Assessment of stereoelectronic factors that influence the CO ₂ fixation ability of N-heterocyclic carbenes: A DFT study

The CO ₂ fixation ability of N-heterocyclic carbenes (NHC) has been assessed on the basis of electronic and steric properties of the N- and C-substituents, measured in terms of molecular electrostatic potential minimum, observed at the carbene lone pair region of NHC (V _min1) as well as at the carboxylate region of the NHC-CO ₂ adduct (V _min2). Both V _min1 and V _min2 are found to be simple and efficient descriptors of the stereoelectronic effect of NHCs. The V _min-based analysis also proved that the stereoelectronic effect of N- and C-substituents is additive. When only C-substituents are present in NHC, its CO ₂ affinity solely depends on the electronic effect, whereas if the N-center bears the substituents, the steric factor plays a major role in the carboxylation/decarboxylation process. For standard substituents, maximum CO ₂ binding energy of 18.0 kcal/mol is observed for the most electron-donating combination of NMe ₂ as the C-substituent and Me as the N-substituent. Introduction of ring strain through five-membered ring fusion at the NC bond slightly increased the electron-rich character of the carbene lone pair and also enhanced the CO ₂ binding energy to 20.9 kcal/mol. To further improve the CO ₂ fixing ability of NHCs, we have proposed the use of CH ₂OH, CH ₂NHCOMe, and CH ₂NHPh as N-substituents, as they participate in intramolecular hydrogen bond interaction with the carboxylate. With the new strategy, considerable improvement in the CO ₂ binding energy (26.5 to 33.0 kcal/mol) is observed.