Stanford PULSE Institute, SLAC National Accelerator Laboratory, Stanford University, Menlo Park, California 94025, United States.
Theory Institute for Materials and Energy Spectroscopies, SLAC National Accelerator Laboratory, Stanford University, Menlo Park, California 94025, United States.
Inorg Chem. 2022 Jul 4;61(26):9868-9876. doi: 10.1021/acs.inorgchem.2c00167. Epub 2022 Jun 22.
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) (bpy = 2,2'-bipyridine; pyS = pyridine-2-thiolate) identifies the structure of the fully protonated catalyst as a solvated [Ni(bpy)(DMF)] (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.
使用 X 射线吸收光谱（XAS）研究了几种 Ni 中心吡啶-2-硫醇光催化剂的质子化。虽然先前认为吡啶硫醇配体的质子化会导致吡啶 N 位的部分脱螯合，但我们观察到质子化配体完全解离，并被溶剂分子取代。催化剂 Ni(bpy)(pyS)（bpy = 2,2'-联吡啶；pyS = 吡啶-2-硫醇）的 Ni K 边和 S K 边 XAS 的组合确定了完全质子化催化剂的结构为溶剂化的[Ni(bpy)(DMF)]（DMF = 二甲基甲酰胺）配合物和解离的配体作为 N-质子化的 2-硫代吡啶酮（pyS-H）。这一惊人的结果得到了紫外可见吸收光谱和 DFT 计算的进一步支持，并通过 XAS 和紫外可见光谱的组合，在其他催化剂结构和溶剂环境中得到了证明。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作为主要的电子接受体，导致游离的配体和还原的 Ni 物种。在析氢反应（HER）机制的更大背景下，考虑了配体解离的作用。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，因此突出了配体配位的关键作用。这表明通过添加过量的质子化配体（2-硫代吡啶酮）使平衡向结合物种移动，可能会提高含吡啶硫醇催化剂的性能。
