Thermal properties of complexes of lanthanides(III) and some of d-block elements with 5-chloro-2-methoxybenzoic acid

Article abstract of DOI:10.1023/B:JTAN.0000005189.34087.b5

The thermal properties of 5-chloro-2-methoxybenzoates of lanthanides(III) and Mn(II), Co(II), Ni(II), Cu(II) and Zn(II) were studied in air and nitrogen atmospheres. The complexes were obtained as mono-, di-, tetra- and pentahydrates with a metal to ligand ratio of 1:3 (in the case of lanthanides(III)) and 1:2 (in the case of d-block elements). They have colours typical for Ln³⁺ and M²⁺ ions. All complexes are polycrystalline compounds. When heated they dehydrate to form anhydrous salts which next in air are decomposed to the oxides of the respective metals while in nitrogen to the mixtures of metal oxides, oxychlorides and carbon.

Full text of DOI:10.1023/B:JTAN.0000005189.34087.b5

Journal of Thermal Analysis and Calorimetry, Vol. 74 (2003) 521–527
THERMAL PROPERTIES OF COMPLEXES OF
LANTHANIDES(III) AND SOME OF d-BLOCK ELEMENTS
WITH 5-CHLORO-2-METHOXYBENZOIC ACID
Wieslawa Ferenc* and Beata Bocian
Department of General Chemistry, Faculty of Chemistry, Maria Curie-SkÓodowska University,
20-031 Lublin, Poland
Abstract
The thermal properties of 5-chloro-2-methoxybenzoates of lanthanides(III) and Mn(II), Co(II),
Ni(II), Cu(II) and Zn(II) were studied in air and nitrogen atmospheres. The complexes were obtained
as mono-, di-, tetra- and pentahydrates with a metal to ligand ratio of 1:3 (in the case of
lanthanides(III)) and 1:2 (in the case of d-block elements). They have colours typical for Ln3+ and
M
2+ ions. All complexes are polycrystalline compounds. When heated they dehydrate to form anhy-
drous salts which next in air are decomposed to the oxides of the respective metals while in nitrogen
to the mixtures of metal oxides, oxychlorides and carbon.
Keywords: 5-chloro-2-methoxybenzoates, complexes of d-block elements, complexes of
lanthanides(III), thermal stability of compounds in air and nitrogen atmospheres
Introduction
5-Chloro-2-methoxybenzoic acid is a white crystalline solid sparingly soluble in cold water.
Its melting point temperature changes from 77 to 98°C depending on the solvents used for its
crystallization. The complexes of lanthanides(III) and some of d-block elements with
5-chloro-2-methoxybenzoic acid have not been prepared and investigated so far. Therefore
we decided to obtain them as solids, to examine and compare some of their physico-chemical
properties. In our previous papers [1–6] we characterised the complexes of
5-chloro-2-methoxybenzoic acid with lanthanides(III) and Mn(II), Co(II), Ni(II), Cu(II) and
Zn(II) by elemental analysis, FTIR and Far-IR spectra, X-ray diffraction, thermogravimetric
studies and magnetic measurements. Taking into account the kind of cation we decided to
compare the thermal properties of 5-chloro-2-methoxybenzoates of f-block and of some of
d-block elements in air and nitrogen during heating to 1273 K (air) and 1173 K (nitrogen) in
order to investigate the influence of central ion on the properties of complexes.
Experimental
The complexes of 5-chloro-2-methoxybenzoic acid with lanthanides(III) and Mn(II),
Co(II), Ni(II), Cu(II), Zn(II) were prepared by the addition of equivalent quantities of
*
Author for correspondence: E–mail: wetafer@hermes.umcs.lubin.pl
1388–6150/2003/ $ 20.00
Akadémiai Kiadó, Budapest
© 2003 Akadémiai Kiadó, Budapest
Kluwer Academic Publishers, Dordrecht

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0.1 M ammonium 5-chloro-2-methoxybenzoate (pH ~5) to a hot aqueous solution con-
taining the nitrates of those elements and crystallizing at 293 K. The solids were filtered
off, washed with hot water and methanol to remove ammonium ions and dried at 303 K.
The thermal stability and decomposition of the prepared complexes were deter-
mined using Paulik–Paulik–Erdey Q-1500D derivatograph with Derill converter, re-
cording TG, DTG and DTA curves. The measurements were made at a heating rate of
10 K min–1 with a full scale. The samples were heated in platinum crucibles in static
air to 1273 K. TG sensitivity was 100 mg, DTG and DTA sensitivities were regulated
by a Derill computer programme. The initial mass of samples used in measurements
were equal to 100 mg. The measurements in nitrogen were made on an OD-102
derivatograph at a heating rate of 10 K min–1. The samples were heated to 1173 K at
the following sensitivities: TG–100 mg, DTA–1/10, DTG–1/5. The nitrogen flowed
through gas washers filled with pyrogallol and silica gel at a rate of 115 cm3 min–1.
The thermogravimetric analysis of DSC/TG was performed at temperatures
293–973 K using a differential thermoanalyser Netzsch STA 409 C 3F at a heating rate
of 1 K min–1. The experiments were carried out under argon flow of 75 cm3 min–1.
The gaseous decomposition products were analysed in argon atmosphere over
the range 4000–400 cm–1 using the Bruker IFS 66 spectrometer.
Results and discussion
The complexes of 5-chloro-2-methoxybenzoic acid with lanthanides(III) were obtained
as crystalline products with a metal to ligand ratio of 1:3 and a general formula
Ln(C8H6ClO3)3·nH2O (where Ln=La–Lu, and n=5 for La–Nd, n=1 for Sm–Gd, n=2 for
Tb, Dy and n=4 for Ho–Lu). The complexes of 5-chloro-2-methoxybenzoates of some of
d-block elements were obtained also as crystalline products but with a metal to ligand ra-
tio of 1:2 and a general formula M(C8H6ClO3)2·nH2O, where M=Mn, Co, Ni, Cu, Zn and
n=4 for Mn(II), n=5 for Co(II), Ni(II), n=1 for Cu(II) and n=2 for Zn(II).
The thermal stability of the 5-chloro-2-methoxybenzoates of lanthanides(III)
and Mn(II), Co(II), Ni(II), Cu(II) and Zn(II) was studied in air and nitrogen atmo-
spheres (Table 1, Figs 1, 2). The complexes were found to be hydrates. Heated in air
they decompose in two or three steps. 5-Chloro-2-methoxybenzoates of
lanthanides(III) in air are stable up to 338–413 K. The complexes of Mn(II), Co(II),
Ni(II), Cu(II) and Zn(II) with 5-chloro-2-methoxybenzoic acid in air are stable up to
338–403 K. Next at 338–413 K (complexes of f-block elements) and 338–433 K
(complexes of d-block elements) they lose the water molecules in one step forming
anhydrous compounds. The mass losses calculated from TG curves are equal to
2.64–11.33% (in the case of lanthanides(III)) and 3.96–17.16% (in the case of
Mn(II), Co(II), Ni(II), Cu(II) and Zn(II)). The theoretical values are 2.50–11.00%
(complexes of f-block elements) and 3.98–17.32% (complexes of d-block elements).
These values correspond to the release of 1, 2, 4 or 5 molecules of water. Using the
DSC technique the values of enthalpy of the dehydration process of 5-chloro-2-
methoxybenzoates of lanthanides(III) and Mn(II), Co(II), Ni(II), Cu(II) and Zn(II),
H, were determined. These values are proportional to the energy of the bonding of
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Fig. 1 Relationships between the temperature of initial dehydration of complexes (T1)
(in air and nitrogen) and Z
Fig. 2 Relationships between the temperature of decomposition of anhydrous complex
(T2) (in air and nitrogen) and Z
respective 1, 2, 4 or 5 of water molecules in the appropriate complexes and they are
equal to: 50.60–74.67 kJ mol–1 for monohydrates of Sm(III), Eu(III), Gd(III) and
Cu(II); 103.09–175.26 kJ mol–1 for dihydrates of Tb(III), Dy(III) and Zn(II);
182.85–250.12 kJ mol–1 for tetrahydrates of Ho(III), Er(III), Tm(III), Yb(III), Lu(III)
and Mn(II); 260.92–265.3 kJ mol–1 for pentahydrates of La(III), Ce(III), Pr(III),
Nd(III), Co(II) and Ni(II) (Table 1).
The anhydrous complexes of lanthanides(III) in air are stable up to 493–533 K
and next in the temperature range 493–1435 K they decompose to the oxides of ap-
propriate lanthanides with intermediate formation of LnOCl (with the exception of
cerium(III) complex that directly decomposes to CeO2). The temperatures of oxide
formations change from 1498 K (Pr6O11, Nd2O3) to 763 K (CeO2).
The anhydrous complexes of 5-chloro-2-methoxybenzoates of Mn(II), Co(II),
Ni(II), Cu(II) and Zn(II) in air are stable up to 483–568 K and heated further to
1273 K they decompose to the respective oxides: Mn3O4, Co3O4, NiO, ZnO and a
mixture of Cu2O, CuO. In air the anhydrous 5-chloro-2-methoxybenzoates of Mn(II)
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and Cu(II) decompose with the intermediate formation of Mn2OCl2 and Cu2OCl (Ta-
ble 1). The intermediate and final products of decomposition were identified by
X-ray powder diffraction, elemental and IR spectra analyses.
The dehydration process is connected with an endothermic effect seen on DTA
curves whereas the combustion of the organic ligand with exothermic one.
In the nitrogen atmosphere, similarly as in the air, the complexes of La(III), Ce(III),
Pr(III), Nd(III), Co(II) and Ni(II) were found to be pentahydrates. The 5-chloro-2-
methoxybenzoates of Ho(III), Er(III), Tm(III), Yb(III), Lu(III) and Mn(II) are tetra-
hydrates. The complexes of Tb(III), Dy(III) and Zn(II) were found to be dihydrates while
those of Sm(III), Eu(III), Gd(III) and Cu(II) are monohydrates. 5-Chloro-2-methoxy-
benzoates of f- and of chosen d-block elements heated in nitrogen are stable up to
323–393 K (Table 1). Next they dehydrate in one step losing all molecules of crystalliza-
tion water and form the anhydrous compounds. During the further heating the anhydrous
complexes decompose in the range 473–1173 K. The final products of decomposition of
5-chloro-2-methoxybenzoates of lanthanides(III) and Mn(II), Co(II), Ni(II), Cu(II) and
Zn(II) in nitrogen are the mixtures of the carbon, metal oxides or metal oxychlorides.
The thermal stability of the obtained hydrates (T1) do not change regularly (Ta-
ble 1, Fig. 1). Monohydrates of 5-chloro-2-methoxybenzoates of Sm(III), Eu(III),
Gd(III) and Cu(II) are the most thermally stable (in air T1=388–403 K and in nitrogen
T1=383–393 K), which indicates that the water molecules are the most strongly
bounded in these complexes. The rests of the hydrates for which the values of initial
temperature of dehydration being equal to T1=338–363 K (in air) and T1=323–353 K
(in nitrogen) have the similar thermal stability.
In the series of an anhydrous 5-chloro-2-methoxybenzoates of f- and of some of
d-block elements the most thermally stable in air and nitrogen is the complex of
Ni(II). Its initial temperatures of decomposition in air and in nitrogen, T2, are equal to
568 K and 553 K, respectively. The weakest thermally stable in air are complexes of
Cu(II) and Zn(II) (T2=483 K) but in nitrogen only the complex of Cu(II) (T2=473 K).
Other anhydrous complexes (except those of Mn(II) and Co(II)) whose values of ini-
tial temperatures of decompositions are equal to T2=493–533 K (in air) and
T2=493–523 K (in nitrogen) have the similar stability (Table 1, Fig. 2).
The FTIR spectra of gaseous products evolved during decomposition of 5-
chloro-2-methoxybenzoates of lanthanides(III), Mn(II), Co(II), Ni(II), Cu(II) and
Zn(II) were presented in Table 2. The first product of decomposition of 5-chloro-2-
methoxybenzoates of studied d- and f-block elements are molecules of H2O. The
bands at 4000–3500, 2000–1350 and 1820–1330 cm–1 characteristic for stretching
and bending vibrations of OH confirm their presence in these complexes. The further
heating of anhydrous compounds leads to the decomposition of the organic ligand.
This process is connected with the release of a large amounts of CO2 molecules
whose valence and deformation vibrations absorb in the wavenumber ranges
2400–2200 and 700–670 cm–1, respectively. The bands appearing at 3800–3500 cm–1
may be also connected with the valence vibrations of CO2 molecules. In the FTIR
spectra at 3059–2650 cm–1 the weak bands characteristic for stretching vibrations of
HCl molecules releasing at a higher temperature are also observed. The bands at
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2220–2060 cm–1 are connected with valence vibrations of CO molecules. In the FTIR
spectra the bands in the ranges 3100–1420, 1000 and 915 cm–1 are characteristic for
CH stretching vibrations which is connected with the occurrence of the gaseous hy-
drocarbon molecules [7–12].
Conclusions
On the basis of the obtained results it appears that 5-chloro-2-methoxybenzoates of some
of d- and f-block elements are hydrated, crystalline complexes. When heated in air and in
inert atmospheres they decompose in two (in the case of Ce(III), Co(II), Ni(II) and Zn(II)
complexes) or three steps. The enthalpy values of the dehydration process are propor-
tional to the energy of the bonding of respective water molecules in the appropriate com-
plexes. The complexes are more stable in air than in nitrogen atmosphere. The mono-
hydrates of 5-chloro-2-methoxybenzoates of Cu(II), Sm(III), Eu(III) and Gd(III) are the
most thermally stable of complexes under study in air and nitrogen. The molecules of
H2O, HCl, CO2, CO and hydrocarbons are the gaseous products evolved during decom-
position of 5-chloro-2-methoxybenzoates of d- and f-block elements.
* * *
This work is financially supported by the grant (No 3 T09A 092 16) from Committee of Scientific
Researches in Poland.
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