A mononuclear ruthenium complex showing multiple proton-coupled electron transfer toward multi-electron transfer reactions

Abstract

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.