Green Synthesis of NiFe₂O₄ Nanoparticles Using Combretum Indicum Leaf Extract for the Synthesis of 2-Aryl Benzimidazole

The key advantage of magnetic nanoparticles is that they can be easily removed from the reaction by using an external magnet. Also, this magnet assisted catalyst recovery saves catalyst loss, time, and energy [1]. These days, a lot of research is going on to produce nanoscale metals by physical, chemical, and environmentally friendly methods [2,3].

Green synthesis is gradually replacing physical and chemical methods due to the excessive energy consumption [3,4], the release of toxic and harmful chemicals [5], and the complexity of the equipment and synthesis conditions [6,7], Extracts from leaves are currently the most common green source for synthesis [8,9,10], flower roots [11,12,13], peelings [14], fruits [15] and seeds of different plants [16,17]. However, the wide range of phytochemicals in plant extracts can effectively reduce a great number of metal ions and also aid in the green synthesis of nanoparticles [18].

Plant extracts are a key component in the preparation of nanoparticles due to their benefits and special properties, and are a more economical, efficient, and environmentally friendly way of producing nanoparticles than by chemical and physical methods [19,20]. Several phytochemicals, including terpenoids, flavonoids, ketones, aldehydes, amides, and carboxylic acids, are present in the leaves and act as stabilizers and reducing agents during the synthesis of metal and metal oxide nanoparticles [19]. In particular, plant extracts are often employed in nanoparticle synthesis due to their affordability, environmental friendliness, and ability to produce biocompatible, enhanced nanoparticles. Plant extracts play a key role in the preparation of nanoparticles [21,22,23]. So, plant extract-induced synthesis of NiFe₂O₄ nanoparticles was considered in this study. Plant extracts contain phytochemicals that act as natural reducing agents and stabilizers to form toxic free biocompatible nanoparticles.

Due to their variety of pharmacological actions and medicinal chemistry applications, 2-aryl benzimidazole synthesis is important [24,25]. These substances have been widely studied due to their various biological activities, including antibacterial, antifungal, antiviral, anticancer, and antihistamine properties [26]. Several benzimidazole compounds have been synthesized with antimicrobial [27], anticancer [28], anti-inflammatory [29], analgesic [30], antimalarial [31], antitubercular [32], antiviral [33], antihistaminic [34], anti-HIV [35], acetylcholinesterase [36], and antiprotozoal [37] activities. Further investigation into 2-aryl benzimidazoles might result in the development of more potent and therapeutically useful substances for the treatment of a range of diseases [38]. The popularity of these molecules in drug discovery is enhanced by the development of eco-friendly and efficient synthesis methods using ionic liquids, AlCl₃, or magnetic nanocatalysts [39,40,41].

While plant-extract-mediated green synthesis of magnetic ferrite nanoparticles is well established, reports specifically employing Combretum indicum leaf extract to fabricate NiFe₂O₄ remain scarce, underscoring the novelty of the approach described here. Bio-inspired synthesis of related binary and multicomponent oxide nanomaterials, including zinc ferrite, zinc tin oxide, nickel ferrite, bismuth ferrite, and zinc zirconate, has likewise been demonstrated using natural plant extracts as green chelating agents [42,43,44,45,46], further supporting the broader applicability of phyto-mediated routes to magnetically active spinel oxides such as the NiFe₂O₄ nanocatalyst reported in the present work.

The catalyst loading had a marked impact on the model reaction of 4-hydroxybenzaldehyde and o-phenylenediamine. Increasing the NiFe₂O₄ from 5 to 20 mg also increased the yield (62–95%) and reduced the reaction time (90–40 min) since more active sites were available. The best catalytic activity was found to be 20 mg NiFe₂O₄ (Table 1 entry 4). No further improvement was made with 25 mg of catalyst, so 20 mg is the best catalyst loading.

Table 2 shows that aldehydes containing both electron-donating and electron-withdrawing groups produced the desired products with good to excellent yields (87–95%) within 30–45 min, indicating broad substrate scope and efficiency of the reaction. The observed melting points closely matched the reported values, confirming both the purity and identity of the synthesized compounds.

Next, we have compared the catalyst with other reported catalysts (Table 3). The NiFe₂O₄ catalyst is more efficient than the reported catalysts, with yields of 87–95% in 35–50 min under mild conditions. In contrast, other catalysts require much longer reaction times and much harsher reaction conditions.

Protic solvents play a beneficial effect in encouraging condensation and cyclization; ethanol produced the highest yield (95%) in the shortest time (40 min), followed by methanol (88%, 50 min) and water (82%, 52 min). Less successful were acetonitrile (75%, 60 min) and toluene (80%, 55 min). As a result, ethanol was chosen as the best solvent, taking into account green chemistry, yield, and efficiency (Table 4).

The recyclability of the NiFe₂O₄ catalyst was investigated by the same model reaction. It was easily recovered by filtration and washed for reuse (Table 5 and Figure 1). The catalyst remained active in six cycles without deactivation. Consistent high yields were obtained, similar to those with a new catalyst.

Nickel(II) nitrate hexahydrate (Ni(NO₃)₂·6H₂O, ≥99%) and iron(III) nitrate nonahydrate (Fe(NO₃)₃·9H₂O as precursor salts. The reagents were obtained from Sigma-Aldrich (St. Louis, MO, USA)) and used without further purification. The reaction progress was monitored by Thin Layer Chromatography (TLC). Melting points of the products were measured by a digital melting point apparatus. The synthesized products were characterized by ¹H NMR (500 MHz, CDCl₃/DMSO, TMS) on a Bruker Avance Neo 500 spectrometer at the Sophisticated Analytical Instrumentation Facility (SAIF), Chandigarh University, Mohali, India. The mass spectra were obtained on a Shimadzu-GCMS-QP2010 spectrometer operating at 70 eV.3.2. Preparation of Plant Extract.

The leaves of Combretum indicum were sourced from a local garden and washed with deionized water. About 10 g of leaves were cut into pieces and placed in 100 mL of deionized water, then heated at 70 °C. After cooling, the mixture was filtered, and the leaf extract was used for the synthesis of the nanoparticles.

Combretum indicum leaf extract appears to have a diverse phytochemical profile. Primary constituents of the extract include phenolic compounds and flavonoids such as gallic acid and rutin, along with alkaloids, saponins, tannins, and flavonoids like vitexin and orientin [50]. Metal ion coordination can also be achieved with the hydroxyl, aromatic, and carbonyl groups that are present in these phytochemicals. This suggests that the earliest stage of nanoparticle formation may involve the chelation of metal ions with the phenolics and flavonoids of the leaves via ion coordination, similar to previous reports of chelation-induced formation of metal oxide nanoparticles [51]. During the sol-gel auto-combustion of the metal–phytochemical complex, it is likely that the complex will undergo thermal decomposition, allowing the ligands to be expelled as gases and crystallizing the spinel NiFe₂O₄ structure. FT-IR spectra show a broad absorption near 3240 cm⁻¹ along with weak bands near 1182 and 979 cm⁻¹ (Section 4.4), which suggest that phytochemicals are present on the surface of the nanoparticles, where they may have a functional role.

Nickel ferrite nanoparticles were synthesized by sol–gel auto combustion using nickel nitrate hexahydrate [Ni(NO₃)₂·6H₂O] and ferric nitrate. The metal nitrates were dissolved in 50 mL of deionized water, and 1 drop of Combretum indicum leaf extract was added dropwise while stirring at 80 °C, then heated until a gel. The dried gel was then annealed at 600 °C for 4 h to produce the final nickel ferrite nanoparticles.

Catalyst was analyzed by various methods through XRD, FT-IR, SEM, TEM, and BET analysis to check their structure, shape, and surface features. XRD confirmed phase purity and crystallite size, FT-IR spotted the key functional groups and the metal–oxygen bonds, while SEM and TEM showed their overall shape and nanoscale details. BET gave information about surface area and porosity, hinting that these particles could work well as catalysts.

A mixture of o-phenylenediamine (1 mmol) and the chosen aryl aldehyde (1 mmol) in a 25 mL round-bottom flask. Then, 20 mg of NiFe₂O₄ magnetic nanoparticles and 10 mL of ethanol as the solvent (Scheme 1). Then the mixture was stirred on a magnetic stirrer at room temperature, with progress monitored by TLC at regular intervals. Once the reaction finished, separated the catalyst with a magnet, washed it with ethanol, and dried it. After recrystallizing from ethanol gave good to excellent yields (Table 3, Entries 3a–e).

2-(2-Nitrophenyl)-1H-benzimidazole (3a): mp 117–119 °C; yield 92%; FT-IR (KBr, ν, cm⁻¹): 3739, 3040, 2526, 1797, 1604, 1519, 1341, 1421, 1267, 1223, 1066, 979, 916, 853, 764, 734. ¹H NMR (500 MHz, DMSO-d₆, δ ppm):13.05 (s, 1H, N–H), 9.04–7.94 (m, 4H, Ar–H), 7.56–7.35 (m, 4H, Ar–H), 1.06 (s, 2H).

2-(4-Nitrophenyl)-1H-benzimidazole (3b): mp 309–311 °C; yield 93%; FT-IR (KBr, ν, cm⁻¹): 3711, 3649, 3297, 3014, 2906, 2680, 2320, 1653, 1595, 1506, 1432, 1328, 1096, 1009, 955, 844, 740, 693. ¹H NMR (500 MHz, DMSO-d₆, δ ppm):13.29 (s, 1H, N–H), 8.83–8.44 (m, 4H, Ar–H), 8.40–7.60 (m, 4H, Ar–H).

2-(4-Hydroxyphenyl)-1H-benzimidazole (3c): mp 195–197 °C; yield 95%; FT-IR (KBr, ν, cm⁻¹): 3921, 2880, 2596, 1887, 1594, 1443, 1381, 1243, 1159, 1107, 893, 824, 736. ¹H NMR (500 MHz, DMSO-d₆, δ ppm): 10.55 (s, 1H, N–H), 9.80 (s, 1H, O–H), 8.10–7.60 (m, 4H, Ar–H), 7.25–6.40 (m, 4H, Ar–H).

From the XRD pattern of synthesized NiFe₂O₄ nanoparticles, the diffracted peaks are obtained at 2θ ≈ 30.2°, 35.5°, 43.2°, 53.6°, 57.2°, and 62.8°, corresponding to the (220), (311), (400), (422), (511), and (440) planes of the cubic spinel structure (JCPDS 10-0325). The absence of impurity peaks in the XRD pattern indicates the phase purity of the synthesized material (Figure 2).

SEM micrographs show particle distribution and surface morphology of NiFe₂O₄ nanoparticles at different magnifications. The images reveal that NiFe₂O₄ nanoparticles generally exhibited non-uniform, aggregated-nanostructured appearance with particle size distribution, and it resembles to most of the spinel ferrites (Figure 3).

HR-TEM images of synthesized nickel ferrite nanoparticles shown quasi-spherical particles with a little aggregation owing to magnetic interaction. The nanoparticles show a uniform distribution with an average size of 12–18 nm. High resolution lattice fringe images show the clear appearance of planes. The obtained values correspond to the characteristic d-values of the NiFe₂O₄ spinel structure. The formation of the sharp and continues lattice fringes showed good crystallinity of the obtained nanoparticles (Figure 4).

The FT-IR spectra show a broad band at about ~3240 cm⁻¹ due to adsorptive water/surface hydroxyls, and a band near ~1502 cm⁻¹ may attributed to H–O–H bend or residual organic from the process. There were found very weak peaks at about ~1182 and ~979 cm⁻¹, attributed to surface organic group or adsorbed anions (Figure 5). Metal-oxygen characteristic vibration of spinel ferrite is located between the wave-number 400–600 cm⁻¹ (Fe–O/Ni–O) located at the octahedral and tetrahedral sites, which confirms the formation of NiFe₂O₄. Adsorption water, surface hydroxyl groups, and surface organics on the surface show that there may exist incomplete surface coverages, which would affect its magnetic property and catalytic performance.

NF1 had the nitrogen adsorption–desorption isotherm with a typical mesoporous type (Figure 6). BET analysis over a limited P/P₀ range (0.05–0.30) linear portion yielded a surface area of 49.82 m²g⁻¹ with a high correlation coefficient (r = 0.999987), confirming an excellent fit to the BET. There was no microporosity on the t-plot, which confirmed the material was mainly mesoporous. BJH desorption analysis showed a pore diameter of 6.17 nm and pore volume of 0.254 cc·g⁻¹, with the total pore volume found at P/P₀ = 0.99356 was 0.251 cc·g⁻¹. These results demonstrate that NF1 has a moderate surface area and well-defined mesoporosity, making it suitable for catalytic applications. Although 49.82 m²g⁻¹ is modest relative to high-surface-area supports such as mesoporous silica or carbon, it is comparable to, and slightly higher than, values reported for other plant-extract-mediated NiFe₂O₄ nanoparticles, such as the 46.73 m²g⁻¹ obtained for NiFe₂O₄ synthesized using Hydrangea paniculata flower extract [52]. This shows that the catalytic effectiveness found here is due to the high density and accessibility of active surface sites rather than an extremely large surface area, which is consistent with the mesoporous structure revealed by the BJH and t-plot analyse.

The room-temperature magnetization–field (M–H) hysteresis loop of the NiFe₂O₄ nanoparticles, as illustrated in Figure 7, demonstrates a sigmoidal shape indicative of ferrimagnetic ordering, recorded over a magnetic field range of ±2 T. The sample shows a saturation magnetization (M_s) of approximately 1.6 emu/g, with a low coercivity (Hc) of about 100–150 Oe and a small remanence (M_r) of approximately 0.12 memu, resulting in a squareness ratio (Mr/Ms) of roughly 3–4%. The observed reduction in M_s compared to that of bulk NiFe₂O₄, which typically ranges from 50 to 55 emu/g, can be attributed to the surface spin disorder that is characteristic of nanoscale ferrites. Furthermore, the low values of Hc and the M_r/M_s ratio suggest that the material exhibits soft ferrimagnetic properties, approaching near-superparamagnetic behavior, which is consistent with a particle size that is near the single-domain threshold.

In this study, we have developed a green and efficient protocol for the synthesis of NiFe₂O₄ nanocomposites using extract of Combretum indicum leaf, which is cost effective and easy to prepare. The synthesized catalyst was well-characterized by FT-IR, XRD, SEM, TEM, and BET. The catalytic system was highly active in the condensation of aromatic aldehydes with o-phenylenediamine in mild conditions at room temperature, providing high yields and short reaction times. This eco-friendly approach not only simplifies operations and allows for easy product separation but also uses a reusable and sustainable catalyst in line with green chemistry principles. The small, quasi-spherical particle size (12–18 nm) (Section 4.3), together with the moderate BET surface area of 49.82 m²g⁻¹ (Section 4.5), is consistent with a high density of accessible surface coordination sites, which is expected to favor substrate binding and facilitate the condensation/cyclization steps of the benzimidazole-forming reaction. A systematic study of how particle size and shape independently influence surface coordination and catalytic rate (e.g., by comparing catalyst batches annealed at different temperatures to vary crystallite size) was beyond the scope of the present work and is suggested as a direction for future studies.

During the preparation of this manuscript, the authors used ChatGPT (OpenAI), QuillBot, and Claude in order to improve the language, grammar, and readability of the manuscript. After using these tools/services, the authors reviewed and edited the content as needed and take full responsibility for the content of the published article.

The authors are thankful to the Principal, MSS’s Arts, Science and Commerce College, Ambad, Dist. Jalna, Maharashtra, India, for providing laboratory facilities.

Conceptualization, K.T. and S.K.; Methodology, K.T.; Investigation, K.T. and S.K.; Writing—Original Draft Preparation, K.T.; Writing—Review & Editing, K.T. and S.K.; Supervision, S.K. All authors have read and agreed to the published version of the manuscript.

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The data supporting the findings of this study are available within the article.

This research received no external funding.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.