Solid-state 2-methoxybenzoates of light trivalent lanthanides : SSSynthesis, characterization and thermal behaviour

Article abstract of DOI:10.1007/s10973-007-8506-9

Solid-state LnL₃ compounds, where L is 2-methoxybenzoate and Ln is light trivalent lanthanides, have been synthesized. Thermogravimetry (TG), differential scanning calorimetry (DSC), X-ray powder diffractometry, infrared spectroscopy and elemen

Full text of DOI:10.1007/s10973-007-8506-9

Journal of Thermal Analysis and Calorimetry, Vol. 91 (2008) 3, 897–902
SOLID-STATE 2-METHOXYBENZOATES OF LIGHT TRIVALENT
LANTHANIDES
Synthesis, characterization and thermal behaviour
A. B. Siqueira*, G. Bannach, E. C. Rodrigues, C. T. Carvalho and M. Ionashiro
Instituto de Química, UNESP, C.P. 355, CEP 14801-970 Araraquara, SP, Brazil
Solid-state LnL3 compounds, where L is 2-methoxybenzoate and Ln is light trivalent lanthanides, have been synthesized. Thermo-
gravimetry (TG), differential scanning calorimetry (DSC), X-ray powder diffractometry, infrared spectroscopy and elementary analysis
were used to characterize and to study the thermal behaviour of these compounds. The results led to information on the composition, de-
hydration, thermal stability and thermal decomposition of the isolated compounds. On heating these complexes decompose in three (Ce,
Pr) or five (La, Nd, Sm) steps with the formation of the respective oxide: CeO2, Pr6O11 and Ln2O3 (Ln=La, Nd, Sm) as final residues. The
theoretical and experimental spectroscopic study suggests predominantly the ionic bond between the ligand and metallic center.
Keywords: light trivalent lanthanides, 2-methoxybenzoate, thermal decomposition, thermal stability
Introduction
Experimental
Materials and methods
Benzoic acid and some of their derivatives have been
used as conservant, catalyst precursor polymers in
pharmaceutical industries, beyond other applications.
A survey of literature shows that the complexes of
rare earth and d-block elements with benzoic acid and
some of its derivatives have been investigated in
aqueous solutions and in the solid-state.
The 2-methoxybenzoic acid (HL) 99% was obtained
from Aldrich and purified by recrystallization. Aque-
ous solution of NaL 0.1 mol L–1 was prepared from
aqueous HL suspension by treatment with sodium hy-
droxide solution 0.1 mol L–1.
Lanthanide chlorides were prepared from the corre-
sponding metal oxides (except for cerium) by treatment
with concentrated hydrochloric acid. The resulting solu-
tions were evaporated to obtain an almost dry substance.
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 by adding diluted sodium hydroxide solution. Ce-
rium(III) was used as its nitrate 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 chloride or nitrate
solutions. No precipitate was observed during the ad-
dition of sodium benzoate; however the precipitate
was obtained when the solution was evaporated in a
water bath. The precipitates were washed with dis-
tilled water until elimination of the chloride (or ni-
trate) ions, filtered through and dried on Whatman
n° 42 filter paper, and kept in a desiccator over
anhydrous calcium chloride.
In aqueous solutions, the papers reported the
magnetic, physical and chemical properties, thermo-
dynamics of complexation, the spectroscopic study
and the influence of pH surfactant and synergic agent
on the luminescent properties of terbium chelates
with benzoic acid derivatives [1–3].
In the solid-state the papers reported the prepara-
tion, thermal behaviour, thermal decomposition, vi-
brational and electronic spectroscopy, spectral and
magnetic studies of several metal-ion complexes of
phenyl substituted derivatives of benzoates [4–17].
In an endeavour to extend the literature works, the
present paper deals with the synthesis of La(III),
Ce(III), Pr(III), Nd(III) and Sm(III) 2-methoxy-
benzoates, studied by means of infrared spectroscopy,
X-ray powder diffractometry, thermogravimetry (TG),
differential scanning calorimetry (DSC) and other ana-
lytical methods. The results permitted to obtain infor-
mation concerning the thermal stability, thermal de-
composition and information on the bond between the
ligand and metallic center.
In the solid-state compounds metal ions, hydra-
tion water and 2-methoxybenzoate contents were de-
*
Author for correspondence: massaoi@iq.unesp.br
1388–6150/$20.00
Akadémiai Kiadó, Budapest, Hungary
Springer, Dordrecht, The Netherlands
© 2008 Akadémiai Kiadó, Budapest

SIQUEIRA et al.
Results and discussion
termined from TG curves. The metal ions were also
determined by complexometry with standard EDTA
solution [18, 19] after igniting the compounds to the
respective oxides and their dissolution in hydrochlo-
ric acid solution.
X-ray powder patterns were obtained by using a
Siemens D-5000 X-ray diffractometer using CuKa ra-
diation (l=1.541 ) and setting of 40 kV and 20 mA.
Infrared spectra for sodium benzoate as well as
for its metal-ion compounds were recorded on a
Nicolet model Impact 400 FTIR instrument, within
the 4000–400 cm–1 range. The solid samples were
pressed into KBr pellets.
TG curves were obtained by using a Mett-
ler TA-4000 thermal analysis system. The purge gas
was air at a flow rate of 100 mL min–1 applying
20 K min–1 of heating rate and about 7 mg of initial
sample masses in alumina crucibles.
DSC curves were obtained with thermal analysis
system model DSCQ10 from TA Instruments. The
purge gas was air (flow rate: 50 mL min–1). The heat-
ing rate was 20 K min–1 and the sample mass was about
5 mg in aluminium crucibles with perforated cover.
The X-ray diffraction powder patterns, Fig. 1 show
that the 2-methoxybenzoates of the metal ions consid-
ered in this work have a crystalline structure without
evidence for formation of an isomorphous series.
Infrared spectroscopic data on 2-methoxy-
benzoate and their compounds with trivalent ions
considered in this work are shown in Table 1. The in-
vestigation was focused mainly within the
1700–1400 cm–1 range because this region is the po-
tentially most informative to assign coordination
sites. In sodium 2-methoxybenzoate strong band at
1576 cm–1 and a medium intensity band located at
1404 cm–1 are attributed to the anti-symmetrical and
symmetrical frequencies of the carboxylate groups,
respectively [8]. In all the complexes considered in
this work the symmetrical and anti-symmetrical vi-
brations of the COO– groups when compared to the
sodium salt, suggest that the compounds have a
bidentate bond with an incomplete equalization of
bond lenghts in the carboxylate anion, which is in
agreement with the literature [5].
Computational strategy
a
b
c
Calculation of theoretical infrared spectrum of lantha-
num 2-methoxybenzoate has been necessary to evalu-
ate the structure and wave function computed by the
ab initio SCF Hartree–Fock–Roothan method [20] us-
ing a split valence (3–21 g) basis set [21]. The per-
formed molecular calculations in this work were done
by using the Gaussian 98 routine [22] and the hard-
ware IBM power 3. The geometry optimization was
carried out without any constraints. The molecule of
2-methoxybenzoate contains rings with conforma-
tional flexibility and all variables optimized. The op-
timization proceeded more uniformly when all vari-
ables were optimized.
d
e
10
20
30
40
50
60
70
2q/degree
Fig. 1 X-ray powder diffraction patterns of the compounds:
a – LaL3·4H2O; b – CeL3·4H2O; c – PrL3;
d – NdL3·4H2O; e – SmL3·4H2O
Table 1 Spectroscopic data for Na(I) and some light lanthanide ions (III) 2-methoxybenzoate. IR spectra cm–1
c
c
n(O–H)H2Ob
Dn(nasym–nsym)
nasym(COO–
nsym(COO –
Compound
NaL
)
)
–
1576s
1541s
1544s
1544s
1545s
1546s
1404m
1398m
1396m
1404m
1398m
1398m
172
143
148
140
147
148
LaL3·4H2O
CeL3·4H2O
PrL3
3489br
3489br
3471br
3483br
3481br
NdL3·4H2O
SmL3·4H2O
L=2-methoxybenzoate; am – medium; s – strong; br – broad; bn(O–H): hydroxyl group stretching frequency;
cn
and n
: symmetrical and antisymmetrical vibrations of the COO– group, respectively.
sym(COO–
)
asym(COO–
)
898
J. Therm. Anal. Cal., 91, 2008

2-METHOXYBENZOATES OF LIGHT TRIVALENT LANTHANIDES
The theoretical infrared spectrum of the LaL3
A comparative analysis between the experimen-
tal and theoretical spectrum permitted to verify that:
(a) the first assignment shows a strong contribution at
was calculated by using an anharmonic field [23] and
the obtained frequencies were not scaled. The geome-
try optimization was computed by optimized algo-
rithm of Berny [24]. The obtained geometry from cal-
culations is presented in Fig. 2 and Table 2.
The theoretical infrared spectrum of the LaL3
(electronic state 1A) was obtained with frequency val-
ues (cm–1), relative intensities, assignments and de-
scription of vibrational modes. The theoretical fre-
quency, assignments and description was visualized
by using graphic software Molden 4.2 for Linux [25].
It was compared to the experimental infrared spec-
trum of the LaL3.
1541 cm–1 suggesting a n
assignment, while
)
asym(COO –
the theoretical results show the corresponding peak at
1550 cm–1 with discrepancies of 0.5 %; (b) the second
assignment shows a strong contribution at 1398 cm–1
suggesting a n
assignment, while the theoreti-
)
sym(COO –
cal results show the corresponding peak at 1410 cm–1
with discrepancies of 0.8%. The theoretical data are
in agreement with experimental ones.
The analytical and thermoanalytical (TG) data
are shown in Table 3. These results establish the
stoichiometry of these compounds, which are in
agreement with the general formula LnL3·nH2O,
where Ln represents La, Ce, Pr, Nd or Sm, L is
2-methoxybenzoate and n=4, 4, 0, 4, 4, respectively.
The TG curve of these compounds shows that the
thermal decomposition of the anhydrous compounds oc-
curs in consecutive and/or overlapping steps and the
evolved product in the first step was identified by infra-
red spectrum and qualitative test as 2-methoxybenzoic
anhydride. For the lanthanum, neodymium and samar-
ium compounds the formation of dioxycarbonate as in-
termediate was observed during the thermal decomposi-
tion of these compounds. All the residues were con-
firmed by X-ray powder diffractometry.
Fig. 2 Proposed structure 3D of solid-state anhydrous compound
of lanthanum 2-methoxybenzoate (optimized using
Hartree–Fock–Roothan method, LanL2DZ basis set of
Gaussian 98)
The thermal behaviour of the compounds is
heavily dependent on the nature of the metal ion and
so the features of each of these compounds are dis-
cussed individually. The mass losses and temperature
ranges observed for each step of the TG curves are
shown in Table 4.
Table 2 Theoretical geometries parameters of LaL3 compound
d
d
d
d
d
<
<
<
<
La – O
2.49 
1.25 
1.49 
1.40 
1.07 
95.01°
118.97°
51.59°
119.85°
COO –
C
C
C
C
C
– O
COO –
COO –
– Cring
COO –
Lanthanum compound
ring – Cring
ring – Hring
TG curve is shown in Fig. 3a. The TG curve indicates
mass losses in five steps. The first mass loss observed
between 316 and 383 K is due to dehydration with loss
of 4H2O (calcd.=10.85%; TG=10.33%). The anhy-
drous compound is stable up to 523 K and above this
temperature up 823 K the thermal decomposition oc-
curs in three steps being the two first overlapping ones.
– O
– C
– La
COO –
COO –
O
O
– O
COO –
COO –
COO –
COO –
– La – O
COO –
Cring – Cring – Cring
La=lanthanum; L=2-methoxybenzoate; d=atoms distance;
<=atoms angle; COO–=carboxylate; ring=benzene ring
Table 3 Analytical and thermoanalytical (TG) data of the compounds
Metal/%
DL/%
H2O/%
Final
residue
Compound
Calcd.
TG
EDTA
Calcd.
TG
Calcd.
TG
LaL3·4H2O
CeL3·4H2O
PrL3
20.41
21.05
23.71
21.54
22.25
21.14
21.27
23.81
21.51
22.21
21.11
21.46
23.71
22.22
22.62
64.63
63.31
71.36
64.12
63.54
64.89
63.67
71.17
64.37
63.45
10.85
10.83
–
10.33
10.21
–
La2O3
CeO2
Pr6O11
Nd2O3
Sm2O3
NdL3·4H2O
SmL3·4H2O
10.76
10.66
10.67
10.78
L means 2-methoxybenzoate
J. Therm. Anal. Cal., 91, 2008
899

SIQUEIRA et al.
Table 4 Temperature ranges q and mass losses (%) observed for each step of TG curves of the compounds
Steps
Compound
first
second
third
fourth
fifth
q/K
loss/%
316–383
10.33
523–573
28.06
573–703
8.18
703–823
25.73
823–1013
2.92
LaL3·4H2O
CeL3·4H2O
PrL3
q/K
loss/%
333–393
10.21
523–573
26.87
573–683
36.8
–
–
–
–
q/K
loss/%
503–573
29.61
573–693
9.35
693–838
32.21
–
–
–
–
q/K
loss/%
323–383
10.67
508–573
23.78
573–653
10.44
653–853
28.42
853–1058
1.73
NdL3·4H2O
SmL3·4H2O
q/K
loss/%
333–393
10.78
493–573
28.26
573–663
8.08
663–843
25.38
843–1063
1.73
L=2-methoxybenzoate
Praseodymium compound
a
b
c
d
e
TG curve is shown in Fig. 3c. The thermal decomposi-
tion of this compound occurs in three steps. The anhy-
drous compound is stable up to 503 K and above this
temperature the thermal decomposition occurs in three
overlapping steps, being the first and third ones fast pro-
cesses and the second step slow process. The total mass
loss up to 838 K is in agreement with the formation of
Pr6O11, as final residue (calcd.=71.36%; TG=71.17%).
Neodymium compound
273
473
673
873
1073
TG curve is shown in Fig. 3d. The thermal decomposi-
tion of this compound occurs in five steps. The first
mass loss observed between 323 and 383 K, is due to
dehydration with loss of 4H2O (calcd.=10.76%;
TG=10.67%). The anhydrous compound is stable up to
508 K and above this temperature up to 1058 K the
mass loss occurs in four steps being the three first up to
853 K overlapping ones. Tests with hydrochloric acid
solution on samples heated up to 853 K, confirmed the
elimination of CO2 and calculations based on the mass
loss suggest the formation of a mixture of neodymium
oxide and dioxycarbonate in no simple stoichiometric
relation. The last step that occurs between 853 and
1058 K is ascribed to the thermal decomposition of
dioxycarbonate to neodymium oxide Nd2O3, as final
residue (calcd.=74.88%; TG=75.04%).
Temperature/K
Fig. 3 TG curves of the compounds: a – LaL3·4H2O
(m=7.250 mg); b – CeL3·4H2O (m=7.250 mg);
c – PrL3 (m=7.390 mg); d – NdL3·4H2O (m=7.290 mg);
e – SmL3·4H2O (m=7.325 mg)
Tests with hydrochloric acid solution on samples
heated up to 823 K confirmed the elimination of CO2
and calculations based on the mass loss up to this tem-
perature suggest the formation of lanthanum dioxy-
carbonate (La2O2CO3) as intermediate. The last step
that occurs between 823 and 1013 K is attributed to the
thermal decomposition intermediate to the lanthanum
oxide as final residue (calcd.=75.48%; TG=75.22%).
Cerium compound
TG curve is shown in Fig. 3b. The TG curve shows
three mass loss steps. The first mass loss observed be-
tween 333 and 393 K is due to dehydration with loss
of 4H2O (calcd.=10.83%; TG=10.21%). The anhy-
drous compound is stable up to 523 K and the thermal
decomposition occurs in two consecutive steps be-
tween 523 and 683 K. The total mass loss up to 683 K
is in agreement with the formation of CeO2 as final
residue (calcd.=74.14%; TG=73.98%).
Samarium compound
TG curve is shown in Fig. 3e. The thermal decomposi-
tion of this compound also occurs in five steps. The first
mass loss observed between 333 and 393 K, is due to
dehydration with loss of 4H2O (calcd.=10.66%;
TG=10.78%). The anhydrous compound is stable up to
493 K and between 493–1063 K the thermal decompo-
sition occurs in four steps being the three first overlap-
900
J. Therm. Anal. Cal., 91, 2008

2-METHOXYBENZOATES OF LIGHT TRIVALENT LANTHANIDES
Conclusions
ping ones. Tests with hydrochloric acid solution on
samples heated up to 843 K, confirmed the elimination
of CO2 and calculations based on the mass loss up to this
temperature also suggest the formation of a mixture of
samarium oxide and dioxycarbonate in no simple
stoichiometric relation. The last step that occurs be-
tween 843 and 1063 K corresponding to the thermal de-
composition of the dioxycarbonate to samarium oxide
Sm2O3 as final residue (calcd.=74.20%; TG=74.23%).
DSC curves of the compounds are shown in
Fig. 4. These curves show thermal events correspond-
ing to the mass losses observed in TG curves or due to
physical phenomenon.
From analytical and thermoanalytical (TG) results a
general formula could be established for these com-
pounds in the solid state.
The thermal decomposition of the compounds
occurred in three (Ce, Pr) or five (La, Nd, Sm) steps
with formation of the respective oxide: La2O3, CeO2,
Pr6O11, Nd2O3 and Sm2O3 as final residue.
The X-ray powder patterns pointed out that the
2-methoxybenzoates of the metal ions have a crystal-
line structure, without evidence concerning the for-
mation of isomorphous compounds.
The theoretical and experimental infrared spec-
troscopic data suggest that the ligand acts as a
bidentate bond with an incomplete equalization of
bond lenghts in the carboxylate anion.
a
TG and DSC provided previously unreported in-
formation about the thermal stability, thermal decompo-
sition and physical phenomenon of these compounds.
b
c
d
Acknowledgements
e
The authors thank FAPESP, CNPq and CAPES Foundations
(Brazil) for financial support and computational facilities of
IQ-UNESP and CENAPAD-UNICAMP.
273
373
473
573
Temperature/K
673
773
873
Fig. 4 DSC curves of the compounds: a – LaL3 (m=4.959 mg);
b – CeL3 (m=4.917 mg); c – PrL3 (m=5.132 mg) ;
d – NdL3 (m=4.800 mg); e – SmL3·4H2O (m=4.930 mg)
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Received: April 12, 2007
Accepted: June 10, 2007
DOI: 10.1007/s10973-007-8506-9
902
J. Therm. Anal. Cal., 91, 2008