The many faces of FOX-7: A precursor to high-Performance energetic materials

Article abstract of DOI:10.1002/anie.201501973

New derivatives of 1,1-diamino-2, 2-dinitroethene (FOX-7) are reported. These highly oxygen- and nitrogen-rich compounds were fully characterized using IR and multinuclear NMR spectroscopy, elemental analysis (EA), and differential scanning calorimetry (DSC). X-ray structure determination of (E)-1,2-bis{(E)-2-chloro-1-(chloroimino)-2,2-dinitroethyl}diazene) (10), N1, N2-dichloro-1, 2-diazenedicarboximidamide (11), and (E,E)-N,N′-1,2-ethanediylidenebis(2, 2-dinitro-2-chloro-ethanamine) (12) was helpful in their characterization. Heats of formation (HOF) were calculated (Gaussian03) and combined with experimental densities to estimate the detonation velocities (D) and pressures (P) of the high-energy-density materials (HEDMs) (EXPLO5, v6.01). The compounds exhibit good thermal stability, high density, positive HOF, acceptable oxygen balances, and excellent detonation properties, which often are superior to that of 1,3,5-trinitroperhydro-1,3,5-triazine (RDX). Fox in a box: New derivatives created from 1,1-diamino-2, 2-dinitroethene (FOX-7) often have properties superior to those of RDX. The compounds exhibit good thermal stability, high density, positive heats of formation, an acceptable oxygen balance, and excellent detonation properties. These findings contribute to the value of FOX-7 as an energetic precursor.

Full text of DOI:10.1002/anie.201501973

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201501973
German Edition: DOI: 10.1002/ange.201501973
Energetic Materials
The Many Faces of FOX-7: A Precursor to High-Performance
Energetic Materials**
Haixiang Gao* and Jeanꢀne M. Shreeve*
Abstract: New derivatives of 1-diamino-2, 2-dinitroethene
(FOX-7) are reported. These highly oxygen- and nitrogen-rich
compounds were fully characterized using IR and multinuclear
NMR spectroscopy, elemental analysis (EA), and differential
scanning calorimetry (DSC). X-ray structure determination of
(E)-1,2-bis{(E)-2-chloro-1-(chloroimino)-2,2-dinitroethyl}-
diazene) (10), N1, N2-dichloro-1, 2-diazenedicarboximida-
mide (11), and (E,E)-N,N’-1,2-ethanediylidenebis(2, 2-dinitro-
2-chloro-ethanamine) (12) was helpful in their characteriza-
tion. Heats of formation (HOF) were calculated (Gaussian03)
and combined with experimental densities to estimate the
detonation velocities (D) and pressures (P) of the high-energy-
density materials (HEDMs) (EXPLO5, v6.01). The com-
pounds exhibit good thermal stability, high density, positive
HOF, acceptable oxygen balances, and excellent detonation
properties, which often are superior to that of 1,3,5-trinitro-
perhydro-1,3,5-triazine (RDX).
new HEDMs include high density, positive heats of formation
(HOF), positive oxygen balance (OB), high detonation
velocity and pressure, high thermal stability, simple synthesis,
low sensitivity toward external forces, such as impact and
friction, and environmental friendliness.[1–5]
1,1-Diamino-2,2-dinitroethene (FOX-7) was first synthe-
sized nearly 20 years ago.[7] Recently, this compound has
emerged as a potential candidate for use as an insensitive
HEDM, attracting substantial interest because its perfor-
mance as an explosive is comparable to RDX. Furthermore,
the sensitivity to impact and friction of FOX-7 and, thus to
involuntary detonation, is markedly lower.[8] These properties
render FOX-7 attractive because the low sensitivity of this
HEDM will reduce the risk of serious and fatal accidents
during its handling and application. For future applications,
FOX-7 is considered an important preferred component of
propellants and has attracted interest from researchers for its
high potential in diverse applications. Many studies have been
performed on the synthesis,[9] molecular structure,[10] reac-
tions,[11] theoretical calculations,[12] thermal behavior,[13]
explosive performance,[14] and applications of FOX-7.[15]
In a continuing effort to seek less sensitive, more power-
ful, eco-friendly HEDMs, our interest focused primarily on
the modification of the molecular structure of FOX-7 to
obtain new HEDMs that contain a high percentage of oxygen
and nitrogen and lower amounts of carbon and hydrogen. We
noted that the molecular structure of FOX-7 contains the
T
he design, synthesis, and characterization of high-energy-
density materials (HEDMs) have attracted considerable
interest from researchers around the world.[1] In general, the
raison dꢀetre for the search for new HEDMs is the desire to
achieve a detonation performance comparable to those of the
current benchmark HEDMs, such as octahydro-1,3,5,7-tetra-
nitro-1,3,5,7-tetrazocine (HMX) and 2,4,6,8,10,12-hexanitro-
2,4,6,8,10,12-hexaazaisowurtzitane (CL-20).[2,3] Furthermore,
there are several criteria that new HEDMs should satisfy,
such as inexpensive synthesis, safe handling, and good
environmental compatibility, among others. In the quest for
higher detonation-performing HEDMs, the field of HEDMs
chemistry must expand the boundaries of the energy capacity
of compounds, which requires new classes of functionalized
compounds,[4] advanced theoretical prediction techniques,[5]
and new synthetic strategies.[6] Desirable characteristics for
functional group C(NH2)2, similar to nitroguanidine.[16] This
=
fact suggests that FOX-7 could be a good intermediate for
reaction with nucleophiles and electrophiles, enabling the
design and optimization of the molecular structure to enhance
performance with respect to high energy and insensitivity.
Our earlier research showed that the design approach is
a practical method to improve the detonation performance
properties relative to those of FOX-7 by modifying its
structure with metal,[17] nitro,[18] or azo functional groups.[19]
Pursuing our continuing interest to extend the chemistry of
FOX-7 and its derivatives, in this study, we report the
synthesis of a series of FOX-7 derivatives that display
potentially significant physical and energetic properties.
The nitro group is a vital constituent of HEDMs; its
presence greatly contributes to the overall energetic perfor-
mance. Additionally, the nitro group enhances the OB
(oxygen balance) and density which improves the detonation
performance (pressure and velocity).[20] Initially, our strategy
was to introduce additional nitro groups to modify FOX-7.
Hexahydro-3-tert-butyl-2,2-dinitromethylene-1, 3, 5-triazine
(2) was isolated as a light yellow crystalline solid in 91% yield
from the Mannich condensation of FOX-7 (1), formaldehyde,
and tert-butyl amine (Scheme 1). Compound 2 was nitrated
[*] Prof. Dr. H. Gao
Department of Applied Chemistry, China Agricultural University
Beijing, 100193 (China)
E-mail: hxgao@cau.edu.cn
Prof. Dr. J. M. Shreeve
Department of Chemistry, University of Idaho
Moscow, ID 83844-2343 (USA)
E-mail: jshreeve@uidaho.edu
[**] We are grateful for the support of the Office of Naval Research
(N00014-12-1-0536), the Defense Threat Reduction Agency
(HDTRA1-11-1-00340) and Grant of China Scholarship Council
(CSC No, 201306355006). We are indebted to Dr. Orion Berryman
(NSF CHE-1337908) and Dr. Brendan Twamley for considerable
assistance with crystal structuring.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201501973.
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Scheme 1. Derivatives of FOX-7.
Table 1: Properties of the derivatives of FOX-7.[a]
using mixed nitric acid and acetic
anhydride as the reaction medium
at 08C to give 1, 2, 3, 4-tetrahydro-1,
3-dinitro-6-trinitromethyl-1, 3, 5-tri-
azine (3) in 89% yield. The struc-
ture of 3 is supported by 1H and
13C NMR spectroscopic data as well
as elemental analysis (EA) (see the
Supporting Information). The nitra-
tion of other cyclic amines derived
from 1 gave similar trinitromethyl
derivatives, as shown in Scheme 1.
Thus, 2-(dinitromethylene)-1,3-dia-
zacyclopentane (4) and 4-(dinitro-
methylene)- tetrahydro-4H-1, 3, 5-
oxadiazin(6) gave 3-3-nitro-2-(trini-
tromethyl)-l,3-diazacyclopent-1-
ene (5) and 4-(trinitromethyl)-3-
nitro-dihydro-2H-1, 3, 5-oxadiazin
(7), respectively. Nitration of 2, 4,
and 6 increased the number of nitro
groups in forming 3, 5, and 7. Thus,
compound 3 has a positive OB of
19.7%; and 5 and 7 have 12.1% and
17.1%, respectively, which are
superior to RDX (0%) and HMX
Td[b]
[8C]
d[c]
OB[d]
[%]
DfH[e]
D[f]
P[g]
[GPa]
IS[h]
[J]
FS[i]
[N]
[gcmÀ3
]
[kJmolÀ1/kJgÀ1
]
[msÀ1
]
1
2
3
4
274.0
180.7
141.2
257.0
(261–262)[j]
129.8
209.2
140.4
127.0
139.0
137.7
159.6
120.7
124.5
270.0
123.5
125.6
204
1.89
1.46
1.87
1.69
0
À53.1/À0.36
125.4/0.39
8930
6903
9112
8394
34.0
15.2
38.1
28.6
60
>40
6
>350
>360
80
À75.0
19.7
À60.7/À0.25
À27.6
À5.7/À0.02
>40
>360
5
6
7
1.75
1.79
1.83
12.1
À16.8
17.1
7.4
14.9
14.9
À35.0
8.8
À4.9
À10.7
À15.8
À9.2
0
256.2/1.47
À74.2/À0.27
À148.6/À0.78
274.1/1.26
1010.3/2.35
1060.1/2.47
348.3/1.91
À94.3/0.26
71.6/0.43
104.8/0.57
304.2/1.30
348.2/1.04
80.0/0.36
8504
8280
8807
8389
8348
7851
6460
8250
8803
7252
8492
8862
8748
9320
31.9
28.9
34.9
32.5
32.4
30.1
16.4
30.8
33.8
22.1
29.4
35.2
34.9
39.5
3.5
>40
3.5
10
3.5
3.5
>40
5
6
4
8
7.5
7.4
7.4
40
>360
60
8[k]
9
1.86/1.84[l]
1.97/1.99[l]
1.95/2.00[l]
1.71/1.77[l]
1.95
–
40
40
10
11
12
13[m]
14
15
16
RDX
HMX
>360
240
–
240
360
160
120
120
1.78
1.88
1.73
1.85
1.82
1.91
280
0
104.8/0.36
[a] All compounds are anhydrous except 14, which is monohydrate. [b] Thermal decomposition
temperature (onset) under nitrogen gas (DSC, 58CminÀ1). [c] Density measured by gas pycnometer
(258C). [d] OB=oxygen balance (%); for CaHbOcNd: 1600(cÀaÀb/2)/Mw; Mw =molecular weight of
compound. [e] Calculated HOF. [f] Detonation velocity. [g] Detonation pressure. [h] Impact sensitivity.
[i] Friction sensitivity. [j] Ref. [5b]. [k] Ref. [19]. [l] Crystal density. [m] Ref. [22c].
(0%). OB is calculated assuming all carbon is converted into
carbon monoxide and all hydrogen to water. The oxygen
percentages of 3, 5, and 7 are 49.4%, 48.5%, and 51.4%,
respectively, which are superior to RDX (43.2%) and HMX
(43.2%) and nearly competitive with ammonium perchlorate
(54.5%).
Although 5 was previously synthesized from 1,1-diiodo-
2,2-dinitroethylene, its physical and detonation properties
were not studied.[5] In our work, 5 as well as the other newly
synthesized compounds were characterized using IR and
NMR spectroscopy, EA, and differential scanning calorim-
etry (DSC). The detonation velocities and pressures of the
compounds were calculated with EXPLO5 v6.01. To further
evaluate these energetic materials, impact sensitivities and
friction sensitivities were measured using a BAM Fallhammer
apparatus and a BAM friction tester, respectively (Table 1).
The densities of these polynitro derivatives of FOX-7 fall
in the range 1.75–1.89 gcmÀ3 (compound 3), which equal or
exceed those of common explosives. The calculated detona-
tion pressures of 3, 5, and 7 are over the range of P 31.9–
38.1 GPa (RDX 34.9 GPa; HMX 39.5 GPa). The detonation
velocities fall between 8504 and 9112 msÀ1 (RDX 8748 msÀ1
;
HMX 9320 msÀ1).The calculated detonation properties cou-
pled with the relatively high hydrolytic and thermal stabilities
2
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suggest that these oxygen-rich materials may be attractive
candidates for energetic applications.
Several azo-based HEDMs have been investigated in
recent years[21] for use in applications, such as insensitive
munitions, gas-generating substances (for example for use in
airbags) and propellant additives. The presence of the azo
À = À
( N N ) moiety generally imparts a high endothermic HOF
to a compound; thus, the decomposition energy is largely
derived from this source rather than from the fuel-oxidizer
reaction. The reactions of FOX-7 with chlorine-based oxidiz-
ing reagents trichloroisocyanuric acid (TCICA) and N-
chlorosuccinimide[19] were previously studied carefully by
our group, and the reagents proved successful in the synthesis
of the previously known dichloro substrate, 1-chloro-1,1-
dinitro-2-(N-chloroamidino)ethane (8) and in the conversion
of a single diamine group in FOX-7 to an azo-bridged
product, (E)-1,2-bis{(E)-2-chloro-1-(chloroimino)-2,2-dini-
troethyl}diazene (9).
These halogenated derivatives of FOX-7 have good
detonation properties, and certain derivatives have extraordi-
nary properties as hypergolic oxidizers. However, the low
yield of the reactions and the expensive oxidizing reagents
required for the reactions preclude their applications and
further study. Now, we have used commercial bleach (sodium
hypochlorite) as an oxidizing reagent with FOX-7 to give
a higher yield of 9 (42.5% vs 11% with TCICA as oxidizer)[19]
and simultaneously obtained a novel isomer azo-bridged
product
(E)-1,2-bis{(Z)-2-chloro-1-(chloroimino)-2,2-dini-
troethyl}diazene) (10) (yield 21.0%) (Scheme 1). Because
10 has good solubility in n-hexanes and 9 is insoluble, the two
isomers could be easily separated by washing the products
with n-hexanes. Compound 10 forms a brownish prism-
shaped crystal, and its structure was obtained by X-ray
crystallography (Figure 1). Compound 10 has physical prop-
erties similar to 9 (thermal stability, density, impact, and
friction sensitivity) but has a slightly higher HOF. The ignition
delay (ID) test shows that 10 has a shorter ID time with
UDMH (unsymmetric dimethylhydrazine) as fuel of 3 ms
compared with 11 ms for 9 (Figure 2). This result suggests that
10 would perform as a superior hypergolic oxidizer.
Figure 1. ORTEPs of compounds a) 10, b) 11, and c) 12. Ellipsoids are
set at 50% probability.[26]
Further treatment of 9 with ammonia or dimethyl sulfide
leads to N1, N2-dichloro-1, 2-diazenedicarboximidamide (11)
or (E, E) -N, N’-1, 2-ethanediylidenebis (2, 2-dinitro-2-chloro-
ethanamine, 12; Scheme 1). These structures were deter-
mined by X-ray crystallography (Figure 1). Compared with
their parent molecule 9, compounds 11 and 12 have superior
stability (thermal, impact, and friction) with less favorable
detonation properties (Table 1).
Recently, we also examined the reactivity of FOX-7 and
its hydrazine derivative 1-amino-1-hydrazino-2,2-dinitroe-
thene (13) with a variety of bases, which led to a series of
isolated salts containing FOX-7 and 13 anions.[22] Compound
13 also acts as a synthon for a number of dinitromethylene
compounds.[23] The condensation of 13 with glyoxal in
aqueous KOH solution gives the heterocyclic 3-(dinitro-
methyl)-1, 2, 4-triazine potassium salt (14),[24] which proved to
be a reactive substrate in a condensation reaction with 1-
nitro-3-aminoguanidine, forming the corresponding deriva-
Figure 2. Hypergolic test of compound 10 with unsymmetric dimethyl-
hydrazine (UDMH).
zino-[6,5-e][1,2,4]triazine (15).[24] Similarly, a similar reaction
of 14 with 13 led to the new symmetric heterobicyclic
material,
3,7-bis(dinitromethylene)-octahydro-[1,2,4]tria-
zino-[6,5-e][1,2,4]triazine (16), which exhibits better detona-
tion properties (density 1.85 gcmÀ3; pressure, 35.2 GPa,
velocity, 8862 msÀ1, IS = 7.5 J) than 15, and compares favor-
ably with RDX (density 1.80 gcmÀ3; P = 34.9 GPa, D =
8748 msÀ1, IS = 7.4 J), and thus may have potential as an
HEDM (Table 1).
The syntheses of HEDMs which contain polynitro, azo,
and heterobicyclic functional groups from FOX-7 were
successful. The structures of these compounds were deter-
tive,
3-nitrimino-7-dinitromethylene-octahydro-[1,2,4]tria-
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measured or calculated. The new compounds exhibit good
physical and detonation properties, such as high densities,
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Received: March 2, 2015
Published online: && &&, &&&&
4
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Chemie
Communications
Energetic Materials
Fox in a box: New derivatives created
from 1-diamino-2, 2-dinitroethene (FOX-
7) often have properties superior to those
of RDX. The compounds exhibit good
thermal stability, high density, positive
heats of formation, an acceptable oxygen
balance, and excellent detonation prop-
erties. These findings contribute to the
value of FOX-7 as an energetic precursor.
H. Gao,* J. M. Shreeve*
&&&& — &&&&
The Many Faces of FOX-7: A Precursor to
High-Performance Energetic Materials
Angew. Chem. Int. Ed. 2015, 54, 1 – 5
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!