Crystalline 1H-1,2,3-triazol-5-ylidenes: new stable mesoionic carbenes (MICs).

Until recently, the availability of neutral carbon-based κC ligands was limited to carbon monoxide, isocyanides, and carbenes. Compared to phosphorus-based ligands, carbenes tend to bind more strongly to metal centers, avoiding the necessity for the use of excess ligand in catalytic reactions. The corresponding complexes are often less sensitive to air and moisture, and are remarkably resistant to oxidation. As the robustness of carbene complexes is largely due to the presence of strong carbon–metal bonds, other types of carbon-based ligands are highly desirable. It is noteworthy that, although complexes between a carbene and a transition metal have been known for a long time, the recent developments in their application in catalysis have been greatly facilitated by the availability of carbenes that are stable enough to be bottled. Moreover, carbenes, especially imidazol-2-ylidenes and 1,2,4-triazol-5-ylidenes , are also excellent organocatalysts (Scheme 1).