SYNTHESIS, REACTIVITY, STRUCTURAL AND CATALYTIC ASPECTS OF VANADIUM COMPLEXES

Abstract

Vanadium was named after Vanadis, the most aristocratic of Scandinavian goddess, who symbolizes beauty and fertility - essential features of vanadium chemistry. Vanadium may be beneficial and possibly essential in humans, but certainly essential for certain organisms, including tunieates, bacteria and some fungi. Vanadium with atomic number 23 and electronic configuration [Ar]3d 4s , exhibits formal oxidation states from +V down to -III. Vanadium makes more stable compounds in the oxidation states of +IV and +V under normal conditions, and generally stabilizes through V-0 bond. The oxocations [VO]2+, [VO] 4 and [V02]+ are most common for biological systems. The discovery of vanadium(V) in vanadium based enzymes e.g. vanadatedependent haloperoxidases, attracted attention of researcher to develop coordination chemistry of vanadium(V) in search of good models for vanadiumcontaining biomolecules. Vanadium compounds have also been found to promote a peroxide-driven oxidation of organic substrates. As zeolite encapsulated metal complexes enjoy the advantageous features of homo- as well as heterogeneous catalysts, these materials have provided opportunities to develop catalytic processes for various reactions such as alkylation, hydrogenation. dehydrogenation, amination, oxidation, cyclization, acylation, isomerisation etc. All these clearly indicated the need for further development of coordination chemistry of vanadium In view of the above it may clearly be conceived that the coordination complexes of vanadium and their encapsulation in the nano- cavity of zeolite-Y arc oi' increasing potential interest. The present thesis is therefore, aimed to H describe the coordination chemistry of vanadium and study catalytic potential of neat as well as encapsulated complexes. Stability, structural and reactivity studies have been carried out to model the role of vanadium in vanadium based enzymes. For convenience the work presented in the thesis has been divided in the following chapters: First chapter is introductory one and deals with the general remarks on vanadium and its occurrence in biological systems. Literature on vanadium complexes, and their catalytic applications (neat as well as zeolite-Y encapsulated) for various organic transformations have also been reviewed. Reaction between [VIV0(acac)2] and the ONN donor Schiff base Hpydxaepy (2.1) (Hpydx-aepy = Schiff base obtained by the condensation of pyridoxal and 2-aminoethylpyridine) results in the formation of a complex |VlvO(acac)(pydx-aepy)] (2.1). Addition of aqueous 30 %H202 to 2.1 yields the poor stable oxidoperoxidovanadium(V) complex [V 0(02)(pydx-aepy)] (2.2). Its formation has also been demonstrated in solution by treating 2.1 with H202 in methanol. Reaction of vanadium exchanged zeolite-Y with 2.1 in methanol followed by aerial oxidation gave zeolite-Y encapsulated dioxidovanadium(V) complex, abbreviated as [Vv02(pydx-aepy)]-Y (2.3). Characterization of all these complexes by metal analysis, spectroscopic (IR, electronic, 'H and 5IV NMR) studies, scanning electron micrographs and X-ray diffraction patterns are presented in Second chapter. The molecular structure of 2.1 confirms its distorted octahedral geometry with the ONN binding mode of the tridentate ligand while one acetylacetonato group remains bound to the VlvO-centre. The encapsulated complex [Vv02(pydx-aepy)]-Y (2.3) catalyzes the oxidation of styrene, cyclohexene, methyl phenyl sulfide and diphenyl sulfide using H202 as oxidant in good yield. Styrene under optimized reaction conditions gave four reaction products namely, styrene oxide, benzaldehyde. l-phenylethane-l,2-diol and benzoic acid while cyclohexene gave cyclohexene epoxide, 2-cyclohexene-l-one, iii 2-cyclohexene-l-ol and cyclohexane-l,2-diol. Organic sulfides gave the corresponding sulfoxide as the major product. Neat complex [VlvO(acac)(pydxaepy)] has been used as catalyst precursor to compare its catalytic activities with the encapsulated one. Third chapter is based on the Schiff bases obtained by the condensation of pyridoxal and 2-aminoethylbenzimidazole (Hpydx-aebmz, 3.1) or 2-aminomethylbenzimidazole (Hpydx-ambmz, 3.II) and their vanadium complexes. Thus, complexes [VlvO(acac)(pydx-aebmz)] (3.1) and [V,vO(acac)(pydx-ambmz)| (3.2) have been isolated with these ligands. The aerobic oxidation of the methanolic solution of 3.1 yielded [Vv02(pydx-aebmz)] (3.3) and its reaction with aqueous H202 gave the oxidoperoxidovanadium(V) complex, [VvO(02)(pydx-aebmz)] (3.4). The formation of 3.4 in solution has also been established by titration of methanolic solutions of 3.1 with H202. The full geometry optimization of all species envisaged was done using DFT methods for suitable model complexes. The * V NMR chemical shifts (8 ) have also been calculated, the theoretical data being used to support assignments of the experimental chemical shifts. The 3lV hyperfine coupling constants have been calculated for 3.1, the obtained values being in good agreement with the experimental EPR data. Reaction between the VIV0 + exchanged zeolite-Y and Hpydx-aebmz and Hpydx-ambmz in refluxing methanol followed by aerial oxidation results in the formation of the encapsulated complexes, fVv02(pydx-aebmz)]-Y (3.5) and [Vv02(pydx-ambmz)]-Y (3.6). The molecular structure of 3.1, determined by single crystal X-ray diffraction, confirms its distorted octahedral geometry. Oxidation of styrene is investigated using some of these complexes as catalyst precursors with H202 as oxidant. Under optimized reaction conditions for the conversion of styrene in acetonitrile, a maximum of 68 % conversion of styrene (with [V 02(pydx-aebmz)]-Y) and 65 % (with [Vv02(pydx-ambmz)]-Y) is achieved in 6 h of reaction time. The selectivity of the various products is similar for both catalysts and follows the order: IV benzaldehyde >l-phenylethane-l,2-diol > benzoic acid > styrene oxide > phenyl acetaldehyde. Speciation of the systems and plausible intermediates involved in the catalytic oxidation processes are established by UV-Vis, EPR, 5IV NMR and DFT studies. Both non-radical (Sharpless) and radical mechanisms of the olefin oxidations have been theoretically studied, and the radical pathway has been found to be more favorable than the Sharpless mechanism. Fourth chapter describes the reaction between [VIV0(acac)2] and the ONN donor ligand (E)-4-f(2-(dimethylamino)ethyliminolmethyl-5-(hydroxymethyl)-2- methylpyridin-3-ol (Hpydx-dmen, 4.1) that resulted in the formation of the complex |V vO(acac)(pydx-dmcn)] (4.1). Structure of [VlvO(acac)(pydx-dmen)J (4.1) has been confirmed by single crystal X-ray study. The poor stable oxidoperoxidovanadium(V) complex [VvO(02)(pydx-dmen)] (4.2) has been prepared by the treatment of 4.1 with aqueous 30 % H202. Its formation has also been demonstrated in solution by treating 4.1 with H202 in methanol. Treatment of 4.1 with vanadium(IV) exchanged zeolite-Y followed by aerial oxidation gave dioxidovanadium(V) complex encapsulated in the nano-cavity of zeolite-Y, |Vv()2(pydx-dmen)]-Y (4.3). The encapsulated complex |Vv02(pydx-dmen)J-Y (4.3) catalyzes the oxidation of styrene, methyl phenyl sulfide, diphenyl sulfide and cyclohexene using H202 as oxidant in good yield. Fifth chapter describes the reaction of [VIV0(acac)2] with 03N2 donor ligands H3sal-dahp (5.1) and H3hap-dahp (5.II) (Hsal = salicylaldehyde, Hhap = 2- hydroxyacetophenone and Hdahp = 1,3-diamino-2-hydroxypropane) that resulted in the formation of complexes [VlvO(sal-dahp)j (5.1) and [VvO(hap-dahp)J (5.2), respectively. Treatment of 5.1 and 5.II with vanadium(IV) exchanged zeolite-Y gave encapsulated [VO(sal-dahp)]-Y (5.3) and [VO(hap-dahp)]-Y (5.4) complexes in zeolite-Y. Structure of [VO(hap-dahp)] (5.2) has been confirmed by single crystal X-ray study. The encapsulated complexes [VO(sal-dahp)]-Y (5.3) and [VO(hap-dahp)]-Y (5.4) catalyze the oxidation of styrene, methyl phenyl sulfide and diphenyl sulfide using H202 as oxidant in good yield. Styrene under optimized reaction conditions gave five reaction products namely, styrene oxide, benzaldehyde, l-phenylethane-l,2-diol, benzoic acid and phenylacetaldehyde while organic sulfides gave the corresponding sulfoxides as the major product. Neat complexes [VIV0(sal-dahp)] (5.1) and [VvO(hap-dahp)] (5.2) also exhibit good catalytic activity for these substrates. Sixth chapter describes the reactivity of oxidovanadium(IV) complexes |VlvO(acac)(pydx-aebmz)], [V[v0(acac)(pydx-dmen)] and fVlvO(acac)(pydxaepy)] prepared in chapters 2 to 4 towards catechol, benzohydroxamic acid and 8- hydroxyquinoline. Reaction of ligands catechol and benzohydroxamic acid causes immediate oxidation of vanadium(IV) complexes and the formation of vanadium(V) complexes of the types [VvO(cat)L] and [VvO(bha)L], respectively. Octahedral structure of one of the complexes [VvO(bha)(pydx-dmen)] has been confirmed by single crystal X-ray study. These complexes are, though, stable in the solid state, they are poor stable in solution and convert slowly into the corresponding dioxido species. Their stabilities in solution have been studied by monitoring spectral changes (UV-vis and 5IV NMR). Reaction of 8- hydroxyquinoline with [VlvO(acac)(pydx-aebmz)] proceeds through the formation of a green oxidovanadium(IV) complex, [VIV0(hq)2)], which is isolable from acetone. However, in methanol this product oxidizes fast to give [VvO(OMe)(hq)2)]; structure of which has been confirmed by single crystal X-ray study.