N-Heterocyclic carbenes as tunable ligands for catalytic metal surfaces

Heterogeneous catalysts represent the backbone of many industrial processes and thus have a considerable societal impact. The active sites of these heterogeneous catalysts are often metal surfaces, which consist of a large ensemble of metal atoms, and their properties differ significantly from metal complexes bearing highly developed ligands typically used in homogeneous catalysis. A highly interesting approach is the combination of these sophisticated ligand systems from homogeneous catalysis with metal surfaces to improve parameters such as stability, reactivity and selectivity. One class of these advanced ligands is N-heterocyclic carbenes (NHCs), which have been increasingly applied as molecular modifiers for metal surfaces. This review summarizes developments in this area, covering fundamental properties, preparation methods, important analytical techniques and reactions of NHC-modified metal surfaces. Emerging strategies for NHC design are categorized and future directions are outlined. N-Heterocyclic carbenes are versatile ligands that can be used to modulate the properties of metallic surfaces. This Review focuses on efforts to tune the reactivity of metal-based heterogeneous catalysts by such molecular species, providing an overview of the techniques to study carbene–surface interaction and examples of successful catalytic reactions based on this approach.