Electrochemistry of [Mo2Cp2(CO)4{μ-η2:η3-HC⋮C−C(R1)(R2)}]+Complexes (R1 = H, R2 = H, Me, Et, Fc; R1 = Me, R2 = Me, Ph). Control of the Reduction Process (Two-ElectronvsOne-Electron) by the Substituents R1 and R2: EHMO Rationalization

Abstract

The electrochemical reduction of [Mo2Cp2(CO)4{μ-η2:η3-HC⋮C−C(R1)(R2)}]+ complexes has been investigated by cyclic voltammetry, controlled-potential electrolysis, and coulometry. On the cyclic voltammetry time scale, the complexes with R1 = H, R2 = H (1+), Me (2+), Et (3+) undergo an irreversible or a quasi-reversible one-electron reduction whereas the analogues with R1 = H, R2 = Fc (4+) and R1 = Me, R2 = Me (5+) and Ph (6+) reduce in a single-step, reversible or quasi-reversible, two-electron process. Two different chemical reactions are involved in the overall reduction mechanism. The first chemical step is assigned as a structural rearrangement, responsible for slowing down the heterogeneous electron transfer. Extended Hückel MO calculations indicate that in the case of the complexes with R1 = H, R2 = Fc and R1 = Me, R2 = Me or Ph, a small increase in the distance between one metal center and the carbon of the C(R1)(R2) group could trigger the two-electron transfer process. The second chemical reaction leading to the final product(s) of the reduction involves radical species, even when a two-electron transfer is observed by cyclic voltammetry. The final products formed in these processes have been identified either by 1H NMR spectroscopy of the compounds extracted from the catholyte after controlled-potential electrolyses or from a comparison of their characteristic redox potentials with those of authentic samples. The nature of the final product(s), either a dimer or μ-alkyne and μ-enyne complexes, is also dependent on the nature of R1 and R2.