Preparation, crystal structures, thermal decomposition and explosive properties of two novel energetic compounds M(IMI)4(N 3)2 (M = CuII and NiII, IMI = imidazole): The new high-nitrogen materials (N > 46%)

Article abstract of DOI:10.1002/ejic.201100054

Two novel multiligand coordination complexes of imidazole-copper(II) azide and imidazole-nickel(II) azide, Cu(IMI)₄(N₃)₂ and Ni(IMI)₄(N₃)₂, were synthesized and characterized by elemental analysis and FTIR spectroscopy. The crystal structures were determined by single-crystal X-ray diffraction. The results show that the crystals of Cu(IMI)₄(N₃)₂ and Ni(IMI)₄(N₃)₂ crystallize in the orthorhombic Pna2₁ and monoclinic P2₁/n space groups, respectively. The metal cations are six-coordinate, and are bound to two azido ligands by μ₁-azido bridges and to four imidazole molecules through nitrogen atoms. Under nitrogen, with a heating rate of 5 Kmin^-1,these compounds go through two main exothermic stagesbetween 420 K and 750 K during their thermal decompostion, as seen in the differential scanning calorimetry (DSC) and thermogravimetry-differential thermogravimetry (TG-DTG) curves. The nonisothermal kinetic parameters were calculated by the Kissinger and Ozawa methods. The sensitivity properties of M(IMI)₄(N₃) ₂ were also determined by standard methods, and the results showed that Cu(IMI)₄(N₃)₂ had high impact and friction sensitivities and low flame sensitivity. Ni(IMI)₄(N ₃)₂ had low sensitivity to external stimuli, but it had a very high energy of combustion. The results of all the studies showed that these M(IMI)₄(N₃)₂ (M = Cu^II and Ni ^II) compounds have potential application as energetic materials. Cu(IMI)₄(N₃)₂ (IMI = imidazole) crystallizes in the orthorhombic space group Pna2₁. The metal cations are six-coordinate, and are linked to two azido ligands by μ₁-azido bridges and to four imidazole molecules through nitrogen atoms. In addition, this compound has high impact and friction sensitivity and therefore has potential application as an energetic material.

Full text of DOI:10.1002/ejic.201100054

FULL PAPER
DOI: 10.1002/ejic.201100054
Preparation, Crystal Structures, Thermal Decomposition and Explosive
II
Properties of Two Novel Energetic Compounds M(IMI) (N ) (M = Cu and
4
3 2
II
Ni , IMI = Imidazole): The New High–Nitrogen Materials (N Ͼ 46%)
[a]
[a]
[a]
[a]
[a]
Bi-dong Wu, Shi-wei Wang, Li Yang,* Tong-lai Zhang, Jian-guo Zhang,
[a]
[a]
Zun-ning Zhou, and Kai-bei Yu
Keywords: Copper / Nickel / Azides / Imidazole / Decomposition / Sensitivity
Two novel multiligand coordination complexes of imidazole–
copper(II) azide and imidazole–nickel(II) azide, Cu(IMI)
and Ni(IMI) (N , were synthesized and characterized
postion, as seen in the differential scanning calorimetry
(DSC) and thermogravimetry–differential thermogravimetry
(TG-DTG) curves. The nonisothermal kinetic parameters
were calculated by the Kissinger and Ozawa methods. The
4
-
(
N
3
)
2
4
3 2
)
by elemental analysis and FTIR spectroscopy. The crystal
structures were determined by single-crystal X-ray diffrac-
sensitivity properties of M(IMI)
by standard methods, and the results showed that Cu(IMI)
and mo- (N had high impact and friction sensitivities and low flame
/n space groups, respectively. The metal cations sensitivity. Ni(IMI) (N had low sensitivity to external stim-
are six-coordinate, and are bound to two azido ligands by μ uli, but it had a very high energy of combustion. The results
azido bridges and to four imidazole molecules through nitro-
4 3 2
(N ) were also determined
tion. The results show that the crystals of Cu(IMI)
Ni(IMI) (N crystallize in the orthorhombic Pna2
noclinic P2
4
(N
3
)
2
and
4
-
4
3
)
2
1
3 2
)
1
4
3 2
)
1
-
II
of all the studies showed that these M(IMI)
4
3 2
(N ) (M = Cu
–
1
II
gen atoms. Under nitrogen, with a heating rate of 5 Kmin , and Ni ) compounds have potential application as energetic
these compounds go through two main exothermic stages
between 420 K and 750 K during their thermal decom-
materials.
Introduction
their π-electron configuration. Moreover, the role of the
imidazole ring as a ligand is well known, and it has two
potential N coordination sites that enable it to form ex-
tended structures, and its role as a ligand in a variety of
coordination compounds has been well documented.
Many IMI derivatives have been studied in recent years
Energetic materials chemists have been interested in envi-
ronmentally compatible green energetic materials, and this
research field has proven to be fertile. In order to meet the
requirements of future civil and military applications it is
imperative to develop new energetic materials that have
good thermal stability, mechanical insensitivity, good per-
[24–43]
[24]
[25]
such as 2-azidoimidazole, imidazole-4-acetic acid, 1,4-
[26]
[27]
dinitroimidazole,
azole,
imidazole (MTNI),
2,4-dinitroimidazole,
2,4,5-trinitroimidazole,
4,5-nitroimid-
1-methyl-2,4,5-trinitro-
formance, with economic and environmentally friendly syn-
[28]
[29]
thesis processes.[
1,2]
Therefore, a number of novel high-en-
[30]
[31]
2,4,5-tri(4-pyridyl)imidazole,
ergy-density materials (HEDM) have been synthesized in
recent years, and they have been suggested as suitable can-
didates for a range of applications including explosives, py-
rotechnics, propellant formulations, and so forth.
Five-membered nitrogen-rich heterocyclic compounds
are traditionally sources of energetic materials.[3–23] Imid-
azole (IMI) is a pentacyclic heterocyclic compound contain-
ing two nitrogen atoms and three carbon atoms and has a
relatively small volume that may reduce steric hindrance
and increase the material density. The functional group is –
N=C–C=N–, and the key feature of these heterocycles is
[32]
1
1
,2,4,5-tetranitroimidazole,
4,4Ј,5,5Ј-tetranitro-2,2Ј-bi-
[34–35]
[36–43]
H-imidazole (TNBI)
and so on.
IMI and IMI
derivative complexes are widely used in energetic materials,
agriculture, medicine and other fields. In particular, studies
of the coordination properties of IMI and its derivatives are
[44]
of interest. For example, Tufan et al. have reported four
new clathrates with the general formula M(IMI) Ni(CN) ·
2
4
II
II
II
II
2
C H (M = Mn , Co , Ni and Cd ) and characterized
6 12
them by IR and Raman spectroscopy. Their spectral results
suggested that these compounds belong to the Hofmann
[45]
type of clathrates. In 2009, Yazicilar et al. reported four
novel transition metal benzenesulfonate (BS) complexes of
[
a] State Key Laboratory of Explosion Science and Technology,
imidazole with general formulae [M(IMI) (H O) ]·(BS) [M
3
2
3
2
Beijing Institute of Technology,
II
II
II
Beijing 100081, China
= Mn , Ni and Co ] and [Cu(BS)(IMI) ]·(BS). The results
3
Fax: +86-10-68-911-202
showed that these complexes exhibit different decomposi-
tion characteristics. Magnetic susceptibility measurements
E-mail: yanglibit@bit.edu.cn
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201100054.
II
showed that the Co complex had orbital interactions. In
2616
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2011, 2616–2623

4 3 2
Energetic Compounds M(IMI) (N )
(M = CuII and Ni , IMI = Imidazole)
II
Figure 1. Different azido ligand coordination modes.
2
(
010, Tong et al.[46] reported a density functional theory analyses based on the optimized structure were carried out
DFT) study of [Mo O (IMI) ]. The introduction of the by B3LYP functional analyses with the 6-311++g(d,p) basis
2
6
4
IMI ligand into this complex resulted in a significant weak- set. The optimized structure was characterized to be associ-
ening of the important Mo–ligand interactions. ated with a true local energy minimum on the potential en-
Researchers have also paid intense attention to the azido ergy surface without an imaginary frequency component.
ligand due to its various coordination modes[
47–51]
(Fig- The highest occupied molecular orbitals (HOMOs) and
ure 1). The various intriguing structures of azido–metal lowest unoccupied molecule orbitals (LUMOs) of the IMI
complexes, ranging from discrete structures to three-dimen- group are shown in Figure 2. In the HOMO, the C=N
sional arrays, have promoted the development of coordina- double bond of the azo group may form a large π-orbital.
tion chemistry. Moreover, the azido ion has been studied The delocalized bonding orbitals show that the IMI group
due to its explosive nature, its ability to form pure and has a strongly conjugated system.
mixed-ligand complexes, its ambidentate nature and bridg-
ing capacity, and so on. In particular, azide coordination
compounds have moderate thermal stability. Consequently,
many scientists have performed considerable research on
energetic azide coordination compounds. This can be dem-
[
52]
onstrated by consideration of some examples: Patil et al.
[
53]
and Narang et al. have systemically studied the synthesis
of hydrazine–transition-metal azides by different methods
that were developed between 1980 and 2000. At the begin-
ning of the 21st century, Bose et al. reported the synthe-
sis, characterization, and single-crystal XRD analyses of
[54]
[
Zn(dpa)(N ) ] and [Zn(dpa)(N )(NO )] (dpa = 2,2Ј-dipyr-
3 2 3 3 2
Figure 2. HOMO (a) and LUMO (b) of the IMI group.
idylamine), and discussed the role of dpa in crystal engi-
neering and in the luminescence behavior of these com-
pounds. Das et al.
The peaks in the calculated IR data can be assigned to
four kinds of stretching and bending modes: symmetric and
[
55]
II
have studied two new series of Zn
–
complexes containing imidazole derivatives and N groups. asymmetric stretching (υ , υas), in-plane bending (δ), out-of-
3
s
[
56]
Zhu et al. have reported the optimization of the synthetic plane bending (γ), and in-plane rocking (ω). The asymmet-
method for, and the explosive properties of, hydrazine– ric N–C=N stretching mode of the ring and the C–N(=N)
nickel(II) azide. They testified that this material is a poten- stretching mode of the azo group were observed. In the
tially powerful primary explosive that possesses high flame compounds, the νas(N–H) and ν
sensitivity. Our research group have studied the synthesis, IMI group were shifted to lower wavenumbers than in the
(N–H) frequencies of the
s
–
thermal decomposition, and explosive properties of ener- IMI spectrum because of the coordination effect of N
3
[57]
[58]
getic complexes [Cd (N H ) (N ) ] ,
Mn(CHZ) (N )
with the central metal ion.
2
59]
2
4 2
3 4 n
2
3 2
[
and [Cd(en)(N ) ] ,
and have found that [Cd(en)(N3)2]n
has good impact, friction, and flame sensitivities, and that
Cd (N H ) (N ) ] and Mn(CHZ) (N ) have good impact
3
2 n
Molecule Structure
[
2
2
4 2
3 4 n
2
3 2
sensitivity. Such azido compounds have potential applica-
tion as energetic materials.
The structures of the compounds are shown in Figure 3,
and selected bond lengths and angles of these compounds
are listed in Tables 1 and 2.
In order to further the studies of imidazole–metal azide
complexes, in this contribution we report two novel multili-
gand coordination complexes, Cu(IMI) (N ) and Ni(IMI) -
From Figure 2, it can be seen that the electronic density
of the HOMO at the N1 atom is relatively high. The forma-
tion of the coordination bond between the IMI N atom and
4
3 2
4
(
N ) , which have been synthesized and their crystal struc-
3 2
tures, the thermal decomposition mechanisms, and sensitiv-
ity properties evaluated.
II
II
the Cu or Ni ions is easy to understand, because the
metal ions have unoccupied 3d orbitals. In the Cu(IMI)4-
(
N ) and Ni(IMI) (N ) molecules, there are single cop-
3 2 4 3 2
Results and Discussion
per(II) and nickel(II) cations, respectively, and four imid-
azole molecules and two azido groups. The metal(II) cat-
ions contribute six empty orbitals to accommodate the
Vibrational Spectroscopy
To gain a good understanding of the properties of the lone-pair electrons of the ligands, that is to coordinate with
IMI group, molecular-orbital and IR absorption-frequency the six N atoms of the four imidazole molecules and two
Eur. J. Inorg. Chem. 2011, 2616–2623
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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2617

L. Yang et al.
FULL PAPER
4 3 2 4 3 2 4
Figure 3. Molecular structures of M(IMI) (N ) . (a) Cu(IMI) (N ) , thermal ellipsoids drawn at 30% probability level; (b) Ni(IMI) -
(N ) , thermal ellipsoids drawn at 10% probability level.
3 2
Table 1. Selected bond lengths [nm] and bond angles [°] for Cu(IMI)
4
(N ) .
3 2
Bond
[nm]
Bond
[nm]
Bond
[nm]
Cu1–N1
Cu1–N3
Cu1–N5
Cu1–N7
Cu1–N9
0.2017(3)
0.2017(3)
0.2015(2)
0.2010(2)
0.2459(3)
Cu1–N12
N1–C3
N1–C1
N2–C3
N2–C2
0.2846(4)
0.1317(5)
0.1381(5)
0.1333(5)
0.1376(5)
N2–H2
0.094(5)
N9–N10
N10–N11
N12–N13
N13–N14
0.1161(4)
0.1194(4)
0.1174(4)
0.1185(4)
Angle
[°]
Angle
[°]
Angle
[°]
N1–Cu1–N3
N7–Cu1–N5
N9–Cu1–N12
N5–Cu1–N1
N7–Cu1–N1
N5–Cu1–N3
N7–Cu1–N3
176.20(16)
176.47(16)
177.78(10)
89.45(13)
89.72(14)
89.23(14)
91.37(14)
N1–Cu1–N9
N3–Cu1–N9
N5–Cu1–N9
N7–Cu1–N9
N1–Cu1–N12
N3–Cu1–N12
90.80(13)
92.82(10)
92.65(13)
90.79(13)
89.91(11)
86.43(12)
N5–Cu1–N12
N7–Cu1–N12
N10–N9–Cu1
N9–N10–N11
N13–N12–Cu1
N12–N13–N14
85.25(13)
91.31(12)
130.5(3)
177.8(4)
125.0(3)
178.7(4)
Table 2. Selected bond lengths [nm] and bond angles [°] for Ni(IMI)
4
(N
3
)
2
.
Bond
[nm]
Bond
[nm]
Bond
[nm]
Ni1–N1
Ni1–N1A
Ni1–N3
Ni1–N3A
Ni1–N5
0.2092(2)
0.2092(2)
0.2121(2)
0.2121(2)
0.2129(2)
Ni1–N5A
N5–N6
N6–N7
N1–C1
N1–C3
0.2129(2)
0.1178(2)
0.1161(2)
0.1365(2)
0.1318(2)
N2–C2
N2–C3
C1–C2
N2–H2
0.1352(3)
0.1331(3)
0.1346(3)
0.083(2)
Angle
[°]
Angle
[°]
Angle
[°]
N1A–Ni1–N1
N3A–Ni1–N3
N5A–Ni1–N5
N5–N6–N7
180.00(8)
180.00
180
N6–N5–Ni1
N1–Ni1–N3
N1–Ni1–N3A
N1–Ni1–N5
121.39(13)
91.17(6)
88.83(6)
87.91(6)
N1–Ni1–N5A
N3–Ni1–N5
N3A–Ni1–N5
92.09(6)
90.41(6)
89.59(6)
178.0(2)
azido groups. The central metal(II) cations are coordinated
In the Cu(IMI) (N ) molecule, the axial Cu–N bonds
4 3 2
to two azido groups that act as μ -bridging ligands [Cu–N are ca. 0.04–0.08 nm longer than the four equivalent equa-
1
bond lengths are 0.2459(3) and 0.2846(4) nm, and the Ni– torial Cu–N bonds. This distorted lengthening of the bonds
II
N bond lengths are 0.2092(2) 0.2121(2) nm], and to four N incorporating the Cu center can be attributed to the Jahn–
atoms from imidazole ligands. Conforming to the general Teller effect. The bond angles involving atoms N1 (N5) and
characteristics of coordinated azides, the μ -azido groups N3 (N7) from two contraposition imidazole ligands and the
1
are nearly linear and asymmetric with ϽN9–N10–N11 copper(II) cation are ϽN1–Cu1–N3 176.20(16)° and ϽN5–
177.8(4)°, N9–N10 0.1161(4) nm, N10–N11 0.1194(4) nm, Cu1–N7 176.47(16)°. The bond angles involving nitrogen
ϽN12–N13–N14 178.7(4)°, N12–N13 0.1174(4) nm, N13– atoms from two adjacent imidazole ligands and the cop-
N14 0.1185(4) nm in the Cu(IMI) (N ) molecule, and per(II) cation are ϽN1–Cu1–N5 89.45(13)°, ϽN1–Cu1–N7
4
3 2
ϽN5–N6–N7 178.0(2)°, N5–N6 0.1178(2) nm, N4–N5 89.72(14)°, ϽN7–Cu1–N9 90.79(13)° and ϽN5–Cu1–N12
.1161(2) nm for the Ni(IMI) (N ) molecule. 85.25(13)°, all of which deviate from 90°. The distances be-
0
4
3 2
2618
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© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2011, 2616–2623

4 3 2
Energetic Compounds M(IMI) (N )
(M = CuII and Ni , IMI = Imidazole)
II
tween the copper(II) cation and the nitrogen atoms of the
The angles between planes F and G, planes F and H,
imidazole ligands are approximately equal [the Cu–N bond and planes G and H are 98.6°, 31.3° and 97.9°, respectively,
lengths are between 0.2010(2) and 0.2017(3) nm]. Therefore, which all deviate from 90°. However, the face-to-face imid-
the four nitrogen atoms of the imidazole ligands and the cop- azole rings are coplanar. The distance between N5 from an
per(II) cation exhibit a slightly distorted square configura- azido group and plane F is 0.2129(2) nm. All the above data
II
tion; the equation for the plane of these atoms (denoted demonstrate that the Ni cation exhibits a slightly distorted
plane A) is –0.983x – 7.099y – 8.663z = –0.3851. The dis- octahedral configuration.
tances between atoms N9, N12 from two azido groups and
Weak N–H···N hydrogen bonds between the azido
plane A are 0.2453 and 0.2831 nm, respectively. All the above groups and the imidazole ligands occur between the mole-
II
data demonstrate that the Cu cation exhibits a slightly dis- cules in the solid state, and it can be seen from the crystal
torted octahedral configuration. The equations for the four packing diagrams of these complexes (see Supporting Infor-
planes formed by the imidazole rings are as follows:
mation) that all of these intermolecular hydrogen bonds ex-
tend the structures to give 3D supermolecular structures,
and make an important contributions towards enhancing
the thermal stability of the complexes.
N1–C1–N2–C2–C3 (plane B): 13.894x + 0.001y – 1.175z = 5.0225;
N3–C4–N4–C5–C6 (plane C): 13.687x – 1.196y + 1.917z = 5.5089;
N5–C7–N6–C8–C9 (plane D): 0.019x + 8.850y + 5.912z = 6.8743;
N7–C10–N8–C11–C12 (plane E): 0.784x + 6.414y + 9.427z =
Thermal Decomposition
7.4631.
The angles between planes A and B, planes A and D,
and planes B and D are 97.9°, 105.8° and 92.6°, respec- compounds they were analyzed by DSC and TG-DTG, with
tively, which all deviate from 90°. Therefore, planes B and a linear heating rate of 5 Kmin in a flowing N atmo-
In order to investigate the thermal behaviors of the title
–1
2
–1
C as well as planes D and E are not superposed, and the sphere (flow rate 20 mLmin ). The data curves from these
angles between them are 16.1° and 21.8°, respectively.
analyses are shown in Figures 4 and 5. There are two main
In the Ni(IMI) (N ) molecule, the bond angles involving exothermic stages observed between 420 K and 750 K in
4
3 2
atoms N1 (N3) and N1A (N3A) from two contraposition the DSC curves. In the TG-DTG curves there are two main
imidazole ligands and the nickel(II) cation are both 180.00°
[
ϽN1–Ni1–N1A 180.00(8)°, ϽN3–Ni1–N3A 180.00°]. The
bond angles involving nitrogen atoms from two adjacent
imidazole ligands and the nickel(II) cation are ϽN1–Ni1–
N3 91.17(6)° and ϽN1–Ni1–N3A 88.83(6)°, both of which
deviate from 90°. The distances between the nickel(II) cat-
ion and the nitrogen atoms of the imidazole ligands are
all equal [Ni–N 0.2129(2) nm]. Therefore, the four nitrogen
atoms of the imidazole ligand and the nickel(II) cation exhi-
bit a slightly distorted quadrilateral configuration. The six
coordination atoms and the central nickel(II) cation form
three planes with the following plane equations:
Ni1–N1–N3–N1A (plane F): –2.011x – 0.057y + 10.754z = 4.3432;
N1–C1–N2–C2–C3 (plane G): 8.106x + 3.956y – 2.885z = 4.5896;
N3–C4–N4–C5–C6 (plane H): –3.492x + 4.929y + 9.524z = 5.4744.
Figure 4. DSC curves of M(IMI)
sphere and with a heating rate of 5 Kmin .
4
(N
3
)
2
recorded in an N
2
atmo-
–
1
–
1
Figure 5. TG-DTG curves of M(IMI)
Ni(IMI) (N
4
(N
3
)
2
recorded in an N
2
atmosphere and with a heating rate of 5 Kmin . (a) Cu(IMI)
4
3 2
(N ) ; (b)
4
3 2
) .
Eur. J. Inorg. Chem. 2011, 2616–2623
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
2619

L. Yang et al.
FULL PAPER
successive mass losses that correspond to the successive the energy of combustion of Ni(IMI) (N ) is higher than
4
3 2
exothermic stages seen in the DSC curves.
that of RDX, HMX and TNT.
These two metal coordination complexes should be ther-
For the Cu(IMI) (N ) compound the melting point was
4
3 2
not observed in the DSC curve. The exothermic stages were modynamically relatively stable structures. The standard en-
slow decomposition stages, and peaks were observed at thalpies of formation of the M(IMI) (N ) complexes were
4
3 2
463 K and 671 K. The TG-DTG curves showed a mass loss back-calculated from the energies of combustion on the ba-
of 50.67% in the first stage of decomposition that occurred sis of Equations (1) and (2), and with Hess’s Law as given
over this temperature range, and reached the highest rate of in Equation (5). With the known enthalpies of formation of
–
1
mass loss at 455 K when the mass loss percentage equalled copper oxide {Δ H° [CuO(s)] = –155.2 kJmol }, nickel
f
298
–
1
–1
1
5.14%min . The second mass-loss stage occurred very oxide {Δ H° [NiO(s)] = –224.3 kJmol }, carbon dioxide
f 298
–1
slowly, and the total mass loss was 23.48%.
{Δ H° [CO (g)]
=
393.5 kJmol }
and
water
f
298
2
–1
In the DSC curve of Ni(IMI) (N ) a melting point peak {Δ H° [H O(l)] = –285.8 kJmol }, the enthalpies of for-
4
3
f
298
2
with a corresponding endothermic stage can be seen in the mation of Cu(IMI) (N ) and Ni(IMI) (N ) can be calcu-
4
3 2
4
3 2
420–450 K range. An exothermic slow decomposition stage lated:
is seen in the DCS data between 550 K and 565 K. The TG-
DTG curve for this complex shows a mass loss of 60.68%
in the first stage of decomposition over the 420–450 K tem-
perature range, and reached the greatest rate of loss at
Δ
f
H°{[M(IMI)
4
(N
3
)
2
](s)}
H°{[M(IMI)
=
Δ
f
H°[MO(s)]
(N ](s)}
+
8Δ
f
H°[H
2
O(l)]
+
(5)
12Δ H°[CO (g)] – Δ
f
2
c
4
3
)
2
–
1
Δ
f
f
H°298{[Cu(IMI)
4
(N
3
)
2
](s)} = –2402.6 kJmol ;
–1
4
51 K with a mass-loss percentage of 6.82%min . The
main successive exothermic stages between 640 K and
90 K correspond to a decomposition stage. The corre-
Δ
H°298{[Ni(IMI)
4
(N
3
)
2
](s)} = –4731.2 kJmol–1
.
6
Non-Isothermal Kinetics Analysis
sponding TG-DTG curves show that the second mass loss
was 36.42%, and reached the greatest rate of loss at 689 K
with mass a loss percentage of 8.20%.
Kissinger’s[61] and Ozawa’s methods[62] are widely used to
determine the Arrhenius equation for a given material. The
Kissinger [Equation (6)] and Ozawa equations [Equa-
tion (7)] are as follows:
In order to study the energies of combustion and the
enthalpies of formation of the title compounds, constant-
volume energies of combustion (Q ) for Cu(IMI) (N ) and
v
4
3 2
Ni(IMI) (N ) were measured by oxygen bomb calorimetry,
4
3 2
–
1
and were determined to be 11.09 MJkg and 16.71
MJkg , respectively. The bomb Equations (1) and (2) are
as follows:
–
1
(6)
CuC12
H
16
N14 + 16.5O
2
Ǟ CuO + 8H
2
O + 12CO
2
+ 7N
2
(1)
(
7)
NiC12
H
16
N14 + 16.5O
2
Ǟ NiO + 8H
2
O + 12CO
2
+ 7N
2
(2)
T is the temperature [K] at which the exothermic peak
The energies of combustion are given by Equations (3)
and (4):
p
occurs in the DSC curve; A is the pre-exponential factor
–
1
–1
[
s ]; E is the apparent activation energy [kJmol ]; R is the
ΔHCu(IMI)4(N3 = Q
p
=
Q
v
+
ΔnRT = 4683.05 kJmol–1
=
–1
–1
)
2
gas constant (8.314 JK mol ); β is the linear heating rate
Kmin ]; and G(α) is the reaction-mechanism function.
1
1.15 MJkg–1
(3)
–1
[
Based on the temperatures at which the first exothermic
ΔHNi(IMI)4(N3)2
6.73 MJkg–1
=
Q
p
=
Q
v
+
ΔnRT
=
6942.52 kJmol–1
=
(4)
peaks occur in the DSC curves measured with four different
1
–1
heating rates (5, 10, 15 and 20 Kmin ), Kissinger’s and
The energies of combustion of cyclotrimethylenetrinitr- Ozawa’s methods were applied to study the kinetics param-
amine (RDX), cyclotetramethylenetetranitramine (HMX) eters of the title compounds. From these data, the apparent
and 2,4,6-trinitrotoluene (TNT) have been reported in the activation energies EK and EO (E is the average value of
–
1
–1
literature and are 9.60 MJkg , 9.44–9.88 MJkg and EK and E ), pre-exponential factor A , linear correlation
O
K
–
1 [60]
1
5.22 MJkg , respectively. Therefore, the energy of com- coefficients R and R , and standard deviations S and S
K O K O
bustion of Cu(IMI) (N ) is lower than that of TNT, yet were determined and are listed in Table 3.
4
3 2
Table 3. Temperatures of the first main exothermic peaks in the DSC curves of the compounds recorded at different heating rates, and
chemical kinetics parameters for these complexes.
Cu(IMI)
[K] values recorded with heating rates 5, 10, 15, 20 Kmin–1 457.45, 462.75, 463.95, 465.75
Kissinger’s method
4
(N
3
)
2
4 3 2
Ni(IMI) (N )
T
p
637.35, 669.65, 690.05, 692.85
73.9, 3.31, –0.9819, 0.1219
80.8, –0.9861, 0.05316
–1
E
K
[kJmol ], lnA
k
, R
k
, S
k
286.1, 30.79, –0.9852, 0.1229
Ozawa’s method
E [kJmol ], R , S
O o o
–1
279.4, –0.986, 0.05338
2620
www.eurjic.org
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2011, 2616–2623

4 3 2
Energetic Compounds M(IMI) (N )
(M = CuII and Ni , IMI = Imidazole)
II
Accordingly, the Arrhenius equations for the M(IMI)4- gen-rich materials, with their nitrogen contents being higher
N ) complexes can be expressed as Equations (8) and (9): than 46%. Furthermore, these compounds have lower oxy-
(
3
2
gen balance values than TNT (Ω = –74.0%).
3
lnk = 30.79 – 282.75ϫ10 /(RT) for Cu(IMI)
4
(N
3
)
2
(8)
3
lnk = 3.31 – 77.35ϫ10 /(RT) for Ni(IMI)
4
(N
3
)
2
(9)
Conclusions
Two novel high-nitrogen and environmental friendly
energetic complexes of imidazole–copper(II) azide and
Sensitivity Tests
By applying the China National Military Standard, the imidazole–nickel(II) azide, Cu(IMI)
(N ) and Ni(IMI) -
4
3 2 4
impact, friction, and flame sensitivities of the complexes (N , were synthesized and characterized. The crystal
)
3 2
[
63]
II
II
were determined.
The impact sensitivities of the complexes were deter- plexes are six-coordinate, have slightly distorted octahedral
mined with a fall-hammer apparatus. A sample of M(IMI)4- geometries, and are bound to two azido ligands in μ -modes
N ) (30 mg) was placed between two steel poles and hit and four imidazole molecules. The results from the thermal
structures show that the Cu and Ni ions in these com-
1
(
3
2
with a 2.0 kg drop hammer from a starting height of 25 cm. analysis of these compounds indicated that they undergo
The test results showed that the firing rate was 100%, and two main exothermic stages, as seen in the DSC and corre-
the 50% firing height (h ) was 21.2 cm (4.16 J) for Cu(IMI) - sponding TG-DTG curves. The energies of combustion and
5
0
4
(
N ) . For Ni(IMI) (N ) , h = 74.8 cm (14.7 J).
the enthalpies of formation of Cu(IMI)
(N ) and Ni(IMI) -
4 3 2 4
3
2
4
3 2 50
Friction sensitivities were determined with an MGY-1 (N have been determined experimentally; the combustion
)
3 2
pendular friction sensitivity apparatus by a standard pro- energies for these copper and nickel complexes are
cedure with 20 mg of sample. When Cu(IMI) (N ) was 11.15 MJkg–1 and 16.73 MJkg , respectively, and their en-
–1
4
3 2
–
1
compressed between two steel poles with mirror surfaces at thalpies of formation are –2402.6 kJmol
a pressure of 1.23 MPa and then hit with a 1.5 kg hammer –4731.2 kJmol , respectively. Non-isothermal kinetics
at a 70° angle relative to the horizontal, the firing rate was analyses results indicated that the Arrhenius equation of
and
–1
1
00%. Conversely, when Ni(IMI) (N ) was compressed at Cu(IMI)
4
(N
a pressure of 3.92 MPa and hit at a 90° angle relative to the = 30.79 – 282.75ϫ10 /(RT) and lnk = 3.31 – 77.35ϫ10 /
horizontal, the firing rate was 48%.
(RT), respectively. The sensitivity measurement results for
Flame sensitivities were determined by a standard Cu(IMI) (N showed that the 50% firing heights for im-
method in which the sample was ignited by a standard pact and flame sensitivities were 21.2 and 13.94 cm, respec-
black powder pellet. Cu(IMI) (N ) and Ni(IMI) (N )
tively, and the friction sensitivity firing rate was 100% with
20 mg) were compacted into a copper cap under a pressure a 1.5 kg hammer, 70° angle, and 1.23 MPa pressure. There-
of 58.8 MPa before being ignited by a standard black pow- fore, Cu(IMI) (N has a good mechanical sensitivity and
der pellet. The test results showed that the 50% firing good combustion performance. Although Ni(IMI) (N
height (h ) of Cu(IMI) (N ) was 13.94 cm, and that there has a lower sensitivity than the corresponding copper com-
3 2 4 3 2
) and Ni(IMI) (N ) can be expressed as lnk
4
3 2
3
3
)
3 2
4
4
3 2
4
3 2
(
)
3 2
4
)
3 2
4
5
0
4
3 2
was no fire at a minimum height of 6 cm for Ni(IMI)4- plex, it has a high heat of combustion that is higher than
N ) .
those of RDX, HMX and TNT. In other words, Ni(IMI)
Therefore, compared to hydrazine–nickel(II) azide,[56] (N
can be regarded as an energy additive that can be
Cu(IMI) (N ) has a very high sensitivity to external stim- applied to improve the explosion performance of traditional
(
-
4
3
2
)
3 2
4
3 2
uli.
explosives and propellant formulations. In conclusion, the
results of these studies have shown that M(IMI) (N ) (M
4
3 2
II
II
=
Cu and Ni ) complexes have potential application as
Physicochemical Properties
energetic materials.
The physicochemical properties of the M(IMI) (N3)2
4
complexes are given in Table 4. These compounds are nitro-
Experimental Section
Table 4. Physicochemical properties of M(IMI)
4
(N
3
)
2
complexes.
(N
Caution: The title compounds are energetic materials and tend to
explode under certain conditions. Appropriate safety precautions
(safety glasses, face shield, leather coat and ear plugs) should be
taken, especially when these compounds are prepared on a large
scale and in dry state.
Cu(IMI)
4
(N
3
)
2
Ni(IMI)
4
3 2
)
[
a]
T
T
N [%]
m
[K]
407
637; 653
47.27
–127.20
77.35
6942.5
–4731.2
14.7
[
b]
d
[K]
457; 666
46.70
–125.74
282.75
4683.1
–2402.6
[
c]
[
d]
Ω [%]
E [kJmol ]
–
1 [e]
Materials and Physical Techniques: The copper sulfate pentahydrate
(CuSO ·5H O), nickel acetate hexahydrate [Ni(OAc) ·6H O] and
4 2 2 2
–1 [f]
ΔH° [kJmol ]
H°298 [kJmol ]
Impact sensitivity [J] 4.16
–1 [g]
Δ
f
imidazole reagents were all analytically pure commercial products.
The starting material sodium azide was a commercial product that
was purified by recrystallization prior to use. Elemental analyses
were performed with a Flash EA 1112 fully automatic trace ele-
ment analyzer. FTIR spectra were recorded with a Bruker Equinox
[
a] Melting point/DSC endothermic peak. [b] Thermal degradation/
DSC main exothermic peak. [c] Nitrogen content. [d] Oxygen bal-
ance. [e] Activation energy. [f] Experimental energy of combustion.
–
1
[g] Molar enthalpy of formation.
55 infrared spectrometer (KBr pellets) over the 4000–400 cm
Eur. J. Inorg. Chem. 2011, 2616–2623
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
2621

L. Yang et al.
FULL PAPER
4 3 2
Table 5. Crystal data and structure refinement details for M(IMI) (N ) complexes.
Cu(IMI)
4
(N
3
)
2
4 3 2
Ni(IMI) (N )
Empirical formula
Formula mass
Crystal dimensions [mm]
Crystal system
Space group
Z
C
419.93
0.33ϫ0.27ϫ0.07
orthorhombic
12
H
16CuN14
12 16
C H N14Ni
415.08
0.33ϫ0.23ϫ0.23
monoclinic
Pna2
1
1
P2 /n
4
2
a, b, c [nm]
β [°]
1.3958(3), 1.0110(2), 1.2228(3)
0.8777(3), 1.0464(3), 1.0847(4)
90
110.728(3)
h, k, l
Temperature [K]
Unit cell volume, V [nm ]
c
D [gcm ]
–13 to 18, –13 to 13, –15 to 12
93(2)
1.7255(7)
1.616
–10 to 11, –11 to 13, –14 to 14
295(2)
0.9317(5)
1.480
3
–3
λ [nm]
μ (Mo-K
F(000)
0.071073
1.299
860
0.071073
1.072
428
–
1
α
) [mm ]
θ range [°]
3.33–27.47
0.9146, 0.6716
13356
3.15–27.45
0.7884, 0.7167
7072
Max., min. transmission
Measured reflections
Unique data
3600 (Rint = 0.0206)
2090 (Rint = 0.0267)
[
[
a]
a]
[b]
R
R
1
, wR
, wR
2
[IϾ2σ(I)]
(all data)
R
R
1
= 0.0402, wR
= 0.0374, wR
2
= 0.0821
= 0.0802
R
R
1
= 0.0332, wR
= 0.0459, wR
2
= 0.0654
= 0.0713
[b]
1
2
1
2
1
2
Goodness of fit
δpmax, δpmin [enm ]
1.001
419, –370
0.998
0.316, 259
–3
2
2
2
2
2
2)2/∑w(F
2)]1/2, P = (F
2
[
a] w = 1/[σ2(F
o
) + (0.0346P) + 1.6900P]. [b] w = 1/[σ2(F
o
) + (0.0269P) + 0.3160P]. wR
2
= [∑w(F
o
– F
c
o
o
+
2
c
2F )/3.
–
1
range with a resolution of 4 cm . DSC and TG-DTG measure-
ments were carried with a Pyris-1 differential scanning calorimeter
and Pyris-1 thermogravimetric analyzer (Perkin–Elmer, USA) with
the sample under a dry nitrogen atmosphere (flow rate =
X-ray Data Collection and Structure Refinement: Blue crystals were
chosen for XRD analysis. The XRD data collections were per-
formed with a Rigaku AFC-10/Saturn 724+ CCD detector dif-
fractometer with graphite-monochromated Mo-K
0.071073 nm) in multi-scan mode for Cu(IMI) (N
scan mode for Ni(IMI) (N . The structures were solved by direct
methods with SHELXS-97
α
radiation (λ =
–1
2
0 mLmin ). For the DSC andTGA analyses the crystalline sam-
ples were powdered and placed in aluminum and open platinum
pans, respectively; the samples were then heated from 323 K to
4
3 2
)
and in φ-ω
4
3 2
)
[
64]
and refined by full-matrix least-
–1
2
[65]
8
73 K with a linear heating rate of 10 Kmin . The combustion
squares methods on F with SHELXL-97.
All non-hydrogen
heats of the title compounds were measured by oxygen bomb calo-
rimetry (Parr 6200, USA).
atoms were located from the difference Fourier maps and subjected
to anisotropic refinement. Detailed information concerning the
crystallographic data collections and structures refinements is
Synthesis of Cu(IMI)
4
(N
3
)
2
and Ni(IMI) : A solution of
4
(N
3
)
2
summarized in Table 5. CCDC-796440 [for Cu(IMI)
4 3 2
(N ) ] and
CuSO ·5H O (2.5 g, 10 mmol) or Ni(OAc)
4
2
2
·6H O (2.8 g, 10 mmol)
2
-
796439 [for Ni(IMI) (N ] contain the supplementary crystallo-
4
3 2
)
in deionized water (100 mL) was placed in a glass reactor with a
thermo-water bath. The reaction solution was stirred with a me-
chanical agitator and heated to 60–70 °C. Imidazole (2.72 g,
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
40 mmol) and sodium azide (1.30 g, 20 mmol) dissolved in deion-
ized water (50 mL) were added to the CuSO or Ni(OAc) aqueous
4
2
Supporting Information (see footnote on the first page of this arti-
cle): Hydrogen bond lengths and bond angles, metal cations’ octa-
hedral coordination structures, and the coplanar structures and the
packing of the unit cell of M(IMI) (N ) .
solution with continuous stirring and with the reaction solution
maintained at 60–70 °C over the course of 30 min. Then the solu-
tion was cooled to room temperature with stirring. The precipitate
that formed was collected by filtration, washed with ethanol, and
dried under vacuum in an explosion-proof water-bath dryer. The
4
3 2
yield was 69.0% for Cu(IMI)
4
(N
3
)
2
and 72.0% for Ni(IMI)
4
(N
3
)
2
.
Acknowledgments
Single crystals suitable for XRD measurements were obtained by
recrystallizing the products from this hydrothermal method from
deionized water at room temperature over a period of 1 month.
The project was supported by the Science and Technology on Ap-
plied Physical Chemistry Laboratory (9140C3703051105), the State
Key Laboratory of Explosion Science and Technology
Elemental Analyses: Cu(IMI)
.84, N 46.70; found C 34.41, H 3.74, N 46.93. Ni(IMI)
415.08): calcd. C 34.72, H 3.89, N 47.27; found C 35.06, H 3.67,
N 47.10. IR (KBr pellets): Cu(IMI) (N : ν˜ = 3571 [w, vs (NH)],
310 [vs (υ NH)], 2091 [vs (υas )], 1620 [s (δ NH)], 1291
4
(N
3
)
2
(419.93): calcd. C 34.32, H
(
No.YBKT10-05 and ZDKT10-01b) and the Program for New
Century Excellent Talents in University (NCET-10-0051, CNET-
9-0051).
3
(
4
(N )
3 2
0
4
3 2
)
–
3
N
3
–
–
[w (υ N
3
)], 834 [w (δ imidazole)], 681 [w (δ N
3
)]. Ni(IMI)
4 3 2
(N ) :
[
1] J. Giles, Nature 2004, 427, 580–581.
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ν˜ = 3133 [vs (υ NH)], 2056 [vs (υas
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3
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–
–1
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)], 819 [w (δ imidazole)], 624 [w (δ N )] cm .
2622
www.eurjic.org
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2011, 2616–2623

4 3 2
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Received: January 16, 2011
Published Online: April 28, 2011
Eur. J. Inorg. Chem. 2011, 2616–2623
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
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