Schiff base Mn(III) and Co(II) complexes coated on Co nanoparticles: An efficient and recyclable magnetic nanocatalyst for H2O2 oxidation of sulfides to sulfoxides

Article abstract of DOI:10.1039/c7ra11225d

In this paper, an effective and selective heterogeneous catalyst was produced by immobilization of manganese and cobalt Schiff base-complexes on Co magnetite nanoparticles (MNP). The catalysts Co@SiO₂[(EtO)₃Si-L³]/M (M = Mn(iii) and Co(ii)) were synthesized using Co@SiO₂ core-shell nanoparticles and amino-functionalized Co@SiO₂. The Schiff base ligand Co@SiO₂[(EtO)₃Si-L³] was synthesized by reacting Co@SiO₂ core-shell nanoparticles with 2-hydroxy 1-naphthaldehyde for the synthesis of Co@SiO₂[(EtO)₃Si-L³]/M. The catalysts were characterized by several techniques, such as FT-IR, TEM, XRD, TGA and VSM. The catalytic activities of the prepared catalysts were studied by oxidation of sulfides to the sulfoxides under different conditions. These catalysts can be easily recovered and reused in at least seven sequential cycles without considerable leaching and loss of reactivity.

Full text of DOI:10.1039/c7ra11225d

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PAPER
Schiff base Mn(III) and Co(II) complexes coated on
Co nanoparticles: an efficient and recyclable
magnetic nanocatalyst for H O oxidation of
Cite this: RSC Adv., 2018, 8, 3889
2
2
sulfides to sulfoxides
ab
a
c
Shokoufeh Ghahri Saremi,* Hassan Keypour, Mohammad Noroozi
b
and Hojat Veisi
In this paper, an effective and selective heterogeneous catalyst was produced by immobilization of
manganese and cobalt Schiff base-complexes on Co magnetite nanoparticles (MNP). The catalysts
3
Co@SiO
2
[(EtO)
3
Si–L ]/M (M
¼
Mn(III) and Co(II)) were synthesized using Co@SiO
2
core–shell
3
nanoparticles and amino-functionalized Co@SiO
2
. The Schiff base ligand Co@SiO [(EtO)
2
3
Si–L ] was
synthesized by reacting Co@SiO
2
core–shell nanoparticles with 2-hydroxy 1-naphthaldehyde for the
Received 11th October 2017
Accepted 5th January 2018
3
2 3
synthesis of Co@SiO [(EtO) Si–L ]/M. The catalysts were characterized by several techniques, such as
FT-IR, TEM, XRD, TGA and VSM. The catalytic activities of the prepared catalysts were studied by
oxidation of sulfides to the sulfoxides under different conditions. These catalysts can be easily recovered
and reused in at least seven sequential cycles without considerable leaching and loss of reactivity.
DOI: 10.1039/c7ra11225d
rsc.li/rsc-advances
22
Mn(III) complex supported on magnetic cobalt nanoparticles,
Schiff base complexes of Ni, Co, Cr, Cd and Zn supported on
Introduction
23
3 4
Since the oxidation reactions have potential for generating Fe O magnetic nanoparticles, magnetic nanoparticle immo-
24
25
a wide range of drugs and ne chemicals, they are very impor- bilized N-propylsulfamic acid,
Au/CTN–silica catalyst,
tant to the chemical industry. Conventionally, oxidation reac- cobalt(II), copper(II), zinc(II) and palladium(II) Schiff base
26
tions have been carried out using environmentally unfavorable complexes, 1,4-bis(3-methylimidazolium-1-yl) butane ditri-
28
27
reagents, solvents and catalysts. Hence, efforts have been made bromide, carboxylated multi-walled carbon nano tubes,
towards the development of efficient, easily recoverable, highly 1,4-bis(3-methylimidazolium-1-yl) butane ditribromide
ionic liquid reagent and Mo(VI) complex sup-
29
selective and reusable heterogeneous catalysts. Recently [bMImB](Br
magnetic nanomaterials have attracted signicant attention in ported on Fe
oxidation reactions because of their catalytic activity and easy neous nanocatalysts were synthesized by chemical modication
3 2
)
30
3 4
O nanoparticles. In this study, new heteroge-
1–5
separation.
of surface magnetic nanoparticles of Co with Schiff-base
Schiff base transition metal complexes as catalysts have been ligands, followed by complex formation through the reaction
widely used because of their potential use in numerous reac- with Co(II) and Mn(III) salts. Furthermore, the catalytic behavior
6
,7
8
tions, such as hydrogenation of organic substrates, epoxi- of these complexes was investigated by carrying out the oxida-
9
10,11
dation of olens, conversion of epoxides into halohydrines,
asymmetric ring opening of terminal epoxides and oxidation
reactions.
tion of suldes to sulfoxides under mild conditions.
12
13–15
In recent years some publications have been re-
ported suldes-oxidation using phthalazine-based di-iron Experimental
1
6
complexes,
b-brominated
meso-tetraphenylporphyrinato
Materials and equipments
17
18
manganese(III) acetate, copper Schiff base complex, immo-
2 5
All solvents used in this study such as THF, C H OH, DMSO,
19
bilized metalloporphyrins, TsOH by phenyliodine diacetate as
2 2 3
DMF, CH Cl , ethyl acetate, MeOH, CHCl , acetone, and MeCN
2
0
21
an oxidant, dendritic bis(acylamino) pyridines, Schiff base of
(analytical grade) were purchased from Fluka or Merck and
used without further purication. In addition, cobalt chloride
hexahydrate, citric acid, sodium borohydride, 3-aminopropyl-
triethoxysilane (APTES), tetraethoxysilane (TEOS), 2-hydroxy
a
Faculty of Chemistry, Bu-Ali Sina University, Hamedan 65174, Iran
b
Department of Chemistry, Payame Noor University, Tehran, Iran. E-mail: sho.
1
-naphthaldehyde, Mn (acac) , H O (analytical grade 30%
saremi@gmail.com
3 2 2
c
3 2
aqueous solution), and Co(CH COO) were purchased from
Center for Research and Development of Petroleum Technologies at Kermanshah,
Research Institute of Petroleum Industry (RIPI), Iran
Fluka or Merck. HPLC grade methyl phenyl sulde and other
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sulde compounds were purchased from Aldrich in order to test water, 0.07 g of citric acid was added and the solution was
the performance of the catalysts. stirred for 2 min under nitrogen atmosphere. Then, a solution
Nanocatalysts were characterized using a Holland Philips of 3 g sodium borohydride in 100 mL distilled water was added
ꢀ
PW-1840 with monochromatic Cu Ka radiation and l 1.54 A dropwise to the stock solution under nitrogen atmosphere.
4
X-ray powder diffraction (XRD) diffractometer at a scanning During the addition of NaBH , cobalt nanoparticles were
ꢀ
ꢁ1
ꢀ
ꢀ
speed of 2 min from 5 to 80 (2q). The particle size and formed and the color of the solution changed to black. The
morphology were investigated using scanning electron micros- nanoparticles were rst stabilized by the addition of citrate.
copy (SEM) (Holland Philips XL30 microscope) at an accelerating Aer reduction, the particles remained stable for several hours.
ꢁ
1
voltage of 25 kV. The FT-IR measurements (4000–400 cm ) were A perfect monomer formation is generally favored through
performed using KBr disc on a Shimadzu Fourier Transform decreased concentration of cobalt nuclei and good long-time
1
13
Infrared spectra (FT-IR) 8400. The H- and C-spectra were dispersion aer citrate stabilization to ensure that the core
recorded at 90 MHz, on a Bruker DRX 500-Avance FT NMR particles are well separated before coating begins.
instrument with CDCl
3
as the solvent. Elemental analysis for C,
Second, the cobalt magnetic core was coated with the SiO
. The solu-
2
H and N were performed using a Perkin-Elmer 2400 series shell to obtain core–shell nanoparticles Co@SiO
2
analyzer. Thermogravimetric Analysis (TGA) was performed on tion containing Co nanoparticles was stirred for 30 min. Then,
ꢀ
a Mettler Toledo TGA SDTA 85-e instrument at 30–800 C with under nitrogen atmosphere, 200 mL ethanolic solution of
ꢀ
ꢁ1
a temperature gradient of 10 C min
.
3-aminopropyl-triethoxysilane (APTES) and 800 mL tetrae-
Preparation of the magnetic Co@SiO –NH amine (3). The thoxysilane (TEOS) was sequentially added to the solution. The
2
2
overall synthesis process of the nanocatalyst Co@SiO [(EtO) - nitrogen atmosphere was discontinued and the resultant
2
3
3
Si–L ]/M (M ¼ Co(II) and Mn(III)) (6) is shown in Scheme 1. solution was vigorously stirred for 24 h. Finally, the obtained
Magnetic Co@SiO –NH amine was synthesized according to black precipitate was collected with an external magnet,
the procedure previously reported in literature. First, to washed twice with deionized water and ethanol and dried at
2
2
15
ꢀ
a solution of 1 g cobalt chloride hexahydrate in 60 mL distilled 50 C in the oven.
3
Scheme 1 Preparation of Co@SiO
2
[(EtO)
3
Si–L ]/M (M ¼ Mn(III) and Co(II)) nanocatalysts.
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Further, 3.0 g of the resultant Co@SiO
50 mL toluene for 50 min via sonication. Then, 11.25 mL
48.06 mmol) of APTES was added dropwise to the mixture with
2
was dispersed in
1
(
vigorous mechanical stirring. The mixture was reuxed for 15 h.
Eventually, the reaction mixture was cooled and the resultant
precipitate was collected with an external magnet, washed twice
ꢀ
with deionized water and ethanol and the dried at 55 C in the
oven.
3
Preparation of Co@SiO
2
[(EtO)
3
Si–L ] (5). For the preparation
of the Schiff base ligand (5), the obtained amine in the previous
step was reacted with the 2-hydroxy-1-naphthaldehyde.
First, 1.0 g of Co@SiO –NH was dispersed in 60 mL of dry
2
2
methanol by sonication for 20 min. Then, 1.20 g (7 mmol) of 2-
hydroxy 1-naphthaldehyde was added to the mixture. Aer
ꢀ
stirring at 50 C for 24 h, the reaction ask was le undisturbed
to be cooled. Then, the obtained precipitate was collected with
an external magnet and washed twice with dry methanol and
dried under vacuum for 20 h.
3
Preparation of Co@SiO [(EtO) Si–L ]/M (M ¼ Co(II) and
2
3
3
Mn(III)) (6). Initially, 300 mg of Co@SiO [(EtO) Si–L ] was
2
3
ultrasonically dispersed in 30 mL ethanol for 30 min. Then,
Mn (acac) (1 mmol) was dissolved in 10 mL ethanol. This
solution was added to the dispersion of Co@SiO
The mixture was reuxed for 10 h. Then, the Co@SiO
3
3
2
[(EtO)
3
Si–L ].
3
Fig. 1 FT-IR spectra of (a) Co@SiO
2
–NH
2
, (b) Co@SiO
2
[(EtO)
3
Si–L ],
3
2 3 2 3
3
2
[(- (c) Co@SiO [(EtO) Si–L ]/Mn(III) and (d) Co@SiO [(EtO) Si–L ]/Co(II).
3
3
EtO) Si–L ]/Mn(III) was isolated by an external magnet and
washed with ethanol, water and acetone, in sequence. All the
above steps were also performed for the preparation of
ꢁ1
peak at 1082 cm indicates the formation of silica layer on
magnetic cobalt NPs.
3
Co@SiO [(EtO) Si–L ]/Co with Co(CH COO) instead of Mn
2
3
3
2
(
acac)
The Mn content in Co@SiO
determined to be 4.41 wt% by atomic absorption spectroscopic
3
.
In the FT-IR spectrum of 2-hydroxy-1-naphthaldehyde,
a band appears at 1652 cm , which is assigned to (C]O)
vibration of carbonyl group. In the FT-IR spectrum of
Si–L ] (curve b), this band was shied to
(lower wavenumber), which is attributed to the
formation of the imine bond (C]N) on the surface of the
magnetic cobalt NPs. Vibrations in the range of 1480–1600 cm
can be assigned to the aromatic ring.
The IR spectrum of Co@SiO [(EtO) Si–L ]/Mn(III) complex
curve c) shows a vibrational peak at 1638 cm , corresponding
to (C]N) bond. The shi in stretching vibration to lower
frequency in the complex in comparison to free ligand
1641 cm ) indicates the formation of metal–ligand bonds.
2 3
This band in the complex Co@SiO [(EtO) Si–L ]/Co(II) (curve d)
3
2
[(EtO)
3
Si–L ]/Mn(III) catalyst was
ꢁ1
2
(
AAS) analysis. The percent cobalt in the Co@SiO
2
[(EtO)
3
Si–L ]/
3
Co@SiO
2
ꢁ1
[(EtO)
3
Co(II) was determined to be 12.43 wt%. This value is attributed
to the cobalt magnetic core and metal in the complex. The
prepared catalysts were characterized by FT-IR, XRD, FESEM,
EDX, TEM, and VSM.
1641 cm
ꢁ1
General procedure for oxidation of suldes to sulfoxides. In
3
2
3
a typical reaction, a 25 mL reaction ask was charged with 0.02 g
ꢁ1
(
3
of the catalyst Co@SiO
2
[(EtO)
3
Si–L ]/M, 1 mmol of the methyl
phenyl sulde, 3 mL of ethyl acetate and 60 mL H
reaction mixture was stirred at 50 C. The progress of reaction
was monitored by thin layer chromatography (TLC).
2
O
2
(30%). The
ꢀ
ꢁ1
(
3
ꢁ
1
appears at 1637 cm . From the comparison between these
spectra, it can be concluded that rst, the reaction for the
formation of an imine has occurred, and second, the reaction
for the formation of the complex occurred at the surface of the
nanoparticles.
Results and discussion
Characterization of the catalysts
Fig. 1 shows the FT-IR absorption spectrum of Co@SiO
2
–NH
2
Fig. 2 illustrates the TGA curves, showing the mass loss of
the samples as they decompose upon heating. The heating
3
3
(
a), Co@SiO
and Co@SiO
pure Co@SiO
445 and 1627 cm that are characteristic of symmetrical and
asymmetrical modes of the O–H bonds on the surface cobalt
2
[(EtO)
[(EtO)
–NH
3
Si–L ] (b), Co@SiO
2 3
[(EtO) Si–L ]/Mn(III) (c),
3
2
3
Si–L ]/Co(II) (d). The FT-IR spectrum for
2
(curve a) shows a stretching vibration at
ꢀ
ꢁ1
ꢀ
rate is 10 C min between 30 and 800 C. The metal ions
cobalt, manganese and silicone) and organic materials of the
15
2
(
ꢁ
1
3
samples were completely converted into metals, metal oxides
and burn to generate gaseous products at elevated tempera-
tures. The rst mass loss in the thermal analysis of Co@SiO2
curve a) is observed in the temperature range 90–250
4.35%) due to removal water molecules present at the surface
ꢁ1
nanoparticles, respectively. The peaks at 2901 and 2872 cm
are assigned to the methylene (–CH ) of the propyl group,
2
ꢀ
(
(
C
respectively, which indicates that APTES molecules were
attached to the surface of the silica-coated MNPs. The main
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because of the existence of amorphous silica. XRD pattern of
3
Co@SiO2 [(EtO) Si–L ]/Mn(III) complex (curve a) shows three
3
ꢀ
major peaks in the range of 20–40 . XRD pattern of
Co@SiO [(EtO) Si–L ]/Co(II) (curve b) can be assigned to the
diffraction from the three major peaks with 2q at 19 , 30 and
0 reection indexes of (100), (200) and (220) planes, respec-
3
2
3
ꢀ
ꢀ
ꢀ
4
15
tively (card 25-0250 in the JCPDS le).
3
Fig. 4 shows the TEM images of Co@SiO
2
[(EtO)
3
Si–L ]/Mn(III)
3
(
a) and Co@SiO [(EtO) Si–L ]/Co(II) (b). As observed from the
2 3
images, the Schiff base complex Mn(III) and Schiff base complex
Co(II) shell (black color) were surrounded by the Co nano-
particles (ash color) and core–shell-type supports were ob-
tained. This indicates the successful coating on the surface of
the magnetic nanoparticles. It can be observed that the size of
Fig.
Co@SiO
2
TGA spectra of (a) Co@SiO
2
,
(b) Co@SiO
2
–NH
2
(c)
3
3 2 3
3
2
[(EtO) Si–L ]/Co(II) and (d) Mn@SiO [(EtO) Si–L ]/Co(III).
of nano cobalt. The second small weight loss (5.52%) occurred Co nanoparticles is in range of 20–30 nm.
ꢀ
in the region of 280–480 C, which is related to the removal of
2 3
As shown in Fig. 5, the morphology of the Co@SiO [(EtO) Si–
3
3
trapped water molecules from the lattice. The subsequent L ]/Mn(III) and Co@SiO [(EtO) Si–L ]/Co(II) was evaluated using
weight loss (5.16%) appears in the range of 500–750 C that is scanning electron microscopy. SEM images Fig. 5a and b indi-
2
3
ꢀ
related to the decomposition of SiO . In the TGA curve of cate that the catalysts were composed of nanometer-sized
2
Co@SiO –NH (curve b), the small weight loss is observed in particles. It can be observed that the particles are not fully
2
2
ꢀ
ꢀ
the range of 40–150 C (1.10%) and 150–280 C (3.55%), which spherical. In addition, some particle aggregations are observed,
is assigned to the physical loss of the adsorbed or trapped which are likely to be caused by the magneto static interactions
lattice water, respectively. A weight loss is detected in the between particles.
ꢀ
ꢀ
range of 300–450 C and 500–750 C, which are predominantly
2 3
To investigate the catalyst composition, Co@SiO [(EtO) Si–
3
3
attributed to the decomposition of organic substances in L ]/Mn(III) and Co@SiO [(EtO) Si–L ]/Co(II) were analyzed using
magnetic Co nanoparticles.
Co@SiO [(EtO) Si–L ]/Co(II) and Co@SiO [(EtO) Si–L ]/Mn(III) spectrum of Co@SiO [(EtO) Si–L ]/Co(II) showed the presence
2
3
1
5
The loss in mass for energy dispersive X-ray spectroscopy (EDX) (Fig. 6). The EDX
3
3
3
2
3
2
3
2
3
catalysts according to curves c and d is 31.17% and 33.77%, of Si, Co, C, O, and N atoms (curve b). In the EDX spectrum of
3
respectively. These results are related to the loss of [(EtO)
3
Si– Co@SiO [(EtO) Si–L ]/Mn(III), in addition to the mentioned
2
3
3
L ] coated on the Co nanoparticles in the complexes and prove atoms, the signals of manganese is also seen (curve a). These
3
the attachment of [(EtO)
3
Si–L H] moiety on to the surface of results once again conrm that the nanoparticles are
Co nanoparticle.
completely covered with functional groups containing nitrogen,
Therefore, the results of the TG curves correspond to the oxygen, etc.
results of the infrared spectra, which indicate that the surface
Magnetic characterization of the Co@SiO [(EtO) Si–L ]/
of the nanoparticles is covered with the ligand functional Mn(III) and Co@SiO [(EtO) Si–L ]/Co(II) was performed using
3
2
3
3
2
3
groups.
As shown in Fig. 3 (curve a and b), for core magnetic cobalt versus magnetic eld at 300 K for the two catalysts are shown in
a vibrating sample magnetometer (VSM). Plots of magnetization
ꢀ
ꢀ
ꢀ
NPs, diffraction peaks with 2q at 44.2 , 51.5 and 75.9 corre- Fig. 7. As shown in the gure, the plots of the catalysts indicate
spond to (111), (200) and (220) Braggs reection, respectively
superparamagnetic behavior. The measured saturation
indicative of an fcc metallic cobalt structure of the magnetite magnetization value (36.28 emu g for Co@SiO [(EtO) Si–L ]/
a
ꢁ
1
3
2
3
3
ꢁ
1
(
card 15-0806 in the JCPDS le). The XRD patterns of Mn(III) and 35.98 emu g
2 3
for Co@SiO [(EtO) Si–L ]/Co(II))
3
3
Co@SiO
2
[(EtO)
3
Si–L ]/Mn(III) and Co@SiO
2
[(EtO)
ꢀ
3
Si–L ]/Co(II)
show a visible diffusion peak at 2q ¼ 15–25 that appeared
2
3
Fig. 3 The X-ray diffraction patterns of (a) Co@SiO
2
[(EtO)
3
Si–L ]/ Fig.
4
TEM image of (a) Co@SiO
2
[(EtO)
3
Si–L ]/Mn(III) and (b)
2
3
Mn(III) and (b) Co@SiO
2
3
[(EtO) Si–L ]/Co(II).
2 3
Co@SiO [(EtO) Si–L ]/Co(II).
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3
3
Fig. 5 SEM image of (a) Co@SiO
2
[(EtO)
3
Si–L ]/Mn(III) and (b) Co@SiO
2
[(EtO)
3
Si–L ]/Co(II).
3
3
Fig. 6 Energy-dispersive X-ray spectroscopy (EDX) of (a) Co@SiO
2
[(EtO)
3
Si–L ]/Mn(III) and (b) Co@SiO
2
[(EtO)
3
Si–L ]/Co(II).
3
C, H, and N content. Anal. found for Co@SiO
C, 21.53; H, 1.70; N, 2.01 and for Co@SiO [(EtO)
2.08; H, 1.74; N, 2.11. The calculated percentage of elemental
2
[(EtO)
3
Si–L ]/Mn(III):
3
2
3
Si–L ]/Co(II): C,
2
3
analysis for ligand L is: C, 71.00; H, 6.36; N, 5.52. Aer analyzing
these results, it can be concluded that the ratio of the percentage
of the elements in the two complexes corresponds to the ratio of
3
these elements in the ligand L .
Catalytic activities of the synthesized catalysts
3
The catalytic activity of Co@SiO
2
[(EtO)
3
Si–L ]/M (M ¼ Mn(III),
Co(II)) was examined through the oxidation of suldes to sulf-
[(- oxides (Scheme 2).
Fig. 7 Room temperature magnetization curves of (a) Co@SiO
EtO) Si–L ]/Mn(III) and (b) Co@SiO [(EtO) Si–L ]/Co(II).
3 2 3
2
3
3
demonstrate smaller saturation magnetization values of the
ꢁ
1
15
bare Co nanoparticles (39.78 emu g ). This could be due to
3
2 3
SiO [(EtO) Si–L ] coating on the surface of Co NPs. Neverthe-
less, their magnetization is sufficient and these magnetic
nanoparticles can be quickly separated from their dispersion by
using an external magnetic eld.
3
The complexes of Co@SiO [(EtO) Si–L ]/Mn(III) and
2
3
3
Co@SiO [(EtO) Si–L ]/Co(II) were analyzed by a Perkin-Elmer 2400
2
3
3
series analyzer and elemental analysis results showed a signicant Scheme 2 Co@SiO [(EtO) Si–L ]/M (M ¼ Mn(III), Co(II)) catalysts for
2
3
oxidation of sulfide.
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First, the oxidation reaction of the methyl phenyl sulde to calculated. In order to analyze the products formed during the
methyl phenyl sulfoxide was selected as the model reaction. reaction, FT-IR technique was used. FT-IR analysis of the ob-
The catalysts were tested under the following reaction condi- tained product from the oxidation of sulde compounds
ꢁ1
tions: methyl phenyl sulde (0.5 mmol), hydrogen peroxide shows S]O stretching frequency at 1049 cm , indicating the
0% (90 mL, 0.881 mmol), catalyst 0.02 g, reaction time 50 min, formation of sulfoxide from sulde. There is a band at
3
ꢀ
ꢁ1
reaction temperature 50 C and various solvents without 692 cm , corresponding to C–S stretching frequency. Since
optimized amounts. The results are summarized in Table 1. As there is no stretching frequency band observed at 1150 cm
ꢁ
1
,
indicated by the data, the catalysts are highly active and corresponding to sulfone, we understand that sulde is not
selective. In addition, it can be seen that excellent yield of oxidized to sulfone under the present experimental condi-
product was obtained in acetonitrile. To determine the reac- tions. The product analysis study demonstrates that sulfoxide
tion yield, the following procedure was adopted. Aer is the only product formed under the present reaction
completion of reaction and separation of catalyst using conditions.
a magnet, the organic phase was extracted with ethanol. Then,
The effect of temperature was investigated by carrying out
it was dried over sodium sulfate and evaporated. In the next the model reaction at different temperatures in acetonitrile. As
ꢀ
step, the obtained product was recrystallized with ethanol. The shown in Fig. 8a, the best result was achieved at 65 C.
resultant sulfoxide was weighed and the reaction yield was Furthermore, the different reaction times were examined at
a
2 2
Table 1 Catalytic results oxidation of methyl phenyl sulfide to methyl phenyl sulfoxide with H O by various catalysts and different solvents
Entry
Catalyst
Solvent
THF
C H OH
2 5
DMSO
DMF
Selectivity (%)
Isolated yield
3
1
2
3
4
5
6
7
8
9
Co@SiO
Co@SiO
Co@SiO
Co@SiO
Co@SiO
Co@SiO
Co@SiO
Co@SiO
Co@SiO
Co@SiO
Co@SiO
Co@SiO
Co@SiO
Co@SiO
Co@SiO
Co@SiO
Co@SiO
Co@SiO
Co@SiO
Co@SiO
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
[(EtO)
[(EtO)
[(EtO)
[(EtO)
[(EtO)
[(EtO)
[(EtO)
[(EtO)
[(EtO)
[(EtO)
[(EtO)
[(EtO)
[(EtO)
[(EtO)
[(EtO)
[(EtO)
[(EtO)
[(EtO)
[(EtO)
[(EtO)
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
Si–L ]/Mn(III)
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
52
77
45
84
49
69
73
77
89
84
41
72
35
82
48
65
69
75
84
80
3
Si–L ]/Mn(III)
3
Si–L ]/Mn(III)
3
Si–L ]/Mn(III)
3
Si–L ]/Mn(III)
CH
Ethyl acetate
CHCl
2 2
Cl
3
Si–L ]/Mn(III)
3
Si–L ]/Mn(III)
3
3
Si–L ]/Mn(III)
Acetone
MeCN
CH OH
3
THF
MeCN
DMSO
3
Si–L ]/Mn(III)
3
10
11
12
13
14
15
16
17
18
19
20
Si–L ]/Mn(III)
3
Si–L ]/Co(II)
3
Si–L ]/Co(II)
3
Si–L ]/Co(II)
3
Si–L ]/Co(II)
C
CH
2
H
5
OH
Cl
Ethyl acetate
CHCl
3
Si–L ]/Co(II)
2
2
3
Si–L ]/Co(II)
3
Si–L ]/Co(II)
3
3
Si–L ]/Co(II)
Acetone
MeCN
CH OH
3
3
Si–L ]/Co(II)
3
Si–L ]/Co(II)
a
Reaction condition: methyl phenyl sulde (0.5 mmol), hydrogen peroxide 30% (90 mL, 0.881 mmol), catalyst 0.02 g, reaction time: 50 min, reaction
ꢀ
temperature 55 C.
2 3
Fig. 8 Effect of temperature (a) and time (b) on the oxidation of methyl phenyl sulfide to methyl phenyl sulfoxide catalyzed by Co@SiO [(EtO) Si–
3
3
2 3
L ]/Mn(III) (red) and Co@SiO [(EtO) Si–L ]/Co(II) (blue).
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Table 2 The optimized amounts of the catalyst and oxidant in
a
oxidation of methyl phenyl sulfide to methyl phenyl sulfoxide
3
Co@SiO
Mn(III) (g)
2
[(EtO)
3
Si–L ]/
H
(mL)
2
O
2
30%
Selectivity
(%)
Isolated
yield
Entry
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
0.02
0.02
0.02
0.02
0.03
0.03
0.03
0.03
0.04
0.04
0.04
0.04
0.05
0.05
0.05
0.05
0.06
0.06
0.06
0.06
Not used
60
70
80
90
60
70
80
90
60
70
80
90
60
70
80
90
60
70
80
90
90
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
61
73
82
87
70
78
84
92
81
88
90
93
86
92
94
97
91
95
97
99
8
Fig. 9 Recycling of the two synthesized magnetic nanocatalysts used
the oxidation of methyl phenyl sulfide to methyl phenyl sulfoxide.
0
1
2
3
4
5
6
7
8
9
0
1
oxidation reactions (Table 3). The results indicated
that the catalyst is capable of oxidizing of suldes with high
yield.
Very recently some studies have been reported for suldes
17,22
oxidation using manganese(III) complexes.
Therefore, the proposed mechanism for the performance of
these complexes is shown in the Scheme 3.
a
Reaction condition: methyl phenyl sulde (0.5 mmol), reaction time:
0 min, reaction temperature 65 C.
ꢀ
5
Comparison with other catalysts
Comparative data on the performance of various catalysts in
the oxidation of suldes are shown in Table 4. Although Schiff
base complexes are oen used as efficient homogeneous
catalysts for the oxidation reaction, some problems oen exist
in these types of oxidation studies, such as (1) deactivation, (2)
instability and (3) expensive recycling of these homogeneous
systems. Therefore, for improving the performance of catalytic
activity, scientists tried to convert them into heterogeneous
catalysts using a variety of methods. The greatest advantage of
heterogeneous catalysis is the facile separation of catalysts
from the reaction media and products. In order to x catalysts
onto heterogeneous supports, the different methods can be
used, including graing the catalyst onto inorganic supports
such as silica. This is a very good method for synthesis of
heterogeneous catalysts because of it eliminates some of the
synthesis steps. However, a major challenge is the design of
insoluble catalysts with high yields, selectivity, and cost
effectiveness. According to all the above discussion, insoluble
Schiff base complexes with good stability and catalytic activity
are still rare and lack generality.
ꢀ
a constant temperature of 65 C and the reaction progress was
monitored by TLC. Finally, the reaction time of 50 min was
obtained as optimum time (Fig. 8b).
In addition to the solvent, time and temperature of the
reaction, the other conditions such as catalyst amount and
oxidant amount were investigated (Table 2). The amount of
3
Co@SiO
2
[(EtO)
3
Si–L ]/Mn(III) catalyst used in the reaction was
varied in the range 0.02–0.06 (g). As shown in Table 2,
increasing the amount of catalyst improved the yield and the
maximum yield of the product was obtained with 0.06 g of
catalyst. Also increasing the amount of H O (30%), as an
oxidizing agent, to 90 mL, increases the reaction yield (Table 2,
entry 20).
To evaluate reusability of the catalyst, the oxidation of
methyl phenyl sulde was studied under optimum condition.
Aer each cycle, the catalyst was separated by an external
magnet, and washed several times with deionized water and
2
2
ꢀ
ethanol. Then, it was dried in an oven at 50 C and used in the
It is noteworthy that the reaction tolerates oxidation-
sensitive functional groups and the sulfur atom is selec-
tively oxidized. Synthetic processes for magnetic nano-
materials are currently undergoing rapid improvement and
have opened up tremendous possibilities for the fabrication
of magnetically-recoverable catalysts. Separation and
recovery of catalysts by external magnetic eld is an envi-
ronmentally friendly alternative because not only does it
minimize the use of auxiliary materials, but also prevents
mass loss and reduces the operation time. We synthesized
a highly active catalyst based on Schiff base complexes of
Mn(III) or Co(II) ion supported on magnetic cobalt
next run. The results show that the synthesized catalysts can be
reused seven times without any considerable loss in activity
(Fig. 9).
Due to the relatively high magnetic capability of the present
catalysts, it is easy to separate each of them by an external
magnet. Moreover, due to the strong bond between functional
groups with core and the presence of Schiff base groups,
a strong bond with transition metals are formed in the
complexes.
Performance capability of the catalyst Co@SiO [(EtO) -
2
3
3
Si–L ]/Mn(III) was evaluated by using different suldes in
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Table
Co@SiO
3 Oxidation of the variety of sulfides to sulfoxides by Table 3 (Contd.)
3
2 3
[(EtO) Si–L ]/Mn(III) nanocatalyst
Isolated
yield (%)
Isolated
yield (%)
Entry Suldes
Sulfoxides
Entry Suldes
Sulfoxides
1
99
96
14
90
2
3
1
1
5
6
87
92
90
95
91
17
97
4
5
nanoparticles, which have been synthesized in a limited
number in recent years. This synthesized catalysts were
1
5
capable of catalyzing the oxidation reactions of alcohols and
suldes with high yield. Further investigation of other cata-
lytic reactions with this synthesized nanocatalyst is in prog-
ress. Other desirable properties of synthesized catalysts that
are included in this publication are the high yield, short
reaction time, selectivity, mild reaction conditions, and good
catalyst recovery.
The efficiency of this method is demonstrated by comparing
our results on the oxidation of benzyl phenyl sulde with data
from the literature. As shown in Table 4, the previously reported
procedures suffer from one or more disadvantages such as
6
7
8
9
97
90
86
96
89
90
3
1–36
33–35
longer reaction times,
low yield,
using a transition
31–35
metal,
solvents.
and the need for volatile and toxic organic
31–34,37
1
1
0
1
1
1
2
3
93
95
Scheme 3 Electron transfer mechanism for the Co@SiO
2
[(EtO)
3
Si–
3
2 2
L ]/M (M ¼ Mn(III) and Co(II)) catalyzed H O oxidation of organic
sulfides.
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2 2
Table 4 Comparison of the activity of various catalysts in the oxidation of benzyl methyl sulfide using H O
Entry
Catalyst
Conditions
Time (min)
Yield (%)
Ref.
1
2
3
4
5
6
7
8
9
MNPs–PA
SSA
NBS
Solvent-free, rt
10
40
15
20 h
150
330
240
360
180
40
95
96
90
52
96
92
83
53
83
92
100
99
24
37
36
33
34
31
32
35
CH
CH
CH
3
3
3
CN, rt
CN, 35–40 C
CN, 20 C
ꢀ
ꢀ
ꢀ
Cu(II) complex, TEMPO
TaCl
Cp/Mo(CO)
Silica-based ammonium tungstate
ZnBr , pyridine-2-3-carboxylic acid
Schiff base/Cu(II) salen/Fe
Co@SiO @[Mn(III)SBC]
Au/CTN-silica
Co@SiO [(EtO)
5
MeOH, 45 C
3
Cl
CH
CH
3
COCH
Cl :MeOH, rt
2
3
–MeOH
2
2
MeOH, rt
EtOH, 60 C
ꢀ
3
O
4
23
24
22
ꢀ
10
11
12
2
Solvent-free, 45 C
ꢀ
60 C
120
50
3
ꢀ
2
3
Si–L ]/Mn(III)
55 C
This work
4
M. J. Jacinto, H. C. F. Santos, R. F. Jardim, R. Landers and
L. M. Rossi, Appl. Catal., A, 2009, 360, 177.
Conclusion
3
In this study, the catalysts Co@SiO
2
[(EtO)
3
Si–L ]/Mn(III) and
5 R. L. Oliveira, P. K. Kiyohara and L. M. Rossi, Green Chem.,
2010, 12, 144.
6 L. Canali and D. C. Sherrington, Chem. Soc. Rev., 1999, 28, 85.
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101, 37.
3
Co@SiO [(EtO) Si–L ]/Co(II) were successfully prepared via
2 3
3
immobilizing the Schiff base ligand [(EtO) Si–L H] on the Co
3
nanoparticles coated with SiO2 and then reacting them with
metal salts viz. Mn(III) and Co(II). These catalysts were employed
for the oxidation of the suldes to sulfoxides and showed high
catalytic activity and selectivity. It was further found that the
9 B. S. Lane and K. Burgess, Chem. Rev., 2003, 103, 2457.
catalyst can be easily isolated using an external magnet and 10 G. Righi and C. Bonini, Synthesis, 1994, 3, 225.
reused seven times without signicant catalytic deactivation.
11 G. X. Zheng, J. J. Eisch, Z. R. Lui and X. Ma, J. Org. Chem.,
1992, 57, 5140.
These newly developed heterogeneous catalysts were simple
to prepare. These catalysts were found to be easily reusable 12 L. P. C. Nielson, C. P. Stevenson, D. G. Backmond and
several times aer reaction with an external magnet and drying
E. N. Jacobsen, J. Am. Chem. Soc., 2004, 126, 1360.
without signicant catalytic deactivation, which typically occurs 13 J. Lopez, S. Liang and X. R. Bu, Tetrahedron Lett., 1998, 39,
due to the leaching of the active species or degradation of the
4199.
structure. It appears that the Schiff base complex has played an 14 A. M. Daly, C. T. Dalton, M. F. Renehan and D. G. Gilheany,
important role in the stabilization of metal catalyst particles.
Tetrahedron Lett., 1999, 40, 3617.
These unique results open new perspectives for the application 15 H. Keypour, S. G. Saremi, H. Veisi and R. Azadbakht, RSC
of these types of magnetic catalysts in other organic reactions.
Adv., 2016, 6, 77020.
16 M. Szavuly, S. D. Szilvasi, R. Csonka, D. Klesitz, G. Speier,
M. Giorgi and J. Kaizer, J. Mol. Catal. A: Chem., 2014, 393,
Conflicts of interest
3
17.
17 S. Rayati, F. Nejabat and S. Zakavi, Inorg. Chem. Commun.,
014, 40, 82.
There are no conicts to declare.
2
1
8 P. Gogoi, M. Kalita, T. Bhattacharjee and P. Barman,
Tetrahedron Lett., 2014, 55, 1028.
Acknowledgements
1
2
9 X. T. Zhou and H. B. Ji, Catal. Commun., 2014, 53, 29.
0 B. Yu, C. X. Guo, C. L. Zhong, Z. F. Diao and L. N. He,
Tetrahedron Lett., 2014, 55, 1818.
1 Y. Imada, T. Kitagawa, S. Iwata, N. Komiya and T. Naota,
Tetrahedron, 2014, 70, 495.
We thank the Payame Noor University, Bu-Ali Sina University
and Center for Research and Development of Petroleum Tech-
nologies at Kermanshah, Research Institute of Petroleum
Industry (RIPI), Iran for their partial support on this project.
2
2
2
2
2
2 A. R. Judy Azar, E. Safaei and S. Mohebbi, Mater. Res. Bull.,
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