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Theoretical Study of Water Oxidation by the Ruthenium Blue Dimer. II. Proton Relay Chain Mechanism for the Step [bpy 2 (HOO)Ru IV ORu IV (OH)bpy 2 ] 4+ → [bpy 2 (O 2 – )Ru IV ORu III (OH 2 )bpy 2 ] 4+

2013; American Chemical Society; Volume: 117; Issue: 49 Linguagem: Inglês

10.1021/jp406504h

1520-6106

Roberto Bianco, P. Jeffrey Hay, James T. Hynes,

Catalytic Processes in Materials Science

The oxidation of water to O2 by the oxidized species [L2(O)Ru(V)ORu(V)(O)L2]4+ of the Ru blue dimer catalyst (L = bpy, bipyridine) is examined using density functional theory with model ligands and explicit solvent approaches. Following our earlier study of the initial O–O formation by addition of water (step I) ( J. Phys. Chem. A 2011, 115, 8003), we report calculations on the subsequent, penultimate step in the superoxide production (denoted step II), involving proton transfer from the reactant [L2(HOO)Ru(IV)ORu(IV)(OH)L2]4+ to form [L(O2(–))Ru(IV)ORu(III)(H2O)L2]4+. The reaction profile of step II commences with a rearrangement of the HOO and OH groups and associated solvent relaxation in the complex, accompanied by a barrier of ~9 kcal/mol and a free-energy change of +3 kcal/mol. Subsequently, a water molecule connecting these two groups mediates a double proton transfer in a proton relay chain that proceeds spontaneously with a free-energy decrease of 8 kcal/mol to form step II’s product. Comparison with other calculations is made, and the implications for the overall water oxidation to O2 are discussed.