A multidentate copper complex on magnetic biochar nanoparticles as a practical and recoverable nanocatalyst for the selective synthesis of tetrazole derivatives

Waste recycling, novel and easy methods of recycling catalysts, use of green solvents, use of selective catalysts and preventing the production of by-products are the most important principles of green chemistry and modern technology. Therefore, in this work, biochar nanoparticles (B-NPs) were synthesized by the pyrolysis of chicken manure as a novel method for waste recycling. Subsequently, the B-NPs were magnetized by Fe(0) nanoparticles to improve the easy recovery of biochar. Then, the surface of biochar magnetic nanoparticles (FeB-MNPs) was modified by (3-chloropropyl)trimethoxysilane (3Cl-PTMS). Finally, a multidentate copper complex of 2,2′-(propane-1,3-diylbis(oxy))dianiline (P.bis(OA)) was immobilized on the surface of modified FeB-MNPs, which was labeled as Cu-P.bis(OA)@FeB-MNPs. Cu-P.bis(OA)@FeB-MNPs was investigated as a commercial, homoselective, practical, and recyclable nanocatalyst in the synthesis of 5-substituted-1H-tetrazole compounds through the [3 + 2] cycloaddition of sodium azide (NaN3) and organo-nitriles in polyethylene glycol 400 (PEG-400) as a green solvent. Cu-P.bis(OA)@FeB-MNPs was characterized using wavelength dispersive X-ray (WDX) spectroscopy, scanning electron microscopy (SEM), thermogravimetric analysis (TGA), energy-dispersive X-ray spectroscopy (EDS), vibrating-sample magnetometer (VSM), atomic absorption spectroscopy (AAS) and N2 adsorption–desorption (Brunauer–Emmett–Teller (BET) method) techniques. Cu-P.bis(OA)@FeB-MNPs was recovered and reused for several runs in the synthesis of tetrazoles.


Introduction
Chemical sciences is one of the most important elds for development in the world, which provides many applications in the elds of medicine and various industries.However, unfortunately, waste chemical materials have also been introduced into the environment during the growth of chemical science.Therefore, principles of green chemistry were introduced, which minimize the environmental damage from chemical industries and laboratories.2][3][4][5] Acids, bases, and transition metals or metal complexes are among the most wellknown types of catalysts.In general, catalysis systems are divided into two categories: homogeneous catalysis systems and heterogeneous catalysis systems. 6Homogeneous catalysts are known to exhibit several properties such as high performance, selectivity, instability, difficult to recover, and time-consuming product purication. 7,8On the other side, heterogeneous catalysts are known to exhibit different properties such as good recoverability, high stability, low efficiency, and selectivity. 7,8][10][11][12] Because decreasing the particle size provides a high surface area, the catalytic activity and selectivity are increased (e.g., homogeneous catalysts).In addition, nanomaterials have high stability and are heterogeneous in nature, enabling them to be easily recovered and reused like heterogeneous catalysts.In this context, various nanoparticles such as boehmite, [13][14][15] mesoporous silica materials, [16][17][18] graphene oxide, [19][20][21] MOF compounds, [22][23][24][25][26][27][28][29][30][31] carbon nanostructures, 32,33 polymers, 34 biochar, 35,36 and magnetic particles 37 have been reported as catalysts or catalyst supports.However, most of these materials are synthesized from mineral and non-renewable chemical compounds, which are against the principles of green and modern chemistry.9][40] As is known, the use of renewable materials and waste recycling are other principles of green chemistry. 41,42The synthesis of biochar nanoparticles from natural and renewable sources is very important in expanding its application.Considering the increasing importance of the catalysts in various industries and laboratories, the introduction of the catalysts (e.g.biochar) made from renewable sources is a necessity for the future.On the other hand, the increase in the population has led to the accumulation of a large amount of waste, which has become a challenge for the planet and the future of mankind.Therefore, recently, the technology of recycling waste and turning waste into valuable materials is of special interest, so that one of the principles of green chemistry is dedicated to this challenge.Considering that biochar synthesis is a novel method for waste recycling, it doubles the importance of expanding the use of biochar.Therefore, in this work, biochar nanoparticles were synthesized by the pyrolysis of chicken manure as a method for recycling agricultural waste.4][45][46][47] Despite the special advantages of biochar, it has rarely been reported as a catalyst or catalyst support.Despite the unique advantages of biochar, the separation and recycling of biochar nanoparticles require time-consuming and difficult methods such as centrifugation and ltration.9][50][51] For example, in 2024 biochar was magnetized by magnetic Fe 3 O 4 nanoparticles 36 and magnetic Ni nanoparticles. 48But, biochar, and Fe(0) particleswith high surface area and stabilityhave rarely been used for the magnetization of materials. 52Therefore, in this work, a complex of copper on magnetic biochar nanoparticles (Cu-P.bis(OA)@FeB-MNPs)was prepared as a new recyclable nanocatalyst for the homoselective synthesis of 5-substituted-1H-tetrazole compounds through [3 +  2] the cycloaddition of NaN 3 and organo-nitriles in PEG-400 as a green solvent.This is the rst report on a copper complex of 2,2 0 -(propane-1,3-diylbis(oxy))dianiline on magnetic biochar nanoparticles and its introduction as a green catalyst in the synthesis of organic compounds.

Preparation of biochar magnetic nanoparticles (FeB-MNPs)
Biochar nanoparticles were formed by simple pyrolysis of chicken manure.In this context, 500 g of dried chicken manure was heated at 400 °C as the pyrolysis temperature for 1 h under N 2 sweeping.At the end of pyrolysis, the heating was stopped and the Chinese crucibles were cooled with N 2 sweep.The resulting black solid was biochar.Then, 4.5 g of the synthesized biochar was mixed with FeCl 2 $4H 2 O (5.34 g) in ethanol (25 mL) and H 2 O (5 mL), and then, it was stirred at room temperature for 15 min.Then, NaBH 4 (2.5 g in 70 mL of H 2 O) as a reduction agent was injected into the mixture dropwise within 20 min.The obtained mixture was stirred at room temperature for 15 min.The magnetic powder (biochar magnetic nanoparticles) was ltered by magnetic decantation, washed with ethanol and dried for 6 h at 90 °C. 52

Modication of FeB-MNPs with (3-chloropropyl) trimethoxysilane (3Cl-PTMS)
FeB-MNPs (1 g) were dispersed in n-hexane (25 mL) for 30 min by an ultrasonic bath.Then, (3-chloropropyl)trimethoxysilane (1.5 mL) was injected into it and was allowed to stir for 24 h under reux conditions.Aer 24 h, the mixture was cooled to room temperature, and the modied FeB-MNPs (3Cl-PTMS@FeB-MNPs) were separated by magnetic decantation and washed with ethanol.The modied FeB-MNPs were dried at 60 °C (Scheme 1).

Paper
Nanoscale Advances 25 min in an ultrasonic bath.Then, the mixture was stirred for 72 h under reux conditions.The mixture was allowed to cool to room temperature.Aer that, the functionalized FeB-MNPs with P.bis(OA) (P.bis(OA)@FeB-MNPs) were ltered by magnetic decantation and then washed 5 times with dimethyl sulfoxide (DMSO) and ethanol.The obtained P.bis(OA)@FeB-MNPs were dried at room temperature (Scheme 2).
2.4.Synthesis of Cu-P.bis(OA)@FeB-MNPsP.bis(OA)@FeB-MNPs (1 g) was mixed with Cu(NO 3 ) 2 $9H 2 O (2 mmol) in 25 mL of ethanol.The mixture was stirred under reux conditions for 24 h.Aer that, the mixture was cooled.Then, the prepared nal catalyst (Cu-P.bis(OA)@FeB-MNPs)was separated by magnetic decantation and washed several times with water and ethanol to remove excess copper from the mixture.Finally, Cu-P.bis(OA)@FeB-MNPs were kept at a temperature of 50 °C, which was dried (Scheme 3).

Synthesis of tetrazoles catalyzed by Cu-P.bis(OA)@FeB-MNPs
The catalytic application of Cu-P.bis(OA)@FeB-MNPs was investigated in the synthesis of tetrazoles through a ring addition reaction of nitrile and sodium azide (Scheme 4).A mixture of nitrile (1 mmol), sodium azide (1.4 mmol) in PEG-400 solvent, and 30 mg of Cu-P.bis(OA)@FeB-MNPs was stirred at a temperature of 120 °C.The reaction time was monitored by thin-layer chromatography (TLC) using UV wavelengths of 254 and 356 nm in acetone : n-hexane (8 : 2) tank solvent.Aer completion of the reaction, the reaction mixture was cooled and Scheme 4 Synthesis of tetrazoles in the presence of Cu-P.bis(OA) @FeB-MNPs.
diluted with ethyl acetate, distilled water, and HCl solution (4 N).Then, the Cu-P.bis(OA)@FeB-MNPscatalyst was isolated from the mixture and washed with ethyl acetate and HCl solution (4 N).Aer that, tetrazole products were extracted in ethyl acetate solvent.The organic phase was removed by evaporation to obtain 5-substituted 1H-tetrazoles.

Results and discussion
In this work, a heterogeneous catalyst of copper complex was immobilized on FeB-MNPs.Then, this nanocatalyst was characterized by WDX, SEM, TGA, EDS, VSM, AAS, and BET techniques.Then, its catalytic performance was investigated in the homoselective synthesis of tetrazoles.
3.1.Particle size and morphological identication of Cu-P.bis(OA)@FeB-MNPs by SEM Fig. 1 shows the SEM images of Cu-P.bis(OA)@FeB-MNPs.As can be seen in SEM images, Cu-P.bis(OA)@FeB-MNPs have similar spherical shapes and uniform diameters of less than 70 nm.Also, the observed agglomeration of its particles in the SEM images is due to the magnetic nature of these nanoparticles.
3.2.Qualitative studying of the elements content of Cu-P.bis(OA)@FeB-MNPsEDS analysis of Cu-P.bis(OA)@FeB-MNPs is shown in Fig. 2. Magnetic biochar nanoparticles are composed of C, O, and Fe, and aer their surface modication, it is functionalized with (3chloropropyl)trimethoxysilane.Also, the 2,2 0 -(propane-1,3diylbis(oxy))dianiline ligand is composed of N, O, and C, which is complexed with Cu metal.Therefore, the composition Fig. 2 The EDS diagram of Cu-P.bis(OA)@FeB-MNPs.Considering that silicon element was not present in the structure of the magnetic biochar nanoparticles and the elemental composition difference between FeB-MNPs and 3Cl-PTMS@FeB-MNPs is the presence or absence of silicon, therefore, the presence of silicon element in the elemental composition of 3Cl-PTMS@FeB-MNPs shows the successful modication of FeB-MNPs with (3-chloropropyl)trimethoxysilane.The presence of N element in Cu-P.bis(OA)@FeB-MNPs conrmed that the P.bis(OA) ligand was successfully immobilized on the surface of FeB-MNPs.Also, the presence of Cu element conrmed that copper complex was successfully synthesized on the surface of the modied FeB-MNPs.WDX analysis is another method for the qualitative study of the element content and their distribution in a sample.The WDX images of Cu-P.bis(OA)@FeB-MNPs are shown in Fig. 3.The WDX images conrmed the presence of C, N, O, Si, Fe, and Cu elements, which is in agreement with the obtained results Fig. 4 TGA/DTG diagrams for Cu-P.bis(OA)@FeB-MNPs.a The products were separated using thin layer chromatography.Paper Nanoscale Advances from EDS analysis.In addition, the WDX images show a quite homogeneous distribution of C, N, O, Si, Fe, and Cu elements in the structure of Cu-P.bis(OA)@FeB-MNPs.

Nanoscale Advances Paper
Because the copper element is the main catalytic active site of Cu-P.bis(OA)@FeB-MNPs, the exact concentration of Cumetal was determined by AAS analysis, which was found to be 0.72 × 10 −3 mol g −1 .

Studying the amount of organic ligand on FeB-MNPs using TGA
The content of Cl-PTMS and P.bis(OA) as organic layers that immobilized on FeB-MNPs was studied by TGA analysis.TGA and differential thermogravimetry (DTG) diagrams of Cu-P.bis(OA)@FeB-MNPs are outlined in Fig. 4. The TGA diagram of Cu-P.bis(OA)@FeB-MNPs indicated three steps of weight loss.The rst of them is due to the evaporation of the absorbent solvents, which happened below 200 °C (about 3% of weight).It is very important that no weight loss happens up to 200 °C, except for the evaporation of the absorbent solvents, which means that Cu-P.bis(OA)@FeB-MNPs is stable and applicable up to 200 °C.The immobilized organic ligands on FeB-MNPs were decomposed aer 200 °C, that indicated as the second step of weight loss in the TGA diagram.Therefore, P.bis(OA) ligand was successfully immobilized on the surface of FeB-MNPs.Finally, a small weight loss above 700 °C may be due to the continuation of biochar pyrolysis.

N 2 adsorption-desorption isotherms of Cu-P.bis(OA) @FeB-MNPs
The nitrogen adsorption/desorption technique is commonly used for the determination of the pore volume, pore diameter, and surface area of the materials.The resulting isotherms of nitrogen adsorption/desorption for Cu-P.bis(OA)@FeB-MNPs are investigated in Fig. 5.These isotherms display type H3 based on the IUPAC classication, which shows the pores structure of Cu-P.bis(OA)@FeB-MNPs. 52Based on BET results, the specic surface area of Cu-P.bis(OA)@FeB-MNPs is 55.23 m 2 g −1 .Also, the total pore volume of Cu-P.bis(OA)@FeB-MNPs is 0.09 cm 3 g −1 , and the average pore diameter of Cu-P.bis(OA)@FeB-MNPs is 6.81 nm.

VSM curve of Cu-P.bis(OA)@FeB-MNPs
The magnetic property of Cu-P.bis(OA)@FeB-MNPs was studied with VSM by the "Magnetic Kavir Kashan" device, which is outlined in Fig. 6.As shown, Cu-P.bis(OA)@FeB-MNPs showed 0.8 emu g −1 , which is lower than the reported saturation magnetization for FeB-MNPs. 52The decrease in the saturation magnetic property of Cu-P.bis(OA)@FeB-MNPs is due to the graing of Cucomplex, P.bis(OA) ligand and silica layer on FeB-MNPs.

Catalytic performance of Cu-P.bis(OA)@FeB-MNPs in the synthesis of tetrazoles
The catalytic application of Cu-P.bis(OA)@FeB-MNPs was examined for the synthesis of tetrazoles.This catalyst showed a high efficiency and good selectivity in the cyclization reaction of [2 + 3] nitriles with sodium azide in the synthesis of tetrazoles (Scheme 5).

Nanoscale Advances Paper
the optimal conditions.As clearly shown in Table 1, the production of 5-phenyl-1H-tetrazole increased in polar solvents with high boiling point.Finally, the best results were obtained in the PEG-400 solvent.In addition, in the study of the effect of temperature, the best results were indicated at 120 °C.The model reaction was tested on a large scale of the reactants (NaN 3 (14 mmol, 0.9101 g) with benzonitriles (10 mmol, 1.0304 g)).
The reaction yield was approximately 69% aer 10 h.In continuation, various 5-substituted tetrazole compounds were synthesized under the above-dened conditions in the presence of Cu-P.bis(OA)@FeB-MNPs as a catalyst.The details of the experimental results such as reaction time, efficiency, and TOF values are shown in Table 2.In which, the starting material of benzonitriles having an electron-donating or withdrawing groups on meta, ortho, or para position of the aromatic ring were tested and successfully converted to tetrazole products.As displayed in Table 2, the nal 5-substituted tetrazoles were formed in excellent yields within a fast reaction rate in the presence of Cu-P.bis(OA)@FeB-MNPs.
Importantly, Cu-P.bis(OA)@FeB-MNPs exhibit homoselectivity in the formation of 5-substituted tetrazoles.Because, in the reaction of NaN 3 with dicyano-substituted benzonitriles (e.g.terephthalonitrile and phthalonitrile), only mono-addition happened in the presence of Cu-P.bis(OA)@FeB-MNPs (Scheme 6).These starting materials have two quite similar C^N groups in their structure.The homoselectivity of Cu-P.bis(OA)@FeB-MNPs in the formation of 5-substituted tetrazoles through [3 + 2]cycloaddition of NaN 3 with terephthalonitrile and phthalonitrile were investigated using 1 H NMR. When phthalonitrile was used as the starting material, two products -2-(1H-tetrazol-5-yl)benzonitrile (A) or 1,2-di(1H-tetrazol-5-yl) benzene (B)were formed.As mentioned, one of the C^N groups selectively reacted with NaN 3 for the synthesis of 2-(1H-Scheme 7 The expected catalytic mechanism for the synthesis of 5substituted tetrazoles using Cu-P.bis(OA)@FeB-MNPs. of NaN 3 with phthalonitrile in the presence of Cu-P.bis(OA)@FeB-MNPs, three peaks were indicated for aromatic hydrogens (Fig. 7).Therefore, only product A was certainly formed, which indicates the homoselectively of Cu-P.bis(OA)@FeB-MNPs in the synthesis of tetrazoles.
When terephthalonitrile was used as the starting material, two products -4-(1H-tetrazol-5-yl)benzonitrile (C) or 1,4-di(1Htetrazol-5-yl)benzene (D) were formed.As mentioned, one of the C^N groups selectively reacted with NaN 3 for the synthesis of C, the other C^N group did not react, and the D product was not formed.As expected, when terephthalonitrile was used, if product C was synthesized, two peaks should have been observed in the 1 H NMR spectrum for aromatic hydrogens.But if D was synthesized, only one peak should have been observed in the 1 H NMR spectrum for aromatic hydrogens.As shown in the 1 H NMR spectrum of the nal product from the [3 + 2] cycloaddition reaction of NaN 3 with terephthalonitrile in the presence of Cu-P.bis(OA)@FeB-MNPs, two peaks indicated the aromatic hydrogens (Fig. 7).Therefore, only product C was certainly formed, which indicates the homoselectively of Cu-P.bis(OA)@FeB-MNPs in the synthesis of tetrazoles.

Nanoscale Advances Paper
Based on the literature, 53,54,57 an expected mechanism is illustrated in Scheme 7 for the synthesizing of 5-substituted tetrazoles through [3 + 2] cycloaddition reaction of NaN 3 and nitriles in the presence of Cu-P.bis(OA)@FeB-MNPs.In this suggested mechanism, the interaction of the C^N functional group with the active site of the catalyst causes the C^N group to become susceptible to attack by azide ions, and the intermediate II is formed as sodium salt forms.The addition of HCl during workup converts the salt form of intermediate II to nal tetrazole, which forms tetrazoles extracted in ethyl acetate.
3.7.Reusability of Cu-P.bis(OA)@FeB-MNPsPracticability, reusability, stability, and availability are the determinative factors for catalysts.According to the emphasis of green chemistry on the reusability of catalysts, the reusability of Cu-P.bis(OA)@FeB-MNPs was investigated in the [3 + 2]cycloaddition reaction of NaN 3 with Ph-CN toward the formation of 5-phenyl-1H-tetrazole.In this regard, Cu-P.bis(OA)@FeB-MNPs were isolated aer the completion of the reaction and then reused again without double activation.As indicated in Fig. 8, Cu-P.bis(OA)@FeB-MNPscan be recycled up to 6 times.
Also, the recycled Cu-P.bis(OA)@FeB-MNPs nanocatalyst was characterized by SEM (Fig. 9).The SEM images of recycled Cu-P.bis(OA)@FeB-MNPs catalyst showed that the morphology and size of this catalyst did not signicantly change aer reusing, therefore Cu-P.bis(OA)@FeB-MNPs catalyst was stable aer reusing.

Comparison of Cu-P.bis(OA)@FeB-MNPs with other catalysts
The practicability of Cu-P.bis(OA)@FeB-MNPs was compared with other reported catalysts (Table 3).The [3 + 2] cycloaddition reaction of NaN 3 and Ph-CN in the presence of Cu-P.bis(OA) @FeB-MNPs was compared with other catalysts.As indicated, Cu-P.bis(OA)@FeB-MNPs exhibit 98% of the product within only 2 h, which indicates a better yield and reaction time than the other catalysts.Besides, several reports in literature formed from unrenewable materials or used hazard solvents or limited by time-consuming or difficult catalyst recovery.While FeB-MNPs are formed from renewable materials as an ideal waste recycling and Cu-P.bis(OA)@FeB-MNPs can easily be recovered and reused.More addition, 5-substituted tetrazoles were synthesized in the green solvent (PEG-400) in the presence of Cu-P.bis(OA)@FeB-MNPs.

Table 1
Optimization of reaction conditions for the production of tetrazoles in the presence of Cu-P.bis(OA)@FeB-MNPs