Energetic polyazole polynitrobenzenes and their coordination complexes

Article abstract of DOI:10.1039/b915090k

New energetic polyazole polynitrobenzenes exhibit high heats of formation and thermal stabilities, and react as useful ligands with silver dinitramide to give an energetic coordination complex.

Full text of DOI:10.1039/b915090k

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Energetic polyazole polynitrobenzenes and their coordination complexesw
Zhuo Zeng,a Yong Guo,b Brendan Twamleyb and Jean’ne M. Shreeve*b
Received (in Cambridge, UK) 24th July 2009, Accepted 28th August 2009
First published as an Advance Article on the web 9th September 2009
DOI: 10.1039/b915090k
New energetic polyazole polynitrobenzenes exhibit high heats of
formation and thermal stabilities, and react as useful ligands
with silver dinitramide to give an energetic coordination complex.
1,3,5-trinitrobenzenes (1), to form mono-, di-, tri-, and
tetra-azole-substituted 1,3,5-trinitrobenzenes at 25 1C in
one-pot reactions as shown in Scheme 1. This is a new and
practical route to polyazole-substituted 1,3,5-trinitrobenzenes.
Additionally, development of new metal-coordinated
complexes with these polyazole-substituted 1,3,5-trinitrobenzenes
results in high performance energetic materials. 2,4,6-Tri-1H-
triazol-1-yl-1,3,5-trinitrobenzene, 3f, or 2,3,4,6-tetra-1H-triazol-
1-yl-1,3-dinitrobenzene, 4, compounds with high heats of
formation, were coordinated with oxygen-rich silver dinitramide,
to give an unexpected product 2,4,6-tri-1H-triazol-1-yl-3,5-
dinitrophenol silver monohydrate, as well as the desired
2,3,4,6-tetra-1H-triazol-1-yl-1,3-dinitrobenzene silver dinitramide
coordination complex (vide infra).
Polynitroarylenes play important roles as intermediates in the
syntheses of pharmaceuticals, agrochemicals, polymers, and
energetic materials.1a–c Substituted trinitrobenzenes comprise
an important class of explosives for commercial and military
use, e.g., for 2,4,6-trinitrotoluene (TNT) [detonation velocity
(vD)
=
6881
m
sꢀ1
;
density (d)
=
1.65
7350
g
cmꢀ3],
sꢀ1
,
2,4,6-trinitrophenol (picric acid) (vD
=
m
d = 1.71 g cmꢀ3), and 1,3,5-triamino-2,4,6-trinitrobenzene
(TATB) (vD = 8108 m sꢀ1, d = 1.94 g cmꢀ3).1d
The development of new energetic materials continues to
focus on the synthesis of new heterocycles with high
densities, high heats of formation, and good oxygen balance.2
Mono-, di-, or tri-azole-substituted 1,3,5-trinitrobenzene
moieties are good models which enable investigation of the
structure–property relationship in energetic materials.
Reactions of polyhalo-substituted 1,3,5-trinitrobenzene
with strong nucleophiles in dimethylformamide (DMF) at
room or elevated temperature are difficult to control and often
result in unwanted replacement of one or more of the nitro
groups, and/or incomplete substitution of halogen which
complicates isolation and purification.3 Elevated temperature
increases the hazard level in procedures for manufacturing
energetic materials.
2-1H-Imidazol-1-yl-1,3,5-trinitrobenzene (3a) and 2-1H-triazol-
1-yl-1,3,5-trinitrobenzene (3b) have been reported.5
Polynitrobenzene impurities, the by-products from these
syntheses, are difficult to remove from the DMF–H2O
mixture. However, when 2a or 2b was reacted with 2-fluoro-
1,3,5-trinitrobenzene, 1, at 25 1C in benzene, 3a or 3b resulted
immediately which could be purified in high yield.
The reactions of 2,4-difluoro-substituted 1,3,5-trinitrobenzene,
1b (1 mmol), with 2a (2 mmol) and 2b (2 mmol) were
investigated and found to give the disubstituted products
2,4-di-1H-imidazol-1-yl-1,3,5-trinitrobenzene, 3c, in moderate
yield, and 2,4-di-1H-triazol-1-yl-1,3,5-trinitrobenzene, 3d, in
high yield. 1-Amino-3,5-di-1H-1,2,4-triazolyltrinitrobenzene,
3e, was also prepared in high yield by interacting 1-amino-
3,5-difluoro-trinitrobenzene (1c) with 2b. A thermal ellipsoid
plot of 3ez is given in Fig. 1.
Fortunately, fluorosubstituted 1,3,5-trinitrobenzene is relatively
sensitive to nucleophilic substitution.4 1-Trimethylsilylimidazole
and 1-trimethylsilyltriazole are important as nucleophilic
reagents for transfer of imidazole, and triazole groups since
they exhibit better solubility in organic solvents and superior
regioselective nucleophilicity than their respective sodium
salts. The formation of trimethylsilyl fluoride (TMSF; bp 16 1C)
which is a readily volatile product provides the driving force
for reactions of fluorosubstituted 1,3,5-trinitrobenzene with
1-trimethylsilylazoles. This results in smooth and rapid
nucleophilic substitution reactions at 25 1C.
An efficient synthetic method for triazole-(imidazole-)
substituted 1,3,5-trinitrobenzene, 2,4,6-tri-1H-imidazol-1-
yl-1,3,5-trinitrobenzene, and 2,4,6-tri-1H-triazol-1-yl-1,3,5-
trinitrobenzene, 3f, has not been reported.
In this work, we utilize the mild nucleophiles 2a and 2b for
regioselective nucleophilic substitution reactions by taking
advantage of the formation of TMSF with fluorosubstituted
a College of Chemistry & Environment, South China Normal
University, Guangzhou, Guangdong 510006, P. R. China
b Department of Chemistry, University of Idaho, Moscow,
ID 83844-2343, USA. E-mail: jshreeve@uidaho.edu;
Fax: +1 12088859146; Tel: +1 12088856215
w Electronic supplementary information (ESI) available: Experimental
details, differential scanning calorimeter (DSC) figures for 3a–3f, 4, 6, 7.
CCDC 732611–732614. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/b915090k
Scheme 1 Synthesis of polyimidazolyl or triazolyl 1,3,5-trinitrobenzenes.
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Compound 3f was reacted with silver dinitramide in an
acetonitrile–methanol mixture at 25 1C for two weeks to give
red crystals of the unexpected 2,4,6-tri-1H-triazol-1-yl-3,5-
dinitrophenol silver monohydrate, [Ag(TTDP)(H2O)] (6).
Highly nitrated derivatives of benzene readily react with
water to form phenols, e.g., 1,2,3,5-tetranitrobenzene is readily
converted to picric acid by reaction with hot water, and some
hindered nitro groups of hexanitrobenzene are also displaced
smoothly even at 25 1C.9 A possible mechanism is that 3f in
which three nitro groups are hindered by two adjacent triazole
rings initially reacted with water to give the intermediate 5.
The latter subsequently reacted with silver dinitramide to give
the silver coordination complex, [Ag(TTDP)(H2O)] (6), and
dinitramide acid. The immediate conversion of the latter into
N2O and HNO3 would tend to drive the reaction forward.10
The yellow solid, 2,3,4,6-tetra-1H-triazol-1-yl-1,3-dinitro-
Fig. 1 Structure of one of the unique molecules in the asymmetric
unit of 3e (left) and 3f (right) (displacement ellipsoids shown at 30%
probability). Hydrogen atoms represented by spheres of arbitrary
radius.
When 3 mmol 1-trimethylsilylimidazole (2a) was reacted
with 1 mmol of 2,4,6-trifluoro-1,3,5-trinitrobenzene (1d) in an
attempt to prepare 2,4,6-tri-1H-imidazol-1-yl-1,3,5-trinitro-
benzene, a mixture was obtained. The product could not be
isolated using column chromatography. However, synthesis of
the triazole analogue, 3f, occurred in 80% yield, by slowly
dropping 2b into 1d dissolved in benzene (mole ratio 3 : 1) over
12 h at 25 1C. Importantly, if 2b was added too rapidly
or 1d was present in the benzene solution at a concentration
40.1 mmol mLꢀ1, an inseparable mixture was generated.
2,4,6-Tri-1H-triazol-1-yl-1,3,5-trinitrobenzene, 3f, was structured
by single crystal X-ray diffractiony (Fig. 1).
Polynitroarylenes easily undergo nucleophilic attack resulting
in the displacement of one or more of the nitro groups.6
Combining 2b and 1d, in a mole ratio of 4.5 : 1 under the
same reaction conditions, gave, after separation of the reaction
mixture, 3f (30% yield) and 2,3,4,6-tetra-1H-triazol-1-yl-1,3-
dinitrobenzene (4) (50% yield). In the latter case, in addition
to replacement of the fluorine atoms, one nitro group was
also displaced by triazole to give the tetratriazole-substituted
1,3-dinitrobenzene (4) suggesting that nitro groups positioned
meta to one another are rather susceptible to nucleophilic
attack even under mild reaction conditions.
benzene
silver
dinitramide
coordination
complex,
[Ag(TTDB)(CH3CN)][N(NO2)2] (7) was obtained after one
week from 4 and AgN(NO2)2 in acetonitrile–methanol.
That 2,3,4,6-tetra-1H-triazol-1-yl-1,3-dinitrobenzene (4) is
more stable chemically than 2,4,6-tri-1H-triazol-1-yl-1,3,5-
trinitrobenzene (3f) based on the results of the reaction with
silver dinitramide suggests that 4 could be useful as a suitable
ligand to form energetic metal coordination complexes.
The reaction with AgN(NO2)2 to create dinitramide salts of
3f and 4 led to the synthesis and isolation of the unexpected
product, 6z (Fig. 2), and the sought 2,3,4,6-tetra-1H-triazol-1-
yl-1,3-dinitrobenzene silver dinitramide coordination complex,
[Ag(TTDB)(CH3CN)][N(NO2)2] (7)8 (Fig. 2). In 6, a complex
honeycomb structure is formed with each Ag four coordinate
(Ag1–N = 2.285(3)–2.414(3)). The phenolic O– is stabilized by
intramolecular H-bonding (C11–H11ꢂ ꢂ ꢂO1 = 2.699(4) A),
and possibly by the solvent H2O molecules. The higher
nitrogen-containing dinitramide complex
7 is shown in
Fig. 2. The silver atom is five-coordinate, and in a distorted
trigonal bipyramid geometry. The triazoles N10, N15 and N20
form the equatorial plane and the dinitramide, and disordered
solvent MeCN molecules are the apical components of the
coordination geometry (Ag–N = 2.339(2), 2.231(2), 2.324(2),
2.668(2) and 2.599(8)A, respectively).
Metal coordination complexes offer significant energetic
performance improvement because of the presence of both
oxidizing and reducing groups in the same complex molecules.7
Combination of the oxygen-rich silver dinitramide8 with 3f
(DHf = 737.1 kJ molꢀ1) or 4 (DHf = 920.2 kJ molꢀ1) results
in attractive new metal coordination complexes which could
lead to the development of new energetic materials (Scheme 2).
Physical characteristics and some key properties of these
energetic materials are given in Table 1.
To assess the potential of the newly prepared poly-
azole-substituted trinitrobenzene compounds, their energetic
Fig. 2 Packing diagram of 6 (left) showing the honeycomb voids
along the b-axis. Dashed lines indicate close contacts. Structure of 7
(right) (displacement ellipsoids shown at 30% probability). Only the
major disordered component is shown.
Scheme 2 Synthesis of a silver dinitramide polytriazole polynitro-
benzene coordination complex.
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Table 1 Thermal and physical properties of polyazole polynitrobenzene
give an unexpected hydrolysis product, and a five-coordinate
silver dinitramide complex.
Comp
da
Td
DHfc
167.6
361.8
223.5
476.4
463.8
737.1
920.2
Pd
vD
Isp
ISg
b
e
f
The authors gratefully acknowledge the support of
HDTRA1-07-1-0024, NSF (CHE-0315275), and ONR
(N00014-06-1-1032).
3a
3b
3c
3d
3e
3f
4
1.67
1.70
1.68
1.71
1.72
1.75
1.56
1.96
1.83
1.65
1.94
216.0
256.0
156.0
261.0
298.3
228.0
249.5
293.0
164.0
295.0
330.0
19.7
19.5
21.5
22.1
22.4
22.4
15.3
—
7164
7255
7380
7549
7178
7197
6778
—
210.0
216.8
204.1
215.4
212.3
214.8
201.4
—
40
40
35
35
35
35
45
30
20
15
50
Notes and references
z Crystal data for 3e: C12H9N11O6,
M = 403.30, monoclinic,
6
7
—
—
a = 9.6692(4), b = 9.4624(4), c = 19.0356(8) A, b = 97.696(1)1,
U = 1725.95(13) A3, T = 90(2) K, space group = P2(1)/c (no. 14),
Z = 4, 25 293 reflections measured, 3967 unique (Rint = 0.0386) which
were used in all calculations. The final wR(F2) was 0.0975 (all data).
—
19.5
31.1
—
6881
8114
—
213.0
—
TNT
TATB
ꢀ67
ꢀ139.8
y Crystal data for 3f: C12H6N12O6,
M = 414.29, monoclinic,
a
b
c
Density (g cmꢀ3). Decomposition temperature (1C). Heat of
a = 12.2582(17), b = 10.7107(15), c = 24.007(3) A, b = 92.253(2)1,
U = 3149.5(7) A3, T = 90(2) K, space group = P2(1)/c (no. 14),
Z = 8, 72 957 reflections measured, 7224 unique (Rint = 0.0423) which
were used in all calculations. Rotational twin—179.91 rotation
about the reciprocal axis 1.000, 0.000, ꢀ0.001, refined twinning
ratio = 0.3122(9). The final wR(F2) was 0.0909 (all data).
d
e
formation (kJ molꢀ1). Detonation pressure (GPa). Detonation
f
g
velocity (m
(BAM fallhammer) (J).
s
ꢀ1). Isp—specific impulse (s). Impact sensitivity
z Crystal data for 6: C12H12AgN11O8, M = 546.20, monoclinic,
a = 20.041(3), b = 8.2412(8), c = 24.046(3) A, b = 111.131(9)1,
U = 3704.4(8) A3, T = 90(2) K, space group = C2/c (no. 15), Z = 8,
27 473 reflections measured, 3333 unique (Rint = 0.0342) which were
used in all calculations. The final wR(F2) was 0.0886 (all data).
characteristics are compared with 2,4,6-trinitrotoluene (TNT)
and 1,3,5-triamino-2,4,6-trinitrobenzene (TATB). As shown in
Table 1, the density increases with the increasing number
of azolate substituents in the polyazole trinitrobenzenes.
The [Ag(TTDB)(CH3CN)][N(NO2)2] (7) silver dinitramide
coordination complex with a density of 1.83 g cmꢀ3 is higher
8 Crystal data for 7: C16H11AgN18O8,
M = 691.30, triclinic,
a = 8.8611(4), b = 11.1986(5), c = 13.3871(6) A, a = 78.4787(6)1,
b = 74.2808(6)1, g = 84.8365(7)1, U = 1252.07(10) A3, T = 90(2) K,
space group = P1 (no. 2), Z = 2, 18 562 reflections measured,
than the ligand TTDB at 1.56 g cmꢀ3
.
ꢀ
5758 unique (Rint = 0.0354) which were used in all calculations. The
final wR(F2) was 0.0802 (all data).
These polyazole trinitrobenzene compounds exhibit good
thermal stabilities, ranging from 156 to 298 1C. The 1-amino-
3,5-di-1H-1,2,4-triazolyltrinitrobenzene (3e), which decomposes
at 298 1C, has the highest thermal stability due to strong
intramolecular hydrogen bonding. The heats of formation for
3a, 3b, 3c, 3d, 3e, 3f, and 4 were calculated using the Gaussian
03 suite of programs11 (see ESIw), and are summarized in
Table 1. The heat of formation increases with the number of
azolate substituents for 1,3,5-trinitrobenzene. 2,3,4,6-Tetra-
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1H-triazol-1-yl-1,3-dinitrobenzene (4) (DHf = 920.2 kJ molꢀ1
)
has the highest heat of formation. These values are also
considerably more positive than that of TATB (ꢀ139.8 kJ molꢀ1).
Calculation of detonation properties and specific impulse
values was obtained using Cheetah 5.0.12 In general, an
increase in the number of azole substituents in trinitrobenzene
(higher density) results in higher detonation pressures and
velocities. With the exception of 4, all have higher detonation
pressures and velocities than TNT. Their impact sensitivities
(BAM fallhammer) range from 20 to 45 J and are relatively
insensitive. The new polyazole polynitrobenzenes exhibit good
thermal stabilities, and high heats of formation that make
them new energetic materials.
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¨
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In conclusion, the energetic polyazole-substituted poly-
nitrobenzenes were obtained in good yield by the regioselective
nucleophilic substitution of fluorosubstituted 1,3,5-trinitro-
benzene with trimethylsilylazoles at 25 1C in one-pot reactions.
Polyazole polynitrobenzenes with high heats of formation
used as energetic ligands reacted with silver dinitramide, to
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This journal is The Royal Society of Chemistry 2009
6016 | Chem. Commun., 2009, 6014–6016