# Ultrasound-assisted synthesis of NiO nanoparticles and their catalytic application for the synthesis of trisubstituted imidazoles under solvent free conditions

Aravind Gajengi, Shivkumar Chaurasia, Eric Monflier, Anne Ponchel, Jérémy Ternel, Bhalchandra Bhanage

► To cite this version:

Aravind Gajengi, Shivkumar Chaurasia, Eric Monflier, Anne Ponchel, Jérémy Ternel, et al. Ultrasound-assisted synthesis of NiO nanoparticles and their catalytic application for the synthesis of trisubstituted imidazoles under solvent free conditions. Catalysis Communications, 2021, 161, pp.106366. 10.1016/j.catcom.2021.106366. hal-03431242

HAL Id: hal-03431242

https://hal.science/hal-03431242

Submitted on 15 Nov 2023

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>
Short communication

# Ultrasound-assisted synthesis of NiO nanoparticles and their catalytic application for the synthesis of trisubstituted imidazoles under solvent free conditions

Aravind L. Gajengi<sup>a</sup>, Shivkumar Chaurasia<sup>a</sup>, Eric Monflier<sup>b</sup>, Anne Ponchel<sup>b</sup>, Jeremy Ternel<sup>b</sup>, Bhalchandra M. Bhanage<sup>a,*</sup>

<sup>a</sup> Department of Chemistry, Institute of Chemical Technology, Matunga, Mumbai 400 019, India

<sup>b</sup> Univ. Artois, CNRS, Centrale Lille, Univ. Lille, UMR 8181, Unité de Catalyse et Chimie du Solide (UCCS), Lens, F-62300, France

## ARTICLE INFO

### Keywords:

NiO nanosheets

Ultrasound

Substituted imidazoles

Multicomponent reactions

Cyclodextrins

## ABSTRACT

The present protocol reports the ultrasound assisted synthesis of NiO nanoparticles (NPs) using benzylamine as a base and different types of cyclodextrins (CDs) as capping agents. The use of α-CD, β-CD or γ-CD leads to different morphologies of NiO NPs. In particular, NiO nanosheets obtained using β-CD as the capping agent. The synthesized NPs were characterized by FEG-SEM, TEM, XRD and EDS analysis. They showed high catalytic activity towards synthesis of different trisubstituted imidazoles under solvent free conditions. In addition, NiO NPs could be recycled and reused consecutively up to four recycle runs without much loss of their catalytic activity.

## 1. Introduction

Metals and metal oxide nanomaterials has played an important role in the different fields because of their size, shape and higher surface to volume ratio [1]. Various methods are available for the synthesis of metal and metal oxide nanomaterials based on the microwave, solvothermal, solar assisted, electro deposition, thermal decomposition based techniques [2–5]. The synthesis and application of NiO nanoparticles (NPs) are in demand for its applications in various fields such as catalysts in the organic synthesis [2], sensors [3], photo catalysts [6] etc.

Multicomponent reactions (MCRs) are the reactions in which two or more molecules react together in a one-pot fashion to give the desired products. MCRs showed high atom economy as most of the atoms of reactant will present in the desired product. They avoid the handling loss of compounds as compared with the convergent methods of the synthesis and the methodology is important for the synthesis of biological active and complex compounds [7]. Synthesis of imidazoles is achieved by one-pot synthesis, which containing nitrogen atom at 1,3 positions of the five member ring. Substituted imidazoles are useful in biological and therapeutic applications such as antimicrobial activity, anticancer, inhibitors such as P38 MAP kinase, B-Raf Kinase, calcium antagonist and inhibitors of thromboxane A2 synthesase anti-retro viral [8–11]. A part from these biological and therapeutic actions, they are

also used in the photographic sensitizers [12]. Some important moieties of imidazoles derivatives such trifenagrel is a potent arachidonate cyclooxygenase inhibitor that reduces platelet aggregation, Lophine which is used chemiluminescent molecules, P38 map inhibitor [13] as shown in the Fig. 1.

The imidazoles and its derivatives can be synthesized by condensation of benzil, aldehyde and ammonium acetate in the presence of suitable catalyst. The different catalysts are used for the synthesis of substituted pyrroles such as ionic liquids [14], supported Fe₃O₄ [15], K₅CoW₁₂O₄₀·3H₂O [16], Zeolite [17], MOFs [18] Ni₀.₅Zn₀.₅Fe₂O₄ [19], Modified SBA-15 [20], MoO₃/SiO₂ [21], ZrO₂-Al₂O₃ [22], NiCl₂/Al₂O₃ [23,26], heteropolyacids [24] nano-SnCl₄·SiO₂ [25] etc. These systems have their own advantages as well as they also suffer from one or more limitations such as difficulty in the preparation of the catalyst, recovery of the catalyst, long reaction times, moisture sensitive catalyst, facing problems during the work up of the reaction etc. Hence, there is a need to develop a greener protocol which can avoid such disadvantages.

In the present protocol, we have utilized the ultrasound for the synthesis of NiO NPs by using benzylamine as a base and various CDs as capping agents. The synthesis protocol is schematically depicted and described in the following experimental section. CDs are a class of naturally occurring receptors, which are cyclic oligosaccharides, constituted of n D-glucopyranose units (n = 6, 7 and 8 for α-, β- and γ-CD

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: bm.bhanage@ictmumbai.edu.in (B.M. Bhanage).</footnote>
respectively). Herein, the purpose is to take advantage of their topologies (rigid structure, large number of hydroxyl groups and ability of forming inclusion complexes with organic molecules [26,27] and supramolecular adducts with inorganic metal salts) [28] in order to intimately interact with the reagents during the ultrasound-assisted stage and control the final structure of NiO NPs after calcination. Recently we have reported that, during the preparation of Al₂O₃-supported Ni catalysts by wet impregnation, the controlled addition of native CD to Ni (NO₃)₂ allowed generating stable supramolecular adducts with the Ni cations, based on the ion-molecule interaction with the OH groups present on the CD rims. Such complexation resulted in beneficial impacts on the final structure of the catalysts in terms of dispersion and reducibility of the surface-active Ni species [29].

In this paper, we report that the use of different capping agents such as α-CD, β-CD and γ-CD leads to different morphologies of NiO NPs. Among them β-CD gave NiO nanosheets which possesses acidic sites [2] and hence showed high catalytic activity for the synthesis of 2,4,5-tris trisubstituted imidazoles.

## 2. Experimental

### 2.1. Chemicals and reagents

All other chemicals were purchased from reputed manufacturers like Sigma Aldrich, Alfa Aesar, SpectrochemPvt. Ltd. India and were used without further purification.

### 2.2. Characterization techniques of NiO NPs

The prepared NiO NPs were characterized using various techniques such as FEG-SEM by Tescan MIRA 3 model. The energy dispersive X-ray spectrum (EDS) was recorded by using INCA x-act Oxford instrument (Model 51-ADD0007), XRD analysis was done by using Shimadzu XRD-6100, BET surface area of NiO analyzed by using Smart Instruments India Pvt. Ltd.

### 2.3. The typical procedure for the synthesis of 2,4,5-trisubstituted imidazoles

The mixture of 1 mmol 1,2-diketone, 1 mmol of aldehyde, 2.5 mmol of ammonium acetate was taken in a reaction vial and 10 mg of NiO NPs was added as a catalyst. The reaction vial was placed in an oil bath and heated at 120 °C. The progress of the reaction was monitored by Thin Layer Chromatography (TLC). After completion of the reaction the solid product was dissolved in 20 mL ethyl acetate and the catalyst was separated by centrifugation followed by washing with 10 mL water and 10 mL ethanol. The product was concentrated and recrystallised from ethanol, which gives the corresponding of 2,4,5-trisubstituted imidazoles.

## 3. Results and discussion

### 3.1. Mechanism of interaction of CD with benzylamine during the formation of NPs

¹H NMR spectroscopic studies were performed to provide information on the interactions between the different components of the system (nickel acetate, benzylamine and β-CD) in aqueous medium (Fig. S1). For comparison, the control spectra of the compounds alone were also shown (Fig. S1(a), S1(b) and S1(c)).

No significant chemical shift variation can be detected for β-CD and Ni acetate when they were mixed together in D₂O (Fig. S1-d), indicating the absence of contact between the inner protons of the cavity (H5 and H3) with those of the acetate group. This behaviour is not the result of an inclusion complex, but is rather a result of a β-CD-Ni adduct where the Ni (II) cation are coordinated to the oxygen atoms of the hydroxyl groups at the β-CD rim, as previously evidenced by ESI-MS and DFT calculations. In contrast, for the mixture of β-CD and benzylamine (Fig. S1(e)), the changes in the aromatic and β-CD proton patterns is indicative of an inclusion complex between these two compounds. Even though the chemical shifts measured on the internal protons of β-CD are fairly small (from 3.78 to 3.74 ppm for H5 and from 3.89 to 3.86 ppm for H3) suggesting relatively low binding affinity, the distortion of patterns is obvious, especially in the 3.0–4.0 ppm region (see the inset in Fig. S1). Accordingly, ITC studies have provided further data arguing that host-guest complexation between benzylamine and CD gives rise to weak association constants, typically in the range of 100 to 200 M⁻¹. However, the ROESY spectrum of the β-CD/benzylamine mixture (1:1) provides definitive proof that the aromatic ring penetrates into the β-CD cavity, as shown by the presence of cross-correlation peaks between the H3 proton of β-CD and the aromatic protons of benzylamine (Fig. S2). Note also that the stoichiometry of the inclusion complex between β-CD and benzylamine has been determined by a Job's plot, revealing a maximum at r = 0.5 along with a symmetrical shape, which unambiguously confirms the formation of a true 1:1 host-guest complex the (Fig. S3). Concentrating our attention now to the control Ni acetate/benzylamine mixture in the absence of β-CD (Fig. S1(f)), the shift of the signal of CH₂ to lower fields (from 3.72 to 4.08 ppm), presumably due to decreased electron density on the methylene group adjacent to the nitrogen atom, is a strong indication of the coordination of the amine on the nickel(II) species. Notably, the coordination properties of benzylamine on nickel acetate are still readily apparent when β-CD is added to this solution (Fig. S1-g). Except for the loss of resolution observed for the proton signals of β-CD, the spectrum obtained in the presence of 1equivalent of β-CD is substantially similar to that obtained without β-CD. Therefore, it can be assumed that, in this concentration range, the transition metal (Ni) has, in addition to its primary sphere of ligand (benzylamine), a second-sphere coordination (β-CD) [29,30].

The significant line-broadening could originate from an increase in the exchange rate between the second-sphere coordination adduct [Ni-benzylamine-β-CD)] and free β-CD, but also from an exchange between

<figure><img src="image_2.png" /><figcaption>**Fig. 1.** Biologically active compounds containing imidazole moiety.</figcaption></figure>
multiple conformations of the nickel-based adduct. Beyond this, it
should be noted that the effects observed between cyclodextrin, Ni ac-
etate and benzylamine in solution are not limited to β-CD but have also
been observed using α-CD (Fig. S4) and γ-CD (Fig. S5). In light of these
results showing that the coordination ability of benzylamine to nickel
cations can be preserved within the CD cavity, it appears interesting to
exploit these supramolecular assemblies as potential templates in the
synthesis of NiO materials assisted by ultrasound.

3.2. Synthesis of NiO NPs

In a 100 mL beaker Ni(OAc)₂ (500 mg), α-CD (100 mg) were dis-
solved in 10 mL of distilled water. Then drop wise addition of 10 mL of
benzylamine gave formation of a light green coloured precipitate. Then
the solution was irradiated under ultrasound using an ultrasonic probe
for 90 pulses at 30 amplitude ((PCI Analytics Pvt. Ltd., India). Green
coloured Ni(OH)₂ was formed and thereafter it was separated by
centrifugation, washed twice with distilled water (2 × 10 mL) followed
by ethanol (1 × 10 mL). The product was dried at 80 °C for 2 h. TheNi
(OH)₂ prepared by using α-CD as a capping was designated as Ni(OH)₂-
α. The latter gave nickel oxide NPs, which was designated as NiO-α, after
calcination at 500 °C for 5 h. The same procedure was followed with
β-CD and γ-CD. Ni(OH)₂-β and Ni(OH)₂-γ gave after calcination nickel
oxide NPs which were designated as NiO-β, NiO-γ respectively (Fig. 2).

3.3. Characterization of NiO NPs

The morphology of NiO-α, NiO-β and NiO-γ was analyzed by using FEG-SEM which confirmed that synthesized NPs are in the nano size. The change in the capping agent has an impact on the changes of morphology of these particles. The use of α-CD and γ-CD as capping agents results in agglomerated nanoparticles as observed in Fig.S6(a) and S6(c), whereas the use of β-CD provides stabilization of NiO particles in the form of nanosheets [Fig.S6(b)]. Notably, the TEM images of NiO-β as shown in the Fig. S7 also reveal that an aerated nano-structure is obtained with NiO domains in the nano range. Such a structure is assumed to lead to enhanced specific surface area. The elemental dispersive spectrum (EDS) of NiO-α, NiO-β and NiO-γ shows the presence of only Ni and O elements (Fig.S8). Note that the presence of the carbon peak is due to the carbon tape on which is sample was placed.

After the ultrasound treatment, all nickel samples are shown to
consist of a Ni(OH)₂ phase with a hexagonal lattice. The X-ray diffrac-
tion patterns of the samples are presented in Fig. S9.

After calcination of Ni(OH)₂, the XRD pattern of NiO-α, NiO-β and NiO-γ gives lattice planes of (111), (200), (220), (311) and (222)

<figure><img src="image_3.png" /><figcaption>Fig. 2. Schematic representation for the synthesis of NiO NPs.</figcaption></figure>

corresponding to the hexagonal lattice of NiO, which is in good agree-
ment with the JCPDS 78-0429 file (Fig.S10) [2]. It shows that average
crystallite size of NPs which was determined according to the Scherrer's
equation, was found to be small (3 nm for α-CD, 3.5 nm for β-CD and 4.5
nm for γ-CD).

3.4. Catalytic activity of NiO NPs for the synthesis of 2,4,5-trisubstituted imidazoles

The NiO NPs were used as catalysts for the synthesis of 2,4,5-trisub-
stituted imidazoles 4a (Scheme 1). Initially, we tested the catalytic ac-
tivity of NiO-α, NiO-β and NiO-γ of 10 mg each for the model reaction of
benzil (1 mmol), benzaldehyde (1 mmol) and ammonium acetate (2.5
mmol) under solvent free condition at 120 °C for one hour. The 61%
yield of the product 4a obtained when NiO-α used (Table 1, entry 1),
97% and 69% yields of product 4a were obtained when we utilized NiO-
β and NiO-γ (Table 1, entries 2–3). The significant higher catalytic ac-
tivity of NiO-β may be due to higher specific surface area. So we choose
NiO-β as a catalyst for further optimization for the synthesis of 4a. When,
the model reaction was carried out using bulk NiO as control catalyst
and without adding catalyst, we got 65% and 13% yield of the product
4a respectively (Table 1, entries 4–5).

The Further, we investigated the effect of the reaction time, tem-
perature, type of solvent and catalyst loading for the synthesis of 4a. The
reaction time has a significant influence on the yield of 4a. When the
reaction was carried for 20 min, only 53% yield of the 4a, was obtained
(Table 1, entry 6) while the further increase in the reaction time up to 50
min provided the increase in the yield of 4a up to 91% (Table 1, entries
7–9). The maximum yield of 4a, was obtained when the reaction was
carried for 60 min (Table 1, entry 2). The increase in the reaction tem-
perature was also found to be effective for the synthesis of 4a. When, the
reaction carried out at 80 °C, a lower yield of 51% of 4a, was observed
(Table 1, entry 10) and the 4a product yield increased when the reaction
temperature rose from 90 °C to 110 °C (Table 1, entries 11–13). The
highest yield of 97% of 4a, was observed when the reaction was carried
out at 120 °C (Table 1, entry 2). The nature of the solvent has also a
marked effect on the final yield of the product. Thus, the use of water,
acetonitrile and ethanol were evaluated as solvents under their reflux
conditions and only gave a yield of 4a of 28%, 55% and 53%, respec-
tively (Table 1, entries 14–16), as compared to the value of 97% ob-
tained under neat conditions (Table 1, entry 2). In case of neat condition
as there is no solvent and other reagents including catalysts are same,
effective catalyst concentration is very high and therefore it gives higher
yield. Finally, we studied the effect of the catalyst loading. When the
reaction was carried out using 5, 10 mg of catalyst, we observed 90%,
97% yield of the product 4a respectively (Fig. 3). A Further increase in
the catalyst loading from 15 to 20 mg leads to a marginal increase in the
yield of the product. So, the 10 mg of the catalyst was the optimum
loading for the synthesis of product 4a.

Hence, the optimized reaction conditions are: catalyst (NiO-β),
catalyst loading: 10 mg, time: 60 min, temperature: 120 °C under sol-
vent free conditions. The above optimized reaction conditions were
readily extended to the one-pot synthesis of other substrates of 2,4,5
trisubstituted imidazoles in a one pot synthesis using different de-
rivatives of aldehydes, and 1,2-diketones in the presence of ammonium
acetate as the starting nitrogen source. The reaction of benzil 1a,
benzaldehyde 2a gave excellent of 2,4,5 trisubstituted imidazole 4a
(Table 3, entry 1).Then, keeping benzil 1a different derivatives of
benzaldehyde were also investigated in this MCR. Thus, with benzal-
dehyde substrate bearing an electron donating groups such as 2- hy-
droxyl 2b, 4-N,N-dimethyl 2c, 4-methoxy 2d, 3,4-dimethoxy 2e gave the
excellent yield of the product 4b-4e (Table 3, entries 2–5). In the same
vein, the presence of electron withdrawing groups on benzaldehyde
such as 4-fluoro 2f, 2-chloro 2 g also gave excellent yields of their cor-
responding products 4f-4 g (Table 2, entry 6–7). Finally, the methyl
derivatives of benzil 1b gave the 85% yield of their corresponding
<figure><img src="image_4.png" /><figcaption>**Scheme 1.** Synthesis of 2,4,5-trisubstituted imidazoles catalysed by NiO NPs.</figcaption></figure>

<table><caption>**Table 1**
Optimisation of reaction for the synthesis of substituted imidazoles.ª</caption><thead><tr><th>Entry</th><th>Catalyst</th><th>Temp.(°C)</th><th>Solvent</th><th>Time (min.)</th><th>Yield [%]<sup>b</sup></th></tr></thead><tbody><tr><td>1</td><td>NiO-α</td><td>120</td><td>-</td><td>60</td><td>61</td></tr><tr><td>2</td><td>NiO-β</td><td>120</td><td>-</td><td>60</td><td>97</td></tr><tr><td>3</td><td>NiO-γ</td><td>120</td><td>-</td><td>60</td><td>69</td></tr><tr><td>4</td><td>Bulk NiO<sup>c</sup></td><td>120</td><td>-</td><td>60</td><td>65</td></tr><tr><td>5</td><td>-</td><td>120</td><td>-</td><td>60</td><td>13</td></tr><tr><td>6</td><td>NiO-β</td><td>120</td><td>-</td><td>20</td><td>53</td></tr><tr><td>7</td><td>NiO-β</td><td>120</td><td>-</td><td>30</td><td>60</td></tr><tr><td>8</td><td>NiO-β</td><td>120</td><td>-</td><td>40</td><td>74</td></tr><tr><td>9</td><td>NiO-β</td><td>120</td><td>-</td><td>50</td><td>91</td></tr><tr><td>10</td><td>NiO-β</td><td>80</td><td>-</td><td>60</td><td>51</td></tr><tr><td>11</td><td>NiO-β</td><td>90</td><td>-</td><td>60</td><td>65</td></tr><tr><td>12</td><td>NiO-β</td><td>100</td><td>-</td><td>60</td><td>72</td></tr><tr><td>13</td><td>NiO-β</td><td>110</td><td>-</td><td>60</td><td>83</td></tr><tr><td>14</td><td>NiO-β</td><td>120</td><td>Water</td><td>60</td><td>28</td></tr><tr><td>15</td><td>NiO-β</td><td>120</td><td>Acetonitrile</td><td>60</td><td>58</td></tr><tr><td>16</td><td>NiO-β</td><td>120</td><td>Ethanol</td><td>60</td><td>53</td></tr></tbody></table>

<figure><img src="image_5.png" /><figcaption>**Fig. 3.** Effect of catalyst loading for the synthesis of 4a (Reaction conditions: time: 60 min., neat, temp: 120 °C).</figcaption></figure>

product 4 h (Table 2, entry 8).

The comparison of catalytic activity of NiO NPs with reported catalytic systems for the synthesis of 2,4,5-trisubstituted imidazoles is shown in Table 3. It can be seen that the reported protocols have certain limitations such as longer reaction time, lower yields of product. The catalyst Yb(OPf)₃, Fe₃O₄@CS gave good yields of the products, but has longer reaction time as compared with reported protocol.

### 3.5. Recyclability study

The present protocol is based on Lewis acid based NiO NPs which are heterogeneous in nature and hence recyclable. These NPs could be were

<table><caption>**Table 2**
Synthesis of 2,4,5-trisubstituted imidazoles catalysed by NiONPs.ª</caption><thead><tr><th>Entry</th><th>1,2-diketone</th><th>Aldehyde</th><th>Products</th><th>Yield [%]<sup>b</sup></th></tr></thead><tbody><tr><td>1</td><td><img src="image_1.png" alt="1a structure"><br>1a</td><td><img src="image_2.png" alt="2a structure"><br>2a</td><td><img src="image_3.png" alt="4a structure"><br>4a</td><td>97</td></tr><tr><td>2</td><td><img src="image_4.png" alt="1a structure"><br>1a</td><td><img src="image_5.png" alt="2b structure"><br>2b</td><td><img src="image_6.png" alt="4b structure"><br>4b</td><td>81</td></tr><tr><td>3</td><td><img src="image_7.png" alt="1a structure"><br>1a</td><td><img src="image_8.png" alt="2c structure"><br>2c</td><td><img src="image_9.png" alt="4c structure"><br>4c</td><td>88</td></tr><tr><td>4</td><td><img src="image_10.png" alt="1a structure"><br>1a</td><td><img src="image_11.png" alt="2d structure"><br>2d</td><td><img src="image_12.png" alt="4d structure"><br>4d</td><td>85</td></tr><tr><td>5</td><td><img src="image_13.png" alt="1a structure"><br>1a</td><td><img src="image_14.png" alt="2e structure"><br>2e</td><td><img src="image_15.png" alt="4e structure"><br>4e</td><td>98</td></tr><tr><td>6</td><td><img src="image_16.png" alt="1a structure"><br>1a</td><td><img src="image_17.png" alt="2f structure"><br>2f</td><td><img src="image_18.png" alt="4f structure"><br>4f</td><td>82</td></tr><tr><td>7</td><td><img src="image_19.png" alt="1a structure"><br>1a</td><td><img src="image_20.png" alt="2g structure"><br>2g</td><td><img src="image_21.png" alt="4g structure"><br>4g</td><td>87</td></tr><tr><td>8</td><td><img src="image_22.png" alt="1b structure"><br>1b</td><td><img src="image_23.png" alt="2a structure"><br>2a</td><td><img src="image_24.png" alt="4h structure"><br>4h</td><td>85</td></tr></tbody></table>

separated from reaction mixture and recycled. After the completion of the reaction, the catalyst was readily separated by ultracentrifugation and the recovered catalyst was thoroughly washed with water-and ethanol- and finally dried in the oven (80 °C, overnight) before reusing it. Thus, the reusability of the β-NiO catalyst was examined and the tests showed that the catalyst was reusable up to four times without much loss of catalytic activity (Fig. S11).

<footnote>ªReaction conditions: Benzil (1.0 mmol), Benzaldehyde (1.0 mmol), NH₄OAc (2.5 mmol), NiO NPs (10 mg), ᵇIsolated Yield, ᶜcommercial, neat.</footnote>

<footnote>ª Reaction conditions: 1,2-diketone (1.0 mmol), aldehyde (1.0 mmol), NH₄OAc (2.5 mmol), NiO NPs (10 mg), temperature (120 °C), time: 60 min.,</footnote>

<footnote>ᵇIsolated Yield.</footnote>
<caption>**Table 3**</caption>

The comparison of catalytic activity of NiO NPs for the synthesis of 2,4,5-trisubstituted imidazoles.

<table><thead><tr><th>Entry</th><th>Catalyst</th><th>Condition</th><th>Time [min.]</th><th>Yield</th><th>Ref.</th></tr></thead><tbody><tr><td>1</td><td>Yb(OPf)<sub>3</sub></td><td>Fluorous solvents, 80 °C</td><td>360</td><td>97</td><td>[31]</td></tr><tr><td>2</td><td>InCl<sub>3</sub>·3H<sub>2</sub>O</td><td>MeOH, RT</td><td>720</td><td>70</td><td>[32]</td></tr><tr><td>3</td><td>Zr(acac)<sub>4</sub></td><td>EtOH, reflux</td><td>150</td><td>90</td><td>[33]</td></tr><tr><td>4</td><td>N-acetyl glycine</td><td>Neat, 150 °C</td><td>120</td><td>78</td><td>[34]</td></tr><tr><td>5</td><td>Fe<sub>3</sub>O<sub>4</sub>@CS</td><td>EtOH, reflux</td><td>120</td><td>95</td><td>[15]</td></tr><tr><td>6</td><td>NiCl<sub>2</sub>·6H<sub>2</sub>O/ Al<sub>2</sub>O<sub>3</sub></td><td>EtOH, reflux</td><td>60</td><td>89</td><td>[23]</td></tr><tr><td>7</td><td>NiO NPs</td><td>Neat, 120 °C</td><td>60</td><td>97</td><td>This work</td></tr></tbody></table>

### 3.6. The catalytic mechanism of NiO NPs for the synthesis of 2,4,5-trisubstituted imidazoles

The proposed reaction mechanism for the synthesis of 2,4,5-trisubstituted imidazoles 4a catalysed by NiO NPs is shown in Fig. S12. NiO NPs coordinate with the carbonyl group of an aldehyde thereby increasing the electrophilicity of this aldehyde, which then allowed the subsequent coupling (or the condensation, the insertion) with two ammonia molecules to form the diamine intermediate A. Then, the intermediate A combines with the benzil to form the intermediate B which undergoes rearrangement to form 2,4,5-trisubstituted imidazoles 4a [35].

### 3.7. Characterization data of products

2,4,5-triphenyl-1H-imidazole 4a (Table 2, entry 1).

¹H NMR (400 MHz, DMSO-d₆, ppm) δ 12.65 (s, 1H), 8.05 (d, J = 7.3 Hz, 2H), 7.51 (d, J = 7.5 Hz, 2H), 7.43 (dt, J = 11.6, 7.5 Hz, 6H), 7.34 (t, J = 7.3 Hz, 2H), 7.27 (t, J = 7.4 Hz, 2H), 7.19 (t, J = 7.1 Hz, 1H).

2-(4,5-diphenyl-1H-imidazol-2-yl)phenol 4b (Table 2, entry 2).

¹H NMR (400 MHz, DMSO-d₆, ppm) δ 13.00 (s, 1H), 12.91 (s, 1H), 8.00 (d, J = 7.8 Hz, 1H), 7.59–7.08 (m, 10H), 6.92 (dd, J = 16.5, 8.1 Hz, 2H).

4-(4,5-diphenyl-1H-imidazol-2-yl)-N,N-dimethylaniline 4c (Table 2, entry 3).

¹H NMR (400 MHz, DMSO-d₆, ppm) δ 12.32 (s, 1H), 7.91 (d, J = 8.6 Hz, 2H), 7.53–7.17 (m, 10H), 6.79 (d, J = 8.8 Hz, 2H), 2.96 (s, 6H).

2-(4-methoxyphenyl)-4,5-diphenyl-1H-imidazole 4d (Table 2, entry 4).

¹H NMR (400 MHz, DMSO-d₆, ppm) δ 12.48 (s, 1H), 7.98 (d, J = 8.8 Hz, 2H), 7.49 (s, 2H), 7.47 (s, 2H), 7.29 (d, J = 26.8 Hz, 6H), 7.01 (d, J = 8.8 Hz, 2H), 3.78 (s, 3H).

2-(3,4-dimethoxyphenyl)-4,5-diphenyl-1H-imidazole 4e (Table 2, entry 5).

¹H NMR (400 MHz, DMSO-d₆, ppm) δ 12.54 (s, 1H), 7.66 (d, J = 7.3 Hz, 2H), 7.52 (dd, J = 16.1, 7.1 Hz, 4H), 7.46–7.20 (m, 6H), 7.06 (d, J = 8.5 Hz, 1H), 3.85 (s, 3H), 3.80 (s, 3H).

2-(4-fluorophenyl)-4,5-diphenyl-1H-imidazole 4f (Table 2, entry 6).

1H NMR (400 MHz, DMSO-d₆, ppm) δ 12.66 (s, 1H), 8.17–7.11 (m, 14H).

2-(2-chlorophenyl)-4,5-diphenyl-1H-imidazole 4 g (Table 2, entry 7).

¹H NMR (400 MHz, DMSO-d₆, ppm) δ 12.64 (s, 1H), 7.79 (dd, J = 5.9, 3.4 Hz, 1H), 7.62–7.58 (m, 1H), 7.54 (d, J = 8.1 Hz, 2H), 7.50–7.41 (m, 6H), 7.37–7.29 (m, 3H), 7.22 (t, J = 7.4, 1H).

2-phenyl-4,5-di-p-tolyl-1H-imidazole 4 h (Table 2, entry 8).

¹H NMR (400 MHz, DMSO-d₆, ppm) δ 12.58 (s, 1H), 8.07 (d, J = 7.4 Hz, 2H), 7.50–7.42 (m, 4H), 7.37 (t, J = 9.3 Hz, 3H), 7.24 (d, J = 7.9 Hz, 2H), 7.10 (d, J = 7.9 Hz, 2H), 2.34 (s, 3H), 2.24 (d, J = 25.3 Hz, 3H).

## 4. Conclusion

The present protocol reports ultrasound assisted synthesis of the NiO NPs using Ni(OAc)₂ as a precursor, benzylamine as a base and cyclodextrins as a capping agent. The nature of capping agent such as α-CD, β-CD or γ-CD leads to different morphology of NiO NPs. The NiO nanosheets were formed with β-CD while the agglomerated NiO NPs were observed with α-CD and γ-CD. The catalytic activity of NiO NPs was evaluated for the synthesis of 2,4,5-trisubstituted imidazoles. The excellent yields of the products were obtained for different types of trisubstituted imidazoles. Also, the NiO NPs shows the excellent recyclability up to four recycles without significant loss of its catalytic activity.

## Author contributions

Aravind L. Gajengi: Experiments, Data Analysis, Writing Manuscript.

Shivkumar Chaurasia: Data Analysis.

Eric Monflier: Experiments, Writing, Cyclodextrin preparation and Characterization.

Anne Ponchel: Experiments, Writing, Cyclodextrin preparation and Characterization.

Jeremy Ternel: NMR Analysis.

Bhalchandra M. Bhanage: Manuscript Writing, Experiment Design, Funding.

All authors commented on and amended both the manuscripts. All Authors discussed and contributed to the work.

## Declaration of Competing Interest

None.

## Acknowledgement

The author Aravind L. Gajengi acknowledges to the University Grants Commission (UGC), for providing the Senior Research Fellowship (SRF). The authors are grateful to Prof. David Landy [UCEIV, University of the Littoral Opal Coast (ULCO)] for the determination of association constants by ITC measurements.

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.catcom.2021.106366.

## References

[1] B. Liu, H.C. Zeng, Mesoscale organization of CuO nanoribbons: formation of “dandelions” J, Am. Chem. Soc. 126 (2004) 8124–8125, https://doi.org/10.1021/ja048195o.

[2] A.L. Gajengi, T. Sasaki, B.M. Bhanage, 6. NiO nanoparticles catalyzed three component coupling reaction of aldehyde, amine and terminal alkynes, Catal. Commun. 72 (2015) 174–179, https://doi.org/10.1016/j.catcom.2015.09.031.

[3] C. Liu, L. Zhao, B. Wang, P. Sun, Q. Wang, Y. Gao, X. Liang, T. Zhang, Lu G, acetone gas sensor based on NiO/ZnO hollow spheres: fast response and recovery, and low (ppb) detection limit, J. Colloid Interface Sci. 495 (2017) 207–215, https://doi.org/10.1016/j.jcis.2017.01.106.

[4] A.B. Patil, B.M. Bhanage, Solar energy assisted synthesis of palladium nanoplates and its application in 2-phenoxy-1,1'-biphenyls and N,N-dimethyl-[1,1'-biphenyl] derivatives synthesis, J. Mol. Catal. A Chem. 379 (2013) 30–37, https://doi.org/10.1016/j.molcata.2013.07.012.

[5] Y. Zhang, L.Z. Xie, C.X. Yuan, C.L. Zhang, S. Liu, Y.Q. Peng, H.R. Zhang, LM, Preparation of 3D rose-like nickel oxide nanoparticles by electrodeposition method and application in gas sensors, J. Mater. Sci. Mater. Electron. 27 (2016) 1817–1827, https://doi.org/10.1007/s10854-015-3959-2.

[6] Z. Abolfazl, B. Alireza, M.Z. Ghodsi, E. Hamed, Simultaneous photocatalytic and catalytic activity of p-n junction NiO@anatase/rutile-TiO₂ as a noble-metal free reusable nanoparticle for synthesis of organic compounds, Catal. Commun. 95 (2017) 77–82, https://doi.org/10.1016/j.catcom.2017.02.023.

[7] S. Samai, G.H. Nandi, P. Singh, M.S. Singh, I-Proline: an efficient catalyst for the one-pot synthesis of 2,4,5-trisubstituted and 1,2,4,5-tetrasubstituted imidazoles, Tetrahedron 65 (2009) 10155–10161, https://doi.org/10.1016/j.tet.2009.10.019.
[8] S.E. Wolkenberg, D.D. Wisnoski, W.H. Leister, Y. Wang, Z. Zhao, C.W. Lindsley, Efficient synthesis of imidazoles from aldehydes and 1,2-diketones using microwave irradiation, Org. Lett. 6 (2004) 1453–1456, https://doi.org/10.1021/ol049682b.

[9] G. Haberhauer, F. Rominger, Syntheses and structures of imidazole analogues of lissoclinum cyclopeptides, Eur. J. Org. Chem. (2003) 3209–3218, https://doi.org/10.1002/ejoc.200300207.

[10] J.C. Lee, J.T. Laydon, P.C. McDonnell, T.F. Gallagher, S. Kumar, D. Green, D. McNulty, A protein kinase involved in the regulation of inflammatory cytokine biosynthesis, Nature 739 (1994) 739–746, https://doi.org/10.1038/372739a0.

[11] A.K. Takle, M.J.B. Brown, S. Davies, D.K. Dean, G. Francis, A. Gaiba, A.W. Hird, F. D. King, P.J. Lovell, A. Naylor, A.D. Reith, J. Steadman, D.M. Wilson, The identification of potent and selective imidazole-based inhibitors of B-Raf kinase, Bioorg. Med. Chem. Lett. 16 (2006) 378–381, https://doi.org/10.1016/j.bmcl.2005.09.072.

[12] A. Keivanloo, M. Bakherad, E. Imanifar, M. Mirzaee, Boehmite nanoparticles, an efficient green catalyst for the multi-component synthesis of highly substituted imidazoles, Appl. Catal. A. 467 (2013) 291–300, https://doi.org/10.1016/j.apcata.2013.07.027.

[13] M.J. Climent, A. Corma, S. Iborra, Homogeneous and heterogeneous catalysts for multicomponent reactions, RSC Adv. 2 (2012) 16–58, https://doi.org/10.1039/C1RA00807B.

[14] M.A. Zolfigol, A. Khazaei, A.R. Moosavi-Zare, A. Zare, Z. Asgari, V. Khakyzadeh, A. Hasaninejad, Design of ionic liquid 1,3-disulfonic acid imidazolium hydrogen sulfate as a dual-catalyst for the one-pot multi-component synthesis of 1,2,4,5-tetrasubstituted imidazoles, J. Ind. Eng. Chem. 19 (2013) 721–726, https://doi.org/10.1016/j.jiec.2012.10.014.

[15] Z. Zarnegar, J. Safari, Fe₃O₄@chitosan nanoparticles: a valuable heterogeneous nanocatalyst for the synthesis of 2,4,5-trisubstituted imidazoles, RSC Adv. 4 (2014) 20932–20939, https://doi.org/10.1039/C4RA03176H.

[16] L. Nagarapu, S. Apuri, S. Kantevari, Potassium dodecatugstocobaltate trihydrate (K5CoW12040·3H2O): a mild and efficient reusable catalyst for the one-pot synthesis of 1,2,4,5-tetrasubstituted imidazoles under conventional heating and microwave irradiation, J. Mol. Catal. A. Chem. 266 (2007) 104–108, https://doi.org/10.1016/j.molcata.2006.10.056.

[17] S. Balalaei, A. Arabianian, One-pot synthesis of tetrasubstituted imidazoles catalyzed by zeolite HY and silica gel under microwave irradiation, Green Chem. 2 (2000) 274–276, https://doi.org/10.1039/B0062010.

[18] H. Ramezanalizadeh, F. Manteghi, Mixed cobalt/nickel metal-organic framework, an efficient catalyst for one-pot synthesis of substituted imidazoles, Monatsh. Chem. 148 (2017) 347–355, https://doi.org/10.1007/s00706-016-1776-9.

[19] A. Khazaei, H.A.A. Nik, A. Ranjbaran, A.R.M. Zare, Synthesis, characterization and application of Ni0.5Zn0.5Fe2O4 nanoparticles for the one pot synthesis of triaryl-1H-imidazoles, RSC Adv. 6 (2016) 78881–78886. https://doi.org/10.1039/C6RA05158H.

[20] M. Hajjami, F. Ghorbani, Z. Yousofv, Copper(I) complex of 1,3-DimethylBarbituric acid modified SBA-15 and its catalytic role for the synthesis of 2,3-Dihydroquinazolin-4(1H)-ones and Imidazoles, Appl. Organomet. Chem. 9 (2017) 1–9, https://doi.org/10.1002/aoc.3843.

[21] S.V. Bhosale, M.B. Kalyankar, S.V. Nalage, D.S. Bhosale, S.L. Pandhare, T. V. Kotbagi, S.B. Umbarkar, M.K. Dongare, One-pot synthesis of 2,4,5-tri substituted

imidazoles using MoO₃/SiO₂, an efficient and recyclable catalyst, Synth. Commun 41 (2011) 762–769, https://doi.org/10.1080/0039791100364415.

[22] N. Thimmaraju, S.Z.M. Shamshuddin, Synthesis of 2,4,5-trisubstituted imidazoles, quinoxalines and 1,5-benzodiazepines over an eco-friendly and highly efficient ZrO₂–Al₂O₃ catalyst, RSC Adv. 6 (2016) 60231–60243, https://doi.org/10.1039/C6RA13956F.

[23] M.M. Heravi, K. Bakhtiari, H.A. Oskooie, S. Taheri, J. Mol. Catal. A: Chem. Synthesis of 2,4,5-triaryl-imidazoles catalyzed by NiCl₂·6H₂O under heterogeneous system 263, 2007, pp. 279–281, https://doi.org/10.1016/j.molcata.2006.08.070.

[24] M.M. Heravi, F. Derikvand, F.F. Bamoharram, Highly efficient, four-component one-pot synthesis of tetrasubstituted imidazoles using Keggin-type heteropolyacids as green and reusable catalysts, J. Mol. Catal. A. Chem. 263 (2007) 112–114, https://doi.org/10.1016/j.molcata.2006.08.048.

[25] B.F. Mirjalili, A. Bamoniri, M.A. Mirhoseini, Nano-SnCl₄·SiO₂: an efficient catalyst for one-pot synthesis of 2,4,5-tri substituted imidazoles under solvent-free conditions, Sci. Iran. 20 (2013) 587–591, https://doi.org/10.1016/j.sciient.2013.02.008.

[26] J. Szejtli, Introduction and general overview of cyclodextrin chemistry, Chem. Rev. 98 (1998) 1743–1754, https://doi.org/10.1021/cr970022c.

[27] R. Breslow, S.D. Dong, Biomimetic reactions catalyzed by cyclodextrins and their derivatives, Chem. Rev. 98 (1998) 1997–2012, https://doi.org/10.1021/cr970011j.

[28] E. Norkus, Metal ion complexes with native Cyclodextrins, J. Incl. Phenom. Macrocycl. Chem. 65 (2009) 237–248, https://doi.org/10.1007/s10847-009-9586-x.

[29] A. Tomer, F. Wyrwalski, C. Przybylski, J.F. Paul, E. Monflier, M. Pera-Titus, A. Ponchel, Facile preparation of Ni/Al2O3 catalytic formulations with the aid of cyclodextrin complexes: Towards highly active and robust catalysts for the direct amination of alcohols, J.Catal 356 (2017) 111–124, https://doi.org/10.1016/j.jcat.2017.10.006.

[30] F. Hapiot, S. Tilloy, E. Monflier, Cyclodextrins as supramolecular hosts for organometallic complexes, Chem. Rev. 106 (2006) 767–781, https://doi.org/10.1021/cr050576c.

[31] M.G. Shen, C. Cai, W.B. Yi, Ytterbium perfluorooctanesulfonate as an efficient and recoverable catalyst for the synthesis of trisubstituted imidazoles, J. Fluor. Chem. 129 (2008) 541–544, https://doi.org/10.1016/j.jfluchem.2008.03.009.

[32] S.D. Sharma, P. Hazarika, D. Konwar, An efficient and one-pot synthesis of 2,4,5-tri substituted and 1,2,4,5-tetra substituted Imidazoles catalysed by InCl₃·3H₂O, Tetrahedron Lett. 49 (2008) 2216–2220, https://doi.org/10.1016/j.tetlet.2008.02.053.

[33] A.R. Khosropour, Ultrasound-promoted greener synthesis of 2,4,5-trisubstituted imidazoles catalyzed by Zr(acac)(4) under ambient conditions, Ultrason. Sonochem. 15 (2008) 659–664, https://doi.org/10.1016/j.ultsonch.2007.12.005.

[34] M. Yar, M. Bajda, S. Shahzad, N. Ullah, M.A. Gilani, M. Ashraf, A. Rauf, A. Shaukat, Organocatalyzed solvent free an efficient novel synthesis of 2,4,5-trisubstituted imidazoles for α-glucosidase inhibition to treat diabetes, Bioorg. Chem. 58 (2015) 65–71, https://doi.org/10.1016/j.bioorg.2014.11.006.

[35] E. Kolvari, S. Zolfagharinia, A waste to wealth approach through utilization of nano-ceramic tile waste as an accessible and inexpensive solid support to produce a heterogeneous solid acid nanocatalyst: to kill three birds with one stone, RSC Adv. 6 (2016) 93963–93974, https://doi.org/10.1039/C6RA11923A.