Multiple Interactions between Cations and Anions Achieve a High Density of Cyclo-N5̅ Salts

The first stable cyclo-pentazolate (cyclo-N5–) salt has been synthesized exceeding a six-year period, and the relevant high-energy density salts are much anticipated in all-nitrogen materials. However, until now, all reported nonmetal cyclo-N5– salts suffer from a low density of less than 1.7 g·cm–3. Herein, three nonmetallic cyclo-N5– salts containing different numbers of nitro groups, 3,3,3-trinitropropan-1-amine pentazolate (TNPAP), 3,3-dinitroazetidine pentazolate (DNAP), 5-(nitromethylene) hexahydroimidazo [4,5-d] iminazol-2(1H)-iminium pentazolate (NMHIP), were synthesized based on the consideration of chemical-bonding interactions and π-stacking interactions. Their structures were characterized by single-crystal X-ray diffraction analysis. The TNPAP bearing three nitro groups has the highest density up to 1.734 g·cm–3 in known nonmetallic cyclo-N5– salts. Both crystal structure and theoretical calculation demonstrate the contributions of the hydrogen-bond network and intermolecular nitro-π interaction in the three-dimensional stacking structure to the density. In contrast to DNAP with two nitro groups and NMHIP with only one, the detonation velocity and detonation pressure of TNPAP increased by 0.07/0.2 km·s–1 (0.8%/2.3%) and 2.8/5.2 GPa (9.2%/18.6%), respectively. Our work further reveals the effects of nitro-containing cations on the density and properties of nonmetallic cyclo-N5– salts.