ARTICLE pubs.acs.org/Langmuir
MCM-41-Supported Oxo-vanadium(IV) Complex: A Highly Selective Heterogeneous Catalyst for the Bromination of Hydroxy Aromatic Compounds in Water Susmita Bhunia, Debraj Saha, and Subratanath Koner* Department of Chemistry, Jadavpur University, Kolkata 700 032, India
bS Supporting Information ABSTRACT: An ecofriendly solid catalyst has been synthesized by anchoring vanadium(IV) into organically modified MCM-41. First, the surface of Si-MCM-41 was modified with 3-aminopropyl-triethoxysilane (3-APTES), the amine group of which upon condensation with orthohydroxy-acetophenone affords a N2O2-type Schiff base moiety in the mesoporous matrix. The Schiff base moieties were used to anchor oxovanadium(IV) ions. The prepared catalyst has been characterized by UVvis, IR spectroscopy, small-angle X-ray diffraction (SAX), nitrogen sorption, and transmission electron microscopy (TEM) studies. It is observed that the mesostructure has not been destroyed in the multistep synthesis procedure, as evidenced by SAX and TEM measurements. The catalyst has shown unprecedented high conversion as well as para selectivity toward the bromination of hydroxy aromatic compounds using aqueous 30% H2O2/KBr in water. The reaction proceeds according to the stoichiometric ratio, and the monobrominated product was obtained as the major product using a stoichiometric amount of the bromine source. The immobilized complex does not leach or decompose during the catalytic reactions, showing practical advantages over the free metal complex.
1. INTRODUCTION Over the years, the peroxidative bromination reaction has been an ongoing area of research for chemists because bromoaromatics are versatile intermediates in the manufacture of pharmaceuticals, agrochemicals, and other specialty chemical products. Flame retardants, disinfectants, and antibacterial and antiviral drugs also involve bromination.1 Bromophenols, in particular, are important to the synthesis of flavor compounds.2,3 A number of methods of bromination of aromatic compounds have been reported in previous studies.411 The traditional procedure for direct bromination is the use of elemental bromine, a pollutant and health hazard for which half of the bromine ends up as hydrogen bromide waste (eq 1). ArH þ Br2 f ArBr þ HBr
ð1Þ
Therefore, for large-scale operations it creates an enormous environmental and economical problem. The reoxidation of HBr (e.g., with H2O2) can partially solve this problem because highly toxic, corrosive HBr increases the reactor costs that exceed the cost of purchasing more Br2. Again, the transportation and storage of large quantities of molecular bromine or HBr are extremely hazardous. Marine enzyme haloperoxidase catalyzes the two-electron oxidation of X (X = Cl, Br, or I) in the presence of aqueous hydrogen peroxide under seawater conditions.12 However, a large-scale enzymatic reaction is problematic.13 In view of this, r 2011 American Chemical Society
there has been an upsurge in the design of bromination protocols based on bromide salt oxidation with hydrogen peroxide similar to the haloperoxidase enzyme. For a metal halideH2O2 system, it requires a stoichiometric amount of mineral acids,14 and it is believed that the bromination reaction proceeds via the formation of hypobromous acid, which is more unstable because of its ionic nature and thus more reactive toward the aromatic nucleus. Many vanadium complexes are employed in the oxidative halogenation reaction1518 in the homogeneous phase, but there is one serious problem in the recovery of products from the reaction mixture as they undergo decomposition during the catalytic reactions. To avoid this problem, considerable attention has been paid to the heterogeneous catalytic process because it allows the production and ready separation of a large quantity of the desired product with the use of a small amount of recyclable catalyst. Different approaches such as encapsulation or the immobilization of a catalytically active metal complex in a solid support such as a zeolite19 and the covalent grafting of such an active complex onto reactive polymer surfaces20 or inorganic porous matrices21 have been used to develop the efficient heterogeneous catalyst. Ordered mesoporous silica, for example, MCM-4122 with a high surface area, high thermal stability, and Received: June 4, 2011 Revised: October 24, 2011 Published: October 25, 2011 15322
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Scheme 1. (a) Modification of the Si-MCM-41 Channel Wall with Aptes/Chloroform, (b) Condensation with ortho-HydroxyAcetophenone in Methanol, and (c) Metal Complex Formation of VOSO4/WaterMethanol
attractive pore structure, is the natural choice to use as the matrix. Vanadium species could be functionalized into the mesoporous matrix either by direct hydrothermal synthesis or by an impregnation method.2325 In this study, we report the simple, low cost, highly para selective MCM-41 supported oxo-vanadium(IV) complex-catalyzed bromination of hydroxyaromatic compounds in water at room temperature. Herein bromides ions are used as halogenating agents, and aqueous 30% H2O2 is used as the oxidant; thus this method may be considered to be a functional model for enzyme vanadate-dependent haloperoxidase (V-HalPO)17 and offers a green bromination option compared to the classical electrophilic substitutions.
2. EXPERIMENTAL SECTION 2.1. Materials. Cationic surfactant cetyltrimethylammonium bromide (CTAB, 98%), tetraethyl ortho-silicate (TEOS, 98%), oxovanadium(IV) sulfate (VOSO4 3 5H2O), phenol red, salicylaldehyde, ortho-hydroxy-acetophenone, phenol, para-chloro phenol, toluene, benzaldehyde, and hydrogen peroxide (30% aqueous) were purchased either from Sigma-Aldrich or Spectrochem (India) and were used as received without further purification. The solvents were purchased from Merck (India) and were distilled and dried before use. 2.2. Catalyst Preparation. Mesoporous Si-MCM-41 was synthesized according to the reported procedure26 by the hydrolysis of structure-directing agent CTAB (cetyltrimethylammonium bromide) and TEOS as the silica source in basic solution with a reactant molar composition of 1.0:7.5:1.8:500 CTAB/TEOS/NaOH/H2O. The gel mixture was then hydrothermally held at 110 °C for 60 h in a Teflonlined autoclave. After being cooled to room temperature, the resultant solid was recovered by filtration, washed with deionized water, and dried in air. The collected product was calcined at 550 °C for 12 h to remove the occluded polymeric surfactants. This mesoporous material is designated as Si-MCM-41.
Postsynthesis organic modification of the mesoporous material was performed by stirring 0.1 g of Si-MCM-41 with 0.18 g (0.81 mmol) of 3-APTES in 10 mL of dry chloroform for 12 h under a nitrogen atmosphere. The resulting material, MCM-41-(SiCH2CH2CH2NH2)x, was vacuum-filtered, washed with dry chloroform, and dried under vacuum. The white solid was then refluxed with ortho-hydroxy-acetophenone (8.18 mmol, 1 g) dissolved in 10 mL of methanol for 5 h at 60 °C. The resulting yellowish solid was filtered, washed with methanol, and dried in a desiccator. Finally, catalyst V-MCM-41 was prepared by refluxing the above-mentioned yellowish solid in 10 mL of methanol with VOSO4 3 5H2O (0.405 mmol, 0.10 g) dissolved in 2 mL of water at 60 °C for 12 h. The resulting light-greenish solid was recovered by vacuum filtration, washed with methanol using Soxhlet for 12 h to remove any unreacted vanadyl species, and dried under vacuum (Scheme 1). The atomic absorption spectrometric result showed the vanadium content of the catalyst to be ca. 0.65 wt %. The elemental analysis yields molar ratios of N/V ≈ 2.4 and for C/V ≈ 24.3, representing the fact that oxo-vanadium(IV) ions are in a N2O3 ligand environment as shown in Scheme 1. 2.3. Catalyst Characterization. Fourier transform infrared (FTIR) spectra of KBr pellets were obtained on a Perkin-Elmer RX1 FTIR spectrometer. Electronic spectra were measured on a Shimadzu CP-3101 UVvis spectrophotometer. The vanadium content of the sample was estimated on a Perkin-Elmer A-Analyst 200 atomic absorption spectrometer. Elemental analysis, for carbon, hydrogen, and nitrogen (CHN), was undertaken on a Perkin-Elmer 240C elemental analyzer. The powder X-ray diffraction (XRD) patterns of the samples were recorded with a Scintag XDS-2000 diffractometer using Cu Kα radiation. A surface area measurement was carried out by nitrogen adsorption at 77 K using a Quantachrome Autosorb iQ surface area analyzer. Prior to sorption experiments, samples were outgassed at 80 °C under vacuum until a final pressure of 102 Torr was reached. Transmission electron microscopy (TEM) images were recorded with a transmission electron microscope (FEI, model STWIN) operating at an accelerating voltage of 200 kV. The samples were ground and sonicated in isopropanol and dispersed over carbon-coated copper grids. The products of the catalytic reactions were identified and quantified by 15323
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Figure 1. Small-angle XRD patterns of (a) Si-MCM-41, (b) MCM41-(SiCH 2 CH2 CH 2NH 2)x , and (c) V-MCM-41. using a Varian CP-3800 gas chromatograph equipped with an FID detector. 1H NMR spectra were obtained on a Bruker Avance DPX 300 NMR spectrometer using TMS as the internal standard. Other instruments used in this study were the same as reported earlier.27,28
2.4. General Procedure for the Bromination of Hydroxy Aromatic Compounds. We have followed this general procedure for all catalytic reactions. Catalytic reactions were carried out in a 50 mL reaction flask. In a typical reaction, 5 mmol of the substrate was added to 5 mL of H2O containing 5 mmol of KBr. After the mixture was stirred for 1 min at room temperature, 30 mmol of 30% H2O2 was added dropwise using a pressure-equalizing dropping funnel under stirring. Catalyst, V-MCM-41 (25 mg) followed by 70% HClO4 (2 mmol) was added to the above mixture when the reaction began. The progress of the reaction was monitored by thin layer chromatography (TLC). After the disappearance or no change in the starting material on TLC plates, the catalyst was filtered and the reaction contents were subjected to multiple ether extractions. The combined filtrates were washed with saturated sodium bicarbonate solution. The organic extract was dried over anhydrous sodium sulfate, and the solvent was evaporated under reduced pressure. The crude product was then purified by crystallization or by column chromatography over silica gel (60120 mesh). The purities of the products were confirmed by 1H NMR spectra and quantified by gas chromatography.
3. RESULTS AND DISCUSSION 3.1. XRD Studies. The small-angle X-ray diffraction patterns of Si-MCM-41, MCM-41-(SiCH2CH2CH2NH2)x, and catalyst V-MCM-41 are shown in Figure 1. Pristine Si-MCM-41 shows a typical three-peak pattern,22a,b a very strong reflection at 2θ ≈ 2.3° with d100 = 38.36 Å, and additional peaks with low intensities at 2θ ≈ 4.1 and 4.75° for d110 and d200, respectively, for the quasiregular arrangement of mesopores with hexagonal symmetry. A comparison of the X-ray powder diffraction patterns of Si-MCM41, MCM-41-(SiCH2CH2CH2NH2)x, and V-MCM-41 shows that the typical three-peak pattern of MCM-41 has been retained after organofunctionalization as well as metal complex formation in Si-MCM-41. However, diffraction lines were shifted to higher angles upon organic modification of MCM-41 and subsequently vanadium incorporation into modified mesoporous silica MCM-41. A similar type of behavior was observed by Burkett et al. in
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Figure 2. IR spectra of (a) Si-MCM-41, (b) MCM-41-(SiCH2CH2CH2NH2)x, and (c) V-MCM-41.
phenyl-modified mesoporous sieves29 and by Lim et al. for directly synthesized thiol-MCM-41.30 After postsynthesis grafting, an overall decrease in the intensity of the diffraction lines was noticed. This result could be attributed to the lowering of local order, such as variations in the wall thickness, or might be due to the reduction of scattering contrast between the channel wall of the silicate framework and the vanadium complex present in V-MCM-41, as previously mentioned by Lim et al.30 Marler et al. have reported that the intensity of the diffraction lines decreases systematically on increases in the concentration of organic sorbates in boron-containing MCM-41.31 Therefore, the shift in the diffraction lines to the higher angle and the decrease in the intensity of the peaks upon organic modification of Si-MCM-41 as well as complex formation with vanadium(IV) are not inconsistent. 3.2. Spectroscopic Measurements. The FTIR spectra of SiMCM-41, MCM-41-(SiCH2CH2CH2NH2)x, and catalyst V-MCM41 are shown in Figure 2. The stretching vibrational band for SiO Si of Si-MCM-41, MCM-41-(SiCH2CH2CH2NH2)x, and V-MCM41 samples appeared at around 1050 cm1, indicating that the silica framework remain unaffected upon modification. The IR spectrum of MCM-41-(SiCH2CH2CH2NH2)x exhibits two bands at 2919 and 1466 cm1 that are due to both the asymmetrical stretching vibration of the CH bond in the CH2 unit and in methylene and the CH2 bending vibration, respectively. The appearance of bands in the range of 32003400 cm1 can be attributed to the NH stretching frequency of the primary amine. The appearance of these bands in the IR spectra of MCM-41-(SiCH2CH2CH2NH2)x indicates the attachment of amino-propyl groups to the solid surface that are absent in SiMCM-41. In catalyst V-MCM-41, the characteristic band of the azomethine group of the vanadium(IV) complex moiety appears at ca. 1616 cm1 in the IR spectra of V-MCM-41. The IR band of the free azomethine group appears at 1643 cm1 whereas in V-MCM-41 this band is shifted to a lower frequency, indicating Schiff base formation between ortho-hydroxy-acetophenone and APTES-modified MCM-41 as well as the coordination of azomethine nitrogen with vanadium(IV). The characteristic peak of oxo-vanadium (VdO) that appears at 956 cm1 was obscured by the strong SiOSi vibrational band of V-MCM-41.32 The solid-state diffuse reflectance UVvis spectrum of V-MCM41 is shown in Figure 3. The catalyst exhibits one broad band between 301 and 352 nm that may be assigned either to a ligand-to-metal 15324
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Figure 3. UVvis spectra of the V-MCM-41 catalyst.
Figure 5. Transmission electron micrographs of (a) Si-MCM-41 and (b) V-MCM-41.
Figure 4. (a) N2 adsorption/desorption isotherms of (a) Si-MCM-41, (b) MCM-41-(SiCH2CH2CH2NH2)x, and (c) V-MCM-41. Adsorption points are marked by filled circles, and desorption points are marked by open circles. (b) Pore size distributions of (a) Si-MCM-41, (b) MCM41-(SiCH2CH2CH2NH2)x and (c) V-MCM-41.
charge-transfer band or to a Π f Π* transition originating mainly in the azomethine chromophore.33 Other bands appearing
at 234286 and 201233 nm can be attributed to intraligand transitions. A relatively weak, broad band appeared in the range of 460615 nm and had an absorption maximum at 515 nm that was due to characteristic dd transitions of (VO)2+ in the visible region.34 The CT and dd transitions confirm the presence of anchored vanadium in the mesoporous matrix. 3.3. N2 Sorption Studies. The nitrogen sorption isotherms of Si-MCM-41, MCM-41-(SiCH2CH2CH2NH2)x, and the V-MCM41 catalyst are shown in Figure 4. A gradual decrease in the BET surface area, pore volume, and pore diameter could be observed at each stage of modification as expected because the organic fragments or metal complexes entered the channels. However, all of the materials had type IV isotherms (according to the IUPAC nomenclature) with hysteresis loops, indicating that the mesoporous structure remained. The nitrogen sorption study exhibits that the BET surface area of Si-MCM-41 is 1178 m2 g1 and the mesopore volume is 0.67 cm3 g1. The average pore diameter is calculated to be 33.1 Å using the NLDFT method. All calculated values are in agreement with those reported for good-quality mesoporous silica. MCM-41-(SiCH2CH2CH2NH2)x shows smaller N2 uptake (BET surface area 835 m2g1), pore volume (0.49 cm3g1), and pore diameter (29.4 Å) whereas the V-MCM-41 catalyst shows even smaller N2 uptake (BET surface area 634 m2g1), and pore volume (0.31 cm3 g1) and 15325
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Figure 6. Oxidative bromination of phenol red catalyzed by V-MCM-41.
pore diameter (27.2 Å). As the pores become bulkier, hysteresis occurs at lower P/P0 with a concomitant decrease in pore size.23b This indicates that the immobilization of the oxo-vanadium(IV) complex has taken place substantially inside the pore channels of SiMCM-41. The average density of the attached APTES moiety in MCM-41-(SiCH2CH2CH2NH2)x was ∼1.45 molecules/nm2. 3.4. TEM Studies. Figure 5A,B shows TEM micrographs of SiMCM-41 and the V-MCM-41 catalyst. Both Si-MCM-41 and V-MCM-41 feature an open-ended lamellar-type arrangement of hexagonal porous tubules. When the electron beam falls on the catalyst perpendicular to the pore axis, the pores are seen to be arranged in patches composed of regular rows, as has been interpreted by Chenite et al.35 TEM micrograph of Si-MCM-41 viewed along the pore axis reveals a hexagonal array having a channel dimension of ∼3.9 nm, which is consistent with the XRD results. V-MCM-41 also exhibits a similar type of array of regular rows in TEM micrographs. Therefore, it can be concluded that both Si-MCM-41 and V-MCM-41 have a similar type of external morphology. In our case, the morphology of the tubules in Si-MCM-41 and in V-MCM-41 is rectilinear, and the tubules are 12001700 Å long. The TEM image provides strong evidence that the long-range ordering of the support framework is retained even after the immobilization of the oxo-vanadium(IV) Schiff base complex in the mesoporous silica matrix. 3.5. Bromination Activity Test for Catalyst V-MCM-41. To test the activity of the V-MCM-41 catalyst, the peroxidative bromination of phenolsulfonaphthalein (phenol red) to tetrabromo-phenolsulfonaphthalein (bromophenol blue) was used. This is a facile method and easy to monitor by a UVvis spectroscopic technique.36 λmax values of pure phenol red and bromophenol blue were first determined from the UVvis spectra of the substrates. Phenol red traps the active bromine species without influencing the rate of reaction until it is
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exhausted. The reaction mixture contains 10 mL of a 0.5 M KBr solution, 10 mL of 30% H2O2, 10 mL of 0.1 mM phenol red, and 20 mL of distilled water. The redox activity was tested by adding 50 mg of the V-MCM-41 catalyst to it and by monitoring the spectral changes at regular intervals. From the spectral data (shown in Figure 6), it is evident that as the reaction proceeds, the peak at about λmax ≈ 432 nm corresponds to the phenol red decreases whereas the peak at about λmax ≈ 592 nm corresponds to the bromophenol blue increase, thus indicating the progress of the reaction. After ∼1 h, no further changes in the absorption peaks were observed, confirming the completion of the reaction. 3.6. Catalytic Activity Studies. Oxo-vanadium compounds are able to mimic the vanadium-dependent haloperoxidase enzyme by catalyzing the bromination of organic substrates in the presence of H2O2 and bromide.37 During oxidation, vanadium coordinates with 1 or 2 equiv of H2O2, forming the monoperoxo {VO(O2)+} or bis(peroxo) {VO(O2)2} species that oxidize bromide, possibly via a hydroperoxo intermediate. Oxidized bromine species Br2, Br3, or most likely HOBr ultimately brominates the organic substrates. In this investigation, the catalytic efficacy of V-MCM-41 in the bromination reaction was studied using KBr and H2O2 as the bromide source and oxidizing agent, respectively. The catalyst shows high selectivity toward the monobromination of hydroxy aromatic compounds. It is noteworthy that the catalyst is highly para selective. However, in the substrate in which the para position is blocked, bromination takes place at the ortho position (entry 4, Table 1). To achieve optimum reaction conditions, oxidative bromination reactions were studied by varying parameters such as the nature of the solvent and the volumes of acid and H2O2 at room temperature. It was observed that for a fixed amount of substrate (5 mmol), a catalyst (25 mg), KBr (5 mmol), and 30% H2O2 (30 mmol) in 5 mL of H2O in the presence of HClO4 (2 mmol) are found to be the best conditions under which to achieve 100% conversion. The addition of HClO4 at a time may cause the slow decomposition of the catalyst. To stop the decomposition of the catalyst, HClO4 was successively added to the reaction mixture. The catalytic results are shown in Table 1. The bromination of salicylaldehyde undergoes 100% conversion with a 99% monobrominated product, 5-bromo salicylaldehyde, in a short time under heterogeneous conditions. In the recent past, the bromination of salicylaldehyde over vanadium-based catalysts under heterogeneous conditions has been studied (Table 2). Maurya et al. studied the oxidative bromination of salicylaldehyde mediated by zeolite-Y-encapsulated dioxo-vanadium(V) complexes, which shows a maximum of 39% conversion with 34% 5-bromosalicylaldehyde.38 When oxo-vanadium(IV)-based coordination polymers having a bridging methylene group are employed in the oxidative bromination of salicylaldehyde, the conversion increases to 92% but the selectivity of 5-bromosalicylaldehyde remains low (28%) and 3,5-dibromosalicylaldehyde was formed as a major product (65%).39 Chloromethylated polystyrenesupported oxo-vanadium(IV) complexes have also been used as catalysts in the oxidative bromination of salicylaldehyde, which gives a maximum 73% conversion with ∼81% product (5bromosalicylaldehyde) selectivity, and a dibrominated product was also formed in this case.40 Recently, Maurya et al. have studied this reaction over polymer-anchored dioxo-vanadium(V) complexes, which shows a maximum of ∼85% conversion with ∼90% product (5-brompsalicylaldehyde) selectivity.41 To extend the scope of this reaction, similar reaction conditions were applied to other aromatic compounds. Phenol and 15326
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Table 1. Bromination of Hydroxyaromatic Compounds Using V-MCM-41 as a Catalysta
a Reaction conditions: Substrate, 5 mmol; KBr, 5 mmol; solvent, 5 mL; catalyst, 25 mg; H2O2, 30 mmol; and HClO4, 2 mmol. b Acetonitrile (1 mL) was added to 4 mL of H2O to dissolve the substrate. c Conversion of the reactant is determined by GC. d Isolated yields were calculated from the mass of the product after separation by column chromatography. All of the isolated products showed more than 99% GC purity. e TOF (turnover frequency) = moles converted/(moles of active site time).
Table 2. Heterogeneous Oxybromination of Salicylaldehyde Catalyzed by Other Vanadium-Based Catalysts conversion (wt %)
% selectivity of 5-bromo salicylaldehyde
TOFe
NH4[VO2(sal-inh)]Y
39.3
34.0
52.0
38
[CH2{VO(sal-1,3-pn)}]nb
92.0
28.0
33.8
39
polymer-supported-[VO(fsal-ohyba) 3 DMF]c
73.0
81.36
100.0
40
85.2
90.4
775.0
41
100.0
99.0
448.0
this study
catalyst a
polymer-supported K[VO2(sal-inh)(im)]d V-MCM-41
ref
a
H2 sal-inh is N-isonicotinamidosalicylaldimine. b Oxovanadium(IV) complex of the polymeric Schiff base derived from 5,5-methylenebis(salicylaldehyde) [CH2(Hsal)2] and 1,3-diaminopropane (1,3-pn). c H2fsal-ohyba is the Schiff base derived from 3-formylsalicylic acid and ohydroxybenzylamine. d H2 sal-inh is the Schiff base derived from salicylaldehyde and isonicotinolhydrazide. e TOF (turnover frequency) = moles converted/(moles of active site time).
para-chloro-phenol give monobrominated products with high yields in a short time. The bromination of ortho-hydroxy-
acetophenone proceeds efficiently and furnishes the respective monobrominated product using a stoichiometric amount of 15327
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Table 3. Effect of Solvents on the Oxidative Bromonation of ortho-Hydroxy-acetophenone Catalyzed by V-MCM-41a
a
Reaction conditions: Substrate, 5 mmol; KBr, 5 mmol; solvent, 5 mL; catalyst, 25 mg; H2O2, 30 mmol; and HClO4, 2 mmol. b Conversion of the reactant is determined by GC. c Isolated yields were calculated from the mass of the product after separation by column chromatography. All of the isolated products showed more than 99% GC purity. d Without the addition of HClO4.
determine if vanadium was leaching out of the catalyst, the reaction mixture was filtered out after the reaction and was subjected to atomic absorption spectroscopic analysis. The analysis showed the absence of vanadium in the filtrate. Besides that, the filtrate mixture did not show any catalytic activity toward the bromination reaction. These results indicate that the catalyst is stable and heterogeneous in nature, which has advantages over the homogeneous counterpart. Figure 7. Chart showing the recyclability of the V-MCM-41 catalyst.
potassium bromide as the bromine source. We have also examined the behavior of benzaldehyde and toluene toward the oxidative bromination reaction under similar reaction conditions using V-MCM-41 as the catalyst, but they remained completely inactive. Therefore, it can be concluded that the V-MCM-41 catalyst is highly selective toward hydroxyaromatic compounds. Recently, Sharma et al. have studied the oxidative bromination reaction of various aromatic compounds using Cu2+-perfluorophthalocyanine-immobilized silica gel as the catalyst. Although the yield is good, the reactions occur only in an acetic acid medium at 60 °C.42 At variance with the abovementioned catalytic systems, V-MCM-41 showed the best catalytic activity at room temperature and, even more importantly, in an ecofriendly water medium. Solvent plays an important role in the catalytic bromination reaction. It was observed that among different solvents the maximum conversion takes place in a water medium. The effect of different solvents on the oxidative bromination of ortho-hydroxy-acetophenone is shown in Table 3. The catalyst can be easily recovered after the reaction by simple filtration. After recovery, the catalyst was thoroughly washed with ether and then methanol and dried at room temperature. The recovered catalyst showed almost the same catalytic activity in successive runs shown in Figure 7. To
4. CONCLUSIONS We have successfully immobilized the vanadium(IV) Schiff base complex into the Si-MCM-41 matrix via covalent bonds. The mesoporosity of the material is retained after the immobilization of the vanadium complex. The catalyst efficiently and selectively catalyzes the bromination of hydroxy aromatic compounds using KBr as the bromide source and 30% H 2 O 2 as the oxidizing agent in water at room temperature. This methodology of bromination offers simple reaction conditions, commercial availability of the reagents, excellent product yield, no evolution of hydrogen bromide, and an environmentally more benign alternative in comparison to the hazardous classical bromination protocols, thus affording an opportunity for easy access to a variety of bromo-organics. ’ ASSOCIATED CONTENT
bS
1
H NMR spectra of the compounds used in this study. This material is available free of charge via the Internet at http://pubs.acs.org. Supporting Information.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. 15328
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’ ACKNOWLEDGMENT We acknowledge the Department of Science and Technology, Government of India, for funding this project (through author S.K., SR/S1/IC-01/2009). E. M. SINP is acknowledged for TEM measurement (on a chargeable basis). S.B. thanks CSIR (ref. no. 09/096(0671)2k11-EMR-I) for a senior research fellowship. ’ REFERENCES (1) Ullmann’s Encyclopedia of Industrial Chemistry, 6th ed.; WileyVCH: Weinheim, Germany, 1998; electronic release. (2) Silva, V. M.; Lopes, W. A.; Andrade, J. B.; Veloso, M. C. C.; Santos, G. V.; Oliveira, A. S. Quim. Nova 2007, 30, 629. (3) Hassenkl€over, T.; Predehl, S.; Pilli, J.; Ledwolorz, J.; Assmann, M.; Bickmeyer, U. Aquat. Toxicol. 2006, 76, 37. (4) Lambert, F. L.; Ellis, W. D.; Parry, R. J. J. Org. Chem. 1965, 30, 304. (5) Smith, K.; Bahzad, D. J. Chem. Soc., Chem. Commun. 1996, 467. (6) Paul, V.; Sudalai, A.; Daniel, T.; Srinivasan, K. V. Tetrahedron Lett. 1994, 35, 7055. (7) Singh, A. P.; Mirajkar, S. P.; Sharma, S. J. Mol. Catal. A: Chem. 1999, 150, 241. (8) Auerbach, J.; Weissman, S. A.; Blacklock, T. J.; Angelss, M. R.; Hoogsteen, K. Tetrahedron Lett. 1993, 34, 931. (9) Oberhauser, T. J. Org. Chem. 1997, 62, 4504. (10) Barhate, N. B.; Gajare, A. S.; Wakharkar, R. D.; Badekar, A. V. Tetrahedron Lett. 1998, 39, 6349. (11) Goldberg, Y.; Alper, H. J. Mol. Catal. A: Chem. 1994, 88, 377. (12) Butler, A.; Walker, J. V. Chem. Rev. 1993, 93, 1937. (13) Sels, B.; De Vos, D.; Butinx, M.; Pierard, F.; Kirsch-De Mesmaeker, A.; Jacobs, P. Nature 1999, 400, 855. (14) (a) Rothenberg, G.; Clark, J. H. Green Chem. 2000, 2, 248. (b) Meister, G.; Butler, A. Inorg. Chem. 1994, 33, 3269. (15) Clague, M. J.; Keder, N. N.; Butler, A. Inorg. Chem. 1993, 32, 4754. (16) Conte, V.; Di Furia, F.; Licini, G. Appl. Catal., A 1997, 157, 335. (17) Ligtenbarg, A. G. J.; Hage, R.; Feringa, B. L. Coord. Chem. Rev. 2003, 237, 89. (18) Bolm, C. Coord. Chem. Rev. 2003, 237, 245. (19) (a) De Vos, D. E.; Dams, M.; Sels, B. F.; Jacobs, P. A. Chem. Rev. 2002, 102, 3615. (b) Davis, M. E. Microporous Mesoporous Mater. 1998, 21, 173. (c) Bedioui, F. Coord. Chem. Rev. 1995, 144, 39. (d) Dutta, B.; Jana, S.; Bera, R.; Saha, P.; Koner, S. Appl. Catal., A 2007, 381, 89. (e) Murphy, E. F.; Ferri, D.; Baiker, A.; Doorslaer, S. V.; Schweiger, A. Inorg. Chem. 2003, 42, 2559. (20) Miller, M. M.; Sherrington, D. C. J. Catal. 1995, 152, 368. (21) (a) Lee, C.-H.; Wong, S.-T.; Lin, T.-S.; Mou, C.-Y. J. Phys. Chem. B 2005, 109, 775. (b) Jana, S.; Dutta, B.; Bera, R.; Koner, S. Langmuir 2007, 23, 2492. (22) (a) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C.T.-W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834. (b) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (c) Asefa, T.; MacLachlan, M. J.; Coombs, N.; Ozin, G. A. Nature 1999, 402, 867. (23) (a) Reddy, K. M.; Moudrakovski, I.; Sayari, A. J. Chem. Soc., Chem. Commun. 1994, 1059. (b) Parida, K. M.; Singha, S.; Sahoo, P. C. J. Mol. Catal. A: Chem. 2010, 325, 40. (c) Baleiz~ao, C.; Garcia, H. Chem. Rev. 2006, 106, 3987. (24) Reddy, J. S.; Sayari, A. J. Chem. Soc., Chem. Commun. 1995, 2231. (25) Gontier, S.; Tuel, A. Microporous Mater. 1995, 5, 161. (26) Zhang, W.-H.; Shi, J.-L.; Wang, L.-Z.; Yan, D.-S. Chem. Mater. 2000, 12, 1408. (27) Koner, S.; Chaudhari, K.; Das, T. K.; Sivasanker, S. J. Mol. Catal. A: Chem. 1999, 150, 295. (28) Koner, S. Chem. Commun. 1998, 593.
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