Surface palladium nanoparticles in ionic liquids modified with phosphorus ligands for enhanced catalytic semi-hydrogenation

Thermodynamic stability of nanoparticles necessitates the use of stabilizing agents to provide steric and electronic protection. However, their activity and selectivity remain suboptimal under moderate reaction conditions. In this study, we present high-performance Pd nanoparticles featuring a distinctive Pd-phosphine surface, akin to a "quasi nano-frustrated Lewis pair" architecture. In this system, electron donation from ionophilic phosphine species enhances the electron density of the Pd nanoparticles, contributing to their improved performance. Solid-state NMR and XPS analyses disclose the strong coordination of phosphine species on the Pd NPs. DFT calculations reveal the geometry and conformations of the coordinated phosphine, showing that one of the phenyl rings aligns nearly parallel to the nanoparticle facets. This interaction occurs through the six carbon atoms of the π system. We investigate the structure-activity relationships (SARs) exhibited by these NPs in the efficient semi-hydrogenation of phenylacetylene (TOF = 3.85 s^-1), 2-cyclohexen-1-one (TOF = 0.8 s^-1), and 1,3-cyclohexadiene (TOF = 12.82 s^-1) at 40 °C and 2-4 bar of H₂ in BMIm·NTF₂ ionic liquid. The enhanced activity and selectivity are attributed to (i) the formation of ionic liquid cages/membranes around the NPs akin to catalytically active membranes that tune the diffusion affinity of reactants, reactive intermediates, and products for catalytically active sites and (ii) the steric hindrance provided by the Pd-P bonds.