Thermochemical studies on the lanthanoid complexes of trifluoroacetic acid

Article abstract of DOI:10.1016/j.jallcom.2004.12.082

The thermal decomposition of the lanthanoid complexes of trifluoroacetic acid (Ln(CF₃COO)₃·3H₂O; Ln = La-Lu) was studied by TG and DTA methods. The Ln(CF₃COO) ₃·3H₂O complexes decompose in several stages; first dehydrate to the anhydrous state, then followed by decomposition of the anhydrous salt to a stable product of LnF₃. From the endothermic and exothermic data of Ln(CF₃COO)₃·3H₂O complexes, pyrolysis behavior of the complexes is classified into three groups: (1) La-Pr salts; (2) Nd-Gd salts; (3) Tb-Lu salts. It has been shown that all the final decomposition products were found to result in the formation of LnF₃.

Full text of DOI:10.1016/j.jallcom.2004.12.082

Journal of Alloys and Compounds 408–412 (2006) 573–576
Thermochemical studies on the lanthanoid complexes of
trifluoroacetic acid
∗
Y. Yoshimura , K. Ohara
Department of Applied Chemistry, National Defense Academy, 1-10-20 Hashirimizu, Yokosuka, Kanagawa 239-8686, Japan
Received 30 July 2004; received in revised form 21 October 2004; accepted 15 December 2004
Available online 17 June 2005
Abstract
The thermal decomposition of the lanthanoid complexes of trifluoroacetic acid (Ln(CF
and DTA methods. The Ln(CF COO) O complexes decompose in several stages; first dehydrate to the anhydrous state, then followed
·3H
by decomposition of the anhydrous salt to a stable product of LnF . From the endothermic and exothermic data of Ln(CF COO)
·3H
complexes, pyrolysis behavior of the complexes is classified into three groups: (1) La–Pr salts; (2) Nd–Gd salts; (3) Tb–Lu salts. It has been
shown that all the final decomposition products were found to result in the formation of LnF
2005 Elsevier B.V. All rights reserved.
3
COO)
3
2
·3H O; Ln = La–Lu) was studied by TG
3
3
2
3
3
3
2
O
3
.
©
Keyword: Thermal analysis
1
. Introduction
studies about the trifluoroacetates of praseodymium, samar-
ium and erbium. According to their results, the compounds
areisomorphouswithintheirrespectiveseriesanddecompose
in several stages. However, systematic studies including all
lanthanoid members have been still lacking. It is of impor-
tance to determine the thermal decomposition processes of
all the series, since new insights could be obtained from the
pyrolysis behavior of the complexes.
It is well known that lanthanoids form a variety of com-
plexes of higher coordination numbers ranging from 7 to 12
because of the large size of ions [1,2]. Many thermal stud-
ies on the lanthanoid complexes have been reported so far
[3–8]. Recently, we have reported the thermal decomposition
of the lanthanoid trifluoromethanesulfonate (triflate) com-
plexes (Ln(CF3SO3)3·9H2O) [9]. From the endothermic and
exothermic data of Ln(CF3SO3)3·9H2O complexes, pyroly-
sis behavior of the complexes is classified into three groups:
2. Experimental
(
1) La–Nd salts; (2) Sm–Ho salts; (3) Er–Lu salts. It has also
2
.1. Sample preparation
been shown that all the final decomposition products were
found to result in the formation of LnF3.
The lanthanoid complexes of trifluoroacetic acid
In this study, as an extension of the previous work [9], the
techniques of thermogravimetry (TG) and differential ther-
mal analysis (DTA) were applied to the study of the thermal
decomposition of the lanthanoid complexes of trifluoroacetic
acid. It is to be noted that Rillings and Roberts [10] have
already performed on powder X-ray diffraction and thermal
(Ln(CF3COO)3; Ln = La–Lu) used in this study were pre-
pared from Ln2O3 (Santoku Metals, 99.9%) and trifluo-
roacetic acid (CF3COOH; Wako Pure Chemicals, 98%>).
The oxide was mixed with the slight excess of dilute tri-
fluoroacetic acid at room temperature and then heated on a
◦
hot plate at about ∼50 C to synthesize the Ln(CF3COO)3
∗
and evaporate the solution to dryness. The resulting salt
was recrystallized from aqueous solution followed by dry-
ing in vacuum. The composition of all the complexes as
Corresponding author. Tel.: +81 46 841 3810x3583;
fax: +81 46 844 5901.
E-mail address: muki@nda.ac.jp (Y. Yoshimura).
0
925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2004.12.082

5
74
Y. Yoshimura, K. Ohara / Journal of Alloys and Compounds 408–412 (2006) 573–576
precipitated from the solution revealed to exist as trihydrates
Ln(CF3COO)3·3H2O). The crystals were preserved in a
dessicator before use as samples.
(
2
.2. TG–DTA measurements
The thermogravimetric (TG) and differential thermal anal-
ysis (DTA) measurements were carried out with a Mac Sci-
ence WS-002 TG–DTA system in static air. About 20 mg of
the sample was taken for each run. The sample pan was an
open aluminum type. An ␣-Al2O3 powder was used as a ref-
erence material. TG and DTA curves were obtained up to
◦ ◦
550 C. The heating rate was 5 C/min.
∼
3
. Results and discussion
As examples of the typical TG and DTA curves of lan-
Fig. 2. The TG–DTA trace of the Tm(CF3COO)3·3H2O.
thanoid complexes of trifluoroacetic acid, Figs. 1 and 2
show the results for lanthanum complex of trifluoroacetic
acid (La(CF3COO)3·3H2O) and thulium complex of trifluo-
roacetic acid (Tm(CF3COO)3·3H2O), respectively. As seen
in Fig. 1, the DTA curve for La(CF3COO)3·3H2O gives
three endothermic peaks and two exothermic peaks, whereas
that for Tm(CF3COO)3·3H2O (Fig. 2) is composed of two
endothermic peaks and one sharp exothermic peak. The
respective peaks on the DTA curve correspond to the suc-
cessive steps on the gravimetric change on the TG curve.
La(CF3COO)3·3H2O compound began to lose hydrate
(
La(CF3COO)3) was probably formed. According to Rillings
and Roberts [10], there is the “solution effect”, when the com-
plex loses hydrated water molecules. The “solution” here is
not a result of melting but a result of the anhydrous compound
dissolving in its own evolved water of hydration. The large
◦
sharp exotherm at ∼287 C is due to the decomposition of
the compound. It is clear that no weight change is indicated
◦
in an air atmosphere between ∼300 and ∼550 C.
In the case of Tm(CF3COO)3·3H2O (Fig. 2), hydrate
◦
waters began to lose at ∼108 C giving two breaks in the
◦
◦
water at ∼94 C giving a small broad peak in the
TG curve. The first break at 108 C corresponds to the loss
DTA curve, i.e. this stage corresponds to the dihy-
of two water molecules with the resultant formation of the
drate (La(CF3COO)3·2H2O), whereas the second small
monohydrate (Tm(CF3COO)3·H2O), while the second break
◦
◦
peak at around 135 C appeared to be the monohydrate
at ∼153 C is due to the anhydrous state (Tm(CF3COO)3).
(
La(CF3COO)3·H2O). Finally, whenthetemperaturereached
The anhydrous compound is stable up to a temperature of
◦
◦
the third large sharp peak of ∼162 C, the anhydrous state
∼
300 C.
Fig. 3 compiles the temperatures of all endothermic
peaks (Tnx; x = 1–3) appeared in the DTA curves of
Fig. 3. The series behavior of the temperatures of the endothermic peaks.
The peak temperatures revealed on the DTA curve are designated to Tnx
(x = 1–3, ᭹: Tn1, ꢀ: Tn2, ꢀ: Tn3) for the onset endothermic temperatures.
Fig. 1. The TG–DTA trace of the La(CF3COO)3·3H2O.

Y. Yoshimura, K. Ohara / Journal of Alloys and Compounds 408–412 (2006) 573–576
575
Fig. 5. The series behavior of the temperatures of the exothermic peaks. The
peak temperatures revealed on the DTA curve are designated as Tdx (x = 1–2,
Fig. 4. A schematic diagram for the dehydration processes of
Ln(CF3COO)3·3H2O. Ln = La–Lu (᭹) shows the hydration number (n) of
the Ln(CF3COO)3·nH2O (n = 0–3) at first dehydration step. (ꢀ) and (ꢁ)
denote the hydration numbers of second and third dehydration processes,
respectively.
(᭹) Td1, (ꢀ) Td2) for the onset exothermic temperatures.
salts; (3) Tb–Lu salts. Considering the TG data for the com-
plexes, the reaction scheme for each group of the lanthanoid
trifluoroacetates can be expressed as follows:
Ln(CF3COO)3·3H2O across the series. In case of the La–Dy
trifluoroacetate salts, the third endothermic peak (ꢀ: Tn3)
is observed. After Ho salts in the series, first (᭹: Tn1) and
second (ꢀ: Tn2) endothermic peaks are observed. These
values revealed rather complex behavior in the temperature
region at which dehydration takes place in that three distinct
endotherms are present.
Ln(CF3COO)3·3H2O → Ln(CF3COO)3·2H2O
→ Ln(CF3COO)3·H2O → Ln(CF3COO)3
→
Ln(CF2COO)3 → LnF3
(1)
The Ln(CF3COO)3·3H2O release an appreciable amount
of hydrate water depending on the lanthanoid ions. Fig. 4
is a schematic representation on the hydration number of
Ln(CF3COO)3·3H2O complexes in the course of dehydration
processes upon heating. Y axis indicates the hydration num-
ber (n) of the Ln(CF3COO)3·nH2O (n = 0–3) at each step, that
is (᭹) denotes the total hydration number of the complexes
at first dehydration process, and (ꢀ) and (ꢁ ) correspond to
the numbers of the second and third dehydration processes,
respectively.
The complex begins to lose one H O molecule when
heated to a certain temperature. Then comes another dehydra-
2
tion step and two remaining H O molecules are released com-
2
pletely. In addition to the “solution effect” mentioned above,
the decomposition of Ln(CF COO) to Ln(CF COO) may
3
3
2
3
occur considering the TG data for the complexes. However,
this assignment is rather speculative. We suspect that the
decomposition product of Ln(CF COO) is not stable, if it
2
3
exists. On further heating, finally comes the decomposition
to LnF rather rapidly with the rise of temperature.
3
Fig. 5showsthetemperaturesofallexothermicpeaks(Tdx;
x = 1,2) appeared in the DTA curves of lanthanoid complexes
of trifluoroacetic acid. The trifluoroacetates of La–Gd are
observed to show two exothermic peaks(᭹: T , ꢀ: T ), but
Ln(CF3COO)3·3H2O → Ln(CF3COO)3·1.5H2O
→
→
Ln(CF3COO)3·H2O → Ln(CF3COO)3
Ln(CF2COO)3 → LnF3
d1
d2
(2)
for Tb–Lu acetates, only one exothermic temperature (T ) is
d1
observed. Td1 values show extended s-shaped behavior with
the increase in the atomic number, which is common behav-
ior for thermodynamic and transport properties [11–13] of
lanthanoid electrolyte solutions.
The weight loss at the end of the decomposition (final
weight) is in good agreement with the formation of LnF3.
In addition, in order to confirm that the final decomposition
products were not Ln2O3, we dissolved the final decom-
position products into the nitric acid. It is found that the
decomposition products do not dissolve in the nitric acid.
From all the data obtained in the present study, the pyroly-
sis behavior of the lanthanoid trifluoroacetate complexes can
be classified into three groups, i.e. (1) La–Pr salts; (2) Nd–Gd
The differences in the pyrolysis behavior between the pro-
cess (1) and (2) are present in the number of H2O molecules
at each dehydration process.
Ln(CF3COO)3·3H2O → Ln(CF3COO)3·H2O
→
Ln(CF3COO)3 → LnF3
(3)
Taking the occurrence of one less set of endothermic and
exothermic peaks in the DTA curve and the corresponding
TG curve into account, the decomposition process for the
Tb–Lu complexes is different from those for the groups of
(1) La–Pr complexes and (2) Nd–Gd complexes. The charac-
teristic difference in the pyrolysis behavior is mainly present

5
76
Y. Yoshimura, K. Ohara / Journal of Alloys and Compounds 408–412 (2006) 573–576
in that the Ln(CF2COO)3 is not the intermediate substance
in the course of the decomposition.
in the electron density distribution of a molecule or molecu-
lar ion.
These differences in the pyrolysis behavior are possibly
due to the size differences between lanthanoid ion and its
ligands. There should be the mismatch between the size of
lanthanoid ion and that of the coordination sphere of water
molecules and the trifluoroacetate ions. Their coordination
spheres are not perfectly packed with three H2O molecules,
so that the interactions will be relatively weaker than those
in the complexes with smaller Ln3+ ions.
In conclusion, the present TG–DTA data provides detailed
information concerning the thermal stability and ther-
mal decomposition of the Ln(CF3COO)3·3H2O complexes
across all the rare earth series. The Ln(CF3COO)3·3H2O
complexes decompose in several stages; first dehydrate to
the anhydrous state, then followed by decomposition of the
anhydrous salt to a stable product of LnF3. Both TG and DTA
analyses revealed the decomposition of the salts to be highly
exothermic. An important point is that the pyrolysis behav-
ior of the Ln(CF3COO)3·3H2O complexes can be classified
into three groups as (1) La–Pr salts; (2) Nd–Gd salts; and (3)
Tb–Lu salts, though all the final decomposition products are
As mentioned briefly in the introduction section, Rillings
and Roberts [10] carried out thermal studies on only three lan-
thanide salts (Pr, Sm, Er) of trifluoroacetic acid. They heated
◦
the compounds to higher temperature of ∼1200 C. It was
◦
reported that slow decomposition was observed from 680
LnF3 up to ∼550 C. This means that the thermal decom-
◦
to 890 C resulting in the formation of PrOF as an unstable
position behavior of the lanthanoid complexes, as well as
thermodynamic properties observed in aqueous lanthanoid
salt solutions, is not the same throughout the series and is
considerably dependent on the size of the lanthanoid ion.
intermediate. Further heating in the wet atmosphere produced
◦
additional hydrolysis until ∼1100 C. In general, all of the
anhydrous compounds decomposed initially to the LnF3 and
then the LnOF and Ln2O3 products were formed upon fur-
ther decomposition under the conditions described in their
paper. They also stated that the compounds, which have iso-
morphous structures within the series with similar d spacings
and line intensities [10,14], dehydrate in definite steps.
They suggested the general decomposition scheme for the
trifluoroacetates to proceed as follows:
References
[1] V. Alexander, Chem. Rev. 95 (1995) 273–342.
[
2] M. Leskel a¨ , L. Niinist o¨ , in: K.A. Gschneidner Jr., L. Eyring (Eds.),
Handbook on the Physics and Chemistry of Rare Earths, Elsevier,
1
986 (chapter 56).
3] R.C. Paul, M.S. Bains, J.S. Ghotra, Indian J. Chem. 7 (1964)
14–519.
4] G.A.-H. Hussen, H.M. Ismail, Bull. Chem. Soc. Jpn. 67 (1994)
634–2638.
[
[
Ln(CF3CO2)3 → LnF3 + (CF3CO)2O + CO2 + CO
5
Secondary decomposition of the anhydride also occurred.
2
[
[
5] A.M. Soto, N. Yanagihara, T. Ogura, Anal. Chem. 63 (1991) 1178.
6] N. Yanagihara, S. Nakamura, M. Nakayama, Chem. Lett. (1995)
(
CF3CO)2O → CF3COF + COF2 + CO
There have been a number of thermal decomposition stud-
5
55–556.
[
7] H. Aono, M. Tsuzaki, A. Kawaura, M. Sakamoto, E. Traversa, Y.
Sasaoka, Chem. Lett. 11 (1999) 1175–1176.
8] M.A. Carvalho Filho, N.S. Fernandes, M.I.G. Leles, R. Mendes, M.
Ionashiro, J. Therm. Anal. Calorim. 59 (2000) 669–674.
[9] K. Egashira, Y. Yoshimura, H. Kanno, Y. Suzuki, J. Therm. Anal.
Calorim. 71 (2003) 501–508.
[10] K.W. Rillings, J.E. Roberts, Thermochim. Acta 10 (1974) 285–298.
ies on rare earth acetates [15–21]. Adachi and Secco [15]
reported the thermal transformation in anhydrous acetates.
The anhydrous acetates (Ln(CH3COO)3) prepared from the
decomposition of the hydrated rare earth acetates were stable
and decomposed only above 400 C. The anhydrous acetates
also show two stages of decomposition; At the first stage
[
◦
[
11] F.H. Spedding, M.J. Pikal, B.O. Ayers, J. Phys. Chem. 70 (1966)
440–2447.
12] J.A. Rard, F.H. Spedding, J. Phys. Chem. 79 (1975) 257–262.
2
◦
between 420 and 460 C, anhydrous acetates decompose to
[
Ln2O3·CO2 and at the second stage to the formation of the
[13] H. Kanno, Y. Akama, J. Phys. Chem. 91 (1987) 1263–1266.
[14] J.E. Roberts, J. Am. Chem. Soc. 83 (1961) 1087–1088.
Ln2O3. General decomposition paths are as follows:
[
[
15] G. Adachi, E.A. Secco, Can. J. Chem. 50 (1972) 3100–3103.
16] K.C. Patil, G.V. Chandrashekhar, M.V. George, C.N.R. Rao, Can. J.
Chem. 46 (1968) 257–265.
2
Ln(CH3CO2)3 → Ln2O3·CO2 + 2CO2 + 3CH3COCH3
Ln2O3·CO2 → Ln2O3 + 2CO2
[17] K. Manabe, M. Ogawa, Nihonkagakukaishi (1982) 694–696.
[18] K. Manabe, M. Ogawa, Y. Eizuka, Nihonkagakukaishi (1983)
As compared with the results of trifluoroacetates
Ln(CF3COO)3), the anhydrous Ln(CH3COO)3 complexes
decompose at relatively lower temperatures. This might
be due to the effect of the charge diminution at oxygen
of the acetate ion by fluorine substitution [22]. Electron-
withdrawing groups such as F produce significant changes
450–453.
[
[
19] K. Manabe, M. Ogawa, Nihonkagakukaishi (1983) 1092–1095.
20] D.D. Ensor, G.R. Choppin, J. Inorg. Nucl. Chem. 42 (1980)
(
1477–1480.
[
[
21] I. Mayer, F. Kassierer, J. Inorg. Nucl. Chem. 28 (1966) 2430–2432.
22] J. Barthel, H.-J. Gores, L. Kraml, J. Phys. Chem. 100 (1996)
1283–1287.