Characterization of novel pyridine-derived N-heterocyclic silylenes via density functional theory perspective

We theoretically searched new derivatives of N-heterocyclic silylenes (NHSis) by substituting of pyridine and phosphinine rings, compared to the reference carbenes (N-heterocyclic carbenes-NHCs) including benzopyridine-4-ylidene (1) and dibenzopyridine-4-ylidene (2), at M06-2X/AUG-cc-pVTZ and B3LYP/AUG-cc-pVTZ. Regardless of how organized NHSis are, fusion of 1 six-membered ring leads to more vibration frequency (upsilon(min)), stabilizing effect, and polarizability than 2 six-membered rings. In all cases, singlet (3s-10s) and triplet (3t-10t) NHSis emerge as ground state, showing more stability of singlet states than their corresponding triplet congeners. From thermodynamic and kinetic viewpoints, the scrutinized singlet NHSis show more stability than the parent singlet NHCs (1s and 2s), but the studied triplet NHSis exhibit less stability than triplet NHCs (1t and 2t). Regarding the orientation of silylenic center and nitrogen or phosphor heteroatoms of fused rings, the stabilizing energy for para-fused rings is more than ortho ones. The most stability is confirmed by one para-substituted pyridine NHSi (singlet-triplet energy difference; Delta Epsilon(s-t) = 47.3 and band gap; Delta EHOMO-LUMO = 85.0 kcal/mol; highest occupied molecular orbital [HOMO] and lowest unoccupied molecular orbital [LUMO]) and the least stability is demonstrated by two substituted benzene NHC (Delta Epsilon(s-t) = 11.9 and Delta EHOMO-LUMO = 58.6 kcal/mol). In contrast to pure NHCs, triplet NHSis (3t-10t) show lower Delta Epsilon(HOMO-LUMO), higher nucleophilicity, lower electrophilicity, and lower chemical hardness than their corresponding singlet states (3s-10s).