an eco‐friendly and efficient approach for the synthesis of...

z Organic & Supramolecular Chemistry

An Eco-Friendly and Efficient Approach for the Synthesis ofTetrazoles via Fe3O4/HT-GLYMO-TA as a New RecoverableHeterogeneous Nanostructured CatalystMaryam Sadat Ghasemzadeh and Batool Akhlaghinia*[a]

Tannic acid immobilized on functionalized magnetic hydro-talcite (Fe3O4/HT-GLYMO-TA) was synthesized as a durable,environmentally-friendly and magnetic nanostructured catalystfor the synthesis of 5- and 1-substituted-1H-tetrazoles. Thenanocatalyst was characterized by several methods, Fouriertransform infrared (FT-IR), X-ray diffraction (XRD) analysis,transmission electron microscopy (TEM), field emission scan-ning electron microscopy (FE-SEM), energy-dispersive X-rayspectroscopy (EDS), EDS-map, thermogravimetric analysis

(TGA), vibrating sample magnetometry (VSM) and CHNSanalysis. The results displayed that the superparamagneticcatalyst with a mean particle size of approximately 23–27 nmhas a plate-like shape. The catalytic activity of this newnanostructured catalyst resulted in the preparation of tetrazoleswith high yields. Additionally, this novel procedure offers aplethora of benefits including short reaction times, thesimplicity of operation, the ease of purification procedure andthe reusability of nanocatalyst for at least eight cycles.

Introduction

There has recently been explosive growth in the field of greenchemistry mainly in two areas i. e. the preparation of greennanostructured catalysts and providing green conditions dur-ing the industrial reactions. The majority of research in greenchemistry aims to reduce the consumption of energy neededto produce the desired products. At the same times it strives tolessen or even eliminate any chance of producing potentiallyharmful by-products and attempts to maximize the yieldwithout compromising with the environment, as well. One ofthe substantial efforts to such purposes consists of performingone-pot reactions (using covalent bonds to combine more thantwo reactants in a single step) and using eco-friendly catalystsin green reaction conditions. In recent years, there has beengrowing interest in the usage of sustainable and recyclablecatalysts to decrease the number of harmful substances.

Heterogeneous catalysts (such as zeolites, expanded perlite,hydrotalcites, nanoparticle/metal-organic frameworks (MOF),carbon-based-materials and mesoporous materials etc.) havethe benefits of recovery and reuse after completion of thegreen chemical processes. However, each designed heteroge-neous catalyst also has some disadvantages, which need to beovercome by more developed techniques. One of the disadvan-tages of heterogeneous catalysts is that they require high-speed centrifugation step or tedious workup procedures.Magnetic nanoparticles (MNPs) have been widely used indesigning environmentally friendly heterogeneous nanocata-

lysts. The reason for the above choice is their super magnetism,large surface areas as well as non-toxicity. A prominent featureof magnetic nanocatalysts is that they can be easily separatedfrom the reaction mixture by an external magnetic field, whichachieves a simple separation of the nanocatalyst without usingfiltration or centrifugation. Furthermore, magnetic nanopar-ticles provide high potential active sites for loading otherfunctional groups. To prevent magnetic nanoparticles fromundergoing agglomeration and oxidation, a protective shell, forexample, silica, carbon, metal, metal oxide, polymer can beformed onto their surface. Hydrotalcites as a heterogeneouscatalyst have a great potential to exchange anions and behaveas solid bases. The active basic sites of hydrated hydrotalcitematerials are mainly hydroxyl anions. Compositional andstructural parameters can influence on their basicity. Besides,hydrotalcites can be used in three different types of catalystsystems: catalyst support, redox catalyst and acid/basecatalyst.[1–9] To improve the permanence and long-termdurability of hydrotalcite, magnetic hydrotalcites (MHTs) hasbeen explored to be used as catalyst or potentially effectivesupport material in organic reactions due to its simple recoveryand reusability. Herein, based on the experience gained in ourformer studies in magnetic heterogeneous nanostructuredcatalysts[10–27] we report the preparation and characterization oftannic acid immobilized on functionalized magnetic hydro-talcite (Fe3O4/HT-GLYMO-TA) (IV) as a green heterogeneousand magnetic reusable nanostructured catalyst (Scheme 1). Forthis purpose, functionalization of Fe3O4/HT (II) was done using(3-glycidyloxypropyl)trimethoxysilane and later it was reactedwith tannic acid to produce Fe3O4/HT-GLYMO-TA (IV). Tannicacid (TA) as a natural compound existing in fruit and plants hasexcellent antibacterial effectiveness. It has been approved bythe FDA to be used as food by humans.[28, 29] The structure oftannic acid has a central carbohydrate molecule (glucose),

[a] M. S. Ghasemzadeh, Prof. Dr. B. AkhlaghiniaDepartment of Chemistry, Faculty of Science, Ferdowsi University ofMashhad, Mashhad 9177948974, IranE-mail: [email protected]

Supporting information for this article is available on the WWW underhttps://doi.org/10.1002/slct.202000641

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which is esterified by phenol groups. In addition, the presenceof 3,4,5-trihydroxybenzoyl groups in the tannic acid molecularstructure verified its antibacterial performance.[30,31]

Tetrazoles are nitrogen containing heterocyclic compoundswith a wide-ranging of usages in cis-peptide bond mimics,explosives, ion and peptide chelating agents, informationrecording systems, pharmaceuticals as lipophilic spacers, and

Scheme 1. The preparation of tannic acid immobilized on functionalized magnetic hydrotalcite (Fe3O4/HT-GLYMO-TA (IV)).




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as catalysts in asymmetric synthesis.[32–36] Drugs - basedtetrazole possess numerous biological activities, for example,antiallergic, antibacterial, anticonvulsant, anti-inflammatory,antifungal and antitubercular activities[37] and have presentedthe results in treatment of some diseases, such as AIDS andcancer.[38] Therefore, the synthesis of this heterocyclic nucleuscan be of prime importance.

The conventional method of synthesizing 5-substituted-1H-tetrazoles is by adding azide ions to organic nitriles. Severalhomogenous and heterogeneous catalysts (ZnBr2,

[39] BF3.OEt2,[40]

AlCl3,[41] ZnO,[42] Pd(OAc)2/ZnBr,

[43] ZrOCl2,[44] ZnCl2/ tungstates,

[45]

Zn/Al hydrotalcite,[46] nano CuFe2O4,[47] ZnS,[48] Cu2O,

[49] Zn(OTf)2,

[50] natural zeolite,[51] Zn hydroxyapatite,[52] nano ZnO/Co3O4,

[53] Fe(OAc)2,[54] CdCl2,

[55] FeCl3@SiO2,[56] Ln(OTf)3,

[57] Zeoliteand sulfated zirconia,[58] γ-Fe2O3

[59] and nano CSMIL[60]) havebeen reported for reaction of nitrile and TMS-N3 or NaN3.

1-Substituted-1H-1,2,3,4-tetrazoles can be synthesized bydifferent methods including: the reaction between hydrazoicacid and isocyanides,[61] the acid-catalyzed cycloaddition be-tween isocyanides and trimethysilyl azide,[62] the cyclizationbetween amines with sodium azide /or orthocarboxylic acidesters[63] and the cyclizations of an amine, triethyl orthoformate,and sodium azide using AcOH,[64] PCl5,

[65] In(OTf)3,[66] Yb(OTf)3,

[67]

SSA,[68] [HBIm]BF4,[69] natrolite zeolite,[70] Cu NPs/bentonite,[71]

ZnS nanoparticles,[72] SiO2-H3BO3.[73]

Although each of the methods used for the syntheses of 5-substituted-1H-tetrazoles and 1-substituted-1H-1,2,3,4-tetra-zoles definitely offers certain benefits, many of them havesome of the following drawbacks: long reaction times, hightemperature reactions, low yields, the exhaustive nature ofreaction mixture purification and reusability of the catalyst.Therefore, choosing the optimal method plays an importantrole in achieving good results so Fe3O4/HT-GLYMO-TA (IV) hasbeen prepared to be used in the preparation of tetrazoles (seeScheme 2). Additionally, the obtained products have beencharacterized utilizing 1HNMR and 13CNMR data, FT-IR, massand elemental analysis.

Results and Discussion

Characterization of Fe3O4/HT-GLYMO-TA (IV)

Structural and chemical characterizations of Fe3O4/HT-GLYMO-TA (IV) were done by several methods such as FT-IR, XRD, TEM,FE-SEM, EDS, EDS-map, TGA, VSM and elemental analysis. FT-IRspectroscopy (Figure S1, Supporting Information file, page 3)confirmed the preparation and successful surface functionaliza-tion of the magnetic hydrotalcite. The FT-IR spectra of the HT(I), Fe3O4/HT (II), Fe3O4/HT-GLYMO (III), Fe3O4/HT-GLYMO-TA(IV), the 8th recovered Fe3O4/HT-GLYMO-TA (IV) from thesynthesis of 5-substituted-1H-tetrazoles and 1-substituted-1H-1,2,3,4-tetrazoles are demonstrated in Figure S1 (a-f). As can beclearly observed from Figure S1a, the O� H symmetric stretch-ing vibrations of Mg-OH and Al-OH on the surface of thehydrotalcite structure were revealed by the broad band at3460–3400 cm� 1.[9] In hydrotalcite structure, the bendingvibration of water molecules appeared at 1625 cm� 1. Addition-ally, the absorption bands of asymmetric stretching vibration ofC� O (representing the existence of CO3

2� ) and the angularbending mode of carbonate species existed at 1371 and670 cm� 1, respectively.[9] This means that the carbonate anionsexist between the hydrotalcite layers (HT (I)). The intensity ofthe absorption band at 1371 cm� 1 became weaker in the FT-IRspectrum of Fe3O4/HT (II), representing intercalation of Fe3O4

into the HT (I) structure.[9] Also, the presence of Fe3O4 NPs inthe HT (I) structure can be verified by broad band stretchingvibrations of Al� O and Mg� O bonds which appeared around780–566 cm� 1.[9] This shows that stretching vibrations of Al� Oand Mg� O bonds covered the stretching vibration of Fe� Obond (Figure S1b).[9] In the FT-IR spectrum of Fe3O4/HT-GLYMO(III) (Figure S1c), grafting of (3-glycidyloxypropyl)trimeth-oxysilane on the surface of Fe3O4/HT (II) is recognized by theabsorption bands at 2940, 2883 (associated to the stretchingvibrations of C� H), and 1110 cm� 1 (associated to the stretchingvibration of C� C). It is noteworthy that the symmetricstretching vibration of Si-O� Si bond at 680–665 cm� 1 is coveredby stretching vibration of metal-oxygen so that the bondcannot be distinguished.[9] Finally, the existence of the graftedtannic acid in the FT-IR spectrum of Fe3O4/HT-GLYMO-TA (IV)has been proved by increasing the frequency and intensity ofOH groups (Figure S1d).

The XRD patterns of HT (I), Fe3O4/HT (II), Fe3O4/HT-GLYMO-TA (IV) and the 8th recovered Fe3O4/HT-GLYMO-TA (IV) from thesynthesis of 1-substituted-1H-1,2,3,4-tetrazoles are presented inFigure 1. In Figure 1a, the synthesized nanomaterials demon-strated characteristic diffraction peaks at 2θ=11.21, 22.61,34.51, 38.01, 45.71, 60.31, and 61.51 that are related to thecrystal planes (003), (006), (012), (015), (018), (110), and (113) ofhydrotalcite (Ref. Code: 04–015-1684).[2] The results indicatethat no impurity phases (MgAl2(OH)8 or Mg5(CO3)4(OH)2-4H2O)were revealed in the hydrotalcite structure.[74] In addition, thepresence of the Fe3O4 NPs phase as a cubic lattice in thestructure of hydrotalcite is confirmed by the peaks at 2θ=

29.81, 35.41, 43,31, 57.31, and 62.91 that can be assigned tothe (220), (311), (400), (511), and (440) planes (Ref. Code: 04–002-3668) (Figure 1b).[27] Furthermore, the equation of the

Scheme 2. Synthesis of different structurally 5-substituted-1H-tetrazoles and1-substituted-1H-1,2,3,4-tetrazoles in the presence of Fe3O4/HT-GLYMO-TA(IV) in aqueous media.




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Debye-Scherrer revealed that the size of Fe3O4/HT-GLYMO-TA(IV) was estimated to be 27 nm.

The size and morphology of the fresh Fe3O4/HT-GLYMO-TA(IV) and the 8th recovered Fe3O4/HT-GLYMO-TA (IV) from thereaction of 1-substituted-1H-1,2,3,4-tetrazoles synthesis wereinvestigated by TEM technique (Figure 2a, b). As can berecognized from TEM images, the Fe3O4 NPs entrapped in thematrix of hydrotalcite structure with plate-like in shape. Addi-tionally, a distribution histogram of Fe3O4/HT-GLYMO-TA (IV)indicated that the mean diameter of the nanoparticles is

27 nm, which is in agreement with the mean particle sizecalculated from XRD (Figure 2c).

The morphologies of HT (I), Fe3O4/HT-GLYMO-TA (IV) andthe 8th recovered Fe3O4/HT-GLYMO-TA (IV) from the (c) reactionof 5-substituted-1H-tetrazoles and (d) 1-substituted-1H-1,2,3,4-tetrazoles synthesis were determined by FE-SEM (Figure 3 (a-d)). Figure 3a shows that the synthesized hydrotalcite has aplate-like structure, demonstrating the character of layeredmaterials. As can be seen from the FE-SEM image of Fe3O4/HT-GLYMO-TA (IV), the Fe3O4 NPs were well scattered on the

Scheme 3. Proposed mechanism for the preparation of 5-substituted-1H-tetrazoles and 1-substituted-1H-1,2,3,4-tetrazoles using Fe3O4/HT-GLYMO-TA (IV).




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surface of plate-like layers of HT (I). It is worth mentioning thatthe nanostructured catalyst has a plate-like shape with anaverage particle size of around 27 nm.

The EDS analysis confirmed the existence of oxygen,carbon, iron, magnesium, aluminum and silicon in the structureof Fe3O4/HT-GLYMO-TA (IV) which shown no impure elementsin the nanocatalyst structure (Figure S2, Supporting Informa-tion file, page 3).

Additionally, the EDS-map images proved the uniformlydispersed oxygen, carbon, iron, magnesium, aluminum andsilicon on the surface of nanostructured catalyst (Figure 4).

The thermal stability of Fe3O4/HT (II), Fe3O4/HT-GLYMO (III),Fe3O4/HT-GLYMO-TA (IV) and the 8th recovered Fe3O4/HT-GLYMO-TA (IV) from the reaction of 1-phenyl-1H-tetrazolesynthesis was investigated by TGA analysis (Figure S3, Support-ing Information file, page 4). The TGA of the Fe3O4/HT (II)displays two major weight losses. The adsorbed water on thesurface of the nanocatalyst or between the layers of nano-catalyst was lost during the first step (50-200 °C, 10.75%). Thesecond step (210-580 °C, 15.12%) is associated with the loss ofthe interlayer carbonate ions.[75] In addition, de-carbonationoccurred at 210 °C instead of 440 °C, which proved thepresence of Fe3O4 NPs in the HT (I) structure (Figure S3a).[75] TheTGA thermogram of Fe3O4/HT-GLYMO (III) shows two weightlosses (Figure S3b). It is evident that the first decompositionstep is associated with the removal of adsorbed water from thesurface of Fe3O4/HT-GLYMO (III) (45-210 °C, 10%). Additionally,the removal of the organic fragment occurred at 220–550 °C(25.23%). Based on these results, 0.58 mmol/g of organicsegments is grafted on the surface of Fe3O4/HT (II). Moreover,based on CHNS analysis data, the loading amount of organicfragment supported on Fe3O4/HT (II) was 0.6 mmol/g accordingto carbon content (C= 14.90%).

Similarly, the TGA thermogram of Fe3O4/HT-GLYMO-TA (IV)demonstrates two weight losses steps. The adsorbed water

evaporated from the surface of nanostructure catalyst at 55–199 °C (11.22%). The main weight loss (35.02%) at 200–720 °Ccan be attributed to the total decomposition of the organicsegments that were grafted on the surface of the nano-structured catalyst (Figure S3c). Based on TGA thermogramresults (Figure S3c), 1.39 mmol g � 1 was estimated for theamount of organic linkers anchored on the surface of Fe3O4/HT(II). The result was also in good accordance with the obtainedCHNS analysis data (C= 28.90%). The back-titration analysiswas performed to investigate the number of acidic sites(phenol groups) in Fe3O4/HT-GLYMO-TA (IV). For this purpose,100 mg of the prepared nanostructured catalyst was dispersedin a solution of aqueous NaOH (0.1 M, 15 mL). It was stirred atambient temperature overnight. Next, the filtration of suspen-sion and later the neutralization of filtrate was carried out withan HCl solution (0.1 M). The amount of loaded phenol groupsper 1.000 g of Fe3O4/HT-GLYMO-TA (IV) was determined usingthe consumed volume of HCl (11.25 mL) (1.131 mmol of phenolgroups per 1.000 g of catalyst). Additionally, the result of back-titration analysis is in good accordance with the achieved resultfrom TGA.

Room temperature magnetization curves of Fe3O4/HT (I),Fe3O4/HT-GLYMO-TA (IV) and the 8th recovered Fe3O4/HT-GLYMO-TA (IV) from the reactions of 5-substituted-1H-tetra-zoles and 1-substituted-1H-1,2,3,4-tetrazoles synthesis are dis-played in (Figure 5 (a-d)), respectively. Based on the resultobtained from Figure 5, the amount of saturation magneticmoment of Fe3O4/HT-GLYMO-TA (IV) is Ms= 32.33 emu g� 1

whose amount is less than that of Fe3O4/HT (I) nanoparticles(Ms= 58.12 emu g� 1). Decreasing in the saturation magnet-ization of Fe3O4/HT-GLYMO-TA (IV) after surface grafting, canbe attributed to the contribution of the non-magnetic materi-als.

Catalytic Synthesis of tetrazoles

Due to our substantial interest in developing new methods fortetrazoles synthesis,[76–81] a new investigation on the synthesisof tetrazole derivatives was performed using Fe3O4/HT-GLYMO-TA (IV) as a magnetically eco-friendly heterogeneous nano-structured catalyst in green media (Scheme 2).

Primary experiments were done to investigate the mostoptimal reaction conditions. Recently, we have discovered thatutilizing [bmim][N3] as an azide ion source rather than harmfulreagents, such as TMSN3 and NaN3, is in far more agreementwith sustainable chemistry.[79–81] Firstly, some factors such asloading of catalyst, molar ratios of reactants, the influence oftemperature and solvents in terms of time and yield of modelreactions in the presence of Fe3O4/HT-GLYMO-TA (IV) wereinvestigated. Table 1 and 2 show the summarized results. Asseen in Table 1 and 2 (entry 1), the desirable products wereobtained only if the nanostructured catalyst was added intothe reaction mixture. In other words, without the nanostruc-tured catalyst, the whole experiment was futile. Additionally,the reactions were conducted in CH3CN, DMF, EtOAc, EtOH,toluene, 1,4-dioxane and water as solvents and under solvent-free conditions. Among the different types of solvent, water is

Figure 1. XRD patterns of (a) HT (I), (b) Fe3O4/HT (II), (c) Fe3O4/HT-GLYMO-TA(IV) and (d) the 8th recovered Fe3O4/HT-GLYMO-TA (IV) from the reaction of 1-substituted-1H-1,2,3,4-tetrazoles synthesis.




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https://www.google.com/search?client=firefox-b-d&q=TGA+thermogram&spell=1&sa=X&ved=0ahUKEwiO0qeRzebgAhVHKFAKHbXjDzkQkeECCCgoAA&biw=1508&bih=749

https://en.wikipedia.org/wiki/Phenol

Figure 2. TEM images of (a, b) Fe3O4/HT-GLYMO-TA (IV), (c) the 8th recovered Fe3O4/HT-GLYMO-TA (IV) from the reaction of 1-substituted-1H-1,2,3,4-tetrazolessynthesis and (d) particle size distribution histogram of Fe3O4/HT-GLYMO-TA (IV).

Table 1. Effect of different parameters for the synthesis of 5-phenyl-1H-tetrazole.

Entry Molar ratio (benzonitrile/[bmim][N3]) Cat. amount (g) Solvent Reaction Temp. (°C) Reaction Time (min) Isolated Yield (%)

1 1 :1.1 No catalyst - r.t 24 h -2 1 :1.1 0.005 - r.t 24 h 253 1 :1.1 0.005 - 50 24 h 454 1 :1.1 0.005 - 60 24 h 755 1 :1.1 0.005 H2O 60 1 h 856 1 :1.1 0.005 H2O 65 35 907 1 :1.1 0.005 H2O 70 20 958 1 :1.1 0.006 H2O 70 20 959 1 :1.1 0.004 H2O 70 40 8010 1 :1.1 0.005 EtOH 70 24 h 5011 1 :1.1 0.005 DMF 70 24 h 6512 1 :1.1 0.005 EtOAc 70 24 h 3513 1 :1.1 0.005 CH3CN 70 24 h 5514 1 :1.1 0.005 Toluene 70 24 h 7515 1 :1.1 0.005 1,4-Dioxane 70 24 h 2516a 1 : 1.1 0.005 H2O 70 24 h -17b 1 :1.1 0.005 H2O 70 24 h 1518c 1 : 1.1 0.005 H2O 70 24 h 2519d 1 :1.1 0.005 H2O 70 24 h 3520e 1 : 1.1 0.005 H2O 70 24 h 65

The model was carried out reaction using [a] Fe3O4 [b] HT (I), [c] Fe3O4/HT (II), [d] Fe3O4/HT-GLYMO (III) and [e] Tannic acid.




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the best option for the synthesis of 5-substituted-1H-tetrazolesand 1-substituted-1H-1,2,3,4-tetrazoles (Table 1 and 2).

Based on the results from Table 1, by applying 1 :1.1 molarratio of benzonitrile:[bmim]N3 the standard reaction produced

Figure 3. The FE-SEM images of (a) HT (I), (b) Fe3O4/HT-GLYMO-TA (IV), the 8th recovered Fe3O4/HT-GLYMO-TA (IV) from the reactions of (c) 5-substituted-1H-tetrazoles and (d) 1-substituted-1H-1,2,3,4-tetrazoles synthesis.

Table 2. Effect of different parameters for the synthesis of 1-phenyl-1H-tetrazole.

Entry Molar ratio of PhNH2/HC(OEt)3/[bmim]N3 Cat. amount (g) Solvent Reaction Temp. (°C) Reaction Time (min) Isolated Yield (%)

1 1 :1.2 : 1 No catalyst - 100 24 h -2 1 :1.2 : 1 0.03 - 100 24 h 653 1 :1.2 : 1 0.03 - 90 24 h 654 1 :1.2 : 1 0.03 H2O 90 45 955 1 :1.2 : 1 0.03 H2O 80 60 856 1 :1.2 : 1.2 0.03 H2O 90 45 957 1 :1.2 :1 0.02 H2O 90 45 958 1 :1.2 : 1 0.01 H2O 90 1.35 h 759 1 :1.2 : 1 0.02 EtOH 90 24 h 3010 1 :1.2 : 1 0.02 MeOH 90 24 h 3511 1 :1.2 : 1 0.02 DMF 90 24 h 7512 1 :1.2 : 1 0.02 CH3CN 90 24 h 6013 1 :1.2 : 1 0.02 Toluene 90 24 h 4514 1 :1.2 : 1 0.02 1,4-Dioxane 90 24 h 5015a 1 : 1.2 : 1 0.02 H2O 90 24 h -16b 1 :1.2 : 1 0.02 H2O 90 24 h 1017c 1 : 1.2 : 1 0.02 H2O 90 24 h 1518d 1 :1.2 : 1 0.02 H2O 90 24 h 2519e 1 : 1.2 : 1 0.02 H2O 90 24 h 55

The model was carried out reaction using [a] Fe3O4 [b] HT (I), [c] Fe3O4/HT (II), [d] Fe3O4/HT-GLYMO (III) and [e] Tannic acid.




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high yield (95%) in a short reaction time (20 min) using 0.005 gof Fe3O4/HT-GLYMO-TA (IV) in H2O at 70 °C using (entry 7). Asshown from Table 2, the best result was obtained when thereaction was performed with Fe3O4/HT-GLYMO-TA (IV) (0.02 g),aniline (1 mmol), triethylorthoformate (1.2 mmol) and 1-butyl-3-methylimidazolium azide (1 mmol) in water at 90 °C (entry 7).To get a deeper understanding of the role of nanostructuredcatalyst in the preparation of 5-phenyl-1H-tetrazole and 1-

phenyl-1H-tetrazole, the model reactions were repeated in theabsence of nanostructured catalyst as well as in the presenceof Fe3O4, HT (I), Fe3O4/HT (II), Fe3O4/HT-GLYMO (III) and tannicacid (Table 1 and 2, entries 16–20 and 15–19, respectively).When the reaction was done without the nanostructuredcatalyst and also in the presence of HT (I), Fe3O4/HT (II), Fe3O4/HT-GLYMO (III) and tannic acid, unsatisfactory yields of targetcompounds were produced.

To generalize the present method for the synthesis ofdifferent 5-substituted-1-H-tetrazoles, a variety of differentnitriles and [bmim][N3] in the presence of Fe3O4/HT-GLYMO-TA(IV) under the optimal reaction conditions were employed(Table 3). Aromatic nitriles bearing both electron-donating andelectron-withdrawing groups underwent cycloaddition, produc-ing high to excellent yields of the desired products. Thereaction time was influenced by the nature of the substituentson the benzonitriles. Reactions of electron-poor aromatic andheteroaromatic nitriles (for example: thiophene-2-carbonitrile,4-pyridinecarbonitrile and

2-pyridinecarbonitrileare) were completed in a few minutes(Table 3, entries 1–5 and 13–15). Evidently, nitriles withelectron-donating substituents require longer reaction times(Table 3, entries 6–11). In a similar way, the reaction ofphenanthrene-9-carbonitrile resulted good yield although alonger reaction time was needed, which may be owing to thesteric hindrance (Table 3, entry 12). It is worth mentioning thatnot only aromatic nitriles but also aliphatic nitriles could alsobe used to produce the products in excellent yields (Table 3,entries 16, 17 and 18). With the purpose of establishing thestructure of 5-substituted-1-H-tetrazoles, a comparison ofmelting points was carried out with the known compounds.The results were in good accordance with values expected.

Afterward to clarify the structure of these compounds evenmore, spectroscopic methods, for example, FT-IR, 1HNMR and13CNMR spectroscopy and also mass spectrometry were used.FT-IR spectra of the compounds showed the absorption bandsat: 3485–3329 cm� 1 (owing to the N� H stretching vibration),1469–1430 cm� 1 (owing to scissoring bending C� H), 1293–1233 cm� 1 (owing to N-N=N-), 1189–1110 and 1106–1041 cm� 1

(owing to tetrazole ring).[80] In addition, 1HNMR and 13CNMRspectra revealed the signals of N� H and quaternary carbon NH-C=N at 10.98-7.80 and 164–151 ppm, respectively.[80] (seeSupporting Information file)

The catalytic activity of Fe3O4/HT-GLYMO-TA (IV) was alsostudied in the preparation reaction of 1-substituted-1H-1,2,3,4-tetrazoles synthesis after optimizing the reaction conditionsusing some amines, triethyl orthoformate and [bmim]N3. Thesummarized results are shown in Table 4. Under similarconditions, it was witnessed that amines bearing severalfunctional groups such as methyl, hydroxyl, bromo, chloro andnitro underwent the cyclization reactions in short reactiontimes. All high-yield products were obtained via heating themixture of reaction at 90 °C. In addition, the para-substitutedanilines provided better results than the ortho-substitutedones, because of the less steric hindrance of the para-substituted anilines (Table 4, entries 6 and 10 vs. entries 5 and9). Furthermore, the nanostructured catalyst also functioned

Figure 4. EDS-map images of Fe3O4/HT-GLYMO-TA (IV).

Figure 5. Magnetization curves of (a) Fe3O4/HT (I), (b) Fe3O4/HT-GLYMO-TA(IV), the 8th recovered Fe3O4/HT-GLYMO-TA (IV) from the reactions of (c) 5-substituted-1H-tetrazoles and (d) 1-substituted-1H-1,2,3,4-tetrazoles synthe-sis.




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well with aliphatic and heterocyclic amines, for example, benzylamine, ethylamine, ethylenediamine and 2-amino pyridine toproduce the corresponding 1-substituted-1H-1,2,3,4-tetrazolesin good to excellent yields within short reaction times. (Table 4,entries 12–15).

The structural of the all synthesized 1-substituted-1H-1,2,3,4-tetrazoles has been determined based on their massspectral studies and melting points which were in goodaccordance with the recommended structures.[81] Additionally,the 1HNMR 13CNMR and FT-IR spectral data of synthesized 1-substituted-1H-1,2,3,4-tetrazoles were also reported. FT-IR spec-tra of the synthesized compounds showed the absence of NH2

absorption bands and appearance of stretching and scissoringbending of =CH at 3150–3007 and 833–710 cm� 1,respectively.[81] Also, the absorption bands of C=N and N-N=Nin the tetrazole ring were showed at around 1691–1596 cm� 1

and 1286–1204 cm� 1, respectively.[81] 1HNMR spectra showed a

singlet signal at 9.94-8.96 ppm, attributed to =C� H of thetetrazole ring.[81] In 13CNMR spectra, the signal of the sp2

hybridized carbon of the tetrazole ring was seen at 159–140.4 ppm which is revealing of the formation of tetrazolering.[81] (see Supporting Information file)

Proposed catalytic mechanism for the synthesis of tetrazoles

In accordance with our previous studies,[82,83] the probablereaction mechanism for the preparation of tetrazoles in thepresence of Fe3O4/HT-GLYMO-TA (IV) is presented in Scheme 3.Performing the standard reactions while the catalyst is absent,the presence of HT (I), Fe3O4/HT (II), Fe3O4/HT-GLYMO (III) andtannic acid does not afford a reasonable yield of targetcompounds (Table 1 and 2). Thus, it can be concluded thatFe3O4/HT-GLYMO-TA (IV) has a significant effect on all steps oftetrazoles synthesis as a solid acidic nanocatalyst.

Table 3. Catalytic efficiency of Fe3O4/HT-GLYMO-TA (IV) in the synthesis of 5-substituted-1H-tetrazoles.

Entry Nitriles Reaction Time (min) Isolated Yield (%)

1 C6H5CN1a

20 95

2 4-BrC6H4CN1b

30 95

3 4-ClC6H4CN1c

20 90

4 4-NO2C6H4CN1d

15 95

5 4-CNC6H4CN1e

15 90

6 2-NH2-5-NO2C6H3CN1f

30 95

7 3,5-Di-MeOC6H4CN1g

2 h 90

8 3-MeC6H4CN1h

1.55 h 95

9 4-OHC6H4CN1 i

55 90

10 2-OHC6H4CN1 j

45 95

11 4-EtOC6H4CN1k

45 90

12 9-Cyanophenanthrene1 l

2 h 85

13 4-Pyridinecarbonitrile1m

35 95

14 2-Pyridinecarbonitrile1n

45 90

15 2-Thiophenecarbonitrile1o

30 95

16 C6H5CH2CN1p

2 h 85

17 (CH3)2CHCH2CN1q

1.50 85

18 (CH3)2CH(CH2)2CN1r

1. 15 95




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The proposed mechanism for the synthesis of 5-substi-tuted-1H-tetrazoles can be explained, through the activation ofnitrile functionality by the formation of hydrogen bondbetween nitrogen atom of nitrile group and hydroxyl group ofnanostructured catalyst. Next, the intermediate II is producedfrom the [3 + 2] cycloaddition reaction between activatednitrile and azide ion. The protonolysis of the intermediate IIproduces III and IV. Finally, the stable tautomer IV is obtainedfrom the equilibrium of 5-substituted-3H-tetrazole to 5-sub-stituted-1H-tetrazole. Next, nanostructured catalyst re-enters tothe catalytic run by making the active sites for furtherturnovers.

Subsequently, plausible reaction pathway for 1-substituted-1H-1,2,3,4-tetrazoles synthesis can be described through theinitial formation of carbenium ion A, which is resonancestabilized using the oxygen atom of another ethoxy group A’. Itis suggested that, cleaving the C� O bond and subsequentelimination of ethoxy group of HC(OEt)3 were facilitated via ananomeric process in triethoxymethane (nO!σ*C-O). Afterwards,the nucleophilic attack of amine to carbenium ion A andsuccessive extrusion of the second ethoxy group via ananomeric process (nO!σ*C-O, and nN!σ*C-O) produced thecarbenium ion B which in turn is resonance stabilized through

the formation of either the oxonium B’ or iminium B’’ ions.Then, carbenium ion B converted to intermediate C throughthe reaction with [bmim]N3. Similarly, the third ethoxy group isreleased from intermediate C through the interaction betweena lone pair electrons of nitrogen atom with the anti-bondingorbital of C� O bond (nN!σ*C-O) in order to generate carbeniumion D and D’.[84–91] Cyclized 1-substituted-1H-1,2,3,4-tetrazoles Ewas formed from the catalytic cyclization of the carbenium ionD. Next, the nanocatalyst is re-introduced into the catalyticcycle.

Reusability of the Fe3O4/HT-GLYMO-TA (IV)

The catalyst recycling test was performed in the synthesis of 5-substituted-1H-tetrazoles and 1-substituted-1H-1,2,3,4-tetra-zoles. After the reactions were completed, the Fe3O4/HT-GLYMO-TA (IV) was separated via a magnet bar, washed withEtOAc (3×30) and dried at 60 °C overnight. It is worthmentioning that the reused magnetic nanostructured catalystcan be recycled for eight runs with reasonable yields ofproducts compared to the fresh nanocatalyst (Figure 6).

To confirm the stability of Fe3O4/HT-GLYMO-TA (IV) aftereight recycle runs, the FT-IR, XRD, TEM, FE-SEM, TGA and VSM

Table 4. Catalytic efficiency of Fe3O4/HT-GLYMO-TA (IV) in the synthesis of 1-substituted-1H-1,2,3,4-tetrazoles.

Entry Amines Reaction Time (min) Isolated Yield (%)

1 C6H5NH2

2a45 95

2 4-Me–C6H4NH2

2b35 95

3 3-Me–C6H4NH2

2c40 90

4 3,4-Me2-C6H3NH2

2d55 90

5 4-HO-C6H4NH2

2e40 90

6 2-HO-C6H4NH2

2f75 90

7 4-Br-C6H4NH2

2g55 90

8 3-Br-C6H4NH2

2h60 90

9 4-Cl–C6H4NH2

2 i80 90

10 2-OMe-4-Cl–C6H3NH2

2 j1.30 h 90

11 4-NO2-C6H4NH2

2k65 90

12 2-C5H4NNH2

2 l50 95

13 C6H5CH2NH2

2m60 90

14 CH3CH2NH2

2n1.20 h 80

15 NH2CH2CH2NH2

2o1.40 h 70




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were investigated. In the FT-IR spectrum of the 8th recoverednanostructured catalyst from the reactions of 5-substituted-1H-tetrazoles and 1-substituted-1H-1,2,3,4-tetrazoles synthesis (Fig-ure 7e and f), two absorption bands were present at 1363–1360 cm� 1 and 665–660 cm� 1 which related to the stretchingmodes of carboxylate groups, indicating that the nanostruc-tured catalyst remained stable even after eight runs.

Furthermore, the achieved results from the XRD pattern(Figure 2d), TEM image (Figure 3b), FE-SEM images (Figure 4cand 4d), TGA analysis (Figure S3) and VSM analysis (Figure 5cand 5d) of the 8th recovered nanostructured catalyst from thesynthetic reactions of 5-substituted-1H-tetrazoles and 1-sub-stituted-1H-1,2,3,4-tetrazoles clearly indicated that there wasnot any notable change in the structure of the nanocatalystafter eight runs. Besides, the TGA analysis of 8th recoveredFe3O4/HT-GLYMO-TA (IV) from the synthetic reaction of 1-substituted-1H-1,2,3,4-tetrazoles showed that the weight lossof the grafted organic motif in the second step was reducedfrom 35.02% (in the fresh nanostructured catalyst) to 33.14% inthe 8th recovered nanostructured catalyst, which might beattributed to the diminutive leaching of organic segmentduring reusing processes (Figure 7d).

The saturation magnetization values (Ms) of the 8th recycledFe3O4/HT-GLYMO-TA (IV) from the synthetic reactions of 5-substituted-1H-tetrazoles and 1-substituted-1H-1,2,3,4-tetra-zoles are 7.88 and 5.12 emu g� 1, respectively (Figure 8c and8d). It is worth mentioning that after eight runs the super-paramagnetic behavior of the recovered Fe3O4/HT-GLYMO-TA(IV) was still present. Back-titration analysis of the 8th reusedFe3O4/HT-GLYMO-TA (IV) nanoparticles from the synthetic

reactions of 5-substituted-1H-tetrazoles and 1-substituted-1H-1,2,3,4-tetrazoles displayed that the number of loaded acidicsites (phenol groups) per 1.000 g of nanostructured catalystdecreased to 1.33 and 1.31 mmol, respectively. These resultsclearly indicated that 5% and 7% of active acidic sites (aninsignificant amount) leached out during eight catalyticreaction runs (the fresh catalyst containing 1.4 mmol of acidicgroups per 1.000 g).

In order to show the importance of the current method forthe synthesis of tetrazoles, we have compared the results usingFe3O4/HT-GLYMO-TA (IV) for the model reactions with thoseobtained using other catalysts published recently (Table S1,Supporting Information file, page 4). Using high temperatures(Table S1, entries 1–5, 7–15, 17, 18 and 20), harmful solvents(Table S1, entries 1–12, 14, 19 and 21) and long reaction times(Table S1, entries 1–5, 7–10, 17 and 18) to obtain the desiredproducts are the drawbacks of some of these methods.Therefore, Fe3O4/HT-GLYMO-TA (IV) as green and heteroge-neous nanostructured catalyst is more effective than most ofthe catalysts in terms of the product yields and the reactiontimes.

Conclusion

In summary, we have successfully synthesized tannic acidimmobilized on functionalized magnetic hydrotalcite (Fe3O4/HT-GLYMO-TA (IV)) as an efficient and reusable nanostructuredcatalyst for the one-pot syntheses of 5-substituted-1H-tetrazoleand 1-substituted-1H-1,2,3,4-tetrazoles. Careful characterizationof Fe3O4/HT-GLYMO-TA (IV) by spectroscopic and microscopic

Figure 6. The preparation of 5-substituted-1H-tetrazoles and 1-substituted-1H-1,2,3,4-tetrazoles in optimized reaction conditions using the 8th reused Fe3O4/HT-GLYMO-TA (IV).




2021 / 167782 [S. 6450/6452] 1

techniques revealed that the new nanostructured catalyst has aplate-like shape, an average particle size of 23–27 nm andsuperparamagnetic behavior. There is a myriad of advantagesusing this method such as thermal stability, heterogeneousnature, short reaction times, clean and simple procedure,excellent yields, and easy product separation and purification.These factors turn this method into a superior alternative tothe previous methodologies for the scale-up of those reactions.The nanostructured catalyst can also be successfully recycled atleast eight times with an insignificant drop in its catalyticperformance.

Supporting Information Summary

Physicochemical characterization data of products (includingcolour, melting point, mass, 1HNMR and 13CNMR spectra) andthe characterization data of the nanocatalyst such as FT-IRspectra, EDS analysis and TGA thermogram were described.

Acknowledgements

The authors gratefully acknowledge the partial support of thisstudy by Ferdowsi University of Mashhad Research Council (Grantno. p/3/43366).

Conflict of Interest

The authors declare no conflict of interest.

Keywords: green chemistry · heterogeneous catalysis · tannicacid · tetrazole

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Submitted: February 15, 2020Accepted: May 19, 2020




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