Mesoionic 1,2,3-Triazoles and 1,2,3-Triazole Carbenes

Routine access to 1,2,3-triazoles through the copper-catalyzed azide-alkyne “click” cycloaddition reaction has promoted the rapid development of 1,2,3-triazolylidenes as ligands for transition metals. The organometallic complexes containing this kind of N-heterocyclic carbene ligands (NHCs) have shown to possess unique structural characteristics, including a relatively high covalent contribution to the metal–NHC bond, a strong donor ability, and a mesoionic character. Complexes of mesoionic triazolylidene carbenes (MICs) are readily available, among other methods, by metallation of N3-substituted 1,2,3-triazolium salts, usually followed by transmetallation reactions. A diverse array of monodentate, polydentate, and bridged ligands containing the 1,2,3-triazolylidene motif has been described, and the structures of complexes containing them have been characterized in detail. Promising applications in organometallic catalysis involving C–C and C–X bond formation and redox reactions have emerged in recent years for 1,2,3-triazolylidene metal complexes, and, owing to the modular and multiple functionalization possibilities around the 1,2,3-triazole scaffold, novel applications are expected in upcoming years.