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 Ru25+/4+ and Ru25+/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.