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

We compared and contrasted the Δ Ε s-t , band gap (Δ Ε 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 Δ Ε 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 Δ Ε s-t of frNHC is 15.65 kcal/mol; (3) apart from phosphole frNHSis, the order of Δ Ε 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 Δ Ε HOMO-LUMO is demonstrated by ortho -pyrrole frNHSi (95.65 kcal/mol) while the lowest Δ Ε 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. Graphical abstract