VANADIUM, MOLYBDENUM AND TUNGSTEN COMPLEXES, THEIR REACTIVITY AND CATALYTIC APPLICATIONS

Abstract

Among the transition metal complexes, complexes of vanadium, molybdenum and tungsten derived from ONO donor ligands are of much significance due to their vital role in several organic transformations such as oxidation of alcohols, epoxidation of olefins, oxidative bromination of organic substrates, oxidation of methyl phenyl sulfide, multicomponent reactions etc. as well as biological and therapeutic reasons. These complexes have been extensively used as catalysts in both homogeneous and heterogeneous phases in the presence of a suitable oxidant. However, vanadium, molybdenum and tungsten complexes immobilized on amine-functionalized titania and organic polymer are not much reported. Therefore, this thesis aims to synthesize heterogeneous complexes by immobilizing them on solid supports like titania and chloromethylated polystyrene and explore their catalytic potentials in various organic transformations, multicomponent reaction and biomimicking catalysts. The thesis entitled “Vanadium, molybdenum and tungsten complexes, their reactivity and catalytic applications”, describes the synthesis of heterogeneous vanadium(V), molybdenum(VI) and tungsten(VI) complexes containing ONO donor tridentate organic ligands. These ligands and complexes have been characterized by various spectroscopic techniques and single crystal X-ray studies. These heterogeneous complexes have been used to carry out several organic transformations and reaction conditions were optimized for maximum conversion of these substrates. For convenience, the work presented in this thesis has been divided into the following five chapters: Chapter 1 is the introductory one and discusses the historical background on vanadium, molybdenum and tungsten as the active metal centre present in various organisms. The pros and cons of heterogeneous and homogeneous catalysts in the catalytic reactions are described. A brief discussion is done on various types of solid inert supports used in literature for the immobilization of homogeneous catalysts. A particular emphasis has been given to functionalized titania (TiO2) and polystyrene for the immobilization of homogeneous complexes. A brief and updated literature on titania and polymer immobilized metal complexes and their catalytic applications in various organic transformations has also been reviewed. A final section on the research gap in this particular area and the objective of the present work have been presented in this chapter.Chapter 2a describes the synthesis of two dioxidomolybdenum(VI) complexes, [MoVIO2{H2dfba(bhz)2}(H2O)] (2a.1) and [MoVIO2{H2dfba(fah)2}(H2O)] (2a.2) of ligands H4dfba(bhz)2 (2a.I) and H4dfba(fah)2 (2a.II) (H2dfba = 3,5-diformyl-4-hydroxybenzoic acid, Hbhz = benzoylhydrazide and Hfah = 2-furanoylhydrazide), respectively and their corresponding heterogenized complexes [MoVIO2{Hdfba(bhz)2}(H2O)]@APTMS-TiO2 (2a.3) and [MoVIO2{Hdfba(fah)2}(H2O)]@APTMS-TiO2 (2a.4) supported on amine functionalized titanium dioxide (APTMS-TiO2, 2a.III). These complexes have been isolated and characterized by various spectroscopic techniques (FT-IR, UV-vis, diffusion reflectance, 1H and 13C NMR), elemental analysis (C, H and N), thermal, TEM and powder XRD studies. Single-crystal X-ray study of [MoVIO2{H2dfba(bhz)2}(EtOH)]·EtOH (2a.1a·EtOH) confirms the octahedral structure. Both homogeneous and heterogeneous complexes have been explored as phenoxazinone synthase mimicking catalysts in the oxidation of 2-aminophenol to 2-aminophenoxazine-3-one in acetonitrile in the presence of aq. H2O2. Chapter 2b presents the synthesis of two mononuclear cis-[MoVIO2] complexes with Schiff base ligands, H4L1 (2b.I) and H4L2 (2b.II) (derived from the condensation of 3,5-diformyl-4-hydroxybenzoic acid with isonicotinoylhydrazide and nicotinoylhydrazide), synthesized using [MoVIO2(acac)2] (Hacac = acetylacetone) and ligand in 1:1 molar ratio in methanol. The obtained homogeneous complexes [MoVIO2(H2L1)]n (2b.1) and [MoVIO2(H2L2)(MeOH)] (2b.2) were supported on titania functionalized with amine group (APTMS-TiO2) to obtain heterogeneous complexes [MoVIO2(HL1)(MeOH)]@APTMS-TiO2 (2b.3) and [MoVIO2(HL2)(MeOH)]@APTMS TiO2 (2b.4), respectively. All the isolated complexes were characterized by various spectral techniques (FT-IR, UV-visible, 1H and 13C NMR), elemental (CHN) and thermogravimetric analyses; heterogeneous complexes were additionally characterized by transmission electron microscopy, diffuse reflectance, X-ray photoelectron spectroscopy and Powder-X-ray diffraction studies. The single-crystal X-ray diffraction analysis of complexes 2b.1 and 2b.2 confirms the distorted octahedral geometry for both complexes, where ligands coordinate to the metal centre through Ophenolate, Nazomethine, and Oenolate. Complex 2b.1 has a 3D polymeric structure, where the sixth site of one molecule of Mo is coordinated to the pyridinic nitrogen of the other molecule, while 2b.2 is monomeric. These complexes are explored as bio-mimicking model catalysts for the type II copper site in phenoxazinone synthase. These complexes catalyze the oxidative condensation of o aminophenol (OAP) into 2-aminophenoxazine-3-one (APX) utilizing aq. H2O2 in the MeCN–MeOH mixture. Chapter 3 includes the synthesis of mononuclear dioxotungsten(VI) complex [WVIO2(CH3-hptb)(H2O)] (3.1) of deferasirox (4-[3,5-bis(2-hydroxyphenyl)-1H-1,2,4 triazol-1-yl]benzoic acid, H3hptb, 3.I), which is synthesized by the reaction of [WVIO2(acac)2] with 3.I in MeOH and successfully characterized by various spectral techniques (FT-IR, UV-visible, 1H and 13C NMR), elemental (CHN) and thermogravimetric analyses. During the metal complex formation, the esterification of the carboxylic group of ligand occurs. A similar reaction of [WVIO2(acac)2] with heterogeneous ligand [H2hptb]@APTMS-TiO2 (3.II) (i.e. ligand 3.I immobilized on amine-functionalized titania) in MeOH produced the heterogeneous complex [WVIO2(hptb)(H2O)]@APTMS TiO2 (3.2), which was characterized by additional techniques such as DRS, P-XRD, TEM and XPS. The thermal stability and metal content of complex 3.2 have been achieved through thermogravimetric analysis and MP-AES, respectively. Both complexes have been utilized in bio-mimicking the type II copper-containing enzyme phenoxazinone synthase (PHS). In the presence of a green oxidant (aq. H2O2), both complexes produce 2 aminophenoxazine-3-one (APX) through the catalytic oxidative coupling of o aminophenol (o-AP) in MeOH. Chapter 4 elaborates on the synthesis of cis-[MoVIO2(CH3-hptb)(MeOH)] (4.1), synthesized by the reaction of [MoVIO2(acac)2] and deferasirox [ICL670: 4-[3,5-bis(2 hydroxyphenyl)-1H-1,2,4-triazol-1-yl]benzoic acid, H3hptb, 4.I] in 1:1 molar ratio in MeOH. Complex 4.1 has been characterized successfully by several spectral techniques, viz. FT-IR, UV-visible, 1H and 13C NMR, elemental (CHN) and thermogravimetric analysis. The esterification of the carboxylic group of ligand was observed during the complexation process with molybdenum. The analogous heterogeneous complex [MoVIO2(hptb)(MeOH)]@APTMS-TiO2 (4.2) has been synthesized by immobilizing 4.I on aminofunctionalized TiO2, {[H2hptb]@APTMS-TiO2 (4.II)} and then reacting it with [MoVIO2(acac)2] in MeOH. Characterization of the immobilized complex was achieved through FT-IR, DRS, P-XRD, and TEM, thermogravimetric analysis and MP-AES. Both complexes have been utilized in synthesizing organobromine compounds through oxidative bromination of styrene and thymol in the presence of KBr, oxidant (30 % aq. H2O2) and 70 % aq. HClO4 at room temperature. Chapter 5 consists of the synthesis of oxidovanadium(V) complex [VVO(OEt)(µ Hhptb)]2 (5.1) of 4-[3,5-bis(2-hydroxyphenyl)-1H-1,2,4-triazol-1-yl] benzoic acid (H3hptb), prepared by the reaction of VVO(OEt)3 with ligand in EtOH. Further, complex 5.1 has been immobilized on chloromethylated polystyrene (PS–Cl) to obtain PS [VVO(OEt)(hptb)(EtOH)] (5.2). The homogeneous complex 5.1 was characterized by elemental analysis, spectroscopic techniques (FT-IR, UV-visible, 1H and 13C NMR) and thermogravimetric analysis, while the heterogeneous complex was additionally studied by field emission-scanning electron microscopy, energy dispersive spectroscopy and atomic force microscopy. These complexes have been utilized in catalyzing one-pot three component (benzaldehyde or its derivatives, urea and ethyl acetoacetate) Biginelli reaction producing multifunctionalized 3,4-dihydropyrimidin-2-(1H)-ones (DHPMs) under solvent-free conditions. It was observed that H2O2 helps in improving the yield of the products. Various parameters, such as the amount of catalyst and oxidant, time, temperature, and solvent effects, were optimized for the maximum yield of DHPMs. A suitable reaction pathway for this reaction has been proposed with the help of DFT computational studies. Finally, the summary and all the conclusions based on the achievements are mentioned.