A novel method to synthesize stable nitrogen-rich polynitrobenzenes with π-stacking for high-energy-density energetic materials

Article abstract of DOI:10.1039/c8cc05413d

Two nitrogen-rich energetic compounds with π-stacking, 1,1′-dichloro-2,2′,3,3′,6,6′-hexanitro-5,5′-dihydroxyazobenzene (1) and 8-(2,4,6-triazido-3,5-dinitrophenyl)-8H-[1,2,3]triazolo[4′,5′:5,6]benzo[1,2-c:3,4-c′]bis([1,2,5]oxadiazole) 1,4-dioxide (2), were prepared by a novel synthesis method, and their structures were determined by single-crystal X-ray diffraction analysis. The decomposition temperature of 1 is 336 °C and 2 exhibits excellent heat of formation of 1160.5 kJ mol^?1 (2.21 kJ g^?1). The condensation reaction of the coupling of a N═N bond and an azido from 1 to 2 was proved to be an efficient method to synthesize benzotriazole. This synthetic strategy for benzotriazole may arouse considerable interest in the area of organic synthesis.

Full text of DOI:10.1039/c8cc05413d

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A novel method to synthesize stable nitrogen-rich
polynitrobenzenes with π-stacking for high-energy-density
energetic materials
Received 00th January 20xx,
Accepted 00th January 20xx
Xiaoming Yang,a Xinyu Lin,‡a Li Yang‡a and Tonglai Zhang*a
DOI: 10.1039/x0xx00000x
www.rsc.org/
among these compounds.9 Azoles that consider as anionic or cationic
Two nitrogenꢀrich energetic compounds with πꢀstacking, 1,1'ꢀ
dichloroꢀ2,2',3,3',6,6'ꢀhexanitroꢀ5,5'ꢀdihydroxyazobenzene (1) and 8ꢀ
(2,4,6ꢀtriazidoꢀ3,5ꢀdinitrophenyl)ꢀ8Hꢀ[1,2,3]triazolo[4',5':5,6]benzo
[1,2ꢀc:3,4ꢀc']bis([1,2,5]oxadiazole) 1,4ꢀdioxide (2), were prepared by
a novel synthesis method, and their structures were determined by
singleꢀcrystal Xꢀray diffraction analysis. The decomposition
temperature of 1 is 336 °C and the 2 exhibits excellent heat of
formation of 1160.5 kJ/mol (2.21 kJ/g). The condensation reaction of
coupling of N=N bond and azido from 1 to 2 was proved to be an
efficient method to synthesize benzotriazole. This synthetic strategy
for benzotriazole may be arouse considerable interest in the area of
organic synthesis.
moieties play a vital role in the synthesis of energetic salts. Many
researchers have synthesized numerous energetic salts through weak
intermolecular interactions to improve the physicochemical
properties. These salts with high energy and low sensitivity are
important in energetic materials.10
In the area of energetic materials, most organic explosives with
excellent performance are nitramine compounds, such as RDX,
HMX et al. However, few organic compounds exhibit good balance
between performance and sensitivity that can be used in military and
civilian applications. Therefore, investigation on novel CHNO based
explosives has significant meanings to material chemists throughout
the world.
By analyzing the structures of reported in a large number of
literatures about explosives so far, we can find out that the high
performance organic energetic materials are mostly polyꢀnitro
compounds. The presence of highly energetic group of azido group
(−N3) and furoxan ring can increase the heats of formation. The
macromonomer can be synthesized by small molecules through the
N=N coupling reaction, and this method has been used to synthesize
normal energetic materials. Meanwhile, the stability of nitro
compounds can be improved by introducing the conjugate systems
(Figure 1).
In the past few decades, increasing research interest has been
focused on the syntheses of the high density energetic materials
(HEDMs). To acquire advanced HEDMs, many famous scientists
such as Prof. Klapötke and Prof. Shreeve have devoted themselves to
design and synthesize nitrogenꢀrich compounds with high energy.1, 2
For modern HEDMs, the high performance materials usually possess
properties of good oxygen balance, superior thermal stability, high
nitrogen content and density, predominant heat of formation, and
release enormous energy during deflagration or explosion.3
Nitrogenꢀrich energetic materials functionalized with energetic
groups (NO2, NHNO2, N3, N=N, etc.) are rich in N–O, N–N, C–N,
N=N bonds as well as extensive hydrogen bonds.4 The energy
contains in these compounds is derived mostly from their high
positive heats of formation, i.e. the energy is correlated to the
number of nitrogen atoms in the compound.5 However, in explosives,
the contradictory nature of high energy and stability, making the
development of new HEDMs become an interesting and challenging
problem.6ꢀ8 The nitrogenꢀrich energetic salts are the hottest topics
In a continuing effort to seek powerful and low sensitive
HEDMs, we are interested in coupling aromatic compounds that
Figure 1 Advantages of poly-nitro compound with multi-nitro, multi-
azide and multi-oxidized furazan ring.
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contain nitro, azido, and furoxan group by N=N linker and fulfill the
design strategies mentioned above. Herein, we synthesized 1,1'ꢀ
dichloroꢀ2,2',3,3',6,6'ꢀhexanitroꢀ5,5'ꢀdihydroxyazobenzene
(1),
which was connected by N=N to increase the stability. The organic
compound 8ꢀ(2,4,6ꢀtriazidoꢀ3,5ꢀdinitrophenyl)ꢀ8Hꢀ[1,2,3]triazolo[4',
5':5,6]benzo[1,2ꢀc:3,4ꢀc']bis([1,2,5]oxadiazole) 1,4ꢀdioxide (2) was
designed and synthesized by combining 1 and sodium azide via
nucleophilic substitution and condensation reaction. These
compounds were characterized by singleꢀcrystal Xꢀray diffraction,
1
infrared spectroscopy, HꢀNMR, 13CꢀNMR, elemental analysis, and
DSC. This method was also proved to be a novel good reaction
yields and high reaction selectivity method to synthesis
benzotriazole. To understand its unique structure, the theoretical
calculation was also used to investigate the unique structure of
compounds 1 and 2.
The synthesis of the first two steps is similar to the method of
Mehilal et al.11 However, the product of the third synthesis step, 1,1'ꢀ
dichloroꢀ2,2',3,3',6,6'ꢀhexanitroꢀ5,5'ꢀdihydroxyazobenzene (1), is
confirmed to a completely novel compound. Interestingly, the nitro
groups are replaced by the hydroxyls and the synthesis of 1 is shown
in Scheme 1. The light yellow product was obtained with a high
yield of 90 %.
Figure 2 (a) The molecular structure of 1, (b) The planarity of 1, (c)
The stacking crystal structure of 1
single crystal Xꢀray diffraction analysis. The spectrum of 13C NMR
indicates that the compound 2 may be a symmetric isomer. The
spectral information, crystallographic data and refinement details can
be found in the electronic supplementary information.
Crystals of 1 and 2 suitable for singleꢀcrystal Xꢀray diffraction
were obtained by slow evaporation method, and 1 was from its
acetone solution and 2 was from acetic acid solution at room
temperature, respectively. The obtained results of 1 show that it
crystallizes in the monoclinic space group P21/c and a calculated
density of 1.933 g/cm3 at 153 K. Its high density may be attributable
to intensive zigꢀzag packing mode and Cl atom as well as hydrogen
bon ds. The molecular structure of 1, perpendicular and lateral views
of the rings demonstrated the planarity of the unit, have been shown
in Figure 2. The molecular unit contains two aromatic rings of which
are linked by N=N forms a center symmetric structure (Figure 2(a)).
Except the oxygen atoms in the nitro, all atoms are coplanar (Figure
2(b)). The length of the N=N bond that connects the aromatic rings
is 1.265(3) Å, which is longer than the normal N=N bond (1.25 Å).
The length of the CꢀN bond that connects the aromatic rings is
1.401(3) Å, which is shorter than the normal C–N single bond (1.490
Å). This phenomenon may be attributed to the conjugation effect.
The stacking crystal structures of 1 show that the molecules are
staggered and vertical arrangement thus ensured the molecular
accumulation intensive (Figure 2(c)).
As the chlorine substituents, hydroxyl groups, and the
nitro groups that near the N=N group are very lively and easy to
react with azido group via nucleophilic substitution. Inspired by
the synthesis method reported by Li and Lü,12 a new simple
synthesis route for the formation of compound
proposed (Scheme 2). The compound was treated with NaN3
in acetic acid solution to synthesize the pure compound . It is
2 has been
1
2
interesting to find that furazan formation only occurs on one
aryl ring. We had considered this issue and conducted further
exploration. We propose possible product structures (3) and
optimize the structure with Gaussian. It was found that the
product is planar structure, and it is difficult to change the
conformation again on the basis of compound
The structures of and were determined by FTꢀIR, Hꢀ
NMR and 13CꢀNMR spectroscopy
elemental analysis as well as
2.
1
1
2
,
Compound 2 crystallizes in the orthorhombic space group Pbca
Scheme 1 Synthesis of 1 from 4-chlorobenzoic acid
Figure 3 (a) A view of the molecular unit of compound 2. (b) The
single vertical arrangement of the molecular unit of compound 2.
(c) The stacking crystal structure of compound 2. (d) The π–π
stacking structure of compound 2.
Scheme 2 Synthesis of 2 from 1
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with a crystal density of 1.877 g/cm3 at 153 K. The asymmetric unit decomposition temperature of 336°C and showed a sharp exothermic
of 2 is presented in Figure 3. The connection structure of two decomposes at 153.0°C and 165.1°C with two intense exothermic
aromatic rings occurred conformational change. Further signals, which implicates the rapid decomposition of the compound
functionalization one of the two benzene rings by construction of the in two steps (see Fig. S9†). Apparent activation energies (Ea),
furoxan rings and construction of the benzotriazole after nucleophilic determined by Kissinger's method14 and Ozawa's method,15 are
substitution (Figure 3(a)). It is interesting to find that all the 126.1 and 155.4 kJ/mol, respectively (see Table S13†), which further
functional group atoms perfectly situate the plane of the ring of illustrate the moderate activity of thermoꢀkinetic of the two
benzotriazole, which nearly mutuallyꢀperpendicular with another compounds.
benzene ring plane (Figure 3(b)). The bond lengths show nitrogenꢀ
The enthalpy of formation (ꢀfH) of 1 and 2 were calculated by
nitrogen bonds of N12–N13 and N13–N14 range in 1.33–1.35 Å, Gaussian 09 (Revision D.01) suite program 30, and were estimated
which are consistent with bond lengths characteristic of aromatic to be ꢀ190.2 kJ/mol (ꢀ0.34 kJ/g) and 1160.5 kJ/mol (2.21 kJ/g),
nitrogenꢀnitrogen bonds (1.35 Å). The azido groups on the benzene respectively (Table 1). Compound 2 has high positive enthalpies of
ring present disordered arrangement and are all outside the plane. formation due to the large number of N−N, NꢀO bonds and two
The stacking crystal structure of 2 shows the benzene rings with furazan backbones. Compare with current used explosives, the heat
azido groups parallel to each other by faceꢀtoꢀface. Meanwhile, the of formation of compound 2 is much higher.
molecules are staggered and vertical arranged in different directions,
With the data of roomꢀtemperature density and heats of
classifying the whole crystal to arrange waveꢀlikeꢀtype stacking formation, the detonation properties16 of 1 and 2 was evaluated by
(Figure 3c)). These arrangements of π–π stacking (Figure 3(d)) using EXPLO 5 6.01 program. As it can be seen in Table 1, the
facilitate the formation of stable conjugated systems, increasing the calculated detonation velocity of compound 1 is 8082 m/s, and
stability of this molecular. The distances between benzene ring detonation pressure is 30.8 GPa. The calculated detonation velocity
centroids of compound 2 (4.01 Å), approximately belong to typical of compound 2 is 8094 m/s, and detonation pressure is 26.5 GPa.
geometrical parameters of aromatic π–π interactions (3.65ꢀ4.00 Å).13 The value of detonation velocity of compound 2 dramatically
The high density of this compound can be rationalized in terms of exceeds TNT (6881 m/s) but lower than RDX (8748 m/s). Obviously,
the intensive packing mode.
although the compounds 1 and 2 have high density and high
Based on the experimental observations, we suggest the enthalpy of formation, the detonation pressure is relatively low. This
following plausible mechanism (Scheme 3). Superficially, the may due to the presence of the benzene rings in the molecule,
annulation reaction implementation is by N=N reacted with −NO2 whereas energetic materials with benzene rings generally have a low
group, however, actually come true by nucleophilic substitution and detonation velocity and detonation pressure.17
condensation reaction. Initially, the product I is probably formed.
The testing impact sensitivity (IS) and friction sensitivity (FS)
Obviously, the −N3 is a deficient electron group and the azo group is values are listed in Table 1. Of the two newly prepared compounds,
a rich electron group that can provide electron, the azo group reacts compound 1 can be deemed as thermallyꢀstable insensitive high
with the adjacent −N3 to form an intermediate III, the compound 2 explosives. The compound 2 exhibits similar sensitivity to RDX,
was obtained in good yield through thermal decomposition in the suggesting compound 2 can serve as promising candidates for
acetic acid.
Thermal stability, which needs first to be concerned, is a
HEDM.
It has been found that analyses of the molecular orbitals
significant element, because the application of energetic materials provides valuable information on its electronic structure, and then
will be limited by an unacceptably low decomposition temperature. helps us understand molecular reactivity and other molecular
The thermal decomposition temperature of compounds 1 and 2 were properties.20 The imbalance between positive and negative regions
determined by differential scanning calorimetry (DSC) determined the stability of energetic material. The regions of
measurements with a heating rate at 5 °C/min (Table 1). Not
Table 1. Physical Properties of 1 and 2 Compared with Those of TNT
and RDX
surprisingly, the 1 exhibited excellent thermal stabilities with
a
e
Td
N b
D c
ΔfH d
-190.2
(-0.34)
VD
P f
IS g
15
FS h
192
1
336
153
240
210
20.3 1.892
8082 30.8
8094 26.5
6881 19.5
8748 34.9
2
48.1 1.837 1160.5
(2.21)
6.6
15
96
TNT
RDX
18.5
1.65
-115
(-0.26)
80
353
120
37.8
1.82
7.4
(0.36)
a
b
c
Thermal decomposition temperature (°C). Nitrogen content (%). The
room-temperature densities was calculated by the volume expansion
ρ
=
ρT (1 + αV( 298 − T)); αV = 1. 5 × 10−4 K−1
18
equation
.
d Calculated heat of
298K
formation [kJ/mol (kJ/g)]. e Detonation velocity (m/s). f Detonation pressure
(GPa). g Impact sensitivity (J). h Friction sensitivity (N). Properties of TNT and
RDX are taken from ref. 3, 13,19
Scheme 3 Proposed mechanism for the formation of 2
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recent literature, to the best of our knowledge there is no report
related to the method. It suggests the future chemistry and
medicine applications might be interested in this method as
synthesis of benzotriazole.
The authors acknowledge financial support from the State Key
Laboratory of Explosion and Technology (Grant No. ZDKT 17ꢀ01)
and the Fundamental Research Funds for the Central Universities
(Grant No. 2017cx10001).
Conflicts of interest
Figure 4 HOMO, LUMO, and ESP of compounds 1 and 2. Color
coding for ESP are from red (negative) to blue (positive).
There are no conflicts to declare.
Notes and references
stronger positive potential indicate the high impact sensitivity in
theory.21 Figure
4 illustrates HOMO, LUMO, and ESP of
compounds 1 and 2.
1
2
3
4
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4445.
It can be seen that the whole chemical bonds in compound 1
participate in HOMO and LUMO levels, indicating that the balance
of the electrons in the molecule, make sure the stabilization of the
skeleton framework. Meanwhile, the ESP indicates that the negative
charges are concentrated in the N=N bond, which corresponds to the
change in the bond length. Inspection of compound 2, the whole
chemical bonds participate in HOMO or LUMO levels, the large
difference in energy between these two orbits indicates the stability
of the substance. The ESP indicates that the charges tend to average,
neither very positive nor very negative potential except few positive
potential distributes on the C–NO2 moiety or azide group. This very
positive potential from which indicates the instability of a compound,
indicating the possible trigger bond may appear here and contribute
to its thermal stability.13
M. Zhang, H. Gao, C. Li, W. Fu, L. Tang and Z. Zhou,
Mater Chem A, 2016,
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compounds with good property were constructed and confirmed
by single crystal Xꢀray diffraction. Structurally speaking, the
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Stephenson reduction reaction. The compound
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may be produced by
possesses
.
1
good oxygen balance and superior thermal stability, can be
considered as a potential thermally stable insensitive explosive.
The compound
2 possesses good nitrogen content, moderate
decomposition temperatures, acceptable sensitivities, excellent
positive heat of formation at 1160.5 kJ/mol (2.21 kJ/g) and
ascendant density of 1.837 g/cm3 that are higher than RDX.
Theoretical performances of detonation velocity and detonation
pressure are 8094 m/s and 26.5 GPa, respectively. Furthermore,
computational results of HOMO, LUMO and ESP also support
13 J. Zhang, Q. Zhang, T. T. Vo, D. A. Parrish and J. N. M.
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17 G. Subramanian, G. Eck, J. H. Boyer, E. D. S. And and M. L.
nitro, azide group, and the oxidized furazan ring simultaneously
in this molecule promote its detonation performance.
Meanwhile, the benzene ring and oxidized furazan ring increase
its thermal stability as well as reduce sensitivity. The larger
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Trudell, J Org Chem, 1996, 61, 5801ꢀ5803.
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rich CNOꢀcompound
N=N bond and azido, which can also be considered as a novel
method with high yield to synthesize benzotriazole. In the
2 has been synthesized by coupling of
A
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