Catalysis of Mononuclear Aquaruthenium Complexes in Oxygen Evolution from Water: A New Radical Coupling Path using Hydroxocerium(IV) Species |
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Authors: | Masaki Yoshida Dr. Shigeyuki Masaoka Prof. Dr. Jiro Abe Prof. Dr. Ken Sakai |
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Affiliation: | 1. Department of Chemistry, Faculty of Science, Kyushu University, Hakozaki 6‐10‐1, Higashi‐ku, Fukuoka 812‐8581 (Japan), Fax: (+81)?92‐642‐2596;2. PRESTO, Japan Science and Technology Agency (JST), Honcho 4‐1‐8, Kawaguchi, Saitama, 332‐0012 (Japan);3. Department of Chemistry, School of Science and Engineering, Aoyama Gakuin University, Fuchinobe 5‐10‐1, Chuo‐ku, Sagamihara, Kanagawa 252‐5258 (Japan) |
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Abstract: | The mechanism of O2 evolution from water catalyzed by a series of mononuclear aquaruthenium complexes, [Ru(terpy)(bpy)(OH2)]2+, [Ru(tmtacn)(R2bpy)(OH2)]2+ (R=H, Me, and OMe; R2bpy=4,4′‐disubstituted‐2,2′‐bipyridines), and [Ru(tpzm)(R2bpy)(OH2)]2+ (R=H, Me, and OMe), is investigated, where terpy=2,2′:6′,2′′‐terpyridine, bpy=2,2′‐bipyridine, tmtacn=1,4,7‐trimethyl‐1,4,7‐triazacyclononane, and tpzm=tris(1‐pyrazolyl)methane. The kinetics of O2 evolution is investigated as a function of either the catalyst concentration or the oxidant concentration by employing Ce(NH4)2(NO3)6 as an oxidant; these catalysts can be classified into two groups that have different rate laws for O2 evolution. In one class, the rate of O2 evolution is linear to both the catalyst and Ce4+ concentrations, as briefly reported for [Ru(terpy)(bpy)(OH2)]2+ (S. Masaoka, K. Sakai, Chem. Lett. 2009 , 38, 182). For the other class, [Ru(tmtacn)(R2bpy)(OH2)]2+, the rate of O2 evolution is quadratic to the catalyst concentration and independent of the Ce4+ concentration. Moreover, the singlet biradical character of the hydroxocerium(IV) ion was realized by experimental and DFT investigations. These results indicate that the radical coupling between the oxygen atoms of a RuV?O species and a hydroxocerium(IV) ion is the key step for the catalysis of [Ru(terpy)(bpy)(OH2)]2+ and [Ru(tpzm)(R2bpy)(OH2)]2+, while the well‐known oxo‐oxo radical coupling among two RuV?O species proceeds in the catalysis of [Ru(tmtacn)(R2bpy)(OH2)]2+. This is the first report demonstrating that the radical character provided by the hydroxocerium(IV) ion plays a crucial role in the catalysis of such ruthenium complexes in the evolution of O2 from water. |
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Keywords: | cerium molecular catalyst oxygen evolution radical reactions ruthenium |
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