N-Heterocyclic Carbene Coordination to Surface Copper Sites in Selective Semihydrogenation Catalysts from Solid-State NMR Spectroscopy.

Supported metal nanoparticles correspond to a very large class of heterogeneous catalysts. While detailed structure-activity relationships would require a molecular-level description of the interactions between the metal surfaces and ligands/substrates, this description is rarely accessible. Thus, most insights are often derived from model systems based on single crystals. With the goal to understand efficient alkyne semihydrogenation catalysts based on Cu functionalized with N-heterocyclic carbene (NHC), we cross this gap by investigating NHC-stabilized molecular complexes, supported single sites and nanoparticles via solid-state NMR combined with computations. We demonstrate that, in silica-supported Cu single sites, Cu retains the coordination geometry observed in molecular compounds, as evidenced by the measured and computed dipole-quadrupole cross-term interaction at the carbenic carbon bound to Cu. We show that, for supported Cu nanoparticles, which are highly active and selective for the semihydrogenation of alkynes, NHC binding is favored at Cu adatoms atop of copper surface, thus paralleling what has been inferred by surface science studies on single crystals.