Synthesis of (E)-2-(1H-tetrazole-5-yl)-3-phenylacrylenenitrile derivatives catalyzed by new ZnO nanoparticles embedded in a thermally stable magnetic periodic mesoporous organosilica under green conditions

ZnO nanoparticles embedded in a magnetic isocyanurate-based periodic mesoporous organosilica (Fe3O4@PMO–ICS–ZnO) were prepared through a modified environmentally-benign procedure for the first time and properly characterized by appropriate spectroscopic and analytical methods or techniques used for mesoporous materials. The new thermally stable Fe3O4@PMO–ICS–ZnO nanomaterial with proper active sites and surface area as well as uniform particle size was investigated for the synthesis of medicinally important tetrazole derivatives through cascade condensation and concerted 1,3-cycloaddition reactions as a representative of the Click Chemistry concept. The desired 5-substituted-1H-tetrazole derivatives were smoothly prepared in high to quantitative yields and good purity in EtOH under reflux conditions. Low catalyst loading, short reaction time and the use of green solvents such as EtOH and water instead of carcinogenic DMF as well as easy separation and recyclability of the catalyst for at least five consecutive runs without significant loss of its activity are notable advantages of this new protocol compared to other recent introduced procedures.

www.nature.com/scientificreports/ structures are similar to the pharmacological core of the Saran's family, which are in fact angiotensin II receptor blockers (ARB). Angiotensin II is a bioactive peptide that narrows the vessels through the contraction of the muscles around the heart 37 . These drugs are used in to lower blood pressure and heart failure. Among the most important pharmaceuticals in this category of drugs are losartan and valsartan (Fig. 1). Indeed, these two active pharmaceutical ingredients are the first of a new class of drugs, which have been introduced for the clinical use in hypertension 38,39 . Furthermore, many chemical studies on other tetrazole analogue compounds have described antibacterial and antifungal properties. Tetrazole derivatives also show anti-inflammatory, analgesic, anti-cancer, anticonvulsant and antidiabetic kidney disease activities 32 . Therefore, tetrazole chemistry has yet remained as a fascinating research area for different scientists.
Since the preparation of the first tetrazole compound in more than a hundred years ago, many scientists have invented or described different methods for preparation of tetrazole compounds 35 . The most common of these methods is a 1,3-dipolar cycloaddition reaction between different simple nitrile derivatives and azide ion or hydrazoic acid under pressure. This approach has been more developed using different catalytic systems or altering of substrates during different decades 35,[40][41][42] . Some recent examples include the use of L-cysteine complex of palladium onto mesoporous channels of MCM-41 43 , Fe 3 O 4 @L-lysine-Pd(0) 43 , copper(I) salt 26 , copper catalyst on biochar nanoparticles 44 , guanidine complex of copper supported on boehmite nanoparticles 45 , zinc(II) salts 46 , Zn/Al hydrotalcite 47 , NiFe 2 O 4 48 , TMSCl and Bu 2 SnO or N-methyl-2-pyrrolidone 49 , FeCl 3 -SiO 2 50 , Amberlyst-15 28 , MCM-41-SO 3 H 51 , 1-disulfo-[2,2-bipyridine]-1,1-diium chloride ionic liquid 52 mainly at elevated temperatures, or dialkyl aluminum azides as the substrate and catalyst 36 . Furthermore, Sharples and his colleague have reported improved intermolecular [3 + 2] ring closures between an azide and p-toluenesulfonyl cyanide or acyl cyanide under solvent-free conditions at elevated temperatures to afford good to excellent yields of the corresponding products using easy procedurs 53,54 . Moreover, the use of more complex nitrile derivatives such as benzylidenemalononitriles would be more desirable in terms of abundance of functional groups in the corresponding products 55 . Indeed, this route for the synthesis of tetrazoles involves the multicomponent reactions (MCRs) strategy between aldehydes, sodium azide and nitriles. One of the advantages of this strategy is the use of available, easy and inexpensive basic compounds, namely aldehydes and sodium azide along with relatively expensive nitriles, which has made these reactions economically viable and easy to do on an industrial scale [56][57][58][59][60] . Among different catalytic systems for benzylidenemalononitrile derivatives, which are generally prepared by condensation of aldehydes and molononitrile, Fe 3 O 4 @BNPs-CPTMS-Chitosan-Pd(0) 61 , Cu(II) immobilized on Fe 3 O 4 @HNTs-tetrazole nanocomposite 62 , secondary amine/Cu(II) bifunctional magnetic nanoparticles 63 , Fe 3 O 4 magnetic nanoparticles under microwave irradiation 64 , NiO 2 nanoparticles 65 , silica molybdic acid 66 and Cu 2 (BDC) 2 (DABCO) metal-organic framework 67 could be mentioned. In spite of their merits, there are some disadvantages to the most of previously reported protocols for the synthesis of tetrazoles. These include low boiling point of hydrazoic acid (37 °C), the use of expensive catalytic systems, high pressure conditions, toxic and carcinogenic solvents including DMF, or elevated temperature for the explosive azide ion component.
To address the above challenges and in continuation of our studies to explore the catalytic activity of ZnO species 108,109 as well as PMOs 91,94-99 , we wish to report herein the new ZnO nanoparticles embedded in a magnetic isocyanurate-based periodic mesoporous organosilica (Fe 3 O 4 @PMO-ICS-ZnO) for the cascade reaction of different aromatic aldehydes and malononitrile to afford the corresponding Knoevenagel intermediate, and subsequent [3 + 2] cycloaddition with sodium azide (Fig. 2). Indeed, diverse tetrazole derivatives 5a-o were prepared efficiently using low loading of Fe 3 O 4 @PMO-ICS-ZnO, as a magnetically recoverable catalytic system, in EtOH under reflux conditions in short reaction times. www.nature.com/scientificreports/

Results and discussion
After preparation of the Fe 3 O 4 @PMO-ICS-ZnO nanomaterial, its structural, morphological and textural properties were characterized by Fourier transform infra-red (FTIR) and X-ray powder diffraction (XRD) spectroscopy, field emission scanning electron (FESEM) microscopy, thermogravimetric analysis (TGA), N 2 adsorption-desorption isotherms and vibrating sample magnetometer (VSM). The FT-IR spectra of both PMO-ICS and Fe 3 O 4 @PMO-ICS-ZnO have been presented in Fig. 3. It was observed that the free O-H stretching mode of the silanol groups on the PMO surface appear at 3415 cm −1 . Also, sharp absorption bands were observed at 1689 and 1471, which are related to the vibrations of carbonyl and C-N bonds in the isocyanurate ring within the framework of the prepared nanocatalyst, respectively. The signals appearing in 2939 and 2889 cm −1 are related to the stretching vibrations of aliphatic moiety of PMO-ICS. Furthermore, the observed bands at 1108, 1056 and 943 cm −1 correspond to the asymmetric and symmetric stretching vibrations of siloxane Si-O-Si bonds. On the other hand, the signals at 570 cm −1 are related to the The energy dispersive X-ray (EDX) spectrum of the catalyst has been shown in Fig. 4. Indeed, the signals of iron, nitrogen, silicon, carbon, oxygen and zinc atoms were observed in the studied sample. It is well known that three peaks of iron are observed in the EDX spectrum for magnetic iron oxides. Furthermore, this analysis successfully confirms that ZnO nanoparticles are well embedded into the magnetic catalyst.
The structure of Fe 3 O 4 @PMO-ICS-ZnO nanoparticles was also analyzed by XRD spectroscopy. The wide angle XRD pattern shown in The morphology, distribution and size of the particles of Fe 3 O 4 @PMO-ICS-ZnO nanoporous material (1) were analyzed using field emission scanning electron microscope (FESEM, Fig. 6). A uniform and well-defined distribution as well as almost nano-rod morphology were observed for the ZnO nanoparticles embedded in the magnetic periodic mesoporous organosilica. Furthermore, the average particle sizes of Fe 3 O 4 @PMO-ICS-ZnO nanoporous material (1) was found to be in the range of nano scale and about 45-62 nm.  www.nature.com/scientificreports/ Furthermore, the specific surface area and pore size of nano-ordered Fe 3 O 4 @PMO-ICS-ZnO material 1 were determined using N 2 adsorption-desorption isotherms through Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods (Fig. 7). In fact, the observed data demonstrated that this material has a typical mesoporous structure and a type IV isotherm, which is due to the presence of cylindrical pores on the mesoporous scale. Specific surface area, average pore size and total pore volume are approximately 194.9 m 2 g −1 , 7.3 nm, and 0.36 cm 3 g −1 , respectively. Comparison of the obtained BET and BJH data of Fe 3 O 4 @PMO-ICS-ZnO material 1 with the similar data for PMO-ICS 86 and Fe 3 O 4 @PMO-ICS 87 shows decreasment in the amount of surface area and total pore volume (Table 1). Therefore, it can be concluded that ZnO nanoparticles were firmlyembedded and fixed in the magnetic periodic mesoporous organosilica channels.   www.nature.com/scientificreports/ Thermogravimetric analysis (TGA) was measured for the prepared catalyst 1 at temperatures between 40 and 804 °C. Figure 8 shows three distinct weight loss for the Fe 3 O 4 @PMO-ICS-ZnO (1). The first step, with 3.56% weight loss between 40 and 270 °C, is corresponded to the removing of alcoholic or water solvents remaining from the extraction process. The second and main weight loss (14. 20%) at 270 to 570 °C region is attributed to the elimination of 1,3,5-tris(1,3-propylen) isocyanurate bridges incorporated into the PMO framework as well as condensation of its silanol groups. Finally, the last weight loss (2.26%) was observed in the range of 570-804 °C, which is attributed to more condensation of inorganic moieties present in the structure of catalyst 1 including silica and ZnO. These data clearly shows good thermal stability of the Fe 3 O 4 @PMO-ICS-ZnO, which is very important in design and application of recyclable heterogeneous catalytic systems.
The magnetization of Fe 3 O 4 @PMO-ICS-ZnO (1) sample was measured using vibrating sample magnetometer (VSM) experiment. As can be seen in the Fig. 9, the hysteresis phenomenon was not observed. Instead, the observed "S" like curve at room temperature is also a proof of the paramagnetism of the prepared Fe 3 O 4 @PMO-ICS-ZnO. The saturation magnetization value was strongly enhanced by the external magnetic field strength

Optimization of conditions for the synthesis of tetrazole derivatives in the presence of magnetic Fe 3 O 4 @PMO-ICS-ZnO nanocatalyst (1). After characterization of the magnetic Fe 3 O 4 @PMO-
ICS-ZnO nanocatalyst (1), its efficiency was investigated for the synthesis of 1H-tetrazole derivatives. Therefore, the effect of different catalyst loadings, solvent and temperature on the yield and required time of reaction were systematically investigated in this step to find optimal conditions for the synthesis of 1H-tetrazole derivatives. Hence, the reaction of malononitrile (2, 1 mmol) and sodium azide (3, 1.2 mmol) 4-chlorobenzaldehyde (4a, 1 mmol), was selected as the model reaction. The results have been summarized in Table 2. Initially, the model reaction was examined in the absence of any catalyst under various conditions such as in EtOH at ambient temperature or under reflux conditions. The dependence of obtained yield of the model reaction to the catalyst and temperature was evident because of very low yields of the desired product, (E)-3-(4-chlorophenyl)-2-(1H-tetrazol-5-yl)acrylonitrile, 5a under solvent-free conditions at room temperature or reflux conditions after prolonged reaction times (entries 1,2, Table 2). Interestingly, the obtained yield of desired product 5a was significantly improved in the presence of Fe 3 O 4 @PMO-ICS-ZnO nanocatalyst (1) after very short reaction times (entries 3-5, Table 2). These findings demonstrate high catalytic activity of the magnetic nanocatalyst for the synthesis of the desired product 5a, which originates from very good dispersion of active catalytic sites and proper surface area of the Fe 3 O 4 @PMO-ICS-ZnO. In the next experiments, the effect of different solvents such as water, EtOH, water/EtOH mixture, toluene, DMF, EtOAc and CH 3 CN as well as solvent-free conditions on the reaction rate was investigated (entries 6-12, Table 2). Indeed, both water and EtOH solvents afforded excellent yields of the desired product 5a after same reaction time (entries 5, 6, Table 2). However, separation of the magnetic catalyst 1 is much easier in EtOH than water due to higher solubility of the desired 1H-tetrazole products at elevated temperatures and remaining the heterogeneous catalyst 1. Therefore, 10 mg Fe 3 O 4 @PMO-ICS-ZnO loading in EtOH under reflux conditions was selected as the optimized conditions for further experiments (entries 3, 13, 14, Table 2). On the other hand, both pure PMO-ICS and magnetic PMO-ICS afforded lower yields of the desired product 5a under similar conditions compared to the Fe 3 O 4 @PMO-ICS-ZnO nanocatalyst (1) (entries 15, 16, Table 2). All of these data show effective role of ZnO nanoparticles embedded in the thermally stable magnetic periodic mesoporous organosilica.
In the next step, the optimized conditions were expanded to other aromatic aldehydes 4b-o to investigate the scope of reaction for the preparation of diverse 1H-tetrazole derivatives in the presence of magnetic Fe 3 O 4 @ PMO-ICS-ZnO nanocatalyst (1). Excellent yields were obtained from a variety of aromatic carbocyclic or heterocyclic aldehydes 4a-o under optimized conditions. As data in Table 3 show, high to excellent yields of the desired products 5a-o were obtained within short reaction times. In this regard, aromatic aldehydes with carbocyclic ring bearing electron withdrawing groups 4a-g and electron-deficient heterocycle 4h were examined (entries 1-8, Table 3). On the other hand, aromatic aldehydes with carbocyclic ring bearing electron donating groups 4j-n as well as electron-rich heterocycle 4o survived to involve in the optimized conditions to afford the corresponding 1H-tetrazole derivatives 5j-o (entries 10-15, Table 3).
In the first reaction step, the aromatic aldehydes (4)

Investigating of the reusability of magnetic Fe 3 O 4 @PMO-ICS-ZnO (1) for the synthesis of 5-substituted-1H-tetrazole derivatives 5. As a part of our study, the reusability of magnetic Fe 3 O 4 @
PMO-ICS-ZnO (1) for the synthesis of (E)-3-(4-chlorophenyl)-2-(1H-tetrazol-5-yl)acrylonitrile 5a was investigated in the next step. The catalyst 1 was easily separated after completion of the model reaction under optimized conditions by an external magnet. The recycled Fe 3 O 4 @PMO-ICS-ZnO was separately dispersed in EtOH and EtOAc for 15 min, respectively. The recycled catalyst was kept in an oven at 60 °C for 1 h and then reused for the next runs. The results of the catalyst recycling for five consecutive runs are given in Fig. 11. As data in Fig. 10 demonstrate, only a slight decrease in reaction yield was observed after fourth run. These findings indicate the structural stability of magnetic the Fe 3 O 4 @PMO-ICS-ZnO (1) during synthesis of 5-substituted-1H-tetrazole derivatives 5. Table 4 compares the previously reported methods for the synthesis of tetrazole 5a with the present protocol. It is apparent that high to excellent yields, avoiding the use of toxic or carcinogenic solvents such as DMF, short reaction time and easy separation of the catalyst from the reaction mixture are the advantages of the present protocol compared to the most of previously reported methods. www.nature.com/scientificreports/ Experimental Reagents and instruments. All chemical substances and reagents with high purity were purchased from

Comparison of the catalytic activity of nano-ordered Fe 3 O 4 @PMO-ICS-ZnO (1) in the synthesis of tetrazole derivatives with other catalytic systems.
Merck or Aldrich and used as received, except for liquid aldehydes which were distilled before their using. The progress of reactions and the purity of the obtained products were monitored by thin layer chromatography (TLC) using Merck aluminum plates coated with 0.2 mm silica gel F254. Melting points were measured using an Electrothermal 9100 device and are uncorrected. Characterization of the magnetic catalyst 1 as well as identification of products was performed using KBr discs on a Shimaduzu FTIR-8400S spectrometer. A Bruker DRX-500 Avance spectrometer was used for recording of 1 H NMR (500 MHz) and 13 C NMR (125 MHz) spectra of products in DMSO-d 6 at ambient temperature. The BET specific surface area analysis was performed using ASAP 2020™ instrument. Thermal gravimetric analysis data was obtained by a Bahr company STA 504 equipment. X-Ray diffraction pattern was prepared using a STOE apparatus with CuKα radiation source. Field emission scanning electron microscopy images were recorded by a Zeiss (EM10C) device. VSM analysis was performed using a Lakeshore 7410 series instrument (Supplementary file).
Typical procedure for the preparation of PMO-ICS. The periodic mesoporous organosilica denoted PMO-ICS was prepared according to the method introduced by Jaroniec 122 . This PMO-ICS was synthesized by self-assembly of tris [3-(trimethoxysilyl)propyl] isocyanurate (ICS, Aldrich), tetraethyl orthosilicate (TEOS, Aldrich) in the presence of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer (Pluronic 123, Aldrich, average Mw ≅ 5800 Dalton) under acidic conditions. In a typical experiment, P123 (2.0 g) was added into a 250 mL beaker and a mixture of deionized water (15 mL) and hydrochloric acid solution (2.0 M, 60 mL) was slowly added and stirred until P123 is completely dissolved. Then, ICS (0.01 mol, 3.08 g) and TEOS (0.03 mol, 3.12 g) were simultaneously added dropwise into the obtained solution. After that, the obtained white gel and precipitates was stirred at room temperature for 24 h in a round bottom flask.   Table 3. The reaction progress was monitored by TLC (Eluent: EtOAc/n-hexane, 1:3). After completion of Figure 10. A reasonable mechanism for the one-pot preparation of (E)-2-(1H-tetrazole-5-yl)-3-aryl/ heteroarylacrylenenitrile derivatives 5 using the magnetic nano-ordered Fe 3 O 4 @PMO-ICS-ZnO catalyst (1). www.nature.com/scientificreports/ the reaction, Fe 3 O 4 @PMO-ICS-ZnO catalyst was easily separated from the reaction mixture using an external magnet and the desired product 5 was crystallized by dropwise adding of distilled water to the ethanolic solution. The structure of products 5a-o was confirmed by melting point as well as FTIR, 1 H NMR and 13 C NMR spectroscopy.

Conclusions
Novel magnetic Fe 3 O 4 @PMO-ICS-ZnO nano-ordered catalyst was prepared through a modified environmentally-benign procedure and properly characterized. The catalytic activity of the new thermally stable Fe 3 O 4 @ PMO-ICS-ZnO nanomaterial was demonstrated in the one-pot synthesis of tetrazole derivatives through cascade condensation and concerted 1,3-cycloaddition reactions as a representative of the Click Chemistry concept. Different aromatic aldehydes survived to involve smoothly in the optimized conditions affording the corresponding 5-substituted-1H-tetrazole derivatives in high to quantitative yields and good purity. Low catalyst loading, the use of green solvents such as EtOH or water instead of carcinogenic DMF and short reaction time as well as easy separation and recyclability of the catalyst for at least five consecutive runs without significant loss of its activity are notable advantages of this new protocol compared to other recent introduced procedures.   www.nature.com/scientificreports/