Oxygen-balanced energetic ionic liquid

Article abstract of DOI:10.1002/anie.200600735

On the way to the perfect propellant: The energetic oxidizer-balanced ionic liquid 1-ethyl-4,5-dimethyltetrazolium tetranitratoaluminate (see scheme) has been prepared and characterized. Oxidizer-balanced ionic liquids are promising propellants with good

Full text of DOI:10.1002/anie.200600735

Angewandte
Chemie
Propellants
oxygen carrier and the 1-ethyl-4,5-dimethyltetrazolium cation
o
as
a
more energetic counterion (imidazole, DH =
f
o
ꢀ
1 [8]
ꢀ1 [9]
DOI: 10.1002/anie.200600735
+ 49.8 kJmol ;
tetrazole,
DH = + 237.1 kJmol
).
f
These are the first CO-balanced EILs. Although an oxygen-
balanced tetrazolium salt, 5-aminotetrazolium nitrate, was
Oxygen-Balanced Energetic Ionic Liquid**
[
10]
recently reported,
its melting point of 1738C does not
classify it as an ionic liquid.
C. Bigler Jones, Ralf Haiges, Thorsten Schroer, and
Karl O. Christe*
Polynitratoaluminates were first studied in the 1960s in
[
11]
the USA
USSR.
and, subsequently, during the 1970s in the
[
12–23]
[12–21]
+ [22,23]
Several examples of alkali metal,
NO ,
2
[
1–3]
[24]
Energetic ionic liquids (EILs) are of great interest.
They
and ethylammonium salts of tetra-, penta-, and hexanitra-
toaluminate anions are known. The tetranitratoaluminate
anion contains 12 oxygen atoms; of these, 10.5 are available to
oxidize a fuel cation.
offer enhanced stability, higher densities, no vapor pressure,
and, hence, no vapor toxicity. As a general principle, the
stability of energetic ionic compounds can be greatly
enhanced by making the cation the fuel and the anion the
oxidizer. The formal positive charge increases the ionization
potential of the fuel cation, and the formal negative charge
decreases the electron affinity of the anion. In this manner,
the fuel cation becomes more oxidizer-resistant, and the
oxidizer anion is protected against premature reduction by
the cation. For environmental reasons, it is also desirable to
avoid halogen-containing ingredients, such as perchlorates.
The previously known EILs consist of small oxidizing
Alkylated tetrazolium cations were used in this work
because of their large positive heats of formation and their
potential to form ionic liquids. Ionic salts of the tetranitrato-
aluminate anion can be prepared in essentially quantitative
yields in one-pot reactions in nitromethane solution. The
starting materials are the chloride salt of the cation, aluminum
trichloride, and dinitrogen tetroxide. The synthesis of 1-ethyl-
4,5-dimethyltetrazolium tetranitratoaluminate (3) is shown in
Scheme 1. The starting material 1-ethyl-4,5-dimethyltetrazo-
ꢀ
ꢀ
ꢀ
anions, such as ClO4 , NO3 , or N(NO2)2 , and large fuel
cations containing quaternary nitrogen heterocycles with
long, asymmetric, poorly packing side chains. The most
serious drawback of these EILs is that they are under-
oxidized. The small anions do not carry sufficient oxygen for
complete oxidation of the large fuel cations to carbon
monoxide, resulting in poor performance. In rocket propul-
sion, a low molecular weight of the exhaust products is very
[
4,5]
important.
Furthermore, at high flame temperatures CO2
is dissociated almost completely to CO and O (Boudouard
2
[
6]
equilibrium). Therefore, it is often sufficient to oxidize the
carbon content only to CO and not to CO to achieve near-
2
[
5]
maximum performance. The aim of this study was the
preparation of halogen-free, CO-balanced, EILs.
In 1998, the concept of oxidizer-balanced EILs was
proposed, and in 2002, its practicability was shown by the
preparation of 1-ethyl-3-methylimidazolium tetranitratobo-
Scheme 1. Synthesis of 1-ethyl-4,5-dimethyltetrazolium tetranitrato-
aluminate (3).
[
7]
rate, a compound that turned out to be indeed an ionic
liquid with a freezing point of ꢀ258C. However, its energy
content and thermal stability were marginal. Herein, we
report on a significantly improved compound using the
tetranitratoaluminate anion as a thermally more stable high-
lium chloride (1) was prepared by alkylation of 1,5-dimethyl-
tetrazole with ethyl iodide, followed by anion exchange of
iodide for chloride using an anion-exchange resin. The
alkylation places the ethyl group primarily into the 1 position,
but 16% is also found at the 2 position of the tetrazolium
cation. The percentage of the minor isomer was reduced to
[
*] C. B. Jones, Dr. R. Haiges, Dr. T. Schroer, Prof. Dr. K. O. Christe
Loker Research Institute and Department of Chemistry
University of Southern California
6
% by recrystallization from ethanol and might be reduced
Los Angeles, CA 90089-1661 (USA)
Fax: (+1)213-740-6679
E-mail: kchriste@usc.edu
further by additional recrystallizations. This relatively small
isomeric impurity was not removed from the product. It offers
the benefit of lowering the melting point of the salt without
drastically altering its energetic or chemical properties.
The reaction of the tetrazolium chloride in nitromethane
with one equivalent of anhydrous aluminum trichloride gives
the tetrachloroaluminate salt 2, which is a viscous ionic liquid.
This intermediate can then be reacted directly with an excess
[
**] This work was funded by the Office of Naval Research, the Lawrence
Livermore National Laboratory, and the National Science Founda-
tion. We thank Dr. J. A. Boatzand Dr. J. Mills for the theoretical
calculations, and Prof. G. A. Olah and Dr. J. Goldwasser, Dr. C.
Bedford, Dr. M. Berman, Dr. J. Gilje, and Dr. J. Satcher for their
steady support, and Prof. Dr. R. Bau and Dr. S. Schneider, Dr. W. W.
Wilson, and Dr. R. Wagner for their help and stimulating
discussions.
of N O in nitromethane. Compound 3 is obtained as a clear,
2
4
nearly colorless, viscous liquid by pumping off the volatile
Angew. Chem. Int. Ed. 2006, 45, 4981 –4984
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4981

Communications
[
26]
compounds, NOCl, CH NO , and excess N O , at ambient
tate nitrato ligands, both tetranitratoaluminate isomers are
hexacoordinated with two monodentate and two bidentate
ligands in cis configuration. These isomers differ only in the
3
2
2
4
temperature. It is stable in dry air, hydrolyzes in water, and is
soluble in CH NO and moderately soluble in CH Cl . The
3
2
2
2
identity and purity of the product were established by Raman,
orientation of the NO groups of the monodentate ligands
2
1
14
15
13
infrared, H, N, N, and C NMR spectroscopy, and the
observed material balance.
with respect to each other and, at the MP2 level, differ only by
ꢀ
1
2.5 kJmol . The energetically favored C structure agrees
2
1
4
[25]
The N NMR spectrum of 3 shows a strong signal at d =
with the crystal structure of [N(CH ) ][Al(NO ) ], and its
3
4
3 4
ꢀ
25 ppm, which is attributed to the four nitrogen atoms of the
tetranitratoaluminate anion. The signals at d = ꢀ134 and
calculated spectra are in better agreement with the observed
ones.
ꢀ
144 ppm, assigned to N1 and N4, respectively, of the
Individual modes are difficult to assign because of the size
of the anion and strong vibrational coupling. In the region of
the NꢀO stretching modes, three clusters are observed at
tetrazolium cation, are much broader. Additional signals
from N2 and N3 of the tetrazolium cation are obscured by the
strong broad signal at d = ꢀ25 ppm.
ꢀ1
about 1650–1500, 1350–1290, and 1030–990 cm , with each
cluster containing four fundamental vibrations. These clusters
are characteristic for covalently bound mono- and bidentate
1
5
The N NMR spectrum of the neat liquid shows the
expected five signals, at d = ꢀ15.6 (N2), ꢀ18.2 (N3), ꢀ25.3
ꢀ
[27]
(
Al(NO ) ), ꢀ134.2 (N1), and ꢀ145.8 (N4) ppm. The tetra-
nitrato ligands and distinguish them from ionic nitrates. The
3
4
nitratoaluminate anion contains two monodentate and two
bidentate nitrato ligands, as shown by us by a crystal structure
highest-frequency cluster contains the N=O stretching vibra-
tion of the bidentate ligands and the antisymmetric stretching
vibration of the terminal NO2 part of the monodentate
ligands. The medium-frequency cluster contains the OꢀNꢀO
[
25]
analysis of [N(CH ) ][Al(NO ) ]. In this pseudo-octahedral
3
4
3 4
structure, the two monodentate ligands are cis to each other.
The observation of a single nitrogen resonance for the
monodentate and bidentate nitrato groups is attributed to
fast intramolecular exchange. Even at ꢀ308C in CH NO , this
antisymmetric stretching vibration of the bidentate ligands
and the symmetric stretching vibration of the terminal NO2
part of the monodentate ligands, while the lowest-frequency
cluster is composed of the OꢀNꢀO symmetric stretching
3
2
exchange could not be slowed sufficiently to observe line
broadening or separate signals for the nitrato groups. The
vibration of the bidentate ligands and the stretching vibration
1
13
assignments for the H and C NMR signals are given in the
Experimental Section.
of the coordinating NꢀO part of the monodentate ligands.
Although the spectra of the mono- and bidentate ligands
exhibit different intensity patterns and the frequency separa-
tion between the first and the second cluster is somewhat
larger for the bidentate nitrates, distinction between mono-
dentate and bidentate nitrato ligands becomes difficult when
dealing with molecules that contain both types of ligands at
the same time.
The presence of the tetranitratoaluminate anion was also
confirmed by vibrational spectroscopy. The observed infrared
and Raman spectra are shown in Figure 1, and the frequencies
are listed in the Experimental Section. The anion part of the
infrared spectrum is in good agreement with those previously
[
15]
[12]
reported for Rb[Al(NO ) ]
and Cs[Al(NO ) ].
Addi-
3
4
3 4
tional support came from calculations at the MP2/6-311 +
G(d) level of theory. Two minimum-energy structures were
obtained with C and C symmetry, respectively. Contrary to
For energetic materials, stability and physical properties
are very important. The thermal stability of 3 was investigated
with thermogravimetric analysis (TGA) and differential
scanning calorimetry (DSC). The DSC trace showed a glass
2
1
the tetranitratoborate anion, which possesses four monoden-
transition temperature (T ) at ꢀ468C and a strongly exother-
g
mic decomposition with a maximum at 2178C and an onset at
1
838C. In accord with the DSC data, the TGA showed
catastrophic weight loss to start at 1838C; however, very slow
weight loss also occurred at much lower temperatures, but the
exact onset was difficult to ascertain. When a sample of 3 was
held isothermally in the TGA apparatus at 758C for 4 h, a
1
0.4% weight loss was observed. This slow weight loss is
attributed to the loss of NO and oxygen, accompanied by the
2
formation of AlꢀOꢀAl bridges. Similar observations were
previously reported by Shirokova and Rosolovskii for the
[
13]
cesium polynitratoaluminates.
Ignition of EILs often presents major problems. The
ignition of compound 3 and self-sustained burning were
readily achieved by either thermal heating of 3 to about
2
008C or by the use of a hot 40-gauge Ni/Cr wire wrapped
around the sample container, a glass melting-point capillary.
The capillary was filled to the top with the sample, and a
direct current passed through the wire. After a few seconds,
compound 3 ignited quite spectacularly, giving off flames and
light, fluffy alumina, in accord with the predicted idealized
combustion process [Eq. (1)]. Obviously, the composition of
Figure 1. IR (upper trace) and Raman (lower trace) spectra of liquid 3.
The stars indicate bands assigned to the cation.
4
982 www.angewandte.org
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 4981 –4984

Angewandte
Chemie
the combustion products at a given flame temperature would
significantly deviate from those given in Equation (1).
The solid was dissolved in a minimum amount of methanol, and
passed through a column containing 15 g (55.5 meq) AMBER-
JET 4200 (Cl) ion-exchange resin, using methanol as eluent. The
bulk of the methanol was removed under vacuum, and the anion
exchange was repeated until the effluent was free of iodide, as shown
by the absence of an NH -insoluble precipitate with AgNO . All
þ
ꢀ
2
½N C H Š ½AlðNO Þ Š ! Al O þ 8 N þ 11 H O þ 10 CO
ð1Þ
4
5
11
3
4
2
3
2
2
3
3
volatile material was pumped off, and the residue was recrystallized
When samples were heated in the TGA apparatus to their
1
from ethanol. Yield: 1.412 g (31%); H NMR (CD CN): d = 1.56 (t,
3
decomposition temperature, a false small mass increase was
observed right before the catastrophic mass loss, due to the
thrust of the burning liquid pushing down the TGA pan.
The theoretical performance of 3 as a propellant can be
estimated from the calculated heats of formation of the free
3
J = 7.4 Hz, 3H, CH ), 2.95 (s, 3H, CH ), 4.23 (s, 3H, CH ), 4.61 ppm
q, J = 7.4 Hz, 2H, CH ); minor isomer, 2-ethyl-4,5-dimethyltetra-
zolium chloride: H NMR (CD CN): d = 1.60 (t, J = 7.4 Hz, 3H,
3
3
3
3
(
2
1
3
3
3
CH
), 3.00 (s, 3H, CH
), 4.27 (s, 3H, CH
), 4.86 ppm (q, J = 7.4 Hz,
3
3
3
2
H, CH ). Raman (400 mW): n˜ = 2986 (7.4), 2953 (10.0), 2887 (2.5),
2
ꢀ
1
ꢀ1
2814 (1.0), 2763 (0.4), 1589 (1.8), 1530 (1.0), 1474 (sh), 1456 (1.8), 1416
gaseous cation (836 kJmol ) and anion (ꢀ1486 kJmol ),
calculated at the MP2/6-311 + G(d) level of theory, an
(
(
1.0), 1396 (0.8), 1363 (2.9), 1324 (0.5), 1307 (0.5), 1287 (0.7), 1229
0.2), 1116 (0.3), 1088 (0.3), 1065 (0.8), 1041 (0.5), 980 (0.8), 808 (0.3),
[
28]
estimate of the Coulomb energy of the ions in the liquid
of about 419 kJmol , using publicly available performance
7
3
88 (0.2), 747 (1.0), 724 (3.9), 699 (0.6), 655 (0.6), 594 (1.1), 503 (0.7),
88 (0.6), 296 (1.5), 246 (0.8), 229 (0.8), 153 (sh) cm . IR (KBr): n˜ =
ꢀ
1
ꢀ
1
[
29]
calculation codes.
Based on these estimates, the perfor-
2997 (m), 2946 (w), 2888 (w), 1629 (br), 1587 (s), 1526 (m), 1467 (sh),
1453 (m), 1409 (w), 1388 (w), 1360 (m), 1323 (w), 1287 (w), 1229 (w),
1181 (w), 1151 (w), 1113 (w), 1084 (vw), 1058 (sh), 1034 (s), 975 (m),
mance of this system significantly exceeds those of state-of-
[
30]
the-art materials, such as hydrazine.
ꢀ
1
805 (w), 745 (s), 720 (w), 696 (vw), 648 (w) cm
.
1-Ethyl-4,5-dimethyltetrazolium tetrachloroaluminate (2): In the
glove box, compound 1 (3.25 mmol) and AlCl3 (3.25 mmol) were
placed into a 9-mm (outer diameter) glass ampoule. The ampoule was
connected to the vacuum line, and evacuated at ꢀ1968C. Nitro-
methane ( ꢁ 1 mL) was added at ꢀ1968C, and the ampoule was
allowed to warm slowly to room temperature, which led to an orange
solution. The nitromethane was removed under a dynamic vacuum
overnight at ambient temperature, giving a quantitative yield of 2 as
an amber viscous liquid.
Experimental Section
Caution! Although no difficulties were encountered when handling
these materials, they are highly energetic and potentially explosive!
They should be handled on a small scale while using appropriate safety
precautions (safety shields, face shields, leather gloves, protective
clothing, such as heavy leather welding suits and ear plugs).
Materials and apparatus: All reactions were carried out in Pyrex
glass ampoules that were closed by Teflon/glass high-vacuum valves.
Volatile materials were handled in a Pyrex glass vacuum line.
Nonvolatile materials were handled in the dry argon atmosphere of
a glove box.
1-Ethyl-4,5-dimethyltetrazolium tetranitratoaluminate (3): In the
glove box, compound 2 (1.68 mmol) was loaded into a 9-mm glass
ampoule. The ampoule was connected to the glass vacuum line and
evacuated. After cooling to ꢀ1968C, N O (39.92 mmol) was
2
4
Raman spectra were recorded directly in the glass reactors in the
condensed in, followed by CH NO (27.06 mmol). The mixture was
3
2
ꢀ1
range 3600–80 cm on a Bruker Equinox 55 FRA 106/S FT-RA
spectrometer, using a Nd-YAG laser at 1064 nm with power levels of
allowed to slowly warm to room temperature and stirred for 1.5 h.
The volatile material was removed under a dynamic vacuum for 24 h,
giving an almost colorless clear viscous oil. Expected mass 0.674 g,
ꢀ1
4
00 mW. Infrared spectra were recorded in the range 4000–400 cm
on a Midac, M Series, FT-IR spectrometer. For liquid samples, a Wilks
minicell with AgCl windows was used. Solids were recorded as AgCl
or KBr pellets. The cells were filled inside the glove box using an
Econo minipress (Barnes Engineering Co.) and transferred in a
closed container to the spectrometer before placing them quickly into
the sample compartment which was purged with dry nitrogen to
minimize exposure to atmospheric moisture and potential hydrolysis
1
3
found mass 0.651 g. H NMR (CD NO ): d = 1.62 (t, J = 7.4 Hz, 3H,
3
2
3
CH ), 2.95 (s, 3H, CH ), 4.90 (s, 3H, CH ), 4.67 ppm (q, J = 7.4 Hz,
3
3
3
2
H, CH ); minor isomer, 2-ethyl-4,5-dimethyltetrazolium tetranitra-
2
1
3
toaluminate: H NMR (CD NO ): d = 1.70 (t, J = 7.4 Hz, 3H, CH ),
2
CH ); C NMR: d = 8.77, 13.95, 37.53, 47.84, 153.87 ppm; N NMR
3
2
3
3
.82 (s, 3H, CH ), 4.33 (s, 3H, CH ), 4.91 ppm (q, J = 7.4 Hz, 2H,
3
3
13
14
2
14
(
CD NO ): d = ꢀ144, ꢀ134, ꢀ25 ppm; N NMR (CD Cl ): ꢀ141,
3
2
2
2
1
4
15
of the sample. N and N NMR spectra were recorded at 36.13 and
0.68 MHz, respectively, on Bruker AMX 500 spectrometer.
Samples were either externally referenced to CH NO or dissolved
15
ꢀ
26 ppm; N NMR (neat liquid): d = ꢀ145.8 (s, 1N, N4), ꢀ134.2 (s,
5
a
1
N, N1), ꢀ25.3 (s, 4N, Al(NO ) ), ꢀ18.2 (s, 1N, N3), ꢀ15.6 ppm (s,
3
4
3
2
1N, N2). Raman (400 mW): n˜ = 2973 (9.9), 2951 (10.0), 2887 (1.0),
in CH NO . TGA thermograms were measured on a Shimadzu TGA-
3
2
2761 (0.5), 1630 (1.0), 1611 (1.0), 1585 (1.6), 1547 (0.9), 1453 (1.6),
ꢀ
1
5
0 instrument using a flow rate of 20 mLmin of nitrogen. DSC
1422 (1.0), 1387 (1.0), 1367 (2.3), 1321 (2.0), 1092 (0.9), 1057 (sh), 1021
measurements were recorded on a Shimadzu DSC-50(SH); the
temperature was ramped at a rate of 10 Kmin . Densities were
measured with a pycnometer.
(
(
7.0), 970 (0.8), 808 (0.8), 775 (0.7), 743 (0.9), 715 (3.0), 703 (2.0), 703
ꢀ1
ꢀ1
2.0), 589 (1.2), 508 (0.5), 317 (1.3), 300 (1.2), 271 (0.9), 237 (1.1) cm
.
IR (AgCl plates): n˜ = 3032 (sh), 2995 (w), 2944 (w), 2850 (w,br), 2627
(w,br), 2547 (w,br), 2282 (w,br), 2004 (w,br), 1945 (sh), 1694 (sh), 1630
The starting materials, N O (Matheson), ethyl iodide, AlCl , and
2
4
3
5
-methyltetrazole (Aldrich), were used without further purification.
(sh), 1611 (s), 1583 (s), 1550 (s), 1466 (w), 1451 (m), 1388 (w), 1320 (s),
Deuterated solvents (Cambridge Isotopes) were dried using standard
methods. Nitromethane (Fisher) was dried over CaCl , vacuum-
distilled and stored over 4-Š molecular sieves before use. All volatile
materials were handled using standard high-vacuum techniques. 1,5-
dimethyltetrazole was prepared by a literature method; its identity
1275 (s), 1182 (w), 1142 (w), 1093 (w), 1047 (sh), 1021 (sh), 1001 (s),
2
806 (sh), 796 (m), 773 (w), 739 (m), 711 (w), 664 (w), 516 (sh), 489 (m),
ꢀ1
450 (sh) cm
.
[
31]
Received: February 25, 2006
Published online: July 3, 2006
1
and purity were confirmed by IR, Raman, and H NMR spectroscopy.
1-Ethyl-4,5-dimethyltetrazolium chloride (1): 1,5-dimethyltetra-
zole (28 mmol) and ethyl iodide (15 mL) were placed into a 250-mL
glass ampoule equipped with a high-vacuum valve and degassed by
three freeze-pump-thaw cycles. The ampoule was immersed into a hot
water bath at 1008C for 10 h. The volatile material was removed
under vacuum at room temperature, to give a yellow solid (5.466 g).
Keywords: energetic ionic liquids · propellants ·
tetranitratoaluminate · tetrazoles · tetrazolium ions
.
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 4981 –4984
www.angewandte.org 4983

Communications
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[
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