ZnO Nanoparticles - A Recyclable Catalyst for the Synthesis of Bis(Pyrazolyl)Methanes

Article abstract of DOI:10.1007/s10593-015-1635-3

A one-pot condensation between a wide range of aryl aldehydes and 5-methyl-2-phenyl-2,4-dihydro-3H-pyrazol-3-one by employing ZnO nanoparticles as recyclable and highly efficient catalyst to produce new and known bis(pyrazolyl)methanes in aqueous media is

Full text of DOI:10.1007/s10593-015-1635-3

DOI 10.1007/s10593-015-1635-3
Chemistry of Heterocyclic Compounds, Vol. 50, No. 12, March, 2015 (Russian Original Vol. 50, No. 12, December, 2014)
ZnO NANOPARTICLES – A RECYCLABLE CATALYST
FOR THE SYNTHESIS OF BIS(PYRAZOLYL)METHANES
1
1
2
*
K. Eskandari , B. Karami , and S. Khodabakhshi
A one-pot condensation between a wide range of aryl aldehydes and 5-methyl-2-phenyl-2,4-dihydro-
3H-pyrazol-3-one by employing ZnO nanoparticles as recyclable and highly efficient catalyst to
produce new and known bis(pyrazolyl)methanes in aqueous media is introduced. This efficient and new
method to bis(pyrazolyl)methanes has some advantages, such as the use of a safe and reusable catalyst,
simple operation, short reaction times, and good to excellent yields of the products.
Keywords: aldehyde, 2,4-dihydro-3H-pyrazol-3-one, ZnO nanoparticles, Knoevenagel condensation,
Michael addition.
Recently pyrazole derivatives have received much attention from chemists and pharmacologists, since it
has been shown that these compounds possess important biological activities such as antianxiety, antipyretic,
analgesic, and anti-inflammatory activity [1-4]. Therefore, the synthesis of bis(pyrazolyl)methanes has attracted
considerable interest from organic and pharmaceutical chemists [5-7]. So far, several synthetic routes to
bis(pyrazolyl)methanes have been presented in the literature. In recent studies, some research groups have
focused on a catalyst-promoted synthesis of bis(pyrazolyl)methanes where an aromatic aldehyde is condensed
with 5-methyl-2-phenyl-2,4-dihydro-3H-pyrazol-3-one under various conditions [8-17]. Despite several
advantages, some of the reported methods suffer from disadvantages such as long reaction times, low product
yield, and use of extra tools and unrecyclable catalysts.
According to the green chemistry principles, the use of recyclable catalysts and replacement of toxic
organic solvents by safe and clean ones are effective ways to prevent waste production in chemical reactions
[18-20]. Nanocatalysis has recently attracted much attention from chemists because of its notable features. For
example, nanosized systems dramatically increase the contact between reactants and catalyst [21]. Among safe
and environmentally friendly nanomaterials, ZnO nanoparticles have emerged as safe, recyclable, and highly
efficient catalysts in organic reactions [22-24].
Following our previous studies on the synthesis of new heterocyclic compounds using various catalysts
with prominent virtues [25-29], in this work we demonstrate the use of ZnO nanoparticles (ZnO NPs) as an eco-
friendly and reusable catalyst for the effective synthesis of a wide range of novel and known
_______
*To whom correspondence should be addressed, e-mail: khalileskandari@yahoo.com.
1Department of Chemistry, Yasouj University, P. O. Box 353, Yasouj 75918-74831, Iran.
2Nanotechnology Research Center, Research Institute of Petroleum Industry, P. O.Box 14665-137, Tehran
1485733111, Iran; е-mail: saeidkhm@yahoo.com.
_________________________________________________________________________________________
Published in Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1807-1813, December, 2014.
Original article submitted June 14, 2014.
1658
0009-3122/15/5012-1658©2015 Springer Science+Business Media New York

bis(pyrazolyl)methane derivatives 3 from the condensation reaction of aromatic aldehydes 1 with 5-methyl-
2-phenyl-2,4-dihydro-3H-pyrazol-3-one (2) (Scheme 1).
ZnO NPs were prepared via a slightly modified previously reported method [30]. The advantages of this
procedure are effectiveness, convenience, shorter preparation time, avoiding the use of organic solvents, and
savings in labor.
Scheme 1
The morphology of the ZnO nanoparticles was studied by scanning electron microscopy (SEM). Figure
1 shows the typical SEM image of ZnO nanoparticles, synthesized by the thermal decomposition method, which
have a diameter of about 20 nm. The X-ray diffraction (XRD) spectrum of the ZnO nanoparticles is shown in
Figure 2. ZnO nanoparticles exhibited prominent (100), (002), and (101) peaks corresponding to the ZnO
wurtzite structure. The average crystal size D (Å) can be calculated from the XRD pattern using the Debye–
Scherrer equation [20] D = 0.9λ / βcosθ, where λ (nm) is the X-ray wavelength used, β (radian) is the angular
line width at half maximum intensity, and θ (degrees) i the Bragg angle. For the (101) reflection (2θ = 36.5, β =
0.011 radians, λ = 1.78897 Å), the size of ZnO NPs obtained was around 15 nm, which is close to the estimated
size from the SEM image.
Fig. 1. SEM image of ZnO NPs.
Furthermore, transmission electron microscopy (TEM) analyses and selected area electron diffraction
(SAED) pattern were obtained for a better characterization of the catalyst (Fig. 3). The TEM image shows that
the spherical ZnO nanoparticles have a granular nature with average size around 10-20 nm, which agrees with
the average size obtained from the Debye–Scherrer equation. From the TEM image it was concluded that the
ZnO NPs are crystalline with a wurtzite structure, and no impurities were observed. These observations are in
close agreement with the results obtained from XRD data. In addition, the rings with a dotted pattern in SAED,
shown in the inset of Figure 3, confirm the size of ZnO nanoparticles.
To find the optimal reaction conditions, the treatment of benzaldehyde (1a) with pyrazolone 2 was
selected as the model reaction. The reaction initially was performed under reflux in H2O–EtOH, 1:1, in the
presence of various amounts of ZnO NPs. The results outlined in Table 1 show that the reaction proceeds
efficiently in the presence of 2 mol% of ZnO NPs (entry 3). In addition, the use of excess catalyst did not
improve the reaction rate or product yield (entry 5). The reaction was also studied in the absence of catalyst
where it did not show any appreciable progress even after 120 min (entry 1).
1659

Fig. 2. X-ray diffraction pattern of ZnO NPs.
Fig. 3. TEM image for ZnO NPs.
The reaction was also carried out at room temperature in H2O/EtOH (1:1) in the presence of 2 mol% of
ZnO NPs. The results showed that the progress of the reaction at room temperature is not as good as at reflux
conditions.
After optimization, the generality of the reaction was studied with various aromatic aldehydes 1. The
results are summarized in Table 2. As can be seen the nature of the substituents on the aromatic ring of the
aldehyde 1 shows no important effects in terms of reaction time or product yields. However, when aliphatic
aldehydes were employed instead of aromatic ones, the reaction did not take place.
TABLE 1. Optimization of Catalyst Amount in the Synthesis of
Compound 3a
Entry
Catalyst amount, mol%
Reaction time, min
Yield, %
1
2
3
4
5
—
1
120
20
64
89
92
92
90
2
20
3
20
4
20
1660

TABLE 2. Synthesis of Bis(pyrazolyl)methanes 3a-p Using ZnO NPs
Product
Ar
Reaction time, min
Yield, %
Mp, °C
Ph
20
20
30
30
15
30
30
25
30
20
15
20
30
15
20
30
92
90
88
93
92
90
90
90
91
90
92
92
88
94
85
92
168-170 (171-172 [17])
190-192
3a
3b
3c
3d
3e
3f
2,4-(MeO)2C6H3
3-EtO-4-HOC6H3
Naphthalen-1-yl
4-PhC6H4
212-213
228-230
218-220
3-Indolyl
242-244
2-Furyl
190-191 (189-191 [17])
184-186 (181-183 [8])
231-233 (230-232 [17])
228-230 (227-229 [8])
153-154 (151-153 [17])
226-228 (225-227 [17])
203-205 (202-204 [17])
238-240 (235-237 [17])
174-176 (176-177 [16])
174-176 (173-175 [17])
3g
3h
3i
2-Thienyl
2-Py
2,4-Cl2C6H3
3-O2NC6H4
4-O2NC6H4
4-MeC6H4
2-ClC6H4
3j
3k
3l
3m
3n
3o
3p
4-MeOC6H4
3-BrC6H4
The data presented in Table 3 demonstrate the merit of this method for the synthesis of bis-
(pyrazolyl)methane 3a in comparison with previously reported results. With a brief overview, it can be concluded
that the reaction efficiently proceed under our suggested conditions with respect to the reaction time and product
yield.
The recyclability of the ZnO NPs was investigated for the synthesis of compound 3a. At the end of
reaction in each run, NPs were filtered, collected without significant loss in catalyst mass, washed with diethyl
ether, dried at 120°C for 1 h, and applied to run the next reaction. After four runs, the yield of compound 3a
was reduced to just 90% and after fifth run it was 88%. These results show that the catalytic activity of ZnO
NPs remains high even after five cycles.
Mechanistically, it seems that, firstly, the Knoevenagel condensation occurs between aldehyde 1 and
pyrazole 2 to generate the α,β-unsaturated adduct. Subsequent Michael addition of pyrazole 2 to the
α,β-unsaturated adduct followed by 1,3-proton shift leads to the formation of product 3 [11, 14]. ZnO NPs in
this reaction act as a Lewis acid catalyst with high surface area by activating not only the carbonyl group of the
aldehyde as electrophile, but also pyrazolone for nucleophilic attack.
TABLE 3. Comparison of ZnO NPs with Other Methods for the Synthesis
of Bis(pyrazolyl)methane 3a
Time,
min
Entry
Conditions
Yield, %
1
2
Silica-bonded S-sulfonic acid (0.1 g), EtOH, 
120
80 [8]
90 [9]
74 [10]
86 [11]
82 [12]
87 [13]
92 [14]
92 [15]
81 [16]
92 [17]
91 [5]
92
Sulfuric acid ([3-(3-silicapropyl)sulfanyl]propyl)ester (0.1 g), EtOH,  180
3
Cellulose sulfuric acid (0.2 g), H2O–EtOH, 
1,3-Disulfonic acid imidazolium tetrachloroaluminate (1 mol%), 50°C
Electrolysis, EtOH, NaBr (0.1 g), 20°C
120
60
4
5
33
6
Sodium dodecyl sulfate (5 mol%), H2O, 
60
15
7
Ceric ammonium nitrate (5 mol%), H2O, room temperature
Poly(ethylene glycol)-bound sulfonic acid (1.5 mol%), H2O, 
LiOH–H2O (10 mol%), H2O, 90°C
8
30
75
9
10
11
12
Poly(ethylene glycol)-400, 110°C
120
30
(2-Hydroxyethyl)ammonium propionate (10 mol%), solvent-free, 90°C
ZnO NPs (2 mol%), H2O–EtOH, 
20
1661

In conclusion, we have demonstrated the use of ZnO NPs as a highly efficient and reusable catalyst in
the three-component condensation reaction of 5-methyl-2-phenyl-2,4-dihydro-3H-pyrazol-3-one (2 equiv.) and
aryl aldehyde (1 equiv.) to give bis(pyrazolyl)methanes in high yields and short reaction times.
EXPERIMENTAL
1
IR spectra were recorded on a JASCO FT-IR-680 plus spectrometer in the matrix of KBr. H and 13C
NMR spectra were recorded on a FT-NMR Bruker Avance Ultrashield spectrometer at 400, 300, and 250 MHz
1
for H nucleus and 100, 75, and 62 MHz for 13C nucleus in DMSO-d6 as solvent in the presence of TMS as
internal standard. Scanning electron microscopy (SEM) studies of the nanostructures were carried out with a
JEOL JEM 3010 instrument operating at an accelerating voltage of 300 kV. X-ray diffraction (XRD, D8,
Advance, Bruker, AXS) patterns were obtained for characterization of the heterogeneous catalyst. Melting
points were measured on an electrothermal KSB1N apparatus. TLC was performed on TLC-Grade silica gel-
G/UV 254 nm plates (EtOAc–n-hexane, 1:1). Elemental analyses were performed with a Heraeus CHN-O rapid
analyzer (Wellesley, MA). Chemicals were purchased from Merck and Aldrich.
Preparation of ZnO NPs. Zinc acetate dihydrate (2.19 g, 10 mmol) and oxalic acid (1.08 g, 12 mmol)
were reacted by grinding in an agate mortar for 30 min at room temperature, then washed with water (10 ml)
and ethanol (10 ml) on a filter. The obtained ZnC2O4·2H2O nanoparticles were calcinated at 450°C for 30 min
to produce ZnO nanoparticles (0.78 g).
Synthesis of Bispyrazoles 3a-p Using ZnO NPs. A solution of aromatic aldehyde 1 (1 mmol),
pyrazolone 2 (348 mg, 2 mmol), and ZnO NPs (1.62 mg, 2 mol%) in EtOH–H2O (1:1, 10 ml) was stirred under
reflux for a stipulated time. The progress of the reaction was checked by TLC. After reaction completion, the
reaction mixture was cooled to room temperature and the solvent was evaporated under reduced pressure. The
precipitate was dried and dissolved in hot EtOH (20 ml) to separate the catalyst. The product 3 was obtained
after recrystallization from EtOH.
4,4'-(Phenylmethanediyl)bis(3-methyl-1-phenyl-1H-pyrazol-5-ol) (3a). Light-yellow crystals.
1
IR spectrum, ν, cm-1: 3424, 3062, 2917, 1598, 1498, 1415, 1284, 1186, 1027, 755, 692. H NMR spectrum
(400 MHz), δ, ppm (J, Hz): 13.96 (1H, s, OH); 12.39 (1H, s, OH); 7.71 (4H, d, J = 8.4, H Ar); 7.45 (4H, t,
J = 7.2, H Ar); 7.31-7.24 (6H, m, H Ar); 7.20-7.17 (1H, m, H Ar); 5.00 (1H, s, CH); 2.33 (6H, s, 2CH3).
13C NMR spectrum (100 MHz), δ, ppm: 157.6; 146.4; 140.7; 136.9; 128.8; 128.3; 127.1; 126.4; 126.2; 121.3;
105.7; 33.6; 11.5.
4,4'-[(2,4-Dimethoxyphenyl)methanediyl]bis(3-methyl-1-phenyl-1H-pyrazol-5-ol) (3b). Yellow
crystals. IR spectrum, ν, cm-1: 3428, 2996, 2958, 2839, 1613, 1503, 1460, 1406, 1294, 1209, 1122, 1041, 839,
1
757, 580. H NMR spectrum (300 MHz), δ, ppm (J, Hz): 14.35 (1H, s, OH); 12.38 (1H, s, OH); 7.69 (4H, d,
J = 7.8, H Ar); 7.50-7.39 (5H, m, H Ar); 7.22 (2H, t, J = 6.9, H Ar); 6.46 (2H, t, J = 9.6, H Ar); 5.09 (1H, s,
CH); 3.79 (3H, s, OCH3); 3.69 (3H, s, OCH3); 2.26 (6H, s, 2CH3). 13C NMR spectrum (75 MHz), δ, ppm: 158.8;
156.7; 146.1; 137.6; 137.4; 137.3; 137.1; 136.7; 133.6; 131.9; 128.8; 125.4; 122.9; 120.5; 104.1; 98.2; 55.4;
55.0; 26.9; 11.6. Found, %: C 70.22; H 5.62; N 11.25. C29H28N4O4. Calculated, %: C 70.15; H 5.68; N 11.28.
4,4'-((3-Ethoxy-4-hydroxyphenyl)methanediyl)bis(3-methyl-1-phenyl-1H-pyrazol-5-ol)
(3c).
Chocolate crystals. IR spectrum, ν, cm-1: 3420, 3219, 2985, 2927, 1596, 1498, 1400, 1275, 1214, 1126, 1043,
1
811, 753, 691. H NMR spectrum (300 MHz), δ, ppm (J, Hz): 13.97 (1H, s, OH); 12.36 (1H, br. s, OH); 8.67
(1H, s, OH); 7.69 (4H, d, J = 8.1, H Ar); 7.42 (4H, t, J = 7.8, H Ar); 7.22 (2H, t, J = 7.2, H Ar); 6.82 (1H, s,
H Ar); 6.66 (2H, s, H Ar) 4.82 (1H, s, CH); 3.90 (2H, q, J = 6.9, CH2); 2.29 (6H, s, 2CH3); 1.25 (3H, t, J = 6.9,
CH3). 13C NMR spectrum (75 MHz), δ, ppm: 146.1; 145.1; 142.6; 137.3; 137.0; 133.1; 131.6; 128.9; 125.5;
120.5; 119.7; 115.2; 113.4; 63.9; 32.7; 14.7; 11.6. Found, %: C 70.19; H 5.57; N 11.26. C29H28N4O4.
Calculated, %: C 70.15; H 5.68; N 11.28.
1662

4,4'-(Naphthalen-1-ylmethanediyl)bis(3-methyl-1-phenyl-1H-pyrazol-5-ol) (3d). Navajo-white
crystals. IR spectrum, ν, cm-1: 3419, 3062, 2922, 1608, 1542, 1497, 1402, 1370, 1132, 829, 783, 756, 689.
1H NMR spectrum (300 MHz), δ, ppm (J, Hz): 13.15 (1H, s, OH); 12.19 (1H, s, OH); 8.10-8.00 (1H, m, H Ar);
7.92 (1H, d, J = 6.6, H Ar); 7.80 (1H, d, J = 7.2, H Ar); 7.75-7.62 (5H, m, H Ar); 7.57-7.34 (7H, m, H Ar);
7.25-7.15 (2H, m, H Ar); 5.61 (1H, s, CH); 2.25 (6H, s, 2CH3). 13C NMR spectrum (75 MHz), δ, ppm: 146.0;
144.1; 140.6; 137.3; 136.7; 133.6; 130.7; 128.8; 128.7; 127.0; 125.9; 125.7; 125.3; 125.2; 123.5; 119.9; 105.6;
30.9; 11.9; 11.8. Found, %: C 76.57; H 5.33; N 11.48. C31H26N4O2. Calculated, %: C 76.52; H 5.39; N 11.51.
4,4'-(Biphenyl-4-ylmethanediyl)bis(3-methyl-1-phenyl-1H-pyrazol-5-ol) (3e). Navajo-white crystals.
1
IR spectrum, ν, cm-1: 3444, 3026, 2922, 1599, 1580, 1499, 1405, 1294, 818, 751, 692. H NMR spectrum
(300 MHz), δ, ppm (J, Hz): 14.05 (1H, s, OH); 12.48 (1H, s, OH); 7.74 (4H, d, J = 7.8, H Ar); 7.62-7.55 (4H,
m, H Ar); 7.46-7.34 (9H, m, H Ar); 7.23 (2H, t, J = 7.2, H Ar); 5.01 (1H, s, CH); 2.35 (6H, s, 2CH3). 13C NMR
spectrum (75 MHz), δ, ppm: 146.3; 141.5; 140.0; 137.9; 137.4; 137.3; 128.9; 128.8; 127.8; 127.1; 126.5; 125.5;
120.5; 104.9; 104.6; 32.8; 11.6. Found, %: C 77.38; H 5.43; N 10.84. C33H28N4O2. Calculated, %: C 77.32;
H 5.51; N 10.93.
4,4'-((1H-Indol-3-yl)methanediyl)bis(3-methyl-1-phenyl-1H-pyrazol-5-ol) (3f). Yellow crystals.
1
IR spectrum, ν, cm-1: 3470, 3042, 2920, 1618, 1540, 1488, 1405, 1370, 1136, 830, 786, 755, 689. H NMR
spectrum (400 MHz), δ, ppm (J, Hz): 12.65 (2H, s, OH); 9.85 (1H, s, NH); 8.13-8.11 (2H, m, H Ar); 8.06 (1H,
s, H Ar); 8.05-8.01 (2H, m, H Ar); 7.60-7.58 (1H, m, H Ar); 7.44-7.40 (4H, m, H Ar); 7.32-7.29 (4H, m, H Ar);
7.17-7.13 (1H, m, H Ar); 3.49 (1H, s, CH); 2.39 (6H, s, 2CH3). 13C NMR spectrum (100 MHz), δ, ppm: 162.7;
150.8; 138.9; 138.2; 136.9; 136.4; 128.6; 128.1; 123.8; 123.4; 122.0; 118.5; 118.0; 112.8; 112.2; 18.5; 12.9.
Found, %: C 73.31; H 5.23; N 14.66. C29H25N5O2. Calculated, %: C 73.25; H 5.30; N 14.73
4,4'-((2,4-Dichlorophenyl)methanediyl)bis(3-methyl-1-phenyl-1H-pyrazol-5-ol) (3j). Bisque crystals.
IR spectrum, ν, cm-1: 3420, 3057, 2920, 1597, 1572, 1500, 1470, 1380, 1298, 1189, 845, 792, 752, 690;
1H NMR spectrum (250 MHz), δ, ppm (J, Hz): 13.95 (1H, s, OH); 12.67 (1H, s, OH); 7.74-7.65 (5H, m, H Ar);
7.53 (1H, d, J = 2.0, H Ar); 7.44-7.36 (5H, m, H Ar); 7.22 (2H, t, J = 7.2, H Ar); 5.05 (1H, s, CH); 2.26 (6H, s,
2CH3). 13C NMR spectrum (62 MHz), δ, ppm: 148.3; 147.1; 146.0; 137.3; 134.9; 128.8; 125.4; 120.5; 119.2;
111.6; 111.5; 104.9; 104.6; 31.7; 11.6.
4,4'-((4-Nitrophenyl)methanediyl)bis(3-methyl-1-phenyl-1H-pyrazol-5-ol) (3l). Yellow crystals.
1
IR spectrum, ν, cm-1: 3068, 2920, 1598, 1579, 1524, 1500, 1415, 1348, 785, 753, 691. H NMR spectrum
(400 MHz), δ, ppm (J, Hz): 13.90 (1H, s, OH); 12.49 (1H, s, OH); 8.18 (2H, d, J = 8.8, H Ar); 7.73 (4H, d,
J = 8.0, H Ar); 7.54 (2H, d, J = 8.8, H Ar); 7.45 (4H, t, J = 8.0, H Ar); 7.25 (2H, t, J = 8.0, H Ar); 5.14 (1H, s,
CH); 2.37 (6H, s, 2CH3). 13C NMR spectrum (100 MHz), δ, ppm: 150.3; 146.2; 145.8; 137.1; 129.1; 128.9;
128.5; 125.6; 123.3; 120.5; 104.0; 33.1; 11.5.
1
The Supplementary file containing H and 13C NMR spectra of compounds 3a-f,j,l are available for
authorized users.
This work was carried out with the financial support of the grant Yasouj University of Iran.
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