New Insight into the Aromaticity of cyclo-N5- by Constructing 3D Arrays in Crystal Structures

Pentazole (HN5) has been actively pursued due to its intriguing aesthetic structure and potential applications as ultrahigh energetic materials. It is generally accepted that cyclo-N-5(-) is a highly symmetric ring with good aromaticity. In this work, we designed two innovative pentazolate salts with 3D arrays in their crystal structures and found that the aromaticity of the cyclo-N-5(-) anion could be damaged by specific weak interactions. In such fascinating crystal structures, multiple cyclo-N-5(-) anions are regularly stacked with two modes (antiparallel and vertical) in the parallelepiped spaces constructed by chain cations, forming 3D arrays of cations and anions, respectively. Notably, the aromaticity of vertical cyclo-N-5(-) anions is damaged due to the geometric changes caused by strong local hydrogen bonds at room temperature; meanwhile, the unstable vertical cyclo-N-5(-) can be stabilized by sufficiently strong pi-pi interactions. This is the first time that the aromaticity of a pentazole ring has been damaged by weak interactions. Such findings not only widen the understanding of aromaticity but also deepen the fundamental understanding of cyclo-N-5(-) and contribute to the emerging pentazole chemistry.

Automated summary

Two innovative pentazolate salts with 3D arrays in their crystal structures have been designed, showing that the aromaticity of cyclo-N-5(-) anions can be damaged by specific weak interactions. The geometric changes caused by strong local hydrogen bonds at room temperature impair the aromaticity of vertical cyclo-N-5(-) anions, but they can be stabilized by sufficiently strong pi-pi interactions.