Lanthanide trifluoroacetate complexes with 2-azacyclononanone. Thermal and kinetic studies

Article abstract of DOI:10.1023/A:1013983303200

The synthesis and characterization of Lanthanide trifluoroacetate compounds by microanalytical procedures, infrared absorption spectroscopy, X ray powder diffraction and thermogravimetric analysis were discussed. A kinetic study using thermogravimetric curves obtained for La, Sm and Er was also carried out. The first and second derivatives reactions constituting thermal decomposition of the complexes indicated a high quality optimization of the mass curves. The decomposition occurred in five steps with similar activation energies and pre-exponential factors.

Full text of DOI:10.1023/A:1013983303200

Journal of Thermal Analysis and Calorimetry, Vol. 67 (2002) 397–402
LANTHANIDE TRIFLUOROACETATE COMPLEXES
WITH 2-AZACYCLONONANONE
Thermal and kinetic studies
A. M. Garrido Pedrosa, P. M. Pimentel, D. M. Araújo Melo*,
H. Scatena Jr., F. M. M. Borges, A. G. Souza and L. B. Zinner
Federal University of Rio Grande do Norte, Department of Chemistry, C.P. 1662,
59078-970 Natal, RN, Brazil
Abstract
Ln(TFA)3·3AZA (Ln=La, Sm, Er; TFA=trifluoroacetate and AZA=2-azacyclononanone) compounds
were synthesized and characterized by microanalytical procedures, IR spectroscopy, X-ray powder
diffraction, and thermal analysis. A kinetic study using La, Sm and Er thermogravimetric curves was
carried out aiming to proposing a mechanism for the thermal decomposition of such complexes.
Keywords: 2-azacyclononanone, lanthanide, trifluoroacetate
Introduction
Lanthanide complexes with lactam 2-azacyclononanone (AZA) have been described
in literature [1, 2]. AZA is a traditional ligand containing two cis-trans isomers that
may coexist [3, 4]. The interest in trifluoroacetates comes from the fact that they may
act as monodentates, bidentates, or bridges, between two central atoms, either coordi-
nated or not [5].
In this article we describe the characterization of Ln(TFA)3·3AZA compounds
by microanalytical procedures, infrared absorption spectroscopy, X-ray powder dif-
fraction and thermogravimetric analysis. A kinetic study using thermogravimetric
curves obtained for La, Sm and Er was also carried out.
Experimental
The complexes were prepared from the reaction of hydrated lanthanide trifluoro-
acetates with AZA in ethanol (molar ratio 1:3). Carbon, hydrogen and nitrogen con-
tents were determined using a Perkin Elmer CHN analyser model 240. Thermogravi-
metric curves were obtained on a Perkin Elmer TGA-7 instrument at a heating rate of
10 K min–1 in nitrogen flowing at a rate of 50 cm3 min–1.
*
Author for correspondence: Phone/Fax: + 55(84) 2119241, E-mail: dmelo@matrix.com.br
1418–2874/2002/ $ 5.00
Akadémiai Kiadó, Budapest
© 2002 Akadémiai Kiadó, Budapest
Kluwer Academic Publishers, Dordrecht

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GARRIDO PEDROSA et al.: LANTHANIDE TRIFLUOROACETATE COMPLEXES
Kinetic theoretical basis
Several methods are used to obtain kinetic parameters regarding TG curves [6, 7]. In gen-
eral it is not trivial to obtain TG curves with full plateau. In this work, it was possible by
simulating TG curves using the equations presented here in. Reaction of order one was
proposed as any other would be order difficult to be theoretically justified. Vyazovkin
and Lesnikovich [8] studied the difficulty to obtain kinetic parameters with minimum un-
certainty. It is possible to simulate the curves with reaction of order two, for instance, but
the intermediate molecular masses would have to be the same in all cases.
The quality of the theoretical approach to fit the observed curves is visually estab-
lished and the first and second derivatives of TG curves are particularly helpful in this
sense.
Gathering such parameters is important in order to establish mechanisms, as
they offer information on intermediates, and a likely reaction path.
A theoretical kinetic study was carried out feeding TG data in the QBasic program.
For each step of the process it is possible to apply the following kinetic equation:
k
−Eai
M
mi,j −1∆Tj + i+1 ∆mi,j–1
Mi
∆mi,j = − 0,i exp
(1)
β
RT
where ∆mi,j represents the mass increment of the i-th compound at the j-th step, k0,i is
the pre-exponential factor, Ea,i is the activation energy of the i-th compound, Mi,j rep-
resents the molar mass of the compound i, and β is the heating rate.
The initial mass of the first component is the initial mass m1 of the sample to be
analyzed. For the other components the initial mass is null.
Using a set of activation energies and pre-exponential factors, it is possible to visu-
ally approximate the theoretical mass to the experimental one. The theoretical mass is
given by:
m =
m
(2)
∑
j
i,j
i
where mj is the total mass at the j-th step.
The advantages of determing the kinetic parameters this way are:
• it is possible to study the whole temperature range of the thermogravimetric
curve even with parallel or consecutive reactions;
• it is possible to predict the quantity and the molar mass of the reaction inter-
mediates and
• using the mechanistic proposal it is possible to determine the experimental
curve, and test the initial hypothesis.
Results and discussion
The analytical results are in agreement with Ln(TFA)3·3AZA (Ln=La, Sm, Er;
TFA=trifluoroacetate and AZA=2-azacyclononanone) composition. IR data indicate
J. Therm. Anal. Cal., 67, 2002

GARRIDO PEDROSA et al.: LANTHANIDE TRIFLUOROACETATE COMPLEXES
399
that the trifluoroacetate counter-ions are bound to the central ion as monodentate,
bidentate or even bridges between two lanthanide ions. According to X-ray powder
patterns, the lanthanum compound is different from the isomorphous Sm and Er ones.
Fig. 1 Thermogravimetric curves experimental and calculated obtained for La, Sm and
Er complexes. TG curves experimental (a, d and e) and calculated (b, c and f) by
La, Sm and Er compounds respectively
Thermogravimetric curves were recorded to evaluate the thermal stability of the
compounds and to assist in the kinetic study. The heating rate and atmosphere were
chosen according to preliminary investigations. Figure 1 shows the curves obtained
in N2 atmosphere, at 10 K min–1. They are similar to the curves recorded in air.
Table 1 Summary of thermogravimetric results of Ln(TFA)3·3AZA compounds, in flowing N2
(50 cm3 min–1). Heating rate: 10 K min–1
Mass loss/%
Temperature
range/K
Ln
La
Attribution
theor.
exp.
377– 551
551–668
668 –1170
1170
–
–
33.81
38.19
10.11
18.20
AZA decomposition
AZA and TFA decomposition
TFA decomposition
LaOF
–
18.34
393–556
556–673
673–1170
1170
–
–
31.52
38.24
10.75
19.16
AZA decomposition
AZA and TFA decomposition
TFA decomposition
SmOF
Sm
Er
–
19.51
343– 553
553–669
669–1170
1170
–
–
32.98
37.92
8.59
AZA decomposition
AZA and TFA decomposition
TFA decomposition
ErOF
–
21.34
21.75
The first step of the kinetic process occurred at about 373 K with the decomposi-
tion of AZA, producing C6H12. If thermal decomposition occurred with the evolution
of two AZA molecules, the error in the molar mass of the intermediary would be
higher than the expected one. The second step, between 553–673 K, corresponded to
J. Therm. Anal. Cal., 67, 2002

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GARRIDO PEDROSA et al.: LANTHANIDE TRIFLUOROACETATE COMPLEXES
the loss of H2CCONH molecules and gaseous products due to the decomposition of
TFA. According to Rillings and Roberts [9], it consists in the formation of CO, CO2,
CF3OF and (CF3CO)2O. The decomposition continued up to 1273 K, with formation
of lanthanide oxyfluorides (LnOF) as final residues. Table 1 summarizes the obtained
thermogravimetric data.
Fig. 2 First (a) and second (b) derivative TG curves experimental (a, d and e) and cal-
culated (b, c and f) by La, Sm and Er compounds respectively
Figure 2 shows the first and second derivatives of both calculated and observed
TG curves for the three complexes. Eai, k0i and intermediate molar masses were deter-
mined fitting the observed curves. The global process was consistent with the follow-
ing decomposition reactions:
Ln(TFA)3·3AZA → Ln(TFA)3·3H2CCONH+3C6H12
Ln(TFA)3·3H2CCONH → LnO(OCOCF3)+(CF3CO)2O+3H2CCONH
LnO(OCOCF3) → LnO(COF)+OCF2
LnO(COF) → LnOF+CO
The kinetic parameters obtained for each thermal decomposition step are pre-
sented in Table 2.
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GARRIDO PEDROSA et al.: LANTHANIDE TRIFLUOROACETATE COMPLEXES
401
Table 2 Kinetic parameters for Ln(TFA)3·3AZA compounds and mass percentage of intermedi-
ate products
Molar mass calculated
k0/s–1
Ea/kJ mol–1
Compound
Step
m/%
a
b
1
2
3
4
5
1
2
3
4
5
1
2
3
4
5
1.1·105
3.0·109
1.5·106
9.0·107
2.0·101
1.1·105
3.0·109
3.0·105
9.0·107
2.0·101
1.1·105
3.0·109
1.9·105
9.0·107
2.0·101
67.7
132.0
116.0
134.0
450.0
69.0
100
72
30
22
20
100
72
31
23
21
100
73
31
25
22
901
649
268
202
180
912
660
279
213
191
929
677
288
230
202
901
649
268
202
174
912
660
279
213
185
929
677
296
230
202
La
133.2
113.5
135.0
450.0
68.7
Sm
133.3
110.0
140.0
450.0
Er
a – molar mass obtained by simulation and b – molar mass calculated adding atomic masses
Conclusions
The proposed mechanism for the observed thermal decomposition consists of consec-
utive polymerization reactions of the organic intermediates. The method previously
introduced [10] showed a good agreement with the plotted curves and suggests that
each peak correspond to more than one reaction. The first and second derivatives in-
dicated the high quality of the optimization of the mass curves. The decomposition
occurs in five steps with similar activation energies and pre-exponential factors.
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