Conjugated Energetic Salts Based on Fused Rings: Insensitive and Highly Dense Materials

Nitroamino-functionalized 1,2,4-triazolo[4,3-b][1,2,4,5]tetrazine (1), when combined with intermolecular hydrogen bonds (HBs) and strong noncovalent interactions between layers, results, for example, in an interlayer distance of 2.9 Å for dihydroxylammonium 3,6-dinitramino-1,2,4-triazolo[4,3-b][1,2,4,5]tetrazine (2c) with a packing coefficient of 0.805. For dihydroxylammonium 6,6′-dinitramino-3,3′-azo-1,2,4-triazolo[4,3-b][1,2,4,5]tetrazine (3b), two fused rings are linked by an azo group, which expands the conjugated system resulting in an even shorter interlayer distance of 2.7 Å and a higher packing coefficient of 0.807. These values appear to be the shortest interlayer distances and the highest packing coefficients reported for tetrazine energetic materials. With high packing coefficients, both possess high densities of 1.92 g cm^-3 and 1.99 g cm^-3 at 293 K, respectively. Compared with its precursor, the hydroxylammonium moiety serves as a buffer chain (H-N-O-H), connecting the anion and cation through hydrogen bonds, giving rise to more favorable stacking, and resulting in higher density and lower sensitivity. The sensitivities of all the hydroxylammonium salts are lower than that of their neutral precursors, such as compound 2 (3 J, >5 N) and compound 2c (25 J, 360 N). The detonation properties of 2c (detonation velocity v_D = 9712 m s^-1 and detonation pressure P = 43 GPa) and 3b (v_D = 10233 m s^-1 P = 49 GPa) exceed those of present high explosive benchmarks, such as octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) and hexanitrohexaazaisowurzitane (CL-20). The molecular structures of several of these new energetic materials are confirmed by single-crystal X-ray diffraction measurements. Using calculated and experimental results, the fused ring with a planar large π-conjugated system results in a compromise between desirable stabilities and high detonation properties, thus enhancing future utilization in the design of energetic materials.