SYNTHESIS, CHARACTERIZATION AND CATALYTIC APPLICATIONS OF DIOXIDOMOLYBDENUM (VI) AND DIOXIDOURANIUM (VI) COMPLEXES

Abstract

Molybdenum metal belongs to the 6th group of Modern Periodic table, and has the symbol Mo with atomic number 42. It is a silvery white metal with a gray cast. It has sixth highest melting point in periodic table. The trace element molybdenum is one of the few second transition row metals which is known to involve in more than 50 molybdenum-containing enzymes till date and has well defined functions in biology. Most mononuclear molybdenum enzymes are unique in terms of their active site structure and consist of a Mo center bound to a novel pyranopterin cofactor via dithiolene moiety, except the nitrogenases where Mo is present along with iron in a sulfur linked cluster. The mononuclear molybdenum enzymes based on their active site structure has classified as: (i) molybdenum hydroxylases, (ii) eukaryotic oxotransferases and (iii) prokaryotic oxotransferases. These Mo enzymes are involved in various metabolic processes of the host organism and the products of the catalytic cycles play vital roles in metabolism of the carbon, nitrogen and Sulphur. Molybdenum has been discovered as the essential element for animals and plants. Several physiological abnormalities are produced in human like cancer from its deficiency and excess supply. Molybdenum complexes exhibit anticancer, antibacterial, antifungal and antifertility activity. The presence of at least one Mo=O unit in the active sites of molybdo-enzymatic systems, they are referred as oxido-molybdenum enzymes and this inspired researchers to focus their research on the synthetic oxidomolybdenum complexes in homogeneous or heterogeneous systems in order to mimic the biological systems. Among actinides, uranium is a silvery-white element having symbol U and atomic number 92. Uranium is a naturally occurring element which is found in low levels within all rock, soil, and water. The most common isotopes of naturally occurring uranium are U238 and U235. Most uses of uranium employ its unique nuclear properties like in nuclear power plants and nuclear weapons. The most important oxidation states exhibited Ph.D. Thesis **** Bekele Mengesha Dogie **** I.I.T.Roorkee *** 2019 ii by uranium are uranium (IV) and uranium (VI), and exists in these states in the form of oxides, i.e. uranium (IV) dioxide (UO2); and uranium (VI) trioxide (UO3), respectively. The coordination chemistry of uranium had been studied in nineties extensively but in the past one decade the coordination chemistry of uranium could not develop much like other transition metal ions possibly due to not much scope of the applications of coordination compounds of uranium. The most commonly used is no doubt the uranyl ion, [UO2]2+. The bonding in [UO2]2+ unit is very different, and is mainly due to the combination of d–p and f –p π interactions. The uranyl unit is highly stable to moisture and oxygen. The extremely good catalytic activities rendered by molybdenum complexes and some recent catalytic results shown by uranium complexes encourages us to explore the chemistry and applications of molybdenum and uranium complexes built on aminobisphenol ligands. The coordination chemistry of aminobisphenol based ligands with different transition metals is well documented in the literature but their bio-catalytic/ enzyme mimetic activity and catalytic activities are less explored. The structures and properties of the complexes vary greatly depending upon the precursors/conditions employed during synthesis; giving scope for exploring different geometries and coordination behavior around the metal center. For convenience the work embodied in the thesis has been divided into following chapters: First chapter is the introductory one and presents general remarks on synthesis, characterization and catalytic applications of molybdenum and uranium complexes of aminobis (phenol) based tetradentate ligands along with updated literature survey. Chapter 2 presents the reaction of dibasic tetra dentate ONNO donor Mannich bases derived from ethylenediamine and 2,4–di–tert–butylphenol (H2L1) (I), 2,4–di–methylphenol (H2L2) (II), 2–tert–butyl–4–methylphenol (H2L3) (III), 2,4–di–chlorophenol (H2L4) (IV) and 2-naphthol (H2L5) (V) with [MoVIO2(acac)2] (Hacac = acetylacetone) in a 1:1 molar ratio in refluxing MeOH. The obtained cis-dioxidomolybdenum (VI) complexes Ph.D. Thesis **** Bekele Mengesha Dogie **** I.I.T.Roorkee *** 2019 iii are: [MoVIO2(L1)] (1), [MoVIO2(L2)] (2), [MoVIO2(L3)] (3), [MoVIO2(L4)] (4), and [MoVIO2(L5)] (5), respectively. All complexes were characterized by elemental analysis, various spectroscopic (FT-IR, UV/Vis, 1H and13C NMR) techniques and single-crystal X-ray analysis (of 1, 2, 3 and 5). These complexes adopt a distorted six-coordinated octahedral geometry where ligands act as tetradentate, coordinating through the two Ophenolate and two Namine atoms in a cis-α type binding mode involving coordination of one of the Namine atom in the apical position and one Ooxido terminal oxygen atom in the equatorial position. These complexes catalyze oxygen atom transfer between benzoin and dimethyl sulfoxide (DMSO) in acetonitrile at 80 °C. The formation of benzil could easily be monitored by HPLC. The electronic effect caused by the para substituent and the steric effect caused by the ortho substituent influence the formation of benzil and therefore conversion varied between 66 - 99% in 18 h of reaction time under optimized conditions; most active catalyst being [MoVIO2(L5)]. Almost similar trend has also been obtained with 4-chlorobenzoin. The pseudo first order rate constant for [MoVIO2 (L5)] was found to be 0.0998 h–1. During catalytic reaction, the formation of the binuclear intermediate and its fast decay into the initial dioxidomolybdenum (VI) complex was established by time dependent UV/Vis studies. The reaction of dibasic tetra dentate ONNO donor Mannich bases derived from ethylene diamine and 2,4–di–tert–butylphenol(H2L1) (I) and 2,4–di–methylphenol (H2L2) (II), 2–tert–butyl–4–methylphenol (H2L3) (III), and 2,4–di–chlorophenol (H2L4) (IV) with UVIO2(CH3COO)2·2H2O in a 1:1 molar ratio in refluxing MeOH gave the corresponding mononuclear trans-dioxidouranium(VI) complexes, [UVIO2(L1)·MeOH] (6), [UVIO2(L2)·MeOH] (7), [UVIO2(L3)·MeOH] (8) and [UVIO2(L4)·MeOH] (9), respectively. The synthesized complexes are stable in air, reddish-brown in color and soluble in most solvents. There complexes are characterized by elemental analysis, various spectroscopic (FT-IR, UV/Vis, 1H and13C NMR) techniques and single-crystal X-ray analysis of 8 and 9. The complexes adopt distorted pentagonal bipyramidal geometry around of metal centre. The ligand acts as tetra dentate, coordinated through two phenolato oxygen and two imino Ph.D. Thesis **** Bekele Mengesha Dogie **** I.I.T.Roorkee *** 2019 iv nitrogen atoms; two oxido groups are trans to each other. There complexes are used as catalysts to study the oxidative bromination of thymol and styrene. The catalytic oxidative bromination of thymol resulted in the formation of three products namely, 2-bromothymol, 4-bromothymol and 2, 4-dibromothymol while oxidative bromination of styrene gave two products, 2-bromo-1-phenylethanol and 1-phenylethane-1, 2-diol. In order to find out the optimized reaction conditions for the fixed concentration (10 mmol) of substrate, effects of different amounts of catalyst, KBr, HClO4, and oxidant (H2O2) have been investigated. Under the optimized reaction conditions, all the complexes have shown good catalytic potentials for the oxidative bromination of substrates, establishing the functional similarity to vanadium dependent haloperoxidases. All these details are presented in Chapter 3. The stable dibasic tetra dentate ligand 1,4-bis-(2-hydroxy-3,5-dimethylbenzyl)piperazine (H2pip-2,4-dmp, VI) prepared by reacting 2,4-dimethylphenol with piperazine in the presence of formaldehyde reacts with [MoVIO2(acac)2] and [UVIO2(CH3COO)2] in equimolar ratio to give neutral hexa-coordinated [MoVIO2(pip-2,4-dmp)] (10) and hepta-coordinated [UVIO2(pip-2,4-dmp)(MeOH)] (11), respectively. Chapter 4 presents details of these complexes. After characterizing these complexes by spectroscopic (IR, UV/Vis, 1H and 13C NMR) data, elemental and thermal analyses (and single crystal X-ray diffraction study of uranium complex), they are used as catalysts to study the oxidative bromination of thymol. Such catalytic reactions are observed by many model vanadium complexes and are considered as a functional mimic of haloperoxidases. The catalytic oxidation resulted in the formation of three products namely, 2-bromothymol, 4-bromothymol and 2, 4-dibromothymol. The optimized reaction conditions are obtained considering concentration of substrate, KBr, HClO4, and oxidant for the maximum yield of brominated products. Under the optimized reaction conditions, the product selectivity for both the prepared complexes is investigated. They are found to be competent homogeneous catalysts to afford the products in good yield. Finally, summary and over all conclusions based on the achievements are presented.