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 D2O 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. Visible‐light‐driven semihydrogenation of alkynes with water as the hydrogen source was achieved over single‐atom Ni supported on carbon nitride (CN). The Ni site, paired up with the pyridinic N of the CN scaffold, mediated proton/deuterium transport from water/deuterium oxide (H2O/D2O) to alkynes, producing a diverse range of (deuterated) alkenes with high selectivity.