The H-Pd···N-H metal-ligand dual-atom sites synergistically catalyzed alkyne semi-hydrogenation with complete Z-selectivity.

Abstract

In single-atom catalysts, the coordination microenvironment surrounding transition-metal centers is widely recognized as a key factor shaping their electronic structures and geometries, thereby governing catalytic behaviors. However, the synergetic catalysis between metal center and ligating atoms remains underexplored in single-atom catalysis. Herein, we construct the Pd(II)-N₄ sites on carbon nitride to achieve photocatalytic semi-hydrogenation of alkynes. Under light irradiation, the Pd-N sites could transform to H-Pd···N-H moieties. The Pd-H center enhances alkyne insertion while the adjacent N-H group facilitates intramolecular proton transfer. This metal-ligand cooperativity of H-Pd···N-H sites, together with the steric-hindrance imposed by carbon nitride scaffold, lowers the energy barrier for Z-alkene formation and ensures exclusive stereoselectivity. Simultaneously, the H-Pd···N-H sites exhibit stronger adsorption toward alkynes than alkenes, effectively suppressing over-hydrogenation to alkanes. Across all evaluated substrates, Z-alkenes are the only isomers detected, excluding E-isomer and alkane. Notably, in a 5:95 mixture of diphenylacetylene and Z-stilbene, this method selectively converts only the alkyne to the Z-isomer without over-hydrogenation, highlighting its potential for product purification.