Assessment of stereoelectronic factors that influence the CO2 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(2)OH, CH(2)NHCOMe, and CH(2)NHPh 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.