CHEN Li-Zhen LIU Wei WANG Jian-Long CAO Duan-Lin

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Crystal Structure and Thermal Decomposition Kinetics of 3-Nitro-4-diazo-5-oxygenpyrazole

CHEN Li-Zhen LIU Wei WANG Jian-Long①CAO Duan-Lin

(030051)

A novel compound 3-nitro-4-diazo-5-oxypyrazole was synthesized by the nitra- tion of 4-amino-3,5-dinitropyrazole using nitrification agents of fuming nitric acid and tri- fluoroacetic anhydride. The compound was purified by column chromatography and characterized by IR, NMR, MS and elemental analysis. Two different single crystals obtained by culturing with ethyl acetate as a solvent were measured by X-ray single-crystal diffractometer. The molecular weight of C3HN5O3is 155.09 and the two crystals belong to monoclinic system, space groups21/and21/. For 1:= 5.5007(2),= 9.0691(4),= 11.4158(4) ?,= 92.710°,= 568.85 ?3,= 4,D= 1.811 g/cm3,= 0.162 mm-1,(000) = 312 and the final deviation factor is 0.0213. Crystals 1 and 2 have similar unit cell parameters, except that= 10.1828(12),= 5.5925(6),= 10.5574(10) ? and= 108.330(4)° in crystal 2. The thermal behavior of the compound was studied by TG-DSC and melting endothermic peak and decomposition exothermic peak are at 425.7 and 534.5 K in DSC curve. The activation energy and pre-exponential factor of the exothermic decomposition reaction of the title compound obtained by Kissinger method and Flynn-Wall-Ozawa method are 50.38 kJ/mol, 4.59 × 1022s-1and 55.89 kJ/mol.

3-nitro-4-diazo-5-oxygenpyrazole, crystal structure, thermal decomposition kinetics, apparent activation energy;

1 INTRODUCTION

Pyrazole is a class of five-membered heterocyclic compounds containing multiple C–N, N–N and other energetic bonds, so it is a good energetic material carrier[1, 2]. Pyrazole also has pharmacological properties[3, 4]. The introduction of different groups can make it have different pharmacological pro- perties and explosion characteristics. Energetic groups such as nitro, amino and diazo groups[5-8]are introduced to increase the energy of pyrazole com- pounds. 4-Amino-3,5-dinitro-1H-pyrazole (LLM- 116), a novel energetic material, was synthesized firstly in 2001[9-11]. It has good explosive perfor- mance, such as a density of 1.90 g/cm3, 90% the energy density of HMX, and low mechanical sen- sitivity[12]. 3-Nitro-4-diazo-5-oxypyrazole was syn- thesized by nitration[13]of LLM-116. In this paper, the energetic material 3-nitro-4-diazo-5-oxypyrazole was synthesized and characterized by different methods, such as NMR, crystal analysis[14]and so on. The activation energy and pre-exponential factor were calculated by Kissinger and Flynn-Wall-Ozawa methods[15, 16].

2 EXPERIMENTAL

2. 1 General information

Trifluoroacetic anhydride (AR, 98%) and nitric acid (Tech, 98%) were purchased from commercial sources of Aladdin and Institute of Chemical Reagents in Tianjin, respectively. All regents used in experiments such as ethyl acetate, ether and petro- leum ether were analytically pure and purchased from Institute of Chemical Reagents in Tianjin and used without further purification.

The elemental analysis was carried out on PE instrument (Model CHNO-2000).1H-NMR spectra were recorded on a 300 MHz Bruker instrument with Aceton-d6solvents. The FTIR spectra were measured on a VERTEX 80 produced by Bruker in Germany. MS spectra were carried on a Q Exactive Mass spectrometer produced by Thermo Fisher Scientific Company in Germany. Thermal analysis was carried out on a NETZSCH STA 409 PC/PG Germany instrument. Single-crystal X-ray diffrac- tion data were collected on an Agilent Xcalibur & Gemini made by Agilent in America.

2. 2 Synthesis of 3-nitro-4-diazo-5-oxygenpyrazole

Trifluoroacetic anhydride (9.2 mL) was added to reaction temperature below 5 ℃ and stirred for 10 min. Then 4-amino-3,5-dinitropyrazole (0.36 g) was carefully added in small portions with stirring at 0～5 ℃. The reaction mixture was warmed to 15 ℃, stirred for 4 h, and then poured into crushed ice (200 g) and extracted with ether (3 × 30 mL). The organic extract was combined, washed with water, and dried over anhydrous Na2SO4. The solvent was evaporated in vacuo. The crude product was purified on silica gel (ethyl acetate/petroleum ether, 3/1, v/v) to yield 55 mg of yellow solid (Scheme 1). m.p.: 152～154 ℃. FT-IR (KBr): 1554.17, 1355.95 (C–NO2), 1672.87(C=O), 2139.47 cm?1(N≡N).1H NMR (aceton-d6): 12.2 (s, 1H). EI-MS: m/z 155 (M-). Anal. Calcd. (%) for C3HN5O3(155): C, 23.24; H, 0.65; N, 45.16; O, 30.95. Found (%): C, 23.45; H, 0.63; N, 44.32; O, 31.6.

Scheme 1. Syntheses of the title compound

2. 3 Crystal data and structure determination

Two different crystals were obtained by solvent evaporation of a solution of the title compound dissolved in ethyl acetate/petroleum ether at room temperature. The single crystals 1 (0.40mm × 0.30mm × 0.20 mm) and 2 (0.25mm × 0.15mm × 0.14mm) were used for X-ray data collection performed on a Bruker SMART APEX CCD area- detector diffractometer equipped with a graphite- monochromated Moradiation (= 0.71073 ?) at 106 K for data collection. For 1, 7221 reflections were collected in the range of 3.57≤≤25.98°, among which 1110 were independent (int= 0.0213) and 1053 were observed (> 2()) and used for crystal structure analysis. For 2 , 3228 reflections were collected in the range of 3.948≤≤25.337°, including 1047 independent ones (int= 0.0191) and 736 observed ones (> 2()) which were used for data collection. All parameters were corrected byfactors and empirical absorption. The crystal struc- ture was analyzed using the SHELXS-97 program[17]and optimized using the SHELXL-97 program[18]. The details of data collection and refinement are given in Table 1.

2. 4 Thermal analysis

Thermal analysis was performed using a NETZSCH STA 409 PC/PG instrument. The sample (~2 mg) was placed in a crucible and heated from 30to 350 ℃ at 5, 10, 15 and 20 ℃/min with nitrogen as the protective gas.

3 RESULTS AND DISCUSSION

3. 1 Crystal structure description

Single-crystal analysis shows this compound crystallizes in monoclinic space group21/or21/, with= 4. The molecular structures and crystal packing are shown in Fig. 1～3. Bond lengths, bond angles, torsion angles, and hydrogen bond parameters are shown in Tables 2～4.

Table 1. Crystal Data and Structure Refinement for the Title Compounds

Fig. 1. Molecular structure of the title compound

Fig. 2. Molecular arrangement in crystal 1

Fig. 3. Dimers of crystal 2

Table 2. Bond Lengths (?) and Bond Angles (°) of the Title Compound

Table 3. Torsion Angles (°) of the Title Compound

Table 4. Hydrogen Bonding Geometry for the Title Compound

Symmetry codes:a2–, 2–, 1–;b1–, 0.5+, 1.5–

Fig. 1 shows that the nitro, diazo and oxygen are connected with three different carbon atoms of C(3), C(2) and C(1). And the O(2)–N(3)–C(3)–N(2), O(1)– C(1)–C(2)–N(4), O(3)–N(3)–C(3)–C(2), N(2)– C(3)–C(2)–C(1) and N(5)–N(4)–C(2)–C(1) torsion angles are 8.00(2)°, 4.0(3)°, 4.1(2)° and 5(2)°, respectively. It shows that three groups are located nearly in the same plane as the pyrazole ring. The bond lengths of C(1)–O(1), C(2)–N(4) and N(4)– N(5) are 1.234, 1.334 and 1.115 ?, respectively, which are in agreement with the values of C=O (1.233 ?), C=N (1.3400 ?), N≡N (1.124 ?). Due to the steric effect of nitro group, the bond angle of N(5)–N(4)–C(2) is 175.8(2)° which is less than 180°, so N(5) departs from the nitro group. The bond length of C(3)–N(3) is 1.445 ? and shorter than the average bond length of C–NO2(1.468 ?), indicating that the nitro group is more stable[19].

In the title compound, the oxygen and hydrogen atoms of the adjacent molecules form intermolecular hydrogen bonds. There are two different types of hydrogen bonds, N(1)–H(1)···O(1) and N(1)– H(1)···O(3), in crystal 1(Fig. 2), but only N(1)– H(1)···O(1) hydrogen bond in crystal 2. The measured N(1)???O(1) distance of crystal 1 is 2.863(2) ? and is longer than the N(1)???O(1) distance (2.794(3) ?) in crystal 2. It shows strong inter- molecular N–H???O bonding interactions. The mole- cules of crystal 2 are linked together by pairs of intermolecular hydrogen bonds to form the dimers (Fig. 3) and the torsion angle of two molecules in the dimers is 15.9(4)°[20], but bimolecules linked by N(1)–H(1)···O(1) hydrogen bonds are connected to each other by N(1)–H(1)???O(3) hydrogen bonds to form a three-dimensional structure in crystal 1.

3. 2 Thermal behavior of 3-nitro-4-diazo-5-oxygenpyrazole

The DSC-TG curve (Fig. 4) revealed two signals corresponding to an endothermic peak due to the melting of the compound at 425.7 K followed by an exothermic decomposition peak at 534.5 K. Kissinger and Flynn-Wall-Ozawa methods were used to calculate the kinetic parameters of the title compound[21, 22]. The magnitude of the rate constant () is determined by temperature () and is given by the Arrhenius equation:

Fig. 4. DSC-TG curves of the title compound

whereis the gasconstant,is the Kelvin temperature andandEare constants that are properties of the material. TheE, called the activation energy, is often interpreted as the energy barrier opposing the reaction. The constant, most often called the pre-exponential factor, is a measure of the probability that a molecule with energywill participate in a reaction.andEcan be related to the conversion function() as

When the decomposition is under non-isothermal conditions (heating rate=/), equation (2) can be transformed into:

The Kissinger method is a differential processing method and the Flynn-Wall-Ozawa method is an integral method. Different temperature of the peak values of the DTG curves at multiple heating rates can be used to calculate the activation energyEand pre-exponential constant. Kissinger equation and Flynn-Wall-Ozawa equation are as follows:

whereTis the peak temperature of the DTG curves andis the linear heating rate. The data ofandTare shown in Table 5.

Table 5. Kinetics Basic Data of TG

Figs. 5 and 6 are obtained from the data in Table 5. We can find that the plots of lnβ/T2and lgversus 103/Tgive straight lines.The slop and intercept of the line in Fig. 5 are –6.06024 and 1.90264, respec- tively. The correlation coefficient () of it is 0.97, so the activation energyEand pre-exponential cons- tantobtained by Kissinger method are 50.38 kJ/mol and 4.59 × 1022s-1. At the same time, we can get the slop and intercept of the line in Fig. 7 to be –3.07 and 7.105, respectively. The correlation coefficient () is 0.98. Finally, the activation energyEis 55.89 using Flynn-Wall-Ozawa method. The data calculated above are shown in Table 6.EandAare obtained by Kissinger method andEwas obtained by Flynn-Wall-Ozawa method. The activa- tion energyEis small, indicating that the title compound has poor thermal stability.

4 CONCLUSION

In this study, energetic material 3-nitro-4-diazo- 5-oxypyrazole was synthesized and characterized. Its crystal structures and thermal behavior have been studied.In the crystal structure, the molecule is a planar structure and the dimers are formed by hydrogen bonds of oxygen and hydrogen atoms in the adjacent molecules. The activation energy required for the decomposition of compound accor- ding to Kissinger and Flynn-Wall-Ozawa methods have been found to be 50.38 and 55.89 kJ/mol respectively.

Fig. 5. Plots of ln(/Tp2)-1/Tpby Kissinger method

Fig. 6. Plots of ln-1/Tpby FWO method

Table 6. Kinetics of the Title Compound

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18 January 2018 (CCDC 1583710 for 1 and 1583709 for 2)

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10.14102/j.cnki.0254-5861.2011-1881