Thermal analysis study of some transition metal complexes by TG and DSC methods

Article abstract of DOI:10.1023/B:JTAN.0000017341.20105.22

The thermodynamic and thermal properties of [Cu(L)₂·Cl₂], [Ni(L)₂]·Cl₂, [Co(L)₂·Cl₂]; L=1,2-bis(o-aminophenoxy)ethane (BAFE), complexes have been investigated. The thermal decomposition of

Full text of DOI:10.1023/B:JTAN.0000017341.20105.22

Journal of Thermal Analysis and Calorimetry, Vol. 75 (2004) 189–195
THERMAL ANALYSIS STUDY OF SOME TRANSITION
METAL COMPLEXES BY TG AND DSC METHODS
M. Sekerci 1 and F. Yakuphanoglu 2*
1Firat University Faculty of Arts and Sciences, Department of Chemistry, Elazig, Turkey
2Firat University Faculty of Arts and Sciences, Department of Physics, Elazig, Turkey
(Received June 26, 2003; in revised form August 30, 2003)
Abstract
The thermodynamic and thermal properties of [Cu(L)2⋅Cl2], [Ni(L)2]⋅Cl2, [Co(L)2⋅Cl2]; L=1,2-
bis(o-aminophenoxy)ethane (BAFE), complexes have been investigated. The thermal decomposi-
tion of the complexes took place in two distinct steps in endothermic reaction up to 700°C. The acti-
vation energy E, the entropy change ∆S#, enthalpy ∆H change and Gibbs free energy change ∆G#
were calculated from the results of thermogravimetry analysis (TG) and heat capacity from the re-
sults of differential scanning calorimetry (DSC). It was found that the thermal stabilities and activa-
tion energies of the complexes follow the order Ni(II)>Cu(II)>Co(II) and ECo<ENi<ECu, respectively.
Keywords: thermal analysis, thermodynamic parameters, transition metal complexes
Introduction
The thermal analysis techniques were extensively applied in studying of the thermal
behavior of metal complexes [1–9]. The preparation and characterization of metal com-
plexes by elemental analysis, IR spectroscopy, UV-visible spectroscopy and magnetic
susceptibility studies were reported earlier [1–7]. In the literature, their thermal decom-
position behavior occurred in one or more steps depending on the type of the com-
plexes investigated and decomposition ends with the formation of the metal oxides. It is
also found that thermal behavior of the complexes investigated is related to the decom-
position of the ligand molecule. In order to check the thermal stability of the complexes
in solid state, initial temperatures of the decomposition of the complexes were com-
pared. The main basic of these studies is to determine the thermal behavior of the metal
complexes. Thus, the differential thermal analysis and other thermoanalytical methods
are used to determine kinetic parameters of thermal decomposition such as activation
energy, enthalpy, entropy and heat capacity. In general thermoanalytical methods are
used to constantly measure the changes occurring in the physical properties of a mate-
rial, such as mass as a function of temperature or time. Thermogravimetry is a process
*
Author for correspondence: E-mail: fyhanoglu@firat.edu.tr
1388–6150/2004/ $ 20.00
Akadémiai Kiadó, Budapest
© 2004 Akadémiai Kiadó, Budapest
Kluwer Academic Publishers, Dordrecht

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SEKERCI, YAKUPHANOGLU: TRANSITION METAL COMPLEXES
in which a substance is decomposed in the presence of heat, which causes bonds the
molecules to be broken [10, 11]. Differential scanning calorimetry (DSC) method have
been widely used to determine non-isothermal transformation kinetics. Differential
thermal analysis (DTA) and DSC are of particular importance. Both procedures permit
the amount of heat to be determined that are taken up from or emitted to the surround-
ings per unit time during isothermal procedures or during heating and cooling. In this
manner heat capacities, melt enthalpies, transition temperatures, etc., can be measured
and from this information further indications regarding phase transitions, crystalliza-
tion processes etc. can be derived. The thermodynamic parameters are very important
for the studying of heat transport phenomena mechanism in various solid state com-
pounds.
In this study, we have investigated thermal and thermodynamic properties of
synthesized complexes to understand the mechanism of decomposition and thermo-
dynamic parameters.
Experimental
Synthesis of the Cu(II), Ni(II) and Co(II) complexes
The 1,2-bis(o-aminophenoxy)ethane (0.43 g, 0.02 mol) ligand (L) [12] was dissolved
in 10 mL of absolute ethanol in a 50 mL round-bottom flask. A solution of metal salts
of CuCl2⋅6H2O (0.24 g, 0.01 mol), NiCl2⋅6H2O (0.24 g, 0.01 mol), CoCl2⋅6H2O (0.24 g,
0.01 mol), in 5 mL of absolute ethanol was added dropwise with continuous stirring at
room temperature for 15 min and the reaction continued for 12 h at room temperature.
The resulting precipitates were filtered off, washed with absolute ether and dried at
room temperature. The synthesized complexes [Cu(L)2⋅Cl2], [Ni(L)2]⋅Cl2, [Co(L)2⋅Cl2]
were characterized by 1H-NMR, FT-IR, UV spectroscopy and elemental analysis and
were described elsewhere [12, 13]. Their structural formulas are given in Figs 1a–c.
Fig. 1 The structural formulas of the complexes. a – [Cu(L)2⋅Cl2], b – [Ni(L)2]⋅Cl2 and
c – [Co(L)2⋅Cl2]
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SEKERCI, YAKUPHANOGLU: TRANSITION METAL COMPLEXES
191
Thermal analysis
The DSC and TG curves were obtained using DSC-50 and TG-50 Schimadzu appara-
tus (heating rates of 10 and 20°C min–1, aluminum crucible, mass 20 mg, in nitrogen
atmosphere) respectively. In TG apparatus, the aluminum crucible is then adjusted to
another crucible support platform, which gives a proportional signal to the recorder
and computer interface to plot the mass loss of complex vs. temperature.
Results and discussion
Thermodynamic and thermal studies
Figure 2 shows DSC curves of the complexes. The complexes exhibited an endother-
mic process. The area of the endothermic peak corresponds to the heat of fusion and
the peak temperature corresponds to the melting point. The melting (Tp) and transi-
tion temperatures (T1, T2) of the complexes are given in Table 1. The heat capacities
Cp of the complexes were calculated from DSC results and are given in Table 1.
Fig. 2 The DSC curves of the complexes
The TG curves of the complexes are shown in Fig. 3. The complexes are thermal
stable up to (Ts) temperature (Table 1). Co(II) complex starts to decompose at 132°C
and this step continues up to 382.6°C. The second step starts after 500°C and was
completed at 619.4°C. The final solid product of thermal decomposition is CoO
(calc./found: 7.71/11.46%) (Table 2).
The Ni(II) complex was stable up to 209.5°C. The first step of thermal decomposi-
tion was completed at 377.2°C. The next decomposition step starts at 557.7 and contin-
ues up 623.1. The stable oxide NiO is formed after 623.1°C (calc./found: 8.75/12.09%)
(Table 2).
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SEKERCI, YAKUPHANOGLU: TRANSITION METAL COMPLEXES
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SEKERCI, YAKUPHANOGLU: TRANSITION METAL COMPLEXES
193
Table 2 The TG data for Cu(II), Ni(II) and Co(II) complexes
First step/
Mass loss, ∆m Second step/
Calc./found/
Complex
Residue
°C
calc./found/%
°C
%
[Cu(L)2⋅Cl2]
[Ni(L)2]⋅Cl2
[Co(L)2⋅Cl2]
215.2–358.6
239.5–377.2
132–382.6
88.00 (87.22)
400–569.2
CuO
NiO
CoO
12.00 (12.78)
8.75 (12.09)
7.71 (11.46)
91.25 (87.91) 557.7–623.1
92.29 (88.54) 500–619.4
The Cu(II) complex was stable up to 215.2°C and decomposition started at this tem-
perature and was completed at 358.6°C. The second step temperatures are in the range of
400 and 569.2°C. The solid residue was CuO (calc./found: 12.00/12.78%) (Table 2).
The kinetic of heterogeneous condensed phase reactions that occur in non-
isothermal conditions is usually described by equation:
dα
dT
E




β
= A (α)exp −
(1)

f
RT

where α is the degree of conversion, β is the linear heating rate, and Af(α) is the dif-
ferential conversion function.
For calculate kinetic parameters, the different reaction models may be used. The
fraction mass loss (α) and corresponding (1–α)n are calculated from TG curves,
where n depends upon the reaction model. We choose Coats–Redfern model and it
may be expressed by the following relation [14–16],
1–n


1− (1−α)
T 2 (1−n)
AR
2RT
E




ln
≡ ln
1−
−
(2)




βE
E
RT


A graphical representation of ln[–ln(1–α)/T2] function of 1/T gives a straight
line with inclination –E/R. The activation energies of the complexes were calculated
from Fig. 4 and given in Table 1.
Fig. 3 The TG curves of the complexes
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SEKERCI, YAKUPHANOGLU: TRANSITION METAL COMPLEXES
Fig. 4 Coats and Redfern plots for the complexes
The enthalpy ∆H, activation entropy ∆S#, and the free energy of activation ∆G#,
of the complexes were calculated using the following relations [17]:
Ah




∆S # =2.303lg
R
(3)
kT


∆H=E−RT
∆G# =∆H−T∆S #
(4)
(5)
where h is the Planck constant and T is the temperature, A is the Arrhenius constant.
The thermodynamic parameters calculated were reported in Table 1.
The kinetic parameters, especially activation energy E and activation entropy ∆S#
are helpful in assigning the strength of the bonding of ligand moieties with the metal
ion [17]. The activation energies corresponding decomposition steps are in the range of
91.38–148.87 kJ mol–1 which indicates bonding degree of ligand bound to metal ion. It
was found that the thermal stability increases in this order: Ni(II)>Cu(II)>Co(II). It is
also found that all the complexes have a negative entropy, which indicates that the stud-
ied complexes have more ordered systems than reactants. The activation energy and
Gibbs free energy of the complexes also increase due to increase of atomic number in
the 3rd series of the d-block elements.
The experimental data suggest that studied complexes decompose in two-step pro-
cess. All the complexes show almost a similar decomposition when heated above 600°C.
The solid products of thermal decomposition were metal oxides (CuO, NiO and CoO).
Conclusions
The complexes prepared with different metals decompose in two-step process. The
kinetic parameters in non-isothermal conditions are investigated and thermodynamic
parameters such as enthalpy, entropy and specific heat capacity of the complexes
were calculated. The highest thermal stability is displayed by the Ni(II) complex and
thermal stabilities of the complexes follow the order Ni(II)>Cu(II)>Co(II). In litera-
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SEKERCI, YAKUPHANOGLU: TRANSITION METAL COMPLEXES
195
ture, it is found by Arshad et al. that thermal stabilities of 1,2-dipiperidinoethane
complexes of cobalt, nickel, copper, zinc and cadmium, increase in the following se-
quence: Ni(II)<Co(II)<Cu(II)<Zn(II)<Cd(II) [8]. On the other hand, in another study,
the thermal stabilities of metal complexes of the 6-(2-pyridylazo-acetamidophenol
were found as Ni(II)>Cu(II)>Zn(II)>Fe(II)>Co(II)>Cd(II) [9]. Consequently, it is
evaluated that the coordination of metal ion to ligand is responsible for thermal stabil-
ities of the metal complexes.
References
1 M. A. Zayed and F. A. Nour El-Died, Thermochim. Acta, 114 (1987) 359.
2 M. Zayed, M. M. Khater, M. S. Rizk and H. M. Abd El-Fattah, Thermochim. Acta, 73 (1984) 217.
3 A. A. Soliman, J. Therm. Anal. Cal., 63 (2001) 221.
4 A. F. Petrovic, D. M. Petrovic, V. M. Zeovac and M. Budimir, J. Therm. Anal. Cal., 58 (1999) 589.
5 W. W. Wendlandt and T. D. George, J. Inorg. Chem., 21 (1961) 69.
6 E. L. Simmons and W. W. Wendlandt, J. Inorg. Nucl. Chem., 27 (1965) 2325.
7 N. Deb, S. D. Baruah, N. Sen Sarma and N. N. Dass, Thermochim. Acta, 320 (1998) 53.
8 M. Arshad, Saeed-ur-Rehman, S. Ali Khan, K. Masud, N. Arshad and A. Ghani, Thermochim.
Acta, 364 (2002) 143.
9 G. G. Mohamed, F. A. Nour-El Dien and E. A. El-Gamel, J. Therm. Anal. Cal., 67 (2002) 135
10 C. L. Albano, R. Sciamanna, T. Aquino and J. J. Martinez, Europan Congress on Computional
Methods in Applied Sciences and Engineering, ECOMAS 2000, Barcelona,
11–14 September, 2000.
11 F. Carrasco, Thermochim. Acta, 213 (1993) 115.
12 M. Sekerci, M. Boybay, Ü. Demirtaê and H. Temel, XVI. National Chemistry Congress, 2002.
13 F. Yakuphanoglu, Ph. D. Thesis, Firat University, Elazig, Turkey 2002.
13 M. L. Dhar and O. Singh, J. Thermal Anal., 37 (1991) 499.
14 H. H. Horowitz and A. Metzger, Anal. Chem., 35 (1963) 1464.
15 A. W. Coats and J. P. Redfren, Nature, 201 (1964) 68.
16 R. M. Mahfouz, K. A. Al-Farhan, G. Y. Hassen, A. I. Al-Wassil, S. M. Alshehri and
A. A. Al-Wallan, Synth. React. Inorg. Met-Org. Chem., 32 (2002) 489.
J. Therm. Anal. Cal., 75, 2004