Synthesis,crystal structure,and properties of energetic copper(II) compound based on 3,5-dinitrobenzoic acid and imidazole

Abstract

Energetic copper(II) compound was synthesized based on 3,5-dinitrobenzoic acid (HDNBA) and imidazole (IMI), and characterized by elemental analysis and FTIR characterization. Single-crystal X-ray diffraction analysis revealed that [Cu(IMI)₂(DNBA)₂] (1) belongs to monoclinic, pertains to P21/c space group, β= 96.195(2) and D_c= 1.742 g cm^–3. Cu(II) ion was coordinated to a plane tetragon, by two oxygen atoms and two nitrogen atoms from different DNBA ions and IMI ligands, which of them present typical monodentate coordination mode. Differential scanning calorimetry (DSC) was applied to assess the thermal decomposition behavior of 1. The non-isothermal kinetics parameters were calculated by the Kissinger’s method, Ozawa’s method and Starink’s method, respectively. Impact sensitivity and friction Sensitivity were also determined by standard method. In the end, the catalytic effects on the decomposition of ammonium perchlorate (AP), HMX and RDX of 1 were studied by DSC. All results supported the potential applications of the energetic complex 1 as additive of solid rocket propellant.

Keywords

Copper(II)imidazole 3,5-dinitrobenzoic acid crystal structure thermal behaviour

1 Introduction

High-energetic metal-organic frameworks (HE-MOFs) with high density, good thermal stability, low sensitivity and better performance are a topic of intense research worldwide, which can be used as high explosive, primary explosive, combustion catalyst of solid rocket propellants, etc [1 –6]. Ammonium perchlorate (AP), RDX and HMX are a famous oxidizing agent and importance explosives compositions that widely used in solid rocket propellants, and its thermal decomposition performances directly influence the combustion behavior. As is already known, HE-MOFs were added into energetic material (such as AP) to improve its thermal decomposition behavior based on its energetic and catalytic performances.

Therein, it is particularly important for the choice of metal ions and high-nitrogen energy ligands, which will significantly influence architecture and performance of HE-MOFs. Up to now, Cu(II) ion exhibits good coordination ability to different kinds of ligands, and more importantly, is an environmentally-friendly and good catalytic effect ion compared to heavy metal ions such as lead [2 , 8]. In addition, high-nitrogen energy ligands, such as azole derivatives [9 –11], azido (or azo-) ligand [12 –15] and nitro-compounds [16, 17] and nitramino-compounds [17 –19], can pose danger to spot operator and environment during both their synthesis and performance testing. Therefore, we choose two energy ligands for 3,5-dinitrobenzoic acid (HDNBA) and imidazole (IMI), which of advantages are as follows: (1) HDNBA has two nitro-groups with better explosive properties and high oxygen content, a carboxyl-group with high oxygen content and various coordination modes, and a benzene-ring with good thermal stability [7 , 20–22]; (2) IMI has two potential coordination nitrogen ions with lower energy than tetrazoles, which is beneficial to reduce system sensitivity and improve thermal stability [13]. Zhi-min Li reported two [Cu(H₂O)₂(DNBA)₂] (2) and [Cu(DAT)₂(DNBA)₂] (3, DAT = 1,5-diaminotetrazole), which can advanced 48 °C and 68 °C for the decomposition temperature of AP, respectively [22]. However, the thermal decomposition onset temperature of 3 was only 177 °C, which can affect the thermal stability of the system. Herein, we report a coordination compounds [Cu(IMI)₂(DNBA)₂] (1), which of synthesis, crystal structure, the thermal decomposition mechanism, impact sensitivity and catalytic effects on the decomposition of AP, RDX and HMX, were studied in the present work.

2 Experimental section

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 PerkinElmer Spectrum 100 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 DSC-131 differential scanning calorimeter (SETARAM).

2.2 Synthesis

Cu(NO₃)₂· 3H₂O (0.60 g, 2.5 mmol) was dissolved in distilled water (10 ml) and charged into a glass reactor with a water bath. It was kept under mechanical stirring and heated to the temperature of 60 °C. A solution of HDNBA (0.84 g, 4 mmol, dissolved in 10 mL ethanol) and imidazole (0.27 g, 4 mmol, dissolved in 10 mL deionized water) were added dropwise to copper solution during 5 min, respectively. The pH value of the mixture was adjusted to 6–7 with 1 M NaOH solution. The reaction at the same temperature for 40 min with stirring resulted in a dark blue precipitate. In the end, the solution was cooled to room temperature naturally. Yield: 78%. Single crystals suitable for X-ray measurement were obtained by evaporation of the mother liquor at room temperature for about one week. Elemental analysis calcd. for CuC₂₀H₁₂N₈O₁₂ (1, molar mass 619.92 g/mol) (%): N 18.08, C 38.75 H 1.95; found (%): N 18.09, C 38.72, H 1.96. IR (KBr, cm^–1): 3368 w, 1626 s, 1524 s, 1455 m, 1340 s, 1258 w, 1189 w, 1058 s, 926 m, 852 w, 762 m, 717 s, 650 s, 607 s.

2.3 X-ray data collection and structure refinement

The X-ray diffraction data collection was performed with a Bruker Smart-Apex-II diffractometer at 296(2)K. Detailed information concerning crystallographic data collection and structure refinement are summarized in Table 1.

Table 1
Crystallographic data and structures determination details for 1

Compound 1

Empirical formula CuC₂₀H₁₂N₈O₁₂

Formula weight 619.92

Crystal system Monoclinic

Space group P21/c

a/Å 5.5725(5)

b/Å 10.0201(9)

c/Å 21.2933(19)

β/(°) 96.195(2)

V/Å³ 1182.01(18)

Z 2

D_c/(g cm^–3) 1.742

θ/(°) 1.9∼25.5

h, k, l –6∼6, –12∼11, –23∼25

Reflections collections 6164

Independent reflection (R_int) 2189 (0.023)

S 1.052

R₁, wR₂[I>2σ(I)] R₁= 0.0386, wR₂= 0.0990^[a]

R₁, wR₂(all data) R₁ = 0.0524, wR₂ = 0.1059 ^[a]

F(000) 626

CCDC 2009261

Compound	1
Empirical formula	CuC₂₀H₁₂N₈O₁₂
Formula weight	619.92
Crystal system	Monoclinic
Space group	P21/c
a/Å	5.5725(5)
b/Å	10.0201(9)
c/Å	21.2933(19)
β/(°)	96.195(2)
V/Å³	1182.01(18)
Z	2
D_c/(g cm^–3)	1.742
θ/(°)	1.9∼25.5
h, k, l	–6∼6, –12∼11, –23∼25
Reflections collections	6164
Independent reflection (R_int)	2189 (0.023)
S	1.052
R₁, wR₂[I>2σ(I)]	R₁= 0.0386, wR₂= 0.0990^[a]
R₁, wR₂(all data)	R₁ = 0.0524, wR₂ = 0.1059 ^[a]
F(000)	626
CCDC	2009261

^[a]w = 1/[σ²(F_o²)+(0.0534P)² +0.6217P], wR₂ =[Σw(F_o²–F_c²)²/ Σw(F_o²)]^1/2, P=(F_o² +2F_c²)/3.

3 Results and discussion

3.1 Structure description

The crystal structure of 1 belongs to monoclinic, pertains to P21/c space group, a = 5.5725(5) Å, b = 10.0201(9) Å, c = 21.2933(19) Å, β= 96.195(2) and D_c= 1.742 g cm^–3 from Table 1. In 1, there are one copper(II) cation, two DNBA anion and two imidazole (IMI) molecules from Fig. 1. Noticeably, each imidazole and DNBA ligands show monodentate coordination mode. In Table 2, the distance of Cu-N3 bond (1.99 Å) is approximately equal with that of Cu-O6 bond (1.94 Å). The bond angles of two N (or O) atoms of two contraposition IMI (or DNBA) ligands and Cu(II) cation are all 180° (<O6-Cu1-O6A and <N3-Cu1-N3A). The nitrogen (or oxygen) atoms from two border upon ligands and Cu(II) cation’s bond angles are all close to 90° (<O6-Cu1-N3 = 90.34°). Therefore, all above data demonstrate that the each Cu(II) cations exhibit a plane tetragon configuration with the lengths of side are from 2.7688 Å to 2.7852 Å (Fig. 2), which was similar to 3. From Fig. 3, the equation of the plane A formed by Cu1, O6, O6A, N3, N3A was calculated as 3.560x –5.646y + 9.620z = 3.7670 without deviation. Atoms of benzene rings are in planes B1:3.220x + 8.178y –1.296z = 4.9107 with the mean deviation of 0.0097 Å (C1 to C7) and B2:3.220x + 8.178y –1.296z = 5.1915 (C1A to C7A, deviation is 0.0097 Å). Angles between planes A and B1(B2) is 95.2°, and planes B1 and B2 are parallel. From the packing diagram (Fig. 4), intermolecular hydrogen bonds of C-H ... O (C atoms of imidazole groups and O atoms of carboxyl groups and nitro groups) can form a cage structure and make an important contribution to enhance the thermal stability.

Fig. 1

Molecular structure of 1, thermal ellipsoids drawn at 30% probability level.

Table 2

Selected bond lengths and angles for 1

bond	lengths/ Å	bond	lengths/ Å
Cu1-O6	1.9368(18)	N2-O4	1.220(4)
Cu1-N3	1.990(3)	C7-O5	1.219(4)
N1-O1	1.227(3)	C7-O6	1.273(4)
N1-O2	1.225(3)	C8-N3	1.337(4)
N2-O3	1.217(4)	C10-N3	1.360(4)
bond	angle/°	bond	angle/°
O6-Cu1-O6A	180.0	C10-N3-Cu1	126.6(3)
N3-Cu1-N3	180.0	C7-O6-Cu1	115.36(18)
O6-Cu1-N3	90.34(10)	O5-C7-O6	126.3(3)
C8-N3-Cu1	127.7(2)	O6-C7-C4	114.1(2)

Fig. 2

Coordination tetragon for the central Cu(II) atom of 1 by O and N atoms from DNBA and IMI.

Fig. 3

Plane formed by different DNBA ions of 1.

Fig. 4

Packing plot of 1 along c-axis.

3.2 Thermal decomposition analysis

In order to investigate the thermal behavior, it was analyzed by DSC with four linear heating rates in a N₂ atmosphere, and shown in Fig. 5. In 1, there is one endothermic process and three exothermic processes in the DSC curves. Corresponding to the linear heating rates of 5, 10, 15 and 20 °C min^–1, an endothermic process was more than 240 °C with peak temperatures of 255.0, 264.0, 266.2 and 269.6°C, respectively. Then, it gets unstable and begins to decompose with two continuous exothermic peaks for breaking of imidazole groups, nitro groups, carboxyl groups and Cu-O(N) bonds. The peaks of first exothermic decomposition stages were observed at 255.6, 263.1, 267.2 and 269.6 °C, and the peaks of second exothermic decomposition stages were observed at 271.7, 280.8, 289.3 and 294.8°C, respectively. In the end, there is one exothermic process for breaking of benzene-ring with peak temperatures of 320.2, 336.5, 344.6 and 352.0°C, respectively.

Fig. 5

DSC curves for 1 with β= 5, 10, 15 and 20 C min^–1 in N₂ atmosphere.

3.3 Non-isothermal kinetics analysis

From the DSC curves, it can be inferred that the first exothermic peak make a dominant effect on the decomposition of 1. Based on the DSC curves measured at different heating rates of 5, 10, 15, and 20 K · min^–1, it is widely used to determine the Arrhenius Equation with the apparent activation energy E, pre–exponential factor A, linear coefficient R and standard deviation S corresponding to Kissinger’s method [23], Starink’s method [24], and Ozawa’s method [25], respectively (See Supporting Information), and showed in Table 4. So, the Arrhenius equation of 1 can be expressed as follows: (E is the average number): lnk = 42.38-223.15×10³/(RT).

Table 3
Selected hydrogen bond for 1

D-H ... A D-H/(nm) H ... A/(nm) D ... A/(nm) D-H ... A/(°)

C8-H8-O6 0.9300 2.5400 2.954(4) 107.00

C9-H9-O4 #1 0.9300 2.3700 3.189(5) 147.00

C10-H10-O5 #2 0.9300 2.5500 3.194(4) 127.00

C10-H10-O6 #3 0.9300 2.5200 2.934(4) 107.00

D-H ... A	D-H/(nm)	H ... A/(nm)	D ... A/(nm)	D-H ... A/(°)
C8-H8-O6	0.9300	2.5400	2.954(4)	107.00
C9-H9-O4 #1	0.9300	2.3700	3.189(5)	147.00
C10-H10-O5 #2	0.9300	2.5500	3.194(4)	127.00
C10-H10-O6 #3	0.9300	2.5200	2.934(4)	107.00

Symmetry Codes: #1 -x,-1/2 + y,1/2-z; #2 -x,1-y,1-z; #3 1-x,1-y,1-z.

3.4 Impact sensitivity and friction sensitivity tests

Impact sensitivity was determined by fall hammer apparatus. Sample (30 mg) was placed between two steel poles and was hit by a 5.0 kg drop hammer at a height of 100 cm. The test results showed that the firing rate was 0%.

Friction sensitivity was determined with a pendular friction sensitivity apparatus by a standard procedure. When 1 (20 mg) was compressed between two steel poles with mirror surfaces at the pressure of 3.92 MPa, and then was hit horizontally with a 1.5 kg hammer from 90° angle, the firing rate was 0 %.

3.5 physicochemical properties

The physicochemical properties are tabulated in Table 5. And critical temperature of thermal explosion (T_b), entropy of activation (ΔS^≠), enthalpy of activation (ΔH^≠), and free energy of activation (ΔG^≠) of 1 were calculated (See Supporting Information).

Table 4
Peak temperatures of the main exotherm and the chemical kinetics parameters for 1

Heating rates T_p / Chemical kinetics Kissinger’s Ozawa’s Starink’s

β / K · min^–1 C parameters method method method

5 255.6 E/kJ·mol^–1 224.36 226.54 218.54

10 263.1 lgA 18.84 – –

15 267.2 R –0.96296 –0.99577 –0.99549

20 269.6 S 0.01242 0.0115 0.01233

Heating rates	T_p /	Chemical kinetics	Kissinger’s	Ozawa’s	Starink’s
5	255.6	E/kJ·mol^–1	224.36	226.54	218.54
10	263.1	lgA	18.84	–	–
15	267.2	R	–0.96296	–0.99577	–0.99549
20	269.6	S	0.01242	0.0115	0.01233

Table 5

Physicochemical properties of 1, [Cu(H₂O)₂(DNBA)₂] and [Cu(DAT)₂(DNBA)₂]

Compound	N	OB	D_c /	E	T _b	ΔS^≠	ΔH^≠	ΔG^≠	IS (J)
(%)	(%)	(g cm^–3)	(kJ mol^–1)	(C)	(J·K^–1·mol^–1)	(kJ·mol^–1)	(kJ·mol^–1)
1	18.08	–90.33	1.742	223.15	253.33	102.94	218.86	165.72	>24.5
2	10.73	–61.33	1.970	247.2	312.54	107.42	242.43	180.75	35
3 [22]	32.67	–65.31	1.869	185.4	182.3	112.58	181.69	131.46	12.5

Fig. 6

DSC curves of AP and AP+1 (a), HMX and HMX+1 (b), RDX and RDX+1 (c).

3.6 Catalytic effects on the decomposition of AP, HMX and RDX

To investigate the catalytic effects of 1 on the thermal decomposition of AP, HMX and RDX, the DSC measurements of as-prepared AP+1, HMX+1 and RDX+1 as well as pure AP, HMX and RDX were performed at a heating rate of 10 C min^–1 in the range of 35 °C to 500 °C. Compound 1 and AP were mixed with a mass ratio of 1:1 by grinding, and results the mixture AP+1. The mixtures HMX+1 and RDX+1 were obtained by a similar method.

As shown in Fig. 6(a), every DSC curve has an endothermic peak with the peak temperature of 247.8 °C for AP and 246.2 °C for AP+1, which is caused by the crystal transformation of AP from orthorhombic to cubic. The crystal transformation of AP is not affected by the presence of 1. Then, a sharp exothermic peak is found with a peak temperature of 277.5 °C, and is advanced by 18.5 °C corresponding to the first exothermic peak of AP with 296.0 °C. In the DSC curves of HMX+1 in Fig. 6(b), it has two main exothermic peaks with 259.8 and 280.7°C, and is advanced by 27.4 and 6.5°C corresponding to the main exothermic peak of HMX with 287.2 °C. In Fig. 6(c), there is one endothermic process with a peak temperature of 206.4 °C and one exothermic process with a peak temperature of 217.6 °C for RDX+1, which is advanced by 1.8 and 27.8°C corresponding to the main endothermic and exothermic peaks of RDX with 208.2 and 245.4 °C. Therefore, it can be concluded that 1 has obvious catalytic effects on the decomposition of AP, HMX and RDX, and the result of 1 is better than 2 and 3.

4 Conclusions

Synthesis and characterization of a coordination compound [Cu(IMI)₂(DNBA)₂] (1) were reported, which has 1.742 g cm^–3 of crystal density, 18.08% of nitrogen content, and –90.33% of oxygen balance. In 1, metal Cu(II) ion was four-coordinated with two DNBA anions and two IMI molecules, and exhibits a plane tetragon configuration. Noticeably, each carboxyl group of DNBA and imidazole-ring show monodentate coordination mode. Complex 1 possesses a good thermal stability with the onset decomposition temperature of 240 °C, and lower impact sensitivity. DSC experiments reveal that 1 accelerate the decomposition of AP, HMX and RDX. The decomposition first temperatures of AP, HMX and RDX were advanced 18.5, 27.4 and 27.8 °C, respectively. The studied copper compound provides a possible application in the field of energetic materials for the combustion modifiers of solid rocket propellants.

Supporting Information

Non-isothermal kinetics analysis

It is widely used to determine the Arrhenius Equation for a given material by Kissinger’s method, Ozawa’s method and Starink’s method, which equations are as follows: $\ln (β / - T^{s}) = - (BE / - RT) + C$ (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 K min^–1. B and C are constant. When s = 2, B = 1 and C = ln(RA/E), 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 exothermic peak temperature measured with four different heating rates of 5, 10, 15 and 20 K 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 standard deviation S were determined based on the relationship of ln(β/T²), ln(β/T^1.8) and lnβ to 1/T, corresponding to Kissinger’s method, Starink’s method, and Ozawa’s method, respectively.

Calculation of critical temperature of thermal explosion, ΔS^≠, ΔH^≠ and ΔG^≠

The value of the peak temperature 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 (2), 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).

$\begin{matrix} T_{i} = T_{0} + a β + b β^{2} \\ T_{b} = (E - \sqrt{E^{2} - 4 ER T_{0}}) / 2 R \\ A = (k_{B} T_{0} / h) exp (1 + Δ S^{\neq} / R) \\ Δ H^{\neq} = E - R T_{0} \\ Δ G^{\neq} = Δ H^{\neq} - T_{0} Δ S^{\neq} \end{matrix}$ (2)

Footnotes

Acknowledgments

The project was supported by the National Natural Science Foundation of China (No.22005275), Scientific and Technologial Innovation Programs of Higher Education Institutions in Shanxi (STIP, No.2019L0584), Equipment Pre-research Weapons Industry Joint Fund (No.6141B012896), Equipment Pre-research Key Laboratory Fund (No.6142020305), and the Advantage Disciplines Climbing Plan of Shanxi Province.

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Synthesis,crystal structure,and properties of energetic copper(II) compound based on 3,5-dinitrobenzoic acid and imidazole

Abstract

Keywords

1 Introduction

2 Experimental section

2.1 Materials and physical techniques

2.2 Synthesis

2.3 X-ray data collection and structure refinement

3.1 Structure description

Table 3 Selected hydrogen bond for 1 D-H ... A D-H/(nm) H ... A/(nm) D ... A/(nm) D-H ... A/(°) C8-H8-O6 0.9300 2.5400 2.954(4) 107.00 C9-H9-O4 #1 0.9300 2.3700 3.189(5) 147.00 C10-H10-O5 #2 0.9300 2.5500 3.194(4) 127.00 C10-H10-O6 #3 0.9300 2.5200 2.934(4) 107.00

3.5 physicochemical properties

4 Conclusions

Supporting Information

Non-isothermal kinetics analysis

Calculation of critical temperature of thermal explosion, ΔS≠, ΔH≠ and ΔG≠

Footnotes

Acknowledgments

References

Table 3
Selected hydrogen bond for 1

D-H ... A D-H/(nm) H ... A/(nm) D ... A/(nm) D-H ... A/(°)

C8-H8-O6 0.9300 2.5400 2.954(4) 107.00

C9-H9-O4 #1 0.9300 2.3700 3.189(5) 147.00

C10-H10-O5 #2 0.9300 2.5500 3.194(4) 127.00

C10-H10-O6 #3 0.9300 2.5200 2.934(4) 107.00

Calculation of critical temperature of thermal explosion, ΔS^≠, ΔH^≠ and ΔG^≠