Controlled Ni-doped cobalt ferrite nanoparticles: a green and sustainable heterogeneous catalyst for the synthesis of 5-substituted 1H-tetrazoles via a [3+2] cycloaddition reaction

Abstract

This study explores the catalytic potential of sustainable Ni_xCo_1−xFe₂O₄ (x = 0.0, 0.4, 0.6, 1.0) nanoparticles for the efficient synthesis of bioactive 5-substituted-1H-tetrazoles. The impact of Ni substitution on cobalt ferrite samples was investigated via analytical techniques such as SEM, PXRD, HRTEM, EDX, IR, and VSM. XRD analysis results confirmed the synthesis of ferromagnetic cubic nanostructured material. Furthermore, the formation of a well-defined cubic spinel-structured nanomaterial with an average size of 15.73 nm was revealed by the SEM and TEM micrographs. Tetrazoles represent a vital class of heterocyclic scaffolds with diverse applications in high-energy material science, medical chemistry, biochemistry, and pharmacology, among other domains. Leveraging their importance, we developed a high-yielding facile protocol for tetrazole synthesis via a [3+2] cycloaddition reaction between sodium azide and aromatic nitriles catalyzed by Ni_xCo_1−xFe₂O₄ (x = 0.4) under optimized reaction conditions. The catalytic protocol demonstrated exceptional efficiency, achieving up to 93% isolated yield with broad functional group tolerance, surpassing various existing catalytic systems. The key advantage of this synthetic protocol is the magnetic nature of the catalyst, which allows for easy separation and superior recyclability with insignificant activity loss, which are in accordance with green chemistry principles. This catalytic system is a prospective substitute for medicinal and industrial applications because of its high catalytic efficiency, reusability, and sustainability. This study demonstrates biogenic nickel-doped cobalt ferrite nanoparticles as economical, environmentally benign catalysts for tetrazole scaffold synthesis, providing a scalable and sustainable method of producing bioactive compounds.

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1. Introduction

The invention of newer materials with enhanced properties and novel synthesis measures is a challenge for scientists with a materials background seeking to fulfil recent technological demands. Spinel ferrite nanomaterials have attracted immense popularity among the metal oxides in the present decade.^1–4 They have emerged as an advanced class of nanostructured materials because of their extremely prominent features at the nanometric scale. The recovery of magnetic metal oxide catalysts from reaction mixtures with negligible loss of catalytic activity via the application of an external magnet is possible, which enhances the efficient recyclability of the catalyst.^5–8 The demand for developing sustainable and safer synthesis methods for the synthesis of nanoparticles is increasing continuously, and these synthesis procedures reveal attractive advantages, such as less toxic and harmful ingredients, cost-effectiveness, mild reaction conditions and low power and time consumption. Undeniably, there is a need to develop sustainable and safer synthesis protocols that eradicate the complications and hazardousness of commonly used physicochemical methods. Avoiding the application of surfactants significantly simplifies the creation of nanomaterials with properties such as simplicity, cost-effectiveness and an environmentally benign nature. Thus, the elimination of surfactants is extremely necessary for the sustainable synthesis of nanomaterials.^9–13

The doping of a metal on the surface of spinel ferrite modifies its surface area and generates additional active sites on the surface.^14–16 A synergistic effect can be created between the metal ions and ferrite matrix by doping, which can amend the electronic structure of spinels, thus prompting the redox properties and hence enhancing the selectivity and catalytic activity of the material. The magnetic characteristics of doped ferrites enable facile separation, which simplifies recycling and diminishes waste, increasing the sustainability of the catalytic process.

Tetrazoles were first synthesized by J. A. Bladin in 1885, and are a vital class of poly-aza-heterocyclic moieties mostly found in the living world. Lately, tetrazoles have gained great popularity because of their wide range of applications in numerous domains such as pharmacology, high-energy materials science, biochemistry, and medicinal chemistry.^17–27 Owing to the compelling applications of numerous tetrazole moieties in diverse fields, various synthetic protocols for tetrazole scaffolds have been developed. Tetrazoles are generally synthesized either via a one-pot multicomponent addition method combining an organic nitrile/amine, triethyl orthoformate/aldehyde and sodium azide in the presence of suitable solvent conditions²⁸ and a catalyst, or a [3+2] cycloaddition reaction between isocyanides and hydrazoic acid/trimethyl azide. Because of the biological significance of tetrazoles, the 1,3-dipolar cycloaddition of aryl cyanides and azides to form 5-substituted tetrazoles is a valuable reaction in pharmaceutical and medicinal chemistry (Fig. 1). However, the use of traditional homogeneous catalysts, such as palladium–phosphine complexes, for such syntheses presents drawbacks such as thermal instability, toxicity, air/moisture sensitivity, and catalyst deactivation. The past ten years have seen an increase in the investigation of alternate catalytic techniques due to the drawbacks of homogeneous systems, including metal leaching, poisoning, non-recyclability, and product contamination. On the other hand, for applications requiring high-purity products, like pharmaceuticals and food chemistry, heterogeneous catalysts, offer significant advantages, such as ease of recovery, improved stability, reduced environmental impact, and minimised metal loss. These factors make them less toxic and more sustainable.

Fig. 1 Chemical structures of some pharmaceutically important tetrazole moieties.

Many heterogeneous catalytic systems,^29–32 for instance, ZnO nanoparticles, boehmite-based nano-catalysts³³ including Pd-Arg@boehmite, Pd-SMTU@boehmite, boehmiteNps@Cur-Ni, etc., various copper-based nano-catalysts such as Cu(II)/Fe₃O₄@APTMS-DFXn, Fe₃O₄@SiO₂/ligand/Cu(II), CuO/aluminosilicate, etc., and magnetic nanocatalysts such as Fe₃O₄@tryptophan-La, Fe₃O₄@Nd, Fe₃O₄-adenine-Zn, Fe₃O₄@tryptophan@Ni, and CoFe₂O₄@glycine-Yb are available in the literature.

The synthesis of nanomaterials via green protocols offers relatively reduced energy and lower production costs than other conventional synthetic measures. This enhances the commercial-scale creation of nanomaterials. The biomolecules and phytochemicals extracted from plants play a role as stabilizing and reducing agents for the green or biogenic synthesis of nanoparticles. The application of plant-derived extracts for the production of metal-based hybrid nanoparticles exemplifies a novel green synthetic route that has achieved noteworthy research attention.

Therefore, the aim of the present study is to synthesize Ni-doped cobalt ferrite and investigate the effect of Ni substitution on the morphology, magnetic and structural properties of nickel-doped cobalt ferrite nanoparticles (Ni_xCo_1−xFe₂O₄) with the compositions x = 0.0, 0.4, 0.6 and 1 synthesized via a co-precipitation route. The designed Ni-doped cobalt ferrite catalytic systems were applied fruitfully to obtain 5-substituted 1H-tetrazoles from aromatic nitriles.

2. Experimental section

2.1. Materials and methods

2.1.1. Chemicals. Cobaltous nitrate hexahydrate (Co(NO₃)₂·6H₂O, SRL, 99.5%), nickel chloride hexahydrate (NiCl₂·6H₂O, SRL, 99%), ferrous sulphate heptahydrate (FeSO₄·7H₂O, SRL, 99%), sodium hydroxide pellets (Qualikems Lifesciences Pvt. Ltd), nitriles (Spectrochem, 95%), sodium azide extrapure AR, ACS (NaN₃, SRL, 99%), ammonium acetate extrapure AR (C₂H₇NO₂, SRL, 98%), boronic acids (Spectrochem, 95%), arylhalides (SRL, 99%).

2.1.2. Preparation of Dillenia indica plant extract. The Dillenia indica fruits were collected from the area neighbouring Dibrugarh University campus. They were washed rigorously with deionized water. The pulp extract of Dillenia indica was prepared using 10 g of finely cut pulp. Then, 200 mL of sterile distilled water was added to it in an Erlenmeyer flask, the mixture was boiled for 10 min, and the solution was filtered. The as-prepared filtrate was used as a stabilizing and reducing agent for the synthesis.

2.1.3. Synthesis of Ni_xCo_1−xFe₂O₄ nanoparticles. Ni_xCo_1−xFe₂O₄ nanoparticles (x = 0.0, 0.4, 0.6 and 1.0) were synthesized via a co-precipitation route. Equimolar (0.1 M) starting materials cobaltous nitrate hexahydrate, nickel chloride hexahydrate, and ferrous sulphate heptahydrate were dissolved in pulp extract solution (50 mL) prepared in proper stoichiometric ratios. The solutions of cobaltous nitrate hexahydrate, nickel chloride hexahydrate, and ferrous sulphate heptahydrate were mixed under ultrasonication. 5 mL of NaOH solution (0.1 M) was added to the mixture, the pH was maintained at 10, and the reaction mixture was shifted to a magnetic stirrer at room temperature for 1 h. The precipitate was then collected, centrifuged, washed rigorously with water and ethanol and oven dried. In the subsequent step, annealing of the dried powder was done at 400 °C for 2 h, which led to the formation of biogenic Ni_xCo_1−xFe₂O₄ MNPs (Scheme 1).

Scheme 1 Schematic representation of the synthesis of Ni_xCo_1−xFe₂O₄ nanoparticles.

2.2. Characterization of Ni_xCo_1−xFe₂O₄ nanoparticles

The synthesized MNPs were characterized using scanning electron microscopy (SEM), high resolution transmission electron microscopy (TEM) and selected area electron diffraction (SAED) patterns, elemental mapping, X-ray diffraction (XRD) analyses, X-ray photoelectron spectroscopy (XPS), vibrating sample magnetometry (VSM), infrared spectroscopy, etc.

3. Results and discussion

All four of the synthesized samples were examined for composition and phase purity via powder X-ray diffraction (PXRD) analysis using a Bruker D8 Advance X-ray diffractometer with monochromatic Cu Kα radiation (λ = 1.5406 Å) at room temperature. The XRD patterns of the as-prepared Ni_xCo_1−xFe₂O₄ (x = 0.0, 0.4, 0.6 and 1.0) nanoparticles are shown in Fig. 2, and matched well with JCPDS card no. 22-1086, face centred cubic crystal structure (Fd3m space group). The peak positions in the PXRD pattern shows little alteration with doping, since the scattering factors of cobalt and nickel are nearly the same.^34,35

Fig. 2 XRD patterns of Ni_xCo_1−xFe₂O₄ nanoparticles with nominal compositions x = 0.0, 0.4, 0.6 and 1.0.

The XRD pattern confirmed the cubic spinel structure of Ni_xCo_1−xFe₂O₄ (x = 0.0, 0.4, 0.6 and 1.0) nanoparticles. The extremely intense diffraction peak for the (3 1 1) plane at around 35° for all samples indicates the high crystallinity.

The diffraction pattern of the synthesized material at x = 0.4 concentration showed diffraction peaks at 2θ values of 30.37°, 35.72°, 37.20°, 43.41°, 53.66°, 57.22°, 62.82°, and 74.56°. These peaks correspond to the crystallographic planes (2 2 0), (3 1 1), (2 2 2), (4 0 0), (4 2 2), (5 1 1), (4 4 0), and (5 3 3).^35–38 The absence of additional peaks associated with impurities in the diffraction pattern confirms the formation of the cubic spinel structure of the nanomaterial. Using the Scherrer equation, the crystallite sizes of the synthesized Ni_xCo_1−xFe₂O₄ nanomaterials with nominal compositions of x = 0, 0.4, 0.6, 1.0 were found to be 6.83, 4.35, 4.18 and 3.81, respectively.

To obtain more insight into the morphological and microstructural data of Ni_xCo_1−xFe₂O₄ nanoparticles, SEM and HR-TEM analysis was performed. The SEM analysis was carried out using a Jeol JSM-IT300, Zeiss SIGMA VP scanning electron microscope and is shown in Fig. 3(a). The SEM micrograph of the synthesized Ni_xCo_1−xFe₂O₄ nanoparticles with a nominal composition of x = 0.4 demonstrates a cubic-structured morphology.

Fig. 3 (a) SEM micrograph and (b) EDAX spectrum of the as-synthesized Ni_xCo_1−xFe₂O₄ nanoparticles with a nominal composition of x = 0.4.

To obtain insight into the compositions of the synthesized doped ferrite samples, energy dispersive X-ray spectroscopy (EDAX) equipped within the scanning electron microscope was performed. The EDAX spectrum and elemental mapping are shown in Fig. 3(b) and 4. The existence of the elements Co, Ni, Fe and O in the Ni_xCo_1−xFe₂O₄ samples (x = 0.4) was confirmed by the compositional analysis, and elemental mapping shows the distribution of nickel in the cobalt ferrite samples.

Fig. 4 Elemental mapping of the as-synthesized Ni_xCo_1−xFe₂O₄ nanoparticles with a nominal composition of x = 0.4.

TEM analysis (Fig. 5(a)) shows that samples with a composition of x = 0.4 are cubic in shape. Fig. 5(b) shows their structure with irregular cubic-shaped nanoparticles.

Fig. 5 (a) TEM, (b) HR-TEM, and (c) SAED pattern of the as-synthesized Ni_xCo_1−xFe₂O₄ nanoparticles with a nominal composition of x = 0.4.

For the x = 0.4 samples, a measured discrete lattice fringe with a d-spacing value of 0.27 nm, which corresponds to the (3 1 1) lattice plane, was shown by an HR-TEM image taken from a particular region (Fig. 5(b)).

The SAED pattern (Fig. 5(c)) demonstrates the formation of crystalline-nature nanoparticles for the nominal composition x = 0.4. It reveals several diffraction spot circles because of the poly-crystallinity of the sample.¹⁵

From the relevant TEM images, the sizes of the nanocrystals were found to be in the nano-scale range, and the size distribution curve revealed that the size of nanoparticles fell in the range of 15.73 nm (Fig. 6(a)). These morphological data is well supported by some previously reports in the literature.^36,37

Fig. 6 (a) Average size distribution and (b) hysteresis loops of Ni_xCo_1−xFe₂O₄ nanoparticles with nominal compositions of (a) x = 0.0 (b) x = 0.4, (c) x = 0.6 and (d) x = 1.0.

The room-temperature magnetization of the Ni_xCo_1−xFe₂O₄ nanoparticle samples with different Ni contents (x = 0.0, 0.4, 0.6, 1.0) was measured with a Lakeshore 7400 series vibrating sample magnetometer (VSM) in the range of H = ±15 kOe.

The synthesized Ni-doped CoFe₂O₄ nanoparticles display a ferromagnetic/ferrimagnetic nature, which indicates the existence of magnetic structure in the spinel systems.^36–39 The measured magnetic parameters of the Ni_xCo_1−xFe₂O₄ samples (x = 0.0, 0.4, 0.6 and 1.0) are shown in Fig. 6(b). Notably, it was found that the saturation magnetization value (M_s) decreases with increasing concentration of nickel.³⁹ From the M–H curve, it was noticed that with applied magnetic field, the saturation magnetization (M_s) increased and the remanence (M_r) also increased with increasing Co²⁺ content. The characteristics of saturation magnetization (M_s), retentivity (M_r) and coercivity (H_c) were determined from the hysteresis loop (Fig. 6(b)); the values are listed in Table 1. The distribution of cations between the tetrahedral and octahedral sites of Ni_xCo_1−xFe₂O₄ spinel and Ni²⁺, Co²⁺ and Fe³⁺ has the possibility of affecting the magnetic properties. The M_s value decreased as the concentration of nickel was increased; the reason for this is the fact that the magnetic moments of nickel (3μ_B) and Fe³⁺ (4.9μ_B) are lower than that of cobalt (5μ_B). Hence, the saturation magnetization showed an increasing trend with substitution of the nickel cations in the place of cobalt cations.

Table 1 Magnetic properties of Ni_xCo_1−xFe₂O₄ nanoparticles

x	0.0	0.4	0.6	1.0
M s (emu g^−1)	28.43	26.15	22.63	21.18
M r (emu g^−1)	6.99	8.53	4.27	7.42
H c (kOe)	711.35	1005.47	27.35	355.67
M r/Ms	0.25	0.32	0.19	0.35

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In the AB₂O₄ spinel structure, “A” refers to the tetrahedral site and “B” refers to the octahedral site, and magnetization of the spinel mainly depends on the octahedral site (B). Hence, the “B” site was occupied with the Co²⁺ and Fe³⁺ ions and thus, M_s and M_r increased with increasing Co²⁺.

Néel's theory can clarify the distribution of cations between the octahedral and tetrahedral sites of the spinel. The migration of Co²⁺ ions from the octahedral sites to tetrahedral sites of the CoFe₂O₄ matrix takes place because of the occupancy of Ni²⁺ ions in the octahedral sites. To overcome the disorder generated by Ni²⁺ ion substitution, an equal number of Fe³⁺ ions present in the B-sites of the lattice start to move towards the A-sites.

Replacing Co²⁺ with Ni²⁺ lowers the net moment on the B-site. This directly reduces the net magnetic moment, and Ni²⁺ weakens B–O–A superexchange interactions compared to Co²⁺. This causes less efficient magnetic coupling between A and B sites, and thus reduces the saturation magnetization value.

The increase of the M_r value may result in the distribution of cations of with the two unpaired electrons of nickel and the three unpaired electrons of cobalt at the octahedral site. The coercivity values H_c were found to be highest at x = 0.4 from the M–H curves, whereas the sample with x = 0.6 shows the least coercive field. The M–H curves of the samples exhibited an S-shaped nature. The M_r/M_s ratios, which is also called the squareness and reduced remanence (S), were found and they is displayed in Table 1. The highest coercivity among the Ni_xCo_1−xFe₂O₄ samples was shown for x = 0.4, whereas the highest magnetization was observed for x = 0.

The elemental composition and the valence state of Ni, Co and Fe in the as-synthesized Ni_0.4Co_0.6Fe₂O₄ samples were established using XPS measurements with a Thermo Fisher Scientific Pvt. Ltd (Model: ESCALAB Xi+) X-ray photoelectron spectrometer.

The existence of Co, Ni, Fe and O as the main elements of the sample (Fig. 7) was confirmed from the survey scan. The XPS spectra of Ni shows peaks at 855.11 and 872.86 eV with satellite peaks at 861.31 and 879.20 eV for Ni 2P_3/2 and Ni 2P_1/2 respectively, indicating that Ni is in the +2 valence state. Co shows peaks at 780.16 and 795.46 eV, with satellite peaks at 785.01 and 801.71 eV for Co 2P_3/2 and Co 2P_1/2, indicating that Co is in +2 valence state. Fe shows peaks at 711.41 and 723.81 eV, with satellite peaks at 716.31 and 732.01 eV for Fe 2P_3/2 and Fe 2P_1/2. This is well supported by some previous literature.^9,36–38

Fig. 7 Wide survey scan XPS spectrum of Ni_0.4Co_0.6Fe₂O₄, XPS spectrum of Ni 2p, XPS spectrum of Co 2p, XPS spectrum of Fe 2p, XPS spectrum of O 1s.

The FTIR spectra of pure cobalt ferrite and Ni-doped cobalt ferrite nanoparticles with x = 0.0, 0.4, 0.6 and 1 are presented in Fig. 8. With increasing Ni-doping, both stretching vibrations modes of the metal complex in the octahedral site, ν_B(M_B–O), and tetrahedral site, ν_A(M_A–O), shift slightly toward higher frequencies. For x = 0 and x = 1 the ν_B, ν_A values are 559, 401 and 562, 419 cm⁻¹, respectively, which are almost equal to the typical values for cobalt and nickel spinel ferrites, respectively. With increasing Ni content, the octahedral band in particular shifts toward higher wavenumber since the ionic radius of Co²⁺ (0.74 Å) is lower than that of Ni²⁺ (0.69 Å), which indicates a stronger Ni–O bond. For x = 0.4 and x = 0.6 Ni_xCo_1−xFe₂O₄ nanoparticles ν_B, ν_A values are 563, 404 and 560 and 410 cm⁻¹, respectively. This clearly indicates the substitution of Co²⁺ ions by Ni²⁺ ions in the nanostructured Ni-doped cobalt ferrite.^38–40

Fig. 8 FTIR spectra of Ni_xCo_1−xFe₂O₄ nanoparticles with nominal compositions of (a) x = 0.0 (b) x = 0.4, (c) x = 0.6 and (d) x = 1.0.

In the IR spectra of the four synthesized samples, the stretching vibration bands appearing at around 3343 cm⁻¹ are due to –OH stretching interaction with the nanoparticle surface via H-bonding due to presence of various hydroxyl groups in the Dillenia indica plant extract. The stretching vibrations appearing at 797 cm⁻¹ and 908 cm⁻¹ are ascribed to C–H out-of-plane bending vibrations, whereas the vibrations at 1088 cm⁻¹ appear due to the alkoxy C–O from the O-containing functional groups, e.g., carboxylic, epoxy and carbonyl groups. The emergence of a stretching vibration band around 1244 cm⁻¹ is generally due to C–O stretching vibrations for aryl ether. The stretching frequencies around 1320 cm⁻¹, 1423 cm⁻¹, 1552 cm⁻¹ and 1652 cm⁻¹ are assigned respectively to phenolic O–H bending vibrations, –CH₂/–CH₃ bending vibrations, aromatic C=C stretching and C=O stretching vibrations in flavonoid molecules present in the plant extract.⁹

Correlating the FTIR spectra with the XRD spectra, appearance of metal–oxygen bond stretching in the tetrahedral (569–563 cm⁻¹) and octahedral (401–419 cm⁻¹) sites confirms the formation of spinel-structured nanoparticles, and this information is in agreement with the XRD patterns of synthesized spinel nano-samples. Due to lattice contraction, with increasing Ni doping concentrations, alteration in peak positions was clearly visible in both the powder XRD and FTIR spectra. The crystallite size also changes with doping, with the Scherrer equation indicating that smaller-sized nanoparticles generally show broader peaks in XRD, whereas the FTIR peaks also shift due to shorter and stronger bonds. From the Scherrer equation, the crystallite sizes of the synthesized Ni_xCo_1−xFe₂O₄ nanomaterials with nominal compositions of x = 0, 0.4, 0.6, 1.0 were found to be 8.83, 12.35, 11.18 and 10.81, respectively. The sharpest peak for the synthesised Ni_xCo_1−xFe₂O₄ nanoparticles was obtained for a nominal composition of x = 0.4. The XRD and FTIR spectra clearly indicate the high crystallinity of the sample at this nominal composition.

4. Catalytic study

Initially, our as synthesized nanomaterial was used to investigate its catalytic performance towards our most anticipated 5-substituted-1H-tetrazole synthesis. For the screening of satisfactory reaction conditions for the synthesis of 5-substituted-1H-tetrazole, such as catalyst loading, reaction temperature and solvent, 4-fluorobenzonitrile was selected as the architype in the presence of sodium azide in dimethylformamide (DMF) as the reaction medium.

Out of all the synthesized catalytic systems, Ni_xCo_1−xFe₂O₄ (x = 0.4) showed the highest catalytic activity, whereas the other two catalytic systems CoFe₂O₄ and NiFe₂O₄ showed relatively lower catalytic activity in comparison to Ni_xCo_1−xFe₂O₄ (x = 0.4) and Ni_xCo_1−xFe₂O₄ (x = 0.6) in terms of product yield and reaction time (Table 2). Upon performing the reaction in the presence of 15 mg of the catalyst, the yield was improved to 93%. Further, no significant improvement in product yield or reaction time was observed upon increasing the amount of catalyst.

Table 2 Optimization of catalyst

Entry	Catalyst	Catalyst loading (mg)	Time (h)	Yield^a (%)
(NixCo1−xFe2O4 (x = 0), NixCo1−xFe2O4 (x = 0.4), NixCo1−xFe2O4 (x = 0.6), NixCo1−xFe2O4 (x = 1.0) are denoted as NCF-0, NCF-1, NCF-2 and NCF-3, respectively). Reaction conditions: 4-fluorobenzonitrile (0.121 g, 1 mmol), NaN3 (0.065 g, 1 mmol), NH4OAc (0.092 g, 1.2 mmol), DMF (3 mL), 90 °C.a Isolated yield.
1	NCF-0	10	5	40
2	NCF-0	15	5	62
2	NCF-0	20	5	70
3	NCF-1	10	2	83
4	NCF-1	15	0.5	93
5	NCF-1	20	0.5	93
6	NCF-1	5	0.5	72
7	NCF-2	10	0.5	76
8	NCF-2	15	0.5	88
9	NCF-2	20	0.5	88
10	NCF-3	20	5	58
11	NCF-3	30	5	68
12	—	—	12	No reaction

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To establish the most appropriate solvent, different types of solvents ranging from non-polar to polar as well as green solvents were applied (Table 3), but best outcomes were obtained on using polar solvent DMF, which is indisputably considered to be an excellent organic solvent (Scheme 2). During the optimization of the amount of NaN₃ and NH₄OAc, 1 mmol of NaN₃ and 1.2 mmol of NH₄OAc was found to be sufficient for the smooth progress of the reaction (Table 4).

Table 3 Optimization of solvent and reaction temperature

Entry	Solvent	Temperature (°C)	Yield^a (%)
Reaction conditions: 4-fluorobenzonitrile (0.121 g, 1 mmol), NaN3 (0.065 g, 1 mmol), NH4OAc (0.092 g, 1.2 mmol), NixCo1−xFe2O4 (x = 0.4) catalyst (15 mg).a Isolated yield.
1	Cyclohexane	80	20
2	Toluene	110	32
3	PEG-400	100	35
4	Acetonitrile	80	28
5	1,4-Dioxane	100	—
6	Acetone	50	—
7	Water	100	68
8	Ethanol	70	42
9	DMSO	120	82
10	THF	60	11
11	DMF	120	93
12	DMF	110	93
13	DMF	100	93
14	DMF	90	93
15	DMF	80	89
16	DMF	70	80
17	DMF	RT	51
18	Chloroform	60	58

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Scheme 2 Optimized reaction conditions for the synthesis of 5-substituted 1H-tetrazoles.

Table 4 Optimization of amount of NaN₃ and NH₄OAc

Entry	Amount of NaN3 (mmol)	Amount of NH4OAc (mmol)	Yield^a (%)
Reaction conditions: 4-fluorobenzonitrile (0.121 g, 1 mmol), NaN3 (1 mmol), NH4OAc (1.2 mmol), NixCo1−xFe2O4 (x = 0.4) catalyst (15 mg).a Isolated yield. DMF (3 mL).
1	2	1	91
2	1.2	1	91
3	1	1	92
4	1	1.2	93
5	1	1.5	93
6	1	—	81
7	0.5	1.5	84
8	2	—	85
9	3	—	89

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An investigation of the catalytic activity and efficiency was carried out with a diverse number of aromatic nitrile substrates, and it was noted that both electron-withdrawing and electron-donating substituents, along with sterically hindered aromatic nitriles, underwent the reaction smoothly, and the corresponding 5-substituted 1H-tetrazole derivatives were obtained in good-to-excellent yields (81–93%).

The kind of substituent, i.e., electron-releasing or electron-withdrawing group, has a little influence on the product yields in the 5-substituted 1H-tetrazoles. An electron-withdrawing group at the para position gives better yields than at the ortho position, whereas an electron-donating group gives reduced yield in comparison to an electron-withdrawing group. However, surprisingly, aromatic nitriles containing –NH₂, –NO₂, –OCH₃, and –CHO groups show lower yields in comparison to the other groups (Fig. 9).

Fig. 9 Substrate scope for 5-substituted 1H-tetrazoles synthesis. Reaction conditions: aromatic nitriles (1 mmol), NaN₃ (1 mmol), NH₄OAc (1.2 mmol), Ni_xCo_1−xFe₂O₄ (x = 0.4) catalyst (15 mg), DMF (3 mL). Yields are isolated yields.

It was observed that in the presence of 2 mmol of sodium azide, isophthalonitrile and 4,5-dichlorophthalonitrile undergo a [3+2] cycloaddition reaction, thus forming two products by addition to one position as well as at both the two positions (information is available in the ESI†).

4.1. Mechanism of action

Based on some previously reported mechanistic pathways, a plausible reaction pathway for the synthesis of our highly anticipated tetrazole derivatives using the nickel-doped cobalt ferrite nano-catalyst is shown in Scheme 3. In the very first step, the co-ordination the aromatic nitrile and the azide compound (ammonium azide) with the Ni(II) nano-catalyst forms complex A. The formation of complex A accelerates the cyclization step. This fact was well established by performing the model reaction in the absence of the catalyst. Even after a longer period of time, the cycloaddition reaction was not completed in the absence of the catalyst (Table 2, entry 12).

Scheme 3 Plausible mechanism for the formation of 5-substituted 1H-tetrazoles.

The [3+2] cycloaddition reaction between the nitrile group and the azide ion occurs to produce the intermediate B. via the generation of H⁺ from NH₄OAc, protonolysis of intermediate B occurs, which results in the desired 5-substituted 1H-tetrazoles.

The ammonium acetate in the reaction acts as a proton source to afford pure tetrazole derivatives and the in situ formation of ammonium azide by the reaction between sodium azide and ammonium acetate, which increases the accessibility of azide ions for [3+2] cycloaddition with aromatic nitriles.^41–44

4.2. Recyclability of the Ni_xCo_1−xFe₂O₄ (x = 0.4) catalyst for 5-substituted 1H-tetrazole synthesis

One of the most important characteristics of a heterogeneous catalyst^45–47 is its recyclability. This catalyst can be efficaciously applied up to the fifth catalytic cycle with insignificant loss in the yield of the product, which demonstrates robust characteristics along with reusability and recyclability (Fig. 10).

Fig. 10 Schematic and graphical representation of recyclability of the catalyst.

For the recyclability test, we selected 4-fluorobenzonitrile as the model substrate. The magnetic nano-catalyst was isolated via magnetic separation, which was followed by centrifugation. The catalyst was washed with ethanol, dried and to for an additional cycle by sustaining the same stoichiometry of the reaction. This method was repeated for four consecutive cycles. Upon conducting VSM, SEM and TEM analysis of the reused catalyst after the 5th cycle, a slight change in the saturation magnetization value was observed from the hysteresis loop (25.89 emu g⁻¹) along with a small change in the morphology (Fig. 11). This observation clearly suggests the potential of this catalyst for consecutive use of in successive catalytic runs. PXRD analysis (Fig. S1, ESI†) showed the existence of all the same planes observed for the fresh catalyst in the recycled (reused) catalyst.

Fig. 11 (a) SEM image, (b) TEM image, and (c) VSM spectrum of the recycled catalyst.

4.3. Brief comparison

A synthetic protocol for facile access to 5-substituted-1H-tetrazoles was reported using a nickel-doped cobalt ferrite nano-catalyst. Tetrazole moieties have enormous importance as well, as they have made significant contributions towards the advancement of numerous domains, ranging from pharmacology to materials science.

Here, using our approach, exciting results were provided using their respective developed methodologies. During the synthesis of 5-substituted 1H-tetrazoles, DMF provided the most suitable medium, whereas Ni_0.4Co_0.6Fe₂O₄ showed best catalytic activity.

While some of the other reported catalysts for the synthesis of 5-substituted 1H-tetrazoles often suffer from issues such as longer reaction time and tedious synthetic procedures for the catalyst, our as-synthesized Ni-doped cobalt ferrite nanoparticles stand out as a superior and more sustainable alternative (Table 5). The sustainability of the catalytic process was determined via green chemistry matrix calculations such as E-factor and reaction mass efficiency calculations (calculations are available in the ESI†).

Table 5 Comparative study of catalytic efficacy for 5-substituted 1H-tetrazole synthesis across a number of previously reported systems

Sl no.	Catalyst	Reaction conditions and time	Yield (%)	Ref.
1	CoFe2O4@glycine-Yb (70 mg)	PEG, 120 °C, 2.17–2.58 h	92–96	48
2	Chitosan supported magnetic ionic liquid nanoparticles (CSMIL) (2.5 mol%)	Solvent-free, 70 °C, 6–9 h	81–90	49
3	CuO/aluminosilicate (35 g)	DMF, 110 °C, 2–13 h	80–93	50
4	Fe3O4@SiO2/ligand/Cu(II) (0.4 mol%)	DMF, 110 °C, 4–10 h	83–96	51
5	Fe3O4@SiO2/salen Cu(II)	DMF, 110 °C, 6–12 h	80–92	52
6	ZnO (100 mg)	DMF, 120–130 °C, 14 h	69–82	53
7	ZnO/Co3O4 (50 mg)	DMF, 120–130 °C, 12 h	84–94	54
8	NiFe2O4 (5 mol%)	DMF, 100 °C, 1–3 h	80–98	41
9	MnCl2·4H2O (10 mol%)	DMSO, 120 °C, 1 h	84–92	55
10	Fe3O4@SiO2-AAPA/Imid-Pd (7 mol%)	PEG, 100 °C, 2 h	85–99	56
11	ZnFe2O4@SiO2@n-Pr@xanthine-Pr (50 mg)	PEG, 120 °C, 2–5.66 h	88–97	57
12	CoFe2O4/SBA-15/dithizone/Gd (60 mg)	PEG, 120 °C, 0.33–4 h	75–90	58
13	Boehmite@Schiff-base-Cu (40 mg)	PEG, 120 °C, 1.17–3.33 h	70–98	59
14	Fe3O4@SiO2-LY-C-D-Pd (30 mg)	PEG, 120 °C, 0.5–2 h	89–98	60
15	Ni–ascorbic acid MOF (30 mg)	DMF, HCl, 100 °C, 8–30 h	84–97	61
16	Ni x Co 1−x Fe 2 O 4 (x = 0.4) (15 mg)	DMF, 90 °C, 0.5 h	81–93	This work

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5. Conclusion

This study reported the exploration of the development and assessment of nano-magnetic Ni-doped cobalt ferrite [Ni_xCo_1−xFe₂O₄ (x = 0.0, 0.4, 0.6 and 1.0)] via a co-precipitation route using Dillenia indica pulp extract as a stabilizing and reducing agent. In this work, we investigated how the structural and magnetic properties of nickel-doped cobalt ferrites were affected when Co²⁺ ions were substituted with Ni²⁺ ions. Morphological data from scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), and X-ray diffraction (XRD) patterns were used to evaluate the morphology and size of the particles. These data validated the nanomaterial's nanostructure. All of the produced materials were investigated for our most anticipated synthesis of bioactive 5-substituted 1H-tetrazole moieties that are significant in industry and medicine. Remarkably, Ni_xCo_1−xFe₂O₄ (x = 0.4) demonstrated the highest efficiency among all the catalytic systems in the synthesis of 5-substituted 1H-tetrazole moieties from aromatic nitriles. To our knowledge, the aforementioned Ni–CoFe₂O₄-catalysed synthesis of 5-substituted 1H-tetrazole moieties from aromatic nitriles is one of the best procedures ever documented in terms of product yield, reaction conditions, and reaction time.

Additionally, this catalytic system offers the advantages of a highly cost-effective synthetic protocol, superior recyclability, superficial magnetic retrievability and wider functional group tolerance.

Author contributions

Jyotismita Bora: design, investigation, methodology, data curation, visualization and writing – original draft. Mayuri Dutta: data curation and visualization. Amar Jyoti Kalita: visualization and data curation. Aquif Suleman: visualization and data curation. Ujaswi Chutia: data curation. Dr Bolin Chetia: supervision, conceptualization, resources, investigation, methodology, resources, visualization.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting the findings of this study are available within the article and its ESI.† Additional datasets are available from the corresponding author upon reasonable request.

Acknowledgements

J. B. is grateful to the Department of Science and Technology (DST), India for financial assistance and research fellowship under PURSE programme [No. SR/PURSE/2022/143 (C)] and DST-FIST programme [No. SR/FST/CS-I/2020/152]. The authors would like to acknowledge Dibrugarh University for providing infrastructural facility, CSIC, Dibrugarh University for NMR, Department of Chemistry, Dibrugarh University for LCMS analysis, CIF-IIT Guwahati for VSM analysis, SRIC-IIT-Roorkee for VSM and XPS analysis, STIC- Cochin for HRTEM, PXRD and SEM-EDX analysis, CIF-IASST for TEM and SEM-EDX analysis.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5nj01418b

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