Single‐Atom Nickel on Carbon Nitride Photocatalyst Achieves Semihydrogenation of Alkynes with Water Protons via Monovalent Nickel

Prospects in light-driven water activation have prompted rapid progress in hydrogenation reactions. We describe a Ni2+ -N4 site built on carbon nitride for catalyzed semihydrogenation of alkynes, with water supplying protons, powered by visible-light irradiation. Importantly, the photocatalytic approach developed here enabled access to diverse deuterated alkenes in D2 O with excellent deuterium incorporation. Under visible-light irradiation, evolution of a four-coordinate Ni2+ species into a three-coordinate Ni+ species was spectroscopically identified. In combination with theoretical calculations, the photo-evolved Ni+ is posited as HO-Ni+ -N2 with an uncoordinated, protonated pyridinic nitrogen, formed by coupled Ni2+ reduction and water dissociation. The paired Ni-N prompts hydrogen liberation from water, and it renders desorption of alkene preferred over further hydrogenation to alkane, ensuring excellent semihydrogenation selectivity.