Mechanism of Water Oxidation Catalyzed by a Dinuclear Ruthenium Complex Bridged by Anthraquinone

Tohru Wada,Shunsuke Nishimura,Taro Mochizuki,Yuji Miyazato,Tomohiro Ando

doi:10.3390/catal7020056

Tohru Wada, Shunsuke Nishimura + Show 3 more

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https://doi.org/10.3390/catal7020056

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Abstract

We synthesized 1,8-bis(2,2′:6′,2″-terpyrid-4′-yl)anthraquinone (btpyaq) as a new dimerizing ligand and determined its single crystal structure by X-ray analysis. The dinuclear Ruthenium complex [Ru2(µ-Cl)(bpy)2(btpyaq)](BF4)3 ([3](BF4)3, bpy = 2,2′-bipyridine) was used as a catalyst for water oxidation to oxygen with (NH4)2[Ce(NO3)6] as the oxidant (turnover numbers = 248). The initial reaction rate of oxygen evolution was directly proportional to the concentration of the catalyst and independent of the oxidant concentration. The cyclic voltammogram of [3](BF4)3 in water at pH 1.3 showed an irreversible catalytic current above +1.6 V (vs. SCE), with two quasi-reversible waves and one irreversible wave at E1/2 = +0.62, +0.82 V, and Epa = +1.13 V, respectively. UV-vis and Raman spectra of [3](BF4)3 with controlled-potential electrolysis at +1.40 V revealed that [Ru(IV)=O O=Ru(IV)]4+ is stable under electrolysis conditions. [Ru(III), Ru(II)] species are recovered after dissociation of an oxygen molecule from the active species in the catalytic cycle. These results clearly indicate that an O–O bond is formed via [Ru(V)=O O=Ru(IV)]5+.

Highlights

Artificial photosynthesis is the conversion process from solar to chemical energy and includes carbon dioxide, proton, and nitrogen reduction by visible light irradiation, with water as the electron source [1,2,3,4,5,6,7,8,9,10]
Many complexes containing Ru, Mn, Ir, Fe, Cu, and Co have been reported as molecular water oxidation catalysts [12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29]
One advantage of using molecular catalysts is the possibility of elucidating the reaction mechanism

Summary

Introduction

Artificial photosynthesis is the conversion process from solar to chemical energy and includes carbon dioxide, proton, and nitrogen reduction by visible light irradiation, with water as the electron source [1,2,3,4,5,6,7,8,9,10]. Four-electron water oxidation to dioxygen is, an essential process for artificial photosynthesis, while there are several types of reduction processes. Many complexes containing Ru, Mn, Ir, Fe, Cu, and Co have been reported as molecular water oxidation catalysts [12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29]. A mechanism involving O–O bond formation from two water molecules is an especially attractive aspect from the viewpoint of fundamental chemistry. For Ru-complex catalysts, the O–O bond formation mechanisms proposed by different research groups can be roughly classified into two types:

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Discussion

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