Electrochemical oxidation of half-open ruthenocene compounds. Role of acyclic ligands on acetonitrile coordination

The electrochemical oxidation of half-open Ru(II) compounds Cp∗Ru(2,4-Me2-η5-pentadienyl) (1), Cp∗Ru(2,4-Me2-η5-oxopentadienyl) (2) and Cp∗Ru(2,4-t-Bu2-η5-oxopentadienyl) (3) was carried out in acetonitrile. The relative reactivities of the resulting oxidized forms of Ru(III) toward acetonitrile addition reactions have been examined. The overall oxidation reaction for 1, 2 and 3 is explained in terms of a square mechanism. It is shown that three kinetic products [Cp∗Ru(CH3CN)2(2,4-Me2-η3-pentadienyl)]2+, [Cp∗Ru(CH3CN)2(2,4-Me2-η3-oxopentadienyl)]2+ and [Cp∗Ru(CH3CN)2(2,4-t-Bu2-η3-oxopentadienyl)]2+ are formed via ECE-DISP two-electron mechanism. The electrolysis results revealed that a common thermodynamic product [Cp∗Ru(CH3CN)3]+ (4) is also formed by oxidation of compounds 1–3. The monocationic compound 4 is obtained by means of either two alternative pathways: one-electron and two-electron mechanism. The first one involves the homolytic cleavage of the σ-Ru-η1-acyclic ligand bond; meanwhile an heterolytic rupture of the π-Ru-η3-acyclic ligand bond is involved in the second mechanism. The coulometry experiences additionally revealed a self-consumption step which prevailed at long experimental times of electrolysis. The voltammetric simulation of the proposed mechanism allowed us the determination of the kinetic and thermodynamic viability of the steps involved in the overall mechanism. It was found that the reactivity of the oxodienyl ligand derivatives is higher than that of the corresponding pentadienyl ligand.