Electron spin plays a critical role in chemical processes, particularly in reactions involving metal complexes with unpaired electrons. However, more definitive state‐to‐state experiments are needed to better elucidate the role of electronic spin. Herein, we chose nickel (II) 5,10,15,20‐tetrakis(pentafluorophenyl) porphyrin 1 as a catalyst, which allows switching from a low spin to a high spin state of Ni(II) center through an axial pyridine coordination, for electrocatalytic hydrogen evolution reaction (HER). When pyridine is present, we observed β‐hydrogenation of porphyrin through electron transfer followed by proton transfer. In contrast, hydrogen evolution mainly occurs via the concerted proton‐coupling electron transfer without pyridine coordination. Similar distinct spin‐dependent selectivity was also observed in chemical reduction of 1 by CoCp2 with subsequent addition of pyridinium p‐toluenesulfonate. Computational calculations using density functional theory demonstrated that the transition from low spin to high spin state enriches the ligand’s electron density after one‐electron reduction, leading to preferential protonation of β‐periphery rather than meso‐position or metal center.

中文翻译：

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配位诱导的自旋态转变开关镍 （II） 卟啉在析氢反应中的反应性


电子自旋在化学过程中起着关键作用，特别是在涉及具有不成对电子的金属络合物的反应中。然而，需要更明确的状态间实验来更好地阐明电子自旋的作用。在此，我们选择镍 （II） 5,10,15,20-四（五氟苯基）卟啉 1 作为催化剂，它允许通过轴向吡啶配位从 Ni（II） 中心的低自旋状态切换到高自旋状态，用于电催化析氢反应 （HER）。当存在吡啶时，我们观察到卟啉通过电子转移和质子转移进行β氢化。相比之下，析氢主要通过没有吡啶配位的协同质子耦合电子转移发生。在 CoCp2 化学还原 1 并随后添加对甲苯磺酸吡啶时，也观察到类似的不同自旋依赖性选择性。使用密度泛函理论的计算计算表明，在单电子还原后，从低自旋到高自旋态的转变丰富了配体的电子密度，导致β外围优先质子化，而不是介观位置或金属中心。