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

Abstract

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, single-crystal 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.