Catalytic Oxidation of Cyclooctene by Methoxy Bridged Dinuclear Oxidovanadium(V) Complex

Vanadium plays a vital role in biological systems since the active sites of some enzymes such as haloperoxidases, which are able to catalyze oxidation reactions by hydrogen peroxide, consist of vanadate moieties [1]. The chemistry of vanadium(V) is dominated by the stable oxidovanadium (VO3+) and dioxidovanadium (VO2+) cations, which remain intact during many reactions [2]. Vanadium participates in many important catalytic redox processes and its complexes take part in oxo-transfer and oxidation reactions [3]. The synthesis and utility of new vanadium complexes in oxidation reactions is one of the attractive research fields in inorganic and coordination chemistry. In this work we report the synthesis, characterization, crystal structure and catalytic activity of a new neutral dinuclear vanadium complex, [(VO)2(L)(μ-OCH3)2(OCH3)2]∙CH3OH (1). Complex 1 was synthesized by the reaction of VO(acac)2 and tetradentate N2O2-donor ligand [H2L, where H2L = 1,1 -((1E,1 E)-hydrazine-1,2-diylidenebis(methanylylidene))bis(naphthalen-2-ol)] in methanol (Scheme 1a). The complex was characterized by spectroscopic methods (FT-IR, UV-Vis, NMR) and single crystal X-ray analysis. The molecular structure of complex 1 was determined by single-crystal X-ray analysis which indicated complex 1 is a neutral methoxy bridged dinuclear oxidovanadium(V) complex. During the formation of complex the V4+ is oxidized to V5+ by aerial oxygen. The catalytic Complex 1 was employed as catalyst for oxidation of cis-cyclooctene in the presence of H2O2 as a green oxidant (Scheme 1b). Liquid phase catalytic oxidation of cis-cyclooctene was carried out under air (atmospheric pressure) in a 25 mL round bottom flask equipped with a magnetic stirrer. In a typical experiment, aqueous H2O2 (1-4 mmol) was added to a flask containing complex 1 (1-3 mg) and cyclooctene (1 mmol) in a solvent (3 mL). The course of the reactions was monitored using a gas chromatograph equipped with a capillary column and a flame ionization detector. In order to obtain the suitable condition of catalytic oxidation, the effect of various parameters including the amount of the catalyst, the ratio of H2O2 : cyclooctene, the effect of temperature and also solvent was studied. The results of catalytic reactions indicate that complex 1 is a highly active and selective catalyst in the clean epoxidation reaction of cis-cyclooctene using aqueous hydrogen peroxide.