Green synthesis of palladium nanoparticles using Pistacia atlantica kurdica gum and their catalytic performance in Mizoroki-Heck and Suzuki-Miyaura coupling reactions in aqueous solutions

Article abstract of DOI:10.1002/aoc.3325

A one-pot green method for the synthesis of palladium nanoparticles (Pd-NPs) supported on Pistacia atlantica kurdica (P. a. kurdica) gum is described. This natural gum is used as a reducing and stabilising agent. The formation of the Pd-NPs/P. a. kurdica gum catalyst was verified using several techniques, such as Fourier transform infrared spectroscopy, ultraviolet-visible spectrophotometry, scanning and transmission electron microscopies, X-ray diffraction, energy-dispersive X-ray spectroscopy, dynamic light scattering and wavelength-dispersive X-ray spectroscopy. The Pd-NPs stabilised by P. a. kurdica gum were employed as a heterogeneous catalyst in Mizoroki-Heck and Suzuki-Miyaura cross-coupling reactions at low palladium loading (0.1 mol%) under aerobic, phosphine-free and ligand-free conditions in water. Product yields of up to 98%, a facile work-up, no evidence of leached palladium from the catalyst surface and smooth recovery of the catalyst, which can be reused at least eight times, confirm the efficiency of the catalysts in the reactions investigated.

Full text of DOI:10.1002/aoc.3325

Full paper
Received: 23 January 2015
Revised: 20 March 2015
Accepted: 5 April 2015
Published online in Wiley Online Library: 11 May 2015
(wileyonlinelibrary.com) DOI 10.1002/aoc.3325
Green synthesis of palladium nanoparticles
using Pistacia atlantica kurdica gum and their
catalytic performance in Mizoroki–Heck and
Suzuki–Miyaura coupling reactions in
aqueous solutions
a
b
a
c
Hojat Veisi *, Ali Reza Faraji , Saba Hemmati and Antonio Gil
A one-pot green method for the synthesis of palladium nanoparticles (Pd-NPs) supported on Pistacia atlantica kurdica (P. a. kurdica)
gum is described. This natural gum is used as a reducing and stabilising agent. The formation of the Pd-NPs/P. a. kurdica gum catalyst
was verified using several techniques, such as Fourier transform infrared spectroscopy, ultraviolet–visible spectrophotometry,
scanning and transmission electron microscopies, X-ray diffraction, energy-dispersive X-ray spectroscopy, dynamic light scattering
and wavelength-dispersive X-ray spectroscopy. The Pd-NPs stabilised by P. a. kurdica gum were employed as a heterogeneous
catalyst in Mizoroki–Heck and Suzuki–Miyaura cross-coupling reactions at low palladium loading (0.1 mol%) under aerobic,
phosphine-free and ligand-free conditions in water. Product yields of up to 98%, a facile work-up, no evidence of leached palladium
from the catalyst surface and smooth recovery of the catalyst, which can be reused at least eight times, confirm the efficiency of the
catalysts in the reactions investigated. Copyright © 2015 John Wiley & Sons, Ltd.
Keywords: biosynthesis; palladium; nanoparticles; Pistacia atlantica kurdica gum; coupling
carbon–carbon bonds, and allow the arylation, alkylation or vinylation
of various alkenes through their reactions with aryl, vinyl, benzyl or allyl
Introduction
The utilisation of natural resources has forced scientists into looking
for innovative, eco-friendly techniques and procedures. The tech-
niques and procedures are those which protect human health
and have potential for effective use in the environment, thus
minimising pollution and by-products. Among these procedures,
halides in the presence of a suitable base. These reactions are usually
catalysed by homogeneous palladium complexes, where palladium is
coordinated to phosphine ligands to form water-soluble catalysts.
The main disadvantages of these catalytic systems are catalyst recovery
[1–9]
and generally unstable and toxic ligands.
For many catalytic sys-
[
1]
the synthesis of efficient and green catalysts is a critical challenge.
tems, the role of these ligands is to activate and stabilise the palladium
against agglomeration and formation of palladium black. Achieving a
palladium nanoparticle (Pd-NP) zero-valent oxidation state requires
the use of a reducing agent (e.g. hydrazine, sodium borohydride,
dimethylformamide, polyols or molecular hydrogen) or high tempera-
In order to separate homogeneous catalysts from solution, sev-
eral techniques such as chromatography, distillation and extraction
are used, but removal of the catalyst from a reaction mixture is not
efficient. This limitation in recovering catalysts is related to metal
contamination, corrosion and deposition on the reactor walls.
These disadvantages can be reduced if homogeneous complexes
are bonded to insoluble solid supports, such as ordered or
amorphous silicates, mixed oxides, clay minerals, zeolites, colloids,
molecular sieves, ion-exchange resins, polymer membranes or
[10]
tures in the gas phase. Cheaper and environmentally benign hetero-
geneous systems, mild reaction conditions and the use of renewable
materials are attractive developments; and following new and impor-
tant commercial opportunities, notable progress has been made in
these areas.
[
1,2]
bio-supports.
Needless to say, biodegradable natural com-
[3]
[4]
[5]
[6]
pounds, such as gellan, arabinogalactan, agarose, pectin,
[7]
[8]
starch, chitosan and various plant extracts, provide environ-
mentally friendly catalysts that have attracted a large number of
studies in basic research and the chemical industry to reduce or
*
Correspondence to: Hojat Veisi, Department of Chemistry, Payame Noor University,
Tehran, Iran. E-mail: hojatveisi@yahoo.com
[3]
eliminate the use of hazardous materials.
In addition, to improve the selectivity of materials produced
using heterogeneous catalysts compared with homogeneous cata-
lysts, nanostructures anchored on bio-supports have been
a Department of Chemistry, Payame Noor University, Iran
b Young Researchers and Elite Club, Pharmaceutical Sciences Branch, Islamic Azad
University, Tehran, Iran
[9]
developed.
Cross-coupling reactions, such as Mizoroki–Heck and Suzuki–Miyaura
c Department of Applied Chemistry, Los Acebos Building, Public University of
reactions, are powerful synthetic procedures for the formation of
Navarra, Campus of Arrosadia, E-31006, Pamplona, Spain
Appl. Organometal. Chem. 2015, 29, 517–523
Copyright © 2015 John Wiley & Sons, Ltd.

H. Veisi et al.
each sample. The size distribution was measured in order to num-
ber the nanoparticles in reverse microemulsion using a Zetasizer
Nano-ZS-90 (ZEN 3600, Malvern Instruments).
Catalytic Mizoroki–Heck coupling
A mixture of aryl bromide and iodide (1.0 mmol), n-butyl acrylate
(
(
(
2 3
1.2 mmol), K CO (2.0 mmol), Pd-NPs/P. a. kurdica gum catalyst
0.002 g of composite, containing 0.010 mmol of Pd) and water
3
5 cm ) at 80°C was stirred. The progress of the reaction was mon-
itored using TLC. After completion of the reaction, aqueous ethanol
3
(
10 cm , 50%) was added to the reaction mixture, which was stirred
for 5 min. The catalyst was separated by filtration and recovered.
Finally, evaporation of the solvents afforded the desired pure prod-
ucts. After complete drying, the catalyst was reused for the same
reaction. This process was repeated for six runs. The amount of cat-
alyst leached was determined using inductively coupled plasma
atomic emission spectrometry (ICP-AES).
Figure 1. Images of Pistacia atlantica kurdica tree and gum.
Pistacia atlantica kurdica (P. a. kurdica; Fig. 1) is found widely in
the Zagrous Mountains, and particularly in western and northern
Iran, eastern and northern Iraq, southern Turkey and northern Syria
in so-called Kurdistan. P. a. kurdica is irregularly distributed over the
region and is an important constituent of the natural vegetation. It
is the major source of a gum not very well known in the world, re-
Catalytic Suzuki–Miyaura coupling
A mixture of aryl halide and iodide (1.0 mmol), phenylboronic acid
(
2 3
1.1 mmol), K CO (2 mmol), Pd-NPs/P. a. kurdica gum (0.002 g of
[
11]
3
ferred to here as kurdica.
P. atlantica has antibacterial effects,
composite, containing 0.010 mmol of Pd) and water (5 cm ) was
stirred at 60°C. The progress of the reaction was monitored using
TLC. After completion of the reaction, 5 cm of EtOH was added
and is used in eczema treatments, for throat infections, kidney
stones, asthma and stomach ache and as an astringent, antipyretic,
anti-inflammatory, antiviral, antimicrobial, pectoral and stimulant.
Phytochemistry studies of this plant show that it may have fatty
acids, flavonoids, phenolic compounds and triterpenoids, α-pinene,
3
and the Pd-NPs/P. a. kurdica gum was separated by filtration. After
evaporation of the solvent, the resulting crude products were dried
and reused for seven runs. The amount of catalyst leached was
determined using ICP-AES.
[
12]
terpinolene and starch. Here we report a simple and eco-friendly
procedure for the green synthesis of Pd-NPs, using P. a. kurdica
gum as a reductant, stabiliser and capping agent. The catalytic
performance as a heterogeneous catalyst in Mizoroki–Heck and
Suzuki–Miyaura cross-coupling reactions in aqueous solutions was
also investigated.
Procedure for reusing catalysts
3
After the reaction time, 5 cm of ethanol was added to the reaction
mixture and stirred for 5 min. Then, the catalyst was separated by
filtration. The recovered solid was then washed using ethyl acetate
3
(
5 cm ) and dried under vacuum for 10 min. The recovered catalyst
Experimental
was then used for another run.
Extraction of gum from P. a. kurdica tree
Results and discussion
P. a. kurdica gum was collected from Kermanshah (western part of
Iran) in the summer of 2013 as an exudate from the trunk and
branches of the tree.
The loading of palladium in the heterogeneous catalyst was deter-
mined using ICP and EDX analyses. The final palladium content is
ꢀ
1
5
.2 mmol g . The UV–visible spectra of P. a. kurdica gum and
Preparation of Pd-NPs/P. a. kurdica gum
Pd-NPs/P. a. kurdica gum show a side-band absorption near 259 nm.
The UV–visible spectrum of the Pd-NPs/P. a. kurdica gum catalyst
is significantly changed after reduction, with the disappearance of
the peak at around 420 nm (Fig. 2). This change confirms the com-
3
P. a. kurdica gum (1 g) was dissolved in 50 cm of EtOH at room
3
temperature, and this solution was refluxed with 10 cm of EtOH
containing 10 mg of PdCl at 80°C for 2 h. The solution was allowed
2
2+
plete conversion of Pd to Pd. During this step, the colour of the
catalyst changes from yellow to deep black. Since P. a. kurdica
gum contains phenolic and triterpenoidic groups, they can form
to cool to room temperature and the solvent was evaporated. The
dark grey solid left after this process was dried overnight and then
under vacuum for 48 h.
2+
complexes with Pd ions in solution and change the oxidation
state of the palladium. It seems that triterpenoids and phenolic
compounds in the plant’s biological gum are responsible for the
Catalyst characterisation
2+
Diffuse reflectance spectra were recorded using a JASCO 550
UV–visible spectrophotometer equipped with a diffuse reflectance
reduction of Pd and the starchy compound, also present in this
gum, could act as a stabilising agent to prevent the aggregation of
[
12,13]
attachment, with BaSO
4
as the reference material. FT-IR spectra
the synthesised Pd-NPs in water.
were recorded using KBr pellets with a JASCO FT/IR (680 plus) spec-
trometer. Scanning electron microscopy (SEM) and transmission
electron microscopy (TEM) were performed using a Philips 501
microscope and a Tecnai F30 operating at 300 kV, respectively. In
addition, energy-dispersive X-ray (EDX) analysis was conducted on
Further characterisations by FT-IR measurements entirely con-
firm the previous results (Fig. 3). Indeed, FT-IR measurements were
carried out to identify the functional groups present in the P. a.
kurdica gum responsible for the reduction and stabilisation of the
Pd-NPs. Sharp peaks and bands at 3443, 2949, 1654, 1705, 1654,
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Copyright © 2015 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2015, 29, 517–523

Biosynthesis of palladium nanoparticles using P. a. kurdica gum
Figure 4. EDX spectrum of Pd-NPs/P. a. kurdica gum catalyst.
2
Figure 2. UV–visible spectra: (a) PdCl ; (b) P. a. kurdica gum; (c) Pd-NPs/P. a.
kurdica gum.
Figure 3. FT-IR spectra: (a) P. a. kurdica gum; (b) Pd-NPs/P. a. kurdica gum
catalyst.
Figure 5. (a) TEM images of Pd-NPs/P. a. kurdica gum catalyst. SEM images
of (b) P. a. kurdica gum and (c) Pd-NPs/P. a. kurdica gum catalyst.
ꢀ
1
1
458, 1375, 1247, 1030, 609 and 462 cm are characteristic of the
pure sample of P. a. kurdica gum. The FT-IR spectra of P. a. kurdica
gum and Pd-NPs/P. a. kurdica gum show strong signals from the
carboxylic groups, which are related to the C¼O stretching modes
to the small nanoparticle sizes and ligand capping as an obstacle
to agglomeration, the Pd-NPs/P. a. kurdica gum could be used as
a suitable catalyst for Mizoroki–Heck and Suzuki–Miyaura cross-
coupling reactions.
ꢀ
1 [14]
of –COOH, and appear at 1706 cm
.
The IR absorption corre-
sponding to C–O–H in-plane bend of the hydroxyl groups in the
The powder X-ray diffraction (XRD) pattern of the Pd-NPs/P. a.
kurdica gum catalyst is presented in Fig. 7, confirming the formation
of Pd-NPs. The powder XRD diffraction pattern obtained for the
Pd-NPs/P. a. kurdica gum material shows a broad reflection corre-
sponding to the amorphous gum. The catalyst can be characterised
by four reflections, which involve a strong d111 reflection at 39.4°
and three other weaker reflections at 47.1° (d200), 67.2° (d220) and
82.1° (d311). In addition, the characteristic XRD peaks show crystallo-
graphic planes of Pd-NPs. Solid material can be classified as being
either amorphous or crystalline. In a crystalline solid, the ions
occupy specific locations in a regular crystal lattice. The simplest
lattice structures are face-centred cubic and body-centred cubic.
The face-centred cubic structure consists of eight ions at the
corners of a cube and additional ions located at the centres of each
of the six faces of the cube. The XRD pattern of the resulting
Pd-NPs/P. a. kurdica gum catalyst further shows evidence of the
ꢀ
1 [15]
gum is observed at 1458 cm
cm can be observed due to OH absorptions in polyols, while a
.
A single broad band at 3434
ꢀ
1
ꢀ
1
single sharper band at 1030 cm can be assigned to the C–O–C
[16,17]
bending mode.
On the basis of the IR data, it may be con-
cluded that the phenolic hydroxyls present in the gum are respon-
sible for the reduction of the palladium. Based on these data, the P.
a. kurdica gum has a dual role of a reducing agent as well as a
stabilising agent for Pd-NPs.
The EDX spectrum of Pd-NPs/P. a. kurdica gum is shown in Fig. 4;
signals relating to carbon, oxygen and palladium are observed. The
existence of a palladium signal in the spectrum results from the pal-
ladium complexation with active sites of the organic functional
groups that increase the catalytic performance of the synthesised
catalyst, in comparison with P. a. kurdica gum. The TEM images of
Pd-NPs depicted in Fig. 5(a) show the size of the Pd-NPs to be about
[
18]
10 nm. Figures 5(b) and (c) seem to show that the presence of pal-
formation of face-centred cubic structured Pd-NPs.
ladium in the Pd-NPs/P. a. kurdica gum increases the electrical con-
duction and improves the quality of the SEM micrograph.
The average particle size in reverse microemulsion solution of
the Pd-NPs/P. a. kurdica gum is around 4–7 nm (Fig. 6). According
In combination with SEM, wavelength-dispersive X-ray spectros-
copy (WDX) can provide qualitative information about the distribu-
tion of various chemical elements in the catalyst matrix. Figure 8
shows a representative SEM image and corresponding elemental
Appl. Organometal. Chem. 2015, 29, 517–523
Copyright © 2015 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

H. Veisi et al.
Figure 8. SEM image of Pd-NPs/P. a. kurdica gum and elemental maps of C,
O, N, Pd and C/Pd atoms in the catalyst.
Figure 6. Particle size distribution for Pd-NPs/P. a. kurdica gum catalyst.
Scheme 1. Suzuki–Miyaura cross-coupling reaction.
The synthesis of biaryl using various solvents and co-solvents
was investigated. The results are summarised in Table 1. The sol-
vents considered were DMF, 1,4-dioxane, EtOH, water, THF, toluene
and mixtures of two solvents (DMF/H O, DMF/EtOH and H O/EtOH).
2
2
It is evident from the results in Table 1 that the solvent plays an im-
portant role in the cross-coupling system. The use of aprotic
Table 1. Coupling of 4-iodobenzene with ArB(OH)
kurdica gum catalyst
2
using Pd-NPs/P. a.
a
Entry
Solvent
Base
T (°C)
Yield (%)
15 min 60 min 120 min
1
DMF
K
2
CO
CO
CO
CO
CO
CO
CO
Na CO
CO
CO
CO
CO
CO
CO
CO
CO
PO
3
80
80
80
80
60
80
80
80
60
60
80
60
40
RT
60
60
60
80
24
49
47
87
83
32
20
17
84
90
98
98
80
37
91
13
57
—
51
52
92
85
—
—
—
85
92
98
98
81
45
91
22
62
—
—
54
52
93
85
—
—
—
85
93
98
98
81
45
92
25
62
—
2
DMF/H
2
O (1:1)
K
2
3
3
DMF/EtOH (1:1)
EtOH
K
2
3
Figure 7. XRD spectrum of Pd-NPs/P. a. kurdica gum catalyst.
4
K
2
K
2
3
5
EtOH
3
maps (WDX) for the synthesised catalyst. It can be seen that Pd
metal particles are well dispersed in the catalyst, which agrees well
with the XRD and TEM analyses. The selected-area elemental anal-
ysis figure reveals the presence of C, O, N and Pd throughout the
sample in a homogeneous manner, which confirms the regular uni-
formity of the prepared sample.
6
1,4-Dioxane
Toluene
K
2
3
7
K
2
3
8
Toluene
2
3
9
H
2
H
2
H
2
H
2
H
2
H
2
H
2
H
2
H
2
H
2
O/EtOH (1:1)
K
2
K
2
K
2
K
2
K
2
K
2
3
10
11
12
13
14
15
16
17
18
O/EtOH (1:2)
3
O
O
O
O
O
O
O
O
3
[
19]
Encouraged by our previous work on C–C coupling reactions,
3
the catalytic performance of Pd-NPs/P. a. kurdica gum in
Mizoroki–Heck and Suzuki–Miyaura cross-coupling reactions was
investigated in the present study. To find the best Suzuki coupling
conditions, the reaction between 4-iodobenzene and phenylboronic
acid was chosen as a model; and the effects of various parameters,
namely the reaction temperature, the nature of the solvent and
the base, were initially tested (Scheme 1). The control experiments
performed in the absence of the catalyst after 24 h give a 0.0%
yield, which confirms the role of Pd-NPs/P. a. kurdica gum.
3
3
Cs
Na
2
3
2
3
K
3
4
—
Trace
a
Reaction conditions: aryl iodide (0.116 g, 1 mmol); phenylboronic acid
3
(
0.134 g, 1.1 mmol); base (2 mmol); solvent (5 cm ).
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Appl. Organometal. Chem. 2015, 29, 517–523

Biosynthesis of palladium nanoparticles using P. a. kurdica gum
solvents (DMF, 1,4-dioxane and THF) or non-polar solvents (tolu-
ene) is not favourable for the reaction, even after prolonging the re-
action time (Table 1, entries 1 and 6–8). However, the biaryl yield
on R–X decrease the reaction rate. The Suzuki–Miyaura cross-
coupling reactions of R–X with electron-withdrawing groups
(Table 2, entries 12, 13, 16 and 17) proceed in shorter reaction times
and give coupled products in very good yields. These results reflect
the reactivity towards oxidative addition. In the case of R–Cl with
phenylboronic acid, many catalytic systems do not provide efficient
cross-coupling, even under harsh conditions of high temperature
2 2
increases when H O, EtOH and H O/EtOH are used as co-solvents.
An evaluation of various solvents at 60°C proves that the use of wa-
ter as the protic solvent gives satisfactory results. Furthermore, the
reaction rate also increases with temperature from 40 to 80°C. At
room temperature, almost no biaryl can be synthesised, even
though the mixture is stirred for a prolonged reaction. Finally, the
role of a base (Et N, K PO , Na CO , Cs CO and K CO ) and its
[
20]
and extended reaction times of up to 20–24 h.
This research
indicates that R–Cl with phenylboronic acid couples without any
limitation. The cross-coupling reactions of R–Cl with phenylboronic
acid require longer times and give moderate yields, compared with
R–Br and R–I (Table 2, entries 3, 6 and 9). In addition, 2-
bromothiophene and 2-iodothiophene are coupled with
phenylboronic acid and give the desired products with no poison-
ing of the catalyst (Table 2, entries 20 and 21).
3
3
3
2
3
2
3
2
3
amount in the cross-coupling reaction were also explored. Of these,
K CO shows the best results. In the absence of K CO , the coupling
2
3
2
3
reaction stops completely (Table 1, entry 18). This result indicates
that the prepared catalyst has excellent performance for activating
II
Ar–Pd –X in the presence of K CO . It also seems that the reaction
2
3
rate is related closely to the amount of K CO . The yield increases
Concerning the catalytic mechanism of Pd-NPs/P. a. kurdica gum,
a reasonable proposed mechanism for the Suzuki–Miyaura reaction
is presented in Scheme 2. Based on previous literature, the ‘boo-
merang’ mechanism of catalysis in the cross-coupling reaction
employing a supported palladium source as the catalyst has been
2
3
when the amount of K CO increases from 1 to 2 mmol; no differ-
2
3
ence is observed when the amount of K CO is higher than 2 mmol.
2
3
In our optimisation studies, the results show that the amount of
K CO is 2 eq. to aryl halides. Indeed, it is found that the type of base
2
3
[
21]
and its amount play a large role in the catalytic performance.
With the optimised reaction conditions in hand, the scope and
generality of the green process was examined with aryl iodides,
bromides and chlorides with various substituent groups in the
Suzuki–Miyaura cross-coupling reaction (Table 2). The results con-
firm that the relative rate of reaction of organic halides in the
Pd-NPs/P. a. kurdica gum-catalysed reactions is in the following or-
der: R–Cl < R–Br < R–I. Electron-withdrawing substituents on R–X
increase the reaction rate, while electron-donating substituents
debated.
The results presented indicate that the dynamics of
the molecular and size interconversion of palladium species deter-
mine the type of mechanism. To determine the behaviour of palla-
dium, a filtration test was carried out using Pd-NPs/P. a. kurdica gum
catalyst in the Mizoroki–Heck and Suzuki–Miyaura reactions. The
cross-coupling reaction of iodobenzene with phenylboronic acid
is 98% complete in 15 min at 60°C. Therefore, to investigate the role
and behaviour of palladium in the catalytic process, the solid phase
of the reaction mixture is separated from the liquid phase after 5
min. The solid phase is collected after centrifugation for 3 min at
1
6 000 rpm. The filtrate is then allowed to continue to react under
Table 2. Heterogeneous Suzuki–Miyaura reaction of aryl halides with
phenylboronic acid catalysed by Pd-NPs/P. a. kurdica gum
a
the same conditions for another 20 min, until the substrate is
completely consumed. Chemical analysis of the filtrate shows the
presence of palladium, which proves that palladium leaches from
our catalyst surface. Meanwhile, after the end of this reaction under
the same conditions, a black precipitate of palladium is observed in
this solution around the wall of the container after 40 min. These
two observations verify the leaching of palladium from the catalyst
b
c
ꢀ1 d
6 4
Entry RC H X
X
Time (min) Yield (%) TON TOF (min )
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
H
H
H
I
15
40
98
98
75
98
96
75
98
96
70
98
98
98
95
98
92
96
92
98
96
98
96
326.7
326.7
250.0
326.7
320.0
250.0
326.7
320.0
233.3
326.7
326.7
326.7
316.7
326.7
306.7
320.0
306.7
326.7
320.0
326.7
320.0
21.78
8.167
0.834
10.90
5.334
0.596
7.260
3.555
0.389
16.33
5.445
13.07
5.757
8.167
3.407
5.333
1.278
6.534
2.667
6.534
2.667
Br
Cl
I
300
4-CH
4-CH
4-CH
4-COCH
4-COCH
4-COCH
3
30
60
[
22]
3
Br
Cl
I
surface (cycles B and C).
3
420
In another study after the completion of the reaction, the Pd-
NPs/P. a. kurdica gum catalyst is separated from the reaction mix-
ture, and the amount of palladium in the aqueous solution is eval-
uated. There is no black precipitation of palladium around the walls
of the container and only a trace amount of palladium from the ini-
tially added Pd-NPs/P. a. kurdica gum catalyst is detected. To ex-
plain the increased palladium during the first stage of the
experiment and its decrease at the end stages of the experiment,
we can conclude that the palladium re-deposits onto the gum after
3
45
90
3
Br
Cl
I
3
600
0
1
2
3
4
5
6
7
8
9
0
1
4-CH
4-CH
4-NO
4-NO
2-CH
2-CH
3
O
O
20
60
25
55
40
90
60
3
Br
I
2
2
Br
I
3
[
23]
3
Br
I
the completion of the reaction. This study indicates that, in the
4-CHO
4-CHO
Br
I
240
50
120
50
120
1-Naphthyl
1-Naphthyl Br
2-Thienyl
2-Thienyl
I
Br
a
3
Reactions carried out under aerobic conditions in 5 cm of H
2
O, 1.0
CO
mmol aryl halide, 1.1 mmol phenylboronic acid and 2 mmol K
in the presence of catalyst (0.002 g, 0.1 mol% Pd) at 60°C.
2
3
b
c
Isolated yield; average of two runs.
Turnover number: moles of aryl halide converted per mole of Pd.
d
Scheme 2. Proposed mechanism for Suzuki–Miyaura cross-coupling of
ethylbenzene catalysed by Pd-NPs/P. a. kurdica gum.
Turnover frequency.
Appl. Organometal. Chem. 2015, 29, 517–523
Copyright © 2015 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

H. Veisi et al.
catalytic process, the prepared catalyst behaves like a heteroge-
neous catalyst, which can be recovered and reused.
The catalytic activities of the Pd-NPs/P. a. kurdica gum were then
examined in the Mizoroki–Heck cross-coupling reaction (Scheme 3).
The coupling reaction of iodobenzene with n-butyl acrylate was
used as a model reaction to investigate the catalytic performance
of the nanocatalyst.
To optimise the effectiveness of the reaction conditions for the
Mizoroki–Heck reaction in the presence of the Pd-NPs/P. a. kurdica
gum catalyst, various parameters were investigated – such as the
amount of catalyst, temperature and nature of the solvent – to
study the performance of the heterogeneous nanocatalyst.
Employing water as the solvent in the presence of K CO at 80°C
2
3
Figure 9. Reusability of Pd-NPs/P. a. kurdica gum catalyst in Suzuki–
Miyaura (SM) and Mizoroki–Heck (MH) cross-coupling reactions.
gives a 98% yield after 15 min. For comparison purposes, other sol-
vents (DMSO, DMF, toluene and EtOH) were investigated. After 10
min, the amount of product decreases in the following order with
the various solvents: 98.0% (DMSO) > 92.0% (DMF) > 81.0%
apparently effective. Consequently, the temperature effect (at 40,
60, 80 and 90°C) on the Mizoroki–Heck reaction was also monitored
after 30 min in the presence of the nanocatalyst. Generally speak-
ing, increasing the reaction temperature (80°C) and reaction time
give higher yields (98%). The reaction of iodobenzene with n-butyl
acrylate (at 1:1 eq.) in the presence of K CO (2 eq.) in water at 80°C
(H2O) > 79.0% (ethanol) > 6.0% (toluene). However, water is partic-
ularly attractive as a reaction solvent. It is inexpensive, non-toxic,
non-flammable and easily separated from organic products. Com-
parison of the results indicates that the Mizoroki–Heck cross-
coupling reaction in water proceeds much better than in organic
solvents, with an excellent yield and shorter reaction time. There-
fore, reactions were conducted in water when we next explored
the efficiency of the catalyst with other aryl halides with n-butyl ac-
rylate. Increasing the temperature to accelerate the reaction is
2
3
provides a 98% yield of the coupling product (Table 3, entry 1);
these reaction conditions were chosen for further experiments.
The reusability of the immobilised Pd-NPs/P. a. kurdica gum
catalyst was confirmed by performing a series of consecutive exper-
iments in which the used catalyst was filtered, washed with fresh
solvent and used in another run without any further treatment.
The results depicted in Fig. 9 clearly show that a slight decline of ac-
tivity occurs after the sixth and seventh runs for the Mizoroki–Heck
and Suzuki–Miyaura reactions, respectively. Consequently, the
results clearly suggest that the Pd-NPs/P. a. kurdica gum efficiently
catalyses the Suzuki–Miyaura and Mizoroki–Heck cross-coupling
reactions.
Scheme 3. Mizoroki–Heck cross-coupling reaction.
Conclusions
Table 3. Heterogeneous Mizoroki–Heck reaction of aryl halides with
n-butyl acrylate catalysed by Pd-NPs/P. a. kurdica gum
a
We have shown that Pd-NPs stabilised by P. a. kurdica gum are effi-
cient and stable catalysts in the Suzuki–Miyaura and Mizoroki–Heck
cross-coupling reactions. The advantage of Pd-NPs/P. a. kurdica
gum compared to previously reported catalysts is the use of inex-
pensive, non-toxic, non-flammable solvents, a natural support
medium and a simple nanoparticle preparation route, thus elimi-
nating the use of reducing agents such as those mentioned earlier.
The reactions are conducted in the open air, which indicates that
the Pd-NPs/P. a. kurdica gum catalyst is highly stable and not sensi-
tive to oxygen. The catalytic system is applicable to a wide range of
aryl halides (iodide, bromide and chloride), and can be easily
separated from the products and reused without significant loss
in activity.
b
c
ꢀ1 d
Entry
RC
6
H
4
X
X
Time (h) Yield (%)
TON
TOF (h )
1
2
3
4
5
6
7
8
9
H
H
H
I
0.25
5
98
90
70
96
80
60
98
96
98
96
96
92
96
96
326.7
300.0
233.3
320.0
266.7
200.0
326.7
320.0
326.7
320.0
320.0
306.7
320.0
320.0
1307
60.00
9.722
640.0
44.45
8.334
1089
106.7
2178
213.4
914.3
153.3
640.0
128.0
Br
Cl
I
24
0.5
6
4-CH
4-CH
4-CH
4-CH
4-CH
3
3
3
3
3
Br
Cl
I
24
0.3
3
O
O
Br
I
4-NO
4-NO
2
0.15
1.5
0.35
2
10
11
12
13
14
2
Br
I
2-CH
2-CH
3
3
Br
I
Acknowledgments
1-Naphthyl
1-Naphthyl
0.5
2.5
The authors thank Payame Noor University for the financial support
of this work.
Br
a
3
Reactions carried out under aerobic conditions in 5 cm of H
2
O, 1.0
mmol aryl halide, 1.1 n-butyl acrylate and 2 mmol K CO in the
2 3
presence of catalyst (0.002 g, 0.1 mol% Pd) at 80°C.
b
c
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