2.1. The preparation and characterization of the catalyst
The catalyst was prepared according to Scheme 1. The nano magnetite (Fe3O4 NPs) was synthesized according to the literature.[13] The agar polysaccharide was immobilized around the prepared Fe3O4 NPs. Then, copper was incorporated successfully among the agar linker at the same time. The catalyst structure was characterized using different analyses, including FT-IR, XRD, SEM, TEM, VSM, EDX, and TGA.
In Fig. 2, the FT‐IR spectrum demonstrated the absorptions of Agar (1070 cm-1) appear along with a peak at 582 cm−1 which corresponds to the stretching vibration band of the Fe-O group in Fe3O4@Agar. This indicates that magnetic Fe3O4 NPs were coated by Agar. Possible bindings of copper with OH group in Fe3O4@Agar-Cu NPs have decreased the combined intensity of hydroxyl peaks at 3356 cm−1. Splitting of the bending bands of hydroxyl at 1614 cm−1 and 1352 cm−1 also indicate the bonding of Cu with OH moiety. The variations in the region of 1350 cm−1 and 1000 cm−1 can be attributed to the perturbation in C-O vibrations induced by Agar-Cu complexation (Fig. 2).
The XRD patterns help define the crystal structures of Fe3O4@Agar NPs and Fe3O4@Agar-Cu NPs. Sharp peaks vouch for the excellent crystallinity of the prepared samples (Fig. 3). For Fe3O4@Agar NPs, the outcome is in agreement with the standard patterns of inverse cubic spinel magnetite (Fe3O4) crystal structure, showing six diffraction peaks at 2θ about 35.5◦, 43.3◦, 56.9◦, 62.6◦, 62.8◦, and 74.1◦ marked by their corresponding indices (3 1 1), (4 0 0), (3 3 3), (4 0 4), (4 0 4), and (5 3 3), respectively. The small and weak broad bands in the span of 21◦–28◦ indicate the existence of Agar. Diffraction patterns of the Fe3O4@Agar-Cu NPs exhibit three additional peaks at 2θ about 43.3◦, 50.2◦, and 74.1◦; corresponding to (1 1 1), (2 0 0), and (2 0 2) planes of face-centered cubic (fcc) copper crystal structure. No impurities in the XRD patterns infer the formation of net Fe3O4 and Cu nanoparticles.
SEM imaging of the nanoparticles shows nanometer-sized particles of less than 25 nm in diameter. Fig. 4 shows the morphology of the Fe3O4@Agar-Cu nanoparticles with a core–shell structure and spherical form.
TEM image of the catalyst has been shown in Fig. 5. The spherical shape of each nanoparticle corresponded to the core of the catalyst, similar to SEM image which can be observed at a scale of less than 25 nm. Also, TEM images show that magnetic nanoparticles of Fe3O4 have been encapsulated by the biopolymeric network of Agar.
Magnetic hysteresis measurements of Fe3O4@Agar (Fig. 6a) and Fe3O4@Agar-Cu (Fig. 6b) NPs were created in the limited area −15000 to 15000 Oe using VSM. As shown in Fig. 6, the saturation magnetization of Fe3O4@Agar-Cu NPs is anent 35 emu g−1, lower than that of Fe3O4@Agar (33 emu g−1). The magnetization curve displays that the Fe3O4@Agar-Cu NPs have paramagnetic attributes in which the nanoparticles can be easily separated from the reaction melange using an external magnet.
To specify the elemental composition of Fe3O4@Agar-Cu NPs, EDX analysis was fulfilled (Fig. 7). The EDX pattern supports excellent dispersion of Fe3O4@Agar-Cu NPs. Chemical characterization of the nanoparticles showed that they were composed of iron, carbon, oxygen, and copper elements, and this analysis detect the presence of 7.21 mol% Cu in Fe3O4@Agar-Cu NPs.
TGA of Fe3O4@Agar-Cu NPs was manipulated in the range of 20–550 ºC (Fig .8). The first mass loss of Fe3O4@Agar-Cu NPs at below 190 ºC is due to the removal of physically adsorbed water. The second and the major weight loss of Fe3O4@Agar-Cu NPs in the range of 200oC to 380oC is ascribed to Agar as the organic moiety.
2.2. Catalytic application of Fe3O4@Agar-Cu catalyst
After the characterization of the catalyst structure, the new prepared catalyst efficiency has been investigated in the Buchwald-Hartwig reaction. Initially, in order to optimize the model reaction conditions of iodobenzene with aniline, some parameters, including solvents, amounts of catalyst, temperature and the time of reaction were scrutinized thoroughly. (Table 1) The impact of the catalyst was inspected with different amounts of Fe3O4@Agar-Cu. The reaction did not proceed in the absence of the catalyst even after 15 h. The effect of solvents was also examined by polar and nonpolar solvents, including DMSO, DMF, Toluene and water. Fortunately, a high yield was observed in H2O as a green and sustainable solvent. The effect of time duration and temperature on the model reaction was also evaluated, and it was found the best time and temperature are considered as 12 h in 100 ºC with excellent yield.
Next, in order to expand the scope of this reaction, various derivatives of C-N bond cross-coupling reactions have been represented in Table 2. Considering both primary and secondary arylamines was the wisdom of this catalyst that is not previously usual. There are not any significant differences in reaction yields considering various amines (primary or secondary) bearing electron-donating or electron-with drawing groups, and all related products were obtained in good to excellent yields. Also, for improving the validity of the synthesis field and larger scale of the reaction, coumarins (entry 16 and 17) were considered as complicated amine structures. They were conducted in Buchwald-Hartwig amination reaction using Fe3O4@Agar-Cu NPs as the catalyst. The results showed magnificent success and remarkable yields based on confirmation by Mass spectroscopy, 1H, and 13C NMR analyses.
As it is presented in Table 3, the comparison of this catalyst with some recently published catalysts has been performed. Various conditions have been applied, but the use of green and nontoxic reaction conditions has not been reported. Employing reachable and no harmful materials, achieving high yields of product, and mild reaction condition is the art of this study.
To check the catalyst's reusability on the model reaction, the nanocatalyst was removed easily from the mixture after the termination of the reaction, washed with ethanol and deionized water successively, and dried in vacuum oven. Soon afterward, the catalyst applied directly for the next run. Providentially, this catalyst was reused for five times and shown good revenue in the reaction process by the use of ICP analyses without significant leaching of Cu NPs (Fig. 9).
The possible mechanism of C-N cross-coupling represents in Fig. 10. The proceed mechanism is the same as C-C cross-coupling reactions.[13] Including oxidative addition of the aryl halide to a Cu0 nanoparticle, the addition of the amine to the oxidative addition complex, deprotonation followed by reductive elimination of the species intermediate releases the desired product amines and complete the reaction cycles.