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 (Vmin1) as well as at the carboxylate region of the NHC–CO2 adduct (Vmin2). Both Vmin1 and Vmin2 are found to be simple and efficient descriptors of the stereoelectronic effect of NHCs. The Vmin-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.