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Energetic Nitrogen-Rich Salts and Ionic Liquids.

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Reviews
J. M. Shreeve et al.
DOI: 10.1002/anie.200504236
Explosives
Energetic Nitrogen-Rich Salts and Ionic Liquids**
Rajendra P. Singh, Rajendar D. Verma, Dayal T. Meshri, and Jeanne M. Shreeve*
Keywords:
density · energetic salts · heat of
formation · ionic liquids ·
thermochemistry
Angewandte
Chemie
3584
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2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 3584 – 3601
Angewandte
Chemie
Energetic Materials
Energetic salts offer many advantages over conventional
energetic molecular compounds. The use of nitrogen
containing anions and cations contributes to high heats of
formations and high densities. Their low carbon and hydrogen
content gives rise to a good oxygen balance. The decomposition of these compounds is predominantly through the
generation of dinitrogen which makes them very promising
candidates for highly energetic materials for industrial or
military applications.
1. Introduction
More energetic materials are consumed in peaceful
applications than in armed conflict; modern advances and
modern life would be impossible without such energetic
species. Energetic materials store relatively large amounts of
energy in a compact and readily deliverable form. The power
output is a function of the rate at which energy is liberated.
Wherever a readily controllable source of energy is required
for periods of time ranging from milliseconds (in guns) to
many seconds (in rockets), propellants which evolve gases are
employed. They are used for propelling projectiles and
rockets, shearing bolts and wires, driving turbines, operating
pumps, moving pistons, and starting engines. When very rapid
rates of energy application and high pressures are essential,
explosives are employed. They are used to produce highintensity shock waves in air, water, and rock; for blasting,
mining, cratering, and other civil engineering purposes; for
cutting, metal welding and forming, and fragmentation; in
shaped charges and many specialty devices requiring high
rates of energy transmission; and for initiation of detonation
phenomena. More than 90 % of the energetic materials used
for industrial purposes are ammonium nitrate based.[1] The
single-component explosives most commonly used for military compositions are HMX (1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane), RDX (1,3,5-trinitro-1,3,5-triazacyclohexane), or TNT (2,4,6-trinitrotoluene), nitroglycerin, and nitrocellulose.[1–12]
There are research programs worldwide to develop highenergy density materials (HEDM) with higher performance
and/or enhanced insensitivity with respect to thermal shock,
friction, and electrostatic discharge. These modern HEDMs
derive most of their energy either from: 1) oxidation of the
carbon backbone, as with traditional energetic materials[13, 14]
or 2) their very high positive heats of formation. Examples of
the first class are: TNT, RDX, and HMX.[15] Examples of the
second class of energetic materials include modern nitrocompounds, such as: CL-20 (2,4,6,8,10,12-(hexanitrohexaaza)cyclododecane),[16, 17] TNAZ (1,3,3-trinitroazetidine),
FOX-7 (1,1-diamino-2,2-dinitroethene),[18] hepta- and octanitrocubanes (which have high densities and energies arising
from their substantial cage strain), 3,3’-azobis(6-amino1,2,4,5-tetrazine), and tetrazole azide.
Recently, a new group of energetic ionic salts containing a
large number of nitrogen atoms has been studied. These socalled “high-nitrogen” compounds form a unique class of
Angew. Chem. Int. Ed. 2006, 45, 3584 – 3601
From the Contents
1. Introduction
3585
2. Triazolium-Based Ionic Salts
3586
3. Tetrazole-Based Salts
3591
4. Urotropinium-Based Salts
3595
5. Tetrazine-Based Salts
3596
6. Azetidinium-Based Salts
3597
7. Picrate-Based Salts
3597
8. Polynitrogen-Containing Salts
3598
9. Imidazolium-Based Salts
3599
10. Miscellaneous
3599
11. Conclusion
3600
energetic materials whose energy is derived from their very
high heats of formation rather than from the overall heat of
combustion. The high heat of formation is directly attributable to the large number of energetic N N and C N bonds.[19]
Generally, heterocyclic molecular compounds have been
utilized in energetic roles owing to their higher heats of
formation, density, and oxygen balance compared to those of
their carbocyclic analogues. Heterocyclic rings (containing
amino, nitro, or azide substituents) paired with nitrate,
perchlorate, dinitramide, or picrate anions form highly
energetic salts. These energetic ionic salts are more environmentally acceptable (with the exception of perchlorate), since
a higher percentage of their decomposition products is
dinitrogen. Another area of increasing interest is based on
high-energy ionic salts in which both the cation and the anion
are high-nitrogen species. These energetic ionic salts possess
advantages over non-ionic molecules owing to their lower
vapor pressures and higher densities. While the use of highnitrogen compounds is likely to be limited because of cost, the
[*] Prof. R. D. Verma, Prof. J. M. Shreeve
Department of Chemistry
University of Idaho
Moscow, ID 83844-2343 (USA)
Fax: (+ 1) 208-885-9146
E-mail: jshreeve@uidaho.edu
Dr. R. P. Singh, Dr. D. T. Meshri
Advance Research Chemicals, Inc.
1110 W. Keystone Avenue, Catoosa, OK 74015 (USA)
[**] J.M.S. acknowledges the AFOSR (F49620-03-1-0209), NSF
(CHE0315275), and ONR (N00014-02-1-0600) for support of work
accomplished at the University of Idaho.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3585
Reviews
J. M. Shreeve et al.
main advantages of such compounds would be for tactical
missiles (low signature) and for applications where low flame
temperature (low gun-barrel corrosion) or low smoke production are advantageous.
Although ionic liquids have been known for nearly a
century, interest in exploring ionic liquids as high-energy
materials, explosives, and propellants has grown markedly in
recent years. The tunable properties associated with ionic
liquids, such as melting point < 100 8C, low vapor pressure,
long liquid range, high thermal and hydrolytic stabilities, and
high density make them likely candidates for energetic
applications. Most hydrolytically stable ionic liquids have
organic cations and inorganic polyatomic anions. Ionic liquids
have been used as solvents and catalysts in green chemistry,
and their chemistry has been summarized in several
reviews.[20–25]
Over the past few years a large number of publications
describing energetic ionic salts and energetic ionic liquids
have appeared but no comprehensive Review of these
materials is currently available. Our continuing interest in
this subject prompts us to review this burgeoning field. This
Review covers the chemistry of energetic ionic salts, including
energetic ionic liquids, from 1999 to the end of 2005.
positions in the ring. A large number of ionic salts that contain
a triazole derivative have been reported to be energetic
materials.
2.1. 1,2,4- and 1,2,3-Triazolium Salts
1,2,4-Triazole and 1,2,3-triazole have positive heats of
formation of 109 kJ mol 1 and 272 kJ mol 1, respectively.[26, 27]
The ionic salts consisting of either of the protonated heterocyclic cations paired with nitrate, perchlorate, and dinitramide
anions were the first to be synthesized.[28] Reactions of 1,2,4triazole or its 1,2,3-isomer with concentrated nitric acid,
perchloric acid, and dinitramidic acid gave excellent yields of
1,2,4- or 1,2,3-triazolium nitrate (1 a) or (2 a), perchlorate (1 b)
or (2 b), and dinitramide (1 c) or (2 c), respectively
(Scheme 1). All the salts were characterized by IR, Raman,
2. Triazolium-Based Ionic Salts
Triazoles are five-membered aromatic heterocycles that
contain three nitrogen atoms located at the 1,2,4- or 1,2,3-
Scheme 1.
Rajendra P. Singh was born in India and
obtained his Ph.D. in 1985 from Banaras
Hindu University. After several teaching and
research projects in Japan and India, he
worked for two years as an ERATO
researcher with Prof. Ryoji Noyori. In 1995,
he became a post-doctoral fellow with Professor D. S. Matteson at Washington State
University before he joined the University of
Idaho’s fluorine research group in 1998.
Since March 2003, he has been associate
research director at Advance Research
Chemicals, Inc. His research interests center
around the development of new synthetic
methodologies in organofluorine chemistry.
Rajendar D. Verma was born in Amritsar
(India). He received his M.Sc. from Birla
Institute of Technology and Science, Pilani in
1955. He taught in a degree college for
8 years before joining Panjab University,
Chandigarh as a research student in 1963.
He was appointed a Lecturer in Chemistry
in 1966 and received his Ph.D. in 1968. He
established a fluorine research group at
Panjab University Department of Chemistry.
Professor Verma retired in 1996. He had the
privilege of collaborating with Professors
George H. Cady, Charles B. Colburn, and
Jean’ne M. Shreeve.
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Dayal Meshri was born in British India. He
received his B.Sc. from Gujarat University,
M.Sc. from the MR Institute of Science, his
Ph.D. from the University of Idaho, and did
postdoctoral work at Cornell University. He
then joined Ozark-Mahoning Co.-Pennwalt
Corp. as Head of Research and Research
Director. In 1987, he founded Advance
Research Chemicals, Inc. (ARC) where he is
President and CEO. ARC has become one of
the largest inorganic fluoride specialty chemical producers in the world and delivers to
automobile, electronic, semiconductor, plastic, and pharmaceutical industries.
Jean’ne M. Shreeve is a Montana native.
She received a B.A. in chemistry from the
University of Montana, an M.S. in analytical
chemistry from the University of Minnesota,
and a Ph.D. in inorganic chemistry from the
University of Washington, Seattle. She
joined the University of Idaho faculty in
1961, became department head in 1973,
and in 1987, assumed the role of VicePresident for Research and Graduate Studies. Since July 2004, she has been the
Jean’ne M. Shreeve Professor of Chemistry.
Her research interests include the syntheses,
characterization, and reactions of new fluorine-containing compounds as
well as energetic compounds.
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Chemie
Energetic Materials
and NMR spectroscopy, material balance, density determination, elemental analysis, DSC (differential scanning calorimetry), TGA (thermogravimetric analysis), and initial
safety testing. 1,2,3-Triazolium salts melt at lower temperatures than their 1,2,4-triazolium analogues (Table S1 in the
Supporting Information). Some of these salts (1 b,c, 2 a–c)
have melting points < 100 8C, categorizing them as ionic
liquids. These salts are energetic and are stable at moderate
temperatures, with the two perchlorate salts the most
thermally stable and the dinitramides the least. A singlecrystal X-ray diffraction study of 1 b showed significant
hydrogen bonding between the perchlorate anion and the
protonated 1,2,4-triazolium ring, which contributes to its
relatively high density of 1.96 g cm 3. Interestingly, the 1,2,3triazolium perchlorate is nearly 0.2 g cm 3 less dense, which is
attributable to less efficient packing in the crystal.
2.2. Triazolium Salts Containing Amino Substituents
From the analysis of the structures of thermally stable
explosives, it has been found that the incorporation of amino
groups into a heterocyclic triazole ring is one of the simplest
routes to enhance thermal stability.[6] The N-amino group
behaves as an electron withdrawing group in these highnitrogen heterocycles that, when paired with nitrate, perchlorate or dinitramide anions, form energetic salts. 4-Amino1,2,4-triazolium salts (3 a–c; Scheme 2), are formed by the
reaction of 4-amino-1,2,4-triazole with concentrated HNO3,
HClO4, and HN(NO2)2, respectively, under reaction conditions as utilized for 1 a–c and 2 a–c.[28]
1-Amino-1,2,4-triazole with concentrated HNO3 or
HClO4 led to the formation of the corresponding salts (4 a)
and (4 b) in excellent yields (Scheme 2).[29] When 1-amino1,2,4-triazole was treated with iodomethane, a quaternary salt
(5) was produced that underwent metathesis with AgNO3 or
AgClO4 to give high yields of the corresponding nitrate (6 a)
and perchlorate (6 b) salts, respectively. Similarly when 1methyl-4-amino-1,2,4-triazolium iodide (7) under went metathesis with AgNO3 or AgClO4, the nitrate (8 a) and perchlorate (8 b) were formed.[30] The syntheses of 1,5-diamino-1,2,4triazolium nitrate (9 a) and perchlorate (9 b) were also
reported (Scheme 2).[29] Characterization by elemental analysis, and vibrational, and multinuclear NMR spectroscopy,
and mass spectrometry confirmed these structures. Singlecrystal X-ray analysis of 8 b showed the existence of
significant hydrogen bonding between the perchlorate anion
and the amino group and that methylation had occurred at
N1. (Physical and thermal characteristics of these salts are
tabulated in Table S2 of the Supporting Information.)[30]
Inspection of these data reveals certain interesting facts
regarding thermal characteristics of the compounds. For
example, the position of the methyl group on the ring does
not influence the melting points (glass transition temperatures) of 1-amino-4-methyl-1,2,4-triazolium nitrate (6 a)
(Tg = 62 8C) and the 1-methyl-4-amino derivative 8 a (Tg =
60 8C). The corresponding perchlorate salts 6 b and 8 b melt
at 91 8C and 86 8C, respectively. However, the nitrate salts of
the 1-amino derivative 3 a (Tm = 69 8C) and the 4-amino
Angew. Chem. Int. Ed. 2006, 45, 3584 – 3601
Scheme 2.
derivative 4 a (Tm = 121 8C), are considerably higher melting
than their methyl-substituted analogues. Clearly, the opportunity for hydrogen bonding, as well as modified packing
effects and reduced lattice energies, is markedly reduced in
the methylated salts. Thermal degradation temperatures for
both nitrate and perchlorate salts are increased when a methyl
group is present in the ring. Introduction of a second amino
functionality causes the melting point (4 b versus 9 b) to
increase and the enthalpy of formation to become less
positive, but appears to have essentially no impact on other
properties. The combustion energies and standard molar
enthalpies of formation for perchlorate salts are higher than those of
nitrates.
The synthesis and characterization of a series of nitrate salts, based
on the 1-alkyl-4-amino-1,2,4-triazolium cations 10 a–d, have been
reported.[31] Single-crystal X-ray
diffraction studies of 1-isopropyl-4amino-1,2,4-triazolium (10 b; tri-
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J. M. Shreeve et al.
clinic, P1̄ symmetry) and methylcyclohexyl-4-amino-1,2,4triazolium (10 d; monoclinic, P21/n symmetry) suggested that
in both cations, the amino group is twisted and does not
straddle the triazole ring in a symmetric fashion. This
arrangement is most likely due to extensive hydrogen
bonding within the crystal structure which, however, does
not affect the bond lengths in the structures in a significant
manner.[31]
2.3. Triazolium Salts Containing Azido Substituents
Hydrogen atoms of azoles can be substituted readily by
various energetic functional groups. When endothermic
moieties, such as the azido group, are incorporated into
azoles, the enthalpies of formation of the azoles are
increased.[30, 32, 33] For example, the standard heat of formation
for 3-azido-1,2,4-triazole (DH of = + 458 kJ mol 1)[33] is approximately four-times larger than that of 1-H-1,2,4-triazole.[28]
These azido-substituted azoles, when paired with nitrate,
perchlorate, or azolate (e.g., 4,5-dinitroimidazolate or 5nitrotetrazolate anions), formed energetic salts.[30, 34]
Various energetic salts based on azido-1,2,4-triazoles
(11 a–d; Scheme 3)[30] were readily quaternized in good
the protonated 1,2,4-triazole ring for 11 a. This hydrogen
bonding accounts for the higher density, (1.76 g cm 3) and
melting point (Tm = 147 8C) of 11 a, compared with that of 14 a
(1 = 1.58 g cm 3, Tm = 98 8C), in which the presence of the
methyl group on the triazole ring reduces the opportunity for
hydrogen bonding (Table S3 of the Supporting Information).
The chemistry of energetic ionic salts and ionic liquids
containing azidoethyl substituents on the triazolium rings was
examined.[34] Reaction of 1-(2-azidoethyl)-1,2,4-triazole with
nitric acid, perchloric acid, 4,5-dinitroimidazole, or 5-nitrotetrazole gave the corresponding1-(2-azidoethyl)-1,2,4-triazolium salts 15 a–d, in over 97 % yield (Scheme 4). 1-(2Azidoethyl)-1,2,4-triazole was also treated with iodomethane
Scheme 4.
Scheme 3.
yields with concentrated nitric or perchloric acid in methanol.
Under similar reaction conditions, 1-methyl-3-azido-1,2,4triazole reacted with concentrated nitric or perchloric acids to
give 1-methyl-3-azido-1,2,4-triazolium nitrate (12 a), and
perchlorate (12 b), respectively. When 1-methyl-3-azido1,2,4-triazole was quaternized with iodomethane the salt 13
formed. Subsequent metathesis with AgNO3 or AgClO4 led to
the 1,4-dimethyl-3-azido-1,2, 4-triazolium nitrate (14 a) and
perchlorate (14 b), salts respectively (Scheme 3).
These azido triazolium salts were characterized by
elemental analysis, IR, 1H and 13C NMR spectroscopy, mass
spectrometry, and DSC studies. Examination of the crystal
structures of 11 a and 14 a illustrated the influence of
significant hydrogen bonding between the nitrate anion and
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to form 1-(2-azidoethyl)-4-methyl-1,2,4-triazolium iodide
(16), which upon metathesis with the silver salts of nitric or
perchloric acids formed the nitrate 17 a, and perchlorate 17 b,
respectively, in very good yields (Scheme 4). The melting
points of all these salts were found to be lower than 100 8C,
and most of them are liquids at room temperature. The
standard heats of formation for the salts with perchlorate
anion are more positive than those of the nitrate analogues.
Most of these salts showed good thermal stability and high
density (Table S3 of the Supporting Information).
The reaction of 1-(2-azidoethyl)-3-azido-1,2,4-triazole
with concentrated nitric acid or 5-nitrotetrazole gave 1-(2azidoethyl)-3-azido-1,2,4-triazolium nitrate (18 a) and the 5nitro tetrazolate 18 b, respectively (Scheme 5). Metathesis of
1-(2-azidoethyl)-4-amino-1,2,4-triazolium bromide or 1methyl-4-(2-azidoethyl)-1,2,4-triazolium
bromide
with
AgNO3 or AgClO4 gave good yields of the nitrates 19 a and
20 a and the perchlorates 19 b and 20 b, respectively
(Scheme 5).[34]
Density and thermal characteristics of azido- and azidoethyl-containing triazolium derivatives are summarized in
Table S3 of the Supporting Information. The data show that
the influence of the position of a substituent group on the
triazolium ring plays an important role, for example the
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Energetic Materials
substituted triazoles 21 a–c with iodomethane to form quaternary salts 22 a–c, metathesis of which with various metal salts
resulted in the ionic liquids and energetic salts 23 a–c
(Scheme 6). The yields of products are usually excellent.
Scheme 5.
Scheme 6.
Tg values for 1-(2-azidoethyl)-4-methyl-1,2,4-triazolium nitrate (17 a; 57 8C)) and 1-methyl-4-(2-azidoethyl)-1,2,4-triazolium nitrate (20 a; 56 8C) are essentially the same, but
their decomposition temperatures (17 a, Td = 119 8C; 20 a,
Td = 143 8C) differ markedly.[34] For the analogous perchlorate
salts, the melting points and thermal degradation temperatures are Tg = 52 8C and Td = 192 8C for 17 b, and Tm =
63 8C and Td = 152 8C for 20 b, respectively.
Density and enthalpy of formation are important characteristics of energetic salts and are governed by their molecular
structures. In general, the densities for the salts having
perchlorate as anions are higher than those of the analogous
nitrates (see the data in Table S3 of the Supporting Information). Comparing the standard molar enthalpy (DH of) of the
salts, when perchlorate is used as an anion, the positive heats
of formation as well as heats of combustion are higher than
those of the corresponding nitrates. Considering 15 a–d, the
impact of the anions on heats of formation of 1-(2-azidoethyl)
1,2,4-triazolium salts decreases in the order: 5-nitro-tetrazolate > perchlorate > 4,5-dinitro-imidazolate > nitrate.
2.4. Triazolium Salts Containing Fluoroalkyl Substituents
Incorporation of substituents at carbon atoms of 1,2,4triazoles was efficiently accomplished by construction of
triazoles with the required substituents in place.[35] By
utilizing this process various fluoroalkyl-substituted 1,2,4triazolium salts were obtained,[36] first by reaction of 1,3,5Angew. Chem. Int. Ed. 2006, 45, 3584 – 3601
These ionic salts were characterized by elemental analyses, IR and multinuclear NMR spectroscopy, and GC mass
spectrometry. The bis(trifluoromethanesulfonyl)amide salts
23 (Y = N(SO2CF3)2) are insoluble in water, which helps in
purification. The solubilities of these salts in organic solvents
increase with increasing dielectric constant of the solvent.
Their melting points decrease in the order SO3CF3 > BF4 >
ClO4 > N(SO2CF3)2 ,[36] which could be attributed to the
diffuse charge on the latter anion. Ionic salts with larger
cations have a lower melting point owing to less-efficient
packing in the solid phase.
Reactions of 1-alkyl-1,2,4-triazoles with polyfluoroalkyl
halides resulted in quaternization at N4 of the triazole ring to
yield 24 a–n. Metathesis of 24 a–n with various metal salts
gave excellent yields of ionic salts 25 a–n (Table 1).[37] Ionic
salts containing longer alkyl and polyfluoroalkyl substituents
have lower melting points because of less-efficient packing in
the solid. The densities were found to be higher with
elongation of the fluoroalkyl substituent and with higher
fluorine concentration. Cations with bulkier alkyl groups
resulted in salts of lower density.
In another study,[38] a high-yield, efficient procedure to
make functionalized alkyl-/fluoroalkyl-containing triazolium
salts and triazolium ionic liquids led to triazolium cations that
contained covalently bound anionic groups, such as sulfonate,
fluorocarboxy, fluorohomoallylic, and fluoroalkanol. These
were converted by metathesis with fluorine-containing anions
into low-melting ionic salts (26 a,b, 27, 28 a,b, 29 c,d, 30;
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Table 1: Synthesis of triazolium salts 25 containing fluorinated side
chains.
25 a
25 b
25 c
25 d
25 e
25 f
25 g
25 h
25 i
25 j
25 k
25 l
25 m
25 n
R
RF
Y
CH3
(CH2)3CH3
(CH2)6CH3
(CH2)9CH3
(CH2)2CF3
(CH2)2CF3
(CH2)2CF3
(CH2)2CF3
N(SO2CF3)2
N(SO2CF3)2
N(SO2CF3)2
N(SO2CF3)2
Tg [8C]
R
RF
Y
(CH2)3CH3
(CH2)6CH3
(CH2)9CH3
CH3
(CH2)3CH3
(CH2)6CH3
(CH2)3CH3
(CH2)6CH3
(CH2)3CH3
(CH2)3CH3
(CH2)2(CF2)3CF3
(CH2)2(CF2)3CF3
(CH2)2(CF2)3CF3
(CH2)2(CF2)5CF3
(CH2)2(CF2)5CF3
(CH2)2(CF2)5CF3
(CH2)2(CF2)3CF3
(CH2)2(CF2)3CF3
(CH2)2(CF2)5CF3
(CH2)2(CF2)3CF3
N(SO2CF3)2
N(SO2CF3)2
N(SO2CF3)2
N(SO2CF3)2
N(SO2CF3)2
N(SO2CF3)2
PF6
PF6
PF6
SO3CF3
Td [8C]
58
67
67
68
376
395
402
400
Tm [8C]
Td [8C]
69
55
53
62
53
22
296
298
289
173
394
396
396
381
396
405
357
354
361
391
2.5. 1,2,4-Triazolium Azolate Salts
The need for new, safe, high-performance propellants and
other industrial applications, coupled with the desire to
reduce environmental footprints and overall exposure to
hazardous and potentially toxic materials, has led to renewed
efforts to explore new, advanced, energetic salts. Lithium
dicyanotriazolate was reported as a useful electrolyte (38).[39]
1-Butyl-3-methylimidazolium 3,5-dinitro-1,2,4-triazolate (39;
Tm = 32 8C, Td = 239 8C) resulted from the metathesis of 1butyl-3-methylimidazolium halide with potassium 3,5-dinitro1,2,4-triazolate.[40] However, no energetic characterization
was carried out for this material. Some novel ionic liquids
made up of azolium cations and azolate anions were also
reported.[41] These salts are: 1-ethyl-3-methylimidazolium
1,2,4-triazolate (40) (Tg = 76 8C, Td = 207 8C, h = 60.2 cP at
25 8C) and tetrazolate (41) (Tg = 89 8C, h = 42.5 cP at 25 8C).
Compounds 40 and 41 were prepared by coupling of 1-ethyl3-methylimidazolium hydroxide with triazole or tetrazole,
respectively.[41]
Scheme 7).[38a] These functionalized triazolium salts have many characteristics
of excellent ionic liquids with respect to
air, water, and thermal stability, and all
are liquids at 25 8C.
A method for the syntheses of 1alkyl-1,2,4-triazolium 4-nitroimides was
developed based on alkylation of the
metal salts of 4-nitramino-1,2,4-triazole
with halo-and dihaloalkanes (Scheme 8).[38b] The salts 31–37 are solid and
characterized by IR and NMR spectroscopy and elemental analysis. In the IR
spectra the characteristic absorption
bands arising from an N-nitroimido
group bound to a heterocycle are
observed at 1280–1300 and 1390–
1415 cm 1. The nitramino group that
would have given absorption bands at
1550–1620 cm 1 is absent. In the
1
H NMR spectra, the signals for the
protons of the triazole ring are nonequivalent and shifted downfield relative to the signals for the protons in the
starting salt, evidence that supports the
imide structure. This conclusion is supported by the 13C NMR spectroscopic
data. No thermal or physical properties
were reported.
Scheme 7.
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Scheme 9.
Scheme 8.
By taking advantage of the fact that both 4,5-dinitroimidazole and 5-nitrotetrazole are strong NH acids (pKa = 0.8 for
5-nitrotetrazole),[42] energetic salts with various 1,2,4-triazolium derivatives as cations and 4,5-dinitroimidazolate and 5-
When 5-nitrotetrazolate was used as the anion, the
melting points of the compounds were lower than those of
the corresponding 4,5-dinitroimidazolates, while the thermal
decomposition temperatures were higher (Table S4 of the
Supporting Information). In addition to modified packing
effects and reduced lattice energies, the opportunity for
hydrogen bonding with the anion is reduced in methylsubstituted compounds, which results in lower melting points.
The densities exceed 1.45 g cm 3 for all the compounds.
Energetic salts that incorporate a nitro group have a
substantially improved oxygen balance which results in
higher heats of combustion and possible detonation processes.
The oxygen coefficients (a)[44] for the salts are between 0.18
and 0.33 and are within the range reported for energetic
compounds.[45] Of all the azolium azolate compounds, 1methyl-3-azido-1,2,4-triazolium 5-nitrotetrazolate (43 d) has
the highest heat of formation.
3. Tetrazole-Based Salts
nitrotetrazolate as anions (Scheme 9) were obtained.[43] 4,5Dinitroimidazole and 5-nitrotetrazole were treated with
variously 4-substituted derivatives of 1,2,4-triazoles to produce energetic ionic salts 42 a–d and 43 a–d (Scheme 9).
Angew. Chem. Int. Ed. 2006, 45, 3584 – 3601
Tetrazoles are unsaturated five-membered heterocycles
with four nitrogen atoms in the ring. The enthalpies of
energetic chemical systems are governed by their molecular
structure. From imidazole (DH of = + 58.5 kJ mol 1) to 1,2,4triazole (DH of = + 109.0 kJ mol 1) to tetrazole (DH of =
+ 237.2 kJ mol 1),[32] the heats of formation get increasingly
positive. Since the generation of molecular nitrogen as an
end-product of propulsion or explosion is highly desired to
avoid environmental pollution and health risks, as well as to
reduce detectible plume signatures, compounds containing a
backbone of directly linked nitrogen atoms (catenated nitrogen) are of great interest. The high nitrogen content of
tetrazole and its derivatives has led to investigations for their
use as potential energetic materials.
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3.1. Tetrazolium Salts Containing Amino Substituents
Aminotetrazoles have a high nitrogen content and,
despite their large positive enthalpies of formation,[46] are
thermally stable.[47] Aminotetrazoles are, therefore, prospective high-energy materials. The synthesis of various tetrazolium salts containing amino substituents on the ring through
the reactions of 1-amino-5-methyltetrazole or 2-amino-5methyltetrazole with iodomethane resulted in quaternary
salts 44 and 46. Metathesis with AgNO3 or AgClO4 formed 1amino-4,5-dimethyltetrazolium nitrate (45 a) or perchlorate
(45 b), and 2-amino-4,5-dimethyltetrazolium nitrate (47 a) or
perchlorate
(47 b),
respectively,
in
high
yield
(Scheme 10).[29, 30] Proof of synthesis was supported by elemental analyses, IR and multinuclear NMR spectroscopy and
mass spectrometry.
A high-yield synthesis of 5-aminotetrazolium nitrate (48)
is reported from the reaction of 5-aminotetrazole with nitric
acid (Scheme 11).[48] Its characterization was by IR, Raman,
and multinuclear (1H, 13C, 15N) NMR spectroscopy, and DSC.
Bomb calorimetry, sensitivity measurements, and ab initio
calculations were also performed. From the combined
experimental and theoretical calculations, 48 is predicted to
be a powerful and promising explosive with a good oxygen
balance and low sensitivity.
Recently the syntheses of 1,5-diamino-4-methyltetrazolium salts were realized by quaternization, for example,
treatment of 1,5-diamino-1H-tetrazole with iodomethane to
form 1,5-diamino-4-methyltetrazolium iodide (49) in 86 %
yield.[49] Subsequent metathesis of 49 with silver nitrate, silver
dinitramide, or silver azide led to the 1,5-diamino-4-methyltetrazolium nitrate (50 a), dinitramide (50 b), or azide (50 c) in
good yields. Most of these salts were thermally stable. The salt
50 b melts at < 100 8C which places it in the category of an
energetic ionic liquid. The enthalpies of combustion (DH oc) of
50 a–c were determined experimentally using oxygen bomb
calorimetry, and the values are
2456 cal g 1 for 50 a,
2135 cal g 1 for 50 b, and 3594 cal g 1 for 50 c. The detonation velocities (D) and detonation pressure (P) for 50 a–c
were calculated using empirical equations of Kamlet and
Jacob; the values are D = 7682 m s 1, P = 23.4 GPa for 50 a,
D = 8827 m s 1, P = 33.66 GPa for 50 b, D = 7405 m s 1, P =
20.8 GPa for 50 c (Table S5 of the Supporting Information).
Scheme 11.
These salts were characterized by IR, Raman, and NMR
spectroscopy, mass spectrometry, elemental analysis, X-ray
diffraction. In initial safety testing low impact sensitivities
were demonstrated. Compounds 45 a,b, and 47 a also fall into
the ionic-liquid class. Densities and thermochemical characteristics of substituted amino-, aminomethyl-, and polymethyltetrazolium salts are summarized in Table S5 of the
Supporting Information. While all of these new salts exhibit
thermal stabilities > 170 8C based on DSC/TGA studies
(except the azide) and positive heats of formation, the
perchlorates 45 b, 47 b, as well as the dinitramide 50 b and
the azido 50 c are highest. The densities of 1-amino-4,5dimethyltetrazolium perchlorate (45 b) and 1-methyl-4,5diaminotetrazolium dinitramide (50 b) are markedly higher
than the others.
3.2. Energetic Salts Containing 5,5’-Azotetrazolate Anions
In the 1890s, Thiele first prepared sodium 5,5’-azotetrazolate (51) from the oxidation of 5-amino-tetrazole with
potassium permanganate.[50] Reaction of strontium and
barium chlorides with an aqueous solution of 51 gave
strontium 5,5’-azotetrazolate (52 a) and barium 5,5’-azotetrazolate (52 b), respectively.[51] Both salts were insoluble in
water. Salt 52 b, was used with various metal sulfates to
Scheme 10.
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produce the corresponding 5,5’-azotetrazolate salts 53 a–d,
54 a,b, and 55 a–f (Scheme 12). These metal salts often contain
water of crystallization, and loss of water has been observed
during storage.[51] Upon loss of water, the sensitivity of the
gas generators and explosives.[52] Salts of the 5,5’-azotetrazolate dianion with various methylated ammonium (56 a–e) and
hydrazinium (57 a–d,f) cations were obtained from the
reactions of barium 5,5-azotetrazolate (52 b) with appropriate
ammonium and hydrazinium sulfates.[52, 53] N,N,N-Trimethylhydrazinium 5,5’-azotetrazolate (57 e) was obtained by the
metathesis of trimethylhydrazinium iodide and silver 5,5azotetrazolate (Scheme 13).
Scheme 13.
Scheme 12.
compounds to shock and friction increases drastically. The
salts were characterized by IR, Raman, and NMR spectroscopy, and their thermal properties were studied by DSC and
TGA. X-ray crystal studies on the pentahydrates 51 and 52 b,
hexahydrate 53 a, dihydrate 53 c, octahydrate 54 b, docosahydrate 55 b, and gadolinium 5,5’-azo-tetrazolate hydrate (55 f)
show that the water molecules were either in the coordination
sphere of the cation or bound by hydrogen bonds. The
azotetrazolate ion is not connected to the harder cations, such
as calcium or yttrium, and the salt decomposed in water to
form tetrazolhydrazine with concomitant evolution of nitrogen. The free acid 5,5’-azotetrazolate, synthesized from the
sodium salt with HBF4, was decomposed within seconds at
room temperature but could be retained at 30 8C.
Salts of 5,5’-azotetrazolate with protonated nitrogen bases
(e.g., ammonium, guanidinium, and triaminoguanidinium)
are unique gas-generating agents producing little smoke or
residue, which may lead to a variety of applications, including
Angew. Chem. Int. Ed. 2006, 45, 3584 – 3601
The 5,5’azotetrazolates 57 g–n are salts that contain
organic nitrogen-base cations.[53b, 54a] These salts were characterized by IR, Raman, NMR spectroscopy, and elemental
analyses; X-ray crystal structures of 56 a, 56 c, 56 e, 57 a, 57 e,
57 j, and 57 l were determined.
The salts were insensitive to shock, friction, or electric
discharge ( 20 kV). Detonation was not observed in either
the drop hammer test (5 kg, 50 cm) or when the salts were
ground forcefully in a mortar. None of these salts melts, but
rather they decompose at specific temperatures with rapid gas
evolution. Above the decomposition point, explosion occurred upon rapid heating to give nitrogen gas as the main
product, especially if the compound was compressed before
heating. The decomposition temperature decreased for salts
with NH+ groups as the number of methyl groups increased.
The hydrazine salts also produced large amounts of hydrogen,
which decreased with the increasing number of methyl
groups. The ammonium compounds (56 a–c) produced only
small amounts of hydrogen, whereas none was detected for
56 d,e.[53b] The formation of methane was observed for all salts.
Heats of formation of hydrazinium,[53a] guanidinium,[54b]
and ammonium 5,5’-azotetrazolates[54a] were found to be
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DH of = + 858, + 410, and + 443 kJ mol 1, respectively. To
compare these properties with azotetrazolates that contain
heterocyclic cations, imidazolium, triazolium, and tetrazolium
derivatives
were
synthesized
and
characterized
(Scheme 14).[55] Surprisingly, while most azotetrazolates
exhibit melting points in excess of 160 8C, [bis(1-butyl-3methyl-imidazolium)] 5,5’-azotetrazolate (58 a) was an ionic
liquid at 25 8C with a melting point of 3 8C, similar to its 3,5dinitrotetrazolate analogue.[40a] This compound is of little
value with respect to energetic contributions, given its density
Scheme 14.
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of 1.26 g cm3. This is different from [bis(1,3-dimethyl-4-nitroimidazolium)] 5,5’azotetrazolate (58 b), which has a positive
heat of formation, higher than that of bis(triaminoguanidinium) 5,5’azotetrazolate (57 m).
Although the quaternized salts of 4-amino-1,2,4-triazole,
that is, bis(1-methyl-4-amino-1,2,4-triazolium) (59 a), and
bis(1,4-diamino-1,2,4-triazolium) (59 b) 5,5’-azotetrazolate
have lower nitrogen content than the bis(triaminoguanidinium) derivative 57 m, their heats of formation are much
higher at + 1580 and + 1705 kJ mol 1, respectively, with
densities of approximately 1.57–
1.59 g cm 3. The structure of 59 a,
determined by single-crystal X-ray
analysis, shows that the unit cell
was packed as a layered structure
with hydrogen bonds and an interlayer distance of 3.04 M. Perhaps
somewhat surprisingly, bis(1,2,5trimethyltetrazolium) 5,5’-azotetrazolate (60) has a lower density
and much lower positive heat of
formation. In contrast to the metal
azotetrazolates,[51] none of these
salts (except 60) was solvated.
Compounds 58 b–60 decomposed
violently upon melting. Microwaving 59 a at 200 8C caused violent
decomposition leading to the formation of a carbon black powder.
With the exception of 59 b, which
spontaneously evolved nitrogen
gas, most of the salts were stable
at room temperature for at least
two months.
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3.3. Energetic Salts of Bistetrazolates
Although there are several reports of 5,5’-bistetrazolate[56]
and iminobis(5-tetrazolate)[57] salts, most are described in the
patent literature. Some processes have been developed for
preparing various 5,5’-tetrazolate salts, which were very useful
as low toxic and easy to handle gas-generating agents for air
bags and as high-molecular-weight foaming agents. In methanol at reflux, bistriazole or iminobis(5-tetrazole) readily
quaternized 4-amino-1,2,4-triazole to give [bis(4-amino-triazolium)] 5,5’-bistetrazolate (61 a) or the iminobis(5-tetrazolate) 61 b, respectively (Scheme 15).[55] While the densities of
these two salts are still in the same range as the azotetrazolates, and, although the heat of formation of 61 a decreased
somewhat, the values for 60 and 61 b are dramatically lower.
methylurotropinium azide (67 a; white solid, Tm = 165–170 8C
(dec.), 1 = 1.4 g cm 3), N-methylurotropinium dinitramide
(67 b; white solid, Tm = 121–124 8C (dec.), 1 = 1.46 g cm 3),
and N-methylurotropinium azotetrazolate (67 c; yellow crystals, Tm = 181–184 8C (dec.), 1 = 1.46 g cm 3) were prepared
from either the corresponding iodide or sulfate (Scheme 16).
Scheme 15.
The diammonium (62), disodium (63), disilver (64), and
manganese(ii) (65) 5,5’ bistetrazolates are also known.[56]
Chemistry of copper complexes with bis(tetrazolyl)amine
has also been studied and some of them are of interest as
additives in pyrotechnics and ammonium perchlorate-based
propellants.[56e]
4. Urotropinium-Based Salts
The chemistry of high-energy density materials (HEDM)
containing only C, H, N, and O atoms is of great interest.
Urotropine is a nitrogen-rich cage molecule, which when
paired with energetic anions forms energetic salts. The initial
preparation of urotropinium salts was carried out in the
1950s,[58] and subsequently the syntheses of a variety of them
were reported.[59] Urotropinium nitrate (66; colorless crystals,
Tm = 157–161 8C (dec.), 1 = 1.47 g cm 3(from X-ray)) was
prepared by the reaction of urotropine with nitric acid. NAngew. Chem. Int. Ed. 2006, 45, 3584 – 3601
Scheme 16.
Owing to the high sensitivity and explosive nature of
anhydrous silver azide, an alternative route for 67 a using
sodium azide with N,N-dimethylurotropinium diiodide was
utilized.[59] All the salts were handled easily, were insensitive
to air or light, and were soluble in polar organic solvents. They
were characterized by analytical and spectroscopic (IR,
Raman, 1 H, 13C, 14N, NMR) methods and X-ray diffraction
techniques. The crystal structure of 66 (monoclinic, space
group P21/C) consisted of an urotropinium cation linked to a
planar nitrate group by hydrogen bonds. The structures of
67 a–c revealed that there is no contact between the anions
and cations. The azide ion in 67 a (monoclinic, space group
P21/m) is linear; the dinitramide ion in 67 b (monoclinic, space
group P21) is asymmetric; and the azotetrazolate anion in 67 c
(monoclinic, space group C2/m) is planar.
Syntheses of urotropinium and N-methylurotropinium
salts were broadened to include several other energetic
organic and inorganic anions, such as 3,5-dinitropyrazolate,
4,5-dinitroimidazolate, 3,5-dinitro-1,2,4-triazolate, 5-nitrotetrazolate, perchlorate, nitrate, and azide.[60] In methanol
solution, urotropine was found to react readily with 3,5dinitropyrazole, 4,5-dinitroimidazole, 3,5-dinitro-1,2,4-triazole, and 5-nitrotetrazole to form urotropinium salts 68 a–
d. Reaction of silver salts of 3,5-dinitro-1,2,4-triazole, nitric,
perchloric, and hydrofluoric acid with N-methylurotropinium
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iodide led to N-methylurotropinium 3,5-dinitro-1,2,4-triazolate (70 a), nitrate (70 b), perchlorate (70 c), and fluoride
(70 d) salts. N-Methylurotropinium azide (67 a) is also accessible by the reaction of 69 d with (CH3)3SiN3 (Scheme 17).[60]
Scheme 17.
Most of the new salts exhibit high positive heats of
formation, which are higher for salts with organic anions
(68 a–d, 70 a) than those of their inorganic anion (70 b–d, 67 a)
analogues (Table S7 of the Supporting Information). The
structure of 70 b was confirmed by single crystal X-ray
analysis.
5. Tetrazine-Based Salts
There has been considerable interest in the study of the
reactivity and properties of various tetrazine derivatives. The
1,2,4,5-tetrazine ring system is electroactive and has a high
electron affinity. Tetrazines possess high positive heats of
formation and large crystal densities—properties important
in energetic materials applications. Additionally, they seem to
be insensitive friction, impact, and electrostatic discharge.
The synthesis and properties of various ionic 1,2,4,5tetrazine explosives and energetic materials such as 71–76
have been reported.[61] Energetic salts 71 a,b, 72 a,b, and 75 a,b
were synthesized by the reaction of nitric acid and perchloric
acid with 3,6-diguanidino-1,2,4,5-tetrazine, 3,6-diguanidino1,2,4,5-tetrazine-1,4-di-N-oxide, and 3,6-dihydrazino-1,2,4,5tetrazine, respectively, whereas 75 c was made by the reaction
of HN(NO2)2 with 3,6-dihydrazino-1,2,4,5-tetrazine. Synthesis
of 73 and 74 involved the reaction of the disodium salt of 3,6bis(nitroguanyl)tetrazine with AgNO3 and triaminoguanidinium hydrochloride, respectively.
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Poly-rho tests, which are single shot experiments to
determine detonation velocity as a function of density, were
performed on 71 a,b. When 71 a is formulated with 5 wt % of
Kel-F 800 binder, a maximum pellet density of 1.79 g cm 3 is
obtained. At this density, the detonation velocity measured
was 8.07 km s 1. The dinitrate derivative (71 b) was formulated with 3 wt % Estane binder and 3 wt % nitro-plasticizer.
At a maximum pellet density of 1.60 g cm 3, a detonation
velocity of 7.31 km s 1 was obtained. Salt 72 b was found to be
unstable based on DSC analysis, whereas salt 72 a showed
improved thermal stability but less than 71 a.[61a] The crystal
density of 74 was reported to be 1.61 g cm 3, and the heat of
formation was 300 2 kcal mol 1.[61c] The densities of 75 a–c,
76 are reported to be in the range of 1.80 to 1.96 g cm 3.[61b]
When these salts were heated over the temperature range 40–
500 8C at a scan rate of 20 8C min 1, 75 a–c left no residue,
whereas 76 left an orange powder. In DSC studies, 75 a and
75 c exhibited their major exotherm between 152 and 164 8C,
respectively. Salt 75 b exhibited an exotherm at 190 8C and 76
showed the highest exotherm (220 8C).[61b] All tetrazine-based
salts seem to have interesting explosive performance and
extraordinary combustion properties.[61]
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6. Azetidinium-Based Salts
Azetidine-based explosives, such as 1,3,3-trinitroazetidine[62] demonstrate excellent performance partly because of
the high strain associated with the four-membered ring. The
basicity (pKb = 6.5) of 3,3-dinitroazetidine[63] allowed the
preparation of a variety of solid energetic 3,3-dinitroazetidinium salts 77–79 with high oxygen-balance.[64] These salts
were synthesized in 82–95 % yields, either by mixing the freebase 3,3-dinitroazetidine with the appropriate acid or by
metathesis of 3,3-dinitroazetidinium trifluoromethane sulfonate[63] with ammonium salts of the acid. They were characterized by elemental analyses, IR and 13C NMR spectroscopy.
Densities and thermal characteristics are tabulated in
Table S8 of the Supporting Information. All the salts were
subjected to small-scale thermal and sensitivity tests.[64]
The single-crystal X-ray structures of 3,3-dinitroazetidinium dinitramide (77 c; orthorhombic, space group Cmc21) and
1-isopropyl-3,3-dinitroazetidinium dinitramide (78; orthorhombic, space group Pbca) were also reported.[65] The
latter was formed during an attempt to crystallize 3,3dinitroazetidinium nitrate (77 a) from acetone. X-ray structures have confirmed that the dinitramide ions in 77 c and 78
have quite different conformations with different bend, twist,
and torsion angles. The possible reason for these dramatic
differences is claimed to be due to the different symmetries
for this ion found in the two structures, as well as the absence
of hydrogen-bonding interactions in 78.[65] Dehydration of
77 a with acetic anhydride provided an alternate route for the
synthesis of 1,3,3-trinitroazetidine.[64] The synthesis and
characterization of 15N-labeled isotopomers of 77 a was also
reported.[66]
7. Picrate-Based Salts
Although anhydrous picric acid is unstable, and its impact
and friction sensitivities are higher than those of trinitrotoluene, many organic and inorganic picrates have been
Angew. Chem. Int. Ed. 2006, 45, 3584 – 3601
reported.[7] The picrate anion, when combined with highnitrogen azolium cations, forms energetic salts with high
positive heats of formation. By taking advantage of the acidic
properties of picric acid, energetic salts containing picrate and
dipicrate anions have been prepared.[67] Scheme 18 depicts
synthetic routes to salts 80–83 composed of azolium cations
with picrate as the anion. Scheme 19 shows the synthetic
routes to the energetic bisazolium dipicrate salts 84 and
85 a,b.[67]
Scheme 18.
Triazolium or substituted triazolium picrates were first
prepared by direct reaction of the triazole with picric acid in
methanol or by quaternization with methyl iodide and
treatment with silver picrate (Scheme 18). Bridged bis(imidazolyl)- or bis(tetrazolyl)methane compounds were treated
with picric acid to form dipicrates, or they were quaternized
and then metathesized to yield the desired salts (Scheme 19).
All of these salts were well-characterized, and X-ray
crystal structures of 81 a and 81 b were also determined. Their
physical characteristics and thermochemical properties, along
with those of other energetic materials, are given in Table S9
of the Supporting Information. Most of the picrates have good
thermal stabilities and relatively high densities and oxygen
balance. The bridged azolium ion picrates are more stable
thermally than their monocationic analogues. In general, the
majority of these picrates were found to be more stable
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Scheme 20.
Scheme 19.
thermally than their nitrate analogues and less than the
perchlorates. Not surprisingly, the azido derivative (83 b)
exhibited the highest enthalpy of formation but also one of
the lowest thermal stabilities of this family of picrates,
decomposing at 176 8C. 5-Aminotetrazolium picrate (81 b)
was the densest of the picrate salts at 1.85 g cm 3 (determined
by X-ray), which places it between picric acid (1.77 cm g 3)
and HMX (1.90 g cm 3). Protonation in 81 a occurred at one
of the ring nitrogen atoms rather than on the N-bound amino
group. However, in contrast, in the reaction of picric acid with
the C-amino triazole, a high melting ammonium salt 82 was
formed.
Compound 86 was a white solid that was sparingly soluble
in anhydrous HF and marginally stable at 22 8C. However, it
was storable for weeks at 78 8C without noticeable decomposition. With water, 86 was violently explosive. The high
energy density of N5+ was confirmed by the G2 method of
calculation, and the enthalpies of formation were DH of = 1478
1
and DH 298
for free gaseous N5+.
f = 1469 kJ mol
+
The stable salt N5 [SbF6] (87) was also synthesized using
a similar procedure and characterized.[70] When 87 was
treated with an equimolar amount of SbF5 in anhydrous HF
at room temperature, the salt N5+[Sb2F11] (88) was obtained
(Scheme 20). Both 87 and 88 are colorless, hygroscopic solids
that are soluble in anhydrous HF and stable at room
temperature. Based on DSC measurements, thermal decomposition started at 70 8C. The structure for 88 has been
obtained using single-crystal X-ray analyses.
A large number of compounds with the N5+ ion have
subsequently been synthesized in anhydrous HF
(Scheme 21).[71] Salts 89 and 90 are friction-sensitive white
solids. Compounds 91 and 92 are extremely shock-sensitive
and explode upon warming to room temperature. Compound
93 is a clear, colorless liquid and a very useful intermediate in
the synthesis of other N5+ salts. Salts 94, 95, and 96 are
marginally stable white solids that were characterized by
vibrational (IR, Raman), and NMR spectroscopy.
8. Polynitrogen-Containing Salts
Polynitrogen compounds are of significant interest as
high-energy materials for propulsion or explosive applications.[68] In spite of numerous theoretical studies, in which
certain all-nitrogen compounds have been predicted to be
stable, only a few unsuccessful experimental studies aimed at
their actual syntheses have been reported. Although the two
simplest, N2 and N3 , are well known, some other species such
as N4 ! , N3+, N4+ have been observed only as free gaseous or
matrix-isolated ions or radicals. Theoretically, some polynitrogen species, such as N4, N8, [N(N3)2] , N(N3)3, and
[N(N4)4]+ should be vibrationally stable. In the past few
years, the single most important breakthrough has been the
synthesis and study of the N5+ ion.[69]
The synthesis of N5+[AsF6] (86) was accomplished by the
reaction of [N2F]+[AsF6] and HN3 in the presence of HF
(Scheme 20).[69] In this reaction a small excess of HN3 is used
to ensure the complete conversion of the [N2F]+[AsF6] . The
only by-product was protonated HN3 (less than 20 %).
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Scheme 21.
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Theoretical studies on the N62+ and N42+ ions and their N10
and N12 azido derivatives were carried out.[72] Enthalpies of
formation of gas-phase N3, N3 , N5+, and N5 have been
calculated from ab initio molecular orbital theory. The salts
N5+N3 and N5+N5 were shown to be unstable and should
decompose spontaneously into N3C and N2.[73]
9. Imidazolium-Based Salts
Imidazolium salts with fluorine-containing anions have
dominated the field of ionic liquids.[74] However, nitro- and
azido-substituted imidazoles, when paired with nitrate or
perchlorates, form solid energetic salts 97 a–h in excellent
yields.[30] Salts 97 a–f were made by the metathesis of the
Scheme 22.
(102) is made from B(N3)3 and PPh4(N3) and characterized by
X-ray crystal structure.[77] When 2,2,6,6-tetramethylpiperidinoboron diazide was treated with HN3, the salt 2,2,6,6tetramethylpiperidinium tetraazidoborate (103) resulted.[78]
Various metal azides and their binary azide salts have
been also reported.[79] Reaction of metal fluorides with
trimethylsilylazide led to the formation of 104 a,b and 105.
When these metal azides were further treated with tetraphenylphosphonium azide or tetramethylamonium azide, 106 a–d
and 107 are formed (Scheme 23).
corresponding iodide salt with AgClO4 and AgNO3, whereas
97 g,h are formed by the reactions of 2-azidoimidazole with
HClO4 and HNO3, respectively. All these compounds are
well-characterized solids. In general, the nitrate salts have
lower melting points and thermal stabilities than the corresponding perchlorates. Energetic ionic salts containing imidazolium triazolate or tetrazolate, and tetrazolium imidazolate are described in Section 2.5.
10. Miscellaneous
Scheme 23.
Crystal structures of hydrazinium dinitramide,
[NH2NH3]+[N(NO2)2] , (98) and hydroxylammonium dinitramide , [NH3OH]+[N(NO2)2] , (99) salts show the presence
of protonated amine cations and dinitramide anions that are
linked by hydrogen bonding.[75] In addition, in 99 there are
both neutral and zwitterionic hydroxylamine moieties
involved in the hydrogen-bonding scheme. The actual formula for 99 was claimed to be [NH3+OH]2[N3O4 ]2·
(NH2OH)·[NH3+O ] in which, the hydroxylamine exists in
its three possible forms: protonated, neutral, and zwitterionic.
Reaction of tert-butylhydrazine with HN3 is reported to
yield tert-butylhydrazinium azide, [(CH3)3CNH2NH2]+N3
(100), and N,N,N-trimethylhydrazinium azide, [NH2N(CH3)3]+N3 (101) is obtained by the reaction of silver azide
with N,N,N-trimethylhydrazinium iodide.[76] Both of these
salts are characterized by X-ray structural analysis, IR,
Raman, and multinuclear NMR spectroscopy. Compound
100, unlike 101, is very hygroscopic.
There is a report on high energetic tetraazidoborate salts
(Scheme 22).[77, 78] Tetraphenylphosphonium tetraazidoborate
Angew. Chem. Int. Ed. 2006, 45, 3584 – 3601
The salts 104 a,b, 106 a–d are obtained as dark red solids
whereas 105 and 107 are bright yellow and orange solids,
respectively. All these polyazide metal salts are very shock
sensitive and can explode violently. Binary salts 106 a–d, 107
are claimed to be more shock-sensitive than their corresponding metal azide salts 104 a,b and 105.
Very recently the synthesis of highly energetic, tetrazolium polynitratoaluminate 108 which has a good oxygen
balance was reported.[80]
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
3599
Reviews
J. M. Shreeve et al.
11. Conclusion
Energetic ionic salts, whether solid or liquid, offer many
advantages over traditional molecular energetic compounds.
Salt-based energetic materials tend to exhibit lower vapor
pressures and higher densities and, in many cases, an
enhanced thermal stability compared to their atomically
similar non-ionic analogues. The use of heterocyclic nitrogencontaining cations and anions contributes to higher heats of
formation and higher densities, and the low amounts of
carbon and hydrogen also allow for a good oxygen balance to
be achieved more readily than with their carbocyclic analogues. Because a higher percentage of the decomposition
products will be dinitrogen, these nitrogen-rich compounds
are promising high-energetic materials that may be more
acceptable than their alternatives for both industrial and
military uses.
Received: November 29, 2005
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