C8N12O8: A promising insensitive high-energy-density material

Article abstract of DOI:10.1021/acs.cgd.8b01016

It is desirable to consider crystal engineering in the research of energetic materials. Herein, we report the synthesis, structure, and energetic properties of a promising explosive, 1,2-bis(3-(4-nitro-1,2,5-oxadiazol-3-yl)-1,2,4-oxadiazol-5-yl)diazene (5). Based on the single-crystal diffraction data, 5 is nearly coplanar and features face-to-face and wavelike crystal stackings, thereby exhibiting an excellent density of 1.92 g cm^-3. Meanwhile, the enthalpy of 5 is as high as 962.4 kJ/mol owing to its extensive high-energy N-N, C-N, and N-O bonds. Compound 5 featuring such a characteristic demonstrates high thermal stability (T_d = 256 °C), good mechanical sensitivity (IS, 18 J; FS, 220 N), and superior detonation performance (D, 9240 m s^-1; P, 37.5 GPa). The combination of advanced performance and desirable security make it a potential replacement for the commonly used explosives RDX and HMX.

Full text of DOI:10.1021/acs.cgd.8b01016

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Article
C8N12O8: A Promising Insensitive High-energy-density Material
Qian Wang, Yanli Shao, and Ming Lu
Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b01016 • Publication Date (Web): 11 Sep 2018
Downloaded from http://pubs.acs.org on September 12, 2018
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C8N12O8: A Promising Insensitive Highꢀenergyꢀ
density Material
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Qian Wang, Yanli Shao, Ming Lu*
School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing
210094, China.
ABSTRACT: It is desirable to consider crystal engineering in the research of energetic
materials. Herein we report the synthesis, structure and energetic properties of a promising
explosive, 1,2ꢀbis(3ꢀ(4ꢀnitroꢀ1,2,5ꢀoxadiazolꢀ3ꢀyl)ꢀ1,2,4ꢀoxadiazolꢀ5ꢀyl)diazene (5). Based on
the singleꢀcrystal diffraction data, 5 is nearly coplanar and features faceꢀtoꢀface and wavelike
crystal stackings, thereby exhibiting an excellent density of 1.92 g cmꢀ3. Meanwhile, the enthalpy
of 5 is as high as 962.4 kJ/mol owing to its extensive highꢀenergy NꢀN, CꢀN and NꢀO bonds.
Compound 5 featuring such a characteristic demonstrates high thermal stability (Td = 256 °C),
good mechanical sensitivity (IS: 18 J, FS: 220 N), and superior detonation performance (D: 9240
m sꢀ1, P: 37.5 GPa). The combination of advanced performance and desirable security make it a
potential replacement for the commonly used explosives RDX and HMX.
Introduction
In the last few decades, accompanied by the extensive demand in the military and civilian
fields, the development of highꢀenergyꢀdensity materials (HEDMs) has been of broad interest.1ꢀ2
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Preparing safer, cheaper, greener and more powerful energetic materials is a key target for many
synthetic and theoretical researchers.3ꢀ4 However, these interrelated requirements are often
contradictory to each other, especially energy and safety. Therefore, some suitable strategies are
necessary to guide the investigation of new HEDMs.
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In the design of modern energetic materials, one of the most acceptable strategies is the crystal
engineering. Abundant studies have illustrated that microcosmic structures are closely related to
macrocosmic properties in the field of energetic materials. 5ꢀ6 In general, there are four types of
crystal packing: faceꢀtoꢀface, wavelike, crossing, and mixing (Figure 1).7 Thereinto, faceꢀtoꢀface
and wavelike crystal stackings are the most satisfactory types due to their high crystal densities
and good molecular stabilities. For example, 1,3,5ꢀtriaminoꢀ2,4,6ꢀtrinitrobenzene (TATB) with
the faceꢀtoꢀface style shows a high density (1.93 g cmꢀ3) and is very insensitive (IS: 50 J, FS: 360
N).8 Another example, 1,1ꢀdiaminoꢀ2,2ꢀdinitroethene (FOXꢀ7) has the wavelike stacking,
thereby exhibiting an excellent mechanical sensitivity (IS: 60 J, FS: >360 N).9ꢀ10 Unfortunately,
TATB and FOXꢀ7 do not have favorable detonation performance due to their negative heats of
formation. Their enthalpies are as low as ꢀ139.3 kJ/mol and ꢀ53.1 kJ/mol, respectively. Thus,
their detonation velocities (TATB: 8144 m sꢀ1; FOXꢀ7: 8930 m sꢀ1) are less than the secondary
explosive HMX (9144 m sꢀ1). This greatly limits their practical applications.
Figure 1. Four types of crystal packing in the field of energetic materials.
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To avoid above drawback, an effective strategy is to introduce nitrogenꢀrich backbones (such
as, triazole, tetrazole, and oxadiazole) into the design of HEDMs owing to their high heats of
formation.11ꢀ13 With our continuing interest, herein we report the synthesis, structure and
energetic performance of 1,2ꢀbis(3ꢀ(4ꢀnitroꢀ1,2,5ꢀoxadiazolꢀ3ꢀyl)ꢀ1,2,4ꢀoxadiazolꢀ5ꢀyl)diazene
(5), which shows the faceꢀtoꢀface crystal stacking and a relatively high enthalpy. Furthermore,
quantum chemical calculations are employed to better understand the intrinsic structureꢀproperty
relationship.
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Scheme 1. The synthetic pathway of 5.
Synthesis
The synthetic pathway is shown in Scheme 1. Compounds 1ꢀ3 were synthesized according to
the literature.14 Bicyclic compound 4 was obtained by treating 3 with 50% H2O2, 98% H2SO4
and Na2WO4 in high yield (about 85%) (different from the reported method in Ref. 14). Through
a direct oxidative coupling reaction of 4, the target molecule 5 was formed over a period of 4 h at
the ambient temperature. Compounds 4 and 5 were well characterized by infrared (IR) and
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multinuclear nuclear magnetic resonance (NMR) spectroscopy, elemental analysis, differential
scanning calorimetry (DSC) and singleꢀcrystal Xꢀray diffraction analysis.
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Notably, starting from commercially available material malononitrile, the synthesis process
could be divided into four steps. All these reagents used are cheap and the overall yield is as high
as 40.33%. The preparation conditions (reaction temperature: ꢀ5 ꢀ 100 °C and reaction pressure:
ambient pressure) are mild. Additionally, the target compound 5 is nonhygroscopic and exhibits
a very poor solubility in water. These positive results make it competitive in practical
applications.
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Structure discussions
In general, the planarity of a molecule is closely related to its crystal packing.5ꢀ7 Compound 4
crystallizes in the monoclinic space P 21/c with a low density of 1.738 g cmꢀ3 at 296K (different
from the Ref. 14 (150 K)) (Figure 2). The two oxadiazole rings of 4 are nonꢀcoplanar,
confirming by the dihedral angle of N4ꢀC3ꢀC1ꢀN2 (ꢀ134.85°). Thus, the crystal packing of 4 is
not faceꢀtoꢀface or wavelike.
Figure 2. (a) ORTEP representation of molecular structure of 4. Displacement ellipsoids are
shown at 50% probability level. (b) Crystal packing of 4 viewing down the unit cell axis a.
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Crystal 5 suitable for single crystal Xꢀray diffraction was obtained by slow evaporation from
dimethyl sulfoxide (DMSO) at room temperature. It crystallizes in the orthorhombic space group
P bca with a relatively high density of 1.916 g cmꢀ3 at 296K (Figure 3). In comparison to 4,
compound 5 is more coplanar, which is evident from the dihedral angles of N3ꢀO2ꢀC3ꢀN6
(179.61°) and N2ꢀC2ꢀC4ꢀN3 (165.72°). The azo double bond of 5 is 1.248 Å, in accord with the
general bond distance for N=N (1.20 Å).15
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Figure 3. ORTEP representation of molecular structure of 5. Displacement ellipsoids are shown
at 50% probability level.
Further, the crystal packing diagrams of 5 observing from the aꢀc axes are shown in Figure 4,
which reveal the structural details from three directions. Along the b and c axes, the tetracyclic
compound 5 features a basic faceꢀtoꢀface stacking. Recent studies indicate that faceꢀtoꢀface
stacking often possesses an excellent mechanical performance owing to free interlayer sliding,
such as TATB.16ꢀ17 Regarding the aꢀaxis, 5 is wavelike πꢀstacked which is like the lowꢀsensitivity
explosive FOXꢀ7. With such an outstanding crystal performance, it is not surprising that 5
demonstrates desirable energetic properties, such as high density, good impact and friction
sensitivities (ρ = 1.92 g cmꢀ3, IS = 18 J, FS = 220 N), which is superior to the benchmark
secondary explosive HMX (ρ = 1.91 g cmꢀ3, IS = 7 J, FS = 120 N).
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Figure 4. (a) Crystal packing of 5 viewing down aꢀaxis and its simplified drawing. (b) Crystal
packing of 5 viewing down bꢀaxis and its simplified drawing. (c) Crystal packing of 5 viewing
down cꢀaxis and its simplified drawing (Blue line: oxadiazole rings; Red line: NO2; Yellow star:
hot spot).
Energetic properties
In addition to crystal analysis, its physical properties (such as thermal stability, density, and
detonation performance) were also measured (Table 1). The thermal stability is an important
physical factor of safety level. The melt point and decomposition temperature of 5 were
confirmed by DSC at a scan rate of 5 °C minꢀ1. Compound 5 melts at 157 °C and has a high
decomposition point at 256 °C, which is higher than RDX (204 °C).
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Table 1. Physiochemical properties of 4ꢀ5, HMX, RDX, TNT, FOXꢀ7, and TATB.
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Compound
RDXk
–
HMXk
–
TNTl
81
FOXꢀ7l TATBl
4
5
a
Tm [°C]
–
157
–
–
9
b
Td [°C]
89
1.74
74.7
ꢀ8.1
269.8
8155
27.3
35
256
1.92
75.5
0
204
1.80
81.1
0
280
1.91
81.1
0
295
1.65
60.8
ꢀ18.6
ꢀ59.4
7303
21.3
15
274
1.89
81.1
0
324
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ρc [g cmꢀ3]
N+Od [%]
OBcoe [%]
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69.8
ꢀ18.6
ꢀ139.3
8144
31.2
50
ꢁHf [kJ molꢀ1]
962.4
9240
37.5
18
70.3
8795
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7
70.4
9144
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7
ꢀ53.1
8930
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f
Dg [m sꢀ1]
Ph [GPa]
ISi [J]
FSj [N]
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220
120
120
>353
>360
360
[a] Melting point. [b] Thermal decomposition temperature (onset) under nitrogen (determined by
the DSC exothermal peak, 5 °C minꢀ1). [c] Xꢀray densities at 296K and the corresponding RT
Values using the equation (ρ298k=ρT/(1+av(298ꢀT0)); av=1.5×10ꢀ4 Kꢀ1). [d] Content of nitrogen and
oxygen. [e] Oxygen balance [%] Based on CO for CaHbNcOd: OB[%]=1600×(dꢀaꢀb/2)/MW. [f]
Calculated molar enthalpy of formation in solid state. [g] Detonation velocity. [h] Detonation
pressure. [i] Impact sensitivity. [j] Friction sensitivity. [k] Ref. [3]. [l] Ref. [10].
Interestingly, nitroꢀbased compound 5 does not have intraꢀ and intermolecular hydrogen
bonds, however, it still has such a remarkable thermal behavior. The single crystal data indicates
two possible explanations, trigger bond dissociation enthalpy (TBDE) and noncovalent
interaction analysis (NCI).18ꢀ19 The TBDE represents the lowest energy to break the first
chemical bond during thermal decomposition, in this case, calculations show that the Cꢀnitro
bond is the weakest. The TBDE of 5 is as high as 295.6 kJ molꢀ1, which significantly
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outperforms the powerful explosive RDX (152.8 kJ molꢀ1). Meanwhile, the NCI plots were
evaluated by using Multiwfn20 (Figure 5). Compound 5 shows edgeꢀtoꢀface πꢀπ stackings
between two independent molecules, thereby increasing the thermostability and the density.
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Figure 5. Noncovalent interaction analysis, including hydrogen bonds (no signal in this case)
and πꢀπ interactions for 5 (blue, strong attraction; green, weak interaction; and red, strong
repulsion).
Density is an important physical factor of energy level. The density of 5 was measured by
using single crystal analysis at 296K and converted into the corresponding RT values using the
equation (ρ298k=ρT/(1+av(298ꢀT0)); av=1.5×10ꢀ4 Kꢀ1).21 Tetracyclic compound 5 shows a
remarkable density at 1.92 g cmꢀ3 (a measured density of 1.9151 g cmꢀ3 determined by a gas
pycnometer), which is higher than that of HMX and FOXꢀ7 (1.91 cmꢀ3 and 1.89 cmꢀ3,
respectively). This value is due to its prominent crystal packing (Figure 4), extensive
noncovalent interactions between molecules (Figure 5) and high packing coefficient. Its packing
coefficient was evaluated by PLATON and is up to 74.3%, which is slightly lower than TATB
(78.1%).
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Another physical factor of energy level is heat of formation. Generally, higher heats of
formation always result in better detonation performance. The enthalpy of 5 was calculated based
on compatible isodesmic reactions by using Gaussian0922 program (more details in the
Supporting information). As anticipated, due to the existence of extensive highꢀenergy NꢀN, CꢀN
and NꢀO bonds, 5 exhibits a high positive enthalpy at 962.4 kJ/mol (a calculated enthalpy of
903.86 kJ/mol determined by the bomb calorimetry), greatly superior to FOXꢀ7 (ꢀ53.1 kJ/mol)
and TATB (ꢀ139.3 kJ/mol). With the enthalpy and calculated density in hand, the detonation
performance was determined by using EXPLO5.23 As for detonation velocity, the value of 5
(9240 m sꢀ1) remarkably exceeds HMX (9144 m sꢀ1) and FOXꢀ7 (8930 m sꢀ1). In terms of
detonation pressure, the value of 5 is observed at 37.5 GPa, which is slightly lower than HMX
(39.2 GPa) while transcends RDX (34.9 GPa) and FOXꢀ7 (34.0 GPa). Furthermore, on the basic
carbon monoxide (CO), 5 exhibits an oxygen balance value of 0%, equal to HMX (0%) and
FOXꢀ7 (0%).
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The mechanical sensitivity of 5 was measured to evaluate its safety to external stimuli.24 The
IS and FS of 5 are estimated to be 18 J and of 220 N, respectively. These values are much higher
than those of HMX (IS: 7 J, FS: 120 N) and comparable to TNT (IS: 15 J, FS: >353 N), which
further suggests its promising application potential.
To obtain a better understanding of good mechanical properties, the twoꢀdimensional (2D)ꢀ
fingerprint of crystal and the associated Hirshfeld surfaces were employed to show the
intermolecular interactions.25 Generally, blockꢀshaped surfaces contributes more efficiently to
faceꢀtoꢀface π–π stacking and lead to lower IS. These noncovalent interactions are attributed by
red spots on the Hirshfeld surfaces.26 As for 5, the molecule presents a nearly planar structure
and a plate shape (Figure 6a). At the same time, most red dots locate on the surface edges. With
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respect to those red dots on the plate faces, they denote π–π stacking, such as CꢀN and OꢀC
interactions (about 22.8%) (Figure 6b, 6c). The N-N interactions is about 20.7%, which
demonstrates the weak interactions between molecules. Moreover, the ratio of NꢀO interactions
is as high as 39.4%, which reflects in high percentage of interlayer contact, thereby improving
the mechanical stability.
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Figure 6. (a) Hirshfeld surfaces calculation of 5 (white, distance d equals the van der Waals
distance; blue, d exceeds the van der Waals distance; red, d is less than van der Waals distance).
(b) Twoꢀdimensional fingerprint plots in crystal stacking for 5 as well as the associated Hirshfeld
surfaces. (c) The individual atomic contact percentage contribution to the Hirshfeld surface.
Conclusion
In conclusion, a promising energetic compound featuring faceꢀtoꢀface and wavelike crystal
stackings was prepared. TBDE, NCI analysis and the Hirshfeld surfaces analysis were studied to
better understand its intrinsic structureꢀproperty relationship. Tetracyclic compound 5 exhibits
excellent thermostability (Td = 256 °C), desirable detonation performance (D: 9240 m sꢀ1, P: 37.5
GPa) and moderate mechanical sensitivity (IS: 18 J, FS: 220 N). Additionally, the preparation of
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5 is within four steps from commercially available reagents in high overall yields. All these
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positive contributions highlight its potential application as a nextꢀgeneration energetic material.
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ASSOCIATED CONTENT
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CCDC 1845822 and 1851593
AUTHOR INFORMATION
Corresponding Author
*Ming Lu, Email: luming@mail.njust.edu.cn.
Present Addresses
†If an author’s address is different than the one given in the affiliation line, this information may
be included here.
Author Contributions
Q. Wang and M. Lu designed the study. Q. Wang performed most of the reactions. Y. Shao
synthesized part of the precursor. Q. Wang finished the DFT calculations. Q. Wang and Y. Shao
prepared the manuscript.
ACKNOWLEDGMENT
This work is supported by the National Natural Science Foundation of China (NSAF, NO.
U1530101).
REFERENCES
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A promising energetic compound featuring faceꢀtoꢀface and wavelike crystal stackings was
prepared. This compound exhibited high crystal density, excellent thermal stability, high
detonation velocity, and good sensitivities, which makes it a potential insensitive highꢀenergyꢀ
density Material.
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