Bis(N-cyclopropenio)-imidazol-2-ylidene: An N-Heterocyclic Carbene Bearing Two N-Cationic Substituents

Thanks to the onio-substitution strategy, tricationic imidazolium salt (1 center dot H)(OTf)(3) featuring two N-cyclopropenio substituents was readily prepared in a two-step procedure from 1H-imidazole. The influence of the two cyclopropenium moieties on the electronic properties of the related dicationic NHC 1(2+) was evaluated on the basis of the (1)J(CH), delta Se-77, and Tolman electronic parameter (TEP) values obtained, respectively, from NHC precursor (1 center dot H)(OTf)(3), NHC=Se adduct (5)(OTf)(2), and NH-CRh(CO)(2) complex (7)(OTf)(2), leading to the conclusion that carbene 1(2+) is one of the least-donating cationic NHCs reported to date. The coordinating ability of 1(2+) was extended to Pd(II) and Au(I) centers, providing dicationic [PdCl(allyl)(1)] (OTf)(2)(8)(OTf)(2) and [AuCl(1)](OTf)(2) (10)(OTf)(2) complexes, respectively, which were fully characterized. While Pd complex (8)(OTf)(2) was found to exhibit the lability of a cyclopropenium substituent upon the cleavage of a C+-N bond, Au complex (10)(OTf)(2) was implemented in model Au-catalyzed intramolecular cyclization reactions, showing moderate activity.