Dioxidomolybdenum(VI) complexes chelated with N4-(3-methoxyphenyl)thiosemicarbazone as molybdenum(IV) precursors in oxygen atom transfer process and oxidation of styrene

Abstract

Four new mononuclear dioxidomolybdenum(VI) complexes of the type [MoO2LD] (where D = methanol (1), ethanol (2), propanol (3) and [MoO2LD]2·D (where D = 4-picoline (4)) have been synthesized by the reaction of [MoO2(acac)2] with the thiosemicarbazone (H2L) derived from 3-ethoxy-2-hydroxybenzaldehyde and N4-(3-methoxyphenyl)thiosemicarbazone in presence of donor solvents like methanol, ethanol, propanol and 4-picoline. Crystal and molecular structures of the complexes were determined by single crystal X-ray diffraction analysis. All the complexes portrayed similar mononuclear structures where the thiosemicarbazone is bonded to molybdenum(VI) ion as a binegative tridendate agent. The molybdenum(VI) centre in complexes 1–4 is six coordinate by the thiosemicarbazone, two oxido groups and an oxygen or nitrogen atom from the donor solvent molecules. The complexes 1–4 exhibit oxygen atom transfer to PPh3 in acetonitrile medium in presence of NN bidendate donors to form complexes of the form [Mo(IV)OL(NN)] (where NN = 2-2′ bipyridine (5) or 1,10 phenanthroline (6)). The complexes were further characterized by elemental analysis, spectroscopic methods (IR, UV–Vis and 1H NMR) and thermogravimetric analysis. The electrochemical behaviour of these complexes have been investigated for an insight into the redox behaviour of the molybdenum(VI) centres in these complexes. Hirshfeld surface analysis was successfully employed for exploring the coordination geometries and various non-covalent interactions present in their crystal structures. Furthermore, the catalytic abilities of 1–4 were tested for the oxidation of styrene using aqueous H2O2 as oxidant and NaHCO3 as co-catalyst. The reaction condition for all-out catalytic proficiency of the catalysts 1–4 were examined by studying the effect of various parameters such as the amount of catalyst, H2O2, co-catalyst (NaHCO3) and solvent (CH3CN) as well as temperature of the reaction. Almost 97–98% product selectivity was attained for the oxidation of styrene to styrene oxide.