Schiff base complexes of rare earth metal ions: Synthesis, characterization and catalytic activity for the oxidation of aniline and substituted anilines

Article abstract of DOI:10.1016/j.jorganchem.2013.12.014

Several new lanthanide complexes of Pr(III), Sm(III), Gd(III), Tb(III), Er(III) and Yb(III) with the sodium salt of the Schiff base, 2-[(5-bromo-2-hydroxy-benzylidene)-amino]-5-methyl-pentanoic acid, derived from leucine and 5-bromosalicylaldehyde have been synthesized. These complexes having general formula [Ln(HL)(NO₃)₂(H₂O)] ·NO₃ were characterized by elemental analysis, UV-vis., FT-IR, EPR, Mass spectrometry and Thermal analysis. The FT-IR spectral data suggested that the ligand behaves as a tridentate ligand with one nitrogen and two oxygen donor atoms, sequence towards central metal ion. From the analytical data, the stoichiometry of the complexes was found to be 1:1 (metal:ligand). The physico-chemical data suggested eight coordination number for Ln(III) Schiff base complexes. Thermal behaviour (TGA/DTA) and fluorescence nature of the complexes were also studied. The Gd(III) Schiff base complex was found to be an efficient catalyst for the oxidation of aniline and substituted anilines under mild conditions.

Full text of DOI:10.1016/j.jorganchem.2013.12.014

Journal of Organometallic Chemistry 753 (2014) 72e80
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Schiff base complexes of rare earth metal ions: Synthesis,
characterization and catalytic activity for the oxidation of aniline
and substituted anilines
,
L. Lekha b, K. Kanmani Raja c, G. Rajagopal d, D. Easwaramoorthy a
*
a Department of Chemistry, B.S. Abdur Rahman University, Vandalur, Chennai 600 048, Tamil Nadu, India
b Department of Chemistry, Saveetha School of Engineering, Thandalam, Chennai 602 105, Tamil Nadu, India
c Department of Chemistry, Government Arts College for Men (A), Nandanam, Chennai 600 035, Tamil Nadu, India
d Department of Chemistry, Madras Medical College, Chennai 600 003, Tamil Nadu, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Several new lanthanide complexes of Pr(III), Sm(III), Gd(III), Tb(III), Er(III) and Yb(III) with the sodium salt
of the Schiff base, 2-[(5-bromo-2-hydroxy-benzylidene)-amino]-5-methyl-pentanoic acid, derived from
leucine and 5-bromosalicylaldehyde have been synthesized. These complexes having general formula
[Ln(HL)(NO3)2(H2O)]$NO3 were characterized by elemental analysis, UVevis., FT-IR, EPR, Mass spec-
trometry and Thermal analysis. The FT-IR spectral data suggested that the ligand behaves as a tridentate
ligand with one nitrogen and two oxygen donor atoms, sequence towards central metal ion. From the
analytical data, the stoichiometry of the complexes was found to be 1:1 (metal:ligand). The physico-
chemical data suggested eight coordination number for Ln(III) Schiff base complexes. Thermal behav-
iour (TGA/DTA) and fluorescence nature of the complexes were also studied. The Gd(III) Schiff base
complex was found to be an efficient catalyst for the oxidation of aniline and substituted anilines under
mild conditions.
Received 5 November 2013
Received in revised form
29 November 2013
Accepted 5 December 2013
Keywords:
Schiff base
Amino acid
Lanthanide(III) Schiff base complexes
Fluorescence
Catalytic activity and oxidation of aniline
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
[11,12]. Further the lanthanide complexes were found to exhibit
extremely sharp emission [13].
Schiff base ligands have gained paramount importance due to
their versatility like straight forward synthesis, electron donor
property and multidentate nature, which results in very high
binding constants for d- and f-block metals [1,2]. The condensation
between eNH2 group of the amino acids and carbonyl group of the
aldehydes or ketones needs a special condition due to Zwitter ionic
effect of amino acids. It has been observed that pH plays a vital role
in the process of condensation [3,4]. The trivalent lanthanide ion,
complexes with strongly chelating species containing highly elec-
tronegative donor atoms, forms stable complexes [5]. Lanthanide
and lanthanide complexes have attracted a great deal of interest in
recent years because they have applications as antioxidants [6], in
medicinal inorganic chemistry [7,8], catalysis [9], luminescence
chemical probes and sensors [10] and pharmacological activities
Catalytic oxidation has gained much interest among the aca-
demic and industrial chemists [14]. Especially, oxidation of organic
substrates by inexpensive, readily abundant, terminal and envi-
ronmental friendly oxidants like aqueous H2O2, molecular oxygen
are very attractive from the view point of industrial technology and
synthetic purposes [15]. Concerning the green oxidant, hydrogen
peroxide is one of the most powerful candidates besides oxygen,
because of its high atom efficiency, and water is expected as the
only by-product to be generated from the reaction [16,17].
Catalytic oxidation of amines to their corresponding oxygen
containing derivative has attracted much attention during the past
few decades [18]. Only few examples have been reported [19,20],
among which, it was found that transition-metal-catalyzed oxida-
tive reaction of anilines to corresponding azobenzene is highly
desirable under atmospheric conditions [21].
Anilines are most widespread and principal contaminants of
industrial waste waters. These comprise an important class of
environmental contaminants and they are the building blocks for
many textile dyes, agrochemicals and other class of synthetic
chemicals. The reaction pathways of aromatic amines in natural
systems are dominated by redox reactions with soil and sediment
* Corresponding author. Tel.: þ91 44 22751347x261, þ91 9444075620 (mobile);
fax: þ91 4422750520.
E-mail
addresses:
easwaramoorthi@yahoo.com,
easwar@bsauniv.ac.in
(D. Easwaramoorthy).
0022-328X/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2013.12.014

L. Lekha et al. / Journal of Organometallic Chemistry 753 (2014) 72e80
73
constituents. Oxidation of such compounds to harmless products is
the important goal for basic research and industrial applications
[22]. Oxidation of anilines leads to the formation of azobenzene
[23], azoxybenzene [24,25], nitrobenzene [26] and nitrosobenzene
[27] by organic [28] and inorganic oxidants [23]. The product
composition in the oxygenation of amines depends on the oxidant,
catalyst and reaction condition employed. The oxidation of aniline
leading to the azo compound as a single product becomes a
research interest for chemists.
As a result of the above facts, and in view of the diversified roles
of lanthanide Schiff base metal complexes, in continuation of our
work [29,30], we have synthesized and studied the physicochem-
ical properties of lanthanide(III) complexes derived from Schiff base
ligand with N, O donor atoms. The structures of the isolated com-
plexes have been established using spectral techniques like UVe
vis., FT-IR, 1H NMR, Mass Spectrometry and EPR. The fluorescence
study and investigation on the oxidation of aniline into their cor-
responding azobenzene in the presence of rare earth metal Schiff
base complexes were made using H2O2 as an oxygen source.
room temperature. A fine yellow precipitate of the solid complex
formed was filtered off, washed with ethanolewater mixture and
stored in a vacuum desiccator over anhydrous calcium chloride. All
other Ln(III) complexes were prepared using the same procedure.
2.5. Procedure for oxidation of aniline
To a stirred solution of the catalyst (0.015 mmol) and 30% of
H2O2 (10 mmol) in CH2Cl2 (7 mL), aniline (8 mmol) was added and
the reaction mixture was stirred at room temperature for the
required time. Progress of the reaction was monitored by TLC at
regular intervals and continued for mentioned reaction time. After
removal of the solvent, the residue was purified by Silica gel flash
chromatography to afford azobenzene. All the azobenzene thus
obtained were identified by comparing 1H NMR data with values
reported in the literature and those of the authentic samples. The
desired 3,30-azotoluene was obtained (yield: 66%), for which, 1H
NMR (CDCl3, 200 MHz):
d
(ppm) ¼ 2.44 (s, 6H), 7.27e7.73 (m, 14H).
13C NMR (CDCl3, 100 MHz):
138.0, 152.4.
d
(ppm) ¼ 20.9, 119.7, 123.4, 128.7, 131.4,
2. Experimental
3. Results and discussion
2.1. Materials
In the present investigation, the Schiff base ligand and its six
new lanthanide metal complexes were synthesized and charac-
terized by different analytical techniques. Scheme 1 summarizes
the multi-step procedure leading to the target complexes. The
ligand was synthesized by the one step condensation of leucine and
5-bromosalicyaldehyde and characterized by UVevis., FT-IR, 1H
NMR, Mass and TG Analysis.
All the Ln(III) complexes were synthesized by the multi-step
procedure in which Ln(NO3)3$xH2O was used as the source of metal
and they correspond to the formula [Ln(HL)(NO3)2(H2O)]$NO3. All
the complexes were stable in air and non-hygroscopic powders,
soluble in DMSO, DCM and DMF, sparingly soluble in methanol,
ethanol, acetonitrile and ethylacetate.
All the chemicals used were of analytical grade purchased from
Aldrich chemical Company and were used without further
purification.
2.2. Physical measurements
IR spectra (KBr pellets) were recorded in the region 4000e
400 cmꢀ1 on a FT-IR spectrum BX-II spectrophotometer. 1H NMR
spectrum was recorded with a model Bruker Advance DPZ-300
spectrometer operating at 300 MHz using DMSO-d6 as a solvent
and TMS as an internal standard. Electronic spectra were recorded
on Perkin Elmer LS25 spectrophotometer using ethanol as a sol-
vent. EPR spectra were recorded on an E4-EPR spectrometer. The
measurements were taken in the X-band, on microcrystalline
powder at liquid nitrogen temperature using DPPH as standard. The
mass spectra were recorded on a GCMS-QP2010 Shimadzu mass
spectrometer with DI (Direct Inlet) and CI (Chemical Ionization).
Fluorescence spectra were measured in DMF using Perkin Elmer
LS45 Fluorescence Spectrophotometer.
3.1. Elemental analysis
Table 1 shows the list of the elemental analysis of the Schiff base
ligand and synthesized Ln(III) Schiff base complexes which are in
good agreement with the calculated values.
A mass spectrum of the newly synthesized Schiff base ligand
confirms the proposed formula by detecting the molecular frag-
ments. Mass spectrometry chemical ionization of the ligand shows
2.3. Synthesis of Schiff base ligand
The sodium salt of the Schiff base ligand was synthesized as per
the following literature procedure [31]. To an aqueous solution of
leucine (0.002 mol) in 10 mL water containing NaOH (0.002 mol), 5-
bromosalicyaldehyde (0.002 mol) in 10 mL ethanol was added drop
wise with constant stirring and heated under reflux for 3e5 h on a
mantle at 50 ꢁC. Then, the reaction mixture was cooled to room
temperature. Fine shining yellow precipitate of the Schiff base
ligand formed was filtered off, washed with ethanolewater
mixture and stored in a vacuum desiccator over anhydrous calcium
chloride.
O
HO
+
HO
O
Br
NH2
leucine
5 bromosalicylaldehyde
ethanol-water mix.
NaOH
reflux
2.4. Synthesis of [Pr(HL)(NO3)2(H2O)]$NO3 complex
Br
COONa
The rare earth Schiff base complex was synthesized as per the
following general literature procedure [31]. A 0.002 mol of a Schiff
base ligand was dissolved in 10 mL of ethanol. To this solution, a
solution of 0.002 mol Pr(NO3)3$xH2O in 10 mL water, was added
drop wise with constant stirring and finally heated under reflux for
3e5 h on a hot plate at 50 ꢁC. The reaction mixture was cooled to
Ln(NO3)3.xH2O
N
[Ln(HL)(NO3)2(H2O)].NO3.
CH
OH
Sodium salt of Schiff base ligand
Scheme 1. Synthesis of Schiff base ligand and Ln(III) complexes.
reflux

74
L. Lekha et al. / Journal of Organometallic Chemistry 753 (2014) 72e80
Table 1
Elemental analysis of the ligand and Ln (III) Schiff base complexes.
Compound
m/z
Found (calc.)%
C
Colour
Yield (%)
H
N
Schiff base ligand
336
596
605
612
613
621
628
46.45(46.65)
26.16(26.18)
25.75(25.76)
25.48(25.47)
25.40(25.40)
25.05(25.06)
24.81(24.83)
4.50(4.49)
2.85(2.85)
2.78(2.80)
2.76(2.77)
2.74(2.76)
2.74(2.73)
2.69(2.70)
4.17(4.15)
7.03(7.04)
6.93(6.93)
6.84(6.85)
6.85(6.84)
6.73(6.74)
6.65(6.68)
Bright Yellow
Yellow
Yellow
Yellow
Yellow
86
96
94
96
95
91
89
[Pr(HL)(NO3)2(H2O)]NO3
[Sm(HL)(NO3)2(H2O)]NO3
[Gd(HL)(NO3)2(H2O)]NO3
[Tb(HL)(NO3)2(H2O)]NO3
[Er(HL)(NO3)2(H2O)]NO3
[Yb(HL)(NO3)2(H2O)]NO3
Yellow
Yellow
m/z at 336. The HRMS of the complexes show the molecular ions
corresponds to the methine protons associated with two methyl
groups. Multiplets at 2.43e2.56 ppm are due to methylene
hydrogen atom associated with an azomethine group [36]. The
multiplets in 6.59e7.49 ppm region are assigned to the protons of
the benzylidene ring groups [37]. The singlet at 3.32 ppm is due to
the water impurity of the deuterated DMSO, and the strong singlet
at 8.34 ppm is assigned to the azomethine proton [38]. This is in
[M]þ at various m/z values. Some isotopic peaks are also found
d
a, b
ꢂ
around the fragment peaks [32].
d
d
3.2. Thermogravimetric analysis
d
The presence of water molecules and the thermal behaviour of
the Schiff base ligand and the complexes were investigated by
thermal analysis. Thermogravimetric curves, TGA and DTA for the
ligand and [Ln(HL)(NO3)2(H2O)]$NO3 complexes have been ob-
tained in nitrogen atmosphere between 0 and 800 ꢁC. Fig. 1 rep-
resents the TGA and DTA curves for Pr(III) Schiff base complex. The
appearance of small endothermic peak on DTA curve around 100 ꢁC
corresponds to the loss of the water molecule [33]. The corre-
sponding weight loss on TGA curve is 2.7% due to the release of
water molecule. A further increase of temperature up to 200 ꢁC,
leads to the elimination of HNO3 molecule, 28.31%. Further
enhancement of temperature up to 400 ꢁC leads to the decompo-
sition of organic moiety with the weight loss of 40.23% [34]. The
final residue of 28.76% corresponds to the Pr2O3 [35].
The thermal behaviour of the other lanthanide complexes is
similar to [Pr(HL)(NO3)2(H2O)]$NO3 which indicates that the
complexes undergo two major steep degradation steps. In the first
stage, the coordinated water molecules are eliminated followed by
the elimination of nitric acid, the second stage decomposition
corresponding to the elimination of the Schiff base ligand and
beyond 640 ꢁC the TGA curve show no change, possibly due to the
complexes being decomposed to its corresponding Ln2O3 [35].
accordance with the IR data.
3.4. Spectroscopic characterization of the Schiff ligand and the
Ln(III) complexes
The IR spectral data for the Schiff base ligand and Ln(III) com-
plexes, are listed in Table 2 and representative spectrum of the
complex is shown in Fig. 3. In order to study the binding mode of
the Schiff base ligand to the metal ion in the complexes, the IR
Spectrum of the free Schiff base ligand was compared with the
spectra of the complexes. The change in profile of stretching fre-
quencies in the complexes, while compared with those observed
for the isolated ligand confirms the coordination of the ligand with
the central metal ion. The IR spectral shifts of different complexes
are similar, indicating similar structures of the Ln(III) complexes.
The IR spectrum of the ligand shows a strong absorption band
around 1665 cmꢀ1, which may be attributed to the azomethine
group and it was found that this absorption frequency shifted to
1640 cmꢀ1 in the spectra of the complex indicates a stronger double
bond character of the imine bond and a coordination of the azo-
methine nitrogen atom to the Ln(III) ion [39,40]. This coordination is
further confirmed through the appearance of a medium intensity
band around 420 cmꢀ1 assigned to an Ln-N vibration. The IR spec-
trum of the free Schiff base ligand exhibits a broad band around
3400 cmꢀ1, which is attributed to the stretching frequency of the
aromatic hydroxyl substituent group, perturbed by intramolecular
hydrogen bonding [OeH/N] between phenolic hydrogen and
3.3. 1H NMR spectra
The 1H NMR Spectra of Schiff base ligand in DMSO-d6 is shown
in Fig. 2, which shows signal of multiplets at
equivalent to the 6H of two methyl groups. Singlet at
d
0.84e0.88 ppm
1.69 ppm
d
Fig. 1. Representative TG/DTA curves of the [Pr(HL)(NO3)2H2O]$NO3 complex.

L. Lekha et al. / Journal of Organometallic Chemistry 753 (2014) 72e80
75
Fig. 2. 1H NMR spectra of the Schiff base ligand.
Table 2
Significant infrared spectral data for the ligand and the Ln(III) complexes.
-
-
-
nsym ðCOO
Þ
Compounds
nOH
nC]
nAreO
nNO
nasym ðCOO
nLneO
nLneN
N
Þ
3
n1
n2
n3
n4
n0
Schiff base ligand
3400
3490
3485
3495
3492
3480
3498
1665
1640
1642
1640
1638
1641
1640
1260
1268
1270
1270
1268
1270
1270
e
e
e
e
e
e
e
e
e
[Pr(HL)(NO3)2(H2O)]NO3
[Sm(HL)(NO3)2(H2O)]NO3
[Gd(HL)(NO3)2(H2O)]NO3
[Er(HL)(NO3)2(H2O)]NO3
[Tb(HL)(NO3)2(H2O)]NO3
[Yb(HL)(NO3)2(H2O)]NO3
1470
1471
1470
1471
1470
1470
1028
1029
1029
1028
1028
1029
825
827
825
826
825
826
1290
1292
1290
1292
1292
1292
1383
1383
1383
1383
1383
1383
1505
1515
1505
1500
1510
1525
1380
1390
1380
1380
1390
1400
540
548
545
540
546
545
420
421
420
422
420
418
Fig. 3. IR Representative spectrum of [Yb(HL)(NO3)2H2O]$NO3complex.

76
L. Lekha et al. / Journal of Organometallic Chemistry 753 (2014) 72e80
Table 3
azomethine nitrogen atoms [41,42]. Further, the appearance of the e
OH band around 3490 cmꢀ1 in the spectra of the complexes with an
increase in intensity indicates that the hydroxyl oxygen is coordi-
nated to the Ln(III) ion without proton displacement [43]. The
appearance of band near 1260 cmꢀ1 in the free Schiff base ligand is
associated with the AreO band of the phenolic hydroxyl substituent
[44]. The shift in the stretching frequency of AreO band around
1270 cmꢀ1 in the spectra of the Ln(III) complexes suggest the coor-
dination of Ln(III) ion occurs through the oxygen atom of the hy-
droxyl benzene of the Schiff base ligand [45]. The complexes show a
strong band in the region 1500e1525 cmꢀ1 and aweaker band in the
region 1380e1400 cmꢀ1 assigned to asymmetric and symmetric
stretching frequencies of COOꢀ group respectively. The frequency
difference between the asymmetric and symmetric stretching fre-
quencies comes around 215 cmꢀ1, which suggests the monodentate
coordination of the carboxyl group of amino acid with the Ln(III) ion
[46]. This is further confirmed through the appearance of a medium
intensity band around 540 cmꢀ1, which may be assigned to LneO
vibration [47]. The infrared spectra of the Ln(III) complexes displays
UVevis. spectral data lmax (nm) for the ligand and the complexes.
Compound
lmax (nm)
ε ꢄ 104(cm2 molꢀ1
)
Band
assignment
Schiff base ligand
203,241,334
424
200,229,364
273
429
202,242,359
280
442
200,230,369
276
428
201,229,361
273
433
203,229,364
273
429
200,222,339
252
5.05,4.52,3.28
1.61
3.58,2.41,0.31
0.47
0.05
3.17,0.79,0.15
0.20
0.07
3.40,1.36,0.0
0.34
0.094
4.09,3.30,0.52
0.7
0.085
3.58,1.59,0.21
0.34
0.05
3.49,1.77,0.21
0.83
p
n /
p /
/
p
*
*
p*
[Pr(HL)(NO3)2(H2O)]NO3
p
LMCT
n / p*
p / p*
LMCT
n / p*
p / p*
LMCT
n / p*
p / p*
LMCT
n / p*
p / p*
LMCT
n / p*
p / p*
LMCT
n / p*
[Sm(HL)(NO3)2(H2O)]NO3
[Gd(HL)(NO3)2(H2O)]NO3
[Tb(HL)(NO3)2(H2O)]NO3
[Er(HL)(NO3)2(H2O)]NO3
[Yb(HL)(NO3)2(H2O)]NO3
several intense band at 1470 cmꢀ1
( ( (n3)
n1),1028 cmꢀ1 n2), 825 cmꢀ1
and 1290 cmꢀ1
(
n4), which represents the coordinated nitrate ions
407
0.071
with the central metal ion. The difference in frequency separation is
approximately 180 cmꢀ1, which suggests that the mode of coordi-
nation of NOꢀ3 ion with the central metal ion is bidentate in nature
[48,49]. The IR band at 1383 cmꢀ1 in the spectrum of the complex
indicates the existence of the free nitrate group in the coordination
sphere. The existence of medium intensity band around 1615 cmꢀ1 is
due tothe symmetric and the asymmetric OeH stretching vibrations
of inner-sphere water molecule [50]. The presence of coordinated
water is also established and supported by TG/DTA analysis of these
complexes.
transition of unpaired electron between these split levels with g ꢃ 2
and g < 2 are observed. The presence of these lines indicates the case
of the existence of a strong crystal field.
The UVevis. absorption spectra for the Schiff base ligand and the
Ln(III) complexes were carried out in DMF at room temperature.
The numerical values of the maximum absorption wavelength
(lmax) are listed in Table 3 and shown in Fig. 4. The absorption of the
ligand is characterized by four main absorption bands in the re-
Electron Paramagnetic Resonance is being a very powerful
method of studying the magnetic properties and crystal field sym-
metry of the lanthanide(III) complexes. Since, spin-lattice relaxation
times are shortened due to the very strong spineorbit coupling in
Ln(III) ion, the EPR spectra can be observed at very low temperature.
The X-band EPR spectrum of the [Gd(HL)(NO3)2(H2O)]$NO3 complex
exhibits the following lines with effective g values of 4.55, 2.12 and
1.64, which represents gx s gy s gz; shows the rhombic spectral
nature of [Gd(HL)(NO3)2(H2O)]$NO3 complex [51]. As a result, the
gions 200e500 nm. The band at the lmax ¼ 203 nm and at 241 nm is
attributed to
p / p* transition state [52,53]. The band at the
lmax ¼ 334 nm corresponds to the
azomethine group and the lmax ¼ 424 nm is attributed to the
n / * transition state associated with the azomethine group with
p / p* transition state of the
p
an intra-molecular charge transfer [44].
The UV spectra of the complexes generally show the charac-
teristic bands of the free ligands with some changes both in
Fig. 4. UV spectra of the Schiff base ligand and Ln(III) Schiff base complexes in DMF solution (1.0 ꢄ 10ꢀ6 M) at room temperature.

L. Lekha et al. / Journal of Organometallic Chemistry 753 (2014) 72e80
77
frequencies and intensities. These modifications of the shifts and
the intensity of the absorption bands confirmed the coordination of
the ligand to the metal ion. The UVevis. absorption spectra of all
the Ln(III) Schiff base complexes show similarities, which indicates
similarity in their structures. In general, the lanthanide ions do not
appreciably contribute to the spectra of their complexes, since fef
transitions are Laporte-forbidden and very weak in nature [54]. The
absorption bands around 200, 330, 420 nm are slightly shifted to
lower frequencies upon complexation. The appearance of a new
band around 270 nm, being absent in the ligand gives further evi-
dence for the complexation. The band at 273 nm is attributed to the
ligand metal charge transfer [55] and the one around 360 nm may
lanthanide ions [58]. Besides, the direct coordination of an organic
ligand to a lanthanide ion can further improve the energy-transfer
rates by reducing the distance between the ligand and the
lanthanide ion [59,60]. This energy excites the ligand to the excited
singlet (S1) state, followed by an energy migration via intersystem
crossing from the S1 state to a ligand triplet (T) state. The energy is
then transferred from the triplet state of the ligand to a resonance
state of the coordinated lanthanide ion without radiation, which in
turn undergoes subsequent emission in the visible region [61e64].
Inspection of emission spectra at lexc. 395 nm exhibits the
characteristic emission spectra of Sm(III) and Tb(III) complexes.
This indicates the coordination nature of the ligand with rare earth
metal (III) ions for absorption and transfer of energy. The sensitized
emission spectra of Sm(III) Schiff base complexes display two
luminescence bands at 565 nm and 600 nm corresponding to
assigned to the
p / p* transition state of the azomethine group
coupled with charge transfer from ligand to central metal ion [56].
Photophysical studies of the ligand and its rare earth metal
complexes have been explored in this article. Fig. 5 gives the
emission spectrum of the Samarium and Terbium Schiff base
complexes in DMF solution at room temperature. Fluorescence
properties refer to any process, which increases, decreases the
fluorescence intensity or a shift of the maximum absorption of a
sample. These include excited state reaction, molecular rear-
rangement, energy transfer, ground state complex formation and
quenching due to collision [34].
The emission spectra of the complexes were recorded upon the
excitation at 395 nm. The ligand present fluorescence band at
485 nm. The fluorescence spectra of the complexes present bands
that indicate the formation of complexes and fluorescence property
of the metallic ions. The strongest emission was located around
the 4G5/2 / 6H5/2 and 4G5/2 / 6H7/2 respectively and a shoulder at
4
642 nm is due to G5/2
/
6H9/2. Similarly, the Tb(III) Schiff base
complexes also display two fluorescence bands around 556 nm and
590 nm, this may attributed to 5D4 7F5 and 5D4 7F4 respec-
tively and a shoulder at 624 nm due to 5D4 7F3. This indicates
/
/
/
that the energy-transfer chain is successfully completed from the
excitation of the ligand to the emission of the coordinated Sm(III)
and Tb(III) central metal ions [52]. The appearance of small band at
840 nm denotes the secondary derivatives of the rare earth Schiff
base complexes.
3.5. Catalytic oxidation of aniline
485 nm for the free ligand, may be attributed to
p / p* transition
The oxidation of aniline and its derivatives to the corresponding
azo compound have been carried out smoothly in an almost
quantitative yield using H2O2 as an oxidant and rare earth metal
Schiff base complexes as catalyst.
state, which shifts upon the complexation. Due to the shielding of
4f orbitals from the environment by an outer shell of 5s and 5p
orbitals, the fef absorption bands are very narrow, which causes
the minimally perturbed spectral properties of the lanthanide ions
by the external field generated by the ligands [57]. An organic
ligand is necessary to improve the quantum yield of luminescent
emission for the lanthanide ions, since the absorption coefficients
of the ligand are many orders of magnitude larger than the
intrinsically low molar absorption coefficients of the trivalent
Table 4 gives the results of aniline oxidation into their corre-
sponding azobenzene as the product. All the reactions were carried
out at room temperature, by using 30% H2O2 as an oxidant com-
bined with the catalytic amount of rare earth metal Schiff base
complexes. The azobenzene was produced in good yield at room
temperature in CH2Cl2 with the reaction time less than an hour.
Initially, the reaction was carried out without catalyst (Table 4,
entry 1) and enhanced amount of oxidant, it proceeds with the
formation of product with poor yield and greater reaction time.
Then six types of catalyst were tested (Table 4, entries 2e7), 70e90%
yields of azobenzene with excellent conversions were obtained. To
our surprise, among the various catalyst used, the conversion of
aniline to corresponding azobenzene in the presence of
[Gd(HL)(NO3)2(H2O)]NO3 with 10 mmol of H2O2, results 85% of yield
within 15 min of reaction time at reflux (Table 4, entry 3). Elevation
of reaction temperature has no effect on the aniline oxidation with
the above Gd(III) Schiff base complex as a catalyst. The other tested
catalyst also gave moderate to good yield of oxidation product
(Table 4, entries 4e6). Although, the [Sm(HL)(NO3)2(H2O)]NO3 and
Table 4
Oxidation of aniline catalyzed by various Ln(III) complexes in CH2Cl2 at RT.
Entry
Catalyst
Oxidant
Yield
(%)
Reaction
time (min)
1
e
Enhanced
conc. H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
e
30
180
2
3
4
5
6
7
8
[Sm (HL) (NO3)2(H2O)]NO3
[Gd (HL) (NO3)2(H2O)]NO3
[Pr (HL) (NO3)2(H2O)]NO3
[Eu (HL) (NO3)2(H2O)]NO3
[Tb (HL) (NO3)2(H2O)]NO3
[Yb (HL) (NO3)2(H2O)]NO3
[Gd (HL) (NO3)2(H2O)]NO3
90
85
70
80
75
90
e
30
15
25
30
25
Fig. 5. Emission spectra of Sm(III) and Tb(III) Schiff base complexes in DMF solution
60
240
(1.0
ꢄ
10ꢀ6 M) at room temperature. Emission spectrum is obtained with
lexc. ¼ 395 nm.

78
L. Lekha et al. / Journal of Organometallic Chemistry 753 (2014) 72e80
Table 5
Oxidation of aniline and substituted anilines catalyzed by Gd(III) complex in CH2Cl2 at RT.
Entry Anilines
Product
Yield (%)
NH2
N N
1
85
Diphenyl-diazene
N N
Phenylamine
NH2
2
88
H3C
CH3
CH3
Di-p-tolyl-diazene
p-Tolylamine
NH2
N N
3
30
Cl
Cl
Cl
Bis-(4-chloro-phenyl)-diazene
N N
4-Chloro-phenylamine
O2N
NH2
4
12
O2N
NO2
Bis-(4-nitro-phenyl)-diazene
4-Nitro-phenylamine
NH2
N
N
OCH3
H3CO
5
88
OCH3
Bis-(4-methoxy-phenyl)-diazene
4-Methoxy-phenylamine
O2N
N N
NO2
H2N
NH2
6
7
60
62
Bis-(3-nitro-phenyl)-diazene
m-phenylenediamine
NH2
H3CO
OCH3
N N
OCH3
Bis-(3-methoxy-phenyl)-diazene
3-Methoxy-phenylamine
Cl
NH2
Cl
N
N
8
20
Cl
3-Chloro-phenylamine
Bis-(3-chloro-phenyl)-diazene

L. Lekha et al. / Journal of Organometallic Chemistry 753 (2014) 72e80
79
Table 5 (continued )
Entry
Anilines
Product
Yield (%)
NH2
N N
9
66
H3C
CH3
CH3
m-Tolylamine
Di-m-tolyl-diazene
Cl
Cl
NH2
10
11
12
25
N N
Cl
2-Chloro-phenylamine
Bis-(2-chloro-phenyl)-diazene
OMe MeO
NH2
N N
OMe
2-Methoxy-phenylamine
Bis-(2-methoxy-phenyl)-diazene
HO
O
NH2 OH
O
N
N
12
12
HO
azodibenzoicacid
O
Amino-benzoic acid
[Yb(HL)(NO3)2(H2O)]NO3 complexes gave the good yield of the
product (90%), the reaction time (30 min and 60 min respectively)
was the fact that carries less importance in the purview of industrial
and academics. The effect of catalyst without oxidant (Table 4, entry
8) was also tested. No starting material was converted into the
product in the absence of H2O2. After these testing,
[Gd(HL)(NO3)2(H2O)]NO3 was chosen as the catalyst and 10 mL of
H2O2 as an oxidant as the best reaction conditions.
simple and general procedure for the preparation of azo derivatives
that is difficult to prepare by conventional methods. The result
clearly demonstrates that this oxidation system is more efficient in
the oxidation of aniline to azobenzene with H2O2 as an oxidant and
[Gd(HL)(NO3)2(H2O)]$NO3 complex as a catalyst.
4. Conclusion
To our delight 85% of azobenzene was produced in CH2Cl2 with
total conversion of staring material and only 2 mol % of catalyst
were utilized for the oxidation purpose. With the best reaction
condition (Table 4, entry 3) the experiment was carried out to check
the generality and efficiency of this methodology with the various
kinds of substituted anilines are shown in Table 5. Under the
optimized conditions, aniline (entry 1) and various substituted
anilines were oxidized to its corresponding products in good to
excellent yields. The oxidative reaction of aromatic aniline with an
electron withdrawing substituted group such as nitro, acid and
chloro group on the phenyl ring proceeds with the yield of trace
amount (entries 3, 4, 8, 10 and 12) under the reaction conditions.
This was suspected due to the low electron density of phenyl ring.
Further, among the various ortho, meta and para substituted ani-
lines, para substituted anilines results excellent yield of the
oxidative products (entries 2 and 5) while compared with meta
substituted aniline, which results good yield of the desired product
(entries 6, 7 and 9) and ortho substituted aniline, which results a
moderate yield of the desired product (entry 11). The desired
products were characterized by 1H NMR.
In this work the tridentate Schiff base ligand and its Ln(III)
complexes of the type [Ln(HL)(NO3)2(H2O)]$NO3 were synthesized
and characterized by elemental analysis, TGA/DTA, spectral analysis
like 1H NMR, UV, FT-IR, EPR and Mass Spectrometry. These
analytical and spectral data reveal that the ligand coordinate to the
central Ln(III) ion by its phenolic oxygen, azomethine nitrogen and
the carboxylic oxygen with 1:1 stoichiometry and their coordina-
tion number is eight. The samarium and terbium complexes pre-
sent interesting fluorescence properties. The newly prepared Ln(III)
complexes possesses good catalytic ability for oxidation of aniline
under mild conditions and [Gd(HL)(NO3)2(H2O)]$NO3 complex was
utilized to check the generality and efficiency of oxidation of
various substituted anilines.
Acknowledgements
The authors thank the authorities of B.S. Abdur Rahman Uni-
versity, Chennai 600048, The Dean, Madras Medical College, Gov-
ernment Rajivgandhi General Hospital, Chennai 600003, Saveetha
School of Engineering, Chennai and Department of Chemistry,
Government Arts College for Men (A), Nandanam, Chennai 600 035,
for providing research facilities and moral support.
In summary, a general rare earth metal Schiff base catalyzed
oxidation of aniline and substituted aniline to the corresponding
azo compounds have been developed. The reaction proceeds at
room temperature and use H2O2 as the green oxidant, provides a

80
L. Lekha et al. / Journal of Organometallic Chemistry 753 (2014) 72e80
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