Molybdenum Schiff base complex covalently anchored to silica-coated cobalt ferrite nanoparticles as a novel heterogeneous catalyst for the oxidation of alkenes

Article abstract of DOI:10.1007/s10562-012-0770-z

Silica-coated cobalt ferrite nanoparticles were prepared and functionalized with Schiff base groups to yield immobilized bidentate ligands. The functionalized magnetic nanoparticles were then treated with Mo (O ₂)₂(acac)₂, resulting in the novel immobilized molybdenum Schiff base catalyst. The as-prepared catalyst was characterized by X-ray powder diffraction, transmission electron microscopy, vibrating sample magnetometry, thermogravimetric analysis, Fourier transform infrared, and inductively coupled plasma atomic emission spectroscopy. The immobilized molybdenum complex was shown to be an efficient heterogeneous catalyst for the oxidation of various alkenes using t-BuOOH as oxidant. This catalyst, which is easily recovered by simple magnetic decantation, could be reused several times without significant degradation in catalytic activity.

Full text of DOI:10.1007/s10562-012-0770-z

Catal Lett (2012) 142:319–325
DOI 10.1007/s10562-012-0770-z
Molybdenum Schiff Base Complex Covalently Anchored
to Silica-Coated Cobalt Ferrite Nanoparticles as a Novel
Heterogeneous Catalyst for the Oxidation of Alkenes
•
M. Kooti M. Afshari
Received: 15 November 2011 / Accepted: 14 January 2012 / Published online: 24 January 2012
Ó Springer Science+Business Media, LLC 2012
Abstract Silica-coated cobalt ferrite nanoparticles were
prepared and functionalized with Schiff base groups to
yield immobilized bidentate ligands. The functionalized
magnetic nanoparticles were then treated with Mo
(O2)2(acac)2, resulting in the novel immobilized molyb-
denum Schiff base catalyst. The as-prepared catalyst was
characterized by X-ray powder diffraction, transmission
electron microscopy, vibrating sample magnetometry,
thermogravimetric analysis, Fourier transform infrared, and
inductively coupled plasma atomic emission spectroscopy.
The immobilized molybdenum complex was shown to be
an efficient heterogeneous catalyst for the oxidation of
various alkenes using t-BuOOH as oxidant. This catalyst,
which is easily recovered by simple magnetic decantation,
could be reused several times without significant degra-
dation in catalytic activity.
homogeneous catalysts on a solid support, however, can
provide easy recovery and recycling of the catalyst, as well
as product separation which are of great importance in
chemical processes [2–5]. Several insoluble solid materials
have been frequently applied as support for Schiff base
complexes [6–10]. Inorganic matrices show some advan-
tages over organic supports such as high thermal, chemical,
and mechanical stability.
Nanoparticles have recently attracted much attention as
alternative support materials for homogeneous catalyst
immobilization [11–13]. The use of nanoparticles as a
support can lead to significant enhancement of catalyst
activity but the problem of catalyst recovery from the
reaction mixture still remains because nanoparticle mate-
rials are easily dispersed in solution and cannot be effi-
ciently filtered out of the reaction medium. This issue can
be addressed by using magnetic instead of nonmagnetic
supports. Catalysts supported on magnetic nanoparticles
(MNPs) can be readily isolated from the product solution
merely by applying an external magnetic field. Therefore,
magnetic catalytic systems developed on MNPs supports
have been successfully used in catalyzing a wide range of
organic reactions including C–C coupling [14–17], hydro-
genation [18], oxidation [19, 20] and polymerization [21]
with excellent performance. Some instructive reviews have
recently reported on magnetic nanocatalysts and their
applications in various fields [22–24].
Keywords Cobalt ferrite Á Alkenes oxidation Á Magnetic
nanoparticles Á Schiff base Á Surface functionalization Á
Molybdenum catalyst
1 Introduction
Schiff base transition metal complexes have been widely
used as homogenous or heterogeneous catalysts in various
organic transformations [1]. Although homogenous cata-
lysts exhibit high catalytic activity in many organic reac-
tions, these systems have several drawbacks such as
the difficulty of catalyst recycling. Immobilization of
The most common type of MNPs are spinel ferrites with
the general formula of MFe2O4 (M is a divalent cation).
One of the well-known ferrites is CoFe2O4, which has
gained extensive attention due to its prominent chemical
stability and unusual properties [25–27]. This ferrite, along
with other ferrites, such as Fe3O4, is prone to aggregation
and show poor dispersion in liquid media because of their
large specific surface area and magnetization. Furthermore,
M. Kooti (&) Á M. Afshari
Department of Chemistry, Shahid Chamran University,
Ahvaz 6135-743135, Iran
e-mail: m_kooti@scu.ac.ir
123

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M. Kooti, M. Afshari
grafting the surface of these ferrite MNPs with organic
molecules or metal complexes is also not an easy task. In
order to overcome these disadvantages, the MNPs must be
coated with a thin layer of an inert material, such as silica.
Silica coating shields the magnetic dipole interaction
between the MNPs and can prevent their agglomeration
[28–30]. Moreover, the silica coating provides an inert
barrier between the reaction solution and the MNP’s core
while maintaining the capacity for surface modifications
[31].
modification. In this procedure, 1 g of cobalt ferrite was
dispersed by ultrasonic vibration in a mixture of 20 mL of
water and 80 mL of ethanol. After one hour of sonification,
2.4 mL of ammonium hydroxide (25%) was added, fol-
lowed by gradual addition of 2 mL of tetraethoxysilane
(TEOS) to the dispersed nanoparticles and the mixture was
stirred mechanically for 24 h. The resulted black precipi-
tate of
Si-MNPs were collected by applying a permanent magnet
and rinsed three times with ethanol. The product was dried
in an oven at 120 °C for 2 h.
There are many reports in the literature on the prep-
aration and applications of silica-coated MNPs in various
fields [32–35]. However, to the best of our knowledge,
no example of using silica-coated cobalt ferrite nano-
particles as support for Schiff base complexes has been
reported to date. In this study, we wish to present a novel
method for the immobilization of a Schiff base molyb-
denum complex on silica-coated cobalt ferrite nanopar-
ticles. The as-fabricated Schiff base molybdenum
complex anchored on silica-coated MNPs was examined
towards oxidation of alkenes which showed excellent
catalytic efficiency.
2.3 Synthesis of Molybdenum Schiff Base Complex
Immobilized on Si-MNPs
The Schiff base used in this study was prepared by the
addition of 1.77 g (8 mmol) of amiopropyltriethoxysilane,
NH2 (CH2)3Si (OEt)3, to a stirred methanolic (20 mL)
solution of salicylaldehyde (0.98 g, 8 mmol). The resulting
bright -yellow solution was stirred at room temperature for
2 h. Removal of the solvent under vacuum afforded an oily
yellow liquid of silylethoxy-modified 5-boromosalicyl-
aldimine (salenSi) product [39]. The obtained salenSi was
dissolved in 15 mL of dried toluene and then added to
0.250 g of Si-MNPs dispersed in 25 mL of dried toluene.
After sonication for 30 min, this mixture was refluxed for
2 days and then cooled to room temperature to yield a dark
black solid material. The product, salenSi@Si-MNPs, was
separated by a magnetic field, washed with toluene and
ethanol and dried at room temperature in a vacuum
dessicator.
2 Experimental
2.1 General
All chemicals were purchased from Sigma-Aldrich or
Merck and used as received without further purification.
The used t-BuOOH was 70% w/w solution in water. Mo
(O2)2(acac)2 complex was synthesized according to the
published procedure [36]. X-ray diffraction (XRD) patterns
of the samples were taken with a Philips X-ray diffrac-
tometer (Model PW1840) over a 2h range from 10° to 70°
using Cu Ka radiation (k = 1.54056 A°). The morphology
and size of the particles were determined by transmission
electron microscopy (TEM) using a Philips CM10-HT
100 kV microscope. IR spectra were recorded on a BO-
MEM MB-Series 1998 FT-IR spectrometer. GC analysis
was performed with a Shimadzu GC-16A instrument using
a 2 m column packed with silicon DC-200 or Carbowax
20 m. NMR spectra were recorded in CDCl3 on a Bruker
Advanced DPX 400 MHz spectrometer using TMS as
internal standard.
Finally, the immobilized Schiff base, salenSi@Si-MNPs,
was dispersed in 25 mL of DMF and added to a solution
containing 0.33 g (1 mmol) of Mo (O2)2(acac)2 dissolved in
10 mL of DMF. The mixture was then stirred vigorously at
70 °C under an N2 atmosphere for 2 h. After that, the solid
product was isolated by magnetic decantation and washed
with DMF, ethanol and diethyl ether and dried under vac-
uum at room temperature to afford the immobilized
Molybdenum catalyst (Mo-salenSi@Si-MNPs).
2.4 Catalytic Studies
The catalytic oxidation reactions of alkenes were carried
out in a 25 mL flask equipped with a magnetic stirrer and a
reflux condenser. In a typical run, the reaction vessel was
charged with: Mo-salenSi@Si-MNPs catalyst (50 mg), 1,2-
dichloroethane (5 mL), alkene (1 mmol), t-BuOOH (70%,
3 mmol). This mixture was heated in an oil bath at 70 °C
and the progress of the reaction was monitored by GC. At
the end of the reaction, the catalyst was fixed magnetically
at the bottom of the flask and the solution was taken off
with a pipette. The solutions were purified on a silica-gel
2.2 Preparation of Silica-Coated Cobalt Ferrite
(Si-MNPs)
Cobalt ferrite MNPs were prepared by the co-precipitation
method according to the procedure reported by Maaz et al.
[37]. Silica-coated magnetic nanoparticles (Si-MNPs) were
synthesized by the Stober method [38] with some
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Molybdenum Schiff Base Complex
321
plate to obtain the pure product. The identities of the
1
products were confirmed by FT-IR and H NMR spectral
higher frequencies and appears at 1,651 cm-1 (see Fig 1d)
in the spectrum of Mo-salenSi@Si-MNPs (4), indicating
the coordination of the imine moiety through nitrogen atom
to molybdenum metal [41]. This finding reveals that the
Mo–Schiff base complex (Mo-salenSi) has been success-
fully grafted onto the surface of Si-MNPs.
data.
3 Results and Discussion
The XRD patterns of MNPs, Si-MNPs and Mo-salen-
Si@Si-MNPs are shown in Fig. 2. The position and rela-
tive intensities of all peaks are compatible with cubic
CoFe2O4 crystals (JCPDS PDF #221086), indicating the
retention of a crystalline cubic reverse spinel structure
during functionalization of CoFe2O4. A weak broad hump
appeared in the spectra of Si-MNPs and Mo-salenSi@
Si-MNPs (Fig. 2b, c) at 2h = 18–28°, which could be
assigned to an amorphous silica phase in the shell of
CoFe2O4. Moreover, compared to the uncoated MNPs, the
intensities of all peaks in the coated MNPs samples are
slightly decreased. These results can be used to prove the
preparation of coated MNPs, i.e. Si-MNPs and Mo-sal-
enSi@Si-MNPs. The average crystallite size of MNPs,
Si-MNPs and Mo-salenSi@Si-MNPs is calculated to be
about 25.5, 34.7, and 34.8 nm, respectively, using the
Debye-Scherer formula. Therefore, the mean crystallite
size of the MNPs is slightly increased on coating them
with silica or functionalizing with molybdenum Schiff
base complex.
3.1 Characterizations of Immobilized Molybdenum
Complex (Mo-salenSi@Si-MNPs)
Magnetic nanoparticles can be easily coated with a layer of
SiO2 through chemical bonds to yield Si-MNPs (see
Scheme 1). Further surface modification of Si-MNPs was
achieved by the addition of excess salenSi (1) to yield sal-
enSi@Si-MNPs (3). Finally, during the reaction of com-
posite salenSi@Si-MNPs (3) with Mo (O2)2(acac)2, the
coordinated acetylacetonato (acac) ligands are replaced with
two salenSi moieties to produce the immobilized molybde-
num Schiff base complex, Mo-salenSi@Si-MNPs, (4).
The FT-IR spectrum of MNPs (Fig. 1) shows the
expected Fe–O stretching absorption at 577 cm-1 and,
after coating MNPs with silica, this band shifts to high
wavenumbers of 588.9 cm-1 (see Fig. 1b). The strength-
ening of this bond can be attributed to the formation of Fe–
O–Si bonds in the coated MNPs (Si-MNPs) where Fe–O–H
groups on the surface of the CoFe2O4 nanoparticles are
replaced by Fe–O–Si–O [40]. In addition, we observed a
new broad and strong band at 1,088 cm-1 and a weak
absorption peak at 802 cm-1, which are characteristic
peaks of the symmetrical and asymmetrical vibrations of
the Si–O–Si bond [40]. The FT-IR spectrum of Schiff base
anchored on silica-coated CoFe2O4 (salenSi@ Si-MNPs)
shows all the expected principal bands for the free Schiff
base ligand. The C–H deformation band is visible at
1,478 cm-1 and the sharp band at 1,634 cm-1 was
assigned to –C=N stretching vibration of the imine group
of the ligand. This last band undergoes a slight shift to
The TEM images of the as-prepared MNPs, Si-MNPs and
Mo-salenSi@Si-MNPs, are presented in Fig. 3, which show
that most of the particles are quasi-spherical. The size of
these nanoparticles can be estimated from the images of
Fig. 3 to be in the range of 20–30 nm, which is in a close
agreement with the sizes obtained from XRD studies. Inter-
estingly, the magnetic core is visible as a dark spot inside the
bright spherical SiO2 thin shell in the TEM images of Si-
MNPs and Mo-salenSi@Si-MNPs samples (see Fig. 3b, c).
Magnetic measurements of all the prepared MNPs were
carried out at room temperature using a vibrating sample
Scheme 1 Schematic representation of the formation of Mo-salenSi@Si-MNPs catalyst
123

322
M. Kooti, M. Afshari
magnetometer (VSM) with a peak field of 8 kOe and the
hysteresis loops were obtained. As seen in Fig. 4, the
hysteresis curves of Si-MNPs and Mo-salenSi@Si-MNPs
are more or less the same but slightly different from that of
MNPs. Figure 4 showed that all the fabricated samples had
good magnetic properties at room temperature and the
saturation magnetization (Ms) values of the coated samples
are slightly less than that of uncoated MNPs. This is
probably caused by the formation of amorphous silica on
the surface of CoFe2O4 nanoparticles as well as the
restriction of domain wall motion due to the magnetic
dilution effect of inert silica. Even with this reduction in
the saturation magnetization, the coated and functionalized
samples could still be efficiently separated from solution
with a permanent magnet.
TGA was performed to determine percent weight loss
of the Si-MNPs and salenSi@Si-MNPs samples. The
weight decrease observed in the slop of diagram (b) in
Fig. 5, in the range of 300–600 °C, must be related to the
loss of organic group (Schiff base ligand). Therefore, the
difference between the remaine weights of Si-MNPs and
salenSi@Si-MNPs at 600 °C can be hypothetically
attributed to the loss of Schiff base adsorbed on the sur-
face of Si-MNPs. Since this difference was 9.35%, the
amount of Schiff base loaded on Si-MNPs is estimated to
be 0.36 mmol/g.
Fig. 1 FT-IR spectra of a MNPs, b Si-MNPs, c salenSi@Si-MNPs,
and d Mo-salenSi@Si-MNPs
Elemental analysis of the catalyst was carried out and its
nitrogen content was 0.542%. According to the obtained
nitrogen content, the amount of grafted ligand was esti-
mated (0.38 mm/g) which is in a good agreement with the
data obtained from TGA studies. The content of molyb-
denum in the Mo-salenSi@Si-MNPs catalyst, as deter-
mined by inductively coupled plasma atomic emission
spectroscopy (ICP-AES), was 0.98 wt%. This is another
proof for the fact that molybdenum complex was be
immobilized onto the silica-coated MNPs.
Fig. 2 XRD pattern of a MNPs, b Si-MNPs, and c Mo-salenSi@
Si-MNPs
Fig. 3 TEM images of a MNPs, b Si-MNPs, and c Mo-salenSi@Si-MNP
123

Molybdenum Schiff Base Complex
323
Scheme 2 Oxidation of alekenes using Mo-salenSi@Si-MNPs
catalyst
The as-prepared Mo-salenSi@Si-MNPs catalyst was
used towards oxidation of various alkenes, under the
optimized conditions obtained for oxidation of styrene (see
Scheme 2).
Under the optimized conditions which obtained for oxi-
dation of styrene, various alkenes such as cyclohexene,
a-methylstyrene, 4-methoxystyrene, 4-nitrostyrene, and
1-heptene were oxidized. The results are shown in Table 1.
Oxidation of cyclohexene gave an allylic oxidation product,
2-cyclohexene-1-one with 95% yield. With styrene and its
derivatives, however, the major products were the corre-
sponding carbonyl compounds with high yield and selec-
tivity. The explanation given by Moghadam et al.
(Scheme 3) might be applicable for our catalysis system
[42]. It was proposed that epoxide is formed in the first step
of the reaction. The epoxide is subsequently attacked by
t-BuOOH to give aldehyde or keteone products. Apparently,
in these cases the conjugation of aromatic ring with the
carbonyl group can stabilize the products. The great feature
of the catalytic oxidation with Mo-salenSi@Si-MNPs is that
non-activated terminal olefin, 1-octene, could be proficiently
transformed to the corresponding epoxide in high yield.
Compared with the homogeneous and heterogeneous
MoO2-salen catalysts, used by Moghadam et al. [42], our
novel catalyst (Mo-salenSi@Si-MNPs)has shown muchmore
superiority.The observed high activity of the catalyst can be
attributed to the nanosized character of the support which
increases surface area for more interaction with substrates
This new catalytic system has some remarkable features
including that the catalyst, Mo-salenSi@Si-MNPs, is stable at
used conditions and can be readily isolated from the reaction
mixture by means of an external magnet. Neither obvious loss
of activity nor leaching of the catalyst was observed when the
catalyst had been reused in five consecutive runs.
Fig. 4 Hysteresis loops of a MNPs, b Si-MNPs, and c Mo-salenSi@Si-
MNPs
Fig. 5 TGA of a Si-MNPs and b salenSi@Si-MNPs
3.2 Oxidation of Alkenes
In order to find the optimized reaction conditions, styrene was
examined as model substrate using t-BuOOH as
oxidant in the presence of catalytic amount of Mo-salen-
Si@Si-MNPs. The optimum conditions used for oxidation of
styrene by this catalytic system are: catalyst 50 mg, oxidant
and substrate in a molar ratio of 150:50, respectively. In this
reaction, benzaldehyde was produced in 95% conversion.
Different solvents, such as acetone, chloroform, acetonitrile
and 1,2-dichloroethane were also examined for the oxidation
of styrene by t-BuOOH and 1,2-dichloroethane gave the
highest conversion of styrene. Beside t-BuOOH, other oxi-
dants, such as H2O2 and H2O2/Urea (UHP), were also
investigated for the oxidation of styrene and the results
showed that t-BuOOH is the best source of oxygen. The
reaction temperature was set at 70 °C, since the observed
conversion of styrene was very low at room temperature and
reaches 95% at 70 °C. It is worth mentioning that only trace
styrene conversion was observed when the reaction was
carried out in the absence of catalyst or oxidant, keeping the
rest of the reaction conditions constant.
3.3 Catalyst Recycling
Catalyst reusability is of major concern in heterogeneous
catalysis. The recovery and reusability of the catalyst was
investigated using styrene as model substrate.
Catalyst recycling experiments were achieved by fixing
the catalyst magnetically at the bottom of the flask, after
which the solution was taken off with a pipette, the solid
washed with 1,2-dichloroethane twice, and the fresh sub-
strate dissolved in the same solvent was introduced into
the flask, allowing the system to proceed for next run. The
123

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M. Kooti, M. Afshari
Table 1 Oxidation of some alkenes by t-BuOOH catalyzed by Mo-salenSi@Si-MNPs
Entry
1
Alkenes
Producta
Conversion %b
Selectivity
Time (h)
8
95
85
O
CH3
CH3
2
100
90
8
O
3
4
100
100
85
90
8
8
O
H3C
H3C
O
H3CO
H3CO
5
6
70
65
95
8
6
O
O2N
O2N
O
100
O
7
68
100
6
a
Reaction conditions: alkene(1mmol), t-BuOOH(3mmol), catalyst (40mg), 1,2-dichloroethane(5ml), T = 70 °C
GC yields based on starting alkenes
b
Scheme 3 Proposed mechanism for oxidation of styrene and its
derivatives
Fig. 6 Catalyst recycling experiments for styrene oxidation
catalyst was consecutively reused five times without any
noticeable loss of its catalytic activity (see Fig. 6). Our
catalyst is highly magnetic and its saturation magnetization
value (36.88 emu/g) is much higher than other reported
magnetic catalysts [43, 44]. Therefore, it could be easily
and almost completely separated by an external magnet
which is of a great advantage for a heterogeneous catalyst.
was successfully achieved by a multiple synthetic proce-
dure and the product was confirmed by XRD, FT-IR, TGA,
TEM, ICP-AES, and VSM. This novel immobilized
molybdenum Schiff base complex was examined as an
efficient catalyst for the oxidation of various olefins in the
presence of t-BuOOH. The catalyst has showed high
catalytic activity in these reactions, including with non-
reactive terminal olefins. Moreover, immobilized molyb-
denum Schiff base catalyst could be easily recovered by
simple magnetic decantation and reused five times without
significant loss of activity. The excellent catalytic
4 Conclusions
In summary, covalent functionalization of Schiff base
molybdenum complex onto silica-coated CoFe2O4 MNPs
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Molybdenum Schiff Base Complex
325
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Acknowledgment The authors wish to acknowledge the support of
this study by the Research Council of Shahid Chamran University,
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