Preparation, crystal structure, and thermal decomposition of the intriguing five-coordinated compound [Cu(IMI)4Cl]Cl (IMI = Imidazole)

Article abstract of DOI:10.1002/zaac.201200520

The intriguing multi-ligand compound [Cu(IMI)₄Cl]Cl (1) with the ligand imidazole (IMI) was synthesized and characterized by elemental analysis and FT-IR spectroscopy. The crystal structure was determined by X-ray single crystal diffraction and the crystallographic data showed that the compound belongs to the monoclinic P2₁/n space group [α = 8.847(2) A, b = 13.210(3) A, c = 13.870(3) A, and β = 90.164(3)°]. Furthermore, the Cu^II ion is five-coordinated by four nitrogen atoms from four imidazole ligands and a chlorine atom. The thermal decomposition mechanism was determined based on differential scanning calorimetry (DSC) and thermogravimetric (TG-DTG) analysis. The non-isothermal kinetics parameters were calculated by the Kissinger's method and Ozawa's method, respectively. The energy of combustion, enthalpy of formation, critical temperature of thermal explosion, entropy of activation (ΔS ^≠), enthalpy of activation (ΔH^≠), and free energy of activation (ΔG^≠) were measured and calculated. Copyright

Full text of DOI:10.1002/zaac.201200520

ARTICLE
DOI: 10.1002/zaac.201200520
Preparation, Crystal Structure, and Thermal Decomposition of the
Intriguing Five-coordinated Compound [Cu(IMI)4Cl]Cl (IMI = Imidazole)
Bi-Dong Wu,[a] Zun-Ning Zhou,[a] Yan-Gang Bi,[a] Li Yang,[a] Jian-Guo Zhang,[a] and
Tong-Lai Zhang*[a]
Keywords: Imidazole; Copper; X-ray diffraction; Thermal properties; Impact sensitivity
Abstract. The intriguing multi-ligand compound [Cu(IMI)4Cl]Cl (1) thermal decomposition mechanism was determined based on differen-
with the ligand imidazole (IMI) was synthesized and characterized by tial scanning calorimetry (DSC) and thermogravimetric (TG-DTG)
elemental analysis and FT-IR spectroscopy. The crystal structure was
analysis. The non-isothermal kinetics parameters were calculated by
determined by X-ray single crystal diffraction and the crystallographic the Kissinger’s method and Ozawa’s method, respectively. The energy
data showed that the compound belongs to the monoclinic P21/n space of combustion, enthalpy of formation, critical temperature of thermal
group [α = 8.847(2) Å, b = 13.210(3) Å, c = 13.870(3) Å, and β =
90.164(3)°]. Furthermore, the CuII ion is five-coordinated by four
nitrogen atoms from four imidazole ligands and a chlorine atom. The
explosion, entropy of activation (ΔS϶), enthalpy of activation (ΔH϶),
and free energy of activation (ΔG϶) were measured and calculated.
their nitrogen contents are 46.70%, 47.27%, 25.23%, 33.17%,
and 24.74%, respectively.
In order to deepen the studies on the imidazole compound,
[Cu(IMI)4Cl]Cl (1) was synthesized. The intriguing five-coor-
dinated crystal structure and the thermal decomposition
mechanism were studied in the presented work.
Introduction
The heterocyclic compounds have attracted considerable
interest due to their rather large densities, good oxygen bal-
ance, high performance and environmental compatibility.[1,2]
Klapötke, Shreeve and other chemists have done studies about
high energy density materials (HEDMs) on the basis of imid-
azole,[3–16] triazole,[17–21] and tetrazole[22–32]
.
Imidazole (IMI) is a pentacyclic heterocyclic compound
containing two potential nitrogen coordination atoms. The ad-
vantages of imidazole include the high nitrogen content (N =
41.15%), ready availability, high positive heat of formation
and good thermal stability. What is particularly important, de-
composition of imidazole results in the generation of large vol-
umes of environmentally friendly nitrogen (N2). Imidazole de-
rivative compounds can be obtained in three different ways.
Firstly, a new compound was synthesized by introducing
azido or nitro explosive groups in the imidazole group, such
as 2-azido-imidazole,[11] nitro-imidazole,[9,10,13] and so on.
Secondly, the preparation of a salt by neutralization or meta-
thesis reaction.[11,33]. Furthermore, the role of the imidazole
ring as metal binding site in compound is well-known. A series
of compounds have been developed in our laboratory based on
the imidazole and azide compounds. Cu(IMI)4(N3)2,[16]
Results and Discussion
Crystal Structure Description
In our previous study, the electronic density of the highest
occupied molecular orbitals (HOMOs) at the N(1) atom of the
imidazole group is relatively higher compared with the lowest
unoccupied molecular orbitals (LUMOs).[15,16] It is easy to
understand the coordination bond formation between the nitro-
gen atom and the CuII ion with unoccupied 3d orbitals.
In 1, there are one copper(II) cation, four imidazole mole-
cules, a coordination chlorine atom, and an external chlorine
atom (Figure 1). The four basically equivalent Cu–N bonds are
approximately equal (2.0 Å) (see Table 1). The angles between
two nitrogen atoms from two contraposition imidazole ligands
and CuII cations are 174.75(8)° (N1–Cu1–N3) and 158.33(8)°
(N5–Cu1–N7), whereas the bond angles between nitrogen (or
chlorine) atoms from two borders upon ligands and CuII cat-
ions are all close to 90°. Therefore, all data of the above dem-
onstrate that the CuII cation exhibits a distorted pentahedron
configuration (Figure 2). From the packing diagram, all of
these intermolecular hydrogen bonds are to extend the struc-
ture into the 3D super-molecular structure, and make an impor-
tant contribution to enhance the thermal stability of the com-
pound (see Supporting Information).
Ni(IMI)4(N3)2,[16] [Ni(IMI)6](ClO4)2,[15] [Ni(IMI)6](NO3)2,[15]
[14]
and [Cu(IMI)4](PA)2
are the nitrogen-rich materials and
* Prof. Dr. T. L. Zhang
Fax: +86-10-68911202
E-Mail: ztlbit@bit.edu.cn
[a] State Key Laboratory of Explosion Science and Technology
Beijing Institute of Technology
Beijing 100081, P. R. China
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/zaac.201200520 or from the au-
thor.
Z. Anorg. Allg. Chem. 2013, 639, (5), 799–803
© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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T.-L. Zhang et al.
ARTICLE
Table 1. Selected bond lengths /Å and bond angles /° for compound 1.
Bond
Bond
Bond
Cu1–N1
Cu1–N3
1.997(2)
2.005(2)
Cu1–N7
Cu1–N5
2.0089(19)
2.0148(19)
Cu1–Cl1
Cu1···Cl2
2.6154(7)
4.1249(9)
Angle
Angle
Angle
N1–Cu1–N3
N1–Cu1–N7
N3–Cu1–N7
N1–Cu1–N5
N3–Cu1–N5
174.75(8)
88.97(8)
89.37(8)
90.12(7)
89.59(7)
N7–Cu1–N5
N1–Cu1–Cl1
N3–Cu1–Cl1
N7–Cu1–Cl1
N5–Cu1–Cl1
158.33(8)
92.31(6)
92.91(6)
103.58(6)
98.09(6)
N1-Cu1···Cl2
N3–Cu1···Cl2
N7–Cu1···Cl2
N5–Cu1···Cl2
Cl1–Cu1···Cl2
87.78(6)
87.03(6)
79.81(6)
78.52(6)
176.604(17)
Figure 3. DSC curve of 1 in a nitrogen atmosphere with a heating rate
of 10 K·min–1.
Figure 1. Molecular structure of 1, thermal ellipsoids drawn at 30%
probability level.
Figure 4. TG-DTG curve of 1 in a nitrogen atmosphere with a heating
rate of 10 K·min–1.
Figure 2. Pentahedral coordination around the copper atom in 1.
In the DSC curve of 1, a melting point peak (506.25 K) with
a corresponding endothermic stage can be seen in the range of
460–530 K. TG-DTG curves showed the first mass-loss stage
occurred very slowly (total mass loss 13.12%). The exother-
Thermal Decomposition Mechanism
In order to investigate the thermal behavior, compound 1 mic decomposition stages were seen in the DSC data, and
was analyzed by DSC and TG-DTG, with a linear heating rate peaks were observed at 555.05 K and 765.15 K. The TG-DTG
of 10 K·min–1 in a flowing nitrogen atmosphere (flow rate curves showed a mass loss of 70.61%, and reached the highest
20 mL·min–1). The data curves from these analyses are shown rate of mass loss at 519.88 K when the mass loss percentage
in Figure 3 and Figure 4.
has a value 14.21%·min–1 in the first stage of decomposition.
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The Intriguing Five-coordinated Compound [Cu(IMI)4Cl]Cl
Non-isothermal Kinetics Analysis
The energy of combustion is as follows (T = 298.15 K):
ΔH = Qp = Qv + ΔnRT = –7391.90 kJ·mol–1 = –18.17 MJ·kg–1
Kissinger’s method and Ozawa’s method, respectively, are
widely used to determine the Arrhenius equation for a given
material. The Kissinger Equation (1)[34] and Ozawa Equation
(2)[35] are as follows:
(4)
So the energy of combustion 1 was –18.17 MJ·kg–1, which
was higher than the energy of combustion of Cu(IMI)4(N3)2
(–11.15 MJ·kg–1).[16]
The metal coordination compound should have relatively
thermodynamically stable structure. The standard enthalpy of
formation was back calculated from the energy of combustion
on the basis of Equation (3), and Hess’s Law as applied in
thermochemical Equation (5). With the known enthalpies
2
ln β / Tp = ln [RA / Ea] – Ea / (RTp)
(1)
(2)
lg β = lg [AEa / RG(α)] – 2.315 – 0.4567Ea / RTp
Tp is the peak temperature (K), at which the exothermic peak
occurs in the DSC curve. A is the pre-exponential factor (s–1).
Ea 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
(K·min–1), and G(α) is the reaction mechanism function.
Based on the first and second exothermic peak temperatures
measured with four different heating rates of 5, 10, 15, and
20 K·min–1, Kissinger’s method and Ozawa’s method were ap-
plied to study the kinetics parameters of the title compound.
From the original data, the apparent activation energy Ek and
Eo, the pre-exponential factor Ak, and the linear coefficients Rk
and Ro were determined and are showed in Table 2. Accord-
ingly, the Arrhenius Equation of the different thermal decom-
position reaction stages can be expressed as follows (E is the
average of Ek and Eo):
of formation of copper oxide [ΔfH°298 (CuO, s)
=
=
–155.2 kJ·mol–1], carbon dioxide [ΔfH°298 (CO2, g)
–393.5 kJ·mol–1], water [ΔfH°298 (H2O, l) = –285.8 kJ·mol–1],
and hydrogen chloride [ΔfH°298 (HCl, g) = –92.3 kJ·mol–1],
the enthalpy of formation of 1 can now be calculated as:
ΔfH° (1, s) = ΔfH° (CuO, s) + 7ΔfH° (H2O, l) +
12ΔfH° (CO2, g) + 2ΔfH° (HCl, g) – ΔcH° (s) (5)
ΔfH°298 (1, s) = +323.54 kJ·mol–1 = +0.41 MJ·kg–1.
lnk1 = 3.145–60.865ϫ103 / (RT) and lnk2 = 2.743–
80.755ϫ103 / (RT)
Calculation of Critical Temperature of Thermal Explosion,
ΔS϶, ΔH϶, and ΔG϶
The value of the peak temperature corresponding to βǞ0
obtained according to the following Equation (6)[36] is
428.15 K, where a, b, c, and d are coefficients:
Table 2. Peak temperatures of the first exotherm at different heating
rates and kinetic parameters.
1
Tpi = Tp0 +aβ +bβ2 + cβ3
(6)
Heating rates β /K·min–1
5
10
15
20
Peak temperatures Tp /K
519.55
555.05
561.15
564.35
718.45
The corresponding critical temperature of thermal explosion
(Tb) obtained is 447.69 K by the following Equation (7):[36]
765.15
767.15
791.85
Kissinger’s method
Ek /kJ·mol–1
(7)
58.02
3.145
76.70
2.743
ln (Ak/s–1)
Linear correlation coefficient (Rk)
Standard deviation
–0.9348
0.2301
–0.9569
0.1859
The entropy of activation (ΔS϶), enthalpy of activation
(ΔH϶), and free energy of activation (ΔG϶) of the decomposi-
tion reaction of 1 corresponding to T = Tp0 and A = AK (ob-
tained by Kissinger’s method), obtained by the following
Equation (8), Equation (9), and Equation (10)[36] are
–230.09 kJ·mol–1, 57.31 J·K–1·mol–1, and 155.82 kJ·mol–1,
respectively.
Ozawa’s method
Eo /kJ·mol–1
63.71
–0.9496
0.1002
84.81
–0.9675
0.0808
Linear correlation coefficient (Ro)
Standard deviation
(8)
Energy of Combustion and Enthalpy of Formation
In order to study the energy of combustion and the enthalpy
of formation of 1, constant volume energy of combustion (Qv)
was measured with an oxygen bomb calorimeter and showed
a value of –18.16 MJ·kg–1.
ΔH϶ = E – RT
(9)
ΔG϶ = ΔH϶ – TΔS϶
(10)
The bomb equation is as follows:
where kB is the Boltzmann constant (1.381ϫ10–23 J·K–1) and
CuC12H16N8Cl2 + 16O2 Ǟ CuO + 7H2O(l) +
12CO2 + 4N2 + 2HCl(g) (3) h is the Planck constant (6.626ϫ10–34 J·s).
Z. Anorg. Allg. Chem. 2013, 799–803
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801

T.-L. Zhang et al.
ARTICLE
charged into a glass reactor with a water bath. It was kept under me-
chanical stirring and heated to the temperature of ca. 60–70 °C. Imid-
azole (40 mmol) was dissolved in distilled water (20 mL), and sub-
sequently it was added to the CuII aqueous solution during 25–30 min
with continuous stirring. Finally the solution was cooled to room tem-
perature naturally. IR (KBr): ν˜ = 2948 (N–H), 1604 (C=N), 1531
(C=C), 1328 (C–N), 1255 (C–N), 1063 (C–N), 772 (C–H) cm–1.
CuC12H16N8Cl2: calcd. C 35.43; H 3.96; N 27.55%; found: C 35.48;
H 3.42; N 27.47%.
Physicochemical Properties
The physicochemical properties of 1 are tabulated in
Table 3.
Table 3. Physicochemical properties of 1.
1
a)
Td /K
555.05, 765.15
27.55
–125.9
60.865, 80.755
–7391.90
+323.54
428.15
447.69
–230.09
57.31
155.82
Ͼ12.25
b)
N
Ω
E
/%
/%
c)
d)
/kJ·mol–1
X-ray Data Collection and Structures Refinement: A crystal was
chosen for X-ray determination. The X-ray diffraction data collection
were 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-97[37] (Sheldrick, 1990) and refined by full-matrix least-
squares methods on F2 with SHELXL-97[38] (Sheldrick, 1997). 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 4.
ΔH° /kJ·mol–1
e)
ΔfH°298 /kJ·mol–1
f)
Tp0 /K
Tb /K
ΔS϶ /kJ·mol–1
ΔH϶ /J·K–1·mol–1
ΔG϶ /kJ·mol–1
Impact sensitivity /J
a) Thermal degradation/DSC main exothermic peak. b) Nitrogen con-
tent. c) Oxygen balance. d) Activation energy. e) Experimental energy
of combustion. f) Molar enthalpy of formation.
Table 4. Crystal data and structure refinement for 1.
[Cu(IMI)4Cl]Cl (1)
Conclusions
Empirical formula
Formula mass
Temperature /K
Color
Crystal dimensions /mm
Crystal system
Space group
Z
CuC12H16N8Cl2
406.77
293(2)
The novel copper(II) imidazole chloride compound,
[Cu(IMI)4Cl]Cl (1), was synthesized and characterized. The
surprise is that the CuII ion was intriguing five-coordinated
in a distorted pentahedron arrangement with four imidazole
molecules and a coordination chlorine atom. In 1, thermal
analysis indicated that there were one endothermic process and
two main exothermic processes, with peaks at 506.25 K,
555.05 K, and 765.15 K, respectively, as shown in the DSC
blue
0.40ϫ0.38ϫ0.16
monoclinic
P21/n
4
a /Å
b /Å
8.847(2)
13.210(3)
13.870(3)
90.164(3)
–12 Յ h Յ 9
–17 Յ k Յ 18
–17 Յ l Յ 18
1621.0(6)
1.667
1.688
828
2.77~29.00
0.7681, 0.5516
11488
curve corresponding to the TG-DTG curves. Non-isothermal c /Å
β /°
hkl-ranges
kinetics analysis results indicated that the Arrhenius Equation
of 1 can be expressed as follows: lnk1 = 3.145–60.865ϫ103/
(RT) (the first exothermic process) and lnk2 = 2.743–
80.755ϫ103/(RT) (the second exothermic process). The ex-
periment found the energy of combustion and the enthalpy of
formation of 1 are –18.17 MJ·kg–1 and +323.54 kJ·mol–1
(+0.41 MJ·kg–1), respectively. It is more important, 1 was re-
garded as the less toxicity energy additive which will be ap-
Unit cell dimensions V /Å3
Dc /g·cm–3
μ (Mo-Kα) /mm–1
F(000)
θ Range /°
Max. and min. transmission
plied to improve the explosion performance of the traditional Measured reflections
Unique data
4261 (Rint = 0.0348)
explosives and propellant formulations.
a)
R1, wR2 [I Ͼ 2σ(I)]
R1, wR2 (all data)
Goodness of fit
0.0380, 0.0936
0.0433, 0.0976
a)
0.999
0.537, –0.575
Experimental Section
δpmax, δpmin /e·Å–3
Materials and Physical Techniques: All the reagents and solvents
were of analytical grade and used without further purification as com-
mercially obtained.
[∑w(Fo –Fc ) /∑w(Fo )]1/2
,
P
=
(Fo +2Fc )/3;
w
=
2
2 2
2
2
2
a) wR2
=
1/[σ2(Fo )+(0.0485P)2+1.3960P].
2
Elemental analysis was 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 and TG measurements were
carried out with a Pyris-1 differential scanning calorimeter and a Pyris-
1 thermogravimetric analyzer (Perkin-Elmer, USA). The combustion
heat was measured by oxygen bomb calorimetry (Parr 6200, USA).
Crystallographic data (excluding structure factors) for the structure in
this paper have been deposited with the Cambridge Crystallographic
Data Centre, CCDC, 12 Union Road, Cambridge CB21EZ, UK. Copies
of the data can be obtained free of charge on quoting the depository
number
CCDC-910441
(Fax:
+44-1223-336-033;
E-Mail:
deposit@ccdc.cam.ac.uk, http://www.ccdc.cam.ac.uk).
Synthesis of [Cu(IMI)4(Cl)]Cl (1): The synthesis of 1 was as follows:
CuCl2·2H2O (10 mmol) was dissolved in distilled water (30 mL), and
Supporting Information (see footnote on the first page of this article):
Hydrogen bond lengths /Å and bond angles /° for 1.
802
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Z. Anorg. Allg. Chem. 2013, 799–803

The Intriguing Five-coordinated Compound [Cu(IMI)4Cl]Cl
[19] Q. Ma, M. Zhu, L. Lu, S. Feng, J. Yan, Inorg. Chim. Acta 2011,
370, 102.
Acknowledgements
[20] C. Qi, S. H. Li, Y. C. Li, Y. Wang, X. K. Chen, S. P. Pang, J.
Mater. Chem. 2011, 21, 3221.
[21] V. Thottempudi, H. Gao, J. M. Shreeve, J. Am. Chem. Soc. 2011,
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[22] T. M. Klapötke, J. Stierstorfer, B. Weber, Inorg. Chim. Acta 2009,
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The projects were supported by Science and Technology on Applied
Physical Chemistry Laboratory (Nos. 9140C3703051105 and
9140C370303120C37142); Key Support Foundation of State Key Lab-
oratory of Explosion Science and Technology (Nos. QNKT12–02,
YBKT 10–05 and ZDKT10–01b).
[23] Y. Guo, G. H. Tao, Z. Zeng, H. Gao, D. A. Parrish, J. M. Shreeve,
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Received: November 27, 2012
Published Online: February 18, 2013
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Z. Anorg. Allg. Chem. 2013, 799–803
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