Thermal stability of 2,3,4-, 2,4,5- and 3,4,5-trimethoxybenzoates of light lanthanides

Article abstract of DOI:10.1023/A:1010188905767

The physico-chemical properties and thermal stability in air of light lanthanide 2,3,4-, 2,4,5- and 3,4,5-trimethoxybenzoates were compared and the influence of the position of -OCH₃ substituent on their thermal stability was investigated. The complexes of these series are crystalline, hydrated or anhydrous salts with colours typical of Ln³⁺ ions. The carboxylate group is a bidentate, chelating ligand. The thermal stability of 2,3,4-, 2,4,5- and 3,4,5-trimethoxybenzoates of rare earth elements was studied in the temperature range 273-1173 K. The positions of methoxy groups in benzene ring influence the thermal properties of the complexes and their decomposition mechanism. The different thermal properties of the complexes are connected with various influence of inductive and mesomeric effects of -OCH₃ substituent on the electron density in benzene ring.

Full text of DOI:10.1023/A:1010188905767

Journal of Thermal Analysis and Calorimetry, Vol. 60 (2000) 131–138
THERMAL STABILITY OF 2,3,4-, 2,4,5- AND 3,4,5-TRI-
METHOXYBENZOATES OF LIGHT LANTHANIDES
W. Ferenc and B. Bocian
Department of Inorganic and General Chemistry, Faculty of Chemistry, Maria Curie-Sk³odowska
University, 20-031, Lublin, Poland
Abstract
The physico-chemical properties and thermal stability in air of light lanthanide 2,3,4-, 2,4,5- and
3,4,5-trimethoxybenzoates were compared and the influence of the position of –OCH3 substituent on
their thermal stability was investigated. The complexes of these series are crystalline, hydrated or
anhydrous salts with colours typical of Ln3+ ions. The carboxylate group is a bidentate, chelating
ligand. The thermal stability of 2,3,4-, 2,4,5- and 3,4,5-trimethoxybenzoates of rare earth elements
was studied in the temperature range 273–1173 K. The positions of methoxy groups in benzene ring
influence the thermal properties of the complexes and their decomposition mechanism. The different
thermal properties of the complexes are connected with various influence of inductive and
mesomeric effects of –OCH3 substituent on the electron density in benzene ring.
Keywords: complexes of rare earth elements, influence of methoxy-substituents, light lanthanide
trimethoxybenzoates, properties of trimethoxybenzoates, thermal stability of tri-
methoxybenzoates, 2,3,4-, 2,4,5 and 3,4,5-trimethoxybenzoates
Introduction
2,3,4-, 2,4,5- and 3,4,5-trimethoxybenzoic acids having the formula C10H11O5 are white,
crystalline solids, sparingly soluble in cold water. Their solubility in water increases with
the rise of temperature [1–3]. The literature survey shows that there is no information
about the complexes of 2,3,4-trimethoxybenzoic acid with various cations, but it informs
about the complexes of 2,4,5-trimethoxybenzoates with K+, Na+, Ca2+ which have been
obtained and investigated in solid state [2, 4, 5] and those of Cu2+, Be2+, UO22+ , La3+, Ce3+,
Pr3+, Nd3+, Sm3+, Gd3+ and Yb3+ to have been studied in solution [6]. The salts of
3,4,5-trimethoxybenzoic acid only with the following cations: K+, Cu2+, Ag+, Ca2+ and
Ba2+ have been also obtained as solids [3].
In the literature there is no information about these acid complexes with the light
lanthanides. Therefore, we decided to obtain them as solids, to examine some of their
physico-chemical properties and to compare them. In our previous papers [7–9] we char-
acterized these complexes by elemental analysis, IR spectra data, thermogravimetric
studies and X-ray diffraction measurements but now taking into account the presence and
positions of three methoxy groups in benzene ring we decided to compare the properties
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of trimethoxybenzoates of light lanthanides in order to investigate the influence of
substituent positions on their properties.
Experimental
2,3,4-, 2,4,5- and 3,4,5-trimethoxybenzoates of light lanthanides were prepared by add-
ing the equivalent quantities of 0.1 M ammonium 2,3,4-, 2,4,5- and 3,4,5-trimethoxy-
benzoates (pH~5) to a hot solution containing the light lanthanide nitrates and by crystal-
lization at 293 K. The solids formed were filtered off, washed with hot water to remove
ammonium ions and dried at 303 K to a constant mass. The carbon, and hydrogen con-
tents were determined by elemental analysis with V2O5 as oxidizing agent. The rare earth
element contents were established by the oxalic acid method. The IR spectra of the com-
plexes, the spectra of 2,3,4-, 2,4,5- and 3,4,5-trimethoxybenzoic acids and their sodium
salts were run in the range 4000–400 cm–1 using KBr discs on an M-80 spectro-
photometer. Some of the results are presented in Fig. 1. X-ray powder patterns were
taken on a DRON-2 diffractometer using Ni-filtered CuKα radiation within the range
2Θ=5–80° by Debyé-Scherrer-Hull method. The thermal stability of these complexes in
air in the range 273–1173 K was studied by the use of TG, DTG and DTA techniques. In
the case of 2,3,4- and 2,4,5-trimethoxybenzoates of light lanthanides the measurements
Fig. 1 Infrared spectra of 2,3,4-, 2,4,5- and 3,4,5-trimethoxybenzoates of Nd
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133
were made with an OD-102 derivatograph (Firm MOM-Budapest) at a heating rate of
10°C min–1. The test samples were heated at a sensitivity: TG – 100 mg, DTA – 500 µV,
DTG – 500 µV [7, 9]. The thermal stability of 3,4,5-trimethoxybenzoates of light
lanthanides were made with a Q-1500D derivatograph at a heating rate of 10°C min–1.
The test samples were heated at a sensitivity TG – 50 mg, DTA – 500µV, DTG – 500µV
[8].
Results and discussion
2,3,4-, 2,4,5- and 3,4,5-trimethoxybenzoates of light lanthanides were obtained as
crystalline products with a metal to ligand ratio of 1:3. 2,3,4-Trimethoxybenzoates
form anhydrous salts of general formula Ln(C10H11O5)3 for Ln=La–Gd [7].
3,4,5-Trimethoxybenzoates of Ce–Gd were obtained as monohydrates Ln(C10H11O5)3
H2O while that of lanthanum forms anhydrous salt [8]. The light lanthanide
2,4,5-trimethoxybenzoates form trihydrates: Ln(C10H11O5)33H2O. In these three se-
ries of trimethoxybenzoates being compared the number of crystallization water mol-
ecules changes with the change of –OCH3 group positions in benzene ring. The
colours of the complexes are typical of the appropriate trivalent lanthanide ions
which is connected with the structure of molecules and electron density in benzene
ring. In these molecules the f→f electronic transitions of the central ions are those of
the lowest energy and absorption occurs at relatively high wavelengths which de-
pends on the nature of metal ion [10, 11]. The compositions of the complexes were
established on the basis of elemental analysis and IR spectra data and their external
crystalline forms were also estimated [7–9]. From the IR spectra analysis of light
lanthanide 2,3,4-, 2,4,5- and 3,4,5-trimethoxybenzoates it appears that the carboxy-
late ion is a symmetrical, bidentate chelating ligand [7–9, 12–14]. The magnitudes of
separation, ∆ν
(∆ν
=ν
–ν
) which characterize the metal– oxygen
sOCO–
OCO–
OCO–
asOCO–
bond change in the range from 171–160 cm–1 for 2,4,5-trimethoxybenzoates to 110–
105 cm–1 for 3,4,5-trimethoxybenzoates. In the case of 2,3,4-trimethoxybenzoates of
light lanthanides the magnitude of separation, ∆ν
is equal to 140 cm–1. In these
OCO–
complexes being compared the participation of M–O ionic bond is presumably the
largest in the case of 2,4,5-trimethoxybenzoates while the least one in the case of
3,4,5-trimethoxybenzoates. The bands of metal-oxygen group are being changed de-
pending on the position of –OCH3 substituents in aromatic ring. Accordingly in the
case of 3,4,5-trimethoxybenzoates of light lanthanides these bands appear in the
range 488–472 cm–1 while for 2,3,4- and 2,4,5-trimethoxybenzoates they are shifted
to higher frequencies to be equal to 525–520 cm–1. The frequencies of the bands due
to ν
vibration are changed depending on the position of –OCH3 groups in aro-
asOCO–
matic ring and they are at 1540 cm–1 for 2,3,4-trimethoxybenzoates, at 1540–
1530 cm–1 for 3,4,5-trimethoxybenzoates and at 1535–1528 cm–1 for 2,4,5-tri-
methoxybenzoates. Similarly, the frequencies of the bands of ν
vibrations have
sOCO–
various values being equal to: 1400, 1430–1425 and 1370–1364 cm–1 for 2,3,4-,
2,4,5- and 3,4,5-trimethoxybenzoates of light lanthanides, respectively. The changes
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in the values of these band frequencies result from the various position of –OCH3
groups in aromatic ring and various influences of inductive and mesomeric effects on
electron densities in the molecules of those complexes [15–17]. The X-ray spectra of
2,3,4-, 2,4,5- and 3,4,5-trimethoxybenzoates of light lanthanides were recorded.
Their analysis suggests that the complexes are polycrystalline compounds with low
symmetry, large size of the unit cells and different structures [18].
The thermal stability of these complexes was studied in air in the range 273–
1173 K by TG, DTG and DTA techniques. The obtained results are presented in
Figs 2–5. When heated at 273–1173 K, the complexes decmpose in various steps. The
anhydrous complex of lanthanum 3,4,5-trimethoxybenzoate is decomposed in two
steps: first the oxycarbonate is formed which next it is decomposed to La2O3.
Monohydrates of Pr(III), Nd(III), Sm(III) Eu(III) and Gd(III) are stable up to
336–358 K and then losing one molecule of crystallization water in one step (336–
395 K) they form anhydrous complexes (380–395 K) that at 573–903 K are decom-
posed to the oxides of the respective metals with the intermediate formation of the
oxycarbonates. Monohydrate of cerium 3,4,5-trimethoxybenzoate expels one water
molecule at 344–392 K to form anhydrous complex which is then directly decom-
posed to CeO2 (768 K) which is the final product of decomposition. The oxides of
lanthanides to be the final products 3,4,5-trimethoxybenzoate decompositions are
formed at 768 (CeO2)–1008 K (La2O3) (Figs 2, 5). Trihydrates of La(III), Pr(III),
Nd(III), Eu(III) and Gd(III) 2,4,5-trimethoxybenzoates decompose in three steps
while those of Ce(III) and Sm(III) in the two ones. The dehydration process is con-
nected with the endothermic effect in DTA curves. In the range 453–1023 K the an-
hydrous complexes are decomposed to oxides of the respective metals with the inter-
Fig. 2 Dependence of initial dehydration and oxide formation temperatures of 2,4,5-
and 3,4,5-trimethoxybenzoates of light lanthanides and Z
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mediate formation of the oxycarbonates. 2,4,5-Trimethoxybenzoates of Ce(III) and
Sm(III) decompose in two steps. In the first stage at 333–453 K they lose crystalliza-
tion water and form anhydrous complexes which are next decomposed to oxides
(533–973 K). The oxides of the appropriate metals are formed in the range 953
(Pr6O11, Eu2O3)–1073 K (Nd2O3) (Figs 2, 4).
The complexes of La(III) and Nd(III) with 2,3,4-trimethoxybenzoic acid decom-
pose in two steps. In the temperature range 533–953 K they are decomposed to
oxycarbonates that next decomposes at 893–1093 K forming La2O3 and Nd2O3 which
are the ultimate produts of the decomposition. The complexes of Ce(III), Pr(III),
Sm(III) and Eu(III) with 2,3,4-trimethoxybenzoic acid decompose only in one step.
The anhydrous salts are directly decomposed to the oxides of respective light
lanthanides (523–963 K). The temperatures of the oxide formation change in the se-
ries of light lanthanides. The lowest temperature of oxide formation has cerium oxide
(943 K) while the highest one has lanthanum (1093 K) (Fig. 3). In the complexes be-
ing compared the dehydration process is connected with an endothermic effect in
DTA curves whereas the combustion of the organic ligand with an exothermic one.
Considering the temperatures at which the dehydration process of the 2,4,5- and
3,4,5-trimethoxybenzoates of the light lanthanides takes place and the way by which
it proceeds, it is possible to assume that the molecules of water are differently bound
in the outer-sphere of complex coordination. According to Nikolaev et al. [19] and
Singh et al. [20] water eliminated below 413–423 K can be considered as water of
crystallization and water eliminated above 423 K may be that coordinated to the cen-
tral ion [21]. In our complexes water is generally eliminated below 413 K. Therefore,
it is assumed to be crystallization water. From the comparison of the mechanism of
2,3,4-, 2,4,5- and 3,4,5-trimethoxybenzoates of light lanthanide decompositions it ap-
Fig. 3 TG, DTG and DTA curves of 2,3,4-trimethoxybenzoate of Nd
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pears that thermal decompositions of 2,3,4-, 2,4,5- and 3,4,5-trimethoxybenzoic ac-
ids with La(III) and Nd(III) proceed through oxycarbonates (Figs 3–5). The anhy-
drous complex of these acids with Ce(III) are directly decomposed to CeO2. In the se-
ries of trimethoxybenzoates being compared the temperatures of the oxide formations
are the highest for La2O3 (1008–1093 K) while the lowest ones are for CeO2
(768–943 K) formed by the cerium 2,3,4- and 3,4,5-trimethoxybenzoate decomposi-
tions (Fig. 2). In the series of rare earth element 2,4,5-trimethoxybenzoates the lowest
temperature of oxide formation falls to Pr6O11 (Fig. 2).
Fig. 4 TG, DTG and DTA curves of 2,4,5-trimethoxybenzoate of Nd
Fig. 5 TG, DTG and DTA curves of 3,4,5-trimethoxybenzoate of La
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From the comparison of the decomposition mechanism results for the three series of
trimethoxybenzoates of light lanthanides it follows that the various position of –OCH3
substituents in benzene ring influences the decomposition process being strongly con-
nected with the various participations of the inductive and mesomeric effects of methoxy
groups in the electron density of the system. The thermal stability of anhydrous
trimethoxybenzoates increases in order: 2,4,5-<2,3,4-<3,4,5-. The various position of
methoxy groups influence the number of crystallization water molecules in the com-
plexes and the positions of the bands of M–O and asymmetrical vibrations of carboxylate
groups in their IR spectra. The number of crystallization water molecules and the posi-
tions of asymmetrical vibration bands of carboxylate groups in the IR spectra of
trimethoxybenzoates increase in the following sequence: 2,3,4-<3,4,5-< 2,4,5-. The posi-
tions of the bands of M–O vibrations change in the order: 3,4,5-<2,3,4-≈2,4,5-. The par-
ticipation of the ionic bonds in trimethoxybenzoates of light lanthanides decreases in the
way: 2,4,5->2,3,4->3,4,5-.
The changes in the values presented above are probably connected with the vari-
ous influences of inductive and mesomeric effects of methoxy groups on the electron
density of the system depending on their position in benzene ring. The inductive ef-
fects of each methoxy groups cause the delocalization of the electrons in the molecule
and the change of its energy state brought about the conjugation of electrons. It leads
to the stabilization of the system. The appearence of the mesomeric effect causes the
characteristic changes in the molecule of organic compound. Therefore, the lenghts
of some of the σ bonds may be changed. This can be observed in the positions of the
bands of the characteristic group vibrations in the IR spectra of complexes and in
their various thermal stability in air during heating from 273 to 1173 K.
The obtained results also reveal that irrespective of the methoxy- group posi-
tions in benzene ring the ratio of metal: ligand in the complexes, their colours and the
dentate of carboxylate group stay the same for all 2,3,4-, 2,4,5- and 3,4,5-trimeth-
oxybenzoates of light lanthanides.
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