Energy Decomposition Analysis of the Activation of CO2 by Frustrated Lewis Pairs.

Numerous frustrated Lewis pairs (FLPs) have been developed for activating small molecules such as H2 and CO2 in metal-free catalysis. In this work, we applied our energy decomposition scheme based on the block-localized wave function (BLW) method to explore the governing forces in the CO2 activations by a series of intramolecular FLPs where both the Lewis acid (with B as the center) and Lewis base (with N or P as the center) are attached to a benzene ring. The uniqueness of the BLW method is the self-consistent derivation of the diabatic state where the charge transfer interactions between FLPs and CO2 are completely quenched, allowing us to evaluate the impact of the charge transfer on both geometries and energetics. We showed that the structural rearrangements prepared for the activated states cost considerable energy, leading to the small overall binding energies, which are often even positive (destabilizing). Thus, one key suggestion for the rational design of FLPs is the fixation of the Lewis acid and base centers, together with the substituent groups bonding to these centers. All FLPs studied in this work exhibit strong chemical interactions with CO2 in the activated states by donating electrons from FLPs to CO2, leading to the activation of CO2 with a bent geometry. Approximate linear correlation between the charge transfer energies and the activated CO bond lengths in CO2 is observed. Without the charge transfer, FLPs would be unable to absorb and activate CO2 with their intramolecular electric fields alone.