Evolving the Scope of 5,5’-Azobistetrazoles in the Search for High Performing Green Energetic Materials

Article abstract of DOI:10.1002/ejoc.202100747

The azotetrazole moiety represents a great platform for energetic materials, it offers a planar and nitrogen-rich backbone, combined with a high heat of formation, which easily can be functionalized and tuned. Herein, we start from sodium 5-aminotetrazolate and obtain two isomers by substitution reaction with 2-chloroethanol. Azidoethyl and nitratoethyl substituted azo- tetrazoles were finally synthesized by oxidative azo coupling of the respective N-ethyl functionalized 5-aminotetrazole precursors using tert-butyl hypochlorite as reagent. All compounds were analyzed through multicore NMR and IR spectroscopy as well as mass spectrometry. All solid compounds were further investigated using low-temperature X-ray crystallography. The purity was verified by CHNO elemental analysis and the decomposition temperature (DTA) and sensitivities toward impact, friction and electrostatic discharged were determined. Based on the CBS-4M calculation results, the energetic properties were calculated using the EXPLO5 code.

Full text of DOI:10.1002/ejoc.202100747

European Journal of Organic Chemistry
Accepted Article
Accepted Article
Title: Evolving the Scope of 5,5’-Azobistetrazoles in the Search for
High Performing Green Energetic Materials
Authors: Maximilian Benz, Michael S. Gruhne, Thomas M. Klapötke,
Nina Krüger, Tobias Lenz, Marcus Lommel, and Joerg
Stierstorfer
This manuscript has been accepted after peer review and appears as an
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of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.202100747
Link to VoR: https://doi.org/10.1002/ejoc.202100747
01/2020

10.1002/ejoc.202100747
European Journal of Organic Chemistry
COMMUNICATION
Evolving the Scope of 5,5’-Azobistetrazoles in the Search for High
Performing Green Energetic Materials
Maximilian Benz,[a] Michael S. Gruhne,[a] Thomas M. Klapötke,*[a] Nina Krüger,[a] Tobias Lenz,[a] Marcus
Lommel,[a] and Jörg Stierstorfer[a]
Dedicated to Professor Konstantin Karaghiosoff on the occasion of his 65th birthday
[a]
Maximilian Benz, Michael S. Gruhne, Prof. Dr. Thomas, M. Klapötke, Nina Krüger, Tobias Lenz, Marcus Lommel and Dr. Jörg Stierstorfer
Department of Chemistry, University of Munich (LMU)
Butenandtstr. 5‒13 (D), 81377 München, Germany
Email: tmk@cup.uni-muenchen.de
www.hedm.cup.uni-muenchen.de
Supporting information for this article is given via a link at the end of the document.
Abstract: The azotetrazole moiety represents a great platform for
energetic materials, it offers a planar and nitrogen-rich backbone,
combined with a high heat of formation, which easily can be
functionalized and tuned. Herein, we start from sodium 5-
aminotetrazolate and obtain two isomers by substitution reaction with
2-chloroethanol. Azidoethyl and nitratoethyl substituted azo-
tetrazoles were finally synthesized by oxidative azo coupling of
N-ethyl functionalized 5-aminotetrazole precursors using tert-butyl
hypochlorite as reagent. All compounds were analyzed through
multicore NMR and IR spectroscopy as well as mass spectrometry.
All solid compounds were further investigated using low-temperature
X-ray crystallography. The purity was verified by CHNO elemental
analysis and the decomposition temperature (DTA) and sensitivities
toward impact, friction and electrostatic discharged were determined.
Based on the CBS-4M calculation results, the energetic properties
were calculated using the EXPLO5 code.
spectroscopy by Bannert et al.[12] However, tetrazoles represent
a more stable alternative. Currently, these are not considered
toxic and are even used in various medicines.[1, 13] Tetrazoles
have already found their way into the field of energetic materials,
with the best-known representatives in this field probably being
TKX-50 or DBX-1.[14-15] Furthermore, their use as ligands in
nitrogen-rich complexes as lead azide or lead styphnate
substitutes is being investigated.[16-17] A particularly interesting
moiety because of its detonation parameters improving character,
is the azo bridge. In pyrazoles and triazoles, this building block is
already widely used.[18-19] However, since tetrazoles are more
limited with respect to their derivatizability, this unit is hardly found
so far. Substituents are usually introduced before the azo coupling,
and the neutral 5,5’-azobistetrazole (often called simply
azotetrazole) is primarily used for the preparation of nitrogen-rich
salts. Therefore, only methyl, amino, hydroxy or nitramino
derivatives have been prepared by azo coupling of the respective
5-aminotetrazoles (Figure 1) yet.[20-24]
In the field of energetic materials, the most common approaches
for the synthesis of new molecules are the introduction of cage or
ring strain, the oxidation of the carbon backbone, or a raise of the
molecule’s nitrogen content.[1] Widely used examples of merging
these models are mainly 2,4,6-trinitrotoluene (TNT),
cyclotrimethylenetrinitramine (RDX), or hexanitrohexaaza-
isowurtzitane (CL-20).[2-3] However, since TNT in particular, as
well as the decomposition products of RDX, turned out to be toxic,
an intensive search for a substitute is underway.[4-5] Suitable
substitutes should not only surpass the performance of their
predecessors, but also exhibit high thermal stability with low
sensitivity.[6-7] Furthermore, the synthesis should be carried out in
a few steps with simple processing while being inexpensive and
environmentally friendly. However, the opportunities available
here are looking increasingly exhausted. CL-20 for example is
currently part of many studies, trying to compensate the high
sensitivity by co-crystallization of insensitive molecules.[8-9]
Figure 1. A) 1,1’-Diaminoazotetrazole, B) 1,1’-dihydroxyazotetrazole, C) 1,1’-
dimethylazotetrazole, D) 1,1’-dinitraminoazotetrazole, E) bisguanidinium
azotetrazolate.
In this work, we present the synthesis of four new symmetric
energetic azotetrazoles, containing azidoethyl or nitratoethyl
functionalities. The main access to this substance class was
In addition to trying to improve known molecules, increasing
the nitrogen content also provides a good starting point.[5] The
building blocks represented in this area are primarily azoles. The
most nitrogen-rich azole, pentazole, is currently only used as a
salt and is extremely difficult to access.[10-11] The first
5-aminopentazole derivative was only recently detected by NMR
generated through
a
well-known reaction of sodium
5-aminotetrazolate with 2-chloroethanol resulting in the formation
of the two isomers 1a and 1b (Scheme 1 and 2).[25] For 1a and 2a
an additional selective route is the reaction of cyanogen azide with
2-aminoethanol
and
2-chloroethylamine
hydrochloride,
respectively.[26] Straightforward chlorination of 1a and subsequent
1
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10.1002/ejoc.202100747
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COMMUNICATION
chlorine-azide exchange formed the azidoethyl unit of 3a.[27] In
order to introduce energetic groups only in the last synthesis step,
2a was azo-coupled to the corresponding 1,1'-chloroethyl-
azotetrazole (7, for more information see the SI), but the
compound decomposed during the attempt of a chloro-azide
exchange. With 1a and 3a in hands undergoing oxidative azo
coupling with tert-butyl hypochlorite (t-BuOCl)[28] in acetonitrile,
1,1’-bishydroxyethyl-azotetrazole (4a) and 1,1’-bisazidoethyl-
azotetrazole (6a) could be synthesized in good yields. Compound
4a was further nitrated in 100% nitric acid to yield the bis-
nitratoethyl compound 5a. The complete synthetic protocol
presented is featuring the use of only industrially accessible
chemicals as there are 5-aminotetrazole, chloroethanol, thionyl
chloride, sodium azide, t-BuOCl and nitric acid, making all
products easily available.
Single crystals suitable for low temperature X-ray diffraction were
obtained directly from the reaction mixture (4a, 6a, 6b) or after
recrystallization from dichloromethane (5a) or a mixture of
acetonitrile and water (5b). Details on the structures of the
compounds 4a (CCDC no 2090053) together with measurement
and refinement data of all crystallographically investigated
compounds can be found in the Supporting Information (Figures
S1–2, Table S1). The datasets were uploaded to the CSD
database under the CCDC nos. 2090058 (5a), 2090057 (6a),
2090056 (5b) and 2090054 (6b) and can be obtained free of
charge. The bond lengths and angles in the 5,5’-azobistetrazole
moieties, as well as in the nitratoethyl and azidoethyl groups as
substituents on tetrazoles are in accordance with similar
structures already described in the literature and are therefore not
part of the discussion.[20-21]
Both nitratoethyl derivatives crystalize in the monoclinic space
group P21/c with respectively two (5a) or four (5b) formula units
per unit cell. In both structures, the ethyl groups are rotated out of
the plane at approximately the same angle (5a: C1–N1–C2–
C3 = 92.4(3) °; 5b: N1–N2–C3–C4 = 83.21(18) °). The invasion
symmetry in 5a results in an opposite position of the two ethyl
groups, whereas in 5b, due to the lack of symmetry, both residues
stand away in roughly the same direction (Figure 2).
Scheme 1. Synthetic pathway for the synthesis of 1-substituted isomers starting
from 1-(2-hydroxyethyl)-5-aminotetrazole (1a).
Despite
the
long
knowledge
of
hydroxyethyl-5-
aminotetrazoles, hardly any reactions are known about 2-(1-
hydroxyethyl)-5-aminotetrazole (1b), whereas there are plenty of
references about the corresponding 1-isomer 1a and its
derivatives.[25] Nonetheless, chloromethyl (2b) and azidoethyl
(3b) derivatives could be synthesized in the same manner as for
the other isomer (2a and 3a). Oxidation of 1b followed by nitration
results in the formation of 5b, which was purified by
recrystallization (Scheme 2). To avoid recrystallisation, 1b was
protected with a TBS group to prevent the formation of by-
products during azo formation or nitration (for more information
see the SI compound 8). However, the above-mentioned
reactions led to the cleavage of the protective group. Following
the procedure for the preparation of compound 6a, bis(2,2’-(2,2’-
azidoethyl)-azotetrazole (6b) was prepared in an identical manner
by oxidative dimerization of 3b using t-BuOCl as reagent.
Figure 2. Low temperature crystal structures of the compounds 5a and 5b.
Selected bond lengths (Å) of 5a: N5–N5i 1.259(3), N1–C2 1.460(3), O1–C3
1.456(3). Selected bond lengths (Å) of 5b: N5–N6 1.258(2), N2–C3 1.461(2),
C4–O1 1.453(2). Selected bond angles (°) of 5a: N5i–N5–C1 111.7(2), C1–N1–
C2 130.69(18), C3–N6–N7 115.66(19). Selected bond angles (°) of 5b: N6–N5–
C1 112.20(15), N1–N2–C2 123.19(15), C4–O1–N11 113.20(14).
The azidoethyl derivatives crystallize in a monoclinic space group
(6b: P21/n) and a triclinic space group (6a: P−1). As already
observed for compounds 5a and 5b, the arrangement of the side
chains of the molecule is represented by the torsion angle created
by the tetrazole and ethyl groups. Since in this case both
molecules possess an inversion symmetry, the azidoethyl
Scheme 2. Synthetic pathway for the synthesis of 2-substituted isomers starting
from 2-(2-hydroxyethyl)-5-aminotetrazole (1b).
2
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10.1002/ejoc.202100747
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COMMUNICATION
moieties are located in opposite directions. The torsion angles,
however, differ more clearly for 6a (C1–N1–C2–C3 = 90.4(3) °)
and 6b (N1–N2–C2–C3 = 39.959(7) °) compared to 5a and 5b
(Figure 3).
Figure 4 Proton coupled 15N NMR spectra of 6a (A) and 5a (B) measured in
DMSO-D6.
Depending on the molecule, additional resonances for the
respective OH or NH2 function are detectable. Successful azo
coupling is easily detectable by the disappearance of the NH2
singlet signal. All investigated compounds show three signals in
the 13C spectrum. The signals for the tetrazole carbon of the
monomers occurs at around 156 ppm for the 1-substituted
tetrazoles 1a–3a and 167 ppm for 2-substituted derivatives (1b–
3b) while the respective azo coupled products show a slight deep-
field shift for this signal. Furthermore, two signals for the ethyl rest
are detectable, depending on the particular substitution thereof.
As a result of inversion symmetry, for azo coupled compounds
4a,b–6a,b only the half of the expected number of different
resonances is obtained.
Figure 3. Low temperature crystal structures of the compounds 6a and 6b.
Selected bond lengths (Å) of 6a: N5–N5i 1.263(4), N1–C2 1.469(3), N6–C3
1.480(3). Selected bond lengths (Å) of 6b: N5–N5i 1.2536(2), N2–C2
1.46606(17), C3–N6 1.4870(2). Selected bond angles (°) of 6a: N5i–N5–C1
111.4(3), C1–N1–C2 131.0(2), C3–N6–N7 114.3(2). Selected bond angles (°)
of 6b: N5i–N5–C1 112.452(8), N1–N2–C2 123.753(10), C3–N6–N7
115.139(11).
All compounds were investigated using 1H and 13C NMR
spectroscopy. The CH2 signals in the proton spectra for the
respective ethyl functions, splitting up to doublet of doublet (dd)
or multiplet (m) signals, appear, depending on the functional
group (‒Cl, ‒OH, ‒ONO2, ‒N3), in a region of 3.76‒5.29 ppm.
Due to poor solubility, 15N NMR spectra could only be measured
for 5a and 6a (Figure 4). Assignments were performed on basis
1
of spectra of several azotetrazole[21] derivatives and 2D H, 15N
HMBC spectra (for more details see the Supporting Information).
Azotetrazole resonances N1-N5 appear homogenously for both
compounds with the nitrogen N5, involved in the azo bridge, as
the most deep-field shifted signal (117.7 ppm for 6a and
118.3 ppm for 5a). Signals N1-N4 arise in the regular range as
expected for 1-alkyl-5-aminotetrazoles. In addition, 5a shows a
signal at −43.6 ppm representing the organic nitrate N6. Nitrogen
resonances of the azido function can be detected at −134.8 ppm
(Nβ), −171.6 ppm (Nγ) and −322.6 ppm (Nα) in the spectrum of 6a.
3
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10.1002/ejoc.202100747
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COMMUNICATION
Table 1. Energetic properties and detonation parameters of azo compounds 5a, 5b, 6a and 6b.
5a
C6H8N12O6
344.21
2
5b
C6H8N12O6
344.21
˂ 1
6a
C6H8N16
304.24
4
6b
C6H8N16
304.24
˂ 1
TNT
C7H5N3O6
227.13
15
Formula
M [g mol−1
IS [J][a]
]
FS [N][b]
ESD [J][c]
60
20
6
0.2
353
0.1
1.5
0.25
0.75
-
 [g cm−3 [d]
]
1.702
48.8
1.676
48.8
1.605
73.7
1.539
1.648
18.5
N [%][e]
Ω [%][f]
73.7
−18.6/−46.5
167/179
630.1
662.3
−18.6/−46.5
-/97
−52.6/−84.1
147/188
1456.2
1486.0
−52.6/−84.1
72/140
1439.2
1468.9
−24.7/−74.0
290
Tmelt /Tdec [°C][g]
ΔfH° [kJ mol−1 [h]
ΔfU°[kJ mol−1 [i]
Explo5 V6.05.02
−ΔExU [kJ kg−1 [j]
Tdet [K][k]
]
613.0
645.3
−55.5
−168.0
]
]
5195
3608
782
5135
3598
785
4759
3175
777
4702
3181
782
4427
3222
633
V0 [L kg−1 [l]
PCJ [kbar][m]
]
266
256
233
207
194
Vdet [m s−1 [n]
]
8179
8060
8148
7801
6824
[a] Impact sensitivity (BAM drophammer (1 of 6)). [b] Friction sensitivity (BAM friction tester (1 of 6)). [c] Electrostatic discharge device (OZM research). [d] From
X-ray diffraction analysis recalculated to 298 K. [e] Nitrogen content. [f] Oxygen balance with respect to CO/CO2 [g] Melting/Decomposition temperature (DTA;
β = 5 °C min−1). [h] Calculated enthalpy of formation. [i] Calculated energy of formation. [j] Energy of explosion. [k] Detonation temperature. [l] Volume of
detonation products (assuming only gaseous products). [m] Detonation pressure at Chapman-Jouguet point. [n] Detonation velocity.
Generally, compounds 5a, 5b, 6a and 6b can be classified[29] as
sensitive to very sensitive energetic materials with high heats of
formation (Table 1). The crystallographically determined room
temperature densities range from 1.702 g cm−3 (5a) to
1.539 g cm−3 (6b), with the 2-substituted isomers each being less
dense than the 1-substituted ones. The same applies to the
comparison of nitrates and corresponding azides within the same
substitution position. The calculated detonation parameters,
which are closely linked to the respective density and heat of
formation, respond analogously. Nitrate esters 5a (8179 m s−1)
and 5b (8060 m s−1) both show higher values for detonation
velocity then the respective azidomethyl derivatives. Azido
compound 6a (8148 m s−1) is just slightly below the value of 5a,
whereas 6b (7801m s−1) differs about 250 m s−1 from the nitrate
5b due to a higher difference in density. The 1-isomeres also
achieve higher decomposition temperatures than the 2-isomeres.
5a and 6a decompose at around 180 °C, whereas 5b already
decomposes shortly below 100 °C. Sensitivity values toward
impact and friction for 5a (IS = 2 J, FS = 60 N) are comparable
with those of PETN. As a trend for both, the respective azides and
the 2-substituted isomers are more sensitive compared to the 1-
substituted homologs. Azide 6b is the most sensitive compound
in this study with IS ˂ 1 J and FS = 0.2 N, which is in the range for
primary explosives.
Summarized, we report the synthesis by oxidative azo coupling of
several 1- and 2-substituted 5-aminotetrazoles using tert-butyl
hypochlorite as oxidation agent and characterization (XRD, NMR
etc.) of 1- and 2-substituted nitratoethyl and azidoethyl substituted
5,5’-azotetrazoles and the examination of the utility as energetic
materials. The 1-substituted isomers showed more promising
results in term of detonation properties and thermal stabilities
compared with the respective 2-substituted derivatives. In terms
of sensitivity toward external stimuli and the respective calculated
energetic parameters azidomethyl derivative 6a and 6b could be
further investigated for potential application as primary explosives.
Nitrate ester compound 5a shows the most promising
characteristics as it outperforms TNT with a detonation velocity of
about 8200 m s−1 and a thermal stability as well as sensitivity
values in the same range then PETN. Due to the simple and
4
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Impact: Insensitive > 40 J, less sensitive ≥ 35 J, sensitive
≥ 4 J, very sensitive ≥ 3 J; friction: Insensitive > 360 N,
less sensitive = 360 N, sensitive < 360 N, > 80 N, very
sensitive ≥ 80 N, extreme sensitive ≥ 10 N; according to
the UN recommendations on the transport of dangerous
goods.
Acknowledgements
For financial support of this work by Ludwig-Maximilian University
(LMU), the Office of Naval Research (ONR) under grant no. ONR
N00014-19-1-2078 and the Strategic Environmental Research
and Development Program (SERDP) under contract no.
W912HQ19C0033 are gratefully acknowledged. We thank Prof.
Konstantin Karaghiosoff for NMR measurements.
Keywords: Azo compounds • Crystallography • Energetic
Material • NMR spectroscopy • Tetrazoles
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5
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Entry for the Table of Contents
Tert-butyl hypochlorite is a powerful, cheap and handy reagent for performing oxidative azo coupling of amines. By the formation of
differently substituted azotetrazoles, new energetic molecules were created through an elegant synthetic route.
Twitter: https://twitter.com/HEDM_at_LMU
6
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