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

Abstract

The electrochemical reduction of isomeric complexes, [Mo₂Cp₂(μ-SMe)₃(μ-η¹:η¹-HCCPh)]⁺ (1⁺) and [Mo₂Cp₂(μ-SMe)₃(μ-η¹:η²-CCHPh)]⁺ (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-CHCHPh)] (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.