Mn-Schiff base complex supported on magnetic nanoparticles: Synthesis, crystal structure, electrochemical properties and catalytic activities for oxidation of olefins and sulfides

Article abstract of DOI:10.1016/j.poly.2017.05.049

A novel, magnetically recoverable Mn-Schiff base catalyst has been prepared by the adsorption on silica-coated magnetic nanoparticles. X-ray crystallography studies reveal that the Mn-complex adopt a distorted octahedral six-coordinate configuration formed by the tetradentate Schiff base ligand and two oxygen atoms of acetate anion. The heterogeneous catalyst was characterized by powder X-ray diffraction, transmission electron microscopy, scanning electron microscopy, vibrating sample magnetometer, thermo-gravimetric analysis, the energy-dispersive X-ray spectroscopy and FTIR spectroscopy. The catalyst loading on the magnetic support was determined by atomic absorption spectroscopy. Highly chemoselective oxidation of sulfides with excellent yield to the corresponding sulfoxides, and epoxidation of olefins in ethanol at room temperature were efficiently enhanced under the influence of the magnetically separable catalyst. The separation and recycling of the catalyst is simple, effective, and economic in this clean oxidation method. The electrochemical properties of the Mn-complex were investigated.

Full text of DOI:10.1016/j.poly.2017.05.049

Accepted Manuscript
Mn-Schiff base complex supported on magnetic nanoparticles: Synthesis, crys-
tal structure, electrochemical properties and catalytic activities for oxidation of
olefins and sulfides
Saeed Rayati, Elham Khodaei, Majid Jafarian, Andrzej Wojtczak
PII:
S0277-5387(17)30405-9
DOI:
http://dx.doi.org/10.1016/j.poly.2017.05.049
POLY 12668
Reference:
Polyhedron
To appear in:
Received Date:
Revised Date:
Accepted Date:
13 April 2017
23 May 2017
24 May 2017
Please cite this article as: S. Rayati, E. Khodaei, M. Jafarian, A. Wojtczak, Mn-Schiff base complex supported on
magnetic nanoparticles: Synthesis, crystal structure, electrochemical properties and catalytic activities for oxidation
of olefins and sulfides, Polyhedron (2017), doi: http://dx.doi.org/10.1016/j.poly.2017.05.049
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Mn-Schiff base complex supported on magnetic nanoparticles:
Synthesis, crystal structure, electrochemical properties and
catalytic activities for oxidation of olefins and sulfides
Saeed Rayati,a,* Elham Khodaei,a Majid Jafarian,a Andrzej Wojtczakb
a Department of Chemistry, K.N. Toosi University of Technology, P.O. Box 16315-1618, Tehran
15418, Iran
b Faculty of Chemistry, N. Copernicus University, Gagarina 7, 87-100 Torun, Poland
Abstract
A novel, magnetically recoverable Mn-Schiff base catalyst has been prepared by the adsorption
on silica-coated magnetic nanoparticles. X-ray crystallography studies reveal that the Mn-
complex adopt a distorted octahedral six-coordinate configuration formed by the tetradentate
Schiff base ligand and two oxygen atoms of acetate anion. The heterogeneous catalyst was
characterized by powder X-ray diffraction, transmission electron microscopy, scanning electron
microscopy, vibrating sample magnetometer, thermo-gravimetric analysis, the energy-dispersive
X-ray spectroscopy and FTIR spectroscopy. The catalyst loading on the magnetic support was
determined by atomic absorption spectroscopy. Highly chemoselective oxidation of sulfides with
excellent yield to the corresponding sulfoxides, and epoxidation of olefins in ethanol at room
temperature were efficiently enhanced under the influence of the magnetically separable catalyst.
The separation and recycling of the catalyst is simple, effective, and economic in this clean
oxidation method. The electrochemical properties of the Mn-complex were investigated.
Keywords: Green oxidation, Magnetic nanoparticles, Heterogeneous nanocatalyst, Olefins,
Sulfides
1

* Corresponding author. Tel.: +98 21 22850266; Fax: +98 21 22853650.
E-mail addresses: rayati@kntu.ac.ir, srayati@yahoo.com (S. Rayati).
1. Introduction
Complexation of Schiff base ligands with metal ions is attractive because of physicochemical
properties of metal complexes and broad range of utilization in various areas of science such as
electrochemistry, biological studies and catalytic fields [1-4]. Catalytic oxidation of organic
compounds is an important domain of chemical research for producing valuable oxidation
products. Since sulfoxides have many applications in biological systems such as antifungal,
antibacterial, anti-ulcer, anti-hypertensive and anti-atherosclerotic, the selective oxidation of
sulfides to sulfoxides is an important transformation in organic chemistry [5,6]. Also epoxidation
is one of the typical reactions in organic synthesis which is important to produce various
synthetic products such as drugs, pesticides, epoxy paints, rubber promoters and dyestuffs [7, 8].
Although transition metal complexes are successful homogeneous catalysts, one of the major
disadvantages of homogeneous catalysis systems is the problem of separation and recovery of
the catalysts from the reaction mixture at the end of the process [9–14]. In this sense much effort
has been done to develop adequate supports and heterogeneization methods as various solid
supports include organosilica materials [15, 16], carbon materials [17, 18], silica materials [19–
21], zeolites [22, 23] and layered compounds [24]. However, immobilization of homogeneous
catalysts usually declines the efficiency or selectivity of catalyst. Nanoparticles (NPs) as catalyst
supports could solve this problem by creating semi-homogeneous system. Although surface of
nanoparticles (NPs) is a well-designed method to decrease the distance between heterogeneous
and homogeneous catalysis, but nanoparticles are difficult to be removed from the reaction
2

mixture. This drawback can be solved by magnetic nanoparticles (MNPs), which can be easily
separated. In addition, the notable advantages of MNPs such as easy preparation, low cost, high
surface area, low toxicity and super paramagnetism properties is provoking us to use magnetic
nanoparticles as an ideal support for heterogenization of homogeneous catalysts [25-27]. To
benefit from easy recovery of the catalyst, we decided to immobilize Mn-Schiff base complex on
silica coated magnetic nanoparticles (Fe3O4@SiO2–MnL(OAc)). Selective epoxidation of olefins
to epoxides and oxidation of sulfides to sulfoxides were enhanced by using [Fe3O4@SiO2–
MnL(OAc)] as a novel, magnetically recoverable nanocatalyst in ethanol.
2. Experimental
2.1. Instruments and reagents
Infrared spectra were recorded as KBr pellets using Unicam Matson 1000 FT-IR. A Varian (AA
240) atomic absorption spectrometer was used for manganese determination. X-ray powder
diffraction (XRD) was carried out on a Philips X'Pert diffractometer with CuKα radiation.
Scanning electron microscopy (SEM) images were obtained on a Hitachi S-1460 field emission
scanning electron microscope using Acc voltage of 15 kV. Energy dispersive spectroscopy
(EDX) was obtained on a VEGA3 TESCAN field emission scanning electron microscope using
HV voltage of 20 kV.Transmission electron microscopy (TEM) images were obtained on a Zeiss
EM-900 transmission electron microscopy device using Acc voltage of 80 kV. The oxidation
products were analyzed by HP Agilent 6890 gas chromatograph equipped with a HP-5 capillary
column (phenyl methyl siloxane 30 m × 320 µm× 0.25µm) and a flame ionization detector.
Magnetic properties were obtained on a BHV-55 vibrating sample magnetometer (VSM).
Electrochemical studies of catalysts were carried out in a conventional three-electrode cell
3

powered by an electrochemical system consisting of an EG&G model 273
potentiostat/galvanostat and a solartone model 1255 frequently response analyzer. The system
was run by a pc using the M270 and M398 commercial software packages via a GPIB interface
for impedance measurements. A glassy carbon (GC) disc electrode and graphite electrode were
used as working and counter electrode, respectively. The potentials are relative to the aqueous
Ag/AgCl placed in separate compartment containing 0.1M tetrabutylammonium
hexafluorophosphate (TBAH) in anhydrous acetonitrile. 2,2΄-Dimethylpropylenediamine, 5-
bromo-2-hydroxybenzaldehyde, manganese(II) acetate tetrahydrate, hydrogen peroxide (solution
30% in water) and urea hydrogen peroxide (UHP) were used as received from commercial
suppliers. Solvents were purchased from Merck or Fluka chemical companies.
.
2.2. Preparation of the Schiff base ligand (H2L)
H2L was prepared according to the described procedure [28-29]. To a stirred ethanolic solution
(30 mL) of 2, 2-dimethylpropylenediamine (0.254 g, 2.487 mmol), 5-bromo-2-
hydroxybenzaldehyde (1 g, 4.97 mmol) was added. The mixture was stirred and heated to reflux
for 1 h and the solvent was evaporated to one-fourth. A yellow precipitate was filtered and
washed with ethanol (Yield: 92 %).
2.3. Preparation of manganese (III) complex
An ethanolic solution of manganese(II) acetate (0.294 g, 1.2 mmol) was added to the ethanolic
solution (20 mL) of the Schiff base ligand ( 0.58 g, 1 mmol) and the reaction mixture was
refluxed for 2 h. The solvent was evaporated to one-fourth and the brown powders were filtered
and washed with cold ethanol (Yield: 83 %). The complex [MnL(OAc)] (1mmol) was dissolved
in 15 mL of ethanol and kept for slow evaporation at room temperature. After 5 days, well-
4

shaped dark brown X-ray quality single crystals deposited at the bottom of the vessel. Figure 1
shows the crystal structure of [MnL(OAc)].
Fig. 1. Crystal structure of [MnL(OAc)] complex. Both molecules constituting the asymmetric unit are displayed.
The atom displacement ellipsoids are plotted at 30% probability level. Hydrogen atoms are omitted for clarity.
2.4. X-ray structure of the complex
Brown crystals of [MnL(OAc)] were obtained from the ethanol solution. X-ray diffraction data
were collected at 293(2) K with an Oxford Sapphire CCD diffractometer using MoKα radiation λ
= 0.71073 Å and ω-2θ method. Obtained crystals have been affected by heavy twinning. The
twinning analysis with CrysAlisPro 1.171.38.43 (Rigaku OD, 2015) [30] gave 3 significant
crystal components, with 41797 composite observations collected (12800 fully isolated, 28997
overlapped). The best results of the structure determination have been obtained with data for the
major component. Structure was solved by direct methods and refined with the full-matrix least-
squares method on F2 with the use of SHELX2014 program package [31]. The analytical
absorption correction was applied with CrysAlisPro 1.171.38.43 software [30]. Positions of all
hydrogen atoms have been found from the electron density maps and hydrogen atoms were
constrained during refinement using the appropriate riding mode in SHELXL. The data
collection and refinement processes are summarized in Table 1. Structure has been
deposited with CSD, the deposition number being 1525285
.
5

Table 1
Crystal data and structure refinement for [MnL(OAc)].
Compound [MnL(OAc)]
Formula
Formula weight
T (K)
C21H21Br2MnN2O4
580.16
293(2) K
Wavelength (Å)
Crystal system
Space group
0.71073
Monoclinic
P2(1)/c
Unit cell dimensions
a (Å)
21.218(4)
b (Å)
12.222(2)
c (Å)
20.012(4)
90
117.71(3)
90
α (o)
β (o)
γ (o)
V (Å3)
4594.5(18)
Z
8
Dcalc (Mg/m3)
µ (mm-1)
1.677
4.082
F(000)
2304
Crystal size (mm)
θ Range for data collection (o)
Index ranges
Reflections collected / unique
Completeness to theta = 25.00
Absorption correction
Tmax/Tmin
Refinement method
Data / restraints / parameters
Goodness-of-fit on F2
Final R indices [I>2σ(I)]
R indices (all data)
Difference peak and hole (e Å3)
0.557 x 0.378 x 0.210
2.035 to 28.455
-26<=h<=27, -15<=k<=15, -26<=l<=21
22853 / 9612 [R(int) = 0.1683]
94.4 %
Analytical
0.4859 and 0.2276
Full-matrix least-squares on F2
9612 / 0 / 541
0.925
R1 = 0.0900, wR2 = 0.2044
R1 = 0.2088, wR2 = 0.2515
0.809 and -0.616
2.5. Preparation of the Fe3O4@SiO2–[MnL(OAc)]
Fe3O4 magnetic nanoparticles were prepared by a method reported in the literature [32]. Briefly,
FeCl2.4H2O (2 g, 10 mmol) was dissolved in 30 mL HCl (1M). To this solution 30 mL NaOH (3
M) was added drop wise with continuous stirring until black precipitate was formed. The
precipitate was then washed several times with distilled water. The precipitate was then
separated out using external magnetic field and dried at 100 ºC. The prepared Fe3O4
nanoparticles were then coated with tetraethyl orthosilicate (TEOS). The synthesized Fe3O4 was
suspended in ethanol (35 mL) for 30 min at room temperature. Then, TEOS (4 mL) and aqueous
6

ammonia (25 %, 12 mL) were added slowly to the mixture and sonicated for 10 min. The dark
brown precipitate, Fe3O4@SiO2 was obtained after 24 h stirring at room temperature. The
obtained MNPs (1 g) were dispersed in 50 mL chloroform with sonication. To this mixture
excess amount (1.2 g, 2.06 mmol) of [MnL(OAc)] was added and the resulted mixture stirred for
12 h at room temperature then filtered to obtain a black precipitate, which was washed with
chloroform and finally dried in air [32]. The preparation of Fe3O4@SiO2–[MnL(OAc)] has been
shown in figure 2.
3. Result and discussion
3.1. Characterization of the manganese Schiff base complex [MnL(OAc)]
Comparison of the IR spectra of the complex with the ligand provides evidence for the
coordination mode of ligand to the manganese center. A sharp band appearing at 1628 cm-1 due
to υ(C=N) (azomethine), shifts to lower wave appears at 1614 cm-1. This indicates that
azomethine nitrogen atoms are involved in coordination to the manganese center. IR spectra of
the Mn complex showed two characteristic strong bands in the regions 461 and 413 cm-1, which
assigned to the Mn-O and Mn-N bonds respectively [33]. Electronic spectral data of the ligand
and complex in DMF are summarized in Table 2. The Mn(III) complex shows a weak broad
band in 590 nm, due to the expected d-d transition, and a more intense band in 400 nm due to the
charge transfer band which are characteristics of an octahedral geometry around Mn center [34,
35].
Table 2
UV-Vis data for the ligand and Mn(III) complex.
Compound
H2L
λmax (nm)
(ε, mol−1dm3cm−1)
264(26620)
Assignmen
t
π→π*
333(5060)
n→π*
π→π*
MnL(OAc)
283(31400)
7

372(7300)
400(5240)
590(220)
n→π*
MLCT
d-d
3.2. X-ray structure of [MnL(OAc)]
The asymmetric part of the reported structure consists of two complex molecules.
The molecules are related by the non-crystallographic symmetry determined basing on the
non-H atoms: [0.253 1.000 0.102] 150.86º, shift 6.817 Å [36]. In both complex
molecules, the octahedral MnN2O4 coordination sphere is formed by the four-dentate
Schiff base ligand and bidentately coordinated acetate. Due to the Schiff base
architecture, the imine N atoms occupy the cis positions and the phenolate O also are
positioned cis. One of the phenolate O atoms (O1, O21) is positioned trans to the acetate
oxygen (O3, O23), while the other (O2, O22) is trans to the imine N1, N21. In both
molecules, the shortest coordination bonds are formed by the phenolate O atoms O1, O2
and O21, O22 and range between 1.869(6) and 1.908(6)
Å (Table 3). In each molecule, the
Mn-N bonds are not equivalent, with the bond Mn1-N1 1.972(7) and Mn2-N21 1.984(7) Å
trans to the phenolate O2/O22 being significantly shorter than these Mn1-N2 and Mn2-
N22 trans to acetate O4/O24. In turn, these acetate atoms form Mn-O bonds exceeding
2.27 Å, much longer than the other pair O3, O23 (Table 3). There is a slight difference
between the molecules in the acetate position relative to the Schiff base. In particular, the
angles C21-C20-Mn1 and C51-C50-Mn2 differ significantly, being 175.0(12) and 169.8(11)º,
respectively. The O1-Mn1-O3 and O21-Mn2-O23 angles also differ, being 157.5(3) and
159.1(3)º, respectively. The O2-Mn1-O4 and O1-Mn1-O4 are 94.1(3) and 98.6(3)º, respectively,
while their equivalents in molecule 2 are 91.2(3) and 100.8(3)º.
8

Geometry of the Schiff base ligands is similar in both molecules and is typical for
such systems [37, 38]. The C-Br distances range from 1.890(10) to 1.906(9) Å. However,
two complex molecules differ slightly in their conformation. For molecule 1, the consecutive
torsion angles in the bridging fragment C7-N1-C8-C9-C10-N2-C11 are -109.0(9), -71.1(10),
60.3(11) and 135.6(9)º, while the corresponding values for molecule2 C27-N21-C28-C29-C30-
N22-C31 fragment are -100.0(9), -75.6(9), 60.4(10) and 142.0(8)º. Orientation of the phenolate
moieties relative to the imine group can be described with C1-C6-C7-N1 and N2-C11-C12-C17
of -7.7(15) and -20.5(14)º, respectively, while for molecule 2 the corresponding values are -
6.7(13) and -16.0(13)º. In such conformation, the dihedral angle between two phenolate rings is
74.0(5) and 77.6(5)º, for molecule 1 and 2, respectively.
Analysis of the crystal packing revealed the C-H…π interactions involving phenolic
rings: C10-H10B with C12--C17[-x,1-y,1-z] and C40-H40B with C32--C37[1-x,2-y,1-z], both
the H…Cg distances to the ring gravity centers being 2.56 Å. Also the C-Br…π interactions are
found C4-Br1…Cg(C31--C36)[x,y,z], C14-Br2…Cg(C42--C47)[x,3/2-y,1/2+z] and C34-
Br21…Cg(C1--C6)[x,3/2-y,-1/2+z] with the Br…Cg distances of 3.719(5), 3.809(4) and
3.731(4) Å, respectively.
9

Table 3
Selected bond lengths [Å] and angles [º] for [MnL(OAc].
Molecule 1
Molecule 2
Mn1-O2
1.870(6)
1.908(6)
1.972(7)
2.097(7)
2.117(7)
89.2(3)
176.7(3)
89.7(3)
90.1(3)
157.5(3)
92.1(3)
88.8(3)
114.6(3)
88.9(3)
87.9(3)
94.1(3)
98.6(3)
89.1(3)
58.9(3)
146.6(3)
Mn2-O22
1.869(6)
1.896(7)
1.984(7)
2.060(7)
2.153(7)
88.8(3)
176.6(3)
90.6(3)
89.6(3)
159.1(3)
92.2(3)
89.4(3)
112.6(3)
87.8(3)
88.3(3)
91.2(3)
100.8(3)
92.1(3)
58.4(3)
146.6(3)
Mn1-O1
Mn2-O21
Mn1-N1
Mn2-N21
Mn1-O3
Mn2-O23
Mn1-N2
Mn2-N22
O2-Mn1-O1
O2-Mn1-N1
O1-Mn1-N1
O2-Mn1-O3
O1-Mn1-O3
N1-Mn1-O3
O2-Mn1-N2
O1-Mn1-N2
N1-Mn1-N2
O3-Mn1-N2
O2-Mn1-O4
O1-Mn1-O4
N1-Mn1-O4
O3-Mn1-O4
N2-Mn1-O4
O22-Mn2-O21
O22-Mn2-N21
O21-Mn2-N21
O22-Mn2-O23
O21-Mn2-O23
N21-Mn2-O23
O22-Mn2-N22
O21-Mn2-N22
N21-Mn2-N22
O23-Mn2-N22
O22-Mn2-O24
O21-Mn2-O24
N21-Mn2-O24
O23-Mn2-O24
N22-Mn2-O24
3.3. Characterization of the heterogeneous catalyst (Fe3O4 @ SiO2–[MnL(OAc)])
[MnL(OAc)] is anchored onto the Fe3O4@SiO2 through covalent or hydrogen bonding (Figure
2). Hydrogen bond formation between the OH groups of the silica layer and the O and N atoms
10

of the Schiff base and the coordination of hydroxyl groups to the Mn center were considered as
the possible interactions leading to the immobilization of the Schiff base complex on the silica
coated Fe3O4.
OAC
Br
O
N
O
N
Br
Mn
H
H
Me
Me
Stirred for 24 h
at room temperature
CHCl3
Br
Br
H
H
H
O
O
N
N
Me
Me
Mn
O
O
N
N
Me
Me
OAC
Mn
OR
H
Br
Br
Fig. 2. Schematic representation of the preparation of Fe3O4@SiO2-[MnL(OAc)].
The morphology and shape of Fe3O4, Fe3O4@SiO2 and Fe3O4@SiO2-[MnL(OAc)] have been
investigated by scanning electron microscopy (SEM) (Figure 3a-c). The results show the
spherical morphology of the particles and catalyst were well distributed and the average particle
size was from 53 to 90 nm for Fe3O4@SiO2-[MnL(OAc) [39]. The average grains size of the
Fe3O4 prticles is around 46 nm which increase to 66 nm and 78 nm after covering with SiO2 and
Schiff base complex.
(c)
(a)
(b)
11

Fig. 3. SEM Photographs of (a) Fe3O4 (b) Fe3O4@SiO2 (c)
Fe3O4@SiO2-[MnL(OAc)].
Figure 4a shows transmission electron microscopy (TEM)
images of Fe3O4@SiO2-[MnL(OAc)]. TEM micrograph of
the prepared catalyst shows magnetite nanoparticles were
approximately spherical in nature with sizes ranging from 60 to 80 nm that is in agreement with
other results [40]. The magnetic properties of Fe3O4, Fe3O4@SiO2 and Fe3O4@SiO2-
[MnL(OAc)] were also investigated at room temperature using a vibrating sample magnetometer
(VSM) and gave magnetization saturation values for Fe3O4, Fe3O4@SiO2 and Fe3O4@SiO2-
[MnL(OAc)] of 61.3, 39.93 and 19.3 emu/g, respectively. Hence, the magnetization of Fe3O4
decreased considerably with the attachment of SiO2 and the Schiff base complex (The hysteresis
loops are shown in Figure 4b). Nevertheless, these nanoparticles with paramagnetic
characteristics and high magnetization values can quickly respond to an external magnetic field
and quickly re-disperse when the external magnetic field is removed. This supports their
potential application for targeting and separation [41].
80
(a)
(b)
60
40
20
0
-10000
-5000
0
5000
10000
-20
-40
-60
-80
a
b
c
Applied magnetic field(Oe)
12

Fig. 4. (a) TEM image of the Fe3O4@SiO2-[MnL(OAc)] (b) Magnetization curves at 300 k for (a) Fe3O4, (b)
Fe3O4@SiO2 and(c) Fe3O4@SiO2-[MnL(OAc)].
Figure 5a and b show thermo-gravimetric analysis (TGA) of [MnL(OAc)] and Fe3O4@SiO2-
[MnL(OAc)]. The TGA curve of the bare [MnL(OAc)] shows two steps, a small weight loss
attributed to the loss of the acetate group and a sharp intense peak is assigned for decomposition
of the complex. In addition, the TGA of Fe3O4@SiO2-[MnL(OAc)] indicates weight loss below
200 °C which may be due to the loss of adsorbed water. The mass loss in the range of 250–692
°C is attributed to decomposition of the Schiff base and organic volatile compounds, (Figure 5b).
On the basis of these results grafting of the Schiff base onto the Fe3O4@SiO2 is verified [42].
9
8.5
8
16
15
14
13
12
11
(b)
(a)
Fe3O4@SiO2-MnL(OAc)
MnL(OAc)
7.5
7
6.5
6
0
200
400
600
800
0
200
400
Temprature(°C)
600
800
Temprature (°C)
Fig. 5. (a) TGA curves of the MnL(OAc), (b) Fe3O4@SiO2-[MnL(OAc)].
Figure 6 shows the FTIR spectra of Fe3O4@SiO2 and Fe3O4@SiO2-[MnL(OAc)].
13

Fig. 6. IR spectra of (a) Fe3O4@SiO2 and (b) Fe3O4@SiO2-[MnL(OAc)].
Broad peak in the region 1100-1162 cm-1 is assigned to the stretching vibrations of Si-O-Si. By
immobilization of the complex on the surface of Fe3O4@SiO2, peaks in 1633 cm-1and 1530 cm-
1can be related to the C=N stretching vibrations and C=C bonds respectively [43]. The Mn-
content of the supported catalyst determined by atomic absorption spectroscopy revealed that the
metal content of the catalyst is 0.6 mmol per gram of the catalyst.
311
511
(a)
(b)
220
422
400
Fig. 7. (a) The powder XRD pattern and (b) The energy-dispersive X-ray spectroscopy (EDX) spectrum of
Fe3O4@SiO2-[MnL(OAc)].
The crystalline structure of the prepared magnetic nanoparticles was characterized with XRD
(Figure 7a). The relatively strong diffraction peaks at 2θ: 31ᵒ, 36ᵒ, 44ᵒ, 58ᵒ, 63ᵒ and 75ᵒ could be
assigned to the (220), (311), (400), (422), (511) and (440) crystallographic faces of magnetite
(reference JCPDS card no. 19-629), which agreed well with the crystallographic planes of Fe3O4
and indicated the presence of the Fe3O4 core with a highly crystalline cubic spinel structure [44].
14

EDX spectrum of Mn-Salen-MNPs (Figure 7b), showed the presence of N, O and Fe as well as
Mn, species in the catalyst.
3.4. General procedure for catalytic oxidation of sulfides with urea-hydrogen peroxide
In a typical procedure, to a mixture of methyl phenyl sulfide (0.27 mmol, 31.72 µL) and catalyst
(0.0027 mmol, 0.0045 g) in ethanol (1 mL), imidazole (0.351 mmol, 0.0238 g), UHP (0.054
mmol, 0.05 g) and acetic anhydride (0.81 mmol, 76 µL) was added. The reaction mixture was
stirred for a 5 min at room temperature. The progress of the reaction was monitored by GC.
3.5. General procedure for catalytic oxidation of alkenes with hydrogen peroxide
In a typical procedure, to a mixture of cyclooctene (0.081 mmol, 11.72 µL) and catalyst (0.0027
mmol, 0.0045 g):) in ethanol (1 mL), imidazole (0.21 mmol, 0.0137 g), H2O2 (0.81mmol, 63.3
µL) and acetic anhydride (1.21 mmol, 114 µL) was added. The reaction was stirred for 4h at
room temperature. The progress of the reaction was monitored by GC.
3.6. Catalytic activity of the synthesized catalyst
The catalytic activity of the Fe3O4@SiO2-[MnL(OAc)] for oxidation of sulfides and alkenes with
hydrogen peroxide or urea hydrogen peroxide were investigated.
3.6.1. Oxidation of sulfides
Blank experiment (no catalyst) showed that oxidation of methyl phenyl sulfide with urea
hydrogen peroxide (UHP) led to the formation of only 7% product.
In order to achieve the appropriate condition for the oxidation of sulfides, different reaction
conditions such as amount of imidazole as co-catalyst, amount of acetic anhydride as activator of
oxidant and the concentration of oxidant on the reaction conversion has been investigated.
15

Methyl phenyl sulfide was used as sample and ethanol was selected as green solvent for catalytic
reactions.
The effect of imidazole concentration on the oxidation of methyl phenyl sulfide with UHP in the
presence of Fe3O4@SiO2-[MnL(OAc)] has been studied (Fig. 8). An increase in Im/Catalyst
molar ratio, the rate of conversion of methyl phenyl sulfide remarkably increased. The highest
sulfide conversion was observed in 130:1 (Im/Catalyst) molar ratio. However, addition of ImH
beyond the 130:1 ratio leads to no observable change in conversions for both neat [MnL(OAc)]
or Fe3O4@SiO2-[MnL(OAc)].
100
Heterogeneous Catalyst
80
60
40
20
0
0
25
50
75 100 125 130 150
mmol imidazole
Fig. 8. Effect of various ImH/catalyst molar ratios on the oxidation of methyl phenyl sulfide catalyzed by
Fe3O4@SiO2-[MnL(OAc)]. Reaction conditions is catalyst (0.0027 mmol, 0.0045 g):imidazole (X):methyl phenyl
sulfide (0.27 mmol, 31.72 µL):UHP (0.054mmol, 0.05 g):acetic anhydride (0.81 mmol, 76 µL at room temperature
for a 5 min reaction time.
In the absence of an oxidant activator, the reaction does not proceed, while in the presence of
acetic anhydride 100 % methyl phenyl sulfoxide was obtained. In order to find the optimum
amount of acetic, oxidation of methyl phenyl sulfide was performed using different molar ratios
of acetic anhydride and 1:1.5 molar ratio was chosen on the oxidation of methyl phenyl sulfide
(Table 4).
Table 4
16

Illustrates the influence of acetic anhydride/oxidant molar ratio on the oxidation of methyl phenyl sulfide by
Fe3O4@SiO2-[MnL(OAc)].a
Acetic
anhydride/Oxidant
Conversion %
(Neat
[MnL(OAc)])
Conversion %
(Fe3O4@SiO2-
[MnL(OAc)])
1:0
0
0
1:1
58
98
65
100
1:1.5
aReaction conditions: catalyst (mmol):imidazole:sulfide:UHP:acetic anhydride: is 1:130:100:200:X at room
temperature for 5 min reaction time
The effect of different amounts of oxidant has been examined and the results indicate that
maximum conversion was obtained at 2:1 molar ratio of UHP to sulfide (Figure 9).
Fe3O4@SiO2-[MnL(OAc)]
100
80
60
40
20
0
1:1
1:2
1:3
sulfide/UHP
Fig. 9. The influence of sulfide/UHP molar ratio on the oxidation of methyl phenyl sulfide by Fe3O4@SiO2-
[MnL(OAc)]. Reaction conditions: catalyst (mmol): imidazole:sulfide:UHP:acetic anhydride is 1:130:100:X:300 at
room temperature for a 5 min reaction time.
The heterogeneous catalyst can be magnetically separated and reused multiple times without
significant loss of catalytic activity. The stability of the magnetic nanocatalyst was monitored
using a multiple sequential oxidation reaction. For each one of the repeated reactions, the catalyst
was separated, washed with chloroform and ethanol and dried before being used. The catalyst
was consecutively reused eight times without detectable catalyst leaching or significant loss of
activity (Table 5).
Table 5
17

Recycling of the catalytic system in the oxidation of thioanisole.a
Run
Conversiona %
Selectivity %
100
Cycle 1
Cycle 2
Cycle 3
Cycle 4
Cycle 5
Cycle 6
Cycle 7
Cycle 8
100
100
100
100
98
100
100
100
100
100
97
95
93
100
100
aReaction conditions: catalyst:imidazole:sulfide:UHP:acetic anhydride is 1:130:100:200:300 for a 5 min reaction
time.
The oxidation activity of this clean catalytic system towards organic sulfides has been evaluated
(Table 6). This catalytic system is very efficient for the oxygenation of sulfides to sulfoxides. It
leads to the complete conversion of thioanisole, 2-chlorothiosanisole, 4-chlorothiosanisole,
diphenyl and benzyl phenyl sulfides to the corresponding sulfoxides in excellent yields (100%)
and excellent selectivities (100%) within 5 min at room temperature.
Table 6.
Oxidation of various sulfides by Fe3O4@SiO2–[MnL(OAc)]a
Substrates
Conversion %
Selectivity %
(Sulfoxide)
S
H3C
100
100
Cl
S
100
100
H3C
S
H3
C
100
93
100
100
Cl
S
H3
C
F
18

S
CH2
29
100
100
100
98
S
100
S
OCH3
98
89
100
100
H3
C
S
CN
H3C
S
aReaction conditions: The molar ratio for catalyst:imidazole:sulfide:UHP:acetic anhydride is 1:130:100:200:300.
The reactions were run for 5 min at room temperature.
3.6.2. Oxidation of alkenes
Blank experiments show that in the oxidation of cyclooctene with hydrogen peroxide proceeded
sluggishly and gave only 9 % yield of cyclooctene oxide after 4 h at 25 ᵒC. Also we performed a
blank experiments in the presence of neat Fe3O4 and Fe3O4@SiO2 at 25 °C, which afforded 5 %
and 4 % cyclooctene oxide respectively after 4 h (See S1). Oxidation of cyclooctene with UHP
or H2O2 in the presence of catalytic amounts of Fe3O4@SiO2-[MnL(OAc)] led to the formation
of 59% or 73% cyclooctene oxide as the sole product (Table 7). Therefore, H2O2 was selected as
oxidant for further investigation.
Table 7
Oxidation of cyclooctene with UHP or H2O2 in the presence of Fe3O4@SiO2-MnL(OAc).a
mmol
alkene/oxidant
Conversiona
Conversion %
(H2O2)
%
(UHP)
11
21
37
1:1
1:2
1:3
1:4
18
30
39
56
45
19

1:5
59
73
aReaction conditions: catalyst (mmol):imidazole:cyclooctene:oxidant:acetic anhydride is 1:75:30:X:450 at room
temperature for a 4 h reaction time.
Our investigation showed that the efficiency of the epoxidation reaction was affected
significantly by amount of imidazole, acetic anhydride, oxidant, and reaction time. The effect of
various ImH/catalyst molar ratios on the oxidation of cyclooctene has been studied (See S2,
Supplementary materials). In the absence of imidazole, the reaction proceeds only 17%, while an
increase in Im/catalyst molar ratio up to 75 remarkably improved the rate of cyclooctene
epoxidation. However, its co-catalytic activity decreased at Im/catalyst ratio >75.
Using different amounts of acetic anhydride show that in the 1:1.5 molar ratio of hydrogen
peroxide to acetic anhydride, complete conversion of cyclooctene to cyclooctene oxide will
occur (See S3).
In order to find a desired reaction time for the oxidation of cyclooctene, several reaction times
were investigated (See S3) and the highest conversion was obtained after 4 h.
Recovery of the Fe3O4@SiO2-[MnL(OAc)] was easy and efficient. The catalyst was recovered
by decantation of the reaction mixture in the presence of an external magnet. After washing with
ethanol, it was used directly in the next reaction without further purification. The ease of
recovery, combined with the intrinsic stability of the silica-protected Fe3O4 nanoparticle
component, allows the catalyst to be recovered efficiently at least eight times in the oxidation of
different substrates (See S4).
The scope of the protocol was subsequently extended to a range of olefins, as shown in figure 10.
Excellent conversions and selectivities to epoxide were generally obtained within 4h. Complete
conversion of cyclooctene, cyclohexene, 1-methyl cyclohexene, 4-methyl styrene and 4-chloro
styrene was achieved with the formation of the corresponding epoxides as the sole products
20

(entries 1–3, 5, 7). A notable feature of the nanocatalyst is the desired epoxide yield obtained in
the oxidation of linear alkene (1-octene) (entry 9, 83%). The higher conversion was obtained in
the presence of Fe3O4@SiO2–[MnL(OAc)] with respect to the neat [MnL(OAc)] confirms the
superiority of the supported catalyst over the free catalyst.
100
O
O
O
CHO
H3C
O
CH3
CH3
O
CHO
CHO
Cl
CHO
5
80
60
40
20
0
H3CO
H3C
CH3
Cl
H3C
H3CO
1
2
3
4
5
6
7
8
9
10
a
b
100
100
100
100
100
89
100
100
100
100
90
83
56
100
100
100
100
100
100
100
Fig. 10. Epoxidation of alkenes by (a) Fe3O4@SiO2–[MnL(OAc)] and b is the selectivity of heterogeneous catalysts
respectively. Reaction conditions: catalyst (mmol):imidazole: cyclooctene:H2O2:acetic anhydride is 1:75:30:300:450
at room temperature for a 4 h reaction time.
3.7. Electrochemical studies
The electrochemical properties of the manganese complex heterogeneous catalyst for the
oxidation of methyl phenyl sulfide or cyclooctene in the potential range of -1 to 3 V in CH3CN
containing 0.1 M tetrabutylammonium hexafluorophosphate (TBAH) as the supporting electrode
at scan rate of 100 mVs-1 have been investigated (Figure 11). An increase in the current intensity
21

in the presence of methyl phenyl sulfide and Fe3O4@SiO2-[MnL(OAc)] (Fig. 11a and Fig. 12 a)
compared to the blank experiments indicates the catalytic activity of the catalysts.
Chronoamperometry is also be used to confirm the catalytic activity of the catalysts.
Chronoamperograms of the acetonitrile solution of the Fe3O4@SiO2-[MnL(OAc)] (red line) in
the peak potential and in the presence of methyl phenyl sulfide (Figure 11b) or cyclooctene
(Figure 12b) indicate the higher catalytic activity of the supported catalyst compare to the
unsupported one.
350
Fe3O4@SiO2-[MnL(OAc)]
blank
900
700
500
300
100
-100
Fe3O4@SiO2-[MnL(OAc)]
blank
300
250
200
150
100
50
0
-1
0
1
2
3
0
10
20
30
40
50
-50
V
(a)
(b)
t/s
Fig. 11. (a) the voltammetric behavior of the Fe3O4@SiO2-[MnL(OAc)] and blank between -1 and 3 V and (b)
chronoamperograms of Fe3O4@SiO2-MnL(OAc) and blank in acetonitrile solution containing. 0.1 M TBAH as the
supporting electrolyte. Reaction conditions is catalyst (0.0027 mmol, 0.0045 g):imidazole (0.351 mmol, 0.0238
g):methyl phenyl sulfide (0.27 mmol, 31.72 µL):UHP (0.054mmol, 0.05 g):acetic anhydride (0.81 mmol, 76 µL at
room temperature for a 5 min reaction time.
Similar results were obtained in the oxidation of the cyclooctene and in the presence of
Fe3O4@SiO2-[MnL(OAc)]. The growth of current for heterogeneous catalyst with respect to the
homogeneous one indicates the superiority of the supported catalyst over the free catalyst (Se
S5).
22

1200
1000
800
600
400
200
0
Fe3O4@SiO2-[MnL(OAc)] MnL(OAc)
blank
Fe3O4@SiO2-[MnL(OAc)] MnL(OAc)
blank
3.00E-05
2.70E-05
2.40E-05
2.10E-05
1.80E-05
1.50E-05
1.20E-05
9.00E-06
6.00E-06
3.00E-06
0.00E+00
-1
0
1
2
3
0
10
20
30
40
50
-200
(a)
V
t/s
(b)
Fig. 12. (a) the voltammetric behavior of the Fe3O4@SiO2-[MnL(OAc)] and blank between -1 and 3 V and (b)
chronoamperograms of Fe3O4@SiO2-MnL(OAc) and blank in acetonitrile solution containing. 0.1 M TBAH as the
supporting electrolyte. Reaction conditions is catalyst (0.0027 mmol, 0.0045 g):imidazole (0.21 mmol, 0.0137
g):cyclooctene (0.081 mmol, 11.72 µL):H2O2 (0.81mmol, 63.3 µL):acetic anhydride (1.21 mmol, 114 µL at room
temperature for a 1:75:30:300:450 at room temperature time between -1 and 3 V.
4. Conclusion
In this study, a tetradenate Schiff base (N2O2) ligand and its corresponding manganese(III)
complex ([MnL(OAc)]) synthesized and characterized. [MnL(OAc)] was supported on the
surface of silica coated magnetic noparticles which led to the formation of Fe3O4@SiO2-
[MnL(OAc)]. The potential catalytic activity of Fe3O4@SiO2 MNPs-[MnL(OAc)] as
heterogeneous catalyst for oxidation of sulfides and alkenes was examined by UHP and H2O2 as
green oxidants in ethanol by approaching to the green chemistry. The heterogeneous catalyst was
reused multiple consecutive times without loss in activity or selectivity of nanocatalyst. In
addition, the electrochemical behavior of Fe3O4@SiO2-[MnL(OAc)] was investigated in
different condition by cyclic voltammetry and chronoamperometry.
Acknowledgements
23

The financial support of this work by K.N. Toosi University of Technology research council is
acknowledged.
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25

Mn-Schiff base complex supported on magnetic nanoparticles: Synthesis,
crystal structure, electrochemical properties and catalytic activities for
oxidation of olefins and sulfides
Saeed Rayati, Elham Khodaei, Majid Jafarian, Andrzej Wojtczak
R’
O
S
C
C
O
R
R’
C
S
C
R
26

Mn-Schiff base complex supported on magnetic nanoparticles: Synthesis,
crystal structure, electrochemical properties and catalytic activities for
oxidation of olefins and sulfides
Saeed Rayati, Elham Khodaei, Majid Jafarian, Andrzej Wojtczak
The synthesis, crystal structure, electrochemistry behaviour and catalytic performance of a novel
magnetically separable Mn-Schiff base complex in the selective oxidation of olefins and sulfides
in ethanol are described.
27