Department of Chemistry, University of Utah , Salt Lake City, Utah 84112-0850, United States.
J Am Chem Soc. 2017 Jun 28;139(25):8586-8600. doi: 10.1021/jacs.7b03278. Epub 2017 Jun 15.
Deployment of solar fuels derived from water requires robust oxygen-evolving catalysts made from earth abundant materials. Copper has recently received much attention in this regard. Mechanistic parallels between Cu and single-site Ru/Ir/Mn water oxidation catalysts, including intermediacy of terminal Cu oxo/oxyl species, are prevalent in the literature; however, intermediacy of late transition metal oxo species would be remarkable given the high d-electron count would fill antibonding orbitals, making these species high in energy. This may suggest alternate pathways are at work in copper-based water oxidation. This report characterizes a dinuclear copper water oxidation catalyst, {[(L)Cu(II)]-(μ-OH)}(OTf) (L = MeTMPA = bis((6-methyl-2-pyridyl)methyl)(2-pyridylmethyl)amine) in which water oxidation proceeds with high Faradaic efficiency (>90%) and moderate rates (33 s at ∼1 V overpotential, pH 12.5). A large kinetic isotope effect (k/k = 20) suggests proton coupled electron transfer in the initial oxidation as the rate-determining step. This species partially dissociates in aqueous solution at pH 12.5 to generate a mononuclear {[(L)Cu(II)(OH)]} adduct (K = 0.0041). Calculations that reproduce the experimental findings reveal that oxidation of either the mononuclear or dinuclear species results in a common dinuclear intermediate, {[LCu(III)]-(μ-O)}, which avoids formation of terminal Cu(IV)═O/Cu(III)-O intermediates. Calculations further reveal that both intermolecular water nucleophilic attack and redox isomerization of {[LCu(III)]-(μ-O)} are energetically accessible pathways for O-O bond formation. The consequences of these findings are discussed in relation to differences in water oxidation pathways between Cu catalysts and catalysts based on Ru, Ir, and Mn.
从水中获得的太阳能燃料的应用需要由丰富的地球材料制成的强大的氧析出催化剂。在这方面，铜最近受到了广泛关注。在文献中，Cu 和单原子 Ru/Ir/Mn 水氧化催化剂之间存在着相似的机制，包括末端 Cu 氧/氧物种的中间产物；然而，鉴于高 d 电子数会填充反键轨道，使这些物种能量很高，晚期过渡金属氧物种的中间产物将是显著的。这可能表明在基于铜的水氧化中存在替代途径。本报告描述了一种双核铜水氧化催化剂，{[(L)Cu(II)]-(μ-OH)}(OTf)（L = MeTMPA = 双((6-甲基-2-吡啶基)甲基)(2-吡啶基甲基)胺），其中水氧化以高法拉第效率（>90%）和中等速率（在 pH 值为 12.5 时在约 1 V 过电势下 33 s）进行。大的动力学同位素效应（k/k = 20）表明质子耦合电子转移是初始氧化的速控步骤。该物种在 pH 值为 12.5 的水溶液中部分解离，生成单核 {[(L)Cu(II)(OH)]} 加合物（K = 0.0041）。重现实验结果的计算表明，无论是单核还是双核物种的氧化都会导致一个共同的双核中间体，{[LCu(III)]-(μ-O)}，这避免了末端 Cu(IV)═O/Cu(III)-O 中间体的形成。计算还进一步表明，{[LCu(III)]-(μ-O)}的分子间水亲核攻击和氧化还原异构化都是 O-O 键形成的能量可及途径。这些发现的结果与 Cu 催化剂和基于 Ru、Ir 和 Mn 的催化剂之间的水氧化途径的差异有关。
