Schiff base complexes of Ni, Co, Cr, Cd and Zn supported on magnetic nanoparticles: As efficient and recyclable catalysts for the oxidation of sulfides and oxidative coupling of thiols

Article abstract of DOI:10.1016/j.ica.2015.07.004

Abstract Oxidation of sulfides to sulfoxides and oxidative coupling of thiols into their corresponding disulfides were carried out using hydrogen peroxide (H₂O₂) as oxidizing agent in the presence of immobilized Ni, Co, Cr, Zn or Cd complexes on Fe₃O₄ magnetic nanoparticles (M-Salen-MNPs) as stable, heterogeneous, efficient and magnetically recoverable nanocatalysts under mild reaction conditions. These supported complexes were characterized by FT-IR spectroscopy, thermogravimetric analysis (TGA), powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). A variety of aromatic and aliphatic sulfides and thiols with different functional groups were successfully oxidized with short reaction times in good to excellent yields. Recovery of the catalyst is easily achieved by magnetic decantation and reused for several consecutive runs without significant loss of its catalytic efficiency.

Full text of DOI:10.1016/j.ica.2015.07.004

Accepted Manuscript
Schiff base complexes of Ni, Co, Cr, Cd and Zn supported on magnetic nano-
particles: as efficient and recyclable catalysts for the oxidation of sulfides and
oxidative coupling of thiols
Arash Ghorbani-Choghamarani, Zahra Darvishnejad, Bahman Tahmasbi
PII:
S0020-1693(15)00329-1
http://dx.doi.org/10.1016/j.ica.2015.07.004
ICA 16597
DOI:
Reference:
Inorganica Chimica Acta
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Received Date:
Revised Date:
Accepted Date:
9 June 2015
8 July 2015
9 July 2015
Please cite this article as: A. Ghorbani-Choghamarani, Z. Darvishnejad, B. Tahmasbi, Schiff base complexes of Ni,
Co, Cr, Cd and Zn supported on magnetic nanoparticles: as efficient and recyclable catalysts for the oxidation of
sulfides and oxidative coupling of thiols, Inorganica Chimica Acta (2015), doi: http://dx.doi.org/10.1016/j.ica.
2015.07.004
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Schiff base complexes of Ni, Co, Cr, Cd and Zn supported on magnetic
nanoparticles: as efficient and recyclable catalysts for the oxidation of sulfides
and oxidative coupling of thiols
Arash Ghorbani-Choghamarani∗, Zahra Darvishnejad, Bahman Tahmasbi
Department of Chemistry, Faculty of Science, Ilam University, P.O. Box 69315516, Ilam, Iran.
Abstract
Oxidation of sulfides to sulfoxides and oxidative coupling of thiols into their corresponding
disulfides were carried out using hydrogen peroxide (H2O2) as oxidizing agent in the presence of
immobilized Ni, Co, Cr, Zn or Cd complexes on Fe3O4 magnetic nanoparticles (M-Salen-MNPs)
as stable, heterogeneous, efficient and magnetically recoverable nanocatalysts under mild
reaction conditions. These supported complexes were characterized by FT-IR spectroscopy,
thermogravimetric analysis (TGA), powder X-ray diffraction (XRD), scanning electron
microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). A variety of aromatic and
aliphatic sulfides and thiols with different functional groups were successfully oxidized with
short reaction times in good to excellent yields. Recovery of the catalyst is easily achieved by
magnetic decantation and reused for several consecutive runs without significant loss of its
catalytic efficiency.
Keywords
Magnetic Fe3O4; Nanoparticles; Hydrogen peroxide; Schiff base; Sulfoxide; Disulfide.
∗Address correspondence to A. Ghorbani-Choghamarani, Department of Chemistry, Faculty of Science,
Ilam University, P.O. Box 69315516, Ilam, Iran; Tel/Fax: +98 841 2227022; E-mail address:
a.ghorbani@mail.ilam.ac.ir
1

1. Introduction
The immobilization of homogeneous catalysts on various support materials to facilitate catalysts
separation and recycling is of great importance in catalyst science [1, 2]. However,
immobilization of homogeneous catalysts usually decreases the catalytic activity or selectivity
[3]. This could be solved using nanoparticles (NPs) as catalyst supports. Nanoparticulate
supports form a ‘‘semihomogeneous’’ system and can serve as an intermediate between
homogeneous and heterogeneous catalysis [4]. However, nanoparticles are difficult to separate
by filtration techniques [5]. This drawback can be overcome by magnetic nanoparticles (MNPs),
which can be easily removed from the reaction mixture by magnetic separation. More important,
magnetic separation of the MNPs is easier and more effective than filtration or centrifugation,
simple, economical and promising for industrial applications [3, 6]. One of the most promising
MNPs supports for the development of high-performance catalyst supports is superparamagnetic
iron oxide [6, 7]. The notable advantages of Fe3O4 MNPs are simple in synthesis, readily
available, low cost, high surface area, low toxicity and superparamagnetism properties [8].
Therefore, Fe3O4 MNPs are considered as ideal supports for the heterogenization of
homogeneous catalysts [9].
Recently, metal complexes of Schiff-bases have been emerged for the development of efficient
variety of catalysts [7, 10] however, to clean and facilitate catalysts separation, Schiff-base
compounds have been supported onto heterogeneous materials [11, 12]. Among them,
salicylidene Schiff-base (salen) can be easily prepared using condensation of salicylaldehyde and
amine groups. On the other hand, salen complexes of transition metals are stable compounds that
have been used as catalysts for various organic reactions. Among transition metals, Ni, Co, Cr,
2

Cd and Zn Schiff-base complexes are amongst the most versatile catalysts known for organic
transformations [13, 14].
The selective oxidation of organosulfur compounds is of great importance in synthetic,
biological, and industry [14]. Disulfides are important in both biological (including DNA
cleavage, stabilisation of peptides in proteins,) and industrial processes (e.g. applications in
vulcanization of rubbers and elastomers) [14]. Furthermore, disulfides are important reagents in
organic synthesis, and can be used as a protecting group for thiol [15], sulfonylation of enolates
and other anions [16], synthesis of organo-sulfur compounds via C–S bond formation [17] and
they are also important starting materials in the preparation of sulfenyl and sulfinyl reagents [18].
Likewise, the selective oxidation of sulfides to sulfoxides is an important transformation in
organic chemistry [19] Some of biologically active sulfoxides play an important role as
therapeutic agents such as antifungal, antibacterial, anti-atherosclerotic, anti-ulcer,
antihypertensive and anti-atherosclerotic as well as psychotropics and vasodilators [20, 21].
Furthermore, Omeprazole and the pesticide Fipronil are two typical examples of the extensive
application of these intermediates in pharmaceutical and fine chemical industries [22, 23].
Additionally, sulfoxides are also valuable in the C-C bond-formation and molecular
rearrangements [24].
Therefore, many methods including different catalysts (homogeneous and heterogeneous
catalysts) and various oxidants (e.g. metal oxidants, organic oxidant, peroxides, halogens, and
air) have been used for the selective oxidation of organic sulfides and thiols [15-25]. Concerning
the green oxidant, H2O2 (with only H2O as a byproduct) in particular have received attention
3

because of their environmental implications, readily available, high atom efficiency and lower
costs involved than when using other oxidizing agents [26].
Owing to the context of green chemistry and reuse of catalyst, herein Ni, Cd, Co, Cr and Zn-
based heterogeneous catalysts have been reported for oxidative coupling of thiols into
corresponding disulfides and also oxidation of sulfides to sulfoxides under mild condition.
2. Experimental Section
2.1. Materials:
Chemicals and solvents used in this work were purchased from Sigma-Aldrich, Fluka or Merck
chemical companies and used without further purification.
2.2. Preparation of Fe3O4 magnetic nanoparticles:
The free Fe3O4 MNPs were prepared using chemical precipitation of ions Fe3+ and Fe2+ with a
molar ratio of 2:1. Typically, FeCl3.6H2O (5.838 g, 0.0216 mol) and FeCl2.4H2O (2.147 g,
0.0108 mol) were dissolved in 100 mL deionized water at 80 °C under N2 atmosphere and
vigorous mechanical stirring conditions. Then, 10 mL of 25% NH3 (aq) was added to the reaction
mixture. After 30 minutes, the reaction mixture was cooled down to room temperature, and black
NPs (Fe3O4) were washed two times with distilled water and solution of NaCl (0.02 M), and each
time was decanted with external magnet.
2.3. Preparation of magnetic nanoparticles bonded several transition metals (e.g. Ni, Cd, Co, Zn
and Cr) complexes (M-Salen-MNPs):
The obtained Fe3O4 NPs (1.5 g) was dispersed in 100 mL ethanol/water (volume ratio, 1:1)
solution by sonication for 30 min, then (3-aminopropyl)-trimethoxysilane (APTES) (2.5 mL)
4

was added to the reaction mixture. The reaction mixture was stirred using mechanical stirring
under N2 atmosphere at 40 ºC for 8 h. Subsequently, nanoparticles were re-dispersed in ethanol
via sonication for 5 times and separated through magnetic decantation. The nanoparticles product
(APTES–Fe3O4) was dried at room temperature. In order to prepare Salen-Fe3O4, the above
mentioned solid (1 g) was refluxed with 0.122 g (1 mmol) of salicylaldehyde in ethanol for 8h
under N2 atmosphere. The resulting solid (Salen-Fe3O4) was separated by magnetic decantation,
washed with ethanol and dried at room temperature. Finally, for the preparation of M-Salen-
MNPs, the Salen-Fe3O4 (1.0 g) was dispersed in 25 mL of dry ethanol and was mixed with 1.0
mmol of metal salts (Ni(NO3)2.6H2O, Co(NO3)2.6H2O, CrCl3.6H2O, CdCl2 or Zn(NO3)2.4H2O)
under N2 atmosphere. The mixture was stirred at 70 ºC for 15 h. The solid product (M-Salen-
MNPs) was obtained by magnetic decantation, washed with ethanol and dried at 50 ˚C.
2.4 General procedure for the oxidation of sulfides to sulfoxides using H2O2 in the presence of
M-Salen-MNPs:
A mixture of sulfide (1 mmol), H2O2 (0.4 mL) and M-Salen-MNPs (0.02 g) was stirred at 35 ºC
under solvent-free condition and the progress of the reaction was monitored by TLC. After
completion of the reaction, catalyst was separated using external magnet and washed with ethyl
acetate, and next, the product was extracted with ethyl acetate. The organic layer was dried over
anhydrous Na2SO4 (1.5 g). Finally, the organic solvents were evaporated, and products were
obtained in good to high yield.
2.5 General procedure for the oxidative coupling of thiols into disulfides using H2O2 in the
presence of M-Salen-MNPs:
5

M-Salen-MNPs (0.01 g) was added to a mixture of thiol (1 mmol) and H2O2 (0.4 mL) in ethanol
(5 mL). Then the mixture was stirred for the appropriate time at room temperature. The progress
was monitored by TLC. After completion of the reaction, the catalyst was separated by an
external magnet and the mixture was washed with ethyl acetate. The product was extracted with
ethyl acetate. The organic layer was dried over anhydrous Na2SO4 (1.5 g). In some cases, the
product was recrystalized from ethanol for further purification and products were obtained in
good to high yield.
2.6 Selected Spectral data
1
Methyl(p-tolyl)sulfane (Table 1, entry 2): H NMR (400 MHz, CDCl3): δ = 2.23 (s, 3H), 2.56 (s,
3H), 7.18 (d, J= 3.2 Hz, 2H), 7.40 (d, J= 3.2 Hz, 2H) ppm.
2-((Methylsulfinyl)methyl)furan (Table 1, entry 4): 1H NMR (400 MHz, CDCl3): δ = 2.58 (s, 3H),
4.06 (d, J= 14 Hz,1H), 4.15 (d, J= 14 Hz,1H), 6.43 (d, J= 3.2 Hz,1H), 7.14-7.57 (m, 2H) ppm.
1
Tetrahydrothiophene 1-oxide (Table 1, entry 5): H NMR (400 MHz, CDCl3): δ = 2.26 (t, J= 7.6
Hz, 2H), 3.05 (t, J= 7.6 Hz, 2H) ppm.
1-(Propylsulfinyl)propane (Table 1, entry 8): 1H NMR (400 MHz, CDCl3): δ = 1.12 (t, J= 6 Hz,
6H), 1.85-1.94 (m, 4H), 2.64-2.96 (m, 4H) ppm.
1,2-Di-p-tolyldisulfane (Table 2, entry 1): 1H NMR (400 MHz, CDCl3): δ = 2.34 (s, 6H), 7.13 (d, J=
7.9 Hz, 4H), 7.41 (d, J= 7.9 Hz, 4H) ppm.
1,2-Di(naphthalen-2-yl)disulfane (Table 2, entry 3): 1H NMR (400 MHz, CDCl3): δ = 7.48 (m,
4H), 7.64 (m, 2H), 7.75 (m, 2H), 7.80 (m, 4H), 8.1 (s, 2H) ppm.
1,2-Bis(4-bromophenyl)disulfane (Table 2, entry 4): 1H NMR (400 MHz, CDCl3): δ = 7.34 (d, J=
8.2 Hz, 4H), 7.43 (t, J= 8.2 Hz, 4H) ppm.
6

1,2-Bis(benzo[d]thiazol-2-yl)disulfane (Table 2, entry 7): 1H NMR (400 MHz, CDCl3): δ = 7.37
(t, J= 3.2 Hz, 2H), 7.47 (t, J= 3.2 Hz, 2H), 7.79 (d, J= 3.2 Hz, 2H), 7.95 (d, J= 3.2 Hz, 2H) ppm.
1,2-Bis(4,6-dimethylpyrimidin-2-yl)disulfane (Table 2, entry 8): 1H NMR (400 MHz, CDCl3): δ
= 2.39 (s, 12H), 6.75 (s, 2H) ppm.
2,2'-Disulfanediyldiethanol (Table 2, entry 10): 1H NMR (400 MHz, CDCl3): δ = 2.59 (br, 2H),
2.90 (t, J= 5.6 Hz, 4H), 3.92 (t, J= 5.6 Hz, 4H) ppm.
3. Results and discussion
All of the catalysts were prepared by the concise route outlined in Scheme 1. In order to perform
immobilization of Ni, Co, Cr, Cd and Zn on magnetic nanoparticles, initially, naked Fe3O4
MNPs were prepared and functionalized with (3-aminopropyl)-triethoxysilane (APTES) by a
previously procedure according to the literature method [27]. Subsequently, the metal-free salen
supported on Fe3O4 (Salen-Fe3O4) was formed using condensation of salicylaldehyde and
terminal amine. The chromium salen complex immobilized on Fe3O4 MNPs (Cr-Salen-MNPs)
(I) was prepared using reaction of Salen-Fe3O4 with CrCl3.6H2O. Also, other catalyst including
Zn-Salen-MNPs (II), Cd-Salen-MNPs (III), Co-Salen-MNPs (IV) and Ni-Salen-MNPs (V) were
prepared by the same procedure using Zn(NO3)2.4H2O, CdCl2, Co(NO3)2.6H2O and
Ni(NO3)2.6H2O respectively. These nanocatalysts have been characterized by scanning electron
microscopy (SEM), Energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD),
thermogravimetric analysis (TGA) and Fourier transform infrared spectroscopy (FT-IR). Finally,
we examined the catalytic activity of all catalysts (I-V) for the oxidative coupling of thiols into
disulfides and also oxidation of sulfides to sulfoxides using H2O2 as oxidant.
Scheme 1 here
7

The size of the catalysts were evaluated using scanning electron microscopy. It can be seen that
most of the particles formed were nanometer-sized with an average diameter about 15 nm. The
SEM images of Cd-Salen-MNPs and Zn-Salen-MNPs are shown in Figure 1. To investigate the
catalyst characterization for the supporting transition metals species, the synthesized catalysts
were analyzed using EDX. For example as shown in Figure 2, the EDX spectrum of Ni-Salen-
MNPs, Co-Salen-MNPs and Cr-Salen-MNPs showed the presence of N, O, Si and C and as well
as Ni, Co and Cr species in the catalyst, respectively.
Figure 1 here
Figure 2 here
The formation of a magnetite crystal phase in the all catalysts aggregate powder was identified
from the X-ray diffraction pattern. Figure 3 shows XRD pattern for Ni-Salen-MNPs, Co-Salen-
MNPs, Cr-Salen-MNPs, Cd-Salen-MNPs and Zn-Salen-MNPs. As seen in Figure 3, the iron
oxide phase was identified from the XRD patterns by the peak positions at 30.5 (2 2 0), 35.7 (3 1
1), 43.5 (4 0 0), 53.9 (4 2 2), 57.3 (5 1 1) and 63 (4 4 0), which were in agreement with magnetite
standard data [28].
Figure 3 here
One indication of bond formation between the Fe3O4 and the salen complexes can be inferred
from thermogravimetric analysis (TGA). Figure 4 shows the TGA curves for bare Fe3O4 MNPs
(black curve), APTES-Fe3O4 (red curve), Cr-Salen-MNPs (blue curve) and Cd-Salen-MNPs
(green curve). The TGA curve of the bare Fe3O4 MNPs shows the mass loss of the hydroxyl
groups as it decomposes upon heating (Figure 4). The small amount of weight loss below 200 °C
8

is due to the removal of physically adsorbed solvent and surface hydroxyl groups [9]. Organic
groups have been reported to desorb at temperatures above 260 °C. The organic groups on the
APTES-Fe3O4 are found to have a mass percentage loss about 5%, while the Cr-Salen-MNPs and
Cd-Salen-MNPs catalysts have the greatest mass loss, at 13% and 10%, respectively.
Thermal stability of the magnetic catalysts was also determined by this TGA analysis, since
synthesis of many organic compounds was usually carried out at high temperature. Based on
TGA diagram of the catalyst, no weight loss of catalyst was observed by increasing the
o
temperature up to 300 C (except for the removal of the solvents). As shown in Figure 4, Cr-
Salen-MNPs and Cd-Salen-MNPs catalysts were stable even at 300 ºC.
Figure 4 here
Figure 5 shows FT-IR spectra for Fe3O4, APTES-Fe3O4, Zn-salen-MNPs, Ni-salen-MNPs, Cd-
salen-MNPs, Co-salen-MNPs and Cr-salen-MNPs. The FT-IR spectrum of the all catalysts shows
several peaks that are characteristic of functionalized salen complexes, which clearly differs
from that of the unfunctionalized Fe3O4 MNPs and aminopropyl-functionalized magnetic
nanoparticles.
The FT-IR spectrum for Fe3O4 alone shows a stretching vibration at 3390 cm−1 which
incorporates the contributions from both symmetrical and asymmetrical modes of the O-H bonds
which are attached to the surface iron atoms. The bands at low wavenumbers (about 560 cm−1)
come from vibrations of Fe-O bonds of iron oxide [6]. In the FT-IR spectra of APTES-Fe3O4, the
presence of the anchored n-propylamine groups are confirmed by C–H stretching vibrations that
appear at 2920 and 2855 cm−1 and also N-H stretching vibration modes as a broad band that
appear at 3440 cm-1 [27]. The FT-IR spectrum for the APTES-Fe3O4 and catalysts shows two
9

peaks about of 781 and 1046 cm-1 corresponding to the symmetric and asymmetric Si-O–Si
vibrations [29].
Reaction of APTES-Fe3O4 with salicylaldehyde produces Salen-Fe3O4 in which the presence of
C=N moiety is asserted about 1630 cm−1 bands in FT-IR spectra, this band shifts to lower
frequency in the catalysts, which indicates the formation of metal-ligand bonds [9, 14]. Also,
vibrations in the 3420 cm-1 are attributed to the hydroxyl groups.
Figure 5 here
As a part of our ongoing program directed towards the catalytic activity of modified magnetic
nanoparticles in organic reactions [8, 27], we were interested in finding a simple and efficient
method for the oxidation of sulfides into sulfoxides using magnetic nanoparticles bonded with
several transition metals (e.g. Ni, Cd, Co, Zn and Cr) forming complexes (M-Salen-MNPs) as a
magnetically recoverable nanocatalyst (Scheme 2).
Scheme 2 here
The generality of this approach has been demonstrated by a facile oxidation of various sulfides
as shown in Table 1. The sulfoxides were obtained in high yields. As shown, a variety of sulfides
including dialkyl sulfides (Table 1, entries 6-10), alkylaryl sulfides (Table 1, entries 1-4), cyclic
sulfides (Table 1, entry 5) and other sulfides with different functional groups (Table 1, entries 3,
9 and 10) were successfully employed to prepare the corresponding sulfoxides. In all of the
reactions no any byproducts was observed, and all products were obtained in high to excellent
yields in the short reaction times. Therefore, the results revealed that this methodology is
effective for the oxidation of a wide range of sulfides.
10

Table 1 here
In order to explore the activity of these catalysts in organic transformations we report an efficient
method for the oxidative coupling of thiols into corresponding disulfides as shown in Scheme 3.
The experimental procedure is very simple and convenient, and has the ability to tolerate a
variety of thiols including aromatic thiols (Table 2, entries 1-6), aliphatic thiols (Table 2, entries
10 and 11), benzilic thiol (Table 2, entry 9) and heteroaromatic thiols (Table 2, entries 7 and 8)
including other functional groups such as alkyl, halide, carboxylic acid, amine, and hydroxyl
groups under reaction conditions. Interestingly other functional groups in these substrates
remained intact during the oxidative coupling reaction. These results indicated that the present
protocol could be applicable to the chemoselective coupling of thiols in the presence of other
functional groups. The results of these studies were shown in Table 2. Therefore, the results
revealed that this methodology is effective for the oxidative coupling of a wide range of thiols.
Scheme 3 here
Table 2 here
Because of mild conditions of described heterogeneous systems, there is no overoxidation to
sulfone (for oxidation of sulfides) or thiosulfinates, disulfoxides, sulfinyl sulfones or disulfones
(for the oxidative coupling of thiols) was observed (Scheme 4).
Scheme 4 here
We examined the recycling of Cr-Salen-MNPs as a model catalyst for the oxidative coupling of
2-mercapto benzothiazole. After completion of the reaction, the catalyst was separated easily and
rapidly from the product by exposure to an external magnet, and the reaction solution was
11

decanted, magnetically. The remaining magnetic nanoparticles were further washed with the
ethyl acetate, and subjected to the next run. The efficient separation using this method minimizes
the loss of catalyst during separation. The recycled catalyst was used for up 10 runs without any
significant loss of its catalytic activity (Figure 6). The average isolated yield for 10 successive
runs was 99%, which clearly demonstrates the practical recyclability of this catalyst.
Figure 6 here
In order to examine the efficiency of this procedure, we compared the results of the preparation
of (methylsulfinyl) benzene (Table 3, entry 1–9) and 1,2-bis(benzo[d]thiazol-2-yl)disulfane
(Table 3, entry 10–18) with the previous methods in the literature. It is obvious that described
procedure in this research project shows good reaction time and higher yield than the other
catalysts, which has been reported previously. Also these new catalysts are comparable in terms
of price, non-toxicity, stability and easy separation. In addition, the number of the recyclability
of these catalysts are more than the previously reported catalysts.
Table 3 here
4. Conclusion
In conclusion, we successfully anchored Ni(NO3)2.6H2O, Co(NO3)2.6H2O, CrCl3.6H2O, CdCl2
and Zn(NO3)2.4H2O onto functionalized Fe3O4 MNPs. These catalysts were characterized by FT-
IR spectroscopy, thermogravimetric analysis (TGA), powder X-ray diffraction (XRD), scanning
electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). These catalysts
employed for the oxidation of sulfides to sulfoxides and oxidative coupling of thiols into their
corresponding disulfides using H2O2 as oxidant. The notable advantages of these methods are
12

operational simplicity, use of cheap and chemically stabile oxidant, simple work-up procedure,
good to high yields of products and short reaction times. More importantly, the catalysts can be
easily separated using external magnet and reused for several times without significant loss of
their catalytic activity.
Acknowledgements
Authors thank the research facilities of Ilam University, Ilam, Iran, for financial support of this
research project.
References
[1] L. Guanghua, W. Mai, R. Jin, L. Jao, Synlett. 9 (2008) 1418-1422.
[2] T.J. Yoon, W. Lee, Y.S. Oh, J.K. Lee, New J. Chem., 27 (2003) 227-229.
[3] Y. Zhu, L.P. Stubbs, F. Ho, R. Liu, C.P. Ship, J.A. Maguire, N.S. Hosmane, Chemcatchem. 2
(2010) 365-374.
[4] S. Shylesh, V. Schunemann, W.R. Thiel, Angew. Chem. Int. Ed. 49 (2010) 3428-3459.
[5] A. Rostami, B. Tahmasbi, A. Yari, Bull. Korean Chem. Soc. 34 (2013) 1521-1524.
[6] A. Rostami, B. Tahmasbi, F. Abedi, Z. Shokri, J. Mol. Catal. A: Chem. 378 (2013) 200-205.
[7] S.M. Sadeghzadeh, F. Daneshfar, M. Malekzadeh, Chin. J. Chem. 32 (2014) 349-355.
[8] A. Ghorbani-Choghamarani, G. Azadi, RSC Adv. 5 (2015) 9752-9758.
[9] B. Atashkar, A. Rostami, B. Tahmasbi, Catal. Sci. Lett. 3 (2013) 2140-2146.
[10] Z. Abbasi, M. Behzad, A. Ghaffari, H. Amiri Rudbari, G. Bruno, Inorg. Chim. Acta. 414
(2014) 78-84.
[11] F. Dehghani, A.R. Sardarian, M. Esmaeilpour, J. Organomet. Chem. 743 (2013) 87-96.
[12] V. Ayala, A. Corma, M. Iglesias, F. Sanchez, J. Mol. Catal. A: Chem. 221 (2004) 201-208.
13

[13] V. Polshettiwar, R. Luque, A. Fihri, H. Zhu, M. Bouhrara, J.M. Basset, Chem. Rev. 111
(2011) 3036–3075.
[14] A. Ghorbani-Choghamarani, Z. Darvishnejad, M. Norouzi, Appl. Organometal. Chem. 29
(2015) 170–175.
[15] S. Thurow, V.A. Pereira, D.M. Martinez, D. Alves, G. Perin, R.G. Jacob, E.J. Lenardão,
Tetrahedron Lett. 52 (2011) 640-643.
[16] B. V. Tamhankar, Int. J. Res. in Org. Chem. 4 (2014) 4-6.
[17] Y. Liu, H. Wang, C. Wang, J.P. Wan, C. Wen, RSC Adv. 3 (2013) 21369-21372.
[18] R. Hekmatshoar, S. Sajadi, M.M. Heravi, F.F. Bamoharram, Molecules 12 (2007) 2223-
2227.
[19] A. Ghorbani-Choghamarani, B. Ghasemi, Z. Safari, G. Azadi, Catal. Commun. 60 (2015)
70-75.
[20] A. Shaabani, A.H. Rezayan, Catal. Commun. 8 (2007) 1112-1116.
[21] B. Sreedhar, P. Radhika, B. Neelima, Neha Hebalkar, A.K. Mishra, Catal. Commun. 10
(2008) 39-44.
[22] G.P. Romanelli, P.I. Villabrille, C.V. Cáceres, P.G. Vázquez, P. Tundo, Catal. Commun.
12 (2011) 726-730.
[23] L.Villalobos, T. Ren, Inorg. Chem. Commun. 28 (2013) 52-54.
[24] D. Habibi, M.A. Zolfigol, M. Safaiee, A. Shamsian, A. Ghorbani-Choghamarani, Catal.
Commun. 10 (2009) 1257–1260.
[25] X.B. Li, Z.J. Li, Y.J. Gao, Q.Y. Meng, S. Yu, R.G. Weiss, C.H. Tung, L.Z. Wu, Angew.
Chem. Int. Ed. 53 (2014) 2085–2089.
[26] S. Rayati, F. Nejabat, S. Zakavi, Inorg. Chem. Commun. 40 (2014) 82–86.
14

[27] A. Ghorbani-Choghamarani, M. Norouzi, J. Mol. Catal. A: Chem. 395 (2014) 172–179.
[28] X.N. Zhao, H.C. Hu, F.J. Zhang, Z.H. Zhang, Appl. Catal. A: Gen. 482 (2014) 258–265.
[29] A. Ghorbani-Choghamarani, F. Nikpour, F. Ghorbani, F. Havasi, RSC Adv. 5 (2015)
33212–33220.
[30] A. Ghorbani-Choghamarani, G. Azadi, B. Tahmasbi, M. Hadizadeh-Hafshejani, Z. Abdi,
Phosphorus Sulfur Silicon Relat. Elem. 189 (2014) 433-439.
[31] M. Hajjami, S. Rahmani, J. Porous. Mater. (2015) In Press.
[32] A. Jabbari, H. Mahdavi, M. Nikoorazm, A. Ghorbani-Choghamarani, J. Porous. Mater. 22
(2015) 1111–1118.
[33] M. Nikoorazm, A. Ghorbani-Choghamarani, H. Mahdavi, S.M. Esmaeili, Micropor.
Mesopor. Mat. 211 (2015) 174-181.
[34] M. Nikoorazm, A. Ghorbani-Choghamarani, N. Noori, J. Porous. Mater. 22 (2015) 877–
885.
15

FeCl3.6H2O
+
FeCl2.4H2O
NH2OH
Fe3O4
NP
N2, rt, 30 min
O
N
O
O
N
N
O
Ni
O
Cr
O
Si
O
O
N
O
Si
Si
O
O
O
O
Si
O
O
I
V
O
Fe3O4
NP
O
Fe3O4
NP
O
C
H
Si(H2C)3N
O
HO
O
O
O
O
O
O
O
N
N
O
O
N
Si
Si
Fe3O4
NP
Si
Si
O
O
O
O
Fe3O4
NP
Fe3O4
NP
Si
O
Cd
N
Co
O
O
O
O
O
O
Zn
O
Si
O
O
N
N
O
IV
III
II
Scheme 1. Synthesis of M-Salen-MNPs.
Scheme 2. M-salen-MNPs catalyzed the Oxidation of sulfides into sulfoxides using H2O2.
16

Scheme 3. M-salen-MNPs catalyzed the oxidative coupling of thiols into disulfides using H2O2.
O
O
O
H2O2 /Cat.
S
S
S
+
R
R
R
R
R
R
88-99%
0%
O
O
O
S
+
S
R
R
R
S
R
S
H2O2 /Cat.
S
R
R
0%
0%
+
R SH
S
90-99%
O
O
O
S
S
R
R
+
R
S
R
S
O
O
O
0%
0%
Scheme 4. Chemoselective oxidative coupling of thiols and oxidation of sulfides in the presence
M-salen-MNPs using H2O2 without overoxidation.
17

Figure 1. SEM images of Cd-Salen-MNPs (left) and Zn-Salen-MNPs (right).
18

Figure 2. EDX spectrum of Ni-Salen-MNPs (a), Co-Salen-MNPs (b) and Cr-Salen-MNPs (c).
19

Figure 3. The XRD pattern of Zn-Salen-MNPs (a), Ni-Salen-MNPs (b), Co-Salen-MNPs (c),
Cd-Salen-MNPs (d) and Cr-Salen-MNPs (e).
20

110
105
100
95
90
85
80
25
125
225
325
425
525
625
725
Temperature(◦C)
Figure 4. TGA diagram for Fe3O4 MNPs (black curve), APTES-Fe3O4 (red curve), Cr-Salen-
MNPs (blue curve) and Cd-Salen-MNPs (green curve).
21

Figure 5. FT-IR spectra of Fe3O4 (a), APTES-Fe3O4 (b), Zn-salen- MNPs (c), Ni-salen- MNPs
(d), Cd-salen- MNPs (e), Co-salen- MNPs (f) and Cr-salen- MNPs (g).
22

100
90
80
70
60
50
40
30
20
10
0
1
2
3
4
5
Run number
6
7
8
9
10
Figure 6. Reuse of the M-salen-MNPs for the oxidative coupling of 2-mercapto benzothiazole.
23

Table 1. Oxidation of sulfides into sulfoxides using H2O2 in the influence of M-salen-MNPs at 35 ºC under solvent-free
condition.
Entry
1
Sulfide
Catalyst [Time (min)] Catalyst [Yield (%)a] Mp (oC) found [Reference]
I [20]
II [15]
III [20]
IV [60]
V [60]
I [25]
II [40]
III [20]
IV [30]
V [30]
I [5]
I [95]
II [95]
III [95]
IV [96]
V [96]
I [92]
II [95]
III [94]
IV [94]
V [99]
I [94]
II [94]
III [94]
IV [94]
V [94]
I [95]
II [96]
III [96]
IV [96]
V [96]
I [92]
II [88]
III [90]
IV [91]
V [90]
I [94]
II [92]
III [96]
IV [95]
V [94]
I [97]
II [96]
III [95]
IV [99]
V [96]
I [95]
Ph-S-Me
30-32 [19]
119-123 [30]
Oil [31]
Oil [19]
Oil [19]
Oil [32]
60-65
2
3
p-MePh-S-Me
Ph-S-CH2CH2OH
II [5]
III [5]
IV [5]
V [5]
I [10]
II [15]
III [10]
IV [10]
V [10]
I [5]
4
Methyl furfuryl sulfide
Tetrahydrothiophene
II [5]
5
III [5]
IV [5]
V [5]
I [30]
II [30]
III [30]
IV [30]
V [30]
I [30]
II [60]
III [15]
IV [120]
V [60]
I [10]
6
CH3(CH2)2CH2-S-CH2(CH2)2CH3
CH3(CH2)10CH2-S-CH3
CH3CH2CH2-S-CH2CH2CH3
CH3-S-CH2CH2OH
7
II [10]
III [10]
IV [5]
V [10]
I [5]
II [93]
III [94]
IV [93]
V [93]
I [97]
II [95]
III [91]
IV [96]
V [94]
I [94]
8
Oil [14]
Oil [14]
Oil [19]
II [5]
9
III [5]
IV [5]
V [5]
I [5]
II [5]
III [5]
IV [5]
V [5]
II [95]
III [93]
IV [91]
V [93]
10
CH3-S-CH2CH2CO2CH3
a Isolated yields
24

Table 2. Oxidative coupling of thiols into disulfides using hydrogen peroxide in the influence of M-salen-MNPs in ethanol at
room temperature.
Entry
1
Thiol
Catalyst [Time (min)] Catalyst [Yield (%)a] Mp (oC) found [Reference]
I [10]
II [25]
III [15]
IV [60]
V [30]
I [45]
II [60]
III [60]
IV [60]
V [60]
I [60]
I [98]
II [99]
III [99]
IV [98]
V [99]
I [99]
II [97]
III [97]
IV [97]
V [98]
I [99]
p-MePh-SH
36-38 [14]
48-53 [16]
2
3
4
5
6
7
Ph-SH
2-naphthylthiol
p-BrPh-SH
II [60]
III [60]
IV [60]
V [60]
I [30]
II [30]
III [30]
IV [30]
V [30]
I [60]
II [99]
III [99]
IV [99]
V [99]
I [99]
II [99]
III [99]
IV [99]
V [99]
I [99]
134-136 [14]
86-90 [14]
II [60]
III [60]
IV [60]
V [60]
I [2]
II [99]
III [99]
IV [99]
V [99]
I [99]
II [99]
III [99]
IV [99]
V [99]
I [99]
o-COOHPh-SH
o-NH2Ph-SH
278-284 [14]
87-89 [14]
II [2]
III [2]
IV [2]
V [2]
I [10]
II [30]
III [15]
IV [40]
V [40]
I [20]
II [99]
III [99]
IV [99]
V [99]
I [99]
2-mercaptobenzothiazole
174-176 [14]
II [20]
III [20]
IV [20]
V [20]
I [60]
II [60]
III [60]
IV [60]
II [99]
III [99]
IV [99]
V [99]
I [99]
II [99]
III [99]
IV [99]
8
9
4,6-Dimethyl-2-mercaptopyrimidine
161-165 [14]
56-58 [14]
PhCH2-SH
25

V [60]
I [2]
II [5]
III [5]
IV [2]
V [15]
I [10]
V [99]
I [92]
II [90]
III [92]
IV [91]
V [92]
I [96]
10
11
HOCH2CH2-SH
HO2CCH2-SH
Oil [14]
Oil [14]
II [10]
III [10]
IV [10]
V [10]
II [95]
III [96]
IV [97]
V [97]
a Isolated yields
Table 3. Comparison of M-salen-MNPs for the oxidation of methyl phenyl sulfide and oxidative coupling of 2-
mercaptobenzothiazole with previously reported procedure.
Entry
Substrate
p-MePh-SH
p-MePh-SH
Catalyst
Time (min) Yield (%)a [Reference]
1
2
VO-salen-MCM-41
Ni-salen-MCM-41
Cd-salen-MCM-41
SSA, KBr, wet SiO2
Cr-Salen-MNPs
Zn-Salen-MNPs
Cd-Salen-MNPs
Co-Salen-MNPs
Ni-Salen-MNPs
Ni-salen-MCM-41
Cd-salen-MCM-41
Cu(II) Schiff base
complex MCM-41
[bmim][SeO2(OCH3)]
Cr-Salen-MNPs
Zn-Salen-MNPs
Cd-Salen-MNPs
Co-Salen-MNPs
Ni-Salen-MNPs
120
156
150
120
20
95 [32]
95 [33]
98 [33]
3
p-MePh-SH
p-MePh-SH
4
55 [24]
5
p-MePh-SH
p-MePh-SH
95 [this work]
95 [this work]
95 [this work]
96 [this work]
96 [this work]
93 [33]
6
15
20
7
p-MePh-SH
p-MePh-SH
8
60
60
9
p-MePh-SH
2-mercaptobenzothiazole
2-mercaptobenzothiazole
10
11
45
20
93 [33]
12
2-mercaptobenzothiazole
155
74 [34]
13
14
15
16
17
18
2-mercaptobenzothiazole
2-mercaptobenzothiazole
2-mercaptobenzothiazole
2-mercaptobenzothiazole
2-mercaptobenzothiazole
2-mercaptobenzothiazole
120
10
30
15
40
40
99 [15]
99 [this work]
99 [this work]
99 [this work]
99 [this work]
99 [this work]
a Isolated yield
26

Graphical abstract
Schiff base complexes of Ni, Co, Cr, Cd and Zn supported on Fe3O4 magnetic
nanoparticles: as efficient and recyclable catalysts for the oxidation of sulfides and
oxidative coupling of thiols
Arash Ghorbani-Choghamarani*, Zahra Darvishnejad and Bahman Tahmasbi
Ni, Co, Cr, Zn or Cd complexes on Fe3O4 magnetic nanoparticles (M-Salen-MNPs), as
recyclable and efficient catalysts, have been applied for the chemoselective oxidation of sulfides
into sulfoxides and oxidative coupling of thiols into disulfides.
O
Si
N
O
Cd
O
O
O
S
O
H2O2, Cat.
SF, r.t.
O
S
O
Si
R2
R1
R2
R1
O
N
O
Ni
N
Co
O
reusable
of catalyst
...
....
...
....
.
...
.
...
....
Si
O
O
H2O2, Cat.
Ethanol, r.t
R
S
R
SH
R
S
O
O
O
Si
O
Cr
N
Zn
N
O
O
O
Si
O
27

Highlights
Fe3O4-M-Schiff base prepared via post-grafting method
Fe3O4-M-Schiff base characterized by, FT-IR, XRD, TGA, EDX and SEM techniques
Fe3O4-M-Schiff base catalyzed oxidation of sulfides and oxidative coupling of thiols
Catalysts could be easily recovered using a magnet and recycled
28