Ultrasonic-assisted green synthesis of palladium nanoparticles and their nanocatalytic application in multicomponent reaction

Article abstract of DOI:10.1039/c4nj01543f

This paper describes a novel one pot synthesis of palladium nanoparticles (PdNPs) using a green method. An aqueous extract of Perilla frutescens leaf was used as a bioreductant to reduce Pd²⁺ ions to Pd⁰ without any extra surfactants or capping agents. Polyphenols and flavonoids are believed to be actively involved in the reduction process. The formation of PdNPs was confirmed by UV-Vis spectra. Transmission electron microscopy revealed spherical nanoparticles, ranging in size between 10 and 17 nm (average ～13 nm). X-ray diffraction revealed reflections from the (111), (200), (220), (311) and (222) planes confirming the crystallinity of the nanoparticles with a face centred cubic (fcc) structure. Energy dispersive X-ray spectroscopy confirmed the presence of palladium. Fourier transform infrared spectroscopy suggested the role of polyols present in the P. frutescens leaf extract. The synthesized nanoparticles were used as catalysts for the synthesis of pyrazolylphosphonate derivatives in high yield. The excellent catalytic performance of the synthesized PdNPs can be associated with their extremely small size and high dispersity, so the synthesized PdNPs will be applicable for future industrial processes.

Full text of DOI:10.1039/c4nj01543f

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Ultrasonic-assisted green synthesis of palladium
nanoparticles and their nanocatalytic application
in multicomponent reaction†
Cite this:DOI: 10.1039/c4nj01543f
Nagaraj Basavegowda, Kanchan Mishra and Yong Rok Lee*
This paper describes a novel one pot synthesis of palladium nanoparticles (PdNPs) using a green method. An
aqueous extract of Perilla frutescens leaf was used as a bioreductant to reduce Pd2+ ions to Pd0 without any
extra surfactants or capping agents. Polyphenols and flavonoids are believed to be actively involved in
the reduction process. The formation of PdNPs was confirmed by UV-Vis spectra. Transmission electron
microscopy revealed spherical nanoparticles, ranging in size between 10 and 17 nm (average B13 nm).
X-ray diffraction revealed reflections from the (111), (200), (220), (311) and (222) planes confirming the
crystallinity of the nanoparticles with a face centred cubic (fcc) structure. Energy dispersive X-ray spectro-
scopy confirmed the presence of palladium. Fourier transform infrared spectroscopy suggested the role of
polyols present in the P. frutescens leaf extract. The synthesized nanoparticles were used as catalysts for
the synthesis of pyrazolylphosphonate derivatives in high yield. The excellent catalytic performance of
the synthesized PdNPs can be associated with their extremely small size and high dispersity, so the
synthesized PdNPs will be applicable for future industrial processes.
Received (in Montpellier, France)
10th September 2014,
Accepted 13th November 2014
DOI: 10.1039/c4nj01543f
www.rsc.org/njc
hydrogen storage10 and sensing applications11 owing to their
tendency to adsorb hydrogen. Several synthetic methods have
Introduction
Noble metal nanoparticles are gaining increasing importance been developed for the preparation of metal nanoparticles
because of their applications in diverse fields as well as their including chemical, physical, and biological techniques.12
prominent stability. PdNPs have a range of applications in the However, biological techniques have several advantages over
field of both homogeneous (high activity) and heterogeneous physical and chemical methods due to being simple, cost
(separation and reusability) catalysis.1 The much larger surface- effective, and environmentally friendly by using bacteria, fungi,
to-volume ratio of nanoparticles, providing many highly active yeast as well as plants.13 The rate of reduction of metal ions in
metal uncoordinated sites compared to their bulk counterparts, the process using a plant extract is more stable, and the rate of
has attracted considerable interest for potential catalytic appli- synthesis is faster than that of microorganisms.14 The organic
cations.2 PdNPs are particularly interesting because of their biomolecules like polysaccharides, amino acids, lipids, pro-
applicability to all arrays of palladium-catalysed reactions, such teins, and peptides are responsible for capping agents for the
as hydrogenation,3 Suzuki reaction,4 Heck reaction,5 C–N cou- synthesis of nanoparticles.15
pling,6 Stille coupling7 and in the oxidation of hydrocarbon.8
Moreover, the steady fabrication of the PdNPs is necessary
The size effects and shape of nanoparticles play crucial roles in for their exploitation, which remains an area of active research.
the catalytic performance, which are strongly dependent on the The synthesis of PdNPs has been revealed in several papers
crystallographic planes that abort the nanoparticle surface.9 using the plant leaf extracts of Pulicaria glutinosa(d), Glycine max
Such catalysts have been used because of their excellent activity (soya bean),16 Cinnamomum camphora,17 and Gardenia jasminoides.18
and high selectivity owing to their small size (1–100 nm) and In the green synthesis of metal nanoparticles, the reduction rate of
exposed active metallic or organometallic compounds. In metal ions by plants is much faster with the aid of ultrasonication.19
addition to their catalytic applications, they can be used in
Perilla frutescens L. (Lamiaceae) is cultivated in East Asia,
and both the seed and leaves are used as ingredients in Korean
cuisine. This plant is commonly known for its antiallergic,
antitumor, and antioxidant properties.20 Recently, our group
synthesized gold and silver nanoparticles in an aqueous
solution using the P. frutescens leaf extract.21 Organophos-
phorus compounds continue to attract considerable attention
School of Chemical Engineering, Yeungnam University, Gyeongsan 712-749,
Republic of Korea. E-mail: yrlee@yu.ac.kr; Fax: +82-53-810-4631;
Tel: +82-53-810-2529
† Electronic supplementary information (ESI) available: Synthesis and character-
ization (UV and XRD) of PdNPs using naringenin and gallic acid. See DOI:
10.1039/c4nj01543f
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necessary to cap the nanoparticles was analysed by FT-IR analysis
(JASCO FT-IR spectrophotometer at the range of 400–4000 cmÀ1).
The morphology and size of the synthesized particles were
analysed by FE-TEM (FEI Tecnai G2 F20 ST operated at 200 kV,
a point resolution of 0.24 nm and a Cs of 1.2 mm) and coupled
with EDAX. The TGA experiments were performed to monitor the
% weight loss of the nanoparticles, using TG-DTA, SDT-Q600
V20.5 Build 15, heated under a N2 atmosphere from room
Scheme 1 Schematic diagram of the green synthesis of PdNPs using an
aqueous extract of P. frutescens leaf and their catalytic activity for multi-
component reaction.
temperature to 1000 1C at a heating rate of 10 1C minÀ1
.
Catalytic activity of PdNPs
because of their range of biological activities.22 Phosphonyl
compounds have received wide attention in modern medicinal
chemistry. They are ideal for use in drug design owing to their
good bioactivity,23 low toxicity, and the ease of substitution
with conventional heterocyclic ring groups.24 We report the eco-
friendly synthesis of PdNPs using P. frutescens and their cata-
lytic applications in multi component reaction, as illustrated in
Scheme 1.
The synthesized PdNPs were used as catalysts for the synthesis
of variety of pyrazolylphosphonates with multi substituents on
the pyrazolone ring.
General procedure for the synthesis of compounds 4a–4j.
To a mixture of 3-methyl-1-phenyl-5-pyrazolone (1.0 mmol,
174.2 mg), arylaldehyde (1.2 mmol), and triethylphosphite
(2.0 mmol, 0.347 mL) in ethanol (10 mL) were added the PdNPs
(5 mol%, 5.321 mg) at room temperature. The reaction mixture
was stirred at room temperature for 6–8 h. The progress of the
reaction was monitored by TLC. After completion of the reac-
tion, the solvent was removed using a reduced pressure eva-
porator. The residue was purified by column chromatography
on silica gel to give the product.
Experimental section
Chemicals
Palladium chloride (PdCl2), pyrazolone derivatives, benzaldehyde
derivatives, and triethyl phosphite were purchased from Sigma-
Aldrich and used as received. The leaves of P. frutescens are widely
available in Asian countries. Air dried leaves of P. frutescens were
obtained at a local market in Yeongchon, South Korea.
Characterization data
Diethyl[(5-hydroxy-3-methyl-1-phenyl-1H-pyrazol-4-yl)(phenyl)-
methyl]phosphonate (4a). Yield: 93%, yellow solid: m.p. 71–73 0C;
1H-NMR (300 MHz, CDCl3) d 11.45 (1H, s), 7.67 (2H, d, J = 7.8 Hz),
7.27–7.22 (4H, m), 7.18–7.06 (4H, m), 4.06 (1H, d, J = 27.9 Hz),
3.98–3.90 (2H, m), 3.76–3.68 (1H, m), 3.43–3.35 (1H, m), 1.98
(3H, s), 1.10 (3H, t, J = 7.1 Hz), 0.89 (3H, t, J = 7.1 Hz); 13C-NMR
(75 MHz, CDCl3) d 151.2 (d, Jc-p = 5.0 Hz), 148.0 (d, Jc-p = 12.0 Hz),
139.2, 136.4 (d, Jc-p = 3.8 Hz), 133.1, 130.1, 129.1 (d, Jc-p = 6.0 Hz),
128.9, 128.4, 127.7 (d, Jc-p = 3.0 Hz), 125.9, 122.0, 93.9, 64.5
(d, Jc-p = 5.0 Hz), 63.7 (d, Jc-p = 7.5 Hz), 41.2 (d, Jc-p = 135.8 Hz), 16.5
(d, Jc-p = 6.0 Hz), 16.2 (d, Jc-p = 5.3 Hz), 12.8; IR (KBr) 2989, 2911,
2733, 2602, 1720, 1593,1517,1503, 1449, 1418,1360, 1193, 1174,
1027, 978, 762, 698, 562 cmÀ1; HRMS: m/z [M+] calcd for
C21H25N2O4P: 400.1552; found: 400.1553.
Preparation of the P. frutescens leaf extract
The dried leaves were first ground to a fine powder. 6 g of
crushed powder was added to 300 mL of deionized Milli-Q
water followed by boiling for 10 min. The broth solution
obtained was filtered through a 0.2 mm filter paper and stored
at 4 1C for further use.
Biosynthesis of PdNPs using the aqueous P. frutescens leaf
extract
A volume of 5 mL of the P. frutescens extract (about 20 mg after
drying) was mixed with 50 mL of an aqueous solution of 2 mM
palladium chloride (PdCl2). The synthesis of PdNPs with the
P. frutescens leaf extract was carried out by sonicating the
solution for 2 h at 60 1C in an ultrasonic bath (Fischer Scientific,
FS-60), operated at 50 Hz with a maximum power of 260 W.
Diethyl[(5-hydroxy-3-methyl-1-phenyl-1H-pyrazol-4-yl)(4-methoxy-
phenyl)methyl]phosphonate (4b). Yield: 95%, light yellow solid:
m.p. 4300 1C; 1H-NMR (300 MHz, CDCl3) d 7.74 (2H, d, J =
8.7 Hz), 7.35–7.23 (4H, m), 7.16–7.11 (1H, m), 6.76 (2H, d, J =
8.7 Hz), 4.09 (1H, d, J = 27.6 Hz), 4.02–3.96 (2H, m), 3.84–3.76
Characterization of PdNPs
The reduction of palladium ions by the P. frutescens leaf extract (1H, m), 3.67 (3H, s), 3.51–3.45 (1H, m), 2.05 (3H, s), 1.16 (3H, t, J =
was monitored by the changes in colour. The absorption 7.1 Hz), 1.00 (3H, t, J = 7.1 Hz); 13C-NMR (75 MHz, CDCl3) d 162.4,
spectrum of this solution was recorded using a UV-visible 159.1 (d, Jc-p = 3.1 Hz), 151.0 (d, Jc-p = 4.9 Hz),147.9, 147.7, 139.0,
spectrometer (Optizen 3220, Double beam) at wavelengths 130.1 (d, Jc-p = 5.9 Hz), 128.9, 128.1 (d, Jc-p = 4.7 Hz), 125.8, 121.8,
between 300 and 600 nm. The spectra were recorded using 114.2 (d, Jc-p = 2.5 Hz), 94.2 (d, Jc-p = 4.1 Hz), 64.5 (d, Jc-p = 7.4 Hz),
deionized Milli-Q water as a blank, and the data were plotted. 63.5 (d, Jc-p = 7.5 Hz), 55.3, 40.0 (d, Jc-p = 136.5 Hz), 16.4 (d, Jc-p
The crystallinity of the nanoparticles was determined by powder 5.6 Hz), 16.2 (d, Jc-p = 5.8 Hz), 12.7; IR (KBr) 2986, 2911, 2838, 2745,
XRD (PANalytical X’Pert MRD at 30 kV, 40 mA, with Cu Ka 2623, 1722, 1595, 1511, 1252, 1178, 1029, 972, 757, 695, 573 cmÀ1
(1.5418 Å) radiation). The presence of bioactive molecules HRMS: m/z [M+] calcd for C22H27N2O5P: 430.1658; found: 430.1659.
=
;
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Diethyl[(5-hydroxy-3-methyl-1-phenyl-1H-pyrazol-4-yl)(3-methoxy- 63.6 (d, Jc-p = 7.5 Hz), 35.6 (d, Jc-p = 137.0 Hz), 16.5 (d, Jc-p
=
phenyl)methyl]phosphonate (4c). Yield: 93%, light yellow solid: 5.5 Hz), 15.7 (d, Jc-p = 5.6 Hz), 13.0; IR (KBr) 3058, 2983, 2911,
m.p. 4300 1C; 1H-NMR (600 MHz, CDCl3) d 7.79 (2H, d, J = 1593, 1500, 1404, 1202, 1027, 966, 776, 696, 535 cmÀ1; HRMS:
7.8 Hz), 7.38 (2H, t, J = 7.8 Hz), 7.21–7.18 (2H, m), 6.99–6.97 m/z [M+] calcd for C25H27N2O4P: 450.1708; found: 450.1709.
(2H, m), 6.77 (1H, d, J = 7.8 Hz), 4.18 (1H, d, J = 27.6 Hz), 4.11–
Diethyl[(5-hydroxy-3-methyl-1-(p-tolyl)-1H-pyrazol-4-yl)(phenyl)-
4.01 (2H, m), 3.91–3.85 (1H, m), 3.74 (3H, s), 3.59–3.53 (1H, m), methyl]phosphonate (4g). Yield: 94%, light yellow solid: m.p.
1
2.12 (3H, s), 1.22 (3H, t, J = 7.1 Hz), 1.05 (3H, t, J = 7.1 Hz); 13C- 73–75 1C; H-NMR (300 MHz, CDCl3) d 7.59 (2H, d, J = 8.1 Hz),
NMR (150 MHz, CDCl3) d 159.9 (d, Jc-p = 3.8 Hz), 151.1, 147.8 7.33 (2H, d, J = 6.6 Hz), 7.25–7.12 (5H, m), 4.14 (1H, d, J = 27.9 Hz),
(d, Jc-p = 20.2 Hz), 139.0, 137.7 (d, Jc-p = 5.6 Hz), 129.8, 128.7 4.05–3.94 (2H, m), 3.85–3.72 (1H, m), 3.52–3.84 (1H, m), 2.27
(d, Jc-p = 31.2 Hz), 125.8, 121.9, 121.3 (d, Jc-p = 11.0 Hz), 114.8 (3H, s), 2.06 (3H, s), 1.17 (3H, t, J = 7.1 Hz), 0.96 (3H, t, J = 7.1 Hz);
(d, Jc-p = 11.0 Hz), 112.9 (d, Jc-p = 5.6 Hz), 93.9, 64.5 (d, Jc-p
12.8 Hz), 63.6 (d, Jc-p = 11.0 Hz), 55.3, 41.0 (d, Jc-p = 217.2 Hz), 16.3 12.4 Hz), 136.4, 136.4, 136.3, 135.7, 130.0, 129.4, 129.0 (d, Jc-p
=
13C-NMR (75 MHz, CDCl3) d 151.0 (d, Jc-p = 5.3 Hz), 147.5 (d, Jc-p
=
=
(d, Jc-p = 5.6 Hz), 16.1 (d, Jc-p = 5.8 Hz), 12.7; IR (KBr) 2983, 2921, 6.1 Hz), 128.9 (d, Jc-p = 2.5 Hz), 127.6 (d, Jc-p = 3.2 Hz), 122.1, 93.8
2839, 2747, 2623, 1722, 1595, 1495, 1450, 1261, 1175, 1030, 971, (d, Jc-p = 4.5 Hz), 64.4 (d, Jc-p = 7.4 Hz), 63.6 (d, Jc-p = 7.4 Hz), 41.0
757, 694, 545 cmÀ1; HRMS: m/z [M+] calcd for C22H27N2O5P: (d, Jc-p = 135.9 Hz), 21.1, 16.4 (d, Jc-p = 5.6 Hz), 16.1 (d, Jc-p = 5.7 Hz),
430.1658; found: 430.1655.
12.6; IR (KBr) 2985, 2925, 2754, 2633, 1715, 1583, 1518, 1449,
Diethyl[(5-hydroxy-3-methyl-1-phenyl-1H-pyrazol-4-yl)(4-nitro- 1416, 1365, 1217, 1182, 1028, 973, 819, 562 cmÀ1; HRMS: m/z [M+]
phenyl)methyl]phosphonate (4d). Yield: 88%, light yellow solid: calcd for C22H27N2O4P: 414.1708; found: 414.1709.
1
m.p. 4300 1C; H-NMR (300 MHz, CDCl3) d 11.42 (1H, s), 7.66
Diethyl[(5-hydroxy-3-methyl-1-(4-nitrophenyl)-1H-pyrazol-4-yl)-
(2H, d, J = 7.8 Hz), 7.24 (2H, t, J = 7.8 Hz), 7.17–7.03 (5H, m), 4.05 (phenyl)methyl]phosphonate (4h). Yield: 89%, light yellow solid:
(1H, d, J = 27.9 Hz), 3.98–8.3.85 (2H, m), 3.75–3.67 (1H, m), 3.42– m.p. 4300 1C; 1H-NMR (300 MHz, CDCl3) d 8.19 (2H, d, J =
3.33 (1H, m), 1.97 (3H, s), 1.09 (3H, t, J = 7.1Hz), 0.88 (3H, t, J = 9.0 Hz), 8.07 (2H, d, J = 9.0 Hz), 7.34–7.19 (5H, m), 4.17–3.99
6.9 Hz); 13C-NMR (75 MHz, CDCl3) d 162.5, 151.2,147.9 (d, Jc-p
=
(3H, m), 3.84–3.76 (1H, m), 3.50–3.41 (1H, m), 2.05 (3H, s), 1.21
12.5 Hz), 139.1, 136.4 (d, Jc-p = 4.3 Hz), 129.1 (d, Jc-p = 6.1 Hz), (3H, t, J = 7.1 Hz), 0.97 (3H, t, J = 7.1 Hz); 13C-NMR (75 MHz,
128.9, 128.9, 127.6 (d, Jc-p = 3.1 Hz), 125.9, 121.9, 93.9, 64.5 CDCl3) d 152.3 (d, Jc-p = 8.5 Hz), 150.2, 150.1, 144.5 (d, Jc-p
(d, Jc-p = 7.4 Hz), 63.7 (d, Jc-p = 7.4 Hz), 41.1 (d, Jc-p = 135.8 Hz), 10.9 Hz), 135.9 (d, Jc-p = 7.1 Hz), 131.9, 129.1, 129.1, 129.0, 128.0
16.4 (d, Jc-p = 5.6 Hz), 16.1 (d, Jc-p = 5.8 Hz), 12.8; IR (KBr) 2807, (d, Jc-p = 5.0 Hz), 124.8, 120.5, 95.2 (d, Jc-p = 7.0 Hz), 64.9 (d, Jc-p
=
=
2747, 2625, 1722, 1593, 1499, 1175, 1027, 970, 757, 697, 12.0 Hz), 63.7 (d, Jc-p = 12.1 Hz), 41.1 (d, Jc-p = 218.0 Hz), 16.4
562 cmÀ1; HRMS: m/z [M+] calcd for C21H24N3O6P: 445.1403 (d, Jc-p = 9.1 Hz), 16.2 (d, Jc-p = 9.0 Hz), 12.9; IR (KBr) 2983, 2923,
found: 445.1403.
2720, 2542, 1718, 1592, 1514, 1334, 1172, 1025, 979, 852, 748,
Diethyl[(2-chlorophenyl)(5-hydroxy-3-methyl-1-phenyl-1H-pyrazol- 565 cmÀ1; HRMS: m/z [M+] calcd for C21H24N3O6P: 445.1403;
4-yl)methyl]phosphonate (4e). Yield: 95%, light yellow solid: m.p. found: 445.1405.
4300 1C; 1H-NMR (300 MHz, CDCl3) d 11.64 (1H, s), 7.66 (2H, d, J =
Diethyl[(1-(4-chlorophenyl)-5-hydroxy-3-methyl-1H-pyrazol-4-
7.5 Hz), 7.52–7.50 (1H, m), 7.26 (3H, m), 7.06 (3H, m), 4.84 (1H, d, J = yl)(phenyl)methyl]phosphonate (4i). Yield: 96%, light yellow
1
27.9 Hz), 4.00–3.94 (2H, m), 3.75–3.67 (1H, m), 3.47–3.42 (1H, m), solid: m.p. 4300 1C; H-NMR (300 MHz, CDCl3) d 11.69 (1H, s),
1.99 (3H, s), 1.11 (3H, t, J = 6.9 Hz), 0.87 (3H, t, J = 6.9 Hz); 13C-NMR 7.73 (2H, d, J = 8.4 Hz), 7.34–7.15 (7H, m), 4.13 (1H, d, J = 27.9 Hz),
(75 MHz, CDCl3) d 151.4, 148.2 (d, Jc-p = 12.5 Hz), 139.1, 134.8 4.07–3.96 (2H, m), 3.83–3.75 (1H, m), 3.39–3.41 (1H, m), 2.05
(d, Jc-p = 3.7 Hz), 133.4 (d, Jc-p = 8.1 Hz), 131.1 (d, Jc-p = 4.5 Hz), 129.5 (3H, s), 1.19 (3H, t, J = 7.1 Hz), 0.98 (3H, t, J = 7.1 Hz); 13C-NMR
(d, Jc-p = 2.3 Hz), 128.9, 128.8, 127.8 (d, Jc-p = 3.1 Hz), 125.9, 121.9, (75 MHz, CDCl3) d 151.0, 148.1 (d, Jc-p = 19.3 Hz), 137.5, 135.9
93.8, 64.5 (d, Jc-p = 7.6 Hz), 63.9 (d, Jc-p = 7.4 Hz), 36.6 (d, Jc-p
=
(d, Jc-p = 7.0 Hz), 130.8, 129.8 128.8, 128.7, 128.7, 128.2, 127.5
137.6 Hz), 16.4 (d, Jc-p = 5.6 Hz), 16.1 (d, Jc-p = 5.6 Hz), 12.9; IR (KBr) (d, Jc-p = 4.4 Hz), 122.6, 93.9, 64.4 (d, Jc-p = 12.2 Hz), 63.4 (d, Jc-p
=
3065, 2985, 2926, 1725, 1593,1501, 1447, 1412, 1179, 1028, 971, 11.5 Hz), 40.8 (d, Jc-p = 217.0 Hz), 16.2 (d, Jc-p = 8.8 Hz), 15.9
751, 692, 566 cmÀ1; HRMS: m/z [M+] calcd for C21H24ClN2O4P: (d, Jc-p = 9.7 Hz), 12.5; IR (KBr) 2991, 2912, 2723, 2609, 1592, 1497,
434.1162; found: 434.1163.
Diethyl[(5-hydroxy-3-methyl-1-phenyl-1H-pyrazol-4-yl)(naph- calcd for C21H24ClN2O4P: 434.1162; found: 434.1164.
1447, 1412, 1171, 1023, 979, 833, 795, 562 cmÀ1; HRMS: m/z [M+]
thalen-1-yl)methyl]phosphonate (4f). Yield: 90%, light yellow
Diethyl[(5-hydroxy-3-methyl-1-(4-nitrophenyl)-1H-pyrazol-4-yl)
solid: m.p. 4300 1C; 1H-NMR (300 MHz, CDCl3) d 11.90 (1H, s), (3-methoxyphenyl) methyl]phosphonate (4j). Yield: 93%, light
1
8.25 (1H, d, J = 8.4 Hz), 7.90 (3H, d, J = 7.8 Hz), 7.82 (2H, m), yellow solid: m.p. 4300 1C; H-NMR (600 MHz, CDCl3) d 12.19
7.64 (1H, t, J = 7.4 Hz), 7.57–7.43 (4H, m), 7.29–7.24 (1H, m), (1H, s), 8.25 (2H, d, J = 8.8 Hz), 8.11 (2H, d, J = 8.8 Hz), 7.24–7.21
5.25 (1H, d, J = 27.9 Hz), 4.26–4.11 (2H, m), 3.71–3.58 (1H, m), (1H, m), 6.97–6.94 (2H, m), 6.79 (1H, d, J = 8.4 Hz), 4.17–4.07
3.10–2.97 (1H, m), 2.09 (3H, s), 1.35 (3H, t, J = 6.9 Hz), 0.62 (3H, m), 3.90–3.86 (1H, m), 3.76 (3H, s), 3.56–3.52 (1H, m), 2.12
(3H, t, J = 6.9 Hz); 13C-NMR (75 MHz, CDCl3) d 151.7 (d, Jc-p
5.5 Hz), 148.1 (d, Jc-p = 12.5 Hz), 139.3, 134.4, 132.4 (d, Jc-p
=
=
(3H, s), 1.25 (3H, t, J = 7.0 Hz), 1.06 (3H, t, J = 7.0 Hz); 13C-NMR
(150 MHz, CDCl3) d 160.1, 152.2,150.1, 150.0, 144.5 (d, Jc-p
=
5.3 Hz), 131.5 (d, Jc-p = 5.8 Hz), 129.1 (d, Jc-p = 26.0 Hz), 128.4 5.6 Hz), 137.3 (d, Jc-p = 7.4 Hz), 130.0 (d, Jc-p = 3.6 Hz), 124.8, 121.4
(d, Jc-p = 3.8 Hz), 126.6, 126.0 (d, Jc-p = 3.9 Hz), 125.9 (d, Jc-p (d, Jc-p = 9.4 Hz), 120.4, 115.0 (d, Jc-p = 9.4 Hz), 113.0, 95.1 (d, Jc-p
3.2 Hz), 123.0, 122.0, 94.6 (d, Jc-p = 4.4 Hz), 64.5 (d, Jc-p = 5.3 Hz), 7.2 Hz), 65.0 (d, Jc-p = 11.0 Hz), 63.7 (d, Jc-p = 11.0 Hz), 55.4, 41.0
=
=
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(d, Jc-p = 217.2 Hz), 16.4 (d, Jc-p = 9.4 Hz), 16.2 (d, Jc-p = 9.2 Hz),
12.9; IR (KBr) 2987, 2928, 2834, 2615, 1593, 1518, 1338, 1256,
1189, 1014, 856, 761, 690, 546 cmÀ1; HRMS: m/z [M+] calcd for
C22H26N3O7P: 475.1508; found: 475.1508.
Results and discussion
Characterization of PdNPs
The synthesis of PdNPs using the P. frutescens leaf extract was
initially monitored visually and then by UV-visible spectroscopy
(Fig. 1). During visual monitoring, the colour of the reaction
mixture changed gradually from yellowish into dark brown
within 2 h after sonication at 60 1C indicating the formation
of PdNPs. In UV-visible spectroscopy, the PdCl2 solution
showed a distinct peak approximately at 425 nm indicating
the existence of Pd2+ ions. During the formation of PdNPs, the
peak of the Pd2+ ions present at 425 nm began to disappear and
vanished within 2 h. Generally PdNPs do not show any distinct
peak due to the surface plasmon.25 This is because of the
reduction of Pd2+ ions to Pd0. Similarly, Kesarla et al.26 and
Khan et al.27 reported that the disappearance of an absorption
band at approximately 425 nm confirmed the complete
reduction of Pd(II) to PdNPs. In order to find active components
of the reduction process, some control experiments were carried
out. In the presence of naringenin of flavonoid or gallic acid of
polyphenol as a reducing reagent, PdNPs were synthesized.
These results indicated that flavonoid and polyphenol were
responsible to reduce Pd2+ ions to Pd0 (Fig. S1 and S2, the ESI†).
The morphology and size of the PdNPs produced were
analysed by transmission electron microscopy. Aliquots of the
Pd nanoparticle solutions were placed on a carbon coated
copper grid and allowed to dry under ambient conditions.
The TEM images were recorded at different magnifications.
Fig. 2a shows that PdNPs were almost spherical with a few
irregularly shaped particles due to the agglomeration of small
PdNPs dispersed over a thin layer of bioorganic compounds.
Fig. 2b shows a particle size distribution histogram. Fig. 2c
presents a magnified overview image of a single particle and
the inset shows the selected area electron diffraction (SAED)
patterns corresponding to the (111), (200), (220), (311), and
Fig. 2 TEM images of the PdNPs (a) scale bar at 20 nm, (b) size distribu-
tion histogram, (c) 5 nm (inset: SAED pattern), and (d) EDAX results showing
the formation of PdNPs.
(222) face centred cubic planes, which confirmed the crystalline
nature of PdNPs. The size of the PdNPs ranged from 10 to
17 nm with a mean of B13 nm. Energy dispersive X-ray analysis
(EDAX) revealed a strong signal in the palladium region and
confirmed the formation of PdNPs (Fig. 2d), which showed a
typical optical absorption peak at B2.8 keV because of surface
plasmon resonance.28 The C and Cu peaks were probably due to
the carbon coated copper grid used for sample preparation.
Fig. 3 presents XRD patterns of the synthesized PdNPs. The
scanning range selected was between 201 and 901 2y. The XRD
peaks were observed at 39.921, 46.431, 67.771, 81.651, and 86.131
2y, which were assigned to the (111), (200), (220), (311), and
(222) Bragg’s reflections of face-centred PdNPs, which are
consistent with JCPDS (no. 05-0681). A few additional sharp
unassigned peaks at 27.871 and 32.301 2y were also observed. These
were attributed to the crystallization of other bioorganic compounds
on the surfaces of PdNPs, which agreed with a previous report using
the Delonix regia leaf extract.28 Similar unassigned peaks were found
for silver nanoparticles synthesized using a Pelargonium graveolens
leaf extract.29 Interestingly, the XRD patterns of PdNPs synthesized
Fig. 1 UV-visible spectra and the inset of (a) PdCl2 solution, and (b)
biosynthesized PdNPs.
Fig. 3 XRD patterns of PdNPs.
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This suggests that bioactive molecules were capped on the
PdNPs and were degraded completely at high temperatures,
leaving a residual mass of 46.74%.
Catalytic activity
Recently, it was reported that the nanoparticles possess
complex hybrid structures, so called multiple-species catalysis,
which facilitate the progress of organic reactions.31 Impor-
tantly, three-component reactions of pyrazoles with aromatic
aldehydes and triethyl phosphite for pyrazolylphosphonate deri-
vatives using nanoparticles as multiple-species catalysis have not
been developed so far. To assess the possible applications of
PdNPs as efficient catalysts, the synthesized PdNPs were used next
for the preparation of a variety of pyrazolylphosphonate deriva-
tives. To examine a multi-component reaction for the synthesis of
pyrazolylphosphonates, reactions of pyrazolone derivatives with
several arylaldehydes and triethylphosphite in the presence of
PdNPs in ethanol at room temperature for 6–8 h were carried out.
The results are summarized in Table 1. Treatment of 1
(3-methyl-1-phenyl-5-pyrazolone) with benzaldehyde and triethyl-
phosphite in the presence of 5.0 mol% of PdNPs for 6 h gave
compound 4a in 93% yield, whereas that with p-anisaldehyde and
m-anisaldehyde afforded the desired product 4b and 4c in 95%
and 93% yield, respectively. Similarly, reaction of 1 (3-methyl-1-
phenyl-5-pyrazolone) with 4-nitrobenzaldehyde provided 4d in
88% yield and that with o-chloro benzaldehyde gave 4e in 95%
yield. Interestingly, with 1-naphthaldehyde, the desired product
4f was isolated in 90% yield. The pyrazolones with an electron
Fig. 4 FT-IR spectra of the P. frutescens leaf extract and PdNPs synthe-
sized from the P. frutescens leaf extract.
by naringenin or gallic acid showed less crystallinity compared
to the nanoparticles synthesized using the P. frutescens extract
(Fig. S3, the ESI†).
FT-IR spectroscopy was carried out to identify possible
bioactive molecules like polyphenols and flavonoids30 in the
P. frutescens leaf extract responsible for the reduction and
stabilization of PdNPs. The spectrum of the P. frutescens leaf
extract revealed absorption bands at 3402 and 1613 cmÀ1
,
representing the O–H and CQC stretching in polyols or flavo-
noids, respectively (Fig. 4). The vibrations at 1070 and 1395 cmÀ1
were assigned to C–O stretching and the C–H bending of polyols
or flavonoids. After the bioreduction of PdCl2 by the leaf extract,
some changes were noticed in positions and the magnitude of
the stretching vibrations, indicating the participation of poly-
phenols or flavonoids. The band at 3403 cmÀ1 indicated the O–H
group of polyols participating in the bioreduction process. The
small peaks at 2929 and 1717 cmÀ1 were attributed to C–H and
CQO stretching vibrations. The peaks at 1068 cmÀ1 were
assigned to the C–O stretching vibration in polyols or flavonoids.
TGA characterization was performed at a heating rate of
10 1C minÀ1 in a N2 atmosphere. The TGA trace of PdNPs
(Fig. 5) revealed three stages of weight loss from room tem-
perature to 1000 1C. The first stage weight loss at B30–150 1C
was 6.88%. The weight loss was found to be 33.13% at B150–
650 1C. Finally, at B650–950 1C, the weight loss was 13.25%.
Table 1 Synthesis of various pyrazolylphosphonates 4a–4j using PdNPs
as catalystsa
a
Reaction conditions: 1 (1.0 mmol), 2 (1.2 mmol), 3 (2.0 mmol), PdNPs
Fig. 5 TGA traces of the PdNPs.
(5.0 mol%) in ethanol (10 mL) rt for 6–8 h.
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Spherical shaped palladium nanoparticles were prepared by
a simple, inexpensive, and environmentally benign method
using the Perilla frutescens leaf extract as a bioreductant. The
synthetic method was highly crystalline without using any
harmful reducing or capping agents and also overcame the
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This work was supported by Priority Research Centers Program
through the National Research Foundation of Korea (NRF)
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