Abstract
Multicomponent reactions (MCRs) offer a significant opportunity for simplicity, efficiency, and sustainability in the development of catalytic protocols for synthesizing medicinally important heterocycles. By minimizing environmental impact through the use of green solvents and recyclable heterogeneous catalysts, researches contribute directly to the principles of green chemistry and sustainable industrial practices. In this article, we present the development of a new multifunctional heterogeneous nanocatalyst, Cu@MIL-125-NH-Ac-Met, which was created by sequentially modifying MIL-125-NH2 with bromoacetyl bromide and metformin ligands to enable the efficient immobilization of copper nanoparticles. A variety of characterization techniques, including XRD, FESEM, FT-IR, EDS, ICP-OES, BET, TGA, and TEM, were employed to analyze the structural properties of the nanocomposite. The results confirmed the successful formation of uniformly distributed Cu nanoparticles (9–23 nm) with a 3.92% Cu loading. The Cu@MIL-125-NH-Ac-Met nanocomposite exhibited outstanding catalytic performance in the synthesis of tetrahydrobenzo[b]pyrans and in Cu-catalyzed click reactions. These reactions were conducted in a water/ethanol (1:1) medium at 50 °C, achieving high product yields (85–99%) under mild conditions. The catalyst retained excellent stability and activity over at least five consecutive cycles, with Cu leaching below 1%. This work demonstrates an efficient, recyclable, and environmentally sustainable catalytic system with significant potential for diverse organic transformations and green synthetic applications.
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Introduction
The use of catalysts in various organic transformations has led to high efficiency and environmental friendliness1. Heterogeneous catalysts offer several advantages over homogeneous catalysts, particularly in creating catalytic processes that are not only environmentally friendly but also reusable. One of the key benefits of using a heterogeneous catalyst is the ease and speed with which it can be removed from the reaction medium, eliminating the need for a neutralizing agent2,3. This characteristic enhances the efficiency and sustainability of chemical processes. Recently, metal-organic frameworks (MOFs) have gained popularity as effective catalysts in organic synthesis. In terms of environmental impact, MOFs serve as cost-effective and environmentally friendly solid supports4,5,6.
MOFs are a significant class of highly porous crystalline materials that include bridging organic ligands as linkers and clusters or metal ions as nodes7,8,9,10. They have emerged as valuable structures in the development of advanced multifunctional materials. These frameworks are typically synthesized using hydrothermal or solvothermal methods, where crystals form gradually from a heated solution. Since gaining scientific attention in the 1990s, MOFs have proven to be highly effective in applications such as heterogeneous catalysis and significantly enhancing efficiency11,12. MOFs can be prepared with a diverse range of metal species and organic ligands. The materials exhibit an exceptionally high surface area, frequently surpassing that of conventional porous materials such as zeolites and activated carbons13,14,15. This remarkable surface area facilitates numerous active sites, making these materials particularly effective for processes like adsorption, catalysis, and gas storage. As a result, MOFs are considered ideal platforms for these applications. This class of materials offers a range of beneficial features, including remarkable structural diversity and versatility. They possess high porosity and surface area, which enhances their functionality, while their low density contributes to lightweight applications. Additionally, these materials exhibit good thermal stability and present opportunities for post-synthetic modification, allowing for tailored properties to meet specific needs16,17,18,19,20. MOFs are excellent supports for metallic nanoparticles due to their high surface area, adjustable pore structures, and ability to stabilize these nanoparticles. Incorporating nanoparticles, such as palladium (Pd) or gold (Au), into metal-organic frameworks (MOFs) creates composite materials that enhance catalytic effectiveness. The presence of the MOF not only provides structural support but also significantly improves the dispersion and stability of the nanoparticles21,22,23. This arrangement effectively prevents aggregation and fosters an accessible environment where reactants can efficiently interact with the catalytic sites, ultimately leading to improved catalytic performance. This synergy between MOFs and metallic nanoparticles leads to the development of highly robust and durable catalysts. MOFs have shown great potential in the synthesis of nanoparticles, although they do encounter some challenges. Issues such as a broad particle size distribution, weak reactant affinity, and limited alloying capabilities can arise. However, these challenges can be effectively addressed by implementing surface modification on MOFs. Enhancing binding efficiency through modification can facilitate controlled growth and bolster the stability of nanoparticles on the substrate, while still preserving the bulk properties of the material. Various physical and chemical methods have been reported for post-synthetic modification (PSM) of MOFs. Although these strategies may present complexities and specific chemical requirements, they offer promising pathways for optimizing nanoparticle synthesis24,25,26,27.
MCRs are a strong synthetic tool in organic chemistry, where at least three components are compounded in one pot to generate a product that incorporates all reactants. MCRs represent a novel method for the efficient, easy, automated, and high-throughput synthesis of a broad variety of organic compounds. MCRs offer significant advantages over traditional step-by-step methodologies. They can align with green chemistry principles and are characterized by low costs, short reaction times, mild conditions, high efficiency, process simplicity, and excellent yields. Additionally, they utilize eco-friendly solvents and catalysts, enabling the synthesis of the desired scaffold in a single synthetic step. Over the past decade, this valuable technology has emerged as one of the most efficient and cost-effective methods for combinatorial synthesis28,29,30,31.
Pyrans are crucial heterocycles that contain oxygen and have been extensively utilized as antimicrobial, antiproliferative, antioxidant, and anticancer agents. Furthermore, pyrans may serve as potential calcium channel antagonists and find applications in cosmetics, pigments, and agrochemicals. Tetrahydrobenzo[b]pyrans demonstrate a wide range of anticancer activities and are particularly effective against cancer cells. Furthermore, these compounds may serve as cognitive enhancers in the treatment of neurodegenerative conditions such as Alzheimer’s, Huntington’s, Parkinson’s, and Down syndrome diseases32,33,34,35.
The concept of “click chemistry,” introduced by Sharpless and colleagues in 2001, denotes a class of chemical reactions characterized by their high yield, selectivity, and favorable thermochemical properties. These processes are designed to operate under straightforward reaction conditions and leverage easily accessible starting materials, making them highly efficient in various applications36,37. The azide-alkyne cycloaddition (AAC) reaction is a classical one-pot “click” conversion for synthesizing triazole compounds, serving as an efficient organic transformation method for preparing different bioactive compounds38,39,40. The Huisgen 1,3-dipolar cycloaddition reaction between azides and alkynes is an efficient method for synthesizing a significant class of nitrogen heterocycles known as triazoles. Triazoles are intriguing compounds that feature a five-membered aromatic ring composed of three nitrogen atoms and two carbon atoms. They are known for their diverse range of functional activities, making them valuable in multiple fields. Their applications extend to being effective pesticides, herbicides, and insecticides, as well as serving as sensitizers, developers, and corrosion inhibitors41,42,43. Furthermore, triazoles play a role in the production of copolymers, dyes, and pharmaceuticals, highlighting their versatility and importance in various industries. These protocols offer specific advantages; however, many are limited by the use of costly and harmful catalysts that contain halides and toxic metals, as well as issues such as low product yields, extended reaction times, challenging purification processes, and tedious work-up procedures. Thus, exploring unconventional methods to construct these valuable scaffolds using environmentally friendly reaction media and suitable catalysts is essential44.
Chemical processes often rely on hazardous and toxic solvents, which pose significant challenges. However, a promising solution lies in green solvents. This approach not only reduces the reliance on harmful substances but also aligns with principles of sustainability by minimizing raw material usage and decreasing waste. Furthermore, the use of multicomponent reactions can lead to more efficient protocols, allowing for faster synthesis, lower energy consumption, improved product yields, and reactions carried out under milder conditions. By adopting these methods, safety and efficiency in chemical processes can be increased45,46.
Recently, we have been investigating new methods for incorporating pendant amines into MOF pores47. A promising approach that warrants further exploration is the direct covalent attachment of amines to the MOF ligands. This method has the potential to minimize amine ligand and Cu nanoparticle loss, offering a valuable alternative in our research. In this study, a method for post-synthesis grafting of alkylamine onto MIL-125-NH2 is presented.
A constructive approach was developed for modifying MIL-125-NH₂ through reaction with bromoacetyl bromide and metformin. This modification aims to present an additional amine group, which is designed to improve the binding affinity for Cu metal. After immobilization of copper nanoparticles, the catalytic properties of the resulting nanocomposite were evaluated in tetrahydrobenzo[b]pyran and click reactions. The results revealed that the introduced nanocatalyst Cu@MIL-125-NH-Ac-Met is potentially useful in catalytic applications under mild conditions.
Experimental
Preparation of MIL-125-NH2
The synthesis of MIL-125-NH2 was carried out using the previously established method48. First, 0.7 g (0.4 mmol) of 2-aminoterephthalic acid was dissolved in a mixture of 11.2 mL of anhydrous dimethylformamide (DMF) and 2.8 mL of methanol at room temperature. The resulting solution was then sonicated for 180 s. Following sonication, 0.36 g (1.2 mmol) of titanium isopropoxide was incorporated into the solution, and the mixture was stirred vigorously for 5 min. The next step involves transferring the mixture to a steel autoclave and placing it in an oven at 150 °C for 20 h. After the reaction, the sample was separated by centrifugation and washed with methanol and DMF.
The procedure for MIL-125-NH-AcBr preparation
0.1 g of MIL-125-NH2 was dispersed in 10 mL of THF. The mixture was then placed in water at 0 °C for 60 s. After that, 0.1 g (0.5 mmol) of bromoacetyl bromide (BrAcBr) was slowly added to the mixture and stirred for 20 h at room temperature. In the final step, the precipitate was washed twice with THF solvent49.
Preparation of MIL-125-NH-Ac-Met
The obtained MIL-125-NH-AcBr (0.4 g) was dispersed in 15 mL of toluene using sonication for 15 min. Then, 2 mmol (0.202 g) of triethylamine and 1 mmol (0.129 g) of metformin were added to the solution. After refluxing for 24 h, the MIL-125-NH-Ac-Met precipitate was obtained. The filtered product was washed with ethanol50.
Preparation of Cu@MIL-125-NH-Ac-Met
MIL-125-NH-Ac-Met (0.1 g) in 10 mL methanol was added and sonicated for 5 min at room temperature. Then Cu(OAc)2.2H2O (0.008 g) was added to the reaction mixture. The mixture was stirred at room temperature for 20 h, and the precipitate was separated by centrifugation and washed with methanol. The resulting precipitate was then dried in a vacuum oven at 80 °C (Scheme 1).
Synthesis of Cu@MIL-125-NH-Ac-Met nanocatalyst.
Catalytic activity
The synthesis of tetrahydrobenzo[b]pyran derivatives was achieved using a mixture of dimedone (1 mmol), aromatic aldehydes (1 mmol), malononitrile (1 mmol), 15 mL solution of ethanol and water (1:1), and 15 mg of Cu@MIL-125-NH-Ac-Met nanocatalyst was heated with stirring at 50 °C. Thin-layer chromatography (TLC) showed the reaction’s completion time. Upon completing the reaction, the catalyst was carefully filtered and thoroughly washed with ethanol before being dried under vacuum for reuse. The filtrate was then evaporated to yield the crude product, which was subsequently purified through recrystallization in ethanol. To ensure the quality and integrity of the products, they were analyzed using 13C NMR and 1H NMR spectroscopy. All of the compounds of this study were known materials.
Click reaction: In a 25 ml round bottom flask equipped with a magnetic stirrer, a mixture of halogenated compounds (1 mmol), acetylene (1 mmol), sodium ascorbate (0.08 g), sodium azide (1 mmol) in water and ethanol solvent in a ratio of 1:1 was prepared, to the above mixture was added 0.015 g of nanocatalyst and the resulting mixture was stirred at 50 ℃. The progress of the reaction was followed by TLC and was investigated using a solvent tank of hexane and ethyl acetate in a ratio of 3:1. After the reaction was completed, distilled water was added to separate the product, and then the product was extracted with dichloromethane. The obtained organic phase was collected, and the reaction product was isolated by evaporation of the solvent. The structure of the products was confirmed by 1H NMR and 13C NMR analyses (Supplementary Data) is included in the file.
Spectral data for selected compounds
2-amino-7,7-dimethyl-4-(2-nitrophenyl)-5-oxo-5,6,7,8-tetrahydro-4 H chromene-3-carbonitrile
Rf = 0.65, M. P.–213–215 °C IR (KBr, cm− 1): 3470, 3334 (NH2), 2956 (C-Haliphatic), 2194 (CN), 1216 (CO). 1H NMR (300 MHz, DMSO-d6): δ (ppm) 0.896 (s, 3 H, CH3), 1.028 (s, 3 H, CH3), 2.03 (AB system, J AB = 15.9 Hz, 1H, CH2), 2.22 (AB system, J AB = 16.2 Hz, 1H, CH2), 2.44–2.59 (m, 2 H, CH2), 4.962 (s, 1H, CH), 7.227 (s, 2 H, NH2), 7.36–7.39 (dd, 1H, Ar–H), 7.42–746 (m, 1H, Ar–H), 7.65–7.70 (m, 1H, Ar–H), 7.82–7.85(dd, 1H, Ar–H).13C NMR (DMSO-d6, 300 MHz), δC (ppm): 27.16, 28.78, 30.4, 32.34, 40.27, 50.03, 56.8, 112.81, 119.58, 124.21, 128.34, 130.77, 133.85, 139.47, 149.45, 159.68, 163.21, 196.31.
2-amino-7,7-dimethyl-4-(4-methoxyphenyl)-5-oxo-5,6,7,8-tetrahydro-4 H chromene-3-carbonitrile
Rf = 0.67, M. P. − 196–198 °C IR (KBr, cm-1): 3321, 3373 (NH2), 2965 (C-Haliphatic), 2193 (CN), 1212 (CO); 1H NMR (300 MHz, DMSO-d6): δ (ppm) 0.963 (s, 3 H, CH3), 1.05 (s, 3 H, CH3), 2.11 (AB system, J AB = 15.9 Hz, 1H, CH2), 2.26 (AB system, J AB = 16.2 Hz, 1H, CH2), 2.45–2.58 (m, 2 H, CH2), 3.727 (s, 3 H, OCH3) 4.14 (s, 1H, CH), 6.86 (d, 2 H, Ar–H), 6.98 (s, 2 H, NH2), 7.07 (d, 2 H, Ar–H). 13C NMR (DMSO-d6, 300 MHz), δC (ppm): 27.24, 28.9, 32.26, 35.23, 40.06, 50.67, 55.20, 113.47, 114.14, 120.3, 128.7, 137.33, 158.39, 158.89, 162.6, 196.14.
2-amino-7,7-dimethyl-4-(4-cyanophenyl)-5-oxo-5,6,7,8-tetrahydro-4 H chromene-3-carbonitrile
Rf = 0.7, M. P. − 205–207 °C, IR (KBr, cm-1): 3475, 3353 (NH2), 2960 (C-Haliphatic), 2192 (CN), 1214 (CO); 1H NMR (300 MHz, DMSO-d6): δ (ppm) 0.966 (s, 3 H, CH3), 1.049 (s, 3 H, CH3), 2.12 (AB system, J AB = 15.9 Hz, 1H, CH2), 2.27 (AB system, J AB = 16.2 Hz, 1H, CH2), 2.51–2.54 (m, 2 H, CH2), 4.31 (s, 1H, CH), 7.2 (s, 2 H, NH2), 7.38 (d, 2 H, Ar–H), 7.78 (d, 2 H, Ar–H). 13C NMR (300 MHz, DMSO-d6): δ (ppm): 27.25, 28.9, 32.27, 35.24, 40.82, 50.49, 55.47, 59.03, 113.47, 114.15, 120.3, 128.7, 137.34, 158.39, 158.89, 162.62, 196.145.
1-(4-Bromobenzyl)-4-phenyl-1 H-1,2,3-triazole
Rf = 0.64, M.P: -150–151 °C. IR (KBr): νmax = 2972, 2838, 1622, 1587, 1160, 980, 742 cm− 1.1H NMR (400 MHz, DMSO-d6): δ (ppm) = 5.64 (s, 2 H), 7.32 (tt, J = 8.4, 3 H), 7.44 (tt, J = 7.6, 2 H), 7.59 (d, J = 8.4, 2 H), 7.83–7.85 (m, 2 H), 8.64 (s, 1H). 13C NMR (100 MHz, DMSO-d6): δ (ppm) = 52.73, 121.9, 122.09, 125.62, 128.39, 129.36, 130.63, 131.06, 132.19, 135.86, 147.15.
Results and discussion
The Cu@MIL-125-NH-Ac-Met nanocatalyst was characterized by utilizing some characterization methods like FT-IR, FE-SEM, EDX, TGA, XRD, and ICP. The FT-IR spectra of MIL-125-NH2, MIL-125-NH-AcBr, MIL-125-NH-Ac-Met, and Cu@MIL-125-NH-Ac-Met are illustrated in Fig. 1. All of the characteristic peaks of MIL-125-NH2, 1500–1700 cm− 1 range, 3447 and 3364 cm− 1, 1383 and 1253 cm− 1 corresponded to carbonyl stretching vibrations, asymmetric and symmetric stretching vibrations of N-H bond, and C–N stretching vibrations, respectively. Also, the adsorption band at 770 cm− 1 relates to the Ti–O–Ti band in MIL-125-NH251,52,53. Moreover, the infrared vibration bands for MIL-125-NH-AcBr, MIL-125-NH-Ac-Met, and Cu@MIL-125-NH-Ac-Met closely align with those observed in the original MIL-125-NH2. This strong correspondence suggests that these compounds retain the same chemical structure as the pristine MIL-125-NH2, highlighting their structural integrity and consistency.
FT-IR spectra of MIL-125-NH2, MIL-125-NH-AcBr, MIL-125-NH-Ac-Met and Cu@MIL-125-NH-Ac-Met.
X-ray diffraction (XRD) analysis is a vital technique for investigating the structural properties of materials. XRD patterns of MIL-125-NH2, MIL-125-NH-AcBr, MIL-125-NH-Ac-Met, and Cu@MIL-125-NH-Ac-Met are shown in Fig. 2. For the MIL-125-NH2 sample, all the diffraction peaks are in good agreement with prior reports54,55, indicating excellent crystallization. The characteristic peaks of all compounds at 2Ɵ = 6.5°, 9.5°, 11.5°, 13.5°, 14.7°, 16.3°, 17.7°, 18.9°, 22.29°, and 23.44° attributed to the (100), (200), (211), (220), (310), (222), (312), (400), (204), and (422) planes, respectively. After modification with BrAcBr, metformin, and Cu, the characteristic diffraction patterns of MIL-125-NH-AcBr, MIL-125-NH-Ac-Met, and Cu@MIL-125-NH-Ac-Met correspond to the XRD pattern of MIL-125-NH2. The XRD patterns of these compounds indicate that there is no significant loss of crystallization.
XRD patterns of simulated, as-synthesized, and modified samples of MIL-125-NH2; MIL-125-NH-AcBr, MIL-125-NH-Ac-Met, and Cu@MIL-125-NH-Ac-Met.
The energy-dispersive X-ray spectrometer (EDX) analyzer, used in conjunction with the microscope, was employed for elemental analysis56. The element mapping diagram (Fig. 3) of Cu@MIL-125-NH-Ac-Met indicates that elements including Ti, O, N, C, and Cu are uniformly distributed throughout Cu@MIL-125-NH-Ac-Met. The ICP-OES analysis successfully quantified the amount of Cu nanoparticles that were effectively loaded onto the support. The nanocatalyst contained 3.92 wt% Cu.
Elemental mapping and EDX spectrum of the Cu@MIL-125-NH-Ac-Met.
To investigate the morphologies of synthesized MIL-125-NH2, two key techniques, scanning electron microscopy (SEM) and transmission electron microscopy (TEM), were utilized, as shown in Fig. 4. The FESEM image in Fig. 4 (a) reveals granular-shaped agglomerated nanoparticles with a rough surface, highlighting their unique structural characteristics57. Furthermore, Fig. 4 (b) illustrates that the modifications effectively preserve the structural morphology and porosity of the MOF. Additionally, the TEM images (Fig. 4c) further corroborate these findings by showcasing the dense agglomeration of Cu nanoparticles. The consistent results from both FESEM and TEM techniques provide a comprehensive understanding of the material’s morphology.
FESEM images (a) MIL-125-NH2; (b) Cu@MIL-125-NH-Ac-Met, and (c) TEM of Cu@MIL-125-NH-Ac-Met.
The nitrogen adsorption–desorption isotherms for the samples, covering all stages of synthesis and modification as well as the final catalyst, were systematically recorded and are presented in Fig. 5. The BET surface area of MIL-125-NH2 was determined to be approximately 720.63 m²/g58. Upon functionalization with BrAcBr and metformin, this surface area decreased to 655.71 m²/g and 281.76 m²/g, respectively, aligning with expectations. This value further decreased to 105.98 m²/g following the loading of copper nanoparticles.
Nitrogen adsorption–desorption isotherms of MIL-125-NH2, MIL-125-NH-AcBr, MIL-125-NH-Ac-Met, and Cu@MIL-125-NH-Ac-Met.
The TGA analysis presented in Fig. 6 has been conducted to assess the thermal stability of the material and to explore how Ac-Met and nanoparticles influence the properties of the MOF.
In the TGA thermogram, the initial weight loss observed at around 170 °C indicates the release of solvent molecules held within the material’s pores. Following this, a considerable weight loss occurring between 350 and 600 °C is likely due to the decomposition of the framework itself56,59. The improved thermal stability of MIL-125-NH-Ac-Met, compared to the unmodified MOF, can be attributed to the strong binding of Ac-Met to the MOF structure. The weight losses observed for MIL-125-NH2, MIL-125-NH-AcBr, MIL-125-NH-Ac-Met, and Cu@MIL-125-NH-Ac-Met between 170 and 600 °C were found to be 57 wt%, 53 wt%, 43 wt% and 44 wt%, respectively. After the immobilization of nanoparticles, the thermal degradation pattern closely resembles that of MIL-125-NH-Ac-Met, with a slightly higher residual amount, confirming the presence of Cu nanoparticles. Based on the TGA results, it can be concluded that the designed catalyst is thermally stable and suitable for catalytic applications.
The thermogravimetric analysis (TGA) curves of MIL-125-NH2, MIL-125-NH-AcBr, MIL-125-NH-Ac-Met, and Cu@MIL-125-NH-Ac-Met.
Tetrahydrobenzo[b]pyran derivatives
After confirmation of the desired structure, the multicomponent reaction has been explored to demonstrate the catalytic activity of the synthesized Cu@MIL-125-NH-Ac-Met nanocatalyst.
For the tetrahydrobenzene synthesis, dimedone (1 mmol), 4-chlorobenzaldehyde (1 mmol), and malononitrile (1 mmol) were used as a representative reaction to investigate various parameters such as solvent, temperature, and catalyst amount (Table 1). Initially, to optimize the solvent, a variety of solvents, including EtOH, CH3CN, CH2Cl2, THF, H2O, and water/ethanol were tested (entries 1–6). According to the experimental results, water/ethanol (1:1) was chosen as the most suitable green solvent for this reaction (entry 6). Then, temperature effects were investigated on the reaction yield. The product yield improved significantly by raising the temperature from room temperature to reflux temperature (entries 7–10). In the next step, the effect of different amounts of nanocatalyst (10, 15, and 20 mg) in the model reaction was checked. The results showed that 15 mg of Cu@MIL-125-NH-Ac-Met is adequate to complete the reaction, and no significant addition in efficiency was seen with increasing the nanocatalyst amount to 20 mg (entries 10–12). Thus, the optimal tetrahydrobenzene reaction was conducted at 50 °C using 1 mmol of dimedone, 1 mmol of 4-chlorobenzaldehyde, 1 mmol of malononitrile, and 15 mg of nanocatalyst Cu@MIL-125-NH-Ac-Met in ethanol/water (1:1) as solvent (entry 9).
Various substituted aryl aldehydes were used under optimized reaction conditions to verify the protocol’s generality and versatility (Table 2). The reaction gave excellent yields of the desired tetrahydrobenzo[b]pyran derivatives.
To highlight the significant advantages of Cu@MIL-125-NH-Ac-Met, its catalytic activity was compared with some other catalysts previously reported for the synthesis of tetrahydrobenzo[b]pyran. While all the catalysts demonstrate effectiveness, the Cu@MIL-125-NH-Ac-Met nanocatalyst stands out for its superior performance in yield, reaction time, and optimal reaction conditions, including solvent choice and temperature (Table 3).
The synthesis of tetrahydrobenzo[b]pyran derivatives can be effectively achieved through a well-defined mechanism, as outlined in Scheme 21,45,63. The process begins with the condensation of activated aldehyde with malononitrile, resulting in the formation of the intermediate arylidene malononitrile (I) through dehydration. Next, a nucleophilic addition occurs, where the enolizable dimedone interacts with the arylidene malononitrile, resulting in the production of intermediate (II). Finally, a series of consecutive intermolecular cyclization and rearrangement steps culminate in the formation of the desired tetrahydrobenzo[b]pyran.
The possible mechanism for the synthesis of tetrahydrobenzo[b]pyran derivatives.
Click reaction
Organic azides typically exhibit stability in the presence of water and oxygen; however, low molecular weight organic azides can pose explosive risks and are challenging to handle safely. To enhance safety in synthetic procedures, it is advantageous to develop protocols that eliminate the need for isolating azides. In this regard, chemists have made significant progress by synthesizing triazoles through a three-component reaction involving an organic bromide, sodium azide, and an alkyne, all catalyzed by copper. This approach not only improves safety but also streamlines the synthesis of valuable triazole compounds.
To optimize the click reaction conditions, the reaction of benzyl bromide with phenylacetylene and sodium azide was chosen as the model reaction (Table 4). Then, the parameters affecting the reaction were investigated. To investigate the effect of the solvent, water, ethanol, methanol, water/ethanol, DMF, THF, and EtOAc were tested (entries 1–7). Water/ethanol (1:1) showed the highest effectiveness as a solvent, which resulted in the highest yield (entry 7). To check the effect of temperature on the reaction, different temperatures have been tested. The conversion rate was low at 25 and 45 °C, but at 50, 60, and reflux temperatures, the efficiency reached 98% (entries 7–11). In the last step, the model reaction was made at 50 °C in a solvent of water/ ethanol (1:1) the different amounts of nanocatalyst. As the results in the table showed, increasing the catalyst amount increased the product yield by 15 mg, but using a higher amount of catalyst, there was no further increase in product yield (entries 10, 12, 13).
To study the generality of this protocol and based on the optimized terminations, the scope and limitations of this reaction were investigated by changing the alkyne structure and benzyl halide components. In this effort, triazole derivatives were successfully produced in the presence of a catalyst in excellent yields (Table 5).
The catalytic performance of the prepared nanomaterials was compared with previously reported methods. The superiority of the present catalyst in terms of reaction time and efficiency percentage is exactly apparent. In addition, the environmentally safe nature of the catalyst, reaction conditions, and economic aspects are other significant advantages of the present methodology (Table 6).
The proposed reaction mechanism, depicted in Scheme 3, has been extensively explored in various studies to elucidate its pathway69,70,71. The process initiates with the alkyne coordinating to the Cu (I) center of Cu@MIL-125-NH-Ac-Met, acting as a π-ligand. This coordination effectively enhances the acidity of the C-H bond. Following deprotonation, a σ, π-di(copper) acetylide complex (I) is formed. Concurrently, a reaction occurs between the benzyl halide and sodium azide, leading to the generation of a benzyl azide intermediate that interacts with the Cu (I) center to create a dicopper intermediate. The subsequent acetylide-azide reaction proceeds through an oxidative addition step, resulting in a six-membered intermediate (II). This intermediate then undergoes a reductive elimination process, yielding the Cu (I)-triazolide intermediate (III). The final triazole product is produced through a protonation step, which also serves to regenerate the catalyst, effectively completing the catalytic cycle.
Probable mechanism for the click reaction.
Reusability performance of Cu@MIL-125-NH-Ac-Met nanocatalyst
Reusability and stability are crucial attributes for catalysts in advancing green chemistry. Therefore, these properties were evaluated for the Cu@MIL-125-NH-Ac-Met nanocatalyst in the model reaction under optimal conditions. Following the completion of each reaction cycle, the Cu@MIL-125-NH-Ac-Met nanocatalyst was efficiently removed by filtration, followed by thorough washing with water and ethanol, and subsequently dried under vacuum. The dried nanocatalyst was then utilized in the next runs. Remarkably, this nanocatalyst demonstrated impressive reusability, maintaining its catalytic activity over five consecutive reaction cycles (Fig. 7). To elucidate the nature of the catalyst—whether heterogeneous or homogeneous—a hot filtration test was conducted based on established methodologies72. Each reaction commenced under optimal conditions, and at the midpoint, when the reaction had progressed to 50%, the catalyst was removed from the reaction vessel. The continuation of the reaction without the catalyst provided insights into its heterogeneity, reinforcing the effectiveness of the Cu@MIL-125-NH-Ac-Met nanocatalyst in these reactions. The results showed that the product yield was the same as 50%, and the reaction did not occur without the presence of the catalyst. These observations, without a doubt, confirm that the catalyst structure is very stable and no leaching of copper metal into the solution has occurred. The nanocatalyst was thoroughly examined following five consecutive runs using techniques such as FT-IR, XRD, and FESEM. The FT-IR spectra and XRD patterns of both the fresh and used nanocatalysts exhibited remarkable similarities, suggesting that the surface morphology of the reused catalyst remains consistent with that of the fresh catalyst (Fig. 8). In addition, FE-SEM of the catalyst after the reusability test illustrates that the uniform and rather homogeneous morphology of the nanocatalyst is kept (Fig. 9). This stability underscores the durability and potential for repeated use of the nanocatalyst in various applications.
Reusability of the Cu@MIL-125-NH-Ac-Met in tetrahydrobenzo[b]pyran (a) and click reaction (b).
XRD patterns (A) and FT-IR images (B) of reused nanocatalysts.
SEM images reused nanocatalysts after five cycles in click reaction (a) and synthesis of tetrahydrobenzo[b]pyran derivatives (b).
Conclusions
In this study, a heterogeneous nanocatalyst was successfully developed, which exhibited remarkable efficiency and thermal stability through a carefully designed three-step PSM procedure. The use of alkyl amines in the modification is critical for preventing the accumulation and release of Cu from the surface of the MOF. The nanocatalyst was employed in the preparation of 4 H-benzopyran derivatives and in click reactions under environmentally friendly conditions. The development of the new nanocatalyst demonstrated impressive catalytic activity, indicating a significant step forward compared to earlier studies. This advancement can be attributed to two primary factors. First, the incorporation of amine groups improves the loading capacity of the metal nanoparticles. Second, the high porosity and unique surface area of the MIL-125-NH2 framework can enhance the distribution of nanoparticles, leading to more effective performance. In comparison, the unmodified MOF showed limited metal loading and experienced aggregation of nanometals under identical conditions. As a result of these developments, the Cu@MIL-125-NH-Ac-Met nanocatalyst was successfully synthesized. This innovative design provides outstanding reactivity and recovery while also presenting several notable benefits. It boasts impressive stability, low production costs, and moderate reaction conditions, making it highly efficient. Furthermore, the straightforward modification processes and ease of reaction work-up make it potential in catalysis applications. The introduced nanocatalyst offers a versatile solution for various catalytic reactions and industrial applications, owing to its cost-effectiveness and exceptional catalytic performance.
Data availability
The data that support the findings of this study are available from the corresponding author upon reasonable request.
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We would like to thank the University of Mazandaran for financial support (Research Council Grant).
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Z. Sadati and H. Alinezhad wrote the main manuscript text and M. Tajbakhsh analyzed the data and results. All authors reviewed the manuscript.
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Sadati, Z., Alinezhad, H. & Tajbakhsh, M. Copper immobilized MIL-125-NH2 as an efficient nanocatalyst for click reaction and synthesis of benzo[b]pyrans. Sci Rep 16, 3201 (2026). https://doi.org/10.1038/s41598-025-33201-5
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DOI: https://doi.org/10.1038/s41598-025-33201-5
