Reversible conversion between chemical and electrical energies catalyzed by Ru complexes aimed to construct sustainable society

Reversible conversion between carbon dioxide and MeOH through six-electron redox reactions is one of the feasible pathway to construct renewable societies. Metal complexes that can oxide MeOH at potentials more negative than four-electron reduction of dioxygen are candidates for the electrode materials in fuel-cells. An aqua-Ru complex having a quinone ligand (pKa = 5.5) exists as the hydroxy analog (pKa ca. 11) in neutral water, and the latter is converted to an oxyl radical complex with a semiquinone ligand in highly basic solutions due to the electron flow from the deprotonated hydroxy group to the quinone ligand. On the other hand, an Ru-aminyl radical complex with quinone derived from the corresponding Ru-amine one works as an active catalyst in the electrochemical oxidation of MeOH in the presence of base at 0.3 V (vs. SCE). We will also discuss the electrochemical reduction of carbon dioxide catalyzed by Ru complexes in the symposium.