Mixed-valence properties of ruthenium-polypyridine dimers bridged by imidazolate and triazolate ligands

The dinuclear complex cis,cis+[(bpy)(2)ClRu(mu-bim)RuCl(bpy)(2)](n+) (bpy=2,2'-bipyridine; bim=benzimidazolate; n = 1, 2, or 3) was synthesized, isolated as a hexafluorophosphate salt, and investigated in organic solutions by cyclic voltammetry and UV/visible/NIR spectroelectrochemistry. The mixed-valent species (n=2) displays significant metal-metal electronic coupling in the ground state but exhibits localized Ru(III) and Ru(II) oxidation states, as deduced from its intervalence charge transfer (IVCT) band and redox parameters. On the basis of the resonance energy (H-AB) estimated in the context of Hush's semiclassical theory, the extent of intermetallic communication was found to be larger than that recently reported for the bta-bridged analog (bta = benzotriazolate). Some differences between the IVCT features of these systems have been rationalized in terms of the degree of sigma,pi-basic character of the bridging ligands, according to an electron super-exchange mechanism of the "hole-transfer" type. The stabilization of the mixed-valent complexes is attributed mainly to cooperative metal-to-ligand/ligand-to-metal charge-transfer effects. The combined pi-acceptor and sigma,pi-donor abilities of the ancillary (bpy) and bridging (bim or bta) ligands, respectively, are also responsible for the high stability of the fully oxidized (Ru-III-L-Ru-III) and fully reduced (Ru-II-L-Ru-II) isovalent species.