Assessing the Potential of Zwitterionic NHC·CS₂ Adducts for Probing the Stereoelectronic Parameters of N‐Heterocyclic Carbenes

Abstract

Abstract Five imidazol(in)ium‐2‐dithiocarboxylates bearing cyclohexyl, mesityl, or 2,6‐diisopropylphenyl substituents on their nitrogen atoms were prepared from the corresponding N‐heterocyclic carbenes (NHCs) by reaction with carbon disulfide. They were characterized by IR, UV/Vis, and NMR spectroscopy, 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 NHC · CS 2 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 NHC · CO 2 adducts were deemed more appropriate for evaluating the σ‐donating properties of carbene ligands.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)