N-heterocyclic carbenes with negative-charge tags: direct sampling from ionic liquid solutions

Abstract

Herein we report on the sampling and characterization via electrospray ionization (tandem) mass spectrometry of free, long-lived N-heterocyclic carbenes (NHC) bearing negative-charge tags. To facilitate electrospray “ion fishing” via electrostatic ejection directly to the gas phase, negative-charge tagged NHC were formed in imidazolium-based ionic liquid solutions via double deprotonation of imidazolium cations bearing acid side groups, viz. CH₂CO₂H or (CH₂)₃SO₃H. Via ESI-MS/MS experiments, the gaseous N-heterocyclic carbenes were found to display structurally diagnostic dissociations and bimolecular reactions. In perfect parallel to solution chemistry, the gaseous negative-charge tagged NHC were found to react promptly with CO₂ by carboxylation to form negative-charge tagged imidazolium carboxylates. Neutral carbenes were inaccessible for mass spectrometry, but the charge tag strategy opens many new possibilities to explore the intrinsic chemistry of these key but elusive species.