Copper based on diaminonaphthalene-coated magnetic nanoparticles as robust catalysts for catalytic oxidation reactions and C–S cross-coupling reactions

Abstract

In this work, the immobilization of copper(II) on the surface of 1,8-diaminonaphthalene (DAN)-coated magnetic nanoparticles provides a highly active catalyst for the oxidation reaction of sulfides to sulfoxides and the oxidative coupling of thiols to disulfides using hydrogen peroxide (H₂O₂). This catalyst was also applied for the one-pot synthesis of symmetrical sulfides via the reaction of aryl halides with thiourea as the sulfur source in the presence of NaOH instead of former strongly basic and harsh reaction conditions. Under optimum conditions, the synthesis yields of sulfoxides, symmetrical sulfides, and disulfides were about 99%, 95%, and 96% respectively with highest selectivity. The heterogeneous copper-based catalyst has advantages such as the easy recyclability of the catalyst, the easy separation of the product and the less wastage of products during the separation of the catalyst. This heterogeneous nanocatalyst was characterized by FESEM, FT-IR, VSM, XRD, EDX, ICP and TGA. Furthermore, the recycled catalyst can be reused for several runs and is economically effective.

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1. Introduction

Catalysis is the master key to chemical transformations.¹ Almost all biological reactions and more than 90% of industrial processes need catalysts.² It is known that homogeneous catalysts, because of the solubility in the reaction medium, have good performance.^3–5 However, these materials have disadvantages such as the generation of highly toxic wastes⁶ and time-consuming catalyst separation.⁷ Additionally, metal catalysts are recycled with difficulty, and they produce high amounts of wastes. In industrial applications, homogeneous catalysts in comparison with heterogeneous catalysts have a contribution of less than 20%.³ From the standpoint of green chemistry, using heterogeneous catalysts to run procedures for making fine chemical products has attracted remarkable attention from industrial and academic researchers. However, the catalytic activity of heterogeneous catalysts is reduced over time.^2,8,9 Magnetic nanoparticle (MNP) catalysts can address the recovery and separation problems encountered in many catalytic reactions. Furthermore, these catalysts show not only high catalytic activity, but also a high degree of chemical stability. MNP catalysts can be recycled with an external magnetic field.^10–13

DAN has been widely utilized in the structure of inorganic and organometallic complexes as a suitable ligand because of having bidentate nucleophilic centers. This ligand has a unique structure and properties that make it an interesting choice for a diverse range of applications over recent years. DAN complex has been used in various fields such as optical devices, biological applications, conducting polymers,¹⁴ sensors, and electronic science.¹⁵ This ligand has been recently considered because of its unique properties in heterocyclic synthesis.¹⁶

Noble metals such as Pt, Cu, Au and Rh have been incorporated into the structure of nanoparticles.^17–21 Among the mentioned metals, copper nanoparticles have been paid attention due to their catalytic activity, suitable treating costs, and high conductivity, which make them useful in nanoscience.^22–24 Cu-based materials, because of the several oxidation states of this metal, can catalyze many oxidative reactions via both one- and two-electron pathways. The feasible modification of the properties of these materials via different synthetic policies and post-synthetic chemical treatments has engendered considerable interest in the field of catalysis. Furthermore, Cu's high boiling point makes it a good candidate for reactions under hard conditions such as high temperature and pressure.²⁵

It is clear that sulfides are significant precursors for the synthesis of sulfoxides. Sulfoxides have been reported to possess widespread pharmaceutical functions and biological properties such as anticancer, antifungal and anti-atherosclerotic activities.^26,27 Furthermore, they have been extensively utilized for the preparation of carbon–carbon bond and rearrangement reactions.^28,29

Moreover, disulfides have important applications in biological and industrial fields such as protecting agents²⁵ stabilization of protein structures,³⁰ and vulcanizing entities.^31,32 The catalytic formation of the C–S bond is a basic transformation in synthetic reactions. Their importance is based on the application in pharmaceutical industries and preparing a scaffold for aryl sulfides as intermediates in synthetic organic chemistry.^33,34 Although the sulfide and disulfide oxidation reactions have been studied extensively,^35–44 it is necessary to introduce procedures that are more simple, efficient and selective. Herein, a magnetically separable heterogeneous catalyst was fabricated by anchoring a Cu complex supported on functionalized CoFe₂O₄ nanoparticles. This catalyst exhibits efficient oxidation reactions of sulfides to sulfoxides and oxidative coupling of thiols to disulfides utilizing hydrogen peroxide. It is necessary to introduce procedures that are more simple, efficient and selective. Moreover, we used the prepared nanoparticle as a catalyst for the odorless C–S cross-coupling reaction of aryl halides under mild conditions. Furthermore, the catalyst is separated from the reaction media via magnetic decantation and the nanocatalyst can be recycled several times.

2. Experimental

2.1 Materials and methods

Chemical materials were supplied from Sigma-Aldrich and Merck and used as received. The X-ray powder diffraction (XRD) data of the CoFe₂O₄-DAN-Cu(II) nanocatalyst were obtained using a Co radiation source with a wavelength of λ = 1.78897 Å, 40 kV. The morphology of the nanocatalyst was investigated by FESEM-TESCAN MIRA3, operating at a voltage of 30 kV, which was gold-coated using a sputtering coater. FT-IR spectra of the prepared samples were recorded from a KBr disc using a VRTEX 70 model BRUKER FT-IR spectrophotometer, in the range of 400 and 4000 cm⁻¹. ¹H NMR spectra were recorded using a Bruker 400 MHz NMR AVANCE 300 spectrometer. TGA was performed using a Shimadzu DTG-60 instrument in the temperature range 25–800 °C. Energy-dispersive X-ray spectroscopy (EDX) was used for the elemental analysis. The copper content in the catalyst was evaluated by inductively coupled plasma-optical emission spectrometry (ICP-OES; 730-ES Varian). TEM analysis of the catalyst was performed using a Zeiss-EM10C transmission electron microscope.

2.2 Preparation of CoFe₂O₄-DAN-Cu(II) nanocatalysts

First, the mixture of Fe(NO₃)₃·6H₂O (16 mmol, 5.584 g) and CO(NO₃)₂·6H₂O (8 mmol, 2.328 g) was dissolved in deionized water (20 mL) under vigorous stirring conditions until the components were dissolved. A brown precipitate was produced by adding sodium hydroxide (15 mL, 3 M) under magnetic stirring. After adjusting the pH to 11, the solution was heated at 80 °C for an hour. The obtained black magnetic nanoparticles were separated, washed five times with distilled water/ethanol mixture solution, and dried in an oven at 60 °C for 14 h. To prepare CoFe₂O₄–Cl, 1 g of MNP powder was dispersed in 50 mL of toluene under sonication for 30 min, then (1 mL) of 3-chloropropyltrimethoxysilane (CPTMS) was added to the reaction mixture and refluxed for 24 h. The obtained nanoparticles were separated by an external magnet, washed with ethanol/water mixture solution and dried in an oven at 80 °C. For the functionalization of CoFe₂O₄–Cl with 1,8-diaminonaphthalene, 1 g of the suspension of CoFe₂O₄–Cl in 70 mL of acetonitrile was added to a mixture of KI (6 mmol, 0.996 g) and K₂CO₃ (6 mmol, 0.828 g). The obtained mixture was stirred magnetically at room temperature for 35 min. Afterwards, 1,8-diaminonaphthalene (18 mmol, 2.84 g) was added to the solution and the mixture was refluxed for 24 h. The resulting mixture was then filtered, washed with 95% ethanol and dried at 60 °C to obtain CoFe₂O₄-DAN. Finally, the CoFe₂O₄-DAN-Cu(II) nanoparticles were afforded by mixing CoFe₂O₄-DAN (2 g) and CuCl₂·2H₂O (1 g) in 20 mL of ethanol and stirring under reflux for 18 h. The obtained product was then magnetically separated, washed with ethanol and dried at 50 °C.

2.3 General procedure for oxidation of sulfides to sulfoxides with H₂O₂ in the presence of CoFe₂O₄-DAN-Cu(II)

In a 25 mL round-bottom flask equipped with a magnetic stirrer and heater, a suspension of 1 mmol sulfide and 0.025 g catalyst in ethanol (3 mL) was stirred for 2 min at room temperature. Then, H₂O₂ (0.5 mL, 4.90 mmol) was added dropwise to the reaction mixture and the obtained blend was stirred at 25 °C for appropriate time (Table 1). After reaction completion, ethyl acetate (2 × 5 mL) was added to the mixture, a heterogeneous catalyst was separated from the mixture using an external magnet. After that, the filtered solution was dried over Na₂SO₄ (2 g). The products were obtained by evaporating the organic solvent.

Table 1 Optimization of conditions for oxidation of sulfides to sulfoxides in the presence of CoFe₂O₄-DAN-Cu(II)

Entry	Catalyst (mg)	Oxidation agent	Solvent	Temp. (°C)	Time (min)	Yield^a (%)
a Isolated yields.
1	25	H2O2 (0.5 mL)	Acetonitrile	25	20	85
2	25	H2O2 (0.5 mL)	n-Hexane	25	120	25
3	25	H2O2 (0.5 mL)	EtOAc	25	25	80
4	25	H2O2 (0.5 mL)	H2O	25	30	75
5	25	H2O2 (0.5 mL)	Ethanol	25	15	99
6	—	H2O2 (0.5 mL)	Ethanol	25	140	Trace
7	25	H2O2 (0.55 mL)	Ethanol	25	15	98
8	25	H2O2 (0.4 mL)	Ethanol	25	25	85
9	25	H2O2 (0.3 mL)	Ethanol	25	25	79
10	25	H2O2 (0.2 mL)	Ethanol	25	25	68
11	25	H2O2 (0.1 mL)	Ethanol	25	25	43
12	25	—	Ethanol	25	140	Trace
13	7	H2O2 (0.5 mL)	Ethanol	25	25	55
14	9	H2O2 (0.5 mL)	Ethanol	25	25	68
15	10	H2O2 (0.5 mL)	Ethanol	25	25	85
16	CuCl2·2H2O	H2O2 (0.5 mL)	Ethanol	25	65	40
17	25	NaIO4 (2 mmol)	CH3CN/H2O	25	100	85
18	25	Oxone (0.3689 g, 0.6 mmol)	Ethanol	60	720	90
19	25	O2 (2 MPa)	PEG	100	720	10

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2.4 Typical procedure for the synthesis of derivative sulfides in the presence of CoFe₂O₄-DAN-Cu(II)

In a 25 mL round-bottom flask equipped with a magnetic stirrer and heater, a mixture of the aryl halide (1 mmol), thiourea (1 mmol, 0.076 g), NaOH (1 mmol, 0.04 g), and CoFe₂O₄-DAN-Cu(II) (0.03 g) nanocatalyst were mixed under stirring at 130 °C in 3 mL DMSO for an appropriate time specified in Table 2. After reaction completion (monitored by TLC), CH₂Cl₂ was added to the reaction mixture, the heterogeneous catalyst was magnetically decanted. The filtered solution was washed with H₂O and dehydrated over Na₂SO₄. After organic solvent volatilization, the corresponding sulfide was obtained.

Table 2 Oxidation of sulfides with CoFe₂O₄-DAN-Cu(II) in the presence of H₂O₂^a

Entry	Substrate	Product	Time (min)	Yield^b (%)	Mp (°C)
a Reaction conditions: catalyst (0.025 g), sulfide (1 mmol), 30% H2O2 (0.5 mL) and solvent (3 mL) at 25 °C.b Isolated yields.
1			15	99	Oil^51
2			15	95	Oil^52
3			2	93	Oil^53
4			27	95	Oil^54
5			6	94	Oil^53
6			20	93	112–114 (ref. 11)
7			85	88	130–133 (ref. 55)
8			100	87	117–119 (ref. 55)
9			105	85	Oil^47

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2.5 Typical procedure for oxidative coupling of thiols to disulfides with H₂O₂ in the presence of CoFe₂O₄-DAN-Cu(II)

In a typical experiment, thiol (1 mmol), CoFe₂O₄-DAN-Cu(II) (0.025 g) and ethanol (3 mL) were taken in a 25 mL round-bottom flask, equipped with a magnetic stirrer and heater. The solution was stirred for 2 min at room temperature. After that, H₂O₂ (0.4 mL, 3.92 mmol) was added dropwise to the reaction mixture and the obtained blend was stirred at 25 °C for an appropriate time (Table 3). The reaction process was monitored by TLC (ethyl acetate/n-hexane: 1 : 4). Upon completion of the reaction, the nanocatalyst was magnetically isolated and the product was extracted with ethyl acetate. The product was obtained, dried over anhydrous Na₂SO₄ and evaporated to yield pure products.

Table 3 Optimization of conditions for sulfide synthesis in the presence of CoFe₂O₄-DAN-Cu(II)

Entry	Catalyst (mg)	Base	Thiourea (mmol)	Solvent	Temp. (°C)	Time (h)	Yield^a (%)
a Isolated yields.
1	30	NaOH	1	H2O	130	4	N.R.
2	30	NaOH	1	PEG	130	4	N.R.
3	30	NaOH	1	DMF	130	3.5	52
4	30	NaOH	1	DMSO	130	2.5	95
5	30	NaOH	1	Toluene	130	3	35
6	—	NaOH	1	DMSO	130	5	Trace
7	30	NaOH	0.5	DMSO	130	3.5	67
8	30	NaOH	0.8	DMSO	130	3.5	82
9	8	NaOH	1	DMSO	130	3.5	37
10	10	NaOH	1	DMSO	130	3.5	52
11	20	NaOH	1	DMSO	130	3.5	79
12	30	NaOH	1	DMSO	45	3.5	40
13	30	NaOH	1	DMSO	65	3.5	65
14	30	NaOH	1	DMSO	85	3.5	75
15	30	NaOH	1	DMSO	100	3.5	85
16	30	KOH	1	DMSO	130	3.5	Trace
17	30	Na2CO3	1	DMSO	130	3.5	Trace
18	CuCl2·2H2O	NaOH	1	DMSO	130	4	Trace

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2.6 Selected spectral data

2.6.1 Methyl phenyl sulfoxide. ¹H-NMR (300 MHz, CDCl₃, ppm): δ 2.98 (s, 3H), 7.46–7.87 (m, 5H); ¹³C-NMR (75 MHz, CDCl₃, ppm): δ 44.15, 124.44, 129.60, 131.45, 144.29. IR (KBr) (cm⁻¹): ν (S=O): 1079.

2.6.2 Benzyl phenyl sulfoxide. ¹H-NMR (400 MHz, DMSO, ppm): δ 4.68 (s, 2H), 7.14–7.34 (m, 4H), 7.53–7.75 (m, 4H). ¹³C-NMR (75 MHz, DMSO, ppm): δ 61.15, 128.51–128.81, 129.16, 129.60–131.45, 138.83. IR (KBr) (cm⁻¹): ν (S=O): 1068.

2.6.3 Dibenzyl sulfoxide. ¹H-NMR (400 MHz, DMSO, ppm): δ 4.16 (S, 4H), 7.29–7.45 (m, 10H); ¹³C-NMR (75 MHz, DMSO, ppm): δ 58.03, 127.57, 129.03, 130.18, 130.91. IR (KBr) (cm⁻¹): ν (S=O): 1026–1089.

2.6.4 Diphenyl sulfide. ¹H-NMR (400 MHz, DMSO, ppm): δ 7.72–7.32 (m, 4H), 7.32–7.38 (m, 4H); ¹³C-NMR (75 MHz, CDCl₃, ppm): δ 127.51, 129.66, 131.01, 134. 83. IR (KBr) (cm⁻¹): 3421, 3080, 1592, 1333, 1108, 1104, 838, 631.

2.6.5 Chemical 4,4′-thiodiphenol. ¹H-NMR (400 MHz, CDCl₃, ppm): δ 6.63–6.68 (m, 4H), 7.713–7.732 (m, 4H); ¹³C-NMR (75 MHz, CDCl₃, ppm): δ 123.64, 128.81, 158.83. IR (KBr) (cm⁻¹): 3421, 2822, 1625, 1498, 1117, 933, 797, 501.

2.6.6 Di-p-tolylsulfane. ¹H-NMR (400 MHz, DMSO, ppm): δ 2.43 (s, 6H), 7.26–7.32 (m, 4H), 7.33–7.49 (m, 4H); ¹³C-NMR (400 MHz, CDCl₃, ppm): δ 22.15, 131.46, 131.69, 132. 83, 139.16. IR (KBr) (cm⁻¹): 3333, 2854, 1625, 1425, 1303, 1073, 838, 697.

2.6.7 Dibenzylsulfane. ¹H-NMR (300 MHz, DMSO, ppm): δ 3. 377 (s, 4H), 7.261–7.412 (m, 4H); ¹³C-NMR (100 MHz, DMSO, ppm): δ 35.15, 127.165, 128.69, 138.16. IR (KBr) (cm⁻¹): ν (S–S): 1069.

2.7 Procedure for the recovery of the CoFe₂O₄-DAN-Cu(II) nanocatalyst

For recycling of the catalyst in the mentioned reactions, methyl phenyl sulfide, iodobenzene, and 4-methylthiophenol as model substrates were used under optimized conditions. After completion of the reaction, the CoFe₂O₄-DAN-Cu(II) nanocatalyst was separated from the reaction mixture by a magnet, washed, and dried at 80 C. The obtained nanocatalyst was used for the next reaction under optimum conditions. This procedure was repeated for nine runs. Selected spectral data using ¹H-NMR spectra for sulfoxides, symmetrical sulfides, and disulfides are given in the ESI.†

3. Results and discussions

CoFe₂O₄-DAN-Cu(II) nanoparticles were synthesized according to a three-step procedure (Scheme 1). Cobalt ferrite was prepared by the co-precipitation of aqueous Fe³⁺ and Co²⁺ salt solutions in the presence of a base at 80 °C.⁴⁵ The functionalization of the hydroxyl groups of CoFe₂O₄ with CPTMS yielded CoFe₂O₄–Cl. Finally, CoFe₂O₄-DAN was treated with CuCl₂·2H₂O to give CoFe₂O₄-DAN-Cu(II) via an electrostatic interaction between copper ions and functional groups of CoFe₂O₄-DAN. The catalytic performance of the synthesized heterogeneous nanocatalyst was evaluated in the oxidation of sulfides, thiols and C–S cross-coupling reactions of aryl halides (Scheme 1).

Scheme 1 Preparation of CoFe₂O₄-DAN-Cu(II) nanocatalyst and its application in the oxidation of sulfides, thiols and C–S cross-coupling reactions.

3.1 Catalyst characterization

The obtained CoFe₂O₄-DAN-Cu(II) nanocatalyst was characterized by FESEM, FT-IR, EDX, TGA, VSM, XRD and ICP-OES analysis. Fig. 1 depicts the particle shape and surface morphology of CoFe₂O₄ and CoFe₂O₄-DAN-Cu(II). According to the SEM images, prepared nanoparticles were made up of homogeneous and quite spherical particles ranging about 21 nm in size. Moreover, grafting copper complex onto nanoparticles did not cause any significant change in the morphology and structure of CoFe₂O₄-DAN-Cu(II) nanoparticles. Transmission electron microscopic (TEM) study of the CoFe₂O₄-DAN-Cu(II) nanocatalyst is also shown in Fig. 1d. This image shows that CoFe₂O₄ was encapsulated by copper complexes, and the particles show an average diameter of about 21 nm, like the sizes resulting from the SEM measurements.

Fig. 1 SEM images of CoFe₂O₄ at (a) 200 nm and CoFe₂O₄-DAN-Cu(II) at (b) 200 nm, (c) 20 μm, and TEM image of CoFe₂O₄-DAN-Cu(II) (d).

The EDX spectrum can provide qualitative information about the types of different chemical elements in the catalyst. Fig. S1† shows an EDX spectrum of CoFe₂O₄-DAN-Cu(II) and Fe, Si, N, C, O, and Cu were detected. On the basis of these results, it is demonstrated that copper(II) was immobilized on the surface of diaminonaphthalene-coated MNPs. Moreover, the elemental mapping images indicate the uniform dispersion of copper in the nanocatalyst. This has been further confirmed from the EDX spectrum of the nanocatalyst. The ICP-OES technique was studied for the measurement of Cu amounts loaded onto the modified surface of the nanoparticles, from which the exact amount was found to be 0.43 mmol g⁻¹.

The magnetic properties of bare CoFe₂O₄ (a) and CoFe₂O₄-DAN-Cu(II) (b) were studied using a vibrating sample magnetometer (VSM) with a peak field of 15 kOe (Fig. 2). The saturation magnetization value (M_s) of these nanoparticles was found to be 53.7 and 36.6 emu g⁻¹ respectively. As seen, the magnetization of the synthesized nanocatalyst is decreased because of the copper complex coated on CoFe₂O₄ MNPs; however, this reduction is insignificant for the separation of this nanocatalyst using an external magnetic field.

Fig. 2 VSM curves of CoFe₂O₄ (a) and CoFe₂O₄-DAN-Cu(II) nanocatalyst (b).

Fig. S2† shows the thermogravimetric analysis (TGA) curve of CoFe₂O₄-DAN-Cu(II) at a temperature ranging from 25 °C to 800 °C. The first weight loss was about 2 wt%, until the temperature of 150 °C, which is attributed to the removal of adsorbed solvents and surface hydroxyl groups.⁴⁷ In the range of 200–500 °C, decomposition of the organic layer and Cu complex grafted onto the surface of magnetite nanoparticles was shown. On the basis of these results, CoFe₂O₄-DAN-Cu(II) has high thermal stability that spreads its application for several types of organic reactions.

The FT-IR spectra shown in Fig. S3† exhibit a comparison of bare CoFe₂O₄, CoFe₂O₄Cl, CoFe₂O₄-DAN and CoFe₂O₄-DAN-Cu(II) nanoparticles. The pure nanoparticles exhibit bands at 430 and 586 cm⁻¹ characteristic of the Fe–O stretching vibration in the tetrahedral and octahedral sites of the CoFe₂O₄, respectively. The broad band at 3403 cm⁻¹ can be related to characteristic –OH bands of CoFe₂O₄ (Fig. S3a†).⁴⁸ The band located at 2986 cm⁻¹ in the spectrum of CoFe₂O₄–Cl could be attributed to the stretching vibration of C–H (Fig. S3b†). The sharp peak at 1101 cm⁻¹ is attributed to the Si–O stretching vibration, and these bands are assigned to the characteristic absorptions of the linker CPTMS attached on the cobalt ferrite surface (Fig. S3b†).⁴⁹ The peaks at about 1440–1660 cm⁻¹ are due to the amine group bending vibration of the 1,8-diaminonaphthalene ring and also the band at 3400 cm⁻¹ is assigned to the amine stretching band of the synthesized nanoparticles (Fig. S3c†).⁵⁰ In the spectrum of CoFe₂O₄-DAN-Cu(II), however, the shift absorption of amine to a lower wave number occurs, which is attributed to a robust interaction between the N group of the copper complex on CoFe₂O₄ (Fig. S3d†).

Fig. 3 represents the XRD patterns of CoFe₂O₄ and CoFe₂O₄-DAN-Cu(II). The diffraction peaks related to Bragg's reflections from the (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1), (4 4 0), and (5 3 3) planes correspond to the standard spinel structure of CoFe₂O₄ (JCPDS 02-8517) (Fig. 3a).⁴⁶ Furthermore, on the basis of the Debye–Scherrer equation, the crystallite size of CoFe₂O₄ was about 21 nm. The XRD pattern of CoFe₂O₄-DAN-Cu(II) exhibits characteristic peaks whose relative intensities match well with the standard XRD data of CoFe₂O₄. These results indicate that the spinel structure of the CoFe₂O₄ framework was well maintained during the process of catalyst preparation.

Fig. 3 XRD patterns of CoFe₂O₄ (a) and CoFe₂O₄-DAN-Cu(II) (b) nanocatalysts.

3.2 Catalytic activity

After synthesis and characterization of the nanocatalyst, we decided to study the catalytic activity in the oxidation of sulfides. Therefore, for optimizing the reaction conditions for the oxidation of sulfides, methyl phenyl sulfide was used as a model substrate and different amounts of the catalyst, H₂O₂ and various solvents were studied. The results are summed up in Table 1. In order to obtain the optimum solvents of the reaction, the oxidation reaction was performed in different solvents. It was observed that polar aprotic solvents such as acetonitrile, ethyl acetate, water and ethanol obtained 85%, 80%, 75% and 99% conversion of sulfide respectively (Table 1, entries 1 and 3–5). Moreover, the reaction was performed in non-polar solvents like n-hexane that gave only 25% yield (Table 1, entry 2). These results indicated that ethanol is the appropriate solvent for this reaction. Moreover, it is seen that selectivity remained the same in all solvents and sulfoxide is the only product of this system. Then, different oxidation agents such as NaIO₄, oxone (2KHSO₅·KHSO₄·K₂SO₄) and O₂ were studied (Table 1, entries 17–19). The oxidant H₂O₂ showed higher activity than other used oxidants, while the sulfoxide selectivity was kept above 100%. The results also indicate that the reaction did not perform in the absence of H₂O₂ and very low yield was obtained (Table 1, entry 12). The effect of changing catalyst loaded (0.007, 0.009, 0.010 and 0.025 g) on methyl phenyl sulfide conversion was studied (Table 1, entries 13–15 and 5), and it indicates that the existence of catalyst is vital for this reaction. The highest yield was obtained in the presence of 0.025 g CoFe₂O₄-DAN-Cu(II) nanocatalyst. It is shown that no detectable product was observed when the reaction was carried out in the absence of the CoFe₂O₄-DAN-Cu(II) catalyst (Table 1, entry 6). It should be mentioned that only 40% was obtained in the presence of CuCl₂·2H₂O used as a catalyst (Table 1, entry 16). As shown, using ethanol as the solvent in the presence of catalytic amounts of CoFe₂O₄-DAN-Cu(II) (0.025 g) and H₂O₂ (0.5 mL) at 25 °C was realized to be the optimized conditions (Table 1, entry 5).

Because of the successful oxidation of methyl phenyl sulfide, the reactions of different sulfides under optimized conditions were examined (Table 2). Sulfoxides were prepared at excellent conversion in a short reaction time. It can be seen that phenyl sulfide with an electron-withdrawing group showed a longer reaction time and a lower yield (Table 2, entries 8 and 9). Researches show that in this case, the resonance of the sulfur electron pair with the aromatic ring is effective. Moreover, alkyl sulfides with longer alkyl chains proceeded in higher reaction times (Table 2, entries 4 and 6). These results are probably due to the substrate's insolubility in ethanol that decreased the substrate concentration in the reaction mixture and lowered the reaction rate. The selectivity of the catalyst is highly important in the industry and in the mentioned reaction, oxidation of sulfides with the optimal conditions is thoroughly selective and the sulfone was not observed as a by-product.

Sulfoxide usually acts as an electrophile, while sulfide is a nucleophilic reductant. This dual performance of the sulfur atom in the sulfide and the sulfoxide makes it an appropriate system to investigate nucleophilic behavior versus oxidant electrophilic behavior. H₂O₂ as a mild oxidizing agent reacts slowly and must be activated by homogeneous or heterogeneous catalysts. The catalytic cycle for the oxidation of sulfide by H₂O₂ catalyzed by the CoFe₂O₄-DAN-Cu(II) catalyst is proposed in Scheme 2. The explanation for this transformation is the formation of the intermediate A using the reaction of CoFe₂O₄-DAN-Cu(II) with H₂O₂, followed by the conversion of the intermediate A to the active oxidant. Then, nucleophile sulfide attacks active oxidants and with heterolytic cleavage of the Cu–O bond yields sulfoxide.^56,57

Scheme 2 Proposed mechanism for the oxidation of sulfides in the presence of CoFe₂O₄-DAN-Cu(II).

In continuation of our experiment, the catalytic activity of CoFe₂O₄-DAN-Cu(II) was examined for the synthesis of symmetrical sulfides. The effect of catalyst dosage, the nature of the solvents and bases, and the reaction temperature were optimized (Table 3). Initially, the reaction of the coupling of iodobenzene (1 mmol), thiourea (1 mmol) as a sulfur source and NaOH was chosen for a model reaction. In order to investigate the influence of the solvents, we utilized DMF, toluene, water, DMSO and PEG as solvents (Table 3, entries 1–5). As shown, the reaction did not proceed in PEG, H₂O and yielded only 35% in toluene (Table 3, entries 1, 2 and 5). The sulfide synthesis reaction was accomplished in excellent yields in DMSO as the reaction solvent (Table 3, entry 4), whereas the product yield prepared in DMF under the same condition was 52% (Table 3, entry 3). Additionally, different bases for the sulfide synthesis were checked (Table 3, entries 16, 17 and 4), and 0.4 g of NaOH was found to be the desired amount of the base (Table 3, entry 4). The reaction was carried out for catalytic amounts of thiourea (Table 3, entries 7, 8 and 4). The highest yield of diphenyl sulfide was achieved using 1 mmol of thiourea without increasing the catalyst loading (Table 3, entry 4). In order to gain the best reaction temperature, the mentioned reaction was investigated at several temperatures (Table 3, entries 12–15 and 4), and the results indicate that increasing temperature from 45 to 130 °C increased the yield. Therefore, the optimized temperature was found to be 130 °C. The effect of the catalyst amount was also checked (Table 3, entries 9–11 and 4), and the results show that with the increase in the amount of catalyst from 0.008 to 0.05, the yield also increased from 37% to 95%. The control experiment showed that catalyst-free conditions yielded trace amounts of products even after 5 hours (Table 3, entry 6). Furthermore, the reaction was carried out in the presence of CuCl₂·2H₂O as the catalyst, which afforded diphenyl sulfide in low yields (Table 3, entry 18).

Several derivatives of aryl halides with electron-donating and electron-withdrawing groups were examined under optimal conditions (Table 4). In the synthesis of symmetrical sulfides, used haloarenes follow ArI > ArBr > ArCl (Table 4, entries 1–3). Aromatic haloarenes with electron-withdrawing groups are more reactive in comparison to electron-donating groups (Table 4, entries 4 and 7). Selectivity is one of the most notable advantages of this system by which the coupling reaction of 1-bromo-4-nitrobenzene, 1-chloro-4-nitrobenzene, and 1-iodo-4-nitrobenzene resulted in the favorite product without extra changes.

Table 4 Synthesis of symmetrical sulfides in the presence of CoFe₂O₄-DAN-Cu(II) nanocatalysts^a

Entry	Substrate	Product	Time (h)	Yield^b (%)	MP (°C)
a Reaction conditions: catalyst (0.030 g), aryl halide (1 mmol), CH4N2S (1 mmol), base (0.040 g) and solvent (3 mL) at 130 °C.b Isolated yields.
1			2.5	95	Oil^55
2			4	88	Oil^55
3			8	40	Oil^58
4			7	69	Oil^58
5			5.5	75	158–160 (ref. 56)
6			8	58	Oil^58
7			5	75	157–160 (ref. 58)
8			9.5	45	158–160 (ref. 59)
9			4.5	78	44–46 (ref. 56)
10			10.5	43	150–153 (ref. 26)

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A plausible mechanism for this transformation is outlined in the presence of the CoFe₂O₄-DAN-Cu(II) nanocatalyst (Scheme 3). First, the oxidative addition of Cu nanoparticles to the aryl halide forms intermediate (I). Then, the coupling reaction of the aryl halide with thiourea generates intermediate (II), which is transmitted to a thiol anion in the presence of NaOH. Finally, thiol anion reacts with intermediate (I) to produce the sulfide product as well as the initial catalyst.⁶⁰

Scheme 3 Plausible mechanism for the symmetrical sulfide synthesis by CoFe₂O₄-DAN-Cu(II) nanocatalysts.

At last, the CoFe₂O₄-DAN-Cu(II) catalytic system was examined for oxidative coupling of thiols to disulfides using 4-methylthiophenol as the model substrate. To find out the optimized reaction conditions, the presence of various catalytic amounts of CoFe₂O₄-DAN-Cu(II) and several organic solvents using different amounts of H₂O₂ was tested (Table 5). Initially, the model reaction was carried out in several solvents such as acetonitrile, n-hexane, EtOAc, CH₂Cl₂, EtOH and H₂O. Among them, acetonitrile (85%), EtOAc (87%) and EtOH (96%) showed better conversion for selective disulfide synthesis. However, because of the nontoxic and availability properties, EtOH was chosen as a green solvent (Table 5, entries 1–6). In order to study the effect of the oxidant nature, the reaction was also investigated in the presence of several oxidants such as NaClO₂, NaIO₄ and 1,3-dibromo-5,5-dimethyl-hydantoin (DBDMH) instead of H₂O₂ (Table 5, entries 17–19). Compared with the other oxidants, H₂O₂ showed higher activity. Then, the effect of the catalyst amount (0.008, 0.010, 0.020 and 0.025 g) on 4-methylthiophenol conversion was studied, and it was observed that the activity of the catalyst was affected by the amount of the catalyst (Table 5, entries 13–15 and 6). The highest yield was obtained in the presence of 0.025 g of CoFe₂O₄-DAN-Cu(II). The blank reaction showed that catalyst-free conditions produced trace amounts of products even after 140 minutes (Table 5, entry 7). When the catalyst CoFe₂O₄-DAN-Cu(II) was replaced with CuCl₂·2H₂O, the desired oxidation product was obtained in lower yields (45%) (Table 5, entry 16).

Table 5 Optimization of conditions for oxidative coupling of thiols in the presence of CoFe₂O₄-DAN-Cu(II)

Entry	Catalyst (mg)	Oxidation agent	Solvent	Temp. (°C)	Time (min)	Yield^a (%)
a Isolated yields.
1	25	H2O2 (0.4 mL)	Acetonitrile	25	35	85
2	25	H2O2 (0.4 mL)	n-Hexane	25	180	25
3	25	H2O2 (0.4 mL)	CH2Cl2	25	40	60
4	25	H2O2 (0.4 mL)	EtOAc	25	35	87
5	25	H2O2 (0.4 mL)	H2O	25	30	75
6	25	H2O2 (0.4 mL)	Ethanol	25	20	96
7	—	H2O2 (0.4 mL)	Ethanol	25	140	Trace
8	25	H2O2 (0.45 mL)	Ethanol	25	25	97
9	25	H2O2 (0.3 mL)	Ethanol	25	40	80
10	25	H2O2 (0.2 mL)	Ethanol	25	40	75
11	25	H2O2 (0.1 mL)	Ethanol	25	40	65
12	25	—	Ethanol	25	140	Trace
13	8	H2O2 (0.4 mL)	Ethanol	25	40	58
14	10	H2O2 (0.4 mL)	Ethanol	25	40	72
15	20	H2O2 (0.4 mL)	Ethanol	25	25	86
16	CuCl2·2H2O	H2O2 (0.4 mL)	Ethanol	25	75	45
17	25	NaClO2 (1 mmol, 0.090 g)	Methanol	5	20	93
18	25	NaIO4 (1 mmol, 0.213 g)	H2O	25	25	100
19	25	DBDMH (0.2 eq.)	CHCl3	25	2	96

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Utilizing the optimum reaction condition, a variety of substituted thiols were selected for the synthesis of disulfides (Table 6). The results indicate that the CoFe₂O₄-DAN-Cu(II) nanocatalyst exhibited efficient catalytic activity with good to high yields in the reaction time. It was observed that aliphatic thiols oxidized more slowly than aromatic ones (Table 6, entries 1 and 9). It is also shown that aromatic thiols including electron-donating groups are more reactive than thiols with electron-withdrawing groups (Table 6, entries 1 and 7). This system shows good chemoselectivity by which the hydroxyl group of 2-mercaptoethanol remains constant, and a thiol functional group of these molecules was oxidized to disulfide. Furthermore, selectivity is one of the main important features of this system. Because of the selectivity of the mentioned heterogeneous catalyst, there is no overoxidation to thiosulfinates, disulfoxides, sulfinyl sulfones, or disulfones.

Table 6 Synthesis of disulfides in the presence of CoFe₂O₄-DAN-Cu(II) nanocatalysts^a

Entry	Substrate	Product	Time (min)	Yield^b (%)	MP (°C)
a Reaction conditions: catalyst (0.025 g), thiol (1 mmol), 30% H2O2 (0.4 mL) and solvent (3 mL) at 25 °C.b Isolated yields.
1			20	96	38–40 (ref. 26)
2			40	92	65–71 (ref. 55)
3			25	96	58–60 (ref. 53)
4			30	95	134–136 (ref. 26)
5			45	88	276–278 (ref. 26)
6			20	90	Oil^55
7			60	86	88–90 (ref. 26)
8			65	90	98–99 (ref. 55)
9			35	95	Oil^26
10			35	90	55–57 (ref. 55)

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A plausible mechanism for this transformation is shown in Scheme 4, on the basis of the literature works.²³ A comparison for the efficiency of the catalytic activity of CoFe₂O₄-DAN-Cu(II) for the oxidation of sulfides to sulfoxides (Table 7, entries 1–7), synthesis of symmetrical sulfides (Table 7, entries 8–11), and oxidative coupling of thiols to disulfides (Table 7, entries 12–15) with several previously reported methods is presented. Recently, the use of Co@SiO₂[(EtO)₃Si–L₃]/Mn(III) (Table 7, entry 1), FeNi₃/SiO₂ (Table 7, entry 3), Mn(III)-binapthyl Schiff base diamine-SBA-15 (Table 7, entry 4), K₆H₈[(SeV₁₀O₂₈(SeO₃)₃)₂(M(H₂O)₄)]·24H₂O (Table 7, entry 5), VO-TAPT-2,3-DHTA COF (Table 7, entry 6), CuFe₂O₄ (Table 7, entry 12), Pd-isatin Schiff base@KIT-6 (Table 7, entry 13), and TiO(O₂CCF₃)/NaI/thiol (Table 7, entry 14) was reported for oxidation reactions. These methods need toxic organic solvents with high reaction times. The use of Mn(III)-binapthyl Schiff base diamine-SBA-15 (Table 7, entry 4), K₆H₈[(SeV₁₀O₂₈(SeO₃)₃)₂(M(H₂O)₄)]·24H₂O (Table 7, entry 5), VO-TAPT-2,3-DHTA COF (Table 7, entry 6), and Pd-isatin Schiff base@KIT-6 (Table 7, entry 13) as the catalyst for oxidation reactions was reported, requiring high reaction times. Moreover, harsh conditions are found when using TiCl₃(O₃SCF₃) (Table 7, entry 14), TiO(O₂CCF₃)₂ (Table 7, entry 14), CuFe₂O₄ (Table 7, entry 12), and ethyl 2-oxocyclohexanecarboxylate as catalysts (Table 7, entry 8) in oxidation reactions and sulfide synthesis, respectively. FeNi₃/SiO₂ (Table 7, entry 3) causes to use of m-CPBA instead of green oxidant H₂O₂. Accordingly, this catalyst affords an absolutely green procedure, and these results clearly accentuate the efficiency of this applied methodology.

Scheme 4 Plausible mechanism for the oxidative coupling of thiols by CoFe₂O₄-DAN-Cu(II) nanocatalysts.

Table 7 Comparison of CoFe₂O₄-DAN-Cu(II) in the catalytic oxidation reactions and synthesis of sulfide derivatives with other catalysts^a

Entry	Substrate	Catalyst	Time (min)	Catalyst loading	Condition	Yield^b (%)	Ref.
a Abbreviations: ^1Schiff-base Mn(III) and Co(II) complexes coated on Co nanoparticles, ^2tungstate ions loaded onto triazine-based ionic liquid-functionalized magnetic nanoparticle, ^3FeNi3 nanoparticle conjugated tetraethyl orthosilicate; ^4chloro(S,S)(−)[N-3-tert-butyl-5-chloromethylsalicylidene]-N′-[3′,5′-ditert-butylsalicylidene]1,1′-binapthyl-2,2′-diamine manganese(III) complex over modified surface of SBA-15, ^5transition metal-sandwiched heteropolyoxovanadate complexes, ^6complex vanadium of Schiff base of 2,4,6-tris(4-aminophenyl)-1,3,5-triazine–2,3-dihydroxyterephthaldehyde (TAPT–2,3-DHTA), ^8ethyl 2-oxocyclohexanecarboxylate ligand in the presence of Cu2O as catalyst, ^9nickel(II) complex supported on modified surface of Fe3O4. ^10Magnetite/silica nanoparticles supported N-heterocyclic carbene nickel catalyst, ^12copper ferrite nanoparticles, ^13Pd(II)-isatin Schiff base complex immobilized into three-dimensional mesoporous silica KIT-6, ^14TiCl3(O3SCF3) and TiO(O2CCF3)2 as the catalyst.b Isolated yields.
1	^1Ph–S–CH3	Co@SiO2[(EtO)3Si–L3]/Mn(III)	50	0.06 g	Methyl phenyl sulfide (0.5 mmol), acetonitrile, 90 μL H2O2 and 65 °C	99	61
2	^2Ph–S–CH3	MNP@TA-IL/W	60	0.4 mol%	H2O, 1.5 mmol H2O2 and 25 °C	99	62
3	^3Ph–S–CH3	FeNi3/SiO2	60	0.04	DCM (2.0 mL), m-CPBA (2.0 mmol) and r.t	99	63
4	^4Ph–S–CH3	Mn(III)-binapthyl Schiff base diamine-SBA-15	300	50 mg	CH2Cl2 and 25 °C	94	64
5	^5Ph–S–CH3	M^2+-sandwiched POVs: K6H8[(SeV10O28(SeO3)3)2(M(H2O)4)]·24H2O	60	2 μmol	CH3OH and 25 °C	97.9	65
6	^6Ph–S–CH3	VO-TAPT-2,3-DHTA COF	240	20 mg	CH3CN and 25 °C	95	66
7	Ph–S–CH3	CoFe2O4-DAN-Cu(II)	15	0.025 g	C2H5OH and 25 °C	99	This work
8	^8Iodobenzene	Ethyl 2-oxocyclohexanecarboxylate	1200	0.1 mmol	80 °C and under Ar	96	67
9	^9Iodobenzene	Fe3O4@SBTU@Ni(II)	210	0.030	DMSO and 130 °C	94	60
10	^10Iodobenzene	MNP-Si-NHC(Pyr)-Ni	600	10 mol%	DMF, 100 °C, base (2 mmol) and thiol (1 mmol)	92	68
11	Iodobenzene	CoFe2O4-DAN-Cu(II)	150	0.030 g	DMSO and 130 °C	95	This work
12	^12Thiophenol	CuFe2O4	24 h	10 mol%	1.80 mmol of halide, 2.0 eq. of base, 5 mL of 1,4-dioxane and under N2 atmosphere	95	69
13	^134-Methyl thiophenol	Pd-isatin Schiff base@KIT-6	30	0.025 g	CH3CN, 25 °C and 5 mmol H2O2	95	70
14	^14Thiophenol	TiO(O2CCF3)2/NaI/thiol	150	1 mmol	CH3CN and under reflux conditions	100	71
15	Thiophenol	CoFe2O4-DAN-Cu(II)	20	0.025 g	C2H5OH and 25 °C	96	This work

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3.3 Catalyst recycling

Recycling as one of the main important properties of nanomagnetic catalysts was also investigated by the synthesis of methyl phenyl sulfoxide, diphenyl sulphide, and 1,2-di-p-tolyldisulfane under optimized conditions. After completion of the reactions, the CoFe₂O₄-DAN-Cu(II) nanocatalyst was removed from the reaction mixture by a magnet and used in the next cycle under mentioned optimum conditions. As shown in Fig. 4, the catalyst can be recycled for nine runs without any considerable loss of catalytic activity.

Fig. 4 Reusability of the CoFe₂O₄-DAN-Cu(II) nanocatalyst in the sulfide oxidation, sulfide synthesis and oxidative coupling of thiols.

In order to clarify the changes in the chemical structure of the prepared catalyst towards oxidation during the nine cycles, FE-SEM and FT-IR analyses were performed. SEM and FT-IR analyses of this catalyst after nine runs show no significant changes during the reaction time (Fig. 5). Moreover, the amount of copper on recovered CoFe₂O₄-DAN-Cu(II) was measured by ICP-OES analysis for the conversion of methyl phenyl sulfide to methyl phenyl sulfoxide in the fresh catalyst and after nine consecutive cycles, it was found to be 0.43 mmol g⁻¹ and 0.32 mmol g⁻¹, which indicates the minimum amount of Cu leaching in the catalytic process. Furthermore, AAS analysis of the reaction mixture after catalyst removal showed no considerable amount of copper. AAS analyses of the recycled CoFe₂O₄-DAN-Cu(II) catalyst also exhibited no significant (<0.001 mmol g⁻¹ of Cu) differences compared with the fresh catalyst, presenting that no considerable copper leaching occurred during the catalytic processes. Moreover, regarding the heterogeneous nature of CoFe₂O₄-DAN-Cu(II), the oxidation of methyl phenyl sulfide has been examined by a hot filtration experiment under optimal reaction conditions. In this test, we found the yield of product in a half time of the reaction to be 65%. Then, the reaction was repeated and at half time of the reaction, the catalyst was separated using a magnet and allowed to react further. The yield of the reaction in this stage was 69%, which confirmed that the reaction proceeded heterogeneously and after hot filtration, the reaction of the residual mixture was completely stopped.

Fig. 5 FT-IR and SEM analyses of the CoFe₂O₄-DAN-Cu(II) nanocatalyst after nine runs.

3.4 Catalyst poisoning test

Another heterogeneity test was performed with Hg poisoning. Hg(0) can poison catalysts, either by amalgamating the applied metal or adsorbing on the metal surface.⁷² The sulfide oxidation was conducted under the same mentioned conditions, except the addition of elemental Hg (1 mmol) to the reaction mixture at 50% conversion of methyl phenyl sulfide to methyl phenyl sulfoxide. No product was gained with this procedure, which is strong evidence that the catalyst nature is heterogeneous.

4. Conclusion

An efficient CoFe₂O₄-DAN-Cu(II) nanocatalyst has been successfully synthesized using an anchored Cu complex on the stable and bidentate ligand 1,8-diaminonaphthalene-modified CoFe₂O₄ nanoparticles. Then, the prepared nanostructure has been characterized by techniques such as SEM, EDX, ICP-OES, TGA, XRD, VSM, FT-IR and TEM. The immobilized Cu complex exhibits remarkable catalytic activity in the oxidation of sulfides and thiols in a green catalytic system (C₂H₅OH as a solvent and H₂O₂ as an oxidant). All studied sulfides and thiols were oxidized to their corresponding sulfoxides and disulfides without production of any by-product. Moreover, the C–S cross-coupling reaction produced in the clean suspension of a variety of diarylsulfides appropriate to good yields. This system has many benefits such as high yields, chemical stability and ease of catalyst separation using an external magnet. The nanocatalyst can be recycled and reused for next runs without any significant change in its catalytic activity.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors wish to thank the financial support from the University of Kurdistan.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1ra01029h

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