Photo- and Electrocatalytic Hydrogen Production Using Valence Isomers of N₂S₂-Type Nickel Complexes

Three Schiff-base-type nickel(II) complexes (1a-3a) and the corresponding noninnocent-type complexes (1b-3b) were synthesized, and the equilibria between these valence isomers were observed in tetrahydrofuran (THF) at room temperature. The electronic state of the noninnocent-type nickel complex was also confirmed by isolation of the one-electron-reduced species. The catalytic ability for the photogeneration of hydrogen from water was examined about 1a-3a and 1b-3b in the presence of a photosensitizer and a sacrificial electron donor. Then, a Schiff-base-type complex with chlorine atoms (2a) and a noninnocent-type complex with methyl groups (3b) on the pendant phenyl rings being present as the minor species in THF exhibited high activity of over 400 turnover numbers. The dynamic light scattering and transmission electron microscopy measurements suggested the formation of NiS_x-like aggregate species under photocatalytic conditions. The electrocatalytic activities of the nickel complexes for hydrogen production were also investigated, and a plausible reaction mechanism was proposed on the basis of a combined electrochemical and density functional theory study.