Reversible CO2 fixation by N-heterocyclic imines forming water-stable zwitterionic nitrogen-base-CO2 adducts

Zwitterionic Lewis base adducts between nitrogen bases and CO2 play a pivotal role as transient intermediates in many projects aiming at CO2 capture, storage and utilization. Yet, fundamental questions about the required parameters for the formation of isolable adducts remain and only a single adduct (TBD-CO2) has been characterized unequivocally. Using a combination of NMR spectroscopy, singlecrystal X-ray diffraction, IR spectroscopy and quantum chemistry, we systematically explore the formation and properties of the CO2 adducts with amidines (DBN), guanidines (MTBD) and N-heterocyclic imines. Spectroscopic and theoretical results show that the stability of the NHI-CO2 adducts is largely governed by the nature of the N-heterocycle and the substituent at the exocyclic nitrogen atom. The surprising stability of some NHI-CO2 adducts towards hydrolysis can be ascribed to the hydrophobic CO2 binding site of the nitrogen bases and offers new opportunities in the field of CO2 capture and utilization.