5,6-Fused bicyclic tetrazolo-pyridazine energetic materials

Article abstract of DOI:10.1039/c9cc08782f

This study presents the synthesis of 5,6-fused bicyclic conjugated energetic compounds through a combined strategy of anchoring the catenated nitrogen-atom chain and introducing vicinal C-amino and C-nitro groups into a tetrazolo-pyridazine ring. Their crystal structures were confirmed by single crystal X-ray diffraction. Both compounds display good thermal stability, high energetic properties and low sensitivities as energetic materials.

Full text of DOI:10.1039/c9cc08782f

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S. J. Connon, M. O. Senge et al.
Conformational control of nonplanar free base porphyrins:
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5,6-Fused Bicyclic Tetrazolo-Pyridazine Energetic Materials
Sitong Chena, Yuji Liua, Yongan Fenga, Xianjin Yangb and Qinghua Zhang*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
This study presents the synthesis of 5,6-fused bicyclic conjugated these fused explosives, vicinal C-amino and C-nitro groups were
energetic compounds through a combined strategy of anchoring found to play an important role in both increasing thermal stability
the catenated nitrogen-atom chain and introducing vicinal C- and decreasing sensitivities through inter- and intramolecular
amino and C-nitro groups into a tetrazolo-pyridazine ring. Their hydrogen bonding. For example, 4-amino-3,7,8-trinitropyrazolo[5,1-
crystal structures were confirmed by single crystal X-ray c][1,2,4]triazine (PTX) and 4-amino-3,7-dinitro [1,2,4]triazolo[5,1-
diffraction. Both compounds display good thermal stability, high c][1,2,4]triazine (DPX-26 or TTX) displayed better energy and
energetic properties and low sensitivities as energetic materials.
safety properties than that of RDX1, 19-20. 4,7-Diamino-3,8-dinitro-
pyrazolo[5,1-c][1,2,4]triazine had a high decomposition temperature
(355°C)21. Moreover, it has been recognized that the conjugated
fused rings containing catenated nitrogen-atom chains often have
high heats of formation and high detonation performances18. Two
typical examples are 1,2,9,10-tetranitrodipyrazolo[1,5-d:5’,1’-
f][1,2,3,4]tetrazine and 2,9-dinitrodi([1,2,4]triazolo)[1,5-d:5',1'-
f][1,2,3,4]tetrazine, both of which had high heats of formation (758
kJ·mol-1 and 787 kJ·mol-1)18, 22. Another two examples with high
detonation velocities (vD) and pressures (P) are mono- and dioxide
of 6-aminotetrazolo[1,5-b][1,2,4,5]tetrazine (vD >9300 m·s-1, P >35
The development of high-energy and low-sensitivity energetic
materials is a long-term objective from the academic community in
materials science1. The main challenge in this area is how to
overcome the energy-and-safety contradiction in the design of high-
performing explosive molecules2-5
. Recently, many energetic
material researchers have focused on the development of nitrogen-
rich fused-ring energetic compounds due to their promising
properties and performances6-14. As compared to traditional high
explosives such as cyclo-1,3,5-trimethylene-2,4,6-trinitamine (RDX)
and 1,3,5,7-tetranitrotetraazacyclooctane (HMX), these fused-ring
energetic compounds are often flat conjugated molecules that tend to
display high density and high thermal stability through strong
hydrogen bonding and π-π interactions15-16. The crystal packing
driven by both inter- and intramolecular hydrogen bonding as well as
π-π interactions allows the sliding and compression between their
molecular layers, thereby decreasing their mechanical sensitivities
toward mechanical stimuli17. In addition, the high nitrogen content
of N-heterocyclic fused ring skeleton ensures the large amount of
N=N and N-N bonds in the molecular structure, which contributes a
high positive heats of formation for the target energetic molecules18.
Thus, the development of fused-ring energetic materials is thought to
be a promising strategy to reconcile the energy-safety contradiction,
achieving a good balance between high energy, good thermal
stability and low sensitivities in the next-generation energetic
materials.
GPa)23-24
. Based on above analyses, the design strategy of
integrating nitrogen-atom chains and vicinal C-amino/C-nitro groups
into nitrogen-rich fused rings should be a promising way to reduce
the energy-and-safety contradiction of energetic molecules.
In recent years, encouraging advances in development of fused-
ring energetic materials have been occurring at a rapid rate1, 18-24, and
various conjugated fused-ring explosives have been synthesized. In
Scheme 1 Synthesis of 1, 2 and 3
a Institute of Chemical Materials, China Academy of Engineering Physics (CAEP),
Mianyang, 621900, P. R.China. E-mail: qinghuazhang@caep.cn.
With these molecular features as guidelines for the design of
new fused-ring explosives, we now wish to report the synthesis of
two 5,6-fused tetrazolo-pyridazine energetic molecules, viz. 6,8-di-
amino-7-nitrotetrazolo[1,5-b]pyridazine (1) and 8-amino-6,7-
b
School of Chemistry and Molecular Engineering, East China University of
Science and Technology, Shanghai, P. R.China.
†
Electronic supplementary information (ESI) available: CCDC 1947089,
1947090, 1952870 and 1952871. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/x0xx00000x
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dinitro-tetrazolo[1,5-b]pyridazine (2). We began with the nitration of
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DOI: 10.1039/C9CC08782F
3,6-di-
Fig. 1 (a) Molecular structure of 1; (b) horizontal view of 1; (c) hydrogen bonding networks of 1·0.5H2O; (d) crystal stacking and interlayer
spacing of 1·0.5H2O, colored by symmetry operation; (e) crystal structure of 2; (f) horizontal views of 2; (g) crystal stacking of 2, colored by
symmetry equivalence
chloropyridazin-4-amine (p1) using nitric-sulfuric mixed-acid
system, which produced the nitro intermediate compound: 4-
amino-3,6-dichloro-5-nitropyridazine (p2, 91% yield). Subsequent
amina-tion of p2 with aqueous ammonia yielded 3,5-diamino-6-
chloro-4-nitropyridazine (p3, 95% yield). After that, 6,8-diamino-
7-nitrotetr-azolo[1,5-b]pyridazine (1, 87% yield), was obtained
through the cyclization reaction between p3 and NaN3 in DMF. 8-
Amino-6,7-dinitrotetrazolo[1,5-b]pyridazine (2, 18% yield) was
synthesized through the oxidation of 1 using HOF·CH3CN complex
as oxidant. We also tried to introduce N-oxide into 2 by using
excessive HOF·CH3CN complex but failed. Only trace amount of 3
(<1wt%) was detected by HRMS (Fig. S10). Both the structures of
1 and 2 were characterized by single-crystal X-ray diffraction,
nuclear magnetic resonance (NMR), high resolution mass spectra
(HRMS), elemental analysis (EA), and infrared spectra (IR),
respectively.
Compound 1 crystallizes in C2/c space group and monoclinic
crystal system. It has a co-planar molecular structure (Fig. 1a, 1b)
with all the torsion angles (Table S5) within 6° around 0° or 180°,
which may be attributed to its conjugated fused-ring skeleton and
two strong intramolecular hydrogen bonds N3-H3A···O2 (1.93(3)
Å) and N1-H1B···O1 (1.94(3) Å). Influenced by these strong
intramolecular hydrogen bonds, the bond lengths are 1.333(3) Å for
C1-N1 (C-NH2), 1.423(3) Å for C2-N2 (C-NO2) and 1.324(3) Å for
C3-N3 (C-NH2), which are much shorter than those in the C-
nitro/C-amino compounds without strong intramolecular hydrogen
bonds. Besides, there are face-to-face π-π interactions in its
molecular stacking. The interlayer spacings are 3.027 Å for x, y, z/-
x, -y, -z and 2.915 Å for -x, y, 0.5-z/x, -y, 0.5+z (Fig. 1d), which
are much shorter than that of typical π-stacked porphyrin-porphyrin
stacking in its crystal (3.4 Å)17. Due to these strong inter- and
intramolecular interactions, the packing coefficient of the crystal is
0.7750, even higher than those of the typical explosives TATB
(1,3,5-triamino-2,4,6-trinitro-benzene, 0.7722), RDX (0.7524) and
FOX-7 (1,1-diamino-2,2-dinitroethene, 0.7734). Thus, the mea-
sured density at room temperature of 1 is as high as 1.869 g·cm-3.
Compound 2 crystallizes in P21/c space group and monoclinic
crystal system. There are two stacking patterns in the crystal of 2
(Fig. 1(e~g); named as 2-A and 2-B). Both 2-A and 2-B have co-
planar molecular structures except for the nitro groups connected to
C4 or C8 (Fig. 1f). The lengths of the intramolecular hydrogen
bonds N1-H1A···O1 and N9-H9B···O5 are 2.05 Å and 2.07 Å
(Table S8), which are longer than those in 1. The bond lengths of
C2-NH2 (1.302(4) Å), C3-NO2 (1.424(4) Å), C6-NH2 (1.317(4) Å),
and C7-NO2 (1.439(4) Å) are also shorter than those in the C-
nitro/C-amino compounds without intramolecular hydrogen bonds.
And the bond lengths of C4-NO2 and C8-NO2 are 1.494(4) Å and
1.489(4) Å (Table S6), respectively. Interestingly, 2-A are layered
stacking, while 2-B are wave-like stacked in the rest space (Fig. 1g).
Due to the larger amount of N and O atoms than 1, the density of 2
(1.886 g·cm-3) is higher than 1 although the packing coefficient for
2 is lower (0.7431). In addition, it is the anchoring strategy that
ensures the continuous sites for vicinal C-NH2/C-NO2 forming the
intramolecular hydrogen bonds.
The thermal stability of 1 and 2 was evaluated by differential
thermal analysis (DTA), differential scanning calorimetry (DSC)
and thermogravimetric analysis (TGA). The onset decomposition
temperature (Td, Table 1) of 1 is up to 287°C (Fig. S15), which is a
potential heat-resistant explosive. The Td value of 2 is 202°C (Fig.
S16), which is also acceptable. The apparent activation energies (1:
721.5 kJ·mol-1 > 2: 235.8 kJ·mol-1) were calculated using
2 | J. Name., 2012, 00, 1-3
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Kissinger’s method. No obvious weight loss was observed before
their thermal decomposition (TG-DTG curves: Fig. S17, S18).
detonation pressure (P) are 8899 m·s-1 and 30.3 GPa for 1, and
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9021 m·s-1 and 34.8 GPa for 2, respectively (Table 1). Thus, both 1
DOI: 10.1039/C9CC08782F
and 2 showed higher detonation velocities (vD) than TATB (8114
m·s-1), RDX (8795 m·s-1) and FOX-7 (8800 m·s-1). And the
detonation pressure (P) of 2 is very close to RDX (34.9 GPa) and
FOX-7 (35.0 GPa).
Table 1 Properties of the energetic materials
1
2
TATB
RDX
204
FOX-7
238
Tda (°C)
287
202
360
ρb (g·cm-3)
Ωd (%)
1.869
-65.3
1.940c
-55.8
69.75
-139
8114
32.4
1.800 1.885
-21.6 -21.6
81.06 81.06
1.886c
The impact sensitivities (IS) were measured to be > 40 J for 1
and 18 J for 2 based on a BAM fall hammer. And the friction
sensitivities (FS) were measured to be > 360 N for 1 and 112N for
2 based on a BAM friction tester. Thus, both 1 and 2 displayed the
desired low mechanical sensitivities. The bond dissociation
enthalpies (BDEs) for C-NO2 or N-NO2 were calculated to explain
their low sensitivities, as C-NO2/N-NO2 is the weakest bond in this
kind of compounds27. The higher C-NO2/N-NO2 BDEs usually
correspond to more chemically stable compounds. The BDE for C-
NO2 bond in 1 is 286.0 kJ·mol-1, which is very close to that in
TATB (287.2 kJ·mol-1). This could be used to explain the
insensitivity of 1 towards impact and friction, which is similar to
that of TATB. The lowest BDE for C-NO2 in 2 is 201.1 kJ·mol-1
(201.1 kJ·mol-1 for C4/8-NO2, 249.2 kJ·mol-1 for C3/7-NO2),
which is much higher than that of RDX (145.6 kJ·mol-1). Thus, the
IS of 2 are obviously lower than that of RDX. Table 1 lists the main
properties of 1, 2, TATB, RDX and FOX-7 as energetic materials.
Two-dimensional (2D) fingerprint plots and the associated
Hirshfeld surfaces were employed to study the intermolecular
interactions (Fig. 2). In the crystals of 1, all the red dots are related
to intermolecular hydrogen bonds (N-H···N, N-H···O and O-
H···N), the percentage of which are 48.0% of all the intermolecular
interactions. And the N···N and O···N interactions belong to π-π
interactions, the percentage of which are 6.9% and 15.3%,
respectively. This means hydrogen bonding and π-π interactions
constitute 70.7% of the total weak interactions (Fig. 2d), which is
-35.4
N+O%
73.44 77.86
△fHe (kJ·mol-1)
vDf (m·s-1)
Pg (GPa)
ISh (J)
398
470
9021
34.8
18
70
-134
8800
35.0
>40
8899
30.3
>40
8795
34.9
7.4
>40
FSi (N)
>360
112
>360
120
>360
201.1
249.2
BDEj (kJ·mol-1)
286.0
287.2
145.6 280.2
EKk (kJ·mol-1)
721.5 235.8
-
-
-
a
b
Onset decomposition temperature (10 °C·min-1); Measured density by a gas
c
d
pycnometer at 298 K; Crystal density; Oxygen balance based on CO2 for
CaHbNcOd: Ω(%)=1600×(d−2a−b/2)/Mw;
Detonation velocity calculated using EXPLO5/6.02;
calculated using EXPLO5/6.02; Impact sensitivity evaluated by a standard
BAM fall-hammer; Friction sensitivity evaluated by a BAM friction tester;
Bond dissociation enthalpies for C-NO2 or N-NO2; Apparent activation energy
calculated using Kissinger’s method.
e
f
Calculated heat of formation;
g
Detonation pressure
h
i
j
k
The calculated heats of formation (△fH) in solid phase were
398 kJ·mol-1 and 470 kJ·mol-1 for 1 and 2, respectively (Table 1),
both of which were much higher than those of TATB (-139 kJ·mol-
1), RDX (-70 kJ·mol-1) and FOX-7 (-134 kJ·mol-1). The high △fH
may be influenced by the highly energetic nitrogen-atom chain
anchored in the fused ring23-26. And the detonation performances
were calculated based on the calculated △fH and densities (ρ)
using EXPLO5/6.02 code. The detonation velocity (vD) and
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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Fig. 2 (a) Hirshfield surfaces of 1 and 2; (b) fingerprint plot of 1; (c) fingerprint plots of 2; (d) populations of molecular interactions of 1, 2,
RDX and TATB in crystal stacking
6
7
8
9
Y. Liu, G. Zhao, Y. Tang, J. Zhang, L. Hu, G. H. Imler, D.
A. Parrish, J. M. Shreeve, J. Mater. Chem. A, 2019, 7, 7875.
L. Hu, P. Yin, G. H. Imler, D. A. Parrish, H. Gao and J. M.
Shreeve, Chem. Commun., 2019, 55, 8979.
T. M. Klapötke, P. C. Schmid, S. Schnell and J. Stierstorfer,
Chem. Eur. J., 2015, 21, 9219.
similar to that of TATB (71.9%) and much higher than that of RDX
(58.7%). Besides, the population of the O···O interactions (0.7%)
of 1 is even less than TATB (13.6%). In the crystals of 2, the
population of the O···O interactions are 15.3% for 2-A and 11.7%
for 2-B, which are close to that of TATB (13.6%). However, the
sum population of the O···H and N···H interactions are 32.9% and
30.0%, both of which are less than that of TATB (51.1%) and that
of 1 (48.0%), indicating fewer intermolecular hydrogen bonds in
the crystals of 2. These results are also in agreement with the
experimental result that 2 has the relatively higher impact and
friction sensitivities than 1.
C. Bian, X. Dong, X. Zhang, Z. Zhou, M. Zhang and C. Li,
J. Mater. Chem. A, 2015, 3,3594.
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12 L. Zhai, F. Bi, H. Huo, Y. Luo, X. Li, S. Chen, B. Wang,
Front. Chem., 2019, 7, 559.
In summary, two 5,6-fused bicyclic explosives 6,8-diamino-7-
nitrotetrazolo[1,5-b]pyridazine (1) and 8-amino-6,7-dinitrotetr-
azolo[1,5-b]pyridazine (2) were synthesized and characterized.
Both compounds possessed promising combination properties
including low mechanical sensitivities (IS: >40 J for 1, 18J for 2;
FS: >360 N for 1, 112 N for 2), high detonation performances (vD:
8899 m·s-1 for 1, 9021 m·s-1 for 2; P: 30.3 GPa for 1, 34.8 GPa for
2) and good thermal stability (Td: 287 °C for 1, 202 °C for 2).
Additionally, the strategy of anchoring nitrogen-atom chains in the
tetrazolo-pyridazine ring, in combination with the introduction of
vicinal C-amino and C-nitro groups, has demonstrated great
promise for the research and development of new achieve energetic
materials with a good balance between high energy and good
stability.
13 L. Hu, P. Yin, G. Zhao, C. He, G. H. Imler, D. A. Parrish, H.
Gao and J. M. Shreeve, J. Am. Chem. Soc., 2018, 140,
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Conflicts of interest
There are no conflicts to declare.
23 H. Wei, J. Zhang and J. M. Shreeve, Chem. Asian J., 2015,
10, 1130.
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2732.
Acknowledgement
This work was supported by the Science Challenge Project
(TZ2018004), the National Natural Science Foundation of
China (No.21875228, 21905257), and the China Postdoctoral
Science Foundation (No. 2019M650251).
27 S. Zeman, Struc. Bond., 2007, 125, 195.
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DOI: 10.1039/C9CC08782F
Two 5,6-fused tetrazolo-pyridazine compounds were synthesized and characterized, which
exhibited high thermal stability, excellent energetic property and low mechanical sensitivity.