A thermal decomposition study on cobalt(II) complexes of 1,2-bis(imino-4′-Antipyrinyl)ethane

Article abstract of DOI:10.1023/B:JTAN.0000032266.26941.20

The phenomenological, kinetic and mechanistic aspects of thermal decomposition of perchlorate, nitrate, chloride, bromide and iodide complexes of cobalt(II) with the Schiff base 1,2-bis(imino-4′ antipyrinyl)ethane (GA) have been studied by TG and DTG analyses. The kinetic parameters like the activation energy, pre-exponential factor and entropy of activation were calculated. The decomposition reactions follow 'random nucleation with one nucleus on each particle - Mampel model'.

Full text of DOI:10.1023/B:JTAN.0000032266.26941.20

Journal of Thermal Analysis and Calorimetry, Vol. 76 (2004) 813–821
A THERMAL DECOMPOSITION STUDY ON
COBALT(II) COMPLEXES OF
1,2-BIS(IMINO-4’-ANTIPYRINYL)ETHANE
N. T. Madhu1, P. K. Radhakrishnan2, M. Grunert1, P. Weinberger1 and
W. Linert1*
1Department of Chemistry, Institute for Applied Synthetic Chemistry 163/AC, Vienna University
of Technology, Getreidemarkt 9/163-AC, A-1060 Vienna, Austria
2School of Chemical Sciences, Mahatma Gandhi University, Kottayam – 686 560, Kerala, India
(Received October 9, 2003; in revised form March 17, 2004)
Abstract
The phenomenological, kinetic and mechanistic aspects of thermal decomposition of perchlorate,
nitrate, chloride, bromide and iodide complexes of cobalt(II) with the Schiff base 1,2-bis(imino-4’-
antipyrinyl)ethane (GA) have been studied by TG and DTG analyses. The kinetic parameters like the
activation energy, pre-exponential factor and entropy of activation were calculated. The decomposi-
tion reactions follow ‘random nucleation with one nucleus on each particle – Mampel model’.
Keywords: cobalt(II) complexes, Coats–Redfern, kinetics, thermal decomposition
Introduction
A plethora of references describing the metal complexes of Schiff bases have ap-
peared in the literature during the past few decades [1–7]. However, little is known
on the thermal studies of such complexes [4, 5]. In view this and as part of our
continuing interest on the thermal studies of Schiff base-metal complexes [6], we re-
port here the phenomenological, kinetic and mechanistic aspects of thermal decom-
position of the perchlorate, nitrate, chloride, bromide and iodide complexes of co-
balt(II)
with
the
Schiff
base
derived
from
antipyrine,
viz.,
1,2-bis(imino-4’-antipyrinyl)ethane (GA), empirical formula: C24H24N6O2.
Experimental
The Schiff base, 1,2-bis(imino-4’-antipyrinyl)ethane(GA) and its cobalt(II) complexes
were prepared and characterized as reported earlier [7]. Thermogravimetric analyses
were undertaken on Delta Series TGA 7 thermal analyzer in nitrogen atmosphere,
*
Author for correspondence: E-mail: wlinert@mail.zserv.tuwien.ac.at
1388–6150/2004/ $ 20.00
Akadémiai Kiadó, Budapest
© 2004 Akadémiai Kiadó, Budapest
Kluwer Academic Publishers, Dordrecht

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MADHU et al.: 1,2-BIS(IMINO-4’-ANTIPYRINYL)ETHANE
sample mass: 10 mg, heating rate: 10°C min–1. The kinetic evaluation of the thermal
decomposition of the complexes was done using a computer program in Qbasic.
Results and discussion
The elemental analysis, electrical conductance in non-aqueous media, magnetic
moments and infrared as well as electronic spectra show [7] that the five complexes have
the formulae: [Co(GA)(ClO4)]ClO4, [Co(GA)(NO3)]NO3, [Co(GA)Cl2], [Co(GA)Br2]
and [Co(GA)I2]. GA acts as a neutral tetradentate ligand coordinating through both
carbonyl oxygens and both azomethine nitrogens in all these complexes. In the per-
chlorate and nitrate complexes, one of the anions is coordinated in a bidentate fashion,
whereas in the halide complexes both the anions are coordinated to the metal, generating
Fig. 1 TG and DTG curves of [Co(GA)ClO4]ClO4
Fig. 2 TG and DTG curves of [Co(GA)NO3]NO3
J. Therm. Anal. Cal., 76, 2004

MADHU et al.: 1,2-BIS(IMINO-4’-ANTIPYRINYL)ETHANE
815
Fig. 3 TG and DTG curves of [Co(GA)Cl2]
Fig. 4 TG and DTG curves of [Co(GA)Br2]
an octahedral geometry around the Co(II) ion. The TG and DTG curves of all five
complexes are given in Figs 1–5.
Phenomenological aspects
The phenomenological aspects of the present complexes are presented in Table 1.
The perchlorate complex, Co(GA)(ClO4)]ClO4, undergoes a three-stage decomposi-
tion in the range 252–641°C. The first stage starts at 252°C and ends at 374°C with a
mass loss of (31.42%) is attributed to the decomposition of half a molecule of GA.
The infrared spectrum of the residue after this stage shows the presence of GA indi-
cating only partial removal of the ligand. The DTG peak corresponding to this stage
is observed at 305°C. The second stage starts at 374°C and ends at 540°C with a DTG
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MADHU et al.: 1,2-BIS(IMINO-4’-ANTIPYRINYL)ETHANE
Fig. 5 TG and DTG curves of [Co(GA)I2]
Table 1 Phenomenological data for the thermal decomposition of cobalt(II) complexes
Stage of
decomposition
Mass loss found
Complex
TG plateaux/°C DTG peak/°C
(calcd.)/%
31.42
(31.21)
I
II
III
I
252–374
374–540
540–641
156–327
327–423
423–499
172–463
463–636
305
431
580
270
389
440
354
610
18.56
(18.65)
[Co(GA)(ClO4)]ClO4
31.78
(31.21)
35.07
(35.03)
19.92
(20.28)
[Co(GA)(NO3)]NO3
II
III
I
35.35
(35.03)
38.64
(38.37)
[Co(GA)Cl2]
[Co(GA)Br2]
[Co(GA)I2]
38.36
(38.37)
II
33.60
(33.09)
I
II
I
283–450
450–
372
–
continuous
59.56
(59.98)
179–461
326
35.71
(35.53)
II
461–539
483
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MADHU et al.: 1,2-BIS(IMINO-4’-ANTIPYRINYL)ETHANE
817
peak at 431°C. The corresponding mass loss (18.56%) is due to the conversion of two
perchlorates into chlorides. Absence of perchlorate ion after this stage is confirmed
by infrared spectral analysis of the intermediate compound obtained at 540°C. The
third stage, which occurs in the temperature range 540–641°C with a DTG peak
at 580°C, corresponds to a mass loss of (31.78%) corresponding to the decomposi-
tion of the remaining half of the GA molecule. The final residue is anhydrous cobalt
chloride as confirmed by qualitative analysis.
The nitrate complex is formulated as [Co(GA)(NO3)]NO3. It decomposes in
three stages. The first stage starts at 156°C and ends at 327°C with a DTG peak
at 270°C. The corresponding mass loss (35.07%) is due to the decomposition of half
a molecule of GA. The infrared spectrum of the residue after this stage shows the
presence of GA indicating only a partial removal of the ligand. The second stage of
decomposition starts at 327°C and ends at 423°C with the DTG peak at 389°C. The
corresponding mass loss (19.92%) is due to the decomposition of two nitrate ions.
The third stage starts at 423°C and ends at 499°C with the DTG peak at 440°C. The
corresponding mass loss (35.35%) is attributed to the removal of the remaining half
of the molecule of GA. The decomposition complete at 499°C and the final residue is
anhydrous cobalt oxide as confirmed by qualitative analysis.
The chloride complex, [Co(GA)Cl2], undergoes a two-stage decomposition. The
first stage begins at 172°C and ends at 463°C. The corresponding mass loss (38.64%)
is due to the decomposition of half a molecule of GA. The infrared spectrum of the
residue after this stage shows the presence of GA, indicating only a partial removal of
the ligand. The second stage starts at 463°C and ends at 636°C and the corresponding
mass loss (38.36%) is due to the removal of the remaining half of the molecule of GA,
yielding anhydrous cobalt chloride as the final residue as confirmed by qualitative
analysis. The rate of mass loss is found to be maximum at 354 and 610°C, respectively,
for the first and second stages as indicated by the corresponding DTG peaks.
The bromide complex is represented as [Co(GA)Br2]. It undergoes decomposi-
tion in two stages. The first stage starts at 283°C and ends at 450°C. The correspond-
ing mass loss (33.60%) is attributed to the decomposition of half a molecule of GA.
The infrared spectrum of the residue after this stage shows the presence of GA, indi-
cating only a partial removal of the ligand. The DTG peak of this stage is observed
at 372°C. The second stage of decomposition starts at 450°C and is a continuous one.
The steady mass loss observed in this stage may be due to the expulsion of the re-
maining half of the molecule of GA together with volatilization of the residue of an-
hydrous cobalt bromide [8] above 678°C.
The iodide complex, [Co(GA)I2], undergoes a two-stage decomposition. The
first stage begins at 179°C and ends at 461°C with the DTG peak at 326°C. The cor-
responding mass loss (59.56%) is due to the decomposition of the GA molecule. The
second stage starts at 461°C and ends at 539°C with the DTG peak at 483°C. The cor-
responding mass loss (35.71%) is due to the decomposition of two iodide ions. The
decomposition complete at 539°C and the final residue is anhydrous cobalt oxide as
confirmed by qualitative analysis.
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MADHU et al.: 1,2-BIS(IMINO-4’-ANTIPYRINYL)ETHANE
Structure and thermal stability correlation
The TG and DTG data reveal that although the molecular composition of the com-
plexes is the same, viz., Co(GA)X2 (X=ClO−, NO−, Cl– , Br– or I–), the nature of the
4
3
decomposition pattern is different. The perchlorate and nitrate complexes exhibit a
three-stage decomposition whereas a two-stage decomposition is observed in the
case of chloride, bromide and iodide complexes. Within the series the nitrate com-
plex is the least stable and the bromide complex is the most stable. The difference in
thermal stability of the complexes reveals that the anion may have some marked in-
fluence on the thermal stability of the complexes which is in the order:
Bromide > Perchlorate > Iodide > Chloride > Nitrate
Kinetic aspects
All the well-defined stages were selected for the study of the kinetics of decomposi-
tion of the complexes. The kinetic parameters (the activation energy E and the
pre-exponential factor A) were calculated using the Coats–Redfern equation [9].
g(α)
T 2
AR
2RT
E




log
=log
1−
−




φE
E
2.303RT




In the present case, a plot of L.H.S of this equation vs. 1/T gives straight line
(Fig. 6) whose slope and intercept are used for calculate the kinetic parameters by the
least-square method. The goodness of fit was checked by calculating the correlation
coefficient.
The entropy of activation ∆S can be calculated using the equation
kTs
A=
e∆S/ R
h
where k=Boltzmann constant, h=Planck’s constant and Ts=peak temperature.
The various kinetic parameters calculated are given in Table 2. The activation
energy (E) in the different stages are in the range 30.16–306.99 kJ mol–1. The
respective values of the pre-exponential factor (A) vary from 2.72⋅10–2 to 7.84⋅107 s–1.
The corresponding values of the entropy of activation (∆S) are in the range –281.02 to
–100.89 J mol–1 K–1. There is no definite trend either in the values of A or ∆S among the
different stages in the present series. But the activation energy for the second stage is
greater than that of the first stage, may due to the less steric strain occurring at that
stage. However, the negative values of entropy of activation indicate that the activated
complex has a more ordered structure than the reactants [6].
Mechanistic aspects
The assignment of the mechanism is based on the assumption that the form of g(α)
depends on the reaction mechanism. Here in the present investigation, nine forms of
g(α), suggested by Šatava [10] are used to enunciate the mechanism of thermal
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MADHU et al.: 1,2-BIS(IMINO-4’-ANTIPYRINYL)ETHANE
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Table 2 Kinetic parameters for the thermal decomposition of cobalt(II) complexes
Complex
Stage
E/kJ mol–1
A/s–1
∆S/J mol–1 K–1
I
II
III
I
97.94
174.90
232.48
66.49
45.79
1.01
1.46×103
–218.61
–251.88
–173.89
–238.94
–218.07
–100.89
–281.02
–200.45
–266.63
–275.78
–162.08
[Co(GA)(ClO4)]ClO4
3.73
[Co(GA)(NO3)]NO3
II
III
I
114.15
306.99
30.16
55.96
7.84×107
2.72×10–2
6.33×102
0.15
4.84×10–2
5.30×104
[Co(GA)Cl2]
[Co(GA)Br2]
[Co(GA)I2]
II
I
187.53
49.88
I
35.66
II
221.64
Fig. 6 log[g(α)/T 2] vs. 1000/T plots for the thermal decomposition of cobalt(II)
complexes. a – [Co(GA)(ClO4)]ClO4, b – [Co(GA)(NO3)]NO3, c – [Co(GA)Cl2],
d – [Co(GA)Br2], e – [Co(GA)I2]
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MADHU et al.: 1,2-BIS(IMINO-4’-ANTIPYRINYL)ETHANE
821
decomposition in each stage. The correlation coefficient for all these nine forms were
calculated (Table 3) and the form of g(α) for which the correlation has a maximum
value is chosen as the mechanism of reaction. In the present investigation, the highest
value of correlation coefficient is obtained for
g(α) = –ln(1–α)
in all stages of decomposition. Hence the mechanism is ‘random nucleation with one
nucleus on each particle’ representing ‘Mampel model’.
* * *
We thankfully acknowledge financial support by the Austrian Science Foundation (FWF,
project 15874-NO3) and to the "Jubil¬umsfondes" of the Austrian National Bank (Project 10668)
References
1 T. Radhakrishnan, P. T. Joseph and C. P. Prabhakaran, J. Inorg. Nucl. Chem., 38 (1976) 2217.
2 R. K. Agarwal, P. Garg, H. Agarwal and S. Chandra, Synth. React. Inorg. Met.-Org. Chem.,
27 (1997) 251.
3 S. Mayadevi and K. K. M. Yusuff, Synth. React. Inorg. Met.-Org. Chem., 27 (1997) 319.
4 B. Ferenc and B. Bocian, J. Therm. Anal. Cal., 74 (2003) 521.
5 B. Ma³ecka, E. D. Ciesla and P. K. Olszewski, J. Therm. Anal. Cal., 74 (2003) 485.
6 C. R. Vinodkumar, M. K. M Nair and P. K. Radhakrishnan, J. Therm. Anal. Cal., 61 (2000) 143.
7 N. T. Madhu and P. K. Radhakrishnan, Trans. Met. Chem., 25 (2000) 287.
8 J. A. Dean, Section-4 Inorganic Chemistry. Langes Handbook of Chemistry,
McGraw-Hill 1987, p. 4.
9 A. W. Coats, J. P. Redfern, Nature, 201 (1964) 68.
10 V. Satava, Thermochim. Acta, 2 (1971) 423.
J. Therm. Anal. Cal., 76, 2004