Bidentate Versus Monodentate Phosphines on Nickel Phosphide Nanoparticles for the Semi‐Hydrogenation of Phenylacetylene at 0 °C

Selective alkyne hydrogenation under ultra‐mild conditions is a growing objective for sustainable fine‐chemical synthesis and industrial processes. However, achieving significant activity at very low temperatures remains challenging, especially for non‐noble metal catalysts whose surfaces are not reactive without additional promoters. Moreover, very mild reaction conditions provide an opportunity to gain a better understanding of the specific role of ligand denticity in the reaction. This study investigates how phosphine ligand structure—specifically monodentate versus bidentate coordination—affects the low‐temperature hydrogenation of phenylacetylene catalyzed by nickel phosphide nanoparticles. While as‐prepared nanoparticles show no activity at 0 °C, adding carefully selected phosphine ligands enables partial conversion to styrene with complete selectivity. Sixteen phosphines were screened to assess how steric and electronic factors tune surface reactivity. Among monodentate ligands, only tri‐ n ‐butylphosphine (P ⁿ Bu ₃ ) and tricyclohexylphosphine (PCy ₃ ) significantly promoted hydrogenation, with P ⁿ Bu ₃ giving the highest conversion (22%). For bidentate ligands, 1,4‐bis(diphenylphosphino)butane ( dppb ) exhibited the best performance (14%), while others with shorter, longer, or more rigid linkers showed a lower activity. A stereoelectronic map correlating Tolman cone angles and Tolman electronic parameters revealed that ligands with intermediate steric hindrance and electron‐donating ability occupy an optimal region for reactivity. Stoichiometry studies showed that both P ⁿ Bu ₃ and dppb reach a conversion plateau, though P ⁿ Bu ₃ was required in higher amount to activate the catalyst surface, consistent with its non‐chelating nature. The results suggest that ligand–surface interactions (chelation strength, residence time, and hemilability) govern the activation of H ₂ and phenylacetylene at 0 °C. Overall, the work establishes structure–performance relationships that guide the design of phosphine‐modified nanoparticle catalysts for ultra‐mild alkyne hydrogenation, with potential application for enantioselective catalysis.