Dissociation of Pyridinethiolate Ligands during Hydrogen Evolution Reactions of Ni-Based Catalysts: Evidence from X-ray Absorption Spectroscopy

Abstract

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.

Core-Excited States and X-ray Absorption Spectra from Multireference Algebraic Diagrammatic Construction Theory

“…While this assumption is valid for many closed-shell molecules at (or near) equilibrium geometries, XAS is increasingly being used to probe core-level excited states of chemical systems with more challenging electronic structures, such as radicals, − transition-metal complexes, − and along the dissociative reaction pathways − where multireference effects can be significant. However, in contrast to single-reference theories, the range of available multireference methods is much narrower, including implementations of multiconfigurational self-consistent field, − multireference perturbation theories, − configuration interaction, − coupled cluster, ,− and driven similarity renormalization group approaches. − …”

Mononuclear Iron Pyridinethiolate Complex Promoted CO2 Photoreduction via Rapid Intramolecular Electron Transfer

“…The transfer of electrons from the pyS ligand to the bpy ligand via the Fe metal center. Meanwhile, the pyS ligand is completely dissociated, [22] and the generated pyS * radicals are further coupled to afford 2,2'-dithiodipyridine (pyS-Spy), which is verified by the control experiments, i. e., replacing the pySH ligand in the system in an equal proportion to the pyS-Spy can obtain a similar photocatalytic performance (Figure S20).…”

Light-induced hydrogen production from water using nickel(ii) catalysts and N-doped carbon-dot photosensitizers: catalytic efficiency enhancement by increase of catalyst nuclearity

“…80 Noticeably, according to a recent report, full dissociation of one protonated thioamidato ligand may also take place. 81 The preferential protonation of the pyrimidine N-site, and not the S-site, is also supported by the fact that the low valent Ni( i ) species formed after the first reduction would be better stabilized by coordination of the soft S atom of the thioamidato L N S − ligand. 82 The hemilabile (or labile) character of the ligands and the presence of proton binding sites on them facilitate the formation of a Ni–H intermediate and the subsequent H 2 formation.…”