Mechanisms of Water Oxidation from the Blue Dimer to Photosystem II

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Abstract
The blue dimer, cis,cis-[(bpy)2(H2O)RuIIIORuIII(H2O)(bpy)2]4+, is the first designed, well-defined molecule known to function as a catalyst for water oxidation. It meets the stoichiometric requirements for water oxidation, 2H2O O−O, by utilizing proton-coupled electron-transfer (PCET) reactions in which both electrons and protons are transferred. This avoids charge buildup, allowing for the accumulation of multiple oxidative equivalents at the Ru−O−Ru core. PCET and pathways involving coupled electron–proton transfer (EPT) are also used to avoid high-energy intermediates. Application of density functional theory calculations to molecular and electronic structure supports the proposal of strong electronic coupling across the μ-oxo bridge. The results of this analysis provide explanations for important details of the descriptive chemistry. Stepwise e−/H+ loss leads to the higher oxidation states [(bpy)2(O)RuVORuIV(O)(bpy)2]3+ (RuVORuIV) and [(bpy)2(O)RuVORuV(O)(bpy)2]4+ (RuVORuV). Both oxidize water, RuVORuIV stoichiometrically and RuVORuV catalytically. In strongly acidic solutions (HNO3, HClO4, and HSO3CF3) with excess CeIV, the catalytic mechanism involves O---O coupling following oxidation to RuVORuV, which does not build up as a detectable intermediate. Direct evidence has been found for intervention of a peroxidic intermediate. Oxidation of water by RuVORuIV is far slower. It plays a role late in the catalytic cycle when CeIV is depleted and is one origin of anated intermediates such as [(bpy)2(HO)RuIVORuIV(NO3)(bpy)2]4+, which are deleterious in tying up active components in the catalytic cycle. These intermediates slowly return to [(bpy)2(H2O)RuIVORuIII(OH2)(bpy)2]5+ with anion release followed by water oxidation. The results of a recent analysis of water oxidation in the oxygen-evolving complex (OEC) of photosystem II reveal similarities in the mechanism with the blue dimer and significant differences. The OEC resides in the thylakoid membrane in the chloroplasts of green plants, and careful attention is paid in the structure to PCET, EPT, and long-range proton transfer by sequential local proton transfers. The active site for water oxidation is a CaMn4 cluster, which includes an appended Mn site, Mn(4), where O---O coupling is thought to occur. Photochemical electron transfer results in oxidation of tyrosine YZ to YZ·, which is ∼7 Å from Mn(4). It subsequently oxidizes the OEC through the stepwise stages of the Kok cycle. O---O coupling appears to occur through an initial peroxidic intermediate formed by redox nucleophilic attack of coordinated OH− in Ca−OH− on MnIV═O.