Palladium nanoparticles immobilized on sepiolite–cyclodextrin nanosponge hybrid: Efficient heterogeneous catalyst for ligand- and copper-free C─C coupling reactions

Article abstract of DOI:10.1002/aoc.4508

A novel hybrid system composed of sepiolite clay and cyclodextrin nanosponge (CDNS) was prepared via reaction of Cl-functionalized sepiolite with amine-functionalized CDNS. CDNS–sepiolite was then applied for immobilization of Pd(0) nanoparticles. The resulting hybrid system, Pd@CDNS-sepiolite, was characterized using various techniques and successfully used as an efficient and heterogeneous catalyst for ligand- and copper-free Sonogashira and Heck coupling reactions under mild reaction conditions. Recycling experiments confirmed that Pd@CDNS-sepiolite was recyclable and could be used for several consecutive reaction runs with slight Pd leaching and loss of catalytic activity.

Full text of DOI:10.1002/aoc.4508

Received: 13 February 2018
DOI: 10.1002/aoc.4508
Revised: 27 May 2018
Accepted: 18 June 2018
F U L L P A P E R
Palladium nanoparticles immobilized on sepiolite–
cyclodextrin nanosponge hybrid: Efficient heterogeneous
catalyst for ligand‐ and copper‐free C─C coupling reactions
Samahe Sadjadi1
| Majid M. Heravi2
| Maryam Raja2 | Fatemeh Ghoreyshi Kahangi3
1 Gas Conversion Department, Faculty of
Petrochemicals, Iran Polymer and
A novel hybrid system composed of sepiolite clay and cyclodextrin nanosponge
(CDNS) was prepared via reaction of Cl‐functionalized sepiolite with amine‐
functionalized CDNS. CDNS–sepiolite was then applied for immobilization of
Pd(0) nanoparticles. The resulting hybrid system, Pd@CDNS‐sepiolite, was
characterized using various techniques and successfully used as an efficient
and heterogeneous catalyst for ligand‐ and copper‐free Sonogashira and Heck
coupling reactions under mild reaction conditions. Recycling experiments con-
firmed that Pd@CDNS‐sepiolite was recyclable and could be used for several
consecutive reaction runs with slight Pd leaching and loss of catalytic activity.
Petrochemicals Institute, Tehran, Iran
2 Department of Chemistry, School of
Science, Alzahra University, Vanak,
Tehran, Iran
3 Department of Chemistry, University of
Gilan, Rasht, 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;
s.sadjadi@ippi.ac.ir
KEYWORDS
C─C coupling reactions, cyclodextrin nanosponge, Pd nanoparticles, sepiolite
Majid M. Heravi, Department of
Chemistry, School of Science, Alzahra
University, PO Box 1993891176, Vanak,
Tehran, Iran.
Email: mmh1331@yahoo.com;
m.heravi@alzahra.ac.ir
Funding information
Iran Polymer and Petrochemical Institute
and Alzahra University; Iran National
Science Foundation
1 | INTRODUCTION
the applications of CDNSs in various research fields such as
drug delivery[11–13] and catalysis have been growing.[1,14–18]
Cyclodextrin nanosponges (CDNSs), three‐dimensional poly-
meric network systems, are derived from the reaction of a
cross‐linker[1,2] such as carbonyl diimidazole and
diarylcarbonates[3] with cyclodextrins (CDs) as monomers.
The presence of CDs, which naturally have cavities, as well
as the porous structure of a polymeric network render CDNSs
promising candidates for accommodating diverse range of
guest molecules with various sizes, shapes and polarities from
drugs and organic compounds to catalytic active spe-
cies.[1,2,4,5] The high thermal and pH stability of CDNSs as
well as biocompatibility expand their utility.[1,6–10] Recently,
Sepiolite is a naturally occurring hydrated Mg–Al silicate
fibrous clay with the general formula of Mg8Si12O30
(OH)4(OH2)4⋅nH2O (Figure S1 in supporting informa-
tion).[19] As sepiolite is an assembly of needle‐like particles
separated by parallel channels,[20] it possesses an open
porous framework. Each structural unite of sepiolite is
composed of two tetrahedral silica sheets and a central
octahedral sheet containing magnesium.[21] The porous
nature of sepiolite as well as its high surface area make it a
suitable candidate for supporting various catalytic species
and developing novel heterogeneous (photo)catalysts.[22–24]
Appl Organometal Chem. 2018;e4508.
wileyonlinelibrary.com/journal/aoc
© 2018 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.4508

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SADJADI ET AL.
Among various organic transformations, C─C coupling
(OAc)2, NaBH4, (3‐chloropropyl) trimethoxysilane, N‐[3‐
(trimethoxysilyl)propyl] ethylenediamine (AEAPTMS), tol-
uene and MeOH. All were of analytical grade and purchased
from Sigma‐Aldrich and used without further purification.
Sepiolite clay, PANGEL S9, was provided from TOLSA.
The organic reagents for performing Heck and
Sonogashira coupling reactions included aryl halides,
acetylenes, K2CO3, alkenes and EtOH. All were of analyt-
ical grade and purchased from Merck and used without
further purification.
reactions such as Heck and Sonogashira reactions are of
great importance. These Pd‐catalysed reactions are potent
tools for the synthesis of natural products or complicated
chemicals with biological activities.[25] Classically, these
coupling reactions proceed in the presence of phosphine
ligands and copper co‐catalysts.[26,27] Development of cop-
per‐ and ligand‐free protocols[28] with the use of heteroge-
neous catalysts in aqueous media is considered as a
turning point in this field and can facilitate the applications
of such processes for industrials purposes.[26,29–40] As an
example, immobilization of Pd on cellulose–aluminium
oxide composite[41] and biopolymers[42–45] can be men-
tioned. In this context, use of biopolymers such as wool, chi-
tosan and cellulose as catalyst supports has attracted much
attention due to their biodegradability, biocompatibility,
chemical and physical versatility, availability and relatively
low cost. Notably, most biopolymers can be chemically
modified or hybridized with other materials.[42–45]
In the course of our research on heterogeneous cataly-
sis,[46–49] recently we reported the utility of CDNS and nat-
ural clays for immobilizing catalytic active species.[50] In
this line, it was found that the hybridization of inorganic
supports with CDNS can lead to an improvement of the cat-
alytic performance of the resulting catalyst.[51] Considering
this issue, herein we report the utility of a CDNS–sepiolite
hybrid system for immobilization of Pd(0) nanoparticles
and development of a heterogeneous catalyst, Pd@CDNS‐
sepiolite, with the utility of promoting Sonogashira and
Heck reactions (Schemes 1 and 2). Furthermore, the recy-
clability of Pd@CDNS‐sepiolite and Pd leaching upon
recovery and recycling were investigated.
The structure of the hybrid catalyst was confirmed using
various characterization techniques including X‐ray diffrac-
tion (XRD), Fourier transform infrared (FT‐IR) spectroscopy
and thermogravimetric analysis (TGA). The morphology of
the catalyst was studied by recording scanning electron
microscopy (SEM)/energy‐dispersive spectroscopy (EDS)
and transmission electron microscopy (TEM) images. More-
over, the content of Pd nanoparticles in the catalyst was esti-
mated using inductively coupled plasma atomic emission
spectrometry (ICP‐AES). The textural properties of the cata-
lyst were investigated using Brunauer–Emmett–Teller mea-
surements. SEM/EDS images of Pd@CDNS‐sepiolite and
its components were recorded with a Tescan instrument. A
BELSORP Mini II apparatus was used for studying the tex-
tural properties of the catalyst. To degas the catalyst, it was
heated at 423 K for 4 h. XRD patterns of sepiolite and
Pd@CDNS‐sepiolite were recorded using a Siemens D5000
with Cu Kα radiation from a sealed tube. A Mettler Toledo
TGA instrument was employed for accomplishing TGA,
using nitrogen atmosphere and a heating rate of 10 °C min
−1 in the range 30–700 °C. FT‐IR spectra were recorded with
a PerkinElmer Spectrum 65 instrument. TEM analyses were
performed using a Philips CM30300Kv field emission trans-
mission electron microscope. To perform this analysis, the
samples were coated with gold. To perform ICP analysis,
an ICP analyser (Vista‐pro, Varian) was employed.
2 | EXPERIMENTAL
2.1 | Materials and Instrumentation
Heck and Sonogashira coupling reactions were moni-
tored using HPLC (Agilent‐1100). As all the organic products
of Heck and Sonogashira reactions were previously reported,
their formation was validated by comparing their melting
points, determined in open capillaries using an Electrother-
mal 9100 without further corrections, and FT‐IR spectra with
those of known samples. To further confirm the structure of
the products, some selected products were characterized
To prepare the catalyst, the following reagents were used:
sepiolite clay, diphenyl carbonate, β‐cyclodextrin, Pd
1
SCHEME 1 Sonogashira reaction.
using H NMR and 13C NMR spectroscopy, with a Bruker
DRX‐400 spectrometer at 400 and 100 MHz, respectively.
2.2 | Synthesis of CDNS
To prepare CDNS (1), the melting approach was
applied.[4,5,9] Briefly, cross‐linking agent diphenyl carbon-
ate (8 mmol) was melted. Then, β‐cyclodextrin (1 mmol),
which served as monomer, was added to the melted
SCHEME 2 Heck reaction.

SADJADI ET AL.
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cross‐linking agent. The polymerization reaction
proceeded by stirring the mixture at 120 °C for 12 h. At
the end of the reaction, the obtained white solid was
cooled to room temperature and milled. Subsequently, to
purify CDNS and remove phenol by‐product that formed in
the course of the reaction, a solution NaOH was added to
an aqueous suspension of CDNS and the resulting mixture
was stirrer for 5 h to let the phenol become soluble sodium
phenoxide. Then, the CDNS was filtered off and repeatedly
washed with acetone and distilled water. Further
purification was performed by Soxhlet extraction with EtOH
for 4 h. To ensure successful removal of phenol, the purified
CDNS was suspended in basic aqueous medium and stirred
for 3 h. Then, CDNS was filtered and the filtrate was
subjected to UV–visible spectral and HPLC analyses. The
results confirmed that CDNS did not contain phenol. The
pure CDNS was then dried in an oven at 90 °C for 12 h.
nucleophilic substitution reaction of sepiolite‐Cl and
CDNS‐N. More precisely, the electron pair of amine on
CDNS‐N attacked the Cl‐sepiolite and substituted Cl (Cl
acted as an active leaving group).
2.6 | Immobilization of Pd Nanoparticles
on CDNS–sepiolite
CDNS–sepiolite (1.2 g) was suspended in dry toluene
(20 ml) and stirred vigorously for 15 min. Then, a solu-
tion of Pd (OAc)2 (0.02 g) in MeOH (12 ml) was slowly
added. The obtained suspension was then stirred at ambi-
ent temperature for 10 h. To obtain Pd(0) nanoparticles,
NaBH4 was used as reducing agent. More precisely,
NaBH4 was dissolved in a mixture of toluene and MeOH
(10 ml, 0.2 N) and introduced to the abovementioned sus-
pension. Then, the suspension was stirred for 4 h. Finally,
Pd@CDNS‐sepiolite was obtained (70% yield) by washing
with MeOH and drying in an oven at 60 °C for 12 h
(Figure 1).
2.3 | Synthesis of Amine‐Functionalized
CDNS (CDNS‐N)
To prepare Pd@sepiolite and Pd@CDNS, similar
methodologies were used, except that sepiolite and
CDNS‐N were used, respectively, for immobilization of
Pd nanoparticles.
A solution of AEAPTMS (4 ml in 20 ml of dry toluene) was
added to a suspension of CDNS (1.2 g) in dry toluene
(40 ml). The mixture was then subjected to ultrasonic irradi-
ation at a power of 100 W for 30 min. Subsequently, the
obtained suspension was refluxed for 24 h. Upon completion
of the process, the white solid (2) was filtered and washed
repeatedly with dry toluene and dried at 80 °C overnight.
2.7 | Typical Procedure for Sonogashira
Reaction
To a mixture of the organic reagents, i.e. aryl halide
(1.0 mmol) and acetylene (1.2 mmol) in EtOH (20 ml),
Pd@CDNS‐sepiolite (25 mg) and K2CO3 (2.0 mmol) were
added. Then, the resulting mixture was heated at 50 °C
with stirring. HPLC was used for monitoring the progress
of the reaction. Upon completion of the reaction, the cat-
alyst was filtered off and the reaction mixture was cooled
to room temperature. Then, the organic layer was
extracted with diethyl ether and purified by column chro-
matography over silica gel using hexane–ethyl acetate
(4:1) as eluent.
2.4 | Synthesis of Cl‐Functionalized
Sepiolite (Sepiolite‐Cl)
Sepiolite (1 g) was suspended in xylene (100 ml) and
mixed vigorously for 15 min. Subsequently, 3‐
chloropropyltrimethoxysilane (2.5 ml) was added
dropwise to the solution. The resulting suspension was
then stirred for 24 h. Sepiloite‐Cl was obtained by drying
in a vacuum oven at 90 °C overnight.
2.5 | Synthesis of CDNS–Sepiolite Hybrid
2.8 | Typical Procedure for Catalytic
Mizoroki–Heck Reaction
To a mixture of sepiolite‐Cl (2.25 g) in dry toluene
(100 ml), ammonia (0.5 ml) as a catalyst was added. Sub-
sequently, CDNS‐N (0.75 g) was crushed and well dis-
persed under ultrasonic irradiation of 100 W in toluene
and then added to the aforementioned mixture. Subse-
quently, the suspension was refluxed for 24 h. Upon com-
pletion of the reaction, the solid was filtered and poured
in water. Then the aqueous suspension was centrifuged.
Sepiolite–CDNS that was at the top of the centrifuged
mixture was gathered, washed with toluene repeatedly
and dried in a vacuum oven at 100 °C. The conjugation
of sepiolite and CDNS was achieved through routine
To a mixture of aryl halide (1.0 mmol) and alkene
(1.5 mmol) in EtOH (20 ml), K2CO3 (2 mmol) and
Pd@CDNS‐sepiolite (30 mg) were added and the
resulting mixture was stirred at 100 °C for the appropriate
reaction time. Upon completion of the reaction,
Pd@CDNS‐sepiolite was filtered, washed with EtOH
and dried at 70 °C for further use. The organic layer,
extracted with diethyl ether, was washed with water, sep-
arated and dried over anhydrous Na2SO4. Recrystalliza-
tion was used for further purification.

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SADJADI ET AL.
FIGURE 1 Synthetic procedure for preparation of Pd@CDNS‐sepiolite
shift is observed for the CDNS‐N spectrum. Again, the
similarity of these two spectra can be attributed to the
low amount of organosilane on CDNS. The FT‐IR spectra
of CDNS–sepiolite and Pd@CDNS‐sepiolite are also
depicted in Figure 2. The characteristic bands of CDNS‐
N and sepiolite are present in these spectra. This observa-
tion can confirm the formation of CDNS–sepiolite.
The XRD patterns of sepiolite and Pd@CDNS‐sepio-
lite are illustrated in Figure 3. According to the litera-
ture,[20] the reflection bands observed at 2θ = 7.6°,
19.6°, 20.70°, 23.9°, 28°, 27.38°, 34.4°, 37.54° and 40.1°
can be attributed to the sepiolite structure.[20] Other
observed peaks can be attributed to the impurities of sepi-
olite. As depicted, the characteristic peaks of Pd(0) nano-
particles cannot be detected as the Pd characteristic peaks
are generally small and the amount of Pd nanoparticles is
very low, as confirmed by ICP analysis (discussed below).
Considering the literature, a high distribution of Pd nano-
particles can also justify the fact that the characteristic
peaks of Pd nanoparticles were not observed.[53]
3 | RESULTS AND DISCUSSION
3.1 | Catalyst Characterization
In Figure 2 the FT‐IR spectra of CDNS, CDNS‐N, sepio-
lite, sepiolite‐Cl, CDNS–sepiolite and Pd@CDNS‐sepiolite
are depicted. The FT‐IR spectrum of sepiolite exhibits
characteristic bands at 3561, 3453 and 645 cm−1, which
can be assigned to the stretching and bending vibrations
of ─OH groups. These functional groups are attached to
octahedral Mg ions located in the interior blocks of sepi-
olite.[52] The observed band at 1016 cm−1 is representative
of Si─O stretching. The FT‐IR spectrum of sepiolite‐Cl is
very similar to that of pristine sepiolite. This observation
is not beyond expectation, as the content of organosilane
is very low (estimated to be about 5 wt% via TGA,
discussed below). The FT‐IR spectrum of CDNS showed
a characteristic band at 3350 cm−1 that is due to the
─OH functional group. The band at 2923 cm−1 can be
assigned to the ─CH2 functional groups. Moreover, the
band at 1775 cm−1 in the spectrum of CDNS‐N can be
attributed to the C═O functional group. Comparing the
FT‐IR spectrum of CDNS with that of CDNS‐N, it can
be seen that both spectra are similar and only a small
The SEM images of CDNS, CDNS‐N, sepiolite,
CDNS–sepiolite and Pd@CDNS‐sepiolite are depicted in
Figure 4. As shown, CDNS exhibited aggregate‐like mor-
phology. Upon functionalization with organosilanes, the

SADJADI ET AL.
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FIGURE 2 FT‐IR spectra of CDNS, sepiolite, CDNS‐N, sepiolite‐Cl, CDNS–sepiolite and Pd@CDNS‐sepiolite
FIGURE 3 XRD patterns of
Pd@CDNS‐sepiolite and sepiolite
morphology of CDNS changed to some extent. However,
CDNS‐N still showed aggregate‐like morphology. The
SEM image of sepiolite is in good accordance with the lit-
erature[52] and exhibited needle‐like morphology. As
shown, upon conjugation with CDNS, sepiolite preserved
its needle‐like morphology. Incorporation of Pd nanopar-
ticles also did not alter markedly the needle‐like mor-
phology. However, some small aggregates can be
detected in the SEM images of Pd@CDNS‐sepiolite.
The EDS spectrum of Pd@CDNS‐sepiolite (Figure 4)
showed Si, O and Mg atoms, which are mainly represen-
tative of sepiolite. Moreover, the presence of Pd atoms
can confirm the incorporation of Pd(0) nanoparticles in
the hybrid catalyst. Also, the observation of C and N
atoms can be attributed to the presence of CDNS‐N. How-
ever, EDS analysis cannot solely confirm the formation of
CDNS‐N. Hence, the formation of CDNS and the final
catalyst was further confirmed using other techniques
(discussed below).
In Figure 5, TEM images of sepiolite and
Pd@CDNS‐sepiolite are depicted. As shown, the TEM
image of pristine sepiolite exhibited needle‐like mor-
phology, which is in good agreement with previous
reports.[54,55] A comparison of the TEM images of the
catalyst and that of pristine sepiolite showed that the
catalyst is slightly different from the sepiolite and exhib-
ited a slightly more compact morphology. In the TEM
images of Pd@CDNS‐sepiolite, it can be seen that the
sepiolite needles are covered with CDNS to some extent.
This surface functionalization resulted in the slightly
aggregated morphology. Considering the TEM images,
in which the Pd nanoparticles are not detectable, it
can be concluded that Pd nanoparticles can locate
within the interlayer of sepiolite. Moreover, according
to the literature, Pd can also be accommodated within
the cavities of CDs.[56] Hence, another possibility for
accommodation of Pd nanoparticles is the cavities of
CDs in CDNS.

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SADJADI ET AL.
FIGURE 4 SEM images of (a) CDNS, (b) CDNS‐N, (c) sepiolite, (d) CDNS–sepiolite and (e, f) Pd@CDNS‐sepiolite. (g) EDS analysis of
Pd@CDNS‐sepiolite

SADJADI ET AL.
7 of 14
FIGURE 5 TEM images of (a) Pd@CDNS‐sepiolite and (b) sepiolite
ICP‐AES analysis was applied to calculate the con-
tent of Pd nanoparticles in Pd@CDNS‐sepiolite. To pre-
pare a sample for analysis, a known amount of
Pd@CDNS‐sepiolite (0.002 g) was digested in a concen-
trated 1:5 mixture (18 ml) of acidic solution of HNO3
and HCl (6 M) and then the obtained extract was
analysed using ICP‐AES. Using this methodology and
knowing the starting amount of the catalyst, the con-
tent of Pd nanoparticles was estimated to be about
0.5 wt%.
In Figure 6, the nitrogen adsorption–desorption iso-
therm of Pd@CDNS‐sepiolite is depicted. As illustrated,
the obtained isotherm is of type II with H3 hysteresis
loops.[57] This observation can imply that Pd@CDNS‐
sepiolite is of a porous nature. It is worth mentioning that
the nitrogen adsorption–desorption isotherm of
Pd@CDNS‐sepiolite is distinguished from that of CDNS
(Figure S2 in supporting information).
of sepiolite (161 m2 g−1) and higher than that of CDNS
(ca 10 m2 g−1).
The catalyst was also investigated using TGA
(Figure 7). To estimate the content of organic functional-
ities, the thermograms of sepiolite‐Cl and CDNS–sepiolite
were also recorded (Figure 7). Sepiolite‐Cl exhibited two
weight losses: the first one at about 120 °C can be
assigned to the loss of water and the second at about
330 °C is due to loss of organosilane. Using this thermo-
gram, the content of organosilane on the sepiolite was
estimated to be about 5 wt%.
The thermogram of CDNS–sepiolite showed three
weight losses. The first was at about 110 °C, which
can be due to loss of water, the others at about 320
and 560 °C. These weight losses can be due to the
degradation of CDNS‐N.[50] Using this analysis, the
content of CDNS‐N was calculated to be about
15 wt%.
The specific surface area of Pd@CDNS‐sepiolite was
Considering the above discussion, the weight losses in
the thermogram of the catalyst can be attributed to the
calculated to be 130 m2 g−1, which was lower than that

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SADJADI ET AL.
FIGURE 6 Nitrogen adsorption–
desorption isotherms of Pd@CDNS‐
sepiolite
FIGURE 7 TGA curves of Pd@CDNS‐
sepiolite, sepiolite‐Cl and CDNS–sepiolite
loss of water (about 150 °C) and degradation of CDNS
and organosilanes (about 320 and 530 °C).
while the reaction of methyl acrylate and iodobenzene
was chosen for the Heck reaction. First, the model reac-
tions were performed in the presence of cost‐effective
K2CO3 as base and water as solvent at room temperature.
The results established that, although the resections
proceeded in the absence of copper and ligand, the yields
of the products were moderate (Tables S1 and S2 in
supporting information). Hence, to improve the yield of
the desired products, the reactions were performed in var-
ious solvents, such as CH3CN, toluene and CHCl3
(Tables S1 and S2 in supporting information). Gratify-
ingly, use of EtOH as solvent markedly increased the
yield of the reaction. To further improve the yield of
the products, the reaction was examined at elevated
3.2 | Catalytic Activity
After verifying the structure of Pd@CDNS‐sepiolite, the
study of its catalytic activity was undertaken. In this line,
the utility of Pd@CDNS‐sepiolite for promoting C─C
coupling reactions, Heck and Sonogashira reactions, was
investigated. Initially, two model reactions were selected
to study the effects of reaction variables and to optimize
the reaction conditions.
For the Sonogashira reaction, the reaction of
iodobenzene with acetylene was selected as a model,

SADJADI ET AL.
9 of 14
TABLE 1 Sonogashira reaction of aryl halides with alkynesa
Entry
Aryl halide
Terminal alkyne
Product
Time (h:min)
Yield (%)b
1
1:45
94
2
3
4
3:30
3:00
2:00
88
93
92
5
3:30
80
6
7
8
9
3:30
3:50
3:20
4:40
70
66
73
60
10
11
12
13
3:30
3:00
3:30
4:30
93
90
92
90
14
4:30
85
15
16
4:30
5:15
93
80
aReactions were run with 1 mmol of aryl halide, 1.2 mmol of acetylene and 2 mmol of K2CO3 in the presence of the catalyst in EtOH.
bIsolated yield.
temperatures (50 and 70 °C; Tables S1 and S2 in
supporting information). It was found that an increase
of the temperature to an optimum amount (50 °C for
Sonogashira reaction and 70 °C for Heck reaction)
increased the yields of the products. However, further
increase of the temperature had a detrimental effect
and decreased the yields of the reactions. The effect of
base was also investigated using various bases, such as
NaOH, Cs2CO3 and KOH (Tables S1 and S2 in
supporting information). It was found that K2CO3 was

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SADJADI ET AL.
TABLE 2 Heck reaction of aryl halides with alkenesa
Entry
Aryl halide
Alkene
Product
Time (h)
Yield (%)b
1
3
86
2
3
2
2
90
80
4
5
6
2.5
2
90
90
92
2
7
3
85
88
70
73
8
3
9
4.2
4
10
aReaction conditions: aryl halide (1.0 mmol) and alkene (1.5 mmol) in EtOH (20 ml) in the presence of K2CO3 (2 mmol) and catalyst.
bIsolated yield.
the best base for these protocols. Moreover, optimization
of the amount of the base was performed and the opti-
mum amount of K2CO3 was estimated to be 2 mmol.
Finally, the optimum amount of catalyst was deter-
mined using various amounts of Pd@CDNS‐sepiolite
(0.01–0.03 mg; Tables S1 and S2 in supporting informa-
tion). It was found that using 0.02 g catalyst led to the
highest yields of the products.
Next, to elucidate whether these protocols could be
generalized, various Heck and Sonogashira products
were synthesized using a selection of alkenes, alkynes
and aryl halides with a variety of electronic and steric
properties (Tables 1 and 2). All employed reagents
could undergo the corresponding coupling reactions
and resulted in products in good to high yields. Of
note, use of aryl iodides led to higher yields compared
to aryl chlorides and aryl bromides. Furthermore,
reagents with less steric hindrance resulted in higher
yields.
To study the effect of hybridization of CDNS and sepi-
olite, two control catalysts, Pd@CDNS and Pd@sepiolite,
were prepared (Section 2) and then their Pd content was
estimated via ICP and compared with that of
Pd@CDNS‐sepiolite. The results confirmed that the con-
tent of Pd in Pd@CDNS‐sepiolite was higher than that
in Pd@CDNS and Pd@sepiolite, indicating that hybridiz-
ing CDNS and sepiolite can improve the anchoring of Pd
species. Notably, due to the higher content of Pd in
Pd@CDNS‐sepiolite, it was more efficient compared to
Pd@sepiolite and Pd@CDNS.
According to the literature, the main catalytic active
species for the coupling reactions is Pd.[58] The proposed
mechanism for the ligand‐ and copper‐free Sonogashira
coupling reaction in the presence of the catalyst is as follows.

SADJADI ET AL.
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First, the reaction is commenced by activation of terminal
C─ H of alkyne by the catalyst. Subsequently, deprotonation
by K2CO3 and formation of an intermediate, potassium
acetylide, occur (Figure 8). Aryl halide is activated by
Pd@CDNS‐sepiolite to afford another intermediate, ArPdX,
that undergoes halide displacement to form arylpalladium
acetylide. C─C coupling product as well as Pd@CDNS‐sepi-
olite are produced by reductive elimination.
To further investigate the effect of recycling, ICP‐AES
analysis was used to investigate the leaching of Pd upon
recycling. Low leaching of Pd was observed in the case
of the catalyst recycled for the fifth time. This can justify
the observed decrease in the yield of the products upon
catalyst reuse.
Next, the stability of Pd@CDNS‐sepiolite upon
recycling was investigated by comparing the FT‐IR
spectra of fresh and recycled Pd@CDNS‐sepiolite. As
shown in Figure 10, the spectra of fresh and recycled
catalyst are very similar, implying that the structure
of Pd@CDNS‐sepiolite was preserved upon recycling.
It is worth noting that recycling of the catalyst may
result in partial hydrolysis of CDNS. Hence, the
observed decrease of the yield of the model reaction
can be attributed to both Pd leaching and hydrolysis
of CDNS.
3.3 | Catalyst Recyclability
In the last part of the investigation, the recyclability of
Pd@CDNS‐sepiolite was studied. In this line, after the
end of Sonogashira model reaction, the catalyst was fil-
tered, washed and dried (Section 2) and subjected to the
next run of the model reaction. This cycle was repeated
for five consecutive reaction runs. A comparison of the
yields of the product in the case of fresh and recycled cata-
lyst established that Pd@CDNS‐sepiolite could be success-
fully recycled with slight loss of catalytic activity (Figure 9).
The morphology of the recycled Pd@CDNS‐sepiolite
was also studied by recording the SEM image of the
recycled catalyst. As shown in Figure 11, the morphology
FIGURE 8 Plausible mechanism for
promotion of Sonogashira coupling
reaction
FIGURE 9 Efficiency of catalyst for
promoting model reaction after several
reaction runs

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SADJADI ET AL.
FIGURE 10 FT‐IR spectra of fresh and
recycled Pd@CDNS‐sepiolite
FIGURE 11 SEM image of recycled
catalyst.
of the recycled catalyst is slightly more aggregated but
still very similar to that of fresh catalyst, implying that
recycling did not markedly affect the morphology.
immobilization of Pd nanoparticles. The catalytic activ-
ity of Pd@CDNS‐sepiolite for ligand‐ and copper‐free
Sonogashira and Heck coupling reactions was con-
firmed. Moreover, it was proved that the catalyst was
recyclable and could be easily recovered and recycled
for several consecutive reaction runs with only slight
loss of the catalytic activity and slight leaching of
Pd(0) nanoparticles.
4 | CONCLUSIONS
CDNS‐N reacted with sepiolite‐Cl to afford CDNS–sepio-
lite which subsequently served as an efficient support for

SADJADI ET AL.
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ACKNOWLEDGEMENTS
The authors are grateful for partial financial support from
Iran Polymer and Petrochemical Institute and Alzahra
University. MMH is also grateful to Iran National Science
Foundation for an individual grant. The contribution of
Professor Atai is gratefully appreciated for providing sepi-
olite clay.
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Samahe Sadjadi http://orcid.org/0000-0002-6884-4328
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How to cite this article: Sadjadi S, Heravi MM,
Raja M, Kahangi FG. Palladium nanoparticles
immobilized on sepiolite–cyclodextrin nanosponge
hybrid: Efficient heterogeneous catalyst for ligand‐
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Organometal Chem. 2018;e4508. https://doi.org/
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