Acidic Stabilization of the Dual-Aromatic Heterocyclic Anions

Recently, we discovered that the delocalization of nitrogen lone-pair electrons (NLPEs) in five-membered nitrogen heterocycles created a second sigma-aromaticity in addition to the prototypical pi-aromaticity. Such dual-aromatic compounds, such as the pentazole anion, were proved to have distinct chemistry in comparison to traditional pi-aromatics, such as benzene, and were surprisingly unstable, susceptible to electrophilic attack, and relatively difficult to obtain. The dual-aromatics are basic in nature, but prefer not to be protonated when confronting more than three hydronium/ammonium ions, which violates common sense understanding of acid-base neutralization for a reason that is unclear. Here, we carried out 63 test simulations to explore the stability and reactivity of three basic heterocycle anions (pentazole anion N-5 over bar , tetrazole anion N4C1H1 over bar , and 1,2,4-triazole anion N3C2H2 over bar ) in four types of solvents (acidic ions, H3O+ and NH4+, polar organics, THF, and neutral organics, benzene) with different acidities and concentrations. By quantum mechanical calculations of the electron density, atomistic structure, interatomic interactions, molecular orbital, magnetic shielding, and energetics, we confirmed the presence of dual aromaticity in the heterocyclic anions, and discovered their reactivity to be a competition between their basicity and dual aromaticity. Interestingly, when the acidic ions H3O+/NH4+ are three times more in number than the basic heterocyclic anions, the anions turn to violate acid-base neutralization and remain unprotonated, and the surrounding acidic ions start to show a significant stabilization effect on the studied heterocyclic anions. This work brings new knowledge to nitrogen aromatics and the finding is expected to be adaptable for other pnictogen five-membered ring systems.

Automated summary

Recent research has found that nitrogen lone-pair electron delocalization in nitrogen heterocycles can lead to a second sigma-aromaticity, resulting in dual-aromatic compounds with distinct chemistry compared to traditional pi-aromatics. Despite being basic in nature, these compounds are surprisingly unstable and susceptible to electrophilic attack. Quantum mechanical calculations revealed a competition between their basicity and dual aromaticity. Furthermore, when the number of acidic ions is three times higher than the basic heterocyclic anions, the anions violate acid-base neutralization and remain unprotonated, with the surrounding acidic ions showing a stabilizing effect.