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. Accepted version Financial support from the National Natural Science Foundation of China (Nos. 11604017, 11572053, and U1730244), the Science Challenging Program of China (No. TZ2016001), and the National Supercomputing Center (Shenzhen) and helpful discussions with B.C. Hu, M. Gozin, M. Š ob, E. Hayward, C.Y. Zhang, J.G. Zhang and H.H. Zong are gratefully acknowledged.