A computational perspective of novel N-heterocyclic silylenes using density functional theory

In this theoretical survey, we inspect substituent effects of two fused benzene, pyrrole, phosphole, furan, and thiophene rings on the stability, polarity, charge distribution, nucleophilicity, electrophilicity, and aromaticity of singlet (s) and triplet (t) silylenes, at density functional theory (DFT). Singlet silylenes as ground state exhibit more stability than their corresponding triplet congeners. In contrast to previous reports on Hammick carbenes, here higher thermodynamic stability (singlet-triplet energy difference or Delta Es-t = E-t - E-s) is considered for silylene situated between two sulfur heteroatoms of two substituted thiophene rings, in a "chair" arrangement in the direction of the silylenic center. Regardless of how arranged, the order of stabilizing effect for fused rings is thiophene > furan > pyrrole > phosphole > benzene. The substituted Hammick silylenes with two fused heterocyclic rings show more stability than synthesized silylenes by Denk and Kira. Also, higher kinetic stability (the frontier molecular orbitals energy difference or Delta EHOMO-LUMO) is revealed by silylene situated between two oxygen heteroatoms of two substituted furan rings, in a "W" arrangement to the divalent center. The scrutinized singlet structures display wider band gap than their triplet states. Every triplet silylene shows higher nucleophilicity (about 1/5 times) than its corresponding singlet state. Commonly, six-membered aromatic N-heterocyclic silylenes (NHSis) show higher N and lower omega than that of five-membered aromatic NHSis. The highest N and the least omega character is anticipated for the substituted pyrrole NHSis in either "W" arrangement or "chair" orientation toward the silylenic center, among the fused species.