Chelating energetic material Zn(SCZ) 2 (H 2 O) 2 ·2HTNR with semicarbazide as ligand and styphnic acid as counterion

Abstract

A novel nitrogen and oxygen rich chelating energetic material (CEM) Zn(SCZ)₂(H₂O)₂·2HTNR (1, SCZ = semicarbazide, TNR = styphnic acid) was prepared and characterized by elemental analysis, IR spectra and differential scanning calorimetry (DSC). Its crystal structure was determined through X-ray single crystal diffraction at 153(2) K. The crystal belongs to triclinic. Space group is P-1 with crystal parameters of the unit cell dimensions: a = 0.8284(2) nm, b = 1.1848(4) nm, c = 1.3411(4) nm, α = 99.617(4)°, β = 92.593(4)°, γ = 97.148(4)°, V = 1.2642(6) nm³, Z = 2 and D_c =1.943 g·cm^–3. The kinetic parameters of the first exothermic process of 1 were studied by the Kissinger’s method and Ozawa-Doyle’s method. Sensitivity tests revealed that 1 is insensitive to mechanical stimuli. Energy of combustion and enthalpy of formation of 1 were calculated.

Keywords

Zinc(II)semicarbazide crystal structure thermal analysis sensitivity

1 Introduction

For the time being, insensitive high-energy-density materials (HEDMs) is a topic of interest that is receiving significant attention in the field of energetic materials [1 –6]. The development of insensitive HEDMs continues to focus on coordination energetic materials [7 –12]. On one hand, nitrogen heterocyclic rings have obtained much attention as ligands to construct energetic coordination complexes, such as 3-azido-1,2,4-triazole, 5-nitrotetrazole and 1,5-diaminotetrazole, due to the positive heat of formation and thermal stability. On the other hand, a new class of transition metal complexes containing chelating groups with chain ligands, we noted as chelating energetic materials (CEMs), has also attracted researcher’s interests.

Carbohydrazide based CEMs have been studied by many researchers worldwide [13 –17]. Among them the perchlorate CEMs are shown to be potential primary explosives. Particularly, cadmium tri(carbohydrazide) perchlorate and zinc tri(carbohydrazide) perchlorate both turn out to be lead-free primary explosives in industrial detonators. 3-Amino-1-nitroguanidine is also an excellent bidentate ligand, and some perchlorate containing CEMs are found out to be laser ignitable [18]. SCZ based coordination CEMs were reported extensively [19 –24], but its energetic characteristics didn’t arouse attention until recently [25, 26].

Researchers have also paid intense attention to the green zinc coordination compounds. In 2013, a new energetic zinc-FOX-7 (FOX-7 = 1,1-diamino-2,2-dinitroethylene) complex, Zn(NH₃)₂(FOX-7)₂ was synthesized and structurally characterized by Zhe et al. [27]. In the same year, We prepared and characterized a green nitrogen-rich coordination compound Zn₃(ATZ)₆(N₃)₆ (ATZ = 4-amino-1,2,4-triazole, N% = 61.7%), which has the highest nitrogen content reported for a coordination compound based on 4-amino-1,2,4-triazole being reported [28]. In 2014, we have reported the structure of [Zn(SCZ)₃](NO₃)₂ and its potential to be energetic material [26].

In order to deepen the study on SCZ based compounds, herein we report a novel zinc coordination CEM Zn(SCZ)₂(H₂O)₂·2HTNR (1), which was studied by its preparation, crystal structure, thermal decomposition and sensitivities.

2 Experimental

2.1 Materials and physical techniques

All reagents (analytic grade) and chemicals (chemically pure) were purchased commercially and used without further purification. 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^–1with a resolution of 4 cm^–1. DSC measurement was performed with a Pyris-1 differential scanning calorimeter in a dry nitrogen atmosphere with flowing rate of 20 mL·min^–1. The condition for the thermal analysis was as follow: the sample was powdered and sealed in the aluminum pans with a linear heating rate of 10°C·min^–1 from 50°C to 500°C.

2.2 Synthesis

Semicarbazide hydrochloride (3.33 g, 30 mmol) was dissolved in distilled water (30 mL) and the pH value was adjusted to 6-7 using 10% NaOH solution. Zinc carbonate (10 mmol, 1.14 g) was added to a solution of 2,4,6-trinitroresorcinol (20 mmol, 4.90 g) in deionized water (40 mL), and the mixture was stirred at 60–65°C until a clear solution resulted. The solution containing SCZ was added and the mixture was kept at 60–65°C for 15 min. Then the solution was cooled to room temperature. The precipitate was collected by filtration, washed with ethanol, and dried under vacuum in an explosion-proof water-bath dryer. Yield: 42% (3.11 g). Yellow prism single crystals suitable for X-ray measurements were obtained by recrystallization of the products from deionized water at room temperature over half month. Anal. Calcd (%) for C₁₄H₁₈N₁₂O₂₀Zn: C, 22.73; H, 2.45; N, 22.72. Found (%): C, 22.85; H, 2.39; N,22.86. IR (KBr): 3453, 3321, 3235, 2371, 1634, 1537, 1437, 1382, 1305, 1269, 1170, 1087, 928, 788, 746, 689, 560, 474 cm^–1.

2.3 X-ray data collection and structure refinement

The X-ray diffraction data collection was performed with a Rigaku AFC-10/Saturn724⁺CCD detector diffractometer with graphite monochromated Mo-Kα radiation (λ = 0.71073) with φ and ω modes at 153(2) K. The structure was solved by direct methods using SHELXS-97 and refined with SHELXL-97 [29, 30]. 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.

Table 1
Crystallographic data and structure determination details.

1

CCDC 1002241

Empirical formula C₁₄H₁₈N₁₂O₂₀Zn

Formula weight 739.77

T/K 153(2)

Crystal system Triclinic

Space group P-1

a/nm 0.8284(2)

b/nm 1.1848(4)

c/nm 1.3411(4)

α/(°) 99.617(4)

β/(°) 99.502(4)

γ/(°) 97.148(4)

V/nm³ 1.2642(6)

Z 2

D_c/(g·cm^–3) 1.943

θ/(°) 2.12∼31.00

h, k, l –12∼12,–17∼17,–19∼19

Reflections collections 7936

R _int 0.0330

S 1.001

R₁,wR₂ 0.0395, 0.0844

R₁,wR₂(all data) 0.0556, 0.0913

μ(MoKα)/mm^–1 1.094

F(000) 752

	1
CCDC	1002241
Empirical formula	C₁₄H₁₈N₁₂O₂₀Zn
Formula weight	739.77
T/K	153(2)
Crystal system	Triclinic
Space group	P-1
a/nm	0.8284(2)
b/nm	1.1848(4)
c/nm	1.3411(4)
α/(°)	99.617(4)
β/(°)	99.502(4)
γ/(°)	97.148(4)
V/nm³	1.2642(6)
Z	2
D_c/(g·cm^–3)	1.943
θ/(°)	2.12∼31.00
h, k, l	–12∼12,–17∼17,–19∼19
Reflections collections	7936
R _int	0.0330
S	1.001
R₁,wR₂	0.0395, 0.0844
R₁,wR₂(all data)	0.0556, 0.0913
μ(MoKα)/mm^–1	1.094
F(000)	752

3 Results and discussion

3.1 Structure description

The coordination environment of the zinc cation, the molecular structure and the packing plot of 1, are demonstrated in Fig. 1 and Fig. 2 Selected bond lengths and angles are listed in Table 2.

Fig.1

Molecular structure of 1.

Fig.2

Packing diagram of 1.

Table 2

Selected bond lengths and angles for 1

bond	length/Å	bond	length/Å	bond	length/Å
Zn1–O1	2.0761(15)	Zn1–O2	2.1497(16)	Zn1–N1	2.1107(17)
Zn1–O1#1	2.0761(15)	Zn1–O2#1	2.1497(16)	Zn1–N1#1	2.1107(17)
Zn2–O4#2	2.1609(16)	Zn2–N4#2	2.1010(18)	Zn2–O4	2.1609(16)
Zn2–O3#2	2.0828(15)	Zn2–N4	2.1010(18)	Zn2–O3	2.0828(15)
N3–C1	1.331(2)	O1–C1	1.261(2)	N4–N5	1.412(2)
bond	angle/°	bond	angle/°	bond	angle/°
O1–Zn1–O2	90.64(6)	O4#2–Zn2–N4	89.90(6)	O1–Zn1–N1	79.48(6)
N4–Zn2–N4#2	180	O1–Zn1–O1#1	180	O1–Zn1–O2#1	89.37(6)
O1–Zn1–N1#1	100.52(6)	O2–Zn1–N1	87.66(6)	O1#1–Zn1–O2	89.37(6)
O2–Zn1–O2#1	180	O2–Zn1–N1#1	92.34(6)	O1#1–Zn1–N1	100.52(6)
O2#1–Zn1–N1	92.34(6)	O1#1–Zn1–O2#1	90.64(6)	N1–Zn1–N1#1	180
O1#1–Zn1–N1#1	79.48(6)	O2#1–Zn1–N1#1	87.66(6)	Zn1–O1–C1	113.21(11)
Zn1–O2–H02A	111.6(19)	Zn1–O2–H02B	115(2)	Zn1–N1–N2	107.96(11)
O3–Zn2–N4#2	100.71(6)	O3–Zn2–N4	79.29(6)	O3#2–Zn2–O4	91.20(6)
O3–Zn2–O4#2	91.20(6)	O3–Zn2–N4#2	100.71(6)	O4–Zn2–N4	90.10(6)

Symmetry code: #1 -x, 1-y, 1-z; #2 -x, -y, -z.

The central Zn(II) ion has sp³d² hybridization, contributing six empty orbits to accommodate the lone pair electrons from ligands. Molecular unit of CEM 1 contain one zinc cation, two neutral SCZ bidentate ligands, two water molecules and two HTNR^- ions. The central zinc cation coordinates with six ligand atoms, which are two oxygen atoms from two water molecules, two nitrogen atoms and two oxygen atoms from two SCZ ligands. SCZ shows typical bidentate coordination mode. These selected bond angles O1-Zn1-O1#1, N1–Zn1–N1#1 and O2-Zn1-O2#1 are 180°. The bond lengths of the central Zn(II) atom to coordinated atoms range from 2.07 Å to 2.17 Å, which demonstrate that the zinc(II) cation is coordinated to form a slightly distorted octahedral configuration. Two SCZ molecules coordinated to the central zinc atom to form two chelates and these two chelates are almost in the same plane. Two water molecules are in sides of the plane, ensuring the minimum of the space steric hindrance, benefiting to the structural stability. CEM 1 crystallizes with a triclinic unit cell in the space group P-1 with two formula units in the unit cell, which is different with [Zn(SCZ)₃](NO₃)₂ with monoclinic P2₁/c space group. The crystal density of 1 is 1.943 g·cm^–3.

The chelate effect, electrostatic attractions, and intermolecular and intramolecular hydrogen bonds (Table 3) extend the structure into a 3D supramolecular structure and make an important contribution to enhance the thermal stability of the complex.

Table 3

Hydrogen bonds for 1

D–H…A	D–H/(Å)	H…A/(Å)	D…A/(Å)	D–H…A/(°)
N1–H1A…O18ⁱ	0.89(2)	2.54(3)	3.130(2)	125(2)
N1–H1A…O19ⁱ	0.89(2)	2.19(2)	3.024(2)	156(2)
N1–H1B…O14ⁱⁱ	0.95(2)	2.24(3)	3.058(2)	144(2)
N1–H1B…O15ⁱⁱ	0.95(2)	2.44(2)	3.117(2)	128.3(18)
N2–H2N…O13ⁱⁱⁱ	0.86(2)	2.09(2)	2.924(2)	163(2)
O2–H02B…O12	0.74(3)	2.18(3)	2.892(2)	163(3)
N3–H3A…O13ⁱⁱⁱ	0.82(2)	2.33(2)	3.046(2)	147(2)
N3–H3A…O14ⁱⁱⁱ	0.82(2)	2.20(3)	2.838(2)	135(2)
N3–H3B…O17	0.87(3)	2.43(2)	3.047(2)	129(2)
N6–H6B…O9	0.84(2)	2.50(2)	3.040(2)	122(2)
O10–H10O…O3	0.81(3)	2.46(2)	3.094(2)	136(3)
O10–H10O…O9	0.81(3)	1.92(3)	2.589(2)	139(3)
O10–H10O…N8	0.81(3)	2.54(3)	2.925(2)	111(2)
C11–H11…O20^iv	0.950(4)	2.545(3)	3.480(3)	168.0(2)

Symmetry codes: (i) -x, 1-y, 1-z; (ii) -1+x, -1+y, z; (iii) x, -1+y, z; (iv) 1+x, y, z.

3.2 Thermal decomposition and non–isothermal kinetics analysis

In order to investigate the thermal behavior of 1, DSC curve with a linear heating rate of 10°C·min^–1 was recorded (Fig. 3). In the DSC curve, there is one endothermic peak and two continuous exothermic peaks. The endothermic stage occurs at 150°C with a peak temperature of 179°C. The peak temperature of the first and the second exothermic processes are 232°C and 317°C, respectively. Comparing with [Zn(SCZ)₃](NO₃)₂, CEM 1 shows lower thermal stability, which is probably due to that the central ion coordinated with bidentate SCZ ligand in [Zn(SCZ)₃](NO₃)₂ is more stable than the central ion coordinated with water molecule ligands in 1.

Fig.3

DSC curve for 1 with β = 10°C·min^–1 in a nitrogen atmosphere.

In the present works, Kissinger’s method and Ozawa-Doyle’s method are widely used to determine the apparent activation energy (E) and the pre-exponential factor (A) [31, 32]. The Kissinger and Ozawa-Doyle equations are as follows, respectively. $In β / T_{p}^{2} = In [RA / E] - E / ({RT}_{p})$ (1) $Ig β = Ig [AE / RG (α)] - 2.315 - 0.4567 E / RTp$ (2)

Where T_p is the peak temperature [°C], A is the pre-exponential factor [s^–1], E is the apparent activation energy [kJ mol^–1], R is the gas constant (8.314 J K^–1 mol^–1), β is the linear heating rate [°C min^–1], and G(α) is the reaction mechanism function.

Based on the first exothermic peak temperatures measured at four different heating rates of 5, 10, 15 and 20°C/min, Kissinger’s method and Ozawa-Doyle’s method were applied to study the kinetics parameters of 1. From the original data, the apparent activation energy E, pre-exponential factor A, linear coefficient R and standard deviations S were determined and shown in Table 4.

Table 4

Peak temperatures of the first exotherm and the chemical kinetics parameters

β/(°C min^–1)	T_p/°C	Prameter	Kissinger’s method	Ozawa’s method
5	228	E/kJ mol^–1	239.3	235.5
10	232	lgA	22.98	—
15	236	R_c	–0.9778	–0.9793
20	240	S	0.1492	0.0648

So, the Arrhenius equation of 1 can be expressed as follows: (E is the average of E_k and E_o): $lnk = 52.85 - 237.4 \times 10^{3} / (RT)$ (3)

The equation can be used to estimate the rate constants of the initial thermal decomposition process of the title compound.

3.3 Sensitivity test

Impact sensitivity was determined by a 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 showed that the 50% firing height (h₅₀) was 50 cm (= 22.5 J).

Friction sensitivity was determined using 20 mg sample. 1 was compressed between two steel poles with mirror surfaces at the pressure of 3.92 MPa, and was hit horizontally with a 1 kg hammer from 90°angle. The test showed that the title compound did not fire.

According to the method of flame sensitivity test, 20 mg of 1 was compacted to a copper cap under the press of 39.2 MPa and was ignited by black powder pellet. The test result demonstrated that 1 was not fire at 6 cm minimum height.

To sum up, both CEM 1 and [Zn(SCZ)₃](NO₃)₂ are insensitive complexes.

3.4 Energy of combustion and enthalpy of formation

In order to study the energy of combustion (ΔH) and the enthalpy of formation (Δ_fH°₂₉₈), constant-volume energy of combustion (Q_v) was measured by oxygen bomb calorimetry and was –8.54 MJ·kg^–1.

The bomb equation is as follow: $C_{14} H_{18} {ZnN}_{12} O_{20} + 9 O_{2} \to {ZnO + 9 H}_{2} {O + 14 CO}_{2} + 6 N_{2}$ (4)

And the energy of combustion is as follow (T = 298.15 K): $Δ H_{1} = Q_{p 1} = Q_{v 1} + Δ nRT = - 6298.61 kJ \cdot {mol}^{- 1} = - 8.51 MJ \cdot {kg}^{- 1}$ (5)

So the energies of combustion of 1 is –8.51 MJ·kg^–1. The literature had reported the combustion energies of RDX (1,3,5-trinitro-1,3,5-triaza-cyclohexane), HMX (1,3,5,7-tetranitro-1,3,5,7-tetraazocane) and TNT (2,4,6-trinitrotoluene) were –9.60 MJ·kg^–1, –9.44 to –9.88 MJ·kg^–1 and –15.22 MJ·kg^–1, respectively [33]. Consequently, the energy of combustion of 1 is approximately equal to the energies of combustion of RDX and HMX, and almost half of TNT.

The metal coordination compound should have relatively thermodynamically stable structure. The standard enthalpy of formation was back calculated from the heat of combustion on the basis of Equation (4), and Hess’s Law as applied in thermochemical Equation (5). With the known enthalpies of formation of zinc oxide [Δ_fH°₂₉₈(ZnO, s) = –348.28 kJ·mol^–1], carbon dioxide [Δ_fH°₂₉₈(CO₂, g) = –393.5 kJ·mol^–1] and water [Δ_fH°₂₉₈(H₂O, l) = –285.8 kJ·mol^–1], the enthalpy of formation of 1 can now be calculated as: $\begin{matrix} Δ_{f} H^{\circ} 298 (1, s) = Δ_{f} H^{\circ} (ZnO, s) + 9 Δ_{f} H^{\circ} (H_{2} O, 1) + 14 Δ_{f} H^{\circ} ({CO}_{2}, g) - Δ_{c} H^{\circ} (1, s) \\ = - 2131.27 kJ \cdot {mol}^{- 1} = - 2.88 kJ \cdot g^{- 1} \end{matrix}$ (6)

4 Conclusions

A novel CEM Zn(SCZ)₂(H₂O)₂·2HTNR (1, SCZ = semicarbazide, TNR = styphnic acid) was prepared and characterized. In 1, Zn^2 + was six-coordinated with two SCZ groups and two water molecules. SCZ also presents typical bidentate coordination modes. Non-isothermal kinetics analysis reveals that the Arrhenius equation of 1 can be expressed as follows: lnk = 52.85–237.4×10³/(RT). Sensitivity tests revealed that 1 is insensitive to mechanical stimuli. Energy of combustion and enthalpy of formation of 1 were –8.51 kJ·g^–1 and –2.88 kJ·g^–1, respectively. Besides, SCZ based CEMs are worthy of further experimentation aimed at exploring novel modern pyrotechnical composition.

Footnotes

Acknowledgments

We gratefully acknowledge financial support from the State Key Laboratory of Explosion Science and Technology (No. YBKT16-17), Science and Technology on Applied Physical Chemistry Laboratory (No. 9140C370101150C37170).

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