Effects of α-mono heteroatoms (N vs. P), and β-conjugation on cyclic silylenes

Investigation of fused remote N-heterocyclic silylenes (frNHSis), at DFT

We compared and contrasted the ΔΕ_s-t, band gap (ΔΕ_HOMO-LUMO), aromaticity, charge distribution, and reactivity of singlet (s) and triplet (t) benzopyridine-4-ylidene as the fused remote N-heterocyclic carbene (frNHC) and frNHSis with different fused aromatic rings, at (U)B3LYP/AUG-cc-pVTZ and (U)M06-2X/AUG-cc-pVTZ levels of theory. In this investigation, we found (1) all s and t divalent states appear as minimum structures, for having no negative force constant. Nonetheless, only singlets present more thermodynamic stability than their triplet analogous; (2) the trend of ΔΕ_s-t in kcal/mol is ortho-pyrrole (52.94) > ortho-furan (51.84) > ortho-thiophene (50.38) > para-furan (49.36) > para-pyrrole (49.00) > para-phosphole (48.67) ≥ para-thiophene (48.64) > benzene (44.33) > ortho-phosphole frNHSi (27.50), while ΔΕ_s-t of frNHC is 15.65 kcal/mol; (3) apart from phosphole frNHSis, the order of ΔΕ_s-t in a "ortho position or zigzag array" about 1.8-4.0 kcal/mol is more than that of in a "para position or chair array"; (4) the highest ΔΕ_HOMO-LUMO is demonstrated by ortho-pyrrole frNHSi (95.65 kcal/mol) while the lowest ΔΕ_HOMO-LUMO is verified by the reference frNHC (63.44 kcal/mol); (5) in contradiction of frNHC, all singlet frNHSis reveal higher band gap and lower global reactivity than their triplet congeners; (6) charge distribution along with MEP maps indicate differentially electronic cloud in middle of rings frNHSis vs. frNHC; (7) we anticipate higher nucleophilicity and lower electrophilicity of triplet frNHSis than singlet congeners, will make them worthy of synthetic surveys.

A quest for stable 2,5-bis(halobora)cyclopentenylidene and its Si, Ge, Sn and Pb analogs at theoretical levels

Interesting π- and σ-bond hyperconjugations cause the unusual stability of the puckered Arduengo type divalents with electron deficient boron atoms.