Introduction

Since the first report of periodic mesoporous silica designated as MCM-41 in the early 1990s ordered mesoporous silica materials have attracted much attention1. Many new synthetic methods based on heterogenization of homogeneous catalysts have become a major area of research since the potential advantages of these materials such as simplified recovery and reusability over homogeneous systems can have positive environmental effects2. Many porous materials have technical applications as heterogeneous catalysts and excellent adsorbents3,4. In particular mesoporous compounds, have gained a special place5,6. In the last decade, among the different types of mesoporous materials used in heterogeneous catalysis, interesting research was carried out on MCM-41 materials7,8,9, because of some valuable properties, such as larger surface area, high reactivity, good accessibility, and suitable pore structure, chemical, and thermal stability. Much research has been done on the use of these compounds in biomass10. for example, Wang et al.11 prepared a series of Sn-containing mesoporous MCM-41 catalysts for the conversion of pyruvaldehyde to ethyl lactate. Multicomponent reactions, such as Ugi12, Gewald13 or coupling reactions4, and redox9,14 have also been reported using these functionalized MCM-41 catalysts.

Transition metal-catalyzed carbon–oxygen and carbon–sulfur bond formation with aryl halides and aromatic phenols and thiols as electron-pair donors is a powerful tool to prepare O- and S-containing compounds that have high applicability in synthetic, biological, pharmaceutical, and materials science15. Many metals such as palladium, nickel, copper, iron, and gold have been able to catalyze this type of reaction16. Traditional methods for the synthesis of aryl-sulfur bonds are different and often require harsh reaction conditions17. For example, the coupling of copper thiolates with aryl halides requires polar solvents such as HMPA and temperatures around 200 °C. Reduction of aryl sulfones or aryl sulfoxides requires strong reducing agents such as DIBAL-H or LiAlH418.

Copper-catalyzed coupling reactions of aryl halides with nucleophiles, so-called Ullmann-type reactions19, are well-established methods for preparing pharmaceutically and materially important compounds. Many scientists have reported different conditions for these types of reactions in recent years using copper-based catalysts20. This reaction was also carried out in 2014 using thioamides as a source of sulfur21,22. In 2021, the Ullmann reaction was used to produce endochin-like quinolone compounds, which are safe treatments for a range of important human and animal afflictions in Sovitj Pou research group23. Also, cellulose-supported poly(hydroxamic acid) − Cu(II) complex was successfully applied to the Ullmann etherification24. Various functionalized 2-aminobenzo[b]thiophenes have been synthesized at room temperature by the Ullmann coupling reaction in the presence of different Cu salts and 1 10 phenanthroline as ligand25. Ge research group prepared three different types of functionalized chitosan and anchored with copper salts for use as the catalyst for the Ullmann C–X coupling reaction26. Following the interest in Ullmann-type reactions and mesoporous materials, in this work, we report a new recoverable catalyst based on modified MCM-41 anchored with copper ions, which is an efficient catalyst for the Ullmann type reactions.

Experimental

Materials

All commercially available chemicals, solvents, reagents and were purchased from Sigma-Aldrich and Merck company, briefly cetyltrimethylammonium bromide (CTAB) (Sigma-Aldrich, ≥ 99%), ammonium hydroxide solution (Sigma-Aldrich, ≥ 99.99%), tetraethyl orthosilicate (TEOS) (ACROS, 98%), EtOH (Sigma-Aldrich, ≥ 99.8%), anhydrous N, N-dimethylformamide (Sigma-Aldrich ≥ 99.8%), metformin hydrochloride (Merck, ≥ 99.9%).

The melting points of the prepared derivatives were measured by an Electrothermal 9100 apparatus, which was reported without any correction. The FT-IR spectra were recorded in the range of 400–4000 cm−1 using a Shimadzu IR-470 spectrometer by using KBr pellets. 1H-NMR spectra were recorded using the Bruker DRX-500 and 300 AVANCE spectrometer. Elemental analysis was provided by EDX analysis, which was recorded by TESCAN4992. The morphology of the synthesized catalyst was studied by SEM using VEGA2 TESCAN instrument. TGA of the prepared catalyst was obtained by an STA504. The XRD measurement of the catalyst was recorded with the X′ Pert Pro diffractometer operating with (40 mA, 40 kV). N2 adsorption–desorption isotherms of Cu@MCM-41-HPr-Met nanocomposite were measured at the temperature of liquid nitrogen with a Micromeritics system. The surface area of the nanoparticles was calculated using the Brunauer–Emmett–Teller (BET) method. All products we compared based on their spectra and physical data recorded in the references.

Catalyst preparation

The catalyst has been prepared according to Scheme 1, as follows. The given protocol has been used for the preparation of the catalyst on a 5 g scale.

Scheme 1
scheme 1

Schematic representation of the synthesis of heterogeneous catalyst Cu@MCM-41-HPr-Met.

Full size image

Preparation of MCM-41

The MCM-41 synthesis is performed according to the reported procedure by Zanjanchi27. In brief, 2.7 g ethylamine was added to 42 ml of deionized water and the mixture was stirred at room temperature for 10 min. The amount of 1.47 g of the surfactant cetyltrimethylammonium bromide (CTAB) was gradually added to the above solution under stirring for 30 min. After further stirring for 30 min, a clear solution was obtained. Then, 2.1 g of TEOS solution was added dropwise to the solution. The pH of the reaction mixture was adjusted to 8.5 by the slow addition of the hydrochloric acid solution (1 M) to the mixture. After precipitate formation, slow stirring for 2 h is necessary, and then the precipitate was separated by centrifuge and washed 3 times. The product was dried at 45 °C for 12 h and calcined at 550 °C for 5 h to decompose the surfactant to obtain the white powder. This powder was used as the parent material to prepare the main catalyst.

Preparation of MCM-41-EPTMS

In a typical procedure, a round-bottom flask charged with 0.5 g MCM-41 and 10 mL of n-hexane was added, then 0.5 g (2.11 mmol) [3-(2,3-epoxypropoxy)-propyl]-trimethoxysilane added into the reaction mixture. Reaction after stirring for 24 h under inert N2 atmosphere and reflux condition in oil bath, was cooled to room temperature. The solid product MCM-41-EPTMS filtered off and was washed twice with n-hexane, dried in an oven at 70 °C for 24 h.

Preparation of MCM-41-HPr-Met

The obtained MCM-41-EPTMS (0.4 g) was dispersed in toluene (15 ml) by sonication for 15 min. Triethylamine (2 mmol, 0.202 g) and metformin (1 mmol, 0.129 g) were added to the above solution. Finally, MCM-41-HPr-Met precipitate was obtained after 24 h under reflux condition and washing filtered product with EtOH.

Preparation of Cu@MCM-41-HPr-Met

The MCM-41-HPr-Met precipitate (0.25 g) was added in EtOH (25 mL), then copper (II) acetate monohydrate (1 mmol, 0.199 g) was added to the reaction mixture and stirred under N2 atmosphere at 70 °C for 20 h. Finally obtained Cu@MCM-41-HPr-Met precipitate was washed with EtOH and dried at 70 °C for 12 h.

General procedure for diaryl sulfide derivatives catalyzed by Cu@MCM-41-HPr-Met

A mixture of aryl halide (1 mmol), thiophenol (1.2 mmol), K2CO3 (2 mmol), and 30 mg Cu@MCM-41-HPr-Met as a catalyst in 3 ml DMSO/EtOH (2:1) was stirred for 6 h at 90 °C. The test tube was filled with inert N2 gas and sealed. The progress was monitored by TLC, after the reaction was complete, the test tube was cooled to room temperature. First, the EtOAc solvent was added to the reaction mixture to separate catalyst by filtration, the mixture of reaction was poured in distilled water and extracted with EtOAc (3 \(\times\) 15 ml). The organic phase was separated with a separatory funnel and the solvent was evaporated with rotary. The final net product was obtained by column chromatography (EtOAc: n-hexane).

General procedure for diaryl ether derivatives catalyzed by Cu@MCM-41-HPr-Met

A mixture of aryl halide (1 mmol), phenol (1.5 mmol), K2CO3 (2 mmol), and 50 mg Cu@MCM-41-HPr-Met as a catalyst in 3 ml DMF/EtOH (2:1) was stirred for 6 h at 90 °C under inert N2 gas in a sealed tube. The progress was monitored by TLC, after the reaction was complete, the test tube was cooled to room temperature. To separate catalyst by filtration the EtOAc solvent was added to the reaction mixture, after catalyst filtration, the mixture was poured in distilled water and extracted with EtOAc (3 × 15 ml). The organic phase was separated with a separatory funnel and the solvent was evaporated with rotary. The final net product was obtained by column chromatography (EtOAc: n-hexane).

Result and discussion

In this research, the preparation of Cu@MCM-41-HPr-Met was done as outlined in Scheme 1. The surface of MCM-41 has many hydroxyl groups, which can be functionalized with alkoxysilane reagents. We used [3-(2,3-epoxypropoxy)-propyl]-trimethoxysilane as a valuable linker. Metformin was used then to open the unstable epoxy ring. Catalyst structure can attach to metals because of its NH and OH groups that can chelate the metal cation easily. Copper (II) cation was chosen because it can catalyze the Ullmann Type reactions.

Characterization of the catalyst

Spectroscopic and analytical techniques FT-IR, TGA, EDX, SEM, TEM, BET, and XRD were used to determine the structural properties of the catalyst Cu@MCM-41-HPr-Met.

XRD analysis

To identify the crystal structure of the synthesized catalyst, we investigated the low-angle XRD patterns of the modified MCM-41 and Cu@MCM-41-HPr-Met, both were shown in Fig. 1A. The wide-angle XRD pattern of the Cu@MCM-41-HPr-Met was shown in Fig. 1B. The intense 2θ peak at around 2.4° should be attributed to the (100) plane of the MCM-41 and the very weak peak between 4° and 5° was characteristic of its long-range hexagonal structure28,29. Comparison of patterns a, b, and c in Fig. 1A shows that the hexagonal structure of the MCM-41 was not destroyed by functionalization, but decreased.

Figure 1
figure 1

(A) The low-angle XRD patterns of a: MCM-41-EPTMS, b: MCM-41-HPr-Met c: Cu@MCM-41-HPr-Met, (B) XRD pattern of Cu@MCM-41-HPr-Met.

Full size image

FT-IR spectroscopy

The FT-IR spectra of the synthesis steps are shown in Fig. 2. In spectrum a, the absorption bands at 1074 and 3432 cm−1 are related to the stretching vibrations of the O–Si groups, and the OH bond stretching vibrations of MCM-41. In spectrum b, the band observed at 2941 cm−1 is attributed to the stretching vibrations of the aliphatic C–H bonds, approving the addition of the propyl group. The wideband observed at 3432 cm−1 is due to stretching vibrations of the OH bond. The ether C–O group appears at 1080 cm−1. In the c spectrum, the band appearing at 3434 cm−1 is attributed to the stretching vibrations of the N–H and O–H groups, and the bands at 1479 and 1645 cm−1 represent the C = N imino groups. In the d spectrum, the band shifting to the 1556 cm−1, indicates that the copper ion has been successfully doped on the MCM-41-HPr-Met.

Figure 2
figure 2

FT-IR spectra of a) MCM-41, b) MCM-41-EPTMS, c) MCM-41-HPr-Met, d) Cu@MCM-41-HPr-Met.

Full size image

Scanning electron microscopies (SEM)

SEM images of the Cu@MCM-41-HPr-Met catalyst are presented in three scales: 5, 10, and 20 µm are shown in Fig. 3. This analysis shows the morphology and size of the synthesized particles. These images show that the particles have spherical morphology as well as a layered structure. As we can see in Fig. 3, we can estimate the particle size between 0.37–0.64 µm.

Figure 3
figure 3

SEM images of Cu@MCM-41-HPr-Met catalyst.

Full size image

Transmission electron microscopies (TEM)

TEM analysis was performed to more accurately study the morphology and particle size of the mesostructured catalyst (Fig. 4A, B). The TEM micrograph in Fig. 4B indicates the ordered mesoporous channels of silica after modification.

Figure 4
figure 4

TEM images of Cu@MCM-41-HPr-Met catalyst.

Full size image

EDX analysis

As is shown in Fig. 5A, the presence of expected O, N, Si, C, and Cu elements in the structure of the synthesized nanoparticles were approved by energy dispersive X-ray (EDX) analysis. The presence of the copper ions on the catalyst was confirmed with the bands of 8.04, 8.90 keV (K lines), and 0.92 keV (L line). Moreover, the distribution of the elements in this mesoporous catalyst is shown in the EDX mapping images in Fig. 5B.

Figure 5
figure 5

The EDX analysis and mapping images of the Cu@MCM-41-HPr-Met catalyst.

Full size image

TGA analysis

Thermal gravimetric analysis (TGA) can study the behavior of matter versus temperature. In Fig. 6, the downward trend diagram illustrates the fact that the sample mass decreases with increasing temperature. As shown, weight loss below 200 °C corresponds to the removal of the adsorbed water and organic solvents (about 2 wt.%). The second region is mainly related to the thermal decomposition of organic ligands in the temperature range between 200 °C and 700 °C (about 20 wt.%).

Figure 6
figure 6

TGA spectrum of Cu@MCM-41-HPr-Met catalyst.

Full size image

BET analysis

The N2 adsorption–desorption isotherm and BJH pore size distribution of the Cu@MCM-41-HPr-Met are shown in Fig. 7A,B. The isotherm is classified as type IV, characteristic of mesoporous materials, with a sharp capillary condensation of nitrogen into the mesoporous channels at high relative pressure and H1 hysteresis loop, which reveals the presence of large channel-like pore structures. Also, based on the shape of its hysteresis, Cu@MCM-41-HPr-Met catalyst has cylindrical pores and the initial structure is retained after functionalization. The structural data of the Cu@MCM-41-HPr-Met catalyst nanoparticles are summarized in Table 1.

Figure 7
figure 7

(A) Adsorption/desorption N2 isotherms and (B) pore size distribution of Cu@MCM-41-HPr-Met.

Full size image
Table 1 Surface area, pore volume, and pore diameter of Cu@MCM-41-HPr-Met.
Full size table

Application of catalyst in Ullmann type reaction

We applied MCM-41-HPr-Met to Ullmann-type reactions to show the catalytic utility of the newly constructed structure. We assume that, as shown in Scheme 2, the copper first enters the C-X bond of the aryl halide and produces the intermediate A. (Thio)phenol is converted to its anion in the presence of potassium carbonate as a base, then reacts with the intermediate A and produces intermediate B. When the catalyst leaves, the desired product diaryl sulfides or diaryl ethers are produced.

Scheme 2
scheme 2

Proposed mechanism for the catalytic activity of Cu@MCM-41-HPr-Met in O-arylation and S-arylation reactions.

Full size image

S-Arylation of thiols

To begin with, the reaction between 4-nitro-1-bromobenzene and thiophenol was selected as the model reaction. Then, by optimizing the amount of catalyst and selecting the appropriate solvent and base, accurate time, and temperature measurement, the further reaction progression and product yield increased. As seen in Table 2, by repeating the experiment under different conditions, K2CO3 was selected as the appropriate base and DMSO/EtOH as a solvent for the reaction. After testing different Cu@MCM-41-HPr-Met catalyst amounts, 30 mg was selected as the optimum amount (Table 3).

Table 2 Effect of solvents and different bases in C-S bond formation in a reaction between 1-bromo-4-nitrobenzene and thiophenol as a model reaction.
Full size table
Table 3 Optimization of Cu@MCM-41-HPr-Met catalyst in model reaction.
Full size table

After determining the optimal conditions for the reaction of the model, to prove the repeatability of this method and the efficiency of the catalyst, the reaction of derivatives of aryl halides and thiophenols was performed under optimal conditions and diaryl sulfide products were obtained with a high yield. As shown in Table 4, the reactivity with the derivatives of iodobenzene and bromobenzene is higher than that of chlorobenzene. However, studies have shown that doing a reaction with chlorobenzene derivatives with good yield shows Cu@MCM-41-HPr-Met catalyst's high efficacy. In general, the placement of electron-donating groups on aryl halide derivatives increases reactivity, and placing electron-withdrawing groups decreases it.

Table 4 Reactions of aryl halides with thiophenols in the presence of Cu@MCM-41-HPr-Met.
Full size table

O-Arylation of phenols

In this type of reaction, as before, the model reaction was investigated in the presence of DMF/EtOH as the solvent, and K2CO3 as the base, and reaction time, temperature, and Cu@MCM-41-HPr-Met catalyst amount was optimized (Table 5).

Table 5 Optimization of solvents and bases in C-O bond formation in the reaction between Bromobenzene and phenol as a model reaction.
Full size table

The reactivity of aryl halides with electron-withdrawing groups in the para position is better than aryl halides with the electron-withdrawing group in the ortho position. Also, the order of reactivity of aryl halides is that aryl bromides are more reactive than aryl chlorides (Table 6).

Table 6 The reaction of aryl halides with phenols in the presence of Cu@MCM-41-HPr-Met as a catalyst.
Full size table

To show the capability and efficiency of this method and the Cu@MCM-41-HPr-Met as a catalyst, a comparison has been summarized in Table 7 with the previous methods of synthesis diaryl sulfides and diaryl ethers reported in some literature.

Table 7 Comparison of the results obtained in the synthesis of diaryl sulfides (1–5) and diaryl ethers (6–10) in the presence of Cu@MCM-41-HPr-Met and other catalysts.
Full size table

Reusability of the catalyst

One of the most important issues with heterogeneous catalysts is their effective lifespan and their ability to be recycled and reused. Therefore, this was also examined in the present catalytic system. For this purpose, after the reaction was complete, the Cu@MCM-41-HPr-Met catalyst was separated using a strainer and washed several times using ethyl acetate. The Cu@MCM-41-HPr-Met catalyst was then placed in an oven to dry. We used the catalyst again in the (thio)phenol's reaction with 1-bromo-4-nitrobenzene as a model reaction. This operation was repeated 5 times and they give the results for both types of reactions in Fig. 8. As you can see, there were no significant changes in the efficiency or activity of the catalyst after repeated use.

Figure 8
figure 8

The number of Cu@MCM-41-HPr-Met catalyst recovery in the synthesis of diaryl sulfides and diaryl ether.

Full size image

Conclusion

In this paper, we were able to synthesize a new functionalized catalyst based on mesoporous silicates and investigate its reactivity in C–S and C–O bond formation. Considering the advantages of heterogeneous catalysts, Cu@MCM-41-HPr-Met catalyst has such as ease of use, easy separation of products, adaptability to the environment, mild reaction conditions, and most importantly, recyclability of the catalyst.