(Imidazol-2-ylidene) → S coordination interactions 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 become explicit after S-oxidation in the (NHC) → SR⁽⁺⁾ systems. To explore this hypothesis, we performed quantum chemical design and synthesis of (NHC) → SR⁽⁺⁾, (NHC) → S(O)R⁽⁺⁾, and (NHC) → S(O)₂R⁽⁺⁾ complexes in which the ligands are imidazol-2-ylidene derivatives. Eight derivatives of the (imidazol-2-ylidene) → SR⁽⁺⁾ systems were generated, and their sulfoxide and sulfone derivatives were obtained by oxidation using urea–H₂O₂ and mCPBA, respectively. The crystal structures of three compounds belonging to a series were determined. 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.