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 -N 5 –, 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 -N 5 –, 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 -N 5 – highly reactive to electrophiles and easily broken. Only in sufficiently acid solution can the cyclo -N 5 – become unsusceptible to the electrophilic attack and gain extra stability through the formation of hydrogen-bonded complex from surrounding electrophiles; otherwise, the cyclo -N 5 – cannot be productively isolated. The dual aromaticity discovered in cyclo -N 5 – is expected to be universal for pnictogen five-membered ring systems.