Ni–citric acid coordination polymer as a practical catalyst for multicomponent reactions

Coordinative polymers (CP) are a subclass of Metal–organic frameworks (MOFs) with porous microstructures which have been widely synthesized in recent years and applied in various fields especially in catalysis science. In this work Coordinative polymers (CP) of nickel and citric acid (CA) was prepared as a new catalyst (Ni-CP) and applied in organic multicomponent reactions. The obtained catalyst was characterized by SEM, WDX, EDS, AAS, FT-IR, XRD and BET analysis. N2 adsorption–desorption isotherms indicate good BET surface area for Ni-CP; therefore can be employed as an efficient catalyst in multicomponent reactions for the synthesis of polyhydroquinoline and 2,3-dihydroquinazolin-4(1H)-one derivatives. Finally, this catalyst was recovered and reused several consecutive times.


Result and discussion
Herein coordination polymer of nickel and Citric acid (Ni-CP) is reported as an efficient catalyst in multicomponent reactions for the synthesis of polyhydroquinoline and 2,3-dihydroquinazolin-4(1H)-one derivatives. Ni-CP was characterized by AAS, EDS, WDX, SEM, TGA, XRD, FT-IR and BET analysis. The surface morphological features of synthesized Ni-CP were studied by scanning electron microscope. Figure 2 shows the SEM image of Ni-CP with a magnification of 5000. The instrumental parameters, accelerating voltage, spot size, and magnification and working distances are indicated on the SEM image. The results indicate that mono-dispersive and highly crystalline Ni-CP is obtained. The appearance of some particles is spherical shape. We can observe that the Ni-CP is highly agglomerated and they are essentially cluster of Ni-CP. The SEM picture indicates the size of polycrystalline particles. The observation of some larger Ni-CP may be attributed to the fact that Ni-CP tends to agglomerate due to their high surface energy and high surface tension of the ultrafine Ni-CP. The fine particle size results in a large surface area that in turn, enhances the Ni-CP catalytic activity. So, we can conclude that the prepared Ni-CP particles are in the nanometer range. The average diameter of the particle observed from SEM analysis is 30 nm, which is larger than the diameter predicted from X-Ray broadening. The FT-IR spectrum of the citric acid had absorption bands in the regions of 1703 cm −1 and 1751 cm −1 respectively, which is related to the stretching vibration of the C=O bonds of aliphatic carboxylic acid groups (Fig. 3b). These absorption peaks were not observed in the spectra of the prepared Ni-CP, while the spectra of the Ni-CP presented two absorption peaks related to the vibration of the C=O bonds of aliphatic carboxylic acid groups at 1589 cm −1 . The high shift to the lower wavenumber of the absorption peak of carbonyl demonstrates the existence of the metal coordination bonding and also confirms the complexation reaction between Ni and citric acid (Fig. 3c). In addition, the peaks at the region of 2855-2928 cm −1 can be regarded as the characteristic of the stretching vibrations of aliphatic C-H 15 . The presence of aliphatic C-H stretching vibration indicates that the organic ligand used in the final sorbent structure remained and was not destroyed. In addition, various peaks were presented such as 400-1000 cm −1 for Ni-O bonds and 3000-3600 cm −1 for OH of carboxylic acid 40,41 . Based on the FT-IR results, we can also observe that the Ni-CP obtained from Nickel Nitrate shows sharp characteristic peaks, suggesting the high crystalline nature of Ni-CP (Fig. 3).
The elemental content of Ni-CP was obtained by EDS (energy-dispersive X-ray spectroscopy) analysis (Fig. 4). Based on the EDS results, the presence of oxygen (A%:45), carbon (A%:47), and as well as nickel (A%:8) species have been confirmed in the structure of the catalyst.
The homogeneous distribution of these elements has been studied by X-Ray Mapping (WDX analysis) in this catalyst which is shown in (Fig. 5). Also, the exact amount of nickel in the structure of this CP catalyst was calculated by atomic absorption spectroscopy (AAS) analysis which was found to be 2.07 × 10 −3 mol g −1 .
To investigate the porous nature and the efficient surface area of Ni-CP, nitrogen adsorption-desorption measurement (BET) was performed. The BET isotherms of Ni-CP and the corresponding Barrett-Joyner-Halenda (BJH) pore size distributions plot are shown in (Fig. 6). As could be seen from this Figure, the observed type-H3 hysteresis loop in the partial pressure range from 0.3 to 1.0, indicates the slit-shaped pores. The isotherm revealing type-H3 does not show any limiting adsorption at high P/P0, which is observed with spherical in shape particles 42 . Based on BET results, this CP catalyst has a high surface area of about 22.65 m 2 g −1 . Also, pore volumes and pore diameters of this CP catalyst are 0.11 cm 3 g −1 and 19.34 nm respectively. The textural properties of Ni-CP which were obtained from N 2 adsorption-desorption analysis are summarized in (Table 1). The synthesized CP materials were characterized by PXRD analysis using PW1730 instrument from Philips Company having CuKα (λ = 1.540598 Å) radiation at 40 kV and 30 mA with 2θ = 20°-100°. The XRD pattern of Ni-CP is shown in (Fig. 7). According to powder PXRD standards (PXRD, Ref. No. 01-087-0718), the crystalline peaks appearing at 42.2°, 51.3°, 75 °, 90.3° can be attributed to the (111), (300), (320), and (350) crystallographic planes of nickel crystals, which are in agreement with the previously reported works of literature 43,44 . The PXRD patterns shown in (Fig. 7) confirm the successful coordination of nickel ions within the prepared framework.  www.nature.com/scientificreports/ In addition, the (111) Ni diffraction peak with appreciable intensity further confirms the presence of Ni metal in the prepared Ni-CP 45 .
The mass ratios and the thermal stability of Ni-CP were examined by the thermogravimetric analysis (TGA) (Fig. 8). In the TGA curve, the two obvious weight losses were found in the temperature range of 50-270 °C, which can be attributed to the release of the physically adsorbed moisture (water) and DMF solvents from the sample 15 . In this sense, it was at above 270 °C that the framework degradation started. The main weight loss at 270-340 °C was caused by the decomposition of citric acid ligand 46 . This result confirms the successful synthesis of Ni-CP and also indicates that the thermal stability of the sample is about 270 °C. The DSC results also support the TGA data, based on weight loss of the sample, and approve the range of temperature stability of sample.
Catalytic study of Ni-CP. The catalytic activity of Ni-CP was investigated in the multicomponent reaction for the synthesis of polyhydroquinoline and 2,3-dihydroquinazolin-4(1H)-one derivatives. In the synthesis of   www.nature.com/scientificreports/ polyhydroquinolines, 4-chlorobenzaldehyd was selected as a model substrate to obtain the best reaction conditions. The selected model reaction was tested in various conditions ( Table 2). As shown, the model reaction did not proceed in the absence of Ni-CP ( Table 2, entry 1). Meanwhile, the model reaction was completed in the presence of 5 mg of this catalyst with a 97% yield ( Table 2, entry 2). Also, the model reaction was performed in a wide range of solvents and temperature. As shown, the reaction is dependent on solvent and temperature in which ethanol at 80 °C shows the best results for the synthesis of polyhydroquinolines (Table 2).
To extend the application of Ni-CP as a catalyst, the various aldehydes were investigated under obtained optimizing reaction conditions (Table 3). All products having electron-withdrawing groups or electron-donating groups were synthesized with good to excellent yields. In this investigation, electron-donating groups (such as OH, Me, OMe and Nme 2 ) and electron-withdrawing groups (such as Cl, F, Br, CN, NO 2 and CF 3 ) on the aromatic ring of benzaldehydes were examined under obtained optimizing reaction conditions for the synthesis of the wide range of polyhydroquinoline derivatives.
In continuation, the catalytic activity of Ni-CP was examined for the synthesis of 2,3-dihydroquinazolin-4(1H)-one derivatives as an efficient, recyclable, stable, and commercially available CP catalyst. The synthesis of 2,3-dihydroquinazolin-4(1H)-ones via cyclocondensation of anthranilamide and aldehydes in the presence of Ni-CP is shown in Table 4. In order to find the best reaction conditions, cyclocondensation of 4-chlorobenzaldehyde and anthranilamide was selected as model reaction (Table 4). At first, the effect of solvent was studied in    (Table 4, entries 3-5). Then, the amount of Ni-CP and the effect of temperature were also examined. As shown in Table 4, this reaction requires 6 mg of Ni-CP as a catalyst in the best conditions. Also, the best results were obtained at 80 °C. Meanwhile, lower yields were observed at 25 °C, 50 °C and 70 °C (Table 4, entries 10-12). b No reaction. After optimization of the reaction conditions, we have investigated the synthesis of a series of 2,3-dihydroquinazolin-4(1H)-ones to explore the scope of this procedure (Table 5). In these studies, various functional groups on the aromatic ring of an aldehyde such as OH, Me, OMe, Cl, Br, F and NO 2 were well tolerated. Therefore, this procedure is efficient for the synthesis of a wide range of 2,3-dihydroquinazolin-4(1H)-ones including electron-donating and electron-withdrawing substituents.
Based on the previous reports, the possible mechanism for the synthesis of 2,3-dihydroquinazolin-4(1H)one derivatives in the presence of Lewis acid can be suggested in (Fig. 9) 28 . This mechanism has several steps in which the first step involves activation of aldehyde by Ni-CP catalyst. The second step includes attacking active aldehyde by the amine group of 2-amino benzamide which followed by the dehydration reaction led to the synthesis of imine I. Intermediate I is converted to intermediate II by an intermolecular reaction. This step is included tautomerization of the amide and an intramolecular nuclear attack on carbon of imine. Finally, 2,3-dihydroquinazolin-4(1H)-one derivatives were synthesized by the protons transfer and Ni-CP catalyst was regenerated for new cycle 28 .

Recycling ability and leaching study of the catalyst.
Recyclability is one of the most important properties of heterogeneous catalysts. The recyclability of the Ni-CP catalyst was analyzed in a typical [2+3] cycloaddition of anthranilamide and 4-chloro-benzaldehyde under the optimized reaction conditions. Figure 10 shows that Ni-CP catalyst can be reused up to 5 times without any significant loss of its activity.
The structure of reused catalyst was considered using FT-IR and XRD analysis. The FT-IR (Fig. 11) and XRD (Fig. 12) did not show any significant change compared to the fresh catalyst. All of the peaks of fresh catalyst are existed in the recovered catalyst.
In order to indicate the heterogeneous nature of Ni-CP, nickel leaching of this catalyst was studied. In this regard, the synthesis of 2,3-dihydroquinazolin-4(1H)-one using condensation of 4-chlorobenzaldehyde with 2-amino benzamide was selected as a model reaction. After completion of the reaction, the catalyst was removed by simple filtration and the amount of nickel was calculated in filtered solution by AAS which was found to be 0.0000370 mmol mL −1 . As known, this very small amount of nickel doesn't have a significant effect on the reaction progress. Therefore, the completion of the reaction could be dependent on the heterogeneous nickel species. It was also proved by the hot filtration test that the Ni-CP played a catalytic role in the reaction without the Ni leaching into the solution.    Table 6). The products were obtained in higher yields over faster times in the presence of Ni-CP. Also, this catalyst has several advantages in terms of non-toxicity, price, stability and easy separation.

Conclusion
In this work, nickel nitrate and citric acid nitrate were used to synthesis a new CP catalyst (Ni-CP). This CP catalyst was characterized by AAS, WDX, EDS, SEM, TGA, XRD and N 2 adsorption-desorption analysis. Ni-CP was successfully described as an efficient and recyclable catalyst in the synthesis of polyhydroquinoline and 2,3-dihydroquinazolin-4(1H)-one derivatives in which all products were prepared in good yields. This catalyst has several advantages such as non-toxicity, price, stability and easy separation.