Synthesis of 5-(1H-pyrazol-1-yl)-2H-tetrazole-derived energetic salts with high thermal stability and low sensitivity

Abstract

In this study, a novel energetic compound 3-nitro-1-(2H-tetrazol-5-yl)-1H-pyrazole-4-carboxylic acid (1) and a series of corresponding energetic salts (2–4) were designed and synthesized. Their chemical structures were determined through NMR spectra, elemental analysis, FT-IR, and single crystal X-ray diffraction. All of these energetic compounds displayed low sensitivity to impact (IS > 40 J) and friction (FS > 240 N) and acceptable detonation properties. Compound 1–3 showed high thermal stability, with decomposition temperatures higher than 300 °C. In addition, the structure–property relationship of these compounds was elucidated by crystal stacking analyses combined with energetic performance parameters. In this way, a new approach for the development of novel energetic materials can be provided given the high performance of these energetic compounds.

Full text

Synthesis of 5-(1H-pyrazol-1-yl)-2H-tetrazole-derived energetic salts with
high thermal stability and low sensitivity
Yue Zheng
a
,
b
, Xia Zhao
a
,
b
, Xiujuan Qi
a
,
b
, Kangcai Wang
a
,
*
, Tianlin Liu
a
,
**
a
Institute of Chemical Materials, China Academy of Engineering Physics, Mianyang, 621999, China
b
School of Material Science and Engineering, Southwest University of Science and Technology, Mianyang, 621900, China
ARTICLE INFO
Keywords:
Energetic materials
Low sensitivity
Energetic salts
High thermal stability
ABSTRACT
In this study, a novel energetic compound 3-nitro-1-(2H-tetrazol-5-yl)-1H-pyrazole-4-carboxylic acid (1) and a
series of corresponding energetic salts (2–4) were designed and synthesized. Their chemical structures were
determined through NMR spectra, elemental analysis, FT-IR, and single crystal X-ray diffraction. All of these
energetic compounds displayed low sensitivity to impact (IS >40 J) and friction (FS >240 N) and acceptable
detonation properties. Compound 1–3showed high thermal stability, with decomposition temperatures higher
than 300 C. In addition, the structure–property relationship of these compounds was elucidated by crystal
stacking analyses combined with energetic performance parameters. In this way, a new approach for the devel-
opment of novel energetic materials can be provided given the high performance of these energetic compounds.
1. Introduction
The development of novel advanced energetic materials has been a
high-priority task primarily due to their significant applications in na-
tional defense and aerospace technologies [1–3]. The desirable charac-
teristics of advanced energetic materials include high detonation
performance, high density, excellent thermal stability, good environment
compatibility, and low sensitivity to impact and friction [4–6]. The re-
quirements of being a high energy material yet having low sensitivity to
impact and friction are generally contradictory to each other, making the
development of new high-performance energetic materials a great chal-
lenge [7–10].
In the design of new energetic materials with high energy and low
sensitivity, several efficient strategies have been widely employed, such
as the use of energetic co-crystals [11,12] and energetic metal-organic
frameworks (EMOFs) [13,14]. Recently, the formation of energetic
salts based on nitrogen-rich heterocycles has emerged as another
powerful and straightforward approach to the development of new en-
ergetic materials with desired high energy and low sensitivity [15,16]. In
general, this approach expands the utility of common acidic
polynitro-substituted heteroacycles by including a large series of related
salts. These salts share major parts of their high-energy parent hetero-
cyclic molecules. Meanwhile, cationanion interactions, especially
hydrogen bonding interactions, decrease the salts’sensitivity to thermal
and mechanical stimuli [17,18]. Therefore with this approach, achieving
a good balance between high energy and low sensitivity is practical. A
typical example of the balance between high energy and low sensitivity
compounds is dihydroxylammonium 5,50-bistetrazole-1,10-diolate
(TKX-50), which is a well-known energetic salt with high energy (v
D
:
9696 m/s) and low sensitivity (IS: 20 J) [19]. Its high energy is derived
from a nitrogen-rich bitetrazole skeleton with high positive heat of for-
mation, while its low sensitivity is mainly attributable to extensive
hydrogen bond (N/O⋯H) interactions between the bitetrazole anions
and hydroxylamine cations [19].
There is considerable interest in the formulation of energetic salts
through rational combination of nitrogen-rich bases with acidic energetic
groups such as nitroamine (–NHNO
2
)[20–22], dinitromethyl
(–CH(NO
2
)
2
)[23,24], N-hydroxyl (–NOH) [25,26], and tetrazole
[27–29], which enhance the density and decrease the mechanical
sensitivity. Among the developed energetic salts, nitroamine-derived
energetic salts have been demonstrated to enjoy the advantages of high
density and excellent detonation performance. However, most of
nitroamine-derived energetic salts have significantly high sensitivity,
which limit their practical applications [20–22]. The tetrazole, acting as
both backbone and anion, has great potential for the development of
high-performance energetic materials due to its high nitrogen content
* Corresponding author.
** Corresponding author.
E-mail addresses: wangkangcai@caep.cn (K. Wang), hollandtian@caep.cn (T. Liu).
Contents lists available at ScienceDirect
Energetic Materials Frontiers
journal homepage: www.keaipublishing.com/en/journals/energetic-materials-frontiers
https://doi.org/10.1016/j.enmf.2020.08.004
Received 8 July 2020; Received in revised form 18 August 2020; Accepted 19 August 2020
Available online 6 September 2020
2666-6472/©2020 Institute of Chemical Materials, China Academy of Engineering Physics. Production and hosting by Elsevier B.V. on behalf of KeAi. This is an open
access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Energetic Materials Frontiers 1 (2020) 83–89

and regular planar structure. For instance, Zhang et al. reported a series
of tetrazolate-functionalized energetic compounds with well-balanced
energy and sensitivity [28]. Notably, carboxyl-containing heterocyclic
anions display great promise in the fabrication of traditional
metal-organic frameworks (MOFs) [30,31]. As for the developed func-
tional MOFs containing carboxyl groups, numerous examples have
demonstrated their excellent thermal stability due to their strong coor-
dination ability and extensive hydrogen bond networks [32]. In contrast,
there are limited studies on carboxyl-functionalized heterocyclic com-
pounds in the field of energetic materials. Therefore, introducing
carboxyl groups into energetic anions may be a promising approach to
developing novel high-performance energetic materials.
In the study, a novel carboxyl-containing compound (1) was designed
and synthesized based on the energetic salt 5-(1H-pyrazol-1-yl)-2H-tet-
razole. In the design strategy of this energetic salt, both nitro and
carboxyl groups were introduced into the pyrazole ring to combine with
nitrogen-rich bases to improve the compound’s energy. To adjust the
energy and sensitivity properties of compound 1, an energetic potassium
metal-organic framework (compound 2) and two nitrogen-rich energetic
salts (hydroxylammonium salt 3and hydrazinium salt 4) were prepared
accordingly. Meanwhile, comparative studies on the properties of the
compounds were conducted.
2. Experiments
2.1. Materials and methods
All the chemicals used in this study were obtained from commercial
sources and used “as is”. NMR spectra were measured on a Bruker 400
MHz spectrometer (
1
H NMR at 400 MHz and
13
C NMR at 100 MHz).
Elemental analysis was conducted on a Vario Micro cube elemental
analyzer, and FTIR spectra were recorded on a PerkinElmer Spectrum II
IR Spectrometer. The decomposition temperatures of the energetic ma-
terials were measured by employing DTA strategy on an OZM DTA 552-
Ex differential thermal analyzer at a scan rate of 10 C min
1
. The impact
sensitivity and friction sensitivity were measured on a standard BAM fall
hammer tester and a BAM friction tester, respectively [22]. The
single-crystal X-ray diffraction experiments were performed using the
multi-scan technique at room temperature on a Rigaku Oxford XtaLAB
Synergy/four-circle diffractometer with mirror-mono-chromated Cu K\
α
radiation (λ¼1.54184 Å).
Cyanogen azide is documented as being extremely toxic and
dangerous. Therefore, it was always dissolved in a solvent to attain a
dilute solution during this study. Proper safety precautions were taken
during the preparation, characterization, and handling of all the ener-
getic materials. Lab personnel and the equipment were properly groun-
ded, and protective equipment including protective coat, Kevlar gloves,
and face shield were used.
2.2. Synthesis of 3-nitro-1-(2H-tetrazol-5-yl)-1H-pyrazole-4-carboxylic
acid (1)
BrCN (265 mg, 2.5 mmol) was dissolved in anhydrous CH
3
CN
(10 mL) using an ice bath, and then NaN
3
(650 mg, 10 mmol) was added.
The reaction mixture was stirred for 4 h at 0–5C and then was filtered.
The filtrate was added to the aqueous solution of potassium 4-carboxy-
lato-5-nitropyrazol-1-ide (126 mg, 0.5 mmol, in 4 mL H
2
O). After the
mixture was maintained at room temperature for 24 h, the solvent was
removed in air. The residue was washed with CH
3
CN and cold water
separately. The solid residue was dispersed in 10 mL of concentrated HCl
and stirred at room temperature for 1 h. After the reaction mixture was
filtered and washed with cold water, pure compound 1was obtained as
white powder (63 mg, yield: 56%). FT-IR (KBr,
ν
/cm
1
): 3576, 3450,
3113, 3079, 2977, 1913, 1721, 1705, 1579, 1543, 1384, 1347, 1250,
1214, 1127, 1097, 1022, 960, 842, 765, 742, 595;
1
H NMR (400 MHz,
DMSO‑d
6
)δ: 9.03;
13
C NMR (100 MHz, DMSO‑d
6
)δ: 160.76, 158.19,
155.68, 136.10, 111.35; Elemental analysis, calcd (%): C 26.88, H 1.34,
N 43.55; found: C 26.02, H 1.21, N 42.98.
2.3. Synthesis of potassium 3-nitro-1-(2H-tetrazol-5-yl)-1H-pyrazole-4-
carboxylate (2)
Compound 1(122 mg, 0.5 mmol) was dissolved in 10 mL methanol,
and then KOH (62 mg, 1.1 mmol) was added. The reaction mixture ob-
tained was stirred at 60 C for 1 h and then was cooled to room tem-
perature, filtered, and washed with cold methanol to obtain compound 2
(138 mg, yield: 92%). FT-IR (KBr,
ν
/cm
1
): 3550, 3468, 3416, 3134,
1618, 1548, 1400, 1385, 1340, 1112, 963, 851, 799, 617;
1
H NMR (400
MHz, DMSO‑d
6
)δ: 8.16;
13
C NMR (100 MHz, DMSO‑d
6
)δ: 162.63,
159.99, 155.31, 131.57, 105.48; Elemental analysis, calcd (%): C 19.93,
H 0.33, N 32.54; found: C 18.52, H 0.22, N 31.08.
2.4. Synthesis of hydroxylammonium 3-nitro-1-(2H-tetrazol-5-yl)-1H-
pyrazole-4- carboxylate (3)
Compound 3was synthesized according to the procedure similar to
that of compound 2, except that hydroxylamine water solution (50%, 70
mg, 1.1 mmol) was used to substitute KOH. From this process, the
compound 3was obtained as white solid (134 mg, yield: 82%). FT-IR
(KBr,
ν
/cm
1
): 3422, 3254, 3146, 1876, 1701, 1558, 1483, 1406,
1385, 1363, 1334, 1273, 1209, 1110, 958, 846, 785, 767, 595;
1
H NMR
(400 MHz, DMSO‑d
6
)δ: 8.73, 7.30;
13
C NMR (100 MHz, DMSO‑d
6
)δ:
161.30, 159.46, 154.88, 134.77, 110.91; Elemental analysis, calculated
(%): C 20.62, H 3.12, N 43.29; found: C 20.41, H 2.99, N 42.98.
2.5. Synthesis of hydrazinium 3-nitro-1-(2H-tetrazol-5-yl)-1H-pyrazole-4-
carboxylate (4)
Compound 4was synthesized according to the procedure similar to
that of the compound 2, except that hydrazine hydrate (98%, 110 mg,
2.2 mmol) was used to substitute KOH. It was obtained as yellow solid
(136 mg, yield: 94%). FT-IR (KBr,
ν
/cm
1
): 3478, 3327, 3244, 3139,
3107, 3033, 2985, 2698, 2583, 1614, 1584, 1510, 1398, 1385, 1367,
1344, 1257, 1105, 1082, 995, 850, 799, 768, 535;
1
H NMR (400 MHz,
DMSO‑d
6
)δ: 8.25, 7.42;
13
C NMR (100 MHz, DMSO‑d
6
)δ: 163.82,
159.81, 155.31, 132.07, 118.01; Elemental analysis, calculated (%): C
20.76, H 3.83, N 53.27; found: C 20.02, H 3.42, N 52.84.
3. Results and discussion
3.1. Synthesis
The energetic parent compound 1was synthesized according to the
routes shown in Scheme 1. In the compound 1, one nitro group was
introduced to increase energy and one carboxyl group was employed to
promote the formation of hydrogen bonds with nearby molecules in the
crystals. To further enhance the energy and reduce sensitivity, relevant
EMOF (2) and energetic salts (3and 4) were also prepared. The CCDC
numbers of compounds 1–4were 2002614, 2002615, 2002616, and
2002617, respectively.
3.2. Crystal structures
The crystals of the compound 1used for single X-ray diffraction were
obtained through volatilization of its ethyl acetate solution. 1⋅H
2
O
crystallized in the monoclinic space group P2
1
/c (No. 14), with a crystal
Y. Zheng et al. Energetic Materials Frontiers 1 (2020) 83–89
84

density of 1.732 g cm
3
. There was one energetic molecule and one water
molecule in an asymmetric unit of 1⋅H
2
O(Fig. 1). As for the structure of
energetic molecule of 1⋅H
2
O, all the atoms except oxygen atoms from
nitro groups (O3 and O4) stayed in a plane. In contrast, O3 and O4 hung
on both sides of the pyrazole-ring plane at different dihedral angles,
which were 28.09 (1)
o
and 30.33 (1)
o
for O3–N1–C3–C2 and
O4–N1–C3–N2, respectively (Figure S5).
In the structure of 1⋅H
2
O, each energetic molecule interacted with
two nearby energetic molecules through hydrogen bonds to form an
energetic band-shaped structure with a width of about 10 Å (Fig. 1a).
This band-shaped structure is rarely observed in the energetic materials.
The lengths of N⋯H and O⋯H hydrogen bonds were 1.904 (1) Å and
2.477 (1) Å, respectively. The shorter N⋯H distance indicates the exis-
tence of strong interactions among the nearby energetic molecules. Most
especially, the adjacent parallel energetic bands were connected by water
molecules through hydrogen bonding. Thus a plane-like structure was
formed. Each water molecule interacted with three energetic molecules,
with a hydrogen bond length in the range of 1.555 (1) Å - 2.259 (1) Å
(Fig. 1b). The layered structures were further packed to form the 3D
supramolecular structure of 1⋅H
2
O. The distance between two adjacent
layered structures was about 4.7 Å, as calculated based on the shortest
distance between two atoms that were separately contained in the nearby
layers (Fig. 1c). The unique supramolecular structure and existence of
strong hydrogen bond interactions among the nearby molecules may
have a significant impact on the performance of the compound 1in terms
of energy, sensitivity, and thermal stability.
The crystals of 2⋅2H
2
O were obtained by the volatilization of its water
solution. 2⋅2H
2
O crystallized in the triclinic space group P-1 (No. 2), with
a crystal density of 1.905 g cm
3
. There were one energetic molecule,
two potassium ions, and two water molecules in an asymmetric unit of
2⋅2H
2
O(Fig. 2a). In the energetic molecule, all of the atoms except the
oxygen atoms from carboxyl groups (O1 and O2) stayed in a plane. In
contrast, O1 and O2 hung on both sides of the pyrazole-ring plane at
different dihedral angles, which were 45.33 (1)
o
and 30.33 (1)
o
for
O1–C1–C2–C4 and O2–C1–C2–C3, respectively (Figure S6). These results
are significantly different from those of 1⋅H
2
O.
In the 3D structure of 2⋅2H
2
O, each energetic molecule interacted
with four nearby potassium ions through K–O and K–N bonds, with the
bond length of K–O/N in the range of 2.672 (1) Å - 2.839 (1) Å. The
adjacent four potassium ions were connected with oxygen atoms to form
a{K
4
O
8
} cluster. The bridge oxygen atoms were from four water mole-
cules and four carboxylate groups, with the K–O bond length in the range
of 2.672 (1) Å 2.935 (1) Å. Adjacent clusters were connected by
carboxylate groups to form a ladder-like structure (Fig. 2b). Most espe-
cially, adjacent parallel ladder-like structures were further connected by
the energetic molecules to form the 3D structure of 2⋅2H
2
O(Fig. 2d). If
the energetic molecule is considered as a four connected linker and K2
atom is considered as a seven connected node, the 3D structure of 2⋅2H
2
O
can be simplified as a 4,7-connected topological structure, as shown in
Fig. 2c.
3⋅2H
2
O crystallized in the monoclinic space group P2
1
(No. 4), with a
crystal density of 1.672 g cm
3
. An asymmetric unit contained one en-
ergetic molecule, two hydroxylammonium ions, and two water molecules
(Fig. 3). The structure of the energetic molecule in 3⋅2H
2
O was signifi-
cantly different from that of 1⋅H
2
O and 2⋅2H
2
O. In 3⋅2H
2
O, the skeleton
of the ligand was greatly twisted, and even the tetrazole ring could not
stay in the plane of the pyrazole ring. The dihedral angle of the ring was
18.27(1)
o
for N2–N3–C5–N4 (Figure S7). Meanwhile, the nitro and
carboxylate groups were also located out of the pyrazole-ring plane, with
dihedral angles of 25.06 (1)
o
for O2–N1–C3–N2 and 53.78 (1)
o
O3–C1–C2–C4 (Figure S7). The twisted structure of the energetic mole-
cule may be due to intensive hydrogen bond interactions in the crystal.
In the crystal structure of 3⋅2H
2
O, each energetic molecule interacted
with eight hydroxylammonium ions through N⋯H and O⋯H hydrogen
bonds (Fig. 3a), with the lengths of N/O⋯H hydrogen bonds in the range
of 1.812 (1) Å –2.519 (1) Å. The eight hydroxylammonium ions sur-
rounded the energetic molecule from different directions and interacted
Fig. 1. (a) Energetic band-shaped structure in 1⋅H
2
O; (b) layered structure of
1⋅H
2
O; (c) 3D supramolecular structure of 1⋅H
2
O.
Scheme 1. Synthesis route of energetic compound 1and corresponding ener-
getic materials.
Y. Zheng et al. Energetic Materials Frontiers 1 (2020) 83–89
85

with the energetic molecule through intensive hydrogen bonds. Mean-
while, each hydroxylammonium ion interacted with four energetic
molecules through hydrogen bonds, with the lengths of the hydrogen
bonds in the range of 2.104 (1) Å 2.900(1) Å. Through this interaction
mode, a 3D supramolecular structure was obtained. The crystal structure
of 3⋅2H
2
O can also be considered to be formed from the packing of planar
layer structure composed of energetic molecules, hydroxylammonium
ions, and water molecules (Fig. 3b). Water molecules were filled into the
layered structures, acting as the bridge molecules to connect two adja-
cent layers.
The crystals of the compound 4used for single X-ray diffraction were
obtained by the volatilization of the methanol solution of the compound.
The compound 4crystallized in the triclinic space group P-1 (No. 2), with
a crystal density of 1.710 g cm
3
. There were one energetic molecule, one
hydrazinium ion, and two half of hydrazine molecules in an asymmetric
unit of the compound (Fig. 4). In the structure of energetic molecule, the
oxygen atoms from carboxylate hung on both sides of pyrazole-ring plane
at different dihedral angles, which were 69.48 (1)
o
and 69.22 (1)
o
for
O3–C1–C2–C3 and O4–C1–C3–C4, respectively (Figure S8).
In the crystal structure of the compound 4, two adjacent energetic
molecules interacted with each other through hydrogen bonds to form a
dimer structure. Meanwhile, the dimers were connected through
hydrazinium ions and hydrazine atoms to form a planar layered structure
(Fig. 4a). The lengths of N/O⋯H hydrogen bonds ranged from 1.913 (1)
Å–2.980 (1) Å. Then the planar layers were packed to form the 3D su-
pramolecular structure of the compound 4(Fig. 4b). The existence of
intensive hydrogen bonds and planar layered structure in the compound
4may have contributed significantly to enhancement of the stability and
safety.
To gain insights into the intra- and intermolecular interactions among
the molecules in 1⋅H
2
O, 3⋅2H
2
O, and the compound 4, the Hirshfeld
surfaces and their relevant 2D fingerprint spectra were studied. Red and
blue spots on the Hirshfeld surfaces denote the high and low close contact
populations, respectively [33–35]. In 1⋅H
2
O(Fig. 5a), red spots repre-
sented strong interactions of N/O⋯H occurring between energetic mol-
ecules and their surrounding water molecules. The ratio of red spot area to
the total Hirshfeld surface area of the energetic molecules was up to 51.7%
(Fig. 5d), indicating the existence of strong interactions between the
molecules in 1⋅H
2
O crystals. In contrast, only blue spots were observed on
the plate area of the Hirshfeld surface (Fig. 5a). They denoted
σ
-
π
in-
teractions of N⋯O, N⋯C, N⋯C, etc., which may account for the regular
plane packing mode of 1⋅H
2
O. There were no strong, close contact
populations between two adjacent layers, which may be one of the main
reasons for the formation of the planar molecular structure of energetic
molecules in 1⋅H
2
O. In 3⋅2H
2
O and the compound 4, the red spots rep-
resenting the strong interactions of N/O⋯H hydrogen bonds were
observed in the plate areas of Hirshfeld surfaces, indicating the presence
of strong interactions between adjacent layers in 3⋅2H
2
O and the com-
pound 4(Fig. 5b and c).This observation correlated well with the twisted
molecular structure of energetic molecules. Furthermore, the ratios of
N/O⋯H interactions in these two kinds of crystals were much higher than
that in 2⋅H
2
O, which may be caused by higher hydrogen content of
hydroxylammonium and hydrazinium ions compared to water molecules.
3.3. Energetic properties
The main properties of compounds 1–4such as density, thermal
stability, and heat of formation are important for evaluating the appli-
cation potential of energetic materials. As shown in Table 1, the four
compounds were nitrogen-rich, with the compound 4having highest
nitrogen content of 49.02%. The compounds 1–4all featured negative
oxygen balance based on carbon monoxide, with the specific data of
17.78%, 8.00%, 19.24%, and 28.02%, respectively. Most espe-
cially, the compounds 1–3had excellent thermal stability and their
decomposition temperatures were higher than 300 C, even up to 315 C
(compound 2). It is higher than decomposition temperatures of most
typical explosives, such as TNT (290 C) [36], RDX (204 C) [37], and
HMX (280 C) [37]. The high thermal stability of the compounds 1,2,
and 3may be due to the planar structure of their parent energetic mol-
ecules and the presence of intensive hydrogen bond nets in their crystal
structures. In general, the energetic salts that are formulated with
hydrazinium cations have poor thermal stability [38,39]. In this study,
the decomposition temperature of the compound 4was only 195 C,
which may be caused by the reductive or nucleophilic behavior of
hydrazinium cations. Furthermore, the compounds 1,3, and 4exhibited
Fig. 2. (a) Symmetric unit of 2⋅2H
2
O; (b) ladder-like structure fabricated from
{K
4
O
8
} clusters; (c) topological structure; (d) 3D structure of 2⋅2H
2
O.
Fig. 3. (a) Layered structure in 3⋅2H
2
O; (b) 3D supramolecular structure
of 3⋅2H
2
O.
Y. Zheng et al. Energetic Materials Frontiers 1 (2020) 83–89
86

positive heat of formation, which is vital for energetic performance,
especially for the I
sp
of energetic materials.
The detonation performance of the compounds was evaluated using
program EXPLO 5 (version 6.02), while the calculated heat of formation
was obtained by using Gaussian 09 program. The calculated detonation
velocities of the compounds 1–4were in the range of 7288–8285 m⋅s
1
,
and the detonation pressure of the compounds varied from 16.92 GPa
to 28.81 GPa. The detonation velocity of the compound 4was up to
8285 m⋅s
1
, which was much higher than that of its parent molecule 1
(7747 m⋅s
1
) and TNT (6950 m⋅s
1
)[36]. In other words, the association
of the employed parent energetic compound with nitrogen-enriched
cations can result in a remarkable increase in the detonation velocity.
Also, the I
sp
of compounds 3and 4were much higher than that of the
compound 1, which should be attributable to the introduction of
nitrogen-enriched cations into the crystal structures of compounds 3and
4. Most especially, all of the compounds exhibited excellent impact
sensitivity, which was higher than 40 J. Meanwhile, all the compounds
displayed good friction sensitivity, which was 360 N, 288 N, and 288 N,
respectively for compounds 2–4, demonstrating the effectiveness of the
employed strategies to develop insensitive energetic materials.
4. Conclusions
In this study, a novel energetic compound 3-nitro-1-(2H-tetrazol-5-
yl)-1H-pyrazole-4-carboxylic acid (1), and corresponding energetic
salts (2–4) were designed and prepared. The structures of these energetic
compounds were determined through NMR spectra, elemental analysis,
FT-IR, and single crystal X-ray diffraction. The compound 2is an ener-
getic MOF with a regular 3D structure and displayed excellent thermal
stability (T
d
¼315 C) and low sensitivity to impact and friction
(IS >40 J, FS ¼360 N) due to strong rigid restraints in the structure.
The compounds 3and 4are nitrogen-rich energetic salts with high
detonation performance (v
D
>8143 m⋅s
1
,p¼25.28 Gpa) and low
sensitivity (IS >40 J, FS ¼288 N) due to extensive hydrogen bond in-
teractions in the crystals. As demonstrated by the excellent thermal sta-
bility and low sensitivity of these energetic materials, introducing
carboxylate group into energetic skeleton is an effective strategy to
develop novel energetic materials with desired properties and
performance.
Fig. 4. (a) Layered structure in the compound 4; (b) 3D supramolecular struc-
ture of the compound 4.
Fig. 5. (a)–(c) Hirshfeld surfaces of 1⋅H
2
O, 3⋅2H
2
O, and 4, respectively; (d) percentages and types of interactions in the total Hirshfeld surfaces, calculated from 2D
fingerprint spectra.
Y. Zheng et al. Energetic Materials Frontiers 1 (2020) 83–89
87

Declaration of competing interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Acknowledgements
The authors acknowledge the support from the National Natural
Science Foundation of China (21805255, 21702196).
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://do
i.org/10.1016/j.enmf.2020.08.004.
References
[1]. Gao H, Shreeve JM. Azole-based energetic salts. Chem. Rev. 2011;111(11):
7377–7436.
[2]. Zhang Q, Shreeve JM. Energetic ionic liquids as explosives and propellant fuels: a
new journey of ionic liquid chemistry. Chem. Rev. 2014;114(20):10527–10574.
[3]. Qu Y, Babailov SP. Azo-linked high-nitrogen energetic materials. J. Mater. Chem. A.
2018;6(5):1915–1940.
[4]. Tian Y, Li J. New development of Ammunition safety: robust munitions. Chin. J.
Energetic Mater. 2017;25(2):91–93.
[5]. Zhang C. On the energy &safety contradiction of energetic materials and the
strategy for developing low-sensitive high-energetic materials. Chin. J. Energetic
Mater. 2018;26(1):2–10.
[6]. Huynh MHV, Coburn MD, Meyer TJ, Wetzler M. Green primary explosives: 5-
Nitrotetrazolato-N2-ferrate hierarchies. Proc. Natl. Acad. Sci. U.S.A. 2006;103(27):
10322–10327.
[7]. Yin P, He C, Shreeve JM. Fused heterocycle-based energetic salts: alliance of
pyrazole and 1,2,3-triazole. J. Mater. Chem. A. 2016;4(4):1514–1519.
[8]. B
elanger-Chabot G, Rahm M, Haiges R, Christe KO. Ammonia-(dinitramido)
boranes: high-energy-density materials. Angew. Chem. Int. Ed. 2015;54(40):
11730–11734.
[9]. Zhang W, Zhang J, Deng M, Qi X, Nie F, Zhang Q. A promising high-energy-density
material. Nat. Commun. 2017;8:181.
[10]. Thottempudi V, Gao H, Shreeve JM. Trinitromethyl-substituted 5-nitro- or 3-Azo-
1,2,4-triazoles: synthesis, characterization, and energetic properties. J. Am. Chem.
Soc. 2011;133(16):6464–6470.
[11]. Bennion JC, Chowdhury N, Kampf JW, Matzger AJ. Hydrogen peroxide solvates of
2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane. Angew. Chem. Int. Ed.
2016;55(42):13118–13121.
[12]. Yang Z, Li H, Zhou X, Zhang C, Huang H, Li J, Nie F. Characterization and
properties of a novel energeticenergetic cocrystal explosive composed of HNIW
and BTF. Cryst. Growth Des. 2012;12(11):5155–5158.
[13]. Zhang J, Du Y, Dong K, Su H, Zhang S, Li S, Pang S. Taming dinitramide anions
within an energetic metal–organic framework: a new strategy for synthesis and
tunable properties of high energy materials. Chem. Mater. 2016;28(5):1472–1480.
[14]. Wang Q, Wang S, Feng X, Wu L, Zhang G, Zhou M, Wang B, Yang L. A heat-resistant
and energetic metal–organic framework assembled by chelating ligand. ACS Appl.
Mater. Interfaces. 2017;9(43):37542–37547.
[15]. Liu Y, Zhao G, Tang Y, Zhang J, Hu L, Imler GH, Parrish DA, Shreeve JM.
Multipurpose [1,2,4]-triazolo-[4,3-b][1,2,4,5]-tetrazine-based energetic materials.
J. Mater. Chem. A. 2019;7(13):7875–7884.
[16]. Zhang J, Mitchell LA, Parrish DA, Shreeve JM. Enforced layer-by-layer stacking of
energetic salts towards high-performance insensitive energetic materials. J. Am.
Chem. Soc. 2015;137(33):10532–10535.
[17]. Singh RP, Verma RD, Meshri DT, Shreeve JM. Energetic nitrogen-rich salts and
ionic liquids. Angew. Chem. Int. Ed. 2006;45(22):3584–3601.
[18]. Armand M, Endres F, MacFarlane DR, Ohno H, Scrosati B. Ionic-liquid materials for
the electrochemical challenges of the future. Nat. Mater. 2009;8:621–629.
[19]. Fischer N, Fischer D, Klap€
otke TM, Piercey DG, Stierstorfer J. Pushing the limits of
energetic materials-the synthesis and characterization of dihydroxylammonium
5,5’-bistetrazole-1,1-diolate. J. Mater. Chem. 2012;22(38):20418–20422.
[20]. Rudakov GF, Ustinova TV, Kozlov IB, Zhilin VF. Synthesis and properties of
alkylnitramino-1,2,4,5-tetrazines. Heterocycl. Compd. 2014;50:53–64.
[21]. Tang Y, Mitchell LA, Imler GH, Parrish DA, Shreeve JM. Ammonia oxide as a
building block for high-performance and insensitive energetic materials. Angew.
Chem. Int. Ed. 2017;56(21):5894–5898.
[22]. Fischer D, Klap€
otke TM, Stierstorfer J. 1,5-Di(nitramino)tetrazole: high sensitivity
and superior explosive performance. Angew. Chem. Int. Ed. 2015;54(35):
10299–10302.
[23]. Haiges R, Christe KO. Energetic high-nitrogen compounds: 5-(trinitromethyl)-2H-
tetrazole and -tetrazolates, preparation, characterization, and conversion into 5-
(dinitromethyl)tetrazoles. Inorg. Chem. 2013;52(12):7249–7260.
[24]. Liu T, Liao S, Song S, Wang K, Jin Y, Zhang Q. Combination of gem-dinitromethyl
functionality and a 5-amino-1,3,4-oxadiazole framework for zwitterionic energetic
materials. Chem. Commun. 2020;56(2):209–212.
[25]. Dippold AA, Klap€
otke TM. A study of dinitro-bis-1,2,4-triazole-1,1’-diol and
derivatives: design of high-performance insensitive energetic materials by the
introduction of N-oxides. J. Am. Chem. Soc. 2013;135(26):9931–9938.
[26]. He P, Zhang J, Yin X, Wu J, Wu L, Zhou Z, Zhang T. Energetic salts based on
tetrazole N-oxide. Chem. Eur J. 2016;22(23):7670–7685.
[27]. Klap€
otke TM, Sabat
e CM. Nitrogen-rich tetrazolium azotetrazolate salts: a new
family of insensitive energetic materials. Chem. Mater. 2008;20(5):1750–1763.
[28]. Wang B, Qi X, Zhang W, Wang K, Li W, Zhang Q. Synthesis of 1-(2H-tetrazol-5-yl)-
5-nitraminotetrazole and its derivatives from 5-aminotetrazole and cyanogen
azide: a promising strategy towards the development of C-N linked bistetrazolate
energetic materials. J. Mater. Chem. A. 2017;5(39):20867–20873.
[29]. Xia H, Zhang W, Jin Y, Song S, Wang K, Zhang Q. Synthesis of thermally stable and
insensitive energetic materials by incorporating the tetrazole functionality into a
fused-ring 3,6-Dinitropyrazolo-[4,3-c]Pyrazole framework. ACS Appl. Mater.
Interfaces. 2019;11(49):45914–45921.
[30]. Liu X, Gao W, Sun P, Su Z, Chen S, Wei Q, Xie G, Gao S. Environmentally friendly
high-energy MOFs: crystal structures, thermostability, insensitivity and remarkable
detonation performances. Green Chem. 2015;17(2):831–836.
[31]. Xu Y, Liu W, Li D, Chen H, Lu M. In situ synthesized 3D metal–organic frameworks
(MOFs) constructed from transition metal cations and tetrazole derivatives: a
family of insensitive energetic materials. Dalton Trans. 2017;46(33):11046–11052.
[32]. Ding M, Cai X, Jiang H. Improving MOF stability: approaches and applications.
Chem. Sci. 2019;10(44):10209–10230.
[33]. Spackman MA, Byrom PG. A novel definition of a molecule in a crystal. Chem. Phys.
Lett. 1997;267(3):215–220.
[34]. Spackman MA, McKinnon JJ. Fingerprinting intermolecular interactions in
molecular crystals. CrystEngComm. 2002;4(66):378–392.
[35]. McKinnon JJ, Jayatilaka D, Spackman MA. Towards quantitative analysis of
intermolecular interactions with Hirshfeld surfaces. Chem. Commun. 2007;37:
3814–3816.
[36]. Johnson EC, Sabatini JJ, Chavez DE, Sausa RC, Byrd EFC, Wingard LA, Guzm
an PE.
Bis(1,2,4-oxadiazole)bis(methylene) dinitrate: a high-energy melt-castable
Table 1
Physiochemical properties of compounds 1–4compared with TNT and RDX.
Compound Formula
a
N content
b
/%
Ω
c
/%
ρ
d
/g⋅cm
3
T
de
/
o
C
Δ
f
H
f
/kJ⋅mol
1
v
Dg
/m⋅s
1
p
h
/GPa
I
sp
/s
IS
i
/J
FS
j
/N
1C
5
H
3
N
7
O
4
43.55 17.78 1.754 312 75.71 7747 23.03 196 >40 240
2C
5
HN
7
O
4
K
2
32.54 8.00 1.923 315 353.72 7288 16.92 164 >40 360
3C
5
H
9
N
9
O
6
43.29 19.24 1.692 305 30.91 8143 25.28 220 >40 288
4C
5
H
7
N
9
O
4
49.02 28.02 1.750 195 211.45 8285 28.81 209 >40 288
TNT [36]C
7
H
5
N
3
O
6
18.50 73.96 1.649 290 59.3 6950 20.5 205 15 353
RDX [37]C
3
H
6
N
6
O
6
37.84 0 1.80 204 70.3 8795 34.9 268 7.4 120
a
Molecular formula.
b
Nitrogen content.
c
OB for C
a
H
b
O
c
N
d
, 1600(c-a-b/2)/Mw (based on CO).
d
Density measured using a gas pycnometer at ambient temperature.
e
Decomposition temperature.
f
Heat of formation.
g
Detonation velocity.
h
Detonation pressure.
i
Impact sensitivity.
j
Friction sensitivity.
Y. Zheng et al. Energetic Materials Frontiers 1 (2020) 83–89
88

explosive and energetic propellant plasticizing ingredient. Org. Process Res. Dev.
2018;22(6):736–740.
[37]. Bian C, Feng W, Lei Q, Huang H, Li X, Wang J, Li C, Xiao Z. A facile synthesis of
energetic salts based on fully nitroamino-functionalized [1,2,4]triazolo[4,3-b]
[1,2,4]triazole. Dalton Trans. 2020;49(2):368–374.
[38]. Klap€
otke TM, Petermayer C, Piercey DG, Stierstorfer J. 1,3-Bis(nitroimido)-1,2,3-
triazolate anion, the N-nitroimide moiety, and the strategy of alternating positive
and negative charges in the design of energetic materials. J. Am. Chem. Soc. 2012;
134(51):20827–20836.
[39]. Klap€
otke TM, Piercey DG, Rohrbacher F, Stierstorfer J. Synthesis and
characterization of energetic salts of the (C
4
N
12
2–
) dianion. Z. Anorg. Allg. Chem.
2012;638(14):2235–2242.
Yue Zhen received her BS from Southwest University of Sci-
ence and Technology in 2018. She is currently perusing her
master’s research in chemistry under the supervision of both
Prof. Qinghua Zhang and Prof. Yaping Zhang. Her research
interests mainly focus on the synthesis and characterization of
novel energetic materials.
Kangcai Wang received his Ph.D. at Sichuan University (SCU)
in 2015 under the supervision of Prof. Zhien Lin and Prof.
Dingguo Xu. From 2015 to 2017, he worked in the group of
Prof. Qinghua Zhang as a postdoctoral fellow at the Institute of
Chemical Materials, China Academy of Engineering Physics
(CAEP). From 2017 to 2018, he joined the group of Prof.
Michael Gozin as a postdoctoral fellow at Tel Aviv University.
Since 2017, he has worked as an assistant research fellow in the
Institute of Chemical Materials, China Academy of Engineering
Physics (CAEP). His research interests mainly focus on ener-
getic ionic liquids and energetic metal-organic frameworks.
Tianlin Liu received his Ph.D. at Tianjin University (TJU) in
2014 under the supervision of Prof. Jun’an Ma and Prof. Fa
Cheng. Since 2014, he has worked as an assistant research
fellow in the Institute of Chemical Materials, China Academy of
Engineering Physics (CAEP). His research interests mainly focus
on low-sensitivity high-energy energetic materials and ener-
getic ionic liquids.
Y. Zheng et al. Energetic Materials Frontiers 1 (2020) 83–89
89