Synthesis, Structural Characterization and Properties of Bis(1,5-diamino-1H-tetrazolium) 3,3?-bis(nitramino)-4,4?-azofurazan

XU Cheng BI Fu-Qiang ZHANG Min LIU Qing DING Ke-Wei GE Zhong-Xue

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

1 INTRODUCTION

Because of high positive heats of formation and active-oxygen-bearing cycle, furazan derivates had attracted great interest in the field of energy chemistry for decades[1-4]. In order to increase the energy content, energy-containing groups such as nitro, diazo, amino and nitramino had been introduced into the furazan ring[5-9]. For example, one promising furazan derivative with both diazo and nitramino groups is 3,3?-bis(nitramino)-4,4?-azofurazan (DNAAF)[10], which has high density (1.996 cm-3at 110 K and 1.919 cm-3at 295 K) and has been successfully designed and synthesized. However, the acidic and low decomposition temperature (122 ℃)properties of DNAAF had badly limited its application. Fortunately, due to electron with-drawing effect of the nitro group bearing on the N–H, DNAAF could be used as acid to react with bases to form neutral ionic energetic salts. Because ionic energetic materials often possess advantages of lower vapor pressures and higher stabilities over their molecular analogues[11,12], we expected that the energy salts containing nitrogen-rich tetrazolium cation and DNAAF anion would possess both high nitrogen content and concomitant high energetic properties.

Herein, we report the first energetic salt containing DNAAF as the anion, and bis(1,5-diamino-1H-tetrazolium) 3,3?-bis(nitramino)-4,4?-azofurazan as the cation (compound 1). It could be synthesized with high yield using 3,4-diaminofurazan (DAF) as the starting material. Its structure was confirmed by IR,1H NMR,13C NMR, elemental analysis and single-crystal X-ray diffraction. And its thermal and energetic properties were also studied from experimental and theoretical aspects.

2 EXPERIMENTAL

2.1 Materials and measurements

1,5-diamino-1H-tetrazole (DAT) was prepared by literature method[13]. Commercially obtained reagents were used without further purification.1H NMR and13C NMR spectra were recorded on a Bruker 500 MHz and 125 MHz spectrometer in DMSO-d6. Infrared spectra were recorded as 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 VARIEl-3 elemental analyzer. Differential scanning calorimetric (DSC) measurements were carried out on a TA 901s instrument over the range from 40 to 400 ℃ at the heating rate with 5 ℃/min.

2.2 Synthesis and characterization

Compound 1 was synthesized as Scheme 1.

Scheme 1. Synthesis of the title compound

2. 2. 1 3,3?-Bisamino-4,4?-azofurazan (2)

NaHCO3(3.36 g, 40 mmol) was added to a solution of DAF (2.0 g, 20 mmol) in water (40 mL)at room temperature, followed by the addition of sodium hypochlorite solution (50 mL 5% content of active chlorine) with drops. The mixture was stirred for 15 min and then filtered. The crude product was washed with cold water and dried in vacuo to yield 1.66 g of 2 (86% yield). Anal. Calcd. (%) for C4H4N8O2: C, 24.50; H, 2.06; N, 57.13. Found (%):C, 24.55; H, 2.10; N, 56.98.1H NMR (500 MHz,DMSO-d6): δ 6.90;13C NMR (125 MHz, DMSO-d6):δ.155.20, 150.08; IR (KBr, cm-1): v 3439, 3331, 1617,1494, 1420, 1296, 1033, 777 cm-1.

2. 2. 2 3,3?-Bis(nitramino)-4,4?-azofurazan (3)

2 was slowly added to the nitric acid (100%, 3.0 mL) at –2～–5 ℃ after being stirred for 1 h at 0～5 ℃. The mixture was poured into ice water, and then the resulting precipitate was collected by filtration and washed with cold water to yield 1.14 g 3 (1.14 g, 80%). Anal. Calcd. (%) for C4H2N10O6: C,16.79; H, 0.70; N, 48.95. Found (%): C, 16.82; H,0.74; N, 48.63.1H NMR (500 MHz, DMSO-d6): δ 10.45;13C NMR (125 MHz, DMSO-d6): δ 158.75,145.46. IR (KBr, cm-1): v 3153, 1604, 1497, 1361,1289, 1232, 954, 750 cm-1.

2. 2. 3 Bis(1,5-diamino-1H-tetrazolium)3,3?-bis(nitramino)-4,4?-azofurazan (1)

Compound 1,5-diamino-1H-tetrazole (0.70 g, 7.0 mmol) was added to 3 (1 g, 3.5 mmol) in water (5 mL). The mixture was subsequently stirred for 15 min and the solvent was evaporated in vacuum to yield 1 (1.53 g, 89.7%). Anal. Calcd. (%) for C6H10N22O6: C, 14.82; H, 2.07; N, 63.37. Found (%):C, 14.70; H, 1.98; N, 62.79.1H NMR(DMSO-d6,500 MHz): δ 7.1(brs, 5H);13C NMR(DMSO-d6, 125 MHz): δ 147.9, 153.8, 159.6. IR (KBr, cm-1): v 3373,3252, 3156, 2959, 2853, 2631, 1716, 1618, 1544,1469, 1427, 1400, 1324, 1289, 1266, 1235, 1077,1051, 992, 944, 814, 777, 689, 589 cm-1.

2.3 Crystal structure determination

Crystals of 1 were obtained as orange block by slowly evaporating its aqueous solution at room temperature. The crystal with approximate dimensions of 0.36mm × 0.30mm × 0.28mm was mounted on the top of a glass fiber in a random orientation. The determination of unit cell and data collection were performed on a Bruker Smart APEⅡ X-ray diffractometer equipped with a MoΚα radiation (λ = 0.71073 ?) using a ?-ω scan mode at 296(2) K. A total of 5020 reflections were collected in the range of 3.0≤θ≤27.0°, of which 1992 were independent (Rint= 0.022) and 1661 were observed with I > 2σ(I). The data collection and procession were performed with programs SMART and SAINT.The structure was solved by direct methods and refined by full-matrix least-squares/difference Fourier techniques with SHELXS-97 and SHELXL-97 programs[14,15]. All non-hydrogen atoms were refined with anisotropic displacement parameters.After that, all hydrogen atoms were located theoretically and refined with riding model position parameters and fixed isotropic thermal parameters.The final full-matrix least-squares refinement gave R= 0.042, wR = 0.121 (w = 1/[σ2(Fo)2+ (0.0837P)2+0.0239P], where P = (Fo2+ 2Fc2)/3), (Δ/σ)max<0.001, S = 1.04, (Δρ)max= 0.25 and (Δρ)min= –0.33 e/?3. All calculations were performed on a BRUKER SMATR by using SHELXTL.

3 RESULTS AND DISCUSSION

3.1 Crystal structure description

The crystal belongs to orthorhombic with space group P21/c. The molecular structure of 1 is shown in Fig. 1, and intermolecular hydrogen bonding interactions in the crystal structure are shown in Fig. 2.Selected bond lengths and bond angles are listed in Table 1. Hydrogen bond lengths and bond angles are shown in Table 2.

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

Table 2. Hydrogen Bond Lengths (?) and Bond Angles (°)

Fig. 1. Molecular structure of the title compound

Fig. 2. Molecular packing of the unit cell of compound 1 (Dashed lines indicate hydrogen bonds)

As seen in Fig. 1, all the atoms of DNAAF anion are in the same plane, and a symmetric centre could be found at the mid-point of N=N bond. In contrast to the mother molecule DNAAF, nitramino groups of DNAAF2-are not tilted out of the furazan plane.The C–N and N–N bond lengths within the Ar–(NNO2)-fragments are 1.3213 and 1.3684 ?, shorter than the ones in DNAAF which are 1.376 and 1.371 ?, respectively. This dissimilarity indicated the presence of a delocalized π-electron system.

In the crystal, each DNAAF anion is surrounded by eight DAT cations and each DAT cation is surrounded by four DNAAF2-via strong hydrogen bonds. Among these hydrogen bonds, the strongest one comes from N(9) of the DAT+and N4 in the nitrimino group of DNAAF2-N(9)-H(9)···N(4) (x,y+1, z) (H···N, 1.96, N···N, 2.7809(16) ?, N-H···N angle is 158°). Four weaker hydrogen bonds could been found between DNAAF anion and DAT cation,namely N(10)-H(3)···N(3) (-x, y+1/2, -z+3/2),N(10)-H(3)···O(2) (-x, y+1/2, -z+3/2), N(10)-H(4)···O(3) (x, -y+1/2, z-1/2) and N(11)-H(1)···O(3)(-x+1, y+1/2, -z+3/2). All hydrogen bond lengths(Table 2) lie well within the sum of van der Waals radii (rw(O) + rw (N) = 3.07 ?, rw(N) + rw (N) =3.20 ?)[16]. Besides hydrogen bonds, the short N···N and N···O contacts also could be observed, which bring additional stability to the crystal structure.

According to the crystal data, its density is 1.735 g·cm-3, lower than that of DNAAF. The reason might be that the presence of DAT cations makes the layered structure looser. However, due to the presence of more H-bonds compared to DNAAF, compound 1 is expected to show an improved thermal stability.

3.2 Thermal and energetic properties

Thermostability of an energetic compound is important especially for processing and storing. In order to investigate its thermostability, differential scanning calorimetry (DSC) was employed on 1 in a temperature range from 50 to 400 ℃, and the result is shown in Fig. 3. Two exothermic processes could be observed from Fig. 3 regarded as a sharp peak and a broad peak at 142.1 and 214.2 ℃, respectively.It could be concluded that the main decomposition occurred at about 142 ℃ without previous melting.The coacer phase decomposed subsequently with the temperature increasing. As expected, compound 1 shows higher decomposition temperature than DNAAF (122 ℃) , indicating more thermal stability.

Fig. 3. DSC curve of compound 1

In order to obtain the detonation properties of 1,the density of 1 was measured with a gas pycnometer. The gas phase enthalpies of formation were calculated with the CBS-4M method using the Gaussian G09W program package[17]. The calculated gas-phase enthalpies of formation (ΔfH°(g)) was converted into the solid state enthalpies of formation(ΔfH°(s)) by Born-Haber energy cycles and the equations suggested by Jenkins et al[18].

By using the density and calculated heat of formation, the detonation pressures (P) and velocities(D) were calculated based on the VLW equation[19].The predicted performance data of compound 1 are summarized in Table 3. Compound 1 has the high heat of formation at 1614.23 kJ·mol-1due to the large number of nitrogen atoms presented in the molecule. The detonation velocity and detonation pressure of compound 1 are 8.781 km·s-1and 30.7 GPa, respectively, which are much higher than those of TNT and comparable to those of RDX. These properties coupled with the rather high thermal stabilities make compound 1 an attractive candidate for energetic applications.

Table 3. Property of Compound 1

4 CONCLUSION

An energetic salt of bis(1,5-diamino-1H-tetrazolium) 3,3?-bis(nitramino)-4,4?-azofurazan was synthesized using as the starting material DAF. Its structure was confirmed by NMR, IR, elemental analysis and single-crystal X-ray diffraction.Thermal and energetic properties were also investigated by DSC and theoretical calculation. The results showed that it exhibits good thermal stability,detonation pressure and velocity, suggesting it is a potential energetic material.

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