Yolk-shell structured nanoparticles (NPs) are hybrid materials in which a core is encapsulated in a hollow shell and can move freely within this shell, commonly demonstrated as core/void/shell. In this structure, the core is not blocked and thus provides an effective active site for the chemical processes (Kim et al., 2002; Kamata et al., 2003; Yin et al., 2004; Gao et al., 2007; Liu et al., 2013; Purbia and Paria, 2016; Xie et al., 2017). The unique properties of yolk-shell structured materials, such as low density, high surface area, permeable shells, high thermal stability, and interstitial hollow spaces, make them powerful platforms for biotechnology/biomedicine, controlled release, magnetic resonance imaging, data storage, catalysis, environmental remediation, etc. (Kresge et al., 1992; Vartuli et al., 1994; Holmes et al., 1998; Vartuli et al., 1998; Tsuji et al., 1999; Morishita et al., 2006; Puanngam and Unob, 2008; Du and He, 2011; Zhao et al., 2011; Ghaedi et al., 2013; Zhang, 2013; Nasab and Kiasat, 2016; Purbia and Paria, 2016). Among the various categories of yolk-shells (YSs), magnetic composites with Fe₃O₄ cores and nano-silica shells are very attractive due to their advantages such as good magnetic properties, high chemical and thermal stability, non-toxicity, high adsorption capacity, high surface area, high biocompatibility, and high accessibility of–OH groups on their surface for any modification (Arruebo et al., 2006; Liu et al., 2011; Yang et al., 2015). Recently, the catalytic application of YS-structured magnetic mesoporous silica nanocomposites has received much attention. Some of the newly developed systems in this regard are Au@Void@PMO (Yang et al., 2015), Fe₃O₄@SiO₂@Pd/HSPMO (Dai et al., 2017), PMO-MHS (Zhang et al., 2008), and Fe₃O₄@void@mSiO₂ (Qiu et al., 2015).

Periodic mesoporous organosilica (PMOs), a desirable class of organic-inorganic composite materials with great properties such as high surface area, high lipophilicity, and high thermal and mechanical stability, have emerged as an ideal support (Wang et al., 2015; Yu et al., 2019; Norouzi et al., 2020; Neysi and Elhamifar, 2023). In particular, bifunctional PMOs (BPMOs), which contain organic functionalities on both the mesoporous walls and channels, are highly attractive for catalytic processes

On the other hand, ionic liquids (ILs) have attracted tremendous attention in chemistry and materials science in the last decade owing to their unique characteristics, such as low vapor pressure, high chemical and thermal stability, and their capability to dissolve a variety of compounds. In particular, recently, imidazolium-based ILs have been widely used as linkers for the effective immobilization of catalytic active sites on solid supports (Neysi et al., 2020; Jangra et al., 2021; Veysipour et al., 2021; Neysi and Elhamifar, 2022).

The preparation of pyranopyrazole derivatives has emerged as a powerful tool in organic synthesis because they are an important class of biologically active compounds. Some biological properties of pyranopyrazoles are anticancer, antifungal, anti-anxiety, antiviral, and anti-AIDS (Babaie and Sheibani, 2011; Moosavi-Zare et al., 2013; Zolfigol et al., 2013; Ali et al., 2014; Gujar et al., 2014; Pandit et al., 2015). To date, many homogeneous and heterogeneous catalysts have been reported for the synthesis of pyranopyrazoles under different conditions. However, some of these systems suffer from the problems of high catalyst loading, harsh conditions, and the use of toxic organic solvents. Therefore, design a green and efficient catalytic system to overcome the above limitations is a significant subject between chemists. Given the above and continuing our recent studies on the design and preparation of novel magnetic and mesoporous catalytic systems, herein novel magnetic methylene and ionic liquid-based bifunctional periodic mesoporous organosilica (BPMO) supported copper with yolk-shell structure (YS-Fe₃O₄@PMO/IL-Cu) was prepared and applied as an effective and recoverable catalyst for the synthesis of pyranopyrazoles under green conditions. In this BPMO, methylene functional groups are incorporated into the mesoporous walls, while ionic liquid functions are located in the mesoporous channels.

The synthesis of YS-Fe₃O₄@PMO/IL-Cu NPs is presented in Scheme 1. Initially, the surface of Fe₃O₄ NPs was coated with a periodic mesoporous organosilica shell via hydrolysis and co-condensation of TEOS and BTEM in the presence of CTAB and pluronic P123 surfactants. To obtain a yolk-shell structure, the CTAB and pluronic P123 templates were removed by Soxhlet extraction. Subsequently, the surface of YS-Fe₃O₄@PMO/IL NPs was modified with a complex of ionic liquid and copper salt to obtain YS-Fe₃O₄@PMO/IL-Cu catalyst. It is important to note that the YS-Fe₃O₄@PMO/IL-Cu catalyst is a multifunctional material that contains the advantages of magnetic NPs, supported ionic liquids and YS-structured mesoporous materials. For example, as shown in Scheme 1, IL moieties play a key role in the immobilization and stabilization of catalytic copper species.

Figure 1 demonstrates the FT-IR of Fe₃O₄@surfactants@PMO, YS-Fe₃O₄@PMO, and YS-Fe₃O₄@PMO/IL-Cu NPs. For all samples, the peaks observed at 588 and 3300–3450 cm^-1 are related to Fe-O and O-H bonds, respectively. Also, the signals observed at 823 and 1078 cm^-1 are assigned to the asymmetric and symmetric vibrations of the Si-O-Si bond, respectively (Figures 1A,B). It should be noted that before surfactant extraction, the sharp peaks at 2923 and 2855 cm^-1 are due to C-H stretching vibrations of CTAB and P123 (Figure 1A). After the Soxhlet extraction, the intensity of these peaks is significantly decreased, confirming the successful elimination of surfactants (Figure 1B). In Figure 1C, the peaks at 1418 and 1625 cm^-1 are related to C=C and C=N of imidazolium rings, respectively.

The XRD analysis of the Fe₃O₄, YS-Fe₃O₄@PMO, and YS-Fe₃O₄@PMO/IL-Cu catalysts is displayed in Figure 2. This clearly illustrates six signals at 2Ɵ = 30.3, 35.7, 43.4, 53.8, 57.7, and 63.0°, which is in agreement with the standard XRD pattern of Fe₃O₄ NPs. This confirms that the Fe₃O₄ NPs are very stable during the preparation of the YS-Fe₃O₄@PMO/IL-Cu catalyst. It is also important to note that for YS-Fe₃O₄@PMO and YS-Fe₃O₄@ PMO/IL-Cu materials, the intensity of PXRD peaks is decreased, indicating the successful modification of magnetite NPs with Me-PMO, IL, and copper moieties. (Figure 2).

The N₂ adsorption–desorption isotherm of the YS-Fe₃O₄@PMO/IL-Cu showed a type IV isotherm with an H1 hysteresis loop, which is characteristic of ordered mesostructures with high regularity (Figure 3). Also, the BET surface area, average pore size, and total pore volume of the designed YS-Fe₃O₄@PMO/IL-Cu nanocomposite were found to be 659 m²/g, 7.6 nm, and 1.30 cm³/g, respectively. These results demonstrate the good formation of an ordered PMO shell for YS-Fe₃O₄@PMO/IL-Cu.

The SEM image of the YS-Fe₃O₄@PMO/IL-Cu catalyst showed the presence of uniform particles with spherical structure and an average size of 70 nm (Figure 4). These are very good NPs for catalytic and adsorption processes.

The VSM analysis showed a saturation magnetization of about 30 emu·g⁻¹ for the designed YS-Fe₃O₄@PMO/IL-Cu nanocatalyst, lower than that of pure magnetic iron oxide NPs (Figure 5) (Norouzi et al., 2020). This proves the successful coating of PMO shell on magnetite NPs and also confirms the high magnetic properties of the catalyst, which is an excellent characteristic in the catalytic field.

The EDX pattern confirmed the presence of the desired elements in the YS-Fe₃O₄@PMO/IL-Cu catalyst (Figure 6). This analysis illustrated the signals of C, Si, N, Cu, Fe, and O elements in the catalyst, proving the successful incorporation and immobilization of the expected inorganic and organic moieties into/onto Fe₃O₄ NPs.

In the next step, TGA analysis was conducted to evaluate the thermal stability of the YS-Fe₃O₄@PMO/IL-Cu catalyst (Figure 7). The first weight loss below 120°C is due to the loss of water and alcoholic solvents left over from the synthesis process. Another weight loss at 200°C–320°C is related to the decomposition of the remaining CTAB and P123 surfactants. The highest weight loss, observed at 325°C–650°C, is attributed to the removal of methylene and ionic liquid functional groups, which are incorporated/immobilized in/on the structure of YS-Fe₃O₄@PMO/IL-Cu nanocomposite.

After characterizing the YS-Fe₃O₄@PMO/IL-Cu catalyst, its application in the synthesis of dihydropyrano [2, 3-c]pyrazoles was investigated. For this part of the study, the condensation between malononitrile, PhCHO, ethyl acetoacetate, and hydrazine hydrate was selected as a model reaction. The effects of the solvent and catalyst loading were studied at RT under ultrasonic conditions. As displayed in Table 1, the effects of different solvents such as EtOH, CH₃CN, n-Hexane, DMF, H₂O, and solvent-free media were studied, and the best results were obtained in H₂O at 25°C (Table 1, entries 1–6). The effect of catalyst loading was also investigated, with the best yield obtained in the presence of 0.36 mol% of YS-Fe₃O₄@PMO/IL-Cu. According to these results, the use of 0.36 mol% of YS-Fe₃O₄@PMO/IL-Cu in H₂O at 25°C under ultrasonic irradiation was chosen as the optimum condition. In order to prove whether the cupper centers act as catalytic sites or not, in the next study the reaction was carried out using Cu-free Fe₃O₄, YS-Fe₃O₄@PMO, and YS-Fe₃O₄@PMO/IL materials under the same conditions as YS-Fe₃O₄@PMO/IL-Cu (Table 1, entries 10–12). The result showed that for all Cu-free samples, only a low yield of the desired product was obtained, indicating that the reaction is mainly catalyzed by immobilized copper sites.

After optimizing the different parameters, the efficiency of the YS-Fe₃O₄@PMO/IL-Cu nanocatalyst was evaluated by using different aldehyde substrates for the preparation of pyrazole derivatives. As seen in Table 2, all investigated aldehydes were converted to their corresponding products in high yields. These results confirm the high efficiency of YS-Fe₃O₄@PMO/IL-Cu for the preparation of a wide range of biologically active pyranopyrazoles.

The recoverability and reusability of the YS-Fe₃O₄@PMO/IL-Cu catalyst were investigated in the condensation of malononitrile, ethyl acetoacetate, benzaldehyde, and hydrazine hydrate under optimized conditions. For this purpose, at the end of the reaction, the catalyst was magnetically removed, washed, and reused in the next run under the same conditions as in the first step. The results indicated that the synthesized catalyst can be recovered and reused at least 9 times without significant loss of efficiency (Figure 8).

A leaching test was then performed to investigate the nature of the catalyst under the reaction conditions. For this purpose, the model reaction was selected as the test. After about 50% of the process was completed, the catalyst was removed using an external magnet, and the reaction progress of the residue was monitored for 60 min. The result demonstrated no further progress of the reaction, confirming no leaching of the active catalytic species and also the heterogeneous nature of the designed catalyst. This result confirms the successful immobilization of the copper moieties on the material framework.

Next, a comparative study was performed between the activity of the YS-Fe₃O₄@PMO/IL-Cu catalyst and several identified catalysts in the synthesis of pyranopyrazoles (Table 3). The results showed that our designed catalyst is better than other catalysts in terms of catalyst loading, reaction time, and recovery numbers. These findings are attributed to the magnetic nature, mesoporous structure, supported ionic liquids, and high stability of the designed YS-Fe₃O₄@PMO/IL-Cu nanocatalyst.

In this study, the magnetic YS-Fe₃O₄@PMO/IL-Cu catalyst was prepared and identified by using PXRD, FT-IR, TGA, EDX, ICP, SEM, nitrogen sorption, and VSM analyses. The TGA, EDX, and FT-IR analyses demonstrated the high chemical and thermal stability of YS-Fe₃O₄@PMO/IL-Cu. The VSM and PXRD analyses showed very good magnetic properties of the material. The nano dimensions and particle size of this catalyst were confirmed using SEM analysis. The nitrogen-sorption diagram also showed a mesoporous structure for the designed catalyst. The YS-Fe₃O₄@PMO/IL-Cu nanocomposite was used as a powerful catalyst in the synthesis of biologically active pyranopyrazoles, giving the desired products in high yields and selectivity. Moreover, the YS-Fe₃O₄@PMO/IL-Cu catalyst was easily recovered and reused at least 9 times without any significant decrease in its efficiency.