Challenging the Limits of Nitro Groups Associated with a Tetrazole Ring

Article abstract of DOI:10.1021/acs.orglett.9b01565

To challenge the limits of nitro groups associated with a tetrazole ring, the polynitrotetrazoles, 2,5-bis(dinitromethyl)-2H-tetrazole, and 2-(dinitromethyl)-5-(trinitromethyl)-2H-tetrazole as well as their salts, were designed, synthesized, and characterized. The neutral compounds were also studied by natural bonding orbital (NBO) and Hirshfeld surface calculations. The heats of formation were calculated with Gaussian 03 and combined with experimentally determined densities in order to obtain the detonation properties of the energetic materials with the EXPLO5 program. They exhibit high densities, good oxygen balances, positive heats of formation, and excellent detonation properties.

Full text of DOI:10.1021/acs.orglett.9b01565

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Challenging the Limits of Nitro Groups Associated with a Tetrazole
Ring
Qiong Yu,† Gregory H. Imler,‡ Damon A. Parrish,‡ and Jean’ne M. Shreeve*,†
†Department of Chemistry, University of Idaho, Moscow, Idaho 83844-2343, United States
‡Naval Research Laboratory, 4555 Overlook Avenue, Washington, D.C. 20375, United States
S
* Supporting Information
ABSTRACT: To challenge the limits of nitro groups associated with a tetrazole ring, the
polynitrotetrazoles, 2,5-bis(dinitromethyl)-2H-tetrazole, and 2-(dinitromethyl)-5-(trinitro-
methyl)-2H-tetrazole as well as their salts, were designed, synthesized, and characterized.
The neutral compounds were also studied by natural bonding orbital (NBO) and Hirshfeld
surface calculations. The heats of formation were calculated with Gaussian 03 and combined
with experimentally determined densities in order to obtain the detonation properties of the
energetic materials with the EXPLO5 program. They exhibit high densities, good oxygen
balances, positive heats of formation, and excellent detonation properties.
olynitro compounds are of particular interest in the
development of high energy density materials (HEDM)
here have more positive O
(Figure 2, Supporting Information). Compound 1 was
̅
̧nitro values indicating lower stabilities
P
since they possess high densities and good oxygen balances.1
Recently, some polynitro compounds of different heterocyclic
rings, such as furoxan,2 triazole,3 and 1,3,4-oxadiazole,4 were
reported (Figure 1). Compounds i, ii, and iii exhibit high
Figure 1. Polynitro compounds with different heterocyclic rings.
Figure 2. Average negative nitro charge (Onitro) of polynitro
compounds based on the number of nitro groups associated with
the tetrazole ring.
̅
densities and excellent energetic properties. Tetrazole has been
used widely in high-nitrogen energetic materials owing to its
high nitrogen content and high heat of formation.5 5-
(Trinitromethyl)-2H-tetrazole (1) is a combination of
tetrazole and a trinitromethyl group and has a density of
1.92 g cm−3.6 In our continuing efforts to introduce as many
nitro groups associated with a tetrazole ring as possible,
compound 1 was selected as the building block. The molecular
stability of polynitro compounds can be evaluated by the
obtained in one step by nitrating ethyl 2-(2H-tetrazol-5-yl)
acetate in good yield. To introduce additional nitro groups
associated with the tetrazole, the acidic proton of the tetrazole
was replaced by a carbonyl-containing group having an
activated methylene moiety which can be nitrated with an
acidic nitrating mixture.8 Compound 10 was obtained by
average negative nitro charge (-O
̅
̧nitro). The more negative the
O
̧
nitro value, the more stable the compound.7 Compared with 1,
Received: May 4, 2019
the structures of the compounds that are under investigation
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b01565
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
neutralizing its potassium salt (9) which was obtained by
displacing one nitro group of a trinitromethyl group from 3
under basic conditions. The pure potassium salt was prepared
from impure 3 which was made by nitrating 2 with 100%
HNO3/98% H2SO4. In order to obtain pure 3, a 90% HNO3/
90% H2SO4 mixture was used (Table 1). The attempt to
a
Table 1. Nitration of 2 with Mixed Acid
b
c
entry
nitrate
products
Figure 3. TLC result of the reaction of 3 in mixed acid after 3 h at
room temperature. The left spot is compound 3. The right column is
the extracted products from the reaction.
1
2
3
4
70% HNO3 (0.5)/90% H2SO4 (0.5)
80% HNO3 (0.5)/90% H2SO4 (0.5)
90% HNO3 (0.5)/90% H2SO4 (0.5)
100% HNO3 (0.5)/98% H2SO4 (0.5)
2, 3
2, 3
3
disappeared after 1−2 min on the TLC plate (Figure 3b). 14
N
d
3, 4
a
NMR was used to characterize the different chemical
environments where nitro groups are found. It is very sensitive
to the nitrogen of the nitro group. Both 3 and 4 have two
different signals from nitro groups in the 14N NMR spectra. To
compare the difference between 3 and 4, the 14N NMR
spectrum of pure compound 3 was run in CDCl3 giving peaks
at −35.68 ppm and −39.76 ppm (Figure 4a). The extracted
All reactions were carried out at rt. A 1 mmol amount of 2 was used
b
as substrate in every reaction. Reaction time is 3 h. Volume of mixed
c
acid is given in parentheses (mL). Products were characterized by 1H
d
NMR and 13C NMR. Compound 4 was observed by TLC and 14N
NMR.
identify 4 was more difficult because, although it was observed,
it could not be isolated or characterized. As the number of
̅
nitro groups increases, the negative O̧nitro values decrease, with
the resulting compound being less stable, and the synthesis
becoming more difficult.
Starting from 1, which is easily obtained from ethyl 2-(2H-
tetrazol-5-yl)acetate in good yield,6 reaction with bromoace-
tone under alkaline conditions gave 2 (Scheme 1). Nitration of
Scheme 1. Synthesis of 2-(Dinitromethyl)-5-
(trinitromethyl)-2H-tetrazole (3) and 2,5-
Bis(trinitromethyl)-2H-tetrazole (4)
Figure 4. (a) 14N spectrum of 3. (b) 14N spectrum of products of the
reaction of 3 in mixed acid after 3 h at room temperature. (c) 14N
spectrum of sample b which stayed for 3 min.
products in dichloromethane were diluted with CDCl3 in order
to run the14N NMR spectrum. As shown in Figure 4b, two
peaks at −38.41 ppm and −41.75 ppm corresponding to
compound 4 were observed; unfortunately, after 3 min, these
two peaks disappeared (Figure 4c). Concomitantly compound
3 was also detected which is much more stable than 4.
A series of nitrogen-rich salts 5−9 were obtained by treating
3 with different bases at 10 °C (Scheme 2). The potassium salt
5 and ammonium salt 6 were obtained by reaction of 3 with an
excess of potassium bicarbonate and ammonium bicarbonate
in acetonitrile. The excess base was removed by filtration.
Compound 3 decomposed if the hydroxylamine or hydrazine
was dropped directly. Fortunately, these two bases were diluted
with acetonitrile and dropped into the reaction to yield the
energetic salts, 7 and 8.
2 was attempted by using different concentrations of mixed
acids (HNO3/H2SO4) such as 70% HNO3/90% H2SO4, 80%
HNO3/90% H2SO4, 90% HNO3/90% H2SO4, or 100%
HNO3/98% H2SO4, but interestingly compound 3 was the
only product obtained when 90% HNO3/90% H2SO4 was the
nitrating reagent (Table 1). When 70% HNO3 or 80% HNO3
was used, the products were a mixture of 2 and 3. When 2 was
treated with 100% HNO3/98% H2SO4, the reaction gave 3 and
4 simultaneolusly. Compound 4 was observed using TLC by
nitrating 3 with a mixture of pure nitric and concentrated
sulfuric acids at room temperature. Dichloromethane was used
to extract the products from the reaction mixture. As shown in
Figure 3a, compound 4 was observed at first but then
When 3 was treated with hydroxylamine hydrochloride and
potassium hydroxide, the trinitromethyl group attached to
carbon was reduced to the dinitromethyl group to form the
B
DOI: 10.1021/acs.orglett.9b01565
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Organic Letters
Letter
Scheme 2. Synthesis of Energetic Salts 5−9
dipotassium (2H-tetrazole-2,5-diyl)bis(dinitromethanide) (9).
A similar reaction was reported previously.9 The potassium salt
9 was acidified with sulfuric acid to give 2,5-bis-
(dinitromethyl)-2H-tetrazole (10) which is a white solid
(Scheme 3). The energetic salts of 10 were obtained from the
reaction of its silver salt and corresponding hydrochloride salts.
Figure 5. Single crystal X-ray structures of (a) 3, (b) 5·CH2Cl2, (c) 6·
CH3CN, (d) 9·H2O, (e) 10, and (f) 12·H2O.
Oxygen balance (OB) is an index of the deficiency or excess
of oxygen in a compound.10 All the compounds except 14
exhibit positive oxygen balances ranging from 5.1% to 32.2%
(Table 1). It is important to note that the OB value of 4 is
32.2% which is higher than the oxidizer ammonium
dinitramide (ADN 25.8%).11 Interestingly, 14 has an oxygen
balance of zero. The energetic salts of 2-(dinitromethyl)-5-
(trinitromethyl)-2H-tetrazole have higher heats of formation
than the salts of 2,5-bis(dinitromethyl)-2H-tetrazole arising
from the presence of one more nitro group.
Scheme 3. Synthesis of 2,5-Bis(dinitromethyl)-2H-tetrazole
(10) and Its Energetic Salts, 11−14
Thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC) of all compounds were carried out at a
heating rate of 5 °C min−1 using dry nitrogen as an atmosphere
to determine their thermal behavior. They decomposed
between 73 and 128 °C (onset temperature) without melting
(Table 2). The densities of these compounds ranged between
1.75 to 2.05 g cm−3 which were obtained from gas pycnometer
measurements.
The heat of formation is a very important parameter in
evaluating the performance of energetic materials. To
investigate the energetic properties of these new compounds,
the heats of formation (HOF) were computed by using the
method of isodesmic reactions with the Gaussian 03 suite of
programs (Scheme S1).13 Compound 9 has a negative HOF of
−0.98 kJ g−1 while the other compounds have positive HOFs
(0.05−1.08 kJ g−1) owing to the high HOF value of the
tetrazole ring.
With the densities and HOFs in hand, the detonation
properties were calculated by using the EXPLO5 6.01 program
(Table 2).14 The calculated detonation velocities lie in the
range between 7394 and 9590 m s−1, and detonation pressures
range from 23.9 to 42.4 GPa, respectively. Among them,
compound 13 exhibits the highest detonation velocity and
pressure (9590 m s−1, 42.4 GPa) which is higher than those of
HMX (9320 m s−1, 39.5 GPa). Compound 3 has a lower
detonation velocity and pressure (8511 m s−1, 31.2 GPa) than
9 (9123 m s−1, 37.7 GPa), which indicates that the presence of
one more nitro group from the trinitromethyl fragment
decreases the energetic power of 3. This decrease is also
found for the energetic salts of 2-(dinitromethyl)-5-(trini-
tromethyl)-2H-tetrazole which have lower detonation veloc-
All compounds were characterized by IR, 1H NMR, and 13
C
NMR spectroscopy as well as elemental analysis (Supporting
Information). The structures of 3, 5·CH2Cl2, 6·CH3CN, 9·
H2O, 10, and 12·H2O were further confirmed by single-crystal
X-ray diffraction (Figure 5, Supporting Information). Crystals
of 3 and 10 were obtained by recrystallization from
dichloromethane with calculated high densities of 1.896 g
cm−3 and 1.923 g cm−3 at 296(2) K, respectively. The crystals
of 5·CH2Cl2, 6·CH3CN, 9·H2O, and 12·H2O suitable for
crystal structure analyses were obtained from a mixture of
methanol and dichloromethane, acetonitrile, water, and
methanol, respectively with solvent molecules in the crystal
structures. As is known, the introduction of nitro groups
usually increases compound density; however, compound 3
has a lower density than 10. This apparently arises from the
presence of the trinitromethyl group bonded to 3 which leads
to greater disorder in the crystal packing compared to that of
10.
C
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Organic Letters
Letter
Table 2. Properties of Energetic Compounds (3, 5−10, and 12·H2O−14)
a
b
c
d
e
f
g
h
i
compds
Td [°C]
ρ
[g cm−3
]
ΔHf [kJ g−1
]
Dv [m s−1
]
P [GPa]
IS [J]
FS [N]
OB [%]
Isp [s]
3
5
6
7
8
9
73
1.88
1.98
1.79
1.92
1.81
2.05
1.92
1.75
1.91
1.83
1.81
1.82
1.90
0.88
0.30
0.70
0.78
1.08
−0.98
0.89
0.05
0.30
0.95
−
8511
7910
8617
9054
8920
7394
9123
8655
9590
9230
−
31.2
27.6
31.6
36.3
34.6
23.9
37.7
31.8
42.4
37.9
−
2
1
1
1
1
4
3
8
5
1
3−5
7.4
80
40
120
20
10
32.2
28.0
23.5
27.0
20.3
22.6
23.0
5.1
13.9
0
25.8
0
245
219
258
256
266
197
259
262
272
274
118
121
107
97
128
88
114
89
115
159
210
280
80
10
12·H2O
13
14
ADN
120
240
80
120
64−72
120
120
j
m
202
−
−
k
l
RDX
HMX
0.36
0.36
8748
9320
34.9
39.5
k
7.4
0
c
a
d
h
b
Decomposition temperature (onset) under nitrogen gas (DSC, 5 °C/min). Density measured by gas pycnometer (25 °C). Heat of formation.
Detonation pressure (calculated with Explo5 v6.01). Detonation velocity (calculated with Explo5 6.01). Impact sensitivity. Friction sensitivity.
Oxygen balance (based on CO) for CaHbOcNd, 1600(c − a − b/2)/MW, MW = molecular weight. Specific impulse (values obtained from Explo5
v6.01 and calculated at an isobaric pressure of 70 bar and initial temperature of 3300 K). Reference 10. Reference 12. Idaho exptl value.
e
f
g
i
j
k
l
m
Calculated with Explo5 v6.01.
ities and pressures than the salts of 2,5-bis(dinitromethyl)-2H-
tetrazole except for the potassium salt. The hydrazinium salt 14
also exhibits good detonation velocity of 9230 m s−1 and
pressure of 37.9 GPa which are comparable with those of
HMX. For initial safety testing, the impact and friction
sensitivities of 3, 5−10, and 12·H2O-14 were measured by
following BAM standard methods15 and are given in Table 1.
Compounds 3, 5, 7, and 8 are very sensitive toward impact and
friction. Compounds 6, 10, and 14 are very sensitive toward
impact but exhibit moderate sensitivities toward friction.
Compound 12·H2O has an impact sensitivity of 8 J and
friction sensitivity of 240 N which are comparable with those
of RDX and HMX (IS 7.4 J, FS 120 N). The specfic impulse
(Isp), a measure of a propellant’s efficiency (in seconds), was
also calculated (Explo 5 v6.01) for all compounds. The Isp
values range from 197 to 274 s, which, except for 9, are higher
than that of ADN (Table 2).
As shown in Figure 6, two-dimensional (2D)-fingerprints of
crystals 3 and 10 and the associated Hirshfeld surfaces were
employed to show their intermolecular interactions.16 The red
and blue regions on the Hirshfeld surfaces represent high and
low close contact populations, respectively. Both 3 and 10 have
the highest ratios of O···O interactions of all kinds of
interactions. The trinitromethyl moiety of 3 displays a
molecular geometry with a propeller-type orientation of the
nitro groups, increasing the chance to form O···O interactions.
As a result, the value of 3 (60.5%) is higher than that of 10
(43.9%). It is found that relative frequencies of close O···O
contacts can increase the possibility of unexpected explo-
sions.17 Block-shaped surfaces contribute more efficiently to
face-to-face π−π stacking and result in better mechanical
stability in which π−π stacking is often present as C···O and
O···C interactions.18 Unfortunately, only 0.9% and 1% C···O
and O···C interactions were observed in 3 and 10, respectively.
According to these results, it is not surprising that the
compounds reported in this work all have high sensitivities
toward impact and friction. It can be seen, from Figure 6c and
6d, a pair of remarkable spikes on the bottom left (O···H and
H···O interactions) in the 2D fingerprint plots of both crystals
denote the presence of hydrogen bonds among neighboring
molecules.19 The thicker spikes of 10 indicate that more
Figure 6. Two-dimensional fingerprint plots in crystal stacking for 3
and 10 as well as the associated Hirshfeld surfaces. Images (a) and (b)
showing the Hirshfeld surfaces that use color coding to represent the
proximity of close contacts around 3 and 10 molecules (white,
distance d equals the van der Waals distance; blue, d exceeds the van
der Waals distance; red, d is less than the van der Waals distance).
The fingerprint plots in crystal stacking for 3 (c) and 10 (d). In
images (e) and (f), the individual atomic contacts percentage
contribution to the Hirshfeld surface are shown in the pie graphs
for 3 and 10, respectively.
hydrogen bonds are observed. It is also found that the value of
O···H and H···O interactions in 10 (20.5%) is almost twice
that of 3 (11.0%). Hydrogen bonds are helpful in increasing
the density which is in agreement with the fact that 10 (1.92 g
cm−3) has a higher density than 3 (1.88 g cm−3).
In summary, a family of polynitromethyl functionalized
derivatives of 2,5-bis(dinitromethyl)-2H-tetrazole (3) and 2-
(dinitromethyl)-5-(trinitromethyl)-2H-tetrazole (10) as well
as their energetic salts were synthesized and characterized. 2,5-
Bis(dinitromethyl)-2H-tetrazole (4), which reaches the limit of
the number of nitro groups possible associated with one
tetrazole ring, was observed by treating 3 with a mixture of
100% nitric acid and concentrated sulfuric acid. Unfortunately,
D
DOI: 10.1021/acs.orglett.9b01565
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(5) (a) Zhao, B.; Li, X.; Wang, P.; Ding, Y.; Zhou, Z. New J. Chem.
it is too unstable to be isolated or characterized. Compared to
10, 3 has one additional nitro group on the trinitromethyl
group but exhibits a lower density, heat of formation,
detonation velocity and pressure, decomposition temperature,
and higher sensitivities. The energetic salts of 3 also have lower
detonation velocities and pressures than those of 10 except for
the potassium salt 5. Compound 3 has one of the highest
oxygen balances (32.2%) of all tetrazole compounds which is
higher than ADN (25.8%) and comparable to ammonium
perchlorate (AP 34.0%). Compound 13 has the best
detonation properties (9590 m s−1 and 42.4 GPa) which are
superior to HMX; however, use will be limited owing to its low
decomposition point and high sensitivities. The two-dimen-
sional fingerprint plots in crystal stacking for 3 and 10 as well
as the associated Hirshfeld surfaces indicate that the high
sensitivities are a result of the high ratios of O···O interactions.
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ASSOCIATED CONTENT
■
(13) Frisch, M. J.; Trucks, G. W.; Schlegel, H. G.; Scuseria, G. E.;
Robb, M. A.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant,
J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci,
B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.;
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M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.;
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Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.;
Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.;
Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.;
Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.;
Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, A. L.
G.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-
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M. W.; Johnson, B.; Chen, W.; Wong, M.; Gonzalez, C.; Pople, J. A.
Gaussian 03, revision D.01; Gaussian, Inc.: Wallingford, CT, 2004.
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b01565.
Crystal refinements (PDF)
Accession Codes
CCDC 1903374−1903379 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
́
(14) Suceska, M. EXPLO5, 6.01; Brodarski Institure: Zagreb,
■
Croatia, 2013.
Corresponding Author
*E-mail: jshreeve@uidaho.edu.
ORCID
(15) (a) http://www.bam.de. (b) A 20 mg sample was subjected to
a drop-hammer test with a 5 or 10 kg dropping weight. The impact
sensitivity was characterized according to the UN recommendations
(insensitive, >40 J; less sensitive, 35 J; sensitive, 4 J; very sensitive, 3
J).
Jean’ne M. Shreeve: 0000-0001-8622-4897
Notes
(16) Spackman, M. A.; Jayatilaka, D. CrystEngComm 2009, 11, 19−
32.
The authors declare no competing financial interest.
(17) Zhang, J.; Zhang, Q.; Vo, T. T.; Parrish, D. A.; Shreeve, J. M. J.
J. Am. Chem. Soc. 2015, 137, 1697−1704.
ACKNOWLEDGMENTS
(18) Tian, B.; Xiong, Y.; Chen, L.; Zhang, C. Relationship between
the crystal packing and impact sensitivity of energetic materials.
CrystEngComm 2018, 20, 837−848.
■
This work was supported by the Office of Naval Research
(N00014-16-1-2089) and the Defense Threat Reduction
Agency (HDTRA 1-15-1-0028). We are also grateful to the
Murdock Charitable Trust, Vancouver, WA, Reference No.
2014120:MNL:11/20/2014 for funds supporting the purchase
of a 500 MHz nuclear magnetic resonance spectrometer.
(19) Zhang, C.; Xue, X.; Cao, Y.; Zhou, Y.; Li, H.; Zhou, J.; Gao, T.
CrystEngComm 2013, 15, 6837−6844.
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DOI: 10.1021/acs.orglett.9b01565
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