DataSheet2_Mononuclear manganese complexes as hydrogen evolving catalysts.pdf

Molecular hydrogen (H2) is one of the pillars of future non-fossil energy supply. In the quest for alternative, non-precious metal catalysts for hydrogen generation to replace platinum, biological systems such as the enzyme hydrogenase serve as a blueprint. By taking inspiration from the bio-system, mostly nickel- or iron-based catalysts were explored so far. Manganese is a known oxygen-reducing catalyst but has received much less attention for its ability to reduce protons in acidic media. Here, the synthesis, characterization, and reaction mechanisms of a series of four mono-nuclear Mn(I) complexes in terms of their catalytic performance are reported. The effect of the variation of equatorial and axial ligands in their first and second coordination spheres was assessed pertaining to their control of the turnover frequencies and overpotentials. All four complexes show reactivity and reduce protons in acidic media to release molecular hydrogen H2. Quantum chemical studies were able to assign and interpret spectral characterizations from UV–Vis and electrochemistry and rationalize the reaction mechanism. Two feasible reaction mechanisms of electrochemical (E) and protonation (C) steps were compared. Quantum chemical studies can assign peaks in the cyclic voltammetry to structural changes of the complex during the reaction. The first one-electron reduction is essential to generate an open ligand-based site for protonation. The distorted octahedral Mn complexes possess an inverted second one-electron redox potential which is a pre-requisite for a swift and facile release of molecular hydrogen. This series on manganese catalysts extends the range of elements of the periodic table which are able to catalyze the hydrogen evolution reaction and will be explored further.

分子氢（H₂）是未来非化石能源供给的重要支柱之一。在研发替代铂的非贵金属产氢催化剂的过程中，氢化酶（hydrogenase）这类生物系统可作为设计蓝本。受该生物系统启发，目前已探索的产氢催化剂多为镍基或铁基体系。锰（Manganese）是已知的氧还原催化剂，但其在酸性介质中还原质子的能力却鲜受关注。本文报道了四种单核Mn(I)配合物（mono-nuclear Mn(I) complexes）的合成、表征方法，以及与其催化性能相关的反应机制研究。本研究评估了该系列配合物第一、第二配位层中赤道配体与轴向配体的变化对周转频率与过电位的调控作用。四种配合物均表现出催化活性，可在酸性介质中还原质子并释放分子氢（H₂）。量子化学研究可对紫外-可见（UV–Vis）光谱与电化学表征结果进行归属与解析，并阐明反应机制。本文对比了两种包含电化学（E）与质子化（C）步骤的可行反应路径。量子化学分析可将循环伏安法（cyclic voltammetry）中的特征峰归因为反应过程中配合物的结构变化。第一步单电子还原是为质子化形成开放配体位点的关键步骤。扭曲八面体构型的锰配合物具有反转的第二单电子氧化还原电位，这是实现分子氢快速、温和释放的先决条件。该锰基催化剂系列拓展了可催化析氢反应（hydrogen evolution reaction, HER）的元素周期表元素范围，后续将对其开展更深入的研究。