Synthesis, Structural Studies and Catalytic Activity of a Series of Dioxidomolybdenum(VI)-Thiosemicarbazone Complexes

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Reaction of the thiosemicarbazone ligands, [4-(p-bromophenyl)thiosemicarbazone of salicylaldehyde (H2L1), 4-(p-X-phenyl)thiosemicarbazone of o-vanillin {X = F (H2L2), X = Cl (H2L3) and X = OMe (H2L4)}, 4-(p-bromophenyl)thiosemicarbazone of 5-bromosalicylaldehyde (H2L5), and 4-(p-chlorophenyl)thiosemicarbazone of o-hydroxynaphthaldehyde (H2L6)] with [MoO2(acac)2] afforded a series of new oxidomolybdenum(VI) complexes [Mo(VI)O2L1–6(solv)] (1–6) {where solv (solvent) = DMSO (1, 3, 5 & 6) and H2O (2 & 4)}. The molecular structures of 2 and 3 were determined by X-ray crystallography, demonstrating the dibasic tridentate behavior of ligands. The cyclic voltammogram pattern is similar for 1–6, which includes two irreversible reduction processes within the potential window −0.71 to −0.66 V and −0.92 to −0.85 V corresponding to the metal centered reduction from Mo(VI)/Mo(V) and Mo(V)/Mo(IV) respectively. Catalytic potential of 1–6 was tested for the oxidation of styrene and cyclohexene. The effect of various parameters such as the amount of catalyst, oxidant, NaHCO3, and solvent was checked to optimize the conditions for the best performance of the catalyst. 100% product selectivity for the formation of cyclohexene oxide from cyclohexene and ∼98–99% product selectivity for the oxidation of styrene to styrene oxide was observed.