Preparation, thermoanalytical and IR study of mixed-ligand complexes formed in water-1,2-ethanediol-cobalt(II)sulfate systems
J. Therm. Anal. Cal.
PREPARATION, THERMOANALYTICAL AND IR STUDY OF MIXED-LIGAND COMPLEXES FORMED IN WATER-1,2-ETHANEDIOL-COBALT(II)SULFATE SYSTEMS
I. Labádi 1 2
L. Horváth 1 2
G. Kenessey 0 1
G. Liptay 0 1
0 Department of Inorganic Chemistry, Technical University of Budapest , Gellért tér 4, 1521 Budapest , Hungary
1 Akadémiai Kiadó , Budapest, Hungary Springer, Dordrecht , The Netherlands
2 Department of Inorganic and Analytical Chemistry, University of Szeged , Dóm tér 7, 6720 Szeged , Hungary
Parent and mixed-ligand cobalt(II) complexes of different compositions were prepared with water, sulfate ion and 1,2-ethanediol as ligands. The magnetic susceptibility data, the IR spectra and the thermoanalytical curves of the complexes were recorded. Oxygen atoms bound by one or two coordinate bonds to the metal ion, or by hydrogen-bonds were observed in the crystals pace. used to obtain solid complexes. The solid complexes were stored under CaCl2 or P2O5 in a desiccator. The preparation was repeated several times to check the reproducibility. The details were as follows:
Co(II) complexes; 1; 2-ethanediol; IR spectra; mixed-ligand complexes; thermoanalytical study
It is well-known that a hydroxy (OH) group, in an
aliphatic organic compound is a relatively weak
donor group in respect of complexation of metal ions.
However, some natural or artificial polyalcohols (e.g.
sugars or cyclodextrins) can often absorb and
transport metal ions in vivo [
], which suggests the
important coordinating ability of the OH groups. The
most important chemical factor in these processes is
complexation through the occurence of coordination
between OH groups and metal ions. To clarify the
nature of the coordination between OH groups and
metal ions, complexes of 1,2-ethanediol(Gl) with
Cu(II), Fe(II), Zn(II), Ca(II), Mg(II), Cd(II), Mn(II)
and Ni(II) ions were earlier prepared and investigated
by thermoanalytical, magnetic susceptibility, IR
spectroscopic and X-ray diffraction methods [
To complete this study from the aspect of
theoretical considerations of the Irving-Williams series of
metal ions [
], Co(II) complexes of 1,2-ethanediol
have now been prepared and investigated by the
above-mentioned methods. The metal ion source was
CoSO4⋅7H2O, so that our earlier studies the
coordination ability of 1,2-ethanediol, water and sulfate ion
could be continued [
Preparation of complexes
For the preparation of the complexes, CoSO4⋅7H2O,
1,2-ethanediol and water were mixed, the solid
compound was dissolved, and different methods were
Co(Gl)1–1.5(H2O)4–5SO4 (complex 1)
20 g of CoSO4⋅7H2O was dissolved in 10 cm3 of
1,2-ethanediol and 2 cm3 of water. In a refrigerator,
solid crystals separated out during 1 day, the
composition of which was close to that of the starting
material, CoSO4⋅7H2O, together with a small amount of
1,2-ethanediol. This solid was filtrated off and
discarded. On the continued cooling, solid crystals
precipitated out from the filtrate, these were separated by
filtration, washed with ethanol and stored over CaCl2
in a desiccator.
Co(Gl)1.5(H2O)4SO4 (complex 2)
5 g of CoSO4⋅7H2O and 5 cm3 of 1,2-ethanediol were
mixed at room temperature. After some hours the
mixture became a slurry. The precipitated solid
compound was separated by filtration, washed with
ethanol, dried and stored over CaCl2 in a desiccator.
Co(Gl)2(H2O)2–3SO4 (complex 3)
5 g of CoSO4⋅7H2O and 10 cm3 of 1,2-ethanediol were
mixed at room temperature. The mixture was filtered
to remove the undissolved solid material. A mixture
of ethanol (10 cm3) and benzene (20 cm ) was then
added to the filtrate during stirring. The precipitated
solid compound was separated by filtration, washed
with ethanol, dried and stored over CaCl2 or/and P2O5
in a desiccator.
Co(Gl)3(H2O)3SO4 (complex 4)
5 g of CoSO4⋅7H2O was dissolved in 10 cm3 of
1,2-ethanediol by heating (70–80°C). The solution
was stored over CaCl2 in a vacuum desiccator. The
pink crystals that precipitated from the solution after
some days, were separated by filtration and washed
with ethanol and ether. The solid material was stored
over CaCl2 in a desiccator.
Co(Gl)3SO4 (complex 5)
5 g of CoSO4⋅7H2O was dissolved in 10 cm3 of
1,2-ethanediol by heating (70–80°C). The solution
was heated to evaporate the water content of the
mixture, and then stored over P2O5 in a vacuum
desiccator. Solid pink crystals precipitated from the solution
after some days; they were separated by filtration and
washed with ethanol and ether. The solid material was
stored over P2O5 in a desiccator.
Determination of composition
The metal, 1,2-ethanediol and water contents of the
complexes were determined by classical analytical
methods (complexometry, Malaprade reaction and
Karl–Fischer method). The analytically determined
compositions were checked thermoanalytically.
The thermal decompositions of the complexes were
investigated with a MOM OD-2 derivatograph at a
heating rate of 2°C min–1. The mass of the
investigated samples was 100–200 mg. The measurements
were carried out in Pt crucibles in an air or nitrogen
atmosphere and α-Al2O3 was used as reference
substance. Decomposition intermediates were prepared
by stopping the heating process at appropriate
temperatures and their compositions were determined by
The IR spectra of the complexes were taken in nujol
in the interval 400–4000 cm–1 with a DIGILAB
instrument. The characteristic bands were utilized to
determine the coordination modes of the ligand, water
molecules and sulfate ion.
Magnetic susceptibility measurements
The magnetic susceptibilities of the complexes were
measured by the Faraday method, using CuSO4 as a
reference compound. A Bruker M15 magnet and a
Sartorius microbalance were used.
Results and discussion
Preparation and compositions of complexes
To prepare the cobalt – 1,2-ethanediol complexes, we
primarily used the common methods, applied earlier to
prepare other 1,2-ethanediol complexes [
]. The basic
idea was to vary the temperature of the preparation, the
ratio of the reactants, and the conditions of the storage of
the prepared complexes. In contrast with the other
1,2-ethanediol metal complexes (e.g. those of Cu(II),
Fe(II), Zn(II) and Ni(II) [
]), these methods of
preparation led to several complexes. Accordingly, we
attempted a new way to obtain complexes: by adding an
ethanol+benzene mixture (as a water-attractive agent) to
a solution of cobalt sulfate and 1,2-ethanediol, the water
content of the complexes could be decreased. This mode
of preparation resulted in a complex with special
thermal behaviour (complex 3).
The compositions of complexes 1 and 3 are
uncertain, varying somewhat in the course of repeated
experiments, and the composition was also strongly
dependent on the condition of storage.
Magnetic susceptibility measurements
The magnetic data on the complexes are shown in
Table 1. The experimentally determined μeff values are
higher than the theoretical value for high-spin
complexes of the cobalt(II) ion (3.83), and an octahedral
coordination sphere around the cobalt(II) ion may be
suggested for all of the complexes [
It was found that the magnetic susceptibility data
followed the Curie–Weiss law. According to the
Curie constant (Θ) values, all the complexes participate
in antiferromagnetic interactions. This magnetic
Bands of CO bonds
Bands of H2O
haviour suggests a polymeric structure for all the
complexes. Complex 5 is too sensitive to moisture for
its magnetic behaviour to be determined.
IR spectroscopic measurements
In the IR spectra of the complexes, the bands of the
water molecules, of the sulfate ion and the stretching
vibration of the C–O bonds could be identified, the values of
the band maxima are presented in Table 2. For
comparison, the IR bands of CoSO4⋅7H2O are also listed.
Bands of the sulfate ion
A sulfate ion with Td symmetry could be observed in
CoSO4⋅7H2O, which correlates well with the structure
of the molecule. In cobalt(II) sulfate heptahydrate, the
sulfate ion is not coordinated to the cobalt(II) ions, it
bound by hydrogen-bonds to water molecules [
some 1,2-ethanediol complexes containing water
molecules, the ν2 and ν4 stretching bands of the
sulfate ion are split into two bands, indicating the C3v
symmetry of the sulfate ion. This means that the
sulfate ion is coordinated by 1 oxygen atom to the metal
ion in these complexes.
Bands of the water molecules
The coordinated water molecules in the hexa- and
heptahydrate give a band with maximum at 1650 cm–1
(deformation vibration). This band could be observed
in all off the water-containing complexes too, which
show that coordinated water molecules are present in
these complexes. The weak band at 1730 cm–1
indicates the presence of water bound by hydrogen-bonds
Bands of the 1,2-ethanediol molecule
1,2-Ethanediol gives 2 strong bands, with maxima at
1085 and 1045 cm–1, assigned predominantly to the
stretching vibration of the CO bond. Decreases in
these band maxima are expected on coordination of
the oxygen atom. This phenomenon can be observed
in Table 2. In the IR spectra of complexes 1, 2, 3
and 4, one or both of the CO bands of free
1,2-ethanediol are to be seen, with 2 other bands close each of
these bands (1070, 1060, 1040 and 1030 cm–1). These
bands point to the presence of free and coordinated
OH groups, i.e. the 1,2-ethanediol molecules act as
monodentate and bidentate ligands.
The structure of the tris(1,2-ethanediol)
complexes (like complex 5) is known [
1,2-ethanediol molecules act as bidentate chelate
ligands and the sulfate ion is located in the outer
coordination sphere, bound by hydrogen-bonds. In
accordance with this structure, the 1070 and 1030 cm–1
bands could be assigned to a bidentate 1,2-ethanediol.
Thermoanalytical behaviour of complexes
On the basis of the thermoanalytical behaviour, the
investigated complexes can be divided into three
groups (A, B and C).
Decomposition of type A
Complexes 1 and 2 lose most of their water and
1,2-ethanediol molecules below 100 and 150°C,
respectively (Fig. 1). Analysis of the intermediates
prepared at 200°C, showed that approximately 0.5
molecule of ethyleneglycol and 1 molecule of water
remained in the solid compound. When the heating
was stopped at 250°C, the product contained only
1 water molecule. The decomposition was
accompanied by endothermic effects and it was the same in air
and in nitrogen atmosphere. The suggested
decomposition scheme is as follows:
Decomposition of type B
Complexes 3 and 4 lose water molecules (two and
three, respectively) and 1,2-ethanediol (one in the
case of the complex 4) molecules below 150°C and
Co(Gl)2(H2O)0–1SO4 bis complexes are formed. This
step is endothermic. The bis complexes decompose
between 180 and 250°C in endothermic process to
give Co(Gl)xSO4 (x=0.5 and 0.66) and between 250
and 300°C in exothermic process to give CoSO4. The
ratio of losses of masses is 2 to 1 in the case of the
Fig. 2 Thermoanalytical curves of complexes of type C
complex 4. The decomposition scheme for complex 3
is shown on Fig. 1 and for complex 4 is as follows:
In a nitrogen atmosphere, the decomposition is
the same as in air, but the exothermic effect is then not
observed. This points to the oxidation of
1,2-ethanediol in air at this temperature.
Decomposition of type C
The tris(1,2-ethanediol) cobalt complex (5) decomposes
in 3 steps: one molecule of the 1,2-ethanediol leaves the
solid phase in each step. The third step is exothermic
(Fig. 2). The decomposition scheme is as follows:
The magnetic and IR data indicate that in all the
investigated cobalt complexes the sulfate ion, the water
and the1,2-ethanediol molecules form an octahedral
coordination sphere around the cobalt(II) ion. The
water and the sulfate ion are coordinated as
monodentate ligands, while in the case of 1,2-ethanediol
there are several possibilities. The 1,2-ethanediol
molecule may coordinate to a metal ion through 1 or 2
oxygen atoms (as a mono- or bidentate ligand). In the
latter case, bridging or chelating binding occurs.
Earlier studies found that, in the presence of water,
1,2-ethanediol always acts as a bridging ligand:
chelation exists only in the water-free complexes
]. The suggested polymeric structure for the
mixed-ligand complexes (complexes 1–4) could be
explain by the bridging coordination. This was also
observed in other mixed-ligand metal complexes of
1,2-ethanediol (M=Mn, Zn, Cu and Ni) [
the tris(1,2-ethanediol) complex, the molecule forms
a chelate and the structure is not polymeric [
It is known that cobalt sulfate heptahydrate loses
6 water molecules at approximately 100°C (depending
on the heating rate). Only 1 water molecule can bind to
metal sulfates above 150°C; the water oxygen atom
and the sulfate oxygen atoms complete the octahedral
coordination sphere. In the mixed-ligand complexes,
besides the water, the 1,2-ethanediol molecules can
compete with the sulfate oxygen atoms for the
coordination sites. This competition depends on the amounts
of water and 1,2-ethanediol, and on the binding modes
of water and 1,2-ethanediol in the complex.
From the aspect of thermal behaviour, complex 4
seems to be characteristic. On heating, this complex
loses all its water below 150°C and a bis(1,2-ethanediol)
complex is formed, which decomposes in 2 steps
between 180 and 250°C. In the first step, 4/3 molecules of
1,2-ethanediol are released from the solid phase. Similar
behaviour was observed in the case of
] and pyridine or picoline complexes
of nickel, cobalt, zinc and cadmium halides . Such
compositions of the intermediate were interpreted in
terms of a polymeric and layered structure.
In complexes 1 and 2, there is not enough
1,2-ethanediol to form a bis complex on heating, so
the intermediate (above 150°C) contains both water
and 1,2-ethanediol, in approximately the same ratio as
present in the complexes (0.5–1).
In complex 4, there are more than 2 molecules
1,2-ethanediol, and thus the bis(1,2-ethanediol)
intermediate complex is formed on heating above 150°C.
On further heating, this intermediate decomposes in a
similar way as complex 3.
In complex 5, there is no water and the
1,2-ethanediol molecules are bound via chelation. On
heating, the bis(1,2-ethanediol) complex also forms
as an intermediate, which decomposes in a different
way from the intermediates of complexes 3 and 4.
Because of the chelation of 1,2-ethanediol, there is no
possibility for the formation of polymeric structure,
which was suggested for the decomposition products
of complexes 3 and 4.
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Received: March 30 , 2005 In revised form: May 20 , 2005 DOI : 10.1007/s10973-005-7011-2