Characterization of GO@Fe3O4@Cur–Cu nanocatalystFigure 4 shows the FT-IR absorption spectra of GO@Fe3O4, Cur@propylamide, GO@Fe3O4@ Cur, GO@Fe3O4@Cur–Cu, and recycled GO@Fe3O4@Cur–Cu. In the FT-IR absorption spectra of GO@Fe3O4, the stretching vibrations at 3396 cm−1 indicate the symmetric modes of the O–H bonds25. In addition, the absorption bands at 1726, 1618, 1225, and 1051 cm−1 belong to the stretching vibrations of the C=O bond of acid, stretching vibration C=C of aromatic rings and stretching vibration C–O of epoxy, respectively. In addition, the absorption bands at 400–700 cm−1 are related to Fe–O vibrations (Fig. 4a)30. Figure 4b shows the FT-IR absorption spectra of Curcumin@propylamide. The absorption peak at 3355, 1722, and 1704 cm−1 corresponds to the symmetric modes of the O–H/NH bonds, stretching vibrations of C=O bond of amide, and stretching vibrations of the C=O bond of ester. Moreover, the characteristic peaks of Si–O which appear at 1091 cm−1 are attributed to Si–O stretching vibrations of silicapropylisocyanate linker. Considering the GO@Fe3O4@Cur catalyst spectra, the absorption bands at 1728, 1578, 1461, 1120, and 1074 cm−1 belong to the stretching vibrations of the C=O, C–O, Si–O stretching vibration, respectively (Fig. 4c). The addition of copper species to the structure of GO@Fe3O4@Cur significantly shift the FTIR spectra in stretching vibrations of C=O bond of amide 1722 to 1729 cm−1 (Fig. 4d). The FT-IR image showed that the structure of the recovered GO@Fe3O4@Cur–Cu NPs did not change significantly, indicating that the catalyst is robust and recyclable (Fig. 4e).Figure 4FT-IR spectra of GO@Fe3O4 (a), Cur@propylamide (b), GO@Fe3O4@Cur (c), GO@Fe3O4@Cur–Cu (d), and recycled GO@Fe3O4@Cur–Cu (e).X-ray diffraction (XRD) analysis was utilized to examine the crystal structure of two samples: GO, and GO@Fe3O4@Cur–Cu (Fig. 5). The primary diffraction peaks of graphene oxide were observed at 2θ angles of 26.39°, and 35.49°, which align with the patterns previously reported in the literature22 (Fig. 5a). In the XRD spectrum of GO@Fe3O4@Cur–Cu, the presence of sharp peaks within the 2θ range of 12.84°, 17.89°, and 22.29° indicates the successful functionalization of GO@Fe3O4 with curcumin31. The diffraction peaks of 25°, and 36.24° indicates the presence of GO in the GO@Fe3O4@Cur–Cu nanocomposite. Furthermore, the presence of Fe in the structure is confirmed by the appearance of peaks at 2θ = 44.7°, and 65.6°32 (Fig. 5b).Figure 5XRD pattern of GO (a), and GO@Fe3O4@Cur–Cu nanocomposite (b).The size and morphological characteristics of the catalyst were investigated using FE-SEM. As can be seen, the catalyst has an almost wrinkled layer structure and a uniform surface. The SEM images revealed that the NPS exhibit good consolidation with a smooth surface, indicating their high quality and uniform distribution (Fig. 6A). The HRTEM images revealed that the Fe3O4 MNPs had a relatively spherical shape and illustrated a uniform and smooth morphology in the GO@Fe3O4@Cur–Cu, as shown in Fig. 6B. Some regions of the sample appeared darker in the HRTEM image, possibly due to variation in thickness or high mass density of particles, and Fe3O4 MNPs. The HRTEM image analysis showed that the nanoparticles had a median diameter of 6 ± 1.25 nm (Fig. 7).Figure 6FESEM images (A), and HRTEM images (B) of GO@Fe3O4@Cur–Cu.Figure 7Particle size distribution of GO@Fe3O4@Cur–Cu.EDX analysis was carried out over a selected zone of GO@Fe3O4@Cur–Cu to study its elemental composition (Fig. 8). Elemental mapping verified the presence and uniform distribution of all elements, including O, C, Fe, N, Si and Cu (Fig. 9). The existence of Cu in the EDS spectrum is indicative of successful immobilization of copper on the surface of GO@Fe3O4@Cur. These results confirmed the purity of the sample and indicated the absence of any impurities or foreign elements (Figs. 8 and 9).Figure 8EDX spectrum of GO@Fe3O4@Cur–Cu.Figure 9Elemental mapping of the C, N, O, Si, Cu, and Fe atoms in GO@Fe3O4@Cur–Cu MNPs.Figure 10 shows the magnetic properties of the GO@Fe3O4@Cur–Cu nanocatalyst (Fig. 10A) and recycled nanocatalyst (Fig. 10B) as determined using VSM. According to the figure, due to the s-shape of the curve, the synthesized materials are super-magnetic. The results of VSM analysis of the recovered catalyst after five successive cycles demonstrate that the GO@Fe3O4@Cur–Cu exhibits durability and stability as well.Figure 10VSM analysis of GO@Fe3O4@Cur–Cu (A), and recycled nanocatalyst (B).Figure 11 illustrates the thermogravimetric analysis (TGA) curves depicting the residual masses of GO@Fe3O4@Cur–Cu within the temperature range of 25–600 °C. The TGA curve reveals an initial negligible weight loss (2%) occurring between 90 and 180 °C, which confirms the evaporation of the solvent absorbed on the surface of GO@Fe3O4@Cur–Cu. Subsequently, a significant weight loss of 17% is observed in the 200–320 °C range, attributed to the decomposition of the immobilized ligand present on the surface of prepared support. Based on the thermal characterization data of GO@Fe3O4@Cur–Cu, it can be inferred that the catalyst remains stable up to 320 °C. However, as the temperature increases beyond 320 °C, the thermal stability of GO@Fe3O4@Cur–Cu significantly declines due to the decomposition of its structure.Figure 11TGA curve of GO@Fe3O4@Cur–Cu.Investigating the catalytic activity of GO@Fe3O4@Cur–Cu nanocatalyst for preparation of polyhydroquinolines and sulfoxidesThe reaction of dimedone (1mmol), 4-chlorobenzaldehyde (1 mmol), ammonium acetate (1 mmol), and ethylacetoacetate (1mmol) was considered as a model reaction for polyhydroquinoline synthesis in the presence of GO@Fe3O4@Cur–Cu nanocatalyst, and the effect of different amounts of nanocatalyst on the reaction efficiency was investigated. The results shown in Table 1 and Fig. 12. As can be seen in this Table, using of 7 mg nanocatalyst, the yield of desired product has increased to 96% (entry 5), and the increase in amount of nanocatalyst has not affected on the product yield. In the absence of the catalyst (entry1), the reaction efficiency in 4 h is very small, which justifies the need to use the above-mentioned catalyst in the study of this reaction.Table 1 Survey of the catalyst amount for the synthesis of (4b).Figure 12Catalytic performances in polyhydroquinoline synthesis catalyzed by GO@Fe3O4@Cur–Cu at different amounts of catalyst. Error bars indicate the range of data based on repeat experiments.The role of solvents in the reaction was also screened. As shown in Table 2, it was found that solvent-free is the best condition to produce the target product (4b) in high yield in comparison with other solvents (Table 2, entry 7).Table 2 Survey of solvent type in synthetic reaction of compound (4b).In continuation, the effect of temperature on the desired product yield was investigated. For this purpose, the four-component reaction of dimedone (1mmol), 4-chlorobenzaldehyde (1 mmol), ammonium acetate (1 mmol), and ethylacetoacetate (1mmol) was investigated in the presence of 7 mg of nanocatalyst GO@Fe3O4@Cur–Cu at different temperatures. The results are summarized. According to Table 3, the yield of desired product (4b) increased to 96% at the temperature of 100 °C. Also, increasing the reaction temperature has not increased the product formation time (entries 5 vs 4). The optimum time to complete the reaction was 240 min (entry 4).Table 3 Study of temperature and time effect for the synthesis of product (4b).Due to confirmation of the generality of this green approach, various benzaldehyde derivatives bearing electron-donating, withdrawing groups, and aliphatic groups were selected and reacted with dimedone, ammonium acetate, and ethylacetoacetate in the presence of novel nanocatalyst GO@Fe3O4@Cur–Cu under optimized reaction conditions. These results are summarized in Table 4.Table 4 The functionalized GO@Fe3O4@Cur–Cu catalyzed one-pot synthesis of various polyhydroquinoline compounds.As can be seen from the table, by using GO@Fe3O4@Cur–Cu nanocatalyst, the desired products were obtained with excellent yields. Also, by using benzaldehyde with electron withdrawing groups, the yield of the products increased to the highest value. The structure of synthetic products (4a-l) display in Fig. 13.Figure 13Structure of products (4a-l) synthesized in the presence of the GO@Fe3O4@Cur/Cu nanocatalyst.In continuation of our research33,34,35, to investigate the efficiency of GO@Fe3O4@Cur/Cu as nanocatalyst in organic transformations, the oxidation of sulfides to sulfoxides was examined. First, to optimize the reaction conditions for oxidation of sulfides, methylphenyl sulfide (1 mmol) and 30% H2O2 (0.3 mL) was used as a model reaction in the presence of different amount of catalyst and organic solvents (Tables 5 and 6). Effect of catalyst in synthesis of sulfoxides was studied by varying the amount of GO@Fe3O4@Cur/Cu (Fig. 14). Table 5 entry 3 shows 0.01 gr catalyst was found to be the most effective and product was obtained in 96% yield.Table 5 Survey of the catalyst amount for oxidation of sulfides (5a).Table 6 Optimization of the solvent for the synthesis of (5a).Figure 14Catalytic performances in oxidation of sulfides catalyzed by GO@Fe3O4@Cur–Cu at different amounts of catalyst. Error bars indicate the range of data based on repeat experiments.The role of solvents in the reaction was also screened. As shown in Table 5, it was found that solvent-free is the best condition.Due to confirmation of the generality of this green approach, various sulfides oxidized with H2O2 in the presence of novel nanocatalyst GO@Fe3O4@Cur–Cu under optimized reaction condition. These results were summarized in Fig. 15 and Table 7.Figure 15The structure of products (5a-e).Table 7 The functionalized GO@Fe3O4@Cur–Cu catalyzed oxidation of sulfides to sulfoxides.The proposed mechanism for polyhydroquinoline synthesisAs shown in Fig. 16, the carbonyl groups in benzaldehyde and ethylacetoacetate are activated through interaction with the nanocatalyst23. As a result, the condensation reaction of benzaldehyde with ethylacetoacetate or dimedone is accelerated and the desired alpha–beta unsaturated intermediate is formed in a short time. Also, by using of this nanocatalyst, Michael addition reaction of intermediate (II) with alpha–beta unsaturated intermediate accelerated. Finally, the interamolecule cyclization of intermediate leads to forming of the desired product36.Figure 16Proposed mechanism for the synthesis of polyhydroquinoline derivatives using GO@Fe3O4@Cur–Cu nanocatalyst.Reusability potential of GO@Fe3O4@Cur–Cu nanocatalystIn organic synthesis, the ability to recycle a catalyst is essential. To test the reusability of biosynthesized GO@Fe3O4@Cur–Cu NPs as a catalyst, the NPs were separated from the reaction medium using super magnet and washed multiple times with ethanol and water to obtain a clean catalyst without contamination. The results depicted in Fig. 17 demonstrate that the catalytic performance of GO@Fe3O4@Cur–Cu nanocomposite was not significantly affected after being subjected to multiple catalytic runs (up to five). Consequently, the reused GO@Fe3O4@Cur–Cu NPs are able to accelerate the production of the polyhydroquinoline in a proficient yield. Also, to calculate the accurate loading of copper ions in the GO@Fe3O4@Cur sample, ICP-OES analysis is applied. The amount of copper in fresh and recycled catalyst after 5 times recycling is 0.364 and 0.357 mmolg−1, respectively.Figure 175-run recovery of GO@Fe3O4@Cur–Cu nanocomposite.In order to examine the stability of GO@Fe3O4@Cur–Cu after recovering and reusing, recovered catalyst was characterized by TEM and XRD techniques. The TEM image showed that the morphology of the recovered GO@Fe3O4@Cur–Cu NPs did not change significantly, indicating that the catalyst is robust and recyclable (Supplementary Fig. 22). In the XRD spectrum of reused GO@Fe3O4@Cur–Cu (Fig. 18), the presence of sharp peaks within the 2θ ranges of fresh GO@Fe3O4@Cur–Cu nano catalyst (Fig. 5) clearly indicated the structural stability of catalyst after reused. The position and relative intensities of all peaks confirm well.Figure 18The XRD pattern of reused catalyst.In order to show the merit of GO@Fe3O4@Cur–Cu in comparison with other reported system catalysts, synthesis of ethyl 4-(4-chlorophenyl)-2,7,7-trimethyl-5-oxo-1,4,5,6,7,8-hexahydroquinoline-3-carboxylate was compared (Table 8). It is important to point out that in present work, our catalytic system has benefit such as: mild reaction conditions, high yields and good reaction times.Table 8 Comparison results of GO@Fe3O4@Cur–Cu nanocomposite with other catalysts in synthesis of ethyl 4-(4-chlorophenyl)-2,7,7-trimethyl-5-oxo-1,4,5,6,7,8-hexahydroquinoline-3-carboxylate.
