Bis(1,2,4-oxadiazolyl) Furoxan: A Promising Melt-Castable Eutectic Material of Low Sensitivity

Article abstract of DOI:10.1002/cplu.201800563

A scalable synthesis of bis(1,2,4-oxadiazoyl) furoxan, C₆H₂N₆O₄, its physical properties, and its theoretical performance values are described. Previous attempts to synthesize this compound required expensive reagents, and/or time-consuming synthesis processes and low overall yields. In addition to disclosing a streamlined synthesis of bis(1,2,4-oxadiazolyl) furoxan, we report its molecular configuration and crystal structure, as well as its correct melting point. Bis(1,2,4-oxadiazolyl) furoxan exhibits a very insensitive behavior to impact, friction, and electrostatic discharge, with a calculated detonation pressure 20 % higher than that of TNT. Given its physical properties and theoretical performance values, this material can be classified as a promising ingredient in the development of melt-castable eutectic technology.

Full text of DOI:10.1002/cplu.201800563

Communications
DOI: 10.1002/cplu.201800563
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Bis(1,2,4-oxadiazolyl) Furoxan: A Promising Melt-Castable
Eutectic Material of Low Sensitivity
Eric C. Johnson,[a] Eric J. Bukowski,[a] Jesse J. Sabatini,*[a] Rosario C. Sausa,[b]
Edward F. C. Byrd,[b] Melissa A. Garner,[c] and David E. Chavez[d]
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lower performing counterpart DNAN (dinitroanisole)[2] are the
A scalable synthesis of bis(1,2,4-oxadiazoyl) furoxan, C6H2N6O4,
benchmark examples of a standalone melt-castable materials.
However, recently, high-performing nitrate ester-based materi-
als have been developed that also fall into the standalone melt-
castable range.[3]
its physical properties, and its theoretical performance values
are described. Previous attempts to synthesize this compound
required expensive reagents, and/or time-consuming synthesis
processes and low overall yields. In addition to disclosing a
streamlined synthesis of bis(1,2,4-oxadiazolyl) furoxan, we
report its molecular configuration and crystal structure, as well
as its correct melting point. Bis(1,2,4-oxadiazolyl) furoxan
exhibits a very insensitive behavior to impact, friction, and
electrostatic discharge, with a calculated detonation pressure
20% higher than that of TNT. Given its physical properties and
theoretical performance values, this material can be classified as
a promising ingredient in the development of melt-castable
eutectic technology.
On the other hand, a melt-castable eutectic material
°
typically has a melting point just north of 100 C. Such a
material cannot be melted alone using steam heating, but is
still potentially useful in melt-castable technology when
combined with other energetic materials. This practice of
combining two or more ingredients in a formulation to achieve
an overall melting point that becomes amenable to steam
heating is the basic definition of melt-castable eutectic science.
DEMN (a mixture of 34.9 wt.% diethylenetriamine trinitrate/
33.4 wt.%ethylenediamine dinitrate/25.4 wt.%methyl nitrogua-
nidine/6.3 wt.% nitroguanidine)[4] is an example of one such
°
formulation in which an optimal melting point below 100 C has
Introduction
been achieved, with a recent example of a potential melt-
castable eutectic ingredient being bisisoxazole tetrakis(methyl
nitrate).[5] Despite the aforementioned research examples, the
number of melt-castable materials available for practical use
remains quite limited on the bases of cost, performance and
sensitivity. Therefore, syntheses of melt-castable ingredients
(eutectic or standalone) that have a high degree of scalability,
with low sensitivities and high performance are still very much
desired.
Melt-castable explosive technology is an area of significant
importance in civilian and military applications. Such a technol-
ogy must display a high performance, a low sensitivity, and
must possess specific characteristics. An energetic material that
°
melts between 70–120 C is typically defined as being in the
melt-castable range,[1] with a melting point below 100 C being
°
ideal. This feature allows for the use of steam at ambient
pressures during casting operations, which is critical in minimiz-
ing manufacturing costs.
There are two classes of melt-castable technologies; stand-
alone and eutectic materials.
In this paper, we report the synthesis, crystal structure, and
proper physical properties of bis(1,2,4-oxadiazolyl) furoxan, (5).
Further, we report its sensitivity to external stimuli and its
calculated performance properties.
A standalone melt-castable
energetic material possesses melt-castable physical properties
without the need to employ any other ingredients. TNT and its
Results and Discussion
[a] E. C. Johnson, Dr. E. J. Bukowski, Dr. J. J. Sabatini
U.S. Army Research Laboratory
Energetic Technology Branch
Aberdeen Proving Ground, MD 21005 (USA)
E-mail: jesse.j.sabatini.civ@mail.mil
[b] Dr. R. C. Sausa, Dr. E. F. C. Byrd
U.S. Army Research Laboratory
Energetic Materials Science Branch
Aberdeen Proving Ground, MD 21005 (USA)
[c] M. A. Garner
Upon reviewing the literature for melt-castable compounds, it
was found that Yan et al. prepared bis(1,2,4-oxadiazolyl) furoxan
(5) from glyoxal in 5 steps in a low yield of 4% (Scheme 1).[6]
A
DSC of 5 was reported as having an onset temperature of
°
°
melting of 98 C, with a peak melting temperature of 114 C,
°
and an onset decomposition temperature of 208 C. As this
reported melting temperature range was quite broad, it
indicates that an impurity is likely present in the final product.
Two years prior to the aforementioned synthesis, 5 was
synthesized by Fershtat et al. utilizing a scandium triflate-
mediated double cyclization reaction of bis-amidoxime 7 and
trimethyl orthoformate (Scheme 2).[7] Their synthesis was not
only simpler than the route by Yan et al., but was also higher
University of Maryland
Department of Chemistry and Biochemistry
College Park, MD 20742 (USA)
[d] Dr. D. E. Chavez
Los Alamos National Laboratory
Los Alamos, NM 87545 (USA)
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/cplu.201800563
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Scheme 3. Double cyclization of 7 to give 5.
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catalyst is commonly used to synthesize monosubstituted 1,2,4-
oxadiazoles from amidoximes, such a transformation often
[10,11]
Scheme 1. Synthesis of 5 by Yan et al.[6]
requires temperatures of 70 C or higher.
Unfortunately,
°
when heat was applied to this cyclization reaction, the reaction
decomposed and failed to yield any product. Fershtat et al.
reported that BF3 ·Et2O was not an effective catalyst in the
presence of trimethyl orthoformate to convert amidoxime-
substituted furoxans to the monosubstituted 1,2,4-oxadiazole
products at room temperature.[7] In our case however, BF3 ·Et2O
proved to be a very effective catalyst at room temperature,
converting 7 to 5 in the presence of triethyl orthoformate in
near quantitative yield. Hence, the overall yield of 5 from
cyanoacetic acid was further improved to 34% by this latter
method. The BF3 ·Et2O-mediated double cyclization reaction
detailed in Scheme 3 has been carried out on 25 g scale without
difficulty. No exothermic event was observed throughout the
course of the reaction.
Scheme 2. Synthesis of 5 by Fershtat et al.[7]
Interestingly, a DSC of 5, synthesized by the cyclization
method described in Scheme 3 showed an onset temperature
yielding overall. By the methodology of Fershtat et al., 5 was
afforded in 28.5% overall yield. While the authors did not report
any decomposition temperature for 5, they listed a melting
°
of decomposition of 227 C, which is significantly higher than
°
the 208 C value reported by Yan et al. The onset melting
°
°
point of 84–86 C. This melting point discrepancy was signifi-
temperature of 5 was determined to be 114.3 C, with a peak
°
cantly different than the melting point reported for 5 by Yan
et al. While it was not clear as to why such a difference in
melting points for 5 existed when examining the two reported
methods, the material appeared to be a possible melt-castable
melting point of 117.8 C. Such a sharp melting point range is
indicative of a material of high purity. Hence, the melting point
of 5 is quite different than the broad melting point of 98–114 C
observed by Yan et al., as well as the sharp melting point of 84–
86 C reported by Fershtat et al. Although the 114.3–117.8 C
melting point range falls outside of the standalone melt-
castable range, the melting point range of 5 is such that
classifies it as a potential melt-castable eutectic ingredient.
Hence, it could be mixed with other energetic materials to
°
°
°
°
material. In fact, a melting point of 84–86 C would fit the
definition of a standalone melt-castable material.
Despite the simpler synthesis and increase in overall yield of
5 by Fershtat et al., the cyclization step, while produced under
mild conditions, is still problematic from the scale-up perspec-
tive. The reason centers on the use of scandium triflate.
Scandium is a rare-earth metal of relative low abundance, and
as such, the price of scandium-based reagents is high. There-
fore, in developing a more cost-efficient route for 5 that is
amenable for scalability, a cheaper cyclization method from 7
was desired.
Intermediates 6 and 7 were synthesized in equivalent yields
as the procedure by Fershtat et al. outlined in Scheme 2.[8,9]
Instead of relying on Sc(OTf)3 to promote cyclization to give 5,
we chose to employ some cost-efficient catalysts in the
presence of an excess of triethyl orthoformate (Scheme 3).
When p-toluenesulfonic acid monohydrate (PTSA) was used,
bis-amidoxime 7 was converted to bis(1,2,4-oxadiazolyl) furoxan
(5) in 81% yield. We also examined the effectiveness of boron
trifluoride diethyl etherate in this reaction. Although this latter
°
depress the melting point below 100 C so that the resulting
formulation can be used in casting operations.
The physical properties and explosive performances of 5 are
provided in Table 1.[12] 5 outperforms TNT in oxygen balance,
density, detonation velocity, and heat of formation by signifi-
cant margins. 5 also has a predicted detonation pressure that is
20% higher than that of TNT.
The sensitivity of 5 toward impact, friction, and electrostatic
discharge (ESD) were determined and compared to TNT, which
is considered the benchmark insensitive melt-castable explosive
(Table 1). 5 was found to be more insensitive than TNT to
impact and friction, while exhibiting an equivalent ESD
sensitivity.
X-ray diffraction data analysis confirms the molecular
structure of 5, reveals its molecular configuration, and indicates
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(R2), and O4 (R3), respectively]. The 1,2,5-oxadiazole-N-oxide
ring presents an O2 atom appendant forming a near planar
functionalized moiety, M1 [r.m.s. deviation=0.014 (1) Å]. Plane
Table 1. Physical properties, calculated performance, and sensitivities of 5
compared to TNT.
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5
6
7
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9
Data category
5
TNT
°
°
M1 subtends dihedral angles of 3.8 (5) and 11.2 (6) in relation
to the planes R2 and R3, respectively, whereas planes R2 and R3
[a]
°
T
m [ C]
114.3
227
À 64.9
À 21.6
1.79
24.1
7577
220
80.4
295
[b]
°
Tdec [ C]
ΩCO2 [%][c]
ΩCO [%][d]
À 74
À 24.7
1.65
20.5
6950
–
°
subtend a dihedral angle of 9.9 (6) . The R3 moiety twists
slightly from the near planar R1 and M1 moieties perhaps
because of the N4···N5 [2.83 (1) Å] and N6···O2 [2.91 (1) Å]
intramolecular interactions.
1 [gcmÀ 3
]
[e]
Pcj [GPa][f]
Vdet [msÀ 1
Isp [s][h]
°
[g]
]
Intermolecular interactions play a key role in the crystal
packing and stability of 5. Contacts between the H atoms and O
or N atoms of adjacent molecules [H6···O2i =2.37 (1) Å or
H4···N1ii =2.56 (2) Å; symmetry codes: (i) 1À x, À 1/2+y, À z; (ii)
À 2À x, À 1/2+y, À 2À z] dominate the intermolecular interac-
tions. In addition, contacts between the O3 atoms and O1
atoms of adjacent atoms [O3···O1iv =2.93 (1) Å; symmetry code:
(iv) À x, À 1+y, z], the N5 atoms and N1 atoms of nearby
molecules [N5···N1v =3.07 (2) Å; symmetry code: (v) À 1À x, À 1/2
+y, À 1À z], and the C3 atoms and O2 atoms of adjacent
molecules [C3···O2iii =3.19 (1) Å; symmetry code: (iii) À 2À x, À 1/
2+y, À 1À z] contribute to its crystal packing (see Figure 1 in
the Supporting Information). The C···O and N···N interactions
help link the molecules along the a axis, whereas the O···H,
N···H, and O···O interactions contribute to its network of near
planar sheets almost parallel to (010) (see Figure 1 in the
Supporting Information). Based on its molecular mass and
lattice constants listed in Table 1 in the Supporting Information,
we obtain a density of 1.79 gcmÀ 1 at 297 K.
In summary, an efficient, scalable route to bis-1,2,4-
oxadiazolyl furoxan (5) has been developed, its crystal structure
determined, and its explosive performance and sensitivity
evaluated. It features a three step synthesis in an overall yield of
34%, which is an improved overall yield over previously existing
syntheses. 5 was found to have a low sensitivity to impact,
friction and ESD, and was estimated to be safer to handle than
TNT. The characteristics and performance properties of 5
suggest that this material is not a standalone melt-castable as
previously reported, but instead a potential high-energy melt-
castable eutectic material. Combining 5 with other high-energy
À 1 [i]
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ΔfH [kJmol
IS[j] [J]
]
357
À 59.3
>34.7
>360
>0.25
15
FS[k] [N]
240
0.25
ESD[l] [J]
[a] Tm =onset temperature of melting. [b] Tdec =onset temperature of
decomposition. [c] ΩCO2 =CO2 oxygen balance. [d] ΩCO =CO oxygen
balance. [e] 1=derived density from X-ray data. [f] Pcj =detonation
pressure. [g] Vdet =detonation velocity. [h] Isp =specific impulse. [i] ΔfH
=molar enthalpy of formation. [j] IS=impact sensitivity. [k] FS=friction
sensitivity [l] ESD=electrostatic discharge sensitivity.
°
key intra- and intermolecular interactions. The details of the
data collection, cell constants determination, as well as the
structure solution and refinement are provided in the Support-
ing Information.[13–15] 5 presents one molecule in the asymmetric
unit (Figure 1). Its bond length and angles are in the usual
range, consistent with those reported for other oxadiazoles and
their derivatives.[3,16–22] Overall, 5 is near planar [r.m.s.=0.082
(5) Å] with a maximum out-of-plane deviation of 0.021 (2) Å for
the N5 atom. The molecule comprises three near planar
pentagonal oxadiazole rings [r.m.s. deviation=.0026 (2), 0.0060
(5), and 0.023 (4) Å for rings containing the atoms O1 (R1), O3
°
ingredients may depress the melting point below 100 C, thus
making it amenable for high-energy melt-casting formulations.
Experimental Section
Chemicals and solvents were used as received from Oakwood
Chemicals. Dicyanofuroxan (6) and bis-amidoxime 7 were synthe-
sized according to the literature.[8,9] 1H and 13C NMR spectra of bis-
oxadiazole 5 were recorded using an Anasazi 90 MHz and 22.5 MHz
instrument, respectively. The chemical shifts quoted in ppm in the
text refer to typical standard tetramethylsilane in CDCl3 as the
solvent. Infrared spectra were measured with a Bruker Alpha-P FTIR
instrument. Melting and decomposition temperatures were meas-
°
ured at a heating rate of 5 C/min using a TA Instruments Q10 DSC
instrument. Single-crystal X-ray diffraction (XRD) studies were
performed with a SuperNova Dualflex diffractometer containing an
EosS2 charge-coupled device detector and a molybdenum Mo-Kα
(λ=0.71073 Å) radiation source.
Figure 1. (A) Molecular configuration and atom-numbering scheme for 5.
Non-hydrogen atoms are shown as 50% probability displacement ellipsoids.
(B) Selected atom-atom intramolecular and intermolecular interactions for 5,
where the contacts are depicted as dashed lines.[15]
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Caution! Although we did not experience any problems handling
the compounds described in this paper, proper laboratory
precautions should be taken. Laboratories and personnel should be
properly grounded, and safety equipment such as heavy Kevlar/
steel gloves, reinforced Kevlar coat, ballistic face shield, ear plugs,
and blast shields are necessary for all energetic transformations,
and in handling any material that is determined to be energetic.
Keywords: energetic materials
explosives · heterocycles · organic synthesis
·
eutectic technology
·
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[1] P. Ravi, D. M. Badgujar, G. M. Gore, S. P. Tewari, A. K. Sikder, Review on
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Bis(1,2,4-oxadiazolyl) Furoxan (5)
Method A: Bis-amidoxime 7 (505 mg, 2.50 mmol, 1.00 eq)) was
placed in a 10 ml round bottomed flask. Triethylorthoformate
(1.45 g, 13.7 mmol, 5.48 eq)) was then added to the reaction flask
and the mixture stirred at ambient temperature. p-Toluenesulfonic
acid monohydrate (47 mg, 0.25 mmol, 0.100 eq)) was then added
and the reaction stirred at room temperature for 8 h. After
1.5 hours, the mixture becomes a homogenous solution after which
a precipitate began to form. The mixture was poured into water
(10 mL), triturated, filtered, washed with water and air dried to
provide 450 mg of bis-1,2,4-oxadiazolyl furoxan (5) (81%) as an off-
white solid.
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Method B: To a 1 L round-bottom flask equipped with a stir bar
charged with 206 mL of triethyl orthoformate (183.5 g, 1.24 mol,
10 eq) was added bis-amidoxime 7 (25.0 g, 123.8 mmol, 1.00 eq). To
the resulting stirred suspension was then added 1 mL of boron
trifluoride diethyl etherate (1.41 g, 9.90 mmol, 0.08 eq). After
10 minutes, the solid went into solution, and the reaction mixture
was stirred further at ambient temperature for 8 h. The reaction
mixture was then poured onto crushed ice and stirred vigorously
for 2 h, during which time the ice melted. The resulting white solid
was collected by Büchner filtration, washed with cold two 500 mL
portions of cold water, suction dried for 1 h, and was transferred to
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°
crystallization dish. Drying of the white solid overnight at 60 C
afforded 26.7 g (97%) of bis(1,2,4-oxadiazolyl) furoxan (5) as an off-
Crystallogr. 2015, C71, 3–8.
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°
°
°
white solid. Tmelt =114.3 C (onset), 117.8 C (peak); Tdec =227.0 C
1
°
(onset), 268.2 C; H NMR (90 MHz, DMSO-d6) 10.02 (s, 1 H), 9.95 (s,
1 H); 13C NMR (22.5 MHz, DMSO-d6) δ 169.62, 169.34, 169.70, 168.40,
167.76, 167.48; IR (neat) 3108.99 (w), 1604.49, (m), 1578.19 (m),
1546.95, 1529.96 (m), 1270.35 (s), 1091.82 (w), 1017.00 (s) cmÀ 1
.
Acknowledgements
The authors thank the U.S. Army for financial support in carrying
out this work.
Conflict of Interest
Manuscript received: October 31, 2018
Revised manuscript received: December 1, 2018
The authors declare no conflict of interest.
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COMMUNICATIONS
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A melt-castable eutectic: A scalable
synthesis of bis(1,2,4-oxadiazolyl)
furoxan, C6H2N6O4, its physical prop-
erties, and its theoretical perform-
ance values are described. This
material displays a calculated deto-
nation pressure 20% higher than
TNT, and is very insensitive to
impact, friction, and electrostatic
discharge. The material is classified
as an insensitive explosive ingredient
that could find application in melt-
castable eutectic formulations.
E. C. Johnson, Dr. E. J. Bukowski,
Dr. J. J. Sabatini*, Dr. R. C. Sausa,
Dr. E. F. C. Byrd, M. A. Garner, Dr. D. E.
Chavez
1 – 5
Bis(1,2,4-oxadiazolyl) Furoxan: A
Promising Melt-Castable Eutectic
Material of Low Sensitivity
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