CATALYTIC ACTIVITIES OF POLYMER -ANCHORED METAL COMPLEXES

Abstract

Catalysts have played a vital role in organic transformations as they have wide range of applications in chemical industries and have major impact on the quality of human life as well as economic progress. More than 90 % of the processes in the petroleum, petrochemical and fertilizer industries are catalytically induced. Most of the catalytic processes, widely engaged in the manufacture of bulk as well as fine chemicals, are homogenous in nature, producing large amount of side waste materials and imposing hazardous impact on the surrounding environment. Awareness of environmental issues has put a major stress on the development of such catalytic processes that are beneficial from both, industries and environmental point of view. With the advent of Green Chemistry movement there has been continuing efforts in developing catalytic protocols that provide the better process selectivity, easy separation with re-use ability along with no production of hazardous side waste materials; this is the root concept of Chemical Clean Technology or Green Chemistry. A new emerging class of solid catalyst named, 'heterogenized homogeneous catalyst', has played a considerable role in the research field of catalyst in the past few years. During heterogenization, homogenous catalysts are anchored over some solid inert support where it combines the features of both homogeneous as well as heterogeneous catalyst. Various heterogeneous systems such as polymer-anchored metal complexes, zeolite-encapsulated transition metal complexes, and MCM-41, heteropolyacids, alumina, silica and metal oxides have been developed. After the discovery of Merrifield resin (chloromethylated polystyrene crosslinked with divinylbenzene), in the solid-phase peptide synthesis, the polymeric supports have become the major driving force in combinatorial chemistry. Functionalized resins have found enormous use in solid-phase organic synthesis, asymmetric synthesis, as an ion exchange resin in chromatography, metal ion u removal, in biological and pharmaceutical systems and in immobilization ofenzymes and catalysts. Among the different functionalized resin, the most commonly used one is chloromethylated polystyrene which may be micro porous /gel-type resin (Merrifield resin) or macro porous in nature. Besides, divnylbenzene (DVB), ethyleneglycol, dimethacrylate (EGDMA), trimethylpropane trimethacrylate (TRIM) etc. are introduced as novel cross-linkers that exert strong influence on the swelling properties ofresin beads. Anchoring ofthe homogeneous catalysts on insoluble polymer support is specialized method because this enhances the thermal stability, selectivity, recycle ability and easy separation of the catalyst from reaction products leading to the operational flexibility. All these encouraged us to design new polymer-anchored complexes and used them as catalyst for the oxidation ofvarious organic substrates. The present thesis, thus, describes the syntheses of metal complexes of Schiff base ligands anchored onto chloromethylated polystyrene (cross-linked with 5 % divnylbenzene) and their characterisation by various physicochemical techniques. Different types of oxidation reactions have been carried out and suitable reaction conditions have been obtained to achieve maximum oxidation of organic substrates. The obtained reaction products have been analysed by GC and their identities confirmed by GC-MS. For convenience the work presented in the thesis has been divided in the following chapters. First chapter is the introductory part of the research work that along with the history and development of catalysts also deal with the major aspects of heterogeneous catalysts; brief introduction of different heterogeneous systems and catalytic reactions mainly oxidation reactions facilitated by these catalysts have been reviewed. The later part ofthis chapter provides details about the polymer-support, their origin and synthesis, and different uses. Asmall section ofthis part also deals with their mode of preparation, structure, and morphology. Details of the various oxidation reactions like epoxidation of olefins, hydroxylation of alkylaromatic compounds, oxidation of alcohols, oxidation of phenol and hydroquinone and in bromination of salicylaldehyde, catalyzed by polymer-supported catalysts have been mentioned. Second chapter describes the syntheses of 2-(a-hydroxymethyl)benzimidazole (Hhmbmz) and 2-(a-hydroxyethyl)benzimidazole (Hhebmz) ligands and their polymer-supported analogue, PS-Hhmbmz and PS-Hhebmz, respectively. Syntheses of oxovanadium(IV), copper(II) and dioxomolybdenum(VI) of PS-Hhmbmz, and oxovanadium(IV) and copper(II) complexes of PS-Hhebmz have been described. Syntheses of neat complexes have also been reported. These polymer-anchored metal complexes have been characterised by various physico-chemical techniques such as elemental analyses, IR, EPR and UV-Vis spectroscopy, scanning electron micrograph and TGA-DTA, to ensure the loading of the metal complexes on polymer-support. Catalytic activity of the anchored complexes, PS-[VO(hmbmz)2] and PS- [Cu(hmbmz)2) for the oxidation of styrene and ethylbenzene while PS-[VO(hmbmz)2] and PS-[Mo(hmbmz)2)] for the oxidative bromination of salicylaldehyde have been studied. The EPR study of both freshly prepared and spent catalysts indicate that metal-Schiff base moiety is intact and coordination environment are not altered during catalytic reaction and catalysts have recycle ability up to three cycles. Catalytic potentials of polymer-anchored complexes of 2-(ahydroxyefhyl) benzimidazole, PS-[VO(hebmz)2] and PS-[Cu(hebmz)2] have also been explored. Kinetic study of the oxidative coupling of 2-aminophenol (OAP) to 2- aminophenoxazine (APX) catalysed by PS-[Cu(hebmz)2] in the presence of air at 70 °C in DMF has been studied and different kinetic parameters have been calculated. The anchored vanadium complex, PS-[VO(hebmz)2] has been used for the oxidation of benzoin , an a- hydroxy ketone, in the presence of 70 % tert-butylhydroperoxide as an oxidant. Effect of base (KOH) on the yield and the selectivity of product have been studied. Synthesis of dibasic tetradentate ligand, N,N'-bis(salicylidene) diethylenetriamine (H2saldien), polymer-anchored ligand (PS-H2saldien) and their copper(II) and oxovanadium(IV) complexes have been described in the third chapter. The polymer-anchored ligand and metal complexes have been characterised IV through various techniques. Oxidation of phenol has been optimized using PS- [Cu(saldien)] and PS-[VO(saldien)] as a supported catalyst in presence of30 %H202 in water. The PS-[Cu(saldien)] has been found to be selective for benzoquinone and hydroquinone while neat complex, [Cu(saldien)] gave hydroquinone and catechol as major products in aqueous medium. Here, the PS-[Cu(saldien)] has been found to be more active than PS-[VO(saldien)]. The oxidation ofhydroquinone to benzoquinone has also been carried out under the optimized reaction condition by using PS- [Cu(saldien)] as catalyst. Fourth chapter deals with the synthesis of dibasic tridentate ligand, H2fsal-ala derived from 3-formyl salicylic acid and alanine supported on chloromethylated polystyrene and its copper(II) and oxovanadium (IV) complexes, PS-[VO(fsalala). DMF] and PS-[Cu(fsal-ala)]. Their respective neat complexes, [VO(fsalala). H20] and [Cu(fsal-ala)] have also been synthesized. The characterization of the anchored metal complexes has been carried out by various spectroscopic techniques, scanning electron micrograph, thermal analysis and elemental analyses. Catalytic activity of PS-[VO(fsal-ala).DMF] and PS-[Cu(fsal-ala)] has been tested for the epoxidation of olefins and oxidation/hydroxylation of alkylaromatic compounds. First, the epoxidation of styrene has been optimized in the presence of 30 %H202 using PS-[VO(fsal-ala).DMF] as catalyst and then oxidation ofvarious substrates like, cyclohexene, trans-stilbene and limonene has been carried out under the optimized reaction condition. Similarly, the reaction condition for the oxidation of benzene has been optimized in the presence of PS-[VO(fsal-ala).DMF] using 30 %H202 as an oxidant and then under the optimized reaction condition, the hydroxylation of other substrates e.g. cyclohexane, ethybenzene and naphthal ene has also been reported. Catalytic activity of the PS-[Cu(fsal-ala)] has also been tested for all the substrates described above under the same optimized reaction condition. The intermediate species ofthe complexes formed during the catalytic reactions have been identified by studying the interaction of [VO(fsal-ala).H20] and [Cu(fsal-ala)] with 30 %H202 in methanol and monitoring the changes by electronic absorption spectroscopy.