Influence of the initial bonding mode of the hydrocarbyl bridge on the mechanisms and products of the electrochemical reduction of alkyne- and vinylidene dimolybdenum tris(µ-thiolate) complexes

International audienceThe electrochemical reduction of isomeric complexes, [Mo2Cp2(μ-SMe)3(μ-η1:η1-HCCPh)]+ (1+) and [Mo2Cp2(μ-SMe)3(μ-η1:η2-C[double bond, length as m-dash]CHPh)]+ (3+), where the hydrocarbyl bridges in a η1:η1- or a η1:η2 mode, has been studied by cyclic voltammetry and controlled-potential electrolysis in thf–[NBu4][PF6] and CH2Cl2–[NBu4][PF6], in the absence and in the presence of acid. The binding mode of the CC fragment induces different electrochemical behaviour of the complexes in acid-free solutions since 1+ reduces in two diffusion-controlled one-electron steps while the first reduction of 3+ is characterized by slow electron transfer kinetics. Controlled-potential reduction of both 1+ and 3+ produces a mixture of the acetylide [Mo2Cp2(μ-SMe)3(μ-η1:η2-CCPh)] (2) and alkylidyne complexes [Mo2Cp2(μ-SMe)3(μ-η1-CCH2Ph)] (4). In the presence of acid, the electrochemical reduction of 1+ and of 3+ occurs according to ECE processes. The nature of the products formed by controlled-potential reduction of 1+ depends on the nature of the acid and of the solvent. The transient formation of a complex with a μ-alkenyl ligand, either [Mo2Cp2(μ-SMe)3(μ-η1:η2-CH[double bond, length as m-dash]CHPh)] (7) or an isomer, is suggested by the oxidative electrochemistry of 7 and by its reaction with acids. In thf–[NBu4][PF6] in the presence of an excess of acid (HBF4/Et2O) and of phenylacetylene, electrolysis of 1+ gives rise to catalytic reduction of phenylacetylene to styrene. However, unidentified reactions limit the efficiency of this process. The reduction of 3+ in acidic medium produces the alkyl complex [Mo2Cp2(μ-SMe)3(μ-CH2CH2Ph)] (6) through alkylidyne [Mo2Cp2(μ-SMe)3(μ-η1-CCH2Ph)] (4) and alkylidene [Mo2Cp2(μ-SMe)3(μ-η1-CHCH2Ph)]+ (5+) intermediates. Some ethylbenzene was formed after reduction of 5+ in the presence of acid. These results show an effect of the binding mode of the hydrocarbyl bridge on the mechanism and products of the reduction of the corresponding complexes