Energetic mono-, di-, and trisubstituted nitroiminotetrazoles

Article abstract of DOI:10.1002/anie.200804755

(Chemical Equation Presented) A bundle of energy: The title compounds were synthesized in good yield from aminotetrazoles (obtained from the reaction of cyanogen azide with primary amines) by treatment with 100% nitric acid and were fully characterized by

Full text of DOI:10.1002/anie.200804755

Communications
DOI: 10.1002/anie.200804755
Energetic Materials
Energetic Mono-, Di-, and Trisubstituted Nitroiminotetrazoles**
Young-Hyuk Joo and Jeanꢀne M. Shreeve*
Over the last decade, the synthesis of energetic heterocyclic
compounds has attracted considerable interest.[1–3] Environ-
mental contamination by nitro compounds is associated
principally with the explosives industry and military testing
of explosives.[2] Compounds with a high nitrogen-atom con-
tent are potential candidates for the replacement trinitroto-
luene (TNT) and other common explosives, such as 1,3,5-
trinitro-1,3,5-triazinane (RDX), 1,3,5,7-tetranitro-1,3,5,7-tet-
razocane (HMX), 2,4,6,8,10,12-(hexanitrohexaaza)cyclodo-
decane (CL-20), 1,3,3-trinitroazetidine (TNAZ), and 1,1-
diamino-2,2-dinitroethene (FOX-7), which have high densi-
ties and energies because of substantial cage strain, or for use
in propellants when combined with a suitable oxidizer.[2,3] The
combination of a tetrazole ring with energetic groups
containing oxygen atoms, such as nitro groups, nitrate
esters, or nitramines, is of particular interest.[1d,4] To meet
the continuing need for improved energetic materials, the
synthesis of energetic heterocyclic compounds has attracted
considerable interest as a result of their rather large densities,
good oxygen balance, and high heats of formation.[1,2]
system was prepared nearly 60 years ago by treatment of
nitroaminoguanidine with KNO2 and concentrated HCl.[6a,b]
In 1957, the synthesis of a 1-alkyl-substituted 5-nitroiminote-
trazole was investigated extensively by two different meth-
ods:[6c] 1) the direct nitration of 1-methyl-5-aminotetrazole
with nitric acid, and 2) the reaction of potassium methylnitr-
amine with cyanogen bromide to form methylnitrocyanamide.
When methylnitrocyanamide was treated with hydrazoic acid,
1-methyl-5-nitroiminotetrazole was isolated.
More recently, the complete characterization of nitro-
iminotetrazole and its salts as HEDMs was reported
(Scheme 1).[4] The heat of formation was determined for
Scheme 1. Simplest nitroiminotetrazoles.
Five-membered nitrogen-containing heterocycles are tra-
ditional sources of energetic materials. Much attention has
been focused on azoles, and in particular tetrazoles, as
energetic compounds.[1,2] Energetic materials based on tetra-
zoles show the desirable properties of high N-atom content
and thermal stability (due to aromaticity).[5] Tetrazole com-
pounds containing nitroimino groups have been investigated
intensively as energetic materials both theoretically and
experimentally, as the nitroimino group can offer improved
density and oxygen balance, and a high heat of formation.[4]
Additionally, the decomposition of these compounds results
in the generation of nitrogen gas. Therefore, they are very
promising candidates for applications requiring environmen-
tally friendly energetic materials.[1b] The high energetic
density materials (HEDMs) with the best performance
(RDX, HMX) belong to the class of typical organic cyclic
and cage molecules.
each nitroiminotetrazole compound by bomb calorimetric
measurements, and the density in the crystalline state was
determined by single-crystal X-ray diffraction.[4a] Further-
more, a complex of copper nitroiminotetrazole, which was
formed in high yield by the reaction of 1-methyl-5-nitro-
iminotetrazole with copper(II) nitrate in aqueous solution,
was synthesized and investigated as a new primary explosi-
ve.[4c]
Our research group recently reported the preparation of
mono-, di-, and trisubstituted 5-aminotetrazole compounds by
a convenient method based on the reaction of cyanogen
azide[7] with primary amines[8a] or hydrazines.[8b] The nitration
of these aminotetrazoles with 100% nitric acid without a
solvent has now been shown to give mono-, di-, and
trisubstituted nitroiminotetrazole derivatives (Scheme 2).
The treatment of aminotetrazole compounds, which were
synthesized from primary amines and cyanogen azide, with
excess 100% nitric acid[9] led to nitroiminotetrazoles 1–9 in
good yields (1: 88%, 2: 89%, 3: 84%, 4: 67%, 5: 64%, 6:
88%, 7: 67%, 8: 74%, 9: 72%).[10] When the reaction was
complete, the reaction mixture was poured into ice water and
stirred for 1–3 h to give a white solid. In the case of
compounds 1, 2, and 3, the white solid did not precipitate
from ice water, but was obtained when the mixture was dried
with air. Compounds 1 and 2 were formed as a mixture and
separated by crystallization from water. The structures of 2, 4,
and 9 are supported by IR, 1H NMR, 13C NMR, and 15N NMR
spectroscopic data as well as elemental analysis (Table 1).
The physical data for 1, 3, 5·H2O, 5, 6·H2O, 6, 7·2H2O, 7,
and 8 are summarized in the Supporting Information. The
15N NMR spectra of 6 and 8 could not be recorded owing to
the poor solubility of these compounds in dimethyl sulfoxide
Nitroiminotetrazoles are of special interest because they
combine both the oxidizer and energetic nitrogen-rich back-
bone in one molecule. The simple 5-(nitroimino)tetrazole
[*] Dr. Y.-H. Joo, Prof. Dr. J. M. Shreeve
Department of Chemistry, University of Idaho
Moscow, ID 83844-2343 (USA)
Fax: (+1)208-885-9146
E-mail: jshreeve@uidaho.edu
[**] We gratefully acknowledge the support of the DTRA (HDTRA1-07-1-
0024), the NSF (CHE-0315275), and the ONR (N00014-06-1-1032).
We are grateful to Dr. D. A. Parrish, Naval Research Laboratory
(NRL), for determining the single-crystal X-ray structures.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200804755.
564
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2009, 48, 564 –567

Angewandte
Chemie
imino-1H-tetrazole (1) and 1,2-bis(4,5-dihydro-5-nitroimino-
1H-tetrazol-1-yl)ethane (4; I-8: Idaho) are shown in
Figure 1.[11] Structural details are given in the Supporting
Information. The colorless crystals are stable at room
temperature and not hygroscopic.
Figure 1. Molecular structures (hydrogen atoms shown as spheres of
arbitrary radius, and thermal displacement set at 50% probability) of 1
(top) and 4 (bottom).
Scheme 2. Synthesis of nitroiminotetrazoles.
Figure 2 shows the 15N NMR spectra of 7 (with six signals
at d = À173.8, À155.2, À150.6, À44.3, À25.3, and À15.4 ppm)
and 9 (with seven signals at d = À351.7, À168.9, À156.3,
À155.1, À26.1, À25.0 and À15.1 ppm). The signals for the
tertiary amine group in compound 9 appear as expected at
Table 1: Selected physical data of nitroiminotetrazole derivatives 2, 4,
and 9.[a]
2: colorless crystal; IR (KBr): n˜ =3545, 3446, 3019, 2980, 2851, 2739,
2641, 2581, 2478, 1642, 1583, 1493, 1451, 1414, 1350, 1314, 1291, 1259,
1
1059, 897, 853 cmÀ1; H NMR ([D6]DMSO): d=4.57–4.60 (m, 2H),
4.88–4.92 (m, 2H), 7.02 ppm (br s, 3H); 13C NMR ([D6]DMSO):
d=44.2, 69.5, 150.7 ppm; 15N NMR ([D6]DMSO): d=À171.3 (m, N1),
3
À155.4, À153.1, À40.2 (t, 3JN,H =3.3 Hz, N7), À25.0 (t, JN,H =2.0 Hz,
N2), À23.7 (N3), À15.4 ppm (NO2).
4: colorless crystal; IR (KBr): n˜ =3430, 3196, 3107, 3031, 1581, 1489,
1308, 1261, 1025 cmÀ1; 1H NMR ([D6]DMSO): d=4.66 (s, 4H),
11.65 ppm (br s, 2H); 13C NMR ([D6]DMSO): d=44.6, 150.9 ppm;
15N NMR ([D6]DMSO, 558C): d=À170.6, À156.8, À154.3, À24.7,
À20.1, À15.5 ppm.
9: white solid; IR (KBr): n˜ =3431, 3258, 3075, 2843, 1583, 1492, 1449,
1
1308, 1256, 1040, 719 cmÀ1; H NMR ([D6]DMSO): d=2.99 (t,
3
3J=5.2 Hz, 6H), 4.12 (t, J=5.2 Hz, 6H), 12.71 ppm (br s, 3H);
13C NMR ([D6]DMSO): d=44.7, 49.5, 149.8 ppm; 15N NMR ([D6]DMSO):
d=À351.7, À168.9, À156.3, À155.1, À26.1, À25.0, À15.1 ppm.
[a] 1H, 13C, and 15N NMR (external standard: CH3NO2) spectra were
recorded at 300.1, 75.5, and 50.7 MHz, respectively. The data for 1, 3,
5·H2O, 5, 6·H2O, 6, 7·2H2O, 7, and 8 are summarized in the Supporting
Information.
(DMSO). Single crystals suitable for X-ray crystal-structure
determination were obtained from aqueous solution. X-ray
crystal structures of 4,5-dihydro-1-(2-hydroxyethyl)-5-nitro-
Figure 2. 15N NMR spectra of 7 (top) and 9 (bottom): delay of 10 s
between the pulses.
Angew. Chem. Int. Ed. 2009, 48, 564 –567
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
565

Communications
high field in the H-decoupled 15N NMR spectrum. The N1,
For initial safety testing, the impact sensitivity was tested
according to BAM methods (BAM Fallhammer).[14] A range
of impact sensitivities was found, from the insensitive com-
pound 2 (40 J) to the sensitive nitroiminotetrazoles (1, 3–6, 8,
and 9: 10–20 J) and the very sensitive compound 7 (3 J;
Table 2). HMX and RDX have an impact sensitivity of 7.4 J.[2]
The physical values for the decomposition temperature,
density, oxygen balance, heat of formation (in kJgÀ1), and
detonation properties of 4, 5, and 6
N4, and N5 signals for both compounds were observed at
d values between À151 and À174 ppm. The signals were
assigned on the basis of literature values for the chemical
shifts of substituted nitroiminotetrazoles.[4a]
Density is one of the most important physical properties
of all energetic materials. As is shown in Table 2, the densities
of the new nitroiminotetrazoles range from 1.454 to
decrease as the number of methyl-
ene groups increases, although their
Table 2: Physical properties of nitroiminotetrazoles 1–9 and comparison with those of RDX and HMX.
shock sensitivity is essentially con-
stant at 10–15 J.
[c]
Compound
Td[a]
[8C]
Density[b]
DfHꢀ298
P[d]
[GPa]
nD[e]
IS[f]
[J]
OB[g]
[%]
[gcmÀ3
]
[kJmolÀ1 (kJgÀ1)]
[msÀ1
]
Safety precautions: Although
we have experienced no difficulties
with the shock instability of nitro-
iminotetrazoles 1–9, these com-
pounds must be synthesized only
in 2–3 mmol amounts, and extreme
care is absolutely necessary, partic-
ularly with compound 7. Manipula-
tions must be carried out in a fume
1
2
3
4
5
6
7
8
158
117
124
194
173
145
139
233
182
230
287
1.722
1.712
1.454
1.858
1.658
1.579
1.759
1.564
1.616
1.816
1.910
258.7 (1.49)
350.1 (1.60)
398.8 (2.01)
1038.3 (3.63)
1032.0 (3.44)
989.6 (3.15)
922.5 (2.55)
1022.2 (3.00)
1649.7 (3.40)
92.6 (0.42)
28.23
30.88
17.70[h]
38.19
28.06
24.71
33.02
23.61
25.96
35.17
39.63
8465
8496
20
À55.2
>40
20
10
10
15
3
15
10
7.4
7.4
À23.6
À121
7358[h]
9329
8374
7963
8741
7667
8185
8977
9320
À39.1
À53.3
À66.2
À28.8
À84.6
À64.3
À21.6
À21.6
9
RDX[i]
HMX[i]
hood behind
a
safety shield.
104.8 (0.35)
Leather gloves must be worn.
[a] Temperature of thermal decomposition under nitrogen gas (DSC, 108CminÀ1). [b] The density at
258C was determined by using a gas pycnometer. [c] Heat of formation calculated by using Gaussian 03
(with 83.68 kJmolÀ1 as the enthalpy of sublimation for each compound). [d] Calculated detonation
pressure. [e] Calculated detonation velocity. [f] Impact sensitivity (determined by a BAM drop-hammer
test). [g] Oxygen balance (%) for CaHbOcNd: 1600ꢀ(cÀ2aÀb/2)/Mw; Mw =molecular weight of the
nitroiminotetrazole. [h] Values were determined by using Cheetah 4.0. [i] Data from reference [2].
Received: September 29, 2008
Published online: December 12, 2008
Keywords: energetic materials ·
.
explosives · heats of formation ·
nitrogen heterocycles · tetrazoles
1.858 gcmÀ3 (RDX: 1.816; HMX: 1.910 gcmÀ3). The decom-
position temperatures (without melting) fall between 117 and
2338C. (Compounds 4, 5, 8, and 9 explode at their decom-
position temperatures (as determined by differential scanning
calorimetry (DSC)).
[1] For reviews, see: a) R. P. Singh, R. D. Verma, D. T. Meshri, J. M.
Shreeve, Angew. Chem. 2006, 118, 3664 – 3682; Angew. Chem.
Int. Ed. 2006, 45, 3584 – 3601; b) G. Steinhauser, T. M. Klapꢀtke,
Angew. Chem. 2008, 120, 3376 – 3394; Angew. Chem. Int. Ed.
2008, 47, 3330 – 3347; c) R. P. Singh, H. Gao, D. T. Meshri, J. M.
Shreeve in High Energy Density Materials (Ed.: T. M. Kla-
pꢀtke), Springer, Berlin, 2007, pp. 35 – 83; d) T. M. Klapꢀtke in
High Energy Density Materials (Ed.: T. M. Klapꢀtke), Springer,
Berlin, 2007, pp. 85 – 122.
The remaining task was to determine the heats of
formation of the substituted nitroiminotetrazoles 1–9. These
values were computed by using the method of isodesmic
reactions (see the Supporting Information). Calculations
were carried out by using the Gaussian 03 suite of pro-
grams.[12] The geometric optimization of the structures and
frequency analyses were carried out by using the B3LYP
functional with the 6-31 + G** basis set, and zero-point
energies were calculated at the MP2/6-311 + + G** level. All
1-substituted nitroiminotetrazoles exhibited positive heats of
formation, with the highest calculated for 4, 5, and 9 (3.63,
3.44, and 3.40 kJgÀ1, respectively; RDX: 0.417; HMX:
0.354 kJgÀ1).
By using the experimental values for the densities of
nitroiminotetrazoles 1–9, we calculated the detonation pres-
sures (P) and velocities (D) on the basis of traditional
Chapman–Jouget thermodynamic detonation theory by using
Cheetah 4.0 and 5.0.[13] For compounds 1–9, the calculated
detonation pressures lie in the range between 17.70 and
38.19 GPa (RDX: 35.17; HMX: 39.63 GPa). The detonation
velocities lie in the range between 7358 and 9329 msÀ1 (RDX:
8977; HMX: 9320 msÀ1).
[2] H. H. Klause in Energetic Materials (Ed.: U. Teipel), VCH,
Weinheim, 2005, pp. 1 – 25.
[3] N. Kubota, Propellants and Explosives, VCH, Weinheim, 2007.
[4] a) T. M. Klapꢀtke, J. Stierstorfer, Helv. Chim. Acta 2007, 90,
2132 – 2150; b) T. M. Klapꢀtke, H. Radies, J. Stierstorfer, Z.
Naturforsch. B. 2007, 62, 1343 – 1352; c) G. Geisberger, T. M.
Klapꢀtke, J. Stierstorfer, Eur. J. Inorg. Chem. 2007, 4743 – 4750;
d) T. M. Klapꢀtke, J. Stierstorfer, A. U. Wallek, Chem. Mater.
2008, 20, 4519 – 4530; e) H. Gao, Y. Huang, C. Ye, B. Twamley,
J. M. Shreeve, Chem. Eur. J. 2008, 14, 5596 – 5603; f) H. Xue, H.
Gao, B. Twamley, J. M. Shreeve, Chem. Mater. 2007, 19, 1731 –
1739.
[5] a) S. V. Levchik, A. I. Balabanovich, O. A. Ivashkevich, A. I.
Lesnikovich, P. N. Gaponik, L. Costa, Thermochim. Acta 1993,
225, 53 – 65; b) A. I. Lesnikovich, O. A. Ivashkevich, S. V.
Levchik, A. I. Balabanovich, P. N. Gaponik, A. A. Kulak,
Thermochim. Acta 2002, 388, 233 – 251.
[6] a) T. E. OꢁConnor, G. Fleming, J. Reilly, J. Soc. Chem. Ind.
London Trans. Commun. 1949, 68, 309 – 310; b) E. Lieber, E.
Sherman, R. A. Henry, J. Cohen, J. Am. Chem. Soc. 1951, 73,
566
www.angewandte.org
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2009, 48, 564 –567

Angewandte
Chemie
2327 – 2329; c) J. A. Garrison, R. M. Herbst, J. Org. Chem. 1957,
22, 278 – 283.
1475 observed (I > 2sI) reflections and 0.0419 for all 1616
reflections, goodness-of-fit = 1.065, 110 parameters. Crystallo-
graphic data for 4: C4H6N12O4: Mr = 286.21; crystal size: 0.25 ꢃ
0.12 ꢃ 0.11 mm3; monoclinic, P21/n, a = 8.182(3), b = 6.614(2),
c = 10.463(3) ꢄ, a = 90, b = 112.380(4), g = 908, V= 523.6(3) ꢄ3,
Z = 2, 2qmax = 56.68, 1calcd = 1.815 mgmÀ3, m = 0.159 mmÀ1, F-
(000) = 292, R1 = 0.0808 for 1124 observed (I > 2sI) reflections
and 0.0868 for all 1265 reflections, goodness-of-fit = 1.228, 91
parameters. CCDC 703780 (1) and CCDC 703781 (4) contain
the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif..
[7] a) F. D. Marsh, M. E. Hermes, J. Am. Chem. Soc. 1964, 86, 4506 –
4507; b) F. D. Marsh, J. Org. Chem. 1972, 37, 2966 – 2969; c) J. E.
McMurry, A. P. Coppolino, J. Org. Chem. 1973, 38, 2821 – 2827;
d) K. Banert, Y.-H. Joo, T. Rꢂffer, B. Walfort, H. Lang, Angew.
Chem. 2007, 119, 1187 – 1190; Angew. Chem. Int. Ed. 2007, 46,
1168 – 1171.
[8] a) Y.-H. Joo, J. M. Shreeve, Org. Lett. 2008, 10, 4665 – 4667;
b) Y.-H. Joo, B. Twamley, S. Garg, J. M. Shreeve, Angew. Chem.
2008, 120, 6332 – 6335; Angew. Chem. Int. Ed. 2008, 47, 6236 –
6239.
[9] An alternate method with 70% HNO3 was also used for the
synthesis of nitroiminotetrazoles 4 and 5. The reaction mixture
was stirred for 3 days and then dried in air, and the white solid
product was recrystallized from water. However, the products
were obtained by this method in just 30–34% yield.
[10] The 1-substituted aminotetrazole (2 mmol) was added in small
portions to 100% HNO3 (10 mL) at 08C. The reaction mixture
was stirred at ambient temperature for 18 h and then poured into
ice water (20 g) and stirred for a further 3 h. The product was
precipitated, filtered, washed with water, and dried in air at
room temperature.
[12] Gaussian 03 (Revision D.01), M. J. Frisch et al.; see the
Supporting Information.
[13] a) L. E. Fried, K. R. Glaesemann, W. M. Howard, P. C. Souers,
CHEETAH 4.0 Userꢀs Manual, Lawrence Livermore National
Laboratory, 2004; b) S. Bastea, L. E. Fried, K. R. Glaesemann,
W. M. Howard, P. C. Souers, P. A. Vitello, CHEETAH 5.0 Userꢀs
Manual, Lawrence Livermore National Laboratory, 2007.
[14] a) www.bam.de. b) A portion of nitroiminotetrazoles 1–9
(20 mg) was subjected to a drop-hammer test, in which a 5 or
10 kg weight was dropped. For the categorization of impact
sensitivities (insensitive: > 40 J; less sensitive: ꢁ 35 J; sensitive:
ꢁ 4 J; very sensitive: ꢂ 3 J), see: J. C. Galvez-Ruiz, G. Holl, K.
Karaghiosoff, T. M. Klapꢀtke, K. Loehnwitz, P. Mayer, H.
Noeth, K. Polborn, C. J. Rohbogner, M. Suter, J. J. Weigand,
Inorg. Chem. 2005, 44, 4237 – 4253.
[11] Crystallographic data for 1: C3H6N6O3: Mr = 174.14; crystal size:
0.88 ꢃ 0.48 ꢃ 0.34 mm3; triclinic, P1, a = 7.1095(13), b =
¯
7.1116(13), c = 7.8928(15) ꢄ, a = 89.235(2), b = 66.303(2), g =
66.804(2)8, V= 330.86(11) ꢄ3, Z = 2, 2qmax = 56.68, 1calcd
=
1.748 mgmÀ3
, , F(000) = 180, R1 = 0.0393 for
m = 0.153 mmÀ1
Angew. Chem. Int. Ed. 2009, 48, 564 –567
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
567