Examination of Water Oxidation by Catalysts Containing Cofacial Metal Sites

A series of bimetallic complexes were developed to examine how the distance between two juxtaposed {Ru(tpy)(bpy)Cl}(1+) (2; tpy = 2,2';6',2 ''-terpyridine; bpy = 2,2'-bipyridine) units affect catalytic behavior in the context of water oxidation. The directionality imposed by the organic backbone for each of these "Pacman" complexes elicits idealized M center dot center dot center dot M distances of ca. 5, 10, and 12 angstrom for 4-6, respectively. Cyclic voltammetry indicates that intramolecular electronic communication occurs between the metal sites of 4, while the metal fragments are essentially electrochemically isolated for 5 and 6. Despite the disparate distances between the metal sites, each of these complexes are shown to mediate the Ce(IV)-driven oxidation of water in acidic media. Dioxygen evolution was monitored with an optical probe in the headspace of the reaction vessel, and independently verified by the detection of appropriate ratios of (18)O-(18)O, (18)O-(16)O, and (18)O-(16)O in GC-MS experiments using (18)O-labeled water. A fitting of the dioxygen evolution data indicates catalytic behavior that is first-order with respect to catalyst concentration in all cases. This finding rules out the possibility that dioxygen formation occurs independently at each metal site simultaneously, and leaves open the possibility that O-O bond formation occurs by either both metal sites acting cooperatively, or at one metal site independent of the other. Taking into account the significant electrostatic repulsion between the two metal fragments in higher oxidation levels, we contend that the latter mechanism is the more likely scenario. Notwithstanding, complex 4 generates the largest k(obs) values among the bimetallic series, which suggests that the close proximity of another metal may benefit catalysis. A comparison of k(obs) and catalytic turnover numbers (TONs) of the bimetallic systems to the mononuclear complex 2, however, does not indicate an improvement in catalytic behavior. This is an important observation in the collective pursuit of efficient water oxidation catalysts.