IMMOBILIZED VANADIUM COMPLEXES AND THEIR CATALYTIC ROLE IN OXIDATION REACTIONS

Abstract

Catalysts have played key role in the development of modern chemical technologies. They activate the chemical reaction at milder conditions through the bonding of reactant molecules with definite functional groups called 'active sites', where they react and finally detach from the catalyst for the next cycle. Modern catalytic science faces big challenges as most of the catalytic processes, which are widely engaged in the manufacture of bulk as well as fine chemicals, are homogeneous in nature. Production of a large amount of side waste materials by these processes has imposed a hazardous impact on the environment. Homogeneous catalysts also face the problem of separation from the substrate and products, and very often decompose or polymerize during catalytic action. There is, therefore, necessity to create new highly effective industrial processes, which are selective, ecologically safe and consume minimum energy. In this direction, immobilization ofhomogeneous catalysts to facilitate easy product separation, catalysts recovery and recycle ability has been the major concerns of industries as well as researchers. Alumina and silica gels are readily available inorganic compounds and have been modified to immobilize various catalysts by direct reaction of surface hydroxyl groups with reactive species. Even, mesoporous molecular sieves such as MCM-41 and SBA-15 have also been used for the immobilization of homogeneous catalysts. Transition metal complexes having good catalytic properties have also been encapsulated in the super cages of zeolites to give them solid support. After the discovery of Merrifield resin i.e. chloromethylated polystyrene crosslinked with divinylbenzene, attention of researchers was diverted to use functionalized polymers for the immobilization of homogeneous materials. This method has now become a very specialized method as this enhances the thermal stability, selectivity, and recycles ability of the catalyst. In addition, separation of the catalyst from the reaction products also becomes easy. Polymer immobilized (also called polymer-anchored) complexes have provided opportunity to develop catalytic n process in the synthesis of fine chemicals and being used in various types ofoxidation as well as hydrogenation reactions. Though, different types of ligands and various transition metal ions have been used to develop polymer-anchored catalysts, polymeranchored vanadium based catalysts have not been studied in detail. Discovery of vanadium(V) in the active site of vanadate-dependent haloperoxidases and its importance in various catalytic reactions has stimulated research on the catalytic potentials of vanadium complexes. Many vanadium complexes have been reported to show potential catalytic activity towards the oxidative halogenations and sulfoxidations along with other oxidation reactions. All these encouraged us to design polymer-anchored vanadium based catalysts and use them for various oxidation reactions. The thesis entitled "Immobilized vanadium complexes and their catalytic role in oxidation reactions" mainly deals with the syntheses of vanadium complexes of Schiff bases anchored onto chloromethylated polystyrene (cross-linked with 5% divinylbenzene) and their characterization by various physico-chemical techniques. Catalytic activities towards oxidation and oxidative animation of various substrates have also been carried out under optimized reaction conditions to achieve maximum oxidation products. For convenience the work presented in the thesis has been divided in the following five chapters and abstract of each chapter is given below: First chapter is the introductory one and describes various types of solid inert support that have been used for immobilization of homogeneous catalysts. Abrief introduction on functionalized polymers and different modes to use them for immobilization of catalysts has been described. Literature on polymer-immobilized metal complexes and their catalytic applications for various reactions have also been reviewed. Second chapter describes the anchoring of the Schiff base derived from 3- formylsalicylic acid and o-hydroxybenzylamine (H2fsal-ohyba) to the chloromethylated polystyrene cross-linked with 5%divinylbenzene (abbreviated as PS-H2fsal-ohyba, 2.1). Treatment of [VO(acac)2] with PS-H2fsal-ohyba in dimethylformamide (DMF) gave oxovanadium(IV) complex, PS-[VO(fsalin ohyba)-DMF] (2.1). Complex 2.1 can be oxidized to dioxovanadium(V) species, PSK[ V02(fsal-ohyba)] (2.2) on aerial oxidation in presence of KOH and to oxoperoxo species, PS-K[VO(02)(fsal-ohyba)] (2.3) in presence of H202 and KOH in DMF suspension. All these complexes have been characterized by various techniques. These complexes catalyze oxidation of styrene, ethylbenzene and phenol efficiently. Styrene gives five reaction products namely, styrene oxide, benzaldehyde, 1- phenylethane-1,2-diol, benzoic acid and phenylacetaldehyde, while ethylbenzene gives benzaldehyde, phenyl acetic acid, styrene and 1-phenylethane-1,2-diol. The oxidation products of phenol are catechol and />-hydroquinone. These catalysts are also able to catalyse oxidative bromination of salicylaldehyde to 5- bromosalicylaldehyde with ca. 80 % selectively in presence of aqueous 30 % H202/KBr, a reaction similar to that exhibited by vanadate-dependent haloperoxidases. Their corresponding neat complexes have also been prepared and catalytic activities have been compared. Syntheses of polymer-anchored ligands PS-H2fsal-ea (3.IV), PS-H2fsal-pa (3.V) and PS-H2fsal-amp (3.VI) from the Schiff bases, H2fsal-ea (3.1), H2fsal-pa (3.II) and H2fsal-amp (3.III) (Hsal = salicylaldehyde, ea = 2-aminoethanol, pa = 3- aminopropanol and amp = 2-amino-2-mefhylefhanol), respectively and their oxovanadium(IV) complexes have been described in Chapter third. The polymeranchored ligands PS-H2fsal-ea (3.IV), PS-H2fsal-pa (3.V) and PS-H2fsal-amp (3.VI) on treatment with [VO(acac)2] in dimethylformamide (DMF) gave oxovanadium(IV) complexes, PS-[VO(fsal-ea)-DMF] (3.4), PS-[VO(fsal-pa)-DMF] (3.5) and PS- [VO(fsal-amp)-DMF] (3.6), respectively. The corresponding neat complexes, [VO(fsal-ea)]2 (3.1), [VO(fsal-pa)]2 (3.2) and [VO(fsal-amp)]2 (3.3) have also been prepared similarly. All these complexes exhibit a medium intensity band between 964 - 993 cm-1 region in their IR spectra due to v(V=0) stretch. The EPR spectra of polymer-anchored complexes are characteristics of monomeric V(IV) centres with a simple S = Vi electronic spin with axial pattern typical for square pyramidal geometry. Broad features for neat complexes along with magnetic susceptibility study suggest the presence of antiferromagnetic exchange interaction between two vanadium IV centres in close proximity. These catalysts have been tested for the oxidation of styrene and cumene and found to be efficient. Styrene gives five reaction products namely styrene oxide, benzaldehyde, l-phenylethane-l,2-diol, benzoic acid and phenylacetaldehyde, while cumene gives acetophenone, 2-phenylpropanal, oc-methyl styrene oxide, 2-phenyl-2-propanol, 2-isopropyl-l,4-benzoquinone and a-methyl styrene. The polymer-anchored heterogeneous catalysts are recyclable. The catalytic activities of neat complexes have also been carried out and compared with the corresponding anchored ones. Bidentate ligand 2-(2'-hydroxyphenyl)bezimidazole (Hhpbmz, 4.1) has been covalently bonded to chloromethylated polystyrene cross-linkined with 5 % divinylbenzene (abbreviated as PS-Hhpbmz, 4.II). Reaction between PS-Hhpbmz and [VO(acac)2] (Hacac = acetylacetone) gave polymer-anchored complex, PS- [VO(hpbmz)2] (4.1). Characterization of the catalyst using scanning electron micrographs, spectroscopic (infrared and electronic), thermogravimetric and elemental analyses studies and its catalytic activities has been included in the fourth Chapter. Corresponding neat complex, [VO(hpbmz)2] with the ligand Hhpbmz has also been prepared to compare the spectral properties and catalytic activities. Catalytic activities of both the complexes have been tested for the oxidation of styrene, ethylbenzene and methyl phenyl sulfide using H202 as oxidant. Reaction conditions for the maximum oxidation of these substrates have been optimized by considering the concentration of oxidant, amount of catalyst, volume of solvent and temperature of the reaction mixture. Under the optimized conditions, oxidation of styrene by PS-[VO(hpbmz)2] gave a maximum of 70.8 % conversion with five reaction products namely styrene oxide, benzaldehyde, l-phenylethane-l,2-diol, benzoic acid, and phenyl acetaldehyde. Ethylbenzene gave 31.2 %conversion with five reaction products namely benzaldehyde, phenyl acetic acid, styrene, 1- phenylethanel,2-'diol and benzoic acid. Benzaldehyde has been obtained in highest yield in both cases. Under the optimized reaction conditions, PS-[VO(hbmz)2] exhibited a maximum of 76.5 % conversion of methyl phenyl sulfide to the corresponding sulfoxide and sulfone. The selectivity for the formation of sulfoxide has been found to be. 75 %. Catalyst PS-[VO(hpbmz)2] does not leach metal ion during catalytic activity and is recyclable up to two cycles. Catalytic activity of neat analogue has been found to be lower in all three substrates than that of the anchored one. The dibasic tridentate ligand H2sal-cys (5.1) derived from salicylaldehyde and L-cysteine has been covalently bonded to the chloromethylated polystyrene crosslinked with 5 % divinylbenzene through carboxylate group. Synthesis of oxovanadium(IV) complex, PS-[VO(sal-cys)-DMF] (5.1) from the resulting ligand PS-H2sal-cys (5.II) has been reported in Chapter fifth. The corresponding neat complex, [VO(sal-eta)]2 (5.2) (H2sal-eta = Schiff base derived from salicylaldehyde and 2-aminoethanethiol), has been prepared by reacting [VO(acac)2] with H2sal-eta (5.III) in acetonitrile. These complexes have been characterised by IR, and electronic spectroscopic studies, magnetic susceptibility measurements, and thermal as well as scanning electronic micrographs. A medium intensity band at ca. 980 cm-1 due to v(V=0) stretch in their IR spectra suggests the presence of oxovanadium(IV) centre. Magnetic susceptibility study of 5.2 suggests the presence of antiferromagnetic exchange interaction between two vanadium centres in the close proximity. Complex PS-[VO(sal-cys)-DMF] (5.1) catalyzes the oxidative amination of styrene with secondary amines like diethylamine, imidazole and benzimidazole in mild basic conditions. The polymer-anchored heterogeneous catalyst is recyclable. The catalytic activity of neat analogue for the oxidative amination has also been carried out and compared with the anchored catalyst.