Polymer-anchored peroxo compounds of molybdenum and tungsten as efficient and versatile catalysts for mild oxidative bromination

Polyhedron 52 (2013) 246–254

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Polymer-anchored peroxo compounds of molybdenum and tungsten as efficient and versatile catalysts for mild oxidative bromination Jeena Jyoti Boruah, Siva Prasad Das, Rupam Borah, Sandhya Rani Gogoi, Nashreen S. Islam ⇑ Department of Chemical Sciences, Tezpur University, Napaam, Tezpur 784028, Assam, India

a r t i c l e

i n f o

Article history: Available online 28 September 2012 We dedicate this paper to the memory of Professor Alfred Werner for his enormous contributions to the field of inorganic chemistry. Keywords: Polymer immobilized peroxometallates Molybdenum(VI) catalyst Tungsten(VI) catalyst Peroxidative bromination Bromoperoxidase activity Functional mimic

a b s t r a c t A polymer supported peroxomolybdate(VI) compound of the type [MoO2(O2)(CN)2]–PAN [PAN = poly(acrylonitrile)] (PANMo) was obtained by reacting H2MoO4 with 30% H2O2 and the macromolecular ligand, PAN at near neutral pH. The macrocomplex has been characterized by elemental analysis (CHN and EDX analysis), spectral (IR, UV–Vis and 13C NMR, 95Mo NMR), thermal (TGA-DTG) as well as SEM studies. The catalytic activity of PANMo and its previously reported tungsten containing analog PANW, in oxidative bromination of organic substrates has been explored. The supported complexes could serve as efficient heterogeneous catalysts for the oxidative bromination of a variety of structurally diverse aromatic compounds, with H2O2 as terminal oxidant, to afford bromo organics in impressive yields under environmentally clean conditions. The catalysts afforded regeneration and could be reused for a minimum of six reaction cycles. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Bromination of organic compounds has been receiving tremendous attention in recent years, mainly owing to the utility of bromo-organics as fine chemicals, therapeutic agents, agrochemicals and also as valuable intermediates for construction of chemically and biologically active molecules [1–3]. The halogenated organic compounds are in fact, being recognized as designer molecules for material science [4]. The traditional bromination protocols require elemental bromine and halogenated solvents which are toxic, corrosive and environmentally hazardous [4–14]. Alternative safer stoichiometric brominating agents such as, N-bromosuccinimide (NBS), N-bromoacetamide (NBA), or bromodimethylsulfonium bromide are although useful [11,15–18], however, high cost and organic waste generation associated with these reagents limit their synthetic utility. A few polymeric reagents were reported to be effective as halogenating agents under relatively milder condition, nevertheless, their preparation require specific polymer backbone and direct contact with bromine [19,20]. Therefore, search for alternative environmentally benign bromination protocols, which can mimic the biological bromoperoxidation for the synthesis of brominated organics continues unabated. The enzymes Vanadium Bromoperoxidases (V-BPO) catalyze bromination by using H2O2 and bromide salts instead of Br2 ⇑ Corresponding author. Tel.: +91 3712 267007 (Off.), +91 9435380222; fax: +91 3712 267006. E-mail addresses: [email protected], [email protected] (N.S. Islam). 0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.poly.2012.09.036

[14,21], leading to the biosynthesis of a variety of naturally occurring brominated products. These enzymes, present in several marine organisms, function explicitly in catalyzing rate determining bromide oxidation to generate an oxidized bromine species capable of transferring bromine atoms to acceptor molecules with electron rich p bonds [22]. Aqueous H2O2, recognized as an ideal clean and ‘‘green’’ oxidant [23], is capable of oxidizing bromide on its own in highly acidic medium (pH < 3), but is ineffective in solution at pH > 5.0 and often require to be activated by homogeneous or heterogeneous catalysts. This feature has provided impetus for the development of myriad useful catalysts or reagents including vanadium [12,24– 29], tungsten [11,30–32], molybdenum [31,33–36] and rhenium [37] based systems for oxidative bromination by H2O2. Nevertheless, scope for improvement still remains owing to some of the disadvantages associated with the available protocols. For instance, in contrast to natural V-BPO which is most efficient at pH 5.5–7 most of model complexes reported, were found to be catalytically active only in presence of acid [4,12,38–42]. These acids, although readily available and cheap, usually suffer from the drawbacks such as difficulty in separation from organic products and production of large volumes of hazardous wastes [13,43–45]. These disadvantages are becoming increasingly conspicuous in view of the growing ecological awareness in recent years [4]. During the past decade, our efforts have been directed towards developing functional mimics of haloperoxidases with an ability to mediate organic oxidations under mild conditions [46–53]. It is

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pertinent here to mention that we have generated a number of monomeric and dimeric peroxovanadium (pV) and peroxotungsten (pW) complexes [46–53] and a polymer-anchored pV compound [54], which exhibited good activity as bromide oxidant under mild condition viz., ambient temperature and neutral pH, considered as essential requirements of a biomimetic model [46,51–53]. Furthermore, we have developed a heterogeneous catalyst based on a poly(acrylonitrile) supported pW complex, for selective and mild oxidation of sulfides by hydrogen peroxide [55]. There has been continued interest in peroxo complexes of Mo and W, mainly due to their applications as versatile catalysts or oxidants in a variety of organic oxidations [23,56–62], including bromide oxidation [30,31,35,36]. However, a perusal of literature reveals that few groups have till date investigated the potential of polymer-immobilized peroxometallates as environmentally benign homogeneous or heterogeneous catalysts in organic oxidations [30,63–66]. There is also a dearth of information pertaining to well defined peroxo metallates anchored to polymer matrices in general, notwithstanding the advantages associated with converting selective homogeneous catalysts to heterogeneous polymer supported systems such as, enhancement of stability, easy work up of reaction mixture, and regeneration and reusability of such systems [67–69]. Formation of a robust polymer–metal linkage which can withstand repeated catalytic cycles is an essential requirement for regeneration of supported complex catalysts. Therefore, appropriate choice of ligand groups in the polymeric support is an important prerequisite in order to establish stable and easy attachment to metals [54,68,69]. As coordination chemistry plays a central role in metal–ligand interaction in systems ranging from simple inorganic complexes, catalysts, advanced materials to complex biomolecules, it is relevant in this context, to recall the tremendous contributions of Alfred Werner to the domain of modern inorganic chemistry and all other related fields. Inspired by the above findings, in the present work we have directed our efforts to explore the synthetic scope of polymeranchored pMo and pW complexes as heterogeneous catalysts in oxidative bromination of organic substrates under environmentally benign reaction conditions. We have recently gained an access to a set of pMo complexes supported on water soluble polymers which exhibited unique biochemical properties [70]. For the present investigation, since our focus was to obtain heterogeneous catalytic systems, poly(acrylonitrile) was chosen for immobilization of the pMo species due to its being an insoluble, non-toxic, cheap and commercially available reagent. Applications of acrylonitrile polymers in diverse areas such as, medicine [71], antioxidants [71], surface coatings [72], catalysis [73], textiles treatment [71], binders [71] and as adsorbant for removal of heavy metal ions from water [73,74] have been well documented. As far as we are aware, the compound PANW reported previously by us, is the only example where PAN has been used as polymeric support to obtain an immobilized peroxometal compound [55]. In the present study, synthesis of a new polymer-bound pMo compound was achieved by attaching pMo species to poly(acrylonitrile) matrix. The two compounds viz., PANW and PANMo, having similar macro ligand environment, enabled us to draw comparison on their efficiency as catalyst in the chosen reactions. 2. Experimental 2.1. Materials The sources of chemicals used, all reagent grade products, are given below: molybdic acid, 70% perchloric acid (E. Merck, Mumbai, India), acetone, hydrogen peroxide, acetonitrile, methanol, ethylacetate, petroleum ether (RANKEM), silica gel (60– 120 mesh), sodium hydroxide, sodium sulfate (E. Merck, India),

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phenol red (Merck, India Ltd.), KBr, K2HPO4 (SD Fine chemicals, India). Poly(acrylonitrile) (PAN) (Mw = 48 200), was obtained from Sigma–Aldrich Chemical Company, Milwaukee, USA. The compound, [WO2(O2)(CN)2]–PAN (PANW) was prepared by the method described in our earlier paper [55]. The water used for solution preparation was deionized and distilled. 2.2. Synthesis of [MoO2(O2)(CN)2]–PAN [PAN = poly(acrylonitrile)] (PANMo) Molybdic acid (3.05 g, 18.86 mmol) was dissolved in 30% H2O2 (20 mL, 176.40 mmol) by maintaining the temperature at 30– 40 °C, to obtain a clear solution of pH ca. 1. Subsequently, pH of the reaction solution was raised to 5.0, by the addition of concentrated sodium hydroxide solution (ca. 8 M) dropwise with constant stirring. Keeping the temperature of the reaction mixture below 4 °C in an ice bath, 1.0 g of poly(acrylonitrile) was added to it. The suspended polymer beads were allowed to swell in the reaction mixture under continuous stirring for 24 h. The white residue obtained was separated by decanting off the supernatant liquid and was repeatedly washed with pre-cooled acetone. The product was dried in vacuo over concentrated sulfuric acid. Anal. Calc.: C, 59.58; H, 4.96; N, 23.17; Mo, 7.33; O22 , 2.44. Found: C, 59.76; H, 4.58; N, 23.86; Mo, 7.38; O22 , 2.44%. The metal loading calculated from the observed molybdenum content is 0.77 mmol g 1 of polymer for [MoO2(O2)(CN)2]–PAN. 2.3. Elemental analysis The compounds were analyzed for C, H, and N by using an elemental analyzer Perkin-Elmer 2400 series II. Molybdenum content was estimated gravimetrically as molybdenum oxinate, MoO2(C9H6ON)2 [75]. Molybdenum, C, H, and N contents were also obtained from EDX analysis. Peroxide content for the compounds was determined by adding a weighed amount of the compound to a cold solution of 1.5% boric acid (W/V) in 0.7 M sulfuric acid (100 mL) and titration with standard cerium(IV) solution [76]. 2.4. Physical and spectroscopic measurements The IR spectra were recorded with samples as KBr pellets at ambient temperature, in a Nicolet model 410 FTIR spectrophotometer. Electronic spectral studies were carried out using a Cary 100 Bio, Varian spectrophotometer equipped with a peltier controlled constant temperature cell. The absorbance values are denoted as, e.g., A592, at the wave lengths indicated. Thermogravimetric analysis was done in SHIMADZU TGA-50 system using aluminum pan, at a heating rate of 10 °C/min under an atmosphere of nitrogen. The SEM characterization was carried out by using the JEOL JSM6390LV Scanning Electron Micrograph attached with an energydispersive X-ray detector. Scanning was done at 1–20 lm range and images were taken at a magnification of 15–20 kV. Data were obtained using INCA software. The EDX analysis was carried out focusing multiple regions over the surface of the polymer and the data are the averaging out of the data from these regions. The standardization of the data analysis is an integral part of SEM-EDX instrument employed. The 13C NMR spectra for PAN and PANMo were recorded in a JEOL JNM-ECS400 spectrometer at carbon frequency 100.5 MHz, 32 768 X-resolution points, number of scans 10 000, 1.04 s of acquisition time and 2.0 s of relaxation delay with 1H NMR decoupling method in (DMSO-d + DMF) (1:4). The 95Mo NMR spectra were recorded in JEOL JNM-ECS400 spectrometer at a molybdenum frequency of 26.07 MHz with sample in a 10 mm spinning tube with a sealed coaxial tube in (DMSO-d + DMF) (1:4), where DMSO-d provided the lock signal. The chemical shift data are recorded as negative values of ppm

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(d) in the low-frequency direction with reference to 1 M Na2MoO4
2H2O solution at 298 K. The 1H and 13C NMR of organic substrates and their corresponding brominated products were recorded in a JEOL JNM-ECS400 spectrometer (CDCl3 as solvent and TMS as an internal standard). Melting points were determined in open capillary tubes on a Büchi Melting Point B-540 apparatus and are uncorrected. HPLC analyses were performed using a Waters Tm 2487 dual k detector and assayed at fixed wavelengths using a C18, column (Nova-Pak C18, 3.9  150 mm, Waters).

consisting of peroxomolybdenum species, generated in situ by reacting H2MoO4 with H2O2 at pH 5.0. The strategically maintained pH of ca. 5 was found to be optimum for the formation of the peroxomolybdenum species and their anchoring to the pendant nitrile groups of PAN. The factors such as maintenance of required contact time of 24 h and temperature at <4 °C were also found to be important for the desired synthesis. The compound PANMo, like the W containing analog PANW, is stable, non-hygroscopic and can be stored for a prolonged period without any decrease in its catalytic efficiency.

2.5. Measurement of bromination activity in water 3.2. Characterization The method of de Boer et al. [77] of introducing four bromine atoms into the molecule of phenol red (e433 mM = 19.7) to form a bromophenol blue (e592 mM = 67.4) was used to measure bromination activity. To study the catalytic oxidative bromination by the pMo and pW compounds viz., PANMo and PANW, the compound (0.08 mmol metal) and H2O2 (16–80 mM) were added to the reaction mixture containing phosphate buffer (50 mM, pH 5.5), KBr (2 M) and phenol red (1.6 mM). The volume of the reaction mixture was kept at 25 mL. The redox activity was monitored by measuring the change in the absorbance at 592 nm at 30 °C by withdrawing required amount of aliquots from the reaction mixture and diluting it to 100 times at an interval of 5 min. 2.6. Bromination of organic substrates with H2O2 catalyzed by PANMo and PANW and product analysis Organic substrate (1.0 mmol) was added to a solution of acetonitrile:water (1:1, 5 mL) containing KBr (4.0 mmol) and 30% H2O2 (16.0 mmol) in a 50 mL two necked round-bottomed flask. A weighed amount of the solid compound viz., PANMo (0.129 g, 0.1 mmol Mo) or PANW (0.263 g, 0.1 mmol W) maintaining metal:substrate, substrate:Br and Br :H2O2 at 1:10, 1:4 and 1:4, respectively was then added to the reaction mixture at room temperature under continuous stirring. The progress of the reaction was monitored by TLC. After completion of the reaction the products as well as unreacted organic substrate were then extracted with diethyl ether and dried over anhydrous Na2SO4. Products were then separated by TLC and HPLC. 1H NMR spectroscopy and melting point determinations were made to interpret the products (see Supplementary data). 2.7. Regeneration Regeneration of the insoluble compounds, PANMo and PANW was achieved by separating the spent catalyst, after completion of the reaction, by centrifugation followed by washing with acetone and drying in vacuo over concentrated sulfuric acid. The catalyst was then placed into a fresh reaction mixture containing the substrate (1.0 mmol), KBr (4.0 mmol) and 30% H2O2 (16.0 mmol) in acetonitrile:water (1:1, 5 mL). The process was repeated up to 6th reaction cycle. In an alternative, yet equally effective, procedure adopted for regeneration of the catalysts, the spent reaction mixture remaining in the reaction vessel after separating the organic reaction product, was charged with fresh substrate, KBr, H2O2 and acetonitrile and then the experiment was repeated. 3. Results and discussion 3.1. Synthesis The synthesis of the immobilized compound PANMo was achieved by allowing the polymer to swell in a reaction mixture

The elemental analysis data for the compound PANMo indicated a ratio of Mo:O22 as 1:1. The molybdenum loading on the compound PANMo was found to be 0.77 mmol per gram of the polymeric support, which was calculated on the basis of Mo content, obtained from gravimetric analysis and confirmed by EDX spectral analysis. 3.2.1. SEM and energy dispersive X-ray (EDX) analysis Information regarding the change in particle size, as well as morphological changes occurring on the surface of the polymer after incorporation of the peroxomolybdates into the polymer matrix, was derived from scanning electron microscopic study. It was evident from the micrograph that the metal ions are distributed across the surface of the polymeric peroxometal compound, PANMo (Fig. 1.), leading to considerable roughening of its surface in contrast to the smooth and flat surface of the pristine polymer. Also, enhancement of the average particle size after incorporation of the pMo moieties into the polymer beads in comparison to the size of the PAN beads was noted. The results of energy dispersive X-ray spectral analysis on the composition of the compound, were in agreement with the elemental analysis values. Absence of sodium as counter ion in the compound was confirmed from the EDX spectrum. The observation is in accord with the charge neutrality of the nitrile bound monoperoxomolybdenum(VI) species. 3.2.2. IR spectral studies The polymer-anchored complex PANMo displayed characteristic spectral patterns in the infrared region involving absorptions due to m(Mo@O) and coordinated peroxide ligand as summarized in Table 1 and presented in Fig. 2. The intense absorption at ca. 960 cm 1 in the spectrum of PANMo was consistent with the presence of a terminally bonded Mo@O group [78,79]. The presence of side-on bound peroxo ligand in the compound was evident from the observance of the expected m(OAO), masym(MoAO2) and msym(MoAO2) modes in the range of ca. 860, 610 and 530 cm 1, respectively [78,79]. On the basis of available literature data on metal compounds with coordination environment comprising of nitrile ligand as well as poly(acrylonitrile) [80–82], empirical assignments could be derived for the IR bands observed for the compound PANMo. The IR spectrum of the neat poly(acrylonitrile), exhibits a strong m(C„N) absorption observed at 2247 cm 1, in addition to the typical bands at 2938 and 2870 cm 1 due to masym(CH2) and msym(CH2), respectively. It has been established previously that in simple N-bonded nitrile complexes there is usually an increase in m(C„N) upon coordination [80–82]. In the spectrum of PANMo, apart from the band at 2247 cm 1 for the free nitrile groups, a new medium intensity band was observed at 2367 cm 1. This latter band is attributable to a shift of m(C„N) to a higher frequency, resulting from coordination of the Mo(VI) ion with the nitrile groups of PAN. The IR spectrum of PANMo resembled closely the spectral pattern observed for PANW [55].

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249

Fig. 1. Scanning electron micrographs of (A) PAN and (B) PANMo. EDX spectra of (C) PANMo.

Table 1 Infrared spectral data for PANMo. PANMo

Assignments

531(m) 639(m) 864(m) 950(m)  2247(vs) 2367(m) 2870(m) 2938(m)

msym(MoAO2) masym(MoAO2) m(OAO) m(Mo@O) m(C„N) msym(CH2) masym(CH2)

3.2.3. 13C NMR studies The coordination induced 13C NMR chemical shift has been widely utilized as a convenient tool in understanding the mode of coordination of the co-ligands in peroxo metal compounds [70,83–88]. The 13C NMR spectra of the pristine PAN and the compound PANMo are presented in Fig. 3. The assignments of major peaks were made on the basis of available reports [89]. The 13C NMR spectrum of the pure poly(acrylonitrile) exhibits, in addition to the peak due to pendant nitrile groups at 120.21 ppm, the characteristic signals corresponding to chain carbon atoms at 33.35 (for CH2) and 27.97 (for CH) ppm. The spectrum of PANMo on the other hand, apart from the signal at 120.28 ppm owing to free nitrile group of the polymer, showed a new signal at 129.02 ppm which may be ascribed to molybdenum bound nitrile group. On coordination to a metal, the nitrile carbon resonance has been known to undergo a downfield shift [90]. The spectrum thus provided clear evidence for the existence of both coordinated as well as free nitrile groups in PANMo. Presence of strong metal–ligand interaction was indicated by the significant downfield shift, Dd (dcomplexed dfree nitrile)  8 ppm in the metal anchored compound relanitrile tive to the free nitrile peak of the pristine polymer. The 13C NMR spectral information also provided evidence, inter-alia, in support of the stability of the compound in solution.

Fig. 2. FTIR spectra of (A) PAN and (B) PANMo.

3.2.4. TGA-DTG analysis That the compound PANMo gradually undergoes multistage decomposition on heating up to a final temperature of 750 °C, was evident from the TG-DTG plots presented in Fig. 4. It is significant to note that the compound remains stable upto a temperature of 112 °C. Moreover, unlike some monomeric peroxomolybdenum compound [91] or the neat polymer (PAN), PANMo does not explode on heating [92]. A close analogy was noted in the

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Fig. 5.

95

Mo NMR spectra of PANMo in DMSO-d + DMF (1:4).

decomposition process on heating the compound up to a final temperature of 750 °C was recorded to be 39.69%. Thermogravimetric analysis data of the compound was thus consistent with its composition and formula assigned.

Fig. 3.

13

C NMR of (A) PAN and (B) PANMo in DMSO-d + DMF (1:4).

thermal degradation pattern of PANMo and the previously reported PANW [55]. The first decomposition stage in PANMo, has been observed in the temperature range of 112–157 °C, with the corresponding weight loss of 2.14%, consistent with the total loss of coordinated peroxo groups from the complexes. IR spectrum of the decomposition product, isolated at this stage, showed the complete absence of peroxide in it. The next decomposition step, appearing as a single peak in DTG, between 306 and 359 °C with a weight loss of 13.44%. It has been reported previously that the degradation of PAN below 400 °C is accompanied by elimination of HCN, NH3 and H2O and concomitant intramolecular polymerization of nitrile groups to form conjugated polyamine (AC@N)n and that the loss of nitrogen commences at 750 °C [93]. In the present study, the residue obtained at 359 °C was subjected to IR spectral analysis. The IR spectrum displayed bands at 1578 and 1625 cm 1 typical of m(C@C) and m(C@N) stretching in addition to m(Mo@O) at 950 cm 1. The observations are in accord with the previous findings on thermal degradation pattern of PAN [92], as well as PANW [55]. The degradation further continued up to 750 °C. The total weight loss which occurred during the course of the overall

3.2.5. 95Mo NMR studies The 95Mo NMR analysis has been used as a sensitive and useful technique for study of the structure of molybdenum peroxo complexes in solution. The spectrum of PANMo displayed (Fig. 5.) a lone resonance at 114 ppm (relative to [MoO4]2 ), characteristic of monoperoxomolybdate species [94], in agreement with the formula assigned to the compound. The appearance of a single characteristic peak in the 95Mo NMR spectrum of the compound confirmed the presence of a single coordination environment for the peroxomolybdenum species present in solution. On the basis of the above results, the proposed structure of PAN anchored pMo complex, PANMo is shown schematically in Fig. 6, that includes a dioxomonoperoxo molybdenum(VI) moiety bonded to the polymer matrix via the N atom of the pendant nitrile group. 3.3. Catalytic activity of the supported complexes, PANMo and PANW 3.3.1. Activity in H2O2 induced bromination of organic substrates Catalytic performances of the immobilized complexes PANMo, as well as PANW as catalysts in oxidative bromination of a series of aromatic compounds have been investigated. Based on the results of trial runs, the reaction conditions were optimized by using aniline as the model substrate and PANMo as catalyst (Table 2). The reactions were conducted at room temperature (RT) under magnetic stirring. The reaction of the substrate with bromide (2 equivalents) in presence of the terminal oxidant H2O2 (2 equivalents) and Mo:substrate molar ratio maintained at 1:10, proceeded smoothly in CH3CN:H2O (1:1) with 100% conversion of the substrate (Table 2, entry 1) to afford a mixture of ortho- and para-bromo aniline, with the p-substituted aniline as the predominant product. Enhancement of the rate of conversion was observed on increasing the substrate:bromide ratio to 1:4 leading to an increase

CH

H2 C

CH

C

C O

N O

H2 C

N

Mo O O

Fig. 4. TGA-DTG plot of PANMo.

Fig. 6. Proposed structure of PANMo (‘‘

’’ represents polymer chain).

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J.J. Boruah et al. / Polyhedron 52 (2013) 246–254 Table 2 Optimization of reaction conditions for PANMoa catalyzed oxidation of bromide by H2O2.

NH2

NH2

NH2 Br

PANMo

+

CH3CN/H2O(1:1), 30% H2O2, Br

Br

Entry

Molar ratio Mo:S

S:Br

Br:H2O2

1 2 3 4 5 6 7 8 9 10 11

1:10 1:10 1:10 1:10 1:5 1:20 1:10 1:10 1:10 – –

1:2 1:4 1:4 1:4 1:4 1:4 1:4 1:4b 1:4c 1:4d 1:4e

1:1 1:1 1:2 1:4 1:4 1:4 1:4 1:4 1:4 1:4 1:4

Temperature

Time (min)

Isolated yield (%)

TOF (h

RT RT RT RT RT RT 78 °C RT RT RT RT

40 35 30 25 20 60 20 55 15 30 30

96 95 96 97 95 96 98 95 96 <5 34

14.40 16.28 19.20 23.27 14.24 18.60 29.40 10.36 38.40 – –

1

)

a All the reactions were carried out using 0.129 g catalyst (0.1 mmol Mo), aniline as substrate, KBr as bromide source and CH3CN/H2O (1:1, 5 mL) as solvent unless otherwise stated. b Et4NBr used as bromide source. c Using 4.0 mmol HClO4. d Control reaction: aniline (1.0 mmol), KBr (4.0 mmol), H2O2 (16.0 mmol) and CH3CN/H2O (1:1, 5 mL). e Control reaction with HClO4: aniline (1.0 mmol), KBr (4.0 mmol), H2O2 (16.0 mmol), HClO4 (4.0 mmol) and CH3CN/H2O (1:1, 5 mL).

of TOF to 16.28 h 1 (Table 2, entry 2). The TOF could further be improved by increasing the amount of H2O2 to 4 equivalents with respect to bromide (Table 2, entry 4). However, it was observed that increase in molar ratio of Br–:H2O2 beyond 1:4 had no observable effect on rate of reaction. The effect of catalyst amount has also been evaluated. A 2-fold increase in the amount of the catalyst although slightly speeded up the reaction, the corresponding TOF was observed to decrease (Table 2, entry 5). On the other hand, decreasing the amount of catalyst reduced the rate, as well as TOF of reaction (Table 2, entry 6). It is noteworthy that on increasing the reaction temperature up to 78 °C (Table 2, entry 7), a significant increase in TOF was noted. The rate of the reaction was however, found to be substantially reduced when Et4NBr was used as source of bromide instead of KBr (Table 2, entry 8). Importantly, the TOF was found to be nearly double when the reaction was conducted in presence of perchloric acid, without altering the other parameters (Table 2, entry 9). That the pMo or pW catalyst plays a crucial role in facilitating the desired reactions was apparent from a control experiment conducted in absence of the catalyst. Very little conversion of aniline was recorded (<5%) under the optimized condition in absence of the catalyst (Table 2, entry 10). When the control reaction was carried out in presence of perchloric acid, a relatively higher yield was obtained (34%) (Table 2, entry 11), however, the reaction remained incomplete even after 1 hour. In order to explore the scope of the methodology, a series of structurally diverse activated aromatics was subjected to the oxidative bromination by H2O2–Br system into their corresponding bromo-organics under the optimized reaction conditions. The data presented in Table 3 show that both the compounds, PANMo as well as PANW, efficiently catalyzed the bromoperoxidation of the chosen substrates to the corresponding brominated products in impressive yield. Preferential bromination at either ortho or para position of the aromatic ring in each of the substrates, leading to mono substitution, indicates an electrophilic bromination mechanism. From the findings (Table 3) it is evident that the immobilized compound PANW exhibit somewhat superior catalytic activity over the Mo containing analog in terms of TOF.

The remarkable feature of the present procedure is that the reaction takes place under mild condition at near neutral pH or the natural pH of the reaction mixture, and no extra addition of acid or alkali is required for the bromination reaction. Although a significant increase in TOF of the reaction was observed when conducted in presence of perchloric acid at a highly acidic pH (Table 2, entry 9), in the present study, we have strategically maintained a mild reaction condition by avoiding addition of acid or other additives, as far as possible. 3.3.2. Test for heterogeneity of the reaction In order to confirm the heterogeneous nature of the insoluble catalysts and to examine the leaching of the metal complexes from the polymer-bound catalysts into the reaction medium during the oxidation reactions, separate experiments were carried out using aniline as the substrate and PANMo as catalyst. The filtrate obtained by separating the solid catalyst after completion of the reaction was transferred to a reaction vessel and the reaction was allowed to continue for another 5 h, by adding fresh aniline, KBr and H2O2. The reaction afforded product yield of <5%, in line with the conversion obtained in absence of any catalyst. It has thus been confirmed that the reaction did not proceed on the removal of the solid catalyst. The results are in accord with the occurrence of a purely heterogeneous catalytic process [64,95] and further indicate the absence of catalyst leaching. 3.3.3. Regeneration of the reagents The heterogeneous catalyst, PANMo could be recovered and recycled without further conditioning, after separating it from the spent reaction mixture after completion of each cycle of bromination, by charging it with H2O2, a fresh lot of substrate and bromide. It has been found that the catalyst remains effective upto a minimum of six reaction cycles without further treatment with any reagent (Table 3, entries 9c, 10c and 12c). However, it is notable that, the activity of the catalyst decreased gradually in case of highly basic substrates like aniline and nitroaniline. This may be explained on the basis of the fact that nitrile groups of PAN are susceptible to undergo hydrolysis to form amide at pH > 7.5 [96,97].

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Table 3 Bromination of organic substrates with 30% H2O2 catalyzed by PANMo or PANW.a Entry

Substrate

Product

1

Aniline

2 3

p-Aminophenol m-Aminophenol

4 5

o-Aminophenol p-Nitroaniline

6

m-Nitroaniline

7

o-Nitroaniline

8 9

Quinol Pyrogallol

4-Bromoaniline 2-Bromoaniline 4-Amino-3-bromophenol 5-Amino-2-bromophenol 3-Amino-4-bromophenol 2-Amino-5-bromophenol 2-Bromo-4-nitroaniline 2,6-Bibromo-4-nitroaniline 2-Bromo-5-nitroaniline 4-Bromo-3-nitroaniline 4-Bromo-2-nitroaniline 2-Bromo-6-nitroaniline 2-Bromobenzene-1,4-diol 4-Bromobenzene-1,2,3-triol

10

Resorcinol

4-Bromobenzene-1,3-diol

11 12

Acetanilide Salicylaldehyde

N-(4-Bromophenyl)acetamide 5-Bromo-2-hydroxybenzaldehyde

13 14

o-Methoxytoluene Catechol

1-Methoxy-2-methyl-4-bromobenzene 4-Bromo-1,2-dihydroxybenzene

PANMo

PANW b

Time (h)

Yield (%)

TOFb (h

23.27

0.16

61.87

12.66 14.39

0.50 0.41

12.93 9.70

0.50 0.83

11.80

0.75

9.60

0.83

4.85 7.29 7.21 2.71 2.68 6.46 3.88 3.80 8.18 4.70

1.50 1.00 1.00 3.00 3.00 1.33 2.00 2.00 0.83 1.67

87 12 97 73 25 96 62 34 73 25 82 15 96 95 93c 96 94c 96 97 96c 95 96

Time (h)

Yield (%)

TOF (h

0.41

83 14 95 75 20 97 84 13 83 15 85 11 97 97 96c 95 94c 97 97 95c 95 94

0.75 0.66 0.75 1.00 0.83 1.00 2.00 1.33 1.33 3.50 3.50 1.50 2.50 2.50 1.16 2.00

1

)

1

)

19.40 23.90 19.20 11.56 13.06 11.68 6.40 9.50 9.30 3.20 3.13 7.21 4.85 4.80 11.44 5.74

a Reaction conditions: substrate(1.0 mmol), KBr(4.0 mmol), 30% H2O2(16.0 mmol) and PANMo (0.129 g, 0.1 mmol Mo) or PANW (0.263 g, 0.1 mmol W) in CH3CN/H2O (1:1, 5 mL) at RT. b TOF (turnover frequency) = mmol of product per mmol of metal per hour. c Yield of 6th reaction cycle.

The resulting structural change in the polymer pendant functional groups of the catalyst, is likely to lead to leaching of the pMo moieties causing a fall in its catalytic activity.

C

SO3H

KBr, H2O2 PANMo or PANW

Br

Br

O Br

C

OH

Br SO3H

592

1.2 1.0

Absorbance

3.3.4. Immobilized complexes as catalysts in H2O2 mediated bromination in water Water being the prospective environment friendly solvent for ‘green’ chemistry purposes, we were particularly interested to examine the catalytic activity of the complexes in water. The method of de Boer et al. [77] was used to assess the bromination activity of the compounds in water, in line with our earlier work on peroxovanadium and peroxotungsten mediated bromination reactions [46,51,52,54,98]. Phenol red undergoes facile bromination reaction, which can be monitored conveniently using electronic spectroscopy (Figs. 7 and 8). The compounds efficiently catalyzed the oxidation of phenol red to bromophenol blue in conjunction with H2O2 as the terminal oxidant. The yellow color of the standard reaction solution containing phenol red and bromide in phosphate buffer gradually changed to blue on addition of the solid compound PANMo or PANW at concentrations indicated in Table 4. The spectrum recorded showed a decrease in absorbance of the peak at A433 and a new peak at A592 characteristic of the product bromophenol blue (Fig. 7). In this case also, PANW displayed superior activity compared to PANMo. A TOF of 28.23 h 1 was recorded in a reaction conducted by maintaining molar ratio of Mo:PR at 1:20 and PR:H2O2 at 1:20 (Table 4). It was found that with increase in concentration of H2O2, rate of reaction increases. Thus the TOF could be improved to 60 h 1 by using a 50-fold excess of H2O2 (Table 4). The catalysts could be regenerated in situ and recycled after completion of each cycle of bromination by adding a fresh lot of phenol red, bromide and H2O2. It is noteworthy that, a reaction conducted under identical condition without the substrate phenol red, displayed a peak at 262 nm with a shoulder at 237 nm on addition of solution of the compound PANW or PANMo. Interestingly, addition of phenol

OH

O

0.8 0.6 433 0.4 0.2 0 350

400

450

500

550

600

650

Wavelength (nm) Fig. 7. PANMo catalyzed bromination with H2O2 in water. The reaction mixture contained phosphate buffer (0.05 M, pH 5.5), KBr (2 M), phenol red (1.6 mM), PANMo (0.08 mM Mo) and H2O2 (80 mM). Absorbance was recorded by withdrawing required amount of aliquots from the reaction mixture and diluting it to 100 times at an interval of 5 min.

red to this solution led to the formation of bromophenol blue, as indicated by the appearance of peak at 592 nm and concomitant decrease in A262 nm. It is thus inferred that the 262 nm peak corresponds to an oxidized species of bromide, active as brominating agent, probably an equilibrium mixture of BrOH, Br2 and Br3 as proposed earlier [22,31,34,99,100].

3.3.5. The proposed mechanism A scheme of reactions, shown in Fig. 9 using PANMo as a representative, is proposed which is based on our results and work of some other laboratories [31,34,99–103]. The first step is likely to be the formation of diperoxomolybdate species II from the monoperoxomolybdate species I, in presence of H2O2 (reaction a).

J.J. Boruah et al. / Polyhedron 52 (2013) 246–254

1.4 1.2

(b)

Absorbance

1.0

(a)

0.8 0.6 0.4 0.2

PANMo PANW PAN

0 0

10

20

30

40 50 Time (min)

60

70

80

90

Fig. 8. Rate of bromination with (a) PANMo + H2O2, (b) PANW + H2O2. PANMo, PANW and PAN were ineffective in bromination. Absorbance were recorded at 592 nm. The reaction mixture contained phosphate buffer (0.05 M, pH 5.5), KBr (2 M), phenol red (1.6 mM), PANMo (0.08 mM Mo) or PANW (0.08 mM W) and H2O2 (80 mM). Absorbance was recorded by withdrawing required amount of aliquots from the reaction mixture and diluting it to 100 times at an interval of 5 min.

Compound

Concentration (mM of metal)

Phenol red (mM)

TOF (h

PANMo

0.08 0.08 0.08 0.08

1.6 1.6 1.6 1.6

28.23 60.00b 34.28 68.57b

1

)

a All reactions were carried out with phosphate buffer (0.05 M, pH 5.5), KBr (2 M), phenol red (1.6 mM), compound (0.08 mM metal) and 32 mM H2O2 unless otherwise indicated. b Using 80 mM H2O2.

H2 C CH

ABr

CH C

C N

[c]

(OBr

N

Mo

AH -

O O

' Br+' Br2 Br3-)

O O

H2O2

I [b]

[a]

H2 C

Br -

C N

Acknowledgments

C

O

Anchoring of pMo moiety, in its monoperoxodioxo form, to the pendant nitrile groups of poly(acrylonitrile) afforded a stable and well-defined supported complex of molybdenum, PANMo. The compound PANMo and the previously reported W(VI) containing analog PANW were tested for their activity in organic bromination. The macromolecular complexes heterogeneously catalyzed the oxidative bromination of a variety of aromatic substrates by H2O2 in high yields under environmentally acceptable reaction conditions. The attractive features associated with the methodology include: (i) use of harmless bromide salt instead of bromine as source of bromide; (ii) avoidance of acid, halogenated solvent or any other additive; (iii) easy regeneration and reusability of the catalyst, and (iv) the reactions occur at ambient temperature. The developed protocol thus conforms to several guiding principles of ‘‘green’’ chemistry. It may therefore be hoped that the supported complexes, PANMo and PANW may be useful additions to the range of bioinspired catalysts for organic oxidations. It is pertinent to mention that efficiency of PANW as effective catalyst for mild and selective oxidation of organic sulfides has already been established [55]. The work on other applications of the complexes is now underway in our laboratory.

H2O

CH

CH

tion of bromo-organics. Subsequently, a catalytic cycle is generated when species I combines with peroxide in the presence of excess H2O2 to afford the diperoxomolybdate complex II (reaction a). Although further studies are necessary to establish the identity of the reactive diperoxo metal species II, the proposed reaction pattern is in agreement with our previous findings [46,47] as well as observations of others that for a peroxomolybdate or peroxotungsten complex to be active in oxidation an oxo-diperoxo configuration may be a principal requirement [34,46,55,104]. Involvement of monoperoxo Mo(VI) or W(VI) as intermediate has been previously proposed by Reynolds et al. [34] as well as by Butler and co-workers [31] in the Mo(VI) and W(VI) catalyzed bromide oxidations. The aforementioned considerations lend further credence to the proposed scheme. It is reasonable to expect that bromide would attack an edge-bound peroxo group in preference to a hepta coordinated molybdenum or tungsten centre as observed in some other redox processes involving peroxo compounds of W(VI) and Mo(VI) [32,104,105]. However, in absence of any direct evidence we refrain from drawing any conclusion regarding the exact nature of such an intermediate. 4. Conclusions

Table 4 Bromination of phenol red with 30% H2O2 catalyzed by PANMo or PANW.a

PANW

253

N

O Mo O O O II Fig. 9. The proposed mechanism. (a) In presence of excess H2O2 the polymer-bound monoperoxo species I combines with peroxide to form the reactive diperoxomolybdate intermediate II; (b) reaction of the diperoxo intermediate II with bromide to yield oxidized bromine with concomitant regeneration of the original catalyst; (c) transfer of bromine from the active species to acceptor AH. The monoperoxo species I formed combines with H2O2 to regenerate the active diperoxomolybdate intermediate II giving rise to a catalytic cycle. No attempt is made to show the exact stoichiometry of the reaction.

The reaction of the diperoxomolybdenum species II with bromide yield an oxidized bromine species, with concomitant regeneration of the original monoperoxomolybdate species I (reaction b). Transfer of the bromine atom to the substrate AH from the oxidized bromine intermediate, likely to be an equilibrium mixture of BrOH, Br2 and Br3 , takes place (reaction c) leading to the forma-

Financial support from the Department of Science and Technology, New Delhi, India, is gratefully acknowledged. We are also grateful to the Council of Scientific and Industrial Research (CSIR), New Delhi, India for providing Research Fellowship to J.J.B. (SRF) as well as University Grants Commission (UGC), New Delhi, India for providing Research Fellowship to S.P.D. (RGNSRF). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.poly.2012.09.036. References [1] A. Butler, J.V. Walker, Chem. Rev. 93 (1993) 1937. [2] I. Cabanal-Duvillard, J.-F. Berrien, J. Royer, H.-P. Husson, Tetrahedron Lett. 39 (1998) 5181. [3] D. Wischang, O. Brucher, J. Hartung, Coord. Chem. Rev. 255 (2011) 2204. [4] A. Podgorsek, M. Zupan, J. Iskra, Angew. Chem., Int. Ed. 48 (2009) 8424. [5] K. Kikushima, T. Moriuchi, T. Hirao, Tetrahedron 66 (2010) 6906.

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