Nurdin Lucie, Spasyuk Denis M, Fairburn Laura, Piers Warren E, Maron Laurent
Department of Chemistry , University of Calgary , 2500 University Drive NW , Calgary , Alberta T2N 1N4 , Canada.
LPCNO, Université de Toulouse, INSA, UPS, LPCNO , 135 avenue de Rangueil , F-31077 Toulouse , France , and CNRS, LPCNO, F-31077 Toulouse, France.
J Am Chem Soc. 2018 Nov 28;140(47):16094-16105. doi: 10.1021/jacs.8b07726. Epub 2018 Nov 15.
In reactions of significance to alternative energy schemes, metal catalysts are needed to overcome kinetically and thermodynamically difficult processes. Often, high-oxidation-state, high-energy metal oxo intermediates are proposed as mediators in elementary steps involving O-O bond cleavage and formation, but the mechanisms of these steps are difficult to study because of the fleeting nature of these species. Here we utilized a novel dianionic pentadentate ligand system that enabled a detailed mechanistic investigation of the protonation of a cobalt(III)-cobalt(III) peroxo dimer, a known intermediate in oxygen reduction catalysis to hydrogen peroxide. It was shown that double protonation occurs rapidly and leads to a low-energy O-O bond cleavage step that generates a Co(III) aquo complex and a highly reactive Co(IV) oxyl cation. The latter was probed computationally and experimentally implicated through chemical interception and isotope labeling experiments. In the absence of competing chemical reagents, it dimerizes and eliminates dioxygen in a step highly relevant to O-O bond formation in the oxygen evolution step in water oxidation. Thus, the study demonstrates both facile O-O bond cleavage and formation in the stoichiometric reduction of O to HO with 2 equiv of Co(II) and suggests a new pathway for selective reduction of O to water via Co(III)-O-O-Co(III) peroxo intermediates.
在对替代能源方案具有重要意义的反应中，需要金属催化剂来克服动力学和热力学上困难的过程。通常，高氧化态、高能量的金属氧代中间体被认为是涉及O - O键断裂和形成的基元步骤中的介质，但由于这些物种的短暂性质，这些步骤的机制很难研究。在这里，我们使用了一种新型的双阴离子五齿配体系统，该系统能够对钴（III）-钴（III）过氧二聚体的质子化进行详细的机理研究，钴（III）-钴（III）过氧二聚体是氧还原催化生成过氧化氢过程中的一种已知中间体。结果表明，双质子化迅速发生，并导致一个低能量的O - O键断裂步骤，生成一个钴（III）水合络合物和一个高活性的钴（IV）氧基阳离子。通过计算对后者进行了探测，并通过化学截获和同位素标记实验在实验上证明了其存在。在没有竞争性化学试剂的情况下，它会二聚并消除氧气，这一步骤与水氧化中析氧步骤中的O - O键形成高度相关。因此，该研究证明了在使用2当量的Co（II）将O化学计量还原为HO的过程中，O - O键的断裂和形成都很容易，并提出了一条通过Co（III）-O - O-Co（III）过氧中间体将O选择性还原为水的新途径。
