Syntheses, structures, and characterization of three new complexes constructed from triazolyl n-heterocyclic ligand

Article abstract of DOI:10.1080/15533174.2013.783857

Treatment of triazolyl ligand 3-(3-oxyphenyl)-5-(pyrazine-2-yl)-1,2,4- triazole (opt) with Cu(NO3)2·3H2O, Co(NO3)2·3H2O or NiCl2·6H2O resulted in three new complexes, [Cu(opt)2] (1), [Co(opt)2(H2O)2] (2), [Ni(opt)2(H2O)2] (3). Single-crystal X-ray crystallographic analyses reveal that all complexes 1-3 present mononuclear structures. Interestingly, various π···π interactions and hydrogen bonds interactions are discovered and extend these mononuclear structures to 3-D supramolecular networks. Notably, complex 1 presents the cavity construction with the approximate channel diameter 11.835 A. Furthermore, the thermal analyses results are in reasonable agreement with the crystal structure analyses of complexes 1-3.

Full text of DOI:10.1080/15533174.2013.783857

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Synthesis and Reactivity in Inorganic, Metal-Organic,
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Syntheses, Structures, and Characterization of
Three New Complexes Constructed From Triazolyl N-
Heterocyclic Ligand
Qinqin Yuan a , Ming Zhao b , Songhe Zhang a , Hongwei Hou a , Jinpeng Li c , Shujun Zhu a ,
Xiaoli Gao a , Huan Sun a , Yaoting Fan a & Junbiao Chang a c
a Department of Chemistry , Zhengzhou University , Henan , P. R. China
b Experimental Management Centre , Zhongzhou University , Henan , P. R. China
c School of Pharmaceutical Sciences , Zhengzhou University , Henan , P. R. China
Accepted author version posted online: 22 Oct 2013.Published online: 21 Nov 2013.
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To cite this article: Qinqin Yuan , Ming Zhao , Songhe Zhang , Hongwei Hou , Jinpeng Li , Shujun Zhu , Xiaoli Gao , Huan
Sun , Yaoting Fan & Junbiao Chang (2014) Syntheses, Structures, and Characterization of Three New Complexes Constructed
From Triazolyl N-Heterocyclic Ligand, Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, 44:4,
558-566, DOI: 10.1080/15533174.2013.783857
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Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, 44:558–566, 2014
Copyright ꢀ Taylor & Francis Group, LLC
C
ISSN: 1553-3174 print / 1553-3182 online
DOI: 10.1080/15533174.2013.783857
Syntheses, Structures, and Characterization of Three New
Complexes Constructed From Triazolyl N-Heterocyclic
Ligand
Qinqin Yuan,1 Ming Zhao,2 Songhe Zhang,1 Hongwei Hou,1 Jinpeng Li,3 Shujun
Zhu,1 Xiaoli Gao,1 Huan Sun,1 Yaoting Fan,1 and Junbiao Chang1,3
1Department of Chemistry, Zhengzhou University, Henan, P. R. China
2Experimental Management Centre, Zhongzhou University, Henan, P. R. China
3School of Pharmaceutical Sciences, Zhengzhou University, Henan, P. R. China
for designing novel coordination complexes since it can adjust
the coordination mode, flexibility of the molecular backbone,
configurational preference, type and topology of the products
by coordinating directly to the metal centers.[9] While in var-
ious organic ligands, the derivatives of triazole are excellent
nitrogen-heterocyclic ligands and often used to construct com-
plexes with various configurations.[10–12] More importantly, their
aromatic rings attract our interest because of their importance in
crystal engineering, which may form strong π-π stacking inter-
actions, and may lead to variable π-π interactions modes and ar-
chitectures. These interactions have great importance influence
on controlling the conformation of molecules,[13] crystal pack-
ing,[14] molecular assembly into an organized supramolecular
network,[15–17] and the structure of proteins and DNA.[18] Based
on the foregoing reasons and as an extension of this research, we
utilized the triazolyl ligand 3-(3-oxyphenyl)-5-(pyrazine-2-yl)-
1,2,4-triazole (opt) to react with metal ions and prepared three
new Cu(II), Co(II) and Ni(II) complexes. Herein, the synthesis,
crystal structures, and thermal properties of the three complexes
were reported.
Treatment of triazolyl ligand 3-(3-oxyphenyl)-5-(pyrazine-2-
yl)-1,2,4-triazole (opt) with Cu(NO3)2·3H2O, Co(NO3)2·3H2O or
NiCl2·6H2O resulted in three new complexes, [Cu(opt)2] (1),
[Co(opt)2(H2O)2] (2), [Ni(opt)2(H2O)2] (3). Single-crystal X-ray
crystallographic analyses reveal that all complexes 1–3 present
mononuclear structures. Interestingly, various π···π interactions
and hydrogen bonds interactions are discovered and extend these
mononuclear structures to 3-D supramolecular networks. Notably,
complex 1 presents the cavity construction with the approximate
channel diameter 11.835 Å. Furthermore, the thermal analyses re-
sults are in reasonable agreement with the crystal structure analy-
ses of complexes 1–3.
Keywords cavity constructure, crystal structure, π···π interaction,
synthesis
INTRODUCTION
In recent years, the rational design and preparation of met-
allosupramolecular complexes have attracted intense interest
for their unfamiliar architectures and potential applications.[1–6]
Various novel supramolecular architectures have been success-
fully built by the rational combination of preselected organic
ligands and elaborately choosing metal ions with definite co-
ordination geometry.[7,8] In this field, the preselected organic
ligand, to a certain extent, is crucial and usually the first choice
EXPERIMENTAL
General Details
The ligand opt and other staring materials were of reagent
grade quality and were obtained from commercial sources with-
out further purification. Carbon, hydrogen and nitrogen analyses
were carried out on a FLASH EA 1112 elemental analyzer. IR
data were recorded on a BRUKER TENSOR 27 spectropho-
tometer with KBr pellets in the 400–4000 cm–1 region. UV-vis
absorption spectra were recorded on a Unico UV-2102 PC spec-
trometer. TG-DSC measurements were performed by heating
the samples from 20–780◦C for 1, 20–650◦C for 2 and 3 at a
rate of 10◦C/min in air on a NETZSCH STA 409PC differential
thermal analyzer.
Received 31 January 2013; accepted 5 March 2013.
The authors are thankful for financial support from the National
Outstanding Young Scholarship (Nos. 30825043) and National Natural
Science Foundation (Nos. 20971110, J0830412 and 91022013), China
Postdoctoral Science Foundation (Nos. 20110491005), Program for
New Century Excellent Talents of Ministry of Education of China
(NCET-07-0765), and the Outstanding Talented Persons Foundation of
Henan Province.
Address correspondence to Jinpeng Li, Department of Chem-
istry, Zhengzhou University, Zhengzhou 450052, P. R. China. E-mail:
ljp-zd@zzu.edu.cn
558

TRANSITIONAL METAL COMPLEXES
559
RESULTS AND DISCUSSION
Preparation of [Cu(opt)2] (1)
The DMF (2 mL) solution of ligand opt (2.4 mg, 0.01 mmol)
was added dropwise to a methanol solution (4 mL) of
Cu(NO3)2·3H2O (4.8 mg, 0.02 mmol). The resulting mix-
ture was kept at room temperature. Four days later, the
light blue crystals suitable for X-ray diffraction were ob-
tained from the resultant green solution. Yield: 43%. Anal.
Calcd. for C108H72Cu4.5N45O9: C, 53.33; H, 2.96; N, 25.93.
Found C, 50.04; H, 3.47; N, 24.28%. IR(cm–1, KBr): 3017
(s), 2698(w), 1589(s), 1438(s), 1425(s), 1358(m), 1313(w),
1231(m), 1205(m), 1186(w), 1145(m), 1041(m), 1018(w),
885(m), 745(w), 684(w), 419(w).
Structural Description of [Cu(opt)2] (1)
The structure determination of 1 reveals this molecule crys-
tallizes in the space group R-3. A perspective view of the molec-
ular structure with atom labeling scheme is shown in Figure 1.
The Cu(II) center is tetracoordinated with four nitrogen atoms
from two opt ligands. The bond lengths of Cu-N vary from
1.924(4) to 2.041(4) Å, which are in good accordance with the
reported Cu(II) complexes with N donors.[20] The N(1)-Cu(1)-
N(1)#1 and N(5)#1-Cu(1)-N(5) angles in 1 are 179.996(1) and
179.997(1), respectively, which is very close to 180º. The other
bond angles around Cu(II) vary from 81.31(18) to 98.70(18),
respectively. The two opt ligands link one Cu(II) atom to form
a quadrangular constructure. It should be pointed out that there
are affluent aromatic ring systems in 1, which may form vari-
ous π···π stacking interactions. Analysis of the crystal packing
of 1 revealed that an infinite 1-D layered supramolecular array
(Figure 1) was formed by the head-to-tail π-π stacking inter-
actions between any two adjacent triazole rings with the inter-
planar separation of 3.240 Å (center-to-center separation: 3.932
Å).[11,21,22] While between any adjacent triazole and pyrazine
rings, the π-π interactions with the inter-planar separation of
3.298 Å (center-to-center separation: 4.069 Å) is slightly weaker
than that of two adjacent triazole rings. Additionally, the inter-
molecular O. . .H. . .N hydrogen bonds are also observed in 1.
The N. . .O distance of the hydrogen bonds is 2.753(6) Å and
the bond angle around O. . .H. . .N is 164.4◦, which further sta-
bilizes the crystal structure and lead to the cavity constructure
with the approximate channel diameter 11.835 Å in 1 (Figure 2).
These aforementioned weak interactions result in an infinite 3-D
supramolecular network (Figure 2). It is noteworthy that, from
the actual elemental and thermogravimetric analysis of complex
1, there should be some water molecules in the cavities of 1.
However, we cannot find them from its crystal structure analy-
ses. This is because the skeleton of 1 is of high symmetry and
porous nature, which led to water molecules being disordered.
Preparation of [Co(opt)2(H2O)2] (2)
The DMF (2 mL) solution of ligand opt (7.2 mg,
0.03 mmol) was added dropwise to the H2O (5 mL) solution of
Co(NO3)2·3H2O (11.6 mg, 0.04 mmol). Then the mixture was
sealed in 18 mL Teflon-lined stainless steel container, which
was heated to 150◦C under autogenously pressure for four days.
After slow cooling to room temperature with the rate 10◦C/h, the
angered rod-like crystals suitable for X-ray diffraction were ob-
tained. Yield: 56%. Anal. Calcd. for C24H18CoN10O4: C, 50.58;
H, 3.16; N, 24.59. Found: C, 50.41; H, 3.02; N, 24.49%. IR
(cm–1, KBr): 3398(s), 1595(m), 1530(w), 1460(s), 1352(w),
1228(s), 1136(w), 883(m), 749(m), 679(w), 425(w).
Preparation of [Ni(opt)2(H2O)2] (3)
The DMF (2 mL) solution of ligand opt (7.2 mg, 0.03 mmol)
was added dropwise to a mixture of NiCl2·6H2O (7.1 mg,
0.03 mmol) and H2O (6 mL). Then the mixture was sealed
in an 18 mL Teflon-lined stainless steel container, which was
heated to 130◦C under autogenously pressure for three days.
After slow cooling to room temperature with the rate 10◦C/h,
the green crystals suitable for X-ray diffraction were obtained.
Yield: 61%. Anal. Calcd. for C24H18N10NiO4: C, 50.60; H,
3.16; N, 24.60. Found: C, 50.44; H, 3.01; N, 24.52%. IR
(cm–1,KBr): 3423(s), 1596(m), 1533(w), 1464(s), 1422(w),
1371(w), 1353(s), 1229(s), 1193(w), 1140(w), 1039(m), 884(s),
864(w), 750(w), 685(w), 524(w), 430(w).
Structural Description of [Co(opt)2(H2O)2] (2)
and [Ni(opt)2(H2O)2] (3)
X-Ray Crystallography
The X-ray diffraction analysis reveals that both complexes
2 and 3 crystallize in the monoclinic space group P21/n
and exhibit similar mononuclear structures. Herein, we only
describe the structure of complex 2 in detail. The Co(II)
ion in 2 is six-coordinated in a slightly distorted octahedron
geometry, which is provided by four N donor atoms from
four opt ligands and two oxygen atoms from two coordinated
water molecules (Figure 3). The four N donor atoms occupy
the equatorial plane and two oxygen atoms take up the apical
positions, which result in a mononuclear constructure. The
bond length of Co-O is 2.1276(7) Å and the Co-N lengths
vary from 2.0610(9) Å to 2.1022(9) Å, which are comparable
to those reported cobalt(II) complexes [Co(plbbm)Cl2]n
[plbbm = 1,1-(1,3-propylene)bis-1H−benzimidazole],[23]
The diffraction intensity data of 1, 2 and 3 were collected
at room temperature on a Rigaku RAXIS-IV and SATURN-
724 imaging plate area detector with graphite monochromated
Mo-Kα radiation (λ = 0.71073 Å). The structures were solved
by direct methods and expanded with Fourier techniques. The
non-hydrogen atoms were refined with anisotropic thermal pa-
rameters. Hydrogen atoms were included but not refined. The
final cycle of full-matrix least-squares refinement was based on
observed reflections and variable parameters. All calculations
were performed using the SHELX-97 crystallographic software
package.[19] Crystallographic parameters and structural refine-
ment are summarized in Table 1. Selected bond lengths and
angles are listed in Table 2.

560
Q. YUAN ET AL.
TABLE 1
Crystallographic data for complexes 1–3
1
2
3
Formula
fw
C108H72Cu4.5N45O9
2430.08
293(2)
0.71073
Hexagonal
R-3
36.151(4)
36.151(4)
5.3013(11)
90
90
120
6000.0(16)
1
1.345
C24H18CoN10O4
569.41
293(2)
0.71073
Monoclinic
P21/n
8.4249(4)
7.347(4)
18.526
90
94.10
90
1143.8(6)
2
C24H18N10NiO4
569.19
293(2)
0.71073
Monoclinic
P21/n
8.5138(3)
7.303(2)
18.319
90
93.58
90
1136.9(3)
2
Temp (K)
Wavelength (Å)
Crystal syst
Space group
a (Å)
b (Å)
c (Å)
α (◦)
β (◦)
γ (◦)
V(Å3)
Z
Dc (g·cm–3)
1.653
582
1.663
584
F(000)
2474
θ range for data collection(deg) collection(deg)
Reflections collected /unique
Data / restraints / params
1.13–25.00
19215 / 2345
2345 / 0 / 177
1.052
2.98–26.36
5101 / 2327
2327 / 0 / 180
1.021
3.00–26.36
5023 / 2315
2315 / 0 / 180
1.034
Goodness-of-fit on F2
a
b
Final R1 , wR2
0.0597, 0.1521
0.0396, 0.0857
0.0363, 0.0871
2
2
2
2
aR1 = ||Fo|− |Fc||/|Fo|. b wR2 = [w(|Fo | − |Fc |)2/w|Fo2|2]1/2. w = 1/[σ2(Fo)2 + 0.0297P2 + 27.5680P], where P = (Fo + 2Fc )/3.
TABLE 2
Selected bond lengths (Å) and angles (◦) for complexes 1–3
Complex 1
Cu(1)-N(1)
Cu(1)-N(5)
N(1)-Cu(1)-N(5)
N(5)#1-Cu(1)-N(5)
Complex 2
1.924(4)
2.041(4)
81.31(18)
179.997(1)
Cu(1)-N(1)#1
1.924(4)
Cu(1)-N(5)#1
N(1)-Cu(1)-N(5)#1
N(1)#1-Cu(1)-N(5)
2.041(4)
98.69(18)
98.70(18)
N(1)-Cu(1)-N(1)#1
N(1)#1-Cu(1)-N(5)#1
N(2)-N(1)-Cu(1)
179.996(1)
81.31(18)
137.4(3)
Co(1)-N(1)
Co(1)-O(3)
2.100(2)
2.1276(17)
91.38(8)
91.38(8)
102.60(8)
87.44(8)
Co(1)-N(1)#1
Co(1)-N(4)#1
N(1)#1-Co(1)-O(3)#1
O(3)#1-Co(1)-O(3)
O(3)#1-Co(1)-N(4)
O(3)#1-Co(1)-N(4)#1
2.0999(19)
2.154(2)
88.62(8)
180
87.44(8)
92.56(8)
Co(1)-O(3)#1
2.1276(17)
180
88.62(8)
77.40(8)
92.56(8)
180
N(1)-Co(1)-N(1)#1
N(1)-Co(1)-O(3)
N(1)-Co(1)-N(4)
O(3)-Co(1)-N(4)
N(4)-Co(1)-N(4)#1
N(1)-Co(1)-O(3)#1
N(1)#1-Co(1)-O(3)
N(1)#1-Co(1)-N(4)
O(3)-Co(1)-N(4)#1
Complex 3
Ni(1)-N(1)#1
Ni(1)-N(4)#1
N(1)#1-Ni(1)-N(1)
N(1)-Ni(1)-N(4)#1
N(1)-Ni(1)-O(4)#1
N(1)#1-Ni(1)-N(4)#1
2.0610(9)
2.1022(9)
180
100.99(7)
91.50(7)
79.01(7)
Ni(1)-N(1)
Ni(1)-O(4)#1
N(1)-Ni(1)-N(4)
O(4)#1-Ni(1)-O(4)
N(1)-Ni(1)-O(4)
N(1)-Ni(1)-O(4)#1
2.0610(9)
2.1231(16)
79.01(7)
180
88.50(7)
91.50(7)
Ni(1)-N(4)
Ni(1)-O(4)
N(1)#1-Ni(1)-N(4)
N(4)-Ni(1)-N(4)#1
N(4)#1-Ni(1)-O(4)
N(4)#1-Ni(1)-O(4)#1
2.1022(9)
2.1231(16)
100.99(7)
180
87.11(7)
92.89(7)
Symmetry transformations used to generate equivalent atoms:
For 1: #1 –x + 1/3, –y + 2/3, –z + 5/3.
For 2: #1 –x, –y + 2, –z.
For 3: #1 –x + 1, –y + 1, –z + 1.

TRANSITIONAL METAL COMPLEXES
561
FIG. 1. (a) Perspective view of [Cu(opt)2] (1) with atom-labeling scheme; (b) One-dimensional layered supramolecular network of complex 1. The dashed lines
represent the π···π stacking interactions between two adjacent aromatic rings (color figure available online).
[Co(2gb)3]3+ (2gb = 2-guanidinobenzimidazole),[24] {[Co(II)
For 2 and 3, there are the same aromatic ring systems as
(OOCH)2(phen)(H2O)]·H2O}n,[25] [Co(Pht)(4-MeIm)2(H2O)]n
that in 1, in which they form similar π-π stacking interactions.
(Pht2– dianion of o-phthalic acid; 4-Meim = 4-methylimida-
Figure 3 reveals that an infinite 2-D layer of 2 (3) was formed
zole),[26] {[Co(H2O)3(BPDC)]n·nH2O (H2BPDC = 2-bipyri-
by the π-π stacking interactions between two adjacent triazole
dyl-3,3-dicarboxylic acid)[27] and {[Co(NO3)(pbbm)2]NO3·1/
and pyrazine rings with the interplanar separations of 3.334
Å (2) and 3.340 Å (3) [center-to-center separations: 3.874 Å
2H2O}n [pbbm = 1,1’-(1,5-pentamethylene)bis-1H-benzimi-
dazole].[28]
(2) and 3.964 Å (3)], two adjacent triazole and benzene rings

562
Q. YUAN ET AL.
FIG. 2. Top: the cavity with the approximate channel diameter 11.835 Å forming by hydrogen bonds and π···π stacking interactions along c axis; Bottom: the
space-filling representation of 3-D supramolecular network of [Cu(opt)2] (1) (hydrogen atoms omitted for clarity) (color figure available online).

TRANSITIONAL METAL COMPLEXES
563
FIG. 3. (a) ORTEP view with atom labeling of the mononuclear fragment for complexes 2 and 3 (M = Co for 2 and M = Ni for 3); (b) two-dimensional layered
supramolecular network of complexes 2 or 3. The dashed lines represent the π-π stacking interactions between two adjacent aromatic rings (color figure available
online).
mol/L mol·L–1 DMF solution. The UV-Vis spectra of the free
with the inter-planar separations of 3.3715 Å and 3.417 Å (3)
ligand opt exhibit two intense absorption peaks at 264 nm and
295 nm, respectively. The absorption peak at 264 nm can be
ascribed to the π-π∗ transition of the benzene rings, while the
absorption peak at 295 nm should be assigned to the intra-
ligand n-π∗ transition.[29–31] In the spectra of complexes 1–3,
the dominant absorption peaks are 265 and 296 nm for 1, 267
and 296 nm for 2, and 271, 300 and 355 nm for 3, which
is basically coincident with that of opt ligand and only these
peaks are red-shifted to a certain degree. The foregoing two
absorption peaks can also be ascribed to the intraligand π-π∗
or n-π∗ transitions. Additionally, the latter absorption peak at
296 nm (3 for 300 nm) undergoes a gradual fall (from 1 to
3) when compared with that of the free ligand, indicating the
coordination of Cu(II), Co(II), and Ni(II) atoms with opt ligand.
While for complex 3, a newly-produced moderate absorption
peak is observed at 355 nm, which probably arises from the
transition of the bonding electrons between metal ion and opt
ligand, namely arises from the metal to ligand charge transfer
transition.[32,33]
[center-to-center separations: 3.673 Å (2) and 3.549 Å (3)].
While between two adjacent pyrazine and benzene rings, the
inter-planar separations of π-π stacking interactions are 3.367
Å for 2 and 3.489 Å for 3 [center-to-center separation: 3.776
Å (2) and 4.000 Å (3)]. Moreover, the intermolecular O. . .H. . .O
and O. . .H. . .N hydrogen bonds are also observed in complexes 2
and 3, which further stabilize their crystal structures and lead to
a 3-D supramolecular network (Figure 4). The O. . .O and O. . .N
distances of the hydrogen bonds are 2.852(2) Å, 2.863(3) Å for 2
and 2.872(3), 2.883(3) Å for 3, the bond angles around O. . .H. . .O
and O. . .H. . .N are 164.7◦, 164.3◦ for 2 and 165.6◦, 159.6◦ for
3. These foregoing facts specify that the π-π interactions are
very important in all three complexes, where they contribute
significantly to molecular self-assembly processes.
UV-Vis Spectra
Figure 5 presents the UV-/Vis absorption spectra of the free
ligand opt and its corresponding complexes 1–3 in 2 × 10–5

564
Q. YUAN ET AL.
FIG. 4. Packing of 2 and 3 viewed along b axis, showing the hydrogen bonds, π···π stacking interactions and three-dimensional supramolecular networks of 2
and 3 (hydrogen atoms omitted for clarity) (color figure available online).
Thermogravimetric Analysis (TGA)
To characterize the thermal stability of the different self-
assemblies constructed by the same ligands and different met-
als, the complexes 1, 2, and 3 were investigated by the TG-DSC
techniques. As shown in Figure 6, the TG-DSC of 1 is deter-
mined in the range 20–780◦C in the air. The TG curve of com-
plex 1 exhibits two continuous weight loss stages in the ranges
of 77–192◦C and 332–657◦C, corresponding to the decompo-
sitions of the water molecule and opt ligand. Finally, a plateau
region is observed from 660 to 780◦C. A black residue of CuO
(obsvd. 13.96%, calcd. 13.80%) was remained. There are two
weak endothermic peaks (329.6◦C and 391.0◦C) and one strong
exothermic peak (540.6◦C) on the DSC curve of 1. The thermal
decomposition behavior of complex 2 is similar to that of 3. On
the DSC curve of 2 (or 3), there are one big exothermic peaks
at 511.4◦C (2) [512.2◦C (3)] and two weak endothermic peaks
at 222.4◦C and 452.5◦C (2) [240.7◦C (3)], respectively. The
TG curves of complexes 2 and 3 both exhibit two continuous
FIG. 5. The UV-Vis absorption spectra of opt ligand and its corresponding
weight loss stages in the ranges of 194–230◦C and 334–530◦C
(2) [197–262◦C and 404–534◦C (3)] corresponding to the con-
comitant release of the coordination water molecule and the
complexes 1, 2 and 3 in 2 × 10–5 mol·L–1 DMF solution (color figure available
online).

TRANSITIONAL METAL COMPLEXES
565
FIG. 6. The thermal decomposition curves of [Cu(opt)2] 1 (a), [Co(opt)2(H2O)2] 2 (b), and [Ni(opt)2(H2O)2] 3 (c).
decomposition of opt ligand. Finally, in complex 2, a plateau REFERENCES
region is observed from 530 to 650◦C. A brown amorphous
residue of CoO (observed 14.44%, calculated 13.16%) was re-
mained. While in complex 3, the plateau region is between 534
and 650◦C, the decomposition process was completed at 650◦C
giving dark green nickel oxide (NiO) as the final decomposition
product (obsvd. 13.60%, calcd. 13.12%).
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