Mn(III) complex supported on Fe 3O 4 nanoparticles: Magnetically separable nanocatalyst for selective oxidation of thiols to disulfides

Article abstract of DOI:10.1080/00958972.2013.821699

A manganese(III) complex, [Mn(phox)₂(CH₃OH) ₂]ClO₄ (phox = 2-(2′-hydroxyphenyl)oxazoline), was immobilized on silica-coated magnetic Fe₃O₄ nanoparticles through the amino propyl linkage usi

Full text of DOI:10.1080/00958972.2013.821699

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Mn(III) complex supported on Fe3O4
nanoparticles: magnetically separable
nanocatalyst for selective oxidation of
thiols to disulfides
Mojtaba Bagherzadeha, Mohammad Mehdi Haghdoosta, Firouz
Matloubi Moghaddama, Behzad Koushki Foroushania, Setareh
Saryazdia & Ebrahim Payaba
a Chemistry Department, Sharif University of Technology, Tehran,
Iran.
Accepted author version posted online: 04 Jul 2013.
To cite this article: Mojtaba Bagherzadeh, Mohammad Mehdi Haghdoost, Firouz Matloubi
Moghaddam, Behzad Koushki Foroushani, Setareh Saryazdi & Ebrahim Payab (2013) Mn(III)
complex supported on Fe3O4 nanoparticles: magnetically separable nanocatalyst for selective
oxidation of thiols to disulfides, Journal of Coordination Chemistry, 66:17, 3025-3036, DOI:
10.1080/00958972.2013.821699
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Journal of Coordination Chemistry, 2013
Vol. 66, No. 17, 3025–3036, http://dx.doi.org/10.1080/00958972.2013.821699
Mn(III) complex supported on Fe3O4 nanoparticles:
magnetically separable nanocatalyst for selective oxidation of
thiols to disulfides
MOJTABA BAGHERZADEH*, MOHAMMAD MEHDI HAGHDOOST,
FIROUZ MATLOUBI MOGHADDAM, BEHZAD KOUSHKI FOROUSHANI,
SETAREH SARYAZDI and EBRAHIM PAYAB
Chemistry Department, Sharif University of Technology, Tehran, Iran
(Received 3 March 2013; in final form 21 June 2013)
A manganese(III) complex, [Mn(phox)2(CH3OH)2]ClO4 (phox = 2-(2′-hydroxyphenyl)oxazoline),
was immobilized on silica-coated magnetic Fe3O4 nanoparticles through the amino propyl linkage
using a grafting process in dichloromethane. The resulting Fe3O4@SiO2–NH2@Mn(III) nanoparti-
cles are used as efficient and recyclable catalysts for selective oxidation of thiols to disulfides using
urea-hydrogen peroxide as the oxidant. The nanocatalyst was recycled several times. Leaching and
recycling experiments revealed that the nanocatalyst can be recovered, recycled, and reused more
than five times, without the loss of catalytic activity and magnetic properties. The recycling of the
nanocatalyst in six consecutive runs afforded a total turnover number of more than 10,000. The
heterogeneous Fe3O4@SiO2–NH2@Mn(III) nanoparticle shows more selectivity for the formation
of disulfides in comparison with the homogeneous manganese complex.
Keywords: Mn(III) complex; Fe3O4 nanoparticle; Magnetically separable nanocatalyst; Thiol
oxidation; Disulfides
1. Introduction
Synthesizing catalysts with the desired properties is a highly sought-after and challenging
goal. A number of catalysts with various characteristics have been developed [1].
Homogeneous catalysts typically operate at relatively mild conditions and exhibit high
activities and selectivities, but it is difficult to separate and regenerate the catalyst from the
reaction mixture. Covalent grafting of a homogeneous catalyst on solid supports [2] results
in relatively good outcomes, but decreases the active surface area and the reactivity of
immobilized catalysts.
Immobilized homogeneous catalysis has gained a new impulse with the advent of
nanotechnology [3]. Because of the large surface area of nanoparticles, high loadings of
catalytically active sites are guaranteed and, therefore, nanoparticle-supported homogeneous
catalysts exhibit high catalytic activity and selectivity [4]. However, nanometer-sized
particles are difficult to separate by traditional filtration techniques and expensive
*Corresponding author. Email: bagherzadeh@sharif.edu
Ó 2013 Taylor & Francis

3026
M. Bagherzadeh et al.
ultracentrifugation is often needed to have good separation of the catalyst from the reaction
mixture. This shortcoming could be obviated using magnetic nanoparticle (MNP) supports.
An external magnet could easily separate the immobilized catalysts from the reaction
mixture, which results in more effective separation than conventional methods [4–10].
We have focused on using magnetically separable transition metal complexes as catalysts
for the synthesis of organic compounds [11]. In this paper, we synthesized the first magneti-
cally recyclable manganese complex through covalent anchoring of [Mn(phox)2(CH3OH)2]
ClO4 (phox = 2-(2′-hydroxyphenyl)oxazoline) on silica-coated Fe3O4 nanoparticles. This
catalyst (hereafter called Fe3O4@SiO2–NH2@Mn(III)) efficiently catalyzes the selective
oxidation of thiols to disulfides using urea-hydrogen peroxide (UHP) as the environmental-
friendly oxidant. In addition, the present Fe3O4@SiO2–NH2@Mn(III) catalyst was easily
recovered and reused without the loss of activity or selectivity.
2. Experimental
2.1. Methods
1
The H NMR spectra were recorded at room temperature with a Bruker FT NMR 500
(500 MHZ) spectrophotometer using CDCl3 as solvent and chemical shifts were reported
in ppm with tetramethylsilane as an internal standard. Scanning electron microscopy
(SEM) was carried out on Philips XL30. IR spectra were recorded as KBr pellets using a
ABB FT–IR spectrophotometer. Measures of pH were carried out by a Mettler Toledo S40
SevenMulti™ pH-meter. A Varian (AA220) flame atomic absorption spectrometer (air/acet-
ylene flame) was used for the determination of manganese ion. A Hewlett-Packard (HP,
Palo Alta, USA) HP 6890 plus series GC equipped with a split/splitless injector and a HP
5973 mass-selective detector system were used for GC-MS analysis. The MS was operated
in the EI mode (70 eV) and separations were performed on a 30 m  0.25 mm HP-5 MS
column (0.25 μm film thickness). The UV–Vis DRS spectra of the samples were recorded
with a PE Lambda 20 spectrometer.
2.2. Synthesis of [Mn(phox)2(CH3OH)2]ClO4 complex
Hphox (2-(2′-hydroxyphenyl)oxazoline) was prepared in a similar manner according to the
reported procedure [12]. Manganese complexes, bis[2-(2′-hydroxyphenyl)-oxazolinato]
dimethanol manganese(III) perchlorate, [Mn(phox)2(CH3OH)2]ClO4 were synthesized
according to previously reported work [13].
2.3. Synthesis of Fe3O4 nanoparticles
Under N2, 2 g (10.0 mM) of FeCl2·4H2O, 5.2 g (19.3 mM) of FeCl3·6H2O, and 0.85 mL of
concentrated HCl were dissolved in 25 mL of degassed water. This solution was added
dropwise at room temperature to 250 mL of sodium hydroxide solution (1.5 M) under N2.
After the mixture was stirred for 30 min (1300 rpm), the formed black precipitates were
separated using a 0.5 T magnet and washed several times with degassed water. Finally,
for storage of MNPs, Fe3O4 nanoparticles were dispersed in 100 mL degassed water under
N2 [14].

Mn(III) on Fe3O4 Nanoparticles
2.4. Synthesis of Fe3O4@SiO2 nanoparticles
3027
One gram of freshly prepared Fe3O4 nanoparticles was added to 30 mL of an aqueous
solution of citric acid (0.02 g/mL), then the pH was adjusted to 5.2 using ammonia and the
mixture was heated to 80–90 °C for 1.5 h. After heating, the pH of the reaction mixture
was increased with ammonia to pH = 11, and 1.25 mL of tetraethylorthosilicate (TEOS)
dissolved in ethanol (12.5 mL) was added dropwise into the suspension of particles. The
mixture was stirred at room temperature for 24 h to let the base-catalyzed hydrolysis and
the condensation of TEOS monomers on the nanoparticles surface go to completion.
Finally, the dark brown Fe3O4@SiO2 nanoparticles were separated using a magnet and
were washed with distilled water and ethanol.
2.5. Synthesis of Fe3O4@SiO2–NH2
Five milliliter of 3-aminopropyltriethoxysilane (APS) dissolved in 100 mL ethanol was
added dropwise to the suspension of one gram of silica-coated Fe3O4 nanoparticles in
100 mL of distilled water. The pH of the reaction mixture was increased with KOH to
pH = 11 and the reaction mixture was stirred at 70 °C for 5 h. Finally, the brown
precipitates were separated using a 0.5 T magnet and were thoroughly washed with
distilled water (five times, 150 mL) to remove any unbound APS.
2.6. Synthesis of Fe3O4@SiO2–NH2@Mn(III) nanoparticles
Under N2, one gram of Fe3O4@SiO2–NH2 nanoparticles was added to a solution of
[Mn(phox)2(CH3OH)2]ClO4 complex (100 mg) in 100 mL of dry dichloromethane and
the suspension was refluxed for 24 h. After cooling, the brown Fe3O4@SiO2–NH2@Mn
(III) nanoparticles were separated using a magnet and were washed several times with
dichloromethane and toluene.
2.7. General conditions for catalytic oxidation of thiols
A typical reaction using the Fe3O4@SiO2–NH2@Mn(III) nanoparticles as catalyst and thiols
as substrate is described as follows. To a solution of thiol (1 mM) and catalyst (0.01 g,
containing 0.0002 mM of manganese complex) in a (1 : 1) mixture of CH3OH–CH2Cl2
(2 mL) was added UHP (0.093 g, 1 mM) as the oxidant. The mixture was stirred at room
temperature for 2 h. The catalyst particles were then collected at the bottom of the test tube
using a magnet, the supernatant was carefully decanted, and the completion of oxidation
reaction was followed by TLC (petroleum ether/ethyl acetate, 4 : 1). After the separation of
catalyst nanoparticles, the solvent supernatant was removed under vacuum and the
remaining was purified by column chromatography using petroleum ether/ethylacetate, 4 : 1.
1
Isolated products were weighted and also analyzed by H NMR. Washing several times
with methanol and dichloromethane, the catalyst particles were dried in vacuum and could
be reused.
3. Results and discussion
The methodologies used in functionalization of Fe3O4 nanoparticle surfaces and
subsequent Mn(III) complex anchoring are presented in scheme 1. We started with Fe3O4

3028
M. Bagherzadeh et al.
Scheme 1. Step-by-step synthesis of Fe3O4@SiO2–NH2@Mn(III).
nanoparticles of 10–50 nm average diameters which were prepared by facile and
convenient co-precipitation method [14]. The SEM image of the dispersed MNPs in water
depicts relatively uniform Fe3O4 nanoparticles with dimensions less than 50 nm and rather
high surface area (Supplementary material). The high surface area of nanoparticles is
beneficial for the high loading of catalyst during the complex anchoring step.
In the XRD pattern of the prepared Fe3O4 (figure 1), six characteristic peaks (2h = 30.1,
35.7, 43.1, 54.0, 57.1, and 63.6), corresponding to (220), (311), (400), (422), (511), and
(440) Bragg reflections, respectively, were observed. These peaks are consistent with the
database in JCPDS file (PCPDFWIN v.2.02, PDF No. 85-1436) and reveal that the
resultant nanoparticles were pure Fe3O4 without impurity phases. The average particle size
of the sample was estimated by Scherrer’s equation to be 15.9, which is in good agreement
with that observed in the SEM image (Supplementary material).
Although Fe3O4 nanoparticles can be easily prepared, they have the following
disadvantage of being fairly reactive to acidic and oxidative environments [15]. In the
presence of oxygen, the Fe3O4 nanoparticles rapidly oxidize to Fe2O3 which shows low
magnetic properties [16]. Also, direct grafting of metal complex on the surface of Fe3O4
nanoparticles is almost impossible due to relative inertness of the surface. There are only a
limited number of molecules that can be bonded to MNP surfaces [17]. Freshly prepared
Fe3O4 nanoparticles will aggregate rapidly into large clusters and thus will lose their nano
properties [4]. In order to prevent these problems, the nanoparticles have to be coated.
Among various coating agents, silica offers unique advantages for applications,
particularly, in catalysis. For doing so, the silica-coated MNPs (Fe3O4@SiO2) were
Figure 1. XRD patterns of the as-synthesized Fe3O4 nanoparticles.

Mn(III) on Fe3O4 Nanoparticles
3029
synthesized by basic hydrolysis and condensation of TEOS on the surface of the Fe3O4
nanoparticles [17]. The obtained Fe3O4 and Fe3O4@SiO2 nanoparticles were characterized
in our previous work [11].
In previous work, a vanadium complex was covalently grafted via the reaction of chloro
complex and silanol groups of surface with concomitant release of HCl [11]. Presence of a
halide on metal center, which is a necessity in this process, is limited to a few complexes.
A possible way to overcome this drawback is the surface derivatization with a variety of
functional groups. One of the preferred functional groups for bonding different metal
centers to nanoparticles is NH2. In this work, Fe3O4@SiO2 nanoparticles were
functionalized by bonding 3-aminopropylsilane (APS) to their surfaces as described
elsewhere [17]. Because of the presence of the magnetic core inside, MNPs cannot be
analyzed by solid-state NMR spectroscopy. Therefore, amino-functionalized nanoparticles
(Fe3O4@SiO2–NH2) were characterized by FT–IR and elemental analysis. Figure 2 shows
the FT–IR spectra of the core-shell MNPs before and after being functionalized with ASP.
Compared with the FT–IR spectrum of Fe3O4@SiO2, the characteristic peaks at 2861 and
2926 cmÀ1 ascribed to C–H stretch of the propyl in the pendant APS can be clearly
observed in the FT–IR spectrum of Fe3O4@SiO2–NH2, which confirms that APS
molecules have bonded to the surface of the silica-coated MNPs [18]. This conclusion is
further supported by elemental analysis which gave the percentages of C, H, and N to be
5.10, 1.54, and 1.53%, respectively. The wave numbers of N–H asymmetric and symmetric
stretching vibrations of the amine group are at 3300–3400 cmÀ1 and are obscured by water
bands [19].
The SEM image of Fe3O4@SiO2–NH2 nanoparticles, as shown in figure 3, clearly
suggests a smooth morphology and confirms that the particles fall in the nano size range
after silica coating and amine functionalization. Comparison of the SEM image of
these nanoparticles with those of bare Fe3O4 (Supplementary material) reveals that
Fe3O4@SiO2–NH2 exhibits particles having smooth surfaces, while the Fe3O4 particles
exhibit rough surfaces. It must be emphasized that the smoothness of the surface is due to
the presence of silica shell on the particles. This comparison also shows that the final
Fe3O4@SiO2–NH2 nanoparticle size is very close to the size of the initial Fe3O4 particles,
indicating that the silica layer is very thin.
After functionalization by APS, Fe3O4@SiO2–NH2 nanoparticles offer NH2 binding sites
for immobilization of many metal complexes. We treated Fe3O4@SiO2–NH2 nanoparticles
with the [Mn(phox)2(CH3OH)2]ClO4 complex in dry CH2Cl2 to synthesize Fe3O4@SiO2–
NH2@Mn(III) nanoparticles. Methanols in the axial positions are highly labile toward
Figure 2. FT–IR spectra of Fe3O4@SiO2 (up) and Fe3O4@SiO2–NH2 (down) nanoparticles.

3030
M. Bagherzadeh et al.
Figure 3. SEM image of the Fe3O4@SiO2–NH2 nanoparticles.
substitution. Therefore, we proposed that methanol axial ligands of the complex could be
replaced by the amine group of the surface, which is a stronger nucleophile than methanol.
Comparison of the FT–IR spectra of [Mn(phox)2(CH3OH)2]ClO4 complex [figure 4(a)]
with those of the Fe3O4@SiO2–NH2@Mn(III) nanoparticles [figure 4(b)] revealed that
signals at 758, 868, 932, 1332, 1399, 1477, and 1541 cmÀ1 corresponding to the vibration
modes of manganese complex are present in the FT–IR spectra of Fe3O4@SiO2–NH2@Mn
(III) nanoparticles.
Figure 5 shows the diffuse reflectance UV–Vis (DRS) spectrum of freshly prepared
Fe3O4@SiO2–NH2@Mn(III) nanoparticles (curve c). The UV–Vis spectra of the [Mn
(phox)2(CH3OH)2]ClO4 complex in methanol solution are given for comparison (curve a).
The curve of manganese complex in methanol solution exhibits ligand-to-metal charge
transfer and π→π⁄ transition with absorption maxima around 289 and 237 nm, respectively
[13]. The DRS spectrum of the Fe3O4@SiO2–NH2@Mn(III) nanoparticles shows
comparable absorption behavior, but exhibits a slight red shift due to the interaction of the
metal complex with the surface.
The thermal behavior of the Fe3O4@SiO2–NH2@Mn(III) nanoparticles was evaluated
by TG-DTA. The TGA and DTA curves shown in Supplementary material indicate the
weight loss of Fe3O4@SiO2–NH2@Mn(III) nanoparticles is a multistage process that
Figure 4. FT–IR spectra of (a) [Mn(phox)2(CH3OH)2]ClO4 and (b) Fe3O4@SiO2–NH2@Mn(III) nanoparticles.

Mn(III) on Fe3O4 Nanoparticles
3031
Figure 5. (a) The UV–Vis spectrum of [Mn(phox)2(CH3OH)2]ClO4, (b) diffuse reflectance UV–Vis spectrum of
Fe3O4@SiO2–NH2, and (c) diffuse reflectance UV–Vis spectrum of Fe3O4@SiO2–NH2@Mn(III) nanoparticles.
corresponds to three peaks in the DTA curves. In the first step, the nanoparticles show a
small weight loss of 3.38% from 50 to 150 °C, mainly due to the loss of physically
adsorbed water. The second and third weight loss peaks at 300–600 °C are due to the loss
of organic groups (phox and amino propyl linkage).
The FT–IR, UV–Vis, and TGA studies provide evidence for anchoring of the manga-
nese(III) complex. Elemental analysis of Fe3O4@SiO2–NH2@Mn(III) nanoparticles gave
the percentages of carbon and nitrogen to be 6.01 and 1.59%, respectively. The significant
increase in C/N ratio from 3.3 in Fe3O4@SiO2–NH2 to 3.7 in Fe3O4@SiO2–NH2@Mn(III)
strongly justifies this conclusion that complex immobilization has been successful. The
loading of manganese(III) complex was 0.0208 mM gÀ1, determined by AAS test of
manganese after acid digestion.
Selective oxidation of sulfur-containing organic molecules, such as thiols and sulfides,
without the formation of over-oxidized products is one of the most important transforma-
tions in total synthesis, biological chemistry, and petroleum industries, and it has recently
received attention from several research groups [20–30]. These considerations led us to test
the Fe3O4@SiO2–NH2@Mn(III) nanocatalyst as a magnetically recyclable catalyst for
selective oxidation of thiols to disulfides (table 1). As proof of principle, 0.01 g of
Fe3O4@SiO2–NH2@Mn(III) nanocatalyst (0.0002 mM of manganese loading) was tested.
Catalysis in CH2Cl2/MeOH using 1 mM of both thiophenol and UHP was run to
selectively oxidize thiophenol to diphenyldisulfide in 90% yield (table 1, entry 5). Several
thiols, such as 4-methylthiophenol (table 1, entry 8), benzylthiol (table 1, entry 9),
4-bromothiophenol (table 1, entry 10), and naphthalenethiol (table 1, entry 11), underwent
oxidation and produced the corresponding symmetric disulfides. In all cases, only selective
formation of disulfides was observed and no thiolsulfonate, an over-oxidized side product,
was detected.
The heterogeneous Fe3O4@SiO2–NH2@Mn(III) nanoparticle is a more selective catalyst
than the homogeneous [Mn(phox)2(CH3OH)2]ClO4 complex (table 1, entry 5 versus 6).
Oxidation with [Mn(phox)2(CH3OH)2]ClO4 proceeded with a 55% yield of disulfides
accompanied by a significant amount of a side product (table 1, entries 6 and 7). The
structure of the side product was established as α-diphenyldisulfone on the basis of
elemental analysis and GC–MS (Supplementary material). Diphen is one of the various
possible oxidized forms of disulfides that still retain the S–S bond (scheme 2) [31]. It is

3032
M. Bagherzadeh et al.
Table 1. Oxidation of thiols by Fe3O4@SiO2–NH2@Mn(III) nanocatalyst.a
SH
Fe3O4@SiO2-NH2@Mn(III)/UHP
room temperature, 2 h
R
S
R
R
S
Selectivity to
disulfide (%)
Melting point of
product (°C)e
Entry
1
Thiol
Catalyst
Yield (%)b
SH
–
Trace
Trace
Trace
Trace
90
–
–
SH
SH
SH
SH
SH
SH
2
Fe3O4
–
–
3
Fe3O4@SiO2
–
–
4
Fe3O4@SiO2–NH2
–
–
5
Fe3O4@SiO2–NH2@Mn(III)
[Mn(phox)2(CH3OH)2]ClO4
[Mn(phox)2(CH3OH)2]ClO4
Fe3O4@SiO2–NH2@Mn(III)
Fe3O4@SiO2–NH2@Mn(III)
Fe3O4@SiO2–NH2@Mn(III)
100
80
82
100
100
100
59–61 (59–60)
59–61 (59–60)
59–61 (59–60)
53–54 (52–56)
67–69 (69–70)
91–93 (91–93)
6
54c
7
55d
SH
8
83
9
81
SH
SH
10
88
Br
SH
11
Fe3O4@SiO2–NH2@Mn(III)
87
100
141–143 (142–145)
aReaction conditions: thiol (1 mM), UHP (1 mM), catalyst (0.01 g, 0.0002 mM of Mn(III) complex), 2 mL solvent
(CH2Cl2–CH3OH (1 : 1 v/v)), and room temperature, 2 h.
bAll yields refer to isolated yields.
c0.025 mM of catalyst were employed.
d0.05 mM of catalyst were employed.
eValues in parentheses are the melting points reported [32, 33].
obvious that the formation of this over-oxidized product could be responsible for the low
yield of disulfide in Mn(III) homogeneous catalytic system. Direct oxidation of disulfides
to disulfones has not been reported until now.

Mn(III) on Fe3O4 Nanoparticles
3033
Scheme 2. Various oxidized forms of diphenyldisulfide.
Control experiments indicate that no oxidation occurred in the absence of catalyst (table 1,
entry 1). The use of Fe3O4, Fe3O4@SiO2, and Fe3O4@SiO2–NH2 nanoparticles in place of
Fe3O4@SiO2–NH2@Mn(III) led to the formation of the disulfide in very poor yield and
selectivity (table 1, entries 2–4).
Based on previously reported studies [25, 34], it is possible to postulate two mecha-
nisms for the catalytic oxidation of thiols to disulfides. In the first mechanism, the catalytic
reaction is mediated by one-electron reduction of the manganese complex, resulting in the
formation of Mn(II) intermediate and thiol radical [25]. The formation of S–S bond is
likely to take place via coupling of thiol radicals. The Mn(III) complex is then regenerated
with the reaction of Mn(II) intermediate and UHP, which re-enters the cycle. To test the
validity of this mechanism, a catalytic run was carried out in the presence of a radical
scavenger, 2,6-di-tert-butyl-p-cresol (0.5 equiv.), to retard the formation of thiol radicals. It
was observed that in the presence of a radical scavenger, the conversion did not decrease
at all. Hence, this radical mechanism is ruled out by our data. Oxidation of thiols is
assumed by the second mechanism to occur via reactive Mn(V)-oxo intermediates, which
are produced by the reaction of the hydrogen peroxide with Mn(III) and which then react
with the thiol in a fashion analogous to that observed previously for the oxidation of
sulfides [34]. We tentatively propose the latter mechanism for the oxidation of thiols by
Fe3O4@SiO2–NH2@Mn(III) catalyst (scheme 3).
One of the main objectives of this study was to synthesize a catalyst with high stability
and simple reusability. The recyclability of Fe3O4@SiO2–NH2@Mn(III) nanoparticle as
catalyst for the oxidation of thiols was surveyed. In total, six consecutive cycles were
carried out. Work-up in between cycles involved the separation of the catalyst particles
from the reaction mixture using a magnet and the removal of the clear supernatant solution
(figure 6). The catalyst particles were washed several times with CH2Cl2 and CH3OH,
dried, and then reused in the subsequent cycle. The results show that Fe3O4@SiO2–
NH2@Mn(III) nanocatalyst can be recovered, recycled, and reused more than five times
without any deterioration of conversion, selectivity, and magnetic properties (figure 7). In
addition, AAS analysis of the supernatant did not show any leaching of manganese species

3034
M. Bagherzadeh et al.
Scheme 3. Possible mechanisms for the oxidation of thiols by Fe3O4@SiO2–NH2@Mn(III).
Figure 6. (a) Fe3O4@SiO2–NH2@Mn(III) nanocatalyst dispersion in the reaction mixture and (b) Fe3O4@SiO2–
NH2@Mn(III) nanocatalyst adsorbed on the magnet.

Mn(III) on Fe3O4 Nanoparticles
3035
Figure 7. Reuse of [Mn(phox)2(CH3OH)2]ClO4 (black) and Fe3O4@SiO2–NH2@Mn(III) nanocatalyst (gray).
into the solution (detection limit: 0.1 ppm), which indicated that the manganese complex
was tightly bound to the surface. A total turnover number of 10,000 is possible after four
runs, demonstrating the efficient use of the catalyst in this highly selective reaction
(TON = (mol of product)/mol of manganese complex). This is not surprising because the
surface –NH2 shows considerable stability against substitution. On the other hand, all
attempts for the separation of homogeneous [Mn(phox)2(CH3OH)2]ClO4 catalyst from the
reaction mixture by solvent extraction method were unsuccessful, another advantage of
Fe3O4@SiO2–NH2@Mn(III) nanocatalyst over the homogeneous manganese complex.
Comparing our results on thiol oxidation with those of other similar studies [20–29]
reveals that the catalyst reported here is superior to some of the previously reported
catalysts and reagents in terms of conversions and reaction conditions. In contrast to
previously reported work, Fe3O4@SiO2–NH2@Mn(III)/UHP does not suffer from
drawbacks such as high reaction temperature [27], long reaction time [20], using large
amounts of hazardous materials [24, 29], and using stoichiometric amounts of expensive
reagents [22, 23, 28]. The attractive features of this catalytic system, such as low cost and
simple recyclability, make it particularly suitable for oxidation reactions.
In summary, a method has been presented for the immobilization of a manganese(III)
complex on silica-coated MNPs. The nanoparticles can be used as efficient, easily
recoverable catalysts for the selective oxidation of thiols to disulfides. With this catalytic
system, various thiols were transformed to structurally diverse disulfides in high yields and
selectivities. The catalyst particles can be easily collected at the bottom of the test tube
using a magnet and reused. The present protocol offers a straightforward path for the
immobilization of many other types of catalysts to MNPs, representing a useful alternative
to existing catalysts and immobilization methodologies.
Acknowledgments
We acknowledge the Research Council of Sharif University of Technology for the research
funding of this project. Omid Zandi and Meghdad Hosseini are kindly acknowledged for
the SEM analysis.

3036
M. Bagherzadeh et al.
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