A magnetic supported iron complex for selective oxidation of sulfides to sulfoxides using 30% hydrogen peroxide at room temperature

Abstract

A magnetic supported iron (iron(II) acetylacetonate) was synthesized to be used as an efficient and recyclable heterogeneous catalyst for the selective oxidation of sulfides to corresponding sulfoxides using H₂O₂ as a green oxidant at room temperature. The synthesized Fe₃O₄@SiO₂-APTES(Fe(acac)₂) catalyst presented excellent sulfide conversion and good sulfoxide selectivity. It can be easily recovered and reused for 8 reaction cycles without considerable loss of activity. The facile recovery of the catalyst is carried out by applying an external magnet device. The catalyst was fully characterized by techniques of TEM, SEM, XRD, EDS, FTIR, TGA, ICP-AES, VSM and elemental analysis (CHN).

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Introduction

The integration of nanotechnology with green chemistry offers innovative prospects to meet stringent demands for ecological and economical sustainability. The significant advancement in nanotechnology over the past decades has led to potential applications of magnetic nanoparticles (MNPs) in numerous scientific fields, including catalysis, biomedicine, environmental protection and energy storage.^1–4 Magnetic nanoparticles (MNPs) as catalyst supports have been attracting more and more attention because they are readily dispersed in reaction solution with intrinsically high surface area rending the efficient accessibility of substrates to the surface. In addition, they are super-paramagnetic and can be easily recovered from the reaction mixture using an external magnet.^5–7

Unprotected MNPs are often unstable and tend to aggregate during the catalytic transformations. Therefore, MNPs are coated by silica which acts as a stabilizer, limiting the effect of the outside environment on the core particles. Encapsulation of MNPs with amorphous silica not only contributes to the amelioration of chemical stability and dispersibility but also combines the advantageous properties of the magnetically responsive core and possible further surface-functionalized silica shell.^8,9 At this time, silica-coated MNPs have appeared as versatile supports for the immobilization of active sites to form magnetically recyclable heterogeneous catalysts.⁶

Magnetically recoverable heterogeneous catalysts have been developed and applied in versatile organic synthesis for a wide range of catalytic reactions, including coupling reactions,^10–12 Friedlander reaction,¹³ hydrogenation, hydroformylation, epoxidation reactions,¹⁴ selective oxidation of sulfides to sulfoxides.¹⁵

The selective oxidation of sulfides to sulfoxides is a great important in synthesis of chemically useful and biologically active molecules such as drugs.^16–18 A stoichiometric amount of organic or inorganic oxidizing agents is required to affect this oxidation process and thus are dangerous and a large amount of toxic waste would be generated.^19,20 In recent years, the procedures employing molecular oxygen or hydrogen peroxide as a primary oxidant in the presence of a catalyst have been found to be promising. Hydrogen peroxide is a non-toxic, cheap and effective oxidizer reagent. In addition, the oxidation reaction by hydrogen peroxide can be controlled easier than molecular oxygen and air.^21–23

So far, some interesting transition metal catalysts have been reported for the sulfide oxidation, including Cu,²⁴ Mo,¹⁶ Ti,²⁵ V²⁶ and Fe²⁷ in the presence of a suitable oxidant such as hydrogen peroxide. Owing to low price and low toxicity of iron, Fe complexes have been widely used as homogenous catalysis. Very recently, the elegant work of He and co-workers²⁸ has shown that Fe(acac)₂ can be effectively applied for selective oxidation of sulfide to sulfoxide using oxygen in polyethylene glycol (PEG 1000) as solvent.

Although this protocol has shown remarkable properties such as high activity and selectivity in oxidation of sulfide to sulfoxide, but its use has been limited due to the solubility of catalyst in the liquid phase along with reactants. Therefore, the catalyst separation from the reaction products, recycling and reuse is difficult.

One way to solve this drawback is to immobilize catalytic system onto a large surface area solid carrier. One of the solid carriers is magnetic nanoparticles. The use of magnetic nanoparticles enables the separation from reaction mixture by an external magnet and reuse of the catalyst itself. In this study, we report our results about the preparation of Fe₃O₄@SiO₂-APTES(Fe(acac)₂) composite as a recoverable heterogeneous nanocatalyst for selective oxidation of sulfides to sulfoxides using 30% hydrogen peroxide as oxidant under mild reaction conditions in ethanol as green solvent.

Experimental

Materials and methods

All chemicals were purchased from Sigma-Aldrich and used without further purification. X-ray diffraction (XRD) patterns were obtained with a APD 2000, using Cu Kα radiation (50 kV, 150 mA). Fourier transform infrared (FT-IR) spectra of KBr powder-pressed pellets were recorded on a ABB Bomem MB100 spectrometer. A TGA-Q5 thermogravimetric analyzer was used to study the thermal properties of the compounds under an inert N₂ atmosphere (at 20 mL min⁻¹) and heating at a rate of 10 °C min⁻¹. The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were performed on KYKY-EM3200 and Philips CM30, respectively. An energy dispersive detector (EDS) coupled to the microscope was used to identify chemical elements of the prepared catalyst.

The room-temperature magnetization in the applied magnetic field was performed by a homemade vibrating sample magnetometer (Meghnatis Daghigh Kavir Company, Iran) from −10 000 to +10 000 Oersted. The immobilized iron content on the catalyst was measured by inductively coupled plasma atomic emission analysis (ICP-AES, PerkinElmer Optima 3300 DV). Elemental analysis (CHN analysis) was carried out on PerkinElmer, USA (2400, Series II).

Synthesis of Fe₃O₄ magnetic nanoparticles (MNPs)

The Fe₃O₄ magnetic nanoparticles were prepared as reported in literature.²⁹ Briefly, FeCl₃·6H₂O (4.8 g, 0.018 mol) and FeCl₂·4H₂O (1.8 g, 0.0089 mol) were added to 100 mL deionized water and vigorously stirred (700 rpm) under Ar atmosphere for 1 h until the salts dissolved completely (Scheme 1). Then, 10 mL of 25% NH₄OH was added dropwise into the reaction mixture in 10 min and MNP black precipitate was formed immediately. After continuously mechanical stirring for 1 h, the precipitate was separated by an external magnet and washed with the double distilled water in five times, and then vacuum-dried at 50 °C overnight.

Scheme 1 Preparation of magnetically recoverable heterogeneous nanocatalyst Fe₃O₄@SiO₂-APTES(Fe(acac)₂).

Synthesis of silica coated Fe₃O₄ magnetic nanoparticles (Fe₃O₄@SiO₂)

1 g of Fe₃O₄ magnetic nanoparticles was ultrasonically dispersed in a solution containing 40 mL ethanol and 10 mL water, and then loaded into a three-necked bottle. The pH value was adjusted to 10 with an ammonia solution and 0.5 mL TEOS was dropwise added and stirred at 50 °C for 6 h to obtain magnetic nanoparticles of Fe₃O₄@SiO₂. After washing with ethanol and water for several times, magnetic nanoparticles of Fe₃O₄@SiO₂ were dried at 60 °C overnight.

Synthesis of amino-functionalized Fe₃O₄@SiO₂

3-Aminpropyltriethoxysilane (APTES) (1 mL) was added to a suspension of Fe₃O₄@SiO₂ (1 g) in dry toluene (25 mL) and refluxed for 24 h under N₂ atmosphere. Afterwards, the sample was collected by magnetic separation, washed with toluene and anhydrous ethanol several times, and then dried under vacuum in an oven at 60 °C overnight. The resulting material was denoted as Fe₃O₄@SiO₂-APTES.

Immobilization of [Fe(acac)₂] onto Fe₃O₄@SiO₂-APTES

Fe₃O₄@SiO₂-APTES (1 g) was dispersed into 20 mL dry dichloromethane (CH₂Cl₂) under mechanical stirring. [Fe(acac)₂] (1 mmol) was then added into the above mixture and then refluxed for 16 h. The resulting nanocomposites (nominated as Fe₃O₄@SiO₂-APTES(Fe(acac)₂)) were magnetically collected, washed with dichloromethane, ethanol and water and dried under vacuum (Scheme 1).

General procedure for the oxidation of sulfides to sulfoxides

A mixture of sulfide (1 mmol), 30% H₂O₂ (1.5 equiv.), Fe₃O₄@SiO₂-APTES(Fe(acac)₂) catalyst in ethanol (3 mL) was stirred at 25 °C. Completion of the reaction was indicated by TLC (n-hexane/ethylacetate 10 : 1) and gas chromatography (GC). After completion of the reaction, the catalyst was separated by an external magnet and then water (10 mL) was added to the mixture and extracted by ethyl acetate (3 × 5 mL). After drying with anhydrous sodium sulfate, the organic residue was analyzed by GC and then purified by column chromatography on silica gel with ethyl acetate/hexane (1 : 10) to afford the desired product. The separated catalyst was washed with EtOH and used directly for a subsequent round of reaction without further purification. All reaction products were identified by GC chromatogram, IR spectra and melting point as compared with authentic samples.

Results and discussion

Catalyst characterization

The crystalline structure of the synthesized Fe₃O₄@SiO₂-APTES(Fe(acac)₂) catalyst was identified by wide angle XRD. As shown in Fig. 1, six characteristic peaks (2θ = 30.8, 35.4, 43.6, 53.5, 57.7, and 62.4) corresponding to (220), (311), (400), (422), (551) and (440) diffraction planes of Fe₃O₄ MNPs are in good agreement with the standard XRD data for the cubic Fe₃O₄ phase of inverse spinel crystal structure (JCPDS card numbers 89-43191, 19-0629, 79-0419).^30,31

Fig. 1 The wide-angle XRD pattern of the Fe₃O₄@SiO₂-APTES(Fe(acac)₂) catalyst.

Typical magnetization curves as a function of the applied field at room temperature are shown in Fig. 2. As seen from Fig. 2, all of the samples exhibit superparamagnetic behavior and had little hysteresis, remanence and coercivity. Low decreasing of saturation magnetization of Fe₃O₄ from about 70 emu g⁻¹ (a) to almost above 59 emu g⁻¹ (b) for Fe₃O₄@SiO₂ illustrated that the coated silica on Fe₃O₄ is thin layer and then the weight contribution from nonmagnetic portion is low. Also, the Fe₃O₄@SiO₂-APTES(Fe(acac)₂) catalyst could be easily separated by applying an external magnet (Fig. 2). This property provides an easy and efficient way to separate and recycle the catalyst from heterogeneous systems, which minimizes the loss of catalyst during the separation stage.

Fig. 2 Magnetization curves of Fe₃O₄ (a), Fe₃O₄@SiO₂ (b), and Fe₃O₄@SiO₂-APTES(Fe(acac)₂) catalyst (c). The inset is a photograph of Fe₃O₄@SiO₂-APTES(Fe(acac)₂) catalyst under an external magnetic field.

The transmission electron microscopy (TEM) and scanning electron microscopy (SEM) images for the Fe₃O₄@SiO₂-APTES(Fe(acac)₂) catalyst are shown in Fig. 3. Both the TEM and SEM showed that the Fe₃O₄@SiO₂-APTES(Fe(acac)₂) nanoparticles were present as spherical and uniform particles and the size of nanoparticles was less than 50 nm. The EDS analysis also confirms the presence of C, N, O, Si, and Fe atoms in the structure of Fe₃O₄@SiO₂-APTES(Fe(acac)₂) catalyst (Fig. S1, see ESI†).

Fig. 3 Transmission electron microscopy (TEM) image (a), scanning electron microscopy (SEM) image (b).

In order to confirm the existence of organic groups on the surface of Fe₃O₄@SiO₂, FTIR spectra of Fe₃O₄@SiO₂-APTES and the Fe₃O₄@SiO₂-APTES(Fe(acac)₂) catalyst are shown in Fig. 4. As shown in Fig. 4a, there are two characteristic absorption bands at 623 cm⁻¹ and 1098 cm⁻¹ which correspond to the stretching vibration of Fe–O and Si–O, respectively.³² The bands at 807 cm⁻¹ and 467 cm⁻¹ are due to the deformation of Si–O bond.³³ Fig. 4a also exhibits absorption bands in the 2855 cm⁻¹ and 2927 cm⁻¹ which are assigned to the stretching of C–H bonds and the broad peak at 3423 cm⁻¹ is due to the O–H and N–H vibrations. The band around 1620 cm⁻¹ is due to the bending vibration of water molecules adsorbed on the surface (Fig. 4).

Fig. 4 The FTIR spectra of Fe₃O₄@SiO₂-APTES (a) and Fe₃O₄@SiO₂-APTES(Fe(acac)₂) catalyst (b).

Fig. 5 The recycling experiment of catalyst in the oxidation reaction of methyl phenyl sulfide to methyl phenyl sulfoxide under optimized condition; the oxidation reaction was quenched after 120 min at each step.

The FT-IR spectrum of Fe₃O₄@SiO₂-APTES(Fe(acac)₂) catalyst shows peaks which are clearly different from those of Fe₃O₄@SiO₂-APTES. The FT-IR spectrum of Fe₃O₄@SiO₂-APTES(Fe(acac)₂) catalyst (Fig. 4b) shows several new sharp bands in the range of 1300–1600 cm⁻¹ which don't exist in the spectrum of Fe₃O₄@SiO₂-APTES.³⁴

The peaks at 1523 cm⁻¹ and 1573 cm⁻¹ correspond to the double bond of C=C and carbonyl group, respectively. The peak at 932 cm⁻¹ and relatively small peak at 1630 cm⁻¹ are also corresponding to stretching vibrations of C–O and C=N bonds, respectively. By going from Fe₃O₄@SiO₂-APTES to Fe₃O₄@SiO₂-APTES(Fe(acac)₂) catalyst, the intensity of stretching vibration of C–H bonds at 2863 cm⁻¹, 2932 cm⁻¹ and also the bending vibration of H–C–H bond at 1384 cm⁻¹ increases.^34,35 Thus, it can be confirmed that surface modification of the Fe₃O₄@SiO₂-APTES by anchoring of Fe(acac)₂ was successful.

Thermogravimetric analysis (TGA) of Fe₃O₄@SiO₂-APTES and Fe₃O₄@SiO₂-APTES(Fe(acac)₂) catalyst is seen in Fig. S2.† The first weight loss curve below 200 °C in thermogravimetric analysis (TGA) of Fe₃O₄@SiO₂-APTES and Fe₃O₄@SiO₂-APTES(Fe(acac)₂) catalyst is attributed to residual physisorbed water and/or organic solvents, which was applied during their preparation. TGA diagrams of Fe₃O₄@SiO₂-APTES and Fe₃O₄@SiO₂-APTES(Fe(acac)₂) show that the largest weight loss occurs between 200 °C and 600 °C. This is due to the decomposition of the grafted organic molecule on the surface which is consistent with covalently bonded organic groups to the surface of MNPs.

The largest weight loss is corresponding to 2.6% and 5.5% of the initial sample weight for Fe₃O₄@SiO₂-APTES and Fe₃O₄@SiO₂-APTES(Fe(acac)₂), respectively. The small amount of 5.5% weight loss in the range of 200–600 °C indicates that the Fe₃O₄@SiO₂-APTES(Fe(acac)₂) catalyst have a good thermal stability up to 200 °C.

The difference between the largest weight loss for Fe₃O₄@SiO₂-APTES and Fe₃O₄@SiO₂-APTES(Fe(acac)₂) could be attributed to the amount of iron acetylacetonate grafted on the surface. A loading 0.114 mmol g⁻¹ for Fe grafted on the Fe₃O₄@SiO₂-APTES obtains from this difference. The Fe content was also measured by elemental analysis (CHN analysis) which indicated a loading of 0.1 mmol g⁻¹. In order to obtain an insight into the accurate amount of Fe grafted on the Fe₃O₄@SiO₂-APTES, inductively coupled plasma atomic emission analysis (ICP-AES) was applied on the Fe(acac)₂ solution before and after grafting on the Fe₃O₄@SiO₂-APTES. The Fe content grafted on the Fe₃O₄@SiO₂-APTES was measured about 0.11 mmol g⁻¹. The loading of APTES on the Fe₃O₄@SiO₂ was measured by TGA (Fig. S2a†) and CHN analysis about 0.44 and 0.38 mmol g⁻¹, respectively.

Oxidation of sulfides to sulfoxides by using Fe₃O₄@SiO₂-APTES(Fe(acac)₂) catalyst

To investigate the catalyst activity of Fe₃O₄@SiO₂-APTES(Fe(acac)₂), we have chosen the oxidation of sulfides to sulfoxide. In order to get an insight into the optimum catalytic conditions for the selective oxidation of sulfides to sulfoxides, the oxidation reaction of methyl phenyl sulfide by 30% H₂O₂ and in the presence of Fe₃O₄@SiO₂-APTES(Fe(acac)₂) catalyst as a model experiment was investigated. In this study the effect of solvent, temperature and the amount of Fe₃O₄@SiO₂-APTES(Fe(acac)₂) catalyst on the conversion and selectivity toward sulfoxide was examined. The results of the solvent effect on the oxidation reaction are summarized in Table 1. It was found that conversion of sulfide in less-polar solvents (entries 2 and 9) is low. In addition, selectivity to the corresponding sulfoxide was decreased to 54% in high-polar solvent (water). Among different solvents, EtOH (entry 5) was found to be the best solvent in terms of conversion and selectivity toward sulfoxide. The amount of Fe₃O₄@SiO₂-APTES(Fe(acac)₂) catalyst and the effect of temperature was also investigated after choosing EtOH as the best solvent for the oxidation reaction.

Table 1 Optimization of the reaction conditions for selective oxidation of methyl phenyl sulfide (1 mmol) to methyl phenyl sulfoxide in the presence of H₂O₂ (1.5 equiv.) and Fe₃O₄@SiO₂-APTES(Fe(acac)₂) (1 mol%) catalyst at room temperature

Entry	Solvent	Time (h)	Con.^a (%)	Yield^a (%)	Sel.^b (%)
a Determined by GC with area normalization.b Sulfoxide selectivity.
1	EtOAc	6	37	34	92
2	Toluene	6	21	20.5	98
3	H2O	6	67	36	54
4	CH2Cl2	6	70	61	87
5	EtOH	6	88	85	97
6	MeOH	6	81	73	90
7	THF	6	77	72.4	94
8	CH3CN	6	79	74	94
9	n-Hexane	6	17	15	88

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The amount of catalyst was also considered. It was found that the amount of catalyst could remarkably promote conversion of sulfide to sulfoxide. As shown in Table 2, no significant amount of sulfoxide was produced in absence of Fe₃O₄@SiO₂-APTES(Fe(acac)₂) catalyst (entry 1). The conversion of sulfide was 72% when the catalyst amount was 0.5 mol% (45 mg). With the increase of the catalyst amount from 0.5 mol% to 2 mol% (180 mg), conversion value was increased from 72% to 97%. When more amounts of catalyst (3 mol% and 4 mol%) were used, the conversion of sulfide and selectivity for sulfoxide did not change remarkably. These results show that 2 mol% of the catalyst was sufficient for oxidation of sulfides to corresponding sulfoxides.

Table 2 Optimization of the reaction conditions in EtOH^a

Entry	Catalyst (mol%)	Time (h)	T (°C)	Con.^b (%)	Yield^b (%)	Sel.^b (%)
a Reaction conditions: methyl phenyl sulfide (1 mmol), 30% H2O2 (1.5 equiv.), indicated amount of catalyst, ethanol (3 mL) as solvent at reaction temperature; the amount of Fe grafted on the surface was determined by ICP-AES analysis (loading = 0.11 mmol g^−1).b Determined by GC with area normalization.
1	—	24	25	14	13.6	97
2	0.5	9	25	72	69	96
3	1	6	25	88	85	97
4	2	2	25	97	94	96
5	3	2	25	97.7	91.8	94
6	4	2	25	97	92	95
7	2	2	40	99	73	74
8	2	2	60	>99	47.5	48

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In the next step, we examined the effect of temperature on the oxidation reaction. At room temperature, the obtained conversion and selectivity toward sulfoxide were 97% and 96%, respectively. However, a quantitative conversion was generally observed with the increase of temperature to 40 and 60 °C but over-oxidation of the sulfoxide product to sulfone occurred which reduced the selectivity of the target product (Table 2, entries 7 and 8). Therefore, the best selectivity was obtained in room temperature.

Catalytic activity of Fe₃O₄@SiO₂-APTES(Fe(acac)₂) for the oxidation of sulfides to sulfoxides

The applicability of this protocol was studied by oxidation of several types of sulfides with different electronic and steric effects to corresponding sulfoxides under the optimized conditions. As shown in Table 3, the sulfides with less steric hindrance were converted to the corresponding sulfoxides in good to excellent yields. Also, the influence of electronic effect was found in the case of 4-nitrophenyl sulfide, which has a negative effect on the reaction time. The chemoselectivity of the presented protocol was also investigated in the selective oxidation of sulfides containing hydroxyl, ester, and carbon–carbon double bond groups. These substrates selectively underwent oxidation at the sulfur atom without undergoing further structural changes in their functional group. For example, in the case of allylic sulfides, no over oxidation to the sulfone or epoxidation of the double bond was observed and only the corresponding sulfoxide were obtained in excellent yields (Table 3, entries 11). Also, the presence of ester group did not interfere with the oxidation process of the sulfide, and desired sulfoxides were obtained in excellent yield (Table 3, entry 10). It is clear that these kinds of sulfides are completely unaffected under the reaction conditions, indicating the good ability of this protocol in oxidation of different types of sulfides.

Table 3 Oxidation of sulfides to the corresponding sulfoxides by using 30% H₂O₂ and Fe₃O₄@SiO₂-APTES(Fe(acac)₂) catalyst^a

Entry	Substrate	Product	Time (h)	Con.^b (%)	Yield^b (%)	Sel.^b (%)
a Reaction conditions: sulfide (1 mmol), 30% H2O2 (1.5 equiv.), Fe3O4@SiO2-APTES(Fe(acac)2) catalyst (2 mol%, 180 mg), ethanol (3 mL) as solvent at room temperature.b Determined by GC with area normalization.
1			2.7	96	91	95
2			2.5	98	96	98
3			2.5	95	93	98
4			8	84	80	95
5			6	86	80	93
6			4	>99	96	97
7			4.5	96	92	96
8			3	95	90	95
9			3	97	88	90
10			3.5	96	92	96
11			4	86	76	89

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It would be interesting to know that there is not a clear mechanism for iron-catalyzed sulfoxidation at the present stage. An electrophilic Fe(IV)=O (one redox equivalent above the Fe(III) state) has been proposed to be the active oxidizing species involved in oxidation by Fe complexes and O₂ or H₂O₂. Therefore, the presence of Fe(IV)=O complex (high-valent iron oxo complex) is essential in oxidation reactions. The first stage for the formation of Fe(IV)=O complex is performed in situ by the reaction of Fe(II) complex (1) with H₂O₂ as oxidizing agent to form compound 2 (Scheme 2). Then, O–O bond homolysis/heterolysis of compound 2 results in the generation of compound 3, which is involved in oxygen atom transfer reaction for the oxidation of sulfides to sulfoxides.^28,36,37

Scheme 2 Proposed mechanism for the oxidation of sulfides to sulfoxide using 30% H₂O₂ in the presence of Fe₃O₄@SiO₂-APTES(Fe(acac)₂) catalyst.

In order to show the efficiency of our methodology, our results in oxidation of methyl phenyl sulfide was compared to literature reports (Table 4). Table 4 clearly point out the efficiency of the proposed methodology (Table 4, last entry) in both activity and reusability of the Fe₃O₄@SiO₂-APTES(Fe(acac)₂) catalyst as compared to literature reports involving several homogeneous and heterogeneous systems under various conditions (Table 4, entries 1–7).

Table 4 Comparison of the activity of various catalysts in oxidation of methyl phenyl sulfide using 30% H₂O₂

Entry	Catalyst	Solvent	Temp (°C)	Time (min)	Yield %	Reusability^a [ref]
a The amounts of catalyst and oxidizing agent (H2O2) are 3 mol% and (3 equiv.), respectively.
1	Polymer anchored Cu(II) complex	CH3CN	25	180	83	5 (ref. 38)
2	Ionic liquid-based polyoxometalate salts	MeOH	25	30	94.7	5 (ref. 39)
3	Silica-based tungstate interphase	CH2Cl2:MeOH	25	90	82	8 (ref. 23)
4	Peroxotungstate supported on silica	CH2Cl2:MeOH	8	150	91.9	6 (ref. 40)
5	Fe/SBA-15	H2O	25	120	99	10 (ref. 41)
6	Bis-[N-(propyl-1-sulfoacid)-pyridinium] hexafluorotitanate	[BPy][BF4]	25	120	95	6 (ref. 25)
7	Molybdate-based Fe3O4 catalyst	CH3CN	25	90	91	7 (ref. 15)
This work	Fe3O4@SiO2-APTES(Fe(acac)2)	EtOH	25	120	92	8

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Entry 5 indicates that heterogeneous system (Fe/SBA-15) have higher catalytic activity, better reusability than Fe₃O₄@SiO₂-APTES(Fe(acac)₂) catalyst but these kinds of heterogeneous catalysts have a problem in recovery in such a way that most heterogeneous systems require a filtration or centrifugation step or a tedious workup of the final reaction mixture to recover the catalyst, although Fe₃O₄@SiO₂-APTES(Fe(acac)₂) catalyst can be easily recovered from the reaction mixture only using an external magnet.

A comparison between Fe₃O₄@SiO₂-APTES(Fe(acac)₂) catalyst (this work) and molybdate-based Fe₃O₄ catalyst (entry 7) which are recyclable heterogeneous catalysts by external magnet indicates that the oxidation of sulfides to sulfoxides in EtOH as a green solvent gives almost better results in yield and reusability of the catalyst. On the other hand, less amounts of Fe₃O₄@SiO₂-APTES(Fe(acac)₂) catalyst (2 mol%) and H₂O₂ (1.5 equiv.) as oxidizing agent are used in this work. In addition, the results in Table 4 indicate that the amount of yield and reusability of Fe₃O₄@SiO₂-APTES(Fe(acac)₂) catalyst is comparable with the other results summarized in Table 4.

Catalyst recycling

The recycling properties of catalyst were investigated in methyl phenyl sulfide oxidation at room temperature employing aqueous 30% H₂O₂ for 1 mmol of substrate. After the completion of first oxidation reaction of methyl phenyl sulfide to afford the methyl phenyl sulfoxide under optimized condition, the catalyst was magnetically isolated after each cycle, washed with EtOH, and then placed into a fresh reaction mixture. Under the described conditions, the catalyst exhibited high activity but the yield of reaction in 8 run decreased with respect to 1 run at the time of 120 min (Fig. 5).

Conclusion

We have prepared recyclable catalyst of iron(II) acetylacetonate immobilized on amine-modified magnetic nanoparticles (Fe₃O₄@SiO₂-APTES(Fe(acac)₂)) for sulfide oxidation using H₂O₂ as oxidant at room temperature. Under the mild conditions, a wide range of sulfides with different steric/electronic effects and containing the oxidation-sensitive groups (such as OH or C=C double bond) could be converted into their corresponding sulfoxide with high chemoselectivity in ethanol as green solvent. This catalyst can be considered as heterogeneous version of Fe(acac)₂ and easily prepared from commercially available starting materials. The properties of high activity, high stability, easy recoverability, and reusability of the catalyst render it potentially valuable catalyst in industrial applications.

Acknowledgements

We gratefully acknowledge financial support from the Research Council of Sharif University of Technology.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: The EDS analysis and thermogravimetric analyses (TGA) of Fe₃O₄@SiO₂-APTES and Fe₃O₄@SiO₂-APTES(Fe(acac)₂) catalyst. See DOI: 10.1039/c4ra07356h

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