†Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973-5000, United States.
‡Department of Chemistry, University of Memphis, Memphis, Tennessee 38152, United States.
Inorg Chem. 2015 May 4;54(9):4310-21. doi: 10.1021/ic5031137. Epub 2015 Apr 22.
The ability of cobalt-based transition metal complexes to catalyze electrochemical proton reduction to produce molecular hydrogen has resulted in a large number of mechanistic studies involving various cobalt complexes. While the basic mechanism of proton reduction promoted by cobalt species is well-understood, the reactivity of certain reaction intermediates, such as Co(I) and Co(III)-H, is still relatively unknown owing to their transient nature, especially in aqueous media. In this work we investigate the properties of intermediates produced during catalytic proton reduction in aqueous solutions promoted by the (DPA-Bpy)Co(OH2) (DPA-Bpy = N,N-bis(2-pyridinylmethyl)-2,20-bipyridine-6-methanamine) complex (Co(L)(OH2) where L is the pentadentate DPA-Bpy ligand or Co(OH2) as a shorthand). Experimental results based on transient pulse radiolysis and laser flash photolysis methods, together with electrochemical studies and supported by density functional theory (DFT) calculations indicate that, while the water ligand is strongly coordinated to the metal center in the oxidation state 3+, one-electron reduction of the complex to form a Co(II) species results in weakening the Co-O bond. The further reduction to a Co(I) species leads to the loss of the aqua ligand and the formation of Co(I)-VS) (VS = vacant site). Interestingly, DFT calculations also predict the existence of a Co(I)(κ(4)-L)(OH2) species at least transiently, and its formation is consistent with the experimental Pourbaix diagram. Both electrochemical and kinetics results indicate that the Co(I) species must undergo some structural change prior to accepting the proton, and this transformation represents the rate-determining step (RDS) in the overall formation of Co(III)-H. We propose that this RDS may originate from the slow removal of a solvent ligand in the intermediate Co(I)(κ(4)-L)(OH2) in addition to the significant structural reorganization of the metal complex and surrounding solvent resulting in a high free energy of activation.
钴基过渡金属配合物能够催化电化学质子还原生成氢气，这导致了大量涉及各种钴配合物的机理研究。虽然钴物种促进质子还原的基本机理已经很清楚，但某些反应中间体的反应性，如 Co(I) 和 Co(III)-H，由于其瞬态性质，特别是在水介质中，仍然相对未知。在这项工作中，我们研究了在水溶液中由(DPA-Bpy)Co(OH2)（DPA-Bpy = N,N-双(2-吡啶基甲基)-2,20-联吡啶-6-甲胺）配合物（Co(L)(OH2)，其中 L 是五齿 DPA-Bpy 配体或Co(OH2)作为简写）促进的催化质子还原过程中产生的中间体的性质。基于瞬态脉冲辐射和激光闪光光解方法的实验结果，以及电化学研究，并得到密度泛函理论（DFT）计算的支持，表明尽管水配体在氧化态 3+中与金属中心强烈配位，但配合物的单电子还原形成 Co(II)物种会削弱 Co-O 键。进一步还原为 Co(I)物种会导致水配体的丢失和Co(I)-VS)（VS = 空位）的形成。有趣的是，DFT 计算还预测至少在瞬态存在Co(I)(κ(4)-L)(OH2)物种，其形成与实验 Pourbaix 图一致。电化学和动力学结果均表明，Co(I)物种在接受质子之前必须经历某种结构变化，这种转变代表了整体Co(III)-H形成的速率决定步骤（RDS）。我们提出，除了金属配合物和周围溶剂的显著结构重排导致高活化能外，这种 RDS 可能源自中间态Co(I)(κ(4)-L)(OH2)中溶剂配体的缓慢去除。
