Catalysis of Mononuclear Aquaruthenium Complexes in Oxygen Evolution from Water: A New Radical Coupling Path using Hydroxocerium(IV) Species

Abstract

The mechanism of O(2) evolution from water catalyzed by a series of mononuclear aquaruthenium complexes, [Ru(terpy)(bpy)(OH(2))](2+), [Ru(tmtacn)(R(2)bpy)(OH(2))](2+) (R=H, Me, and OMe; R(2)bpy=4,4'-disubstituted-2,2'-bipyridines), and [Ru(tpzm)(R(2)bpy)(OH(2))](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 O(2) evolution is investigated as a function of either the catalyst concentration or the oxidant concentration by employing Ce(NH(4))(2)(NO(3))(6) as an oxidant; these catalysts can be classified into two groups that have different rate laws for O(2) evolution. In one class, the rate of O(2) evolution is linear to both the catalyst and Ce(4+) concentrations, as briefly reported for [Ru(terpy)(bpy)(OH(2))](2+) (S. Masaoka, K. Sakai, Chem. Lett. 2009, 38, 182). For the other class, [Ru(tmtacn)(R(2)bpy)(OH(2))](2+), the rate of O(2) evolution is quadratic to the catalyst concentration and independent of the Ce(4+) 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 Ru(V)=O species and a hydroxocerium(IV) ion is the key step for the catalysis of [Ru(terpy)(bpy)(OH(2))](2+) and [Ru(tpzm)(R(2)bpy)(OH(2))](2+), while the well-known oxo-oxo radical coupling among two Ru(V)=O species proceeds in the catalysis of [Ru(tmtacn)(R(2)bpy)(OH(2))](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 O(2) from water.