Cu(II)-Schiff base complex-functionalized magnetic Fe3O4 nanoparticles: A heterogeneous catalyst for various oxidation reactions

Article abstract of DOI:10.1002/aoc.3266

Cu(II)-Schiff base complex-functionalized magnetic Fe₃O₄ nanoparticles were prepared and characterized using Fourier transform infrared spectroscopy, thermogravimetric analysis and scanning electron microscopy techniques. This compound acts as a highly active and selective catalyst for the oxidation of sulfides and thiols. These reactions can be carried out in ethanol or solvent-free conditions in the presence of hydrogen peroxide with complete selectivity and very high conversion under mild reaction conditions. The designed catalytic system prevents effectively the over-oxidation of sulfides to sulfones. Separation and recycling can also be easily done using a simple magnetic separation process.

Full text of DOI:10.1002/aoc.3266

Full Paper
Received: 18 October 2014
Revised: 26 November 2014
Accepted: 28 November 2014
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/aoc.3266
Cu(II)–Schiff base complex-functionalized
magnetic Fe3O4 nanoparticles: a heterogeneous
catalyst for various oxidation reactions
Arash Ghorbani-Choghamarani*, Zahra Darvishnejad
and Masoomeh Norouzi
Cu(II)–Schiff base complex-functionalized magnetic Fe3O4 nanoparticles were prepared and characterized using Fourier transform
infrared spectroscopy, thermogravimetric analysis and scanning electron microscopy techniques. This compound acts as a highly
active and selective catalyst for the oxidation of sulfides and thiols. These reactions can be carried out in ethanol or solvent-free
conditions in the presence of hydrogen peroxide with complete selectivity and very high conversion under mild reaction condi-
tions. The designed catalytic system prevents effectively the over-oxidation of sulfides to sulfones. Separation and recycling
can also be easily done using a simple magnetic separation process. Copyright © 2015 John Wiley & Sons, Ltd.
Keywords: magnetic Fe3O4; nanoparticles; hydrogen peroxide; oxidation; thiol; sulfide
The oxidation reaction of organic sulfides is also of potential use
Introduction
in the decontamination of toxic chemicals and facile electron trans-
fer reactions.[20,21] Metal–salen complexes (metal = Mn, Cr, Fe, Ru,
In recent years, reactions catalysed by chiral salen-based transi-
tion metal complexes have become a matter of interest.[1] Stable
Co, V and Ti) have been used extensively as efficient homogeneous
complexes are easily formed with most transition metal ions with
different oxidation states. In many cases, metal–salen complexes
have been immobilized on different materials, such as polymers[2]
or inorganic solids such as silica[3] and zeolites.[4] Interest in them
is related to the fact that they perform highly selective processes
in a multi-phase system, where the catalysts can be easily
recovered.[5]
In this regard, magnetic nanoparticle-supported catalysts have
been efficiently employed as heterogeneous catalysts. The main
advantage of a catalytic system based on magnetic nanoparticles
is that the nanoparticles can be efficiently isolated from the
product solution through a simple magnetic separation process
after completion of the reactions.[6] Additionally, magnetic
nanoparticle-supported catalysts also show high dispersion and
reactivity with a high degree of chemical stability. Because of these
advantages of magnetic nanoparticles over other supporting
materials, various catalysts and ligands have been immobilized
on these particles.[7]
catalysts for the selective oxidation of organic sulfides and
sulfoxides.[22–28]
Disulfides play interesting roles in both biological (DNA cleavage
properties and stabilization of peptides in proteins) and
chemical[29] (protecting groups, vulcanizing agents, and oils for rub-
ber and elastomers) processes. The easy interconversion between
thiols and disulfides and the higher stability of the latter mean that
disulfides are frequently used as a source for thiols.[30]
Thiols are among the functional groups which can be easily
over-oxidized; therefore, extensive studies have been carried out
for their controlled oxidation with molecular oxygen,[31] peroxide[32]
and metal oxidants.[33] Methods using iodine/hydrogen
iodide,[34] bromine,[35] potassium dichromate[36] and potassium
permanganate/copper(II) sulfate[37] are also known for performing
this oxidative transformation.[38]
Here we report copper(II)-coordinated magnetic nanoparticles as
a catalyst for the oxidation of a wide range of organic compounds
in the presence of H2O2. The magnetic catalyst could be readily sep-
arated from the reaction media by simple magnetic decantation,
and could be reused without significant degradation in activity
and to minimize environmental damage.
So, magnetic catalytic systems have been introduced as very
powerful, clean, recoverable supports for a variety of catalytic reac-
tions. The use of nanoparticles as catalyst support has some disad-
vantages. One of them is their highly active surface which leads to
the agglomeration of the catalyst particles. Coating the catalyst sur-
face with an organic or an inorganic shell is an appropriate strategy
to prevent agglomeration.[8–12]
The selective oxidation of organic molecules is fundamentally
vital in life sciences and immensely useful in industry.[13] Selective
oxidation of sulfides to sulfoxides and sulfones under adjusted reac-
tion conditions is an important process in the decontamination of
sulfur mustard (a chemical warfare agent)[14–18] and in the produc-
tion of organosulfurous compounds of medicinal importance.[19]
*
Correspondence to: Arash Ghorbani-Choghamarani, Department of Chemistry,
Faculty of Science, Ilam University, PO Box 69315516, Ilam, Iran. E-mail: a.
ghorbani@mail.ilam.ac.ir
Department of Chemistry, Faculty of Science, Ilam University, PO Box 69315516,
Ilam, Iran
Appl. Organometal. Chem. (2015)
Copyright © 2015 John Wiley & Sons, Ltd.

A. Ghorbani-Choghamarani et al.
accelerating voltage of 200 kV. FT-IR measurements were per-
formed using KBr discs with a Thermo IR-100 infrared spectrom-
eter (Nicolet). Thermogravimetric analysis (TGA) curves were
recorded using a PL-STA 1500 device (Thermal Sciences).
Preparation of Magnetic Fe3O4 Nanoparticles
FeCl3ꢀ6H2O (5.838 g, 0.0216 mol) and FeCl2ꢀ4H2O (2.147 g,
0.0108 mol) were dissolved in 100 ml of deionized water in a
three-necked flask (250 ml) under nitrogen atmosphere. Thereaf-
ter, under rapid mechanical stirring, 10 ml of NH3 solution (32%)
was added into the solution within 30 min. After being rapidly
stirred for 30 min, the resultant black dispersion was heated to
80 °C for 30 min. The black precipitate formed was isolated by
magnetic decantation, washed with double-distilled water until
neutrality, and further washed twice with ethanol and dried at
80°C in vacuum.
Preparation of Schiff Base Complex of Metal Ions
The Schiff base was prepared as follows. To a solution of 3-
aminopropyl(triethoxy)silane (1mmol, 0.176g) in 25 ml of etha-
nol was added a solution of salicylaldehyde (1mmol, 0.122 g) in
ethanol (5 ml) at room temperature. The colour of the reaction
solution changed to yellowish, due to imine formation. The
resulting Schiff base ligand, as a bright yellow precipitate, was
separated by filtration. This yellow precipitate was collected and
was washed with ethanol. The precipitate was dried under vacuum
at room temperature. The crude product was recrystallized from
ethanol to obtain the pure product.
Scheme 1. Synthesis of Fe3O4–Schiff base of Cu(II).
The Schiff base complex of metal ions was prepared as follows. A
solution of the corresponding Schiff base (2 mmol) in ethanol and
anhydrous Cu(NO3)2 (0.178 g, 1 mmol) were mixed at room temper-
ature and allowed to stand for 4 h. A colour change from yellow to
light green was observed. Then the complex was filtered and was
washed with a large excess of ethanol. The greenish blue precipi-
tate was collected and dried under vacuum at room temperature.
Then the product was purified by recrystallization from ethanol
affording the pure Schiff base complex.
Preparation of Fe3O4–Schiff Base of Cu(II)
The copper(II) complex was prepared by adding an ethanol sus-
pension of magnetic nanoparticles (2g) to a vigorously stirred
Figure 1. FT-IR spectra of (a) Fe3O4 nanoparticles and (b) Fe3O4–Schiff base
of Cu(II).
Experimental
Materials
Chemicals and solvents used in this work were obtained from
Sigma-Aldrich, Fluka or Merck and were used without further
purification.
Instrumentation
The particle size and morphology were investigated using a JEOL
JEM-2010 scanning electron microscopy (SEM) instrument, at an
Figure 2. TGA curves of (a) Fe3O4 and (b) Fe3O4–Schiff base of Cu(II).
wileyonlinelibrary.com/journal/aoc
Copyright © 2015 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. (2015)

Cu(II)-Schiff base-Fe3O4 nanoparticles in various oxidation reactions
Scheme 2. Oxidation of sulfides to the corresponding sulfoxides using
H2O2 catalyzed by Fe3O4-Cu(II)-Schiff base complex.
mixture with stirring. The conversion and selectivity were deter-
mined using TLC analysis.
Oxidation of thiols
Figure 3. SEM images of Fe3O4–Schiff base of Cu(II) at different
magnifications.
Cu(II)–Schiff base complex-functionalized magnetic Fe3O4 nanopar-
ticles (0.01 mmol) were suspended in a solution of a thiol (1 mmol)
in ethanol (5 ml). Then H2O2 (0.4 ml) added to the reaction mixture .The
mixture was stirred at room temperature to achieve pure products.
Schiff complex of metal ions (1 mmol). The resulting mixture was
refluxed for 4 h. The solvent was removed and the resulting solid
was dried at 80 °C overnight. The FT-IR spectrum of the catalyst
showed the expected bands, including a distinctive band due to
C¼N stretching.
Results and Discussion
Characterization of Cu(II)–Schiff Base Complex-functionalized
Magnetic Fe3O4 Nanoparticles
Catalytic Oxidation of Organic Compounds
Scheme 1 shows the preparation route of the functionalized Fe3O4
nanoparticles. In this scheme, the most probable modes for coordi-
nation of Cu to imine-modified magnetic nanoparticles are shown.
First, imines are typically prepared by the condensation of primary
Oxidation of sulfides
Cu(II)–Schiff base complex-functionalized magnetic Fe3O4 nanopar-
ticles (0.02 mmol) were added to sulfides (1mmol) under solvent-
free conditions. Then the mixture was allowed to equilibrate at a
reaction temperature of 40 °C. Then H2O2 (0.4ml) added to the
Table 2. Oxidation of sulfides to corresponding sulfoxides using
Fe3O4–Cu(II)–Schiff base complexa
Substrate Product Time (min) Yield (%)b
M.p. (°C)
Oil
Ref.
[14]
Entry
Table 1. Optimization of the amount of Fe3O4–Schiff base of Cu(II)a
1a
2a
60
98
1
2
1b
1c
1d
1e
1f
2b
2c
2d
2e
2f
35
45
99
90
48
90
96
99
93
96
90
97
Solid (white) [14]
Entry
Catalyst
0
Yield (%)b
3
Oil
[14]
[14]
[14]
c
—
4
1440
45
66–69
Oil
1
2
5
0.005
0.01
42
52
65
97
97
6
20
Oil
3
4
5
6
7
1g
1h
1i
2g
2h
2i
180
60
127–129
Oil
[14]
[15]
[15]
[14]
[16]
0.015
0.02
8
9
45
Oil
0.03
10
11
1j
2j
10
Oil
aReaction conditions: methylphenyl sulfide (1 mmol), H2O2 (0.4 ml,
1 mmol) and solvent free at 40 °C.
1k
2k
10
Oil
bIsolated yields.
aSulfide/Fe3O4–Schiff base of Cu(II)/H2O2 (1 mmol/0.02 g/0.4 ml).
bIsolated yields.
cReaction proceeds in the absence of the catalyst.
Appl. Organometal. Chem. (2015)
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A. Ghorbani-Choghamarani et al.
well-formed, spherical particles having rather uniform coating of
organic layers.
Application of Fe3O4–Cu(II)–Schiff Base Complex in Oxidation
Reaction
We investigated the catalytic activity of Fe3O4–Schiff base of Cu(II)
for oxidation of organic substrates such as sulfides and thiols. First,
the reaction parameters such as catalyst amount, kind of solvent
and effect of temperature were optimized in the oxidation reaction
of sulfides to sulfoxides.
In order to optimize the catalyst amount, various quantities of
catalyst were used in the oxidation of methylphenyl sulfide
(1mmol) with H2O2 (0.4 ml). For oxidation of methylphenyl sulfide
in the absence of catalyst, no formation of product occurs after
60min (Table 1, entry 1). To improve the yield and optimize the re-
action conditions, the same reaction was carried out in the pres-
ence of a catalytic amount of Fe3O4–Cu(II)–Schiff base complex
under similar conditions. Surprisingly, a significant improvement
is observed and the yield of methylphenyl sulfoxide is markedly in-
creased to 42% after stirring; the mixture was stirred for only 60 min
(Table 1, entry 2). An increase in the quantity of Fe3O4–Schiff base of
Cu(II) from 0.005 to 0.02 g increases the product yield slightly from
42 to 97% (Table 1, entry 5). Using more than 0.02 g of Fe3O4–Schiff
base of Cu(II) has less effect on the yield and time of the reaction.
Scheme 3. Oxidative coupling of thiols to the corresponding disulfides
using H2O2 catalyzed by Fe3O4-Cu(II)-Schiff base complex.
amines and aldehydes. Then, the copper(II) complex is reacted with
the magnetic nanoparticles to obtain the amine-modified magnetic
nanoparticles. The prepared catalyst was characterized using FT-IR
spectroscopy, TGA and SEM.
Table 3. Oxidation of thiols to corresponding disulfides using Cu(II)–
Schiff base complex-functionalized magnetic Fe3O4 nanoparticlesa
Substrate Product Time (min) Yield (%)b M.p. (°C) Ref.
Entry
FT-IR spectra of the Fe3O4 nanoparticles and Fe3O4–Schiff base of
Cu(II) are shown in Figs 1(a) and (b), respectively. The strong absorp-
tion at 580 cmꢁ1 is characteristic of the Fe–O stretching vibration but
for Fe3O4 nanoparticles is at 572 cmꢁ1 as a blue shift, due to the size
reduction. The broad feature at 3500–3000 cmꢁ1 for magnetic nano-
particles corresponds to the absorption of –OH stretching of hydroxyl
groups. In the spectrum of Fe3O4–Schiff base of Cu(II) in Fig. 1(b), the
band appearing at 1620 cmꢁ1 is due to imine (mainly ascribed to the
C¼N stretching band). The spectra display a number of absorption
peaks ascribed to 3-aminopropyl(triethoxy)silane. The absorptions
at around 1111 and 1001 cmꢁ1 correspond to Si–O stretching vibra-
tion. The FT-IR spectra have a broad band at 3394 cmꢁ1, representing
bonded –NH2 groups. The band observed at about 2854–2922 cmꢁ1
can be assigned to the aliphatic C–H group.
TGA was performed and the profiles obtained are exhibited in
Fig. 2. The profile of magnetic nanoparticles shows a very small
weight loss at about 126 °C owing to the removal of physically
adsorbed solvent and surface hydroxyl groups (Fig. 2(a)). In addi-
tion, there is a weight loss of about 2.5% for the magnetic nanopar-
ticles in the temperature range between 260 and 355 °C which may
be associated with the thermal crystal phase transformation from
Fe3O4 to γ-Fe2O3.
3a
4a
95
99
174–176 [39]
1
2
3b
3c
3d
3e
3f
4b
4c
4d
4e
4f
10
60
99
99
99
53
93.4
98
99
92
99
99
36–38
56
[38]
[39]
3
4
5
161–165
87–91
Oil
5
10
[39]
[40]
[38]
[40]
[38]
6
10
7
3g
3h
3i
4g
4h
4i
120
10
86–90
87–89
Oil
8
9
10
10
11
3j
4j
120
120
278–284
3k
4k
134–136 [38]
aThiol/ Fe3O4-Schiff base of Cu(II)/ H2O2 (1 mmol/ 0.01 g/ 0.4 ml).
bIsolated yields.
The TGA curve of Fe3O4–Schiff base of Cu(II) (Fig. 2(b)) initially
shows a very similar weight loss of 4.54% between 80 and 110°C,
which corresponds to removal of water molecules. Fe3O4–Schiff
base of Cu(II) shows a weight loss of 21.56% between 110 and
480 °C which is similar to the decomposition of organic chemicals.
SEM images of the Fe3O4–Cu(II)–Schiff base complex at different
magnifications are shown in Fig. 3. Fe3O4–Schiff base of Cu(II) ex-
hibits a cluster of aggregated spherical particles with an average
size larger than 70–80 nm in diameter. The surface morphology of
the supported Schiff base complex of Cu(II) is practically identical
to that of supported molecular sieves and is composed of relatively
Figure 4. Reuse of the nanocatalyst in the oxidation of 4-methylthiophenol
to 1,2-di-p-tolyldisulfane.
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Appl. Organometal. Chem. (2015)

Cu(II)-Schiff base-Fe3O4 nanoparticles in various oxidation reactions
Table 4. Comparison of Fe3O4–Schiff base of Cu(II)/H2O2 for oxidation of methylphenyl sulfide and 4-methylbenzenethiol with previously reported
procedures
Entry
Substrate
Catalyst
Time (min)
60
Yield (%)a
98
Ref.
Methylphenyl sulfide
Fe3O4–Cu(II)–Schiff base complex/H2O2
This work
1
2
Methylphenyl sulfide
Methylphenyl sulfide
Methylphenyl sulfide
Methylphenyl sulfide
Methylphenyl sulfide
4-Methylbenzenethiol
4-Methylbenzenethiol
4-Methylbenzenethiol
4-Methylbenzenethiol
4-Methylbenzenethiol
4-Methylbenzenethiol
Thiourea dioxide/TBHP
Alumina-supported nanoruthenium/H2O2
2NaBO3ꢀ4H2O(I)/KBr
210
120
120
300
150
10
93
92
57
35
90
99
99
85
82
91
84
[42]
3
[28]
4
[43]
5
Clay-supported ceric ammonium nitrate (CAN)/O2
SiO2–W2–Py/H2O2
[44]
6
[45]
7
Fe3O4–Cu(II)–Schiff base complex/H2O2
NaI/H2O2
This work
[37]
8
30
9
Fe(NO3)3ꢀ9H2O/Fe(HSO4)3
40
[45]
10
11
12
(CH3)4 N+CrO3Fꢁ
75
[46]
Fe(TPP)Cl/UHP
(C2H5)3NH+[CrO3F]ꢁ
10
[47]
115
[48]
aIsolated yield.
In order to optimize reaction conditions, methylphenyl sulfide
was subjected to oxidation in various solvents but the reaction
did not complete after 24h under these conditions. Eventually,
we decided to carry out the oxidation reaction in the absence of
solvent (i.e. solvent-free conditions) for all reactions. Interestingly
we observe that the oxidation reaction of methylphenyl sulfide
completes within 60 min in 97% yield.
In order to investigate the efficiency of this procedure in compar-
ison with known methods reported in the literature, the results for
the preparation of (methylsulfinyl)benzene (Table 4, entries 1–6)
and 1,2-di-p-tolyldisulfane (entries 7–12), as representative examples,
are compared with the best of well-known data from the literature.
Conclusions
In the next step, the influence of temperature on methylphenyl
sulfide conversion and product selectivity in the presence Fe3O4–
Schiff base of Cu(II) (0.02g) was examined. When the reaction tem-
perature is increased gradually from room temperature to 40°C, the
yield of the reaction increases from 0 to 97% in 60min.
Fe3O4–Schiff base of Cu(II) can function as a heterogeneous oxida-
tion catalyst with H2O2 for oxidation of organic compounds. The
large surface area of the magnetic nanoparticles makes Fe3O4–
Schiff base of Cu(II) a good catalyst. Due to its operational simplicity,
generality and efficacy, this material is applicable to the oxidation of
a variety of organic compounds. It is important to mention that the
morphology of Fe3O4–Schiff base of Cu(II) does not change after is
use in oxidation reactions which is a key factor for its reusable prop-
erty. The main advantage of a catalytic system based on magnetic
nanoparticles is that it can be efficiently isolated from the product
solution through a simple magnetic separation process after com-
pletion of the reactions.
Oxidation of Sulfides to Sulfoxides
After optimization of the reaction parameters, the wider applicability
of the catalyst was tested for oxidation of a variety of substituted
aromatic and aliphatic sulfides (Scheme 2). The results are summa-
rized in Table 2. We used several sulfides as substrates for oxidation
and obtained corresponding sulfoxides without by-products such as
sulfones.
Oxidation of Thiols to Disulfides
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thiols to corresponding disulfides (Scheme 3). Oxidation reactions
were performed under mild and completely heterogeneous condi-
tions at room temperature with H2O2. Thiols are easily oxidized to
produce disulfides in ethanol solvent (Table 3). After reaction com-
pletion, the product is extracted by simple filtration and dichloro-
methane is removed by evaporation.
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