Nickel Schiff base complex anchored on Fe3O4@MCM-41 as a novel and reusable magnetic nanocatalyst and its application in the oxidation of sulfides and oxidative coupling of thiols using H2O2

Article abstract of DOI:10.1080/10426507.2018.1455200

A novel and reusable nanocatalyst was synthesized by anchoring a nickel Schiff base complex onto Fe₃O₄@MCM-41 (Fe₃O₄@MCM-41@Ni-P2C) and characterization was accomplished with Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), powder X-ray diffraction (XRD), vibrating sample magnetometry (VSM), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and atomic absorption spectroscopy (AAS) techniques. This catalytic system was efficiently used for oxidation of sulfides to sulfoxides and oxidative coupling of thiols to corresponding disulfides using H₂O₂ as green oxidant at room temperature. These reactions were carried out in a green solvent (ethanol) and/or under solvent-free conditions with short reaction time, complete selectivity and very high conversion under mild reaction conditions. More importantly, separation and recycling of this magnetic catalyst can be easily done through a simple and low cost magnetic separation process.

Full text of DOI:10.1080/10426507.2018.1455200

Phosphorus, Sulfur, and Silicon and the Related Elements
ISSN: 1042-6507 (Print) 1563-5325 (Online) Journal homepage: http://www.tandfonline.com/loi/gpss20
Nickel Schiff base complex anchored on
Fe3O4@MCM-41 as a novel and reusable magnetic
nanocatalyst and its application in the oxidation of
sulfides and oxidative coupling of thiols using H2O2
Mohsen Nikoorazm, Farshid Ghorbani, Arash Ghorbani-Choghamarani &
Zahra Erfani
To cite this article: Mohsen Nikoorazm, Farshid Ghorbani, Arash Ghorbani-Choghamarani &
Zahra Erfani (2018): Nickel Schiff base complex anchored on Fe3O4@MCM-41 as a novel and
reusable magnetic nanocatalyst and its application in the oxidation of sulfides and oxidative
coupling of thiols using H2O2, Phosphorus, Sulfur, and Silicon and the Related Elements, DOI:
10.1080/10426507.2018.1455200
To link to this article: https://doi.org/10.1080/10426507.2018.1455200
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Publisher: Taylor & Francis
Journal: Phosphorus, Sulfur, and Silicon and the Related Elements
DOI: https://doi.org/10.1080/10426507.2018.1455200
Nickel Schiff base complex anchored on Fe3O4@MCM-41 as a novel
and reusable magnetic nanocatalyst and its application in the
oxidation of sulfides and oxidative coupling of thiols using H2O2
Mohsen Nikoorazm1,*. Farshid Ghorbani 2. Arash Ghorbani-Choghamarani1. Zahra
Erfani1
1Department of Chemistry, Faculty of Science, Ilam University, P. O. Box69315516, Ilam, Iran.
2Department of Environment, Faculty of Natural Resource, University of Kurdistan, 66177-15177 Sanandaj, Iran.
Abstract: A novel and reusable nanocatalyst was synthesized by anchoring a nickel Schiff base
complex onto Fe3O4@MCM-41 (Fe3O4@MCM-41@Ni-P2C) and characterization was
accomplished with Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis
(TGA), powder X-ray diffraction (XRD), vibrating sample magnetometry (VSM), transmission
electron microscopy (TEM), scanning electron microscopy (SEM) and atomic absorption
spectroscopy (AAS) techniques. This catalytic system was efficiently used for oxidation of
sulfides to sulfoxides and oxidative coupling of thiols to corresponding disulfides using H2O2 as
green oxidant at room temperature. These reactions were carried out in a green solvent (ethanol)
and/or under solvent-free conditions with short reaction time, complete selectivity and very high
conversion under mild reaction conditions. More importantly, separation and recycling of this
magnetic catalyst can be easily done through a simple and low cost magnetic separation process.

Graphical Abstract

Keywords: Schiff base complex, Fe3O4@MCM-41@Ni-P2C, oxidation, sulfoxides, disulfides.
1. Introduction
The applications of conventional homogeneous catalysis in organic synthesis because of
difficulties in separation, reusability and contamination of the products by residual catalyst have
been restricted [1]. In the fine chemical industries, convert of homogeneous catalytic reactions to
heterogeneous versions through different methods is one of the major research directions. For
this reason, immobilization of homogeneous catalysts on various solid supports with the
advantage of catalysts separation and recycling is one strategy that developed in recent years [2].
However, immobilization of homogeneous catalysts decrease the activity and selectivity of
catalysts. This difficulty can be overcome using nanomaterials as desirable heterogeneous
supports. When the size of the support is decreased to the nanometer scale, the nano-sized
catalysts can be used as efficient bridge and fill the split between homogeneous and
heterogeneous catalysts [3-6]. Among the different types of supports used in heterogeneous
catalysis, mesoporous silica MCM-41 has been employed as useful and versatile solid support
for homogeneous catalyst immobilization. MCM-41 belongs to the M41S mesoporous molecular
sieve family, with hexagonal array of unidirectional pore structures which reported by Beck, et al
[7, 8]. These materials owing to their high thermal stability [9], high pore volume [10], high
surface area [11], very narrow pore size distribution [12] and chemical and mechanical stability
[13] are ideal to use as support.
However, mesoporous silica powders are difficult to separate by filtration techniques because of
their small sizes, which hinder their utilization in liquid–solid, processes [14]. This could be
solved using magnetic nanoparticles (MNPs), which can be quickly and easily removed from the
reaction mixture by magnetic separation process [15]. In the other hand, magnetic separation is
easier and faster than filtration or centrifugation, economical, and promising era for industrial
applications [6]. A novel material can be obtained by integration magnetic nanoparticles with
hexagonal mesoporous silica spheres to form porous magnetic nanocomposites. This type of
magnetic nanostructures, have the properties and advantage of both mesoporous silica and
magnetic nanoparticles [16, 17].
Recently, Schiff base transition metal complexes have been emerged for the development of a
variety of catalysts because of their excellent catalytic performances in a wide range of organic
transformations [18]. Among transition metals, Ni complexes of salen type in comparison with
traditional catalysts are safe, non-toxic, and being cheap, require only mild reaction conditions
for good yields of products in short reaction times and have recently been employed as efficient
catalysts [19].

Catalytic conversion of organosulfur compounds such as sulfides and thiols into their
corresponding sulfoxides and disulfides respectively is a topic of interest in several chemical
areas, due to their applications in biological, medical, synthetic and petroleum industry [20, 21].
For example, pantoprazole, omeprazole, lansoprazole and pesticide fipronil are typical examples
of the pharmaceutically important sulfoxide derivatives in pharmaceutical and fine chemical
industries [22-24]. Likewise Disulfide bonds play special roles in both biological (DNA
cleavage, stabilization of peptides) and chemical (protecting groups, vulcanizing agents
sulfonylation of enolates and other anions) processes [25, 26]. In this study, we wish to disclose
an efficient and rapid protocol using cheap, easily recyclable of new Schiff-base complexes of
nickel anchored to magnetic nanoporous MCM-41 as a heterogeneous nano-organometallic
catalyst for the oxidation of sulfides to sulfoxides and also oxidative coupling of thiols into the
corresponding disulfides by H2O2 as oxidant under mild reaction conditions.
2. Result and discussion
In this research we describe the preparation of Ni (II) Schiff-base complex anchored to
Fe3O4@MCM-41 as a novel and reusable magnetic nanocatalyst (Scheme 1). The structure of
this catalyst was studied and fully characterized by FT-IR, VSM, EDS, TEM, SEM, XRD, and
TGA techniques.

OH
OH
OH
FeCl3.6H2O
FeCl2.6H2O
NH4OH, CTAB
N2 atmosphere
EtOH
NH3
CTAB
TEOS
Deionized water
Magnetic Fe3O4
Fe3O4@MCM-41
O
O
Si
NH2
N
O
Si
N
O
CHO
H
O
O
N
H
O
O
O
Si
NH2
H
O
Si
N
N
O
n-hexane, 24 h reflux,
N2 atmosphere
O
EtOH, 3 h reflux,
N2 atmosphere
Fe3O4@MCM-41-NH2
F
e
O
@
M
C
M
-4
1
@
P
2C
3
4
O
2
H
6
.
x
)
2
u
l
f
3
O
e
r
N
E
(
i
.
N
h
N
0
N
O
2
,
Si
O
O
H
O
t
Ni
O
N
N
Si
O
O
Fe3O4@MCM-41@Ni-P2C
Scheme 1. Synthesis of Fe3O4@MCM-41@Ni-P2C.

2.1. Characterization of Fe3O4@MCM-41@Ni-P2C
The low angle XRD pattern of Fe3O4@MCM-41 and Fe3O4@MCM-41@Ni-P2C ( Fig. 1a )
shows three reflection peaks of (d100), (d110) and (d200) at 2θ = 2.52°, 3.94° and 4.38°,
respectively, indexing to the hexagonal regularity of Fe3O4@MCM-41 mesostructure.
Comparison of X-ray powder diffraction patterns of Fe3O4@MCM-41 and Fe3O4@MCM-
41@Ni-P2C shows after the anchoring of Nickel (II) complex onto Fe3O4@MCM-41 the
intensity of the main peak (d100) was decreased which means that the hexagonal mesostructures
are less ordered due to incorporation of Ni (II) Schiff base complex into the network of
Fe3O4@MCM-41.
The high angle XRD pattern for the Fe3O4@MCM-41 and Fe3O4@MCM-41@Ni-P2C samples
are shown in Fig. 1b. The characteristic peaks at 2θ = 30.26°, 35.61°, 42.96°, 54.1°, 57.1° and
62.86° assigned to magnetite were observable for samples which in good agreement with the
standard Fe3O4 XRD spectrum and demonstrated that catalyst had been successfully synthesized
[27]. The broad peak appeared in around 2θ = 20-30o are related to amorphous silica [28].

Fig. 1. XRD patterns of (a) low angle and (b) high angle of Fe3O4@MCM-41 and
Fe3O4@MCM-41@Ni-P2C.

Fig. 2 shows the Fourier-transform infrared (FT-IR) spectra for the synthesized (a) Fe3O4, (b)
Fe3O4@MCM-41,(c) Fe3O4@MCM-41-NH2, (d) Fe3O4@MCM-41@P2C, (e) Fe3O4@MCM-
41@Ni-P2C and (f) Recovered Fe3O4@MCM-41@Ni-P2C. The FT-IR spectrum for Fe3O4
nanoparticles shows stretching vibration at 3200–3500 cm-1 which proofs the contributions of
both symmetrical and asymmetrical modes of O–H bonds attached to the surface of the magnetic
nanoparticles. The strong absorption near 579 cm-1 is characteristic of Fe–O bond stretching
vibration. In the spectrum of Fe3O4@MCM-41 sample three peaks at 460, 958 and 1081cm-1
corresponded to bending, symmetric stretching and asymmetric stretching of Si-O–Si vibrations
respectively. The Fe3O4@MCM-41-NH2 sample has typical NH2 bending at 1546 cm-1, confirms
the successful functionalization of the Fe3O4@MCM-41 with 3-APTES (spectrum c). The band
at 1637 cm-1 was assigned to C=N stretching, after the anchoring of Nickel (II) into the network
of Fe3O4@MCM-41@ P2C, this band shifts to lower frequency and appears at 1625 cm-1, which
indicates the formation of Ni -ligand bonds [3]. the spectrum of the recovered Fe3O4@MCM-
41@Ni-P2C, shows that the catalyst is stable during the oxidation reactions.

Fig 2. FT-IR spectra of (a) Fe3O4 (b) Fe3O4@MCM-41(c) Fe3O4@MCM-41@nPr-NH2 (d)
Fe3O4@MCM-41@P2C (e) Fe3O4@MCM-41@Ni-P2C and (f) Recovered Fe3O4@MCM-
41@Ni-P2C.

The morphologies and size of Fe3O4@MCM-41 and Fe3O4@MCM-41@Ni-P2C are shown in
Fig. S 1 (Supplemental Materials). The SEM images of Fe3O4@MCM-41@Ni-P2C show that
the catalyst consists of uniform spherical particles with uniform nanometer size. To characterize
the catalyst in terms of the nickel metal anchored to Fe3O4@MCM-41 surface, EDS analysis of
Fe3O4@MCM-41@Ni-P2C was conducted. As shown in Fig. 3, the EDS spectrum of the catalyst
showed the presence of, Fe, Si, O, N, C and Ni species in the catalyst.

Fig 3. The EDS spectrum of Fe3O4@MCM-41@Ni-P2C.
FeK
2000
1500
1000
500
0
SiK
O K
N K
FeL
FeK
NiL
C K
NiK
NiK
keV
10.00
0

The TEM images of the Fe3O4@MCM-41@Ni-P2C are shown in Figure S 2 (Supplemental
Materials). The TEM micrograph showed agglomeration of many ultrafine spherical-like
particles which display dark magnetite cores surrounded by a mesoporous shell.
To investigate the magnetic properties of (a) Fe3O4@MCM-41 and (b) Fe3O4@MCM-41@Ni-
P2C, magnetic measurements were carried out in an applied magnetic field at room temperature
and the obtained results are presented in Fig. 4. The saturation magnetization (Ms) observed for
Fe3O4@MCM-41 and Fe3O4@MCM-41@Ni-P2C, were found to be 13.06 and 8.90 emu g-1,
respectively. Decreasing the magnetic properties of the Fe3O4@MCM-41@Ni-P2C (13.06
→8.90 emu g-1), compared with the Fe3O4@MCM-41 is due to the coating of the Fe3O4@MCM-
41 by organic layers and a nickel complex.

Fig 4. Magnetic curves of (a) Fe3O4@MCM-41 and (b) Fe3O4@MCM-41@Ni-P2C.

The thermogravimetric analysis (TGA) curves of Fe3O4@MCM-41 and Fe3O4@MCM-41@Ni-
P2C are shown in Fig. 5. The TGA curves of Fe3O4@MCM-41 show a small amount of weight
loss below 200 °C which can be attributed to evaporation of physically adsorbed water and
solvent inside the pores channel. TGA curve of Fe3O4@MCM-41@Ni-P2C shows three step
weight losses. In the first step, Fe3O4@MCM-41@Ni-P2C showed 2.9 % weight loss between
25-200 ˚C. This mass loss seems to be due to removal of adsorbed water and solvent, the second
mass loss about 16% appearing at 200 to 300 ˚C is due to decomposition of the organically
modified framework. Upon further heating, the TGA analysis data of Fe3O4@MCM-41@Ni-P2C
sample indicated the third 17% weight loss between 300 to 800 ˚C is likely attributed to
decomposition of silanol groups.

Fig. 5. TGA curves of Fe3O4@MCM-41 and Fe3O4@MCM-41@Ni-P2C.

Finally, the exact loading of Ni on Fe3O4@MCM-41 has been determined by atomic absorption
spectroscopy (AAS) technique. Based on AAS, the Ni amount of the immobilized catalyst on
Fe3O4@MCM-41 was found to be 2.8 × 10-4 mole g−1.
2.2. Evaluation of the catalytic activity of Fe3O4@MCM-41@Ni-P2C in oxidation reaction
After full characterization of the prepared nanocatalyst, in the present work we wish to describe
highly efficient and simple method for the oxidation of sulfides and oxidative coupling of thiols
using hydrogen peroxide in the present of these catalysts (Scheme 2).

O
S
Fe3O4@MCM-41@Ni-P2C
solvent-free, H2O2, r.t.
S
R1
R1
R2
R2
Fe3O4@MCM-41@Ni-P2C
EtOH, H2O2, r.t.
R3
S
R3-SH
R3
S
Scheme 2. Fe3O4@MCM-41@Ni-P2C catalyzed oxidation of sulfides to sulfoxides and
oxidative coupling of thiols into disulfides using H2O2.

2.3. Oxidation of sulfides to sulfoxides
In order to determine the best reaction conditions, different parameters were tested for oxidation
of methyl phenyl sulfide as model reaction, including catalyst amounts, use of different amounts
of H2O2, and various solvents (Table 1). To determine the role of catalyst, the model reaction
was carried out in the absence of catalyst under solvent-free conditions. It was observed that the
formation of product did not occur after 12 h (Table 1, entry 1). To improve the yield and
optimize the reaction conditions, the same reaction was carried out in the presence of catalyst
amount, it was observed that the best results were obtained with (10 mg, 0.28 mol%) catalyst
(Table 1, entry 2). Subsequently, the model reaction was performed in different solvents such as
acetonitrile, ethanol, ethyl acetate, water, dichloromethane and also solvent-free conditions
(Table 1, entries 2-7). It was found that the reaction under solvent-free conditions lead to the
formation of product in higher yield after shorter reaction time. Finally, the reaction was
performed using different amounts of H2O2, which the best results were observed when using 0.4
mL of H2O2 .

Table 1. Comparison between various conditions for the oxidation of methyl phenyl
sulfide in 90 min at room temperature
Entry
Amount of catalyst (mg)
Amount of
oxidant (mL)
Solvent
Time
(min)
Yield (%)a
1
2
3
4
None
10
0.4
0.4
0.4
0.4
Solvent free
Solvent free
Acetonitrile
12h
Trace
98
90
10
90
86
10
Dichloromet
hane
90
84
5
10
10
10
8
0.4
0.4
0.4
0.4
0.4
0.3
0.5
Ethanol
Ethyl acetate
H2O
90
90
90
90
90
90
90
92
45
35
92
82
70
85
6
7
8
Solvent free
Solvent free
Solvent free
Solvent free
9
6
10
11
10
10
Reaction conditions: Methyl phenyl sulfide (1 mmol), H2O2 (0.4 mL) and catalyst (10 mg) under solvent-free at room
temperature.
a Isolated yield.

After optimization of the reaction conditions, to determine the efficiency and the scope of this
method, a variety of substituted aromatic and aliphatic sulfides was tested for oxidation under the
optimized condition and the results are summarized in Table 2. As it was shown, all the reactions
occurred with high selectivity for sulfoxide formation; no over oxidation products such as
sulfones were obtained.

Table 2. Oxidation of sulfides to the sulfoxides with H2O2 in the presence of a catalytic
amount of Fe3O4@MCM-41@Ni-P2C under solvent-free at room temperature.
Time
(min)
Yield
(%)a
m. p. (°C)
[Ref.]
Entry
1
Substrate
Product
S
S
O
100
30
94
96
133-135 [29]
Oil [6]
O
S
S
2
3
O
S
S
OH
OH
45
10
90
91
92
98
oil [6]
Oil [30]
30-32 [6]
O
4
5
S
S
S
O
S
O
6
7
10
60
93
92
Oil [17]
S
S
121-122 [6]
S
S
O
S
8
9
480
300
90
88
115-117 [31]
87-89 [32]
S
O
O
S
C12H25
C12H25
S
C12H25
C12H25

O
S
S
10
15
94
103-105 [32]
Reaction conditions: substrate (1 mmol), H2O2 (0.4 mL) and catalyst (10 mg) under solvent-free at room
temperature.
a Isolated yield.

2.4. Oxidative coupling of thiols to disulfide
In second part of this study to show the general applicability of Fe3O4@MCM-41@Ni-P2C, we
decided to examine the oxidative coupling of thiols to disulfides. In order to find optimal
reaction conditions, 4-methylthiophenol was selected as model substrate and influence of various
reaction parameters such as different amount of oxidant, catalyst and nature of the solvent have
been studied (Table 3). Among the solvents investigated, ethanol was chosen as the best solvent
due to the high yield and shorter reaction time (Table 3, entry 2). To investigate the effect of
different amount of oxidant, the reaction was performed using different amounts of H2O2, the best
results were obtained with 0.4 mL of H2O2. The effect of catalyst amount on the outcome of the
reaction was also evaluated and the best result was obtained with 0.01 g of catalyst. With optimal
conditions in hand, a variety of thiols with different functional groups converted into
corresponding disulfides using H2O2 and catalytic amounts of Fe3O4@MCM-41@Ni-P2C in
ethanol with good to excellent yields (Table 4).

Table 3. Effect of catalyst, oxidant and solvents variation on the oxidative coupling of 4-
methylthiophenol in 25 min at room temperature.
Entry
Amount of catalyst (mg)
Amount of
oxidant (mL)
Solvent
Ethanol
Time
(min)
Yield (%)a
1
2
3
4
None
10
0.4
0.4
0.4
0.4
12h
Trace
96
Ethanol
25
10
Acetonitrile
25
88
10
Dichloromet
hane
25
85
5
6
10
10
10
8
0.4
0.4
0.4
0.4
0.4
0.3
0.5
n-hexane
Ethyl acetate
H2O
25
25
25
25
25
25
25
91
90
75
90
82
78
86
7
8
Ethanol
Ethanol
Ethanol
Ethanol
9
6
10
11
10
10
Reaction conditions: 4-methylthiophenol (1 mmol), H2O2 (0.4 mL) and catalyst (10 mg) in EtOH (4 mL) at room
temperature.
a Isolated yield.

Table 4. Oxidative coupling of thiols into disulfides with H2O2 in the presence of a catalytic
amount of Fe3O4@MCM-41@Ni-P2C in EtOH at room temperature.
E
ntry
Time
(min)
Yield
(%)a
m. p. (°C)
[Ref.]
Substrate
Product
SH
S
S
1
2
35
25
92
97
66-69 [33]
SH
S
S
39-41 [17]
H3C
CH3
H3C
S
S
S
SH
S
S
S
S
3
4
45
55
98
172-175 [17]
N
N
N
O
O
N
O
N
SH
SH
86
76-79[34]
N
5
45
35
97
94
60-62 [34]
Oil [6]
S
S
HOOC
HO
S
S
HOOC
HO
SH
SH
S
COOH
OH
6
7
S
25
60
45
96
Oil [17]
COOH
COOH
COOH
S
S
SH
8
9
98
276-280[6]
133-136 [6]
SH
S
S
96

SH
S
S
10
20
97
90-91[17]
Br
Br
Br
Reaction conditions: substrate (1 mmol), H2O2 (0.4 mL) and catalyst (10 mg) in EtOH (4 mL) at room
temperature.
a Isolated yield.

2.5. Recyclability of the catalyst
The simple recovery and easy reusability of catalysts is one of their most important advantages,
making them useful for commercial applications. In this purpose, the recyclability of
Fe3O4@MCM-41@Ni-P2C was studied in oxidation of methyl phenyl sulfide and oxidative
coupling of 4-methylthiophenol. After completion of the reaction, catalyst was separated by an
external magnet. The remaining catalyst after washing with ethanol and drying reused in the next
run without any purification. As shown in Figure S 3 (Supplemental Materials), the catalyst
could be recycled over six runs without significant loss of catalytic activity. Also, the nature of
the recovered catalyst was investigated by FT-IR spectra (Fig. 2 f).
In order to investigate the efficiency of this procedure, we compared the results on the oxidation
of the methyl phenyl sulfide (entries 1–6) and oxidative coupling of 4-methylthiophenol (entries
7–12) with the previously reported procedures in the literature (Table 5). It is obvious that
described procedure in this study clearly shows good reaction time and higher yield than the
other catalysts which have been reported previously.

Table 5. Comparison of Fe3O4@MCM-41@Ni-P2C for the oxidation of methyl phenyl
sulfide and oxidative coupling of 4-methylthiophenol with previously reported procedure.
En
try
Substrate
Catalyst
Time
(min)
Yield
(%)
Ref.
This
1
Methylphenylsulfide
Fe3O4@MCM-41@Ni-P2C
90
98
work
2
3
4
5
6
7
Methylphenylsulfide
Methylphenylsulfide
Methylphenylsulfide
Methylphenylsulfide
Methylphenylsulfide
4-methylthiophenol
Bromide ion
Ion Exchange Resin
NBS
VO2F(dmpz)2
Thiourea dioxide/TBHP
Fe3O4@MCM-41@Ni-P2C
165
180
270
300
210
25
85
98
93
95
93
97
35
36
37
38
39
This
work
8
9
10
11
12
4-methylthiophenol
4-methylthiophenol
4-methylthiophenol
4-methylthiophenol
4-methylthiophenol
(C2H5)3NH+[CrO3F]-
[bmim][SeO2(OCH3)]
Co-Salophen
115
120
180
1440
40
84
99
96
93
85
40
41
42
43
44
NaI
Fe(NO3)3_9H2O/Fe(HSO4)3