Electrochemical and DFT studies of the oxidative decomposition of the trihydride complexes Cp*M(dppe)H3 (M = Mo, W) in acetonitrile

Abstract

A detailed electrochemical study of the oxidative decomposition of the trihydride complexes Cp*M(dppe)H₃ (M = Mo, W) in acetonitrile is presented. For the Mo complex, the decomposition occurs by four different pathways involving classical and non-classical tautomers, whereas only the classical form is accessible for the W derivative. Each of the decomposition pathways has been quantitatively assessed by analyses of the linear sweep voltammograms. In addition to the previously established (B. Pleune, D. Morales, R. Meunier-Prest, P. Richard, E. Collange, J. C. Fettinger and R. Poli, J. Am. Chem. Soc., 1999, 121, 2209–2225) deprotonation, disproportionation, and H₂ reductive elimination occurring via the non-classical tautomer of the 17-electron complex [Cp*Mo(dppe)H₃]⁺ (obtained by oxidation at E_1/2 = −0.33 V vs. Ag/AgCl), a new decomposition pathway from the more stable classical tautomer has been identified following a second oxidation process. In addition, the oxidatively induced H₂ reductive elimination, previously evidenced only in THF or CH₂Cl₂, has been quantitatively assessed in MeCN. This process occurs preferentially by an associative mechanism (k = 0.020(4) M⁻¹s⁻¹) via the 19-electron [Cp*Mo(dppe)H(H₂)(MeCN)]⁺ intermediate and is therefore in direct competition with the disproportionation mechanism. The resulting 17-electron [Cp*Mo(dppe)H(MeCN)]⁺ product is further oxidized at ca. 0.2 V. The oxidation of [Cp*Mo(dppe)H₃]⁺ occurs at ca. 1.0 V and the resulting 16-electron [Cp*Mo(dppe)H₃]²⁺ complex immediately delivers a proton to the starting material, giving [Cp*Mo(dppe)H₄]⁺ and [Cp*Mo(dppe)H₂]⁺. The latter coordinates MeCN in a rate determining step to afford [Cp*Mo(dppe)H₂(MeCN)]⁺. The mechanistic details are consistent with studies at different scan rates and different MeCN concentrations, and are backed up by DFT calculations.