Pushing the limits of energetic materials - The synthesis and characterization of dihydroxylammonium 5,5′-bistetrazole-1,1′- diolate

Article abstract of DOI:10.1039/c2jm33646d

The safe preparation and characterization (XRD, NMR and vibrational spectroscopy, DSC, mass spectrometry, sensitivities) of a new explosive dihydroxylammonium 5,5′-bistetrazole-1,1′-diolate (TKX-50) that outperforms all other commonly used explosive materials is detailed. While much publicized high-performing explosives, such as octanitrocubane and CL-20, have been at the forefront of public awareness, this compound differs in that it is simple and cheap to prepare from commonly available chemicals. TKX-50 expands upon the newly exploited field of tetrazole oxide chemistry to produce a material that not only is easily prepared and exceedingly powerful, but also possesses the required thermal insensitivity, low toxicity, and safety of handling to replace the most commonly used military explosive, RDX (1,3,5-trinitro-1,3,5-triazacyclohexane). In addition, the crystal structures of the intermediates 5,5′-bistetrazole-1,1′-diol dihydrate, 5,5′-bistetrazole-1,1′-diol dimethanolate and dimethylammonium 5,5′-bistetrazole-1,1′-diolate were determined and presented.

Full text of DOI:10.1039/c2jm33646d

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PAPER
Pushing the limits of energetic materials – the synthesis and characterization
of dihydroxylammonium 5,50-bistetrazole-1,10-diolate†
*
Niko Fischer, Dennis Fischer, Thomas M. Klapotke, Davin G. Piercey and Jorg Stierstorfer
€
€
Received 6th June 2012, Accepted 9th August 2012
DOI: 10.1039/c2jm33646d
The safe preparation and characterization (XRD, NMR and vibrational spectroscopy, DSC, mass
spectrometry, sensitivities) of a new explosive dihydroxylammonium 5,50-bistetrazole-1,10-diolate
(TKX-50) that outperforms all other commonly used explosive materials is detailed. While much
publicized high-performing explosives, such as octanitrocubane and CL-20, have been at the forefront
of public awareness, this compound differs in that it is simple and cheap to prepare from commonly
available chemicals. TKX-50 expands upon the newly exploited field of tetrazole oxide chemistry to
produce a material that not only is easily prepared and exceedingly powerful, but also possesses the
required thermal insensitivity, low toxicity, and safety of handling to replace the most commonly used
military explosive, RDX (1,3,5-trinitro-1,3,5-triazacyclohexane). In addition, the crystal structures of
the intermediates 5,50-bistetrazole-1,10-diol dihydrate, 5,50-bistetrazole-1,10-diol dimethanolate and
dimethylammonium 5,50-bistetrazole-1,10-diolate were determined and presented.
capacity of compounds, which requires new classes of
Introduction
compounds,2,11 new synthetic strategies,12 and advanced
computational techniques. For example, the high nitrogen
content of many advanced explosives has led to the preparation
of new nitrogen–nitrogen bond forming reactions12 and new
heterocyclic systems13 in the quest for even higher performance.
In both civilian and military circles, the highest performing
explosives make use of the same strategy: cyclic and caged
nitramines. Belonging to the oldest class of explosives,
those derive their energy from the oxidation of a carbon
backbone by containing the oxidizer in the same molecule;
RDX (1,3,5-trinitro-1,3,5-triazacyclohexane), HMX (1,3,5,7-
tetranitro-1,3,5,7-tetraazacyclooctane) and CL-20 (2,4,6,8,10,12-
hexanitro-2,4,6,8,10,12-hexaza-isowurtzitane) all have fatal
flaws that mandate replacement with modern explosives.
Advanced energetic strategies allow for retention or improve-
ment of the explosive performance, while avoiding the multi-
tude of downsides present in these compounds: toxicity to
living organisms (all), difficult and expensive synthesis (HMX,
CL-20), high sensitivity to mechanical stimuli (all), and spon-
taneous changing of properties (CL-20).2 New strategies in the
design of energetic materials include those with ring or cage
strain, high heat of formation compounds, and compounds
containing strong dipoles or zwitterionic structures.14
The rational design of new energetic materials is a rapidly
exploding field1–7 with a long traditional rooting in the chemical
sciences8,9 and a complexity that rivals that of the drug design.
While the field has come a long way since the days of Liebig,
Berzelius and Gay-Lussac, and the concept of isomerism being
determined from explosive silver fulminate and non-explosive
silver cyanate,10 current work in this field still follows the trend of
its historic beginnings; that of simultaneous academic and
practical interest and advances. In the quest for higher-per-
forming, safer, cheaper, greener, explosive materials, energetic
materials chemistry must push the boundaries of the energy
Energetic Materials Research, Department of Chemistry, University of
Munich (LMU), Butenandtstr. 5-13, D-81377, Germany. E-mail: tmk@
cup.uni-muenchen.de; Fax: +49 (0)89 2180 77492; Tel: +49 (0)89 2180
77491
† Electronic supplementary information (ESI) available: 1. Materials and
methods; 1.1. NMR spectroscopy; 1.2. vibrational spectroscopy; 1.3.
mass spectrometry and elemental analysis; 1.4. differential scanning
calorimetry; 1.5. sensitivity testing; 2. experimental work; 2.1. synthesis
via oxidation of 5,50-bistetrazole with potassium peroxymonosulfate;
2.2. synthesis via cyclization of diazidoglyoxime; 2.3. safer synthesis
including a multi-step one pot reaction; 3. X-ray diffraction; 3.1.
instrument and refinement software; 3.2. crystallographic data and
refinement parameters; 3.3. bond lengths, bond angles and hydrogen
bonding of TKX-50; 3.4 crystal structures of 5,50-bistetrazole-1,10diol;
Unfortunately, the known materials with the highest detona-
tion energy are often highly sensitive due to their unprecedented
energy content,5 and are made via long and expensive pathways
with a multitude of steps, making industrial scale-up infeasible.
For example, both DDF (dinitroazofuroxane) and ONC (octa-
nitrocubane) possess detonation velocities at the limit of known
performances (around 10 000 m sꢀ1), however both are highly
3.5.
crystal
structure
of
dimethylammonium
5,50-bistetrazole-1,10diolate; 4. explosive performance; 4.1. heat of
formation calculations; 4.2. small scale shock reactivity test; 4.3 flame
test; 4.4 hot plate test; 5. toxicity assessment; 6. Fast Cook-Off test. cif
files. CCDC 872230, 872231, 872232, 884559, 884560 and 884561. For
ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c2jm33646d
20418 | J. Mater. Chem., 2012, 22, 20418–20422
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sensitive and have more than 10 synthetic steps with exotic,
expensive reagents used.15
A very promising explosophoric moiety in the design of new
energetic materials is the tetrazole ring; the carbon on position 5
of the ring allows the facile attachment of various substituents
for energetic tailorability, and the high nitrogen content and heat
of formation of the heterocycle lead to high energetic perfor-
mances. In order to improve the energetic properties of tetra-
zoles, several recently published studies showed that introduction
of N-oxides yields compounds with even higher densities and
stabilities, lower sensitivities and better oxygen balances.2,7,11
Combining these principles with practical considerations in
mind, a simple and secure synthetic pathway to the high per-
forming energetic material dihydroxylammonium 5,50-biste-
trazole-1,10-diolate (TKX-50) was devised.
Scheme 2 Synthesis of 5,50-bistetrazole-1,10-diol in a one pot reaction
from dichlororglyoxime.
Scheme 3 Synthesis of TKX-50 from 5,50-bistetrazole-1,10-diole isolated
from the one pot reaction described in Scheme 2. DMF is cleaved under
the acidic conditions to form the dimethylammonium salt of 5,50-biste-
trazole-1,10-diol, which is then converted into TKX-50.
Results and discussion
Synthesis (simple and scalable)
There are two major routes (A and B) to the 5,50-bistetrazole-
1,10-diol (1,1-BTO) moiety (Scheme 1). The first (A) of which, the
oxidation of the parent heterocycle with aqueous potassium
peroxymonosulfate only leads to 1,1-BTO in poor yield (11%).
The oxidation of the 5,50-bistetrazolate anion with peroxy-
monosulfate was carried out in a manner similar to that we have
previously reported for 5-nitro- and 5-azidotetrazoles.2,7 Unfor-
tunately, this reaction was found to produce the 2,20 isomer as
the major product, with only traces of the 1,10 isomer which
crystallized upon adding aqueous hydroxylamine.
HCl gas is bubbled through (Scheme 2). After cyclization of the
azidooxime in the acidic medium the dimethylammonium salt of
5,50-bistetrazole-1,10-diol is formed by a reaction with dime-
thylamine (formed by hydrolysis of DMF). After isolation and
recrystallization of dimethylammonium 5,50-bistetrazole-1,10-
diolate, it is dissolved in a sufficient amount of boiling water and
combined with a solution of hydroxylammonium chloride, from
which TKX-50 crystallizes first (Scheme 3).
An alternative procedure using NMP (N-methyl-2-pyrroli-
done) instead of DMF for the chloro–azido exchange, followed
by the same treatment, leads to the free acid 5,50-bistetrazole-
1,10-diol which is then isolated as its sodium salt tetrahydrate
upon the addition of aqueous sodium hydroxide and subse-
quently treated with hydroxylammonium chloride in water.
Starting from dichloroglyoxime, the overall yields of both
procedures are very high with 72 % (DMF-route) and 85 %
(NMP-route) for the synthesis of TKX-50. For a detailed
description of all synthetic routes yielding TKX-50 and for all
analytical data please refer to the ESI.†
After discovering the outstanding characteristics of TKX-50 as
a high explosive, a different route to the precursor 5,50-biste-
trazole-1,10-diol was necessitated. Tselinskii et al.16 reported on
the synthesis of the mentioned precursor 1,1-BTO from the
cyclization of diazidoglyoxime under acidic conditions for the
first time. Diazidoglyoxime is prepared from dichloroglyoxime in
a chloro–azido exchange reaction in DMF with more than 80 %
yield, whereas dichloroglyoxime is prepared from glyoxime via
chlorination in ethanol in high yield.
The problematic step here is the isolation of the highly friction
and impact sensitive compound diazidoglyoxime, mandating a
revised procedure before industrial-scaled use. The problem was
overcome by a procedure combining the formation and cycliza-
tion of diazidoglyoxime in one step in solution. Starting from
commercially available glyoxal, the reaction process was trans-
formed into a five step, four pot synthesis to isolate TKX-50.
The prepared solution of diazidoglyoxime in DMF (impure
with sodium chloride) is directly poured into diethylether and
X-ray diffraction
The crystal structure of TKX-50 was determined at three
temperatures (100 K, 173 K, 298 K) in order to detect potential
low temperature phase transitions and obtain precise densities
(for explosive performance calculations). In addition the crystal
structures of the intermediates 5,50-bistetrazole-1,10-diol dihy-
drate (recryst. from either water, MeCN, EtOH or glacial acetic
acid), 5,50-bistetrazole-1,10-diol dimethanolate (recryst. from
methanol) and dimethylammonium 5,50-bistetrazole-1,10-diolate
(crystallized from H2O) were determined and are presented in the
ESI.† Detailed crystallographic data and parameters of the
measurements and solutions are given in Table S1.† The lack of
observed phase transitions between 100 K and 298 K is advan-
tageous for energetic materials use as constant properties upon
temperature changes result. The density follows the expected
trend of decreasing with increased temperature (100 K: 1.918 g
cmꢀ3 > 173 K: 1.915 g cmꢀ3 > 298 K: 1.877 g cmꢀ3). TKX-50
crystallizes in the monoclinic space group P21/c with two
anion–cation moieties in the unit cell. The molecular moiety of
Scheme 1 Synthesis of TKX-50 via oxidation of 5,50-bistetrazole (A)
and via cyclization of diazidoglyoxime (B).
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It uses Becker–Kistiakowsky–Wilson’s equation of state (BKW
EOS) for gaseous detonation products and Cowan–Fickett’s
equation of state for solid carbon (see ESI†). The input is based
on the sum formula, calculated heats of formation (see ESI†) and
the maximum densities according to their crystal structures (ESI,
Table S1†).
Withrespecttothedetonationvelocity(Table1),TKX-50shows
higher calculated values than all other mass-produced and used
explosives like 2,4,6-trinitrotoluene (TNT), RDX, HMX and CL-
20. Lookingatthe detonation pressure, TKX-50 exceedsthe values
calculated for TNT and RDX and is comparable to HMX, but is
slightly lower than for CL-20. Also in terms of potential use as a
propellant mixture ingredient TKX-50 shows promising values
due to its high nitrogen content. The calculated specific impulse
using 60 bar isobaric rocket conditions is 261 seconds, which is
slightly better than those of the other compounds in Table 1.
To assess the explosive performance of TKX-50 on a small
laboratory scale, a small-scale reactivity test (SSRT) was carried
out (see ESI†) in comparison to CL-20 and RDX. Here, a defined
volume of the explosive is pressed into a perforated steel block,
which is topped with a commercially available detonator (Orica,
DYNADET-C2-0ms). Initiation of the tested explosive results in
denting a separate aluminium block, which is placed right
underneath the steel block (Fig. 2). From measuring the volumes
of the dents (CL-20 $ TKX-50 [ RDX) (Table 2 in the ESI†), it
can be concluded that the small scale explosive performance of
TKX-50 exceeds the performance of commonly used RDX and is
comparable to that of CL-20.
Fig. 1 Representation of the solid state molecular structure of TKX-50
at 100 K. Thermal ellipsoids are drawn at the 50% probability
level; symmetry codes: (i) 2 ꢀ x, ꢀy, 2 ꢀ z; (ii) x, 0.5 ꢀ y, ꢀ0.5 + z; and
(iii) ꢀ1 + x, 0.5 ꢀ y, ꢀ0.5 + z.
TKX-50 at 100 K is depicted in Fig. S1.† Its density of 1.918 g
cmꢀ3 is significantly higher than that of non-oxide dihydroxyl-
ammonium 5,50-bistetrazolate (1.742 g cmꢀ3) recently pub-
lished.17 The reason for this may be the strong hydrogen bond
network (involving all four hydrogen atoms of the hydroxyl-
ammonium cations) (Fig. 1).
The performance and safety characteristics for shipping of an
explosive can be related to the data obtained from the Koenen
test.18,19 The explosive is placed in an open-ended, flanged steel
tube, which is locked up with a closing plate with variable orifice
(0–10 mm), through which gaseous decomposition products are
Energetic performance
The performance data (Table 1) were calculated with the
computer code EXPLO5.05 (latest version). EXPLO5.05 is based
on the chemical equilibrium, a steady state model of detonation.
Table 1 Energetic properties and detonation parameters of prominent high explosives in comparison to TKX-50
2,4,6-TNT
RDX
b-HMX
3-CL-20
TKX-50
Formula
Molecular mass [g molꢀ1
IS [J]a
C7H5N3O6
227.13
15 21
353
—
C3H6N6O6
222.12
C4H8N8O8
296.16
C6H6N12O12
438.19
4 21
C2H8N10O4
236.15
20
120
0.10
]
7.5 21
7 21
FS [N]b
120 21
0.20
112 21
0.20
48 21
ESD-test [J]c
N [%]d
0.13
38.3
18.50
ꢀ73.96
37.84
ꢀ21.61
37.84
ꢀ21.61
59.3
ꢀ27.10
U [%]e
ꢀ10.95
Tm [ꢁC]f
81
205 22
275 24
—
—
Tdec. [ꢁC]f
290
210 24
279 24
215 23
221
Density [g cmꢀ3 g
]
1.713 (100 K)24
1.648 (298 K)25
ꢀ55.5
1.858 (90 K)26
1.806 (298 K)27
86.3
1.944 (100 K)28
1.904 (298 K)28
116.1
2.083 (100 K)29
2.035 (298 K)29
365.4
1.918 (100 K)30
1.877 (298 K)30
446.6
Theor. DfHꢁ [kJ molꢀ1 h
Theor. DfUꢁ [kJ kgꢀ1 i
EXPLO5.05 values
]
]
ꢀ168.0
489.0
492.5
918.7
2006.4
ꢀDEUꢁ [kJ kgꢀ1 j
]
5258
3663
235
7459
569
205
6190
4232
380
8983
734
258
6185
4185
415
9221
729
258
6406
4616
467
9455
666
251
6025
3954
424
9698
846
261
TE [K]k
pCꢀJ [kbar]l
D [m sꢀ1 m
]
Gas vol. [L kgꢀ1 n
]
IS [s]o
a
Impact sensitivity (BAM drophammer (1 of 6)). b Friction sensitivity (BAM friction tester (1 of 6)). c Electrostatic discharge device (OZM research).
d
e
f
g
Nitrogen content. Oxygen balance (U ¼ (xO ꢀ 2yC ꢀ 1/2zH)M/1600). Decomposition temperature from DSC (b ¼ 5 ꢁC minꢀ1). From X-ray
h
diffraction. Calculated (CBS-4M method) enthalpy of formation. Calculated energy of formation. Energy of explosion. Explosion
temperature. Detonation pressure. Detonation velocity. Volume of detonation gases (assuming only gaseous products). Specific impulse using
isobaric (60 bar) conditions.
i
j
k
l
m
n
o
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TNT destroys the steel tube up to an orifice width of 6 mm,
RDX even up to 8 mm.20 In order to get an ‘‘Interim Hazard
Classification’’ also a ‘‘Fast Cook-Off test’’ (UN test 3d) was
performed in which TKX-50 underwent controlled deflagration
(no explosion occurred).
High safety – low sensitivity
Impact sensitivity is a high priority in explosive devices used in
the military due to the range of stresses devices may be exposed
to. The impact sensitivity of TKX-50 is 20 J which is much lower
than those for RDX, HMX and CL-20, which range from 4 to 7.5
J, and all three of which need desensitizing components added for
practical use. The low impact sensitivity of TKX-50 shows that it
can be used without desensitization.
Friction sensitivity is more important in the manufacturing
context, where TKX-50 with 120 N is of comparable or lower
sensitivity than any of RDX, HMX or CL-20, increasing the
margin of safety in the industrial context. Both the impact and
friction sensitivities of TKX-50 as compared to 2,4,6-trinitro-
toluene (TNT), RDX, HMX and CL-20 are presented in Table 1.
The human body can generate up to 25 mJ of static electricity,
which can easily set off the most sensitive explosives such as lead
azide or silver fulminate. TKX-50 has an electrostatic sensitivity
of 0.100 J, which is far higher than the human body can generate,
Fig. 2 Small-scale reactivity test of TKX-50, RDX and CL-20. Above
pictures: test setup for the SSRT. Below: dented aluminium blocks after
initiation of the explosive with a commercial detonator.
allowing
a comparable margin of safety when handling,
vented. A defined volume of 25mL of the compound is loaded into
the flanged steel tube and a threaded collar is slipped onto the tube
from below. The closing plate is fitted over the flanged tube and
secured with a nut. The decomposition is initiated via thermal
ignition using four Bunsen burners, which are ignited simulta-
neously. The test is completed when either rupture of the tube or
no reaction is observed after heating the tube for a minimal time
period of at least 5 min. In the case of the tube’s rupture, the
fragments are collected and weighed. The reaction is evaluated as
an explosion if the tube is destroyed into three or more pieces. The
Koenen test was performed with 23.0 g of TKX-50 using a closing
plate with an orifice of 10 mm and caused the rupture of the steel
tube into approximately 100 pieces, the size of which reached
down to smaller than 1 mm from 40 mm (Fig. 3).
comparable to RDX or HMX.
Thermal stability is important for any explosive in practical use
as demanding military requirements need explosives that can
withstand high temperatures. For example, a munition sitting in
ꢁ
the desert can exceed 100 C and for general use a component
ꢁ
explosive must be stable above 200 C. TKX-50 with a decom-
position onset of 222 ꢁC easily surpasses this requirement (Fig. 4,
inset). This stability hasbeen confirmed using a long-term stability
test, where the sample is heated in an open glass vessel to a
ꢁ
temperature of 75 C over 48 h to ensure safe handling of the
material even at elevated temperatures (Fig. 4, outer curve). The
lack of exothermic or endothermic events in the sample temper-
ature or heat flow curve implies that the compound is stable.
Fig. 4 Outer curve: long term stability (TSC plot) of TKX-50 at a
temperature of 75 ꢁC over a period of 48 h. Inner plot: thermal stability of
TKX-50 and RDX shown in the DSC plot (heating rate 5 ꢁC minꢀ1).
Toxicity – environmentally friendly
One of the major aims in our search for new ‘‘green’’ energetic
materials is the low toxicity of the newly investigated compound
Fig. 3 a) Koenen test experimental setup. (b) Parts of the Koenen steel
sleeve before and (d) after the test. (c) Moment of detonation.
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J. Mater. Chem., 2012, 22, 20418–20422 | 20421

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Acknowledgements
Financial support of this work by the Ludwig-Maximilian
University of Munich (LMU), the U.S. Army Research Labo-
ratory (ARL), the Armament Research, Development and
Engineering Center (ARDEC), the Strategic Environmental
Research and Development Program (SERDP) and the Office of
Naval Research (ONR) is acknowledged. Furthermore, the
authors acknowledge the contributions of Stefan Huber, Sus-
anne Scheutzow, Dr Karin Lux, Sebastian Rest, Marius Rey-
mann and Fabian Wehnekamp.
Fig. 5 Toxicity assessment of TKX-50 using a luminescent bacteria
inhibition test. Plot of log G against log c for determination of the EC50
value.
Notes and references
€
1 M. Gobel, B. H. Tchitchanov, J. S. Murray, P. Politzer and
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T. M. Klapotke, Nat. Chem., 2009, 1, 229–235.
€
€
2 M. Gobel, K. Karaghiosoff, T. M. Klapotke, D. G. Piercey and
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itself, and of its degradation and decomposition products. In
recent times the toxicity of energetic materials is a growing concern
due to new understandings of the fate of explosives in the envi-
ronment. The nitramine content of the ubiquitous RDX as well as
less used HMX and CL-20 has been shown to be toxic to vital
organisms at the base of the food chain, and in addition RDX is a
probable human carcinogen. To assess the toxicity of TKX-50 to
aquatic life, diluted aqueous solutions of the high explosive were
subjected totheluminescentmarinebacteriumVibriofischeri using
the commercially available bioassay system LUMIStoxꢀ. Vibrio
fischeri is a representative species for other aquatic life and there-
fore a useful indicator when it comes to groundwater pollution.
Being the most important toxicological parameter, the EC50 value
of the sample was determined. EC50 is the effective concentration
of the examined compound, at which the bioluminescence of the
strain Vibrio fischeri is decreased by 50% after a defined period of
exposure as compared to the original bioluminescence of the
samplebeforebeingtreatedwiththedifferentlydilutedsolutions of
the test compound. For RDX we observe an EC50 value of 91 ppm
after an incubation time of 30 minutes. The herein determined
EC50 value of TKX-50 of 130 ppm (Fig. 5 and ESI†) lies signifi-
cantly above the EC50 value found for RDX indicating a lower
toxicity to Vibrio fischeri, and as such, other aquatic life.
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Conclusions
We have detailed the preparation of a new explosive, TKX-50 or
dihydroxylammonium 5,50-bistetrazole-1,10-diolate. This mate-
rial has exemplified the utility of the tetrazole N-oxide chemistry
by providing a new explosive material that is of very high
performance (as calculated and demonstrated by SSRT testing),
pushing the limits towards the most powerful explosives known,
and synthesized in an industrially viable process. Additionally,
TKX-50 is of lower sensitivity (mechanically and thermally) than
its contemporaries in currently used explosives such as RDX,
HMX and CL-20, making increased margins of safety when
applied in practical use and devices. Finally, we have demon-
strated the lower toxicity of TKX-50 compared to the nitramine
RDX, as determined by the EC50 value for the decrease in
luminescence of Vibrio fischeri. All of the characteristics of TKX-
50 make it appropriate and exemplary to not just fulfill the long-
standing goal of a ‘‘green’’ RDX replacement, but also to replace
it with a material of superior performance.
€
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