Substituent influence of heteroatom on the novel remote N-heterocyclic silylenes (RNHSis) using density functional theory

Abstract

In this work, we have compared and contrasted the stability, polarity, polarizability, frequency, band gap, charge distribution, and global reactivity of singlet (s) and triplet (t) states of the fused benzo-4-sila-1,4-dihydropyridine-4,4-diyl as the reference structure (1-s and 1-t) as well as the fused pyrrole congeners (2x-s, and 2x-t, x = NH, PH, AsH, O, S, and Se), using density-functional theory (DFT). According to frequency data, all singlet and triplet remote N-heterocyclic silylene (RNHSi) structures appear as minimum displaying a positive force constant. Every singlet silylene as ground state exhibits more stability than its corresponding triplet congener. Consistent with Hoffmannʼs findings, the fusion of one pyrrole ring with different heteroatoms stabilizes the corresponding RNHSi more than the fusion of one benzene ring. In going from 1-s to 2x-s species and substituting of NH, PH, AsH, O, S, and Se groups, higher singlet–triplet energy difference (ΔE s-t = E t – E s) and the higher band gap is found for 2NH-s and 2 O-s structures. Interestingly, the higher thermodynamic and kinetic stability of the optimized RNHSis than the synthesized Kiraʼs silylene makes them worthy of synthetic exploration. According to the distributed charge and FMO (the frontier molecular orbital) shapes, the more negative region is localized on pyridineʼs nitrogen and the less negative region is confined over pyrroleʼs nitrogen, while the positive charge is distributed on silylenic center. This phenomenon is inconsistent with the expected canonical forms for RNHSis.