A novel chiral manganese-tetraamide macrocycle complex covalently attached to magnetite as recyclable catalyst for aerobic asymmetric epoxidation of olefins

Article abstract of DOI:10.1016/j.ica.2017.03.019

A novel Mn complex containing N₄-tetradentate tetraamide macrocyclic ligand (L) derived from chiral diethyl-2,3-benzylidene-L-tartrate and polyamidoamine dendrimer on Fe₃O₄@SiO₂ surface was synthesized. The nanocomposite particles were investigated by SEM, XRD, VSM, EPR and FTIR. The nanocomposite showed high catalytic activity and selectivity for the epoxidation of linear terminal, cyclic and most of the aromatic olefins by O₂ in the presence of isobutyraldehyde under mild conditions; epoxide selectivity 87–100%, enantiomeric excess 53–100%. The catalyst could be separated and recovered from the reaction system by applying an external magnetic field and reused for four cycles without the loss of activity after each cycle. Total TON of 16957 was obtained after four cycles.

Full text of DOI:10.1016/j.ica.2017.03.019

Accepted Manuscript
Research paper
A novel chiral manganese-tetraamide macrocycle complex covalently attached
to magnetite as recyclable catalyst for aerobic asymmetric epoxidation of olefins
Leila Hadian-Dehkordi, Hassan Hosseini-Monfared, Pavlo Aleshkevych
PII:
S0020-1693(17)30310-9
http://dx.doi.org/10.1016/j.ica.2017.03.019
ICA 17479
DOI:
Reference:
Inorganica Chimica Acta
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1 March 2017
11 March 2017
13 March 2017
Please cite this article as: L. Hadian-Dehkordi, H. Hosseini-Monfared, P. Aleshkevych, A novel chiral manganese-
tetraamide macrocycle complex covalently attached to magnetite as recyclable catalyst for aerobic asymmetric
epoxidation of olefins, Inorganica Chimica Acta (2017), doi: http://dx.doi.org/10.1016/j.ica.2017.03.019
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A novel chiral manganese-tetraamide macrocycle complex covalently attached
to magnetite as recyclable catalyst for aerobic asymmetric epoxidation of
olefins
Leila Hadian-Dehkordia, Hassan Hosseini-Monfareda,*, Pavlo Aleshkevychb
a Department of Chemistry, University of Zanjan 45195-313, Zanjan, Islamic Republic of Iran.
Email: monfared@znu.ac.ir
b Institute of Physics, Polish Academy of Sciences (PAN), Al. Lotnikow 32/46, PL-02-668
Warsaw, Poland. Email: pavloa@ifpan.edu.pl
Abstract
A novel Mn complex containing N4-tetradentate tetraamide macrocyclic ligand (L) derived from
chiral diethyl-2,3-benzylidene-L-tartrate and polyamidoamine dendrimer on Fe3O4@SiO2
surface was synthesized. The nanocomposite particles were investigated by SEM, XRD, VSM,
EPR and FTIR. The nanocomposite showed high catalytic activity and selectivity for the
epoxidation of linear terminal, cyclic and most of the aromatic olefins by O2 in the presence of
isobutyraldehyde under mild conditions; epoxide selectivity 87-100%, enantiomeric excess 53-
100%. The catalyst could be separated and recovered from the reaction system by applying an
* Corresponding author. Tel.: +98 24 33052576; fax: +98 24 33283203. E-mail address: monfared@znu.ac.ir (H.
Hosseini-Monfared).
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external magnetic field and reused for four cycles without the loss of activity after each cycle.
Total TON of 16957 was obtained after four cycles.
Keywords: Nanocomposite; Mn(III) complex; Magnetite; Enantioselective epoxidation;
Tetraamide macrocyclic ligand
1. Introduction
Catalytic asymmetric epoxidation of olefins is a very important organic transformation since the
resulting enantiomerically pure epoxides are highly useful intermediates and building blocks
[1,2]. Numerous asymmetric catalysts have been designed for producing enantioenriched
compounds in the past four decades [3,4,5] and the main emphasis has been on catalyst design to
get the targeted epoxide with high enantioselectivity and yield [1]. Metal complexes of first row
transition elements, such as manganese and iron being less toxic [6] and biologically abundant
[7], are ideal when developing ‘‘green’’ catalysts. Among existing catalytic approaches,
manganese is particularly attractive candidate in account of its cost and natural abundance (the
3rd transition metal in the earth crust after iron and titanium) and biocompatibility, which is
particularly valuable for the pharmaceutical industry [8]. The potential of manganese is seen in a
number of biochemical redox processes such as peroxidases [9], catalases [10] and in
photosystem II (PSII) [11] where it is involved in the oxidation of water to dioxygen [12].
Macrocyclic ligands are considerably attractive in the quest for new chemistry, because
they offer a wide variety of donor atoms, ionic charges, coordination numbers and geometry of
the resultant complexes [13,14]. In addition, they impart thermodynamic and kinetic stabilities to
their metal complexes uncommon or non-existent with ligands of lesser types. Macrocyclic
2

chelators can form highly stable complexes with transition metals and lanthanides and the
complexes of macrocyclic ligands are used in imaging as MRI contrast agents,
radiopharmaceuticals and luminescent probes [15]. Macrocyclic ligand complexes are of great
importance in a number of biological processes such as photosynthesis and dioxygen transport
[16]. Many metal complexes of naturally occurring porphyrins, corrins and phthalocyanines
macrocycles have been investigated because of their potential as dyestuffs or pigments [17].
Several contributions have been made regarding the oxidation with macrocyclic ligand
complexes. Manganese meso-tetraaryl porphyrins have been studied intensively as catalysts for
epoxidation of olefins [18]. Tetrapyrrolic macrocycles (namely porphyrins and phthalocyanines)
supported on micro and mesoporous inorganic supports have been used as oxidation catalysts
[19]. Manganese complexes with polydentate N-donor ligands catalyze olefin epoxidations with
percarboxylic acids or H2O2 in a chemo- and sometimes enantioselective fashion with high
efficiency [20]. Nevertheless, these N4 ligands have shown very interesting results in asymmetric
epoxidation of olefins with ee up to 96% with selected substrates, but mostly at temperatures of
−20 °C or below. Moreover, the synthetic protocols of these ligands are tedious, multi-step and
often require expensive chiral starting materials. Metal(III) complex of a biuret-amide based
macrocyclic ligand exhibits both excellent reactivity for the activation of H2O2 and high stability
at low pH and high ionic strength [21]. Maity et al. reported manganese complexes with non-
porphyrin N4 ligands as recyclable catalyst for the asymmetric epoxidation of olefins [22]. One
class of these macrocycles is those incorporating amide groups. The amide groups can bind with
different metal ions via nitrogen and/or oxygen atoms [23]. Also, macrocyclic amides are
ubiquitous in biochemistry, because they provide the linkages that held together two of the most
important types of biopolymers, nucleic acids and proteins [24]. Collins et al. have developed
3

robust tetraamido macrocyclic ligands (TAMLs) for iron for water oxidation [25]. They used
very strongly donor deprotonated amide ligands to allow access to high oxidation state iron and
resulting in a series of extremely robust macrocycles.
On the other side, the soluble catalysts suffer from problems associated with the
instability, separation and recovery of the expensive catalyst. Immobilization of catalysts on
solid supports allows easy separation from the reaction mixture by simple filtration and thus
greatly facilitates the handling of such catalysts. Using magnetic nanoparticles as support for
immobilization of catalytically active species presents the opportunity to solve a range of catalyst
recovery and diffusion of substrates problems [26]. As prepared nanocomposite nanoparticles act
under favorable quasi-homogeneous conditions and can be recycled by simple magnetic
decantation rather than time-consuming and energy-intensive filtration or centrifugation [27].
Magnetically recoverable nanocatalysts have been used in organic synthesis for a wide range of
catalytic reactions [28,29].
It is believed that substantial room for improvement remains in terms of catalyst cost and
use of environmentally friendly oxidants with high atom efficiency. With this backdrop, we have
come up with a simple methodology to synthesize a chiral tetraamide N4 ligand (L) immobilized
on Fe3O4@SiO2 with readily available chiral diethyl-L-(+)-tartrate, ethylene diamine, methyl
acrylate as starting materials in three convenient steps. We then used its manganese complex in
the asymmetric epoxidation of olefins with O2 as environmentally friendly oxidant with high
atom efficiency at room temperature.
2. Experimental
2.1. Materials and instrumentation
4

L-(+)-Tartaric acid (>99%, Merck), iron(III) chloride hexahydrate (98%, Fluka), diethylene
glycol (99%, Merck), manganese(II) chloride tetrahydrate (>99%, Merck), methyl acrylate
(>99%, Merck), (3-aminopropyl)triethoxysilane (Fluka, 98%), ammonia (25%, Mojallali, Iran),
tetraethoxysilane (99%, Merck), polyvinylpirolidone (PVP) (Mr 55,000 Da, Merck),
cyclohexene (99%, Fluka), 1-methyl-1-cyclohexene (97%, Sigma-Aldrich), alpha-methyl styrene
(>99%, Merck), cis-stilbene (>95%, Merck), trans-stilbene (96%, Sigma) and other reagents
were obtained from commercial sources and used as received without further purification. (+)-
Diethyl-L-tartrate and diethyl-2,3-O-benzylidene-L-tartrate were prepared from L-(+)-tartaric
acid following previously reported procedure [30]. Fe3O4/tart-NPs (MNs) were prepared
according to our reported procedure [31].
The reaction products of the oxidation were determined and analyzed by HP Agilent 6890 gas
chromatograph equipped with flame-ionization detector. The enantiomeric excess (ee%) was
obtained by chiral GC column. To determine the conversion an HP-5 capillary column (phenyl
methyl siloxane 30 m × 320 µm × 0.25 µm) was used. The carrier gas (N2) flow rate was 0.7
mL/min. The column was held at 90 °C for 0.1 min, ramped at 10 °C/min to 190°C, and then
held 20 min at this temperature, before the temperature program was stopped. The enantiomeric
excess (ee%) was determined using a SGE-CYDEX-B capillary column (25 m × 0.22 mm ID×
0.25 µm). The carrier gas (N2) flow rate was 0.7 mL/min. The oven temperature was held at 50
°C for 0.10 min, then heated to 150°C at a rate of 10 °C min-1 and kept for 15 min. Then the
temperature program was stopped. 1H- and 13C-NMR spectra of the reaction mixture (without
purification) were recorded on a Bruker 250 MHz spectrometer. Fourier transform infrared (FT-
IR) spectra were run using a Perkin-Elmer 597 spectrophotometer after making pellets with KBr
powder. Powder X-ray diffraction patterns were collected at the X'Pert Pro, Herzog, Germany,
5

wavelength 1.5419 Å (Cu Kα), voltage: 40 kV, current, 40 mA. The size and morphology of
solid compounds were recorded by using a Hitachi F4160 scanning electron microscope (SEM)
operated at an accelerating voltage of 10 KV. Magnetization measurement was performed at
room temperature using a vibrating sample magnetometer (VSM) device in the Development
Center of the University of Kashan (Kashan, Iran). EPR measurements were carried out using a
Bruker EMX spectrometer working at a fixed frequency of 9.38 GHz (X-band) with an Oxford
Instruments helium-flow cryostat.
2.2. Synthesis of silica coated magnetite, Fe3O4@SiO2
Similar to reported procedure [32], Fe3O4/tart-NPs solution (34 mL, 20 mg MNs mL -1 solution
in deionized water) was added to PVP solution (0.64 mL, 25.6 gL-1 in deionized water) and the
mixture stirred for 1 day at room temperature. The PVP-stabilized nanoparticles were separated
by addition of aqueous acetone (H2O/acetone=1/10, v/v) and centrifugation at 4000 rpm for 10
min. The PVP-coated magnetite nanoparticles was suspended in 50 mL of 2-propanol containing
34 mL of concentrated ammonia (25%) [33] and sonicated for 1 h. After that, 3.4 mL (15.2
mmol) of tetraethoxysilane (TEOS) in 40 mL of 2-propanol were added dropwise within 3 h to
the magnetite suspension while it was stirring with a mechanical stirrer. The stirring was
continued for another 3 h, and then the silica-coated nanoparticles were separated magnetically,
washed five times with DI water and dried at room temperature (1.37 g).
2.3. Amine functionalization of Fe3O4@ SiO2 (Fe3O4-NH2)
6

1.28 g of dry silica-coated magnetite powder was suspended in 26 mL of dry toluene. After
sonication for 35 min, (3-aminopropyl)triethoxysilane (APTES) (1.37 mmol) was added under
mechanical stirring. The solution was heated at 105 °C for 20 h. The particles were separated by
an external magnet after cooling to room temperature, washed three times with dry methanol and
dried at room temperature (1.24 g).
2.4. Synthesis of Fe3O4-polyamidoamine dendrimer (Fe3O4-en)
1.14 g of amine functionalized nanoparticles (Fe3O4-NH2) were suspended in 17 mL methanol
and sonicated for 35 min. After that, 1.4 mL methyl acrylate (15.4 mmol) was added and the
mixture heated at 50 °C for 5 days. The resulting mixture was cooled to room temperature, the
particles separated magnetically, washed three times with methanol and dried at room
temperature. As a result 1.04 g brown powder of Fe3O4-COOMe was obtained. Dried Fe3O4-
COOMe (0.94 g) was then suspended again in 14 mL of methanol and sonicated for 30 min.
After dropwise addition of 2.0 mL (30.18 mmol) ethylenediamine at room temperature, the
solution was heated at 50 °C for 5 days. The magnetic material was magnetically separated,
washed 3 times with dry methanol, and dried at room temperature. As synthesized Fe3O4-
polyamidoamine dendrimer (Fe3O4-en) weight was 0.94 g [33].
2.5. Synthesis of Fe3O4-[MnCl(L)] (L = a chiral tetraamide macrocyclic ligand)
7

The prepared Fe3O4-en (0.405 g) was stirred with 4.55 mmol (1.34 g) of diethyl-2,3-O-
benzylidene-L-tartrate in 10 ml of ethanol for 5 days at room temperature [34] to give a chiral
macrocycle anchored onto magnetite (Fe3O4-L, Scheme 1) (0.26 g).
The immobilized [MnIIICl(L)] was prepared by refluxing as prepared Fe3O4-L (0.26 g)
with MnCl2·4H2O (1.31 g, 6.60 mmol) in dried toluene for 14 h. The resulting solid was
magnetically filtered and washed three times with MeOH. The recovered powder was extracted
in a Soxhlet using refluxing MeOH (750 mL) for 24 h [35].
2.6. Oxidation of olefins
The oxidation of olefins was performed in a 25-mL round-bottom flask equipped with a small
magnetic bar. In a typical experiment the flask was charged with the suspension of 2.0 mmol
substrate, 1.0 mg Fe3O4-[MnCl(L)] nanocomposite (contains 0.46 mmol Mn/g composite), 5.0
mmol isobutyraldehyde and 3 mL acetonitrile as solvent. The reaction was performed under
oxygen atmosphere by using O2 balloon. At the end of 8 h, the catalyst was removed by an
external magnet (1.2 T) and the product mixture analyzed by gas chromatography. Conversions
and yields were calculated with respect to the starting substrate. The reaction products were
quantified by gas chromatography and identified by comparison with the retention time and
spectral data to those of an authentic sample. To ensure reproducibility each catalytic reaction
was carried out at least two times.
For recycling experiments, after completion of the reaction, the nanocatalyst was recovered
using a magnet, washed with acetonitrile, dried at 80 ºC for 1 h and reused.
2.7. Proof of stereochemistry
8

Prochiral olefins were oxidized by Fe3O4-[MnCl(L)] and RCHO/O2. Analysis of the
corresponding chiral products by chiral GC was used to determine the enantioselectivity
of the reaction. The absolute configuration was established by comparing the GC data
with those observed for R-(+)-limonene.
3. Results and discussion
3.1. Synthesis and characterization
Magnetite nanoparticles (Fe3O4/tart-NPs) with an average size of 14.1 nm synthesized in
diethylene glycol in the presence of L-(+)-tartaric acid (Fe3O4/tart) (Eq. 1) [31], were stabilized
with PVP to make them homogeneously dispersed in 2-propanol. The nanoparticles were coated
with a shell of stable silica (SiO2). The PVP method has been applied to magnetite nanoparticles
having ionic surface charges to generate a sol–gel silica coating with TEOS [36].
The crystallinity and magnetic properties of the core material did not change upon
coating with silica. Fig. 1 shows the representative XRD pattern of Fe3O4-NPs synthesized in the
presence of tartaric acid. The intense peak at 2θ = 35.87, FWH = 0.413, demonstrates the
crystallite size of 20.2 nm by X’Pert program. The sharp and intense peaks indicate the good
crystallinity of the sample. The diffraction peaks can be well corresponded to the standard
magnetite Fe3O4 (Ref. code 01-075-0449). Comparison of the patterns of Fe3O4/tart and Fe3O4-
[MnCl(L)] shows the amorphous silica coating of the magnetite in Fe3O4-[MnCl(L)]composite as
a broad peak about 2θ = 23 º.
9

< Fig. 1 >
Enantiomerically pure (+)-diethyl-L-tartrate was synthesized by treatment of L-(+)-
tartaric acid with ethanol, and then its dihydroxyl groups protected by benzaldehyde in the
presence of p-toluenesulfonic acid to produce diethyl-2,3-benzylidene-L-tartrate in high yield
(Eq. 2) [30].
Polyaminoamido dendrimer was constructed on the surface of magnetite nanoparticles.
The introduction of amino groups onto the magnetite surface for the preparation of an initiator
site was achieved by the treatment of Fe3O4@SiO2 surface silanol groups with APTES. The
amino propionate ester (Fe3O4-CO2Me) was formed by a Michael-type addition of the magnetite
immobilized aminopropyl (Fe3O4-NH2) amino group to methyl acrylate. Subsequent amidation
of the ester moiety with ethylenediamine completed the first generation of polyaminoamido
dendrimer (Fe3O4-en). Dendrimers are highly branched macromolecules and are of great interest
as carriers of functional groups [37]. Consequently, dendritic catalysts have found numerous
applications including metal complexation and occasional use as catalysts [38]. Dendrimer
building methods pioneered by Tsubokawa and co-workers [39] were employed to propagate the
dendrimer generation.
The chiral tetraamide macrocyclic ligand anchored to magnetite (Fe3O4-L) was obtained
by the condensation reaction of Fe3O4-en with diethyl-2,3-O-benzylidene-(+)-tartrate in a
10

macrocyclization reaction (Scheme 1). The macrocyclic ligand was metallated with MnCl2·4H2O
and during the reaction Mn(II) oxidized to Mn(III) by air. The macrocyclic ligand L provides a
N4 coordination sphere through two amide-N and two deprotonated amide-N ligations to the
Mn(III) ion, stabilizing its 3+ oxidation state in the complex (Scheme 1). The deprotonated
secondary amides provide an appropriate ligand field, since these are comparatively strong s-
donors, negatively charged, and have proven to be sufficiently oxidation resistant [40].
< Scheme 1>
Recently, a manganese(III) complex of an acyclic amido-amine ligand has been reported
by Ramidi et al [41]. It has been also found that when manganese(III) tetraamido-N macrocyclic
ligand and related complexes are treated with peroxides the Mn(V)-monooxo complexes were
produced [42]. One method to achieve increased stability of the metal complexes is to rigidify
the chelator thus slowing down the kinetics of dissociation [43]. This work has mainly been
aimed at copper(II) complexes which are often thermodynamically stable but kinetically labile
[44]. Tetraazamacrocycles incorporating both amide groups and a backbone benzene ring were
synthesized as chelators for copper radionuclides. When deprotonated, the 13-membered
macrocycle forms a highly stable copper(II) complex and the 67Cu labelled compound is stable in
serum [45].
The FT-IR spectra of Fe3O4@SiO2, the intermediates and Fe3O4-[MnCl(L)] are shown in
Fig. 2. The bands at 3429 and 1629 cm-1 are assignable to the ν(O–H) and δ(O–H) mode [46] of
water molecules, respectively. The amine and amide N-H bending of the compounds appear
about 1630 cm-1 which are overlapping with the water O-H bending mode.
11

The characteristic antisymmetric (νas) Si–O–Si vibration bands [47] of the [SiO4] unit is observed
around 1218 and 1093 cm−1 for the core-shell Fe3O4@SiO2. The band at 799 cm-1 is assigned to
νs(Si–O–Si) mode. Formation of the amino propionate ester (Fe3O4-CO2Me) was confirmed by
the appearance of the CO2 band at 1736 cm-1. Furthermore, disappearance of the ester CO2 band
in intermediate Fe3O4-en and its appearance in Fe3O4-L at 1745 cm-1 strongly approved the
formation of the macrocyclic tetraamide ligand. The bending mode of the amide N-H at 1615
cm-1 was also confirmed the formation of Fe3O4-L. The insertion of Mn into the macrocycle
caused a shift in the amide N-H bending mode from 1615 to 1629 cm-1.
< Fig. 2 >
The thermal stability of Fe3O4-L and Fe3O4-[MnCl(L)] nanocomposite were examined by
means of TGA and the results are shown in Fig. 3. The nanocomposite containes 6.7% of organic
material which belongs to the PVP adsorbed on the magnetite nanoparticles and immobilized
macrocyclic ligand. The organic content for Fe3O4-L was 14.9%. The remarkable parallelism of
the two TGA curves confirms the presence of the same organic material. The lower weight loss
of the nanocomposite with respect to Fe3O4-L can be due to the formation manganese oxides,
probably Mn3O4, by increasing the temperature up to 570 ºC. The content of manganese in
Fe3O4-[MnCl(L)] sample was confirmed with measuring Mn of the sample by ICP analysis (2.5
wt%, 0.46 mmol Mn/g composite).
< Fig. 3 >
Morphology of the Fe3O4-[MnCl(L)] nanoparticles was examined by SEM (Fig. 4a). The
nanocomposite particles have nearly disk shape and uniform size distribution with an average
size of 95 nm. The grain sizes of the samples estimated from the SEM picture are larger than
those obtained from XRD data. This means that, the SEM picture indicates the size of
12

polycrystalline particles. The observation of some larger nanoparticles may be attributed to the
fact that Fe3O4-[MnCl(L)] nanoparticles have the tendency to self-agglomeration due to their
high surface energy of the fine nanoparticles. The nanoparticles of Fe3O4-[MnCl(L)] are stable
and preserve their shape and uniform distribution after catalysis. After four times recycle in the
catalytic oxidation of α-methylstyrene, the nanoparticles are well separated and their size
increased to an average of 118 nm (Fig. 4b and c).
< Fig. 4 >
The magnetic properties of the synthesized Fe3O4-[MnCl(L)] indicated a
superparamagnetic nature of the material (Fig. 5). The magnetization value of Fe3O4-[MnCl(L)]
(15 emu g−1) is lower than the value of Fe3O4-tart-NPs (55 emu g−1) but is basically close to
other results [48]. On the one hand, this feature allows catalyst separation under relatively low
external magnetic field. The negligible coercivity indicates the superparamagnetic property of
this sample, which is available for re-dispersion of the catalysts in solution without the
occurrence of severe assembly and/ or aggregation usually appearing for ferromagnetic
nanoparticles [49]. After removing the magnet from the batch a few minutes later the catalysts
were re-dispersed completely as shown before adding the magnet.
< Fig. 5 >
Fig. 6 shows the electron spin resonance (ESR) spectra recorded for the powder of the
Fe3O4–[MnCl(L)] composite at room and liquid helium temperatures. The big broad absorption
is dominated at all temperatures. The integral intensity of this absorption is barely changed
between 300 K and 10 K, whilst the resonance locus is shifted to the direction of lower magnetic
field at lower temperature. The non paramagnetic behavior allow us to assign the resonance
absorption in Fig. 6 with ferromagnetic resonance of magnetite nanoparticles. Since it is powder,
13

the particles are here randomly orientated with respect to the magnetic field and the spectrum
represents the envelope of all resonance lines summed over all possible orientations. The
temperature shift toward lower magnetic field of the resonance absorption reflects an increase of
magnetic anisotropy of single particles or even inter-particle interactions [50]. Given the
ferromagnetic resonance from magnetite is dominant, the open question remains about resonance
absorption of manganese. Because of large ZFS splitting, the Mn(III) ions are “silent” in the X-
band (9.3 GHz) ESR spectroscopy. The only signal that was expected to be seen is Mn(II). We
do not see this signal, easily detectable in paramagnetic compounds, however strong dipolar
interaction with magnetite particles could make Mn(II) absorption too broad for detection.
< Fig. 6>
3.2. Catalytic oxidation
Having synthesized and characterized Fe3O4-[MnCl(L)] nanocomposite, its role as a
heterogeneous catalyst was then evaluated for the oxidation of olefins. We first examined
the reaction using the nanocomposite as a catalyst in air oxidation of phenylcyclohexene
(Table 1). The oxidation of 1-phenylcyclohexene by aqueous H2O2 as oxidant was not
successful, since H2O2 was decomposed by Fe3O4-[MnCl(L)] in early minutes of the reaction
(Table 1, entry 1). Therefore, molecular O2 was used as a green and economic oxidant in the
oxidation reaction. Presence of a co-oxidant such as isobutyraldehyde [51] or other reducing
agent [52,53] is essential for the activation of dioxygen. The mixture of O2/iprCHO without
catalyst showed poor oxidation catalysis (Table 1, entry 2), and using the combination of
O2/iprCHO and support Fe3O4/tart-NPs increased the conversion to 37% with enantiomeric
excess of 62% (entry 3). Interestingly, the nanocomposite was a very efficient catalyst and
14

catalyzed the oxidation by O2/ iPrCHO with 100% conversion and 82% ee within 8 h (entry 4).
Co-oxidation of 1-phenylcyclohexene and isobutyraldehyde under the investigated conditions led
to the formation of 1-phenylcyclohexene oxide and isobutyric acid. The reaction was performed
directly under oxygen atmosphere by using O2 balloon and high pressure of oxygen was not
necessary. Both O2-balloon and O2 gas with 2 bar pressure in an autoclave, gave the similar
results (Table 1, entry 5); for simplicity O2-balloon was selected. The reaction temperature
showed a negative effect on the oxidation. A temperature increase from 25 °C to 45 or 60 ºC led
to a decrease in the conversion and ee (Table 1, entries 6, 7). Probably, the active species is not
stable at higher temperatures. Thereby the room temperature was chosen as optimized
temperature.
Furthermore, cyclooctene, cyclohexene and tetralin were also examined to explore the
scope of the catalytic activity of the nanocomposite (not shown in the Table). Under optimized
conditions, the Fe3O4-[MnCl(L)]/O2/iprCHO/CH3CN system oxidized cis-cyclooctene to
cyclooctene oxide with 100% conversion and 94% selectivity within 6 h; cyclohexene to
cyclohexene oxide with 100% conversion and 89% selectivity within 4 h; tetralin to 1-tetralol
with 43% yield within 9 h and benzyl alcohol to benzaldehyde with 66% yield in 9 h. Tetralin is
a convenient substance to choose for studying the autoxidation of the CH2 group [54].
Acetonitrile as solvent provided better medium than methanol, n-hexane or ethylacetate.
In fact, the catalyst did not work in methanol or n-hexane. Although, the oxidation of α-
methylstyrene in ethylacetate was high (conversion 100%), epoxide selectivity as low as 60%
with 70% ee was achieved. However, the activity of the nanocomposite in acetonitrile was
remarkable and it could oxidize α-methylstyrene quantitatively with 90% epoxide selectivity and
71% ee under the same reaction conditions. Acetonitrile did not involve through peroxymidic
15

acid, since no MeCONH2 under the catalytic oxidations was detected [55]. These findings
suggest that the polarity [56], low coordinating ability and aprotic nature of acetonitrile play the
main roles in improving the activity of the catalyst.
< Table 1>
Under the optimized conditions, aerobic oxidation of various prochiral olefins was
studied. The catalyst Fe3O4-[MnCl(L)] exhibited broad substrate scope and was highly selective
toward epoxidation with O2/iPrCHO. It remarkably catalyzed the epoxidation of linear terminal
olefins, cycloolefins and aromatic olefins with excellent selectivity of 87 to 100%, except styrene
(Table 2). 1-Decene and 1-octene were oxidized by Fe3O4-[MnCl(L)]/O2/iPrCHO with 34% and
70% conversions, respectively, and more than 99% ee (Table 2, entries 1, 3).
By increasing the temperature from 25 to 60 ºC, the conversion of 1-decene increased
from 34% to 69% (entry 2). Probably there are different mechanisms for conjugated (1-
phenylcyclohexene) and unconjugated (1-decene) olefins oxidation by Fe3O4-
[MnCl(L)]/O2/iPrCHO/CH3CN system. While the oxidation of 1-phenylcyclohexene was
decreased by increasing the reaction temperature (Table 1), the oxidation of 1-decene increased
(Table 2) by going from 25 to 60 ºC.
In addition to the main product of the corresponding epoxides, the oxidation of styrene,
α- and β-methylstyrene were occurred partially by breaking the double bond and benzaldehyde
or acetophenone obtained (entries 4-6). The stability of the plausible intermediate benzyl radical
favors breaking of the olefin double bond in styrene, α- and β-methylstyrene. In α- and β-
substituted styrene, presence of the electron donor substituent (methyl) increased the epoxide
selectivity with respect to styrene. Enantioselectivity in the oxidation of β-methyl styrene was the
16

highest. These findings show the importance of steric hindrance effect during the oxygen transfer
step and electrophilic nature of the active oxygen transfer intermediate.
Catalyst Fe3O4-[MnCl(L)] was also tested for the oxidation of cis- and trans-
stilbene (entries 8, 9). cis-Stilbene is a frequently used substrate for the study of olefin
epoxidation mechanism [57] because of mechanistic information associated with the ratio of cis-
and trans-isomers in the stilbene oxide product. Trans-stilbene was oxidized quantitatively to
trans-stilbene oxide, but cis-stilbene epoxide (46%) and thermodynamically stable trans-stilbene
oxide (54%) were obtained by the oxidation of cis-stilbene. This finding also emphasizes on the
critical role of steric effect in determining the product selectivity by this catalyst. The oxidation
reaction probably proceeds through a benzyl radical intermediate, which allows free rotation
about the C-C bond axis. In addition, our preliminary experiments in the presence of
hydroquinone as a radical scavenger proved the involvement of a radical intermediate in the
oxidation of styrene by Fe3O4-[MnCl(L)]/O2/iPrCHO/CH3CN system.
Indene was oxidized by O2/isobutyraldehyde in the presence of Fe3O4-[MnCl(L)] to
indene oxide with 100% selectivity and 53% ee in 9 h (entry 10). Epoxidation of indene
usually leads to the formation of various products but the selectivity to one of the
products, viz., the epoxide is of great importance for the chemical industry [58].
The enantioselectivity of the present Fe3O4-[MnCl(L)] catalyst are comparable and
sometimes better than chiral Mn complexes with complicated and difficult to synthesis N4
macrocylic ligands in the oxidation of styrene with H2O2 [59,60,61,62,63].
< Table 2>
17

3.3. Recyclability study
The catalyst Fe3O4-[MnCl(L)] was tested for its reusability in the epoxidation of α-methylstyrene
with O2/aldehyde as an oxidant under the optimized reaction conditions. After the first catalytic
run, the catalyst was separated from the reaction mixture using an external magnet and the
residue was washed with acetonitrile to remove the product, dried at 80 ºC for 1 h, and used as
such for the subsequent catalytic runs without further purification (Fig. 7). The FT-IR spectra
(Fig. S3) and SEM images (Fig. 4) of the initial catalyst and the recycled catalyst showed the
integrity of the catalyst after three catalytic cycles. Total TON of 16957 was obtained after four
cycles. The activity of the catalyst decreased after 4 times use because of the complex
degradation.
Although the conversion was almost constant during the experiments, but selectivity to
acetphenone was increased and the selectivity to α-methylstyrene oxide decreased with each
recycling step. These findings may be related to the adsorption of moisture by the catalyst. The
nanocomposite is highly hygroscopic and this property is clearly seen in its TGA spectrum (Fig.
3). Drying of the used catalyst before each experiments slightly improved its activity, however
its selectivity was lower than that of the fresh one yet.
< Fig. 7 >
3.4. Possible Mechanism
The key role of Fe3O4-[MnCl(L)] catalyst is evident in the oxidation of olefins. In the absence of
catalyst almost no reaction occurred under our mild operating conditions with O2/aldehyde
(Table 1, entry 2). It showed high catalytic activity and selectivity for the epoxidation of various
18

olefins by molecular oxygen/aldehyde. Additionally, cyclohexene was oxidized mainly to
cyclohexene oxide (89%) by this system. It has often been assumed that high-valent metal oxo
species are the most likely active intermediates when olefins are selectively oxidized to the
corresponding epoxides in metal complex-catalyzed oxygen atom transfer reactions [64,65].
Additional evidence for this was obtained from stereochemical investigations of the
epoxidation of cis/trans stilbene. Cis-Stilbene was converted to cis- and trans-epoxide almost
with 50% retention of configuration. Furthermore, electron-rich α- and β-substituted styrene
with the electron donor substituent (methyl) increased the epoxide selectivity with respect to
styrene. These reflect the electrophilic nature of the oxygen transfer from the plausible
intermediate to the olefin. Enantioselectivity in the oxidation of β-methyl styrene was the higher
than α-isomer. It shows the importance of steric hindrance effect during the oxygen transfer step.
Although suggesting a mechanism required further detailed studies in future, we suppose
that the oxygenation of substrates with Fe3O4-[MnCl(L)] and O2/aldehyde occurs by a
mechanism proposed to the oxidation with O2/aldehydes catalyzed by metal complexes as is
given in Scheme 2 [66]. In this mechanism, the supported Mn-macrocycle complex ([MnIII])
reacts with the aldehyde to generate an acyl radical (RC(O)•). The acyl radical then reacts with
dioxygen to give an acylperoxy radical (RC(O)OO•). The acylperoxy radical acts as a carrier in a
chain mechanism by reacting with another aldehyde molecule to give the peroxyacid, thereby
generating another acyl radical. Oxygenation of substrate is assumed by this mechanism to occur
via a reactive high-valent Mn-oxo intermediate, which is produced by the reaction of the
peroxyacid with the Mn-L of Fe3O4-[MnCl(L)] and which then reacts with the substrate (olefin)
to give the oxygenated product. The induction time of about 4 h in the oxidation of 1-
19

phenylcyclohexene can be considered the required time for the formation of acylperoxy radical
and/or Mn-oxo active intermediate (Fig. 8).
<Scheme 2>
<Fig. 8>
4. Conclusions
A novel tetraamide macrocyclic ligand complex of Mn(III) was synthesized onto magnetite and
applied in the catalytic epoxidation of various olefins with O2/aldehyde. To the best of our
knowledge, this is the first report on the chiral tetramide macrocyle complex of the manganese
catalyzed asymmetric aerobic oxidation, allowing for general epoxidation of a relatively wide
variety of olefins in good yield and ee values up to greater than 99%. Moreover, the synthesized
nanocatalyst can be recovered by simple magnetic decantation. In addition, the heterogeneous
nanocatalysts are relatively stable and can be reused four times
Acknowledgments
We are grateful for financial support from the University of Zanjan and the Iran National Science
Foundation (INSF 95820950) and Polish Academy of Sciences (PAN) for financial support of
this study.
References
[1]
[2]
[3]
G. D. Faveri, G. Ilyashenko, M. Watkinson, Chem. Soc. Rev. 40 (2011) 1722.
W. Dai, G. Li, B. Chen, L. Wang, S. Gao, Org. Lett. 17 (2015) 904.
P. McMorn, G. J. Hutchings, Chem. Soc. Rev. 33 (2004) 108.
20

[4]
[5]
D. Sinou, Adv. Synth. Catal. 344 (2002) 221.
R. V. Ottenbacher, D. G. Samsonenko, E. P. Talsi, K. P. Bryliakov, ACS Catal. 4
(2014) 1599.
[6]
[7]
H. Hasan, Manganese, The Rosen Publishing Group, New York, 2008
K. Barnese, E.B. Gralla, J. S. Valentine, D.E. Cabelli, Proc. Natl. Acad. Sci. U. S. A. 109
(2012) 6892.
[8]
[9]
D.A. Valyaev, G. Lavigne, N. Lugan, Coord. Chem. Rev. 308 (2016) 191.
X.B. Cheng, R. Jia, P.S. Li, S.Q. Tu, Q. Zhu, W.Z. Tang, X.D. Li, Enzyme
Microb. Technol. 41 (2007) 258.
[10] L. Dubois, J. Pecaut, M.-F. Charlot, C. Baffert, M.-N. Collomb, A. Deronzier, J.-M.
Latour, Chem.–Eur. J. 14 (2008) 3013.
[11] S. Mukhopadhyay, S.K. Mandal, S. Bhaduri, W.H. Armstrong, Chem. Rev. 104 (2004)
3981.
[12] K. Wieghardt, Angew. Chem., Int. Ed. Engl. 33 (1995) 725.
[13] S.A. Zarei, A. Khandar, M. Khatamian, S.A. Hosseini-Yazdi, I. Dechamps, E. Guillon,
M. Piltan, Inorg. Chim. Acta 394 (2013) 348.
[14] N.E. Borisova, M.D. Reshetova, Y.A. Ustynyuk, Chem. Rev. 107 (2006) 46.
[15] R.E. Mewis, S.J. Archibald, Coord. Chem. Rev. 254 (2010) 1686.
[16] S. Chandra, K. Gupta, Transit. Met. Chem. 27(2002) 196.
[17] S.K. Gupta, Y.S. Kushwah, Polyhedron 20 (2001) 2019.
[18] H. Hosseini-Monfared, V. Aghapoor, M. Ghorbanloo, P. Mayer, Appl. Catal. A 372
(2010) 209.
21

[19] M.J.F. Calvete, M. Silva, M.M. Pereira, H.D. Burrows, RSC Adv. 3 (2013) 22774.
[20] I. Garcia-Bosch, L. Gómez, A. Polo, X. Ribas, M. Costas, Adv. Synth. Catal. 354 (2012)
65.
[21] S. Sengupta, C. Panda, M. Ghosh, US8754206 B2.
[22] N.Ch. Maity, P.K. Bera, D. Ghosh, S.H.R. Abdi, R.I. Kureshy, N.H. Khan, H.C.
Bajajab, E. Suresh, Catal. Sci. Technol. 4 (2014) 208.
[23] C.W. Yeh, K.H. Chang, C.Y. Hu, W. Hsu, J.D. Chen, Polyhedron 31 (2012) 657.
[24] A.K. Sharma, S. Biswas, S.K. Barman, R. Mukherjee, Inorg. Chim. Acta 363 (2010)
2720.
[25] A. Chanda, D.-L. Popescu, F.T. de Oliveira, E.L. Bominaar, A.D. Ryabov, E. Munck,
T.J. Collins, J. Inorg. Biochem. 100 (2006) 606.
[26] L.M. Rossi, N.J.S. Costa, F.P. Silva, R. Wojcieszak, Green Chem. 16 (2014) 2906.
[27] R. Mrówczyński, A. Nan, J. Liebscher, RSC Adv. 4 (2014) 5927.
[28] V. Polshettiwar, R. Luque, A. Fihri, H. Zhu, M. Bouhrara, J.M. Basset, Chem. Rev.
111 (2011) 3036.
[29] J. Sun, G. Yu, L. Liu, Z. Li, Q. Kan, Q. Huo, J. Guan, Catal. Sci. Technol. 4 (2014)
1246.
[30] S. Alavi, H. Hosseini-Monfared, P. Aleshkevych, RSC Adv. 4 (2014) 48827.
[31] L. Hadian-Dehkordi, H. Hosseini-Monfared, Green Chem. 18 (2016) 497.
[32] T. Yoon, J.S. Kim, B.G. Kim, K.N. Yu, M. Cho, J. Lee, Angew. Chem. Int. Ed. 44 (2005)
1068.
[33] R. Abu-Reziq, H. Alper, D. Wang, M.L. Post, J. Am. Chem. Soc. 128 (2006) 5279.
[34] S. Ghosh, A.K. Banthia, B.G. Maiya, J. Am. Chem. Soc. 4 (2002) 3603.
22

[35] M. Ghorbanloo, H. Hosseini-Monfared, C. Janiak, J. Mol. Cat. A 345 (2011) 12.
[36] C. Graf, D.L.J. Vossen, A. Imhof, A. van Blaaderen, Langmuir 19 (2003) 6693.
[37] S. Hecht, J.M.J. Frechet, Angew. Chem. Int. Ed. 40 (2001) 75.
[38] D. Astruc, F. Chardac, Chem. Rev. 101 (2001) 2991.
[39] N. Tsubokawa, H. Ichioka, T. Satoh, S. Hayashi, K. Fujiki, React. Funct. Polym. 37 (
1998) 75.
[40] T.J. Collins, Acc. Chem. Res. 35 (2002) 782.
[41] P. Ramidi, C.M. Felton, B.P. Subedi, H. Zhou, Z.R. Tian, Y. Gartia, B.S. Pierce, A.
nGhosh, J. CO2 Util. 9 (2015) 48.
[42] T.J. Collins, S.W. Gordon-Wylie, J. Am. Chem. Soc. 111 (1989) 4511.
[43] E. Faggi, R. Gavara, M. Bolte, L. Fajarí, L. Juliá, L. Rodríguez, I. Alfonso, Dalton Trans.
44 (2015) 12700.
[44] T.J. Wadas, E.H. Wong, G.R. Weisman, C.J. Anderson, Curr. Pharm. Des. 13 (2007) 3.
[45] P. Antunes, R. Delgado, M.G.B. Drew, V. Felix, H. Maecke, Inorg. Chem. 46 (2007)
3144.
[46] T. Rajh, L.X. Chen, K. Lukas, T. Liu, M.C. Thurnauer, D.M. Tiede, J. Phys. Chem.
B 106 (2002) 10543.
[47] A. Jitianu, M. Crisan, A. Meghea, I. Rau, M. Zaharescu, J. Mater. Chem. 12 (2002)
1401.
[48] S.F. Si, C.H. Li, X. Wang, D.P. Yu, Q. Peng, Y.D. Li, Cryst. Growth Des. 5 (2005) 391.
[49] C.T. Yavuz, J.T. Mayo, W.W. Yu, A. Prakash, J.C. Falkner, S. Yean, L. Cong, H.J.
Shipley, A. Kan, M. Tomson, D. Natelson, V. L. Colvin, Science 314 (2006) 964.
23

[50] N. Guskos, E.A. Anagnostakis, A. Guskos, J. of Achievements in Materials and
Manufacturing Engineering 24 (2007) 26.
[51] R.A. Sheldon, A history of oxygen activation: 1773-1993, in The activation of
dioxygen and homogeneous catalytic oxidation, ed. D.H. Barton, A.E. Martell,
D.T. Sawyer, Plenum Press, New York, 1993, p. 11.
[52] T. Mukaiyama, T. Yamada, Bull. Chem. Soc. Jpn. 68 (1995) 17.
[53] I. Tabushi, N. Koga, J. Am. Chem. Soc. 101 (1979) 6456.
[54] H. Hock, W. Susemihl, Ber. 66 (1933) 61.
[55] G.B. Payne, P.H. Deming, P.H. Williams, J. Org. Chem. 26 (1961) 659.
[56] V. Gutmann, Coord. Chem. Rev. 18 (1976) 225.
[57] W. Adam, K.J. Roschmann, C.R. Saha-Möller, D. Seebach, J. Am. Chem. Soc.
124 (2002) 5068.
[58] V. Ramaswamya, P. Awatia, A.V. Ramaswamy, Top. Catal. 38 (2006) 251.
[59] F. Bruyneel, C. Letondor, B. Basturk, A. Gualandi, A. Pordea, H. Stoeckli-Evans, R.
Neier, Adv. Synth. Catal. 354 (2012) 428.
[60] M. Wu, B. Wang, S. Wang, C. Xia, W. Sun, Org. Lett. 11 (2009) 3622.
[61] J. Rich, M. Rodriguez, I. Romero, X. Fontrodona, W. N. M. Piet, V. Leeuwen, Z. Freixa,
X. Sala, A. Poater, M. Sola, Eur. J. Inorg. Chem. (2013) 1213.
[62] N.C. Maity, P.K. Bera, D. Ghosh, S.H.R. Abdi, R.I. Kureshy, N.-U. H. Khan, H.C.
Bajaj, E. Suresh, Catal. Sci. Technol. 4 (2014) 208.
[63] I. Garcia-Bosch, L. Gomez, A. Polo, X. Ribas, Adv. Synth. Catal. 354 (2012) 65.
[64] N.S. Finney, P.J. Pospisil, S. Chang, M. Palucki, R. G. Konsler, K.B. Hansen, E.N.
Jacobsen, Angew. Chem. Int. Ed. Engl. 36 (1997) 1720.
24

[65] J.T. Groves, J. Lee, S.S. Marla, J. Am. Chem. Soc. 119 (1997) 6269.
[66] L.I. Simandi, Ed., Catalytic Activation of Dioxygen by Metal Complexes, Kluwer
Academic Publishers: Dordrecht, The Netherlands, 1992.
Fig/Scheme captions
Fig. 1 XRD patterns of the synthesized (a) Fe3O4/tart-NPs and (b) Fe3O4-[MnCl(L)]
Scheme 1 The synthesis of Fe3O4-[MnCl(L)]
Fig. 2 FT-IR spectra of the synthesized nanocomposite and the support.
Fig. 3 TGA analyses of the synthesized Fe3O4-L and Fe3O4-[MnCl(L)]
Fig. 4 SEM images of (a) Fe3O4-[MnCl(L)] as-synthesized, (b) and (c) recovered Fe3O4-
[MnCl(L)] after 4 times use as catalyst in the oxidation of α -methylstyrene with O2 (see the next
section).
25

Fig. 5 Magnetization curve of (a) Fe3O4/tart and (b) Fe3O4-[MnCl(L)]. The inset shows
photograph of the separation of the dispersed catalyst Fe3O4-[MnCl(L)] in acetonitrile (left) by
using an external magnet (right).
Fig. 6 EPR spectra for powdered Fe3O4–[MnCl(L)] complex, recorded at room and liquid helium
temperatures.
Fig. 7 Reuse of the catalyst in the aerobic oxidation of α-methylstyrene. Conditions as given in
the footnote of Table 2. SO, α-methylstyrene oxide; BA, benzaldehyde; AP, acetophenone. The
epoxide configuration was S. Total TON for four cycle was 16957.
Fig. 8 Kinetic plot for the oxidation of 1-phenylcyclohexene by the nanocomposite under the
conditions given in Experimental.
Table 1 Oxidation of 1-phenylcyclohexene a
Table 2 Asymmetric oxidation of unfunctionalized olefins catalyzed by Fe3O4-[MnCl(L)] a
26

Fig. 1
27

Scheme 1
28

Fig. 2
29