Preparation and thermal properties of lanthanide complexes with 2,3-dichlorobenzoic acid and 1,10-phenanthroline

Article abstract of DOI:10.1016/j.tca.2010.09.011

Three lanthanide complexes with a general formula [Ln(2,3-DClBA) ₃phen]₂ (Ln(III) = Eu(1), Tb(2), Ho(3); 2,3-DClBA = 2,3-dichlorobenzoate; phen = 1,10-phenanthroline) were synthesized and characterized by elemental analysis, molar conductance, infrared and ultraviolet spectra and powder X-ray diffraction (XRD). The luminescent properties of the complexes 1 and 2 were studied. The thermal behaviors of the complexes were also discussed by thermogravimetric (TG), differential thermogravimetric (DTG) and infrared spectra (IR) techniques. The heat capacities of the complexes were measured from 259.15 to 493.02 K by means of Differential scanning calorimeter (DSC). The dependence of heat capacity on the reduce temperature x (x = [T - (T_max + T_min)/2]/[(T_max - T_min)/2]) was fitted to a polynomial equation with the least squares method for each complex. Furthermore, based on the fitted polynomial, the smoothed heat capacities and the derived thermodynamic functions (H_T - H _{298.15 K}), (S_T - S_{298.15 K}) and (G_T - G_{298.15 K}) in the measured temperature range were obtained with an interval of 10 K.

Full text of DOI:10.1016/j.tca.2010.09.011

Thermochimica Acta 512 (2011) 118–123
Contents lists available at ScienceDirect
Thermochimica Acta
journal homepage: www.elsevier.com/locate/tca
Preparation and thermal properties of lanthanide complexes with
2,3-dichlorobenzoic acid and 1,10-phenanthroline
Juan-Fen Wanga,b, Ning Renc, Fan-Tao Mengd, Jian-Jun Zhanga,b,∗
a Experimental Center, Hebei Normal University, Shijiazhuang 050016, PR China
b College of Chemistry & Material Science, Hebei Normal University, Shijiazhuang 050016, PR China
c Department of Chemistry, Handan College, Handan 056005, PR China
d College of Geography and Environment Science, Northwest Normal University, Lanzhou 730070, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 12 July 2010
Received in revised form 6 September 2010
Accepted 9 September 2010
Available online 17 September 2010
Three lanthanide complexes with a general formula [Ln(2,3-DClBA)3phen]2 (Ln(III) = Eu(1), Tb(2), Ho(3);
2,3-DClBA = 2,3-dichlorobenzoate; phen = 1,10-phenanthroline) were synthesized and characterized by
elemental analysis, molar conductance, infrared and ultraviolet spectra and powder X-ray diffraction
(XRD). The luminescent properties of the complexes 1 and 2 were studied. The thermal behaviors of the
complexes were also discussed by thermogravimetric (TG), differential thermogravimetric (DTG) and
infrared spectra (IR) techniques. The heat capacities of the complexes were measured from 259.15 to
493.02 K by means of Differential scanning calorimeter (DSC). The dependence of heat capacity on the
reduce temperature x (x = [T − (Tmax + Tmin)/2]/[(Tmax − Tmin)/2]) was fitted to a polynomial equation with
the least squares method for each complex. Furthermore, based on the fitted polynomial, the smoothed
Keywords:
Lanthanide complexes
2,3-Dichlorobenzoic acid
1,10-Phenanthroline
Luminescence
heat capacities and the derived thermodynamic functions (HT − H298.15 K), (ST − S298.15 K) and (GT − G298.15 K
)
in the measured temperature range were obtained with an interval of 10 K.
Heat capacity
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
general formula of [Ln(2,3-DClBA)3phen]2 (Ln(III) = Eu(1), Tb(2),
Ho(3)). XRD patterns demonstrate that the three complexes are
isostructural. The thermal decomposition processes of the com-
plexes were discussed by TG technique and the final residues were
verified by IR technique. Furthermore, the heat capacities Cp,m of the
three complexes were determined by means of DSC from 259.15 to
493.02 K and the thermodynamic parameters, such as the enthalpy,
entropy and Gibbs free energy of the complexes relative to the stan-
dard reference temperature 298.15 K were established from the
smoothed heat capacities.
Many of stable ternary lanthanide complexes with aromatic acid
and nitrogen-containing ligands were obtained for their certain
conjugate system. They show interesting and various structures
due to the different coordination models of the carboxyl groups and
have been widely applied as functional materials [1–8]. In addition,
the lanthanide complexes with distinct luminescent properties
[9–16] may be used as luminescent probes in biomedical assays,
as new fluorescence materials, as emitters in white LED devices
[8]. However, their thermodynamic properties have less or not
been studied. As we known, these properties are important not
only for the theoretical study, but also the practical applications.
Differential scanning calorimetry is a modern instrument analysis
technology which can be used to determine the continuous heat
capacities of materials quickly and precisely. In previous works,
parts of binary lanthanide complexes with dichlorobenzoic acid
have been reported by other researchers, focusing on the ther-
mal behaviors of the complexes [17–19]. In this paper, we have
synthesized and characterized a new series of complexes with a
2. Experimental
2.1. Materials
All chemicals were achieved from commercial sources as ana-
lytical grade and used without further purification. LnCl3·6H2O
(Ln(III) = Eu, Tb, Ho) were prepared by dissolving their respective
oxide in hydrochloric acid, and then drying the solution by water-
bath heating.
2.2. Preparation of the complexes
∗
Corresponding author at: College of Chemistry & Material Science, Hebei Normal
LnCl3·6H2O [Ln(III) = Eu, Tb, Ho] (0.5 mmol) was dissolved in
distilled water. 2,3-dichlorobenzoic acid (1.5 mmol) and phen
(0.5 mmol) were dissolved together in 95% ethanol solution. The pH
University, Shijiazhuang 050016, PR China. Tel.: +86 31186269386;
fax: +86 31186268405.
E-mail address: jjzhang6@126.com (J.-J. Zhang).
0040-6031/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.tca.2010.09.011

J.-F. Wang et al. / Thermochimica Acta 512 (2011) 118–123
119
Table 1
Data of elementary analyses and molar conductance of the complexes.
Complex
1
C%
H%
N%
Ln%
ꢀm (S cm2 mol−1
)
Found
Calcd
Found
Calcd
Found
Calcd
44.30
43.93
43.67
43.60
42.96
43.31
1.97
1.90
1.84
1.89
1.82
1.87
3.23
3.11
3.11
3.08
3.08
3.06
17.00
16.84
17.65
17.48
18.30
18.02
30.45
29.47
29.46
2
3
Table 2
Frequencies (cm−1) of the absorption bands for the ligands and complexes.
−
−
)
Compound
ꢁC=N
ꢁC=O
ꢁas(COO
ꢁs(COO
ꢂꢁ(ꢁas − ꢁs)
ıC–H
ꢁLn–O
)
Phen
2,3-DClHBA
1561
–
–
–
–
–
–
–
–
854,738
–
–
–
1692
1
2
3
1520
1520
1519
–
–
–
1620
1624
1626
1409
1410
1411
211
214
215
846,728
843,729
843,729
418
419
419
of the solution was controlled in 6–7 using 1 mol L−1 NaOH solution.
The ethanol solution of the two ligands then was added dropwise
into the LnCl3·6H2O aqueous solution under stirring. The reaction
mixture was continually stirred about 10 h at room temperature,
and then deposited overnight. Subsequently, the precipitates were
filtered out, washed with 95% ethanol and dried in a far infrared
dryer. The powders finally were stored in a silica-gel desiccator for
the later characterization.
mass was about 11 mg based on their molar mass of 1804.36,
1818.27 and 1830.28 g mol−1, respectively.
3. Results and discussion
3.1. Elemental analysis
The results of elementary analyses are listed in Table 1. From
Table 1, it can be seen that the experimental data are consistent
well with the theoretical values, indicating that the complexes have
a composition of [Ln(2,3-DClBA)3phen]2 [(where Ln(III) = Eu(1),
Tb(2), Ho(3)] as anticipated.
2.3. Apparatus and conditions of experiment
The carbon, hydrogen, and nitrogen contents in the complexes
were determined on a Vario-EL III elemental analyzer and the metal
content was assayed using an EDTA titration method. Infrared
spectra were recorded at room temperature with KBr discs on a
BRUKER TENSOR27 spectrometer from 4000 to 400 cm−1. Ultra-
violet spectra were depicted in a SHIMADZU 2501 spectrometer.
The fluorescent spectra were measured on an F-4500 Hitachi
spectrophotometer in the solid state at room temperature. Molar
conductivity was measured by DDS-307 conductometer (Shanghai
Precision & Scientific Instrument CO.LED). X-ray diffraction iden-
tification was carried out for the crystalline analyses by a Bruker
D8 ADVANCE X-ray diffractometer in a scanning range of 5–50◦
3.2. Molar conductance
The prepared complexes were dissolved in DMSO (dimethyl sul-
foxide) solution and their molar conductivity was determined with
DMSO as a reference. The values of molar conductance given in
Table 1 are all less than 40 S cm2 mol−1, indicating that the three
complexes are non-electrolyte in DMSO [21].
3.3. Infrared spectra
˚
Table 2 presents the important infrared frequencies of the
organic ligands and the complexes. The band of ꢁC=O at 1692 cm−1
due to the COOH group in the free acid ligand disappears completely
in the spectra of the complexes, whereas there appear two bands
around 1620–1626 and 1409–1411 cm−1 that arise from asymmet-
ric and symmetric vibrations of the COO− group, which indicates
that the oxygen atoms of carboxylic groups have formed coordi-
native bonds with Ln(III) ion [22]. Meanwhile, the appearance of
absorption band at 418–419 cm−1 attributed to the ꢁLn–O may also
suggest that the oxygen atoms from the carboxylic groups are coor-
dinated to the Ln(III) ion [23]. The bands of ꢁC=N (1561 cm−1) and
ıC–H (854 and 738 cm−1) in the spectrum of 1,10-phenanthroline
ligand are observed to shift to lower wavenumbers in the spectra
of the complexes, suggesting the coordination of the two nitrogen
atoms of the neutral ligand to Ln(III) ion [24].
(2ꢃ) with CuK␣ radiation (ꢄ = 1.5418 A). The thermal stability of the
complexes was conducted with a Perkin Elmer TGA7 thermogravi-
metric analyzer and TG-DTG curves were recorded. The samples
(about 3 mg) were heated in a platinum crucible from 298 to 1173 K
under a dynamic N2 atmosphere at a heating rate of 5 K min−1
.
Heat capacities of the prepared complexes were performed on a
NETZSCH DSC 200 F3 in the temperature range of 259.15–493.02 K
at a heating rate of 10 K min−1 using indirect measurement method.
The atmosphere was nitrogen gas and the flow rate is 40 mL min−1
.
The heat capacity of the reference standard material sapphire was
measured to verify the reliability of the heat capacity measurement
method by DSC and the relative deviations of our experimental
results were within 0.50%, compared with the recommended val-
ues by NIST [20] in the entire measured temperature range. The heat
capacity of the material can be determined by creating a blank base-
line profile, a reference standard substance (sapphire) profile and
then a sample profile. All experiments were carried out in almost
the same conditions. The mass of the reference standard substance
sapphire (12.75 mg) or a sample loaded in an aluminum crucible
sealed with a pierced lid was accurately weighted on heating. Here,
the weights of aluminum crucibles are as equal as possible. The
apparatus has an automatic data processing program, from which
we can obtain the heat capacity values of the sample at different
temperature by the indirect measurement method. The samples
3.4. Ultraviolet spectra
The ligands and complexes were recorded in DMSO solution by
UV spectra with DMSO as a reference, respectively. The spectra data
are listed in Table 3. The maximum absorption wavelength of the
free acid ligand 2,3-DClHBA is 256.0 nm with an absorbency of 0.02.
The main absorption peaks of the complexes are 264.2–265.1 nm,
which draws a conclusion that the free acid ligands were coor-

120
J.-F. Wang et al. / Thermochimica Acta 512 (2011) 118–123
Table 3
UV absorption of the ligands and complexes (ꢄ (nm), Amax).
Compound
␭
max
(nm)
Amax
Phen
2,3-DClHBA
[Eu(2,3-DClBA)3phen]2
[Tb(2,3-DClBA)3phen]2
[Ho(2,3-DClBA)3phen]2
265.0
256.0
265.1
264.2
265.1
0.31
0.02
0.88
1.20
0.80
dinated to the metal ion forming a ␲-conjugated system [25].
Moreover, compared with the complexes, the maximum absorp-
tion peak of phen at 265.0 nm is similar to that of in the complexes,
indicating that the formation of the coordination bonds of Ln–N
should have no significant influence on the UV absorption of the
phen [12]. However, the absorbency is enhanced from 0.31 to 0.88,
1.2 or 0.80 for three complexes, respectively, suggesting a bigger
chelating ring is formed.
Fig. 2. Excitation spectra (ꢄ = 545nm) and luminescence spectra (ꢄ = 320 nm) of the
complex 2.
3.5. Fluorescence spectra
The excitation and emission spectra of the complexes 1 and
2 were recorded in solid state at room temperature. The excita-
tion wavelengths of the complexes were performed in the range of
200–400 nm. The fluorescent spectra were recorded in the range
of 400–700 nm at a selective excitation wavelength 320 nm, and
they exhibit strong characteristic luminescence of europium and
terbium ions, as shown in Figs. 1 and 2. In addition, the complexes
1 and 2 exhibit red and green luminescence, respectively, under the
radiation of UV light. For the complex 1, three main emission peaks
of the Eu(III) ion are observed at 579.6, 591.6 and 617.4 nm, cor-
chlorides is quite weak, but from Figs. 1 and 2 we can see that
the luminescence intensity of Ln(III) (Ln = Eu, Tb) complexes are
remarkable strong.
3.6. XRD
The powder diffraction patterns of the complexes have been
recorded and depicted in Fig. 3. From the patterns, it is evident
that the three complexes are isostructural.
7
7
7
responding to the 5D0 → F0, 5D0 → F1 and 5D0 → F2 transitions,
7
respectively. The 5D0 → F1 magnetic dipole transition occurs at
3.7. Thermal decomposition processes of the complexes
591.6 nm and the three peaks 614.8, 617.4 and 619.6 nm belong to
7
the 5D0 → F2 electric dipole transition which appears the dividing,
The thermal analytical data of the complexes 1–3 are summa-
rized in Table 4. The TG–DTG curves of the complexes at a heating
rate of 5 K min−1 under a dynamic N2 atmosphere are shown in
Figs. 4–6, respectively.
As seen from the DTG curves, the thermal decomposition
processes of the three complexes present three main stages, respec-
tively. For the complex 1 [Eu(2,3-DClBA)3phen]2 the first stage
mass loss is 19.75% between 514.14 and 627.84 K, which coincides
with the loss of two phen molecules with a calculated mass loss
of 19.97%. The second and third stages take place at 627.84 K and
continue to 1045.83 K with a loss of 61.24% (theoretical mass loss
is 60.54%), corresponding to the loss of the rest groups. The charac-
teristic absorptions of the final residue are similar to IR spectrum of
which is similar to the complex in the literature [26]. The 5D0 → F2
7
7
transition is stronger than the 5D0 → F1 and its rate is about 2.5,
indicating that the symmetry of the Eu(III) ion in the coordination
compound is lower and have no symmetry centre [27,28]. For the
complex 2, four main emission peaks in the luminescence spectrum
at about 490, 545, 585 and 620 nm are the corresponding charac-
teristic peaks of Tb(III) ion, which are assigned to the 5D4 → F6,
7
5D4 → F5, 5D4 → F4 and 5D4 → F3, respectively. The most intense
7
7
7
emission band arises from the 5D4 → F5 transition, which is the
7
preferred transition in terbium-containing complexes [29]. As we
known the luminescence intensity of solid Ln(III) (Ln = Eu, Tb)
Fig. 1. Excitation spectra (ꢄ = 615nm) and luminescence spectra (ꢄ = 320 nm) of the
complex 1.
Fig. 3. XRD patterns of the complexes.

J.-F. Wang et al. / Thermochimica Acta 512 (2011) 118–123
121
Table 4
Thermal decomposition data for the complexes 1–3 from TG-DTG analysis.
Complex
Step
Temperature range (K)
DTG Tp (K)
Mass loss rate (%)
Found
Probable
Residue(%)
Obs.
Calcd.
Expelled groups
Calcd.
I
II
III
514.14–627.84
627.84–725.53
725.53–1045.83
595.81
684.70
779.18
19.75
30.71
30.53
80.99a
25.17
33.31
20.61
79.09a
23.21
36.44
19.89
79.54a
19.97
–
2phen
–
–
1
2
3
x(2,3–DClBA)
(6 − x)(2,3-DClBA)–3O
–
–
Eu2O3
19.02
–
–
1/2Tb4O7
20.47
–
80.51a
19.49
20.56
20.64
I
II
III
514.72–655.53
655.53–749.80
749.80–1044.96
603.09
711.86
807.03
–
–
–
2phen+ x(2,3-DClBA)
(6 − x)(2,3-DClBA)-3.5O
2phen+ x(2,3-DClBA)
79.44a
I
II
III
517.60–667.88
667.88–776.97
776.97–1038.56
603.32
753.59
812.59
–
–
–
(6 − x)(2,3-DClBA)–3O
Ho2O3
20.31
79.36a
–
Tp is the peak temperature of DTG.
a
The total loss rate.
the standard sample Eu2O3. Therefore, Up to 1045.83 K, the com-
plex 1 was completely degraded into Eu2O3 with a total loss of
80.99% (theoretical loss is 80.51%). Based on the above analysis, the
thermal decomposition of the complex 1 can be described as the
following process:
For the complexes 2 and 3, they have similar thermal decompo-
sition stages. Here, the complex 2 is selected as a representative for
the thermal analysis. The first mass loss stage takes place between
514.72 and 655.53 K with a mass loss of 25.17%, which corresponds
to the loss of two phen molecules and partial 2,3-DClBA ligands.
[Eu(2, 3-DClBA)3phen]2 → [Eu(2, 3-DClBA)3]2 →→ Eu2O3
Fig. 6. TG–DTG curves of the complex 3 at a heating rate of 5 K min−1
.
Fig. 4. TG–DTG curves of the complex 1 at a heating rate of 5 K min−1
.
Fig. 7. Molar heat capacity Cp,m of the complexes as functions of temperature T in
the temperature range.
Fig. 5. TG–DTG curves of the complex 2 at a heating rate of 5 K min−1
.

122
J.-F. Wang et al. / Thermochimica Acta 512 (2011) 118–123
Complex 1 [Eu(2,3-DClBA)3phen]2
Cp,m/J·mol-1·K-1 = 1866.53481 +538.5433x+509.79171x2+210.38569x3-1161.58492x4
-78.04671x5+686.47247x6 R2 = 0.9997 SD = 6.0565
Complex 2 [Tb(2,3-DClBA)3phen]2
Cp,m/J·mol-1·K-1 = 1938.12532 +399.58347x+253.92084x2+84.57623x3-575.23617x4
-4.46018x5+285.04963x6
R2 = 0.9996 SD = 5.5512
Complex 3 [Ho(2,3-DClBA)3phen]2
Cp,m/J·mol-1·K-1 = 1956.74226 +552.28398x+352.6551x2+10.53234x3-754.95227x4
+68.28757x5+406.56437x6 R2 = 0.9998 SD = 4.40656
Scheme 1.
The second and third stages belong to the loss of the remaining
2,3-DClBA ligands which undergo the decomposition with a mass
loss of 53.92% from 655.53 to 1044.96 K. Up to 1044.96 K, there is
no mass change with the increase of temperature, suggesting that
a final residue of Tb4O7 has been formed, which is verified by the
residue IR spectrum that has similar characteristic absorptions as
the standard sample spectra of Tb4O7. So, on the basis of the above
analytical results, the thermal decomposition of the complexes 2
and 3 are presented as the following:
tive upper limit (493.02 K) and lower limit (259.15 K) in the above
measured temperature region.
3.9. Thermodynamic functions of the complexes
Thermodynamic functions of the complexes were calculated on
the basis of the fitted polynomial of the heat capacities as a function
of the reduced temperature (x) according to the following thermo-
dynamic equations:
[Tb(2, 3-DClBA)3phen]2 → [Tb2(2, 3-DClBA)6−x] →→ (1/2)Tb4O7
ꢀ
T
HT − H295.15 K
=
Cp,mdT
(1)
(2)
(3)
[Ho(2, 3-DClBA)3phen]2 → [Ho2(2, 3-DClBA)6−x] →→ Ho2O3
298.15 K
We can draw a conclusion from the above analyses that the
three complexes all have good thermostability due to the higher
decomposed temperature until 514 K and the final residues are
their respective oxide. In addition, Ln–N bonds of the complexes
break firstly in the TG experiments, which may be explained by
the longer bond of Ln–N than that of Ln–O because of the stronger
coordination ability of oxygen atom than nitrogen atom.
ꢀ
T
ST − S295.15 K
=
Cp,mT
−1dT
298.15 K
ꢀ
ꢀ
T
T
GT − G295.15 K
=
Cp,mdT − T
Cp,mT
−1dT
298.15 K
298.15 K
Smoothed values of Cp,m and the derived values of fundamental
thermodynamic functions of the complexes relative to the stan-
dard reference temperature 298.15 K are listed in Tables 2–4 (see
Supplementary data), respectively, with an interval of 10 K.
3.8. Heat capacities of the prepared complexes
From the thermal analytical data, all of the three complexes
have no mass loss until 514 K at a liner heating rate of 5 K min−1
,
indicating they are thermostable. So we measured the molar
heat capacities of the three complexes by DSC from 259.15 to
493.02 K. Four times parallel experiments were carried out and
the average molar heat capacity values (see Table 1 in supple-
mentary data) of each complex were obtained from the measured
values. The curves of molar heat capacity values Cp,m vs temper-
ature for the three complexes are plotted in Fig. 7. It can be seen
from Fig. 7 the Cp,m of the complexes with temperature increas-
ing have a similar changing trend, which also demonstrates the
three complexes are isostructural. Over the measured tempera-
ture range there is no obvious endothermic or exothermic peak
existing in the Cp,m curves. However, there is a phenomenon
that the curves are not smooth at temperature from 300 K to
350 K and the causes are under further study. The average val-
ues of the parallel experimental heat capacities for each complex
were fitted to a polynomial, respectively, in the reduced tempera-
ture (x) (x = [T − (Tmax + Tmin)/2]/[(Tmax − Tmin)/2]) by means of the
least-squares method [30,31] and the corresponding equations,
correlation coefficients R2 and standard deviations of the fitting
results are given in Scheme 1, in which x = (T − 376.085)/116.935,
T is the experimental temperature, Tmax and Tmin are the respec-
4. Conclusions
In summary, the title complexes have been successfully synthe-
sized. The complexes 1 and 2 show strong emission. The thermal
decomposition processes of the three complexes present three
stages and they are all thermally stable. Heat capacities of the
complexes were measured by a differential scanning calorimeter
over the temperature range from 259.15 to 493.02 K and the aver-
age values of experimental heat capacities of the three complexes
were fitted to a polynomial equation, respectively. Additionally, the
thermodynamic functions of the complexes were derived from the
fitted polynomial equations of heat capacities Cp,m relative to the
standard reference temperature 298.15 K.
Acknowledgement
This project was supported by the National Natural Science
Foundation of China (Nos. 20773034 and 21073053), and the Nat-
ural Science Foundation of Hebei Province (No. B2007000237).

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