A Cationic N-Heterocyclic Carbene Containing an Ammonium Moiety

The dicationic 4-ammonio-substituted imidazolium salt (2 center dot H)(OTf)(2) is readily obtained from its 4-amino-substituted imidazolium precursor (1 center dot H)(OTf) by methylation of the exocyclic nitrogen. The cationic NHC (2)(OTf) is generated in situ by deprotonation of (2 center dot H)(OTf)(2) with 1 equiv of potassium bis(trimethylsilyl)amide and is trapped with sulfur or selenium. The direct linkage of the cationic ammonium moiety to the carbenic heterocycle through a sigma bond has a profound effect on the electronic properties of the NHC by significantly decreasing its sigma-donor ability and, to a lesser extent, increasing its pi-acidity. These features make the 4-ammonio-substituted NHC 2(+) an overall weak electron donor. To evaluate the coordination abilities of carbene 2(+), several transition-metal complexes supported by this ligand have been prepared, including the cationic rhodium(I) complexes [RhCl(cod)(2)](OTf) and [RhCl(CO)(2)(2)](OTf), the cationic palladium(II) complexes [PdCl(eta(3)-allyl)(2)](OTf) and [PdCl2(CH3CN)(2)](OTf), the zwitterionic palladium(II) complex [PdCl3(2)], and the tricationic silver(I) complex [Ag(2)(2)](OTf)(3).