Theoretical Study of Water Oxidation by the Ruthenium Blue Dimer. II. Proton Relay Chain Mechanism for the Step [bpy₂(HOO)Ru^IVORu^IV(OH)bpy₂]⁴⁺ → [bpy₂(O₂^–)Ru^IVORu^III(OH₂)bpy₂]⁴⁺

The oxidation of water to O2 by the oxidized species [L2(O)­RuVORuV(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)­RuIVORuIV(OH)­L2]4+ to form [L­(O2–)­RuIVORuIII(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.