Intramolecular Assembly of Nitrobiazoles and an Ether Bridge: Toward Energetic Materials with Enhanced Energy and Safety.

Recently, the construction of novel fused-ring frameworks has become one of the most significant innovative approaches to access high-energy and thermostable energetic molecules. In this work, an ether bridge was utilized as a building block to construct energetic fused-ring skeletons for the first time. Two new [5,7,5]-tricyclic N-heterocycle-based backbones, ditriazole-1,3,6-oxadiazepine and pyrazole-triazole-1,3,6-oxadiazepine, were synthesized via a straightforward one-step synthetic route and the energetic performances of their derivatives were further evaluated. Containing an additional oxygen atom, high-density pyrazole-triazole backbone, and high crystal packing coefficient, the asymmetric molecule 2,10,11-trinitro-5 H ,7 H -pyrazolo[1,5- c ][1,2,4]triazolo[5,1- e ][1,3,6]oxadiazepine ( NOB-3 ) features a high crystal density of 1.825 g cm -3 , much superior to those of the symmetrical analogues 2,10-dinitro-5 H ,7 H -bis([1,2,4]triazolo)[1,5- c :5',1'- e ][1,3,6]oxadiazepine ( NOB-4 , d = 1.758 g cm -3 ) and D ( d = 1.634 g cm -3 ). Meanwhile, the compounds NOB-3 and NOB-4 exhibit better thermal stability than the parent molecule DNBT ( T d = 251 °C), and their decomposition temperatures reach up to 303 and 294 °C, respectively. The remarkable overall performance of NOB-3 and NOB-4 strongly suggests them as appropriate candidates for heat-resistant explosives. Our study may give new insights into the close correlation of the structural properties of energetic fused-ring frameworks, and the universality of the asymmetric heterocycles combination strategy for designing advanced high-energy density materials (HEDMs) was emphasized again.