Manipulating the electrolyte medium to favor either one-electron or two-electron oxidation pathways for (fulvalendiyl)dirhodium complexes

The anodic oxidations of three compounds having two Rh-containing moieties linked by a fulvalendiyl (Fv) dianion have been studied in varying electrolyte media. Following the integrated approach to manipulation of solvents and supporting electrolytes (Barriere, F.; Geiger, W. E. J. Am. Chem. Soc., in press), Delta E-1/2 values (Delta E-1/2 = E-1/2(2) - E-1/2(1)) of the successive one-electron oxidations of the neutral compounds to the corresponding dications were altered to either favor or disfavor the disproportionation of the mixed-valent intermediate. In the cases of RhF2(CO)(4) (1) and Rh(2)Fv(CO)(2)(mu-dppm) (2) [Fv = C10H8 dppm = bis(diplienylphosphino)methane], replacing the [PF6]- Supporting electrolyte anion with [B(C6F5)](-) in CH2Cl2, changed the voltammograms from those of a single two-electron wave (having '' inverted '' E-1/2 potentials) to those of two distinct '' normal '' one-electron waves. In the case of Rh(2)Fv-(COD)(2) (3, COD = C8H12 both the solvent and supporting electrolyte were changed to manipulate Delta E-1/2 between normal and inverted values. Changes of up to 330 mV in Delta E-1/2, were observed, resulting in modifications of over 10(5) in the disproportionation equilibrium constant, K-disp, of the mixed-valent intermediate 3(+). The thermodynamic stabilization of 1(+) and 2(+) allowed for their IR characterization and confirmed the previously postulated formation of Rh-Rh bonds in the cation radicals. An integrated approach to medium effects is shown to be a powerful method for manipulating the equilibrium concentrations of the various redox states that make up a multiple-electron-transfer process.