Cu(II) Schiff base complex supported on Fe3O4 nanoparticles as an efficient nanocatalyst for the selective aerobic oxidation of alcohols

Article abstract of DOI:10.1002/aoc.4433

Herein, we have prepared a new Cu(II) Schiff base complex supported onto the surface of modified Fe₃O₄ nanoparticles as highly stable, heterogeneous and magnetically recyclable nanocatalyst for the selective aerobic oxidation of different alcohols. The structure, morphology, chemical composition and magnetic property of the nanocatalyst and its precursors were characterized using FT-IR, TGA, AAS, ICP-AES, XRD, SEM, EDS, VSM and N₂ adsorption–desorption analyses. Characterization results exhibited the uniform spherical morphology for nanocatalyst and its precursors. A promising eco-friendly method with short reaction time and high conversion and selectivity for oxidation of various primary and secondary alcohols under O₂ atmosphere condition was achieved. The synthesized nanocatalyst could be recovered easily by applying an external magnetic field and reused for least eight subsequent reaction cycles with only negligible deterioration in catalytic performance.

Full text of DOI:10.1002/aoc.4433

Received: 26 December 2017
DOI: 10.1002/aoc.4433
Revised: 28 April 2018
Accepted: 2 May 2018
F U L L P A P E R
Cu(II) Schiff base complex supported on Fe3O4
nanoparticles as an efficient nanocatalyst for the selective
aerobic oxidation of alcohols
Milad Aghajani | Niaz Monadi
Department of Inorganic Chemistry,
Herein, we have prepared a new Cu(II) Schiff base complex supported onto the
surface of modified Fe3O4 nanoparticles as highly stable, heterogeneous and
magnetically recyclable nanocatalyst for the selective aerobic oxidation of dif-
ferent alcohols. The structure, morphology, chemical composition and mag-
netic property of the nanocatalyst and its precursors were characterized using
FT‐IR, TGA, AAS, ICP‐AES, XRD, SEM, EDS, VSM and N2 adsorption–desorp-
tion analyses. Characterization results exhibited the uniform spherical mor-
phology for nanocatalyst and its precursors. A promising eco‐friendly method
with short reaction time and high conversion and selectivity for oxidation of
various primary and secondary alcohols under O2 atmosphere condition was
achieved. The synthesized nanocatalyst could be recovered easily by applying
an external magnetic field and reused for least eight subsequent reaction cycles
with only negligible deterioration in catalytic performance.
Faculty of Chemistry, University of
Mazandaran, P.O. Box 47416‐95447,
Babolsar, Iran
Correspondence
Niaz Monadi, Department of Inorganic
Chemistry, Faculty of Chemistry,
University of Mazandaran, P.O. Box
47416‐95447 Babolsar, Iran.
Email: nimonadi@umz.ac.ir
Funding information
University of Mazandaran Research
Councils
KEYWORDS
cu(II) complex, magnetic nanocatalyst, oxidation of alcohols, Schiff base ligand
1 | INTRODUCTION
is unacceptable in terms of green and sustainable chemis-
try.[14–16] Hence, a practical method to solve aforemen-
The selective oxidation of alcohols to the corresponding
carbonyl compounds is one of the most fundamental
reactions in organic chemistry because aldehydes and/or
ketones are considerably ubiquitous intermediates for
pharmaceuticals, agricultural chemicals and fine
chemicals.[1–5] Traditionally, this oxidation process is
performed with stoichiometric amounts of toxic oxidants
and/or homogeneous catalysts, but most of these reagents
generate a large amount of hazardous waste causing seri-
ous environmental problems.[6,7] So, it is necessary to
replace such hazardous oxidants with clean environmen-
tally friendly reagents[8–13] Although homogeneous cata-
lysts can often perform oxidation reactions efficiently,
they are associated with drawbacks including difficulty
of separation from products, deactivation of catalyst via
dimerization and instability at high temperatures, which
tioned problems is to immobilize a homogeneous active
molecule on a solid support.[17–22] Among various solid
supports, Fe3O4 is valuable support for the immobiliza-
tion of inorganic and organic catalysts due to its unique
properties including the high surface area, low toxicity,
high dispersion, biocompatibility and super‐paramagnetic
behavior, which make it an important alternative to
porous materials.[23–26] Fe3O4 nanoparticles can be easily
functionalized and separated from the reaction media by
magnetic decantation rather than filtration and/or centri-
fugation process, which require time‐consuming and
tedious separation procedures.[27–30] However, bare
Fe3O4 nanoparticles tend to aggregate in a liquid media
due to the anisotropic dipolar attraction. A solution for
this problem is to protect the magnetic core with an outer
sell such as silica[31] or polymers.[32] Mostly, silica is used
Appl Organometal Chem. 2018;e4433.
wileyonlinelibrary.com/journal/aoc
Copyright © 2018 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.4433

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AGHAJANI AND MONADI
to protect the magnetic core providing appropriate
properties including preventing aggregation, protecting
against oxidation, ease of functionalization, conservation
of crystal structure and improving thermal and chemical
flame‐ionization detector (FID). The textural properties
of the synthesized compounds were measured from
nitrogen adsorption–desorption analysis 77 k (Belsorp,
BELMAX, Japan). The content of copper was deter-
mined by inductively coupled plasma atomic emission
analysis (ICP‐AES, Varian company VISTA‐PRO model).
stability[33]
.
Schiff base complexes of transition metals are impor-
tant homogeneous catalysts in the oxidation of organic
compounds by various oxygen atom donors.[34–37] How-
ever, to prevent the formation of oxo and peroxodimeric
and other polymeric species and facilitate catalysts sepa-
ration, Schiff base complexes have been immobilized
onto solid supports.[38–40] The magnetic support offers
high catalytic efficiency and better recovery without
having the intrinsic obstacles of leaching during the
reaction. Thus to date, a number of reports have been
published on the use of Cu(II) Schiff base complexes
supported on functionalized Fe3O4 nanoparticles as
heterogeneous nanocatalysts for various organic trans-
formations.[41–46]
Herein, we have prepared a new Cu(II) Schiff base
complex supported onto modified Fe3O4 nanoparticles
(Cu(II)‐5CML Schiff base/Fe3O4@SiO2) with a stepwise
procedure as an efficient, selective and recyclable
nanocatalyst for the oxidation of different alcohols in
the presence molecular oxygen.
2.2 | Preparation of the precursors and
nanocatalyst
2.2.1 | Preparation of Fe3O4@SiO2‐amine
Fe3O4 nanoparticles and silica‐coated on Fe3O4 nanopar-
ticles (Fe3O4@SiO2) were prepared based on our previous
work.[40] For the synthesis of Fe3O4@SiO2‐amine,
Fe3O4@SiO2 (1.0 g) was dispersed in ethanol (40.0 ml)
and 3‐aminopropyltriethoxysilane (APTES) was added
drop wise to the mixture under vigorous stirring and
refluxed for 12 h under N2 atmosphere. The dark brown
precipitate (Fe3O4@SiO2‐amine) was collected by mag-
netic decantation and washed sequentially with ethanol/
water (3 × 10 mL) to remove the unreacted silylating
agent. The product was dried under vacuum.
2.2.2 | Preparation of Schiff base ligand
(5CML Schiff base)
2 | EXPERIMENTAL DETAILS
2.1 | General information
5‐Chloromethyl‐2‐hydroxybenzaldehyde was prepared
according to the reported method in the literature.[47]
The tridentate Schiff base ligand was synthesized by the
addition of the solution 5‐Chloromethyl‐2‐hydroxy-
benzaldehyde (1.5 mmol, 0.25 g) in ethyl acetate (5.0 ml)
to the solution of 2‐aminophenol (1.5 mmol, 0.16 g) in
ethyl acetate (5.0 ml). The reaction mixture was stirred
for 2 h at room temperature, giving yellow powder (69%
yield). The product was washed several times with CH2Cl2
and ethyl acetate and dried under vacuum. The yellow
sample was denoted as 5CML Schiff base.
All the chemicals and solvents were purchased from
commercial suppliers and were used without further
purification. Infrared spectra were recorded in KBr disks
by a Bruker FT‐IR instrument in the range of 400 to
1
4000 cm−1. H NMR and 13C NMR spectra for samples
were recorded on a Bruker 400 MHz Ultrashieldavance
III Spectrometer. Magnetic properties of samples were
detected at room temperature from −10000 to
+10000 Oe using a vibrating sample magnetometer
(VSM, Meghnatis Kavir Kashan Co., Kashan, Iran). The
morphology of the nanostructures was examined using
a field emission scanning electron microscopy (FE‐
SEM; TE SCAN‐Mira3‐XMCL) equipped with energy dis-
persive X‐ray Spectroscopy (EDS). Thermogravimetric
analysis (TGA) was conducted from room temperature
to 800 °C at a heating rate of 10 °C min−1 under N2
atmosphere using a NETZSCH STA 409 PC/PG instru-
ment. The X‐ray powder patterns (XRD) were recorded
with a Bruker, D8 ADVANCE (Germany) diffractometer
(Cu Kα radiation, λ = 1.5404 Å) in the 2θ range of 10°‐
70° at room temperature. The products of the alcohols
oxidation were analyzed using a VARIAN CP‐3800 gas
chromatograph equipped with a capillary column and a
2.2.3 | Preparation of 5CML Schiff base/
Fe3O4@SiO2
The prepared Fe3O4@SiO2‐amine magnetic nanoparticles
(1.0 g) were dispersed in methanol (50.0 ml) by ultrasonic
irradiation for 20 min. Then, 5CML Schiff base ligand
(1.5 mmol, 0.39 g) in methanol (20.0 ml) was added to
the mixture and stirred for 8 hr at room temperature
under N2 atmosphere. Finally, the resulting sample was
separated using an external magnetic field, washed
sequentially with dichloromethane, ethyl acetate and
methanol, and then dried under vacuum.

AGHAJANI AND MONADI
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was collected with a syringe and analyzed using GC
equipped with a capillary column.
2.2.4 | Preparation of Cu(II)‐5CML Schiff
base/Fe3O4@SiO2
0.5 g of prepared 5CML Schiff base/Fe3O4@SiO2 nano-
particles were dispersed in methanol (50.0 ml). Then, a
solution of CuCl2.3H2O (1.5 mmol, 0.26 g) in methanol
(10.0 ml) was added to the mixture and refluxed under
N2 atmosphere for 24 hr. Then, the prepared nanocatalyst
was separated using an external magnet and washed sev-
eral times with ethyl acetate, and ethanol. Subsequently,
to remove the unreacted materials, Soxhlet extraction
was carried out with ethanol, and the final product was
dried under vacuum.
3 | RESULTS AND DISCUSSION
3.1 | Preparation and characterization of
the nanocatalyst and its precursors
The Cu(II)‐5CML Schiff base/Fe3O4@SiO2 was prepared
as shown in Scheme 1. First, Fe3O4@SiO2‐amine was ini-
tially prepared using FeCl3.6H2O and FeCl2.4H2O salts
via the co‐precipitation method, subsequently encapsu-
lated with silica using tetraethyl orthosilicate (TEOS),
and functionalized with APTES. The resulting
Fe3O4@SiO2‐amineas support was then allowed to react
with as‐prepared Schiff base ligand through a condensa-
tion reaction between amine group of APTES and
chloromethyl group of the Schiff base ligand. Finally,
metallation was carried out with CuCl2.3H2O in metha-
nol to furnish Cu(II) Schiff base complex supported on
magnetic Fe3O4@SiO2‐amine.
2.3 | General procedure for oxidation of
alcohols
The oxidation process was carried out under identical
conditions in a two‐necked flask (10 ml) under an atmo-
spheric pressure of molecular oxygen (from a balloon).
Typically, a mixture of alcohol (1 mmol), TEMPO
(0.1 mmol), nanocatalyst (2 mol%, 14 mg based on
AAS) and H2O/CH2Cl2 (2:1 ratio) were added in a two‐
necked flask equipped with a tubular reflux condenser.
The flask was then filled with pure oxygen (balloon
filled) and heated to a target temperature by a tempera-
ture‐controlled oil bath under magnetic stirring. The
progress of the reaction was monitored by using TLC.
After the completion of the reaction, the nanocatalyst
was separated by magnetic decantation, and washed
with ethyl acetate, water and ethanol. The resulting
solution was extracted with ethyl acetate (2 × 5 ml),
and then dried with Na2SO4. Finally, the organic layer
The prepared nanocatalyst and its precursors were
characterized by FT‐IR, TGA, SEM, EDS, XRD, VSM
and N2 adsorption–desorption.
3.1.1 | FT‐IR studies
To confirm the formation of the nanocatalyst and sur-
face modification, the FT‐IR spectra of the nanostruc-
tures are investigated (Figure 1). The FT‐IR spectrum
of Fe3O4 shows a strong absorption band at 589 cm−1
corresponding to Fe − O stretching vibrations and a
SCHEME 1 Synthetic route for the
formation of Cu(II)‐5CML Schiff base/
Fe3O4@SiO2 nanocatalyst, (1) NH3(25%),
co‐precipitation method/nSonication/ N2
(2) TEOS, Stöber process/ N2 (3) APTES/
EtOH (reflux)/ N2

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AGHAJANI AND MONADI
FIGURE 1 FT‐IR spectra for Fe3O4 (a), Fe3O4@SiO2 (b), Fe3O4@SiO2‐amine (c), 5CML Schiff base/Fe3O4@SiO2 (d) and Cu(II)‐5CML
Schiff base/Fe3O4@SiO2 (e)
broad band around 3300–3400 cm−1 attributed to the O‐
3.1.2 | TGA analysis
H
on the surface (Figure 1a). The spectrum of
TGA curves of Fe3O4 and Cu(II)‐5CML Schiff base/
Fe3O4@SiO2 are exhibited in Figure 2. The TGA curve
of Fe3O4@SiO2 shows a small weight loss (~4%) below
150 °C related to the removal of physically adsorbed
water. As it can be seen, there is no significant weight loss
above 150 °C (Figure 2a). However, TGA curve of Cu(II)‐
5CML Schiff base/Fe3O4@SiO2 shows two weight losses
(Figure 2b). There is an initial weight loss of 6% below
180 °C, which is ascribed to the loss of trapped and phys-
ically adsorbed water. A main weight loss in the range of
Fe3O4@SiO2 (Figure 1b) shows Fe‐O stretching vibra-
tions at 593 cm−1 and also two new peaks at 810 and
1101 cm−1 corresponding to the symmetric and asym-
metric Si‐O‐Si vibrations, which is evidence for the
formation of silica layer on Fe3O4 nanoparticles. After
surface modification with APTES, the presence of
anchored aminopropyl groups are confirmed by appear-
ance of new bands at 2900–3000 cm−1 due to C‐H
stretching vibrations and also a strong peak at 1629 cm
−1
assigned to the –NH2 vibration mode (Figure 1c).
The FT‐IR spectrum of 5CML Schiff base/Fe3O4@SiO2
(Figure 1d) and Cu(II)‐5CML Schiff base/Fe3O4@SiO2
(Figure 1e) show several new peaks that are characteris-
tic of functionalized Schiff base complex, which clearly
differs from that of the functionalized Fe3O4 by APTES
groups. The spectrum of 5CML Schiff base/Fe3O4@SiO2
(Figure 1d) shows new absorption peaks at 1480–
1530 cm−1 corresponding to the stretching vibrations of
aromatic rings and appearance a new absorption at
1641 cm−1 relating to the C=N stretching vibrations.
While this C=N band shifted to a lower frequency in
the spectrum of Cu(II)‐5CML Schiff base/Fe3O4@SiO2
and appears at 1632 cm−1, could be due to the coordina-
tion of azomethine nitrogen with the Cu(II) center.
Comparison of all the FT‐IR data confirms that Schiff
base complex is successfully immobilized onto the sur-
face of functionalized magnetic support.
FIGURE 2 TGA diagram for Fe3O4 (red curve) and Cu(II)‐5CML
Schiff base/Fe3O4@SiO2 (green curve)

AGHAJANI AND MONADI
5 of 14
280–410 °C (~ 36%) may be associated to thermal destruc-
tion of Schiff base complex. Based on the weight losses,
the amount of loaded Schiff base complex is estimated
to be 0.91 mmol g−1. Also the amount of loaded Cu on
magnetic support was found to be 1.18 mmol g−1 based
on AAS. This shows that there is a significant amount
of loaded metal on the magnetic support.
3.1.3 | VSM studies
The magnetic properties of the nanostructures were eval-
uated using VSM technique at room temperature and
their magnetization curves are depicted in Figure 3. As
it can be seen, the plots of magnetization versus magnetic
field show the absence of hysteresis phenomenon and
indicate that nanocatalyst has superparamagnetic prop-
erty at room temperature. The magnetic saturation values
for Fe3O4, Fe3O4@SiO2 and Cu(II)‐5CML Schiff base/
Fe3O4@SiO2 nanocatalyst are 63.6, 44.2 and 32.8 emu g−1
,
respectively. Magnetization of Fe3O4 was decreased con-
siderably with the increase of silica coated and copper
Schiff complex, which provides clear evidence of the
existence of some organic compound on the surface of
the magnetic core.[42] In addition, the nanocatalyst can
still be separated from the solution by using an external
FIGURE 3 Magnetization curves for Fe3O4, Fe3O4@SiO2, Cu(II)‐
5CML Schiff base/Fe3O4@SiO2 and recycled nanocatalyst after 8th
run at room temperature. The inset is a photograph of nanocatalyst
under an external magnetic field
FIGURE 4 SEM images for Fe3O4 (a), Fe3O4@SiO2 (b), Cu(II)‐
5CML Schiff base/Fe3O4@SiO2(c) and recycled nanocatalyst after
8th run (d)

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AGHAJANI AND MONADI
magnetic field on the sidewall of the reactor and can be
redispersed after the removal of the external magnet
(inset of Figure 3).
of Cu(II)‐5CML Schiff base/Fe3O4@SiO2 is 80 nm.
These results confirmed that coating process and
grafting of metal complex do not have a significant
effect on the spherical morphology. EDS spectrum of
Cu(II)‐5CML Schiff base/Fe3O4@SiO2 confirms the
presence of C, N, O, Si, Fe and Cu on the nanocatalyst
surface (Figure 5).
3.1.4 | SEM and EDS studies
The morphology of the nanostructures were evaluated
using SEM. According to the SEM images, the synthe-
sized Fe3O4 and Fe3O4@SiO2 were essentially fine with
spherical morphology and the nanoparticles are roughly
in the range of 35–45 and 50–60 nm, respectively
(Figure 4a‐b). After the anchoring of the Schiff base
complex onto magnetic support, SEM images of the
synthesized nanocatalyst (Figure 4c) indicates uniformly
spherical morphology and an increase the nanoparticles
size can be seen at this stage. The average particles size
3.1.5 | XRD studies
Figure 6 Shows the XRD patterns of the Fe3O4,
Fe3O4@SiO2 and the Cu(II)‐5CML Schiff base/
Fe3O4@SiO2 nanocatalyst. As it can be seen in Figure 6
a, the diffraction peaks (220), (311), (400), (422), (511)
and (440) are attributed to the standard Fe3O4 sample
(JCPDS file no. 19–0629) with an inverse cubic spinal
structure without any impurity. Also, the broad peak at
2θ from 20° to 24° indicates the existence of amorphous
silica layer (Figure 6b) [43]. A comparison between the
XRD patterns of Fe3O4 versus the nanocatalyst shows
that the pattern of immobilized Schiff base complex
includes six peaks related to Fe3O4 nanoparticles with
reduced intensity and a broadening of the Bragg peaks
(Figure 6c). This confirms that grafting of Schiff base
complex on the magnetic support does not change the
crystallinity and morphology of the nanocatalyst. How-
ever, the change in the peak intensities can be related
to the shielding effect on the surface of Fe3O4
nanoparticles.
FIGURE
5
EDS spectrum of Cu(II)‐5CML Schiff base/
Fe3O4@SiO2
FIGURE 7 N2 adsorption–desorption isotherm for Cu(II)‐5CML
Schiff base/Fe3O4@SiO2 and the inset is the corresponding pore
size distribution
FIGURE 6 XRD patterns of Fe3O4 (a), Fe3O4@SiO2 (b) and
Cu(II)‐5CML Schiff base/Fe3O4@SiO2 (c)

AGHAJANI AND MONADI
7 of 14
the Supporting Information). The calculated BET surface
3.1.6 | N2 adsorption–desorption studies
areas for Fe3O4, Fe3O4@SiO2 and Cu(II)‐5CML Schiff
The N2 adsorption–desorption isotherms and the corre-
sponding pore size distribution curves of nanostructures
at 77 k were evaluated. This analysis for all nanostruc-
tures (Fe3O4, Fe3O4@SiO2 and Cu(II)‐5CML Schiff base/
Fe3O4@SiO2) showed a typical type‐IV isotherm pattern
with H3 hysteresis loops (Figure 7 and Figures S1‐S4 in
base/Fe3O4@SiO2 are 80.24, 215.30 and 44.31 m2 g−1
,
respectively. The large surface area of Fe3O4@SiO2 indi-
cated small and uniform particle size of the nanoparticles.
On the other hand, the drastic decrease in surface area for
Cu(II)‐5CML Schiff base/Fe3O4@SiO2 suggested the suc-
cessful attachment of Schiff base complex to the function-
alized magnetic support by covalent grafting.
3.1.7 | Effect of reflux time on loading of
copper content
To study the effect of reflux time on the copper content
immobilized on magnetic support, the reaction mixture
was refluxed for different periods of time in MeOH
(Figure 8). As it can be seen, by increasing the reflux
time to 24 h, the amount of loaded copper on the sup-
port has been increased from 0.17 to 1.18 mmol g−1
.
Further increase in the reflux time is not effective on
the metal loading. The best metal loading was
1.18 mmol g−1after 24 h.
FIGURE 8 The effect of reflux time on loading of copper content
over support
3.2 | Catalytic performance
The oxidation of alcohols to carbonyl compounds is a
fundamental class of organic transformation that has
great potential for biological systems as well as industrial
processes. Hence, many efforts have been devoted to
selective oxidation of alcohols with eco‐friendly oxidants,
such as hydrogen peroxide (H2O2), molecular oxygen (O2)
and air.[48–52] Hence, the catalytic performance of Cu(II)‐
5CML Schiff base/Fe3O4@SiO2 for the oxidation of
various alcohols under O2 atmosphere as oxidant was
SCHEME 2 General procedure for the aerobic oxidation of
alcohols
TABLE 1 Experiments to Study of different screening conditions for oxidation of benzyl alcohol as model substratea
b
c
Entry
Catalyst (14 mg)
Time
Conversion%
Selectivity% to (A)
1
2
3
4
5
6
7
8
No catalyst
24 h
2 h
41
91
Nanocatalyst (2 mol%)
Fe3O4
100
trace
trace
trace
51
>99
18 h
18 h
18 h
12 h
6 h
‐
Fe3O4@SiO2
‐
‐
Fe3O4@SiO2‐amine
Cu@Fe3O4@SiO2
87
>99
‐
d
Nanocatalyst (2 mol%)
32
e
No catalyst
24 h
trace
aReaction conditions: Alcohol (1 mmol), TEMPO (0.1 mmol), O2 (balloon), H2O/CH2Cl2 (2:1 ratio, 1.5 ml), Temperature (45 °C).
bDetermined by GC.
cDetermined by GC.
dUnder Argon balloon.
eIn the absence of TEMPO.

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AGHAJANI AND MONADI
studied. In order to evaluate optimum reaction condition,
benzyl alcohol was chosen as a model substrate for the
oxidation process (Scheme 2).
(a)
The model reaction was carried out in the absence of
catalyst at 45 °C for 24 h, but the reaction did not com-
plete whereas in the presence of the nanocatalys
(Table 1, entry 1), the oxidation product, benzaldehyde
was formed in 100% conversion after only 2 hr, indicating
the rule of the nanocatalyst for this reaction (Table 1,
entry 2). Hence, the synthesized nanocatalyst is crucial
for this model reaction. Furthermore, the influence of var-
ious factors such as reaction time, various solvents,
amount of the nanocatalyst and temperature in the oxida-
tion of benzyl alcohol were investigated and the results are
depicted in Figure 9. The impact of various solvents such
as methanol, ethanol, acetonitrile, water, toluene, dichlo-
romethane, ethyl acetate and also solvent‐free condition
were examined; H2O/CH2Cl2 (2:1) was the best solvent
among the tested solvents within 2 hr (Figure 9a). As seen
in Figure 9b, increasing the amounts of the nanocatalyst
from 1 mol% to 2 mol% led to higher conversion. The max-
imum conversion of benzaldehyde was observed with
2 mol% (14 mg) of the nanocatalyst and benzaldehyde
was the only product (Figure 9b). In order to investigate
the impact of temperature on the efficiency of the reaction,
the oxidation of benzyl alcohol was also carried out in the
range of 25–80 °C by employing 14 mg of the nanocatalyst.
The results show that conversion and selectivity are
substantially influenced by temperature. As expected, the
conversion improved when the reaction temperature was
raised; however, at low temperatures (between 25 to
35 °C) no significant conversion of benzaldehyde was
observed. Raising the temperature to 45 °C resulted a con-
sistent enhancement in conversion in short time. At the
above 45 °C, lower selectivity was observed. Therefore,
45 °C was chosen as optimized temperature (Figure 9c).
It was also found that the addition of TEMPO is essential
for Cu Schiff base complex activation. Therefore, different
amounts of TEMPO were examined and the best result
was achieved by addition only 0.1 mmol of TEMPO.
Further increase of TEMPO decreases the conversion,
which is probably due to block the active site (Figure 9d).
Furthermore, the model reaction was examined in the
presence of different sample to realize the effect of the
nanocatalyst in more detail and the results are shown in
Table 1. Among Fe3O4, Fe3O4@SiO2, Fe3O4@SiO2‐amine,
Fe3O4@SiO2@Cu nanostructures and Cu(II)‐5CML Schiff
base/Fe3O4@SiO2 nanocatalyst examined, only Fe3O4@-
SiO2/5CMLSchiff base‐Cu nanocatalyst indicated an effi-
cient catalytic activity in the oxidation reaction (Table 1,
entries 3–6 and 2). This certainly shows the rule of the
nanocatalyst in the oxidation reaction. When the model
reaction was carried out under optimal condition under
(b)
(c)
(d)
FIGURE 9 The effect of various solvents (a), different amounts
of the nanocatalyst (b), temperature (c) and TEMPO (d) in the
oxidation of benzyl alcohol as a model reaction

AGHAJANI AND MONADI
9 of 14
Argon atmosphere, only 32% conversion was obtained
after 6 hr (Table 1, entry 7), indicating the necessity of
O2 as oxidant for oxidation reaction. On the other hand,
a series of blank experiments (Table 1, entries 1 and 8)
were carried out. The model reaction in the absence of
the nanocatalyst and in the presence of TEMPO give only
41% conversion of oxidation product after 24 hr. The
results indicate that the presence of the nanocatalyst and
TEMPO as co‐catalyst are necessary for an effective cata-
lytic performance.
a
TABLE 2 Aerobic oxidation of various alcohols using Cu(II)‐5CML Schiff base/Fe3O4@SiO2
c
e
Entry
Alcohol
Time
Conversionb% (Yield%)
Selectivity% (A)
TON d/TOF (h−1
)
1
2 h
100 (92)
>99
680/340
2
3
2:30 h
2:30 h
100
>99
>99
680/272
639/256
94 (85)
4
4 h
100
>99
780/170
5
6
7
8
4 h
89
100 (89)
100 (88)
100
>99
97
605/151
680/389
680/340
680/340
1:45 h
2 h
>99
95
2 h
9
3:30 h
5:30 h
2:30 h
100 (91)
100
>99
>99
>99
680/194
680/124
680/272
10
11
100 (92)
12
13
8 h
8 h
79 (71)
87 (77)
>99
>99
537/67
592/74
14
15
7 h
7 h
83 (72)
77
>99
>99
564/81
524/75
aReaction conditions: Substrate (1 mmol), Nanocatalyst (2 mol%, 17 mg), TEMPO (0.1 mmol), O2 (balloon), H2O/CH2Cl2 (2:1 ratio, 1.5 ml), Temperature
(45 °C).
bDetermined by GC (The yield in parenthesis refer to the isolated product).
cDetermined by GC.
dTON = moles of product formed/ moles of metal in the catalyst (based on AAS).
eTOF = TON/time(h).

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AGHAJANI AND MONADI
After achieving optimal conditions, 2 mol% (14 mg)
and 0.1 mmol of nanocatalyst and TEMPO, respectively
under O2 atmosphere in H2O/CH2Cl2 (2:1 ratio) as the
reaction media at 45 °C, the scope and limitation of this
protocol for other substrates were examined, and the
results are summerized in Table 2. As illustrated in
Table 2, various types of primary and secondary benzylic
alcohols, including those with both electron‐withdrawing
and ‐donating groups, were selectively converted into the
corresponding benzaldehydes and ketones in excellent
conversions under the optimal reaction conditions. As
expected, for benzyl alcohols with electron‐donating
groups such as p‐methoxy, p‐methyl, and p‐isopropyl
the reaction proceed in short reaction time (Table 2,
entries 6–8), while benzyl alcohols with electron‐with-
drawing groups including p‐Cl, o‐Cl, m‐Cl, p‐NO2, o‐
NO2 and 2,4‐dichloro substituents require longer reaction
time to rich satisfactory conversion (Table 2, entries 2–5,
9 and 10). However, the electronic effects exhibit a minor
impact on the reaction conversion and this oxidation pro-
tocol tolerates various electron‐rich and electron‐deficient
benzylic substrates. It is noteworthy over oxidation to car-
boxylic acid has not been observed by GC and IR analysis
of the reaction mixture and corresponding benzaldehydes
were the sole products which indicate a selective oxida-
tion protocol. The aerobic oxidation of aliphatic second-
ary alcohols such as: cyclopentanol, cyclohexanol,
heptan‐2‐ol and octan‐2‐ol have also been investigated
under optimal conditions (Table 2, entries 12–15). The
results show that these alcohols could effectively be oxi-
dized to corresponding carbonyl products in good conver-
sion with high selectivity.
FIGURE 10 Reuse of the nanocatalyst in the aerobic oxidation of
benzyl alcohol
synthesized nanocatalyst has high stability and rules out
the possibility of the oxidation by leached Cu complex.
The structure of the reused nanocatalyst after eight
consecutive trials was evaluated by FT‐IR, FE‐SEM, VSM
and N2 adsorption–desorption. The FT‐IR spectrum of
the nanocatalyst after 8th run exhibited Fe‐O, Si‐O, C=N,
C–H aliphatic and aromatic vibration peaks, which is sim-
ilar to that of the fresh catalyst (Figure S5 in the supporting
information). FE‐SEM images of the reused nanocatalyst
show that the morphology of the nanocatalyst has been
preserved during recycling reactions (Figure 4d). Further-
more, VSM curve of the nanocatalyst after eight cycles
shows very slight decrease in the magnetization value
(Ms = 29.9 emu g−1) with superparamagnetic behavior
and so there is no considerable change in its magnetic
property (Figure 3). On the basis of these results, it can
be concluded that the synthesized nanocatalyst acts as an
efficient recyclable magnetic nanocatalyst in the selective
3.3 | Reusability
The reusability and recovery of the catalysts are very
important issues from both the practical and environ-
mental points of view. Since one of the main targets of
this study was to develop a highly simple and efficient
procedure, the recyclability of the nanocatalyst in the
model reaction was investigated by isolating the
nanocatalyst from the reaction media by external magnet.
Then, the nanocatalyst was washed with ethyl acetate/
water and ethanol, dried in vacuum and reused directly
in the next run. Pleasingly, it was found that the perfor-
mance of the nanocatalyst was almost maintained after
at least 8 consecutive reaction runs with a negligible
decrease in catalytic activity (Figure 10). After the
nanocatalyst was separated, the filtrate was analyzed by
ICP‐AES for leaching of Cu. The amounts of copper
leaching after the 1st and 8th runs of used nanocatalyst
were 0.81% and 2.66%, respectively (Table S1 in the
supporting information), which indicates that the
FIGURE 11 Sheldon's test for study of heterogeneous or
homogeneous reaction pathway

AGHAJANI AND MONADI
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oxidation of alcohols with its high stability. Additionally,
the N2 adsorption–desorption isotherm for the reused
nanocatalyst after eight runs was the same as the fresh
one and exhibited a type‐IV isotherm pattern with H3 hys-
teresis loops (Figure S6 in the supporting information).
The BET surface area for reused nanocatalyst was
46.12 m2 g−1. The increase in BET surface area of the
reused nanocatalyst suggests more fine dispersion of the
immobilized Cu(II) complex on the support surface.
presented in Figure 11. After 1 hr (48% conversion) the
nanocatalyst was separated by an external magnet and
reaction was continued for 12 hr. The progress of the
reaction was monitored by measuring the reaction con-
version by GC. The reaction conversion was increased
only 6%, indicating negligible leaching of the catalytic
active species to the solution and confirms the heteroge-
neous character of the nanocatalyst, in good agreement
with the ICP analysis.
3.4 | Sheldon's test
3.5 | Proposed mechanism
In order to ensure whether the nanocatalyst truly pro-
ceeds from heterogeneous pathway or not, a hot filtration
test for the oxidation of benzyl alcohol under optimal
reaction conditions was performed and the results are
The mechanism of the TEMPO‐catalyzed oxidation of
alcohols with the supported Cu(II) Schiff base complex
has not been investigated in study. However, it is assumed
that reaction could proceed according to the mechanisms
SCHEME 3 A plausible reaction mechanism for aerobic oxidation of alcohols

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AGHAJANI AND MONADI
proposed by Sheldon[11,53] or by Semmelhack.[54] In the
proposed mechanism by Sheldon, the key step is the
re‐oxidation of Cu(I) to Cu(II) by TEMPO radical and
copper‐centered dehydrogenation of the alcohol. The
resulting Cu(I) is rapidly re‐oxidized by TEMPO radical
to afford Cu(II) and hydroxylamine. The role of oxygen is
to re‐oxidize the hydroxylamine to TEMPO radical.
(Scheme 3, Mechanism 1). In the proposed pathway by
Semmelhack, Cu(II) complex can oxidize TEMPO radical
to the nitrosonium ion and, reduce to Cu(I). Then, Cu(I)
complex is oxidized again to the initial Cu(II) complex by
O2. The alcohol is then oxidized by nitrosonium ion,
generating the aldehyde and hydroxylamine. In addition,
rapid synproportionation of nitrosonium ion with hydrox-
ylamine regenerates TEMPO radical (Scheme 3, Mecha-
nism 2).Since no basic compound has been used in the
reaction media presented in this study, mechanism 2 is
probably closer to our catalytic system.
3.6 | Comparison
To evaluate the efficiency of our protocol with respect to
other reported systems, we compared the oxidation of
benzyl alcohol in Table 3.[55–66] As it can be seen from
Table 3, it is obvious that the described procedure in this
research has attractive benefits, including high conver-
sion and selectivity, short reaction time, easy separation
of the nanocatalyst from reaction media with high TOF
under mild condition. In addition, the number of the
recyclability for present work is more than the previously
reported catalysts, which indicates that the synthesized
nanocatalyst has good stability and recoverability.
4 | CONCLUSION
In this work,
a new Cu(II)Schiff base complex
immobilized on the surface of magnetic support was pre-
pared and fully characterized by various spectroscopic
and microscopic techniques. We have employed this
as‐prepared nanomaterial as a magnetically recyclable
nanocatalyst for a mild, environmentally friendly and
selective aerobic oxidation of primary and secondary
alcohols to corresponding aldehydes or ketones in the
presence of TEMPO as a redox mediator at 45 °C.
Quantitative analysis of the products was conducted
using GC and characterized by FT‐IR and NMR spectros-
copy. The present work offers several advantages such as
short reaction time, environmentally benign mild reac-
tion conditions, good to excellent conversion and
selectivity for variety of alcohols and good stability of
the nanocatalyst. Leaching and Sheldon's test confirmed
a strong interaction between Schiff base complex and

AGHAJANI AND MONADI
13 of 14
[18] Y. Kuang, N. M. Islam, Y. Nabae, T. Hayakawa, M. A.
magnetic support. Most importantly, we were able to
separate nanocatalyst easily from the reaction media by
applying an external magnet and reusing up to 8 runs
without significant loss of its catalytic activity. The
catalytic activity of this nanocatalyst in other organic
transformation reactions is under investigation in our
laboratory.
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We gratefully acknowledge the University of Mazandaran
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How to cite this article: Aghajani M, Monadi N.
Cu(II) Schiff base complex supported on Fe3O4
nanoparticles as an efficient nanocatalyst for the
selective aerobic oxidation of alcohols. Appl
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