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
vD = 9712 m s–1 and detonation pressure P = 43 GPa) and 3b (vD = 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.
