A new silver coordination polymer based on 4,6-diamino-2-pyrimidinethiol: synthesis, characterization and catalytic application in asymmetric Hantzsch synthesis of polyhydroquinolines

A highly efficient and stable heterogeneous coordination polymer (CP) was successfully prepared by hydrothermal combination of silver and 4,6-diamino-2-pyrimidinethiol. The prepared coordination polymer was characterized by FT-IR, XRD, TGA, SEM, EDX, X-ray mapping and Nitrogen adsorption–desorption analysis. The prepared Ag–CP exhibit excellent catalytic activity in multicomponent Hantzsch synthesis of polyhydroquinolines under mild reaction conditions in relatively short reaction times. The heterogeneity of the catalyst was confirmed by the hot filtration test; also, the catalyst was reused for at least four times under the optimized reaction conditions without any significant loss of its catalytic activity.

General procedure for the catalytic synthesis of polyhydroquinolines. A mixture of aromatic aldehydes (1.0 mmol), ethyl acetoacetate (1 mmol), dimedone (1 mmol), NH 4 OAc (1.2 mmol) and Ag-CP (8 mg) was stirred in EtOH under reflux conditions for the required time. The progress of reaction was monitored by TLC. After completion of the reaction, the mixture was cooled down to room temperature. Afterwards, the catalyst was separated using simple filtration and, then, washed by hot ethyl acetate. Finally, the solvent was evaporated and the pure polyhydroquinoline products was obtained through recrystallization in ethanol.

Results and discussion
Structural and chemical composition analysis. The FT-IR spectrum of the 4,6-Diamino-2-pyrimidinethiol ( Fig. 1a) before of the CPs complexion had absorption bands in the regions of 3339 cm −1 and 3435 cm −1 respectively, related to the stretching vibration of the N-H bonds of free NH 2 groups. This bands disappeared in the FT-IR spectra of the Ag-CP (Fig. 1c), which is indicative of the fact that the NH 2 groups of 4,6-diamino-2-pyrimidinethiol have been deprotonated and coordinated to the Ag atoms. Moreover, a shift on the bending vibration of NH 2 near 1628 cm −1 in the Ag-CP to more higher wavenumbers in comparison to the 4,6-Diamino-2-pyrimidinethiol demonstrates the existence of the metal coordination bonding and confirms the successful complexion of Ag ions with the nitrogen atoms and thiol atoms of ligand. In the curve of the Ag-CP (Fig. 1c), the strong C=C stretching vibration band at 1460 cm −1 , and C-N stretch band at 1092 cm −1 provide evidences confirming the successful synthesis of Ag-CP. On the basis of the FT-IR, we can also observe that the FT-IR spectrum of Ag-CP obtained from silver nitrate (Fig. 1b) shows sharp characteristic peaks suggesting the high crystalline nature of Ag-CP. On the basis of the FT-IR results Silver ions were coordinated to the amine and thiol functional groups of 4,6-Diamino-2-pyrimidinethiol as a tridentate ligand and confirm the suggested structure in Scheme 1.
The synthesized CPS 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 = 10°-80°. The XRD pattern of Ag-CP is shown in Fig. 2. According to powder PXRD standards (PXRD, Ref. No. 01-087-0718), the crystalline peaks occurring at 2θ = 38.48°, 44.77°, 64.99°and 77.92° can be attributed to the (111), (200), (220), and (311) crystallographic planes of silver crystals, which are in agreement with the previously reported literatures 54 . The PXRD patterns shown in Fig. 2 confirm the successful coordination of silver ions within the prepared framework. In addition, the (111) Ag diffraction peak with appreciable intensity further confirms the presence of Ag metal in the prepared Ag-CP 55 . www.nature.com/scientificreports/ The mass ratios and the thermal stability of Ag-CP were examined by the thermogravimetric analysis (TGA) (Fig. 3). In all of the TGA curve, the weight loss which was occurred below 200 °C was attributed to the release of physically adsorbed moisture and water and organic solvents from the sample 56 . It was at above 200 °C that the framework degradation started. The main weight loss at 250-600 °C was caused by the decomposition of 4,6-Diamino-2-pyrimidinethiol ligand 57,58 . This result confirms the successful synthesis of Ag-CP, and the fact that the temperature stability of the sample is about 200 °C. The DSC results which support the TGA data, based on weight loss of the sample, approve the range of temperature stability of the sample.
As shown in the Fig. 4, the morphology of the surfaces of the obtained Ag-CP porous material was further characterized by scanning electron microscopy. A closer look at the SEM images of the obtained porous material indicates that the Ag-CP have been formed in uniform nanometer-sized particles.
The EDX spectrum showed that a lot of Ag species was observed on the surface of the obtained Ag-CP porous catalyst in Fig. 5. The presence of C, S and N elements in the Ag-CP was also confirmed by energy dispersive X-ray (EDX) measurements.
The X-ray mapping technique was performed in order to complete the characterization of the Ag-CP elements and how they are distributed in the CPs structure. Figure 4 shows the elemental X-ray mapping of Ag-CP. The uniform distribution of the index elements (C, S, N, and Ag) is observed at the obtained porous material. The  www.nature.com/scientificreports/ uniform distribution of C, N and S elements indicated the presence of the 4,6-diamino-2-pyrimidinethiol scaffold, which has been used as a ligand. In addition, the uniform distribution of the Ag element can be seen in the X-ray-mapping images. It can be concluded from the figure that the Ag has evenly coordinated to 4,6-diamino-2-pyrimidinethiol scaffold, and a good catalytic surface has been formed via the uniform incorporation of Ag catalytic species with nitrogen and sulfur groups (Fig. 6). The surface area and pore size distribution of Ag-CP are studied by BET analysis which exhibits type IV adsorption isotherm according to IUPAC classification. The appeared characteristic hysteresis loop is in good agreement with CPs material (Fig. 7). Regarding the BET analysis, the calculated surface area of Ag-CP is 3.59 m 2 /g. The pore volumes and pore size distribution of Ag-CP are calculated by BJH analysis and the values are 0.02 cm 3 g -1 , and 23.45 nm, respectively. Which indicated that the obtained Ag-CP is a mesoporous material. These results are in agreement with the reported coordination compounds in literature 59,60 . Catalytic study. After the successful characterization of the synthezed Ag-CP, its catalytic activity was evaluated in multicomponent Hantzsch condensation of polyhydroquinolines under diverse conditions (Table 1). Firstly, the Hantzsch reaction was carried out between para-chlorobenzaldehyde and dimedone, ethyl acetoacetate and ammonium acetate as a model reaction. Afterwards, we aimed at investigating the optimum reaction conditions in presence of the newly synthesized Ag-CP catalyst. Table 1 clearly depicts that the reaction progress is highly affected with catalyst loading, solvent and temperature. At the first step, the effect of Ag-CP loading to catalyze the reaction was examined by varying the amount of Ag-CP in the model reaction. It was observed that the yield of the polyhydroquinoline product enhanced with increasing the amount of the catalyst from 1 to 7 mg (Table 1, entries 4-10). The best result in an appropriate time was obtained using 7 mg of the catalyst (Table 1,     www.nature.com/scientificreports/ that 4,6-diamino-2-pyrimidinethiol and Ag(NO 3 ) 2 cannot efficiently catalyze the reaction and, as a result, the product is obtained in low yields in 85 min. After optimization of the reaction conditions, we have explored the scope of the reaction with various electron-donating and electron-withdrawing groups of aldehydes (Table 2). Both these substituents gave excellent yield of the product.
A plausible reaction mechanism for the Hantzsch synthesis of polyhydroquinoline derivatives in the presence of Ag-CP is depicted in Scheme 2. Initially, the carbanion was formed by proton abstraction from the active methylene compounds (ethyl acetoacetate or dimedone) which underwent Knoevenagel condensation reaction with aldehyde to form an α,β-unsaturated compound. In the second part of the reaction, ammonium acetate gave acetic acid and ammonia and, then, the ammonia combined with the active carbonyl compounds (ethyl acetoacetate or dimedone) and attained imine derivatives. Finally, the Michael addition of imine derivatives on the α,β-unsaturated carbonyl compounds (which were activated by Ag-CP) was followed by cyclization reaction. Besides, dehydration gave the final polyhydroquinoline products (Scheme 2).
Hot filtration. The hot filtration test was another analysis to approve the heterogeneous nature of the Ag-CP in the Hantzsch synthesis of polyhydroquinolines. On this basis, the model reaction was studied again under the optimized reaction condition. After 43 min (59% conversion), the Ag-CP were removed from the reaction by simple filtration. Afterwards, the rest of the reaction was stirred in the absence of the catalyst for a further 43 min. The obtained results show that the Ag-based framework played a catalytic role in the reaction without the Ag leaching into the solution or framework degradation.
Recyclability study. Considering the environmental and economic factors, and also the principles of green chemistry, the long-term durability experiment of the prepared catalyst has been investigated. In this sense, the model reaction was carried out under the optimized reaction conditions to test the reusability behaviour of the Ag-CP (Fig. 8). Briefly, after the finalization of each run of the Hantzsch reaction, which was monitored by the thin-layer chromatography, the reaction mixture was cooled to room temperature and, then, the heterogeneous catalyst was separated by simple filtration, washed with Acetone, EtOAc and H 2 O and, finally, dried in an oven at 60 °C overnight to be used in the next run. As shown in Fig. 8, no significant decrease in the yield of the reactions was observed even after four runs. These results confirmed the high catalytic activity and long-term durability of the Ag-CP in the Hantzsch synthesis of polyhydroquinolines.
The Ag-CP have been characterized by FT-IR before and after the reaction (Fig. 9a, b). These spectra showed the similar peaks and indicating the stability of this catalyst under applied reaction conditions.  www.nature.com/scientificreports/ Furthermore, no noticeable changes in crystalline phase can be found from comparison of PXRD of the spent (Fig. 10a) and fresh (Fig. 10b) catalyst. Which shows an excellent physicochemical stability during Hantzsch reaction.
Comparison of the proposed catalyst with the previously reported catalysts for the Hantzsch condensation. Finally, to evaluate the performance of the present catalytic activity in the unsymmetrical Hantzsch reaction, the current protocol was compared with some of the previously reported catalysts in the synthesis of polyhydroquinolines (Table 3). There is no doubt that all of the listed catalysts in Table 3 can significantly produce the desired product in good to excellent yield. But, it was found out that the Ag-CP as a heterogeneous catalyst can be regarded as superior to almost all of the methods presented in Table 3. These observations may be attributed to the synergistic effect between the Ag and the basic cites in the prepared Ag-CP catalytic system.

Conclusion
In summary, a novel Ag-CP catalytic system was successfully prepared using the reaction of 4,6-diamino-2-mercaptopyrimidine which contains a pyrimidine ring, two amine and one thiol functional groups with strong chelating ability and silver nitrate solutions as commercial available starting materials. Besides, it was characterized by a progressive mode for the preparation. The Ag-CP has an efficient catalytic activity for Hantzsch Synthesis of polyhydroquinolines with various aldehyde-bearing aromatic hydrocarbons in green media and in low reaction times. More importantly, the prepared catalyst affords several advantages such as ease of synthesis, wide range of carriers, ligand-free protocol and easy separation. Simultaneously, the catalyst has remarkable stability and recyclability, and the catalytic activity of the Ag-CP without any significant loss of its activity after four times continuous service.   www.nature.com/scientificreports/ Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/.