Zhang Lei, Yao Chuang, Yu Yi, Jiang Sheng-Li, Sun Chang Q, Chen Jun
Software Center for High Performance Numerical Simulation , Institute of Applied Physics and Computational Mathematics , Beijing 100088 , China.
Laboratory of Computational Physics , Institute of Applied Physics and Computational Mathematics , Beijing 100088 , China.
J Phys Chem Lett. 2019 May 16;10(10):2378-2385. doi: 10.1021/acs.jpclett.9b01047. Epub 2019 Apr 29.
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, 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, 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 highly reactive to electrophiles and easily broken. Only in sufficiently acid solution can the cyclo-N become unsusceptible to the electrophilic attack and gain extra stability through the formation of hydrogen-bonded complex from surrounding electrophiles; otherwise, the cyclo-N cannot be productively isolated. The dual aromaticity discovered in cyclo-N is expected to be universal for pnictogen five-membered ring systems.
戊唑阴离子是全氮含能材料的最佳候选物，但由于不明原因，只能从酸性溶液中分离出来，这阻碍了高能量密度全氮物质的高产率实现。在此，我们首次报道了环氮（cyclo-N）的双重芳香性（π和σ）的发现，这使得该阴离子在本质上不稳定，但在酸性环境中具有额外的稳定性。除了与典型苯类似的常见π芳香性外，五个孤对电子在环氮的赤道平面上离域，形成了额外的σ芳香性。正是σ芳香体系中孤对电子间排斥和吸引的兼容共存，使得裸环氮对亲电试剂具有高反应性且容易断裂。只有在足够酸性的溶液中，环氮才能不易受到亲电攻击，并通过与周围亲电试剂形成氢键复合物而获得额外的稳定性；否则，环氮无法有效地分离出来。环氮中发现的双重芳香性预计在氮族元素五元环体系中具有普遍性。
