Palladium nanoparticles immobilized on cyclodextrin-decorated halloysite nanotubes: Efficient heterogeneous catalyst for promoting copper- and ligand-free Sonogashira reaction in water–ethanol mixture

Article abstract of DOI:10.1002/aoc.4211

Combining the excellent features of halloysite nanoclay and cyclodextrin, a novel hybrid system was designed and synthesized based on covalent attachment of tosylated cyclodextrin to thiosemicarbazide-functionalized halloysite nanoclay and used for the im

Full text of DOI:10.1002/aoc.4211

Received: 9 October 2017
DOI: 10.1002/aoc.4211
Revised: 3 November 2017
Accepted: 6 November 2017
F U L L P A P E R
Palladium nanoparticles immobilized on cyclodextrin‐
decorated halloysite nanotubes: Efficient heterogeneous
catalyst for promoting copper‐ and ligand‐free Sonogashira
reaction in water–ethanol mixture
Samahe Sadjadi
Gas Conversion Department, Faculty of
Combining the excellent features of halloysite nanoclay and cyclodextrin, a
novel hybrid system was designed and synthesized based on covalent attach-
ment of tosylated cyclodextrin to thiosemicarbazide‐functionalized halloysite
nanoclay and used for the immobilization of Pd nanoparticles. The resulting
hybrid, Pd@HNTs‐T‐CD, was then characterized using various techniques,
and successfully used for promoting copper‐ and ligand‐free Sonogashira
coupling reactions of halobenzenes and acetylenes in a mixture of water and
ethanol. Notably, under Pd@HNTs‐T‐CD catalysis, the reaction could proceed
in relatively short reaction time to furnish the corresponding products in high
yields. Additionally, the catalyst was recyclable and could be simply recovered
and reused for several reaction runs. Results also established negligible
leaching of Pd, indicating the efficiency of HNTs‐T‐CD for embedding Pd
nanoparticles.
Petrochemicals, Iran Polymer and
Petrochemicals Institute, PO Box 14975‐
112, Tehran, Iran
Correspondence
Samahe Sadjadi, Gas Conversion
Department, Faculty of Petrochemicals,
Iran Polymer and Petrochemicals
Institute, PO Box 14975‐112, Tehran, Iran.
Email: samahesadjadi@yahoo.com
KEYWORDS
cyclodextrin, halloysite, heterogeneous catalyst, Pd nanoparticles, Sonogashira coupling reaction
1 | INTRODUCTION
and purification of products. To circumvent these prob-
lems, the development of heterogeneous catalysts for
The Sonogashira coupling reaction, the reaction of termi-
nal alkynes and vinyl or aryl halides, is a potent tool for
the formation of C─C bonds and the synthesis of compli-
cated organic compounds such as natural products and
macrocycles.[1] Mostly, this reaction is catalysed by phos-
phine‐based Pd catalysts together with copper as co‐cata-
lyst in toxic solvents and under degassed conditions.[2,3]
As the use of copper‐based co‐catalysts can induce the for-
mation of by‐products through promoting homocoupling
of alkynes, the development of ligand‐ and copper‐free
Sonogashira reactions has received growing attention. In
this line, various Pd‐based homogeneous catalytic systems
have been developed.[2] However, they suffer from some
drawbacks such as tedious recovery of precious catalyst
Sonogashira reactions has been suggested.[4–6] Moreover,
the use of green procedures, especially use of aqueous
media, for promoting C–C coupling reactions has been a
focus.[7–15]
Halloysite nanotubes (HNTs) are a form of natural clay,
mostly found in New Zealand, France, USA and China in
large quantities, with tubular morphology and general for-
mula Al2(OH)4Si2O5⋅2H2O, in which the octahedral
gibbsite Al(OH)3 sheet forms the inner surface, while tetra-
hedral SiO4 is in the outer surface.[16] Similar to kaolin, the
ratio of Al to Si in HNTs is 1:1. However, a monolayer of
H2O molecules separates the unit layers in HNTs.[17–19]
High mechanical strength, large surface area and large
internal and outer diameters (10–100 and 30–190 nm,
Appl Organometal Chem. 2018;e4211.
wileyonlinelibrary.com/journal/aoc
Copyright © 2018 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.4211

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SADJADI
respectively) render HNTs a promising candidate for devel-
oping (photo)catalysts,[18,20] drug carriers,[20] polymeric
matrices and state‐of‐the‐art materials with the utility for
separation,[21] waste treatment[22,23] and electrical
applications.[24]
SCHEME 1 Sonogashira reaction
To manipulate the properties of HNTs and improve
their potential for immobilizing catalytic species, surface
functionalization is suggested.[19] In this line, HNTs have
been functionalized with various functionalities such as
SCHEME 2 Mizoroki–Heck reaction
N‐b‐aminoethyl‐γ‐aminopropyltrimethoxysilane,[25]
aminopropyltriethoxysilane[26]
and
γ‐
poly(N‐
2 | EXPERIMENTAL
isopropylacrylamide).[27] Recently, functionalization of
HNTs has been comprehensively reviewed by Riela and
co‐workers.[28] It has been shown that functionalized
HNTs can be successfully used for the immobilization of
various catalytically active species[29–31] such as nanopar-
ticles,[18,27,32–36] heteropolyacids,[37] ionic liquids, com-
plexes,[38,39] etc.[19] It has been confirmed that
functionalized HNTs not only can effectively suppress
the leaching of supported catalytically active species but
also can exert synergistic effects and improve the catalytic
performance of a hybrid catalyst.
2.1 | Materials and instrumentation
The chemicals used for the preparation of the catalyst
included
halloysite
nanoclay,
(3‐chloropropyl)
β‐CD, p‐
trimethoxysilane,
thiosemicarbazide,
toluenesulfonyl chloride (p‐TsCl), triethylamine, toluene
and Pd(OAc)2, all obtained from Sigma‐Aldrich.
The reagents used for the investigation of catalytic
activity and performing Mizoroki–Heck and Sonogashira
coupling reactions were acetylenes, halobenzenes,
K2CO3, styrene, distilled water and ethanol. All were pur-
chased from Merck and used as received.
Cyclodextrins (CDs) are water‐soluble and nontoxic
cyclic oligosaccharides with hydrophilic exteriors and
hydrophobic interior cavities,[40] which can host various
guest molecules with suitable shape and size and form
inclusion complexes. Hence, CDs can act as transfer shut-
tles to promote biphasic processes.[41] The utility of CDs
for developing heterogeneous and homogeneous catalysts
is well established.[42–44] Moreover, CDs can be consid-
ered as useful capping agents and stabilizers for tuning
synthesis of nanoparticles.[45–50]
To the best of the author's knowledge, there are only a
few reports on the design and preparation of HNT–CD
hybrid systems, mostly used for drug delivery or removal
purposes.[51–53] Moreover, there are a few reports on the
catalytic utility of Pd species immobilized on functional-
ized HNTs.[35,54,55]
In continuation of our studies on HNTs as catalyst
supports[37,38,56] and considering the efficiency of
CDs[57–59] and HNTs for immobilization of Pd,[35] herein
we report, for the first time, a novel hybrid system based
on covalent conjugation of tosylated CD to
thiosemicarbazide‐functionalized HNTs (HNTs‐T‐CD)
for immobilization of Pd nanoparticles and the develop-
ment of a novel catalyst, Pd@HNTs‐T‐CD. Considering
the synthetic importance of C–C coupling reactions, we
report the catalytic activity of Pd@HNTs‐T‐CD for pro-
moting ligand‐ and copper‐free Sonogashira coupling
reactions (Scheme 1) and a model Mizoroki–Heck reac-
tion (Scheme 2) in a mixture of water and ethanol. More-
over, the recyclability of the catalyst and Pd leaching were
studied.
The progress of Sonogashira and Mizoroki–Heck cou-
pling reactions was monitored by TLC using commercial
aluminium‐backed plates of silica gel 60 F254, visualized
using ultraviolet light. The coupling products were identi-
fied by comparing their melting points, determined in
open capillaries using an Electrothermal 9100 without
further corrections, and Fourier transform infrared (FT‐
IR) spectra. For some selected samples, 1H NMR and
13C NMR spectra were also obtained with a Bruker
DRX‐400 spectrometer at 400 and 100 MHz, respectively.
To characterize the structure of the catalyst,
Brunauer–Emmett–Teller (BET) measurements, X‐ray
diffraction (XRD), scanning electron microscopy (SEM)/
energy‐dispersive X‐ray spectroscopy (EDS), transmission
electron microscopy (TEM), thermogravimetric analysis
(TGA), FT‐IR spectroscopy and inductively coupled
plasma atomic emission spectroscopy (ICP‐AES) were
employed. TEM images were recorded with a CM30300Kv
field emission microscope. SEM/EDS analyses were
recorded with a Tescan instrument, using Au‐coated sam-
ples and an acceleration voltage of 20 kV. BET analyses of
pure HNTs and Pd@HNTs‐T‐CD were carried out using a
Belsorp Mini II apparatus. To degas the samples, they
were pre‐heated at 423 K for 4 h prior to BET analyses.
FT‐IR spectra of fresh and recycled catalyst were recorded
using a PerkinElmer Spectrum 65 instrument. TGA was
carried out using a Mettler Toledo thermogravimetric
apparatus with a heating rate of 10 °C min−1 from 35 to
750 °C under nitrogen atmosphere. Room temperature

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powder XRD patterns were obtained using a Siemens
D5000 with Cu Kα radiation from a sealed tube. The
ultrasonic apparatus used for the synthesis of the catalyst
was a Bandelin HD 3200 with output power of 150 W and
TT13 tip.
2.5 | Synthesis of HNTs‐T‐CD (4)
To synthesize HNTs‐T‐CD, CD‐OTs (0.5 g) was dis-
persed in dry toluene (20 ml) under stirring and the
pH value of the suspension was kept at ca 7–8. Subse-
quently, the resulting mixture was added to a suspen-
sion of HNTs‐T (1 g) in 20 ml of dry toluene and
refluxed overnight. Finally, the product (4) was filtered,
washed with dry toluene repeatedly and dried in an
oven at 80 °C for 12 h.
2.2 | Functionalization of HNTs with Cl:
Synthesis of HNTs‐Cl (1)
To prepare HNTs‐Cl, HNTs (1.2 g) were dispersed in dry
toluene (60 ml) and stirred to furnish a well‐dispersed sus-
pension. Afterwards, (3‐chloropropyl)trimethoxysilane
(4 ml) was added dropwise into the suspension and then
the resulting mixture was sonicated under ultrasonic irra-
diation at a power of 100 W for 0.5 h. Notably, the ultra-
sonic apparatus was equipped with a temperature
sensor, so the temperature could be monitored in the
course of the reaction. As the ultrasonic irradiation could
increase the temperature of the reaction medium, the
temperature was controlled and kept constant at 25 °C
by using a cold water bath. Subsequently, the well‐dis-
persed suspension was refluxed under inert (nitrogen)
atmosphere at 110 °C overnight. Upon completion of the
process, the precipitate was separated by simple filtration,
washed with dried toluene repeatedly and dried at 100 °C
for 24 h.
2.6 | Doping of Pd Nanoparticles:
Synthesis of Pd@HNTs‐T‐CD (5)
To dope Pd nanoparticles, HNTs‐T‐CD (1.2 g) was dis-
persed into 12 ml of dry toluene containing Pd(OAc)2
(0.02 g). The suspension was then stirred at room temper-
ature for 8 h. Subsequently, a solution of NaBH4 in a mix-
ture of toluene and methanol (10 ml, 0.2 N) was added
dropwise into the suspension. After 2 h, product 5 was
obtained by washing with methanol and drying in an
oven at 60 °C for 12 h. A schematic of the procedures
for the preparation of Pd@HNTs‐T‐CD is shown in
Figure 1.
To prepare Pd@HNTs‐T, a similar procedure was
applied, in which the steps in Sections 1.4 and 1.5 were
omitted. In the case of the synthesis of Pd@CD, a proce-
dure similar to that in Section 1.6 was employed, in which
CD and water were used as support and solvent instead of
HNTs‐T‐CD and toluene.
2.3 | Incorporation of Thiosemicarbazide:
Synthesis of HNTs‐T (2)
2.7 | Typical Procedure for Sonogashira
Reaction
In
order
to
further
functionalize
HNTs‐Cl,
thiosemicarbazide (0.5 g) was added to a suspension of
HNTs‐Cl (1 g) in dry toluene (50 ml) in the presence of
a catalytic amount of Et3N (1 ml). Subsequently, the
resulting mixture was refluxed at 110 °C. After 24 h, the
precipitate was filtered off, washed with dry toluene sev-
eral times and dried at 100 °C overnight.
Halobenzene (1.0 mmol) and acetylene (1.2 mmol) were
added to a flask containing Pd@HNTs‐T‐CD (6 mol%) as
catalyst, K2CO3 (2.0 mmol) as base and 5 ml of water–eth-
anol (4:1) as solvent. The flask was then heated at 60 °C
under stirring condition. The progress of the reaction
was monitored by TLC. Upon completion of the reaction,
the catalyst was filtered off and the mixture was cooled to
room temperature. The organic layer was extracted with
diethyl ether and purified by column chromatography
over silica gel using hexane–ethyl acetate (4:1) as eluent
to afford the desired product.
2.4 | Synthesis of Tosylated CD (CD‐OTs; 3)
Tosylation of CD with p‐TsCl was carried out according to
a previous report.[60] Typically, p‐TsCl (7.9 mmol) was
added to a solution of CD (15.86 mmol) in pyridine. Then,
the resulting mixture was cooled to 0 °C and kept for 48 h.
Afterwards, an oily product was obtained by adding dis-
tilled water (20 ml) to the mixture and evaporating the
solvent. The oily product transformed into a white precip-
itate upon addition of icy water. Finally, the desired prod-
uct was furnished by filtration, recrystallization from
water and drying.
2.8 | Typical Procedure for Catalytic
Mizoroki–Heck Reaction of 4‐Iodobenzene
and Styrene
A solution of K2CO3 (2 mmol) in 1 ml of ethanol was
added to a mixture of 4‐iodobenzene (1.0 mmol), styrene
(1.5 mmol) and Pd@HNTs‐T‐CD catalyst (6 mol%) in

4 of 16
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FIGURE 1 Preparation of Pd@HNTs‐
T‐CD
ethanol (3.0 ml). The resulting mixture was then stirred at
130 °C for 2.5 h. Upon completion of the reaction (moni-
tored by TLC), the catalyst was filtered, washed with eth-
anol and dried at 100 °C under air for use in the next run.
The organic layer, extracted with diethyl ether, was
washed with deionized water, separated and dried over
anhydrous Na2SO4. Finally, the pure product was
obtained by recrystallization.
FIGURE 2 FT‐IR spectra of Pd@HNTs‐
T and pure HNT

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with the characteristic bands of HNTs), which can be
assigned to ―CH2 stretching and Si―O stretching,
respectively. The characteristic bands of CD are at
3452 cm−1 (―OH group), which overlap with the char-
acteristic band of HNTs, the band at 2852 cm−1, which
3 | RESULTS AND DISCUSSION
3.1 | Catalyst Characterization
FT‐IR spectroscopy was employed to establish the struc-
ture of Pd@HNTs‐T‐CD (Figure 2). The bands at 3693
and 3626 cm−1 are indicative of vibration of hydroxyl
groups in the inner surface of HNTs. Moreover, the
bands at 538 and 1022 cm−1 which are representative
of Al―O―Si vibration and Si―O stretching can be
attributed to the structure of HNTs. The conjugation of
(3‐chloropropyl)trimethoxysilan can be confirmed by
observing the bands at 2920 and 1022 cm−1 (overlapped
is due to ―CH2 stretching, and the band at 1674 cm−1
.
The characteristic bands of thiosemicarbazide over-
lapped with those of HNT. However, the shoulder at
1113 cm−1, which is representative of C¼S functionality,
can indicate the presence of thiosemicarbazide on the
structure of the catalyst. The incorporation of
thiosemicarbazide was further confirmed by other analy-
ses (discussed below).
FIGURE 3 SEM image and EDS analysis of (A, B) HNTs‐T‐CD and (C, D) Pd@HNTs‐T‐CD

6 of 16
SADJADI
FIGURE 4 Elemental mapping of
Pd@HNTs‐T‐CD
FIGURE 5 TEM images of Pd@HNs‐T‐
CD catalyst
FIGURE 6 XRD patterns of (a) pure
HNTs and (b) Pd@HNTs‐T

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7 of 16
FIGURE 7 TGA analysis of (a) HNTs‐T
and (b) Pd@HNTs‐T
T‐CD is distinguished from that of HNTs‐T‐CD, indicating
that introduction of Pd nanoparticles could affect the
morphology.
The EDS analysis of HNTs‐T‐CD (Figure 3) clearly
showed Si, Al and O atoms, which can be representative
of HNT structure. Moreover, the observation of C, O, N,
S atoms can indicate the functionalization of HNTs with
thiosemicarbazide and CD. Besides the abovementioned
atoms, Pd atoms can also be detected in the EDS trace of
Pd@HNTs‐T‐CD, confirming the incorporation of Pd
nanoparticles in the structure of the catalyst.
Next, to study the distribution Pd nanoparticles, ele-
mental mapping analysis of Pd@HNTs‐T‐CD was per-
formed (Figure 4). As can be seen, Pd nanoparticles
were dispersed almost uniformly on the catalyst and no
aggregation was observed.
TEM images of Pd@HNTs‐T‐CD are illustrated in
Figure 5. In these images the tubular morphology of
HNTs is clearly observable. Additionally, it can be seen
that the surface of HNTs are decorated with CD. Pd nano-
particles, darker spots, can also be detected on the surface
of HNTs‐T‐CD.
To study the phase and composition of Pd@HNTs‐T‐
CD, the XRD technique was employed (Figure 6). Accord-
ing to the literature,[17,26] the peaks observed at 2θ = 8°,
13°, 23°, 28°, 31°, 58° and 67° (JCPDS card no. 29‐1487)
(labelled as H) are indicative of HNTs. Also, the peaks at
40°, 49°, 68°, 82.1° and 86° (labelled as P) correspond to
the {111}, {200}, {220}, {311} and {222} planes of Pd, respec-
tively. The reflection peaks are consistent with those of a
face‐centred cubic crystalline structure of bulk Pd (JCPDS
card no. 46‐1043).[61]
FIGURE 8 (a) Nitrogen adsorption–desorption and (b) BJH
isotherms of Pd@HNTs‐T‐CD
The SEM images and EDS analyses of HNTs‐T‐CD
and Pd@HNTs‐T‐CD are depicted in Figure 3. Comparing
the tubular morphology of pure HNTs (Figure S1 in
supporting information) with that of HNTs‐T‐CD
established that the functionalization of HNTs with
silanic compound and subsequent introduction of
thiosemicarbazide and CD could alter the morphology of
HNTs and lead to a more compact morphology. It is pos-
tulated that functionalization of HNTs with
thiosemicarbazide and CD can result in some
noncovalent interactions such as hydrogen bonding and
electrostatic interactions between the functionalities of
HNTs and consequently a more aggregated morphology
can be observed. Notably, the morphology of Pd@HNTs‐
TGA of Pd@HNTs‐T‐CD over the range 35–750 °C
(Figure 7) exhibited four weight loss steps. The first,
below 150 °C, is due to the loss of water. The successive

8 of 16
SADJADI
steps observed at 240, 390 and 570 °C can be assigned to
the loss of the functionalities on the HNTs. To determine
the content of CD, the thermogram of HNTs‐T was also
obtained (Figure 7) and compared with that of the cata-
lyst. In this way the content of CD and other functionali-
ties (thiosemicarbazide and silane compound) were
calculated to be ca 18 and ca 19 wt%, respectively.
the organic moiety. Regarding the pore volume, the
results showed that upon functionalization of the HNTs
with organic moiety, this value decreased. This observa-
tion can confirm that the functionalities not only were
positioned on the surface of the HNTs, but also could
locate inside the cavity of HNTs. This can be further
proved by comparing the average pore diameter of HNTs
and Pd@HNTs‐T‐CD. This value is higher for the latter,
indicating the presence of the functionalities inside the
cavity of HNTs.
To explore the content of Pd nanoparticles on the cat-
alyst, ICP‐AES analysis was applied. to this end, a known
amount of Pd@HNTs‐T‐CD was digested in concentrated
hydrochloric and nitric acids solution. The ICP‐AES anal-
ysis of the resulting extract established that the content of
Pd nanoparticles was about 0.2 wt%.
Nitrogen
adsorption–desorption
isotherm
of
Pd@HNTs‐T‐CD is depicted in Figure 8. Considering the
shape of the isotherm, which is of type II nitrogen adsorp-
tion–desorption isotherms with H3 hysteresis loops,[26] it
can be concluded that Pd@HNTs‐T‐CD possesses a
porous structure. Next, to study the effect of
functionalization of HNTs and incorporation of Pd nano-
particles on the textural properties of HNTs, specific sur-
face area, average pore diameter and total pore volume
of HNTs and Pd@HNTs‐T‐CD were calculated and com-
pared (Table 1). Comparing the surface areas of HNTs
and Pd@HNTs‐T‐CD, it can be seen that on
functionalization of the surface of HNTs with
thiosemicarbazide and CD, the surface area decreased
markedly, indicating the coverage of HNT surface with
3.2 | Catalytic Activity
In the next step, the potential of Pd@HNTs‐T‐CD for the
development of copper‐ and ligand‐free Sonogashira cou-
pling reactions was examined. Initially, the reaction of
TABLE 1 Textural properties of Pd@HNTs‐T and HNTs‐T
Catalyst
SBET (m2 g−1
)
Total pore volume (cm3 g−1
)
Average pore diameter (nm)
Pd@HNTs‐T‐CD
12.0
0.09
30.5
HNTs
51
0.2
15
TABLE 2 Optimization of reaction conditions for Sonogashira coupling reaction of iodobenzene with phenylacetylenea
Entry
Loading of catalyst (mol%)
Condition: solvent/temperature (°C)
Base
Time (h)
Yield (%)b
1
6
H2O–EtOH (4:1)/r.t.
K2CO3
3
60
2
6
4
9
6
6
6
6
6
6
6
6
6
6
6
6
H2O–EtOH (4:1)/100
H2O–EtOH (4:1)/60
H2O–EtOH (4:1)/60
H2O–EtOH (4:1)/60
H2O–EtOH (2.5:2.5)/60
H2O–EtOH (4:1)/60
H2O/60
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
KOH
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
3
80
75
95
95
85
80
75
70
78
60
55
65
50
90
95
3
4
5
6
7
8
9
EtOH/60
10
11
12
13
14
15
17
Toluene/reflux
CHCl3/60
4
CH3CN/60
5
H2O–EtOH (4:1)/60
H2O–EtOH (4:1)/60
H2O–EtOH (4:1)/60
H2O–EtOH (4:1)/60
3
NaOH
Cs2CO3
K2CO3
3
1.15
1.5
aReaction were run in 5 ml of solvent with 1 mmol of iodobenzene, 1.2 mmol of phenylacetylene and 2 mmol of base.
bIsolated yield.

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TABLE 3 Pd@HNTs‐T‐CD‐catalysed Sonogashira reaction of various halides with terminal alkynesa
Entry
Halide
Terminal alkyne
Product
Time (h)
Yield (%)b
1
1.5
95
2
3
2.15
2.25
93
90
4
5
6
7
2
90
85
87
90
2.5
2.4
2.5
8
9
2
91
83
3.5
10
11
12
13
2.15
2.5
90
90
85
90
2.45
3.5
14
15
3.5
91
92
3.45
16
17
18
19
20
4
80
76
73
72
83
4.5
4.5
5
4
(Continues)

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TABLE 3 (Continued)
Entry
Halide
Terminal alkyne
Product
Time (h)
Yield (%)b
21
4.25
81
22
23
24
4.5
4.2
4.5
70
80
76
25
25
5
5
68
51
27
5.45
45
aReaction conditions: aryl halide (1.0 mmol), terminal alkyne (1.2 mmol), Pd@HNTs‐T (6 mol%), K2CO3 (2.0 mmol) in water–EtOH (4:1/5.0 ml) at 60 °C.
bIsolated yield.
phenylacetylene and iodobenzene was selected as a model
reaction and performed in the presence of K2CO3 as base
and Pd@HNTs‐T‐CD as catalyst. As the development of a
green procedure for Sonogashira coupling reactions was
the aim, water was first tested as reaction medium. Unfor-
tunately, using water as solvent did not furnish the prod-
uct in high yield (only 75%). Then the reaction in ethanol
was examined. Gratifyingly, the desired product was
obtained in high yield, much higher than that of water
or other classic solvents such as toluene, acetonitrile and
chloroform (Table 2). Also, a mixture of ethanol and
water was tested as reaction medium. Among various
ratios of water to ethanol, a value of 4:l led to a yield sim-
ilar to that when using ethanol. Hence, a mixture of water
and ethanol in a ratio of 4:1 was selected as the reaction
medium. By comparing the yield of the model product
in the presence of various amounts of the catalyst at dif-
ferent temperatures, the optimum values of reaction tem-
perature and amount of catalyst were determined to be
60 °C and 6 mol%, respectively. Moreover, among various
examined bases, including K2CO3, Cs2CO3, KOH and
NaOH, K2CO3 was found to be the base of choice. Under
optimum reaction conditions, i.e. using 6 mol% of the cat-
alyst, K2CO3 as base in a mixture of ethanol and water
(4:1) at 60 °C, the model reaction was finished within
1.5 h to furnish the desired product in 95% yield.
through formation of inclusion complexes with hydro-
phobic guests[62–65] and improving their solubility in
aqueous phase and consequently increasing the yield of
the reaction. On the other hand, HNTs have been
reported as efficient supports for immobilization of nano-
particles.[19] Hence, it is expected that synergistic effects
between CD and HNTs can improve the catalytic activity
of Pd@HNTs‐T‐CD compared to that of Pd@HNTs‐T and
Pd@CD. To elucidate this assumption, Pd@HNTs‐T and
Pd@CD were prepared and their catalytic activities for
the synthesis of the model product were compared with
that of Pd@HNTs‐T‐CD. The results confirmed the supe-
rior catalytic activity of the latter compared to
Pd@HNTs‐T and Pd@CD, indicating the synergistic
effects between CD and HNTs.
In the next step, the generality of the protocol was
confirmed by using various acetylenes and halobenzenes.
As evident from Table 3, various halobenzenes could be
used for performing Sonogashira reactions successfully.
However, aryl iodides reacted more rapidly and efficiently
compared to bromobenzenes and chlorobenzenes to
afford the corresponding products in excellent yields. Aryl
bromide and chloride led to high and moderate yields,
respectively. Among aryl iodides, sterically demanding
derivative naphthyl iodide resulted in lower yield and lon-
ger reaction time.
The high catalytic activity of the catalyst can be attrib-
uted to the presence of both HNTs and CD. According to
previous reports, CD can promote the reaction rate
Finally, to elucidate the merit of Pd@HNTs‐T‐CD for
promoting Sonogashira coupling reactions, the yield of
the model product under the presented protocol was

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compared with those of some previously reported meth-
odologies (Table 4). Various procedures making use of dif-
ferent bases and reaction conditions have been developed
for promoting Sonogashira coupling reactions. Compared
to most of the previous reports (Table 4), Pd@HNTs‐T‐CD
led to the desired product in shorter reaction time. More-
over, the yield of the coupling product under Pd@HNTs‐
T‐CD catalysis was higher than or comparable to that of
the other catalysts listed in Table 4. Notably, in the pres-
ent protocol the reaction could proceed in aqueous
medium (ethanol–water mixture) in the presence of
Pd@HNTs‐T‐CD and K2CO3 with no need for toxic sol-
vent or base. Additionally, compared to most of the previ-
ous protocols, the required reaction temperature for
Pd@HNTs‐T‐CD was relatively low and the reaction
could proceed at 60 °C. It is worth noting that compared
to homogeneous catalysts such as classic Pd(OAc)2,
Pd@HNTs‐T‐CD was easily recoverable and recyclable.
All these results established that Pd@HNTs‐T‐CD could
be considered as an efficient catalyst for promoting the
Sonogashira coupling reaction.
3.3 | Reaction Mechanism
According to the literature,[79] a plausible reaction mech-
anism for promoting the Sonogashira reaction under
Pd@HNTs‐T‐CD catalysis can be proposed as depicted
as Scheme 3. More precisely, the terminal C─H in alkyne
could be activated by the catalyst and deprotonated by
K2CO3 to form potassium acetylide. Reaction of aryl
halide with the catalyst could also promote formation of
an intermediate, ArPdX. The latter undergoes halide dis-
placement to form arylpalladium acetylide. Finally,
reductive elimination furnishes the corresponding prod-
uct and regenerates Pd(0).
TABLE 4 Comparison of catalytic activity of present catalyst with that of other reported catalysts in the Sonogashira coupling reaction
between iodobenzene and phenylacetylenea
Time Yield
Entry Catalyst
Reaction conditions
(h)
(%)
Ref.
[66]
1
Pd(0)/MCoS‐1
Et3N/H2O/90 °C
6
94
[67]
[68]
[69]
[70]
[71]
[72]
[73]
[74]
[75]
[76]
[13]
[2]
2
NiCl2.6H2O
n‐Bu4‐NBr/NaOH/EG/120 °C
CH3CN, Et3N, 82 °C
Hexamine/H2O/reflux
DMSO/K3PO4/100 °C
DMF/K2CO3/90 °C
2
86
76
85
81
93
80
83
90
3
Nano‐Pd/AT‐Mont
Pd‐LHMS‐3
3
4
10
1
5
NHC–palladium complex
Fe3O4@SiO2/Schiff base/Pd(II)
I‐Pd
6
1
7
CH3CN/Et3N/reflux
DMF/piperidine/110 °C
DMF/Et3N/80 °C
24
24
10
8
Nano‐Pd@Fe3O4
Pd/MgLa
9
10
11
12
13
14
15
Polymeric N‐heterocyclic carbene–Pd complex‐grafted silica DMF/K2CO3/120 °C
1.25 93
Pd(OAc)2
MeCN/DABCO/air
18
6
100
Nanopolymer‐supported palladium(II) complex
SiO2‐acac‐Pd NPs
Water/TBAB/Et3N/70 °C
Water/piperidine/reflux
Water/K2CO3/80 °C
94
90
91
82
0.5
4
[11]
[77]
Pd NPs reduced by extract of Piper longum
Thiazolidine‐based heterogeneous palladium catalytic
Water/CH3CN/N‐methylpiperazineN2/
5
system
90 °C
[78]
16
17
18
PdNPs@EDACs
Water/K2CO3/CuI/80 °C
7
98
Pd@HNTs‐T
Pd@CD
H2O–EtOH (4:1)/60 °C
H2O–EtOH (4:1)/60 °C
H2O–EtOH (4:1)/60 °C
2
90
86
95
This
work
This
work
This
work
19
20
3
Pd@HNTs‐T‐CD
1.5
aTBAB: tetrabutylammonium bromide; Pd(0)/MCoS‐1: Pd‐grafted porous metal–organic framework; PdNPs@EDACs: Pd NPs supported on ethylenediamine‐
functionalized cellulose.

12 of 16
SADJADI
SCHEME 3 Mechanism of Sonogashira
reaction
To further investigate the catalytic activity of
leaching of Pd@HNTs‐T‐CD was carried out to elucidate
whether HNTs‐T‐CD could effectively immobilize Pd
nanoparticles and suppress Pd leaching. For this purpose,
the model reaction was performed in the presence of fresh
Pd@HNTs‐T‐CD. Upon the completion of the reaction,
the catalyst was filtered, washed with ethanol and dried
in an oven at 100 °C. Then, the recovered catalyst was
subjected to the next run of the reaction for up to 13 con-
secutive reaction runs. A comparison of the yield of the
model product using fresh and recycled catalysts
(Figure 9) demonstrated that recycling of the catalyst for
up to four reaction runs resulted in the desired product
in comparative yield, indicating that the catalyst pre-
served its catalytic activity. Recycling of the catalyst for
the fifth reaction run, however, led to a slight loss of the
catalytic activity (8%). Upon further recycling of the cata-
lyst, the decreasing trend of the yield of the model product
continued with a very slight slope (2–3% decrease in each
run) and the after 13 recycles of the catalyst, the yield of
the product decreased to 69%.
Pd@HNTs‐T‐CD, the efficiency of this catalyst for pro-
moting a model Mizoroki–Heck reaction, reaction of sty-
rene and iodobenzene, was investigated. It was found
that using 6 mol% Pd@HNTs‐T‐CD as catalyst and
K2CO3 as base, the reaction could proceed in ethanol at
60 °C to afford the desired product in 90% yield. Of note,
the reaction was efficient neither in pure water nor in a
mixture of water and ethanol, and ethanol was found to
be the best solvent. This result demonstrated that
Pd@HNTs‐T‐CD not only could be useful for promoting
the Sonogashira reaction but also could be used for other
C–C coupling reactions such as the Heck reaction.
3.4 | Catalyst Recyclability
Facile recovery, efficient recyclability and low leaching of
catalytic species are some of the most important features
of heterogeneous catalysts, which facilitate their large‐
scale uses. Hence, a study of the recyclability and Pd
FIGURE 9 Recyclability of the
Pd@HNTs‐T‐CD catalyst in Sonogashira
coupling reaction

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13 of 16
In the next step, Pd leaching was calculated by using
TEM images, an indication that Pd aggregation did not
occur. However, recycling of the catalyst for 13 reaction
runs led to aggregation of Pd nanoparticles (Figure 10).
These results indicated that the decrease in the catalytic
activity of the catalyst recycled for 13 reaction run not
only was due to Pd leaching but also arose from aggrega-
tion of the Pd nanoparticles.
Next, to study the stability of the catalyst in the course
of the reaction and to elucidate whether recovery and
recycling of Pd@HNTs‐T‐CD could affect its structure,
FT‐IR spectra of fresh and recycled Pd@HNTs‐T‐CD were
recorded and compared. As is obvious from Figure 11, the
spectra of fresh and recycled catalysts are identical and
exhibit the characteristic bands of the catalyst. This obser-
vation confirmed that the catalyst preserved its structure
in the course of the reaction, and recycling of the catalyst
did not lead to the collapse of its structure.
ICP‐AES analysis. It was found that the leaching of Pd
for up to four reaction runs was negligible and for the fifth
run this value was 0.01. Pd leaching for the recycled cata-
lyst for 13 reaction runs was 0.08. To elucidate whether
the low leaching of Pd nanoparticles is due to their depo-
sition on the product, four products (Table 3, entries 1–4)
were subjected to ICP‐AES analysis. For this purpose, the
products were digested in a mixture of concentrated HCl
and HNO3 and then the filtrates were analysed using
ICP‐AES. The results confirmed that the products were
not contaminated by Pd nanoparticles. This finding can
establish the heterogeneous nature of the catalysis. In
other words, the reaction did not proceed through
leaching of Pd into the solution and its re‐deposition on
the support.
To study whether recycling of Pd@HNTs‐T‐CD
resulted in a morphology change and Pd agglomeration,
SEM and TEM images of the recycled catalyst after 4
and 13 reaction runs were obtained (Figure 10). The
SEM images of fresh catalyst and catalyst recycled four
times are very similar, while recycled catalyst after 13
reaction runs exhibited more aggregated morphology.
Regarding TEM images, similar results were observed.
More precisely, recycling of the catalyst for up to four
reaction runs did not cause any significant change in
3.5 | Spectral Data for Selected
Compounds
3.5.1 | 1,2‐Diphenylethyne (Table 3, entry 1)[80]
1
White solid, m.p. 63 °C. H NMR (400 MHz, CDCl3, δ,
ppm): 7.60–7.57 (d, 4H, J = 12), 7.43–7.36 (m, 6H). 13C
NMR (100 MHz, CDCl3, δ, ppm): 132.1, 128.8, 128.8,
FIGURE 10 Recycled catalyst: TEM
images after (A) 4 and (B) 13 reaction runs;
SEM images after (C) 4 and (D) 13 reaction
runs

14 of 16
SADJADI
FIGURE 11 FT‐IR spectra of (a) fresh
Pd@HNTs‐T‐CD and (b) recycled catalyst
123.7, 89.9. FT‐IR (KBr, cm−1): 3413, 3062, 2921, 1952,
1630, 1490, 1442, 1161, 916, 754.
3.5.5 | 1‐(Phenylethynyl)naphthalene
(Table 3, entry 9)[82]
1
Yellow oil. H NMR (400 MHz, CDCl3, δ, ppm): 8.57 (d,
1H, J = 8.3), 7.97–7.84 (m, 3H), 7.79–7.67 (m, 3H), 7.62
(t, 1H, J = 7.4), 7.57–7.44 (m, 4H). 13C NMR (100 MHz,
CDCl3, δ, ppm): 133.3, 133.3, 131.7, 130.4, 128.8, 128.5,
128.5, 128.4, 126.9, 126.5, 126.3, 125.4, 123.5, 121.0,
94.4, 87.6.
3.5.2 | 1‐Methyl‐4‐(phenylethynyl)ben-
zene (Table 3, entry 2)[80]
White solid, m.p. 70–72 °C. 1H NMR (400 MHz, CDCl3, δ,
ppm): 7.57–7.54 (m, 2H), 7.47–7.45 (d, 2H, J = 8), 7.40–
7.35 (m, 3H), 7.20–7.18 (d, 2H, J = 8), 2.40 (s, 3H). 13C
NMR (100 MHz, CDCl3, δ, ppm): 138.9, 132.0, 132.0,
129.6, 128.8, 128.5, 123.9, 120.6, 90.0, 89.2, 22.0. FT‐IR
(KBr, cm−1): 3552, 3474, 3414, 2958,1626, 1500, 1261,
1098, 867, 751, 626.
3.5.6 | 3‐Phenylprop‐2‐yn‐1‐ol (Table 3,
entry 10)[83]
Yellow oil. 1H NMR (400 MHz, CDCl3, δ, ppm): 7.48–7.46
(m, 2H), 7.36–7.33 (m, 3H), 4.53 (s, 2H), 1.73 (s, 1H). 13C
NMR (100 MHz, CDCl3, δ, ppm): 132.1, 129.0, 128.8,
123.0, 52.2, 87.6, 86.2. FT‐IR (KBr, cm−1): 3300, 3065,
2922, 2859, 2237, 1959, 1888, 1745, 1597, 1488, 1441,
1363, 1259, 1027, 955, 800, 756, 690, 571, 520.
3.5.3 | 1‐Methoxy‐4‐(phenylethynyl)ben-
zene (Table 3, entry 4)[80]
White solid, m.p. 65–67 °C. 1H NMR (400 MHz, CDCl3, δ,
ppm): 7.57–7.55 (d, 2H, J = 8), 7.54–7.51 (d, 2H, J = 8),
7.40–7.29 (m, 3H), 6.93–6.91 (d, 2H, J = 8), 3.05 (s, 3H).
13C NMR (100 MHz, CDCl3, δ, ppm): 160.1, 133.5, 131.9,
128,8, 128.4, 124.1, 115.8, 114.5, 89.8, 88.5, 55.8. FT‐IR
(KBr, cm−1): 3472, 3414, 2925, 1895, 1647, 1597, 1502,
1540, 1245, 1023, 821, 751, 685.
4 | CONCLUSIONS
HNTs were functionalized with thiosemicarbazide and
then conjugated to tosylated CD to form HNTs‐T‐CD.
This hybrid system could be successfully used for the
immobilization of Pd nanoparticles and formation of
Pd@HNTs‐T‐CD. The latter could efficiently catalyse the
copper‐ and ligand‐free Sonogashira coupling reactions
of halobenzene and acetylenes in a mixture of water and
ethanol as solvent. Additionally, the catalyst was recycla-
ble and could promote the reactions for up to five reaction
runs with a slight loss of the catalytic activity and Pd
leaching.
3.5.4 | 1‐(4‐(Phenylethynyl)phenyl)ethan‐
1‐one (Table 3, entry 7)[81]
White solid, m.p. 95–97 °C. 1H NMR (250 MHz, CDCl3, δ,
ppm): 7.86 (d, 2H, J = 7.5, Ar‐H), 7.45–7.54 (m, 4H, Ar‐H),
7.16–7.33 (m, 3H, Ar‐H), 2.51 (s, 3H, CH3). 13C NMR
(62.9 MHz, CDCl3, δ, ppm): 197.6, 136.5, 132.2, 132.0,
129.5, 128.8, 128.3, 128.2, 123.0, 92.9, 88.9, 27.0.

SADJADI
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[24] Y. Zhang, A. Tang, H. Yang, J. Ouyang, Appl. Clay Sci. 2016,
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ACKNOWLEDGEMENTS
The author appreciates partial financial support from Iran
Polymer and Petrochemical Institute. Moreover, the
support of Prof. Heravi is cordially appreciated.
[25] M. Zou, M. Du, M. Zhang, T. Yang, H. Zhu, P. Wang, S. Bao,
Mater. Res. Bull. 2014, 61, 375.
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Antill, C. J. Kepert, J. Phys. Chem. C 2008, 112, 15742.
ORCID
[27] M. Massaro, V. Schembri, V. Campisciano, G. Cavallaro, G.
Lazzara, S. Milioto, R. Noto, F. Parisi, S. Riela, RSC Adv. 2016,
6, 55312.
Samahe Sadjadi http://orcid.org/0000-0002-6884-4328
[28] M. Massaro, G. Lazzara, S. Milioto, R. Noto, S. Riela, J. Mater.
Chem. B 2017, 5, 2867.
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How to cite this article: Sadjadi S. Palladium
nanoparticles immobilized on cyclodextrin‐
decorated halloysite nanotubes: Efficient
heterogeneous catalyst for promoting copper‐ and
ligand‐free Sonogashira reaction in water–ethanol
mixture. Appl Organometal Chem. 2018;e4211.
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