Abstract
A nitrogen-rich coordination compound [Zn(ATZ)2(N3)2]3(H2O)2 (1, ATZ = 4-amino-1,2,4-triazole) was prepared and fully characterized. The crystal structure is triclinic, space group P21/n. Thermal decomposition mechanisms of the compound were predicted based on DSC, TG-DTG and FTIR analyses. The kinetic parameters of the first exothermic process of 1 were studied both by the Kissinger’s method and Ozawa-Doyle’s method. The critical temperature of thermal explosion, entropies of activation (ΔS≠), enthalpies of activation (ΔH≠), and free energies of activation (ΔG≠) were calculated.
Keywords
Introduction
The synthesis and development of new energetic materials continues to focus on high energy density materials (HEDMs) with high densities, high heats of formation and good detonation properties. Nitrogen-rich energetic salts based on C-N and N-N heteroaromatic rings with high nitrogen content are among the most recent HEDMs. A number of novel HEDMs, such as CL-20, various triazole- and tetrazole- based salts, nitrated cubanes, furazans, and furoxans, have been synthesized in recent years, and they have been suggested as suitable candidates with a range of applications [1–3].
Azido ligand, due to its various coordination modes, has attracted a vast amount of research attentions [4, 5]. Azido anions can coordinate with the metal via μ-1 mode as a monodentate bridging ligand, μ-1,1 and μ-1,3 modes as a bidentate ligand, or μ1,1,1 and μ-1,1,3 modes as tridentate (Fig. 1). The various intriguing structures of azido-metal complexes, ranging from discrete structures to MOFs, have promoted the development of coordination chemistry. Consequently, many scientists have performed considerable research on energetic azide coordination compounds.
Five-membered nitrogen-rich heterocyclic complexes are traditional sources of energetic materials. Research groups worldwide have gained abundant significant findings in regards to nitrogen-rich energetic salts on the basis of imidazole, triazole, tetrazole, and their derivatives [6–18]. As 4-amino-1,2,4-triazole(ATZ) can be both mono-dentate and bidentate ligands, its derivatives constitute a versatile types, which can coordinate with metal ions to form stable compounds with various structures and enjoyable properties.
In order to deepen the studies on nitrogen-rich metal ATZ azides, we report herein the preparation, crystal structure and thermal behavior of [Zn(ATZ)2(N3)2]3(H2O)2 (1).
Experimental
Materials and physical techniques
All reagents (analytic grade) were purchased commercially and used without further purification. Elemental analyses were performed with a Flash EA 1112 full automatic trace element analyzer. The FT-IR spectra were recorded with a Bruker Equinox 55 infrared spectrometer (KBr pellets) in the range of 4000-400 cm–1 with a resolution of 4 cm–1. DSC and TG-DTG measurements were performed with a Pyris-1 differential scanning calorimeter and Pyris-1 thermogravimetric analyzer (Perkin-Elmer, USA) in a dry nitrogen atmosphere with flowing rate of 20 mL·min–1.
Synthesis
A solution containing Zn(OAc)2·2H2O (0.43 g, 2 mmol) in distilled water (10 mL) was charged into a glass reactor with a thermo-water bath. The reaction solution was stirred with a mechanical agitator and heated to 333 K for using. A solution of 4-ATZ (purity: 98%, 0.42 g, 5 mmol) dissolved in distilled water (10 mL) was slowly added into the Zn(OAc)2·2H2O solution. And then sodium azide (purity: 98%, 0.27 g, 4 mmol), dissolved in distilled water (10 mL), was added to the base solution during 20 min with continuous stirring and keeping at 333 K for another 15 min. Afterwards, the solution was cooled to the room temperature naturally. The precipitate was collected by filtration, washed with distilled water and ethanol, and the products were dried in an explosion-proof dryer. Yield: 59%. Single crystals suitable for X-ray measurement were obtained by evaporation of the mother liquor at room temperature. Elemental analysis for 1 (molar mass 988.86 g/mol) (%):calcd. C 14.58, H 2.85, N 59.49; found C 14.50, H 2.87, N 58.99. IR (cm–1, KBr pellets): 3243, 3113 (NH2, CH2), 2054 (N3–), 1617 (NH), 1196(N-N).
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) with φ and ω modes at 153(2) K. The structure was solved by direct methods using SHELXS-97 [19] and refined by full-matrix least-squares methods on F2 with SHELXL-97 [20]. All non-hydrogen atoms were obtained from the difference Fourier map and subjected to anisotropic refinement by full-matrix least squares on F2. Detailed information concerning crystallographic data collection and structure refinement are summarized in Table 1.
Results and discussion
Vibrational spectroscopy
To gain a good understanding of the properties of the ATZ group, molecular-orbital and IR absorption-frequency analyses based on the optimized structure were carried out by B3LYP functional analyses with the 6–311++g(d,p) basis set. The optimized structure was characterized to be associated with a true local energy minimum on the potential energy surface without an imaginary frequency component. The highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecule orbitals (LUMOs) of the IMI group are shown in Fig. 2. The electronic densities of the HOMOs at N1 (or N2) atoms of the ATZ group are relatively higher. That’s the reason why the coordination bond formation between the N atom and the zinc ion.
Structure description
The coordination environment of 1 is shown in Fig. 3. Selected bond lengths and angles are listed in Tables 2 and 3.
The molecular unit of 1 contains three central zinc cations, six ATZ molecules, six azido groups and two crystal water, which is centrosymmetric. The ZnII ion has sp3d2 hybridization, contributing six empty orbits to accommodate the lone pair electrons from ligands. Central Zn1 was coordinated with four ATZ molecules and two azides to form an octahedron configuration. The terminal zinc atoms Zn2 and Zn2A are hexacoordinated with three azido ligands and three ATZ ligands (Zn-N = 0.2.1158(17) nm ∼ 0.22240(17) nm), composing two distorted octahedral structures. In the molecular structure, these metal ions are bridged by ATZ and azide via μ-1,2 and μ-1,1 coordination modes, while azide acted as monodentate and bidentate ligand.
Weak N–H···N hydrogen bonds between azido groups and ATZ ligands were observed in 1. It can be seen from the packing diagram (Fig. 4) that all intermolecular and intramolecular hydrogen bonds extend the structure into a 3D supramolecular structure, which make an important contribution to enhance the thermal stability of the complex.
Thermal decomposition mechanism of 1
The DSC and TG-DTG curves with the linear heating rate of 10C·min–1 in a nitrogen atmosphere are shown in Fig. 5 and Fig. 6, respectively.
In the DSC curve, there are two endothermic peaks and two exothermic peaks. The endothermic stage represents the volatilization of the two crystal water molecules with a peak temperature of 65.8C. Another endothermic process was observed at a peak temperature of 142.4C, presenting the fusion of 1. Meanwhile, referring to the TG-DTG curve, the structure of 1 gets broken, resulting in further decomposition. The exothermic stage occurs in the range of 172.1∼320.2C with a peak temperature of 249.9C. Corresponding to this process, there is a mass loss of 52.1% in the TG-DTG curves, which roughly coincides with the value of 51.0%, calculated for the loss of six ATZ molecules from the complex. According to the TG-DTG curve, beyond the measurable ceiling of DSC, there is still another decomposition stage with the onset temperature of 439.2C, corresponding to another process with a mass loss of 25.2% in the TG-DTG curve. The loss percentage is near the loss of six azido ligands from the complex (calcd. 25.5%). From the TG-DTG curves it can be seen that the mass loss of 1 ends at 700C and the mass of the final residue is 22.8% of the initial mass, coincident with the calculating value of ZnO, 24.5%.
Non-isothermal kinetics analysis
In the present works, Kissinger’s method [21] and Ozawa-Doyle’s method [22] are widely used to determine the apparent activation energy (E) and the pre-exponential factor (A). The Kissinger and Ozawa-Doyle equations are as follows, respectively:
Where Tp is the peak temperature [C], A is the pre-exponential factor [s–1], E is the apparent activation energy [kJ mol–1], R is the gas constant (8.314 J K–1 mol–1), β is the linear heating rate[C min–1], and G(α) is the reaction mechanism function.
Based on the first exothermic peak temperatures measured at six different heating rates of 5, 10, 15 and 20C/min, Kissinger’s method and Ozawa-Doyle’s method were applied to study the kinetics parameters of 1. From the original data, the apparent activation energy E, pre-exponential factor A, linear coefficient R and standard deviations S were determined and shown in Table 4.
So, the Arrhenius equation of 1 can be expressed as follows: (E is the average of Ek and Eo):
The equation can be used to estimate the rate constants of the initial thermal decomposition process of the title compound.
Calculation of critical temperatures of thermal explosion, ΔS≠, ΔH≠ and ΔG≠
The value of the peak temperature (Tp0) corresponding to β → 0 obtained according to the following equation [23] is 242C, where a, b and c are coefficients.
The corresponding critical temperatures of thermal explosion(Tb) obtained by the following equation [23] is 262C.
The entropies of activation (ΔS≠), enthalpies of activation (ΔH≠), and free energies of activation (ΔG≠) of the decomposition reaction corresponding to T = Tp0, Ea = EK and A = AK, obtained by the following equations [23] are 178.8 J·K–1·mol–1, 113.52 kJ·mol–1, and 205.63 kJ·mol–1, respectively.
Conclusions
A nitrogen-rich coordination compound [Zn(ATZ)2(N3)2]3(H2O)2 (1, ATZ = 4-amino-1,2,4-triazole) was prepared and fully characterized. The structure was solved by using SHELXS-97 and refined by full-matrix least-squares methods on F2 with SHELXL-97. Its crystal belongs to triclinic, space group P21/n. Thermal analysis indicates that there are two endothermic processes and two main exothermic processes as shown in the DSC curve, and the final decomposed residue mass is ZnO. Non-isothermal kinetics analysis reveals that the Arrhenius equation of 1 can be expressed as follows: lnk = 21.88-117.8×103/(RT). The critical temperature of thermal explosion, entropies of activation (ΔS≠), enthalpies of activation (ΔH≠), and free energies of activation (ΔG≠) calculated were 178.8 J·K–1·mol–1, 113.52 kJ·mol–1, and 205.63 kJ·mol–1, respectively.