Communication pubs.acs.org/JACS
Pushing the Limits of Oxygen Balance in 1,3,4-Oxadiazoles Qiong Yu,‡,† Ping Yin,‡ Jiaheng Zhang,∥ Chunlin He,‡ Gregory H. Imler,§ Damon A. Parrish,§ and Jean’ne M. Shreeve*,‡ ‡
Department of Chemistry, University of Idaho, Moscow, Idaho 83844-2343, United States School of Chemical Engineering, Nanjing University of Science & Technology, Nanjing 210094, China ∥ School of Materials Science and Engineering, Harbin Institute of Technology, Shenzhen 518055, China § Naval Research Laboratory, 4555 Overlook Avenue, Washington, D.C. 20375, United States †
S Supporting Information *
oxadiazole rings have been reported and shown to demonstrate excellent performance (Figure 1).
ABSTRACT: Gem-trinitromethyl groups were introduced into a 1,3,4-oxadiazole ring to give the first example of a bifunctionalized single five-membered ring with six nitro groups. 2,5-Bis(trinitromethyl)-1,3,4-oxadiazole (12) has a high calculated crystal density of 2.007 g cm−3 at 150 K (1.941 g cm−3 at 293 K) and a very high positive oxygen balance (39.12%), which makes it a strong candidate as a high energy dense oxidizer. The dihydroxylammonium and dihydrazinium salts of bis(trinitromethyl)-1,3,4-oxadiazole (5 and 6) exhibit excellent calculated detonation properties (5, vD = 9266 m s−1, P = 38.9 GPa; 6, vD = 8900 m s−1, P = 36.3 GPa) and acceptable impact sensitivities (5 20 J, 6 19 J), which are superior to those of RDX (7.4 J) and HMX (7.4 J). Such attractive features support the application potential of the gem-polynitromethyl group in the design of advanced energetic materials. Surprisingly, 2,5-bis(trinitromethyl)-1,3,4-oxadiazole (12) is more thermally stable and less sensitive than its bis(dinitromethyl) analogue, 8.
Figure 1. Selected derivatives of polynitromethyl (i−iv) and target molecules (8 and 12). aref 6a. bref 7a. cref 7b. dref 6c. eExperimental density (g cm−3). fOxygen balance based on CO (%). gDetonation velocity (m s−1). hDensity calculated from X-ray at 293 K.
Considering the advantages of oxadiazole and gem-polynitromethyl groups in enhancing the properties of energetic materials, their combination is an efficient way to design highly dense energetic materials. Oxadizaole has four isomers: 1,2,4oxadiazole, 1,2,5-oxadiazole, 1,3,4-oxadiazole, and 1,2,3-oxadiazole. Although 1,2,5-oxadiazole is often the first choice of the oxadizole family in the design of energetic materials,8 introduction of two vicinal bulky groups into a single 1,2,5oxadiazole ring has not been reported. Two such substituents at neighboring positions on the 1,2,5-oxadiazole ring would lead to the structure being unstable, if not impossible. Compared to the 1,2,5-oxadiazole ring, 1,3,4-oxadiazole has been studied rarely as a component in energetic materials.9 It is symmetric with the same elemental composition as the 1,2,5-oxadiazole ring. The two positions available for substitution are not vicinal and thus steric hindrance would not be an issue which should make a more stable species possible. Therefore, we were encouraged to synthesize polynitromethyl derivatives based on 1,3,4-oxadiazole which would have high positive oxygen balances and contain only very small amounts of carbon and hydrogen (Scheme 1). The high nitrogen and oxygen content of these compounds should lead
O
ver the last few decades, the development of high-energy density materials (HEDMs) has been very rapid.1 However, the demands of both military and civilian communities require the generation of HEDMs which exhibit more highly dense, thermally stable, environmentally friendly, and nitrogen- and oxygen-rich properties. This continues to be a challenging and ongoing task for chemists and materials scientists. In the search for high-performing energetic materials, the basic strategy is to combine various energetic moieties with a variety of backbones. Because of their usual high density, high positive heat of formation, good oxygen balance, and thermal stability, nitrogen- and oxygen-rich rings are attractive targets in the design of novel HEDMs.2 Oxadiazole is a good example of a nitrogen- and oxygen-rich ring that has attracted considerable interest in recent years.3 Explosophore groups such as nitramino,4 trinitroethyl,5 dinitromethyl,6 trinitromethyl,7 dinitromethyl, and trinitromethyl groups have been found to exhibit increasing roles in the field of energetic materials. The presence of gem-polynitromethyl fragments enhance the oxygen balance values and density of the resulting compounds which improve their detonation velocities and pressures. Recently, the gem-polynitromethyl energetic derivatives based on triazole and © 2017 American Chemical Society
Received: May 24, 2017 Published: June 19, 2017 8816
DOI: 10.1021/jacs.7b05158 J. Am. Chem. Soc. 2017, 139, 8816−8819
Communication
Journal of the American Chemical Society
All the compounds were thoroughly characterized by IR, 1H NMR, and 13C NMR spectroscopy and elemental analysis (Supporting Information). Crystals of 3·H2O, 5·acetonitrile, 8, 12, and 13 suitable for single-crystal X-ray diffraction were obtained at room temperature by slow evaporation of aqueous 3·H2O, acetonitrile (5·acetonitrile), dichloromethane (8 and 12) or methanol solutions (13), respectively. The crystal structures are shown in Figure 2.
Scheme 1. Synthesis of 2,5-Bis(dinitromethyl)-1,3,4oxadiazole (8) and Its Energetic Salts (3−7, 9−11) and 2,5Bis(trinitromethyl)-1,3,4-oxadiazole (12)
to high crystal densities. Experimental thermal stabilities and sensitivities toward impact and friction, as well as detonation properties, suggest that these new moieties may have potential applications as oxidizers or high performance energetic materials. In this work, the starting material, 1,3,4-oxadiazolyl-2,5diacetic acid diethyl ester (1) was easily synthesized.10 As shown in Scheme 1, 1 was treated with a mixture of 100% nitric acid and 98% sulfuric acid at 0 °C to form the gem-dinitro compound 2 as a yellow oil. When 2 was reacted with an excess of aqueous ammonia for 12 h, the light-yellow solid, 3·H2O, precipitated. The stable disilver salt of 4 was isolated from water in good yields by reacting 3·H2O with silver nitrate. It can be stored at room temperature without decomposition. Because the reaction of 2 with hydroxylamine and hydrazine hydrate led to unidentified mixtures of products, the pure dihydroxylammonium or dihydrazinium salts (5 and 6) were obtained alternatively by a metathesis reaction of 4 with hydroxylamine hydrochloride or hydrazine hydrochloride, respectively. When 5 was recrystallized from methanol, one of the hydroxylammonium cations was converted to the ammonium cation (13, Scheme 2), which was characterized by single-crystal X-ray
Figure 2. Single crystal X-ray structures of (a) 3·H2O, (b) 5· acetonitrile, (c) 8, (d) 12, and (e) 13.
The densities of all the new compounds range between 1.65 and 1.92 g cm−3, which were measured by using a gas pycnometer (25 °C). Oxygen balance (OB) is the index of the deficiency or excess of oxygen in a compound (Table 1). Compound 12 exhibits the highest oxygen balance of 39.1%, which is higher than that of the oxidizers ammonium dinitramide (ADN + 25.8) and ammonium perchlorate (AP + 34.04);12 8 has an oxygen balance of 23.0%. Salts 5 and 6 have oxygen balances of 4.9% and 14.0%, respectively. However, salts 7 and 9−11 have negative OB values ranging from −19.7 to −6.7%. It is important to note that the OB value of the hydrazinium salt is zero, which indicates that it can fully use its chemical energy. All compounds decompose between 86 and 235 °C (onset temperature) without melting except for 12, which melts at 80 °C. The heats of formation of 8, 12, and the energetic anion were obtained by using the isodesmic reaction approach with the Gaussian 03 suite of programs.14 The solid-phase heats of formation of energetic salts were calculated on the basis of the Born−Haber energy cycle (Supporting Information).15 The detonation properties and of 3 and 5−13 were evaluated by using the EXPLO5 6.01 program (Table 1).16 The calculated detonation velocities lie in the range from 8045 to 9266 m s−1, and detonation pressures from 24.7 to 38.9 GPa, respectively. Compound 5 shows remarkable values (vD = 9266 m s−1, P = 38.9 Pa) that are comparable to those of HMX (vD = 9320 m s−1, P = 39.5 Pa). This is due to its high density and acceptable heat of formation. Compounds 8 and 13 also exhibit excellent detonation properties (8 vD = 8967 m s−1, P = 36.9 Pa; 13 vD = 8904 m s−1, P = 35.0 Pa) that are superior to those of RDX (vD = 8748 m s−1, P = 34.9 Pa). For safety testing, the sensitivities toward friction and impact were measured by using standard BAM methods.17 The neutral compounds 8 (3 J) and 12 (4 J) are relatively sensitive to impact; however, the ionic derivatives 5−7 have lower impact sensitivities of 20, 19, and 28 J, respectively. Compounds 9 and 10 are insensitive toward impact and friction but show unsatisfactory detonation
Scheme 2. Synthesis of Hydroxylammonium Ammonium 2,5-Bis(trinitromethyl)-1,3,4-oxadiazolate (13)
diffraction (Supporting Information). Fluorination of the dipotassium salt of bi(dinitromethyl)-1,3,4-oxadiazole resulted in 2,5-bis(fluorodinitromethyl)-1,3,4-oxadiazole that was reported earlier; however, there are no details of the synthesis or properties.11 The neutral compound, 2,5-bis(dinitromethyl)1,3,4-oxadiazole (8), was obtained by acidifying 3·H2O with concentrated sulfuric acid. When 8 was reacted with guanidine carbonate, aminoguanidine carbonate or 1,5-diaminotetrazole, a series of nitrogen-rich salts, (9−11) were obtained in high yields. Nitration of 8 was carried out by using 100% nitric acid and 98% sulfuric acid. 8817
DOI: 10.1021/jacs.7b05158 J. Am. Chem. Soc. 2017, 139, 8816−8819
Communication
Journal of the American Chemical Society Table 1. Properties of Energetic Compounds (3·H2O and 5−13) comp
Tma [°C]
Tdb [°C]
dc [g cm−3]
ΔHfd [kJ g−1]
vDe [m s−1]
Pf [GPa]
ISg [J]
FSh [N]
OBi[%]
Ispj [s]
3·H2O 5 6 7 8 9 10 11 12 13 ADNl RDXm HMXm
− − − − − − − − 80
188 146 190 178 86 235 145 142 102 149 159 204 280
1.75 1.89 1.84 1.67 1.91 1.71 1.65 1.70 1.92 1.86 1.81 1.82 1.91
−0.90 −0.58 0.03 0.91 −0.20 −0.69 −0.02 2.07 0.08 −0.79 − 0.36 0.36
8326 9266 8900 8510 8967 8050 8055 8623 8229 8904 − 8748 9320
29.4 38.9 36.3 27.7 36.9 25.4 24.7 31.0 29.2 35.0 − 34.9 39.5
10 20 19 28 3 >40 >40 11 4 18 3−5 7.4 7.4
240 360 80 240 160 >360 >360 120 240 160 64−72 120 120
4.9 14.0 0 −19.7 23.0 −12.1 −15.0 −6.7 39.1 9.8 25.8 0 0
249 267 262 243 255 214 227 268 225 267 202k − −
93 − −
Melting point. bDecomposition temperature (onset) under nitrogen gas (DSC, 5 °C/min). cDensity measured by gas pycnometer (25 °C). dHeat of formation. eDetonation pressure (calculated with Explo5 v6.01). fDetonation velocity (calculated with Explo 6.01). gImpact sensitivity. hFriction sensitivity. iOxygen balance (based on CO) for CaHbOcNd, 1600(c − a− b/2)/MW, MW = molecular weight. jSpecific impulse (values obtained from Explo5 v6.01 and calculated at an isobaric pressure of 70 bar and initial temperature of 3300 K). kCalculated via Explo 5 v6.01. lRef 12. mRef 13. a
properties. The dihydrazinium salt 6 is the most sensitive toward friction of all the energetic compounds. Salts 3·H2O and 11 are more sensitive to impact than the other energetic salts. The specfic impulse (Isp), a measure of a propellant’s efficiency (in seconds), was also calculated (Explo 5 v6.01) for all compounds. The Isp values range from 214 to 267 s, which are higher than that of ADN. They are included in Table 1. In summary, derivatives with gem-polynitromethyl substituents bonded to 1,3,4-oxdiazole (3·H2O, 5−12) were synthesized and fully characterized. The structures of 3·H2O, 5, 8, and 12 were confirmed by single-crystal X-ray analysis. Compound 12 has a high crystal density of 2.007 g cm−3 at 150 K and an oxygen balance of 39.1%, sensitivities (IS = 4 J, FS = 240 N), as well as good calculated detonation properties (vD = 8229 m s−1, P = 29.2 Pa), suggesting that this molecule has a high potential as an oxidizer for applications in solid rocket propellants and missiles. Compared to 12, 8 with a single hydrogen atom bonded to the dinitromethyl group is less thermally stable (86 °C) and more sensitive (IS = 3 J, FS = 160 N). Of all the new salts, the dihydroxylammonium salt 5 and dihydrazinium salt 6 have high experimental densities (5, 1.89 g cm−3; 6, 1.84 g cm−3), moderate decomposition temperatures (5, 146 °C; 6, 190 °C), good calculated detonation properties (5, vD = 9266 m s−1, P = 38.9 GPa; 6, vD = 8900 m s−1, P = 36.3 GPa and acceptable sensitivities toward impact (5, 20 J; 6, 19 J), which suggests potential application as environmentally friendly and high-performing nitrogen- and oxygen-rich energetic materials. Interestingly, compound 13, with one hydroxylammonium and one ammonium cation, exhibits moderate density, thermal stability, detonation properties, and sensitivities, which are between those of the ammonium salt, 3·H2O, and the hydroxylammonium salt, 5.
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X-ray crystallographic file for 12 (TXT) X-ray crystallographic file for 13 (TXT)
AUTHOR INFORMATION
Corresponding Author
*
[email protected] ORCID
Jiaheng Zhang: 0000-0002-2377-9796 Jean’ne M. Shreeve: 0000-0001-8622-4897 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Office of Naval Research (N00014-16-1-2089) and the Defense Threat Reduction Agency (HDTRA 1-15-1-0028). We are also grateful to the Murdock Charitable Trust, Vancouver, WA, Reference No.: 2014120:MNL:11/20/2014 for funds supporting the purchase of a 500 MHz nuclear magnetic resonance spectrometer. Qiong Yu appreciates a Chinese government scholarship.
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REFERENCES
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b05158. Synthesis, characterization data, calculation detail, and crystallographic data (PDF) X-ray crystallographic file for 3·H2O (TXT) X-ray crystallographic file for 5·acetonitrile (TXT) X-ray crystallographic file for 8 (TXT) 8818
DOI: 10.1021/jacs.7b05158 J. Am. Chem. Soc. 2017, 139, 8816−8819
Communication
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DOI: 10.1021/jacs.7b05158 J. Am. Chem. Soc. 2017, 139, 8816−8819
