Synthesis, Crystal Structure, Thermal Decomposition and Sensitive Properties of a New Complex [Cu(IMI) 4 ](PA) 2

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1 CHEM. RES. CHINESE UNIVERSITIES 2012, 28(4), Synthesis, Crystal Structure, Thermal Decomposition and Sensitive Properties of a New Complex [Cu(IMI) 4 ](PA) 2 WANG Shi-wei, WU Bi-dong, YANG Li *, ZHANG Tong-lai, ZHOU Zun-ning and ZHANG Jian-guo State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology, Beijing , P. R. China Abstract A new coordination complex [Cu(IMI) 4 ](PA) 2 had been synthesized with imidazole(imi) as ligands and picrate(pa ) groups as outer anions, and characterized by Fourier transform infrared(ftir) spectrum and elemental analysis. Its crystal structure was determined by single crystal X-ray diffraction(xrd) analysis. The crystallographic data show that the crystal belongs to monoclinic, C2/c space group, a=2.542(5) nm, b= (18) nm, c=1.3778(3) nm, β= (3) and Z=4. Furthermore, the central copper(ii) ion is coordinated by four N atoms from four imidazole ligands. All the molecular units are linked into a zigzag pattern along a-axis by the hydrogen bonds, and extended to the distance regularly. Thermal decomposition mechanisms were determined based on differential scanning calorimetry(dsc) and thermogravimetry-differential thermogravimetry(tg-dtg) analysis, and kinetic parameters of the first exothermic process were studied using Kissinger s and Ozawa-Doyle s method, respectively. Sensitivity tests show that the title complex has low sensitivity to external stimulus, but it has a higher energy of combustion of 14.2 kj/g due to which it may be used as the additives of energetic materials to improve the explosive performance. Keywords Copper; Imidazole; Crystal structure; Thermal decomposition; Sensitivity Article ID (2012) Introduction In recent years, as a new kind of energetic material, high-nitrogen energetic compounds have attracted considerable attention. Owing to their high positive heat of formation and thermal stability characteristics [1 5], they are mainly used as high-energy insensitive explosives, small-scale solid fuel propulsion system, non-smoking pyrotechnics, gas generating agent, flameless low-temperature fire-fighting agents, almost referring to each area of energetic materials. Many heterocyclic compounds are high in nitrogen and have a high enthalpy of formation for the existence of N N or N C high-energy chemical bonds in the molecule. Azole-based compounds are very good energetic ligands because the nitrogen atoms in the ring have lone pair of electrons and provided necessary conditions for them to coordinate to the central metal irons. Therefore, as a result of the stability of the heterocyclic ring, azide groups or nitro groups can be introduced in order to enhance the combustion heat and the detonation performance. Imidazole(IMI) is a nitrogen-containing five-numbered heterocyclic compound, whose derivatives were widely used in agriculture [6] and medicine [7,8] in the past. And imidazole contains two potential coordination N atoms that enable it to form coordination compounds [9 22]. However, less efforts have been made to study its utilization in energetic materials at the beginning. Until the 1970s, researchers began to study the application of imidazole ligands in the field of energetic materials. During this period, nitro-imidazole such as 2-nitroimidazole [23], 1,4-dinitro-imidazole [24,25], 2,4-dinitro-imidazole [26 28], 4,5-dinitro-imidazole [29], 2,4,5-trinitro-imidazole [30,31], 2-azido-imidazole [32 34] and related complexes have been synthesized and applied in energetic materials. Moreover, imidazole-based energetic materials have lots of advantages, such as high nitrogen content, high bond energy and heat of formation, simple synthesis, appropriate mechanical sensitivity and so on. Recently, in our group, Fan [35] and Gao [36] have synthesized a series of coordination compounds with imidazole as ligand, manganese, cadmium and copper as central metal cations, perchlorate, picrate and styphnate as outer anions. Wu et al. [37] had reported two imidazole energetic complexes [Cu(IMI) 4 (N 3 ) 2 ] and [Ni(IMI) 4 (N 3 ) 2 ] which have potential application as energetic materials. In this paper, we will report the synthesis, crystal structure, thermal decomposition and sensitive properties of copper(ii) imidazole picrate complex [Cu(IMI) 4 ](PA) 2 (PA=picric acid), so as to enrich the studies on imidazole ligands. 2 Experimental 2.1 Materials and General Methods All the reagents and solvents were of analytical grade and used without further purification as commercially obtained. The fourier transform infrared(ftir) spectra were recorded on a Bruker Equinox 55 infrared spectrometer with KBr *Corresponding author. yanglibit@bit.edu.cn Received June 13, 2011; accepted September 29, Supported by the Project of Science and Technology of Applied Physical Chemistry Laboratory of China (No.9140C ) and the Project of State Key Laboratory of Explosion Science and Technology, China(Nos. QNKT12-02, ZDKT10-01b).

2 586 CHEM. RES. CHINESE UNIVERSITIES Vol.28 pellet method in a range of cm 1 with a resolution of 4 cm 1. Differential scanning calorimetry(dsc) and thermogravimetry(tg) measurements were carried out via a Pyris-1 differential scanning calorimeter and a Pyris-1 thermogravimetric analyzer(perkin Elmer, USA), respectively, in dry nitrogen atmosphere at a flowing rate of 20 ml/min. The conditions for the thermal analysis were as follows: for Pyris-1 DSC, the crystal sample was powdered and sealed in aluminum pans at a linear heating rate of 5, 10, 15 or 20 C/min respectively from 50 C to 600 C; for Pyris-1 TGA, the crystal sample was powdered and put in the platinum open pans at a heating rate of 10 C/min from 50 C to 800 C. The combustion heat of the title compound was measured by oxygen bomb calorimetry (Parr 6200, USA). 2.2 Synthesis of [Cu(IMI) 4 ](PA) 2 The preparative route of [Cu(IMI) 4 ](PA) 2 is shown in Scheme 1, and the synthesis was as follows. CuSO 4 5H 2 O (0.25 g, 1 mmol) was dissolved in distilled water(30 ml), and then charged into a glass reactor with a water bath. The solution was mechanically stirred and heated to a temperature of 70 C. Imidazole(0.41 g, 6 mmol) and picric acid (PA, 0.46 g, 2 mmol) were dissolved in distilled water(20 ml), respectively, and subsequently they were added into the CuSO 4 5H 2 O aqueous solution simultaneously with continuous stirring over a period of 25 min. The suspension was stirred for an additional period of 20 min at 70 C and then cooled to room temperature. The resultant mixture was filtered and left at room temperature for 2 d, and single crystals suitable for X-ray analysis were obtained. FTIR(KBr), /cm 1 : 3142, 1625, 1554, 1430, 1320, 1072, 745. Elemental analysis(%) calcd. for CuC 24 H 20 N 14 O 14 (M r =792.08): C 36.36, H 2.52, N 24.74; found: C 36.45, H 2.33, N X-Ray Data Collection and Structure Refinement A gray single crystal with dimensions of 0.33 mm 0.27 mm 0.13 mm was selected for X-ray diffraction analysis. The data collection was performed on a Rigaku AFC-10/Saturn CCD diffractometer with graphite monochromated Mo Kα radiation(λ= nm) at 103(2) K in ψ and ω scan modes. A total of reflections(3478 unique, R int =0.0308) were measured in a range of 3.05 θ 27.49, of which 3121 were observed with I>2σ(I). The scaled maximum and minimum transmission factors were and , respectively. A semi-empirical absorption correction(sadabs) was applied to the raw intensities. The structure was solved by means of direct method and SHELXS-97 program [38] and refined by full-matrix least-squares method on F 2 with SHELXL-97 program [39]. All non-hydrogen atoms were obtained from the difference Fourier map and refined anisotropically. The hydrogen atoms were obtained geometrically and treated by a constrained refinement. The detailed crystallographic data are listed in Table 1. The selected bond lengths, bond angles and hydrogen bond lengths and bond angles are listed in Tables 2 and 3, respectively. Scheme 1 Preparative route of [Cu(IMI) 4 ](PA) 2 Table 1 Crystal data and structure refinement parameters of [Cu(IMI) 4 ](PA) 2 Empirical formula CuC 24 H 20 N 14 O 14 M r Temperature/K 103(2) Crystal system, space group Monoclinic, C2/c Crystal size 0.33 mm 0.27 mm 0.13 mm a/nm 2.542(5) b/nm (18) c/nm (3) β/( ) (3) V/nm 3, Z 3.059(6), 4 Limiting index 31 h 32, 11 k 11, 17 l 17 D c /(g cm 3 ) λ/nm Absorption coeff. /mm F(000) 1612 θ range for data collection/( ) Measured reflection Unique datum, R int 3478, R 1, wr 2 [I >2σ(I)] , R 1, wr 2 (all data) , Goodness-of-fit (Δρ) max, (Δρ) min /(e nm 3 ) 428, 427 Table 2 Selected bond lengths(nm) and bond angles( ) of [Cu(IMI) 4 ](PA) 2 * Cu1 N1# (19) O1 C (3) Cu1 N (2) O2 N (3) N1 C (4) O3 N (2) N1 C (3) O4 N (2) N2 C (3) O5 N (3) N2 C (4) O6 N (3) C1 C (3) O7 N (3) N1 Cu1 N1# (11) N3#1 Cu1 N (14) N1 Cu1 N3# (7) C1 N1 Cu (17) N1#1 Cu1 N3# (10) C6 N3 Cu (15) N1 Cu1 N (10) C4 N3 Cu (14) N1#1 Cu1 N (7) * Symmetry transformation used to generate equivalent atoms: #1: x, y, z+3/2.

3 No.4 WANG Shi-wei et al. 587 Table 3 Hydrogen bond lengths and bond angles of [Cu(IMI) 4 ](PA) 2 * D H A d(d H)/nm d(h A)/nm d(d A)/nm (D H A)/( ) N2 H2 O1# N2 H2 O7# N4 H4 O1# N4 H4 O2# C5 H5 O3# C5 H5 N5# C6 H6 O4# C9 H9 O * Symmetry transformations used to generate equivalent atoms: #2: 1/2+x, 1/2+y, z; #3: 1/2+x, 1/2+y, z; #4: 1/2+x, 1/2 y, 1/2+z; #5: x, y, 1 z. 3 Results and Discussion 3.1 Crystal Structure Description There is one copper(ii) cation, four coordination IMI ligands, and two PA anions in the molecule of [Cu(IMI) 4 ](PA) 2 (Fig.1). In our previous studies [37], the hybrid density functional of B3LYP with the G** basis set was employed for the calculations and it was found that the electronic density of the highest occupied molecular orbital(homo) at the N1 atom was relatively high which means it would be the possible coordination site. In the crystal structure of the title compound, the Fig.1 Molecular structure of [Cu(IMI) 4 ](PA) 2 central copper(ii) cation is coordinated with four N1 atoms of four IMI ligands which is consistent with the computational results. In addition, the central copper(ii) cation and the four N1 atoms are configured into a tabular tetrahedron structure. The Cu N distances are approximately equal which are localized in a range of (19) (2) nm. The angles between two adjacent Cu N bonds are for N1 Cu1 N3 and N1A Cu1 N3A, for N1 Cu1 N1, and for N3 Cu1 N3, respectively, which slightly derivate from 90. Four coordinated N atoms and the central copper(ii) cation form two planes: N1 Cu1 N3A(plane 1) and N1A Cu1 N3(plane 2), and the plane equations can be expressed as follows: N1 Cu1 N3A(plane 1): x 6.383y 6.691z = ; N1A Cu1 N3(plane 2): x y 6.961z = The angle between planes 1 and 2 is 88.13, and the angles between every two symmetrical imidazole rings are all 87.62, all of which are close to 90, lead to the minimum steric hindrance. From Fig.2, it can be seen that lots of intermolecular and intramolecular hydrogen bonds link the molecules into a 3D network. What s more, the intermolecular ones can be separated into three types. The first type of hydrogen bonds, formed between the hydroxyl-group of PA and N2 atom of imidazole, N2 H2 O1 and N4 H4 O1, and the hydrogen bond lengths are and nm, respectively. The second type of ones formed between the nitro-group of PA and N2 atom of imidazole, N2 H2 O7 and N4 H4 O2, and the hydrogen bond lengths can be observed to be and nm, respectively. The third type of ones consist of C atoms of imidazole and N or O atoms of PA, C5 H5 O3, C5 H5 N5 and C6 H6 O4, the lengths of which are slightly longer and therefore the strength is weaker than that of the former types. These extensive hydrogen bonds not only play an important role in enhancing the stability of the crystal structure of [Cu(IMI) 4 ](PA) 2, but also contribute to the formation of an interesting structure. It can be seen from Fig.3 that the central Cu atoms arrange in a straight line along the b-axis. From the view of a-axis, the molecular units are configured into a zigzag pattern, and extend to the distance regularly. Fig.2 Packing plot of [Cu(IMI) 4 ](PA) 2 viewed along the b-axis of the unit cell Fig.3 Packing plot of [Cu(IMI) 4 ](PA) 2 viewed along the a-axis of the unit cell 3.2 Thermal Decomposition In order to investigate the thermal behavior of the title

4 588 CHEM. RES. CHINESE UNIVERSITIES Vol.28 compound, the DSC and TG-DTG experiments were carried out. The DSC curve of the title coordination compound at a heating rate of 10 C/min is shown in Fig.4 and the TG-DTG curves at a heating rate of 10 C/min are illustrated in Fig.5. cm 1 in the FTIR spectrum of the residue also proved that the final residue was Cu 2 O. In order to study the energies of combustion of the title complex, constant-volume energies of combustion(q v ) for [Cu(IMI) 4 ](PA) 2 was measured by means of an oxygen bomb calorimeter, and was determined to be 14.2 kj/g. The bomb equation was as follows: CuC 24 H 20 N 14 O O 2 CuO+10H 2 O+24CO 2 +7N Non-isothermal Kinetics Analysis Fig.4 DSC curve of [Cu(IMI) 4 ](PA) 2 at a heating rate of 10 C/min Fig.5 TG-DTG curves of [Cu(IMI) 4 ](PA) 2 at a heating rate of 10 C/min It can be seen that there is one endothermic process and four main exothermic processes in the DSC curve, and four successive mass losses in the TG-DTG curves. According to the DSC curve, the endothermic process occurrs between and C which represent the melting of the complex. Then, two successive exothermic processes appeare in a temperature range of C with the peak temperatures at C and C, respectively. Corresponding to the endothermic and the two exothermic processes, the TG-DTG curves show two consecutive mass losses in a range of C which correspond to the release of two PA molecules, as a mass loss of 58.4% vs. a calculated value of 57.6%. In addition, there is another successive exothermic stage in the DSC curve between C and C with the peak temperature at C and C, respectively. The peak area of this exothermic process is much larger than that of the previous one, which means much more heat is released in this stage. Corresponding to this process, another two consecutive mass losses of 32.9% in the TG-DTG curves can be seen which is approximately consistent with the loss of four imiazole ligands(calcd. 34.3%). As everyone knows, the cleavage of coordination bonds would release more energy than the cleavage of electrovalent ones, which explains the latter peak area is larger than the former one in the DSC curve clearly. The mass fraction of the final residue is 8.8%, coincident with the calculated value of Cu 2 O, 9.1%. The absorption band at 620 In the present work, the values of the apparent activation energy E and the pre-exponential factor A were calculated via Kissinger s method [40] and Ozawa-Doyle s method [41]. Based on the first exothermic peak temperatures(263.6, 277.2, and C) measured at four different heating rates of 5, 10, 15 and 20 C/min, respectively, Kissinger s method and Ozawa-Doyle s method were used to study the kinetics parameters of complex [Cu(IMI) 4 ](PA) 2, and the preexponential factor and linear correlation coefficient were determined. The Arrhenius equation can be expressed with the calculators E a (the average of E K and E O, in kj/mol) and ln A K (in s) as follows: lnk= /RT for the first exothermic process. This equation can be used to estimate the rate constant of the initial thermal decomposition process of [Cu(IMI) 4 ](PA) 2 (Table 4). The calculated results with both the methods were similar and they were all in the normal range of kinetic parameters for the thermal decomposition reaction of solid materials. Table 4 Kinetic parameters obtained by Kissinger s and Ozawa-Doyle s methods E K /(kj mol 1 ) ln(a K /s 1 ) R K E O R O Sensitivity Test According to the China National Military Standard(GJB 772A-97), the impact, friction and flame sensitivities of the title complex were measured. Impact sensitivity of [Cu(IMI) 4 ](PA) 2 was determined with a fall-hammer apparatus. Briefly, 30 mg of the complex was placed between two steel poles and hit with a 5.0 kg drop hammer. The test results show that 50% firing height(h 50 ) was cm. Friction sensitivity was determined with an MGY-1 pendular friction sensitivity apparatus. When 20 mg of the complex was compressed between two steel poles with mirror surfaces at a pressure of 3.92 MPa and then hit with a 1.5 kg hammer at an angle of 90, the title complex did not fire. Flame sensitivity was determined by a standard method in which the sample was ignited by a standard black powder pellet. First 20 mg of the complex was compacted into a copper cap under a pressure of 58.8 MPa and then ignited by black powder pellet. The test results show that the complex has a smaller ignition probability at a minimum height of 6 cm. 4 Conclusions A new coordination complex [Cu(IMI) 4 ](PA) 2 was

5 No.4 WANG Shi-wei et al. 589 synthesized and characterized. The crystal structure show that the central copper(ii) cation is four-coordinated into a tabular tetrahedron geometry by four N atoms from four IMI molecules, and the molecular units are linked together by both the intermolecular and intramolecular hydrogen bonds into a zigzag figure. Thermal analysis indicates that when heated, the complex first lost two outer PA anions and then four imiazole ligands and the final decomposed residue mass fraction was 8.8%, coincident with Cu 2 O, 9.1%. Non-isothermal kinetics analysis results reveal that the Arrhenius equation of [Cu(IMI) 4 ](PA) 2 was lnk= /RT. The combustion energy of the title complex measured by oxygen bomb calorimeter was 14.2 kj/g. Sensitivity tests show that the title complex had a certain impact sensitivity, and was insensitive to friction and flame. 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