CATALYTIC ROLE OF IMMOBILIZED VANADIUM COMPLEXES IN OXIDATION REACTIONS

Abstract

A growing interest in the chemistry of vanadium is based on the recognition of its importance from the biological and pharmacological perspective. The potential therapeutic use of vanadium compounds in the treatment of parasitic diseases, diabetes and cancer as well as capability of cleaving DNA in visible light and cellular proteins on irradiation with light additionally stimulated the coordination chemistry of vanadium. Vanadium compounds have also been found to act as catalyst precursor for the oxidation of organic substrates. Basic chemistry and potential applications of vanadium complexes in diverse fields have been discussed time to time in International Vanadium Chemistry symposium held biannually. Several research papers and review articles have appeared in the literature in recent years on the catalytic applications of vanadium complexes. The functionalized polymers (cross-linked as well as non-cross-linked) have widely been used as support to immobilize metal complexes through covalent bonding. These polymer-supported transition metal complexes have widely been used as catalyst for many organic transformations due to their additional advantages over simple transition metal complexes like operational flexibility due to their insolubility, recycle ability, better product selectivity and activity due to active site isolation and high surface area. Polymer-supported vanadium complexes have shown better results not only in modeling oxidative halogenations reactions but in other vanadium mediated oxygen transfer reactions as well. In view of the above it was considered desirable to study catalytic role of polymer-supported vanadium complexes in oxidation reactions. For convenience, the work embodied in the thesis has been divided into following chapters: First chapter is introductory one and presents general remarks on supported complexes and in particular polymer-supported vanadium complexes. Updated literature survey has also been included here. Second chapter is based on polymer supported vanadium complexes with 1-(2- pyridylazo)-2-naphthol (Hpan) and their catalytic activities. Monobasic tridentate ONN donor ligand, 1-(2-pyridylazo)-2-naphthol [Hpan (2.I)] reacts with [VIVO(acac)2] in dry ii methanol to yield the oxidovanadium(IV) complex [VIVO(acac)(pan)] (2.1). The dioxidovanadium(V) complex [{VVO(pan)}2(μ-O)2] (2.2) is obtained by aerial oxidation of 2.1 in methanol. Complex 2.2 can also be prepared directly by reacting [VIVO(acac)2] with 2.I followed by aerial oxidation in methanol. Treatment of 2.1 or 2.2 in methanol with H2O2 yields the oxidomonoperoxidovanadium(V) complex [VVO(O2)(pan)(MeOH)] (2.3). Reaction of imidazolomethylpolystyrene cross-linked with 5 % divinylbenzene (PS–im) with 2.2 in DMF resulted in the formation of polymer-grafted dioxidovanadium(V) complex, PS–im[VVO2(pan)] (2.4). All these complexes are characterized by various spectroscopic techniques (IR, electronic, NMR (1H and 51V), and electron paramagnetic resonance (EPR)), thermal, field-emission scanning electron micrographs (FE–SEM) as well as Energy dispersive X-ray (EDX) studies. The crystal and molecular structure of 2.3 has been determined, confirming the ONN binding mode of 2.I. The polymer-grafted complex 2.4 has been used for the oxidative bromination of styrene, salicylaldehyde and trans–stilbene. Various parameters, such as amounts of catalyst, oxidant (aqueous 30 % H2O2), KBr and aqueous 70 % HClO4 have been optimized to obtain the maximum oxidative bromination of substrates. Under the optimized reaction conditions, styrene gave a maximum of 99 % conversion after 2 h of reaction with the main products having a selectivity order of: 1-phenylethane-1,2-diol (75 %) > 2-bromo-1-phenylethane-1-ol (20 %) > 1,2-dibromo-1-phenylethane (1.2 %). With nearly same conversion in same time, oxidative bromination of salicylaldehyde gave three products with the selectivity order: 5-bromosalicylaldehyde > 2,4,6-tribromophenol > 3,5-dibromosalicylaldehyde. A maximum of 91 % conversion of trans–stilbene has been obtained in 2 h of reaction time where selectivity of the obtained reaction products varied in the order: 2,3- diphenyloxirane (trans–stilbene oxide) > 1,2-dibromo-1,2-diphenylethane > 2-bromo- 1,2-diphenylethanol. Catalytic activity of nonpolymer grafted complex 2.2 is lower than that of the polymer-grafted one. In addition, the recycle ability of grafted complex makes it better over neat one. Three neat complexes [VVO2(acpy-bhz)] (3.1) [VVO2(acpy-inh)] (3.2) and [VVO2(acpy-nah] (3.3) and the corresponding polymer-supported (PS) dioxidovanadium(V) complexes having monobasic tridentate ONN donor ligands, iii abbreviated herein as PS-im[VVO2(acpy-bhz)] (3.4) PS-im[VVO2(acpy-inh)] (3.5) and PS-im[VVO2(acpy-nah] (3.6) have been isolated through covalent bonding of imidazolomethylpolystyrene, obtained by reacting chloromethylated polystyrene crosslinked with 5 % divinylbenzene with imidazole, with the corresponding neat complexes 3.1, 3.2 and 3.3. All compounds are characterized in solid state and in solution, namely by spectroscopic techniques (IR, UV-Vis, 51V NMR, thermal and scanning electron micrograph studies) and whole studies are reported in Chapter 3. The monomeric form {[VO2(acpy-nah)]·DMSO (3.3·DMSO)} (3.3a) of complex 3.3 has also been isolated from its solution in DMSO and its molecular structure is confirmed by single crystal Xray diffraction. Polymer-supported as well neat complexes have been used as catalyst precursors for the oxidative bromination of styrene and trans-stilbene using 30 % aqueous H2O2 as an oxidant, the compounds acting as functional models of vanadium dependent haloperoxidases. 1-phenylethane-1,2-diol, 2-bromo-1-phenylethane-1-ol (bromohydrin) and 1,2-dibromo-1-phenylethane are the reaction products of styrene after 1 h of reaction, while those of trans-stilbene are: 2,3-diphenyloxirane (transstilbene oxide), 2-bromo-1,2-diphenylethanol and 1,2-dibromo-1,2-diphenylethane. It has also been shown that all these compounds are catalyst precursors for the catalytic oxidation of benzoin by peroxide, the products being benzil, methylbenzoate, benzoic acid and benzaldehyde-dimethylacetal. An outline of the mechanism has been proposed and plausible intermediates involved in the catalytic processes are proposed/ established by UV-Vis and 51V NMR studies. Fourth chapter deals with polymer supported as well as neat vanadium complexes of 2-benzoylpyridine based ONN donor ligands. Reaction between monobasic tridentate ONN donor ligands, Hbzpy-tch (4.I) and Hbzpy-inh (4.II) with [VIVO(acac)2] in dry methanol gives two different types of complexes, [VOIV(acac)(bzpy-tch)] (4.1) and [VOIV(OMe)(bzpy-inh)] (4.2), respectively. Irrespective of their nature both complexes upon aerial oxidation in methanol give dimeric [{VVO(bzpy-tch)}2(μ-O2)] (4.3) and [{VVO(bzpy-inh)}2(μ-O2)] (4.4). These complexes can also be prepared directly by reacting [VIVO(acac)2] with these ligands followed by aerial oxidation in methanol. Treatment of 4.1 or 4.2 in methanol with H2O2 yields the oxidomonoperoxidovanadium(V) complexes [VVO(O2)(bzpyiv tch)(MeOH)] (4.5) and [VVO(O2)(bzpy-inh)(MeOH)] (4.6). Reaction of 4.3 and 4.4 with imidazolomethylpolystyrene cross-linked with 5 % divinylbenzene (PS–im) in DMF gives polymer-supported dioxidovanadium(V) complex, PS–im[VVO2(bzpy-inh)] (4.7) and PS–im[VVO2(bzpy-tch)] (4.8). The complexes have been characterized by various spectroscopic techniques (IR, electronic, NMR (1H and 51V), and electron paramagnetic resonance (EPR)), ESI-MS, thermal, atomic force microscopy (AFM), field-emission scanning electron micrographs (FE–SEM) as well as energy dispersive X-ray (EDAX) studies. The crystal and molecular structures of 4.3 and 4.4 have been determined confirming the μ-bis(O) and ONN binding mode of 4.II in dimeric structure. The polymer-grafted complexes 4.7 and 4.8 have been used for the oxidation of isoeugenol. Intermediate peroxide species involved during catalytic action have also been isolated in the solid state as well as generated in solution and studied. Catalytic activity of nonpolymer supported complexes has also been carried out which show lower conversion than that of the polymer-grafted one. The recycle ability of grafted complex makes them better over neat ones. Chapter 5 describes peroxidase mimicking property of polymer-supported complex PS-[VIVO(sal-dahp)] at pH 7 in aqueous medium. Polymeric complex [VIVO(Hsal-dahp)]n has been prepared by the reaction of VOSO4 or [VIVO(acac)2] with dibasic pentadentate ligand H3sal-dahp (sal = salicylaldehyde and dahp = 1,3-diamino- 2-hydroxypropane) in methanol. The polymeric complex upon dissolving in hot DMSO yields the monomeric complex [VIVO(Hsal-dahp)(dmso)] (5.1); single crystal X-ray analysis of which confirms the coordination of two phenolic oxygen and two imine nitrogen atoms of the ligand to the vanadium center while hydroxyl group of the ligand does not participate in coordination. Reaction of 5.1 with chloromethylated polystyrene crosslinked with 5 % divinylbenzene (abbreviated as PS-Cl) in DMF in the presence of triethylamine and K2CO3 gives stable polymer-supported oxidovanadium(IV) complex PS-[VIVO(sal-dahp)] (5.2). Both complexes have been characterized by IR, electronic and EPR spectral studies, thermogravimetric analysis, field-emission scanning electron micrographs (FE-SEM) and energy dispersive X-ray (EDAX) as well as atomic force microscopy (AFM) studies. The polymer-supported complex 5.2 has been successfully used for the peroxidase-like oxidation of pyrogallol. The plausible intermediate species v formed during peroxidase mimicking activity has been established in solution electronic absorption spectrophotometrically. A good peroxidase mimicking property of polymersupported complex 5.2 at pH 7 in aqueous medium, its stability in a wide range of pHs, easy separation from the reaction medium and reusability without considerable decrease in activity i.e. maintaining its heterogeneity makes it better over its natural counterparts i.e. biological systems in terms of its greener application in industry. Finally, summary and over all conclusions based on the achievements are presented.