6533b828fe1ef96bd1288548

RESEARCH PRODUCT

The reduction of tris-dithiolene complexes of molybdenum(vi) and tungsten(vi) by hydroxide ion: kinetics and mechanism

subject

description

The kinetic study of the spontaneous reduction of some neutral tris-dithiolene complexes [ML3] of molybdenum(VI) and tungsten(VI), (L = S2C6H42−, S2C6H3CH32− and S2C2(CH3)22−; M = Mo or W) by tetrabutylammonium hydroxide in tetrahydrofuran-water solutions demonstrates that OH− is an effective reductant. Their reduction is fast, clean and quantitative. Depending upon both the molar ratio in which the reagents are mixed and the amount of water present, one- or two-electron reductions of these tris-dithiolene complexes were observed. If Bu4NOH is present in low concentration or/and at high concentrations of water, the total transformation of the neutral M(VI) complex into the monoanionic M(V) complex is the only observed process. Stopped-flow kinetic data for this reaction are consistent with the rate law: –d[ML3]/dt = d[ML3−]/dt = k[ML3][Bu4NOH]. The proposed mechanism involves nucleophilic attack of OH− to form a mono-anionic seven-coordinate intermediate [ML3OH]−, which interacts with another molecule of [ML3] to generate the monoanionic complex [ML3]− transfering the oxygen from coordinated OH− to water. Hydrogen peroxide was identified as the reaction product. The molybdenum complexes are more difficult to reduce than their corresponding tungsten complexes, and the values of k obtained for the molybdenum and tungsten series of complexes increase as the ene-1,2-dithiolate ligand becomes more electron-withdrawing (S2C6H42− > S2C6H3CH32− > S2C2(CH3)22−). This investigation constitutes the only well-established interaction between hydroxide ion and a tris(dithiolene) complex, and supports a highly covalent bonding interaction between the metal and the hydroxide ion that modulates electron transfer reactions within these complexes.