A comparative study on insensitive energetic derivatives of 5-(1,2,4- Triazol-C-yl)-tetrazoles and their 1-Hydroxy-tetrazole analogues

Article abstract of DOI:10.1002/zaac.201300304

The synthesis and characterization of selected nitrogen-rich salts based on 5-(1,2,4-triazol-C-yl)tetrazoles and their 1-hydroxytetrazole analogues is presented. The combination with guanidinium, triaminoguanidinium, and hydroxylammonium cations leads to enhanced performance and sensitivities. The main focus of this work is on the energetic properties of those ionic derivatives in comparison to the neutral compounds. Additionally, the positive influence of the introduction of N-oxides in energetic materials is shown. Structural characterization was accomplished by means of Raman, IR, and multinuclear NMR spectroscopy. The standard enthalpies of formation were calculated for selected compounds at the CBS-4M level of theory, the detonation parameters were calculated using the EXPLO5.05 program. Additionally, thermal stability was measured via DSC and sensitivities against impact, friction, and electrostatic discharge were determined.

Full text of DOI:10.1002/zaac.201300304

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ARTICLE
DOI: 10.1002/zaac.201300304
A Comparative Study on Insensitive Energetic Derivatives of 5-(1,2,4-
Triazol-C-yl)-tetrazoles and their 1-Hydroxy-tetrazole Analogues
Manuel Dachs,[a] Alexander A. Dippold,[a] Jakob Gaar,[a] Marcel Holler,[a] and
Thomas M. Klapötke*[a]
Keywords: Triazoles; Tetrazoles; Heterocycles; Energetic materials; Nitrogen-rich
Abstract. The synthesis and characterization of selected nitrogen-rich introduction of N-oxides in energetic materials is shown. Structural
salts based on 5-(1,2,4-triazol-C-yl)tetrazoles and their 1-hydroxy-
characterization was accomplished by means of Raman, IR, and multi-
tetrazole analogues is presented. The combination with guanidinium, nuclear NMR spectroscopy. The standard enthalpies of formation were
triaminoguanidinium, and hydroxylammonium cations leads to en-
hanced performance and sensitivities. The main focus of this work is
on the energetic properties of those ionic derivatives in comparison
to the neutral compounds. Additionally, the positive influence of the
calculated for selected compounds at the CBS-4M level of theory, the
detonation parameters were calculated using the EXPLO5.05 program.
Additionally, thermal stability was measured via DSC and sensitivities
against impact, friction, and electrostatic discharge were determined.
the corresponding salts. Cations like guanidinium, triaminogu-
anidinium or hydroxylammonium not only increase the overall
nitrogen content and thus the heat of formation, but also im-
Introduction
In the last decades, research in the field of energetic materi-
als faced a profound change. Numerous studies raised aware-
ness of the toxicity of widely-used substances like TNT, RDX,
and HMX and their degradation products towards humans and
the environment.[1] Additionally, modern safety requirements
of the armed forces[2] cause a growing demand for material
less vulnerable to stimuli like shock, heat, and bullet impact.
Research around the globe focuses strongly on compounds
based on nitrogen-rich heterocycles, since those liberate
mostly molecular nitrogen as innoxious product of combustion
or explosion. Furthermore, attractive energetic properties due
to substantial ring strain and highly positive heats of formation
attract notice to the research of environmentally friendly high-
prove the performance characteristics.[4a,6]
.
The focus of this study is on the synthesis and full charac-
terization of energetic salts of 5-(1,2,4-triazol-C-yl)tetrazoles
(1, 2) and their 1-hydroxy-tetrazole analogues (3, 4). The ener-
getic performance and sensitivity data of the ionic compounds
are presented and compared to the neutral precursors. Ad-
ditionally, the positive influence of the introduction of N-ox-
ides in energetic materials is shown.
Results and Discussion
power energetic materials.[3] Recent studies on C–C connected Synthesis
heterocycles like bistriazoles and bistetrazoles revealed excel-
All four neutral 5-(1,2,4-triazol-C-yl)tetrazoles (1, 2) and 5-
lent characteristics regarding stability and detonation proper-
ties.[3a,4] The connection via C–C bond of a triazole ring with
its opportunity to introduce a large variety of energetic moie-
ties and a tetrazole or a N-hydroxy-tetrazole ring implying a
large energy content leads to energetic materials with tunable
properties.[5] Due to the fact that nitrogen-rich salts of ener-
getic compounds show an increased stability compared to the
uncharged compounds, we present the treatment of energetic
triazole compounds with several nitrogen-rich bases to form
(1,2,4-triazol-C-yl)tetrazol-1-oles (3, 4) were synthesized as
published recently starting from 5-amino-1H-1,2,4-triazole-3-
carbonitrile.[5] In the case of compounds 1 and 2, first of all
the tetrazole ring was built up by a cycloaddition with sodium
azide, followed by introduction of the energetic moieties via
Sandmeyer reaction or nitration with nitric acid (Scheme 1).[5a]
* Prof. Dr. T. M. Klapötke
Fax: +49-89-2180-77492
E-Mail: tmk@cup.uni-muenchen.de
[a] Department of Chemistry
Ludwig-Maximilian University of Munich
Butenandtstr. 5–13, Haus D
81377 Munich, Germany
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/zaac.201300304 or from the au-
thor.
Scheme 1. Synthesis of NTT (1) and NATT (2).
Z. Anorg. Allg. Chem. 0000, ᭿,(᭿), 0–0
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M. Dachs, A. A. Dippold, J. Gaar, M. Holler, T. M. Klapötke
ARTICLE
A different approach was used for the synthesis of com-
pounds 3 and 4. The energetic moieties were primarily intro-
duced by modification of the amine group of 5-amino-1H-
1,2,4-triazole-3-carbonitrile. Multiple reaction steps including
the formation of an amidoxime, chlorination, chlorine to azide
exchange and finally cyclization lead to the 1-hydroxy-tetra-
zole compounds 3 and 4 (Scheme 2).[5b]
Scheme 2. Synthesis of 5-(3-nitro-1H-1,2,4-triazol-5-yl)tetrazol-1-ol
(NTTO) (3) and 5-(5-nitrimino-1,4H-1,2,4-triazol-3-yl)tetrazol-1-ol
(NATTO ) (4).
Preparation of the corresponding salts of compounds 1–4
was accomplished by diluting the neutral compound in ethanol
and addition of two equivalents of the corresponding organic
base (Scheme 3). This step benefits from the very poor solubil-
ity of the ionic target molecules, contrary to the neutral ones,
which dissolve readily in ethanol. Precipitation of the desired
ionic compounds occurred almost quantitative and led to high
purities.
Scheme 3. Synthetic route to the nitrogen-rich salts derived from 1–4
using the corresponding nitrogen-rich bases (hydroxylamine, guanidi-
nium carbonate, triaminoguanidine).
comparison with similar molecules and additional theoretical
calculations using Gaussian 09 (MPW1PW91/aug-cc-
pVDZ).[7] The signals of the triazole nitrogen atoms as well as
the nitro group can be found in both cases in the expected
range very similar to the recently published 3,3Ј-dinitro-5,5Ј-
bistriazolate anion.[8] Two well resolved resonances are ob-
served in the 15N NMR spectrum of the tetrazolate compound
1c at –7.3 (N5/6), and –67.6 ppm (N4/7), which is in good
agreement with the resonances of the 5,5Ј-bistetrazolate anion
(–3.0, –66.0).[3a] The tetrazole-N-oxide ring of compound 3c
shows four well resolved resonances. The signals can be ob-
served at shifts of –82.4 (N4), –20.2 (N5), –17.0 (N6), and
–54.0 ppm (N7), comparable with the signals of the similar
5,5Ј-bistetrazole-1,1Ј-diolate anion.[9]
Multinuclear NMR Spectroscopy
All compounds were investigated using 1H, 13C, and
14N NMR spectroscopy. The 13C{1H}-spectra show three sig-
nals for the carbon atoms in the expected range, deprotonation
with nitrogen-rich bases shifts the signals of all carbon atoms
to lower field in comparison to the uncharged compounds. The
signal of the carbon atom next to the energetic moiety is
shifted furthest downfield for all compounds in the range of
157.3 ppm (2c) and 165.9 ppm (3b).
The signal of the bridging carbon of the triazole ring can be
observed in the range of 151.4 ppm (2a) to 157.3 ppm (1c) and
the corresponding signal of the tetrazole carbon atom is located
in the range of 148.4 ppm (2a) to 156.4 ppm (1c) for the tria-
zolyl-tetrazole compounds. In the case of the tetrazole-1 N-
oxide compounds, all signals of the bridging carbon atoms are
shifted to higher field. The signal of the triazole ring can be
Single Crystal X-ray Structure Analysis
All compounds were recrystallized from water, which
observed in the range of 147.8 ppm (4a) to 152.8 ppm (3c) and mainly led to the formation of microcrystalline material not
the corresponding signal of the tetrazole carbon atom is located suitable for X-ray analysis. Only crystals of compounds 3b
in the range of 137.7 ppm (4c) to 139.2 ppm (3a). The were appropriate and the crystal structure is discussed in the
14N{1H} NMR spectra of all compounds show a broad signal following. The bond lengths and torsion angles within the
for the nitro group between –10 ppm and –25 ppm. Based on azole rings are all in the range of formal C–N and N–N single
comparable ionic compounds,[4a,6] the proton signals of all cat- and double bonds (C–N: 1.47 Å, 1.22 Å; N–N: 1.48 Å,
ions can be found in the expected range.
1.20 Å).[10] The C3–N5–N6 angle of the N-oxide anion has an
Due to the insufficient solubility of compound 2a–c and 4a–c, value of 108.7(1)° as compared to 109.9(2)° for the neutral
15N{1H} NMR spectra were recorded for compounds 1c and compound 3. As expected the N5–O3 bond length is shortened
3c, as illustrated in Figure 1. The assignments are based on to 1.317(2) Å upon deprotonation [1.345(2) Å in 3]. The tor-
2
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Energetic Derivatives of 5-(1,2,4-Triazol-C-yl)-tetrazoles and their 1-Hydroxy-tetrazole Analogues
Figure 2. Molecular structure of guanidinium 5-(3-nitro-1,2,4-triazo-
late-5-yl)tetrazol-1-olate (3b). Thermal ellipsoids are set to 50% prob-
ability.
the oxygen atom O1 is involved in a electrostatic interaction
with the π electrons of the overlying tetrazole ring, which sup-
ports the stacking of the layers.
Figure 1. 15N{1H}NMR spectra of compounds 1c (bottom) and 3c
(top) recorded in [D6]DMSO. The x axis represents the chemical shift
δ in ppm.
Figure 3. Formation of planes in the crystal structure of 3b. Thermal
ellipsoids are set to 50% probability. Symmetry codes: (i) 1/2–x,
1/2+y, 1/2–z; (ii) x, –1+y, z; (iii) 1–x, –y, –z; (iv) 1/2+x, 1/2–y, –1/2+z;
(v) 1–x, 1–y, –z.
sion angle between both heterocycles and the one of the nitro
group is very small [2.7(2)° and 0.8(2)°], which leads to a
nearly planar assembly. Compound 3b crystallizes as a mono-
hydrate in the monoclinic space group P21/n with a density of
1.639 g·cm–3, the formula unit is shown in Figure 2.
Due to the planarity of both cation and anion, the crystal
structure of 3b is built up by planes that are kept together by
a strong network of hydrogen bonds. A shown in Figure 3,
each NTTO2– anion is surrounded by five guanidinium cations
Theoretical Calculations, Performance Characteristics, and
Stabilities
The heats of formation of all compounds were calculated on
via strong hydrogen bonds towards the atoms of the azole rings the CBS-4M level of theory using the atomization energy
and the oxygen O1 of the nitro group (Table 1). It is remark- method and utilizing experimental data (for further details and
able to note that all accessible nitrogen atoms (and the N-oxide results refer to the Supporting Information). All compounds
O3) act as acceptor for hydrogen bonds, which is merely pos- show highly endothermic enthalpies of formation in the range
sible due to the several N–H groups of the surrounding guani- of 234 kJ·mol–1 (1b) to 1009 kJ·mol–1 (4c), all by far outper-
dinium cations. All contacts lie well within the sum of van der forming RDX (70 kJ·mol–1).
Waals radii [rw(O) + rw(N) = 3.07 Å, rw(N) + rw(N) =
In order to estimate the detonation performances of the pre-
3.20 Å][11] with a D···A length between 2.801(2) Å and pared compounds selected key parameters were calculated
3.199(2) Å. Most of the hydrogen bonds are strongly directed with EXPLO5 (version 5.05)[20] and compared to RDX. The
with D–H···A angles between 155(2)° and 175(2)°. In addition, calculated detonation parameters using experimentally deter-
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M. Dachs, A. A. Dippold, J. Gaar, M. Holler, T. M. Klapötke
ARTICLE
Table 1. Hydrogen bonds present in 3b.
D–H···A
d(D–H) /Å
d(H···A) /Å
d(D–A) /Å
Ͻ (D–H···A) /°
O4i–H4b···N2
0.87(2)
0.86 (2)
0.85(2)
0.90(2)
0.86 (2)
0.86(2)
0.86(2)
0.86(2)
0.89(2)
0.89(2)
0.87(2)
0.87(2)
2.06(2)
1.98(2)
2.10(2)
2.20(2)
2.32(2)
2.32(2)
2.10(2)
2.40(2)
2.16(2)
2.32(2)
2.19(2)
2.37(2)
2.916(2)
2.801(2)
2.920(2)
3.095(2)
3.175(2)
3.053(2)
2.814(2)
3.199(2)
3.040(2)
3.152(2)
3.044(2)
3.172(2)
169(2)
158(2)
165(2)
175(2)
173(2)
143(1)
140 (2)
155(2)
170(2)
154(2)
164(2)
154(2)
N9ii–H9a···O4
N9iv–H9b···N3
N10iv–H10a···N8
N11–H11a···N6
N11ii–H11b···O4
N12i–H12a···O3
N12v–H12b···N7
N13i–H13a···N1
N13i–H13b···N2
N14iii–H14a···O1
N14v–H14b···N7
Symmetry operators: (i) 1/2–x, 1/2+y, 1/2–z; (ii) x, –1+y, z; (iii) 1–x, –y, –z; (iv) 1/2+x, 1/2–y, –1/2+z; (v) 1–x, 1–y, –z.
mined densities (gas pycnometry at 25 °C with dried com-
pounds) and calculated heats of formation are summarized in
Table 2 and Table 3.
Sensitivity
Regarding the precursor compounds NATT (2) and NATTO
(4), both show very high sensitivity towards impact, friction,
and electrostatic discharge. One of the key aspects of this study
was the synthesis of ionic derivatives that are safer to handle,
while being at least equally energetic. In the case of com-
pounds 2a–c, the sensitivity towards impact is reduced from
1 J to 40 J and the sensitivity towards friction could be lowered
to 160 N (2a), 324 N (2b) and 360 N (2c). Compounds 4a–c
also show lower sensitivity values [8 J (4a), 40 J (4b), 10 J
(4c)] in comparison to the neutral compound 4, however the
low sensitivities of the corresponding compounds bearing no
N-oxide are only reached for compound 4b.
Figure 1 shows the thermal decomposition of the ionic de-
rivatives 2a–c (solid lines) and 4a–c (dashed lines).
As expected, the thermal stability of all ionic compounds
mostly depends on the cation, however the ionic N-oxide com-
pounds (4a–c) all show a lower decomposition temperature in
comparison to compounds 2a–c as it is expected for N-hydroxy
azoles.[12] With a decomposition temperature of 238 °C (2b)
and 212 °C (4b), only the guanidinium salts show a higher
onset temperature compared to RDX (Figure 4).
Figure 4. DSC plots of ionic derivatives 2a–c (solid lines) and 4a–c
(dashed lines). DSC plots were recorded with a heating rate of
5 K·min–1.
Performance
The results of theoretical calculations regarding perform-
The sensitivity of the neutral NTT (1) and NTTO (3) ance are summarized in Table 2 and Table 3. In general, the
towards external stimuli could also be further decreased by triazol-C-yl-tetrazoles show lower performance values in com-
deprotonating with organic bases. All compounds (except 3a) parison to their 1-hydroxy-tetrazole analogues. As expected,
show an impact sensitivity of 40 J and friction sensitivity of the additional oxygen atom generally leads to increased ener-
360 N. As shown in Figure 3, the guanidinium salt (3b) shows getic properties due to a higher density and an even greater
a remarkably high decomposition temperature (296 °C) in energy output.[12b,14] In comparison to the ionic derivatives of
comparison to the neutral compound 1, whereas compound 3c compounds 1 and 2, a marked performance increase is seen.
starts to decompose at 190 °C and 3a at 182 °C.
The detonation velocities of the hydroxyl ammonium salts 3a
Again, the ionic N-oxide compounds (3a–c) show lower de- and 4a are increased by about 500 ms–1. For the guanidinium
composition temperatures in comparison to compounds 1a–c, salts 3b and 4b, the influence of the additional oxygen is
only the guanidinium salt 3b exceeds 200 °C with a thermal slightly lower; however, the detonation velocity is still in-
stability of 269 °C (Figure 5).
creased by about 270 m·s–1. The introduction of the N-oxide
4
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