Synthesis, characterization, and energetic properties of diazido heteroaromatic high-nitrogen C-N compound

Article abstract of DOI:10.1021/ja0509735

The synthesis, characterization, and energetic properties of diazido heteroaromatic high-nitrogen C-N compound, 3,6-diazido-1,2,4,5-tetrazine (DiAT), are reported. Its normalized heat of formation (NδH_f), experimentally determined using an additive method, is shown to be the highest positive NδH_f compared to all other organic molecules. The unexpected azido-tetrazolo tautomerizations and irreversible tetrazolo transformation of DiAT are remarkable compared to all other polyazido heteroaromatic high-nitrogen C-N compounds, for example, 2,4,6-triazido-1,3,5- triazine; 4,4′,6,6′-tetra(azido)hydrazo-1,3,5-triazine; 4,4′,6,6′-tetra(azido)azo-1,3,5-triazine; and 2,5,8-tri(azido)-1,3, 4,6,7,9,9b-heptaazaphenalene (heptazine).

Full text of DOI:10.1021/ja0509735

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Synthesis, Characterization, and Energetic Properties of
Diazido Heteroaromatic High-Nitrogen C-N Compound
My Hang V. Huynh,*,† Michael A. Hiskey,*,† David E. Chavez,† Darren L. Naud,† and
Richard D. Gilardi‡
Contribution from the Dynamic Experimentation DiVision, DX-2: Materials Dynamics, MS
C920, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, and Laboratory for
the Structure of Matter, The NaVal Research Laboratory, Washington, D.C. 20375
Received February 15, 2005; E-mail: huynh@lanl.gov; hiskey@lanl.gov
Abstract: The synthesis, characterization, and energetic properties of diazido heteroaromatic high-nitrogen
C-N compound, 3,6-diazido-1,2,4,5-tetrazine (DiAT), are reported. Its normalized heat of formation (N∆Hf),
experimentally determined using an additive method, is shown to be the highest positive N∆Hf compared
to all other organic molecules. The unexpected azido-tetrazolo tautomerizations and irreversible tetrazolo
transformation of DiAT are remarkable compared to all other polyazido heteroaromatic high-nitrogen C-N
compounds, for example, 2,4,6-triazido-1,3,5-triazine; 4,4′,6,6′-tetra(azido)hydrazo-1,3,5-triazine; 4,4′,6,6′-
tetra(azido)azo-1,3,5-triazine; and 2,5,8-tri(azido)-1,3,4,6,7,9,9b-heptaazaphenalene (heptazine).
Scheme 1. C-N Heteroaromatic High-Nitrogen Compounds
Introduction
Energetic organic compounds with high-nitrogen content1,2
have currently attracted significant attention from many re-
searchers because of their novel properties, for example, high
density,3,4 high positive heat of formation (+∆Hf),5-7 and high
thermal stability,8-10 that result in numerous unique applications
including effective precursors for carbon nanospheres11,12 and
carbon nitride nanomaterials,13-16 solid fuels in micropropulsion
systems,17,18 gas generators,19 smoke-free pyrotechnic fuels,20,21
and fire extinguishers onboard military aircraft.22,23 These high-
† Los Alamos National Laboratory.
‡ The Naval Research Laboratory.
(1) Ritter, S. Chem. Eng. News 2004, 82 (41), 44.
(2) Benson, F. R. The High-Nitrogen Compounds; Wiley-Interscience: New
York, 1984.
nitrogen compounds form a unique class of C-N heteroaromatic
compounds, for example, triazine,24 heptazine,25-27 triazole,28-31
tetrazine,32,33 and tetrazole,34 Scheme 1.
(3) Chavez, D. E.; Hiskey, M. A.; Gilardi, R. D. Angew. Chem., Int. Ed. 2000,
39, 1791-1793.
(4) Kerth, J.; Lobbecke, S. Propellants, Explos., Pyrotech. 2002, 27, 111-
(19) Jadhav, H. S.; Dhavale, D. D.; Krishnamurthy, V. N. Theory Pract. Energ.
Mater. 2001, 4, 493-504.
(20) Chavez, D. E.; Hiskey, M. A.; Naud, D. L. J. Pyrotech. 1999, 10, 17-36.
(21) Chavez, D. E.; Hiskey, M. A. J. Pyrotech. 1998, 7, 11-14.
(22) Bennett, G.; Kolleck, M. L.; Bennett, J. M. Fire in the Air. Military
Aerospace Technology, Dec 31, 2003.
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Proceedings of Halon Options Technical Working Conference, 2000; pp
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(24) Ott, E.; Ohse, E. Ber. Dtsch. Chem. Ges. 1921, 54B, 179-186.
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(26) Finkelshtein, A. I.; Spiriddonova, N. V. Russ. Chem. ReV. 1964, 33, 400-
405 and references therein.
118.
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R. D. Angew. Chem., Int. Ed. 2004, 43, 4924-4928.
(6) Zhou, G.; Zhang, J.-L.; Wong, N.-B.; Tian, A. J. Mol. Struct. Theochem.
2004, 668, 189-195.
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Propellants, Explos., Pyrotech. 2003, 28, 181-188.
(8) Huynh, M. H. V.; Hiskey, M. A.; Pollard, C. J.; Montoya, D. P.; Hartline,
E. L.; Gilardi, R. D. J. Energ. Mater. 2004, 22, 217-229.
(9) Churakov, A. M.; Smirnov, O. Y.; Ioffe, S. L.; Strelenko, Y. A.;
Tartakovsky, V. A. Eur. J. Org. Chem. 2002, 14, 2342-2349.
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R. D. Angew. Chem., Int. Ed. 2004, 43, 5658-5661.
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Chem., Int. Ed. 2005, 44, 737-739.
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J. AM. CHEM. SOC. 2005, 127, 12537-12543
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A R T I C L E S
Huynh et al.
Scheme 2. Azido Heteroaromatic Compounds
Materials. House water was purified with a Barnstead E-Pure
deionization system. High-purity acetonitrile, chloroform, methylene
chloride, tetrahydrofuran, dimethylformamide, and dimethyl sulfoxide
were used as received from Aldrich. Hydrazine hydrate and sodium
nitrite were purchased from Fisher and were used without further puri-
fication. Deuterated solvents were purchased from Cambridge Isotope
Laboratories and were used as received. TBAH was recrystallized three
times from boiling ethanol and was dried under vacuum at 120° for 2
days. Other chemicals employed in the preparation of compounds were
reagent grade and were used without further purification.
Instrumentation and Measurement. Electronic absorption spectra
were acquired by using a Hewlett-Packard Model 8453 diode array
UV-visible spectrophotometer in quartz cuvettes. Elemental analyses
were performed by Atlantic Microlabs (Norcross, GA) and Los Alamos
National Laboratory. FT-IR spectra were recorded on a Nexus 670 FT-
IR spectrophotometer at 4 cm-1 resolution interfaced with an IBM-com-
patible PC. IR measurements were made in Nujol mulls. 1H NMR and
13C NMR spectra were obtained in DMSO-d6, CD3CN, (CD3)2CO, CD3-
OD, and CDCl3 recorded on a JEOL Eclipse 300 Fourier transform
spectrometer. Melting points were determined by differential scanning
calorimetry (DSC Exo.) at 5 °C/min using TA instruments 2920
Modulated DSC. Heats of formation (∆Hf) were measured using a Parr
Adiabatic Calorimeter model 1720. All reported values are the average
of at least three independent experiments and are within (2 kJ/mol.
Cyclic voltammetric experiments were carried out at scan rates 100,
200, 400, and 800 mV/s with the use of a PAR model 263 potentiostat.
All measurements were conducted in a three-compartment cell in
chloroform and dimethyl sulfoxide with 0.2 M TBAH as the supporting
electrolyte. A 1.0-mm platinum-working electrode was used for these
measurements. All potentials are referenced to the saturated sodium
chloride calomel electrode (SSCE, 0.236 V versus NHE) at room
temperature and at 80 °C and are uncorrected for junction potentials.
In all cases, the auxiliary electrode was a platinum wire. The solution
in the working compartment was deoxygenated by N2 bubbling. All
redox potentials are the average of at least three independent experi-
ments and are within (2 mV. Los Alamos is at the elevation of 7500
ft and atmospheric pressure of 580 Torr (11.2 psi or 0.76 atm). Humidity
is normally below 20% during spring.
In particular, polyazido heteroaromatic compounds have
recently been extensively studied for their useful applications
either in material science35,36 or as high-density energetic
materials.3,4 In 1963, Marcus reported the preparation of 3,6-
di(azido)-1,2,4,5-tetrazine (DiAT) with its melting point of 130
°C; however, no other physical properties or crystal structure
were available.37
As part of our continual effort in high-nitrogen energetic
materials, we herein report on an improved synthetic pathway
for DiAT and its full detailed characterization. Its unexpected
azido-tetrazolo tautomerizations and irreversible tetrazolo trans-
formation are also described. From our characterization and
property data, DiAT possesses the highest positive normalized
heat of formation (N∆Hf ) ∆Hf divided by the number of atoms
in a molecule), that was experimentally determined using an
additive method, compared to all other organic molecules. As
shown in Scheme 2, it is remarkable that neither 2,4,6-tri(azido)-
1,3,5-triazine (TAT);38 4,4′,6,6′-tetra(azido)hydrazo-1,3,5-tri-
azine (TAHT);5,8 4,4′,6,6′-tetra(azido)azo-1,3,5-triazine (TAAT);5,8
nor 2,5,8-tri(azido)-s-heptazine (TAH)16 has been reported to
undergo azido-tetrazolo tautomerization and tetrazolo transfor-
mation.
Synthesis and Characterization. Since 7 is extremely sensitive to
friction, electrostatic discharge, and impact,39 one should always handle
7 wet, use thick gloves behind a blast shield, and limit the amount to
less than 300 mg. Compounds 1,40 2,41 3,41,42 4,40 and 540 were prepared
by literature procedures.
3-Azido-6-(3,5-dimethylpyrazol-1-yl)-1,2,4,5-tetrazine (6). To a
100-mL-jacketed beaker containing 40 mL of 3 M HCl was added 2.06
g (10 mmol) of 2, and the suspension was stirred until complete
dissolution occurred. The temperature was adjusted to -5 °C, while a
solution of NaNO2 (0.76 g, 11 mmol) in 10 mL of water was added
dropwise with vigorous stirring while maintaining the temperature
below 3 °C. The solution was then extracted four times with 50 mL
75:25 (v/v) tert-butylmethyl ether:hexane mixture. The combined
organic layer was dried with Na2SO4 and was concentrated to give 6
as a low melting orange solid (80 °C), yield 1.6 g (75%). Caution:
although 6 does not possess any sensitivity problems, care should always
be taken when working with compounds containing azide, for example,
gently transfer the compound using plastic spatulas. 1H NMR (CDCl3)
δ 2.23 (s, 3H), 2.54 (s, 3H), 6.05 (s, 1H); 13C NMR (CDCl3) δ 13.18,
13.94, 111.80, 143.68, 154.53, 159.34, 164.46. Anal. calcd for
C6H2N9: C, 38.71; H 3.25; N, 58.04. Found: C, 38.85; H, 3.36; N,
57.88.
Experimental Section
The compounds appearing in this study are shown in Scheme 3.
3,6-Bis(3,5-dimethylpyrazol-1-yl)-1,2,4,5-tetrazine (1); 3-hydrazino-
6-(3,5-dimethylpyrazol-1-yl)-1,2,4,5-tetrazine (2); 3,6-di(hydrazino)-
1,2,4,5-tetrazine (3); 3-amino-6-(3,5-dimethylpyrazol-1-yl)-1,2,4,5-
tetrazine (4); 3,6-di(amino)-1,2,4,5-tetrazine (5); 3-azido-6-(3,5-
dimethylpyrazol-1-yl)-1,2,4,5-tetrazine (6); 2,3-(tetrazolo)-6-(3,5-
dimethylpyrazol-1-yl)-1,2,4,5-tetrazine (6A); 3,6-di(azido)-1,2,4,5-
tetrazine (7); 3-azido-1,6-(tetrazolo)-1,2,4,5-tetrazine (7A); 1,6:2,3-
bis(tetrazolo)-1,2,4,5-tetrazine (7B).
Abbreviations and formulas used in the text include the following:
TBAH ) [Bu4N]PF6 ) tetrabutylammonium hexafluorophosphate and
DMP ) 3,5-dimethylpyrazol-1-yl.
(39) Paine, R. T.; Koestle, W.; Borek, T. T.; Duesler, E.; Hiskey, M. A. Inorg.
Chem. 1999, 38, 3738-3743.
(35) Freudenberger, J.; Gaganov, A.; Hickman, A. L.; Jones, H. Cryogenics
2003, 43, 133-136.
(40) Coburn, M. D.; Buntain, G. A.; Harris, B. W.; Hiskey, M. A.; Lee, K.-Y.;
Ott, D. G. J. Heterocycl. Chem. 1991, 28, 2049-2050.
(36) Raj, B.; Mudali, U. K.; Vasudevan, M.; Shankar, P. Trans. Indian Inst.
Met. 2002, 55, 131-147.
(41) Chavez, D. E.; Hiskey, M. A. J. Heterocycl. Chem. 1998, 35, 1329-1332.
(42) Latosh, N. I.; Rusinov, G. L.; Ganebnykh, I. N.; Chupakhin, O. N. Russ.
J. Org. Chem. 1970, 35, 1363-1371.
(37) Marcus, J. H.; Remanick, A. J. Org. Chem. 1963, 28, 2372-2375.
(38) Ott, E. U.S. Pat. 1,390,378, 1921.
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Diazido Heteroaromatic High-Nitrogen C−N Compound
A R T I C L E S
Scheme 3. Compounds in the Study
3,6-Di(azido)-1,2,4,5-tetrazine (7). To a 100-mL-jacketed beaker
containing 40 mL of 3 M HCl was added 0.5 g (3.52 mmol) of 3, and
the suspension was stirred until complete dissolution occurred. The
temperature was adjusted to -5 °C, while a solution of NaNO2 (0.61
g, 8.84 mmol) in 10 mL of water was added dropwise with vigorous
stirring while maintaining the temperature below 3 °C. The reaction
was allowed to proceed at 0 °C for 30 min during which time a bright
orange solid precipitated. It was filtered using a Bu¨chner funnel, washed
thoroughly with cold water, and air-dried. Without isolation, this crude
product was washed into a plastic beaker with methylene chloride or
chloroform leaving an insoluble impurity behind. The filtrate was slowly
evaporated to give a crystalline product in a 54% yield. Caution: 7 is
extremely sensitive to spark, friction, and impact! The material should
be gently handled with plastic spatulas. Grounding straps, thick gloves,
and face shield should be used at all times. This reaction should be
done inside a hood and behind a thick blast shield. Differential scanning
calorimetry (DSC Exo.): onset of decomposition at 130 °C; 13C NMR
(300 MHz, CDCl3, 25 °C): 164.2 ppm; IR (Nujol mull): ν (N3) 2169
(vs) and 2142 (vs); ν(tetrazine) 1460 (vs), 1193 (vs), 1065 (vs), 927
(vs), 817 (vs), 546 (vs) cm-1; UV-vis (CHCl3) (λmax, nm (ꢀ, M-1 cm-1):
541 sh (4.64 × 102), 521 (6.50 × 102), 373 (1.79 × 103), and 268
(1.94 × 104). Cyclic voltammetry in 0.2 M Bu4NPF6/CHCl3 (V vs
SSCE): E1/2 ) +0.518 V. Because of its extreme sensitivity, elemental
analysis was not performed. Two azido substituents on the tetrazine
parent ring have made 7 to be a primary explosive. Its explosive
sensitivities are (1) friction < 10 g (BAM), (2) impact ∼2-3 cm (type
12), and (3) spark << 0.25 J.43
hydrazine was used as the solvent. Furthermore, no details on
the physical as well as chemical characteristics and energetic
properties of 7 were provided other than just a melting point of
130 °C.
In our improved synthetic pathway shown in eq 2, bright
magenta 1 rapidly reacted with 2 equiv of hydrazine hydrate in
CH3CN to give deep red 3 which underwent diazotization by
NaNO2 to give 7. The bright orange solid precipitated which
was filtered, washed with fresh cold water, air-dried, and
recrystallized using either CH2Cl2 or CHCl3.
Results
Synthesis. Monosubstituted azido 1,2,4,5-tetrazines have been
previously reported in the literature.11,23,37,44 In 1963, Marcus
reported the preparation of 7 in which 5 underwent a nucleo-
philic substitution reaction to form 3 followed by diazotization
by NaNO2, eq 1.37
Reminiscent of the preparation of 7, the preparation of 6 is
summarized in eq 3. A rapid reaction between 1 and 1 equiv of
hydrazine hydrate in CH3CN to form red 2 which was diazotized
to give deep red 6.
In our reaction pathways, the inexpensive hydrazine mono-
hydrate was used in the preparations of 2 and 3, and they both
gave nearly quantitative yields.
Infrared Spectroscopy and Heat of Formation (∆Hf). The
infrared spectra of compounds 6 and 7 were obtained in Nujol
mulls. In these spectra, significant differences between the two
compounds appear in the region 2100-2300 cm-1. For 7, two
bands assigned to νasym(N3) are at 2169 and 2207 cm-1. The
ν
asym(N3) for 6 appears at lower energy, 2152 cm-1. The
This reaction pathway was rather inefficient and costly
because the reaction yield was low, and the expensive anhydrous
experimental ∆Hf values are +615 kJ/mol for 1, +465 kJ/mol
for 4, +307 kJ/mol for 5, and +858 kJ/mol for 6.
UV-Visible Spectroscopy. UV-visible spectra for 1, 6, and
7 in chloroform are shown in Figure 1.
(43) Huynh, M. H. V.; Hiskey, M. A.; Hartline, E. L.; Montoya, D. P.; Gilardi,
R. D. Angew. Chem. Int. Ed. 2004, 43, 4924-4928.
(44) Licht, H.-H.; Ritter, H. J. Energ. Mater. 1994, 12, 223-235.
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A R T I C L E S
Huynh et al.
for all compounds were obtained at room temperature, while
the data for 7B were collected at 80 °C.
Discussion
Heats of Formation. The azido group is one of the most
energetic functional groups known and has previously been
reported to add 293 kJ/mol to the energy content of a molecule.45
In addition, the tetrazine ring system contains a large inherent
positive ∆Hf.3 Predictably, the combination of doubly substituted
covalent azides with the tetrazine ring will lead to a molecule
with a very large ∆Hf. However, an experimental ∆Hf for 7
cannot be measured because it is extremely sensitive to spark,
friction, and impact.
To derive a reasonable ∆Hf value for 7, we proposed to use
an additive method which appears to be valid by the ∆Hf values
of 1, 4, and 5, eq 4.
Redox Chemistry. All cyclic voltammograms of tetrazine
compounds provide evidence for chemically reversible couples
on the cyclic voltammetric time scale at a scan rate from 200
to 800 mV/s at room temperature and at 80 °C. The redox
potentials are -0.617 V for 1, -0.145 V for 6, -0.534 V for
6A, +0.518 V for 7, and -0.106 V for 7A. The redox potential
of -0.472 V for 7B was collected at 80 °C.
13C NMR Spectroscopy. The resonances of 6, 6A, 7, 7A,
and 7B are strongly dependent upon the polarity of solvents
used. In nonpolar solvents such as CD2Cl2 or CDCl3, the azido
substituents retain their own identity. In polar solvents such as
CD3CN, (CD3)2CO, and CD3OD, 6 is converted to 6A, whereas
7 undergoes azido-tetrazolo equilibrium. In a more polar solvent
such as DMSO-d6, 7 irreversibly transforms into the tetrazolo
isomer. Selected resonances of the tetrazine parent ring are
158.98 ppm for 1 in CDCl3, 164.46 and 159.34 ppm for 6 in
CDCl3, 152.12 and 150.35 ppm for 6A in DMSO-d6, 164.19
ppm for 7 in CDCl3, 157.94 and 150.41 ppm for 7A in DMSO-
d6, and 144.89 ppm for 7B in DMSO-d6. The 13C NMR data
In this stepwise amino substitution, the ∆Hf values for 1,
4, and 5 were experimentally measured as +615, +465, and
+307 kJ/mol, respectively. Their normalized heats of forma-
tion (N∆Hf) are correspondingly calculated as +18.09 kJ/
atom for 1, +20.22 kJ/atom for 4, and +25.58 kJ/atom for
5. As seen in eq 4, the first substitution of DMP by NH2
decreases ∆Hf by 150 kJ/mol and increases N∆Hf by 2.13
kJ/atom. The second substitution further reduces ∆Hf by 158
kJ/mol and raises N∆Hf by 5.36 kJ/atom. These data confirm
that ∆Hf of a symmetrically disubstituted tetrazine can be
predicted if the heat of reaction of the first substituted step is
known.
Extending this logic to 7, we measured the ∆Hf value for 6
from which the ∆Hf value for 7 is derived accordingly. It is
predicted to be +1101 kJ/mol. As shown in eq 5, the N∆Hf
value for 7 is +91.75 kJ/atom which is the highest positive
N∆Hf reported for all known organic molecule. Notably, the
replacement by the first N3 increases N∆Hf by 19.21 kJ/atom,
while the second N3 increases N∆Hf by an additional 54.45
kJ/atom. Compared to the stepwise amino-substituted data for
Figure 1. Absorption spectra in CHCl3: Black ) 7, red ) 6, and blue )
1 The spectral features are summarized in Table 1. The π f π* (tetrazine)
band appears in the UV region at 265 nm for 7, at 289 nm for 6, and at 299
nm for 1. Two additional transitions appear in the visible region at 373 and
521 nm for 7, at 383 and 530 nm for 6, and at 383 and 536 nm for 1. In
both UV and visible regions, all transitions shift to higher energy in the
order of 7 > 6 > 1.
(45) Haiges, R.; Boatz, J. A.; Vij, A.; Gerken, M.; Schneider, S.; Schroer, T.;
Christe, K. O. Angew. Chem., Int. Ed. 2003, 42, 5847-5851.
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