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Keywords:

  • Azides;
  • Copper;
  • Crystal;
  • Thermal;
  • Sensitivities

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements

The multi-ligand coordination compound copper(II) 1,2-diaminopropane (pn) azide, [Cu(pn)(N3)2]n (1), was synthesized using pn and azido groups. It was characterized by X-ray single crystal diffraction, elemental analysis, and FT-IR spectroscopy. The crystal structure of 1 belongs to the monoclinic system, space group C2/c. The copper(II) cation is six-coordinated by one pn molecule and four azido ligands with μ-1 and μ-1,1,3 coordination modes. Thermogravimetric investigations with a heating rate of 10 K·min–1 under nitrogen showed one main exothermic stage with a peak temperature of 215.7 °C in the DSC curve. The non-isothermal kinetics parameters were calculated by Kissinger and Ozawa methods, respectively. The heat of combustion was measured by oxygen bomb calorimetry, and the enthalpy of formation, the critical temperature of thermal explosion, the entropy of activation (ΔS), the enthalpy of activation (ΔH), and the free energy of activation (ΔG) were calculated. The measurements showed that 1 has very high impact, friction, and flame sensitivities.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements

High-energy-density-materials (HEDM) with high performance have attracted worldwide research groups over the last decades.1,2 Nitrogen-rich heterocyclic complexes and salts are able to fulfill many requirements in the challenging field of energetic materials research, because they are generally highly endothermic with high densities and proper sensitivities. Klapötke, Shreeve and others research groups performed various studies on high-nitrogen modern energetic complexes and salts based on imidazole derivatives,35 triazole derivatives,68 and tetrazole derivatives.918

In recent years, a series of energetic coordination compounds were synthesized using transition metal ions such as MnII, FeII, CoII, NiII, CuII, ZnII, and CdII as central metal cations, and chloride,19 perchlorate,20 nitrate,20 picrate,2123 and azido2431 ions as anions. The perchlorate metal compounds had better sensitivity properties than chloride and nitrate compounds. The picrate and styphnate compounds are more flame sensitive. Especially azide compounds have been studied because of the versatile coordination modes of the azido anions (N3). The azido ion can bind the metal atoms by μ-1 mode as a monodentate bridging ligand, μ-1,3 and μ-1,1 modes as a bidentate ligand, and μ-1,1,3 mode as tridentate ligand. In addition, the azide compounds can release a great amount of energy, which can be concluded from the average bond energies of N–N (160 kJ·mol–1), N=N (418 kJ·mol–1), and N≡N (954 kJ·mol–1). To demonstrate it by some examples, the azido groups of compounds [Mn3(N3)6(admtrz)4]n27 (admtrz = 4-amino-3,5-dimethyl-1,2,4-triazole) and [Cd3(N3)6(admtrz)4]n30 have three different coordination modes: μ-1, μ-1,3 and μ-1,1 modes. Zhu reported the nickel(II) hydrazine azide, which is a potential powerful primary explosive.32 Sheng also reported tetraamminediazido cobalt(III) perchlorate (DACP), which is very sensitive to laser excitation with 635 nm.33 We synthesized and investigated the crystal structures, the thermal decomposition mechanisms, and sensitivity properties of [Zn(HZ)2(N3)2]n (HZ = hydrazine),25 [Cd2(HZ)2(N3)4]n,29 Mn(CHZ)2(N3)2 (CHZ = carbohydrazide),34 [Cu(en)(N3)2]n (en = ethylenediamine),24 Co(en)2(N3)2(NO3),24 [Cd(en)(N3)2]n,28 Cu(IMI)4(N3)2 (IMI = imidazole),26 and Ni(IMI)4(N3)2,26 and found that the azido groups show different coordination modes. These compounds have potential application as energetic materials.

In order to deepen the studies on the metal azide compounds, a new multi-ligand coordination compound, [Cu(pn)(N3)2]n (1), was synthesized, and the crystal structure, the thermal decomposition mechanism, and sensitivity properties were studied in the presented work.

Results and Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements

Molecule Structure

The crystal structure of 1 belongs to the monoclinic system, space group C2/c (Table 1). In 1, there is one CuII cation, one 1,2-diaminopropane molecule, and four azido ligands (Figure 1). In addition, the Cu1 cation is coordinated by one azido group, which acts as μ-1 bridge (N3–N4–N5), and three azido groups, which act as μ-1,1,3 bridges (N6–N7–N8, N6A–N7A–N8A, and N6B–N7B–N8B). The axial Cu1–N6B (2.516 Å) and Cu1–N8A bonds (2.659 Å) are about 0.51–0.66 Å longer than the other four basically equivalent equatorial Cu–N bonds (ca. 2.0 Å). The bond angle of two opposite nitrogen atoms and the Cu1 cation (N6B–Cu1–N8A) is 173.45°. The bond angles of two neighbored nitrogen atoms and the Cu1 cation deviate from 90° (Table 2) and reveal that the Cu1 cations exhibit distorted octahedral configuration.

Table 1. Crystal data and structure refinement for 1.
 1
  1. a

    a) w = 1/[σ2(Fo2)+(0.0413P)2+0.1060P], wR2 = [∑w(Fo2Fc2)2/∑w(Fo2)]1/2, P = (Fo2 + 2Fc2)/3.

Empirical formulaCu0.4C1.2N3.2H4
Formula mass88.69
Temperature /K143(2)
Crystal dimensions /mm0.50 × 0.40 × 0.08
Crystal systemmonoclinic
Space groupC2/c
Z20
a17.928(5)
b6.910(2)
c13.536(4)
β94.480(4)
h, k, l–18 ≤ h ≤ 26,
 –10 ≤ k ≤ 10
 –18 ≤ l ≤ 19
Unit cell dimensions V31671.6(9)
Dc /g·cm–31.762
μ (Mo-Kα) /mm–12.574
F(000)904
θ Range /°3.02–31.50
Measured reflections8053
Unique data2707 (Rint = 0.0434)
R1, wR2 a) [I > 2σ(I)]0.0348, 0.0803
R1, wR2 a) (all data)0.0459, 0.0857
Goodness of fit1.003
δpmax, δpmin (e·Å–3)0.680, –0.730
thumbnail image

Figure 1. The molecule structure of 1, thermal ellipsoids drawn at 50 % probability level.

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Table 2. Selected bond lengths /Å and bond angles /° for 1.
Bond Bond Bond 
Cu1–N12.0146(17)Cu1–N8A2.659N3–N41.200(2)
Cu1–N22.0054(18)Cu1–N6B2.516N4–N51.157(2)
Cu1–N31.9935(17)N1–C11.454(4)N6–N71.163(2)
Cu1–N82.0150(18)N2–C21.488(3)N7–N81.197(2)
Angle Angle Angle 
N6B–Cu1–N8A173.45N6B–Cu1–N395.42N6B–Cu1–N889.16
N6B–Cu1–N192.67N6B–Cu1–N288.88N8A–Cu1–N181.18
N8A–Cu1–N888.85N8A–Cu1–N390.85N8A–Cu1–N292.73
N3–Cu1–N292.58(7)N2–Cu1–N8176.18(7)N4–N3–Cu1119.30(14)
N3–Cu1–N1171.25(7)N1–Cu1–N892.56(7)N7–N8–Cu1117.55(14)
N2–Cu1–N184.26(7)C1–N1–Cu1107.64(15)N3–N4–N5177.6(2)
N3–Cu1–N890.87(7)C2–N2–Cu1108.95(14)N6–N7–N8178.5(2)

In compound 1, the planes are formed by the N8 atoms and two neighbored CuII cations, and the plane equations are as follows: Cu1–Cu1A–N8–N8A (plane A): 14.710x + 3.514y + 2.655z = 6.3129 (R = 0.000); and Cu1B–Cu1C–N8AA–N8C (plane B): 14.710x + 3.514y + 2.655z = 9.8267 (R = 0.000). Thus, planes A and B are parallel. Moreover, the copper cations and the 1,2-diaminopropane molecule form stable five-membered rings. The four CuII ions and four azido groups form large sixteen-membered rings (Figure 2), and extend the structure into a 3D supermolecular structure. Weak N–H···N hydrogen bonds occur between the azido groups and the amino groups of pn ligands, which enhance the thermal stability (Table 3).

thumbnail image

Figure 2. The packing of unit cells view for 1 (omit pn group and μ-1 azido ligands).

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Table 3. Hydrogen bond lengths /Å and bond angles /° for 1.
D–H···Ad(D–H)d(H···A)d(D···A)D–H···A
N1–H1A···N3 #10.92002.36003.230(2)159.00
N1–H1B···N5 #20.92002.16003.075(3)172.00
N2–H2A···N6 #30.92002.18003.075(2)165.00
N2–H2B···N3 #40.92002.21003.055(2)152.00

Thermal Decomposition and Non–isothermal Kinetics Analysis

The thermal behavior of 1 was investigated, the DSC curve with a linear heating rate of 10 K·min–1 is shown in Figure 3. For compound 1, a melting point could not be observed in the DSC curves. The exothermic stage exhibits one tardigrade decomposition stage with a peak temperature of 215.7 °C. Subsequently, the first exothermic peak temperatures with four different heating rates of 5, 10, 15, and 20 K·min–1 were measured. From the original data, the apparent activation energy (Ek and Eo), the pre-exponential factor (Ak) and the linear correlation coefficients (Rk and Ro) were determined by Kissinger's method35 and Ozawa's method,36 and are shown in Table 4. Accordingly, the Arrhenius Equation of compound 1 was lnk = 16.28 –168.9 × 103/RT, where E = 1/2 (Ek + Eo).

thumbnail image

Figure 3. DSC curve for 1 with β = 10 K·min–1 in a nitrogen atmosphere.

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Table 4. The peak temperatures at different heating rates and the chemical kinetics parameters.
Heating rates β /K·min–1Tp /KParametersKissinger's methodOzawa's method
5479.55E /kJ·mol–1169.2168.6
10488.85ln(A/s–1)16.28
15492.85R–0.9834–0.9848
20494.05s0.12790.05557

Heat of Combustion and Enthalpy of Formation

The heat of combustion (ΔH = Qp) was back calculated from the constant volume heat of combustion (Qv) on the basis of Equation (1) and Equation (2). The value of Qv was measured by oxygen bomb calorimetry and was –4.42 MJ·kg–1.

The bomb combustion reaction equation and the heat of combustion result as follows:

  • equation image((1))
  • equation image((2))

Consequently, the heat of combustion of 1 is lower than that of RDX (–9.60 MJ·kg–1), HMX (–9.44 to –9.88 MJ·kg–1) and TNT (–15.22 MJ·kg–1).

The standard enthalpy of formation (ΔfH°298) was back calculated from Equation (1), and Hess's Law as applied in thermochemical Equation (3). The enthalpies of formation of copper oxide, carbon dioxide, and water are –155.2 kJ·mol–1, –393.5 kJ·mol–1, and –285.8 kJ·mol–1, respectively. So, compound 1 has a relatively thermodynamically stable structure and the enthalpy of formation can be calculated as:

  • equation image((3))

Calculation of the Critical Temperature of Thermal Explosion, ΔS, ΔH, and ΔG

The value of the peak temperatures corresponding to β[RIGHTWARDS ARROW]0 (Tp0), the corresponding critical temperature of thermal explosion (Tb), entropy of activation (ΔS), enthalpy of activation (ΔH), and free energy of activation (ΔG) were obtained by the following Equation (4), where a, b, and c are coefficients, kB is the Boltzmann constant (1.381 × 10–23 J·K–1) and h is the Planck constant (6.626 × 10–34 J·s) 37,38.

  • equation image((4))

The physicochemical properties of 1 are tabulated in Table 5. Compound 1 is a nitrogen-rich material with a nitrogen content of more than 50 %. It has a smaller oxygen balance than TNT (Ω – 74.0 %). In addition, the value of Tb (473.49 K) shows that the transition from thermal decomposition to thermal explosion is not easy to take place compared to copper(II) ethylenediamine azide (425.28 K).24 The positive value of ΔG (221.26 kJ·mol–1) indicates that the exothermic decomposition reaction must proceed under the heating condition.

Table 5. Physicochemical properties of 1.
 1
  1. a

    a) Thermal degradation / DSC main exothermic peak. b) Nitrogen content. c) Oxygen balance. d) Activation energy. e) Experimental heat of combustion. f) Molar enthalpy of formation.

Td /°C a)215.7
N /% b)50.54
Ω /% c)–86.6
E /kJ·mol–1 d)168.9
ΔH° /kJ·mol–1 e)–982.44
ΔfH°298 /kJ·mol–1 f)–2320.76
Tp0 /K462.45
Tb /K473.49
ΔS /J·K–1·mol–1–121.52
ΔH /kJ·mol–1165.06
ΔG /kJ·mol–1221.26

Sensitivity Tests

Impact sensitivity was determined by Fall Hammer Apparatus. The compound (20 mg) was placed between two steel poles and was hit by a 5.0 kg drop hammer. The test results showed that the 50 % firing height (h50) was 5.2 cm (= 2.55 J).

Friction sensitivity was determined on a MGY-1 pendular friction sensitivity apparatus by a standard procedure using a 20 mg sample. When the compound was compressed between two steel poles with mirror surfaces at a pressure of 1.23 MPa, and was hit horizontally with a 1.5 kg hammer from 70° angle, the firing rate was 100 %.

Flame sensitivity was determined by following a standard method, in which the sample was ignited by standard black powder pellet. The compound (20 mg) was compacted to a copper cap under the pressure of 58.8 MPa and was ignited by standard black powder pellet. The test results showed that the 50 % firing height (h50) was 20.2 cm.

Therefore, compared to copper(II) ethylenediamine azide24, compound 1 has very higher impact and friction sensitivities, however, it has lower flame sensitivity.

Conclusions

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements

The new high-nitrogen energetic compound of copper(II) 1,2-diaminopropane (pn) azide, [Cu(pn)(N3)2]n (1), was synthesized and characterized. The crystal structure shows that the CuII ion is six-coordinated in a distorted octahedral arrangement, and the azido groups display different coordination modes (μ-1 and μ-1,1,3 modes). The experiments found the heat of combustion of 1 is –4.43 MJ·kg–1, which is lower than that of RDX, HMX, and TNT. Non-isothermal kinetics analysis results indicated that the Arrhenius equations can be expressed as follows: lnk = 16.28 –168.9 × 103/RT. The critical temperature of thermal explosion (Tb), entropy of activation (ΔS), enthalpy of activation (ΔH), and free energy of activation (ΔG) of the decomposition reaction of 1 are 473.49 K, –121.52 J·K–1·mol–1, 165.06 kJ·mol–1, and 221.26 kJ·mol–1, respectively. The impact, friction, and flame sensitivities measuring results showed that 1 has extreme potential application as energetic material.

Experimental Section

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements

Materials and Physical Techniques: All reagents and solvents were all analytically pure commercial products. 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 measurement was carried with a Pyris-1 differential scanning calorimeter (Perkin-Elmer, USA) in a dry nitrogen atmosphere with a flowing rate of 20 mL·min–1 and a linear heating rate of 10 K·min–1 from 50 °C to 600 °C. The constant-volume heat of combustion was measured by oxygen bomb calorimetry (Parr 6200, USA).

Synthesis of 1: A solution containing Cu(OAc)2·H2O (2.00 g, 10 mmol) in deionized water (30 mL) was charged into a glass reactor with a thermo-water bath. The reaction solution was stirred with a mechanical agitator and heated to 60–70 °C for preparation. The compounds pn (0.78 g, 10 mmol) and sodium azide (1.30 g, 20 mmol) were dissolved in deionized water (10 mL), respectively, and the pn solution was adjusted to pH 8.0–9.0 with acetic acid. Afterwards, they were added to the Cu(OAc)2 aqueous solution during 10–15 min with continuous stirring. Half an hour later, the solution was cooled to room temperature whilst stirring naturally. The precipitate was collected by filtration, washed with ethanol, dried in an explosion-proof water-bath dryer. The yield was 54 %. Single crystal suitable for X-ray measurement was evaporated slowly at room temperature for a week. Elemental analysis for Cu0.4C1.2N3.2H4: calcd. C 16.25; H 4.55; N 50.54 %; found: C 16.20; H 4.23; N 50.13 %. IR (KBr): equation image = 3500 (N–H), 2051(N–N), 1629 (N–H), 1394 (C–N), 1085 (C–N), 617(N–N), 528(N–N) cm–1.

X-ray Data Collection and Structure Refinement: The X-ray diffraction data collection was performed with a Rigaku AFC–10/Saturn 724+ CCD detector diffractometer with graphite monochromated Mo-Kα radiation (λ = 0.71073 Å). The structure was solved by direct methods using SHELXS–9739 and refined by full-matrix least-squares methods on F2 with SHELXL-97.40 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 structures refinement are summarized in Table 1.

Acknowledgements

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements

The projects were supported by the State Key Laboratory of Explosion Science and Technology (No. QNKT12-02 and ZDKT10-01b), Science and Technology on Applied Physical Chemistry Laboratory (9140C3703051105 and 9140C370303120C37142).