Design concepts for N-heterocyclic carbene ligands

N-heterocyclic carbenes (NHCs) are a widely used class of ligands that have enabled an exciting variety of organometallic chemistry and catalysis. As researchers design and customize these ligands for specific applications, various design concepts have emerged. This review covers several of these design concepts and their impact on and promise for organometallic chemistry and catalysis. These concepts are bulky ligands that remain flexible for substrate binding and reactivity, ring-expanded NHCs with different steric and electronic properties, NHCs appended to supramolecular assemblies that control the microenvironment around the metal center; and NHCs with pendant labile functional groups.