The oxidation of water is of great importance for artificial photosynthetic systems that capture and store solar energy as renewable and sustainable fuel. [1,2] To closely mimic the function of the Mn 4 CaO 5 cluster, which is at the heart of photosystem II, [3] intensive research effort has been devoted to the development of catalysts for the oxidation of water. [4,5] In addition to metal oxides, homogeneous molecular catalysts have attracted considerable attention in recent years, mainly because of their tunable structures and rapid oxygen evolution reletive to that observed for heterogeneous catalysts. Among these catalysts, ruthenium complexes have been most successfully used. A few monomeric and polymeric ruthenium catalysts have shown remarkable activities up to thousands of turnover numbers (TONs) for the chemically driven oxidation of water by Ce(NH 4 ) 2 (NO 3 ) 6 (Ce IV ) as sacrificial oxidant. [6][7][8][9] However, in view of their application as devices for water splitting it is essential to immobilize homogeneous catalysts on surfaces of heterogeneous electrodes without losing their reactivity and stability. [10][11][12][13][14][15][16][17] Therefore, proper methods for combining solid conductors/semiconductors and suitable transition-metal complexes should be developed to efficiently drive the oxidation of water at a minimum overpotential and under ambient conditions.
Along these lines, polypyridyl ruthenium catalysts [10][11][12][13] as well as inorganic molecular catalysts [14][15][16] have been anchored on metal oxide electrode surfaces, which successfully supports [*] Dr.