Synthesis and application of [Zr-UiO-66-PDC-SO3H]Cl MOFs to the preparation of dicyanomethylene pyridines via chemical and electrochemical methods

A metal–organic framework (MOF) with sulfonic acid tags as a novel mesoporous catalyst was synthesized. The precursor of Zr-UiO-66-PDC was synthesized both via chemical and electrochemical methods. Then, zirconium-based mesoporous metal–organic framework [Zr-UiO-66-PDC-SO3H]Cl was prepared by reaction of Zr-UiO-66-PDC and SO3HCl. The structure of [Zr-UiO-66-PDC-SO3H]Cl was confirmed by FT-IR, PXRD, FE-SEM, TEM, BET, EDX, and Mapping analysis. This mesoporous [Zr-UiO-66-PDC-SO3H]Cl was successfully applied for the synthesis of dicyanomethylene pyridine derivatives via condensation of various aldehyde, 2-aminoprop-1-ene-1,1,3-tricarbonitrile and malononitrile. At the electrochemical section, a green electrochemical method has successfully employed for rapid synthesis of the zirconium-based mesoporous metal–organic framework UiO-66-PDC at room temperature and atmospheric pressure. The synthesized UiO-66-PDC has a uniform cauliflower-like structure with a 13.5 nm mean pore diameter and 1081.6 m2 g−1 surface area. The described catalyst [Zr-UiO-66-PDC-SO3H]Cl was also employed for the convergent paired electrochemical synthesis of dihydropyridine derivatives as an environmentally friendly technique under constant current at 1.0 mA cm−2 in an undivided cell. The proposed method proceeds with moderate to good yields for the model via a cooperative vinylogous anomeric based oxidation.

On the other hand, chemical reactivity is very complex. Numerous factors control the reaction mechanisms which are subject to everyday experiences. According to the alabugin's theory, one of the most effective factors is stereoelectronic effects, which are the stabilizing interactions of orbitals in space, are based on the quantum nature of molecular bonding but express this nature in a set of simple and intuitive practical rules that build a bridge between structure and reactivity 28,29 . Anomeric effect (AE) has been divided to different kinds such as geminal (Endo, Exo and reverse), vinylogous and so on (Fig. 1, Part I) 28 .
The relationship and detail of the impact of AE as an oldest stereoelectronic effect on structure and reactivity is also our main interest. This paper attempts to describe the role of vinylogous AE in the course of the synthesis of target molecules. Recently, we have introduced and developed a new term entitled "anomeric based oxidation" (ABO) in the course of special reactions [30][31][32][33][34][35][36] . Cooperative geminal and vinylogous ABO has been reviewed ( Fig. 1, Parts II and III) 37, 38 . Behind the chemical studies, many efforts have been done to access a mild and green condition for the electrosynthesis and application of MOFs [39][40][41][42][43][44][45][46][47][48][49][50] . Anodic and cathodic electrosynthesis methods are the important techniques that recently have been used for the preparation of various kinds of MOFs [39][40][41][42][43][44] . Even though the significant chemical [51][52][53] and electrochemical synthesis [54][55][56] of Zr based MOFs have been dedicated www.nature.com/scientificreports/ to the UiO-66 MOF, there are not any reports using cathodic electrosynthesis of UiO-66 derivatives at room temperature and pressure.
In this work, we wish to report the electrosynthesis of the mesoporous UiO-66-PDC (Zr-mMOF) via a reductive electrosynthesis technique as the first example. It was found that the electrosynthesis of Zr-mMOF by this method is rapid and could be done at room temperature and pressure without the need for any base or pre-base additive for activation of the ligand.
These results proved by the Fourier Transforms Infrared (FT-IR) spectroscopy, Field Emission Scanning Electron Microscopy (FE-SEM) and N 2 adsorption-desorption isotherm. At the second step and after treatment of electro-synthesized UiO-66-PDC by the SO 3 HCl, the catalyst, [Zr-UiO-66-PDC-SO 3 H]Cl was employed in a green procedure for convergent paired electrosynthesis of dihydropyridine compounds. "Paired electrosynthesis" have been successfully employed for the synthesis of organic and inorganic compounds 40,[57][58][59] . This positive glance comes from the improved energy efficiency, enhanced atom economy, time-saving, and increasing electrochemical yield [60][61][62][63] . We imagined that the convergent pairing of two electrochemical reactions would provide a promising protocol towards the green chemistry principles. In other words, by the implementation of this strategy, cooperative anodic and cathodic reactions lead to a one-step process at green solvent and room temperature and without the need for any ex-situ base additive and replacement of the electrodes.
According to the above concepts, after preparation of Zr-metal-organic frameworks [Zr-UiO-66-PDC-SO 3 H] Cl as a mesoporous catalyst, it was employed for the synthesis and electrosynthesis of special dicyanomethylene pyridines by condensation of various aldehydes (bearing electron-donating and electron-withdrawing groups), malononitrile and 2-aminoprop-1-ene-1,1,3-tricarbonitrile under solvent-free conditions at 100 °C (method A) and constant current electrolysis via the convergent paired electrosynthesis in the ethanol at room temperature and pressure (method B) (Fig. 2).

Results and discussion
Nowadays inter and multidisciplinary researches and investigations are a great demand both for academics and industries researchers. On the other hand, making bridges between basic to advanced concepts are necessary for the development of knowledge. In this research, chemical and electrochemical methods for the preparation of designed molecules were applied. Stereoelectronic effects as a bridge between structure and reactivity was also considered in the course of reactions 28 . With this aim, we have studied reactions and the obtained results are presented.
Chemical and electrochemical preparation of Zr-mMOF. Metal Fig. 3. The broad peak of O-H stretching related to SO 3 H group at 2700-3500 cm −1 and peaks observed at 1186 and 1063 cm −1 were related to stretching O-S and N-S respectively 16 . The peak 1733 cm −1 in H 2 PDC was related to stretching C = O bond. Also, the PXRD pattern indicates the crystallinity of synthesized [Zr-UiO-66-PDC-SO 3 H]Cl (Fig. 3) 13 .
The materials in the structure of [Zr-UiO-66-PDC-SO 3 H]Cl and Zr-UiO-66-PDC were characterized by energy-dispersive X-ray spectroscopy (EDX) (Fig. 4). The  In order investigation, the structural and thermal stability of [Zr-UiO-66-PDC-SO 3 H]Cl was also determined using the technique of the thermal gravimetric (TG), derivative thermal gravimetric (DTG), as well as differential thermal analysis (DTA) (Fig. 6). Initial stage weight loss is between room temperature up to 100 °C, associated with organic solvents and H 2 O which have been applied in the course of preparation of [Zr-UiO-66-PDC-SO 3 H] Cl. In continued, twice steps of weight loss (includes about 30% weight loss) has occurred at about 300 °C which is linked to breaking the band of N-S of the structure of the catalyst. Therefore, according to literature survey 13 , the structure of [Zr-UiO-66-PDC-SO 3 H]Cl is stable, even after adding sulfonic acidic functional groups. www.nature.com/scientificreports/ Features of the prepared functionalized MOFs such as Surface area, pore volumes and pore size distribution were obtained using N 2 adsorption-desorption isotherm (Fig. 6). The calculated surface areas using the BET equation, total pore volume and average pore size are 15.24 m 2 g −1 , 0.1914 cm 3 g −1 and 50.22 nm respectively.
We also employed a cathodic (reductive) electrochemical technique for the preparation of Zr-UiO-66-PDC. As shown in Fig. 7. Our procedure involves immersing a carbon electrode in a solution containing pyridine-2,5-dicarboxylic acid (H 2 PDC) as a ligand, zirconium tetrachloride as a cation source and potassium nitrate as a supporting electrolyte. In-situ electrogeneration of hydroxide ions generated by electroreduction of water (as a co-solvent), NO 3 − (as a counter ion) and/or direct deprotonation of ligand at 30.0 mA cm −2 for 1800s is an essential requirement in this method [39][40][41][42]44 . An increase in the local pH at the cathode surface causes activation of the ligands (deprotonation), and consequently formation of Zr-UiO-66-PDC through the coordination of activated ligands with zirconium cations (Fig. 7) 55,56 . The nucleation rate and growth of Zr-UiO-66-PDC are only controlled by the cathodic reaction without the need for any ex-situ base/probes additive, at room temperature, atmospheric pressure, and short time.
Characterization of the prepared Zr-UiO-66-PDC was examined by the FT-IR, PXRD, FE-SEM, BET, EDX and mapping analysis. To confirm the functionality and bonding groups of the electrosynthesized UiO-66-PDC MOF, FT-IR analysis was performed. The obtained pattern is consistent with the reported pattern of chemical procedure 13 (Fig. 8A). Also, the recorded PXRD pattern proved the purity and high crystallinity of the electrosynthesized Zr-UiO-66-PDC MOF that is consistent with the reported pattern 13 and the Zr based MOFs synthesized by electrochemical method 55,56 (Fig. 8B). FE-SEM images of electrosynthesized (UiO-66-PDC) under constant current conditions shows the uniform cauliflower-shaped nanoparticles with an average diameter size of around 25.0 nm (Fig. 8C). This result is consistent with the Zr based MOFs synthesized by electrochemical method 55,56 . The N 2 adsorption/desorption isotherm of Zr-UiO-66-PDC is shown in Fig. 8D. A "type IV" isotherm with a hysteresis loop (between p/p 0 = 0.4 and 1) which is characteristic of mesoporous materials is observed. www.nature.com/scientificreports/ Furthermore, the pore size distribution obtained by the Barrett-Joyner-Halenda (BJH) method shows two peaks of 1.2 and 18.9 nm but the average pore size is 13.4 nm. Besides, the specific surface area measured from the N 2 isotherms is 1081.6 m 2 g −1 that is higher than the obtained amount of above mentioned hydrothermal and microwave methods and is consistent with the Zr based MOFs prepared electrochemically 55,56 . Finally, the Zr-UiO-66-PDC structure was characterized by energy-dispersive X-ray spectroscopy (EDX) and mapping analysis (Fig. 8E). The obtained EDX pattern was confirmed the simultaneous existence of Zr, C, O, and N elements for the configuration of the Zr-UiO-66-PDC structure. Furthermore, mapping analysis was indicated the homogeneous and well-dispersed distribution of the above-mentioned elements in the Zr-UiO-66-PDC structure (Fig. 8E).
Chemical and electrochemical synthesis of dicyanomethylene pyridine derivatives. Lately, a wide range of dicyanomethylene pyridines was prepared via one-pot multi-component condensation reactions in the presence of a catalytic amount described [Zr-UiO-66-PDC-SO 3 H]Cl as a mesoporous catalyst. The condensation of 4-chloro benzaldehyde, 2-aminoprop-1-ene-1,1,3-tricarbonitrile and malononitrile was selected as a model for optimization of the reaction conditions. As shown, the best condition reaction for the synthesis of 3,5-diaminobiphenyl-2,4,6-tricarbonitrile was achieved in the presence of 10.0 mg [Zr-UiO-66-PDC-SO 3 H]Cl in refluxing water (Table 1 entry 10). Different amount of catalyst, temperature and solvent were not improved in the yield and time (Table 1 entries 1-17 except 10). The optimization of reaction conditions along with the isolated yields of products are summarized in Table 1.
After optimization reaction condition, the scope and limitations of [Zr-UiO-66-PDC-SO 3 H]Cl as a novel catalyst was investigated in the preparation of dicyanomethylene pyridine via a condensation reaction of widespread analogue of aldehyde (mono, bis and tris substituted C=O) which are bearing electron-donating and electron-withdrawing groups, malononitrile and 2-aminoprop-1-ene-1,1,3-tricarbonitrile.
As shown in Table 2, the results indicated that this strategy is appropriate for the synthesis of dicyanomethylene pyridine ( Table 2). The proposed mechanism for the synthesis of dicyanomethylene pyridine derivatives using [Zr-UiO-66-PDC-SO 3 H]Cl was summarized in Fig. 9 (Fig. 9) 31 . www.nature.com/scientificreports/ The obtained results from model reaction under argon and nitrogen atmospheres verified abovementioned our suggestion for the latter step. Literature survey shows that there is no rational and clear stepwise mechanism for synthesis of the presented molecules 64,65 . Herein, we wish to present a comprehensive mechanism for a mentioned reaction so that its final step might progress through a cooperative vinylogous ABO in the absence of oxygen molecules. In the intermediates III and IV, sharing the electron density from the Endo and Exo nitrogens lone pairs to the vacant antibonding σ* orbital of SP 3 C-H bond through vinylic C=C double bonds support the unusual hydride transfer for releasing the molecular hydrogen (H 2 ). Recently, we have been named this phenomenon a new term entitled a cooperative vinylogous ABO and its development for various catalytic systems is our main research interest [30][31][32][33][34][35][36][37][38] . The described ABO mechanism is in good agreement with the "vinylogous anomeric effect" concept which at first, had been introduced by Katritzky 66 .
At the second step of this work, [Zr-UiO-66-PDC-SO 3 H]Cl was employed for the preparation of dihydropyridine compounds via convergent paired electrosynthesis as a green and sustainable technique. To shed light on  It is noteworthy to mention that, the higher applied currents lead to the occurrence of side reactions like polymerization of malononitrile and the lower applied currents lead to prolonged reaction time or low yields due to the inactivation of malononitrile. Table 3 indicate the optimization of applied current density at the electrolysis condition. Table 4 indicates the results of the model reactions based on the electronic properties of substitute groups on the benzaldehyde. The data presented in Table 4 shows the applied procedure have a satisfying performance for electrosynthesis of related dihydropyridine derivatives in a one-pot reaction with a 63-85% overall yield. It is noteworthy to mention that, the higher product yield for aldehydes with electron-withdrawing groups, 4-Chlorobenzaldehyde, is comparable and even greater than that of their simple (benzaldehyde) and/or electrondonating (4-methylbenzaldehyde) group homologues. These results may be caused by more efficient deprotonation of related aldehydes bearing electron-withdrawing groups lead to an intermediate under the proposed mechanism. So, the obtained trend may be repeated for the other homologue aldehydes based on the electron characteristics of substituted groups with acceptable tolerance.

Conclusions
In this study, we have introduced a novel Zr-metal-organic frameworks [Zr-UiO-66-PDC-SO 3 H]Cl as a mesoporous catalyst. This catalyst was tested for the preparation of various novel dicyanomethylene pyridine via a cooperative vinylogous anomeric based oxidation mechanism. The topology of the [Zr-UiO-66-PDC-SO 3 H]Cl was also characterized by SEM and TEM images. As well as, the thermal and solvent stability of the catalyst is high after the reaction of SO 3 HCl with [Zr-UiO-66-PDC]. Furthermore, the major advantages of the presented work are mild and green conditions, high yields, short reaction times, facile workup and reusability of the described [Zr-UiO-66-PDC-SO 3 H]Cl. Also, this paper provides a green and promising electrochemical procedure for the preparation of mesoporous (UiO-66-PDC). From the standpoint of environmental issues, synthesis of these compounds can be performed without the need for any ex-situ chemical agent such as a base or probase. On the other hands, the use of electricity eliminates the need for high temperature and pressure, which is the most outstanding features of this study. Furthermore, the convergent paired electrosynthesis of dicyanomethylene pyridine derivatives with the prepared catalyst was performed as an environmentally friendly technique under green conditions, at room temperature and pressure. We think that the present work is a promising insight for inter and multidisciplinary research, rational design, syntheses and applications of task-specific MOFs and bioactive molecules.    PerkinElmer PE-1600-FTIR device was recorded for infrared spectra of compounds. SEM was performed using a scanning electron microscope for field publishing made by TE-SCAN. Thermal gravimetry (TG), differential thermal gravimetric (DTG) and differential thermal (DTA) were analyzed by a Perkin Elmer (Model: Pyris 1). BET and BJH were analyzed by BELSORP-mini ii high precision Surface area and pore size. www.nature.com/scientificreports/ reaction mixture was cool down to room temperature. Then, the mixture was added 10 mL ethanol and the catalyst was subsequently removed by centrifugation (1000 rpm). Finally, the product was recrystallized with EtOH ( Fig. 2). Characteristic of the products. 6