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

We report homogeneous electrocatalytic and photocatalytic H evolution using two Ni(II) complexes with SN-type tetradentate ligands bearing two different sizes of chelate rings as catalysts. A Ni(II) complex with a five-membered SCS-Ni chelate ring (1) exhibited higher activity than that with a six-membered SCS-Ni chelate ring (2) in both electrocatalytic and photocatalytic H 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-H) species. We also clarified that Ni(II) complexes can act as homogeneous catalysts in the electrocatalytic H evolution, in which complex 1 exhibited higher reactivity than that of 2. In the photocatalytic system using [Ru(bpy)] 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-H intermediate should be involved in the rate-limiting step as evidenced by kinetic isotope effects observed in both photocatalytic and electrocatalytic H evolution. Kinetic analysis and density functional theory calculations indicated that the difference in H evolution activity between the two complexes was derived from that of activation barriers of the reactions between the Ni-H intermediates and proton, which is consistent with the fact that increase of proton concentration accelerates the H evolution.