The different kinetic and mechanistic behaviors of molybdenum and tungsten in the reduction of tris(benzene-1,2-dithiolato)Mo(VI) and W(VI) complexes by ascorbic acid in aqueous media

The mono-electronic reduction of tris(benzene-1,2-dithiolato)Mo(VI) and W(VI) complexes (ML3: M = Mo, W; L = S2C6H2−4, S2C6H3CH2−3) to their anionic forms ML−3 by L(+)-ascorbic acid (H2A) has been studied in tetrahydrofurane (THF):water and THF:methanol by means of diode-array, stopped-flow, and mass spectrometry–electrospray ionization (MS-ESI) spectroscopy. The kinetic study in methanol demonstrates that the reaction is first order in each reactant, the electron transfer being rate limiting. This fact was assessed by the absence of a primary saline effect and by the correlation observed between the activation free enthalpy (ΔG≠) and the reduction potentials measured by cyclic voltamperometry. In aqueous media, Mo(VI)-tris(dithiolenes) also reduce to their Mo(V) anionic forms. The reaction obeys the rate law − d[ML3]/dt = (kS+kA[H2A]T)[ML3] (M = Mo), in agreement with a parallel kinetic scheme involving the reduction of complexes by ascorbic acid (kA) and by interaction with the solvent (kS). Unexpectedly, the W(VI) complexes were not reduced by excess hydrogen ascorbate in the presence of water. These compounds underwent an extremely rapid autoreduction, which initially yielded an oxo W(VI)-dithiolene and [W(S2C6H4)3]−, as assessed by the MS-ESI spectra. This observation suggests that tungsten tris(dithiolenes) are capable of coordinating water efficiently, undergoing further reduction after ligand displacement. © 2011 Wiley Peiodicals, Inc. Int J Chem Kinet 43: 279–291, 2011