ELECTROCHEMISTRY OF SYMMETRICAL AND ASYMMETRICAL DINUCLEAR RUTHENIUM, OSMIUM, AND MIXED-METAL 2,2'-BIPYRIDINE COMPLEXES BRIDGED BY 2,2'-BIBENZIMIDAZOLATE

Electrochemical studies in n-butyronitrile (propyl cyanide) at platinum and glassy-carbon electrodes over the temperature range -70 to +25-degrees-C on a series of 2,2'-bibenzimidazolate- (BiBzIm-) bridged complexes of the kind [(R1(2)bpy)2M1(BiBzIm)M2-(R2(2)bpy)2](ClO4)2 (M1 and M2 = Ru and/or Os; R1(2)bpy and R2(2)bpy = bypyridine ligand) reveal the presence of an extensive series of reversible one-electron-oxidation and -reduction charge-transfer processes. The combined use of mixed-metal and symmetric and asymmetric 2,2'-bipyridine ligands enables considerable information to be obtained concerning the nature of the electron-transfer processes. The reversible half-wave potentials of the oxidation processes are dependent on the metal and involve generation of oxidation state (II-III), (III-III), and (III-IV) dinuclear complexes. The potential differences in the first two oxidation processes correlate with the intervalence transfer energies. The third oxidation process leads to the formation of a reactive (III-IV) complex from which the (III-III) complex is regenerated by reaction with the solvent, light, or impurity. The predicted fourth one-electron process leading to the formation of the fully oxidized (IV-IV) dinuclear complex is not observed and is believed to be outside the available potential range in the n-butyronitrile solvent. The one-electron-reduction charge-transfer processes, which vary in number from four to six, are essentially 2,2'-bipyridine ligand based. Substantial changes in the potentials, observed in changing from symmetrical to asymmetrical complexes but with a relative independence on the metal, readily enable the ligand-based nature of the reduction processes to be identified in a systematic manner. The potentials for the reduction processes are discussed in terms of the pi* orbital energy levels of the 2-2'-bipyridine ligands and the strength of ligand-ligand, metal-mediated ligand-ligand, or bridging-ligand-mediated ligand-ligand interactions arising from increased electron repulsion terms associated with the reduction process.