Substituent effects on the stability of cyclic - unsaturated remote N-heterocyclic Hammick carbenes using density functional theory

Abstract

We investigated the structure, stability and chemical properties of novel singlet (s) and triplet (t) Hammick carbenes with different fused aromatic rings, at (U)B3LYP/6‐311+G*, (U)M06-2X/6‐311+G*, (U)B3LYP/AUG-cc-pVTZ and (U)M06-2X/AUG-cc-pVTZ levels of theory. It is observed that: (1) All singlet and triplet structures appear as minima on their energy surfaces and each singlet carbene displays more thermodynamic stability than its corresponding triplet state; (2) Regarding to the relationship among carbenic center and substituted heteroatom, the order of thermodynamic stability based on singlet-triplet energy differences (ΔΕs-t) for fused rings is ortho-pyrrole > furan > thiophene > phosphole in a “W” arrangement; (3) The highest kinetic stability based on band gap (ΔΕHOMO-LUMO) is demonstrated by substituted ortho-pyrrole carbene in a "W" arrangement; (4) Regardless of how orchestrated, the order of stabilization for fused rings is pyrrole > furan > thiophene > phosphole; (5) The ortho-phosphole ring in a “W” arrangement destabilizes carbene more than benzene ring.