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

The mechanism of O-2 evolution from water catalyzed by a series of mononuclear aquaruthenium complexes, [Ru(terpy)(bpy)(OH2)(2+), [Ru-(tmtacn)(R(2)bpy)(OH2)(2+) (R = H, Me, and OMe; R(2)bpy = 4,4'-disubstituted-2,2'-bipyridines), and [Ru(tpzm)-(R(2)bpy)(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 O-2 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 O-2 evolution. In one class, the rate of O-2 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)(R(2)bpy)(OH2)](2+), the rate of O-2 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 Ru-v=O species and a hydroxocerium(IV) ion is the key step for the catalysis of [Ru(terpy)(bpy)(OH2)](2+) and [Ru(tpzm)(R(2)bpy)(OH2)](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)-(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 O-2 from water.