Intermolecular Weak Hydrogen Bonding (Het-H-N/O): An Effective Strategy for the Synthesis of Monosubstituted 1,2,4,5-Tetrazine-Based Energetic Materials with Excellent Sensitivity

Article abstract of DOI:10.1021/acs.joc.9b02484

A series of monosubstituted 1,2,4,5-tetrazine-based energetic materials was effectively synthesized and fully characterized with IR, multinuclear nuclear magnetic resonance (NMR), and elemental analyses. Heats of formation and detonation performances were determined using Gaussian 03 and EXPLO5 v6.01 programs, which show that 5 and 9 as secondary explosives have detonation velocities superior to the current secondary-explosive benchmark, triaminotrinitrobenzene (TATB). Importantly, compounds 2, 5, and 9 were first characterized with single-crystal X-ray diffraction and Hirshfeld surface calculations, and some intermolecular weak hydrogen bonds (Het-H-N/O) among these compounds illustrate the relationship between these weak interactions and excellent sensitivity of energetic materials. This design method for next-generation energetic materials by incorporating intermolecular weak hydrogen bonds may be of future importance.

Full text of DOI:10.1021/acs.joc.9b02484

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Article
Intermolecular weak hydrogen-bonding (Het-H-N/O): an
effective strategy for the synthesis of monosubstituted 1,2,4,5-
tetrazine-based energetic materials with excellent sensitivity
Yingle Liu, Gang Zhao, Qiong Yu, Yongxing Tang, Gregory
H Imler, Damon A. Parrish, and Jean'ne M. Shreeve
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.9b02484 • Publication Date (Web): 21 Nov 2019
Downloaded from pubs.acs.org on November 23, 2019
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Intermolecular weak hydrogen-bonding (Het-H-
N/O): an effective strategy for the synthesis of
monosubstituted 1,2,4,5-tetrazine-based energetic
materials with excellent sensitivity
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Yingle Liu,*a,b, Gang Zhao, b Qiong Yu,b Yongxing Tang,b Gregory H. Imler,c Damon A. Parrish,c
Jean'ne M. Shreeve*b
a School of Chemistry and Environmental Engineering, Sichuan University of Science &
Engineering, 180 Xueyuan Street, Huixing Lu, Zigong, Sichuan 643000 (China)
b Department of Chemistry, University of Idaho, Moscow, ID 83844-2343, USA.
c Naval Research Laboratory, Code 6910, 4555 Overlook Ave., Washington, DC 20375 USA.
E-mail: liuyingleyou@163.com; jshreeve@uidaho.edu
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ABSTRACT: A series of monosubstituted 1,2,4,5-tetrazine-based energetic materials were
effectively synthesized, and fully characterized with IR, multinuclear NMR, and elemental
analysis. Heats of formation and detonation performances were determined using Gaussian 03 and
EXPLO5 v6.01 programs, which show that 5 and 9 as secondary explosives have detonation
velocities superior to the current secondary-explosive benchmark, TATB. Importantly, compounds
2, 5 and 9 were first characterized with single-crystal X-ray diffraction and Hirshfeld surface
calculations, and some intermolecular weak hydrogen bonding (Het-H-N/O) among these
compounds illustrate the relationship between these weak interactions and excellent sensitivity of
energetic materials. This design method for next-generation energetic materials by incorporating
intermolecular weak hydrogen bonds may be of future importance.
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KEYWORDS: Energetic materials, Tetrazine, Weak hydrogen-bonding, Sensitivity, N-oxide.
Abstract graphic
3
3
7
2.50
6
.
2
H
N
N
N
N
N
3
3
7
.
2
2.506
2.670
2.670
N
N
N
N
N
N
N
N
H
H
IS = 42 J
FS > 360 N
Weak Hydrogen-bonding
Intermolecular weak hydrogen-bonding improve the sensitivity of energy materials.
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INTRODUCTION
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Over many years, nitrogen-rich energetic compounds have attracted significant interest from many
researchers because they can be widely used as propellants,1,2 explosives,3-6 airbag inflators7-8 and
pyrotechnics9 in satisfying the growing demand for military and civilian uses. The combination of
highly endothermic heterocycles with energetic units has become a widely accepted concept for
the design of nitrogen-rich energetic compounds in the past few decades.10,11 Among them,
energetic units, including nitro, nitroamino, dinitromethyl, and trinitromethyl groups, can greatly
enhance the oxygen balance and the density of materials which leads to an increase in detonation
performance. However, compounds with these explosophoric groups nearly always exhibit high
sensitivities towards destructive stimuli and have low thermal decomposition temperatures. A
major challenge for designing modern nitrogen-rich energetic materials is to overcome the
sensitivity versus performance trend observed for conventional explosives. 12,13
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1,2,4,5-Tetrazines, also known as s-tetrazines, has been studied for more than a hundred years.14-
16 Some 1,2,4,5-tetrazine-based energetic materials which contain various substituents at positions
3 and 6 show desirable properties associated with high N-atom content, positive heats of formation,
and thermal stabilities.17,18 Particularly, some N-oxide 1,2,4,5-tetrazine-based nitrogen-rich
energetic compounds, including 6-aminotetrazolo[1,5-b]-1,2,4,5-tetrazine-7-N-oxide,19,20 2,4-di-
N-oxide 1,2,4,5-tetrazine,21 6-amino-[1,5-b]tetrazolo-1,2,4,5-tetrazine mono-N-oxide and di-N-
oxide,22,23 have excellent detonation performance. In addition, in recent years, some 1,2,4,5-
tetrazine-based fused rings with outstanding density, insensitivity and detonation performance due
to the larger conjugated system and better stacking efficiency have been synthesized and have
attracted the wide attention of researchers.24,25 In contrast, while a considerable amount of work
has demonstrated that monosubstituted 1,2,4,5-tetrazine is a suitable starting material for the
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synthesis of heterocyclic compounds that can be used as biologically active agents for
bioorthogonal chemistry, very limited studies on monosubstituted 1,2,4,5-tetrazine-based
energetic materials have been reported.26,27 Recently fused ring-based energetic materials that were
synthesized from monosubstituted 1,2,4,5-tetrazine, III, show excellent insensitivity toward
external stimuli (IS = 31 J, FS > 240 N) which is much better than the value for I (IS = 3 J, FS >
5 N) and II (IS = 1 J, FS > 40 N) which were obtained from disubstituted 1,2,4,5-tetrazine (Fig
1).28
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Considering that different substituents on the 1,2,4,5-tetrazine ring have important effects on
the mechanical sensitivity of energetic materials and that we were inspired by the excellent thermal
behavior of monsubstituted tetrazine-based energetic compounds,26 we now report the synthesis
of 4,4'-(diazene-1,2-diyl)bis(N-(1,2,4,5-tetrazin-3-yl)-1,2,5-oxadiazol-3-amine) (2), 1,2-bis(4-
((1,2,4,5-tetrazin-3-yl)amino)-1,2,5-oxadiazol-3-yl)diazene oxide (3), N-6-amino-1,2,4,5-
tetrazine 1,5-dioxide (5), and tri(1,2,4,5-tetrazin-3-yl)amine (9) (Figure 1). Some of them show
good detonation velocities and pressures, and excellent decomposition temperatures and
sensitivities. More importantly, we obtained three monosubstituted 1,2,4,5-tetrazine crystals
a. Previous work
NHNO2
N
NHNO2
N
NHNO2
N
N3
O2NHN
N
N
N
N
N
N
N
N
N
N
N
IS = 31 J
FS > 240 N
Td = 132 o
IS = 1 J
IS = 3 J
N
N
N
N
FS > 40 N
FS > 5 N
III
Td = 150 o
C
Td = 138 o
C
I
II
C
b. This work
N
N
N
N
O(n)
NH2
H
N
H
N
N
N
N
N
N
O
O
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
O
O
5
2
3
9
n = 1
IS = 33 J
FS > 360 N
Td = 232 o
n = 0
IS = 29 J
FS > 360 N
Td = 219 o
IS = 42 J
FS > 360 N
Td = 231 o
IS = 32 J
FS > 360 N
Td = 278 o
C
C
C
C
Figure 1. Monosubstituted tetrazine-based energetic materials in previous studies and this study.
containing no solvent molecules which illustrate the relationship between the intermolecular weak
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hydrogen-bonding and mechanical sensitivity of energetic materials. It is our intention to provide
more information for designing next-generation energetic materials with excellent sensitivity and
thermal stability.
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RESULTS AND DISCUSSION
Synthesis
The synthetic pathways to monosubstituted energetic materials 2, 3, 5 and 9 are shown in Scheme
1. 3-(3,5-Dimethyl-1H-pyrazol-1-yl)-1,2,4,5-tetrazine (1) was synthesized according to the
literature.26 It was treated with 4,4'-(diazene-1,2-diyl)bis(1,2,5-oxadiazol-3-amine) or 1,2-bis(4-
amino-1,2,5-oxadiazol-3-yl)diazene oxide in acetonitrile with K2CO3 or Cs2CO3 as base,
respectively, to obtain the corresponding 4,4'-(diazene-1,2-diyl)bis(N-(1,2,4,5-tetrazin-3-yl)-
1,2,5-oxadiazol-3-amine) (2) and 1,2-bis(4-((1,2,4,5-tetrazin-3-yl)amino)-1,2,5-oxadiazol-3-
yl)diazene oxide (3). Compounds 5 and 9 were first synthesized by Hiskey21 and Rudakov,26
respectively, but crystal structures, and the physicochemical and energetic properties
O(n)
Me
N
N
N
H2N
NH2
NH2
NH2
(CF3CO)2O,
H2O2
N
N
Me
N
O
N
O
NH3.H2O
CH3CN
N
O
O
N
N
N
N
N
N
N
N
DCM, rt
N
N
N
N
1) K2CO3/Cs2CO3
CH3CN, 80 o
2) HCl, rt
C
4
5
1
N2H4.H2O
CH3CN, rt
N
N
N
N
N
O(n)
N
N
N
N
N
N
N
N
N2H4
H
N
H
N
Cl
N
N
N
N
N
N
N
N
H
N
N
8
TCCA
CH3CN
N
N
N
N
N
N
N
N
N
N
N
N
N
N
1) K2CO3, CH3CN,
80 o
2) HCl, rt
N
N
N
O
O
N
N
N
N
C
7
2
3
6
n=0
n=1
9
Scheme 1. Preparation of monosubstituted 1,2,4,5-tetrazine energetic materials 2, 3, 5 and 9.
of these compounds have not been reported until now. Now we have improved the synthetic
methods for 5 and 9. As shown in Scheme 1, via replacing the hard-to-obtain compound
peroxytrifluoroacetic acid with a mixture of the common compounds trifluoroacetic acid anhydride
and hydrogen peroxide, 1,2,4,5-tetrazin-3-amine was easily oxidized to the yellow N-6-amino-
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1,2,4,5-tetrazine 1,5-dioxide (5) in high yield. In addition, taking into account the toxicity of
chlorine gas and the inconvenience of its use, trichloroisocyanuric acid (TCCA) was used as a
chlorinating reagent to transform 3-hydrazinyl-1,2,4,5-tetrazine (6) into 3-chloro-1,2,4,5-tetrazine
(7). Finally, 7 was reacted with di(1,2,4,5-tetrazin-3-yl)amine (8) to give the desired tri(1,2,4,5-
tetrazin-3-yl)amine (9).
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Spectral studies of compounds
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13
Compounds 2, 3, 5 and 9 were fully characterized by H and C{1H} NMR and IR spectral
analysis as well as by elemental analysis. In the 13C{1H} NMR spectra of compounds 2, 3 and 9,
two signals were observed in the range of 162.6–164.6 ppm for C3 and 150.7–158.2 ppm for C6
corresponding to the different types of carbon atoms in the monosubstituted 1,2,4,5-tetrazine rings.
However, due to the electron-donating effect of the oxygen atoms, the carbon signals for the
monosubstituted 1,2,4,5-tetrazine ring in 5 shifts to high field. The carbon resonance of C3 in 5
was observed at 144.5 ppm and the carbon resonance of C6 at 149.4 ppm.
Single crystal X-ray diffraction studies
Single-crystals suitable for X-ray diffraction studies were obtained by slow evaporation of a mixed
solution of DMSO (2, 5), DMF (5) or CH3CN (9). Crystal structures of these compounds are given
in Figure 2-5. The detailed crystallographic refinement parameters and structural data for these
compounds can be found in the SI.
Compound 2 crystallizes in the monoclinic space group P21/c with a density of 1.665 g cm −3 at
20 °C (Figure 2). The two furazan rings, two secondary amino groups and the azo group are nearly
coplanar, which is shown by the torsion angles N(7)–C(8)–C(12)–N(13) = 4 .07(3)° and N(13)–
N(13)–C(12)–C(8) = -1.12(2)°. However, there is a twist angle between the plane of two
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Figure 2. (a) Molecular structure of 2. (b) Packing diagram of 2. (c) Weak hydrogen-bonding of 2.
monosubstituted tetrazine rings and two furazan rings, which is seen by the torsion angle C(6)–
N(7)–C(8)–N(9) = 27.24(2)°. The lengths of the N–C, N=C, N-N or N=N bonds in the tetrazine
ring (C(6)–N(5): 1.340 Å; N(5)-N(4): 1.319 Å; N(4)-C(3): 1.327 Å; C(3)–N(2): 1.317 Å; N(2)-
N(2): 1.317 Å; N(2)-N(1): 1.324 Å; N(1)-C(6): 1.330 Å) are slightly different, and the average
bond length in the monosubstituted 1,2,4,5-tetrazine ring is 1.326 Å which is shorter than the
values of all the other 3,6-disubstituted tetrazines. For example, 1.335 Å in N3, N6-di(1H-1,2,4-
triazol-5-yl)-1,2,4,5-tetrazine-3,6-diamine perchlorate salt17 and 1.336 Å in N3,N6-bis(3-azido-1H-
1,2,4-triazol-5-yl)-1,2,4,5-tetrazine-3,6-diamine.18 Unexpectedly, in addition to the hydrogen
bond formed by the secondary amine with nitrogen atoms and weak interaction formed among C,
N and O atoms in different molecules, a weak hydrogen bond (Het-H-N)29,30 exists between the C-
H of a monosubstituted 1,2,4,5-tetrazine and a nitrogen atom belonging to the furan ring of another
molecule. From this view, the monosubstituted tetrazine ring should be more aromatic and less
sensitive toward external stimuli than the 3,6-disubstituted tetrazine.
A colorless crystal of 5•DMSO was obtained by slow evaporation of a DMSO solution that
contained 5 (Figure 3). 5.DMSO crystallizes in the orthorhombic P212121 space group with four
molecules in the unit cell with a calculated density of 1.567 g cm-3 at 293 K. Considering the weak
interactions and the arrangement of the molecules in space are very important parameters for
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understanding the properties of energetic materials. A crystal which contains no solvent molecules
is often very important in determining the properties of energetic materials.31 Subsequently, 5
was dissolved in various other solvents, including MeOH, CH3CN or DMF, and a single crystal
was grown by slowly evaporating the corresponding solvent. Ultimately, a yellow crystal of 5
which contained no solvent was successfully obtained from the solution of DMF (Figure 4).
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(a)
(b)
Figure 3. (a) Molecular structure of 5.DMSO. (b) Packing diagram of 5.DMSO.
Compound 5 is in the orthorhombic Iba2 space group with eight molecules in the unit cell and a
crystal density of 1.770 g cm−3 at 20 °C. The H, O, C and N atoms in 5 are all nearly planar, which
is supported by the torsion angles H(4A)–N(4)–C(3)–N(5) = -0.1(0)°, C(3)–N(5)–N(7)–C(8) =
0.3(4)°, H(8)-C(8)-N(9)-N(2) = -179.5(6)° and O(1)-N(2)-C(3)-N(4) = 1.6(5)°. The average length
of the N–C, N=C, N-N or N=N bonds in the monosubstituted 1,2,4,5-tetrazine ring are 1.339 Å,
falling within the range of similar 3,6-disubstituted 1,2,4,5-tetrazine rings (1.335 Å -1.344 Å).
However, the average length of the N-oxide bond (1.271 Å) in 5 is longer than all of the reported
similar N-oxide bonds in 3,6-disubstituted 1,2,4,5-tetrazine rings (1.259 Å -1.269 Å).22, 23
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Figure 4. (a) Molecular structure of 5. (b) Hydrogen bond and weak hydrogen bonds in the crystal of
5. (c) Packing diagram of 5.
It is easy to speculate that the electron density in monosubstituted 1,2,4,5-tetrazine 1,5-oxides is
higher than the corresponding value in 3,6-disubstituted 1,2,4,5-tetrazine 1,5-oxides. Additionally
each NH2 group interacts with H(4), O(6), N(9) in two other molecules through hydrogen-bonding,
and C-H in the monosubstituted 1,2,4,5-tetrazine ring interacts with the N or the O atom in the
other two molecules via two intermolecular weak hydrogen bonds (Het-H-N/O). The lengths of
these two Het-H-N/O bonds are 2.632 Å and 2.628 Å, which are longer than the values of other
normal hydrogen bonds (2.074 Å - 2.222 Å). Under the influence of these interactions, five
molecules create a wave-like stacking arrangement (interlayer distance = 2.935 Å) (Fig. 4). As a
result, the packing coefficient of 5 is 0.73.
Tri(1,2,4,5-tetrazin-3-yl)amine (9) crystallizes in the monoclinic P21/c space group with four
formula units per cell and a crystal density of 1.729 g cm−3 at 293 K (Figure 5). The lengths of the
N–C, N=C, N-N or N=N bonds in the monosubstituted 1,2,4,5-tetrazine ring (C(8)–N(13): 1.329
Å; N(13)-N(12): 1.324 Å; N(12)-C(11): 1.321 Å; C(11)–N(10): 1.322 Å; N(10)-N(9): 1.321 Å;
N(9)-C(8):1.331 Å) are nearly the same, so this compound should possess very good aromaticity.
Unexpectedly, the three 1,2,4,5-tetrazine rings interact with each other with strong intramolecular
N–N interactions in the range of 2.749-2.910 Å. Because of this kind of directional interaction, the
three 1,2,4,5-tetrazine rings are distorted with a dihedral angle C(8)–
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(b)
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.84.444
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2.506
2.670
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3
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7
.
2
2.506
2.670
Figure 5. (a) Molecular structure of 9. (b) N–N interactions in the crystal of 9. (c) N-H weak hydrogen-
bonding in crystal of 9. (c) Packing diagram of 9.
N(7)–C(6)–N(5) = 36.38(3)°. In the vertical direction, there are four N–N interactions between the
monosubstituted 1,2,4,5-tetrazine ring of one molecule and two other molecules in the range of
3.012-3.064 Å, and the distance between two center nitrogen atoms is 4.444 Å. In the horizontal
direction, there are six Het-H…N weak hydrogen bonds between the 1,2,4,5-tetrazine ring of one
molecule and other four molecules in the range of 2.506-2.733 Å. Based on these weak
interactions, the crystal 5 is stacked like a superimposed flower with a packing coefficient of 0.72.
Physicochemical and energetic properties
The onset decomposition temperatures and thermal stabilities of compounds 2, 3, 5 and 9 were
evaluated using differential scanning calorimetric (DSC) measurements with a heating rate of 5
C min−1 (Table 1). All of compounds (2, 3, 5 and 9) decomposed without melting. Among them,
the monosubstituted 1,2,4,5-tetrazine-1,5-dioxide (5) exhibits the best thermal stability with an
onset decomposition temperature of 278 C, which is higher than the corresponding values of all
reported 3,6-disubstituted 1,2,4,5-tetrazine-1,5-dioxides (Td: 110-252 C)22,23 and N-oxide 1,2,4,5-
tetrazine-based fused rings (Td: 140-185 C).19,20 This can be explained by the higher electron
density and better aromaticity in monosubstituted 1,2,4,5-tetrazine-1,5-dioxides
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Table 1. Energetic properties of compounds 2, 3, 5 and 9.
5
6
7
8
a
Pb
△Hfc
Dvd
Pe
ISf
(J)
29
33
32
42
15
50
FSg
(N)
Tdec
Comp.
(g cm-3) (kJ mol-1/kJ g-1) (m s-1) (GPa)
(C)
219
232
278
231
295
360
2
3
5
1.67
1.75
1.77
1.73
1.65
1.930
1654.3/4.65
1517.6/4.08
237.2/1.84
1386.2/5.39
-59.4/-0.26
-154.2/-0.60
8075
8401
8640
8322
6881
8114
25.1
28.3
30.2
26.4
19.5
31.2
>360
>360
>360
>360
>353
360
9
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9
TNT[h]
TATB[i]
LLM-
116[j]
RDX [k]
178
1.90
96.3/0.56
8488
34.2
>40
360
204
1.80
70.3/0.36
8795
34.9
7.4
120
[a] Decomposition temperature (onset). [b] Density measured by a gas pycnometer at 25 ℃. [c]
Calculated molar enthalpy of formation in solid state. [d] Calculated detonation velocity. [e]
Calculated detonation pressure. [f] Impact sensitivity. [g] Friction sensitivity. [h] Ref.32. [i] Ref.
33. [j] Ref. 34. [k] Ref. 35.
than the corresponding value in 3,6-disubstituted 1,2,4,5-tetrazine-1,5-dioxides. Other compounds
have thermal decomposition temperatures which fall between 219 C and 232 C.
The experimental densities of all monosubstituted 1,2,4,5-tetrazine compounds were obtained
by using a gas pycnometer (25 C) and found to range between 1.67 and 1.77 g cm-3. These are
higher than the density of TNT (1.65 g cm-3)32 which is a classic explosive, and the density (1.77
g cm-3) of 6-amino-1,2,4,5-tetrazine-1,5-dioxide, 5, is comparable to the currently used typical
HEDMs, such as RDX (1.80 g cm-3). The remarkably higher density of 5 may be attributed to its
planar conjugated system and a wave-like stacking arrangement. Heats of formation of 2, 3, 5 and
9 were calculated with Gaussian 03 software36 based on isodesmic reactions (SI). Due to the presence
of a large number of N–N or C–N bonds in the monosubstituted 1,2,4,5-tetrazine rings, all compounds
have relatively high positive heats of formation (≥ 1.84 kJ g-1) exceeding the values for TNT (-0.26 kJ
g-1),32 TATB (-0.60 kJ g-1),33 LLM-116 (0.56 kJ g-1)34 and RDX (0.36 kJ g-1)35 markedly.
Based on the measured densities and calculated heats of formation, detonation performances of
2, 3, 5 and 9 were calculated by using EXPLO5 (v 6.01).37 As shown in Table 1, the calculated
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detonation velocities are found in the range 8075 to 8640 m s-1. Among them, 5 has the highest
detonation velocity (8640 m s-1) which exceeds that of LLM-116 (8488 m s-1) and is comparable
to RDX (8795 m s-1). The detonation pressures fall between 25.1 and 30.2 GPa with values in
some cases exceeding TNT (19.5 GPa) and approaching TATB (31.2 GPa). Sensitivities toward
impact (IS) and friction (FS) were obtained using BAM technology.38 Not surprisingly these four
monosubstituted 1,2,4,5-tetrazine compounds exhibit low impact and friction sensitivities (IS, 29–
42 J; FS > 360 N) because of the good aromaticity of the monosubstituted 1,2,4,5-tetrazine rings
and intermolecular weak hydrogen bonds. It should be noted that tri(1,2,4,5-tetrazin-3-yl)amine
(9) has the lowest sensitivities (IS = 42 J; FS > 360 N), and the monosubstituted 1,2,4,5-tetrazine-
1,5-oxide, 5, shows lower sensitivities (IS = 32 J; FS > 360 N) than the corresponding values of
all reported 3,6-disubstituted 1,2,4,5-tetrazine-1,5-dioxides (IS: 3-27 J; FS: 10-360 N)22 and N-
oxide 1,2,4,5-tetrazine-based fused rings (IS: 1.5-10 J; FS: 10-160 N),19,20 but it is slightly higher
than 3-amino-6-(3,5-dimethylpyrazol-l-yl)-1,2,4,5-tetrazine 2,4-dioxide (IS = 35 J; FS > 360
N)11a and the recently reported polycyclic N-oxides based on 3,6-disubstituted-1,2,4,5-tetrazine
(IS: 18.5-78.4 J; FS>360 N).23
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To gain further information about sensitivity of this series of compounds, weak interactions in
representative compounds 5 and 9 were investigated via Hirschfeld surfaces and 2D fingerprint
plots.39 As shown in Fig. 6a and b, the surface of 5 is approximately horizontal and the surface of
compound 9 is slightly distorted. Furthermore, some red dots mainly caused by the intermolecular
normal hydrogen bonding and/or weak hydrogen bonding (Het-H-N) were found on the edges of
each surface. The populations of these weak interactions can also be directly achieved by the 2D
fingerprint plots. From Fig. 6c and d, the normal hydrogen bonding and weak hydrogen bonding
possess 56.0% of the total weak interactions for 5 (Figure S22), and weak hydrogen bonding
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possesses 44.3% of the total weak interactions for 9. Remembering that the intermolecular
hydrogen bonding has a stabilizing action toward impact and friction,28,40 it is readily understood
that 5 and 9 have good sensitivities (IS ≥ 32 J, FS > 360 N). In addition, the sum of proportions
of N/N, N/C and C/N for 9 is 50.6% which mainly comes from the intramolecular interaction a a
among three monosubstituted 1,2,4,5-tetrazine rings and intermolecular interactions caused by
different monosubstituted 1,2,4,5-tetrazine rings. Mechanical stimuli toward 9 from any
orientation can be buffered. So it is not surprising that the 9 exhibits superior sensitivity values (IS
= 42J; FS > 360N).
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(a)
(b)
(c)
(d)
N...
C
dr
dr
O...
N
C...
N
H...
N
H...
N
N...
N...
O
H...
O
H
N...
N
O...
H
H...
H
N...
H
di
di
(f)
(e)
N
C
...C
...N
...H 2.4%
N...
H...
H
N
N...
O
H
30.5%
25.5%
N
H
...H
...N
O...
30.5%
N
%
13.7
%
44.3
C...
H
C
&
2.4%
1.2%
H...
C...H & H...C 1.8%
C...O & O...C 7.5%
C...
C
O...
H...
H
N...
N
%
36.9
O...O 2.4%
O
H...
H
N...N 2.7%
C...N & N...C 2.5%
9.2%
Figure 6. Two-dimensional fingerprint plots in crystal stacking for 5 and 9 as well as the associated
Hirshfeld surfaces. Images (a) and (b) show the Hirshfeld surfaces that use color coding to
represent the proximity of close contacts around 5 and 9 molecules (white, distance d equals the
van der Waals distance; blue, d exceeds the van der Waals distance; red, d is less than the van der
Waals distance). The fingerprint plots in crystal stacking for 5 (c) and 9 (d). In images (e) and (f),
the individual atomic contacts percentage contribution to the Hirshfeld surface are shown in the
pie graphs for 5 and 9, respectively.
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CONCLUSION
5
6
7
8
In conclusion, a series of monosubstituted 1,2,4,5-tetrazine-based energetic materials was
synthesized. All the new compounds were fully characterized with IR, multinuclear NMR, and
elemental analysis. Three monosubstituted 1,2,4,5-tetrazine structures were confirmed via X-ray
diffraction. Some intermolecular weak hydrogen bonds (het-H-N/O) among these compounds
have been discovered. Based on this weak hydrogen bonding, these derivatives of monosubstituted
1,2,4,5-tetrazine exhibit excellent low sensitivity. Especially, 9, similar to TATB, is insensitive to
mechanical stimuli and its calculated detonation velocities and pressures are comparable to the
current secondary-explosive benchmark TATB. The monosubstituted 1,2,4,5-tetrazine 1,5-dioxide
(5) was found to possess good density, excellent thermal stability, and good detonation
performance which outperform commonly used explosives, for example, TNT, TATB, and LLM-
116. Compounds 5 and 9 have significant application potential as secondary explosives.
9
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EXPERIMENTAL SECTION
General methods
All reagents were purchased from AKSci, VWR or Alfa Aesar in analytical grade and were used as
supplied. 1H and 13C NMR spectra were recorded on a Bruker 300 MHz NMR spectrometer or 500
MHz (Bruker AVANCE 500) nuclear magnetic resonance spectrometer. Chemical shifts for 1H, and
13C NMR spectra are reported relative to (CH3)4Si. [D6]DMSO was used as a locking solvent unless
otherwise stated. Infrared (IR) spectra were recorded on an FT-IR spectrometer (Thermo Nicolet
AVATAR 370) as thin films using KBr plates. Density was determined at room temperature by
employing a Micromeritics AccuPyc II 1340 gas pycnometer. Melting and decomposition (onset) points
were recorded on a differential scanning calorimeter (DSC, TA Instruments Q2000) at a scan rate of 5
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oC min-1. Elemental analyses (C, H, N) were performed on a Vario Micro cube Elementar Analyser.
Impact and friction sensitivity measurements were made using a standard BAM Fall hammer and a
7
8
9
BAM friction tester.
determination.
A Waters Q-Tof Premier mass spectrometer was used for HRMS
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Computational methods
The gas phase enthalpies of formation were calculated based on isodesmic reactions (Scheme S1). The
enthalpy of reaction is obtained by combining the MP2/6-311++G** energy difference for the reactions,
the scaled zero point energies (ZPE), values of thermal correction (HT), and other thermal factors. The
solid state heats of formation of 2, 3, 5 and 9 were calculated with Trouton's rule according to eqn (1)
(T represents either the melting point or the decomposition temperature when no melting oC occurs prior
to decomposition).41
△Hsub =188/J mol-1 K-1× T
(1)
Experimental methods
Compounds 4, 6, 8 and 9 were synthesized according to the reported method and the spectral data
of 1H NMR are the same as reported literature.26 1H and 13C{H} NMR spectra of compound 5 are
identical with the data reported in the literature.21
4,4'-(Diazene-1,2-diyl)bis(N-(1,2,4,5-tetrazin-3-yl)-1,2,5-oxadiazol-3-amine) (2).
A mixture of 4,4'-(diazene-1,2-diyl)bis(1,2,5-oxadiazol-3-amine) (3.0 mmol, 0.59 g), 1,2,4,5-
tetrazine 1 (6.0 mmol, 1.06 g), K2CO3 (6.0 mmol, 0.83 g) and MeCN (15 mL) was heated at reflux
(80 oC) in an oil bath, checking the efficiency of the reaction by TLC, until the starting material 1
had totally disappeared (~ 4 h). The reaction mixture was cooled and diluted with ice water (30
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mL). The mixture was acidified to a pH of 1 by dropwise addition of 5% HCl to generate a fine
red precipitate that was collected by vacuum filtration. The red precipitate was washed with cold
water (3 × 10 mL) and dried overnight under high vacuum to provide pure red solid 2 (0.91 g) in
85% yield. Td 219 oC; 1H NMR (300 MHz, [D6]DMSO) δ 12.13 (s, 2H), 10.28 (s, 2H) ppm; 13C{1H}
NMR (75 MHz, [D6]DMSO) δ 162.8, 159.2, 156.8, 145.6 ppm; IR (KBr pellet): ṽ 3193, 3068,
3024, 2435, 1597, 1455, 1399, 1251, 1122, 1043, 936, 821, 604 cm-1. Crystal structure
confirmation is given.
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1,2-Bis(4-((1,2,4,5-tetrazin-3-yl)amino)-1,2,5-oxadiazol-3-yl)diazene oxide (3).
A mixture of 1,2-bis(4-amino-1,2,5-oxadiazol-3-yl)diazene oxide (3.0 mmol, 0.64 g), 1,2,4,5-
tetrazine 1 (6.0 mmol, 1.06 g), Cs2CO3 (6.0 mmol, 1.96 g) and MeCN (15 mL) was heated to reflux
(80 oC) in an oil bath, checking the efficiency of the reaction by TLC, until the starting material 1
had totally disappeared (~ 6 h). The reaction mixture was cooled and diluted with ice water (30
mL). The mixture was acidified to a pH of 1 by dropwise addition of 5% HCl to generate a fine
red precipitate that was collected by vacuum filtration. This red precipitate was washed with cold
water (3 × 10 mL) and dried overnight under high vacuum to leave a pure red solid 3 (0.87 g) in
78% yield. Td 232 oC; 1H NMR (300 MHz, [D6]DMSO) δ 12.25 (s, 2H), 10.29 (s, 1H), 10.25 (s,
1H) ppm; 13C{1H} NMR (75 MHz, [D6]DMSO) δ 162.9, 162.6, 157.0, 156.9, 148.4. 146.6 pm;.
IR (KBr pellet): ṽ 3102, 3030, 2852, 2788, 2543, 2438, 1519, 1420, 1345, 1265, 1201, 1138, 1074,
1013, 935, 764, 608 cm-1; HRMS (ESI-TOF) m/z: [M + H]+ Calcd for C8H5N16O3: 373.0550;
Found 373.0737.
6-Amino-1,2,4,5-tetrazine 1,5-dioxide (5). Trifluoroacetic anhydride (4 mL, 28 mmol) was added
to a slurry of 50% hydrogen peroxide (1.3 mL, 25 mmol) in methylene chloride (20 mL) with
stirring at <10 oC. The tetrazine compound 4 (7 mmol) was added at 0 oC and stirred for 30 min;
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then for 12 hours at room temperature. The solvent was removed and the residue was washed with
5
6
o
ether and then air-dried to give a yellow solid 5 (0.68 g) in 75% yield. Td 278 C; 1H NMR (d6-
7
8
9
DMSO) δ 8.62 (s, 2H), 8.52 (s, 1H) ppm. 13C{1H} NMR (d6-DMSO) δ 149.4, 144.5 ppm. IR (KBr
pellet): ṽ 3259, 3092, 3048, 3007, 2429, 1874, 1592, 1493, 1450, 1387, 1259, 1121, 1050, 934,
807, 608 cm-1. 1H NMR spectra given in SI.
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3-Chloro-1,2,4,5-tetrazine (7). A slurry of 3-hydrazinyl-1,2,4,5-tetrazine (8 mol, 0.90 g) in
acetonitrile (16 mL) was cooled to 0 oC. After cooling, excess trichloroisocyanuric acid (9.6 mol,
2.23 g) dissolved in acetonitrile (20 mL) was added to the slurry dropwise. During the addition,
the solvent changed to a bright orange color, and gas evolution was observed. After the addition,
the system was warmed to room temperature and then stirred for an additional 30 min. The
precipitate was removed by vacuum filtration, and the solvent was removed from the filtrate via
o
rotary evaporation (the temperature of the bath should be < 25 C) leaving behind an orange
powder 7 (0.59 g) in 64% yield. The product was utilized without further purification since the
NMR spectral data are the same as reported literature.26 1H NMR (d6-DMSO) δ 10.30 (s, 1H) ppm;
13C{1H} NMR (d6-DMSO) δ157.8, 170.8 ppm.
Tri(1,2,4,5-tetrazin-3-yl)amine (9) was obtained according to the literature.26 Red solid (60%);
Td 231 oC; 1H NMR (d6-DMSO) δ 10.82 (s, 3H) ppm. 13C{1H} NMR (d6-DMSO) δ 158.2, 164.6
ppm. IR (KBr pellet): ṽ 3012, 1427 cm-1. 1H NMR spectrum provided in SI.
ASSOCIATED CONTENT
Supporting Information.
The Supporting Information is available free of charge on the ACS Publications website at DOI:
xxxxxxxxxxxxxx. NMR spectra and CIF files; Theoretical calculations, X-ray crystallography of
2, 5.DMSO, 5 and 9.
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AUTHOR INFORMATION
5
6
7
Corresponding Authors
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9
* (Y. L) E-mail: liuyingleyou@163.com.
* (J. M. S.) E-mail: jshreeve@uidaho.edu
Notes
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The authors declare no competing financial interest.
Caution!
Although no explosions were observed during the syntheses and handing of these compounds in this
study, all manipulations should be carried out in a hood behind a safety shield. Mechanical actions
involving scratching or scraping must be avoided. Eye protection and leather gloves should be worn.
All of the energetic compounds must be synthesized on a small scale.
ACKNOWLEDGMENTS
This work was supported by the Office of Naval Research (N00014-16-1-2089), the Defense
Threat Reduction Agency (HDTRA 1-15-1-0028), Science and Technology Department of
Sichuan Province (No. 2016JY0246), Education Department of Sichuan Province (No. 16ZB0247)
and Open Project Program of Chemical Synthesis and Pollution Control Key Laboratory of
Sichuan Province (No. CSPC201906).
REFERENCES
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Thermo-Mechanical Properties of NCO-fMWCNTs/HTPB PU and Alkyne-fMWCNTs/acyl-GAP
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Based Nanocomposite and its Propellants. RSC Adv. 2016, 6, 49101–49112; (b) Hiskey, M. A.;
Chavez, D. E.; Naud, D. L. Propellant Containing 3; 6-Bis (1H-1, 2, 3, 4-Tetrazol-5-ylamino)-1, 2,
4, 5-tetrazine or Salts thereof. U.S. Pat. 6, 458, 227, 2002.
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(2) Zhang, X.; Shen, L.; Luo, Y.; Jiang, R.; Sun, H.; Liu, J.; Fang, T.; Fan, H.; Liu, Z. Synthesis and
Ignition Properties Research of 1,5-Diazabicyclo[3.1.0]Hexane Type Compounds as Potential
Green Hypergolic Propellants. Ind. Eng. Chem. Res. 2017, 56, 2883–2888.
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