Catalytic Water Oxidation Involving Ruthenium Polypyridyl Complexes

Light-driven water oxidation occurs in oxygenic photosynthesis in Photosystem II where reductive equivalents are produced to ultimately convert carbon dioxide into carbohydrates. This process effectively stores solar energy in the form of chemical bonds. Water oxidation is a key component in schemes for artificial photosynthesis, such as solardriven water splitting into hydrogen and oxygen... which could provide much needed clean, renewable fuels. The ... cis,cis-[(bpy)2(H2O)RuIIIORuIII(OH2)(bpy)2]4+, is the first well characterized molecule known to catalyze water oxidation. It meets the stoichiometric requirements for water oxidation ... by utilizing proton-coupled electron transfer (PCET) reactions in which both electrons and protons are transferred. In the key step, oxidation to the catalytically-active state, [(O)RuVORuV(O)]4+, results in nucleophilic water attack to form the O-O bond, producing a peroxidic complex. The mechanism of blue dimer catalyzed water oxidation has been resolved to its clearest understanding yet, yielding new insights and opportunities for rational catalyst design. Following this foray into the complexities of the blue dimer, a plethora of single-site (one aqua ligand) ruthenium monomers has been developed, each of which are capable of catalytic water oxidation, driven electrochemically or under acidic conditions using Ce(IV) as a sacrificial oxidant. These homogeneous catalysts have been incorporated into devices by the synthesis of their phosphonic acid derivatized analogues to provide stable interfacial attachment to metal oxide surfaces. Low overpotentials for the electrocatalysis of water oxidation have been achieved with high turnover numbers. Furthermore, a strategy for enhancing rates of water oxidation has been developed using a series of kinetically facile electron transfer mediators with varying thermodynamic driving force. Rate enhancements by factors of up to 30 have been obtained in solution and with surface-modified electrodes. An electrochemical kinetic analysis has been applied for homogeneous water oxidation with surface-modified electrodes. The incorporation of catalysts with electron transfer mediators, which have been studied extensively as chromophores for excited state electron transfer reactions, has led to the design and synthesis of assemblies for electrocatalytic water oxidation, providing new insights into their application toward solar energy conversion. Another approach toward assemblies of this kind has been pursued by exploiting the pH dependence of phosphonic acid derivatized complexes by electrostatic association of cationic water oxidation catalysts to deprotonated, anionic redox mediator-chromophore adsorbates on electrode surfaces. This negates the need for difficult synthetic procedures and bridge design for attaching the necessary components for an artificial photosynthetic apparatus