Highly efficient epoxidation of alkenes with m-chloroperbenzoic acid catalyzed by nanomagnetic Co(III)@Fe3O4/SiO2 salen complex

Article abstract of DOI:10.1007/s12039-017-1229-y

A new type of heterogeneous Co(III) complex was synthesized by covalent grafting of homogeneous Co(III) salen complex onto the surface of Fe₃O₄/SiO₂ nanoparticle (NP). The heterogeneous nanocatalyst was characterized by X-

Full text of DOI:10.1007/s12039-017-1229-y

J. Chem. Sci. ꢀc Indian Academy of Sciences.
DOI 10.1007/s12039-017-1229-y
REGULAR ARTICLE
Highly efficient epoxidation of alkenes with m-chloroperbenzoic acid
catalyzed by nanomagnetic Co(III)@Fe O /SiO salen complex
3
4
2
∗
ALI ALLAHRESANI and MOHAMMAD ALI NASSERI
Department of Chemistry, Faculty of Sciences, University of Birjand, P. O. Box 97175-615, Birjand, Iran
Email: a_allahresani@birjand.ac.ir; manaseri@birjand.ac.ir
MS received 25 August 2016; revised 7 November 2016; accepted 29 December 2016
Abstract. A new type of heterogeneous Co(III) complex was synthesized by covalent grafting of homoge-
neous Co(III) salen complex onto the surface of Fe3O4/SiO2 nanoparticle (NP). The heterogeneous nanocata-
lyst was characterized by X-ray diffraction (XRD), thermogravimetric analysis (TGA), transmission electron
microscopy (TEM), Fourier transform infrared spectra (FT-IR), atomic absorption spectroscopy (AAS), vibrat-
ing sample magnetometer (VSM) and nitrogen adsorption–desorption isotherm (BET). The catalytic activity
was investigated for the epoxidation of alkenes using m-chloroperbenzoic acid as oxidant at room tempera-
ture and the corresponding epoxide was achieved with excellent yields and selectivity. In addition, the effect of
axial ligand was studied on the epoxidation reaction and pyridine N-oxide (PNO) was chosen as an excellent
axial ligand in dichloromethane. Furthermore, the heterogeneous catalyst showed good stability and the mag-
netic properties (which made possible the easy recovery of catalyst with external magnet) without significant
decrease in the activity in the epoxidation reaction.
Keywords. Co (III) salen complex; Fe3O4/SiO2; Olefins; epoxidation; m-chloroperbenzoic acid.
1
. Introduction
high catalyst loading capacity due to high surface area,
high dispersion, convenient recycling of catalyst and
Epoxidation of terminal alkenes to the corresponding
epoxides is especially interesting because epoxides are
valuable intermediates for the synthesis of fine chem-
icals, pharmaceuticals and agrochemicals.1 Schiff
bases and their stable complexes with most of transition
outstanding stability, have been extensively employed
as a good candidate for catalyst supports.2
4,25
In
addition, they possess attractive properties (low toxicity
and high biocompatibility, super paramagnetism, high
magnetic susceptibility and saturation magnetization)
that could potentially be used in catalysis, magnetic res-
onance imaging contrast enhancement, drug delivery,
data storage, magnetic bioseparation, gene manip-
ulation, targeted drug carrier, enzyme immobiliza-
tion, immunoassay and environmental remediation.2
However, pure magnetic particles such as Fe O NPs
,2
3
6
metals have a key role in coordination chemistry.
They can be prepared simply and cheaply for industrial
applications. A number of homogeneous and heteroge-
neous Schiff base complexes were developed for the
oxidation reactions and, in particular, epoxidation of
alkenes. Although, great attention was shown to the
epoxidation of alkenes using Mn(III) salen complex,
several investigations have reported that Co(III) com-
plexes are good catalysts for the epoxidation of
6 33
3
4
7
14
usually suffer from a large aggregation which alters
magnetic properties. To overcome this problem, an
appropriate coating material such as silica was applied
15 18
34
alkenes.
Covalent attachment of Co(III) complexes
to avoid such limitations. Several types of function-
alized Fe O NPs and studies on immobilization of
on a solid support such as polymer, zeolite and silica
have recently been reported as innovation in the cat-
alytic properties of such compounds.1
Recently, magnetic nanoparticles with special prop-
erties, such as ease of separation by external magnet,
3
4
organo-catalysts and metal on silica-coated Fe O NPs
3
4
9 23
26,35 37
have also been recently reported.
of the study of metal salen complexs,
In continuation
38,39
herein, we
synthesized Co(III) salen complex anchored onto the
Fe O /SiO NPs (Co@Fe O /SiO ). The heterogeneous
3
4
2
3
4
2
catalyst was used in the epoxidation of alkenes using
m-chloroperbenzoic acid (m-CPBA) as an oxidant in
dichloromethane (Scheme 1).
∗
For correspondence

Ali Allahresani and Mohammad Ali Nasseri
◦
◦
◦
◦
◦
◦
O
2θ = 30.2 , 35.4 , 43.3 , 53.6 , 57.5 , 63.1 . Fe3O4/SiO2:
−
1
◦
IR/cm : 3446, 1059, 954, 755, 580; XRD: 2θ = 15–27 ,
O
OOH
◦ ◦ ◦ ◦ ◦ ◦
0.2 , 35.4 , 43.3 , 53.6 , 57.5 , 63.1 .
3
Cl
DCM
Co@Fe O /SiO
r.t., 3h
3
4
2
2.3 Synthesis of ligand salen
O
OH
N,N’-Bis(2,4-di-hydroxybenzaldehyde)-1,2-cyclohexanediamine
42
was synthesized as described in the literature. N,N’-Bis
2,4-di-hydroxybenzaldehyde)-1,2-cyclohexanediamine: Light
(
Cl
◦
−1
:
brown Solid, yield 85%, M.p. = 187–189 C, IR/cm
Scheme 1. Schematic illustration for the epoxidation of 2600–3450(br, OH), 2920(CH2), 2880(CH2), 1615(vs,
alkenes.
C=N), 1496(s), 1425(s), 1380(vs), 1247(m), 1170(s, C–
1
O), 970(s), 925(m), 835(s), 780(m), 730(vs). H-NMR
(
(
DMSO, 250 MHz) ppm :1.5 (4H, CH2), 1.8 (4H, CH2), 3.1
2H, CH), 6.37–7.43 (C, 6H, CHarom), 8.43 (s, 1H,
2. Experimental
CH=N), 8.50 (s, 1H, CH=N), 9.8-10.20 (br, 2H, OH),
2.1 Materials
13
1
2.72 (s, 1H, OH), 13.16(s, 1H, OH); C-NMR(DMSO,
Tetraethoxysilane (TEOS), FeCl3·6H2O, FeCl2·4H2O, styrene, 62.7 MHz) ppm: 23.6, 29.2, 71.8, 104.5, 109.2, 118.1,
+
α-methylstyrene, 4-chlorostyrene, cyclooctene, indene, cis 133.0, 161.0, 162.5, 163.1; Ms, m/z (%), 355.16(M + 1,
&
trans stilbene, cyclohexene, 1-octene, pyridine N-oxide 2), 354.16(M+, 20), 277(10), 262(38), 227(41), 183(39),
(
PNO), pyridine (Py), N-methylmorpholine N-oxide (NMNO), 122(base peak), 109(29), 77(55).
-methyl imidazole (MI), imidazole (IM), ethylacetate
EtOAc), dichloromethane (DCM), CH3CN, toluene, CHCl3,
1
(
2.4 Synthesis of Co(III) salen complex
ethanol (EtOH), tetrahydrofuran (THF), m-chloroperbenzoic
acid (m-CPBA), NaIO4, NH4OAc, tert-butylhydrogenperoxide
Co(III) salen complex was synthesized as described in
(TBHP), PhI(OAc)2, H2O2 (30%), (R,R)-1,2-cyclohexane-
43
the literature. Co(III) salen complex: Dark brown Solid,
diamine, 2,4-dihydroxybenzaldehyde and oxone were pur-
chased from Merck Company and used without purification.
The IR spectra were recorded on a Perkin-Elmer 783
Infrared spectrophotometer in a KBr pellet, scanning from
−1
yield 90%, IR/cm ), 2600–3450(br, OH), 2920(CH2),
2880(CH2), 1590(vs, C=N), 1496(s), 1425(s), 1380(vs),
1247(m), 1170(s, C–O), 970(s), 925(m), 835(s), 780(m),
730(vs).
−1
4
000 to 600 cm at room temperature. The XRD mea-
surements were recorded using a Bruker D8-advance X-
ray diffractometer with Cu Kα radiation (k = 1.5406 Å).
The TEM images were obtained using the Philips CM10
instrument. Magnetization measurements were carried out at
2
.5 Synthesis of Co@Fe O /SiO nanocatalyst
3 4 2
Homogeneous Co(III) salen complex was chemically immo-
bilized onto the Fe3O4/SiO2. In brief, 2 g of Fe3O4/SiO2 was
dispersed in 100 mL of dry toluene under magnetic stirring
followed by the addition of 0.5 g of homogeneous Co(III)
complex (Scheme 1) and refluxed for 24 h under inert atmo-
sphere. After the completion of the reaction, the heteroge-
neous catalyst (Co@Fe3O4/SiO2) was washed in turn with
dry toluene, EtOH and extracted repeatedly on a Soxhlet
extractor with methanol and dichloromethane until the wash-
ing became colorless. The heterogeneous catalyst was dried
3
7
00 K on a vibrating sample magnetometer (VSM Leak shore
200). TGA curves were obtained using a Perkin–Elmer Dia-
mond TG/DTA thermal analyzer by heating the samples in
−
1
an Argon flow at a rate of 100 mL min with a heating rate
◦
−1
of 10 C min . The yields and selectivity of the products
were obtained by GC-17A Shimadzu with capillary column
(Shimadzu, CBP5, 30 m × 25 mm × 0.25 μm).
2
.2 Synthesis of Fe O /SiO nanocatalyst
◦
3
4
2
at 70 C under vacuum for 6 h. The Co@Fe3O4/SiO2 was
characterized by FT-IR, TEM, XRD, VSM, BET and TGA.
Black powder of magnetic Fe3O4 NPs was synthesized as
described in the literature.40 The Fe3O4/SiO2 NPs were syn-
thesized by the Stober method with some modification.
Briefly, 0.5 g of Fe3O4 (2.1 mmol) was dispersed in the
41
2.6 General procedure for the epoxidation of alkenes
solution containing the mixture of ethanol/deionized water Typically, 2.5 mol% of Co@Fe3O4/SiO2 as catalyst (based
80:20 mL), followed by the addition of 3 mL of NH3. Then, on Co element) was dispersed in dichloromethane (3 mL)
.5 mL of TEOS was added dropwise to the mixture and for 20 min. Styrene (1 mmol), toluene (internal standard,
(
1
stirred mechanically for 20 h at room temperature. The core- 40 μL), m-CPBA (2 mmol) as oxidant and PNO (0.5 mmol)
shell Fe3O4/SiO2 was separated by an external magnet, as additive were added to the mixture. The reaction mix-
washed with deionized water and ethanol three times and ture was stirred at room temperature for appropriate times
◦
−1
dried at 80 C for 8 h. Fe3O4: IR/cm : 3446, 580; XRD: and the progress of the reaction was monitored by TLC.

Nanomagnetic Co(III)@Fe3O4/SiO2 salen complex
After the completion of the reaction, the catalyst was sep- These vibration bands are due to Fe–O, Si–OH, and O–
arated using external magnet and the solution was washed
with NaOH (1 M, 8 mL) and brine (8 mL) and dried over
MgSO4. Finally, the solution was concentrated to 1 mL and
the yield and selectivity of products were determined by GC.
The heterogeneous catalyst was washed with ethanol and
reused.
H, respectively. Also, the vibration bands at 755 and
−1
1
100 cm are due to Si–O–Si. The vibration bands
−1
at 755, 954 and 1100 cm confirmed that the silica
shell was coated on the surface of the Fe O NPs. The
3
4
IR spectra of Co@Fe O /SiO NPs showed a strong
3
4
2
−1
vibration band at 1595 cm . This vibration band is
assigned coordination to Co through C=N functional
groups (Figure 1c). Furthermore, new stretching, vibra-
3
. Results and Discussion
−1
tion bands at 1590, 2880 and 2920 cm confirmed the
immobilization of homogeneous Co(III) salen complex
on the Fe O /SiO NPs (Figure 1c).
Co@Fe O /SiO NPs was synthesized according to the
3
4
2
procedure illustrated in Scheme 2. Initially, Fe O NPs
3
4
3
4
2
was synthesized and coated with silica (Fe O /SiO ).
3
4
2
The TGA curves of Fe O /SiO and Co@Fe O /SiO
3
4
2
3
4
2
The homogeneous Co(III) salen complex was immo-
bilized onto the Fe O /SiO NPs afforded the desired
NPs are shown in Figure 2. In the TG cure of
Fe O /SiO NPs, a weight loss over the range of 90–
3
4
2
3
4
2
Co@ Fe O /SiO nanoparticle.
◦
3
4
2
1
65 C was observed (3%). This weight loss is attributed
to the loss of adsorbed water and dehydroxylation of
3.1 Characterizations of the catalyst
The magnetic properties of the Fe O , Fe O /SiO and
3
4
3
4
2
Co@Fe O /SiO NPs were investigated by different
3
4
2
techniques (FT-IR, TEM, VSM, XRD, BET, AAS and
TGA). The IR spectrum of Fe O NPs shows impor-
3
4
−
1
tant vibration bands in 560–590 and 3400 cm which
are due to Fe–O and O–H, respectively (not listed).
Salen ligand shows important band at 3450 cm which
belongs to the O–H group (Figure 1a). Also, the bands
at 2920 and 2880 cm are assigned to the aliphatic
CH bond. Furthermore, the band at 1615 cm corre-
−
1
−1
−1
sponds to the C=N bond. This band shifts to the lower
−1
position (25 cm ) in homogeneous Co(III) salen com-
plex, which suggests the coordination of Co through
C=N bond (Figure 1b). In Figure 1(d) which belongs
to the Fe O /SiO NPs, several important vibration
3
4
2
Figure 1. FT-IR spectra. (a) salen, (b) Co complex, (c)
−1
bands at 560–590, 954, and 3400 cm are observed. Co@Fe O /SiO , and (d) Fe O /SiO NPs.
3
4
2
3
4
2
OEt
OEt
Si
EtO
OH
OH
OEt
Fe O
Fe O
3 4
3
4
Fe3O4
Fe O /SiO
3
4
2
HO
N
N
Cl
Co
N
N
Co
Cl
OH
OH
HO
OH
Fe O
Fe O
O
3
4
3
4
7
7
Fe O /SiO
3
4
2
3 4 2
Co@Fe O /SiO
Scheme 2. Schematic illustration for the preparation of Co@Fe3O4/SiO2 catalyst.

Ali Allahresani and Mohammad Ali Nasseri
−
internal OH groups. The second weight loss over the 70.495, 38.30 and 34.30 emu/g, respectively. By immo
◦
range 250–590 C wa observed. This weight loss is bilization of SiO
and Co(III) salen complex onto
, the saturation magnetization values of Fe
and Co@Fe /SiO catalyst were decreased.
Figure 2a). As shown in Figure 2b, the first step of Despite the considerable decrease in the magnetization
weight loss is decreased after the immobilization of of Co@Fe /SiO , this heterogeneous catalyst can
Co(III) salen complex onto the Fe O /SiO NPs which still be separated from the solution by using an external
2
ascribed to even further decomposition of the mate- Fe
rial. The total weight losses were approximately 10% SiO
O
4
O /
3 4
3
O
2
3
4
2
(
O
4
3
2
3
4
2
indicated that the Co(III) salen complex has been suc- magnetic field on the side wall of the reactor.
cessfully immobilized onto the Fe O /SiO . The TGA
Figure 4 shows the XRD patterns of Fe
curve of Co@Fe O /SiO has two weight losses over SiO and Co@Fe /SiO NPs which were determined
the range of 90–160 C and 250–650 C which are by powder X-ray diffraction (XRD). The XRD pat-
similar to the Fe O /SiO . The total weight losses tern of Fe NPs (Figure 4A) indicates a crystalline
O , Fe O /
4 3 4
3
4
2
3
O
4
3
4
2
2
3
2
◦
◦
O
4
3
4
2
3
◦
◦
◦
◦
◦
were approximately 13% (Figure 2b). The loading of structure at 2θ: 30.2 , 35.4 , 43.3 , 53.6 , 57.5 and
Co(III) salen complex is 0.40 mmol g based on the 63.1 which are assigned to the (2 2 0), (3 1 1), (4 0 0),
difference in the weight losses for Fe O /SiO and (4 2 2), (5 1 1) and (4 4 0) crystallographic faces of
−1
◦
3
4
2
◦
Co@Fe O /SiO .
magnetite. A broad peak at 2θ = 15–27 in the XRD
3
4
2
The vibrating sample magnetometer (VSM) record- patterns of Fe O /SiO and Co@Fe O /SiO NPs are
3 4 2 3 4 2
ings of Fe O , Fe O /SiO NPs and Co@Fe O /SiO
2
assigned to amorphous silica Figure 4(B, C). Using
3
4
3
4
2
3
4
are shown in Figure 3. As obvious in Figure 3, all the Debye–Scherrer equation (1), the size of these particles
nanoparticles have superparamagnetism property at could be estimated by determining the width of the (3 1
3
00 K and no hysteresis phenomenon was observed. 1) Bragg reflection (1).
The values of saturation magnetization for Fe O ,
kλ
3
4
D =
(1)
Fe O /SiO and Co@Fe O /SiO NPs catalyst are
3
4
2
3
4
2
β cos θ
where, λ is the wavelength of the X-ray, k is the Scherrer
constant, β is the half-width of the peak and θ is half
of the Bragg angle. Therefore, the size (D) of the parti-
cles could be estimated. The average diameter of Fe O4
3
NPs was 15 nm, while the diameter of Fe O /SiO NPs
3
4
2
was about 20 nm which is due to the agglomeration of
Fe O inside nanosphere and surface growth of silica on
3
4
the shell.
Figure 2. Thermogravimetric weight loss pattern of (a)
Fe3O4/SiO2NPs and (b) Co@ Fe3O4/SiO2 with temperature
◦
raised of 10 C/min.
8
6
4
2
0
0
0
0
0
0
0
0
0
20000
10000
0
10000
20000
2
4
6
8
(a)
(b)
(c)
Filed (G)
Figure 3. Magnetization curves of (a) Fe3O4, (b) Fe3O4/ Figure 4. XRD pattern of (A) Fe3O4, (B) Fe3O4/SiO2 NPs
SiO2 NPs, (c) Co@Fe3O4/SiO2 NPs at 300 K. and (C) Co@Fe3O4/SiO2 NPs.

Nanomagnetic Co(III)@Fe3O4/SiO2 salen complex
Figure 5. TEM images of (a) Fe3O4, (b) Fe3O4/SiO2 and (c) Co@Fe3O4/SiO2.
Figures 5a and 5b show the TEM images of Fe O
oxide, the effects of different amounts of catalyst,
oxidant, axial ligand, solvent, temperature and time
3
4
and Fe O /SiO NPs. Considering the TEM of Fe O
3
4
2
3
4
and Fe O /SiO , the average size of 16 and 21 nm were were investigated for the test reaction.
3
4
2
obtained which are similar to the results of XRD pat-
terns. The TEM images of Fe O /SiO NPs indicated
3
4
2
3
.2b The effect of catalyst amount on the epoxi-
the successful coating of silica shell on the magnetic
Fe O NPs (Figure 5b).
dation of styrene catalyzed by Co@Fe O /SiO : To
optimize the amount of catalyst, different amounts
of Co@Fe O /SiO (0.5–3 mol% based on Co ele-
3
4
2
3
4
The TEM image of Co@Fe O /SiO showed the suc-
3
4
2
3
4
2
cessful immobilization of Co(III) salen complex on the
Fe O /SiO NPs (Figure 5c). The BET specific surface
ment) were studied in the epoxidation of styrene
1 mmol) with m-CPBA (2 mmol) at room temperature
in 3 mL DCM and the results showed that 2.5 mol%
of Co@Fe O /SiO is the optimum amount of cata-
3
4
2
(
area of the Co@Fe O /SiO was measured at room tem-
3
4
2
perature by nitrogen physisorption and the surface area
2
of was found to be 20 m /g. The loading of Co(III) salen
3
4
2
lyst (Table 1, entry 5). The epoxidation reaction of
styrene was carried out over parent of Co@Fe O /SiO
complex on the Fe O /SiO was 0.42 mmol/g based on
3
4
2
Co elemental analysis by AAS.
3
4
2
(
Fe O and Fe O /SiO ) to determine which part of the
3 4 3 4 2
Co@Fe O /SiO has catalytic activity and the results
3
4
2
3
.2 Catalytic activity studies
showed that low yields were observed (20 and 42%,
respectively) (not listed). In the absence of catalyst,
since there are no active sites, negligible conversion was
3.2a Epoxidation of unfunctionalized alkenes: The
catalytic activity of Co@Fe O /SiO was studied for
3
4
2
the epoxidaton of terminal alkenes. The activity and found which demonstrates that the epoxidation reaction
selectivity of Co@Fe O /SiO was compared with the is a catalytic process (Table 1, entry 8). The effect of
3
4
2
respective homogeneous Co salen complex. Initially, axial ligand (PNO) was also investigated and the results
the reaction of styrene as the test reaction was investi- showed that in the absence of PNO an obvious decrease
gated using m-CPBA as the oxidant and PNO as axial in the yield (70%) was observed (not listed). Also, in
ligand at room temperature in dichloromethane (DCM) the homogeneous phases, the Co(III) complex was used
as solvent. In search of optimal reaction conditions under the same reaction conditions and the time of
to achieve maximum selectivity and yield of styrene reaction is shortened to 2 h (Table 1, entry 8).

Ali Allahresani and Mohammad Ali Nasseri
3.2c The effect of different solvents and time on the
The progress of the conversion of styrene was also
epoxidation of styrene catalyzed by Co@Fe O /SiO : investigated at different times (Figure 6). As shown in
3
4
2
The selected solvents should be stable and dissolve the the Figure 6, the conversion increased linearly with the
substrate and oxidant. So, different types of solvent reaction time and reached up to 70% at 2 h. Increas-
with a range of polarity were investigated in the reac- ing the time of reaction to 3 h gradually increased the
tion under the same conditions. Considering the results, yield of styrene oxide to 95% with constant selectivity
the polarity of solvents was effective on the catalytic of 99%.
activity of Co@Fe O /SiO (Table 2). Under the men-
3
4
2
tioned conditions, the reaction did not proceed well in 3.2d The effect of different oxidants/additives in the
water which can be due to the low dispersion of the epoxidation of styrene: Co(III) is a good Lewis acid,
catalyst and collision with the substrate (Table 2, entry which makes it able to withdraw electron from the per-
44
1
). In the case of ethanol and methanol, the low yiled oxidic oxygen and susceptible to be attacked by nucle-
of styrene oxide was observed which may be because of ophilic alkene in DCM medium. The conversion and
the coordination of solvent with Co(III) ions and com- selectivity of the reaction were investigated in the pres-
45,46
peting with the oxidation reaction.
Furthermore, ence of various oxidants (m-CPBA, urea hydrogen per-
poor conversion of styrene oxide was observed in the oxide (UHP), NaIO , Oxone, H O , PhI(OAc) , PhIO
4
2
2
2
nonpolar media (Table 2, entry 5). Finally, the excel- and TBHP). As can be seen from Table 3, in the absence
lent yield of the styrene oxide was observed in DCM of oxidant, no yield was observed (Table 3, entry 1).
(
Table 2, entry 9).
In the presence of some oxidants, such as UHP, H O ,
2 2
PhIO, PhI(OAc) , low yield of products was observed
2
(
Table 3, entries 2–5). Oxone and NaIO (for both of
4
Table 1. Optimization of different amount of Co@Fe3O4/
SiO2 nanocatalyst on the reaction of styrene with m-CPBA
as a model reaction.
them, the 2:1 mixture of acetonitrile: water was used
as the solvent) gave better yields (Table 3, entries 6,
a
7
). The best yields were observed in the presence of
b
Entry
Catalyst amount (mol%)
Yield (%)
m-CPBA (Table 3, entry 9). Different amount of the
oxidant was also tested and the best result was obtained
1
2
3
4
5
6
7
8
0.5
1.0
1.5
2.0
2.5
3.0
20
41
64
80
95
97
98
0
120
100
80
60
Yield (%)
homogeneous Co (III) complex (2.5 mol%)
–
40
Selectivity (%)
20
0
aReaction conditions: substrate (1 mmol), DCM (3 mL), oxi-
dant (2 mmol) and Co@Fe3O4/SiO2 (2.5 mol% based on
Co) at r.t. Duration: 3 h. b Determined by GC with a Shi-
madzu CBP5 column (30 m × 0.32 mm × 0.25 mm).
0
1
2
3
4
Time (h)
Figure 6. Change of conversion with reaction time for the
model reaction catalyzed by fresh Co@Fe3O4/SiO2 catalyst.
Table 2. The effect of different solvents on the epoxida-
tion of styrene.a
Table 3. The effect of different oxidants on the epoxida-
tion of styrene catalyzed by Co@Fe3O4/SiO2.
a
b
Entry
Solvent
Yield (%)
Entry
Oxidant
Yield (%)
1
2
3
4
5
6
7
8
9
H2O
EtOH:H2O
MeOH
EtOH
13
22
28
30
40
50
65
80
95
1
2
3
4
5
6
7
8
9
–
0
H2O2
PhIO
25
27
22
30
50
60
85
95
THF
PhI(OAc)2
UHP
NaIO4
Oxone
TBHP
m-CPBA
CHCl3
CH3CN
EtOAc
DCM
aReaction conditions: substrate (1 mmol), m-CPBA
2 mmol) and Co@Fe3O4/SiO2 (2.5 mol% based on Co
(
aReaction conditions: Styrene (1 mmol), Oxidant (2 mmol),
PNO (0.5 mmol) and Co@Fe3O4/SiO2 (2.5 mol% based on
Co) at r.t. duration: 3 h.
b
element) at r.t. duration: 3 h. Determined by GC with a
Shimadzu CBP5 column (30 m × 0.32 mm × 0.25 mm).

Nanomagnetic Co(III)@Fe3O4/SiO2 salen complex
with 2 equivalents of the m-CPBA which provides mod- in Table 4, poor conversion was observed in the pres-
erate source of oxygen for the catalytic reaction (not ence of NH OAc (Table 4, entry 1). Using some axial
4
listed). Consequently, the optimum molar ratio of olefin ligand such as Py, MI, imidazole and NMNO, the
to oxidant is 1:2.
yield increased up to 45, 56, 68 and 80%, respec-
The effect of different axial ligands such as NH OAc, tively (Table 4, entries 2–5). Finally, the best yield
4
PNO, NMNO, Py, MI and imidazole was investigated was observed in the presence of PNO as axial lig-
in the epoxidation of styrene in DCM/m-CPBA system and (Table 4, entry 6, yield: 95%). Under the same
(
Table 4). Despite of some reports which stated that conditions, a new reaction was carried out in the pres-
axial ligands decrease the yield of reaction, our results ence of homogeneous Co(III) complex and styrene was
showed that some axial ligands obviously increased the converted to the styrene oxide in 2 h with 97% yield
yields and shortened the time of reaction. As shown (not listed).
◦
The reactions were also performed at 40 C and obvi-
ous increase in the conversion was observed while
the selectivity of the reaction decreased to 85% (not
listed). Therefore, we employed the optimized condi-
Table 4. The effect of different axial ligand on the epoxi-
dation of styrene catalyzed by Co@Fe3O4/SiO2.
a
Entry
Axial ligand
Yield (%) tions (Co@Fe O /SiO , 2.5 mol%), m-CPBA (2 mmol)
3
4
2
◦
and PNO (0.5 mmol) in 3 mL DCM at 25 C for the
conversion of several olefins into the corresponding
products. Table 5 lists a group of alkenes that
1
2
3
4
5
6
NH4OAc
Py
15
45
56
65
80
95
MI
imidazole
NMNO
PNO
were investigated using Co@Fe O /SiO . The cata-
3 4 2
lyst showed excellent activity toward alkene oxida-
tion with an average isolation yield of 93%. The
yield and selectivity of the products are 91–95%
and 99%, respectively. The efficiency of the cata-
lyst was investigated for the epoxidation of substrates
aReaction conditions: Styrene (1 mmol), m-CPBA (2 mmol),
axial ligand (0.5 mmol) and Co@Fe3O4/SiO2 (2.5 mol%
based on Co) at r.t.
Table 5. Epoxidation of different alkenes using m-CPBA catalyzed by Co@Fe3O4/SiO2.a
Time (h) Yield (%) Selectivity (%) TONd TOF (h
−1
)
b
c
e
Entry
Alkene
1
3
95
99
3800
1266.7
2
3
4
3
3
3
95
93
94
99
99
99
3800
3720
3760
1266.7
1240.0
1253.3
Cl
5
6
3
3
94
92
99
99
3760
3680
1253.3
1226.7
7
3
91
99
3640
1213.3
8
9
3
3
92
90
99
99
3680
3600
1226.7
1200.0
a
Reaction conditions: Substrate (1 mmol), DCM (3 mL), Oxidant (2 mmol), PNO (0.5 mmol)
b
and Co@Fe3O4/SiO2 (2.5 mol% based on Co). Determined by GC with a Shimadzu CBP5
c
d
column (30 m × 0.32 mm × 0.25 mm). Selectivity based on alkene conversion. TON =
e
mmol of converted alkene/mmol Co; TOF = TON/time of reaction.

Ali Allahresani and Mohammad Ali Nasseri
Themagnetic nanocomposite showed excellent catalytic
1
9
00
0
0
0
0
0
0
0
0
0
activity toward the epoxidation of alkenes and the
best conversion was achieved in the presence of m-
CPBA as oxidant and PNO as the axial ligand in
DCM. The selectivity of the reaction was also inves-
tigated and excellent selectivity was observed (99%).
Co@Fe O /SiO showed comparable conversion and
8
7
6
5
4
3
2
1
Selectivity (%)
Conversion (%)
3
4
2
selectivity as that of homogeneous counterpart for the
epoxidation of several alkenes. In addition, the hetero-
geneous Co@Fe O /SiO is relatively stable and can
0
1
2
3
4
5
Run
3
4
2
be reused five times with constant catalytic activity and
no metal leaching. Therefore, absence of side reac-
tions, short reaction times, high activity and stability,
easy recoverability with external magnet and reusabil-
ity render it a potentially valuable catalyst for industrial
applications.
Figure 7. The recycles of the catalyst for the epoxidation
styrene with m -CPBA (2 mmol) and PNO (0.25 mmol)
in DCM (3 mL) catalyzed by Co@Fe3O4/SiO2 (2.5 mol%
based on Co elemental content) at r.t.
having sensitive groups (substrates that have electron-
withdrawing group likes 2-cyclohexen-1-one) in which
lower yields and selectivity were observed. It could be
deduced that Co@Fe O /SiO possessed efficient cat-
Acknowledgements
3
4
2
alytic activity, when the axial ligand PNO was used.
The TON and TOF of the reactions are 3600–3800 and
The authors are grateful to the University of Birjand for
financial support.
−1
1
213.3–1266.7 h , respectively. The conditions used
◦
for styrene: injector temperature, 220 C; detector tem-
perature, 250 C, injection volume, 0.2 μL, and the oven
temperature program was initially started at 150 C;
raised to 220 C at 20 C min ; and was set at 220 C
constantly for 5 min.
◦
References
◦
◦
◦
−1
◦
1
2
3
. Xia Q H, Ge H Q, Ye C P, Liu Z M and Su K X 2005
Advances in homogeneous and heterogeneous catalytic
asymmetric epoxidation Chem. Rev. 105 1603
. Lane B S and Burgess K 2003 Metal-catalyzed epoxida-
tions of alkenes with hydrogen peroxide Chem. Rev. 103
To study the reusability of Co@Fe O /SiO , the cat-
3
4
2
alyst was separated by external magnet, washed with
ethanol, dried under vacuum and then subjected to a
subsequent round of reaction under the same reaction
conditions with no significant loss of activity, which
validated its recyclability. The yield and selectivity
of the final products were comparable to that of the
original one (Figure 7). The leaching of Co ion a into
the solution was checked during the reaction by AAS
and no significant Co was detected in the filtrate. As
shown in the Figure 7, the catalytic activity did not
change significantly after five repeated runs (Yield:
from 95 to 90%, Selectivity from 99 to 95%). The slight
decrease in the activity and enantioselectivity of prod-
ucts may be due to the aggregation of Fe O or the
2457
. Dede B, Karipcin F and Cengiz M 2009 Novel
homo- and hetero-nuclear copper (II) complexes of
tetradentate schiff bases: Synthesis, characterization,
solvent-extraction and catalase-like activity studies J.
Hazard. Mater. 163 1148
4
. Ghosh T and Pal S 2015 Synthesis and characterization
of dodecahedral cerium (IV) and gadolinium (III) com-
plexes with a tetradentate schiff base J. Chem. Sci. 127
1201
5
. Gupta M, Sharma P, Gupta M and Gupta R 2015 Silica
functionalized Cu (II) catalysed selective oxidation of
benzyl alcohols using TEMPO and molecular oxygen as
an oxidant J. Chem. Sci. 127 1485
3
4
leaching of silica.
6. Sandaroos R, Taghi Goldani M, Damavandi S and
Mohammadi A 2012 Efficient asymmetric Baeyer–
Villiger oxidation of prochiral cyclobutanones using
new polymer-supported and unsupported chiral Co
4
. Conclusions
(
salen) complexes J. Chem. Sci. 124 871
7. Zhang W, Loebach J L, Wilson S R and Jacobsen E N
990 Enantioselective epoxidation of unfunctionalized
Novel Co@Fe O /SiO was synthesized by grafting
3
4
2
1
hemogeneous Co(III) salen complex onto the Fe O
3
4
olefins catalyzed by salen manganese complexes J. Am.
Chem. Soc. 112 2801
@
SiO NPs. The characterization results confirmed that
2
hemogeneous Co(III) salen complex has been suc-
cessfully incorporated onto Fe O @SiO NPs. This
nanocatalyst has strong magnetic responsivity due to
8
. Liao S and List B 2010 Asymmetric counteranion-
directed transition-metal catalysis: Enantioselective
epoxidation of alkenes with manganese (III) salen phos-
phate complexes Angew. Chem. Int. Ed. Engl. 49 628
3
4
2
−
1
a high saturation magnetization value (34.3 emu g ).

Nanomagnetic Co(III)@Fe3O4/SiO2 salen complex
9
. Zhang H, Wang Y M, Zhang L, Gerritsen G, Abbenhuis 25. Ko S and Jang J 2006 A Highly efficient palladium nano-
H C, Van Santen R A and Li C 2008 Enantioselective
catalyst anchored on a magnetically functionalized polymer-
epoxidation of β-methylstyrene catalyzed by immobi-
nanotube support Angew. Chem. Inter. Edit. 45 7564
lized Mn (salen) catalysts in different mesoporous silica 26. Polshettiwar V, Luque R, Fihri A, Zhu H, Bouhrara M
supports J. Catal. 256 226
0. Qi B, Wang Y, Lou L, Huang L, Yang Y and Liu S 2013
and Basset J-M 2011 Magnetically recoverable nanocat-
alysts Chem. Rev. 111 3036
1
1
1
Solvent-free aerobic oxidation of alcohols over palla- 27. Cai W and Wan J Q 2007 Facile synthesis of super-
dium supported on MCM-41 J. Mol. Catal. A: Chem.
70 95
paramagnetic magnetite nanoparticles in liquid polyols
3
J. Colloid. Interf. Sci. 305 366
1. Kureshy R I, Roy T, Khan N-uH, Abdi S H, Sadhukhan 28. Yan F, Li J, Zhang J J, Liu F Q and Yang W S 2009
A and Bajaj H C 2012 Reusable chiral macrocyclic Mn
III) salen complexes for enantioselective epoxidation of
Preparation of Fe O /polystyrene composite particles
3 4
(
3 4
from monolayer oleic acid modified Fe O nanoparti-
nonfunctionalized alkenes J. Catal. 286 41
cles via miniemulsion polymerization J. Nanopart Res.
1 289
2. Maity N C, Abdi S H, Kureshy R I, Khan N-u H,
1
Suresh E, Dangi G P and Bajaj H C 2011 Chiral macro- 29. Gu S C, Onishi J Y, Kobayashi Y, Nagao D and Konno
cyclic salen Mn (III) complexes catalyzed enantiose-
lective epoxidation of non-functionalized alkenes using
NaOCl and urea-H2O2 as oxidants J. Catal. 277 123
3. Ren W, Fu X, Bao H, Bai R, Ding P and Sui B 2009
Enantioselective epoxidation of unfunctionalized olefins
catalyzed by chiral salen Mn (III) catalyst immobi-
lized on zirconium oligostyrenylphosphonate-phosphate
Catal. Commun. 10 788
M 2005 Preparation and colloidal stability of monodis-
perse magnetic polymer particles J. Colloid. Interf. Sci.
289 419
1
3
0. Wang X, Ji H, Zhang X, Zhang H and Yang X
2
010 Hollow polymer microspheres containing a gold
nanocolloid core adsorbed on the inner surface as a
catalytic microreactor J. Mater. Sci. 45 3981
31. Liao Z, Wang H, Lv R, Zhao P, Sun X, Wang S, Su W,
Niu R and Chang J 2011 Polymeric liposomes-coated
superparamagnetic iron oxide nanoparticles as contrast
agent for targeted magnetic resonance imaging of cancer
cells Langmuir 27 3100
1
4. Barbaro P and Liguori F 2009 Ion exchange resins:
Catalyst recovery and recycle Chem. Rev. 109 515
5. Tang Q, Zhang Q, Wu H and Wang Y 2005 Epoxi-
dation of styrene with molecular oxygen catalyzed by
cobalt(II)-containing molecular sieves J. Catal. 230 384
6. Tang Q, Wang Y, Liang J, Wang P, Zhang Q and Wan
1
3
2. Weertman J R and Koch C C 2002 In Nanostruc-
1
1
1
tured materials: processing, properties and applications
2+
H 2004 Co -exchanged faujasite zeolites as efficient
heterogeneous catalysts for epoxidation of styrene with
molecular oxygen Chem. Commun. 440
(Norwich, NY: William Andrews Publishing)
3
3. Nakayama H, Arakaki A, Maruyama K and Takeyama
H 2003 Single-nucleotide polymorphism analysis using
fluorescence resonance energy transfer between DNA-
labeling fluorophore, fluorescein isothiocyanate, and
DNA intercalator, POPO-3, on bacterial magnetic parti-
cles Biotechnol. Bioeng. 84 96
4. Jia H F, Zhu G Y and Wang P 2003 Catalytic behav-
iors of enzymes attached to nanoparticles: The effect of
particle mobility Biotechnol. Bioeng. 84 406
7. Sebastian J, Jinka K M and Jasra R V 2006 Effect of
alkali and alkaline earth metal ions on the catalytic epox-
idation of styrene with molecular oxygen using cobalt
(II)-exchanged zeolite X J. Catal. 244 208
8. Cui H, Zhang Y, Zhao L and Zhu Y 2011 Adsorption
synthesized cobalt-containing mesoporous silica SBA-
3
15 as highly active catalysts for epoxidation of styrene
with molecular oxygen Catal. Commun. 12 417
9. Jinka K M, Pai S M, Newalkar B L, Choudary N V and
Jasra R V 2010 Co-SSZ-51 as a catalyst for styrene epoxi-
dation using molecular oxygen Catal. Commun. 11 638
0. Jain S and Reiser O 2008 Immobilization of cobalt(II)
schiff base complexes on polystyrene resin and a study
of their catalytic activity for the aerobic oxidation of
alcohols Chem. Sus. Chem. 1 534
3
5. Lei Z, Li Y and Wei X 2008 A facile two-step
modifying process for preparation of poly (SStNa)-
grafted Fe3O4/SiO2 particles J. Solid. State. Chem. 181
1
480
2
3
6. Shylesh S, Schunemann V and Thiel W R 2010 Magnet-
ically separable nanocatalysts: Bridges between homo-
geneous and heterogeneous catalysis Angew. Chem.
Inter. Edit. 49 3428
7. Govan J and Gun’ko Y K 2014 Recent advances in the
application of magnetic nanoparticles as a support for
homogeneous catalysts Nanomater. 4 222
8. Nasseri M A, Allahresani A and Raissi H 2014 Graft-
ing of a chiral Mn (III) complex on graphene oxide
nanosheets and its catalytic activity for alkene epoxida-
tion RSC Adv. 4 26087
2
1. Kowalski G, Pielichowski J and Jasieniak M 2003 Poly-
mer supported cobalt (II) catalysts for alkene epoxida-
tion Appl. Catal. A: Gen. 247 295
3
2
2. Jin C, Fan W B, Jia Y J, Fan B B, Ma J H and Li R F
3
2
006 Encapsulation of transition metal tetrahydro-schiff
base complexes in zeolite Y and their catalytic proper-
ties for the oxidation of cycloalkanes J. Mol. Catal. A:
Chem. 249 23
2
3. Bhunia S, Jana S, Saha D, Dutta B and Koner S 2014 39. Allahresani A and Nasseri M A 2014 A magneti-
Catalytic olefin epoxidation over cobalt(II)-containing
mesoporous silica by molecular oxygen in dimethylfor-
mamide medium Catal. Sci. Technol. 4 1820
cally recyclable Fe3O4@SiO2/Mn (III) chiral salen com-
plex as a highly selective and versatile heterogeneous
nanocatalyst for the oxidation of olefins and sulfides
RSC Adv. 4 60702
2
4. Amarasekara A S, Oki A R, McNeal I and Uzoezie U
2
007 One-pot synthesis of cobalt-salen catalyst immo- 40. Gawande M B, Branco P S and Varma R S 2013 Nano-
bilized in silica by sol–gel process and applications
in selective oxidations of alkanes and alkenes Catal.
Commun. 8 1132
magnetite (Fe3O4) as a support for recyclable catalysts
in the development of sustainable methodologies Chem.
Soc. Rev. 42 3371

Ali Allahresani and Mohammad Ali Nasseri
4
1. Tang J, Dong W J, Wang G, Yao Y Z, Cai L M, Liu
Y, Zhao X, Xu J Q and Tan L 2014 Efficient molybde-
num (VI) modified Zr-MOF catalysts for epoxidation of 44. Yang Y, Hao S, Zhang Y and Kan Q 2011 Oxovanadium
catalytic applications in aerobic oxidation RSC Adv. 4
55028
olefins RSC Adv. 4 42977
2. Qi Y, Luan Y, Yu J, Peng X and Wang G 2015
Nanoscaled copper metal–organic framework (MOF)
(IV) and dioxomolybdenum (VI) salen complexes teth-
ered onto amino-functionalized SBA-15 for the epoxi-
dation of cyclooctene Solid State Sci. 13 1938
4
based on carboxylate ligands as an efficient heteroge- 45. Lopez J, Liang S and Bu X R 1998 Unsymmetric chi-
neous catalyst for aerobic epoxidation of olefins and oxi-
dation of benzylic and allylic alcohols Chem. Eur. J. 21
ral salen schiff bases: A new chiral ligand pool from
bis-schiff bases containing two different salicylaldehyde
units Tetrahedron Lett. 39 4199
1589
4
3. Yu J, Luan Y, Qi Y, Hou J Y, Dong W J, Yang M and 46. Vezin H, Lamour E and Catteau J P 2002 Free radi-
Wang G 2014 Hierarchical PS/PANI nanostructure sup-
ported Cu (II) complexes: Facile synthesis and study of
cal production by hydroxy-salen manganese complexes
studied by ESR and XANES J. Inorg. Biochem. 92 177