In the last decade, magnetic nanoparticles (MNPs) have been noted by many researchers owing to their advantages of high stability, magnetic and electrical properties, high surface area, and special optical properties (Ichiyanagi et al., 2004; Zhang et al., 2006; Bisht and Rajeev, 2011; Ghasemzadeh et al., 2017; Ramamoorthy and Rajendran, 2017; Yin et al., 2017; Vennela et al., 2019). Among different magnetic nanoparticles, cobalt oxide (Co₃O₄) NPs are very interesting due to their high chemical stability, good reactivity, high surface area, excellent semiconductivity, easy synthesis, high catalytic performance, and superior magnetic properties. These nanoparticles have many applications, such as in energy storage, sensors, anodic supplies, drug delivery, and catalysis (Kumar et al., 2008; Koza et al., 2012; Wang et al., 2015; Agilandeswari and Rubankumar, 2016; Ivetić et al., 2016; Pourzare et al., 2017; Galini et al., 2018; Shi et al., 2019; Tahanpesar et al., 2019; Mohammadi et al., 2020; Tonelli et al., 2020; Al-Qasmi, 2022; Anele et al., 2022; Bilge et al., 2022; Farrag and Ali, 2022; Mohammadpour-Haratbar et al., 2022; Saeed et al., 2022). The chemical formula of these NPs is Co²⁺(Co³⁺)₂O₄ with a normal spinel structure in which Co²⁺ ions are in tetrahedral interstices and Co³⁺ ions are in the octahedral interstices of the cubic close-packed lattice of oxide anions (Salavati-Niasari et al., 2009; Thota et al., 2009). It is believed that the shape and size of NPs affect their properties; thus, the morphology of these nanoparticles must be controlled. Co₃O₄ NPs have been synthesized in various morphologies such as nanorods (Lou et al., 2007; Meng et al., 2015), nanotubes (Chen et al., 2015; Wang et al., 2019), nanowires (Mahmoud and Al-Agel, 2011; Yao et al., 2012), cubic (Ghiasi et al., 2016; Liu et al., 2020), spheres (Chen et al., 2007; Wang et al., 2017), and mesoporous (Qiu et al., 2014; Zhao et al., 2019). Since Co₃O₄ NPs and their nanocomposites are resistant to corrosion, they have been used as powerful catalysts in different chemical reactions. Some recent reports in this matter are Co₃O₄–SiO₂ nanocomposites for CO oxidation (Jia et al., 2011), Co₃O₄@SiO₂ NPs for the preparation of 3,4-dihydroquinoxalins (Ghasemzadeh et al., 2016), Co₃O₄ for the reduction of 4-nitrophenols (Al Nafiey et al., 2017), Co₃O₄@SiO₂ for the preparation of indazoles (Ghasemzadeh et al., 2017), Co₃O₄ NPs as a photocatalyst for methylene blue degradation (Vennela et al., 2019), and Co₃O₄@SiO₂ core/shell–nylon as an adsorbent for the removal of Congo red from wastewater (Mohammadi et al., 2020). There are also several methods for the synthesis of these NPs, including sol–gel, chemical pyrolysis, microemulsion, chemical vapor deposition (CVD), coprecipitation, microwave, decomposition of organic precursors, and hydrothermal methods (He et al., 2004; Yoshikawa et al., 2004; Ştefănescu et al., 2008; Ozkaya et al., 2009; Gupta et al., 2011; Alagiri et al., 2013; Salavati-Niasari and Khansari, 2014; Gopinath et al., 2016; Diallo et al., 2017; Izu et al., 2017; Jamil et al., 2018; Tan et al., 2018; Yetim, 2021). However, some of the aforementioned methods face problems of high reaction temperature, high pressure, and the use of expensive precursors. Therefore, designing a novel method to overcome the aforementioned limitations is an important subject in this matter.

On the other hand, multicomponent reactions are one of the very important chemical processes because of their key role in the synthesis of biologically active heterocyclic organic compounds (Fotouhi et al., 2007; Altass et al., 2021; Alshorifi et al., 2022; Altass et al., 2022; El-Yazeed et al., 2022). Tetrahydrobenzo[b]pyrans are one of these compounds that are synthesized via a three-component reaction. They have good biological activity and are used in the pharmaceutical field, cosmetics, agriculture, pigment, etc. (Hekmatshoar et al., 2008). To date, many catalysts have been reported for the preparation of tetrahydrobenzo[b]pyrans. Some of the recently developed catalysts are choline hydroxide-based ionic liquid [Ch][OH] (Hu et al., 2014), nano-structured diphosphate (Na₂CaP₂O_7, DIPH) (Maleki et al., 2016), dihydrogen phosphate-supported silica-coated magnetic nanoparticles (H₂PO₄–SCMNPs) (Saadati-Moshtaghin and Zonoz, 2017), Fe₃O₄@Ph–SO₃H (Elhamifar et al., 2018), Preyssler heteropoly acid on Ni0.5Zn0.5Fe₂O₄ magnetic nanoparticles (MNPs) (Javid and Moeinpour, 2018), nickel Schiff base complex immobilized on silica-coated Fe₃O₄ (Fe₃O₄@SiO₂@NiSB) (Maleki et al., 2020), and Eu/IDA/CPTS@CoFe₂O₄ (Tamoradi et al., 2020).

In view of the aforementioned characteristics, herein, for the first time, a novel surfactant-assisted method is presented for the preparation of magnetic cobalt oxide nanoparticles. In this method, NaBH₄ has been used as a reducing agent and the pluronic P123 surfactant has been employed as a stabilizer. Moreover, the reaction was performed in EtOH at room temperature. The Co₃O₄ NPs were characterized and employed as a powerful nanocatalyst in the synthesis of tetrahydrobenzo[b]pyrans.

The water- and air-stable magnetic Co₃O₄ nanoparticles were easily and rapidly prepared at RT through a novel reduction method in the presence of pluronic P123 as a stabilizer and NaBH₄ as a reducing agent (Scheme 1).

The Co₃O₄ NPs were characterized using FT-IR, PXRD, VSM, EDX, XPS, and SEM techniques. In the FT-IR spectrum (Figure 1) of Co₃O₄ NPs, the stretching vibration of the Co–O bond was observed at 619 cm⁻¹. The absorption peaks that appeared at 1,636 and 3,416 cm⁻¹ are related to bending and stretching vibrations of OH, respectively. Furthermore, the band at 1,412 cm⁻¹ is for the B–O bond, resulting from the hydrolysis of the borohydride ion (Medina et al., 2019).

The PXRD analysis shows nine peaks with low intensity at 2θ = 19, 32, 35, 37, 45, 56, 60, 65°, and 72° corresponding to the spinel crystalline structure of magnetic Co₃O₄ NPs, which is in agreement with the previous literature (Figure 2) (He et al., 2004; Merino et al., 2012; Mujtaba et al., 2016; Hu et al., 2018).

The XPS analysis of cobalt oxide nanoparticles clearly shows the presence of Co and O elements (Figure 3). The most significant peaks related to O 1s, Co 2p_3/2, and Co 2p_1/2 appeared at 529.1, 780.1, and 795.8 eV, respectively. The energy difference between the Co 2p_3/2 and Co 2p_1/2 splitting is 15.7 eV, indicating the presence of Co²⁺ and Co³⁺ in the prepared material. These results are in good agreement with the previous reports and confirm the Co₃O₄ structure for the prepared material (Nwanya et al., 2017; Qiu et al., 2017; Zhang et al., 2017).

The morphology of the catalyst was studied by SEM. This analysis showed sponge-like spherical particles for the designed material (Figure 4). According to the SEM images, the average size of the designed Co₃O₄ NPs was about 36 nm.

In the next study, the magnetic properties of Co₃O₄ NPs were investigated using a vibrating sample magnetometer (VSM). Importantly, herein, two other methods were used to prepare magnetic Co₃O₄ NPs, and the magnetic properties of the resulting products were compared with those of our method (Figure 5). These methods were a) the milling process (Medina et al., 2019), b) EtOH/NaBH₄ without a stabilizer (our test), and c) ETOH/NaBH₄ in the presence of pluronic P123 as a stabilizer agent (our method). The VSM analysis showed the products with a magnetization of 28, 33, and 56 for the a, b, and c methods, respectively (Figure 5). This confirms the very good efficiency of our novel procedure to prepare Co₃O₄ NPs with high magnetic properties. This result is attributed to the key role of pluronic P123 as a surfactant in the stabilization of magnetic NPs during their preparation. It is also important to note that the prepared black Co₃O₄ NPs were easily separated using an external magnet (Figure 5, inset figure).

The EDX analysis was used to distinguish the elements in the structure of the Co₃O₄ material. This analysis showed the presence of O and Co in the Co₃O₄ structure (Figure 6). Moreover, the EDX mapping analysis also showed the uniform distribution of these elements in the material network (Figure 7).

After successful characterization, the Co₃O₄ NPs were used as an efficient catalyst in the synthesis of tetrahydrobenzo[b]pyrans. For this, the reaction between dimedone, benzaldehyde, and malononitrile was selected as a test model, and the effect of different parameters was studied (Table 1). In the absence of a catalyst, no product was found (Table 1, entry 1), while by adding the catalyst, the product yield was increased, and the best result was obtained using 0.015 g of Co₃O₄ NPs (Table 1, entries 2–4). Screening different solvents showed that a low yield in toluene and acetonitrile, a moderate yield in EtOH and THF, and the highest yield in water were obtained (Table 1, entries 3, 5–8). Increasing temperature from RT to 35°C and 50°C showed no significant change in the product yield (Table 1, entries 3, 9, 10). According to these results, 0.015 g of Co₃O₄, water solvent, and RT were chosen as optimum conditions (Table 1, entry 3).

With the optimal conditions in hand, the catalytic activity of Co₃O₄ was investigated for different aldehyde substrates to produce the tetrahydrobenzo[b]pyran derivatives. As shown, benzaldehyde (Table 2, entry 1), electron-donating containing aldehydes (Table 2, entries 2–4), and electron-withdrawing bearing aldehydes (Table 2, entry 5) are converted to the corresponding products in high yields at short times. Halogen-substituted aldehydes also delivered a high yield of the corresponding adducts (Table 2, entries 6, 7). In addition, the reaction between 4-nitrobenzaldehyde, malononitrile, and coumarin in the presence of the Co₃O₄ nanocatalyst also gave a high yield of the corresponding product (Table 2, entry 8). These results confirm the high activity and efficiency of the Co₃O₄ catalyst for the preparation of a wide range of biologically active tetrahydrobenzo[b]pyrans.

The recoverability and reusability of the designed catalyst were investigated in the reaction model of benzaldehyde, dimedone, and malononitrile under optimal conditions. It was found that the Co₃O₄ nanocatalyst can be recovered and reused six times with no noticeable decrease in its activity (Figure 8).

Finally, the catalytic activity of the designed Co₃O₄ nanoparticles was compared with former heterogeneous catalysts in the synthesis of tetrahydrobenzo[b]pyrans (Table 3). It was found that the present catalyst has a high ability to synthesize desired products with higher efficiency and more recovery times than previous catalysts. These findings are attributed to the high surface area, excellent magnetic properties, and good stability of Co₃O₄ MNPs.

In this study, a novel and interesting method for the preparation of magnetic cobalt oxide (Co₃O₄) NPs was developed. These NPs were synthesized under green conditions at RT in a short time. The FT-IR, PXRD, SEM, VSM, XPS, and EDX analyses confirmed the good preparation, high stability, and good magnetic properties of Co₃O₄ NPs. These magnetic NPs were used as a powerful and efficient nanocatalyst for the synthesis of tetrahydrobenzo[b]pyrans. The desired products were obtained in water as an environmentally friendly solvent in a short time. The Co₃O₄ nanocatalyst was separated easily using an external magnet and recovered at least six times with no significant decrease in its activity. Due to the simplicity and eco-friendliness of the designed method, the preparation of other magnetic metal oxide NPs using this strategy is underway in our laboratory.