Synthesis and characterization of molybdenum (VI) complex immobilized on polymeric Schiff base-coated magnetic nanoparticles as an efficient and retrievable nanocatalyst in olefin epoxidation reactions

Article abstract of DOI:10.1002/aoc.5410

In this study, a new polymeric functionalized magnetic nanocatalyst containing a molybdenum Schiff base complex was prepared using a few consecutive steps. Poly (methylacrylate)-coated magnetic nanoparticles were synthesized via radical polymerization of methyl acrylate onto modified magnetic nanoparticles followed by the amidation of the methyl ester groups with hydrazine. Polymeric functionalization efficiently provides the advantage that more catalytic units can be grafted on the surface of magnetic nanoparticles. The functionalization process was continued with salicylaldehyde which introduced Schiff base groups on to the surface of the polymeric support. In the final step, the desired catalytic system was prepared via complexation of the Schiff base groups with MoO₂(acac)₂. The resulting nanoparticles were characterized by infrared spectroscopy, powder X-ray diffraction, scanning and transmission electron microscopy, elemental analysis, inductively coupled plasma optical emission spectrometry, vibrating sample magnetometry and thermogravimetric analysis. This heterogenized catalytic system was also found to be highly active, sustainable and recyclable nanocatalyst in alkene epoxidation. Eventually, the attractive features of the synthesized catalyst such as simple work-up, good stability, magnetic separation, high TOF and high surface area; make it appropriate for oxidation reactions.

Full text of DOI:10.1002/aoc.5410

Received: 17 September 2019
DOI: 10.1002/aoc.5410
Revised: 30 October 2019
Accepted: 11 November 2019
F U L L P A P E R
Synthesis and characterization of molybdenum (VI)
complex immobilized on polymeric Schiff base-coated
magnetic nanoparticles as an efficient and retrievable
nanocatalyst in olefin epoxidation reactions
Anahita Mortazavi-Manesh
| Mojtaba Bagherzadeh
Chemistry Department, Sharif University
of Technology, P.O. Box 11155-3615
Tehran, Iran
In this study, a new polymeric functionalized magnetic nanocatalyst con-
taining a molybdenum Schiff base complex was prepared using a few consecu-
tive steps. Poly (methylacrylate)-coated magnetic nanoparticles were
synthesized via radical polymerization of methyl acrylate onto modified mag-
netic nanoparticles followed by the amidation of the methyl ester groups with
hydrazine. Polymeric functionalization efficiently provides the advantage that
more catalytic units can be grafted on the surface of magnetic nanoparticles.
The functionalization process was continued with salicylaldehyde which intro-
duced Schiff base groups on to the surface of the polymeric support. In the
final step, the desired catalytic system was prepared via complexation of the
Schiff base groups with MoO (acac) . The resulting nanoparticles were charac-
Correspondence
Mojtaba Bagherzadeh, Chemistry
Department, Sharif University of
Technology, PO Box 11155-3615, Tehran,
Iran.
Email: bagherzadeh@sharif.edu
Funding information
Iran National Science Foundation (INSF),
Grant/Award Number: 96008692
2
2
terized by infrared spectroscopy, powder X-ray diffraction, scanning and trans-
mission electron microscopy, elemental analysis, inductively coupled plasma
optical emission spectrometry, vibrating sample magnetometry and ther-
mogravimetric analysis. This heterogenized catalytic system was also found to
be highly active, sustainable and recyclable nanocatalyst in alkene epoxida-
tion. Eventually, the attractive features of the synthesized catalyst such as sim-
ple work-up, good stability, magnetic separation, high TOF and high surface
area; make it appropriate for oxidation reactions.
K E Y W O R D S
epoxidation, immobilization, magnetically separable nanocatalyst, molybdenum, polymer coated
magnetic nanoparticles
[
7,8]
1
| INTRODUCTION
selective oxidation reactions.
In this regard, very effi-
cient and selective catalytic systems employing a mild
oxidant combined with a Schiff base complex have been
Metal catalyzed oxidation of olefins constitutes an impor-
tant area in modern chemistry from the academic and
[
9–13]
reported in recent years.
Despite of the advantages
[1–4]
industrial point of view.
Among transition metal
such as accessibility of catalytic sites, high turnover num-
bers, high selectivity and yields, some problems including
recyclability and reusability issues have limited the appli-
complexes which provide a convenient route for oxygen
[5,6]
transfer processes,
Schiff base complexes have been
[
14,15]
found to be very effective homogeneous catalysts for the
cation of the mentioned homogeneous catalysts.
Appl Organometal Chem. 2019;e5410.
wileyonlinelibrary.com/journal/aoc
© 2019 John Wiley & Sons, Ltd.
1 of 10
https://doi.org/10.1002/aoc.5410

2
of 10
MORTAZAVI-MANESH AND BAGHERZADEH
In order to overcome the separation problem, the
tetraethyl orthosilicate (TEOS), and 3-(trimethoxysilyl)
propylmethacrylate (MPS, 98%) were obtained from
Merck. 2,2 -Azobisisobutyronitrile (AIBN) was purchased
from Daejung chemical & metal company in Korea. Ana-
lytically pure methyl acrylate (MA) was purchased from
concept of heterogenizing of homogeneous catalysts
has emerged to combine the advantages of both homo-
0
[16–20]
geneous and heterogeneous catalysts.
Commonly,
heterogenization is carried out by supporting the homo-
geneous catalyst onto the surface of an insoluble organic
East
of
China
Chemical
Corporation.
Bis
[21–23]
or inorganic solid such as mesoporous silica,
poly-
and carbon-based
However, these heterogeneous systems
require filtration steps to recover the catalysts leading to
(acetylacetonato)
dioxomolybdenum
(VI),
[24,25]
[26,27]
[40]
mer backbones,
materials.
zeolites
MoO (acac)2
was prepared as described in the litera-
2
[
28]
ture. Other reagents and solvents were obtained from
Merck or Aldrich chemical companies. All commercially
available solvents and tested compounds were of analyti-
cal-reagent grade and used without further purification.
FT-IR spectra were recorded as KBr pellets using
ABB Bommem MB-100 FT-IR spectrophotometer. Scan-
ning electron microscopy (SEM) was carried out on
Philips XL30. Transmission electron microscopy (TEM)
analysis was acquired with a Zeiss EM 900 microscope.
X-ray diffraction (XRD) pattern was obtained by D4
ENDEAVOR diffractometer (Bruker AXS Inc.) with Cu
Kα as a radiation source. Elemental analysis (CHN)
was performed using a Heraeus Elemental Analyzer.
The metal loading of the catalyst and leaching of
molybdenum into the solution were determined by
inductively coupled plasma atomic emission spectros-
copy (ICP-AES) on a Perkin-Elmer AA-300 spectropho-
tometer. A vibration sample magnetometer (VSM) was
used to measure the magnetic properties of the samples
[29]
an activity loss during continuous cycles.
More recently, among the numerous types of support
materials, silica-coated magnetic nanoparticles (MNP)
have attracted considerable attention due to their super-
paramagnetism property, relatively high surface area, dis-
tribution of the active sites on the outer surface of the
support,
functionalization.
good
stability,
easy
synthesis
and
[30–34]
All these features made magnetic
nanoparticles a promising alternative to other heteroge-
neous catalysts most of which require a tedious work-up
[35–37]
procedures to recover the catalyst.
The introduction
of functional organic groups on magnetic nanoparticles'
surfaces followed by supporting the catalyst is an elegant
way to bridge the gap between heterogeneous and homo-
[38,39]
geneous catalysis.
In this manner, functionalized
polymer-based magnetic nanoparticles are gaining much
attention compared MNPs modified by small organic
molecules, because of the presence of abundant active
sites and large surface area. Polymeric functionalization
obtains the advantage that more catalytic units can be
grafted to the surface of magnetic nanoparticles and thus
provides a higher catalyst loading.
Accordingly in the present work, with the aim of
increased ease of recovery, increased loading and stabil-
ity of catalyst, we have reported the preparation of poly
at room temperature.
(TGA) were recorded from room temperature to 600 C
at a heating rate of 10 C min
Thermogravimetric analyses
ꢀ
ꢀ
−1
under a nitrogen
atmosphere by a TGA Q 50 thermogravimetric ana-
lyzer. Gas chromatographic (GC) analyses were per-
formed on an Agilent Technologies 6890 N, 19019 J-
413 HP-5, capillary 60 m × 250 μm × 1 μm.
(methylacrylate)-coated magnetic nanoparticles con-
taining a molybdenum Schiff base complex via a few
steps. Moreover, catalytic performance of this hetero-
genized catalytic system was investigated in liquid
phase epoxidation of olefins with tert-butyl hydroperox-
ide (TBHP) as an oxidant. The catalyst could be conve-
2.2 | Synthesis of Fe O magnetic
nanoparticles
3
4
Fe O magnetic nanoparticles were prepared on the basis
3
4
[41,42]
niently separated
using an external magnet and
of the procedure reported in previous studies.
In
efficiently recovered several times without a significant
loss in catalytic activity.
brief, 5.2 g (19.3 mmol) of FeCl .6H O and 2 g
3
2
(10.0 mmol) of FeCl .4H O were dissolved in 100 ml
2
2
deionized water at room temperature under N atmo-
2
sphere. Then, 10 ml of 25% aqueous ammonia was added
dropwise into the mixture at room temperature, which
the black precipitates of Fe O nanoparticles were formed
2
2
| EXPERIMENTAL SECTION
.1 | Materials and characterization
3
4
immediately. The resultant mixture was vigorously
stirred for 1 hr. The formed precipitates were separated
using a strong magnetic field (0.5 T magnet), washed sev-
techniques
ꢀ
Ferric chloride hexahydrate (FeCl .6H O), ferrous chlo-
eral times with distilled water, and then dried at 50 C
3
2
ride
tetrahydrate (FeCl .4H O), ammonia (30%),
overnight.
2
2

POLYMERIC FUNCTIONALIZED MAGNETIC NANOCATALYST
3 of 10
2
.3 | Synthesis of Fe O magnetic
2.6 | Synthesis of hydrazine-
functionalized magnetic nanoparticles
3
4
nanoparticles coated by silica
The resulting Fe O nanoparticles (2 g) were ultrasoni-
cally dispersed in 100 ml of an ethanol-water mixture
0.2 g of MNP@PMA nanoparticles were homogeneously
dispersed in 20 ml of methanol. Excess amount of hydra-
zine (1 ml) was added to the mixture, and the mixture
was refluxed under continuous stirring for 48 hr. The
obtained nanoparticles were magnetically separated and
then extensively washed with methanol and dried at
3
4
(
(
4
4: 1) for 10 min. Then, 4 ml of tetraethyl orthosilicate
TEOS) was added dropwisely to the suspension at
ꢀ
0 C under stirring for 30 min. The pH of this solu-
tion was adjusted to 10 by slow addition of NH solu-
3
[43]
ꢀ
tion.
The reaction mixture was refluxed for 8 hr to
60 C for being used in the next step.
allow the base-catalyzed hydrolysis and condensation
of TEOS monomers on the nanoparticle surface go to
completion. As a result, the silica coated magnetic
nanoparticles (MNP) were magnetically separated
and then washed with distilled water and ethanol.
2.7 | Synthesis of polymeric Schiff base-
coated magnetic nanoparticles
ꢀ
The final dark brown product was dried at 60 C for
0.15 g of MNP@PMA-NHNH2 nanoparticles were
suspended in 20 ml of ethanol. Afterwards, 0.5 ml
2
4 hr.
(
(
4.6 mmol) of salicylaldehyde dissolved in ethanol
10 ml) was added dropwisely to the solution while stir-
ꢀ
2.4 | Synthesis of the modified magnetic
ring at 70 C. The resultant mixture was refluxed for
24 hr and finally the solid product was separated by a
magnet, washed with ethanol for several times and dried
nanoparticles
ꢀ
In a typical reaction, magnetic nanoparticles (1 g) were
dispersed in 40 ml of a 4: 1 mixture of ethanol–water,
and then 2 ml of ammonium hydroxide 25% solution
was added to the flask. Then, an excess amount
at 60 C overnight.
2.8 | Synthesis of MNP@PMA-SB-Mo
nanocatalyst
(10 mmol per 1 g of MNP) of the 3-(trimethoxysilyl)
propylmethacrylate (MPS) was added dropwise over a
ꢀ
period of 10 min, and the mixture was stirred at 50 C
The desired nanocatalyst was prepared by refluxing the
Schiff base-functionalized magnetic nanoparticles
(0.1 g) in absolute methanol (20 ml) with a methanol
solution (2 ml) of MoO (acac) (0.1 mmol, 0.032 g),
[44,45]
for 48 hr.
The MPS coated magnetic nanoparticles
were separated by an external magnet. After washing
with methanol for several times to remove any excess
2
2
ꢀ
of reagents, the modified nanoparticles were dried at
bis (acetylacetonato)dioxomolybdenum (VI), at 60
C
ꢀ
5
0 C overnight.
for 24 hr. The resulting solid was magnetically col-
ꢀ
lected, washed with methanol and dried at 60 C for
24 hr.
2
.5 | Synthesis of poly (methylacrylate)-
coated magnetic nanoparticles
2.9 | General oxidation procedure
0
.5 g of MNP@MPS nanoparticles were homogenized
by ultrasonication in 25 ml dried chloroform for
0 min. The mixture was completely deoxygenated by
The liquid-phase olefin epoxidation reactions were car-
ried out in air and with autogenous pressure in a 5 ml
batch microreactor equipped with a stir bar and a sam-
pling valve, and immersed in a thermostated oil bath.
Typically, a mixture of the substrate (1 mmol), catalyst
(0.004 g, containing 2.2 × 10− mmol of Mo complex) and
C H Cl (1 ml) was stirred in a 5 ml tube for 10 min at
84 C. Then 182 μL of 5.5 M solution of the oxidant t-
BuOOH in decane (1 mmol) was added and the system
was stirred for 1–3 hr under reflux. The progress of the
reaction was monitored by GC. All the reactions were
performed at least two times and the products were com-
pared with standard samples.
1
bubbling purified argon for 15 min. 3 g of methyl
acrylate (MA) was then added dropwise into the sus-
pension of particles. Afterwards, 0.07 g of AIBN was
added to the flask to initiate
The reaction mixture was performed under stirring at
3
polymerization.
2
4
2
ꢀ
ꢀ
6
0
C
during 24 hr. The resulting poly
(methylacrylate)-coated magnetic nanoparticles were
magnetically collected and washed with methanol to
remove excess MA monomers and other excess
reactants.
[44]

4
of 10
MORTAZAVI-MANESH AND BAGHERZADEH
2
.10 | Reusability of the catalyst
In the following step, hydrazine is immobilized on
the MNP@PMA nanoparticles through the reaction
between amine group of hydrazine and ester functional-
ity (-CO CH ) on the surface of MNP@PMA to form
hydrazine-functionalized magnetic nanoparticles. After-
wards, the obtained nanoparticles have been surface-
modified with salicylaldehyde which introduced Schiff
base group on to the surface of support.
The reusability and the stability of the catalyst were stud-
ied in repeated oxidation reactions. At the end of each
reaction, the catalyst was magnetically separated from
the reaction mixture, washed with C H Cl and then
dried in vacuum at room temperature for 4 hr before
reusing in the subsequent oxidation reaction. The catalyst
was consecutively reused about six times without detect-
able catalyst leaching or significant loss of activity.
2
3
2
4
2
Eventually, the molybdenum-Schiff base nanocatalyst
was prepared via complexation of MoO (acac) and Schiff
2
2
base groups on the surface of MNP@PMA-SB
nanoparticles.
3
3
| RESULTS AND DISCUSSION
.1 | Catalyst preparation and
The X-ray diffraction analysis was carried out for the
nanocatalyst which reveals the presence and degree of
crystallinity of Fe O magnetic nanoparticles. As illus-
3
4
characterization
trated in Figure 1, in the XRD pattern of the synthesized
nanoparticles, six characteristic peaks (2θ = 30.28, 35.59,
43.27, 53.75, 57.18, and 62.86), corresponding to (220),
(311), (400), (422), (511), and (440) Bragg reflections,
respectively, were observed. These diffraction peaks are
in good agreement with the database in JCPDS file
(PCPDFWIN v.2.02, PDF No. 85–1436) and indicate that
the Fe O structure was not changed after all modifica-
A
schematic illustration of the functionalized
nanoparticles synthesis is shown in Scheme 1. First, the
co-precipitation method was used for the synthesis of
Fe O nanoparticles with sufficient magnetism to allow
3
4
easy catalyst recovery. Coating of the Fe O surface with
3
4
a silica shell was carried out by TEOS to increase the
functionality and stability of the nanoparticles. In the
next step, surface modification of MNPs with MPS before
polymer coating obtains effective polymerization and
3
4
tions. This XRD pattern is consistent with the pattern pre-
[
47]
viously reported for Fe O samples.
3
4
To confirm the structure of products, the reactions
were monitored by FT-IR, as shown in Figure 2. The FT-
IR spectrum (Figure 2a) represents significant absorption
bands at about 592 and 623 cm− which corresponds to
Fe–O vibration modes of Fe O . Furthermore, a strong
[
46]
covalent grafting of polymer chains onto the surface.
The synthetic procedure for the poly (methylacrylate)-
coated magnetic nanoparticles, noted as MNP@PMA, is
depicted in Scheme 1. Coating of PMA onto the MNPs
has some advantages over conventional grafting of mate-
rial onto the surface of MNPs. This approach efficiently
endows the resulting nanoparticles with a series of active
catalyst sites, providing higher catalyst loading. On the
other hand, polymer chains will prevent aggregation of
nanocatalysts in high temperature applications.
MNP@PMA was synthesized by free radical initiation
polymerization of methyl acrylate as a monomer and
AIBN as an initiator in the presence of MNP@MPS
nanoparticles.
1
3
4
−1
absorption band at around 1083 cm (between 1000 and
1200 cm ), which can be assigned as vibration mode of
indicates that the Fe O core is successfully
3 4
coated by silica shell. Another band at around 3420 cm
−
1
[
48,49]
Si–O–Si,
−
1
is associated with the vibration of absorbed water
molecules.
The MPS coating on the MNP surface was confirmed
by the peaks observed at 2925 and 1701 cm− in Figure 2b,
which were attributed to C–H and ester C=O bonds
1
[
44]
respectively.
The FT-IR spectrum of MNP@PMA
SCHEME 1 Synthetic pathways for the
preparation of polymeric Schiff base-coated
magnetic nanoparticles

POLYMERIC FUNCTIONALIZED MAGNETIC NANOCATALYST
5 of 10
Figure 2c) shows a strong vibration band in 1735 cm−
1
(
corresponding to the ester C=O of poly-(methylacrylate).
Conversion of the CO Me groups in the polymer shell
2
to CONHNH2 and the formation of the hydrazine-
functionalized magnetic nanoparticles were approved by
−
1
the appearance of an amide C=O band at 1638 cm and
elimination of the ester C=O peak in the FT-IR spectrum
of MNP@PMA, as shown in Figure 2d (and compared to
Figure 2c). This conclusion is also confirmed by the ele-
mental analysis which gave the percentages of C, N, and
H to be 16%, 2.8%, and 4.4%, respectively.
Additionally as illustrated in Figure 2e, the presence
of vibration band of C=N at 1622 cm−
1 [50]
indicates the
FIGURE 1 XRD pattern of MNP@PMA-SB-Mo nanocatalyst
functionalization process with salicylaldehyde which
introduced Schiff base groups on to the surface of the
polymeric support. The FT-IR spectrum of the
MNP@PMA-SB-Mo nanoparticles (Figure 2f) clearly
showed the formation of molybdenum complex, as exem-
plified by the appearance of symmetrical and anti-sym-
metrical stretch of two cis doubly bonded oxygens in 914
and 945 cm− at the molybdenum core. Moreover, the
loading of molybdenum complex on the surface was
1
0
.56 mmol/g, determined by ICP-AES test of
molybdenum.
The morphology and textural properties of the
nanocatalyst were examined by scanning and transmis-
sion electron microscopy (Figure 3). The SEM and TEM
images of MNP@PMA-SB-Mo nanoparticles clearly
depict a smooth morphology and uniform distribution of
magnetic nanoparticles.
Thermogravimetric analysis (TGA) is a simple
method for studying the decomposition pattern and the
thermal stability of the compounds. TGA curves of
MNP@MPS, MNP@PMA-NHNH2 and MNP@PMA-SB
nanoparticles are depicted over the temperature range of
ꢀ
ꢀ
2
5–600 C in Figure 4. The weight loss at around 200 C is
mainly due to the loss of physically adsorbed water or sol-
[
51]
ꢀ
The weight loss between 200 and 600 C is attrib-
vent.
FIGURE 2 FT-IR spectra of: (a) MNP (b) MNP@MPS (c)
MNP@PMA (d) MNP@PMA-NHNH
MNP@PMA-SB-Mo
2
(e) MNP@PMA-SB (f)
uted to the thermal degradation of organic groups coated
on the surface of magnetic nanoparticles, which was
FIGURE 3 (a) SEM and (b) TEM
image of MNP@PMA-SB-Mo

6
of 10
MORTAZAVI-MANESH AND BAGHERZADEH
TABLE 1 Screening of solvents and reaction conditions in the
oxidation of cyclooctene in the presence of MNP@PMA-SB-Mo
a
nanocatalyst
Amount of catalyst
a,b
Entry
(g)
Solvent
Conversion%
1
2
3
4
5
6
7
8
-
C2H4Cl2 24
0.004
0.004
0.004
0.004
0.004
0.002
0.01
CHCl3
68
54
32
21
CH2Cl2
CH3OH
CH3CN
C2H4Cl2 98
C2H4Cl2 67
C2H4Cl2 99
FIGURE 4 TGA analysis of (a) MNP@MPS (b) MNP@PMA-
NHNH (c) MNP@PMA-SB
2
a
Reaction condition: 1 mmol cyclooctene: 1 mmol TBHP. The reactions were
estimated at about 10, 19.6 and 30.4 wt% for MNP@MPS,
run under reflux for 1 hr.
MNP@PMA-NHNH and MNP@PMA-SB nanoparticles,
b
2
Determined by GC.
respectively. Therefore, it can be found that the loading
amount of MPS on the surface of MNPs is about
were studied while keeping
a fixed amount of
0
.8 mmol/g. Also, estimated loading material for
cyclooctene (1 mmol) and TBHP (1 mmol). By reducing
the amount of catalyst from 0.004 to 0.002 g, the conver-
sion shows a significant reduction from 98% to 67%
(Table 1, entries 6, 7). On the other hand, increasing of
catalyst amount from 0.004 to 0.01 g does not have a
noticeable effect on the percent of conversion under the
reaction conditions (Table 1, entries 6, 8).
MNP@PMA-NHNH nanoparticles is about 0.92 mmol/g.
2
Comparing the TGA curves of MNP@PMA-SB and
MNP@PMA-NHNH2 nanoparticles indicates that the
loading amount of Schiff base ligand on the surface of
MNPs is about 1.1 mmol/g.
Figure 5 shows kinetic profile of cyclooctene epoxida-
tion using MNP@PMA-SB-Mo as the catalyst under
reflux in C H Cl as a solvent. It can be clearly observed
3.2 | Catalytic oxidation of olefins
2
4
2
Catalytic activity of the designed system was investigated
in the liquid-phase epoxidation of olefins. At first, we
examined the oxidation of cyclooctene, used as a model
substrate, with the tert-butyl hydroperoxide (TBHP) as
the mono-oxygen source under different conditions and
solvents to form epoxide as the main product (Table 1).
Blank experiment (in the absence of catalyst) gave low
yield of cyclooctene epoxide after 1 hr under the same
conditions (Table 1, entries 1).
that oxidation in this system proceeds rapidly in first
30 min as the conversion reaches 91%, but then it con-
tinues more slowly until almost all of the substrate is
consumed after 1 hr. This behavior has been attributed to
the formation of tert-butyl alcohol, a by-product of the
decomposition of TBHP, which may act as a competitive
inhibitor with the oxidant for coordination to the molyb-
[
53,54]
denum center.
For comparison, effect of solvent on cyclooctene epox-
idation was studied by carrying out the reaction in differ-
ent solvents (CHCl , CH Cl , C H Cl , CH OH, CH CN).
3
2
2
2
4
2
3
3
As shown in Table 1, the efficiency of the nanocatalyst
was affected by the nature of the solvent and the quantity
of the catalyst used in the reaction. 1,2-Dichloroethane
was found to be the best solvent for optimizing yields. It
should be mentioned that highly coordinating solvents
such as CH OH which compete with TBHP for binding
3
to the molybdenum center, cause a significant decrease
[52]
in the catalytic activity.
The poorer yield obtained with
FIGURE 5 Time-dependence of the conversion in the
catalytic cyclooctene oxidation catalyzed by MNP@PMA-SB-Mo.
Reaction conditions: 1 mmol cyclooctene, 1 mmol TBHP, 0.004 g
catalyst (containing 2.2 × 10− mmol of Mo complex), 1 ml
dichloroethane, reflux
CH Cl is probably due to the lower reaction temperature
under reflux conditions.
The effect of catalyst amount on the conversion is also
illustrated in Table 1. Three different amounts of catalyst
2
2
3

POLYMERIC FUNCTIONALIZED MAGNETIC NANOCATALYST
7 of 10
In order to show the general applicability of
MNP@PMA-SB-Mo nanocatalyst for epoxidation proto-
col, various alkenes were subjected to the oxidation
reaction with the catalyst under the optimized reaction
conditions. This heterogenized catalytic system converts
the olefins to the corresponding epoxides in high to
excellent yields similar to that obtained for
cyclooctene, as summarized in Table 2. The selective
conversion of cyclohexene to the epoxidized product
reached 96% after 2 hr. This observation indicates that
cyclohexene oxidation reaction occurs via non-radical
that the catalytic activity tends to decrease as the ole-
fin becomes conjugated with phenyl group.
The least reactive aliphatic terminal alkenes such as
1-octene and 1-heptene were catalytically oxygenated in
desired times with moderate yields and excellent
selectivities.
A proposed mechanism of molybdenum-catalyzed
epoxidation reaction, based on the result of previously
[
52,55]
reported investigations;
is shown in scheme 2.
The catalytic reaction pathway involves transfer of one
tert-BuOOH proton to one oxo group of the dioxo
complex and coordination of tert-BuOO− anion to the
Lewis acidic metal center. The subsequent step pro-
ceeds by oxygen transfer to the olefin, thus generating
the epoxidized product. Nucleophilic attack of the ole-
fin to one oxygen of the peroxo ligand yields
corresponding epoxide and tert-butyl alcohol as a by-
product.
[52]
processes.
The reactions of styrene and its deriva-
tives were not selective to the corresponding epoxide
under the applied reaction conditions, giving a wide
spectrum of products via double bond cleavage includ-
ing
benzaldehyde,
acetophenone
and
4-meth-
ylbenzaldehyde from styrene, α-methylstyrene and 4-
methylstyrene respectively. The results demonstrate
a
TABLE 2 Catalytic oxidation of various alkenes with TBHP by MNP@PMA-SB-Mo nanocatalyst
a,b
TOF (h−1
)
Entry
Alkene
Conversion%
Selectivity to epoxide%
Time (hr)
1
98
100
1
445.4
2
3
4
96
92
82
100
72
2
3
3
218.2
139.4
124.2
78
5
76
83
3
115.1
6
7
63
58
100
100
3
3
95.4
87.8
a
Reaction condition: 0.004 g catalyst: 1 mmol alkene: 1 mmol TBHP. The reactions were run under reflux in 1 ml dichloroethane.

8
of 10
MORTAZAVI-MANESH AND BAGHERZADEH
SCHEME 2 Proposed mechanism for oxidation of alkenes with TBHP catalyzed by MNP@PMA-SB-Mo\
3
.3 | Catalyst separation, reuse, and
complete epoxidation of cyclooctene, washed with
C H Cl , dried at room temperature and then reused in
another catalytic cycle.
stability
2
4
2
The stability and the catalytic recyclability were exam-
ined by reusing the nanocatalyst isolated from a previous
run in the next run of cyclooctene epoxidation under the
optimized conditions. For the reusability test, the catalyst
was separated by using an external magnet after
The synthesized MNP@PMA-SB-Mo nanocatalyst
possesses a relatively high superparamagnetic behavior
(Figure 6). Magnetic measurements were performed in
an applied magnetic field sweeping from −10 to +10 kOe
using vibrating sample magnetometer (VSM) at room
temperature. Also, the magnetic property of the reused
nanocatalyst was investigated and no considerable
change was observed. This observation represents that
FIGURE 8 Reusability of the MNP@PMA-SB-Mo
nanocatalyst in the oxidation of cyclooctene with TBHP. Reaction
FIGURE 6 Magnetization curves of fresh (black curve) and
reused (blue curve) MNP@PMA-SB-Mo nanocatalyst
conditions: 1 mmol cyclooctene, 1 mmol TBHP, 0.004 g catalyst,
ꢀ
1 ml C H Cl , 84 C, 1 hr
2 4 2
FIGURE 7 (a) FT-IR spectrum and
b) XRD pattern of reused MNP@PMA-
(
SB-Mo nanocatalyst after sixth reaction
cycle

POLYMERIC FUNCTIONALIZED MAGNETIC NANOCATALYST
9 of 10
TABLE 3 Comparison of literature reports of molybdenum catalysts on the selective oxidation of cyclooctene under various conditions
ꢀ
Entry
Catalyst
Molar ratio cat: sub: ox
1:454.5:454.5
1:100:120
Temp. ( C)
Reflux
Reflux
Reflux
55
Time (hr)
Conversion%
Refs.
This work
[29]
1
2
3
4
5
6
7
MNP@PMA-SB-Mo
1Mo@SMNP
2Mo@SMNP
1
98
6
96
1:100:120
6
58
[29]
MCM-41-MoO
2
Cl(THF)
2
1:89:134.1
2
31
[53]
MCM-41-L/MoO
Mo-MCM-41
2
Cl
2
1:89:134.1
55
2
29
[53]
1:68:205
40
3
97
[56]
Mo-Fe
3
O
4
@SiO
2
@P
4
VP
1: 59.3: 372.8
60
12
85.6
[57]
the structure of the magnetic nanoparticles is remained
unchanged during the consecutive recycle steps.
nanoparticles bearing the Mo (VI) complex, as a novel
and efficient catalyst for oxidation of olefins, have
been presented. The most important advantage of this
heterogenized catalytic system is the simplicity of the
catalyst separation in which the catalyst can be sepa-
rated by applying a magnet from the reaction mixture.
The MNP@PMA-SB-Mo nanocatalyst can be reused for
six additional runs with nearly constant catalytic activ-
ity; no metal leaching was detected after the consecu-
tive catalytic reactions.
The attractive features of this catalytic system such as
easy work-up, chemical stability, simple recyclability,
high TOF and high surface area which is guaranteed high
loading of catalytically active sites; make it particularly
suitable and cost-effective for oxidation reactions.
The FT-IR spectrum of the MNP@PMA-SB-Mo
nanoparticles after six reuses (Figure 7a) shows no
noticeable changes (compared to Figure 2f). Further-
more, the XRD pattern of the reused nanocatalyst after
the sixth run (Figure 7b) indicated that the crystalline
structure of Fe O nanoparticles was maintained during
3
4
the course of the reaction.
As shown in Figure 8, the conversions at 1 hr were
similar for all six consecutive runs and selectivity to the
epoxide remained very high (about 100%). After each cat-
alytic reaction, the catalyst nanoparticles were collected
using a magnet, supernatant carefully, decanted and ana-
lyzed by ICP-AES analysis to determine the amount of
molybdenum leaching. ICP-AES analysis did not show
any leaching of molybdenum species into the solution
ACKNOWLEDGEMENTS
(
<0.01 ppm).
This work was supported by Iran National Science Foun-
dation (INSF) for post-doctoral research No. of 96008692.
For comparison, some of the most relevant systems
reported to date have been included in Table 3. From
these results, it is clear that MNP@PMA-SB-Mo
nanocatalyst is more effective than other heterogenized
catalytic system under various conditions. Moreover,
comparison of the catalytic performances presented in
this work with those of other similar studies reveals that
our catalytic system is superior to some of the previously
ORCID
Anahita Mortazavi-Manesh https://orcid.org/0000-
0
002-1502-2371
Mojtaba Bagherzadeh https://orcid.org/0000-0003-
009-3693
3
[56,57]
reported catalysts
in terms of catalytic activity
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How to cite this article: Mortazavi-Manesh A,
Bagherzadeh M. Synthesis and characterization of
molybdenum (VI) complex immobilized on
polymeric Schiff base-coated magnetic
nanoparticles as an efficient and retrievable
nanocatalyst in olefin epoxidation reactions. Appl
Organometal Chem. 2019;e5410. https://doi.org/10.
6
336.
36] V. Polshettiwar, R. Luque, A. Fihri, H. Zhu, M. Bouhrara,
J. M. Basset, Chem. Rev. 2011, 111, 3036.
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1002/aoc.5410