Development of CuMnxOy (x = 2, and y = 4)-GO heterostructure for the synthesis of pyranoquinoline derivatives

The pyranoquinoline derivatives are synthetically important due to their biological properties. In this research, these derivatives were produced through an environmentally friendly method. This method includes the use of CuMnxOy (x = 2, and y = 4)-GO as a nanocatalyst, which is easy to produce, has excellent performance, cost-effectiveness, and recyclability among its features, and also the use of water as a green solvent. Pyranoquinolines through the one-pot, the multi-component reaction between different derivatives of aryl glyoxal, ethyl cyanoacetate, and 4-hydroxyquinoline-2(1H)-one were synthesized using nanocatalyst, K2CO3, and H2O. Also, the structure of the CuMnxOy-GO nanocatalyst was evaluated and confirmed via different analyses. The distinguishing features of this work compared to previous works are easy workup, recyclability of nanocatalyst, facile synthesis process, and provide high yields of products.

Preparation of CuMn x O y (x = 2, and y = 4)-GO nanocatalyst. GO was produced according to standard Hummer's procedure and then it was delaminated under sonication in the existence of polyethylene glycol ) PEG) as a stabilizer 17,18 (Support Information in Text S1). Firstly, GO (50 mg) and CuMn x O y (50 mg) was added in two different flasks in 25 mL of deionized water and placed under ultrasonic conditions for 1.5 h. Then, they were added to a 100 mL flask and subjected to ultrasonication for 50 min. Next, the reaction mixture was placed under stirrer conditions for 24 h for better placement of CuMn x O y nanoparticles between GO sheets. After that, the resulting mixture was filtered and rinsed with water, and dried at 120 °C. Lastly, the CuMn x O y -GO nanoccatalyst was obtained.

Results and discussion
Based on our research in the development of nanostructures [26][27][28][29][30][31] . Many reports recorded on the preparation of pyranoquinolines using different catalysts 10,[32][33][34][35][36][37][38][39] . In this study, the CuMn x O y -GO nanocatalyst was produced through a new strategy (Fig. 2). In this nanocatalyst, Cu and Mn nanoparticles have been grown on GO sheets. GO can be a good choice as a substrate due to its high electronic properties, easy preparation method, and large surface area for placing copper and manganese sites 40 . Then, CuMn x O y -GO was checked and confirmed through various methods. In the next step, this nanocatalyst was used in the synthesis and identification of  Characterization. The specifications of the devices used are explained in Text S2 of Support Information.
The FT-IR analysis was performed to investigate the intermolecular bonds and identify the type of functional groups present in the synthesized nanocatalyst (Fig. 3). The peaks around 510-607 cm −1 can be allocated to Cu-O and Mn-O bonds 44 . The CH 2 stretching peaks were detected at 2879-2915 cm −1 . The peaks were observed at 1036 and 1629 cm −1 corresponding to C-O-C and C=C, respectively. Also, the broad peak at 3398 cm −1 indicates the hydroxyl group, which is due to the absorption of a large amount of water on the nanocatalyst surface 45 .
The XRD pattern was used to identify the crystallinity and mineral phase of the CuMn x O y and CuMn x O y -GO nanocatalyst (Fig. 4). The pattern involves the five considerable various peaks at 30.8°, 36.1°, 55.2°, 58.2°, and 63.04° could be ascribed to (220), (316), (430), (507), and (445) in CuMn x O y crystal planes (JCPDS No. 84-0543) 46 , respectively. The peak at 26.72° can be assigned to GO in CuMn x O y -GO nanocatalyst 47 (Fig. 6c). The peak at 531.24 eV is attributed to the oxygens C-O/C=O/O-C=O in GO (O3), 530.89 eV is to the absorbed oxygens on the plane of CuMn x O y (O2), and 529.46 eV is related to the lattice oxygens (O1) 51 . The high-resolution XPS spectrum of Cu 2p is indicated in Fig. 6d. Two peaks at 957 eV and 936 eV matched Cu 2p1/2, and Cu 2p3/2. Cu in CuMn x O y has oxidation states Cu 2+ and Cu 1+ . The peak at 932.15 eV is attributed to Cu 1+ , while two peaks at 936.4 and 954.17 eV can correspond to Cu 2+ . In addition, the existence of a potent satellite peak at 946.28 eV indicates a high percentage of Cu 2+ species 52 . The results obtained of the high-resolution spectrum of Mn 2p implied that the major peaks at 656 eV to Mn 2p 1/2 and 646 eV are related to Mn 2p 3/2 (Fig. 6e). The spectrum of Mn 2p 3/2 fractures into three detached peaks at energies of 644.28, 642.7 and 641.16 eV, which respectively belong to Mn 4+ , Mn 3+ , and Mn 2+ species in CuMn x O y -GO. However, the peak at 656.20 eV energy pertains to Mn 3+ species sans any cleaving. Cu 1+ is formed resulting from the equivalence reaction of Mn 3+ + Cu 2+ −→ Mn 4+ + Cu + . The atomic ratio of Mn/Cu is equal to 3.1, which discloses some surplus Mn on the surface of the nanocatalyst 53 .
TEM spectrum was used to image the surface of the CuMn x O y and CuMn x O y -GO with higher magnification (Fig. 7a,b). Also, this analysis has investigated the internal structure of the nanocatalyst. The images obtained from TEM analysis provided more and better details of the nanocatalyst surface, based on which the GO sheets are in the form of layers with spherical nanoparticles placed between them. AFM analysis was utilized to measure the dimensions of CuMn x O y -GO nanostructured plates. This analysis provided more information than FESEM about the morphology and roughness of the nanocatalyst surface (Fig. 7c). The obtained results showed that the nanoparticles are hill-like with a roughness size of 2.62 nm, which indicates the successful loading of nanoparticles on the GO plates.   (Fig. 8). The effect of the CuMn x O y was investigated in the reaction. The results displayed that the reaction proceeded at 4 h with 78% efficiency, while this reaction took place in the presence of CuMn 2 O 4 /GO nanocatalyst with 96% efficiency for 2.5 h. Meanwhile, the reaction did not proceed in the absence of the CuMn x O y -GO nanostructure. Also, the reaction was studied in the presence and absence of K 2 CO 3 co-catalyst. The study indicated that the derivatives    (Fig. 8a). Derivatives of pyranoquinolines have been prepared in the presence of H 2 O with high efficiency compared to other solvents. The synthesis of pyranoquinolines using other catalysts was checked. As shown in Fig. 8b, the conversion of pyranoquinolines increased from 68 to 96% within 2.5 h, due to the presence of Cu and Mn active nanoparticles in GO sheets. Also, the impact dosage of nanocatalyst was checked from 20 to 40 mg on the prepared pyranoquinolines (Fig. 8c). In addition, the effective dosage of K 2 CO 3 has been studied within the range of 20-40 mg (Fig. 8d). The most optimal conditions include the reaction of phenyl glyoxal (0.1 mmol, 10 mg), ethyl cyanoacetate (0.1 mmol, 10 mg), and 4-hydroxyquinoline (0.1 mmol, 16 mg) in the presence of CuMn x O y -GO nanocatalyst (20 mg), water (6 mL), and K 2 CO 3 (0.3 mmol, 40 mg) in 2.5 h with 96% efficiency ( Table 2). In the preparation of pyranoquinolines, the aryl glyoxals with different electron-donating and electron-withdrawing substituents were used. The structure of pyranoquinolines was investigated through different analyses (the results are shown in Text S3, Support Information). Among different derivatives, aryl glyoxal with H and NO 2 substitution caused the synthesis of pyranoquinolines with 96% and 85% efficiency, respectively (Table 3). Meantime, the results displayed that the production of pyranoquinoline derivatives is not affected by the effects of electron-donating, electron-withdrawing, and steric hindrance of substitutions.
Proposed mechanism of pyranoquinolines synthesis. The one-pot, three-component process between various aryl glyoxal derivatives, 4-hydroxyquinolin-2(1H)-one, ethyl cyanoacetate and using H 2 O, CuMn x O y -GO, and K 2 CO 3 under reflux condition for synthesis pyranoquinolines with 85-96% yields as shown in Fig. 9. The proposed mechanism includes the coupling of copper and manganese active sites on the nanocatalyst with carbonyl groups in aryl glyoxal. Next, the acidic hydrogen of ethyl cyanoacetate is taken up by K 2 CO 3 . Then, Knoevenagel condensation was performed between aryl glyoxal and ethyl cyanoacetate as active methylene 54 . Afterwards, the hydrogen of the hydroxy group of 4-hydroxyquinoline is taken by K 2 CO 3 and it Reusability studies. Reusability ability is one of the important and effective factors in choosing the suitable catalyst for a chemical reaction. Here, the recycling process of the catalyst after the completion of the reaction was investigated by separating it from the reaction mixture using a centrifuge (2800 rpm for 7 min) then washing it with deionized water, and finally drying it in an oven. The CuMn x O y -GO was recovered during 6 cycles.
The results obtained are shown below (Fig. 11a). During the recycling process, no significant change in catalyst activity was observed. The images obtained through TEM (Fig. 11b), and FESEM (Fig. 11c) analyses showed that the structure of the catalyst is almost stable.

Conclusions
In this study, the CuMn x O y -GO nanocatalyst has been prepared by loading CuMn x O y nanoparticles between GO sheets. The obtained results from the CuMn x O y -GO of the structure indicated that its synthesis was successful. Pyranoquinoline derivatives have been synthesized using CuMn x O y -GO nanocatalyst. The high activity of the catalyst causes a fast and effective reaction between aryl glyoxal, 4-hydroxyquinolin-2(1H)-one, and ethyl cyanoacetate in the presence of K 2 CO 3 , water and produced the desired derivatives with high yields. The features of this work include the use of water as a green solvent, economics, the reusability of nanocatalyst after several times without losing its catalytic activity, and high yields of products. In the future, this work can be a good model for the synthesis of heterocyclic derivatives in the presence of nanocatalysts based on carbon material.