Click functionalization of magnetite nanoparticles: A new magnetically recoverable catalyst for the selective epoxidation of olefins

Article abstract of DOI:10.1002/aoc.4064

A new magnetically recoverable heterogeneous molybdenum catalyst was developed by means of a click chemistry approach. First, silica-coated magnetite nanoparticles were functionalized using a bidentate ligand via thiol–ene click reaction of mercaptopropyl-modified magnetite nanoparticles with acrylic acid. Then, a molybdenum complex was covalently supported on the surface of the clicked silica-coated magnetite nanoparticles. The prepared catalyst was characterized using Fourier transform infrared and inductively coupled plasma optical emission spectroscopies, X-ray diffraction, vibrating sample magnetometry and transmission electron microscopy. The catalytic performance of the prepared heterogeneous catalyst was investigated in the epoxidation of olefins with tert-butyl hydroperoxide as oxidant. This catalyst could be reused for five runs without significant loss of activity and selectivity.

Full text of DOI:10.1002/aoc.4064

Received: 15 June 2017
DOI: 10.1002/aoc.4064
Revised: 2 August 2017
Accepted: 4 August 2017
F U L L P A P E R
Click functionalization of magnetite nanoparticles: A new
magnetically recoverable catalyst for the selective
epoxidation of olefins
Majid Masteri‐Farahani
| Samaneh Shahsavarifar
Faculty of Chemistry, Kharazmi
University, Tehran, Islamic Republic of
Iran
A new magnetically recoverable heterogeneous molybdenum catalyst was
developed by means of a click chemistry approach. First, silica‐coated magne-
tite nanoparticles were functionalized using a bidentate ligand via thiol–ene
click reaction of mercaptopropyl‐modified magnetite nanoparticles with acrylic
acid. Then, a molybdenum complex was covalently supported on the surface of
the clicked silica‐coated magnetite nanoparticles. The prepared catalyst was
characterized using Fourier transform infrared and inductively coupled plasma
optical emission spectroscopies, X‐ray diffraction, vibrating sample magnetom-
etry and transmission electron microscopy. The catalytic performance of the
prepared heterogeneous catalyst was investigated in the epoxidation of olefins
with tert‐butyl hydroperoxide as oxidant. This catalyst could be reused for five
runs without significant loss of activity and selectivity.
Correspondence
Majid Masteri‐Farahani, Faculty of
Chemistry, Kharazmi University, Tehran,
Islamic Republic of Iran.
Email: mfarahany@yahoo.com
KEYWORDS
click chemistry, epoxidation, heterogeneous catalysis, magnetite, molybdenum
1 | INTRODUCTION
with the aid of appropriate linkers or ligands on the sur-
face of magnetite nanoparticles.[9–15]
Catalytic reactions have a critical role in promoting the
development of the chemical industry.[1,2] Since many
catalysts are expensive and pollute the environment, the
development of well‐planned processes for reusing cata-
lysts is essential from an economic point of view. One
way to address this problem is to employ solid supports
for the immobilization of homogeneous catalysts.[3,4]
The selection of support is critical, as it affects catalytic
efficiency.[5–8]
Recent studies have indicated that magnetite nanopar-
ticles with superparamagnetic behaviour are appropriate
supports for immobilizing catalysts and may facilitate iso-
lation of catalysts from reaction mixtures with an external
magnetic field. Functionalization of the surface of magne-
tite nanoparticles is a well‐known method for the prepa-
ration of magnetically recyclable heterogeneous
catalysts. Homogeneous catalysts can be immobilized
One of the efficient methods for the covalent
functionalization of solid supports is the application of
the click chemistry approach introduced by Sharpless
and co‐workers.[16] Because of their interesting benefits,
click reactions are robust tools for combining various
groups to afford new functional materials.[17] Among
the various click processes, the thiol–ene click reactions
have attracted considerable attention in materials sci-
ence.[18–21] Thiol–ene click reactions involve the reaction
of S―H groups with C═C double bonds which proceeds
by a radical mechanism.
Although several efforts have been made for the
immobilization of molybdenum complexes onto various
solid supports,[8,22–26] the use of the click chemistry
approach as a new and efficient method for preparing
heterogenized molybdenum catalysts has not been
reported so far. The special and interesting magnetic
Appl Organometal Chem. 2017;e4064.
wileyonlinelibrary.com/journal/aoc
Copyright © 2017 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.4064

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MASTERI‐FARAHANI AND SHAHSAVARIFAR
properties encouraged us to employ the thiol–ene click
reaction on the surface of magnetite nanoparticles for
the design of a new heterogenized molybdenum catalyst
for the epoxidation of olefins.
cooling to room temperature, the solid was magnetically
separated, washed several times with methanol to remove
the unreacted residue of silylating reagent and then vac-
uum‐dried at 80 °C.
In this paper, modification of the surface of magnetite
nanoparticles with mercaptoacetic acid via a thiol–ene
click reaction and immobilization of a molybdenum com-
plex is reported. The prepared magnetically recoverable
molybdenum catalyst showed high catalytic activity and
excellent selectivity (100%) in the epoxidation of olefins
and allylic alcohols with tert‐butyl hydroperoxide (TBHP)
as oxidant. As one advantage, the catalyst can be easily
recovered by magnetic separation. Also, the catalyst can
be reused several times without considerable loss of cata-
lytic activity and showed high stability under the reaction
conditions.
2.3 | Click reaction of SCMNPs‐SH with
acrylic acid
To a solution of acrylic acid (2 mmol) in chloroform
(30 ml) was added SCMNPs‐SH (1 g) under dry nitrogen
atmosphere. After 15 min, benzoyl peroxide (0.1 mmol)
was added and the mixture was refluxed for 2 h. The pre-
pared SCMNPs‐S‐COOH was then magnetically sepa-
rated, washed with chloroform to remove the unreacted
reagents and then vacuum‐dried at 80 °C.
2.4 | Immobilization of molybdenum
complex on surface of SCMNPs‐S‐COOH
2 | EXPERIMENTAL
A solution of MoO2(acac)2 (4 mmol, prepared according
to a literature method[28]) in ethanol (50 ml) was added
to SCMNPs‐S‐COOH (2 g, dried in a vacuum oven at
80 °C) and, after sonication, the mixture was refluxed
for 12 h under nitrogen atmosphere. The product was sep-
arated with an external magnet and washed with metha-
nol to remove unreacted MoO2(acac)2. The obtained
SCMNPs‐S‐COO‐MoO2 catalyst was then dried under vac-
uum at 80 °C.
2.1 | Materials and instrumentation
Chemicals were provided by Merck and were of high
purity. All of the solvents were distilled and purified using
standard procedures.
Fourier transform infrared (FT‐IR) spectra were
acquired using a PerkinElmer Spectrum RXI FT‐IR spec-
trometer. Chemical analysis of the prepared catalyst was
done using a Varian VISTA‐MPX inductively coupled
plasma optical emission spectrometry (ICP‐OES) instru-
ment. Transmission electron microscopy (TEM) images
were recorded using a Philips EM 208S instrument with
an accelerating voltage of 100 kV. Scanning electron
microscopy (SEM) images were obtained with a Zeiss
DSM 960A microscope. Magnetic susceptibility analyses
were done using a vibrating sample magnetometer
(BHV‐55, Riken, Japan) at room temperature. X‐ray dif-
fraction (XRD) patterns were acquired using Cu Kα radi-
ation (λ = 1.54 Å) with a Siefert XRD 3003 PTS
diffractometer. The progress of the catalytic epoxidation
of olefins was determined using a gas chromatograph
(Agilent 6890 N) equipped with an HP‐5 capillary column
and flame ionization detector.
2.5 | Catalytic epoxidation of olefins in
presence of SCMNPs‐S‐COO‐MoO2
In a typical experiment for the epoxidation reaction,
100 mg of SCMNPs‐S‐COO‐MoO2 catalyst was added to
a solution of olefin (8 mmol) in chloroform (10 ml). After
addition of TBHP (80% in di‐tert‐butyl peroxide; 14 mmol)
the mixture was refluxed under nitrogen atmosphere.
Samples were collected at given times and were analysed
using GC to determine the conversion and selectivity of
the epoxidation reaction. At the end of each epoxidation
reaction, the catalyst was magnetically separated, washed
with chloroform and then dried under vacuum. The cata-
lyst was reused five times in the epoxidation of
cyclooctene under the same experimental conditions.
2.2 | Preparation of mercaptopropyl‐
modified silica‐coated magnetite
nanoparticles (SCMNPs‐SH)
3 | RESULTS AND DISCUSSION
Silica‐coated magnetite nanoparticles (SCMNPs) were
prepared according to a reported method.[27] SCMNPs
(1 g) were dispersed in toluene (50 ml) with an ultrasonic
homogenizer and then 3‐mercaptopropyltrimethoxysilane
(1 ml) was added to the mixture. The mixture was
refluxed for 24 h under nitrogen atmosphere and, after
3.1 | Preparation of SCMNPs‐S‐COO‐MoO2
catalyst
The details of the synthetic pathway adopted for the con-
struction of the molybdenum catalyst are illustrated in
Scheme 1. Initially, SCMNPs were prepared according to

MASTERI‐FARAHANI AND SHAHSAVARIFAR
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MeO
MeO
MeO
SiO2
SiO2
Fe3O4
Si
SH
O
O
Si
SH
Fe3O4
Toluene
MeO
CHCl3
COOH
benzoyl peroxide
SiO2
SiO2
O
O
O
O
O
MoO2(acac)2
EtOH
C
Si
S
COOH
Si
Fe3O4
S
Fe3O4
O
MeO
MeO
O
Mo
O
O
O
SCHEME 1 Procedure for construction of SCMNPs‐S‐COO‐MoO2
an earlier report.[27] Then, the surface of SCMNPs was
modified with thiol groups via treatment of surface silanol
(Si―OH) groups with 3‐mercaptopropyltrimethoxysilane
to produce SCMNPs‐SH nanomaterial. In the next step,
the thiol–ene click reaction was conducted between
unsaturated C═C bonds of acrylic acid and thiol func-
tional groups of SCMNPs‐SH in the presence of benzoyl
peroxide as radical initiator to afford SCMNPs‐S‐COOH,
as a bidentate ligand for the subsequent immobilization
of molybdenum species. Eventually, complexation of
SCMNPs‐S‐COOH was carried out with an excess of
MoO2(acac)2 to give the SCMNPs‐S‐COO‐MoO2 catalyst.
Afterwards, Soxhlet extraction of the product was done
to remove the physically adsorbed molybdenum species
from the final catalyst.
3.2 | Characterization of SCMNPs‐S‐COO‐
MoO2 catalyst
In order to confirm the modification of the surface of
SCMNPs in each step, the FT‐IR spectra of the obtained
materials were acquired and are shown in Figure 1. The
bands at around 450–590 and 1000–1100 cm−1 in the spec-
tra of all the materials are due to the stretching vibrations
of Fe―O and Si―O in the SCMNPs core.[29] The tethering
of mercaptopropyl groups on the surface of SCMNPs can
be established by the observation of C―H stretching
vibration band appearing at about 2928 cm−1 and a weak
band at 2298 cm−1 due to the S―H stretching vibration in
the FT‐IR spectrum of SCMNPs‐SH.[30] In the FT‐IR spec-
trum of SCMNPs‐S‐COOH, the observed new band at
FIGURE 1 FT‐IR spectra of (a) SCMNP,
(b) SCMNP‐SH, (c) SCMNP‐S‐COOH and
(d) SCMNP‐S‐COO‐MoO2

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MASTERI‐FARAHANI AND SHAHSAVARIFAR
1735 cm−1 can be assigned to C═O stretching vibration of
supported carboxyl group and confirms the successful
click reaction between S―H group of SCMNPs‐SH and
C═C bond of acrylic acid. The presence of cis‐MoO2 group
in SCMNPs‐S‐COO‐MoO2 can be deduced from the
appearance of two bands at 903 and 946 cm−1 due to the
stretching vibrations of the cis‐MoO2 fragment.[31]
According to the ICP‐OES results, the molybdenum
content of SCMNPs‐S‐COO‐MoO2 catalyst was found to
be 0.06 mmol g−1 which further indicated the
functionalization with the molybdenum complex.
In order to reveal the crystal phase of the prepared
SCMNP‐S‐COO‐MoO2, powder XRD analysis was con-
ducted. The XRD pattern of SCMNPs‐S‐COO‐MoO2
(Figure 2) was found to be in accordance with standard
XRD data (JCPDS card number 19–0629) for reverse spi-
nel structure of magnetite.[32] As can be seen in the
XRD pattern, there are six characteristic Bragg peaks cor-
responding to the (220), (311), (411), (422), (511) and (440)
crystallographic planes of magnetite.
The broadness of the peaks is a consequence of the
nanocrystalline nature of the prepared catalyst. The resul-
tant catalyst exhibited only the diffraction peaks of mag-
netite nanoparticles which indicate that, during the
functionalization steps, the crystal phase of the magnetite
core is maintained. Moreover, there is no additional peak
due to the molybdenum complex which reveals the good
dispersion of the molybdenum complex on the surface of
SCMNPs.
The particle size and morphology of the SCMNPs‐S‐
COO‐MoO2 catalyst were explored by means of SEM
and TEM. The SEM image (Figure 3a) of SCMNPs‐S‐
COO‐MoO2 revealed the smooth surface of the nearly
spherical nanoparticles with diameters of about 15 nm.
The TEM image (Figure 3b) also confirmed that the diam-
eters of SCMNPs‐S‐COO‐MoO2 nanoparticles are about
15–20 nm.
The magnetization behaviours of MNPs, SCMNPs
and SCMNPs‐S‐COO‐MoO2 were studied using vibrating
sample magnetometry. Superparamagnetic behaviour
can be seen for all of the prepared nanomaterials as both
their magnetization and demagnetization curves pass
through the origin and there is no any hysteresis loop
in these curves (Figure 4). Also, the saturation magneti-
zation values decreased with the progress of
functionalization of SCMNPs. However, in spite of a
lower magnetization value, the net magnetism exhibited
by SCMNP‐S‐COO‐MoO2 is enough for its efficient isola-
tion from dispersed mixtures with employment of a per-
manent magnet.
FIGURE 2 XRD pattern of as‐prepared SCMNP‐S‐COO‐MoO2
catalyst
FIGURE 3 (a) SEM and (b) TEM images of as‐prepared SCMNP‐
S‐COO‐MoO2 catalyst

MASTERI‐FARAHANI AND SHAHSAVARIFAR
5 of 7
FIGURE
4
Magnetization curves of MNPs, SCMNPs and
SCMNP‐S‐COO‐MoO2 (inset: Magnetic separation of SCMNPs‐S‐
COO‐MoO2 with permanent magnet)
3.3 | Catalytic epoxidation of olefins in
presence of SCMNPs‐S‐COO‐MoO2 catalyst
The prepared SCMNPs‐S‐COO‐MoO2 catalyst was
employed in the catalytic epoxidation of olefins and
allylic alcohols with TBHP as oxidant. The results are
shown in Figure 5. It was found that the olefins were
selectively (>99%) converted to their corresponding epox-
ides. Examination of the results reveals that the activity
of the catalytic system is olefin‐dependent, with cyclic
olefins exhibiting higher conversions. The sequence of
decreasing reactivities based on conversions is:
cyclooctene > cyclohexene > 1‐hexene > 1‐octene > 1‐
decene. To account for this trend, two governing factors,
i.e. electronic and steric effects, should be taken into con-
sideration. As indicated in our earlier works,[10,12] the
mechanism of the epoxidation reaction with alkyl hydro-
peroxides in the presence of molybdenum compounds is
electrophilic addition of electron‐deficient peroxidic oxy-
gen to the olefin C═C double bond as nucleophile
(Figure 6). Thus, increasing the electron donation ability
of C═C double bond in cyclic olefins is expected to pro-
mote the rate of the epoxidation reaction in comparison
with terminal olefins.
FIGURE 5 Results of epoxidation of olefins (top) and allylic
alcohols (bottom) with TBHP in the presence of catalyst
(selectively > 99%). Reaction conditions: Catalyst (100 mg), olefin
(8 mmol), TBHP (14 mmol), refluxing chloroform (10 ml)
catalyst clearly shows the importance of the catalyst in
the epoxidation reaction.
At the end of each reaction, the SCMNPs‐S‐COO‐
MoO2 catalyst could be easily isolated from the mixture
by applying an external magnetic field. The inset of
Figure 4 obviously shows that the reaction mixture
quickly becomes transparent after applying an external
magnet.
Also, the reusability of SCMNPs‐S‐COO‐MoO2 as cat-
alyst was explored by its reuse in the successive epoxida-
tion of cyclooctene with TBHP under the same reaction
conditions. In each cycle, the SCMNPs‐S‐COO‐MoO2
was magnetically isolated, washed with methanol and
dried at 80 °C under vacuum. As is evident from Table 2,
In order to reveal the important role of the catalyst in
this reaction, two experiments were carried out in which
cyclooctene was reacted with TBHP in the absence of
the catalyst or in the presence of SCMNPs and the results
are given in Table 1. As can be seen, no significant reac-
tion occurred in the absence of the catalyst. The signifi-
cant decrease of olefin conversion in the absence of the

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MASTERI‐FARAHANI AND SHAHSAVARIFAR
FIGURE 6 Mechanism of epoxidation of cyclooctene with TBHP in the presence of SCMNPs‐S‐COO‐MoO2
by magnetic decantation and used for epoxidation of ole-
TABLE 1 Epoxidation of cyclooctene with TBHP in the absence
of catalyst
fins and allylic alcohols for up to five times without loss
of activity and selectivity.
Entry
Time (h)
Conversion (%)
Selectivity (%)a
1b
8
9
35
2c
8
5
39
ACKNOWLEDGEMENT
Reaction conditions: catalyst (100 mg), olefin (8 mmol), TBHP (14 mmol),
refluxing chloroform (10 ml).
aSelectivity to epoxycyclooctane.
The authors are grateful to Kharazmi University for
financial support.
bReaction carried out in the presence of SCMNPs.
cReaction carried out in the absence of catalyst.
ORCID
Majid Masteri‐Farahani
http://orcid.org/0000-0001-9770-
9566
TABLE 2 Epoxidation of cyclooctene with TBHP in the presence
of recycled catalyst and ICP‐OES analysis of recycled catalyst
Mo content
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How to cite this article: Masteri‐Farahani M,
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magnetite nanoparticles: A new magnetically
recoverable catalyst for the selective epoxidation of
olefins. Appl Organometal Chem. 2017;e4064.
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