A Cationic N-Heterocyclic Carbene Containing an Ammonium Moiety

Abstract

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(η 3 -allyl)( 2 )](OTf) and [PdCl 2 (CH 3 CN)( 2 )](OTf), the zwitterionic palladium(II) complex [PdCl 3 ( 2 )], and the tricationic silver(I) complex [Ag( 2 ) 2 ](OTf) 3 .