Molecular electrostatic potential at nuclear position as a new concept in evaluation of the substitution effects of intramolecular B/N frustrated Lewis pairs in H2 splitting and CO2 reduction

Abstract

AbstractThis study aims to investigate the efficiency and effectiveness of some of the intramolecular boron‐nitrogen frustrated Lewis pairs (B/N‐FLPs) as metal‐free organocatalysts in CO2 conversion. A two‐step mechanism was considered for the catalytic reaction, including the hydrogen splitting by the FLPs (step 1) and reduction reaction by the hydrogenated FLPs (FLPH2s) (step 2). The boron atom in the studied FLPs is bonded to various substituents, which have substantial effects on the performance of the B atom as a Lewis acid. The studied FLPs are classified into two distinguished categories. The quantum theory of atoms in molecules method and natural bond orbital analyses showed that more occupation of the p orbital of boron by the substituted groups causes the reluctance of the boron atom in hydrogen splitting and acceptation of electron density of the hydride ion. However, molecular electrostatic potential at the nuclear positions was used as a novel descriptor in justification of the kinetic behavior of FLPs in hydrogen splitting and reduction reactions. Obtained ΔΔVn values in the donor‐acceptor interacting system, as an outcome of the electrostatic potential concept, show a remarkable linear correlation with the calculated barrier energies (∆G≠) of hydrogen splitting and reduction reactions. This linear correlation can be observed for ΔΔVn values vs electron localization function and localized orbital locator of the developing bond at transition states 1 and 2. Finally, it is concluded that similar activation energies for hydrogen splitting and reduction reactions can be an appropriate criterion for the performance and efficacy of the studied FLPs in the overall reaction.