Department of Chemistry and Institute for Sustainable Energy, Environment & Economy, University of Calgary, 2500 University Drive N.W., Calgary, Canada T2N-1N4.
J Am Chem Soc. 2010 Nov 17;132(45):16094-106. doi: 10.1021/ja106108y. Epub 2010 Oct 26.
The mechanistic details of the Ce(IV)-driven oxidation of water mediated by a series of structurally related catalysts formulated as Ru(tpy)(L)(OH(2)) [L = 2,2'-bipyridine (bpy), 1; 4,4'-dimethoxy-2,2'-bipyridine (bpy-OMe), 2; 4,4'-dicarboxy-2,2'-bipyridine (bpy-CO(2)H), 3; tpy = 2,2';6'',2''-terpyridine] is reported. Cyclic voltammetry shows that each of these complexes undergo three successive (proton-coupled) electron-transfer reactions to generate the Ru(V)(tpy)(L)O (Ru(V)=O) motif; the relative positions of each of these redox couples reflects the nature of the electron-donating or withdrawing character of the substituents on the bpy ligands. The first two (proton-coupled) electron-transfer reaction steps (k(1) and k(2)) were determined by stopped-flow spectroscopic techniques to be faster for 3 than 1 and 2. The addition of one (or more) equivalents of the terminal electron-acceptor, (NH(4))(2)[Ce(NO(3))(6)] (CAN), to the Ru(IV)(tpy)(L)O (Ru(IV)=O) forms of each of the catalysts, however, leads to divergent reaction pathways. The addition of 1 eq of CAN to the Ru(IV)=O form of 2 generates Ru(V)=O (k(3) = 3.7 M(-1) s(-1)), which, in turn, undergoes slow O-O bond formation with the substrate (k(O-O) = 3 × 10(-5) s(-1)). The minimal (or negligible) thermodynamic driving force for the reaction between the Ru(IV)=O form of 1 or 3 and 1 eq of CAN results in slow reactivity, but the rate-determining step is assigned as the liberation of dioxygen from the Ru(IV)-OO level under catalytic conditions for each complex. Complex 2, however, passes through the Ru(V)-OO level prior to the rapid loss of dioxygen. Evidence for a competing reaction pathway is provided for 3, where the Ru(V)=O and Ru(III)-OH redox levels can be generated by disproportionation of the Ru(IV)=O form of the catalyst (k(d) = 1.2 M(-1) s(-1)). An auxiliary reaction pathway involving the abstraction of an O-atom from CAN is also implicated during catalysis. The variability of reactivity for 1-3, including the position of the RDS and potential for O-atom transfer from the terminal oxidant, is confirmed to be intimately sensitive to electron density at the metal site through extensive kinetic and isotopic labeling experiments. This study outlines the need to strike a balance between the reactivity of the Ru═O unit and the accessibility of higher redox levels in pursuit of robust and reactive water oxidation catalysts.
报道了一系列结构相关的催化剂Ru(tpy)(L)(OH(2))（L=2,2'-联吡啶（bpy），1；4,4'-二甲氧基-2,2'-联吡啶（bpy-OMe），2；4,4'-二羧酸-2,2'-联吡啶（bpy-CO(2)H），3；tpy=2,2',6'',2''-三联吡啶）介导的 Ce(IV)-驱动的水氧化的机理细节。循环伏安法表明，这些配合物中的每一种都经历三个连续的（质子耦合）电子转移反应，生成Ru(V)(tpy)(L)O（Ru(V)=O）基序；每个这些氧化还原偶对的相对位置反映了 bpy 配体上取代基的供电子或吸电子性质。通过停流光谱技术确定前两个（质子耦合）电子转移反应步骤（k(1)和 k(2)）对于 3 比 1 和 2 更快。然而，向每个催化剂的Ru(IV)(tpy)(L)O（Ru(IV)=O）形式添加一个（或多个）当量的末端电子受体（(NH(4))(2)[Ce(NO(3))(6)]（CAN））导致不同的反应途径。然而，向 2 的Ru(IV)=O形式添加 1 eq 的 CAN 会生成Ru(V)=O（k(3)=3.7 M(-1) s(-1)），其继而与底物缓慢形成 O-O 键（k(O-O)=3×10(-5) s(-1)）。对于 1 或 3 与 1 eq 的 CAN 之间的反应，Ru(IV)=O形式的最小（或可忽略不计）热力学驱动力导致缓慢的反应性，但反应速率决定步骤被指定为在每个配合物的催化条件下从Ru(IV)-OO水平释放氧气。然而，2 会先经过Ru(V)-OO水平，然后才快速失去氧气。为 3 提供了竞争反应途径的证据，其中Ru(V)=O和Ru(III)-OH氧化还原水平可以通过催化剂的Ru(IV)=O形式的歧化来生成（k(d)=1.2 M(-1) s(-1)）。在催化过程中，还涉及到从 CAN 中提取 O-原子的辅助反应途径。通过广泛的动力学和同位素标记实验证实，1-3 的反应性的可变性，包括反应的决定步骤和从末端氧化剂转移 O-原子的能力，与金属位点的电子密度密切相关。这项研究表明，需要在Ru═O单元的反应性和更高氧化态的可及性之间取得平衡，以追求强大和反应性的水氧化催化剂。
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