droplets
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Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65
and pH-triggered emulsion inversion. 0.46% Pd0-Oct/triN-SiO2 fabricated with a triamine/octylsilane ratio of 1.04 was recycled in the hydrogenation of styrene in ethyl acetate–water mixture at pH 3– 4. The separation of product and catalyst was achieved by adjusting the pH to 7–8 [Eq. (3)]. The upper organic phase is decanted and then the subsequent reaction is performed at pH 3–4. Ethylbenzene yields increased then changed significantly in 36 runs (entry 4). Products from previous cycles result in yields higher than 100%. Pd loss in cycle 1 was 0.1 ppm and then below 0.1 ppm in the last cycle. 0.55% Rh0-Oct/triN-SiO2 exhibited similar features in a 9-run test (entry 5).
same transformation [177]. Reactions, in this case, were performed at pH 7–8 and demulsifying was achieved by changing the pH to 3– 4. A yield of 83% in run 1 increased to 100% (run 10) and changed little afterward (entry 6). The rate of hydrogenation was found to be 5.14 times higher than that performed in a conventional biphasic system using isopropyl alcohol as demulsifier. In another approach, a large liquid–liquid–solid reaction interface was created between the organic phase and the Pickering emulsion phase with the nanoparticle catalyst by vigorous stirring [178]. At the end of the reaction, phase separation occurs by stopping the stirring without the need of adjusting the pH. Three catalysts have been
Catalyst 1% Pd0-Oct/triN-SiO2 was prepared using silica nanospheres with a triamine/octylsilane ratio of 0.39 and tested in the
tested in recycling in the hydrogenation of styrene. Significantly prolonged reaction times from 65 to 425 min were needed to
Table 22 Reuse studies with other catalysts of silicon-based supports. Catalysta
Conditions
Number of cycles
Yield (%)b
Ref.
1
0.32% Pd0-87-SiliaCat (0.1)
K2CO3, MeOH, reflux, 2h
7
99.4 (99.6–99.1)
[173]
2
0.67% Pd0-87-SiliaCat (0.5)
H2 balloon, MeOH, rt, 30 min
8
99.5 (99.7–98.9)
[174]
3
10.7% Pd0-pmim-POSS (0.8)
50 °C, K2CO3, H2O/EtOH (1:1), 1 h
10
100
[175]
4
0.46% Pd0-Oct/triN-SiO2 (0.5)
36
95 (77–108)
[176]
5
0.55% Rh0-Oct/triN-SiO2 (0.1)
9
80 (74–101)
[176]
6
1% Pd0-Oct/triN-SiO2 (2000)c
15
95 (83–100)
[177]
7
0.54% Pd0-Oct/SiO2 (8000)c
10
Pd0-Si/nw (0.3)
10
100d 94 (64–100) 97 (98–95)
[178]
8
1 atm, ethyl acetate/ H2O (2.6:3), pH 3–4, rt, 0.5–2 h 1 atm, ethyl acetate/ H2O (2.6:3), pH 3–4, rt, 2.6–4 h 3.5 bar, ethyl acetate/ H2O (1:1), 40 °C, 115 min 6 bar, H2O, 40 °C, 50– 140 min Et3N, TBAAc, butanol, 100 °C, 24 h
9 10 11 12
0.021% Pd0-PVPy/[bmim][PF6]/LC (1) 0.021% Pd0-PVPy/[bmim][PF6]/LC (1) 0.02% Pd0-HDA/[bmim][PF6]/LC (1) Pd0-glassA (5)
Et3N, 100 °C, 20 h Et3N, 100 °C, 4 h Et3N, 100 °C, 20 h Et3N, MeCN, 120 °C, 3 h
15 18 17 40
95.4 (100–43)e 96.3 (100–55)f 96.3 (100–21)g 98
[180] [180] [180] [181]
13
(PdCl2/4-bpy)10-quartz
EtOH, 1 bar, 35 °C
10
100d
[182]
14
K10 montmorillonite (20)h
toluene, rt, 6–300 mini
14
92 (99–15)
[183]
Entry
a
Reaction
[179]
Percentage data show Pd loading (wt%) when available; data in parentheses indicate mol% of metal used. Average of yields. Data in parentheses indicate range of yields. Substrate/catalyst ratio. d Conversion. e Runs 1–12 = 100%, run 13 = 95%, run 14 = 96%. f Runs 1–14 = 100%, run 15 = 98%, run 16 = 100%, run 17 = 89%. g Runs 1–14 = 100%, run 15 = 98%, run 16 = 58%. h 20 mg catalyst, 1 mM substrate. i Despite increasing reaction times from 6 min to 300 min, yield decreased to 15% in run 5. Activation after runs 5, 8 and 11, however, restored activity (yields of 98–99% in 6 min). b
c
Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65
maintain full conversion when a delayed condensation method was applied to fabricate a mesoporous support material with a core–shell structure (0.54% Pd0-88-triN-SiO2, data not shown). Whereas Pd losses were below 0.12 ppm, significant aggregation of Pd particles was detected. Better stability was found when Pd particles were generated inside the mesopores using silica with an octyl-functionalized shell (0.54% Pd0-Oct/SiO2) (entry 7).
29
washing the catalyst after each run (entry 14, last three runs). Calcination of the catalyst (400 °C, 1 h) resulted in the collapse of the interlayer structure with diminishing Brønsted acidity and the concomitant increase in Lewis acidity. This was taken as evidence that edge-surface Lewis acid sites are the probable active species inducing the reaction. 4.2. Use of oxide-based catalysts
In a recent report Yamada and co-workers described the fabrication of a hybrid catalyst composed of Pd nanoparticles (5–10 nm) and a silicon-nanowire-array (Pd-Si/nw) [179]. It showed extremely high activity (TON = 2 106, TOF = 4 104 h1) and excellent stability in 10 cycles in the Heck coupling (entry 8). Neither Pd leaching nor further reaction in hot filtration test was detected. The confined nanosized reaction spaces around the Pd particles are accounted for these excellent features. García and co-workers prepared Pd nanoparticles stabilized with poly(Nvinyl-2-pyrrolidone) (Pd-PVPy) or hexadecylamine (Pd-HDA) [180]. Laponite clay (LC, a synthetic crystalline layered Na/Li/Mg silicate) loaded with [bmim][PF6] served as the support material. Heck reactions proceeded smoothly with the resulting catalysts at 100 °C in solventless conditions. 0.021% Pd0-PVPy/[bmim] [PF6]/LC (mean particle diameter = 3.5 nm) gives high yields in 20-h runs with large drops in the last cycles (entry 9). Shortening the time to 4 h was found, however, to be beneficial with consistently high yields in 16 cycles with drops in the last two runs (entry 10). The catalyst with hexadecylamine as stabilizer exhibited similar features (entry 11). Hot filtration indicated that soluble species are not involved in catalysis. However, significant Pd losses up to 90% could be detected. Nishiwaki et al. prepared two glass-supported catalysts by treating test-tubes with a dilute Pd(OAc)2 solution in acetonitrile (3.3 102 mM) [181]. According to ICP analysis 75% of Pd was adsorbed on the glass surface. An XPS study revealed that PdII species were completely reduced at 120 °C in 3 h. In the case of Pd0-glassA 50 equiv of Et3N was used as reductant (120 °C, 12 h) resulting in the formation of large Pd aggregates. In the other case (Pd-glassB) Et3N was not applied and Pd nanoparticles were not observed. Surface OH groups were suggested to assist the reduction of Pd(OAc)2 physically adsorbed on the glass surface. Sample Pd-glassA proved to be highly stable in recycling (entry 12). The Pd content of the used catalyst decreased to 62%. No changes in catalytic activities were measured after using the catalyst kept in air in one month. Catalytic multilayer films were fabricated by immersing cleaned quartz slides alternately into aqueous PdCl2 and ethanolic 4,40 -bipyridine solutions [182]. Sample (PdCl2/4bpy)10-quartz with 10 bilayers created by this layer-by-layer assembly process has a Pd loading of 7.2 105 mM. The catalytic film has high activity and outstanding stability in the hydrogenation of styrene (entry 13). During hydrogenation PdII ions were slowly transformed to Pd0 forming small Pd clusters reaching the size of 5 nm of the catalyst recovered after the last cycle. Montmorillonite K10, a commercial smectite phyllosilicate was found to be highly active in cyanosilylation of ketones [183]. Activates, however, decreased significantly in recycling (entry 14). Repeated washing with dilute HCl solution, however, restored activities. Full restoration of activates could be obtained by
4.2.1. Oxides as catalyst support The photocatalytic degradation of dye pollutants was studied with TiO2 (anatase) doped with well-dispersed Au nanoparticles of 2–3 nm (2% Au-TiO2) [184]. XRD, XPS and FT-IR indicated strong interactions between Au particles and TiO2. Excellent performance was found in 11 repeated catalyst uses in the degradation of methyl orange (MO) (Table 23, entry 1). Zhang and Zhu fabricated composite particles of a TiO2 core with a porous SiO2 shell then dissolved them in CH2Cl2–dimethyl carbonate under sonication followed by casting. The process resulted in embedding SiO2/TiO2 particles into polycarbonate (polyCarb) [185]. The resulting porous membrane (SiO2/TiO2-polyCarb) has high porosity and efficient mass transfer properties. Photocatalytic degradation of sewage oil was chosen to test recyclability with satisfactory stability (entry 2). Moreover, the membrane could be successfully cleaned by alkaline solution to partially restore activity. Bimetallic supported AuPt catalysts fabricated by mixing a Au/ PVA sol with TiO2 support material followed by adding K2[PtCl4]/ PVA with hydrogen in aqueous solution (1% Au60Pt40-TiO2) [186]. It was tested in the oxidation of sorbose to 2-ketogulonic acid (a precursor for vitamin C) exhibiting high but decreasing activities. However, activities could be restored by a thorough washing of the recovered sample with distilled water (entry 3). A heterogeneous catalyst bearing trilacunary polyoxotungstate Na12[a-P2W15O56]24 H2O (P2W15) supported on c-alumina (P2W15-Al2O3) has been fabricated by Chen, Song and coworkers [187]. Major features are excellent stability, reactivity and selectivity in the oxidation of sulfides to sulfoxides under mild conditions (entry 4). A solution of Ti(OBu)4 in ethanol–water mixed with the solution of IL [bmim][FeCl4] in ethanol was kept overnight at room temperature then dried (60 °C and 100 °C). Then the resulting powder was calcined (250 °C, 3 h) to get a sample with [bmim][FeCl4] immobilized on amorphous TiO2 ([bmim][FeCl4]-TiO2) [188]. The catalyst was utilized in sulfur removal from a model diesel oil [dibenzothiophene (DBT) in octane] to transform DBT to the corresponding sulfone conducted in the presence of IL [omim][PF6] and acetonitrile (extraction-coupled oxidative desulfurization). The material showed excellent catalytic features when reused in 25 runs (entry 5). A similar study was performed with the use of catalyst HPW-NH2-Al2O3 fabricated by treating aminopropylfunctionalized c-alumina beads with heteropoly acid H3[PW12O40] [189]. The recycling performance of this sample with the immobilized highly-dispersed acidic component was somewhat lower (entry 6). A series of well-characterized Ru catalysts supported on TiO2 (164 m2 g1) were made by loading RuCl3 followed by reduction with hydrogen (300 °C, 2 h) [190]. Catalysts were tested in the hydrogenation of CO2 to formic acid. The utility of the sample with a Ru loading of 1 wt% (1% Ru0-TiO2, total metal surface area = 1.23 m2 g1 cat) in IL [dami][CF3(CF2)3SO3] and water was tested in a recycling study affording high TON values (entry 7). Both free formic acid and its salt with the dami anion of IL exist in equilibrium. Product formic acid was isolated with a nitrogen flow at 75–80 °C. Slight decreases of activities are mainly due to Rh loss and particle aggregation. Rhodium single-atom catalysts with different metal loadings were made by treating ZnO nanowires with RhCl3. The samples
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Table 23 Use of oxide-supported catalysts. Entry
Reaction
Catalysta
Conditions
Number of cycles
Yield (%)b
Ref.
1 2 3
Photocatalytic degradation of MO Photocatalytic degradation of sewage oil
2% Au0-TiO2 (1) SiO2/TiO2-polyCarb 1% Au60Pt40-TiO2 (0.4)
H2O, UV irradiation, 1 h H2O, 30 °C, UV irradiation, 3 h 2 bar, H2O, pH 7.5, 50 °C, 8 h
11 20 10
99 (100–98) 52 (55–50)c 80 (84–85)d
[184] [185] [186]
4
P2W15-Al2O3 (2.5)
methanol, 25 °C, 35 min
10
94.8 (95–94.5)
[187]
5
[bmim][FeCl4]TiO2
30% H2O2, [omim][PF6], octane, 60 °C, 1 he
25
100f
[188]
6 7
HPW-NH2-Al2O3 1% Ru0-TiO2 (1)h
MeCN, 60 °C, 2 h g [dami][CF3(CF2)3SO3], 40 bar (CO2/ H2 = 1:1), H2O, 80 °C, 5 h
10 10
[189] [190]
8
0.006% Rh0-ZnO/ nw (0.004) 1% Pt0-89-TiO2 (500)j
16 bar, (CO/H2 = 1:1), toluene, 100 °C, 12 h ethyl acetate/H2O (1:1), 30 bar, 50 °C, 300–360 min
5
98.6 (99.2–98.4)f 2.3 104 (2.33– 2.28 104)i 3745 (4000–3455)i 99.3 (100–99)k 93 (69–99)
0.5% Pd1TiO2 (0.01)
EtOH, 1 atm, 28 °C, 60 min
20
9
10
a b c d e f g h i j k
10
100k
[191] [178]
[192]
Percentage data show Pd loading (wt%) when available; data in parentheses indicate mol% of metal used. Average of yields. Data in parentheses indicate range of yields. Degradation efficiency. Run 7 = 60%, run 8, after washing = 85%. 5 mg catalyst, 5 mL model oil, H2O2/DBT = 4. Efficiency of sulfur removal. 60 mg catalyst, 40.6 mg DBT in 20 g octane, H2O2/DBT = 8. 1 mg Ru. TON values. Substrate/catalyst ratio. Conversion.
after in situ reduction (0.006% Rh0-ZnO/nw) proved to be highly active in hydroformylation to provide TON values (4 104), which are better than that of the Wilkinson complex (1.9 104) [191]. In a recycling study, hydroformylation of styrene yielded isomeric aldehydes with a cumulative TON of 1.87 104 in five reuses (entry 8). The lack of leaching and inactivity of the filtrate demonstrate the strong binding of Rh atoms to the support. The method involving a Pickering emulsion phase discussed earlier (Table 22, entry 6) was further extended to the use of a TiO2 nanoparticle system. It was made by loading Pt particles (2– 3 nm) to commercial TiO2 followed by hydrophobization with (MeO)3SiMe to ensure phase separation (1% Pt0-89-TiO2) [178]. In fact, this sample gave better results (entry 9) with a minimal Pt loss of 0.1 ppm detected in every cycle. An atomically dispersed stable palladium–titanium dioxide catalyst (Pd1/TiO2) was prepared by depositing Pd by ultraviolet irradiation onto ultrathin TiO2 nanosheets stabilized by ethylene glycol induced [192]. High activity (an average TOF of 9 103 h1) and excellent stability were reported in the hydrogenation of styrene in 20 cycles (entry 10) without any change in FT-EXAFS spectrum. These features testify to the robust character of atomically dispersed palladium under reaction conditions.
4.2.2. Oxides serving as catalytically active phase A simple commercial TiO2 gave scattered results in the aerobic, light-mediated oxidative aza-Henry reaction in five runs (Table 24, entry 1) [193]. Carniti and coworkers embarked on the development of niobium oxide catalysts and to find an appropriate solvent system to increase their stability and durability. It their latest contribution, unsupported Nb2O5 was shown to exhibit high activity in the dehydration of xylose to furfural in a water–CPME (cyclopentyl methyl ether) mixed solvent [194]. Activities, however, decreased in repeated uses and increase in temperature and reaction time was needed to maintain activity with the concomitant increase in selectivity (entry 2). Another sample with Nb2O5 supported on silica-zirconia showed better stability (entry 3). Overall, however, pure Nb2O5 gave the highest furfural yields. A CuCr2O4 spinel catalyst (particle size: 20–50 nm) prepared by a hydrothermal method with the use of cationic surfactant CTAB proved to be active and stable in the hydroxylation of benzene to phenol with better than 91% selectivities (entry 4) [195]. The sample was also tested in the oxidation of varied cycloalkanes [196] exhibiting, however, lower stability (data not shown). CuII was suggested to be the catalytically active species participating in a radical pathway. Both the Biginelli and Hantzsch syntheses that are condensations to form 1,4-dihydropyridines and 3,4dihydropyrimidin-2(1H)-ones, respectively, have been accomplished with the use of an a-Al2O3 woodpile catalyst fabricated by 3D printing [197]. It works as a Lewis acid inducing these reactions under microwave heating with high, stable activities in 10run reuses (entries 5, 6).
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Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65 Table 24 Catalysts with oxides as the active phase in recycling. Catalysta
Conditions
Number of cycles
Yield (%)b
Ref.
1
TiO2 (1 equiv)
EtOH, 11 W lamp, 40 h
5
87 (83–92)
[193]
2
Nb2O5c
CPME/H2O (2.33:1), 180–190 °C, 1–4 h d
16
52 (34–65)
[194]
3
CPME/H2O (2.33:1), 180 °C, 6 h
7
45 (38–53)
[194]
4
Nb2O5-SiO2/ Zr2O5c CuCr2O4e
50% H2O2, MeCN, 80 °C, 10 h
10
66.5 (68–66)f 62.5 (64–61)
[195]
5
a-Al2O3
100 °C, 10 min
10
94.5 (95–94)
[197]
6
a-Al2O3
120 °C, 30 min
10
90 (91–89)
[197]
7
TiO2(cement)
30% H2O2, UV light, H2O, rt
50
73 (83–70)g 63 (67–60)h
[198]
Entry
a b c d e f g h
Reaction
Percentage data show Pd loading (wt%) when available; data in parentheses indicate mol% of metal used. Average of yields. Data in parentheses indicate range of yields. 10 wt% xylose, 5 wt% catalyst. Runs 1–7 = 180 °C, 1 h, runs 8, 9 = 180 °C, 2 h, runs 10, 11 = 180 °C, 4 h, runs 12, 13 = 190 °C, 2 h, runs 14–16 = 190 °C, 4 h. 1 g benzene, 80 mg catalyst. Conversion. Fresh beads. Old beads reactivated = 100 °C, 1 h or sun-light irradiation, 1–2 h.
Table 25 Recycling of polymer-based catalysts in coupling reactions. Entry
Reaction
1
Catalysta
Conditions
Number of cycles
Yield (%)b
Ref.
0.29% PdII-90-PS (0.1)
Cs2CO3, H2O, 70 °C, 2 min
15
97 (100–96)
[199]
1.14% Pd0-(PVA) (2) 16.3% PdII-91-PAMAM (GMA) (1) 15.05% PdII-92-POP (0.8)
K2CO3, EtOH, 80 °C, 2 h KOH, iPrOH/H2O (1:1), 70 °C, 15–40 minc K2O3, EtOH/H2O (1:1), 80 °C, 30 min K2CO3, H2O, rt, 1.5 h
30 10
99 (100–95) 99.3 (100–98)
[201] [202]
15
98 (99–97)
[203]
15
96.3 (99–94)d
[204]
X= Br, R= Ac 2 3
X = I, R = H X = Br, R = MeO
4
X = Br, R H
5
a b c d
6.9% Pd0-93-HBPE (0.1)
Percentage data show Pd loading (wt%) when available; data in parentheses indicate mol% of metal used. Average of yields. Data in parentheses indicate range of yields. Run 1 = 15 min, run 2 = 20 min. Conversion.
Verma and coworkers have prepared spherical cement beads coated with TiO2 and studied their applicability in the oxidative degradation of the antibiotic Cephalexin induced with UV light [198]. The freshly-coated photocatalyst was utilized in 50 cycles with excellent performance achieving an average degradation efficiency of 73% (entry 7). Mineralization was determined by measuring the concentration of nitrate, nitrite and sulfate ions. Reactivated beads showed somewhat lower activity.
4.3. Polymer-supported catalysts 4.3.1. Catalysts based on synthetic polymers Catalyst 0.29% PdII-90-PS was prepared by reacting aminomethylated polystyrene with a bis-carbaldehyde to form a polystyrene-anchored Schiff base and then loading PdCl2 [199]. It is a robust catalyst in Suzuki coupling in 15 runs (Table 25, entry 1) with a low level of Pd leaching (0.3%) and small increases in
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Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65
conversion in a hot filtration test. In an interesting approach, Hariprasad and Radhakrishnan fabricated free-standing, multilayer thin-film ‘‘dip catalysts” containing metal particles [200]. The method included spin-coating and annealing of polyvinyl alcohol (PVA) containing K2[PdCl4]. This was followed by spin-coating and annealing of PVA and, finally, generating another Pdcontaining layer to get a layered Pd-PVA/PVA/Pd-PVA structure abbreviated as Pd0-(PVA). Catalyst 1.14% Pd0-(PVA) with spherical Pd nanoparticles (15–30 nm) has been applied in Suzuki coupling [201]. The film could be simply removed from the reaction vessel and reused. Yields of 100% were observed in 22 cycles with drops of a few percents in additional reuses (entry 2). The cumulative TON and TOF values, respectively, are 4.96 104 and 1102 h1.
PAMAM, a hyperbranched poly(amidoamine) polymer attached to a glycidyl methacrylate (GMA) resin, was used as support material to synthesize an active and recyclable Pd catalyst. Multiple NH2 anchoring sites allow a high degree of Pd loading [16.3% PdII-91PAMAM(GMA)] [202]. Reactions could be repeated with high efficiency in 10 runs, albeit with increases in reaction time (entry 3). A robust porous organic polymer (POP) with an embedded bipyridyl ligand was reacted with PdCl2 to have sample 15.05% PdII-92-POP [203]. It induced Suzuki coupling to provide high yields and showed an efficient performance in a recycling study (entry 4). PdCl2 was found to be strongly coordinated (negative hot filtration test) without reduction (XPS).
Catalyst 6.9% Pd0-93-HBPE was utilized in the coupling of 4methyliodobenzene and methyl acrylate at room temperature with high activity and satisfactory stability (entry 5) [204]. It was prepared using a hyperbranched aliphatic polyester (HBPE) functionalized with poly(caprolactone) as hydrophobic core and PEG. This water-soluble polymer with micellar properties served as anchoring site encapsulated Pd particles (1–2 nm) generated by reducing PdCl2 with NaBH4.
Additional reactions resulting in the formation of new C–C bonds are collected in Table 26. An insoluble porous organic polymer of high surface area (959 m2 g1) has been synthesized by free-radical polymerization
of vinyl-functionalized diphosphane monomer dppe [1,2-bis(diphe nylphosphano)ethane] [205]. This was then treated with Rh(acac) (CO)2 to have sample 2% RhIII-POP/dppe. It has excellent swelling properties in organic solvents and affords higher activity in hydroformylation than the corresponding homogeneous complex. Very similar activities and selectivities could be maintained in six runs (Table 26, entry 1) with Rh leaching of <10 ppb in each run. These excellent features are attributed to high ligand concentration in the porous polymer and strong interactions between Rh species and ligand as demonstrated by XPS. Aldol condensations of varied aldehydes and ketones could be induced with the solid base poly (N-vinylimidazole) (PVIm) under ultrasonic irradiation [206]. It gave high yields in the reaction of acetophenone and benzaldehyde in a 10-run experiment with small increases in reaction time (entry 2). Corma and co-workers have synthesized porous aromatic frameworks by reacting tetrakis-(4-halophenyl)methanes (I or Br) with 1,4-phenylenediboronic acid followed by functionalization with acidic and basic functions (94a and 94b) [207]. When this mixture was used in one-pot cascade reactions the two antagonistic functions worked independently that is neutralization of the active sites did not occur. In step 1, transformation of benzaldehyde dimethylacetal to benzaldehyde is catalyzed by acidic 94a and then, in step 2, benzaldehyde undergoes condensation induced by basic catalyst 94b to give the final product (entry 3). Increasing selectivities with full conversion were found in seven cycles. The recovered catalyst mixture was washed with buffer AcONa/AcOH before reuse.
Pericàs et al. reported the use of a polystyrene-supported tripodal tris(triazolyl)methanol CuI complex in alkyne–azide cycloadditions [208]. It exhibited high activity and good stability in recycling; however, it had to be recharged by treatment with CuCl in THF after a few runs because of leaching. The cationic complex 95 reported recently, in turn, has significantly improved characteristics [209]. It could easily be prepared by reacting the immobilized ligand with [Cu(MeCN)4][PF6] to furnish catalyst 2.15% CuI-95-PS. Results with the best stability found in carbene transfer reactions by ethyl diazoacetate are shown in entry 4. The complex was also tested in the same reaction in a 48-h continuous flow experiment (entry 5).
Hashimoto and co-workers accomplished an outstanding achievement by preparing and using mixed dirhodiumII tetracarboxylate complex 96⁄ immobilized on a PS-based crosslinked polymer matrix through covalent bonding (RhIII-96⁄-poly with a
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Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65 Table 26 Polymer-based catalysts reused in other reactions to form new C–C bonds. Catalysta
Conditions
Number of cycles
Yield (%)b
Ref.
1
2% RhIII-POP/dppe (0.05)
toluene, 80 °C, 10 bar, 18 h
6
99.8 (100–99.5)c 100d 88e
[205]
2
PVIm (50)f
ultrasonic irradiation, rt, 40–44 ming
10
93 (94–92)
[206]
3
94a + 94b
toluene, 90 °C, 1 h, under argon
7
80 (78–85)
[207]
4
2.15% CuI-95-PS (5.2)
neat, 6 hh
5
89 (82–95)
[209]
5 6
2.15% CuI-95-PS (5.2) RhIII-96*-poly (2)
EtOH, CH2Cl2, 48 hi toluene, –78 °C, 4 h
14j 20
94.5 (99–92)c,k 83 (85–81) 95–94l
[209] [210]
7
RhIII-96*-poly (2)
toluene, –60 °C, 2 h
15
82 (87–80) 91–90l
[210]
8
RhIII-96*-poly (2)
CH2Cl2, 23 °C, 20 min
100
88 (86–90) 91–92l
[210]
9
3.2% CuII-97-poly (2.5)
1 atm O2, DMF, 120 °C, 40 min
10
98.5 (99–97)
[211]
10
98*-PS (20)
neat, rt, 48 h
5
99 99–88l
[212]
11
Amberlyst 15 (200)
Zn (3 equiv), H2O, 70 °C, 2 h
10
78 (82–80)m
[213]
12
2.5% RuII-99-FPA (0.5/1)n
PhCF3/CH2Cl2 (1:19), 50 °C, 1–2 h
20
97.5 (>98–94)c
[214]
13
2.5% RuII-99-FPA (1)
PhCF3/CH2Cl2 (1:19), 50 °C, 1.5 h
12
97.5 (>98–95)c
[214]
Entry
a b c d e f g h i j k l m n o
Reaction
Percentage data show Pd loading (wt%) when available; data in parentheses indicate mol% of metal used. Average of yields. Data in parentheses indicate range of yields. Conversion. Yield of aldehydes. Yield of the branched aldehyde. 50 mg catalyst for 10 mM of benzaldehyde. Runs 1–6 = 40 min, runs 7,8 = 42 min, runs 9,10 = 44 min. Ethyl diazoacetate added slowly. Full operation time; residence time = 1 min. 14 samplings. Conversion dropped to 83% after 36 h and then increased after the catalyst was washed with CH2Cl2. Range of enantiomeric excess. Run 7 = 72%, run 10, after acid washing = 80%. Runs 1–15 = 1 mol%, runs 16–20 = 0.5 mol%. Runs 1–15 = 1 h, runs 16–20 = 2 h.
loading of 0.27 mM g1) [210]. The complex tethered to the polymer chain with a long spacer is uniformly distributed. Consequently, the active metal center is rather flexible and easily accessible. The catalyst was tested in intramolecular C–H insertion reactions showing good stability with high, consistent enantioselectivities in the insertion of an a-diazo ester (entry 6) and aryldiazoacetate (entry 7). The intramolecular aromatic insertion of a-diazo-b-ketoester took place with excellent durability in 100 cycles with a Rh leaching of 28 ppm (0.0019%) in the first run (entry 8). The performance of the immobilized catalyst matches that of the homogeneous complex with somewhat longer reactions.
o
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A polymer of hierarchical porosity, large pore volume, and high thermal and chemical stability functionalized with a phenanontroline moiety was treated with Cu(OAc)2 to form catalyst 3.2% CuII-97-poly [211]. It displays a good performance in the Glaser homocoupling of acetylenes to give 1,4-diphenylbut-1,3-diyne in high yields in 10 cycles without any Cu loss (entry 9). A swellable pearl-like copolymer of styrene and thiourea-pyrrolidine (98⁄-PS) was explored as an organocatalyst in asymmetric Michael additions [212]. Addition products were delivered in stable yields with small decrease in enantioselectivity in a 5-run study (entry 10). Amberlyst 15 a commercially available acidic resin in combination with zinc has been found to be a regenerable catalyst system in pinacol coupling of aromatic aldehydes [213]. Decreasing activities could be offset by acidic washing (entry 11).
transfer hydrogenation of 3-phenyl-2H-1,4-benzoxazine with the use of Hantzsch ester (diethyl 1,4-dihydro-2,6-dimethyl-3,5-pyridi nedicarboxylate, 101) as the hydrogen source (entry 4).
Kamer and Heutz used solid-phase synthesis to prepare a library of phosphane–phosphite ligands immobilized on Merrifield resin [219]. [Rh(cod)2][BF4] in combination with these ligands exhibited good performance in the asymmetric hydrogenation of a,b-unsaturated amino acid esters under mild conditions. Ligand 102⁄ was successfully reused in 11 cycles with moderate enantioselectivities (entry 5).
The Hoveyda–Grubbs second generation RuII complex anchored on a fluorous polyacrylate (PA) polymer was applied in RCM in a fluorous mixed solvent [214]. 2.5% RuII-99-FPA exhibited high activity and excellent stability in the transformation of allylsulfonamides substrates (entries 12, 13). Catalyst reuse was performed by dissolving the reaction mixture in EtOAc and extraction with FC-22.
In the final part of this subsection, examples of recycling studies in additional organic transformations induced by polymersupported catalysts are discussed. Candida antarctica lipase immobilized on an acrylic resin (Novozyme 435Ò) has been tested in esterification of oleic acid [215]. Consistently high yields were obtained with primary alcohols (ethanol, propanol, butanol) in experiments repeated 10 times (Table 27, entry 1), whereas complete loss of catalytic activity was found with methanol. Another example is the synthesis of lutein dipalmitate through esterification of lutein with vinyl palmitate [216]. High conversions with slowly decreasing tendency were observed (entry 2). Arnold and coworkers applied a useful strategy to fabricate a Pd catalyst by dissolving Pd(OAc)2 in a commercial epoxy resin and initiating polymerization with IL [emim][OAc]. Formation of Pd particles was shown by XRD and XPS in catalyst 5.43% Pd0-poly/epox [217]. Excellent features were found in 10 consecutive uses in the hydrogenation of ethyl cinnamate (entry 3). On average, 0.006% of the original Rh loading was lost in every run. Asymmetric transfer hydrogenation was catalyzed with microporous binol-derived phosphoric acid (binol-100-poly) [218]. It was made by oxidative coupling at the indicated positions (see arrows) of the intermediate phosphoric acid derivative forming a microporous polymer network. It is a highly efficient and highly stable catalyst without changing activity and selectivity in the
The McGowan group used Wang resin to immobilize Cp⁄ Rh and Ir complexes by reacting the resin with the corresponding triflate of hydroxyl-tethered intermediates formed in situ [220]. The catalysts are efficient in the transfer hydrogenation of benzaldehyde, albeit exhibiting lower activity than the corresponding homogeneous complexes. Nevertheless, 6.9% IrII-103-PS could be recycled 21 times without significant activity losses (entry 6), whereas activities decreased slightly then dropped to 79.8% in run 26.
Flexible, porous polymeric materials 104 and 105 with a 3D network and granular morphology, respectively, have been synthesized by click reaction and then loaded with palladium nanoparticles. PdII ions undergo in situ reduction when the samples are applied in the hydrogenation of alkenes and nitrobenzene to form catalysts 3.2% Pd0-104 and 6.8% Pd0-105 [221]. Pd particles are located on the external surface of the former sample (6.7 nm). In contrast, a dual distribution was found for the other catalyst: particles of 3.4 nm are found in the interior of pores, whereas larger particles (8.2 nm) are located on the external surface. High, stable activities with increasing selectivities were found in recycling (entries 7, 8).
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Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65 Table 27 Polymer-supported catalysts reused in additional organic transformations. Entry
Reaction
Catalysta
Conditions
Number of cycles
Yield (%)b
Ref.
1
Esterification of oleic acid with alkan-1-ols
CA/Novozyme (5)c
10 10 10
96 (95–96.9) 94.7 (94.2–95.2) 90.9 (90.1–91.5)
[215]
2 3
Esterification of lutein with vinyl palmitate
10 10
84 (87–81)e 99 (>99–99)
[216] [217]
4
CA/Novozymed 5.43% Pd0-poly/epox (3) binol-100-poly (5)
32 °C, 24 h ethanol propanol butanol toluene, 60 °C, 8 h 2.5 bar, MeOH, rt, 2 h CHCl3, rt, 2 h
10
99f 98
[218]
5
[Rh(cod)2][BF4] (3.3)
102*, 1 atm H2, THF, 25 °C, 20 h
11
97.1f 46 (48.5–43.3)g
[219]
6
6.9% IrII-103-PS (7.4)
tBuOK, 60 °C, 48 h
25
97.5 (98.4–93.7)h
[220]
7
3.2% Pd0-104 (1)
1.5 bar, MeOH, 40 °C, 1 h
6
100f 99.5 (97–100)
[221]
8
6.8% Pd0-105 (1)
1.5 bar H2, MeOH, 40 °C, 1 h
6
[221]
9
0.014% Ag0-(PVA) (0.45)
H2O, 25 °C, 15 min
30
100f 96.5 (85–100) 97.5 (99.9–95.3)
[200]
10 11
26.7% Pt0-HIPE (3.4) TEMPO-106-poly (1)
H2O, rt, 30–45 min, under nitrogen KBr, CH2Cl2, pH 8.6, 0 °C, 10 min
20 6
>97 94–97
[222] [224]
12
TEMPO-106-poly (1)
CuCl, DMF, 60 °C, 1 h
6
90–92
[224]
13
Ni90Pd10-PVPy (5)
air bubbling, H2O, 80 °C, 4 h
10
100 46i
[225]
14
1.59% CuI-107-poly
cyclohexane, reflux, 2 h
6
98
[226]
15
108*-PS (10)
toluene, rt, 5 h
9
90 (94–85)j 91 (90–92)g
[227]
16
2.15% CuI-95-PS (5.2)
neat, 3 h
5
99
[209]
17
2.3% Co -109-poly (0.03)
1 atm CO2, TBAB, rt, 30 h
15
96 (93–98)
[228]
18
3.2% CuII-97-poly (2.5)
1 atm O2, DMF, 120 °C, 40 min
10
98.4 (99–97)
[211]
19
1% Au0-PTFE (0.75)
50% H2O2, green light, tBuOK, 32 °C, 45 min
10
>99
[229]
graph-C3N4-PET (50)k 7.1% MoVI-110-PS (0.3)
solar irradiation, pH 5, 2.5 h iPrOH, 80 °C, 6 h
15 4l
100 95 ± 4%m 82 ± 4%n
[230] [231]
5.7% Au0-111-poly (0.1)
BuOH, rt, 9 h
10
100f
[232]
20 21
22
a b c d e f g h i j k l m n
II
Degradation of sulfaquinoxaline
Percentage data show loading (wt%) of active species when available; data in parentheses indicate mol% used. Average of yields. Data in parentheses indicate range of yields. wt% relative to the acid. 20 mg mL1 enzyme, 20 mg mL1 lutein. Yields decreased to 81% in run 3 and then increased. Conversion. Range of enantiomeric excess. A yield of 79.8% in run 26. Isolated yields. Yields increased to 94% in run 6 and decreased. 50 mg catalyst, 30 mL solution (2 105 mL L1). Six-our continuous operation repeated 4 times. Steady-state conversion of TBHP. Steady-state epoxide yield.
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Another example of a free-standing thin-film ‘‘dip catalyst” is a sample with Ag nanoparticles embedded in a thin multilayer polymer film [0.014% Ag0-(PVA)] [200] [see also the use of related catalyst 1.14% Pd0-(PVA), Section 4.3.1, Table 25, entry 2]. It afforded high, slightly decreasing activities in the reduction of 4nitrophenol with NaBH4 (entry 9). AFM images clearly showed the swelling of the film in the aqueous solution thereby allowing access of the reactants to Ag nanoparticles. An amphiphilic polymer material composed of a poly(styreneco-2-ethylhexyl acrylate) shell with a hyperbranched PEI core served to deposit Pt particles (average size of 32 nm) by reducing K2[PtCl4] with NaBH4 [222]. Then the rubbery material was treated with a biphasic water/oil mixture to form a HIPE elastomer decorated by Pt nanoparticles. The product was transformed with repeated compressions to the final catalyst (26.7% Pt0-HIPE). Features found in the reduction of 4-nitrophenol shown in entry 10 indicate high activity and excellent durability. In a follow-up study, on the basis of platinum leaching, the authors estimated that their catalyst may be recycled over 10 million times [223]. A test by soaking the sample in a solution of NaBH4 for 30 days without detecting Ostwald ripening resulted in an estimate of 1500 possible runs. These conclusions, however, are certainly exaggerated. It is obvious that in addition to metal loss, other features, such as poisoning, catalyst loss because of disaggregation and incomplete recovery, aging in long runs, etc. can result in decreasing activities. Nitroxide block polymer brush TEMPO-106-poly was fabricated by polymerizing cross-linked polystyrene microspheres with polymethacrylate grafted with TEMPO via controlled radical polymerization [224]. The resulting polymer exhibited high stability in six-run tests oxidizing benzyl alcohol to benzaldehyde with both NaOCl and O2 (entries 11, 12). Singh and coworkers prepared Ni– Pd alloys stabilized by PVPy by reducing NiCl2 and K2[PdCl4] with NaBH4 in aqueous solution [225]. According to TEM-EDS analysis, the two metals are uniformly distributed in the nanoparticles (2– 4 nm). The alloy catalysts were used in air-oxidation of biomassderived furans to the corresponding carboxylic acids. About 5% NaBO2 in the as-prepared samples serving as base was found to be sufficient to achieve satisfactory performance. Consequently, as-prepared catalysts were used without washing. Ni90Pd10-PVPy showed an excellent activity toward the oxidation of furfuryl alcohol in 10 cycles, albeit giving low isolated yields (entry 13). Characterization of the spent catalyst recovered by centrifugation indicated practically no structural changes.
Regular microspheres of a melamine–formaldehyde resin made in the presence of PEG2000 were reacted with 1,4-butane sultone and then H2SO4 to produce polymeric immobilized Brønsted acidic IL. This was further treated with K2CuI3 to form catalyst 1.59% CuI107-poly [226]. It efficiently mediated acetalization to form 1,3dioxacycloalkanes exhibiting high stability in a reuse experiment (entry 14). The Pericàs group has prepared a polystyrene-supported bifunctional organocatalyst with (1R,2R)-2-(piperidin-1-yl)cyclo hexanamine moiety attached to a thiourea unit [227]. 108⁄-PS with a functionalization degree of 97% (0.426 mM g1) was successfully tested in a-amination of 1,3-dicarbonyl compounds with azodicar-
boxylates. It is noteworthy that this immobilized chiral organocatalyst does not undergo irreversible deactivation in contrast to its homogeneous counterpart. Good characteristics are found in a recycling study with high, stable enantioselectivities (entry 15). After each run the catalyst was treated in Et3N for 1 h. In a continuous flow experiment, conversions decreased slowly from 99% to 88% in 7.5 h. Catalyst 2.15% CuI-95-PS discussed above (Table 26, entries 4, 5) could be used in carbene transfer with ethanol to form an O–C bond showing high, stable activities in consecutive applications (entry 16) [209]. The robustness of the catalyst was also demonstrated in continuous flow with ethanol. Conversions slowly decreased to a value of 83% after 38 h. The original activity, however, could be restored by washing with CH2Cl2 resulting in reswelling of the catalyst to have a final conversion of 92% at operation time 49 h.
A novel tetraphenylporphyrin-based supercrosslinked organic polymer of high surface area (1360 m2 g1) with numerous micro- and macropores was synthesized with dicholormethane as crosslinker under AlCl3 catalysis [228]. Treatment with Co (OAc)2 resulted in the desired product 2.3% CoII-109-poly, which was used as catalyst to couple epoxides with CO2. The porous structure enhances CO2 interaction with the polymer and, consequently, improves its uptake. Reactions could be repeated with high efficiency in 15 runs (entry 17). Catalyst 3.2% CuII-97-poly [211] applied in acetylene homocoupling (Table 26, entry 9) exhibited a similar performance in the click reaction of phenylacetylene and benzylazide (entry 18). Gold nanoparticles supported on PTFE (1% Au0-PTFE) were used in oxidative esterification of aldehydes with H2O2 under irradiation with green light [229]. During the recycling study, the catalyst was treated with H2O2 in aqueous acetone for 2 h between runs to maintain activity (entry 19). Graphitic carbon nitride (graph-C3N4) was dispersed to polyethylene terephthalate via electrospinning followed by a hydrothermal treatment [230]. Catalyst graph-C3N4-PET thus prepared was used in photocatalytic degradation of the antibiotic sulfaquinoxaline over solar irradiation exhibiting excellent performance in 15 2.5-h cycles (entry 20). No changes in the structure of the used sample could be detected by XRD and FESEM.
A MoVI complex immobilized on polystyrene functionalized with 2-(aminomethyl)pyridine (7.1% MoVI-110-PS) is an efficient and selective catalyst in epoxidations [231]. When applied in a
Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65
continuous flow study, it transformed 4-vinylcyclohex-1-ene to the corresponding epoxide with tert-butyl hydroperoxide (TBHP) to afford high, steady-state TBHP conversion and product yields (entry 21). Greiner, Schmalz and coworkers fabricated patchy hybrid nonwovens as an efficient catalyst platform [232]. Polymer fibers serving as carrier were composed of PS-block-PE-block-poly (methyl methacrylate) functionalized by introducing diisopropylamino groups (111-poly). Then the polymer was dissolved in DMSO followed by quenching resulting in patchy, worm-like crystalline-like micelles forming a shell. This material after electrospinning was dipped into a citrate-stabilized dispersion of Au nanoparticles (11 ± 3 nm) to give 5.7% Au0-111-poly. After swollen in butanol it was tested in alcoholysis of dimethylphenylsilane with butanol, showed high, stable activities in 10 cycles (entry 22). No Au leaching was found by ICP-OES measurements.
37
A support material was made by grafting hyperbranched polyethyleneimine (HPEI) onto polyacrylonitrile fiber (PANf, 114) [236]. Then PANf fibers were mixed with an aqueous solution of HAuCl4 and the resulting material was reduced with NaBH4 to have Au0-114-PANf catalysts. The best results in the reduction of 4nitrophenol were achieved with supports of low HPEI contents (weight gains of PANf in fabricating the support are 0.31% and 0.58%). Catalyst stabilities were characterized by measuring the time needed to achieve 100% conversion (Fig. 4). In all these cases continued use of catalysts required prolonged reactions to maintain activities. 0.71% Au0-1140.58-PANf, however, exhibited relatively stable activities in 39 cycles, whereas significant increases in reaction time were needed after the 28th run with sample 0.31% Au0-1140.31-PANf. A TON close to 7 104 was realized with 0.71% Au0-1140.58-PANf.
The Schulz group has developed and tested salen complexes in a range of organic transformations. Polymeric chiral catalyst CrIII-112⁄-poly [233] was applied in four asymmetric syntheses in nine cycles [234]. It shows very low activity in the addition of dimethyl zinc to benzaldehyde (Table 28, cycle 1). However, yields in further applications in hetero-Diels–Alder reaction (cycle 2), Henry reaction (cycle 3) and ring opening of cyclohexene oxide (cycle 4) are similar to or better than those found by using a fresh catalyst (yields of 63, 92 and 75%, respectively). Features are similar in the second reuse sequences (cycles 5–8). A comparison of activities in runs 2 and 9 testifies to satisfactory catalyst stability.
Luis, García-Verdugo and co-workers completed a detailed recycling study with a polymer mixture composed of supported precatalyst PdII-113a-PS, supported ionic liquid-like catalyst Pd0113b-PS as well as bases 113c-PS and 113d-PS [235]. The latter play a dual role as base being involved in scavenging and stabilizing the active Pd species. PdII-113a-PS in combination with either base gave high stability tested in five Heck reactions followed by Sonogashira coupling, Suzuki reaction and, finally Heck coupling again (Table 29). The polymer after each reaction was washed with NaOAc solution and then used again.
4.3.2. Catalyst supported on natural polymers Starch-capped Ni nanoparticles were shown to act as catalysts in transesterifications in the presence of amine additives [237]. The catalyst was prepared by reducing Ni(AcO)2 with NaBH4 in liquid ammonia as complexing agent at pH 10. The catalyst and the amine additive were sonicated before the reaction and applied in the transesterification diethyl malonate at room temperature with a 20-fold excess of methanol. A constant yield of 96% with increasing reaction time in 24 repeated reactions was observed (Table 30, entry 1). The nanoparticles were separated by centrifugation and reused. Oxidized Ni species were not detected after reaction, which can be attributed to the protecting effect of starch. The Pérez-Juste group has fabricated Pd and Au nanoparticles loaded on cellulose filter paper and tested their preparations in reuse studies in the form of 1.3 by 4 cm strips. First, oleylaminecapped monodisperse metal nanoparticles were prepared and then cellulose paper was dipped into the solution of concentrated metal dispersions. According to XPS analysis, catalyst Pd0-paper with particles of 3.0 ± 0.4 nm contains varied Pd species (Pd0 = 77%, PdII = 18%, PdIV = 5%) [238]. Pd loadings varied between 0.37 mg (1 dipping) and 0.87 ± 0.08 mg (5 dippings). Recycling tests in the oxidative homocoupling of 4-carboxyphenylboronic acid (entry 2) and the reduction by NaBH4 of 4-nitrophenol (entry 3) indicate good stability. The paper strips were washed with borate buffer
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Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65
Table 28 Iterative study with catalyst CrIII-112*-poly in varied asymmetric syntheses. Reaction
Cycle
Yield/ee (%)
Cycle
Yield/ee (%)
1
20/24
5
9/6
2
93/63 63b
6 9
66/65 64/64
3
86/51 92b
7
94/29
4a
83/3 75b
8a
79/3
Reaction conditions: 10 mol% catalyst, rt, 23 h. a Reaction time = 72 h. b Yields with a fresh catalyst.
Table 29 Coupling of iodobenzene in Heck, Sonogashira and Suzuki reaction.
Cycle
a b c d
Reactant
Product
Yield (%)a
Yield (%)b
1
99
78c
2 3 4 5 6 7 8
99 85 96 94 92 91c 99
82d 58d 99d 99d 99c 59c 99d
PdII-113a-PS, 113c-PS. Pd0-113b-PS, 113d-PS. Reaction time = 24 h. Reaction time = 160 min.
and dried overnight at 50 °C before reuse. A homogeneous pathway could be ruled out by a hot filtration test. Catalyst Au0-paper (average particle size of 12 nm) exhibited a similar performance in a longer study (entry 4) [239]. Another gold preparation, 0.723% Au0-collagen/EGCG afforded similar results (entry 5) [240]. The catalyst was made by depositing Au nanoparticles of 10.7 nm onto collagen fibers modified with EGCG, a natural polyphenol through crosslinking with glutaraldehyde. Nagashima et al. used a different approach to have catalysts based on cotton and filter paper [241]. First Pd2(dba)3 was reacted with the ammonium salt of hyperbranched polystyrene [HPS-N+(C12H25)3Cl] in DMF to have an egg-shell structure of Pd particles (3 nm) surrounded with HPS salt. Immobilization was performed by treating a mixture of this material and cotton or filter paper with citric acid in DMF. Pd-HPS/cotton (1.65 mg Pd) could be repeatedly used in Heck coupling in 16 cycles (entry 6) with significant amounts of Pd leaching (run 1: 1.1 mg, run 5: 3.3 mg). Product yields dropped in run 17 and 18 but increased to 95% in run 19 in a prolonged reaction (24 h). Pd-HPS/paper (0.17–0.66 mg
Pd), in contrast, proved to be more resistant. Five-run tests in coupling reactions (entries 7, 8), intramolecular coupling (entry 9) and alkene hydrogenation (entry 10) testify to excellent stability. A minimal Pd leaching of 0.15–2.0 ppm was attributed to mechanical abrasion. Au nanoparticles generated on the surface of yeast cells by biosorption followed by bioreduction have a bimodal sizedistribution of 14.8 ± 3.9 and 30 nm [242]. Full conversions were measured in a 10-run recycling study in reducing nitrobenzene (entry 11). Chitosan (115, R = H or Ac) loaded with an IL, a Pd salt and a ligand (PdII-115-[bmim][BF4]) was applied in varied allylation reactions [243]. It gave excellent, higher than 98% conversion values and proved to be highly stable in the reaction of morpholine with (E)-1,3-diphenyl-3-acetoxyprop-1-ene in 10 runs in the presence of ionic phosphane ligand TPPTS (entry 12) despite a significant Pd leaching of 19.3%. Qi, Zhang and co-workers have fabricated porous chitosan microspheres by adding NaOH solution into a mixture of chitosan, PEG 20 M and glutaraldehyde as crosslinking agent followed by extraction of PEG and deposition of Na2[PdCl4] [244]. In situ reduction gives catalyst Pd0-115/116 with Pd particles (2–4 nm), which exhibits excellent stability in Heck coupling in 13 repeated reactions (entry 13).
A silica-based palladium catalyst has been fabricated by depositing SiO2 nanofilaments on glass surfaces. This was followed by O2 plasma treatment to generate OH groups and then by coating with polydopamine (PDA) via polymerization of dopamine. In the final step treatment with K2[PdCl4] and reduction by catechol moieties in PDA give catalyst Pd0-SiO2/PDA/glass [245]. This so-called mussel-inspired method relies on the observation that mussel foot proteins containing catechol found in DOPA (3,4-dihydroxy-L-
Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65
39
140
Time for full conversion (min)
120 100
80
60
40 20
0 1
3
5
7
9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 Cycyle num ber
Fig. 4. Reduction of 4-nitrophenol with NaBH4 induced by catalysts 0.31% Au0-1140.31-PANf (open bars) and 0.71% Au0-1140.58-PANf (shaded bars). Reaction conditions: 0.0037 mol% Au, H2O, 35 °C. Reprinted (adapted) from Ref. [236], Copyright (2017), with permission from The Royal Society of Chemistry.
phenylalanine) and lysine have strong adhesive properties and play crucial roles in strong adherence to surfaces [246]. The catalyst deposited into a laboratory round-bottom glass flask with 0.22 mg Pd was used as a reactor to run 20 reactions with high activity and slow, gradual decreases (entry 14). 4.4. Varied carbon materials serving as catalyst support Carbon nanotubes and other ordered carbon materials have become rather popular supports in recent years [247] including industrial applications [248]. Relevant examples are included in this subsection. Results of carbon–carbon cross-coupling reactions are collected in Table 31, whereas additional examples are shown in Table 32. Philippot, García-Suárez and co-workers generated homogeneously distributed Pd nanoparticles by decomposing Pd2(dba)3 in the presence of phenolic resin-based mesoporous carbon beads (C) [249]. Two other supports were prepared by oxidation with oxygen plasma of commercial beads (oxC) and the sample after heat-treatment at 2000 °C (2000-oxC). The resulting catalysts have decreasing BET surface area (999, 878 and 130 m2 g1) and slightly decreasing particle mean diameters (2.5 ± 0.68, 2.4 ± 0.46 and 2.0 ± 0.38 nm). All three catalysts were tested in Suzuki coupling in the aqueous phase. 1.35% Pd0-2000-oxC gives high, stable conversions in 10 runs (Table 31, entry 1) with decreasing Pd content to 1.12 wt% lost mainly in the first two cycles. All three catalysts exhibited lower stability when used five times under microwave irradiation (data not shown). Park, Joo and coworkers made a colloidal solution of Pd particles (2.9 ± 0.6 nm) in the presence of oleylamine [250]. The particles thus produced were incorporated into hexagonally ordered mesoporous carbon support material CMK-3 to afford catalyst 3.33% Pd0-OA/CMK-3 with oleylamine as capping agent. When compared to samples with other capping agents, this catalyst showed the highest stability in recycling (entry 2) with a small increase in particle size to 3.0 ± 0.5 nm. A dual-porous carbon support was fabricated by treating a mixture of FeCl3, 1,3,5-triphenylbenzene, and formaldehyde dimethyl
acetal in CH2Cl2 (45 °C, 5 h, 80 °C, 19 h) to form a polymer network [251]. The isolated solid was impregnated with PdCl2 in acetonitrile and the resulting powder after centrifugation underwent carbonization and reduction (500 °C, N2, 400 °C, H2). The narrow pore sizes and complicated pore structure of the microporous organic polymer prevent aggregation/sintering of metal particles during high-temperature treatments. The catalyst with a microporous carbon support (mpC) has a Pd loading of 1.1 wt% with an average particle size of 7.6 nm and affords good performance in multiple repeating reactions (entry 3). The result of a hot filtration test and a 2.2 ppm of leached Pd indicate that dissolved Pd species are not involved in the reaction. Diao and co-workers have prepared a hematite–carbon nanocomposite by treating a mixture of D-glucose and a-Fe2O3 nanoparticles (200 °C, 12 h) [252]. After removing hematite with HCl the resulting hollow carbon nanonets (HCnano) were used as support material to deposit PdCl2. Pd nanoparticles (8 nm) were generated on the surface of nanonets by reduction with hydrazine hydrate to get catalyst 25.5% Pd0-HCnano. High catalyst stability was found in the Heck reaction (entry 4) in contrast to Suzuki coupling. It is despite the observation that significant particle agglomeration was observed in the former reaction. Srivastava fabricated a Pd0-rGO sample by treating a mixture of reduced graphene oxide (rGO) and Pd(OAc)2 with sodium dodecyl sulfate serving as both surfactant and reducing agent [253]. Good stability was observed when the catalyst was applied in Heck reaction in IL [bmim][NTf2] (entry 5). Drops of a few percents of the yield with increasing reuse number were attributed to aggregation of Pd particles from 5 nm to 20 nm in run 14. The product was isolated with extraction and the dried IL–catalyst system was recycled. Cu deposited on activated carbon (1.6% Cu0-C) proved to be highly active and recyclable in click chemistry to form 1,2,3triazoles [254]. The catalyst was prepared by treating CuCl2 and 4,40 -di-tert-butylbiphenyl in anhydrous THF at room temperature under argon atmosphere to form Cu nanoparticles (6 ± 2 nm). This was followed by adding activated carbon to have a catalyst providing high activity and stability in a three-component cycloaddition in triazole synthesis (Table 32, entry 1).
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Table 30 Recycling of catalysts with natural polymers. Catalysta
Conditions
Number of cycles
Yield (%)b
Ref.
1
Ni0-starch
ethylenediamine, rt, 30–120 minc
24
96
[237]
2
Pd0-paper
borate buffer (pH 8.7), 70 °C, 250 min, under air
10
6.3 (6.7–5.8)d
[238]
3
Pd0-paper
H2O, rt, 14 min
11
6.65 (6.8–5.8)d 93.5 (97–92)
[238]
4 5
Au0-paper 0.723% Au0-collagen/ EGCG (300) 2.9% Pd0-HPS/cotton (3.12)
H2O, rt, 30 min H2O, 25 °C e
20 21
95 (96–93) 99.5 (100–99)
[239] [240]
20
94 (>99–50)
[241]
0.55% Pd0-HPS/paper (1.47) 0.94% Pd0-HPS/paper (2.5) 0.57% Pd0-HPS/paper (1.52)
K2CO3, H2O, 80 °C, 24 h
5
99
[241]
K2CO3, H2O, 50 °C, 24 h
5
99
[241]
KOAc, DMAc, 120 °C, 24 h
5
99
[241]
0.5% Pd0-HPS/paper (0.67) 4.8% Au0-yeast (0.012)
2 bar, EtOAc, rt, 5 h
5
99
[241]
H2O, 30 °C, 5 min
10
100
[242]
12
PdII-115-[bmim][BF4] (5)
TPPTS, rt, 1 h, under nitrogen
10
>98g
[243]
13
Pd0-115/116 (1)
KOAc, DMF/ethylene glycol (10:1), 110 °C, 5 h
13
97 (96–100)
[244]
14
Pd0-SiO2/PDA/glass (0.02)
Et3N, TBAAc, butanol, 100 °C, 12 h
20
97 (99–90)
[245]
Entry
Reaction
6
K2CO3, DMF, 80 °C, 12–24 h
f
R1= MeO, R2= tBu 7 8 9
10 11
a b c d e f g
R1 = H, R2 = tBu 1
2
R = H, R = Bu
Percentage data show loading (wt%) of active species when available; data in parentheses indicate mol% used. Average of yields. Data in parentheses indicate range of yields. Run 23 = 90 min, run 24 = 120 min. Rate constant: 104 s1. Reaction time not reported. Run 17 = 85%, run 18 = 68%, run 19 = 95% (24 h), run 20 = 50% (24 h). Conversion.
Rezaeifard and Jafarpour immobilized an FeIII porphyrin complex (117) on multiwall carbon nanotube (Cnt) through coordinative anchoring of the hydroxyl functionality of the nanotube [255]. The resulting catalyst (5.12% FeIII-117-porph/Cnt) showed high activity in the oxidation of sulfides, alkanes and alkenes with the use of tetrabutylammonium peroxomonosulfate (TBAOX). The catalyst applied in a reuse study in epoxidation of cyclooctene surpassed the activity of pure parent FeIII porphyrin. It could be easily recovered by filtration in 10 cycles (entry 2). Moreover, tetrabutylammonium monosulfate could also be recovered by lyophilization. Much higher efficiency and better stability were observed with the use of bimetallic nanoparticles supported on functionalized carbon nanotubes with loadings of 5.46% Pt and 3.44% Pd (Pt74Pd26-118-Cnt) [256]. Nitrobenzene was hydrogenated to aniline in 99% yield in recycling, albeit an increase in reaction time to 30 min was needed in run 11 (entry 3). FDU-type ordered mesoporous carbon (FDU = Fu-Dan University) was doped with nitrogen by treatment with ammonia and the resulting material (FDU-N) was used to deposit Pd nanoparticles of about 2.3 ± 0.5 nm
(H2[PdCl4], NaBH4) [257]. The multifunctional catalyst showed excellent performance in the selective hydrogenation of phenol to cyclohexanone with high stability in recycling (entry 4).
A family of catalyst materials fabricated by functionalizing carbon nanohorns (Cnh) with bis-imidazolium salts was applied in the synthesis of cyclic carbonates [258]. Two samples with different loadings (IL-119a-Cnh 1.93 mM g1, IL-119b-Cnh 2.93 mM g1) used in recycling studies allowed the synthesis of the target
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Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65 Table 31 Recycling of carbon-based catalysts in coupling reactions. Entry
Reaction
1
Catalysta
Conditions
Number of cycles
Yield (%)b
Ref.
1.35% Pd0-2000-oxC (1.5)
K2O3, H2O, PEG, 50 °C, 10 min
10
96c
[249]
3.33% Pd0-OA/CMK-3 (0.02) 1.1% Pd0-mpC (0.002)
K2CO3, DMF/H2O (5:1), 150 °C, 10 min K3PO4, DMF, 120 °C, 2 h, in air
6 10
>99c 94.5 (95–93)
[250] [251]
25.5% Pd0-HCnano (0.032) Pd0-rGO (0.25)
K2CO3, DMF, 120 °C, 0.5 h [bmim][NTf2], K2CO3, 100 °C, 1 h
6 14
100 95.5 (98–90)
[252] [253]
X= Br, R= CHO 2 3
X = Br, R = OH
4 5
R = Me R = Et
R= H
a b c
Percentage data show loading (wt%) of active species when available; data in parentheses indicate mol% used. Average of yields. Data in parentheses indicate range of yields. Conversion.
Table 32 Reuse of carbon-based catalysts in varied organic transformations. Catalysta
Conditions
Number of cycles
Yield (%)b
Ref.
1
1.6% Cu0-C (0.5)
H2O, 70 °C, 3 h
5
100c
[254]
2
5.12% FeIII-117-porph/Cnt (0.05)
70 °C, 15 min, under air
10
92.5 (94–91)
[255]
3
Pt74Pd26-118-Cnt (1)
EtOH, 1 bar, rt, 20 mind
11
99
[256]
4
2.5% Pd0-FDU-N (5)
1 bar, H2O, 100 °C, 2 h
6
>98
[257]
5
IL-119a-Cnh (0.23)
40 bar, 150 °C, 3 h
6
135 (101–175)e
[258]
6 7
IL-119b-Cnh (0.34) 5.62% MnO2-GO (0.1)
40 bar, 150 °C, 3 h air, 110 °C, 3 h
6 6
147 (100–183)e 95 (97.5–96.5)c
[258] [259]
8
3.2% MnII-GO (0.1)
H2O2 (3 equiv), NaHCO3, DMF, 0 °C, 30 min
10
97.5 (98–96)
[260]
9
9.3% PdII-120a-rGO (1)
1 atm, Cs2CO3, toluene, 100 °C, 15 min
10
100
[261]
10
9% RuII-120b-rGO (2)
Cs2CO3, toluene, 100 °C, 12 h
10
100
[261]
11
PdII/RuII-120a + 120b-rGO (2)
tBuONa, iPrOH, 80 °C, 3 h
13
98.5 (100–87)f
[262]
12
AgAu-rGOg
H2O, 3.5 h
15
>99
[263]
13
RhIII-121-diamond (1)
40 bar, MeOH, NaOH (aq. solution, Na/Rh = 5) rt, 4 h
5
>99
[264]
Entry
a b c d e f g
Reaction
Percentage data show loading (wt%) of active species when available; data in parentheses indicate mol% used. Average of yields. Data in parentheses indicate range of yields. Conversion. 30 min in run 11. TON values. Run 8 = 95%, runs 10 and 12 = 100%, run 13 = 87%. 1.8 mL CrVI (0.1 mM aqueous solution), 0.5 mg catalyst.
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Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65
compound with gradually increasing efficiencies (entries 5, 6). Since Cnt-based catalysts did not show similar behavior, the authors emphasized the beneficial effect of the support material.
4.5. Magnetic and magnetically tagged catalysts
Graphene oxide (GO) decorated with ultrafine MnO2 nanoparticles of 3 nm prepared by a simple and facile in-situ growing method showed excellent characteristics in the selective oxidation of benzyl alcohol to benzaldehyde in a six-run study (entry 7) [259]. Stability was attributed to strong interactions between MnO2 and the oxygen-containing functional groups of GO. Another graphene oxide-based catalyst prepared by treating GO with Mn (OAc)2 was suggested to have homogeneously dispersed MnII ions coordinated to oxygenated functional groups [260]. Epoxidation of styrene with hydrogen peroxide could be efficiently mediated with this robust catalyst (entry 8). The palladium complex of N-heterocyclic carbene ligand with a pyrene tag was immobilized on the surface of rGO by p-stacking [261]. Catalyst PdII-120a-rGO (9.3 wt% loading of the complex) afforded full conversion and 100% selectivity in the hydrogenation of styrene in 10 cycles without detectable Pd leaching (entry 9). The related Ru complex RuII-120b-rGO (9.0 wt% loading) showed similar properties in the oxidant-free dehydrogenation of benzyl alcohol to benzaldehyde with the loss of 1.3 wt% of the complex (entry 10). Since Cs2CO3 was found to adsorb on the surface of rGO, addition of the base was needed only in the first run. These two complexes were subsequently co-immobilized onto rGO and employed in hydrodefluorination of fluoroarenes [262]. Catalyst PdII/RuII-120a + 120b-rGO (5.19 wt% Pd and 4.97 wt% Ru loading) gave stable activities in 12 runs (entry 11). The synergistic action of the two metals was suggested by the authors to account for catalyst performance.
A range of mono- and bimetallic catalysts supported on rGO generated by using Albizia Saman leaf extract as reducing and stabilizing agent was synthesized in one pot [263]. AgAu-rGO was successfully used in reducing toxic CrVI ions with formic acid affording high stability and excellent efficiency (entry 12). A Wilkinson-type catalyst was prepared by modifying nanodiamond particles (7 ± 3 nm) with catechol phosphane ligands and then reacting with [Rh(cod)Cl]2 (catalyst RhIII-121-diamond) [264]. It induced the hydrogenation of chalcone in five successive runs (entry 13). Both a hot filtration test and ICP-AES (0.1 ppm Rh loss) indicated the heterogeneous nature of the reaction.
The use of nanosize magnetic catalysts emerged in the last decade as an efficient solution for the separation and reuse of catalyst materials. Magnetite (Fe3O4) and hematite (c-Fe2O3) are applied most prolifically to fabricate magnetic nano-catalysts. These can be conveniently and efficiently collected using a permanent magnet and then separate the reaction mixture by decantation or using a syringe. Magnetic and magnetically tagged catalysts combine the properties of highly active homogeneous catalysts with easy separation representing bridges between homogeneous and heterogeneous catalysis because the nano nature is considered as a quasihomogeneous phase [265]. A further advantage is easy surface functionalization, which allows fine-tuning catalytic properties. A number of reviews treat varied applications of magnetic nanoparticles [265,266]. In this subsection recent examples are discussed to highlight their successful use as recyclable support materials in catalytic studies. Organization has been made according to the type of transformations. 4.5.1. Recycling in hydrogenation and hydrogenolysis A few results about the recycling of magnetic catalysts in hydrogenation and hydrogenolysis are collected in Table 33. Rossi and co-workers fabricated a catalyst composed of silicacoated magnetic particles and Ni deposited by decomposing Ni [cod]2 (2% Ni0-Fe3O4/SiO2) [267]. Then they studied the hydrogenation of cyclohexene to find high catalyst stability in 15 repeated uses (entry 1). An additional advantage of this catalyst is the easy reduction of oxidized surface Ni species under reaction conditions. They have also prepared and tested two Rh catalysts. In an earlier study they reported the first magnetically recoverable Rh catalyst (Rh particles of 3–5 nm) made with the use of Fe3O4 coated with silica and functionalized with (3-aminopropyl)triethoxysilane (1.55% Rh0-Fe3O4/SiO2) [268]. It proved to be highly active giving a wide range of TOF values and a high cumulative TON of 1.8 105 in 18 reuses (entry 2). The other catalyst with silicacoated magnetic particles was impregnated with the colloidal suspension of Rh particles to synthesize 0.09% Rh0-Fe3O4/SiO2 [269]. High activities decreased significantly already in the fourth run (entry 3). Nevertheless, a high cumulative TON was achieved in seven runs with a Rh loss of 7%. Nanosized magnetic FeCo alloy particles with a graphitic shell (FeCo/graph) were embedded into mesoporous silica spheres. The product was treated with (3-amino propyl)trimethoxysilane and then reacted with K2[PtCl4] in ethanol to generate Pt nanoparticles (3.5 nm), which became immobilized on the amine-functionalized silica surface (5.6% Pt0-FeCo/ graph/SiO2) [270]. The catalyst maintained excellent activity in five reuses in the hydrogenation of cyclohexene (entry 4). Reiser and coworkers have fabricated Pd catalysts using a magnetic support composed of carbon-coated Co nanobeads functionalized with an imidazolium-based ionic liquid. The sample with an unusually high Pd loading of 34 wt% deposited by microwave decomposition of Pd2(dba)3CHCl3 proved to be the best catalyst in hydrogenations (34% Pd0-122-IL/C/Co) [271]. Results in the hydrogenation of trans-stilbene to reach full conversion with the
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Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65 Table 33 Hydrogenation of C@C and C@O bond and hydrogenolysis with magnetic catalysts. Catalysta
Conditions
Number of cycles
Yield (%)b
Ref.
1
2% Ni0-Fe3O4/SiO2 (0.33)
6 bar, 75 °C, 25 min
15
>99c
[267]
2 3 4 5
1.55% Rh0-Fe3O4/SiO2 (0.053) 0.09% Rh0-Fe3O4/SiO2 (0.002) 5.6% Pt0-FeCo/graph/SiO2 (0.12) 34% Pd0-122-IL/C/Co (0.1)
6 bar, 75 °C, 25 min 6 bar, 75 °C, 25 min 1 atm, acetone, 25 °C, 10 h 10 bar, iPrOH, rt, 2 hf
18 7 5 12
2.7 104 (4.1–1.5)d 3.0 105 e 99.9 100
[268] [269] [270] [271]
6
3% Pd0-Ni0/CeO2–x (0.0016)
1 atm, EtOH, 25 °C, 20 min
8
100c
[272]
7
1.55% Rh0-Fe3O4/SiO2 (0.053)
6 bar, 75 °C, 25 min
21
0.7 103 (1.25–0.6)d
[268]
8
1% Rh0-c-Fe2O3 (1)
1 atm, H2O, 30 min, rt
5
600g
[273]
9
1.95% Pt0-Fe3O4/SiO2 (0.083)
6 atm, 75 °C, H2O, min, rt
14
99c1.56 104
10
20% Ni0(Ce)-Al2O3
70 bar, Ca(OH)2, 240 °C, 12 h
18
93 (98–86)c
Entry
a b c d e f g
Reaction
e
[274] [275]
Percentage data show metal loading (wt%) when available; data in parentheses indicate mol% used. Average of yields. Data in parentheses indicate range of yields. Conversion. TOF values (h1). Cumulative TON value. 2.5 h in the last run. Average TOF (h1).
use of 0.1 mol% of Pd are shown in entry 5. 13% of Pd was lost with significantly increased values in the last two runs (64 and 48 ppm, respectively) and a cumulative Co loss of 180 ppm was also measured. A novel magnetically recoverable nanocatalyst is composed of active Pd nanoparticles deposited onto a support material with magnetic Ni nanoparticles (61 wt%) and oxygen-deficient cerium oxide (3% Pd0-Ni0/CeO2–x) [272]. Full conversions in the hydrogenation of styrene could be maintained with increasing reaction times with a very low palladium loading of 0.0016 mol% (entry 6).
The performance of catalyst 1.55% Rh0-Fe3O4/SiO2 tested in the hydrogenation of benzene in 21 runs (entry 7) giving a cumulative TON of 1.16 104 is similar to that found in cyclohexene hydrogenation (see entry 2). Catalyst 1% Rh0-c-Fe2O3 with Rh nanoparticles of 2.2 nm was applied in the hydrogenation of arenes under mild conditions (room temperature, atmospheric pressure) [273]. In a recycling study to transform toluene to methylcyclohexane, consistently high TOF values were measured in five runs with magnetic recovery (entry 8). An Fe3O4 core covered with an aminopropylated silica layer was loaded with K2[PtBr4]. The resulting catalyst undergoes in situ reduction when used in the hydrogenations of alkenes, aromatics and carbonyl compounds [274]. 1.95% Pt0-Fe3O4/SiO2 with particles of 2.5 nm showed an excellent performance in the hydrogenation of pentan-3-one in 14 runs without loss of activity and a high cumulative TON (entry 9). A cerium-doped Ni on Al2O3 magnetic catalyst [20% Ni0(Ce)-Al2O3] made by co-precipitation works efficiently in the hydrogenolysis of sorbitol to form a mixture of
C2–C4 diols of practical importance (entry 10) [275]. A slow decrease in activity caused by particle agglomeration could be offset by adding a small amount of fresh catalyst. The overall glycol selectivity was about 55–60%. A benchmark reaction in testing metal nanoparticle catalyst materials is the reduction of nitroarenes, in particular, that of nitrobenzenes and nitrophenols. Readers are advised to consult a related recent review [276]. Support material 123 was fabricated by coating Fe3O4 particles with oleic acid followed by functionalization with 3mercaptopropionic acid. Reduction of PdCl2 with NaBH4 resulted in the formation of Pd particles (catalyst 5.3% Pd0-123-Fe3O4/olac) [277]. Pt particles were deposited in a similar way to the surface of carbon-coated magnetic Fe3O4 microspheres prepared by carbonization of glucose (0.58% Pt0-C/Fe3O4) [278]. Both catalysts showed high efficiency in the catalytic hydrogenation of nitrobenzene (Table 34, entries 1, 2). A Thales Nano H-Cube continuous flow reactor was utilized in the hydrogenation of nitroarenes, azides and alkenes [279]. The catalyst is composed of Pd nanoparticles (1–3 nm) and maghemite (c-Fe2O3) spheres (10–30 nm). XPS spectra indicate that palladium is dominantly metallic with minor amounts of PdO. High catalyst activity with sufficient stability in the hydrogenation of 4-methoxynitrobenzene was detected in multiple repeating reactions (entry 3).
Wang et al. prepared a hierarchical catalyst material by first modifying Fe3O4 with polyacrylic acid followed by treatment with 4-vinylpyridine and divinylbenzene to form a polymeric shell. Finally, Au nanoparticles (average size = 5 nm) were generated by
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reduction with NaBH4 of HAuCl4 to give catalyst 18% Au0-124Fe3O4/poly [280]. It was applied in the reduction of 4-nitrophenol with NaBH4 (entry 4). High activity and a very low amount of Au leaching (37.9 ppb) were reported. Another Au catalyst (23.14% Au0-125-Fe3O4) was somewhat less effective showing a large drop in the final cycle of an 11-run test [281]. In the synthesis, first Fe3O4 nanoparticles were surface functionalized by a treatment with hexadecyltrimethoxysilane followed by a reaction with chitosan to form a surface layer via ionotropic gelation. After Au seeding, Au nanoparticles were deposited by reduction of HAuCl4 with glucose. The catalyst was thoroughly characterized by various methods (TEM, HRTEM, SEM, FT-IR, XRD, XPS and TGA) and tested in a 30-day period. This was done by preparing an aqueous catalyst solution (1 mg mL1) and testing it every day for the first five days followed by additional tests every 5th day (10 samplings). Only very small activity drops were detected (entry 5).
Li et al. fabricated novel magnetic double-shelled hollow microspheres by starting with the synthesis of C-SiO2-Fe3O4 [282]. First, a mixture of Fe3O4 nanoparticles, TEOS, CTAB, FeCl3, and HCl was aerosolized and the droplets underwent hydrolysis and condensation in a furnace to form spherical particles. These were then coated with a resin layer by the polymerization of 2,4dihydroxybenzoic acid and formaldehyde. Finally, carbonization and removal of silica allowed the transfer of the magnetic core to the outer carbon shell. Structural features were demonstrated by instrumental studies (TEM, HRTEM, and SEM). The support material loaded with Pt particles (K2[PtCl6] in ethanol) gave catalyst Pt0-C/C/Fe3O4, which maintained full conversion in five cycles (entry 6). Silica microspheres were hydrothermally treated with ferrocene to generate particles coated with a double shell of magnetic Fe3O4 and carbon followed by partial etching the silica core with NH3 (C/Fe3O4/SiO2). Aminopropylation, impregnation with HAuCl4 and, finally, reduction with NaBH4 afforded catalyst 3.4% Au0-C/Fe3O4/ SiO2 with a double-shelled yolk-like structure. The silica core is movable, the shell is mesoporous, while Au nanoparticles of 2 nm are located in both the interior cavity and the mesoporous shell [283]. Increasing reaction times were needed to maintain high activity in recycling (entry 7). Reasons are the loss of Au particles from the surface and adsorption of product 4-aminophenol. Interestingly, Au content increased slightly to 3.9%, which was attributed to the partial etching of silica by NaBH4 under reaction conditions. An excellent, highly active and reusable gold catalyst has been made by first fabricating magnetic polymer beads by coating Fe3O4 particles with a polymer layer of 1,4-divinylbenzene methyl methacrylate. These were then mixed with a solution of Au nanoparticles (10 nm) to have catalyst 0.43% Au0-Fe3O4/poly [284]. A sample with a high gold particle density of 4292 lg g1 showed high stability in 23 runs in the reduction of 4-nitrophenol (entry 8). The same activity was measured using the sample after stored for 3 months. The activity of this sample is close to that of a quasi-homogeneous isolated Au nanoparticle catalyst. A composite catalyst with gold nanoparticles was prepared and tested in the same reaction under similar conditions [285]. First Fe3O4 particles were generated and deposited on the surface of rectorite a layered silicate clay material. Further treatment with
chitosan in an acidic suspension was followed by reaction with HAuCl4 and finally, reduction with NaBH4 to form gold nanoparticles of <10 nm. 14.3 wt% Au0-Fe3O4/115/rect showed high, stable activities in a longer study (entry 9). 3% Pd0-Ni0/CeO2–x, the novel magnetically recoverable nanocatalysts used in the hydrogenation styrene (Table 33, entry 6) was tested in the reduction of 4nitrophenol as well [272]. High stability and excellent activity with an ultralow metal loading of 1 105 mol% was reported under ultrasonic irradiation (entry 10). Shafiee et al. applied a multistep process to functionalize magnetic GO nanosheets [286]. Modifications included stepwise treatment with 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide, N-hydroxysuccinimide, functionalized diethylene glycol (DEG) H2N-DEG-OTs and, finally, aminoguanidin hydrochloride. Pd loading gave catalyst 9.6% PdII-126-Fe3O4/GO with magnetic particles (<10 nm) and PdII stabilized in a five-membered chelate ring. It is a catalyst water-tolerant and robust catalyst and reveals high activity and durability in 20 cycles (entry 11).
A supported catalyst bearing spherical bimetallic particles (10– 12 nm) with an Fe core coated with Au shell has recently been developed. An aqueous dispersion of particles (10–12 nm) mixed with the aqueous dispersion of graphene oxide functionalized with 4-aminothiophenol gave catalyst Fe0Au0-127-GO [287]. It was applied in 10 repeated experiments in the catalytic reduction of both 4- and 2-nitrophenol with NaBH4 reused after magnetic separation (entries 12 and 13).
A Ni catalyst was prepared by reducing Ni(AcO)2 in the presence of Fe3O4 and starch as capping agent [288]. Catalyst 0.45% Ni0Fe3O4/starch with a core–shell structure was tested in the reduction of 4-nitroaniline in 30 cycles (entry 14). Whereas increases in reaction time were needed to maintain high product yields, an estimated cumulative TON of 3.5 103 could be achieved. Kim, Hyeon and co-workers have synthesized 1.09% Rh0-Fe3O4 heterodimer magnetic nanocrystals by controlled thermolysis of a mixture of Rh(acac)3 and Fe(acac)3. The resulting catalyst is composed of Rh (average diameter = 2–3 nm) and Fe3O4 particles (16 nm) [289]. It showed consistently high activities in the reduction with hydrazine of nitrobenzene (entry 15) without significant changes in shape and size of the particles. 4.5.2. Reuse studies in oxidations Immobilization of TEMPO on magnetic polystyrene nanospheres and the application of the resulting catalyst sample (10.6% TEMPO-128-Fe3O4/PS) in the Montanari oxidation of alcohols have been successfully achieved by Wang and co-workers [290]. High and constant activities were reported in 20 repeated reactions to selectively transform benzyl alcohol to benzaldehyde (Table 35, entry 1). Similar features were found in an iterative study to oxidize 20 alcohols of varied structure (yields = >99– 96%, reaction time = 5–40 min).
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Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65 Table 34 Hydrogenation and reduction of nitrobenzenes with magnetic catalysts. Entry
a b c d e f g h i
Reaction
Catalysta 0
Conditions
Number of cycles
Yield (%)b
Ref.
1
5.3% Pd -123-Fe3O4/olac (1.25)
EtOH, 1 bar, rt, 1 h
10
>97.5 (>99–>95)
[277]
2 3
0.58% Pt0-C/Fe3O4 (0.12) 3.92% Pd0-c-Fe2O3 (6.2)c
EtOH, 1 bar, rt, 1 h EtOH, 30 °C, 10–15 mind
10 12
>98 (>99–96) 93.5 (95–92)
[278] [279]
4
18% Au0-124-Fe3O4/poly (0.018)
EtOH, rt, 3 min
10
97.5 (100–97)e
[280]
5 6 7 8 9 10 11 12 13
23.14% Au0-125-Fe3O4 (0.0023) 20.3% Pt0-C/C/Fe3O4 3.4% Au0-C/Fe3O4/SiO2 0.43% Au0-Fe3O4/poly 14.3% Au0-Fe3O4/115/rect 3% Pd0-Ni0/CeO2–x (1 105) 9.6% PdII-126-Fe3O4/GO (0.026) Fe0Au0-127-GO Fe0Au0-127-GO
H2O, H2O, H2O, H2O, H2O, H2O, H2O, H2O, H2O,
10 5 9 23 14 20 20 10 10
97 (98–96) 100 98.5 (99–97.5)e 100e 99.5 (100–97.5)g 100–96 (99)e 99 99 (100–98.1)e 98.7 (100–97.6)e
[281] [282] [283] [284] [285] [272] [286] [287] [287]
14
0.45% Ni0-Fe3O4/starch (1.06)
H2O, rt, 12–140 mini
30
95.9 (96–95)
[288]
15
1.09% Rh0-Fe3O4 (1)
EtOH, 80 °C, 1 h
8
99
[289]
rt, rt, rt, rt rt, rt, rt, rt rt
12 minf 8 min 100–350 s 30 min 100 sh 200 s
Percentage data show metal loading (wt%) when available; data in parentheses indicate mol% used. Average of yields. Data in parentheses indicate range of yields. 6.2 wt% relative to substrate. Time of the full recycling study; residence time = 0.75 min. Conversion. See text. Degradation efficiency. Ultrasonic irradiation. Run 15 = 24 min, run 25 = 47 min, run 29 = 70 min.
Rezaeifard, Jafarpour and co-workers fabricated a catalyst by immobilizing a CuII phthalocyanine-tetrasulfonic acid tetrasodium complex (0.044% CuII-129-Fe3O4/SiO2) through ionic interaction on a silica-coated magnetic support functionalized with (3aminopropyl)triethoxysilane [293]. It exhibited excellent catalyst stability in the oxidation of methyl phenyl sulfide to the corresponding sulfone in seven runs (entry 4). DABCO tribromide immobilized on magnetic Fe3O4 particles (DABCO-74b-Fe3O4) gave constant yields in the oxidation of methyl phenyl sulfide to the corresponding sulfoxide in the first nine cycles under solvent-free conditions [291]. Yields, however, decreased significantly in subsequent uses (entry 2). Catalyst 0.024% CuII-129-Fe3O4 with an immobilized CuII salen complex showed similar features in the oxidation of benzyl methyl sulfide in 12 repeated reactions (entry 3) [292]. The catalyst was easily prepared by refluxing the N-triethoxysilylpropyl-substituted salen complex and magnetic particles in ethanol.
Entry 5 presents results found with the use of a multicomponent magnetic TiO2–graphene photocatalyst to remove herbicides from water [294]. Silica-coated Fe3O4 particles were treated with (BuO)4Ti and then calcined to form a TiO2 layer. The particles thus prepared were grafted with (3-aminopropyl)trimethoxysilane and then, after mixing with graphite oxide, were reduced with hydrazine hydrate to form rGO. During the final step, electrostatic interaction between particles and rGO sheets gives catalyst TiO2-SiO2/ Fe3O4-131-rGO. Catalyst performance was tested by illuminating
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Table 35 Reuse studies with magnetic catalysts applied in oxidations. Catalysta
Conditions
Number of cycles
Yield (%)b
Ref.
1
10.6% TEMPO-128-Fe3O4/ PS (1)
NaBr, NaOCl, CH2Cl2, pH 9.1, 10 °C, 5 min
20
99 (99.5–96)c
[290]
2
DABCO-73b-Fe3O4
30% H2O2, rt, 20 min
15
87 (97–78)d
[291]
3
0.024% CuII-129-Fe3O4 (0.5)
33% H2O2, EtOH, 60 °C, 1.2 h
12
97.5 (99–92)
[292]
4
0.044% CuII-130-Fe3O4/SiO2 (0.5)
Bu4NHSO5, H2O/EtOH (1:2), rt, 1.5 h
7
100
[293]
5
TiO2-SiO2/Fe3O4-131-rGO (20)
500 watt xenon lamp (290–800 nm), H2O, rt, 180 min
9
99 (100–97.1)
[294]
Entry
a b c d
Reaction
Percentage data show loading of active species (wt%) when available; data in parentheses indicate mol% used. Average of yields. Data in parentheses indicate range of yields. Conversion. Runs 1–9 = 97%.
the aqueous solution of the typical herbicide 2,4-dichlorophenoxyacetic acid with a 500W xenon arc lamp. The removal efficiency could be maintained at a very high level in eight runs regaining a value of 99.1% in the ninth reuse after sonication (entry 5). Moreover, after laying aside the catalyst for one year, an efficiency of 95.6% was measured after an ultrasound treatment. Scheme 2. Epoxidation induced by varied magnetic catalysts.
An ionic liquid-type peroxometalate was immobilized on silicacoated magnetic nanoparticles either through hydrogen-bonding interactions (sample 4.7% IL-132a-Fe3O4/SiO2) or by covalent bonding (16.7% IL-132b-Fe3O4/SiO2) [295]. A third sample (36.8% IL-132c-Fe3O4/SiO2) was prepared from a zwitterion and Na7[PW11O39]. All three catalysts exhibited high, constant activities in the epoxidation of cyclooctene (Scheme 2) with selectivities higher than 99%. Low levels of leaching were observed in the mixed solvent methanol–H2O (11–19 ppm) whereas 25–48 ppm of W was lost in pure methanol. In all reactions, epoxide selectivities of >99% were measured.
4.5.3. Recycling of magnetic catalysts in asymmetric syntheses Examples for the recovery and reuse of magnetic catalysts in asymmetric syntheses are listed in Table 36.
Li et al. reported a unique method for catalyst synthesis applying enantioselective bioreduction [296]. This was a multistep process first coating iron oxide nanoparticles with a poly(glycidyl methacrylate) (PGMA) shell modified with epoxy rings followed by further modification with formyl groups. Finally, functionalized particles 133-Fe3O4/PGMA were loaded with alcohol dehydrogenase enzyme RDR from Devosia riboflavina. The particles form reversible clusters, which dissociate inducing bioreduction under reaction conditions (catalyst RDR-133⁄-Fe3O4/PGMA). The catalyst was tested in the enantioselective reduction of 7-methoxy-2tetralone with NADH as cofactor and isopropyl alcohol regenerating the cofactor. High, slowly decreasing yields and high ee values were observed (entry 1) with NADH undergoing 6000–7700 recycling in each run.
A spherical siliceous mesocellular foam was first grafted with magnetic c-Fe2O3 nanoparticles followed by functionalization with TsDPEN to have immobilized hybrid ligand 134⁄-Fe2O3/SiO2 [297]. It was applied in asymmetric transfer hydrogenation by forming the active catalyst in situ in the presence of [RuCl2(p-cymene)]2. Whereas full conversion required increasing reaction time in nine runs, high enantioselectivities could be maintained (entry 2). The decrease in activity is attributed to catalyst loss caused by stirring and Ru leaching (11 mol% in nine runs). Another example for
Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65
47
asymmetric transfer hydrogenation is the use of magnetic chiral catalysts 135⁄ prepared by grafting silica-coated Fe3O4 nanoparticles with the corresponding functionalized chiral diamine derivatives [298]. Both catalysts exhibited high activities and enantioselectivities in aqueous medium (entries 3, 4).
nanocrystals (30 nm) decorated each with a single Pd nanosphere of 6 nm. Increasing activities were found in recycling in the Suzuki coupling (Table 37, entry 1). Catalyst 2.23% PdII-138Fe3O4/SiO2 was prepared by anchoring the functionalized b-ketoiminatophosphanyl palladium complex into silica-coated
Phenylene-coated magnetic nanoparticles were prepared by the co-condensation of chiral 4-(trimethoxysilyl)ethyl)phenylsulfonyl1,2-diphenylethylene-diamine and 1,4-bis(triethyoxysilyl)benzene onto Fe3O4. This was followed by complexation with [{Cp⁄RhCl2}2] to have catalyst 1.16% RhIII-136⁄-Fe3O4/phen [299]. It induced asymmetric transfer hydrogenation of acetophenone to give the (S)alcohol with high activity and high ee values in 10 cycles (entry 5).
Fe3O4 [302]. In recycling experiments carried out with 4chloroanisole the catalytic system could be reused 10 times delivering the coupling product in excellent yields (entry 2). A minimal Pd loss (0.06 ppm) was measured by ICP–AES and neither Pd aggregation nor any activity of the filtered solution could be detected after the last use. These excellent features are attributed to the high durability of the silica-coated support and the robust nature of the palladium complex. Last but not least, this catalyst exhibits high activities in the coupling of unrecative chloroaromatics.
A chiral analog of 4-N,N-dimethylaminopyridine immobilized on magnetic particles (2.84% 137⁄-Fe3O4) proved to be a highly active catalyst material in asymmetric acylation of racemic secondary alcohols allowing kinetic resolution [300]. Major features found in a 20-run recycling are high, varying conversions and high enantioselectivities under mild conditions (entry 6). Then the same catalyst sample was applied in additional 12 runs in the kinetic resolution of five other ester derivatives affording similar conversions (89–62%) and ee values (99–70%). In the final, 32nd run the original alcohol was tested again to give reasonably high conversion (64%) and ee (93%). A comparison of these values to those in entry 6 clearly indicates excellent stability.
4.5.4. C–C bond forming reactions induced by magnetic catalysts The results of recycling studies reported in coupling reactions are collected in the following two tables. Table 37 lists information with respect to coupling reactions with the majority of data acquired in Suzuki coupling, whereas the results of multicomponent processes forming new C–C bonds are treated in Table 38. Heterodimer nanoparticles were fabricated by Kim and coworkers by thermal decomposition of a solution of Pd(acac)2, Fe(acac)3, oleylamine and oleic acid [301]. TEM images of the resulting 1.10% Pd-Fe3O4 sample showed the presence of Fe3O4
Excellent stabilities in long recycling experiments have been demonstrated with diphosphane-functionalized robust palladadendrons grafted onto magnetic c-Fe2O3 particles coated with a carboxyl-functionalized polymer [303]. Four catalysts thus prepared (PdII-139a–139d-Fe3O4/poly) afforded full conversions in Suzuki coupling in the first seven runs of 25-run tests (entry 3). In further reuses scattered values with slightly decreasing tendencies were measured. Leaching resistance of these samples (14 ppm or 0.36% Pd loss per cycle) is higher than that of related catalyst systems.
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Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65
Table 36 Recycling of magnetic catalysts in asymmetric syntheses. Catalysta
Conditions
Number of cycles
Yield (%)b
Ref.
1
RDR-133*-Fe3O4/ PGMA
NADH (0.011 mol%), iPrOH, Tris buffer (pH 8), 30 °C, 20 min
15
80 (90–72) >99c
[296]
2
0.6% RuII-134*Fe2O3/SiO2 (0.5)
Et3N (1.4:1), CH2Cl2, 40 °C, 1.5–7 h
9
99d 94–90c
[297]
3
2.77% IrIII-135a*Fe3O4/SiO2 (0.2)
H2O, TBAB, 40 °C, 8 h
10
98 (99.2–95.9) 89.6–86c
[298]
4
1.25% RhIII-135b*Fe3O4/SiO2 (0.2) 1.16% RhIII-136*Fe3O4/phen (1)
H2O, TBAB, 40 °C, 8 h
10
[298]
H2O, 40 °C, 0.5 h
10
2.84% 137*-Fe3O4 (5)
Et3N, toluene, rt, 16 h, under argon
20
98.7 (99.9–96.9) 87.6–83.8c 99.7 (100–97.8)d 95.1–95.9c 200–196e 66 (72–59)d 99–82f
Entry
Reaction
5
6
a b c d e f
[299]
[300]
Percentage data show loading of active species (wt%) when available; data in parentheses indicate mol% used. Average of yields. Data in parentheses indicate range of yields. Range of enantiomeric excess. Conversion. Range of TON values. Enantiomeric excess of the recovered alcohol.
Table 37 Catalyst reuse in coupling reactions with magnetic separation. Entry
Reaction
1
Catalysta
Conditions
Number of cycles
Yield (%)b
Ref.
1.10% Pd-Fe3O4 (0.1)
Na2CO3, DME/H2O (3:1), reflux, 24 h, under argon
10
92 (90–96)
[301]
2.23% PdII-138-Fe3O4/ SiO2 (0.5) PdII-139-Fe3O4/poly (1.96) c 1.31% Pd0-140-Fe3O4/ SiO2 (0.5) 0.89% Pd0-141-Fe3O4/ SiO2 (0.025) 2.45% PdII-142-Fe3O4/ poly (1) 2.83% Pd0-143-Fe3O4 (0.2) 2.75% Pd-144-Fe3O4 (0.2) PdII-145-C/Co (0.5) PdII-145-C/Co (0.5) 1.36% PdII-146-Fe3O4/ 115 (0.23)
K2CO3, TBAB, H2O, 60 °C, 5 h
10
95.5 (96–95)
[302]
NaOH, THF/TritonX 405 (1:9), H2O, 65 °C, 1 h, under nitrogen KOH, toluene, 100 °C, 6 h
25
93–97d
[303]
10
92
[304]
K2CO3, H2O, 80 °C, 12 h, under argon
10
99.5 (100–99)
[305]
K2CO3, H2O/EtOH (1:3), 70 °C, 12 h
21
98 (99–96)
[306]
K2CO3, H2O/EtOH (1:1), rt, 0.66 h
8
>97 (98–97)
[307]
K2CO3, H2O/EtOH (1:1), rt, 1 h, under air
8
>99 (100–99)
[308]
Na2CO3, H2O/THF (5:2), 60 °C, 14 h Na2CO3, H2O/THF (5:2), 60 °C, 14 h Et3N, DMF, 90 °C, 1.5 h
8 12 66
95 100 (100–98)e 78.5 (84–72)
[309] [309] [310]
2.23% PdII-138-Fe3O4/ SiO2 (0.5)
piperidine, TBAB, H2O, 60 °C, 6 h
10
95 (96–94)
[302]
X= I, R1= R2= H 2
X = Cl, R1 = MeO, R2 = H
3
X = I, R1 = MeO, R2 = H
4
X = Br, R1 = OMe, R2 = H
5
c d e
X = Br, R = OMe, R = H 1
2
X = Br, R = Ac, R = H
7
X = Br, R1 = R2= H
8
X = Br, R1 = R2= H
12
a
2
6
9 10 11
b
1
1
2
X = Br, R = R = H X = Br, R1 = CH2COOH, R2 = H
Percentage data show metal loading (wt%) when available; data in parentheses indicate mol% used. Average of yields. Data in parentheses indicate range of yields. Four catalysts (139a–139d). Average conversions of four catalysts. 100% yields were found in the first 11 runs followed by the coupling of 4-bromoacetophenone giving yield of 98%.
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Angurell, Rossell et al. have grafted phosphane-functionalized dopamine onto Fe3O4 particles followed by loading K2[PdCl4] and, finally, generating Pd particles by reduction with NaBH4 (catalyst 1.31% Pd0-140-Fe3O4/SiO2) [304]. No change in activity was detected in 10 cycles in Suzuki coupling (entry 4). Silica-coated magnetic nanoparticles with anchored imidazolium ionic liquid bearing triethylene glycol moieties have been fabricated by Karimi and co-workers. Treatment with Na2[PdCl4] gave a precatalyst [305]. An induction period and the almost complete loss of activity in the presence of Hg0 testify to the formation of Pd particles being the actual catalytic species. Catalyst 0.89% Pd0-141-Fe3O4/SiO2 exhibits high activity including the coupling of heteroaromatic compounds and shows excellent stability in 10 runs (entry 5). After product extraction with hexane the aqueous phase with the catalyst was reused. High activity is assumed to originate from the hydrophilic nature of the catalyst allowing the formation welldistributed Pd particles and exposing the active sites to substrates.
hydrazine hydrate to deliver magnetic catalyst 2.83% Pd0-143Fe3O4 with Pd particles of 2 nm [307]. A wide range of physical methods was used for catalyst characterization (XRD, XPS, EDS, FT-IR, ICP, SEM, TEM, and VSM). Only minimal changes were measured in an eight-run test of catalyst stability (entry 7). In another contribution a similar method for the synthesis of catalyst 2.75% Pd0-144-Fe3O4 was reported except by using magnetic particles functionalized with 3-(cyanoethyl)triethoxysilane [308]. Catalyst characteristics shown in entry 8 are similar to those of the previous samples.
Majoral and coworkers have prepared dendritic phosphane ligand 145 in a multistep process and used it with Pd(OAc)2 and magnetic Co/C particles [309]. 145 and Pd(OAc)2 non-covalently attached to particles exhibit high activity and excellent stability in Suzuki coupling (catalyst PdII-145-C/Co, entry 9). 428 ppm Pd leached during 10 cycles (entry 10), whereas a much higher value (895 ppm) was measured when the catalyst system was used without Co/C particles. It worked in similar efficiency in the synthesis of Felbinac (biphen-4-ylacetic acid) a non-steroidal anti-inflammatory drug.
A miniemulsion polymerization of divinylbenzene and 4chloromethylstyrene in the presence of a magnetic fluid gave a magnetic support, which was further reacted with 1-(2,6diisopropylphenyl)-1H-imidazole and Pd(OAc)2 to have the supported Pd NHC complex (2.45% PdII-142-Fe3O4/poly) [306]. It showed excellent recyclability in 21 runs (entry 6). A hot filtration test indicated the heterogeneous nature of the reaction and Pd particles were not detected in the used catalyst. The Veisi group performed functionalization with diaminoglyoxime of Fe3O4 particles followed by treatment with PdCl2 and then reduction with Table 38 Catalyst reuse with magnetic separation in multicomponent reactions. Catalysta
Conditions
Number of cycles
Yield (%)b
Ref.
1
1.93% Pd0-147-Fe3O4/PEI/rGO (0.9)
PPh3, H2O, 100 °C, 1 h, in air
30
97 (100–92)
[311]
2
GSH-148-Fe3O4
H2O, 80 °C, 15 min
8
92.5 (93.8–89.8)
[312]
3
9.6% HPA-149-Fe3O4 (4 104)
100 °C, 30 min
8
92
[313]
4
CuFe2O4 (10)
1 atm O2, 100 °C, 24 h
10
88 (92–84)
[314]
5
mpSiO2NHAc/npSiO2SO3H/Fe3O4 (10)
DMF/MeOH (5:1), 40 °C, 1h
10
98
[315]
Entry
a b
Reaction
Percentage data show metal loading (wt%) when available; data in parentheses indicate mol% used. Average of yields. Data in parentheses indicate range of yields.
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A magnetite powder coated with chitosan was the support material for depositing Pd to form 1.36% PdII-146-Fe3O4/115 [310]. It exhibits outstanding stability upon reuse in the Heck coupling of iodobenzene and acrylic acid. An average yield of 78.6% was measured in 60 runs before dropping to about 35% in the 66th use (cumulative TON = 1.13 105) (entry 11). The performance of 2.23% PdII-138-Fe3O4/SiO2 in Sonogashira coupling (entry 12) is similar to that found in the Suzuki reaction (entry 2).
Wang et al. deposited Pd nanoparticles onto reduced graphene oxide nanosheets strongly coupled with Fe3O4 particles [311]. First, graphene oxide was functionalized in reactions with polyethyleneimine and then with 3,4-dihydroxybenzaldehyde. This was followed by generating Pd nanoparticles (4.31 ± 0.40 nm) applying PdCl2 and NaBH4. Finally, the material thus obtained was reacted with Fe3O4 to have 1.93% Pd0-147-Fe3O4/PEI/rGO. The catalyst was fully characterized and proved to be highly active in Tsuji–Trost allylation performed in water under air to have mixtures of mono- and diallylated products. Selective diallylation, however, could be achieved with increasing reaction time. Consistently high product yields in 30 repeated reactions of ethyl acetoacetate and allyl ethyl carbonate were demonstrated (Table 38, entry 1). In contrast, rapid deactivation was experienced with the use of Pd/C and Pd/rGO. According to TEM of the used catalyst, particle agglomeration did not occur but 7.6% of palladium was lost.
Magnetic particles grafted with glutathione (GSH-148Fe3O4) served as a highly efficient nanocatalyst in the synthesis of spirooxindoles to afford excellent yields in short reactions in aqueous medium with similar features in reuses (entry 2) [312]. It is even more important that excellent TON (8.5 105) and TOF values (3.4 106 h1) could be realized. Magnetic Fe3O4 particles modified with pyridine moieties was loaded with Wells–Dawson heteropoly acid H6[P2W18O62] to form catalyst 9.6% HPA-149-Fe3O4 [313]. It gave high, constant yields in the three-component synthesis of 1-amidoalkyl-2-naphthols without leaching of the active species (entry 3).
Copper ferrite (CuFe2O4) an analog of magnetite was used successfully in unique oxidative cross-dehydrogenative coupling reactions [314]. It was probed in a reuse study to perform a Csp3–Csp3 coupling of 2-phenyl-1,2,3,4-tetrahydroisoquinoline and nitromethane (entry 4). The result of a hot filtration test and a low level of Cu leaching (0.39 ppm) strongly indicate a heterogeneous mechanism. The particle size of 34 ± 11.6 nm changed slightly to 31 ± 12.6 nm after 10 cycles. An acid-base three-component bifunctional catalyst with a magnetic Fe3O4 core, a nanoporous silica inner shell (npSiO2SO3H) and a microporous silica outer shell (mpSiO2NHAc) has been designed for the synthesis of 5-aryl1,2,3-triazoles [315]. Uniform egg-like particles of 400 nm with three-layered structure could be detected by TEM with the corresponding layered distribution of the functional groups (EDS). Aromatic aldehydes reacted with nitromethane and NaN3 to afford high yields and stable activities in a 10-run recycling (entry 5). 4.5.5. Reuse in hydrogen generation Significant efforts have been made to develop suitable hydrogen storage and releasing materials with the hydrolytic dehydrogenation of both ammonia–borane (H3N:BH3, AB) as one of the best candidates and methylamine–borane (MeH2N:BH3, MeAB). Results of recycling studies are collected in Table 39. Most catalysts discussed here have cobalt in the support loaded with metal particles generated by reducing the corresponding metal ions with hydrogen formed in the decomposition of ammonia–borane. A number of successful attempts have been published by the Özkar group. A catalyst with copper loading (1.32% Cu0-150CoFe2O4/SiO2) was fabricated by adsorbing CuCl2 on silica-coated magnetic cobalt ferrite particles and then reducing in situ [316]. In the last cycle in a 10-run experiment, 98% remaining activity was measured (entry 1) with an initial TOF of 2400 h1. This is the highest value found with a non-noble metal catalyst. Particle size increased slightly from 0.7 ± 0.3 nm to 1.1 ± 0.6 nm. A similar strategy for catalyst synthesis was used to prepare samples 1.98% Pd0-150-CoFe2O4/SiO2 [317] and 1.96% Ru0-150-CoFe2O4/SiO2 [318]. Catalytic activities (TOF values) of the Pd-loaded sample increased continuously in a 10-run study with a small drop in the last reuse (entry 2). The sample with Ru nanoparticles, in turn, retained 94% of the original activity in the 10th run (entry 3). Metal leaching and sintering of metal particles were not detected in either system. Catalyst 0.98% Ag0-150-CoFe2O4/SiO2 made by in situ reduction of AgNO3 maintained the original activity in seven cycles (entry 4) [319].
A one-step seeding-growth method was used by Chen, Ku and coworkers to prepare magnetically recyclable Cu33Fe67 alloy nanoparticles (3 nm) by reducing a mixture of CuCl2 and FeCl2 with an aqueous solution of ammonia–borane [320]. The activity
Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65
51
of this sample is one of the highest among non-noble metal catalysts (entry 5). Magnetic Ag-CoFe/graphene prepared in situ by reduction with MeAB gave the best result in the hydrolysis of MeAB by retaining 96.6% of its original activity (entry 6) [321]. 4.5.6. Recycling of magnetic catalysts in other transformations In an earlier study, a magnetic support with an immobilized active species analogous to 4-N,N-dimethylaminopyridine (Damp) was probed in a recycling study in a range of organic transformation of synthetic interest [322]. Catalyst Damp-151aFe3O4/SiO2 with a loading of 20 mM g1 proved to be robust affording high yields in the acetylation of 1-phenylethanol (Table 40, entry 1). Similar features were found when the same catalyst sample after recovery was further tested in bishydroxylation, O- and N-acylation and azalactone synthesis in triplicate in each reaction.
a-Cyanation of tertiary amines with trimethylsilyl cyanide can be promoted with high efficiency using catalyst 12.4% AuIII-152-Fe3O4/SiO2 loaded with a goldIII-bipyridyl complex (entry 2) [323]. A magnetic catalyst with an immobilized guanidine moiety as the functional group of high basicity (Gnd-151bFe3O4/SiO2) proved to be efficient in a recycling study [324]. Full conversions were achieved in the first 17 cycles in the cyanosilylation of benzaldehyde followed by small drops in subsequent reuses (entry 3).
Immobilization of the enzyme Mucor javanicus lipase onto a microsphere support composed of a magnetic core coated with oleic acid and a porous polymer shell gave a catalyst material (sample MJ-153-Fe3O4/olac, Fig. 5) with improved activity and selectivity in solvent-free esterification [325]. Both high activity and high regioisomeric excess could be maintained in a 30-run recycling study (entry 4). A magnetic catalyst composed of zinc porphyrin-based organic polymer 154 fabricated by reacting terephthalaldehyde and pyrrole in the presence of Fe3O4 core and silica shell particles was applied in the cycloaddition of CO2 to propylene oxide (entry 5) [326]. High catalytic activity is attributed to covalently highly cross-linked metalloporphyrin sites, whereas slight decreases in the last cycles result from the loss of 21% Zn.
Catalyst mim-155-Fe3O4 with a loading of 0.60 mM g1 was employed in cycloadditions [327]. Stability was tested in the reaction of epichlorohydrin with CO2 at low pressure to form the corresponding cyclic carbonate with slight decreases in activity in 11 cycles (entry 6). Fe3O4 was mixed with the aqueous solution of PtCl2 and then the mixture was basified (pH 13) and stirred for one day to produce catalyst 2.3% PtO/PtO2Fe3O4 [328]. The resulting catalyst efficiently mediated the hydrosilylation of varied alkynes with high activity and stability in recycling (entry 7).
Another catalyst was made by anchoring a ligand suitable for reacting with first-generation Hoveyda–Grubbs Ru complex to aminopropyl-modified Fe3O4 particles [329]. The resulting supported catalyst complex (0.25% RuII-156-Fe3O4) proved to be efficient in RCM of two substrates, albeit with increasing reaction times (entries 8, 9). Activity was also tested by recycling a single batch of catalyst sample in the transformation of nine dienes (Table 41).
A supported catalyst system with second-generation Hoveyda–Grubbs RuII complex (0.23% RuII-157-Fe3O4/SiO2) was synthesized by Lee, Zhong and coworkers. The metathesis catalyst functionalized with an imidazolium salt serving as suitable linker was immobilized on silica-coated magnetic particles [330]. It exhibited high activity and could be recycled in RCM in 13 runs (entry 10). In the next reuse, however, reaction time had to be increased to 24 h to have a conversion of 92%. In sharp contrast, the conversion achieved with the related catalyst without the silica shell dropped to 76% in run seven.
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Table 39 Hydrogen generation by the hydrolysis of boranes.
Entry 1 2 3 4 5 6 a b c d e f g h i
Catalysta
Number of cycles
Yield (%)b
Ref.
10
99 (100–98)
[316]
25 °C, 5 mind
10
172 (112–206)e
[317]
25 °C, 5–6 min
10
96 (100–94)
[318]
25 °C, 5–6 min
7
73f
[319]
Conditions 0
1.32% Cu -150-CoFe2O4/SiO2 (1.4) 1.98% Pd0-150-CoFe2O4/SiO2 (0.19) 1.96% Ru0-150-CoFe2O4/SiO2 (0.74) 0.98% Ag0-150-CoFe2O4/SiO2 (0.28) Cu33Fe67 Ag-CoFe/graphene (0.05)h
25 °C, 50 min
c
25 °C 30 °C, 25–27 min
8 5
22 (10–27) 3i
g
[320] [321]
Percentage data show loading of active species (wt%) when available; data in parentheses indicate mol% used. Average of yields. Data in parentheses indicate range of yields. 15 mL of 100 mM solution. 10 mL of 100 mM solution. TOF values (min1). mL H2 formed. Reaction time (min) to achieve full conversion. Catalyst/MeAB. n(H2)/n(MeAB).
In an earlier study, Zhu et al. reported the fabrication of a magnetic catalyst by immobilization of the same RuII complex on a starch-modified support [331]. 2.8% RuII-158-Fe3O4/starch afforded an excellent performance in the self-metathesis of methyl oleate with a low Ru leaching of 4 ppm (entry 11). The catalyst, however, exhibits lower activity than that of the Grubbs catalyst (TOF of 700 h1).
A Co0Fe0-CoFe2O4 magnetic composite material with CoFe nanoparticles (10 nm) was made by treating a solution of FeSO4 and CoCl2 in the presence of KOH at 140 °C [333]. The product is composed of spinel-type CoFe2O4 and cubic CoFe alloy. It was applied as a Fenton-like catalyst to induce the decomposition of varied dye pollutants (Methyl orange, Congo red, Rhodamine B, Methylene Blue) with Oxone. Satisfactory stability was demonstrated in the decomposition of Orange II (2-naphthol orange) in recycling (entry 13). The SEM image of the used catalyst was very similar to that of the pristine sample, but leaching of both Co and Fe was detected. Superparamagnetic c-Fe2O3 nanoparticles (mean particle size = 10.5 ± 1.4 nm) with a high surface area of 147 m2 g1 prepared by calcination of Fe3O4 proved to be efficient and recyclable in the degradation of PET with ethylene glycol (entry 14) [334]. 4.6. Catalyst based on MOF materials
In another catalyst, Rh nanoparticles stabilized by Oct4NBr surfactant were deposited onto a magnetic support functionalized with hyperbranched polymer 159 with terminal phosphane moieties [332]. 1% Rh0-159-PPh2/Fe3O4/SiO2 was tested in hydroformylation including a recycling study with estragol. Because of double bond migration, three isomeric aldehydes are formed in consistently high yields in six runs even in shorter reactions (entry 12) with linear/branched ratios of 1.8–2.
Metal–organic frameworks (MOFs) are crystalline, porous materials composed of inorganic (metal) ions or clusters and organic linkers [335]. Also known as porous coordination polymers, they are of recent high interest because of their unique properties including extremely high porosity and internal surface area as well as wide variability forming varied structures. They have been used, among others, as catalyst materials or supports. Examples with respect to their use in recycling studies are collected in Table 42.
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Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65 Table 40 Miscellaneous transformations promoted by magnetic catalysts. Catalysta
Conditions
Number of cycles
Yield (%)b
Ref.
1
Damp-151a-Fe3O4/SiO2 (5)
Et3N (1.5 equiv), CH2Cl2, rt, 16–1 hc
14
97 (>98–94)
[322]
2
12.4% AuIII-152-Fe3O4/SiO2 (10)
TBHP, MeOH, 50 °C, 6 h
10
94.5 (96–93)
[323]
3
Gnd-151b-Fe3O4/SiO2
CH2Cl2, rt, 5 min
20
99 (100–92)d
[324]
4
MJ-153-Fe3O4/olac
4 Å MS, 37 °C, 1 h
30
[325]
5
0.025% ZnII-154-Fe3O4/SiO2 (4.6 104)
30 bar, KI, 120 °C, 2 h
16
97.5 (100–95)e 92–86f 94 (97–85)
[326]
6
mim-155-Fe3O4 (1)
10 bar, 140 °C, 4 h
11
98 (100–95)
[327]
7
2.3% PtO/PtO2-Fe3O4 (0.6)
130 °C, 15 min
10
99.6 (100–96)g
[328]
8
0.25% RuII-156-Fe3O4 (2.5)
CH2Cl2, 40 °C, 2–10 hh
22
>97 (>98–95)
[329]
9
0.25% RuII-156-Fe3O4 (2.5)
CH2Cl2, 40 °C, 2–6 hi
18
>97 (>98–96)
[329]
10
3.23% RuII-157-Fe3O4/SiO2 (0.85)
CH2Cl2, rt, 0.5–1 h
14
94 (>99–75)j
[330]
11
2.8% RuII-158-Fe3O4/starch (2008)k
50 °C, 3 h, under argon
5
486 (484–488)l
[331]
12
1% Rh0-159-PPh2/Fe3O4/SiO2 (0.5)
40 atm (CO/H2 = 1:1), toluene, 80 °C
6
100m (70–73)n,o (66–72)n,p
[332]
13
Co0Fe0-CoFe2O4
10
98 (100–96)r
[333]
14
c-Fe2O3 (0.5)
phosphate buffer (pH 7), 20 °C, 5 min H2O, 300 °C, 60 min
10
89.6 (87.4–91.7)
[334]
Entry
a b c d e f g h i j k l m n o p r s
Reaction
s
Percentage data show loading of active species (wt%) when available; data in parentheses indicate mol% used. Average of yields. Data in parentheses indicate range of yields. Runs 1–12 = 16 h, run 13 = 3, run 14 = 1 h. Runs 1–17 = 100%. Residual activities. Regioisomeric excess of the 1,3-isomer. 96% in run 5. Runs 1–13 = 2 h, runs 14–18 = 4 h, runs 19–22 = 10 h. Runs 1–10 = 2 h, runs 11–13 = 4 h, runs 14–18 = 6 h. See text. Substrate/catalyst ratio. TOF values (h1). Conversion. Aldehyde selectivity. Reaction time = 6 h. Reaction time = 3 h. Degradation efficiency. Catalyst/PET ratio.
A catalyst synthesis was started by depositing Au particles onto Fe3O4 nanoparticles by reduction with L-ascorbic acid. These were then functionalized with mercaptoacetic acid followed by repeated treatments with FeCl3 and benzene-1,3,5-tricarboxylic acid in ethanol [336]. Au0-Fe3O4/MIL-100(Fe) core–shell material (MIL = Matérial Institut Lavoisier) with uniformly dispersed Au nanoparticles of 3–5 nm required increasing reaction times in the reduction
of nitrobenzene in 30 runs (entry 1). Kinetic constants dropped from 2.92 min1 (run 1) to 0.51 min1 (run 30). This, that is decreasing rates, however, appears to be a universal feature of this reaction (see also Section 4.5.1, Table 34). A magnetic Fe3O4 and a polyacrylic acid (PAA) core with a PVPy middle layer was covered with a porous ZIF-8 layer. This MOF material was generated by the reaction of Zn(NO3)2 and 2-methylimidazole. In the final step, the
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Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65
Fig. 5. Catalyst MJ-153-Fe3O4/olac used in esterification of oleic acid. Reprinted (adapted) from Ref. [325], Copyright (2017), with permission from Elsevier.
composite product was doped with copper to have 5.45% CuIIFe3O4/PAA/PVPy [337]. High and stable activities were recorded in the oxidation of benzyl alcohol to benzaldehyde with TEMPO and O2 (entry 2). A novel magnetic composite material has recently been fabricated in a multistep process and used in the same reaction. First, Fe3O4 nanoparticles were modified with PAA and then coated with PVP. The resulting core–shell Fe3O4/PVP microspheres were reacted with FeCl3 and 1,3,5-benzenetricarboxylic acid generating a MIL-100(Fe) shell [Fe3O4/PVP/MIL-100(Fe)] [338]. Reactions could be repeated 10 times with similar high efficiencies in the formation of benzaldehyde (entry 3). No further activity was found in a hot filtration test and FTIR and XRD of the recovered sample showed no structural changes. Cokoja, Kühn and coworkers fabricated a range of UiO MOF materials (UiO = Universitetet i Oslo) with the use of ZrCl4 as linker [339]. The best performance in recycling was found with UiO-67 made by reacting a mixture of 2-aminobiphenyl-4,40 -dicarboxylate and biphenyl-4,40 -dicarboxylic acid followed by modification with salicylic acid and, finally, functionalization with [MoO2(acac)2] (catalyst 2.43% MoVI-160-UiO-67). It afforded high, varying activities and selectivities in the epoxidation of cis-cyclooctene (entry 4). A UiO-66 MOF catalyst, bearing aromatic sulfonic acid groups (HSO3-161-UiO-66), obtained by modifying UiO-66-NH2 (the product of 2-aminoterephthalic acid and ZrCl4) with orthosulfobenzoic acid anhydride showed stable activity in acetalization of benzaldehyde (entry 5) [340]. MOF material 16.2% Fe3O(BPDC)3 a product of the reaction between biphenyl-4,40 -dicarboxylic acid (BPDC) and FeCl3 was successfully used in the formation of coumarin (entry 6) [341]. Leaching tests indicated that both catalysts were stable under reaction conditions. All three MOF samples were thoroughly characterized by means of varied instrumental techniques.
Martín-Matute, Zou and coworkers have functionalized a MIL101(Cr) preparation modified with amino groups by nitration and subsequent reduction [342]. Impregnation with Pd[Cl2(MeCN)2] followed by reduction with NaBH4 gave Pd-MIL-101(Cr)NH2 samples with Pd loadings of 4–16 wt%. All products were wellcharacterized (SEM, TEM, FT-IR, XRD and TGA) and used in Suzuki reaction to learn the effect of loadings. 8% Pd0-MIL-101(Cr)NH2 with Pd particles of 2.6 nm proved to be the best catalyst to provide very high activities in the coupling of varied substrate/reagent combinations under mild conditions. It exhibited excellent features in multiple repeating reactions of 4-bromotoluene and pinacol phenylboronate (entry 7). Only limited particle aggregation and a loss of 0.96 wt% Pd were detected after 10 uses. A highly durable catalyst based on MIL-88 B(Cr)NH2 made by reacting CrCl3 with 2-aminoterephthalic acid was fabricated by first depositing Pd particles of 2–3 nm (Na2[PdCl4], NaBH4) followed by encaging the resulting material in a silica coating [343]. The final catalyst has particles of 80 80 220 nm with a ricelike shape and homogeneously distributed Pd particles. It was shown by TGA to have an improved thermal stability when compared to a sample without silica. The catalyst [0.51% Pd0-MIL88 B(Cr)NH2/SiO2] was applied in the oxidation of 1phenylethanol in a continuous flow study. First, a 40 mm long reactor packed with the catalyst (800 mg, 0.0385 mM Pd) was flushed with a mixture of toluene and methanol. This was found to prevent acetalization presumably with methanol reacting with Lewis acidic chromium clusters. A mixing device (1.6 m long microchannel, 1 mL) served to effectively mix the liquid phase (1-phenylethanol and toluene) and synthetic air. A continuous increase in yields was observed resulting from the reduction of unreacted PdII present and a few percents of ethylbenzene byproduct was also detected (entry 8). Pd leaching was below the detection limit (<0.1 ppm), while a constant CrIII level of 0.2 ppm was measured. These results indicate the highly beneficial effect of the protecting silica layer.
Table 41 RCM of varied dienes induced with catalyst 0.25% RuII-156-Fe3O4.
Cycle
X
Y
n
Time (h)
Conversion (%)
1 2 3 4 5 6 7 8 9
TsN (EtO2C)2C O O TsN (EtO2C)2C TsN (EtO2C)2C O
CH2 CH2 C10H21CH C10H21CHCH2 CH2 CH2 CH2 CH2 C10H21CH(CH2)2
1 1 1 1 2 2 3 3 1
2 2 2 12 12 12 12 12 12
>98 >98 98 95 95 95 98 95 96
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Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65 Table 42 Recycling of MOF-based catalysts. Catalysta
Conditions
Number of cycles
Yield (%)b
Ref.
1
Au0-Fe3O4/MIL-100(Fe) (3.4)c
rt
30
5.5 (1–9.5)d
[336]
2
5.45% CuII-Fe3O4/PAA/PVP (1)
1 atm, TEMPO, MeCN, NaHCO3, 60 °C, 12 h
15
97 (98–95)
[337]
3
Fe3O4/PVP/MIL-100(Fe) (5)
10
98
[338]
4
2.43% MoVI-160-UiO-67 (1)
1 atm, TEMPO, KNO2, MeCN, 60 °C, 12 h DMF, 50 °C, 4 h
10
97.5 (100– 92)e
[339]
5
HSO3-161-UiO-66 (0.1)
THF, 23 °C, 2 h
5
98
[340]
6
16.2% Fe3O(BPDC)3 (5)
DMF, 60 °C, 3 h
6
93 (91–95)
[341]
7
8% Pd0-MIL-101(Cr)NH2 (3)
K2CO3, H2O/EtOH (1:1), rt, 0.5 h, under air
10
99 (>99– 97)
[342]
8
0.51% Pd0-MIL-88 B(Cr)NH2/ SiO2 (3)
air, toluene, 110 °C
24f
75.5 (25– 79)g
[343]
9
Al-IQT-HB (2)
2 h, rt
10
97 (97–99)
[344]
Entry
a b c d e f g
Reaction
Percentage data show loading (wt%) of active species when available; data in parentheses indicate mol% used. Average of yields. Data in parentheses indicate range of yields. 3.4 mg catalyst for 0.18 mM substrate. Reaction time (min) for full conversion. Run 3 = 95%, run 8 = 92%. 24 samplings in 168 h. The highest yield of 79% was measured at 144 h.
Al-ITQ-HB is a metal–organic hybrid material synthesized by the solvothermal reaction of aluminum chloride and p-heptylbenzoic acid in dimethylformamide [344]. It represents a novel family developed by the Corma group (ITQ = Instituto de Tecnología Química) based on ordered aluminum cluster-type sheets with Lewis acidic character and hydrophobic pockets [345]. Significant rate accelerations were demonstrated in the synthesis of benzimidazoles and cyanosilylation of ketones. In addition, high catalyst stability was found in 10 cycles in the latter reaction at room temperature under solvent-less conditions (entry 9). 5. Additional examples A few other results performed with catalysts hard to categorize are collected in this subsection. A range of bimetallic PdAg alloy nanoparticles were prepared using a simple one-pot method by co-reducing Pd(NO3)2 and AgNO3 in acidic solution with citric acid at room temperature [346]. The prepared samples of highly crystalline nanoporous structure are composed of nanocrystals of 2–4 nm interconnected three-dimensionally to form crystals of 60–100 nm. XPS firmly confirmed the alloy structure. When used in the catalytic decomposition of formic acid to generate hydrogen, Pd50Ag50 exhibited the highest activity with high stability in recycling (Table 43, entry 1). It was separated by ultracentrifugation, redispersed in water and reused in the next cycle. Czaun, Olah, Prakash and coworkers have developed a quasi carbon neutral energy storage system based on formic acid as a hydrogen/energy carrier [347]. The
catalyst system derived from IrCl3 and 1,3-bis(20 -pyridyl-imino)-i soindoline was applied in generating hydrogen from formic acid with very low concentrations of CO impurity. The solid catalyst material, an iridium carbonyl derivative, formed under reaction conditions (Ir-IndH) can be recovered and reused. Upon repeated addition of formic acid to this system, scattered production levels of the H2/CO2 mixture were observed. In contrast, high activities with small decreases could be maintained in 20 days in a continuous operation (entry 2). Moreover, gas flows of 101.5 mL min1 and 76 mL min1 could be measured after storing the catalyst for one and four years, respectively. Compare these results to those in Table 7, entry 10. Since formic acid was fully consumed and the selectivity is close to 100%, the hydrogen yield can also be considered to be 100%. Miras, Song and coworkers performed intercalation of a series of polyoxometalate salts into layered double hydroxide (LDH) modified with tris(hydroxymethyl)aminomethane (Tris) and used the prepared composites for the degradation of dyes [348]. The best catalyst with the [PW12O40]3 anion intercalated (PW12-Tris/ LDH) showed excellent durability in 10 cycles in the decomposition of methylene blue (entry 3). XRD and FT-IR spectra show no sign of deterioration of the used sample. }llo }si and coworkers have immobilized b-isocupreidine (162) Szo on an anion-exchanger LDH and used this inorganic–organic hybrid material (162-LDH) in the asymmetric Michael addition of ethyl 2-fluoroacetoacetate and trans-b-nitrostyrene [349]. According to XRD, 162-LDH was deposited on the surface of LD with non-covalent interactions without intercalation. The catalytic
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Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65
Table 43 Additional catalysts reused in varied reactions. Entry
Reaction
1
HCOOH
H2 + CO2
2 3
a
c d e f g h i j k l m n
Conditions
Number of cycles
Yield (%)b
Ref.
Pd50Ag50 (10.7)c
60 °C, 10 h, under argon
5
229d
[346]
109 (120– 101)f 99
[347]
Ir-IndH (0.0029) Degradation of methylene blue
PW12-Tris/LDH (50)
g
e
100 °C
5
30% H2O2, H2O, 30 °C, 140 min heptane, 20 °C, 16 h
10 9
99 (99.5– 98.3) 89–76h 67–40i 2.45–1.6j 99.9% (100– 99.8)
[348]
4
162-LDH (2.5)
5
Janus nanosheets based on [bmim]3[PW12O40]
50 °C, 3 h
6
6
[Cp2Zr(H2O)2][OSO2C4F9]2 (2)
rt, 30 min
5
92.5 (92–93)
[351]
7
2.7% Ru-163-PQS (2)
H2O, rt, 2 h
10
97 (>99–92)
[352]
8
Rh0/TPGS-1000 (0.5)
5 atm H2, Na2CO3, H2O, 50 °C, 1.5 h
15
97 (100–85)k,l
[353]
9
PdII-164 (5)
K3PO4, DMF, 100 °C, 48 h
10
92.3 (96–91)k
[354]
10
hollow-channel Au nanoparticles (0.15) Pd(OAc)2/S-Au (0.15)
0.1 M KOH, 0.5 M EtOH, H2O, rt K2CO3, solvent X = Im X = Brn
500
91 (100–86.7)
[355]
10 10
98.5 (>99–97) 99 (>99–99)
[357]
11
b
Catalysta
[349]
[350]
Percentage data show Pd loading (wt%) when available; data in parentheses indicate mol% of catalyst used. Average of yields. Data in parentheses indicate range of yields. mg catalyst used for 10 mL 0.5 M formic acid. mL H2 gas formed in each cycle. Five samplings in day 1, 2, 17, 19, 20. Gas flow (mL min1). mg catalyst used for 50 mL methylene blue solution (10 mg L1). Range of enantiomeric excess of the major (R,R) isomer. Range of enantiomeric excess of the major (S,R) isomer. Range of diastereomeric ratios. Conversion. TPGS added after run 15 increased the conversion to 92%. Microwave heating; step 1: EtOH, 80 °C, 60 min, 200 W; step 2: EtOH, 80 °C, 60 min, 500 W; see also text. Microwave heating; step 1: DMF, 90 °C, 50 min, 300 W; step 2: toluene/H2O (2:1), 104 °C, 60 min, 500 W; see also text.
performance was similar to that of the corresponding homogeneous organocatalyst. Significant deactivation was observed in reactions carried out in dichloroethane. In less polar heptane, in contrast, high, consistent yields and decreasing enantioselectivities were found in an eight-run reuse study (entry 4).
Janus nanosheets were made first by preparing an aqueous solution of [bmim]Cl and a hydrolyzed styrene–maleic anhydride copolymer at 70 °C and pH 2.5 [350]. This was then mixed with the solution of TEOS, triethoxy-(3-(2-imidazolin-1-yl)propyl)silane and (EtO)3SiPh in paraffin at 70 °C. The two solutions were mixed to form and oil-and-water emulsion with a paraffin/silica coreshell Janus structure. Finally, Janus hollow spheres obtained by dissolution of the paraffin core were crushed with an ultrasonic cell
crusher to form Janus nanosheets with imidazolyl on one side and phenyl on the other side acting as both catalyst and emulsifier. Nanosheets after exchanging the anion of the IL for a heteropolyanion were tested successfully in deep desulfurization with a sample based on [bmim]3[PW12O40] affording the best features (entry 5). A mixture of dibenzothiophene dissolved in octane and H2O2 forms an emulsion in the presence of the nanosheets at 50 °C. After complete reactions, the emulsion was broken by centrifugation, the upper oily phase was analyzed, while the lower IL phase was separated from Janus nanosheets with filtration, washed, dried and then reused. Zirconocene complex [Cp2Zr(H2O)2][OSO2C4F9]2 is an air-stable, moisture insensitive strong Lewis acid and an efficient catalyst for the synthesis of N-heterocycles under solvent-free conditions [351]. Five-run recycling studies under mild conditions revealed high stability. The best example is shown in entry 6. Lipshutz and Ghorai designed the amphiphile platform PQS (PEG ubiquinol succinate) with a lipophylic side-chain and a hydrophilic (Me-PEG) component [352]. Then Grubbs first generation Ru complex was attached and the resulting catalyst (2.7% Ru-163-PQS) was used in RCM including a recycling test (entry 7).
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halide was first irradiated (step 1) to release the catalytically active Pd species for the oxidative addition. Then the mixture was transferred to the reaction chamber to complete the catalytic cycle (step 2). Slightly different reaction conditions were necessary to maintain consistently high yields for iodobenzene and bromobenzene in 10-run recycling studies (entry 11). A few selected results with the use of this catalyst in Buchwald–Hartwig amination of aryl halides testifying to high catalyst stability are collected in Scheme 3 [358]. In a follow-up study, an improved and safer method to prepare sulfur-modified Au mesh was disclosed applying aqueous Ru0 nanoparticles stabilized by D-a-tocopheryl polyethylene glycol 1000 succinate (TPGS-1000) micelles with a lipophilic core and hydrophilic shell form an active catalyst system in the hydrogenation of a-pinene to cis-pinane [353] with a good performance in a 15-run recycling (entry 8). The use of basic conditions was crucial to prevent aggregation of Ru0 particles. Drops in product yields in the last cycles were attributed to traces of ethyl acetate used for product extraction. It diffuses into the micelles thereby decreasing the access of substrate molecules. Only a minimal loss of 0.2% Ru was measured after the recycling study. The insoluble pyridyl–triazole complex PdII-164 synthesized by Astruc et al. gave slowly decreasing then stable activities in the Heck reaction in 10 runs (entry 9) [354]. Evans and coworkers reported the fabrication of hollow gold nanoflowers [355]. These were made by depositing Au nanoparticles onto a methyl orange–FeCl3 nanofiber as template in a single step. Primary Au particles are formed by reduction of HAuCl4 with ascorbic acid. In a large excess of the reducing agent, secondary nucleation takes place resulting in the formation of urchin-like particles with spike lengths of 20–30 nm. At the same time, the template undergoes auto-degradation induced by gold particles leaving a cavity. These hollow-channel catalyst particles were tested in the electrocatalytic oxidation of ethanol in a cyclic voltametric study exhibiting high activities and stabilities by retaining 86.7% of the original activity after 500 cycles (entry 10).
Arisawa and co-workers have developed and applied a highly stable, low-leaching Pd catalyst in varied coupling reactions [356]. A gold mesh was modified with sulfur by treatment with piranha solution (conc. H2SO4/30% H2O2 = 3:1) followed by loading Pd(OAc)2 [Pd(OAc)2/S-Au]. It was used in Suzuki couplings under microwave heating [357]. The solution of the catalyst and organic
Scheme 4. A two-step oxidation/asymmetric reduction sequence.
Table 44 Catalysts with satisfactory stability in 10–16 reuses. Catalyst
Ref.
Catalyst
Ref.
Homogeneous catalysts 0.19% PdII-3-HBPE 4.26% PdII-5-PIB 11.4% IrIII-9/poly AuIII-14 [Mo(O)(O2)2(H2O)n] Pd(OAc)2 + IL [{Rh(l-OSiMe3)(cod)}2] + IL CuII-26*-CTC
[25] [28] [31] [39] [46] [48] [55] [60]
Cu(OAc)2-27a* saccharin + DMAP CuI + 43 Pd0 + 52 Rh0 + 55a Rh0 + 55b 16.7% Pt0-56
[61] [79] [81] [95] [98] [98] [99]
[118] [129] [132] [145] [148] [157] [160] [161] [161] [166] [167] [168] [169]
[Rh(cod)][BF4] + 102* Ni90Pd10-PVPy 2.3% CoII-109-poly 1% Au0-PTFE graph-C3N4-PET 5.7% Au0-111-poly 4.8% Au0-yeast 2.9% Pd0-HPS/cotton PdII-115-[bmim][BF4] Pd0-115/116 1.35% Pd0-2000-oxC Pt74Pd26-118-Cnt PdII-120a-rGO
[219] [225] [228] [229] [230] [232] [242] [241] [243] [244] [249] [256] [261]
[169]
RuII-120b-rGO
[261]
Heterogeneous catalysts PdII-60-SiO2 4% Pd0-SBA-15 1.06% PdII-65-PMO 4.7% Ru0-MCM-41 + IL 2.1% PdCl2(phen)/NaY 72-SiO2 2.23% Rh0-76-SiO2 1.28% Rh0-77a-SiO2 2.78% Rh0-77b-SiO2 (0.1) 81a+82-SiO2 Im-83 a-SiO2 2% Pd0-Im-83b-SiO2 5.43% Pd0-HS-SiO2/[bmim] [BF4] 5.23% Pd0-HS-SiO2/[bmim] [PF6] 0.06% [RhCl(PPh3)3]-SiO2/ [bmim][PF6] 0.29% PdII-90-PS 10.7% Pd0-pmim-POSS
[170]
AgAu-rGO
[263]
[199] [175]
[267] [302]
1% Pd0-Oct/triN-SiO2 0.54% Pd0-Oct/SiO2 1% Pt0-89-TiO2
[177] [178] [178]
(PdCl2/-bpy)10/quartz
[182]
P2W15-Al2O3
[187]
HPW-NH2-Al2O3 Nb2O5
[189] [194]
15.05% PdII-92-POP
[203]
2% Ni0-Fe3O4/SiO2 2.23% PdII-138-Fe3O4/ SiO2 34% Pd0-122-IL/C/Co 1.95% Pt0-Fe3O4/SiO2 14.3 wt% Au0-Fe3O4/115/ rect 1.31% Pd0-140-Fe3O4/ SiO2 0.89% Pd0-141-Fe3O4/ SiO2 PdII-145-C/Co mpSiO2NHAc/ npSiO2SO3H/Fe3O4 1.96% Ru0-150-CoFe2O4/ SiO2 Fe3O4/PVP/MIL-100(Fe) Al-IQT-HB Pd(OAc)2/S-Au CuCl2/PdCl2 + [(R)-ADH]
0
Scheme 3. Recycling studies with Pd(OAc)2/S-Au catalyst.
6.9% Pd -93-HBPE CA/Novozyme 5.43% Pd0-poly/epox binol-100-poly
[204] [215] [217] [218]
[271] [274] [285] [304] [305] [309] [315] [317] [338] [344] [357,358] [360]
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Table 45 Catalysts with satisfactory stability in 18 and more reuses. Catalyst
Ref.
Catalyst
Ref.
Homogeneous catalysts 4-poly Rh0 (PEG4000)
[27] [89]
AgI + Cs2CO3 Au0-58a-HPEIa
[80] [102]
[133] [133] [151] [154] [163] [172] [176] [180]
RhIII-96*-poly 0.71% Au0-1140.58-PANfc Au0-paper 3% Pd0-Ni0/CeO2–x (1 105) 0.43% Au0-Fe3O4/poly 9.6% Pd0-126-Fe3O4/GO 10.6% TEMPO-128-Fe3O4/PS 2.45% PdII-142-Fe3O4/poly
[210] [236] [239] [272] [284] [286] [290] [306]
Heterogeneous catalysts (HCA)Au0-HS-SBA-15 (HCA)Au0-H2N-SBA-15 AO/peroxidase-MCF CR/glymo-SiO2(HIPE)b IrIII-80a*-SiO2 0.02% RhIII-86-SiO2 + IL 0.46% Pd0-Oct/triN-SiO2d 0.021% Pd0-PVPy/[bmim] [PF6]/LCe 0.02% Pd0-HDA/[bmim][PF6]/ LCf Pd-glassA [bmim][FeCl4]/TiO2 TiO2 0.014% Ag0-(PVA) 1.14% Pd0-(PVA)
[180]
PdII-139a–139d-Fe3O4/poly
[303]
[181] [188] [198] [200] [201]
[310] [324] [325] [329] [343]
26.7% Pt0-HIPE Ni0-starch
[222] [237]
1.36% PdII-146-Fe3O4/115 151b-Fe3O4/SiO2 MJ-153-Fe3O4 0.25% RuII-156-Fe3O4 0.51% Pd0-MIL-88B(Cr)NH2/ SiO2g Ir-IndH hollow-channel Au nanoparticles
Pd0-SiO2/PDA/glass
[245]
[347] [355]
a
Reaction time had to be increased significantly in runs 18 and 19. A yield of 57% in run 21. Small changes in activity in the first 33 runs and larger decreases to a final activity of about 50% in run 45. d With increases in reaction time. e Run 17 = 89%, run 18 = 55%. f Run 16 = 58%, run 17 = 21%. g Continuous flow experiment. b
c
Na2S2O8 in H2SO4 [359]. This second generation catalyst is of low leaching and recyclable giving identical activity range in the amination of bromobenzene with morpholine (93–91%). Gröger and coworkers have developed a rather unique method to combine Wacker oxidation induced by CuCl/PdCl2 and subsequent enzymatic reduction to transform styrene to 1phenylethanol in ‘‘one pot” [360]. Called compartmentalization, Wacker oxidation was performed in a thimble made of polydimethylsiloxane placed in a flask. Organic products formed in oxidation can diffuse out through the thimble membrane into the
flask containing the enzyme. Asymmetric biocatalytic reduction in the outer phase is induced by alcohol dehydrogenase [(R)-ADH] from Lactobacillus kefir (L. kefir). In this arrangement, site-isolation prevents poisoning of the enzyme by metal ions and isolation of the intermediate ketone is not necessary. The system proved to be robust affording high conversions and excellent enantioselectivities (Scheme 4).
6. Summary analysis The literature abounds in publications claiming the development of recyclable catalysts. In close scrutiny, however, it turns out that only a few repeated runs are tried, which is insufficient to arrive at any definite conclusion. Moreover, decreasing conversions and yields are reported in most cases. That is, only a fraction of these studies have real practical significance. Therefore, it may be useful and instructive to sum up a few important conclusions of the varied examples collected in this review. Over 300 catalyst preparations with the corresponding information of their performance in recycling studies have been selected and discussed. Of these catalysts about 150 have been tested at least in 10–16 repeated runs. Many of these, however, exhibit insufficient stability that is they show decreasing activities in repeated cycles. Therefore, only catalyst samples exhibiting stable or increasing activities or show only a maximum overall decrease of 1–1.5% in conversions or yields have been selected and collected in Table 44. Catalysts that underwent more exhausting tests (18 or more reuses) are found in Table 45. Here, the rule is a maximum of 0.5% decrease per cycle. Naturally, such a degree of cumulative deactivation (9% loss of conversions or yields in 18 cycles) is a sign of unsatisfactory stability in the long run. Consequently, it is obvious that only a handful of catalysts can fulfill even such a modest selection condition. A few heterogeneous catalysts have also been included, which exhibit full conversions and high yields in long runs but activity drops in the last cycles. Finally, because of the significance of high turnover numbers in assessment of catalyst reuse related examples are collected in Table 46. In comparison, the table also lists catalysts, which are not treated in the manuscript because their recycling performance has not met the conditions set for selection (references indicated by an asterisk).
Table 46 Catalysts providing high cumulative TON values. Catalyst III
Rh -38 Rh(acac)(CO)2 + 1a Ir(acac)(cod) + 1a IrIII-39 fluorous-47 16.7% Pt0-56 Au0-58b-HPEI/dendr 4.7% Ru0-MCM-41 + IL 1.81% RhIII-70a-SBA-15 0.02% RhIII-86-SiO2 + IL 1% Ru-TiO2 0.006% Rh0-ZnO/nw 1.14% Pd0-(PVA) 1.55% Rh0-Fe3O4/SiO2 0.09% Rh0-Fe3O4/SiO2 1.95% Pt0-Fe3O4/SiO2 1.36% PdII-146-Fe3O4/115 Perfluoro-tagged Pd particles on fluorous SiO2 0.076% Pd0-alginate/gellan [IrCp*Cl2] 2,20 -bipyridine complex on porous triazine framework *
Continuous flow experiment.
Reaction
No of cycles
Hydroformylation Hydroformylation Hydroformylation H2 from formic acid Singlet oxidation of citronellol Hydrogenation of methyl crotonate Reduction of 4-nitrophenol Hydrogenation of CO2 Hydroformylation Asymmetric hydrogenation Hydrogenation of CO2 to formic acid Hydroformylation Suzuki coupling Hydrogenation of cyclohexene Hydrogenation of cyclohexene Hydrogenation of pentan-2-one Heck coupling Suzuki coupling Suzuki coupling Hydrogenation of CO2
a
5 3 11 25 14 19 10 17 a
10 5 30 18 7 14 66 4 4 5
Cumulative TON 4
1.29 10 2.5 105 1.1 105 3.85 104 2.7 104 2.2 104 2.3 104 1.77 105 6.9 104 1.43 105 2.3 104 1.87 104 4.96 104 1.8 105 3.0 105 1.56 104 1.13 105 3.33 105 1.9 105 2.8 104
Ref. [71] [72] [72] [73] [86] [99] [103] [145] [146] [172] [190] [191] [201] [268] [269] [274] [310] [361]* [362]* [363]*
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In summary, 128 catalysts are listed in the above three tables and only about half of these show real promising properties. The great majority of these systems are supported catalysts. This is not surprising since one of the main advantages of heterogeneous catalysts is easy separation. In fact, great efforts have been devoted to develop such catalyst systems with improved stability and recyclability in recent years. This is the final tally of our survey for valuable recyclable catalysts from the last few years. 7. Conclusions and outlook 7.1. Successful examples of recycling On the basis of the above data it is easy to corroborate a conclusion of recyclable catalysts. As it has recently been formulated by these authors [364], unsatisfactory stability in the long run is the rule rather than the exception. And then an obvious question arises: is it necessary to study catalyst recycling at all and make any efforts to develop recyclable catalysts? Considering practical applications, a high cumulative TON is more crucial than good recycling ability of a catalyst that is yields as a function of cycles. As stated by Gladysz ‘‘the pursuit of higher-turnover-number catalysts represents a more important research direction than recoverable catalysts” [7]. However, there is an endless list of reactions where highly productive catalysts, that is, catalysts affording high TON values do not exist because the necessary efforts simply have not been made. Sections 3 and 4 cover a wide range of reactions with a number of catalysts with sufficiently high stability and low TON numbers (see Section 6 with the related summary analysis). A few examples may be illustrative here. Very high productivity and activity data in the Suzuki coupling with the use of [PdCl(g3-C3H5)]2 together with a-cyclodextrin ligand are unprecedented: a TON of 3.4 1011 and a TOF of 1 109 h1 were achieved using 1 1010 mol% of catalyst [365]. However, a mere 0.84 mM of product was formed in 14 days! In comparison, PdCl2 reused 140 times in a mixture of ILs bis(diphenylphosphano)cobal tocenium hexafluorophosphate and [bmim][PF6] gave 31.5 mM of product in 280 h (cumulative TON = 1.3 104) [366]. Magnetic catalyst 1.36% PdII-146-Fe3O4/115 gave an even better performance [310]: it was reused in 66 cycles in the Heck reaction with a cumulative TON of 1.13 105 allowing the isolation of 259 mM of product (0.23 mol% Pd, 99 h). 7.2. The advantage of recycling There is an ongoing debate in the catalysis community about of the usefulness and significance of catalyst recycling. There is no doubt that the development of robust catalysts requires dedication and hard work. It is especially true for the immobilization of metal complexes. Since the activity of a modified complex is often lower, when compared to that of the original homogeneous catalyst, efforts appear to be useless. Prime examples are oxide-supported catalysts fabricated with the use of surface organometallic chemistry pioneered by Copéret and Basset [367,368] as well as polymer-supported catalysts developed by Buchmeiser [369]. Only a handful of rather unsuccessful attempts are referred to in these reviews. In 2007, Copéret and Basset declared [368]: ‘‘From all available literature data, it is clear that the supported system have never out-performed the homogeneous systems (TON ranges from 100–2000 in RCM, and after 5–7 recycling using 1–5 mol%, most systems show a sharp drop in activity).” However, Buchmeiser using a monolithic catalyst preparation already reported in 2001 [370]: ‘‘This results in turnover frequencies (TOFs) up to 25 min1, thus even exceeding homogeneous analogues. In comparison, the TOF for diethyl diethylmalonate using [Cl2Ru(Mes2-NHC)(PCy3) (CHPh)] is 4 min1.” Another conclusion by Brookhart, Scott and
59
coworkers in 2009 [371]: ‘‘The simplest of these approaches, adsorption on c-Al2O3, is also found to be the most effective, yielding catalysts that are. . .comparable to or even more active than the corresponding species in solution.” And then, in 2011, an interesting remark by Basset [372]: ‘‘. . . the catalyst [a single-site tantalum hydride supported on fibrous silica (KCC-1)] proved to be quite regenerable and using it in a second run yielded similar or even better results than the first run.” It is certain, that immobilization of metal complexes to fabricate robust catalysts can be successfully accomplished applying appropriately selected support materials. As demonstrated in this review, the use of varied carbon/graphene preparations, dendrimers, mesoporous solids and polymers appears to be a more promising approach. A remark for illustration by Marks and Zhao [373]: ‘‘In many cases, these catalysts [polymer-supported organolanthanides] exhibit activities comparable to the homogeneous precursors and are recoverable/recyclable with only minor to moderate loss of activity, depending on the particular resin amino substituents.” According to data collected and analyzed in this review, the search for stable, recyclable catalysts should not be unproductive. Catalyst recycling, in general, is of enormous importance from the point of view of industrial applications and the environment. Environmental concerns and economic consideration and, in particular, the limited availability of expensive noble metals, necessitate the development of catalyst with good recycling ability that is it is an important issue worth pursuing. Catalysts including both homogeneous complexes and supported/immobilized samples with extremely high productivity and activity do exist. There are a few examples highlighted in this review when catalysts exhibited excellent features up to 100 reuses without significant catalyst deterioration. These recent accomplishments can serve as a good basis for additional studies and justify the usefulness of efforts to achieve further development. Within the context of this review, catalyst use in the fine chemical industry appears to be highly relevant. However, the use of heterogenized catalysts in industrial practice is still rare. One of the reasons is economic: the high value of organic products prepared usually in multistep processes compares unfavorably with the relatively small cost of investments. Consequently, there was no stimulus for development of immobilized catalysts. Furthermore, the fabrication of the required complexes tailored for specific processes and their subsequent heterogenization are laborious and time-consuming. With the advent of green chemistry, however, requiring among others waste minimization and the avoidance of hazardous reagents, the scenario has changed. Consider also the dwindling supply and high price of precious metals used in many synthetic operations. The high price of organometallic complexes should make their reuse even more desirable. Heterogeneous catalysis appears to be the obvious solution. Consequently, there is a high demand for further efforts to develop durable catalysts and suitable means of their recovery, reactivation and regeneration. Naturally, catalyst deactivation in industrial processes requires special attention and regeneration is a key issue with respect to economy and sustainability [374]. 7.3. The problem of catalyst deactivation There are a number of reasons resulting in decreasing or loss of catalytic activity and/or selectivity in repeated uses. In the case of immobilized complexes, the dissolution of active species may take place and these may be recaptured as another, less active species. Detachment of active species from the support surface and their decomposition may also take place. Fouling that is coverage of the surface with deposits such as carbonaceous residues (coke formation) and thermal degradation are two rather common reasons for deactivation, in particular, at high temperature.
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When working with heterogeneous catalysts, there are multiple reasons for deactivation [375]. In the case of metal nanoparticles, the increasing particle size because of Ostwald ripening can be a serious problem. The driving force of the process is the higher stability of larger particles having a lower surface to volume ratio in comparison to smaller particles, which have higher surface energy and higher total Gibbs energy. As a consequence, the number of energetically more stable larger particles increases at the expense of smaller particles resulting in re-dispersion. It is well-known that larger particles with a lower number of available surface atoms exhibit lower activities. A similar problem is sintering resulting, again, in the increase in particle sizes and the concomitant decrease in active surface area. Additional reasons are poisoning caused by strong chemisorption of impurities and mechanical failure (attrition, erosion). The latter leads to fracture of catalyst particles to form ever smaller particles, which may be lost. Such incomplete catalyst recovery is an important problem, in particular, if simple filtration is used. Importantly, however, there are a number of remedies from organic solvent nanofiltration, ultraand size-exclusion filtration, centrifugation through phase separation and precipitation to the use of multiphase reaction media and magnetically recyclable catalyst preparations. 7.4. What to do? This is a ‘‘what to do list” for further advancement of recyclable catalysis. (i) Surface and structural modifications and appropriate functionalization have resulted in catalysts with improved stability and durability upon reuses. Following this line with valuable data in hand further efforts with the use of the great variety of experimental possibilities available for such manipulations will allow fine-tuning of properties and enhancement of both catalyst lifetime and stability for prolonged use. In addition, the widening of the reaction scope is also expected. (ii) It should also be an obvious necessity to characterize and evaluate catalysts before and after reaction and monitor continuously the working catalyst systems to collect information of recyclability, catalyst recovery and deactivation. Mechanistic studies can contribute important basic information. In particular, exploring the nature of the true catalytic species is of utmost importance. (iii) Simple methods often neglected such as poisoning and filtration tests as well as detecting the leaching of active species may supply useful data to accomplish further improvements. In short, the full use of techniques available for catalyst characterization to elucidate more fully the working catalysts is absolutely necessary to further the development of the field. (iv) Continuous-flow processes offer a number of advantages such as high efficiency, easy scale up and their possible application for both homogeneous and heterogeneous catalytic systems. Flow processes are faster, more efficient and scalable. All in all, this technique has real promise as an alternative to efficient catalyst reuse. Note, that sequential reactions in a flow can also be operated. References [1] (a) W.A. Cornils, I.T. Herrmann, Horváth, W. Leitner, S. Mecking, H. OlivierBourbigou, D. Vogt (Eds.), Multiphase Homogeneous Catalysis, Wiley-VCH, Weinheim, 2005; (b) M. Lombardo, A. Quintavalla, M. Chiarucci, C. Trombini, Synlett (2010) 1746–1765. [2] A.E.C. Collis, I.T. Horváth, Catal. Sci. Technol. 1 (2011) 912–919.
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Review
Catalyst recycling—A survey of recent progress and current status Árpád Molnár a,⇑, Attila Papp b a b
Department of Organic Chemistry, University of Szeged, Szeged, Hungary Soneas Research Ltd, Hungary
a r t i c l e
i n f o
Article history: Received 22 June 2017 Accepted 14 August 2017
Keywords: Complex Heterogeneous catalyst Nanoparticle Recycling Ionic liquid Magnetic catalyst
a b s t r a c t This review gives a comprehensive treatment to show and illustrate current efforts in the field of recyclable catalysis. Results of recycling studies performed with a wide range of soluble homogeneous and immobilized complexes as well as heterogeneous catalysts developed in recent years have been collected and discussed. Among others, transformations including hydrogenation, reduction, oxidation, varied asymmetric syntheses and coupling reactions are covered. A thorough analysis of the available data and discussion of issues related to recyclability in general are also given. The review includes selected literature examples until the beginning of 2017. Ó 2017 Elsevier B.V. All rights reserved.
Contents 1. 2. 3.
4.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 About valid assessment of recycling ability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Catalyst recycling from homogeneous systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 3.1. Temperature-controlled phase-separable catalyst recycling. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 3.2. Aqueous biphasic catalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 3.3. Ionic liquid-based systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 3.4. Recycling of soluble catalysts used in monophasic reactions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 3.5. Stabilized soluble metal nanoparticles in reuse studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Recycling of solid catalysts applied in heterogeneous systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 4.1. Catalysts systems based on siliceous support materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 4.1.1. The use of supports with ordered structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 4.1.2. Catalysts with amorphous silica support . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Abbreviations: AAS, atomic absorption spectroscopy; AB, ammonia–borane (H3N:BH3); acac, acetylacetonate; BAIB, [bis(acetoxy)iodo]benzene; bdmi, 1-butyl-2,3dimethylimidazolium; BARF, tetrakis-[3,5-bis(trifluoromethyl)phenyl]borate; BINAM, 1,10 -binaphthyl-2,20 -diamine; BINAP, 2,20 -bis(diphenylphosphano)-1,10 -binaphthyl; bmim, 1-butyl-3-methylimidazolium; BOX, bisoxazoline; BWT, black wattle tannin; C10mim, 1-decyl-3-methylimidazolium; Cnh, carbon nanohorn; Cnt, carbon nanotube; DABCO, 1,4-diazabicyclo[2.2.2]octane; dansyl, [5-(dimethylamino)naphthalene-1-sulfonyl]; dba, dibenzylideneacetone; DMAc, N,N-dimethylacetamide; DMF, N,Ndimethylformamide; dmpz, 3,5-dimethylpyrazole; DA, dopamine; DPEN, 1,2-diphenyl-1,2-diaminoethane; ee, enantiomeric excess; emim, 1-ethyl-3-methylimidazolium; FESEM, field emission scanning electronic microscopy; FSG, fluorous silica gel; F-SPE, fluorous solid-phase extraction; GO, graphene oxide; rGO, reduced graphene oxide; graph, graphite; HBPE, hyperbranched polyethylene; HMS, hexagonal mesoporous silica; HPEI, hyperbranched polyethylenimine; HRTEM, high-resolution transmission electron microscopy; ICP-AES, inductively coupled plasma atomic emission spectroscopy; ICP-OES, inductively coupled plasma optical emission spectroscopy; IL, ionic liquid; l/br, linear to branched ratio; LC, laponite clay; MCF, mesocellular foam; MAB, methylamine–borane (MeH2N:BH3); mim, methylimidazolium; MTBE, methyl tert-butyl ether; MW, microwave heating; NHC, N-heterocyclic carbene; NMP, N-methyl-2-pyrrolidone; Oct, octyl; olac, oleic acid; omim, 1-methyl-3-octylimidazolium; PAA, polyacrylic acid; PE, polyethylene; PEI, polyethyleneimine; PEG, poly(ethylene glycol); PET, polyethylene terephthalate; PIB, polyisobutylene; pmim, 1-propyl-3-methylimidazolium; PMO, periodic mesoporous organosilica; poly, polymer; ppy, 2-phenylpyridine; PS, polystyrene; PVP, poly(4-vinylpyridine); PVPy, poly(N-vinyl-2-pyrrolidone); RCM, ring-closing metathesis; rGO, reduced graphene oxide; scCO2, supercritical scCO2; SFM, scanning force microscopy; TBAAc, tetrabutylammonium acetate; TBAB, tetrabutylammonium bromide; TBHP, tert-butyl hydroperoxide; TEOS, tetraethyl orthosilicate; TEMPO, 2,2,6,6-tetramethylpiperidine 1-oxyl; TfO, triflato (CF3SO3, trifluoromethylsulfonato); TGA, thermogravimetric analysis; TOF, turnover frequency; TON, turnover number; TPPTS, P(C6H4-mSO3Na)3; TRPTC, thermoregulated phase-transfer catalysis; TsDPEN, N-(p-toluenesulfonyl)-1,2-diphenylethylenediamine; VSM, vibrating sample magnetometer. ⇑ Corresponding author at: Dóm tér 8, H-6720 Szeged, Hungary. E-mail addresses: [email protected] (Á. Molnár), [email protected] (A. Papp). http://dx.doi.org/10.1016/j.ccr.2017.08.011 0010-8545/Ó 2017 Elsevier B.V. All rights reserved.
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4.1.3. Catalysts with other silicon-based supports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Use of oxide-based catalysts. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.1. Oxides as catalyst support . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.2. Oxides serving as catalytically active phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Polymer-supported catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.1. Catalysts based on synthetic polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.2. Catalyst supported on natural polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4. Varied carbon materials serving as catalyst support. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5. Magnetic and magnetically tagged catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.1. Recycling in hydrogenation and hydrogenolysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.2. Reuse studies in oxidations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.3. Recycling of magnetic catalysts in asymmetric syntheses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.4. C–C bond forming reactions induced by magnetic catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.5. Reuse in hydrogen generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.6. Recycling of magnetic catalysts in other transformations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.6. Catalyst based on MOF materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Additional examples. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Summary analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1. Successful examples of recycling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2. The advantage of recycling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3. The problem of catalyst deactivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4. What to do?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.
5. 6. 7.
1. Introduction ‘‘Green” has become a buzzword in recent decades for sustainability. Sustainable chemistry is of extremely high importance stimulating the development of clean processes and technologies. It includes waste reduction, minimization of materials and energy, renewability, introduction and use of environmentally benign reagents and efficient processes. In particular, recycling, that is the reuse of catalysts, the topic of this review, is an important issue when considering the limited availability and dwindling supply of expensive noble metals. A long catalyst lifetime and the ability to easily recycle the catalyst are highly desirable for industrial applications. That is, both environmental and economic considerations push for the development of processes that enable the separation and recovery as well as reuse of the catalyst. The general requirements of any catalyst are high activity, selectivity and stability for prolonged use. Furthermore, a catalytic process should have low environmental impact. Homogeneous (soluble) metal complexes are well-known for their high, in certain instances, extremely high activities and selectivities to induce varied transformations. No wonder that they are widely used in both academia and industry in synthetic organic chemistry and commercial chemical technologies. The downside, however, is often low stability and the difficulty in reutilization because of the complexity of their separation being time consuming and requiring high energy. These problems may be solved using ‘‘non-conventional” solvents (ionic liquids, water, fluorinated and supercritical solvents) and multiphase homogeneous catalysis [1]. Heterogenization that is immobilization of catalytically active species on suitable carriers may provide another solution to these needs [2]. Solid heterogeneous catalysts are easy to handle and can easily be separated by simple techniques (filtration, centrifugation, magnetic separation, etc.) allowing catalyst recycling. Immobilization, however, introduces additional synthetic steps in catalyst preparation. Furthermore, the activity and selectivity of immobilized catalysts are frequently lower compared to the corresponding soluble homogeneous complexes. One of the main reasons is the much lower number of available active sites, compared to the single-site nature of homogeneous complexes. Additionally, deactivation resulting from poisoning, metal leaching
27 29 29 30 31 31 37 39 42 42 44 46 47 50 51 52 55 58 59 59 59 59 60 60
and decomposition take place (see a detailed discussion in Section 7.3). A further major problem is environmental concerns arising from the use of costly and often poisonous materials applied in catalyst synthesis. To compile a general review of the field of catalyst recycling is an immense task. A review by Cole-Hamilton [3], as well as two excellent books edited by Cole-Hamilton and Tooze [4] and de Vos, Vankelecom and Jacobs [5] have been published. However, all three have focused only on soluble and immobilized homogeneous complexes. Additional related reviews covering even narrower fields are also available for interested readers [1,6]. Gladysz has discussed general problems of catalyst recycling in two relevant papers [7]. A comprehensive review about catalyst recycling comprising varied possibilities of catalyst recovery and reuse and, at the same time, covering diverse transformations has not been compiled before. This review is intended to be illustrative by presenting and discussing a selection of varied examples of the last 6–7 years with a handful of results from 2017. Nevertheless, it is a comprehensive treatment to show and illustrate current efforts in the development of the field. In addition to recent achievements, a few noteworthy examples from earlier studies will also be treated here as appropriate. A thorough analysis of the available data and discussion of issues related to recyclability in general are also included. The aim of this review is to show a wide spectrum of reactions and catalyst systems with the potential of catalyst recycling. Still, the strict selection rules applied in related previous reviews by the principal author are also applicable here [8]. Specifically, the general rule for data selection is that decrease in percentage activity (conversion or yield) cannot be higher than the number of cycles. Of studies with 5–9 runs only results without decreases in activity are treated. There are, however, a few notable exceptions (for example, novel catalysts and rarely studied reactions). A further precondition for selection, in all cases, is the availability of appropriate catalyst characterization information. Whereas consistently high yields in repeated runs cannot be taken as satisfactory evidence for high catalyst stability, they testify to the robust nature of a catalyst system, which is a necessary requirement for long-term uses (see also Section 2).
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Strictly speaking, studies about continuous flow systems do not belong to the topic of this review. Nevertheless, a handful of excellent examples with high catalyst stability affording high productivities (TON numbers) are included to demonstrate the merits of such approaches. Readers may find it useful to consult a recent review [9]. 2. About valid assessment of recycling ability Before undertaking the elaboration of the topic, it appears to be important to start with a short discussion of this closely related, important question of recycling chemistry. Characteristically, the consistently high yields of a catalyst in a few repeated runs are taken as evidence to prove its high recycling potential. In the great majority of published data, however, small but consistent drops of yields are experienced. Naturally, this is a clear sign of unsatisfactory catalyst stability in the long run. A number of examples exist when steadily and significantly decreasing tendencies in yields are erroneously characterized by the authors to state satisfactory catalytic activities. Kinetic data may provide important pieces of information about the nature of active species. As pointed out by Gladysz [10], an induction period and catalytic activity of the separated reaction mixture as well as induction periods in repeated runs of the recovered catalyst often with decreased activity are clear signs indicating that the ‘‘recycled” species is certainly not the active catalyst. Induction periods detected in successive catalytic cycles show the regeneration of the active catalyst in every new cycle that is the catalyst resting state is actually recycled. It is quite unfortunate that kinetic analysis has been rarely performed in recycling studies. An additional key problem is the efficacy of catalyst recovery, which is seldom determined. Considering practical applications, a high cumulative TON is more crucial than good recycling ability of a catalyst that is yields as a function of cycles. Furthermore, average TOF values giving the rate of product formation measured in each cycle are valid and meaningful data. Data sets collected by such careful and thorough studies will certainly allow the unequivocal assessment of catalyst systems with respect to recyclability, stability and deactivation if any. See also a related excellent discussion by Jones [11]. 3. Catalyst recycling from homogeneous systems A basic and important necessity of an efficient and recyclable catalytic process is the easy and, as far as possible, complete separation of the catalyst from the reaction system. Considering homogeneous complexes, either heterogenization or the use of two-phase systems affords a suitable solution. In the latter case, the catalyst is dissolved in one phase and the substrate/product in the other. Phase-separation at the end of the reaction allows reusing the catalyst-containing phase in the next run. The most frequently applied methods are organic/organic separation including fluorous biphasic chemistry, the use of aqueous biphasic catalysis and ionic liquid-based systems. These are discussed in Sections 3.1–3.3. Other specific options are recycling of soluble solid catalysts used in monophasic reactions (3.4) and the use of stabilized soluble metal nanoparticles (3.5). For readers interested in pursuing the subject further, a few additional sources may prove valuable to find important background information [12]. Additional readings are about practical aspects of these methods [13] including industrial applications [14].
3
[15]. A liquid–liquid system can be transformed into a single phase by changing the temperature of the reaction mixture. After reaction, the biphasic system is formed again by temperature change with one phase containing the product and the other the catalyst. Phase separation may also be achieved by perturbing the reaction mixture by the addition of water. Another possibility is to have a homogeneous monophasic reaction system, which upon cooling results in precipitation of the solid catalyst. Interested readers are advised to consult recent publications for further details [16]. As mentioned in Introduction, this review covers primarily results of recent years. However, this subsection cannot be treated without briefly mentioning the seminal work by Horváth and Rábai who developed the fluorous biphasic catalyst recovery concept [17] with the main motivation to carry out hydroformylation of high-molecular-weight alkenes. They reported the use of a rhodium catalyst prepared in situ from Rh(acac)(CO)2 and fluoroussoluble phosphane P[CH2CH2(CF2)5CF3]3. When applied in a 1:1 t oluene–perfluoromethylcyclohexane (C6F11CF3) solvent mixture in the hydroformylation of dec-1-ene, a homogeneous singlephase system is formed at reaction temperature (100 °C). There are two great advantages of this catalyst system: (i) it can be utilized for the hydroformylation of alkenes of both low and high molecular weights and (ii) the biphasic system formed at room temperature allows facile catalyst separation since the product phase is easily separated from the catalyst-containing fluorous phase, which is then recycled. Recent results with respect to the application of thermomorphic solvent systems are collected in Table 1. These afford catalysis to take place in a single phase followed by catalyst recovery via phase separation. Behr and co-workers have developed thermomorphic multicomponent solvent systems and performed varied transformations [6a,18]. Upon studying in detail the ternary model systems DMF–decane–dodec-1-ene and DMF–decane–dodecanal, hydroformylation of dodec-1-ene was carried out in a 1:1 DMF– decane mixture containing 16 wt% dodec-1-ene induced by the Rh(acac)(CO)2/Biphephos (1) (1:20) catalyst system [19]. The polar DMF phase separating at 10 °C was reused in an eight-run recycling study (entry 1). The original l/br (linear/branched) ratio of 98:2 decreased to 84:16 in run 5 but increased upon adding fresh ligand 1a. Cumulative losses of Rh and ligand in eight runs were, respectively, 40 and 229 ppm. Significantly lower levels of leaching were reported in a follow-up study when the catalyst system was reused in 30 runs (entry 2) [20]. The first efficient recycling of an iridium catalyst used in hydroformylation of oct-1-ene has recently been reported [21]. The reaction was induced by Ir(cod) (acac) in the presence of sulfonated triphenylphosphane ligands in the mixed polar solvent DMF and water under homogeneous conditions. Product separation was achieved by extraction with the nonpolar solvent 2,2,4-trimethylpentane. Recycling data using TPPMS (monosulfonylphenylphosphane) are collected in entry 3.
3.1. Temperature-controlled phase-separable catalyst recycling Catalyst recovery from thermoregulated or thermomorphic solvent systems is easily performed by a temperature change
Recent efforts by the Behr group with respect to hydroformylation to explore other possibilities in catalyst recycling including
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Table 1 Catalyst recycling in thermoregulated systems. Entry
Reaction
1
Catalysta
Conditions
Number of cycles
Yield (%)b
Ref.
Rh(acac)(CO)2 (0.1)
1a, 20 bar (CO/H2 = 1:1), DMF/decane (1:1), 100 °C, 2 h
8
74.7 (78–72)
[19]
Rh(acac)(CO)2 (0.1) Ir(cod)(acac) (0.8) PdII-2 (1)
1a, 20 bar (CO/H2 = 1:1), DMF/decane (1:1), 100 °C, 2 h 30 bar (CO/H2 = 2:1), TPPMS, DMF/H2O (39:1), 100 °C, 5 h DABCO, CuI, DMF/FC 77 (1:1), 135 °C, 4 h c
30
71 (79–54)
[20]
5
72 (67–75.54)
[21]
8 8
99.9 (100 ? 99) 98 (100–92)d
[24]
PdII-2 (1)
DABCO, CuI, DMF, 135 °C, 4 h
96 (100–74) 100e >99f
[24]
R= C10H21 2
R = C10H21
3
R = C6H13
4 O2N
Ph
I + Ph O2N
Et3N, heptane/DMF (1:1), 100 °C, 16 h
7
0.19% Pd -3HBPE (0.05) 4-poly (2)
14 8 10
(Boc)2O, heptane/EtOH (1:1), rt, 1 h
20
91
[27]
8
4-poly (5)
Heptane/EtOH (1:1), 80 °C, 24 h
20
91
[27]
9
4.26% PdII-5PIB (1)
Et3N, heptane/DMF (1:1), 75 °C, 46 h, under nitrogen
10
>97f
[28]
10
CrIII-6a*-PIB (1)
Heptane/EtOH (1:1), rt, 24 h, under nitrogen
5
85 (70–99)
[29]
CrIII-6b*-PIB (5)
Heptane/EtOH (1:1), rt, 12 h, under nitrogen
6
58.5 (42–95) 10–22g
[29]
12
CrIII-7-PE (5)
Toluene, 75 °C, 3 h, under nitrogen
6
96 (88–100)
[29]
13
RuII-8-PE (5)
Toluene, PEG, 70 °C, 8 min, under nitrogen
5
86
[30]
14
11.4% IrIII-9PIB (1)
Bu3N (2 equiv), Hantzsch ester (2 equiv), MeCN/heptane, 85 °C, 3 h
10
91 (86–96)
[31]
15
11.4% IrIII-9PIB (2.5)
Bu3N (10 equiv), HCCOH (10 equiv), MeCN/ heptane, 85 °C, 3 h
10
67.5 (60–76)
[31]
16
11.4% IrIII-9PIB (0.023)
iPr2NEt, MeCN/heptane, 90 °C, 30 min
11h
83:17–81:19i
[31]
5
II
6
OH
11
O + Me3SiN3
a b c d e f g h i
[25]
N3
Percentage data show loading of active species (wt%) when available; data in parentheses indicate mol% used. Average of yields. Data in parentheses indicate range of yields. The last run in 6 h. Without CuI; the last two runs in 8 h. Without CuI; the first two runs in 2 h. Conversion. Range of enantiomeric excess. Number of samplings. Range of Z/E ratios.
the use of solvent systems, organic solvent nanofiltration and varied technical solutions are useful further readings [22]. A review about nanofiltration is also advised [23]. Lu and co-workers applied the fluorous biphasic concept by conducting Sonogashira reactions under two conditions [24]. Catalyst PdII-2 with fluorous ponytails applied in the mixture of DMF and perfluorinated solvent FC 77 could be separated by cooling
the reaction mixture to 0 °C to have the fluorous phase with the dissolved catalyst (entry 4). When working in DMF alone, the catalyst precipitates upon cooling the reaction mixture and then it is separated by centrifugation. High yields decreased in the last three runs, when the reaction was performed in the presence of CuI in 14 cycles, whereas the reaction time was increased to maintain full conversion in the absence of CuI (entry 5).
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5
monophasic reaction mixture by addition of 10 vol% of water and then reusing the heptane phase. Significantly increasing yields were observed in oxirane ring opening with 4-methoxythiophenol (entry 10) and Me3SiN3 with low enantioselectivity (entry 11). In contrast, thermomorphic phase separation could be used when ring opening of cyclohexene oxide was catalyzed with complex 7 bearing a polyethylene (PE) oligomer support. Applied in toluene at 75 °C, the solid catalyst precipitates from the cold reaction mixture. After separation by centrifugation, it works with increasing activity and leaches 0.3% of metal (entry 12). A hyperbranched polyethylene (HBPE) with disulfide functionalities as binding sites for PdII–diimine moiety 3 proved to be a highly stable catalyst (0.19% PdII-3-HBPE) in Heck coupling in heptane–DMF (entry 6) [25]. The reaction mixture separates at room temperature allowing the reuse of the catalyst-containing heptane phase. Pd leaching after the first three runs (3.88 ppm) becomes negligible in further reuses (0.7 and 0.6 ppm). High conversions of >99% were also measured in runs 1 and 10 in 4 h. Formation of small Pd clusters was detected in the first several cycles. Another recycling application of an oligomeric PE-bound catalyst is the use of immobilized Hoveyda–Grubbs second generation Ru complex in RCM [30]. The reactant and catalyst RuII-8-PE are dissolved in toluene containing PEG as a cosolvent. This reaction mixture gives a gel upon cooling. The product is dissolved with toluene and the gel is recycled by adding fresh substrate. An average yield of 86% was reported with 0.19% Rh leaching in run 4 (entry 13). A number of soluble polymer systems have been developed and applied in thermoresponsive separating mode by the Bergbreiter group [26]. Terpolymer 4 containing dodecyl, dansyl [5-(dimethyla mino)naphthalene-1-sulfonyl] and N-butyl-N-dansylaminomethyl groups attached to polystyrene was found to significantly increase phase-selective solubility in alkane. Accordingly, functionalized polymer 4 (catalyst 4-poly) with immobilized organocatalyst 4dimethylaminopyridine has been tested in acylation and transesterification in a heptane–ethanol mixture [27]. Reactions carried out at room temperature are monophasic but the addition of 5 vol% water perturbs the system and induces separation of the heptane phase containing the polymeric catalyst. High average yields were reported in 20-run studies (entries 7, 8) with 0.09% and 0.002% catalyst leaching, respectively. Polyisobutylene (PIB) is another soluble catalyst support used as the handle for immobilizing NHC (N-heterocyclic carbene) ligands. The formed catalyst 4.26% PdII-5-PIB was successfully applied in the Heck reaction in DMF–heptane exhibiting high and stable activities in 10 cycles (entry 9) [28]. After removing the DMF phase the remaining heptane solution of the catalyst was reused.
A homogeneous, soluble polyisobutylene-tagged iridium complex (11.4% IrIII-9/poly) has been employed in three studies in the acetonitrile–heptane thermomorphic solvent system [31]. It was used in the photocatalytic deiodination at elevated temperatures (entry 14) and in deiodination–cyclization (entry 15) exhibiting high stability upon repeating the reactions 10 times. The catalyst was recovered by cooling the reaction mixture and then reusing the acetonitrile phase containing the catalyst. In another application, photocatalytic E/Z alkene isomerization was performed with continuous recycling of the catalyst phase in a flow process in a 30-min experiment. The heptane-saturated solution of the substrate and the catalyst dissolved in heptane was pumped into a microreactor and irradiated. Stable Z/E ratios (entry 16) with the loss of 2.6% of Ir in the first mmol of converted substrate were measured. Ir loss leveled off well below 0.1% (2 ppm).
3.2. Aqueous biphasic catalysis The immobilized CrIII-salen catalysts 6⁄ anchored to PIB prepared by Bergbreiter et al. have also been used in heptane–ethanol [29] (here and in similar anchored catalysts, only a single moiety is shown). Again, the catalyst is separated by perturbing the
As already mentioned, the development of aqueous biphasic catalysis has been motivated by easy catalyst recycling. At an appropriately selected temperature the reaction mixture is
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Table 2 Catalyst recycling in aqueous biphasic solvent systems. Catalysta
Conditions
Number of cycles
Yield (%)b
Ref.
1
[RuCl2(PTA)4] (5)
H2O/tert-BuOH (1:1), 100 °C, 24 h
7
86.4 (69–93)c
[36]
2
[Ir(cod)Cl]2 (0.2)
(L)-cysteine, H2O/toluene (4:2), buffer (pH 10), 50 bar, 80 °C, 21 h
5
77.4 (67–82)
[37]
3
Rh(acac)(CO)2 (0.2)
10, H2O/toluene/(CH2OH)2 (8:4:1), buffer (pH 7.2), 50 bar, 80 °C, 24 h
5
79.6 (66–80)
[37]
4
Rh(acac)(CO)2 (0.2) [Rh(cod)2] [BF4] (0.5)
10, H2O/(CH2OH)2 (9:1), buffer (pH 7.2), 50 bar, 80 °C, 24 h 11, H2O/scCO2, 30 bar, 56 °C, 2 h
5
67.6 (61–72)
[37]
5
>94 (81 ? 99)c
[38]
[Rh(cod)2] [BARF] (0.5) [Rh(cod)2] [BARF] (0.5) AuIII-14 (2.5)
12*, H2O/scCO2, 30 bar, 56 °C, 2 h
5
[38]
13*, H2O/scCO2, 30 bar, 56 °C, 2 h
5
H2O/toluene (1:1), rt, 2 h
10
100c 39–43d 100c 97–99d 100
[39]
Rh(acac)(CO)2 (0.1)
Me(OCH2CH2)nPPh2, 40 bar, pentane, H2O, 90 °C, 4 h c
8
98.6 (99.9–97.3)
[40]
Ref.
Entry
Reaction
5
6 7 8
9
a b c d
[38]
Percentage data show loading of active species (wt%) when available; data in parentheses indicate mol% used. Average of yields. Data in parentheses indicate range of yields. Conversion. Range of enantiomeric excess.
Table 3 Catalyst recycling in IL-based multiphase solvent systems. Entry 1
a b c d e f g
Reaction
Catalysta
Conditions
Number of cycles
Yield (%)b
RhCl3 (0.1)
15, TPPTS, 50 atm (H2/CO = 1:1), 85 °C, 5 h
35
81 (34.9–96.6)c
2
Rh(acac)(CO)2 (0.1)
16, [bmim][Tf2N], Tf2NH, MeOH, 20 bar, 80 °C, 2h
17
3
[Rh(cod)2] [BF4] (1)
17*, [bmim][BF4], MeOH, 1.5 atm, 25 °C, 2.5 h
8
4
[Mo(O) (O2)2(H2O)n]
[omim][PF6], dmpz, H2O, 60 °C, 4 h
5
RuCl3 (0.1)
6
Pd(OAc)2 (0.3)
d
[43]
97 (98–92) 10–30e 88–51f 99.75 (100–98) 91–84g
[44]
10
95 (91–99)
[46]
35 atm, [Oct3NMe]Cl, H2O/isooctane (1:1), 150 °C, 16 h
6
93 (60–100)
[47]
18, DTBPMB, MeOH, 22 bar (CO/C2H4/ Ar = 2:2:1), 80 °C, 20 min
15
98.5 (100–97.5)
[48]
[45]
Percentage data show loading of active species (wt%) when available; data in parentheses indicate mol% used. Average of yields. Data in parentheses indicate range of yields. Run 5 = 96.5, run 16 = 96.6, run 23 = 95.4, run 32 = 84.7%. Conversions. Aldehyde selectivities. Dimethylacetal selectivities. Range of enantiomeric excess.
monophasic but it becomes biphasic at room temperature. Another possibility called thermoregulated (temperature-controlled) phase-transfer catalysis (TRPTC) was developed by Jin and coworkers [32,33]. In a biphasic aqueous–organic TRPTC system at high temperatures above the cloud point, the catalyst is transferred from the aqueous into the organic phase thus allowing a homogeneously catalyzed process to take place. At lower temperatures it returns into the aqueous phase and the latter can be recycled.
The method, for example, allows the hydroformylation of water-immiscible alkenes. A prime example is the Ruhrchemie–R hône-Poulenc technology for the hydroformylation of C2–C5 alkenes operating with a Rh complex with sulfonated triarylphosphane as ligand [34]. For further information about this and related reactions, interested readers are directed to book chapters [35]. [RuCl2(PTA)4] (PTA = 1,3,5-triaza-7-phosphaadamantane) was found by Frost and Lee to be effective in nitrile hydration in a
Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65
biphasic mixture of water and tert-butyl alcohol [36]. High but significantly changing conversions were observed (Table 2, entry 1) with increasing amounts of Ru leaching into the organic phase (run 1 = 2.9 ppm, run 2 = 24.5 ppm, run 7 = 77.2 ppm). Although the monophasic aqueous reaction was much faster and gave constant conversions (99%), yields were significantly lower (average yield = 53%, range of yields = 18–67%). Paganelli et al. successfully applied (L)-cysteine and (S)captopril (10) as thioligands and Rh(acac)(CO)2 and [Ir(cod)Cl]2 in selective hydrogenation of a,b-unsaturated carbonyl compounds [37]. In recycling studies, the Ir–(L)-cysteine system worked satisfactorily in the hydrogenation of 2-cyclohexen-1-one (entry 2). The Rh complex, in turn, performed better with ligand 10 in the selective transformation of trans-cinnamaldehyde either in water–toluene–ethylene glycol (entry 3) or in water–ethylene glycol (entry 4).
Leitner and co-workers used a so-called inverted supercritical CO2 (scCO2)–aqueous solvent system for Rh-catalyzed hydrogenations [38]. This allows the separation of the aqueous phase containing the products and the reuse of the catalyst dissolved in scCO2 without depressurizing the autoclave. Methyl 2acetamidoacrylate was tested to show increasing and then stable activities in a five-run study induced by catalyst [Rh(cod)2][BF4] and phosphane ligand 11 (entry 5). Catalyst [Rh(cod)2][BARF] {BARF = tetrakis-[3,5-bis(trifluoromethyl)phenyl]borate} in combination with the chiral ligand (R)-Cl-MeO-BIPHEP (12⁄) afforded full conversions but moderate enantioselectivities (entry 6). Neither Rh nor P leaching was observed. Finally, quantitative conversions and excellent enantioselectivities were measured in the presence of chiral ligand (R,S)-3-H2F6-Binaphos (13⁄) (entry 7). Loss of Rh and P in the first run is 1.4 ppm and 5.2 ppm, respectively; however, leaching was not measurable in further uses. These favorable characteristics are attributed to the use of the BARF ligand: it is not prone to hydrolysis, increases the solubility of the Rh complex in scCO2 and enhances catalyst stability.
The novel AuIII-NHC complex 14 existing as an equilibrium mixture [Eq. (1)] was shown to induce cyclization of c-alkynoic acids to enol-lactones in aqueous biphasic media [39]. Surprisingly, byproduct formation from alkyne hydration was not observed. Complete conversion and 100% selectivity were observed when used in 10 repeated reactions in the cycloisomerization of a 1,6diyne at room temperature when isomer 14b is the major catalytic species (entry 8).
7
Bönnemann and co-workers prepared a RhIII complex by reacting Rh(acac)(CO)2 and thermoregulated ligand Me(OCH2CH2)nPPh2 (molar mass = 918) and used it in hydroformylation in a TRPTC system [40]. It exhibited high activity in the biphasic hydroformylation of oct-1-ene, albeit with a gradual decrease in aldehyde yield in reuse (entry 9). This was attributed to the formation and subsequent agglomeration of colloidal Rh particles. 3.3. Ionic liquid-based systems Ionic liquids (ILs) used in catalysis are characteristically salts with a melting point typically <100 °C. It is a diverse family with rich possibilities for functionalization with a wide range of anion and cation combinations. This allows fine-tuning their physical and chemical properties. Catalysis in ionic liquids offers special advantages. Namely, reactions can be carried out either in monophasic systems or, more characteristically, in liquid–liquid biphasic conditions. Furthermore, ILs greatly simplify product isolation by simple decantation or extraction. In fact, the use of ILs combines catalysis taking place in the homogeneous phase with heterogeneous catalysis providing easy product isolation, catalyst recovery and reuse. In addition, they often increase catalyst stability and affect selectivity. For comprehensive treatments and references readers are advised to consult relevant literature affording additional useful information [41] including practical applications [42]. In Tables 3–5, examples for catalyst recycling of multiphase solvent systems based on ILs are collected. Guanidinium methanesulfonates with a polyether tag were used in Rh-catalyzed biphasic hydroformylation of higher olefins in the presence of TPPTS [tris(m-sulfonylphenyl)phosphane] [43]. At reaction temperature, it is a liquid–liquid biphasic system. Upon cooling, the IL phase solidifies allowing easy separation and catalyst recycling. When applied in a reuse study of the hydroformylation of oct-1-ene with RhCl3 and IL 15, it exhibited high levels of recyclability with small drops after run 29 (Table 3, entry 1). Rh (and P) leaching after runs 1, 7 and 16, respectively, are 0.06% (0.16%), 0.07% (0.11%) and 0.04% (0.13%).
Hydroformylation of oct-1-ene was studied by applying Rh (acac)(CO)2 and ligand 16 with a lysine tag in the mixed solvents [bmim][Tf2N] (bmim = 1-butyl-3-methylimidazolium) and methanol [44]. A recycling study of 17 runs was executed in the presence of Tf2NH, which protonates ligand and, therefore, decreases Rh leaching to 0.3–0.4%. In addition to isomeric aldehydes, large amounts of dimethyl acetals are formed (entry 2). Rh complexes [Rh(cod)2][BF4] and [Rh(cod)Cl]2 in combination with ligand 17⁄ were shown to provide high ee values in the asymmetric hydrogenation of methyl (Z)-2-acetamidocinnamate with the former showing higher stability [45]. The reaction was performed in [bmim][BF4]–methanol (entry 3) with Rh leaching of 0.1% in the first runs.
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Table 4 Catalyst recycling in hydroaminomethylation of alkenes.a.
a b c d e f g
Entry
Cycle
R
Rh (mol%)
Conv. (%)
Sel. (%)a
l/br
1b 2 3 4 5
1 2 3 4 5
C6H13 C8H17 C4H9 C10H21 C6H13
0.056 0.022 0.022 0.028 0.028
96.4 88.4 92.7 87.8 89.1
90.8 96.5 92.4 96.8 97.3
35.1 44.4 33.5 20.2 18.1
6c 7 8 9 10
1c 2d 3e 4e 5g
C6H13 C6H13 C6H13 C6H13 C6H13
0.13 0.13 0.13 0.13 0.13
94.3 94.1 90.1 91.2 89.2
94.9 93.4 87.7 78.9 77.3
28.1 44.8 68.5f 72.8f 78.4f
Selectivity of amine formation. 19/Rh = 4.5 in entries 1–5. 19/Rh = 4.0 in entries 6–10. 110 °C. 110 °C, 6 h. Calculation includes enamine intermediate. 110 °C, 4 h.
Table 5 Cycling studies with ILs in monophasic solvent systems. Catalysta
Conditions
Number of cycles
Yield (%)b
Ref.
1
20 (1)
(DHQ)2-PHAL, NMO, MeCN/H2O (4:1), rt, 0.5–2 h, under argon
5
97.2 (95–99) 70–67c
[50]
2
20 (1)
(DHQ)2-PHAL, NMO, MeCN/H2O (4:1), rt, 0.5–2 h, under argon
5
91.6 (85–95) 81–74c
[50]
3
21
205 °C, 8 h
7
[51]
4
[HO3S-C3mim] [HSO4] g
H2O, 170 °C, 5 h
6
100d 94.9 (95.3– 94.3)e 95.0 (95.7–94.3)f 51.5% (58–66)
[52]
5
[H3NCH2CH2NH3] [HSO4]2 (0.2)
70 °C, 2 h
7
96h
[53]
6
22 (1)
Oct2ImBr, DAB (2 mol%) 50 °C, 3 h
9
90 (89–92)
[54]
7
[{Rh(l-OSiMe3) (cod)}2] (0.001)
23, 120 °C, 1 h, under argon
10
96.5 (99–97)
[55]
8
PdII-24 (0.05)
[Bpy][BF4], Et3N, 100 °C, 1.5 h
5
91.5 (86–93)
[56]
Entry
a b c d e f g h
Reaction
Percentage data show loading of active species (wt%) when available; data in parentheses indicate mol% used. Average of yields. Data in parentheses indicate range of yields. Range of enantiomeric excess. Conversion. Yield of dibutyl terephthalate. Yield of ethylene glycol. 550 mg cellulose, 1 g IL. Major isomer = o,p0 (45% selectivity).
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Alkene epoxidation in an aqueous–IL biphasic system was developed by Montilla, Galindo and co-workers [46]. Catalyst [Mo(O)(O2)2(H2O)n] is applied in [omim][PF6] with aqueous H2O2 and 3,5-dimethylpyrazole (dmpz) an N-donor base additive, which inhibits hydrolysis and enhances activity. The results acquired in a 10-run study with dmpz added after each cycle show high, stable activity (entry 4). Upgrading biomass-derived levulinic acid to c-valerolactone via hydrogenation was accomplished in an aqueous–organic (isooctane)–IL {trioctlymethylammonium chloride, [Oct3NMe]Cl} multiphase system [47]. Quantitative conversion with full selectivity
could be achieved either with Ru/C or using RuCl3. Whereas the supported catalyst gave decreasing activities in recycling, RuCl3 after an initial yield of 60% afforded yields of 100% (entry 5). Since the catalyst is completely segregated into the IL phase, recycling was conducted by removing the lower product-containing aqueous phase and then adding the reactant dissolved in water. Brønsted acid ILs prepared by Riisager and coworkers have been applied as both promoters and reaction media in the methoxycarbonylation of ethylene to synthesize methyl propanoate [48]. Reactions were catalyzed by Pd(OAc)2 and phosphane ligand 1,2-bis(di-tert-butylphosphinomethyl)benzene (DTBPMB). The reaction system with the use of IL [1-(4-sulfonylbutyl)-3-methyli midazolium][pTsO] (18) reveals high durability in 15 cycles (entry 6). After runs 5, 10 and 15, the upper product layer was decanted, the solvent evaporated and the IL phase containing the catalyst was reused.
Table 6 Recycling of BOX complexes in organic transformations. Entry
Reaction
Catalysta II
*
Conditions
Number of cycles c
Yield (%)b
Ref. d
1
Cu -25 -CTC (10)
CH2Cl2, 50 °C, 44–19 h, under argon
10
97.4 (95–76) 92–64e
[58]
2
CuII-25*-CTC (10)
CH2Cl2, 20 °C, 6–24 hf
10
87 (92–76) 70–64g
[59]
3
CuII-26*-CTC (10)
CH2Cl2, 20 °C, 1.5–2 h,h under argon
14
84.3 (92–40)i 73–87j
[60]
CuII-26*-CTC (10)
CH2Cl2, 50 °C, 3–1.5 h, under argon
6k
[60]
5
Cu(OAc)2-27a* (5)
iPrOH, rt, 24 h, under argon
11
87.8 (84–92) 85–89g 87 (77–96)l 89–75g
6
Cu(OAc)2-27b* (5)
iPrOH, rt, 24 h, under argon
8
[62]
7
Cu(OAc)2-28* (5)
iPrOH, rt, 24 h, under argon
14
8
Cu(OAc)2-29* (5)
iPrOH, rt, 24 h, under argon
13
K2CO3, DMF/H2O (1:1), 70 °C, 1 h
12
82 (78–78)m 26–44g 70 (86–69)n 66–74g 63 (51–50)o 92–86g 96 (98.5–94)
R= Me 4
9
a b c d e f g h i j k l m n o
R=H
II
Pd -31 (0.4)
[61]
[62] [62] [64]
Percentage data show loading of active species (wt%) when available; data in parentheses indicate mol% used. Average of yields. Data in parentheses indicate range of yields. Runs 1, 2 = 44 h, run 3 = 30 h, run 4 = 20 h, runs 5–10 = 19 h. Run 7 = 96%. Range of enantiomeric excess of the major anti isomer. Run 1 = 12 h, runs 2–7 = 6 h; runs 8 (12 h) and 9 (24 h) were performed at 20 °C, run 10 = 6 h. Range of enantiomeric excess. Run 12 = 18 h, –10 °C, run 14 = 44 h, 30 °C. Yields changed from 73% to 92% and then dropped to 40% in run 14. Enantiomeric excess of the major endo isomer. Continued use of catalyst applied in entry 3. A yield of 83% in run 1 dropped to 77% in run 2 then increased to 89% (run 3), dropped again to 80% (run 5) and then increased to 90, 95 and 96% in the final runs. Values increased to 91% (run 4) then decreased. Values increased to 95% (run 2), decreased to 47% (run 8) then increased again to 89% (run 12). Values increased to 75% (run 7) then decreased.
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Vogt and co-workers conducted hydroaminomethylation of alkenes induced by [Rh(cod)2][BF4] in the presence of Sulfoxantphos ligand 19 and [pmim][BF4] (pmim = 1-propyl-3-methylimidazolium) under biphasic conditions [49]. Table 4 includes the results of two separate recycling studies. In entries 1–5, varied alkenes were reacted with changing metal loadings. The first three runs were performed by adding new alkene to the reaction mixture without removing the product. In contrast, the upper product layer was removed after every reaction in entries 6–10. In this case, because of shorter reactions and lower temperatures, the enamine intermediate could not be fully hydrogenated. Decreasing regioselectivities shown in the first five entries may be attributed to the formation of isomeric alkenes that remained in the IL phase. This is in accordance with the opposite trend in the second set of experiments allowing much slower isomerization of the alkene.
Additional examples of recycling studies performed in or induced by ILs carried out in monophasic reactions are collected in Tables 5 and 6. The third-generation dendritic osmium catalyst 20 was successfully applied in asymmetric dihydroxylations. Increasing, slightly fluctuating yields and high enantioselectivities induced by biscinchona alkaloid 1,4-bis(9-O-dihydroquininyl)phthalazine [(DHQ)2-PHAL] and N-methylmorpholine N-oxide (NMO) as reoxidant were reported in five-run studies (entries 1, 2) [50]. The residue after evaporation of the reaction mixture was dissolved in acetone and the catalyst and (DHQ)2-PHAL were precipitated by the addition of water.
depolymerization of cellulose to levulinic acid performed in a Teflon-lined SS autoclave [52]. Yields increased in recycling (entry 4). The aqueous solution of IL was extracted with MIBK, dried under vacuum, adjusted to 1 g (adding 5%) and then reused. A range of C1–C4 alkylammonium hydrosulfates were evaluated in the condensation of phenol with formaldehyde to form bisphenol isomers [53]. Of these acidic ILs, [H3NCH2CH2NH3][HSO4]2 afforded the best performance (entry 5).
Degradation (depolymerization) of natural rubber, a process of practical importance to produce high-value products, has been accomplished by applying Hoveyda–Grubbs second generation Ru complex 22 in IL Oct2ImBr in the presence of 1,4-diacetoxy-2butene (DAP) as chain transfer agent [54]. Recycling experiments showed that the catalytic system could be reused nine times in this metathetic degradation with stable activity (entry 6). Varied allyland vinyl-ammonium-based ILs were used by Maciejewski and coworkers in hydrosilylations catalyzed by Ru complexes [55]. The best result in the hydrosilylation of oct-1-ene with IL trimethylvinylammonium ethanesulfonate (23) is shown in entry 7.
The phosphane-ligated ionic palladium complex PdII-24, prepared by reacting the corresponding bisphosphanoimidazolium derivative with [Pd(MeCN)Cl2], was well-characterized by means of XRD and 31P NMR spectroscopy [56]. It works as a precatalyst in carbonylative Sonogashira coupling in IL [Bpy][BF4] (Bpy = N-butylpyridinium) with increasing yields in five runs (entry 8).
The Brønsted–Lewis acidic IL 21 was applied in the butanolysis of polyethylene terephthalate (PET) affording full conversion and high yields of dibutyl terephthalate and ethylene glycol (entry 3) [51]. [HO3S-C3mim][HSO4] [C3mim = 1-methyl-3-(3-sulfopropyl)i midazolium hydrogen sulfate] showed lower activity in 3.4. Recycling of soluble catalysts used in monophasic reactions
Scheme 1. Use of diazabisoxazolin and triazabisoxazolin CuII complexes in cyclopropanation.
Catalyst separation from monophasic systems, as illustrated in this section, can easily be made by precipitating the catalyst or removing the product by distillation and then reuse the catalyst. Extracting either the catalyst or the product can also be applied. An easy possibility when solid catalysts soluble at high temperatures precipitate at room temperature. However, some unique examples have also been collected here. BOX complexes, for example, can be precipitated at the end of reaction by forming charge transfer complexes, which are reused in subsequent runs. The use of novel self-assembled and self-supported coordination polymer materials links homogeneous and heterogeneous catalyses. When the reactant and reagent are added to the reaction mixture, the polymers break up and dissolve. Upon complete consumption of the educts the polymers reassemble and precipitate allowing
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Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65 Table 7 Nobel metal derivatives reused in varied organic transformations. Catalysta
Conditions
Number of cycles
Yield (%)b
Ref.
1
RhIII-32* (5)
H2O, 60 °C, 5–7 h
7
78 (58–88) 58–74c
[65]
2
RhIII-33 (0.5)
NaOH, H2O, 100 °C, 8 h
20
94 (98–83)d
[66]
3
Pt0-34a (0.5)
H2O, 30 °C, 6 h
8
>95e 55:45–60:40f
[67]
4
Pt0-34b (0.5)
H2O, 30 °C, 6 h
9
>95e 56:44–65:35f 76 (83–79)g
[67]
55.7 (71–56)h 2.3 104– 3.7 104 i 98.5 (100– 94.5)
[69]
Entry
a b c d e f g h i j k l m
Reaction
II
5
Pd -35 (1)
KOH, propan-2-ol/H2O (1:1), 70 °C, 1 h
10
6
RuII-36 (0.002)
70 atm (CO2/H2 = 1:1), DMF, 110 °C, 5–7 h
12
7
RuII-37 (0.1)
PPh3 (0.1 mol%), tBuOK (0.5 mol%), toluene/ tBuOH (9:1), rt, 24 h
25j
8
RhIII-38
30 bar (CO/H2 = 1:1), 50 °C, 120 h
40j
63 (68–60)e,k e
[68]
[70]
[71]
9
Rh(acac)(CO)2 (2 105)
1a, 20 bar (CO/H2 = 1:1), toluene, 120 °C, 72 h
5
100 93–90l
[72]
10
IrIII-39 (2 105)
20 bar He, 50 °C, 20 h
11
235 (208– 245)m
[73]
Percentage data show loading of active species (wt%) when available; data in parentheses indicate mol% used. Average of yields. Data in parentheses indicate range of yields. Enantiomeric excess of the trans isomer. Run 19 = 93%, run 20 = 83%. Conversion. Range of b-(E)/a ratios. Run 3 = 57%, run 7 = 88%, run 9 = 65%. Run 2 = 44%, run 3 = 67%, run 7 = 47%, run 9 = 63%. Range of TON values. Number of samplings. Run 1 = 28%, run 2 = 59%, run 3 = 68%. Selectivity for hydroformylation vs isomerization and hydrogenation. Pressure (bar).
Table 8 The reuse of metallopolymer NiII-40* in Michael additions.
a b c d e
Entry
R1
R2
R3
Ar
Number of cycles
Conv. (%)a
ee
1 2c 3 4 5 6d 7e
MeO
Me
H
Ph
EtO MeO EtO Me Me
Et Me Et Me Me
H H Me H H
Ph 4-BrC6H4 Ph Ph Ph
8 13 8 7 8 7 8
99 99 (98–95) 99 99 99 99 98.8 (99–97)
89.5 (92–76) 83 (85–76) 86 (92–61) 83.5 (93–56) 87.3 (92–69) 87.7 (91–75) 85 (91–58)
Average of conversions. Data in parentheses indicate range of conversions. Average of ee values. Data in parentheses indicate range of ee values. Room temperature. dr = 1.05:1. dr = 3.5:1.
b
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easy recovery and reuse. This dynamic self-supported catalysis has emerged as a new strategy for catalyst reuse. A recent review summarizes the advances of the field [57]. Results reported with the use of BOX complexes are collected in Table 6 and Scheme 1. Additional studies about the application of other catalysts in varied reactions are found in Tables 7–11. In an earlier study, Schulz and co-workers prepared and applied complex 25⁄ in Diels–Alder reactions [58]. Catalyst recovery was performed by adding 2,4,7-trinitrofluoren-9-one to the reaction mixture to form a brown charge transfer complex (CTC) precipitating with pentane. In further cycles, consequently, this charge transfer complex (25⁄-CTC) was reused. Yields are lower at the end of the recycling study because of shorter reactions (Table 6, entry 1;
only the major endo isomer is shown). Complex 25⁄-CTC was also used in ene reactions [59] with similar catalyst performance (entry 2). Note, however, that both reaction time and temperature were changed. Complex 26⁄ (more precisely, the corresponding CTC complex) was also reused in the Diels–Alder reaction of Nacyloxazolidinones with cyclopentadiene [60]. Yields with the methyl-substituted dienophile changed significantly in 14 cycles and dropped to 40% in the last run when the reaction was performed at 30 °C (entry 3). The catalyst was then further tested with the unsubstituted compound affording satisfactory stability and enantioselectivity (entry 4).
29⁄ exhibited similar behavior that is they afforded widely changing activities and enantioselectivities (entries 6–8) [62].
Cu(TfO)2 complexes of the other two diazabisoxazolin ligands [Cu(TfO)2-27c⁄ and Cu(TfO)2-27d⁄] and that of triazabisoxazolin 30⁄ were applied in cyclopropanation carried out in CH2Cl2 [63]. These work as discussed above with activities varying significantly, whereas changes in selectivities are minimal (Scheme 1). Pd-BOX complex PdII-31 applied in Suzuki reaction shows high activity and stability in coupling reactions performed in DMF–water solution (entry 9) [64]. Recycling is made by extracting the product with hexane and then charging the catalyst phase with substrates and base.
Table 9 Hydroformylation of oct-1-ene with Rh(acac)(CO)2 and ligand 41.
The García group has developed a number of multitopic oxazoline-based chiral ligands. These form coordination polymers of chain and layered structures in noncoordinating solvents when reacted with CuII salts. Bis(oxazoline) complex 27a⁄ formed with Cu(OAc)2 was applied in the asymmetric Henry reaction [61]. High activities changed during reuse (entry 5). Complexes 27b⁄, 28⁄ and
a
Cycle
Time (h)
Conversion (%)
l/br
Isomers (%)
Rh leach (%)
P leach (%)
1 2 3 4 5 6a 7 8
18 18 19 19 25 17 18 20
90 73 75 60 63 77 61 68
43.3 42.3 13.9 7.6 3.9 34.8 40.4 22.7
5.7 4.5 39.5 50.9 78.6 14.0 6.1 25.5
0.6 0.8 7.4 3.5 1.3 2.1 1.2
20 15 18 11 11
Ligand 41 is added.
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Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65 Table 10 Reuse studies induced with various other catalyst preparations. Entry
Reaction
1
Catalysta
Conditions
Number of cycles
Yield (%)b
Ref.
KI (1.88), K2CO3 (0.45)
25 bar, 120 °C, 1 h
12
84.5 (87–82.5)
[76]
MeOH, 60 °C, 30 min
12
86 (89–83.5)
[76]
2
3
4-poly (5)
heptane, 80 °C, 12 h
12
90
[27]
4
4-poly (5)
heptane, 90 °C, 12 h
8
87
[27]
5
42 (1.5)
30% H2O2, MeCN, 60 °C, 1 h
98 (99–97) 99.5 (100–98)
[77]
6
saccharin + DMAP
25 °C, 2 h
10 42a 42b 10
>99
[79]
7
saccharin + DMAP
25 °C, 2–4 h
10
>99
[79]
8
AgI (2.5), Cs2CO3
DMF, H2O, 25 °C, 16 h
CuI (0.1)
43 (1 mol%), H2O, 25 °C, 3 h
80 78 80 81 85 90 89 91
[80]
9
R = H, X = none 55 R = 4-F, X = none 24 R = MeO, X = none 26 R = H, X = (CH2)2 32c R = H, X = CH2O 30 R = 3-OH, X = none 21 R = 2-Cl, X = none 20c 10
[81]
10
CuII-DC/ Br-poly
[PEGMA500]/[EBPA]/[HCc]/[CQ]/[TEA] = 100/2/1/0.5/1, heptane/EtOH (4:3), white LED (9.6 W); 25 °C, 22 h
5
97 (96.5–97.4)d
[82]
11
CuBr2/45
[MMA]/[EBPA]/[CuBr2]/[PIL]/[AIBN] = 200:1:1:4:0.8; 70 °C, 6 h
10
62 (62.5–67.8)d,e 98 (100–95)f
[83]
R = Me, iPr
a b c d e f
Percentage data show loading of active species (wt%) when available; data in parentheses indicate mol% used. Average of yields. Data in parentheses indicate range of yields. 50 °C, 24 h. Conversion. Run 2 = 51.8%, run 5 = 69.1%, run 10 = 67.8%. Recycling efficiency: percentage of residual catalyst in solution.
Candeias, Afonso and co-workers have prepared and applied homochiral dirhodiumII complexes in asymmetric intramolecular C–H insertion of a-diazo acetamides in aqueous solution [65]. Water-soluble complex RhIII-32⁄ could be recycled after product extraction with diethyl ether. Increasing yields, changing stereoselectivities (cis/trans 1:0.3–1:0.6) and moderate enantioselectivities were measured (Table 7, entry 1). Binuclear complex RhIII-33 soluble in water is an active catalyst in the aerobic oxidation of alcohols [66]. In a recycling study, 1-(4-methoxyphenyl)ethanol was transformed to the corresponding ketone in aqueous solution. The catalyst exhibited high, slowly decreasing activities with a significant drop in the last run (entry 2).
Two water-soluble NHC Pt0 complexes (Pt0-34a, Pt0-34b) have been applied in hydrosilylation of alkynes in aqueous solution [67]. Recycling studies were performed until conversions of >95%
were achieved (entries 3, 4). Activities changing in a wide range were found with the Pd NHC complex Pd(bmim-y)Br2(pyridine) (PdII-35) (bmim-y = 1-butyl-3-methylimidazol-2-ylidene) applied in Suzuki coupling (entry 5) [68]. Pd particles formed under reaction conditions were shown by the Hg0 test not to be involved in the coupling process.
Ru pincer complex RuII-36 was used in N-formylation of amines with CO2 and H2 [69]. In the reaction of dimethylamine, at a catalyst loading of 9.3 105 mol%, a TON of 6 105 was achieved. Varied yields were found with dimethylammonium dimethylcarbamate to form dimethyl formamide (entry 6). Recycling was made by removing the product with vacuum distillation and recharging the residue. Another Ru catalyst, diphosphane/diamine complex
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Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65
catalyst. Under different conditions, a TON value of 5 106 was measured in a single run and the catalyst was active in continuous operation in about 3.5 months. The CO content of the product mixture is below the detection limit (less than 6 ppm), which makes this system suitable for delivering hydrogen to fuel cell applications.
Table 11 Recycling of catalyst 14.9% 46*-4G3 in Michael additions.
Cycle
R1
R2
Yield (%)
ee
1 2 3 4 5 6 7
Et Pent Pr Me Me Me Me
Ph Ph Ph Ph 4-MeOC6H4 2-furyl 4-BrC6H4
99 99 99 99 98 97 98
99 99 99 99 96 93 95
syn/anti ratios = 97:3–90:10.
RuII-37, was applied in the asymmetric hydrogenation of a-tetralone [70]. It was used in a small-scale continuous nanofiltration study, with co-feeding the solution of catalyst, substrate (S/C = 9400:1) and PPh3 as well as the base into the reactor filled with catalyst solution. At 18 h, when the conversion at the reactor outlet and that in the retentate decreased a few percents, the substrate feed was cut into half (entry 7). A cumulative TON of 4750 corresponding to a TON of 250 indicates that the catalyst was reused in 15 cycles. The system performance is the equivalent of 60 recycles.
(Diphenylphosphinoethyl)dimethylethoxysilane and silanolterminated polydimethyl-siloxane were used in the synthesis of low-molecular weight ligand 38. This was then mixed with Rh (acac)(CO)2 to produce homogeneous complex RhIII-38 with a P/Rh ratio of 3 after washing out nonbound Rh with toluene [71]. A continuous hydroformylation of oct-1-ene in toluene (30:70 v/v, feed rate = 13 mL h1) was performed to have a stabilized conversion of 60%, an aldehyde selectivity of 97%, a l/br ratio of 2.5 and a cumulative TON 1.29 104 (entry 8). The stabilized Rh loss in the permeate after 40 h on stream was 1–2 ppm with a cumulative loss of 11%. Controlled tandem hydroformylation and isomerization of fatty substrates including undec-10-enonitrile were induced by Rh(acac)(CO)2 and Ir(acac)(cod) with the former exhibiting much better stability [72]. Linear aldehydes were formed with high selectivity (l/br ratio = 99:1) and the catalyst showed high levels of recyclability (entry 9). Catalyst recycling was carried out reusing the residue after vacuum distillation. Both catalysts gave high cumulative TON values [Rh(acac)(CO)2 = 2.5 105 (5 runs), Ir(acac)(cod) = 1.1 105 (3 runs)]. The homogeneous Ir complex of 1,10-phenanthroline-4,7-diol (39) works as a highly efficient catalyst in the generation of hydrogen from formic acid under pressure [73]. Catalyst IrIII-39 delivered a mixture of H2 and CO2 affording a cumulative TON of 3.85 104 when reused in 11 runs (entry 10). It precipitates after reaction upon cooling to 20 °C that is it can be recycled as a heterogeneous
Chiral NiII-40⁄ is a dynamic, self-supported catalyst with Evans NiII complex existing as a polymer in its resting state [74]. When used in enantioselective Michael additions, it dissolves in the reaction medium and disaggregates to release the active complex bearing only a single diamine ligand. The catalyst is insensitive to air and moisture and displays high activity and enantioselectivity. It is also robust when reused up to 13 cycles (Table 8). It can be easily recovered by precipitating with the addition of diethyl ether.
Kluwer, Reek et al. applied a reversible catalyst adsorption strategy with non-covalent interactions. Hydroformylation of oct1-ene was carried out using the Rh(acac)(CO)2 complex and ligand 41 [75]. The catalyst system provided the corresponding aldehydes with high productivity (TON = 6 104, 5 104 mol% Rh) and a selectivity of 93.8% in a batch process. When performed in a flow reactor, the system after reaction was cooled to room temperature and the reaction mixture was pumped through a column filled with end-capped silica-alumina scavenging the RhIII complex and ligand through specific non-covalent adsorption mode. This is reversible and, therefore, when the column is flushed with THF in a reverse flow the catalyst desorbs and can be used again. Small Rh leaching and a significant loss of the ligand resulted in large drops in linear/branched ratios in an eight-run recycling study (Table 9). In run 5 the ratio is close to that of the ligand-free system. Addition of ligand in run 6 restores both activity and selectivity.
Cheng, Zhang and co-workers used the aqueous solution of a simple binary salt mixture in the synthesis of dimethyl carbonate [76]. First ethylene oxide was reacted with CO2 in the presence of KI and K2CO3 (Table 10, entry 1) and then the product mixture
Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65
was further treated with methanol to perform transesterification of intermediate ethylene carbonate (entry 2). The salt mixture was isolated and reused after evaporating the liquid phase. Functionalized polymer 4 (catalyst 4-poly) discussed earlier as a thermoresponsive catalyst applied in a heptane–ethanol mixture (see Section 3.1) has also been used in heptane to induce alcohol acetylation and cyclopropane ring opening with concomitant acetylation [27]. In these reactions products were extracted with acetonitrile and then the heptane phase with dissolved catalyst was reused in the next cycle. High average yields were reported (entries 3, 4) with metal leaching of 0.015 and 0.005%, respectively. Zhou, Kühn, and coworkers synthesized octamolibdate-based solids (42) and used them in alkene epoxidation as ‘‘reactioncontrolled phase-transfer catalysts” [77]. The term was introduced by Xi et al. [78]. The catalyst systems are soluble under reaction conditions, that is, they are transferred from the solid to the liquid phase and then self-separate at the end of reaction and can be reused after washing with water. The two catalysts showed similar performance in the epoxidation of cis-cyclooctene (entry 5).
The salt of saccharin and 4-N,N-dimethylaminopyridine (DMAP) as catalyst has been used in the acylation of alcohols with acid anhydrides [79]. The salt precipitates upon addition of hexane to the reaction mixture. Varied alcohols and phenols undergo clean transformation under base- and solvent-free conditions with repeated uses of the catalyst. The best examples are shown in entries 6 and 7. In an interesting study, Hong and coworkers have performed the carboxylation of terminal acetylenes with carbon dioxide exhaust gas [80]. CO2 generated by a burning candle was captured in ethanolamine and then reacted with AgI and Cs2CO3 in aqueous DMF. The reaction solution could be recycled with maintained activity. Selected examples are summarized in entry 8. Astruc and coworkers have fabricated amphiphilic dendrimer 43 bearing 27 triethylene glycol (TEG) termini and nine intradendritic triazole groups (G) [81]. The resulting catalyst after treatment with CuSO4 was used in Huisgen-type alkyneazide cycloaddition (CuAAC) ‘‘click” reaction in water induced by CuI ions formed under reaction conditions. The catalyst system is highly stable and efficient in 10 reuses without drop in activities (entry 9). The TEG termini make the dendrimer water-soluble allowing product extraction and reuse of the aqueous phase after drying. Moreover, a reaction performed with 1 ppm CuI gave a TON of 5.1 105 and a TOF of 2.12 104 h1 (a yield of 50% in 24 h).
A recycling study of an atom-transfer radical polymerization (ATRP) process induced by a CuII complex was accomplished by Zhang, Cheng and coworkers [82]. The hybrid catalyst complex (HCc) was fabricated by carrying out the polymerization of poly (ethylene glycol) methyl ether methacrylate (PEGMA500),
15
2-bromophenylacetate (EBPA) as ATRP initiator, N,Ndibutyldithiocarbamate copperII [Cu(DC)2], a random copolymer of octadecyl acrylate (OA) and MA-Ln [2-(bis(pyridin-2-ylmethyl) amino)ethyl acrylate] [POA-ran-P(MALn), 44] and (2,4,6trimethylbenzoyl) diphenylphosphane oxide (TPO). The resulting complex CuII-DC/Br-poly (HCc) formed upon irradiation with a mercury lamp for 10 h was dissolved with heptane and used to catalyze the polymerization of the above mixture. The involvement of copper catalyst DC–Cu–Br/CuBr/CuBr2 = 0.6:0.33:0.07, a ternary complex with POA-ran-P(MA-Ln) was speculated. HCc induced polymerization upon irradiation with visible light exhibiting high stability (entry 10). Adding water to the reaction mixture allowed easy product separation. In a subsequent study they developed a catalyst system and used it again in ATRP [83]. It is a thermoregulated random copolymer polyionic liquid (PIL) polyelectrolyte with side-chains bearing both PEG-based ILs and ATRP ligands (45). It was used with CuBr2 in the polymerization of methyl methacrylate (MMA) affording moderate, varying conversions and high catalyst recycling efficiency (entry 11). Catalyst separation and recycling, in this case, were achieved by decreasing polymerization temperature to room temperature. CuI and CuII complexes in a reversible, dynamic equilibrium play a pivotal role in the mechanism. A recent review addressing the progress in separation and recycling of transition metal catalysts in ATRP is highly advised to interested readers [84].
Proline-derived Jørgensen–Hayashi organocatalyst (S)-a,a-dip henylprolinol trimethylsilyl ether (46⁄) was grafted to azidoterminated phosphorus dendrimers and the resulting catalysts were applied in asymmetric Michael addition of aldehydes and nitroalkenes [85]. The third-generation dendrimer (14.9% 46⁄4G3) displayed high stability in seven consecutive reactions with varied educts (Table 11). The catalyst was recovered by precipitation with pentane after evaporating the solvent.
Bourne, George and co-workers reported a process combining a monophasic reaction with biphasic separation. The organic substrate is mixed with fluorous solvent 3-ethoxy-2-(trifluorome thyl)dodecafluorohexane (HFE) and a porphyrin-based catalyst modified with fluorous ponytail fluorous-47 [86]. The reaction studied was photo-oxidation with singlet oxygen of citronellol (48) to form the corresponding isomeric peroxides 49a and 49b in scCO2 [Eq. (2)]. In this process with continuous recycling of the fluorous phase containing the catalyst, conversions decreased slowly in a 20-h test reaction (Fig. 1) and a TON of 2.7 104 was calculated. This, however, is an apparent value since about 12% of the catalyst remained in the product phase for each cycle.
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70
Conversion (%)
60 50 40 30 20 10 0 1
3
5
7
9
11
13
15
17
19
21
23
25
Cycle number
yields also increased from 27 to 40% (3 h) and 60 to 66% (20 h). A combined Rh loss of 24 ppm was measured in six cycles. The Wang group tested Rh nanoparticles in a number of reactions. Hydrogen reduction of RhCl3 in the presence of PEG4000 gave nanoparticles of 1.7 ± 0.4 nm, which were applied in a toluene–heptane mixture at 120 °C in hydroformylation of oct-1ene [89]. The lower PEG phase containing Rh particles was reused in 21 runs giving high, randomly changing yields (entry 2). Particle size did not change, which was attributed by the authors to the pseudocrown ether interactions between PEG4000 and Rh particles. Stabilized Rh nanoparticles of 1.3 nm were also prepared in IL 50 [90]. This system reveals high stability giving full conversions in six runs with high aldehyde yields (entry 3). This is despite an increase in particle size to 2.4 nm and a Rh loss of 0.3%.
Fig. 1. Continuous recycling of the fluorous phase containing catalyst fluorous-47 in the singlet oxidation of citronellol (2 104 mol% catalyst, flow rate = 0.1 mL min1, full reaction time = 20 h; see Eq. (2). Reprinted (adapted) from Ref. [86], Copyright (2017), with permission from The Royal Society of Chemistry.
3.5. Stabilized soluble metal nanoparticles in reuse studies The primary motive driving the use of varied solvent systems is, naturally, efficient catalyst recovery and reuse. In most cases, the recycling performance of mixed biphasic thermoregulated solvents consisting of an organic solvent in combination with water, PEG, polymers or another organic solvent proved to be quite satisfactory. Catalyst reuse in other organic–aqueous or IL solvents was accomplished by recharging the reaction mixture with fresh educts after extracting or decanting the product. In the first part of this subsection, examples of recycling of organo-soluble nanoparticles are discussed. In Table 12, results with respect to hydroformylation and hydrogenations are collected. In these cases, particles applied in thermoregulated biphasic solvent systems transfer from one phase to the other at reaction temperature. After complete reaction, they return to the original phase, which is separated and then reused in the next cycle. Reactions performed in other solvent systems are listed in Table 13. A recent review about the use of homogeneous recyclable metal nanoparticles is advised [87]. Behr and co-workers studied the hydroformylation of dodec-1ene by using Rh nanoparticles of 3 nm prepared by hydrogenating Rh(acac)(CO)2 dissolved in DMF [88]. Reactions were carried out in a mixed DMF–decane solution finding slightly increasing conversions by recycling the DMF phase (Table 12, entry 1). Aldehyde
Rh nanoparticles (2.4 ± 0.3 nm) were also generated from RhCl3 treated under hydrogen pressure (40 bar, 70 °C, 2 h) in the presence of phosphane ligand Ph2P(CH2CH2O)16Me in a butanol–water mixture (1:1) [91]. This system worked efficiently in the hydrogenation of alkenes [92]. Product yields in the hydrogenation of cyclohexene were found to be 100% in a seven-run study (entry 4). Particle aggregation was not detected and only a slight increase in the mean diameter from 2.6 to 3.2 nm was measured. A Rh leaching of 1.1 wt% in run 1 decreased significantly in further cycles (cumulative Rh loss in six runs = 1.67 wt%). Additional successful applications of Rh and Pd metal nanoparticles in varied hydrogenations have also been reported. Decreasing yields were found in the selective hydrogenation of cyclooctadiene to cyclooctene in the presence of PEG-based IL 51 [93], which may be due to an increase in particle size from 1.7 ± 0.2 nm to 2.5 ± 0.4 nm (entry 5). Alkyne semihydrogenation was promoted with Ru particles prepared by hydrogen treatment of RuCl3 in PEG2000 [94]. Catalyst recycling was carried out in a toluene–heptane biphasic system (entry 6). A small increase in particle size from 1.6 ± 0.4 to 1.8 ± 0.4 nm was detected after 10 cycles and a significant metal leaching of 1.7 wt% per cycle were measured. Elemental mercury completely stopped the reaction testifying to the heterogeneous nature of the reaction.
Hou and co-workers fabricated Pd particles by reducing Pd (OAc)2 with NaBH4 in the presence of IL 52 in CH2Cl2 [95]. According to data acquired by NMR spectroscopy TPPTS anions surround Pd particles and IL cations form an outer layer thereby providing stabilization. After complete phase separation at 0 °C the IL phase is recycled. The stabilized nanoparticles gave 100% yield in the hydrogenation of styrene under mild conditions without particle aggregation (entry 7).
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Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65 Table 12 Recycling of soluble metal nanoparticles in thermoregulated systems. Entry
Reaction
1
Catalysta
Conditions
Number of cycles
Yield (%)b
Ref.
Rh0 (0.1)
30 bar, DMF/decane (1:1), 110 °C, 3 h or 20 h
6
62 (59–66)c,d 93 (92–95)c,e
[88]
Rh0 (0.1)
60 bar, PEG4000, toluene/heptane (3:1), 120 °C, 3h 50, 50 bar, cyclohexane, 120 °C, 4 h
21
93.5 (99–88)f
[89]
R= C10H21 2 3
a b c d e f g
R = C6H13
0
R = C6H13
Rh (0.25) 0
6
c
100 99.5 (100–99) 100
[90]
4
Rh (0.1)
Ph2P(CH2CH2O)16Me, 10 bar, butanol/H2O (1:1), 60 °C, 1 h
7
[92]
5
Rh0 (0.05)
51, 10 bar, toluene/heptane (4:1), 60 °C, 6 h
11
95 (99–92)c 84.5 (89–82)
[93]
6
Rh0 (0.1)
11
92 (94–91)
[94]
7
Pd0 (0.1)
20 bar, PEG2000, toluene/heptane (3:1), 100 °C, 1h 52, 1 atm, EtOAc, 45 °C, 3 h
10
100
[95]
8
3.47% Pd0-53PIB (1)
54-PIB, K2CO3, THF/DMF/Polywax (4:4:3), 90 °C, 12 h, under nitrogen
5
89
[96]
9
Au0-EOEOVE (2)
KOH, H2O, 27 °Cg
6
>99
[97]
Percentage data show metal loading (wt%) when available; data in parentheses indicate mol% used. Average of yields. Data in parentheses indicate range of yields. Conversions. Three reactions in 3 h. Three reactions in 20 h. Yields decreased from 99% to 88% in run seven and then increased to 94%. Reaction time not reported.
Table 13 Recycling soluble metal nanoparticles in other solvent systems. Entry
a b c d
Reaction
Catalysta 0
Conditions
Number of cycles
Yield (%)b
Ref.
1
Rh (1)
55a, 40 bar, [bdmi][Tf2N], 75 °C, 3 h
10
95.6 (72–100)c
[98]
2 3
Rh0 (1) 16.7% Pt0-56 (0.065)
55b, 40 bar, [bdmi][Tf2N], 75 °C, 3 h 1.1 bar, MeOH/heptane (1:5), 20 °C
10 14
90.1 (32–100)c 100c
[98] [99]
4
Pd0 (1)
57, 3 bar, glycerol, 100 °C, 3 h
10
94 (95–92)d
[100]
5
27.7% Pd0-(SL) (0.003)
K2CO3, H2O, 60 °C, 6 h
8
97 (98–96)
[101]
Percentage data show metal loading (wt%) when available; data in parentheses indicate mol% used. Average of yields. Data in parentheses indicate range of yields. Conversions. Run 4 = 96%, run 5 = 97%, run 9 = 91%, run 10 = 92%.
A polyisobutylene-bound ortho-metallated Pd catalyst system developed by Bergbreiter and co-workers (3.74% Pd0-53-PIB) was applied in heptane–DMF in Heck and Suzuki couplings [96]. Average yields of 89–96% in five runs were observed requiring, however, the increase in reaction times from 6 h to 12 h. The reason is a loss of about 7% Pd to the reaction mixture. This problem was addressed by performing the reaction with added polyethylene oligomer (Polywax) and PIB-bound phosphane 54 to afford high, stable yields (entry 8). In this case, the hot THF–DMF–Polywax monophasic mixture was perturbed by adding water. Cooling results in the separation of a Polywax phase containing both PIBbound components. A minimal Pd leaching of 31 ppm (0.12%) was measured. Pd particles of about 50 nm are formed during reaction and effectively recycled. Au nanoclusters were prepared in the presence of a star polymer consisting of 2-(2-ethoxy)ethoxyethyl vinyl ether (EOEOVE) in water and used in the aerobic oxidation
of benzyl alcohol with high efficiency and stability (entry 9) [97]. The catalyst was recovered by decantation at 60 °C.
Additional reactions, performed in other solvent systems including organic–aqueous or IL solvents are collected in Table 13. Catalyst reuse is accomplished by recharging the reaction mixture with fresh educts after extracting or decanting the product. A mixture of Rh(allyl)3 and phosphane-functionalized ILs was treated with hydrogen in [bdmi][Tf2N] (bdmi = 1-butyl-2,3dimethylimidazolium) to form stabilized Rh particles with mean particle diameters of 2.3 ± 0.5 nm (in 55a) and 1.7 ± 0.5 nm
Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65
(in 55b) [98]. The formed solutions were used in the hydrogenation of toluene to find induction periods in the first few cycles and increasing conversions (entries 1, 2). This is caused by incomplete reduction of the precursor allowing the formation of nanoparticles during reaction. In addition, a small decrease in particle size was also observed in the case of the catalyst made in IL 55a. Catalyst recycling was performed by extracting the product with pentane and drying the catalyst phase after each use.
Platinum particles of 3 nm were fabricated by reducing H2[PtCl6] with NaBH4 in aqueous solution in the presence of a polymer with dendritic core–multishell architectures [99]. It is composed of a hydrophilic core (hPG3000 = hyperbranched polyglycerol) and a hydrophilic outer shell of polyethylene glycol with an inner hydrophobic shell of alkyl chains in between connected through carboxamide bonds (56). Catalyst 16.7% Pt0-56 containing stabilized particles was used in the hydrogenation of methyl crotonate in a methanol–heptane two-phase system (entry 3) to have full conversions but significantly decreasing TOF values. Pt leaching into the product phase in the first, second and fifth run, respectively, was determined to be 0.22, 0.38 and 0.16 ppm. After removing the heptane phase, the methanol solution with the catalyst showing no sign of particle aggregation was recycled. A cumulative TON of 2.2 104 was measured in 14 cycles.
Gómez and coworkers have generated Pd nanoparticles by treating Pd(cod)Cl2 in glycerol in the presence of N-benzylated PTA-based ligand 57 (PTA = 1,3,5-triazaphosphaadamantane) as stabilizer under hydrogen (3 bar, 60 °C) [100]. The black colloidal solution was used directly in the hydrogenation of (E)-4phenylbut-3-en-2-one in 10 runs with randomly changing yields and reused after product extraction with CH2Cl2 (entry 4). Interestingly, the particle size of 3.2 ± 0.8 nm decreased to 1.7 ± 0.6 nm during reuse. Monodispersed Pd nanoparticles were generated by reducing PdCl2 with the use of the extract of herbal tea Stachys lavandulifolia (SL) serving as reductant, capping agent and stabilizer [101]. Water-soluble spherical Pd particles (5–7 nm) were used as a homogeneous catalyst [27.7% Pd0-(SL)] in aqueous solution for Suzuki coupling. The solid formed by adding ethyl acetate to the reaction mixture was separated by filtration and was used in subsequent runs in a recycling study (entry 5). Pd leaching was below the detection limit of 0.1 ppm. An amphiphilic hyperbranched copolymer bearing a hydrophilic hyperbranched polyethylenimine (HPEI) core and a hydrophobic shell formed by linear palmitamide (C16) (58a) were loaded with Au nanoparticles prepared by reducing HAuCl4 with NaBH4 (catalyst Au0-58a-HPEI) [102]. The polymer-stabilized organosoluble particles were applied in the reduction of 4-nitrophenol. Catalyst stabilities in recycling studies were characterized by measuring the time necessary to achieve full conversion. The best result is an average reaction time of 3 min in 17 runs (range of time = 2–7 min; Fig. 2, shaded bars). However, significantly longer times of 34 min and 60 min were required in the 18th and 19th
cycles. The average particle diameter increased from 3.2 nm to 15 nm during recycling. These studies were extended to the synthesis and use of catalysts with dendritic shells [103]. Another sample with particles of 3.0 ± 0.5 nm and a generation 2 dendritic shell (catalyst Au0-58b-HPEI/dendr) exhibited the best performance applied with a lower Au loading (Fig. 2, open bars). In the first nine runs full conversions were accomplished in 3–50 min. Again, significant further increases in reaction times were necessary in subsequent cycles (run 10 = 125 min, run 11 = 290 min). More importantly, however, an unprecedented cumulative TON of 2.3 104 could be achieved.
4. Recycling of solid catalysts applied in heterogeneous systems Heterogeneous catalysis has been used overwhelmingly in the chemical industry [104]. Suggested additional readings are general treatments [105], and reviews and books about other, more specific topics discussing the use of ILs [106], nanoparticles [107], functional resins [108], and supported organocatalysts [109]. A multi-author book about asymmetric reactions over varied support materials with chiral active species may be of special interest [110]. Readers’ attention is also directed to an early review [111] and a book about dendrimers [112], in particular, to chapters about homogeneous enantioselective (Chapter 9) and heterogeneous catalysts (Chapter 11). An important chapter summarizes
350
Time for full conversion (min)
18
300 250 200 150 100 50 0 1
3
5
7
9
11
13
15
17
19
Cycle numbers Fig. 2. Reduction of 4-nitrophenol induced by organo-soluble Au nanoparticle catalysts. Shaded bars: Au0-58a-HPEI (8.3 mol%), H2O/CH2Cl2 (10:2.5), 32 °C; open bars: Au0-58b-HPEI/dendr (0.21 mol%), H2O/CHCl3 (10:2), 32 °C. Reprinted (adapted) from Refs. [102,103], Copyright (2017), with permissions from Wiley and The Royal Society of Chemistry, respectively.
Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65
industrial applications [113]. Three books focusing on the fine chemical industry are also advised [114]. Varied possibilities are available to separate and then reuse solid catalysts from simple filtration through other methods requiring more sophisticated techniques such as ultracentrifugation and membrane separation. In a few rare cases—monolith, mesh, membrane, thin film samples, dendrimers—the catalyst can be very simply removed (see examples in the following subsections). Magnetic catalyst separation has recently become a popular and widely studied research area (Section 4.5). Note, that examples for catalyst recovery accomplished by precipitation including soluble polymers were treated separately (Sections 3.1 and 3.5). Organization in this part of the manuscript is made according to the type of support materials.
19
sample delivered diaryl sulfides in high yields by the coupling of halobenzenes and thiophenols with high activity and stability (entry 5) [121]. Similar performances were reported for catalysts with an immobilized Rh complex (RhIII-62a-MCM-41) used in the hydrothiolation of alkynes (entry 6) [122] and sample 2.98% RhIII-62bMCM-41 inducing the hydrophosphanylation of alkynes with Ph2P(O)H (entry 7) [123]. The latter reaction was also accomplished with 4% RhIII-62a-MCM-41 (entry 8) [124]. Similar results in a C–S coupling with a higher Rh loading are shown in entry 9 [125].
4.1. Catalysts systems based on siliceous support materials 4.1.1. The use of supports with ordered structure Porous solids with ordered structure—silica-based mesoporous materials (MCM, SBA, and others) synthesized using surfactantdirected self-assembly, zeolites (microporous crystalline aluminosilicates), and recent discoveries including periodic mesoporous organosilicas (PMOs)—have attracted great attention. One of their most useful application possibilities is catalysis. Important features are high surface area, uniform pore size distribution, easy functionalization and high stability to generate readily accessible, covalently bound active sites. Suggested readings provide rich information about ordered mesoporous materials [115] and industrial application of zeolites [116]. Results achieved with ordered mesoporous silica materials are collected in Tables 14–16, whereas Table 17 shows recycling studies with zeolites and related crystalline materials. The Veisi group fabricated Pd catalysts starting with aminopropyl-functionalized SBA-15 and studied them in Suzuki coupling. The best sample was synthesized by anchoring melamine–pyridine to the amino group followed by treatment with PdCl2 to get immobilized complex 3.1% PdII-59-SBA-15 [117]. The catalyst characterized by means of appropriate physical methods (UV–Vis, FT-IR, TEM, SEM, XRD, EDX, XPS and ICP-AES) was tested in Suzuiki coupling showing high stability in 8 runs (Table 14, entry 1). Pd leaching was not detected by either hot filtration tests or a poisoning experiment with mercaptopropyl-functionalized silica. Immobilized catalyst complex PdII-60-SiO2, synthesized with the use of azido-functionalized mesoporous silica followed by click reaction with ethynylpyridine and, finally, a treatment with PdCl2 [118], gave consistently high yields in a 10-run recycling study (entry 2).
Complexes anchored to MCM-41 developed by Cai et al. were successfully applied in C–C couplings. Sample PdII-61a-MCM-41 with a Pd loading of 2.87 wt% was efficiently recycled in the coupling of 4-chlorobenzoyl chloride with BiPh3 in N-methyl-2pyrrolidone (NMP) as solvent and Bu3N as the base (entry 3) [119]. Catalyst 2.86% CuI-61b-MCM-41, in contrast, has somewhat increased stability in homocoupling (entry 4) [120]. A NiII-MCM-41
In a recent contribution the Cai group reported the synthesis of an immobilized CuI bipyridine complex by reacting MCM-41 with bis-[3-(triethoxysilyl)propylaminomethyl]-2,20 -bipyridine and then with CuI (6.7% CuI-63-MCM-41) [126]. It was successfully applied in a less frequently studied coupling reaction of arylboronic acids with diaryl diselenides to form organic selenides with satisfactory stability in 10 runs (entry 10). Hot filtration test and ICP-AES analysis indicated the heterogeneous nature of the reaction. In a synthesis process aminopropylated MCM-41 was reacted with pyridine-2-carboxaldehyde and then the product was loaded with CuI [127]. The resulting catalyst bearing a Schiff base–pyridine bidentate CuI complex (2.6% CuI-64-MCM-41) exhibited similar features in Sonogashira reaction (entry 11). Related sample 2.86% CuI-61b-MCM-41 with an immobilized bidentate CuI complex exhibited satisfactory stability in Buchwald N-arylation (entry 12) [128].
Data for miscellaneous other reactions induced by catalysts with ordered mesoporous silica supports are found in Table 15 and shown in Fig. 3. Catalyst 4% Pd0-SBA-15 prepared by depositing Pd particles into the nanocages of aminopropylated SBA-16 was also applied in aerobic alcohol oxidations with an impressive stability in 12 runs (entry 1) [129]. The mixture of trioctylphosphane, octadecene and Pd(OAc)2 was heated at 260 °C for 30 min to generate Pd nanoparticles. These were immobilized by mixing with SBA-15 in toluene (rt, 24 h) and then calcination gave PdO/SBA-15 [130]. It exhibited high, stable activity in seven cycles (entry 2). No further reaction was detected in a hot filtration experiment. Karimi and co-workers have fabricated a periodic mesoporous organosilica (PMO) material by hydrolysis–polycondensation of 1, 3-bis(3-trimethoxysilylpropyl)imidazolium chloride (65) and tetramethoxysilane in the presence of Pluronic P123 as structure directing agent followed by adsorbing Pd(OAc)2 (1.06% PdII-65PMO) [131]. Decrease in BET surface area and pore volume testified to the incorporation of IL into the silica wall. Intensity loss of diffraction peaks indicated that Pd species were located in the
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Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65
Table 14 Reactions forming C–C and C–heteroatom bonds. Catalysta
Conditions
Number of cycles
Yield (%)b
Ref.
1
3.1% PdII-59-SBA-15 (0.3)
K2CO3, EtOH/H2O (2:1), 50 °C, 0.8 h, in air
8
98
[117]
2 3
PdII-60-SiO2 (2) 2.87% PdII-61a-MCM-41 (1.5)
K2CO3, EtOH, 80 °C, 6 h Bu3N, NMP, 80 °C, 3 hc
10 10
>99 93.5 (95–92)
[118] [119]
4
2.86% CuI-61b-MCM-41 (1)
piperidine, CH2Cl2, 25 °C, 4 h
10
94.5 (96–93)
[120]
5
NiII-MCM-41 (5)
NaOH, DMF, 70 °C, 1 h
5
94
[121]
6
4% RhIII-62a-MCM-41 (0.3)
EtOH, 40 °C, 24 h, under argon
10
94 (95–93)
[122]
7
2.98% RhIII-62b-MCM-41 (3)
toluene, 80 °C, 4 h, under argon
10
88.5 (90–87)
[123]
8 9 10
4% RhIII-62a-MCM-41 (2) 4% RhIII-62a-MCM-41 (5) 6.7% CuI-63-MCM-41 (5)
toluene, 70 °C, 3 h, under argon CH2Cl2, Et3N, 30 °C, 24 h, under argon DMSO/H2O (2:1), 110 °C, 14 h, in air
10 10 10
90 (91–89) 93 (95–92) 94.6 (96–93)
[124] [125] [126]
11
2.6% CuI-64-MCM-41 (5)
PPh3, K2CO3, DMF, 100 °C, 6 h, under argon
10
95 (96–94)
[127]
12
2.86% CuI-61b-MCM-41 (4)
K3PO4, toluene, 110 °C, 24 h
10
96.5 (98–95)
[128]
Entry
Reaction
(PhSe)2 + MeO
a b c
B(OH)2
SePh
MeO
Percentage data show metal loading (wt%) when available; data in parentheses indicate mol% used. Average of yields. Data in parentheses indicate range of yields. 1 h in run 1.
pores of PMO [132]. Increasing reaction time was needed to achieve close to complete conversions in the oxidation of 1phenylethanol with better than 99% selectivity (entry 3). The negative result of the hot filtration test and Pd content below the detection limit indicate that leached Pd is not involved in the reaction. Poisoning tests provided additional proof.
Jeong and co-workers immobilized human carbonic anhydrase (HCA) onto gold nanoparticles deposited either to aminopropylated or mercaptopropylated SBA-15 (H2N-SBA-15, HS-SBA-15). The thiol-functionalized sample exhibited somewhat better stability than the amino-functionalized catalyst in the hydrolysis of 4nitrophenyl acetate in 20 cycles [133] (entries 4 and 5; 4% and 7% loss of the original activity, respectively). In contrast, a significant loss of 21% was measured for the Au-free catalyst. The higher stability of the Au-loaded samples was attributed to the strong electrostatic interaction of the enzyme with Au particles. The catalysts were separated by centrifugation and were washed with the buffer before reuse. Much larger differences in stability were detected with the use of Pseudomonas fluorescens lipase immobilized on unmodified and aminopropylated SBA-15 [134]. Significant drops in activity are seen after the 10th run when PF/SBA-15 is reused in the solvent-
free esterification with ethanol of sunflower oil (biodiesel production; Fig. 3, open bars). PF/H2N-SBA-15, in turn, showed still satisfactory activity in the 20th cycle (shaded bars), albeit with a higher enzyme loading (502 ± 24 vs. 256 ± 15 mg g1 support). Interestingly, PF/SBA-15 proved to be more active in the hydrolysis of tributyltin. SBA-15 with the immobilized CoII Schiff base complex with low Co loading (1.8% CoII-66-SBA-15) developed by Luque and coworkers is a useful catalyst of high activities in organic transformations. High activity was reported in 12 runs when used in solvent-free Pechmann condensation of resorcinol and ethyl acetoacetate (entry 6) with small decreases in yields [135]. It showed lower stability in the formation of benzimidazoles through oxidative condensation (data not shown) [136].
The Cai group conducted tandem reactions to form 2-aminosubstituted benzo[d]thiazoles using catalyst 2.7% CuII-61c-MCM41 with immobilized CuSO4 and found satisfactory stability (entry 7) [137]. A 2% loss of Cu was measured by ICP analysis. In a recent
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Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65 Table 15 Use of ordered mesoporous silicas in other reactions. Catalysta
Conditions
Number of cycles
Yield (%)b
Ref.
1
4% Pd0-SBA-16 (1)
1 bar, K2CO3, H2O, 50 °C, 4 h
12
99 (100–99)
[129]
2
PdO/SBA-15 (0.66)
7
95
[130]
3
1.06% PdII-65-PMO (0.25)
K2CO3, toluene, 70 °C, 4 h, aerobic conditions K2CO3, trifluoromethyltoluene, 95 °C, 3–5.5 h
10
99
[132]
4
(HCA)Au0-HS-SBA-15
MeCN, Tris buffer (pH 6.4), 25 °C, 15 min
20
97 (100–96)c
[133]
5
(HCA)Au0-H2N-SBA15 1.8% CoII-66-SBA-15 (0.1)
MeCN, Tris buffer (pH 6.4), 25 °C, 15 min
20
96 (99–92)c
[133]
110 °C, 3 h
12
96.5 (98–92)
[135]
Entry
6
a b c d e f
Reaction
7
2.7% CuII-61c-MCM41 (0.25)
Et3N, DMSO, 80 °C, 8 h
10
93.5 (95–92)
[137]
8
3% CuI-67-MCM-41 (10)
Cs2CO3, DMF, 60 °C, 12 h, under argon
10
94 (95–93)
[138]
9
10% CuII-SBA-15 (8)
CH2Cl2, rt, 16 h
5
91.5 (85–97)
[139]
10
H5[PW10V2O40]/ MCM-48 (0.25)
100 °C, 1 h
10
69 (70–68)
[140]
11
HPW-mim/KCC-1
10 bar, 90 °C, 1.5 h
10
97.5 (98–97)
[141]
12
9.7% Pd0-H2N-PEI/ KCC-1 (5)
cyclohexane, 4 Å MS, 130 °C, 20 h
8
95.5 (94–97)
[142]
13
2.76% Au0-68*-SBA15 (2)
CD2Cl2, rt, 0.5 h
11
92.3 (94–90) 95–97d
[143]
14
2.7% RuII-69*-SiO2 (1)
H2O, 40 °C, 3 h
10
99 (>99–98)e 98–97d
[144]
15
4.7% Ru0-MCM-41
[dami][CF3(CF2)3SO3], 40 bar (CO2/ H2 = 1:1), H2O, 80 °C, 5 h
10
17,640 (17,760– 17,570)f
[145]
Percentage data show loading (wt%) of active species when available; data in parentheses indicate mol% used. Average of yields. Data in parentheses indicate range of yields. Relative activities. Range of enantiomeric excess. Conversion. TON values.
contribution they reported the fabrication of a tridentate CuI complex immobilized on MCM-41 (3% CuI-67-MCM-41) [138]. The synthesis included functionalizing the support with [3-(2-aminoethylamino)-propyl]trimethoxysilane followed by a reaction with pyridine-2-carbaldehyde and CuI. It is a leach-free, highly-active and stable catalyst in a cascade reaction to form quinazolines (entry 8). Cu(NO3)2 loaded onto SBA-15 in an aqueous solution at pH 10.5 gave CuII-SBA-15 with a copper loading of 10% [139]. It induced azide–alkyne cycloaddition affording 1,4-disubstituted triazoles in a regioselective manner with increasing activity upon recycling (entry 9).
Tayebee and Maleki loaded Keggin-type heteropoly acid H5[PW10V2O40] on MCM-48 and recycled the catalyst in the synthesis of 4-nitrophenyl-14H-dibenzo[a,j]xanthene. Constant yields
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Table 16 Recycling of catalysts RhIII-70 in the hydroformylation of oct-1-ene.
Entry
Run
Catalyst
Pressure (bar)
Conversion (%)
Linear
Cumulative TON
TOFa
1 2 3 4 5 6 7 8 9 10 11 12 13
1 2 3 4 5 6 7 8 9 10c 15 16 17
1.81% RhIII-70a-SBA-15
50 50 50 50 50 50 50 20 20 50 20 20 20
90.5 95.8 98.7 86.5 74.4 34.3b 92.6 94.2 91.6 86.9 90.0 78.8 93.4
90 91 88 88 88 88 91 92 94 91 85 76 54
671 1306 1932 2511 3030 3221 3885 4597 14,748 25,589 67,716 68,225 68,640
195
Rh(acac)(CO)2 Rh(acac)(CO)2
50 20
33.9 53.5
93b 95b
0.106% RhIII-70b-glass
50 50 50 50 20 20
79.4 84.0 85.1 44.2 95.0 94.3
74.2 82.6 87.1 92.3 91.3 91.2
14 15 16 17 18 19 20 21 a b c d
1 2d 3d 8d 9 10
95 126 96 306 995 836 226 67 119 120 149
519 1113 1789 4624 5318 5979
76 62 102 15 115
Values were measured at 20–40% conversion. Measured after 2 h. Reaction run in neat oct-1-ene. In toluene/isopropyl alcohol (12:1) solvent mixture.
Table 17 Studies using other ordered porous materials. Catalysta
Conditions
Number of cycles
Yield (%)b
Ref.
1
2.1% PdCl2(phen)/NaY (0.7)
Et2N, DMAc, 20 bar, 130 °C, 1 h
16
98.5 (97–99)
[148]
2
4.06% CoII-SAPO-5 (1.7)
CHP,c air bubbling, DMF, 90 °C, 5 h
8
82 (83–81)
[149]
3
Pd0-zeolite/poly (0.1)d
air, 250 °C
11
91 (92.2–89.7)
[150]
AO/peroxidase-MCF (1)
20
96 (100–95)
[151]
10
97.5 (99–96)
[152]
Entry
5
1.6% Ru -71-MCF (5)
H2O2, citrate buffer (pH 4), 30 °C, 1–10 min CH2Cl2, 50 °C
6
1.6% RuII-71-MCF (5)
CH2Cl2, 50 °C
10
94.5 (99–91)
[152]
7
1.6% RuII-71-MCF
CH2Cl2, 50 °C, 40 mine
10
97 (98–98)
[152]
4
a b c d e
Reaction
Decolorization of RBBR
II
Percentage data show loading (wt%) of active species when available; data in parentheses indicate mol% used. Average of yields. Data in parentheses indicate range of yields. Cumyl hydroperoxide (0.2 mM) as initiator. 0.1 g. 36 mol catalyst, 2.2 mM substrate/run, 0.15 M solution, flow rate: 5 mL min1.
in short reactions could be maintained in a solventless system (entry 10) [140]. A catalyst based on KCC-1, a new class of fibrous silica nanospheres, was fabricated and tested by Sadeghzadeh [141]. KCC-1 functionalized with chloropropyl moieties was reacted with 1-methylimidazole (mim) followed by ion-exchange with H3[PW12O40] to get catalyst HPW-mim/KCC-1. It showed high
activity and stability in the solventless synthesis of ethylene carbonate in 10 reuses (entry 11). A low level of agglomeration of catalyst particles and a loss of 3.6% HPW were detected in the recovered sample. In another study, KCC-1 was modified with 3-glycidoxypropyltrimethoxysilane and then reacted with polyethyleneimine forming a highly-branched network of amino
Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65
functionalities. In the final step, highly dispersed Pd nanoparticles (1.8–3.5 nm) were generated to give sample 9.7% Pd0-H2N-PEI/ KCC-1 [142]. Decarbonylation of 2-naphthaldehyde underwent smoothly with high product yields (entry 12). The Somorjai group has reported the one-step synthesis of immobilized gold complexes by reacting SBA-15 with molecular gold complexes [(phosphane)AuBF4] [143]. Reduction with ascorbic acid resulted in the formation of Au0 species located inside the channels. Regioselective asymmetric lactonization of allenoic acid was induced with catalyst 2.76% Au0-68⁄-SBA-15 synthesized with phosphane 68⁄ affording the highest activity and enantioselectivity. Lactonization could be repeated with similar efficiency in recycling (entry 13). The loss of the molecular complex (37%, measured by FT-IR) and 3.2% Au leaching (ICP-OES) were found after 11 cycles.
A unique material, a chrysanthemum-like mesoporous silica was functionalized with TsDPEN [N-(p-toluenesulfonyl)-1,2-diphe nylethylenediamine] and then treated with [RuCl2(C5Me5)] to generate chiral active sites (catalyst 2.7% RuII-69⁄-SiO2). It afforded high, stable activities and enantioselectivities in transfer hydrogenation of acetophenone (entry 14) [144]. Catalysts composed of silica or MCM-41 loaded with ruthenium were used in combination with ILs to test their performance in the selective hydrogenation of CO2 to formic acid [145]. 4.7% Ru0-MCM-41 and IL [dami] [CF3(CF2)3SO3] [dami = 1,3-di(N,N-dimethylaminoethyl)-2-methyli midazolium] gave the best results in terms of TON and TOF values achieving a cumulative TON of 1.77 105 in 10 cycles (entry 15). After separating the aqueous phase the IL phase containing the catalyst was reused.
Hydroformylation of oct-1-ene was studied by Reek, Marras and co-workers using Nixanthphos-based Rh–diphosphane complexes
Conversion (%)
100 80 60 40 20 0 1
3
5
7
9
11
Run
13
15
17
19
Fig. 3. Ethanolysis of sunflower oil. Open bars: PF/SBA-15, shaded bars: PF/H2NSBA-15. Ethanol/oil ratio: 8:1, 30 °C, 7 h. Reprinted (adapted) from Ref. [134], Copyright (2017), with permission from Wiley.
23
anchored onto solid supports (70; Rh/ligand ratio = 1/15). 1.81% RhIII-70a-SBA-15 was made by grafting the corresponding trimethoxysilyl derivative onto SBA-15 then capping and, finally, reacting the formed support with Rh(acac)(CO)2 [146]. The catalyst exhibited high stability and gave high conversions in 15 runs (Table 16). In runs 16 and 17 (entries 12, 13) linear to branched ratios (l/br) decreased significantly. A remarkable cumulative TON of 6.9 104 could be achieved in 17 cycles. Surprisingly, the supported catalyst exhibited higher activity than the homogeneous complex (compare TOF values of entries 1 and 9 to those in entries 14 and 15, respectively). This was attributed to the formation of an inactive dinuclear complex when the homogeneous catalyst was used. Unfortunately, a second batch of the same catalyst gave inferior results with a drop to 67% of the linear isomer and increased isomerization. In a follow-up study the complex was immobilized on microscope cover slips by a drop by drop dilution method that is, slowly adding the functionalized ligand and the capping agent into refluxing toluene (sample 0.106% RhIII-70b-glass, Rh/ ligand = 1/15) [147]. The method assures a more homogeneous distribution than by immersing the glass into ligand solution and then capping. This catalyst, however, gave only a cumulative TON of 5979 under identical conditions in 10 runs (entries 16–21).
A few examples of recycling studies with other ordered porous materials are collected in Table 17. Encaging the palladium complex of 1,10-phenanthroline [PdCl2(phen)] into the supercage of NaY zeolite gave a catalyst, which is highly active and stable in aminocarbonylation [2.1% PdCl2(phen)/NaY] [148]. After separation by centrifugation, reactions could be repeated with consistently high yields in 16 runs with a low level of Pd leaching of 0.08% (Table 17, entry 1). Pd ions were shown to undergo reduction to Pd0 without forming particles. The complex adsorbed on NaY, Pd/C and Pd/Y exhibited inferior characteristics. SAPO-5, a crystalline silicoaluminophosphate of high silica content prepared by the dry-gel conversion, was treated with Co(OAc)2 to form 4.06% CoII-SAPO-5 through ion-exchange with a Si/Al ratio of 0.6 [149]. High activities were recorded in the epoxidation of various alkenes under air with satisfactory stability in recycling in the reaction of isopropenylbenzene (entry 2). A hybrid composite material with Pd particles (45.3 ± 9.4 nm) and natural zeolite embedded on an electrospun polymer nanofiber membrane was developed and studied for water treatment [150]. Specifically, zeolite microparticles of this multifunctional membrane are capable of adsorbing ammonia and then Pd can induce recovery of N2 by oxidation. With about 5% catalyst loss in 11 uses, high degrees of N2 recovery with slow decreases were recorded (entry 3). A recombinant dye-decolorizing peroxidase was immobilized on a mesocellular silica foam (MCF) and used to decolorize Remazol Brilliant Blue R (RBBR) an anthraquinone dye [151]. The foam was prepared by polymerization of TEOS in the presence of 1,3,5trimethylbenzene to form a mesoporous silica material with large pores (25 nm) and a cage-like structure interconnected through windows of about 10 nm. The peroxidase produced from Aspergillus oryzae was immobilized at pH 5 (catalyst AO/peroxidaseMCF). Complete decolorization took place in the first four cycles
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in a 20-run study in about 1 min. Increasing time, however, was needed in further cycles with slow decreases in activities. The whole recycling experiment took 50 min to complete (entry 4). Another MCF-based catalyst material with an anchored Ru complex was applied in metathesis reactions [152]. Immobilization was performed by reacting MCF partially capped with trimethylsilane with a triazoles-modified ligand followed by treatment with second-generation Hoveyda–Grubbs RuII complex. Supported catalyst 1.6% RuII-71-MCF showed god stability in recycling in RCM (entries 5,6). Even better results were found with diethyl diallyl malonate when the sample was used in a circulating flow reactor (entry 7). Ru leaching in the first four and the following eight runs were 8.4 ppm and 2.9 ppm, respectively. Significant decreases in activity were experienced, however, when the reaction was performed in toluene attributed to higher solubility of ethylene in toluene resulting in slower ethylene removal from the system.
4.1.2. Catalysts with amorphous silica support Oxidative fluoroalkylation of aromatics has been achieved applying an electrogenerated NiIII bipyridyl complex stabilized by silica nanoparticles [153]. In a reuse study, 2-phenylpyridine and perfluoroheptanoic acid reacted in the presence of NiIII(2bpy)xSiO2 (2bpy = 2,20 -bipyridine) affording high, stable yields (Table 18, entry 1). A ‘‘high internal phase emulsion” (HIPE) open-cell hybrid matrix was fabricated by the hydrolysis of TEOS by Backov and co-workers. The monolith thus prepared was grafted with (3-gly cidyloxypropyl)trimethoxysilane (glymo) followed by immobilizing the lipase Candida rugosa [154]. 19 runs could be conducted in the esterification of oleic acid with butan-1-ol with full conversion with drops in activity in the final runs to 98% and then 57% (entry 2). This was ascribed to the partial collapse of the monolith and the loss of enzyme. The catalyst without the stabilizing glymo moiety could only be reused in three cycles with a slower kinetics. The next entry is about oxidative sulfur removal. IL [Oct3MeN]2[W2O11] was incorporated into mesoporous amorphous silica made by polymerizing TEOS in the presence of the IL as template followed by calcination at 450 °C [155]. Desulfurization of dibenzothiophene in octane occurred with high efficiency in recycling (entry 3). 2.5% Pd0-72-Mn3O4/SiO2 was prepared by treating porous hollow silica nanospheres with a Mn3O4 layer on the interior surface with a solution of Na2[PdCl4] at pH 2.4 [156]. The method allowed the generation of Pd particles (2.0 ± 0.4 nm) exclusively on the interior cavity surface with the concomitant partial dissolution of Mn3O4. The catalyst is highly efficient in the hydrolytic oxidation of hydrosilanes to silanols (5 104 mol% Pd, TON = 1.98 105) with sufficient stability in recycling (entry 4) and it outperforms commercial 5% Pd/C (range of yields = 95–33%). Werner and Kohrt treated aminopropyl-functionalized silica with iodoethanol and isolated silica-immobilized ammonium salt 73-SiO2 [157]. It retained high activity in the formation of cyclic carbonates in 13 runs (entry 5). The yield with an analogous sample with polystyrene support dropped to 60% in the eighth run.
Hasaninejad and co-workers performed multicomponent reactions to form 4H-benzo[b]pyrans induced by DABCO immobilized on silica gel (11.7% DABCO-74 a-SiO2) [158]. Increasing reaction time was required to sustain high yields in a 15-run recycling study (entry 6). Acid-treated silica reacted first with (3iodopropyl)trimethoxysilane and then acetylacetonate and, finally, treated with Pt(OAc)2 in acetonitrile gave a black solid with immobilized Pd nanoparticles of 6–12 nm (0.43% Pd0-75-SiO2) [159]. The air- and moisture-insensitive catalyst afforded a cumulative TON of 1.34 104 in the Suzuki coupling of iodobenzene and phenylboronic acid (entry 7).
The Marciniec group developed and tested silica-anchored Rh and Ru complexes. Rh siloxide surface complex 2.23% Rh0-76SiO2 was studied in alkene hydrosilylation exhibiting high activity and excellent recycling properties (entry 8) [160]. Rh siloxide phosphane complexes 77-SiO2 showed similar futures in 10-run recycling (entries 9–11) [161]. RCM of diethyl diallyl malonate was induced by a mixture of Ru carbene siloxide complexes 78 and 79 with varying activities (entry 12) [162]. After 15 runs, 95% of the catalyst could be recovered. In a hot filtration test the reaction mixture after removing the catalyst at 20% conversion showed no further reaction.
Helmchen and Malakar have recently reported the first example of the preparation and use of immobilized iridium catalysts in allylic amination [163]. These were prepared by loading cationic Ir(p-allyl) complexes on silica modified with p-ethylphenylsulfonic acid. The loading process was ion-exchange with the trimethylammonium salt. Thorough recycling studies were performed with catalyst IrIII-80a⁄-SiO2 (dbcot = dibenzooctadiene). Characteristic examples are collected in Table 19. The effect of solvents (entries 1–3), catalyst loading (compare entry 1 and 4), and the substituent (compare entry 1 and 5) were examined. The use of the enantiomer of catalyst IrIII-80a⁄-SiO2 was also tested (compare entries 1 and 6). In most cases, stable activities found in many repeated uses dropped significantly toward the end of the studies attributed to catalyst loss. The quantity of the iridium salt remaining on the catalyst in entries 1, 2 and 3 is, respectively, 0.13, 0.64,
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Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65 Table 18 Varied reactions studied with catalysts supported on amorphous silica. Catalysta
Conditions
Number of cycles
Yield (%)b
Ref.
1
NiIII(2bpy)x-SiO2 (1)
MeCN, rt, 24 h, under argon
5
85
[153]
2 3
CR/glymo-SiO2(HIPE) [Oct3MeN]2[W2O11]/SiO2 (5)c
heptane, 37 °C, 24 h 30% H2O2, octane, 60 °C, 1 h
21 8
98 (100–57) 99.5 (98–100)d
[154] [155]
4
2.5% Pd0-72-Mn3O4/SiO2 (1) 73-SiO2 (2)
H2O, acetone, rt, 30 min
10
97 (98–95)e
[156]
1 bar, 90 °C, 6 h
13
97.5 (98–97)
[157]
Entry
Reaction
5
6
11.7% DABCO-74a-SiO2 (6)
EtOH, rt, 35–55 min
15
93.2 (96–90)
[158]
7
0.43% Pd0-75-SiO2 (1)
NaHCO3, reflux, 6–7 min
6
2254 (2350–2050)f
[159]
8
2.23% Rh0-76-SiO2 (0.01)
100 °C, 1 h, under argon
95 (97–87) 89 (88–91)g
[160]
9
1.28% Rh0-77a-SiO2 (0.1)
toluene 100 °C, 1 h, under argon
99 99
[161]
2.78% Rh0-77b-SiO2 (0.1)
toluene 100 °C, 1 h, under argon toluene 100 °C, 1 h, under argon
R = Bu 15 R = 20 SiMe(OSiMe3)2 R = C14H29 10 R = 10 (3,4-diMeO)Bn 10
99
[161]
10
70 (65–72)h
[161]
15
75.5 (78–73)
[162]
10
a b c d e f g h
R = Ph
11
3.29% Rh0-77c-SiO2 (0.1)
12
1.5% Ru0-78 + 79-SiO2 (5)
CH2Cl2, 40 °C, 12 h, under argon
Percentage data show loading (wt%) of active species when available; data in parentheses indicate mol% used. Average of yields. Data in parentheses indicate range of yields. 5 mg of catalyst. Sulfur removal efficiency. Conversion. TON values. Run 15 = 83%. Run 5 = 84%.
and 1.83 mol%. Note, that although catalyst 80a⁄ was used in these studies, when phenyl-substituted allyl carbonate was reacted (entries 1–4 and 6) it is transformed to IrIII-80b⁄-SiO2, which is the resting state recovered and reused.
Immobilized ILs are useful catalyst materials applied frequently in recycling studies. Here, results with the use of ILs as the active material loaded onto amorphous silica supports are collected (Tables 20 and 21). Immobilization may be achieved through non-covalent interaction or covalent attachment (supported ionic liquid-like phase catalysis, SILC, SILP or SILPC) [164]. A combina-
tion of these two approaches is the confinement of an ionic liquid onto a support modified with covalently-bound IL. Readers are advised to consult recent books about detailed discussions [165]. Note, however, that studies with the use of ILs immobilized on magnetic supports are included in Section 4.5. Bis(imidazolium)-tagged TEMPO derivatives (81) noncovalently immobilized on various supports have been applied in the oxidation of varied alcohols with [bis(acetoxy)iodo]benzene (BAIB) [166]. Both catalysts applied with 82 gave quantitative yields in 10 runs in the oxidation of 1-phenylethanol with 10 mol% loading. Steady performances could also be achieved with 1 mol% catalyst in the oxidation of varied alcohols (Table 20).
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Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65
Table 19 Reuse of catalyst IrIII-80a*-SiO2 in allylic aminations.
a b c d e f g
Entry
R
Runs
Catalyst (mol%)
Solvent
t (min)
Conv. (%)
Yield (%)
bra
ee
1 2 3 4 5 6f
Ph Ph Ph Ph Me Ph
33 43 24 11 23 30
2.4 2.4 2.4 0.42 2.32 2.32
THF toluene hexane THF THF THF
10–200b 10–180c 20 1.33–24d 15–90e 10–30g
100–67 100–69 n.r. n.r. n.r. n.r.
87–57 88–59 87–78 89–70 91–63 89–61
97–89 96–90 95–89 95–85 97–93 98–90
98–92 93–91 90–83 95–96 91–93 99–98
Branched isomer selectivity relative to linear. 10 min in the first 38 runs. 10 min in the first 21 runs. Hour. 15 min in the first 20 runs. The enantiomer of IrIII-80a*-SiO2 was used. 10 min in the first 25 runs.
Table 20 Oxidation of alcohols induced by immobilized TEMPO derivatives 81. Cycle
Reaction
81a + 82-SiO2
81b + 82-SiO2
Time (h)
Yield (%)
Time (h)
Yield (%)
1
0.5
>95
0.5
>95
2
0.5
>95
0.5
>95
3
0.5
>95
>95
4
0.5
>95
>95
5
5
>95
5
>95
6
3
>95
3
>95
7
0.5
92
0.5
92
8
0.5
>95
0.5
>95
9
0.5
>95
0.5
>95
10
0.5
>95
0.5
>95
11
0.5
92
12
0.5
92
Reaction conditions: 1 mol% 81, 25 wt% loading relative to 82, CH2Cl2, BAIB (1.1 equiv), rt.
Additional results are collected in Table 21. Wei and co-workers developed silica-supported multilayer imidazole-based IL brushes (Im-83). Im-83a-SiO2 proved to be a highly stable catalyst in 10 cycles in a nucleophilic substitution (entry 1) [167]. The brush with the PF 6 anion (Im-83b) was loaded with PdCl2 and then Pd nanoparticles with uniform distribution (mean diameter = 0.95 nm) were generated by reduction with NaBH4 (catalyst 2% Pd0-Im-83b-SiO2) [168]. Full conversion could be maintained in a 15-run study in the solventless hydrogenation of nitrobenzene with a slight increase in reaction time to achieve full conversion (entry 2). Whereas the average particle size increased to 1.52 nm, no sign of particle aggregation was detected by TEM analysis.
Hagiwara used mercaptopropyl-functionalized silica gel (HSSiO2) to load 10 wt% [bmim][BF4] and Pd(OAc)2 [169]. Both this sample and a catalyst with supported [bmim][PF6] gave full conversions in the hydrogenation of cyclohexene in 10 uses (entries 3, 4). The hydrogenation catalyst [RhCl(PPh3)3] co-entrapped with IL [bmim][PF6] in silica by the sol–gel method [170] revealed high
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Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65 Table 21 Recycling of ILs immobilized on amorphous silica. Catalysta
Conditions
Number of cycles
Yield (%)b
Ref.
1
Im-83a-SiO2 (0.4)
H2O, 80 °C, 5 h
10
99.3 (99.8–98.8)
[167]
2
2% Pd0-Im-83b-SiO2 (0.029)
1 bar, 30 °C, 8.5–9 h
15
100c
[168]
3
5.43% Pd0-HS-SiO2/[bmim] [BF4] (5)
1 atm, EtOH, rt, 45 mind
10
100c
[169]
4
5.23% Pd0-HS-SiO2/[bmim] [PF6] (5) 0.06% [RhCl(PPh3)3]-SiO2/ [bmim][PF6] (0.001) RuII-84* + 85-SiO2/[bmim] [BF4] (0.1)
1 atm, EtOH, rt, 80 min
10
100c
[169]
27 bar, CH2Cl2, 100 °C, 30 min
10
98c
[170]
KOH, iPrOH, 30 bar, 20 °C, 6 h
5
100 79–75e
[171]
0.02% RhIII-86*-SiO2
[emim][NTf2], scCO2 flow, 120 bar, 40 °C, 81 h
33f
1.43 105 99–70e
Entry
5
6
7
a b c d e f g
Reaction
g
[172]
Percentage data show Pd loading (wt%) when available; data in parentheses indicate mol% of metal used. Average of yields. Data in parentheses indicate range of yields. Conversion. Average time to reach full conversion. Range of enantiomeric excess. See text. Cumulative TON.
stability in the hydrogenation of styrene in 10 runs (entry 5). A threecomponent catalyst composed of chiral Ru complex RuCl2(PPh3)2 (S,S-DPEN) (RuII-84⁄), IL 85 and [bmim][BF4] immobilized on silica could be repeatedly used with high efficiency in the asymmetric hydrogenation of acetophenone (entry 6) [171]. Catalyst samples prepared with the use of other supports (MCM-48, MCM-41 and SBA-15) were less stable with a large drop of yield in the fourth cycle. Since Ru loss was not detected, decreasing activities were attributed to pore blocking. Amorphous silica with larger pores, in turn, is not affected. Moreover, a higher TON of 1000 could be obtained in comparison to that of the homogeneous complex (200).
Asymmetric hydrogenation of dimethyl itaconate has been studied in a continuous flow system over a SILP catalyst with immobilized Schrock–Osborn-type chiral phosphane–phosphora midite 1-naphthyl-QUINAPHOS Rh complex (0.02% RhIII-86⁄-SiO2) in the presence of IL [emim][NTf2] and scCO2 [172]. In an 81-h continuous operation with 33 samplings, a very high cumulative TON value and slowly decreasing enantioselectivities were measured (entry 7).
4.1.3. Catalysts with other silicon-based supports The group of Béland, Pagliaro and coworkers has developed a series of sol–gel entrapped Pd catalysts, which are now commercially available. In the fabrication of a catalyst with Pd particles first a sol is prepared via an alcohol-free hydrolysis–polycondensa tion of MeSi(OEt)3 and Si(OEt)4 followed by adding K2[PdCl4] and then hydrogenation [173]. The siloxane matrix 87 in the resulting catalyst (0.32% Pd0-87-SiliaCat) encapsulates Pd particles of 3.2 nm. An excellent performance in a seven-run reuse study in Suzuki coupling was found with a low leaching of Pd (0.43 ppm; Table 22, entry 1). Another SiliaCat preparation with higher loading (0.67% Pd0-87-SiliaCat) with Pd nanoparticles (mean particle size = 5.7 nm) showed high activity and stability in the hydrogenation of methyl 4-nitrobenzoate at room temperature with a minimal leaching of Pd and Si (1–2 ppm) (entry 2) [174].
Ervithayasuporn et al. fabricated a catalyst by depositing palladium onto polyhedral oligomeric silsesquioxane (POSS) modified with pmim. Palladium ions undergo in situ reduction to form Pd particles (3.0–1.5 nm). Catalyst 10.7% Pd0-pmim-POSS allowed the execution of 10 cycles without any decrease in activity in the coupling of 4-bromoacetophenone and phenylboronic acid (entry 3) [175]. The Yang group has developed a novel method for catalyst recycling by working in a Pickering emulsion catalytic system. This is a particle-stabilized emulsion–organic biphasic system composed of Pd nanoparticles (2–4 nm), silica modified with pH-sensitive triamine (MeO)3Si(CH2)3(NHCH2CH2)2NH2 and hydrophobic octyltrimethoxysilane [176]. The key features are the strong adsorption of catalyst particles on the surface of emulsion droplets
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Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65
and pH-triggered emulsion inversion. 0.46% Pd0-Oct/triN-SiO2 fabricated with a triamine/octylsilane ratio of 1.04 was recycled in the hydrogenation of styrene in ethyl acetate–water mixture at pH 3– 4. The separation of product and catalyst was achieved by adjusting the pH to 7–8 [Eq. (3)]. The upper organic phase is decanted and then the subsequent reaction is performed at pH 3–4. Ethylbenzene yields increased then changed significantly in 36 runs (entry 4). Products from previous cycles result in yields higher than 100%. Pd loss in cycle 1 was 0.1 ppm and then below 0.1 ppm in the last cycle. 0.55% Rh0-Oct/triN-SiO2 exhibited similar features in a 9-run test (entry 5).
same transformation [177]. Reactions, in this case, were performed at pH 7–8 and demulsifying was achieved by changing the pH to 3– 4. A yield of 83% in run 1 increased to 100% (run 10) and changed little afterward (entry 6). The rate of hydrogenation was found to be 5.14 times higher than that performed in a conventional biphasic system using isopropyl alcohol as demulsifier. In another approach, a large liquid–liquid–solid reaction interface was created between the organic phase and the Pickering emulsion phase with the nanoparticle catalyst by vigorous stirring [178]. At the end of the reaction, phase separation occurs by stopping the stirring without the need of adjusting the pH. Three catalysts have been
Catalyst 1% Pd0-Oct/triN-SiO2 was prepared using silica nanospheres with a triamine/octylsilane ratio of 0.39 and tested in the
tested in recycling in the hydrogenation of styrene. Significantly prolonged reaction times from 65 to 425 min were needed to
Table 22 Reuse studies with other catalysts of silicon-based supports. Catalysta
Conditions
Number of cycles
Yield (%)b
Ref.
1
0.32% Pd0-87-SiliaCat (0.1)
K2CO3, MeOH, reflux, 2h
7
99.4 (99.6–99.1)
[173]
2
0.67% Pd0-87-SiliaCat (0.5)
H2 balloon, MeOH, rt, 30 min
8
99.5 (99.7–98.9)
[174]
3
10.7% Pd0-pmim-POSS (0.8)
50 °C, K2CO3, H2O/EtOH (1:1), 1 h
10
100
[175]
4
0.46% Pd0-Oct/triN-SiO2 (0.5)
36
95 (77–108)
[176]
5
0.55% Rh0-Oct/triN-SiO2 (0.1)
9
80 (74–101)
[176]
6
1% Pd0-Oct/triN-SiO2 (2000)c
15
95 (83–100)
[177]
7
0.54% Pd0-Oct/SiO2 (8000)c
10
Pd0-Si/nw (0.3)
10
100d 94 (64–100) 97 (98–95)
[178]
8
1 atm, ethyl acetate/ H2O (2.6:3), pH 3–4, rt, 0.5–2 h 1 atm, ethyl acetate/ H2O (2.6:3), pH 3–4, rt, 2.6–4 h 3.5 bar, ethyl acetate/ H2O (1:1), 40 °C, 115 min 6 bar, H2O, 40 °C, 50– 140 min Et3N, TBAAc, butanol, 100 °C, 24 h
9 10 11 12
0.021% Pd0-PVPy/[bmim][PF6]/LC (1) 0.021% Pd0-PVPy/[bmim][PF6]/LC (1) 0.02% Pd0-HDA/[bmim][PF6]/LC (1) Pd0-glassA (5)
Et3N, 100 °C, 20 h Et3N, 100 °C, 4 h Et3N, 100 °C, 20 h Et3N, MeCN, 120 °C, 3 h
15 18 17 40
95.4 (100–43)e 96.3 (100–55)f 96.3 (100–21)g 98
[180] [180] [180] [181]
13
(PdCl2/4-bpy)10-quartz
EtOH, 1 bar, 35 °C
10
100d
[182]
14
K10 montmorillonite (20)h
toluene, rt, 6–300 mini
14
92 (99–15)
[183]
Entry
a
Reaction
[179]
Percentage data show Pd loading (wt%) when available; data in parentheses indicate mol% of metal used. Average of yields. Data in parentheses indicate range of yields. Substrate/catalyst ratio. d Conversion. e Runs 1–12 = 100%, run 13 = 95%, run 14 = 96%. f Runs 1–14 = 100%, run 15 = 98%, run 16 = 100%, run 17 = 89%. g Runs 1–14 = 100%, run 15 = 98%, run 16 = 58%. h 20 mg catalyst, 1 mM substrate. i Despite increasing reaction times from 6 min to 300 min, yield decreased to 15% in run 5. Activation after runs 5, 8 and 11, however, restored activity (yields of 98–99% in 6 min). b
c
Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65
maintain full conversion when a delayed condensation method was applied to fabricate a mesoporous support material with a core–shell structure (0.54% Pd0-88-triN-SiO2, data not shown). Whereas Pd losses were below 0.12 ppm, significant aggregation of Pd particles was detected. Better stability was found when Pd particles were generated inside the mesopores using silica with an octyl-functionalized shell (0.54% Pd0-Oct/SiO2) (entry 7).
29
washing the catalyst after each run (entry 14, last three runs). Calcination of the catalyst (400 °C, 1 h) resulted in the collapse of the interlayer structure with diminishing Brønsted acidity and the concomitant increase in Lewis acidity. This was taken as evidence that edge-surface Lewis acid sites are the probable active species inducing the reaction. 4.2. Use of oxide-based catalysts
In a recent report Yamada and co-workers described the fabrication of a hybrid catalyst composed of Pd nanoparticles (5–10 nm) and a silicon-nanowire-array (Pd-Si/nw) [179]. It showed extremely high activity (TON = 2 106, TOF = 4 104 h1) and excellent stability in 10 cycles in the Heck coupling (entry 8). Neither Pd leaching nor further reaction in hot filtration test was detected. The confined nanosized reaction spaces around the Pd particles are accounted for these excellent features. García and co-workers prepared Pd nanoparticles stabilized with poly(Nvinyl-2-pyrrolidone) (Pd-PVPy) or hexadecylamine (Pd-HDA) [180]. Laponite clay (LC, a synthetic crystalline layered Na/Li/Mg silicate) loaded with [bmim][PF6] served as the support material. Heck reactions proceeded smoothly with the resulting catalysts at 100 °C in solventless conditions. 0.021% Pd0-PVPy/[bmim] [PF6]/LC (mean particle diameter = 3.5 nm) gives high yields in 20-h runs with large drops in the last cycles (entry 9). Shortening the time to 4 h was found, however, to be beneficial with consistently high yields in 16 cycles with drops in the last two runs (entry 10). The catalyst with hexadecylamine as stabilizer exhibited similar features (entry 11). Hot filtration indicated that soluble species are not involved in catalysis. However, significant Pd losses up to 90% could be detected. Nishiwaki et al. prepared two glass-supported catalysts by treating test-tubes with a dilute Pd(OAc)2 solution in acetonitrile (3.3 102 mM) [181]. According to ICP analysis 75% of Pd was adsorbed on the glass surface. An XPS study revealed that PdII species were completely reduced at 120 °C in 3 h. In the case of Pd0-glassA 50 equiv of Et3N was used as reductant (120 °C, 12 h) resulting in the formation of large Pd aggregates. In the other case (Pd-glassB) Et3N was not applied and Pd nanoparticles were not observed. Surface OH groups were suggested to assist the reduction of Pd(OAc)2 physically adsorbed on the glass surface. Sample Pd-glassA proved to be highly stable in recycling (entry 12). The Pd content of the used catalyst decreased to 62%. No changes in catalytic activities were measured after using the catalyst kept in air in one month. Catalytic multilayer films were fabricated by immersing cleaned quartz slides alternately into aqueous PdCl2 and ethanolic 4,40 -bipyridine solutions [182]. Sample (PdCl2/4bpy)10-quartz with 10 bilayers created by this layer-by-layer assembly process has a Pd loading of 7.2 105 mM. The catalytic film has high activity and outstanding stability in the hydrogenation of styrene (entry 13). During hydrogenation PdII ions were slowly transformed to Pd0 forming small Pd clusters reaching the size of 5 nm of the catalyst recovered after the last cycle. Montmorillonite K10, a commercial smectite phyllosilicate was found to be highly active in cyanosilylation of ketones [183]. Activates, however, decreased significantly in recycling (entry 14). Repeated washing with dilute HCl solution, however, restored activities. Full restoration of activates could be obtained by
4.2.1. Oxides as catalyst support The photocatalytic degradation of dye pollutants was studied with TiO2 (anatase) doped with well-dispersed Au nanoparticles of 2–3 nm (2% Au-TiO2) [184]. XRD, XPS and FT-IR indicated strong interactions between Au particles and TiO2. Excellent performance was found in 11 repeated catalyst uses in the degradation of methyl orange (MO) (Table 23, entry 1). Zhang and Zhu fabricated composite particles of a TiO2 core with a porous SiO2 shell then dissolved them in CH2Cl2–dimethyl carbonate under sonication followed by casting. The process resulted in embedding SiO2/TiO2 particles into polycarbonate (polyCarb) [185]. The resulting porous membrane (SiO2/TiO2-polyCarb) has high porosity and efficient mass transfer properties. Photocatalytic degradation of sewage oil was chosen to test recyclability with satisfactory stability (entry 2). Moreover, the membrane could be successfully cleaned by alkaline solution to partially restore activity. Bimetallic supported AuPt catalysts fabricated by mixing a Au/ PVA sol with TiO2 support material followed by adding K2[PtCl4]/ PVA with hydrogen in aqueous solution (1% Au60Pt40-TiO2) [186]. It was tested in the oxidation of sorbose to 2-ketogulonic acid (a precursor for vitamin C) exhibiting high but decreasing activities. However, activities could be restored by a thorough washing of the recovered sample with distilled water (entry 3). A heterogeneous catalyst bearing trilacunary polyoxotungstate Na12[a-P2W15O56]24 H2O (P2W15) supported on c-alumina (P2W15-Al2O3) has been fabricated by Chen, Song and coworkers [187]. Major features are excellent stability, reactivity and selectivity in the oxidation of sulfides to sulfoxides under mild conditions (entry 4). A solution of Ti(OBu)4 in ethanol–water mixed with the solution of IL [bmim][FeCl4] in ethanol was kept overnight at room temperature then dried (60 °C and 100 °C). Then the resulting powder was calcined (250 °C, 3 h) to get a sample with [bmim][FeCl4] immobilized on amorphous TiO2 ([bmim][FeCl4]-TiO2) [188]. The catalyst was utilized in sulfur removal from a model diesel oil [dibenzothiophene (DBT) in octane] to transform DBT to the corresponding sulfone conducted in the presence of IL [omim][PF6] and acetonitrile (extraction-coupled oxidative desulfurization). The material showed excellent catalytic features when reused in 25 runs (entry 5). A similar study was performed with the use of catalyst HPW-NH2-Al2O3 fabricated by treating aminopropylfunctionalized c-alumina beads with heteropoly acid H3[PW12O40] [189]. The recycling performance of this sample with the immobilized highly-dispersed acidic component was somewhat lower (entry 6). A series of well-characterized Ru catalysts supported on TiO2 (164 m2 g1) were made by loading RuCl3 followed by reduction with hydrogen (300 °C, 2 h) [190]. Catalysts were tested in the hydrogenation of CO2 to formic acid. The utility of the sample with a Ru loading of 1 wt% (1% Ru0-TiO2, total metal surface area = 1.23 m2 g1 cat) in IL [dami][CF3(CF2)3SO3] and water was tested in a recycling study affording high TON values (entry 7). Both free formic acid and its salt with the dami anion of IL exist in equilibrium. Product formic acid was isolated with a nitrogen flow at 75–80 °C. Slight decreases of activities are mainly due to Rh loss and particle aggregation. Rhodium single-atom catalysts with different metal loadings were made by treating ZnO nanowires with RhCl3. The samples
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Table 23 Use of oxide-supported catalysts. Entry
Reaction
Catalysta
Conditions
Number of cycles
Yield (%)b
Ref.
1 2 3
Photocatalytic degradation of MO Photocatalytic degradation of sewage oil
2% Au0-TiO2 (1) SiO2/TiO2-polyCarb 1% Au60Pt40-TiO2 (0.4)
H2O, UV irradiation, 1 h H2O, 30 °C, UV irradiation, 3 h 2 bar, H2O, pH 7.5, 50 °C, 8 h
11 20 10
99 (100–98) 52 (55–50)c 80 (84–85)d
[184] [185] [186]
4
P2W15-Al2O3 (2.5)
methanol, 25 °C, 35 min
10
94.8 (95–94.5)
[187]
5
[bmim][FeCl4]TiO2
30% H2O2, [omim][PF6], octane, 60 °C, 1 he
25
100f
[188]
6 7
HPW-NH2-Al2O3 1% Ru0-TiO2 (1)h
MeCN, 60 °C, 2 h g [dami][CF3(CF2)3SO3], 40 bar (CO2/ H2 = 1:1), H2O, 80 °C, 5 h
10 10
[189] [190]
8
0.006% Rh0-ZnO/ nw (0.004) 1% Pt0-89-TiO2 (500)j
16 bar, (CO/H2 = 1:1), toluene, 100 °C, 12 h ethyl acetate/H2O (1:1), 30 bar, 50 °C, 300–360 min
5
98.6 (99.2–98.4)f 2.3 104 (2.33– 2.28 104)i 3745 (4000–3455)i 99.3 (100–99)k 93 (69–99)
0.5% Pd1TiO2 (0.01)
EtOH, 1 atm, 28 °C, 60 min
20
9
10
a b c d e f g h i j k
10
100k
[191] [178]
[192]
Percentage data show Pd loading (wt%) when available; data in parentheses indicate mol% of metal used. Average of yields. Data in parentheses indicate range of yields. Degradation efficiency. Run 7 = 60%, run 8, after washing = 85%. 5 mg catalyst, 5 mL model oil, H2O2/DBT = 4. Efficiency of sulfur removal. 60 mg catalyst, 40.6 mg DBT in 20 g octane, H2O2/DBT = 8. 1 mg Ru. TON values. Substrate/catalyst ratio. Conversion.
after in situ reduction (0.006% Rh0-ZnO/nw) proved to be highly active in hydroformylation to provide TON values (4 104), which are better than that of the Wilkinson complex (1.9 104) [191]. In a recycling study, hydroformylation of styrene yielded isomeric aldehydes with a cumulative TON of 1.87 104 in five reuses (entry 8). The lack of leaching and inactivity of the filtrate demonstrate the strong binding of Rh atoms to the support. The method involving a Pickering emulsion phase discussed earlier (Table 22, entry 6) was further extended to the use of a TiO2 nanoparticle system. It was made by loading Pt particles (2– 3 nm) to commercial TiO2 followed by hydrophobization with (MeO)3SiMe to ensure phase separation (1% Pt0-89-TiO2) [178]. In fact, this sample gave better results (entry 9) with a minimal Pt loss of 0.1 ppm detected in every cycle. An atomically dispersed stable palladium–titanium dioxide catalyst (Pd1/TiO2) was prepared by depositing Pd by ultraviolet irradiation onto ultrathin TiO2 nanosheets stabilized by ethylene glycol induced [192]. High activity (an average TOF of 9 103 h1) and excellent stability were reported in the hydrogenation of styrene in 20 cycles (entry 10) without any change in FT-EXAFS spectrum. These features testify to the robust character of atomically dispersed palladium under reaction conditions.
4.2.2. Oxides serving as catalytically active phase A simple commercial TiO2 gave scattered results in the aerobic, light-mediated oxidative aza-Henry reaction in five runs (Table 24, entry 1) [193]. Carniti and coworkers embarked on the development of niobium oxide catalysts and to find an appropriate solvent system to increase their stability and durability. It their latest contribution, unsupported Nb2O5 was shown to exhibit high activity in the dehydration of xylose to furfural in a water–CPME (cyclopentyl methyl ether) mixed solvent [194]. Activities, however, decreased in repeated uses and increase in temperature and reaction time was needed to maintain activity with the concomitant increase in selectivity (entry 2). Another sample with Nb2O5 supported on silica-zirconia showed better stability (entry 3). Overall, however, pure Nb2O5 gave the highest furfural yields. A CuCr2O4 spinel catalyst (particle size: 20–50 nm) prepared by a hydrothermal method with the use of cationic surfactant CTAB proved to be active and stable in the hydroxylation of benzene to phenol with better than 91% selectivities (entry 4) [195]. The sample was also tested in the oxidation of varied cycloalkanes [196] exhibiting, however, lower stability (data not shown). CuII was suggested to be the catalytically active species participating in a radical pathway. Both the Biginelli and Hantzsch syntheses that are condensations to form 1,4-dihydropyridines and 3,4dihydropyrimidin-2(1H)-ones, respectively, have been accomplished with the use of an a-Al2O3 woodpile catalyst fabricated by 3D printing [197]. It works as a Lewis acid inducing these reactions under microwave heating with high, stable activities in 10run reuses (entries 5, 6).
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Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65 Table 24 Catalysts with oxides as the active phase in recycling. Catalysta
Conditions
Number of cycles
Yield (%)b
Ref.
1
TiO2 (1 equiv)
EtOH, 11 W lamp, 40 h
5
87 (83–92)
[193]
2
Nb2O5c
CPME/H2O (2.33:1), 180–190 °C, 1–4 h d
16
52 (34–65)
[194]
3
CPME/H2O (2.33:1), 180 °C, 6 h
7
45 (38–53)
[194]
4
Nb2O5-SiO2/ Zr2O5c CuCr2O4e
50% H2O2, MeCN, 80 °C, 10 h
10
66.5 (68–66)f 62.5 (64–61)
[195]
5
a-Al2O3
100 °C, 10 min
10
94.5 (95–94)
[197]
6
a-Al2O3
120 °C, 30 min
10
90 (91–89)
[197]
7
TiO2(cement)
30% H2O2, UV light, H2O, rt
50
73 (83–70)g 63 (67–60)h
[198]
Entry
a b c d e f g h
Reaction
Percentage data show Pd loading (wt%) when available; data in parentheses indicate mol% of metal used. Average of yields. Data in parentheses indicate range of yields. 10 wt% xylose, 5 wt% catalyst. Runs 1–7 = 180 °C, 1 h, runs 8, 9 = 180 °C, 2 h, runs 10, 11 = 180 °C, 4 h, runs 12, 13 = 190 °C, 2 h, runs 14–16 = 190 °C, 4 h. 1 g benzene, 80 mg catalyst. Conversion. Fresh beads. Old beads reactivated = 100 °C, 1 h or sun-light irradiation, 1–2 h.
Table 25 Recycling of polymer-based catalysts in coupling reactions. Entry
Reaction
1
Catalysta
Conditions
Number of cycles
Yield (%)b
Ref.
0.29% PdII-90-PS (0.1)
Cs2CO3, H2O, 70 °C, 2 min
15
97 (100–96)
[199]
1.14% Pd0-(PVA) (2) 16.3% PdII-91-PAMAM (GMA) (1) 15.05% PdII-92-POP (0.8)
K2CO3, EtOH, 80 °C, 2 h KOH, iPrOH/H2O (1:1), 70 °C, 15–40 minc K2O3, EtOH/H2O (1:1), 80 °C, 30 min K2CO3, H2O, rt, 1.5 h
30 10
99 (100–95) 99.3 (100–98)
[201] [202]
15
98 (99–97)
[203]
15
96.3 (99–94)d
[204]
X= Br, R= Ac 2 3
X = I, R = H X = Br, R = MeO
4
X = Br, R H
5
a b c d
6.9% Pd0-93-HBPE (0.1)
Percentage data show Pd loading (wt%) when available; data in parentheses indicate mol% of metal used. Average of yields. Data in parentheses indicate range of yields. Run 1 = 15 min, run 2 = 20 min. Conversion.
Verma and coworkers have prepared spherical cement beads coated with TiO2 and studied their applicability in the oxidative degradation of the antibiotic Cephalexin induced with UV light [198]. The freshly-coated photocatalyst was utilized in 50 cycles with excellent performance achieving an average degradation efficiency of 73% (entry 7). Mineralization was determined by measuring the concentration of nitrate, nitrite and sulfate ions. Reactivated beads showed somewhat lower activity.
4.3. Polymer-supported catalysts 4.3.1. Catalysts based on synthetic polymers Catalyst 0.29% PdII-90-PS was prepared by reacting aminomethylated polystyrene with a bis-carbaldehyde to form a polystyrene-anchored Schiff base and then loading PdCl2 [199]. It is a robust catalyst in Suzuki coupling in 15 runs (Table 25, entry 1) with a low level of Pd leaching (0.3%) and small increases in
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Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65
conversion in a hot filtration test. In an interesting approach, Hariprasad and Radhakrishnan fabricated free-standing, multilayer thin-film ‘‘dip catalysts” containing metal particles [200]. The method included spin-coating and annealing of polyvinyl alcohol (PVA) containing K2[PdCl4]. This was followed by spin-coating and annealing of PVA and, finally, generating another Pdcontaining layer to get a layered Pd-PVA/PVA/Pd-PVA structure abbreviated as Pd0-(PVA). Catalyst 1.14% Pd0-(PVA) with spherical Pd nanoparticles (15–30 nm) has been applied in Suzuki coupling [201]. The film could be simply removed from the reaction vessel and reused. Yields of 100% were observed in 22 cycles with drops of a few percents in additional reuses (entry 2). The cumulative TON and TOF values, respectively, are 4.96 104 and 1102 h1.
PAMAM, a hyperbranched poly(amidoamine) polymer attached to a glycidyl methacrylate (GMA) resin, was used as support material to synthesize an active and recyclable Pd catalyst. Multiple NH2 anchoring sites allow a high degree of Pd loading [16.3% PdII-91PAMAM(GMA)] [202]. Reactions could be repeated with high efficiency in 10 runs, albeit with increases in reaction time (entry 3). A robust porous organic polymer (POP) with an embedded bipyridyl ligand was reacted with PdCl2 to have sample 15.05% PdII-92-POP [203]. It induced Suzuki coupling to provide high yields and showed an efficient performance in a recycling study (entry 4). PdCl2 was found to be strongly coordinated (negative hot filtration test) without reduction (XPS).
Catalyst 6.9% Pd0-93-HBPE was utilized in the coupling of 4methyliodobenzene and methyl acrylate at room temperature with high activity and satisfactory stability (entry 5) [204]. It was prepared using a hyperbranched aliphatic polyester (HBPE) functionalized with poly(caprolactone) as hydrophobic core and PEG. This water-soluble polymer with micellar properties served as anchoring site encapsulated Pd particles (1–2 nm) generated by reducing PdCl2 with NaBH4.
Additional reactions resulting in the formation of new C–C bonds are collected in Table 26. An insoluble porous organic polymer of high surface area (959 m2 g1) has been synthesized by free-radical polymerization
of vinyl-functionalized diphosphane monomer dppe [1,2-bis(diphe nylphosphano)ethane] [205]. This was then treated with Rh(acac) (CO)2 to have sample 2% RhIII-POP/dppe. It has excellent swelling properties in organic solvents and affords higher activity in hydroformylation than the corresponding homogeneous complex. Very similar activities and selectivities could be maintained in six runs (Table 26, entry 1) with Rh leaching of <10 ppb in each run. These excellent features are attributed to high ligand concentration in the porous polymer and strong interactions between Rh species and ligand as demonstrated by XPS. Aldol condensations of varied aldehydes and ketones could be induced with the solid base poly (N-vinylimidazole) (PVIm) under ultrasonic irradiation [206]. It gave high yields in the reaction of acetophenone and benzaldehyde in a 10-run experiment with small increases in reaction time (entry 2). Corma and co-workers have synthesized porous aromatic frameworks by reacting tetrakis-(4-halophenyl)methanes (I or Br) with 1,4-phenylenediboronic acid followed by functionalization with acidic and basic functions (94a and 94b) [207]. When this mixture was used in one-pot cascade reactions the two antagonistic functions worked independently that is neutralization of the active sites did not occur. In step 1, transformation of benzaldehyde dimethylacetal to benzaldehyde is catalyzed by acidic 94a and then, in step 2, benzaldehyde undergoes condensation induced by basic catalyst 94b to give the final product (entry 3). Increasing selectivities with full conversion were found in seven cycles. The recovered catalyst mixture was washed with buffer AcONa/AcOH before reuse.
Pericàs et al. reported the use of a polystyrene-supported tripodal tris(triazolyl)methanol CuI complex in alkyne–azide cycloadditions [208]. It exhibited high activity and good stability in recycling; however, it had to be recharged by treatment with CuCl in THF after a few runs because of leaching. The cationic complex 95 reported recently, in turn, has significantly improved characteristics [209]. It could easily be prepared by reacting the immobilized ligand with [Cu(MeCN)4][PF6] to furnish catalyst 2.15% CuI-95-PS. Results with the best stability found in carbene transfer reactions by ethyl diazoacetate are shown in entry 4. The complex was also tested in the same reaction in a 48-h continuous flow experiment (entry 5).
Hashimoto and co-workers accomplished an outstanding achievement by preparing and using mixed dirhodiumII tetracarboxylate complex 96⁄ immobilized on a PS-based crosslinked polymer matrix through covalent bonding (RhIII-96⁄-poly with a
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Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65 Table 26 Polymer-based catalysts reused in other reactions to form new C–C bonds. Catalysta
Conditions
Number of cycles
Yield (%)b
Ref.
1
2% RhIII-POP/dppe (0.05)
toluene, 80 °C, 10 bar, 18 h
6
99.8 (100–99.5)c 100d 88e
[205]
2
PVIm (50)f
ultrasonic irradiation, rt, 40–44 ming
10
93 (94–92)
[206]
3
94a + 94b
toluene, 90 °C, 1 h, under argon
7
80 (78–85)
[207]
4
2.15% CuI-95-PS (5.2)
neat, 6 hh
5
89 (82–95)
[209]
5 6
2.15% CuI-95-PS (5.2) RhIII-96*-poly (2)
EtOH, CH2Cl2, 48 hi toluene, –78 °C, 4 h
14j 20
94.5 (99–92)c,k 83 (85–81) 95–94l
[209] [210]
7
RhIII-96*-poly (2)
toluene, –60 °C, 2 h
15
82 (87–80) 91–90l
[210]
8
RhIII-96*-poly (2)
CH2Cl2, 23 °C, 20 min
100
88 (86–90) 91–92l
[210]
9
3.2% CuII-97-poly (2.5)
1 atm O2, DMF, 120 °C, 40 min
10
98.5 (99–97)
[211]
10
98*-PS (20)
neat, rt, 48 h
5
99 99–88l
[212]
11
Amberlyst 15 (200)
Zn (3 equiv), H2O, 70 °C, 2 h
10
78 (82–80)m
[213]
12
2.5% RuII-99-FPA (0.5/1)n
PhCF3/CH2Cl2 (1:19), 50 °C, 1–2 h
20
97.5 (>98–94)c
[214]
13
2.5% RuII-99-FPA (1)
PhCF3/CH2Cl2 (1:19), 50 °C, 1.5 h
12
97.5 (>98–95)c
[214]
Entry
a b c d e f g h i j k l m n o
Reaction
Percentage data show Pd loading (wt%) when available; data in parentheses indicate mol% of metal used. Average of yields. Data in parentheses indicate range of yields. Conversion. Yield of aldehydes. Yield of the branched aldehyde. 50 mg catalyst for 10 mM of benzaldehyde. Runs 1–6 = 40 min, runs 7,8 = 42 min, runs 9,10 = 44 min. Ethyl diazoacetate added slowly. Full operation time; residence time = 1 min. 14 samplings. Conversion dropped to 83% after 36 h and then increased after the catalyst was washed with CH2Cl2. Range of enantiomeric excess. Run 7 = 72%, run 10, after acid washing = 80%. Runs 1–15 = 1 mol%, runs 16–20 = 0.5 mol%. Runs 1–15 = 1 h, runs 16–20 = 2 h.
loading of 0.27 mM g1) [210]. The complex tethered to the polymer chain with a long spacer is uniformly distributed. Consequently, the active metal center is rather flexible and easily accessible. The catalyst was tested in intramolecular C–H insertion reactions showing good stability with high, consistent enantioselectivities in the insertion of an a-diazo ester (entry 6) and aryldiazoacetate (entry 7). The intramolecular aromatic insertion of a-diazo-b-ketoester took place with excellent durability in 100 cycles with a Rh leaching of 28 ppm (0.0019%) in the first run (entry 8). The performance of the immobilized catalyst matches that of the homogeneous complex with somewhat longer reactions.
o
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A polymer of hierarchical porosity, large pore volume, and high thermal and chemical stability functionalized with a phenanontroline moiety was treated with Cu(OAc)2 to form catalyst 3.2% CuII-97-poly [211]. It displays a good performance in the Glaser homocoupling of acetylenes to give 1,4-diphenylbut-1,3-diyne in high yields in 10 cycles without any Cu loss (entry 9). A swellable pearl-like copolymer of styrene and thiourea-pyrrolidine (98⁄-PS) was explored as an organocatalyst in asymmetric Michael additions [212]. Addition products were delivered in stable yields with small decrease in enantioselectivity in a 5-run study (entry 10). Amberlyst 15 a commercially available acidic resin in combination with zinc has been found to be a regenerable catalyst system in pinacol coupling of aromatic aldehydes [213]. Decreasing activities could be offset by acidic washing (entry 11).
transfer hydrogenation of 3-phenyl-2H-1,4-benzoxazine with the use of Hantzsch ester (diethyl 1,4-dihydro-2,6-dimethyl-3,5-pyridi nedicarboxylate, 101) as the hydrogen source (entry 4).
Kamer and Heutz used solid-phase synthesis to prepare a library of phosphane–phosphite ligands immobilized on Merrifield resin [219]. [Rh(cod)2][BF4] in combination with these ligands exhibited good performance in the asymmetric hydrogenation of a,b-unsaturated amino acid esters under mild conditions. Ligand 102⁄ was successfully reused in 11 cycles with moderate enantioselectivities (entry 5).
The Hoveyda–Grubbs second generation RuII complex anchored on a fluorous polyacrylate (PA) polymer was applied in RCM in a fluorous mixed solvent [214]. 2.5% RuII-99-FPA exhibited high activity and excellent stability in the transformation of allylsulfonamides substrates (entries 12, 13). Catalyst reuse was performed by dissolving the reaction mixture in EtOAc and extraction with FC-22.
In the final part of this subsection, examples of recycling studies in additional organic transformations induced by polymersupported catalysts are discussed. Candida antarctica lipase immobilized on an acrylic resin (Novozyme 435Ò) has been tested in esterification of oleic acid [215]. Consistently high yields were obtained with primary alcohols (ethanol, propanol, butanol) in experiments repeated 10 times (Table 27, entry 1), whereas complete loss of catalytic activity was found with methanol. Another example is the synthesis of lutein dipalmitate through esterification of lutein with vinyl palmitate [216]. High conversions with slowly decreasing tendency were observed (entry 2). Arnold and coworkers applied a useful strategy to fabricate a Pd catalyst by dissolving Pd(OAc)2 in a commercial epoxy resin and initiating polymerization with IL [emim][OAc]. Formation of Pd particles was shown by XRD and XPS in catalyst 5.43% Pd0-poly/epox [217]. Excellent features were found in 10 consecutive uses in the hydrogenation of ethyl cinnamate (entry 3). On average, 0.006% of the original Rh loading was lost in every run. Asymmetric transfer hydrogenation was catalyzed with microporous binol-derived phosphoric acid (binol-100-poly) [218]. It was made by oxidative coupling at the indicated positions (see arrows) of the intermediate phosphoric acid derivative forming a microporous polymer network. It is a highly efficient and highly stable catalyst without changing activity and selectivity in the
The McGowan group used Wang resin to immobilize Cp⁄ Rh and Ir complexes by reacting the resin with the corresponding triflate of hydroxyl-tethered intermediates formed in situ [220]. The catalysts are efficient in the transfer hydrogenation of benzaldehyde, albeit exhibiting lower activity than the corresponding homogeneous complexes. Nevertheless, 6.9% IrII-103-PS could be recycled 21 times without significant activity losses (entry 6), whereas activities decreased slightly then dropped to 79.8% in run 26.
Flexible, porous polymeric materials 104 and 105 with a 3D network and granular morphology, respectively, have been synthesized by click reaction and then loaded with palladium nanoparticles. PdII ions undergo in situ reduction when the samples are applied in the hydrogenation of alkenes and nitrobenzene to form catalysts 3.2% Pd0-104 and 6.8% Pd0-105 [221]. Pd particles are located on the external surface of the former sample (6.7 nm). In contrast, a dual distribution was found for the other catalyst: particles of 3.4 nm are found in the interior of pores, whereas larger particles (8.2 nm) are located on the external surface. High, stable activities with increasing selectivities were found in recycling (entries 7, 8).
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Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65 Table 27 Polymer-supported catalysts reused in additional organic transformations. Entry
Reaction
Catalysta
Conditions
Number of cycles
Yield (%)b
Ref.
1
Esterification of oleic acid with alkan-1-ols
CA/Novozyme (5)c
10 10 10
96 (95–96.9) 94.7 (94.2–95.2) 90.9 (90.1–91.5)
[215]
2 3
Esterification of lutein with vinyl palmitate
10 10
84 (87–81)e 99 (>99–99)
[216] [217]
4
CA/Novozymed 5.43% Pd0-poly/epox (3) binol-100-poly (5)
32 °C, 24 h ethanol propanol butanol toluene, 60 °C, 8 h 2.5 bar, MeOH, rt, 2 h CHCl3, rt, 2 h
10
99f 98
[218]
5
[Rh(cod)2][BF4] (3.3)
102*, 1 atm H2, THF, 25 °C, 20 h
11
97.1f 46 (48.5–43.3)g
[219]
6
6.9% IrII-103-PS (7.4)
tBuOK, 60 °C, 48 h
25
97.5 (98.4–93.7)h
[220]
7
3.2% Pd0-104 (1)
1.5 bar, MeOH, 40 °C, 1 h
6
100f 99.5 (97–100)
[221]
8
6.8% Pd0-105 (1)
1.5 bar H2, MeOH, 40 °C, 1 h
6
[221]
9
0.014% Ag0-(PVA) (0.45)
H2O, 25 °C, 15 min
30
100f 96.5 (85–100) 97.5 (99.9–95.3)
[200]
10 11
26.7% Pt0-HIPE (3.4) TEMPO-106-poly (1)
H2O, rt, 30–45 min, under nitrogen KBr, CH2Cl2, pH 8.6, 0 °C, 10 min
20 6
>97 94–97
[222] [224]
12
TEMPO-106-poly (1)
CuCl, DMF, 60 °C, 1 h
6
90–92
[224]
13
Ni90Pd10-PVPy (5)
air bubbling, H2O, 80 °C, 4 h
10
100 46i
[225]
14
1.59% CuI-107-poly
cyclohexane, reflux, 2 h
6
98
[226]
15
108*-PS (10)
toluene, rt, 5 h
9
90 (94–85)j 91 (90–92)g
[227]
16
2.15% CuI-95-PS (5.2)
neat, 3 h
5
99
[209]
17
2.3% Co -109-poly (0.03)
1 atm CO2, TBAB, rt, 30 h
15
96 (93–98)
[228]
18
3.2% CuII-97-poly (2.5)
1 atm O2, DMF, 120 °C, 40 min
10
98.4 (99–97)
[211]
19
1% Au0-PTFE (0.75)
50% H2O2, green light, tBuOK, 32 °C, 45 min
10
>99
[229]
graph-C3N4-PET (50)k 7.1% MoVI-110-PS (0.3)
solar irradiation, pH 5, 2.5 h iPrOH, 80 °C, 6 h
15 4l
100 95 ± 4%m 82 ± 4%n
[230] [231]
5.7% Au0-111-poly (0.1)
BuOH, rt, 9 h
10
100f
[232]
20 21
22
a b c d e f g h i j k l m n
II
Degradation of sulfaquinoxaline
Percentage data show loading (wt%) of active species when available; data in parentheses indicate mol% used. Average of yields. Data in parentheses indicate range of yields. wt% relative to the acid. 20 mg mL1 enzyme, 20 mg mL1 lutein. Yields decreased to 81% in run 3 and then increased. Conversion. Range of enantiomeric excess. A yield of 79.8% in run 26. Isolated yields. Yields increased to 94% in run 6 and decreased. 50 mg catalyst, 30 mL solution (2 105 mL L1). Six-our continuous operation repeated 4 times. Steady-state conversion of TBHP. Steady-state epoxide yield.
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Another example of a free-standing thin-film ‘‘dip catalyst” is a sample with Ag nanoparticles embedded in a thin multilayer polymer film [0.014% Ag0-(PVA)] [200] [see also the use of related catalyst 1.14% Pd0-(PVA), Section 4.3.1, Table 25, entry 2]. It afforded high, slightly decreasing activities in the reduction of 4nitrophenol with NaBH4 (entry 9). AFM images clearly showed the swelling of the film in the aqueous solution thereby allowing access of the reactants to Ag nanoparticles. An amphiphilic polymer material composed of a poly(styreneco-2-ethylhexyl acrylate) shell with a hyperbranched PEI core served to deposit Pt particles (average size of 32 nm) by reducing K2[PtCl4] with NaBH4 [222]. Then the rubbery material was treated with a biphasic water/oil mixture to form a HIPE elastomer decorated by Pt nanoparticles. The product was transformed with repeated compressions to the final catalyst (26.7% Pt0-HIPE). Features found in the reduction of 4-nitrophenol shown in entry 10 indicate high activity and excellent durability. In a follow-up study, on the basis of platinum leaching, the authors estimated that their catalyst may be recycled over 10 million times [223]. A test by soaking the sample in a solution of NaBH4 for 30 days without detecting Ostwald ripening resulted in an estimate of 1500 possible runs. These conclusions, however, are certainly exaggerated. It is obvious that in addition to metal loss, other features, such as poisoning, catalyst loss because of disaggregation and incomplete recovery, aging in long runs, etc. can result in decreasing activities. Nitroxide block polymer brush TEMPO-106-poly was fabricated by polymerizing cross-linked polystyrene microspheres with polymethacrylate grafted with TEMPO via controlled radical polymerization [224]. The resulting polymer exhibited high stability in six-run tests oxidizing benzyl alcohol to benzaldehyde with both NaOCl and O2 (entries 11, 12). Singh and coworkers prepared Ni– Pd alloys stabilized by PVPy by reducing NiCl2 and K2[PdCl4] with NaBH4 in aqueous solution [225]. According to TEM-EDS analysis, the two metals are uniformly distributed in the nanoparticles (2– 4 nm). The alloy catalysts were used in air-oxidation of biomassderived furans to the corresponding carboxylic acids. About 5% NaBO2 in the as-prepared samples serving as base was found to be sufficient to achieve satisfactory performance. Consequently, as-prepared catalysts were used without washing. Ni90Pd10-PVPy showed an excellent activity toward the oxidation of furfuryl alcohol in 10 cycles, albeit giving low isolated yields (entry 13). Characterization of the spent catalyst recovered by centrifugation indicated practically no structural changes.
Regular microspheres of a melamine–formaldehyde resin made in the presence of PEG2000 were reacted with 1,4-butane sultone and then H2SO4 to produce polymeric immobilized Brønsted acidic IL. This was further treated with K2CuI3 to form catalyst 1.59% CuI107-poly [226]. It efficiently mediated acetalization to form 1,3dioxacycloalkanes exhibiting high stability in a reuse experiment (entry 14). The Pericàs group has prepared a polystyrene-supported bifunctional organocatalyst with (1R,2R)-2-(piperidin-1-yl)cyclo hexanamine moiety attached to a thiourea unit [227]. 108⁄-PS with a functionalization degree of 97% (0.426 mM g1) was successfully tested in a-amination of 1,3-dicarbonyl compounds with azodicar-
boxylates. It is noteworthy that this immobilized chiral organocatalyst does not undergo irreversible deactivation in contrast to its homogeneous counterpart. Good characteristics are found in a recycling study with high, stable enantioselectivities (entry 15). After each run the catalyst was treated in Et3N for 1 h. In a continuous flow experiment, conversions decreased slowly from 99% to 88% in 7.5 h. Catalyst 2.15% CuI-95-PS discussed above (Table 26, entries 4, 5) could be used in carbene transfer with ethanol to form an O–C bond showing high, stable activities in consecutive applications (entry 16) [209]. The robustness of the catalyst was also demonstrated in continuous flow with ethanol. Conversions slowly decreased to a value of 83% after 38 h. The original activity, however, could be restored by washing with CH2Cl2 resulting in reswelling of the catalyst to have a final conversion of 92% at operation time 49 h.
A novel tetraphenylporphyrin-based supercrosslinked organic polymer of high surface area (1360 m2 g1) with numerous micro- and macropores was synthesized with dicholormethane as crosslinker under AlCl3 catalysis [228]. Treatment with Co (OAc)2 resulted in the desired product 2.3% CoII-109-poly, which was used as catalyst to couple epoxides with CO2. The porous structure enhances CO2 interaction with the polymer and, consequently, improves its uptake. Reactions could be repeated with high efficiency in 15 runs (entry 17). Catalyst 3.2% CuII-97-poly [211] applied in acetylene homocoupling (Table 26, entry 9) exhibited a similar performance in the click reaction of phenylacetylene and benzylazide (entry 18). Gold nanoparticles supported on PTFE (1% Au0-PTFE) were used in oxidative esterification of aldehydes with H2O2 under irradiation with green light [229]. During the recycling study, the catalyst was treated with H2O2 in aqueous acetone for 2 h between runs to maintain activity (entry 19). Graphitic carbon nitride (graph-C3N4) was dispersed to polyethylene terephthalate via electrospinning followed by a hydrothermal treatment [230]. Catalyst graph-C3N4-PET thus prepared was used in photocatalytic degradation of the antibiotic sulfaquinoxaline over solar irradiation exhibiting excellent performance in 15 2.5-h cycles (entry 20). No changes in the structure of the used sample could be detected by XRD and FESEM.
A MoVI complex immobilized on polystyrene functionalized with 2-(aminomethyl)pyridine (7.1% MoVI-110-PS) is an efficient and selective catalyst in epoxidations [231]. When applied in a
Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65
continuous flow study, it transformed 4-vinylcyclohex-1-ene to the corresponding epoxide with tert-butyl hydroperoxide (TBHP) to afford high, steady-state TBHP conversion and product yields (entry 21). Greiner, Schmalz and coworkers fabricated patchy hybrid nonwovens as an efficient catalyst platform [232]. Polymer fibers serving as carrier were composed of PS-block-PE-block-poly (methyl methacrylate) functionalized by introducing diisopropylamino groups (111-poly). Then the polymer was dissolved in DMSO followed by quenching resulting in patchy, worm-like crystalline-like micelles forming a shell. This material after electrospinning was dipped into a citrate-stabilized dispersion of Au nanoparticles (11 ± 3 nm) to give 5.7% Au0-111-poly. After swollen in butanol it was tested in alcoholysis of dimethylphenylsilane with butanol, showed high, stable activities in 10 cycles (entry 22). No Au leaching was found by ICP-OES measurements.
37
A support material was made by grafting hyperbranched polyethyleneimine (HPEI) onto polyacrylonitrile fiber (PANf, 114) [236]. Then PANf fibers were mixed with an aqueous solution of HAuCl4 and the resulting material was reduced with NaBH4 to have Au0-114-PANf catalysts. The best results in the reduction of 4nitrophenol were achieved with supports of low HPEI contents (weight gains of PANf in fabricating the support are 0.31% and 0.58%). Catalyst stabilities were characterized by measuring the time needed to achieve 100% conversion (Fig. 4). In all these cases continued use of catalysts required prolonged reactions to maintain activities. 0.71% Au0-1140.58-PANf, however, exhibited relatively stable activities in 39 cycles, whereas significant increases in reaction time were needed after the 28th run with sample 0.31% Au0-1140.31-PANf. A TON close to 7 104 was realized with 0.71% Au0-1140.58-PANf.
The Schulz group has developed and tested salen complexes in a range of organic transformations. Polymeric chiral catalyst CrIII-112⁄-poly [233] was applied in four asymmetric syntheses in nine cycles [234]. It shows very low activity in the addition of dimethyl zinc to benzaldehyde (Table 28, cycle 1). However, yields in further applications in hetero-Diels–Alder reaction (cycle 2), Henry reaction (cycle 3) and ring opening of cyclohexene oxide (cycle 4) are similar to or better than those found by using a fresh catalyst (yields of 63, 92 and 75%, respectively). Features are similar in the second reuse sequences (cycles 5–8). A comparison of activities in runs 2 and 9 testifies to satisfactory catalyst stability.
Luis, García-Verdugo and co-workers completed a detailed recycling study with a polymer mixture composed of supported precatalyst PdII-113a-PS, supported ionic liquid-like catalyst Pd0113b-PS as well as bases 113c-PS and 113d-PS [235]. The latter play a dual role as base being involved in scavenging and stabilizing the active Pd species. PdII-113a-PS in combination with either base gave high stability tested in five Heck reactions followed by Sonogashira coupling, Suzuki reaction and, finally Heck coupling again (Table 29). The polymer after each reaction was washed with NaOAc solution and then used again.
4.3.2. Catalyst supported on natural polymers Starch-capped Ni nanoparticles were shown to act as catalysts in transesterifications in the presence of amine additives [237]. The catalyst was prepared by reducing Ni(AcO)2 with NaBH4 in liquid ammonia as complexing agent at pH 10. The catalyst and the amine additive were sonicated before the reaction and applied in the transesterification diethyl malonate at room temperature with a 20-fold excess of methanol. A constant yield of 96% with increasing reaction time in 24 repeated reactions was observed (Table 30, entry 1). The nanoparticles were separated by centrifugation and reused. Oxidized Ni species were not detected after reaction, which can be attributed to the protecting effect of starch. The Pérez-Juste group has fabricated Pd and Au nanoparticles loaded on cellulose filter paper and tested their preparations in reuse studies in the form of 1.3 by 4 cm strips. First, oleylaminecapped monodisperse metal nanoparticles were prepared and then cellulose paper was dipped into the solution of concentrated metal dispersions. According to XPS analysis, catalyst Pd0-paper with particles of 3.0 ± 0.4 nm contains varied Pd species (Pd0 = 77%, PdII = 18%, PdIV = 5%) [238]. Pd loadings varied between 0.37 mg (1 dipping) and 0.87 ± 0.08 mg (5 dippings). Recycling tests in the oxidative homocoupling of 4-carboxyphenylboronic acid (entry 2) and the reduction by NaBH4 of 4-nitrophenol (entry 3) indicate good stability. The paper strips were washed with borate buffer
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Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65
Table 28 Iterative study with catalyst CrIII-112*-poly in varied asymmetric syntheses. Reaction
Cycle
Yield/ee (%)
Cycle
Yield/ee (%)
1
20/24
5
9/6
2
93/63 63b
6 9
66/65 64/64
3
86/51 92b
7
94/29
4a
83/3 75b
8a
79/3
Reaction conditions: 10 mol% catalyst, rt, 23 h. a Reaction time = 72 h. b Yields with a fresh catalyst.
Table 29 Coupling of iodobenzene in Heck, Sonogashira and Suzuki reaction.
Cycle
a b c d
Reactant
Product
Yield (%)a
Yield (%)b
1
99
78c
2 3 4 5 6 7 8
99 85 96 94 92 91c 99
82d 58d 99d 99d 99c 59c 99d
PdII-113a-PS, 113c-PS. Pd0-113b-PS, 113d-PS. Reaction time = 24 h. Reaction time = 160 min.
and dried overnight at 50 °C before reuse. A homogeneous pathway could be ruled out by a hot filtration test. Catalyst Au0-paper (average particle size of 12 nm) exhibited a similar performance in a longer study (entry 4) [239]. Another gold preparation, 0.723% Au0-collagen/EGCG afforded similar results (entry 5) [240]. The catalyst was made by depositing Au nanoparticles of 10.7 nm onto collagen fibers modified with EGCG, a natural polyphenol through crosslinking with glutaraldehyde. Nagashima et al. used a different approach to have catalysts based on cotton and filter paper [241]. First Pd2(dba)3 was reacted with the ammonium salt of hyperbranched polystyrene [HPS-N+(C12H25)3Cl] in DMF to have an egg-shell structure of Pd particles (3 nm) surrounded with HPS salt. Immobilization was performed by treating a mixture of this material and cotton or filter paper with citric acid in DMF. Pd-HPS/cotton (1.65 mg Pd) could be repeatedly used in Heck coupling in 16 cycles (entry 6) with significant amounts of Pd leaching (run 1: 1.1 mg, run 5: 3.3 mg). Product yields dropped in run 17 and 18 but increased to 95% in run 19 in a prolonged reaction (24 h). Pd-HPS/paper (0.17–0.66 mg
Pd), in contrast, proved to be more resistant. Five-run tests in coupling reactions (entries 7, 8), intramolecular coupling (entry 9) and alkene hydrogenation (entry 10) testify to excellent stability. A minimal Pd leaching of 0.15–2.0 ppm was attributed to mechanical abrasion. Au nanoparticles generated on the surface of yeast cells by biosorption followed by bioreduction have a bimodal sizedistribution of 14.8 ± 3.9 and 30 nm [242]. Full conversions were measured in a 10-run recycling study in reducing nitrobenzene (entry 11). Chitosan (115, R = H or Ac) loaded with an IL, a Pd salt and a ligand (PdII-115-[bmim][BF4]) was applied in varied allylation reactions [243]. It gave excellent, higher than 98% conversion values and proved to be highly stable in the reaction of morpholine with (E)-1,3-diphenyl-3-acetoxyprop-1-ene in 10 runs in the presence of ionic phosphane ligand TPPTS (entry 12) despite a significant Pd leaching of 19.3%. Qi, Zhang and co-workers have fabricated porous chitosan microspheres by adding NaOH solution into a mixture of chitosan, PEG 20 M and glutaraldehyde as crosslinking agent followed by extraction of PEG and deposition of Na2[PdCl4] [244]. In situ reduction gives catalyst Pd0-115/116 with Pd particles (2–4 nm), which exhibits excellent stability in Heck coupling in 13 repeated reactions (entry 13).
A silica-based palladium catalyst has been fabricated by depositing SiO2 nanofilaments on glass surfaces. This was followed by O2 plasma treatment to generate OH groups and then by coating with polydopamine (PDA) via polymerization of dopamine. In the final step treatment with K2[PdCl4] and reduction by catechol moieties in PDA give catalyst Pd0-SiO2/PDA/glass [245]. This so-called mussel-inspired method relies on the observation that mussel foot proteins containing catechol found in DOPA (3,4-dihydroxy-L-
Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65
39
140
Time for full conversion (min)
120 100
80
60
40 20
0 1
3
5
7
9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 Cycyle num ber
Fig. 4. Reduction of 4-nitrophenol with NaBH4 induced by catalysts 0.31% Au0-1140.31-PANf (open bars) and 0.71% Au0-1140.58-PANf (shaded bars). Reaction conditions: 0.0037 mol% Au, H2O, 35 °C. Reprinted (adapted) from Ref. [236], Copyright (2017), with permission from The Royal Society of Chemistry.
phenylalanine) and lysine have strong adhesive properties and play crucial roles in strong adherence to surfaces [246]. The catalyst deposited into a laboratory round-bottom glass flask with 0.22 mg Pd was used as a reactor to run 20 reactions with high activity and slow, gradual decreases (entry 14). 4.4. Varied carbon materials serving as catalyst support Carbon nanotubes and other ordered carbon materials have become rather popular supports in recent years [247] including industrial applications [248]. Relevant examples are included in this subsection. Results of carbon–carbon cross-coupling reactions are collected in Table 31, whereas additional examples are shown in Table 32. Philippot, García-Suárez and co-workers generated homogeneously distributed Pd nanoparticles by decomposing Pd2(dba)3 in the presence of phenolic resin-based mesoporous carbon beads (C) [249]. Two other supports were prepared by oxidation with oxygen plasma of commercial beads (oxC) and the sample after heat-treatment at 2000 °C (2000-oxC). The resulting catalysts have decreasing BET surface area (999, 878 and 130 m2 g1) and slightly decreasing particle mean diameters (2.5 ± 0.68, 2.4 ± 0.46 and 2.0 ± 0.38 nm). All three catalysts were tested in Suzuki coupling in the aqueous phase. 1.35% Pd0-2000-oxC gives high, stable conversions in 10 runs (Table 31, entry 1) with decreasing Pd content to 1.12 wt% lost mainly in the first two cycles. All three catalysts exhibited lower stability when used five times under microwave irradiation (data not shown). Park, Joo and coworkers made a colloidal solution of Pd particles (2.9 ± 0.6 nm) in the presence of oleylamine [250]. The particles thus produced were incorporated into hexagonally ordered mesoporous carbon support material CMK-3 to afford catalyst 3.33% Pd0-OA/CMK-3 with oleylamine as capping agent. When compared to samples with other capping agents, this catalyst showed the highest stability in recycling (entry 2) with a small increase in particle size to 3.0 ± 0.5 nm. A dual-porous carbon support was fabricated by treating a mixture of FeCl3, 1,3,5-triphenylbenzene, and formaldehyde dimethyl
acetal in CH2Cl2 (45 °C, 5 h, 80 °C, 19 h) to form a polymer network [251]. The isolated solid was impregnated with PdCl2 in acetonitrile and the resulting powder after centrifugation underwent carbonization and reduction (500 °C, N2, 400 °C, H2). The narrow pore sizes and complicated pore structure of the microporous organic polymer prevent aggregation/sintering of metal particles during high-temperature treatments. The catalyst with a microporous carbon support (mpC) has a Pd loading of 1.1 wt% with an average particle size of 7.6 nm and affords good performance in multiple repeating reactions (entry 3). The result of a hot filtration test and a 2.2 ppm of leached Pd indicate that dissolved Pd species are not involved in the reaction. Diao and co-workers have prepared a hematite–carbon nanocomposite by treating a mixture of D-glucose and a-Fe2O3 nanoparticles (200 °C, 12 h) [252]. After removing hematite with HCl the resulting hollow carbon nanonets (HCnano) were used as support material to deposit PdCl2. Pd nanoparticles (8 nm) were generated on the surface of nanonets by reduction with hydrazine hydrate to get catalyst 25.5% Pd0-HCnano. High catalyst stability was found in the Heck reaction (entry 4) in contrast to Suzuki coupling. It is despite the observation that significant particle agglomeration was observed in the former reaction. Srivastava fabricated a Pd0-rGO sample by treating a mixture of reduced graphene oxide (rGO) and Pd(OAc)2 with sodium dodecyl sulfate serving as both surfactant and reducing agent [253]. Good stability was observed when the catalyst was applied in Heck reaction in IL [bmim][NTf2] (entry 5). Drops of a few percents of the yield with increasing reuse number were attributed to aggregation of Pd particles from 5 nm to 20 nm in run 14. The product was isolated with extraction and the dried IL–catalyst system was recycled. Cu deposited on activated carbon (1.6% Cu0-C) proved to be highly active and recyclable in click chemistry to form 1,2,3triazoles [254]. The catalyst was prepared by treating CuCl2 and 4,40 -di-tert-butylbiphenyl in anhydrous THF at room temperature under argon atmosphere to form Cu nanoparticles (6 ± 2 nm). This was followed by adding activated carbon to have a catalyst providing high activity and stability in a three-component cycloaddition in triazole synthesis (Table 32, entry 1).
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Table 30 Recycling of catalysts with natural polymers. Catalysta
Conditions
Number of cycles
Yield (%)b
Ref.
1
Ni0-starch
ethylenediamine, rt, 30–120 minc
24
96
[237]
2
Pd0-paper
borate buffer (pH 8.7), 70 °C, 250 min, under air
10
6.3 (6.7–5.8)d
[238]
3
Pd0-paper
H2O, rt, 14 min
11
6.65 (6.8–5.8)d 93.5 (97–92)
[238]
4 5
Au0-paper 0.723% Au0-collagen/ EGCG (300) 2.9% Pd0-HPS/cotton (3.12)
H2O, rt, 30 min H2O, 25 °C e
20 21
95 (96–93) 99.5 (100–99)
[239] [240]
20
94 (>99–50)
[241]
0.55% Pd0-HPS/paper (1.47) 0.94% Pd0-HPS/paper (2.5) 0.57% Pd0-HPS/paper (1.52)
K2CO3, H2O, 80 °C, 24 h
5
99
[241]
K2CO3, H2O, 50 °C, 24 h
5
99
[241]
KOAc, DMAc, 120 °C, 24 h
5
99
[241]
0.5% Pd0-HPS/paper (0.67) 4.8% Au0-yeast (0.012)
2 bar, EtOAc, rt, 5 h
5
99
[241]
H2O, 30 °C, 5 min
10
100
[242]
12
PdII-115-[bmim][BF4] (5)
TPPTS, rt, 1 h, under nitrogen
10
>98g
[243]
13
Pd0-115/116 (1)
KOAc, DMF/ethylene glycol (10:1), 110 °C, 5 h
13
97 (96–100)
[244]
14
Pd0-SiO2/PDA/glass (0.02)
Et3N, TBAAc, butanol, 100 °C, 12 h
20
97 (99–90)
[245]
Entry
Reaction
6
K2CO3, DMF, 80 °C, 12–24 h
f
R1= MeO, R2= tBu 7 8 9
10 11
a b c d e f g
R1 = H, R2 = tBu 1
2
R = H, R = Bu
Percentage data show loading (wt%) of active species when available; data in parentheses indicate mol% used. Average of yields. Data in parentheses indicate range of yields. Run 23 = 90 min, run 24 = 120 min. Rate constant: 104 s1. Reaction time not reported. Run 17 = 85%, run 18 = 68%, run 19 = 95% (24 h), run 20 = 50% (24 h). Conversion.
Rezaeifard and Jafarpour immobilized an FeIII porphyrin complex (117) on multiwall carbon nanotube (Cnt) through coordinative anchoring of the hydroxyl functionality of the nanotube [255]. The resulting catalyst (5.12% FeIII-117-porph/Cnt) showed high activity in the oxidation of sulfides, alkanes and alkenes with the use of tetrabutylammonium peroxomonosulfate (TBAOX). The catalyst applied in a reuse study in epoxidation of cyclooctene surpassed the activity of pure parent FeIII porphyrin. It could be easily recovered by filtration in 10 cycles (entry 2). Moreover, tetrabutylammonium monosulfate could also be recovered by lyophilization. Much higher efficiency and better stability were observed with the use of bimetallic nanoparticles supported on functionalized carbon nanotubes with loadings of 5.46% Pt and 3.44% Pd (Pt74Pd26-118-Cnt) [256]. Nitrobenzene was hydrogenated to aniline in 99% yield in recycling, albeit an increase in reaction time to 30 min was needed in run 11 (entry 3). FDU-type ordered mesoporous carbon (FDU = Fu-Dan University) was doped with nitrogen by treatment with ammonia and the resulting material (FDU-N) was used to deposit Pd nanoparticles of about 2.3 ± 0.5 nm
(H2[PdCl4], NaBH4) [257]. The multifunctional catalyst showed excellent performance in the selective hydrogenation of phenol to cyclohexanone with high stability in recycling (entry 4).
A family of catalyst materials fabricated by functionalizing carbon nanohorns (Cnh) with bis-imidazolium salts was applied in the synthesis of cyclic carbonates [258]. Two samples with different loadings (IL-119a-Cnh 1.93 mM g1, IL-119b-Cnh 2.93 mM g1) used in recycling studies allowed the synthesis of the target
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Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65 Table 31 Recycling of carbon-based catalysts in coupling reactions. Entry
Reaction
1
Catalysta
Conditions
Number of cycles
Yield (%)b
Ref.
1.35% Pd0-2000-oxC (1.5)
K2O3, H2O, PEG, 50 °C, 10 min
10
96c
[249]
3.33% Pd0-OA/CMK-3 (0.02) 1.1% Pd0-mpC (0.002)
K2CO3, DMF/H2O (5:1), 150 °C, 10 min K3PO4, DMF, 120 °C, 2 h, in air
6 10
>99c 94.5 (95–93)
[250] [251]
25.5% Pd0-HCnano (0.032) Pd0-rGO (0.25)
K2CO3, DMF, 120 °C, 0.5 h [bmim][NTf2], K2CO3, 100 °C, 1 h
6 14
100 95.5 (98–90)
[252] [253]
X= Br, R= CHO 2 3
X = Br, R = OH
4 5
R = Me R = Et
R= H
a b c
Percentage data show loading (wt%) of active species when available; data in parentheses indicate mol% used. Average of yields. Data in parentheses indicate range of yields. Conversion.
Table 32 Reuse of carbon-based catalysts in varied organic transformations. Catalysta
Conditions
Number of cycles
Yield (%)b
Ref.
1
1.6% Cu0-C (0.5)
H2O, 70 °C, 3 h
5
100c
[254]
2
5.12% FeIII-117-porph/Cnt (0.05)
70 °C, 15 min, under air
10
92.5 (94–91)
[255]
3
Pt74Pd26-118-Cnt (1)
EtOH, 1 bar, rt, 20 mind
11
99
[256]
4
2.5% Pd0-FDU-N (5)
1 bar, H2O, 100 °C, 2 h
6
>98
[257]
5
IL-119a-Cnh (0.23)
40 bar, 150 °C, 3 h
6
135 (101–175)e
[258]
6 7
IL-119b-Cnh (0.34) 5.62% MnO2-GO (0.1)
40 bar, 150 °C, 3 h air, 110 °C, 3 h
6 6
147 (100–183)e 95 (97.5–96.5)c
[258] [259]
8
3.2% MnII-GO (0.1)
H2O2 (3 equiv), NaHCO3, DMF, 0 °C, 30 min
10
97.5 (98–96)
[260]
9
9.3% PdII-120a-rGO (1)
1 atm, Cs2CO3, toluene, 100 °C, 15 min
10
100
[261]
10
9% RuII-120b-rGO (2)
Cs2CO3, toluene, 100 °C, 12 h
10
100
[261]
11
PdII/RuII-120a + 120b-rGO (2)
tBuONa, iPrOH, 80 °C, 3 h
13
98.5 (100–87)f
[262]
12
AgAu-rGOg
H2O, 3.5 h
15
>99
[263]
13
RhIII-121-diamond (1)
40 bar, MeOH, NaOH (aq. solution, Na/Rh = 5) rt, 4 h
5
>99
[264]
Entry
a b c d e f g
Reaction
Percentage data show loading (wt%) of active species when available; data in parentheses indicate mol% used. Average of yields. Data in parentheses indicate range of yields. Conversion. 30 min in run 11. TON values. Run 8 = 95%, runs 10 and 12 = 100%, run 13 = 87%. 1.8 mL CrVI (0.1 mM aqueous solution), 0.5 mg catalyst.
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Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65
compound with gradually increasing efficiencies (entries 5, 6). Since Cnt-based catalysts did not show similar behavior, the authors emphasized the beneficial effect of the support material.
4.5. Magnetic and magnetically tagged catalysts
Graphene oxide (GO) decorated with ultrafine MnO2 nanoparticles of 3 nm prepared by a simple and facile in-situ growing method showed excellent characteristics in the selective oxidation of benzyl alcohol to benzaldehyde in a six-run study (entry 7) [259]. Stability was attributed to strong interactions between MnO2 and the oxygen-containing functional groups of GO. Another graphene oxide-based catalyst prepared by treating GO with Mn (OAc)2 was suggested to have homogeneously dispersed MnII ions coordinated to oxygenated functional groups [260]. Epoxidation of styrene with hydrogen peroxide could be efficiently mediated with this robust catalyst (entry 8). The palladium complex of N-heterocyclic carbene ligand with a pyrene tag was immobilized on the surface of rGO by p-stacking [261]. Catalyst PdII-120a-rGO (9.3 wt% loading of the complex) afforded full conversion and 100% selectivity in the hydrogenation of styrene in 10 cycles without detectable Pd leaching (entry 9). The related Ru complex RuII-120b-rGO (9.0 wt% loading) showed similar properties in the oxidant-free dehydrogenation of benzyl alcohol to benzaldehyde with the loss of 1.3 wt% of the complex (entry 10). Since Cs2CO3 was found to adsorb on the surface of rGO, addition of the base was needed only in the first run. These two complexes were subsequently co-immobilized onto rGO and employed in hydrodefluorination of fluoroarenes [262]. Catalyst PdII/RuII-120a + 120b-rGO (5.19 wt% Pd and 4.97 wt% Ru loading) gave stable activities in 12 runs (entry 11). The synergistic action of the two metals was suggested by the authors to account for catalyst performance.
A range of mono- and bimetallic catalysts supported on rGO generated by using Albizia Saman leaf extract as reducing and stabilizing agent was synthesized in one pot [263]. AgAu-rGO was successfully used in reducing toxic CrVI ions with formic acid affording high stability and excellent efficiency (entry 12). A Wilkinson-type catalyst was prepared by modifying nanodiamond particles (7 ± 3 nm) with catechol phosphane ligands and then reacting with [Rh(cod)Cl]2 (catalyst RhIII-121-diamond) [264]. It induced the hydrogenation of chalcone in five successive runs (entry 13). Both a hot filtration test and ICP-AES (0.1 ppm Rh loss) indicated the heterogeneous nature of the reaction.
The use of nanosize magnetic catalysts emerged in the last decade as an efficient solution for the separation and reuse of catalyst materials. Magnetite (Fe3O4) and hematite (c-Fe2O3) are applied most prolifically to fabricate magnetic nano-catalysts. These can be conveniently and efficiently collected using a permanent magnet and then separate the reaction mixture by decantation or using a syringe. Magnetic and magnetically tagged catalysts combine the properties of highly active homogeneous catalysts with easy separation representing bridges between homogeneous and heterogeneous catalysis because the nano nature is considered as a quasihomogeneous phase [265]. A further advantage is easy surface functionalization, which allows fine-tuning catalytic properties. A number of reviews treat varied applications of magnetic nanoparticles [265,266]. In this subsection recent examples are discussed to highlight their successful use as recyclable support materials in catalytic studies. Organization has been made according to the type of transformations. 4.5.1. Recycling in hydrogenation and hydrogenolysis A few results about the recycling of magnetic catalysts in hydrogenation and hydrogenolysis are collected in Table 33. Rossi and co-workers fabricated a catalyst composed of silicacoated magnetic particles and Ni deposited by decomposing Ni [cod]2 (2% Ni0-Fe3O4/SiO2) [267]. Then they studied the hydrogenation of cyclohexene to find high catalyst stability in 15 repeated uses (entry 1). An additional advantage of this catalyst is the easy reduction of oxidized surface Ni species under reaction conditions. They have also prepared and tested two Rh catalysts. In an earlier study they reported the first magnetically recoverable Rh catalyst (Rh particles of 3–5 nm) made with the use of Fe3O4 coated with silica and functionalized with (3-aminopropyl)triethoxysilane (1.55% Rh0-Fe3O4/SiO2) [268]. It proved to be highly active giving a wide range of TOF values and a high cumulative TON of 1.8 105 in 18 reuses (entry 2). The other catalyst with silicacoated magnetic particles was impregnated with the colloidal suspension of Rh particles to synthesize 0.09% Rh0-Fe3O4/SiO2 [269]. High activities decreased significantly already in the fourth run (entry 3). Nevertheless, a high cumulative TON was achieved in seven runs with a Rh loss of 7%. Nanosized magnetic FeCo alloy particles with a graphitic shell (FeCo/graph) were embedded into mesoporous silica spheres. The product was treated with (3-amino propyl)trimethoxysilane and then reacted with K2[PtCl4] in ethanol to generate Pt nanoparticles (3.5 nm), which became immobilized on the amine-functionalized silica surface (5.6% Pt0-FeCo/ graph/SiO2) [270]. The catalyst maintained excellent activity in five reuses in the hydrogenation of cyclohexene (entry 4). Reiser and coworkers have fabricated Pd catalysts using a magnetic support composed of carbon-coated Co nanobeads functionalized with an imidazolium-based ionic liquid. The sample with an unusually high Pd loading of 34 wt% deposited by microwave decomposition of Pd2(dba)3CHCl3 proved to be the best catalyst in hydrogenations (34% Pd0-122-IL/C/Co) [271]. Results in the hydrogenation of trans-stilbene to reach full conversion with the
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Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65 Table 33 Hydrogenation of C@C and C@O bond and hydrogenolysis with magnetic catalysts. Catalysta
Conditions
Number of cycles
Yield (%)b
Ref.
1
2% Ni0-Fe3O4/SiO2 (0.33)
6 bar, 75 °C, 25 min
15
>99c
[267]
2 3 4 5
1.55% Rh0-Fe3O4/SiO2 (0.053) 0.09% Rh0-Fe3O4/SiO2 (0.002) 5.6% Pt0-FeCo/graph/SiO2 (0.12) 34% Pd0-122-IL/C/Co (0.1)
6 bar, 75 °C, 25 min 6 bar, 75 °C, 25 min 1 atm, acetone, 25 °C, 10 h 10 bar, iPrOH, rt, 2 hf
18 7 5 12
2.7 104 (4.1–1.5)d 3.0 105 e 99.9 100
[268] [269] [270] [271]
6
3% Pd0-Ni0/CeO2–x (0.0016)
1 atm, EtOH, 25 °C, 20 min
8
100c
[272]
7
1.55% Rh0-Fe3O4/SiO2 (0.053)
6 bar, 75 °C, 25 min
21
0.7 103 (1.25–0.6)d
[268]
8
1% Rh0-c-Fe2O3 (1)
1 atm, H2O, 30 min, rt
5
600g
[273]
9
1.95% Pt0-Fe3O4/SiO2 (0.083)
6 atm, 75 °C, H2O, min, rt
14
99c1.56 104
10
20% Ni0(Ce)-Al2O3
70 bar, Ca(OH)2, 240 °C, 12 h
18
93 (98–86)c
Entry
a b c d e f g
Reaction
e
[274] [275]
Percentage data show metal loading (wt%) when available; data in parentheses indicate mol% used. Average of yields. Data in parentheses indicate range of yields. Conversion. TOF values (h1). Cumulative TON value. 2.5 h in the last run. Average TOF (h1).
use of 0.1 mol% of Pd are shown in entry 5. 13% of Pd was lost with significantly increased values in the last two runs (64 and 48 ppm, respectively) and a cumulative Co loss of 180 ppm was also measured. A novel magnetically recoverable nanocatalyst is composed of active Pd nanoparticles deposited onto a support material with magnetic Ni nanoparticles (61 wt%) and oxygen-deficient cerium oxide (3% Pd0-Ni0/CeO2–x) [272]. Full conversions in the hydrogenation of styrene could be maintained with increasing reaction times with a very low palladium loading of 0.0016 mol% (entry 6).
The performance of catalyst 1.55% Rh0-Fe3O4/SiO2 tested in the hydrogenation of benzene in 21 runs (entry 7) giving a cumulative TON of 1.16 104 is similar to that found in cyclohexene hydrogenation (see entry 2). Catalyst 1% Rh0-c-Fe2O3 with Rh nanoparticles of 2.2 nm was applied in the hydrogenation of arenes under mild conditions (room temperature, atmospheric pressure) [273]. In a recycling study to transform toluene to methylcyclohexane, consistently high TOF values were measured in five runs with magnetic recovery (entry 8). An Fe3O4 core covered with an aminopropylated silica layer was loaded with K2[PtBr4]. The resulting catalyst undergoes in situ reduction when used in the hydrogenations of alkenes, aromatics and carbonyl compounds [274]. 1.95% Pt0-Fe3O4/SiO2 with particles of 2.5 nm showed an excellent performance in the hydrogenation of pentan-3-one in 14 runs without loss of activity and a high cumulative TON (entry 9). A cerium-doped Ni on Al2O3 magnetic catalyst [20% Ni0(Ce)-Al2O3] made by co-precipitation works efficiently in the hydrogenolysis of sorbitol to form a mixture of
C2–C4 diols of practical importance (entry 10) [275]. A slow decrease in activity caused by particle agglomeration could be offset by adding a small amount of fresh catalyst. The overall glycol selectivity was about 55–60%. A benchmark reaction in testing metal nanoparticle catalyst materials is the reduction of nitroarenes, in particular, that of nitrobenzenes and nitrophenols. Readers are advised to consult a related recent review [276]. Support material 123 was fabricated by coating Fe3O4 particles with oleic acid followed by functionalization with 3mercaptopropionic acid. Reduction of PdCl2 with NaBH4 resulted in the formation of Pd particles (catalyst 5.3% Pd0-123-Fe3O4/olac) [277]. Pt particles were deposited in a similar way to the surface of carbon-coated magnetic Fe3O4 microspheres prepared by carbonization of glucose (0.58% Pt0-C/Fe3O4) [278]. Both catalysts showed high efficiency in the catalytic hydrogenation of nitrobenzene (Table 34, entries 1, 2). A Thales Nano H-Cube continuous flow reactor was utilized in the hydrogenation of nitroarenes, azides and alkenes [279]. The catalyst is composed of Pd nanoparticles (1–3 nm) and maghemite (c-Fe2O3) spheres (10–30 nm). XPS spectra indicate that palladium is dominantly metallic with minor amounts of PdO. High catalyst activity with sufficient stability in the hydrogenation of 4-methoxynitrobenzene was detected in multiple repeating reactions (entry 3).
Wang et al. prepared a hierarchical catalyst material by first modifying Fe3O4 with polyacrylic acid followed by treatment with 4-vinylpyridine and divinylbenzene to form a polymeric shell. Finally, Au nanoparticles (average size = 5 nm) were generated by
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reduction with NaBH4 of HAuCl4 to give catalyst 18% Au0-124Fe3O4/poly [280]. It was applied in the reduction of 4-nitrophenol with NaBH4 (entry 4). High activity and a very low amount of Au leaching (37.9 ppb) were reported. Another Au catalyst (23.14% Au0-125-Fe3O4) was somewhat less effective showing a large drop in the final cycle of an 11-run test [281]. In the synthesis, first Fe3O4 nanoparticles were surface functionalized by a treatment with hexadecyltrimethoxysilane followed by a reaction with chitosan to form a surface layer via ionotropic gelation. After Au seeding, Au nanoparticles were deposited by reduction of HAuCl4 with glucose. The catalyst was thoroughly characterized by various methods (TEM, HRTEM, SEM, FT-IR, XRD, XPS and TGA) and tested in a 30-day period. This was done by preparing an aqueous catalyst solution (1 mg mL1) and testing it every day for the first five days followed by additional tests every 5th day (10 samplings). Only very small activity drops were detected (entry 5).
Li et al. fabricated novel magnetic double-shelled hollow microspheres by starting with the synthesis of C-SiO2-Fe3O4 [282]. First, a mixture of Fe3O4 nanoparticles, TEOS, CTAB, FeCl3, and HCl was aerosolized and the droplets underwent hydrolysis and condensation in a furnace to form spherical particles. These were then coated with a resin layer by the polymerization of 2,4dihydroxybenzoic acid and formaldehyde. Finally, carbonization and removal of silica allowed the transfer of the magnetic core to the outer carbon shell. Structural features were demonstrated by instrumental studies (TEM, HRTEM, and SEM). The support material loaded with Pt particles (K2[PtCl6] in ethanol) gave catalyst Pt0-C/C/Fe3O4, which maintained full conversion in five cycles (entry 6). Silica microspheres were hydrothermally treated with ferrocene to generate particles coated with a double shell of magnetic Fe3O4 and carbon followed by partial etching the silica core with NH3 (C/Fe3O4/SiO2). Aminopropylation, impregnation with HAuCl4 and, finally, reduction with NaBH4 afforded catalyst 3.4% Au0-C/Fe3O4/ SiO2 with a double-shelled yolk-like structure. The silica core is movable, the shell is mesoporous, while Au nanoparticles of 2 nm are located in both the interior cavity and the mesoporous shell [283]. Increasing reaction times were needed to maintain high activity in recycling (entry 7). Reasons are the loss of Au particles from the surface and adsorption of product 4-aminophenol. Interestingly, Au content increased slightly to 3.9%, which was attributed to the partial etching of silica by NaBH4 under reaction conditions. An excellent, highly active and reusable gold catalyst has been made by first fabricating magnetic polymer beads by coating Fe3O4 particles with a polymer layer of 1,4-divinylbenzene methyl methacrylate. These were then mixed with a solution of Au nanoparticles (10 nm) to have catalyst 0.43% Au0-Fe3O4/poly [284]. A sample with a high gold particle density of 4292 lg g1 showed high stability in 23 runs in the reduction of 4-nitrophenol (entry 8). The same activity was measured using the sample after stored for 3 months. The activity of this sample is close to that of a quasi-homogeneous isolated Au nanoparticle catalyst. A composite catalyst with gold nanoparticles was prepared and tested in the same reaction under similar conditions [285]. First Fe3O4 particles were generated and deposited on the surface of rectorite a layered silicate clay material. Further treatment with
chitosan in an acidic suspension was followed by reaction with HAuCl4 and finally, reduction with NaBH4 to form gold nanoparticles of <10 nm. 14.3 wt% Au0-Fe3O4/115/rect showed high, stable activities in a longer study (entry 9). 3% Pd0-Ni0/CeO2–x, the novel magnetically recoverable nanocatalysts used in the hydrogenation styrene (Table 33, entry 6) was tested in the reduction of 4nitrophenol as well [272]. High stability and excellent activity with an ultralow metal loading of 1 105 mol% was reported under ultrasonic irradiation (entry 10). Shafiee et al. applied a multistep process to functionalize magnetic GO nanosheets [286]. Modifications included stepwise treatment with 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide, N-hydroxysuccinimide, functionalized diethylene glycol (DEG) H2N-DEG-OTs and, finally, aminoguanidin hydrochloride. Pd loading gave catalyst 9.6% PdII-126-Fe3O4/GO with magnetic particles (<10 nm) and PdII stabilized in a five-membered chelate ring. It is a catalyst water-tolerant and robust catalyst and reveals high activity and durability in 20 cycles (entry 11).
A supported catalyst bearing spherical bimetallic particles (10– 12 nm) with an Fe core coated with Au shell has recently been developed. An aqueous dispersion of particles (10–12 nm) mixed with the aqueous dispersion of graphene oxide functionalized with 4-aminothiophenol gave catalyst Fe0Au0-127-GO [287]. It was applied in 10 repeated experiments in the catalytic reduction of both 4- and 2-nitrophenol with NaBH4 reused after magnetic separation (entries 12 and 13).
A Ni catalyst was prepared by reducing Ni(AcO)2 in the presence of Fe3O4 and starch as capping agent [288]. Catalyst 0.45% Ni0Fe3O4/starch with a core–shell structure was tested in the reduction of 4-nitroaniline in 30 cycles (entry 14). Whereas increases in reaction time were needed to maintain high product yields, an estimated cumulative TON of 3.5 103 could be achieved. Kim, Hyeon and co-workers have synthesized 1.09% Rh0-Fe3O4 heterodimer magnetic nanocrystals by controlled thermolysis of a mixture of Rh(acac)3 and Fe(acac)3. The resulting catalyst is composed of Rh (average diameter = 2–3 nm) and Fe3O4 particles (16 nm) [289]. It showed consistently high activities in the reduction with hydrazine of nitrobenzene (entry 15) without significant changes in shape and size of the particles. 4.5.2. Reuse studies in oxidations Immobilization of TEMPO on magnetic polystyrene nanospheres and the application of the resulting catalyst sample (10.6% TEMPO-128-Fe3O4/PS) in the Montanari oxidation of alcohols have been successfully achieved by Wang and co-workers [290]. High and constant activities were reported in 20 repeated reactions to selectively transform benzyl alcohol to benzaldehyde (Table 35, entry 1). Similar features were found in an iterative study to oxidize 20 alcohols of varied structure (yields = >99– 96%, reaction time = 5–40 min).
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Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65 Table 34 Hydrogenation and reduction of nitrobenzenes with magnetic catalysts. Entry
a b c d e f g h i
Reaction
Catalysta 0
Conditions
Number of cycles
Yield (%)b
Ref.
1
5.3% Pd -123-Fe3O4/olac (1.25)
EtOH, 1 bar, rt, 1 h
10
>97.5 (>99–>95)
[277]
2 3
0.58% Pt0-C/Fe3O4 (0.12) 3.92% Pd0-c-Fe2O3 (6.2)c
EtOH, 1 bar, rt, 1 h EtOH, 30 °C, 10–15 mind
10 12
>98 (>99–96) 93.5 (95–92)
[278] [279]
4
18% Au0-124-Fe3O4/poly (0.018)
EtOH, rt, 3 min
10
97.5 (100–97)e
[280]
5 6 7 8 9 10 11 12 13
23.14% Au0-125-Fe3O4 (0.0023) 20.3% Pt0-C/C/Fe3O4 3.4% Au0-C/Fe3O4/SiO2 0.43% Au0-Fe3O4/poly 14.3% Au0-Fe3O4/115/rect 3% Pd0-Ni0/CeO2–x (1 105) 9.6% PdII-126-Fe3O4/GO (0.026) Fe0Au0-127-GO Fe0Au0-127-GO
H2O, H2O, H2O, H2O, H2O, H2O, H2O, H2O, H2O,
10 5 9 23 14 20 20 10 10
97 (98–96) 100 98.5 (99–97.5)e 100e 99.5 (100–97.5)g 100–96 (99)e 99 99 (100–98.1)e 98.7 (100–97.6)e
[281] [282] [283] [284] [285] [272] [286] [287] [287]
14
0.45% Ni0-Fe3O4/starch (1.06)
H2O, rt, 12–140 mini
30
95.9 (96–95)
[288]
15
1.09% Rh0-Fe3O4 (1)
EtOH, 80 °C, 1 h
8
99
[289]
rt, rt, rt, rt rt, rt, rt, rt rt
12 minf 8 min 100–350 s 30 min 100 sh 200 s
Percentage data show metal loading (wt%) when available; data in parentheses indicate mol% used. Average of yields. Data in parentheses indicate range of yields. 6.2 wt% relative to substrate. Time of the full recycling study; residence time = 0.75 min. Conversion. See text. Degradation efficiency. Ultrasonic irradiation. Run 15 = 24 min, run 25 = 47 min, run 29 = 70 min.
Rezaeifard, Jafarpour and co-workers fabricated a catalyst by immobilizing a CuII phthalocyanine-tetrasulfonic acid tetrasodium complex (0.044% CuII-129-Fe3O4/SiO2) through ionic interaction on a silica-coated magnetic support functionalized with (3aminopropyl)triethoxysilane [293]. It exhibited excellent catalyst stability in the oxidation of methyl phenyl sulfide to the corresponding sulfone in seven runs (entry 4). DABCO tribromide immobilized on magnetic Fe3O4 particles (DABCO-74b-Fe3O4) gave constant yields in the oxidation of methyl phenyl sulfide to the corresponding sulfoxide in the first nine cycles under solvent-free conditions [291]. Yields, however, decreased significantly in subsequent uses (entry 2). Catalyst 0.024% CuII-129-Fe3O4 with an immobilized CuII salen complex showed similar features in the oxidation of benzyl methyl sulfide in 12 repeated reactions (entry 3) [292]. The catalyst was easily prepared by refluxing the N-triethoxysilylpropyl-substituted salen complex and magnetic particles in ethanol.
Entry 5 presents results found with the use of a multicomponent magnetic TiO2–graphene photocatalyst to remove herbicides from water [294]. Silica-coated Fe3O4 particles were treated with (BuO)4Ti and then calcined to form a TiO2 layer. The particles thus prepared were grafted with (3-aminopropyl)trimethoxysilane and then, after mixing with graphite oxide, were reduced with hydrazine hydrate to form rGO. During the final step, electrostatic interaction between particles and rGO sheets gives catalyst TiO2-SiO2/ Fe3O4-131-rGO. Catalyst performance was tested by illuminating
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Table 35 Reuse studies with magnetic catalysts applied in oxidations. Catalysta
Conditions
Number of cycles
Yield (%)b
Ref.
1
10.6% TEMPO-128-Fe3O4/ PS (1)
NaBr, NaOCl, CH2Cl2, pH 9.1, 10 °C, 5 min
20
99 (99.5–96)c
[290]
2
DABCO-73b-Fe3O4
30% H2O2, rt, 20 min
15
87 (97–78)d
[291]
3
0.024% CuII-129-Fe3O4 (0.5)
33% H2O2, EtOH, 60 °C, 1.2 h
12
97.5 (99–92)
[292]
4
0.044% CuII-130-Fe3O4/SiO2 (0.5)
Bu4NHSO5, H2O/EtOH (1:2), rt, 1.5 h
7
100
[293]
5
TiO2-SiO2/Fe3O4-131-rGO (20)
500 watt xenon lamp (290–800 nm), H2O, rt, 180 min
9
99 (100–97.1)
[294]
Entry
a b c d
Reaction
Percentage data show loading of active species (wt%) when available; data in parentheses indicate mol% used. Average of yields. Data in parentheses indicate range of yields. Conversion. Runs 1–9 = 97%.
the aqueous solution of the typical herbicide 2,4-dichlorophenoxyacetic acid with a 500W xenon arc lamp. The removal efficiency could be maintained at a very high level in eight runs regaining a value of 99.1% in the ninth reuse after sonication (entry 5). Moreover, after laying aside the catalyst for one year, an efficiency of 95.6% was measured after an ultrasound treatment. Scheme 2. Epoxidation induced by varied magnetic catalysts.
An ionic liquid-type peroxometalate was immobilized on silicacoated magnetic nanoparticles either through hydrogen-bonding interactions (sample 4.7% IL-132a-Fe3O4/SiO2) or by covalent bonding (16.7% IL-132b-Fe3O4/SiO2) [295]. A third sample (36.8% IL-132c-Fe3O4/SiO2) was prepared from a zwitterion and Na7[PW11O39]. All three catalysts exhibited high, constant activities in the epoxidation of cyclooctene (Scheme 2) with selectivities higher than 99%. Low levels of leaching were observed in the mixed solvent methanol–H2O (11–19 ppm) whereas 25–48 ppm of W was lost in pure methanol. In all reactions, epoxide selectivities of >99% were measured.
4.5.3. Recycling of magnetic catalysts in asymmetric syntheses Examples for the recovery and reuse of magnetic catalysts in asymmetric syntheses are listed in Table 36.
Li et al. reported a unique method for catalyst synthesis applying enantioselective bioreduction [296]. This was a multistep process first coating iron oxide nanoparticles with a poly(glycidyl methacrylate) (PGMA) shell modified with epoxy rings followed by further modification with formyl groups. Finally, functionalized particles 133-Fe3O4/PGMA were loaded with alcohol dehydrogenase enzyme RDR from Devosia riboflavina. The particles form reversible clusters, which dissociate inducing bioreduction under reaction conditions (catalyst RDR-133⁄-Fe3O4/PGMA). The catalyst was tested in the enantioselective reduction of 7-methoxy-2tetralone with NADH as cofactor and isopropyl alcohol regenerating the cofactor. High, slowly decreasing yields and high ee values were observed (entry 1) with NADH undergoing 6000–7700 recycling in each run.
A spherical siliceous mesocellular foam was first grafted with magnetic c-Fe2O3 nanoparticles followed by functionalization with TsDPEN to have immobilized hybrid ligand 134⁄-Fe2O3/SiO2 [297]. It was applied in asymmetric transfer hydrogenation by forming the active catalyst in situ in the presence of [RuCl2(p-cymene)]2. Whereas full conversion required increasing reaction time in nine runs, high enantioselectivities could be maintained (entry 2). The decrease in activity is attributed to catalyst loss caused by stirring and Ru leaching (11 mol% in nine runs). Another example for
Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65
47
asymmetric transfer hydrogenation is the use of magnetic chiral catalysts 135⁄ prepared by grafting silica-coated Fe3O4 nanoparticles with the corresponding functionalized chiral diamine derivatives [298]. Both catalysts exhibited high activities and enantioselectivities in aqueous medium (entries 3, 4).
nanocrystals (30 nm) decorated each with a single Pd nanosphere of 6 nm. Increasing activities were found in recycling in the Suzuki coupling (Table 37, entry 1). Catalyst 2.23% PdII-138Fe3O4/SiO2 was prepared by anchoring the functionalized b-ketoiminatophosphanyl palladium complex into silica-coated
Phenylene-coated magnetic nanoparticles were prepared by the co-condensation of chiral 4-(trimethoxysilyl)ethyl)phenylsulfonyl1,2-diphenylethylene-diamine and 1,4-bis(triethyoxysilyl)benzene onto Fe3O4. This was followed by complexation with [{Cp⁄RhCl2}2] to have catalyst 1.16% RhIII-136⁄-Fe3O4/phen [299]. It induced asymmetric transfer hydrogenation of acetophenone to give the (S)alcohol with high activity and high ee values in 10 cycles (entry 5).
Fe3O4 [302]. In recycling experiments carried out with 4chloroanisole the catalytic system could be reused 10 times delivering the coupling product in excellent yields (entry 2). A minimal Pd loss (0.06 ppm) was measured by ICP–AES and neither Pd aggregation nor any activity of the filtered solution could be detected after the last use. These excellent features are attributed to the high durability of the silica-coated support and the robust nature of the palladium complex. Last but not least, this catalyst exhibits high activities in the coupling of unrecative chloroaromatics.
A chiral analog of 4-N,N-dimethylaminopyridine immobilized on magnetic particles (2.84% 137⁄-Fe3O4) proved to be a highly active catalyst material in asymmetric acylation of racemic secondary alcohols allowing kinetic resolution [300]. Major features found in a 20-run recycling are high, varying conversions and high enantioselectivities under mild conditions (entry 6). Then the same catalyst sample was applied in additional 12 runs in the kinetic resolution of five other ester derivatives affording similar conversions (89–62%) and ee values (99–70%). In the final, 32nd run the original alcohol was tested again to give reasonably high conversion (64%) and ee (93%). A comparison of these values to those in entry 6 clearly indicates excellent stability.
4.5.4. C–C bond forming reactions induced by magnetic catalysts The results of recycling studies reported in coupling reactions are collected in the following two tables. Table 37 lists information with respect to coupling reactions with the majority of data acquired in Suzuki coupling, whereas the results of multicomponent processes forming new C–C bonds are treated in Table 38. Heterodimer nanoparticles were fabricated by Kim and coworkers by thermal decomposition of a solution of Pd(acac)2, Fe(acac)3, oleylamine and oleic acid [301]. TEM images of the resulting 1.10% Pd-Fe3O4 sample showed the presence of Fe3O4
Excellent stabilities in long recycling experiments have been demonstrated with diphosphane-functionalized robust palladadendrons grafted onto magnetic c-Fe2O3 particles coated with a carboxyl-functionalized polymer [303]. Four catalysts thus prepared (PdII-139a–139d-Fe3O4/poly) afforded full conversions in Suzuki coupling in the first seven runs of 25-run tests (entry 3). In further reuses scattered values with slightly decreasing tendencies were measured. Leaching resistance of these samples (14 ppm or 0.36% Pd loss per cycle) is higher than that of related catalyst systems.
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Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65
Table 36 Recycling of magnetic catalysts in asymmetric syntheses. Catalysta
Conditions
Number of cycles
Yield (%)b
Ref.
1
RDR-133*-Fe3O4/ PGMA
NADH (0.011 mol%), iPrOH, Tris buffer (pH 8), 30 °C, 20 min
15
80 (90–72) >99c
[296]
2
0.6% RuII-134*Fe2O3/SiO2 (0.5)
Et3N (1.4:1), CH2Cl2, 40 °C, 1.5–7 h
9
99d 94–90c
[297]
3
2.77% IrIII-135a*Fe3O4/SiO2 (0.2)
H2O, TBAB, 40 °C, 8 h
10
98 (99.2–95.9) 89.6–86c
[298]
4
1.25% RhIII-135b*Fe3O4/SiO2 (0.2) 1.16% RhIII-136*Fe3O4/phen (1)
H2O, TBAB, 40 °C, 8 h
10
[298]
H2O, 40 °C, 0.5 h
10
2.84% 137*-Fe3O4 (5)
Et3N, toluene, rt, 16 h, under argon
20
98.7 (99.9–96.9) 87.6–83.8c 99.7 (100–97.8)d 95.1–95.9c 200–196e 66 (72–59)d 99–82f
Entry
Reaction
5
6
a b c d e f
[299]
[300]
Percentage data show loading of active species (wt%) when available; data in parentheses indicate mol% used. Average of yields. Data in parentheses indicate range of yields. Range of enantiomeric excess. Conversion. Range of TON values. Enantiomeric excess of the recovered alcohol.
Table 37 Catalyst reuse in coupling reactions with magnetic separation. Entry
Reaction
1
Catalysta
Conditions
Number of cycles
Yield (%)b
Ref.
1.10% Pd-Fe3O4 (0.1)
Na2CO3, DME/H2O (3:1), reflux, 24 h, under argon
10
92 (90–96)
[301]
2.23% PdII-138-Fe3O4/ SiO2 (0.5) PdII-139-Fe3O4/poly (1.96) c 1.31% Pd0-140-Fe3O4/ SiO2 (0.5) 0.89% Pd0-141-Fe3O4/ SiO2 (0.025) 2.45% PdII-142-Fe3O4/ poly (1) 2.83% Pd0-143-Fe3O4 (0.2) 2.75% Pd-144-Fe3O4 (0.2) PdII-145-C/Co (0.5) PdII-145-C/Co (0.5) 1.36% PdII-146-Fe3O4/ 115 (0.23)
K2CO3, TBAB, H2O, 60 °C, 5 h
10
95.5 (96–95)
[302]
NaOH, THF/TritonX 405 (1:9), H2O, 65 °C, 1 h, under nitrogen KOH, toluene, 100 °C, 6 h
25
93–97d
[303]
10
92
[304]
K2CO3, H2O, 80 °C, 12 h, under argon
10
99.5 (100–99)
[305]
K2CO3, H2O/EtOH (1:3), 70 °C, 12 h
21
98 (99–96)
[306]
K2CO3, H2O/EtOH (1:1), rt, 0.66 h
8
>97 (98–97)
[307]
K2CO3, H2O/EtOH (1:1), rt, 1 h, under air
8
>99 (100–99)
[308]
Na2CO3, H2O/THF (5:2), 60 °C, 14 h Na2CO3, H2O/THF (5:2), 60 °C, 14 h Et3N, DMF, 90 °C, 1.5 h
8 12 66
95 100 (100–98)e 78.5 (84–72)
[309] [309] [310]
2.23% PdII-138-Fe3O4/ SiO2 (0.5)
piperidine, TBAB, H2O, 60 °C, 6 h
10
95 (96–94)
[302]
X= I, R1= R2= H 2
X = Cl, R1 = MeO, R2 = H
3
X = I, R1 = MeO, R2 = H
4
X = Br, R1 = OMe, R2 = H
5
c d e
X = Br, R = OMe, R = H 1
2
X = Br, R = Ac, R = H
7
X = Br, R1 = R2= H
8
X = Br, R1 = R2= H
12
a
2
6
9 10 11
b
1
1
2
X = Br, R = R = H X = Br, R1 = CH2COOH, R2 = H
Percentage data show metal loading (wt%) when available; data in parentheses indicate mol% used. Average of yields. Data in parentheses indicate range of yields. Four catalysts (139a–139d). Average conversions of four catalysts. 100% yields were found in the first 11 runs followed by the coupling of 4-bromoacetophenone giving yield of 98%.
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Angurell, Rossell et al. have grafted phosphane-functionalized dopamine onto Fe3O4 particles followed by loading K2[PdCl4] and, finally, generating Pd particles by reduction with NaBH4 (catalyst 1.31% Pd0-140-Fe3O4/SiO2) [304]. No change in activity was detected in 10 cycles in Suzuki coupling (entry 4). Silica-coated magnetic nanoparticles with anchored imidazolium ionic liquid bearing triethylene glycol moieties have been fabricated by Karimi and co-workers. Treatment with Na2[PdCl4] gave a precatalyst [305]. An induction period and the almost complete loss of activity in the presence of Hg0 testify to the formation of Pd particles being the actual catalytic species. Catalyst 0.89% Pd0-141-Fe3O4/SiO2 exhibits high activity including the coupling of heteroaromatic compounds and shows excellent stability in 10 runs (entry 5). After product extraction with hexane the aqueous phase with the catalyst was reused. High activity is assumed to originate from the hydrophilic nature of the catalyst allowing the formation welldistributed Pd particles and exposing the active sites to substrates.
hydrazine hydrate to deliver magnetic catalyst 2.83% Pd0-143Fe3O4 with Pd particles of 2 nm [307]. A wide range of physical methods was used for catalyst characterization (XRD, XPS, EDS, FT-IR, ICP, SEM, TEM, and VSM). Only minimal changes were measured in an eight-run test of catalyst stability (entry 7). In another contribution a similar method for the synthesis of catalyst 2.75% Pd0-144-Fe3O4 was reported except by using magnetic particles functionalized with 3-(cyanoethyl)triethoxysilane [308]. Catalyst characteristics shown in entry 8 are similar to those of the previous samples.
Majoral and coworkers have prepared dendritic phosphane ligand 145 in a multistep process and used it with Pd(OAc)2 and magnetic Co/C particles [309]. 145 and Pd(OAc)2 non-covalently attached to particles exhibit high activity and excellent stability in Suzuki coupling (catalyst PdII-145-C/Co, entry 9). 428 ppm Pd leached during 10 cycles (entry 10), whereas a much higher value (895 ppm) was measured when the catalyst system was used without Co/C particles. It worked in similar efficiency in the synthesis of Felbinac (biphen-4-ylacetic acid) a non-steroidal anti-inflammatory drug.
A miniemulsion polymerization of divinylbenzene and 4chloromethylstyrene in the presence of a magnetic fluid gave a magnetic support, which was further reacted with 1-(2,6diisopropylphenyl)-1H-imidazole and Pd(OAc)2 to have the supported Pd NHC complex (2.45% PdII-142-Fe3O4/poly) [306]. It showed excellent recyclability in 21 runs (entry 6). A hot filtration test indicated the heterogeneous nature of the reaction and Pd particles were not detected in the used catalyst. The Veisi group performed functionalization with diaminoglyoxime of Fe3O4 particles followed by treatment with PdCl2 and then reduction with Table 38 Catalyst reuse with magnetic separation in multicomponent reactions. Catalysta
Conditions
Number of cycles
Yield (%)b
Ref.
1
1.93% Pd0-147-Fe3O4/PEI/rGO (0.9)
PPh3, H2O, 100 °C, 1 h, in air
30
97 (100–92)
[311]
2
GSH-148-Fe3O4
H2O, 80 °C, 15 min
8
92.5 (93.8–89.8)
[312]
3
9.6% HPA-149-Fe3O4 (4 104)
100 °C, 30 min
8
92
[313]
4
CuFe2O4 (10)
1 atm O2, 100 °C, 24 h
10
88 (92–84)
[314]
5
mpSiO2NHAc/npSiO2SO3H/Fe3O4 (10)
DMF/MeOH (5:1), 40 °C, 1h
10
98
[315]
Entry
a b
Reaction
Percentage data show metal loading (wt%) when available; data in parentheses indicate mol% used. Average of yields. Data in parentheses indicate range of yields.
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A magnetite powder coated with chitosan was the support material for depositing Pd to form 1.36% PdII-146-Fe3O4/115 [310]. It exhibits outstanding stability upon reuse in the Heck coupling of iodobenzene and acrylic acid. An average yield of 78.6% was measured in 60 runs before dropping to about 35% in the 66th use (cumulative TON = 1.13 105) (entry 11). The performance of 2.23% PdII-138-Fe3O4/SiO2 in Sonogashira coupling (entry 12) is similar to that found in the Suzuki reaction (entry 2).
Wang et al. deposited Pd nanoparticles onto reduced graphene oxide nanosheets strongly coupled with Fe3O4 particles [311]. First, graphene oxide was functionalized in reactions with polyethyleneimine and then with 3,4-dihydroxybenzaldehyde. This was followed by generating Pd nanoparticles (4.31 ± 0.40 nm) applying PdCl2 and NaBH4. Finally, the material thus obtained was reacted with Fe3O4 to have 1.93% Pd0-147-Fe3O4/PEI/rGO. The catalyst was fully characterized and proved to be highly active in Tsuji–Trost allylation performed in water under air to have mixtures of mono- and diallylated products. Selective diallylation, however, could be achieved with increasing reaction time. Consistently high product yields in 30 repeated reactions of ethyl acetoacetate and allyl ethyl carbonate were demonstrated (Table 38, entry 1). In contrast, rapid deactivation was experienced with the use of Pd/C and Pd/rGO. According to TEM of the used catalyst, particle agglomeration did not occur but 7.6% of palladium was lost.
Magnetic particles grafted with glutathione (GSH-148Fe3O4) served as a highly efficient nanocatalyst in the synthesis of spirooxindoles to afford excellent yields in short reactions in aqueous medium with similar features in reuses (entry 2) [312]. It is even more important that excellent TON (8.5 105) and TOF values (3.4 106 h1) could be realized. Magnetic Fe3O4 particles modified with pyridine moieties was loaded with Wells–Dawson heteropoly acid H6[P2W18O62] to form catalyst 9.6% HPA-149-Fe3O4 [313]. It gave high, constant yields in the three-component synthesis of 1-amidoalkyl-2-naphthols without leaching of the active species (entry 3).
Copper ferrite (CuFe2O4) an analog of magnetite was used successfully in unique oxidative cross-dehydrogenative coupling reactions [314]. It was probed in a reuse study to perform a Csp3–Csp3 coupling of 2-phenyl-1,2,3,4-tetrahydroisoquinoline and nitromethane (entry 4). The result of a hot filtration test and a low level of Cu leaching (0.39 ppm) strongly indicate a heterogeneous mechanism. The particle size of 34 ± 11.6 nm changed slightly to 31 ± 12.6 nm after 10 cycles. An acid-base three-component bifunctional catalyst with a magnetic Fe3O4 core, a nanoporous silica inner shell (npSiO2SO3H) and a microporous silica outer shell (mpSiO2NHAc) has been designed for the synthesis of 5-aryl1,2,3-triazoles [315]. Uniform egg-like particles of 400 nm with three-layered structure could be detected by TEM with the corresponding layered distribution of the functional groups (EDS). Aromatic aldehydes reacted with nitromethane and NaN3 to afford high yields and stable activities in a 10-run recycling (entry 5). 4.5.5. Reuse in hydrogen generation Significant efforts have been made to develop suitable hydrogen storage and releasing materials with the hydrolytic dehydrogenation of both ammonia–borane (H3N:BH3, AB) as one of the best candidates and methylamine–borane (MeH2N:BH3, MeAB). Results of recycling studies are collected in Table 39. Most catalysts discussed here have cobalt in the support loaded with metal particles generated by reducing the corresponding metal ions with hydrogen formed in the decomposition of ammonia–borane. A number of successful attempts have been published by the Özkar group. A catalyst with copper loading (1.32% Cu0-150CoFe2O4/SiO2) was fabricated by adsorbing CuCl2 on silica-coated magnetic cobalt ferrite particles and then reducing in situ [316]. In the last cycle in a 10-run experiment, 98% remaining activity was measured (entry 1) with an initial TOF of 2400 h1. This is the highest value found with a non-noble metal catalyst. Particle size increased slightly from 0.7 ± 0.3 nm to 1.1 ± 0.6 nm. A similar strategy for catalyst synthesis was used to prepare samples 1.98% Pd0-150-CoFe2O4/SiO2 [317] and 1.96% Ru0-150-CoFe2O4/SiO2 [318]. Catalytic activities (TOF values) of the Pd-loaded sample increased continuously in a 10-run study with a small drop in the last reuse (entry 2). The sample with Ru nanoparticles, in turn, retained 94% of the original activity in the 10th run (entry 3). Metal leaching and sintering of metal particles were not detected in either system. Catalyst 0.98% Ag0-150-CoFe2O4/SiO2 made by in situ reduction of AgNO3 maintained the original activity in seven cycles (entry 4) [319].
A one-step seeding-growth method was used by Chen, Ku and coworkers to prepare magnetically recyclable Cu33Fe67 alloy nanoparticles (3 nm) by reducing a mixture of CuCl2 and FeCl2 with an aqueous solution of ammonia–borane [320]. The activity
Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65
51
of this sample is one of the highest among non-noble metal catalysts (entry 5). Magnetic Ag-CoFe/graphene prepared in situ by reduction with MeAB gave the best result in the hydrolysis of MeAB by retaining 96.6% of its original activity (entry 6) [321]. 4.5.6. Recycling of magnetic catalysts in other transformations In an earlier study, a magnetic support with an immobilized active species analogous to 4-N,N-dimethylaminopyridine (Damp) was probed in a recycling study in a range of organic transformation of synthetic interest [322]. Catalyst Damp-151aFe3O4/SiO2 with a loading of 20 mM g1 proved to be robust affording high yields in the acetylation of 1-phenylethanol (Table 40, entry 1). Similar features were found when the same catalyst sample after recovery was further tested in bishydroxylation, O- and N-acylation and azalactone synthesis in triplicate in each reaction.
a-Cyanation of tertiary amines with trimethylsilyl cyanide can be promoted with high efficiency using catalyst 12.4% AuIII-152-Fe3O4/SiO2 loaded with a goldIII-bipyridyl complex (entry 2) [323]. A magnetic catalyst with an immobilized guanidine moiety as the functional group of high basicity (Gnd-151bFe3O4/SiO2) proved to be efficient in a recycling study [324]. Full conversions were achieved in the first 17 cycles in the cyanosilylation of benzaldehyde followed by small drops in subsequent reuses (entry 3).
Immobilization of the enzyme Mucor javanicus lipase onto a microsphere support composed of a magnetic core coated with oleic acid and a porous polymer shell gave a catalyst material (sample MJ-153-Fe3O4/olac, Fig. 5) with improved activity and selectivity in solvent-free esterification [325]. Both high activity and high regioisomeric excess could be maintained in a 30-run recycling study (entry 4). A magnetic catalyst composed of zinc porphyrin-based organic polymer 154 fabricated by reacting terephthalaldehyde and pyrrole in the presence of Fe3O4 core and silica shell particles was applied in the cycloaddition of CO2 to propylene oxide (entry 5) [326]. High catalytic activity is attributed to covalently highly cross-linked metalloporphyrin sites, whereas slight decreases in the last cycles result from the loss of 21% Zn.
Catalyst mim-155-Fe3O4 with a loading of 0.60 mM g1 was employed in cycloadditions [327]. Stability was tested in the reaction of epichlorohydrin with CO2 at low pressure to form the corresponding cyclic carbonate with slight decreases in activity in 11 cycles (entry 6). Fe3O4 was mixed with the aqueous solution of PtCl2 and then the mixture was basified (pH 13) and stirred for one day to produce catalyst 2.3% PtO/PtO2Fe3O4 [328]. The resulting catalyst efficiently mediated the hydrosilylation of varied alkynes with high activity and stability in recycling (entry 7).
Another catalyst was made by anchoring a ligand suitable for reacting with first-generation Hoveyda–Grubbs Ru complex to aminopropyl-modified Fe3O4 particles [329]. The resulting supported catalyst complex (0.25% RuII-156-Fe3O4) proved to be efficient in RCM of two substrates, albeit with increasing reaction times (entries 8, 9). Activity was also tested by recycling a single batch of catalyst sample in the transformation of nine dienes (Table 41).
A supported catalyst system with second-generation Hoveyda–Grubbs RuII complex (0.23% RuII-157-Fe3O4/SiO2) was synthesized by Lee, Zhong and coworkers. The metathesis catalyst functionalized with an imidazolium salt serving as suitable linker was immobilized on silica-coated magnetic particles [330]. It exhibited high activity and could be recycled in RCM in 13 runs (entry 10). In the next reuse, however, reaction time had to be increased to 24 h to have a conversion of 92%. In sharp contrast, the conversion achieved with the related catalyst without the silica shell dropped to 76% in run seven.
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Table 39 Hydrogen generation by the hydrolysis of boranes.
Entry 1 2 3 4 5 6 a b c d e f g h i
Catalysta
Number of cycles
Yield (%)b
Ref.
10
99 (100–98)
[316]
25 °C, 5 mind
10
172 (112–206)e
[317]
25 °C, 5–6 min
10
96 (100–94)
[318]
25 °C, 5–6 min
7
73f
[319]
Conditions 0
1.32% Cu -150-CoFe2O4/SiO2 (1.4) 1.98% Pd0-150-CoFe2O4/SiO2 (0.19) 1.96% Ru0-150-CoFe2O4/SiO2 (0.74) 0.98% Ag0-150-CoFe2O4/SiO2 (0.28) Cu33Fe67 Ag-CoFe/graphene (0.05)h
25 °C, 50 min
c
25 °C 30 °C, 25–27 min
8 5
22 (10–27) 3i
g
[320] [321]
Percentage data show loading of active species (wt%) when available; data in parentheses indicate mol% used. Average of yields. Data in parentheses indicate range of yields. 15 mL of 100 mM solution. 10 mL of 100 mM solution. TOF values (min1). mL H2 formed. Reaction time (min) to achieve full conversion. Catalyst/MeAB. n(H2)/n(MeAB).
In an earlier study, Zhu et al. reported the fabrication of a magnetic catalyst by immobilization of the same RuII complex on a starch-modified support [331]. 2.8% RuII-158-Fe3O4/starch afforded an excellent performance in the self-metathesis of methyl oleate with a low Ru leaching of 4 ppm (entry 11). The catalyst, however, exhibits lower activity than that of the Grubbs catalyst (TOF of 700 h1).
A Co0Fe0-CoFe2O4 magnetic composite material with CoFe nanoparticles (10 nm) was made by treating a solution of FeSO4 and CoCl2 in the presence of KOH at 140 °C [333]. The product is composed of spinel-type CoFe2O4 and cubic CoFe alloy. It was applied as a Fenton-like catalyst to induce the decomposition of varied dye pollutants (Methyl orange, Congo red, Rhodamine B, Methylene Blue) with Oxone. Satisfactory stability was demonstrated in the decomposition of Orange II (2-naphthol orange) in recycling (entry 13). The SEM image of the used catalyst was very similar to that of the pristine sample, but leaching of both Co and Fe was detected. Superparamagnetic c-Fe2O3 nanoparticles (mean particle size = 10.5 ± 1.4 nm) with a high surface area of 147 m2 g1 prepared by calcination of Fe3O4 proved to be efficient and recyclable in the degradation of PET with ethylene glycol (entry 14) [334]. 4.6. Catalyst based on MOF materials
In another catalyst, Rh nanoparticles stabilized by Oct4NBr surfactant were deposited onto a magnetic support functionalized with hyperbranched polymer 159 with terminal phosphane moieties [332]. 1% Rh0-159-PPh2/Fe3O4/SiO2 was tested in hydroformylation including a recycling study with estragol. Because of double bond migration, three isomeric aldehydes are formed in consistently high yields in six runs even in shorter reactions (entry 12) with linear/branched ratios of 1.8–2.
Metal–organic frameworks (MOFs) are crystalline, porous materials composed of inorganic (metal) ions or clusters and organic linkers [335]. Also known as porous coordination polymers, they are of recent high interest because of their unique properties including extremely high porosity and internal surface area as well as wide variability forming varied structures. They have been used, among others, as catalyst materials or supports. Examples with respect to their use in recycling studies are collected in Table 42.
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Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65 Table 40 Miscellaneous transformations promoted by magnetic catalysts. Catalysta
Conditions
Number of cycles
Yield (%)b
Ref.
1
Damp-151a-Fe3O4/SiO2 (5)
Et3N (1.5 equiv), CH2Cl2, rt, 16–1 hc
14
97 (>98–94)
[322]
2
12.4% AuIII-152-Fe3O4/SiO2 (10)
TBHP, MeOH, 50 °C, 6 h
10
94.5 (96–93)
[323]
3
Gnd-151b-Fe3O4/SiO2
CH2Cl2, rt, 5 min
20
99 (100–92)d
[324]
4
MJ-153-Fe3O4/olac
4 Å MS, 37 °C, 1 h
30
[325]
5
0.025% ZnII-154-Fe3O4/SiO2 (4.6 104)
30 bar, KI, 120 °C, 2 h
16
97.5 (100–95)e 92–86f 94 (97–85)
[326]
6
mim-155-Fe3O4 (1)
10 bar, 140 °C, 4 h
11
98 (100–95)
[327]
7
2.3% PtO/PtO2-Fe3O4 (0.6)
130 °C, 15 min
10
99.6 (100–96)g
[328]
8
0.25% RuII-156-Fe3O4 (2.5)
CH2Cl2, 40 °C, 2–10 hh
22
>97 (>98–95)
[329]
9
0.25% RuII-156-Fe3O4 (2.5)
CH2Cl2, 40 °C, 2–6 hi
18
>97 (>98–96)
[329]
10
3.23% RuII-157-Fe3O4/SiO2 (0.85)
CH2Cl2, rt, 0.5–1 h
14
94 (>99–75)j
[330]
11
2.8% RuII-158-Fe3O4/starch (2008)k
50 °C, 3 h, under argon
5
486 (484–488)l
[331]
12
1% Rh0-159-PPh2/Fe3O4/SiO2 (0.5)
40 atm (CO/H2 = 1:1), toluene, 80 °C
6
100m (70–73)n,o (66–72)n,p
[332]
13
Co0Fe0-CoFe2O4
10
98 (100–96)r
[333]
14
c-Fe2O3 (0.5)
phosphate buffer (pH 7), 20 °C, 5 min H2O, 300 °C, 60 min
10
89.6 (87.4–91.7)
[334]
Entry
a b c d e f g h i j k l m n o p r s
Reaction
s
Percentage data show loading of active species (wt%) when available; data in parentheses indicate mol% used. Average of yields. Data in parentheses indicate range of yields. Runs 1–12 = 16 h, run 13 = 3, run 14 = 1 h. Runs 1–17 = 100%. Residual activities. Regioisomeric excess of the 1,3-isomer. 96% in run 5. Runs 1–13 = 2 h, runs 14–18 = 4 h, runs 19–22 = 10 h. Runs 1–10 = 2 h, runs 11–13 = 4 h, runs 14–18 = 6 h. See text. Substrate/catalyst ratio. TOF values (h1). Conversion. Aldehyde selectivity. Reaction time = 6 h. Reaction time = 3 h. Degradation efficiency. Catalyst/PET ratio.
A catalyst synthesis was started by depositing Au particles onto Fe3O4 nanoparticles by reduction with L-ascorbic acid. These were then functionalized with mercaptoacetic acid followed by repeated treatments with FeCl3 and benzene-1,3,5-tricarboxylic acid in ethanol [336]. Au0-Fe3O4/MIL-100(Fe) core–shell material (MIL = Matérial Institut Lavoisier) with uniformly dispersed Au nanoparticles of 3–5 nm required increasing reaction times in the reduction
of nitrobenzene in 30 runs (entry 1). Kinetic constants dropped from 2.92 min1 (run 1) to 0.51 min1 (run 30). This, that is decreasing rates, however, appears to be a universal feature of this reaction (see also Section 4.5.1, Table 34). A magnetic Fe3O4 and a polyacrylic acid (PAA) core with a PVPy middle layer was covered with a porous ZIF-8 layer. This MOF material was generated by the reaction of Zn(NO3)2 and 2-methylimidazole. In the final step, the
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Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65
Fig. 5. Catalyst MJ-153-Fe3O4/olac used in esterification of oleic acid. Reprinted (adapted) from Ref. [325], Copyright (2017), with permission from Elsevier.
composite product was doped with copper to have 5.45% CuIIFe3O4/PAA/PVPy [337]. High and stable activities were recorded in the oxidation of benzyl alcohol to benzaldehyde with TEMPO and O2 (entry 2). A novel magnetic composite material has recently been fabricated in a multistep process and used in the same reaction. First, Fe3O4 nanoparticles were modified with PAA and then coated with PVP. The resulting core–shell Fe3O4/PVP microspheres were reacted with FeCl3 and 1,3,5-benzenetricarboxylic acid generating a MIL-100(Fe) shell [Fe3O4/PVP/MIL-100(Fe)] [338]. Reactions could be repeated 10 times with similar high efficiencies in the formation of benzaldehyde (entry 3). No further activity was found in a hot filtration test and FTIR and XRD of the recovered sample showed no structural changes. Cokoja, Kühn and coworkers fabricated a range of UiO MOF materials (UiO = Universitetet i Oslo) with the use of ZrCl4 as linker [339]. The best performance in recycling was found with UiO-67 made by reacting a mixture of 2-aminobiphenyl-4,40 -dicarboxylate and biphenyl-4,40 -dicarboxylic acid followed by modification with salicylic acid and, finally, functionalization with [MoO2(acac)2] (catalyst 2.43% MoVI-160-UiO-67). It afforded high, varying activities and selectivities in the epoxidation of cis-cyclooctene (entry 4). A UiO-66 MOF catalyst, bearing aromatic sulfonic acid groups (HSO3-161-UiO-66), obtained by modifying UiO-66-NH2 (the product of 2-aminoterephthalic acid and ZrCl4) with orthosulfobenzoic acid anhydride showed stable activity in acetalization of benzaldehyde (entry 5) [340]. MOF material 16.2% Fe3O(BPDC)3 a product of the reaction between biphenyl-4,40 -dicarboxylic acid (BPDC) and FeCl3 was successfully used in the formation of coumarin (entry 6) [341]. Leaching tests indicated that both catalysts were stable under reaction conditions. All three MOF samples were thoroughly characterized by means of varied instrumental techniques.
Martín-Matute, Zou and coworkers have functionalized a MIL101(Cr) preparation modified with amino groups by nitration and subsequent reduction [342]. Impregnation with Pd[Cl2(MeCN)2] followed by reduction with NaBH4 gave Pd-MIL-101(Cr)NH2 samples with Pd loadings of 4–16 wt%. All products were wellcharacterized (SEM, TEM, FT-IR, XRD and TGA) and used in Suzuki reaction to learn the effect of loadings. 8% Pd0-MIL-101(Cr)NH2 with Pd particles of 2.6 nm proved to be the best catalyst to provide very high activities in the coupling of varied substrate/reagent combinations under mild conditions. It exhibited excellent features in multiple repeating reactions of 4-bromotoluene and pinacol phenylboronate (entry 7). Only limited particle aggregation and a loss of 0.96 wt% Pd were detected after 10 uses. A highly durable catalyst based on MIL-88 B(Cr)NH2 made by reacting CrCl3 with 2-aminoterephthalic acid was fabricated by first depositing Pd particles of 2–3 nm (Na2[PdCl4], NaBH4) followed by encaging the resulting material in a silica coating [343]. The final catalyst has particles of 80 80 220 nm with a ricelike shape and homogeneously distributed Pd particles. It was shown by TGA to have an improved thermal stability when compared to a sample without silica. The catalyst [0.51% Pd0-MIL88 B(Cr)NH2/SiO2] was applied in the oxidation of 1phenylethanol in a continuous flow study. First, a 40 mm long reactor packed with the catalyst (800 mg, 0.0385 mM Pd) was flushed with a mixture of toluene and methanol. This was found to prevent acetalization presumably with methanol reacting with Lewis acidic chromium clusters. A mixing device (1.6 m long microchannel, 1 mL) served to effectively mix the liquid phase (1-phenylethanol and toluene) and synthetic air. A continuous increase in yields was observed resulting from the reduction of unreacted PdII present and a few percents of ethylbenzene byproduct was also detected (entry 8). Pd leaching was below the detection limit (<0.1 ppm), while a constant CrIII level of 0.2 ppm was measured. These results indicate the highly beneficial effect of the protecting silica layer.
Table 41 RCM of varied dienes induced with catalyst 0.25% RuII-156-Fe3O4.
Cycle
X
Y
n
Time (h)
Conversion (%)
1 2 3 4 5 6 7 8 9
TsN (EtO2C)2C O O TsN (EtO2C)2C TsN (EtO2C)2C O
CH2 CH2 C10H21CH C10H21CHCH2 CH2 CH2 CH2 CH2 C10H21CH(CH2)2
1 1 1 1 2 2 3 3 1
2 2 2 12 12 12 12 12 12
>98 >98 98 95 95 95 98 95 96
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Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65 Table 42 Recycling of MOF-based catalysts. Catalysta
Conditions
Number of cycles
Yield (%)b
Ref.
1
Au0-Fe3O4/MIL-100(Fe) (3.4)c
rt
30
5.5 (1–9.5)d
[336]
2
5.45% CuII-Fe3O4/PAA/PVP (1)
1 atm, TEMPO, MeCN, NaHCO3, 60 °C, 12 h
15
97 (98–95)
[337]
3
Fe3O4/PVP/MIL-100(Fe) (5)
10
98
[338]
4
2.43% MoVI-160-UiO-67 (1)
1 atm, TEMPO, KNO2, MeCN, 60 °C, 12 h DMF, 50 °C, 4 h
10
97.5 (100– 92)e
[339]
5
HSO3-161-UiO-66 (0.1)
THF, 23 °C, 2 h
5
98
[340]
6
16.2% Fe3O(BPDC)3 (5)
DMF, 60 °C, 3 h
6
93 (91–95)
[341]
7
8% Pd0-MIL-101(Cr)NH2 (3)
K2CO3, H2O/EtOH (1:1), rt, 0.5 h, under air
10
99 (>99– 97)
[342]
8
0.51% Pd0-MIL-88 B(Cr)NH2/ SiO2 (3)
air, toluene, 110 °C
24f
75.5 (25– 79)g
[343]
9
Al-IQT-HB (2)
2 h, rt
10
97 (97–99)
[344]
Entry
a b c d e f g
Reaction
Percentage data show loading (wt%) of active species when available; data in parentheses indicate mol% used. Average of yields. Data in parentheses indicate range of yields. 3.4 mg catalyst for 0.18 mM substrate. Reaction time (min) for full conversion. Run 3 = 95%, run 8 = 92%. 24 samplings in 168 h. The highest yield of 79% was measured at 144 h.
Al-ITQ-HB is a metal–organic hybrid material synthesized by the solvothermal reaction of aluminum chloride and p-heptylbenzoic acid in dimethylformamide [344]. It represents a novel family developed by the Corma group (ITQ = Instituto de Tecnología Química) based on ordered aluminum cluster-type sheets with Lewis acidic character and hydrophobic pockets [345]. Significant rate accelerations were demonstrated in the synthesis of benzimidazoles and cyanosilylation of ketones. In addition, high catalyst stability was found in 10 cycles in the latter reaction at room temperature under solvent-less conditions (entry 9). 5. Additional examples A few other results performed with catalysts hard to categorize are collected in this subsection. A range of bimetallic PdAg alloy nanoparticles were prepared using a simple one-pot method by co-reducing Pd(NO3)2 and AgNO3 in acidic solution with citric acid at room temperature [346]. The prepared samples of highly crystalline nanoporous structure are composed of nanocrystals of 2–4 nm interconnected three-dimensionally to form crystals of 60–100 nm. XPS firmly confirmed the alloy structure. When used in the catalytic decomposition of formic acid to generate hydrogen, Pd50Ag50 exhibited the highest activity with high stability in recycling (Table 43, entry 1). It was separated by ultracentrifugation, redispersed in water and reused in the next cycle. Czaun, Olah, Prakash and coworkers have developed a quasi carbon neutral energy storage system based on formic acid as a hydrogen/energy carrier [347]. The
catalyst system derived from IrCl3 and 1,3-bis(20 -pyridyl-imino)-i soindoline was applied in generating hydrogen from formic acid with very low concentrations of CO impurity. The solid catalyst material, an iridium carbonyl derivative, formed under reaction conditions (Ir-IndH) can be recovered and reused. Upon repeated addition of formic acid to this system, scattered production levels of the H2/CO2 mixture were observed. In contrast, high activities with small decreases could be maintained in 20 days in a continuous operation (entry 2). Moreover, gas flows of 101.5 mL min1 and 76 mL min1 could be measured after storing the catalyst for one and four years, respectively. Compare these results to those in Table 7, entry 10. Since formic acid was fully consumed and the selectivity is close to 100%, the hydrogen yield can also be considered to be 100%. Miras, Song and coworkers performed intercalation of a series of polyoxometalate salts into layered double hydroxide (LDH) modified with tris(hydroxymethyl)aminomethane (Tris) and used the prepared composites for the degradation of dyes [348]. The best catalyst with the [PW12O40]3 anion intercalated (PW12-Tris/ LDH) showed excellent durability in 10 cycles in the decomposition of methylene blue (entry 3). XRD and FT-IR spectra show no sign of deterioration of the used sample. }llo }si and coworkers have immobilized b-isocupreidine (162) Szo on an anion-exchanger LDH and used this inorganic–organic hybrid material (162-LDH) in the asymmetric Michael addition of ethyl 2-fluoroacetoacetate and trans-b-nitrostyrene [349]. According to XRD, 162-LDH was deposited on the surface of LD with non-covalent interactions without intercalation. The catalytic
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Á. Molnár, A. Papp / Coordination Chemistry Reviews 349 (2017) 1–65
Table 43 Additional catalysts reused in varied reactions. Entry
Reaction
1
HCOOH
H2 + CO2
2 3
a
c d e f g h i j k l m n
Conditions
Number of cycles
Yield (%)b
Ref.
Pd50Ag50 (10.7)c
60 °C, 10 h, under argon
5
229d
[346]
109 (120– 101)f 99
[347]
Ir-IndH (0.0029) Degradation of methylene blue
PW12-Tris/LDH (50)
g
e
100 °C
5
30% H2O2, H2O, 30 °C, 140 min heptane, 20 °C, 16 h
10 9
99 (99.5– 98.3) 89–76h 67–40i 2.45–1.6j 99.9% (100– 99.8)
[348]
4
162-LDH (2.5)
5
Janus nanosheets based on [bmim]3[PW12O40]
50 °C, 3 h
6
6
[Cp2Zr(H2O)2][OSO2C4F9]2 (2)
rt, 30 min
5
92.5 (92–93)
[351]
7
2.7% Ru-163-PQS (2)
H2O, rt, 2 h
10
97 (>99–92)
[352]
8
Rh0/TPGS-1000 (0.5)
5 atm H2, Na2CO3, H2O, 50 °C, 1.5 h
15
97 (100–85)k,l
[353]
9
PdII-164 (5)
K3PO4, DMF, 100 °C, 48 h
10
92.3 (96–91)k
[354]
10
hollow-channel Au nanoparticles (0.15) Pd(OAc)2/S-Au (0.15)
0.1 M KOH, 0.5 M EtOH, H2O, rt K2CO3, solvent X = Im X = Brn
500
91 (100–86.7)
[355]
10 10
98.5 (>99–97) 99 (>99–99)
[357]
11
b
Catalysta
[349]
[350]
Percentage data show Pd loading (wt%) when available; data in parentheses indicate mol% of catalyst used. Average of yields. Data in parentheses indicate range of yields. mg catalyst used for 10 mL 0.5 M formic acid. mL H2 gas formed in each cycle. Five samplings in day 1, 2, 17, 19, 20. Gas flow (mL min1). mg catalyst used for 50 mL methylene blue solution (10 mg L1). Range of enantiomeric excess of the major (R,R) isomer. Range of enantiomeric excess of the major (S,R) isomer. Range of diastereomeric ratios. Conversion. TPGS added after run 15 increased the conversion to 92%. Microwave heating; step 1: EtOH, 80 °C, 60 min, 200 W; step 2: EtOH, 80 °C, 60 min, 500 W; see also text. Microwave heating; step 1: DMF, 90 °C, 50 min, 300 W; step 2: toluene/H2O (2:1), 104 °C, 60 min, 500 W; see also text.
performance was similar to that of the corresponding homogeneous organocatalyst. Significant deactivation was observed in reactions carried out in dichloroethane. In less polar heptane, in contrast, high, consistent yields and decreasing enantioselectivities were found in an eight-run reuse study (entry 4).
Janus nanosheets were made first by preparing an aqueous solution of [bmim]Cl and a hydrolyzed styrene–maleic anhydride copolymer at 70 °C and pH 2.5 [350]. This was then mixed with the solution of TEOS, triethoxy-(3-(2-imidazolin-1-yl)propyl)silane and (EtO)3SiPh in paraffin at 70 °C. The two solutions were mixed to form and oil-and-water emulsion with a paraffin/silica coreshell Janus structure. Finally, Janus hollow spheres obtained by dissolution of the paraffin core were crushed with an ultrasonic cell
crusher to form Janus nanosheets with imidazolyl on one side and phenyl on the other side acting as both catalyst and emulsifier. Nanosheets after exchanging the anion of the IL for a heteropolyanion were tested successfully in deep desulfurization with a sample based on [bmim]3[PW12O40] affording the best features (entry 5). A mixture of dibenzothiophene dissolved in octane and H2O2 forms an emulsion in the presence of the nanosheets at 50 °C. After complete reactions, the emulsion was broken by centrifugation, the upper oily phase was analyzed, while the lower IL phase was separated from Janus nanosheets with filtration, washed, dried and then reused. Zirconocene complex [Cp2Zr(H2O)2][OSO2C4F9]2 is an air-stable, moisture insensitive strong Lewis acid and an efficient catalyst for the synthesis of N-heterocycles under solvent-free conditions [351]. Five-run recycling studies under mild conditions revealed high stability. The best example is shown in entry 6. Lipshutz and Ghorai designed the amphiphile platform PQS (PEG ubiquinol succinate) with a lipophylic side-chain and a hydrophilic (Me-PEG) component [352]. Then Grubbs first generation Ru complex was attached and the resulting catalyst (2.7% Ru-163-PQS) was used in RCM including a recycling test (entry 7).
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halide was first irradiated (step 1) to release the catalytically active Pd species for the oxidative addition. Then the mixture was transferred to the reaction chamber to complete the catalytic cycle (step 2). Slightly different reaction conditions were necessary to maintain consistently high yields for iodobenzene and bromobenzene in 10-run recycling studies (entry 11). A few selected results with the use of this catalyst in Buchwald–Hartwig amination of aryl halides testifying to high catalyst stability are collected in Scheme 3 [358]. In a follow-up study, an improved and safer method to prepare sulfur-modified Au mesh was disclosed applying aqueous Ru0 nanoparticles stabilized by D-a-tocopheryl polyethylene glycol 1000 succinate (TPGS-1000) micelles with a lipophilic core and hydrophilic shell form an active catalyst system in the hydrogenation of a-pinene to cis-pinane [353] with a good performance in a 15-run recycling (entry 8). The use of basic conditions was crucial to prevent aggregation of Ru0 particles. Drops in product yields in the last cycles were attributed to traces of ethyl acetate used for product extraction. It diffuses into the micelles thereby decreasing the access of substrate molecules. Only a minimal loss of 0.2% Ru was measured after the recycling study. The insoluble pyridyl–triazole complex PdII-164 synthesized by Astruc et al. gave slowly decreasing then stable activities in the Heck reaction in 10 runs (entry 9) [354]. Evans and coworkers reported the fabrication of hollow gold nanoflowers [355]. These were made by depositing Au nanoparticles onto a methyl orange–FeCl3 nanofiber as template in a single step. Primary Au particles are formed by reduction of HAuCl4 with ascorbic acid. In a large excess of the reducing agent, secondary nucleation takes place resulting in the formation of urchin-like particles with spike lengths of 20–30 nm. At the same time, the template undergoes auto-degradation induced by gold particles leaving a cavity. These hollow-channel catalyst particles were tested in the electrocatalytic oxidation of ethanol in a cyclic voltametric study exhibiting high activities and stabilities by retaining 86.7% of the original activity after 500 cycles (entry 10).
Arisawa and co-workers have developed and applied a highly stable, low-leaching Pd catalyst in varied coupling reactions [356]. A gold mesh was modified with sulfur by treatment with piranha solution (conc. H2SO4/30% H2O2 = 3:1) followed by loading Pd(OAc)2 [Pd(OAc)2/S-Au]. It was used in Suzuki couplings under microwave heating [357]. The solution of the catalyst and organic
Scheme 4. A two-step oxidation/asymmetric reduction sequence.
Table 44 Catalysts with satisfactory stability in 10–16 reuses. Catalyst
Ref.
Catalyst
Ref.
Homogeneous catalysts 0.19% PdII-3-HBPE 4.26% PdII-5-PIB 11.4% IrIII-9/poly AuIII-14 [Mo(O)(O2)2(H2O)n] Pd(OAc)2 + IL [{Rh(l-OSiMe3)(cod)}2] + IL CuII-26*-CTC
[25] [28] [31] [39] [46] [48] [55] [60]
Cu(OAc)2-27a* saccharin + DMAP CuI + 43 Pd0 + 52 Rh0 + 55a Rh0 + 55b 16.7% Pt0-56
[61] [79] [81] [95] [98] [98] [99]
[118] [129] [132] [145] [148] [157] [160] [161] [161] [166] [167] [168] [169]
[Rh(cod)][BF4] + 102* Ni90Pd10-PVPy 2.3% CoII-109-poly 1% Au0-PTFE graph-C3N4-PET 5.7% Au0-111-poly 4.8% Au0-yeast 2.9% Pd0-HPS/cotton PdII-115-[bmim][BF4] Pd0-115/116 1.35% Pd0-2000-oxC Pt74Pd26-118-Cnt PdII-120a-rGO
[219] [225] [228] [229] [230] [232] [242] [241] [243] [244] [249] [256] [261]
[169]
RuII-120b-rGO
[261]
Heterogeneous catalysts PdII-60-SiO2 4% Pd0-SBA-15 1.06% PdII-65-PMO 4.7% Ru0-MCM-41 + IL 2.1% PdCl2(phen)/NaY 72-SiO2 2.23% Rh0-76-SiO2 1.28% Rh0-77a-SiO2 2.78% Rh0-77b-SiO2 (0.1) 81a+82-SiO2 Im-83 a-SiO2 2% Pd0-Im-83b-SiO2 5.43% Pd0-HS-SiO2/[bmim] [BF4] 5.23% Pd0-HS-SiO2/[bmim] [PF6] 0.06% [RhCl(PPh3)3]-SiO2/ [bmim][PF6] 0.29% PdII-90-PS 10.7% Pd0-pmim-POSS
[170]
AgAu-rGO
[263]
[199] [175]
[267] [302]
1% Pd0-Oct/triN-SiO2 0.54% Pd0-Oct/SiO2 1% Pt0-89-TiO2
[177] [178] [178]
(PdCl2/-bpy)10/quartz
[182]
P2W15-Al2O3
[187]
HPW-NH2-Al2O3 Nb2O5
[189] [194]
15.05% PdII-92-POP
[203]
2% Ni0-Fe3O4/SiO2 2.23% PdII-138-Fe3O4/ SiO2 34% Pd0-122-IL/C/Co 1.95% Pt0-Fe3O4/SiO2 14.3 wt% Au0-Fe3O4/115/ rect 1.31% Pd0-140-Fe3O4/ SiO2 0.89% Pd0-141-Fe3O4/ SiO2 PdII-145-C/Co mpSiO2NHAc/ npSiO2SO3H/Fe3O4 1.96% Ru0-150-CoFe2O4/ SiO2 Fe3O4/PVP/MIL-100(Fe) Al-IQT-HB Pd(OAc)2/S-Au CuCl2/PdCl2 + [(R)-ADH]
0
Scheme 3. Recycling studies with Pd(OAc)2/S-Au catalyst.
6.9% Pd -93-HBPE CA/Novozyme 5.43% Pd0-poly/epox binol-100-poly
[204] [215] [217] [218]
[271] [274] [285] [304] [305] [309] [315] [317] [338] [344] [357,358] [360]
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Table 45 Catalysts with satisfactory stability in 18 and more reuses. Catalyst
Ref.
Catalyst
Ref.
Homogeneous catalysts 4-poly Rh0 (PEG4000)
[27] [89]
AgI + Cs2CO3 Au0-58a-HPEIa
[80] [102]
[133] [133] [151] [154] [163] [172] [176] [180]
RhIII-96*-poly 0.71% Au0-1140.58-PANfc Au0-paper 3% Pd0-Ni0/CeO2–x (1 105) 0.43% Au0-Fe3O4/poly 9.6% Pd0-126-Fe3O4/GO 10.6% TEMPO-128-Fe3O4/PS 2.45% PdII-142-Fe3O4/poly
[210] [236] [239] [272] [284] [286] [290] [306]
Heterogeneous catalysts (HCA)Au0-HS-SBA-15 (HCA)Au0-H2N-SBA-15 AO/peroxidase-MCF CR/glymo-SiO2(HIPE)b IrIII-80a*-SiO2 0.02% RhIII-86-SiO2 + IL 0.46% Pd0-Oct/triN-SiO2d 0.021% Pd0-PVPy/[bmim] [PF6]/LCe 0.02% Pd0-HDA/[bmim][PF6]/ LCf Pd-glassA [bmim][FeCl4]/TiO2 TiO2 0.014% Ag0-(PVA) 1.14% Pd0-(PVA)
[180]
PdII-139a–139d-Fe3O4/poly
[303]
[181] [188] [198] [200] [201]
[310] [324] [325] [329] [343]
26.7% Pt0-HIPE Ni0-starch
[222] [237]
1.36% PdII-146-Fe3O4/115 151b-Fe3O4/SiO2 MJ-153-Fe3O4 0.25% RuII-156-Fe3O4 0.51% Pd0-MIL-88B(Cr)NH2/ SiO2g Ir-IndH hollow-channel Au nanoparticles
Pd0-SiO2/PDA/glass
[245]
[347] [355]
a
Reaction time had to be increased significantly in runs 18 and 19. A yield of 57% in run 21. Small changes in activity in the first 33 runs and larger decreases to a final activity of about 50% in run 45. d With increases in reaction time. e Run 17 = 89%, run 18 = 55%. f Run 16 = 58%, run 17 = 21%. g Continuous flow experiment. b
c
Na2S2O8 in H2SO4 [359]. This second generation catalyst is of low leaching and recyclable giving identical activity range in the amination of bromobenzene with morpholine (93–91%). Gröger and coworkers have developed a rather unique method to combine Wacker oxidation induced by CuCl/PdCl2 and subsequent enzymatic reduction to transform styrene to 1phenylethanol in ‘‘one pot” [360]. Called compartmentalization, Wacker oxidation was performed in a thimble made of polydimethylsiloxane placed in a flask. Organic products formed in oxidation can diffuse out through the thimble membrane into the
flask containing the enzyme. Asymmetric biocatalytic reduction in the outer phase is induced by alcohol dehydrogenase [(R)-ADH] from Lactobacillus kefir (L. kefir). In this arrangement, site-isolation prevents poisoning of the enzyme by metal ions and isolation of the intermediate ketone is not necessary. The system proved to be robust affording high conversions and excellent enantioselectivities (Scheme 4).
6. Summary analysis The literature abounds in publications claiming the development of recyclable catalysts. In close scrutiny, however, it turns out that only a few repeated runs are tried, which is insufficient to arrive at any definite conclusion. Moreover, decreasing conversions and yields are reported in most cases. That is, only a fraction of these studies have real practical significance. Therefore, it may be useful and instructive to sum up a few important conclusions of the varied examples collected in this review. Over 300 catalyst preparations with the corresponding information of their performance in recycling studies have been selected and discussed. Of these catalysts about 150 have been tested at least in 10–16 repeated runs. Many of these, however, exhibit insufficient stability that is they show decreasing activities in repeated cycles. Therefore, only catalyst samples exhibiting stable or increasing activities or show only a maximum overall decrease of 1–1.5% in conversions or yields have been selected and collected in Table 44. Catalysts that underwent more exhausting tests (18 or more reuses) are found in Table 45. Here, the rule is a maximum of 0.5% decrease per cycle. Naturally, such a degree of cumulative deactivation (9% loss of conversions or yields in 18 cycles) is a sign of unsatisfactory stability in the long run. Consequently, it is obvious that only a handful of catalysts can fulfill even such a modest selection condition. A few heterogeneous catalysts have also been included, which exhibit full conversions and high yields in long runs but activity drops in the last cycles. Finally, because of the significance of high turnover numbers in assessment of catalyst reuse related examples are collected in Table 46. In comparison, the table also lists catalysts, which are not treated in the manuscript because their recycling performance has not met the conditions set for selection (references indicated by an asterisk).
Table 46 Catalysts providing high cumulative TON values. Catalyst III
Rh -38 Rh(acac)(CO)2 + 1a Ir(acac)(cod) + 1a IrIII-39 fluorous-47 16.7% Pt0-56 Au0-58b-HPEI/dendr 4.7% Ru0-MCM-41 + IL 1.81% RhIII-70a-SBA-15 0.02% RhIII-86-SiO2 + IL 1% Ru-TiO2 0.006% Rh0-ZnO/nw 1.14% Pd0-(PVA) 1.55% Rh0-Fe3O4/SiO2 0.09% Rh0-Fe3O4/SiO2 1.95% Pt0-Fe3O4/SiO2 1.36% PdII-146-Fe3O4/115 Perfluoro-tagged Pd particles on fluorous SiO2 0.076% Pd0-alginate/gellan [IrCp*Cl2] 2,20 -bipyridine complex on porous triazine framework *
Continuous flow experiment.
Reaction
No of cycles
Hydroformylation Hydroformylation Hydroformylation H2 from formic acid Singlet oxidation of citronellol Hydrogenation of methyl crotonate Reduction of 4-nitrophenol Hydrogenation of CO2 Hydroformylation Asymmetric hydrogenation Hydrogenation of CO2 to formic acid Hydroformylation Suzuki coupling Hydrogenation of cyclohexene Hydrogenation of cyclohexene Hydrogenation of pentan-2-one Heck coupling Suzuki coupling Suzuki coupling Hydrogenation of CO2
a
5 3 11 25 14 19 10 17 a
10 5 30 18 7 14 66 4 4 5
Cumulative TON 4
1.29 10 2.5 105 1.1 105 3.85 104 2.7 104 2.2 104 2.3 104 1.77 105 6.9 104 1.43 105 2.3 104 1.87 104 4.96 104 1.8 105 3.0 105 1.56 104 1.13 105 3.33 105 1.9 105 2.8 104
Ref. [71] [72] [72] [73] [86] [99] [103] [145] [146] [172] [190] [191] [201] [268] [269] [274] [310] [361]* [362]* [363]*
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In summary, 128 catalysts are listed in the above three tables and only about half of these show real promising properties. The great majority of these systems are supported catalysts. This is not surprising since one of the main advantages of heterogeneous catalysts is easy separation. In fact, great efforts have been devoted to develop such catalyst systems with improved stability and recyclability in recent years. This is the final tally of our survey for valuable recyclable catalysts from the last few years. 7. Conclusions and outlook 7.1. Successful examples of recycling On the basis of the above data it is easy to corroborate a conclusion of recyclable catalysts. As it has recently been formulated by these authors [364], unsatisfactory stability in the long run is the rule rather than the exception. And then an obvious question arises: is it necessary to study catalyst recycling at all and make any efforts to develop recyclable catalysts? Considering practical applications, a high cumulative TON is more crucial than good recycling ability of a catalyst that is yields as a function of cycles. As stated by Gladysz ‘‘the pursuit of higher-turnover-number catalysts represents a more important research direction than recoverable catalysts” [7]. However, there is an endless list of reactions where highly productive catalysts, that is, catalysts affording high TON values do not exist because the necessary efforts simply have not been made. Sections 3 and 4 cover a wide range of reactions with a number of catalysts with sufficiently high stability and low TON numbers (see Section 6 with the related summary analysis). A few examples may be illustrative here. Very high productivity and activity data in the Suzuki coupling with the use of [PdCl(g3-C3H5)]2 together with a-cyclodextrin ligand are unprecedented: a TON of 3.4 1011 and a TOF of 1 109 h1 were achieved using 1 1010 mol% of catalyst [365]. However, a mere 0.84 mM of product was formed in 14 days! In comparison, PdCl2 reused 140 times in a mixture of ILs bis(diphenylphosphano)cobal tocenium hexafluorophosphate and [bmim][PF6] gave 31.5 mM of product in 280 h (cumulative TON = 1.3 104) [366]. Magnetic catalyst 1.36% PdII-146-Fe3O4/115 gave an even better performance [310]: it was reused in 66 cycles in the Heck reaction with a cumulative TON of 1.13 105 allowing the isolation of 259 mM of product (0.23 mol% Pd, 99 h). 7.2. The advantage of recycling There is an ongoing debate in the catalysis community about of the usefulness and significance of catalyst recycling. There is no doubt that the development of robust catalysts requires dedication and hard work. It is especially true for the immobilization of metal complexes. Since the activity of a modified complex is often lower, when compared to that of the original homogeneous catalyst, efforts appear to be useless. Prime examples are oxide-supported catalysts fabricated with the use of surface organometallic chemistry pioneered by Copéret and Basset [367,368] as well as polymer-supported catalysts developed by Buchmeiser [369]. Only a handful of rather unsuccessful attempts are referred to in these reviews. In 2007, Copéret and Basset declared [368]: ‘‘From all available literature data, it is clear that the supported system have never out-performed the homogeneous systems (TON ranges from 100–2000 in RCM, and after 5–7 recycling using 1–5 mol%, most systems show a sharp drop in activity).” However, Buchmeiser using a monolithic catalyst preparation already reported in 2001 [370]: ‘‘This results in turnover frequencies (TOFs) up to 25 min1, thus even exceeding homogeneous analogues. In comparison, the TOF for diethyl diethylmalonate using [Cl2Ru(Mes2-NHC)(PCy3) (CHPh)] is 4 min1.” Another conclusion by Brookhart, Scott and
59
coworkers in 2009 [371]: ‘‘The simplest of these approaches, adsorption on c-Al2O3, is also found to be the most effective, yielding catalysts that are. . .comparable to or even more active than the corresponding species in solution.” And then, in 2011, an interesting remark by Basset [372]: ‘‘. . . the catalyst [a single-site tantalum hydride supported on fibrous silica (KCC-1)] proved to be quite regenerable and using it in a second run yielded similar or even better results than the first run.” It is certain, that immobilization of metal complexes to fabricate robust catalysts can be successfully accomplished applying appropriately selected support materials. As demonstrated in this review, the use of varied carbon/graphene preparations, dendrimers, mesoporous solids and polymers appears to be a more promising approach. A remark for illustration by Marks and Zhao [373]: ‘‘In many cases, these catalysts [polymer-supported organolanthanides] exhibit activities comparable to the homogeneous precursors and are recoverable/recyclable with only minor to moderate loss of activity, depending on the particular resin amino substituents.” According to data collected and analyzed in this review, the search for stable, recyclable catalysts should not be unproductive. Catalyst recycling, in general, is of enormous importance from the point of view of industrial applications and the environment. Environmental concerns and economic consideration and, in particular, the limited availability of expensive noble metals, necessitate the development of catalyst with good recycling ability that is it is an important issue worth pursuing. Catalysts including both homogeneous complexes and supported/immobilized samples with extremely high productivity and activity do exist. There are a few examples highlighted in this review when catalysts exhibited excellent features up to 100 reuses without significant catalyst deterioration. These recent accomplishments can serve as a good basis for additional studies and justify the usefulness of efforts to achieve further development. Within the context of this review, catalyst use in the fine chemical industry appears to be highly relevant. However, the use of heterogenized catalysts in industrial practice is still rare. One of the reasons is economic: the high value of organic products prepared usually in multistep processes compares unfavorably with the relatively small cost of investments. Consequently, there was no stimulus for development of immobilized catalysts. Furthermore, the fabrication of the required complexes tailored for specific processes and their subsequent heterogenization are laborious and time-consuming. With the advent of green chemistry, however, requiring among others waste minimization and the avoidance of hazardous reagents, the scenario has changed. Consider also the dwindling supply and high price of precious metals used in many synthetic operations. The high price of organometallic complexes should make their reuse even more desirable. Heterogeneous catalysis appears to be the obvious solution. Consequently, there is a high demand for further efforts to develop durable catalysts and suitable means of their recovery, reactivation and regeneration. Naturally, catalyst deactivation in industrial processes requires special attention and regeneration is a key issue with respect to economy and sustainability [374]. 7.3. The problem of catalyst deactivation There are a number of reasons resulting in decreasing or loss of catalytic activity and/or selectivity in repeated uses. In the case of immobilized complexes, the dissolution of active species may take place and these may be recaptured as another, less active species. Detachment of active species from the support surface and their decomposition may also take place. Fouling that is coverage of the surface with deposits such as carbonaceous residues (coke formation) and thermal degradation are two rather common reasons for deactivation, in particular, at high temperature.
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When working with heterogeneous catalysts, there are multiple reasons for deactivation [375]. In the case of metal nanoparticles, the increasing particle size because of Ostwald ripening can be a serious problem. The driving force of the process is the higher stability of larger particles having a lower surface to volume ratio in comparison to smaller particles, which have higher surface energy and higher total Gibbs energy. As a consequence, the number of energetically more stable larger particles increases at the expense of smaller particles resulting in re-dispersion. It is well-known that larger particles with a lower number of available surface atoms exhibit lower activities. A similar problem is sintering resulting, again, in the increase in particle sizes and the concomitant decrease in active surface area. Additional reasons are poisoning caused by strong chemisorption of impurities and mechanical failure (attrition, erosion). The latter leads to fracture of catalyst particles to form ever smaller particles, which may be lost. Such incomplete catalyst recovery is an important problem, in particular, if simple filtration is used. Importantly, however, there are a number of remedies from organic solvent nanofiltration, ultraand size-exclusion filtration, centrifugation through phase separation and precipitation to the use of multiphase reaction media and magnetically recyclable catalyst preparations. 7.4. What to do? This is a ‘‘what to do list” for further advancement of recyclable catalysis. (i) Surface and structural modifications and appropriate functionalization have resulted in catalysts with improved stability and durability upon reuses. Following this line with valuable data in hand further efforts with the use of the great variety of experimental possibilities available for such manipulations will allow fine-tuning of properties and enhancement of both catalyst lifetime and stability for prolonged use. In addition, the widening of the reaction scope is also expected. (ii) It should also be an obvious necessity to characterize and evaluate catalysts before and after reaction and monitor continuously the working catalyst systems to collect information of recyclability, catalyst recovery and deactivation. Mechanistic studies can contribute important basic information. In particular, exploring the nature of the true catalytic species is of utmost importance. (iii) Simple methods often neglected such as poisoning and filtration tests as well as detecting the leaching of active species may supply useful data to accomplish further improvements. In short, the full use of techniques available for catalyst characterization to elucidate more fully the working catalysts is absolutely necessary to further the development of the field. (iv) Continuous-flow processes offer a number of advantages such as high efficiency, easy scale up and their possible application for both homogeneous and heterogeneous catalytic systems. Flow processes are faster, more efficient and scalable. All in all, this technique has real promise as an alternative to efficient catalyst reuse. Note, that sequential reactions in a flow can also be operated. References [1] (a) W.A. Cornils, I.T. Herrmann, Horváth, W. Leitner, S. Mecking, H. OlivierBourbigou, D. Vogt (Eds.), Multiphase Homogeneous Catalysis, Wiley-VCH, Weinheim, 2005; (b) M. Lombardo, A. Quintavalla, M. Chiarucci, C. Trombini, Synlett (2010) 1746–1765. [2] A.E.C. Collis, I.T. Horváth, Catal. Sci. Technol. 1 (2011) 912–919.
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