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Scientific Reports volume 14, Article number: 31510 (2024 ) Cite this article neodymium magnet push pin
In recent two decades, considerable efforts have been devoted to the room-temperature green syntheses of metal-organic frameworks (MOFs) to reduce energy consumption and increase safety. It could improve some properties (e.g., catalysis, gas adsorption) and facilitate the utilities of sensitive compounds. Herein, the magnetic hybrid catalyst (Fe3O4@MOF-5) was synthesized through a mixing procedure at room temperature and confirmed by various techniques. The SEM images exhibit cubic crystals that were uniformly coated by the Fe3O4 cores. Then, the catalytic ability of Fe3O4@MOF-5 was studied in the green synthesis of tetrahydropyridines via a domino multi-component reaction, which led to the desired products with high yield. Magnetic solid properties make it easily separated from the reaction medium, so the proposed catalyst can be reused five times while maintaining the catalytic activity over 80%.
Heterogeneous catalysts have received increasing attention because of their high environmental compatibility, improved efficiency, and ease of recycling. Heterogeneous catalysis shows exceptional properties in terms of activity and selectivity, so it is a suitable alternative to homogeneous catalysis. In recent years, porous solids, due to their unique properties, have gained particular attention as catalysts or catalytic supports in organic synthesis1,2,3,4.
Metal-organic frameworks (MOFs), the term that was first introduced by Yaghi et al. in 1995, are crystalline and sponge-like structures that consist of metallic cations and multi-topic organic linkers by strong coordination bonds. The synthesis of MOFs depends on the precise selection of metal centers and organic ligands, which can be self-assembled in a solid state under specific synthetic conditions. Hence, the shape, size, and function of the pores of the MOFs can be controlled, unlike zeolites5,6. MOFs show larger pores and higher porosity than zeolites due to the expanded dimensions of the network by large metal clusters7. Due to their structural tunability, MOFs have attracted particular attention in chemistry and technology. They can be used in different fields, such as heterogeneous chemical catalysis8,9,10,11,12,13,14, drug delivery15,16,17, gas separation18,19, luminescent materials20,21,22, and biomedical imaging23.
MOFs incorporated with efficient nano/microparticles like metal nanoparticles (MNPs)24,25, quantum dots26,27, graphene28,29, silica30,31, and magnetic beads32 exhibit special properties than their counterparts. Among them, MOFs containing more than one metal somewhere in their structure (as guests with or without interaction with the MOF framework) open new opportunities to fabricate multifunction structures and their tuning of the properties. Due to the synergistic effects derived from the presence of two or more metals, mixed-metal MOFs (MM-MOFs) show better performance in various applications compared to single-metal MOFs, particularly in gas storage, heterogeneous catalysis, sensing, and construction of photoactive materials. Particularly in the field of heterogeneous catalysis, MM-MOFs offer the possibility of having two centers of different catalytic activity that are especially suited to promote tandem or cascade reactions33.
Hence, magnetic MOF composites (MFCs) have gained special attention in recent years because they take advantage of the properties of both MNPs and MOFs, involving high surface area, devisable composition, easy loading, rapid separation, low toxicity, and controllable magnetic. MFCs are categorized into non-core-shell and core-shell34,35,36. Depending on the type of interactions between the MNPs and MOFs, MCFs are synthesized by four different procedures involving:
embedding such as Fe3O4@UiO-66@PPI37.
encapsulation such as Fe3O4@PDA@UiO-66-NH238 and.
mixing such as HPU-13@Fe3O439
Among them, the mixing procedure has two advantages over others. It does not use any reagent to modify the surface of ferrite NPs. In addition, it does not use any polymer coating to stimulate the growth of MOFs39.
Although the solvothermal procedure mainly synthesizes MOFs, this method suffers from disadvantages such as high temperature, high pressure, and toxic solvents40. Hence, the green synthesis of MOFs at room temperature is a challenging and valuable target that leads to decreased energy costs and rising safety conditions41. MOF-5, MOF-74, MOF-177, MOF-199, and IRMOF-0 are synthesized frameworks in this way42.
N-heterocyclic compounds currently occupy a central place in research due to their application in dyes, drugs, agrochemicals, and polymers43,44. Nitrogenous frameworks found the core of various pharmaceutical drugs such as Stivarga, Alatrofloxacin, Zytiga, Caprelsa, Gemifloxacin, Vosaroxin, Erivedge, Zelboraf, Xalkori, Cometriq, etc45,46. A class of N-heterocyclic compounds is tetrahydropyridines (THPs), which are the backbone of many natural and synthetic products, pharmaceuticals, and biologically active compounds47. So far, several of the literature has been reported on the synthesis of THPs, for example, CAN48, hexagonal PbCrxFe12−xO19,49 ETPPBr/THFTCADES,50 BF3.SiO2,51 Bi(NO3)3.5H2O,52 ([Dsbim]Cl),53 Fe/MWCNTs,54 Fe3O4@TDSN-Bi(III),55 chiral phosphoric acid (CAP),56 ZrP2O7, 57 L-proline/TFA,58etc.
Following our previous study in fields of MNPs and MOFs58,62,63,61 and considering the synergistic effects derived from the presence of two or more metals, a magnetic hybrid (Fe3O4@MOF-5) was prepared through a mixing procedure at room temperature (Fig. 1) and studied with various techniques.
Synthesis of magnetic hybrid Fe3O4@MOF-5.
Then, the ability of Fe3O4@MOF-5 to catalyze the one-pot green reaction between β-ketoester 1, anilines 2, and aromatic aldehydes 3 was investigated (Fig. 2).
Synthesis of substitutes THPs catalyzed by Fe3O4@MOF-5.
The commercial reagents were provided by the Merck or Aldrich chemical companies and were used without purification. The reaction progress and purity of products were controlled by TLC-silica gel 60 F-254 plates. FT-IR spectra were recorded on a PerkinElmer Spectrum Version 10.02.00 using KBr pellets. The1H NMR (250 MHz) and8C NMR (62.5 MHz) spectra were recorded on a Bruker spectrometer (δ in ppm) using CDCl3‐d6 as a solvent with chemical shifts measured relative to TMS as the internal standard. Melting points were determined by a BUCHI 510 melting point apparatus. Elemental analysis was performed using an MIRA II analyzer. The FESEM images were taken using a MIRA III analyzer. The XRD analysis was done with an XRD Philips PW1730. The TG-DTA was performed using a TGA STA6000 device. The nitrogen adsorption-desorption isotherms were determined by the BET Mini II analyzer.
Fe3O4@MOF-5 was synthesized in two sequential steps.
Step 1: A mixture of FeCl3 (0.65 g, 4.0 mmol), trisodium citrate (0.20 g, 0.68 mmol), and NaOAc (1.20 g) was dissolved in ethylene glycol (20 mL) and stirred for 30 min. Afterward, the obtained mixture was transferred to the autoclave and heated at 200 °C for 10 h. After reaching ambient temperature, the black solid (Fe3O4@Citrate) was washed several times with deionized water and ethanol62.
Step 2: A mixture of terephthalic acid (BDC) (0.075 g, 0.45 mmol) and triethylamine (1.25 mL) was dissolved in 60 mL of DMF (solution A). Also, Zn(OAc)2.2H2O (0.254 g, 1.1 mmol) was dissolved separately in 75.0 mL of DMF (solution B). In continuing, the prepared Fe3O4@Citrate (0.05 g) from the first step and solution A (4.0 mL) were mixed, sealed in a flask, and stirred for 30 min at room temperature. Then, solution B (4.0 mL) was added to the stirring mixture and stirred again for 150 min. This cycle was repeated 15 times, and the obtained precipitate was separated with supermagnet and immersed in CH2Cl2 for 3 days. The solvent was changed every day. Next, the product was activated at 100 °C41,62.
In an around bottom flask, β-Ketoester (1.0 mmol), aniline (2.0 mmol), aromatic aldehyde (2.0 mmol), and the Fe3O4@MOF-5 catalyst (20 mg) were reacted in EtOH (5.0 mL) under reflux conditions for the required time. The reaction was pursued by TLC (n-hexane/EtOAc). After the end reaction, the crude reaction mixture was diluted with chloroform to dissolve the formed organic products and separate the catalyst from the mixture using an external magnet or centrifuge. The separated catalyst was washed several times with chloroform and air-dried. Eventually, the obtained organic products were purified by washing with ethanol.
White solid, M.P.: 236–239 °C; IR (KBr) ν: 3241, 2961, 2869, 1645, 1602, 1489, 1318, 1255, 1067, 1010 and 799 cm−1;1H NMR (250 MHz, CDCl3) δ = 10.22 (s, 1 H), 7.22–7.01 (m, 12 H), 6.40 (t, 3 H), 6.02 (d, J = 8.2 Hz, 2 H), 5.05 (s, 1 H), 4.54–4.40 (m, 1 H), 4.38–4.23 (m, 1 H), 2.96–2.75 (m, 3 H), 2.63 (d, J = 15.0 Hz, 1 H), 1.47 (t, J = 7.1 Hz, 3 H), 1.23 (d, J = 6.2 Hz, 12 H)8. C NMR (62.5 MHz, CDCl3) δ = 168.2, 155.5, 148.3, 147.0, 146.0, 140.3, 139.7, 137.0, 131.8, 131.5, 127.5, 126.8, 126.4, 126.1, 119.1, 117.0, 114.5, 108.1, 98.8, 59.8, 58.2, 54.8, 33.6, 24.0, 14.7.
White solid, M.P.: 231–233 °C; IR (KBr) ν: 3242, 2961, 2868, 1645, 1604, 1493, 1368, 1318,
1256, 1176, and 754 cm−1;1H NMR (250 MHz, CDCl3) δ = 10.22 (s, 1 H), 7.21–6.97 (m, 12 H), 6.41 (d, J = 26.8 Hz, 3 H), 6.07 (s, 2 H), 5.05 (s, 1 H), 4.38 (d, J = 37.4 Hz, 2 H), 2.96–2.75 (m, 3 H), 2.61 (d, J = 15.4 Hz, 1 H), 1.46 (s, 3 H), 1.23 (d, J = 6.4 Hz, 12 H)8. C NMR (62.5 MHz, CDCl3) δ = 168.23, 155.6, 148.2, 145.6, 139.8, 136.5, 128.8, 128.6, 127.2, 126.8, 126.4, 113.9, 59.8, 58.2, 54.9, 33.6, 24.0, 14.7.
White solid, M.P.: 232–236 °C; IR (KBr) ν: 3233, 3059, 2983, 2872, 1648, 1600, 1497, 1317, 1251, 1183, 796 cm−1;1H NMR (250 MHz, CDCl3) δ = 10.25 (s, 1 H), 7.39 (d, J = 8.1 Hz, 3 H), 7.31–7.23 (m, 9 H), 7.15 (d, J = 8.0 Hz, 2 H), 6.33 (t, 3 H), 5.99 (d, J = 8.1 Hz, 2 H), 5.12 (s, 1 H), 4.55–4.40 (m, 1 H), 4.39–4.27 (m, 1 H), 2.87 (dd, J = 15.1, 5.6 Hz, 1 H), 2.73 (d, J = 15.1 Hz, 1 H), 1.47 (t, J = 6.9 Hz, 3 H).
Cream solid, M.P.: 236–238 °C; IR (KBr) ν: 3238, 3060, 2978, 2872, 1650, 1595, 1500, 1368, 1251, 1175, 1067, and 748 cm−1;1H NMR (250 MHz, CDCl3) δ = 10.30 (s, 1 H), 7.10 (dd, J = 15.1, 5.7 Hz, 10 H), 6.65 (t, J = 7.1 Hz, 1 H), 6.49–6.33 (m, 7 H), 5.10 (s, 1 H), 4.50–4.24 (m, 3 H), 2.85–2.74 (m, 2 H), 1.46 (t, J = 7.0 Hz, 3 H).
Yellow solid, M.P.: 175–178−1 °C; IR (KBr) ν: 3215, 2974, 2915, 1653, 1593, 1520, 1348, 1317, 1255, 1070 and 785 cm;1H NMR (250 MHz, CDCl3) δ = 10.27 (s, 1 H), 8.31 (s, 1 H), 8.09 (d, J = 8.0 Hz, 2 H), 7.95 (s, 1 H), 7.63 (d, J = 7.8 Hz, 1 H), 7.46 (d, J = 4.8 Hz, 3 H), 6.92 (t, J = 8.5 Hz, 4 H), 6.44 (s, 1 H), 6.32 (dd, J = 16.6, 7.8 Hz, 4 H), 5.28 (s, 1 H), 4.59–4.45 (m, 1 H), 4.41–4.26 (m, 1 H), 2.85 (d, J = 4.6 Hz, 2 H), 2.27 (s, 3 H), 2.17 (s, 3 H), 1.51 (t, J = 7.0 Hz, 3 H).
White solid, M.P.: 174–176 °C; IR (KBr) ν: 3248, 3457, 2981, 2872, 1652, 1593, 1500, 1373, 1252, 1172, 1070, and 750 cm−1;1H NMR (250 MHz, CDCl3) δ = 10.29 (s, 1 H), 7.34 (d, J = 8.9 Hz, 6 H), 7.19–7.02 (m, 8 H), 6.67–6.40 (m, 5 H), 6.28 (s, 2 H), 5.14 (s, 1 H), 4.54–4.26 (m, 2 H), 2.93–2.72 (m, 2 H), 1.47 (t, J = 7.2 Hz, 3 H).
White solid, M.P.: 200–203 °C; IR (KBr) ν: 3224, 3073, 2949, 1660, 1605, 1524, 1493, 1349, 1320, 1258, 1190, and 733 cm−1;1H NMR (250 MHz, CDCl3) δ = 10.28 (s, 1 H), 8.13 (t, J = 8.1 Hz, 3 H), 7.94 (s, 1 H), 7.63 (d, J = 7.8 Hz, 1 H), 7.46 (t, 4 H), 7.12 (dd, J = 8.5, 2.6 Hz, 2 H), 7.03 (dd, J = 8.9, 2.6 Hz, 2 H), 6.37 (t, J = 8.9 Hz, 6 H), 5.30 (s, 1 H), 3.98 (d, J = 2.6 Hz, 3 H), 2.85 (s, 2 H).
White solid, M.P.: 223–226 °C; IR (KBr) ν: 3241, 3093, 2978, 2856, 1646, 1603, 1493, 1372, 1255, 1178, 1071, and 727 cm−1;1H NMR (250 MHz, CDCl3) δ = 10.24 (s, 1 H), 7.30 (d, J = 4.8 Hz, 8 H), 7.17 (s, 2 H), 7.02 (dd, J = 13.6, 8.3 Hz, 4 H), 6.42 (t, 3 H), 6.17 (d, J = 8.2 Hz, 2 H), 5.12 (s, 1 H), 4.55–4.43 (m, 1 H), 4.38–4.26 (m, 1 H), 2.86 (dd, J = 15.1, 5.7 Hz, 1 H), 2.69 (d, J = 15.2 Hz, 1 H), 1.47 (t, J = 7.1 Hz, 3 H).
White solid, M.P.: 213–215 °C; IR (KBr) ν: 3238, 3068, 2982, 2968, 1653, 1601, 1495, 1369, 1330, 1256, and 1090 cm−1;1H NMR (250 MHz, CDCl3) δ = 10.25 (s, 1 H), 7.24 (t, J = 6.9 Hz, 6 H), 7.12 (d, J = 8.2 Hz, 2 H), 7.03 (t, J = 7.6 Hz, 4 H), 6.33 (dd, J = 16.6, 7.6 Hz, 5 H), 5.06 (s, 1 H), 4.52–4.41 (m, 1 H), 4.38–4.26 (m, 1 H), 2.82 (dd, J = 15.2, 5.5 Hz, 1 H), 2.68 (d, J = 15.1 Hz, 1 H), 1.45 (t, J = 7.2 Hz, 3 H).
Yellow solid, M.P.: 175–177 °C; IR (KBr) ν: 3241, 3178, 2976, 2865, 1655, 1592, 1522, 1496, 1350, 1247, 1176, and 732 cm−1;1H NMR (250 MHz, CDCl3) δ = 10.34 (s, 1 H), 8.27 (s, 1 H), 8.11 (t, J = 8.0 Hz, 2 H), 7.94 (s, 1 H), 7.60 (d, J = 7.8 Hz, 1 H), 7.48 (t, 3 H), 7.12 (d, J = 8.6 Hz, 2 H), 7.03 (d, J = 9.1 Hz, 2 H), 6.54–6.27 (m, 5 H), 5.30 (s, 1 H), 4.61–4.45 (m, 1 H), 4.43–4.28 (m, 1 H), 2.87 (d, J = 6.0 Hz, 2 H), 1.52 (t, J = 7.0 Hz, 3 H).
White solid, M.P.: 221–224 °C; IR (KBr) ν: 3241, 3087, 2973, 2859, 1646, 1602, 1491, 1372, 1251, 1068, and 800 cm−1;1H NMR (250 MHz, CDCl3) δ = 10.23 (s, 1 H), 7.28 (s, 7 H), 7.16 (dd, J = 18.9, 8.6 Hz, 7 H), 6.39 (d, J = 7.4 Hz, 3 H), 6.11 (d, J = 8.2 Hz, 2 H), 5.09 (s, 1 H), 4.53–4.39 (m, 1 H), 4.38–4.26 (m, 1 H), 2.85 (dd, J = 15.1, 5.6 Hz, 1 H), 2.70 (d, J = 15.2 Hz, 1 H), 1.46 (t, J = 7.1 Hz, 3 H).
White solid, M.P.: 200–203 °C; IR (KBr) ν: 3229, 3068, 2979, 1654, 1590, 1487, 1370, 1250,
1176, and 720 cm−1;1H NMR (250 MHz, CDCl3) δ = 10.25 (s, 1 H), 7.25 (q, J = 7.2 Hz, 9 H), 7.16 (t, 3 H), 7.04 (d, J = 8.2 Hz, 2 H), 6.41–6.23 (m, 6 H), 5.05 (s, 1 H), 4.52–4.40 (m, 1 H), 4.39–4.25 (m, 1 H), 2.82 (dd, J = 15.2, 5.4 Hz, 1 H), 2.70 (d, J = 15.3 Hz, 1 H), 1.44 (t, J = 7.1 Hz, 3 H).
MOF-5 is a cubic metal-organic framework compound with the Zn4O(BDC)3 formula, where BDC2− = 1,4-benzodicarboxylate. Since MOF-5 has one of the highest surface-to-volume ratios among metal-organic frameworks with a surface area of 2200 m2/cm3,64 we used it as a support. Also, they need to be functionalized to attach the magnetic Fe3O4 NPs to MOFs, so Fe3O4 NPs were functionalized with citric acid and attached to MOF-563.
In a comparative study, changes in the Fe3O4@MOF-5 chemical structure than Fe3O4@Citrate and MOF-5 were evaluated by FT-IR analysis (Fig. 3). Considering the FT-IR spectrum of Fe3O4@Citrate, the appeared absorption bands at 3443, 1638, 1405 and 646 cm−1 are related to the stretching vibration of O-H, C = O, C-O and Fe-O groups64. As can be seen from the fingerprint pattern of the MOF-5, the broadband at 3600 –3100 cm−1 is attributed to stretching vibrations of the OH groups from adsorbed moisture. The characteristic peaks at 1607, 1505, 1392, 1019, 750, and 552 cm−1 are assigned to the asymmetric and symmetric vibrations of the carboxylate groups, C = C aromatic, C-C, C-H out-of-plan and Zn-O, respectively65. The spectrum of the Fe3O4@MOF-5 nanocomposite demonstrated all aforesaid typical peaks. Therefore, it can be concluded that the magnetic hybrid Fe3O4@MOF-5 has been successfully synthesized.
FT-IR spectra of Fe3O4@MOF, MOF-5, and Fe3O4@Citrate.
The EDX-mapping technique was used to show elements’ chemical nature and spatial distribution in an MOF magnetic hybrid. The results confirmed the presence of Zn, Fe, C, and O elementals in the structure of Fe3O4@MOF-5 (Fig. 4).
EDX analysis of magnetic hybrid Fe3O4@MOF-5.
Also, the mapping images show the uniform distribution of these elements in the magnetic hybrid MOF (Fig. 5).
Mapping images of magnetic hybrid Fe3O4@MOF-5.
The morphology of the pure Fe3O4@Citrate and nanoporous Fe3O4@MOF-5 catalyst was investigated by SEM analysis. As can be observed, the Fe3O4@Citrate core has spherical particles that are almost uniform in size and shape (Fig. 6a-c). In addition, the Fe3O4@MOF-5 nanocomposites have a cubic morphology that is uniformly coated by spherical Fe3O4 nanoparticles (Fig. 6d-i). The obtained morphology is in agreement with the morphology MOF-566.
SEM images of Fe3O4@Citrate and Fe3O4@MOF-5.
The crystalline phases of magnetic hybrid Fe3O4@MOF-5 were studied using the PXRD method. As demonstrated in Fig. 7, the appearing characteristic peaks at 2θ = 6.7, 9.6, and 13.5 are related to nanoporous MOF-5 that agree with (200), (220), and (400) indices according to previous reports67,72,69. Also, the diffraction peaks at 2θ = 35.2, 57.5, and 62 are associated with the Fe3O4 nanoparticles, which correspond to (311), (511), and (440) Miller indices, respectively62.
The XRD pattern of Fe3O4@MOF-5 nanocomposite.
The magnetic properties of Fe3O4@Citrate and Fe3O4@MOF-5 were determined by VSM analysis. From Fig. 8 it is observed that saturation magnetization Fe3O4@Citrate nanoparticles is 60 emu g−1. When Fe3O4@Citrate is integrated with nonmagnetic MOF-5, saturation magnetization is reduced to 43 emu/g. Despite decreasing the magnetic properties of as-prepared Fe3O4@MOF-5, its magnetization value is sufficient because an external magnetic field can easily separate it.
VSM of Fe3O4@Citrate and Fe3O4@MOF-5 hybrid composite.
In a more detailed study, the porosity of the presented Fe3O4@MOF-5 hybrid composite was also determined by nitrogen adsorption-desorption isotherms (Fig. 9). The resulting data of the BET is listed in Table 1. The calculated BET surface area for Fe3O4@MOF-5 is 19.56 m2 g−1 with a total pore volume of 0.13 cm3 g−1 (Fig. 9a), which is much lower than the value reported for MOF-5 (1077 m2g−1)71,72. The intense reduction observed can be attributed to heavier and nonporous Fe3O4 nanoparticles.
N2 adsorption-desorption isotherms of Fe3O4@MOF-5 measured at 77 K.
In addition, the pore size of the Fe3O4@MOF-5 is about 1.22 nm based on the BJH curve (Fig. 9b).
A TGA-DTG analysis was also conducted to evaluate the thermal stability of the porous Fe3O4@MOF-5 catalyst. As depicted in Fig. 10, the TGA-DTG curves show four-step weight loss on heating in the 25 to 1000 °C temperature range under the N2 atmosphere. The first slight weight loss (3%) before 187 °C could be attributed to the physical surface desorption of residual solvent molecules. The second weight loss (12%) between 187 °C and 441 °C is probably due to the decomposition of citrate molecules71,76,73. The third weight loss (6%) between 441 and 585 °C belongs to the decomposition of the organic linker1,60,74. The last weight loss (7%) at 695 °C is ascribed to the sample decomposition. Moreover, about 72% of the initial mass remains at 800 °C.
TGA-DTG analysis of as-synthesized Fe3O4@MOF-5.
The amount of Zn and Fe in the Fe3O4@MOF-5 catalyst is about 772.92 × 10−6 and 6941.1 × 10−6 mmol/g by the ICP analysis, respectively.
The catalytic activity of the magnetic hybrid Fe3O4@MOF-5 nanocatalyst was surveyed in the one-pot synthesis of diverse THPs. To this aim, the conversion of the three-component reaction of ethyl acetoacetate, benzaldehyde, and 4-chloroaniline to the desired THP was picked as the model reaction to find the optimal reaction conditions. The effect of different factors such as solvent, temperature, and amount of catalyst were investigated. Thus, after extensive tests to find the optimal conditions, we concluded that using EtOH as a solvent at reflux temperature with 20 mg of the Fe3O4@MOF-5 nanocatalyst is the best result (Table 2).
According to the results of the experiments, the solvent had a significant effect (entries 1–6). Polar protic solvents (EtOH, H2O, and EtOH: H2O) are more effective than nonpolar and polar aprotic solvents.
It is necessary to cite that the reaction yield in the presence of 20 mg of catalyst is 95%, whereas in the absence of the catalyst, it is much lower. In addition, when the amount of catalyst was increased to 30 and 40 mg, the reaction yield decreased, probably due to the increase of the reverse reaction.
The obtained results encouraged us to investigate the scope of the reaction using different aldehydes and anilines bearing electron-donating and electron‐withdrawing groups in the aryl moiety to study the effectiveness and efficiency of the Fe3O4@MOF-5 catalyst. As shown in Table 3, the reactions were accomplished under ideal conditions (refluxing ethanol and 20 mg o catalyst) within 30–180 min, and the desired THPs 4(a-l) were obtained in excellent yields (75–95%).
No electronic effects were observed regarding different substituents at the aromatic aldehyde and anilines. Electron-withdrawing substituents (NO2, Cl, Br, and I) and electron‐donating substituents (Me, iPr) are well tolerated in this reaction.
Both the organic linker and the metal nodes can be catalyzed by the reaction as organocatalytic and Lewis acid sites, respectively. Fe3O4 nanoparticles also increase the catalytic activity and facilitate the separation of the catalyst.
Initially, the activated β-ketoester condenses with aniline, forming an intermediate I; on the other hand, the second aniline condenses with activated aldehyde, forming an intermediate II. Condensation steps are facilitated by the addition of the Fe3O4@MOF-5 catalyst, which increases the reaction rate. Then, both formed intermediates react in an intermolecular Michael-type reaction to give intermediate III, which reacts with the second aldehyde to generate an intermediate IV. Afterward, a tautomerization provides an intermediate V to mediate the final ring‐closure reaction by a domino intermolecular Michael/tautomerization sequence, affording the desired cyclic products 4(a-l) with excellent yield (Fig. 11).
Proposed mechanistic pathway for synthesizing THPs 4(a-l) by Fe3O4@MOF-5.
Recently, the recycling of catalysts has attracted significant attention because this process requires less energy75,76,77. The recycling and reuse studies were conducted using a model reaction under optimal conditions. As demonstrated in Fig. 12, the magnetic hybrid Fe3O4@MOF-5 catalyst could be effectively reused for five continuous cycles while maintaining the catalytic activity over 80%.
Reusability of the magnetic hybrid Fe3O4@MOF-5 catalyst.
Furthermore, the stability of the catalyst was studied by the FT-IR and XRD analyses. As shown in Fig. 13, the FT-IR spectrum of the recovered catalyst is like the fresh catalyst, showing the stability of the catalyst during the reaction. Moreover, the XRD patterns of the recovered catalyst demonstrate that the crystalline phase is preserved (Fig. 14).
FT-IR spectra of the fresh and recovery catalyst.
XRD patterns of the fresh and recovery catalyst.
The efficiency of as-prepared the magnetic hybrid Fe3O4@MOF-5 catalyst was also compared with some previously reported catalysts for the synthesis of THPs. As shown in Table 4, the proposed catalyst used in this paper for the synthesis of THPs is much better than the other catalysts. It is accessible, applicable, and reusable with a very short reaction time, high yields, and easy work-up.
An efficient magnetic hybrid Fe3O4@MOF-5 nanocatalyst was synthesized as the catalytically active material by mixing at room temperature to prepare THPs. Different techniques comprehensively studied the structure of the introduced magnetic hybrid framework. According to the BET data, the surface area for Fe3O4@MOF-5 is much lower than MOF-5 due to the presence of nonporous Fe3O4 cores. Likewise, Fe3O4@MOF-5 was used to catalyze the one-pot synthesis of the functionalized THPs exhibiting potential biological activities. The functionalized THPs were synthesized in mild conditions with high efficiency without requiring purification of the intermediates. The magnetic hybrid Fe3O4@MOF-5 catalyst can be reutilized in subsequent reactions without a significant reduction in yield.
All data generated or analysed during this study are included in this published article (and its Supplementary Information files).
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Department of Organic Chemistry, Faculty of Chemistry and Petroleum Sciences, Bu-Ali Sina University, Hamedan, Iran
Hosein Khodakarami, Davood Habibi & Masoumeh Beiranvand
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Hosein Khodakarami did the lab experiment. Davood Habibi wrote the main manuscript text. Masoumeh Beiranvand is a co-author.
The authors declare no competing interests.
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Khodakarami, H., Habibi, D. & Beiranvand, M. Room-temperature synthesis of Fe3O4@MOF-5 magnetic hybrid as an efficient catalyst for the one-pot green synthesis of tetrahydropyridines. Sci Rep 14, 31510 (2024). https://doi.org/10.1038/s41598-024-83092-1
DOI: https://doi.org/10.1038/s41598-024-83092-1
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