The protonation of several Ni-centered pyridine-2-thiolate photocatalysts for hydrogen evolution is investigated using X-ray absorption spectroscopy (XAS). While protonation of the pyridinethiolate ligand was previously thought to result in partial dechelation from the metal at the pyridyl N site, we instead observe complete dissociation of the protonated ligand and replacement by solvent molecules. A combination of Ni K-edge and S K-edge XAS of the catalyst Ni(bpy)(pyS)₂ (bpy = 2,2′-bipyridine; pyS = pyridine-2-thiolate) identifies the structure of the fully protonated catalyst as a solvated [Ni(bpy)(DMF)₄]²⁺ (DMF = dimethylformamide) complex and the dissociated ligands as the N-protonated 2-thiopyridone (pyS-H). This surprising result is further supported by UV–vis absorption spectroscopy and DFT calculations and is demonstrated for additional catalyst structures and solvent environments using a combination of XAS and UV–vis spectroscopy. Following protonation, electrochemical measurements indicate that the solvated Ni bipyridine complex acts as the primary electron-accepting species during photocatalysis, resulting in separate protonated ligand and reduced Ni species. The role of ligand dissociation is considered in the larger context of the hydrogen evolution reaction (HER) mechanism. As neither the pyS-H ligand nor the Ni bipyridine complex acts as an efficient HER catalyst alone, the critical role of ligand coordination is highlighted. This suggests that shifting the equilibrium toward bound species by addition of excess protonated ligand (2-thiopyridone) may improve the performance of pyridinethiolate-containing catalysts.

中文翻译：

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Ni基催化剂析氢反应中吡啶硫醇配体的解离：来自X射线吸收光谱的证据

使用 X 射线吸收光谱 (XAS) 研究了几种以 Ni 为中心的 pyridine-2-thiolate 光催化剂用于析氢的质子化。虽然之前认为吡啶硫醇盐配体的质子化会导致吡啶基 N 位点的金属部分脱螯，但我们观察到质子化配体的完全解离和溶剂分子的置换。催化剂 Ni(bpy)(pyS) ₂ (bpy = 2,2'-bipyridine;pyS = pyridine-2-thiolate) 的 Ni K-edge 和 S K-edge XAS 的组合确定了完全质子化催化剂的结构作为溶剂化 [Ni(bpy)(DMF) ₄ ] ²⁺(DMF = 二甲基甲酰胺) 复合物和解离的配体作为 N-质子化 2-硫代吡啶酮 (pyS-H)。这一令人惊讶的结果得到了紫外-可见吸收光谱和 DFT 计算的进一步支持，并且使用 XAS 和紫外-可见光谱的组合证明了额外的催化剂结构和溶剂环境。质子化后，电化学测量表明溶剂化的 Ni 联吡啶配合物在光催化过程中充当主要的电子接受物质，从而产生单独的质子化配体和还原的 Ni 物质。在析氢反应 (HER) 机制的更大范围内考虑配体解离的作用。由于 pyS-H 配体和 Ni 联吡啶配合物都不能单独作为有效的 HER 催化剂，因此强调了配体配位的关键作用。