Synthesis and characterization of 1,1′-azobis(5-methyltetrazole)

Article abstract of DOI:10.1039/c2nj40731k

A high-nitrogen compound (N₁₀ structure), 1,1′-azobis(5- methyltetrazole) which is relatively stable, was obtained by azo coupling reactions with three different oxidants such as trichloroisocyanuric acid (TCICA), sodium dichloroisocyanurate (SDIC) and tert-butyl hypochlorite (t-BuOCl). In particular, TCICA has been used for the first time to oxidize N-NH₂ to the N-N=N-N linkage. The structural elucidation of the title compound was made by spectral and X-ray crystallographic analyses. The new N₁₀ linkage containing compound exhibits both relative thermal stability and physical stability. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012.

Full text of DOI:10.1039/c2nj40731k

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LETTER
Synthesis and characterization of 1,10-azobis(5-methyltetrazole)w
Yongxing Tang, Hongwei Yang,* Jianhua Shen, Bo Wu, Xuehai Ju, Chunxu Lu
and Guangbin Cheng*
Received (in Montpellier, France) 16th August 2012, Accepted 1st October 2012
DOI: 10.1039/c2nj40731k
A
high-nitrogen compound (N10 structure), 1,10-azobis-
for the first time by employing the same azo coupling reagent
SDIC.7b Unfortunately this material was both thermally and
physically unstable with a decomposition temperature of 80 1C
and it decomposes in solution. The compound, containing eight-
catenated nitrogen atoms, 2,20-azobis(5-nitrotetrazole) (3)
reported by the same group was even more unstable in solution
and difficult to handle with violent explosion when subjected
to mild stimuli.7a In a quest for the synthesis of a highly stable
high-nitrogen containing compound, herein we report the
synthesis and characterization of 1,10-azobis(5-methyltetrazole)
5, a relatively stable ten-nitrogen catenated azo compound.
By following the synthetic procedures used for the generation
of compounds 1–3, we treated 5-methyl-1-aminotetrazole,
4 with sodium dichloroisocyanurate (SDIC) as azo coupling
reagent to afford 5 with 53% yield (entry 1, Table 1). In order to
improve the yield of 5 and to explore other oxidizing agents, we
screened a host of different oxidants which were successfully
used to prepare compounds contain the C–NQN–C structure
from energetic compounds with the C–NH2 group. Unfortunately,
no formation of azo compound 5 was observed following the use
(5-methyltetrazole) which is relatively stable, was obtained by
azo coupling reactions with three different oxidants such as
trichloroisocyanuric acid (TCICA), sodium dichloroisocyanurate
(SDIC) and tert-butyl hypochlorite (t-BuOCl). In particular,
TCICA has been used for the first time to oxidize N–NH2 to
Q
the N–N N–N linkage. The structural elucidation of the title
compound was made by spectral and X-ray crystallographic
analyses. The new N10 linkage containing compound exhibits
both relative thermal stability and physical stability.
The generation of nitrogen gas as an end product of nitrogen-rich
compounds is highly favored for the enhancement of energy and
avoiding environmental pollution.1 Nitrogen-rich compounds have
therefore received increasing attention as promising candidates for
high energy-density materials (HEDM) which might be used as
propellants, explosives or especially as gas generators.2 Over the
past decades, numerous efforts have been devoted to the syntheses
of azo linked compounds especially those containing the
C–NQN–C structure owing to their various roles as nitrogen-
rich compounds,3 important dye materials4 and photochromic
materials.5 The oxidative azo coupling reaction starting from the
C–NH2 functionality using reagents such as KMnO4 in HCl and
10% NaClO solution have been extensively studied.6 Although
azo coupling from N–NH2 is a very efficient approach for the
expansion of the nitrogen atom chain, their synthetic study has
been very limited compared with that of C–NH2.5a,7,8
of oxidants such as KMnO4 in HCl, or NaOH,6,8b H2O2,4a Br2,
8d,9
and 10% NaClO solution6,10 (entries 2–6, Table 1). In our
continuous search for the best azo coupling reagent we employed
tert-butyl hypochlorite (t-BuOCl) for the oxidative coupling of
5-methyl-1-aminotetrazole in acetonitrile at 0 1C to form 5 in 59%
yield. It is noteworthy to mention that tert-butyl hypochlorite has
failed to yield 3 from 1-aminotetrazole.7b,11 A slight increase in
yield from 53 to 59% of azo product 5 (cf. entry 1 and entry 7,
Table 1) by changing the oxidizing agent from SDIC to t-BuOCl
motivated us to explore new azo coupling reagents. Next, we
Azo energetic compounds containing long catenated nitrogen
atom chains (more than 8 nitrogen atoms) have recently
attracted great attention due to their unique properties and high
heats of formation (Scheme 1).5a,7 1,10-Azobis-1,2,3-triazole (1)
with a stable N8 structure was synthesized by treatment of
1-amino-1,2,3-triazole with sodium dichloroisocyanurate
(SDIC) as azo coupling reagent at low temperature, and
12
employed trichloroisocyanuric acid (TCICA) as an oxidant for
the conversion of 4 to 5. To our delight, better yield of 5 was
obtained with TCICA (67%, entry 8, Table 1) than both SDIC and
t-BuOCl under the same reaction conditions. To the best of our
knowledge, TCICA is used successfully for the first time as
oxidative azo coupling reagent to convert N–NH2 to the
N–NQN–N linkage. We have found that the new N10 catenated
well characterized.5a The Klapotke group had synthesized 1,
10-azobistetrazole (2) having the well characterized N10 structure
¨
School of Chemical Engineering, Nanjing University of Science and
Technology, Xiaolingwei 200, Nanjing, Jiangsu, China.
E-mail: hyang@mail.njust.edu.cn, gcheng@mail.njust.edu.cn;
Fax: +86 25 8431 601; Tel: +86 25 8431 5948-8205
w Electronic supplementary information (ESI) available: CCDC
reference number 889575. For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/c2nj40731k
Scheme 1 High-nitrogen compounds with N8 and N10 chain.
c
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Table 1 Azo coupling reaction of 5-methyl-1-aminotetrazole with
different oxidants
1 and 2. Color change, UV–vis spectral (Fig. S6, ESIw) and
Raman spectral (Fig. S7, ESIw) changes were not observed
when a sample of 5 was exposed to xenon light radiation for 1h
or by three days exposure to UV light. Therefore, application
of 5 in the photochromic materials research field is limited
owing to the lack of photochromicity.
Typical TG-DTG and DSC thermographs of 5 are shown in
Fig. 2 with the temperature ranging from 50 1C to 250 1C. The
compound 5 has a narrow exothermicity temperature range
with an onset decomposition temperature of 112.5 1C and an
exothermic peak with its maximum at 127.2 1C, which is 47 1C
higher than that of 2 (80 1C). TG spectra exhibited a loss
amounting to B80% in the temperature range of 116–136 1C
with a DTG peak at 128.1 1C. This mass loss stage corresponds
to the exothermic process at peak temperature 127.2 1C in the
DSC curve, which shows that the introduction of a methyl
group has increased the thermal stability.
Reaction conditionsa
Oxidant
Solvent Temp (1C) Time (h) Yieldb (%)
Entry
1
2
3
4
5
6
7
8
SDIC
KMnO4/HCl
CH3CN
None
0
50
50
0.5
6
6
6
2
2
0.5
0.5
53
0
0
0
0
0
59
67
KMnO4/NaOH None
H2O2
Br2/CH3COOH CH3CN
CH3CN r. t.
0
r. t.
0
10%NaClO
t-BuOCl
None
CH3CN
CH3CN
TCICAc
0
a
Unless otherwise mentioned, reaction conditions: mol ratio of 5-methyl-
1-aminotetrazole to oxidant was 1 : 1. 5-Methyl-1-aminotetrazole:
10.0 mmol. b Isolated yields. c The mole ratio of 5-methyl-1-aminotetrazole
to TCICA is 1 : 2.
Compound 5 possesses a calculated heat of formation of
986 kJ molꢀ1, which is lower than that of the N10 compound 2
as a result of the addition of the methyl group to the carbon
atom of tetrazole ring, and higher than that of the N8
compound 1 due to the longer nitrogen chain. High heat of
formation and the low density of 5 result in a moderate
calculated detonation velocity of 7320 m sꢀ1 and a detonation
pressure of 20.99 GPa, which is lower than that of compound
1–3 and TATB, and superior to TNT (Table 2).13 Although
possessing relative thermal stability, compound 5 is very
sensitive. We also experienced several unexpected explosions
when we took away the flask from the evaporator or while
transferring our sample from the flask.
compound has shown improved physical stability. Unlike the
reported compounds 2 and 3, storage of 5 in various solvents such
as DMSO, CHCl3, acetonitrile did not lead to decomposition
through the release of nitrogen gas.
Single crystals of 5 suitable for X-ray diffraction measurements
have been obtained by slow evaporation overnight from the
solution of 5 in acetone and ethyl acetate mixture. The ORTEP
diagram of 5 is shown in Fig. 1. The bond distances and angles of
the tetrazole moiety are in accordance with values of compounds
1 and 2 reported. The azo bond adopts a stable E configuration
due to lower active energy than the Z configuration. The bond
length of the azo double bond is 1.243 A, similar to that of 1 with
the N8 structure (1.250 A), but longer than that of compound
2 with the N10 structure (1.178 A). The crystal density of the N10
compound 5 with the methyl group at the carbon atom of the
tetrazole ring is 1.482 g cmꢀ3, less than that of the N10 compound
2 (1.774 g cmꢀ3) with hydrogen attached to the carbon atom of
the tetrazole ring (Table S3, ESIw).
In conclusion, 1,10-azobis(5-methyltetrazole), a novel ten-
nitrogen atoms directly linked compound, could be obtained by
three different azo coupling reagents from 5-methyl-1-amino-
tetrazole. The structure of 5 was confirmed by single-crystal
X-ray diffraction and characterized by means of vibration and
The 1H NMR spectrum (Fig. S3, ESIw) of 5 showed, as
expected, only one singlet at a chemical shift of d = 2.91 ppm
that represented the CH3 group. 13C NMR spectroscopic
(Fig. S4, ESIw) studies reveal clearly assignable resonances
for each carbon. A characteristic signal for the tetrazole
carbon was observed at a chemical shift of d = 153.16 ppm
and the other signal for the CH3 at a chemical shift of d =
9.14 ppm. The photochromic features of 5 were investigated
by following the protocol of studies reported for compounds
Fig. 1 ORTEP diagram of the N10 compound 5 with 30% prob-
ability ellipsoids.
Fig. 2 TG-DTG (a) and DSC (b) spectrum of the N10 compound 5.
c
2448 New J. Chem., 2012, 36, 2447–2450
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Table 2 The comparison of detonation properties of some energetic
compounds with 5a
concentrated under vacuum by evaporation (Attention:
although repeated washing with acetonitrile was performed,
the filter cake was spontaneously exploded while the partial
filter cake dried). The crude product obtained after the
removal of the solvent under vacuum was purified by silica
chromatography using 1 : 1 ethyl acetate: petroleum ether as
eluent to afford 0.65 g of 5, a light yellow solid (yield 67%,
ra
Qb
Dc
Pd
DfHm
Tdec
(1C)
e
f
Compd. (g cmꢀ3
)
(J gꢀ1
)
(km sꢀ1
)
(GPa) (kJ molꢀ1
)
5
1
2
3
1.482
1.620
1.774
1.80*g
5078.42 7.320
7.764
9.185
9.184
5390.75 7.840
6000.84 7.190
20.99 986.05
25.24 962.27
36.10 1030.00
39.00 1092.20
27.16 75.86i
21.72 69.75i
127.2
193.8
80.0
50.0 (mp)
1
entry 8, Table 1). H NMR (500 MHz, CDCl3, TMS): d 2.91
TATBh 1.790i
ppm (s, CH3). 13C NMR (126 MHz, CDCl3, TMS): d 153.1,
9.1 ppm. IR: 2832 (vw), 1695 (m), 1542 (m), 1452 (m), 1385
(vs), 1094 (m), 1044 (m), 988 (m), 937 (m), 738 (m), 684 (m)
cmꢀ1. ESI-MS: m/z 193.00 [M-H]ꢀ. Anal. calcd for C4H6N10
(194): C, 24.74; H, 3.11; N, 72.14. Found: C, 24.76; H, 3.10; N,
72.13. Impact sensitivity and friction sensitivity: too sensitive
for measurement; electrostatic sensitivity discharge (ESD): 5 mJ.
Sodium dichloroisocyanurate (SDIC) as an oxidizing agent
(entry 1, Table 1): Acetic acid (0.5 mL) was added to a
solution of SDIC (2.2 g, 10 mmol) in water (4 mL) with
vigorous stirring at room temperature for 30 min. Then this
oxidizing suspension was added to a solution of 4 (0.99 g,
10 mmol) in 20 mL CH3CN. The reaction mixture was further
stirred at 0 1C for 30 min. The solution was neutralized with
solid Na2CO3, then filtered to remove insoluble solids. The
filter cake was washed several times with acetonitrile, and the
filtrate was concentrated. The crude product obtained after the
removal of the solvent under vacuum was purified by silica
chromatography using 1 : 1 ethyl acetate : petroleum ether as
eluent to afford 0.51 g of 5, a light yellow solid (yield 53%).
tert-Butyl hypochlorite as an oxidizing agent (entry 7,
Table 1): 4 (0.99 g, 10 mmol) was dissolved in 20 mL CH3CN.
The solution was cooled at 0 1C and 1.1 mL (1.1 g, 10 mmol)
tert-butyl hypochlorite was added dropwise. The reaction
mixture was further stirred at 0 1C for 30 min. The solution
was neutralized with solid Na2CO3, then filtered to remove
insoluble solids. The filter cake was washed several times with
acetonitrile, and the filtrate was concentrated. The crude
product obtained after the removal of the solvent under
vacuum was purified by silica chromatography using 1 : 1 ethyl
acetate : petroleum ether as eluent to afford 0.57 g of 5, a light
yellow solid (yield 59%).
TNTh
1.650i
a
b
Calculated density from X-ray measurement. Heat of explosion.
c
d
Detonation velocity. Detonation pressure. Calculated molar
e
f
enthalpy of formation. Temperature of decomposition. Estimated.
h
g
TATB: 1,3,5-triamino-2,4,6-trinitrobenzene; TNT: trinitrotoluene.
Ref. 13.
i
multinuclear spectroscopy. Theoretical study shows introduction
methyl at carbon atom of tetrazole ring of 5 decreases detonation
energy and density compared to the N10 compound 2. Never-
theless, the compound exhibits better thermal stabilities with
decomposition temperature of 127.2 1C and physical stability in
various solvents than that of 2.
Experimental
Syntheses
Caution: 1,10-Azobis(5-methyltetrazole) reported in this
publication is very sensitive towards friction, impact, and
electrostatic discharge. Therefore proper safety precautions
should be taken when handling these compounds. Laboratories
and personnel should be properly grounded, and safety equipment
such as Kevlar gloves, leather coats, face shields, and ear plugs are
strongly recommended.
Compound 4. To a mixture of 60 g (0.5 mole) of benzaldehyde
hydrazone, 39 g (0.6 mole) of NaN3, and 150 mL (0.9 mole) of
triethyl orthoacetate was added 250 mL of acetic acid with
stirring, and heated to 80 1C for 6 h. The reaction mixture was
then poured with stirring into 1.2 liters of water, and the
precipitate was removed by filtration and washed with water.
The precipitate was then treated with water (200 mL) and
100 mL of concentrated hydrochloric acid, and the benzaldehyde
was removed from the mixture by steam distillation. The
distillation residue was neutralized with aqueous ammonia
and evaporated under vacuum to remove water. The residue
was extracted with ethyl acetate, and the extract was dried over
MgSO4. Ethyl acetate was removed by evaporation under
vacuum to give 8 g of 5-methyl-1-aminotetrazole in the form
Acknowledgements
This work was supported by the Natural Science Foundation
of Jiangsu Province (BK2011696) and the ‘‘NJUST Research
Funding, No 2011YBXM03’’ of Nanjing University of Science
and Technology.
1
of a clear yellowish liquid (yield 16%). H NMR (500 MHz,
DMSO-d6, TMS): d 6.85 (s, 2H), 2.43 ppm (s, 3H). 13C NMR
(126 MHz, CDCl3, TMS): d 150.5, 7.4 ppm.
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dissolved in 20 mL CH3CN. The solution was cooled at 0 1C
and TCICA (4.65 g, 20 mmol) was added dropwise. The
reaction mixture was further stirred at 0 1C for 30 min. The
solution was neutralized with solid Na2CO3, then filtered to
remove insoluble solids. The filter cake was washed several
times with acetonitrile, and the combined filtrate was
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