This journal is c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012 New J. Chem., 2012, 36, 2447–2450 2447

Cite this: New J. Chem., 2012, 36, 2447–2450

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-

(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–NQQQN–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

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

well characterized.5a The Klapotke group had synthesized 1,

10-azobistetrazole (2) having the well characterized N10 structure

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

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

employed trichloroisocyanuric acid (TCICA) 12 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

Scheme 1 High-nitrogen compounds with N8 and N10 chain.

School of Chemical Engineering, Nanjing University of Science andTechnology, 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 5948-8205w Electronic supplementary information (ESI) available: CCDCreference number 889575. For ESI and crystallographic data in CIFor other electronic format see DOI: 10.1039/c2nj40731k

NJC Dynamic Article Links

www.rsc.org/njc LETTER

Dow

nloa

ded

by U

nive

rsity

of

Oxf

ord

on 2

2 N

ovem

ber

2012

Publ

ishe

d on

03

Oct

ober

201

2 on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/C

2NJ4

0731

KView Article Online / Journal Homepage / Table of Contents for this issue

2448 New J. Chem., 2012, 36, 2447–2450 This journal is c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012

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 cm�3, less than that of the N10 compound

2 (1.774 g cm�3) with hydrogen attached to the carbon atom of

the tetrazole ring (Table S3, ESIw).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

1 and 2. Color change, UV–vis spectral (Fig. S6, ESIw) andRaman 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.

Compound 5 possesses a calculated heat of formation of

986 kJ mol�1, 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 s�1 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.

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

Table 1 Azo coupling reaction of 5-methyl-1-aminotetrazole withdifferent oxidants

Entry

Reaction conditionsa

Oxidant Solvent Temp (1C) Time (h) Yieldb (%)

1 SDIC CH3CN 0 0.5 532 KMnO4/HCl None 50 6 03 KMnO4/NaOH None 50 6 04 H2O2 CH3CN r. t. 6 05 Br2/CH3COOH CH3CN 0 2 06 10%NaClO None r. t. 2 07 t-BuOCl CH3CN 0 0.5 598 TCICAc CH3CN 0 0.5 67

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.

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.

Dow

nloa

ded

by U

nive

rsity

of

Oxf

ord

on 2

2 N

ovem

ber

2012

Publ

ishe

d on

03

Oct

ober

201

2 on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/C

2NJ4

0731

K

View Article Online

This journal is c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012 New J. Chem., 2012, 36, 2447–2450 2449

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

of a clear yellowish liquid (yield 16%). 1H 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.

Compound 5. Trichloroisocyanuric acid (TCICA) as an

oxidizing agent (entry 8, Table 1): 4 (0.99 g, 10 mmol) was

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

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%,

entry 8, Table 1). 1H NMR (500 MHz, CDCl3, TMS): d 2.91

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)

cm�1. 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%).

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.

Notes and references

1 (a) G. Steinhauser and T. M. Klapotke, Angew. Chem., Int. Ed.,2008, 47, 3330–3347; (b) T. Fendt, N. Fischer, T. M. Klapotke andJ. Stierstorfer, Inorg. Chem., 2011, 50, 1447–1458; (c) M. Gobel,K. Karaghiosoff, T. M. Klapotke, D. G. Piercey and J. Stierstorfer,J. Am. Chem. Soc., 2010, 132, 17216–17226; (d) R. Wang, Y. Guo,Z. Zeng and J. M. Shreeve, Chem. Commun., 2009, 2697–2699.

2 (a) N. Fischer, D. Izsak, T. M. Klapotke, S. Rappengluck andJ. Stierstorfer, Chem.–Eur. J., 2012, 18, 4051–4062;(b) A. A. Dippold, T. M. Klapotke, F. A. Martin and S. Wiedbrauk,Eur. J. Inorg. Chem., 2012, 2012, 2429–2443; (c) Y. Zhang, Y. Huang,D. A. Parrish and J. M. Shreeve, J. Mater. Chem., 2011, 21, 6891–6897;

Table 2 The comparison of detonation properties of some energeticcompounds with 5a

Compd.ra

(g cm�3)Qb

(J g�1)Dc

(km s�1)Pd

(GPa)DfHm

e

(kJ mol�1)Tdec

f

(1C)

5 1.482 5078.42 7.320 20.99 986.05 127.21 1.620 7.764 25.24 962.27 193.82 1.774 9.185 36.10 1030.00 80.03 1.80*g 9.184 39.00 1092.20 50.0 (mp)TATBh 1.790i 5390.75 7.840 27.16 75.86i

TNTh 1.650i 6000.84 7.190 21.72 69.75i

a Calculated density from X-ray measurement. b Heat of explosion.c Detonation velocity. d Detonation pressure. e Calculated molar

enthalpy of formation. f Temperature of decomposition. g Estimated.h TATB: 1,3,5-triamino-2,4,6-trinitrobenzene; TNT: trinitrotoluene.i Ref. 13.

Dow

nloa

ded

by U

nive

rsity

of

Oxf

ord

on 2

2 N

ovem

ber

2012

Publ

ishe

d on

03

Oct

ober

201

2 on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/C

2NJ4

0731

K

View Article Online

2450 New J. Chem., 2012, 36, 2447–2450 This journal is c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012

(d) Y. Guo, G. H. Tao, Z. Zeng, H. Gao, D. A. Parrish andJ. M. Shreeve, Chem.–Eur. J., 2010, 16, 3753–3762.

3 (a) E. G. Francois, D. E. Chavez and M. M. Sandstrom, Propellants,Explos., Pyrotech., 2010, 35, 529–534; (b) M. V. Huynh,M. A. Hiskey,E. L. Hartline, D. P. Montoya and R. Gilardi, Angew. Chem., 2004,116, 5032–5036; (c) K. Y. Lee, L. Alamos and N. Mex, US 4623409,1986; (d) A. B. Sheremetev, V. O. Kulagina, N. S. Aleksandrova,D. E. Dmitriev, Y. A. Strelenko, V. P. Lebedev and Y. N. Matyushin,Propellants, Explos., Pyrotech., 1998, 23, 142–149.

4 (a) M. Wang, K. Funabiki and M. Matsui, Dyes Pigm., 2003, 57,77–86; (b) E. V. Brown and G. R. Granneman, J. Am. Chem. Soc.,1975, 97, 621–627.

5 (a) Y. C. Li, C. Qi, S. H. Li, H. J. Zhang, C. H. Sun, Y. Z. Yu andS. P. Pang, J. Am. Chem. Soc., 2010, 132, 12172–12173;(b) P. Gorostiza and E. Y. Isacoff, Science, 2008, 322, 395–399.

6 L. Batog, L. Konstantinova and V. Rozhkov, Russ. Chem. Bull.,2005, 54, 1915–1922.

7 (a) J. Stierstorfer, T. M. Klapotke and D. G. Piercey, DaltonTrans., 2012, 41, 9451–9459; (b) T. M. Klapotke and D. G. Piercey,Inorg. Chem., 2011, 50, 2732–2734.

8 (a) C. Qi, S. H. Li, Y. C. Li, Y. Yuan, X. K. Chen and S. P. Pang,J. Mater. Chem., 2011, 21, 3221–3225; (b) S. H. Li, S. P. Pang,X. T. Li, Y. Z. Yu and X. Q. Zhao, Chin. Chem. Lett., 2007, 18,1176–1178; (c) S. H. Li, H. G. Shi, C. H. Sun, X. T. Li, S. P. Pang,Y. Z. Yu and X. Q. Zhao, J. Chem. Crystallogr., 2009, 39, 13–16;(d) T. M. Klapotke, P. Mayer, A. Schulz and J. J. Weigand,Propellants, Explos., Pyrotech., 2004, 29, 325–332.

9 J. Heppekausen, T. M. Klapotke and S. M. Sproll, J. Org. Chem.,2009, 74, 2460–2466.

10 (a) L. V. Batog, L. S. Konstantinova, O. V. Lebedev andL. I. Khmel’nitskii, Mendeleev Commun., 1996, 6, 193–195;(b) L. V. Batog, L. S. Konstantinova, O. V. Lebedev andL. I. Khmel’nitskii, Mendeleev Commun., 1996, 3.

11 J. C. Bottaro, R. J. Schmitt and P. E. Penwell, US 5889161, 1999.12 (a) J. M. Veauthier, D. E. Chavez and B. C. Tappan, J. Energ.

Mater., 2010, 28, 229–249; (b) D. Chavez, L. Hill and M. Hiskey,J. Energ. Mater., 2000, 18, 219–236.

13 H. S. Jadhav, M. B. Talawar, R. Sivabalan, D. D. Dhavale,S. N. Asthana and V. N. Krishnamurthy, J. Hazard. Mater.,2007, 143, 192–197.

Dow

nloa

ded

by U

nive

rsity

of

Oxf

ord

on 2

2 N

ovem

ber

2012

Publ

ishe

d on

03

Oct

ober

201

2 on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/C

2NJ4

0731

K

View Article Online