Synthesis, Characterization and thermal behaviour of hemimellitic acid complexes with lanthanides(III)

Article abstract of DOI:10.1007/s10973-007-8471-3

The complexes of lanthanides(III) with hemimellitic acid (1,2,3-benzenetricarboxylic acid, H₃btc) of the formula Ln(btc)?nH₂O, where Ln=lanthanide(III) ion and n=2-6 were prepared and characterized by elemental analysis, infrared spe

Full text of DOI:10.1007/s10973-007-8471-3

Journal of Thermal Analysis and Calorimetry, Vol. 91 (2008) 2, 595–599
SYNTHESIS, CHARACTERIZATION AND THERMAL BEHAVIOUR OF
HEMIMELLITIC ACID COMPLEXES WITH LANTHANIDES(III)
R. ˜yszczek*
Department of General and Coordination Chemistry, Maria Curie-SkÓodowska University, M.C. SkÓodowskiej Sq. 2
20-031 Lublin, Poland
The complexes of lanthanides(III) with hemimellitic acid (1,2,3-benzenetricarboxylic acid, H3btc) of the formula Ln(btc)·nH2O,
where Ln=lanthanide(III) ion and n=2–6 were prepared and characterized by elemental analysis, infrared spectra, X-ray diffraction
patterns and thermal analysis. The IR spectra of the complexes indicate coordination of lanthanides(III) through all carboxylate
groups. The complexes of La(III), Ce(III), Pr(III) and Er(III) are amorphous. On heating in air atmosphere all complexes lose water
molecules and next anhydrous compounds decompose to corresponding metal oxides.
Keywords: 1,2,3-benzenetricarboxylic acid, IR spectra, lanthanides(III), TG-DTA analysis
Introduction
solvent-inclusion clathrates with the brick-wall,
herringbone or lamellar architectures [14].
Recently, a great attention has been paid to the
construction of metal-organic frameworks as promising
materials for applications in gas storage, separation and
catalysis [1–4]. Their useful properties arises from
desirable pore shapes and sizes, high porosity and
flexible frameworks. For construction of coordination
polymers are chosen appropriate polydentate ligands
as bridges between several metal centres. The benz-
enepolycarboxylate acids [3] are commonly used in
the synthesis of such new materials. Polycarboxylate
ligands exhibit a variety of coordination fashions and
the capability of forming coordination architectures
of diverse sizes and shapes. Symmetric benzene-
polycarboxylate acids such as 1,4-benzenedicarb-
oxylic acid [5] 2,6-naphthalenedicarboxylic acid [6],
1,3,5-benzenetricarboxylic acid [7, 8] and 1,2,4,5-
benzenetetracarboxylic acid [9, 10] are intensively
employing to construct coordination polymers due to
theirs bridging abilities. On the other hand
asymmetric ligands such as 1,2,4- and 1,2,3-benz-
enetricarboxylic acids can form fascinating structures
by reason of the special orientation of carboxylate
groups. In the molecule of 1,2,3-benzenetricarboxylic
acid (hemimellitic acid) none of carboxyl groups is
coplanar with the benzene ring [11]. In literature are
known only a few examples of coordination polymers
with hemimellitic acid. Two-dimensional coordina-
tion polymer with the zinc ions and three-dimensional
coordination polymer with copper(II) were obtained by
Ling Xu et al. [12, 13]. Both complexes form
networks with channels. Hemimellitic acid form also
The lanthanides are explored in the construction
of coordination polymer due to their high coordination
number and flexible coordination geometry in
comparison to the transition metal frameworks that is
led to unusual molecular architectures.
The aim of this work was to synthesize complexes
of lanthanides(III) with 1,2,3-benzenetricarboxylic
acid as potential coordination polymers and investi-
gations some of their properties.
Experimental
1,2,3-benzenetricarboxylic acid hydrate (98%) and
cerium(III) nitrate hexahydrate (99.99%) were purchased
from Aldrich. Lanthanide chlorides were prepared
from lanthanide oxides (99.9%), Sigma.
The complexes of lanthanide from La(III) to
Eu(III) with hemimellitic acid were prepared by
dropwise adding of aqueous ammonium hemimellitate
solution (2 mmol, 20 mL, pH=5.5) to aqueous
solution of lanthanide chloride(III) (2 mmol, 150 mL,
pH=4.5) (for cerium(III) complex nitrate solution was
used). The precipitates of complexes were left for 2 h
in mother solution at 60°C, and then filtered off,
washed with water and dried at 30°C to constant
mass. The complexes of lanthanide(III) from Gd(III)
to Lu(III) were synthesized by addition of freshly
precipitated lanthanide hydroxide (2 mmol) to aqueous
solution of hemimellitic acid (2 mmol, 100 mL). The
complexes of Gd(III), Er(III), Tm(III), Yb(III) and
*
reniam@hermes.umcs.lublin.pl
1388–6150/$20.00
Akadémiai Kiadó, Budapest, Hungary
Springer, Dordrecht, The Netherlands
© 2007 Akadémiai Kiadó, Budapest

˜YSZCZEK
Lu(III) were immediately precipitated and next were
powder diffractions showed that the complexes of
La(III), Ce(III), Pr(III) and Er(III) are amorphous. The
remaining compounds were obtained as polycrystalline
substances. Depending on number of water molecules
in the complex they are isostructural in several
groups: Nd–Eu, Gd–Ho and Tm–Lu.
kept for an 1 h in mother solution and then isolated by
filtration, washed with water and dried at 30°C to
constant mass. The complexes of Tb(III), Dy(III) and
Ho(III) were obtained by evaporation of reaction
mixture for several days at room temperature. The
prepared compounds were dried at 30°C to constant
mass. The yields of synthesis are in the range: 60–80%.
The prepared complexes were identified by
means of elemental analysis and thermogravimetry.
The contents of C and H were determined by using a
Perkin Elmer 2400 CHN elemental analyzer.
The infrared spectra of the obtained complexes
and 1,2,3-benzenetricarboxylic acid were recorded in
KBr discs on a SPECORD M80 spectrophotometer
over the range 4000–400 cm–1.
The X-ray powder diffraction patterns were
collected on a HZG 4 diffractometer, using Ni filtered
CuKa radiation. Measurements were taken over the
range of 2q=5–70°.
Spectroscopic analysis
The infrared spectra of the 1,2,3-benzenetricarboxylic
acid and its lanthanide complexes were recorded in
purpose to establish ligating behaviour of hemimellitate
ligand. The presence of water molecules in all
investigated complexes is confirmed by broad band in
the region 3800–2500 cm–1 with strong maximum at
about 3500 cm–1 originating from stretching vibrations
of OH group from water molecules influenced by
extensive hydrogen-bonding network. In the spectra
of dihydrate complexes of Nd(III), Sm(III) and
Eu(III) strong absorption peak at 3632 cm–1 was
found besides a broad absorption band about
3420 cm–1. This band corresponds to very weakly
hydrogen bonded OH groups in the water molecules.
Moreover in these spectra are present broad bands
with maxima at 2550 and 2232 cm–1 resulting from
overtones of hydrogen bonds [15]. Appearing of these
band in the studied complexes may point to presence
of water molecules bonding in different way.
Thermal analysis was carried out by the
TG-DTA method using SETSYS 16/18 analyser from
Setaram. Samples of about 7–8 mg were heated in
ceramic crucible up to 850°C at a heating rate of
10°C min–1 in dynamic air atmosphere (0.75 dm3 h–1).
Results and discussion
The
infrared
spectrum
of
free
The studied complexes of lanthanide with hemimellitic
acid have general formula Ln(btc)·nH2O where
Ln=lanthanide(III) ion, btc=C6H3(COO)33– and n=2–6.
The detail formula are given in Table 1. The X-ray
1,2,3-benzenetricarboxylic acid is characterized by
strong double band at 1728 and 1700 cm–1 due to
stretching vibrations of unequivalent C=O groups.
Additionally, carboxylic group COOH give strong
Table 1 Characteristic IR bands (cm–1) of 1,2,3-benzenetricarboxylic acid and its lanthanide(III) compounds
nOH
n
n(C– C)
n
b(C– H)
gOH
g(C– H)
n(C– C)
ar
as(COO–
)
ar
s(COO –
–
)
ar
ar
H3btc
3800–2000
3700–2800
–
1464
1464
1464
1464
1400
1400
1400
1458
1448
1457
1456
1461
1464
1480
1460
1064
1072
1072
1072
1064
1064
1064
1068
1072
1069
1069
1070
1070
1070
1075
–
776, 756
578, 540
544
544
544
568
568
568
572
590
580
581
560
560
570
576
La(btc)·4H2O
Ce(btc)·4.5H2O
Pr(btc)·4H2O
Nd(btc)·2H2O
Sm(btc)·2H2O
Eu(btc)·2H2O
Gd(btc)·5.5H2O
Tb(btc)·6H2O
Dy(btc)·5H2O
Ho(btc)·5H2O
Er(btc)·4.5H2O
Tm(btc)·4H2O
Yb(btc)·4H2O
Lu(btc)·4H2O
1552
1544
1552
1384
1380
1380
1360
1360
1364
1365
1384
1381
1381
1386
1384
1380
1376
940 772, 712
940 772, 712
940 768, 712
936 768, 708
3800–2500
3800–2500
3632, 3000–2300
3632, 3600–3000
3632, 3600–3100
3800–2500
1532, 1484
1532, 1484
1532, 1484
1537
936
704
936 768, 720
938 767, 716
936 768, 712
940 769, 713
940 769, 714
942 769, 708
940 772, 708
936 772, 712
939 771, 699
3800–2500
1552
3800–2500
1556
3800–2500
1559
3800–2500
1553
3800–2500
1552
3800–2500
1536
3800–2500
1537
n – stretching vibrations, b – in-plane bending vibrations, g – out-of-plane deformation vibrations
596
J. Therm. Anal. Cal., 91, 2008

HEMIMELLITIC ACID COMPLEXES
absorption band at 1280 cm–1 corresponds to stretch-
tricarboxylic acid and 4,4`-bipyridine ligand only
carboxylate groups from 1 and 3 position in hemi-
mellitic acid take part in Cu atom binding. Both
carboxylate groups act as bidentate-bridging [13].
Lack of the crystal structure of the investigated
complexes makes interpretation of infrared spectra
hardly possible. It is worth to note that in the case of
dihydrate complexes the band arising from
asymmetric stretching vibrations of COO group is
splitted into the two peaks with maxima at 1532 and
1484 cm–1, respectively. Splitting of nasym (COO)
absorption band has been attributed to many causes
e.g. two coordination modes, polymer–dimer equilibrium
or coupling between neighbouring carboxylate groups
[16]. That suggest quite different mode of carboxylate
coordination in these complexes in comparison to
remaining compounds. The IR spectral data and
possible assignments are listed in Table 1.
ing vibrations of C–OH group. When the metal com-
plexes are formed the absorption peaks arised from
COOH groups are replaced by asymmetric and sym-
metric stretching vibrations of carboxylate group
COO. In the spectra of lanthanide hemimellitates the
asymmetric stretching vibrations of COO group are
hardly distinguishable because in the same wavenumber
range appear vibrations arised from stretching vibra-
tions of benzene ring as well as deformation vibra-
tions of water molecules. It was assumed that asym-
metric stretching vibrations of carboxylate group are
in the range 1552–1532 cm–1 while the symmetric vi-
brations are in range 1360–1384 cm–1. It is not possi-
ble to determine the coordination mode of COO group
based only the IR data. The carboxylate group shows
the significant diversity of mode of metal bonding.
The most utilised the coordination mode by
carboxylate group is: monodentate, bidentate-bridg-
ing and bidentate-chelating [16]. The known crystal
structures of hemimellitate metal complexes present
different types of metal bonding. In the structure of
nickel(II) complex with 1,2,3-benzenetricarboxylic
acid only ionic interactions appears between
hexaaquanickel(II) ion and fully deprotonated anions
of hemimellitic acid [17]. On the other hand in the
copper(II) complex, the investigated acid behaves as
monodentate ligand in which only carboxylate group
in position 2 is engaged in metal bonding while two
remaining COOH groups remain protonated [18]. In
the copper(II) complex with 1,2,3-benzene-
Thermal analysis
Thermal data of the lanthanide 1,2,3-benzenetricarboxylates
are given in Table 2. The studied complexes are stable
up to about 30°C. During heating of the complexes in
the air atmosphere multi-stage decomposition is ob-
served (Figs 1–4). The first mass loss observed on the
TG curve is associated with dehydration process simi-
larly as in 1,2,4-benzenetricarboxylates of lanthan-
ides [19, 20]. The loss of water molecules occurs in
wide temperature range (30–270°C) and in different
way in depending on their number. The shape of TG
Table 2 Thermogravimetric data obtained during heating complexes of lanthanide(III) with 1,2,3-benzenetricarboxylic acid in
air atmosphere
Te*ndo/°C
Mass loss/%
Mass loss/%
Intermediate
solid product
T1/°C
T2/°C
Complex
Residue
found
calcd.
found calcd.
La(btc)·4H2O
Ce(btc)·4.5H2O
Pr(btc)·4H2O
Nd(btc)·2H2O
Sm(btc)·2H2O
Eu(btc)·2H2O
Gd(btc)·5.5H2O
Tb(btc)·6H2O
Dy(btc)·5H2O
Ho(btc)·5H2O
Er(btc)·4.5H2O
Tm(btc)·4H2O
Yb(btc)·4H2O
Lu(btc)·4H2O
30–200
30–180
30–210
40–235
30–255
40–250
30–235
30–270
30–240
40–210
40–190
30–270
30–185
30–105
97
17.50
19.80
16.90
10.00
10.00
9.80
17.22
18.91
17.14
9.28
La(btc)
Ce(btc)
Pr(btc)
Nd(btc)
Sm(btc)
Eu(btc)
Gd(btc)
Tb(btc)
Dy(btc)
Ho(btc)
Er(btc)
Tm(btc)
Yb(btc)
Lu(btc)
390–760 61.36 61.03
335–500 59.47 59.81
380–600 59.07 59.47
380–660 56.20 56.57
380–650 56.45 55.69
350–650 55.32 54.58
400–700 60.76 60.89
380–570 60.97 60.57
410–580 59.51 59.42
400–580 58.54 59.11
380–570 57.35 58.00
400–590 55.24 59.94
400–565 54.66 56.42
390–570 55.55 56.19
La2O3
CeO2
91
95
Pr6O11
Nd2O3
Sm2O3
Eu2O3
Gd2O3
Tb4O7
Dy2O3
Ho2O3
Er2O3
Tm2O3
Yb2O3
Lu2O3
75*, 227
76*, 250
80*, 250
115, 215* 22.00
112, 220* 22.50
9.15
9.11
21.36
22.78
19.58
19.47
17.78
16.07
15.92
15.86
98, 130
105,130
95
20.97
19.24
18.20
17.00
15.90
16.30
95, 240
158
165
T1 – temperature range of dehydration; T2 – temperature range of degradation of anhydrous complexes to suitable oxides;
Te*ndo – very weak endothermic effects
J. Therm. Anal. Cal., 91, 2008
597

˜YSZCZEK
Fig. 3 TG, DTG and DTA curves of thermal decomposition of
Tm(btc)·4H2O
Fig. 1 TG, DTG and DTA curves of thermal decomposition of
La(btc)·4H2O
Fig. 4 TG, DTG and DTA curves of thermal decomposition of
Lu(btc)·4H2O
Fig. 2 TG, DTG and DTA curves of thermal decomposition of
Eu(btc)·2H2O
(Fig. 3). On the DTG and DTA curves recorded at
temperature range of water molecules losing are pres-
ent double peaks at about 90–130°C and very weak
peak at 230°C. Such course of thermal decomposition
indicate multi-step dehydration process. Endothermic
effects recorded at lower temperature probably corre-
spond to water molecules which occupy position in
outer coordination sphere being hydrogen bonded
with organic ligand. The second endothermic effect at
higher temperature is connected with releasing of
small amount of water that is clearly reflected on the
TG curve. Relatively high temperature of water liber-
ating points to tightly bonding of water molecules in
the complexes. In the case of tetrahydrate complexes
of Yb(III) and Lu(III) dehydration process is
characterized by asymmetric broad DTG peak
associated with endothermic effects recorded at 158
and 165°C, respectively (Fig. 4).
and DTG curves of tetrahydrate 1,2,3-benzene-
tricarboxylates of La(III), Ce(III), Pr(III) and Er(III)
indicates a single-step dehydration connected with
endothermic effect at about 95°C (Fig. 1). In the case
of dihydrate hemimellitates of Nd(III), Sm(III) and
Eu(III) dehydration process occurs in entirely differ-
ent way. Heating of the complexes above 30°C result-
ing in slowly mass loss connected with hardly distin-
guishable endothermic effect at about 80°C (Fig. 2).
This DTA effect is connected with liberation of very
weakly hydrogen bonded water molecules. The sec-
ond step of dehydration proceeds with distinct mass
loss detected on the TG curve and strong endothermic
effect at 250°C. Relatively high temperature of water
molecules liberating point to their strong bonding in
structures of complexes. It may suggest that water
molecules appear in inner coordination sphere being
directly bonded with lanthanide ions [21]. In different
ways dehydration of 1,2,3-benzenetricarboxylates of
Gd(III), Tb(III), Dy(III), Ho(III) and Tm(III) occurs
As can be seen from thermal decomposition data
water molecules are removed at different temper-
atures in accordance of strength of their bonding.
Anhydrous hemimellitates are formed in the range
185–270°C. The next decomposition stage is
598
J. Therm. Anal. Cal., 91, 2008

HEMIMELLITIC ACID COMPLEXES
4 T. Sato, W. Mori, Ch. N. Kato, E. Y, T. Kuribayashi,
connected with degradation of anhydrous complexes
that begins in the temperature range 335–400°C. The
decomposition process of the complexes is associated
with burning of organic ligand resulting in strong
exothermic effect. Such way of degradation
mechanism is characteristic for complexes of
lanthanide benzoates [22]. As the final solid products
of thermal decomposition suitable metal oxides
(Ln2O3, CeO2, Pr6O11, Tb4O7) are formed in the range
565–760°C.
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Conclusions
The new complexes of lanthanide(III) with hemimellitic
acid were obtained as hydrates. The 1,2,3-benzene-
tricarboxylate ligand is entirely deprotonated in the
studied complexes. As can be seen from infrared
spectra, the dihydrate complexes show different mode
of coordination in comparison to remaining
compounds. The decomposition process of the
studied complexes in the air atmosphere proceeds in
two main stages: dehydration and degradation of
organic ligand. Amorphous complexes lose water
molecules in one step. Dehydration process in crystalline
compounds occurs in overlapped stages.
Further synthesis in the gel medium and
hydrothermal conditions are provided in purpose to
obtaining monocrystals of lanthanide(III) hemimelli-
tates for X-ray structural investigations. Additionally,
examination of reversibility of dehydration process for
hydrated complexes will be done.
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Received: March 22 , 2007
Accepted: April 12 , 2007
OnlineFirst: July 11, 2007
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