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

Supported metal nanoparticles are a very large class of heterogeneous catalysts. While detailed structure-activity relationships require a molecular-level description of the interactions between the metal surfaces and ligands/substrates, this description is rarely accessible. Thus, most insights are derived from models based on single crystals. With the goal to understand 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 by solid-state NMR combined with computations. We show that in silica-supported Cu single sites, Cu retains the coordination geometry observed in molecular compounds, while, for supported Cu nanoparticles, which are active and selective for the semihydrogenation of alkynes, NHC binding is favored at Cu adatoms atop of copper surface, thus paralleling conclusions of surface science studies on single crystals.