Mechanisms of Water Oxidation from the Blue Dimer to Photosystem II

Mediator-assisted water oxidation by the ruthenium "blue dimer" cis,cis-[(bpy)2(H2O)RuORu(OH2)(bpy)2]4+

Light-driven water oxidation occurs in oxygenic photosynthesis in photosystem II and provides redox equivalents directed to photosystem I, in which carbon dioxide is reduced. Water oxidation is also essential in artificial photosynthesis and solar fuel-forming reactions, such as water splitting into hydrogen and oxygen (2 H(2)O + 4 h nu --> O(2) + 2 H(2)) or water reduction of CO(2) to methanol (2 H(2)O + CO(2) + 6 h nu --> CH(3)OH + 3/2 O(2)), or hydrocarbons, which could provide clean, renewable energy. The "blue ruthenium dimer," cis,cis-[(bpy)(2)(H(2)O)Ru(III)ORu(III)(OH(2))(bpy)(2)](4+), was the first well characterized molecule to catalyze water oxidation. On the basis of recent insight into the mechanism, we have devised a strategy for enhancing catalytic rates by using kinetically facile electron-transfer mediators. Rate enhancements by factors of up to approximately 30 have been obtained, and preliminary electrochemical experiments have demonstrated that mediator-assisted electrocatalytic water oxidation is also attainable.

Photosystem II: The machinery of photosynthetic water splitting

This review summarizes our current state of knowledge on the structural organization and functional pattern of photosynthetic water splitting in the multimeric Photosystem II (PS II) complex, which acts as a light-driven water: plastoquinone-oxidoreductase. The overall process comprises three types of reaction sequences: (1) photon absorption and excited singlet state trapping by charge separation leading to the ion radical pair [Formula: see text] formation, (2) oxidative water splitting into four protons and molecular dioxygen at the water oxidizing complex (WOC) with P680+* as driving force and tyrosine Y(Z) as intermediary redox carrier, and (3) reduction of plastoquinone to plastoquinol at the special Q(B) binding site with Q(A)-* acting as reductant. Based on recent progress in structure analysis and using new theoretical approaches the mechanism of reaction sequence (1) is discussed with special emphasis on the excited energy transfer pathways and the sequence of charge transfer steps: [Formula: see text] where (1)(RC-PC)* denotes the excited singlet state (1)P680* of the reaction centre pigment complex. The structure of the catalytic Mn(4)O(X)Ca cluster of the WOC and the four step reaction sequence leading to oxidative water splitting are described and problems arising for the electronic configuration, in particular for the nature of redox state S(3), are discussed. The unravelling of the mode of O-O bond formation is of key relevance for understanding the mechanism of the process. This problem is not yet solved. A multistate model is proposed for S(3) and the functional role of proton shifts and hydrogen bond network(s) is emphasized. Analogously, the structure of the Q(B) site for PQ reduction to PQH(2) and the energetic and kinetics of the two step redox reaction sequence are described. Furthermore, the relevance of the protein dynamics and the role of water molecules for its flexibility are briefly outlined. We end this review by presenting future perspectives on the water oxidation process.

Ruthenium(II) Complexes of New Chelating Indolizino[2,3-b]pyrazine- and Indolizino[2,3-b]quinoxaline-Derived Ligands: Syntheses, Electrochemistry and Absorption Spectroscopy

The highly conjugated chelating ligands 5-(2-pyridyl)indolizino[2,3-b]pyrazine 1, 5-(2-pyridyl)indolizino[2,3-b]quinoxaline 2, and 8,9-dimethyl-5-(2-pyridyl)indolizino[2,3-b]quinoxaline 3 were prepared in one step, with good yields, from di-2-pyridylmethane and 2,3-dichloropyrazine, 2,3-dichloroquinoxaline, and 8,9-dimethyl-2,3-dichloroquinoxaline, respectively. Compounds 1–3 display long-wavelength absorption maxima in the green (1) and yellow (2 and 3) to give intensely coloured red and purple solutions, respectively. Bis(2,2′-bipyridyl)ruthenium(ii) and bis(4,4′-dimethyl-2,2′-bipyridyl)ruthenium(ii) complexes were prepared in moderate to good yields, characterized by NMR spectroscopy and mass spectrometry, and studied by cyclic voltammetry and absorption spectroscopy. Copper(ii) and silver(i) nitrate complexes of the ligands were prepared and complexes [Cu(NO3)2(1)], [Cu(NO3)2(2)]2, and [Ag(NO3)(3)2] were characterized by X-ray crystallography. These structures revealed the planar nature of the ligands and confirmed the proposed chelating mode.