Preferential Reduction of 2-Phenylpyridine under D ₂ : Palladium Nanoparticles Stabilized by N -Heterocyclic Carbenes Prefer D ₂ to H ₂

Late-stage hydrogen isotope exchange and dearomatization reactions under D₂ have recently been used for preparing labeled compounds using metal nanoparticle catalysts. In the case of dearomatization, lower rates are usually observed under D₂ compared to H₂ as a result of a kinetic isotope effect. Here, we report the synthesis of NHC-stabilized PdNPs, their characterization, and their use for the reduction of 2-phenylpyridine to 2-phenylpiperidine or 2-phenylpiperidine-d₆, which reveals an unexpected preference for D₂ over H₂. This effect is observed using NPs stabilized by polyvinylpyrrolidone (Pd@PVP) but is more pronounced in the presence of NHC ligands (Pd@NHC). DFT calculations reveal that the phenomenon arises from a higher surface concentration of deuterides relative to hydrides in both systems, which is enhanced by the electronic influence of NHC ligands. This study provides the first report and mechanistic insight into an unusual isotopic effect in NP-catalyzed deuteration, highlighting the pivotal role of ligand-NP interactions.