Synthesis, characterization, and thermal decomposition of oxamido heterobinuclear Cu(II)-Ln(III) complexes

Article abstract of DOI:10.1016/j.tca.2003.09.006

Three oxamido-bridged copper(II)-lanthanide(III) heterobinuclear complexes described by the overall formula Cu(oxen)Ln(phen)₂(ClO ₄)₃ (oxen=N,N′-bis(2-aminoethyl)oxamide, Ln=Eu, Gd, and Er, phen=1,10-phenanthroline), have

Full text of DOI:10.1016/j.tca.2003.09.006

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Thermochimica Acta 142 (2004) 97–105
Synthesis, characterization, and thermal decomposition
of oxamido heterobinuclear Cu(II)–Ln(III) complexes
Hai-Dong Wanga,b, Yong Chenc, Yan-Tuan Lia, Xian-Cheng Zenga,∗
a
Faculty of Chemistry, Sichuan Key Laboratory of Green Chemistry and Technology, Sichuan University, Chengdu 610064, PR China
b
Qinghai Institute of Salt Lakes, The Chinese Academy of Sciences, Xining 810008, PR China
c
Department of Applied Oil Engineering, Logistical Engineering University, Chongqing 400016, PR China
Received 11 October 2002; received in revised form 2 September 2003; accepted 8 September 2003
Abstract
Three oxamido-bridged copper(II)–lanthanide(III) heterobinuclear complexes described by the overall formula Cu(oxen)Ln(phen)2(ClO4)3
(oxen = N,Nꢀ-bis(2-aminoethyl)oxamide, Ln = Eu, Gd, and Er, phen = 1,10-phenanthroline), have been synthesized and characterized by
the elemental analyses, IR spectroscopy, X-ray analysis and energy dispersive spectrometer (EDS) analysis. The thermal decomposition of
the complexes was studied by TG, DTG, and DTA technique under dynamic nitrogen atmosphere. The decompositions of these complexes
took place through four stages. The second and third stages were analyzed kinetically by means of the Achar method and the Coats–Redfern
method. The activation energies, pre-exponential factors and the most probable kinetic model function were determined for these stages.
© 2003 Elsevier B.V. All rights reserved.
Keywords: Cu(II)–Ln(III) heterobinuclear complex; Thermal decomposition; Non-isothermal kinetic
1. Introduction
of Cu(oxen)Ln(phen)2(ClO4)3 (Ln = Eu, Gd and Er) by
means of thermal analyses.
The interest in bridged heteroplymetallic systems with
two different paramagnetic centers has risen considerable
in recent years [1]. The synthesis, spectroscopic and mag-
netic investigations of new heterobinuclear complexes are
important not only for gaining some insight into the elec-
tronic and geometric structure of metalloproteins and en-
zymes and thus the correlating structure with biological
function, but also for obtaining information about designing
and synthesizing molecule-based magnets and investigating
the spin-exchange mechanism between paramagnetic metal
ions. But to this end, few studies on thermochemistry, so far
as we know, have been reported.
As part of our continuous work, the coordination envi-
ronment of the title complexes had been measured by dif-
ferent analysis technique. The results indicate that their co-
ordination environment is very similar to that of the single
crystal [Cu(oxap)Ni(phen)2](ClO4)2·2H2O [2]. In this pa-
per, we study the thermal decompositions of the complexes
Corresponding to Scheme 1, the coordination environ-
ment of oxamido heterobinuclear Cu(II)–Ln(III) complex
was drawn below.
2. Experiment
2.1. Materials
All materials were commercial products of chemical
or analytic grade purity and were used without further
purification. The mononuclear complex as “ligands” of
N,Nꢀ-bis(2-aminoethyl)oxamidocopper(II) complex was
prepared by previously published procedures [3]. Three
compounds of the hydrated lanthanide(III) perchlorate was
prepared by general methods.
2.2. Synthesis of binuclear Cu(II)–Gd(III) complexes
The methods used to prepare three copper(II)–lanthanide-
(III) heterobinuclear complexes were virtually identical and
were exemplified by Cu(oxen)Eu(phen)2(ClO4)3·2H2O.
A solution of Cu(oxen) (0.265 g, 1 mmol) in absolute
∗
Corresponding author.
E-mail address: zengxc@pridns.scu.edu.cn (X.-C. Zeng).
0040-6031/$ – see front matter © 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.tca.2003.09.006

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H.-D. Wang et al. / Thermochimica Acta 142 (2004) 97–105
3. Results and discussions
N
N
NH2
Cu
N
3.1. General properties of the binuclear complexes
O
N
N
Ln
(ClO4)2
(ClO4)
These binuclear complexes are very soluble in acetoni-
trile, DMF and DMSO to give stable solutions at room
temperature; moderately soluble in water, methanol and
acetone, and practically insoluble in carbon tetrachloride,
chloroform and benzene. The Cu(oxen)Ln(phen)2(ClO4)3
(Ln = Eu, Gd, and Er) complex can be recrystallized
from a DMF/ethanol (1:2) mixture. In the solid state all
of the Cu(II)–Ln(III) binuclear complexes are fairly stable
in air, thus allowing physical measurements. For the three
Cu(II)–Ln(III) binuclear complexes, the observed molar
conductance values in DMF solution due to 1:2 ion−ic com-
plexes. These values indicated that only one ClO4 anion
exist in the inner-spheres of these complexes. This is con-
sistent with the measured IR data of the heterobinuclear
complexes.
NH2
N
O
Scheme 1. The coordination environment of oxamido heterobinuclear
Cu(II)–Ln(III) complex.
ethanol (15 cm3) was added successively a solution of
Eu(ClO4)3·6H2O (0.56 g, 1 mmol) in absolute ethanol
(15 cm3) with stirring at room temperature. Then the mixed
solution was added an absolute ethanol solution (25 cm3) of
1,10-phenanthroline (0.4 g, 2 mmol). The color of the solu-
tion turned from violet-red to Cambridge blue immediately
and a small amount of precipitate formed. After stirring for
ca. 12 h, the resultant Cambridge blue microcrystals were
filtered off, washed several times with absolute ethanol and
diethyl ether and dried under reduced pressure.
3.2. Infrared spectra
Since the IR spectra of the three binuclear complexes
are similar, the discussion is confined to the most important
vibrations of the 200–4000 cm−1 region in relation to the
structure. The IR absorption bands of the complexes are
given in Table 2. We will only discuss the selected infrared
bands, with most relevant IR absorption bands from the
IR spectra of the binuclear complexes and the mononu-
clear fragment N,Nꢀ-bis(2-aminoethyl)oxamidocopper(II).
The carbonyl stretching vibration at 1585 cm−1 for the
mononuclear fragment Cu(oxen)is considerably shifted to-
wards higher frequencies (ca. 60–70 cm−1) in the binuclear
complexes. Therefore, in general, when the deprotonated
amide nitrogen is coordinated to the metal ion, its amide
band shifts considerably towards lower wavenumbers. In
the case of an oxamide dianion coordinated to two metal
ions as a bridging ligands, the amide band reverts to near
its original position (in the protonated species) [4]. Al-
All analytical data and colors of the binuclear complexes
are collected in Table 1.
2.3. Physical measurements
Carbon, hydrogen and nitrogen elemental analyses were
performed with a Perkin-Elmer elemental analyzer Model
240. Metal contents were determined by EDTA titration.
IR spectra were recorded with a Nicolet FT-IR spectropho-
tometer using KBr pellets. Molar conductances were mea-
sured with a DDS-11A conductometer. TG–DTG–DTA was
carried out on a TGDTA92 simultaneous analyzer (Setaram
Corp.). TG–DTA runs were carried out at a heating rate of
10 ◦C min−1 under a dynamic nitrogen atmosphere using a
flow rate of 40 cm3 min−1, and the temperature range was
20–1300 ◦C. The reference was ␣-Al2O3. Alumina crucibles
were used to hold 2–3 mg samples for analyses. The X-ray
analysis is carried out by means of a Riemens D/Max III B
powder diffractometer, using Cu K␣ radiation. EDS analysis
is carried out by means of KEVEX-Sigma energy dispersive
spectrometer.
=
though the amide is due to a composite N–C O vibration,
=
it can essentially be seen as ν(C O). It is likely that the
=
bond order of C O (carbonyl) in the binuclear complexes is
higher than that in the corresponding mononuclear complex
Cu(oxen). This shift has often been used as a diagnostic
Table 1
Elemental analyses and colors of the binuclear complexes
Complex
Empirical formula (formula weight)
Color
Elemental analyses (calculated) (%)
C
H
N
Cu
Ln
(1)a
(2)b
(3)c
CuEuC30H32N8O16Cl3 (1082.50)
Light-blue
Light-blue
Light-blue
33.12 (33.29)
33.17 (33.13)
33.02 (32.82)
2.70 (2.98)
2.78 (2.97)
2.79 (2.94)
10.29 (10.35)
10.20 (10.30)
10.18 (10.21)
5.90 (5.87)
5.87 (5.84)
5.80 (5.79)
14.21 (14.04)
14.40 (14.46)
15.30 (15.24)
CuGdC30
H32N8O16Cl3 (1087.79)
CuErC30H32N8O16Cl3 (1097.80)
a
Cu(oxen)Eu(phen)2(ClO4)3·2H2O.
Cu(oxen)Gd(phen)2(ClO4)3·2H2O.
Cu(oxen)Er(phen)2(ClO4)3·2H2O.
b
c

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H.-D. Wang et al. / Thermochimica Acta 142 (2004) 97–105
99
Table 2
IR spectra of analytical data of the complexes
Complexes
IR (cm−1
)
−
=
=
ν(C O)
ν(NH2)
ν(ClO4
)
ν(N C)
ν(Ln–N)
Cu(oxen)Eu(phen)2(ClO4)3·2H2O
Cu(oxen)Gd(phen)2(ClO4)3·2H2O
Cu(oxen)Er(phen)2(ClO4)3·2H2O
1645
1650
1655
3250
3250
3250
11301020
11301020
11301020
1550
1530
1530
395
380
382
indicator for oxamido-bridged structures [4]. On−th1e other
3.3. Energy dispersive spectrometer
=
hand, the C O deformation vibration at 720 cm of the
ligand complex, N,Nꢀ-bis(2-aminoethyl)oxamidocopper(II),
disappeared in the spectra of the binuclear complexes.
This fact may be attributed to the coordination of the
carbonyl oxygens to the Ln(III) ion [5–7]. This coordi-
nation mode of the complex ligand, Cu(oxen), has been
revealed by X-ray diffraction analysis of an analogous
The composition of three binuclear complexes were de-
fined by energy dispersive spectrometer (EDS) analysis
(shown in Figs. 1–3), respectively. The analysis results in-
dicate that each complex contains two kinds of different
metal ions. So, from these conclusions can be proved these
complexes are that heterobinuclear complex.
=
complex [5]. In addition, the –N C– stretching vibra-
tion for the terminal ligand (phen) was shifted to higher
frequencies (1530 cm−1) in these binuclear complexes,
suggesting that the N atoms of the terminal ligand co-
ordinated with the Ln(III) ion. The additional band ob-
served at around 380–390 cm−1 due to ν(Ln–N) further
supports this view. However, in general, the difference in
wavenumbers between the two highest frequency (1130
and 1020 cm−1), indicating that the coordinated perchlo-
rate ions is a single dentate [8]. Thus, the above spectral
observations, together with the molar conductance data,
confirm that the perchlorate ion is coordinated to the
Ln(III) ions in a single dentate fashion in these binuclear
complexes.
3.4. Thermal analysis
3.4.1. Cu(oxen)Eu(phen)2(ClO4)3·2H2O
The TG, DTG, and DTA curves of the Cu(oxdn)Eu(phen)2
(ClO4)3·2H2O complex are shown in Fig. 4. Four stages of
the dissociation of complex are indicated in TG and DTG
curve. The decomposition starts from 31 ◦C and ends at
173 ◦C, the mass loss observed is 3.31% against calculated
3.33%, corresponding to the release of 2 mol of water. The
second stage is from 173 to 281 ◦C. The mass loss observed
is 8.91% against calculated 9.19%, showing that 1 mol per-
chlorate ion is expelled. The third stage is in continuation
with the second stage from 281 to 412 ◦C. The mass loss
Fig. 1. Energy dispersive spectrometer of Cu(II)–Eu(III) complex.

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H.-D. Wang et al. / Thermochimica Acta 142 (2004) 97–105
Fig. 2. Energy dispersive spectrometer of Cu(II)–Gd(III) complex.
observed is 18.36% against calculated loss of 18.37%, cor-
responding to the dissociation of 2 mol of perchlorate ions.
The fourth stage is from 412 to 1273 ◦C. The mass loss ob-
served is 39.81%, showing that organic groups are expelled.
We can make out from the TG curve, trend that the mass loss
descends to being slow. The results indicate that the serious
product charcoal appearance has occurred the decomposition
process of chemical compound. The mass loss is measured
cannot to be compared with theory value. The sinters of
thermal decomposition were analyzed by the X-ray powder
diffraction method (shown in Fig. 5). The results shown that
the final product is considered to be Eu2O3, Cu, and C. The
thermoanalytical data for Cu(oxen)Eu(phen)2(ClO4)3·2H2O
complex are listed in Table 3.
There is one weakness endothermic peak and two exother-
mic peaks in DTA curve corresponding to the chemical
events observed in the TG curve. Especially, there is the
strong peak (exothermic peak) in DTA curve, shown that
Fig. 3. Energy dispersive spectrometer of Cu(II)–Er(III) complex.