NHCs

N-heterocyclic carbenes (NHCs) are a class of stable, predominantly neutral singlet carbenes in which the reactive carbene center is embedded in a nitrogen-containing heterocycle, most commonly an imidazole ring. The adjacent nitrogen atoms stabilize the carbene by resonance donation, making it persistent under ordinary conditions. In typical aryl-imidazol-2-ylidene derivatives, the carbene carbon is flanked by two nitrogens, and the substituents on the nitrogens tune steric and electronic properties.

NHCs are among the strongest organic sigma-donor ligands known, with relatively weak pi-acceptor ability. This leads to highly covalent metal–NHC bonds and often enhanced catalytic activity in transition-metal complexes. Free carbenes are sensitive to air and moisture, but many precursors exist as air-stable salts and can be converted to active species in situ or used as isolated complexes.

Preparation commonly involves deprotonation of imidazolium or related salt precursors with strong bases to generate the free carbene, or the synthesis of metal-NHC complexes directly from these precursors. Variants include benzimidazol-2-ylidenes, triazolylidenes, and abnormal NHCs, with a wide range of steric profiles from bulky to compact.

Applications span major areas of homogeneous catalysis and organocatalysis. As ligands, NHCs enable effective catalysts for cross-coupling reactions (such as Suzuki–Miyaura and related methods), olefin metathesis, hydrogenation, and hydrofunctionalization. In metal-free catalysis, NHCs serve as organocatalysts that enable umpolung reactivity of carbonyl compounds (for example, Stetter-type processes). Notable ligand examples include IMes, IPr, and SIPr, among others.

Historical note: stable NHCs were first isolated in the early 1990s, a development that profoundly impacted organometallic chemistry and catalysis.