Thermal and spectral studies of rare earth element 3-methoxy-4-methylbenzoates

Article abstract of DOI:10.1007/s10973-004-0466-8

3-Methoxy-4-methylbenzoates of Y(III) and lanthanide(III) (La-Lu) were prepared as crystalline compounds with molar ratio of metal to organic ligand of 1.0:3.0 and general formula Ln(C₉H₉O₃) ₃.nH₂O, w

Full text of DOI:10.1007/s10973-004-0466-8

Journal of Thermal Analysis and Calorimetry, Vol. 78 (2004) 999–1007
THERMAL AND SPECTRAL STUDIES OF RARE
EARTH ELEMENT
3-methoxy-4-methylbenzoates
W. Brzyska* and W. Oüga
Department of General Chemistry, Faculty of Chemistry, Marie Curie SkÓodowska University,
Pl 20-032 Lublin, Poland
(Received November 24, 2003; in revised form December 23, 2003)
Abstract
3-Methoxy-4-methylbenzoates of Y(III) and lanthanide(III) (La–Lu) were prepared as crystalline
compounds with molar ratio of metal to organic ligand of 1.0:3.0 and general formula
Ln(C9H9O3)3⋅nH2O, where n=2 for Y, La–Er and n=0 for Tm–Lu. IR spectra of the prepared
complexes suggest that carboxylate groups are bidentate chelating. During heating dihydrated
complexes lose crystallization water molecules in one (Y, La, Pr–Er) or two steps (Ce) and then all
the anhydrous complexes decompose directly to oxides Ln2O3, CeO2, Pr6O11 and Tb4O7.
Keywords: complexes, IR spectra, 3-methoxy-4-methylbenzoic acid, rare earth element, thermal
analysis
Introduction
3-Methoxy-4-methylbenzoic acid C6H3(OCH3)(CH3)COOH is a crystalline solid,
sparingly soluble in water [1]. The complexes of 3-methoxy-4-methylbenzoic acid
with rare earth elements were previously unknown. In our previous work [2] we have
studied the complexes of rare earth elements with 4-methoxy-2-methylbenzoic acid.
4-Methoxy-2-methylbenzoates were prepared as crystalline complexes with
general formula Ln(C9H9O3)3. Only La(III) and Pr(III) complexes were prepared as
monohydrates. Monohydrates heated lose crystallization water molecules and then
all the anhydrous complexes decompose directly to oxides. Only La(III) complex de-
compose to oxide with intermediate formation La2O2CO3. The carboxylate group in
the studied complexes is a tridentate chelating – bridging or bidentate chelating (Y).
In the literature there are some papers on the preparation and thermal and spec-
tral properties of rare earth complexes with 2,3- [3, 4], 2,4- [4, 5] and 3,4-dimethoxy-
benzoic [4, 5] acids.
*
Author for correspondence: E-mail: BRZYSKA@hermes.umcs.lublin.pl
1388–6150/2004/ $ 20.00
Akadémiai Kiadó, Budapest
© 2004 Akadémiai Kiadó, Budapest
Kluwer Academic Publishers, Dordrecht

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BRZYSKA, O¨GA: 3-METHOXY-4-METHYLBENZOATES
As a continuation of our work on the rare earth element carboxylate [2, 6–8], we
now report on the preparation, and spectral and thermal studies of rare earth element
complexes with 3-methoxy-4-methylbenzoic acid in solid-state.
Experimental
3-Methoxy-4-methylbenzoates of rare earth element(III) (Y, La–Lu, without Pm)
were prepared by reaction of stoichiometric quantities of 0.2 M solution of ammo-
nium 3-methoxy-4-methylbenzoate (pH 5.5) and 0.2 M solution of rare earth ele-
ment(III) chlorides (Ce was used as its nitrate). The precipitation formed was heating
in the mother solution for 0.5 h at 343–353 K, was then filtered off, washed with hot
water to remove ammonium ions and dried at 303 K to a constant mass. The sodium
salt was prepared by adding 3-methoxy-4-methylbenzoic acid to solution of NaOH
and crystallizing at room temperature.
The carbon and hydrogen in the prepared complexes were determined by ele-
mental analysis on a Perkin Elmer CHN 2400 analyser. The metal content was deter-
mined from TG curve and by ignition of the complexes prepared in air to 1273 K. The
content of crystallization water was determined from TG curve and by heating the
samples isothermally at suitable temperature to a constant mass.
The IR spectra of 3-methoxy-4-methylbenzoic acid, its sodium salt and the separated
rare earth complexes were recorded over the range 4000–400 cm–1 on a Specord M 80
spectrophotometer. The samples were prepared as KBr discs. The X-ray patterns were
recorded with a HZG (Zeiss Jena) diffractometer by Debye–Scherrer powder method using
Ni filtered CuKα radiation. The measurements were made over the range 2θ=5–80°.
The solubility of the prepared complexes in water was determined at 293 K. The
saturated solution were prepared under isothermal conditions. The content of metal(III)
ions was determined by spectrophotometric analysis using UV-VIS Specord M 40
spectrophotometer, with arsenazo III. The solubility of the complexes was calculated
on the basis of the metal(III) concentration in the saturated solution.
The thermal stability of the prepared complexes was determined using a
Paulik–Paulik–Erdey Q 1500 derivatograph with Derill converter. Measurements
were made with a sensitivity of 100 mg (TG). The sensibility of DTG and DTA
curves were recorded by Derill computer program. Samples of 100 mg were heated in
air static atmosphere in platinum crucibles to 1273 K at a heating rate 10 K min–1
with a full scale. The hydrated complexes were also heated isothermally at a set tem-
perature to a constant mass. The products of decomposition were confirmed by the IR
spectra and X-ray diffractograms.
Results and discussion
3-Methoxy-4-methylbenzoates of rare earth elements(III) (Y, La–Lu, without Pm) were
prepared as crystalline solids with a colour characteristic for rare earth element(III) ions
and with molar ratio of metal to organic ligand of 1.0:3.0 and general formula
J. Therm. Anal. Cal., 78, 2004

BRZYSKA, O¨GA: 3-METHOXY-4-METHYLBENZOATES
1001
Ln(C9H9O3)3⋅2H2O. Only the complexes of the heaviest lanthanides, Tm–Lu, were
prepared as anhydrous complexes (Table 1). The complexes prepared are sparingly
soluble in water. Their solubility is of the order 10–4 mol dm–3 and change periodically in
the lanthanide series. It changes from 1.30⋅10–4–3.40⋅10–4 mol dm–3 (only solubility of
Ce(III) complex is equal 5.50⋅10–4 mol dm–3).
Table 1 Analytical data and solubilities in water at 293 K of Y(III) and lanthanide(III)
3-methoxy-4-methylbenzoates
Ln/%
C/%
H/%
Solubility/
10–4 mol dm–3
Complex
calcd.
14.34
20.73
20.88
20.97
21.36
22.06
22.25
22.84
23.03
23.43
23.70
23.95
25.49
25.90
26.11
found
14.3
20.7
20.8
21.0
21.4
22.0
22.0
22.8
23.0
23.4
23.8
24.0
25.5
26.0
26.0
calcd.
52.25
48.35
48.28
48.22
47.98
47.55
47.44
47.07
46.96
46.72
46.56
46.40
48.77
48.50
48.36
found
52.0
48.4
47.9
48.0
47.7
47.5
47.4
47.0
47.0
46.7
46.5
46.3
48.8
48.5
48.4
calcd.
5.00
4.62
4.62
4.61
4.59
4.55
4.54
4.50
4.49
4.47
4.45
4.44
4.06
4.04
4.03
found
4.9
4.5
4.5
4.6
4.6
4.5
4.5
4.5
4.5
4.5
4.4
4.4
4.0
4.0
4.0
YL3⋅2H2O*
LaL3⋅2H2O
CeL3⋅2H2O
PrL3⋅2H2O
NdL3⋅2H2O
SmL3⋅2H2O
EuL3⋅2H2O
GdL3⋅2H2O
TbL3⋅2H2O
DyL3⋅2H2O
HoL3⋅2H2O
ErL3⋅2H2O
TmL3
2.62
1.55
5.50
2.96
3.65
1.30
1.65
2.35
2.20
2.11
3.10
2.58
3.40
2.74
2.02
YbL3
LuL3
*L=[C6H3(OCH3)(CH3)COO]–
The IR spectra of the prepared 3-methoxy-4-methylbenzoates are similar to
each other (except of Tm–Lu) and have many bands. When the acid is converted to
complexes the absorption band of C=O group at 1684 cm–1 disappears and the bands
of asymmetrical (νas) and symmetrical (νs) vibrations of OCO– group appear
at 1525–1520 and 1425–1420 cm–1, respectively (for Ce, Pr and Eu complexes
at 1510 and 1410 cm–1), the bands of OCH3 with max. at 1388–384, 1288–1284,
1248–1244 cm–1, the bands of CH3 group at 2930 and 2830, 1408–1404,
1320–1313 cm–1 and the bands of M–O bond at 570–550 cm–1. In the IR spectra of
the prepared complexes there are the bands of aromatic ring 1172, 1168, 1106, 1104
and 1040 cm–1, and the CH bond at 884–880, 816–804, 776–770 cm–1 [9–12]. These
bands are shifted insignificantly (8–4 cm–1) or do not change their position compared
to the respective bond of 3-methoxy-4-methylbenzoic acid, what indicates that
Ln(III) ions have only a weak influence on the benzene ring.
J. Therm. Anal. Cal., 78, 2004

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BRZYSKA, O¨GA: 3-METHOXY-4-METHYLBENZOATES
The frequencies of M–O bond for all the complexes have similar value
(Table 2). Accordingly, it may be suggested that 3-methoxy-4-methylbenzoates of
rare earth have similar stability [13]. The separation value (∆ν) of νas(OCO) and
νs(OCO) in the spectra of the prepared 3-methoxy-4-methylbenzoates have similar
values (∆ν=100–90 cm–1) and are smaller than for the sodium salt (∆ν=150 cm–1),
what shows on smaller degree of ionic bond in the prepared complexes compared to
the sodium salt [9, 11]. The bands of asymmetric vibrations of OCO group are shifted
to lower frequencies (∆=50–35 cm–1) and the symmetric vibrations of OCO group to
higher frequencies (∆=25–10 cm–1, only for Pr and Eu ∆=0) compared to these bands
for the sodium salt. From the shifts of νas(OCO) and νs(OCO) and value ∆ν in the IR
spectra of the prepared complexes, compared to the bands for sodium salt, it is
possible to suggest that OCO group is bonded as bidentate chelating [9–13]. In the IR
spectra of the prepared complexes of Y and La–Er there are broad bands of ν(OH)
with max. at 3410–3300 cm–1 and narrow bands δ(H2O) at 1630–1620 cm–1, that are
characteristic for hydrates. In the IR spectra of Tm–Lu these bands are absent, what
confirms anhydrous character of these complexes.
The rare earth element 3-methoxy-4-methylbenzoates are crystalline solids of
low symmetry and different structure. They are isostructural in the groups: Y, La,
Ce–Sm, Gd–Er, Tm–Lu.
Table 2 Frequencies of characteristic absorption bands in IR spectra of Na(I) and rare earth
3-methoxy-4-methylbenzoates (cm–1)
Complex
ν(OH)
νas(OCO)
νs(OCO)
νas–νs
ν(M–O)
YL3⋅2H2O*
LaL3⋅2H2O
CeL3⋅2H2O
PrL3⋅2H2O
NdL3⋅2H2O
SmL3⋅2H2O
EuL3⋅2H2O
GdL3⋅2H2O
TbL3⋅2H2O
DyL3⋅2H2O
HoL3⋅2H2O
ErL3⋅2H2O
TmL3
3300
3400
3410
3300
3300
3300
3300
3300
3300
3300
3300
3350
-
1524
1510
1510
1510
1520
1520
1510
1520
1520
1525
1520
1525
1520
1520
1525
1560
1425
1420
1420
1410
1420
1425
1410
1425
1420
1425
1420
1425
1420
1420
1425
1410
99
90
560
560
560
560
570
570
560
570
560
570
560
560
560
560
550
480
90
100
100
95
100
95
100
100
100
100
100
100
100
150
YbL3
-
LuL3
-
NaL⋅nH2O
3400
*L=[C6H3(OCH3)(CH3)COO]–
J. Therm. Anal. Cal., 78, 2004

BRZYSKA, O¨GA: 3-METHOXY-4-METHYLBENZOATES
1003
3-Methoxy-4-methylbenzoates of rare earth elements are stable in air and do not
change their composition. Hydrated complexes are stable up to 323–363 K, whereas
the anhydrous complexes (Tm–Lu) up to 563 K. When heated in air (Table 1, Figs 1–4)
the hydrated complexes of Y, La–Er lose crystallization water molecules in one (Y, La,
Pr–Er) or two (Ce) steps over the range 333–363 to 398–413 K (only La and Ce
complexes up to 508 K, respectively). The water molecules in the hydrated complexes
is probably in inner sphere. According to Nikolaev et al. [14] and Singh et al. [15]
water released below 423 K can be considered crystallization water, whereas that
eliminated above 423 K are chemically bonded to central ions through coordination
bond, but generally it is possible to say that temperature of dehydration is not
connected directly with position of water molecules in inner or outer sphere [10, 11].
All the anhydrous complexes heated decompose directly to oxides Ln2O3, CeO2,
Pr6O11 and Tb4O7 over the range 523–563 to 1078–1173 K. The results indicate that
the thermal decomposition of rare earth(III) 3-methoxy-4-methylbenzoates can be
presented in following way:
Ln(C9H9O3)3⋅2H2O→Ln(C9H9O3)3→Ln2O3, Pr6O11, Tb4O7 Ln=Y, La, Pr–Er,
Ce(C9H9O3)3⋅2H2O→Ce(C9H9O3)3→CeO2
Ln(C9H9O3)3→Ln2O3, Ln=Tm–Lu.
The dehydration and decomposition of the complexes are connected with
endothermic effects at 373–383 and 673–678 K, respectively (for Ce(III) complex
at 373, 473, 673 K), whereas the combustion of the organic ligand and products of its
decomposition shows exothermic effects. The temperature of beginning of dehydration
Fig. 1 TG, DTG and DTA curves of Y(C9H9O3)3⋅2H2O
J. Therm. Anal. Cal., 78, 2004

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BRZYSKA, O¨GA: 3-METHOXY-4-METHYLBENZOATES
Fig. 2 TG, DTG and DTA curves of Ce(C9H9O3)3⋅2H2O
Fig. 3 TG, DTG and DTA curves of Er(C9H9O3)3⋅2H2O
(T1) and of the beginning of decomposition (T2) change insignificantly with increasing
atomic number Z of the metal (Table 3, Fig. 5). The temperature of the beginning of
decomposition for the heaviest rare earth complexes (Tm–Lu) having the smallest ionic
radius attain the highest value (563 K). The temperatures over which oxides (TK) exist
change irregularly in the lanthanide series: Yb2O3 forms at the lowest temperature
(1078 K) and Y2O3 at the highest (1173 K). Generally La2O3 obtained by decomposition
of the complexes with different organic ligands forms at the highest temperature [6–9].
J. Therm. Anal. Cal., 78, 2004

BRZYSKA, O¨GA: 3-METHOXY-4-METHYLBENZOATES
1005
Comparing the obtained results with the properties of rare earth elements with
4-methoxy-2-methylbenzoates [2] it is possible to state that the change of position of
CH3 and OCH3 groups in benzene ring in relation to carboxylate group influences on
number of crystallization water molecules, mode of coordination of OCO– group, the
position of asymmetric and symmetric vibration bands of carboxylate group in their IR
spectra and the temperature over the oxides exist (the TK for 3-methoxy-4-methyl-
benzoates are higher than for 4-methoxy-2-methylbenzoates of suitable elements).
Fig. 4 TG, DTG and DTA curves of Lu(C9H9O3)3
Fig. 5 Relationship between T1, T2, TK and Z
J. Therm. Anal. Cal., 78, 2004

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BRZYSKA, O¨GA: 3-METHOXY-4-METHYLBENZOATES
Table 3 Thermoanalytical data of Y(III) and lanthanide(III) 3-methoxy-4-methylbenzoates
Mass loss/%
Mass loss/%
nH2O
mol
DT1/K
DT2/K
Tk/K
Complex
calcd.
found
5.8
calcd.
found
80.6
*
YL3 ⋅2H2O
333–403
363–508
5.80
5.37
2
2
558–1173 80.65
553–1133 74.29
1173
1133
LaL3⋅2H2O
5.5
74.0
363–413
465–503
2.68
2.68
2.7
2.7
1
1
CeL3⋅2H2O
523–1113 72.89
73.0
1113
PrL3⋅2H2O
NdL3⋅2H2O
SmL3⋅2H2O
EuL3⋅2H2O
GdL3⋅2H2O
TbL3⋅2H2O
DyL3⋅2H2O
HoL3⋅2H2O
ErL3⋅2H2O
TmL3
343–413
343–413
343–413
343–413
343–413
342–413
323–400
323–400
333–398
–
5.35
5.33
5.28
5.27
5.23
5.21
5.20
5.20
5.15
–
5.3
5.3
5.2
5.3
5.2
5.2
5.2
5.2
5.1
–
2
2
2
2
2
2
2
2
2
–
–
–
558–1128 73.23
563–1113 73.68
538–1153 72.98
553–1158 72.80
533–1153 72.21
533–1158 71.41
533–1158 71.63
543–1153 71.36
553–1153 71.12
563–1118 70.89
563–1076 70.50
563–1108 70.30
73.0
74.0
73.0
72.6
72.0
71.4
71.6
71.3
71.1
71.0
70.5
70.3
1128
1113
1153
1158
1153
1158
1158
1153
1153
1118
1078
1108
YbL3
–
–
–
LuL3
–
–
–
*L – [C6H3(OCH3)(CH3)COO]–
∆T1 – temperature range of dehydration
∆T2 – temperature range of decomposition to oxides
Tk – temperature over which oxides exist
Conclusions
3-Methoxy-4-methylbenzoates of rare earth element(III) (Y, La–Lu) were prepared
as crystalline solids with molar ratio of metal to organic ligand of 1.0:3.0. Complexes
of Y(III), La(III) to Er(III) were prepared as dihydrates, whereas the complexes of
Tm–Lu as anhydrous ones. The prepared complexes are sparingly soluble in water
(their solubility is of the order 10–4 mol dm–3). They are stable in air at room tempera-
ture. During heating the hydrated complexes lose crystallization water in one (Y, La,
Pr–Er) or two (Ce) steps. All the anhydrous complexes heated decompose directly to
oxides (Ln2O3, CeO2, Pr6O11, Tb4O7). The temperatures over which exist oxides
changes irregularly in the lanthanide series. The yttrium oxide have the highest for-
mation temperature (1173 K), while the ytterbium oxide has the lowest one (1078 K).
The carboxylate group in the studied complexes acts probably as bidentate chelating
and the water is inner sphere of the complexes.
J. Therm. Anal. Cal., 78, 2004

BRZYSKA, O¨GA: 3-METHOXY-4-METHYLBENZOATES
1007
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J. Therm. Anal. Cal., 78, 2004