In this project, the nanocatalyst was prepared through five steps as shown in Scheme 1. In the first step, the Fe3O4@SiO2 magnetic nanoparticles were synthesized using the Stöber sol–gel method [51]. Then, epichlorohydrin as a spacer was chemically grafted on the surface hydroxyl groups of Fe3O4@SiO2 to obtain Fe3O4@SiO2/EP. In the following step, diethylene triamine (DETA) was grafted on the surface of the functionalized Fe3O4@SiO2/EP to produce Fe3O4@SiO2/EP-DETA. Thereafter, again epichlorohydrin was added to the functionalized Fe3O4@SiO2/EP-DETA. In the final stage, the prepared Fe3O4@SiO2.DIL was designed as a supported dicationic ionic liquid-bearing imidazolium ring. Chemical analysis of prepared Fe3O4@SiO2.DIL was performed by using various standard physicochemical techniques such as FT-IR, XRD, TEM, FE-SEM, EDX, and TGA.
Figure 1 shows the FT-IR spectra of Fe3O4@SiO2, and Fe3O4@SiO2.DIL. For two samples, the FTIR analysis exhibited the strong peak at about 571 cm− 1 is corresponded to the stretching vibration of Fe-O bond. The characteristic peak at 3350 cm− 1 is ascribed to the stretching vibration of hydroxyl groups on the surface of Fe3O4 which affirms the nanostructure of Fe3O4 is preserved throughout the process. The peaks observed at 953, and 1098 cm− 1 are belong to symmetric and asymmetric vibrations of the Si–O–Si bond which indicated silica layer had been successfully loaded on the surface of Fe3O4. The FTIR of the prepared Fe3O4@SiO2/EP-DETA can be seen in the Fig. 1b, the new signals appeared below 3000 cm− 1 were attributed to aliphatic C–H bonds of grafted on the surface of silica-coated magnetic nanoparticles. The band at 1356 cm− 1 can be attributed to the C-H bending whereas aromatic vibration bands of imidazole are observed at 1456 cm− 1. The band appeared at 1616 cm− 1 is belong to C = N stretching vibration in imidazolium rings. Stretching mode of O-H and -NH2 can be seen around 3446 cm− 1.
The TGA curves of Fe3O4@SiO2.DIL are shown in Fig. 2. The thermal stability of the materials was measured in a temperature range of 25 to 800°C under N2 atmosphere at heating rate of 10°C per minute. In the first step, the weight loss takes place up to 150°C indicates the removal of water. The weight losses of about 30%, around 200–800°C was due to the decomposition and elimination of grafted ionic liquid on the surface of Fe3O4@SiO2.
To determine the crystal structure of the Fe3O4@SiO2 and Fe3O4@SiO2.DIL nanocatalys X-ray diffraction (XRD) was applied (Fig. 3). The peaks at 2θ = 30.1°, 35.4°, 43.2°, 53.5°, 56.9° and 62.5° are attributed to the corresponding planes of cubic spinel Fe3O4 (JCPDS no. 01–075‐0449). A broad peak at 23° could be assigned to the existence of amorphous silica loaded on Fe3O4 (curve a). The presence of similar peaks in the spectra of Fe3O4@SiO2.DIL (curve b) demonstrates during surface functionalization the crystalline structure of the nanoparticles stays unchanged.
The surface morphology and particle size of Fe3O4@SiO2.DIL were studied using SEM and TEM analyses (Fig. 4). As shown SEM image, spherical morphology with an average diameter of about 20 nm for catalyst was observed (Fig. 4a). Also, from the resulting TEM image, the core–shell nanoparticles with fairly uniform in shape and size was found (Fig. 4b).
The elemental analysis of Fe3O4@SiO2.DIL was performed using Energy dispersive X-ray analysis (EDX) that is shown in Fig. 5. The presence of peaks related to expected elements of Fe, N, Si, Cl, C, and O in the structure confirmed the successful synthesis of Fe3O4@SiO2.DIL.
The elemental mapping images of Fe3O4@SiO2.DIL are given in Fig. 6. The images confirm a uniform distribution of Fe, Si, N, O throughout the structure.
The magnetic properties were measured using vibrating sample magnetometry and illustrated in Fig. 7. The saturation magnetizations values of the Fe3O4@SiO2 and Fe3O4@SiO2.DIL are found to be 52.8 and 28.8 emu g− 1, respectively. These results show that when Fe3O4 coated with SiO2 and ionic liquid the magnetization of decreases considerably. Nevertheless, the catalyst, is still super-paramagnetic and it’s possible to separate and reuse it from solution using an external magnetic field.
After characterization of novel magnetic catalyst, the catalytic activity of Fe3O4@SiO2.DIL was studied for the synthesis of 1,8-dioxodecahydroacridines.
Initially, to optimize condition reaction for the synthesis of 1,8-dioxo-decahydroacridines, the reaction of benzaldehyde (1mmol), dimedone (2 mmol), and ammonium acetate (1 mmol) was chosen as a model reaction and the reaction was carried out in different conditions (Scheme 2). The results were sumerized in Table 1. First, the amount of catalyst was surveyed. Increasing the amount of nanocatalyst to 0.05 g caused an increase in the yield of the product and the best yield was obtained when 0.05 g of catalyst was used (Table 1, Entries 1–6). The results show that by increasing the amount of the catalyst more than 0.05 g no improvement was detected in the yield of reaction but less amount of Fe3O4@SiO2.DIL led to lower reaction yields at the same reaction conditions. As complementary study of reaction optimization, the reaction was examined in various solvents such as ethanol, dichloromethane, water, acetonitrile, and solvent free condition (Table 1, Entries 5, 7–10). As evident from Table 1, solvent free-conditions is the best of choice for this process and target product was obtained in excellent yield (96%) (Table 1, Entry 5). In the next step, to determine the influence of temperature, the model reaction was conducted in various temperatures (Table 1, Entries 11–15). At temperatures less than 80 °C, the reaction was very slow and did not progress to completion. However, at 100 °C the best yield was obtained in a reaction time of 25 min. Finally, it is found that applying 0.05 g Fe3O4@SiO2.DIL under solvent-free condition at 100°C is the best condition to push the reaction of synthesis of 1,8-dioxo-decahydroacridines forward (Table 1, Entry 5).
Table 1
Catalytic performance of the Fe3O4@SiO2.DIL in the synthesis of 1,8-dioxo-decahydroacridines
Entry | Amount of catalyst (gr) | Solvent | Time (min) | Temperature (°C) | Yield (%) |
1 | - | Solvent free | 180 | 100 | - |
2 | 0.01 | Solvent free | 90 | 100 | 65 |
3 | 0.02 | Solvent free | 75 | 100 | 73 |
4 | 0.03 | Solvent free | 40 | 100 | 86 |
5 | 0.05 | Solvent free | 25 | 100 | 96 |
6 | 0.07 | Solvent free | 25 | 100 | 96 |
7 | 0.05 | Ethanol | 120 | 80 | 70 |
8 | 0.05 | Dichloromethane | 360 | 40 | 55 |
9 | 0.05 | Water | 100 | 100 | 85 |
10 | 0.05 | Acetonitrile | 240 | 80 | 80 |
11 | 0.05 | Solvent free | 85 | 60 | 62 |
12 | 0.05 | Solvent free | 70 | 70 | 75 |
13 | 0.05 | Solvent free | 40 | 80 | 83 |
14 | 0.05 | Solvent free | 35 | 90 | 93 |
15 | 0.05 | Solvent free | 25 | 110 | 96 |
In order to probe the scope and generality of the process, three component coupling reaction between various aromatic aldehydes, ammonium acetate, and dimedone in the presence of Fe3O4@SiO2.DIL as catalyst under the solvent-free condition was investigated. The results summarized in Table 2 clearly indicate the reaction of aromatic aldehydes bearing electron withdrawing groups as well as electron donating groups are equally facile for production the acridine derivatives in high yields. However, the results indicated that the reaction time for aromatic aldehydes bearing substituent at para position is shorter and higher yields of products were obtained with these substrates. The products obtained were fully characterized by spectroscopic methods such as IR and 1H NMR.
To manifest the advantages of this catalytic method, our catalytic system was compared with a few previous catalytic systems, and the comparison list was summarized in Table 3. The results show that our method is comparable or superior to some previous reports and has, short reaction time, high yields, mild reaction condition, uses a recoverable and reusable catalyst, and eliminates the usage of organic solvents that go against green chemistry.
Table 3
Comparison of the efficiencies of Fe3O4@SiO2.DIL with some previous catalytic systems in the synthesis of 1,8-dioxo-decahydroacridines
Entry | Catalyst | Reaction Condition | Time | Yield | Ref |
1 | SMSNP-CA | Reflux in EtOH | 6h | 91 | 52 |
2 | SFexZr | Reflux in CH3CN | 150 min | 80.5 | 53 |
3 | Silica bonded N-propylsulfamic acid | Ethanol/Reflux | 2h | 86 | 54 |
4 | CuBr | CH3CN, 82°C | 18h | 62 | 55 |
5 | CeCl3.7H2O | [bmim]+[BF4]−, 100°C | 3,18h | 88 | 56 |
6 | Fe3O4@SiO2.DIL | Solvent free, 100 °C | 25 min | 96 | This work |
Recyclability and reusability are of important and significant characteristics of a heterogeneous catalyst that should be considered. For this aim, the reusability of the catalyst using a model reaction under optimized conditions was assessed. After completion of the reaction, the catalyst was removed by a permanent magnet and washed with EtOH and distilled water, then dried in the oven at 80°C. The recovered catalyst was used in the next run under the same conditions as the first one. As can be seen in Fig. 8, even after the catalyst had been reused five times, a satisfactory yield was obtained. The FE-SEM image of the recovered catalyst after the 5th run (Fig. 9), did not show remarkable changes than the fresh catalyst and the nanoparticles are still approximately spherical.