Synthesis, characterization and thermal behaviour of solid 2-methoxybenzoates of trivalent metals

Article abstract of DOI:10.1007/s10973-007-8546-1

Solid-state Ln(L)₃ compounds, where Ln stands for trivalent Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Y and L is 2-methoxybenzoate have been synthesized. Simultaneous thermogravimetry and differential thermal analysis (TG-DTA), differential scanni

Full text of DOI:10.1007/s10973-007-8546-1

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Journal of Thermal Analysis and Calorimetry, Vol. 92 (2008) 3, 945–951
SYNTHESIS, CHARACTERIZATION AND THERMAL BEHAVIOUR OF
SOLID 2-METHOXYBENZOATES OF TRIVALENT METALS
A. B. Siqueira1,*, C. T. Carvalho1, E. Y. Ionashiro2, G. Bannach2, E. C. Rodrigues3 and
M. Ionashiro1
1Instituto de Química, UNESP, CP 355, CEP 14801-970, Araraquara, SP, Brazil
2Universidade Estadual de Ponta Grossa, CEP 84010-919, Ponta Grossa, PR, Brazil
3Funda¸±o Educacional de Barretos, CEP 14780-000, Barretos, SP, Brazil
Solid-state Ln(L)3 compounds, where Ln stands for trivalent Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Y and L is 2-methoxybenzoate
have been synthesized. Simultaneous thermogravimetry and differential thermal analysis (TG-DTA), differential scanning
calorimetry (DSC), X-ray powder diffractometry, infrared spectroscopy and complexometry were used to characterize and to study
the thermal behaviour of these compounds. The results provided information on the composition, dehydration, coordination mode,
structure, thermal behaviour and thermal decomposition.
Keywords: characterization, heavier lanthanides, 2-methoxybenzoate, thermal behaviour
Introduction
thermal analysis (TG-DTA), differential scanning
calorimetry (DSC) and other methods of analysis.
Several metal-ion compounds of benzoate,
C6H5–COO– (Bz), and phenyl-substituted derivatives
of benzoate have been investigated in aqueous
solutions [1–3] and in solid state [4–13]. In aqueous
solutions these works reported the thermodynamics of
complexation of lanthanides by some benzoic acid
derivatives; the spectroscopic study of trivalent
lanthanides with several carboxylic acids including
benzoic acid; the influence of pH, surfactant and
synergic agent on the luminescent properties of
terbium chelates with benzoic acid derivatives and
thermodynamic of complexation of lanthanides by
benzoic and isophthalic acids [1–3].
In solid state the works reported the thermal and
spectral studies on solid compounds of benzoate and
derivatives with rare earth elements; the reaction of
divalent copper, cobalt and nickel with 3-hydroxy-
4-methoxy and 3-methoxy-4-hydroxybenzoic acid and
structure for these compounds has been proposed on the
basis of spectroscopic and thermogravimetric data;
the thermal decomposition of thorium salts of benzoic
and 4-methoxybenzoic acids in air atmosphere and
thermal behaviour of solid state methoxybenzoate of
some divalent transition metal ions [4–13].
Experimental
Materials
The 2-methoxybenzoic acid (HL) 99% was obtained
from Aldrich. Aqueous solution of Na–L 0.15 mol L–1
was prepared from aqueous HL suspension by treatment
with sodium hydroxide solution (0.1 mol L–1). Lanthanide
chlorides were prepared from the corresponding
metal oxides by treatment with concentrated hydro-
chloric acid. The resulting solutions were evaporated
to near dryness, the residues were again dissolved in
distilled water, transferred to a volumetric flask and
diluted in order to obtain ca. 0.1 mol L–1 solutions,
whose pH were adjusted to 5.0 by adding diluted
sodium hydroxide or hydrochloric acid solutions. The
solid state compounds were prepared by adding
slowly, with continuous stirring the ligand solution to
the respective metal chloride solutions, until total
precipitation of the metal ions. The precipitates were
washed with distilled water until elimination of the
chloride ions, filtered through and dried on Whatman
n° 42, filter paper, and kept in a desiccator over
anhydrous calcium chloride.
In this work 2-methoxybenzoates of trivalent Eu,
Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Y were synthesized.
The compounds were investigated by means of
infrared spectroscopy, X-ray powder diffractometry,
simultaneous thermogravimetry and differential
In the solid-state compounds, hydrate water,
ligand and metal ion contents were determined from
the TG curves. The metal ions were also determined
by complexometric titrations with standard EDTA
*
Author for correspondence: massaoi@yahoo.com.br
1388–6150/$20.00
Akadémiai Kiadó, Budapest, Hungary
Springer, Dordrecht, The Netherlands
© 2008 Akadémiai Kiadó, Budapest

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SIQUEIRA et al.
Table 1 Analytical and thermoanalytical (TG) data of the compounds
Metal/%
TG
DL/%
EDTA Calcd.
H2O/%
Calcd. TG
8.19 8.49 43.71
C%
H%
Calcd.
Final
Residue
Compound
Calcd.
TG
Calcd.
AE
AE
Eu(L)3·3H2O
Gd(L)3·4H2O
Tb(L)3·4H2O
Dy(L)3·5H2O
23.05 22.27 23.47
23.03 23.27 23.33
23.22 23.99 23.02
23.01 22.87 23.42
65.12
62.90
62.16
60.82
63.02
67.27
67.06
66.70
66.43
69.89
65.00
62.77
61.53
61.03
62.54
67.04
66.80
66.35
65.94
69.78
43.63
42.13
41.68
40.96
41.98
41.45
44.83
44.43
44.56
47.93
4.13
4.29
4.28
4.43
4.15
4.23
3.63
3.60
3.59
4.78
4.28
4.32
4.23
4.45
4.11
4.21
3.67
3.63
3.65
4.63
Eu2O3
Gd2O3
Tb4O7
Dy2O3
Ho2O3
Er2O3
Tm2O3
Yb2O3
Lu2O3
Y2O3
10.56 10.70 42.22
10.53 10.29 42.11
12.76 12.58 40.82
Ho(L)3·3.5H2O 24.14 24.33 24.20
9.25
2.82
2.81
2.80
2.79
9.48 42.30
2.66 41.60
2.93 45.01
2.71 44.72
2.71 44.62
Er(L)3·H2O
Tm(L)3·H2O
Yb(L)3·H2O
Lu(L)3·H2O
Y(L)3·4H2O
26.19 26.32 26.42
26.38 26.76 26.75
26.87 27.22 27.39
27.07 27.60 27.64
14.47 14.77 14.56
11.73 11.46 47.05
L=2-methoxybenzoate
Results and discussion
solution using xylenol orange as indicator [14, 15].
X-ray powder patterns were obtained by using a
Siemens D-5000 X-ray diffractometer, employing
CuKa radiation (l=1.541 ) and setting of 40 kV and
20 mA.
Infrared spectra for Na–2-MeO–Bz as well as for
its metal-ion compounds were recorded on a Nicolet
model Impact 400 FT-IR instrument, within the
4000–400 cm–1 range. The solid samples were
pressed into KBr pellets.
Simultaneous TG-DTA and DSC curves were
obtained with two thermal analysis systems,
model SDT 2960 and DSC Q10, both from TA
Instruments. The purge gas was an air with a flow rate
of 100 mL min–1 for TG-DTA and 50 mL min–1 for
DSC experiments. A heating rate of 20°C min–1 was
adopted, with sample masses about 7 mg for TG-DTA
and 5 mg for DSC runs. Alumina and aluminium
crucibles, the latter with perforated covers, were used
for TG-DTA and DSC, respectively.
The analytical and thermoanalytical (TG) data are shown
in Table 1. These results establish the stoichiometry of
these compounds, which are in agreement with the
general formula Ln(L)·nH2O, where Ln represents
trivalent Eu to Lu or Y, L is 2-methoxybenzoate and n=5
(Dy), 4 (Gd, Tb, Y), 3.5 (Ho), 3 (Eu), 1 (Er, Tm, Yb, Lu).
The X-ray diffraction powder patterns (Fig. 1)
show that all the compounds have a crystalline structure
without evidence for formation of an isomorphous
series, except for ytterbium and lutetium compounds
which are isomorphous.
Infrared spectroscopic data on 2-methoxybenzoate
and its compounds with trivalent metal ions considered
in this work are shown in Table 2. The investigation
Computational strategy
Calculation of theoretical infrared spectrum of yt-
trium 2-methoxibenzoate was necessary to evaluate
the structure and wave function computed by the ab
initio SCF Hartree-Fock-Roothan method [16] using
a split valence (cep-31 g) basis set [17]. The
performed molecular calculations in this work were
done by using the Gaussian 98 routine [18] and the
hardware IBM power 3. The geometry optimization
was carried out without any constraints. The benzoate
molecule contains rings with conformational
flexibility, and all variables optimized. The
optimization proceeded more uniformly when all
variables were optimized.
Fig. 1 X-ray powder diffraction patterns of the compounds: a
– Eu(L)3·H2O; b – Gd(L)3·4H2O; c – Tb(L)3·4H2O;
d – Dy(L)3·5H2O; e – Ho(L)3·3.5H2O; f – Er(L)3·1H2O;
g – Tm(L)3·1H2O; h – Yb(L)3·1H2O; i Lu(L)3·1H2O;
j – Y(L)3·4H2O. L=2-methoxybenzoate
946
J. Therm. Anal. Cal., 92, 2008

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SOLID 2-METHOXYBENZOATES OF TRIVALENT METALS
Table 2 Spectroscopic data (cm–1) taken from IR spectra for sodium 2-methoxybenzoate and compounds with europium(III),
heavy lanthanides(III) and yttrium(III)
c
c
nsym(COO –
nasym(COO–
Compound
n(O–H)H2Ob
–
Dn(nasym–nsym)
)
)
NaL
1576s
1537s
1547s
1548s
1547s
1548s
1549s
1549s
1549s
1553s
1553s
1404s
1402s
1398s
1402s
1404s
1404s
1406s
1420s
1418s
1419s
1402s
172
135
149
146
143
144
143
129
131
134
151
:
Eu(L)3·3H2O
Gd(L)3·4H2O
Tb(L)3·4H2O
Dy(L)3·5H2O
Ho(L)3·3.5H2O
Er(L)3·H2O
3479m
3487m
3489m
3489m
3481m
3483m
3466m
3471m
3504m
3485m
Tm(L)3·H2O
Yb(L)3·H2O
Lu(L)3·H2O
Y(L)3·4H2O
L=2-methoxybenzoate; am – medium; s – strong; bn(O–H): hydroxyl group stretching frequency; cn
symmetrical and antisymmetrical vibrations of the COO– group, respectively
and n
sym(COO–
asym(COO–
)
)
was focused mainly within the 1700–1400 cm–1 range
because this region is potentially the most informative
to assign coordination sites. In the sodium
2-methoxybenzoate strong bands located at 1576 and
1404 cm–1 are attributed to the anti-symmetrical and
symmetrical frequencies of the carboxylate groups,
respectively [19–21]. For the synthesized compounds,
the anti-symmetrical and symmetrical stretching
frequencies are located between 1553–1537 and
1420–1398 cm–1, respectively. Analysis of the
frequencies of the nas(COO – and nsym(COO – bands shows
Table 3 Theoretical geometrical parameters of Y(L)3 compound
dY–a O
dY–b O
2.35 
COO –
2.33 
1.29 
1.31 
1.48 
1.44 
1.36 
1.08 
1.07 
114.96°
55.76°
122.10°
COO –
dC
–a O
–b O
COO –
COO –
COO –
dC
COO –
dC
–Cring
COO –
dOOCH –COCH
3
3
dCring –OOCH
3
dCOCH –HOCH
3
3
)
)
dCring –Hring
that the lanthanides are linked to the carboxylate
group by a bidentate bond with an incomplete
equalization of bond lengths in the carboxylate anion,
which is in agreement with [22].
<O
<O
–C
–O
COO –
COO –
COO –
COO –
–Y–O
COO –
<Cring –OOCH –COCH
3
3
The theoretical infrared spectrum of the Y(L)3
was calculated by using a harmonic field [23] and the
obtained frequencies were not scaled. The geometry
optimization was computed by the optimized algo-
rithm of Berny [23]. The obtained geometry from
calculations is presented in Table 3.
Key: Y=yttrium; L=2-methoxybenzoate; d=atomic
distances; <=angle of atoms; COaOb–=carboxylate;
ring=benzene ring; OCH3=methoxy group
1402 cm–1 suggesting a nsym(COO – assignment, while
)
the theoretical results show the corresponding peak at
1378 cm–1 with discrepancies of 1.71%. The theoreti-
cal results are in agreement with the experimental
data.
Simultaneous TG-DTA curves of the com-
pounds show mass losses in steps and thermal events
corresponding to these losses or due to physical
phenomenon.
Due to the great similarity in the thermoanalytical
curves, consequently in the thermal behaviors, only
the TG-DTA curves of europium, dysprosium,
thulium and lutetium compounds are shown in Fig. 2.
For all the compounds, the first mass loss between
313–383 K with endothermic peaks at 363–383 K is
attributed to the dehydration, which occurs in a single
step. Once dehydrated, the TG-DTA curves also show
The theoretical infrared spectrum of the Y(L)3
1
(electronic state A) was obtained with frequency
values (cm–1), relative intensities, assignments and
description of vibrational modes. The theoretical
frequency, assignments and description was visualized
by using graphic software Molden 4.2 for Linux [24].
It was compared to the experimental infrared
spectrum of Y–(2-MeO–Bz)3.
A comparative analysis between the experimental
and theoretical spectrum permitted to verify that:
(a) the first assignment shows a strong contribution at
1553 cm–1 suggesting a nasym(COO – assignment, while
)
the theoretical results show the corresponding peak at
1558 cm–1 with discrepancies of 0.32%; (b) the sec-
ond assignment shows a strong contribution at
J. Therm. Anal. Cal., 92, 2008
947

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SIQUEIRA et al.
that the anhydrous compounds are stable up to 493 K
(Eu to Ho and Y), 503 K (Er, Tm), and 523 K (Yb,
Lu) and above this temperature up to 818 K (Eu),
783 K (Gd, Dy, Y), 798 K (Tb), 773 K (Ho, Er, Tm,
Lu) and 753 K (Yb), the mass losses occur in three
steps without a plateau between the steps. In corres-
pondence with the mass losses, the DTA curve shows
for the first step a small and broad exothermic and/or
endothermic events attributed to the thermal decom-
position (endo) together the oxidation of evolved
products (exo), where the resulting net heats produce
only small thermal event. For the two last mass losses,
the exothermic peaks are attributed to oxidation of the
organic matter.
TG
DTA
For all the compounds, the TG curve shows that
the mass loss in the last step occurs through a slow
process, corresponding to a small and broad exothermic
events (Gd, Dy–Tm, Y) or without thermal events
(Eu, Tb, Yb, Lu). Tests with hydrochloric acid solution
on samples heated up to the initial temperature of the
last step, as indicated by the corresponding TG curves,
showed in all cases, the presence of carbonaceous
residue and evolution of CO2 (except terbium). The
formation of dioxycarbonate as intermediate, has already
been observed during the thermal decomposition of other
lanthanide compounds [12, 13]. For the terbium com-
pound the thermal decomposition occurs without
formation of the intermediate, probably due to
oxidation of Tb(III) to Tb4O7 (exo), together the oxi-
dation of the organic matter (exo).
TG
DTA
TG
Calculations based on the mass loss observed in
this step of the TG curve, suggest the formation of a
mixture of lanthanide oxide (Ln2O3) and lanthanide
dioxycarbonate Ln2O2CO3, in no simple stoichiometric
relation. Thus, the last step is attributed to oxidation of
the carbonaceous residue and thermal decomposition of
the dioxycarbonate (except terbium), to the respective
oxide, Tb4O7 or Ln2O3 (Ln=Eu, Gd, Dy to Lu and Y).
The mass losses, temperature ranges and the
peak temperatures observed for each step of the
TG-DTA curves are shown in Table 4.
DTA
TG
In all the compounds, the endothermic peak at
491 K (Eu), 492 K (Gd), 483 K (Tb, Dy), 485 K (Ho,
Er) and 490 K (Y) without mass loss in the TG curves
is attributed to the phase transition. For the thulium,
ytterbium and lutetium compounds the endothermic
peak at 473, 522 and 527 K, respectively, is attributed
to the phase transition, which occurs simultaneously
with the beginning of thermal decomposition of the
anhydrous compound. The phase transition was also
observed during the thermal decomposition of lanthanide
4-methoxybenzoate [12, 13].
DTA
Fig. 2 TG-DTA curves of a – Eu(L)3·3H2O (m=7.246 mg);
b – Dy(L)3·5H2O (m=6.913 mg); c – Tm(L)3·H2O
(m=7.010 mg); d – Lu(L)3·H2O (m=6.657 mg);
L=2-methoxybenzoate
A great similarity is also observed in the DSC
profiles of these compounds, therefore only the DSC
curves of europium, dysprosium, ytterbium and
948
J. Therm. Anal. Cal., 92, 2008