SYNTHESIS, CHARACTERIZATION AND CATALYTIC ASPECTS OF DIOXIDOMOLYBDENUM(VI) COMPLEXES

Abstract

Chemical reactions generally proceed through the breaking and formation of bonds between atoms in molecules to produce new compounds. In a general sense, anything that increases the rate of any process is often called “catalyst”, a term derived from Greek ����ατα����ϑǫ����υ, meaning “to annul”, or “to untie”, or “to pick up”. The catalyst decreases the activation energy of a reaction by altering the reaction path but it itself remains unchanged. The word “catalysis” was probably used first time in the sixteenth century by the chemist A. Libavious. These days catalysts are playing a vital role in petrochemicals, fine chemicals, pharmaceuticals, fertilizers and food industries. The biochemically significant processes are also based on catalysts. Sometimes chemists surprise the nature, which is a foundation of motivation, by mimicking its complex reactions like most popular mimetic approach of molybdenum compounds. Initially Bortels highlighted the biological importance of molybdenum compounds in 1930. Oxyanion molybdate is the soluble biological active form of molybdenum. On one hand molybdenum is a minor constituent of the earth's crust and on the other hand it is widely bioavailable due to the high solubility of molybdate salts in water, the most abundant transition metal in seawater. Molybdenum is the single 4d transition metal, present in biological a system which outlines the part of the lively site of molybdoenzymes that accomplish the key transformations in the metabolism of nitrogen, sulfur and carbon compounds. On the basis of amino acid sequences, spectroscopic properties, active site structures and catalyzed reactions molybdenum containing enzymes can be distributed in many classes. R. Hille gave a classification of molybdoenzymes with the help of rapidly growing number of X-ray crystal structures, which is based on structural homology of the active sites. Two different types of molybdoenzymes are acknowledged: Molybdenum nitrogenase which catalyzes the reduction of atmospheric dinitrogen to ammonia and another types of molybdoenzymes are oxidoreductases such as aldehyde oxidase, xanthine oxidase, sulfite oxidase, nitrate reductase and xanthine dehydrogenase that transfer an oxido group or two electrons to from the substrate. The IV enormous majority of these molybdoenzymes acquire at least one Mo=O unit in their active sites and are frequently named as oxidomolybdenum enzymes. Upon going through the literature, it is evident that molybdenum Schiff base complexes have provided opportunities to develop catalytic system for various industrial processes. Particularly, oxidation reactions catalyzed by these specialized complexes are well documented. However, in most cases optimization of the reaction conditions to effect maximum efficiency of the catalysts have not been set out. It was, therefore, reasonable to undertake systematic study on the synthesis and characterization of new molybdenum catalysts and to explore their catalytic potential for the oxidation of organic substrates under optimized reaction conditions. First chapter is the introductory one and describes a variety of molybdenum complexes that have been used as homogeneous catalysts in different types of organic transformations. Literature on the catalytic applications of various dioxidomolybdenum(VI) complexes has also been reviewed. In Second Chapter, the dioxidomolybdenum(VI) complexes [MoVIO2(Hsaldahp)( H2O)] (2.1), [MoVIO2(Hclsal-dahp)(H2O)] (2.2) and [MoVIO2(Hbrsal-dahp)(H2O)] (2.3) have been prepared by the reaction of [MoVIO2(acac)2] (Hacac = acetylacetone) with tribasic pentadentate Schiff bases H3sal-dahp (2.I), H3clsal-dahp(2.II) and H3brsaldahp( 2.III) (sal = salicylaldehyde, clsal = 5-chlorosalicylaldehyde, brsal = 5- bromosalicylaldehyde, dahp = 1,3-diamino-2-hydroxypropane) in methanol at reflux condition. Reactions of these complexes with pyridine result in the formation of [MoVIO2(Hsal-dahp)(py)] (2.4), [MoVIO2(Hclsal-dahp)(py)] (2.5) and [MoVIO2(Hbrsaldahp)( py)] (2.6). These complexes have been used as catalysts for the oxidation of methyl phenyl sulfide, benzoin and oxidative bromination of styrene efficiently using H2O2 as green oxidant. Oxidation of methyl phenyl sulphide under the optimized reaction conditions gave ca. 98 % conversion with two major products methyl phenyl sulfoxide and methyl phenyl sulfone in the ca. 66.8 % and 33.2 % selectivity, respectively. The oxidation of benzoin, catalyzed by MoVIO2 complexes was carried out in refluxing methanol which gave 95% conversion in 4 h of reaction time and the selectivity of the V reaction products varied in the order: benzoic acid (40) > methyl benzoate (32 %), > benzil (15%) > benzaldehyde-dimethylacetal (13 %). The oxidative bromination of styrene using molybdenum complexes as catalyst precursors gave 98% conversion and the selectivity of different major products followed the order: phenylethane-1,2-diol (67%) > 1,2-dibromo-1-phenylethane (21%) > 2-bromo-1-phenylethane-1-ol (3%). Third Chapter describes the synthesis of [MoVIO2{Hdfmp(sbdt)2}(H2O)] (3.1), [MoVIO2{Hdfmp(smdt)2}(H2O)] (3.2) and [MoVIO2{Hdfmp(tsc)2}(H2O)] (3.3) by the reaction of [MoVIO2(acac)2] with the tribasic pentadenate O, N and S donor ligands H3dfmp(sbdt)2 (3.I), H3dfmp(smdt)2 (3.II) and H3dfmp(tsc)2 (3.III) derived from 2,6- diformyl-4-methylphenol and S-benzyldithiocarbazate, S-methyldithiocarbazate, and thiosemicarbazide. These complexes were characterized using spectroscopic studies (IR, UV/Vis and NMR), elemental analyses, thermal studies and single crystal study which reveal that only one set of azomethine nitrogen, enthiolate sulfur and phenolate oxygen atoms of the ligands are coordinated to the molybdenum. Oxidations of styrene and cyclohexene have been investigated using these complexes as catalyst precursors in the presence of H2O2 as oxidant in the presence of NaHCO3. Under the optimized reaction conditions, a maximum of 96 % conversion of styrene has been obtained with 3.1, 98 % conversion with 3.2 and 97 % conversion with 3.3 in 2 h of reaction time. The selectivity of the products is similar for the catalyst precursors (i.e. complexes 3.1 to 3.3) and follows the order: styrene oxide > phenyl acetaldehyde. With cyclohexene, a maximum conversion of 96% has been achieved with 3.1, 94 % with 3.2 and 96 % conversion with 3.3, also in 2.5 h of reaction time and cyclohexene oxide is formed as a product with 100 % selectivity. UV-Vis experiment with all complexes confirm the plausible formation of MoVI O(O2)L as intermediates in the catalytic oxidations. New dioxidomolybdenum(VI) complexes, [MoVIO2{Hdfmp(bhz)2}(MeOH)] (4.1), [MoVIO2{Hdfmp(inh)2}(MeOH)] (4.2) and [MoVIO2{Hdfmp(nah)2}(MeOH)] (4.3) of ligand H3dfmp(L)2 obtained by the condensation of 2,6-diformyl-4-methylphenol (dfmp) and hydrazides (L) [L = benzoylhydrazide (bhz), isonicotinoylhydrazide (inh), and nicotinoylhydrazide (nah)], respectively. Studies on these complexes are described in VI Fourth Chapter. All complexes are characterized by various physico-chemical studies. Oxidation of secondary alcohols: 1-phenyl ethanol, 2-propanol and 2-butanol, catalyzed by these molybdenum complexes, using conventional liquid phase and microwaveassisted methods in the presence of 30 % H2O2 as an oxidant have been tested. The effects of various factors, such as temperature and amounts of catalyst, H2O2 and solvent have been investigated. These alcohols under the optimized reaction conditions gave high yields of the respective ketone. Addition of an N-based additive reduces the reaction time considerably. Amongst the two methods studied, the microwave technique proves to be a time efficient system. In Fifth Chapter the synthesis of dioxidomolybdenum(VI) complexes, [MoVIO2(fhmc-bhz)(MeOH)] (5.1), [MoVIO2(fhmc-inh)(MeOH)] (5.2) [MoVIO2(fhmcnah)( MeOH)] (5.3) and [MoVIO2(fhmc-fah)(MeOH)] (5.4) have been described. These complexes are obtained by the reaction of [MoVIO2(acac)2] and potential ONO tridentate ligands H2fhmc-bhz (5.I), H2fhmc-inh (5.II), H2fhmc-nah (5.III), and H2fhmc-fah (5.IV), derived from condensation of equimolar amount of 8-formyl-7-hydroxy-4- methylcoumarin (fhmc) and hydrazides [benzoylhydrazide (bhz), isonicotinoylhydrazide (inh), nicotinoylhydrazide (nah) and furoic acid hydrazide (fah)] in methanol. The structures of the obtained ligands and their respective metal complexes were elucidated by elemental analyses, spectroscopic techniques (IR, electronic, 1H and 13C NMR) and thermogravmetric analyses. These metal complexes have been tested against oxidative bromination of monoterpene (thymol) by using H2O2 as an oxidant. Therefore, they act as functional models of vanadium dependent haloperoxidases. A maximum of 94% conversion has been achieved where selectivity of different major products follows the order: 2,4-dibromothymol (84.6%) > 2-bromothymol (8.4%) > 4-bromothymol (7%). The effects of various factors, such as amounts of catalyst, oxidant, KBr, HClO4 and different solvents have been considered to optimize the reaction conditions for the maximum brominated products. Finally, summary and over all conclusions based on the achievements are presented.