Enhanced aqueous oxidation activity and durability of simple manganese(III) salen complex axially anchored to maghemite nanoparticles

Article abstract of DOI:10.1039/c6ra10527k

Simple Mn-salen complex was anchored coordinatively to γ-Fe₂O₃ nanoparticles through amine functionality which provide a new magnetically recoverable nanocatalyst with high oxidation activity and stability. Catalyst characterization was performed using FT-IR, UV-Vis, XRD, EDS, TGA and ICP-AES. TEM image revealed a quasi-spherical structure with size smaller than 20 nm for nanocatalyst. A thermal stability up to around 300 °C was verified for prepared nanocatalyst based on thermogravimetric analysis. Finally, the catalytic performance of magnetically recoverable Mn-catalyst was exploited in the green oxidation of different types of functional groups including olefins, alcohols, saturated hydrocarbons and sulfur containing compounds with n-Bu₄NHSO₅ (TBAOX) in water under heterogeneous conditions. The salen catalyst proved to be reusable for at least eight times and the oxidant's by-product (n-Bu₄NHSO₄) could also be recycled.

Full text of DOI:10.1039/c6ra10527k

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Page 1 of 23
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Enhanced aqueous oxidation activity and durability of simple
manganese (III) salen complex axially anchored to maghemite
nanoparticles
Abdolreza Rezaeifard,* Maasoumeh Jafarpour,* Alireza Farrokhi, Sousan Parvin and
Fahimeh Feizpour
Abstract:
Simple Mnꢀsalencomplex was anchored coordinatively to γꢀFe2O3 nanoparticles through
amine functionality which provide a new magnetically recoverable nanocatalyst with high
oxidation activity and stability. Catalyst characterization was performed using FTꢀIR, UVꢀ
Vis, XRD, EDS, TGA and ICPꢀAES. TEM image revealed a quasiꢀspherical structure with
size smaller than 20 nm for nanocatalyst. A thermal stability up to around 300 °C was
verified for prepared nanocatalyst based on thermogravimetric analysis. Finally, the catalytic
performance of magnetically recoverable Mnꢀcatalyst was exploited in the green oxidation of
different types of functional groups including olefins, alcohols, saturated hydrocarbons and
sulfur containing compounds with nꢀBu4NHSO5 (TBAOX) in water under heterogeneous
conditions.The salen catalyst proved to be reusable for at least eight times and the oxidant’s
byꢀproduct (nꢀBu4NHSO4) could also be recycled.
Keywords: Magnetically recoverable nanocatalyst; Oxidation; Water; Heterogeneous
catalyst; TetraꢀnꢀbutylammoniumOxone®
Catalysis Research Laboratory, Department of Chemistry, Faculty of Science, University of Birjand,
Birjand, 97179ꢀ414 Iran
* Corresponding Authors: Tel: +98 56 32202516; Fax: +98 56 32202515.Eꢀmail: rrezaeifard@birjand.ac.ir (A.
Rezaeifard); mjafarpour@birjand.ac.ir (M. Jafarpour)

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1.Introduction
Oxyfunctionalization of organic substrates by practical and sustainable catalytic methods is
of great interest from both an industrial, as well as enzymatic point of view.1 Metalꢀsalen
complexes which are easy to prepare and are consequently cheap; have attracted significant
attention in research areas ranging from catalysis to development of electrochemical sensors
and singleꢀmolecule magnets.2ꢀ4 They are stable in water, even at high pHs; and stable in a
reasonable range of temperatures. Nevertheless, its oxidative degradation and consequently
the drawbacks in separating the products and contamination by residual catalyst is the most
important problem of salen ligands. Besides, the major difficulty of homogeneous catalysis is
the need for separation and recovery of the metalꢀcomplexes from the reaction mixture at the
end of the process. Currently, transition metal complexes immobilized on different supports
to make homogeneous catalysts recyclable and economical. Although, the activity may
decrease initially, immobilization may be preferred due to the increased stability of the
catalyst compared with the homogeneous one which may quickly loss activity after short
periods of use.5ꢀ10 Heterogeneous metalꢀsalen complexes which are readily separable, offer
the opportunity to reduce costs as well as pollutants. A variety of methodologies,11,12
including anchoring of salenꢀbased catalysts through coordinative bond of the spacer to the
metal center,13ꢀ22 have emerged for the immobilization of homogeneous metalꢀsalen
complexes.
Moreover, to increase industrial interest for both homogeneous and heterogeneous catalytic
process; the replacement of harmful organic solvents with those are inherently benign is a
necessity today. Among solvents, water is of particular interest because of nonꢀtoxicity, nonꢀ
flammability, readily availability, and cheapness.23ꢀ29 It has also attracted much attention in
both industry and academia because of facile isolation of organic compounds as well as
different reactivity and selectivity patterns which may be accessed compared with those
observed in common organic solvents.30ꢀ38
Magnetic nanoparticles (MNPs) offer advantages inmultiꢀdisciplinary research
includingbiotechnology, biomedicine, magnetic resonance imaging (MRI), separation and
catalysis.39ꢀ42 It is well known that the activity and selectivityof magnetic nanoꢀcatalysts can
be manipulated by surface modification.43 The use of MNPs as catalysts or supports in
chemical synthesis hasbeen extensively studied in recent years as the
separationofnanosizedcatalysts after their use is one of the most challengingfeatures in the
sustainable process development.44ꢀ47

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Recently we have developed new magnetic nanocatalystsfor efficient oxidation of organic
compoundsbased on porphyrin and phthalocyanine complexes.48ꢀ50 Our promising results for
stability of supported catalyst during aqueous oxidation encouraged us to evaluate the activity
and stability of Mnꢀsalen complex which is more susceptible toward oxidative degradation.
Now, we wish to describe theremarkableoxidative efficiencyand stability of simple and easily
prepared Mn (III)ꢀsalencomplexafter being immobilized on functionalized nanosized γꢀ
Fe2O3.Selective oxidation of olefins, saturated hydrocarbons, alcohols and sulfides using nꢀ
Bu4NHSO5 (TBAOX) in water successfully occurred.The survival of the heterogeneous Mnꢀ
salen catalyst was confirmed by spectroscopic data as well as its significant potential for
recyclability and reuse.
2. Experimental
The procedures for the synthesis of MNP,51 SMNP,52 and Silanation of silicaꢀcoated
maghemite nanoparticles (ASMNP),53 as well as TBAOX,54 are given in supporting
information.
2.1.Synthesis of maghemite nanoparticles supported catalyst(Mn-salen@ASMNP)
4.0 g of dry ASMNP powder were mixed with 200.0 mL ethanol toproduce a homogenously
mixed solution, followed by sonicationthe mixture for 30 min. 0.3 g of Mnꢀsalen complex
was added under mechanical stirring and the mixture was heated to 80 °C for 8 h. The free
Mnꢀsalen was removed through soxhlet extraction with ethanol. The concentrated sample
stored in refrigerator to use.
2.2.General procedure for the epoxidation of olefins and alcohols
In a typical experiment, the reaction mixtures were prepared as follows: to a mixture of 1.00
mmol olefin (or alcohol) and 0.03 g Mn-salen@ASMNP (5. 0 × 10−3 mmol Mnꢀsalen, 0.50
mol%) in 2.00 mL bidistilled water at 60 °C was added 0.70 g (2.00 mmol) freshly prepared
TBAOX. After completion the reaction, the catalyst was removed with an external magnet
and the aqueous suspension was extracted thrice with ethyl acetate (3×2.5 mL). The
combined organic layers were dried over Na2SO4. The solvent was removed under reduced
pressure to give the desired product (Table 2 and 3) and the separated catalyst was washed
with EtOH, H2O and used directly for a subsequent round of reaction without further
purification. The purity of the products was confirmed by GC.
2.3.General procedure for the oxygenation of saturated hydrocarbons
In a typical experiment, the reaction mixtures were prepared as follows: to a mixture of 1.0
mmol of saturated hydrocarbon and 0.075 g of Mn-salen@ASMNP (1.2 × 10−2 mmol Mnꢀ

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salen, 1.2 mol%) in 2.0 mL bidistilled water at 80 °C was added 0.71 g (2.0 mmol) of freshly
prepared TBAOX. The catalyst was removed with an external magnet at the end of the
reaction and the aqueous suspension was extracted thrice with ethyl acetate (3×2.5 mL). The
combined organic layers were dried over Na2SO4. The solvent was removed under reduced
pressure to give the desired product(Table 2) and the separated catalyst was washed with
EtOH, H2O and used directly for a subsequent round of reaction without further purification.
The purity of the products was confirmed by GC.
2.4. General procedure for sulfide oxidation
In a typical experiment, the reaction mixtures were prepared as follows: to a mixture of 1.0
mmol of sulfide and 0.015 g of Mn-salen@ASMNP (2.5 × 10−3 mmol Mnꢀsalen, 0.25 mol
%) in 2.0 mL bidistilled water at 25 °C was added 1.07 g (3.0 mmol) of freshly prepared
TBAOX. At the end of the reaction, the catalyst was removed with an external magnet and
the aqueous suspension was extracted thrice with ethyl acetate (3×2.5 mL). The combined
organic layers were dried over Na2SO4. The solvent was removed under reduced pressure to
give the desired product (Table 3) and the separated catalyst was washed with EtOH, H2O,
and used directly for a subsequent round of reaction without further purification. The purity
of the products was confirmed by GC.
3.Results and discussion
3.1.Preparation and characterization of the Mn-salen@ASMNP
The
magnetically
recoverable
catalyst
Mn-salen@ASMNP,
(salen:N,N'ꢀ
bis(salicylidene)ethylenediamine
)
was prepared by immobilization of simple and easyꢀmade
Mn (III) salen complex on the functionalized nanosized γꢀFe2O3 (Scheme 1). Silica coated
magnetic nanoparticles (SMNP) was prepared by addition of tetraethoxysilane (TEOS) and
ammonia solution to asꢀprepared magnetic nanoparticles (MNP).51,52 The silica layered
system (SMNP) then offers the binding sites (Si–OH units) for the immobilization of the
molecular catalyst. For the grafting on the surface (3ꢀaminopropyl) triethoxysilane (APTES)
as chelating ligand was used.53 It was reacted with Mn (III) salen complex in ethanol to yield
the desired magnetically recoverable catalyst [Mn-salen@ASMNP] (Scheme 1).
The compositional analysis of the asꢀprepared Mn-salen@ASMNP nanocomposite was
examined by energyꢀdispersive Xꢀray spectroscopy (EDS) and inductively coupled plasma

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atomic emission spectroscopy (ICPꢀAES). The distinct peaks of carbon, nitrogen, oxygen,
silica, iron, and manganese in the EDS spectrum (Fig. 1) verified the incorporation of the
Mnꢀsalen complex on the surface of the synthesized nanocatalyst. According to ICPꢀAES
analysis, the Mn loading was found to be 0.17 mmolgꢀ1.
H2N
O
Si
O
O
O
NH2(CH2)3Si(OEt)3
TEOS
Si
NH2
O
O
O
Si
O
O
Fe2O3/SiO2 (SMNP)
Fe2O3
H2N
ASMNP
H2N
Mn(salen)Cl
O
Si
O
O
O
Si
NH2
O
O
O
Si
O
O
H2N
O
N
O
Cl
N
Mn
Mn-salen@ASMNP
Scheme 1. Preparation of Mn-salen@ASMNP
Fig. 1. EDS spectrum of the asꢀprepared Mn-salen@ASMNP

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The FTꢀIR spectra of MNP, SMNP and the Mn-salen@ASMNP are given in Fig. 2. The
bands at 582 and 634 cmꢀ1 substantiate the presence of the maghemite core (Fig. 1a).55 The
broad peaks at about 900–1200 cmꢀ1, assigned to the Si–O–Si and Si–OH stretching
vibrations, showthe silica coating of maghemite nanoparticles (Fig. 2b).56,57 The bands
around 3440 and 1627 cmꢀ1 may be attributedto OꢀH stretching and bending of adsorbed wat
er molecules.58 The FTꢀIR spectra of Mn-salen@ASMNP(Fig. 2c) shows two bands at 2874
and 2961 cmꢀ1 related to the CꢀH stretching vibration of the propyl groupsof APTES.15,59 The
frequency of NꢀH stretching vibrations of the amine group fall in the 3300ꢀ3400 cmꢀ1 range
and are obscured by the water band. Immobilization of Mnꢀsalen on amine functionalized
magnetic nanoparticles (Mn-salen@ASMNP) was demonstrated by appearance of signals at
about 1536, 1467, 1382 and 620 cmꢀ1 (Fig. 2c) corresponding to the vibration modes of neat
Mn(salen)Cl (Fig. 2d).15 The band at 470 cmꢀ1may be assigned to MnꢀN bond of axial
coordination of amino group into Mnꢀsalen complex (Fig 1c).16
Fig. 2.FTꢀIR spectra of MNP (a), SMNP (b), Mn-salen@ASMNP (c) and neat Mn(salen)Cl
complex (d).
XRD revealed a cubic structure for crystalline MNP (Fig. S1).60 The same set of
characteristic peaks was observed for ASMNP (Fig. S2) as well as Mn-salen@ASMNP (Fig.
S3), indicating the stability of the crystalline phase of the nanoparticles during the subsequent

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surface modification.61 The size of the particles was estimated to be 10 nm for Mn-
salen@ASMNP, using the Debye–Scherrer equation according to (311) reflection.
Fig. 3 shows the transmission electron microscopy (TEM) micrograph of Mn-
salen@ASMNP. The size distribution histogram shows that the maximum particlesize is in
the range of 8ꢀ14 nm (Fig. 3). Moreover, the TEM image revealed that the nanocatalyst
preserved its morphology after recovery (Fig. S4).
Fig. 3.TEM micrograph and size distribution histograms of asꢀprepared Mn-salen@ASMNP
The thermal behavior of Mn-salen@ASMNP (Fig. 4A) shows that the catalyst is stable up to
around 300 °C and after that the degradation of Mnꢀsalen occurred (Fig. 4B). A decrease in
the weight percentage at about 100 °C related to desorption of water molecules from the
catalyst surface. The organic parts were decomposed completely at 500 °C. From the TGA,
the amount of Mnꢀsalenwas evaluated to be 0.164 mmolgꢀ1, which is in agreement with the
value determined by ICPꢀAES (0.170 mmolgꢀ1).
Fig. 4. TGA thermograms of Mn-salen@ASMNP (A) and MnꢀSalen (B)

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Fig. 5 shows the hysteretic MꢀH curves of the pristine γꢀFe2O3 and Mn-salen@ASMNP at
298ꢁK. No remanence effect (superparamagnetic property) was observed for MNPs with
saturation magnetization ofabout 60 emu/g. The asꢀprepared Mn-salen@ASMNP
nanocatalyst exhibited superparamagnetic properties with decreased saturation magnetization
about 35 emu/g. The superparamagnetic properties of Mn-salen@ASMNP prevents
aggregation and enables them to redisperse rapidly when the magnetic field is removed.
Fig. 5. Magnetic hysteresis curves of the pristine γꢀFe2O3(a) and Mn-salen@ASMNP(b) at
298ꢀK
3.2. Olefin epoxidation activity
The experiments were initiated with oxidation of cyclooctene (0.20 mmol) with TBAOX
(0.40 mmol) in neat water under air, which did not proceed in the absence of catalyst at room
temperature as well as at higher temperatures. When 0.5 mol% of free Mn(salen)Cl was
added to this aqueous reaction mixture at 60 °C, cyclooctene conversion was 22 and 95%
within 1 and 12 h, respectively. Nevertheless, the use of small amount of supported catalyst
(Mn-salen@ASMNP, 0.2 mol%) improved markedly the reaction rate and 49, 58 and 69%
cyclooctene were converted within 3 h at 25, 40 and 60 °C respectively using 1 equiv.
TBAOX. When the oxidant/cyclooctene molar ratio increased up to 1.5 and 2 the conversion
reached to 73 and 100% at 60 °C respectively using 0.2 mol% of catalyst at the same
conditions. Our investigation revealed that the reaction rate is dependent on the catalyst
concentration (Fig. 6). The best performance was obtained when 0.5 mol% of the supported
catalyst was used. It led to complete conversion of the cyclooctene within 1 h with the
formation of 95% of the pertinent epoxide (12, 46 and 57 cyclooctene epoxide after 15, 30
and 45 min, respectively). The very slow oxidation activity of parent materials, MNP, SMNP,

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ASMNP (10ꢀ15% cyclooctene oxide after 12 h), substantiated that the Mnꢀsalen complex
attached to magnetic support acts as active site of catalyst.
97
95
100
89
74
80
60
48
40
23
20
0
0.01
0.05
0.1
0.2
0.5
1
Catalyst mol%
Fig. 6. Catalyst concentration effect screening on the cyclooctene epoxidation. The reactions
were run for 3 h using 1 mmol cyclooctene, 2 mmol TBAOX, in 2.5 ml water at 60 °C except
for reactions using 0.5 and 1 mol% catalyst which monitored after 1 h.
It should be noted that the addition of additives (10 times vs. catalyst), such as imidazole
(44%, 30 min), pyridine (46%, 30 min), and sodium dodecyl sulfate under CMC conditions
(0.008 mol L−1) (51%, 30 min)], as well as performing the reaction in an inert atmosphere
(Ar, 48 and 94%, after 30 and 60 min) did not change the epoxide yield under optimized
conditions. The standard buffered solutions were also used at 25 °C to screen the pH effect on
the oxidation performance. Cyclooctene conversions were 66, 70, 60, 70 and 77% at pH 2, 4,
6, 7, 9 respectively after 3 h.
The oxidizing potential of other common oxidants was also screened in the oxidation of
cyclooctene under optimized conditions. As depicted in Table 1, actually no activity was
observed using O2, H2O2, perborate, percarbonate and PhI(OAc)2 and only trace amounts of
the cyclooctene oxide was observed in the presence of tꢀbutylhydroperoxide (TBHP),
Oxone®, NaIO4 and UHP. Nevertheless, the addition of an equimolar amount of nꢀ
Bu4NHSO4 (TBAHSO4) to Oxone® improved the oxidation rate and a 31% yield of the
epoxide was obtained within 60 min. When the reaction was carried out using TBAOX, the
conversion of cyclooctene and the yield of its epoxide reached to 100 and 95% respectively.

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The good dispersity of Mn-salen@ASMNP in the aqueous solution of TBAOX seems to be
the most important factor that affects the efficiency as described for the improved catalytic
activity of neat CuPc,62 as well as CuSPcS@ASMNP,49 in water. TBAOX, as a quaternary
salt, acts as a stabilizer which improves the dispersibility of the magnetic nanoparticles which
are prone to aggregate.
Table 1. Screening of various oxidant in the epoxidatin of cyclooctene a
Entry Oxidant
Conversion (%)
1
2
3
4
5
6
7
O2
0
0
0
0
0
5
8
H2O2
NaBO3. 4H2O
Na2CO3. 1.5 H2O2
PhI(OAc)2
TBHP
Oxone®
8
NaIO4
10
9
UHP
13
10
Oxone+Bu4NHSO4
TBAOX
31
100 (95)b
11
a
Reaction were run using cyclooctene (0.2 mmol), oxidant (0.4 mmol),
Catalyst (0.006 g) in water (0.4 ml) at 60 oC for 1 h. b GC yield.
Finally, the influence of organic solvents on the efficiency of cyclooctene oxidation in this
catalytic system was probed. After 1 h, actually trace amount of epoxidation product was
appeared at 60 °C in commonly used solvents such as ethyl acetate (10%), chloroform (23%),
methanol (27%) and acetonitrile (31%). The only exception was ethanol, which gave 85%
cyclooctene oxide under optimized conditions. Obviously, the use of water as a standard
“green” reaction medium is a remarkable advantage, especially due to the very low solubility
of the organic compounds in water, providing easy separation of the products.
Having proved the activity of Mn-salen@ASMNP in themodel reaction, the context of
protocol was subsequently employed to a range of olefins as shown in Table 2. Desired
conversions and remarkable selectivity to epoxides were obtained for different olefins. It led
to complete conversion of cyclooctene, norbornene, indene, αꢀPinene and cyclohexene with
the formationof the corresponding epoxides as soleproducts (entries 1–5). No allylic
oxidation products were detected in the case of cyclohexene. A salient feature of title

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epoxidation system is moderate yields of 1,2ꢀepoxyoctane resulting from less reactive
terminal linear aliphatic olefin (entry 6), while no activity was observed in the presence of
Mnꢀporphyrin and Cuꢀphthalocyanine catalysts in our previous related reports.48,49 The
method possesses novelty regarding chemoselectivity. The hydroxyl group which is sensitive
to oxidation remained completely intactproducing the pertinent epoxides in high yields
(entries 7,8). To confirm our claim, a competitive reaction between cyclooctene and benzyl
alcohol was run. After 1 h, only trace amount of benzaldehyde (4%) was appeared in
association with 93% cycloocteneoxide. However, the oxidation of styrenes gave exclusively
the related carbonyl compounds with excellent conversions, indicating that the epoxides
undergo ringꢀopeningreactions (entries 9ꢀ12). It seems that benzaldehydes and acetophenone
are favorably formedat high temperatures in the oxidation of styrene,4ꢀsubstituted styrenes
and αꢀmethylstyrene, respectively, because the high temperature will supplyenough energy to
break the C=C bond.63 These results arefurther supported by the oxidation of styrene oxide as
a substrateunder the same conditions. Benzaldehyde was formed solely in 93% yield within
60 min.
Table 2. The aqueous oxygenation of olefins using TBAOX catalyzed by Mn-
salen@ASMNPa
Entry
Substrate
Product
Yield%b
95
Tim/h
1
1
O
2
O
85
2
3
4
80
70
3
2
O
O
O
5
97
5

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6
7
8
34
5
5
5
O
OH
O
OH
74
89
OH
OH
O
O
90
9
1
5
CHO
10
94
91
CHO
3
11
12
Cl
Cl
CHO
93
3
a
The reactions were run under air in bidistilled water at 60 °C and the molar ratio of the olefin :TBAOX
b
:catalyst was 200: 400 :1. GCyield based on internal standard. The selectivity of the products were >99%
according to GC analysis.
3.3.Oxidation of alcohols and saturated hydrocarbons
When the above epoxidation protocol (200/400/1 molar ratio for substrate:TBAOX:catalyst,
in water at 60°C) was employed for oxidation of different alcohols, moderate to high yields
and excellent selectivity of pertinent carbonyl compounds were observed (Table 3, entries 1ꢀ
8). 1ꢀphenylethanol as a secondary benzylic alcohol oxidized easily and acetophenone was
obtained quantitatively (10, 30, 60 and 88% yields after 15, 30, 45 and 60 min respectively).
However, less reactivity was observed for primary benzylic alcohols, so thatafter 2 h
oxidation of benzyl alcohol 72% benzaldehyde was observed (entry 4). Inspection of the
results in table 3 clearly showed the significance influence of electronic and steric properties

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of the molecules on the oxidation rates. Sterically demanding alcohols such as benzhydrol
(entry 2, 60%/3 h) and 4ꢀtꢀbutylbenzylalcohol (entry 6, 30%/3h) showed less oxidation
reactivity than the related unhindered molecules (entries1 and 4). Moreover, higher
conversion rate and production were observed for electronꢀrich 4ꢀmethoxybenzyl alcohol
(entry 3, 82%/2h) than that of benzyl alcohol with no group on the ring (entry 4, 71%/2) and
especially substituted with an electronꢀdeficient group (entry 5, 40%/3 h). Moderate to good
yields of carbonyl products were achieved in the oxidation of saturated alcohols (entries 7
and 8, 34ꢀ62%). It should be noted that no trace of ester and benzoic acid was observed
resulting from over oxidation of secondary and primary alcohols, respectively.
We also checked the activity of heterogeneous catalyst towards oxidation of saturated
benzylic hydrocarbon and promising results were obtainedusing 2 equiv. TBAOX in water at
80 °C containing 1 mol% of catalyst (Table 3, entry 9ꢀ14). Under these conditions
ethylbenzene was converted completely to acetophenone within 6 h (30, 45, 57,
80and89%after 1, 2, 3, 4 and 5 h, respectively). 4ꢀNitroethylbenzene, as an electronꢀdeficient
benzylic hydrocarbon gave solely the pertinent ketone in moderate yield (entry 12, 60%),
while, its electronꢀrich counterpart, 4ꢀethylanisole (entry 11) converted to approximately
equal yield of the corresponding alcohol (40%) and ketone (50%). Attractive results were
observed for selectivity of the procedure which are of extreme interest from an industrial
point of view.64 2ꢀPhenyl ethanol selectively oxidized to 2ꢀhydroxy acetophenone while,
hydroxyl group was tolerated in the reaction (entry 13). Moreover, oxidation of adamantan
gave absolutely 1ꢀadamantanol in excellent yield (entry 14). It should be noted that, when the
supported Mnꢀsalen catalyst provided herein was replaced by neat Mn(salen)Cl complex, the
efficiency of oxidation reactions reduced significantly (50 and 58% yield of acetophenone
were detected in the oxidation of 1ꢀphenylethanol and ethylbenzeneafter 1 and 6 h,
respectively).

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Table 3. The aqueous oxygenation of alcohols and hydrocarbons using TBAOX catalyzed by
Mn-salen@ASMNPa
Entry
Substrate
Product
Yield%b
Time (h)
OH
O
1
2
88(80)
1
3
OH
O
60 (54)
CHO
OH
3
4
5
82 (78)
72 (65)
40
2
3
3
MeO
MeO
CHO
OH
CHO
OH
O2N
O2N
OH
6
7
30
62
3
2
CHO
OH
O
8
34
2
OH
O
O
9
90 (86)
94 (88)
6
6
O
10

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O
40
50
MeO
11
12
6
OH
MeO
MeO
O
60 (55)
6
6
O2N
O2N
O
OH
OH
13
14
87 (80)
94
6
OH
a The reactions were run under air at 60 °C for entries 1ꢀ8 with molar ratio of 200: 400 :1 for alcohol
:TBAOX:catalyst and at 80 °C for entries 9ꢀ14 with molar ratio of 100:200:1 for
hydrocarbon:TBAOX:catalyst. b GCyield based on internal standard. The numbers in parentese are the
yield of isolated products. The selectivity of the products were >99% according to GC analysis
3.4. Sulfide oxidation
Aqueous oxidation of thioanisole (1mmol) using 3eqiv. of TBAOX at 25 °C, under catalystꢀ
free conditions gave a mixture of the related sulfoxide (60%) and sulfone (30%) after 3 h.
Nevertheless, small amount of Mn-salen@ASMNP (0.2 mol%) resulted complete
conversion of thioanisole and the pertinent sulfone produced solely within 3 h (67 and 88%
sulfone selectivity after 1 and 2 h). Selective production of sulfoxide product under different
conditions such as lesser amount of catalyst (50% selectivity using 0.1 mol% of catalyst and
3 equiv. TBAOX) and oxidant (65% selectivity using 0.2 mol% catalyst and 1 equiv.
TBAOX) failed. The sulfone selectivity dropped to 20% when the catalyst was replaced by
neat Mn(salen)Cl complex with the same quantity. Under the catalytic influence of
heterogeneous Mn-salen@ASMNP, different sulfides were oxidized completely to sulfone
products with >80% selectivity at 25 ºC (Table 4). The promising results were observed for
the chemoselectivity of the method making it a proper alternative for sulfone production.
Sulfides having a benzylic C–H bond (entry 3), carbon–carbon double bond (entries 4,5)
were transformed into the corresponding sulfones in excellent yield without formation of any
epoxide, alcohol andcarbonyl byꢀproducts. Completive reactions between thioanisole and

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equimolar amount of other substrates used in this study such as cyclooctene, 1ꢀphenylethanol
and ethylbenzene under conditions optimized for sulfide oxidation gave methyl phenyl
sulfone as sole product confirming our claim about chemoselectivity of the procedure.
Finally, under the same conditions, phenyl disulfide (entry 8), and 4ꢀmethylbenzenethiol
(entry 9) were oxidized selectively to the pertinent thiosulfonates which are biologically and
medicinally importantcompounds.65
Table 4. Aqueous oxidation of organosulfur compounds using TBAOX catalyzed by Mn-
a
salen@SMNP
Entry Substrate
Productb
Yield %c,d
O
S
O
S
1
93
O
O
S
2
75
90
S
3
O
O
O
O
S
S
S
S
4
96
S
O
O
5
93
89
91
S
O
O
6
S
S
O
7
S
S
O
O
O
S
8
92
88
S
S
S
SH
O
O
9
S
S
a The reactions were run under air in water at 25°C for 3 h and the molar
b
ratio of substrate/TBAOX/catalyst was 500:1000:1. The products were
identified by comparison with authentic samples.66 cThe yield of isolated
d
products. The selectivity of the products were >99% according to GC
analysis,except for entry 2 and 3 which gave 20 and 5% of the related
sulfoxides, respectively.

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3.5.The catalyst activity, reusability and stability
To evaluate the activity of Mn-salen@ASMNPin this oxidation reactions, TOF of different
types of substrates were determined. The desired TOF/h of 190, 176 and 240 were obtained
in the oxidation of cyclooctene, 1ꢀphenyl ethanol and thioanisole, respectively. Comparison
of the results obtained for cyclooctene epoxidation (190/h) than some of those previously
reported methods demonstrated well that Mn-salen@ASMNP is more efficient (Table 5).67ꢀ
75
Table 5. Comparison of results for epoxidation of cyclooctene catalyzed by catalyst 1 and
comparison with some catalysts reported in theliterature.
Entry
1
Catalyst
Cat. mol %
Time (h)
Yield (%)
95
TOF (hꢀ1)
190
Ref.
This work
21
0.5
1.6
Mn-salen@ASMNP
[Mn (salophen) Cl]@SiImꢀFe
1
2
3
4
5
6
7
8
1
100
62.5
[Mn(TPP)Cl@ABIꢀMWCNT]
[Mn(TPP)Cl@APyꢀMWCNT]
PVMo@MCMꢀ41ꢀIm
3.2
2.5
2.5
25
3
99
12.38
10.26
12.27
27.2
67
68
69
70
71
20
3.8
98
0.6
92.8
98
[W(CO)5@APyꢀMWCNT]
[W(CO)6@DABꢀMWCNT]
1.2
0.6
2
98
81.7
[Mn(salophen)Cl]@amineꢀ
MWCNT
6, 3.3
2.5
97ꢀ100
6.6ꢀ8
9
[Mn(TNH2PP)Cl]@WCNT
5
2
95
97
9.5
72
73
10
Mn(TPyP)/SiO2ꢀFe3O4
1.9
2.75
18.65
Recovery of Mn-salen@ASMNP catalyst was easy and efficient. When the magnetic stirring
was stopped, the catalyst absorbed onto the magnetic stirring bar. The catalyst was recovered
by decantation of the reaction mixture in the presence of an external magnet. It was then
washed with ethyl acetate or ethanol as safe solvents, dried under vacuum. To avoid errors
resulting from loss of the catalyst during work up, the recovered catalyst was weighted before
using in the next run. The ease of recovery, combined with the inherent stability of the silicaꢀ
protected Fe2O3 nanoparticle constituent, allowed the catalyst to be retrieved efficiently over
at least eight times in the oxidation of cyclooctene with only a 7% decreas in the epoxide
yield (Table S1). Using a hot filtration method, the Mn content in the filtrates was analyzed
by ICPꢀAES after each run. Negligible quantities of Mn were found only at the first two runs
(0.87 and 0.12% Mn leached at first and second run respectively), and no trace of Mn
dissolved in the filtrate was detected for next runs (Table S1), confirming that the complex
was firmly anchored to the SMNP support. When the filtrate charged by new samples of
cyclooctene and TBAOX, the conversion was about 9% after 12 h under the optimized
conditions. The comparison of the FTꢀIR spectra of fresh and used Mn-salen@ASMNP (Fig.

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7), UVꢀVis (Fig. S5) and EDS spectrum (Fig. 1 and S6) demonstrated that the catalyst
preserved its structure after multiple reuses. It is worth to mention that, the oxidantbyꢀproduct
(TBAHSO4) could be separated by lyophilizing of the aqueous phase (>90%) and reused in
the preparation of TBAOX. Therefore, the use of water as a standard green media which also
provides easy isolation of hydrophobic organic products, the repeated catalyst recycling and
reuse as well as the absence of additives, toxic reagents, or solvents provide many appropriate
conditions to make a sustainable method. The additional advantage of reported magnetic
nanocatalyst relates to facile and inexpensive synthesis of Mnꢀsalen catalyst compared with
porphyrin and phthalocyanine counterparts used in our previous related works.48ꢀ50 Moreover,
different types of functional groups including olefins, alcohols, saturated hydrocarbons and
sulfur containing compounds were oxidized successfully in aquase media regardless of
reaction type.
Fig. 7. FTꢀIR spectra of fresh Mn-salen@ASMNP(a) and after recovering from cyclooctene
(b) and 1ꢀphenylethanol (c) oxidation reactions according to the procedures reported in
Tables 2 and 3.
4.Conclusion
In summary, axial anchoring of simple Mnꢀsalen complex to amineꢀfunctionalized silica
coated magnetic nanoparticles provided a robust and magnetically recoverable nanocatalyst
(8ꢀ14 nm). Using an aqueous solution of TBAOX, the oxidation of olefins, saturated
hydrocarbons and alcohols proceeded well in the presence of a low catalyst loading of
heterogeneous Mnꢀsalen nanocatalyst. In addition, an appropriate alternative for selective
oxidation of sulfides to the sulfones in neat water was developed using Mnꢀcatalyst provided

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herein. Surfactants, additives, toxic reagents or solvents, and byꢀproducts removed from the
presented procedures. Besides these advantageous, the use of water as a standard “green”
solvent providing easy and safe workꢀup procedure, comfortable and inexpensive synthesis of
Mnꢀsalencomplex, easily catalyst and oxidant separation and recycling, and lack of byꢀ
products, make title methodologies cost effective and ecoꢀfriendly systems which may be
amenable to industrial uses.
Acknowledgements
Support for this work by Research Council of University of Birjandis highly appreciated. We
also thank “Iran Science Elites Federation” for Partial support of this work.
Supporting Information
Experimental procedures, XRD, TEM image, UVꢀvis, EDS and the results for recycling of
catalyst.
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Graphical Abstract
Attachment of simple Mn (III) salen to functionalized magnetic nanoparticles provided a robust
magnetically recoverable nanocatalyst for aqueous oxygenation of various substrates using n-
Bu4NHSO5.
O
Si
O
O
H2
N
O
N
O
Cl
N
Mn
S
O
O
S
S
O
O
O
O
OH