New ecofriendly heterogeneous nano-catalyst for the synthesis of 1-substituted and 5-substituted 1H-tetrazole derivatives

A novel ecofriendly heterogeneous catalyst containing Schiff base coordinated Cu(II) covalently attached to Fe3O4@SiO2 nanoparticles through imidazolium linker [Fe3O4@SiO2-Im(Br)-SB-Cu (II)] was synthesized and characterized by using various techniques. The catalytic efficiency of this nano-catalyst was tested in water in the synthesis of tetrazole derivatives using two one-pot multicomponent reaction (MCR) models: The synthesis of 1-aryl 1H-tetrazole derivatives from the reaction of aniline, triethyl orthoformate, and sodium azide and the synthesis of 5-aryl 1H-tetrazole derivatives from the reaction of benzaldehyde, hydroxy amine hydrochloride, and sodium azide. The investigation showed that (i) The catalyst is highly efficient in the synthesis of tetrazole derivatives with high yield (97%) in aqueous medium and mild temperatures; (ii) The catalytic effectiveness is due to the synergy between the metallic center and the imidazolium ion and (iii) The reuse advantage of the catalyst without contamination or significant loss (12% of loss range) in the catalytic activity.

All chemical reagents were purchased from Sigma-Aldrich chemical company and used without further purification. The progress of reactions was monitored by TLC on Silica-gel Polygram SILG/UV254 plates. Fourier Transform Infrared (FT-IR) spectra were recorded on a PerkinElmer 780 FT-IR spectrometer (KBr tablets). The morphology (SEM) and elemental analysis (EDS) of the catalyst were determined by using the FE-SEM TESCAN MIRA3 instrument. Transmission Electron Microscopy (TEM) images were obtained with a Philips EM208 S electron microscope. X-ray Diffraction (XRD) patterns were collected using a Philips PW 1730 diffractometer using Cu Kα radiation (λ = 1.54 A°). Thermogravimetric Analysis (TGA) was performed on a Q600 TA instrument at 30-700 °C with a heating rate of 20 °C min −1 in an argon atmosphere. Vibrating Sample Magnetometer (VSM) analysis was performed at room temperature using an LBKFB instrument. Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES) analysis was performed using a Simultaneous VISTA-PRO instrument. Atomic Absorption Spectroscopy (AAS) analysis was performed using a Shimadzu AA6200 instrument.

The synthesis of modified silica coated Fe 3 O 4 nanoparticles (Fe 3 O 4 @SiO 2 ). The silica coated
Fe 3 O 4 magnetic nanoparticles were synthesized by previously reported methods 34 . FeCl 3 .6H 2 O (6.8 g) and FeCl 2 .4H 2 O (2.5 g) were added to deionized water (300 mL) and stirred under nitrogen gas at room temperature. Gradually, ammonia solution (25% w/w, 70 mL) was added to the vigorously stirred mixture. As soon as the solution's color turned black, the resulting nanoparticles were separated by an external magnet and washed several times with deionized water.
To synthesize silica-coated nanoparticles, Fe 3 O 4 nanoparticles (3.0 g) were dispersed by sonication in a deionized water/ethanol solvent mixture (1:4 v/v, 500 mL) 30 min. Then a solution of ammonia (25% w/w) was gradually added until the pH reaches 10. The tetraethyl orthosilicate (TEOS, 20 mL) was slowly added to the mixture and stirred three hours at 50 °C. The silica-coated nanoparticles (Fe 3 O 4 @SiO 2 ) were collected by a permanent magnet and washed with deionized water and ethanol several times and dried in a vacuum oven at 50 °C for 24 h. In the final stage, Fe 3 O 4 @SiO 2 (1 g) was sonicated in dry toluene (40 mL) for 30 min. Then, 3-Chloropropyl triethoxysilane (2.0 mL) was added dropwise and refluxed for 20 h. The resulting chloro-modified Fe 3 O 4 @SiO 2 was removed from the reaction mixture by a strong magnet, washed with in toluene, ethanol and diethyl ether for several times. Then dried under vacuum at 60 °C for 12 h 35 . The loading amount of Cl atom was 0.3 mmol per gram catalyst based on EDX.
The synthesis of Fe 3 O 4 @ SiO 2 -Im nanoparticles. Imidazole (0.5 mmol, 0.034 g) was added to the dispersed solution of chloro-modified Fe 3 O 4 @SiO 2 (1.0 g) in dry toluene (40 mL) and triethylamine (NEt 3 , 0.5 mmol, 0.05 g) was added dropwise and refluxed for 24 h. The resulting nanoparticles were separated with the external magnet and washed with distilled water and ethanol. The resulted Fe 3 O 4 @ SiO 2 -Im nanoparticles were dried in a vacuum oven at 80 °C for 12 h.  -SB-Cu (II) nano-catalyst (0.6 mol%, 0.008 g) were stirred at 40 °C. The reaction progression was monitored by thin-layer chromatography (TLC) at different interval of time using n-Hexane/ Ethyl acetate (4:1) as eluent. At the end, the reaction mixture was cooled, and catalyst removed by an external magnet. The reaction mixture was extracted with 3 × 10 mL of ethyl acetate. The organic phase was dried with anhydrous Na 2 SO 4 , filtered and then evaporated. The pure product was obtained by recrystallization in a mixture of n-Hexane/Ethyl acetate. The recovered yield was 97%.
General procedure for the synthesis of 5-substituted 1H-tetrazole. Benzaldehyde (1.0 mmol, 0.1 mL), hydroxylammonium chloride (1.0 mmol, 0.07 g), sodium azide (1.2 mmol, 0.08 g) in water (1.0 mL) in the presence of Fe 3 O 4 @ SiO 2 -Im[Br]-SB-Cu (II) nano-catalyst (0.9 mol%, 0.012 g) were stirred at 40 °C. The reaction was followed by thin-layer chromatography (TLC) at different time intervals in (n-Hexane/Ethyl acetate: 4:1). The reaction mixture was cooled, and the catalyst removed by an external magnet. 5 mL of HCl (5 N) was added to the reaction mixture and extracted with 3 × 10 mL of ethyl acetate. The organic phase was extracted again with the HCl solution (1 N), followed by a saturated solution of NaCl. The organic phase was dried with the anhydrous Na 2 SO 4 , filtered and then evaporated. The pure product was obtained in 97% yield by recrystallization with n-Hexane/Ethyl acetate solvent mixture.

Results and discussion
Catalyst characterization. Fe 3 O 4 @SiO 2 -Im[Br]-SB-Cu(II) nano-catalyst was synthesized as depicted in Fig. 1 and investigated in the synthesis of tetrazole derivatives (Fig. 2). The prepared catalyst is characterized by various methods. The FT-IR spectra of the catalyst synthesis steps are shown in Fig. 3. The spectrum 1a, which corresponds to the Fe 3 O 4 nanoparticles, show the peaks at 571 and 3442 cm −1 corresponding to stretching vibrations of Fe-O and OH groups, respectively 36,37 . The appearance of new peaks at 1077 and 1192 cm −1 are corresponding to Si-O (Symm.) and Si-O (Asymm.), respectively. These peaks are a confirmation that the surface of nanoparticles is protected by silica coating layer (Fig. 3, 1b) 38 . The transmittance of core shelled Fe 3 O 4 nanoparticles was slightly lower than that of Fe 3 O 4 nanoparticles due to silica coating. Absorbed peaks in 2852 (Symm.), 2934 (Asymm.), 1420 (Bending) and 814 cm −1 , respectively, correspond to CH 2 and C-Cl are evidence of the modification of nanoparticles surface (Fig. 3, 1c) 39 . The disappearing C-Cl peak and the appearance of new peaks in 1632 and 1742 cm −1 indicate that the imidazole ring was coupled to the nanoparticles surface ( Fig. 3, 1d) 40 . The spectrum 1e shows peak in 3422 cm −1 , which correspond to NH of amine group. The new peak at 1636 cm −1 is evidence of the formation of imine (Fig. 3, 1f) 41 . The new peaks at 635 and 620 cm −1 correspond to Cu-N and Cu-O. Also, the transfer of imine peak to lower frequencies confirms the formation of the metal complex ( Fig. 3, 1g). The morphology of Fe 3 O 4 @SiO 2 -Im[Br]-SB-Cu (II) nano-catalyst was determined by Scanning Electron Microscopy (SEM). SEM images show spherical and irregular shapes for the nanoparticles (Fig. 6).
The morphology of Fe 3 O 4 @SiO 2 -Im[Br]-SB-Cu (II) nano-catalyst was also determined by Transmission electron microscopy (TEM) (Fig. 7a). Also, according to the histogram diagram of the nano-catalyst, the average particle size was estimated to be about 24 nm (Fig. 7b).
The These results indicate that the crystalline cubic structure of nanoparticles Fe 3 O 4 is preserved during the catalyst preparation process. In the Fe 3 O 4 @SiO 2 spectrum, a broad peak is observed in 2θ = 10-20°, which is related to amorphous silica. This broad peak for the nano-catalyst was shifted to lower angles due to the synergetic effect of amorphous silica and Cu(II)-coordinated Schiff base. The average size of nanoparticles was calculated by the Debye-Scherrer equation (D = K.λ/β.cosθ, λ (wavelength, 0.154 nm), K (a crystallized form factor, 0.94), β (Full width at half maximum, (rad)), θ (Bragg reflection angle, (°)) to be about 28 nm which correspond to the TEM results. The magnetic property of nano-catalyst was measured at different steps of the synthesis by vibrating sample magnetometer (VSM) (Fig. 9). As shown in Fig. 9, the magnetic properties of nanoparticles are gradually reduced by the silica layer coating and by the coupling of the Cu(II)-complex to the surface of the nanoparticles. Although the magnetic saturation values for Fe 3    www.nature.com/scientificreports/ g -1 , respectively, nano-catalyst has still a strong magnetic property for its removal from the reaction mixture by an external magnet. This was confirmed by the catalyst recycling study (see Fig. 15). The thermal stability of Fe 3 O 4 @SiO 2 -Im[Br]-SB-Cu (II) nano-catalyst was examined by TGA technique (Fig. 10). In the thermogram diagram of this catalyst, the maximum weight loss occurs in the range of 414-500 °C (15%), which is related to removing organic compounds from the surface of the catalyst. The 5% weight loss between 138 and 414 °C, is related to removing some organic compounds and adsorbed water molecules on the surface of iron oxide nanoparticles, respectively.
The   -SB-Cu (II) was investigated in the synthesis of 1-aryl and 5-aryl 1H-tetrazole derivatives. The synthesis of 1-aryl 1H-tetrazole derivatives was optimized using the reaction model of aniline, triethyl orthoformate, and sodium azide (Fig. 11). The results of this investigation are summarized in Table 1. Firstly, the reaction efficiency in polar protic solvents (Entries 1-3) is higher than in polar aprotic and non-polar solvents (Entries 4-7). This is probably due to the ionic nature of catalyst. The reaction was run in the presence of different level of catalyst and the best result was obtained with 0.6 mol% of catalyst. A control reaction with 0 mol% of catalyst was run and as expected no product was obtained (Entry 9). Other reactions controls were tried in the presence of Fe 3 O 4 and Fe 3 O 4 @SiO 2 -Im[Br] (Entries 14, 15) with very low efficiency. The effect of the temperature and time on the reaction were also investigated (Entries 16-21) and we concluded that the best conditions are: H 2 O solvent, 0.6 mol% of the catalyst loading, time 20 min and 40 °C. To determine the accuracy of the data generated using the small scale reported in Table 1, the best run (Entry 1) was repeated using 10 mmol scale under the same conditions and the yield of the reaction was reproducible.
After optimizing the reaction conditions, different 1-aryl 1H-tetrazole derivatives were synthesized by using different aniline derivatives under the same conditions ( Table 2). The reaction in the presence of electrondonating and electron-withdrawing groups on benzaldehyde and the spatial barrier on aniline have significant impact on the reaction efficacy (Entries 2, 3 & 7, 8).
The plausible mechanism for the synthesis of 1-aryl 1H-tetrazole derivatives by using Fe 3 O 4 @SiO 2 @Im[Br]-SB-Cu (II) nano-catalyst is depicted in Fig. 12 44 . Triethyl orthoformate is activated by the N 3 -coordinated Cu(II) Nano-catalyst followed by aromatic amine attacks on the triethyl orthoformate resulting in the formation of an amide acetal intermediate. The nucleophilic attack of the azide anion on the amide acetal followed by cyclization lead to the desired tetrazole.   www.nature.com/scientificreports/ The reaction model using benzaldehyde, hydroxy amine hydrochloride and sodium azide was selected to optimize the conditions for the synthesis of 5-aryl 1H-tetrazole derivatives (Fig. 13). The results of this investigation are shown in Table 3. Firstly, water was identified as the solvent of choice (Entries 1-8). As in the case of 5-aryl 1H-tetrazole derivatives, we confirmed that the reaction needs the catalyst to proceed and no product was obtained in the absence of the catalyst (Entries 9-13). In this reaction model, the highest conversion rate was obtained in the presence of 0.9 mol% of catalyst (Entry 1). Also in the presence of the precursor of our catalyst (Fe 3 O 4 and Fe 3 O 4 @SiO 2 -Im[Br]), the reaction conversion rate was very low (Entries 14, 15). The effect of temperature and reaction time was also investigated (Entries [16][17][18][19][20][21] and the best conditions are: H 2 O as solvent, 0.9 mol% of the catalyst, time 20 min and 40 °C. To determine the accuracy of the data generated using the small scale reactions reported in Table 3, the best run (Entry 1) was repeated using 10 × the scale under the same conditions and the yield of the reaction (96was reproducible. Different 5-aryl 1H-tetrazole derivatives were synthesized using different aryl-aldehyde derivatives under the optimized conditions ( Table 4). The results show that the reaction efficiency is impacted by the electronic properties and the position of the substituents groups on the benzaldehyde ring.
The hypothetical mechanism for the formation of 5-aryl 1H-tetrazole derivatives using Fe 3 O 4 @SiO 2 -Im[Br]-SB-Cu (II) nano-catalyst is illustrated in Fig. 14 50 . The carbonyl of the aryl-aldehyde is activated by the Cu(II)-catalyst leading to the oxime formation by the attack of hydroxyl ammonium chloride. The formed Cu(II)-activated       [3 + 2] cycloaddition with the azide to yield the desired 5-aryl tetrazole after elimination of a water molecule. We studied the reusability of Fe 3 O 4 @SiO 2 -Im[Br]-SB-Cu (II) in the synthesis of tetrazole derivatives. After completing the reaction, the catalyst was separated by an external magnet from the reaction mixture, washed with ethyl acetate, dried, and reused in subsequent catalytic cycles under the same reaction conditions. The recycled catalyst was successfully reused for eight runs (Fig. 15) with a maximum loss of ~ 12% of yield. The FT-IR comparison of fresh and reused catalysts is shown in Fig. 16 with no change in the catalyst structure.
To investigate the heterogeneous nature of our catalyst, the hot filtration test was performed. A hot filtration test was performed to evaluate the metal leaching rate and assess if the catalytic activity of our catalyst is not due to leached Cu(II) species in the reaction mixture (Fig. 17). In the model reaction for the synthesis of 1-aryl 1H-tetrazole derivatives after the half reaction time in which the reaction conversion rate is 50%, the reaction was stopped, and the catalyst was removed with an external magnet. The reaction mixture without the catalyst was then allowed to proceed further for 60 min. After the separation of catalyst from the reaction mixture, no increase in conversion was observed. This is a strong indication that the catalytic process is taking place only in the presence of the nano-catalyst and confirms the heterogeneity of the catalytic process. The test also indicates that there is no active copper metal species in the synthesis of tetrazole was leached into the reaction mixture.
Our catalyst was benchmarked against published catalyst for the synthesis of tetrazole derivatives ( Table 5). The data in Table 5 show that our catalyst (Entry 10) is more efficient than the other reported catalysts in terms of yield and reaction time.

Conclusion
We have reported the preparation of novel heterogeneous recoverable and reusable nano-catalyst, Fe 3 O 4 @SiO 2 -Schiff base-Cu(II) complex, which is able to catalyze green formation of 1-and 5-substituted 1H-tetrazoles using multicomponent reaction (MCR) approach between aromatic amines, ethyl orthoformate and sodium azide for the preparation of 1-aryl 1H-tetrazole derivatives and between aryl-aldehydes, hydroxylamine hydrochloride and sodium azide for the synthesis of 5-aryl 1H-tetrazole under mild conditions and short reaction time in water. The catalyst was well characterized by various techniques including FT-IR, VSM, XRD, EDX, FE-SEM, TEM, TGA, ICP and AAS. This nano-catalyst simplicity, high efficiency, convenient reusability, ease of work-up are among its critical advantages. Various aromatic aldehydes and aromatic amines served as suitable substrates for the preparation of substituted tetrazoles via MCR protocol with high to excellent yields. This nano-catalytic system has undeniably proven as an efficient and ecofriendly catalyst for aryl-substituted tetrazoles synthesis. Future investigation in the synthesis of alkyl-substituted tetrazoles using aliphatic aldehydes and amines will determine the scope of this nano-catalyst application.