A Cationic N‑Heterocyclic Carbene Containing an Ammonium Moiety
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The dicationic 4-ammonio-substituted imidazolium salt (2·H)(OTf)2 is readily obtained from its 4-amino-substituted imidazolium precursor (1·H)(OTf) by methylation of the exocyclic nitrogen. The cationic NHC (2)(OTf) is generated in situ by deprotonation of (2·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 σ bond has a profound effect on the electronic properties of the NHC by significantly decreasing its σ-donor ability and, to a lesser extent, increasing its π-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(η3-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.
