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

Abstract

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.