Schiff base complex coated Fe3O4 nanoparticles: A highly reusable nanocatalyst for the selective oxidation of sulfides and oxidative coupling of thiols

Article abstract of DOI:10.1016/j.catcom.2014.11.007

In this work, a new nanomagnetic complex of Cu(II) has been synthesized. The prepared catalyst was characterized by TG/DTG, FT-IR, TEM, VSM, ICP, AAS, XRD, EDS and SEM analyses. It was found to be an efficient catalyst for the oxidation of sulfides and oxidative coupling of thiols. High catalytic activity, ease of recovery from the reaction mixture using an external magnet, and several reuse times without significant losses in performance are additional eco-friendly attributes of this catalytic system.

Full text of DOI:10.1016/j.catcom.2014.11.007

Schiff Base Complex coated Fe3O4 nanoparticles: a highly reusable nanocat-
alyst for the selective oxidation of sulfides and oxidative coupling of thiols
Arash Ghorbani-Choghamarani, Banoo Ghasemi, Zohre Safari, Gouhar
Azadi
PII:
DOI:
Reference:
S1566-7367(14)00460-9
doi: 10.1016/j.catcom.2014.11.007
CATCOM 4129
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
9 August 2014
21 October 2014
8 November 2014
Please cite this article as: Arash Ghorbani-Choghamarani, Banoo Ghasemi, Zohre Sa-
fari, Gouhar Azadi, Schiff Base Complex coated Fe3O4 nanoparticles: a highly reusable
nanocatalyst for the selective oxidation of sulfides and oxidative coupling of thiols, Catal-
ysis Communications (2014), doi: 10.1016/j.catcom.2014.11.007
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ACCEPTED MANUSCRIPT
Schiff Base Complex coated Fe O nanoparticles: a highly reusable
3
4
nanocatalyst for the selective oxidation of sulfides and oxidative coupling of
thiols

Arash Ghorbani-Choghamarani, Banoo Ghasemi, Zohre Safari, and Gouhar Azadi
Department of chemistry, Faculty of Science, Ilam University, P. O. Box69315516, Ilam, Iran.
Abstract
In this work, a new nano magnetic complex of Cu(II) has been synthesized. The prepared catalyst was
characterized by TG/DTG, FT-IR, TEM, VSM, ICP, AAS, XRD, EDS and SEM analyses. It was found to be an
efficient catalyst for the oxidation of sulfides and oxidative coupling of thiols. High catalytic activity, ease of
recovery from the reaction mixture using an external magnet, and several reuse times without significant losses
in performance are additional eco-friendly attributes of this catalytic system.
Keywords
Nanoparticle, Schiff base complex, Sulfoxide, Disulfide.
1
. Introduction
Manufacturing protocols can be made economic, greener, and more sustainable, by designing and vigilant use of
catalysts, which reduces the chemical waste harmful to human health and the environment. Homogeneous
catalysts show higher catalytic activities than their heterogeneous counterparts because of their solubility in
reaction media, which increases catalytic site accessibility for the substrate. But, recycling homogeneous
catalysts is often tedious and time consuming and there is also product contamination observed when these
catalysts are used [1]. Therefore, heterogeneous catalysis is favored over homogeneous catalysis due to its ease
of handling and regenerability [2-6]. In contrast, the recovery of the most heterogeneous catalysts from the final
reaction systems requires a filtration or centrifugation step and/or a tedious workup. In this regard, magnetic

Address correspondence to A. Ghorbani-Choghamarani, Department of Chemistry, , Ilam
University, P.O. Box 69315516, Ilam, Iran; Tel/Fax: +98 841 2227022; E-mail address:
arashghch58@yahoo.com or a.ghorbani@mail.ilam.ac.ir
1

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nanomaterials showed major impacts on catalysis and many other areas [7]. Magnetic nanoparticles are
efficient, readily available, high-surface-area resulting in high catalyst loading capacity and outstanding stability
heterogeneous supports for catalysts [8]. The potential advantage of MNPs is that it can be separated from the
reaction medium by an external permanent magnet without using any filtration or centrifugation after
completion of the reaction. Moreover, they are accessible from inexpensive materials and can be easily tuned by
structural appropriate surface modification [9]. For overcoming the aforementioned drawbacks, herein we
report Schiff base complex of Cu(II) supported on superparamagnetic Fe O as an efficient, selective and
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4
recyclable catalyst for the oxidation of sulfides and oxidative coupling of thiols.
2
2
. Experimental
.1. Material and physical measurements
The reagents and solvents used in this work were all purchased from Aldrich and Merck and used without
further purification. Fe O nanoparticles were prepared via previously reported procedure [17]. IR spectra were
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4
recorded as KBr pellets on a VRTEX 70 model BRUKER FT-IR spectrophotometer. TGA/DTA was carried out
on Shimadzu DTG-60 instrument. Powder XRD was collected with a Rigaku-Dmax 2500 diffractometer with
nickel filtered Cu Ka radiation (λ = 1.5418 A˚, 40 kV). TEM of the NPs were recorded using a Zeiss-EM10C
TEM. The Ms Spectra of ligand and complex were recorded on Ajlinet 597530. The particle morphology was
examined by measuring SEM using FESEM-TESCAN MIRA3. The NMR spectra were recorded on BRUKER
NMR-Spectrometer FX 400Q. The Cu amount of catalyst was measure by Tech flame atomic absorption
spectrometer (CTA, 2000, UK) and Inductively Coupled Plasma Atomic Emission Spectrometer Sare Kelasam,
Perkin Elmer, Optima 8000. Supermagnetic properties of catalyst was measured on Vibrating Sample
Magnetometer (VSM) MDKFD.
2
.2 General procedure for the preparation of the ligand:
To the solution of 3-aminopropyl (triethoxy) silane (1 mmol, 0.221 gr) in 25 mL ethanol, the stoichiometric
amount of 5-bromo-2-hydroxybenzaldehyde (1 mmol, 0.201 gr) in ethanol (25 mL) was added dropwise. The
yellow solution was obtained and stirred at room temperature for 8 h. The residue was evaporated under reduced
pressure, to yield a yellowish ligand [18].
2
.3 General procedure for the preparation of the salen complex of Cu(II):
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Cu(NO ) .9H O (1 mmol) was added to the solution of the ligand (2 mmol) in ethanol (25 mL), and the mixture
3
2
2
was reflux to complete the reaction. The progress of the reaction was monitored by TLC. After completion of
complex formation, the color was changed. Resulting product was filtered and washed with ethanol. Then the
product was purified by recrystallization from ethanol [18].
2
.4 General procedure for the preparation of salen complex of Cu(II) immobilized on the magnetic
nanoparticles:
3+
2+
Fe O nanoparticles were prepared by chemical coprecipitation of Fe and Fe ions with a molar ratio of 2:1.
3
4
The solution of salen complex of Cu(II) (1 mmol) in ethanol (10 mL) was added to Fe O nanopareticle (2 g)
3
4
and the resultant mixture was reflux for 15 h. The resulting was isolated by magnetic decantation, washed twice
with ethanol and dried at 80 °C under vacuum.
2
.5 General procedure for the oxidation of sulfides to sulfoxides:
To a solution of sulfide (1 mmol) and 33% H O (0.5 mL) in ethanol (10 mL), Fe O /salen of Cu(II) as catalyst
2
2
3
4
°
(
0.05 gr) was added and the mixture was stirred at 60 C for the specified time. After completion of the reaction,
the catalyst was separated by an external magnet. The product was extracted with CH Cl , washed with water (5
2
2
mL) and dried over anhydrous Na SO . Finally, the excess of solvent was removed under reduced pressure to
2
4
give the corresponding pure sulfoxide.
2
.6 General procedure for the oxidative coupling of thiols:
A 25 mL round-bottom flask was charged with thiol (1 mmol), H O 33% (0.5 mL), and catalyst (0.01 gr) in
2
2
ethanol as solvent. The reaction mixture was stirred at room temperature, and the progress of the reaction was
monitored by TLC. After completion of the reaction, the catalyst was separated by an external magnet and the
product was extracted with CH Cl . The combined organics were washed with water (5 mL) and dried over
2
2
anhydrous Na SO . Finally, the solvent was removed to give the corresponding pure disulfides.
2
4
Supplementary data associated with this article can be found ate at http://www.sciencedirect.com.
3
3
. Results and discussion
.1. Catalyst characterization
As part of our program in environmentally beneficial catalysis [10-12], we are interested in designing a ‘‘green’’
process to effectively carry out the oxidation reaction. Herein, a green and simple method for selective oxidation
3

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of sulfides to sulfoxides and oxidative coupling of thiols to disulfides with H O (33%) catalyzed by salen
2
2
complex of Cu (II) immobilized on magnetic nanoparticle is reported. Salen complex of Cu(II) was prepared by
the route outlined in Scheme 1.
<
Scheme 1>
The magnetic nanocatalyst, is fully characterized by FT-IR, TGA, and SEM analyses. FT-IR spectrum (Fig. 1)
-
1
of the catalyst showed the expected bands, including the Fe–O stretching vibration near 588 cm and a
2
+
distinctive band due to C=N stretching, which is lowered in frequency on complexation to metal Cu from 1634
-
1
to 1624 cm [13].
<
Fig 1>
The thermogravimetric analysis (TGA) curves of Fe O /salen of Cu(II) show the mass loss of the materials as
3
4
they decompose upon heating (Fig. 2). According to this curve, two weight loss steps were observed. The first
mass loss occurs at temperature range of 100–300 °C that is related to the loss of solvent trap in complex. The
second mass loss occurs at temperature range of 300–600 °C that is related to the thermal decomposition of
complex. Also differential thermogravimetric (DTG) of catalyst has been shown in this figure. The peak in the
DTG curve reveals that the fastest weight loss occurred approximately between at 360 °C to 400 °C.
<
Fig 2>
Fig. 3a-d shows the SEM and TEM images of the synthesized Fe O /salen of Cu(II). This study showed
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4
diameters of approximately 10-20 nm for the magnetic nanoparticles.
The energy dispersive spectrum (EDS) of described magnetic nanoparticles showed that the mass percent of C,
N, O, Si, Fe, Cu, and Br is 20.81%, 6.13%, 50.58%, 13.98%, 6.19%, 0.18 and 2.12% respectively (Fig. 3e).
<
Fig 3>
The Cu (II) amount of the immobilized catalyst on magnetic nanoparticles was found to be 0.45 mmol·g−1
based on inductively coupled plasma atomic emission spectroscopy (ICP-OES). Also, the Cu (II) amount was
measured by atomic absorption spectroscopy (AAS), which the results was very closed to PCI-OES method
(
0.47 mmol·g−1).
4

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Magnetic measurements of Fe O /salen of Cu(II) was performed at room temperature using a Vibrating Sample
3
4
Magnetometer (VSM), (The hysteresis loops are shown in Fig. 4a). The value of saturation magnetic moment of
the catalyst is 32.6 emu/g.
The XRD pattern of the catalyst shows characteristic peaks and relative intensity (Fig 4b), which math well with
reported XRD of Fe O (magnetite). It implies that the spinal structure of Fe O has been retained during the
3
4
3
4
process of catalyst preparation.
<
Fig 4>
3
.2. Catalytic studies
After characterization of the catalyst, we have examined its catalytic activity in the oxidation of sulfides and
oxidative coupling of thiols. Initially, in order to find appropriate condition for the oxidation of sulfides,
different reaction conditions including catalyst amount and temperature were investigated (Scheme 2). The
results of the screening process have been summarized in Table 1 of supporting information.
<
Scheme 2>
As shown in Table 1, the reaction was incomplete in the absence of catalyst (yield after 120 min was only 12
). The results show (Table 1 of supporting information) that the optimize conditions for the complete
conversion of dibenzyl sulfide to dibenzyl sulfoxide are H O (0.5 mL), catalyst (0.05 gr) and EtOH as solvent
%
2
2
at 60 °C. The generality of this approach has been demonstrated by facile oxidation of aryl, benzylic, linear,
cyclic and heterocyclic sulfides as shown in Table 1.
<
Table 1>
As is evident from Table 1, the sulfoxides were obtained in short times and high yields.
Also we evaluated catalytic activity of the Fe O /salen of Cu(II) in oxidative coupling of thiols (Scheme 3). At
3
4
first, we explored the catalyst ability for mentioned reaction. 4-Methyl thiophenol and 2-mercapto benzothiazole
were selected as models to optimize the reaction condition. The results have been summarized in Table 2 of
supporting information.
<
Scheme 3>
The investigation of the reaction conditions in terms of the catalyst amount, reaction time and product yield
demonstrated that 10 mg of the catalyst in ethanol was optimal for the desired reaction (Table 2 in supporting
5

ACCEPTED MANUSCRIPT
information, entry 6). Then to demonstrate the efficiency of this protocol, we explored the generality of our
method by applying these conditions to different thiols. The results are summarized in Table 2.
<
Table 2>
As can be seen from Tables 1 and 2, the catalytic system worked exceedingly well in both oxidation of sulfides
and oxidative coupling of thioles with wide range of substrates under the optimized reaction conditions. Also the
reusability is one of the important properties of this catalyst. The recyclability of Fe O /salen of Cu(II) was
3
4
investigated for these reactions. The separated catalyst was reused for 12 and 10 runs in the oxidation of sulfides
and oxidative coupling of thioles respectively, without any significant loss of its activity and demonstrated
excellent recoverability and reusability (Fig. 5).
<Fig. 5>
4
. Conclusion
In conclusion, a novel and recyclable Fe O /salen of Cu(II) was fabricated. The catalyst is easily synthesized
3
4
and can catalyze the synthesis of sulfoxides and disulfides derivatives via oxidation of sulfides and oxidative
coupling of thioles .The mild conditions and the use of a green, nontoxic, inexpensive, and reusable catalyst in
high yields and in relatively short reaction time are some of the advantages of this method.
Acknowledgements
Authors thank the research facilities of Ilam University, Ilam, Iran, for financial support of this research project.
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Fig 1. The comparative FT-IR spectra for (a) Fe O (b) Fe O /salen of Cu(II).
3
4
3
4
Fig 2. TGA and DTG thermograms of Fe O and Fe O /salen of Cu(II).
3
4
3
4
Fig 3. SEM images of Fe O /salen of Cu(II) (a and b); TEM images of Fe O /salen of Cu(II) (c and d); EDSEDS spectrum
3
4
3
4
of catalyst (e).
Fig 4. a) VSM curve of Fe O /salen of Cu(II); b) XRD pattern of Fe O /salen of Cu(II).
3
4
3
4
Fig 5. Reusability of Fe O /salen of Cu(II) in the oxidation of benzyl methylsulfide to benzyl methylsulfoxide.
3
4
Scheme 1. Preparation process of Fe O /salen of Cu(II).
3
4
Scheme 2. Oxidation of sulfide to sulfoxide catalyzed by Fe O /salen of Cu(II).
3
4
Scheme 3. oxidative coupling of thiols catalyzed by Fe O /salen of Cu(II).
3
4
8

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(
a)
(
b)
C-H
C=N
Fe-o
Fig. 1
9

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1
05
40
20
Fe O
3
4
100
Nano Schiff Base Cat.
9
5
90
85
0
80
75
- 20
70
-
40
65
100
200
300
400
500
600
700
o
Temperature, C
Fig. 2
10

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(a)
(b)
(c)
(d)
7000
6000
5000
4000
3000
2000
1000
0
SiK
O K
N K
FeL
CuL
BrL
AlK
K K
FeK
AuM
AuM
C K
AuLl
K K
FeK
CuK CuK AuL
keV
0
5
10
(e)
Fig. 3
11

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(a)
(b)
Fig. 4
12

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Fig. 5
13

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Scheme 1
O
S
Cat., H O (33 %)
S
2
2
R1
R2
R1
R2
0
EtOH, 60 C
1
2
R , R = Aryl, Heterocyclic, Benzylic and Alkyl
Scheme 2
14

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Cat., EtOH
R
SH
R
S
S
R
r.t.
Scheme 3
15

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Table 1. oxidation of sulfides to the sulfoxides in the presence of Fe O /salen of Cu(II).
3
4
Entry
1
Substrate
Product
Catalyst (gr)
0.05
H O
Time
(h)
Yield
(%)
M.p ° C
2
2
(mL)
O
S
0.5
2
97
138 [14]
S
2
3
0.05
0.05
0.5
0.5
1.20
99
80
Oil [15]
20 min
Brown oil
[16]
4
5
S
0.05
0.05
0.5
0.5
1.50
3
99
83
Oil [17]
32 [14]
S
O
S
O
S
S
6
7
8
0.05
0.05
0.1
0.5
0.5
2
20 min
50 min
72
80
95
58
Oil [19]
Oil [19]
61 [15]
S
O
O
S
O
O
O
S
O
S
9
O
S
0.05
0.05
1.2
1.2
6
98
95
114-116 [14]
85-89 [20]
S
S
O
S
1
0
25min
C12H25
C12H25
C12H25
C12H25
16

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Table 2. Oxidative coupling of thiols into disulfides using H O in the presence of Fe O /salen of Cu (II).
2
2
3
4
Time
min)
Yield
(%)
M.p found
(°C)
Entry
Substrate
Product
a
(
SH
S
S
6
1
9
0
5
0
90
69-70 [21]
38-40 [22]
1
2
3
S
S
9
7
O
N
SH
57
94-96 [22]
174-175 [22]
165-167 [23]
S
N
S
N
S
N
SH
S
S
9
9
8
7
4
5
90
25
15
6
7
HS COOH
HOOC
S
S
COOH
98
103 [24]
Oil [22]
3
0
OH
9
1
HS
COOH
COOH
SH
S
S
1
20
91
90
274-281[15]
134-136 [22]
8
9
COOH
SH
S
S
120
1
0
SH
S
S
1
20
9
9
89-90 [22]
Br
Br
Br
17

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Highlights
Schiff base complex of Cu (II) immobilized on the magnetic
nanoparticles was prepared
Schiff base/Cu (II)/Fe O was fully characterized
3
4
Schiff base/Cu (II)/Fe O was employed for the preparation of
3
4
sulfoxides and disulfiddes
Catalyst could be easily recovered using a magnet and recycled
18