Exothermic or Endothermic Decomposition of Disubstituted Tetrazoles Tuned by Substitution Fashion and Substituents

Article abstract of DOI:10.1021/acs.jpca.7b08078

Nitrogen-rich compounds such as tetrazoles are widely used as candidates in gas-generating agents. However, the details of the differentiation of the two isomers of disubstituted tetrazoles are rarely studied, which is very important information for designing advanced materials based on tetrazoles. In this article, pairs of 2,5- and 1,5-disubstituted tetrazoles were carefully designed and prepared for study on their thermal decomposition behavior. Also, the substitution fashion of 2,5- and 1,5- and the substituents at C-5 position were found to affect the endothermic or exothermic properties. This is for the first time to the best of our knowledge that the thermal decomposition properties of different tetrazoles could be tuned by substitution ways and substitute groups, which could be used as a useful platform to design advanced materials for temperature-dependent rockets. The aza-Claisen rearrangement was proposed to understand the endothermic decomposition behavior.

Full text of DOI:10.1021/acs.jpca.7b08078

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Article
Exothermic or Endothermic Decomposition of Disubstituted
Tetrazoles was Tuned by Substitution Fashion and Substituents
Yu-Hui Jia, Kai-Xiang Yang, Shi-Lu Chen, and Mu-Hua Huang
J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b08078 • Publication Date (Web): 08 Dec 2017
Downloaded from http://pubs.acs.org on December 16, 2017
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Exothermic or Endothermic Decomposition of
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Disubstituted Tetrazoles was Tuned by Substitution
Fashion and Substituents
,
‡
†
‡
†
Yu-Hui Jia , Kai-Xiang Yang , Shi-Lu Chen and Mu-Hua Huang*
†
School of Materials Science and Engineering, Beijing Institute of Technology, Beijing, 100081,
China. Email: mhhuang@bit.edu.cn (M.ꢀH. Huang)
‡
School of Chemistry, Beijing Institute of Technology, Beijing, 100081, China
Abstract: Nitrogenꢀrich compounds such as tetrazoles are widely used as candidates in gas
generating agents. However, the detailed study on the differentiation of the two isomers of
disubstituted tetrazoles are rarely studied, which is very important information for design
advanced materials based on tetrazoles. In this article, pairs of 2,5ꢀ and 1,5ꢀdisubstituted
tetrazoles were carefully designed and prepared for the study on their thermal decomposition
behavior. And the substitution fashion of 2,5ꢀ and 1,5ꢀ and the substituents at Cꢀ5 position were
found to affect the endothermic or exothermic properties. This is for the first time to the best of
our knowledge that the thermal decomposition properties of different tetrazoles could be tuned
by substitution ways and substitute groups, which could be used as a useful platform to design
advanced materials for temperatureꢀdependant rockets. And the AzaꢀClaisen rearrangement was
proposed to understand the endothermic decomposition behavior.
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1. Introduction
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Energy release under control is the central task in the area of energetic materials1ꢀ9. Nitrogenꢀrich
materials are widely used in the field of rockets, missiles, artillery, and gas generating agents,
which require the materials to have low sensitivity and produce hot gases and glowing particles
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in a short time. Tetrazoles are a family of nitrogenꢀrich heterocycles and have attracted
12
considerable attention in many areas including pharmaceuticals10, energetic materials11,
,
porous materials13, fuel cells14 and so on. Particularly, tetrazole moiety has become a strategic
15 16
group in the applications such as gas generating agents7, , , propellants and so forth, since
tetrazoles generate two molecules of nitrogen per ring upon decomposition and have high
thermal stability and nonꢀexplosive character. Take 5ꢀaminoꢀ1Hꢀtetrazole as an example, it is
studied as a burn rate modifier, fire suppressor and an environmental friendly gas generating
agent. However, there’re very few examples of tetrazoles developed for gas generating agent
besides 5ꢀaminoꢀ1Hꢀtetrazole and G15B (Figure 1a).
(a)
NH
N
N
N
N
N
N
N
N
ꢀ
H2N
N
H2N
NH2
N
H
N
H
5-ATZ
G15B
Known tetrazoles used as gas generating agents
3
(b)
N
1, R = Ph;
3
4
4
N
N 2
N
N
2, R = NH2;
3, R = Me;
5
5
2
N
N
R
1
N
R
1
4, R = CH2Cl;
5, R = CH2I.
(b)
1,5-disubstituted
(a)
2,5-disubstituted
Tetrazoles (1a-5a and 1b-5b) designed in this work!
Figure1 Known tetrazoles used as gas generating agents (a); and New tetrazoles designed in this
work for the energy release control (b).
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Considering the gas generator system requires control of temperature and pressure in different
applications, we wonder if we can make the tetrazole derivatives to realize the control. Actually,
the tautomerisation between 1Hꢀ and 2Hꢀ tetrazole was well documented17, and two isomers of
2,5ꢀ and 1,5ꢀdisubstituted tetrazoles exist for the disubstituted tetrazoles. The thermal
decomposition studies on tetrazole materials indicated that Nꢀalkyl tetrazole had better thermal
stability than the original tetrazole18. Moreover, it is reported that 2ꢀalkyl tetrazole was less
thermostable than its isomeric 1ꢀalkyl tetrazole19, which is attributed by the different
decomposition pathway. However, the further differences between the two isomers of
disubstituted tetrazoles were rarely studied.
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In this article, we report the design, syntheses, fully characterization and thermal decomposition
properties of 5 pairs of disubstituted tetrazoles (Figure 2b) with Nꢀallyl group, which can be
readily introduced and converted to many functional group based on the derivatization of C=C
double bond.
2. Experimental Section
Causions: Tetrazoles and their derivatives are potential explosives, which may be sensitive to
environmental stimuli such as impact, friction heat and electrostatic discharge. While we
encountered no problems in handling of these materials, appropriate precausions and proper
protective measures (safety glasses, face shields leather coats, Kevlar gloves and ear protectors)
should be taken when preparing and manipulating these materials.
All reagents were used as received from commercial sources without further purification or prepared
as described in the literature. 5ꢀphenylꢀ1Hꢀtetrazole, 5ꢀaminoꢀ1Hꢀtetrazole and 5ꢀmethylꢀ1Hꢀ
tetrazole are supplied by Zhongshenghuateng Co. Ltd in Beijing, China. 5ꢀchloromethylꢀ1Hꢀ
tetrazole is prepared by following a known procedure. Reactions were stirred using Teflonꢀcoated
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magnetic stir bars. Analytical TLC was performed with 0.20 mm silica gel 60F plates.
Chromatographic purification of products was carried out by flash chromatography on silica gel
(230ꢀ400 mesh). NMR spectra were measured in CDCl3 (with TMS as internal standard) or DMSOꢀ
d6 on a Bruker AV400 or Varian INOVAꢀ400M (1H at 400 MHz, 13C at 100 MHz) magnetic
resonance spectrometer. Chemical shifts (δ) are reported in ppm, and coupling constants (J) are in
Hz. The following abbreviations were used to explain the multiplicities: s = singlet, d = doublet, t =
triplet, q = quartet, m = multiplet. Highꢀresolution EI mass spectra (HRꢀEIꢀMS) were recorded on an
GCT CA127 Micronass UK mass spectrometer, and Highꢀresolution ESI mass spectra (HRꢀESIꢀ
MS) on Thermo Q exactive mass spectrometer. ThermoꢀgravimetricꢀanalysisꢀMass spectrometry
(TGAꢀMS) was measured by NETZSCH STA449C / MS403C.
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2.1 The synthesis of 2ꢀallylꢀ5ꢀphenyltetrazole (1a) and 1ꢀallylꢀ5ꢀphenyltetrazole (1b): 5ꢀphenylꢀ
tetrazole (2 mmol, 292 mg) in acetone (10 ml) was added anhydrous sodium carbonate (2.4 mmol,
255 mg) , the resulting mixture was stirred at room temperature for 30 min, and then treated with
allyl bromide (4.0 mmol, 484 mg) in acetone (10 ml). The reaction mixture was heated to reflux at
o
60 C for 2 h, and then acetone was removed by vacuum, the residue was solved in water and
extracted by ethyl acetate, the combined organic layer was dried with anhydrous sodium sulfate and
concentrated to give oily residue, which purified through column chromatography to afford 2ꢀallylꢀ
5ꢀphenylꢀtetrazole (1a, yield 79%) and 1ꢀallylꢀ5ꢀphenylꢀtetrazole (1b, yield 17%).
2-allyl-5-phenyl-tetrazole(1a): Rf = 0.69 [V (EtOAc)/V (Petroleum Ether) = 1/2]; IR (KBr，cmꢀ1):
3037 (w), 3034 (w), 2984 (w), 1645 (w), 1529 (m), 1465 (s), 1450 (s), 1380 (w), 1336 (s), 1279 (s),
1200 (m), 1177 (w), 1136 (w), 1073 (m), 1044 (m), 1027 (m), 989 (m), 983 (m), 789 (s), 733 (s),
694 (s), 590 (w); 1H NMR (400 MHz，CDCl3) δ (ppm): 8.112 (d, 2H, J = 6.0 Hz), 7.409 (q, 3H, J =
7.4 Hz), 6.004~6.102 (m, 1H), 5.332 (d, 1H, J = 1.2 Hz), 5.299 (d, 1H, J = 5.2 Hz), 5.177 (d, 2H, J =
13
6.0 Hz); C NMR (100 MHz, CDCl3) δ (ppm): 165.09, 130.17, 129.85, 128.75, 127.30, 126.70,
120.57, 55.22; HRMS (EI): m/z 186.0909 [M]+ (C10H10N4, required 186.0905).
1-allyl-5-phenyl-tetrazole (1b): Rf = 0.39 [V (EtOAc)/V (Petroleum Ether) = 1/2]; IR (KBr，cmꢀ1):
3083 (w), 3059 (w), 3034 (w), 1645 (w), 1067 (w), 1576 (w), 1539 (m), 1462 (s), 1426 (w)1407
(m), 1288 (m), 1252 (m), 1141 (m), 1109 (m), 1074 (w), 993 (s), 934 (s), 780 (s), 762 (s), 696 (s),
1
591 (m); H NMR (400 MHz, CDCl3) δ (ppm): 7.670 (d, 2H, J = 6.0 Hz), 7.520 (q, 3H, J = 7.52
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Hz), 5.976~6.071 (m, 1H), 5.332 (d, 1H, J = 10.4 Hz), 5.081 (d, 1H, J = 17.2 Hz), 5.029 (d, 2H, J =
13
5.2 Hz); C NMR (100 MHz, CDCl3) δ (ppm): 154.51, 131.41, 130.63, 129.27, 128.73, 123.70,
119.71, 50.07; HRMS (EI): m/z 186.0908 [M]+ (C10H10N4, required 186.0905).
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2.2 The synthesis of 2ꢀallylꢀ5ꢀaminotetrazole (2a) and 1ꢀallylꢀ5ꢀaminotetrazole (2b)：Achieved
by a similar procedure to 1a and 1b, but using 5ꢀaminoꢀ1Hꢀtetrazole (3.0 mmol, 309 mg) as starting
material.
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2-allyl-5-amino-tetrazole(2a) (117 mg, yield 31%) : Rf = 0.54 [V (EtOAc)/V (Petroleum Ether) =
1/1]; IR (KBr, cmꢀ1)：3331 (s), 3150 (s), 3034 (w), 2979 (w), 1680 (s), 1661 (s), 1606 (s), 1593
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(vs), 1474 (m), 1432 (m), 1332 (m), 1281 (w), 1211 (w), 1134 (m), 1087 (w), 987 (m), 792 (m); H
NMR (400 MHz, CDCl3) δ (ppm): 5.987~6.067 (m, 1H), 5.364(d, J = 10.4 Hz, 1H), 5.344 (d, J =
4.8 Hz, 1H), 5.011 (d, J = 4.8 Hz, 2H), 4.397 (s, 2H); 13C NMR (100 MHz, CDCl3) δ (ppm): 166.44,
130.10, 120.76, 55.29; HRMS (ESI): m/z 126.07739 [M+H]+ (C4H8N5, required 126.07797).
1-allyl-5-amino-tetrazole(2b) (204 mg, yield 54%): Rf = 0.13 [V (EtOAc)/V (Petroleum Ether) =
1/1]; IR (KBr, cmꢀ1)：3090 (w), 2940 (m), 2861 (m), 2274 (vs), 1686 (vs), 1648 (m), 1529 (s), 1468
1
(vs), 1424 (s), 1338 (m), 1252 (m), 1121 (w), 1085 (w), 990 (s), 938 (m),782(m); H NMR (400
MHz, CDCl3) δ (ppm): 5.881 ~ 6.006 (m, 1H), 5.416 (d, J = 10.0 Hz, 1H), 5.172 (d, J = 17.2 Hz,
1H), 4.978 (d, J = 3.2 Hz, 2H), 4.813 (s, 2H); HRMS (ESI): m/z 126.08509 [M+H]+ (C4H8N5,
required 126.07797).
2.3 The synthesis of 2ꢀallylꢀ5ꢀmethyltetrazole (3a) and 1ꢀallylꢀ5ꢀmethyltetrazole (3b): achieved
by a similar procedure to 1a and 1b, but using 5ꢀmethylꢀ1Hꢀtetrazole (168 mg, 2 mmol) as starting
material.
2-allyl-5-methyl-tetrazole (3a) (127.5 mg, yield 37%): Rf = 0.62 [V (EtOAc)/V (Petroleum Ether) =
1/2]; IR (KBr，cmꢀ1): 3090 (w), 2989 (w), 2942 (w), 2860 (w), 1648 (w), 1506 (s), 1424 (m), 1394
(m), 1371 (w), 1335 (w), 1198 (m), 1174 (m), 1077 (m), 1033 (s), 991 (s), 939 (s), 800 (s), 716 (w),
671 (w), 576 (w); 1H NMR (400 MHz, CDCl3) δ (ppm): 5.980~6.079 (m, 1H), 5.363 (d, 1H, J = 4.0
13
Hz), 5.330 (d, 1H, J = 13.2 Hz), 5.133 (d, 2H, J = 6.0 Hz), 2.507 (s, 3H); C NMR (100 MHz,
CDCl3) δ (ppm): 163.19, 129.98, 120.86, 55.20, 10.94; HRMS (ESI): m/z 125.0822 [M+H]+
(C5H8N4, required 125.0827).
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1-allyl-5-methyl-tetrazole (3b) (92.5 mg, yield 51%): Rf = 0.10 [V (EtOAc)/V (Petroleum Ether) =
1/2]; IR (KBr，cmꢀ1): 3088 (w), 3034 (m), 2986 (m), 1647 (w), 1521 (m), 1461 (s), 1424 (s), 1339
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(w), 1243 (w), 1216 (m), 1110 (m), 990 (s), 941 (s), 786 (s),740 (w), 714 (w), 654 (w), 603 (m); H
NMR (400 MHz, CDCl3) δ (ppm): 5.864~5.946 (m, 1H), 5.321 (d, 1H, J = 10.4 Hz), 5.107 (d, 1H, J
= 17.2 Hz), 4.906 (d, 2H, J = 5.6 Hz), 2.498 (s, 3H); 13C NMR (100 MHz, CDCl3) δ (ppm): 151.77,
129.85, 119.90, 49.44, 8.91; HRMS (ESI): m/z 125.0822 [M+H]+ ([C5H8N4, required 125.0827).
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2.4 The synthesis of 2ꢀallylꢀ5ꢀchloromethyltetrazole (4a) and 1ꢀallylꢀ5ꢀchloromethyltetrazole
(4b): achieved by a similar procedure to 1a and 1b, but using 5ꢀchloromethylꢀ1Hꢀtetrazole (337 mg,
2 mmol) as starting material.
2-allyl-5-chloromethyl-tetrazole (4a) (197 mg, yield 62%): Rf = 0.61 [V (EtOAc)/V (Petroleum
Ether) = 1/2]; IR (KBr，cmꢀ1): 3089 (w), 3030 (w), 2986 (w), 1647 (m), 1499 (s), 1430 (s), 1398
(w), 1338 (m), 1269 (s), 1207 (m), 1172 (w), 1080 (w), 1031 (s), 990 (s), 942 (s), 804 (vs), 742 (m),
1
664 (m), 608 (w), 579 (w); H NMR (400 MHz, CDCl3) δ (ppm): 5.981~6.081 (m, 1H), 5.372 (d,
1H, J = 6.0 Hz), 5.338 (d, 1H, J = 12.8 Hz), 5.183 (d, 2H, J = 6.4 Hz), 4.743 (s, 2H); 13C NMR (100
MHz, CDCl3) δ (ppm): 163.24, 129.43, 121.46, 55.68, 34.20; HRMS (ESI): m/z 159.0430 [M+H]+
(CH4N5, required 159.0438).
1-allyl-5-chloromethyl-tetrazole (4b) (83 mg, yield 26%): Rf = 0.20[V (EtOAc)/V (Petroleum Ether)
= 1/2]; IR (KBr，cmꢀ1): 3088 (w), 3034 (m), 2986 (m), 1647 (w), 1521 (m), 1461 (s), 1424 (s), 1339
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(w), 1243 (w), 1216 (m), 1110 (m), 990 (s), 941 (s), 786 (s), 740 (w), 714 (w), 654 (w), 603 (m); H
NMR (400 MHz, CDCl3) δ (ppm): 5.911 ~ 6.065 (m, 1H), 5.377 (d, 1H, J = 10.0 Hz), 5.284 (d, 1H,
13
J = 17.2 Hz), 5.066 (d, 2H, J = 6.0 Hz), 4.789 (s, 2H); C NMR (100 MHz, CDCl3) δ (ppm):
151.52, 129.27, 121.11, 50.42, 31.32; HRMS (ESI): m/z 159.0433 [M+H]+ (CH4N5, required
159.0438).
2.5 The synthesis of 2ꢀallylꢀ5ꢀiodomethyltetrazole (5a) and 1ꢀallylꢀ5ꢀiodomethyltetrazole (5b):
A solution of 1ꢀallylꢀ5ꢀchloromethylꢀtetrazole (2 mmol, 317 mg) in acetone (10 ml) and water (10
o
ml) was added potassium iodide (4 mmol, 664 mg), the resulting mixture was heated at 60 C for 8
h. Then the solvent was removed by vacuum, the residue was solved in water and extracted by ethyl
acetate, the combined organics were dried with anhydrous sodium sulfate, concentrated, the residue
was purified through column chromatography to give 1ꢀallylꢀ5ꢀiodomethylꢀtetrazole (366.5 mg,
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yield 73%). By a similar procedure, 2ꢀallylꢀ5ꢀiodomethyltetrazole was synthesized from 2ꢀallylꢀ5ꢀ
chlroromethyltetrazole (2 mmol, 317 mg).
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2-allyl-5-iodomethyl-tetrazole (5a): Rf = 0.64 [V (EtOAc)/V (Petroleum Ether) = 1/2]; IR (KBr，
cmꢀ1): 3087 (w), 3039 (m), 2985 (m), 1646 (m), 1492 (s), 1424(s), 1390 (m), 1337 (m), 1291 (w),
1206 (m), 1172 (s), 1099 (m), 1078 (m), 1027 (m), 989 (s), 940 (s), 797 (s), 717 (m), 679 (w), 555
(m);1H NMR (400 MHz, CDCl3) δ (ppm): 6.009~6.108(m, 1H), 5.409(d, 1H, J = 10.4 Hz), 5.375(d,
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1H, J = 16.8 Hz), 5.185(d, 2H, J = 4.8 Hz), 4.511(s, 2H); C NMR (100 MHz, CDCl3) δ (ppm):
164.60, 129.55, 121.40, 55.67, ꢀ13.18; HRMS (ESI): m/z 250.9775 [M+H]+ (CH4N5, required
250.9794).
1-allyl-5-iodomethyl-tetrazole (5b) (406.6 mg, yield 81%):: Rf = 0.28 [V (EtOAc)/V (Petroleum
Ether) = 1/2]; IR (KBr，cmꢀ1): 3087 (w), 3035 (m), 2982 (m), 1646 (m), 1516 (s), 1458 (s), 1420
(s), 1338 (w), 1306 (w), 1244 (m), 1166 (s), 1091 (s), 989 (s), 940 (s), 780 (m), 746 (w), 710 (m),
689 (w), 553 (m); 1H NMR (400 MHz, CDCl3) δ (ppm): 6.009~6.108(m, 1H), 5.409(d, 1H, J = 10.4
Hz), 5.375(d, 1H, J = 16.8 Hz), 5.185(d, 2H, J = 4.8 Hz), 4.511(s, 2H); 13C NMR (100 MHz,
CDCl3) δ (ppm): 164.60, 129.55, 121.40, 55.67, ꢀ13.18; HRMS (ESI): m/z 250.9775 [M+H]+
(CH4N5, required 250.9794).
3. Results and Discussions
3.1 The synthesis of disubstituted tetrazoles
A few synthetic strategieswere developed towards tetrazoles, including tetrazole itself, monoꢀ
substituted tetrazole, disubstituted tetrazoles20ꢀ25, ionic tetrazoles26ꢀ28 and tetrazoleꢀbased
complex29ꢀ32. Among them, the Click strategy is the most important way23, 25, 33, which required
the starting materials with cyano (or isocyanate) and azido group. In addition, new chemistry
based on CꢀH and CꢀC cleavage have been reported24, as well as copper catalyzed Nꢀ
arylation22(see Table S5). Recently, some green chemistry has been developed towards the
preparation of tetrazoles34ꢀ39. As for as synthesis of disubstituted tetrazoles is concerned, 1,5ꢀ
33 40
34 41
disubstituted tetrazoles23ꢀ25
,
,
and 2,5ꢀdisubstituted tetrazoles22,
,
are synthesized
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efficiently respectively. We chose the Nꢀallylation reactions20, 21, 41 on 5ꢀsubstituted tetrazoles, in
order to get both 1ꢀallylꢀ5ꢀsubstituted tetrazole and 2ꢀallylꢀ5ꢀsubstituted tetrazole from one
reaction without using noble metal and in low cost. Firstly, we investigated on the reaction
conditions based on Nꢀallylation of 5ꢀphenyltetrazole (see Table S1 in supporting information),
and we found heating was necessary for high conversion of starting materials, as well as at least
2 equivalents of allyl bromide. By doing the reaction in acetone, the 2ꢀallylꢀ5ꢀphenyltetrazole
and 1ꢀallylꢀ5ꢀphenyltetrazole were obtained in good overall yield up to 99% yield, with 2,5ꢀ
disubstituted tetrazole as major product (Table S2 in SI). In order to see if possible to vary the
ratio of two isomers, we also studied the solvent effects on the outcome of the ratio of 1a to 1b
(see table S2 in SI). Among the solvents screened, toluene and 1,2ꢀdichloroethane gave no
products owing to poor solubility of 5ꢀphenyltetrazole in them, other solvents such as DMF, THF
and acetonitrile gave similar results as acetone, ethanol gave the ratio of 3.7 to 1 based on the
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crude HꢀNMR spectra (Figure S1ꢀS4). Considering the ease of the reaction, we chose the
o
condition of Na2CO3/Acetone/60 C, and the Nꢀallylations were achieved on 5ꢀphenyltetrazole,
5ꢀmethyltetrazole and 5ꢀchlorotetrazole (Table 1). Under the reaction conditions, although 5ꢀ
phenyltetrazole gave 1a and 1b in a ratio of 79:17, 5ꢀmethyltetrazole gave 2a and 2b in an
isolated yield of 37% and 51%, 5ꢀchloromethyltetrazole gave 3a and 3b in an isolated yield of
62% and 26%. The total yield was 96%, 88% and 88% respectively for tetrazoles 1, 2 and 3.
Afterwards, chloro in compounds 3a and 3b was substituted by iodide to give the corresponding
iodide 5a and 5b.
3.2 The structure characterization of disubstituted tetrazoles
1
13
All the new tetrazoles were fully characterized by TLC, FTꢀIR, HꢀNMR, CꢀNMR and HRꢀ
MS. During the reactions, we found the two isomers could be separated easily, since they have
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big polarity differences in TLC21 (Table S3 in SI). 2,5ꢀdisubstituted tetrazole a had higher Rf
value [Rf > 0.6, 33% EtOAc in PE) than its corresponding 1,5ꢀdisubstituted tetrazole b (Rf<0.39,
see Table S3). By looking at the 1HꢀNMR and 13CꢀNMR spectra of all the disubstituted tetrazoles
carefully, we are delighted to find some obvious differences between two isomers (Table 2). In
the 1HꢀNMR spectra, the chemical shifts differences between Hꢀ8a and Hꢀ8b were 0.03 ppm for
the 2,5ꢀdisubstituted tetrazoles (0.04 ppm for iodide 5a). The corresponding differences were
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0.08 to 0.25 ppm for 1,5ꢀdisubstituted tetrazoles. In the CꢀNMR spectra, the chemical shifts of
Cꢀ5 in 2,5ꢀdisubstituted tetrazoles were higher than 162 ppm, while less than 155 ppm for 1,5ꢀ
disubstituted tetrazoles. It is characteristic 10 ppm higher for 2,5ꢀdisubstituted tetrazole than its
corresponding 1,5ꢀdisubstituted tetrazole.
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Table 1 The differences of NMR spectra between isomers of disubstituted tetrazoles
3
N
3
4
4
N
N
N
2
N
5
2
7
5
N
N
8
Ha
7
R
N
1
1
R
8
6
Ha
Hb
6
Hb
(a)
(b)
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Compound
2ꢀAllylꢀ5ꢀsubstituted tetrazole (a, ppm)
δH6 δH8a δH8b ∆δ δC5
1ꢀAllylꢀ5ꢀsubstituted tetrazole (b, ppm)
R=
δH6 δH8a δH8b
∆δ
δC5
154.5
NA
5.18
5.01
5.13
5.18
5.18
5.33
5.36
5.36
5.37
5.40
5.30
5.34
5.33
5.34
5.36
0.03 165.1
0.02 166.4
0.03 163.2
0.03 163.3
0.04 164.6
5.03
4.98
4.91
5.07
5.01
5.33
5.42
5.32
5.38
5.42
5.08
5.30
5.11
5.28
5.34
0.25
0.12
0.21
0.10
0.08
1
2
3
4
5
ꢀPh
ꢀNH2
ꢀCH3
ꢀCH2Cl
ꢀCH2I
151.8
151.5
152.7
ꢀδ =δH8a−δH8b
Through the above differences, the differentiation of 1,5ꢀ and 2,5ꢀdisubstituted tetrazole could be
realized easily based on 13CꢀNMR Spectra. To further confirm the substitution fashion, the CꢀH
HMBC experiments of 1a and 1b were performed. There’s no cross peak seen between Cꢀ5 and
H6 for 2,5ꢀdisubstituted fashion (Figure 2a), while obvious cross peak between Cꢀ5 and H6 was
seen in the 1,5ꢀdisubstituted fashion (Figure 2b).
Figure 2 CꢀH HMBC of 2ꢀallylꢀ5ꢀphenyltetrazole 1a and 1ꢀallylꢀ5ꢀphenyltetrazole 1b recorded
in CDCl3
Similarly, the CꢀH HMBC experiments on 4a and 4b confirmed the correct structure (See Figure
S37 and S38 in ESI). In order to further confirm the structure of the disubstituted tetrazoles, we
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tried growing single crystals of this class of compounds. Fortunately, the single crystal of 5ꢀ
aminoꢀ2ꢀallyltetrazole (2a) was obtained by slow evaporation of solvents, and the XꢀRay
diffraction was performed, and its structure was shown as Fig 3.
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(a)
(b)
Figure 3 The XꢀRay determined molecule structure of 5ꢀaminoꢀ2ꢀallyltetrazole (2a) and
molecular packing through intermolecular Hꢀbond between NH2 and N1, N4 of tetrazole.
Compound 2a crystallizes in an orthorhombic P c a 21 space group , the parameters were shown
in Table S4 in SI. Its crystal structure determination revealed clearly that the tetrazole ring was in
a 2,5ꢀsubstitution fashion (see Figure 3a). Interestingly, each two molecules of tetrazoles were
packed in a headꢀtoꢀtail fashion though Hꢀbond between NH2, N1 of a second tetrazole ring and
N4 of the third tetrazole ring (see Figure 3b). The distance between the proton and nitrogen is in
the range of 2.1ꢀ2.3 Å. This kind of intermolecular Hꢀbond may contribute to the crystallization
and packing in the crystalline.
The result of two products 1,5ꢀ and 2,5ꢀdisubstituted tetrazoles could be explained by the
isomerization of 1ꢀNH and 2ꢀNH form of 5ꢀsubstiuted tetrazole17. Especially in the presence of
base, the NH of 5ꢀsubstituted tetrazole was deprotonated to give 1ꢀN－ and 2ꢀN－ of 5ꢀsubstituted
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tetrazole. When they reacted with electrophile, both 1,5ꢀ and 2,5ꢀdisubstituted tetrazoles were
obtained.
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With the confirmation of the above compounds’ chemical structure, we moved to the
investigation of their thermal decomposition properties.
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3.3 The thermal decomposition properties of disubstituted tetrazoles
The thermal decomposition was carried out using TGA under nitrogen purge and with a heating
o
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rate of 10 C/min. As shown in Figure 4, the 5ꢀphenyltetrazole had T5% of 217 C, 1ꢀallylꢀ5ꢀ
phenyltetrazole (1b) has T5% of 202 oC, while 2ꢀallylꢀ5ꢀphenyltetrazole (1a) has much lower T5%
o
o
of 147 C. As far as the Tmax is concerned, 5ꢀphenyltetrazole gave Tmax of 327 C, 1ꢀallylꢀ5ꢀ
o
phenyltetrazole (1b) has Tmax of 297 C, while 2ꢀallylꢀ5ꢀphenyltetrazole (1a) has much lower
o
Tmax of 250 C (Figure S39). It is noteworthy that “heat shielding effect”19 of alkyl group was
introduced to explain why the Nꢀalkyl tetrazoles had better thermal stability than their original
tetrazoles. However, it is not the case here: 1,5-disubstituted tetrazole was similar or less
thermal stable than its original 5-substituted tetrazole, while 2,5-disubstituted tetrazole was
obviously less thermal stable.
100
80
60
40
20
0
100
80
60
40
20
0
1a
2a
3a
4a
5a
1b
2b
3b
4b
5b
100
200
300
400
500
100
200
300
400
500
Temperature /oC
Temperature / o
C
(a) TGA of 1aꢀ5a
(b) TGA of 1bꢀ5b
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5b
4b
1a
exo
exo
5a
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2a
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4a
3a
2b
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500
100
200
300
400
500
o
Temperature / oC
Temperature/ C
(c) DSC of 1aꢀ5a
(d) DSC of 1bꢀ5b
Figure 4 TGA of 1aꢀ5a (a), TGA of 1bꢀ5b (b), DSC of 1aꢀ5a and DSC of 1bꢀ5b.
o
o
2a and 2b have melting point of 67.4 C and 117.4 C respectively (Figure S40), and 2a and 2b
o
o
o
o
started to decompose at 150 C and 187 C respectively, decomposed at 210 C and 272 C
maximally. Interestingly, 2a absorbed heat of 170.8 J/g during decomposition, while 2b released
heat of 246.7 J/g during decomposition.
o
o
3a and 3b started to decompose at 87 C and 167 C respectively (Figure S41). 3a and 3b
Decomposed at 145 oC and 245 oC maximally. Interestingly, both 3a and 3b released heat during
decomposition, the value is 217 and 308 J/g respectively.
o
o
4a and 4b started to decompose at 80 C and 167 C respectively (Figure S42). 4a and 4b
o
o
decomposed at 115 C and 215 C maximally. 4a absorbed heat of 128.4 J/g during
o
decomposition at 200ꢀ300 C, while 4b released heat of 697.0 J/g during decomposition at 100ꢀ
250 oC. It is obvious that the heat change observed from DSC was delayed compared with TGA,
owing to the heat transport.
o
o
5a and 5b started to decompose at 75 C and 172 C respectively (Figure S43). 5a and 5b
o
o
decomposed at 165 C and 220 C maximally. 5a released heat of 527.6.4 J/g during
decomposition at 150ꢀ320 oC, while 5b absorbed heat during decomposition at 100ꢀ400 oC.
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Furthermore, the decomposition of 5ꢀphenyltetrazole was accompanied by two stages, whereas
1ꢀallylꢀ5ꢀphenyltetrazole (1b)only showed one stage of decomposition, while 2ꢀallylꢀ5ꢀ
phenyltetrazole (1a) three stages. The above observation was summarized in Table 2.
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Table 2 Thermal decomposition properties of disubstituted tetrazoles
Energy change c (J/g)
T5%/℃
Tmax/℃
Compound
R
aa
bb
a
b
a
b
ꢀPh
ꢀNH2
130
150
87
202
187
167
167
172
247
210
145
115
165
297
272
245
215
220
+260.1
ꢀ246.7
ꢀ217.3
ꢀ128.4
<0 d
ꢀ205.7
ꢀ170.8
ꢀ308.1
+697
1
2
3
4
5
ꢀCH3
ꢀCH2Cl
ꢀCH2I
80
75
+527.6
a, 2,5ꢀdisubstituted tetrazole; b, 1,5ꢀdisubstituted tetrazole; c, “ꢀ” represents absorb energy
during decomposition, “+” represents release energy during decomposition; d, the peak is too broad to
get the accurate energy value.
It can be seen from Table 2 that the 2,5-disubstituted tetrazoles decomposed at lower
temperature than their corresponding 1,5-disubstituted isomers for each pair. E.g., 1a has T5%
o
o
o
o
of 130 C, which is 72 C lower than 1b, 1a has T5% of 247 C, which is 50 C lower than 1b.
Similarly, the T5% difference of 37 oC, 80 oC, 87 oC and 97 oC was observed for the two isomers
o
o
o
o
o
of 2ꢀ5, respectively. The difference of 50 C, 62 C, 100 C, 100 C and 55 C for the Tmax of
two isomers of 1ꢀ5, respectively. As far as the influence of substituent at Cꢀ5 of tetrazole on the
thermal stability is concerned, amino and phenyl group gave the highest thermal stability
among the disubstituted tetazoles investigated, methyl, chloromethyl and iodomethyl group
gave lower stability. Take the T5% of 2,5ꢀdisubstituted tetrazoles as example, 5ꢀamino and 5ꢀ
o
phenyl tetrazoles have T5% of 150 and 130 C, while 5ꢀmethyl and 5ꢀchloromethyl and 5ꢀ
iodomethyl tetrazoles have T5% of 87, 80 and 75 oC. For the T5% of 1,5ꢀdisubstituted tetrazoles,
o
5ꢀamino and 5ꢀphenyl tetrazoles have T5% of 187 and 202 C, while 5ꢀmethyl and 5ꢀ
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chloromethyl and 5ꢀiodomethyl tetrazoles have T5% of 167, 167 and 172 C. Similarly, for the
Tmax of 2,5ꢀdisubstituted tetrazoles, 5ꢀamino and 5ꢀphenyl tetrazoles have Tmax of 210 and 247
oC, while 5ꢀmethyl and 5ꢀchloromethyl and 5ꢀiodomethyl tetrazoles have Tmax of 145, 115 and
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165 C, for the Tmax of 1,5ꢀdisubstituted tetrazoles, 5ꢀamino and 5ꢀphenyl tetrazoles have Tmax
o
of 272 and 297 C, while 5ꢀmethyl and 5ꢀchloromethyl and 5ꢀiodomethyl tetrazoles have Tmax
of 245, 215 and 220 oC. The high thermal stability of 5ꢀamino tetrazoles 2a and 2b, must be the
result of the intermolecular hydrogen bonding between protons of amino group and nitrogen in
tetrazole rings, as shown by 2a’s crystal structure (Figure 3). The similar higher thermal
stability of 1a and 1b was contributed by the phenyl ring.
The substitution effect at Cꢀ5 of disubstituted tetrazole on the exoꢀ or endoꢀthermic property is
very obvious. When amino group and methyl group were suited at Cꢀ5, both 2,5ꢀ and 1,5ꢀ
disubstituted tetrazoles absorbed heat during decomposition. While when phenyl or chloromethyl
group were situated at Cꢀ5, the two isomers of 2,5ꢀdisubstituted tetrazole and 1,5ꢀdisubstituted
tetrazole performs completely different, one absorbed heat and the other released heat.
In addition, the tetrazoles were found stable when it was exposed to UV light (254 nm) in the air
for 30 mins by TLC analysis. These observations will be very useful to design the tetrazole
materials for energetic applications, such as gas generators, where temperature was required to
adjust. For example, in the cases required exothermic process, 2,5ꢀdisubstituted tetrazole 1a
could be used. In the cases of endothermic process, 1,5ꢀdisubstituted tetrazole 1b could be
chosen, or 5ꢀaminoꢀtetrazole 2a and 2b are good candidates.
3.4 The understanding of thermal decomposition properties of disubstituted
tetrazoles
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For the pair of disubstituted tetrazoles, why they have different decomposition behavior during
decomposition? It must be related to its bondꢀcleavage pathway during decomposition. In order
to get some insight on understanding the possible mechanism, 1a and 1b were subjected to TGꢀ
MS measurement (see Figure S44 and S45 in SI). It showed that both 1a and 1b gave off
nitrogen gas (molecular weight of 28) and phenyl (molecular weight of 77). In addition, very
similar decomposition products were seen from TGꢀMS spectra. Considering the cleavage of
chemical bonds usually gave off heat, the endothermic decomposition must be related to other
process. And the azaꢀClaisen rearrangement is considered for both isomers of tetrazoles to give
1c (Scheme 1) before decomposition, and the azaꢀClaisen rearrangement was endothermic
reaction. And this process made the whole decomposition endothermic. The Gaussian calculation
showed 30.6 kcal/mol for transformation from 1a into 1c (see Figure S46 in SI), which is
possible to occur at 100 oC.
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N
N
N
AzaꢀClaisen rearrangement
N
N
N
N
N
1a
30.6 kcal/mol required
1c
about 100 oC required
59.4 kcal/mol required
about 700 oC required
AzaꢀClaisen rearrangement
N
N
AzaꢀClaisen rearrangement
N
N
N
N
N
N
1b
Scheme 1 The possible azaꢀClaisen rearrangement occurred at high temperature
4. Conclusions
In summary, the preparation of both isomers of disubstituted tetrazoles, precise structure
characterization, as well as study on the thermal decomposition behavior were achieved. Based
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on the Rf value differences and chemical shifts of Cꢀ5 of tetrazoles, it is easy to tell the
difference between the pair of isomer. The thermal decomposition behavior of disubstituted
tetrazoles was dramatically influenced by different substituent group at Cꢀ5 of disubstituted
tetrazoles. In addition, when the substituent group is defined at Cꢀ5 position, the variation
between 2,5ꢀ and 1,5ꢀsubstitution fashion could be used to tune the energy release process. This
observation was reported for the first time to the best of our knowledge, and will open a way to
design gas generating agents in the future. The azaꢀClaisen rearrangement was proposed to help
the endothermic decomposition process. The further study on the understanding the mechanism
and application of these tetrazoles are being carried out in our lab, the relevant results will be
reported in due course.
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Supporting Information. Additional FTꢀIR, 1HꢀNMR, 13CꢀNMR, HRꢀMS, TGA, DSC, TGꢀMS
and Tables for Rf values. This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*Phone: (0086)ꢀ10ꢀ68912658. Email: mhhuang@bit.edu.cn.
Notes
The authors declare no competing financial interest should be addressed.
ACKNOWLEDGMENT
We thank the National Natural Science Foundation of China (No. 21202008, 21772013) and
Beijing Natural Science Foundation (No 2162039 ) for generous support.
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Five pairs of 2,5ꢀ and 1,5ꢀdisubstituted tetrazoles were carefully designed and prepared for the
study on their thermal decomposition behavior. And the substitution fashion of 2,5ꢀ and 1,5ꢀ and
the substituents at Cꢀ5 position were found to affect the endothermic or exothermic properties.
This could be used as a useful platform to design advanced materials such as temperatureꢀ
dependant rockets.
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DSC of 1aꢀ5a
DSC of 1bꢀ5b
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