Synthesis and reactivity of (benz)imidazol-2-ylidenes with exocyclic N-acyl or N-sulfonyl groups

Imidazolium and benzimidazolium salts featuring exocyclic N-acyl or N-sulfonyl groups were prepared and converted in situ to the corresponding N-heterocyclic carbenes by deprotonation. The NHCs were further converted to group six adducts (sulfur and selenium) and metal complexes with Rh(I), Ir(I), Cu(I), Ni(II) and Fe(II). The N-acyl derivatives were found to be much more sensitive towards hydrolytic degradation than the N-sulfonyl compounds. IR spectroscopic analysis of NHC-carbonyl complexes revealed the N-acyl NHCs to be slightly better donors (TEP 2054–2055 cm−1) compared to the N-sulfonyl based ligands (TEP: 2056–2061 cm−1). Both classes are comparable to cyclic mono- and diamido carbenes regarding their donor properties.