Novel Schiff base Mn(III) and Co(II) complexes supported on Co nanoparticles: Efficient and recyclable magnetic nanocatalysts for alcohol oxidation

Article abstract of DOI:10.1039/c6ra15967b

In this study, efficient and highly selective heterogeneous catalysts were developed by immobilization of manganese and cobalt Schiff base complexes on Co magnetite nanoparticles (MNPs). Firstly, Co@SiO₂ core-shell nanoparticles were synthesized through a one-pot reaction. Secondly, the Co@SiO₂ was amino-functionalized using 3-aminopropyl triethoxysilane and then Schiff base ligand Co@SiO₂[(EtO)₃Si-L²] was obtained by the reaction of the amino-functionalized nanoparticles with the dialdehyde of 1,2-bis(2-formyl naphthoxy methyl)benzene. Finally, Co@SiO₂[(EtO)₃Si-L²]/M (M = Mn(iii) and Co(ii)) were successfully synthesized. These surface-modified nanoparticles were characterized using various techniques such as TEM, XRD, TGA, SEM, VSM, XPS, EDX and FT-IR. The catalytic activities of the prepared catalysts were investigated by employing them in the oxidation of various aromatic alcohols with an environmentally friendly oxidant under mild conditions. The catalysts can be readily recovered and reused in at least 5 consecutive cycles without significant leaching and loss their catalytic activity.

Full text of DOI:10.1039/c6ra15967b

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A novel Schiff base Mn(III) and Co(II) complexes supported on Co
nanoparticles: An efficient and recyclable magnetic nanocatalyst for
Alcohol oxidation
Hassan Keypour,a,*Shokoufeh Ghahri Saremi,a Hojat Veisi,b Reza Azadbakhtb
Abstract. In this study, an efficient and highly selective heterogeneous catalyst was
developed by immobilization of a manganese and cobalt Schiff base complexes on Co
magnetite nanoparticle (MNP). Firstly, Co@SiO2 coreꢀshell nanoparticles were synthesized
through a oneꢀpot reaction. Secondly, the Co@SiO2 was aminoꢀfunctionalized using 3ꢀ
aminopropyl triethoxysilane and then Schiff base ligand Co@SiO2[(EtO)3Si–L2] was
obtained by the reaction of this aminoꢀfunctionalized nanoparticles with dialdehyde of 1,2ꢀ
bis(2ꢀformyl naphthoxy methyl)benzene. Finally, Co@SiO2[(EtO)3Si–L2]/M (M= Mn(III)
and Co(II)) were successfully synthesized. This surfaceꢀmodified nanoparticle was
characterized by various techniques such as TEM, XRD, TGA, SEM, VSM, XPS, EDX and
FTꢀIR. The catalytic activities of the prepared catalysts were investigated by employing in
oxidation of various aromatic alcohols with environmentally friendly oxidant under mild
conditions. The catalyst can be readily recovered and reused at least 5 consecutive cycles
without significant leaching and loss its catalytic activity.
Keywords: Schiff base complex, Magnetic cobalt Nanoparticles, Alcohol oxidation
Introduction
One of the most stimulating features of nanotechnology is its potential use for in almost any
field. The “nano” in nanocatalysis refers to very small particles in the nano scale range of
size. Because nanoparticles have a large surfaceꢀtoꢀvolume ratio relative to bulk materials,
they offer an attractive alternative to conventional catalysts.[1ꢀ3] Thus, nanocatalysts benefit
from several advantages over conventional catalyst systems; however, isolation and recovery
of these tiny nanocatalysts from the reaction mixture is not easy. This limitation hampers the
economics and sustainability of these nanocatalytic protocols. To overcome this issue, the use
of magnetic nanoparticles has emerged as a viable solution; their insoluble and paramagnetic
nature enables easy and efficient separation of the catalysts from the reaction mixture with an
external magnet.[4ꢀ11]
Oxidation reactions are vitally important to the chemical industry because of their potential
for generating a variety of fine chemicals and drugs. Conventionally, oxidation has been
achieved noncatalytically using stoichiometric quantities of oxidants (such as chromium and
manganese oxides) in the presence of corrosive mineral acids, which in turn produces huge
amounts of toxic waste. Because of growing environmental concerns, continuous efforts have
been made to develop oxidation protocols using green chemistry principles with a focus on
the development of easily recoverable and reusable heterogeneous catalysts. In light of these
concerns, the use of magnetic nanomaterials as a solid support for heterogeneously catalyzed
oxidation reactions has proven to be highly effective[12ꢀ16]
.
*Correspondence to: Hassan keypour, Faculty of Chemistry, BuꢀAli Sina University, Hamedan 65174, Iran. Eꢀ
mail: haskey1@yahoo.com
aFaculty of Chemistry, BuꢀAli Sina University, Hamedan 65174, Iran
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Schiff base transition metal complexes have been extensively studied because of their
potential use as catalysts in a wide range of reactions[17, 18]. These complexes have been
extensively used for hydrogenation of organic substrates[19], epoxidation of olefins[20]
,
conversion of epoxides into halohydrines[21,22], asymmetric ring opening of terminal
epoxides[23] and oxidation reactions[24,25]
.
In recent years, various supports have been reported for the immobilization of Mn(III)
complexes. These supports are include activated carbon [26,27] and silica [28–32]. Therefore, here
we report the further chemically modification of the surface tethered amino groups with
Schiffꢀbase ligands through reaction between amine and aldehyde groups of 1, 2ꢀbis(2ꢀformyl
naphthoxy methyl)benzene to form corresponding imines. Also, we studied the catalytic
behavior of Co(II) and Mn(III) complexes of the Schiff base ligand as heterogeneous
catalysts in the oxidation of various aromatic alcohols with environmentally friendly oxidant.
Experimental
Materials and equipment's
The reagents and solvents used in this work were obtained from Fluka or Merck and used
without further purification. Nanostructures were characterized using a Holland Philips PWꢀ
1840 with monochromated Cu Kα radiation and λ 1.54 A◦ Xꢀray powder diffraction (XRD)
diffractometer, at a scanning speed of 2°/min from 5° to 80°(2θ). Magnetic measurement of
materials was investigated with a vibrating sample magnetometer (VSM) at room
temperature. The particle size and morphology surface were investigated by scanning
electron microscopy (SEM) of a Holland Philips XL30 microscope with an accelerating
voltage of 25 kV. The FTꢀIR measurements were performed using KBr disc on a Shimadzu
Fourier Transform Infrared spectra (FTꢀIR) 8400. Thermogravimetric Analysis (TGA) was
º
taken on a Mettler Toledo TGA SDTA 85ꢀe instrument at 30ꢀ800ºC with 10 C/Min.
Transmission electron microscopy (TEM) images were obtained on an EM10C (Zeiss)
transmission electron microscope at an accelerating voltage of 80 kV. XPS (small area Xꢀray
photoelectron spectroscopy) data were recorded with the PHIꢀ5702 MultiꢀTechnique System
Preparation of the magnetic Co@SiO2 nanoparticles
Stepꢀbyꢀstep synthesis of the nanocatalyst(6) is shown in Scheme 1. In order to preparation of
the magnetic Co@SiO2 nanoparticles(2) firstly, cobalt nanoparticles (1) were synthesized
according to the procedure previously reported in literature.[33] In a ordinary method, 0.07 g
citric acid was added to 60 ml aqueous solution contain 1 g cobalt chloride hexahydrate under
N2 atmosphere and this solution is stirred for 2 min. Then in a beaker, 3 g sodium borhydride
was dissolved in 100 ml water and drop wise added to the stock solution under nitrogen
atmosphere. With the first drop of NaBH4, the color of the solution was quickly changed to
black.
Secondly, cobalt magnetic core was protected and coated with SiO2 shell to obtain core–shell
nanoparticles Co@SiO2. The solution containing Co nanoparticles was left be stirred for 30
minutes. Then 200 ml ethanolic solution of 200 ml of 3ꢀaminopropylꢀtriethoxysaline
(APTES) and 800 ml tetraethoxysaline (TEOS) was added to the stock solution under
nitrogen atmosphere. The nitrogen atmosphere was cut off and the solution was left be
strongly stirred for 24 h. Finally, a puffy black precipitate with strong magnetic property was
obtained. This precipitate was collected using a magnet and washed for several times with
deionized water and ethanol. Then the precipitate Co@SiO2 was dried under vacuum at room
temperature.
2

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Preparation of Co@SiO2[(EtO)3Si–L2](5)
3 g of magnetic nanoparticles Co@SiO2 was distributed and sonicated in 200 ml toluene for
50 min. 4.2 ml (17.93 mmol) of APTES was added dropwise to the suspended solid under
vigorous mechanical stirring. The mixture was refluxed at 120 °C for 24 h. Eventually, The
the reaction mixture was cooled down to room temperature. The resulting amine
functionalized MNPs were magnetically separated by external magnet and washed for several
times with deionized water and ethanol. Then the precipitate was left to be dried at 55 ºC in
the oven
In this step, in order to Schiff base reaction with Co@SiO2ꢀNH2(3) , dialdehyde1,2ꢀbis(2ꢀ
formyl naphthoxy methyl) benzene (4) was synthesized.
300 mL acetonitrile solution includes 2.62 g (10 mmol) 1,2ꢀBis(bromomethyl)benzene, 3.44
g (20 mmol) 2ꢀhydroxyꢀ1ꢀnaphthaldehyde and 2.79 g(20 mmol) potassium carbonate were
refluxed for 48 h. Then the solution was allowed to cool and the product formed as
colourless crystals. (3.3 g, yield: 72%). Anal. Calcd. for C30H22O4: C, 80.70; H, 4.97. Found:
C, 80.84; H, 5.03. FTꢀIR (KBr), cm−1: 1682 (C=O). 1H NMR (CDCl3, 500 MHz) δ (ppm):
5.34 (s, 4H, OꢀCH2ꢀAr), 7.21ꢀ9.1 (m, 12H, Ar), 10.95 (s, 2H, CH=O).
Typically procedure for the preparation of the Co@SiO2[(EtO)3Si–L2](5), explained as
follow:
1 g of Co@SiO2ꢀNH2 was distributed and sonicated in 80 mL of dry methanol for 20 min,
3.12 g (7 mmol) of 1,2ꢀbis(2ꢀformyl naphthoxy methyl) benzene were added and the mixture
was stirred and heated at 50 °C for 24 h. After cooling the reaction mixture to room
temperature, the magnetic Co@SiO2[(EtO)3Si–L2] nanoparticles were collected using a
magnet and washed for several times with dry methanol and dried under a vacuum for 20 h.
Preparation of Co@SiO2[(EtO)3Si–L2]/M (M= Co(II) and Mn(III))(6)
The Co@SiO2[(EtO)3Si–L2] (300 mg) were dispersed in ethanol (30 mL) by ultrasonic
bath for 30 min. Subsequently, a solution of a ethanolic solution (10 ml) of Mn(acac)3 (1
mmol) was added to dispersion of Co@SiO2[(EtO)3Si–L2] and the mixture was stirred and
refluxed for 12 hours. Then, the Co@SiO2[(EtO)3Si–L2]/Mn(III) was separated by magnetic
decantation and washed by ethanol, water and acetone respectively to remove the unattached
substrates. All the above steps are done for preparation of Co@SiO2[(EtO)3Si–L2]/Co with
Co(CH3COO)2. 4H2O instead of Mn (acac)3
.
The Mn content in Co@SiO2[(EtO)3Si–L2]/Mn(III) catalyst was determined 3.91 wt.%
(amount of Mn = 0.7 mmol/g) by atomic absorption spectroscopic (AAS) analysis. For the
Co@SiO2[(EtO)3Si–L2]/Co(II) catalyst, percent cobalt was determined 11.35 wt.%. This
amount of cobalt is related to the cobalt magnetic core and the cobalt ions in hybride. The
prepared catalysts were characterized by FTIR, XRD, SEM, EDX, TEM, XPS and VSM.
General procedure for alcohol oxidation
In a typical reaction, 0.02 g of the catalyst Co@SiO2[(EtO)3Si–L2]/M was placed in a 25
ml reaction flask. 1 mmol of the benzyl alcohol in 3 ml of ethyl acetate and then 30 µL H2O2
(30%) was added in flask. The mixture was stirred for 50 min at 50 ºC. The reaction was
monitored by thin layer chromatography (TLC).
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SiO2
SiO2
O
O
O
CoCl2. 6H2O
+
Acid Citric
NaBH4
APTES
TEOS
NH2
Co
Si
Co
Co
Co@SiO2(2)
Co(1)
Co@SiO2
NH2(3)
+
O
O
O
O
N
Si
O
O
O
SiO2
O
O
Co
O
O
Si
O
N
1,2-bis(2-formyl naphthoxy methyl) benzene(4)
SiO2
Co
C
o
L2
(
C
H
3
O
O
Co@SiO2[(EtO)3Si-L2](5)
O
3 C
O
O
O
O
O
)
Si
2
Si
N
O
N
O
M
Co@SiO2[(EtO)3Si-L2]/M( Mn(III), Co(II))(6)
Scheme 1. Preparation of Co@SiO2[(EtO)3Si–L2]/M (M= Mn(III) and Co(II)) nanocatalysts
Results and discussion
Characterization of the Catalysts
Figure1 shows the FTꢀIR absorption spectrum of Co@SiO2ꢀNH2 (a), Co@SiO2[(EtO)3Si–
L2](b) and Co@SiO2[(EtO)3Si–L2]/Mn (III)(c). The FTꢀIR spectrum for the pure Co@SiO2ꢀ
NH2 (curve a) shows a stretching vibration at 3475 and 1627cmꢀ1which incorporates the
contributions from both symmetrical and asymmetrical modes of the OꢀH bonds which are
1
attached to the surface cobalt atoms. The peaks at 2901 and 2872 cm− is attributed to the
symmetric and asymmetric vibration of methylene (ꢀCH2) of the propyl in the pendant
APTES which confirms that APTES molecules have bonded to the surface of the silicaꢀ
coated MNPs. The main peak at 1082 cmꢀ1 indicates the formation of silica layer on magnetic
cobalt NPs. The strong IR absorption band at 472 cmꢀ1 comes from vibrations of CoꢀO bonds.
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1
The characteristic vibrations of imine group at 1677 cm− in the FTꢀIR spectra (curve b) of the
grafted material confirmed the presence of the imine group on the surface of the magnetic
cobalt NPs. Shifting stretching vibration of the carbonyl group (1682 (C=O) in the ligand 4 to
1
1677 cm− represents a Schiff base reaction and the formation of the imine bond. Vibrations
1
in the range of 1480ꢀ1600 cm− are attributed to the aromatic ring.
The IR spectra of Co@SiO2[(EtO)3Si–L2]/Mn(III) complex (curve c) show important bands
1
from the Schiff base ligand. The free ligand exhibits a ꢀ(C=N) stretch at 1677 cm− while in
1
the complexes, this band shifts to lower frequency and appears at 1672 cm− due to the
coordination of the nitrogen with the Mn(III). This band in the complex Co@SiO2[(EtO)3Si–
1
L2]/Co(II) (curve d) appears at 1667 cm− .
Fig. 1. FTꢀIR spectra of (a) Co@SiO2ꢀNH2, (b) Co@SiO2[(EtO)3Si–L2], (c) Co@SiO2[(EtO)3Si–L2]/Mn(III) and
(d) Co@SiO2[(EtO)3Si–L2]/Co(II)
Figure 2 illustrates the TGA curves, depicting the variations of residual masses of the
samples with a heating rate of 10 °C /min between 30 and 800 °C temperature. The
magnetite, metal ions (cobalt, manganese and silicone) and organic materials of the samples
were completely converted into metals, metal oxides and burned to generate gas product at
elevated temperatures, respectively. The thermal analysis of Co@SiO2 (curve a) shows
initially a negligible weight loss (4.35%) in the region of 90ꢀ250 ◦C which confirms the loss
of hydrogen bonded water molecule present at the surface of nano cobalt. A second small
weight loss (5.52%) occurred in the temperature range 280–480◦C which is probably due to
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the removal of trapped water molecules from the lattice. Further weight loss of 5.16% is
observed in the range 500–750 ◦C, the decomposition event can be attributed to conversion
SiO2 to SiO. Curve b that related to Co@SiO2ꢀNH2, is similar to curve "a" shows that the
small weights loss (1.10%) in the ranges of 40ꢀ150°C and (3.55%) 150ꢀ280°C are probably
due to the evaporation of physically adsorbed water and trapped water molecules from the
lattice, respectively. A weight loss is detected in the range of 300ꢀ450°C and 500ꢀ750°C
which are predominantly attributed to the decomposition of organic substances in magnetic
Co nanoparticles. Curve c that related to Co@SiO2[(EtO)3Si–L2]/Mn(III), shows residue
66.24%. It is estimated that the weight loss of [(EtO)3Si–L2] coated on the Co nanoparticles is
about 43.76%. These results prove the attachment of [(EtO)3Si–L2H] moiety on to the surface
of Co nanoparticle.
Fig. 2. TGA spectra of (a) Co@SiO2, (b) Co@SiO2ꢀNH2 and (c) Co@SiO2[(EtO)3Si–L2]/Mn(III)
Figure 3 shows the Xꢀray diffraction patterns of Co, Co@SiO2[(EtO)3Si–L2],
Co@SiO2[(EtO)3Si–L2]/Mn(III) and Co@SiO2[(EtO)3Si–L2]/Co(II) in sections a, b, c and d,
respectively.
For magnetic cobalt NPs, diffraction peaks(curve a) with 2θ at 44.2°, 51.5° and 75.9° which
corresponds to (111), (200) and (220) Bragg reflection, respectively were observed,
indicative of a fcc metallic cobalt structure of the magnetite(card 15ꢀ0806 in the JCPDS file).
The same set of characteristic peaks were also observed for curves (b), (c) and (d) indicating
the stability of the crystalline phase of Co nanoparticles during [(EtO)3Si–L2] coating and
Mn(III) and Co(II) complexation. The XRD patterns of Co@SiO2[(EtO)3Si–L2],
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Co@SiO2[(EtO)3Si–L2]/Mn(III) and Co@SiO2[(EtO)3Si–L2]/Co(II) (curves b, c and d) shows
an obvious diffusion peak at 2θ = 15–25◦ that appeared because due to the existence of
amorphous silica. XRD pattern of Co@SiO2[(EtO)3Si–L2]/Mn(III) complex (curve c) is
shown three major peaks in the range of 20ºꢀ40º. XRD pattern of Co@SiO2[(EtO)3Si–
L2]/Co(II) (curve d) can be assigned to the diffraction from the three major peaks with 2θ° at
19, 30 and 40 reflection indexes of (100), (200) and (220) , respectively (card 25ꢀ0250 in the
JCPDS file).
Fig. 3. The Xꢀray diffraction patterns of (a) Co@SiO2, (b) Co@SiO2ꢀNH2, (c) Co@SiO2[(EtO)3Si–L2]/Mn(III)
and (d) Co@SiO2[(EtO)3Si–L2]/Co(II).
The TEM micrographs of Co nanoparticles(a), Co@SiO2[(EtO)3Si–L2]/Mn(III) (b) and
Co@SiO2[(EtO)3Si–L2]/Co(II)(c) are shown in figure 4. We can see that the
Co@SiO2[(EtO)3Si–L2]/Mn(III) and Co@SiO2[(EtO)3Si–L2]/Co(II) are nearly in coreꢀshell
structures, with black color shows Co nanoparticles and ashy color is the Schiff base complex
Mn(III) and complex Co(II) shell. This indicates the successful coating on the surface of the
magnetic particles. . In the transmission electron microscopy images, the diameter of the
cobalt nanoparticles was 20ꢀ25 nm.
Fig. 4. TEM image of (a)Co, (b)Co@SiO2[(EtO)3Si–L2]/Mn(III) and (c) Co@SiO2[(EtO)3Si–L2]/Co(II)
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The morphology of the Co nanoparticles, Co@SiO2[(EtO)3Si–L2]/Mn(III) and
Co@SiO2[(EtO)3Si–L2]/Co(II) are clearly seen from the SEM image shown in Fig. 5. curve a
shows the SEM images of the magnetite nanoparticles. Curves b and c show that the catalysts
were made up of uniform nanometerꢀsized particles. It can be seen that the particles are not
fully spherical. In addition, some particle aggregations are observed, which is likely to be
caused by the magneto static interactions between particles.
Fig.5. SEM image of (a)Co, (b)Co@SiO2[(EtO)3Si–L2]/Mn(III) and (c) Co@SiO2[(EtO)3Si–L2]/Co(II)
Presence of Co atoms in Co@SiO2[(EtO)3Si–L2]/Mn(III) and Co@SiO2[(EtO)3Si–L2]/Co(II)
were confirmed using energy dispersive Xꢀray spectroscopy (EDS), which was recorded at
random points on the surface for qualitative analysis (Fig. 6). The EDX spectrum of
Co@SiO2[(EtO)3Si–L2]/Mn(III) also showed the signals of cobalt, manganese and silicon
atoms, which also indicated their presence in the composite in curve b.
Fig. 6. Energyꢀdispersive Xꢀray spectroscopy (EDX) of (a) Co@SiO2[(EtO)3Si–L2]/Co(II) and
(b)Co@SiO2[(EtO)3Si–L2]/Mn(III)
The magnetic properties of Co nanoparticles, Co@SiO2[(EtO)3Si–L2]/Mn(III) and
Co@SiO2[(EtO)3Si–L2]/Co(II) were characterized by a vibrating sample magnetometer
(VSM) with an applied field of ꢀ8000 Oe< H <8000 Oe at 300 K. These nanoparticles show
high permeability in magnetization and their magnetization is sufficient for magnetic
separation with an external magnetic field. Fig. 7 shows the typical room temperature
magnetization curves of bare Co nanoparticles (a), Co@SiO2[(EtO)3Si–L2]/Mn(III) (b), and
Co@SiO2[(EtO)3Si–L2]/Co(II)(c). All the samples show a typical super paramagnetic
behavior. The saturation magnetization of Co NPs at room temperature by measuring the
magnetization curve (a) is 39.78 emu/g; however, the Ms of Co@SiO2[(EtO)3Si–L2]/Mn(III)
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(curve (b) and Co@SiO2[(EtO)3Si–L2]/Co(II) prepared in this study are 36.78 and 36.43
emu/g, respectively, which are lower than of the bare Co nanoparticles. The reduction of
measured saturation magnetization can be mainly attributed to a SiO2[(EtO)3Si–L2] coating
formation on the surface of Co NPs. Even with this reduction in the saturation magnetization,
the catalysts can still be separated from the reaction medium rapidly and easily with a
magnetic field.
Fig. 7. Room temperature magnetization curves of (a)Co nanoparticles, (b)Co@SiO2[(EtO)3Si–L2]/Mn(III) and
(c) Co@SiO2[(EtO)3Si–L2]/Co(II)
Catalytic activities of the synthesized catalysts
After structural characterization of the prepared Co@SiO2[(EtO)3Si–L2]/M( M=Mn(III),
Co(II)) nanocatalyst, its catalytic activity was investigated as an active and stable
magnetically separable nanocatalyst in the oxidation of alcohols (Scheme 2).
SiO2
Co
O
O
O
O
O
O
Si
Si
N
O
N
O
M
OH
O
M= Mn(III) , Co(II)
Solvent, H2O2, 55 C, 50 min
Scheme 2. Co@SiO2[(EtO)3Si–L2]/M (M=Mn(III), Co(II)) catalysts for oxidation of alcohol
We employed the oxidation reaction of benzyl alcohol to benzaldehyde as a model reaction.
At first test was run with catalysts at the following conditions reaction: benzyl alcohol (0.5
mmol), hydrogen peroxide 30% (40 µl, 0.293 mmol), catalyst 0.02 g, reaction time 45 min,
reaction temperature 40◦C, various solvents without optimized amounts and the results show
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that catalysts are highly active (Table 1). Importantly, these catalysts have different activities
in different solvents. Ethyl acetate was the best choice as solvent and this solvent is used in
other reactions. The yield of the reactions was determined by as follows: After the end of
reaction, the nanocatalyst was separated from the reaction mixture with a magnet. Then the
organic phase was separated by water/ ethyl acetate extraction method and the solvent was
dried. the mixture was stirred in warm nꢀhexane several times. Thus, the aldehyde was
dissolved in nꢀhexane, and isolated from remaining alcohol and then was weighted. On the
basis weight of the obtained aldehyde, the reaction yield can be calculated. In order to
optimize the reaction time and temperature, the reaction progress was monitored by thin layer
chromatography technique (TLC) in a regular alternative time and temperature.
Table 1. Catalytic results oxidation of benzyl alcohol to benzaldehyde with H2O2 by various catalysts and
different solventsa
Entry
Catalyst
Solvent
THF
MeCN
DMSO
DMF
Selectivity (%)
Isolated yield
1
2
3
4
5
6
7
8
9
Co@SiO2[(EtO)3Si–L2]/Mn(III)
Co@SiO2[(EtO)3Si–L2]/Mn(III)
Co@SiO2[(EtO)3Si–L2]/Mn(III)
Co@SiO2[(EtO)3Si–L2]/Mn(III)
Co@SiO2[(EtO)3Si–L2]/Mn(III)
100
100
100
100
100
100
100
100
100
100
100
100
66
71
54
56
51
74
53
64
48
50
45
67
CH2Cl2
Co@SiO2[(EtO)3Si–L2]/Mn(III) Ethyl acetate
Co@SiO2[(EtO)3Si–L2]/Co(II)
Co@SiO2[(EtO)3Si–L2]/Co(II)
Co@SiO2[(EtO)3Si–L2]/Co(II)
Co@SiO2[(EtO)3Si–L2]/Co(II)
Co@SiO2[(EtO)3Si–L2]/Co(II)
Co@SiO2[(EtO)3Si–L2]/Co(II)
THF
MeCN
DMSO
10
11
12
DMF
CH2Cl2
Ethyl acetate
aReaction condition: benzyl alcohol (0.5 mmol), hydrogen peroxide 30% (40 µl, 0.293 mmol), catalyst 0.03 g, reaction time: 45 min,
reaction temperature 40◦C.
In this case the reactions was examined by two catalysts Co@SiO2[(EtO)3Si–L2]/Mn(III) and
Co@SiO2[(EtO)3Si–L2]/Co(II) in ethyl acetate as solvent (Table 1, entries 6, 12). Finally, at
a constant temperature 40˚C, 50 minutes was obtained and applied as the optimum time
reaction. Also, reaction temperature 55 ˚C was obtained as optimum temperature at a constant
time 45 min (Fig.8)
.
Fig. 8. Effect of temperature (a) and time (b) on the oxidation of benzyl alcohol catalyzed by
Co@SiO2[(EtO)3Si–L2]/Mn(III)(red) and Co@SiO2[(EtO)3Si–L2]/Co(II)(blue)
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After optimizing the solvent, time and temperature reaction, the other reaction conditions
such as catalyst amount and oxidant amount were optimized. For this purpose, a series of
reactions was designed and done (Table 2).
The amounts of Co@SiO2[(EtO)3Si–L2]/Mn(III) catalyst was used to ranging from 0.02ꢀ0.04
(g). The evaluation results are shown in Table 2 indicates that increase the amount of catalyst,
increase reaction yield. Also increases the amount of H2O2 (30%) as oxidizing agent to 40 µl,
increases the reaction yield. Optimized amounts of used catalyst and oxidizing agent were
determined 0.04 g and 40 µl, respectively (Table 2, entry 10).
Table 2. The optimized amounts of the catalyst and oxidant in oxidation of benzyl alcohol to benzaldehydea
Entry
Co@SiO2[(EtO)3Si–
H2O2 30% ( µl)
Selectivity (%)
Isolated yield
L2]/Mn(III) (g)
0.02
30
40
50
60
30
40
50
60
30
40
50
60
60
100
100
100
100
100
100
100
100
100
100
100
100
100
64
1
2
3
4
5
6
7
8
9
10
11
12
13
0.02
0.02
0.02
0.03
0.03
0.03
0.03
0.04
0.04
0.04
0.04
Not used
91
92
93
79
92
94
94
89
98
98
98
8
aReaction condition: benzyl alcohol (0.5 mmol), reaction time: 50 min, reaction temperature 55 ◦C
The number of available alcohols were used for the oxidation reaction in the presence of the
catalyst Co@SiO2[(EtO)3Si–L2]/Mn(III) (Table 3). The optimal reaction conditions were
applied and the selectivity was 100% in any case of alcohol. The results in Table 3 show that
the catalytic oxidation reactions of alcohols have done with efficiency and high yield. The
different functional groups on the benzene ring did not seem to have influence on the yield of
the reaction.
Table 3
Oxidation of the variety of alcohols by Co@SiO2[(EtO)3Si–L2]/Mn(III) nanocatalyst
O
Co@SiO2[(EtO)3SiꢀL2]/Mn(III)(0.04 g)
OH
R
H
R
Ethyl acetate, H2O2(30%) 40 µl , 55 C, 50 min
(0.5 mmol)
Entry Alcohols
Productsa
Yield (%)
B.P/M.P (oC)
Found
204ꢀ206
Lit
205[34]
HO
HO
O
1
2
95
94
47ꢀ48
48[35]
O
Cl
Cl
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180[34]
55ꢀ58[36]
248[37]
106[35]
43[38]
HO
HO
HO
HO
O
3
4
5
6
7
99
96
97
94
90
178ꢀ180
56ꢀ58
F
Br
F
Br
O
O
246ꢀ248
104ꢀ105
40ꢀ42
O
O
O
NO2
NO2
HO
O
Cl
Cl
Cl
Cl
8
91
154
155[39]
HO
O
9
89
92
48
94
46ꢀ50[39]
93[39]
OH
O
O
10
OH
In order to investigate the recyclability of the catalyst, reuse experiments of the catalyst were
carried out. The oxidation of benzyl alcohol (0.5 mmol) were studied under reaction time 50
min, reaction temperature 55 ◦C, catalyst amount 0.04 g and hydrogen peroxide 40 µl (Fig. 9).
In order to regenerate the catalyst, after each cycle, it was separated by a magnet, and washed
several times with deionized water and ethanol. Then, it was dried in oven at 50 °C and used
in the next run. The results show that this catalyst can be reused five times without any
significant loss in activity performance. The leaching of Mn into the reaction mixture from
the recovered complex supported is analyzed using ICPꢀMS.
100
80
60
Co@SiO2[(EtO)3Si–L2]/Mn(III)
Co@SiO2[(EtO)3Si–L2]/Co(II)
40
20
0
1
2
3
4
5
6
7
8
9
Cycle
Fig. 9. Recycling of the two synthesized magnetic nanocatalysts for oxidation of benzyl alcohol to benzaldehyde
Typical XPS spectra of Co@SiO2[(EtO)3Si–L2]/Mn(III) catalyst used is shown in Fig 10. The
XP spectroscopic analysis usually employed to detect the chemical state of observed
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element[40,41]. In this work, was studied of oxidation state of immobilized manganese on the
2
surface of Co@SiO2[(EtO)3Si–L2]. The XPS spectrum of Mn catalyst produced a Mn p3/2
2
and a Mn p1/2 peak at 643.3 eV and 653.1 eV, and is depicted in Fig. 10. These results
indicated that most of the Mn ions on the surface of the heterogeneous catalyst were in +3
oxidation state (due to the binding energy (BE) 643.3 eV which are attributed to Mn(III) state
of manganese), and they are in accordance with earlier literature data [42–44]. This result
confirms that during the oxidation of alcohols, parts of the Mn3+ in catalyst are converted to
Mn2+.
.
Fig. 10. XPS image of the Co@SiO2[(EtO)3Si–L2]/Mn(III) catalyst.
Conclusion
In conclusion, we have successfully prepared Co@SiO2[(EtO)3Si–L2]/Mn(III) and
Co@SiO2[(EtO)3Si–L2]/Co(II) catalysts through surface modification of silica coated
magnetic cobalt nanoparticles with [(EtO)3Si–L2H] Schiff base and then interaction with Mn
(acac)3 and Co(CH3COO)2.4H2O respectively. The catalysts were proven to be reusable
easily for five times after recovery by separation and drying without significant catalytic
deactivation which were typically happened due to the leaching of active species or
degradation of the structure. These complexes are good heterogeneous catalysts for oxidation
of the variety of alcohols.
Acknowledgement
We are appreciates the BuꢀAli Sina University and Payame Noor University for their partial
support on this project.
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Graphical Abstract
A novel Schiff base Mn(III) and Co(II) complexes supported on Co
nanoparticles: An efficient and recyclable magnetic nanocatalyst for Alcohol
oxidation
Hassan Keypour, Shokoufeh Ghahri Saremi, Hojat Veisi, Reza Azadbakht
SiO2
Co
O
O
O
O
O
O
Si
Si
N
O
N
M
O
O
M= Mn(III) , Co(II)
OH
R = R, Ar
R
H
R
Ethyl acetate, H2O2(30%) 40 µl , 55 C, 50 min
15