Coordination Chemistry of Highly Hemilabile Bidentate Sulfoxide N-​Heterocyclic Carbenes with Palladium(II)

Imidazolium salts, [RS(O) CH2 (C3 H3 N2 )​Mes]​Cl (R=Me (L1 a)​, Ph (L1 b)​)​; Mes=mesityl)​, make convenient carbene precursors. Palladation of L1 a affords the monodentate dinuclear complex, [(PdCl2 {MeS(O)​CH2 (C3 H2 N2 )​Mes}​)​2 ] (2 a)​, which is converted into trans-​[PdCl2 (NHC)​2 ] (trans-​4 a; N-​heterocyclic carbene) with two rotamers in anti and syn configurations. Complex trans-​4 a can isomerize into cis-​4 a(anti) at reflux in acetonitrile. Abstraction of chlorides from 4 a or 4 b leads to the formation of a new dication: trans-​[Pd{RS(O)​CH2 (C3 H2 N2 )​Mes}​2 ]​(PF6 )​2 (R=Me (5 a)​, Ph (5 b)​)​. The X-​ray structure of 5 a provides evidence that the two bidentate SO NHC ligands at palladium(II) are in square-​planar geometry. Two sulfoxides are sulfur- and oxygen-​bound, and constitute five- and six-​membered chelate rings with the metal center, respectively. In acetonitrile, complexes 5 a or 5 b spontaneously transform into cis-​[Pd(NHC)​2 (NCMe)​2 ]​(PF6 )​2 . Similar studies of thioether-​NHCs have also been examined for comparison. The results indicate that sulfoxides are more labile than thioethers.