Journal of Physical Chemistry B, Vol.117, No.49, 15761-15773, 2013
Theoretical Study of Water Oxidation by the Ruthenium Blue Dimer. II. Proton Relay Chain Mechanism for the Step [bpy(2)(HOO)(RuORuIV)-O-IV(OH)bpy(2)](4+) -> [bpy(2)(O-2(-))(RuORuIII)-O-IV(OH2)bpy(2)](4+)
The oxidation of water to O-2 by the oxidized species [L-2(O)(RuORuV)-O-V(O)L-2](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 [L-2(HOO)(RuORuIV)-O-IV(OH)L-2](4+) to form [L(O-2(-))(RuORuIII)-O-IV(H2O)L-2](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 similar to 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 O-2 are discussed.