1,3,4-Oxadiazole Bridges: A Strategy to Improve Energetics at the Molecular Level

Article abstract of DOI:10.1002/anie.202014207

Many energetic materials synthesized to date have limited applications because of low thermal and/or mechanical stability. This limitation can be overcome by introducing structural modifications such as a bridging group. In this study, a series of 1,3,4-oxadiazole-bridged furazans was prepared. Their structures were confirmed by ¹H and ¹³C NMR, infrared, elemental, and X-ray crystallographic analyses. The thermal stability, friction sensitivity, impact sensitivity, detonation velocity, and detonation pressure were evaluated. The hydroxylammonium salt 8 has an excellent detonation performance (D=9101 m s^?1, P=37.9 GPa) and insensitive properties (IS=17.4 J, FS=330 N), which show its great potential as a high-performance insensitive explosive. Using quantum computation and crystal structure analysis, the effect of the introduction of the 1,3,4-oxadiazole moiety on molecular reactivity and the difference between the sensitivities and thermal stabilities of mono- and bis-1,3,4-oxadiazole bridges are considered. The synthetic method for introducing 1,3,4-oxadiazole and the systematic study of 1,3,4-oxadiazole-bridged compounds provide a theoretical basis for future energetics design.

Full text of DOI:10.1002/anie.202014207

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
Chemie
International Edition
www.angewandte.org
Accepted Article
Title: 1,3,4-Oxadiazole Bridges: A Strategy to Improve Energetics at
the Molecular Level
Authors: Jean'ne M. Shreeve, Jinchao Ma, Ajay Kumar Chinnam,
Guangbin Cheng, Hongwei Yang, and Jiaheng Zhang
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202014207
Link to VoR: https://doi.org/10.1002/anie.202014207

10.1002/anie.202014207
Angewandte Chemie International Edition
RESEARCH ARTICLE
1,3,4-Oxadiazole Bridges: A Strategy to Improve Energetics at the
Molecular Level
Jinchao Ma,[a],[b],[c] Ajay Kumar Chinnam,[b] Guangbin Cheng,*[a] Hongwei Yang,[a] Jiaheng
Zhang*[c],[d] and Jean’ne M. Shreeve*[b]
[a]
[b]
[c]
Dr. J. Ma, Prof. G. Cheng, Prof. H. Yang
School of Chemical Engineering
Nanjing University of Science and Technology
Nanjing, 210000, China
E-mail: gcheng@njust.edu.cn
Dr. J. Ma, Dr. A. K. Chinnam, Prof. J. M. Shreeve
Department of Chemistry
University of Idaho
Moscow, Idaho, 83844-2343 USA
Email: jshreeve@uidaho.edu
Dr. J. Ma, Prof. J. Zhang
Biomaterials Center
Zhuhai Institute of Advanced Technology Chinese Academy of Sciences
Zhuhai, 519003, China
Email: zhangjiaheng@hit.edu.cn
Supporting information for this article is given via a link at the end of the document.
Abstract: Many energetic materials synthesized to date have limited
applications because of low thermal and/or mechanical stability. This
limitation can be overcome by introducing structural modifications
such as a bridging group. In this study, a series of 1,3,4-oxadiazole-
bridged furazans was prepared. Their structures were confirmed by
1H and 13C NMR, infrared, elemental, and X-ray crystallographic
analyses. The thermal stability, friction sensitivity, impact sensitivity,
detonation velocity, and detonation pressure were evaluated. The
hydroxylammonium salt 8 has an excellent detonation performance
(D = 9101 m s-1, P = 37.9 GPa) and insensitive properties (IS = 17.4
J, FS = 330 N), which show its great potential as a high-performance
insensitive explosive. Using quantum computation and crystal
structure analysis, the effect of the introduction of the 1,3,4-oxadiazole
moiety on molecular reactivity and the difference between the
sensitivities and thermal stabilities of mono- and bis-1,3,4-oxadiazole
bridges are considered. The synthetic method for introducing 1,3,4-
oxadiazole and the systematic study of 1,3,4-oxadiazole-bridged
compounds provide a theoretical basis for future energetics design.
tetrazole and triazole) because of their high-energy content and
green explosion products.[10-13] With a combination of various
energetic rings and groups, these compounds are expected to
exhibit excellent performance and moderate sensitivity.
Common ways to modify heterocycles at the molecular scale,
including the introduction of various explosophores, such as nitro,
dinitromethyl, and trinitromethyl, and amino groups.[14-17] In
addition, the introduction of a bridge between two energetic units
has been shown to have a dramatic effect on performance. As
seen in Figure 1, the bridge is mainly a functional group such as
carbonyl, ethylene, azo, and azoxy. Bis(5-(trinitromethyl)-1,3,4-
O2N
O2N
NO2
NO2
Carbonyl-bridges
NO2
O2N
O
O
O2N
O
NO2
NO2
NO2
O
N
N
O2N
N
N
O2N
N
N
N
N
N
N
O2N
NO2
NO2
A
B
Td, IS,
FS
NO2
Ethylene-bridges
O2N
HN
HN
N
N
N
N
N
N
N
N
N
N
N
N
N
N
NH
NH
NO2
O2N
O2N
NH
C
D
Azo/azoxy-bridges
Td, IS,
FS
Hf, D,
P
Introduction
O2N
O
O2N
O
N
O
N
N
NH
N
N
N
N
N
N
N
Research and development of energetic materials have attracted
significant attention over the past few decades. Although
HN
HN
NO2
N
O
N
NO2
O
E
F
1,3,4-Oxadiazole-bridges
N
N
cyclotetramethylenetetranitramine
hexanitrohexaazaisowurtzitane
(HMX)
have
and
been
Td, IS,
(CL-20)
O
FS
N
N
Hf, D,
O
recommended as high-performance compounds for a long time,
their use has diminished owing to their instability.[1-4] As a result,
current research is focused on improving the stability, especially
mechanical and thermal stabilities of these energetic materials.
Modifications at the interface (cladding and mixing) and crystal
levels (co-crystal and host-guest assembly) of HMX and CL-20
have been reported several times.[5-9] Along with the refinement
of classical high-energy explosives, studies on new high energy
density materials are under way focusing on the synthesis of five-
or six-ring heterocyclic systems (including furazan, tetrazine,
?
P
O
N
N
NO2
O
O
N
N
X
O2N
R
N
N
N
Y
N3
NHNO2
,
,
(X=O, N; Y=N, O)
R= NO2, N3, NHNO2
NO2
This work
DPO, TKX-55
Figure 1. Energetic derivatives with different bridges.
1
This article is protected by copyright. All rights reserved.

10.1002/anie.202014207
Angewandte Chemie International Edition
RESEARCH ARTICLE
oxadiazol-2-yl)methanone (A)[18] and bis(3-nitro-1-(trinitromethyl)-
1H-1,2,4-triazol-5-yl)methanone (B),[19] which have a carbonyl
group introduced into the conjugated system, show better stability
o
(Td ≥ 108 C, IS ≥ 4 J, FS ≥ 60 N) than most trinitromethyl
H2N
N
NH2
compounds.
1,2-Bis(4,5-dihydro-5-nitroimino-1H-tetrazol-1-
O
H2N
H2NN
NH2
H
O
yl)ethane (C)[20] and N,N'-(1,1'-(ethane-1,2-diyl)bis(3-nitro-1H-
1,2,4-triazole-5,1-diyl))dinitramide (D)[21] are representatives of
azoles linked by ethylene, which decreases the acidity of the ring
NH and broadens the options for the design of desirable energetic
compounds. Generally, azo or azoxy bridges enhance the heat of
formation as well as detonation performance of energetic
compounds, as demonstrated for 3,3’-dinitramino-3,3’-
azobifurazan (E)[22] and 3,3’-dinitramino-3,3’-azoxybifurazan
(F).[23] In contrast, the thermal and mechanical stabilities of
nitraminofurazans with azo or azoxy bridges are reduced,
especially for compound F (Td = 70 oC, IS = 1 J, FS = 16 N).
Surprisingly, research on 1,3,4-oxadiazole-linked energetic
compounds is limited despite the fact that 2,5-bis-(2,4,6-
trinitrophenyl)-1,3,4-oxadiazole (DPO) and 5,5’-bis(2,4,6-
trinitrophenyl)-2,2’-bi(1,3,4-oxadiazole) (TKX-55) exhibit high
thermal stabilities (DPO, 331 oC; TKX-55, 335 oC) and are
reported as heat-resistant explosives.[24-25]
N
H
H4• H2O
N
N
H
reflux
N
N
N
O
N
O
O
1
2 (70%)
R
R
NH2
2
i
N
N
N
N
or ii
N
N
N
O
O
or iii
O
N
N
O
N
O
3 (73%)
-N3
4 (81%)
-NHNO2
-NO2
R
=
2) AcOH/NaN3
5 (71%)
(failed)
H2SO4/H2O2/NaWO4 or H2SO4/H2O2/(NH4)2S2O8
NO2
N
N
N
NH4
N2H5
NH3OH
N
O
6 (80%)
7 (85%)
8 (88%)
N
O
2 Cation
Scheme 1. Synthesis of compounds 1–8.
1,3,4-Oxadiazole, the only oxadiazole isomer that does not
contain an oxygen-nitrogen bond, exhibits a wide variety of
biological activities.[26-28] To determine the influence of the stable
1,3,4-oxadiazole on the properties of energetic compounds, we
performed a systematic study of a set of 1,3,4-oxadiazole-bridged
energetic derivatives (compounds 3–8 and 11–16). All
compounds are characterized by multinuclear NMR and IR
spectroscopy, elemental analyses, and differential scanning
calorimetry (DSC). Theoretical and crystalline investigations of
these compounds were performed in order to understand their
reaction activity, detonation properties and structural features.
These compounds have the characteristic high detonation
performance and reasonable stability, which highlight the
introduction of 1,3,4-oxadiazole bridges as a promising strategy
to improve energetic compounds.
prepared by a four-step reaction from commercially available
ethyl cyanoacetate according to a reported procedure.[31-32]
Compound 9 is then treated with oxalyl chloride in pyridine to form
N'1,N'2-bis(4-amino-1,2,5-oxadiazole-3-carbonyl)oxalohydrazide
(10, yield: 79%). The dehydration of 10 in 20% oleum results in
the formation of intermediate 11 (yield: 72%). Interestingly, as with
compound 3, attempts to oxidize the amino group in compound
11 by various reagents were unsuccessful. The low reactivity of
the amino group in compounds 3 and 11 may be due to the strong
electron-withdrawing effect of the 1,3,4-oxadiazole ring (as see
below). The nitramino- and azide-functionalized compounds, 12
and 13, (yields: 82% and 74%, respectively) are then synthesized
from compound 11 through nitrification and diazotization-
substitution, respectively. Subsequently, three energetic
derivatives (14–16, yields: 82–89%) are prepared by treating 13
with different bases.
Results and Discussion
O
O
N
aNO2
H+
MeOH
H2SO4
1) NH2OH
2) H+
Synthesis and Crystallographic Analysis. The syntheses of 1–
8 (Scheme 1) start from commercially available malononitrile.
Using this starting compound, 4-amino-1,2,5-oxadiazole-3-
carbohydrazonamide (1) is synthesized sequentially through a
series of four steps.[29-30] Compound 2 (yield: 70%) is synthesized
from 1 in hot hydrochloric acid solution (crystal structure and
proposed mechanism shown in Figure 2 and Scheme 3,
respectively), from which 3 (yield: 73%) is synthesized through
intramolecular cyclization in 20% fuming sulfuric acid.
Unfortunately, treatment of amine 3 with various oxidizing
materials, such as H2O2/H2SO4, H2O2/CF3COOH, or
H2O2/H2SO4/(NH4)2S2O8 does not produce the nitro derivative.
However, when compound 3 is added to a solution of NaNO2 in
concentrated H2SO4, followed by AcOH and a solution of NaN3,
azide 4 (yield: 81%) is obtained. The nitration of 3 is carried out
using 100% HNO3 to yield nitroamine 5 (yield: 77%), whose
energetic salts (6–8, yields: 80–88%) are synthesized by reaction
with different bases.
OH
CN
NOH
O
O
N
NH2
NH2
HN
H2
N
O
N
oxalyl chloride,
pyridine
O
O
H
N
H
N
O
N
NH2
N
N
H
N
H
N
O
reflux
O
O
O
N
N
NH2
N
9
10 (79%)
R
H2
N
i
N
N
N
N
N
O
or ii
O
O
O
O
O
or iii
N
N
N
N
N
N
N
R
11 (72%)
2) AcOH/NaN3
-N3
12 (82%)
-NHNO2
-NO2
(failed)
R
=
13 (74%)
2SO4/H2O2/NaWO4 or H2SO4/H2O2/(NH4)2S2O8
O2N
N
N
N
N
O
N2H5
NH4
NH3OH
O
O
N
14 (82%)
15 (84%)
16 (89%)
N
N
NO2
2 Cation
In Scheme 2, 4-amino-1,2,5-oxadiazole-3-carbohydrazide (9) is
Scheme 2. Synthesis of compounds 9–16.
2
This article is protected by copyright. All rights reserved.

10.1002/anie.202014207
Angewandte Chemie International Edition
RESEARCH ARTICLE
X-ray Crystal Structure Determinations. Compound 2·H2O
crystallizes in the orthorhombic space group P21212 with two
formula moieties in the unit cell. The molecular structure of 2·H2O
is shown in Figure 2a. The N−N distance in 2 (1.3969 Å) is slightly
shorter than the typical N−N single bond distance of about 1.45 Å.
The C−N distance in the hydrazine segment is 1.3349 Å, which is
significantly shorter than the typical C−N single bond distance of
about 1.48 Å and even shorter than the C−N double bond distance
of 1.35 Å.[33] The carbonyl groups are coplanar with the
backbones, and the N−N bond in the hydrazine segment is
rotated to form a cross-shaped molecular structure with a dihedral
angle of 85.07°. As seen in Figure 2b, the 2·H2O molecules are
arranged in wavelike layers along the b-axes. Two “V” shaped
layers of molecules open in opposite directions and assume a
staggered arrangement to form wavelike layers. Owing to the high
degree of molecular distortion, the spacing between two adjacent
wave layers is 4.3252 Å.
Figure 3. (a) Molecular structure. (b) 2D layer-by-layer stacking of 6·H2O. (c)
Hydrogen bonds between molecules in the same layer
between two adjacent molecules in the same row is 3.0781 Å.
Because the molecules do not have hydrogen atoms, there is no
hydrogen bond interaction between them in the structure (Figure
4c).
Figure 2. (a) Molecular structure. (b) Packing diagram of 2·H2O.
Compound 6·H2O crystallizes in the monoclinic space group
C2/c (Figure 3a). Because of the combined effects of steric
hindrance and intermolecular interactions, the dihedral angle
defined by the furazan and 1,3,4-oxadiazole rings in 6 is 10.51°.
The 6·H2O crystal exhibits an ordered planar layer stacking. Two
adjacent layers (spacing of 3.1898 Å) are connected by N(6)-
H(6A)…O(4) and N(6)-H(6C)…O(4) hydrogen bonds, which
propagate throughout the crystal layers to form infinite chains
(Figure 3b). In the same layer, the 6·H2O molecules are regularly
arranged line by line (Figure 3c). The O(5)-H(5)…N(2) and O(5)-
H(5)…O(3) hydrogen bonds connect two adjacent anions in the
same line, and two adjacent lines are connected by hydrogen
bonds with ammonium ions [N(6)-H(6A)…O(1), N(6)-H(6B)…O(3),
N(6)-H(6B)…O(4), N(6)-H(6D)…O(3), N(6)-H(6D)…O(4), and
N(6)-H(6D)…N(5)].
Figure 4. (a) Molecular structure. (b) 3D cube layer-by-layer stacking of 12. (c)
Arrangement of molecules in the same 3D cube layer.
Azide 12 (Figure 4a) crystallizes in the monoclinic space group
P21/c (Z = 4) with a calculated density of 1.792 g cm-3 (173 K).
The C–N3 bond lengths are 1.3913 [C(1)-N(3)] and 1.4002 Å
[C(8)- N(12)], which are nearly identical to the same bond in 2,5,8-
triazido-s-heptazine (1.401 Å).[34] Contrary to expectation, the two
1,3,4-oxadiazole rings and the two 1,2,5-oxadiazole rings are not
coplanar [N(5)-C(2)-C(3)-N(6) = 171.98°, N(7)-C(4)-C(5)-N(8) = -
173.21°, N(9)-C(6)-C(7)-N(10) = -178.59°]. Moreover, the azide
group slightly twists out of plane owing to steric effects [N(2)-N(3)-
C(1)-C(2) = -177.93°, C(7)-C(8)-N(12)-N(14) = -177.06°]. Zigzag-
motif rows are formed by molecules of 12 positioned exactly
opposite one another (84.20°) (Figure 4b).[35] The vertical distance
Compound 14·2H2O crystallizes in the monoclinic space group
P21/n with a calculated density of 1.737 g cm−3 at 220 K, which is
slightly lower than that of the four-ring azide 12 (1.792 g cm−3).The
symmetrical unit of the structure consists of one molecule each of
the oxadiazole-furan nitramine and ammonium cations, as well as
one crystal water (Figure 5a). The entire four-ring system is nearly
coplanar with twist angles of N(2)-N(1)-C(1)-C(2) = -179.17° and
C(1)-C(2)-C(3)-N(5)
= -171.06°. Similar to 1,1-diamino-2,2-
dinitroethene (FOX-7),[36-37] 14·2H2O forms wavelike stacks with
a layer spacing of 2.9216 Å. As in the 6·H2O crystal, the N(7)-
H(7D)…O(5), N(7)-H(7B)…O(5), N(7)-H(7B)…N(5), and N(7)-
3
This article is protected by copyright. All rights reserved.

10.1002/anie.202014207
Angewandte Chemie International Edition
RESEARCH ARTICLE
H(7B)…N(3) hydrogen bonds are connected between the layers
(Figure 5b). In the same layer, a hydrogen bond network
consisting of N(7)-H(7B)…N(3), N(7)-H(7A)…N(6), N(7)-
H(7C)…O(4), N(7)-H(7C)…N(1), O(5)-H(5B)…O(4), and O(5)-
H(5A)…N(5) is formed and connects an anion with 6 other
surrounding anions. (Figure 5c).
oxadiazole bridge on the reactivity of aminofurazans, the natural
charges and frontier orbital distributions of compounds 3 and 11
and DABF were calculated by quantum chemistry (DFT) and
compared.[41] As shown in Figure S2 and Table S2-S4, the
introduction of the 1,3,4-oxadiazole ring reduces both the nitrogen
charges and contribution of the amino groups to the highest
occupied molecular orbital (HOMO), which results in the decrease
of amino reactivity.
NH2
NH2
NH3
NH2
H2
N
N
N
N
O
N
N
O
H
N
O
N
N
N
NH2
N
N
NH2
H3
I-1
1
H2
I-1
N
O
N
H2
N
NH2
N
NH
N
N
O
N
N
O
NH2
N
HN
N
N
-NH3
-NH3
NH3
N
HN
O
N
NH2
NH2
I-3
I-2
H
H
O
H2N
H2
N
N
N
N
N
N
N
N
H2O
O
N
N
H
O
N
N
O
O
N
N
NH
H
HN
H
O
H
NH2
NH2
NH2
I-5
H
I-4
H2
N
N
O
H2N
N
H
N
O
N
N
H
-N2H4
O
N
O
2
Scheme 3. Proposed mechanism for the synthesis of 2 from 1
Physical and Detonation Properties. The physicochemical
properties of compounds 4−8, 12-16 are compared with 1,1-
diamino-2,2-dinitroethene (FOX-7), 1,3,5-trinitro-1,3,5-triazine
(RDX) and HMX in Table 1. The decomposition temperatures
(onset) of all compounds are between 67 and 214 °C. Among
these compounds, nitroamines 5 and 13 have the lowest thermal
stability commencing decomposition at 67 and 90 °C, respectively.
The most thermally stable compound is the ammonium salt 14
(214 °C), which is comparable to RDX (204 °C) and FOX-7
(220 °C). The order of thermal stability for the mono-1,3,4-
oxadiazole-bridged compounds is 5 (67 °C) < hydroxylamine salt
8 (169 °C) = azide 4 (169 °C) < hydrazine salt 7 (190 °C) <
ammonium salt 6 (210 °C). Interestingly, the bis-1,3,4-oxadiazole-
bridged compounds show the same trend. Comparison of the
decomposition temperatures of the two series of 1,3,4-
oxadiazole-bridged compounds leads to a surprising conclusion.
Tetracyclic compounds, 12–16, in general, decompose at higher
temperatures than the analogous tricyclic compounds with the
same groups. This can be rationalized by the superior molecular
stability of the former structures, which is derived from a large
conjugated system. The greater thermal stability of bis-1,3,4-
oxadiazole-bridged compounds compared with that of mono-
1,3,4-oxadiazole-bridged compounds is not accidental and has
been shown by several studies. Recent research by our group
further showed that the bis-1,3,4-oxadiazole-bridged compounds,
[5,5'-bis(4-nitro-1H-pyrazol-3-yl)-2,2'-bi(1,3,4-oxadiazole),
Figure 5. (a) Molecular structure. (b) 2D layer-by layer stacking of 14·2H2O. (c)
Hydrogen bonds between molecules in the same layer..
Analysis of Synthesis Mechanism and Reaction Activity. The
1,2-diacylhydrazine unit (as in compound 2) is generally
constructed through the substitution of hydrazide with ester or
acid chloride.[38-39] There are few reports about building 1,2-
dihydrazine structures from the amidine group. Therefore, it is
necessary and meaningful to predict the mechanism of formation
for compound 2 from 1. First, two protonated products (I-1)
undergo an intermolecular reaction, in which the hydrazine group
of one molecule attacks the amine oxime carbon in the other
molecule (Scheme 3). This leads to the loss of one ammonia
molecule to yield intermediate I-2. Adduct I-2 undergoes
cyclization accompanied by the elimination of a second ammonia
molecule to form 1,4-dihydro-1,2,4,5-tetrazine structure I-3.
Apparently, the formation of compound 2 occurs during the
opening of the protonated dihydrotetrazine ring in I-4, which is
converted from I-3 by acid attack. I-4 is then converted to 2 by the
attack of two water molecules and loss of one hydrazine molecule.
Surprisingly, both compounds 3 and 11, exhibit a strong inactivity
toward oxidizing reagents during synthesis. However, 3,3’-
diamino-4,4’-bifurazan (DABF), which lacks the 1,3,4-oxadiazole
bridge, can be oxidized directly to the nitro derivative by a mixture
of H2O2 and CF3COOH.[40] To study the effect of the 1,3,4-
BNPBO and 5,5'-bis(1,4-dinitro-1H-pyrazol-3-yl)-2,2'-bi(1,3,4-
oxadiazole), BDPBO] have better thermal stabilities than the
mono-1,3,4-oxadiazole bridged [2,5-bis(4-nitro-1H-pyrazol-3-yl)-
4
This article is protected by copyright. All rights reserved.

10.1002/anie.202014207
Angewandte Chemie International Edition
RESEARCH ARTICLE
Table 1. Physical and Detonation Properties of Compounds 4−8, 12-16 compared with DNBF, FOX-7, RDX and HMX
Compd
Tdec[a] [°C]
169
67
ρ[b][g cm-3]
1.76
1.91
1.82
1.79
1.88
1.79
1.92
1.87
1.86
1.87
1.94
1.88
1.80
1.91
ΔHf[c][kJ mol-1/ kJ g-1]
1359.8/4.72
711.8/2.18
420.2/1.17
783.5/2.01
553.9/1.41
1370.2/3.85
793.3/2.01
539.6/1.26
890.2/1.94
680.8/1.48
526.0/2.04
-130.0/-0.88
70.3/0.32
D[d][m s-1]
8728
9211
8700
8914
9101
8545
9058
8745
8993
8926
9086
8870
8795
9144
P[e][GPa]
31.2
38.0
32.2
33.3
37.9
29.8
36.2
33.1
34.9
35.8
40.3
34.0
34.9
39.2
IS[f][J]
21.9
-
FS[g][N]
280
-
4
5
6
210
190
169
191
90
24.5
11.0
17.4
4.9
360
280
330
120
120
240
240
240
48
7
8
12
13
6.9
14
214
200
186
80
9.8
15
7.7
16
6.9
DNBF[h]
FOX-7[i]
RDX[j]
HMX[j]
1.5
220
204
280
25
340
120
120
7.4
74.8/0.25
7.4
[a] Thermal decomposition temperature (onset) under nitrogen gas (DSC, 5 oC/min). [b] Density measured with a gas pycnometer (25 oC). [c] Calculated heat of
formation. [d] Calculated detonation velocity. [e] Calculated detonation pressure. [f] Impact sensitivity. [g] Friction sensitivity. [h] Ref. [22]. [i] Ref. [42]. [j] Ref. [43].
1,3,4-oxadiazole, BNPO and 2,5-bis(1,4- dinitro-1H-pyrazol-3-yl)-
1,3,4-oxadiazole), BDPO].[44] Therefore, we analyzed the
molecular thermal stabilities of 1,3,4-oxadiazole-bridged
compounds by calculating the trigger bond dissociation enthalpies
(TBDE).[45-47] The result is consistent with the trend of the
measured decomposition temperatures (Figure 6a), that is, the
greater the dissociation energy, the higher the decomposition
temperature. Additionally, the TBDE of the unreported 1,3,4-
oxadiazole-bridged compounds with some common energetic
units were calculated and analyzed (Figure 6b). Overall, the
thermal stability of bis-1,3,4-oxadiazole-bridged compounds is
higher than that of mono-1,3,4- compounds both in theory and in
practice.
derivatives, compounds 5 (1.91 g cm-3) and 13 (1.92 g cm-3) have
the highest density, which is comparable to that of HMX (1.91 g
cm-3). The heats of formation were calculated using the Gaussian
09 suite of programs. As expected, the introduction of an
additional 1,3,4-oxadiazole ring results in higher heats of
formation for 12–16 (539.6–1370.2 kJ mol-1) compared with those
for mono-1,3,4-oxadiazole-bridged compounds 4–8 (420.2–
1359.8 kJ mol-1). Using the measured densities and calculated
heats of formation, the detonation performances were calculated
using the EXPLO5_V6.01 program.[48] The detonation velocity
and detonation pressure of compounds 4–8 and 12–16 are in the
range of 8545–9211 m s-1 and 29.8–38.0 GPa, respectively
(Figure 8a). Except for azides 4 (8728 m s-1), 6 (8700 m s-1), 12
(8545 m s-1), and 14 (8745 m s-1), these energetic compounds
outperform FOX-7 (8870 m s-1) and RDX (8795 m s-1) in terms of
detonation velocity. Among these compounds, the detonation
velocities of nitroamine 5 (9211 m s-1), hydroxylammonium salt 8
For the azide compounds, the introduction of one more 1,3,4-
oxadiazole ring increases the density from 1.76 (4) to 1.79 g cm-3
(12). Nitroamine derivatives 5–8 and 13–16 exhibit good to
excellent densities between 1.79 and 1.92 g cm−3. Among these
5
This article is protected by copyright. All rights reserved.

10.1002/anie.202014207
Angewandte Chemie International Edition
RESEARCH ARTICLE
terms of impact sensitivity. However, its stability is better than
those of nitraminofurazans with azo, azoxy, and other bridges. In
terms of friction sensitivity, 12–16 are less sensitive than HMX (FS
= 112 N), and 14–16 (FS = 240 N) are even more stable than RDX
(FS = 120 N).
Figure 8. Hirshfeld surfaces of anions in 6·H2O and 14·2H2O: (a) 3D dnorm
surface, (b) shape index, (c) curvedness, and (d) distortion of anions in 6·H2O
and 14·2H2O.
Figure 6. (a) Decomposition temperatures and trigger bond dissociation
enthalpies (TBDE) of BNPO, BNPBO, BDPO, BDPBO, DPO, and TKX-55. (b)
TBDE of series a–e of 1,3,4-oxadiazole-bridged compounds
The Hirshfeld surfaces and associated fingerprint plots of the
crystals were obtained to demonstrate the intermolecular
interactions.[50] The 3D dnorm surfaces in Figure 8a and Figure S9
show that the oxygen atoms of nitro groups in the anions of 6·H2O
and 14·2H2O act as acceptor sites for hydrogen bonds. The
shape index (Figure 8b) shows that the adjacent anions in
14·2H2O are more in contact than those in 6·H2O. The crystal of
12, which has a complex color distribution, is the bumpiest one in
all crystals (Figure S9). The curvedness analysis in Figure 8c
show that the 14·2H2O anion is divided into several planes. The
dihedral angle between the furan and 1,3,4-oxadiazole rings in
the 6·H2O anion crystal is 10.51° (Figure 8d), which is lower than
that in 14·2H2O (10.90°). The dihedral angle between the
nitrosamino group and furan ring is 1.27°, which is also lower than
that in 14·2H2O (2.22°). Combined crystal analyses of 6·H2O and
14·2H2O shows that the bis-1,3,4-oxadiazole bridge is more
distorted than the mono-1,3,4-oxadiazole bridge, which may
explain the lower mechanical stability of bis-1,3,4-oxadiazole-
bridged compounds compared with that of mono-1,3,4-
oxadiazole-bridged-compounds. In the 6·H2O anion, N∙∙∙O, O∙∙∙O,
and O∙∙∙H interactions lead the way, accounting for 23.7%, 20.6%,
and 19.5% of the interactions, respectively (Figure 9). On the
other hand, in the 14·2H2O anion, O∙∙∙H and N∙∙∙H interactions
account for more than half of the interactions (27.1% and 27.6%,
(9101 m s-1), and nitroamine 13 (9058 m s-1) are close to those of
3,3’-dinitramino-4,4’-bifurazan (DNBF, 9086 m s-1) and HMX
(9144 m s-1).
To study the safety of these energetic compounds as energetic
materials, their impact sensitivities (IS) and friction sensitivities
(FS) were measured.[49] With the exception of 5, the IS values of
all compounds are in the range of 4.0 to 24.5 J, while the FS value
is in the range of 120 to 360 N. Tricyclic energetic compounds, 4
and 6–8, have better mechanical stability (IS = 11.0–24.5 J, FS =
280–360 N) than the high-energy explosives RDX (IS = 7.5 J, FS
= 120 N) and HMX (IS = 7.0 J, FS = 112 N) (Figure 7b). Tetracyclic
energetic compounds, 12–16, are more sensitive (IS = 4.9–9.8 J,
FS = 120–240 N) than the tricyclic series. Among these
compounds, 12 (IS = 4.9 J) is the most unstable compound in
Figure 9. 2D fingerprint plots of the anions in (a) 6·H2O and (b) 14·2H2O. (c)
Population of close contacts of anions in 6·H2O and 14·2H2O in the crystal
stacking.
Figure 7. Comparison of (a) detonation performances. (b) Sensitivities of 4–8,
12–16, FOX-7, RDX, and HMX.
6
This article is protected by copyright. All rights reserved.

10.1002/anie.202014207
Angewandte Chemie International Edition
RESEARCH ARTICLE
[16] J. Stierstorfer, K. R. Tarantik, T. M. Klapötke, Chem. Eur. J. 2009, 15,
5775-5792.
respectively). This means that hydrogen bonds are the main
intermolecular interactions in 14·2H2O.
[17] T. M. Klapötke, B. Krumm, T. Reith, Eur. J. Org. Chem. 2017, 2017,
3666-3673.
[18] Q. Yu, G. H. Imler, D. A. Parrish, J. M. Shreeve,Chem. Eur. J. 2017, 23,
17682-17686.
Conclusion
[19] G. Zhao, P. Yin, D. Kumar, G. H. Imler, D. A. Parrish, J. M. Shreeve, J.
Am. Chem. Soc. 2019, 141, 19581-19584.
In summary, a family of new mono- and bis-1,3,4-oxadiazole-
bridged furazans was synthesized and fully characterized. The
mono-1,3,4-oxadiazole-bridged compounds were prepared on
the basis of a rarely reported reaction starting from amidine. In
addition, the reaction mechanism for the synthesis of 2 was
proposed and the chemical inactivity of 3 and 11 toward oxidation
were studied computationally. Experiments and quantum
chemistry calculations proved that the 1,3,4-oxadiazole ring
improves the stability of energetic compounds. Additionally, the
thermal stability of the bis-1,3,4-oxadiazole-bridges was higher
than that of the mono-1,3,4-oxadiazole bridges. The detonation
[20] Y. H. Joo, J. M. Shreeve, Angew. Chem. Int. Ed. 2009, 48, 564-567.
[21] P. Yin, D. A. Parrish, J. M. Shreeve, Angew. Chem. Int. Ed. 2014, 53,
12889-12892.
[22] D. Fischer, T. M. Klapötke, M. Reymann, J. Stierstorfer, Chem. Eur. J.
2014, 20, 6401-6411.
[23] J. Zhang, J. M. Shreeve, J. Am. Chem. Soc. 2014, 136, 4437-4445.
[24] J. C. Dacons, M. E. Sitzmann, J. Heterocycl. Chem. 1977, 14, 1151-1155.
[25] T. M. Klapötke, T. G. Witkowski, ChemPlusChem 2016, 81, 357-360.
[26] J. Tian, H. Xiong, Q. Lin, G. Cheng, H. Yang, New J. Chem. 2017, 41,
1918-1924.
[27] W. Zhang, J. Zhang, M. Deng, X. Qi, F. Nie, Q. Zhang, Nat. Commun.
2017, 8, 181.
performances of nitroamine
5 (9211 m
s-1, 38.0 GPa),
[28] Y. Tang, C. He, L. A. Mitchell, D. A. Parrish, J. M. Shreeve, J. Mater.
Chem. A 2015, 3, 23143-23148.
hydroxylammonium salt 8 (9101 m s-1, 37.9 GPa), and nitroamine
13 (9058 m s-1, 36.2 GPa) were better than that of RDX (8795 m
s-1, 34.7 GPa) and even close to that of the highly explosive HMX
(9144 m s-1, 39.2 GPa). This systematic study of 1,3,4-
oxadiazole-bridged energetic compounds, analysis of the effect of
the 1,3,4-oxadiazole bridge on the reactivity and performance of
the compounds, and design and synthesis of 1,3,4-oxadiazoxle-
bridged compounds provides a theoretical basis for future
energetics design.
[29] T. Ichikawa, T. Kato, T. Takenishi, J. Heterocycl. Chem. 1965, 2, 253-
255.
[30] A. I. Stepanov, V. S. Sannikov, D. V. Dashko, A. G. Roslyakov, A. A.
Astrat’ev, E. V. Stepanova, Chem. Heterocycl. Com. 2015, 51, 350-360.
[31] A. B. Sheremetev, N. S. Aleksandrova, D. E. Dmitriev, B. B. Averkiev, M.
Y. Antipin, J. Heterocycl. Chem. 2005, 42, 519-525.
[32] J. Ma, J. Tang, H. Yang, Z. Yi, G. Wu, S. Zhu, W. Zhang, Y. Li, G. Cheng,
ACS Appl. Mater. Interfaces 2019, 11, 26053-26059.
[33] F. H. Allen, J. Chem. Soc., Perk. T. 2 1987.
[34] D. R. Miller, D. C. Swenson, E. G. Gillan, J. Am. Chem. Soc. 2004, 126,
5372-5373.
[35] T. M. Klapötke, B. Krumm, F. A. Martin, J. Stierstorfer, Chem. Asian J.
2012, 7, 214-224.
Acknowledgements
[36] N. V. Latypov, J. Bergman, A. Langlet, U. Wellmar, U. Bemm, 1998, 54,
11525-11536.
This work was supported by the Science Challenge Project
(TZ2018004), the National Natural Science Foundation of China
(No. 21676147, 21875110), and the Office of Naval Research
(N00014-16-1-2089).
[37] J. Evers, T. M. Klapötke, P. Mayer, G. Oehlinger, J. Welch, Inorg. Chem.
2006, 45, 4996-5007.
[38] C. Astorga, F. Rebolledo, V. Gotor, Synthesis 1991, 1991, 350-352.
[39] T. Fang, Q. Tan, Z. Ding, B. Liu, B. Xu, Org. Lett. 2014, 16, 2342-2345.
[40] M. D. Coburn, J. Labelled Compd. Rad. 1985, 22, 183-187.
[41] M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb,
J. R. Cheeseman, G. Scalmani, V. Barone, G. A. Petersson, H. Nakatsuji,
X. Li, M. Caricato, A. V. Marenich, J. Bloino, B. G. Janesko, R. Gomperts,
B. Mennucci, H. P. Hratchian, J. V. Ortiz, A. F. Izmaylov, J. L.
Sonnenberg, Williams, F. Ding, F. Lipparini, F. Egidi, J. Goings, B. Peng,
A. Petrone, T. Henderson, D. Ranasinghe, V. G. Zakrzewski, J. Gao, N.
Rega, G. Zheng, W. Liang, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J.
Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T.
Vreven, K. Throssell, J. A. Montgomery Jr., J. E. Peralta, F. Ogliaro, M.
J. Bearpark, J. J. Heyd, E. N. Brothers, K. N. Kudin, V. N. Staroverov, T.
A. Keith, R. Kobayashi, J. Normand, K. Raghavachari, A. P. Rendell, J.
C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, J. M. Millam, M. Klene, C.
Adamo, R. Cammi, J. W. Ochterski, R. L. Martin, K. Morokuma, O.
Farkas, J. B. Foresman, D. J. Fox, Gaussian 09 Rev. C.01, Wallingford,
CT, 2010.
Keywords: Energetic Materials • 1,3,4-Oxadiazole • Thermal
Stability • Synthesis • Quantum Calculation
[1]
[2]
[3]
W. E. Bachmann, W. J. Horton, E. L. Jenner, N. W. MacNaughton, L. B.
Scott, J. Am. Chem. Soc. 1951, 73, 2769-2773.
H. Feuer, J. W. Shepherd, C. Savides, J. Am. Chem. Soc. 1956, 78,
4364-4367.
A. T. Nielsen, R. A. Nissan, D. J. Vanderah, C. L. Coon, R. D. Gilardi, C.
F. George, J. Flippen-Anderson, J. Org. Chem. 1990, 55, 1459-1466.
T. N. Arnold, B. C. Santa, United States Patent 1997. US005693794A.
Y. Wang, X. Song, D. Song, L. Liang, C. An, J. Wang, J. Hazard. Mater.
2016, 312, 73-83.
[4]
[5]
[6]
Y. Liu, C. An, J. Luo, J. Wang, J. Nanosci. Nanotechnol. 2019, 19, 5783-
5789.
[7]
[8]
O. Bolton, A. J. Matzger, Angew. Chem. Int. Ed. 2011, 50, 8960-8963.
T. Fei, P. Lv, Y. Liu, C. He, C. Sun, S. Pang, Cryst. Growth Des. 2019,
19, 2779-2784.
[42] T. Yan, G. Cheng, H. Yang, New J. Chem. 2020, 44, 6643-6651.
[43] J. Shao, X. Cheng, X. Yang, J. Mol. Struc- Theochem 2005, 755, 127-
130.
[9]
J. Zhang, J. M. Shreeve, CrystEngComm 2016, 18, 6124-6133.
[10] H. Gao, J. M. Shreeve, Chem. Rev. 2011, 111, 7377-7436.
[11] H. Wei, C. He, J. Zhang, J. M. Shreeve, Angew. Chem. Int. Ed. 2015, 54,
9367-9371.
[44] M. D. Morse, Acc. Chem. Res. 2019, 52, 119-126.
[45] X. Yao, X. Hou, G. Wu, Y. Xu, H. Xiang, H. Jiao, Y. Li, J. Phys. Chem. A
2002, 106, 7184-7189.
[12] A. A. Dippold, T. M. Klapötke, J. Am. Chem. Soc. 2013, 135, 9931-9938.
[13] T. M. Klapötke, J. Stierstorfer, J. Am. Chem. Soc. 2009, 131, 1122-1134.
[14] P. Yin, Q. Zhang, J. M. Shreeve, Acc. Chem. Res. 2016, 49, 4-16.
[15] D. E. Chavez, D. A. Parrish, L. Mitchell, Angew. Chem. Int. Ed. 2016, 55,
8666-8669.
[46] A. J. Bellamy, In High Energy Density Materials, T. M. Klapötke, Ed.
Springer Berlin Heidelberg: Berlin, Heidelberg, 2007; pp 1-33.
[47] R. Meyer, J. Köhler, A. Homburg, Explosives, 6th ed. WileyVCH:
Weinheim, 2007.
[48] M. Suceska, EXPLO5 6.01, Brodarski Institute: Zagreb, Croatia, 2013.
7
This article is protected by copyright. All rights reserved.

10.1002/anie.202014207
Angewandte Chemie International Edition
RESEARCH ARTICLE
[49] Recommendations on the Transport of Dangerous Goods. In Manual of
Tests and Criteria, 5th rev. ed., Nations, U., Ed. United Nations
Publication: New York, 2009.
[50] M. J. Turner, J. J. McKinnon, S. K. Wolff, D. J. Grimwood, P. R.
Spackman, D. Jayatilaka, M. A. Spackman, CrystalExplorer17,
University of Western Australia, 2017.
8
This article is protected by copyright. All rights reserved.

10.1002/anie.202014207
Angewandte Chemie International Edition
RESEARCH ARTICLE
Entry for the Table of Contents
9
This article is protected by copyright. All rights reserved.