English

• 1.   Introduction

  Melt-cast explosive is one of the most important charge manners in warheads [1]. The most common melt-cast explosives are based on 2, 4, 6-trinitrotoluene (TNT), which has been widely used in mines, mortars, grenades and artillery shells [2]. The properties of TNT, such as high chemical stability, moderate insensitivity to impact and friction, a low melting point, make it suitable for melt-cast explosives [3]. Nevertheless, the requirement of high detonation performance in modern applications promotes the investigation of new energetic compounds with low melting point to replace TNT [4].

  Triazene has been widely used in various fields, for example, as an effective group in drugs [5], as a multifunctional linker in solidphase synthesis [6], and as a protecting group for the synthesis of complex natural products [7]. Particularly, in energetic materials, triazene moiety was also investigated as a nitrogen linkage [8]. In this area, besides several furazan-based triazenes [9], tetrazoles were the major substituents of triazene.

  In 1910, 1, 3-bis(tetrazol-5-yl)triazene was synthesized for the first time, however, it is unstable and decomposes slowly as exposure to air [10]. In contrast, sodium 1, 3-bis(tetrazol-5-yl) triazenate (Na₃BTT) and 1, 3-bis(2-methyltetrazol-5-yl)triazene (BMTT) possess more thermal stabilities and lower sensitivities toward mechanic stimuli [11]. Physical properties and detonation performance of 1, 3-bis(1-methyltetrazol-5-yl)triazene and 1, 3-bis (2-methyltetrazol-5-yl)triazene were reported by Klapötke et al. in 2009 [12]. Both of them exhibit better detonation parameters than TNT. Herein, we report our effort to insight the relationship between the structure and performance of 1, 3-bis(2-alkyltetrazol-5-yl)triazenes with low melting point.

  2.   Results and discussion

  Substituted-5-aminotetrazole (1a–1e) was diazotized by sodium nitrite and hydrochloric acid in water to provide the substituted tetrazolyl diazonium salt [11d], which immediately reacted with another molecular of substituted-5-aminotetrazole (1a–1e) to produce substituted tetrazolyl triazene (2a–2e, Scheme 1). The product was obtained conveniently in a good yield as a result of insolubility of 2a–2e in water. In comparison, 1, 3-bis(2-hydroxyethyltetrazol-5-yl)triazene (2f) is soluble in water because of two hydroxyl groups in the molecular, therefore, the yield of 2f could be retained at higher concentration of the reaction mixture (Scheme 1). In addition, 1, 3-bis(2-azidoethyltetrazol-5-yl)triazene (2g) was derived from 2c by substitution with sodium azide (Scheme 2).

  Scheme1

  

  图 Scheme1  Synthesis of triazenes 2a–2f

  Scheme1.  Synthesis of triazenes 2a–2f

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  Scheme2

  

  图 Scheme2  Synthesis of 2g

  Scheme2.  Synthesis of 2g

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  The structures of all tetrazolyl triazenes (2a–2g) were confirmed by IR spectroscopy, ¹H NMR and ¹³C NMR spectroscopy as well as high resolution mass spectrometer. In IR spectra, two strong absorption bands around 1590 cm^-1 and 3450 cm^-1 are attributed to the N = N and N-H bonds of triazenes. The other intense absorption bands at 1230 cm^-1 and 1460 cm^-1 are assigned to the tetrazole ring. The single characteristic resonances at δ 14.19–14.61 in ¹H NMR spectra belong to the amino groups of triazenes. ¹³C NMR spectra of tetrazole ring show a weak resonance, even not in 2b, ranging from 165.4 to 169.0 ppm. The other resonances in ¹H NMR and ¹³C NMR spectra are produced by substitute groups of tetrazoles.

  The thermal properties of triazenes 2a, 2b and 2d–2g were investigated using differential scanning calorimetry (DSC). Gratifyingly, most of triazenes display suitable melting points (from 81 ℃ to 106 ℃) for melt-cast explosives, except 2e (not observed) and 2f (peak, 135 ℃) (Fig. 1). With a long alkyl chain on tetrazole, the tetrazolyl triazene shows a lower melting point than methyl substituted one. In contrast, vinyl substituent cannot lower the melting point because of the planarity caused by conjugation effect between C=C double bond and tetrazole ring. For triazene 2f, hydroxyl groups enhance the effect of hydrogen bond, which results in a higher melting point than other long alkyl substituted tetrazolyl triazenes. On the other hand, substituents show electronic effect on thermal stability of tetrazolyl triazene since triazene compound always decomposes from the rupture of N-N single bond in the triazene moiety upon heating [13]. Thus, electron-donating substituent causes higher thermal stability (such as 2d, which decomposes onset at 207 ℃), whereas electron-withdrawing one gives lower thermal stability (such as 2e, which decomposes onset at 158 ℃).

  图 1

  

  图 1  DSC thermograms of compounds 2a, 2b and 2d–2g (heating rate of 5 ℃/min)

  Figure 1.  DSC thermograms of compounds 2a, 2b and 2d–2g (heating rate of 5 ℃/min)

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  Impact sensitivities were determined by using the standard BAM techniques. As shown in Table 1, due to lack of hydrogen bond, all 1, 3-bis(2-alkyltetrazol-5-yl)triazenes are very sensitive to impact, similar with 1, 3-bis(2-methyltetrazol-5-yl)triazene, especially 2g (bearing two azido groups) showing an impact sensitivity of 0.75 J.

  表 1

  

  表 1  Physical properties and calculated detonation parameters of 2a, 2b and 2d–2g

  Table 1.  Physical properties and calculated detonation parameters of 2a, 2b and 2d–2g

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  Heats of formation of 1, 3-bis(2-alkyltetrazol-5-yl)triazenes were calculated using the program package Gaussian 09 (Revision B.01) [14]. The geometric optimization of the functional with 6-31 +G** basis set, and single energy points were calculated at the MP2 (full)/6-311++G^** level. The calculated heats of formation of compounds 2a, 2b and 2d–2g range from 26.0 kJ/mol to 949.1 kJ/mol (Table 1). Due to cyano and azido groups, triazenes 2a and 2g exhibit high positive heats of formation (949.1 kJ/mol and 923.2 kJ/mol, respectively). The densities of triazene compounds range from 1.49 g/cm to 1.55 g/cm, except compound 2d (1.32 g/cm), as measured using a gas pycnometer (Table 1).

  Based on heats of formation and densities, the detonation performance of 1, 3-bis(2-alkyltetrazol-5-yl)triazenes was predicted using the EXPLO5 (5.05 version) program (Table 1). Without surprise, long alkyl substituents have a negative effect on detonation performance but a benefit to reducing the melting point. The negative effect of increased molecular weight is more than offset introduction of energetic moieties. The detonation velocities and pressures of most 1, 3-bis(2-alkyltetrazol-5-yl) triazenes (except 2b and 2d) were predicted between 6992 m/s and 7122 m/s, and 16.6 GPa and 18.4 GPa, respectively, which are comparable with TNT but lower than those of 1, 3-bis(2-methyltetrazol-5-yl)triazene.

  3.   Conclusion

  In summary, a series of novel nitrogen-rich energetic compounds based on tetrazolyl triazene were synthesized using a straightforward method. Most of these compounds exhibit low melting points ranging from 81 ℃ to 106 ℃, which are suitable for melt-cast explosives. All compounds show moderate thermal stabilities with onset decomposition temperature between 158 ℃ and 207 ℃, except compound 2a (139 ℃). The detonation performance of most compounds is comparable with TNT, while these compounds are more sensitive to impact stimulation than TNT. Among these compounds, 1, 3-bis(2-azidoethyltetrazol-5-yl) triazene (2g) displays the best performance, including a low melting point (106 ℃), moderate onset decomposition temperature (183 ℃) and good detonation performance (D: 7087 m/s; P: 17.6 GPa), apart from impact sensitivity (0.75 J).

  4.   Experimental

  ¹H NMR and ¹³C NMR spectra were recorded on a 400 MHz (Buruker Avance 400) nuclear magnetic resonance spectrometers operating at 400 and 100 MHz, respectively, by using DMSO-d₆ as solvent and field locking solvent. IR spectra were recorded using KBr pellets for solids on a Bruker Alpha FT-IR-Spektrometer. The melting (peak) and decomposition (onset) points were obtained on a differential scanning calorimeter (METTLER TOLEDO) at a scan rate of 5 ℃/min. The preparation of substituted-5-aminotetrazoles is [50_TD$DIFF]deposited in Supporting information.

  4.1   General procedure for synthesis of triazene 2a–2e

  NaNO₂ (0.690 g, 10 mmol, 0.5 equiv.) in water (40 mL) was added dropwise to a solution of substituted-5-aminotetrazole (1a– 1e, 20 mmol) and concentrated HCl (4 mL, 48 mmol) in water (100 mL) at 0 ℃. After 2 h, the reaction was stirred additional 2 h at room temperature. The precipitate was filtered, washed with water, and dried to provide high purity product.

  1, 3-Bis(2-cyanomethyltetrazol-5-yl)triazene (2a): White solid (2.234 g, 86%); mp 85 ℃, 139 ℃ (dec.); IR (KBr, cm^-1): 3490, 3454, 2999, 1603, 1468, 1378, 1301, 1229, 1041, 936, 828, 776, 741; ¹H NMR (400 MHz, DMSO-d₆): δ 14.61 (s, 1H), 6.24 (s, 4H); ¹³C NMR (100 MHz, DMSO-d₆): δ 166.2, 113.4, 41.1; Elemental analysis: (C₆H₅N₁₃, 259.19) calcd.: C 27.80, H 1.94, N 70.25; found: C 28.22, H 2.15, N 69.62; HRMS: calcd. for C₆H₅N₁₃ [M+H]⁺: 260.0864; found: 260.0864.

  1, 3-Bis(2-methoxycarbonylmethyltetrazol-5-yl)triazene (2b): White solid (2.684 g, 83%); mp 81 ℃, 169 ℃ (dec.); IR (KBr, cm^-1): 3539, 3464, 3007, 2964, 1735, 1583, 1454, 1363, 1241, 1043, 986, 790, 738; ¹H NMR (400 MHz, DMSO-d₆): δ 14.45 (s, 1H), 5.86 (s, 4H), 3.75 (s, 6H); ¹³C NMR (100 MHz, DMSO-d₆): δ 166.4, 53.6, 52.9; HRMS: calcd. for C₈H₁₁N₁₁O₄ [M+H]⁺: 326.1068; found: 326.1067.

  1, 3-Bis(2-bromoethyltetrazol-5-yl)triazene (2c): White solid (3.513 g, 89%); mp 131–133 ℃; IR (KBr, cm^-1): 3034, 2979, 2876, 1587, 1458, 1426, 1308, 1258, 1204, 1040, 959, 874, 744; ¹H NMR (400 MHz, DMSO-d₆): δ 14.34 (s, 1H), 5.14 (t, 4H, J = 5.6 Hz), 4.05 (t, 4H, J = 5.6 Hz); ¹³C NMR (100 MHz, DMSO-d₆): δ 169.0, 54.6, 29.9; HRMS: calcd. for C₆H₉Br₂N₁₁ [M+H]⁺: 395.9467; found: 395.9463.

  1, 3-Bis(2-allyltetrazol-5-yl)triazene (2d): White solid (2.195 g, 84%); mp 86 ℃, 207 ℃ (dec.); IR (KBr, cm^-1): 3497, 3451, 2770, 1639, 1597, 1461, 1227, 1044, 930, 810, 557; ¹H NMR (400 MHz, DMSO-d₆): δ 14.26 (s, 1H), 6.07–6.14 (m, 2H), 5.31–5.36 (m, 8H); ¹³C NMR (100 MHz, DMSO-d₆): δ 168.8, 130.6, 120.3, 55.3; Elemental analysis: (C₈H₁₁N₁₁, 261.25) calcd.: C 36.78, H 4.24, N 58.98; found: C 36.98, H 4.12, N 58.90; HRMS: calcd. for C₈H₁₁N₁₁ [M+H]⁺: 262.1272; found: 262.1270.

  1, 3-Bis(2-vinyltetrazol-5-yl)triazene (2e): White solid (1.980 g, 85%); 158 ℃ (dec.); IR (KBr, cm^-1): 3180, 3135, 3018, 1600, 1488, 1386, 1328, 1228, 1196, 1010, 952, 936, 789, 722; ¹H NMR (400 MHz, DMSO-d₆): δ 14.45 (s, 1H), 7.77 (dd, 2H, J = 15.4, 8.6 Hz), 6.09 (d, 2H, J = 15.2 Hz), 5.49 (d, 2H, J = 7.6 Hz); ¹³C NMR (100 MHz, DMSO-d₆): δ 165.4, 130.1, 109.2; Elemental analysis: (C₆H₇N₁₁, 233.19) calcd. C 30.90, H 3.03, N 66.07; found: C 31.38, H 2.84, N 65.78; HRMS: calcd. for C₆H₇N₁₁ [M+H]⁺: 234.0959; found: 234.0962.

  4.2   Synthesis of 1, 3-bis(2-hydroxyethyltetrazol-5-yl)triazene (2f)

  NaNO₂ (0.690 g, 10 mmol) in water (4 mL) was added dropwise to a solution of 5-amino-2-(2-hydroxyethyl)-2H-tetrazole (1f, 2.582 g, 20 mmol) and concentrated HCl (2 mL, 24 mmol) in water (10 mL) at 0 ℃. After 2 h, the reaction was stirred additional 2 h at room temperature. The precipitate was filtered, washed with filtrate, and dried to provide the product with high purity.

  1, 3-Bis(2-hydroxyethyltetrazol-5-yl)triazene (2f): White solid (1.867 g, 69%); mp 135 ℃, 199 ℃ (dec.); IR (KBr, cm^-1): 3412, 3309, 2931, 2816, 1603, 1472, 1316, 1220, 1068, 966, 862, 790, 744. ¹H NMR (400 MHz, DMSO-d₆): δ 14.19 (s, 1H), 5.09 (s, 2H), 4.70 (t, 4H, J = 5.0 Hz), 3.93 (d, 4H, J = 2.8 Hz). ¹³C NMR (100 MHz, DMSO-d₆): δ 165.6, 59.0, 56.2. HRMS: calcd. for C₆H₁₁N₁₁O₂ [M+H]⁺: 270.1170; found: 270.1172.

  4.3   Synthesis of 1, 3-bis(2-azidoethyltetrazol-5-yl)triazene (2g)

  A solution of 1, 3-bis(2-bromoethyltetrazol-5-yl)triazene (2c, 1.580 g, 4 mmol) and NaN₃ (0.650 g, 10 mmol, 2.5 equiv.) in DMF (10 mL) and water (1 mL) was stirred at 50 ℃. After 12 h, the mixture was poured into water (100 mL) and then extracted with ethyl acetate (50 mL × 2). The organic layers were extracted with water (100 mL × 2) and then concentrated under reduced pressure to obtain crude product. The product was purified by column chromatography on silica gel (petroleum ether:AcOEt = 1:1, v/v).

  1, 3-Bis(2-azidoethyltetrazol-5-yl)triazene (2g): White solid (0.685 g, 53%); mp 106 ℃, 183 ℃ (dec.); IR (KBr, cm^-1): 3127, 3013, 2955, 2882, 2137, 2118, 2095, 1601, 1489, 1466, 1398, 1308, 1217, 1202, 1028. ¹H NMR (400 MHz, DMSO-d₆): δ 14.35 (s, 1H), 4.91 (t, J = 5.0 Hz, 4H), 3.98 (t, J = 5.2 Hz, 4H); ¹³C NMR (100 MHz, DMSO-d₆): δ 163.5, 53.1, 49.5; Elemental analysis: (C₆H₉N₁₇, 319.25) calcd.: C 22.57, H 2.84, N 74.59; found: C 23.18, H 2.29, N 74.52; HRMS: calcd. for C₆H₉N₁₇ [M+H]⁺: 320.1300; found: 320.1298.

  Acknowledgment

  Financial support of this work from the National Natural Science Foundation of China (No. 21372027) is acknowledged.

  Appendix A. Supplementary data

  Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cclet.2017.04.006.

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• 

  Scheme1  Synthesis of triazenes 2a–2f

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  Scheme2  Synthesis of 2g

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  Figure 1  DSC thermograms of compounds 2a, 2b and 2d–2g (heating rate of 5 ℃/min)

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  Table 1.  Physical properties and calculated detonation parameters of 2a, 2b and 2d–2g

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