A systematic investigation of factors influencing the decarboxylation of imidazolium carboxylates.

A series of 1,3-disubstituted-2-imidazolium carboxylates, an adduct of CO(2) and N-heterocyclic carbenes, were synthesized and characterized using single crystal X-ray, thermogravimetric, IR, and NMR analysis. The TGA analysis of the NHC-CO(2)'s shows that as steric bulk on the N-substituent increases, the ability of the NHC-CO(2) to decarboxylate increases. The comparison of NHC-CO(2)'s with and without methyls at the 4,5-position indicate that extra electron density in the imidazolium ring enhances the stability of an NHC-CO(2) thereby making it less prone to decarboxylation. Single crystal X-ray analysis shows that the torsional angle of the carboxylate group and the C-CO(2) bond length with respect to the imidazolium ring is dependent on the steric bulk of the N-substituent. Rotamers in the unit cell of a single crystal of I(t)BuPrCO(2) (2f) indicate that the C-CO(2) bond length increases as the N-substituents rotate toward the carboxylate moiety, which suggests that rotation of the N-substituents through the plane of the C-CO(2) bond may be involved in the bond breaking event to release CO(2).