Trinitromethyl-substituted 5-nitro- or 3-azo-1,2,4-triazoles: Synthesis, characterization, and energetic properties

Article abstract of DOI:10.1021/ja2013455

Various new polynitro-1,2,4-triazoles containing a trinitromethyl group were synthesized by straightforward routes. These high nitrogen and oxygen-rich compounds were fully characterized using IR and multinuclear NMR spectroscopy, elemental analysis, natural bonding orbital (NBO) analysis, and differential scanning calorimetry (DSC) and, in the case of 12, with single crystal X-ray structuring. The heats of formation for all compounds were calculated with Gaussian 03 (revision D.01) and then combined with experimentally determined densities to determine detonation pressures (P) and velocities (D) of the energetic materials (Cheetah 5.0). They exhibit high density, good thermal stability, acceptable oxygen balance, positive heat of formation, and excellent detonation properties, which, in some cases, are superior to those of TNT, RDX, and HMX.

Full text of DOI:10.1021/ja2013455

ARTICLE
pubs.acs.org/JACS
Trinitromethyl-Substituted 5-Nitro- or 3-Azo-1,2,4-triazoles:
Synthesis, Characterization, and Energetic Properties
†
‡
,†
Venugopal Thottempudi, Haixiang Gao, and Jean’ne M. Shreeve*
†
Department of Chemistry, University of Idaho, Moscow, Idaho 83844-2343, United States
Department of Applied Chemistry, China Agricultural University, Beijing 100193, China
‡
S Supporting Information
b
ABSTRACT: Various new polynitro-1,2,4-triazoles containing a
trinitromethyl group were synthesized by straightforward routes.
These high nitrogen and oxygen-rich compounds were fully char-
acterized using IR and multinuclear NMR spectroscopy, elemental
analysis, natural bonding orbital (NBO) analysis, and differential
scanning calorimetry (DSC) and, in the case of 12, with single crystal
X-ray structuring. The heats of formation for all compounds were
calculated with Gaussian 03 (revision D.01) and then combined with
experimentally determined densities to determine detonation pressures (P) and velocities (D) of the energetic materials (Cheetah
.0). They exhibit high density, good thermal stability, acceptable oxygen balance, positive heat of formation, and excellent
detonation properties, which, in some cases, are superior to those of TNT, RDX, and HMX.
5
’
INTRODUCTION
Ammonium perchlorate (AP) is the main oxidizer used in
solid rocket fuels. Oxidizers provide the oxygen needed for
oxidation of the fuel to provide the necessary thrust. The search
for a smokeless propellant has encouraged scientists to look for
chlorine-free oxidizers as a substitute for AP because it con-
tributes to acid rain and ozone layer depletion, in addition to
having deleterious impacts on the human thyroid and being a
persistent contaminant in groundwater. Although it has some
drawbacks, including a low melting point, high impact sensitivity,
and negative heat of formation, ammonium dinitramide (ADN)
After decades of effort in the preparation of energetic com-
pounds, higher performance and lower sensitivity continue to be
1
keen concerns in weapon systems. Desirable characteristics for
new energetic materials include positive heat of formation, high
density, high detonation velocity and pressure, high thermal
stability, and low sensitivity toward external forces such as impact
and friction. Of particular interest are high-nitrogen compounds
(
e.g., azoles) in combination with energetic substituents such as
nitro (ꢀNO ), nitrato (ꢀONO ), and nitramine (ꢀNHNO )
2
2
2
11
functionalities, because these compounds have satisfactory oxygen
is an alternative oxidizer to AP. Current research issues in the
field of high energetic materials include increasing oxygen
content, which may give rise to a suitable replacement for AP.
In a continuing effort to seek more powerful, less sensitive,
eco-friendly energetic materials, we are interested in heterocyclic
compounds that contain a high percentage of both oxygen and
nitrogen, and lower amounts of carbon and hydrogen. Hetero-
cyclic compounds with high nitrogen content are environmen-
tally friendly, have high heats of formation, and are endothermic.
The high nitrogen content of these compounds often leads to
high crystal density, which is associated with increased perfor-
mance. Incorporation of a triazole ring into a compound is a
known strategy for increasing thermal stability. Many triazole
compounds show high thermal sensitivity coupled with low
2
content. However, the requirements of insensitivity and high
energy along with positive oxygen balance are quite often contra-
dictory to each other, making the development of new high energy
3
density materials an interesting and challenging problem.
Several explosives such as triaminotrinitro benzene (TATB),
,3,5-trinitrotriazacyclohexane (RDX), 1,3,5,7-tetranitrotetraa-
1
zacyclooctane (HMX), and 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-
1a
hexaazatetracyclododecane (CL-20) are well-known. Recently,
,1-diamino-2,2-dinitroethene (FOX-7) emerged as a potential
1
candidate for use as an insensitive high explosive attracting
substantial interest because its density and detonation perfor-
mance is superior to that of and its sensitivity is much less than
4
that of RDX. Polynitro azoles have been a recent focus because
12
of their high performance and low sensitivity to friction and
sensitivity to shock and impact. In this Article, we report the
synthesis of various polynitro-1,2,4-triazoles, which contain
trinitromethyl groups and which display potentially significant
physical and energetic properties.
5
ꢀ7
impact.
Azoles with more than two nitro groups are highly
powerful, and, as a result, a large variety of nitroazoles have been
8
prepared. In reaction of the latter with bases such as ammonia,
hydrazine, and guanidine, stable energetic salts are obtained,
9
which may hold promise for future applications. In addition,
nitro azoles are valuable in the pharmaceutical field as nitro-
containing synthetic intermediates and drugs.
Received: February 18, 2011
Published: March 30, 2011
10
r 2011 American Chemical Society
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Journal of the American Chemical Society
ARTICLE
Scheme 1. Synthetic Pathway for Intermediates 4 and 5
Scheme 2. Nitration of Triazole 5
Scheme 3. Synthesis of Trinitromethyl (9) and Dinitromethyl (11) 1,2,4-Triazoles
’
RESULTS AND DISCUSSION
It was thought that a nitro group could prove useful in lieu of
the 5-amino group. Therefore, amino triazole 4 was converted
Azolyl acetic acids can be converted into corresponding
into corresponding nitro compound 7 by reacting with sodium
trinitromethyl azoles by nitration with mixed acids, HNO and
3
14b
1
3
nitrite and HNO (Scheme 3). In the reaction of 7 with mixed
3
H SO . Compounds such as aminotriazolylacetic acids, for
2
4
acids, 5-nitro-3-trinitromethyl-1H-1,2,4-triazole (9) was obtained
as a single product at ambient temperature.
During this process, the proposed intermediate trinitro car-
boxylic acid (8) was not isolated. The reaction was complete in
example, 4 and 5, give the corresponding polynitro compounds
by nitration with mixed acids. Aminotriazolylacetic acid (4) was
synthesized by intramolecular condensation of the malonyl
derivative of aminoguanidine bicarbonate in alkaline medium
1
2 h. Because no precipitate formed when the reaction mixture was
1
4
(
Scheme 1).
Nitration of amino triazole 4 was attempted by using different
nitrating methods such as 70% HNO , 100% HNO , or mixed
added to ice, the aqueous solution was extracted with dichloro-
methane, resulting, after solvent removal, in a colorless crystalline
product (9) in 45% yield (Scheme 3).
3
3
acids (HNO /H SO ), but the product was not isolable from the
3
2
4
Conversion of a trinitromethyl group to the corresponding
dinitromethylide salt and subsequently to the dinitromethylene
reaction mixture. When the ethyl ester of aminotriazole (5) was
treated similarly with nitric acid, the attempted nitration reaction
was not successful. With mixed acids, nitration of the ethyl ester
produced a mixture of unknown compounds, which were difficult
to separate (Scheme 2). Compound 6 is a likely intermediate, but
it was neither observed nor isolated.
13,15
compound has been reported.
Using that methodology,
potassium salt 10 was obtained when 9 was treated with an
alkaline solution of hydroxylamine. Acidification with 50%
sulfuric acid gave 3-dinitromethyl-5-nitro-1H-[1,2,4]triazole
(11) as a yellow solid (Scheme 3).
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N-Methyl azoles are often less sensitive to impact and friction
as compared to the corresponding N-H azoles. The methyl
derivative was difficult to prepare by reacting with methyl iodide
or dimethylsulfate in the presence of bases, for example, triethyl
amine, potassium tert-butoxide, potassium hydroxide, or sodium
hydride. In all of these cases, low yields of the impure methylated
product (12) were obtained. When tetranitro 9 was reacted with
to nitro and azo groups with the N2 signal of the triazole ring
downfield relative to N3 and N1. The N3 nitrogen resonance
falls between N2 and N1 (Figure 1). The resonance peaks for N3
and N1 are nearly overlapping in 14. That area of the spectrum
has been expanded in the inset. The N4 signals of 9 and 14 appear
downfield as compared to the N5 signals. The N5 signal of 12
15
appears slightly downfield from N4. The N NMR spectra for 11
and 15 may be found in the Supporting Information.
14
commercially available trimethylsilyl diazomethane, 1-methyl-
-trinitro-3-trinitromethyl-1H-[1,2,4]triazole (12) was obtained
5
Crystals of 12 suitable for single crystal X-ray diffraction were
obtained by dissolving the compound in a minimum amount of
methanol, held at 0 °C, followed by filtration of crystals. It
in 80% yield (Scheme 4).
Next, similar successful attempts were made to synthesize 14
and 15. The amino triazolylacetic acid 4 was converted into the
azo compound, 13, by treating with alkaline potassium
crystallizes in an orthorhombic crystal system (space group Pna2 ).
1
The N1ꢀN2 bond length is 1.3484(15) Å, which is normal in
triazoles. N1ꢀC5, N4ꢀC5, N4ꢀC3, and N2ꢀC3 bond lengths
are 1.3485(15), 1.3112(16), 1.3518(15), and 1.3298(16) Å,
respectively. The bond lengths of C5ꢀN17 and C6ꢀN1 are
1
4b
permanganate. Compound 13 was reacted with mixed acids
0
0
at room temperature to form 5,5 -bis(trinitromethyl)-3,3 -azo-
,2,4-triazole (14). When an attempt was made to convert the
1
trinitromethyl groups of hexanitro 14 to the corresponding
dinitromethylene groups by treating with alkaline hydroxylamine
followed by acidification, a low yield mixture of compounds was
formed. The reaction also failed with potassium iodide in
methanol. Methylation of hexanitro 14 was carried out using
various reagents; however, reaction was successful with tri-
0
0
methylsilyl diazomethane to produce 1,1 -dimethyl-5,5 -bis-
(
0
trinitromethyl)-3,3 -azo-1,2,4-triazole (15) (Scheme 5).
The structures of the new polynitro triazoles are supported by
1
15
13
IR, H, N, and C NMR spectroscopic data as well as
elemental analysis. In the IR spectra, strong absorption bands
ꢀ
1
at ∼1630ꢀ1610 cm are attributed to the trinitromethyl group.
1
3
In the C NMR spectra, resonance bands for the trinitromethyl
group appear between 120 and 130 ppm. The N NMR spectra
15
of the nitrotriazoles were measured in DMSO[d ] solution, and
6
chemical shifts are given with respect to CH NO as external
3
2
15
standard. In Figure 1, selected N NMR spectra of 9, 12, and 14
are shown. The triazole nitrogen signals are seen upfield relative
Scheme 4. Methylation of Tetranitro 9
1
5
Figure 1. Selected N NMR spectra of polynitro-1,2,4-triazoles.
Scheme 5. Synthesis of Triazoles 14 and 15
6
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Journal of the American Chemical Society
ARTICLE
were accomplished by using the B3LYP with the 6-31þG** basis
17
set, and single-point energies were calculated at the MP2/6-
3
11þþG** level. Atomization energies were calculated by the
18
G2 method. All of the optimized structures were characterized
to be true local energy minima on the potential-energy surface
without imaginary frequencies. The remaining task is to deter-
mine the heats of formation of the polynitro triazoles, which are
computed by using the method of isodesmic reactions (Scheme 6).
Thus, the heat of formation of the species being investigated can be
easily extracted. All of the polynitro triazoles exhibit positive heats
of formation and are comparable to or greater than those of TNT,
RDX, and HMX. Compound 9 is far superior to ADN in all
respects, including detonation velocity and pressure, enthalpy of
formation, impact sensitivity, and density, with the exception of
oxygen balance, although 9 is positive at 9% as compared to 25.8%
19
for ADN. The densities of these compounds are in the range of
3
1
.78ꢀ1.94 g/cm , which equals or exceeds that of common
explosives. Tetranitro triazole (9) has the highest density at 1.94
g/cm . The calculated densities are in good agreement with the
experimentally determined values (Table 1).
Impact sensitivity measurements were made using standard
3
20
21
BAM Fallhammer techniques. For all of the compounds, the
impact sensitivities range from those of the relatively less sensi-
tive 9, 11, 12, and 15 between 5.5 and 13 J to the very sensitive
compound 14 (1.5 J). N-Methyl derivatives (12 and 15) are less
sensitive than the corresponding N-H compounds 9 and 14.
Thermal stabilities of the energetic compounds were determined
with differential scanning calorimetry (DSC) at a scan rate of
ꢀ
1
5
1
°C min . Compound 9 melted at 113 °C and decomposed at
35 °C, whereas its methyl derivative (12) melted at 77 °C and
decomposed at 153 °C. Azo compounds 14 and 15 decomposed
at 150 and 165 °C, respectively, without melting. Surprisingly,
the trinitro compound 11 decomposed at 86 °C, which is
thermally less stable than the tetranitro compound 9.
By using the calculated values of the heats of formation and the
experimental values for the densities (gas pycnometer, 25 °C) of
the new energetic polynitro triazoles, the detonation pressures
Figure 2. (a) A view of the molecular unit of 12. (b) Unit cell view along
the b axis. Selected bond lengths [Å]: N1ꢀN2 1.3484(15), N1ꢀC5
1
1
1
.3485(15), N1ꢀC6 1.4684(16), N2ꢀC3 1.3298(16), N4ꢀC5
.3112(16), C5ꢀN17 1.4555(16), C7ꢀN14 1.5337(15), C7ꢀN11
.5373(17), N8ꢀO9 1.2122(14), N8ꢀO10 1.2138(15).
(
P) and detonation velocities (D) were calculated on the basis of
1
.4555(16) and 1.4684(16) Å, which is in the expec-
ted range. Yet C7ꢀN8, C7ꢀN11, and C7ꢀN14 are 1.5316(15),
.5373(15), and 1.5316(17) Å, respectively, which are longer than
traditional ChapmanꢀJouget thermodynamic detonation theory
19
using Cheetah 5.0 (Table 1). The detonation pressures of
polynitro triazoles lie in the range between P = 33.83 and P =
38.41 GPa (as compared to TNT 19.53 GPa, RDX 35.2 GPa, and
HMX 39.6 GPa). Detonation velocities lie between D = 8742
1
normal CꢀN single bonds (Figure 2). The N17ꢀO18 and N17ꢀ
O19 bond lengths are 1.2172(15) and 1.2216 Å. The NꢀO bond
lengths of the nitro functionality of the trinitromethyl group are
N8ꢀO9 1.2122(14), N8ꢀO10 1.2138(15), N11ꢀO13 1.2073-
ꢀ
1
ꢀ1
and D = 9229 m s (as compared to TNT 6881 m s , RDX
ꢀ
1
ꢀ1
8997 m s , and HMX 9320 m s ). The specific impulse values
(CHEETAH 5.0) of these polynitrotriazoles, which range be-
tween 233 and 264 s, suggest propellant possibilities. The
calculated properties coupled with the rather high thermal and
hydrolytic stabilities suggest that these high nitrogen, oxygen-rich
materials may be attractive candidates for energetic applications.
Oxygen balance (OB) is the index of the deficiency or excess
of oxygen in a compound required to convert all carbon into
carbon dioxide, and all hydrogen into water; for a compound
with the molecular formula of C H N O , OB (%) = (d ꢀ 2a ꢀ
(
1
14), N11ꢀO12 1.2195(14), N14ꢀO15 1.2100(15), N14ꢀO16
.2173(15), N17ꢀO18 1.2172, and N17ꢀO19 1.2216(15) Å, res-
pectively. The triazole ring and three nitro groups are tetragonally
attached to C7. The CꢀN bond length [1.533(15) Å] of the
trinitromethyl group of 12 is slightly longer than the CꢀN bond
4
length [1.525(3) Å] of trinitroethyl-containing molecules. This is
in agreement with the greater stability of the latter compounds.
a
b c d
’
PHYSICAL PROPERTIES
b/2)/MW (Table 1). Positive oxygen balance has significance in
explosives, which can be used as oxidizers. Compound 9 has a
positive oxygen balance of 9.1%; the oxygen balances of 11, 12,
and 14 are ꢀ7.3%, ꢀ3.6%, and ꢀ8.6%, respectively, which are
superior to RDX (ꢀ22%) and HMX (ꢀ22%). The oxygen
percentages of 9, 11, and 12 are 48.65%, 44.02%, and 46.19%,
respectively, whicharesuperior to TNT (42.26%), RDX (43.22%),
and HMX (43.22%) and nearly competitive with AP.
As shown in Table 1, the polynitro triazoles exhibit excellent
energetic properties. The enthalpies of energetic materials de-
pend on the molecular structures of the compounds. Conse-
quently, heterocycles with high nitrogen content exhibit higher
heats of formation. All ab initio calculations were carried out
using the program package Gaussian 03 (revision D.01). The
geometric optimization of the structures and frequency analyses
16
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Table 1. Physical Properties of Polynitro Triazoles
a
b
c
ꢀ3
d
e
f
g
h
i
j
compd
T
m
, °C
T
dec, °C
density, g cm
OB, %
O, %
ΔH
f
°, kJ/mol (kJ/g)
IS, J
P, GPa
D, m/s
I
sp, s
k
9
113
135
87
1.94 (1.92)
þ9.12
ꢀ7.33
ꢀ3.46
ꢀ8.6
48.7
44.0
46.2
41.5
39.2
42.3
43.2
43.2
123.2 (0.46)
84.1(0.38)
9.0
9.5
13.0
1.5
5.5
15
35.51
38.41
36.88
36.65
33.83
19.53
35.17
39.63
8983
9229
9006
8964
8742
6881
8997
233
243
264
264
262
1
1
1
1
1
2
4
5
1.91 (1.85)
1.88 (1.88)
1.83 (1.87)
1.78 (1.77)
1.65
77
81
153
150
165
295
230
287
95.6 (0.34)
555.1 (1.20)
505.8 (1.03)
ꢀ22.8
ꢀ74
TNT
RDX
HMX
ꢀ67.0 (0.30)
92.6 (0.42)
dec
1.82
ꢀ21.6
ꢀ21.6
7.4
7.4
dec
b
1.91
104.8 (0.35)
9320
d
a
ꢀ1
c
Melting point. Thermal decomposition temperature (onset) under nitrogen gas (DSC, 5 °C/min ). Gas pycnometer (25 °C). OB = oxygen
e
f
balance (%) for C
a
H
b
O
c
N
d
: 1600 ꢁ (c ꢀ 2a ꢀ b/2)/M
w w
, M = molecular weight of compound. Oxygen content. Heat of formation (Gaussian 03,
h i j
g
revision D.01). Impact sensitivity (BAM drophammer). Detonation pressure (Cheetah 5.0). Detonation velocity (Cheetah 5.0). Specific impulse
k
(
Cheetah 5.0). Calculated density, ref 20.
Scheme 6. Isodesmic Reactions of Polynitro Triazoles
To obtain a better understanding of the stabilities of the
polynitrotriazoles, a natural bonding orbital (NBO) population
analysis fixed at the optimized structure (DFT/3-21G) was used
to investigate the bonding and hybridization in these molecules
B3LYP functional with the 3-21G basis set. The optimized
structures were characterized as true local energy minima on
the potential energy surface without imaginary frequencies.
Natural bond orbital analysis provides an efficient method for
investigating charge distribution in molecular systems. The
charge distribution data calculated by the NBO method for
optimized geometries of 9 and 12 and for 14 and 15 are tabulated
in Tables S3 and S4 and Tables S5 and S6, respectively (Sup-
porting Information). NBO analysis shows that the CꢀNO2
(
single-point energy calculated at the DFT/3-21G level). The
natural bond orbital (NBO) analysis was carried out with the 3.1
16
version that is included in Gaussian 09 program at the mp2-
full)/6-311þþg(d,p) level. The geometry optimizations of the
structures and frequency analysis were carried out with the
(
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Journal of the American Chemical Society
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negative nitro charges (QNitro) correspond to the more stable
23
nitro compounds, because there is more than one nitro group
in our synthesized molecules. In this Article, the QNitro values for
compounds 9, 12, 14, and 15 are listed in Table S7 (Supporting
Information). From Table S7, we can find that average QNitro
decreases when the methyl group was introduced into the
molecule for compounds 9 (ꢀ0.175 as compared to ꢀ0.179 of
1
2) and 14 (ꢀ0.164 as compared to ꢀ0.166 of 15). The more
negative average Q
value indicates that compounds 12 and
Nitro
1
5 are more stable than compounds 9 and 14, respectively, which
are similar to the results for impact sensitivity tests or thermal
decomposition results (Table 1).
Figure 3. NBO analysis of 9 (charge densities: Table S3 of the Suppo-
rting Information).
QNitro ¼ QN þ QO1 þ QO2
ð1Þ
n
X
1
QNitro ¼ n
Q
Nitro,i
ð2Þ
i¼1
The structure of 12 is compared to the X-ray diffraction data.
Most of the optimized bond lengths by B3LYP are similar to the
experimental values. The triazole ring appears to have a slightly
distorted structure, with N2ꢀC3(1.3325) and C3ꢀN4(1.3500)
bond lengths next to the point of substitution not being equal to
the bonds N4ꢀC5(1.3135) and N1ꢀC5(1.3617), respectively.
This is also evident from the increase and decrease of bond angles
exactly at the point of substitution as shown in Table S2 (Suppor-
ting Information). As is seen from Table S2, theoretical calcula-
tions generally give good values for CꢀN and CꢀC bond lengths
of 9 when compared to experimental data. Although theoretical
results deviate slightly from the experimental values, this may
arise because theoretical calculations were carried out on the gas-
phase molecule, whereas the experimental results were obtained
in the solid state. The calculated geometric parameters also
represent a good approximation, and they can be used as a
foundation to calculate the other parameters for the compound.
Figure 4. NBO analysis of 14 (charge densities: Table S4 of the Suppor-
ting Information).
carbon is more positively charged than CꢀC(NO ) . The C5
2
3
carbon of the triazole ring is more positively charged than the C3
carbon in 9 and 12, while the reverse is the case for 14 and 15. All
of the ring nitrogen atoms have negative charge densities, and the
nitrogen atoms of the nitro groups have essentially the same
’
CONCLUSIONS
positive charge density. Oxygen atoms of the NO group bonded
2
to C5 exhibit a more negative charge density as compared to the
oxygen atoms of the trinitromethyl group (9 and 12). Optimized
The syntheses of high energy density polynitro triazoles,
5-nitro-3-trinitromethyl-1H-[1,2,4]triazole) (9), 3-dinitromethyl-
5-nitro-1H-[1,2,4]triazole (11), 1-methyl-5-nitro-3-trinitromethyl-
9
and 12 have an unsymmetric charge distribution in the entire
0
0
molecule. The electron-withdrawing effect of the NO group
1H-[1,2,4]triazole (12), 5,5 -bis(trinitromethyl)-3,3 -azo-1,2,4-
2
0
0
0
considerably reduces the electronic charge of the connected
atoms, and the negative charge is mainly delocalized over the
carbon atoms in the triazole ring and the methyl group (in case of
triazole (14), and 1,1 -dimethyl-5,5 -bis(trinitromethyl)-3,3 -azo-
1,2,4-triazole (15), were carried out, and their physical and
detonation properties were determined. These compounds exhibit
good physical and detonation properties, such as moderate thermal
stabilities, high densities, high heats of formation, and high deto-
nation pressures and velocities. Calculated detonation values for
these compounds are comparable to those of explosives such as
TNT, RDX, and HMX. Tetranitro triazole 9 has a positive oxygen
balance, 48.6% oxygen content, and a decomposition temperature
of 135 °C, which suggests its use as a new stable oxidizer. In all
aspects, except for oxygen balance, it is far superior to ADN. The
other triazoles have good oxygen balance that is superior to RDX
and HMX. The N-methylated compounds are less sensitive than
the corresponding N-H triazoles. Triazoles 9, 11, and 12 are less
sensitive than RDX and HMX, which suggests that these com-
pounds might be of interest for future applications as environmen-
tally friendly and high-performing nitrogen or oxygen-rich mate-
rials and may serve as a series of promising alternatives to RDX
and HMX.
1
2) on the ring due to the conjugative and resonance effects of
the ring. N1 of 9 and 14 show a higher negative charge density
than the analogous nitrogen of 12 and 15 arising from the
presence of the methyl group (Figures 3 and 4). The azo
nitrogens of both 14 and 15 have nearly equal charge densities.
Nitro compounds are very strong electron acceptors, and this
ability can be represented by the net charges of the nitro group. A
higher negative charge on the nitro group implies lower electron
22
attraction and therefore more stability of the nitro compound.
Recently, a nitro group charge method (NGCM) was established
and applied to predict the properties of nitro compounds or
23
explosives, including molecular stability and impact sensitivity.
The molecular stability of the compounds was evaluated and
compared to the nitro group charges (QNitro). QNitro is the alge-
braic sum of charges of all three atoms of a nitro group (eq 1).
The average value for QNitro can be calculated by eq 2. The more
6
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Journal of the American Chemical Society
ARTICLE
’
EXPERIMENTAL SECTION
combined to yield 11 (54%) and dried in a vacuum desiccator over
P O .
4
10
Safety Precautions. While we have experienced no difficulties in
Yellow solid; Tdec 87 °C (onset). IR (KBr): ν~ 3433, 3314, 1602, 1562,
ꢀ1 13
1
the syntheses and characterization of these materials, proper protective
measures should be used. Manipulations must be carried out in a hood
behind a safety shield. Eye protection and leather gloves must be worn.
Caution should be exercised at all times during the synthesis, character-
ization, and handling of any of these materials, and mechanical actions
involving scratching or scraping must be avoided.
1
544, 1462, 1392, 1318, 1239, 1181, 1058, 840, 777, 682 cm . C{ H}
NMR: δ 163.7, 151, 124.9. Anal. Calcd for C H N O6.5, 227: C, 15.87;
3
3 6
H, 1.33; N, 37.01. Found: C, 16.28; H, 0.95; N, 36.25.
-Methyl-5-nitro-3-trinitromethyl-1H-[1,2,4]triazole (12).
To a solution of 9 (100 mg, 0.38 mmol) in diethyl ether was added a
M solution of trimethylsilyl diazomethane (0.3 mL) dropwise at 0 °C.
1
2
1
13
15
General Methods. H, C, and N NMR spectra were recorded
on 300 MHz (Bruker AVANCE 300) and 500 MHz (Bruker AVANCE
The reaction mixture was stirred for 30 min, the solvent was evaporated,
and the product was purified by column chromatography (eluted with
40% CH Cl /hexane from silica gel) to give 12 (80%).
5
7
00) nuclear magnetic resonance spectrometers operating at 300.13,
5.48, and 50.69 MHz, respectively, by using DMSO[d ] as solvent and
2
2
6
White solid; T 77 °C, T 153 °C (onset). IR (KBr): ν~ 3454, 1619,
m
dec
locking solvent unless otherwise stated. The melting and decomposition
points were obtained on a differential scanning calorimeter (TA Instru-
1598, 1569,1504, 1443, 1400, 1337, 1290, 1281, 1171, 1110, 957, 844,
ꢀ
1 1
13
1
798 cm . H NMR (CDCl ): δ 4.38. C{ H} NMR: δ 153.9, 144.4,
3
ꢀ
1
15
1
ments Co., model Q10) at a scan rate of 5 °C min . IR spectra were
recorded using KBr pellets for solids on a BIORAD model 3000 FTS
spectrometer. Densities of the polynitro triazoles were determined at
121.8, 42.1. N{ H} NMR: δ ꢀ35.02 (NO ), ꢀ35.58 ((NO ) ), ꢀ59.99
2
2 3
4
3 7 8
(N2), ꢀ128.03 (N3), ꢀ165.68 (N1). Anal Calcd for C H N O , 277: C,
17.34; H, 1.09; N, 35.38. Found: C, 17.88; H, 0.99; N, 35.58.
0
0
25 °C by employing a Micromeritics AccuPyc 1330 gas pycnometer.
5,5 -bis(Trinitromethyl)-3,3 -azo-1,2,4-triazole Hemihydrate
(14). To a mixture of sulfuric acid (98%, 6 mL) and nitric acid (100%,
5 mL) was added [5-(5-carboxymethyl-1H-[1,2,4]triazole)-2H-[1,2,4]-
triazol-3-yl]-acetic acid (1 g, 3.57 mmol) at 0 °C. After being stirred for
15 h at room temperature, the reaction mixture was poured into ice water,
and the precipitate was filtered. The mother liquor was extracted with ethyl
Elemental analyses were carried out using an Exeter CE-440 elemental
analyzer. Details of the X-ray diffraction analysis of compound 12 are
presented. Data were collected on a Bruker three-circle platform diffrac-
tometer equipped with a SMART PLATINUM 135 CCD detector.
X-ray Crystallography. A thin colorless plate of dimensions 0.26 ꢁ
3
0
.24 ꢁ 0.01 mm was mounted on a MiteGen MicroMesh using a small
4
acetate, and the extract was dried over MgSO . The solvent was evaporated
to give additional product. The combined solids are 14 (55%).
Yellow solid; Tdec 150 °C (onset). IR (KBr): ν~ 3467, 1630, 1617,
amount of Cargille Immersion Oil. Data were collected on a Bruker
three-circle platform diffractometer equipped with a SMART APEX II
CCD detector. The crystals were irradiated using graphite monochro-
ꢀ
1
1593, 1442, 1342, 1282, 1194, 1097, 1052, 964, 842, 801, 758 cm
.
1
3
1
15
1
mated MoK
R
radiation (λ = 0.71073). An Oxford Cobra low tempera-
C{ H} NMR: δ 165.7, 148.8, 123.8. N{ H} NMR: δ ꢀ31.62
((NO
(N1). Anal. Calcd for C
Found: C, 15.56; H, 0.32; N, 41.50.
ture device was used to maintain the crystals at a constant 100(2) K
during data collection. Data collection was performed, and the unit cell
2
)
3
), ꢀ33.0 (NdN), ꢀ60.35 (N2), ꢀ131.79 (N3), ꢀ132.26
H N O
6 3 14
12.5, 471: C, 15.29; H, 0.64; N, 41.62.
24
was initially refined using APEX2 [v2010.3-0]. Data reduction was
25
26
0
0
0
performed using SAINT [v7.60A] and XPREP [v2008/2]. Correc-
tions were applied for Lorentz, polarization, and absorption effects using
1,1 -Dimethyl-5,5 -bis(trinitromethyl)-3,3 -azo-1,2,4-triazole
(15). To a solution of 14 (100 mg, 0.21 mmol) in diethyl ether was added a
2 M solution of trimethylsilyl diazomethane (0.5 mL) dropwise at 0 °C.
The reaction mixture was stirred for 30 min, filtered, and the precipitate
was washed with diethyl ether (5 mL) to give 15 (76%).
27
SADABS [v2008/1]. The structure was solved and refined with the aid
2
8
of the programs in the SHELXTL-plus [v2008/4] system of programs.
2
The full-matrix least-squares refinement on F included atomic coordi-
nates and anisotropic thermal parameters for all non-H atoms. The H
atoms were included using a riding model.
Yellow solid; Tdec 150 °C (onset). IR (KBr): ~ν 3441, 1625, 1593, 1474,
ꢀ1 1
1
443, 1346, 1290, 1220, 1108, 962, 844, 801, 754 cm . H NMR: δ 4.39.
1
3
1
C{ H} NMR: δ 159.7, 147.7, 122.8. Anal. Calcd for C H N O , 490:
5
-Nitro-3-trinitromethyl-1H-[1,2,4]triazole (9). To a mixture
of 98% sulfuric acid (12 mL) and 100% nitric acid (10 mL) was added
5-nitro-1H-[1,2,4]triazol-3yl)-acetic acid (7) (15 mmol) at 0 °C. After
being stirred for 15 h at room temperature, the solution was poured into
8 6 14 12
C, 19.60; H, 1.23; N, 40.00. Found: C, 19.69; H, 1.13; N, 38.00.
(
’
ASSOCIATED CONTENT
∼
30 g of ice and extracted with dichloromethane (3 ꢁ 15 mL). The
organic layer was dried over MgSO
4
, and the solvent was evaporated to
S
Supporting Information. X-ray crystallographic files in
b
1
13
yield 9 (44%).
CIF format for 12, H, C NMR and IR spectra of all com-
pounds, DSC scans of all compounds, and NBO charge densities.
This material is available free of charge via the Internet at http://
pubs.acs.org.
Colorless solid: T
134, 1632, 1610, 1572, 1519, 1481, 1371, 1280, 1174, 1080, 1041, 962,
m
113.7 °C, Tdec 135 °C (onset). IR (KBr): ν~ 3443,
3
8
ꢀ
1
13
1
39, 800, 634 cm
. C{ H} NMR (CD CN): δ 157.9, 147.2, 122.2.
3
1
5
1
N{ H} NMR: δꢀ22.50 (NO
2
),ꢀ29.33((NO ),ꢀ38.58 (N2), ꢀ54.98
)
2 3
(
N3), ꢀ132.33 (N1). Anal. Calcd for C
3
HN O , 263: C, 13.70; H, 0.38;
7 8
’
AUTHOR INFORMATION
N, 37.27. Found: C, 14.18; H, 0.30; N, 37.52.
-Dinitromethyl-5-nitro-1H-[1,2,4]triazole Hemihydrate
11). To a solution of potassium hydroxide (385 mg, 1.55 mmol) in
ethanol (3 mL) was added a solution of hydroxylamine hydrochloride
239 mg, 3.42 mmol) in water (0.5 mL). After being stirred for 15 min,
3
Corresponding Author
jshreeve@uidaho.edu
(
(
the potassium chloride precipitate was removed by filtration, and the
mother liquor cooled to 0ꢀ5 °C was added dropwise to a solution of
tetranitro triazole (9) (410 mg, 1.55 mmol) in ethanol (2 mL). The
mixture was stirred for 3 h, and the precipitate was separated. The
suspension of potassium salt (8) in water (2 mL) at 0ꢀ5 °C was acidified
with 50% sulfuric acid (0.5 mL). The precipitate was removed by
filtration. The filtrate was extracted with ethyl acetate (3 ꢁ 10 mL),
’ ACKNOWLEDGMENT
We gratefully acknowledge the support of ONR (N00014-10-
-0097).
1
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Journal of the American Chemical Society
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dx.doi.org/10.1021/ja2013455 |J. Am. Chem. Soc. 2011, 133, 6464–6471