Nitrogen-rich energetic ionic liquids based on the N,N-Bis(1H-tetrazol-5- yl)amine anion - Syntheses, structures, and properties

Article abstract of DOI:10.1002/ejic.201300631

N,N-Bis(1H-tetrazol-5-yl)amine anions (HBTA^-/BTA^2-) were used as suitable nitrogen-rich components for the construction of energetic ionic liquids. Seven ionic liquids, namely, 1,3-dimethylimidazolium (1, [C ₁mim]HBTA), 1-ethyl-3-methylimidazolium (2, [C₂mim]HBTA), 1-butyl-3-methylimidazolium (3, [C₄mim]HBTA), 1-hexyl-3- methylimidazolium (4, [C₆mim]HBTA), 1-methyl-3-octylimidazolium (5, [C₈mim]HBTA), and 1,2,3-trimethylimidazolium (6, [C ₁mmim]HBTA) cations with the HBTA^- anion and di-1,2,3-trimethylimidazolium N,N-bis(1H-tetrazol-5-yl)amine (7, [C ₁mmim]₂BTA), were synthesized. All materials were fully characterized by IR and NMR spectroscopy, elemental analysis, and high-resolution mass spectrometry. The crystal structures of 1 and 7 were determined by single-crystal X-ray diffraction [1: monoclinic, P2₁/n, a = 14.0653(4) A, b = 6.9507(2) A, c = 22.4699(7) A, β = 93.996(3)°, V = 2191.38(12) A³, Z = 8, ρ = 1.511 g cm^-3; 7: monoclinic, P2₁/c, a = 17.5869(3) A, b = 12.8757(2) A, c = 21.9203(6) A, β = 125.0580(10)°, V = 4063.15(15) A³, Z = 8, ρ = 1.221 g cm^-3]. The nitrogen contents of 1-7 exceed 40 %, and 1 has the highest value of 61.78 %. The ionic liquids 1-7 are thermally stable to 220 °C, and 2-5 are room-temperature ionic liquids. The heats of formation of 1-7 obtained by both experimental and theoretical methods are all positive. The ionic liquids 1-7 are insensitive towards impact (>40 J) and friction (>360 N). They can be ignited in air. These new energetic ionic liquids contain only C, H, and N and are of interest as potential propellants with high energy, high thermal stability, low sensitivity to impact and friction, and environmentally friendly decomposition gases. Nitrogen-rich energetic ionic liquids based on the N,N-bis(1H-tetrazol-5-yl)amine anion were synthesized. These ionic liquids have high nitrogen contents, high heats of formation, and high stabilities to heat, impact, and friction. These nitrogen-rich energetic ionic liquids combust in air after ignition with a flame. They are potential environmentally friendly liquid propellants. Copyright

Full text of DOI:10.1002/ejic.201300631

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DOI:10.1002/ejic.201300631
Nitrogen-Rich Energetic Ionic Liquids Based on the N,N-
Bis(1H-tetrazol-5-yl)amine Anion – Syntheses, Structures,
and Properties
Lin-Lin Dong,[a] Ling He,[a] Hang-Yu Liu,[a] Guo-Hong Tao,*[a]
Fu-De Nie,[b] Ming Huang,[b] and Chang-Wei Hu*[a]
Dedicated to Professor Jean’ne M. Shreeve on the occasion of her 80th birthday
Keywords: Energetic materials / Propellants / Ionic liquids / Nitrogen heterocycles / Heats of formation
N,N-Bis(1H-tetrazol-5-yl)amine anions (HBTA–/BTA2–) were
used as suitable nitrogen-rich components for the construc-
tion of energetic ionic liquids. Seven ionic liquids, namely,
1,3-dimethylimidazolium (1, [C1mim]HBTA), 1-ethyl-3-meth-
ylimidazolium (2, [C2mim]HBTA), 1-butyl-3-methylimid-
azolium (3, [C4mim]HBTA), 1-hexyl-3-methylimidazolium
6.9507(2) Å,
c
=
22.4699(7) Å,
β
=
93.996(3)°,
V
=
2191.38(12) Å3, Z = 8, ρ = 1.511 gcm–3; 7: monoclinic, P21/
c, a = 17.5869(3) Å, b = 12.8757(2) Å, c = 21.9203(6) Å, β =
125.0580(10)°, V = 4063.15(15) Å3, Z = 8, ρ = 1.221 gcm–3].
The nitrogen contents of 1–7 exceed 40%, and 1 has the
highest value of 61.78%. The ionic liquids 1–7 are thermally
stable to 220 °C, and 2–5 are room-temperature ionic liquids.
The heats of formation of 1–7 obtained by both experimental
and theoretical methods are all positive. The ionic liquids 1–
7 are insensitive towards impact (Ͼ40 J) and friction (Ͼ360
N). They can be ignited in air. These new energetic ionic
liquids contain only C, H, and N and are of interest as poten-
tial propellants with high energy, high thermal stability, low
sensitivity to impact and friction, and environmentally
friendly decomposition gases.
(4,
[C6mim]HBTA),
and
1-methyl-3-octylimidazolium
1,2,3-trimethylimidazolium
(5,
(6,
[C8mim]HBTA),
[C1mmim]HBTA) cations with the HBTA– anion and di-1,2,3-
trimethylimidazolium N,N-bis(1H-tetrazol-5-yl)amine (7,
[C1mmim]2BTA), were synthesized. All materials were fully
characterized by IR and NMR spectroscopy, elemental analy-
sis, and high-resolution mass spectrometry. The crystal struc-
tures of 1 and 7 were determined by single-crystal X-ray dif-
fraction [1: monoclinic, P21/n,
a = 14.0653(4) Å, b =
gible vapor pressure, large liquidus ranges, little or no vapor
toxicity, and improved safety issues related to transpor-
tation, handling, and processing from production to end
use, which originate from the charged structures of the ionic
liquids.[9]
Introduction
In recent years, ionic liquids have received much atten-
tion as interesting modifiable soft materials in a wide range
of applications as solvents,[1] catalysts,[2] electrolytes,[3] lu-
bricants,[4] surfactants,[5] magnetic,[6] and optical fluids.[7]
Some ionic liquids have interesting properties as green li-
quid propellants and explosives for application in energetic
materials.[8] Compared with traditional molecular energetic
compounds, such as 2,4,6-trinitrotoluene (TNT), 1,3,5-trin-
itro-1,3,5-triazacyclohexane (RDX), and 1,3,5,7-tetranitro-
1,3,5,7-tetraazacyclooctane (HMX), many energetic ionic
liquids have advantages of enhanced thermal stability, negli-
Propellants are the most important application of ener-
getic ionic liquids. To date, several kinds of typical energetic
ionic liquid propellants have been reported. Christe and
Shreeve have tried to seek suitable monopropellants con-
taining sufficient oxygen.[10] In contrast, the present work
is primarily concerned with bipropellants in which safety
and stability are enhanced by the segregation of fuel and
oxidizer into separate working fluids. Most known energetic
ionic liquids are underoxidized, because small anions such
–
–
[a] College of Chemistry, Sichuan University,
Chengdu 610064, China
as nitrate (NO3 ), perchlorate (ClO4 ), and dinitramide
–
[N(NO2)2 ] have insufficient oxygen to completely oxidize
E-mail: taogh@scu.edu.cn
the large cations to carbon monoxide.[11] However, only a
part of them may supply energy, as hypergols undergo hy-
pergolic ignition with common propellant oxidizers.[12]
The common oxidizers for hypergols comprise highly
toxic and corrosive chemicals including dinitrogen tetroxide
(N2O4) and fuming nitric acid (HNO3), which are still haz-
gchem@scu.edu.cn
Homepage: http://chem.scu.edu.cn/professor/taoguohong/
index.htm
[b] Institute of Chemical Materials, China Academy of
Engineering Physics,
Mianyang 621900, China
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201300631
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ardous and dangerous. Oxygen gas (O2) is nonhazardous, [C8mim]HBTA), and 1,2,3-trimethylimidazolium (6,
nonflammable, and unreactive, but is a strong oxidizer. [C1mmim]HBTA) cations; the di-1,2,3-trimethylimid-
Thus, in consideration of their environmental friendliness, azolium BTA2– salt (7, [C1mmim]2BTA) was also synthe-
green chemistry, and low vapor toxicity, energetic ionic li- sized. Compounds 1–7 were fully characterized, and single-
quids may also be useful fuels with O2 as the oxidizer in crystal X-ray diffraction analysis of 1 and 7 confirmed the
clean bipropellant applications. Moreover, the propellant structures of these ionic liquids. In addition, their thermal
applications of combustible energetic ionic liquids that can and chemical stabilities and sensitivities along with their
ignited in air may reduce the oxidizer tank in the atmo- combustibility characteristics were also determined.
sphere, which will increase the payload of an aircraft
–
The dicyanamide anion [N(CN)2 ] has been studied as a
stable anion for the construction of hypergolic ionic liquids
Results and Discussion
with good fluidity and thermal stability.[13] Nevertheless,
the demand for new energetic ionic liquids with higher en-
Syntheses
ergy density is constant. Nitrogen-rich anions, including az-
–
Substituted imidazolium cations are typical cations for
the construction of ionic liquids. In this work, we intro-
duced substituted imidazolium cations, especially asymmet-
ric imidazolium cations, to design ionic liquids based on the
HBTA– anion. No C2-symmetric axis exists in the HBTA–
anion, but a C2-symmetric axis may be found in H2BTA or
the BTA2– anion. These asymmetric functional groups or
frameworks will lead to low melting points for the new salts.
The energetic ionic liquids 1–6 with the monoanion HBTA–
were obtained directly after anion metatheses (Scheme 2).
The starting material, N,N-bis(1H-tetrazol-5-yl)amine mon-
ohydrate (H2BTA·H2O), was obtained according to the lit-
erature method.[15] The reaction of H2BTA·H2O with silver
nitrate (1:1 molar ratio) in water results in the formation of
silver N,N-bis(1H-tetrazol-5-yl)amine (AgHBTA) as a white
precipitate. Compounds 1–6 were prepared from reactions
of their corresponding 1,3-dialkylimidazolium iodide/brom-
ide salt precursors with AgHBTA in methanol.
ide (N3 ) and 5-aminotetrazolate, were used to prepare
energetic ionic liquids with high heats of formation and
high energy densities.[14] However, low thermal and impact
–
stabilities limit the applications of N3 -containing energetic
ionic liquids. In addition, the nitrogen content of poly-
nitrogen heterocyclic ring frameworks and the melting
points of the stable energetic salts are directly related, that
is, the rigidity of the heterocyclic ring increases the melting
point.
Recently, N,N-bis(1H-tetrazol-5-yl)amine (H2BTA) was
considered as an interesting nitrogen-rich framework for
energetic salts.[15] H2BTA is the product of the reaction be-
tween NaN(CN)2 and NaN3 under acidic conditions
(Scheme 1). The N,N-bis(1H-tetrazol-5-yl)amine anion
(HBTA–) contains two nitrogen-rich tetrazole rings and
–
–
may combine the virtues of the N(CN)2 and N3 anions
with a balance of stability and energy. Compared with other
sensitive 5-substituted tetrazoles,[16] including 5-nitrotetraz-
ole, 5-cyanotetrazole, 1,5-diaminotetrazole, 5,5Ј-azotetraz-
ole, and 5-azidotetrazoles, the neutral H2BTA possesses
high thermal and mechanical stability with a high nitrogen
content of 82.34%. Some metallic and nonmetallic BTA
salts have been reported as nitrogen-rich compounds/ener-
getic materials with high thermal stability (Ͼ200 °C), high
heat of formation (Ͼ +600 kJmol–1), and low impact
(Ͼ40 J) and friction (Ͼ360 N) sensitivities.[15] However, the
melting points of these salts are all higher than 100 °C. To
the best of our knowledge, no ionic liquids based on
HBTA– or BTA2– anions have been reported.
Scheme 2. Synthesis of the ionic liquids 1–6 with HBTA–
monoanion.
Scheme 1. Synthesis of N,N-bis(1H-tetrazol-5-yl)amine (H2BTA).
Herein, we report the synthesis of a series of nitrogen-
rich ionic liquids based on the HBTA– anion with 1,3-di-
The ionic liquid 7 with the BTA2– dianion was obtained
methylimidazolium (1, [C1mim]HBTA), 1-ethyl-3-methyl- by slow recrystallization of the ionic liquid 6 from a meth-
imidazolium (2, [C2mim]HBTA), 1-butyl-3-methylimid- anol/ethyl acetate solution. Neutral H2BTA was the by-
azolium (3, [C4mim]HBTA), 1-hexyl-3-methylimidazolium product and remained in solution after the crystallization
(4, [C6mim]HBTA), 1-methyl-3-octylimidazolium (5, (Scheme 3).
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and 7 are summarized in Table 5. Selected bond lengths [Å]
and angles [°] of 1 and 7 are listed in Table 1.
Compound 1 crystallizes in the monoclinic space group
P21/n and has a density of 1.511 gcm–3 and a unit cell vol-
ume of 2191.38(12) Å3 (Table 5). In the molecule of 1 (Fig-
ure 1), the N1–C1, N4–C1, N6–C2, N20–C5, N20–C6 bond
lengths in the tetrazole and imidazole rings are 1.331(2),
1.322(2), 1.330(2), 1.322(2), and 1.372(2) Å, respectively
(Table 1). These C–N ring bonds are much shorter than
classical C–N single bonds (1.47 Å), but are longer than
C=N double bonds (1.31 Å),[17] which suggests that a little
multiple bond character is present. A similar trend was ob-
served for the N1–N2 [1.353(2) Å], N2–N3 [1.311(2) Å],
and N3–N4 [1.361(2) Å] bonds, which are significantly
longer than typical N=N bonds (1.24 Å). Compared with
those of the neutral H2BTA, the bond lengths on the tetraz-
ole rings of 1 are similar.[15] However, the N5–C1 bond
length of 1 is 1.388(2) Å, which is a little longer than that
in H2BTA [1.365(2) Å] and N5–C2 [1.347(2) Å]. This differ-
ence originates from the conjugated tetrazolate anion. The
N4–C1–N1 angle in the tetrazole ring of 1 is 113.86(15)°,
which is bigger than that in H2BTA [109.5(2)°],[15] but close
to that in the structure of the 5-aminotetrazolate anion
[112.5(5)°].[14c] Furthermore, the C2–N5–C1 [125.67(15)°],
Scheme 3. Formation of the ionic liquid 7 with the BTA2– dianion.
All of the ionic liquids 1–7 were characterized by IR and
NMR spectroscopy, elemental analysis, and high-resolution
mass spectrometry. Typical peaks of the 1-alkyl-3-methyl-
imidazolium cations were recorded in the 1H and 13C NMR
spectra of all compounds. A signal of HBTA– anion at δ =
156.88–160.78 ppm in the 13C NMR spectra was found for
every HBTA– ionic liquid. No hydrated or solvent water
molecules were found in the ionic liquids from NMR spec-
troscopy and elemental analysis.
X-ray Crystallography
In the reported crystal structures of the energetic metallic
HBTA– salts and energetic BTA2– salts, ligands such as
H2O, NH3, or other organic ligands are necessary to stabi-
lize the crystal structures.[15] Moreover, the single-crystal
structure of a nonmetallic HBTA– salt has not been re-
ported previously. Herein, two crystals of 1 and 7 suitable
for X-ray diffraction were obtained by slow recrystallization
from methanol/ethyl acetate solutions. Both of them con-
tain no water or solvent molecules. The structure of 1 is the
first single-crystal structure of a nonmetallic HBTA– salt.
The crystallographic and structural refinement data of 1
Table 1. Selected bond lengths [Å] and angles [°] of 1 and 7.
1
Bond length [Å]
7
Bond length [Å]
N1–C1
N1–N2
N2–N3
N3–N4
N4–C1
N5–C1
N5–C2
N6–C2
N15–N16
N19–C9
N20–C6
N20–C8
1.331(2)
1.353(2)
1.311(2)
1.361(2)
1.322(2)
1.388(2)
1.347(2)
1.330(2)
1.368(2)
1.465(2)
1.372(2)
1.464(2)
N1–C1
N1–N2
N2–N3
N3–N4
N4–C1
N5–C1
N5–C2
N6–C2
N9–C2
N16–N17
N21–C16
N22–C15
1.334(4)
1.354(4)
1.303(4)
1.358(4)
1.334(4)
1.376(4)
1.383(4)
1.334(4)
1.340(4)
1.309(4)
1.466(5)
1.464(5)
1
Bond angle [°]
7
Bond angle [°]
C1–N1–N2
N3–N2–N1
N2–N3–N4
C1–N4–N3
C1–N5–C2
C2–N9–N8
N4–C1–N5
N4–C1–N1
N9–C2–N5
103.70(14)
109.31(14)
109.94(14)
103.18(14)
125.67(15)
107.67(14)
125.31(16)
113.86(15)
127.48(16)
C1–N1–N2
N3–N2–N1
N2–N3–N4
N4–C1–N1
C1–N5–C2
N6–C2–N9
C2–N6–N7
N6–N7–N8
N7–N8–N9
C5–N19–C7
C5–N20–C6
104.2(2)
108.9(2)
110.9(2)
113.0(3)
127.0(3)
113.3(3)
103.2(2)
110.1(2)
110.3(2)
109.0(3)
108.5(3)
Figure 1. Top: molecular structure of 1 (displacement ellipsoids
shown at 30% probability). Hydrogen atoms represented by spheres
of arbitrary radius. Bottom: packing diagram of 1 viewed down the
b axis.
N20–C5–N19 108.48(16)
C7–C6–N20 107.41(16)
C6–C7–N19 106.92(16)
N20–C5–N19 107.6(3)
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N4–C1–N5 [125.31(16)°], and N9–C2–N5 [127.48(16)°] Table 2. Lengths [Å] and angles [°] for hydrogen bonds in 1 and 7.
angles suggest that the two tetrazole rings in HBTA– ionic
liquids are not coplanar.
D–H···A
1[a]
d(D–H) d(H···A) d(D···A) D–H–A
The packing structure of 1 viewed along the b axis is
built up by hydrogen bonding (Figure 1). Each HBTA–
anion is surrounded by one 1,3-dimethylimidazolium cat-
ion. The imino group and the nitrogen atoms on the tetraz-
ole rings in each HBTA– anion may form intramolecular
[N9–H9···N4, 2.898(2) Å] and intermolecular hydrogen
bonds between the neighboring HBTA– anions [N5–
H5···N15 2.916(2) Å, N9–H9···N10 2.809(2) Å, N14–
H14···N1 2.828(2) Å, and N18–H18···N12 2.877(2) Å;
Table 2]. Charge delocalization over the whole molecule is
accomplished by extensive hydrogen bonding in the ex-
tended structure, which may enhance the decrease of C–N
bond lengths in the HBTA– anion. The combination of this
N5–H5···N151
N9–H9···N4
0.88(2)
0.93(2)
0.93(2)
0.88(2)
0.95(2)
2.04(2)
2.43(2)
1.99(2)
1.95(2)
1.97(2)
2.916(2) 173.2(2)
2.898(2) 111.2(18)
2.809(2) 146(2)
2.828(2) 173.9(2)
2.877(2) 158.9(18)
N9–H9···N102
N14–H14···N13
N18–H18···N124
7[b]
N5–H5···N101
0.86(4)
0.86(3)
2.10(4)
2.11(3)
2.934(4) 165(3)
2.900(4) 152.5(3)
N14–H14···N12
[a] 1: x, –1 + y, z; 2: 3/2 – x, –1/2 + y, 1/2 – z; 3: x, 1 + y, z; 4: 2 –
x, 1 – y, –z. [b] 1: –x, –1/2 + y, 1/2 – z; 2: –x, 1/2 + y, 1/2 – z.
extensive hydrogen bonding in the extended structure re- N5–C2 [1.347(2) Å] bond lengths of the nitrogen atoms
sults in a three-dimensional supramolecular network. linked to the two tetrazole rings in 1, the lengths of those
Compound 7 crystallizes in the monoclinic space group in 7 are nearly the same [N5–C1 1.376(4) Å, N5–C2
P21/c with a density of 1.221 gcm–3 and a unit cell volume 1.383(4) Å]. This is maybe because the two tetrazole rings
of 4063.15(15) Å3 (Table 5). In the crystal structure of 7 in the BTA2– anion are in similar chemical environments.
(Figure 2), the C–N and N–N bond lengths of the tetraz- However, in the HBTA– anion, the two tetrazole rings are
ole rings and imidazole rings also show multiple bond char- different and exist as one neutral tetrazole and one tetrazol-
acter (Table 1). Compared with the N5–C1 [1.388(2) Å] and ate anion.
Figure 2. Top: molecular structure of 7 (displacement ellipsoids shown at 30% probability). Hydrogen atoms represented by spheres of
arbitrary radius. Bottom: packing diagram of 7 viewed down the b axis.
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The packing structure of 7 is built up by hydrogen bond-
ing along the b axis (Figure 2). In contrast to the structure
of 1, every BTA2– anion is surrounded by two imidazolium
cations. There are no intramolecular hydrogen bonds be-
cause only one hydrogen atom in each anion is located on
the amino group, which is far away from the other nitrogen
atoms in the tetrazole rings. Only intermolecular hydrogen
bonds exist between the amino group and the neighboring
nitrogen atoms of the tetrazole rings [N5–H5···N10
2.934(4) Å, N14–H14···N1 2.900(4) Å; Table 2]. The rela-
tively weaker hydrogen-bond interactions in 7 result in a
three-dimensional supramolecular network that is different
from that of 1.
NBO Analysis
H2BTA is a weak acid that may reversibly release one or
two protons to form HBTA– and BTA2– anions (Scheme 4).
This reversible ionization can be controlled by the addition
of different amounts of base, so that HBTA– can be trans-
formed into BTA2– by the change of the basicity of sys-
tem.[15] The natural bond order (NBO) analysis data of six
N-substituted imidazolium cations is shown in Table 3. The
charge densities of C-2 of the imidazole rings was ca.
+0.265 in the five 1-alkyl-3-methylimidazolium cations. No
clear difference was found even as the alkyl chain length-
ened from methyl to octyl. However, when the hydrogen
atom attached to C-2 was substituted by a methyl group
([C1mmim]+), its C-2 charge density increased to +0.472
(Figure 3). An obvious increase of the Lewis basicity of the
1,2,3-trimethylimidazolium cation was confirmed compared
to the other five 1-alkyl-3-methylimidazolium cations. As
ionic liquid 6 slowly recrystallized, the HBTA– ion trans-
formed into BTA2–, and a divalent BTA2– salt of 7 was
obtained.
Figure 3. NBO analysis of [C1mim]+ and [C1mmim]+.
trogen-rich ionic liquids containing the 5-aminotetrazolate
anion.[11d] The high nitrogen content originates from the
nitrogen atoms of the HBTA– and BTA2– anions. The
densities of 1–7 range from 1.22 to 1.51 gcm–3.
Ionic liquids 1–7 are thermally stable to more than
220 °C. No water release was observed in the thermogravi-
metric analysis (TGA) curves. Their considerable thermal
stabilities are comparable to those of neutral H2BTA and
their analogous copper BTA2– salts.[15] Hydrogen bonding
could play the predominant role in determining their ther-
mal decomposition temperatures. As confirmed by the crys-
tal structures, extensive hydrogen bonds exist in the ionic
liquids, and these hydrogen bonds can enhance their ther-
mal stabilities.
Scheme 4. Reversible deprotonation of H2BTA.
Table 3. The C-2 charge densities of different substituted imid-
azolium cations.
[C1mim]+ [C2mim]+ [C4mim]+ [C6mim]+ [C8mim]+ [C1mmim]+
0.264
0.265
0.265
0.265
0.265
0.472
The melting points/glass transition temperatures (Tm/Tg)
were observed by differential scanning calorimetry (DSC)
from the first heating cycle. The Tm of 1 is 48 °C, which
is lower than that of 7 (77 °C). For 2–5, which are room-
Physicochemical Properties
The physicochemical properties of the ionic liquids 1–7 temperature liquids, Tg values instead of Tm values were
are summarized in Table 4. These nitrogen-rich ionic liquids observed. Their Tg values ranged from –31 to –15 °C. The
have high nitrogen contents above 40%. For 1 in particular, liquidus range of the ionic liquids 2–5 may excess 240 °C.
the nitrogen content reaches 61.78%. The nitrogen contents The asymmetry of the cations of these ionic liquids could
of 1–7 are even higher than those of the corresponding ni- lead to low Tm/Tg values. For 6, no obvious Tm or Tg was
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Table 4. Physicochemical properties of 1–7.
1
2
3
4
5
6
7
MW[a]
249.27
48
226
61.78
1.51
700.1
5085.8
5078.9
752.3
3018
Ͼ40
263.3
–15
226
58.52
1.47
676.9
5737.1
5731.6
725.6
2756
Ͼ40
291.32
–22
228
52.89
1.42
642.7
7050.0
7046.9
682.3
2342
Ͼ40
319.37
–20
223
48.24
1.37
614.5
8339.8
8339.2
615.9
1928
Ͼ40
347.42
–31
223
44.35
1.34
584.6
9619.7
9621.6
539.6
1553
Ͼ40
263.13
–
225
58.52
1.44
655.2
5663.9
5658.3
652.3
2478
Ͼ40
373.45
77
228
48.73
1.22
791.9
9523.8
9570.9
774.7
2074
Ͼ40
Tm/Tg [°C][b]
Td [°C][c]
N[%][d]
ρ [gcm–3][e]
ΔfHcal [kJmol–1][f]
–ΔcU [kJmol–1][g]
–ΔcH [kJmol–1][h]
ΔfHexp [kJmol–1][i]
ΔfH [kJ kg–1][j]
IS [J][k]
FS [N][l]
Ͼ360
Ͼ360
Ͼ360
Ͼ360
Ͼ360
Ͼ360
Ͼ360
[a] Molecular weight. [b] Melting point. [c] Decomposition temperature. [d] Nitrogen content. [e] Density at 25 °C. [f] Calculated molar
enthalpy of formation. [g] Experimental (constant-volume) energy of combustion. [h] Experimental molar enthalpy of combustion. [i]
Experimental molar enthalpy of formation. [j] Specific enthalpy of formation. [k] Impact sensitivity. [l] Friction sensitivity.
4: C12H21N11(l) + 69/4O2(g)Ǟ12CO2(g) + 21/2H2O(l) + 11/2N2(g)
observed from –80 to 200 °C. The introduction of imid-
azolium cations improved the properties of the liquids and
enhanced their thermal stabilities effectively. The Coulomb
interactions and hydrogen-bond network mainly affect their
physicochemical property. The high delocalization of the
ionic charges and the asymmetry of the cations results in
weak Coulomb interactions between anions and cations
and, thus, low lattice energies and melting points.
The theoretical heats of formation of the ionic liquids 1–
7 are between 584.6 and 791.9 kJmol–1. The theoretical
heat of formation of 1 is 700.1 kJmol–1, which is the highest
among the six nitrogen-rich ionic liquids with HBTA–
monoanions. Their heats of formation decrease when longer
alkyl groups are introduced into the structures of the imid-
azolium cations.
The experimentally determined constant-volume energies
of combustion (ΔcU) of the ionic liquids 1–7 were measured
by using oxygen bomb calorimetry. The heat of combus-
tion, ΔcHo, was calculated from ΔcU and a correction for
change in gas volume during combustion was included:
ΔcHm = ΔcUm + ΔnRT, in which Δn is the difference in
the number of moles of gas between the products and the
reactants. The standard heats of formation of 1–7, ΔfHo,
were back-calculated from the heats of combustion on the
basis of combustion equations (I)–(VII), Hess’s Law as ap-
plied to thermochemical equations (1)–(7).[18]
(IV)
ΔfHo
(4, l) = 12ΔfHo
(CO2, g) + 21/2ΔfHo
298
(H2O, l) –
(4)
298
298
ΔcHo (4, l)
298
5: C14H25N11(l) + 81/4O2(g)Ǟ14CO2(g) + 25/2H2O(l) + 11/2N2(g)
(V)
ΔfHo
(5, l) = 14ΔfHo
(CO2, g) + 25/2ΔfHo
298
(H2O, l) –
(5)
298
298
ΔcHo (5, l)
298
6: C8H13N11(s) + 45/4O2(g)Ǟ8CO2(g) + 13/2H2O(l) + 11/2N2(g)
(VI)
ΔfHo298 (6, s) = 8ΔfHo298 (CO2, g) + 13/2ΔfHo298 (H2O, l) – ΔcHo
(6, s)
298
(6)
7: C14H23N13(s) + 79/4O2(g)Ǟ14CO2(g) + 23/2H2O(l) + 13/2N2(g)
(VII)
ΔfHo
(7, s) = 14ΔfHo
(CO2, g) + 23/2ΔfHo
298
(H2O, l) –
(7)
298
298
ΔcHo (7, s)
298
From the experimentally determined heats of combus-
tion and the known heats of formation of CO2
(–393.51 kJmol–1) and H2O (–285.83 kJmol–1),[19] the heats
of formation of 1–7 are in the range 539.6–774.7 kJmol–1.
The experimental heats of formation of these ionic liquids
are in good agreement with the theoretical data. Of these
nitrogen-rich ionic liquids, salt 1 possesses the highest spe-
cific heat of formation of 3018 kJkg–1. Although 7 has the
highest heat of formation per mole, its specific heat of for-
mation is relatively low at 2074 kJkg–1 because of its larger
molecular weight owing to its two cations.
1: C7H11N11(s) + 39/4O2(g)Ǟ7CO2(g) + 11/2H2O(l) + 11/2N2(g)
(I)
ΔfHo298 (1, s) = 7ΔfHo298 (CO2, g) + 11/2ΔfHo298 (H2O, l) – ΔcHo
298
(1, s)
(1)
Ionic liquids 1–7 exhibit positive heats of formation,
which illustrates that these ionic liquids may be highly ener-
getic. The high heats of formation of the molecules are de-
rived from the large number of N–N and N–C bonds in
2: C8H13N11(l) + 45/4O2(g)Ǟ8CO2(g) + 13/2H2O(l) + 11/2N2(g)
(II)
ΔfHo298 (2, l) = 8ΔfHo298 (CO2, g) + 13/2ΔfHo298 (H2O, l) – ΔcHo
(2,l)
298
(2) the nitrogen-rich HBTA– anions. The molecules with higher
nitrogen content have increased heats of formation. The
neutral H2BTA has more than 80% nitrogen content and a
high heat of formation (633 kJmol–1). Therefore, these ionic
3: C10H17N11(l) + 57/4O2(g)Ǟ10CO2(g) + 17/2H2O(l) + 11/2N2(g)
(III)
ΔfHo
(3, l) = 10ΔfHo
(CO2, g) + 17/2ΔfHo
(H2O, l) –
298
liquids should have higher values for their heats of forma-
(3) tion. These values are substantially higher than that of
298
298
ΔcHo (3, l)
298
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FULL PAPER
TNT (–67 kJmol–1, –0.29 kJg–1).[20] Compared with other are insensitive toward impact and friction.[23] In the pres-
1-alkyl-3-methylimidazolium cation-based energetic salts, ence of dialkylimidazolium cations, HBTA– and BTA2–
the heats of formation of 1–7 are increased markedly. For anions are potential nitrogen-rich structures for the con-
instance, the heat of formation of 3 with the 1-butyl-3- struction of insensitive/safe energetic ionic liquids.
methylimidazolium cation is ΔfH° = 638.8 kJmol–1, which
is much higher than those of the corresponding ionic li-
quids including dicyanamide (266.4 kJmol–1), nitrocyana-
mide (128.5 kJmol–1), 5-aminotetrazolate (305.9 kJmol–1),
and dinitromethanide (–72.2 kJmol–1).[9,14c]
Conclusions
Nitrogen-rich energetic ionic liquids based on dialkyl-
imidazolium cations and N,N-bis(1H-tetrazol-5-yl)amine
anions (HBTA–/BTA2–) were synthesized under mild condi-
tions. Their structures were verified by IR and NMR spec-
troscopy, elemental analysis, and high-resolution mass spec-
trometry. The single-crystal data of 1 showed the exact
structure of the first nonmetallic salt with the HBTA–
monoanion. The crystal structure of 7 reveals that ionic li-
quids with HBTA– monoanions may convert to those with
BTA2– dianions under certain conditions. Ionic liquids 1–7
are thermally stable to more than 220 °C. Among them, 2–5
are liquid at room temperature with liquid ranges exceeding
240 °C. The nitrogen contents of 1–7 are all higher than
40%. Ionic liquid 1 has the highest nitrogen content
(61.78%). Their experimental standard heats of formation
are between 539.6 and 774.7 kJmol–1, which are concordant
with the theoretical heats of formation. Ionic liquids 1–7
are insensitive to impact (Ͼ40 J) and friction (Ͼ360 N).
These CHN-type ionic liquids showed good combustion
characteristics in air after flame ignition. These new nitro-
gen-rich energetic ionic liquids are of interest as potential
safe and environmentally friendly liquid propellants.
Combustion Test
The ionic liquids 1–7 were examined by the hypergolic
ignition test with white fuming nitric acid (WFNA, 100%
HNO3) to monitor their reactivity and evaluate their per-
formance. After the ionic liquids were dropped into
WFNA, no flames were observed, which indicates that no
hypergolic processes occurred. This result suggests that
these ionic liquids cannot be ignited by the WFNA oxidizer.
In 2006, Rogers et al. revealed the combustible character-
istic of many energetic ionic liquids.[21] Combustible ionic
liquids may be of interest as propellants that can be ignited
in air. Therefore, the combustion tests of 1–7 in air at room
temperature were performed. With ignition by flame, the
ionic liquids 1–7 ignited in a very short time and self-sus-
tained combustion in air. For example, the combustion be-
havior of the ionic liquid 2 before and after ignition is
shown in Figure 4. These ionic liquids contain only C, H,
and N. Thus, no halogen or sulfuric byproducts will be gen-
erated during the combustion. These new nitrogen-rich
energetic ionic liquids are environmental friendly during
their energy release processes and could exhibit good com-
bustible characteristics.
Experimental Section
Caution: We have not experienced any problems in handling these
compounds. Although they are impact- and friction-insensitive ma-
terials, we strongly suggest that they should be handled with ex-
treme care by using all standard safety precautions such as leather
gloves, leather coat, face shields, and ear plugs because of their
high nitrogen content.
General Methods: All chemicals of analytically pure grade were ob-
tained commercially and used as received. 1-Alkyl-3-methylimid-
azolium halides and 1,2,3-trimethylimidazolium iodide were pre-
pared according to the literature procedure.[24] Infrared spectra (IR)
were recorded with a NEXUS 670 FT-IR spectrometer by using
KBr pellets. 1H and 13C NMR spectra were recorded with a Bruker
400 MHz nuclear magnetic resonance spectrometer with samples in
[D6]dimethyl sulfoxide ([D6]DMSO). 1H and 13C chemical shifts are
reported in ppm relative to tetramethylsilane (TMS). Differential
scanning calorimetry (DSC) measurements were performed with a
TA DSC Q200 PC calorimeter equipped with an autocool access-
ory and calibrated by using standard pure indium, which was
gently flooded with N2 with a flow rate of 20 mLmin–1. Samples
were sealed in aluminum pans with a hole (1 μm) on the top under
nitrogen gas. Measurements were performed from –80 to 200 °C at
10 °Cmin–1. Thermogravimetric analysis (TGA) measurements
were accomplished with a NETZSCH TG 209F1 thermogravime-
tric analyzer by heating samples at 10 °Cmin–1 from 25 to 500 °C
in a dynamic nitrogen atmosphere at a flow rate of 70 mLmin–1.
Elemental analyses were performed with a Carlo–Erba 1106 Ele-
Figure 4. The combustion behavior of the ionic liquid 2.
Impact and Friction Sensitivities
The impact and friction sensitivities of the ionic liquids
1–7 were determined by using the standard BAM
method.[22] The collected impact sensitivity (IS) and friction
sensitivity (FS) data are summarized in Table 4. All of the
ionic liquids 1–7 are stable at 40 J (10 kg hammer, 40 cm)
in fall-hammer experiments. Their friction sensitivities ex-
ceed 360 N. Based on the UN standard, these ionic liquids mental Analyzer. High-resolution mass spectrometry (HRMS)
Eur. J. Inorg. Chem. 0000, 0–0
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FULL PAPER
measurements were performed with a Waters Q-TOF Premier mass
spectrometer.
Theoretical Study: Computations were performed by the
Gaussian03 (Revision E.01) suite of programs.[25] The geometric
optimization, frequency analyses, and NBO analyses were per-
formed by using the popular Becke three-parameter hybrid func-
tional Lee–Yang–Parr (B3LYP) analyses with the 6-31+G** basis
set. Single-point energies (of gas-phase geometries) were calculated
at the level of Møller-Plesset second-order perturbation theory
truncated at the second order (MP2) with the 6-311++G** basis
set. All of the optimized structures were characterized to be true
local energy minima on the potential energy surface without imagi-
nary frequencies.
Scheme 6. Born–Haber cycle for the heats of formation of the ionic
liquids 1–7.
The theoretical heats of formation of the cations and anions were
computed by using the method of isodesmic reactions (Scheme 5).
The sources of the energies of the parent ions in the isodesmic
reactions were calculated from protonation reactions [ΔfHo(H+) =
ΔfHo(ionic liquid, 298 K) = ΣΔfHo(cation, 298 K) + ΣΔfHo(anion,
298 K) – ΔHL
(7)
+
+1528.085 kJmol–1].[26] The heats of formation of NH2NH3
where ΔHL is the lattice energy of the ionic salt. ΔHL (kJmol–1)
(+770.0 kJmol–1), the HBTA– anion (+500.1 kJmol–1), and the
BTA2– anion (+630.0 kJmol–1) were taken from ref.[15e] The en-
thalpies of reaction (ΔrHo298) were obtained by combining the
MP2/6-311++G** energy differences for the reaction, the scaled
zero point energies, and other thermal factors.
can be predicted by the formula suggested by Jenkins et al.:[27]
ΔHL = UPOT + [p(nM/2–2) + q(nX/2–2)]RT
(8)
nM and nX depend on the nature of the ions Mp+ and Xq–, respec-
tively, and have a value of six for nonlinear polyatomic ions. The
equation for lattice potential energy UPOT has the form:
The theoretical heats of formation of the ionic liquids at T =
298.15 K were calculated based on a Born–Haber energy cycle
(Scheme 6). In a Born–Haber energy cycle, the heat of formation
of an ionic liquid can be simplified by the expression:
UPOT = γ(ρm/Mm)1/3 + δ
(9)
Scheme 5. The isodesmic and protonation reactions of the ions in ionic liquids 1–7.
Eur. J. Inorg. Chem. 0000, 0–0
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ρm is density (gcm–3) and Mm is the chemical formula mass of the
ionic liquid. For MX (1:1) salts, γ is 1981.2 and δ is 103.8. Thus,
UPOT of ionic liquids 1–6 is given by:
from the average of three separate measurements with standard de-
viations calculated as a measure of experimental uncertainty. The
bomb was examined for evidence of unburned carbon after each
run. In all measurements, the calorimeter was calibrated by the
combustion of certified benzoic acid in an oxygen atmosphere at
the same pressure of 3.05 MPa combined with a correction for
burned wire.
UPOT = 1981.2(ρm/Mm)1/3 + 103.8
(10)
For M2X (1:2) salts, γ is 8375.6 and δ is –178.8. Therefore, UPOT
of ionic liquid 7 is given by:
Impact and Friction Sensitivity: Impact tests were performed with
a BCJ impact sensitivity instrument. Test specimens were kept be-
tween two hardened anvils and a 10 kg drop weight was allowed to
fall freely from different heights. Ten tests were conducted for each
compound. The results are reported in terms of height for 50%
probability of explosion of the sample. The friction tests were per-
formed on a BMJ friction sensitivity instrument. The samples were
kept between a fixed corrugated (rough) stainless steel plate and a
movable stainless steel plate. Friction was generated by pulling the
movable plate at varying lever loads.
UPOT = 8375.6(ρm/Mm)1/3–178.8
(11)
X-ray Crystallography: Single crystals of 1 or 7 were removed from
the flask, and a suitable crystal was selected and attached to a glass
fiber. The data were collected by using an Xcalibur Eos dif-
fractometer with graphite-monochromated Mo-Kα radiation (λ =
0.71073). A Cryocool NeverIce low-temperature device was used to
keep the crystals at a constant 134.95(10) K during data collection.
The data were measured by using omega scans of 0.3° per frame
for 5 seconds, and a full sphere of data was collected. By using
Olex2,[28] the structures were solved with the SHELXS structure
solution program by direct methods and refined with the SHELXL
refinement package by using least-squares minimization.[29] No de-
composition was observed during data collection. Details of the
data collection and refinement are given in Table 5.
1,3-Dimethylimidazolium N,N-Bis(1H-tetrazol-5-yl)amine (1): 1,3-
Dimethylimidazolium iodide (448 mg, 2.0 mmol) was dissolved in
methanol (20 mL). Then, silver N,N-bis(1H-tetrazol-5-yl)amine
(AgHBTA, 780 mg, 3.0 mmol) was added to the solution. The re-
sulting mixture was stirred in the dark overnight. Insoluble excess
AgHBTA along with the precipitated silver iodide byproduct were
removed. The filtrate was collected and dried in vacuo to yield
white solid 1 (467 mg, 94%). Clear colorless plate crystals of 1 suit-
able for X-ray structure determination were obtained after
recrystallization from methanol/ethyl acetate. 1H NMR (400 MHz,
[D6]DMSO, 25 °C, TMS): δ = 9.38 (s, 1 H), 7.67 (s, 2 H), 3.86 (s,
6 H) ppm. 13C NMR (400 MHz, [D6]DMSO, 25 °C, TMS): δ =
Table 5. Crystal data and structure refinement for 1 and 7.
1
7
Formula
FW [gmol–1]
Size [mm]
Crystal system
Space group
a [Å]
b [Å]
c [Å]
α [°]
C7H11N11
249.27
0.34ϫ0.30ϫ0.12
monoclinic
P21/n
14.0653(4)
6.9507(2)
22.4699(7)
90.00
C14H23N13
373.45
0.34ϫ0.30ϫ0.25
monoclinic
P21/c
160.77, 137.86, 123.61, 35.84 ppm. IR (KBr): ν = 3415, 3156, 3111,
˜
1577, 1497, 1415, 1174, 1120, 840, 752, 630 cm–1. C7H11N11
(249.27): calcd. C 33.73, H 4.41; found C 33.67, H 4.67.
17.5869(3)
12.8757(2)
21.9203(6)
90.00
125.0580(10)
90.00
4063.15(15)
8
1.221
1-Ethyl-3-methylimidazolium N,N-Bis(1H-tetrazol-5-yl)amine (2): A
similar procedure was followed as that described above for 1. 1-
Ethyl-3-methylimidazolium bromide (382 mg, 2.0 mmol) and
AgHBTA (780 mg, 3.0 mmol) were reacted in methanol to obtain a
colorless liquid 2 (505 mg, 96%). 1H NMR (400 MHz, [D6]DMSO,
25 °C, TMS): δ = 9.20 (s, 1 H), 7.78 (s, 1 H), 7.69 (s, 1 H), 4.18 (q,
J = 7.2 Hz, 2 H), 3.84 (s, 3 H), 1.40 (t, J = 7.2 Hz, 3 H) ppm. 13C
NMR (400 MHz, [D6]DMSO, 25 °C, TMS): δ = 156.97, 137.48,
β [°]
93.996(3)
90.00
2191.38(12)
8
1.511
134.95(10)
0.111
γ [°]
V [Å3]
Z
ρ [gcm–3]
T [K]
134.95(10)
0.085
1584.0
μ [mm–1]
F [000]
1040.0
135.79, 123.73, 122.15, 44.26, 35.85, 15.33 ppm. IR (KBr): ν =
˜
λ(Mo-Kα) [Å]
Reflections
Rint
0.71073
8643
0.0224
337
1.040
0.0408
0.0889
0.0534
0.0953
0.71073
15294
0.0199
504
1.103
0.0747
0.1686
0.0920
0.1770
3392, 3151, 3105, 2877, 1604, 1571, 1460, 1377, 1256, 1168, 1112,
827, 768, 620 cm–1. HRMS (ESI): calcd. for C8H13N11 [M]+
111.0917; found 111.0916; [M]– 152.0439; found 152.0440.
C8H13N11 (263.13): calcd. C 36.50, H 4.98; found C 36.17, H 5.02.
Parameters
S on F2
R1 [IϾ2σ(I)][a]
wR2 [IϾ2σ(I)][b]
R1 [all data][a]
wR2 [all data][b]
Δρmin and max [eÅ–3]
1-Butyl-3-methylimidazolium N,N-Bis(1H-tetrazol-5-yl)amine (3):
The same procedure was followed as that described above for 1. 1-
Butyl-3-methylimidazolium bromide (438 mg, 2.0 mmol) and
AgHBTA (780 mg, 3.0 mmol) were reacted in methanol to obtain
a pale yellow liquid 3 (548 mg, 94%). 1H NMR (400 MHz, [D6]-
DMSO, 25 °C, TMS): δ = 9.37 (s, 1 H), 7.81 (s, 1 H), 7.74 (s, 1 H),
4.19 (t, J = 7.2 Hz, 2 H), 3.86 (s, 3 H), 1.78 (m, 2 H), 1.23 (m, 2
H), 0.88 (t, J = 6.8 Hz, 3 H) ppm. 13C NMR (400 MHz, [D6]-
DMSO, 25 °C, TMS): δ = 159.95, 137.07, 123.78, 122.50, 48.63,
0.19 and –0.24
0.31 and –0.29
[a] R1 = Σ||Fo| – |Fc||/Σ|Fo|. [b] wR2 = [Σw(Fo – Fc ) /Σw(Fo2)2]1/2
.
2
2 2
CCDC-913585 (for 7) and -913586 (for 1) contain the supplemen-
tary crystallographic data for this paper. These data can be ob-
tained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
36.01, 31.69, 19.00, 13.55 ppm. IR (KBr): ν = 3425, 3141, 3069,
˜
2960, 2871, 1616, 1570, 1464, 1378, 1340, 1170, 1113, 841, 756,
622 cm–1. HRMS (ESI): calcd. for C10H17N11 [M]+ 139.1230; found
139.1230; [M]– 152.0439; found 152.0437. C10H17N11 (291.32):
calcd. C 41.23, H 5.88; found C 41.17, H 6.25.
Bomb Calorimetry: The calorimetric measurements were performed
with a Parr 6725 bomb calorimeter (static jacket) equipped with a
Parr 207A oxygen bomb. The samples were carefully dropped into
the platinum crucible and were subsequently burned in a 3.05 MPa
atmosphere of pure oxygen; Parr 45C10 alloy fuse wire was used
for ignition. The experimental heats of combustion were obtained
1-Hexyl-3-methylimidazolium N,N-Bis(1H-tetrazol-5-yl)amine (4):
The same procedure was followed as that described above for 1. 1-
Hexyl-3-methylimidazolium bromide (495 mg, 2.0 mmol) and
Eur. J. Inorg. Chem. 0000, 0–0
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FULL PAPER
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AgHBTA (780 mg, 3.0 mmol) were reacted in methanol to obtain a
colorless liquid 4 (607 mg, 95%). 1H NMR (400 MHz, [D6]DMSO,
25 °C, TMS): δ = 9.32 (s, 1 H), 7.78 (s, 1 H), 7.71 (s, 1 H), 4.18 (t,
J = 7.2 Hz, 2 H), 3.86 (s, 3 H), 1.74 (m, 2 H), 1.25 (m, 6 H), 0.86
(t, J = 6.8 Hz, 3 H) ppm. 13C NMR (400 MHz, [D6]DMSO, 25 °C,
TMS): δ = 159.49, 137.18, 123.80, 122.48, 48.91, 35.97, 30.83,
29.69, 25.39, 22.15, 14.10 ppm. IR (KBr): ν = 3392, 3147, 3080,
˜
2927, 2856, 1603, 1571, 1464, 1375, 1169, 1114, 841, 754, 654,
623 cm–1. HRMS (ESI): calcd. for C12H21N11 [M]+ 167.1543; found
167.1541; [M]– 152.0439; found 152.0440. C12H21N11 (319.37):
calcd. C 45.13, H 6.63; found C 45.37, H 6.84.
[2]
1-Methyl-3-octylimidazolium N,N-Bis(1H-tetrazol-5-yl)amine (5):
The same procedure was followed as that described above for 1. 1-
Methyl-3-octylimidazolium bromide (547 mg, 2.0 mmol) and
AgHBTA (780 mg, 3.0 mmol) were reacted in methanol to obtain a
colorless liquid 5 (667 mg, 96%). 1H NMR (400 MHz, [D6]DMSO,
25 °C, TMS): δ = 9.35 (s, 1 H), 7.78 (s, 1 H), 7.71 (s, 1 H), 4.16 (t,
J = 7.2 Hz, 2 H), 3.85 (s, 3 H), 1.75 (m, 2 H), 1.23 (m, 10 H), 0.85
(t, J = 6.8 Hz, 3 H) ppm. 13C NMR (400 MHz, [D6]DMSO, 25 °C,
TMS): δ = 159.65, 137.13, 123.79, 122.43, 48.91, 35.97, 31.45,
[3]
[4]
29.76, 28.78, 25.76, 22.34, 14.21 ppm. IR (KBr): ν = 3373, 3253,
˜
3134, 3084, 2959, 2863, 1616, 1570, 1464, 1343, 1169, 1110, 837,
760, 656, 623 cm–1. HRMS (ESI): calcd. for C14H25N11 [M]+
195.1856; found 195.1853; [M]– 152.0439; found 152.0440.
C14H25N11 (347.42): calcd. C 48.40, H 7.25; found C 48.76, H 7.52.
[5]
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1,2,3-Trimethylimidazolium N,N-Bis(1H-tetrazol-5-yl)amine (6):
The same procedure was followed as that described above for 1.
1,2,3-Trimethylimidazolium iodide (476 mg, 2.0 mmol) and
AgHBTA (780 mg, 3.0 mmol) were reacted in methanol to obtain
a white solid 6 (350 mg, 94%). 1H NMR (400 MHz, [D6]DMSO,
25 °C, TMS): δ = 7.55 (s, 2 H), 3.72 (s, 6 H), 2.51 (s, 3 H) ppm.
13C NMR (400 MHz, [D6]DMSO, 25 °C, TMS): δ = 156.88, 145.17,
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[7]
[8]
122.39, 35.12, 9.54 ppm. IR (KBr): ν = 3441, 3072, 2904, 1633,
˜
1518, 1414, 1251, 1135, 759, 652, 564 cm–1. HRMS (ESI): calcd.
for C8H13N11 [M]+ 111.0917; found 111.0915; [M]– 152.0439; found
152.0438. C8H13N11 (263.13): calcd. C 36.50, H 4.98; found C
36.82, H 5.20.
Bis(1,2,3-Trimethylimidazolium)
N,N-Bis(1H-tetrazol-5-yl)amine
(7): Transparent colorless plate crystals of 7 suitable for X-ray
structure determination were obtained after recrystallization of 6
from methanol/ethyl acetate. 1H NMR (400 MHz, [D6]DMSO,
25 °C, TMS): δ = 7.58 (s, 2 H), 3.74 (s, 6 H), 2.54 (s, 3 H) ppm.
13C NMR (400 MHz, [D6]DMSO, 25 °C, TMS): δ = 160.56, 144.63,
[9]
121.92, 34.77, 8.93 ppm. IR (KBr): ν = 3437, 3081, 2893, 1632,
˜
1517, 1254, 1136, 767, 649 cm–1. C14H23N13 (373.45): calcd. C
45.02, H 6.16; found C 45.13, H 6.02.
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Supporting Information (see footnote on the first page of this arti-
1
cle): H and 13C NMR, IR, MS, TGA, and DSC data of 1–7.
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Acknowledgments
G. H. T. gratefully acknowledges the support of the National Natu-
ral Science Foundation of China (NSFC) (grant numbers
21103116, J1210004), and the Specialized Research Fund for the
Doctoral Program of Higher Education (grant number
20100181120042). L. H. gratefully acknowledges the financial sup-
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FULL PAPER
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Received: June 15, 2013
Published Online: ᭿
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FULL PAPER
Energetic Ionic Liquids
L.-L. Dong, L. He, H.-Y. Liu, G.-H. Tao,*
F.-D. Nie, M. Huang, C.-W. Hu* ... 1–12
Nitrogen-rich energetic ionic liquids based
on the N,N-bis(1H-tetrazol-5-yl)amine
anion were synthesized. These ionic liquids
have high nitrogen contents, high heats of
formation, and high stabilities to heat, im-
pact, and friction. These nitrogen-rich
energetic ionic liquids combust in air after
ignition with a flame. They are potential
environmentally friendly liquid propellants.
Nitrogen-Rich Energetic Ionic Liquids
Based on the N,N-Bis(1H-tetrazol-5-yl)-
amine Anion – Syntheses, Structures, and
Properties
Keywords: Energetic materials / Propel-
lants / Ionic liquids / Nitrogen hetero-
cycles / Heats of formation
Eur. J. Inorg. Chem. 0000, 0–0
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© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim