SYNTHESIS, REACTIVITY AND CATALYTIC APPLICATIONS OF MOLYBDENUM AND VANADIUM COMPLEXES

Abstract

Over the years, coordination metal complexes of both vanadium and molybdenum derived form schiff base ligands are of much significance due to their vital role in several organic transformations scuh as oxidation of alcohols, epoxidation of olefins, oxidative bromination of organic substrates, oxidation of methyl phenyl sulpfide etc. as wll as due to biological and therapeutic reasons. These complexes have been extensively used as catalyst in both homogeneous and hetrogeneous phases in presence of suitable oxidant such as H2O2 or molecular oxygen under mild conditions. The thesis entitled “Synthesis, reactivity and catalytic applications of molybdenum and vanadium complexes”, describes synthesis of several types of molybdenum(VI) and vanadium(V) complexes including mmononuclear dioxidomolybdenum(VI), dinuclear dioxidomolybdenum(VI), oxidoperoxidomolybdenum(VI), dioxidovanadium(V), oxidoperoxidovanadium(V) and oxido-bridgedvanadium(V) complexes containing ONO donor tridentate organic ligands. These ligands and complexes have been characterised by various spectroscopic techniques and single crystal X-ray study. These Schiff base complexes have been used to carry out several organic transformations and reaction conditions were optimized for maximum conversion of these substrates. This thesis work has been divided in to following three chapters. First chapter is the introductory one and describes the significance of various types of molybdenum(VI) and vanadium(V) complexes derived from tridentate ONO donor ligands and their catalytic importance. Second chapter delineates four ONO donor ligands isolated from the condensation of 4,6-diacetyl resorcinol with isonicotinoyl hydrazide (H2dar-inh, 2.I), nicotinoyl hydrazide (H2dar-nah, 2.II), benzoyl hydrazide (H2dar-bhz, 2.III) and 2-furoyl hydrazide (H2dar-fah, 2.IV) on refluxing in MeOH. The reaction of in situ generated aqueous K[H2VVO4] with ligands 2.I-2.IV at neutral pH gives complexes, [K(H2O)2][VO2(dar-inh)] (2.1), [K(H2O)2][VO2(dar-nah)] (2.2), [K(H2O)2][VO2(dar-bhz)] (2.3) and [K(H2O)2][VO2(dar-fah)] (2.4), respectively. The reaction of [VIVO(acac)2] (acac = acetylacetonato) with these ligands (2.I-2.IV) under aerobic conditions in methanol yields oxidomethoxidovanadium(V) complexes, [VO(OMe)(MeOH)(dar-inh)] (2.5), [VO(OMe)(MeOH)(dar-nah)] (2.6), [VO(OMe)(MeOH)(dar-bhz)] (2.7) and [VO(OMe)(MeOH)(dar-fah)] (2.8). All the isolated complexes have been characterized by elemental, thermal, electrochemical and spectroscopic techniques [FT-IR, UV-Vis, NMR (1H, 13C and 51V NMR)] and single crystal X-ray diffraction analysis (for 2.1, 2.6, 2.7 and 2.8). X-ray analysis confirms the coordination of the ligands through Ophenolate, Nazomethine and Oenolate to the metal center. In the molecular structure of [K(H2O)(EtOH)][VVO2(darinh)] (abbreviated as 2.1a where one molecule of water is replaced by EtOH), water molecules act as bridges between two K+ ions and the complex shows a dimeric structure due to the presence of electrostatic interactions between V=O oxygen atoms with K+ ions. These complexes are active catalysts for the oxidative bromination of thymol in the presence of KBr, HClO4 and H2O2 and give 2-bromothymol, 4-bromothymol and 2,4- dibromothymol as major products. Complexes 2.1-2.4 were also tested as catalysts for the epoxidation of various alkenes (namely styrene, cyclohexene, cis-cyclooctene, 1-hexene, 1-octene, cyclohexenone and trans-stilbene) with H2O2 in the presence of NaHCO3 as promoter giving the corresponding epoxides selectively. Third chapter accounts the condensation of 2,4-dihydroxyacetophenone with isonicotinoyl hydrazide and nicotinoyl hydrazide in an equimolar ratio in methanol resulting in the formation of two stable ONO donor ligands (H2dhap-inh, 3.I) and (H2dhapnah, 3.II), respectively. These ligands upon reacting with [VIVO(acac)2] (Hacac = acetylacetone) in 1:1 molar ratio give the corresponding neutral dioxidovanadium(V) complexes [VVO2(Hdhap-inh)] (3.1) and [VVO2(Hdhap-nah)] (3.2) after overnight aerial oxidation in the presence of K2CO3. In the absence of K2CO3, they result in μ- O{bisoxidovanadium(V)} complexes, [{VVO(dhap-inh)}2(μ-O)] (3.3) and [{VVO(dhapnah)} 2(μ-O)] (3.4). Treatment of neutral dioxidovanadium(V) complexes, 3.1 and 3.2,with H2O2 yield oxidoperoxidovanadium(V) complexes K[VVO(O2)(dhap-inh)(H2O)] (3.5) and K[VVO(O2)(dhap-nah)(H2O)] (3.6). The ligands also react with [MoVIO2(acac)2] in 1:1 molar ratio and form [MoVIO2(dhap-inh)]n (3.7) and [MoVIO2(dhap-nah)]n (3.8). The reaction of oxidoperoxidomolybdenum(VI) species, generated in situ by the reaction of MoO3 with H2O2, with ligands 3.I and 3.II gives the oxidoperoxidomolybdenum(VI) complexes, [MoVIO(O2)(dhap-inh)(MeOH)] (3.9) and [MoVIO(O2)(dhap-nah)(MeOH)] (3.10), respectively. All synthesized complexes were characterized by elemental analysis, thermogravimetric, electrochemical, spectroscopic (IR, UV–Vis, 1H, 13C and 51V NMR) and single crystal X-ray diffraction (for 3.1, 3.2 and 3.8). These complexes have been explored as catalysts for the oxidative bromination of phenol, a model oxidative halogenation reaction, in the presence of HClO4 using 30% H2O2 in the presence of KBr. Under optimized reaction conditions, the selectivity of products follows the order: 4- bromophenol>2-bromophenol with both molybdenum and vanadium complexes. Products characterized by GC analysis showed very good conversion of phenol. Fourth chapter describes the synthesis of four binuclear dioxidomolybdenum(VI) complexes with Schiff base ligands, H4dar(bhz)2 (4.I),H4dar(inh)2 (4.II), H4dar(nah)2 (4.III) and H4dar(fah)2 (4.IV) (H2dar = 4,6-diacetyl resorcinol and Hbhz = benzoylhydrazide, Hinh = isonicotinoylhydrazide, Hnah = nicotinoylhydrazide, and Hfah = 2-furoylhydrazide) prepared by the reaction of these ligand with [MoVIO2(acac)2] (Hacac = acetylacetone) in 1:2 ratio in methanol. Isolated complexes [(MoVIO2)2{dar(bhz)2}(H2O)2] (4.1), [(MoVIO2)2{dar(inh)2}(H2O)2] (4.2), [(MoVIO2)2{dar(nah)2}(H2O)2] (4.3) and [(MoVIO2)2{dar(fah)2}(H2O)2] (4.4) have been characterized by elemental (CHN) analysis, spectral (FT-IR, UV-vis, 1H and 13C NMR) and thermogravimetric analysis. Single crystal X-ray diffraction analysis of complexes 4.1, 4.2 and 4.4 confirms their binuclear structure and distorted octahedral geometry around each molybdenum where ligands behave as bis(dibasic tridentate) coordinating through phenolic oxygen, azomethine nitrogen and enolic oxygen atoms of each unit. These complexes have been successfully used as catalyst precursors in solvent as well as solvent free conditionsfor the one-pot three components (ethylacetoacetate, benzaldehyde and ammonium acetate) dynamic covalent assembly, via Hantzsch reactionusing hydrogen peroxide as an oxidant. Finally, summary and all the conclusions based on the achievements are mentioned. Keywords: Molybdenum complexes; Vanadium complexes; Spectroscopic studies; Catalytic activities, Oxidative bromination; Epoxidation of organic substrates; Multicomponent reaction.