Transition metal complexes with pyrazole-based ligands Part 12. Characterisation and thermal decomposition of CuCl2 complexes with di- and trisubstituted pyrazoles

Article abstract of DOI:10.1023/A:1013133405261

The synthesis of copper(II) chloride complexes with 3,5-dimethylpyrazole, 1-carboxamidine-3,5-dimethylpyrazole, 5-amino-4-carboxamide-1-phenylpyrazole and 4-acetyl-3-amino-5-methylpyrazole is described. The compounds are characterized by elemental analysis, FT-IR spectroscopy, thermal methods, magnetic moment and molar conductivity measurements. Thermal decomposition of the dichloro-(3,5-dimethylpyrazole)-copper(II) complex results in an unstable intermediate with a stochiometric composition. The decomposition of the other compounds is continuous.

Full text of DOI:10.1023/A:1013133405261

Journal of Thermal Analysis and Calorimetry, Vol. 66 (2001) 573 581
TRANSITION METAL COMPLEXES WITH
PYRAZOLE-BASED LIGANDS
Part 12. Characterisation and thermal decomposition of CuCl2
complexes with di- and trisubstituted pyrazoles
2
1
K. MØszÆros SzØcsØnyi1, V. M. Leovac1, . K. Ja imovi , V. I. ¨ e ljevi ,
A. KovÆcs3 and G. Pokol4
1University of Novi Sad, Faculty of Sciences, Institute of Chemistry, 21000 Novi Sad,
Trg D. Obradovi a 3, Yugoslavia
2Faculty of Metallurgy and Technology, Podgorica, Yugoslavia
3Research Group for Technical Analytical Chemistry of the Hungarian Academy of Sciences at the
Institute for General and Analytical Chemistry, Technical University of Budapest,
H-1521 Budapest, Hungary
4Institute for General and Analytical Chemistry, Technical University of Budapest,
H-1521 Budapest, Hungary
(Received February 26, 2001; in revised form May 24, 2001)
Abstract
The synthesis of copper(II) chloride complexes with 3,5-dimethylpyrazole, 1-carboxamidine-3,5-di-
methylpyrazole, 5-amino-4-carboxamide-1-phenylpyrazole and 4-acetyl-3-amino-5-methylpyra-
zole is described. The compounds are characterized by elemental analysis, FT-IR spectroscopy,
thermal methods, magnetic moment and molar conductivity measurements. Thermal decomposition
of the dichloro-(3,5-dimethylpyrazole)-copper(II) complex results in an unstable intermediate with a
stochiometric composition. The decomposition of the other compounds is continuous.
Keywords: 4-acetyl-3-amino-5-methylpyrazole, 5-amino-4-carboxamide-1-phenylpyrazole,
1-carboxamide-3,5-dimethylpyrazole, 1-carboxamidine-3,5-dimethylpyrazole, cop-
per(II) complexes, 3,5-dimethylpyrazole, FT-IR spectroscopy, magnetic measure-
ments, thermal decomposition
Introduction
Continuing our studies on transition metal complexes with pyrazole type ligands [1 5]
we examined reactions between copper(II) chloride and 5-amino-4-carboxamide-1-
phenylpyrazole, (L2), 1-carboxamide-3,5-dimethylpyrazole, (HL3), 1-carboxamidine-
3,5-dimethylpyrazole, (HL4) and 4-acetyl-3-amino-5-methylpyrazole (HL5) ligands. De-
pending on the ligand type and/or the ligand-to-metal ratio mono- and bis(ligand) com-
plexes are formed. In the reaction of 1-carboxamide-3,5-dimethylpyrazole, (HL3), with
CuCl2 a decarboxamidation of the ligand occurred resulting in mono- and bis(ligand)
1418 2874/2001/ $ 5.00
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M SZ` ROS SZ CS NYI et al.: TRANSITION METAL COMPLEXES
copper(II) chloride complexes of 3,5-dimethylpyrazole (HL1). Complexes of the follow-
ing compositions are obtained: [CuCl2(HL1)]2, CuCl2(HL1)2, CuCl2(L2)2, CuCl2(HL4) and
CuCl2(HL5)2.
Some physico-chemical properties of the compounds were determined. Special
attention was paid to the relationship between the structural characteristics and ther-
mal behaviour of the compounds. The thermal properties of the complexes are com-
pared to the thermal data of similar pyrazole complexes with other cations.
Experimental
All the pyrazole derivatives were commercial products of the Aldrich Company ex-
cept of the 4-acetyl-3-amino-5-methylpyrazole, (HL5). The synthesis of this ligand
has been described elsewhere [1].
The elemental, FT-IR and thermal analyses were performed using the standard
methods and instrumentation described in [2]. In the case of the CuCl2(HL5)2 the de-
composition was also followed by mass spectrometry (Balzers Termostar GSD
300 T, capillary MS).
Magnetic susceptibility measurements were conducted at room temperature us-
ing a magnetic susceptibility balance MSB-MKI, Sherwood Scientific Ltd., Cam-
bridge. The data are corrected for diamagnetic susceptibilities.
Results and discussion
All the complexes were prepared reacting hot ethanolic solutions of the appropriate
ligand and copper(II) chloride dihydrate in a ligand-to-metal ratio of 2:1 except in re-
action with HL3, where a 1:1 ligand-to-metal ratio was also applied. The composition
of the compounds and the elemental analyses data are given in Table 1. All the com-
plexes are coloured and well soluble in DMF. Their solubility is lower in alcohols
(MeOH, EtOH) and significantly lower in water. The effective magnetic moment of
the complexes (1.8>µeff. >1.9) corresponds to monomeric d9 systems except of that of
the [CuCl2(HL1)]2 complex. The lower magnetic moment for this compound
(µeff.=1.55) suggests a dimeric structure [6]. This supposition is supported also with
the compound s lower solubility according to the corresponding bis(ligand)complex.
Low molar conductivity (λ(DMF)<30) is characteristic of all formed compounds refer-
ring to the co-ordination of the chlorine. Some physico- chemical properties of the
compounds are presented in Table 2.
In all the compounds the copper(II) is tetracoordinated with the chloride ions
and the pyridine-type nitrogen of the pyrazole rings and in case of the CuCl2(HL4)
complex with the imido nitrogen of the carboxamidine group. This kind of the
co-ordination is proved by single crystal X-ray analysis of the compound [7].
[CuCl2(HL1)]2 and CuCl2(HL1)2 were prepared reacting 1-carboxamide-3,5-di-
methylpyrazole, (HL3), with copper(II) chloride in a ligand-to-metal ratio of 1:1 and
2:1, respectively. In this reaction an elimination of the carboxamide group from the
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M SZ` ROS SZ CS NYI et al.: TRANSITION METAL COMPLEXES
575
ligand takes place. Hence, instead of the expected CuCl2(HL3)2 compound complexes
containing 3,5-dimethylpyrazole (HL1) moieties are formed. The lost of the carbox-
amide group during the reaction is proved by both the elemental analysis data and the
missing ν(C=O) vibrational band at 1720 cm 1 from the IR spectra of the products.
Exclusively CuCl2(HL1)2 complexes (green crystals) were obtained when the
ligand-to-metal ratio was 2:1, while a mixture of CuCl2(HL1)2 and [CuCl2(HL1)]2
(golden crystals) was found using a ratio of 1:1. The two complexes in the mixture
can be separated using the good solubility of CuCl2(HL1)2 in ethanol.
Table 1 Composition of the compounds and elemental analysis data
Found (Calculated)/%
Compound
C
H
N
1 [CuCl2(HL1)]2
2 CuCl2(HL1)2
3 CuCl2(L2)2
4 CuCl2(HL4)
6 CuCl2(HL5)2
26.23 (26.04)
37.18 (36.76)
44.42 (44.57)
26.08 (26.43)
35.46 (34.92)
3.56 (3.50)
5.02 (4.94)
4.38 (3.74)
3.39 (3.70)
4.81 (4.39)
11.95 (12.15)
17.10 (17.15)
20.29 (20.80)
20.70 (20.55)
20.82 (20.36)
Table 2 Some physico-chemical properties of the complexes
λM(DMF),
µeff., µB
Compound
Color
1
S/cm2 mol
1 [CuCl2(HL1)]2
2 CuCl2(HL1)2
3 CuCl2(L2)2
4 CuCl2(HL4)
8 CuCl2(HL5)2
golden
green
black
blue
1.55
1.81
1.87
1.82
1.81
28.3
24.2
22.0
24.7
23.4
black
A probable reaction mechanism of the complex formation of CuCl2 with HL3 re-
sulting in [CuCl2(HL1)]2 and CuCl2(HL1)2 compounds is presented in Schemes 1 and
2, respectively. The first step is a (N, O) co-ordination of the ligand. This is accompa-
nied by an increased polarisation of the C=O bond followed by a synchronous break
of the N H and N C bonds and by the formation of an unstable isocyanic acid deriva-
tive. In the last step the better Cl nucleofile is remaining as a constituent of the com-
pound giving [CuCl2(HL1)]2 or CuCl2(HL1)2 complexes, while the isocyanic acid is
eliminated. The elimination is promoted by the solvent ethanol forming ethyl-
carbamate. A decarboxamidation process was also observed in the reaction of co-
balt(II) chloride [8] and zinc(II) chloride [9] with the same ligand, while with cop-
per(II), nickel(II) and cobalt(II) perchlorates and cobalt(II) nitrate the expected
bis(HL3) complexes were formed [9].
Mono- and bis(ligand) complexes of copper(II) chloride with 3,5-dimethyl-
pyrazole synthesised using 3,5-dimethylpyrazole ligand are already described in lit-
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M SZ` ROS SZ CS NYI et al.: TRANSITION METAL COMPLEXES
Scheme 1
Scheme 2
erature [6, 10]. However, their synthesis starting with 1-carboxamide-3,5-dimethyl-
pyrazole is described for the first time. On the basis of ESR spectroscopy data and the
strong antiferromagnetic ordering in the crystal for the mono(ligand) complex a
dimeric quasi-octahedral structure may be proposed which is established through the
bridging chloride atoms [6]. The structure of the bis(ligand) CuCl2 complex is pro-
posed on the basis of the structure of an analogous bis(HL1) zinc chloride complex,
ZnCl2(HL1)2, which was determined by X-ray analysis. For this compound a tetrahe-
dral arrangement around Zn(II) was found [11]. Based on the known similarity of Zn
and Cu complexes and on the magnetic moment of CuCl2(HL1)2 (µeff.=1.81) we can
assume a distorted tetrahedral structure for this compound.
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577
In the reaction of the 5-amino-4-carboxamide-1-phenylpyrazole (L2) and CuCl2
a bis(ligand) complex is formed. It has been shown that with a bulky substituent on
the pyrrolic nitrogen atom trans-square-planar complexes are obtained [12]. On this
basis and on the basis of the value of the effective magnetic moment to the CuCl2(L2)2
complex a distorted trans-square-planar structure may be prescribed.
The X-ray analysis data refer to a square-planar geometry of the CuCl2(HL4)
compound, also [7]. For the CuCl2(HL5)2 around copper(II) a very distorted tetrahe-
dral environment was found by X-ray structure analysis [1].
Thermal decomposition of the compounds
The decomposition of the compounds starts with an endothermic process followed
with an exothermic one (DSC curves, Fig. 1). The course of the decomposition up to
550 K does not depend on the gas carrier. The decomposition in argon atmosphere
usually slows down and is not complete up to 1100 K. This is the case for compounds
with HL1, L2 and HL5, while the complex with HL4 decomposes completely in both
atmospheres up to 950 K with the difference that in the air the decomposition is com-
plete at 850 K.
Fig. 1 DSC curves of the complexes
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M SZ` ROS SZ CS NYI et al.: TRANSITION METAL COMPLEXES
Fig. 2 TG and DTG curves of the complexes with HL1
TG and DTG decomposition curves are presented in Figs 2 and 3. All the com-
pounds begin to decompose in the temperature range of 450 480 K (Figs 1 3) which
means that there is not a significant difference in their thermal stability at the beginning
of the decomposition.
To the first decomposition step of the dimeric [CuCl2(HL1)]2 complex very sharp
DTG and DSC peaks belong (Figs 1 and 2). The decomposition begins endothermally
which is followed immediately with an exothermic process. At this stage a decompo-
sition product with a stoichiometric composition is formed that decomposes almost
immediately. By freezing-out technique the intermediate was isolated and investi-
gated by IR spectroscopy. The most characteristic changes in the IR spectrum of the
intermediate with respect to the starting compound are the disappearance of the
ν(NH) stretching and ν(=CH) stretching bands as well as considerable alterations in
the bands of the pyrazole ring vibrations. On the other hand, the characteristic bands
of the methyl groups are unchanged. In spite of this fact, the EGA measurement indi-
cates the departure of an organic fragment. Taking this in consideration, probably be-
side the departure of a HCl molecule a partial demethylation takes place, which was
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M SZ` ROS SZ CS NYI et al.: TRANSITION METAL COMPLEXES
579
Fig. 3 TG and DTG curves of the complexes with L2, HL4 and HL5
observed earlier in pyrazole complexes with a methyl-group in position 5 [2]. This
evidence, IR data and the mass loss (found: 11.0; calcd: 10.95%) suggest the decom-
position mechanism which is presented in Scheme 3:
Scheme 3
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M SZ` ROS SZ CS NYI et al.: TRANSITION METAL COMPLEXES
The degradation of the dimeric [CuCl2(HL1)]2 complex in air is not complete up to
1000 K, while the bis(ligand) complex decomposes completely to CuO (found: 17, calcd:
19.45%).
The thermal stability of the CuCl2(L2)2 with a bulky aryl substituent on the
pyrrolic nitrogen atom is somewhat less according to the other complexes. Its decom-
position most likely begins with the breaking off of the aryl-groups (found: 25, calcd:
28.60%). The end product at 900 K in air atmosphere is probably copper(II) oxide
with some coke residue.
The decomposition of the last two complexes is continuous. The CuCl2(HL4) de-
composition probably begins with the demethylation and/or departure of the
3,5-dimethylpyrazole (found: 40, calcd: 35.26%) ligand. The end product is cop-
per(II) oxide in both atmospheres (found: 28, calcd.: 29.18%).
The MS spectrum of the gaseous decomposition products at the beginning of the
CuCl2(HL5)2 decomposition with 4-acetyl-3-amino-5-methylpyrazole ligand shows peaks
for ammonia, methyl and methylen groups. Thus, in this case probably an NH-bridge for-
mation takes place as was supposed at the decomposition of other pyrazole complexes
with NH2-group in position 3, together with a demethylation, characteristic of the
5-methyl derivatives of pyrazoles [2, 3]. The substituents are breaking off side by side.
Above 700 K the pyrazole skeleton is destroying and a copper(II) oxide end product is
obtained (found: 16.3, calcd: 19.30%).
Conclusions
In reaction of the 1-carboxamide-3,5-dimethylpyrazole with copper(II) chloride a
decarboxamidation takes place. Depending on the ligand-to-metal ion ratio a mono-
or bis(3,5-dimethylpyrazole) complexes are formed.
The geometry of the compounds is determined primarily by the ligand. How-
ever, for the mono(ligand) complex with HL1 an octahedral geometry is prescribed,
while for the bis(ligand) complex with the same ligand probably a tetrahedral struc-
ture is characteristic. A very distorted tetrahedral environment was found around cop-
per(II) in CuCl2(HL5)2. CuCl2(HL4) possess a square-planar geometry. A trans-
square-planar geometry is prescribed to CuCl2(L2)2 complex.
The thermal decomposition pattern of the compounds is also different. The
mono(ligand) complex with HL1 decomposes exothermally very rapidly to an unstable
intermediate with a stoichiometric composition. The decomposition of the other com-
pounds is continuous. However, on the basis of our experience with similar compounds
[2, 5], the IR-spectrum of the intermediate and the MS spectral data we can state, that the
decomposition most probably proceeds through a demethylation when there is a
methyl-group in the position 5, e.g. in complexes with HL4 and HL5. Additionally, in the
case of the compound with the latter ligand at the first step likely an NH-bridge formation
takes place parallelly with the demethylation as was observed in other complexes with a
3-amino-group [2, 5].
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The authors would like to thank the Domus Hungarica Scientiarum Artiumque Foundation for their
support. A. K. thanks the Bolyai Foundation for a fellowship.
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