Activation of carbon dioxide by bicyclic amidines

Activation of the carbon dioxide molecule was achieved using bicyclic amidines (DBU, PMDBD, and DBN). The solution reaction Of CO2 with amidines yielded the corresponding zwitterionic complexes through the formation of a N-CO2 bond. C-13 NMR data confirmed the carbamic nature of the carbamic zwitterions, DBU-CO2 and PMDBD-CO2. However, when these adducts were crystallized, the X-ray analyses of the single crystals were in agreement with bisamidinium bicarbonate salt structures, indicating that structural changes occurred in the crystallization process. The elemental and thermogravimetric analysis data for the carbamic zwitterions, DBU-CO2 and PMDBD-CO2, initially obtained by the direct reaction of amidines with CO2, suggest that these molecules are probably associated with one molecule of water by hydrogen-bond formation (amidinium(+)-COO-...H2O). A correlation was observed between the thermal stability and the transcarboxylating activity for the amidine-CO2 complexes. Theoretical calculations of hardness were performed at the B3LYP/cc-pVTZ level of theory and showed concordance with the experimental reactivity of DBU and PMDBD toward CO2.