The first stable carbenes to be isolated were based on an imidazole ring, with the hydrogen in carbon 2 of the ring (between the two nitrogen atoms) removed, and other hydrogens replaced by various groups. These imidazol-2-ylidenes are still the most stable and the most well studied and understood family of persistent carbenes.
A considerable range of imidazol-2-ylidenes have been synthesised, including those in which the 1,3-positions have been functionalised with alkyl, aryl, alkyloxy, alkylamino, alkylphosphino and even chiral substituents:
In particular, substitution of two chlorine atoms for the two hydrogens at ring positions 4 and 5 yielded the first air-stable carbene. Its extra stability probably results from the electron-withdrawing effect of the chlorine substituents, which reduce the electron density on the carbon atom bearing the lone pair, via induction through the sigma-backbone.
Molecules containing two and even three imidazol-2-ylidene groups have also been synthesised.
Imidazole-based carbenes are thermodynamically stable and generally have diagnostic C NMR chemical shift values between 210 and 230 ppm for the carbenic carbon. Typically, X-ray structures of these molecules show N–C–N bond angles of 101–102°.