3.1 Characterization of the Cu/ZnFe2O4 catalyst
The Cu/ZnFe2O4 catalyst was synthesized by loading Cu species on zinc ferrite particles through ultrasound assisted co-precipitation (Supplementary data). The structure of the fabricated zinc ferrite and Cu/ZnFe2O4 materials were characterized by FT-IR, XRD, TGA, EDS, TEM and SEM.
As shown in Fig. 1 (a), the absorption peak at 556 cm−1 is attributed to the Fe-O vibration from the magnetic basement [23]. The peak near 3440 cm-1 is related to the O-H stretching vibration of coordinated the hydroxyl groups and water molecules in the layers. The peak around 1630-1660 and 1383 cm-1 maybe caused by another absorption band corresponding to the water deformation [24]. The absorption peak at 618 cm-1 indicated the existence of copper nanoparticles [25]. The TGA curves of Cu/ZnFe2O4 and ZnFe2O4 were shown in Fig. 1 (b). The Cu/ZnFe2O4 and ZnFe2O4 showed a mass loss of 7%, which was attributed to the loss of adsorbed water on the sample surface. Furthermore, the mass loss of Cu/ZnFe2O4 and ZnFe2O4 was 2%, which was due to the loss of crystalline water bound within the sample. The mass of Cu/ZnFe2O4 catalyst does not decreased significantly at 700 oC, which indicates that the Cu/ZnFe2O4 catalyst had good thermal stability. The Fig. 1 (c) showed the XRD patterns of Cu/ZnFe2O4, Cu (PDF#03–1015) and ZnFe2O4 (PDF#01–1108). The position and relative intensity of the position and relative intensity of the peaks in the Cu/ZnFe2O4 of XRD pattern were consistent with the standard XRD pattern of ZnFe2O4 and Cu, indicating that the crystal structure of ZnFe2O4 was maintained during Cu loading processes. As shown in Fig. 1 (d) the elemental composition was determined by EDS analysis and the results indicated the existence of O, Cu, Zn and Fe elements and further confirmed the structure of Cu/ZnFe2O4. The SEM and TEM images of the obtained catalysts were showed in Fig. 1 (e) (f) (g) (h). As shown, Cu/ZnFe2O4 and ZnFe2O4 were still irregular shape and nearly uniform size. The surface of ZnFe2O4 is changed after the introduction of Cu nanoparticles, and needle-like Cu single crystals are loaded on the original smooth surface.
3.2 Degradation of aromatic nitro compounds with Cu/ZnFe2O4
After investigation of the morphology of Cu/ZnFe2O4, we continue to study the catalytic performance of Cu/ZnFe2O4. The catalytic activity of Cu/ZnFe2O4 in the reduction process of p-nitrophenol to p-aminophenol was investigated with NaBH4 as the reducing agent. Add p-nitrophenol (2 ml, 400 ppm), NaBH4 and Cu/ZnFe2O4 (3mg, 5mg, 7mg, 10 mg) to the flask, and measure the data with an ultraviolet-visible spectrophotometer with a wavelength of 200-600 nm. The decrease in peak intensity at 400 nm is due to the reduction of p-nitrophenol. At this time, the stability of the peak intensity also indicates the proceeding of the reaction. Fig. 2 (a) depicted the UV spectra of the reduction of p-nitrophenol in terms of different reaction time. Fig. 2 (b) and (c) showed the kinetic experiments of the reduction reactions. As shown in formula (1), Ct is concentration of p-nitrophenol with the passage of time, and Co is concentration of p-acylphenol at the beginning of the reaction.
Ln(Ct/Co) = kt (1)
When the catalyst dosage is increased from 3 mg to 10 mg, the k value increased from 1.31×10-2/s to 4.02×10-2/s, the time required for the complete reduction of p-nitrophenol also decreased from 330 s to 120 s. As the concentration increases, the reaction rate also increases. The rate of reaction depends on concentration of p-nitrophenol, which conforms to the characteristics of a quasi-first order reaction. These studies demonstrated that in the reduction of p-nitrophenol, the Cu/ZnFe2O4 exhibited good catalytic activity.
Subsequently, in order to further explore the application of this catalyst in the reduction of aromatic nitro compounds, an array of substrate scope experiments was carried out under the optimum reaction conditions. As shown in Fig. 2 (d), all of the investigated nitro aromatic compounds are reduced to the corresponding amino compounds with high yields (Supplementary data). A series of functional groups were well tolerated including halogen, hydroxyl, carboxyl, alkoxy groups. Notably, both of the two nitro groups of 1,3-dinitrobenzene substrate could be reduced to amino groups in this transformation in 98% yield.
3.3 Cu/ZnFe2O4 catalyzed C-N bond formation reactions
3.3.1 Optimization of C-N bond formation reaction conditions
To further estimate the application possibility of Cu/ZnFe2O4 in catalysis, the C-N bond formation reactions were further investigated with iodobenzene (3a) and imidazole (4a) as the model substrates. Table 1 shows the reaction condition optimization studies. Generally, the nitrogen-based ligands play significant roles in the interaction with metal centers and affect the efficiency of the catalyst system in the C-N bond formation reactions [26]. Therefore, we firstly screened some commercially available ligands on the reactivity, including 1,10-phenanthroline, p-toluenethiol, thioanisole, and proline (entries 1-4). To our delight, we found that the reaction proceeded well without any extra ligand and the yield reached 94% with Cs2CO3 as the base in ethanol at 120 oC (entry 5). To our disappointment, further screening the base failed to elevate the reaction yield, including K2CO3, NaOH, and KOH (entries 6-8). Increasing the catalyst dosage (entry 9), elevating the reaction temperature (entry 10), and prolonging the reaction time (entry 11) also failed to increase the yield. What’s more, decreasing the temperature (entry 12) or shortening the reaction time (entry 13) the reactivity deteriorated dramatically. In addition, altering the ratio of 3a and 4a had almost no influence on the reactivity (entries 14-15). Hence the optimal conditions for this reaction were as follows: 100 mg of Cu/ZnFe2O4, 120 oC, 1.5 equiv. iodobenzene and 2.0 equiv. of Cs2CO3 were added to ethanol for 12 h.
3.3.2 Substrate scope and recyclability investigations
Under the optimal conditions, the substrate scope of aryl halides and nucleophilic reagents were investigated and the results were shown in Table 2. In general, various aryl halides were transformed into the coupling products with 50-94% yields. The functional groups including methyl, methoxy, nitro, cyanide, bromine and trifluoromethyl were tolerated in this reaction. Then the steric effects were investigated via the exploration of substrates bearing methyl group at different position of aryl iodides (entries 2-4). Results demonstrated that the reaction proceeded mildly and delivered the products with high yields. Subsequently, the electronic effects of p-phenyl substituents were conducted (entries 5-9). Substrates bearing both electron-donating groups and electron-withdrawing groups could be converted to the products in 83-89% yield. It is noteworthy that other nucleophiles such as (benzo)imidazoles, benzotriazoles, pyrazols, and pyrroles were also tolerated in this reaction resulting in the corresponding products with high yields (entries 10-15). In addition, transformations with arylamines and cyclic amines, such as 1-phenylpiperazine, morpholine, and piperidine were also viable and furnished the products in 60-94% yields (entries 14-20). In order to further exploration of the substrate scope, several aryl bromides were also subjected to this transformation and all of the reactions were happened stably in 50-74% yields (entries 21-27). Disappointingly, reaction with aryl chloride as the coupling agent generated the product with only 10% of yield (entry 28).
Since one of the key properties of our catalysts is the magnetic response, the recovery and reuse of catalysts were worth focusing on. We are pleased that the catalysts could be quickly and easily recovered from the product via an external magnetic field. The results show that the catalyst could be recovered quickly and easily in the reaction system and used continuously for 6 times without obvious loss of activity (Figure 3).