Transitionmetal complexes with pyrazole-based ligands: Part 21. Thermal decomposition of copper and cobalt halide complexes with 3,5-dimethyl-1- thiocarboxamidepyrazole

Article abstract of DOI:10.1007/s10973-005-9986-0

The thermal decomposition of Cu₂L₂Cl₄, Cu₂L₂Cl₂, Cu₂L₂Br ₂ and Co₂L₂Cl₄ complexes (L=3,5-dimethyl-1-thiocarboxamidepyrazole) is described. The influence of the central ion to ligand mole ratio on the course of complex formation is examined in reaction of L with copper(II) chloride. In Cu(II):L mole ratio of 1:1, in methanolic solution the reaction yields to yellow-green Cu₂L ₂Cl₄ crystals. In the filtrate a thermodynamically more stable orange Cu₂L₂Cl₂ copper(I) complex is forming. With a Cu(II):L mole ratio of 1:2 only the latter compound is obtained. The composition and the structure of the compounds have been determined on the basis of customary methods. On the basis of FTIR spectrum of the intermediate which is forming during the thermal decomposition of Cu₂L ₂Cl₂ a decomposition mechanism is proposed.

Full text of DOI:10.1007/s10973-005-9986-0

Journal of Thermal Analysis and Calorimetry, Vol. 85 (2006) 2, 289–293
TRANSITION METAL COMPLEXES WITH PYRAZOLE-BASED LIGANDS
Part 21. Thermal decomposition of copper and cobalt halide complexes with
3,5-dimethyl-1-thiocarboxamidepyrazole
1
K. Mészáros Szécsényi , V. M. Leovac , A. Kovács , G. Pokol and ꢀ. K. Jaºimoviº
*
1
2
3
4
1
University of Novi Sad, Faculty of Sciences, Department of Chemistry, 21000 Novi Sad, Trg D. Obradoviºa 3, Serbia and Montenegro
2
Hungarian Academy of Sciences, Budapest University of Technology and Economics, Research Group of Technical Analytical
Chemistry, 1111 Budapest, Szt. Gellért tér 4, Hungary
3
Institute for General and Analytical Chemistry, Budapest University of Technology and Economics, 1521 Budapest,
Szt. Gellért tér 4, Hungary
4
Faculty of Metallurgy and Technology, Podgorica, Serbia and Montenegro
The thermal decomposition of Cu
pyrazole) is described. The influence of the central ion to ligand mole ratio on the course of complex formation is examined in
reaction of L with copper(II) chloride. In Cu(II):L mole ratio of 1:1, in methanolic solution the reaction yields to yellow-green
Cu Cl crystals. In the filtrate a thermodynamically more stable orange Cu Cl copper(I) complex is forming. With a Cu(II):L
mole ratio of 1:2 only the latter compound is obtained. The composition and the structure of the compounds have been determined
on the basis of customary methods. On the basis of FTIR spectrum of the intermediate which is forming during the thermal
decomposition of Cu Cl a decomposition mechanism is proposed.
2 2 4 2 2 2 2 2 2 2 2 4
L Cl , Cu L Cl , Cu L Br and Co L Cl complexes (L=3,5-dimethyl-1-thiocarboxamide-
2
L
2
4
2
L
2
2
2
L
2
2
Keywords: cobalt(II) chloride or bromide complexes, copper(I) chloride or bromide complexes, copper(II) chloride or bromide
complexes, coupled TG-MS measurement, 3,5-dimethyl-1-carboxamidepyrazole, TA
Introduction
mal decomposition of Cu L Cl , Cu L Br and
2 2 4 2 2 2
Co L Cl complexes with the same ligand.
2 2 4
Pyrazole type complexes are extensively studied
mainly because of their biological activity [1]. They
are often used in modeling the active centers of
enzymatic reactions [2, 3]. As these reactions take
place at mild conditions and are usually very selective
there are attempts to use them in chemical industry
Experimental
All chemicals for synthesis were analytical reagent
grade and used without any purification. Physico-
chemical characterizations are conducted by usual
methods and are described in details in [11].
[
4]. There are examples of use of pyrazole complexes
as precursors in processing thin films by metal
organic vapor deposition processes [5–7].
The aim of our systematic studies on pyrazole
type complexes is to determine the most important fac-
tors governing the complex formation reaction and to
examine how the structure affects the thermal stability
of the compounds [8–11]. Namely, all potential practi-
cal applicability of new compounds claims not only on
their thermal stability but also the understanding of the
decomposition mechanism, often determined by cou-
pled thermal and MS or FTIR techniques [12–18]. Be-
sides, during the thermal decomposition stable com-
pounds can be formed, offering thus a way to a solid
phase synthesis of new compounds [19–21].
Syntheses
The synthesis and the structure of the 3,5-dimethyl-
1-thiocarboxamidepyrazole ligand, L [22] and the
complexes of Cu L Cl , Cu L Br and Co L Cl is
2
2
4
2
2
2
2
2
4
described elsewhere [23].
Cu L Cl
2 2 2
3
Methanolic solutions of CuCl ⋅2H O (1 mmol, 5 cm )
2
2
3
and L (2 mmol, 5 cm ) were mixed in a mole ratio of
:2. The mixture was heated at 310 K (~40°C) under
reflux for 1 h. The precipitation of orange micro-
crystalline Cu L Cl (Cu C H N S Cl ) began
1
In this paper we report the synthesis, characteriza-
tion and thermal decomposition of Cu L Cl (L=3,5-di-
2 2 2
2
2
2
2
12 18
6
2
2
methyl-1-thiocarboxamidepyrazole) together with ther-
instantly. After 5 h the precipitate was filtered off,
*
Author for correspondence: mszk@uns.ns.ac.yu
1
388–6150/$20.00
Akadémiai Kiadó, Budapest, Hungary
Springer, Dordrecht, The Netherlands
©
2006 Akadémiai Kiadó, Budapest

MÉSZÁROS SZÉCSÉNYI et al.
washed with cold methanol and air dried.
Yield: 67.1%. Elemental analysis data Found, %
Calcd., %): C, 28.79 (28.35); H, 3.58 (3.57);
N, 16.51 (16.53), Cu, 24.25 (25.00). FTIR-spectrum
(
–
cm , KBr): 3276s, 3105s, 1628s, 1583s, 1153m,
1
(
–
73s. Molar conductivity of 10 mol dm solution in
3
–3
8
2
–1
DMF, λ (DMF), 39.6 S cm mol .
M
Scheme 1 Di-μ-chloro-bis{3,5-dimethyl-1-thiocarboxamide-
copper(I)}
Results and discussion
The structure of Cu L Cl , Cu L Br and Co L Cl has
2 2 2 2 4
2
2
4
2
basicity of the components, the crystal field stabiliza-
tion energy (CFSE), the steric requirement of the
components, redox properties, the interaction of the
components with the solvent, etc. In addition, ligands
with groups like thiocarboxamide moiety may be
deprotonated as a consequence of the Brønsted
basicity of the anion [24] or by adding base to the
reaction mixture [11]. Sometimes, even the CFSE is
enough for the deprotonation of the ligand [25].
been determined by single crystal X-ray diffraction [23].
Cu L Cl and Co L Cl are isostructural. From the three
2
2
4
2
2
4
potential donors of the ligand (the pyrazole ring pyridine
N atom, the N and S atoms of the thiocarboxamide moi-
ety) the coordination is established through the pyridine
N atom of the pyrazole ring and S atom of the thio-
carboxamide group forming neutral centrosymmetric
units in which metal ions are doubly bridged by two
chlorine atoms. In this way, metal atoms are sited in a
distorted trigonal bipyramidal environment. In the bro-
mide complex the coordinating atoms are the same.
During the complex formation reduction of Cu(II) to
The copper complexes were synthesized in
methanolic solution with a Cu(II):L mole ratio of 1:1.
The BrÝnsted basicity of both the chloride and bromide
ion in methanol is too low for the deprotonation of the
thiocarboxamide group. In spite of similar experimen-
tal conditions, the structure of the chlorine and bro-
mine compound is different. The coordinating atoms
are the same (N, S), however, in the case of the bro-
mide complex the reduction of Cu(II) to Cu(I) is taking
Cu(I) occurs. The obtained Cu L Br also has a
2 2 2
centrosymmetric dimer structure with a distorted tetra-
hedral geometry around copper(I). However, instead of
halide bridges, the two coppers are sulfur-bridged.
The FTIR spectrum of Cu L Cl is very similar to
2 2 2
those for the other three compounds. In all of the com-
place. This phenomenon may be explained by the
–
lower standard redox potential of the Br /2Br system
plexes the νCS vibration appears at lower frequency
–1
2
(
873, 848, 870 and 842 cm in Cu L Cl , Cu L Cl ,
2 2 2 2 2 4
–
compared to the Cl /2Cl system. By reduction of
2
Cu L Br and Co L Cl , respectively) than that in the
2 2 2 2 4
2
–
1
Cu(II) to Cu(I) the borderline soft Cu(II) acid trans-
forms to soft Cu(I) promoting the soft–soft interaction
with soft sulfur and bromine bases in accordance with
the Hard Soft Acid Base principle (HSAB, [26–28]).
The formed compound is also sterically stabilized.
Namely, as a consequence of the reduction, the mole-
cule contains only one bromide per copper(I). This sta-
bilization effect in the case of the corresponding cop-
per(I) chloride complex is emphasized in forming a
thermodynamically more stable Cu(I) compound from
the filtrate after precipitating the kinetically favored
Cu(II) complex. Namely, in the reaction of
ligand (880 cm ) suggesting a coordination through
the sulfur atom in Cu L Cl , also. All attempts to pre-
2 2 2
pare Cu L Cl single crystals have failed. The similar
2 2 2
composition of the copper(I) chloride and bromide
compounds may refer to a similar structure for these
two complexes. Powder X-ray diffraction patterns of
Cu L Cl suggest that it is most likely not isostructural
2 2 2
with the corresponding bromo complex. However, nei-
ther the powder X-ray diffraction patterns nor the FTIR
data are supporting or excluding that the two com-
pounds are isosturctural. Beside the sulfur bridged cop-
per atoms, it is possible that in the chlorine compound
the copper centers are doubly bridged by chlorines in-
stead of sulfur atoms, also with a distorted tetrahedral
dimer structure (Scheme 1).
CuCl
solution two compounds are formed. Under kinetic
control the yellow-green Cu(II)-complex, Cu L Cl , is
2
⋅2H O and L in a mole ratio of 1:1 in methanolic
2
2
2
4
crystallizing in form of single crystals. From the filtrate
the thermodynamically more stable microcrystalline
diamagnetic Cu(I)-complex, Cu L Cl has been pre-
Factors determining the course of complex formation
2
2
2
cipitated. When the Cu(II):L ratio is raised to 1:2 and
at somewhat higher temperature (∼ 40°C) the only
product of the reaction is the thermodynamically stable
One of the aims of our research work is to determine
the most important factors that influence the complex
formation. Some of these factors are as follows: the
quality and position of the pyrazole substituents, the
Cu L Cl . The excess of the ligand probably promotes
2 2 2
the reduction of Cu(II).
290
J. Therm. Anal. Cal., 85, 2006

TRANSITION METAL COMPLEXES WTH PYRAZOLE-BASED LIGANDS
The role of the solvent and the anion in complex
position in air is slower than in an inert atmosphere. In
addition, traces of decomposition products in platinum
crucible influence the decomposition in air at higher
temperatures [11].
formation may be presented by the reaction of Co(II)
acetate and sulfate with L in methanolic solution:in
both cases the same pink colored cobalt(III) complex,
Co(L–H)
3
, was obtained [11]. In reaction of CoCl
2
The TG-DTG curves of Cu
L Cl , Cu L Br and
2 2 4 2 2 2
and L under similar conditions no crystallization was
observed. The deep blue color of the methanolic
solution refers to a complex formation of different
type than that observed with Co(Oac) and CoSO . In
Cu L Cl in argon are presented in Fig. 1 which clearly
2 2 2
shows that the thermal stability of the copper com-
plexes is about the same. All three complexes begin to
decompose in a temperature range of 410–430 K. The
decomposition of Cu L Cl and Cu L Br complexes is
2
4
acetone, instead of the tris(ligand) Co(III) compound
blue single crystals of a binuclear Co(II) complex,
Co L Cl , were formed.
2
2
4
2
2
2
continuous in the whole range. The decomposition of
Cu L Cl starts at 425 K and consists of several
2
2
4
2
2
4
superposing processes. The mass loss to the first mini-
mum of the DTG-curve probably belongs to evapora-
tion of 2HCl molecules (12%, calcd: 12.59%). This
proposition is supported by the fact that the yel-
Thermal analysis
The thermal decomposition of the compounds was
studied by TG and DSC in flowing air and argon
–
atmospheres with a heating rate of 10 K min . In the
low-green single crystals of Cu
ange modification spontaneously during long standing
at room temperature. In argon at 850 K the end product
is Cu S (26.5%, calcd: 27.47%) while in air the decom-
position is not completed up to 1000 K. The decompo-
sition of Cu Br starts at 415 K and the steps are
somewhat better separated compared to Cu Cl . The
L Cl transform to or-
2 2 4
1
case of Cu L Cl a coupled TG-MS measurement was
2 2 2
carried out in a helium atmosphere.
2
The decomposition pattern is similar in different
atmospheres. On the contrary of our earlier experi-
ences, in the case of the complexes with L, the decom-
L
2
2
2
L
2
2
4
first superposed mass losses may correspond to loss
of 2HBr molecules (25%, calcd: 27.09%). The decom-
position is completed at 840 K in argon and at 855 K in
air (Cu S%, 27.8 Ar, 25.0 air, calcd: 26.64).
2
The decomposition of Cu L Cl takes a different
2 2 2
course. It begins at 420 K. At 530 K a low stability
intermediate is formed which was isolated and its
FTIR spectrum was recorded. The characteristic
2 2 4 2 2 2 2 2 2
Fig. 1 TG-DTG curves of Cu L Cl , Cu L Br and Cu L Cl
2 2 4
Fig. 2 TG-DTG curves of Co L Cl in argon
in argon
J. Therm. Anal. Cal., 85, 2006
291

MÉSZÁROS SZÉCSÉNYI et al.
vibrations of thioamide-group and the pyrazole ring
are still present in the spectrum of the intermediate
3259b, 1627w, 1552w, 1526m, 1406s, 1042m,
the observed one (25%) referring to compounds with
different structure.
(
The thermal stablilty of Co L Cl (Fig. 2) is
2 2 4
–
80sh, 608m cm ) together with a very strong band
1
9
somewhat higher (445 K) than the stability of
Cu L Cl and Cu L Br with a probably similar
–
1
at 2173 cm referring to the formation of an
SCN-group. To examine the course of decomposition
we carried out a coupled TG-MS measurement in a
He atmosphere in aluminum crucible. However, the
decomposition in helium is even faster than in argon.
The successive decomposition steps are almost
inseparable and in a temperature range of 420–480 K
CuSCN (47.5%, calcd: 47.84%) is formed. During the
decomposition we were able to detect only fragments
with significant intensity belonging to m/z of 64, 28 or
2
2
4
2
2
2
course of decomposition (–2HCl: 10%, calcd:
12.79%). At 850 K in argon (air 900 K) CoS is formed
(Ar 30.8%, air, 34.1%, calcd: 31.89%) which up
to 1040 K in argon decomposes to Co (21.6%, calcd:
20.67%) while in air the end product is an oxide
(27.8%, calcd. (CoO): 26.29%).
The first sharp endothermic DSC peak (Fig. 3)
may refer to a decomposition accompanied with melt-
ing of all four compounds. However, the visual obser-
vation shows that there is no melting in the case of
Cu L Cl . The decomposition of Cu L Cl , Cu L Br ,
C H and N , i.e. they do not help in explaining the
5 4 2
decomposition mechanism. In argon and air up
to 530 K the mass loss is 17.5% which may
correspond to loss of a 3,5-dimethylpyrazole (calcd:
2
2
2
2
2
4
2
2
2
Co L Cl complexes above 500 K is accompanied with
2 2 4
an exothermic effect, while the decomposition of
Cu Cl is endothermic up to 600 K.
1
8.91%) which is consistent with appearing of the
2
L
2
2
SCN-band in the IR spectrum of the intermediate.
It is usual that the thermal decomposition
patterns of isomorphous compounds are very similar.
However, the mass loss for the first decomposition
step of the corresponding bromine compound with
departure of a dimethylpyrazole molecule would
amount only 16.08% which is significantly less than
Conclusions
The 3,5-dimethyl-1-carboxamidpyrazole ligand, L, in
the reaction with CuCl
in a bidentate (N, S) coordination which is found by
single crystal X-ray analysis in Cu L Cl , Cu L Br ,
, CuBr and CoCl takes part
2
2 2
2
2
4
2
2
2
Co L Cl complexes. The same type of coordination
2 2 4
mode is proposed for Cu L Cl on the basis of FTIR
2 2 2
data. However, the X-ray patterns and the FTIR-data
of Cu L Cl and Cu L Br are not relevant to decide
2
2
2
2
2
2
whether the corresponding chloro and bromo com-
plexes are isostructural. For this purpose a more de-
tailed X-ray analysis is proclaimed by pronounced
preferred orientation effects observed in pattern and
cannot be undertaken until powder diffraction data in
capillary mode are collected.
The different thermal decomposition pattern of
Cu L Cl may refer to bridging chloro atoms instead of
2 2 2
sulfurs. Namely, the structure of all three Cu L Cl ,
2 2 4
Cu L Br and Co L Cl complexes includes two termi-
2 2 2 2 2
4
nal halide atoms and as a consequence during their
thermal decomposition the first leaving molecules are
most probable hydrohalides. On the contrary, Cu L Cl
2
2
2
decomposes to an intermediate with SCN-group. This
phenomenon may be explained only with dimethyl-
pyrazole departing fragments instead of hydrochloride.
This would be consistent with the bridging chlorine at-
oms in Cu L Cl .
2
2
2
The reduction of Cu(II) to Cu(I) during complex
formation is spontaneous in the bromine compound due
–
probable to higher reducing ability of Br ion relative to
–
Cl . The reduction of Cu(II) in the presence of chloride
ion is promoted by the excess of L and higher tempera-
ture. The thermodynamically more stable compounds
are those of Cu(I) with a more favorable soft–soft
Fig. 3 DSC curves
292
J. Therm. Anal. Cal., 85, 2006

TRANSITION METAL COMPLEXES WTH PYRAZOLE-BASED LIGANDS
Cu(I)–S (Br) interaction (in the case of bromine com-
pound). These complexes are also sterically stabilized.
11 K. Mészáros Szécsényi, V. M. Leovac, ꢀ. K. Jaºimoviº
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1
1
1
Acknowledgements
The work was financed in part by the Ministry for Science and
Environmental Protection of the Republic of Serbia and Provin-
cial Secretariat for Science and Technological Development of
Vojvodina. Financial support from the Hungarian Scientific Re-
search Foundation (OTKA N° T038189) and computational
time from the National Information Infrastructure Development
Program of Hungary is gratefully acknowledged. A.K. thanks
the Bolyai Foundation for support. The authors would like to
thank I. Radosavljeviæ Evans (Dept. of Chemistry, University
of Durham) for performing powder X-ray experiments and
H. Medzihradszky-Scheweige for elemental analyses (Dept. of
Organic Chemistry, Eötvös University, Budapest).
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