3.1. Structural Analysis of the KCC-1@NH2 as catalyst
Scheme 2 demonstrates the sequential process for immobilizing MnFe2O4 with a Polyhedral Oligomeric Silsesquioxanes-based inorganic-organic hybrid. Initially, the MNPs (MnFe2O4) were synthesized using a co-precipitation method in a basic solution. It is known that the surface of the MNPs can be functionalized using various surface modifiers. Subsequently, octakis (3 chloropropyl) octasilsesquioxane was synthesized through hydrolyzing 3-chloropropyltrimethoxysilane in an acidic environment. This was followed by reacting ClPOSS with 3-aminopropyltriethoxysilane to produce APTPOSS. The final step involved treating MnFe2O4 with an aminopropyltriethoxysilane derivative and TEOS. Characterization of MnFe2O4@SiO2/APTPOSS was conducted using TEM, SEM, VSM, Fourier transform infrared (FT-IR) analyses, TGA, and XRD.
Figure 2 illustrates the X-ray diffraction patterns of the synthesized APTPOSS, MnFe2O4, and MnFe2O4@SiO2/APTPOSS. The XRD patterns of the MnFe2O4 nanoparticles with and without POSS show consistent peaks, indicating that the crystalline spinel structure of MnFe2O4 is preserved during the APTPOSS coating process. Specifically, the X-ray diffraction (XRD) patterns reveal six characteristic peaks corresponding to a cubic phase of manganese iron oxide at 2θ values of 30.18°, 35.97°, 43.22°, 53.92°, 57.42°, and 63.03° for both MnFe2O4 and MnFe2O4@SiO2/APTPOSS in Figs. 2(b) and (c). Additionally, the broad peaks observed in Figs. 2(a) and (c) at 2θ values ranging from 16° to 28° are attributed to the presence of an amorphous Si-O bond.
The Fourier transform infrared spectra presented in Fig. 3 illustrate the characteristic features of ClPOSS, MnFe2O4, APTPOSS, and MnFe2O4@SiO2/APTPOSS. Notable features of the ClPOSS molecules include peaks at 2955 cm− 1 (C-H bond stretching), 1102 cm− 1 (Si-O-Si band stretching), and 810 cm− 1 (C-Cl bond stretching). Figures 3(b) and (d) display common features such as peaks near 489, 576, 3437, and 1627 cm− 1 representing the stretching vibrations of Mn-O bonds, Fe-O bonds, O-H bonds, and O-H bond bending. The successful incorporation of Silica/ POSS-based inorganic-organic hybrid on the surface of MnFe2O4 is indicated by broad peaks around 1000–1160 cm− 1 associated with Si-O bond stretching vibrations. Additionally, the presence of C-H bond stretching at 2915–2965 cm− 1 and C-H bond bending vibration at approximately 1470 cm− 1 is observed in Figs. 3(c) and (d).
Figure 4 illustrates the TGA analysis of MnFe2O4@SiO2/APTPOSS. It demonstrates a decline in weight of approximately 28% between 200 and 500°C, which can associated with the degradation of the organic components. Consequently, the catalyst exhibits stability up to less than 200°C, affirming its suitability for application in organic reactions at temperatures around 80°C.
The FE-SEM images in Fig. 5 compare the magnetic nanoparticles. As depicted in Fig. 5(a), the uncoated MnFe2O4 seeds have a size of about 38 nm and display acceptable uniformity. The SEM image in Fig. 5(b) indicates the MnFe2O4 coated with SiO2/POSS have preserved their morphology, but this does not apply to particle size and aggregation. The observed agglomeration and alterations in particle size lead to the conclusion that the nanocatalyst has been effectively synthesized.
The magnetic characteristics of MnFe2O4 and MNPs@SiO2/APTPOSS were examined using a vibrating sample magnetometer at room temperature (Fig. 6). The magnetization hysteresis loops for two samples are S-like and show superparamagnetic properties for them. The saturation magnetization (Ms) is 32.5 Oe for MnFe2O4 and 21.8 Oe for MNPs@SiO2/APTPOSS. The magnetic saturation value decreases by approximately 10.7 emug− 1 following the coating process, possibly attributed to the enhanced surface area facilitating successful nanoparticle coating with SiO2 and APTPOSS.
Different solvents were used to examine the reactivity of isatin derivatives, 1,3-dicarbonyl compounds (2a-d), and malononitrile 1. A good yield was achieved when the reaction was conducted in ethanol (Fig. 7). Subsequently, the reaction was carried out using different catalysts and without a catalyst. It was found that MnFe2O4@SiO2/APTPOSS was the most effective catalyst, resulting in a 96% yield of product 3a. The impact of catalyst loading was then investigated using 5 mg and 15 mg of MnFe2O4@SiO2/APTPOSS respectively. The experimental findings indicated that the utilization of 10 mg of MnFe2O4@SiO2/APTPOSS as a catalyst yielded the most favorable results. Any deviation from this specific catalyst loading, whether it was decreased or increased, led to a decrease in the overall product yield.
Various reaction conditions were explored in the experimental model involving isatin, dimedone, and malononitrile to achieve optimal production yield and showcase the effectiveness of the catalyst. The results indicated that conducting the reaction at 80 ̊C with 10 mg of the catalyst (MnFe2O4@SiO2/APTPOSS) in ethanol as the solvent leads to an enhanced production yield (Table 1, entry 5).
Table 1
Reported catalytic system for the formation of spirooxindole derivatives a
Entry | Catalyst | Solvent/ Temp. (◦C) | Time (min) | Yieldb(%) | Ref. |
1 | - | EtOH / 80 | 90 | 30 | This work |
2 | MnFe2O4@SiO2 (20 mg) | EtOH / 80 | 40 | 60 | This work |
3 | MnFe2O4@SiO2/APTPOSS (10 mg) | EtOH / 50 | 20 | 75 | This work |
4 | MnFe2O4@SiO2/APTPOSS (10 mg) | Solvent-free / 80 | 15 | 80 | This work |
5 | MnFe2O4@SiO2/APTPOSS (10 mg) | EtOH / 80 | 10 | 97 | This work |
6 | MnFe2O4@SiO2/APTPOSS (10 mg) | EtOH / 90 | reflux | 94 | This work |
7 | MnFe2O4@SiO2/APTPOSS (10 mg) | H2O / 80 | 30 | 70 | This work |
8 | Amino-appended b-cyclodextrin (5 mol%) | H2O / RT | 7h | 91 | 35 |
9 | THAM (30 mol%) | EtOH / RT | 4h | 94 | 36 |
10 | TiO2-CNTs (24 mg) | H2O / RT | 6h | 94 | 37 |
a Reaction conditions: isatin (1mmol), malononitrile (1 mmol), and 1,3 dicarbonyl compounds (1 mmol) in presence MnFe2O4@SiO2/APTPOSS (10 mg) in EtOH under reflux conditions |
b Isolated yields |
Table 2 displays the results obtained from the synthesis of various spiro[indoline3,4'pyrano[3,2-c]chromene], spiro[indoline-3,5'-pyrano[2,3-d]pyrimidine], and spiro[chromene-4,3'-indolines] compounds. The reactions took place in ethanol as a solvent at a temperature of 80 ̊C, using isatin derivatives and different 1,3-dicarbonyls.
The catalyst's reusability is a well-established property of significant importance. Our study focused on the utilization of MNPs@SiO2/APTPOSS as a catalyst in optimized conditions. After the reaction, the catalyst underwent washing with acetone and MeOH followed by drying in an oven at 65°C for 70 minutes. It was then directly used with new substrates under the same conditions. The results demonstrate that the catalyst was effectively employed in six consecutive reaction cycles without a noticeable decrease in product yield, showing initial yields of 97% and 90% by the sixth cycle (Fig. 8).