Kinetic analysis of the thermal decomposition of dialkyldithio-carbamates chelates of indium(III)

Article abstract of DOI:10.1023/A:1013971031425

Kinetic analysis of the thermal decomposition of dialkyldithiocrbamtes chelates of indium(III) was carried out using isothermal and non-isothermal thermogravimetry. The complexes decomposed in the liquid phase with the exception of two compounds after mel

Full text of DOI:10.1023/A:1013971031425

Journal of Thermal Analysis and Calorimetry, Vol. 67 (2002) 359–363
KINETIC ANALYSIS OF THE THERMAL
DECOMPOSITION OF DIALKYLDITHIO-
CARBAMATES CHELATES OF INDIUM(III)
A. G. Souza1*, M. M. Oliveira1, I. M. G. Santos1, M. M. Conceição1,
L. M. Nunes1 and J. C. Machado2
1Departamento de Química, CCEN, Universidade Federal da Paraíba, 58.059-900, João Pessoa,
Paraíba, Brazil
2Departamento de Química, Universidade Federal de Minas Gerais, 31.270-901, Belo Horizonte,
Minas Gerais, Brazil
Abstract
The kinetics of thermal decomposition of solid In(S2CNR2)3 complexes, (R=CH3, C2H5, n-C3H7,
i-C3H7, n-C4H9 and i-C4H9), has been studied using isothermal and non-isothermal thermogravi-
metry. Superimposed TG/DTG/DSC curves show that thermal decomposition reactions occur in the
liquid phase, except for the In(S2CNMe2)3 and In(S2CNPr2i)3 compounds.
Keywords: dialkyldithiocarbamates, indium, kinetics, thermogravimetry
Introduction
Studies concerning the kinetics of thermal decomposition of dialkyldithiocarbamates
metallic complexes are scant in literature. Recently, Machado et al. [1] and Rangel
et al. [2] have studied the kinetics of isothermal and non-isothermal decomposition of
cadmium(II) and palladium(II) complexes, in both these studies TG and DSC tech-
niques were used. These authors showed that there is no direct relationship between
apparent activation energy and the thermal stability of the complexes.
In the present work the kinetics of the thermal decomposition of Indium(III)
dialkyldithiocarbamates, In(S2CNR2)3, with R=CH3, C2H5, n-C3H7, i-C3H7, n-C4H9, or
i-C4H9, using isothermal and non-isothermal heating methods was studied.
Experimental
In(III) dialkyldithiocarbamates complexes were synthesized from direct reaction of
InCl3 with the various dialkyldithiocarbamate dialkylammonium compounds in wa-
ter, in 1:3 molar proportion [3, 4]. After filtration, the crystals were recrystallized in
acetone and dried in vacuum.
*
Author for correspondence: E-mail: gouveia@quimica.ufbp.br
1418–2874/2002/ $ 5.00
Akadémiai Kiadó, Budapest
© 2002 Akadémiai Kiadó, Budapest
Kluwer Academic Publishers, Dordrecht

360
SOUZA et al.: DIALKYLDITHIOCARBAMATES CHELATES
Characterization measurements
The melting points were determined using a Microquimica (model MQAPF-301) ap-
paratus. Infrared spectra were recorded in the range 4000–200 cm–1, using a BOMEM
(model MB-102) FTIR spectrophotometer; mass spectra of the ligands and chelates
were obtained using a Hewlett–Packard (model 5988A) spectrometer, with an ioniza-
tion energy of 70 eV, at 523 K. The thermal decomposition residues were analysed by
X-ray powder diffraction, using an X-ray SIEMENS (model D-5000) diffractometer.
Thermal measurements
Thermogravimetric curves were obtained using a Shimadzu (model TGA-50) thermo-
balance, under nitrogen atmosphere. The carrier gas flow was always 0.83 mL s–1 and the
sample masses were 5.0±0.5 mg. For the non-isothermal experiments, three different
heating rates were used: 5, 10 and 15°C min–1.
DSC curves were obtained under nitrogen atmosphere, using a Shimadzu
(model DSC-50) differential scanning calorimeter, with heating rate of 10°C min–1.
Results and discussion
The elemental analysis and melting points are in good agreement with the expected
values for all compounds. After complex formation, the νCN band assumes a doublet
character reflected in the presence of a stretching band shifted to higher frequency, in
comparison with the respective free ligand. On the other hand, a decrease in the νCS
stretching band was observed and an isolated band near 1000 cm–1 indicated that the
ligand is bound to the metal in a bidentate way [5].
The mass spectra of the complexes indicated the presence of the parent ions, ex-
cept for In(S2CNPr2i)3, with the following stability order: In(S2CNPr2n)3>
In(S2(CNMe2)3>In(S2CNBui2)3>In(S2CNBun2)3>In(S2CNEt2)3>In(S2CNPr2i)3.
The thermogravimetric curves of the complexes presented a very similar behav-
ior. The complexes show one definite step of mass loss, attributed to the ligand loss.
The following order of thermal stability is observed: In(S2CNMe2)3>In(S2CNBun2)3>
In(S2CNEt2)3>In(S2CNPr2n)3>In(S2CNPr2i)3~In(S2CNBu2i )3.
X-ray diffraction analysis of the thermal decomposition residues showed that,
except for the In(S2CNBun2)3 complex, the complexes that decompose in the liquid
phase, In(S2CNEt2)3, In(S2CNPrn)3, In(S2CNBun2)3 and In(S2CNBu2i )3, presented Ino
as residue. The complexes that decompose in the solid phase, In(S2CNMe2)3,
ln(S2CNPr2i )3 and In(S2CNBu2n)3, had In2S2 as residue.
Isothermal decomposition
The experimental data for isothermal decomposition were evaluated with the aid of a
linear regression program for the most commonly proposed models [6–8], using the
relationship:
J. Therm. Anal. Cal., 67, 2002

SOUZA et al.: DIALKYLDITHIOCARBAMATES CHELATES
361
g(α)=kt+ko
(1)
where ko is the induction period.
The temperature ranges for isothermal experiments were obtained from TG
curves for each complex. Experimental data were evaluated in terms of decomposed
fraction (α) as a function of time (t). Experimental temperatures for each complex
were: 323, 329, 332, 335 and 338oC for In(S2CNMe2)3; 283, 290, 293, 297 and 305°C
for In(S2CNEt2)3; 285, 290, 295, 300 and 303°C for In(S2CNPr2n)3; 267, 270, 273, 282
and 281°C for In(S2CNPr2i)3; 297, 303, 309, 312 and 315°C for In(S2CNBu2n)3 and
283, 286, 289, 292 and 295°C for In(S2CNBui2)3.
For In(S2CNEt2)3, In(S2CNPrn)3, In(S2CNBu2n)3 and In(S2CNBu2i )3 chelates, a
linear relationship between α and t was observed, in the range 0.1≤α ≤0.9. The same
behavior was not observed for In(S2CNMe2)3 and In(S2CNPr2i)3. For these two che-
lates α vs. t curves presented a sigmoidal shape with a significant induction period.
This behavior could be due to the beginning of the decomposition already in the solid
phase, which opposite to the behavior observed for other chelates, the decomposition
of which occurs completely in liquid phase.
Using the linear correlation coefficients (r) or the standard deviations (s), as crite-
rion of analysis, the most appropriated model to describe the experimental results for the
complexes In(S2CNR2)3 (with R=Et, Prn, Bun and Bui) was the phase-boundary reaction,
R1. The A3 model seems to be equally good to fit the experimental data for
In(S2CNBun2)3. For the complexes with R=Me and Pri, the model that best adjusted to the
experimental data was the nucleation and nucleus growth models, A3 and A4. For all fit-
tings, linear correlation coefficients (r) were equal or better than 0.998, and standard de-
viations (s) were in the interval (0.09–1.6)·10–2.
The kinetic parameters were calculated according the Arrhenius equation:
k=Aexp(–E/RT)
(2)
where k – is the rate constant, A – is the pre-exponential factor and E – is the apparent
activation energy. The results are summarized in Table 1.
Non-isothermal decomposition
The kinetics of thermal decomposition of the In(S2CNR2)3 chelates was also studied
using the non-isothermal heating method, with heating rates of 5, 10 and 15°C min–1,
according to the expression proposed by Coats and Redfern [9]:
ln[g(α)/T2]=ln[AR/φE]–E/RT
(3)
where T – the absolute temperature, E – the apparent activation energy, A – the
pre-exponential factor, R – the gas constant, φ – the heating rate and g(α) – the kinetic
model. The results of experimental data treatment are presented in Table 2.
In spite of the known limitations of the non-isothermal technique in the study of
thermal decomposition kinetics [10], these results are in reasonable agreement with
the isothermal ones, particularly in the more appropriate models to fit the experimen-
tal data.
J. Therm. Anal. Cal., 67, 2002

Table 1 Kinetic parameters calculated according to the Arrhenius equation, for the isothermal TG experiments
Complexes
Model
R1
Parameter
In(S2CNMe2)3
In(S2CNEt2)3
147.01
In(S2CNPr2n)3
In(S2CNPr2i)3
In(S2CNBun2)3
138.24
In(S2CNBui2)3
132.69
E/kJ mol–1
188.01
A/s–1
4.29E+09
0.9996
1.89E+13
0.9980
5.64E+08
0.9992
1.40E+08
0.9987
r
E/kJ mol–1
181.50
3.15E+12
0.9939
183.78
3.04E+14
0.9991
137.65
A3
A4
A/s–1
4.85E+08
0.9992
r
E/kJ mol–1
A/s–1
219.46
186.78
5.01E+15
0.9924
4.78E+14
0.9992
r
Table 2 Kinetic parameters determined using non-isothermal TG method, according to the equation of Coats–Redfern [9]
Complexes
Parameter
φ/°C min–1
In(S2CNMe2)3
A4 Model
183.60
In(S2CNEt2)3
R1 Model
171.02
In(S2CNPr2n)3
R1 Model
187.67
In(S2CNPr2i)3
A2 Model
200.43
In(S2CNBun2)3
A2 Model
145.50
In(S2CNBui2)3
R1 Model
142.99
E/kJ mol–1
5
A/s–1
6.14E+12
0.9956
7.12E+11
0.9998
1.91E+13
0.9988
1.40E+16
0.9945
6.12E+09
0.9972
1.95E+09
0.9988
R
E/kJ mol–1
A/s–1
10
15
169.63
173.85
179.76
218.19
149.90
148.35
6.05E+11
0.9940
1.22E+12
0.9996
3.75E+12
0.9992
5.11E+17
0.9996
1.60E+10
0.9983
6.09E+09
0.9989
R
E/kJ mol–1
A/s–1
144.18
178.14
188.16
193.83
147.13
159.92
5.42E+09
0.9973
2.84E+12
0.9995
1.76E+13
0.9985
2.84E+15
0.9998
9.27E+09
0.9979
3.98E+10
0.9988
R

SOUZA et al.: DIALKYLDITHIOCARBAMATES CHELATES
363
The kinetic parameters of the complexes Pd(S2CNR2)2, Cd(S2CNR2)2 and
In(S2CNR2)3, where R=Et, Prn, Bun or Bui were analysed with the purpose of compar-
ing the thermal behavior of this class of chelates [1, 2]. All complexes decomposed in
the liquid phase, after melting, and their experimental data of thermal decomposition
were adjusted according to the same phase-boundary reaction model (R1).
The values of apparent activation energy, E, and the pre-exponential factor, A,
did not change significantly for these chelates, except for the In(S2CNPr2n)3 complex.
This behavior suggests that the mechanism of thermal decomposition for at least a
significant number of metal ion complexes of this class of metal complexes is af-
fected neither by the central metal ion nor by the radical.
Conclusions
The In(III) dialkyldithiocarbamate complexes In(S2CNR2)3, where R=Et, Prn, Bun or Bui
decompose in the liquid phase, after melting, according to the same phase-boundary reac-
tion model, R1. The other two complexes, i. e., In(S2CNMe2)3 and In(S2CNPr2i)3, decom-
pose in the solid phase, before melting, according to the nucleus growth controlling pro-
cess (A3 or A4 models).
* * *
The authors acknowledge CAPES and CNPq Foundations (Brazil) for scholarships and financial
support of this work.
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