MOLYBDENUM COMPLEXES OF BIOINORGANIC AND CATALYTIC INTEREST

Abstract

Molybdenum catalysts are gaining popularity because of their catalytic uses in number of scientific and industrial fields. As a transition metal with a variety of oxidation states, molybdenum may participate in a broad range of redox processes. Its function in organic transformations, where molybdenum catalysts power processes like olefin metathesis, olefin epoxidation, oxidative bromination, and multicomponent synthesis are especially noteworthy. Molybdenum catalysts are essential to the hydrodesulfurization process in the petroleum industry, which yields cleaner fuels. Furthermore, studies on molybdenum's potential for artificial nitrogen fixation have been sparked by its importance in biological nitrogen fixation, which support sustainable farming practices. The growing interest in using molybdenum for various purposes is highlighted by its roles in oxidation reactions, car catalytic converters, and biological enzymes. The literature has published a number of research studies and review articles on the catalytic applications of molybdenum complexes in recent years. Keeping in mind the wide catalytic applications of molybdenum complexes, the present Ph.D. thesis concentrates on the synthesis of different types of mono- bi- and trinuclear dioxidomolybdenum(VI) complexes, their characterization by various instrumental techniques, and catalytic applications for various organic transformations. In addition to introduction presented in Chapter 1, the thesis contains four more chapters. Summary of these chapters are as follows: Chapter 2 includes the synthesis of three different type dioxidomolybdenum(VI) complexes of pyrazolone-based ligands, 4-acetyl-3-methyl-1-phenyl-5-pyrazolone (Hmp, I)), 3-methyl-1-phenyl-4-propionyl-5-pyrazolone (Hpp, II), 4-butyryl-3-methyl-1-phenyl-5-pyrazolone (Hbutp, III), and 4-isobutyryl-3-methyl-1-phenyl-5-pyrazolone (isobutp, IV) have been prepared. These complexes are [MoVIO2(mp)(OMe)(MeOH] (1), [MoVIO2(pp)(OMe)(MeOH)] (2), [MoVIO2(butp)(OMe)(MeOH)] (3) and [MoVIO2(isobutp)(OMe)(MeOH)] (4) (type 1), [MoVIO2(mp)2] (5), [MoVIO2(pp)2] (6), [MoVIO2(butp)2] (7) and [MoVIO2(isobutp)2] (8) (type 2) and [(μ-O){MoVIO2(DMF)}2(mp)2] (9) (type 3). These complexes are characterized by various spectroscopic (FT-IR, UV/Vis, 1H and 13C NMR) techniques, thermal analysis and single crystal X-ray analysis. These complexes adopt a distorted six-coordinate octahedral geometry where ligands act as bidentate, coordinating through the two O atoms. These complexes have been used as catalysts to explore a single pot multicomponent (benzaldehyde or its derivatives, urea/thiourea and ethyl acetoacetate/phenyl acetoacatate) Biginelli reaction producing biologically active 3,4-dihydropyrimidin-2-(1H)-one and 3,4-dihydropyrimidin-2-(1H)-thione based biomolecules under solvent-free conditions. Presence of H2O2 improves the yield of dihydropyrimidin-2-(1H)-one but it acts as poison for the later molecule. Epoxidation of internal and terminal alkenes mainly resulted in the formation of the corresponding epoxide. The catalytic oxidative bromination of thymol, a reaction facilitated by vanadium dependent haloperoxidases, resulted in the formation of three product namely 2-bromothymol, 4-bromothymol and 2,4-bromothymol. Other phenol derivatives have also been brominated effectively. In Chapter 3, three different tetradentate ONNO donor ligands H2hz(fp)2 (I), H2hz(butp)2 (II) and H2hz(bp)2 (III) have been isolated by the reaction of hydrazine hydrate with 4-formyl-3-methyl-1-phenyl-5-pyrazolone (fp), 4-butyryl-3-methyl-1-phenyl-5-pyrazolone (butp) and 4-benzoyl-3-methyl-1-phenyl-5-pyrazolone (bp), respectively. These synthesized ligands react with [MoVIO2(acac)2] in 1:2 molar ratio in refluxing methanolic to give binuclear complexes [(μ-O){MoVIO2(H2O)}2hz(fp)2] (1) and [(μ-O){MoVIO2(H2O)}2hz(butp)2] (2) and [(μ-O){MoVIO2(H2O)}2hz(bp)2] (3). All ligands and complexes are characterized by several techniques that include, FT-IR, UV-Vis, 1H NMR, 13C NMR spectroscopy, elemental and thermal analysis and single crystal X-ray analysis of 2 and 3. Ligands coordinate through a set of ON functionalities of the same ligand to two MoVIO2 groups attached through an oxido-bridge. Additional coordination of water making them to maintain an octahedral geometry. Azine moiety coordinates to two molybdenum through the symmetrical μ-{ƞ2-(N–N)}fashion. Catalytic activity of these complexes was studied for the synthesis of styrene carbonate from styrene oxide. A pressure of 20 bar CO2, 3 mg catalyst, 1 mmol of tetrabutylammonium bromide (TBAB) at 120 °C for 6 h were found to be most suited reaction condition for 5 mmol styrene oxide. Under the optimized reaction conditions, the product conversion and selectivity for all the complexes were investigated. Effective coupling of CO2 with styrene oxide to give styrene carbonate is the key point for this catalytic reaction. As high as 99% selectivity for styrene carbonate under these optimized reaction conditions was achieved. Chapter 4 includes the synthesis of tris-{cis-[MoO2]2+} complexes [{MoVIO2(H2O)}3L1] (1), [{MoVIO2(H2O)3}L2] (2) and [{MoVIO2(H2O)3}L3] (3) where H6L1-3 (I–III) are tricompartmental ligands [tris(H2ONO)] derived from benzene-1,3,5,-tricarbohydrazide and 3-acetyl-2-hydroxy-6-methyl-4H-pyran-4-one (I), 3-acetyl-4-hydroxy-2H-chromene-2-one (II) or 3-hydroxy-5-(hydroxymethyl)-2-methylpyridine-4-carbaldehyde (III). Elemental analysis, FT-IR, UV/Vis, NMR (1H and 13C), thermal studies and DFT optimized structure for 1 were used to characterize these ligands and complexes. These complexes demonstrate potential catalytic activity towards the one-pot, three-component synthesis of biologically active 3,4-dihydropyrimidine (DHMP) based molecules using alkyl acetoacetate, benzaldehyde (or its derivatives), and urea via the Beginelli reaction in the presence of H2O2, a green oxidant, under mild, solvent-free conditions. Chapter 5 is about five new tricompartmental ligands, each containing three tridentate dibasic ONO sites, which are derived from the condensation of benzene-1,3,5,-tricarbohydrazide (bthz) with 4-formyl-5-pyrazolone [H6btzh(fp)3 (I)], 4-acetyl-5-pyrazolone ]H6bthz(mp)3 (II)], 4-propionyl-5-pyrazolone [H6bthz(ep)3 (III)], 4-butyryl-5-pyrazolone [H6bthz(pp)3 (IV)] and 4-benzoyl-5-pyrazolone [H6bthz(php)3 (V)]. Reaction of I–V with [MoVIO2(acac)2] (Hacac = acetylacetone) lead to the formation of tris{cis-[MoO2]} complexes [{MoVIO2(H2O)}3bthz(fp)3] (1), [{MoVIO2(H2O)}3bthz(ap)3] (2), [{MoVIO2(H2O)}3bthz(ep)3] (3), [{MoVIO2(H2O)}3bthz(pp)3] (4) and [{MoVIO2(H2O)}3bthz(php)3] (5) having C3 symmetry. All ligands and complexes are characterized by various techniques: FT-IR, UV/Visible, NMR (1H and 13C), elemental analysis and thermal studies. X-ray diffraction studies of complexes 2–5 confirm their distorted octahedral geometry where each pocket of ligands coordinate through enolate oxygen (of pyrazole) enolate oxygen (of hydrazide) and azomethine nitrogen atoms. Catalytic potentials of these complexes were achieved for the (i) oxidation of 2-methylnaphthalene to 2-methyl-1,4-naphthoquinone (vitamin K3), (ii) oxidation of different olefins, and (iii) oxidative bromination of various phenol derivatives including thymol. As high as 99% selectivity of 2-methyl-1,4-naphthoquinone has been obtained in high yield while olefins epoxidation resulted the corresponding epoxide selectively. High yield of brominated products catalyzed by these complexes signify them as useful functional model of vanadium dependent haloperoxidases.