Isomers of Dinitropyrazoles: Synthesis, Comparison and Tuning of their Physicochemical Properties

Article abstract of DOI:10.1002/cplu.201800318

Three isomeric dinitropyrazoles (DNPs) were synthesized starting from readily available 1H-pyrazole by slightly improved methods than described in the literature. 3,4-Dinitropyrazole (3), 1,3-dinitropyrazole (4), and 3,5-dinitropyrazole (5) were obtained and compared to each other with respect to thermal stability, crystallography, sensitivity and energetic performance. Two isomers (3 and 4) show high densities (1.79 and 1.76 g cm^–3) and interesting thermal behavior as melt-castable materials (3: T_melt.=71 °C, T_dec.=285 °C; 5: T_melt. = 68 °C, T_dec.=171 °C). Furthermore, eight salts (sodium, potassium, ammonium, hydrazinium, hydroxylammonium, guanidinium, aminoguanidinium and 3,6,7-triamino-[1,2,4]triazolo[4,3-b][1,2,4]triazole (TATOT) of 3 and 5 were synthesized in order to tune performance and sensitivity values. These compounds were characterized using ¹H, ¹³C, ¹⁴N, ¹⁵N NMR and IR spectroscopy as well as mass spectrometry, elemental analysis and thermal analysis through differential scanning calorimetry. Crystal structures of 14 compounds were obtained (3–7, 10–12 and 15–20) by low-temperature single crystal X-ray diffraction. Impact, friction and electrostatic discharge (ESD) values were also determined by standard methods. The sensitivity values range between 8.5 and 40 J for impact and 240 N and 360 N for friction and show mainly insensitive character. The energetic performances were determined using recalculated X-ray densities, heats of formation and the EXPLO5 code and support the energetic character of the title compounds. The calculated energetic performances (V_D: 6245–8610 m s^?1; p_CJ: 14.1–30.8 GPa) were compared to RDX ((O₂NNCH₂)₃).

Full text of DOI:10.1002/cplu.201800318

Accepted Article
Title: Isomers of Dinitropyrazoles: Synthesis, Comparison and Tuning
of Their Physico-Chemical Properties
Authors: Joerg Stierstorfer, Thomas M. Klapötke, Marc F Bölter, and
Alexander Harter
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█ N-Heterocycles
Isomers of Dinitropyrazoles: Synthesis, Comparison and Tuning of
Their Physico-Chemical Properties
Marc F. Bölter,[a] Alexander Harter,[a] Thomas M. Klapötke,*[a] and Jörg Stierstorfer[a]
Abstract: Three isomeric dinitropyrazoles (DNPs) were 1H, 13C, 14N, 15N NMR and IR spectroscopy as well as mass
synthesized starting from easy available 1H-pyrazole by spectrometry, elemental analysis and thermal analysis
slightly improved methods than described in literature. 3,4- (DSC). Crystal structures could be obtained of 14
Dinitropyrazole (3), 1,3-dinitropyrazole (4), and 3,5- compounds (3−7, 10−12 and 15−20) by low temperature
dinitropyrazole (5) were obtained and compared to each single crystal X-ray diffraction. Impact, friction and ESD
other with respect to thermal stability, crystallography, values were also determined by standard methods. The
sensitivity and energetic performance. The two isomers (3 sensitivity values range between 8.5 and 40 J for impact
and 4) show high densities (1.79 and 1.76 g cm–3) and and 240 N and 360 N for friction and show mainly
interesting thermal behavior as melt-castable materials (3: insensitive character. The energetic performances were
Tmelt. = 71 °C, Tdec. = 285 °C; 5: Tmelt. = 68 °C, Tdec.
=
calculated using recalculated X-ray densities, heats of
171 °C). Further, eight salts (sodium, potassium, formation and the EXPLO5 code and support the energetic
ammonium, hydrazinium, hydroxylammonium, guanidinium, character of the title compounds. The calculated energetic
aminoguanidinium and TATOT) of
3
and
5
were performances (VD: 6245–8610 m s–1; pCJ: 14.1–30.8 GPa)
synthesized in order to tune performance and sensitivity were compared to RDX.
values. The obtained compounds were characterized using
characteristics and achieve the requirements mentioned before
(Figure 1).[10] The nitro groups decrease the electron density
inside the pyrazole, whereas the acidity of the N bonded proton
rises and the pKa value decreases (1H-pyrazole: 14.2; 3-
nitropyrazole 9.81; 3,4-dinitropyrazole: 5.14, 3,5-dinitropyrazole:
3.14 and 3,4,5-trinitropyrazole: 2.35).[11] Hence, only the C
nitrated dinitropyrazoles and trinitropyrazole (TNP) are acidic
enough to get deprotonated by adding common bases (figure 1).
Shreeve et al. reported a series of nitrogen-rich salts of TNP with
rather high energetic properties, low sensitivities and high thermal
stabilities.[8]
Introduction
Modern high energy density materials (HEDMs) have to fulfill
different requirements depending on their application such as
high energetic performance, high density, high heat of formation,
low sensitivity toward external stimuli, thermal stability and
environmental impact to replace the widely used RDX.[1] In
contrast to the carcinogenic and hepatoxic RDX nitrogen-rich
materials release mostly environmentally friendly dinitrogen after
decomposition.[2] These goals could be achieved by azoles, such
as
tetrazoles[3],
triazoles[4],
imidazoles[5],
oxadiazoles[6],
furazanes[7] or pyrazoles[8]. In general the deprotonation of azoles
by bases result in higher performances and thermal stabilities
such as 5,5‘-bistetrazole-1,1-dioxide dihydrate (VD: 8764 m s–1,
Tdec.: 214 °C) and its dihydroxylammonium salt (TKX-50, VD:
9698 m s–1, Tdec.: 221 °C) or its potassium salt (Tdec.: 335 °C).[3, 9]
Nitrated pyrazoles arouse interest in the past due to their different
Figure 1. Overview of nitrated pyrazolates arranged according to their rising
[a]
Prof. Dr. Thomas M. Klapötke,* Marc F. Bölter, Alexander Harter,
Dr. Jörg Stierstorfer
sensitivity and energetic performance.
Department of Chemistry
University of Munich (LMU)
Butenandtstr. 5–13 (D)
81377 München (Germany)
Fax: +49 89 2180 77492
E-Mail: tmk@cup.uni-muenchen.de
Comparing the densities of di- and trinitrated pyrazoles the
calculated density of TNP (1.867 g cm–3) is marginally higher than
the ones of the two DNP isomers (1.76 and 1.79 g cm–3).[8] But
the sensitivity values are with 17 J for impact and 92 N for friction
higher than for TNP.[8] Nitrogen-rich salts of dinitrated pyrazoles
(DNPs) have not been mentioned in the literature yet. In this work
three different isomeric DNPs were synthesized by partly different
*
Supporting information for this article is available on the WWW under
http://www.chemeurj.org/ or from the author.
1
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methods than literature and selected alkaline metal and nitrogen-
rich salts of 3,4-DNP and 3,5-DNP were prepared, intensively
characterized and compared to each other.
Figure 2. Molecular units of 3 (left) and 5 (right). Ellipsoids are drawn at the
50% probability level.
The molecular units of compounds 3–5 are shown in Figures 2
and 3.
Results and Discussion
The synthesis of the three DNPs is described by. Janssen et al.
starting from N-nitration of pyrazole, followed by thermal
rearrangement in anisole (2) (Scheme 1).[12] A selective C4
nitration of compound 2 was carried out by using nitric acid and
sulfuric acid yielding 3,4-DNP (3).[12a] Similar nitration conditions
to the first step were used for the synthesis of 1,3-DNP (4).[12a]
A
further thermal rearrangement in benzonitrile and an easier work-
up from literature gave 5 in good yields.[12a]
Starting from compounds 3 and 5 different ionic derivatives (6–
21) were synthesized by deprotonation. The salts were prepared
using sodium hydroxide, potassium hydroxide, ammonia,
Figure 3. Molecular unit of 4. Ellipsoids are drawn at the 50% probability level.
Compound 4 is disordered along the axis of rotation through atom
N2. The bond lengths within the ring nitrogen atoms of the
nitropyrazoles are between the values of a N−N single bond (1.47
Å) and a N=N double bond (1.25 Å). The nitro groups of 3 are
most twisted toward the planar pyrazole ring due to their steric
hindrance (3: –7.8(2)°, –78.83(18)°; 4: –3.1(5)°, 17.1(8)°; 5: –
1.7(4)°, –8.9(3)°). Further, 3,4-DNP (3) is stabilized due to
hydrogen bonding resulting in a high decomposition temperature
(Tdec.: 285 °C). On the one hand classical hydrogen bonds are
found involving N–H···N/O interactions and showing H···N/O
distances between 2.9540(16) and 3.0572(16) Å.[13] Further, weak
non-classical hydrogen bonds implicating C–H···O correlations
are visible.
hydroxylamine,
hydrazine,
guanidine
bicarbonate,
aminoguanidine bicarbonate and TATOT (3,6,7-Triamino-
[1,2,4]triazolo[4,3-b][1,2,4]triazole). Salts of compound 5 show a
much better and faster crystallization behavior than the salts of
compound 3.
The molecular moieties of water free salts are depicted in Figures
4–7. The remaining crystal structures can be found in the
supporting information. The potassium (15) and ammonium (16)
salt of 3,5-DNP crystallize in the space group P–1. The crystal
density of 15 shows the highest value of all compounds with
2.037 g cm–3 whereas the density of 16 is 1.732 g cm–3 at 173 K.
Scheme 1. Synthesis of 1,3-dinitropyrazole (4), 3,4-dinitropyrazole (3) and 3,5-
dinitropyrazole (5) as well as their ionic derivatives.
Crystal structures
In this work the crystal structures of compounds 3–7, 10–12 and
15–20 were obtained. Selected data and parameters from the low
temperature X-ray data collection and refinements are given in
the supporting information. Further information regarding the
crystal-structure determinations have been deposited with the
Cambridge Crystallographic Data Centre as supplementary
publication Nos. 1835056 (3), 1835054 (4), 1835062 (5), 1835053
(6), 1835066 (7), 1835061 (10), 1835065 (11), 1835063 (12),
1835060 (15), 1835055 (16), 1835057 (17), 1835059 (18),
1835058 (19) and 1835064 (20).
Figure 4. Molecular units of 15 (left) and 16 (right). Ellipsoids are drawn at the
50% probability level.
The bond lengths and angles of 15 and 16 are similar to the
neutral compound 3. Compound 16 is stabilized by various H-
bonds between the ammonium cation and the anion e.g.
Dinitropyrazoles 3–5 crystallize in common space groups (3:
P21/c; 4: P21/c; 5: Pca21) with crystal densities between 1.796
g cm–3 (4) and 1.827 g cm–3 (5) at 173 K.
N5−H5A···O3,
N5−H5A···O1,
N5−H5A···O4,
N5−H5B···O2
N5−H5C···N1, N5−H5D···N2. The nitro groups of 16 (5.3°; 5.1°)
are more twisted toward the planar pyrazole ring system
compared to the ammonium salt (2.8(2)°; 1.3(2)°).
The hydrazinium salts of both isomers (10, 18) are crystallizing in
the monoclinic space groups P21/c and P21/n, respectively. The
nitro groups of hydrazinium salt 10 are further twisted toward the
planar ring. The density of 10 (1.704 g cm–3) is higher than that of
compound 18 (1.682 g cm–3). This relates to the slightly higher
density of the neutral compounds 3 and 5. The nitrogen bond
2
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10.1002/cplu.201800318
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lengths of both hydrazines are very similar (10: N9–N10 1.444(4);
18: N5–N6 1.446(2)) and are in the range of a typical N–N single
bond.
Figure 7. Molecular units of 12 (top) and 18 (bottom). Ellipsoids are drawn at
the 50% probability level.
Figure 5. Molecular units of 10 (left) and 18 (right). Ellipsoids are drawn at the
50% probability level.
The N–N bond lengths of aminoguanidinium cations are close to
a N–N single bond with distances between 1.4030(18) Å (12) and
1.410(2) Å (20). Both structures show hydrogen bonding involving
all protons of the aminoguanidinium and the carbon bonded
hydrogen.
Both hydrazinium cations show hydrogen bonding to the pyrazole
ring nitrogen and to the oxygen of the nitro groups. The better
hydrogen interaction of 18 leads to a higher decomposition
temperature (figure 8).
Figures 6 and 7 depict the molecular units of the guanidinium
derivatives (11, 12 and 20). They all crystallize in the common
space groups P–1 (11), P212121 (12) and C2/c (20) with densities
of 1.657 g cm–3 (11, 173 K), 1.647 g cm–3 (12, 173 K) and
1.695 g cm–3 (20, 100 K). The structure of 11 is dominated by
strong hydrogen bonds involving all guanidinium protons and both
nitro groups are twisted out of the pyrazole plane (O1–N3–C1–
N1: 24.4(2)° and O4–N4–C2–C1: -168.89(16)°) due to their steric
hindrance.
Spectroscopy
The three isomers 3–5 can easily be distinguished by 1H, 13C, 14
N
and 15N NMR spectroscopy as well as IR and Raman
spectroscopy. Further a two dimensional 1H/15N NMR HMBC
spectrum was recorded for compound 4.The NMR shifts of 3–5,
1
8, 11, 16 and 19 are listed in table 1. In the H NMR spectra, two
hydrogen signals were observed for 1,3-DNP (9.09 and
7.44 ppm) and for 3,4-DNP (14.84 and 9.10 ppm) and one for
3,5-DNP (7.92 ppm). The reason for the missing N-H peak of 3,5-
DNP might be the fact that the molecule is deprotonated in
solution because of the high pKa = 3.14 value.[11a] In the 13C NMR
spectra, two signals were determined for 3,5-DNP (151.5 and
98.8 ppm) and three for 1,3-DNP (152.6, 130.0 and 104.7 ppm)
and 3,4-DNP (148.2, 132.7 and 126.3 ppm).
Table 1. NMR signals of compounds 3–5, 8, 11, 16 and 19.
compound
1H [ppm]
13C{1H} [ppm]
15N [ppm]
3
14.84, 9.10
148.2, 132.7,
126.3
−175.5, −85.0,
−26.1, −25.5
−113.5, −95.0,
−63.2, −25.1
−123.4, −26.4
-
Figure 6. Molecular unit of 11. Ellipsoids are drawn at the 50% probability
4
9.09, 7.44
152.6, 130.0,
104.7
level.
5
8
7.92
151.5, 98.9
163.5, 138.0,
125.8
Also the nitro groups of compound 12 are twisted toward the
planar pyrazole ring with torsion angles of 28.28(19)°
8.14, 7.12
(O4 N4 C1 N1) and 24.7(2)° (O1 N3 C2 C3). Comparing the
twist of the nitro groups of compound 12 and 20 they are only
distorted slightly for the latter (-3.1(3)° and 4.3(2)°).
−
−
−
−
−
−
11
8.03, 6.94
157.9, 150.9,
138.1, 125.3
156.4, 98.3
157.9, 156.4,
98.4
-
16
19
7.30, 7.10
7.30, 6.93
-
-
In the 14N NMR spectra the resonances for the nitro groups of 3–
5 are in the typical range between −10 and −40 ppm for carbon
bonded and between −40 and −70 ppm for N nitrated species (4).
The three isomers can further be distinguished by 15N NMR
spectroscopy (supporting information). The 15N spectra of 3,5-
DNP (5) shows two signals (−123.4 and −26.4 ppm) due to its
symmetry and 3 (−175.5 ; −85.0, −26.1 and −25.5 ppm) and 4
(−113.5, −95.0, −63.2 and −25.1 ppm) show four, respectively. In
the two dimensional HMBC NMR spectra the protons can be
assigned to the carbon atoms (Figure 8). Due to the signal
splitting into a doublet of doublet of the nitrogen atom at
−113.5 ppm it can be assigned to the pyrazole nitrogen (N4). The
larger coupling belongs to N C H (2J) and the smaller to the
− −
N C C H (3J) coupling. The 3J coupling of the proton at
− − −
3
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9.09 ppm with the carbon bonded nitro group (N1) indicates the
proton (a) to be bonded at C-4.
Figure 8. Two dimensional 1H, 15N HMBC spectrum of compound 3.
Figure 9. DSC plots of compounds 3–4 and DTA plot of 5 measured with
The characteristic absorption bands for nitro groups can be found
at 1560/1329 cm–1 for (3,5-DNP), 1518/1346 cm-1 for (3,4-DNP)
and 1351/1281 cm–1 for 1,3-DNP. The bands at 1637 and
1548 cm–1 were assigned to the N-nitro group of 1,3-DNP.
a heating rate of 5 °C min-1 Critical temperatures are given as onset
.
temperatures.
As expected all of the DNPs are not or less sensitive with IS
values of 25 to 40 J, 360 N for friction sensitivity and ca. 1.5 J for
electrostatic discharge sensitivity. 1,3-DNP (4) implies the lowest
value of 25 J, which refers to the nitramino part, but is still higher
than RDX (IS: 7.5 J). The densities of 3–5 are in between 1.76
and 1.79 g m–3 at 298 K, which is nearby the density of RDX.[16]
All three DNPs are isomers and therefore have the same nitrogen
content and a negative oxygen balance. The compounds have
positive heat of formations in the range of 73.2 kJ mol−1 to 189.1
kJ mol−1. 3,5-DNP (5) represent the lowest value (73.2 kJ mol−1),
followed by 3,4-DNP 3 (124.2 kJ mol−1) and 5 (189.1 kJ mol−1)
with the highest. The detonation pressures (pCJ) lie between 28.9
and 30.8 GPa increasing from compound (5) to compound (3).
The detonation velocities (VD) show the same trend with values
between 8279 m s–1 and 8459 m s–1. Regarding the melting and
decomposition behavior of the DNP salts they strongly vary with
the corresponding cation (figure 10).
In the 1H NMR spectra of 3,4-DNP salts 6–13 the hydrogen signal
of DNP is observed between 8.03 and 8.30 ppm. For 3,5-DNP
salts 14-21 the shift ranges from 7.28 to 7.30 ppm. In the 13C
NMR spectra, three resonances for the 3,4-DNP anion were
observed and two for 3,5-DNP anion due to its symmetry. The
signals range from 125.3–127.3 ppm (C-H), 136.9–139.1 ppm (C-
NO2) and 150.3–163.5 ppm (C-NO2) for 3,4-DNP and from 98.3–
98.4 ppm (C-H), 156.1–156.4 ppm (C-NO2) for 3,5-DNP.
IR spectra of compounds 6–21 were measured and the
frequencies were assigned according to observed data reported
in the literature.[14] The characteristic absorption bands for nitro
groups of salts 6–21 were found in the region from 1527–
1556 cm-1 and 1335–1369 cm-1. Further the absorption bands of
the primary amines were found for the nitrogen-rich cations (8-13
and 16–21) in the range of 3125 and 3580 cm–1. Triazole ring
vibrations for 13 and 21 were found in the regions between 1583–
1446 cm–1.[1h]
Thermal Analysis, Sensitivities, Physicochemical and
Energetic Properties
The different compounds were investigated in regard to their
thermal behavior, sensitivities as well as their energetic
properties. The decomposition temperatures were measured by
differential scanning calorimetry (DSC) or differential thermal
analysis (DTA) with a heating rate of 5 °C min–1. Heats of
formation were calculated by the atomization method using
electronic energies (CBS-4M method). The energetic parameters
were calculated with EXPLO5 V6.03.[15]
The isomers 3–5 only differ in the position of the nitro groups. The
melting and decomposition temperatures rise from the most
instable 1,3-DNP (4) with 68 °C and 171 °C over compound (3)
with 71 °C and 285 °C to compound (5) with 169 °C and
decomposition temperature of 299 °C (Figure 9).
Figure 10. DSC plots of compounds 10–12 and 18–21 measured with a
heating rate of 5 °C min-1 Critical temperatures are given as onset
.
temperatures.
4
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The sodium (6) and potassium (7) salts of 3,4-DNP compounds
dehydrate at 80 °C or 88 °C followed by decomposition at 142 °C
and 174 °C, respectively.
Beside compound 12, which melts at 124 °C, all other
decompose at temperatures between 101 °C and 180 °C with 13
as the highest. One reason could be the close position of the nitro
groups destabilizing the molecule.
low energetic, trinitropyrazoles are characterized by an intensive
synthetic protocol as well as higher sensitiveness which do not
comply with new insensitive munitions regulations. Therefore in
the present work, the three different dinitropyrazoles (3,4-DNP
(3), 1,3-DNP (4) and 3,5-DNP (5) are synthesized, characterized
and compared to each other. Their interesting thermal behavior,
high detonation properties and rather low sensitivity was
identified. In addition the potassium and sodium as well as six
selected nitrogen-rich salts were synthesized of 3,4- (3) and 3,5-
DNP (5). Densities vary between 1.59 and 1.99 g cm-3. Using
EXPLO5, their detonation parameters were calculated. Their
detonation velocity ranges from 6245 to 8610 m s–1. With
exception of 9 (10 J), 15 (8.5 J) and 18 (10 J) their IS values are
higher than 40 J. According to their rather high thermal stabilities,
low sensitivity values and detonation performances they can be
used as HEDM.
As shown in Table 2 all 3,4-DNP salts are insensitive toward
impact, friction and electrostatic discharge. The two exceptions
are the hydrazinium (IS: 10 J) and TATOT (IS: 30 J) salt, but
nevertheless they are less sensitive than RDX (IS: 7.5 J) and are
classified as less sensitive. All salts have friction sensitivity values
of 360 N and the ESD value varies between 1.0 and 1.5 J, which
also shows their insensitive behavior toward external stimuli. The
heats of formation ΔfH° lie in the range from negative values for
the hydrates (6: 1071 kJ mol–1, 7: 675.4 kJ mol–1) to 221.2
−
−
kJ mol–1 for 10. The sodium (6), potassium (7) and guanidinium
(11) salt have lower heat of formations compared to the neutral
compound 3. The densities lie between 1.61 and 1.74 g cm–3 at
298 K with 6 as the highest. The calculated detonation pressures
(pCJ) are in the range from 14.1 GPa (6) to 27.5 GPa (10). The
highest pCJ value of 275 bar belongs to the hydrazinium salt (10)
as well as the highest detonation velocity (VD) with a value of
8369 m s–1. Table 3 shows the properties of different 3,5-DNP
salts compared to RDX. In contrast to the 3,4-DNP salts they
have higher decomposition temperatures (Figure 10). Compound
15 and 16 show the best thermal stability with values of 307°C
(15) and 300°C (16). One reason for the relatively high
decomposition temperature is the hydrogen bonding of 16. The
Experimental Section
The complete experimental procedures for the synthesis of salts can be
found in the supporting information.
CAUTION! All investigated compounds are potentially explosive energetic
materials, although no hazards were observed during preparation and
handling these compounds. Nevertheless, this necessitates additional
meticulous safety precautions (earthed equipment, Kevlar® gloves, Kevlar®
sleeves, face shield, leather coat, and ear plugs).
1-N-Nitropyrazole (1):[12b] 1H-Pyrazole (30.0 g, 495 mmol, 1.0 eq.) was
dissolved in concentrated acetic acid (90 mL) and was cooled to 10 °C.
Subsequently, fuming nitric acid (100 %, 21 mL, 504 mmol, 1.1 eq.) was added
dropwise over 1 h, while keeping the temperature at 10 °C. The suspension was
stirred for 30 min at 10 °C. Afterwards, the mixture was allowed to warm to
room temperature and acetic anhydride (60 mL, 636 mmol, 1.4 eq.) was added
slowly. The suspension was stirred for 1 h to receive a yellow solution, which
was poured on ice afterwards. The resulting precipitate was filtered and dried
on air to obtain 1 as a colorless solid (49.5 g, 438 mmol, 88 %).
1H NMR (400MHz, DMSO d6): δ (ppm) = 8.79 (dd, 1H, 3J = 3.0 Hz, 3J = 1.7 Hz,
NO2-N-CH), 7.87 (s, 1H, N=CH), 6.70 (dd, 1H, 3J = 3.0 Hz, 3J = 1.7 Hz, CH); 13
NMR (101 MHz, DMSO d6): δ (ppm) = 141.5 (NO2-N-CH), 126.8 (N=CH), 109.7
(C-H). IR (ATR, rel. int.): ṽ (cm–1) = 3150 (w), 3121 (m), 1739 (w), 1608 (m),
1528 (w), 1477 (w), 1404 (w), 1371 (w), 1317 (m), 1283 (m), 1253 (m), 1228
(m), 1160 (m), 1062 (m), 1028 (m), 936 (m), 903 (m), 774 (s), 630 (s), 563 (m),
457 (m).
heats of formation lie between 298.7 kJ mol–1 (19) and 187.8
−
kJ mol–1 (18). Therefore, the hydrazinium (18) and
aminoguanidinium (19) salt have a higher heat of formation than
the neutral compound 5 (Table 3).
Regarding sensitivity values most 3,5-DNP salts are not sensitive
toward friction, impact and electrostatic discharge with exception
of the water free potassium salt (IS: 8.5 J) and the hydrazinium
(IS: 10 J) salt. The densities lie between 1.59 and 2.037 g cm–3 at
298 K. The potassium salt of 3,5-DNP has the highest density,
which is also higher than the neutral compound 5. The calculated
detonation pressures (pCJ) lie in the range from 19.1 GPa (19) to
29.6 GPa (18). The highest pCJ value (29.6 GPa) and the highest
detonation velocity (VD: 8610 m s–1) were achieved for the
hydrazinium salt (18).
Overall both hydrazinium salts (10 and 18) and the ammonium
salt 16 perform nearly as well as RDX regarding its detonation
properties (VD= 8109–8610 m s–1) and are less sensitive. Further
the ammonium salt is not sensitive toward impact, friction and
electrostatic discharge and shows a high thermal stability of
300°C.
C
5-Nitropyrazole (2):[12a] Compound
1 (12.5 g, 111 mmol, 1.0 eq.) was
suspended in anisole (250 mL) and heated for 16 h at 145 °C. Afterwards, the
solution was cooled to 0 °C and the resulting precipitate was filtered, washed
with cold anisole and dried on air. Compound 2 was obtained as a yellowish
solid (9.91 g, 87.7 mmol, 79 %).
1H NMR (400 MHz, DMSO d6): δ (ppm) = 13.94 (s, 1H, NH), 8.02 (d, 1H, 3J =
2.5 Hz, N=CH), 7.03 (d, 1H, 3J = 2.5 Hz, CH); 13C NMR (101 MHz, DMSO d6): δ
(ppm) = 156.3 (C-NO2), 132.3 (N=CH), 101.8 (CH); IR (ATR, rel. int.): ṽ (cm–1) =
3141 (m), 3021 (w), 2974 (w), 2926 (m), 2882 (m), 1556 (s), 1510 (s), 1422 (m),
1379 (s), 1350 (s), 1249 (m), 1209 (m), 1144 (w), 1090 (m), 1048 (m), 990 (m),
928 (w), 903 (w), 821 (s), 783 (s), 752 (s), 613 (m), 537 (w).
Toxicity Assessment
3,4-Dinitropyrazole (3):[12a] Compound
2 (9.0 g, 79.6 mmol, 1.0 eq.) was
To determine the toxicity to aquatic life of 5, 7 and 15 using the
luminescent marine bacterium Vibrio fischeri (see supporting
information) was used.[17] The EC50 value (half-maximal effective
concentration) of compound 5 and 7 are lower than that of RDX
(see Tables 2 and 3), which implies that 5 and 7 is more toxic
than RDX. Hence, the potassium salt 15 has a higher EC50 value,
which means that 15 is classified to be less toxic than RDX.
Interestingly, the potassium salt of 3,5-DNP (15) is less toxic than
the free acid (5).
dissolved in concentrated sulfuric acid (96 %, 15 mL) and the viscous solution
was cooled to 0 °C. Subsequently, concentrated nitric acid (100 %, 10 mL) was
added drop wise over 30 min, while keeping the temperature at 0 °C.
Afterwards, further sulfuric acid (96 %, 30 mL) was added and the solution was
allowed to warm to room temperature and heated at 80 °C for 3 h. The mixture
was poured on ice and was extracted with diethyl ether (3x50 mL). The
combined organic layers were dried over magnesium sulfate and the solvent
was concentrated in vacuum. The residue was allowed to stand for
crystallization. Compound (3) was obtained as yellowish crystals (8.56 g,
54.2 mmol, 68 %).
Conclusion
Nitropyrazoles are valuable energetic materials due to their large
variety of substitution patterns. While mono-nitropyrazoles are
5
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10.1002/cplu.201800318
ChemPlusChem
Table 2. Energetic Properties and detonation parameters of compounds 3-11 compared to RDX.
3
4
5
6∙2H2O
7∙2H2O
8
9
10
11
RDX
Formula
C3H2N4O4
C3H2N4O4
C3H2N4O4
C3H5N4O6
Na
216.08
C3H5N4O6
K
232.19
C3H5N5O4
C3H5N5O5
C3H6N6O4
C4H7N7O4
C3H6N6O6
FW [g mol–1
IS [J]a
FS [N]b
ESD [J]c
N [%]d
]
158.07
40
158.07
25
158.07
25
175.03
40
191.03
10
190.05
40
217.06
40
221.12
7.5
40
360
40
360
360
360
360
360
360
360
360
120
1.5
1.0
1.5
1.0
1.5
1.5
1.0
1.0
1.0
0.2
35.44
−30.36
35.44
–30.36
35.44
–30.36
25.93
−22.1
24.13
−20.7
40.00
−41.1
127
36.65
−29.3
101
44.20
−42.08
117
45.15
−55.26
37.84
–21.6
205[18]
Ω [%]e
Tdec. [°C]f
71 (m.p.)
285 (dec.)
1.79
68 (m.p.)
171 (dec.)
1.76
169 (m.p.)
299 (dec.)
1.78
80 (H2O)
142 (dec.)
1.74
88 (H2O)
174 (dec.)
1.72
139 (m.p.)
156 (dec.)
1.63
ρ [g cm–3
]
1.69o
1.72o
1.67
1.81[16]
(298K)g
ΔfH° [kJ mol–1 h
]
124.2
864
189.1
1274
73.2
542
1071
675.4
–
–
–
–
221.2
1268
40.4
70.3
−
−
ΔfU° [kJ kg–1 i
]
4871
2829
288.7
417.0
−
−
EXPLO5 V6.03 values:g
–ΔEU° [kJ kg–1 j
TE [K]k
pCJ [GPa]l
]
5409
3956
30.6
8426
711
5752
4183
30.8
8459
721
5106
3786
28.9
8279
428
2070
1841
14.1
6245
612
3543
2665
17.8
6800
581
–
–
–
–
–
–
–
–
–
–
5402
3573
27.6
8369
832
3975
2866
21.1
7585
815
5845
3810
34.5
D [m s–1 m
V0 [L kg–1 n
]
8861
785
]
Toxicity:
EC50 (15 min)
[g L–1
–
–
–
–
0.27
–
–
0.10
0.08
–
–
–
–
–
–
0.327
0.239
]
EC50 (30 min)
0.19
–
–
[g L–1
]
a
b
c
d
e
impact sensitivity (BAM drophammer, 1 of 6); friction sensitivity (BAM friction tester, 1 of 6); electrostatic discharge device (OZM); nitrogen content;
f
g
oxygen balance; decomposition temperature from DSC (β = 5°C); recalculated from low temperature X-ray densities (ρ298K = ρT / (1+αV(298-T0); αV = 1.5
h
i
j
k
l
m
10–4 K–1); calculated (CBS-4M) heat of formation; calculated energy of formation; energy of explosion; explosion temperature; detonation pressure;
detonation velocity; n assuming only gaseous products; o measured pycnometrically at room temperature.
Table 3. Energetic Properties and detonation parameters of compounds 12-21.
12
13
14·2H2O
15
16
17
18
19∙H2O
20
21
Formula
C4H8N8O4
C6H8N12O4
C3H5N4O6
Na
216.08
C3HN4O4
K
196.16
C3H5N5O4
C3H8N6O4
C3H6N6O4
C4H9N7O5
C4H8N8O4
C6H8N12O4
FW [g mol–1
IS [J]a
FS [N]b
ESD [J]c
N [%]d
]
232.18
40
312.08
30
175.03
40
191.03
40
190.05
10
217.06
40
232.18
40
312.08
40
40
360
8.5
240
360
360
360
360
360
360
360
360
1.5
1.5
1.5
0.4
1.5
1.0
1.0
1.0
1.0
1.0
48.27
53.84
25.93
28.56
40.00
36.65
44.20
45.15
48.27
53.84
Ω [%]e
55.2
61.5
22.1
25.5
41.1
29.3
42.1
55.3
55.2
61.5
−
−
−
−
−
−
−
−
−
−
Tdec. [°C]f
124 (m.p.)
146 (dec.)
1.61
180
99 (H2O)
297
307
251 (m.p.)
300 (dec.)
1.70
141
200
236 (m.p.)
295 (dec.)
1.59
226 (m.p.)
232 (dec.)
1.64
225 (m.p.)
234 (dec.)
1.62o
ρ [g cm–3
]
1.71o
1.72o
1.99
1.72o
1.74
(298K)g
ΔfH° [kJ mol–1 h
]
149.4
750.3
–
–
–
–
194.8
34.0
–
–
187.8
–298.7
108.1
572.4
–
–
−
ΔfU° [kJ kg–1 i
]
936.1
293.4
1091.8
–1159.4
−
EXPLO5 V6.03 values:g
–ΔEU° [kJ kg–1 j
TE [K]k
pCJ [GPa]l
]
4273
2982
22.2
7788
841
–
–
–
–
–
–
–
–
–
–
4235
3064
22.5
7557
397
4764
3298
25.9
8109
794
–
–
–
–
–
5315
3481
29.6
8610
823
3295
2486
19.1
7319
488
4113
2893
22.6
7848
473
–
–
–
–
–
D [m s–1 m
V0 [L kg–1 n
Toxicity:
EC50 (15 min)
[g L–1
]
]
–
–
–
–
–
1.21
0.95
–
–
–
–
–
–
–
–
–
–
]
EC50 (30 min)
–
–
–
[g L–1
]
a
b
c
d
e
impact sensitivity (BAM drophammer, 1 of 6); friction sensitivity (BAM friction tester, 1 of 6); electrostatic discharge device (OZM); nitrogen content;
oxygen balance; decomposition temperature from DSC (β = 5°C); g recalculated from low temperature X-ray densities (ρ298K = ρT / (1+αV(298-T0); αV = 1.5
f
h
i
j
k
l
m
10–4 K–1
;
calculated (CBS-4M) heat of formation; calculated energy of formation; energy of explosion; explosion temperature; detonation pressure;
detonation velocity; n assuming only gaseous products; o measured pycnometrically at room temperature.
mixture was poured on ice and extracted with ethyl acetate (3x50 mL). The
combined organic layers were dried over magnesium sulfate and the solvent
was concentrated in vacuum. The residue was stand for crystallization to give 4
as yellowish needles (5.77 g, 36.6 mmol, 85 %).
1,3-Dinitropyrazole (4):[12a] Compound
2 (5.0 g, 43 mmol 1.0 eq.) was
suspended in acetic acid (100 %, 31 mL) and concentrated nitric acid (100 %,
4.14 mL) was added at r.t. to the suspension. The reaction was stirred for
30 min. at which the solution turned purple. Afterwards acetic anhydride (10 mL)
was added and the solution was stirred for 24 h at room temperature. The
1H NMR (400 MHz, DMSO d6): δ (ppm) = 9.09 (s, 1H, CH), 7.44 (s, 1H, CH);
13C NMR (101 MHz, DMSO d6): δ (ppm) = 152.55 (N=C-NO2), 129.97 (CH),
6
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10.1002/cplu.201800318
ChemPlusChem
104.73 (CH); 14N NMR (DMSO d6): δ (ppm) = –62.4 (N-NO2), –23.5 (C-NO2);
15N NMR (DMSO d6): δ (ppm) = –113.5 (N), –95.0 (N-NO2), –63.2 (N-NO2), –
25.1 (C-NO2); IR (ATR, rel. int.): ṽ (cm–1) = 3170 (w), 3156 (w), 1740 (w), 1637
(m), 1548 (m), 1515 (w), 1471 (w), 1435 (w), 1398 (m), 1351 (m), 1323 (m),
1281 (s), 1237 (s), 1113 (s), 1039 (s), 983 (m), 961 (m), 899 (w), 810 (s), 781
(s), 753 (s), 616 (w), 567 (m), 523 (w), 485 (w); Raman (1064 nm, 200 mW, cm–
1): ṽ = 3173(17), 3159(11), 2836(8), 2753(5), 1638(16), 1549(22), 1521(17),
1474(14), 1437(58), 1400(100), 1320(21), 1324(21), 1289(31), 1238(16),
1212(10), 1116(13), 1102(11), 1044(10), 986( 25), 964(37), 932(9), 986(9),
823(17), 788(9), 569(9); Elemental analysis: calcd. (%) for C3H2N4O4 (M =
158.07 g mol–1): C 22.80, H 1.28, N 35.44; found: C 23.8, H 1.37, N 35.34.;
DSC (5 °C min–1): Tmelt. = 68 °C, Tdec. = 175 °C. Sensitivities (grain size: <
100 μm): BAM impact: 25 J, BAM friction: 360 N, ESD: 1.0 J.
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4 (9.0 g, 56.6 mmol, 1.0 eq.) was
suspended in benzonitrile (250 mL) and heated at 180 °C for 3 hours. After
cooling down the solution aqueous sodium hydroxide (2 M, 200 mL) was added
and the precipitate was collected by filtration. The precipitate was acidified with
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1H NMR (400 MHz, DMSO d6): δ (ppm) = 7.92 (s, 1H, CH) 13C NMR (101 MHz,
DMSO d6): δ (ppm) = 151.53 (N=C-NO2), 99.78 (CH); 14N NMR (DMSO d6): δ
(ppm) = –24.8 (NO2); 15N NMR (DMSO d6): δ (ppm) = –123.4 (N), –26.4 (NO2);
IR (ATR, rel. int.): ṽ (cm–1) = 3156 (w), 1712 (w), 1640 (m), 1550 (m), 1510 (m),
1436 (w), 1398 (m), 1329 (m), 1282 (s), 1237 (s), 1174 (w), 1111 (s), 1039 (s),
983 (m), 809 (s), 779 (s), 751 (s), 618 (w); Raman (1064 nm, 200 mW, cm–1):
ṽ = 3151(4), 1600(2), 1576(4), 1571(4), 1552(9), 1541(8), 1504(2), 1486(3),
1447(15), 1442(14),1431(11), 1401(100), 1358(4), 1341(5), 1273(4), 1196(4),
1025(1), 1015(3), 1005(3), 985(4), 848(1), 833(1), 762(1), 350(4), 286(6),
96(25), 75(6); Elemental analysis: calcd. (%) for C3H2N4O4 (M = 158.07 g mol–
1): C 22.80, H 1.28, N 35.44; found: C 23.04, H 1.28, N 35.76.; DTA (5 °C min–
1): Tmelt. = 171 °C, Tdec. = 299 °C; Sensitivities (grain size: < 100 μm): BAM
impact: 25 J, BAM friction: 360 N, ESD: 1.0 J.
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Acknowledgements
Financial support of this work by the Ludwig-Maximilian
University of Munich (LMU), the Office of Naval Research
(ONR) under grant no. ONR.N00014-16-1-2062 is gratefully
acknowledged. The authors acknowledge collaborations with
Dr. Mila Krupka (OZM Research, Czech Republic) in the
development of new testing and evaluation methods for
energetic materials and with Dr. Muhamed Suceska
(Brodarski Institute, Croatia) in the development of new
computational codes to predict the detonation and propulsion
parameters of novel explosives. We are indebted to and thank
Drs. Betsy M. Rice, Jesse Sabatini and Brad Forch (ARL,
Aberdeen, Proving Ground, MD) for many inspired
discussions. We thank Mr. Stefan Huber for help with the
sensitivity testing and Mrs. Cornelia C. Unger for the toxicity
measurements.
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
Parts of this work were presented on the NTREM conference
2017.[19]
Received: ((will be filled in by the editorial staff))
Revised: ((will be filled in by the editorial staff))
Published online: ((will be filled in by the editorial staff))
Keywords: Energetic materials • Pyrazoles • Nitration• Structure
Elucidation • Cations
7
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10.1002/cplu.201800318
ChemPlusChem
Table of Contents
FULL PAPER
Three
dinitropyrazoles
different
are
isomeric
synthesized,
█ N-Heterocycles
characterized and compared to each
other. They show good thermal and
Thomas M. Klapötke,* Marc F. Bölter,
Alexander Harter, Jörg Stierstorfer
physical
stability
as
well
as
appropriate energetic performances.
■■ – ■■
Several
additionally
ionic
derivatives
to
were
tune
Isomers of Dinitropyrazoles:
Synthesis, Comparison and Tuning
of Their Physico-Chemical
Properties
prepared
performance and sensitivity values.
8
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