Theoretical investigations in the reactions of group 15 analogues of the monocationic five-membered N-heterocyclic carbenes: interplay of electrophilicity, basicity, and aromaticity governing the reactivity

Abstract

The nature of the group 15 analogs of the five-membered N-heterocyclic carbenes (G15-Rea (G15 = N, P, As, Sb, and Bi)) was studied employing five methodologies: calculations of the NICS (the nucleus-independent chemical shifts), the ACID (anisotropy of the current-induced density), the PA (proton affinity), the electrophilicity, and the Fukui functions using density functional theory (DFT). The deep analysis of the results indicates the reactivity of heavy G15′-Rea (G15′ = P, As, Sb, and Bi) is governed by its lowest unoccupied molecular orbital, a vacant p–π orbital. Conversely, the highest occupied molecular orbital, a nonbonding sp² lone pair orbital, determines the reactivity of N-Rea. To probe the origin of the activation barriers of chemical reactions for the group 15 analogs of G15-Rea, two types of model reactions (C–H bond insertion of methane and alkene cycloaddition) were theoretically examined with the help of the activation strain model (ASM). Theoretical evidence reveals that the origin of the activation energies of the model reactions can be explained in terms of the atomic radii of the pivotal group 15 elements and electronic factors of the five-membered G15-Rea species. Interestingly, our theoretical findings strongly suggest that both aromaticity and singlet–triplet energy splitting ΔE_st of the G15-Rea molecules can be used as a diagnostic tool for the prediction of the reactivity of diverse five-membered monocationic group 15 NHCs with saturated and saturated hydrocarbons.