Synthesis and characterization of lanthanide(III) Schiff base complexes derived from cysteine and benzoin

Article abstract of DOI:10.1081/SIM-120003800

Six new complexes of the Schiff base ligand (HL) derived from benzoin and cysteine with Ln=La(III), Pr(III), Nd(III), Sm(III), Eu(III) and Gd(III) of the type LnLCl₂(H₂O)₂ have been synthesized and characterized by element

Full text of DOI:10.1081/SIM-120003800

SYNTH. REACT. INORG. MET.-ORG. CHEM., 32(3), 617–627 (2002)
SYNTHESIS AND CHARACTERIZATION
OF LANTHANIDE(III) SCHIFF BASE
COMPLEXES DERIVED FROM CYSTEINE
AND BENZOIN
H.-Zh. Ma, B. Wang,* and Q.-Zh. Shi
Department of Chemistry, Northwest University,
Shaanxi Key Laboratory of Physico-Inorganic
Chemistry, 710068, Xi’An, P. R. China
ABSTRACT
Six new complexes of the Schiff base ligand (HL) derived from
benzoin and cysteine with Ln ¼ La(III), Pr(III), Nd(III),
Sm(III), Eu(III) and Gd(III) of the type LnLCl2(H2O)2 have
been synthesized and characterized by elemental analyses,
molar conductance, XPS, IR, thermal analysis, 1H NMR and
electronic spectra.
*Corresponding author. E-mail: wangbo@nwu.edu.cn
617
Copyright # 2002 by Marcel Dekker, Inc.
www.dekker.com

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MA, WANG, AND SHI
INTRODUCTION
The coordination chemistry of lanthanides in biological systems173 and
the development of lanthanide shift reagents4,5 have aroused much interest.
As no studies on lanthanide(III) complexes of benzoin and cysteine have been
reported so far and in continuation of our earlier systematic investigation6 on
lanthanide(III) coordination binding models by XPS spectra, we report here
the synthesis and characterization of some of these new complexes of the
Schiff base ligands (HL) derived from benzoin and cysteine (Fig. 1).
EXPERIMENTAL
Reagents and Solvents
Benzoin was A.R. grade, lanthanide chlorides (LnCl3) were obtained by
treating 0.5 mmol of Ln2O3 with 3.0 mmol of 1:1 hydrochloric acid. The
resulting solution was filtered and evaporated to dryness. The white crystals
were dried in a vacuum desiccator over CaCl2; yields, 85%. All other reactants
were of high purity and were used without any further purification.
Preparation of KL
The white compound KL (potassium salt of Schiff base) was obtained
by mixing benzoin (0.032 mol, 6.73 g) with cysteine (0.032 mol, 5.0 g), and
potassium hydroxide (0.032 mol, 1.8 g) in 100 mL EtOH. The mixture was
refluxed for 2 h and precipitation occurred immediately on cooling the
mixture. The crystals were filtered, recrystallized in EtOH two times and
dried in a vacuum descciator over CaCl2; yield, 9.38 g (83%); m.p.
ꢀ
209.77210.2 C.
Figure 1. Structure of HL.

LANTHANIDE(III) SCHIFF BASE COMPLEXES
Preparation of Complexes LnLCl2(H2O)2
619
To a solution of 0.5 mmol lanthanide(III) chloride in 20 mL of abso-
lute EtOH was added 0.35 g (0.1 mmol) of KL in 20 mL EtOH with stirring.
The mixture was refluxed on a water bath for 1 h and then cooled to room
temperature. The resulting precipitate was separated by filtration, washed
with cold EtOH and finally dried at 110 ꢀC for 1 h; yields, 75785%.
Physical Measurements
The melting points of the ligand and complexes were determined with a
Gallenkamp melting point apparatus. Microanalyses for C, H, N and S were
carried out on a Perkin7Elmer 240C elemental analyzer. The content of Cl
was determined by AgNO3 titration. Lanthanides were determined volume-
trically be EDTA titrations7 using xylenol orange as the indicator. The molar
conductance of the complexes in methanol and in DMSO at 10ꢁ4 M con-
centration was measured with a DDS-IIA molar conductometer at room
temperature as before8. The molar conductances are listed in Table I. The
electronic spectra of the ligand and complexes were recorded in a Shimadzu
UV-240 double-beam spectrophotometer using freshly prepared absolute
methanol solutions of the samples. The infrared spectra (CsI pellets) were
obtained on a NIC-5DX spectrophotometer in the 20074000 cmꢁ1 region.
X-ray photoelectron spectra (XPS) were taken on a PHI-550 apparatus, using
non-monochromatic Mg-Ka radiation from on X-ray gun operated at 15 kV
and 20 mA under a pressure of 10ꢁ9 mbar using the carbon 1s peak at
284.6 eV as standard. The samples were finely pulverized and spread on a gold
holder as thin films. The hemispherical electrostatic analyzer was operated in
the retarding mode with a pass energy of 20 eV. The accuracy of the measured
BE value is ꢂ 0.1 eV. The peak areas for semiquantitative analyses were
determined using a linear background. Thermal analysis (TG and DTA)
measurements were made in nitrogen atmosphere between room temperature
1
and 800 ꢀC using a Thermoflex TG-DTA meter. H NMR spectra were
measured on a Bruker AC 80 spectrometer using DMSO-d6 as solvent.
RESULTS AND DISCUSSION
The formation of the complexes may be shown by the following
representative equation:
KL þ LnCl3 ꢃ6H2O Eꢁt!OH LnLCl2ðH2OÞ þ KCl þ 4H2O
2

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MA, WANG, AND SHI

LANTHANIDE(III) SCHIFF BASE COMPLEXES
621
The complexes are soluble in water, chloroform, ether, EtOH, acetone and
dimethylformamide (DMF). The colors of aqueous solutions are dependent
on the pH value. Between pH ¼ 175 yellow solutions were observed, and at
pH ¼ 677.5 colorless turbid solutions were produced. The molar con-
ductance values in methanol and in DMSO indicate that the complexes are
non-electrolytes in solution8,9, but in methanol the molar conductance
values are relatively higher than those in DMSO and this may be attributed
to methanol coordination. To confirm this result, longer time molar con-
ductance values have been examined. It was found that the change of the
molar conductance values with time increased for about 2 h at room tem-
perature and then the data become constant. These phenomena indicate that
the coordinated sphere or the coordination bond model may be changed due
to replacement by small polar molecules. The TG-DTA diagrams of all
complexes are similar and indicate that the complexes sustain weight loss in
the 1207140 ꢀC region and mass loss was observed at ca. 200 ꢀC, then the
complexes continue to decompose exothermally at 350, 520 and 630 ꢀC and
the decomposition is completed at about 780 ꢀC. Water of crystallization
(extraspheric water) is usually lost over the 707110 ꢀC range while water of
coordination (intraspheric water) is eliminated at higher temperature
(1207150 ꢀC). Together with the corresponding weight losses (water) it can
be concluded that the two water molecules in all complexes are coordinated.
Infrared Spectra
The IR spectra of the potassium salt of the ligand (KL) and its
complexes are complicated. However, attempts have been made to identify
some important bands that furnish information regarding the mode of
bonding. The main IR bands with their tentative assignments are listed in
Table II. The broad band between 3414 and 3386 cmꢁ1 due to the stretching
modes of n(OH) of the ligand remains almost unchanged but becomes
stronger in the complexes, which indicates that the hydroxyl group remains
ꢁ
uncoordinated. Two stretching vibration of CO2ꢁ, nas(CO2ꢁ) and ns(CO2
)
in the spectra of the complexes are observed at ca. 1580 and 1406 cmꢁ1 with
the difference Dn(CO2ꢁ) ¼ 174 cmꢁ1. Compared with the ligand, the two
bands at 1566 and 1400 cmꢁ1 having the difference Dn(CO2ꢁ) ¼ 166 cmꢁ1
ꢁ
suggested that the CO2 group is coordinated with the metal ion in uni-
dentate form. The band at 1625 cmꢁ1 in the ligand attributed to the
n(C¼N)stretching vibration shifts to lower frequencies at 160071605 cmꢁ1
after complexation indicating coordination of the nitrogen atom in the C¼N
group to the metal ion. Furthermore, several new bands observed in the far-
IR spectra at ca. 510, 318, 270 and 225 cmꢁ1 in the complexes are assigned

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MA, WANG, AND SHI

LANTHANIDE(III) SCHIFF BASE COMPLEXES
623
to n(Ln-O), n(Ln-N), n(Ln-Cl) and n(Ln-S) stretching modes, respectively.
All of the complexes exhibit an intense sharp band at 3440 cmꢁ1 in the
aqueous stretching region, suggesting that the water molecules in the com-
plexes are in the coordinated form, which is consistent with the thermal
analysis. On the basis of the discussion referred to above, it is possible to
conclude that the ligand is attached to the metal ion by the carboxyl oxygen,
nitrogen and sulfur atoms. Other groups coordinated to the metal ion are
chlorine ion and oxygen atom in water, and seven-coordinated complexes
are formed.
1
Electronic and H NMR Spectra
The electronic spectra of KL in MeOH show two bands at 224 and
315 nm that are attributed to p ! pꢄ transitions of the phenyl ring and C¼N
bond, but in the complexes the latter band shifted to 3207335 nm (Table I)
indicating that the N atom coordinates to the lanthanide(III) ion. The band
at 224 nm was almost unchanged.
To obtain further insight into the coordination model, 1H NMR
1
spectra have been recorded. The H NMR spectrum of KL displays signals
at 1.78, 6.04 and 7.1078.01 ppm, assignable to -SH, -OH and aromatic
protons, respectively. The observed downfield shifts of the -SH signal to ca.
1.54 ppm in the spectra of the complexes are indicative of involvement of the
sulfur atom in coordination. The deshielding is due mainly to the -SH group
being distant from the crowded coordination sphere. The new band at ca.
5.02 ppm in the spectrum of the complexes indicated the coordination of
water.
X-Ray Photoelectron Spectra (XPS)
In order to obtain further insight into the nature of bonding, XP
spectra of the complexes and the ligand were recorded in powder form at
room temperature. The results of the XPS semi-quantitative analyses of the
complexes are in agreement with the elemental analysis data and are given in
Table III and Fig. 2. The values for the Ln 3d or 4d electron binding energy
(BE) in the complexes are approximately the same and support the oxida-
tion number of three. The N 1s and S 2p electron spectra of the complexes
are symmetric and their deconvolution revealed that relative to the free
ligand, the N 1s and S 2p BEs are all shifted higher. This may be explained
as due to the decrease of the electron density after coordination to the
lanthanide(III) ion and it is consistent with the electronic spectral date. The

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MA, WANG, AND SHI

LANTHANIDE(III) SCHIFF BASE COMPLEXES
625
Figure 2. XPS spectra of the ligand (1) and complexes SmLCl2(H2O)2 (2).
Cl 2p spectra of the complexes are asymmetric and the relative intensity of
the peaks on deconvolution is about 1:1, indicating that the two coordinated
CI atoms are in different chemical environments.
The O 1s electron spectra of the complexes are in a very asymmetric
shape. Compared with the ligand bands at 531.6 and 532.5 eV attributed to
hydroxyl -OH and symmetric carboxylic oxygen atoms, the BE values of the
complexes appear at ca. 531.5, 532.4, 533.2 and 533.6 eV and may be
assigned to hydroxyl -OH, H2O, -COOꢁ (coordinated -C-Oꢁ) and -COOꢁ
(-C¼O), respectively which indicates that four chemical environments were
produced after coordination. Combined with the relative intensity of
the bands which equals 1:2:1:1, it can be deduced that two water
molecules coordinated to the lanthanide ion and the COOꢁ group coordi-
nate with the central metal ion through an oxygen atom in an unidentate
form, but the hydroxyl -OH does not take part in coordination with the
lanthanide(III) ion11.
The atomic ratios of the complexes also have been examined. It
was found that the complexes possess the stoichiometric ratio of
Ln:C:N:O:S:Cl ¼ 0.98:16.8:1.0:5.1:1.0:2.0 which is consistent with the ele-
mental analyses data.

626
MA, WANG, AND SHI
Figure 3. Suggested binding model of LnLCl2(H2O)2.
CONCLUSION
On the basis of the above discussions a seven-coordinated capped-
octahedral structure is suggested for the complexes as shown in Fig. 3.
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Received December 8, 2000
Accepted January 6, 2002
Referee I: J. E. Byrd
Referee II: B. L. Westcott