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 (Barriére, F.; Geiger, W. E. J. Am. Chem. Soc., in press), Δ E 1/2 values (Δ 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 Rh 2 Fv(CO) 4 ( 1 ) and Rh 2 Fv(CO) 2 (μ-dppm) ( 2 ) [Fv = C 10 H 8, dppm = bis(diphenylphosphino)methane], replacing the [PF 6 ] - supporting electrolyte anion with [B(C 6 F 5 )] - in CH 2 Cl 2 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 = C 8 H 12 ), both the solvent and supporting electrolyte were changed to manipulate Δ E 1/2 between normal and inverted values. Changes of up to 330 mV in Δ 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.