SYNTHESIS AND CATALYTIC PERFORMANCE OF IMMOBILIZED VANADIUM AND MOLYBDENUM COMPLEXES

Abstract

Catalytic role of transition metal complexes have played a vital role in various organic transformations e.g. oxidation, oxidative halogenation, polymerization etc. Most of the catalytic processes, widely engaged in the manufacture of fine as well as bulk chemicals are homogeneous in nature and produce a large amount of side waste materials that cause a serious environmental problem. The awareness of environmental issues has put a major impact on the development of such catalytic processes that are beneficial from both, industrial and environmental point of view. Environmental issues associated with the disposal of such homogeneous catalysts, coupled with financial concerns due to their increasing costs, have led to considerable research focused towards the development of green and economically viable alternatives such as immobilization or heterogenization of homogeneous transition metal complexes based catalysts. The efficient use of solid support to immobilize such catalysts can go a long way towards achieving these goals. Various thermally stable solid supports such as chemically modified polymer, silica, alumina, mesoporous molecular sieves, multi-wall carbon nanotubes etc. have been invented for the immobilization of homogeneous transition metal complexes. The catalytic oxidation of organic substrates using transition metal complexes immobilized on polymer as catalyst has been studied well due to commercial and synthetic importance of the resulted functionalized molecules. However, impact of insertion of spacer between polymer support and catalyst has been less explored. Such spacer may allow the better interaction of catalytic center with substrates and oxidant during catalytic reaction which may result in the improved catalytic potential of polymersupported complexes. It was, therefore, reasonable to undertake systematic study on the synthesis and characterization of new vanadium and molybdenum complexes immobilized on polymer with spacer, and to explore their catalytic potential under optimized reaction conditions. v The thesis entitled “synthesis and catalytic performance of immobilized vanadium and molybdenum complexes”, describes the synthesis of oxidovanadium(IV), oxidomethioxidovanadium(V) and dioxidomolybdenum(VI) complexes with potential coordinating organic ligands immobilized on chloromethylated polymer support and their characterization by various physico-chemical techniques. Different types of catalytic oxidation reactions have been carried out and suitable reaction conditions have been obtained for the maximum oxidation of organic substrates. The reaction products have been analyzed by gas chromatograph (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 one and describes various types of solid inert support that have been used for the immobilization of homogeneous catalysts. Literature on the catalytic applications of various vanadium and molybdenum complexes has also been reviewed. Second chapter describes the synthesis of ligand H2sal-iah (I), derived from salicylaldehyde and indole-3-acetic hydrazide, which reacts with [VIVO(acac)2] in methanol to give oxidovanadium(IV) complex [VIVO(sal-iah)(H2O)] (2.1). In the presence of KOH, arial oxidation of 2.1 in methanol yields dioxidovanadium(V) complex K[VVO2(sal-iah)]·H2O (2.2). Complex 2.1 has been grafted via covalent bonding through imino nitrogen of the indole group to chloromethylated polystyrene cross-linked with 5% divinylbenzene {now abbreviated as PS-[VIVO(sal-iah)(H2O)] (2.3)}. Its oxidomethoxidovanadium(V) analog {PS-[VVO(OMe)(sal-iah)] (2.4)} has been obtained by aerial oxidation of methanolic suspension of 2.3. All these complexes have been characterized by various spectroscopic techniques (IR, electronic, 1H and 51V NMR, electron paramagnetic resonance (EPR) and thermal as well as field-emission scanning electron micrographs (FE-SEM) studies. The EPR spectrum of 2.3 indicates that the magnetically dilute VIVO-centers are well dispersed in the polymer matrix while EPR spectrum of 2.1 shows slightly different binding mode around vanadium center. 1H and 51V NMR spectra of 2.2 are compatible with the existence of expected dioxide species as the major product and oxidomethoxido species as a minor component in solution. vi Independent of the species presents in methanol, complexes 2.1 and 2.2 upon treatment with H2O2 both change to same oxidoperoxido species. The polymer-grafted complex 2.4 catalyzes the oxidation, by H2O2, of styrene and cyclohexene. Under optimized reaction conditions, the oxidation of styrene gave 95 % conversion where styrene oxide, benzaldehyde, benzoic acid, 1-phenylethane-1,2-diol and phenylacetaldehyde are products. Oxidation of cyclohexene gave 96 % conversion with cyclohexene oxide, 2- cyclohexene-1-ol, cyclohexane-1,2-diol and 2-cyclohexene-1-one as the major products. Neat complex K[VVO2(sal-iah)]·H2O (2.2) is equally active but the recyclability and heterogeneity tests of polymer-grafted complex makes it better over neat analog. Third chapter presents reaction between [VIVO(acac)2] and ONO donor tridentate ligand H2hap-iah (II) [H2hap-iah = Schiff base obtained by the condensation of equimolar amounts of o-hydroxyacetophenone (hap) and indole-3-acetic hydrazide (iah)] in equimolar ratio under oxygen atmosphere in refluxing methanol which gives [VVO(OMe)(hap-iah)] (3.1). Treatment of 3.1 in methanol with H2O2 in presence of KOH results in the formation of K[VVO(O2)(hap-iah)] (3.2). Complex 3.1 has been grafted in chloromethylated polystyrene cross-linked with 5% divinylbenzene {now abbreviated as PS-[VVO(OMe)(hap-iah)] (3.3)} via covalent bonding through imino nitrogen of the indole group. First two complexes were characterized by various spectroscopic techniques (IR, electronic, 1H and 13C NMR, ESI-MS), analytical and thermal studies. Complex 3.3 was also analyzed by field-emission scanning electron micrographs (FE– SEM) as well as Energy dispersive X-ray (EDAX) studies. 51V NMR spectrum of 3.1 is compatible with the existence of complex in three conformations. The polymer-grafted compound 3.3 was used as catalyst for the peroxidase-like oxidation of pyrogallol to purpurogallin at pH 7 buffer solution. Its high peroxidase mimicking ability, stability in a wide pH range, the easy separation from the reaction medium, and the reusability without considerable decrease in activity, suggest the practical utility of the catalyst. Fourth Chapter presents reaction of [MoVIO2(acac)2] with H2sal-iah (I) in methanol which gives dioxidomolybdenum(VI) complex [MoVIO2(sal-iah)(MeOH)] (4.1). Drop wise addition of 30% aqueous H2O2 to the methanolic solution of 4.1 yields complex vii [MoVIO(O2)(sal-iah)(MeOH)] (4.2). Complex 4.1 has been grafted via covalent bonding through imino nitrogen of the indole to chloromethylated polystyrene cross-linked with 5% divinylbenzene {now abbreviated as PS-[MoVIO2(sal-iah)(MeOH)] (4.3)}. All these complexes have been characterized by various spectroscopic techniques (IR, electronic, 1H and 13C NMR) and thermal as well as field-emission scanning electron micrographs (FE-SEM) studies. The crystal structure of 4.1 has been determined, confirming the ONO binding mode of I. The polymer-grafted complex 4.3 catalyzes the oxidative bromination, by H2O2, of styrene and trans-stilbene. Under the optimized reaction conditions, the oxidative bromination of styrene gave 98 % conversion in 2 h time where 2-bromo-1- phenylethane-1-ol and 1,2-dibromo-1-phenylethane are the main products and 1- phenylethane-1,2-diol is the product obtained by the attack of nucleophile water on the -carbon of 2-bromo-1-phenylethane-1-ol. Oxidative bromination of trans-stilbene gave 96% conversion with 2,3-diphenyloxirane (trans-stilbene oxide), 1,2-dibromo-1,2- diphenylethane and 2-bromo-1,2-diphenylethanol as the products. Suitable reaction mechanisms for both reactions have been suggested. Neat complex [MoVIO2(saliah)( MeOH)] (4.1) is equally active but the recyclability and heterogeneity tests of polymer-grafted complex makes it better over neat analog. Reaction of [MoVIO2(acac)2] with H2hap-iah (II) in 1:1 ratio in refluxing methanol gives [MoVIO2(hap-iah)(MeOH)] (5.1). Complex 5.1 has been grafted in chloromethylated polystyrene cross-linked with 5% divinylbenzene {now abbreviated as PS-[MoVIO2(hap-iah)(MeOH)] (5.2)} via covalent bonding through imino nitrogen of the indole group. Both complexes are characterized by various spectroscopic techniques (IR, electronic, 1H and 13C NMR), analytical and thermal studies. Complex 5.2 is also analyzed by atomic force microscopy (AFM), field-emission scanning electron micrographs (FE–SEM) as well as Energy dispersive X-ray (EDAX) studies. This study has been presented in Fifth Chapter. The polymer-grafted compound 5.2 has used for the catalytic oxidation of styrene and cyclohexene in the presence of NaHCO3 using aqueous H2O2 as oxidant. The intermediate peroxido species, expected to be involved during catalytic action, has also been generated from solution of 5.1 and studied by UVviii Vis. Various reaction conditions were considered to optimize reactions conditions for the maximum oxidation of substrates. Styrene under optimized reaction conditions gave mainly styrene oxide as a major product. Oxidation of cyclohexene gave cyclohexene oxide. The polymer-grafted complex shows higher conversions than its neat counterpart. The polymer-grafted complex provides additional advantage over its homogeneous counterpart in terms of increased catalyst lifetime and easier separation from the reaction mixture and allows for recyclable catalytic system. Finally, summary and over all conclusions based on the achievements are presented.