Chinese Chemical Letters  2017, Vol. 28 Issue (1): 117-120   PDF    
Synthesis and properties of potassium 5, 5'-azobis (1-nitraminotetrazolate): A green primary explosive with superior initiation power
Li Ya-Nana, Wang Bo-Zhoua, Shu Yuan-Jiea, Zhai Lian-Jiea, Zhang Sheng-Yonga, Bi Fu-Qianga, Li Yu-Chuanb     
a Key Laboratory of Fluorine & Nitrogen Chemicals, Xi'an Modern Chemistry Research Institute, Xi'an 710065, China;
b School of Material Science & Engineering, Beijing Institute of Technology, Beijing 100081, China
Abstract: Potassium 5, 5'-azobis (1-nitraminotetrazolate), (K2ABNAT), a new green primary explosive, was synthesized via a safe and convenient synthetic procedure based on methylcarbazate and cyanogen azide. The compound was characterized by single-crystal X-ray diffraction, IR spectroscopy, Raman spectroscopy, multinuclear NMR spectroscopy, elemental analysis, and differential scanning calorimetry (DSC). With the calculated (CBS-4M) heat of formation (617.0 kJ/mol) and the room temperature X-ray density (2.11 g/cm3), impressive values for the detonation parameters such as detonation velocity (8367 m/s) and pressure (31.5 GPa) were computed using the EXPLO5 program. The superior calculated energetic performance show it could serve as a green replacement for the widely used primary explosive, lead (II) azide, which contains toxic ingredient.
Key words: Energetic materials     Tetrazole     Azo compounds     Nitroamines     Primary explosives    
1. Introduction

The explosive chain reaction is typically initiated by detonation of a small quantity of highly sensitive, primary explosive in a variety of military purposes and civilian applications [1]. Primary explosives form a group of explosives that, upon ignition, undergo an extremely rapid deflagration-to-detonation transition (DDT) [2-4]. These compounds are widely used in detonators, primers, blasting caps, and initiators. The primary explosives are commonly lead (Ⅱ) azide, lead styphnate [5], copper (I) 5-nitrotetrazolate (DBX-1) [6], and recently reported potassium 1, 10-dinitramino-5, 50-bistetrazolate (K2DNABT) [7] and potassium 4, 5-bis (dinitromethyl) furoxannate (K2BDNMF) [8] (Fig. 1). Among these compounds, lead (Ⅱ) azide is the most widely used primary explosive today. However, in recent years, lead-based primary explosives have been documented to cause environmental and health related problems. Lead is both an acute and chronic toxin, and the human body has difficulty in eliminating it once it has been absorbed and dissolved in the blood [2]. As environmental regulations and human health problems have called for the replacement of lead (Ⅱ) azide in primary explosive formulations [9], a renewed focus on lead-free-alternative (copper, potassium and silver) energetics has unveiled promising properties for their use as a direct drop-in replacement of the undesirable lead and mercury-based legacy primary explosives [10]. Therefore, there is a need to develop "green" primary explosives to replace lead-based energetic compounds. A "green" replacement of lead-based compounds should possess the following properties: (a) insensitivity to light; (b) sensitivity to detonation (but not too sensitive to handle and transport); (c) stability to at least 150 ℃; (d) stability upon storage for long periods of time; (e) free of toxic metals or other known toxins; (f) possess high detonation performance and (g) an ease, safety, and affordability of synthesis [2, 5].

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Figure 1. Promising replacements of lead-based primary explosives.

In seeking "green" replacements for the lead-based compound, one important strategy we focused on is the use of nitrogen-rich energetic compounds. These are widely considered to be viable environmental energetics as the main detonation product is nitrogen gas. At the same time, they always possess high positive heats of formation, which lead to high energy output [2]. As is wellknown, potassium is an environmentally friendly species with good coordinating ability to energetic ligands [11]. Therefore, nitrogen-rich, energetic potassium salts are considered to be "green" candidates for the replacement of lead-based primary explosives. In our present work, we focus our attention on the "green" primary explosives with high detonation performance. As a result of our continuing efforts, we report, herein, the synthesis of a new green primary explosive, potassium 5, 50-azobis (1-nitraminotetrazolate) (K2ABNAT), which contains only potassium as the metal (Scheme 1).

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Scheme 1. Synthetic pathway of K2ABNAT.

2. Experimental

All chemical reagents and solvents were used as supplied unless otherwise stated. Elemental analyses (C, H and N) were performed on a VARI-El-3 elementary analysis instrument. Infrared spectra were obtained as KBr pellets on a Nicolet NEXUS870 Infrared spectrometer in the range of 4000-400 cm-1. Raman spectra were measured with a RENISHAW® InVia instrument. The 1H NMR and 13C NMR data were obtained in DMSO-d6 on a Bruker AV500 NMR spectrometer. The DSC experiment was performed using a DSCQ200 apparatus (TA Instruments, New Castle, USA) under a nitrogen atmosphere at a flow rate of 50 mL/min with about 0.3 mg of the sample sealed in aluminium pans for the DSC measurements. The TG/DTG experiment was performed using a SDT-Q600 apparatus (TA Instruments, New Castle, USA) under a nitrogen atmosphere at a flow rate of 50 mL/min.

2.1. Preparation of 1-methoxycarbonyl-1, 5-diaminotetrazole (2)

Methylcarbazate (1) (1.65 g, 18.1 mmol) in water (10 mL) was added to a freshly prepared solution of cyanogen azide in acetonitrile (22.3 mmol in 40 mL) at ice water condition, and stirring at r.t. for 24 h. After most of the acetonitrile evaporation, the solution was stirred until a white filterable suspension was formed. The suspension was filtered and washed with a small quantity of ice water. The product was dried in air.

Compound 2: White solid (1.58 g, 54.5%); mp: 180.5 ℃, Tdec: 185.5 ℃ (peak, 5 ℃/min); IR (KBr, cm-1): 3373, 3263, 3200, 3104, 2867, 1743, 1655, 1580, 1485, 1451, 1326, 1198, 1119, 1069, 987, 926, 829, 758, 723; Raman (785 nm, 250 mW, cm-1): 2969, 2873, 1749, 1657, 1582, 1512, 1487, 1453, 1342, 1120, 1073, 989, 927, 831, 773, 758, 666 521, 450, 373; 1H NMR (500 MHz, DMSO-d6): δ 11.21 (s, 1H, NH), 7.05 (s, 2H, NH2), 3.73 (s, 3H, CH3); 13C NMR (125 MHz, DMSO-d6): δ 155.14, 155.05, 53.59; Elemental analysis: calcd. (%) for C3H6N6O2 (158.06): C 22.79, H 3.82, N 53.15, found: C 22.86, H 3.76, N 53.34.

2.2. Preparation of 5, 50-azobis (1-methoxyformamidotetrazole) (3)

Compound 2 (0.79 g, 5 mmol) was added to concentrated hydrochloric acid (12.9 mL) at r.t. with stirring until the solid was dissolved. Potassium permanganate (0.79 g, 5 mmol) in water (14 mL) was added dropwise to the stirred solution of 2 at 10 ℃. After stirring for 10 min, the reaction mixture was heated to 55 ℃ for 5 h. The yellow precipitate was filtered, washed with ice water and dried in air.

Compound 3: Yellow solid (0.70 g, 89.7%); Tdec: 190.9 ℃ (peak, 5 ℃/min); IR (KBr, cm-1): 3242, 3024, 2968, 1767, 1638, 1536, 1504, 1467, 1437, 1309, 1248, 1147, 1074, 1049, 964, 848, 767, 751, 594; Raman (785 nm, 250 mW, cm-1): 2974, 1780, 1508, 1443, 1433, 1247, 1084, 1046, 983, 932, 813, 524, 403; 1H NMR (500 MHz, DMSO-d6): δ 12.84 (s, 2H, 2NH), 3.81 (s, 6H, 2NH2); 13C NMR (125 MHz, DMSO-d6): δ 158.00, 155.24, 54.47; Elemental analysis: calcd. (%) for C6H8N12O4 (312.08): C 23.08, H 2.58, N 53.84; found: C 23.13, H 2.52, N 53.90.

2.3. Preparation of potassium 5, 50-azobis (1-nitraminotetrazolate)(K2ABNAT)

Compound 3 (1.87 g, 6 mmol) was suspended in dry acetonitrile (58 mL) and cooled to 0 ℃, then N2O5 (2.59 g, 24 mmol) in dry acetonitrile (32 mL) was added in one portion and the mixture stirred at 0 ℃ for 4 h. The reaction was quenched by adding KOH (2.69 g, 48 mmol) dissolved in water (24 mL), and stirred vigorously for another one hour. The solvent was removed under high vacuum. The residue was stirred in methanol (35 mL) for several hours. The precipitated solid was filtered and suspended in 20 mL of ice water, stirred for 20 min, filtered again and dried in air.

Compound K2ABNAT: Yellow solid (0.46 g, 21.2%); Tdec: 194.3 ℃ (peak, 5 ℃/min); IR (KBr, cm-1): 1635, 1456, 1431, 1298, 1242, 1226, 1154, 1079, 1015, 927, 775; Raman (785 nm, 250 mW, cm-1): 1491, 1480, 1432, 1400, 1313, 1245, 1227, 1090, 1081, 1017, 997, 927, 879, 822, 684, 430, 372, 291; 13C NMR (125 MHz, DMSO-d6): δ 157.96; Elemental analysis: calcd. (%) for C2K2N14O4 (361.95): C 6.63, N 54.12; found: C 6.71, N 54.06.

3. Results and discussion

K2ABNAT was obtained in three steps from methylcarbazate (1) and cyanogen azide, and the synthetic process was described as follows. Compound 1 was reacted with cyanogen azide to yield Nmethoxycarbonyl protected 1, 5-diaminotetrazole (2), which was oxidized in concentrated hydrochloric acid with KMnO4 to 5, 50-azobis (1-ethoxyformamidotetrazole) (3). Compound 3 was gently nitrated with N2O5 in acetonitrile, and decomposed in solution with aqueous KOH to give the mixture of K2ABNAT and KNO3 as a yellow precipitate, from which K2ABNAT is isolated by stirring in water and methanol, respectively. K2ABNAT is insensitive to light and very stable upon storage under ambient conditions. It is not soluble in methanol or ethanol, but could dissolve in N, N’-dimethyl formamide (DMF) and dimethyl sulfoxide (DMSO).

Single crystals of K2ABNAT suitable for X-ray diffraction measurements were obtained by slow-evaporation from a water solution. The molecular structure of K2ABNAT in the solid state was determined by X-ray diffraction at r.t. to obtain accurate density for performance calculations. The ORTEP diagram of K2ABNAT is shown in Fig. 2, and more structural details are given in the Supporting information. The compound crystallizes in the triclinic space group Pıwith a density of 2.11 g/cm3 and a cell volume of 570.7 Å3 at 296 K. The repeating unit of K2ABNAT contains two potassium ions and one 5, 50-azobis (1-nitramino tetrazolate) (ABNAT) anion. There are two different coordination environment of potassium atoms. The potassium atoms are coordinated irregularly by either the nitrogen atoms N1, N2, N4, N5, N6, N9, N10, N11, N13, N14, or the nitro oxygen atoms O1, O2, O3 and O4. Both tetrazole rings and the azo bond are almost planar to each other. The nitro groups are twisted out of this plane by almost 858 and 878, respectively. For the K2ABNAT molecule, the bond distances for N1-N2 (1.332(6) Å), N2-N3 (1.387(5) Å), N7-N8 (1.263(5) Å), N12-N13 (1.388(5) Å) and N13-N14 (1.323(6) Å) lie between an N-N single bond (1.460 Å) and double bond (1.250 Å) [12, 13]. In addition, the bond distances of C1-N7 (1.397(6) Å) and C2-N8 (1.389(6) Å) are shorter than a C-N single bond (1.470 Å) and longer than C5 5N double bond (1.220 Å) [12], which can be explained by the hyper conjugation effect of good delocalization of π electron in the whole molecule structure (data and parameters of the X-ray measurements and structure refinements are given in Table S1 in Supporting information. File CCDC-1439593 contains supplementary crystallographic data of K2ABNAT which can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.).

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Figure 2. Single crystal structure of K2ABNAT. Thermal ellipsoids represent the 50% probability level.

The thermal stability of K2ABNAT was also investigated using differential scanning calorimetry (DSC) and thermogravimetric (TG/DTG). In the DSC curve (Fig. 3), decomposition started at 189.4 ℃ (onset temperature) and a peak temperature of 194.2 ℃ when the heating rate is 5 ℃/min. There is no endothermic peak in the DSC curve which indicates K2ABNAT melts with concomitant decomposition under the heating condition. The typical TG/DTG curves of K2ABNAT are shown in Fig. 4. As noticeable from the TG curve, there was one main mass-loss stage, corresponding to only one peak in the DSC curve (Fig. 3). A sudden weight loss was observed at 175.0 ℃ that stopped at 200.0 ℃, and the mass-loss peak was at 191.6 ℃.

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Figure 3. DSC curve of K2ABNAT at heating rate of 5 ℃/min.

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Figure 4. TG/DTG curves of K2ABNAT at heating rate of 5 ℃/min.

The impact sensitivity test was carried out according to the Fall Hammer Method using a 2.0 kg drop hammer on a ZBL-B impact sensitivity instrument. The friction sensitivity test was determined using a Julius Peters apparatus, following the BAM method [14]. Compound K2ABNAT is very sensitive towards impact (1-2 J) and friction (≤1 N), these properties are comparable to those of lead (Ⅱ) azide and K2DNABT. Therefore, it should be considered to be a primary explosive with handling by appropriate precautions.

In order to explore the energetic performance of K2ABNAT, several detonation parameters were calculated with the EXPLO5 code [15] in its version 5.05 using density (recalculated from the Xray structure at r.t.) and a calculated heat of formation, and compared to those of lead (Ⅱ) azide and K2DNABT. The crystal density of K2ABNAT calculated at 296 K is 2.11 g/cm3, which is equal in value to K2DNABT. The heat of formation of K2ABNAT was calculated by the atomization method using the Gaussian 09 program package at the CBS-4M level of theory [16, 17]. As can be seen in Table 1, the heat of formation (617.0 kJ/mol) of K2ABNAT is much higher than lead (Ⅱ) azide (450.1 kJ/mol) and K2DNABT (326.4 kJ/mol). Based on good oxygen balance and nitrogen content, K2ABNAT easily outperforms lead (Ⅱ) azide and K2DNABT in all critical detonation parameters (energy of formation, heat of detonation, temperature of detonation, detonation velocity, and gas volume after detonation). Therefore, K2ABNAT displays excellent overall performance as a suitable and non-toxic replacement for lead (Ⅱ) azide, with a safe and convenient synthesis in three steps from commonly available chemicals (Scheme 1), which is comparable with that of K2DNABT.

Table 1
Comparison of physicochemical and energetic properties of lead (Ⅱ) azide, K2DNABT and K2ABNAT.

4. Conclusion

In summary, potassium 5, 50-azobis (1-nitramino tetrazolate) (K2ABNAT), a new green primary explosive, was synthesized in three steps from methylcarbazate and fully characterized. K2ABNAT displays high density (2.11 g/cm3), excellent thermal stability (onset temperature 189.4 ℃), high positive heat of formation (617.0 kJ/mol), remarkable energetic performances (D=8367 m/s, P=31.5 GPa), which easily outperforms lead (Ⅱ) azide. Therefore, K2ABNAT is predicted to be a superior energetic performance and green primary explosive, which is potential green replacement for the widely used primary explosive, lead (Ⅱ) azide.

Acknowledgment

We are grateful of financial support from the National Natural Science Foundation of China (No. 21373157). Dr. Yu-Chuan Li from School of Material Science & Engineering, Beijing Institute of Technology is acknowledged for the calculation of energetic properties (Explore 5.0 (5.05 version)).

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.2016.06.026.

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