Journal of Taibah University for Science 9 (2015) 570–578

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ScienceDirect

ZnO nanorods: Efficient and reusable catalysts for the synthesis ofsubstituted imidazoles in water

Kobra Nikoofar ∗, Maryam Haghighi ∗, Maryam Lashanizadegan, Zeinab AhmadvandDepartment of Chemistry, Faculty of Science, Alzahra University, Vanak, PO Box 1993893973, Tehran, Iran

Available online 28 January 2015

Abstract

A simple procedure has been developed for the synthesis of ZnO nanorods (ZnO NRs). The newly synthesized crystals werecharacterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR) and scanning electron microscopy(SEM). Moreover, the catalytic activity was examined in the one-pot, three-component synthesis of 2,4,5-triaryl-1H-imidazolesin water under reflux conditions. The results confirmed ZnO NRs to be mild, benign and effective catalysts for the preparation

of tri-substituted imidazoles in high yield via an operationally simple and environmentally friendly procedure. Furthermore, thecatalyst can be conveniently recovered and reused for at least three runs without any activity loss.© 2015 Taibah University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/).

yst

Keywords: Tri-substituted imidazoles; ZnO nanorods; Reusable catal

1. Introduction

Substituted imidazoles exhibit a wide-range ofbiological activities, including anti-inflammatory [1],fungicidal [2] and anti-tumour [3]. Some derivatives ofmulti-substituted imidazoles inhibit p38 MAP kinase[4], interleukin-1 (IL-1) biosynthesis [5] and selectiveglucagon receptor antagonists [6]. Recent investigationsin green and organometallic chemistry have extendedthe application of imidazoles as ionic liquids [7] and N-heterocyclic carbenes [8,9]. Some marketed medicines,

∗ Corresponding authors. Tel.: +98 2188041344.E-mail addresses: knikoofar@yahoo.com (K. Nikoofar),

mary haghighi@yahoo.com (M. Haghighi).Peer review under responsibility of Taibah University.

http://dx.doi.org/10.1016/j.jtusci.2014.12.0071658-3655 © 2015 Taibah University. Production and hosting by Elsevier B.V(http://creativecommons.org/licenses/by-nc-nd/4.0/).

such as omeprazole [10], consist of modified imidazolecores in their structures. In addition to being biologi-cally active, the imidazole core acts as a main part insome advanced materials, such as fluorescence labellingagents [11], biological imaging agents [12], and chro-mophores for non-linear optic systems [13]. Due to theirvast range of pharmacological, biological and industrialactivities, the development of protocols or the synthesisof imidazoles is an interesting target for chemists.

A number of methods have been developed for thesynthesis of 2,4,5-triarylimidazoles. The reported proce-dures involve the multi-component cyclo-condensationof 1,2-diketones (benzoins or benzils) using an alde-hyde or an ammonium salt as the nitrogen source (inmost cases ammonium acetate) in the presence of astrong protic acid (such as H3PO4 [14] and H2SO4 [15]),

. This is an open access article under the CC BY-NC-ND license

ionic liquids (such as 1-ethyl-3-methylimidazole acetate([EMIM]OAc) [16] and diethyl ammonium hydrogenphosphate ([(Et2)NH2]H2PO4) [17]), ytterbium tri-fluoromethanesulfonate (Yb(OTf)3) [18], NiCl2·6H2O

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K. Nikoofar et al. / Journal of Taiba

upported onto acidic alumina [19], silica sulphuric acidSSA) [20], ceric (IV) ammonium nitrate (CAN) [21], l-ysteine [22], BF3/SiO2 [23], l-proline [24], nano-silicahosphoric acid (nano-SPA) [25], N-bromosuccinimideNBS) [26], silica-supported SbCl3 [27] and nanocrys-alline MgAlO4 [28].

However, despite their potential utility, some of theeported methods suffer from the use of relatively expen-ive moisture-sensitive reagents, high temperatures, longeaction times, tedious work-up procedures using toxiceagents or solvents, harsh reaction conditions, and largeatalytic loadings, leading to the generation of a largemount of waste. Moreover, the synthesis of these N-earing heterocyclic compounds has been carried out inolar solvents, such as methanol, acetic acid, DMF andMSO, which leads to complex isolation procedures.ence, the development of clean, high-yield, and envi-

onmentally benign approaches for imidazole synthesiss still desirable and in demand.

Recently, organic syntheses involving multi-omponent reactions (MCRs) have receivedonsiderable attention. In this strategy, the targetroducts are often obtained in a one-pot process inhich three or more starting materials with different

unctionalities are incorporated together with high atomconomy and high selectivity. This single-step idealoute minimizes the tedious work-up procedures andnvironmentally hazardous wastes. Many complex andmportant organic molecules have been prepared fromhis simple and powerful protocol [29].

Different nano-metal oxides have been of greatnterest in recent years due to their catalytic, ther-

al, mechanical, optical and biomedical applications30–32]. Various ZnO nanostructures, such as nanopar-icles, nanorods, nanowires, nanobelts, nanotubes,anobridges and nanonails, nanowalls, nanohelixes andolyhedral cages, have been synthesized and character-zed in recent years [33–36]. Nano-zinc oxides showedignificant applications in various areas, such as opto-lectronics [36,37], ferromagnetism [38], piezoelectricransducers [39], surface acoustic wave devices [40] andas sensors [41]. They also showed antibacterial [42] andntioxidant properties [43].

It is noteworthy that among one-dimensional nano-tructures, ZnO nanorods and nanowires have beenidely studied due to their ease of formation andevice applications [44]. These forms exhibit uniquend adjustable properties due to their large sur-

ace area-to-volume ratios. Several methods, includingemplate-assisted sol–gel [45], laser-induced [46],ydrothermal [47], solvothermal [48], precipitation [49]nd chemical vapour deposition (CVD) [50] techniques,

rsity for Science 9 (2015) 570–578 571

have been developed for the production ZnO nanorodsand nanowires.

The growth conditions affect the characteristics andproperties of the ZnO NRs. For this reason, variousstudies have reported the controlled growth of ZnOnanostructures to promote their widespread applications.Recently, ZnO nanorods were prepared in the presenceof different capping agents or surfactants, which wereused to control the particle size and morphology [42].However, although various methods have been proposedfor the synthesis of ZnO nanostructures, most of thesetechniques are expensive and complicated and requireadvanced instruments or high temperatures.

In the last decade various morphologies of nano-ZnO have been reported as catalysts in differentorganic reactions, such as the synthesis of �-phosphonomalonates [51], knoevenagel condensation[52] and the syntheses of tetrazoles [53], coumarines[54], ferrocenylphosphonates [55], 3-indolyl-3-hydroxyoxindoles [56], dihydropyranochromenes [57], 1H-pyrazolo phthalazinediones [58], 1,4-dihydropyridine[59], imidazo[1,2-a]azines [60] and Pyranopyrazoles[61].

In this study, a simple and inexpensive precipitationmethod was used for the synthesis of ZnO nanorods byemploying polyethylene glycol (PEG 2000) as a cap-ping agent. The activity of the synthesized ZnO NRsas an inexpensive and easy-to-handle catalyst for thesynthesis of 2,4,5-triarylimidazoles from the one-pot,three-component cyclo-condensation of benzils, arylaldehydes and ammonium acetate in water under refluxconditions has been examined (Scheme 1).

2. Experimental

2.1. Chemicals and apparatus

All of the chemicals and solvents were purchasedfrom Merck. Melting points were determined by the cap-illary tube method using a Stuart Scientific apparatus andare uncorrected. Fourier transform infrared (FT-IR) spec-tra (KBr discs, 500–4000 cm−1) were recorded using aBruker FT-IR model Tensor 27 spectrometer. 1H NMRspectra were recorded in CDCl3 on a Brucker 400 MHzspectrometer. X-ray diffraction patterns (XRD) wererecorded by a Philips X-ray diffractometer (Cu K�1radiation, cobalt anode, wavelength of 1.7889 A, 40 kV,40 mA). Scanning electron microscopy (SEM, model

EM 3200, kyky) was used to characterize the nanostruc-tures. Thin layer chromatography (TLC) was performedon commercial aluminium-backed plates of silica gel (60F254) to monitor reaction progress.

572 K. Nikoofar et al. / Journal of Taibah University for Science 9 (2015) 570–578

-imidaz

Scheme 1. Synthesis of 2,4,5-triaryl-1H

All of the products were characterized by comparingtheir spectroscopic data (FT-IR and 1H NMR) with thoseof authentic samples in the literature. The yields refer toisolated products.

2.2. Synthesis of ZnO nanorods

ZnO NRs were synthesized according to the previ-ously reported method [55], with some modification.Zn(OAc)2·2H2O (1 g) and PEG 2000 (0.3 g) in 140 mLof doubly distilled water at laboratory temperature weredissolved to obtain a transparent solution. Then, 1.5 mLof NH3 (25%) was added dropwise to the aforemen-tioned mixture and refluxed at 80 ◦C for 6 h. The reactionwas terminated by cooling to room temperature followedby centrifugation and washing the precipitate. In the nextstep, H2O (90 mL) was added to the mixture and refluxedfor 9 h. This solution was then cooled, centrifuged and

washed with deionized water and absolute ethanol. TheZnO NRs were dried at 100 ◦C in an oven. The obtainedZnO NRs were characterized by FT-IR (Fig. 1), XRD(Fig. 2) and SEM (Fig. 3).

Fig. 1. FT-IR spectrum of the

oles under ZnO NRs catalysis in water.

2.3. General procedure for the synthesis of2,4,5-triarylimidazoles

ZnO NRs (20 mol%) were added to a mixture ofbenzils (1 mmol), arylaldehydes (1 mmol) and NH4OAc(4 mmol) in H2O (5 mL). The resulting mixture wasrefluxed in a round-bottom flask equipped with acondenser and magnetic stirrer until the comple-tion of the reaction. The mixture was then cooledand filtered. The solid residue was washed withEtOAc (2× 5 mL). After filtrate extraction, the organiclayer was concentrated by vaporizing the solventunder reduced pressure. The resulting mixture wasthen purified by thin-layer chromatography (eluent,9:1 petroleum ether:EtOAc) to produce pure 2,4,5-triarylimidazoles.

2.3.1. 2-(3-Methoxyphenyl)-4,5-diphenyl-1H-imidazole (4h)

White powder, IR (KBr, υ cm−1): 3430 (NH), 1592(C C), 1515 (C N); 1H NMR (CDCl3, 400 MHz): δ

synthesized ZnO NRs.

K. Nikoofar et al. / Journal of Taibah University for Science 9 (2015) 570–578 573

Fig. 2. XRD pattern of the ZnO nanorods (Wurtzite structure).

of synt

1O

21

(((2

2i

1δ

Fig. 3. SEM image

2.7 (s, IH, NH), 7.8–6.78 (m, 14H, Ar–H), 3,82 (s, 3H,CH3).

.3.2. 2-(4-Chlorophenyl)-4,5-bis(4-chlorophenyl)-H-imidazole (4r)

White powder, IR (KBr, υ cm−1): 3435 (NH), 1609C C), 1517 (C N); 1H NMR (CDCl3, 400 MHz): δ 12.8s, IH, NH), 8.07 (d, 2H, J = 8.4 Hz, Ar H), 7.56–7.51m, 6H, Ar H), 7.3 (t, 2H, J = 8.8 Hz, Ar H), 7.15 (t,H, J = 8.4 Hz, Ar H).

.3.3. 2-p-Tolyl-4,5-bis(4-methoxyphenyl)-1H-

midazole (4x)

Pale yellow powder, IR (KBr, υ cm−1): 3409 (NH),623 (C C), 1526 (C N); 1H NMR (CDCl3, 400 MHz):

12.75 (s, IH, NH), 7.93–8.05 (m, 5H, Ar H), 7.73

hesized ZnO NRs.

(d, 2H, J = 8.4 Hz, Ar H), 7.28–7.44 (m, 5H, Ar H),4.02 (s, 6H, OCH3), 2.37 (s, 3H, CH3).

3. Results

3.1. Characterization of ZnO NRs

The FT-IR spectrum (Fig. 1) of the ZnO NRs shows anabsorption band of approximately 515 cm−1 correspond-ing to Zn–O stretching vibration modes, which is thetypical characteristic band of pure ZnO. The weak peakat 1394 cm−1 may be due to the symmetric stretching

of carboxylate groups, probably from the small amountof residual zinc acetate remaining from the synthesis.The broad absorption peaks at approximately 3437 and1456 cm−1 are attributed to vibrations of OH groups

574 K. Nikoofar et al. / Journal of Taibah University for Science 9 (2015) 570–578

Table 1Screening for optimal conditions for the condensation of benzil (1 mmol), 4-chlorobenzaldehyde (1 mmol) and NH4OAc (4 mmol) in the presenceZnO NRs.

Entry Conditiona Time (h) Yieldb (%)

1 H2O/ZnO NRs (20 mol%)/R.T. 4.5 352 CH3CN/ZnO NRs (20 mol%)/R.T. 6 153 C2H5OH/ZnO NRs (20 mol%)/R.T. 5 384 Solvent-free/ZnO NRs (20 mol%)/R.T. 1.5 285 Solvent-free/ZnO NRs (20 mol%)/90 ◦C 3 65c

6 H2O:C2H5OH (1:1)/ZnO NRs (20 mol%)/R.T. 3 387 H2O/ZnO NRs (20 mol%)/Reflux 1.45 838 H2O/ZnO NRs (30 mol%)/Reflux 1.45 839 H2O/ZnO NRs (10 mol%)/Reflux 2.5 4610 H2O/ZnO NRs (20 mol%)/Refluxd 1.45 2811 H2O/ZnO NRs (20 mol%)/Refluxe 1.45 60

a 5 mL of each solvent was used.b Isolated yields.c A mixture of products were obtained.

d 4 mmol of (NH4)2CO3 was used.e 2 mmol of NH4OAc was used.

corresponding to the absorption of water molecules onthe ZnO surface. The peaks at approximately 2925 and2856 cm−1 are due to the C H stretching of acetate[62,63].

The structures of the ZnO nanocrystals were stud-ied by XRD pattern. Fig. 2 shows the X-ray diffractionpattern of obtained ZnO nanorods. Peaks are observedat angles (2θ) of 37.03, 40.175, 42.335, 55.8, 66.655,74.535, 78.96, 80.89 and 82.315; after converting to Cuvalues by multiplying by 0.86, these angles confirm the1 0 0, 0 0 2, 1 0 1, 1 0 2, 1 1 0, 1 0 3, 2 0 0, 1 1 2 and 2 0 1crystal planes, respectively. The XRD pattern is com-pared with the reference pattern (JCPDS No. 36-1451for ZnO) corresponding to wurtzite-type ZnO.

Fig. 3 shows the SEM image of the ZnO nanorodsprepared using PEG 2000 as a capping agent. The aver-age diameter of these nanorods is in the range 35–70 nm.It should be noted that the average size of most of thesynthesized nanorods estimated from SEM images isapproximately 50 nm [55].

3.2. Synthesis of tri-substituted imidazoles

Initially, the condensation of benzil (1 mmol), 4-chlorobenzaldehyde (1 mmol) and ammonium acetate(4 mmol) was chosen as a model to identify the optimizedreaction conditions (Table 1). The corresponding 2-(4-chlorophenyl)-4,5-diphenyl-1H-imidazole was obtained

in an 83% yield after 1.45 h in the presence of 20 mol%of the nano-catalyst in H2O under reflux (Table 1, entry7). Performing the reaction under solvent-free condi-tions both at room temperature and in the presence of

conventional heating did not produce satisfactory results(entries 4 and 5). The use of CH3CN, C2H5OH andH2O/C2H5OH (1/1, v/v) as the solvent yielded worseresults compared to the use of water under the same con-ditions (entries 1, 2, 3 and 6). Decreasing the amount ofNH4OAc to 2 mmol decreased the yield (entry 11), andthe amount of catalyst used was not suitable in entries 8and 9. Using ammonium carbonate as a source of ammo-nia did not produce satisfactory results (entry 10). Theoptimal conditions were determined to be the utilizationof benzil, aldehydes and NH4OAc (molar ratio of 1/1/4)in the presence of ZnO NRs (20 mol%) in H2O underreflux (entry 7).

In the next step, the general applicability of the ZnONRs as catalysts for the synthesis of various deriva-tives of 2,4,5-triaryl-1H-imidazoles was investigated(Table 2). Benzaldehyde and its activated derivativesas well as deactivated aromatic aldehydes successfullyproduced the corresponding tri-substituted imidazoles4a–m. It should be noted that electron-deficient sub-stituents yielded better results than their correspondingelectron-donating substituents. The reaction times of the3- and 2-substituted arylaldehydes (entries 2 and 7) areslightly longer than those of their 4- substituted anal-ogous (entries 6 and 9). This might be due to indirectinductive and/or resonance effects on the reaction centre.Steric hindrance in 2-substituted derivatives is anotherpotential cause of this sluggish response. Furfural and 1-

naphthaldehyde also reacted successfully (entries 14 and15). The specialty of the catalyst was also observed forthe reactions of unsaturated aldehydes, such as 3-phenyl-2-propenal. In this case, no by-products were obtained

K. Nikoofar et al. / Journal of Taibah University for Science 9 (2015) 570–578 575

Table 2Syntheses of 2,4,5-triaryl-1H-imidazoles by one-pot multi-component reactions of benzils (1 mmol), different aldehydes (1 mmol) and NH4OAc(4 mmol) in the presence of ZnO NRs (20 mol%) in H2O (5 mL) under reflux.

Entry Ar R Product Time (h) Yielda (%) Melting-point(◦C) found

Melting-point(◦C) reported [ref]

1 C6H5 H 4a 2.5 80 264–266 266–267 [22]2 4-ClC6H4 H 4b 1.45 83 268–270 272–273 [27]3 4-BrC6H4 H 4c 1.45 86 198–201 203–205 [27]4 4-N(CH3)2C6H4 H 4d 3.5 75 260–261 256–259 [24]5 4-MeC6H4 H 4e 3.45 81 227–229 231–232 [22]6 4-NO2C6H4 H 4f 1.15 90 196–199 199–201 [24]7 3-ClC6H4 H 4g 2 90 205–207 207–209 [27]8 3-MeOC6H4 H 4h 2.25 81 217–219 220–221 [22]9 2-NO2C6H4 H 4i 1.45 78 230–232 230–231 [24]10 2-OHC6H4 H 4j 3.25 78 280–282 282–284 [27]11 2-MeOC6H4 H 4k 3.45 70 209–211 210–212 [22]12 3,5-(OMe)2C6H3 H 4l 4 78 260–263 269–271 [27]13 2-OH-6-NO2C6H3 H 4m 3.15 80 215–217 219–221 [22]14 1-Naphthyl H 4n 3.45 78 210–213 215 [22]15 2-Furyl H 4o 3.45 79 210–212 213–214 [22]16 C6H5CH CH H 4p 3.5 80 194–196 198–199 [22]17 C6H5 Cl 4q 2.5 82 290–291 289–291 [26]18 4-ClC6H4 Cl 4r 2.25 87 272–274 269–271 [26]19 4-MeOC6H4 Cl 4s 3 85 259–261 260–262 [26]20 4-MeC6H4 Cl 4t 3 84 260–262 263–264 [24]21 4-N(CH3)2C6H4 Cl 4u 3.25 86 245–247 248–250 [24]22 C6H5 CH3 4v 3.5 87 269–271 268–270 [26]23 4-ClC6H4 CH3 4w 2.45 83 262–264 260–262 [26]24 4-MeOC6H4 CH3 4x 4 82 251–253 252–254 [26]2 80 210–212 213–214 [24]

(ua

ctn2fiEacrFoic

sSgia

Fig. 4. Reaction yields for four runs in the presence of the recycled

5 4-MeOC6H4 OCH3 4y 4

a Isolated yields.

entry 16). In addition, substituted benzils successfullynderwent three-component condensation with variousldehydes (Table 2, entries 17–25).

In the next step, the reusability of the recoveredatalyst was studied as another efficient and impor-ant aspect of this protocol. For this reason, the ZnOanorods were recovered from the reaction mixture of-(4-chlorophenyl)-4,5-diphenyl-1H-imidazole (4b) byltration and further washing the solid residue withtOAc (2× 10 mL). Drying overnight at room temper-ture and heating at 70 ◦C for 2 h gave the recoveredatalyst. The obtained nanocatalyst was reused andecycled for four runs with almost no decrease in activity.ig. 4 shows that the recovered nano-ZnO catalyzed thene-pot, three-component reaction to give tri-substitutedmidazole 4b with yields of 83, 83, 81 and 80% for fouronsecutive separations and reuses.

Finally, the proposed mechanism for the synthe-is of 2,4,5-trisubstitiuted imidazoles is outlined incheme 2. The catalyst activates the aldehyde carbonyl

roup to form hydroxylamine intermediate [A], whichs dehydrated to imine [B]. The nucleophilic attack ofnother NH3 yields the diamine intermediate [C]. The

catalyst.

consequent nucleophilic attack of [C] on activated ben-zil [D] gives [E], which releases water to form [F].The intramolecular nucleophilic attack on [F] leads tothe cyclisation of [G]. From this stage, the nanocata-lyst can be utilized for another cycle. Subsequently, the

dehydration of [G] gives the tri-substituted imidazoleproduct.

576 K. Nikoofar et al. / Journal of Taibah University for Science 9 (2015) 570–578

for the s

Scheme 2. The suggested mechanism

4. Conclusions

In summary, a modified route for the synthesis ofZnO nanorods has been introduced. The synthesizednanorods are an effective, eco-friendly and reusableLewis acid catalyst that catalyses the one-pot, three-component reaction of benzils with aldehydes andammonium acetate to form 2,4,5-triaryl-1H-imidazolesin high yields. The simple operation and easy work-up of this reaction procedure along with the reusabilityand efficiency of the synthesized nanocatalysts are thehighlights of this work. The authors expect this protocolto find extensive applications in the fields of heteroge-neous nanocatalysts, drug preparation development andcombinatorial organic synthesis.

Acknowledgment

The authors thank Alzahra University for partialfinancial support.

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