Novel insensitive energetic-cocrystal-based BTO with good comprehensive properties

Article abstract of DOI:10.1039/c7ra11428a

Combining a layer construction strategy with cocrystallization techniques, we designed and prepared a structurally unusual 1H,1′H-5,5′-bistetrazole-1,1′-diolate (BTO) based energetic cocrystal, which we also confirmed by single-crystal X-ray diffraction and powder-crystal X-ray diffraction. The obtained cocrystal crystallizes in a triclinic system, P-1 space group, with a density of 1.72 g cm^-3. The properties including the thermal stability, sensitivity and detonation performance of the cocrystal were analyzed in detail. In addition, the thermal decomposition behavior of the cocrystal was studied by differential calorimetry and thermogravimetry tandem infrared spectroscopy. The results indicated that the cocrystal exhibits strong resistance to thermal decomposition up to 535.6 K. The cocrystal also demonstrates a sensitivity of >50 J. Moreover, its formation enthalpy was estimated to be 2312.0 kJ mol^-1, whereas its detonation velocity and detonation pressure were predicted to be 8.213 km s^-1 and 29.1 GPa, respectively, by applying K-J equations. Therefore, as expected, the obtained cocrystal shows a good comprehensive performance, which proves that a high degree of layer-by-layer stacking is essential for the structural density, thermal stability and sensitivity.

Full text of DOI:10.1039/c7ra11428a

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
RSC Advances
View Article Online
View Journal | View Issue
PAPER
Novel insensitive energetic-cocrystal-based BTO
with good comprehensive properties†
Cite this: RSC Adv., 2018, 8, 1784
a
a
a
Jingjing Tao,a Bo Jin,
Shijin Chu, Rufang Peng, Yu Shanga and Bisheng Tanb
*
*
Combining a layer construction strategy with cocrystallization techniques, we designed and prepared
a structurally unusual 1H,10H-5,50-bistetrazole-1,10-diolate (BTO) based energetic cocrystal, which we
also confirmed by single-crystal X-ray diffraction and powder-crystal X-ray diffraction. The obtained
cocrystal crystallizes in a triclinic system, P-1 space group, with a density of 1.72 g cmꢀ3. The properties
including the thermal stability, sensitivity and detonation performance of the cocrystal were analyzed in
detail. In addition, the thermal decomposition behavior of the cocrystal was studied by differential
calorimetry and thermogravimetry tandem infrared spectroscopy. The results indicated that the cocrystal
exhibits strong resistance to thermal decomposition up to 535.6 K. The cocrystal also demonstrates
a sensitivity of >50 J. Moreover, its formation enthalpy was estimated to be 2312.0 kJ molꢀ1, whereas its
detonation velocity and detonation pressure were predicted to be 8.213 km sꢀ1 and 29.1 GPa,
respectively, by applying K–J equations. Therefore, as expected, the obtained cocrystal shows a good
comprehensive performance, which proves that a high degree of layer-by-layer stacking is essential for
the structural density, thermal stability and sensitivity.
Received 16th October 2017
Accepted 26th December 2017
DOI: 10.1039/c7ra11428a
rsc.li/rsc-advances
and cocrystallization to generate compounds with unique
properties.
Introduction
In our design concept, rationally selecting H-bond donor–
acceptor units is the key to forming a layer-by-layer structure in
cocrystal molecule. 1H,10H-5,50-bistetrazole-1,10-diolate (BTO)
was selected as an H-acceptor group that includes electroneg-
ative atoms. This group is a nearly at building block that
features an excellent detonation performance, low production
cost, environmental friendliness, and favorable thermal
stability. However, the group also displays a comparative
density and sensitivity because of the lack of H-bond donor and
acceptor units in the crystal structure of some energetic-salts-
based BTO. Therefore, the high crystal packing cannot be ach-
ieved, and interlayer sliding is prevented. Additionally, for most
energetic compound, there is an inverse relationship between
energy density and molecular stability. In the quest for ener-
getic compound with better detonation behavior, there have
been reports of CHON-based high explosives. Unfortunately, the
majority of them suffer similar problems, for instance, low
molecular stabilities, complex synthetic routes and poor resis-
The development of high-performance insensitive energetic
materials has been of continued interest due to the increasing
demands for military and civilian applications in the energetic
materials eld.1–3 Currently, the relationship between layer
packing strategy and energetic materials with augmented
density, decreased sensitivity to mechanical stimuli, and
increased performance is paid much attention.4–6 The layer-by-
layer assembly of energetic molecules greatly affects the physi-
cochemical and detonation properties. To our best knowledge,
such face-to-face arrangements are present in the crystal
structure of some cocrystal explosives.7,8 Besides the rational
design and screening of energetic backbones and building
blocks, cocrystallization is a powerful method to improve the
density and ameliorate sensitivity at the molecular level.9 The
most common denition of cocrystallization is the generation
of neutral complexes consisting of two or more components
through non-covalent interactions, such as hydrogen bonding,
p-stacking, and van der Waals forces.10 Meanwhile, hydrogen
bonding has been reported as essential for almost all cocrys-
tals.11–13 These two strategies illuminate the above-mentioned
issue and effectively combine the layer packing strategy
¨
tance to hydrolysis. Since Thomas M. Klapotke reported on BTO
salts, numerous ionic salts based on BTO salts have been
developed as potential energetic explosives.14–17 Nevertheless,
introducing chain cations or cyclic ring cations, including tri-
azole, furazan, and tetrazole, decreases the density and sensi-
tivity of title energetic salts, such as 1,3-diamiꢀn3o-1,2,3-
triazolium 5,50-bis(1-oxidotetrazolate) (r ¼ 1.711 g cm , 2 J),18
1H,10H-5,50-bistetrazole-1,10-diolate$2-(5-amino-2-methyl-2H-
tetrazole) (r ¼ 1.608 g cmꢀ3, 8 J),19 and 1,3-diamino-1,2,3-
aState Key Laboratory Cultivation Base for Nonmetal Composites and Functional
Materials, Southwest University of Science and Technology, Mianyang 621010, China
bInstitute of Chemical Materials, Chinese Academy of Engineering Physics, Mianyang
621010, China
† CCDC 1487781. For crystallographic data in CIF or other electronic format see
DOI: 10.1039/c7ra11428a
1784 | RSC Adv., 2018, 8, 1784–1790
This journal is © The Royal Society of Chemistry 2018

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
View Article Online
Paper
RSC Advances
triazolium nitrotetrazolate-2-oxide (r ¼ 1.716 g cmꢀ3, 3 J).18 from glyoxal, which was treated with hydroxylamine to form
Furthermore, density is a major factor affecting explosive glyoxime. Glyoxime was then chlorinated with Cl2 in ethanol.
performance, which limits the salt's practical application Azido exchange occurred and yielded diazidoglyoxime, which
despite a very high nitrogen content reaching 66.7%, and was cyclized under acidic conditions (HCl gas in diethyl ether)
even more importantly, the detonation pressure increases to produce BTO. The metathesis reaction is analogous to that of
with the square of the density.
non-cocrystal IMBTO.20 Thereaer, a cocrystallization experi-
Consequently, we considered introducing another H2BTO ment was conducted by dissolving the same equivalent of
neutral molecule in the imidazolium 5,50-bis(1-oxidotetrazolate) H2BTO and IMBTO in a minimum amount of distilled water,
(IMBTO) to tune the packing column and increase density via and the mixtures were stirred slowly to dissolve. Then, the
cocrystallization. The assumption was that the IMBTO was solution was ltered and the solvent was evaporated over
chosen as H-bond acceptor, and the H2BTO molecule provides a period of several days at room temperature. A new colorless
both H-bond acceptor and H-bond donor units for pairing. The CIMBTO was then obtained.
combination of the IMBTO and H2BTO molecules was expected
to promote increased packing, augmented nitrogen content,
enhanced energetic properties, and remarkable sensitivities.
Crystal structure
A high-quality cocrystal was formed by slowly growing from
Given the above-mentioned points as guidelines, we present
distilled water at room temperature and then assessed by
the facile synthesis of a unique energetic cocrystal composed of
single-crystal X-ray analysis. The results concerning crystallo-
H2BTO and IMBTO. As a curious-type energetic cocrystal, the
graphic data collection and structural renement are summa-
cocrystal was fully characterized through X-ray diffraction and
rized in Table 1, and the molecular moiety of the CIMBTO is
powder X-ray diffraction and proved the feasibility of our
displayed in Fig. 1. The selected bond lengths and angles are
strategy. The expected cocrystal imidazolium BTO (CIMBTO)
displayed a desirable crystal density of 1.720 g cmꢀ3, which is
given in Table 2, and the hydrogen bond lengths and bond
angles of CIMBTO are listed in Table 3.
signicantly higher than that of the non-cocrystal IMBTO
(1.588 g cmꢀ3) recently published.20 Meanwhile, multinuclear
As expected, the test result is consistent with our previous
design idea. The developed cocrystal crystallizes in a triclinic
nuclear magnetic resonance (NMR) spectroscopy, infrared (IR)
system, P1 space group, with a density of 1.72 g cmꢀ3 at 150 K,
ꢀ
spectroscopy, elemental analysis (EA), differential scanning
which is fairly lower than that of H2BTO (1.811 g cmꢀ3) but
calorimetry (DSC), and thermogravimetric analysis (TGA) were
much superior to that of imidazole (1.030 g cmꢀ3). More
employed to characterize the prepared cocrystal. The resultant
cocrystal also exhibited potentially decent thermal stability,
good detonation properties, and extremely low sensitivity
toward accidental stimuli; these characteristics highlight the
Table 1 Crystal data and structural refinement parameters of CIMBTO
cocrystal's potential energetic application.
Parameter
CIMBTO
C10H12N20O4
476.40
1487781
Empirical formula
Formula weight
CCDC number
Crystal size [mm3]
Crystal system
Space group
Results and discussion
Synthesis
0.19 ꢁ 0.17 ꢁ 0.16
Triclinic
H2BTO was easily prepared in accordance with a previously re-
ported procedure (Scheme 1).19 H2BTO was then synthesized
ꢀ
P1
˚
a [A]
3.6752(10)
9.158(2)
13.799(4)
92.786(3)
97.372(3)
90.753(3)
460.0(2)
1
˚
b [A]
˚
c [A]
a[ꢂ]
b[ꢂ]
g[ꢂ]
3
˚
V [A ]
Z
rcalcd [g cmꢀ3
T [K]
]
1.720
150(2)
244
F(000)
Rint.
Data
0.0505
1616
Restraints
Parameters
1
155
1.046
0.0449
0.1209
0.0478
0.1235
GOFa on F2
b
R1 (I > 2s (I))
uR2 (I > 2s (I))
R1 (all data)
c
uR2 (all data)
P
P
a
2
GOF ¼ Goodness of Fit. b R1 ¼ ||FO ꢀ |FC||/ ||FO. c uR2 ¼ [(u(FO
ꢀ
FC2)2/u(FO2)2)]1/2
.
Scheme 1 Synthesis route of CIMBTO.
This journal is © The Royal Society of Chemistry 2018
RSC Adv., 2018, 8, 1784–1790 | 1785

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
View Article Online
RSC Advances
Paper
Table 3 Hydrogen bond lengths [A˚] and angles [ꢂ] for CIMBTO
D–H
d(D–H)
d(H/A)
<DHA
d(D/A)
A
N9–H9Aa
0.880
0.880
0.880
0.880
0.873
0.873
2.390
2.395
2.241
2.436
1.555
2.314
129.44
131.01
139.21
142.63
170.12
155.73
3.026
3.046
2.964
3.180
2.420
3.130
N2
N6
N8
O2
O1
N1
N9–H9Aa
N10–H10A
N10–H10Ab
O2–H2WA
O2–H2WA
a
[ꢀx + 1, ꢀy + 2, ꢀz + 1]. b [ꢀx, ꢀy + 1, ꢀz + 1].
to the simulated growth conditions and test conditions of the
single crystal are different from the experimental one, variance
was found to some degree in the intensity.
To verify our assumption further, we studied the packing
coefficient, which was calculated as the ratio of the total
molecular volume to the unit cell volume, where 73.64% for
CIMBTO was closer to the value for H2BTO itself (74.35%) and
higher than that of IMBTO (71.53%).20 This result signies that
the volume and amount of the internal cavities were largely
reduced in the cocrystal and that the cocrystal was a new
compound.
Fig. 1 Molecular structure of CIMBTO.
importantly, the cocrystal is also much superior to the non-
cocrystal IMBTO (1.588 g cmꢀ3) as recently published.20 The
graphite-like structure of CIMBTO in the solid state was the
same as that of the predicted mode and highly contributed to
the high density in Fig. 3. By contrast, the favorable density
evidently revealed that the cocrystal was certainly a new
substance and was partially attributed to the good detonation
properties. Additionally, in this work, the energetic cocrystal
was formed by combining an energetic salt with a neutral co-
former, which is different from many of other cocrystals. It is
noted that the imidazole salt of BTO was rst prepared, where
proton transfer has occurred. Aerward, the salt was co-
crystallized with H2BTO. This would mean that the proton
transfer happened before the cocrystallization. Analogously, the
similar example can be found in the U.S. patent which present
In the tetrazolate of BTO anion (Table 2), the N–N bond
˚
˚
lengths varied from 1.296(3) A for N2–N3 to 1.345(3) A for N3–
N4. These lengths were between those of the N–N single bonds
(1.454 A) and N]N double bonds (1.245 A).22 However, the C–C
˚
˚
˚
bond connecting the two tetrazole moieties was 1.446 A long.
˚
Such length was located within the normal range of 1.32 A for
the C]C bond and 1.53 A for the C–C bond.23 Additionally, the
˚
N–O bonds are found between a N–O single bond and a N]O
double bond (d(O001–N001) ¼ 1.331(2) A).24 These observations
˚
indicate the existence of a multiple bond and the conjugation of
the negative charge throughout the aromatic rings caused by
the presence of oxygen atoms. All atoms of the BTO anion were
almost coplanar, and the largest torsional angle was 2.58(0.36)ꢂ,
which was located at O001–N001–C001–C001.
a
novel cocrystal composed of HMX and ammonium
perchlorate.21
Additionally, the phase purity of the cocrystal was veried by
performing the powder X-ray diffraction experiment, and then
compare it with the simulated data from the SC-XRD. As shown
in Fig. 4, the powder X-ray diffraction (PXRD) pattern of the as-
prepared cocrystal is in agreement with the numerical calcula-
tion results converted from the single crystal data, which
identies the cocrystal as a new material with high phase purity.
The characteristic peaks are at 9.66ꢂ, 26.28ꢂ, and 28.17ꢂ, which
A series of strong intermolecular N–H/N and O–H/O
hydrogen bonds formed (Table 3 and Fig. 2). The BTO anion was
a H-bond acceptor, and the imidazole cation was a H-bond
ꢀ
can be attributed to the (010), (111), and (111) respectively. Due
Table 2 Selected bond lengths [A˚] and angles [ꢂ] for CIMBTO
Angle/ꢂ
˚
Bond
Length/A
Bond
N(1)–N(2)
N(1)–O(1)
N(2)–N(3)
N(3)–N(4)
N(5)–O(2)
N(5)–N(6)
N(6)–N(7)
N(7)–N(8)
1.329(3)
1.331(2)
1.296(3)
1.345(3)
1.327(2)
1.333(3)
1.299(3)
1.342(2)
N(2)–N(1)–O(1)
N(2)–N(1)–C(1)
N(3)–N(2)–N(1)
N(2)–N(3)–N(4)
O(2)–N(5)–C(2)
C(2)–N(5)–N(6)
N(7)–N(6)–N(5)
N(6)–N(7)–N(8)
121.94(16)
109.24(16)
106.12(16)
110.94(18)
128.36(17)
109.14(16)
105.93(16)
110.93(18)
Fig. 2 Packing diagram of CIMBTO along the a axis.
1786 | RSC Adv., 2018, 8, 1784–1790
This journal is © The Royal Society of Chemistry 2018

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
View Article Online
Paper
RSC Advances
to the stretching vibrations of C–H.25 In addition, the peak at
757 cmꢀ1 was caused by the out-plane exural vibration
absorption of C–H.25 Consequently, the FT-IR spectrum was
consistent with the structural features of the compound.
1
The H NMR and 13C NMR spectra also veried the correct
1
structure of CIMBTO. In the H NMR spectrum, the signal for
the amino group of salt occurred as a broad single band at d/
ppm ¼ 9.09 ppm [s, CH, NH+] and d/ppm ¼ 7.70 [s, CH]CH–
NH+].25 Three signals in the 13C NMR [DMSO-d6] spectrum of
the cocrystal were found at d/ppm ¼ 134.86 [C]NH+], 119.77
[CH–NH, CH–NH+], and 135.62 [CN4O], respectively.19,25
Fig. 3 Packing diagram of CIMBTO along the b axis.
donor. However, the H2BTO molecule was a donor and an
acceptor of the hydrogen bonds. The packing structure was held
not only by electrostatic attraction through cations and anions
but also by hydrogen bonds and p–p interactions. Strong
hydrogen bonds were formed in the crystal structure of CIMBTO
Energetic performance
HOF is an important parameter that represents the energy
content of an energetic material. Positive values correspond to
enhanced detonation properties. The computations were per-
formed with the Gaussian 09 program.26
Isodesmic reactions were used to obtain the formation
enthalpy of CIMBTO at 298 K. Thus, the empirical Kamlet–
Jacobs equations27 were employed to estimate the values of
detonation velocity (D) and detonation pressure (P) for CIMBTO,
as shown in the following equations:
˚
˚
(N10–H10A/N008 [2.964 A], N9–H9A/N2 [3.026 A], N4–H4/
˚
˚
N2 [2.433 A], N10–H10A/O2 [3.180 A]) (Fig. 1). Its packing
structure of CIMBTO was built and linked to a 2D layer by
various hydrogen bonds. Apart from the hydrogen bonds, the p-
stacking interactions were also signicant in the cocrystal
packing. The structures of CIMBTO and imidazolium BTO salt
along the b axis is depicted in Fig. 3. Aer the layer was
assembled, further interlinking of the 2D layers into an intricate
3D supermolecule was stimulated by hydrogen bonding and p
stacking. Remarkably, these 2D layers in the b axis direction
were arranged in a face-to-face arrangement horizontally. As
such, the arrangement led to a decent thermal stability and
insensitivity.
ꢀ 1/2
D ¼ 1.01(NM
Q
)
1/2 1/2(1 + 1.3r)
(1)
(2)
P ¼ 1.558r NM Q1/2
2
ꢀ 1/2
where D is the detonation velocity (km sꢀ1), P is the detonation
pressure (GPa), N is the explosive detonation that generates gas
Spectroscopy
moles per gram (mol gꢀ1), M is the gaseous product of the
ꢀ
average molecular weight (g molꢀ1), Q is the explosive detona-
tion chemical energy per gram (kJ gꢀ1), and r is the density (g
cmꢀ3). Q should be calculated before D and P are determined.
Moreover, Q is also attained by the DHf of the detonation
reactant and product.
Given the theoretical HOFs and the experimental room
temperature density, the detonation properties for IMBTO were
predicted by K–J equations27 (Table 4). CIMBTO showed
The structure of CIMBTO was supported further by its corre-
sponding FT-IR, 1H NMR, 13C NMR, and elemental analysis
(Experimental section). As shown, in the FT-IR spectrum
(Fig. 5), several sharp characteristic bands at 1631, 1425, and
1288 cmꢀ1 were ascribed to the stretching vibration absorption
peaks of C]N, N]N, and N–O, respectively.19 The intense
broad band at 3410 cmꢀ1 was attributed to the N–H stretching
vibrations of CIMBTO, and the peak at 3145 cmꢀ1 corresponded
Fig. 4 PXRD patterns of CIMBTO and simulated data from SC-XRD.
Fig. 5 FT-IR spectrum of CIMBTO.
This journal is © The Royal Society of Chemistry 2018
RSC Adv., 2018, 8, 1784–1790 | 1787