The development of robust and efficient molecular catalysts based on earth-abundant transition metals for water oxidation reactions is a challenging research target. Our group recently demonstrated the high activity and stability of a pentairon-based water oxidation electrocatalyst (M. Okamura, M. Kondo, R. Kuga, Y. Kurashige, T. Yanai, S. Hayami, V. K. K. Praneeth, M. Yoshida, K. Yoneda, S. Kawata and S. Masaoka, Nature, 2016, 530, 465–468). However, the development of strategies to decrease onset potentials for catalysis remains challenging. In this article, we report the construction of a series of pentanuclear iron complexes by introducing electron-donating (methyl) and electron-withdrawing (bromo) substituents on the ligand. Two newly synthesized complexes exhibited five reversible redox processes, similar to what is seen with the parent complex. These complexes can also serve as homogeneous catalysts for water oxidation reactions, and the faradaic efficiencies of the reactions were high. Additionally, the onset potentials of the newly developed complexes were lower than that of the parent complex. Mechanistic insights revealed that there are two methods for decreasing onset potentials: control of the redox potentials of the pentairon complex and control of the reaction mechanism.

Low-overpotential electrochemical CO₂ reduction by a Ru(ii) polypyridyl complex can be achieved by the simple substitution of pyridine with a phosphine ligand.

Two new ruthenium(II) complexes bearing dissociable protons, [Ru(trpy)(H₂bim)Cl]PF₆ (1) and [Ru(trpy)(H₂bim)(OH₂)](PF₆)₂ (2) (H₂bim = 2,2′-biimidazole and trpy = 2,2′:6′,2′′-terpyridine), were synthesized and characterized, where the H₂bim and M–OH₂ moieties are expected to serve as proton-dissociation sites. Single crystal X-ray diffraction analyses revealed that the H₂bim and M–OH₂ moieties act as proton donors in intermolecular hydrogen bonds. Two pK_a values of 2 (pK_a1 = 9.0 and pK_a2 = 11.3) were spectrophotometrically determined, where the first proton dissociation is assigned to that from H₂bim and the second is from M–OH₂. This assignment was supported by the density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations for two sets of conjugated bases, [Ru(trpy)(Hbim)(OH₂)]⁺ and [Ru(trpy)(H₂bim)(OH)]⁺ for the first proton dissociation, and [Ru(trpy)(Hbim)(OH)]⁺ and [Ru(trpy)(bim)(OH₂)]⁺ for the second dissociation. Electrochemical studies in aqueous solutions under various pH conditions afforded the Pourbaix diagram (potential versus pH diagram) of 2, where the pK_a values found from the diagram agree well with those determined spectrophotometrically. It was also found that 2 demonstrates four-step proton-coupled electron transfer (PCET) reactions to give the four-electron oxidized species, [Ru^IV(trpy)(bim)(O)]²⁺, without electrostatic charge buildup during the reactions. The multiple PCET ability of 2 would be applicable to various multi-electron oxidation reactions. Catalysis of electrochemical water oxidation was indeed evaluated in the initial attempt to demonstrate multi-electron oxidation reactions, revealing that the water oxidation potential for 2 is lower than that for other ruthenium catalysts, [Ru(trpy)(bpy)(OH₂)]²⁺, [Ru(trpy)(bpm)(OH₂)]²⁺ and [Ru(tmtacn)(bpy)(OH₂)]²⁺ (bpy = 2,2′-bipyridine, bpm = 2,2′-bipyrimidine, and tmtacn = 1,4,7-trimethyl-1,4,7-triazacyclononane), which are known to be active catalysts for water oxidation.