Orthopalladated complexes of phosphorus ylide: Poly(N-vinyl-2-pyrrolidone)-stabilized palladium nanoparticles as reusable heterogeneous catalyst for Suzuki and Heck cross-coupling reactions

Article abstract of DOI:10.1002/aoc.3768

The phosphorus ylide [Ph₃PCHC(O)C₆H₄-NO₂–4] reacted with Pd(OAc)₂ to give the C,C-orthometallated complex [Pd{κ²(C,C)-C₆H₄PPh₂C(H)CO(C₆H₄-NO₂–4)}(μ-OAc)]₂, which underwent bridge exchange reaction with NaN₃, NaCl, KBr and KI, respectively, to afford the binuclear C,C-orthopalladated complexes [Pd{κ²(C,C)-C₆H₄PPh₂C(H)CO(C₆H₄-NO₂–4)}(μ-X)]₂ (X?=?N₃ (1), Cl (2), Br (3) and I (4)). The complexes were identified using spectroscopy (infrared and NMR), CHNS technique and single-crystal X-ray structure analysis. Thereafter, palladium nanoparticles with narrow size distribution were easily prepared using the refluxing reaction of iodo-bridged orthopalladated complex 4 with poly(N-vinyl-2-pyrrolidone) (PVP) as the protecting group. The PVP-stabilized palladium nanoparticles were characterized using a variety of techniques including X-ray diffraction, transmission and scanning electron microscopies, energy-dispersive X-ray spectroscopy, inductively coupled plasma analysis and Fourier transform infrared spectroscopy. The catalytic activity of the PVP-stabilized palladium nanoparticles was evaluated in the Suzuki reaction of phenylboronic acid and the Heck reaction of styrene with aryl halides of varying electron densities. This catalyst exhibited excellent catalytic activity for Suzuki cross-coupling reactions in ethanol–water. Notably, aryl chlorides which are cheaper and more accessible than their bromide and iodide counterparts also reacted satisfactorily using this catalyst. After completion of reactions, the catalyst could be separated using a simple method and used many times in repeat cycles without considerable loss in its activity.

Full text of DOI:10.1002/aoc.3768

Received: 12 November 2016
DOI: 10.1002/aoc.3768
Revised: 2 January 2017
Accepted: 21 January 2017
F U L L PA P E R
Orthopalladated complexes of phosphorus ylide: Poly
(N‐vinyl‐2‐pyrrolidone)‐stabilized palladium nanoparticles as
reusable heterogeneous catalyst for Suzuki and Heck
cross‐coupling reactions
Kazem Karami1
| Sedigheh Abedanzadeh1 | Omolbanin Vahidnia1 | Pablo Herves2 |
Janusz Lipkowski3 | Krzysztof Lyczko4
1 Department of Chemistry, Isfahan
University of Technology, Isfahan 84156/
83111, Iran
The phosphorus ylide [Ph3PCHC(O)C6H4‐NO2–4] reacted with Pd(OAc)2 to give
the C,C‐orthometallated complex [Pd{κ2(C,C)‐C6H4PPh2C(H)CO(C6H4‐NO2–4)}
(μ‐OAc)]2, which underwent bridge exchange reaction with NaN3, NaCl, KBr and
KI, respectively, to afford the binuclear C,C‐orthopalladated complexes [Pd{κ2(C,
C)‐C6H4PPh2C(H)CO(C6H4‐NO2–4)}(μ‐X)]2 (X = N3 (1), Cl (2), Br (3) and I
(4)). The complexes were identified using spectroscopy (infrared and NMR), CHNS
technique and single‐crystal X‐ray structure analysis. Thereafter, palladium nano-
particles with narrow size distribution were easily prepared using the refluxing reac-
tion of iodo‐bridged orthopalladated complex 4 with poly(N‐vinyl‐2‐pyrrolidone)
(PVP) as the protecting group. The PVP‐stabilized palladium nanoparticles were
characterized using a variety of techniques including X‐ray diffraction, transmission
and scanning electron microscopies, energy‐dispersive X‐ray spectroscopy,
inductively coupled plasma analysis and Fourier transform infrared spectroscopy.
The catalytic activity of the PVP‐stabilized palladium nanoparticles was evaluated
in the Suzuki reaction of phenylboronic acid and the Heck reaction of styrene with
aryl halides of varying electron densities. This catalyst exhibited excellent catalytic
activity for Suzuki cross‐coupling reactions in ethanol–water. Notably, aryl
chlorides which are cheaper and more accessible than their bromide and iodide
counterparts also reacted satisfactorily using this catalyst. After completion of
reactions, the catalyst could be separated using a simple method and used many
times in repeat cycles without considerable loss in its activity.
2 Departamento de Quimica Fisica,
Universidade de Vigo, 36310 Vigo, Spain
3 Cardinal Stefan Wyszynski University in
Warsaw, Faculty of Mathematical and
Natural Sciences, Wojcickiego 1/302‐093
Warszawa, Poland
4 Institute of Nuclear Chemistry and
Technology, Dorodna 16, Warszawa, Poland
Correspondence
Kazem Karami, Department of Chemistry,
Isfahan University of Technology, Isfahan
84156/83111, Iran.
Email: karami@cc.iut.ac.ir
KEYWORDS
Heck, orthopalladated complex, phosphorus ylide, PVP, Suzuki
1 | INTRODUCTION
first report of the use of a palladacycle for the Suzuki
cross‐coupling reaction,[9] a broad diversity of novel
Palladium complexes are widely used in Suzuki and Heck
cross‐coupling reactions which are well‐established method-
ologies in multiple organic transformations.[1–8] Since the
palladacycles have been effectively employed in order to
study their catalytic activities.[10–15] Cyclopalladated com-
plexes have widespread applications in organic synthesis,
Appl Organometal Chem. 2017;e3768.
wileyonlinelibrary.com/journal/aoc
Copyright © 2017 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.3768

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KARAMI ET AL.
1
materials science, photochemistry and the pharmaceutical
industry.[16–23] Catalytic activity of palladium complexes
can be tuned by the use of ligands such as phosphines,
amines, carbenes, dibenzylideneacetone, etc. Complexes
containing phosphorous ligands are very often used as
catalyst precursors in cross‐coupling reactions.[24,25]
Electron‐rich, bulky phosphines and phosphine oxides have
been reported to be successful ligands.[26–30] Among the sev-
eral classes of phosphine ligands described in the literature
for palladium‐catalysed coupling reactions, we became inter-
ested in the phosphorus ylides which are versatile ligands for
a wide variety of transition metals due to their ambidentate
character.[31–33] With the aim of expanding the scope of this
type of orthometallated derivative and owing to our continu-
ing interest in the synthesis of new palladacycles and devel-
oping new catalytic systems,[34–38] we were encouraged to
study the synthesis and catalytic application of dinuclear
halo‐bridged orthopalladated complexes of phosphorus ylide.
In order to address the limited practical application of
homogeneous palladium catalysts, supported palladium
catalysts can be used with high dispersion and narrow size
distribution of palladium particles which would exhibit high
catalytic activity and recyclability with low palladium
loading.[39–44] Poly(N‐vinyl‐2‐pyrrolidone) (PVP) as a well‐
known polymeric protecting agent would provide stable and
catalytically active colloidal nanoparticles and prevent the
agglomeration of nanoparticles.[45,46] The catalytic activity
of a heterogeneous catalyst increases as well as decreases
with the size of nanoparticles according to the increase in
the fraction of important surface atoms effectively improvinf
the catalytic process.[47]
In the present paper, we report the preparation and
characterization of PVP‐stabilized palladium nanoparticles
derived from the palladacycle of general formula
[Pd2{κ2(C,C)‐C6H4(PPh2CHC(O)C6H5NO2}2(μ‐I)2], synthe-
sized using an alcohol reduction method which involves
refluxing a mixture of metal precursor and PVP in water–
ethanol mixture for 3 h.[48,49] The stabilizer concentration
and other chemicals present in the reaction medium would
essentially have an effect on the particle size, shape and cat-
alytic activity of palladium nanoparticles.[50–52] The applica-
tion of the prepared catalyst was investigated in the Suzuki
cross‐coupling reactions of phenylbronic acid and Heck
cross‐coupling reactions of styrene with various aryl halides
under milder reaction conditions compared to those used in
previous studies.
purification or drying. H NMR and 31P{1H} NMR spectra
were recorded in CDCl3 solutions at room temperature with
a 500 MHz Bruker spectrometer. Chemical shifts (δ) are
reported relative to internal tetramethylsilane and external
85% phosphoric acid. Fourier transform infrared (FT‐IR)
spectra were obtained in KBr pellets with a Jasco FT/IR
680 Plus instrument. Scanning electron microscopy (SEM)
studies were conducted with a Philips (model XL‐30) instru-
ment. Transmission electron microscopy (TEM) was carried
out with a Philips (model CM30) instrument. The palladium
content of the catalyst was measured using inductively
coupled plasma optical emission spectroscopy (ICP‐OES;
PerkinElmer 7300DV). Analytical TLC was performed using
Merck TLC silica gel 60 F254 glass plates. GC analyses were
performed using an Agilent Technologies 6890 N chromato-
graph equipped with a flame ionization detector and an HB
50+ column (length = 30 m, inner diameter = 320 μm, film
thickness = 0.25 μm). The temperature program for the GC
analysis was from 70 to 200 °C at 20 °C min−1, held at
200 °C for 0 min, heated from 200 to 280 °C at 10 °C min−1
and held at 280 °C for 1 min. The inlet and detector
temperatures were set at 260 and 280 °C, respectively.
Products were identified by comparison with authentic
samples. Phosphorus ylide (NO2BPPY) was obtained using
procedures described earlier.[53]
2.2 | Synthesis of binuclear Orthopalladated
complexes [Pd(μ‐X)(NO2BPPY)]2 (1–4)
Pd(OAc)2 (0.224 g, 1.0 mmol) was added to a solution of
phosphorus ylide 2‐bromo‐1‐(4′‐nitro‐[1,1′‐biphenyl]‐4‐yl)
ethanone (NO2BPPY; 0.425 g, 1.0 mmol) in CH2Cl2
(15 ml) and the resulting mixture was refluxed at 60 °C for
24 h. The solvent was then evaporated, filtered, washed with
water (10 ml) and Et2O (5 ml) and dried in vacuum. Dinu-
clear orthopalladated complex [Pd(μ‐OAc)(NO2BPPY)]2
(0.484 g, 0.5 mmol) was dissolved in MeOH (10 ml) and
anhydrous NaN3, NaCl, KBr or KI (excess) was added,
respectively, for the synthesis of complexes 1–4. The mixture
was stirred for 12 h at room temperature. A solid precipitated
which was filtered, washed with water (10 ml) and Et2O
(5 ml) and dried in vacuum.
2.2.1 | [Pd(μ‐N3)(NO2BPPY)]2 (1)
Yield 70%; m.p. 207 °C. FT‐IR (KBr, cm−1): ν(C═O) 1633,
ν(N═N═N) 2052, ν(P─C) 853. Anal. Calcd for
C52H38N8O6P2Pd2 (%): C, 54.52; H, 3.34; N, 9.78. Found
(%): C, 54.50; H, 3.32; N, 9.76. 1H NMR (500 MHz, CDCl3,
δ, ppm): 4.32 (br s, 1H, CHP), 4.37 (br s, 1H, CHP), 7.23–
8.07 (m, 36H, HAr). 31P{1H} NMR (202 MHz, CDCl3, δ,
ppm): 17.60, 17.65, 18.47, 18.77.
2 | EXPERIMENTAL
2.1 | Reagents and equipment
All the reactants were purchased from Aldrich or Merck and
used as received. Solvents were used without further

KARAMI ET AL.
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by filtration, washed with ether, filtered and dried at room
temperature in air. The catalyst was crushed and obtained as
a black powder.
2.2.2 | [Pd(μ‐Cl)(NO2BPPY)]2 (2)
Yield 77%; m.p. 199–205 °C (dec). FT‐IR (KBr, cm−1):
ν(C═O) 1635. Anal. Calcd for C52H38Cl2N2O6P2Pd2 (%):
C, 55.15; H, 3.38; N, 2.47. Found (%): C, 55.23; H,
1
2.5 | Typical procedure for Suzuki cross‐
3.32; N, 2.46. H NMR (500 MHz, CDCl3, δ, ppm): 4.83
coupling reaction
(s, CHP, minor), 4.95 (s, CHP, major), 7.24–8.15
(m, C6H5 + C6H4CO + C6H4, both isomers). 31P{1H}
NMR (202 MHz, CDCl3, δ, ppm): 19.86 (br s, both
isomers).
A round‐bottomed flask equipped with a condenser was
charged with aryl halide (1 mmol), phenylboronic acid
(1.5 mmol), Na2CO3 (3.0 mmol) and Pd catalyst (16.5 mg,
0.1 mol% of Pd) in 60% EtOH (10 ml). The reaction flask
was placed in an oil bath and the mixture was stirred and
heated at 60 °C for 4 h.
2.2.3 | [Pd(μ‐Br)(NO2BPPY)]2 (3)
Yield 62%; m.p. 208 °C. FT‐IR (KBr, cm−1): ν(C═O) 1627.
Anal. Calcd for C52H38Br2N2O6P2Pd2 (%): C, 51.13; H, 3.14;
N, 2.29. Found (%): C, 51.20; H, 3.11; N, 2.31%. H NMR
(500 MHz, CDCl3, δ, ppm): 4.96 (br s, 1H, CHP, minor),
5.13 (br s, 1H, CHP, major), 7.22–8.11 (m, 36H, HAr, both
isomers). 31P{1H} NMR (202 MHz, CDCl3, δ, ppm): 19.51
(s, CHP, minor), 20.08 (s, CHP, major).
1
2.6 | Typical procedure for heck cross‐
coupling reaction
A round‐bottomed flask equipped with a condenser was
charged with aryl halide (1 mmol), styrene (1.5 mmol), KI
(3.0 mmol) and Pd catalyst (0.1 mol% of Pd) in 10 ml of
dimethylformamide (DMF). The reaction flask was placed
in an oil bath and the mixture was stirred and heated at
100 °C for 5 h.
2.2.4 | [Pd(μ‐I)(NO2BPPY)]2 (4)
Yield 52%; m.p. 203 °C. FT‐IR (KBr, cm−1): ν(C═O) 1624.
Anal. Calcd for C52H38I2N2O6P2Pd2 (%): C, 58.83; H, 3.61;
N, 2.64. Found (%): C, 58.50; H, 3.60; N, 2.66%. H NMR
(500 MHz, CDCl3, δ, ppm): 5.11 (s, 1H, CHP, minor), 5.22
1
2.7 | General procedure for recycling
reactions
(s,
1H,
CHP,
major),
7.15–8.24
(m,
36H,
When the corresponding Suzuki and Heck reactions, carried
out according to the procedures described above, were fin-
ished, the catalyst was washed with acetone and water. It
was dried under air and reused without any pretreatment for
repeat cycles.
C6H4CO + C6H4 + PPh2, both isomers). 31P{1H} NMR
(202 MHz, CDCl3, δ, ppm): 18.49 (s, CHP, minor), 19.37
(s, CHP, major).
2.3 | Single‐crystal structure determination
3 | RESULTS AND DISCUSSION
Data collection was performed at 100 K using a Bruker four‐
circle diffractometer with CCD detector. The crystal structure
was solved by direct methods and refined by full‐matrix least
squares on F2 by SHELXL97[54] and analysed graphically
using the ORTEP‐3 crystallographic software package.[55]
All non‐hydrogen atoms were refined anisotropically using
reflections I > 2r(I). Hydrogen atoms were inserted at calcu-
lated positions using a riding model with fixed thermal
parameters.
3.1 | Synthesis and characterization of
Orthopalladated complexes
The phosphorus ylide ligand stabilized by carbonyl group was
synthesized using published methods.[53] As shown in
Scheme 1, the corresponding phosphorus ylide is refluxed with
Pd(OAc)2 (1:1) in CH2Cl2. The synthesized dinuclear acetato‐
bridged intermediate reacts with excess NaN3, NaCl, KBr
and KI, respectively, affording complexes of general for-
mula [Pd{κ2(C,C)‐C6H4PPh2C(H)CO(C6H4‐NO2–4)}(μ‐X)]2
(X = N3 (1), Cl (2), Br (3) and I (4)).
2.4 | Catalyst preparation
The palladium catalyst was prepared using a sequence of
reactions. A round‐bottomed flask equipped with a con-
denser was charged with 0.017 mmol of complex [Pd(μ‐I)
(NO2BPPY)]2, PVP (0.086 mmol; Pd:PVP = 1:5) and
NaBH4 (0.0427 mmol) in 35 ml of H2O–EtOH (4:1). The
flask was placed in an oil bath, and the mixture was stirred
and heated at 78–80 °C for 3 h. The catalyst was collected
Although ylides may undergo a C─H activation process
at two different positions,[56] in this study the stabilized phos-
phorus ylide containing deactivating nitro group at the ben-
zoyl ring is metallated at the phenyl ring of the PPh3 unit.
This fact has been fully established through spectroscopic
characterization. The FT‐IR spectrum of phosphorus ylide
shows a strong peak at 1521 cm−1 due to the carbonyl

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KARAMI ET AL.
frequency with respect to the starting ylide, confirming the
C‐coordination.[57,58] The formation of complex 1 is readily
confirmed by a characteristic stretching vibration band at
2052 cm−1 related to the bridged N3 groups.[59]
In the 1H NMR data of the complexes, the 2JHP values are
in the range 2.0–4.0 Hz that are smaller than those of free
ylides and phosphonium salts[56] due to the changes in the
hybridization of the ylidic carbon (sp2 to sp3) in the
C‐coordination mode. The signals related to the methinic
protons of corresponding ylide (CHP) appear as two broad
singlets at 5.12 ppm (minor) and 5.22 ppm (major). The pres-
ence of two lines of different intensities can originate from
the presence of two diastereoisomers (RR/SS and RS/SR)
due to the presence of chiral centres. In the 31P{1H} NMR
spectra of the complexes, signals related to the CHP group
of corresponding ylide appear as separate broad signals
(major/minor) at 17–20 ppm. Complexes 1–4 can exist as
meso/racemic isomers as well as involve the presence of
two chiral centres, which should give different 31P{1H}
NMR spectra (Scheme 2). 1H NMR and 31P{1H} NMR
signals for the CHP groups of all complexes are shifted to
lower field with respect to the parent ylides corresponding
to the C‐bonding of the ylides.[53]
The 31P{1H} NMR spectrum of 1 shows four signals at
17.60, 17.65, 18.47 and 18.77 ppm. Complex 1 is dinuclear
and it is obtained as a mixture of geometric isomers (cis
and trans), with two chiral carbons in each isomer. A signal
is observed for each one of the four diastereoisomers:
cis(RR/SS), cis(RS/SR), trans(RR/SS) and trans(RS/SR). This
behaviour has already been seen for other halide‐bridged
dinuclear phosphorus ylide complexes.[53,60]
SCHEME 1 Synthetic procedure
stretching vibration. This band appears at lower energies
compared with phosphonium salts, due to the charge delocal-
ization present in such compounds. The FT‐IR spectra of
binuclear complexes show a strong vibration band in the
range 1620–1630 cm−1, which is shifted to a higher
Suitable single crystals of complex 2 were obtained at
room temperature by slow diffusion of n‐hexane into chloro-
form solution. According to the single‐crystal X‐ray analysis,
complex 2 has been crystallized in the triclinic space group
P‐1 in the solid phase. The ORTEP view is shown in
SCHEME 2 Possible isomers

KARAMI ET AL.
5 of 13
TABLE 1 Crystal data and structure refinements of complex 2
Figure 1, and crystal data and structural refinement parame-
ters are summarized in Table 1. Each palladium atom is
located in a slightly distorted square planar environment,
surrounded by the ylidic (C101) and orthometallated (C2)
carbon atoms of NO2BPPY in addition to the chloride
bridging atoms (Cl1, Cl1i). Although the orthometallated
ligand is markedly warped, the environment around the Pd
is planar. Phosphrus ylide molecule acts as the chelating
Chemical formula
C26H19ClNO3PPd
566.24
Formula mass
Crystal system
Monoclinic
9.4726(12)
20.691(2)
11.7615(14)
90.000
a (Å)
b (Å)
c (Å)
ligand to form
a
five‐membered orthometallated
α (°)
palladacycle. Selected bond lengths and angles are listed in
Table 2. The angles subtended by the ligands at the Pd(II)
β (°)
92.552(3)
90.000
centre (C1–Pd1–Cl1
=
177.8(2)° and C3–Pd1–
γ (°)
Cl1i = 171.8(2)°) deviate from linearity indicating the
distorted square planar environments. The distance between
Pd(II) and the orthometallated carbon (1.997(8) Å) is in
accordance with other cyclopalladated compounds.[56] The
non‐equivalent Pd1–Cl1 = 2.395 (2) Å and Pd1–Cl1i = 2.439
(2) Å distances are related to the different donating character
of the carbon atoms, in addition to reflecting the different
influence of the atoms in the trans position of carbons. The
values of bond lengths and bond angles are comparable to
those observed in related complexes.[56]
Temperature (K)
100
_
Space group
P21/c
No. Of formula units per unit cell, Z
No. Of measured reflections
No. Of independent reflections
Rint
4
35 435
2403
0.040
0.047
0.135
Final R1 values (I > 2σ(I))
Final wR(F2) values (I > 2σ(I))
TABLE 2 Selected bond lengths (Å) and angles (°) obtained experi-
mentally from X‐ray crystallography of complex 2
3.2 | Synthesis and characterization of catalyst
PVP‐stabilized nanoparticles were prepared through the
synthetic procedure illustrated in Scheme 3. A mixture of
iodo‐bridged orthopalladated complex 4, PVP and NaBH4
was refluxed in a mixture of 7 ml of water and 28 ml of
ethanol for 3 h under air to synthesize the PVP‐stabilized
Pd nanoparticles.
Pd1–C1
2.108(7)
1.997 (8)
2.395(2)
2.439(2)
86.4(3)
Pd1–C3
Pd1–Cl1
Pd1–Cl1i
C3–Pd1–C1
C3–Pd1–Cl1
C3–Pd1–Cl1i
C1–Pd1–Cl1
C1–Pd1–Cl1i
Cl1–Pd1–Cl1i
92.4(2)
171.8(2)
177.6(2)
95.3(2)
94.50(7)
In the FT‐IR spectrum of catalyst (PVP‐stabilized nano-
particles), the bands at 1666 cm−1 are attributed to the char-
acteristic absorptions of the C═O groups in PVP. The
stretching vibrations of aliphatic CH2 groups appear at
2951 cm−1
.
As reported for similar investigations,[61] the formation of
Pd/PVP nanoparticles is possibly due to the donation of elec-
tron lone pairs on the nitrogen and carbonyl groups of PVP
into an unoccupied orbital of the palladium nanoparticles.
Moreover, addition of NaBH4 leads to the reduction of Pd(II)
to Pd(0) (Scheme 4).
The X‐ray diffraction (XRD) pattern of the fresh Pd
nanoparticles exhibits a broad reflection belonging to PVP
and also shows peaks at 40°, 46° and 68° corresponding to
FIGURE 1 Molecular structure of orthopalladated complex 2 with its
atom labelling scheme. Solvent molecules and hydrogen atoms have been
omitted for clarity

6 of 13
KARAMI ET AL.
SCHEME 3 Preparation of PVP‐
stabilized palladium nanoparticles
equation and is found to be close to the size observed from
TEM images.
To investigate the morphology and particle size of the
catalyst, SEM images of the catalyst were obtained
(Figure 3). The SEM images show the uniform
distribution of palladium onto the polymer. Furthermore,
energy‐dispersive X‐ray spectroscopy (EDX) carried out in
conjunction with SEM shows the presence of Pd atoms as well
as P and I atoms in the structure of fresh PVP‐stabilized
palladium nanoparticles (Figure 4).
According to the TEM images, the Pd nanoparticles are
formed and well dispersed through the support surface. The
size distribution of as‐prepared PVP‐stabilized Pd nanoparti-
cles is shown in Figure 5. The histogram can be fitted to a
Gaussian function, yielding a mean size of 2.7 nm.
SCHEME 4 Interaction between PVP and Pd nanoparticles
(111), (200) and (220) crystallographic planes of the Pd(0)
nanoparticles that are reflections of the face‐centred cubic
Pd lattice which is in good agreement with previous
results.[62] In addition, no characteristic diffraction peaks of
Pd(II) are observed in Figure 2. The size of the Pd nanoparti-
cles is estimated from the XRD pattern using the Scherrer
FIGURE 2 Powder XRD patterns of (a) PVP and (b) PVP‐stabilized
FIGURE 3 SEM image corresponding to PVP‐stabilized palladium
palladium nanoparticles
nanoparticles

KARAMI ET AL.
7 of 13
base, solvent, catalyst dosage and reaction time through a
model reaction of bromobenzene with phenylboronic acid.
As evident from Table 3 (entries 1–5), inorganic base sodium
carbonate (Na2CO3) is more effective than the other bases.
After optimization of the base, various solvent systems were
tested (Table 3, entries 5–15). EtOH–H2O in a ratio of 3:2 is
found to be the best solvent for this reaction. With Na2CO3 as
the base and 60% EtOH as the solvent, the effect of catalyst
dosage was examined next at three different concentrations
(entries 5, 16, 17). Under the otherwise same conditions,
changing the amount of the catalyst from 0.1 to 0.2 and to
0.05 mol% decreases the yield of the desired product from
94 to 92 and to 47%, respectively. Therefore, 0.1 mol% was
taken as the optimal amount of catalyst for the present Suzuki
reaction. It is found that the best system for this reaction is
EtOH–H2O (3:2) in combination with Na2CO3, which yields
a 94% conversion of bromobenzene at 60 °C (Table 3, entry 5).
The kinetics of the reaction of bromobenzene with
phenylboronic acid was investigated. As shown in Figure 6,
the yield of the product increases with reaction time until
reaching 94% at 4 h, indicating that the reaction is complete
in this time.
FIGURE 4 EDX spectrum of fresh PVP‐stabilized palladium
nanoparticles
3.3 | Suzuki reactions catalysed by Pd/PVP
nanoparticles
The Pd/PVP nanoparticle catalyst was then applied to the
Suzuki reaction of aryl halides with phenylboronic acid to
evaluate the catalytic performance. Thus, in our preliminary
investigations, conditions were optimized by varying the
With the optimized conditions in hand, the Suzuki reac-
tions of aryl halides with phenylboronic acid were examined
subsequently. As evident from Table 4, aryl iodides bearing
FIGURE 5 TEM images and corresponding nanoparticle size distribution of PVP‐stabilized palladium nanoparticles

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KARAMI ET AL.
TABLE 3 Optimizing reaction conditions for Suzuki reaction of bromobenzene with phenylboronic acida
Entry
Base
Solvent
Time (h)
Catalyst (Mol%)
Yield (%)
1
NaOAc
Et3N
EtOH 60%
EtOH 60%
EtOH 60%
EtOH 60%
EtOH 60%
EtOH 80%
EtOH 40%
EtOH 20%
EtOH
4
4
4
4
4
4
4
4
4
4
5
4
4
4
5
4
4
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.05
0.2
60
87
91
61
94
67
91
31
9
2
3
KOH
4
K3PO4
5
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
6
7
8
9
10
11
12
13
14
15
16
17
MeOH
50
87
12
˂5
MeOH 60%
Acetone
THF
CHCl3
˂10
37
47
92
Toluene
EtOH 60%
EtOH 60%
aReaction conditions: aryl halide (1.0 mmol), phenylboronic acid (1.5 mmol). All the reactions conducted at 55–60 °C.
either electron‐withdrawing group (entry 4) or electron‐
donating groups (entries 2 and 3) react efficiently, affording
the corresponding coupling products in satisfactory yields
(97–100%). The catalyst also exhibits high activity towards
various aryl bromides (entries 5–10). Excellent yields are
obtained for aryl bromides bearing electron‐withdrawing
substituents (entries 6 and 9). For methyl‐substituted aryl
bromide (entry 7), the position of the methyl group has a
large influence on the reaction, and the yield is only moderate
under standard conditions when the methyl group is at the
meta position. Furthermore, electron‐deficient aryl chlorides
also react satisfactorily under the present conditions (entries
11–14). The coupling of 4‐nitrochlorobenzene with
phenylboronic acid (entry 12) generates the corresponding
product in 94% yield.
3.4 | Heck reactions catalysed by Pd/PVP
nanoparticles
Following their application to the Suzuki reaction, the effec-
tiveness of Pd/PVP nanoparticles for Heck reactions was
investigated next. For optimization of the reaction conditions,
various bases, solvents and catalyst dosages were applied
with various reaction times and temperatures. Among the
variety of bases and solvent systems, using KI in DMF is
found to be the best for these reactions (Table 5). According
to the data of Table 5, increasing the reaction temperature
leads to an increase in the yield of the desired products.
The data clearly show that increasing the reaction time does
not lead to a higher conversion of bromobenzene and
decreases the yield. Similar to the Suzuki reactions, changing
FIGURE 6 Kinetic profiles of Pd/PVP catalyst in Suzuki reaction.
(a) normal reaction kinetics; (b) reaction kinetics after removal of
catalyst from the reaction at 30 min. Reaction conditions: bromobenzene
(1.0 mmol), phenylboronic acid (1.5 mmol), Na2CO3 (3.0 mmol),
EtOH–H2O (3:2; 10 ml), catalyst (0.1 mol% Pd), 60 °C, 4 h

KARAMI ET AL.
9 of 13
TABLE 4 Suzuki reactions of various aryl halides with
TABLE 5 Optimizing of reaction conditions for heck reaction of
bromobenzene with styrenea
phenylboronic acida
Entry
R
X
Yield (%) TONb TOF (h−1 c
)
Temperature Time Catalyst Yield
Entry Base
Solvent
(°C)
(h) (Mol%) (%)
1
2
3
4
5
6
7
8
9
H
I
100
97
2000
1940
1960
2000
2000
1900
1520
1440
400
500
480
490
500
500
470
380
360
100
1
Na2CO3 MeOH
Na2CO3 EtOH
Na2CO3 THF
90
90
8
8
8
8
8
6
6
6
6
6
5
5
5
5
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.05
0.2
˂5
˂5
m‐me
p‐me
p‐NO2
H
I
2
I
98
3
90
˂5
I
100
100
95
4
Na2CO3 Toluene
Na2CO3 CHCl3
Na2CO3 DMF
Na2CO3 DMF
NaOAC DMF
Na2CO3 MeOH
Na2CO3 DMF60%
K3PO4 DMF
90
˂5
Br
Br
Br
Br
5
90
˂5
o‐CHO
m‐me
p‐CHO
6
90
80
45
˂5
76
7
55
72
8
55
2‐Bromo‐1,4‐
20
dimethylbenzene
9
55
57
25
63
93
45
90
10
11
12
13
14
1‐Bromonaphthalene
62
53
94
20
70
1240
1060
1880
400
310
450
470
100
350
10
11
12
13
14
100
100
100
100
100
H
Cl
Cl
Cl
p‐NO2
KI
KI
KI
DMF
DMF
DMF
p‐NH2
Benzyl chloride
1400
aReaction conditions: aryl halide (1.0 mmol), phenylboronic acid (1.5 mmol),
Na2CO3 (3.0 mmol), EtOH–H2O (3:2; 10 ml), catalyst (0.1 mol% Pd), 60 °C,
4 h. Reactions were monitored using GC.
bTON = mmol of products/mmol of catalyst.
cTOF = TON/time.
aReaction conditions: aryl halide (1.0 mmol), styrene (1.5 mmol).
product in excellent yield. Notably, the reaction of
4‐nitrochlorobenzene with styrene proceeds well to give
the coupling product in 90% yield (entry 12).
4‐Methylchlorobenzene and 4‐aminochlorobenzene (entries
13 and 14) react with styrene less satisfactorily, affording
the corresponding product in 45 and 25% yield, respec-
tively. For the coupling of chlorobenzene with styrene
(entry 11), the yield of the coupling product is 55%.
the amount of the catalyst from 0.1 to 0.2 and to 0.05 mol%
decreases the yield of reactions (entries 12–14). Therefore,
0.1 mol% was taken as the optimal amount of catalyst for
the present Heck reaction. The best results for the Heck
reaction of bromobenzene with styrene are obtained using
0.1 mol% of the Pd catalyst in DMF as the solvent in combi-
nation with KI at 100 °C for up to 5 h (Table 5, entry 12).
Furthermore, the reaction kinetics of the Heck reaction of
bromobenzene with styrene with 0.1 mol% catalyst was
investigated and the result is shown in Figure 7. The yield
of product increases quickly with reaction time until it
reaches 93% at 5 h.
To demonstrate the versatility of the catalytic system,
we investigated the reaction using a variety of aryl halides
with styrene under the optimized conditions. Aryl iodides
containing either electron‐withdrawing or electron‐donating
groups react efficiently with satisfactory yields (Table 6,
entries 1–3). Aryl halides with electron‐withdrawing
substituents in para positions react smoothly, but the
efficiencies are lower for substrates with electron‐donating
groups. Unexpectedly, 3‐methylbromobenzene reacts with
styrene with 100% yield (entry 8). 1‐Bromonaphthalene
reacts well with styrene (entry 9), affording the desired
FIGURE 7 Kinetic profiles of Pd/PVP catalyst in heck reaction. (a)
normal reaction kinetics; (b) reaction kinetics after removal of catalyst
from the reaction at 30 min. Reaction conditions: bromobenzene
(1.0 mmol), styrene (1.5 mmol), KI (3.0 mmol), DMF (10 ml), catalyst
(0.1 mol% Pd), 100 °C, 5 h

10 of 13
KARAMI ET AL.
TABLE 6 Heck reactions of various aryl halides with styrenea
Entry
R
X )
Yield (%) TONb TOF (h−1 c
1
2
3
4
5
6
7
H
I
100
90
2000
1800
2000
1880
800
400
360
400
470
160
400
80
p‐me
p‐NO2
H
I
I
100
93
Br
Br
Br
p‐CHO
m‐me
40
100
20
2000
400
FIGURE 8 Recyclability of catalyst in Suzuki cross‐coupling
reaction of iodobenzene with phenylboronic acid (reaction conditions
are the same as given in Table 4)
2‐Bromo‐1,4‐
dimethylbenzene
8
2‐Bromopyridine
50
98
50
55
90
25
45
1000
1960
1000
1100
1800
500
200
390
200
220
360
100
180
9
1‐Bromonaphthalene
10
11
12
13
14
p‐Bromoacetylbiphenyl
H
Cl
Cl
Cl
Cl
p‐NO2
p‐NH2
p‐me
900
aReaction conditions: aryl halide (1.0 mmol), styrene (1.5 mmol), KI (3.0 mmol),
DMF (10 ml), catalyst (0.1 mol% Pd), 100 °C, 5 h.
bTON = mmol of products/mmol of catalyst.
cTOF = TON/time.
FIGURE 9 Recyclability of catalyst in heck cross‐coupling reaction
of styrene and bromobenzene (reaction conditions are the same as
given in Table 6)
3.5 | Heterogeneity and recyclability
The reusability of catalysts is a very important theme and
makes them useful for commercial applications. After the
catalytic reactions, the recovery and reusability of Pd/PVP
nanoparticles were initially investigated in the Suzuki
reaction of iodobenzene with phenylboronic acid under
the optimized conditions. Before running another cycle,
the solid was separated by simple filtration and washed
with acetone and water and then dried. The catalyst is
highly reusable and was used for five runs. It retained its
catalytic activity in these repeat cycles. The results are
shown in Figure 8.
In order to determine the recyclability of the PVP‐
supported catalyst for the Heck reaction, styrene and
bromobenzene were used in the same process for four runs.
The catalyst was used for the reaction and recovered after
reaction completion by centrifugation and was washed with
acetone and water. The catalyst, which is highly reusable,
was used for four cycles. The results are shown in Figure 9.
ICP‐OES analysis shows that the palladium content of
fresh catalyst is about 14 wt%. The palladium content after
consecutive reaction cycles is almost the same as that of the
fresh catalyst.
Further information about the homogeneous or heteroge-
neous nature of the catalyst was obtained using the hot filtra-
tion test which indicates that, when using this catalyst,
reactions proceed mostly under heterogeneous conditions.
The results confirm the heterogeneous character of the cata-
lytically active species.
3.6 | Comparison with other studies
To further evaluate the catalytic performance of Pd/PVP
nanoparticles, a comparison was made with several nanopar-
ticle‐supported catalysts reported in the literature. The reac-
tions of bromobenzene with phenylboronic acid and styrene
were taken as the standard reactions, and the results are listed
in Table 7. It can be clearly seen that for the Suzuki reaction
of bromobenzene with phenylboronic acid, the current Pd/
PVP catalyst is superior to others in terms of catalytic effi-
ciency. Moreover, this catalyst also proves to be the most
active for the Heck reaction of bromobenzene with styrene,
as reflected in its higher turnover frequency (TOF) value. It
should be noted that the Pd/PVP‐catalysed Suzuki reactions

KARAMI ET AL.
11 of 13
TABLE 7 Comparison of reported results for Suzuki/heck cross‐coupling reactions using supported Pd catalysts
Entry
Catalysta
Temp.(°C)/time (h)
Medium
Yield (%)
TOF (h−1
)
Ref.
[64]
[65]
[36]
[66]
1
2
3
4
5
Pd/Fe3O4 (1)
Reflux/24
25/5
DME/H2O
70
95
80
89
96
2.9
320
Pd/bentonite (0.06)
Pd nanoparticles/PS (0.5)
Xerogel gl‐MNPs (1)
Pd/PVP (0.1)
MeOH
110/12
60/5
DMF/H2O
MeOH
160
17.8
500
60/4
H2O–EtOH (2:3)
This work
[67]
[68]
[69]
[70]
6
7
PS‐supported (0.05)
Pd/C (0.1)
130/48
140/20
140/20
140/12
100/5
DMA
NMP
DMAc
DMF
DMF
97
40.4
45.5
20
91
8
Pd/MgO (0.1)
Pd/silica (0.1)
Pd/PVP (0.1)
39.7
38
9
120
372
10
93
This work
aData in parentheses indicate mol% of Pd used.
were carried out in ethanol–water in the absence of organic
solvents, which makes the catalytic capacity more impres-
sive. Thus this catalytic system provides a simple and green
method for the Suzuki reaction of aryl halides. Compared
to previous reports,[62,63] this catalytic system requires a very
small amount of catalyst loading and can be run under
relatively mild reaction conditions.
could be used several times with essentially no loss in
activity. Using a green solvent for the Suzuki cross‐coupling
reactions, high temperature stability, total separation from the
reaction products and recyclability can also be mentioned as
the advantages of this catalyst.
ACKNOWLEDGMENTS
We gratefully acknowledge the funding support received for
this project from the Iranian Nano Technology Initiative
Council. We are also grateful for the partial support of this
study by the Department of Chemistry, Isfahan University
of Technology.
4 | CONCLUSIONS
We have described the synthesis and characterization of
dinuclear halo/azido‐bridged orthopalladated complexes of
phosphorus ylide containing electron‐withdrawing NO2‐sub-
stituent at the para position of the benzoyl fragment. The
molecular structure is fully determined through the X‐ray
crystallography technique. In the next step, PVP‐stabilized
Pd nanoparticles with an average particle size of 2.7 nm were
successfully prepared originating from the newly synthesized
iodo‐bridged C,C‐palladacyclic precursor. Various methods
including XRD, TEM, SEM, EDX, ICP‐OES and FT‐IR
spectroscopy were used to identify the prepared catalyst.
The catalytic efficacy of the nanocatalyst was evaluated in
the Suzuki and Heck cross‐coupling reactions which led to
better yields and TOF values under moderate conditions
and shorter reaction times compared with catalysts reported
in the literature. Moreover, aryl iodides, bromides and
chlorides show satisfactorily high yields in the Suzuki and
Heck cross‐coupling reactions, using the PVP‐stabilized Pd
nanoparticles. Recycling experiments show that catalyst
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