Cobalt salophen complex supported on magnetic nanoparticles as an efficient reusable catalyst for oxidation of benzylic alcohols

Article abstract of DOI:10.13005/ojc/290431

A novel and general method has been developed for oxidation of benzylic alcohols using magnetic nanoparticles immobilized salophen Co(II) as an efficient and recyclable catalyst. The structural and magnetic properties of catalyst are identified by transmission electron microscopy (TEM), vibrating sample magnetometer (VSM) FT-IR and XRD instruments. Nanocatalyst can be easily recovered by a magnetic field and reused for subsequent reactions for at least 5 times with less deterioration in catalytic activity.

Full text of DOI:10.13005/ojc/290431

ISSN: 0970-020 X
CODEN: OJCHEG
2013, Vol. 29, No. (4):
Pg. 1523-1530
ORIENTAL JOURNAL OF CHEMISTRY
An International Open Free Access, Peer Reviewed Research Journal
www.orientjchem.org
Cobalt Salophen Complex Supported on Magnetic
Nanoparticles as an Efficient Reusable Catalyst for
Oxidation of Benzylic Alcohols
MOzHgAN AFSHARI1*, MARYAM gORJIzADEH1, SIMIN NAzARI2
and MOHAMMAD NASEH3
1Department of Chemistry, Shoushtar Branch, Islamic Azad University, Shoushtar, Iran.
2Department of Chemistry, Sousangerd Branch, Islamic Azad University, Sousangerd, Iran.
3Department of Chemistry, Dezful Branch, Islamic Azad University, Dezful, Iran.
*Corresponding author E-mail: mozhganafshari@yahoo.com
http://dx.doi.org/10.13005/ojc/290431
(Received: October 15, 2013; Accepted: December 02, 2013)
ABSTRACT
A novel and general method has been developed for oxidation of benzylic alcohols using
magnetic nanoparticles immobilized salophen Co(II) as an efficient and recyclable catalyst. The
structural and magnetic properties of catalyst are identified by transmission electron microscopy
(TEM), vibrating sample magnetometer (VSM) FT-IR and XRD instruments. Nanocatalyst can be
easily recovered by a magnetic field and reused for subsequent reactions for at least 5 times with
less deterioration in catalytic activity.
Key words: Cobalt ferrite, oxidation, Magnetic nanoparticles,
Surface functionalization, Cobalt salophen complex.
INTRODUCTION
organic polymers or inorganic porous have been
frequently applied as heterogeneous catalytic
support for these complexes8-10. Inorganic matrices
show some advantages over organic supports such
as high thermal, chemical and mechanical stability.
However, due to the diffusion of substrates and
products through the pores of the support materials,
a substantial decrease in reaction rate is frequently
Transition metal Schiff base complexes
have attracted much interest in the field of catalysis
because of their reversible bonding oxygen ability and
high activities in the oxidation reactions1-7. However,
these catalysts are generally homogeneous and are
not easily recoverable after oxidation.Also, the work
up procedure is not convenient and generates a lot
of waste. One of the solutions of these problems
is the immobilization of the catalyst on the solid
support. Several insoluble solid materials either
observed compared to the homogeneous system11-14
.
Thus, it is worthwhile to consider new materials as
support for these homogenous catalysts.

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Nowadays, magnetic nanoparticles
reported by Maaz et al.21. Silica coated CoFe2O4
nanoparticles (Si-MNPs) were made by using
sol-gel method22. X-ray diffraction (XRD) patterns
were recorded with a Philips X-ray diffractometer
(Model PW1840). FT-IR spectra were obtained
using BOMEM MB-Series 1998 FT-IR spectrometer.
Magnetic properties of all nanoparticles were
measured with a vibrating sample magnetometer
(VSM, Meghnatis Daghigh Kavir Company, Iran)
at room temperature. NMR spectra were recorded
in CDCl3 on a Bruker Advanced DPX 400 MHz
spectrometer.
(MNPs) have attracted increasing interest as
promising alternatives for the immobilization of
homogeneous catalysts because of their unique
properties including a large surface-to-volume
ratio, magnetism, low toxicity, biocompatibility and
their potential applications in various fields15-20
.
In this study, we wish to present a novel method for
the immobilization of a Co (II) salophen complex
on imidazole functionalized silica coated cobalt
ferrite nanoparticles (Co (salophen)-imid-Si@Si-
MNPs). The catalytic activity and reusability of this
composite in the oxidation of benzylic alcohols was
investigated.
Synthesis of Co (salophen)-imid-Si@Si-MNPs
A schematic representation for the
EXPERIMENTAL
synthesis of Co (II) salophen complex anchored on
imidazole functionalized silica coated cobalt ferrite
nanoparticles is shown in Scheme 1. In the first
step, imidazole functionalized silica coated magnetic
nanoparticles (imid-Si@ Si-MNPs) were achieved by
using the previously reported method23.
general
Chemical materials were purchased from
the Merck Chemical Company in high purity. Cobalt
ferrite MNPs were prepared using the procedure
Scheme 1: Schematic representation of the formation of
Co (salophen)-imid-Si @ Si-MNPs catalyst
Co (II) complexes of Schiff base ligands
until the washings were colorless and left to dry in
have been synthesized by the reaction of CoCl2 in
ethanolic solutions with the Schiff bases obtained
by the condensation of 1, 2 -phenylenediamine (2
mmol, 0.216 g) with salicylaldehyde (4 mmol, 0.43
ml). Then Co (II) salophen complex was dissolved
to 1, 2 dichloroethane and imid -Si@ Si-MNPs (0.75
g) were dispersed to it and the whole mixture was
refluxed for 2 days. After that, the expected final
product, Co(salophen)-imid-Si @ Si-MNPs , was
separated by magnetic decantation and washed
with 1,2 dichloroethane, methanol and chloroform
a desiccator.
Catalytic Studies
In a typical run, a 25 mL flask was charged
with catalyst (100 mg), acetonitrile (5 mL), alcohol (1
mmol), and H2O2 (3 mmol).This mixture was heated
in an oil bath at 70 ˚C and the progress of the reaction
was monitored by TLC. At the end of the reaction,
the catalyst was simply collected using a magnetic
bar and the reaction mixture was then transferred out
of the flask. The decanted solution was purified on

AFSHARI et al., Orient. J. Chem., Vol. 29(4), 1523-1530 (2013)
1525
a silica-gel plate or a silica-gel column to obtain the
pure product. The structures of the products were
established on the basis of their spectral analysis
(IR, 1H NMR).
The FT-IR spectrum of heterogenised
cobalt salophen shows the expected principal bands
for the free schiff base complex.The C-H deformation
band is visible at 1470 cm-1 and the band at 1611
cm-1 was assigned to –C=N stretching vibration of
the immine group of the ligand that were not in
the imidazole functionalized silica coated cobalt
ferrite. The other bands at 589cm-1, 802 cm-1 and
1088 cm-1 ascribed to Fe-O stretching absorption,
asymmetrical and symmetrical vibrations of the Si-
RESULTS AND DISCUSION
Characterization of Co (salophen)-imid-Si@
Si-MNPs
Thecatalystwassynthesizedbyaprocedure
that shown in Scheme 1, and characterized by
various techniques.
O-Si bond respectively (see Fig. 1)24-30
.
Fig. 1: FT-IR spectra of (a) Si-imid@ Si-MNPs and (b) Co(salophen) -imid-Si @ Si-MNPs
XRD measurements of Co(salophen)-imid-Si @ Si-
MNPs exhibit diffraction peaks corresponding to the
cubic reverse spinel structure of CoFe2O4 (JCPDS
PDF #221086), while the silica layered systems show
an additional broad band at 2θ=20–30 (Fig. 2b and
2c) for the amorphous silica.The average crystallite
size of the Co(salophen)-imid-Si @ Si-MNPs was
calculated to be ~28 nm by Debye-Scherer formula
for the (311) reflection.
Fig. 2: XRD pattern of (a) MNPs, (b) Si-MNPs and (c) Co(salophen) -imid-Si @ Si-MNPs

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The TEM micrograph of the nanoparticles
analysis. Interestingly, the magnetic core is visible
as a dark spot inside the bright spherical SiO2 thin
shell in the TEM image of Co(salophen)-imid-Si @
Si-MNPs sample.
is shown in Fig.3. TEM image indicated that most
of the prepared nanoparticles are spherical shaped
and have size less than 30 nm which show a close
agreement with the values calculated by XRD
Fig. 3:TEM image of Co(salophen) -imid-Si @ Si-MNPs
The magnetic properties of the silica coated
cobalt ferrite (Si-MNPs) and Co (salophen)-imid-Si
@ Si-MNPs were studied by a vibrating sample
magnetometerat300K.Thesaturationmagnetization
(Ms), remanence magnetization (Mr) and coercivity
(Hc) values of these compounds are given in Table
1. As clearly shown, the saturation magnetization of
coated sample is slightly less than that of uncoated
sample, whereas the coercivity of coated sample is
larger than that of uncoated one. This is probably
due to the formation of non-crystalline silica on
the surface of CoFe2O4 nanoparticle as well as the
restriction of domain wall motion by the magnetic
dilution effect of inert silica.
Table 1: Magnetic property of the as-fabricated nanoparticles
Ms (emu/g)
Mr (emu/g) Hc (Oe)
compound
59.49
38.93
16.39
24.63
16.40
6.20
796.1
861.05
900
MNPs
Si-MNPs
Co(salophen)- imid -Si@ Si-MNPs
As can be seen from thermogravimetric
analysis (TGA) the catalyst shows a good thermal
stability (~176ºC), with a weight loss at temperature
between 173-753ºC, which is characteristic of
chemisorbed materials and confirmed that the
organic group is covalently bound on the surface
of Si-MNPs (Fig. 4). On the basis of the TGA an
organic group loading at ca. 0.44 mmol g−1 is
obtained. Moreover, the amount of excess cobalt, in
addition of Fe/Co ratio in cobalt ferrite of the catalyst,
determined by ICP-AES, was 0.3 mmol g−1. This
is another proof for the fact that Co (II) salophen
complex was covalently supported on imidazole
functionalized silica coated CoFe2O4 nanopaticles.
Alcohols
In order to optimize the reaction conditions
for the oxidation of benzylic alcohols various
experiments were carried out at 70°C using
1-phenylethanol in the presence of heterogenised
cobalt salophen as catalyst and H2O2 as an
oxidant.

AFSHARI et al., Orient. J. Chem., Vol. 29(4), 1523-1530 (2013)
1527
Fig. 4:TgA of Co(salophen) -imid-Si @ Si-MNPs
To optimize the catalyst requirement, the
amount of catalyst was changed between 25 and 200
mg per 1 mmol 1-phenylethanol alcohol.Increase in
the yield of acetophenone was observed when the
amount of catalyst was increased from 25 to 100
mg but the yield remained almost same with further
increment of catalyst amount up to 200 mg. So an
optimum of 100 mg catalyst in the reaction mixture
is ideal for achieving the best yield. In addition, in
the absence of catalyst, the reaction was found to
be very slow and was not complete even after 8 h.
We have also accomplished the oxidation of
1-phenylethanol in the different solvents such as
n-hexane, acetone, toluene, cyclohexan, chloroform,
dichloromethane and also acetonitrile. Among the
solvents examined, we found acetonitrile to be the
best for this protocol due to high yield and good
dispersibility of the catalyst in this media. Replacing
H2O2 by t-BuOOH, NaIO4 or H2O2/ Urea (UHP) cause
to decrease the obtained total oxidation yields, so
H2O2 was selected as a best source of oxygen for
this system.
carbonyl compounds in moderate to excellent yields
without over-oxidation to carboxylic acids.
Interesting electronic effect seemed to exist
forthesebenzylicsystems.Forexample,p-nitrobenzyl
alcohol (Table 2, entry 9) required a longer reaction
time and gave a decreased yield compared to more
electron-rich benzylic substrates. For investigating
the chemoselectivity, a mixture of 1-phenylethanol
and 2-phenylethanol was subjected under optimized
condition in the presence of Co(salophen) -imid-
Si @ Si-MNPs. Result indicated that the former
was oxidized to acetophenone in 94% yield
while 2-phenylethanol was recovered completely.
Furthermore, this reaction is highly selective for
vicinal diols in oxidizing only the secondary hydroxy
group α to the benzene ring (Table 2, entries 15).
Therefore, it is a method of choice for the oxidation
of benzylic OH in the presence of non-benzylic OH.
To evaluate the stability and level of
reusability of the catalyst, we conducted experiments
of oxidiation of 1-phenylethanol using the recycled
Co (salophen)-imid-Si@ Si-MNPs catalyst. After
the completion of the first reaction, the catalyst was
concentrated on the sidewall of the reaction vessel
using an external magnet and the solution was
removed by decantation and the left used catalyst
was washed with CH3CN. A new reaction was
then conducted with fresh reactants under similar
conditions. It was found that the developed catalyst
could be used at least five times without any change
in reactivity (Table 2, entries 16).Compared with the
homogeneous and heterogeneous same catalysts
our novel catalyst has shown much more superiority.
At optimum conditions we observed that
when a heterogeneous mixture of this alcohol and
Co(salophen) -imid-Si @ Si-MNPs was heated at 70
˚C in acetonitrile, acetophenone was formed as the
only product in 94% yield after 1.5 h.
The obtained preliminary results for the
oxidation of 1-phenylethanol were encouraging and
to extend the scope of this heterogeneous catalytic
system, the oxidation of further alcohols was carried
out under the optimized conditions. The given results
inTable 2 show that this procedure was clean and all
the alcohols were converted into the corresponding

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AFSHARI et al., Orient. J. Chem., Vol. 29(4), 1523-1530 (2013)
Table 2: Oxidation of alcohols Co (salophen)-imid-Si@ Si-MNPs catalysta
Entry
Alochol
Product
Time (h)
Yieldb (%)
O
OH
1
2
80
O
HO
2
3
1.5
1.5
94
88
OH
O
O
HO
4
1.5
94
Cl
Cl
O
OH
5
6
2
80
95
OH
O
MeO
MeO
MeO
MeO
1.5
O
OH
7
8
9
2
4
2
94
70
65
O
OH
O2N
O2N
OH
O
OH
O
O
O
10
11
1.5
1.5
70
94
OH
O
OH
O
CH3
O
CH3O
CH3O
CH3O
12
13
1.5
2
95
80
O
OH
O
S
O
OH
O
S
14
15
2
2
80
90
O
HO
OH
OH
O
HO
16c
1.5
88
aReaction conditions: allcohol(1mmol), H2O2(3 mmol), catalyst (100 mg), acetonitrile (5 ml), T=70ºC,
bIsolated yields, ccatalyst : Fifth recycled catalyst

AFSHARI et al., Orient. J. Chem., Vol. 29(4), 1523-1530 (2013)
1529
The observed high activity of the catalyst can be
attributed to the nanosized character of the support
which increases surface area for more interaction
as an efficient catalyst for the oxidation of some
benzylic alcohols with hydrogen peroxide.Moreover,
the immobilized catalyst could be easily recovered
by simple magnetic decantation and reused at least
five times without significant loss of activity.
with substrates26-28
.
CONCLUSION
ACKNOWLEDgMENTS
In summary the cobalt salophen complex
has been supported on the surface-modified
CoFe2O4 magnetic. The synthesized catalyst was
confirmed by XRD, FT-IR, TGA, TEM, ICP-AES, and
VSM techniques. This nanocatalyst was employed
Authors are thankful to Research Council
of Shoushtar Branch, Islamic Azad University,
Shoushtar, Iran, for providing financial support for
this project.
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