Mixed Valency across Hydrogen Bonds

An oxo-centered triruthenium cluster with one pyridine-4-carboxylic acid ligand forms a mixed valence monoanionic dicarboxylic acid dimer upon partial reduction. Dimerization is not observed in DMSO or in the deprotonated carboxylate complex. Infrared spectroscopy reveals the mixed valence dimer as a charge localized species, and UV/vis/NIR spectroscopy suggests a large stabilization of the ground state by mixed valency across hydrogen bonds, on the order of 2500 cm(-1), or 7 kcal/mol, relative to the hydrogen bonded but isovalent fully reduced dimer. The stabilization is a combination of hydrogen bonding and electronic coupling.