Nitramino-functionalized tetracyclic oxadiazoles as energetic materials with high performance and high stability: crystal structures and energetic properties

Article abstract of DOI:10.1039/c8ce00857d

A series of tetracyclic energetic materials were developed through the introduction of 1,2,4-oxadiazole into nitramino-1,2,5-oxadiazole. All these thirteen new compounds were fully characterized and seven of them were further confirmed by single crystal X-ray diffraction. Salt formation led to planarization of the parent structure, reduction in lengths, increase in bond-dissociation energies of N-NO₂ bonds, and formation of hydrogen bonds, which significantly increased decomposition temperatures from 84 °C (the neutral compound) to 187-303 °C (the energetic salts). Thus, the increase in thermal stability falls in the range of 103-219 °C. To the best of our knowledge, 219 °C is the largest reported increase in thermal stability due to salt formation. In addition, electrostatic surface potentials and noncovalent interactions (π-π stacking) were analyzed to understand structure-property relationships of these compounds. Both theoretical calculations and practical explosive experiments indicate that the dihydroxylammonium salt exhibits better detonation performance than the powerful explosive RDX. The tetracyclic backbone provides these compounds with enhanced stability. Short synthesis steps, low cost, good thermostability, excellent detonation performance, and acceptable mechanical sensitivity highlight the practical applications of these energetic compounds.

Full text of DOI:10.1039/c8ce00857d

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DOI: 10.1039/C8CE00857D
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ARTICLE
Nitramino-functionalized Tetracyclic Oxadiazoles as Energetic
Materials with High Performance and High Stability: Crystal
Structures and Energetic Properties
Received 00th January 20xx,
Accepted 00th January 20xx
Qi Sun, Qiuhan Lin*, and Ming Lu*
DOI: 10.1039/x0xx00000x
www.rsc.org/
A series of tetracyclic energetic materials are developed through the introduction of 1,2,4-oxadiazole into nitramino-1,2,5-
oxadiazole. All these thirteen new compounds were fully characterized and seven of them were further confirmed by
single crystal X-ray diffraction. Salt formation leads to the planarization of the parent structure, the shortening in length
and increase in bond-dissociation energies for N-NO2 bonds, and the formation of hydrogen bonds, which obviously
increases the decomposition temperatures from 84 °C (the neutral compound) to 187-303 °C (the energetic salts). The
increases fall into the range of 103-219 °C, in which, to the best of our knowledge, 219 °C is the largest reported increase
in thermal stability due to salt formation. In addition, electrostatic surface potentials and noncovalent interactions (π-π
stacking) are analyzed to understand the structure-property relationships of these compounds. Both theoretical
calculations and the practical explosive experiment indicate the dihydroxylammonium salt exhibits better detonation
performance than powerful explosive RDX. The tetracyclic backbones promises these compounds with enhanced security.
The short synthetic steps, low cost, good thermostability, excellent detonation performance, and acceptable mechanical
sensitivities highlight the practical applications of these energetic compounds.
excellent detonation performance (D: 9376 m s-1, P: 40.5
Introduction
GPa).7 In addition, 3,3’-dinitramino-4,4’-bifurazan (
b in Scheme
1) includes two 1,2,5-oxadiazole rings, it still possesses good
detonation performance (D: 9086 m s-1, P: 40.3 GPa).8
High-energy-density materials (HEDMs) are widely used in
military and civilian applications as explosives, propellants, and
pyrotechnics.1 Preparing more powerful, safer, and greener
energetic materials is a key target of researchers around the
world, and the key goal is to balance between the opposing
properties of high performance and high stability.2
However, the mono, bi, and tricyclic nitramino-furazans
show undesirably high sensitivities. For example,
a and b have
high impact sensitivity (IS) of <1 J and 1.5 J, respectively, and
friction sensitivity (FS) of <5 N and 48 N, respectively.
Especially, the tricyclic compound
c
is even unstable in air.9
Combining various energetic groups with different
backbones is a useful strategy for designing new energetic
molecules with tailorable properties.3 Among the candidate
energetic groups, the nitramino group has exceptional
properties, which can impart outstanding detonation
performance to energetic compounds,4 while, among the
heterocyclic rings, 1,2,5-oxadiazole (furazan) is a promising
candidate owing to its high nitrogen and oxygen contents.5
Therefore, nitramino-functionalized furazans are an effective
route toward high-performance energetic materials.6 3,4-
These greatly limit their practical applications. Thus, nitramino-
furazans with relatively higher mechanical and thermal
stabilities are highly sought. In addition, although there are
many reports on mono, bi, and tricyclic nitramino-furazans,
the tetracyclic nitramino-furazans are rarely reported.10
Recent years, introducing different heterocyclic rings into
Di(nitramino)furazan (
a
in Scheme 1), which consists of two
1,2,5-oxadiazole ring, exhibits
nitramino groups and
a
School of Chemical Engineering, Nanjing University of Science and Technology.
Xiaolingwei 200, Nanjing, Jiangsu, China. E-mail: linqh@njust.edu.cn;
luming@njust.edu.cn
Scheme 1. Comparision of the mechanical stabilities of nitramino-
† Footnotes relaꢀng to the ꢀtle and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
functionalized oxadiazoles with or without 1,2,4-oxadiazole.
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DOI: 10.1039/C8CE00857D
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Journal Name
H2N
OH
O
N
NH2
N
O
N
H2N
N
Diethyl oxalate
NaNO2/HCl
NH2OH
N
N
NC
CN
N
N
N
O
N
N
O
H2N
N
N
O
1
NO2
NO2
O
O
N
N
HN
O
O
N
N
N
HNO3
N
N
Bases
N
N
N
N
N
N
O
O
NH
N
N
O
N
O
O2N
O2N
2
2
3-14
2M or M
Cations:
Figure 1. Crystal structure of 2·2H2O.
N
NH2
NH2
NH2
H2N
NH2
NH3NH2
NH4
NH3OH
H2N
N
H
NH2
HN NH
H2N
3
4
5
6
7
8
NH2
N
NH2
H
N
N
O
N
H2N
H3N
NH3
NH3
HN NH
N
H
N
H
N
NH
12
K
N
NH
Na
13
9
10
11
14
3-10: Monovalent cations
11-12: Divalent cations
13-14: Metal cations
Scheme 2. Synthesis routes to compounds 2-14.
the structures has been an effective method to modify the
energetic properties.11 In this study, 1,2,4-oxadiazole was
introduced into the nitramino-furazan, and
a series of
nitramino-functionalized furazans with tetracyclic backbones
are explored.
Preparation
As shown in Scheme 2, 3-amino-4-amidoximinofurazan was
prepared through the reaction of malononitrile, sodium nitrite,
and hydroxylamine; this was then reacted with diethyl oxalate
Figure 2. (a) Crystal structure of
5, (b) Packing diagram of 5 viewed
down the b axis. The sky-blue lines indicate hydrogen bonds.
to obtain compound 1 12
at 0 °C to obtain compound
were prepared in high yields via simple acid-base reactions.
.
1
was then reacted with fuming HNO3
. Twelve energetic salts based on
2
Table 1. Hydrogen bonds for 5.
2
D-H···A
D-H/Å
0.826
0.920
0.920
0.915
0.929
H···A/Å
1.849
2.302
2.407
2.019
1.982
D···A/Å
2.674
3.078
3.079
2.898
2.815
D-H···A/°
178.10
141.88
129.92
160.43
148.16
These salts can be divided into three types according to their
cations, viz., monovalent cations, divalent cations, and metal
cations. These compounds were fully characterized using
infrared spectroscopy, 1H and 13C spectroscopy, and elemental
analysis.
O5-H5···O3
N7-H7A···N1
N7-H7A···N5
N7-H7B···N3
N7’-H7C’···O3’
It is worthy to note that all the reagents are commercially
available and cheap. Furthermore, the synthesis conditions are
mild and the product purification is very simple. These
contributes impart these compounds with considerable
potential for practical applications.
there are four molecules in an asymmetric unit. As can be seen
from Figure 1, the whole molecule is symmetric, in other
words, the left part is equal to the right part. Four oxadiazole
rings are almost coplanar, which is evident from the C3-C4-C1-
N2 (170.45°) and O1-C2-C2’-O1’ (-180.00°) torsion angles. The
N6-N5-C3-C4 (-69.03°) torsion angle indicates the nitro group
is out of the oxadiazole plane.
Crystal structure analysis
Crystals of selected compounds are obtained to confirm the
accuracy of the molecular structures and to explore the
potential relationship between the molecular structures and
physicochemical properties. Crystals were prepared through
Crystal structure and packing diagram of the monovalent
cation organic salt
5
are presented in Figure 2. Different with
, the dihydroxylammonium salt (5)
the parent compound
2
slow evaporation in related solutions (2·2H2O in a mixture of
crystallized in the monoclinic crystal system with the P21c
space group, and there are two molecules in an asymmetric
unit. The whole salt is symmetric too, and the four oxadiazole
methanol and water; 6·2H2O, 10, 11·H2O, and 14 in water)
5,
9,
and were tested at 173 K. CCDC numbers of these crystals are
1816899-1816905 and details of crystallographic data are
provided in supporting information.
is more coplanar than 2, which can be seen from the C3-C4-C1-
N2 (177.35°) and O1-C2-C2’-O1’ (-180.00°) torsion angles. In
addition, the salt formation also makes the nitro group nearly
coplanar with the oxadiazole plane, the torsion angle of N6-
With two crystal water, compound
2 crystallized in the
orthorhombic crystal system with the Pbca space group, and
2 | J. Name., 2012, 00, 1-3
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N5-C3-C4 is 175.78°. As is shown in Figure 2(b), there are five are planar structures and symmetric. Due to the difference of
pairs of hydrogen bonds were found in the packing system. All cations and crystal waters, they exhibit different crystal
hydrogen atoms in the hydroxylamine cation form hydrogen densities, and the values are 1.774, 1.790, 1.762, and 1.827 g
bonds with acceptor atoms (N and O) in the anion. The bond cm-3, respectively.
lengths and angles are provided in Table 1. The shortest
Different with organic salts, the inorganic dipotassium salt
distance of hydrogen···acceptor (H···A) in this salt is H5···O3, 14 (Figure 4) crystallized in the Triclinic P-1 space group, and
and the length is 1.849 Å, which is shorter than the average there is one potassium cation and half anion in each
shortest H···A distance (1.893 Å) in reported energetic salts. asymmetric unit. The crystal density at 173 K is 2.043 g cm-3,
Such a short length of the hydrogen bond reveals the close which is obviously higher than those of organic salts (1.762-
packing of the cations and anions, which then leads to a high 1.880 g cm-3). Each anion is coordinated with ten potassium
crystal density of 1.880 g cm-3. Furthermore, these extensive ions, while each potassium cation is coordinated with five
hydrogen-bonding interactions will also contribute to the anions. The bond lengths of K-N and K-O fall in the range of
improvement of thermal stability and mechanical sensitivities.
Molecular structures of other organic salts, including 14 exhibit a 3D-metal-organic framework (Figure 5b), which
monovalent cation salts ·2H2O, , and 10 and divalent cation promotes the increase of thermal stability and density.
2.665-3.265 Å. The whole packing system indicates compound
6
9
salt 11·H2O, are presented in Figure 3, while the packing
diagrams with hydrogen bonds are presented in supporting
information. It is interesting that all these four salts crystallized
in the same monoclinic crystal system, and the same P21c
Thermal stability
Thermal stability is a key property for energetic materials. In
this study, this was measured using differential scanning
calorimetry (DSC) with a ramp rate of 5 °C min-1. It should be
noted that all the sample were put in the oven at 60 °C for 12
h to make sure that there is no crystal water remained. Values
for decomposition temperatures are provided in Table 2 and
space group with compound 5. Also, the anions in these salts
DSC plots for selected compounds (
provided in Figure 5. None of the compounds melt before
decomposition. Although compound has a decomposition
2, 5, 7, and 13) are
2
temperature of 84 °C, its salts have much better thermal
stabilities (Td: 187-303 °C). Among the organic salts, the
dihydroxylammonium salt (
5
) has the lowest decomposition
) has
temperature (Td: 187 °C), while the diguanidinium salt (
7
the highest decomposition temperature (Td: 263 °C). Both
metal salts have higher thermal stabilities (Td: 274 °C and 303
°C) than the organic salts (Td: 187-303 °C).
Figure 3. (a), (b), (c) and (d) Crystal structures of
11·H2O.
6·2H2O, 9, 10, and
In many cases, salt formation can improve the
thermostability of energetic compounds. In this study, the
increases in decomposition temperature are in the range of
Figure 4. (a) Coordination environment of 14, (b) Packing diagram of 3D
metal-organic-framework of 14.
Figure 5. DSC plots for compounds 2, 5, 7, and 13.
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