Two-Electron HCl to H₂ Photocycle Promoted by Ni(II) Polypyridyl Halide Complexes

Photochemical HX splitting requires the management of two protons and the execution of multielectron photoreactions. Herein, we report a photoinduced two-electron reduction of a polypyridyl Ni­(II) chloride complex that provides a route to H2 evolution from HCl. The excited states of Ni complexes are too short to participate directly in HX activation, and hence, the excited state of a photoredox mediator is exploited for the activation of HX at the Ni­(II) center. Nanosecond transient absorption (TA) spectroscopy has revealed that the excited state of the polypyridine results in a photoreduced radical that is capable of mediating HX activation by producing a Ni­(I) center by halogen-atom abstraction. Disproportionation of the photogenerated Ni­(I) intermediate affords Ni­(II) and Ni(0) complexes. The Ni(0) center is capable of reacting with HX to produce H2 and the polypyridyl Ni­(II) dichloride, closing the photocycle for H2 generation from HCl.