DABCO-modified magnetic core-shell as an efficient nanocatalyst for synthesizing polyhydroquinoline derivatives

The fabrication of core-shell structured magnetic resorcinol-formaldehyde composites has garnered considerable attention within the scientific community in recent years. A key area of focus has been the immobilization of homogeneous catalysts onto the surfaces of these materials and transforming them into heterogeneous catalysts. In this study, a novel quaternary ammonium salt catalyst was synthesized by immobilizing 1,4-diazabicyclo [2.2.2] octane (DABCO) on resorcinol-formaldehyde-modified Fe₃O₄ nanocomposite as a support (Fe₃O₄@RF/Pr-DABCO). The Fe₃O₄@RF/Pr-DABCO nanocomposite was characterized using various physicochemical techniques, including FT-IR, VSM, SEM, XRD, and TGA. The Fe₃O₄@RF/Pr-DABCO nanocomposite was employed as a power nanocatalyst in the Hantzsch reaction for synthesizing polyhydroquinoline derivatives using aromatic aldehydes, ammonium acetate, dimedone and ethyl acetoacetate. Various aromatic aldehydes were used as substrates in the presence of 0.003 g of Fe₃O₄@RF/Pr-DABCO under solvent-free condution at 60 °C, achieving high to excellent yields (90-99%) within short reaction times (5-15 min). Furthermore, this nanocatalyst showed excellent reusability and maintained its catalytic activity for at least eight consecutive cycles without a significant decrease in efficiency. These results demonstrate the potential of the Fe₃O₄@RF/Pr-DABCO nanocomposite as an efficient and sustainable catalyst for the synthesis of polyhydroquinoline derivatives via the Hantzsch reaction.

1 Introduction

Aligned with the principles of green chemistry, the development of recyclable and reusable catalytic systems has garnered significant interest among researchers over the past few decades. This focus on sustainable catalyst design aims to address environmental concerns and promote resource efficiency in chemical processes (Saberi et al., 2020). In recent years, the design of magnetic nanocatalysts is of great interest due to their very easy recovery using a simple magnet without the need for conventional and time-consuming filtration and reactions catalyzed by magnetic nanocatalysts have been widely studied due to the easy purification of products and efficient recovery of the catalyst (Shabir et al., 2020; Yazdani et al., 2025). Various magnetic nanoparticles (NPs) have been investigated for the development of magnetic catalytic systems. Among magnetic NPs, Fe₃O₄ NPs has been widely considered due to its low toxicity, easy and low-cost synthesis from cheap and readily available starting materials. Despite the promising attributes of Fe₃O₄ NPs, their practical applications are hindered by their high sensitivity to oxidation and aggregation, as well as their chemically reactive nature due to their high surface area (Serenjeh et al., 2015; Shaker and Elhamifar, 2020; Barzkar and Beni, 2023). These limitations necessitate the development of strategies to overcome these challenges and unlock the full potential of Fe₃O₄ NPs. One effective approach involves the creation of a suitable organic or inorganic coating on the surface of the magnetic NPs, which can enhance their stability and mitigate the aforementioned issues, thus paving the way for broader applications in various fields. Various materials have been utilized for coating magnetic nanoparticles, including silica, metal oxides, and polymers (Gong et al., 2018; Kilic et al., 2019; Shaker and Elhamifar, 2020; 2021b; Kilic et al., 2022). Notably, resorcinol-formaldehyde (RF) resin has emerged as a particularly promising option due to its exceptional properties. These properties encompass high stability, cost-effectiveness, facile structural manipulation, superior mechanical and thermal characteristics, impressive electrical conductivity, and a large surface area. The unique combination of these properties makes RF resin an attractive choice for coating magnetic nanoparticles, with the potential to enhance their performance and broaden their applications in diverse fields (Barzkar and Beni, 2020). Some of the recently developed reports include Fe₃O₄@RF-Au (Shi et al., 2024), Fe₃O₄@RF@void@PMO (Yu et al., 2019), Fe₃O₄@RF@void@PMO(IL)/Cu (Shaker and Elhamifar, 2021a), Fe₃O₄@SiO₂@RF–SO₃H (Barzkar and Beni, 2020) and Fe₃O₄@RF/Cu₂O (Wang et al., 2017).

On the other hand, 1,4- Diazabicyclo [2.2.2] octane, also recognized as DABCO is known as a valuable ligand and catalyst for organic reactions due to its cost-effectiveness, environmental friendliness, non-toxicity, and high reactivity. However, recovering and removing DABCO in chemical reactions is complex. This problem has been solved by immobilizing DABCO on solid substrates, which can be easily filtered and reused multiple times (Baharfar and Azimi, 2014). Recent developments in this area, including Fe₃O₄@DT-Au (Kardan et al., 2022), Pd–DABCO@SiO₂ (Jadhav et al., 2019), Ni(II)–DABCO@SiO₂ (Hajipour and Abolfathi, 2017), Pd-DABCO-γ-Fe₂O₃ (Sobhani and Pakdin-Parizi, 2014), SB-DABCO (Hasaninejad et al., 2011) and Fe₃O₄@SiO₂@DABCO (Gupta et al., 2023).

Meanwhile, heterocycles are a crucial and valuable group of chemical compounds, forming the basis of many pharmaceutical molecules and antibiotics. Currently, Synthesis of nitrogen-containing heterocycles, such as polyhydroquinoline and its derivatives, is important due to their medicinal properties. Polyhydroquinolines possess anti-diabetic, anti-cancer, anti-tumor, liver-protective, and skin-protective properties and treat cardiovascular diseases and Alzheimer’s (Shinde and Thakur, 2023). Polyhydroquinoline derivatives can be synthesized using the Hantzsch reaction, which has garnered significant attention. This reaction is a four-component reaction and is one of the first and most well-known multi-component reactions (Sajjadifar and Azmoudehfard, 2019; Shaker and Beni, 2021; Norouzi and Beiranvand, 2023). Due to the widespread use of polyhydroquinoline derivatives, various catalysts have been introduced for this reaction such as GuHCl (Baghbanian et al., 2010), ceric ammonium nitrate (CAN) (Ko and Yao, 2006), Sc(OTf)₃ (Donelson et al., 2006), urease (Zhu and Li, 2021), palladium (0) nanoparticles (Saha and Pal, 2011), nickel nanoparticles (Sapkal et al., 2009), SBA-15@AMPD-Co (Ghorbani-Choghamarani et al., 2019), [βCD/Im](OTs)₂ (Moheiseni et al., 2021), NiFe₂O₄MNPs (Ahankar et al., 2016), Fe₃O₄adenineNi (Tamoradi et al., 2018), CuSPATB/Fe₃O₄ (Ghorbani-Choghamarani et al., 2016), Fe₃O₄@SiO₂PEG/NH₂ (Kardooni et al., 2017) and Fe₃O₄ @SiO₂@(CH₂)₃Im}C(NO₂)₃ (Yarie et al., 2016). Although most of the reports mentioned have obvious advantages, they also come with problems, such as toxic catalysts, volatile and harmful solvents, expensive metal precursors, high temperatures, the formation of by-products, and contamination of the final product. In most methods, the catalysts get damaged during the reaction and cannot be separated and reused. Therefore, developing efficient, recyclable, and environmentally friendly catalysts for synthesizing polyhydroquinolines is crucial (Kazemi and Mohammadi, 2020; Sharma et al., 2021).

In light of the significance of heterogeneous catalysis and the unique properties of 1,4-diazabicyclo [2.2.2]octane (DABCO), this study aims to design and synthesize a novel core-shell structured Fe₃O₄@RF/Pr-DABCO catalyst (Scheme 1). DABCO, selected for its crucial role as an organic group in preparing quaternary ammonium salt catalysts, can be readily immobilized on the Fe₃O₄@RF support. This approach offers the potential to create a highly efficient and recyclable catalytic system that capitalizes on the benefits of both the magnetic core and the catalytic activity of DABCO. Also, Fe₃O₄@RF/Pr-DABCO is applied as a powerful catalyst for synthesizing polyhydroquinoline derivatives under mild conditions.

Scheme 1

Scheme 1. Preparation of Fe₃O₄@RF/Pr-DABCO nanocatalyst

2 Experimental section

2.1 Materials

All chemicals are used, such as formaldehyde solution (37 wt%), resorcinol, triethylamine, (3-aminopropyl)-trimethoxysilane, dimedone, ethyl acetoacetate, toluene dried, and ammonium acetate, were purchased from Merck, hydrochloric acid (37%) and Benzaldehyde and its derivatives were purchased from Sigma-Aldrich. The morphology of the particles was evaluated by TESCAN-Vega 3 scanning electron microscope (SEM) (Czech republic). Energy-dispersive X-ray spectroscopy (EDX) was obtained by using TESCAN-Vega 3 apparatus (Czechrepublic). Fourier transform infrared (FT-IR) spectroscopy was recorded on a PerkinElmer Spectrum2 spectrometer (United States). Thermal gravimetric analysis (TGA) and Differential Scanning (DSC) Calorimetry were performed using a SDT 650 (United States). Thermal gravimetric analysis (TGA) and differential thermal analysis (DTA) were performed using a STA6000 (United States). X-ray diffraction (XRD) was obtained using a Rigaku Ultima IV diffractometer (Japan). The magnetic properties of the particles were investigated using a vibrating sample magnetometer (VSM) of MDKB. (Iran). Melting points were determined using a KSB1N, Kruss apparatus in open capillary tubes (Germany). TLC-Grade-silica Gel-G/UV 254 thin layer chromatography (TLC) was used. A mixture of ethyl acetate and normal hexane was used for the mobile phase in the TLC tank. Ultrasonic model Elmasonic P60H was used to disperse the particles and to perform the organic reactions (Germany).

2.2 Synthesis of Fe₃O₄ nanoparticles

In a 100 mL round-bottom flask, 25 mL HCl (2 M), FeCl₃.6H₂O (5.2 g), and FeCl₂.4H₂O (2 g) were combined with a magnetic stirrer under argon atmosphere for 30 min at room temperature to form nanoparticles of Fe₃O₄ magnetite. Next, NH₄OH (25%, 30 mL), was slowly injected into the solution over 30 min at 25°C. The obtained black precipitate was separated by a magnet and washed with water before drying in an oven (Kargar et al., 2024).

2.3 Synthesizing of Fe₃O₄@RF

In a 250 mL round-bottom flask, 1 g Fe₃O₄ particles were dispersed in water (20 mL) and ethanol (100 mL) using ultrasonic. Then (2 mL) of 25% ammonium hydroxide, formaldehyde (0.6 mL), and resorcinol (0.4 g) were added to the solution. The mixture was stirred for 10 h at 30°C under a magnetic stirrer and argon atmosphere, then was collected using a magnet and washed with water and ethanol three times, before finally drying at 60°C for 15 h (Mirbagheri et al., 2021).

2.4 Synthesis of Fe₃O₄@RF/PrCl

For this aim, in a 100 mL round-bottom flask, Fe₃O₄@RF nanoparticles (50 mg) were added to 50 mL of dry toluene and dispersed under ultrasonic waves. After that, 2 mL of (3-chloropropyl) trimethoxysilane was injected into the reaction. After refluxing under a magnetic stirrer and argon atmosphere for 24 h at 110°C, an external magnet filtered the product. The resultant product was washed with toluene and dried at 70°C for 15 h (Zare et al., 2020).

2.5 Synthesis of Fe₃O₄@RF/Pr-DABCO

In a 50 mL round-bottom flask, Fe₃O₄@RF/PrCl (1 g), DABCO (0.5 g), and triethylamine (0.4 mL) were dispersed under ultrasonic in 30 mL of dry toluene for 30 min. After 24 h of refluxing under a magnetic stirrer and argon atmosphere, the mixture was separated using a magnet, washed five times with dry toluene, and dried at 50°C for 5 h (Afsar et al., 2018).

2.6 Synthesis of polyhydroquinoline derivatives using Fe₃O₄@RF/Pr-DABCO

For this, aldehyde (1 mmol), ammonium acetate (1.4 mmol), dimedone (1 mmol), ethyl acetoacetate (1 mmol), and Fe₃O₄@RF/Pr-DABCO nanocatalyst (0.003 g) were added into a 5 mL round-bottom flask. The reaction mixture was located in an oil bath at 60°C, the temperature of which was previously adjusted by a heater-stirrer advice, and stirred under solvent-free condition and air atmosphere. The progress of the reaction was monitored by TLC (eluent; n-hexane: ethyl acetate, 4:6). After completion of the reaction, hot EtOH (10 mL) was added in the reaction vessel and catalyst was removed by an external magnet. Finally, the solvent was evaporated, and pure products resulted after recrystallization in EtOH (Nikoorazm and Erfani, 2019).

3 Results and discussion

3.1 Characterization of Fe₃O₄@RF/Pr-DABCO

The FT-IR spectra of Fe₃O₄, RF, Fe₃O₄@RF, Pr-Cl, Fe₃O₄@RF/Pr-Cl, DABCO, and Fe₃O₄@RF/Pr-DABCO are shown in Figure 1. For Fe₃O₄, Fe₃O₄@RF, Fe₃O₄@RF/Pr-Cl and Fe₃O₄@RF/Pr-DABCO, the observed peak at 574 cm⁻¹ is related to the stretching vibrations of the Fe-O bonds (Figures 1d–g). The FT-IR spectrum of Fe₃O₄@ RF (Figure 1e) exhibits peaks at 3,010, 2,854–2,974 cm⁻¹attributable to the vibrations of C-H aromatic and CH₂ moieties of RF resin in Fe₃O₄@RF, respectively. Also, the absorption bands at 1,615 and 1,453 cm⁻¹ correspond to the aromatic rings in the RF. These results are in perfect agreement with the RF spectrum (Figure 1c), thus confirming the successful formation of RF resin on the Fe₃O₄ surface. (Pol et al., 2014; Barzkar and Beni, 2020). The FT-IR spectrum of Fe₃O₄@RF/Pr-Cl (Figure 1f) shows the C-H stretching vibration at 2,950 and 2,826 cm⁻¹, the C–Cl absorption band at 800 cm⁻¹, and Si-O stretching at 1,000–1,100 cm⁻¹, which perfectly match with the observed peaks in the pure Pr-Cl spectrum (Figure 1b). These results prove the successful formation of the Fe₃O₄@RF/Pr-Cl composite (Hamidinasab et al., 2020; Honari et al., 2022). FT-IR pure DABCO (Figure 1a) exhibits a peak at 1,250 cm⁻¹ (C–N stretching), 1,475 cm⁻¹ (CH₂ bending), and 2,850–2,964 cm⁻¹ (C-H stretching vibrations) (Tafti et al., 2024). Thus, for the Fe₃O₄@RF/Pr-DABCO, the presence of a peak at 1,250 cm⁻¹ (C–N stretching) proved immobilization of DABCO groups on the of Fe₃O₄@RF/Pr-Cl composite (Figure 1g) (Hasaninejad et al., 2011; Nasseri and Sadeghzadeh, 2014).

Figure 1

Figure 1. FT-IR spectrum of (a) DABCO, (b) Pr-Cl, (c) RF, (d) Fe₃O₄, (e) Fe₃O₄@RF, (f) Fe₃O₄@RF/Pr-Cl, (g) Fe₃O₄@RF/Pr-DABCO.

The EDX spectrum of Fe₃O₄ nanoparticles demonstrated the presence of Fe and O elements, proving the successful production of these nanoparticles (Figure 2a). To confirm the successful modification of the Fe₃O₄ surface with RF resin, the Fe₃O₄@RF material was also characterized by EDX analysis. The appearance of C-signal in the latter analysis confirms successful-production of RF resin on Fe₃O₄ surface (Figure 2b). Furthermore, to prove the successful chemical immobilization of (3-chloropropyl) trimethoxysilane on Fe₃O₄@RF nanocomposite, the Fe₃O₄@RF/PrCl nanocomposite was also characterized by EDX analysis. The advent of Si and Cl elements proving successful-immobilization of (3-chloropropyl) trimethoxysilane species on Fe₃O₄@RF nanocomposite (Figure 2c). Finally, to confirm the successful immobilization of DABCO on Fe₃O₄@RF/PrCl nanocomposite, the final material was characterized by EDX and the appearance of N-signal confirmed immobilization of DABCO species (Figure 2d). The EDX mapping analysis was also done to display the dispensation of elements in the Fe₃O₄@RF/Pr-DABCO framework. This analysis demonstrated that all elements are uniformly distributed in the Fe₃O₄@RF/Pr-DABCO framework (Figure 3).

Figure 2

Figure 2. EDX spectrum of the (a) Fe₃O₄, (b) Fe₃O₄@RF, (c) Fe₃O₄@RF/Pr-Cl, and (d) Fe₃O₄@RF/Pr-DABCO.

Figure 3

Figure 3. Element mapping of the Fe₃O₄@RF/Pr-DABCO nanocatalyst.

The morphology of the particles at different steps of nanocomposite preparation was investigated by using SEM (Figure 4). The SEM analysis revealed a consistent spherical morphology with uniform particle size distribution at different stages of the process. Additionally, it was observed that the size of the nanoparticles (NPs) increased incrementally at each step compared to the previous stage.

Figure 4

Figure 4. SEM image of the (a) Fe₃O₄, (b) Fe₃O₄@RF, (c) Fe₃O₄@RF/Pr-Cl, and (d) Fe₃O₄@RF/Pr-DABCO.

The powder X-ray diffraction (PXRD) analysis of Fe₃O₄, Fe₃O₄@RF, Fe₃O₄@RF/Pr-Cl, and Fe₃O₄@RF/Pr-DABCO nanomaterials showed six sharp peaks at 2θ = 30.3, 35.6, 43.4, 53.7, 57.4, and 63.1°, corresponding to Miller indices of 220, 311, 400, 422, 511, and 440, respectively. The results of this analysis prove that the Fe₃O₄ crystalline structure is preserved during the synthesizing processes of Fe₃O₄@RF/Pr-DABCO nanocamposite (Figure 5) (Hashkavayi and Raoof, 2017).

Figure 5

Figure 5. XRD pattern of the (a) Fe₃O₄, (b) Fe₃O₄@RF, (c) Fe₃O₄@RF/Pr-Cl, and (d) Fe₃O₄@RF/Pr-DABCO.

The magnetic properties of Fe₃O₄, Fe₃O₄@RF, Fe3O4@RF, Fe₃O₄@RF/Pr-Cl, and Fe₃O₄@RF/Pr-DABCO nanomaterials were investigated by VSM \analysis at room temperature (Figure 6). The results of this analysis demonstrated that all samples have a superparamagnetic behavior. The magnetic saturation of Fe₃O₄, Fe₃O₄@RF, Fe₃O₄@RF/Pr-Cl, and Fe₃O₄@RF/Pr-DABCO nanomaterials were 61.98, 53.87, 42.23, and 41.96 emu/g, respectively. The decrease in saturation magnetization, after each step, confirms successful chemical immobilization of Resorcinol-Formaldehyde precursors and DABCO moieties on the surface of the Fe₃O₄ NPs. However, the magnetic property of Fe₃O₄@RF/Pr-DABCO is still sufficient, and it can be easily recovered using an external magnet.

Figure 6

Figure 6. VSM (a) Fe₃O₄, (b) Fe₃O₄@RF, (c) Fe₃O₄@RF/Pr-Cl, and (d) Fe₃O₄@RF/Pr-DABCO.

To further investigate the compositional structure and thermal stability, thermogravimetric analysis (TGA), derivative thermogravimetry (DTG), differential thermal analysis (DTA), and Differential Scanning Calorimetry (DSC) were performed at different stages of nanocatalyst synthesis (Figures 7A–D). The TGA diagram of Fe₃O₄ (Figure 7Aa) shows two mass loss stages. The first stage, with a mass loss of 1.66%, at temperatures between 25°C and 200°C, is related to water evaporation, and the second stage, with a mass loss of 1.96%, at temperatures between 200°C and 600°C, is connected to the decomposition of hydroxyl groups on the Fe₃O₄ surface., the final mass loss is 3.62% (Zhang et al., 2021). The mass loss graph of Fe₃O₄@RF (Figure 7Ab) shows two stages of mass loss. In the first stage, at temperatures below 200°C, due to the evaporation of water or residual physically adsorbed solvent, the observed mass loss is 2.55%, While the second mass loss15.13% between 200°C and 469°C is related to the decomposition of the RF shell, the final mass loss is 17.68% (Zheng et al., 2016). The mass loss graph of Fe₃O₄@RF/Pr-Cl (Figures 7Ac) has two stages of mass loss. The first mass loss of about 2.49% is related to the residual other solvents or water on the surface of magnetic nanoparticles, which appears below 200°C. The main weight loss, with a mass loss of 18.59%, from 200°C to 600°C, is due to the decomposition of the chloropropyl group and RF shell; the final mass loss is 21.08% (Dehghani et al., 2025). TGA of Fe₃O₄@RF/Pr-DABCO shows two distinct stages of mass loss (Figures 7Ad). The first stage, with an onset of degradation at 50°C–200°C, with mass loss of 1.85%is caused by the evaporation of H₂O or residual solvent on the catalyst’s surface. The second stage, between 200°C and 700°C, with mass loss of 20.78%, is related to continuous decomposition of the organic components (DABCO, propyl group and RF shell). The final mass loss is 22.63% (Rajabi-Salek et al., 2018; Jadidi Nejad et al., 2020; Jia et al., 2022).

Figure 7

Figure 7. (A) TGA (B) DTG, (C) DTA, and (D) DSC of the (A) Fe₃O₄, (B) Fe₃O₄@RF, (C) Fe₃O₄@RF/Pr-Cl, and (D) Fe₃O₄@RF/Pr-DABCO.

TGA and DTG (Figure 7B) data were supported by DTA and DSC curves. Based on DTA and DSC (Figures 7C, D), the separation process of organic materials attached to the surface of Fe₃O₄@RF/Pr-DABCO nanocatalyst is exothermic, and the different intensity of the peaks indicates the synthesis of different stages of the nanocatalyst.

Comparison of the mass loss curve and thermal stability (TGA) graph analysis of each step of the synthesis of the Fe₃O₄@RF/Pr-DABCO magnetic nanocatalyst confirmed that all four steps of the nanocatalyst synthesis were successfully completed and the alkyl group and DABCO was loaded onto the surface of the Fe₃O₄@RF particles. Also, the TGA thermograms of the product obtained at each stage of synthesis are parallel to the elemental composition obtained by EDX.

3.2 Catalytic activity of the Fe₃O₄@RF/Pr-DABCO

Following the successful characterization of Fe₃O₄@RF/Pr-DABCO, the effectiveness of the catalyst was assessed in the green production of polyhydroquinoline. The condensation between 1.4 mmol of NH₄OAc, 1 mmol of benzaldehyde, 1 mmol of ethyl acetoacetate, and 1 mmol of dimedone was chosen as a model reaction. The findings indicate that the catalyst, solvent, and temperature significantly affect the reaction’s progression. Consequently, the amount of catalyst, choice of solvent, and temperature were evaluated to enhance the reaction conditions.

Initially, to evaluate the effectiveness of the Fe₃O₄@RF/Pr-DABCO catalyst, it was observed that the reaction did not occur in the absence of the catalyst, indicating its essential role in the process (Table 1, entry 1). To further examine the catalyst’s impact, we tested various amounts of the catalyst (1, 3, 5, 7, and 9 mg). As detailed in Table 1 entries 2–6, the reaction yield improved with increased catalyst quantity from 0.001 g to 0.003 g (Table 1, entry 3). The optimal yield was achieved with 0.003 g of the catalyst (Table 1, entry 3). Notably, a catalyst amount of 0.005 g produced the same yield as 0.003 g (Table 1, entry 4). Additionally, increasing the catalyst amount from 0.007 to 0.009 g resulted in a minor reduction in product yield. Studies have shown that excessive amounts of catalyst alter the reaction mechanism and increase side reactions, leading to the formation of unwanted by-products. This change reduces the selectivity towards the desired product and consequently reduces its yield (Chen et al., 2021; Platero et al., 2022).

Table 1

Table 1. The reaction conditions optimized for the synthesis of polyhydroquinolines by Fe₃O₄@RF/Pr-DABCO

The model reaction was subsequently evaluated at various temperatures, specifically 25°C, 40°C, 60°C, 70°C, and 90°C (Table 1, entry 3 vs. entries 7–10). An increase in temperature from 25°C to 60°C resulted in enhanced reaction efficiency, with the optimal yield observed at 60°C (Table 1, entry 3). Additionally, satisfactory efficiency was noted at 70°C (Table 1, entry 9). However, as the temperature rose to 90°C, there was a decline in product yield (Table 1, entry 10). Studies have shown that by-products are formed at 90°C, and therefore the yield of the main product is reduced.

This study examined the impact of various solvents, such as dimethyl sulfoxide, toluene, water, acetonitrile, and ethanol, as well as solvent-free conditions (Table 1, entry 3 vs. entries 11–15). The findings indicated that the highest yield achieved was 97% under solvent-free conditions. Because DABCO is an organic compound and the solvents are all organic, the interaction between DABCO and the solvents increases, and the accumulation of substrate around the catalytic active site decreases, and the catalytic activity of DABCO decreases compared to solvent-free conditions. In the presence of ethanol and water solvents, although these solvents also interact with DABCO and reduce its catalytic activity, these solvents can have hydrogen interactions with the substrates, causing substrate activation and facilitating the reaction conditions. However, in general, solvent-free conditions are the best option for this process and do not reduce catalytic activity (Lu et al., 2012; Varghese and Mushrif, 2019).

To ascertain the pivotal role of DABCO groups in the catalytic cycle, the efficacy of DABCO-free Fe₃O₄, Fe₃O₄@RF, and Fe₃O₄@RF/PrCl nanomaterials was investigated under identical reaction conditions and time as the Fe₃O₄@RF/Pr-DABCO catalyst (Table 1, entry 3 vs. entries 16–18). Notably, the latter catalysts exhibited negligible product yields, unequivocally establishing the indispensable presence of DABCO species as crucial catalytic centers for the reaction.

A diverse range of polyhydroquinolines was synthesized under optimal conditions, utilizing 0.003 g of a solvent-free catalyst at 60°C with Fe₃O₄@RF/Pr-DABCO as a heterogeneous catalyst. As detailed in Table 2, various benzaldehydes featuring both electron-donating and electron-withdrawing groups were employed in the synthesis of polyhydroquinolines, resulting in products characterized by high yields, brief reaction times, and precise melting point measurements (Table 2, entries 1–7).

Table 2

Table 2. Synthesis of polyhydroquinolines utilizing Fe₃O₄@RF/Pr-DABCO under ideal conditionsa

The recovery and reusability of the catalyst were tested for eco-friendly industrial and commercial applications. A test model evaluated the condensation of 0.003 mg of the catalyst with ethyl acetoacetate (1 mmol), dimedone (1 mmol) benzaldehyde (1 mmol), and NH₄OAc (1.4 mmol). The catalyst was isolated from the solution following each run through an external magnet, washed with ethanol, dried, and then used again in another reaction to regenerate it. According to Figure 8, the recycling process can be conducted under identical conditions to the initial run at least eight times without experiencing any substantial loss.

Figure 8

Figure 8. Reusability of the Fe₃O₄@RF/Pr-DABCO

In another experiment, the recoverability and reusability of the Fe₃O₄@RF/Pr-DABCO catalyst were investigated at a fixed time under optimal conditions. In this test, the reaction time for each run was 30 min. Following each run, the catalyst was separated using a magnetic field and subsequently reused in the next run, under the same conditions as the first run. As illustrated in Figure 9, this catalyst can be recovered and reused at least 8 times without a substantial decrease in its performance.

Figure 9

Figure 9. Recoverability and reusability of the Fe₃O₄@RF/Pr-DABCO at a fixed time.

Scheme 2 presents a proposed mechanism for synthesizing polyhydroquinolines using the Fe₃O₄@RF/Pr-DABCO as a catalyst. Initially, the carbanion group (A) is generated through the deprotonation of the α-proton of dimedone by the basic nitrogen group of the catalyst, DABCO. Subsequently, intermediate (B) is formed via a Knoevenagel condensation reaction between carbanion (A) and an aldehyde. The formation of intermediate (C) occurs after the elimination of a water molecule from the reaction involving ammonia (derived from ammonium acetate) and ethyl acetoacetate. The Michael addition of intermediate (B) with (C) results in the formation of intermediate (D). Ultimately, the nanocatalyst facilitates the preparation of the final product by promoting the cyclization of (D) and the removal of a water molecule (Al Anazi et al., 2023).

Scheme 2

Scheme 2. A suitable catalytic mechanism for the production of polyhydroquinolines using Fe₃O₄@RF/Pr-DABCO

We investigated the catalytic performance of magnetic Fe₃O₄@RF/Pr-DABCO in producing polyhydroquinoline derivatives and compared it to previously reported nanocatalysts (Table 3). The results show that our designed catalyst is cost-effective, easy to use, and highly efficient, yielding high amounts of polyhydroquinoline derivatives under typical reaction conditions. Therefore, this catalyst is comparable to or even better than previous catalysts in Hantzsch’s synthesis of polyhydroquinolines in terms of reaction conditions, yield, and ease of magnetic recovery.

Table 3

Table 3. Compare the effectiveness of the current catalyst with other catalysts.

4 Conclusion

In summary, a novel magnetic RF modified with DABCO (Fe₃O₄@RF/Pr-DABCO) was successfully synthesized. The EDX and FT-IR analyses confirmed successful chemical immobilization of resorcinol-formaldehyde precursors and DABCO moieties on the surface of the Fe₃O₄ NPs. Also, TGA analysis proved the good immobilization of resorcinol-formaldehyde resin and DABCO moieties onto Fe₃O₄ NPs and showed the high thermal stability of the Fe₃O₄@RF/Pr-DABCO nanocomposite. The SEM image showed that the Fe₃O₄@RF/Pr-DABCO nanostructures are spherical and regular. Also, The XRD demonstrated the structure of Fe3O4 is not changed under the conditions of the synthesis of Fe₃O₄@RF/Pr-DABCO nanocomposite. Finally, its catalytic application was studied as a catalyst for synthesizing polyhydroquinoline derivatives. This nanocatalyst, showed excellent catalytic activity with a minimum amount (0.003 g) at an optimum temperature of 60°C in only 15 min and effectively led to the formation of products with high yields (97%–99%). Additionally, the Fe₃O₄@RF/Pr-DABCO nanocatalyst can be easily recovered using an external magnet and could be reused up to eight times without any significant loss of its catalytic activity.

Data availability statement

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding authors.

Author contributions

ME: Formal Analysis, Investigation, Resources, Writing–original draft. AS: Conceptualization, Supervision, Visualization, Writing–review and editing.

Funding

The author(s) declare that no financial support was received for the research and/or publication of this article.

Acknowledgments

The authors thank the Yasouj University for supporting this work.

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Generative AI statement

The author(s) declare that no Generative AI was used in the creation of this manuscript.

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