Dinuclear and polynuclear dioxomolybdenum(VI) Schiff base complexes: Synthesis, structural elucidation, spectroscopic characterization, electrochemistry and catalytic property

Abstract

Several new binuclear and polynuclear dioxomolybdenum(VI) Schiff base complexes have been synthesized. The reaction of bis(acetylacetonato)dioxomolybdenum with 5-dichlorosalicylaldehyde 4-ethylthiosemicarbazide (L1/L2) in the presence of 4,4′-bipyridine (4,4′-bpy) or 4,4′-bipyridine N,N′-dioxide (4,4′-dpdo) gave a binuclear cis-dioxomolybdenum complex [(MoO2)2L2(D-D)], in which the bidentate ligand, 4,4′-bipyridine or 4,4′-bipyridine N,N′-dioxide (D-D), formed a bridge between the two molybdenum atoms. A second type of dinuclear dioxomolybdenum(VI) Schiff base complex, [(MoO2)2L′D2], was formed by the reaction of bis(acetylacetonato)dioxomolybdenum with 1,4-bis(3-ethoxy-salicylaldehyde carbohydrazonato)butane (L′) in the presence of ethanol or hexamethylphosphoramide (D). In this case, the two molybdenum atoms are not bridged directly by a bidentate ligand, but are coordinated at each end to the O,N,O donor atoms of the symmetrical hexadentate Schiff base ligand. The syntheses of polynuclear dioxomolybdenum(VI) [(MoO2)L′(D-D)]n complexes were achieved by the reaction of the second type of dinuclear dioxomolybdenum(VI) complex, [(MoO2)2L′D2], with the bidentate ligand 4,4′-bipyridine (D-D). These complexes were characterized by elemental analysis, electronic spectra, IR, 1H and 13C NMR spectroscopies, thermogravimetric analysis and cyclic voltammetry. An X-ray study shows that each molybdenum site in the dinuclear and polynuclear complexes adopts an octahedron geometry. The dioxomolybdenum(VI) complexes show catalytic activity for the oxidation of alcohols with H2O2 as an oxidant.