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Synthesis of methyl palladium complexes on silica as single site catalysts activating CCl bonds in heck reactions
Journal of Catalysis 375 (2019) 257–266
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Synthesis of methyl palladium complexes on silica as single site catalysts activating CACl bonds in heck reactions Christoph Gnad a,b, Oliver Dachwald a,b, Gabriele Raudaschl-Sieber a, Klaus Köhler a,b,⇑ a b
Technische Universität München, Department of Chemistry, Lichtenbergstraße 4, Garching b. München 85747, Germany Technische Universität München, Catalysis Research Center, Ernst-Otto-Fischer-Straße 1, Garching b. München 85747, Germany
a r t i c l e
i n f o
Article history: Received 3 May 2019 Revised 2 June 2019 Accepted 3 June 2019
Keywords: Grafting Single-site catalysts Palladium Heck reaction Aryl chlorides
a b s t r a c t Supported catalysts with molecularly dispersed, uniform palladium species were synthesized by the method of grafting. New specific synthesis protocols were developed in order to exclude self-reduction and agglomeration of palladium during immobilization. The successful synthesis of isolated mononuclear palladium(II) surface complexes of uniform structure was confirmed by MAS NMR, infrared spectroscopy and elemental analysis. The catalytic potential of these single-site catalysts for the Heck reaction was directly compared to well-characterized supported palladium oxide materials known and reported for their extraordinarily high activity in the conversion of aryl bromides. In contrast to those, the immobilized palladium surface complexes were able to quantitatively convert demanding non-activated aryl chlorides as substrates with conversions up to 62% and high selectivity. Although believed to be only pre-catalysts, the ligand sphere of the surface complexes directly affects their activity. The timeresolved reaction of chlorobenzene with styrene revealed that catalysts with supported amine complexes are able to constantly convert chlorobenzene during several hours of reaction time. It is proposed that the catalysts continuously provide active, dissolved Pd(0) species during the whole course of reaction. Ó 2019 Elsevier Inc. All rights reserved.
1. Introduction The palladium catalyzed carbon-carbon bond formation is a widely used tool in organic chemistry for the synthesis of various compounds through the linkage of two molecules. Especially the Heck, Suzuki and Negishi cross-coupling reactions reached high importance and thus Richard F. Heck, Akira Suzuki as well as Eiichi Negishi were honored with the Nobel Prize for the discovery of these reactions in 2010 [1–4]. CAC coupling reactions are also applied in the chemical industry, e.g. for the production of pharmaceuticals, agrochemical compounds as well as fine chemicals [5,6]. The Mizoroki-Heck reaction describes the coupling reaction of an alkene with an aryl halide, vinyl halide or aryl triflate under use of a base and a palladium catalyst to form a substituted alkene (Scheme 1). Besides providing high stereo- and regioselectivity, the Heck reaction is able to couple a wide variety of different substrates [1,7,8]. In the chemical industry, it is applied for the synthesis of several commercial products such as Naproxen or SingulairÒ [9].
⇑ Corresponding author at: Technische Universität München, Department of Chemistry, Lichtenbergstraße 4, Garching b. München 85747, Germany. E-mail address: [email protected] (K. Köhler). https://doi.org/10.1016/j.jcat.2019.06.004 0021-9517/Ó 2019 Elsevier Inc. All rights reserved.
Primary, the Heck coupling was discovered as a homogeneously catalyzed reaction using palladium salts such as palladium acetate and palladium chloride or simply palladium metal [1,10]. Due to increasing interest in this reaction, research headed towards the development of solid palladium catalysts to enable facile separation and recycling. Consequently, several reviews summarized the progress made and the huge literature contributions concerning this topic [11–14]. Supported Pd(0) species [15–18] (e.g. nanoparticles or clusters), Pd(II) compounds encapsulated in zeolites or other mesoporous materials [19,20] as well as Pd(II) compounds supported on oxides [21,22] were successfully applied in a few cases also for the activation of demanding substrates such as non-activated aryl chlorides with moderate to high yields, yet during very long reaction times. There are however numerous literature contributions including reviews reporting that – despite of the heterogeneous character of the catalysts – the active species are molecular Pd species dissolved (‘‘leached”) from the surface during the reaction. This can even be regarded as mostly accepted view at least for Heck reactions and it raises the question of the role of supported molecular Pd species directly bound to a support. The approach of this study is to synthesize catalysts with structurally defined, uniform isolated palladium species immobilized on silicon dioxide. The generation of isolated (transition) metal complexes on oxide supports can be realized with synthesis strategies
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Scheme 1. The Mizoroki-Heck reaction of an aryl halide or aryl triflate with an alkene. X = Cl, Br, I, OTf, OTs.
from the field of Surface Organometallic Chemistry (SOMC). By the application of high vacuum techniques in combination with high temperatures, the distribution and number of hydroxyl groups on oxide surfaces can be controlled. This dehydroxylation can result in a homogeneous distribution of isolated surface hydroxyl groups. These functional groups can be converted with transition metal complexes, which possess at least one ligand being able to react with a hydroxyl group. For example, alkyl, amide, alkoxy or halogenide metal complexes are suitable precursors for the reaction with the hydroxyl groups of the oxide surface. Applying said technique known as Grafting, solid catalysts with molecular defined and isolated metal centers can be synthesized [23–25]. Richmond et al. successfully grafted Pd(II) complexes with the structural motif PdMeCl or dimeric palladium compounds on partially dehydroxylated silicon dioxide and applied them as catalysts for hydroamination reactions [26]. Immobilization of palladium complexes can result in potent catalysts for the Heck coupling reactions [27,28]. In these works, grafting of different Pd(II) complexes on silicon dioxide or on a MOF support provides novel solid catalysts with high to maximal metal dispersions and palladium centers with defined coordination spheres. To achieve exclusively uniform molecular metal centers and to avoid self-reduction of palladium in particular with very sensitive Pd methyl complexes as applied in the present work, the grafting procedure had to be further improved regarding the pre-treatment of the support, the reaction temperature as well as precursor concentrations. In contrast to other approaches in which grafted noble metal catalysts function as precursors in order to generate small particles through defined pre-treatments [29], the focus of this contribution lays upon the isolated structurally defined supported metal complexes themselves. The variation of the ligands allows the investigation of the influence of different stabilities of the surface complexes on their reactivity towards the Heck coupling reaction. The synthesized catalysts were applied for the coupling of aryl bromides as well as chlorides. To get a fair relation to supported palladium and palladium oxide particles, all catalysts were directly compared to well-characterized supported palladium oxide materials known and reported for its extraordinarily high activity in the conversion of aryl bromides.
siloxane groups without significant loss of specific surface area. This process can be monitored by IR spectroscopy (see Fig. 5, experimental). The surface of the resulting material consists of approximately 0.8 isolated silanol groups per nm2 [23–25,30–32]. Silicon dioxide, which was dehydroxylated at 700 °C is denoted as SiO700 2 . To obtain heterogeneous single-site palladium catalysts, the dimethylpalladium(II) complexes dimethyl(N,N,N’,N’-tetramethyle thanediamine)palladium(II) and dimethyl[1,2-bis(diphenyl-phos phino)ethane]palladium(II) were chosen as metal-organic precursors. The synthesis of both precursor compounds was described by the group of de Graaf [33]. The prepared metal precursors were grafted on SiO700 as shown in Scheme 2. 2 A noble metal complex comprising donating ligands like methyl groups is likely to be reduced to the oxidation state of zero. The grafting reaction is always in direct competition with the reductive elimination of ethane from the precursor complex [34]. The danger of this undesired reaction to happen is ubiquitous and can already be observed in the pure solid complex. Quantitative decomposition of PdMe2(L) with L = tmeda and dppe occur at T = 398 K and 439 K respectively [33,35], however partial decomposition can be deduced from formation of palladium black during storage in case of the tmeda-complex even at 253 K [36]. Reductive elimination during the grafting reaction can be suppressed by working at low temperatures with very diluted solutions of the precursor. Also, filtration of precursor solutions removes any metal black which could have formed on storage or dissolution of the complexes. Such low temperatures on the other hand, decrease reaction rates, so that long reaction times are necessary to achieve maximum conversion which in turn enables more undesired side reactions. Grafting alkyl palladium(II) complexes is thus always a balance between undesired and wanted reactivity in order to obtain isolated surface species. The corresponding optimization led to the following reaction parameters (given more detailed in the experimental part): 20 h reaction time at 30 to 40 °C, argon atmosphere (Schlenk techniques). The amount of applied precursor complexes was chosen in order to achieve metal loadings between 0.1 and 1.0 wt% palladium. The prepared white to slightly yellow powders were stored under inert conditions at 30 °C.
2. Results and discussion
2.2. Characterization of the silica-bound species
2.1. Preparation of the silica supported palladium catalysts
2.2.1. Elemental analysis To verify the metal loadings of the prepared catalysts, the palladium content was examined by digestion and subsequent photometric quantification. The same procedure was conducted to quantify the phosphorus content. Carbon, hydrogen and nitrogen contents were determined by combustion analysis (and, if not mentioned otherwise, with the exclusion of air contact to prevent contamination of the samples with carbon-containing substances or water). The results are summarized in Table 1. Since the precursors tend to decompose by a reductive elimination, it is apparent that the lower palladium and consequently carbon, hydrogen and nitrogen values are caused by the loss of precursor complex. To avoid the contamination of the catalysts with palladium metal, the precursor complexes were filtered before the grafting step (and the excessive ligand is removed by careful washing of the catalysts). Consequently, the determined
In order to gain catalysts with isolated surface complexes the method of grafting was applied. It relies on the conversion of surface functional groups with defined organometallic complexes under the formation of a covalent bond (at least in the first immobilization step) [23–25]. To achieve metal site isolation, it is necessary to pre-treat the support. In the case of oxides such as SiO2, Al2O3 or TiO2, the number and properties of certain surface hydroxyl groups can be controlled by dehydroxylation [30]. Silicon dioxide is a frequently applied support for the grafting step due to its well-defined surface properties. The surface hydroxyl groups can be classified into neighboring (vicinal and geminal hydroxyl groups) and isolated hydroxyl groups. By the thermal treatment of AEROSILÒ 200 at 700 °C in high vacuum, the neighboring hydroxyl groups are forced to react with each other and condensate to
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Scheme 2. Synthesis of isolated palladium surface complexes by the conversion of the dimethylpalladium(II) complexes PdMe2(tmeda) and PdMe2(dppe) with the isolated silanol groups of SiO700 2 .
Table 1 Elemental analysis of the palladium catalysts with isolated surface complexes. The results are given in weight percent and as molar ratios. Palladium contents were determined by digestion and subsequent photometric quantification, C, H and N contents by combustion analysis and under inert conditions. Weight% calculated/found Molar ratio calculated/found Sample PdMe(tmeda)/SiO2700 [1.0 wt%Pd] PdMe(tmeda)/SiO700 [0.5 wt%Pd] 2 PdMe(dppe)/SiO700 [1.0 wt%Pd] 2 PdMe(dppe)/SiO700 [0.5 wt%Pd] 2 a
Pd
C
H
N
P
1.00/0.76 1.0/1.0 0.50/0.51 1.0/1.0 1.00/0.97 1.0/1.0 0.50/0.47 1.0/1.0
0.79/0.63 7.0/7.3 0.40/0.54 7.0/9.4 3.05/3.28 27.0/30.0 1.52/1.42 27.0/26.8a
0.18/0.15 19.0/20.8 0.09/0.16 19.0/33.1 0.26/0.30 27.0/32.7 0.13/0.16 27.0/35.9a
0.26/0.23 2.0/2.1 0.13/0.17 2.0/2.4 – – – –
– – – – n/a n/a 0.29/0.29 2.0/2.1a
values were not determined under inert conditions.
molar ratios of carbon, nitrogen and phosphorus fit to the calculated values, which indicate the success of the synthesis. The remaining silanol protons of the SiO700 surface can cause the 2 slightly higher hydrogen values since in the case of a palladium loading of 1.0 wt% approximately 40% of the available isolated silanol groups are occupied (which affects the second decimal place). For the sample that was not measured under inert conditions, the higher content can also be attributed to adsorbed water. From the results of the elemental analysis, it can be concluded that palladium was successfully immobilized on dehydroxylated silica and that the stoichiometric Pd:C:H:N:P-ratios fit to the theoretical values for the optimized synthesis procedure developed.
2.2.2. IR spectroscopy To further investigate the surface complexes, the synthesized catalysts were pressed into self-supporting pellets (in glovebox), which were transferred into a gas tight in situ IR cell. The procedure allows the handling and measuring of the materials under the strict exclusion of air contact. Fig. 1 shows the infrared spectra 700 of SiO700 and PdMe(dppe)/SiO700 2 , PdMe(tmeda)/SiO2 2 . Due to the bonding of the methyl complexes of palladium to the support surface, bands appear in the region between 2700 cm 1 and 3200 cm 1. They can be attributed to the vibrations of the backbone ligands tmeda and dppe as well as the methyl ligands directly
700 Fig. 1. IR spectra of a) SiO700 [1.0 wt%Pd] and c) PdMe 2 , b) PdMe(tmeda)/SiO2 (dppe)/SiO700 [1.0 wt%Pd]. Samples were pressed into self-supporting pellets and 2 transferred to a gas tight in situ IR cell. All spectra were taken under inert conditions.
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bound to palladium. The assignment of these bands is listed in Table 2. The sharp band at 3747 cm 1 can be attributed to the stretch vibration of the isolated silanol groups. As already mentioned, around 40% of the available silanols are occupied by the palladium complexes (for 1.0 wt%Pd). Therefore, the silanol band is still clearly visible in the spectra of the two catalysts. The bands in the region of 3100 cm 1 –2750 cm 1 can be assigned to symmetric and asymmetric CAH stretch vibrations of the ligand. By complementary temperature programmed desorption experiments the presence of any organic substances besides the palladium complexes could be excluded. Therefore, it can be stated that the bands derive solely from the surface complexes. 2.2.3. Solid-state NMR spectroscopy Since organic ligands coordinate the palladium metal centers of the single-site catalysts, it is possible to characterize them by solidstate NMR spectroscopy. Fig. 2 shows the 1H-MAS NMR spectrum of PdMe(dppe)/SiO700 2 , Table 3 compares the chemical shifts of the signals from the liquid state 1H NMR spectrum of the precursor complex with the signal derived from the solid-state spectrum of the surface complex. The distinct signals of the 1H solid-state NMR spectrum exhibit similar chemical shifts as the precursor complex and can be assigned to the protons of the phenyl, ethyl and methyl groups of the surface complexes. This clearly demonstrates the molecular
Table 3 Chemical shifts of the signals from the liquid-phase 1H NMR spectrum of PdMe2(dppe) in C6D6 compared to the signals of the solid-state 1H MAS NMR spectrum of PdMe(dppe)/SiO700 assigned to the corresponding functional groups. 2 Chemical shifts [ppm] Functional group
PdMe2(dppe) in C6D6 liquid phase
PdMe(dppe)/SiO700 2 solid sate
Phenyl P-CH2-CH2-P Pd-CH3 Si-OH
7.62, 7.01 1.88 1.29 –
7.14 1.55 1.14 1.95
character and the uniformity of the metal centers of the palladium catalysts achieved by grafting. A suitable method to demonstrate the bonding of the palladium precursors to the SiO700 surface is 31P-HPDEC NMR spectroscopy. In 2 Fig. 3, the liquid phase 31P NMR spectrum of the precursor PdMe2(dppe) in benzene-d6 (left) as well as the solid-state 31 P-HPDEC NMR spectrum of PdMe(dppe)/SiO700 2 (right) are depicted. The 31P NMR spectrum of PdMe2(dppe) shows one singlet at 39.03 ppm, which is caused by both phosphorus atoms of the dppe backbone ligand. In contrast, the solid-state 31P-HPDEC NMR spectrum reveals two signals, both singlets, at 58.48 ppm and 28.13 ppm. Due to the bonding of the palladium complex to the support, the two phosphorus atoms of the dppe ligand are not
Table 2 List of bands appearing in the IR spectra of PdMe(tmeda)/SiO700 [1.0 wt%Pd] and PdMe(dppe)/SiO700 [1.0 wt%Pd] and their assignments to the corresponding vibrations. 2 2 PdMe(dppe)/SiO700 2
PdMe(tmeda)/SiO700 2
m [cm ]
assignment
ref.
m [cm 1]
assignment
ref.
3747 3100–3040 3000–2900 2870–2840
Si-OH HC = CH H2C-CH Pd-CH
[30,31] [37,38] [37,38] [41]
3747 3040–2900 2892 2845 2795
Si-OH N-CH Pd-CH Pd-CH N-CH
[30,31] [39,40] [41] [41] [39]
1
Fig. 2. 1H MAS NMR spectrum of PdMe(dppe)/SiO700 2 . Sample preparation, transfer and measurement were performed under the exclusion of air contact.
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Fig. 3.
31
P NMR spectra of PdMe2(dppe) (left, liquid phase) in benzene d6 and PdMe(dppe)/SiO700 [1.0 wt%Pd] (right, solid state). 2
equivalent anymore, resulting in two signals with similar integral values in the 31P NMR spectrum. This clearly demonstrates the successful reaction between the surface silanol groups and the palladium complex as well as the isolation of the metal centers. In summary, the application of the characterization methods verified the success of the grafting of dimethylpalladium(II) complexes on SiO700 2 . Furthermore, the results point out the generation of isolated, structurally uniform and well-defined palladium centers on the surface of silicon dioxide. 3. Application in Heck-type CC coupling reactions The prepared catalysts were applied in the Heck reaction of chlorobenzene as well as bromobenzene with styrene. A series of preceding parameter optimizations and careful interpretation of literature and earlier experiments lead to the reaction system, which is displayed in Table 4. Palladium concentrations were kept low in order to disfavor agglomeration and deactivation as well as to reach representative TONs and TOFs. On the other hand, the amount of applied palladium catalysts had to be at a certain level to differentiate from catalysis by ‘‘homeopathic” palladium concentrations. In general, it is possible to catalyze the Heck reaction by ultralow amounts of palladium (ppm and even ppb levels) due to the consequential prevention of agglomeration [7,42,43]. This can also lead to conversions of aryl iodides and bromides due to palladium impurities in reactants or additives. Therefore, blind tests without the addition of the catalysts were performed as well which ensured that the conversions and yields can solely be attributed to the respective catalysts. The temperatures were set between 130 °C and 160 °C in order to effectively couple the substrates in short reaction times. Oxidative addition, reduction of palladium and metal
leaching are favored at higher temperatures (for maximum palladium leaching seems to exist an intermediate temperature range). The respective bases were chosen according to optimal performances in previously conducted parameter tests. Highest conversions and yields were achieved with sodium acetate in the case of bromobenzene, whereas the addition of calcium hydroxide delivered best results for the coupling of chlorobenzene. [19,47] Addition of tetrabutylammonium bromide further reduces reaction times by acceleration of the regeneration as well as stabilization of the active Pd(0) species in solution [44–46]. Conversions of aryl halides and yields of trans-stilbene were determined by GLC. Complementary GC-MS analysis validated the assignment of the signals and identified small amounts of cis-stilbene as well as 1,1-diphenylethylene as biproducts. For details, see experimental section. 3.1. Palladium oxide particles vs palladium surface complexes To investigate the catalytic potential of the palladium surface complexes in the Heck reaction, several experiments were performed with grafted catalysts as well as catalysts with silicasupported palladium oxide particles as reference system. The PdO/SiO2 catalysts are well-known for their application in C-C coupling reactions and are among the most efficient systems ever reported for the Heck reactions of aryl bromides [19,47]. The results are listed in Table 5. Regarding the coupling of bromobenzene with styrene, both catalytic systems – PdO/SiO2 and Pd-SiO2 surface complexes reached full conversion and yields of stilbene up to 82% after 0.5 h at 160 °C (entries 3 and 4). But when the temperature is set 20 K lower to 140 °C, conversion and yield decreased to 52% and 47% after 1 h for the grafted catalyst, whereas the PdO catalyst still
Table 4 Selection of reaction parameters after optimization for the Heck coupling reaction of bromobenzene or chlorobenzene with styrene. All experiments were performed in air
X
Base
Additive
Pd [mol%]
T [°C]
t [h]
Br Cl
1.2 eq NaOAc 0.6 eq Ca(OH)2
0.6 eq TBAB 0.6 eq TBAB
0.05–0.15 0.05
130–160 150
0.5–2 6
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Table 5 Application of different pre-catalysts in the Heck reaction of chloro- or bromobenzene with styrene. For reaction parameters see Table 4. Conversion and yield were determined by gas-liquid chromatography. Entry
Catalyst
Aryl Halide
Base
Reaction Temperature [°C]
Reaction Time [h]
Conversion [%]
Yield [%]
1 2 3 4 5 6 7
PdOxH2O/SiO2 PdMe(tmeda)/SiO700 2 PdOxH2O/SiO2 PdMe(tmeda)/SiO700 2 PdOxH2O/SiO2 PdMe(tmeda)/SiO700 2 PdMe(dppe)/SiO700 2
BrAPh BrAPh BrAPh BrAPh ClAPh ClAPh ClAPh
NaOAc NaOAc NaOAc NaOAc Ca(OH)2 Ca(OH)2 Ca(OH)2
140 140 160 160 150 150 150
1 1 0.5 0.5 6 6 6
99 52 99 99 14 67 55
83 47 81 82 <1 62 45
obtained full conversion and 83% yield (entries 1 and 2). Therefore, it can be stated that the higher temperature is a key factor for the activity of the catalysts with isolated surface complexes. The catalysts were also applied for the coupling of the demanding aryl halide chlorobenzene. Due to the increasing stability of the Ar-X bond in the order ArAI < ArABr < ArACl (dissociation energies [48]: Ar-I 281 kJmol 1, ArABr 346 kJmol 1, ArACl 409 kJmol 1) the activation of aryl chlorides represents the highest challenge [7,49]. It is often neglected in the literature that aryl chlorides are therefore superior test substrates compared to iodides and bromides. Catalyzing the reaction by palladium impurities in reagents can be restricted. Effects on the catalytic performance induced by parameter variations can be observed and interpreted more clearly [43]. Furthermore, the comparatively slow oxidative addition of the aryl chlorides to Pd(0) complexes requires longer reaction times and higher temperatures. Thus, the comparison of bromo and chlorobenzene reactivity can be very useful for mechanistic conclusions. Entries 5–7 of Table 5 reveal that with the applied reaction conditions the conventionally supported palladium oxide catalyst reached chlorobenzene conversions of only 14% and negligible low yields whereas the grafted catalysts showed high chlorobenzene conversions of 55% and 67% as well as stilbene yields up to 62% after 6 h at 150 °C. These results point out the huge impact of the choice of the pre-catalyst (particles vs surface complexes) on the outcome of the experiments for demanding substrates. In the case of CAC coupling reactions, the discussion of how the reaction is catalyzed applying solid palladium catalysts is still ongoing. The two principle possibilities are that either the reaction is catalyzed by a ‘‘truly heterogeneous” surface mechanism or that dissolved palladium, which is generated by metal leaching, catalyzes the reaction following the homogeneous reaction mechanism. In order to target this issue, the palladium content in solution was monitored in several publications. Independent of the catalytic system, dissolved palladium could be detected during the progress of the reactions whenever this had been investigated. The results support that heterogeneous palladium catalysts act as reservoir for highly active, coordinately unsaturated Pd(0) species, which are generated by the dissolution of palladium off the support [19,43,47,50–52]. Therefore, the different performances of the chosen catalysts can be traced back to the process of palladium leaching. The diverse bonding situations and coordination spheres of the palladium of the pre-catalysts influence the generation of the active species. The results from Table 5 indicate that in the case of catalysts with surface complexes sufficient amounts of dissolved palladium species are formed (if at all) apparently only at elevated temperatures. This precondition could lead to a controlled release of palladium species, which is high enough to form adequate amounts of active centers but low enough to prevent agglomeration of Pd(0) and the formation of palladium black. This concept of controlled metal leaching can be the reason for the high activity towards the coupling of deactivated aryl halides [19,22,51]. Previously conducted parameter optimizations underlined the importance of the addition of TBAB. In the case of bromobenzene,
higher amounts of tetrabutylammonium bromide increased conversions and yields, whereas in the case of chlorobenzene, no reaction was observed in the absence of TBAB. Furthermore, NMP appeared to be the most suitable solvent for the Heck reaction. This indicates that the generation and stabilization of active Pd species is promoted by coordinating ligands such as bromides or NMP molecules. Most likely, palladium is dissolved by the formation of L2Pd(II)XMe or L2Pd(II)X2 with L2 = tmeda, dppe and X = Br, NMP, which result from the substitution of the silanol ‘‘surface” group by a coordinating ligand. First tests on dissolved palladium during the reactions were conducted. In the case of chlorobenzene, very low but seriously detectable amounts of leached palladium could be detected (2, 1, 5% of the total Pd at 2, 4, 6 h reaction time and 6, 9, 14% chlorobenzene conversion). During the coupling of bromobenzene, the palladium content in solution was found to be increasing significantly from 43% at 5 min reaction time and 4% conversion to 74% at 30 min and 63% conversion. After complete conversion of bromobenzene (97%) at 60 min reaction time, the palladium concentration dropped to 2% of the total Pd. The amounts of leached palladium indicate the dissolution of palladium by the generation of the priory described Pd(II) complexes as well as the catalysis occurring in the liquid phase. However, more detailed palladium leaching experiments as function of reactions time and conditions are necessary (and in progress) for serious conclusions on the reaction mechanism. 3.2. Effect of the coordination sphere of supported palladium complexes From entries 6 and 7 of Table 5 it is apparent that the ligand sphere of the surface complexes influences the catalytic performance. Amines lead to a higher catalytic activity than phosphanes, which can be correlated with their abilities to stabilize palladium in particular oxidation states. Palladium amine complexes tend to dissociate faster leading to a higher amount of active, unsaturated Pd(0) species in solution. In contrast to the tmeda ligand, phosphanes can also stabilize palladium in the oxidation state 0 [33]. Therefore, in the case of the surface complexes with dppe, the generation of dissolved active centers is suppressed. Furthermore, the backbone ligands must leave the coordination sphere of the surface complexes to enable the coordination of the aryl halides and olefins. Especially in the case of the bulky dppe ligand, a surface mechanism seems very unlikely due to the high steric demand, supporting the conclusion that dissolved, unsaturated Pd(0) species are indeed the active centers for the Heck reaction. 3.3. Kinetics of the Heck coupling of chlorobenzene with styrene To further investigate the coupling of aryl chlorides using supported palladium complexes as catalyst, the time-dependent progress of the reaction of chlorobenzene with styrene was plotted and is shown in Fig. 4. It should be mentioned that due to the experimental set-up for steady sampling and reaction temperatures above the boiling point of chlorobenzene (septum instead
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Fig. 4. Time-dependent conversion and evolution of the yield of stilbene from the reaction of chlorobenzene with styrene using PdMe(tmeda)/SiO700 [0.50 wt%Pd] as catalyst. 2 Reaction set-up was optimized for steady sampling. Reaction conditions: 0.05 mol% Pd, 1.0 eq chlorobenzene, 1.3 eq styrene, 0.6 eq Ca(OH)2, 0.6 eq TBAB, NMP, 150 °C, air.
Table 6 Recycling of PdMe(tmeda)/SiO700 [0.1 wt%Pd] after the reaction of chlorobenzene with styrene. Second runs were performed with chlorobenzene as well as bromobenzene. 2 Reaction conditions entry 4: 150 °C, 6 h, 0.05 mol% [Pd]. For further reaction conditions see Table 4. Entry
Run
Catalyst
Aryl Halide
Conversion [%]
Yield (Stilbene) [%]
1 2 3 4
1 1 2 2
PdMe(tmeda)/SiO700 2 PdMe(tmeda)/SiO700 2 Pd/SiO2-recyc Pd/SiO2-recyc
ClAPh ClAPh ClAPh BrAPh
67 70 0 26
62 60 0 20
of screw cap, see experimental section), evaporation of the aryl halide could not be prevented completely, which misleadingly results in higher conversions. Furthermore, the consequent decline in concentration of chlorobenzene as well as the continuous removal of active Pd(0) species through sampling results in lower yields and slower reaction rates compared to the standard catalytic runs. What must also be considered is the selectivity to benzene (also evaporated and lost), i.e. dehalogenation as side reaction. This side reaction is expected to become more relevant with the presence of palladium(0) particles, i.e. with progressive deactivation of the catalyst. Since the stilbene formation starts only after 1 h of reaction time, an initial period for the transformation of the pre-catalyst can be assumed. Nevertheless, the steady increase of the yield after the induction phase shows that the catalyst is able to constantly provide (dissolved) active Pd(0) species over a longer period of time. The decrease of the formation of stilbene can most probably be assigned to the deactivation of the catalyst due to its transformation under reaction conditions. In CAC coupling reactions, metal leaching can be seen as the generation of active centers. On the other hand, because of the dissolution and re-deposition of metal species, the catalyst changes its initial state highly probably with decreasing Pd dispersion. Consequently, the capability of the catalyst to be recycled was further investigated.
Table 6 shows that it is not possible to reuse the catalyst for the coupling of chlorobenzene with styrene (entry 3) under the reaction conditions applied. However, it is still possible to convert bromobenzene to some extent (entry 4), which can be assigned to the more facile activation and oxidative addition of bromobenzene. The reinsertion of the palladium surface complexes shows that the catalysts lose their high activity after the first run. It can be concluded that the immobilized surface complexes were transformed during the reaction of chlorobenzene with styrene. Decreased Pd dispersion and Pd reduction can be assumed. Unfortunately, neither electron microscopy nor other methods could detect Pd particles, possibly due to the small Pd concentration and very small particle sizes and work-up procedures. Despite thermal decompositions tests, which revealed that the surface complexes are stable up to 170 °C in an inert gas flow, the original Pd complex structures were however definitively destroyed (NMR, IR) under Heck reaction conditions. As discussed previously, coordinating substances such as NMP or bromide ions can promote the dissolution of palladium. These processes as well as redeposition of dissolved, reduced Pd species can explain these the transformation of the catalyst. The detailed mechanism of such dissolution dynamics, especially during the prolonged reaction times of the coupling of aryl chlorides, is subject of ongoing studies.
3.4. Activity of the spent catalysts
4. Conclusion
The catalyst as well as all other solids were separated from the reaction mixture after the first run. This as well as the careful work-up procedure should prevent the loss of any palladium [53] (for details, see experimental section). The collected solids (marked as Pd/SiO2-recyc) were subsequently re-inserted in a second run.
Grafting of dimethylpalladium(II) complexes on dehydroxylated silicon dioxide resulted in solid, but molecular palladium catalysts with structurally uniform surface species. Characterization of the ligand sphere by IR and NMR spectroscopy as well as elemental analysis validated the generation of isolated mononuclear
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palladium surface complexes. In contrast to the vast majority of solid palladium catalysts for the Heck reaction, the synthesized materials can effectively couple even non-activated aryl chlorides with high selectivity in few hours. Since the reaction of aryl chlorides is far slower compared to bromides or iodides, the catalysts are able to provide highly active palladium species over a longer period of time. A higher stability of the surface complexes resulted in a decrease in activity.
5. Experimental 5.1. Catalyst synthesis 5.1.1. Synthesis of isolated surface complexes Support pretreatment: AerosilÒ 200 (Evonik) was agglomerated, crushed and pestled to grain sizes between 100 and 300 mm. The resulting powder was calcined at 500 °C in a muffle oven with air ventilation. For dehydroxylation, the prepared support was placed into a quartz Schlenk-tube connected to a high vacuum Schlenk-line equipped with an oil diffusion pump (Ilmvac 100328). A pressure of 10 5 mbar was applied and the SiO2 was heated with 2 Kmin 1 to 700 °C, which was held constant for 16 h. After cooling down, the as prepared SiO700 was handled under strict inert conditions for 2 the following steps to prevent any rehydroxylation [25]. Precursor synthesis: The palladium(II) complexes PdMe2(tmeda) and PdMe2(dppe) were synthesized according to the procedure of de Graaf [33]. PdMe2(tmeda): Elemental analysis (calc.; found) [m%]: C (38.02; 37.69), H (8.78; 8.74), (11.09; 10.85). 1H NMR (400 MHz, C6D6, 294 K) d (ppm) = 0.51 (s, 6H, Pd(CH3)2), 1.63 (s, 4H, NACH2ACH2AN), 2.02 (s, 12H, 2 N(CH3)2). PdMe2(dppe): Elemental analysis (calc.; found) [m%]: C (62.87; 63.61), H (5.65; 5.63), P (11.58; 11.12), Pd (19.89; 18.19). 1H NMR (400 MHz, C6D6, 300 K) d (ppm) = 1.27–1.32 (m, 6H, Pd(CH3)2), 1.79–1.95 (m, 4H, PACH2ACH2AP), 6.99–7.04 (m, 12H, PPh2), 7.58–7.66 (m, 8H, PPh2). Grafting of PdMe2L (L = tmeda, dppe) on SiO700 2 : All steps of the following synthesis were conducted under the exclusion of air using standard Schlenk techniques. The desired amount of the
precursor was dissolved in the pre-cooled solvent in a Schlenktube. The solution was filtrated onto the desired amount of pretreated support in another pre-cooled Schlenk-tube. The mixture was allowed to react without stirring for 20 h at temperatures between 30 °C and 40 °C. After that time, the solvent was removed by filtration and the solid was washed twice with the original fresh solvent each. After evaporation of solvent residues and drying the material in vacuo, a white to pale yellow solid depending on the palladium loading was obtained. The materials were stored under inert conditions below 30 °C. 5.1.2. Synthesis of silicon dioxide supported palladium oxide particles Support pretreatment: AEROSILÒ 200 was agglomerated, pestled, sieved and calcined according to the previous description. General catalyst synthesis procedure: The silicon dioxide supported palladium oxide particles were synthesized by a deposition-precipitation method based on Pearlman [54]. For 1.0 g of catalyst with a target metal loading of 1.0 wt%Pd, 0.99 g of the SiO2 were suspended in 30 mL water. A solution of PdCl2 in 5% HClaq was added dropwise under stirring. Subsequently, the pH of the suspension was slowly increased to 10 by the addition of 10% NaOHaq under the application of an autotitrator (Metrohm 736 GP Titrino). The resulting solid was filtered, washed several times with water and dried in vacuo. The brown powders were stored at room temperature and in air. 5.2. Catalyst characterization 5.2.1. Elemental analysis Carbon, hydrogen and nitrogen contents were determined by combustion analysis using an elemental analyzer (Eurovector). Inert sample preparation was carried out in a glovebox charging a tin foil mini weighing-boat with 10–15 mg of the sample, which was wrapped into another tin foil weighing boat. Palladium and phosphorus contents were analyzed by photometry. The samples were boiled with concentrated sulfuric acid under low vacuum to remove organic compounds. In order to digest the corresponding elements, the residues were treated with concentrated sulfuric and nitric acid. For the quantification of the palladium content, part of the solution was mixed with a freshly prepared solution of nitroso-R salt. In the case of phosphorus, ammonium vanadate solution was used. The contents were determined photometrically by a UV–Vis spectrometer (Shimadzu UV-160). 5.2.2. IR spectroscopy The samples were pressed to self-supporting pellets in the glovebox and subsequently transferred into a gas-tight IR cell. The IR spectra were collected on a FT-IR spectrometer (BioRad FTS 575C) equipped with a liquid-nitrogen-cooled mercury cadmium telluride (MCT) detector.
Fig. 5. IR spectra of self-supporting pellets of untreated SiO2 as well as dehydroxylated silicon dioxide, SiO500 and SiO700 2 2 . The spectra were taken under strict exclusion of air contact. The sharp band at 3747 cm 1 can be assigned to the isolated silanol groups, the broad band between 3710 cm 1 and 3120 cm 1 to neighboring silanol groups (vicinal and germinal).
5.2.3. NMR spectroscopy Liquid state NMR spectra are recorded by a Bruker AMX400. Samples were dissolved in 0.5 mL of the specified solvent inside standard NMR tubes. All chemical shifts are given in d-values (ppm) with the residual proton signals of the solvent serving as a reference. The solid-state 1H MAS as well as 31P-HPDEC spectra were recorded on a Bruker AV 300 NMR spectrometer. The sample material was placed into a ZrO2 rotor with 4 mm diameter and sealed in a glovebox. The spectra were taken at 303 K with a rotational frequency of 12 kHz and measuring frequencies of 300 MHz for the 1H and 122 MHz for the 31P-spectra. The spectra were referenced to an external standard, which was adamantane for 1H measurements (reference signals: 1H: 2.00 ppm). The adamantane standard itself
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was referenced against tetramethylsilane. The standard for 31 P-spectra was ammonium hydrogenphosphate (1.11 ppm with reference to H3PO4). 5.2.4. Thermal decomposition 200 mg catalyst was placed in a U-shaped quartz reactor, which was connected to a gas supply system. The reactor was positioned in a tubular furnace (Carbolite) and was heated with 5 Kmin 1 from room temperature to 600 °C in 50 mLmin 1 He gas flow. The composition of the out-streaming gas flow was monitored by mass spectrometry (Pfeiffer ThermoStar GSD 320). 5.3. Heck reaction 5.3.1. General procedure The palladium catalyst was weighed into a pressure tube with a Teflon screw cap in a glovebox. All other reagents, additives and the solvent were added under ambient atmospheric conditions. Diethylene glycol dibutyl ether was added as a standard for GC analysis. A typical reaction mixture consisted of 7.5 mg palladium catalyst (1.0 wt%Pd, 0.05 mol% Pd), 168.8 mg chlorobenzene (1.5 mmol), 208.3 mg styrene (2.0 mmol), 66.7 mg calcium hydroxide (0.9 mmol), 290.1 mg tetrabutylammonium bromide (0.9 mmol), 200 mg diethylene glycol dibutylether and 5 mL NMethyl-2-pyrrolidone. Deviations from this standard composition are mentioned in the corresponding sections. The as-charged pressure tubes were placed in pre-heated oil baths at reaction temperature. After the desired reaction time, the mixture was extracted with dichloromethane and water. The organic phase was dried over MgSO4 and analyzed by an Agilent 61530A gas chromatograph. The reactants and products were further verified by GCMS analysis. 5.3.2. Palladium monitoring In the case of contemporaneous palladium monitoring in solution, the reaction was scaled up by the factor of 80 and conducted in a round bottom flask equipped with a reflux condenser and a septum. At suitable reaction times, aliquots of 9 mL reaction mixture were taken with a pre-heated syringe, of which 7 mL were filtered immediately by a syringe filter (0.2 mm PTFE membrane). The remaining 2 mL were further processed to determine the conversion and yield by gas liquid chromatography. To quantify the palladium content in the filtered fraction, all organic substances were removed by evaporation and decomposition at 450 °C followed by the treatment with peroxymonosulfuric acid. Palladium was subsequently digested by heating the sample with a mixture of concentrated sulfuric acid, concentrated nitric acid and concentrated hydrochloric acid. After diluting the samples to a proper extent, the palladium concentration was determined by ICP-MS (Agilent 7900). 5.3.3. Coupling of chlorobenzene and styrene with continuous sampling 37.5 mg PdMe(tmeda)/SiO2 [0.5 wt%Pd] (0.05 mol%Pd), 168.8 mg chlorobenzene (1.5 mmol), 208.3 mg styrene (2.0 mmol), 66.7 mg calcium hydroxide (0.9 mmol), 290.1 mg tetrabutylammonium bromide (0.9 mg), 200 mg diethylene glycol dibutyl ether and 5 mL N-Methyl-2-pyrrolidone were placed in a pressure tube equipped with a septum. The pressure tube was placed in a preheated oil bath at 150 °C. Samples of 0.1 mL were taken during the reaction, which were processed as described in the general procedure and analyzed by gas liquid chromatography. After a reaction time of 28 h, additional 168.8 mg chlorobenzene (1.5 mmol) and 208.3 mg styrene (2.0 mmol) were added to the reaction mixture.
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5.3.4. Catalyst recycling At the desired reaction time, the reaction was quenched by placing the pressure tubes in an ice bath. All solids were separated by centrifugation and washed twice with 10 mL dichloromethane. After the solids were dried at room temperature, they were inserted in a consecutive run following the general procedure described previously. Acknowledgements One of the authors (O.D.) thanks the Studienstiftung des deutschen Volkes e. V. for a scholarship. Two of the authors (O.D., C.G.) thank the TUM Graduate School for financial support. The authors would like to thank Ulrike Ammari, Petra Ankenbauer and Bircan Dilki from the microanalytical laboratory at Technical University of Munich for the conduct of the elemental analysis. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43]
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Journal of Catalysis journal homepage: www.elsevier.com/locate/jcat
Synthesis of methyl palladium complexes on silica as single site catalysts activating CACl bonds in heck reactions Christoph Gnad a,b, Oliver Dachwald a,b, Gabriele Raudaschl-Sieber a, Klaus Köhler a,b,⇑ a b
Technische Universität München, Department of Chemistry, Lichtenbergstraße 4, Garching b. München 85747, Germany Technische Universität München, Catalysis Research Center, Ernst-Otto-Fischer-Straße 1, Garching b. München 85747, Germany
a r t i c l e
i n f o
Article history: Received 3 May 2019 Revised 2 June 2019 Accepted 3 June 2019
Keywords: Grafting Single-site catalysts Palladium Heck reaction Aryl chlorides
a b s t r a c t Supported catalysts with molecularly dispersed, uniform palladium species were synthesized by the method of grafting. New specific synthesis protocols were developed in order to exclude self-reduction and agglomeration of palladium during immobilization. The successful synthesis of isolated mononuclear palladium(II) surface complexes of uniform structure was confirmed by MAS NMR, infrared spectroscopy and elemental analysis. The catalytic potential of these single-site catalysts for the Heck reaction was directly compared to well-characterized supported palladium oxide materials known and reported for their extraordinarily high activity in the conversion of aryl bromides. In contrast to those, the immobilized palladium surface complexes were able to quantitatively convert demanding non-activated aryl chlorides as substrates with conversions up to 62% and high selectivity. Although believed to be only pre-catalysts, the ligand sphere of the surface complexes directly affects their activity. The timeresolved reaction of chlorobenzene with styrene revealed that catalysts with supported amine complexes are able to constantly convert chlorobenzene during several hours of reaction time. It is proposed that the catalysts continuously provide active, dissolved Pd(0) species during the whole course of reaction. Ó 2019 Elsevier Inc. All rights reserved.
1. Introduction The palladium catalyzed carbon-carbon bond formation is a widely used tool in organic chemistry for the synthesis of various compounds through the linkage of two molecules. Especially the Heck, Suzuki and Negishi cross-coupling reactions reached high importance and thus Richard F. Heck, Akira Suzuki as well as Eiichi Negishi were honored with the Nobel Prize for the discovery of these reactions in 2010 [1–4]. CAC coupling reactions are also applied in the chemical industry, e.g. for the production of pharmaceuticals, agrochemical compounds as well as fine chemicals [5,6]. The Mizoroki-Heck reaction describes the coupling reaction of an alkene with an aryl halide, vinyl halide or aryl triflate under use of a base and a palladium catalyst to form a substituted alkene (Scheme 1). Besides providing high stereo- and regioselectivity, the Heck reaction is able to couple a wide variety of different substrates [1,7,8]. In the chemical industry, it is applied for the synthesis of several commercial products such as Naproxen or SingulairÒ [9].
⇑ Corresponding author at: Technische Universität München, Department of Chemistry, Lichtenbergstraße 4, Garching b. München 85747, Germany. E-mail address: [email protected] (K. Köhler). https://doi.org/10.1016/j.jcat.2019.06.004 0021-9517/Ó 2019 Elsevier Inc. All rights reserved.
Primary, the Heck coupling was discovered as a homogeneously catalyzed reaction using palladium salts such as palladium acetate and palladium chloride or simply palladium metal [1,10]. Due to increasing interest in this reaction, research headed towards the development of solid palladium catalysts to enable facile separation and recycling. Consequently, several reviews summarized the progress made and the huge literature contributions concerning this topic [11–14]. Supported Pd(0) species [15–18] (e.g. nanoparticles or clusters), Pd(II) compounds encapsulated in zeolites or other mesoporous materials [19,20] as well as Pd(II) compounds supported on oxides [21,22] were successfully applied in a few cases also for the activation of demanding substrates such as non-activated aryl chlorides with moderate to high yields, yet during very long reaction times. There are however numerous literature contributions including reviews reporting that – despite of the heterogeneous character of the catalysts – the active species are molecular Pd species dissolved (‘‘leached”) from the surface during the reaction. This can even be regarded as mostly accepted view at least for Heck reactions and it raises the question of the role of supported molecular Pd species directly bound to a support. The approach of this study is to synthesize catalysts with structurally defined, uniform isolated palladium species immobilized on silicon dioxide. The generation of isolated (transition) metal complexes on oxide supports can be realized with synthesis strategies
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Scheme 1. The Mizoroki-Heck reaction of an aryl halide or aryl triflate with an alkene. X = Cl, Br, I, OTf, OTs.
from the field of Surface Organometallic Chemistry (SOMC). By the application of high vacuum techniques in combination with high temperatures, the distribution and number of hydroxyl groups on oxide surfaces can be controlled. This dehydroxylation can result in a homogeneous distribution of isolated surface hydroxyl groups. These functional groups can be converted with transition metal complexes, which possess at least one ligand being able to react with a hydroxyl group. For example, alkyl, amide, alkoxy or halogenide metal complexes are suitable precursors for the reaction with the hydroxyl groups of the oxide surface. Applying said technique known as Grafting, solid catalysts with molecular defined and isolated metal centers can be synthesized [23–25]. Richmond et al. successfully grafted Pd(II) complexes with the structural motif PdMeCl or dimeric palladium compounds on partially dehydroxylated silicon dioxide and applied them as catalysts for hydroamination reactions [26]. Immobilization of palladium complexes can result in potent catalysts for the Heck coupling reactions [27,28]. In these works, grafting of different Pd(II) complexes on silicon dioxide or on a MOF support provides novel solid catalysts with high to maximal metal dispersions and palladium centers with defined coordination spheres. To achieve exclusively uniform molecular metal centers and to avoid self-reduction of palladium in particular with very sensitive Pd methyl complexes as applied in the present work, the grafting procedure had to be further improved regarding the pre-treatment of the support, the reaction temperature as well as precursor concentrations. In contrast to other approaches in which grafted noble metal catalysts function as precursors in order to generate small particles through defined pre-treatments [29], the focus of this contribution lays upon the isolated structurally defined supported metal complexes themselves. The variation of the ligands allows the investigation of the influence of different stabilities of the surface complexes on their reactivity towards the Heck coupling reaction. The synthesized catalysts were applied for the coupling of aryl bromides as well as chlorides. To get a fair relation to supported palladium and palladium oxide particles, all catalysts were directly compared to well-characterized supported palladium oxide materials known and reported for its extraordinarily high activity in the conversion of aryl bromides.
siloxane groups without significant loss of specific surface area. This process can be monitored by IR spectroscopy (see Fig. 5, experimental). The surface of the resulting material consists of approximately 0.8 isolated silanol groups per nm2 [23–25,30–32]. Silicon dioxide, which was dehydroxylated at 700 °C is denoted as SiO700 2 . To obtain heterogeneous single-site palladium catalysts, the dimethylpalladium(II) complexes dimethyl(N,N,N’,N’-tetramethyle thanediamine)palladium(II) and dimethyl[1,2-bis(diphenyl-phos phino)ethane]palladium(II) were chosen as metal-organic precursors. The synthesis of both precursor compounds was described by the group of de Graaf [33]. The prepared metal precursors were grafted on SiO700 as shown in Scheme 2. 2 A noble metal complex comprising donating ligands like methyl groups is likely to be reduced to the oxidation state of zero. The grafting reaction is always in direct competition with the reductive elimination of ethane from the precursor complex [34]. The danger of this undesired reaction to happen is ubiquitous and can already be observed in the pure solid complex. Quantitative decomposition of PdMe2(L) with L = tmeda and dppe occur at T = 398 K and 439 K respectively [33,35], however partial decomposition can be deduced from formation of palladium black during storage in case of the tmeda-complex even at 253 K [36]. Reductive elimination during the grafting reaction can be suppressed by working at low temperatures with very diluted solutions of the precursor. Also, filtration of precursor solutions removes any metal black which could have formed on storage or dissolution of the complexes. Such low temperatures on the other hand, decrease reaction rates, so that long reaction times are necessary to achieve maximum conversion which in turn enables more undesired side reactions. Grafting alkyl palladium(II) complexes is thus always a balance between undesired and wanted reactivity in order to obtain isolated surface species. The corresponding optimization led to the following reaction parameters (given more detailed in the experimental part): 20 h reaction time at 30 to 40 °C, argon atmosphere (Schlenk techniques). The amount of applied precursor complexes was chosen in order to achieve metal loadings between 0.1 and 1.0 wt% palladium. The prepared white to slightly yellow powders were stored under inert conditions at 30 °C.
2. Results and discussion
2.2. Characterization of the silica-bound species
2.1. Preparation of the silica supported palladium catalysts
2.2.1. Elemental analysis To verify the metal loadings of the prepared catalysts, the palladium content was examined by digestion and subsequent photometric quantification. The same procedure was conducted to quantify the phosphorus content. Carbon, hydrogen and nitrogen contents were determined by combustion analysis (and, if not mentioned otherwise, with the exclusion of air contact to prevent contamination of the samples with carbon-containing substances or water). The results are summarized in Table 1. Since the precursors tend to decompose by a reductive elimination, it is apparent that the lower palladium and consequently carbon, hydrogen and nitrogen values are caused by the loss of precursor complex. To avoid the contamination of the catalysts with palladium metal, the precursor complexes were filtered before the grafting step (and the excessive ligand is removed by careful washing of the catalysts). Consequently, the determined
In order to gain catalysts with isolated surface complexes the method of grafting was applied. It relies on the conversion of surface functional groups with defined organometallic complexes under the formation of a covalent bond (at least in the first immobilization step) [23–25]. To achieve metal site isolation, it is necessary to pre-treat the support. In the case of oxides such as SiO2, Al2O3 or TiO2, the number and properties of certain surface hydroxyl groups can be controlled by dehydroxylation [30]. Silicon dioxide is a frequently applied support for the grafting step due to its well-defined surface properties. The surface hydroxyl groups can be classified into neighboring (vicinal and geminal hydroxyl groups) and isolated hydroxyl groups. By the thermal treatment of AEROSILÒ 200 at 700 °C in high vacuum, the neighboring hydroxyl groups are forced to react with each other and condensate to
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Scheme 2. Synthesis of isolated palladium surface complexes by the conversion of the dimethylpalladium(II) complexes PdMe2(tmeda) and PdMe2(dppe) with the isolated silanol groups of SiO700 2 .
Table 1 Elemental analysis of the palladium catalysts with isolated surface complexes. The results are given in weight percent and as molar ratios. Palladium contents were determined by digestion and subsequent photometric quantification, C, H and N contents by combustion analysis and under inert conditions. Weight% calculated/found Molar ratio calculated/found Sample PdMe(tmeda)/SiO2700 [1.0 wt%Pd] PdMe(tmeda)/SiO700 [0.5 wt%Pd] 2 PdMe(dppe)/SiO700 [1.0 wt%Pd] 2 PdMe(dppe)/SiO700 [0.5 wt%Pd] 2 a
Pd
C
H
N
P
1.00/0.76 1.0/1.0 0.50/0.51 1.0/1.0 1.00/0.97 1.0/1.0 0.50/0.47 1.0/1.0
0.79/0.63 7.0/7.3 0.40/0.54 7.0/9.4 3.05/3.28 27.0/30.0 1.52/1.42 27.0/26.8a
0.18/0.15 19.0/20.8 0.09/0.16 19.0/33.1 0.26/0.30 27.0/32.7 0.13/0.16 27.0/35.9a
0.26/0.23 2.0/2.1 0.13/0.17 2.0/2.4 – – – –
– – – – n/a n/a 0.29/0.29 2.0/2.1a
values were not determined under inert conditions.
molar ratios of carbon, nitrogen and phosphorus fit to the calculated values, which indicate the success of the synthesis. The remaining silanol protons of the SiO700 surface can cause the 2 slightly higher hydrogen values since in the case of a palladium loading of 1.0 wt% approximately 40% of the available isolated silanol groups are occupied (which affects the second decimal place). For the sample that was not measured under inert conditions, the higher content can also be attributed to adsorbed water. From the results of the elemental analysis, it can be concluded that palladium was successfully immobilized on dehydroxylated silica and that the stoichiometric Pd:C:H:N:P-ratios fit to the theoretical values for the optimized synthesis procedure developed.
2.2.2. IR spectroscopy To further investigate the surface complexes, the synthesized catalysts were pressed into self-supporting pellets (in glovebox), which were transferred into a gas tight in situ IR cell. The procedure allows the handling and measuring of the materials under the strict exclusion of air contact. Fig. 1 shows the infrared spectra 700 of SiO700 and PdMe(dppe)/SiO700 2 , PdMe(tmeda)/SiO2 2 . Due to the bonding of the methyl complexes of palladium to the support surface, bands appear in the region between 2700 cm 1 and 3200 cm 1. They can be attributed to the vibrations of the backbone ligands tmeda and dppe as well as the methyl ligands directly
700 Fig. 1. IR spectra of a) SiO700 [1.0 wt%Pd] and c) PdMe 2 , b) PdMe(tmeda)/SiO2 (dppe)/SiO700 [1.0 wt%Pd]. Samples were pressed into self-supporting pellets and 2 transferred to a gas tight in situ IR cell. All spectra were taken under inert conditions.
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bound to palladium. The assignment of these bands is listed in Table 2. The sharp band at 3747 cm 1 can be attributed to the stretch vibration of the isolated silanol groups. As already mentioned, around 40% of the available silanols are occupied by the palladium complexes (for 1.0 wt%Pd). Therefore, the silanol band is still clearly visible in the spectra of the two catalysts. The bands in the region of 3100 cm 1 –2750 cm 1 can be assigned to symmetric and asymmetric CAH stretch vibrations of the ligand. By complementary temperature programmed desorption experiments the presence of any organic substances besides the palladium complexes could be excluded. Therefore, it can be stated that the bands derive solely from the surface complexes. 2.2.3. Solid-state NMR spectroscopy Since organic ligands coordinate the palladium metal centers of the single-site catalysts, it is possible to characterize them by solidstate NMR spectroscopy. Fig. 2 shows the 1H-MAS NMR spectrum of PdMe(dppe)/SiO700 2 , Table 3 compares the chemical shifts of the signals from the liquid state 1H NMR spectrum of the precursor complex with the signal derived from the solid-state spectrum of the surface complex. The distinct signals of the 1H solid-state NMR spectrum exhibit similar chemical shifts as the precursor complex and can be assigned to the protons of the phenyl, ethyl and methyl groups of the surface complexes. This clearly demonstrates the molecular
Table 3 Chemical shifts of the signals from the liquid-phase 1H NMR spectrum of PdMe2(dppe) in C6D6 compared to the signals of the solid-state 1H MAS NMR spectrum of PdMe(dppe)/SiO700 assigned to the corresponding functional groups. 2 Chemical shifts [ppm] Functional group
PdMe2(dppe) in C6D6 liquid phase
PdMe(dppe)/SiO700 2 solid sate
Phenyl P-CH2-CH2-P Pd-CH3 Si-OH
7.62, 7.01 1.88 1.29 –
7.14 1.55 1.14 1.95
character and the uniformity of the metal centers of the palladium catalysts achieved by grafting. A suitable method to demonstrate the bonding of the palladium precursors to the SiO700 surface is 31P-HPDEC NMR spectroscopy. In 2 Fig. 3, the liquid phase 31P NMR spectrum of the precursor PdMe2(dppe) in benzene-d6 (left) as well as the solid-state 31 P-HPDEC NMR spectrum of PdMe(dppe)/SiO700 2 (right) are depicted. The 31P NMR spectrum of PdMe2(dppe) shows one singlet at 39.03 ppm, which is caused by both phosphorus atoms of the dppe backbone ligand. In contrast, the solid-state 31P-HPDEC NMR spectrum reveals two signals, both singlets, at 58.48 ppm and 28.13 ppm. Due to the bonding of the palladium complex to the support, the two phosphorus atoms of the dppe ligand are not
Table 2 List of bands appearing in the IR spectra of PdMe(tmeda)/SiO700 [1.0 wt%Pd] and PdMe(dppe)/SiO700 [1.0 wt%Pd] and their assignments to the corresponding vibrations. 2 2 PdMe(dppe)/SiO700 2
PdMe(tmeda)/SiO700 2
m [cm ]
assignment
ref.
m [cm 1]
assignment
ref.
3747 3100–3040 3000–2900 2870–2840
Si-OH HC = CH H2C-CH Pd-CH
[30,31] [37,38] [37,38] [41]
3747 3040–2900 2892 2845 2795
Si-OH N-CH Pd-CH Pd-CH N-CH
[30,31] [39,40] [41] [41] [39]
1
Fig. 2. 1H MAS NMR spectrum of PdMe(dppe)/SiO700 2 . Sample preparation, transfer and measurement were performed under the exclusion of air contact.
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Fig. 3.
31
P NMR spectra of PdMe2(dppe) (left, liquid phase) in benzene d6 and PdMe(dppe)/SiO700 [1.0 wt%Pd] (right, solid state). 2
equivalent anymore, resulting in two signals with similar integral values in the 31P NMR spectrum. This clearly demonstrates the successful reaction between the surface silanol groups and the palladium complex as well as the isolation of the metal centers. In summary, the application of the characterization methods verified the success of the grafting of dimethylpalladium(II) complexes on SiO700 2 . Furthermore, the results point out the generation of isolated, structurally uniform and well-defined palladium centers on the surface of silicon dioxide. 3. Application in Heck-type CC coupling reactions The prepared catalysts were applied in the Heck reaction of chlorobenzene as well as bromobenzene with styrene. A series of preceding parameter optimizations and careful interpretation of literature and earlier experiments lead to the reaction system, which is displayed in Table 4. Palladium concentrations were kept low in order to disfavor agglomeration and deactivation as well as to reach representative TONs and TOFs. On the other hand, the amount of applied palladium catalysts had to be at a certain level to differentiate from catalysis by ‘‘homeopathic” palladium concentrations. In general, it is possible to catalyze the Heck reaction by ultralow amounts of palladium (ppm and even ppb levels) due to the consequential prevention of agglomeration [7,42,43]. This can also lead to conversions of aryl iodides and bromides due to palladium impurities in reactants or additives. Therefore, blind tests without the addition of the catalysts were performed as well which ensured that the conversions and yields can solely be attributed to the respective catalysts. The temperatures were set between 130 °C and 160 °C in order to effectively couple the substrates in short reaction times. Oxidative addition, reduction of palladium and metal
leaching are favored at higher temperatures (for maximum palladium leaching seems to exist an intermediate temperature range). The respective bases were chosen according to optimal performances in previously conducted parameter tests. Highest conversions and yields were achieved with sodium acetate in the case of bromobenzene, whereas the addition of calcium hydroxide delivered best results for the coupling of chlorobenzene. [19,47] Addition of tetrabutylammonium bromide further reduces reaction times by acceleration of the regeneration as well as stabilization of the active Pd(0) species in solution [44–46]. Conversions of aryl halides and yields of trans-stilbene were determined by GLC. Complementary GC-MS analysis validated the assignment of the signals and identified small amounts of cis-stilbene as well as 1,1-diphenylethylene as biproducts. For details, see experimental section. 3.1. Palladium oxide particles vs palladium surface complexes To investigate the catalytic potential of the palladium surface complexes in the Heck reaction, several experiments were performed with grafted catalysts as well as catalysts with silicasupported palladium oxide particles as reference system. The PdO/SiO2 catalysts are well-known for their application in C-C coupling reactions and are among the most efficient systems ever reported for the Heck reactions of aryl bromides [19,47]. The results are listed in Table 5. Regarding the coupling of bromobenzene with styrene, both catalytic systems – PdO/SiO2 and Pd-SiO2 surface complexes reached full conversion and yields of stilbene up to 82% after 0.5 h at 160 °C (entries 3 and 4). But when the temperature is set 20 K lower to 140 °C, conversion and yield decreased to 52% and 47% after 1 h for the grafted catalyst, whereas the PdO catalyst still
Table 4 Selection of reaction parameters after optimization for the Heck coupling reaction of bromobenzene or chlorobenzene with styrene. All experiments were performed in air
X
Base
Additive
Pd [mol%]
T [°C]
t [h]
Br Cl
1.2 eq NaOAc 0.6 eq Ca(OH)2
0.6 eq TBAB 0.6 eq TBAB
0.05–0.15 0.05
130–160 150
0.5–2 6
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Table 5 Application of different pre-catalysts in the Heck reaction of chloro- or bromobenzene with styrene. For reaction parameters see Table 4. Conversion and yield were determined by gas-liquid chromatography. Entry
Catalyst
Aryl Halide
Base
Reaction Temperature [°C]
Reaction Time [h]
Conversion [%]
Yield [%]
1 2 3 4 5 6 7
PdOxH2O/SiO2 PdMe(tmeda)/SiO700 2 PdOxH2O/SiO2 PdMe(tmeda)/SiO700 2 PdOxH2O/SiO2 PdMe(tmeda)/SiO700 2 PdMe(dppe)/SiO700 2
BrAPh BrAPh BrAPh BrAPh ClAPh ClAPh ClAPh
NaOAc NaOAc NaOAc NaOAc Ca(OH)2 Ca(OH)2 Ca(OH)2
140 140 160 160 150 150 150
1 1 0.5 0.5 6 6 6
99 52 99 99 14 67 55
83 47 81 82 <1 62 45
obtained full conversion and 83% yield (entries 1 and 2). Therefore, it can be stated that the higher temperature is a key factor for the activity of the catalysts with isolated surface complexes. The catalysts were also applied for the coupling of the demanding aryl halide chlorobenzene. Due to the increasing stability of the Ar-X bond in the order ArAI < ArABr < ArACl (dissociation energies [48]: Ar-I 281 kJmol 1, ArABr 346 kJmol 1, ArACl 409 kJmol 1) the activation of aryl chlorides represents the highest challenge [7,49]. It is often neglected in the literature that aryl chlorides are therefore superior test substrates compared to iodides and bromides. Catalyzing the reaction by palladium impurities in reagents can be restricted. Effects on the catalytic performance induced by parameter variations can be observed and interpreted more clearly [43]. Furthermore, the comparatively slow oxidative addition of the aryl chlorides to Pd(0) complexes requires longer reaction times and higher temperatures. Thus, the comparison of bromo and chlorobenzene reactivity can be very useful for mechanistic conclusions. Entries 5–7 of Table 5 reveal that with the applied reaction conditions the conventionally supported palladium oxide catalyst reached chlorobenzene conversions of only 14% and negligible low yields whereas the grafted catalysts showed high chlorobenzene conversions of 55% and 67% as well as stilbene yields up to 62% after 6 h at 150 °C. These results point out the huge impact of the choice of the pre-catalyst (particles vs surface complexes) on the outcome of the experiments for demanding substrates. In the case of CAC coupling reactions, the discussion of how the reaction is catalyzed applying solid palladium catalysts is still ongoing. The two principle possibilities are that either the reaction is catalyzed by a ‘‘truly heterogeneous” surface mechanism or that dissolved palladium, which is generated by metal leaching, catalyzes the reaction following the homogeneous reaction mechanism. In order to target this issue, the palladium content in solution was monitored in several publications. Independent of the catalytic system, dissolved palladium could be detected during the progress of the reactions whenever this had been investigated. The results support that heterogeneous palladium catalysts act as reservoir for highly active, coordinately unsaturated Pd(0) species, which are generated by the dissolution of palladium off the support [19,43,47,50–52]. Therefore, the different performances of the chosen catalysts can be traced back to the process of palladium leaching. The diverse bonding situations and coordination spheres of the palladium of the pre-catalysts influence the generation of the active species. The results from Table 5 indicate that in the case of catalysts with surface complexes sufficient amounts of dissolved palladium species are formed (if at all) apparently only at elevated temperatures. This precondition could lead to a controlled release of palladium species, which is high enough to form adequate amounts of active centers but low enough to prevent agglomeration of Pd(0) and the formation of palladium black. This concept of controlled metal leaching can be the reason for the high activity towards the coupling of deactivated aryl halides [19,22,51]. Previously conducted parameter optimizations underlined the importance of the addition of TBAB. In the case of bromobenzene,
higher amounts of tetrabutylammonium bromide increased conversions and yields, whereas in the case of chlorobenzene, no reaction was observed in the absence of TBAB. Furthermore, NMP appeared to be the most suitable solvent for the Heck reaction. This indicates that the generation and stabilization of active Pd species is promoted by coordinating ligands such as bromides or NMP molecules. Most likely, palladium is dissolved by the formation of L2Pd(II)XMe or L2Pd(II)X2 with L2 = tmeda, dppe and X = Br, NMP, which result from the substitution of the silanol ‘‘surface” group by a coordinating ligand. First tests on dissolved palladium during the reactions were conducted. In the case of chlorobenzene, very low but seriously detectable amounts of leached palladium could be detected (2, 1, 5% of the total Pd at 2, 4, 6 h reaction time and 6, 9, 14% chlorobenzene conversion). During the coupling of bromobenzene, the palladium content in solution was found to be increasing significantly from 43% at 5 min reaction time and 4% conversion to 74% at 30 min and 63% conversion. After complete conversion of bromobenzene (97%) at 60 min reaction time, the palladium concentration dropped to 2% of the total Pd. The amounts of leached palladium indicate the dissolution of palladium by the generation of the priory described Pd(II) complexes as well as the catalysis occurring in the liquid phase. However, more detailed palladium leaching experiments as function of reactions time and conditions are necessary (and in progress) for serious conclusions on the reaction mechanism. 3.2. Effect of the coordination sphere of supported palladium complexes From entries 6 and 7 of Table 5 it is apparent that the ligand sphere of the surface complexes influences the catalytic performance. Amines lead to a higher catalytic activity than phosphanes, which can be correlated with their abilities to stabilize palladium in particular oxidation states. Palladium amine complexes tend to dissociate faster leading to a higher amount of active, unsaturated Pd(0) species in solution. In contrast to the tmeda ligand, phosphanes can also stabilize palladium in the oxidation state 0 [33]. Therefore, in the case of the surface complexes with dppe, the generation of dissolved active centers is suppressed. Furthermore, the backbone ligands must leave the coordination sphere of the surface complexes to enable the coordination of the aryl halides and olefins. Especially in the case of the bulky dppe ligand, a surface mechanism seems very unlikely due to the high steric demand, supporting the conclusion that dissolved, unsaturated Pd(0) species are indeed the active centers for the Heck reaction. 3.3. Kinetics of the Heck coupling of chlorobenzene with styrene To further investigate the coupling of aryl chlorides using supported palladium complexes as catalyst, the time-dependent progress of the reaction of chlorobenzene with styrene was plotted and is shown in Fig. 4. It should be mentioned that due to the experimental set-up for steady sampling and reaction temperatures above the boiling point of chlorobenzene (septum instead
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Fig. 4. Time-dependent conversion and evolution of the yield of stilbene from the reaction of chlorobenzene with styrene using PdMe(tmeda)/SiO700 [0.50 wt%Pd] as catalyst. 2 Reaction set-up was optimized for steady sampling. Reaction conditions: 0.05 mol% Pd, 1.0 eq chlorobenzene, 1.3 eq styrene, 0.6 eq Ca(OH)2, 0.6 eq TBAB, NMP, 150 °C, air.
Table 6 Recycling of PdMe(tmeda)/SiO700 [0.1 wt%Pd] after the reaction of chlorobenzene with styrene. Second runs were performed with chlorobenzene as well as bromobenzene. 2 Reaction conditions entry 4: 150 °C, 6 h, 0.05 mol% [Pd]. For further reaction conditions see Table 4. Entry
Run
Catalyst
Aryl Halide
Conversion [%]
Yield (Stilbene) [%]
1 2 3 4
1 1 2 2
PdMe(tmeda)/SiO700 2 PdMe(tmeda)/SiO700 2 Pd/SiO2-recyc Pd/SiO2-recyc
ClAPh ClAPh ClAPh BrAPh
67 70 0 26
62 60 0 20
of screw cap, see experimental section), evaporation of the aryl halide could not be prevented completely, which misleadingly results in higher conversions. Furthermore, the consequent decline in concentration of chlorobenzene as well as the continuous removal of active Pd(0) species through sampling results in lower yields and slower reaction rates compared to the standard catalytic runs. What must also be considered is the selectivity to benzene (also evaporated and lost), i.e. dehalogenation as side reaction. This side reaction is expected to become more relevant with the presence of palladium(0) particles, i.e. with progressive deactivation of the catalyst. Since the stilbene formation starts only after 1 h of reaction time, an initial period for the transformation of the pre-catalyst can be assumed. Nevertheless, the steady increase of the yield after the induction phase shows that the catalyst is able to constantly provide (dissolved) active Pd(0) species over a longer period of time. The decrease of the formation of stilbene can most probably be assigned to the deactivation of the catalyst due to its transformation under reaction conditions. In CAC coupling reactions, metal leaching can be seen as the generation of active centers. On the other hand, because of the dissolution and re-deposition of metal species, the catalyst changes its initial state highly probably with decreasing Pd dispersion. Consequently, the capability of the catalyst to be recycled was further investigated.
Table 6 shows that it is not possible to reuse the catalyst for the coupling of chlorobenzene with styrene (entry 3) under the reaction conditions applied. However, it is still possible to convert bromobenzene to some extent (entry 4), which can be assigned to the more facile activation and oxidative addition of bromobenzene. The reinsertion of the palladium surface complexes shows that the catalysts lose their high activity after the first run. It can be concluded that the immobilized surface complexes were transformed during the reaction of chlorobenzene with styrene. Decreased Pd dispersion and Pd reduction can be assumed. Unfortunately, neither electron microscopy nor other methods could detect Pd particles, possibly due to the small Pd concentration and very small particle sizes and work-up procedures. Despite thermal decompositions tests, which revealed that the surface complexes are stable up to 170 °C in an inert gas flow, the original Pd complex structures were however definitively destroyed (NMR, IR) under Heck reaction conditions. As discussed previously, coordinating substances such as NMP or bromide ions can promote the dissolution of palladium. These processes as well as redeposition of dissolved, reduced Pd species can explain these the transformation of the catalyst. The detailed mechanism of such dissolution dynamics, especially during the prolonged reaction times of the coupling of aryl chlorides, is subject of ongoing studies.
3.4. Activity of the spent catalysts
4. Conclusion
The catalyst as well as all other solids were separated from the reaction mixture after the first run. This as well as the careful work-up procedure should prevent the loss of any palladium [53] (for details, see experimental section). The collected solids (marked as Pd/SiO2-recyc) were subsequently re-inserted in a second run.
Grafting of dimethylpalladium(II) complexes on dehydroxylated silicon dioxide resulted in solid, but molecular palladium catalysts with structurally uniform surface species. Characterization of the ligand sphere by IR and NMR spectroscopy as well as elemental analysis validated the generation of isolated mononuclear
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palladium surface complexes. In contrast to the vast majority of solid palladium catalysts for the Heck reaction, the synthesized materials can effectively couple even non-activated aryl chlorides with high selectivity in few hours. Since the reaction of aryl chlorides is far slower compared to bromides or iodides, the catalysts are able to provide highly active palladium species over a longer period of time. A higher stability of the surface complexes resulted in a decrease in activity.
5. Experimental 5.1. Catalyst synthesis 5.1.1. Synthesis of isolated surface complexes Support pretreatment: AerosilÒ 200 (Evonik) was agglomerated, crushed and pestled to grain sizes between 100 and 300 mm. The resulting powder was calcined at 500 °C in a muffle oven with air ventilation. For dehydroxylation, the prepared support was placed into a quartz Schlenk-tube connected to a high vacuum Schlenk-line equipped with an oil diffusion pump (Ilmvac 100328). A pressure of 10 5 mbar was applied and the SiO2 was heated with 2 Kmin 1 to 700 °C, which was held constant for 16 h. After cooling down, the as prepared SiO700 was handled under strict inert conditions for 2 the following steps to prevent any rehydroxylation [25]. Precursor synthesis: The palladium(II) complexes PdMe2(tmeda) and PdMe2(dppe) were synthesized according to the procedure of de Graaf [33]. PdMe2(tmeda): Elemental analysis (calc.; found) [m%]: C (38.02; 37.69), H (8.78; 8.74), (11.09; 10.85). 1H NMR (400 MHz, C6D6, 294 K) d (ppm) = 0.51 (s, 6H, Pd(CH3)2), 1.63 (s, 4H, NACH2ACH2AN), 2.02 (s, 12H, 2 N(CH3)2). PdMe2(dppe): Elemental analysis (calc.; found) [m%]: C (62.87; 63.61), H (5.65; 5.63), P (11.58; 11.12), Pd (19.89; 18.19). 1H NMR (400 MHz, C6D6, 300 K) d (ppm) = 1.27–1.32 (m, 6H, Pd(CH3)2), 1.79–1.95 (m, 4H, PACH2ACH2AP), 6.99–7.04 (m, 12H, PPh2), 7.58–7.66 (m, 8H, PPh2). Grafting of PdMe2L (L = tmeda, dppe) on SiO700 2 : All steps of the following synthesis were conducted under the exclusion of air using standard Schlenk techniques. The desired amount of the
precursor was dissolved in the pre-cooled solvent in a Schlenktube. The solution was filtrated onto the desired amount of pretreated support in another pre-cooled Schlenk-tube. The mixture was allowed to react without stirring for 20 h at temperatures between 30 °C and 40 °C. After that time, the solvent was removed by filtration and the solid was washed twice with the original fresh solvent each. After evaporation of solvent residues and drying the material in vacuo, a white to pale yellow solid depending on the palladium loading was obtained. The materials were stored under inert conditions below 30 °C. 5.1.2. Synthesis of silicon dioxide supported palladium oxide particles Support pretreatment: AEROSILÒ 200 was agglomerated, pestled, sieved and calcined according to the previous description. General catalyst synthesis procedure: The silicon dioxide supported palladium oxide particles were synthesized by a deposition-precipitation method based on Pearlman [54]. For 1.0 g of catalyst with a target metal loading of 1.0 wt%Pd, 0.99 g of the SiO2 were suspended in 30 mL water. A solution of PdCl2 in 5% HClaq was added dropwise under stirring. Subsequently, the pH of the suspension was slowly increased to 10 by the addition of 10% NaOHaq under the application of an autotitrator (Metrohm 736 GP Titrino). The resulting solid was filtered, washed several times with water and dried in vacuo. The brown powders were stored at room temperature and in air. 5.2. Catalyst characterization 5.2.1. Elemental analysis Carbon, hydrogen and nitrogen contents were determined by combustion analysis using an elemental analyzer (Eurovector). Inert sample preparation was carried out in a glovebox charging a tin foil mini weighing-boat with 10–15 mg of the sample, which was wrapped into another tin foil weighing boat. Palladium and phosphorus contents were analyzed by photometry. The samples were boiled with concentrated sulfuric acid under low vacuum to remove organic compounds. In order to digest the corresponding elements, the residues were treated with concentrated sulfuric and nitric acid. For the quantification of the palladium content, part of the solution was mixed with a freshly prepared solution of nitroso-R salt. In the case of phosphorus, ammonium vanadate solution was used. The contents were determined photometrically by a UV–Vis spectrometer (Shimadzu UV-160). 5.2.2. IR spectroscopy The samples were pressed to self-supporting pellets in the glovebox and subsequently transferred into a gas-tight IR cell. The IR spectra were collected on a FT-IR spectrometer (BioRad FTS 575C) equipped with a liquid-nitrogen-cooled mercury cadmium telluride (MCT) detector.
Fig. 5. IR spectra of self-supporting pellets of untreated SiO2 as well as dehydroxylated silicon dioxide, SiO500 and SiO700 2 2 . The spectra were taken under strict exclusion of air contact. The sharp band at 3747 cm 1 can be assigned to the isolated silanol groups, the broad band between 3710 cm 1 and 3120 cm 1 to neighboring silanol groups (vicinal and germinal).
5.2.3. NMR spectroscopy Liquid state NMR spectra are recorded by a Bruker AMX400. Samples were dissolved in 0.5 mL of the specified solvent inside standard NMR tubes. All chemical shifts are given in d-values (ppm) with the residual proton signals of the solvent serving as a reference. The solid-state 1H MAS as well as 31P-HPDEC spectra were recorded on a Bruker AV 300 NMR spectrometer. The sample material was placed into a ZrO2 rotor with 4 mm diameter and sealed in a glovebox. The spectra were taken at 303 K with a rotational frequency of 12 kHz and measuring frequencies of 300 MHz for the 1H and 122 MHz for the 31P-spectra. The spectra were referenced to an external standard, which was adamantane for 1H measurements (reference signals: 1H: 2.00 ppm). The adamantane standard itself
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was referenced against tetramethylsilane. The standard for 31 P-spectra was ammonium hydrogenphosphate (1.11 ppm with reference to H3PO4). 5.2.4. Thermal decomposition 200 mg catalyst was placed in a U-shaped quartz reactor, which was connected to a gas supply system. The reactor was positioned in a tubular furnace (Carbolite) and was heated with 5 Kmin 1 from room temperature to 600 °C in 50 mLmin 1 He gas flow. The composition of the out-streaming gas flow was monitored by mass spectrometry (Pfeiffer ThermoStar GSD 320). 5.3. Heck reaction 5.3.1. General procedure The palladium catalyst was weighed into a pressure tube with a Teflon screw cap in a glovebox. All other reagents, additives and the solvent were added under ambient atmospheric conditions. Diethylene glycol dibutyl ether was added as a standard for GC analysis. A typical reaction mixture consisted of 7.5 mg palladium catalyst (1.0 wt%Pd, 0.05 mol% Pd), 168.8 mg chlorobenzene (1.5 mmol), 208.3 mg styrene (2.0 mmol), 66.7 mg calcium hydroxide (0.9 mmol), 290.1 mg tetrabutylammonium bromide (0.9 mmol), 200 mg diethylene glycol dibutylether and 5 mL NMethyl-2-pyrrolidone. Deviations from this standard composition are mentioned in the corresponding sections. The as-charged pressure tubes were placed in pre-heated oil baths at reaction temperature. After the desired reaction time, the mixture was extracted with dichloromethane and water. The organic phase was dried over MgSO4 and analyzed by an Agilent 61530A gas chromatograph. The reactants and products were further verified by GCMS analysis. 5.3.2. Palladium monitoring In the case of contemporaneous palladium monitoring in solution, the reaction was scaled up by the factor of 80 and conducted in a round bottom flask equipped with a reflux condenser and a septum. At suitable reaction times, aliquots of 9 mL reaction mixture were taken with a pre-heated syringe, of which 7 mL were filtered immediately by a syringe filter (0.2 mm PTFE membrane). The remaining 2 mL were further processed to determine the conversion and yield by gas liquid chromatography. To quantify the palladium content in the filtered fraction, all organic substances were removed by evaporation and decomposition at 450 °C followed by the treatment with peroxymonosulfuric acid. Palladium was subsequently digested by heating the sample with a mixture of concentrated sulfuric acid, concentrated nitric acid and concentrated hydrochloric acid. After diluting the samples to a proper extent, the palladium concentration was determined by ICP-MS (Agilent 7900). 5.3.3. Coupling of chlorobenzene and styrene with continuous sampling 37.5 mg PdMe(tmeda)/SiO2 [0.5 wt%Pd] (0.05 mol%Pd), 168.8 mg chlorobenzene (1.5 mmol), 208.3 mg styrene (2.0 mmol), 66.7 mg calcium hydroxide (0.9 mmol), 290.1 mg tetrabutylammonium bromide (0.9 mg), 200 mg diethylene glycol dibutyl ether and 5 mL N-Methyl-2-pyrrolidone were placed in a pressure tube equipped with a septum. The pressure tube was placed in a preheated oil bath at 150 °C. Samples of 0.1 mL were taken during the reaction, which were processed as described in the general procedure and analyzed by gas liquid chromatography. After a reaction time of 28 h, additional 168.8 mg chlorobenzene (1.5 mmol) and 208.3 mg styrene (2.0 mmol) were added to the reaction mixture.
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5.3.4. Catalyst recycling At the desired reaction time, the reaction was quenched by placing the pressure tubes in an ice bath. All solids were separated by centrifugation and washed twice with 10 mL dichloromethane. After the solids were dried at room temperature, they were inserted in a consecutive run following the general procedure described previously. Acknowledgements One of the authors (O.D.) thanks the Studienstiftung des deutschen Volkes e. V. for a scholarship. Two of the authors (O.D., C.G.) thank the TUM Graduate School for financial support. The authors would like to thank Ulrike Ammari, Petra Ankenbauer and Bircan Dilki from the microanalytical laboratory at Technical University of Munich for the conduct of the elemental analysis. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43]
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