Imidazolium scaffolds: Electrospray mass spectrometric observation of N-heterocyclic carbenes in positive-ion mode and halide ion adducts in negative-ion mode

The gas-phase behavior of dicationic protophanes 3a·2X and [14]imidazoliophanes 4a·2X and 5a·2X were examined by electrospray mass spectrometry in both positive- and negative-ion mode. The neutral analytes underwent deprotonation and the positive-ion mode ESI experiments displayed N-heterocyclic carbenes (NHC) whereas the negative-ion mode response showed a variety of polymolecular self-assemblies due to non-covalent interactions with halide anions. Moreover, positive-ion ESI tandem mass spectrometry of dications 3a·2Cl and 4a·2Cl revealed the formation pathway of the carbene species; the neutral analytes M·2Cl produced loss of one counteranion followed by a neutral loss of HCl, which led to the deprotonated singly-charged imidazolylidene ion [M − H]+.