Assessment of Stereoelectronic Factors That Influence the CO₂ Fixation Ability of N-Heterocyclic Carbenes: A DFT Study

The CO₂ 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–CO₂ 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 CO₂ 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 CO₂ binding energy of 18.0 kcal/mol is observed for the most electron-donating combination of NMe₂ 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 CO₂ binding energy to 20.9 kcal/mol. To further improve the CO₂ fixing ability of NHCs, we have proposed the use of CH₂OH, CH₂NHCOMe, and CH₂NHPh as N-substituents, as they participate in intramolecular hydrogen bond interaction with the carboxylate. With the new strategy, considerable improvement in the CO₂ binding energy (26.5 to 33.0 kcal/mol) is observed.