A manganese(iii) Schiff base complex immobilized on silica-coated magnetic nanoparticles showing enhanced electrochemical catalytic performance toward sulfide and alkene oxidation

Article abstract of DOI:10.1039/d0ra02728f

In this study, a novel Mn(iii)-Schiff base complex was synthesized and characterized. The structure of this complex was determined to be a deformed octahedral coordination sphere by single-crystal X-ray diffraction analysis. The Mn(iii)-Schiff base complex was supported on silica-coated iron magnetic nanoparticlesviaaxial coordination by one-step complex anchoring to produce a heterogenized nanocatalyst. After this, the complex was characterized by Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX), thermogravimetric analysis (TGA), vibrating sample magnetometry (VSM), and powder X-ray diffraction (XRD). Moreover, atomic absorption spectroscopy was used to determine the amount of the loaded metal. The heterogenized nanocatalyst effectively catalyzed the oxidation of a broad range of sulfides and alkenes with H₂O₂in the presence of a glassy carbon electrode, applying voltage to the reaction mixture. The results showed that the application of a potential to the reaction mixture could significantly decrease the reaction time when compared with the case of similar chemical oxidation reactions. In addition, an excellent value of turnover frequency (17?750 h^-1) was achieved for the electrochemical oxidation of styrene. Moreover, the nanocatalyst showed good recoverability without significant loss of its activity within six successive runs in the electrochemical oxidation of methyl phenyl sulfide and cyclooctene. The electrochemical properties and stability of Fe₃O₄?SiO₂-[MnL(OAc)] were investigated by cyclic voltammetry measurements and chronoamperometry technique.

Full text of DOI:10.1039/d0ra02728f

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A manganese(III) Schiff base complex immobilized
on silica-coated magnetic nanoparticles showing
enhanced electrochemical catalytic performance
toward sulfide and alkene oxidation†
Cite this: RSC Adv., 2020, 10, 17026
a
Elham Khodaei,a Parinaz Nafarieh,a Majid Jafarian,a Bahareh Elmia
*
Saeed Rayati,
b
and Andrzej Wojtczak
In this study, a novel Mn(III)–Schiff base complex was synthesized and characterized. The structure of this
complex was determined to be a deformed octahedral coordination sphere by single-crystal X-ray
diffraction analysis. The Mn(III)–Schiff base complex was supported on silica-coated iron magnetic
nanoparticles via axial coordination by one-step complex anchoring to produce a heterogenized
nanocatalyst. After this, the complex was characterized by Fourier transform infrared spectroscopy
(FTIR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive
X-ray spectroscopy (EDX), thermogravimetric analysis (TGA), vibrating sample magnetometry (VSM), and
powder X-ray diffraction (XRD). Moreover, atomic absorption spectroscopy was used to determine the
amount of the loaded metal. The heterogenized nanocatalyst effectively catalyzed the oxidation of
a broad range of sulfides and alkenes with H2O2 in the presence of a glassy carbon electrode, applying
voltage to the reaction mixture. The results showed that the application of a potential to the reaction
mixture could significantly decrease the reaction time when compared with the case of similar chemical
oxidation reactions. In addition, an excellent value of turnover frequency (17 750 hꢀ1) was achieved for
the electrochemical oxidation of styrene. Moreover, the nanocatalyst showed good recoverability
without significant loss of its activity within six successive runs in the electrochemical oxidation of methyl
phenyl sulfide and cyclooctene. The electrochemical properties and stability of Fe3O4@SiO2-[MnL(OAc)]
were investigated by cyclic voltammetry measurements and chronoamperometry technique.
Received 24th March 2020
Accepted 16th April 2020
DOI: 10.1039/d0ra02728f
rsc.li/rsc-advances
One of the widely used models of catalytic systems is the
oxidation of organic substrates using Schiff base complexes
Introduction
immobilized on the surface of a solid support. In addition, solid
supports allow easy separation and recovery of the catalyst from
the reaction mixture such that the catalyst can be reused several
times, which is an approach to the green chemistry.11–14
In recent decades, iron oxide magnetic nanoparticles (Fe3O4)
have attracted interest due to their chemical resistance, super-
paramagnetic features, high level contact, cost-effectiveness,
and environmental friendliness.15–18 Because of the special
features of these magnetic nanoparticles, they can be used in
different elds such as magnetic resonance imaging (MRI),19
sensing,20 pharmacology,21 and catalysis.22–24 Due to these
particular characteristics of the iron oxide magnetic nano-
particles, they can be developed as a practical solid support
having stability under reaction conditions and easy separation
by an external magnet.25–29
Over the past few years, several studies have been reported on
the applications of Schiff-base ligands and their transition
metal complexes in homogeneous and heterogeneous catal-
ysis.1–5 Schiff base complexes show excellent catalytic activity in
various reactions. Since manganese complexes are highly active
in oxidation processes, they have been widely studied as
homogeneous catalysts for olen epoxidation in the presence of
various oxygen donors.6–9 However, they suffer from some
disadvantages such as the problem of separation and recovery
from the reaction mixture at the end of the process.10 Hence,
designing catalytic systems of Schiff base complexes with easy
recyclability is still a challenging task.
aDepartment of Chemistry, K. N. Toosi University of Technology, P. O. Box 16315-1618,
Tehran 15418, Iran. E-mail: rayati@kntu.ac.ir; Fax: +98 21 22853650; Tel: +98 21
22850266
Comprehensively, the electrocatalytic performances of tran-
sition metal complexes in the oxidation processes and electro-
chemical oxidation methods have become popular research
bFaculty of Chemistry, N. Copernicus University, Gagarina 7, 87-100 Torun, Poland
† Electronic supplementary information (ESI) available. CCDC 1977428. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/d0ra02728f
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topics. To date, many reports of electrocatalytis in industrially electrode and graphite electrode were used as the working and
important reactions are available.30–32
counter electrodes, respectively. The potentials are relative to
The study reported herein focused on the synthesis and the aqueous Ag/AgCl placed in a separate compartment con-
immobilization of a Mn–Schiff base complex on the surface of taining 0.1 tetrabutylammonium hexauorophosphate
M
silica-coated Fe3O4 magnetic nanoparticles (MNPs) to obtain (TBAH) in anhydrous acetonitrile.
a heterogenized nanocatalyst. Furthermore, in our pervious
investigation, the catalytic activity of another Mn–Schiff base
complex supported on silica-coated magnetic nanoparticles has
been investigated in the chemical oxidation of suldes and
Preparation of bis(2-hydroxy-3-methoxybenzaldehyde)-2,2-
dimethylpropanediimine (H2L)
alkenes with hydrogen peroxide.32 In this study, the catalytic H2L was prepared according to a previously described proce-
activity of the prepared heterogenized nanocatalyst was exam- dure.33 An ethanolic solution (20 mL) containing 2,2-dimethyl-
ined in the electrochemical oxidation of suldes and alkenes in propylenediamine (1 mmol, 0.102 g) and 2-hydroxy-3-
the presence of a glassy carbon electrode as a new modied methoxybenzaldehyde (2 mmol, 0.304 g) was reuxed for 1 h.
procedure. Moreover, the reusability and the electrochemical The yellow H2L crystals were ltered from the reaction mixture,
ꢂ
properties of the heterogenized nanocatalyst were tested.
washed, and dried at 80 C. Anal. calc. for C21H26N2O4 (MW ¼
370.44 g molꢀ1). Yield: 92%. Mp: 110 ꢂC. Required: C, 68.21; H,
7.16; N, 7.49%. Found: C, 68.28; H, 7.07; N, 7.56%. 1H NMR
(DMSO-d6, 300 MHz), d(ppm): 14.14 (s, 2H, C–OH); 6.7–6.95 (m,
6H, aromatic C–H); 8.32 (s, 2H, N]CH); 3.91 (s, 6H, OCH3); 3.48
(s, 4H, NCH2); 1.07 (s, 6H, CCH3) ppm. 13C NMR (DMSO-d6, 300
MHz), d(ppm): 165.78 (C]N); 113.76–152.02 (aromatic C); 67.29
(OCH3); 56.01 (CH2); 37.48 (CH2CCH3); 24.23 (CH3) ppm. IR,
nmax, cmꢀ1: n(O–H)arom 3510; nsym(C–H)arom 2960; nsym(C]N)
1628; nsym(C]C) 1528; nsym(C–O) 1054 cmꢀ1 (KBr disk). UV-Vis,
lmax, nm (3, molꢀ1 dm3 cmꢀ1): 221 (2900); 270 (1600) (in a 5 ꢁ
10ꢀ4 M CHCl3 solution).
Experimental
Chemicals
The chemicals, reagents, and solvents were purchased from
Aldrich, Merck or Fluka chemical companies. 2,2-Dimethyl-
propylenediamine, 2-hydroxy-3-methoxybenzaldehyde, man-
ganese(II) acetate tetrahydrate, iron(II) chloride tetrahydrate,
hydrogen peroxide (30% solution in water), imidazole, and all
the organic substrates, including suldes and alkenes, were
used as received from commercial suppliers without further
purication.
Preparation of the Mn(III)–Schiff base complex [MnL(OAc)]
Instruments
[MnL(OAc)] was synthesized by adding Mn(II) acetate
The UV-Vis absorption spectra were acquired using the Unico (Mn(OAc)2$4H2O, 1.2 mmol, 0.2 g) to an ethanolic solution (20
1
4802 double beam spectrometer. H NMR and 13C NMR anal- mL) of the prepared Schiff base ligand (H2L, 1 mmol, 0.37 g),
yses were carried out by the 300 MHz BRUKER spectrometer in and the resulting mixture was reuxed for 2 h. The dark green
DMSO. Infrared spectra were obtained using KBr pellets by precipitate of [MnL(OAc)] was ltered, washed with cold
Unicam Matson 1000 FTIR in the 400–4000 cmꢀ1 range. Scan- ethanol, and dried at 80 ꢂC. C23H27MnN2O6 (MW ¼ 482.41 g
ning electron microscopy (SEM) images and energy dispersive molꢀ1). Yield: 92%. Mp: 222 ꢂC. IR, nmax, cmꢀ1: nsym(C]N) 1617;
X-ray (EDX) spectra were obtained by the VEGA3 TESCAN eld nsym(C]C) 1549; nsym(C–O) 1070 cmꢀ1 (KBr disk). UV-Vis,
emission scanning electron microscope using an HV voltage of lmax, nm (3, molꢀ1 dm3 cmꢀ1): 240 (3800); 280 (1400); 390
20 kV. Transmission electron microscopy (TEM) images were (780) (in a 5 ꢁ 10ꢀ4 M CHCl3 solution).
obtained via the Zeiss EM-1030 transmission electron micros-
copy device using a voltage of 100 kV. Thermogravimetric
analysis (TGA/DTGA) was performed using the Mettler Toledo
Crystal structure determination
instrument. Magnetic properties were examined by the BHV-55 Crystals were obtained from the CH2Cl2 : MeOH (1 : 1) solution.
vibrating sample magnetometer (VSM). X-ray powder diffraction X-ray diffraction data were acquired via the Oxford Sapphire
˚
(XRD) was carried out using the Philips PW 3710 diffractometer CCD diffractometer using Mo Ka radiation, l ¼ 0.71073 A, at
with Cu Ka radiation. A Varian (AA240) atomic absorption 293(2) K by the u–2q method. The structure was solved by direct
spectrometer was used for metal determination. The Agilent methods and rened by the full-matrix least-squares method on
7890B instrument with an SAB-5 capillary column (phenyl F2 using the SHELX2017 (ref. 34) program package. The
methyl siloxane; 30 m ꢁ 0.32 mm ꢁ 0.25 mm) and a ame analytical absorption corrections were applied (Table 1) (Cry-
ionization detector was used to perform gas chromatography sAlisPro 1.171.38.43 soware),35 and the maximum and
experiments (GC). The electrochemical studies of catalysts were minimum transmissions were 0.921 and 0.747, respectively.
carried out in a conventional three-electrode cell powered by an The hydrogen atoms were located from the electron density
electrochemical system consisting of an EG&G model 273 maps, and their positions were constrained in the renement
potentiostat/galvanostat and a Solartron model 1255 frequency with appropriate AFIX restraints as implemented in SHELX. The
response analyzer. The system was run by a PC using the M270 [MnL(OAc)] structure has been deposited at the Cambridge
and M398 commercial soware packages via a GPIB interface Crystallographic Data Centre, and the deposition number is
for impedance measurements. A glassy carbon (GC) disc CCDC 1977428.†
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Table 1 Crystal data and structure refinement of [MnL(OAc)]
aqueous solution of NaOH (60 mL, 3 M) was added dropwise
until black precipitates appeared. The resulting precipitates
were ltered, washed several times with distilled water until
Empirical formula
Formula weight
Temperature; K
C23H27MnN2O6
482.40
293(2)
0.71073
ꢂ
neutral pH was achieved, and then dried at 100 C.
˚
In the next step, the prepared Fe3O4 MNPs were dispersed in
ethanol (50 mL) by 30 min sonication. To this suspension, 8 mL
of TEOS and 24 mL of aqueous ammonia (25%) were slowly
added, and the mixture was sonicated for 10 min followed by
stirring for 24 h at room temperature. The dark brown precip-
itates of Fe3O4@SiO2 were separated by an external magnet,
Wavelength; A
Crystal system
Space group
Unit cell dimensions; A,
Monoclinic
P21/c
ꢂ
˚
a ¼ 9.1400(9)
b ¼ 12.8784(12)
c ¼ 19.5628(19)
a ¼ 90
washed with distilled water, and dried at 100 C.36
ꢂ
b ¼ 95.859(11)
g ¼ 90
3
˚
Volume; A
2290.7(4)
Z
4
Preparation of Fe3O4@SiO2-[MnL(OAc)]
Density (calculated); mg mꢀ3
Absorption coefficient; mmꢀ1
F(000)
1.399
0.617
1008
To prepare the heterogenized nanocatalyst, the as-prepared
silica-coated Fe3O4 MNPs (Fe3O4@SiO2, 1 g) were dispersed in
50 mL chloroform by sonication. Subsequently, to this
suspension, 0.6 g of [MnL(OAc)] was added, and the mixture was
stirred for 12 h at room temperature. The Fe3O4@SiO2-
[MnL(OAc)] precipitates were then ltered, washed with chlo-
roform, and nally dried at 80 ꢂC.32 A schematic of the synthesis
routes of Fe3O4@SiO2-[MnL(OAc)] is shown in Scheme 1.
The quantity of [MnL(OAc)] immobilized via axial coordina-
tion was determined by measuring the manganese content of
the prepared catalyst Fe3O4@SiO2-[MnL(OAc)] by AAS. The
average particle size was calculated by the Debye–Scherrer
equation.
Crystal size; mm
Theta range for data collection
Index ranges
0.611 ꢁ 0.411 ꢁ 0.146
2.240–28.656ꢂ
ꢀ12 # h # 11,
ꢀ16 # k # 16, 25 # l # 25
15 348
5222 [R(int) ¼ 0.0377]
25.242ꢂ, 100.0%
Analytical
Reections collected
Independent reections
Completeness to theta
Absorption correction
Max. and min. transmission
Renement method
0.921 and 0.747
Full-matrix least-squares on F2
5222/0/289
Data/restraints/parameters
Goodness-of-t on F2
1.055
Final R indices [I > 2sigma(I)]
R indices (all data)
Largest diff. peak and hole; e A
R1 ¼ 0.0418, wR2 ¼ 0.0949
R1 ¼ 0.0591, wR2 ¼ 0.1020
0.379 and ꢀ0.416
ꢀ3
˚
Catalytic reactions
The oxidation reactions of suldes and alkenes were carried out
in a 25 mL glass beaker containing the prepared heterogenized
catalyst (0.0017 mmol), substrate (0.51 mmol of sulde or
0.255 mmol of alkene), imidazole (0.17 mmol), and H2O2
(2.55 mmol for sulde oxidation and 1.275 mmol for alkene
oxidation) in ethanol (1 mL) in the presence of a glassy carbon
Preparation of Fe3O4@SiO2 MNPs
The Fe3O4 MNPs were synthesized according to the following
procedure. In a typical procedure, FeCl2$4H2O (4 g, 20 mmol)
was dissolved in HCl (60 mL, 1 M), and to this mixture, an
Scheme 1 Schematic of the preparation of Fe3O4@SiO2-[MnL(OAc)].
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electrode, applying a voltage of 60 V to the reaction mixture for positioning of the O5 atom in the corner of the octahedron and
90 second. The progress of the reaction was monitored by GC. rigid valence geometry of the carboxylate group of acetate. Since
the four-dentate coordination of L makes the pairs of the O
and N atoms of L occupy the cis positions in the coordination
sphere, the signicant angular deformations of the octahedral
geometry are related to the position of the acetate ligand. The
largest angular deformation from the ideal octahedral geometry
arises from the bidentate coordination of acetate ion, resulting
in the O5–Mn1–O6 angle of 59.37(6)ꢂ. In addition, the trans
angles involving the acetate O atoms are signicantly deviated
from the expected angle of 180ꢂ, with the O2–Mn1–O5 angle of
159.56(6)ꢂ and N1–Mn1–O6 angle of 150.84(6)ꢂ. Contrarily, the
O1–Mn1–N2 angle of 178.81(7)ꢂ is close to the theoretical value.
The other cis angles within the coordination sphere range from
87.67(7)ꢂ (O2–Mn1–N2) to 108.61(7)ꢂ (O2–Mn1–N1).
Recycling procedure
Aer completion of the reaction, the catalyst was separated
from the reaction mixture by an external magnet, washed 3
times with ethanol, and dried at 80 C.
ꢂ
Results and discussion
Characterization of the Mn(III)–Schiff base complex
[MnL(OAc)]
The results of the diffraction experiment and the structure
renement of [MnL(OAc)] are summarized in Table 1. The
selected bond lengths and angles are presented in Table S1.†
Note that one molecule of the [MnL(OAc)] complex consti-
tutes the asymmetric unit of the reported structure (Fig. 1). The
deformed octahedral coordination sphere of Mn(III) is formed
by the four-dentate L ligand and the bidentate acetate ligand
(OAc). The shortest bonds within the coordination sphere
involve both phenolate O atoms, with the Mn1–O1 and Mn1–O2
distances of 1.8630(14) and 1.9251(15) A, respectively, and the
signicantly longer Mn1–O5 [2.1188(16) A] is formed by the
acetate ligand. The Mn1–N2 and Mn1–N1 bond distances are
1.9925(16) and 2.1289(17) A, respectively, reecting a smaller
electrostatic component when compared with the case of the
Mn–O (phenolate) bonds. Moreover, the Mn1–N1 bond is
signicantly longer than the Mn1–N2 bond, and this seems to
reect the internal rigidity of the L ligand with two imine
moieties. This was further supported by the values of ꢀ15.2(3)ꢂ
and ꢀ16.6(3)ꢂ for the torsion angles C1–C6–C7–N1 and C17–
C12–C11–N2, respectively, reecting the internal strain caused
by the four-dentate coordination of L. The Mn1–O6 bond
involves the second O atom of acetate and is the longest bond
The described deformation of the Mn coordination sphere
strongly affects the conformation of the four-dentate L ligand
and the formed chelate rings. The dihedral angle between the
two phenyl rings of L is 56.36(11)ꢂ. The consecutive torsion
angles describing the conformation of the central bridge moiety
C6–C7–N1–C8–C9–C10–N2–C11–C12 are 172.5, 136.9, 61.9,
ꢀ72.0, and ꢀ115.9ꢂ. The two O–C–C–C–N–Mn1 chelate rings
have an envelope conformation, whereas the Mn1–N1–C8–C9–
C10–N2 ring has a chair conformation. In the detected archi-
tecture of the reported complex, the dihedral angles between
the acetate plane and the O1 and O2 phenolate rings are 62.5
˚
˚
˚
ꢂ
ꢂ
˚
and 69.0 , respectively. The hydrogen bond geometry (A and )
for [MnL(OAc)] is presented in Table S2.†
Characterization of the heterogeneous nanocatalyst
(Fe3O4@SiO2-[MnL(OAc)])
FTIR spectra. The FTIR spectra of Fe3O4, Fe3O4@SiO2, and
the prepared Fe3O4@SiO2-[MnL(OAc)] heterogenized nano-
catalyst are shown in Fig. 2. In the FTIR spectra of Fe3O4
(Fig. 2a), the characteristic peak at around 3400 cmꢀ1 is related
to the stretching vibration of OH. In addition, the absorption
band at 580 cmꢀ1 is attributed to the Fe–O bond in the Fe3O4
˚
(2.2870(16) A) in the coordination sphere. This is due to the
Fig. 1 Crystal structure of the manganese(III) Schiff base complex Fig. 2 FTIR spectra of (a) Fe3O4, (b) Fe3O4@SiO2, and (c) Fe3O4@SiO2-
[MnL(OAc)].
[MnL(OAc)].
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magnetic nanoparticles. In the FTIR spectrum of Fe3O4@SiO2
(Fig. 2b), the characteristic band at 575 cmꢀ1 is attributed to the
Fe–O stretching vibration. The strong absorption band at
around 1000–1100 cmꢀ1 can be assigned to the vibration modes
of Si–O–Si and prove that the Fe3O4 nanoparticles were
successfully coated with silica layers. The band at 3400 cmꢀ1 is
associated with the vibrations of hydrogen bonding related to
the water molecules and the hydroxyl group.37,38 In the FTIR
spectrum of Fe3O4@SiO2-[MnL(OAc)] (Fig. 2c), new bands were
observed at 1600 and 1650 cmꢀ1 for C]N, which could be due
to the coordination of the N atom of the azomethine group to
the Mn metal; on the other hand, the band at 1500 cmꢀ1 can be
assigned to C]C stretching in the L ligand in Fe3O4@SiO2-
[MnL(OAc)]. The band at 480 cmꢀ1 could be assigned to the Mn–
O bond formed by the axial ligation of the hydroxyl group with
the Mn–salen complex.39
Fig. 3 The TGA/DTGA thermogram of Fe3O4@SiO2-[MnL(OAc)].
TGA/DTGA analysis. The thermal stability of the Schiff base
complex immobilized on Fe3O4@SiO2 via axial coordination
was analyzed by TGA/DTGA thermograms. Fig. 3 shows the
thermogravimetric analysis curves, depicting the variations in
the residual mass of the sample with the increasing tempera-
ture. The rst stage of weight loss below 100 ꢂC corresponds to
the removal of physically and chemically adsorbed water. The
absolute mass loss in the range of 240–360 ꢂC can be associated
with the release of the Mn(III)–Schiff base complex from the
surface of Fe3O4@SiO2 and the decomposition of the Mn(III)–
Schiff base complex. Based on these results, the graing of the
Schiff base complex on the surface of Fe3O4@SiO2 was
veried.26
SEM and TEM images. The SEM image of Fe3O4 illustrated
that the nanoparticles have an approximately smooth spherical
morphology (Fig. 4a). Aer coating the Fe3O4 MNPs with the
silica shell (Fig. 4b) and immobilizing [MnL(OAc)] via axial
coordination (Fig. 4c), the morphology of the obtained powder
was preserved.
Moreover, Fig. 5 displays the TEM image of Fe3O4@SiO2-
[MnL(OAc)] with clear evidence of the spherical shape and thus
conrms the well-dened core–shell structure with the shell
thickness of ꢃ30.4 nm.
EDX analysis. Energy-dispersive X-ray spectroscopy (EDX)
was conducted to obtain the local information of the different
Fig. 4 SEM images of (a) Fe3O4, (b) Fe3O4@SiO2, and (c) Fe3O4@SiO2-
[MnL(OAc)].
Fig. 5 TEM image of Fe3O4@SiO2-[MnL(OAc)].
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Fig. 6 The energy-dispersive X-ray spectroscopy (EDX) spectrum of
Fe3O4@SiO2-[MnL(OAc)].
Fig. 8 (a) The powder XRD patterns of Fe3O4@SiO2-[MnL(OAc)]. (b)
The simulated powder diffraction pattern based on the crystal struc-
ture of the [MnL(OAc)] complex.
X-ray diffraction. The X-ray diffraction patterns of Fe3O4@-
SiO2-[MnL(OAc)] in the 2theta range of 5–90ꢂ are displayed in
Fig. 8a. The diffraction peaks at 2theta ¼ 31ꢂ, 36ꢂ, 44ꢂ, 54ꢂ, 58ꢂ,
63ꢂ, 71ꢂ, and 75ꢂ corresponded to (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5
1 1), (4 4 0), (6 2 0), and (5 3 3) planes. The pattern is in
accordance with the standard database of cubic inverse spinel
Fe3O4 in the JCPDS le (JCPDS card no. 19-629); this reveals that
during the preparation of the heterogenized magnetic nano-
catalyst, the structure of Fe3O4 MNPs remains unchanged.42
Based on the Scherrer equation, the average particle size of
Fe3O4@SiO2-[MnL(OAc)] was 54.5 nm. The powder diffraction
pattern of [MnL(OAc)] (Fig. 8b) was simulated using Cu Ka
radiation to ensure the compatibility of the simulated pattern
with the experimental XRD pattern obtained for the nano-
catalyst. A comparison of these patterns conrms the disper-
sion of the catalyst on the Fe3O4@SiO2 support and lack of the
[MnL(OAc)] substructure corresponding to the reported mono-
crystalline state.
Fig. 7 Magnetization curves at 300 K for (a) Fe3O4, (b) Fe3O4@SiO2,
and (c) Fe3O4@SiO2-[MnL(OAc)].
elements of [MnL(OAc)] on the surface of Fe3O4@SiO2 (Fig. 6),
and the results conrmed the tertiary character of Fe3O4@SiO2-
[MnL(OAc)].
VSM analysis. The magnetic hysteresis loops (Fig. 7a–c) were
measured using a vibrating sample magnetometer at room
temperature, and the S-Shaped hysteresis loop, typical of
magnetite, indicated ferromagnetic behavior.40 The plot of
magnetization versus magnetic eld (M–H loop) provided the
magnetization saturation values of 62.21, 49.67, and 30.63 emu
gꢀ1 for Fe3O4, Fe3O4@SiO2, and Fe3O4@SiO2-[MnL(OAc)],
respectively (Fig. 7c). The observed decrease in the saturation
magnetization of Fe3O4@SiO2 as compared to that of the pure
Fe3O4 MNPs could be due to the interaction between the iron
core and the silica shell, which decreased the magnetic
moment.41 Moreover, the saturation magnetization of silica-
coated Fe3O4 signicantly decreased with the attachment of
the Mn–Schiff base complex via axial coordination. However, in
the case of both the core–shell magnetic nanoparticles and the
heterogenized nanocatalyst, these values were still sufficiently
large, facilitating the easy separation of nanoparticles by an
external magnet.
Atomic absorption spectroscopy (AAS). The metal content of
the heterogenized catalyst was determined by the ame atomic
absorption spectroscopy (FAAS), which revealed that each gram
of the nanohybrid catalyst contained 540 mmol of Mn–Schiff
base.
Electrochemical catalytic studies
Electrochemical oxidation of methyl phenyl sulde and
cyclooctene by the synthesized nanocatalyst. In the presence of
an external voltage, the electrochemical oxidations of methyl
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Table 2 The electrochemical catalytic oxidation of methyl phenyl sulfide and cyclooctene
Oxidation of methyl phenyl sulde
Oxidation of cyclooctene
Conversionc (%)
Entry
Catalyst
Conversiona (%)
Selectivityb (%)
Selectivityd (%)
1
2
3
4
No catalyst
Fe3O4@SiO2
[MnL(OAc)]
Fe3O4@SiO2-[MnL(OAc)]
0
0
95
96
0
0
100
100
0
0
95
97
0
0
100
100
a
Reaction conditions: cat. (0.0017 mmol), molar ratio of cat. : imidazole : MePhS : H2O2: is 1 : 100 : 300 : 1500, solvent: EtOH (1 mL), stirred at r.t.,
b
potential: 60 V, and time: 90 s. GC yield based on the starting reactant methyl phenyl sulde. Methyl phenyl sulfoxide is the sole product.
c
Reaction conditions: cat. (0.0017 mmol), molar ratio of cat. : imidazole : cyclooctene : H2O2 is 1 : 100 : 150 : 750, solvent: EtOH (1 mL), stirred
at r.t., potential: 60 V, and time: 90 s. d GC yield based on the starting reactant cyclooctene. Cyclooctene oxide is the sole product.
phenyl sulde and cyclooctene were investigated. Imidazole was obtained aer 90 s (Table 2, entries 1 and 2). In the presence of
used as a co-catalyst of [MnL(OAc)]. In the absence of the unsupported [MnL(OAc)], methyl phenyl sulfoxide and cyclo-
catalyst or in the presence of Fe3O4@SiO2, no signicant octene oxide were obtained in a signicant yield (Table 2, entry
products (methyl phenyl sulfoxide or cyclooctene oxide) were 3). Generally, in the case of methyl phenyl sulde oxidation, two
Table 3 Electrochemical oxidation of various sulfides catalyzed by Fe3O4@SiO2-[MnL(OAc)]
Entry
1
Substrate
Major product
Conversiona (%)
Selectivityb (%)
100
96
2
3
4
93
96
91
81
100
90
5
54
100
6
7
8
95
90
100
89
100
85
9
100
100
100
85
10
11
70
100
a
Reaction conditions: cat. (0.0017 mmol), molar ratio of cat. : imidazole : MePhS : H2O2: is 1 : 100 : 300 : 1500, solvent: EtOH (1 mL), room
b
temperature, potential: 60 V, and time: 90 s. GC yield based on the starting sulde.
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Table 4 Electrochemical oxidation of various alkenes catalyzed by Fe3O4@SiO2-[MnL(OAc)]
Entry
Substrate
Major product
Conversiona (%)
Selectivityb (%)
1
2
97
98
100
100
3
4
100
94
100
100
5
6
7
8
100
100
100
35
96
100
75
100
9
100
97
100
100
10
a
Reaction conditions: cat. (0.0017 mmol), molar ratio of cat. : imidazole : cyclooctene : H2O2 is 1 : 100 : 150 : 750, solvent: EtOH (1 mL), room
b
temperature, potential: 60 V, and time: 90 s. GC yield based on the starting alkene.
products, i.e. methyl phenyl sulfoxide and sulfone, are ob- entry 5) as compared to the case of methyl phenyl sulde (Table
tained.43 However, in the present reaction, methyl phenyl sulf- 3, entry 1). In addition, thiophene had a lower oxidation yield
oxide was obtained as the sole product. Moreover, the Schiff than the linear suldes; this may be due to the presence of the S
base complex still had good activity aer immobilization on the atom in the ring that can make the oxidation of thiophene
Fe3O4@SiO2 MNPs (Table 2, entry 4); thus, the supported cata- difficult (Table 3, entry 11).
lyst was preferred over the unsupported catalyst due to its
Electrochemical oxidation of different olens by the het-
recyclability. Note that in the absence of voltage (potential ¼ erogenized nanocatalyst. Further investigations were carried
0 V), methyl phenyl sulde and cyclooctene were not electro- out for a broad range of olens (Table 4) in the presence of
chemically oxidized to the desired products by Fe3O4@SiO2- Fe3O4@SiO2-[MnL(OAc)]. Almost all the olens were converted
[MnL(OAc)] even aer 100 s.
to their epoxides with good conversions and selectivities. Note
Electrochemical oxidation of different suldes by the het- that the afforded epoxide was the sole product in most cases.
erogenized nanocatalyst. In order to further explore the oxida- The electrochemical oxidation of cis-stilbene (Table 4, entry 8)
tion potential of the heterogenized nanocatalyst, a wide variety led to a signicantly lower yield of the corresponding oxide than
of suldes (Table 3) were oxidized to their corresponding that of trans-stilbene (Table 4, entry 9).
oxidation products in the presence of Fe3O4@SiO2-[MnL(OAc)].
Turn over frequency. In order to estimate the longevity of the
Excellent conversions of suldes and selectivities of sulfoxides proposed heterogeneous catalytic system, the turnover frequency
were obtained in the electrochemical oxidation of suldes (TOF) was studied. The catalyst exhibited excellent activity for the
(Table 3, entries 1–11). According to the results, the presence of oxidation of styrene in an ethanol solution under mild conditions
electron-withdrawing groups, such as a vinyl group in phenyl (the molar ratio of cat. : imidazole : styrene : H2O2 was
vinyl sulde, directly attached to the S atom in the sulde 1 : 13 300 : 20 000 : 100 000) with the TOF of 17 750 hꢀ1 for Fe3-
structure can dramatically decrease the sulfoxide yield of the O4@SiO2-[MnL(OAc)], which is one of the highest values reported
electrochemical oxidation reaction (a 54% conversion) (Table 3, to date for heterogenized Schiff base complexes.44
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Fig. 10 The voltammetric behavior of Fe3O4@SiO2-[MnL(OAc)] and
blank at 100 mV sꢀ1 (a). Cyclic voltammograms of Fe3O4@SiO2-
[MnL(OAc)] at different scan rates (100, 200, 400, 600, and 800 (mV
s
ꢀ1)) (b), chronoamperograms of Fe3O4@SiO2-[MnL(OAc)] and blank at
the oxidation potential (c), and chronoamperograms of Fe3O4@SiO2-
[MnL(OAc)] and blank at the reduction potential in EtOH for the
oxidation of methyl phenyl sulfide (d). Reaction conditions: cat.
(0.0017 mmol), molar ratio of cat. : imidazole : sulfide : H2O2: is
1 : 100 : 300 : 1500, solvent: EtOH (1 mL), room temperature, poten-
tial: 60 V, and time: 90 s.
the catalyst showed remarkable results with no signicant loss
of activity aer 6 runs. These observations illustrate the
advantage of the proposed heterogeneous system over a homo-
geneous system.
Fig. 9 Recycling of the catalytic system for the electrochemical
oxidation of methyl phenyl sulfide (a) (reaction conditions: cat. (0.0017
Plausible mechanism. A mechanism for the electrochemical
oxidation of suldes and alkenes catalyzed by Fe3O4@SiO2-
[MnL(OAc)] in the presence of a potential was proposed (see ESI,
Scheme S1†). However, the precise mechanism of the above-
mentioned electrochemical oxidations remains unclear.
Comparison with previous studies. A comparison of the
obtained results with those reported in the literature and our
previous study shows a higher catalytic activity of the prepared
heterogenized nanocatalyst and a signicantly lower reaction
time (Table 5).
mmol),
molar
ratio
of
cat. : imidazole : sulfide : H2O2:
is
1 : 100 : 300 : 1500, solvent: EtOH (1 mL), room temperature, poten-
tial: 60 V, and time: 90 s) and cyclooctene (b) (reaction conditions: cat.
(0.0017 mmol), molar ratio of cat. : imidazole : alkene : H2O2 is
1 : 100 : 150 : 750, solvent: EtOH (1 mL), room temperature, potential:
60 V, and time: 90 s).
Reusability of the heterogenized nanocatalyst. The recovery
and reusability of the Fe3O4@SiO2-[MnL(OAc)] heterogenized
magnetic nanocatalyst were examined. For this, the catalyst was
used in the electrochemical oxidation of methyl phenyl sulde
(Fig. 9a) and cyclooctene (Fig. 9b). In both catalytic reactions,
Electrochemical studies. Cyclic voltammetry measurements
and chronoamperometry technique were used to investigate the
electrochemical properties of the prepared Fe3O4@SiO2-
Table 5 Comparison of the activities of different complexes in the oxidation reaction
Conversion
(%)
Entry
Catalyst
Substrate
Potential
Oxidant
Time
Ref.
1
2
3
4
5
6
7
8
9
MnP2-S1-Mag
MnP3-S2-Mag
Cyclooctene
Cyclooctene
Cyclooctene
Cyclooctene
Cyclooctene
Methyl phenyl sulde
Cyclooctene
Methyl phenyl sulde
Cyclooctene
None
None
None
None
None
None
None
60 V
PhIO
PhIO
n-Bu4NHSO5
n-Bu4NHSO5
O2
UHP
UHP
H2O2
H2O2
1 h
1 h
3 h
20 h
60 min
5 min
4 h
90 s
90 s
78
51
88.5
97
83
100
100
96
45
45
46
47
26
32
32
Fe3O4@SiO2-Im@[MnT(4-OMeP)P]
Fe3O4@SiO2–NH2@[MnTPFPP]
[Fe3O4@SiO2@NH2@MnTCPP(OAc)]
Fe3O4@SiO2-[MnL(OAc)]
Fe3O4@SiO2-[MnL(OAc)]
Fe3O4@SiO2-[MnL(OAc)]
Fe3O4@SiO2-[MnL(OAc)]
This work
This work
60 V
97
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on the surface of silica-coated Fe3O4 MNPs via an axial coordi-
nation, as proved by standard techniques. The dispersion of
[MnL(OAc)] on the silica-coated Fe3O4 MNPs was conrmed by
the simulation of the powder diffraction pattern of the complex.
The prepared heterogenized nanocatalyst showed a notable
activity in the electrochemical oxidation of suldes and alkenes
with H2O2 in the presence of a glassy carbon electrode, applying
60 V voltage, and the excellent turnover frequency of 17 750 hꢀ1
was obtained for the oxidation of styrene. The ternary Fe3-
O4@SiO2-[MnL(OAc)] catalytic system has at least the same
catalytic efficacy as [MnL(OAc)] itself, having the great advan-
tage of easy recovery from the reaction mixture. The hetero-
genized nanocatalysts were successfully reused for six
consecutive times without loss of activity in the electrochemical
oxidation of methyl phenyl sulde and cyclooctene. Eventually,
Fig. 11 The voltammetric behavior of Fe3O4@SiO2-[MnL(OAc)] and the signicant electrochemical properties and stability of Fe3-
blank at 100 mV sꢀ1 (a). Cyclic voltammograms of Fe3O4@SiO2-
O4@SiO2-[MnL(OAc)] were conrmed by cyclic voltammetry
measurements and chronoamperometry techniques.
[MnL(OAc)] at different scan rates (100, 200, 400, 600, and 800 (mV
s
ꢀ1)) (b). Chronoamperograms of Fe3O4@SiO2-[MnL(OAc)] SiO2 and
blank at the oxidation potential (c) and chronoamperograms of Fe3-
O4@SiO2-[MnL(OAc)] and blank at the reduction potential in EtOH for
the oxidation of cyclooctene (d). Reaction conditions: cat. (0.0017
Conflicts of interest
mmol),
molar
ratio
of
cat. : imidazole : alkene : H2O2
is
There are no conicts of interest to declare”.
1 : 100 : 150 : 750, solvent: EtOH (1 mL), room temperature, potential:
60 V, and time: 90 s.
Acknowledgements
This work was supported by the Center for International
Scientic Studies & Collaboration (CISSC), Ministry of Science
Research and Technology. AW acknowledges the Nicolaus
Copernicus University for partly supporting the research under
the Center for Excellence in Science and the interdisciplinary
BRAIN research team.
[MnL(OAc)] heterogenized nanocatalyst in the oxidation of
methyl phenyl sulde (Fig. 10). CV experiments were performed
for a blank reaction (with no catalyst) and a catalytic reaction (in
the presence of the prepared heterogenized nanocatalyst) by
cycling the potential of the glassy carbon (GC) electrode in the
potential range of ꢀ3 to 3 V in ethanol at the scan rate of 100 mV
sꢀ1 (Fig. 10a). A comparison of the electrochemical oxidation
reaction in the presence of Fe3O4@SiO2-[MnL(OAc)] with the
blank reaction conrms that the presence of the nanocatalyst is
necessary for the electrochemical oxidation of methyl phenyl
sulde. With an increase in the scan rate above 100 mV sꢀ1, the
reversible CV curve shied towards positive potentials
(Fig. 10b). These effects showed that with an increase in the
scan rate, the rate of the electrochemical oxidation of methyl
phenyl sulde decreased. The chronoamperometry experiments
were performed to determine the electrochemical activity and
stability of Fe3O4@SiO2-[MnL(OAc)] in the electrochemical
oxidation of methyl phenyl sulde (Fig. 10c and d). The
comparison of the current intensities at the oxidation or
reduction potentials for blank and catalytic reactions shows
a higher rate of the oxidation reaction in the presence of the
heterogenized nanocatalyst.
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