Stabilities of Immonium Ions Derived from N-Heterocyclic Carbenes Probed by Collision-Induced Dissociation Mass Spectrometry

We report efficient methods for the synthesis of both symmetrically and non-symmetrically substituted bis(imidazol-2-yliden)ammonium ions and a new family of bis(imidazol-2-ylidene)formamidines. The fragmentation pathways of these cations were studied in detail by electrospray ionization combined with tandem mass spectrometry. Interestingly, in these cations direct C–N bond cleavage leading to loss of alkyl radicals can compete with rearrangement reactions leading to closed-shell fragments. The experimental results for the major fragmentation channels are compared with those derived from DFT calculations. Inter alia, a novel rearrangement of the cationic species involving specific alkyl migrations between the two imidazole subunits has been found.