Stabilization of the Dual-Aromatic Cyclo-N5ˉ Anion by Acidic Entrapment

Lei Zhang, Chuang Yao, Yi Yu, Sheng-Li Jiang, Chang Q Sun, Jun Chen

J. Phys. Chem. Lett. 2019, 10, 10, 2378-2385

https://doi.org/10.1021/acs.jpclett.9b01047  JPCL-5N.pdf

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.

1. N sp2轨道杂化：环内N:=:N孤对（超氢键）排斥使其失稳，所以五环仅在富质子酸性环境中稳定；

2. 3H-3H 排斥（反氢键）拉伸N:H-O氢键使H-O缩短并弱化N:=:N排斥并导致N-N共价键缩短，储能。

   
   

   Ref：

   Jiang, C. et al. Response to Comment on "Synthesis and characterization of the pentazolate anion cyclo-N₅⁻ in (N₅)₆(H₃O)₃(NH₄)₄Cl.". Science 359, eaas8953 (2018).

   Zhang, C., Sun, C., Hu, B., Yu, C. & Lu, M. Synthesis and characterization of the pentazolate anion cyclo-N5⁻ in (N₅)₆(H₃O)₃(NH₄)₄Cl. Science 355, 374 (2017).