One-Electron Oxidation of Ruthenocene: Reactions of the Ruthenocenium Ion in Gentle Electrolyte Media

The electrochemical oxidation of ruthenocene, RUCp2 (Cp = eta(5)-C5H5), 1, has been studied in dichloromethane using a supporting electrolyte containing either the [B(C6F5)(4)](-) (TFAB) or the [B(C6H3(CF3)(2))(4)](-) (BArF24) counteranion. A quasi-Nernstian process was observed in both cases, with E-1/2 values of 0.41 and 0.57 V vs FeCp2 in the respective electrolyte media. Them ruthenocenium ion 1(+) equilibrates with a metal-metal bonded dimer [Ru2Cp4](2+), 2(2+), that is increasingly preferred at low temperatures. Dimerization equilibrium constants determined by digital simulation of cyclic voltammetry (CV) curves were in the range of 10(2)-10(4) M-1 at temperatures of 256 to 298 K. Near room temperature, and particularly when BArF24 is the counteranion, the dinuclear species [Ru2Cp2(sigma:eta(5)-C5H4)(2)](2+), 3(2+), in which each metal is sigma-bonded to a cyclopentaclienyl ring, was the preferred electrolytic oxidation product. Cathodic reduction of 3(2+) regenerated ruthenocene. The two dinuclear products, 2(2+) and 3(2+), were characterized by H-1 NMR spectroscopy on anodically electrolyzed solutions of 1 at low temperatures in CD2Cl2/[NBu4][BArF24]. The variable temperature NMR behavior of these solutions showed that 3(2+) and 2(2+) take part in a thermal equilibrium, the latter being dominant at the lowest temperatures. Ruthenocene hydride, [1-H](+), was also identified as being present in the electrolysis solutions. The oxidation of ruthenocene is shown to be an inherent one-electron process, giving a ruthenocenium ion which is highly susceptible to reactions that allow it to regain an 18-electron configuration. In a dry non-donor solvent, and in the absence of nucleophiles, this electronic configuration is attained by self-reactions involving formation of Ru-Ru or Ru-C bonds. The present data offer a mechanistic explanation for the previously described results on the chemical oxidation of osmocene (Droege, MA; Harman, W.D.; Taube, H. Inorg. Chem. 1987, 26, 1309) and are relevant to the manner in which sigma:eta(5)-C5H4-complexes of other second and third-row metals are formed.