CATALYTIC ASPECTS OF IMMOBILIZED METAL COMPLEXES

Abstract

The word catalysis has origin from Greek which means "decomposition" or "dissolution". The catalyst decreases the activation energy of a reaction by altering the reaction path but itself remains unchanged. From the beginning the catalysis is an important area of research and continuous efforts have been made to understand and utilize the phenomenon for practical purposes. Nowadays catalysts are playing a vital role in petrochemicals, fine chemicals, pharmaceuticals, fertilizers and food industries. The biochemically significant processes are also based on catalysis. 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. Most of the catalytic processes, which are widely engaged in the manufacture of bulk as well as fine chemicals, are homogeneous in nature. A large amount of waste materials have been produced during these processes which 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. Therefore, there is a necessity to create new highly effective industrial processes, which are selective, ecologically safe and consume minimum energy. In this direction, immobilization of homogeneous catalysts facilitates the easy product separation, catalysts recovery and recycle ability is 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. Discovery of Merrifield resin i.e. chloromethylated polystyrene cross-linked with divinylbenzene, has attracted researchers to use functionalised polymers for the n heterogenisafion of homogeneous materials. Immobilisation of homogeneous catalysts through covalent bonding with chloromethylated polystyrene cross-linked with divinylbenzene have developed them as environmentally safe heterogeneous catalysts for various catalytic reactions and attracted attention in recent years. This method has the advantages of both homogeneous as well heterogeneous catalysts, as this enhances the thermal stability, selectivity, arid recycles ability of the catalyst and ease separation of catalyst from the reaction mixture. Polymer immobilised (also called polymer-anchored) complexes have provided opportunity to develop catalytic process in the synthesis of fine chemicals and being used in various types of oxidation as well as hydrogenation reactions. Though, different types of ligands and various transition metal ions have been used to develop polymer-anchored catalysts. 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 halogenation and sulfoxidation 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 "Catalytic Aspects of immobilized metal complexes" mainly deals with the syntheses of vanadium, molybdenum and copper complexes of Schiff bases immobilised onto chloromethylated polystyrene (cross-linked with 5 % divinylbenzene) and their characterization by various physico-chemical techniques. Catalytic activities towards oxidation and hydroamination of various substrates have also been carried out under optimized reaction conditions to achieve maximum oxidation products. For convenience the work present in the thesis has been divided in the following five chapters: First chapter is the introductory one and describes various types of solid inert support that have been used for immobilisation of homogeneous catalysts. A brief introduction on functionalised polymers and different modes to use them for immobilisation of catalysts has been described. Literature on polymer-immobilised 111 metal complexes and their catalytic applications for various reactions have also been reviewed. Second chapter describes the anchoring of the Schiff base derived from salicylaldehyde and histamine (Hsal-his) to the chloromethylated polystyrene crosslinked with 5 % divinylbenzene (abbreviated as PS-Hsal-his, 2.II). Treatment of [VO(acac)2] with PS-Hsal-his in dimethylformamide (DMF) gave oxovanadium(IV) complex, PS-[VO(sal-his)(acac)] (2.1). Complex 2.1 can be oxidised to dioxovanadium(V) species, PS-[V02(sal-his)] (2.2) on aerial oxidation in presence of H202. All these complexes have been characterised by various techniques. All these complexes catalyse the oxidation of methyl phenyl sulfide, diphenyl sulfide and benzoin efficiently. Under the optimised reaction conditions, a maximum of 93.8% conversion of methyl phenyl sulfide with 63.7% selectivity towards methyl phenyl sulfoxide and 36.3% towards methyl phenyl sulfone has been achieved in 2 h with PS-[V02(sal-his)]. Under similar conditions, diphenyl sulfide gave 83.4% conversion where selectivity of reaction products varied in the order: diphenyl sulfoxide (71.8%) > diphenyl sulfone (28.2%). A maximum of 91.2% conversion of benzoin has been achieved within 6 h, and the selectivity of reaction products are: methylbenzoate (37.0%) > benzil (30.5%) > benzaldehyde-dimethylacetal (22.5%) > benzoic acid (8.1%). The PS-bound complex, PS-[VIV0(sal-his)(acac)] exhibits very comparable catalytic potential. Their corresponding neat complexes have also been prepared and catalytic activities have been compared. Syntheses of polymer-anchored ligands PS-S-tmbmz (3.II), from the Htmbmz (3.1) (Htmbmz = 2-thiomethylbenzimidazole) and their oxovanadium(IV), dioxomolybdenum(VI) and copper(II) complexes have been described in Chapter third. The polymer-anchored ligands PS-S-tmbmz (3.II), on treatment with [VO(acac)2], [Mo02(acac)2] and Cu(CH3COO)2 in dimethylformamide (DMF) gave oxovanadium(IV), dioxomolybdenum(VI) and copper(II) complexes, PS-[VO(Stmbmz) 2] (3.1), PS-[Mo02(S-tmbmz)2] (3.2) and PS-[Cu(S-tmbmz)2] (3.3), respectively. The corresponding neat complexes, [VO(tmbmz)2] (3.4), [Mo02(tmbmz)2] (3.5) and [Cu(tmbmz)2] (3.6) have also been prepared. EPR was iv particularly useful to characterise the binding modes in PS-[VO(S-tmbmz)2] (3.1) and PS-[Cu(S-tmbmz)2] (3.3), confirming that the vanadium and copper centers are well dispersed in the polymer matrix and supporting the presence of N202 binding modes in both cases and the preservation of the binding mode at the end of the catalytic reactions carried out. The catalytic potential of these complexes was tested for the oxidation of styrene, cyclohexene and ethylbenzene using 30 % H202 as an oxidant. Styrene gave three products with the selectivity order: benzaldehyde > 1- phenylethane-1,2-diol > styrene oxide. Oxidation of cyclohexene gave three products, the order of selectivity being: cyclohexane-1,2-diol > 2-cyclohexene-l-one > cyclohexeneoxide. At least four reaction products with the selectivity order: benzaldehyde > phenylacefic acid > acetophenone > 1-phenylethane-1,2-diol have been obtained on oxidation of ethylbenzene. The recycle ability of polymer-anchored metal complexes was checked; the results confirm that the polymer-bound complexes were not leached during the reaction/recovery procedures. Catalytic activities of the PS-supported complexes were higher than those of the corresponding non-polymerbound complexes. The monobasic tridentate ligand H2fsal-dmen (4.1) derived from 3-formyl salicylic acid and N,N-dimethylethylene diamine has been covalently bonded to the chloromethylated polystyrene cross-linked with 5 % divinylbenzene through carboxylic group. Synthesis of oxovanadium(IV) and dioxo-vanadium(V) complexes, PS-[VO(fsal-dmen)(acac)] (4.1) and PS-[V02(fsal-dmen)] (4.2) from the resulting ligand PS-Hfsal-dmen (4.II) has been reported in Chapter fourth. The corresponding neat complexes, [VO(sal-dmen)(acac)] (4.3) and [V02(sal-dmen)] (4.4) have been prepared by the reaction of [VO(acac)2] with Hsal-dmen (4.III) in methanol. These complexes have been characterised by IR, and electronic spectroscopic studies, magnetic susceptibility measurements, and thermal as well as scanning electronic micrographs. The catalytic oxidative desulfurization of model organosulfur compounds like thiophene (T), dibenzothiophene (DBT), benzothiophene (BT), 2- methyl thiophene (MT) and diesel has been carried out using complexes PS-[VO(fsaldmen)( acac)] (4.1) and PS-[V02(fsal-dmen)] (4.2). The sulfur in model organosulfur compounds and diesel has been oxidised to the corresponding sulfones in presence of H202. The polymer-bound heterogeneous catalysts were free from leaching during catalytic action and recyclable. The tridentate Schiff base ligand derived from 3-formylsalicylic acid and 2-(2- aminoethyl)pyridine (H2fsal-aepy, 5.1) has been covalently bonded to chloromethylated polystyrene cross-linkined with 5 % divinylbenzene (abbreviated as PS-Hfsal-aepy, 5.II). Reaction between PS-Hfsal-aepy and [VO(acac)2] (Hacac = acetylacetone) gave polymer-anchored complex, PS-[VO(fsal-aepy)(acac)] (5.1). Complex PS-[VO(fsal-aepy)(acac)] (5.1) has been oxidised to the dioxidovanadium(V) species, PS-[V02(fsal-aepy)] (5.2) on aerial oxidation in the presence of H202 in MeOH. Characterisation 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 fifth Chapter. Corresponding neat complexes, [VO(sal-aepy)(acac)] (5.3) and [V02(salaepy)] (5.4) with the ligand Hsal-his has also been prepared to compare the spectral properties and catalytic activities. Complexes PS-[VO(fsal-aepy)(acac)] (5.1) and PS- [V02(fsal-aepy)] (5.2) catalyse the hydroamination of styrene and vinyl pyridine with amines (aniline and diethylamine) and gave a mixture of two hydroaminated products in good yields. Amongst the two hydroaminated products, the anti-Markovnikov product is favoured over the Markovnikov product. The polymer-anchored heterogeneous catalyst is recyclable. Catalytic activity of neat analogue has been found to be lower than that of the anchored one.