New mixed ligand oxido-vanadium(V) complexes with O,O-donor and diimine ligands: Synthesis, crystal structure and catalytic activity in eco-friendly oxidation of olefins and phenol

The facile synthesis and structural characterization of a set of three new oxidovanadium(V) complexes of general formula [VO2(L)(diimine)], where L is ethyl-maltol(emal) (1), deferiprone(def) (2) or, maltol(mal) (3), and diimine is 5,6-dimethyl-1,10-phenanthroline(5,6-DMP) are described. The neutral complexes 1-3 have been comprehensively characterized by employing various analytical and spectroscopic techniques including single crystal X-ray diffraction analysis. The coordination geometry around the V(V) in each of the complexes was indicated by XRD-analysis to be distorted octahedral with equatorial plane being occupied by the deprotonated O,O-donor ligands. The complexes 1-3 could be successfully implemented in organic oxidations viz., hydroxylation of phenol (PH) and olefin epoxidation under environmentally clean organic-solvent-free conditions with aqueous H2O2 as oxidant. Phenol was selectively converted into dihydroxybenzene (DHB) products, catechol (CT) and hydroquinone (HQ) (HQ/CT=1.3:1) with aqueous H2O2 as oxidant at room temperature, with a TON value of 266. Moreover, the catalysts showed impressive activity in epoxidation of styrene and a range of cyclic olefins such as cyclohexene, cyclooctene, and norbornene in absence of organic solvent, to provide high conversion (up to 100 %) with excellent epoxide selectivity and good TON values. The catalysts are stable and afford recyclability for multiple cycles of epoxidation with virtually unaltered catalytic activity.