A heterogenized chiral imino indanol complex of manganese as an efficient catalyst for aerobic epoxidation of olefins

Article abstract of DOI:10.1039/c7nj00670e

Herein, a new heterogenized chiral catalyst, GFC-[Mn(L)(OH)], was synthesized by grafting the complex [Mn(L)(OH)] on carbon-coated magnetic Fe₃O₄ nanoparticle-decorated reduced graphene oxide sheets (GFC) through an amine linkage (L = (1R,2S)-1-(N-salicylideneamino)-2-indanol). The catalyst was characterized via FT-IR, UV/vis, XRD, SEM, and vibrating sample magnetometer (VSM) techniques. It exhibited excellent activity and selectivity in the epoxidation of olefins with oxygen in the presence of isobutyraldehyde under mild conditions (conversion 38-98%; selectivity 65-98%; and enantioselectivity 58-100%, except for alpha-methylstyrene). Furthermore, the synergistic effect of the reduced graphene oxide support was observed on the increasing activity, epoxide selectivity, and enantioselectivity. The catalyst can be recovered via magnetic separation from the reaction mixture and recycled five times without any significant loss in its activity. The advantage of this development is the use of both the synergic effect of reduced graphene oxide and the magnetite nanoparticles to obtain an easily recyclable heterogeneous green catalyst. In addition, high asymmetric induction of a rigid indanol-based unit of the ligand results in high enantioselectivity.

Full text of DOI:10.1039/c7nj00670e

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ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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Heterogenized chiral iminoindanol complex of manganese as an
efficient catalyst for aerobic epoxidation of olefins
Vahideh Abbasi, Hassan Hosseini-Monfared* and Seyed Majid Hosseini
Received 00th January 20xx,
Accepted 00th January 20xx
A new heterogenized chiral catalyst, GFC-[Mn(L)(OH)], was synthesized by grafting complex [Mn(L)(OH)] on carbon coated
magnetic Fe3O4 nanoparticles decorated reduced graphene oxide sheets (GFC) through the amine linkage (L = (1R,2S)-1-(N-
salicylideneamino)-2-indanol). The catalyst was characterized by FTꢀIR, UV/Vis, XRD, SEM and vibrating sample
magnetometer (VSM) techniques. It exhibited excellent activity and selectivity in the epoxidation of olefins with oxygen in
the presence of isobutyraldehyde under mild conditions (conversion 38-98%; selectivity 65- 98%; enantioselectivity 58-
100, except alpha-methylstyrene). Furthermore, synergistic effect of reduced graphene oxide support was observed on the
increasing activity, epoxide selectivity and enantioselectivity. The catalyst could be recovered by magnetic separation from
the reaction mixture and recycled five times without any significant loss in its activity. The advantage of this development
is using both the synergic effect of reduced graphene oxide and magnetite nanoparticles to obtain an easily recyclable
heterogeneous green catalyst. In addition, the high asymmetry induction of rigid indanol-based unit of the ligand results to
high enantioselectivity.
DOI: 10.1039/x0xx00000x
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of these justifications for enantioselective catalyst and ligand
recovery are examples of the trend towards ‘green chemistry’
processes.7 A single atom sheet graphene with carbon atoms
Introduction
Chiral epoxides are useful starting reagent for the synthesis of
various desirable chiral products by stereospecific ring-opening
reactions.1 Chiral compounds are commonly required in
pharma, agrochemical and fine chemical sectors. In addition,
to prevent a tragedy such as thalidomide drug, pharmaceutical
companies nowadays have to make sure that both
enantiomers of a drug are tested for their biological activity
and toxicity before they are marketed. Obviously, there is a
strong demand for the pure enantiomers.
in a sp2 hexagonal bonding configuration possesses high
8
thermal conductivity and high surface area of 2630 m2/g
which make it a superior candidate to use as support for
heterogenization of catalysts. Metal-organic ligand complexes
can be linked covalently to oxygen-containing functional
groups of graphene oxide (GO) and in addition to
heterogenization benefit from the enhanced activity by the
synergic interactions between the complex and GO.9
Additionally, the existence of anchored complexes will prevent
the aggregation of GO sheets during the reduction, and keep
the specific surface area of the resultant hybrid materials
large, which is required for catalysis.10 Lately Zheng et al
loaded L-proline onto GO and used in asymmetric aldol
reaction.11 Nasseri et al reported the use of a Mn(III) complex
supported on GO for the epoxidation of olefins using m-CPBA
as an oxidant.12
Before the advent of enantioselective reactions, chiral
compounds had to be biosynthetically made or resolved. This
was an expensive and time consuming process, especially for
industrial applications. Catalytic oxidation of olefins is an
economically favourite protocol to get chiral epoxides.2,3,4,5
Further improvement is to use a heterogeous catalyst. There
are several reasons to consider a heterogenization method for
using chiral metal complexes as catalyst. First the price of the
chiral ligands and complexes are usually high. Second
obtaining pure products without residual catalysts or residual
catalyst ligands, and finally due to the environmental problem
which is associated with catalyst/ligand waste or associated
with the processes of their separation from a product.6 Most
To overcome to the problem of GO-metal complex
separation from a reaction medium a fast and clean method is
to use magnetic nanoparticles. GO-metal complex can be
decorated with magnetic nanoparticles and then easily
separated by magnetic decantation.13 Decorating graphene
sheets with Fe3O4 nanoparticles (NPs) will impart desirable
magnetic properties to the graphene, making the composites
promising for a variety of applications in fields such as catalysis
and environmental remediation.14 Coating Fe3O4 NPs with
carbonaceous shells protect them from being dissolved in
Department of Chemistry, University of Zanjan, 45195-313 Zanjan, Iran.
E-mail: monfared@znu.ac.ir (H. Hosseini-Monfared); Tel.: +98 24 33052576; Fax:
+98 24 33583203.
† Electronic Supplementary Information (ESI) available: FT-IR and NMR spectra and
GC chromatograms. See DOI: 10.1039/x0xx00000x
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acidic or basic media and Fe3O4@C have exhibited high 6890-GC) using a SGE-CYDEX-B capillary column (25 m
stability at high temperature and pressure. Magnetic mm 0.25 m). The configuration (R or S) of the chiral
graphene–Fe3O4@carbon (GFC) has been used successfully for products was assigned by comparison of retention times with
× 0.22
×
ꢀ
dye adsorption.10
Chiral manganese(III)
enantioselective catalytic activity in the epoxidation of
olefins.15 Salicylidene Schiff base derivatives also form stable its functionalization with the amine rGO/Fe3O4@C-NH2 (GFC-
those of R-(+)-limonene. H NMR spectra were recorded on a
show Bruker 250 MHz spectrometer.
1
salen
complexes
The reduced graphene oxide/Fe3O4 hybrid (rGO/Fe3O4) and
transition
metal
complexes
for
polymerization,16 NH2) was synthesized according to our previous report.25
enantioselective and selective epoxidation of olefins.17,18,19,20,21
Cis-aminoindanol as a chiral amino alcohol is a good chiral Synthesis of (1R,2S)-1-(N-salicylideneamino)-2-indanol (H2L)
motif which can be used for the preparation of a chiral Schiff
The mixture of (1R,2S)-(+)-cis-1-amino-2-indanol (0.50 g, 3.35
base. The availability of optically active cis-1-amino-2-indanol
mmol), 2-hydroxybenzaldehyde (0.78 g, 3.35 mmol) and
and the high levels of asymmetry it induces are useful for the
anhydrous Na2SO4 (2.00 g) in dry methanol (25 mL) was
development of economical large-scale processes. Cis-1-
refluxed for 4 h. The yellow solid was separated by filtration
amino-2-indanol-based chiral auxiliaries have been used for
and recrystallized from dry methanol. Yield 70% (0.70 g). IR
(KBr) 3418, 3131, 1638, 744 cm-1. 1H NMR (250.13 MHz,
carbon-carbon and carbon-heteroatom bond formations
reactions.22 The rigidity of indane increases the asymmetric
CDCl3): δ = 12.94 (s, 1H,vb), 8.62 (s, 1H), 7.39-7.16 (m, 6H),
transmission during the catalysis. Condensation of chiral cis-1-
6.99-6.91 (m, 2H), 4.82 (d, 1H, J = 6 Hz), 4.70 (dd, 1H, J = 6 Hz, J
amino-2-indanol with salicylaldehyde results to a non-C2-
= 11 Hz), 3.27 (dd, 1H, J = 6 Hz, J = 16 Hz), 3.11 (dd, 1H, J = 5
symmetric Schiff base ligand.
Hz, J = 16 Hz), 2.16 (s, 2H, vb) ppm. 13C NMR (62.90 MHz,
In continuation of our ongoing research on the application
CDCl3): δ = 167, 161, 141, 133, 132, 129, 127, 126, 125, 119,
of heterogeneous catalysts in organic synthesis and
117, 76, 75, 40 ppm.
development of novel catalytic oxidation methods using
environmentally benign oxidants such as O2,23,24 we now
Synthesis of [Mn(L)(OH)]
report the design and application of the covalent attachment
Following a reported procedure for the synthesis of Schiff
of [Mn(L)(OH)] (L = (1S,2R)-1-(N-salicylideneamino)-2-indanol)
bases,20 MnCl2·4H2O (1.09 g, 5.4 mmol) was added to the
onto carbon coated magnetic Fe3O4 nanoparticles supported
solution of H2L (0.70 g, 2.7 mmol) in dry methanol (15 ml) and
on reduced graphene oxide sheets (GFC). It was proved that
gently refluxed for 8 h. The solvent was evaporated and the
the new catalyst GFC-[Mn(L)(OH)] is efficient for the
resulting black solid washed two times with water. The
enantioselective epoxidation of unfunctionalized olefins with
product was dried overnight at room temperature. Yield 74%
O2 in combination with
a
sacrificial coreductant
(0.69 g). Anal. Calcd for C16H14MnNO3: C, 59.45; H, 4.37; N,
4.33. Found: C, 58.94; H, 4.48; N, 4.00. IR (KBr): ν = 3423 (m, O-
isobutyraldehyde. GFC-[Mn(L)(OH)] was recovered from the
reaction mixture by using an external magnetic field.
̄
H), 3020 (w), 2923 (m), 2853 (w), 1612 (vs, C=N), 1127 (m),
1092 (s), 1306 (s), 1045 (s), 754 (vs), 462 (w) cm-1.
Experimental
Synthesis of nanocomposite GFC-[Mn(L)(OH)]
(1R,2S)-(+)-cis-1-amino-2-indanol and all other reagents with
high purity were purchased from Fluka and used as received.
IR spectra were taken with a Matson FT-IR spectrophotomer in
the range of 400–4000 cm–1 as KBr disks. UV/Vis spectra of
A certain amount of [Mn(L)(OH)] (0.54 g) was added to GFC-
NH2 (0.7 g) in methanol and refluxed under a nitrogen
atmosphere for 24 h. The resulting product was isolated by
magnetic separation, washed with large quantities of
methanol and water.
solutions were recorded with
a
Shimadzu 160
spectrophotometer. The UV/Vis spectra of the solid
nanostructures were evaluated in the range of 190-800 nm by
using spectrophotometer Shimadzu-UV-2550-8030 with slit
width of 5.0 nm and light source with wavelength of 360.0 nm
at room temperature. Powder X-ray diffraction (XRD) data
were recorded on a Bruker D8 ADVANCED diffractometer with
Cu Kα radiation (λ = 1.5418 Å). The surface morphologies of
the nanocomposites were identified by the scanning electron
microscope Stereo scan 360 (leica/Cambridge) and Hitachi
f4160. The Mn content of the catalysts was determined by
Varian AA-220 Atomic absorption spectroscopy (AAS). A HP
Aglient 6890 gas chromatograph which was equipped by a HP-
5 capillary column (phenyl methyl siloxane 30 mm×320
μm×0.25 μm) and flame-ionization detector was used
quantification of the oxidation reaction products. The
enantiomeric excess (ee%) was determined by chiral GC (HP
As
a control experiment, Fe3O4@C[Mn(L)(OH)] was
prepared using a reported procedure25 under the same
conditions without using graphene oxide
Catalytic tests
The oxidation of olefins with O2 was performed in a 25-mL
round-bottom flask equipped with a refluxing condenser and a
small magnetic bar. In a typical experiment the flask was
charged with the suspension of catalyst (10.0 mg containing
0.66 μmol Mn), acetonitrile (3 mL), substrate (2 mmol) and
isobutyraldehyde as oxygen acceptor (5 mmol). Dioxygen, as
oxidant, was filled in the system by using an oxygen balloon.
The flask was placed in a thermostatic oil bath at 25, 40 or 60
°C. Reactions were carried out for 8 h under vigorous magnetic
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stirring. At appropriate intervals, aliquots were removed and nitrogen (O–H···N). In general, hydrogen bonding26 to an X-H
analyzed by GC using a HP-5 capillary column (phenyl methyl molecule results in a decrease in the frequency and a
siloxane 30 mm×320 μm×0.25 μm) and a chiral SGE-CYDEX-B broadening of the absorption band that is assigned to the O-H
capillary column (25 m
× 0.22 mm × 0.25 ꢀm). The reaction stretching vibration. The infrared analysis of the resulting
products were quantified by gas chromatography using manganese complex revealed the weakening of the band at
calibration curve. The reported results are the average of three 3423 cm-1 and the disappearance of the broad band about
repeated experiments. The products were identified with 3166 cm-1 in the free ligand indicating deprotonation of the
authentic samples and/or 1H-NMR spectroscopic data.
ligand and replacement by the metal ion. A shift to lower
Control reactions were carried out in the absence of frequency 1612 cm-1 of the band for the imine group is
catalyst, under the same conditions as the catalytic runs. The observed upon metal coordination relative to the free ligand
recyclability of GFC-[Mn(L)(OH)] catalyst was carried out by 1639 cm-1, indicating participation of the nitrogen atom in
separating the catalyst from the reaction system by magnetic metal complexation.27 The IR spectrum of GFC-[Mn(L)(OH)]
decantation. It was washed with methanol and acetonitrile shows (Fig. 1) new weak peaks in the range of 1600–1100 cm-1
and dried at 60 °C, then reused in the next reaction.
owing to C-O, C-N and aromatic ring vibrations which confirm
the successful immobilization of the complex onto GFC.
The UV/Vis spectrum of GO shows a strong absorption
peak at 231 nm and a shoulder at about 305 nm, which are
Results and discussion
attributed to the π
→π* transition of graphitic C–C bonds and
to the n
→
π* transitions of C=O bonds, respectively.8 After
Synthesis and characterization of GFC-[Mn(L)(OH)]
reduction, the rGO/Fe3O4 absorption peak red shifted to 248
nm because the electronic conjugation in the graphene was
restored by the reduction with sodium acetate.28 The UV/Vis
Graphite powder was oxidized using a modified Hummers’
procedure to produce graphene oxide. FT-IR spectra of the
synthesized graphene oxide (GO) and graphite are shown in
Fig. S1 (in Electronic Supplementary Information, ESI). The FT-
IR spectrum of GO shows an O–H bond as a broad peak at
3422 cm-1. In GO the presence of C=O, C=C, C–OH and C–O
(epoxy) functional groups were appeared as strong bands at
1720, 1619, 1223 and 1054 cm-1, respectively.8 The spectrum
of GO also shows a C=C peak at 1583 cm−1 and a C-O peak at
1054 cm−1 corresponding to the remaining sp2 character and
alkoxy group, respectively. Graphite and GO were grossly
similar, whereas the characteristic peaks in the spectrum of
GO do not appear in the curve of graphite.
spectrum of ligand H2L shows two absorption bands (λmax
=
260 and 323 nm) and the corresponding Mn-L complex shows
intense UV absorption bands (λmax = 261 and 328 nm) and a
weak visible absorption band at 400 nm. The UV bands for
both the ligand and complex have been assigned to ligand-
centred π-π* transitions (Fig. 2) and the visible absorption
band of Mn-L has been assigned to the hydroxyl and phenolate
O (pπ)
→ Mn(dπ*) ligand to metal charge-transfer (LMCT)
transitions.29 Elemental analyses and chloride titration
measurements confirmed the absence of chloride ligand.The
tridentate H2L ligand of the complex adopts a near-planar
geometry around the Mn(III) ion and therefore the complex
containing the hydroxide ion as the fourth ligand approaches
the structure of a distorted square planar. However, such a
complex easily passes at six coordination of the pseudo-
octahedral type by forming adducts with electron donating
atoms from surroundings (e.g. donor atoms of the solvent). It
is known that the complexes of Mn(III) ion in an octahedral
environment should give rise to spin allowed absorption peaks
as well as to spin forbidden ones due to its d4 electronic
structure;30 these should exhibit a broad strong band at 476-
625 nm. Complex [Mn(L)(OH)] does not show absorption in the
visible region, presumably because the weak d–d bands are
submerged in the tail of the charge transfer bands.
Black coloured chiral ligand H2L was prepared in good yield
by condensation of salicylaldehyde and chiral (1R,2S)-(+)-cis-1-
amino-2-indanol in refluxing methanol. Also complex
[Mn(L)(OH)] was synthesized in methanol under reflux
condition in a very good yield (74%) and purity by treatment of
H2L with MnCl2. Reduced GO was organofunctionaized by the
condensation reaction of 3-(aminopropyl)trimethoxysilane and
the GFC active C-OH groups to produce GFC-NH2. Complex
[Mn(L)(OH)] was sequentially immobilized onto the support by
coordination with the NH2 of GFC-NH2 as illustrated in Scheme
1. The main spectroscopic feature of H2L is as follows. In the
infrared spectrum (Fig. 1), an absorption band at 1639 cm-1
shows the characteristic of the imine C=N stretching and broad
bands at 3423 and 3166 cm-1 are due to the phenolic and the
aminoindanol unit proton hydrogen bonded to the imine
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Scheme 1 The syntheses of H2L, chiral complex [Mn(L)(HO)] and its immobilization on GFC.
Fig. 1 FT-IR spectra of chiral H2L, [Mn(L)(OH)] (left) and GFC, complex, GFC-complex (right).
The composite of reduced graphene oxide/Fe3O4@C
containing [Mn(L)(OH)] complex is black-brown in colour, this
colour arising from the strong absorption band of the complex
at low wavelength (< 350 nm) tailing to beyond 400 nm as
shown in Fig. 2 for a solution in ethanol. Diffuse reflectance
spectrum of the investigated composite is also presented in
Fig. 2. The main π-π* peaks shifted toward the higher energies;
the ligand to metal charge transfer (CT) peak at 400 nm shifted
toward the lower energy. These shifts could be consistent with
the complex distortion onto the GFC; in this case, the
spectrum is an evidence for immobilization of the complex on
the support.
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Fig. 3 XRD patterns of graphite, GO, rGO/Fe3O4, GFC and composite [GFC-
[Mn(L)(OH)].
Fig. 2 UV/Vis spectra of (a) H2L and (b) [Mn(L)(OH)] recorded in ethanol solution
and (c) DR-UV of GFC-[Mn(L)(OH)].
XRD analysis
Magnetization studies
The XRD patterns of graphite, GO, rGO/Fe3O4, GFC and
composite GFC-[Mn(L)(OH)] are shown in Fig. 3. Graphite
powder exhibits a high crystalline degree with a strong and
sharp diffraction peak at 2θ = 26.37°. In contrast, the
crystalline degree of graphite oxide is very low due to a
preserved graphene like honeycomb lattice in graphite oxide.
The oxidation of graphite led to the shift of the diffraction
peak position from 2θ = 26.6 to 12.1_11.2 Å, corresponding to
the increase in the inter planar distance because of the
attachment of oxygen functional groups to both sides of the
single graphene layer during its oxidation.31 The diffraction
pattern of the GFC shows that the carbon shell prepared by
this method is amorphous; a similar trace to the graphene–
Fe3O4 hybrid but no obvious sharp diffraction peak for the
Magnetization studies showed that Fe3O4 and GFC-[Mn(L)(OH)]
hybrid are superparamagnetic (Fig. 4) since there were no
hysteresis in the magnetization curves at room temperature
with an applied magnetic field sweeping from -10000 to
Error! Bookmark not defined.
+10000 kOe
.
The specific saturation
magnetization (Ms) is 58.3 emu g-1 for Fe3O4 and 11 emu g-1 for
the GFC-[Mn(L)(OH)] hybrid. For composite GFC-[Mn(L)(OH)],
the decrease in the value of Ms could be attributed to the
reduced amount of Fe3O4. The magnetization value of GFC-
[Mn(L)(OH)] is comparable with previous reports32 and is
strong enough to be easily separated using a magnet.
graphite is observed, confirming that.10 In Fig.
3 by
SEM analysis
immobilization of the homogeneous [Mn(L)(OH)] complex on
graphene oxide, 002 reflections peak of GO was completely
disappeared and structure of Fe3O4@C are not destroyed
during immobilization of the complexes. The crystallite size of
14.1 nm was calculated for GFC-[Mn(L)(OH)] using the X’Pert
program.
The thick cakes of graphite seen in the SEM measurement (Fig.
5a) have been exfoliated into thin large flakes of GO (Fig. 5b)
after oxidation. Graphite shows its flat surface with many small
and dispersed flakes and an ordered layer structure. The
primary graphite structure disrupted into crumpled layer
structure due to its oxidation. Nanoparticles of Fe3O4@C in
GFC are seen as large number of granular particles closely
attach on the surface of rGO sheets (Fig. 5c), indicating the
formation of loosely packed Fe3O4 nanoparticles onto rGO
sheets. The image of the GFC-[Mn(L)(OH)] represented in Fig.
5d, clearly shows that the nanoparticle was immobilized onto
the GFC surface. Graphene sheets dispersed Fe3O4
nanoparticles and prevent their self-aggregation.
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Fig. 4 Magnetization curves of (a) Fe3O4 and (b) GFC-[Mn(L)(OH)].
Fig. 5 SEM images of (a) graphite, (b) GO, (c) rGO/Fe3O4 and (d) GFC-[Mn(L)(OH)]
isobutyraldehyde as co-catalyst (entry 2) are essential for the
oxidation. Replacing O2 by air decreased the conversion (entry
Catalytic activity
The catalytic activity of [Mn(L)(OH)] in solution (homogeneous 3).
catalysis) and in heterogenized form as GFC-[Mn(L)(OH)] were
Co-oxidation of styrene and isobutyraldehyde under the
investigated in the oxidation of styrene using O2 as oxidant. investigated conditions leads to the formation of styrene
Results are presented in Table 1. Control experiments revealed oxide/benzaldehyde (main products). Isobutyraldehyde was
that the reaction is catalytic (Table 1, entry 1) and co-oxidized with styrene to isobutyric acid. Comparison of
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Table 1 Catalytic oxidation of styrene.a
Entry
Time
(h)
Catalyst/cocatalyst/oxidant
Conv.
(%)b
Epoxide
selectivity
(%)b
Ee (%)
(conf.)c
1
2
3
4
5
6
7
8
8
8
8
2
2
4
None/iPrCHO/O2
GFC-[Mn(L)(OH)]/none/O2
GFC-[Mn(L)(OH)]/iPrCHO/air
FC-[Mn(L)]/iPrCHO/O2
< 3
0
0
0
0
0
37
84
37
78
54
60
74 (S)
40 (S)
67 (S)
73 (S)
57 (S)
54
GFC-[Mn(L)(OH)]/iPrCHO/O2
[Mn(L)(OH)]/iPrCHO/O2
[Mn(L)(OH)]/iPrCHO/O2
100
83
100
a Reaction conditions: catalyst 10 mg (containing 0.66 mol Mn), styrene 2 mmol, O2 1 bar, CH3CN 3 mL, iPrCHO 5 mmol and temperature 40° C. Reported results are
ꢀ
the average of at least two experiments. b Conversion and selectivity were determined by GC. c The enantiomeric excess (ee) values were determined by GC analysis
on a chiral stationary phase (see the Experimental section).
Fe3O4@C (shown as FC in Table 1) and GFC as catalyst (Fig. 6). The best conversion, epoxide selectivity and
supports revealed the favourable synergic effect of the enantioselectivity were obtained in acetonitrile with relative
reduced graphene oxide (entries 4 and 5). Epoxide selectivity dielectric constant ε/ε°= 37.5. Although the oxidation in ethyl
of GFC-[Mn(L)(OH)] was higher than that of homogeneous acetate was high (conversion 92%), very low epoxide
[Mn(L)(OH)] (entry 6). Furthermore, although the oxidation of selectivity was observed (selectivity 13%). The catalytic activity
styrene with GFC-[Mn(L)(OH)]/iPrCHO/O2 system was of catalyst GFC-[Mn(salan)Cl] decreased in order acetonitrile >
quantitative within 2 h, it proceeded up to 83% by the ethyl acetate > n- hexane > methanol =THF.
homogeneous [Mn(L)(OH)]/iPrCHO/O2 system. Increasing the
reaction time up to 4 h increased the conversion, but of GFC-[Mn(L)Cl/O2/iPrCHO system was also remarkable. The
decreased ee from 73% to 57% (entry 7). catalyst activity increased by increasing the reaction
Effect of the reaction temperature on the catalytic activity
The superior reactivity of GFC-[Mn(L)(OH)] in the aerobic temperature from 0 to 40 °C (Fig. 7). The catalyst showed
oxidation of styrene is attributed to the promotion role of maximum activity at 40 °C and conversion of 100% was
graphene support in the adsorption of reactant styrene and obtained after 2 h. These findings propose the formation of
oxygen in addition to the site isolation of active species.9 active intermediate at temperature higher than 0 °C.
Assuming site isolation effect by immobilization,33,34 the
manganese catalytic sites can be separated from each other
and keep the high efficiency throughout the whole catalytic
process. Synergistic catalyst–support interactions has also
been reported in
a graphene–Mn3O4 electrocatalyst for
vanadium redox flow batteries.35 What is more, reduced
graphene oxide probably is involved in epoxide selectivity by
increasing the electron cloud density around the complex Mn-
centre. These characters further make for the smooth
proceeding and excellent selectivity of the conversion process.
In the meantime, the covalent linkage of the Mn-L to rGO and
chelate structure of the ligand ensures the robust
immobilization of manganese complex onto the rGO surface,
which accounts for the remarkable reusability and stability of
the immobilized catalyst.
The choice of solvent is crucial in the epoxidation of olefins.
The oxidation of styrene performed in different solvents,
namely acetonitrile, ethyl acetate, methanol and n-hexane
Fig. 6 Effect of solvent on the oxidation of styrene by GFC-[Mn(L)(OH)] and O2.
Reaction conditions: GFC-[Mn(L)(OH)] 0.010 g, styrene 2 mmol, solvent 3 ml, O2
1 bar, iPrCHO 5 mmol at 40 ºC, time 8 h. In acetonitrile the reaction time was 2 h.
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salen derivatives38 with bulky substituents in the oxidation of
styrene. Using green and clean oxidant of molecular oxygen holds
promise for the future of GFC-[Mn(L)(OH)]/O2/aldehyde system
application.
By considering the high epoxide selectivity of catalyst (Table 2),
a radical chain reaction leads to the epoxide (Scheme 2). It is
predicted
that
the
oxidation
of olefins
by
GFC-
[Mn(L)(OH)]/O2/aldehyde system proceeds by the formation of
acylperoxy radical by the reaction of aldehyde (isopropylaldehyde)
with the MnII-complex which then results to the formation of
acylperoxo-Mn or high valent Mn=O intermediate.39,40 Epoxide is
formed by the oxidation of olefin by the MnV=O.
acyl radical
Fig. 7 Effect of the reaction temperature on the oxidation of styrene catalyzed by
GFC-[Mn(L)(OH)] as a function of time. Reaction conditions: GFC-[Mn(L)(OH)]
0.01 g, styrene 2 mmol, CH3CN (3 ml), O2 1 bar, iPrCHO 5 mmol.
RCHO
[MnII] + RCO + H+
O2
Application scope of the catalytic system was then examined
under the optimized conditions, and the results are
summarized in Table 2. The results showed that in addition to
the electronic nature of the olefins the steric property of the
substrates influence their reactivities. Vinyl benzene
[MnIII
]
acylperoxy radical
RCHO
RCO3
product (O)
RCO3H
derivatives (styrene, α-methyl styrene, cis- and trans-stilbene)
[MnV=O]
[MnIII
]
and cyclic olefin cis-cyclooctene are more reactive than linear
terminal olefins such as 1-decene and 1-octene. Lowest
activityies (conversion 38-45%) were obtained in the
oxidations of 1-octene and 1-decene (Table 2, entries 1 and 2).
substrate
RCO2H
Scheme 2 Oxidation of olefins by GFC-[Mn(L)(OH)] and O2/isobtyraldehyde; [MnIII]
represents GFC-[Mn(salan)Cl] and RCHO represents isobutyraldehyde.
Epoxide selectivity and ee of
α-methyl styrene and trans-
stilbene are lower than those of styrene and cis-stilbene,
respectively (Table 2, entries 3-6). The oxidation of cis-stilbene
produced mainly trans-stilbene oxide (Table 2, entry 6). The
oxidation reaction probably proceeds through a benzyl radical
intermediate, which is sufficiently stable and allows free
rotation about the C ̶ C bond axis. The control experiments
proved this assumption. The oxidation of styrene under
optimized conditions and in the presence of hydroquinone as a
radical scavenger almost stopped (conversion 5% after 8 h).
This finding confirmed the radical nature of the reaction
intermediate in the oxidation by GFC-[Mn(L)(OH)]/iPrCHO/O2
system. The catalyst was able to catalyse the oxidation of
linear terminal and cyclic olefins in addition to vinyl benzene
derivatives. Interestingly, the catalyst could also oxidize
tetralin to 1-tetralol with 40% conversion and 28%
enantioselectivity (Table 2, entry 7).
Activity, epoxide selectivity and enantioselectivity of GFC-
[Mn(L)(OH)] are comparable and in some cases better than the
catalytic properties of Mn/GO nanocomposite and supported Mn-
salen catalysts in the oxidation of styrene with meta-
chloroperbezoic acid (m-CPBA) and sodium hypochlorite (Table S1
in the Supplementary).11,12,36,37 The enantioselectivity (67%) and
reaction time (2 h) of GFC-[Mn(L)(OH)] are more favourable than
chiral Mn(III)ꢀsalen complex of covalently grafted onto graphene
oxide in the oxidation of styrene with meta-chloroperbezoic acid
(m-CPBA) and sodium hypochlorite (ee 61ꢀ65% within 4ꢀ6 h).12 The
rigidity of indane-based GFC-[Mn(L)(OH)] catalyst increases the
asymmetric transmission during the catalysis more than salen and
8 | J. Name., 2012, 00, 1-3
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Table 2 Asymmetric oxidation of olefins with O2 catalyzed by GFC-[Mn(L)(OH)].a
ee (%)b
(conf.)
Time
(h)
Product(s)/Yield (%) b
Entry
Substrate
Conv. (%)
Epoxide
selectivity
(%)
b
(configuration)
1
2
38
45
100
100
58 (R)
99 (S)
8
8
3
4
100
100
78
65
67 (S)
8 (S)
2
2
O
O
CHO
35 (S)
30 (R)
35
8
5
100
93
92 (R,R)
6 c
100
40
100
100 (R,R)
28 (R)
8
8
ꢀ
7
ꢀ
8
100
100
8
a
i
Reaction conditions: catalyst GFC-[Mn(L)(OH)] 10 mg, substrate 2 mmol, O2 1 bar, CH3CN 3 mL, PrCHO 5 mmol and temperature 40° C. b Conversion, selectivity and
enantioselectivity were determined by GC. c The substrate was 0.50 mmol.
The recyclability of GFC-[Mn(L)(OH)] was examined using the
oxidation of styrene. The products were styrene epoxide (S-
configuration and ee about 67%) and benzaldehyde. After each
Reusability of the catalyst
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run, the solution was simply decanted with the assistance of
an external magnet. After a simple wash with CH3CN and the
subsequent removal of residual solvent by drying at 60 °C for 6
h, the catalyst was reused for the next run. Interestingly, the
obtained catalyst by immobilization of [Mn(L)(OH)] on a
magnetic support (GFC) showed excellent activity and
reusability for 5 recycles. The conversion was maintained at a
similar level after 5 cycles (Fig. 8), however, the epoxide yield
of the catalyst decreased to 65 % whereas show increasing in
its enantioselectivity. Both epoxide selectivity decrease and
enantioselectivity increase suggesting somewhat [Mn(L)(OH)]
leaching (Table 1, entries 6 and 7). The spectra of the filtrates
after each recycle are shown in Fig. S2. Slight leaching is seen
after 4th recycle. The FE-SEM image of the catalyst after
recycling indicated the catalyst is quite robust and no
x
x
significant aggregation was found (Fig. 9)
.
Fig. 9 FE-SEM of the recovered GFC-[Mn(L)(OH)].
The hot filtration test was used to investigate
heterogeneous nature of GFC-[Mn(L)(OH)] nanocatalyst and to
make sure no leaching of Mn and/or [Mn(L)(OH)] during the
course of catalytic olefin oxidation reaction. In this test, a
mixture of GFC-[Mn(L)(OH)] (10 mg), styrene (2.0 mmol),
iPrCHO (5 mmol), in CH3CN (3 mL) was heated at 40 °C for 30
min. The GFC-[Mn(L)(OH)] catalyst was separated from the hot
reaction mixture after 30 min using magnetic separation
technique. Then, it was observed by using GC that 70%
conversion (epoxide selectivity 74%, ee 53%) was achieved.
The reaction was continued with the filtrate for 2 h at the
same reaction temperature. The oxidation product conversion
of 73% (epoxide selectivity 72%, ee 43%) was observed after 2
h confirmed by GC analysis. After the completion of the
reaction, no detectable leaching of manganese was found by
AAS analysis. The results of this study proved the stability of
GFC-[Mn(L)(OH)] in the oxidation reaction condition for 2 h.
The conversion of styrene in the presence of GFC-[Mn(L)(OH)]
after 2 h was 100% (epoxide selectivity 78%, ee 67%) (Table 2,
entry 3).
The key role of GFC-[Mn(L)(OH)] catalysis is evident in the
oxidation of olefins. In the absence of the catalyst no reaction
occurred under our mild operating conditions with
O2/isobutyraldehyd in the oxidation of styrene (Table 1, entry
1). Probably, the oxygenation of olefins occurs by a reactive
high-valent Mn-oxo intermediate which is produced by the
reaction of the isopropylperoxyacid with GFC-[Mn(L)(OH)].24
Chirality is induced by the chiral (1R,2S)-(+)-cis-1-amino-2-
indanol when the olefin is closing the nanocatalyst surface.
Conclusions
In summary, we have successfully developed
a new
magnetically separable catalytic system by immobilization of
the chiral manganese complex of (1R,2S)-(+)-cis-1-amino-2-
indanol on GFC nanoparticles. The chiral GFC-[Mn(L)(OH)]
showed excellent catalytic activity and enantioselectivity for
the asymmetric epoxidation of various terminal, cyclic and
aromatic olefins by O2/aldehyde and could be reused at least
five times. The main feature of this simple catalyst is in using
almost free and green O2 for the oxidation under mild
conditions and a rigid indanol-based unit in the complex to
enhance the enantioselectivity. Carbon coated magnetic Fe3O4
nanoparticles supported on reduced graphene oxide sheets
(GFC) facilitated the separation of the catalyst, and made the
GFC-[Mn(L)(OH)] catalyst reusable. Remarkable synergistic
effect of reduced graphene oxide support was observed on
increasing activity, epoxide selectivity and enantioselectivity.
The advantages raise the prospect of using this type of simple
supported catalyst for organic synthesis and epoxidation of
olefins.
Acknowledgements
The authors are grateful for the financial support of this study
by the University of Zanjan and the Iran National Science
Foundation under Grant No. INSF 95820950.
Fig. 8 Catalyst recyclability study for model reaction. For reaction conditions see
Table 2 footnote.
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