Assessment of Stereoelectronic Factors That Influence the CO<sub>2</sub> Fixation Ability of N-Heterocyclic Carbenes: A DFT Study


The CO<sub>2</sub> 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 (<i>V</i><sub>min1</sub>) as well as at the carboxylate region of the NHC–CO<sub>2</sub> adduct (<i>V</i><sub>min2</sub>). Both <i>V</i><sub>min1</sub> and <i>V</i><sub>min2</sub> are found to be simple and efficient descriptors of the stereoelectronic effect of NHCs. The <i>V</i><sub>min</sub>-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<sub>2</sub> 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<sub>2</sub> binding energy of 18.0 kcal/mol is observed for the most electron-donating combination of NMe<sub>2</sub> 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<sub>2</sub> binding energy to 20.9 kcal/mol. To further improve the CO<sub>2</sub> fixing ability of NHCs, we have proposed the use of CH<sub>2</sub>OH, CH<sub>2</sub>NHCOMe, and CH<sub>2</sub>NHPh as N-substituents, as they participate in intramolecular hydrogen bond interaction with the carboxylate. With the new strategy, considerable improvement in the CO<sub>2</sub> binding energy (26.5 to 33.0 kcal/mol) is observed

Similar works

Full text

oai:figshare.com:article/2557564Last time updated on 2/12/2018

This paper was published in FigShare.

Having an issue?

Is data on this page outdated, violates copyrights or anything else? Report the problem now and we will take corresponding actions after reviewing your request.