Assessing the potential of zwitterionic NHOCS2 adducts for probing the stereoelectronic parameters of N-heterocyclic carbenes

Five imidazol(in)ium-2-dithiocarboxylates bearing cyclo-hexyl, mesityl, or 2,6-diisopropylphenyl substituents on their nitrogen atoms were prepared from the corresponding N-heterocyclic carbenes (NHCs) by reaction with carbon disul-fide. They were characterized by IR, UV/Vis, and NMR spec- troscopy, and by thermogravimetric analysis. Their molecular structures were determined by X-ray diffraction. For the sake of comparison, tricyclohexylphosphonium dithiocarboxylate was also examined. The data acquired were scrutinized to evaluate their usefulness for assessing the steric and electronic properties of NHC ligands. Because of their outstanding ability to crystallize, the five NHOCS2 betaines were found to be highly suitable for probing the steric influence of nitrogen atom substituents on imidazolylidene-based ligand precursors via XRD analysis, while the corresponding NHOCO2 adducts were deemed more appropriate for evaluating the a-donating properties of carbene ligands.