Acidic Stabilization of the Dual-Aromatic Heterocyclic Anions

Recently, we discovered that the delocalization of nitrogen lone-pair electrons (NLPEs) in five-membered heterocycles created a second σ-aromaticity addition to prototypical π-aromaticity. Such dual-aromatic compounds, such as pentazole anion, were proved have distinct chemistry comparison traditional π-aromatics, benzene, and surprisingly unstable, susceptible electrophilic attack, relatively difficult obtain. The dual-aromatics are basic nature, but prefer not be protonated when confronting more than three hydronium/ammonium ions, which violates common sense understanding acid−base neutralization for reason is unclear. Here, carried out 63 test simulations explore stability reactivity heterocycle anions (pentazole anion N5¯, tetrazole N4C1H1¯, 1,2,4-triazole N3C2H2¯) four types solvents (acidic H3O+ NH4+, polar organics, THF, neutral benzene) with different acidities concentrations. By quantum mechanical calculations electron density, atomistic structure, interatomic interactions, molecular orbital, magnetic shielding, energetics, confirmed presence dual aromaticity heterocyclic anions, their competition between basicity aromaticity. Interestingly, acidic ions H3O+/NH4+ times number turn violate remain unprotonated, surrounding start show significant stabilization effect on studied anions. This work brings new knowledge aromatics finding expected adaptable other pnictogen ring systems.