Preparation,crystal structure and thermal decomposition of nitrogen-rich compound Cu(Trz) 4 Cl 2 (Trz = 1,2,4-Triazole,N%   = 40.92%)

2.1 Materials and physical techniques

All the reagents and solvents were of analytical grade and used without further purification as commercially obtained. Elemental analyze was performed on a Flash EA 1112 full-automatic trace element analyzer. The FT-IR spectra was recorded on a Bruker Equinox 55 infrared spectrometer (KBr pellets) in the range of 4000∼400 cm^-1 with a resolution of 4 cm^-1. DSC measurement was carried by using Pyris-1 differential scanning calorimeter (Perkin Elmer, USA).

2.2 Synthesis of 1

The synthesis of 1 was as follows: CuCl₂·2H₂O (10 mmol) was dissolved in distilled water (30 ml), and then charged into a glass reactor with a water bath. It was kept under mechanical stirring and heated to the temperature of 60C. Trz (40 mmol) was dissolved in distilled water (20 ml), and subsequently it was added to the copper aqueous solution during 10 min with continuous stirring. In the end, the solution was cooled to room temperature naturally. Elemental analysis for CuC₈H₁₂N₁₂Cl₂: calcd. C 23.39; H 2.94; N 40.92%. Found: C 23.31; H 2.67; N 40.68%. IR (KBr, cm^-1) ν= 2847(N-H), 1619(C=N), 1537(C=C), 1287(C=C), 1171(C=C), 1044(C=C), 828(C-H).

2.3 X–ray Data collection and structures refinement

The crystal was chosen for X–ray determination. The X–ray diffraction data collection were performed on a Rigaku AFC–10/Saturn 724⁺ CCD detector diffractometer with graphite monochromated Mo K _α radiation (λ= 0.71073 Å). The structure was solved by direct methods using SHELXS–97 (Sheldrick, 1990) [43] and refined by full–matrix least–squares methods on F² with SHELXL–97 (Sheldrick, 1997) [44]. And 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. Further information regarding the crystal–structure determination has been deposited with the Cambridge Crystallographic Data Centre as supplementary publication. These data can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; Fax: +44 1223–336033; E-mail: deposit@ccdc.cam.ac.uk).

3.1 Crystal structure description

In 1, there are one copper(II) cation, four Trz molecules and two coordination chlorine atoms, however, compound 2 ([Cu(IMI)₄Cl]Cl, IMI = Imidazole) [33] has an external chlorine atom which is non-coordinated with copper(II) cation, that is, Cu–Cl bonds of 1 are 2.8042(9) Å but Cu–Cl bonds of 2 are 2.6154(7) Å and 4.1249(9) Å (Fig. 1). It belongs to tetragonal, pertains to I4₁/acd space group, α = 14.4013(4) Å, b = 14.4013(4) Å and c = 15.7049(10) Å (Table 1). The four basically equivalent Cu–N bonds of 1 are equal about 2.0 Å from the Table 2, which are equal with that of 2. The two N atoms from two contraposition Trz ligands and Cu(II) cation’s bond angles are 173.56(11)° (<N1–Cu1–N1B), which are differ from that of 2 (174.75(8)° for < N1–Cu1–N3 and 158.33(8)° for < N5–Cu1–N7). The nitrogen (or chlorine) atoms from two border upon ligands and Cu(II) cation’s bond angles are all close to 90°. Therefore, all data of the above demonstrate that the Cu(II) cations of 1 exhibit distorted–octahedral configuration, while Cu(II) cations of 2 exhibit a distorted pentahedron configuration (Fig. 2). In addition, the angles of triazole-ring (N1-C1-N2-N3-C2) with triazole-rings (N1A-C1A-N2A-N3A-C2A, N1B-C1B-N2B-N3B-C2B and N1C-C1C-N2C-N3C-C2 C) are 68.3°, 107.5° and 70.7°, respectively. From the packing diagram, intermolecular hydrogen bonds of N2-H···Cl1 (Table 3) make an important contribution to enhance the thermal stability of the compound, and all bonds of C1-Cu-Cl are paralleled each other (Fig. 3).

3.2 Thermal decomposition analysis

In order to investigate the thermal behavior of 1, it was analyzed by DSC with a linear heating rate of 10C min^-1 in a flowing N₂ atmosphere with flowing rate 20 ml·min^-1. The data curve of 1 is shown in Fig. 4. A melting point peak (237.9C) with a corresponding endothermic stage can be seen in the 198∼257C, which was higher than that of 2 (a melting point peak is 233.1C). The exothermic decomposition stage was seen in the DSC data, and peak was observed at 348.3C, which was more stable than that of 2 (exothermic peaks were observed at 281.9C and 492.0C) [33]. So, thermal stability is mainly caused by different structures (distorted–octahedral configuration for 1 and distorted pentahedron configuration for 2).

3.3 Non–isothermal kinetics analysis

They are widely used to determine the Arrhenius equation for a given material by Kissinger’s method [45], Ozawa’s method [46] and Starink’s method [47], which equations are as follows: ln ( β / - T s ) = - ( BE / - RT ) + C(1)

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 K min^-1. B and C are constant. When S = 2 and B = 1, Equation (1) is according to Kissinger’s method. When s = 0 and 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 temperature measured with four different heating rates of 5, 10, 15 and 20C min^-1, three methods were applied to study the kinetics parameters of the title compound. From the original data, the apparent activation energy E, pre–exponential factor A, linear coefficient R and and standard deviation S were determined and showed in Table 4. Figure 5 showed the relationship of ln(β/T²), ln(β/T^1.8) and lnβ to 1/T, corresponding to Kissinger’s, Starink’s, and Ozawa’s methods, respectively. Accordingly, the Arrhenius Equation of 1 can be expressed as follows (E value of Starink’s method is the most accurate): ln k = ln A - E s / ( R T ) = 38.90 - 238.6 × 10 3 / ( R T )(2)

The value of the peak temperatures corresponding to β⟶0 (T₀), the corresponding critical temperature of thermal explosion (T_b), entropy of activation (ΔS^≠), enthalpy of activation (ΔH^≠), and free energy of activation (ΔG^≠) were obtained by the following equations (3) [48], where a, b and c are coefficients, k_B is the Boltzmann constant (1.381×10^-23 J/K) and h is the Planck constant (6.626×10^-34 J·s). The physicochemical properties of 1 and 2 are tabulated in Table 5. Nitrogen content of 1 is 40.92%, and that of 2 is 27.55%, which means that 1 may be more sensitive than 2. However, thanks to the introducing of symmetrical structure, thermal stability of 1 is more stable than 2 form thermodynamic parameters, such as T_m, T_d, E, T_b and ΔG^≠. T i = T 0 + a β + b β 2 T b = ( E - E 2 - 4 ERT 0 ) / 2 R A = ( k B T 0 / h ) exp ( 1 + Δ S ≠ / R ) Δ H ≠ = E - RT 0 Δ G ≠ = Δ H ≠ - T 0 Δ S ≠(3)

The novel nitrogen-rich copper(II) triazole chloride compound, Cu(Trz)₄Cl₂ (1), was synthesized and characterized. Like the most copper compounds, the Cu(II) ion was six-coordinated in a distorted–octahedral geometry with four Trz molecules and two coordination chlorine atoms. As shown in the DSC curve of 1, thermal analysis indicated that they had one endothermic process and one main exothermic process, which peaks were 237.9C and 348.3C. Non–isothermal kinetics analysis result indicated that the Arrhenius Equation of 1 can be expressed as follows: lnk = 38.90 –241.3×10³/(RT). It is more important, 1 was regarded as the less toxicity energy additive which will be applied to improve the explosion performance of the traditional explosives and propellant formulations.