Synthesis, characterization, reactivity and catalytic activity of dioxidomolybdenum(VI) complexes derived from tribasic ONS donor ligands

Abstract

The reaction of three Schiff bases, H3dfmp(sbdt)2 (I), H3dfmp(smdt)2 (II) and H3dfmp(tsc)2 (III) [derived from the condensation of 2,6-diformyl-4-methylphenol (dfmp) with S-benzyldithiocarbazate (sbdt), S-methyldithiocarbazate (smdt) and thiosemicarbazide (tsc)], with [MoVIO2(acac)2] (Hacac=acetylacetone) on stirring in methanol results in the formation of the dioxidomolybdenum(VI) complexes [MoVIO2{Hdfmp(sbdt)2}(H2O)] (1), [MoVIO2{Hdfmp(smdt)2}(H2O)] (2) and [MoVIO2{Hdfmp(tsc)2}(H2O)] (3), respectively. All these complexes have been characterized in the solid state and in solution by elemental analyses, spectroscopic techniques (IR, electronic, 1H and 13C NMR) and thermogravimetric analyses. The structures of complexes 1, 2 and 3, confirmed by a single crystal X-ray study, reveal that only one set of azomethine nitrogen, enthiolate sulfur and phenolate oxygen atoms of the ligands are coordinated to the molybdenum centre. All the complexes form an antiparallel dimer through different π–π interactions. These molybdenum complexes catalyze the oxidation of styrene and cyclohexene using 30% H2O2 as an oxidant in the presence of sodium bicarbonate. Styrene, under optimized reaction conditions, gave two reaction products, namely phenylacetaldehyde as the major product and styrene oxide as the minor product. Oxidation of cyclohexene selectively gave 2-cyclohexene-1-ol.