Synthesis, Crystal Structure and Thermal Decomposition of a New Energetic Potassium Salt of Bis(dinitromethyl)difurazanyl Ether①

ZHAI Lian-Jie WAGN Bo-ZhouFAN Xue-Zhong LI Xiang-Zhi

(Xi’an Modern Chemistry Research Institute, Xi’an 710065, China)

1 INTRODUCTION

Furazanyl ether compounds have become an important research direction in recent years due to their good performances such as high densities, high heats of formation, and good oxygen balance[1-9],which could be used as insensitive high explosives and energetic plasticizers. An intramolecular ether bond is a common method to increase both the flexibility and plasticizer effect of a molecule. When ether bonds and furazan rings are in the same plane,a large p-π conjugated system is thus formed. It is expected to significantly improve the thermal stability. It has been reported that 3,3?-dicyanodi-furazanyl ether (FOF-2) has a high heat of formation(576 kJ·mol-1) and thermal stability (Tdec＞ 250 ℃)[8].However, it was noted that these compounds usually have low melting point and volatility in the previous studies, which restricted their wide applications in solid propellants[6-9].

Many energetic salts often possess advantages over neutral molecules, and these salts usually have lower vapor pressures and higher thermal stabilities than similar nonionic analogues[10-16], making them the most intensively studied energetic materials. In the paper, using 3,3?-dicyanodifurazanyl ether(FOF-2) as a material, we report the new reaction and research of the crystal structure and thermal behaviors of the novel high-energy furazanyl ether potassium salt, bis(dinitromethyl)difurazanyl ether potassium salt [K2(BDFE)], which could be used as detonating explosive and flame suppressor in solid propellant to substitute inorganic potassium salts(KCl, K2SO4, KNO3and K3AlF6) to generate much more energy and clean gas[17-19].

2 EXPERIMENTAL

2. 1 Materials and measurements

Melting point was measured on a XT4A Melting-Point Apparatus with Microscope and uncorrected.1H NMR and13C NMR were obtained in DMSO-d6on a Bruker AV500 NMR spectrometer.Infrared spectra were obtained from KBr pellets on a Nicolet NEXUS870 Infrared spectrometer in the range of 4000～400 cm-1. Elemental analyses (C, H and N) were performed on a VARI-El-3 elementary analysis instrument. Differential scanning calorimetry (DSC) studies were carried out on a Q200 apparatus (TA, USA) at a heating rate of 10 K min-1under dry oxygen-free nitrogen atmosphere with a flowing rate of 50 mL·min-1. The TG-DTG experiment was performed with a SDT-Q600 apparatus(TA, USA) operating at a heating rate of 10 K min-1in a flowing dry oxygen-free nitrogen at 100 mL·min-1.

3,3?-dicyanodifurazanyl ether was prepared according to the published procedures[8]. Other chemicals were obtained from commercial sources and used without further purification.

2. 2 Synthesis and characterization

The title compound [K2(BDFE)] was synthesized by the method shown in Scheme 1.

Scheme 1. Synthetic route of the title compound

Preparation of compound 2: To a mixture of water (50 mL), isopropanol (25 mL), 3,3?-dicyanodifurazanyl ether (4.16 g, 20.0 mmol)(FOF-2) and hydroxylamine hydrochloride (2.85 g, 41mmol),sodium carbonate anhydrous was added in batches,and then the reaction mixture was stirred at room temperature for 1 h. The precipitate was filtered,washed with ice-cold water, and dried in vacuo over P2O5to afford a white solid (5.02 g, 91.2% yield).m.p.: 203～204 ℃.1H NMR (DMSO-d6, 500 MHz):δ = 10.67 (s, 2H, OH), 6.28 (s, 4H, NH2) ppm.13CNMR (DMSO-d6, 125 MHz): δ = 160.31 (C–O),142.23 (C–C=N), 141.33 (C–NH2) ppm; IR (KBr,cm–1): v = 3495, 3454, 3349 (NH2), 3172, 2919(OH), 1101, 1021 (ether bond), 1680, 1656, 1525,969 (furazan ring). Calcd. for C6H6N8O5: C, 26.67;N, 41.18; H, 2.24%. Found: C, 26.22; N, 41.07; H,2.08%.

Preparation of compound 3: Appropriate compound 2 (5.40 g, 20.0 mmol) mentioned above was dissolved in 55 mL of concentrated hydrochloric acid and 30 mL of water at room temperature.Saturated sodium nitrite (2.38 g, 41.0 mmol) in water was added dropwise to a stirred solution of amide oxime. After stirring for 2 h at 273 K, the reaction mixture was heated to 293 K for 1.5 h until the N2evolution stopped. The resulting white precipitate was filtered, washed with water,recrystallized from MeOH/H2O (1:1) and dried in vacuo to yield white solid (5.40 g, 87.5%). m.p.:60～61 ℃.1H NMR (DMSO-d6, 500 MHz): δ =13.71 (s, 2H, OH);13C NMR (DMSO-d6, 125 MHz):δ = 158.99 (C–O), 143.03 (C–C=N), 123.40 (C–Cl).IR (KBr, cm–1): v = 3533, 3167, 3020 (OH), 1126,1024 (ether bond), 657 (C–Cl), 1570, 1518, 942(furazan ring). Calcd. for C6H2N6O5Cl2: C, 23.32; N,27.20; H, 0.65%. Found: C, 23.30; N, 26.85; H, 0.73%.

Preparation of compound 4: To a suspension of the above compound 3 (0.55 g, 1.8 mmol) in 50 mL of CHCl3at 293 K was added N2O5(2.2 g, 20 mmol). The mixture was heated to 318 K and kept at this temperature for 40 min. The solvent was evaporated and the residue was subjected to column chromatography on silica gel to isolate colorless crystals 4 (0.23 g, 30.0%). m.p. for 4: 68～69 ℃.13C NMR (DMSO-d6, 125 MHz): δ = 157.67 (C–O),140.37 (C–C＝N), 112.75 (CCl (NO2)2). IR (KBr,cm–1): v = 1613, 1291 (CCl (NO2)2), 1049 (ether bond),1582, 1515, 971 (furazan ring). Calcd. for C6N8O11Cl2:C, 16.72; N, 26.00. Found: C, 16.92; N, 25.25%.

Preparation of compound K2(BDFE): Compound 4 (1.0 g, 2.3 mmol) was dissolved in MeOH (8 mL)and treated with solution of KI (1.5 g, 9.0 mmol) in MeOH (15 mL) at room temperature. The resulting mixture was stirred at room temperature for 1 h and triturated with Et2O (20 mL). The resulting precipitate was collected, washed with ice-cold water, MeOH, and Et2O to furnish a yellow solid(0.81 g, 85.7%). Dec., 245 ℃ (DSC measurement,10 ℃ min-1).13C NMR (DMSO-d6, 125 MHz): δ =160.77 (C–O), 142.31 (C–C=N), 118.67 (C–(NO2)2);IR (KBr, cm–1): v = 1479, 1239 (C–(NO2)2), 1070(ether bond), 1589, 1526, 997 (furazan ring). Calcd.for C6N8O11K2: C, 16.44; N, 25.57%. Found: C,16.15; N, 24.96%.

2. 3 Structure determination

Single crystals suitable for X-ray measurement were obtained by the slow evaporation of an aqueous solution of K2(BDFE). A bright yellow single crystal with dimensions of 0.36mm ×0.28mm × 0.20mm was selected for X-ray singlecrystal diffraction analysis. The data were collected on a Bruker SMART Apex II CCD X-ray diffractometer equipped with a graphite-monochromatized Mo Kα radiation (λ = 0.71073 ?) using the φ-ω scan mode (2.51＜θ＜25.09°) at 296(2) K. A total of 4077 reflections were collected, of which 1475 were independent (Rint= 0.0285) and 1217 with I ＞ 2σ(I)were considered to be observed and used for the refinement. The structure was solved by direct methods and refined by full-matrix least-squares techniques on F2using SHELXS-97 and SHELXL-97 programs[20-21]. All non-hydrogen atoms were refined anisotropically. A full-matrix least-squares refinement gave the final R = 0.0589 and wR =0.1562 for all data (w = 1/[σ2(Fo2) + (0.0881P)2+3.5129P], where P = (Fo2+ 2Fc2)/3). The largest difference peak and hole are 0.584 and –0.530 e·?-3,respectively. The selected bond lengths and bond angles are listed in Table 1.

Table 1. Selected Bond Lengths (?) and Bond Angles (°)

3 RESULTS AND DISCUSSION

3. 1 Synthetic reactions

Using 3,3?-dicyanodifurazanyl ether (FOF-2) as the starting material, K2(BDFE) was synthesized via a four-step synthetic route in the first with a total yield of 20.5%. The elemental analysis, IR1H NMR and13C NMR of the product are all in good agreement with the assumed structure. It was found that compounds 2, 3 and K2(BDFE) could be obtained in good yields above 80%. However, the nitration of 3 with N2O5gave compound 4 with a low yield of 30% even we changed the concentration of N2O5and the type of medium. In all, a new fourstep synthetic route converting cyano group to dinitromethanide group could be used to convert similar compounds.

3. 2 Crystal structure

K2(BDFE) crystallizes in a yellow octahedron with higher density (2.00 g cm-3) observed in this work in the monoclinic space group C2/c containing four molecular moieties in the unit cell. The crystal structure, coordinated environments of K+ion and crystal packing are illustrated in Figs. 1～3.

Fig. 1. Molecular structure of K 2(BDFE) in the crystalline state. Symmetry code: #5: –x, y, –z+1/2

Fig. 2. Coordination environment of K+ ion in K 2(BDFE).Symmetry codes: #1: x, –y+1, z–1/2; #2: x, y, z–1; #3: –x+1/2, –y+3/2, –z+1

Fig. 3. Crystal packing of K 2(BDFE)

The crystallographic studies reveal that K2(BDFE)shows a symmetry axis through the atom O(8). In the molecular structure, there are two central K+ions, one BDFE–anion and four water solvent molecules, of which two water molecules are coordinated to the potassium center while the others are not. Each K+ion is connected with four adjacent BDFE–anions through eight K–O coordination bonds (K(1)–O(2) (2.747(8) ?), K(1)–O(3) (2.956(3)?), K(1)–O(4) (2.910(3) ?), K(1)–O(4)#1 (2.798(3)?), K(1)–O(5)#1 (2.783(3) ?), K(1)–O(5)#2(2.817(3) ?), K(1)–O(6)#2 (2.970(3) ?) and K(1)–O(6)#3 (2.943(3) ?)), forming a dodecahedral structure with K+ion being a coordination center.Atoms O(2) and O(6)#3 are the two ends of the dodecahedron. But, the dodecahedral structure is badly distorted, according to the selected bond lengths and bond angles (Table 1). Simultaneously,each BDFE–anion interacts with five adjacent K+ions through the same coordination interactions.Herein, we can also observe some weak K–N interactions (K(1)–N(1) (3.312(3) ?), K(1)–N(2)#2(3.291(3) ?), K(1)–N(3)#3 (3.009(3) ?)) and K(1)–O(7)#4 interaction (3.325(4) ?) in the crystal packing of K2(BDFE). All finally expand into a three-dimensional network structure through coordination bonds and electrostatic forces (Fig. 3).

Fig. 4. DSC curve of K 2(BDFE) under N2 atmosphere with a heating rate of 10 K·min-1

To the BDFE–anion, we can see that the bond lengths and bond angles in the furazan rings are generally normal and each ring is almost planar[22-23]. The values of C(3)–O(8) (1.350(4) ?)are shorter than those of the normal C–O single bond (0.142～0.146 nm); this may be due to the p-π conjugative effect and the electron-withdrawing influence of neighboring furazan ring. The bond angle C(3)–O(6)–C(4) (121.67(15)°) indicates an sp2hybridization nature of O atom. The dihedral angle between the furazan ring is 0°, indicating conjugation effect in the BDFE–anion.

3. 3 Thermal decomposition

Typical DSC and TG-DTG curves (Figs. 3 and 4)indicate that the thermal decomposition of K2(BDFE) can be divided into three obvious exothermic decomposition stages. The first exothermic process occurs in a sharp temperature range at 163～183 ℃. It might be attributed to the cleavage of one coordination bond. The released ligand molecules remain in the sample as the TG curve indicates no mass loss in this temperature range.The second exothermic process is an intense decomposition process that ends at 245 ℃. The peak temperature is 236 ℃, and this exothermic process corresponds to the second mass loss stage that starts at 210 ℃, ends at 264 ℃, and reaches the largest rate at 241 ℃ with a mass loss percentage of 36.36%. The third exothermic process is a successive decomposition process that starts at 285 ℃ with the peak temperature at 343 ℃. The TG-DTG curve also shows the third mass loss starts at 299 ℃ and reaches the highest rate at 337 ℃with a mass loss percentage of 20.94%. The final residue at 450 ℃ is about 35.6%.

Fig. 5. TG-DTG curve of K 2(BDFE) under N2 atmosphere with a heating rate of 10 K·min-1

4 CONCLUSION

3,3?-Bis(dinitromethyl)difurazanyl ether potassium salt was synthesized and characterized thoroughly. K2(BDFE) crystallizes in the monoclinic system with space group C2/c containing four molecules per unit cell. The thermal behavior of K2(BDFE) presents mainly two exothermic decomposition processes in the temperature range of 200～370 ℃. The results showed that K2(BDFE)has some explosive characteristics and might be used as a potential energetic material or as an ingredient for priming composition.

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