Synthesis, characterization and thermal behaviour of solid 4-methoxybenzoates of heavier trivalent lanthanides

Article abstract of DOI:10.1016/j.tca.2006.09.014

Solid-state Ln-4-MeO-Bz compounds, where Ln stands for trivalent Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Y and 4-MeO-Bz is 4-methoxybenzoate, have been synthesized. Simultaneous thermogravimetry and differential thermal analysis (TG-DTA), differential scan

Full text of DOI:10.1016/j.tca.2006.09.014

Thermochimica Acta 451 (2006) 149–155
Synthesis, characterization and thermal behaviour of solid
4-methoxybenzoates of heavier trivalent lanthanides
Emanuel C. Rodrigues∗, Adriano B. Siqueira, Elias Y. Ionashiro,
Gilbert Bannach, Massao Ionashiro
Instituto de Qu´ımica, UNESP, C.P. 355, CEP 14801-970, Araraquara, SP, Brazil
Received 14 May 2006; received in revised form 8 September 2006; accepted 26 September 2006
Available online 4 October 2006
Abstract
Solid-state Ln–4-MeO-Bz compounds, where Ln stands for trivalent Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Y and 4-MeO-Bz is 4-
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 led to information about the composition, dehydration, phase transition, coordination mode, structure,
thermal behaviour and thermal decomposition of the isolated compounds. The phase transition observed in the some compounds has been reported
for the first time.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Heavier lanthanides; 4-Methoxybenzoate; Characterization; Thermal behaviour
1. Introduction
derivates with rare earth elements [6–11]; the vibrational and
electronic spectroscopic study of lanthanides and effect of
sodium on the aromatic system of benzoic acid [12,13]; the
reaction of bivalent cooper, cobalt and nickel with 3-hidroxy-4-
methoxy and 3-methoxy-4-hidroxybenzoic acids and a structure
for these compounds has been proposed on the basis of spec-
troscopic and thermogravimetric data [14]; the thermal decom-
position of thorium salts of benzoic and 4-methoxybenzoic
acids in air atmosphere [15] and thermal behaviour of solid-
state methoxybenzoates of some bivalent transition metal ions
[16–18].
In this work 4-methoxybenzoates of Eu(III), Gd(III), Tb(III),
Dy(III), Ho(III), Er(III), Tm(III), Yb(III), Lu(III) and Y(III)
were synthesized. The compounds were investigated by means
of infrared spectroscopy, X-ray powder diffractometry, simul-
taneous thermogravimetry and differential thermal analysis
(TG–DTA), differential scanning calorimetry (DSC) and other
methods of analysis.
Benzoic acid and some of their derivatives have been used
as conservant, catalyst precursors polymers, in pharmaceuti-
cal industries, beyond other applications. A survey of literature
shows that the complexes of rare earth and d-block elements with
benzoic acids and some of its derivatives have been investigated
in aqueous solutions and in the solid-state.
In aqueous solutions, the publications reported the thermo-
dynamics of complexation of lanthanides by some benzoic acid
derivatives [1]; the spectroscopic study of trivalent lanthanides
with several carboxylic acids including benzoic acid [2]; the
influence of pH, surfactant and synergic agent on the lumines-
cent properties of terbium chelates with benzoic acid deriva-
tives [3]; the thermodynamic of complexation of lanthanides by
methoxybenzoates [4] and the synthesis, crystal structure and
photophysical and magnetic properties of dimeric and polymeric
lanthanide complexes with benzoic acid and its derivatives [5].
In the solid-state the publications reported the thermal
and spectral behaviour on solid compounds of benzoates and
2. Experimental
∗
The 4-methoxybenzoic acid (4-MeO–HBz) 98% was
obtained from Acros Organics and purified by recrystallization.
Corresponding author.
E-mail address: emanuelr@posgrad.iq.unesp.br (E.C. Rodrigues).
0040-6031/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.tca.2006.09.014

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E.C. Rodrigues et al. / Thermochimica Acta 451 (2006) 149–155
Fig. 1. X-ray powder diffraction patterns of the compounds: (a) Eu(L)3; (b) Gd(L)3; (c) Tb(L)3; (d) Dy(L)3; (e) Ho(L)3; (f) Er(L)3; (g) Tm(L)3; (h) Yb(L)3; (i)
Lu(L)3 and (j) Y(L)3.
Aqueous solution of Na–4-MeO-Bz 0.1 mol L−1 was pre-
pared from aqueous 4-MeO–HBz suspension by treatment with
sodium hydroxide solution 0.1 mol L−1. Lanthanide chlorides
were prepared from the corresponding metal oxides (except for
yttrium) by treatment with concentrated hydrochloric acid. The
resulting solutions were evaporated to near dryness, the residues
were again dissolved in distilled water, transferred to a volumet-
ric flask and diluted in order to obtain ca. 0.1 mol L−1 solutions,
whose pH were adjusted to 5.5 adding diluted sodium hydrox-
ide or hydrochloric acid solutions. Yttrium (III) was used as
its chloride and ca. 0.1 mol L−1 aqueous solutions of this ion
were prepared by direct weighing of the salt. The solid-state
compounds were prepared by adding slowly, with continuous
stirring, the solution of the ligand to the respective metal chlo-
ride 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 Whatmann
42 filter paper, and kept in a desiccator over anhydrous calcium
chloride, under reduced pressure to constant mass.
In the solid-state compounds, hydration water, ligand and
metal ion contents were determined from TG curves. The
Table 2
Spectroscopic data for sodium 4-methoxybenzoate and compounds with some
trivalent ´ıons
Table 1
Analytical and thermoanalytical (TG) data of the compounds
Compound
νas (COO−) (cm−1
)
νsym (COO−) (cm−1
)
ꢀν
Compounds
Metal (%)
Calcd.
Loss (%)
Calcd.
Residuea
NaL
1543s
1508s
1510s
1510s
1510s
1510s
1512s
1512s
1514s
1518s
1514s
1416s
1393s
1394s
1393s
1400s
1400s
1404s
1404s
1410s
1412s
1410s
127
115
116
117
110
110
108
108
104
106
104
EDTA
TG
TG
Eu(L)3
Gd(L)3
Tb(L)3
Dy(L)3
Ho(L)3
Er(L)3
Tm(L)3
Yb(L)3
Lu(L)3
Y(L)3
Eu(L)3
Gd(L)3
Tb(L)3
Dy(L)3
Ho(L)3
Er(L)3
Tm(L)3
Yb(L)3
Lu(L)3
Y(L)3
25.10
25.75
25.95
26.38
26.67
26.95
27.14
27.62
27.84
16.39
25.24
25.65
26.12
26.48
26.86
26.67
27.49
27.75
27.90
16.50
25.16
25.81
25.87
26.24
26.61
26.78
27.52
27.57
28.34
16.50
70.94
70.32
69.48
69.72
69.45
69.18
69.00
68.55
68.33
79.18
70.88
70.16
69.49
69.85
69.59
69.26
68.57
68.61
67.80
78.86
Eu2O3
Gd2O3
Tb4O7
Dy2O3
Ho2O3
Er2O3
Tm2O3
Yb2O3
Lu2O3
Y2O3
Key: s, strong; L, 4-methoxybenzoate; ꢀν = νas−νsym. νsym (COO−) and νas
(COO−) symmetrical and anti-symmetrical vibrations of the COO− structure,
respectively.
L means 4-methoxybenzoate.
a
All the residues was confirmed by X-ray powder diffractometry.

E.C. Rodrigues et al. / Thermochimica Acta 451 (2006) 149–155
151
metal ions were also determined by complexometric titrations
3. Results and discussion
with standard EDTA solution, using xylenol orange as indica-
tor [19,20]. X-ray powder patterns were obtained by using a
Siemens D-5000 X-ray diffractometer, employing Cu K␣ radi-
The analytical and thermoanalytical (TG) data are shown in
Table 1. These results establish the stoichiometry of these com-
pounds, whichareinagreementwiththegeneralformulaLn(L)3,
where Ln represents trivalent Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb,
Lu or Y and L is 4-methoxybenzoate.
The X-ray diffraction powder patterns (Fig. 1) show that all
the compounds have a crystalline structure, with evidence for
formation of an isomorphous series.
Infrared spectroscopic data on 4-methoxybenzoate and its
compounds with trivalent metal ions considered in this work are
shown in Table 2. The investigation was focused mainly within
the 1700–1400 cm−1 range because this region is potentially
most informative to assign coordination sites. In the sodium 4-
methoxybenzoate strong bands located at 1543 and 1416 cm−1
are attributed to the anti-symmetrical and symmetrical frequen-
cies of the carboxylate groups, respectively [21,22]. In the com-
pounds considered in this work analysis of the frequencies of
the νas and νsym (COO−) bands shows that the lanthanides are
linked to the carboxylic group by a bridge bond and/or a biden-
tate bond with an incomplete equalization of bond lengths in the
carboxylate anion; this is in agreement with the literature [5,12].
All attempts at growing good single crystals of the presently
investigated compounds were unsuccessful. The insolubility in
˚
ation (λ = 1.541 A) and settings of 40 kV and 20 mA.
Infrared spectra for Na–MeO–Bz as well as for its metal–ion
compounds were run on a Nicolet model Impact 400 FT-IR
instrument, within the 4000–400 cm−1 range. The solid sam-
ples were pressed into KBr pellets.
Simultaneous TG–DTA curves were obtained with thermal
analysis system model SDT 2960 from TA Instruments. The
purge gas was an air flow of 100 mL min−1. A heating rate of
20 ◦C min−1 was adopted with samples weighing about 7 mg.
Platinum crucibles were used for recording the TG–DTA curves.
DSC curves were obtained with thermal analysis systems
model Q 10 from TA Instruments. The purge gas was an air flow
of 100 mL min−1. A heating rate of 20 ◦C min−1 was adopted
with samples weighing about 5 mg. Aluminium crucibles, with
perforated cover, were used for recording the DSC curves.
Table 3
Temperature ranges (θ), mass losses (%) and peak temperatures observed for
each step of the TG–DTA curves of the compounds
Compound
Steps
First
Second
Third
EuL3
θ ◦C
Loss (%)
Peak (◦C)
280–393
35.84
393 (exo)
393–467
33.70
466 (exo)
467–705
1.34
–
GdL3
TbL3
DyL3
HoL3
ErL3
TmL3
YbL3
LuL3
YL3
θ ◦C
Loss (%)
Peak (◦C)
280–426
38.15
426 (exo)
426–488
29.52
455 (exo)
488–725
2.49
–
θ ◦C
Loss (%)
Peak (◦C)
280–422
40.68
422 (exo)
422–467
28.81
469 (exo)
–
–
–
θ ◦C
Loss (%)
Peak (◦C)
280–418
45.36
418 (exo)
418–475
22.38
464 (exo)
475–715
2.11
–
θ ◦C
Loss (%)
Peak (◦C)
280–418
45.36
418 (exo)
418–464
22.14
464 (exo)
464–715
2.08
–
Fig. 2. TG–DTA curves of Tb(L)3 (L = 4-methoxybenzoate); m = 7.310 mg.
θ ◦C
Loss (%)
Peak (◦C)
280–426
36.87
426 (exo)
426–480
32.39
480 (exo)
480–710
–
–
θ ◦C
Loss (%)
Peak (◦C)
280–431
42.06
431 (exo)
431–470
26.51
469 (exo)
470–710
–
–
θ ◦C
Loss (%)
Peak (◦C)
280–425
49.49
425 (exo)
425–475
16.29
476 (exo)
475–750
2.83
–
θ ◦C
Loss (%)
Peak (◦C)
280–410
30.04
410 (exo)
410–488
35.56
422, 488 (exo)
488–720
–
2.20
θ ◦C
Loss (%)
Peak (◦C)
280–431
45.99
431 (exo)
431–480
31.61
481 (exo)
480–700
1.26
–
L means 4 methoxybenzoate.
Fig. 3. TG–DTA curves of Eu(L)3 (L = 4-methoxybenzoate); m = 6.981 mg.

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E.C. Rodrigues et al. / Thermochimica Acta 451 (2006) 149–155
Fig. 4. X-ray powder diffraction patterns of the: (a) Er(L)3; (b) Er(L)3 heated
up to 280 ◦C (L = 4-methoxybenzoate).
a wide variety of solvents suggest that these compounds might
be polymeric in nature. It is known that high molecular mass
metal coordination polymers bearing oxygen as donor atoms
are generally hard, fiber- or film-like materials, stable at rel-
atively elevated temperatures [23,24]. The presently prepared
compounds do not display these features. So, if polymeric, they
should be, almost certainly, of low nuclearity.
Fig. 6. X-ray powder diffraction patterns of the: (a) Lu(L)3; (b) Lu(L)3 heated
up to 280 ◦C (L = 4-methoxybenzoate).
Simultaneous TG and DTA curves of the compounds exhibit
mass losses in steps and thermal events corresponding to these
losses or due to physical phenomenon.
intermediate, as indicated by the corresponding TG curves, con-
firmed, in all cases, evolution of CO2. The formation of dioxy-
carbonate as intermediate, had already been observed during the
thermal decomposition of other lanthanide compounds [25,26].
For the terbium compound, the thermal decomposition occurs
without formation of intermediate, probably due to the oxidation
reaction of Tb(III) to Tb4O7 (exothermic) together the oxidation
and combustion of the organic matter.
For all the compounds, except terbium, the TG curve shows
that the mass loss in the last step occurs through a slow process,
without thermal event in the DTA curve. Calculations based on
the mass observed in this step of the TG curve, suggest the for-
mation of a mixture of lanthanide oxide, Ln2O3 and lanthanide
dioxycarbonate, Ln2O2CO3 (Ln = Eu, Gd, Dy–Lu, Y), innosim-
ple stoichiometric relation. Tests with hydrochloric acid solution
on samples heated up to the temperatures of formation of this
The TG and DTA curves also show that the anhydrous
compounds are stable up to 280 ◦C. Two patterns of thermal
behaviour are observed for the compounds: those characterized
Fig. 5. DSC curves of heating and cooling of the compound Er(L)3 (L = 4-
methoxybenzoate).
Fig. 7. DSC curves of heating and cooling of the compound Lu(L)3 (L = 4-
methoxybenzoate).

E.C. Rodrigues et al. / Thermochimica Acta 451 (2006) 149–155
153
by a single phase transition (Tb, Dy, Ho, Er, Tm and Y) and
those showing double one (Eu, Gd, Yb and Lu).
The mass losses, temperature ranges and the peak tempera-
tures observed in each step of the TG–DTA curves are shown in
Table 3.
222 ◦C (Ho), 213 ◦C (Er), 207 ◦C (Tm) and 212 ◦C (Y) is
attributed to the reversible phase transition.
The thermal decomposition of the terbium compound occurs
between280and467 ◦Cintwooverlappingsteps, corresponding
to exothermic peaks attributed to oxidation and combustion of
the organic matter in both mass losses. The total mass loss up
to 467 ◦C is in agreement with the formation of terbium oxide,
Tb4O7, as final residue (Calcd. = 69.48%; TG = 69.49%), which
was confirmed by X-ray powder diffractometry.
For the other compounds the thermal decomposition occurs
in three steps between 280 and 715 ◦C (Dy, Ho), 280–710 ◦C
(Er, Tm) and 280–700 ◦C (Y), with the two first correspond-
ing to exothermic peaks. The last step is ascribed to the
Therefore, the features of these compounds are discussed
based on the similarity of the TG–DTA profiles.
3.1. Terbium, dysprosium, holmium, erbium, thulium and
yttrium compounds
The TG–DTA curves of the terbium compound are shown
in Fig. 2. The endothermic peak at 231 ◦C (Tb), 226 ◦C (Dy),
Fig. 8. DSC curves of the compounds: (a) Eu(L)3 (5.400 mg); (b) Gd(L)3 (5.000 mg); (c) Tb(L)3 (4.700 mg); (d) Dy(L)3 (6.200 mg); (e) Ho(L)3 (5.200 mg); (f)
Er(L)3 (5.600 mg); (g) Tm(L)3 (5.900 mg); (h) Yb(L)3 (5.300 mg); (i) Lu(L)3 (5.600 mg) and (j) Y(L)3 (4.500 mg) (L = 4-methoxybenzoate).

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E.C. Rodrigues et al. / Thermochimica Acta 451 (2006) 149–155
thermal decomposition of the lanthanide dioxycarbonate to
the respective oxides, as final residue (Calcd. (%) TG (%);
Dy2O3 = 69.72–69.85; HO2O3 = 69.45–69.59; Er2O3 = 69.19–
69.26; Tm2O3 = 69.00–68.57 and Y2O3 = 79.18–78.86), which
were confirmed by X-ray powder diffractometry.
For the last step, no endothermic peak is observed in the
DTA curve, probably due to the absorbed heat in this step is not
enough to produce the thermal event.
at 242 ◦C (Eu), 239 ◦C (Gd), 200 ◦C (Yb) and 191 ◦C (Lu) are
due to the irreversible phase transition. The broad exotherms
observed for all compounds between 350 and ≥600 ◦C, with-
out the appearance of definitive peaks, are attributed to the
thermal decomposition of the anhydrous compounds, where
the oxidation of the organic matter takes place in consecutive
steps.
4. Conclusion
3.2. Europium, gadolinium, ytterbium and lutetium
compounds
From analytical and thermoanalytical (TG) results a general
formula could be established for these compounds in the solid-
state.
The X-ray powder patterns pointed out that the synthesized
compounds have a crystalline structure with evidence concern-
ing to the formation of isomorphous compounds.
The infrared spectroscopic data suggest that 4-methoxy-
benzoate act as a bridge and/or bidentate ligand toward the metal
ions considered in this work.
The TG–DTA curves of the europium compound are shown
in Fig. 3. The endothermic peak at 239 ◦C (Eu), 234 ◦C (Gd),
197 ◦C (Yb) and 190 ◦C (Lu) is due to the irreversible phase
transition and the endothermic peak at 264 ◦C (Eu), 260 ◦C (Gd),
212 ◦C (Yb) and 211 ◦C (Lu) is attributed to the reversible phase
transition.
The thermal decomposition of these compounds occurs
in three steps between 280–705 ◦C (Eu), 280–725 ◦C (Gd),
280–750 ◦C (Yb) and 280–720 ◦C (Lu), with the two first
corresponding to exothermic peaks. The last step is ascribed
to the thermal decomposition of the lanthanide dioxycar-
bonate to the respective oxides, as final residue (Calcd.
(%) – TG (%); Eu2O3 = 70.94–70.88; Gd2O3 = 70.32–70.16;
Yb2O3 = 68.55–68.61 and Lu2O3 = 68.33–67.80), which were
confirmed by X-ray powder diffractometry. For these com-
pounds, no thermal event corresponding to the last step is also
observed in the DTA curves.
The TG–DTA and DSC curves provided previously unre-
ported information about the thermal behaviour and thermal
decomposition of these compounds.
Acknowledgements
The authors thank FAPESP (Proc. 97/12646-8), CNPq and
CAPES foundations (Brazil) for financial support.
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reversible phase transition, while the small endothermic peaks

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