(Imidazol-2-ylidene)→S Coordination Interaction and its Modulation Upon S-Oxidation

Abstract

(NHC)→E coordination interactions are being explored in many chemical species, including carbones, and nitreones. (NHC)→S interactions are rare, but increasing attention is being paid to the compounds containing such interactions. The electron deficiency at the S centre is responsible for triggering electron donation from the NHC unit in (NHC)→SR(+) systems. It is well known that the positive charge at the sulfur centre increases upon single oxidation and further increases upon double oxidation. This implies that (NHC)→S interactions may get explicit after S-oxidation in (NHC)→SR(+) systems. To explore this hypothesis, we performed quantum chemical design and synthesis of (NHC)→SR(+), (NHC)→S(O)R(+), (NHC)→S(O)2R(+) complexes in which the ligands are imidazol-2-ylidene derivatives. Eight derivatives of (imidazol-2-ylidene)→SR(+) systems were generated, and their sulfoxide and sulfone derivatives were obtained by oxidising using (Urea-H2O2 and mCPBA, respectively). The crystal structures of three compounds belonging to a series were obtained. A comparison of the geometric, energetic and electronic characteristics confirmed the hypothesis that the (NHC)→S coordination interaction becomes comparatively stronger with an increase in oxygen atoms at the sulfur centre.