Muckerman James T, Polyansky Dmitry E, Wada Tohru, Tanaka Koji, Fujita Etsuko
Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973-5000, USA.
Inorg Chem. 2008 Mar 17;47(6):1787-802. doi: 10.1021/ic701892v.
Tanaka and co-workers reported a novel dinuclear Ru complex, Ru2(OH)2(3,6-Bu2Q)2(btpyan)2 (3,6-Bu2Q = 3,6-di tert-butyl-1,2-benzoquinone, btpyan = 1,8-bis(2,2':6',2''-terpyrid-4'-yl)anthracene), that contains redox active quinone ligands and has an excellent electrocatalytic activity for water oxidation when immobilized on an indium-tin-oxide electrode (Inorg. Chem., 2001, 40, 329-337). The novel features of the dinuclear and related mononuclear Ru species with quinone ligands, and comparison of their properties to those of the Ru analogues with the bpy ligand (bpy = 2,2'-bipyridine) replacing quinone, are summarized here together with new theoretical and experimental results that show striking features for both the dinuclear and mononuclear species. The identity and oxidation state of key mononuclear species, including the previously reported oxyl radical, have been reassigned. Our gas-phase theoretical calculations indicate that the Tanaka Ru-dinuclear catalyst seems to maintain predominantly Ru(II) centers while the quinone ligands and water moiety are involved in redox reactions throughout the entire catalytic cycle for water oxidation. Our theoretical study identifies Ru2(O2(-))(Q(-1.5))2(btpyan) as a key intermediate and the most reduced catalyst species that is formed by removal of all four protons before four-electron oxidation takes place. While our study toward understanding the complicated electronic and geometric structures of possible intermediates in the catalytic cycle is still in progress, the current status and new directions for kinetic and mechanistic investigations, and key issues and challenges in water oxidation with the Tanaka catalyst (and its analogues with Cl(-) or NO(2-)substituted quinones and a species with a xanthene bridge instead an antheracene) are discussed.
田中及其同事报道了一种新型双核钌配合物，即Ru2(OH)2(3,6-Bu2Q)2(btpyan)2（3,6-Bu2Q = 3,6-二叔丁基-1,2-苯醌，btpyan = 1,8-双(2,2':6',2''-三联吡啶-4'-基)蒽），该配合物含有氧化还原活性醌配体，固定在氧化铟锡电极上时对水氧化具有优异的电催化活性（《无机化学》，2001年，40卷，329 - 337页）。本文总结了具有醌配体的双核及相关单核钌物种的新特性，并将它们的性质与用联吡啶配体（bpy = 2,2'-联吡啶）取代醌的钌类似物的性质进行了比较，同时还给出了新的理论和实验结果，这些结果显示了双核和单核物种的显著特征。包括先前报道的氧基自由基在内的关键单核物种的身份和氧化态已被重新确定。我们的气相理论计算表明，田中双核钌催化剂似乎主要保持Ru(II)中心，而醌配体和水部分在整个水氧化催化循环中参与氧化还原反应。我们的理论研究确定Ru2(O2(-))(Q(-1.5))2(btpyan)为关键中间体，也是在四电子氧化发生之前通过去除所有四个质子形成的最还原的催化剂物种。虽然我们对理解催化循环中可能中间体的复杂电子和几何结构的研究仍在进行中，但本文讨论了动力学和机理研究的现状及新方向，以及田中催化剂（及其具有Cl(-)或NO(2-)取代醌的类似物和具有氧杂蒽桥而非蒽桥的物种）水氧化中的关键问题和挑战。
