Thermoanalysis and crystal structure of Ammonium 4-Amino-3-(5-tetrazolate)-furazan (NH 4 AFT)

2.1 Materials and physical techniques

The reagents and solvents were all analytically pure commercial products. IR spectra were recorded with KBr plates using a Bruker Equinox 55 infrared spectrometer (KBr pellets) in the range of 4000∼400 cm⁻¹ with a resolution of 4 cm⁻¹. Elemental analyses were performed with a Flash EA 1112 full automatic trace element analyzer. DSC measurements were performed with a Pyris-1 differential scanning calorimeter in a dry nitrogen atmosphere with flowing rate of 20 mL·min⁻¹. The crystal sample was powdered and put in the aluminum pans and open platinum pans, then heated from 30 to 500°C at 5, 10, 15, 20°C·min⁻¹, respectively.

2.2 Synthesis of NH₄AFT

HAFT was easily synthesized through the reaction of malononitrile, sodium nitrite, and hydroxylamine, followed by oxidation with PbO₂, then cycloaddition with NaN₃, as reported [33].

A solution of 30% ammonia hydroxide (0.116 g, 1.0 mmol) was added dropwise to a solution of HAFT (0.153 g, 1.0 mmol) in deionized water (20 mL) and was stirred at 60∼65 °C for 30 min. Then, the resulting solution was cooled to room temperature with incessant stir. The precipitate was collected by filtration, washed twice with ethanol (5 mL), and dried in an explosion-proof water-bath dryer. Single crystals suitable for X-ray measurements were obtained after crystallization from the filtrate. Experiments use ammonia and HAFT in the molar ratios 1:1. Elemental analysis (%) for C₃H₆N₈O: calcd. C 21.18; H 3.55; N 65.86; found: C 21.08; H 3.50; N 65.17. IR(KBr): v ˜  = 3424 (w), 3320 (w), 3250 (w), 3164 (w), 2990 (w), 2842 (w), 2194(vw), 1935 (w), 1840 (w), 1698 (m), 1630 (s), 1596 (m), 1566 (w), 1445 (s),1360 (m), 1212 (vw), 1158 (s), 1084 (vw), 1032 (m), 982 (s), 894 (m), 866(m), 772 (w), 712 (vw), 619 (m), 482 cm⁻¹ (w).

2.3 X-ray data collection and structure refinement

The X-ray diffraction data collection was performed with a Rigaku AFC-10/Saturn724⁺ CCD detector diffractometer with graphite monochromated Mo-Kα radiation (λ= 0.71073 Å) with φ and ω modes at 153(2) K. The structure was solved by direct methods using SHELXS-97 [34] and refined by full-matrix least-squares methods on F² with SHELXL-97 [35]. All non-hydrogen atoms were obtained from the difference Fourier map and subjected to anisotropic refinement by full-matrix least-squares on F².

Crystallographic data (excluding structure factors) for the structure in this paper have been deposited with the Cambridge Crystallographic Data Centre, CCDC, 12 Union Road, Cambridge CB21EZ, UK. Copies of the data can be obtained free of charge on quoting the depository number CCDC-1443631.

3.1 X-ray crystallography

A crystal of NH₄AFT suitable for X-ray diffraction was obtained by slow evaporating from an aqueous solution at room temperature.

The NH₄AFT contains one 4-amino-3-(5-tetrazolate)-furazan anion (AFT^-) and one ammonium cation without crystal water. The single crystal structure of NH₄AFT was determined by applying X-ray single crystal diffraction, which shows that the crystal belongs to triclinic crystal system with space group Pī, and its crystal parameters: a = 7.244(3) nm, b = 7.000(9) nm, c = 7.280(3) nm, α= 104.317(4)°, β= 108.266(5)°, γ= 92.67°, V = 348(433) nm³, and Z = 2. Its experimental density is 1.622 g·cm⁻³ (calculated 1.61 g·cm⁻³ [31]). The structure is shown in Fig. 1. The packing structure along c axis of the unit cell is shown in Fig. 2. The crystallographic and structural refinement data are listed in Table 1. The bond lengths and bond angles are shown in Table 2.

As shown in Fig. 1, the unit cell consists of one ammonium cation and one HAFT anion. According to Table 2, the N-N bond lengths in the tetrazolate rings of it [1.300(3)–1.300(13) Å], are between N-N single bonds (1.454 Å) and N = N double bonds (1.245 Å). The C1-C2 bond and C2-C3 bond are 1.500(9) Å and 1.400(14) Å, respectively. The O1-N5 [1.400(13) Å] bond is longer than the O1-N6 [1.400(7) Å] bond, which indicates that the O1-N5 bond may be the trigger bond of decomposition. In the HAFT anion, the bond lengths of the tetrazole ring range from 1.300(3) Å to 1.343(4) Å [N1-C1] with an average value of 1.308 Å, which is 0.038 Å longer than the normal C = N bond length (1.270 Å), 0.056 Å longer than the normal N = N bond length of 1.252 Å and 0.162 Å shorter than the normal N-N bond length of 1.470 Å. The bond lengths of the furazan ring range from 1.300(4) Å [N6-C3] to 1.400(14) Å [C2-C3] with an average value of 1.361 Å, which is 0.091 Å longer than the normal C = N bond length (1.270 Å), 0.109 Å longer than the normal N = N bond length of 1.252 Å and 0.109 Å shorter than the normal N-N bond length of 1.470 Å. Additionally, in the title salt the tetrazole ring and the furazan ring lie in different planes, and they are slightly twisted with the dihedral angle between them of 6.6°. The atoms in either tetrazolate or furazan rings are planar with mean deviations from their respective plane of 0.0003, 0.0029 Å. A view along the c axis (Fig. 2) reveals the packing of the anions and cations.

Likewise, ATNF (ammonium 4-nitro-3-(5-tetrazole)-furoxan) [36] is also an energetic salt derived from the combination of tetrazole with furoxan. But parts of its properties can’t match NH₄AFT. For example, its thermal degradation temperature (T_d) is 142°C, which is lower than that of NH₄AFT, and its oxygen balance (OB) is –29.6%, which is higher than that of NH₄AFT. In Table 1, the crystallographic and the structural refinement data of ATNF is also listed as a contrast.

The hydrogen bonds lengths and angles are summarized in Table 3. In the crystal structure, there are three types of intra-molecular hydrogen bonds. The first occurs between the nitrogen atom of the furazan ring and the nitrogen atom of the amino in AFT^- (N7-H7A···N6). The second occurs between the nitrogen atom of the tetrazolate groups and the nitrogen atom of the amino in AFT^- (N7-H7B···N1). The third occurs between the nitrogen atom of the amino group and the nitrogen atoms of tetrazolate group (N8-H8A···N2, N8-H8B···N3, N8-H8C···N4, N8-H8D···N1). Since the amino groups in both AFT^- and ammonium are excellent hydrogen-bonding donors, the discrete AFT^- and ammonium are linked into a 2D double layer by the hydrogen-bonding interactions between cations and anions. It can be seen from the packing diagram (Fig. 2).

3.2 Thermal decomposition

In order to investigate the thermal behaviors, DSC curves of NH₄AFT with the linear heating rate of 5, 10, 15, 20°C·min⁻¹ were recorded in a nitrogen atmosphere (Fig. 3). Each DSC curve shows that there are one endothermic peak and one exothermic peak, and the exothermic peak with the onset temperature ranged from 234.6°C, 234.8°C, 235.1°C, 235.5°C, respectively, and the peak temperature ranged from 260.2°C, 270.6°C, 275.6°C, 285.2°C, severally.

3.3 Non-isothermal kinetics analysis

In the presented works, Kissinger’s method [37], Ozawa’s method [38] and Starink’s method [39] have widely been used to determine the Arrhenius Equation. ln ( β / T s ) = - ( BE / RT ) + C(1)

Where T is the peak temperature in K. E is the apparent activation energy in kJ·mol⁻¹. R is the gas constant (8.314 J·K⁻¹·mol⁻¹). β is the linear heating rate in °C·min⁻¹. B and C are constant. When s = 2 and B = 1, Equation (1) is according to Kissinger’s method. When s = 0, B = 1.0516, Equation (1) is according to Ozawa’s method. When s = 1.8 and B = 1.0037, Equation (1) is according to Starink’s method.

Based on the first exothermic peak temperatures measured with four different heating rates of 5, 10, 15, and 20°C·min⁻¹, Kissinger’s method, Ozawa’s method, and Starink’s method were applied to study the kinetics parameters of the title compound. Based on the original data and the relationship of ln(β/T_P²), ln(β/T_P^1.8), and lnβ to 1/T_p, corresponding to Kissinger’s method, Starink’s method, and Ozawa’s method, the apparent activation energy E, pre-exponential factor A, linear correlation coefficient R, and standard deviation S were determined and shown in Table 4. Accordingly, the Arrhenius Equation can be expressed as follows: ln k = 27.26 - 160.3 × 10 3 / RT(2)

The value of the peak temperature corresponding to β⟶ 0 obtained according to the following Equation (3) [40] is 507.6 K (234.4°C), where a, b and c are coefficients. T pi = T p 0 + a β + b β 2 + c β 3(3)

The corresponding critical temperatures of thermal explosion (T_b) obtained is 523.7 K by the following Equation (4) [36], where R is the gas constant, E is the value of E by Kissinger’s method. T b = ( E - E 2 - 4 ERT p 0 ) / 2 R(4)

The entropy of activation (ΔS^≠), enthalpy of activation (ΔH^≠), and free energy of activation (ΔG^≠) of the decomposition reaction of NH₄AFT corresponding to T = T_p0, E_a = E_K and A = A_K (obtained by Kissinger’s method), obtained by the following Equation (5), Equation (6), and Equation (7) [36] are -51 J·K⁻¹·mol⁻¹,140.7 kJ·mol⁻¹, and 166.6 kJ·mol⁻¹, respectively. A = ( k B T / h ) exp ( 1 + Δ S ≠ / R )(5) Δ H ≠ = E - RT(6) Δ G ≠ = Δ H ≠ - T Δ S ≠(7)

Where k_B is the Boltzmann constant (1.381×10⁻²³ J·K⁻¹) and h is the Planck constant (6.626×10⁻³⁴ J·s).