SYNTHESIS, REACTIVITY AND CATALYTIC ACTIVITY OF METAL COMPLEXES

Abstract

Vanadium exhibits formal oxidation states from +V down to —III. The most stable oxidation states +IV and +V under normal conditions are generally stabilized through V-O bond and oxocations [VO]2+, [VO]3+ and [V02]+ are most common for biological systems. Vanadium in these oxidation states comfortably binds with 0, N and S donor ligands. The discovery of vanadium(V) in vanadium based enzymes e.g. vanadate-dependent haloperoxidases, attracted attention of researcher to develop coordination chemistry of vanadium(V) in search of good models for vanadium-containing biomolecules. Vanadium compounds have also been found to promote a peroxide-driven oxidation of organic substrates. Basic chemistry and potential applications of vanadium complexes in diverse fields have been discussed time to time in International Vanadium Chemistry symposium held biannually (For chemistry discussed in recent symposium please refer to Coord. Chem. Rev., 255 (2011)). In view of the above it may clearly be conceived that the coordination chemistry of vanadium is of increasing potential interest and therefore it was considered desirable to study the coordination chemistry of vanadium with the ligands that would provide structural and functional models of haloperoxidases. Stability, structural and reactivity studies have been carried out to model the role of vanadium in vanadium based enzymes. Coordination chemistry has further been extended to manganese with the idea if they would serve good functional models of haloperoxidases. At the end, looking at the importance of zeolite-Y encapsulated metal complexes (ZEMC) where these complexes are suggested as model compounds for enzyme mimicking, copper(II) complex has been prepared and encapsulated in the cavity of zeolite-Y to study its catalytic activity. For convenient the work embodied in the thesis has been divided into following chapters: ii First chapter is introductory one and deals with the general remarks on vanadium and manganese, their occurrence in nature and biological systems. Applications of vanadium complexes with particular emphasis on structural and functional models of haloperoxidases have been discussed. Applications of manganese complexes as potential catalysts have also been considered. At the end importance of zeolite-Y encapsulated metal complexes (with simple as well as macrocyclic ligands) in the catalytic field has also been presented. • [V'vO(acac)2] reacts with ligands CH2(H2L)2 in refluxing methanol to yield two neutral binuclear V''-complexes formulated as [CH2{V''OL(H20)}2], namely 2.1 and 2.2. Ligands CH2(H2L)2 2.1 and 2.0 are derived from 5,5'-methylenebis(salicylaldehyde) and S-benzyldithiocarbazate [CH2(H2sal-sbdt)2, 2.I] or S-methydithiocarbazate [CH2(H2sal-smdt)2, 2.II]. Aerial oxidation of 2.1 and 2.2 in the presence of KOH or CsOH-H20 results in the formation of dioxidovanadium(V) complexes, K2[CH2{Vv02(sal-sbdt)}2]•2H2O (2.3), Cs2[CH2{Vv02(sal-sbdt)}2]•2H20 (2.4) K2[CH2{Vv02 (sal -smdt)}2]•2H20 (2.5) and Cs2[CH2{V~'O2(sal-smdt)}2]-2H2O (2.6).. Characterization of these complexes, their reactivity and catalytic activity are presented in Chapter 2. These compounds are characterized in the solid state and in solution, namely by spectroscopic techniques (IR, UV-Vis, EPR, 111, 13C and 51V NNIIZ). It is demonstrated that the Vv02-complexes 2.3-2.6 are efficient and selective towards the oxidative bromination by H202 of styrene yielding 1,2-dibromo-l-phenylethane, 1-phenylethane- 1 ,2-diol and 2-bromo-l-phenylethane-1-ol, therefore acting as functional models of vanadium dependent haloperoxidases. Plausible intermediates involved in these catalytic processes are established by UV-Vis, EPR and 51V NMR studies. Chapter 3 considers vanadium chemistry with binucleating ligands derived from 2,6-diformyl-4-methylphenol and various hydrazides. These hydrazones [H3dfmp(inh)2 (3.I), H3dfmp(nah)2 (3.11) and H3dfmp(bhz)2 (3.III); inh = isonicotinoylhydrazide, nah = nicotinoylhydrazide and bhz = benzoylhydrazide] react with [VIVO(acac)2] in refluxing methanol to give oxidovanadium(IV) complexes, [VI"O{Hdfinp(inh)2}] (3.1), [VIVO{Hdfmp(nah)2}] (3.2) and [VIvO{Hdfinp(bhz)2}] (3.3). Aerial oxidation of these complexes in methanol results in the formation of complexes, [VvO(OMe) {Hdfmp(inh)2} (MeOH)] (3.4), [VvO(OMe) {Hdfmp(nah)2} (MeOH)] (3.5) and [VvO(OMe) {Hdfmp(bhz)2} (MeOH)] (3.6). In the presence of KOH, oxidation of 3.1-3.3 results in the formation of complexes K[Vv02{Hdfrnp(inh)2}] (3.7), K[Vv02{Hdfmp(nah)2}] (3.8) and K[Vv02{Hdfmp(bhz)2}] (3.9). These compounds were also prepared by the reaction of aqueous KV03 with ligands at pH ca. 7. All compounds are characterized by IR, electronic, EPR, 1H, 13C and 51V NMR spectroscopy and elemental- analyses. Single crystal X-ray analysis of [VvO(OMe) {Hdfmp(bhz)2} (MeOH)] (6) and K[Vv02{Hdfmp(inh)2}] (3.7) confirm the coordination of the ligand in the dianionic (ONO2-) enolate tautomeric form and one of the hydrazide moieties remains non-coordinated. However, the nitrogen atom of the free inh moiety coordinates to the other vanadium centre in 3.7 giving a polynuclear complex. It has also been shown that the dioxidovanadium(V) complexes are active catalysts in the oxidative bromination of styrene, by H202, therefore acting as functional models of vanadium dependent haloperoxidases. Plausible intermediates involved in the catalytic process are established by W-Vis, EPR and 51V NMR studies. Reaction of MnC104 with 2-[2-(IH-(benzo[d]imidazol-2-yl)ethylimino) methyl]phenol (Hsal-aebmz, 4.I) under aerobic conditions results in the - formation of [Mnm(sal'aebmz)2]•ClO4 (4.1). In the presence of triethylamine and using MnC12 under similar condition 4.I forms [Mnal(sal-aebmz-H)(sal-aebmz)] (4.2). These complexes are characterised on the basis of elemental and electrochemical analyses, spectroscopic (IR and I V Vis) data and thermogravimetric studies and results are presented in Chapter 4. Single crystal X-ray analysis of 4.2 shows that it is stabilized, in the solid state, through inter molecular hydrogen bonding between NH groups of the two benzimidazole moieties after losing one of the hydrogen atoms; the coordination sites of the ligand being imine nitrogen of the benzimidazole ring, the azomethine nitrogen and the deprotonated phenolic oxygen. It has been demonstrated that 4.2 is efficient catalyst for the oxidative iv bromination by H202 of styrene yielding 1,2-dibromo-l-phenylethane, 1-phenylethane-1,2-diol and 2-bromo-l-phenylethane-l-ol... Chapter 5 deals with the reaction between CuC12 and (Z)-2-(1-(2-(1H-benzo Ed] imidazol-2-yl)ethylimino) ethyl)phenol (Hhap-aebmz) derived from o-'L droxyacetophenone (Hhap) and 2-aminoethylbenzimidazole (aebmz) which gives [Cu"(hap-aebmz)Cl]. Elemental analysis, magnetic successibility, spectral (IR and - electronic) data and single crystal X-ray studies confirm the distorted square planar structure of the complex. Complex [Cu"1(hap-aebmz)Cl] has also been encapsulated in the nano cavity of zeolite-Y and its encapsulation ensured by various physico-chemical techniques. The encapsulated complex has been used as catalyst for the oxidation of cyclohexene and phenol in the presence of H202. With nearly quantitative oxidation of cyclohexene, the selectivity of the oxidation products obtained follows the order: 2- cyclohexene- l -ol (44 %) > 2-cyclohexene- l -one (40 %) cyclohexeneoxide (12 %) > cyclohexane-1,2-diol (4%). Oxidation of phenol (65.7%) gives two products with the selectivity order: catechol (66.1%) > hydroduinone (32.9%). Finally, summary and over all conclusions based on the achievements are presented.