Catalysis of mononuclear aquaruthenium complexes in oxygen evolution from water: A new radical coupling path using hydroxocerium(IV) species

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)(R ₂bpy)(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 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 ₂bpy)(OH₂)]²⁺, while the well-known oxo-oxo radical coupling among two Ru^V=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. Water oxidation: The reaction mechanism of O₂ evolution catalyzed by mononuclear aquaruthenium complexes can be classified into two groups that exhibit different paths of O-O bond formation. The radical character provided by the hydroxocerium(IV) ion plays a crucial role in the catalysis by such ruthenium complexes in the evolution of O₂ from water.