Synthesis and characterization of insoluble cobalt(II), nickel(II), zinc(II) and palladium(II) Schiff base complexes: Heterogeneous catalysts for oxidation of sulfides with hydrogen peroxide

Article abstract of DOI:10.1016/j.crci.2015.10.003

The condensation reaction of 1,2-bis(2'-aminophenoxy)benzene with 2-pyridinecarbaldehyde in a mole ratio of 1:2 gives a new Schiff base ligand (L). Four Schiff base complexes, CoL(NO₃)₂ (1), NiLCl₂ (2), ZnL(NO₃)₂ (3) and Pd₂LCl₄ (4) have been prepared by direct reaction of the ligand (L) and appropriate metal salts. The Schiff base ligand (L) has been characterized by IR, ¹H NMR and ¹³C NMR spectroscopy and elemental analysis. Also, all complexes have been characterized by IR and XRD spectroscopy techniques and elemental analysis. The synthesized complexes have very poor solubility in all polar and non-polar solvents such as: H₂O, MeOH, EtOH, CH₃CN, DMSO, DMF, CHCl₃, CH₂Cl₂, THF, etc; therefore, they have been used as heterogeneous catalysts. Catalytic performance of the complexes was studied in oxidation of thioanisole using hydrogen peroxide (H₂O₂) as the oxidant. Various factors including the reaction temperature, amount of oxidant and catalyst amount were optimized. The palladium Schiff base complex, Pd₂LCl₄ (4), shows better catalytic activity than other complexes. Therefore, the Pd(II) Schiff base complex has been used as a catalyst for oxidation of different sulfides to their corresponding sulfones in acetonitrile with hydrogen peroxide as the oxidant. The palladium Schiff base complex, Pd₂LCl₄ (4), has shown a very good recyclability, up to five times, without any appreciable decreases in catalytic activity and selectivity.

Full text of DOI:10.1016/j.crci.2015.10.003

C. R. Chimie xxx (2015) 1e10
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Full paper/Memoire
Synthesis and characterization of insoluble cobalt(II),
nickel(II), zinc(II) and palladium(II) Schiff base complexes:
Heterogeneous catalysts for oxidation of sulfides with
hydrogen peroxide
,
Saeid Menati a, Hadi Amiri Rudbari b *, Banafshe Askari b, Mostafa Riahi
Farsani c, Fariba Jalilian b, Ghasem Dini d
a Department of Chemistry, Khorramabad Branch, Islamic Azad University, Khorramabad, Iran
b Faculty of Chemistry, University of Isfahan, Isfahan 81746-73441, Iran
c Department of Chemistry, Islamic Azad University, Shahrekord Branch, Shahrekord, Iran
d Department of Nanotechnology Engineering, Faculty of Advanced Sciences and Technologies, University of Isfahan,
Isfahan 81746-73441, Iran
a r t i c l e i n f o
a b s t r a c t
1,2-bis(20-aminophenoxy)benzene
with
2-
Article history:
The
condensation
reaction
of
Received 5 July 2015
Accepted 15 October 2015
Available online xxxx
pyridinecarbaldehyde in a mole ratio of 1:2 gives a new Schiff base ligand (L). Four
Schiff base complexes, CoL(NO3)2 (1), NiLCl2 (2), ZnL(NO3)2 (3) and Pd2LCl4 (4) have been
prepared by direct reaction of the ligand (L) and appropriate metal salts. The Schiff base
ligand (L) has been characterized by IR, 1H NMR and 13C NMR spectroscopy and elemental
analysis. Also, all complexes have been characterized by IR and XRD spectroscopy tech-
niques and elemental analysis. The synthesized complexes have very poor solubility in all
polar and non-polar solvents such as: H2O, MeOH, EtOH, CH3CN, DMSO, DMF, CHCl3,
CH2Cl2, THF, etc; therefore, they have been used as heterogeneous catalysts. Catalytic
performance of the complexes was studied in oxidation of thioanisole using hydrogen
peroxide (H2O2) as the oxidant. Various factors including the reaction temperature,
amount of oxidant and catalyst amount were optimized. The palladium Schiff base com-
plex, Pd2LCl4 (4), shows better catalytic activity than other complexes. Therefore, the Pd(II)
Schiff base complex has been used as a catalyst for oxidation of different sulfides to their
corresponding sulfones in acetonitrile with hydrogen peroxide as the oxidant. The palla-
dium Schiff base complex, Pd2LCl4 (4), has shown a very good recyclability, up to five
times, without any appreciable decreases in catalytic activity and selectivity.
Keywords:
Schiff base
Heterogeneous
Oxidation
Hydrogen peroxide
Sulfone
ꢀ
© 2015 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.
1. Introduction
pharmaceuticals, polymer materials and ligands in asym-
metric catalysis. Also, these compounds could be used as
Oxidation of sulfides is the most straightforward
method for the synthesis of sulfoxides and sulfones [1].
Sulfoxides and sulfones are important intermediates for
oxotransfer reagents and biologically active molecules [1,2].
Many kinds of oxidants such as nitric acid, organic
peroxides, and heavy metal oxidants have been employed
as effective oxidants to form sulfoxides and/or sulfones in
high yields [1]. Among various kinds of oxidants, hydrogen
peroxide (H2O2) is one of the most straightforward, clean,
and versatile oxidants from both an environmental and
* Corresponding author.
E-mail addresses: h.a.rudbari@sci.ui.ac.ir, hamiri1358@gmail.com
(H. Amiri Rudbari).
http://dx.doi.org/10.1016/j.crci.2015.10.003
ꢀ
1631-0748/© 2015 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.
Please cite this article in press as: S. Menati, et al., Synthesis and characterization of insoluble cobalt(II), nickel(II), zinc(II) and
palladium(II) Schiff base complexes: Heterogeneous catalysts for oxidation of sulfides with hydrogen peroxide, Comptes Rendus
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2
S. Menati et al. / C. R. Chimie xxx (2015) 1e10
economic perspective because H2O2 has a high content of
active oxygen and its byproduct is water [3]. It is well
known that sulfides are oxidized to generate sulfoxides and
sulfones by H2O2 at an elevated temperature and/or with a
long reaction time without a metal catalyst [4]. However,
highly selective oxidation of sulfides under mild reaction
conditions is achieved by the application of optimized
catalysts and/or reaction systems [5].
organic/inorganic supports. Therefore, these complexes
have been used for selective heterogeneous oxidation of
sulfides in acetonitrile. The results showed high stability
and reusability in oxidation reactions.
2. Experimental
2.1. Physical methods
Schiff bases have played an important role in the
development of coordination chemistry; for example, they
readily form stable complexes with most of the transition
metals. Also, these compounds are often used as catalysts
for oxidation reactions under different conditions [6e10].
Although Schiff base complexes are often used as efficient
homogeneous catalysts for the oxidation reaction, some
problems often exist in these types of oxidation studies
such as (1) deactivation, (2) instability and (3) expensive
recycling of these homogeneous systems [11]. So, for
improving the performance of catalytic activity, scientists
tried to convert them into heterogeneous ones in some
different ways. The greatest advantage of heterogeneous
catalysis is the facile separation of catalysts from the re-
action media and products [12]. In order to fix catalysts into
heterogeneous supports, different ways can be used, clas-
sified as: (1) immobilization in zeolites, (2) grafting onto
inorganic supports such as silica, and (3) copolymerization
and attachment of the catalyst onto an organic polymer and
use of insoluble complexes as heterogeneous catalysts
without any supports [12,13]. The latter is a very good way
for synthesis of heterogeneous catalysts because of removal
of some synthesis steps. However, a major challenge is
design of insoluble catalysts with high yields, selectivity,
and cost effectiveness. According to all the above discus-
sion, insoluble Schiff base complexes with good stability
and catalytic activity are still rare and lack generality. Most
of the Schiff base complexes that are used as heterogeneous
catalysts form covalent bonds or electrostatic interactions
with organic polymers.
Infrared spectra (KBr pellets) were recorded on a JASCO,
FT/IR-6300 instrument. The elemental analysis (CHN anal-
ysis) was carried out on Leco, CHNS-932 and PerkinElmer
7300 DV elemental analyzers. Powder X-ray diffraction
(XRD) data were obtained on a D8 Advanced Bruker using
Cu K
a
radiation (2
q
¼ 5e70ꢀ). The oxidation products were
quantitatively analyzed by gas chromatography (GC) on a
Shimadzu GC-16A instrument using a 2 m column packed
with a silicon DC-200 and an FID detector. 1H and 13C NMR
spectra of the ligand were recorded on a Bruker Avance 400
spectrometer using CDCl3 as the solvent.
2.2. Reagents
All chemicals used were of analytical grade and were
used as received without any further purification and were
obtained from SigmaeAldrich. 1,2-bis(20-aminophenoxy)
benzene was prepared according to literature methods [15].
2.2.1. Synthesis of the Schiff base ligand (L)
2-Pyridinecarboxaldehyde (4 mmol) in absolute EtOH
(50 mL) was added dropwise to a boiling solution of 1,2-
bis(20-aminophenoxy)benzene (2 mmol) in absolute EtOH
(25 mL). The solution was gently refluxed for 12 h. Then, the
solvent volume was reduced by using a rotary evaporator
(~5 mL) and cooled in an ice bath for 5 h. The precipitate was
filtered off and washed with cold EtOH and dried in vacuo.
Yield: 58%. Anal. Calcd for C30H22N4O2 C, 76.4; H, 4.7; N, 11.8.
Found: C, 76.5; H, 4.8; N, 11.8%. 1H NMR dH (300 MHz,
CDCl3): 9.29 (2H, HC]N), 8.87 (2H, pyridine), 8.16 (2H,
pyridine), and 8.01e6.63 (aromatic (12H) and pyridine (4H))
ppm. 13C NMR dC (300 MHz, CDCl3): 163.51 (C]N)imi, and
150.34e115.34 (aromatic and pyridine rings) ppm. IR: 1637
This paper is a continuation of our previous study on
Schiff base ligands and complexes with ortho-aminophenyl
diamines [14]. In our previous study, we reported the
crystal and molecular structure of the precursory 1,2-
bis(20-nitrophenoxy)-4-methylbenzene prepared from an
achiral molecule (4-methylcatechol), in which the two
phenoxy groups are placed to one another in such a way
that the molecule is C1 symmetrical, i.e., chiral. The chiral
conformation of 1,2-bis(20-nitrophenoxy)-4-methylben
zene after reduction is maintained in complexation with
zinc(II) and cobalt(II). This implies that in Schiff base li-
gands derived from ortho-aminophenyl diamines, the two
(CH]N)imi, 1601 (CH]N)py, and 1488 (C]C)py
.
2.2.2. Preparation of complexes
2.2.2.1. Generalsynthesis. 2-Pyridinecarboxaldehyde (4 mmol)
in dry ethanol (50 mL) was added dropwise to a boiling
solution of 1,2-bis(20-aminophenoxy)benzene (2 mmol) in the
same solvent (25 mL). The solutionwas stirred and refluxed for
12 h and then the appropriate salt (2 mmol) in EtOH (20 mL)
was added dropwise. Upon addition, immediate precipitation
of complexes occurs. The solution was refluxed for 6 h, and
concentrated in a rotary evaporator until approximately
10e15 mL. The obtained precipitate was filtered, subsequently
washed with EtOH and then dried in air.
a
-diimine systems form an octahedral cavity having the
same chirality as that of the precursory diamine [14]. In a
continuation of this study, herein we report the synthesis
and characterization of four new Schiff base complexes,
CoL(NO3)2 (1), NiLCl2 (2), ZnL(NO3)2 (3) and Pd2LCl4 (4)
derived from 1,2-bis(20-aminophenoxy)benzene and 2-
pyridinecarbaldehyde. These complexes have very poor
solubility in all polar and non-polar solvents. This param-
eter (insolubility) causes these complexes to be suitable for
acting as heterogeneous catalysts without attachment to
2.2.2.2. Ni complex NiLCl2. Yield: (87%). Anal. Calcd for
C
30H22Cl2N4NiO2: C, 60.04; H, 3.70; N, 9.34. Found: C,
Please cite this article in press as: S. Menati, et al., Synthesis and characterization of insoluble cobalt(II), nickel(II), zinc(II) and
palladium(II) Schiff base complexes: Heterogeneous catalysts for oxidation of sulfides with hydrogen peroxide, Comptes Rendus
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S. Menati et al. / C. R. Chimie xxx (2015) 1e10
3
60.09; H, 3.74; N, 9.39%. IR: 1635 (CH]N)imi, 1595 (CH]
N)py, and 1483 (C]C)py, 404 (M ꢁ N).
spectra of the products (sulfone) are available in
Supplementary data.
2.2.2.3. Co complex CoL(NO3)2. Yield: (81%). Anal. Calcd for
3. Results and discussion
C
30H22CoN6O8: C, 55.14; H, 3.39; N, 12.86. Found: C, 55.06;
H, 3.43; N, 12.79%. IR: 1628 (CH]N)imi, 1595 (CH]N)py
1485 (C]C)py, 1456 and 1318 (NO3), 403 (M ꢁ N).
,
3.1. Characterization of Schiff base ligand and complexes
The procedure for the synthesis of the Schiff base ligand,
L, is reported in Scheme 2. The ligand was obtained by the
self-condensation reaction between 1,2-bis(20-amino-
phenoxy)benzene and 2-pyridinecarboxaldehyde with
ethanol as the solvent under reflux conditions. The chem-
ical structure of the ligand was confirmed by elemental
analysis and IR, 1H NMR and 13C NMR spectroscopy. For-
mation of the Schiff base ligand is evidenced by the pres-
2.2.2.4. Zn complex ZnL(NO3)2. Yield: (94%). Anal. Calcd for
C
30H22N6O8Zn: C, 54.60; H, 3.36; N, 12.73. Found: C, 54.65;
H, 3.41; N, 12.69%. IR: 1633 (CH]N)imi, 1595 (CH]N)py
,
1485 (C]C)py,1456 and 1323 (NO3), 407 (M ꢁ N).
2.2.2.5. Pd complex Pd2LCl4. Yield: (92%). Anal. Calcd for
C
30H22Cl4N4O2Pd2: C, 43.67; H, 2.69; N, 6.79. Found: C,
ence of a strong IR band at 1637 cmꢁ1 due to
no bands attributable to (C]O) or (NH2) have been
n(C]N), while
43.69; H, 2.68; N, 6.83%. IR: 1619 (CH]N)imi, 1587 (CH]
N)py, and 1488 (C]C)py, 412 (M ꢁ N).
n
n
detected. The bands at 1601 and 1488 cmꢁ1 of the pyridine
ring vibrations are also present in the IR spectrum of the
ligand [16]. The 1H NMR spectrum is consistent with the IR
observations. The 1H NMR spectrum in CDCl3 shows a peak
at 9.29 ppm corresponding to the imine protons of the
ligand. Cobalt(II), Nickel(II), Zinc(II) and Palladium(II) Schiff
base complexes, CoL(NO3)2 (1), NiLCl2 (2), ZnL(NO3)2 (3)
and Pd2LCl4 (4), were synthesized by the reaction of
Co(NO3)2.6H2O, NiCl2$6H2O, Zn(NO3)2$6H2O and PdCl2
with the tetradentate Schiff base ligand, L, in ethanol under
reflux conditions, respectively (Scheme 2). All of the Schiff
base complexes are stable in air. Their photographic images
are shown in Fig. 1. The bands in the range of
1619e1635 cmꢁ1 were assigned to the eCH]Ne group.
The imine peak of the complexes showed a red-shift of ca.
2e18 cmꢁ1 compared to that of the ligand, indicating the
coordination of the imine nitrogen to the metal ions
(Scheme 2) [17,18]. This feature can be explained by the
withdrawal of electrons from the nitrogen atom to the
metal ion due to the coordination process. The IR spectra
exhibit medium to strong bands at ~1595 and ~1480 cmꢁ1
as expected for the two highest energy pyridine-ring vi-
brations. The shift of the imine and pyridine bands by
complexation suggests coordination via the imine and
pyridine nitrogen atoms. Another conclusive evidence for
the formation of the MeN bond is also shown by the
appearance of a new band at around 405 cmꢁ1 which could
be assigned to the MeN stretching vibrations [19].
2.3. Catalytic activity and optimization of reaction conditions
After the synthesis of CoL(NO3)2 (1), NiLCl2 (2),
ZnL(NO3)2 (3) and Pd2LCl4 (4) Schiff base complexes, we
decided to investigate the catalytic activity of the com-
plexes in oxidation of sulfides with H2O2 as the oxidant
under different reaction conditions.
2.3.1. Oxidation of thioanisole with H2O2 catalyzed by Co, Ni, Zn
and Pd Schiff base complexes
To find the optimized conditions, we investigated the
oxidation of thioanisole as
a model substrate using
hydrogen peroxide (Scheme 1). Generally, oxidation of
thioanisole gives a sulfoxide and sulfone. In our catalytic
reaction sulfone is formed as the major product (Scheme 1).
The effect of different reaction parameters (H2O2 amount,
temperature and amount of catalyst) was studied on the
thioanisole oxidation.
2.3.2. General procedure for catalytic oxidation of sulfides in
acetonitrile
A mixture of 1 mmol sulfide and H2O2 (3 mmol) was
added to a stirring solution of CoL(NO3)2 (1), NiLCl2 (2),
ZnL(NO3)2 (3) and Pd2LCl4 (4) Schiff base complexes
(0.01 mmol) in acetonitrile (3 ml) at 50 ꢀC for the required
period of time (4 h). After completion of the reaction (TLC),
the catalyst was separated by filtration, washed three times
with acetonitrile and then dried under vacuum and used
for the next oxidation cycle. The products were analyzed by
GC using diphenyl sulfide as the internal standard. NMR
When the fundamental IR bands of the NOꢁ3 group
would be identified, it is possible to tell if the NOꢁ3 group is
ionic or coordinated. The occurrence of two strong
Scheme 1. Oxidation of thioanisole with hydrogen peroxide in the presence of CoL(NO3)2 (1), NiLCl2 (2), ZnL(NO3)2 (3) and Pd2LCl4 (4) Schiff base complexes as
catalysts.
Please cite this article in press as: S. Menati, et al., Synthesis and characterization of insoluble cobalt(II), nickel(II), zinc(II) and
palladium(II) Schiff base complexes: Heterogeneous catalysts for oxidation of sulfides with hydrogen peroxide, Comptes Rendus
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S. Menati et al. / C. R. Chimie xxx (2015) 1e10
Scheme 2. Synthetic routes for the preparation of Schiff base complexes.
absorption bands in the complexes under study, 1 and 2, at
~1450 and ~1320 cmꢁ1 is attributed to n4 and n1 modes of
vibrations of the covalently bonded nitrate groups,
respectively. This suggests that nitrate groups are present
et al. bidentate coordination involves a greater distortion
from D3h symmetry than monodentate coordination;
therefore, bidentate complexes should show a larger sep-
aration of (n1
þ
n4) [25]. By studying the spectra of a
inside the coordination sphere [20,21]. Also, if the (n4
ꢁ
n1)
number of compounds with known crystal structures, Devi
et al. [24] showed that the separation of monodentate ni-
trate groups appeared to be 5e26 cmꢁ1 and that of
bidentate groups to be 25e66 cmꢁ1. However, these ab-
sorptions are very weak and can only be used for diagnosis
when there are no other absorptions in this region. The
authors have tried to apply this method for cobalt (II) and
zinc(II) complexes. In both cases, CoL(NO3)2 (1) and
ZnL(NO3)2 (3), a separation of 20e25 cmꢁ1 in the combi-
difference is taken as an approximate measure of the
covalency of the nitrate group [22,23], a value of ~200 cmꢁ1
for the complexes studied suggests strong covalency of the
metal-nitrate bonding. The strong and sharp band at
~1384 cmꢁ1 is characteristic of ionic nitrate. As depicted in
Fig. 2, we do not observe this peak in the IR spectrum of
zinc(II) and cobalt(II) complexes.
Devi et al. have shown that the number and relative
energies of nitrate combination frequencies (
n
1 þ
n
4) in the
nation bands (n1
concludes the monodentate nitrate coordination. On the
basis of the above discussion, a six-coordinated structure is
þ
n4) in the 1700e1800 cmꢁ1 region
1700e1800 cmꢁ1 region of the infrared spectrum may be
used as an aid to distinguish the various coordination
modes of the nitrate groups [24]. According to Agarwal
Fig. 1. The photographic images of CoL(NO3)2 (1), ZnL(NO3)2 (3) and
Pd2LCl4 (4) Schiff base complexes from left to right. The color of NiLCl2 (2)
and ZnL(NO3)2 (3) Schiff base complexes are identical.
Fig. 2. FT-IR spectra of CoL(NO3)2 (1), NiLCl2 (2), ZnL(NO3)2 (3) and Pd2LCl4
(4) Schiff base complexes. Important peaks are indicated by arrows (see the
text).
Please cite this article in press as: S. Menati, et al., Synthesis and characterization of insoluble cobalt(II), nickel(II), zinc(II) and
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S. Menati et al. / C. R. Chimie xxx (2015) 1e10
5
proposed for cobalt (II), zinc(II) and nickel(II) complexes in
which the metal ions coordinate via four azomethine ni-
trogens and the two remaining coordination sites are
occupied by two oxygen atoms of two monodentate NO3ꢁ
groups in zinc and cobalt complexes and by two Cl atoms in
the nickel complex (Scheme 2). As we know, six-coordinate
structures are very common for Co(II), Ni(II) and Zn(II)
complexes. It should be noted that these structures were
confirmed in our previous work by X-ray crystallography
[14].
keeping all other parameters fixed: catalysts (5 mg), tem-
perature (50 ꢀC) and reaction time (4 h). The results are
shown in Table 1.
For all four complexes, H2O2/thioanisole molar ratios
from 1:1 to 4:1 resulted in 9%e72% conversion. When this
molar ratio was changed to 1:4, conversion increased but
sulfoxide selectivity decreased for four complexes. How-
ever, the conversion was found to decrease for thioanisole/
H2O2 molar ratios of 1:1 and 1:2. Therefore, a 1:3 molar
ratio of thioanisole/H2O2 was found to be the optimum.
Because of the great size of the palladium atom and
impossibility of the octahedral structure for palladium
complex, we suggest different coordination modes for the
Pd(II) complex. This proposed structure was confirmed by
elemental analysis. It should be noted that in our previous
work, we proved that the four azomethine nitrogen atoms
of the ligand are not able to provide a square planar
conformation when the metal atom lies in the ligand cavity
[14]. For this reason, the metal atom must be out of the
ligand cavity if a square planar configuration is assumed for
the metal ion (Scheme 2). As shown in Scheme 2, we sug-
gest a bimetallic structure for the Pd(II) complex. Each
Pd(II) atom is coordinated to one pyridine nitrogen and one
azomethine nitrogen. Two remaining coordination sites are
occupied by two Cl atoms.
3.2.2. Effect of catalyst amounts
The amount of catalyst had a considerable effect on the
conversion of thioanisole to the corresponding sulfone. The
results are given in Table 2. Four various amounts, 1, 5, 10
and 20 mg, were used for all the catalysts and thioanisole
(1 mmol), H2O2 (3 mmol), temperature (50 ꢀC) and the
reaction time (4 h) were fixed. For all complexes the lower
conversion of thioanisole into the corresponding sulfone
with 1 mg of catalysts may be as a result of fewer catalytic
sites. The maximum conversion was observed with 5 mg of
catalysts for all four complexes, but there was no consid-
erable difference in the conversion when 10 and 20 mg of
catalysts were employed. Therefore, 5 mg of catalysts were
taken as the optimum amount.
Fig. 3 shows the X-ray powder diffraction pattern of four
Schiff base complexes. As depicted in Fig. 3, the XRD pat-
terns of CoL(NO3)2 (1) NiLCl2 (2) and ZnL(NO3)2 (3) are
almost identical.
3.2.3. Effect of temperature on the reaction
In the present work for optimization of the reaction
conditions, we also studied the effect of temperature on the
catalytic performance of CoL(NO3)2 (1), NiLCl2 (2),
ZnL(NO3)2 (3) and Pd2LCl4 (4) complexes (Table 3). The
effect of temperature on the catalytic activity of the four
catalysts was studied at four different temperatures, 25 ꢀC,
50 ꢀC, 60 ꢀC and reflux (85 ꢀC) with a constant amount of
thioanisole (1 mmol), H2O2 (3 mmol) and catalyst (5 mg) in
3 mL of acetonitrile (Table 3). The maximum conversion,
77%, with 99% selectivity for sulfones was obtained when
the reaction was carried out at 85 ꢀC for the Pd complex. At
3.2. Application of Schiff base complexes for the oxidation of
sulfides
In order to optimize the reaction conditions, we sur-
veyed the oxidation reaction of thioanisole as a model
compound using 30% H2O2 under various reaction condi-
tions in terms of time and product yield.
3.2.1. Effect of H2O2 amounts on the oxidation of thioanisole
In order to determine the role of H2O2 amounts on the
oxidation of thioanisole to the corresponding sulfone and
sulfoxide, the mole ratio of H2O2/thioanisole was changed,
Table 1
Oxidation of thioanisole with various amounts of H2O2 in the presence of
Co, Ni, Zn and Pd Schiff base complexes.a
Entry
Catalyst
mmol H2O2
Conversion (%)b
Selectivity (%)c
1
2
3
4
5
6
7
8
Co
Co
Co
Co
Ni
Ni
Ni
Ni
Zn
Zn
Zn
Zn
Pd
Pd
Pd
Pd
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
9
14
19
23
24
31
36
41
27
34
39
41
51
57
70
72
60
67
71
80
54
66
70
75
57
64
69
73
80
92
99
99
9
10
11
12
13
14
15
16
a
Reaction conditions: thioanisole (1 mmol), catalyst (5 mg), acetonitrile
(3 mL) and H2O2 30% at 50 ꢀC for 4 h.
b
Fig. 3. XRD pattern of CoL(NO3)2 (1), NiLCl2 (2), ZnL(NO3)2 (3) and Pd2LCl4
(4) Schiff base complexes.
Conversion based on sulfide substrates.
Selectivity for sulfone.
c
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S. Menati et al. / C. R. Chimie xxx (2015) 1e10
Table 2
As shown in Table 4, the sulfides with less and high
steric hindrance were converted to the corresponding sul-
fones in good to excellent yields except DBT (Table 4, entry
9). It is obvious that these types of sulfides are completely
not affected under the reaction conditions. Also, the influ-
ence of electronic effects was found in the case of 4-
nitrophenyl sulfide, which has a negative effect on the re-
action time. The chemoselectivity of this catalytic system
was also investigated in the selective oxidation of sulfides
containing hydroxyl and carbonecarbon double bond
groups. These substrates selectively underwent oxidation
at the sulfur atom without undergoing further structural
changes in their functional group. For example, in the case
of allylic sulfides, epoxidation of the double bond was not
observed and only the corresponding sulfones were ob-
tained in excellent yields (Table 4, entries 6 and 7). Also, the
presence of the hydroxyl group did not interfere with the
oxidation process of the sulfide, and the desired sulfone
was obtained in excellent yield (Table 4, entry 8). It is clear
that these kinds of sulfides are completely unaffected
under the reaction conditions, indicating the good ability of
this protocol in oxidation of different types of sulfides.
The recycling properties of the Pd Schiff base complex
were investigated in methyl phenyl sulfide oxidation at
50 ꢀC using aqueous 30% H2O2 for 1 mmol of substrate.
After the completion of the first oxidation reaction of thi-
oanisole to the methyl phenyl sulfone under optimized
conditions, the catalyst was isolated by filtration, washed
with CH3CN, dried in an oven at 80 ꢀC for 2 h and reused for
the next run. Under the described conditions, the catalyst
exhibited high catalytic activity up to five times reuse
without noticeable decrease in catalytic activity (Fig. 4).
The XRD pattern and FT-IR spectrum of the recovered Pd
Schiff base complex indicated that no significant change in
the catalyst took place even after reusing five times (Figs. 5
and 6).
Oxidation of thioanisole with different amounts of catalyst in the presence
of H2O2.a
Entry Catalyst mg of catalyst Conversion (%)b Selectivity (%)c
1
2
3
4
5
6
7
8
Co
Co
Co
Co
Ni
Ni
Ni
Ni
Zn
Zn
Zn
Zn
Pd
Pd
Pd
Pd
1
5
10
20
1
7
19
21
22
16
36
38
40
19
39
42
45
36
70
70
72
45
71
65
63
51
70
64
61
53
69
66
63
79
99
80
81
5
10
20
1
9
10
11
12
13
14
15
16
5
10
20
1
5
10
20
a
Reaction conditions: thioanisole (1 mmol), acetonitrile (3 mL) and
H2O2 (3 mmol) at 50 ꢀC for 4 h.
b
Conversion based on sulfide substrates.
Selectivity for sulfone.
c
lower temperatures, the conversion was low but the
selectivity for sulfoxide was higher for all complexes. At the
reflux temperature, the initial conversion of thioanisole
was higher than the conversion at 50 ꢀC, but when the
reaction would continue the conversion of thioanisole was
almost the same as the conversion at 50 ꢀC. Therefore, 50 ꢀC
was selected as the optimum temperature and all the cat-
alytic oxidation reactions were carried out at this temper-
ature. These results showed that the Pd Schiff base
complex, Pd2LCl4 (4), under optimal reaction conditions
exhibits much better catalytic activity compared to the
other Schiff base complexes. Therefore, the applicability of
the Pd Schiff base complex was studied by oxidation of
several types of sulfides with different electronic and steric
effects to corresponding sulfones under the optimized
conditions (3 mmol H2O2, 5 mg catalyst at 50 ꢀC) (Table 4).
3.3. Comparison with other catalysts
Comparative data on the performance of various Schiff
base complexes in the oxidation of sulfides are shown in
Table 5. As you can see in most of them, the Schiff base
complexes can do the selective oxidation of sulfide to
sulfoxide. The selective oxidation of sulfides to sulfones
with Schiff base complexes as catalysts is very rare.
Table 3
Oxidation of thioanisole with different catalysts in various temperatures.a
Entry Catalyst Temperature ꢀ
C
Conversion (%)b Selectivity (%)c
1
2
3
4
5
6
7
8
Co
Co
Co
Co
Ni
Ni
Ni
Ni
Zn
Zn
Zn
Zn
Pd
Pd
Pd
Pd
25
50
60
85
25
50
60
85
25
50
60
85
25
50
60
85
7
19
24
32
12
36
38
44
15
39
44
48
34
70
73
77
25
71
76
91
31
70
76
88
30
69
73
89
53
99
99
99
1 Conte et al. reported an experimental and theoretical
study concerning the effects of steric and electronic
modification of the ligands on the catalytic activity of
salophen and salen oxo vanadium(V) complexes in the
oxidation of PhSMe with H2O2. The results indicated
that steric factors play a major role in determining the
outcome of the reaction (Table 5, entries 1e14) [26].
2 Islam et al. synthesized a new Co(III) Schiff base com-
plex, (CoL)Cl.4H2O (L ¼ Schiff base), and used this
complex as homogeneous (without support) and het-
erogeneous (with polymer support) catalysts for the
oxidation of alkenes and sulfides using H2O2 as the
oxygen source (Table 5, entries 15e18) [27]. Compari-
son between catalytic activities of the homogeneous
9
10
11
12
13
14
15
16
a
Reaction conditions: thioanisole (1 mmol), acetonitrile (3 mL) and
H2O2 (3 mmol), 5 mg catalyst for 4 h.
b
Conversion based on sulfide substrates.
Selectivity for sulfone.
c
Please cite this article in press as: S. Menati, et al., Synthesis and characterization of insoluble cobalt(II), nickel(II), zinc(II) and
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S. Menati et al. / C. R. Chimie xxx (2015) 1e10
7
Table 4
The oxidation of various sulfides with Pd Schiff base complex as the catalyst by using 30% aqueous H2O2.a
Entry
Sulfide (Substrate)
Sulfone (Product)
Time (h)
5.5
Conversion (%)b
72
Selectivity(%)c
99
1
S
O
S
O
2
4
70
99
S
O
S
O
3
4
6
73
69
99
98
O
S
S
O
6.5
S
O
S
O
NO2
NO2
5
6
4.15
6.5
74
76
99
99
O
O
S
S
S
O
S
O
7
8
9
8
75
73
21
98
99
99
O
S
S
O
O
4.5
6
S
OH
S
OH
O
O
O
S
S
a
Reaction conditions: catalyst (5 mg), substrate (1.0 mmol), H2O2 (3 mmol), CH3CN (solvent, 3 mL), at 50 ꢀC.
Conversion based on sulfide substrates.
Selectivity for sulfone.
b
c
cobalt complex and the heterogeneous complex was
done and it was shown that the polymer anchored
cobalt complex is more active than the homogeneous
complex. The active sites do not leach out from the
polymer support and thus the polymer anchored co-
balt catalyst can be reused without appreciable loss of
activity, indicating that the anchoring procedure was
effective. The reusability of this catalyst was high and
can be reused seven times without significant decrease
from its initial activity.
3 Rezaeifard et al. investigated oxidation of sulfides using
urea hydrogen peroxide (UHP) under the influence of a
tridentate Schiff base dioxo-molybdenum(VI) complex
catalyst, [MoO2(L)(CH3OH)], in ethanol under mild
Please cite this article in press as: S. Menati, et al., Synthesis and characterization of insoluble cobalt(II), nickel(II), zinc(II) and
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S. Menati et al. / C. R. Chimie xxx (2015) 1e10
Fig. 4. The recycling experiment of the Pd Schiff base complex in the
oxidation reaction of methyl phenyl sulfide to methyl phenyl sulfone under
optimized conditions.
Fig. 6. FT-IR spectrum of the Pd Schiff base complex recovered in run 5.
conditions. Relatively high stability and desired turn-
over numbers have been observed for this Mo-catalyst
in oxidation of sulfides to both sulfoxides and sulfones
(Table 5, entries 19e22) [28].
4 Barman et al. reported selective oxidation of sulfide to
sulfoxide with 30% H2O2 catalyzed by the copper(II)e
Schiff base complex. The reactions proceed under mild
conditions in acetonitrile at room temperature to
provide a variety of aryl and alkyl sulfoxides in excel-
lent yield (Table 5, entries 23e24) [10].
sustainable method using all the catalysts in the pres-
ence of H2O2. Conversions of 20e96%, and selectivities
of 98e100% for sulfones are observed with the four
compounds as catalysts (Table 5, entries 27e32). The
Zn(II) Schiff base complex shows a better catalytic ac-
tivity for the oxidation of sulfide and the Co(II) Schiff
base complex shows a lower catalytic performance
under these reaction conditions. Our study indicated
that Pd(II) Schiff base complex is able to convert thio-
anisol to the corresponding sulfone with mild conver-
sion (74%) and high selectivity (99%) (Table 5, entry 29)
[18].
5 Liu et al. presented two chiral robust porous MOFs
containing
m-oxo-bis[Fe(salen)] dimers and demon-
strated their efficient enantioselective abilities to
catalyze oxidation of sulfides to sulfoxides with com-
parable catalytic performance relative to the homoge-
neous catalysts. These two compounds were efficient
and recyclable heterogeneous catalysts for asymmetric
oxidation of sulfides to sulfoxides with an enantiose-
lectivity up to 96% (Table 5, entries 25e26) [29].
6 In our previous work, we have synthesized a new
bidentate NO donor ligand, 2-tert-butyliminomethyl-
phenol, and its Co(II), Cu(II), Zn(II) and Pd(II) complexes
[18]. Then, we have demonstrated the effectiveness of
these complexes as catalysts for the green oxidation of
sulfides to the corresponding sulfones with hydrogen
peroxide. In this system, the reactions can be carried
So, in comparison with the above catalysts, the present
Pd Schiff base catalyst, Pd2LCl4 (4), provides a desirable
catalytic activity for the oxidation of sulfides to corre-
sponding sulfone compounds than other Schiff base com-
plexes. Although the ability of this Pd(II) Schiff base
complex, Pd2LCl4 (4), in oxidation of sulfides to sulfones is
similar to that of our previously reported Pd Schiff base
complex derived from 2-tert-butyliminomethyl-phenol,
PdL2 [18], the catalyst recyclability and stability of our new
Schiff base complex, Pd2LCl4 (4), are much better than
those of our previously reported Pd Schiff base complex,
PdL2, and it can be used several times without any obvious
loss in activity.
out under solvent-free conditions as
a
green
4. Conclusion
In this study, four new Schiff base complexes, CoL(NO3)2
(1), NiLCl2 (2), ZnL(NO3)2 (3) and Pd2LCl4 (4), have been
prepared by the condensation reaction of 1,2-bis(20-ami-
nophenoxy)benzene with 2-pyridinecarbaldehyde in the
presence of Co, Ni, Zn and Pd metal ions. All complexes
have been characterized by IR and XRD spectroscopy
techniques and elemental analysis. The synthesized com-
plexes have very poor solubility in all polar and non-polar
solvents such as: H2O, MeOH, EtOH, CH3CN, DMSO, DMF,
CHCl3, CH2Cl2, THF, etc. This parameter (insolubility) causes
these complexes to be suitable for acting as heterogeneous
catalysts without any organic/inorganic support. Catalytic
performance of the complexes was studied in oxidation of
thioanisole using hydrogen peroxide (H2O2) as the oxidant.
Fig. 5. XRD pattern of the Pd Schiff base complex recovered in run 5.
Please cite this article in press as: S. Menati, et al., Synthesis and characterization of insoluble cobalt(II), nickel(II), zinc(II) and
palladium(II) Schiff base complexes: Heterogeneous catalysts for oxidation of sulfides with hydrogen peroxide, Comptes Rendus
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S. Menati et al. / C. R. Chimie xxx (2015) 1e10
9
Table 5
Catalytic activity of various catalysts for the oxidation of sulfides.
Entry Catalyst
Substrate
PhSCH3
Solvent
MeCN
Sulfoxide Conv(%)/Selec(%) Sulfone Conv(%)/Selec(%) Ref.
1
[3,30,5,50-Cl4 salophenVVO].CF3SO3
96/100
94/100
100/100
53/100
96/100
62/100
6/100
35/100
99/100
15/100
98/100
[26]
[26]
[26]
[26]
[26]
[26]
[26]
[26]
[26]
[26]
[26]
2
[5,50-Cl2 salophenVVO].CF3SO3
[salophenVVO].CF3SO3
PhSCH3
PhSCH3
PhSCH3
PhSCH3
PhSCH3
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
C2H5OH
C2H5OH
C2H5OH
C2H5OH
MeCN
MeCN
3
4
[5,50-(t-Bu)2 salophenVVO].CF3SO3
[3,30-(OMe)2 salophenVVO].CF3SO3
[5,50-(OMe)2 salophenVVO].CF3SO3
5
6
7
[3,30,5,50-(t-Bu)4 salophenVVO].CF3SO3 PhSCH3
8
[3,30,5,50-Cl4 salenVVO].CF3SO3
[5,50-Cl2 salenVVO].CF3SO3
[salenVVO].CF3SO3
PhSCH3
PhSCH3
PhSCH3
PhSCH3
PhSCH3
PhSCH3
PhSCH3
PhSPh
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
[5,50-(t-Bu)2 salenVVO].CF3SO3
[3,30-(OMe)2 salenVVO].CF3SO3
[5,50-(OMe)2 salenVVO].CF3SO3
[3,30,5,50-(t-Bu)4 salenVVO].CF3SO3
PS-TETA-Co (with polymer support)
(CoL)Cl.4H2O (without support)
PS-TETA-Co (with polymer support)
(CoL)Cl.4H2O (without support)
[MoO2(L)(CH3OH)]
>99/100
[26]
93/100
98/100
90/100
76/100
89/100
73/100
[26]
[26]
[27]
[27]
[27]
[27]
PhSPh
PhSC2H5
PhSC2H5
PhSPh
75/93
75/7
[28]
[28]
[28]
[28]
[10]
[10]
[29]
[29]
[18]
[18]
[18]
[18]
[18]
[18]
[MoO2(L)(CH3OH)]
PhSPh
95/3
95/97
92/0
[MoO2(L)(CH3OH)]
PhSCH3
PhSCH3
PhSCH3
PhSPh
92/100
100/2
[MoO2(L)(CH3OH)]
100/98
CoppereSchiff base
100/82
100/92
100/94.6
94/100
CoppereSchiff base
FeL2(OAc)
PhSCH(CH3) CH2Cl2
PhSCH(CH3) CH2Cl2
Fe(salen)-MOF
CuL2 Schiff base complex
ZnL2 Schiff base complex
PdL2 Schiff base complex
CuL2 Schiff base complex
ZnL2 Schiff base complex
PdL2 Schiff base complex
PhSCH3
PhSCH3
PhSCH3
PhSPh
Solvent-free
80/99
93/99
74/99
83/99
96/99
79/99
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
PhSPh
PhSPh
33
34
Pd Schiff base
Pd Schiff base
PhSPh
PhSCH3
MeCN
MeCN
72/99
70/99
Present work
Present work
Various factors including the reaction temperature, amount
of oxidant and catalyst amount were optimized. Among the
four Schiff base complexes, the Pd(II) Schiff base complex,
Pd2LCl4 (4), showed better catalytic activity in oxidation of
thioanisole compared to methyl phenyl sulfone. Therefore,
the Pd(II) Schiff base complex has been used as the catalyst
for oxidation of different sulfides to the corresponding
sulfones in acetonitrile using hydrogen peroxide as the
oxidant. The Pd(II) complex, Pd2LCl4 (4), showed high ac-
tivity with good selectivity for the oxidation of various
sulfides. This separable catalyst can be prepared by a very
simple procedure, by filtration, and the catalyst can be
reused for five cycles without any significant changes in its
catalytic activity and structure.
Please cite this article in press as: S. Menati, et al., Synthesis and characterization of insoluble cobalt(II), nickel(II), zinc(II) and
palladium(II) Schiff base complexes: Heterogeneous catalysts for oxidation of sulfides with hydrogen peroxide, Comptes Rendus
Chimie (2015), http://dx.doi.org/10.1016/j.crci.2015.10.003

10
S. Menati et al. / C. R. Chimie xxx (2015) 1e10
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Acknowledgments
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817e823.
Support for this research by the University of Isfahan
and Islamic Azad University (Shahrekord Branch) is
acknowledged.
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Appendix A. Supplementary data
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Please cite this article in press as: S. Menati, et al., Synthesis and characterization of insoluble cobalt(II), nickel(II), zinc(II) and
palladium(II) Schiff base complexes: Heterogeneous catalysts for oxidation of sulfides with hydrogen peroxide, Comptes Rendus
Chimie (2015), http://dx.doi.org/10.1016/j.crci.2015.10.003