Effect of Axial Ligands on the Spectroscopic and Electrochemical Properties of Diruthenium Compounds

Three related diruthenium complexes containing four symmetrical anionic bridging ligands were synthesized and characterized as to their electrochemical and spectroscopic properties. The examined compounds are represented as Ru₂(dpb)₄Cl, Ru₂(dpb)₄­(CO), and Ru₂(dpb)₄­(NO) in the solid state, where dpb = diphenylbenzamidinate anion. Different forms of Ru₂(dpb)₄Cl are observed in solution depending on the utilized solvent and the counteranion added to solution. Each Ru₂⁵⁺ form of the compound undergoes multiple redox processes involving the dimetal unit. The reversibility as well as potentials of these diruthenium-centered electrode reactions depends upon the solvent and the bound axial ligand. The Ru₂^5+/4+ and Ru₂^5+/6+ processes of Ru₂(dpb)₄Cl were monitored by UV–vis spectroscopy in both CH₂Cl₂ and PhCN. A conversion of Ru₂(dpb)₄Cl to [Ru₂(dpb)₄­(CO)]⁺ was also carried out by simply bubbling CO gas through a CH₂Cl₂ solution of Ru₂(dpb)₄Cl at room temperature. The chemically generated [Ru₂(dpb)₄(CO)]⁺ complex undergoes several electron transfer processes in CH₂Cl₂ containing 0.1 M TBAClO₄ under a CO atmosphere, and the same reactions were seen for a chemically synthesized sample of Ru₂(dpf)₄(CO) in CH₂Cl₂, 0.1 M TBAClO₄ under a N₂ atmosphere, where dpf = N,N′-diphenylformamidinate anion. Ru₂(dpb)₄­(NO) undergoes two successive one-electron reductions and a single one-electron oxidation, all of which involve the diruthenium unit. The CO and NO adducts of Ru₂(dpb)₄ were further characterized by FTIR spectroelectrochemistry, and the IR spectral data of these compounds are discussed in light of results for previously characterized Ru₂(dpf)₄­(CO) and Ru₂(dpf)₄­(NO) derivatives under similar solution conditions.