Stabilization of the Dual-Aromatic cyclo-N₅^- Anion by Acidic Entrapment

Pentazole anion, the best candidate for full-nitrogen energetic materials, can be isolated only from acidic solution for unclear reasons, which hinders the high-yield realization of a full-nitrogen substance with higher energy density. Herein, we report for the first time the discovery of the dual aromaticity (π and σ) of cyclo-N₅^-, which makes the anion unstable in nature but confers additional stability in acidic surroundings. In addition to the usual π-aromaticity, similar to that of the prototypical benzene, five lone pairs are delocalized in the equatorial plane of cyclo-N₅^-, forming additional σ-aromaticity. It is the compatible coexistence of the inter-lone-pair repulsion and inter-lone-pair attraction within the σ-aromatic system that makes the naked cyclo-N₅^- highly reactive to electrophiles and easily broken. Only in sufficiently acid solution can the cyclo-N₅^- become unsusceptible to the electrophilic attack and gain extra stability through the formation of hydrogen-bonded complex from surrounding electrophiles; otherwise, the cyclo-N₅^- cannot be productively isolated. The dual aromaticity discovered in cyclo-N₅^- is expected to be universal for pnictogen five-membered ring systems.