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. 2022 Dec 1;12:20775. doi: 10.1038/s41598-022-25170-w

A new copper-supported zinc ferrite as a heterogeneous magnetic nanocatalyst for the synthesis of bis(pyrazolyl)methanes and oxidation of sulfides

Arash Ghorbani-Choghamarani 1,✉, Hamid Aghavandi 1, Seyed Mahdi Talebi 1
PMCID: PMC9715624  PMID: 36456752

Abstract

In this paper, we report the synthesis of ZnFe2O4@SiO2@APTES@DHBS-Cu as a novel magnetic nanocatalyst, in a mild and green environment. The structure of the described magnetic compound was characterized by different physicochemical techniques including XRD, EDS, AAS, SEM, FT-IR, X-ray elemental mapping, TGA, and VSM analyses. The prepared magnetic nanoparticles exhibit excellent catalytic activity in synthesizing bis (pyrazolyl)methanes and oxidation of sulfide derivatives under green conditions. The heterogeneous nature of the catalyst was confirmed via the hot filtration experiment. Further, ZnFe2O4@SiO2@APTES@DHBS-Cu showed high efficiency and reusability that could be reused for at least five consecutive runs.

Subject terms: Heterogeneous catalysis, Magnetic materials

Introduction

In the past decade, some specific efforts have been made to design effective magnetic nanoparticles instead of immobilized precious metals on mesoporous silica materials1,2. In catalytic studies, the recovery and reusability of the catalyst due to its adaptability to environmental concerns are important features of the catalytic process that has received much attention in recent years3–7. The recovery and reusability of catalysts is an important challenge in modern research because the employed catalysts are often very cost-effective or the obtained products are often very valuable from the economical and medicinal points of view8–10. Recently, heterogeneous catalysts containing an organic–inorganic material hybrid, have received much attention due to having both the advantages of homogeneous and heterogeneous catalysts11–15. Magnetically separable nanomaterials could be considered as a one of the most important classes of materials with unique physicochemical properties that have attracted the attention of a wide variety of researchers16–18. Regarding the catalytic support materials, spinel ferrite compounds have great potential in industry and technology as green heterogeneous catalysts in various organic functional group transformations and as catalytic supports19–22. Magnetic materials such as zinc ferrite due to their unique and non-toxic magnetic properties23, good biological compatibility24, and adjustable magnetic properties25, have been used in various fields including cancer therapy, drug delivery26, and targeting27. Magnetic nanoparticles have properties such as high active surface area, recyclability, chemical, and thermal stability, which are introduced as separated types of heterogeneous catalysts28–30. Zinc ferrite (ZnFe2O4) has the same properties as homogeneous catalysts and is also facilely extracted from the reaction by an external magnet and does not require more rigorous methods such as filtration or centrifugation. In comparison to homogeneous catalysts, heterogeneous ones have been exclusively studied because of their easy recovery and separation from the reaction mixture31. The use of magnetically separable catalysts is a well-favored and fascinating strategy to bridge the split between heterogeneous and homogenous catalysis processes32. As a main member of the ferrite family, ZnFe2O4 has promising potential for use as novel catalytic support. Additionally, the surface hydroxyl groups over them facilitate their surface modifications with a wide variety of organic compounds.

One of the most essential organic processes is the oxidation of sulfides because the corresponding sulfoxides are valuable mediators for the synthesis of chemical and biological molecules33,34. Furthermore, some of the sulfoxides play leading roles as therapeutic factors such as antibacterial35, anti-ulcer36, antimicrobial37, etc. Sulfide derivatives can be easily oxidized in the presence of a large wide variety of catalysts38–40. Several of these catalysts are not suitable for selective sulfoxidation reactions because of various reasons such as over oxidation to sulfones, low yields, toxicity, use of costly reagents, and low selectivity of products41,42.

The five-membered ring containing two Nitrogen functional groups on positions 1 and 2 generates one of the most important heterocyclic ring systems—pyrazoles. In recent years, pyrazoles and their derivatives have received great attention due to a broad spectrum of biological and pharmacological activities43.

Herein, we report ZnFe2O4@SiO2@APTES@DHBS-Cu MNPs as a green, novel, reusable, and eco-friendly nanocatalyst for the synthesis of bis (pyrazolyl)methanes derivatives and oxidation of sulfide to the sulfoxides. ZnFe2O4@SiO2@APTES@DHBS-Cu has a super magnetic property and thus can be simply separated from the reaction mixture using an external magnet.

Experimental

Materials

All required materials for the synthesis of catalyst, oxidation of sulfides, synthesis of bis (pyrazolyl) methanes, solvents, and reagents have been purchased from Fluka or Merck.

Synthesis of ZnFe2O4

At first, for the synthesis of ZnFe2O4 magnetic nanoparticles, 2.94 g of iron (II) chloride tetrahydrate (FeCl2.4H2O), and 3.00 g of Zinc nitrate hexahydrate (Zn (NO3)2.6H2O) were mixed and vigorously stirred in 100 mL of deionized water. In the next step, 5 g NaOH in 50 mL deionized water was solved and added to the reaction mixture dropwise. Subsequently, the mixture reaction was stirred for 1 h under a nitrogen gas (N2) atmosphere. The final product was separated by a magnet, and washed three times, with hot ethanol (30 mL) and deionized water (30 mL). Finally, polycrystalline spinel magnetite nanoparticles (ZnFe2O4) were dried at 65 °C44.

Preparation of nano-ZnFe2O4@SiO2 core shells

In the second step, 1.0 g of the obtained ZnFe2O4 was dispersed in a mixture of EtOH (50 mL), 5.0 mL of ammonia solution, 10 mL of H2O, followed by the addition of 2.65 g of Polyethylene glycol (PEG-400) and 3 mL of tetraethyl orthosilicate (TEOS). After that, the reaction mixture was stirred at 25 °C for 34 h. Eventually, the product was separated by a magnet and washed with EtOH (30 mL) and deionized water (30 mL) five times, and dried at 25 °C.

Preparation of ZnFe2O4@SiO2@APTES@DHBS-Cu(I)

The novel ZnFe2O4@SiO2@APTES@DHBS-Cu MNPs were readily synthesized according to the route depicted in (Fig. 1). In the next step, ZnFe2O4@SiO2@APTES@DHBS-Cu magnetic nanoparticles were synthesized using the following steps. First for functionalization of ZnFe2O4@SiO2 by 3-(Triethoxysilyl)propylamine, 1 g obtained ZnFe2O4@SiO2 nanoparticles were added to 30 mL of a toluene solution and then 2 mL of 3-(Triethoxysilyl)propylamine was added dropwise to this mixture. The reaction mixture was stirred at reflux condition for 23 h. Then, the resulting solid was filtered, washed with ethanol (30 mL) and water (30 mL) several times, and dried at room temperature. An appropriate amount of ZnFe2O4@SiO2@APTES (1 g) was dispersed in deionized water (100 mL) by sonication for 20 min. Subsequently, 2.5 mmol of 3,5-dichloro-2-hydroxybenzenesulfonyl chloride (DHBS) was added and the reaction mixture was stirred at 85 °C for 23 h. The reaction was performed under a nitrogen gas (N2) atmosphere. Finally, to prepare ZnFe2O4@SiO2@APTES@DHBS-Cu, the obtained ZnFe2O4@SiO2@APTES@DHBS (1 g) was dispersed in 30 mL by EtOH sonication for 20 min. Subsequently, 2.5 mmol of Copper (I) chloride was added to the reaction mixture which was stirred under the nitrogen gas (N2) atmosphere at reflux conditions (90 °C) for 24 h a day. Afterward, the reaction mixture was cooled at 25 °C and, then the final ZnFe2O4@SiO2@APTES@DHBS-Cu (MNPs) were isolated, using a magnet, from the reaction mixture, washed by EtOH (30 mL) and distilled water (30 mL) (several times), to remove the residual impurities and, eventually, and then dried at 75 °C for 20 h45.

Figure 1.

Figure 1

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Synthesis of ZnFe2O4@SiO2@APTES@DHBS-Cu.

General procedure for the synthesis of bis (pyrazolyl) methanes

In a round bottom balloon, a mixture of phenylhydrazine (2 mmol), ethyl acetoacetate (2 mmol), and substituted aromatic aldehydes (1 mmol) and ZnFe2O4@SiO2@APTES@DHBS-Cu (0.03 g) was added, and the reaction mixture was refluxed in ethanol for 60 min. Completion of the following reaction has been analyzed via TLC. After the reaction mixture was diluted with hot ethanol to dissolve the organic products, the catalyst was separated using an external magnet and the resultant unrefined bis (pyrazolyl) methane products, were further purified through recrystallization in an ethanol (Fig. 2).

Figure 2.

Figure 2

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preparation of bis (pyrazolyl)methanes.

A general procedure for the oxidation of sulfides

A combination of sulfide (2 mmol) and H2O2 33% (0.3 mL) was poured into the round-bottomed flask containing ZnFe2O4@SiO2@APTES@DHBS-Cu MNPs (0.02 g). The resulting mixture was stirred under solvent-free conditions at 25 °C. After the completion of the reaction, the ZnFe2O4@SiO2@APTES@DHBS-Cu MNPs were separated by a magnet, and the products were extracted with water and ethyl acetate. Finally, the combined organics were dried over anhydrous sodium sulfate (2 g). Evaporation of the solvent gave the pure corresponding sulfoxides an excellent yield (Fig. 3).

Figure 3.

Figure 3

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Oxidation of sulfides to sulfoxides catalyzed by ZnFe2O4@SiO2@APTES@DHBS-Cu.

Selected NMR data

  • Sulfinyldibenzene: 1H NMR (250 MHz, CDCl3): 7.22–7.65 (m, 10H) ppm.

  • Methylsulfinyl)benzene: 1H NMR (250 MHz, CDCl3):2.7 (s, 3H), 7.48–7.93 (m, 5H) ppm.

  • 1-(Butylsulfinyl)butane: 1H NMR (250 MHz, CDCl3): 0.94(t, J = 7.5 Hz, 6H), 1.40 (m, 4H), 1.70 (m, 4H), 2.66 (t, J = 7.5 Hz, 4H) ppm.

  • 4,4′-((4-Tolyl)methylene)bis(3-methyl-1-phenyl-1H-pyrazol-5-ol): 2.29 (s, 6H), 2.41 (s, 3H), 4.79 (s, 1H), 7.09–7.69 (m, 14H), 7.72 (s, 2H) ppm.

  • 4,4′-((4-Methoxyphenyl)methylene)bis(3-methyl-1-phenyl-1H-pyrazol-5-ol): 1H NMR (250 MHz, CDCl3): δ 2.25 (s, 6H), 3.75 (s, 3H), 4.77 (s, 1H), 6.77–7.95 (m, 14H), 8.57–8.61 (b, 2H) ppm.

Result and discussion

Catalyst characterization

The FT-IR spectra of the (a) ZnFe2O4, (b) ZnFe2O4@SiO2, (c) ZnFe2O4@SiO2@APTES, (d) ZnFe2O4@SiO2@ APTES@DHBS and (e) ZnFe2O4@SiO2@APTES@DHBS-Cu (catalyst) are presented in Fig. 4. Using FT-IR spectroscopy the synthesis of zinc ferrite nanoparticles (ZF-NPS) was confirmed. The two absorption bands at 441 and 587 cm−1 are assigned to the stretching vibrations of the zinc–oxygen and the iron-oxygen bonds, respectively. In Fig. 4a, the bending and stretching vibration of adsorbed water molecules on the surface of the nanoparticles at 1576 and 3473 cm−1 are respectively assigned46. Figure 4b confirms the condensation reaction between hydroxyl groups of ZnFe2O4 (MNPs) and the alkoxysilane molecules of TEOS as the first layer. Absorbed peaks at 3444 and 2956 cm−1 were specified as hydroxide stretching vibration mode. The three absorption peaks around 1074, 584 cm−1, and 471 cm−1 indicated the presence of Si–O–Si asymmetric and symmetric stretching vibrations and bending vibration mode of Si–O–Si, as well as a small peak around 1658 cm−1, was assigned to hydroxide stretching vibration of Silicon-hydroxy and twisting vibration of adsorbed H–O–H in a silica shell47. In Fig. 4c, In ZnFe2O4@SiO2@ APTES, the bands in the range of 2879 cm−1 correspond to the bending vibration of CH2 confirming the attachment of APTES chain molecules48,49. The bands at 1442 cm−1 correspond to NH due to the deformation of the hydrogen-bonded amine groups respectively. NH2 stretching vibrations are present at50,51 3466 cm−1. In the spectra of ZnFe2O4@SiO2@APTES@DHBS (d) the peak in the 1327 cm−1 is attributed to the appearance of sulfone groups (–SO2–) and the peaks in the 1425 and 1605 cm−1 correspond to the carbon–carbon double bond (C=C) in the aromatic ring. Moreover, a certain band in the 3437 cm−1 is due to aromatic O–H45. In Fig. 4e, the spectrum FT-IR shows no significant changes in the ZnFe2O4@SiO2@APTES@DHBS-Cu MNPs vibration bands, and only a slight difference in the intensity of bands can be noticed. These observations demonstrate the immobilization of Cu on the surface of ZnFe2O4@SiO2@APTES@DHBS MNPs (Fig. 4e).

Figure 4.

Figure 4

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FT-IR spectra of (a) ZnFe2O4, (b) ZnFe2O4@SiO2, (c) ZnFe2O4@SiO2@APTES, (d) ZnFe2O4@SiO2@APTES@DHBS, (e) ZnFe2O4@SiO2@APTES@DHBS-Cu.

A comparison of FT-IR spectra of the catalyst after recycling is shown in Fig. 5. As shown in this figure, there are no changes in the FT-IR of ZnFe2O4@SiO2@APTES@DHBS-Cu after recovery, which confirmed the stability of this catalyst under reaction conditions.

Figure 5.

Figure 5

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FT-IR analysis of recovered ZnFe2O4@SiO2@APTES@DHBS-Cu.

X-ray diffraction (XRD) analysis was used to investigate and identify the structure of the zinc ferrite sample. XRD patterns of ZnFe2O4 and ZnFe2O4@SiO2@APTES@DHBS-Cu samples are shown in Figs. 6 and 7. Several strong characteristic peaks are seen in the area of 2θ = 8–80. According to X-ray diffraction peaks at 2θ = 18.2°, 30.5°, 35.7°, 43.4°, 57.4°, 62.9°, 74.6° from ZnFe2O4@SiO2@APTES@DHBS-Cu NPS confirm that the particles have a crystalline structure in the cubic spinel phase52–54.

Figure 6.

Figure 6

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XRD spectrum of ZnFe2O4 (A).

Figure 7.

Figure 7

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XRD spectrum of ZnFe2O4@SiO2@APTES@DHBS-Cu (B).

The TGA results of ZnFe2O4@SiO2@APTES@DHBS-Cu are shown in Fig. 8. Thermal stability and the presence of organic matter fixed on MNPs were investigated using TGA. According to the curve of ZnFe2O4@SiO2@APTES@DHBS-Cu, the weight loss of about 3% below 200 °C is due to the removal of physically adsorbed water and surface hydroxyl groups. In addition, according to the TGA curves, observed the weight loss was about 12% at 200–650 °C for ZnFe2O4@SiO2@APTES@DHBS-Cu that is contributed to the thermal decomposition of immobilized organic components on the ZnFe2O4 surface45.

Figure 8.

Figure 8

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The TGA curves of a ZnFe2O4@SiO2@APTES@DHBS-Cu.

The EDX analysis is brought in Fig. 9 for ZnFe2O4@SiO2@APTES@DHBS-Cu. In another investigation, EDX analysis confirmed the existence of silicon, oxygen, chlorine, sulfur, Iron, Carbon, Nitrogen, and Copper elements in the synthesized magnetic nanoparticles and confirm the synthesis of ZnFe2O4@SiO2@APTES@DHBS-Cu. The existence of Copper peaks in this spectrum indicates the loading of Cu onto the ZnFe2O4@SiO2@APTES@DHBS.

Figure 9.

Figure 9

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EDS spectrum of ZnFe2O4@SiO2@APTES@DHBS-Cu.

Figure 10 shows the comparison of the EDS spectrum of the catalyst after recycling. As shown, there are no changes in the EDS spectrum of ZnFe2O4@SiO2@APTES@DHBS-Cu after recovery, which confirmed the stability of this catalyst under optimal reaction conditions.

Figure 10.

Figure 10

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EDS spectrum of recovered ZnFe2O4@SiO2@APTES@DHBS-Cu.

The X-ray mapping analysis was done to determine the element's contents of ZnFe2O4@SiO2@APTES@DHBS-Cu and their distribution (Fig. 11). The ordered dispensation of the elements (Zn, C, S, Si, Fe, O, N, Cl, and Cu) was observed in the nanocatalyst structure. Thus the conclusion is reached that the Copper has uniformly dispersed on the surface of ZnFe2O4@SiO2@APTES@DHBS.

Figure 11.

Figure 11

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X-ray map spectrum of ZnFe2O4@SiO2@APTES@DHBS-Cu.

Figure 12a–f shows the high-magnification SEM images of the ZnFe2O4@SiO2@APTES@DHBS-Cu MNPs. Based on these images, The ZnFe2O4@SiO2@APTES@DHBS-Cu magnetic nanoparticle is spherical with an almost homogenous size distribution. Furthermore, It is found that ZnFe2O4@SiO2@APTES@DHBS-Cu magnetic nanoparticles are about 73–94 nm in size (Fig. 12b).

Figure 12.

Figure 12

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SEM spectrum of ZnFe2O4@SiO2@APTES@DHBS-Cu.

The VSM pattern of ZnFe2O4@SiO2@APTES@DHBS-Cu and ZnFe2O4 magnetic was shown in Fig. 13. The magnetic values were 41.5 and 31 emu/g for ZnFe2O4 and ZnFe2O4@SiO2@APTES@DHBS-Cu, respectively. Due to the coating of nanoparticles with silica and DHBS-Cu, the magnetic properties value of the final catalyst decreased compared to the bare ZnFe2O4. Due to the relatively high magnetic properties of the prepared nanomaterial, the particles were effectively recycled and removed from the mixture.

Figure 13.

Figure 13

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The magnetic properties of (a) ZnFe2O4 and (b) ZnFe2O4@SiO2@APTES@DHBS-Cu.

Catalytic study

Catalytic activity studies of ZnFe2O4@SiO2@APTES@DHBS-Cu for the synthesis of bis(pyrazolyl) methanes via the pseudo-five-component reaction

In primary research to obtain optimal reaction conditions, the effect of temperature, solvent type, and catalyst quantity outcome of the reaction was checked on the condensation of phenylhydrazine (2 mmol), ethyl acetoacetate (2 mmol), and 4-chloro benzaldehyde (1 mmol). First, the effect of solvents was considered. Surprisingly, EtOH (2 mL) was the most suitable solvent to reach the desired product (Table 1). Then, the effects of reaction temperature were evaluated on the model reaction. Among the various screened temperatures (e.g., 25, 50, 70, 100 °C and reflux conditions), the reflux conditions were the most effective temperature for described reaction (Table 1, entry 5). Then, the influence of the catalyst loading on the outcome of the reaction was studied with different amounts of catalyst. Obtained results revealed that 0.03 g of the catalyst is the most effective amount (Table 1, entry 5). As given in Table 1, entry 1, the reaction was not completed in the absence of the catalyst even after 8 h. Eventually, EtOH at reflux condition in the presence of 0.03 g of ZnFe2O4@SiO2@APTES@DHBS-Cu MNPs was found to be the best choice for the described multicomponent reaction. For indicating the exigency of the presence of Cu metal in the nanocatalyst, the model reaction was undertaken in the presence of ZnFe2O4 and ZnFe2O4@SiO2 instead of ZnFe2O4@SiO2@APTES@DHBS-Cu (Table 1, entries16 and 17) that surprisingly no any product was produced.

Table 1.

The results of optimizing the catalyst quantity, temperature, and solvent on the reaction of 4-chlorobenzaldehyde, phenylhydrazine, and ethyl acetoacetate.

Entry Catalyst (g) Solvent Temperature (°C) Time (min) Yield
1 – Ethanol Reflux 8 h Trace
2 0.005 Ethanol Reflux 60 65
3 0.01 Ethanol Reflux 60 87
4 0.02 Ethanol Reflux 60 93
5 0.03 Ethanol Reflux 60 96
6 0.04 Ethanol Reflux 60 96
7 0.03 Acetonitrile Reflux 60 65
8 0.03 H2O Reflux 60 NR
9 0.03 Dimethyl sulfoxide 100 60 87
10 0.03 PEG-400 100 60 70
11 0.03 Dimethyl formamide 100 60 81
12 0.03 Solvent free 100 60 59
13 0.03 Ethanol R. T 60 NR
14 0.03 Ethanol 50 60 58
15 0.03 Ethanol 70 60 89
16 0.03a Ethanol Reflux 60 N. R
17 0.03b Ethanol Reflux 60 N. R
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aReaction was performed in the presence of ZnFe2O4 NPs.

bReaction was performed in the presence of nano-ZnFe2O4@SiO2.

After optimization, we examined various electron-withdrawing and electron-releasing substituted benzaldehydes in ZnFe2O4@SiO2@APTES@DHBS-Cu catalyzed multicomponent cyclo condensation reaction for the preparation of bis(pyrazolyl) methane derivatives to identify the generality and the high proficiency of catalytic system (Table 2). It is evident from Table 2 that a variety of bis(pyrazolyl) methane derivatives were synthesized with values of melting point, yield, and reaction time. As shown in Table 2, this catalytic system is a suitable method for the efficiency of conditions. All derivatives were obtained with excellent yields (70–96%) and short reaction times (45–95 min). Also, the reaction with electron-withdrawing benzaldehydes (NO2, Cl, and Br) is considered faster than the one with electron-donating benzaldehydes (Me, OMe, and OH).

Table 2.

The one-pot pseudo-five-component production of bis (pyrazolyl)methanes catalyzed by ZnFe2O4@SiO2@APTES@DHBS-Cu.

Entry Ar Product Time (min) Yield (%)b TON TOF (h−1) M.P. (°C)
Measured Literature
1 C6H5 graphic file with name 41598_2022_25170_Figa_HTML.gif 45 92 102 136 169–176 171–17755
2 4-ClC6H5 graphic file with name 41598_2022_25170_Figb_HTML.gif 60 96 106 106 214–217 213–21855
3 2,6-ClC6H5 graphic file with name 41598_2022_25170_Figc_HTML.gif 50 70 77.7 97 247–251 248–25356
4 2,4-ClC6H5 graphic file with name 41598_2022_25170_Figd_HTML.gif 45 84 93.3 124 220–226 223–22757
5 2-BrC6H5 graphic file with name 41598_2022_25170_Fige_HTML.gif 60 92 102 102 197–205 198–20655
6 4-MeC6H5 graphic file with name 41598_2022_25170_Figf_HTML.gif 60 93 155 155 200–207 203–20955
7 4-MeOC6H5 graphic file with name 41598_2022_25170_Figg_HTML.gif 65 89 98.8 91.4 170–178 173–17955
8 4-OHC6H5 graphic file with name 41598_2022_25170_Figh_HTML.gif 55 91 101 112 151–158 153–15855
9 3,4-MeOHC6H5 graphic file with name 41598_2022_25170_Figi_HTML.gif 95 91 101 67 192–195 193–19755
10 2-NO2C6H5 graphic file with name 41598_2022_25170_Figj_HTML.gif 45 88 97.7 129 223–227 225–22855
11 3-NO2C6H5 graphic file with name 41598_2022_25170_Figk_HTML.gif 70 85 94.4 85.4 150–154 151–15455
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aIsolated yield.

bReaction conditions: phenylhydrazine (2 mmol), ethyl acetoacetate (2 mmol), aromatic aldehydes (1 mmol), ZnFe2O4@SiO2@APTES@DHBS-Cu in EtOH (2 mL) under reflux conditions.

Based on our previous studies, a proposed possible mechanism for synthesis of bis (pyrazolyl)methanes using ZnFe2O4@SiO2@APTES@DHBS-Cu nanocatalyst has been presented in Fig. 14. Initially, the carbonyl group in the ethyl acetoacetate was activated by the ZnFe2O4@SiO2@APTES@DHBS-Cu nanocatalyst for the attack of lone pair of nitrogen form phenylhydrazine to form intermediate pyrazolone (I). In the next step, the activated aromatic aldehyde by ZnFe2O4@SiO2@APTES@DHBS-Cu nanocatalyst undergoes a tandem reaction with intermediate (II) (which is the tautomer of intermediate (I)) leading to intermediate (III) after removal of an H2O molecule. The next step is a Michael addition of another intermediate of (III) to (II) to form intermediate (IV). In the last step, after the tautomeric proton shift, afford bis(pyrazolyl)methanes.

Figure 14.

Figure 14

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The possible mechanism for ZnFe2O4@SiO2@APTES@DHBS-Cu catalyzed the pseudo-five-component preparation of bis (pyrazolyl) methanes.

In another part of this research project, the catalytic activity of ZnFe2O4@SiO2@APTES@DHBS-Cu was tested in the oxidation of sulfide. The reaction of the methyl phenyl sulfide with H2O2 (as a green oxidizing agent) was chosen as a model reaction (for optimization). The effect of various solvents with different polarities (including Ethyl acetate, Ethanol, n-Hexane, EtOH: H2O (1:1), H2O) as well as solvent-free conditions, was checked. The best reaction yield was gained under solvent-free conditions at 30 min (Table 3, Entry 5). Then the amount of catalyst was optimized. The results of experiments indicated oxidation reaction didn’t occur in the absence of ZnFe2O4@SiO2@APTES@DHBS-Cu even after 360 min (Table 3, entry 1). The details of reaction parameters and results are brought in in Table 3. As shown in Table 3, (entries 16 and 17), the reaction was performed in the presence of ZnFe2O4 and ZnFe2O4@SiO2@APTES@DHBS (0.02 g) for the oxidation of sulfides. Meanwhile, the best yield of products was obtained in the presence of ZnFe2O4@SiO2@APTES@DHBS-Cu (0.02 g).

Table 3.

Optimizing reaction conditions for oxidation of methyl phenyl sulfide in the presence of ZnFe2O4@SiO2@APTES@DHBS-Cu.

Entrya Catalyst (g) Solvent H2O2 (mg) Time (min) Yield (%)b
1 – Solvent-free 0.3 360 Trace
2 0.003 Solvent-free 0.3 30 75
3 0.007 Solvent-free 0.3 30 85
4 0.01 Solvent-free 0.3 30 87
5 0.02 Solvent-free 0.3 30 97
6 0.03 Solvent-free 0.3 30 97
7 0.02 n-Hexane 0.3 30 58
8 0.02 H2O 0.3 30 Trace
9 0.02 H2O: EtOH 0.3 30 47
10 0.02 EtOAc 0.3 30 92
11 0.02 EtOH 0.3 30 82
12 0.02 Solvent-free 0.1 30 88
13 0.02 Solvent-free 0.2 30 92
14 0.02 Solvent-free 0.3 30 96
15 0.02 Solvent-free 0.4 30 96
16 0.02 Solvent-free 0.3 30 N. Rc
17 0.02 Solvent-free 0.3 30 Traced
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aReaction conditions: sulfide (1 mmol) H2O2 (0.3 mL) and ZnFe2O4@SiO2@APTES@DHBS-Cu at 25 °C under solvent-free conditions.

bIsolated yield.

cThe reaction was catalyzed by ZnFe2O4.

dThe reaction catalyzed by ZnFe2O4@SiO2@APTES@DHBS.

After the optimization, the oxidation of different sulfides was investigated in the presence of ZnFe2O4@SiO2@APTES@DHBS-Cu NPs. In all cases, sulfoxides were produced in high yields at short reaction times, which showed efficiency and excellent catalytic activity of described catalyst. As shown in Table 4, this catalytic system is a suitable method in terms of the efficiency of conditions. All derivatives were obtained with excellent yields and short reaction times. Also, the reaction with electron-withdrawing benzaldehydes is considered faster than the one with electron-donating benzaldehydes. The results including product yields and reaction times obtained are summarized in Table 4.

Table 4.

Oxidation of sulfides into sulfoxides in the presence of ZnFe2O4@SiO2@APTES@DHBS-Cu.

Entrya Substrate Time (min) Yield (%)b TON TOF (h−1) Melting point (°C)
Measured Literature
1 graphic file with name 41598_2022_25170_Figl_HTML.gif 35 96 160 275 31–35 32–3658
2 graphic file with name 41598_2022_25170_Figm_HTML.gif 30 95 158 316 130–132 128–13158
3 graphic file with name 41598_2022_25170_Fign_HTML.gif 90 75 125 83.3 151–156 150–15459
4 graphic file with name 41598_2022_25170_Figo_HTML.gif 75 87 145 116 85–89 83–8660
5 graphic file with name 41598_2022_25170_Figp_HTML.gif 70 90 150 129 Oil Oil
6 graphic file with name 41598_2022_25170_Figq_HTML.gif 35 86 143 246 Oil Oil
7 graphic file with name 41598_2022_25170_Figr_HTML.gif 45 90 150 200 116–120 114–11661
8 graphic file with name 41598_2022_25170_Figs_HTML.gif 35 93 155 267 Oil Oil
9 graphic file with name 41598_2022_25170_Figt_HTML.gif 85 72 120 85.1 67–68 70–7258
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aReaction conditions: sulfide (1 mmol) H2O2 (0.3 mL) and ZnFe2O4@SiO2@APTES@DHBS-Cu (0.02 g) at room temperature under solvent-free conditions.

bIsolated yield.

Based on previous studies, a suggested and possible mechanism for the sulfoxidation reaction catalyzed by ZnFe2O4@SiO2@APTES@DHBS-Cu has been presented in Fig. 15. The role of copper in ZnFe2O4@SiO2@APTES@DHBS-Cu as a magnetic nanocatalyst is to form the active oxidant complex. Based on this mechanism, the transfer of oxygen to sulfur leads to the formation of sulfoxide.

Figure 15.

Figure 15

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The suggested mechanism for the oxidation of sulfide.

Hot filtration

To approve the heterogeneous nature of the ZnFe2O4@SiO2@APTES@DHBS-Cu in the synthesis of bis (pyrazolyl) methane compounds hot filtration experiment was performed using 4-chlorobenzaldehyde as a model substrate under the optimal reaction conditions. After half time of the reaction (30 min), the reaction was terminated, and the yield was found to be 53%. In another experiment, the reaction was designed again, and at half time of reaction, the catalyst was separated and the reaction mixture was stirred further without the catalyst. Herein, we observed that low conversion (< 4%) of the product happened through the heating of the catalyst-free mixture for another 30 min. It can be concluded that nanocatalyst is completely heterogeneous.

Also, copper leaching of ZnFe2O4@SiO2@APTES@DHBS-Cu was studied by atomic absorption spectroscopy (AAS). Based on AAS analysis, the amount of copper in fresh and reused catalysts were 3.6 × 10–4 mol g−1 and 3 × 10–4 mol g−1 respectively, which shows that Cu leaching into reaction media from the ZnFe2O4@SiO2@APTES@DHBS-Cu framework is very low.

Reusability of catalyst

Easy separation of a catalyst is an important point of view in heterogeneous catalysts. Therefore, we next considered the reusability of ZnFe2O4@SiO2@APTES@DHBS-Cu in the oxidation of sulfides (A) and synthesis of bis (pyrazolyl)methanes (B) using methyl phenyl sulfide and 4-chlorobenzaldehyde respectively as a model substrates. These reactions were performed under optimized conditions to test the reusability of the ZnFe2O4@SiO2@APTES@DHBS-Cu (Fig. 16). As can be seen from Fig. 16, no significant decrease in the yield of products was observed after five runs. Naturally, ZnFe2O4@SiO2@APTES@DHBS-Cu showed excellent performance in several successive cycles for each of the above reactions.

Figure 16.

Figure 16

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Reusability of ZnFe2O4@SiO2@APTES@DHBS-Cu in the oxidation of sulfides (A) and synthesis of bis (pyrazolyl)methanes (B).

Comparison of the catalyst

The efficiency of ZnFe2O4@SiO2@APTES@DHBS-Cu was investigated by comparison of our results on the synthesis of bis (pyrazolyl)methanes and oxidation of sulfides in model reactions with the previously reported methods (Table 5). The products were obtained in higher yields over faster times in the presence of ZnFe2O4@SiO2@APTES@DHBS-Cu. Also, this catalyst has several advantages in terms of non-toxicity, price, stability, and easy separation.

Table 5.

Comparison results of ZnFe2O4@SiO2@APTES@DHBS-Cu with other catalysts in the synthesis of bis (pyrazolyl)methanes and oxidation of sulfides.

Entry Product Catalyst Time (min) Yield (%) Refs.
1 4,4′-((4-Chlorophenyl)methylene)bis(3-methyl-1-phenyl-1H-pyrazol-5-ol) Fe3O4@THAM-Pd 55 65 62
2 4,4′-((4-Chlorophenyl)methylene)bis(3-methyl-1-phenyl-1H-pyrazol-5-ol) DCDBTSD 49 80 63
3 4,4′-((4-Chlorophenyl)methylene)bis(3-methyl-1-phenyl-1H-pyrazol-5-ol) [TMEDSA][HSO4]2 30 93 64
4 4,4′-((4-Chlorophenyl)methylene)bis(3-methyl-1-phenyl-1H-pyrazol-5-ol) [Dsim][TFA] 30 93 65
5 4,4′-((4-Chlorophenyl)methylene)bis(3-methyl-1-phenyl-1H-pyrazol-5-ol) ZnFe2O4@SiO2@APTES@DHBS-Cu 60 96 This work
6 Methyl(phenyl)sulfane MCM‐41‐Adenine‐Zr 55 97 66
7 Methyl(phenyl)sulfane Fe3O4-AMPD-Pd 20 95 61
8 Methyl(phenyl)sulfane Fe3O4@chitosan-bound picolinaldehyde Cu 140 93 67
9 Methyl(phenyl)sulfane CoFe2O4@SiO2‐CPTES‐Guanidine‐Cu(II) 20 95 68
10 Methyl(phenyl)sulfane ZnFe2O4@SiO2@APTES@DHBS-Cu 30 97 This work
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Conclusions

In this study, a new type of magnetically recoverable nanocatalyst (ZnFe2O4@SiO2@APTES@DHBS-Cu MNPs) was synthesized. To specify the physicochemical features of the nanocatalyst, various techniques; including, FT-IR, SEM, EDX, XRD, TGA, Map, and VSM analysis were used. This study reported a novel route for the synthesis of an extensive range of bis (pyrazolyl)methanes and sulfoxides with excellent yields. This catalyst can upgrade yields and reaction times in the synthesis of bis (pyrazolyl)methanes and oxidation of sulfides to the sulfoxides compared to many reported methods, with meager leaching amounts of supported catalyst into the reaction mixture. The simple and easy manufacturing method of this catalyst, along with its ability to recover and reuse, makes it economical. The wondrous features of the mentioned organic reactions are high novelty, short reaction times, no use of harmful solvents, simple synthetic procedure, high yields of reactions, facile filtration, and reusability of the catalyst. Furthermore, the synthesized nanocatalyst could be separated easily using an external magnet and reused five times without considerable loss of its activity (Supplementary Information 1).

Supplementary Information

Supplementary Information. (581.2KB, pdf)

Acknowledgements

This work was supported by the research facilities of Bu-Ali Sina University, Hamedan, Iran.

Author contributions

A.G.-C.: funding acquisition, supervision, conceptualization, resources, writing-review and editing. H.A.: methodology, validation, investigation, writing-original draft. S.M.T.: validation, investigation, methodology.

Data availability

All data generated or analysed during this study are included in this published article [and its supplementary information files].

Competing interests

The authors declare no competing interests.

Footnotes

Publisher's note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Supplementary Information

The online version contains supplementary material available at 10.1038/s41598-022-25170-w.

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Supplementary Materials

Supplementary Information. (581.2KB, pdf)

Data Availability Statement

All data generated or analysed during this study are included in this published article [and its supplementary information files].

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