Azo substituted 1,2,4-oxadiazoles as insensitive energetic materials

Abstract

5,5′-Diamino-3,3′-azo-1,2,4-oxadiazole (3) was synthesized by reacting 3,5-diamino-1,2,4-oxadiazole with potassium permanganate in the presence of an organic solvent. Nitration of 5-amino-3-azo-1,2,4-oxadiazole gave rise to the 1,2,4-oxadiazolone (4) directly rather than the nitramine compound. Energetic salts of oxadiazolone 4 were prepared by treating with amine bases. These high nitrogen compounds were fully characterized using IR and multinuclear NMR spectroscopy, elemental analysis, and differential scanning calorimetry (DSC), and, in case of 3, with single crystal X-ray structuring. All the azo-substituted 1,2,4-oxadiazoles are impact insensitive materials.

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Research efforts in the field of energetic materials continue to grow worldwide. Most of the energetic materials that are currently in use were developed in the 20^th century. The search for energetic materials with higher performance and lower sensitivity continue to be keen concerns in weapon systems development.¹ In general, the standards for new energetic materials are high positive heat of formation, high density, high detonation velocity and pressure, high thermal stability and low sensitivity towards external forces such as impact and friction.² These energetic materials are widely used in military, industrial, and civilian applications as controllable storage systems for relatively large amounts of chemical energy.³

High-nitrogen compounds, which derive their energy from their inherently high heats of formation, have attracted significant recent attention from many researchers. Together with their high positive heats of formation and high thermal stability, nitrogen-rich heterocycles, such as triazole, tetrazole, and oxadiazoles offer good backbones for the development of new energetic compounds. Such heterocycles are often modified by functional groups, like amino, azo and azido, which further increase overall nitrogen content. Particularly, azo-substituted azoles were shown to be very promising energetic materials.⁴ In addition, these azoles are valuable to the pharmaceutical field as synthetic intermediates and drugs.⁵

The incorporation of amino groups into the azole rings is one of the best strategies by which to increase the thermal stability and impact sensitivity of an energetic molecule. Azole-based high-nitrogen compounds in combination with energetic substituents such as nitro groups (NO₂), azo (N=N) and nitramine (NHNO₂) functionalities are an important class of energetic materials. In treatment with amines such as ammonia, hydrazine, and hydroxylamine azoles often lead to stable nitrogen rich salts which are suitable for high energetic materials.⁶

By combining an azo group with nitrogen-rich heteroaromatic rings in which the azo group is bonded to a carbon atom (C,C′-azo linkage), a new kind of high-nitrogen backbone is obtained. Moreover, it has been shown that the azo linkage not only desensitizes but dramatically increases heats of formation of high-nitrogen compounds.⁷

Additionally, an oxygen atom bonded to nitrogen is a new approach to the development of energetic oxidizers in the class of oxadiazoles-formally derived simply by replacing the NH-group of any triazole derivative by an oxygen atom in order to increase the oxygen balance of these five-membered ring systems.⁸ Many 1,2,5-oxadiazole compounds show high thermal sensitivity coupled with low sensitivity to shock and impact.⁹ Even though various azo derivatives of 3,4-diamino-1,2,5-oxadiazoles (furazans) are well known in the literature, it is surprising that azo derivatives of 3,5-diamino-1,2,4-oxadiazoles have not been investigated. In this communication, we report the synthesis of azo-substituted 1,2,4-oxadiazoles which display potentially significant physical and energetic properties. The novel derivatives of azo-substituted-1,2,4-oxadiazol-5-ones appear to be excellent candidates for pyrotechnics and propellants applications.

Diamino-1,2,4-oxadiazole (2) was prepared by reacting a suspension of sodium dicyanamide in ethanol with hydroxylamine (Scheme 1). The amino group of diamino azoles can be converted to azo substituted amino-azoles by reacting with potassium permanganate in alkaline or acidic medium.¹⁰ However, when 3,5-diamino-1,2,4-triazole was mixed with alkaline potassium permanganate, the azo compound was not formed. If the 3,5-diamino-1,2,4-triazole was reacted with potassium permanganate in hydrochloric acid, a trace amount of the azo compound was obtained. When acetonitrile was added to the reaction mixture of 3,5-diamino-1,2,4-triazole in acidic potassium permanganate, diazo compound 3 was produced in 55% yield. Amine groups bonded to azoles can be converted to corresponding nitramines by reacting with nitrating reagents. When the azo-substituted amino-1,2,4-oxadiazole 3 was reacted with 100% nitric acid, or a nitric acid and sulfuric acid mixture no nitramine product formation was observed.⁹ When reacted with nitric acid in acetic anhydride, a yellow product –a carbonyl containing 1,2,4-oxadiazolone, 4 was obtained in 54% yield. Electron withdrawing groups at the C5-position of 1,2,4-oxadiazoles tend to be unstable with respect to water to form 1,2,4-oxadiazol-5-ones.¹¹ The intermediate nitramine could be formed and when water was added to the reaction mixture in the workup it may have been replaced by a hydroxyl group resulting in 54% of 1,2,4-oxadiazolone 4.

Scheme 1 Synthetic route to azo-substituted 1,2,4-oxadiazoles 3 and 4.

Due to the presence of acidic hydrogen on the ring, 4 was converted to energetic salts by reacting with basic amines. The diammonium salt (5) was produced when 4 was reacted with excess ammonia. Similarly the dihydrazinium (6) or dihydroxylammonium (7) salts was obtained when 4 was reacted with two equivalents of hydrazine or hydroxylamine. Methanol or water was used as a solvent (Scheme 2). All the salts were produced in high yield (90–94%).

Scheme 2 Preparation of energetic salts of 4.

The structures of azo-substituted 1,2,4-oxadiazoles are supported by IR, ¹H, ¹⁵N, and ¹³C NMR spectroscopic data as well as elemental analysis. In the IR spectra, a strong absorption band at 1700 cm⁻¹ is attributed to the carbonyl group of the 1,2,4-oxadiazol-5-ones (compounds 4–7). In the ¹³C spectra, a resonance band for the carbonyl group of 4 appeared at 165 ppm, while the analogous carbonyl bands of energetic salts 5–7 appeared at 174–176 ppm. The ¹⁵N NMR spectra of 1,2,4-oxadiazoles were measured in DMSO[D6] solution and chemical shifts are given with respect to CH₃NO₂ as external standard. In ¹⁵N NMR spectra of compounds 4–7, the ring nitrogen bonded to oxygen appeared at +121 ppm whereas N4 nitrogen appeared at around −226 ppm. Nitrogen of the azo group was observed at −40 ppm.

Crystals of 3 suitable for single crystal X-ray diffraction, were obtained by dissolving the compound in a minimum amount of DMSO–H₂O (9 : 1 ratio) mixture followed by filtration of crystals. It crystallizes in a monoclinic crystal system (space group P21/n). All bond lengths, angles and torsion angles are listed in Table S1 in ESI.† The bond lengths of O1–N2 is 1.4270 (13), O1–C5 is 1.3612 (2) C3–N2 is 1.3080 (16) and C3–N4 is 1.3576 (15) which are normal in 1,2,4-oxadiazoles (Fig. 1).¹¹

Fig. 1 X-ray crystal structure of compound 3.

As shown in Table 1, all azo-substituted 1,2,4-oxadiazoles exhibit moderate to good energetic properties. The enthalpies of energetic materials depend on molecular structures of the compounds. Consequently, heterocycles with high nitrogen content exhibit higher heats of formation. All ab initio calculations were carried out using the program package Gaussian 03 (Revision D.01).¹² The geometric optimization of the structures and frequency analyses were accomplished by using the B3LYP with the 6-31+G** basis set,¹³ and single-point energies were calculated at the MP2/6-311++G** level. The heat of formation of 4 was calculated using G2 Method and the heat of formation of salts 5, 6, and 7 were calculated using the Born–Haber cycle.¹⁴ 5,5′-Diamino-3,3′-azo-1,2,4-oxadiazole (3) exhibits a positive heat of formation whereas azo compounds 4, 5, 6 and 7 exhibit negative heats of formation.

Table 1 Physical and detonation properties of 1,2,4-oxadiazoles

Comp.	Tm^a °C	Td^b °C	d^c g cm^−3	ΔH^°f^d kJ mol^−1 (kJ g^−1)	IS^e J	P^f GPa	D^g m s^−1
a Melting point.b Thermal decomposition temperature (onset) under nitrogen gas (DSC, 5 °C min^−1).c Density, gas pycnometer (25 °C).d Heat of formation (Gaussian 03).e Impact sensitivity (BAM drophammer).f Detonation pressure (Explo5 version 6.01).g Detonation velocity (Explo5 version 6.01).
3	154	355	1.73	334.3 (1.7)	>40	23.4	7980
4	—	123	1.70	−80.8 (−0.4)	>40	20.7	7363
5	—	237	1.68	−463.9 (−2.0)	>40	16.7	7006
6	95	140	1.69	−115.4 (−0.4)	>40	22.1	7875
7	—	112	1.67	−306.3 (−1.2)	>40	22.7	7222
TNT	81	295	1.65	−67.0 (0.3)	15	19.5	6881
RDX	—	230	1.82	92.6 (0.4)	7.4	35.2	8997

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The densities of these compounds are in the range of 1.67–1.73 g cm⁻³ which equals or exceeds that of common explosives like TNT. Compound 3 has the highest density at 1.73 g cm⁻³. When compared to neutral compounds 3 and 4, salts 5, 6, and 7 have lower densities. Impact sensitivity measurements were made using standard BAM Fallhammer techniques.¹⁵ For all of the compounds, the impact sensitivities are above 40 J, which means that the 1,2,4-oxadiazoles are insensitive materials and are much less sensitive than TNT and RDX. Thermal stabilities of the energetic compounds were studied with differential scanning calorimetry (DSC) at a scan rate of 5 °C min⁻¹. Compound 3 melts at 154 °C and decomposes at 355 °C, whereas the carbonyl derivative 4 decomposes at 153 °C and the ammonium salt 5 at 237 °C without melting. The hydrazinium (6) and hydroxylamine (7) salts decompose at 140 °C and 112 °C respectively, which are less stable than the ammonium compound 5.

By using the calculated values of the heats of formation and the experimental values for the densities (gas pycnometer, 25 °C) of the energetic azo linked 1,2,4-oxadiazoles, the detonation pressures (P) and detonation velocities (D) were calculated based on traditional Chapman–Jouget thermodynamic detonation theory using Explo5 program (version 6.01) (Table 1).¹⁶ The detonation pressures of 1,2,4-oxadiazoles lie in the range between P = 16.71 to P = 23.38 GPa. Detonation velocities lie between D = 7006 and D = 7980 m s⁻¹. The calculated properties coupled with the rather high thermal and hydrolytic stabilities suggest these high nitrogen materials as attractive candidates for insensitive energetic applications.

In summary, the syntheses of novel azo-substituted 1,2,4-triazoles 3 and 4 were carried out using straightforward methods. 3,5-Diamino-1,2,4-oxadiazole was successfully converted to azo substituted 1,2,4-oxadiazole in acetonitrile–water media. N-Nitration of amino groups of 3 produced 1,2,4-oxadiazol-5-one derivative 4. Energetic salts of 4 were prepared using simple methods and the physical and detonation properties of these compounds were determined. These 1,2,4-oxadiazoles are less sensitive than TNT which suggests that they might be of interest for future applications as environmentally friendly insensitive energetic materials.

Acknowledgements

The authors gratefully acknowledge the support of ONR (N00014-12-1-0536).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Detailed experimental protocols and spectroscopic characterization data are provided. CCDC 1015568. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ra10821c

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