Across chemical disciplines, an interest in developing artificial water splitting to O(2) and H(2), driven by sunlight, has been motivated by the need for practical and environmentally friendly power generation without the consumption of fossil fuels. The central issue in light-driven water splitting is the efficiency of the water oxidation, which in the best-known catalysts falls short of the desired level by approximately two orders of magnitude. Here, we show that it is possible to close that 'two orders of magnitude' gap with a rationally designed molecular catalyst [Ru(bda)(isoq)(2)] (H(2)bda = 2,2'-bipyridine-6,6'-dicarboxylic acid; isoq = isoquinoline). This speeds up the water oxidation to an unprecedentedly high reaction rate with a turnover frequency of >300 s(-1). This value is, for the first time, moderately comparable with the reaction rate of 100-400 s(-1) of the oxygen-evolving complex of photosystem II in vivo.

Practical-efficiency catalysis of the water oxidation process (2 H 2 O!O 2 + 4 e À + 4 H + ) is the highly sought element of emerging artificial photosynthetic energy-conversion technology. [1,2] While oxygen evolution in naturally occurring photosynthesis, which supports nearly all existing life forms, relies on a sophisticated Mn-based complex, [1a, 3] the majority of artificial molecular water-oxidation catalysts (WOCs) are Ru-based complexes with relatively simple polypyridyl ligands. [4, 5] Recently emerged evidence in favor of alternative O 2 -evolving mechanisms for Ru-catalyzed water oxidation, [5] such as solvent water nucleophilic attack (WNA) and direct O À O coupling via interaction of two M-O units (I2M), [5a, 6-8] demonstrated dramatic mechanistic consequences of different ligand designs, which are yet to be fully rationalized. Understanding of intricate ligand-dependent preferences for one mechanism over the other is necessary for further progress to be made. [5, 7] Unfortunately, vast structural differences between Ru-bound ligands of WOCs which operate by the WNA or I2M mechanism hampers determinations of ligand influence on catalytic pathways.

Catalytic water oxidation is an essential part of light-driven splitting of water into H 2 and O 2 . [1][2][3][4][5][6][7][8] This process would be a clean, renewable and sustainable solution to the energy demands of humanity.[9] Despite many efforts, the features critical for efficient catalytic water oxidation, are insufficiently understood. A major reason is the difficulty of characterizing the reaction intermediates, which are unstable, presumably because of the presence of high-valent metal centers. Continued efforts to understand the reaction mechanism and to find more efficient and robust catalysts are therefore essential. [10][11][12][13][14][15][16][17][18][19][20][21] Although over the years, six-coordinate dinuclear ruthenium centers have been the staple of mechanistic rationale for a number of reported water oxidation catalysts, [10][11][12][13][14][15][16][19][20][21] involvement of seven-coordinate mononuclear intermediates has recently been considered. [10a, 14] A few months ago, we discovered a new water-oxidation catalyst, the Ru II complex 1 (Figure 1). The kinetics of catalytic water oxidation were second order in complex 1, suggesting that the reaction proceeds via a dimeric complex, such as 2 (Figure 1).[21] In fact, both complex 1 and the uncommon seven-coordinate dimeric Ru IV complex 2, were isolated and structurally characterized by X-ray crystallography.[21] The novel structural aspects of complex 1 and the experimentally verified involvement of seven-coordinate ruthenium dimer (2) in the catalytic water oxidation raise the following key questions: What are coordination geometries of the Ru centers in different oxidation states? What are the redox properties? What is the role of the hydrogenbonding network? In view of the uncommon seven-coordinate Ru centers in complex 2, what is a plausible mechanism of O 2 evolution? Our study proposes answers to these and other central questions based on accurate calculations with hybrid density functional B3LYP within self-consistent reaction field (SCRF) solvent model. [22] Most importantly, we demonstrate that O 2 evolution via the direct interaction of oxygen radicals in the doubly oxidized dimer 2 and the subsequent redox-coupled release of O 2 does not require crossing of prohibitively high potential-energy barriers. We also uncover key electronic and structural aspects of sevencoordinate Ru centers.The starting point of our study is complex 1 in aqueous solution. For all calculations, picoline groups are replaced by pyridine groups (py) for purely computational reasons. According to previously published methodology, [23] the standard Gibbs free energy of the redox half reaction contains the free energy difference of the redox pair in the gas phase and the difference in free energy of solvation of oxidized and reduced species as computed in SCRF calculations (the socalled Born-Haber cycle; details in the Supporting Information). All reported potentials are referenced to the normal hydrogen electrode (NHE).[24] To model proton-coupled redox r...

Specific interactions of the I−/I3 − redox mediators with the reduced and oxidized dye, Ru(4,4′-dicarboxy-2,2′-bipyridyl)2(NCS)2, referred to as N3 or Ru(dcbpy)2(NCS)2, have been studied by means of density functional theory (DFT) with the focus on the charge transfer process involving {dye+ I−} adducts; computations had been performed with a series of density functionals (gradient-corrected density functional BP86, and the hybrid density functionals B3LYP, MPW1K, B3PW1K, and MPW1PW91). Different pathways leading to {dye+ I−} adducts have been studied. First, mechanistic insights into the interaction of I− with RuIII(dcbpy)2(NCS)2 via an SCN− ligand directly giving rise to RuII(dcbpy)2(NCS)2I⌉0 have been obtained with the distinctive S−I bonding. Second, the binding of I− to the N3 dye cation via I−−dcbpy interactions has been analyzed. We also report experimental and computational evidence that sheds light on the interaction of the redox mediator with bipyridyl moieties. Evidence for a charge transfer process in the presence of only one I− anion in the outer coordination sphere of the ruthenium center has been identified. Finally, geometries and electronic structures of plausible intermediates have been computationally analyzed based on an inner-sphere interaction between the metal center and the redox mediator, including a two-step regeneration reaction: RuIII(dcbpy)2(NCS)2⌉+ + I− → RuIII(dcbpy)2(NCS)I⌉+ + SCN−, followed by the interaction of a second I− with the intermediate RuIII(dcbpy)2(NCS)I⌉+ complex. Conclusive evidence of a charge-transfer process that gives rise to the regenerated RuII complex, where I− interacts with the intermediate RuIII(dcbpy)2(NCS)I⌉+ complex has been identified.