Mechanistic Insights into Homogeneous Electrocatalytic and Photocatalytic Hydrogen Evolution Catalyzed by High-Spin Ni(II) Complexes with S₂N₂‑Type Tetradentate Ligands

We report homogeneous electrocatalytic and photocatalytic H₂ evolution using two Ni­(II) complexes with S₂N₂-type tetradentate ligands bearing two different sizes of chelate rings as catalysts. A Ni­(II) complex with a five-membered SC₂S–Ni chelate ring (1) exhibited higher activity than that with a six-membered SC₃S–Ni chelate ring (2) in both electrocatalytic and photocatalytic H₂ evolution despite both complexes showing the same reduction potentials. A stepwise reduction of the Ni center from Ni­(II) to Ni(0) was observed in the electrochemical measurements; the first reduction is a pure electron transfer reaction to form a Ni­(I) complex as confirmed by electron spin resonance measurements, and the second is a 1e^–/1H⁺ proton-coupled electron transfer reaction to afford a putative Ni­(II)-hydrido (Ni^II–H) species. We also clarified that Ni­(II) complexes can act as homogeneous catalysts in the electrocatalytic H₂ evolution, in which complex 1 exhibited higher reactivity than that of 2. In the photocatalytic system using [Ru­(bpy)₃]²⁺ as a photosensitizer and sodium ascorbate as a reductant, complex 1 with the five-membered chelate ring also showed higher catalytic activity than that of 2 with the six-membered chelate ring, although the rates of photoinduced electron-transfer processes were comparable. The Ni–H bond cleavage in the putative Ni^II–H intermediate should be involved in the rate-limiting step as evidenced by kinetic isotope effects observed in both photocatalytic and electrocatalytic H₂ evolution. Kinetic analysis and density functional theory calculations indicated that the difference in H₂ evolution activity between the two complexes was derived from that of activation barriers of the reactions between the Ni^II–H intermediates and proton, which is consistent with the fact that increase of proton concentration accelerates the H₂ evolution.