Crystal Growth & Design
Article
form the 3D-cube packing system. Meanwhile, large amount
hydrogen bonds existing between layers or in the network
could enhances the strength of crystal packing as well as the
stability of compound.
Here, each proton in the cation of compound 8 (N5 and
N5a) has a 50% occupancy due to its symmetry, indicating 8 is
univalent. Compound 8 crystallizes in the C2/m space group
with four molecules per unit cell. The density of 8 is 1.722 g·
cm−3 at 173 K which is the highest density nonmetallic
pentazole salts reported so far. All N−N bond length of cyclo-
N5− of 8 are 1.3158(32), 1.3158(32), 1.3265(64), 1.3265(64),
1.3093(39) Å forming a symmetrical structure. The average
−
bond length is 1.3188 Å lower thancompound 6. Each N5
anion is surrounded by two cations to form hydrogen bonds
with it (N4−H4A···N3:2.1 Å, N5−H5··· N3:2.6 Å) (Figure
5b). Each triazole ring (plane through N5, N6 and N7 or N5a,
N6a and N7a) is basically coplanar (5.591(171)°) with the
N5− anion ring. In Figure 5c, since the nitro group (N9) on the
pyrazole ring is affected by the steric hindrance of the amino
group (N8 and N8a) on the triazole ring, the pyrazole
ring(plane through C1,N10 and N10a) is deflected with 54.9°
of the dihedral angle with triazole (plane through N5,N6 and
N7). As can be seen from Figure 5d and 5e, 8 is layer-by-layer
stacking with 3.6994 Å of the interlayer distance. This crystal
packing may be attributed to its high density and low
sensitivity.
Figure 7. DSC curves of 6, 7, and 8.
effect of furazan and 4-nitropyrazole on the decomposition
temperature of pentazolate salt, 11 reported pentazolate salts
are selected for comparison.12,13,15,16
As shown in Figure 8, all pentazolate salts are arranged from
low to high value by decomposition temperature (red pillar),
and the calculated density (blue pillar) at 273 K is used as the
density change. It can be clearly seen that decomposition
temperature of 6, 7, and 8 are higher than that of
+
−
N(CH3)4 N5 by nearly 30 °C about 38% increase and also
better than that of simple, guanidines and azole pentazlate
salts. Synchronously, the crystal density is also maintained at a
high level (1.615−1.695 g·cm−3) which is more superior to
most reported other pentazolate salts (1.126−1.601 g·cm−3).
Overall, furazan and 4-nitropyrazole skeleton have a good
promotion effect on the decomposition temperature and
density of pentazolate salt.
Vibrational Spectroscopy. Infrared (IR) and Raman
(RA) spectra (785 nm excitation) of compounds 6−8 were
identified to provide valuable insight into the vibrational intra-
and inter- molecular bonds. In the IR spectra (Figure 6a), the
−
cyclo-N5 anion of 6−8 exhibits a sharp peak at 1215−1225
cm−1 which is consistent with the result of (N5)6(H3O)3-
(NH4)4Cl.6 Furthermore, when compared to compound 7, the
−
IR spectra of 6 and 8 shows that the peaks of the cyclo-N5
To furthermore understand the relationship of structure-
properties, two-dimensional (2D) fingerprints and Hirshfeld
surface were used to analyze the weak interactions and
hydrogen bonds of pentazolate salts.25 The sensitivity of 6-8
can be described by the shape of Hirshfeld surfaces, and the
regions of red or blue on which represent high or low close
contact populations.26,27
anion are red-shifted by about 10 cm−1, which maybe result
from the fact that crystal packings of 6 and 8 exhibit stronger
N−H···π interactions than that of compound 7. According to
group theory,12 the planar cyclo-N5 anion‘s stretching modes
−
span A1′ + E1′ + 2E2′ + E2′ in the context of D5h symmetry.
As can be seen from RA spectra (Figure 6b), three
−
characteristic peaks of cyclo-N5 of 6−8 are observed in the
From Figure 9c,i, due to the coplanarity of 6 and 8’s anion
and cation, most of the red dots (intermolecular interaction)
are located at the edge of the surface. Therefore, the molecular
plane can be stacked as much as possible to resist external
stimuli in this way, which results in its low sensitivity and good
stability. In Figure 9a,d,g, the sharp peaks on bottom left (H−
O and O−H, H−N and N−H interactions) of 6, 7, and 8
represent hydrogen bonds between surrounding molecules.28
Owing to the introduction of oxygen atoms, the numerous
hydrogen-bonding interactions (H−O and O−H, N−H and
H−N, 63% for 6, 55% for 7, and 56% for 8) partly contribute
to be insensitive between molecules. Correspondingly, low
sensitivity is also partly attribute to π−π stacking interactions
(N−C and C−N, O−N and N−O interactions, 13% for 6, 11%
for 7, 13% for 8) between layer and layer of molecular.
To further research the interactions among crystals 6, 7, and
8, the noncovalent interaction (NCI) analysis (Figure 10) of 6,
7, and 8 calculated by Multiwfn29,30 were used on the basis of
electron density.31 For the color-filled reduced density gradient
(RDG) iso-surface of the NCI plots, the stronger attractions
are represented by blue surfaces and the π−π interactions are
expressed by large green iso-surfaces. From Figure 10, the π−π
interactions exist in abundance in 6, 7, and 8. Meanwhile,
range of 1150−1176 cm−1 (A1′), 1088−1105 cm−1 (E2′), and
1020−1049 cm−1 (E2′), in basically agreement with the result
of (N5)6(H3O)3(NH4)4Cl.6 The discrepancies among three
compounds can result from the strong intermolecular forces
−
between cations and cyclo-N5 anion.
Thermal Stability. The thermal stability of compounds 6-8
was analyzed via differential scanning calorimetry (DSC)
−1
scanning with a heating rate of 5 °C min
over the
temperature range from 30 to 450 °C. As shown in Figure 7,
salts 6, 7, and 8 all decomposed (6:113.3 °C; 7:116.4 °C;
8:110.5 °C) without melting and step by step in the range of
100−350 °C. They all exhibit thermal stabilities beyond 110
°C (onset), higher than that of other pentazolate salts (Figure
7). The exothermic peak of 6 is observed an obvious
endothermic peak at 244 °C which should be the melting of
cationic azide produced by the decomposition of pentazolate
salts.6,13 Correspondingly, compound 8 immediately has an
endothermic peak which could be the volatilization of
hydragoic acid (HN3) after the decomposition of the
pentazolate anion.12 Therefore, the cation also has a certain
degree of influence on the decomposition products (HN3 or
organic azide salt) of pentazolate. To better demonstrate the
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Cryst. Growth Des. 2021, 21, 2690−2698
Chen, Jieyi
