Comptes Rendus Chimie

Ramin Ghorbani-Vaghei

Narges Sarmast

Farid Rahmatpour

lable at ScienceDirect
C. R. Chimie 21 (2018) 644e651Contents lists avaiComptes Rendus Chimie
www.sciencedirect.comFull paper/MemoireImmobilization of palladium nanoparticles as a recyclable
heterogeneous catalyst for the SuzukieMiyaura coupling
reaction
Ramin Ghorbani-Vaghei*, Narges Sarmast, Farid Rahmatpour
Department of Organic Chemistry, Faculty of Chemistry, Bu-Ali Sina University, 65174 Hamedan, Irana r t i c l e i n f o
Article history:
Received 9 January 2018
Accepted 15 March 2018
Available online 13 April 2018
Keywords:
LDH
SuzukieMiyaura reaction
Palladium
Biaryl
Tris* Corresponding author.
E-mail address: rgvaghei@yahoo.com (R. Ghorba
https://doi.org/10.1016/j.crci.2018.03.004
1631-0748/© 2018 Académie des sciences. Publishea b s t r a c t
This work describes the synthesis of Pd nanoparticles that are stabilized on CaAl-layered
double hydroxide functionalized with Tris (tris(hydroxymethyl)aminomethane). The syn-
thesized catalyst is characterized by several different analyses and has been successfully
applied to the SuzukieMiyaura reaction.
© 2018 Académie des sciences. Published by Elsevier Masson SAS. All rights reserved.1. Introduction
One of the most valuable synthetic methods to prepare
symmetric and nonsymmetrical biaryls is a palladium-
catalyzed Suzuki coupling reaction of aryl halides with
arylboronic acids. So many efforts in the catalysis research
have been allocated to extend efficient, safe, and reusable
palladium catalysts [1]. Nowadays, the immobilization of
palladium active sites onto supports to produce heteroge-
neous and reusable catalysts has attracted much attention
[2,3]. Different inorganic, organic, and hybrid porous sys-
tems, suchas zeolites [4],metaleorganic frameworks (MOFs)
[5], and polymers [6], have been applied for supported
palladium catalysts. Catalyst-based layered double hydrox-
ides (LDHs) have gained considerable attention because, in
addition to their readily dispersed characteristic in a reaction
solutionwith an intrinsically high surface area rending, they
are heterogeneous and can be easily recovered from the re-
action mixture. Thus, a lot of LDH-supported catalysts withni-Vaghei).
d by Elsevier Masson SAS. Ahigh catalytic activities have been extended and used in
different organic synthesis, including transesterification re-
actions [7], hydrogenation and oxidation reactions [8],
polymerization of alkene oxide [9], aldol condensation [10],
and synthesis of heterocyclic compounds [11].
In this study, in continuation of our research on the
stabilization of palladium nanoparticles (NPs) on the novel
supports [12e14], we present LDH/Tris/Pd using LDH as a
support and Tris as a coupling linker. Tris with three hy-
droxyl groups in a molecule can chelate to palladium,
producing a novel and efficient catalyst.2. Experimental setup
2.1. Chemicals and instruments
All commercial materials were purchased from Merck
companies and used without further purifications. Fourier
transform infrared (FT-IR) spectra were recorded using a
Shimadzu 435-U-04 FT spectrophotometer (KBr pellets).
Scanning electron microscopy (SEM) was performed using
field emission SEM instrument (SIGMA, Germany).ll rights reserved.
R. Ghorbani-Vaghei et al. / C. R. Chimie 21 (2018) 644e651 645Transmission electron microscopy (TEM) was performed in
an inert atmosphere (N2) in the temperature range 0
e1000 C at a heating rate of 10 C min1 using Zeiss-
EM10C-100 kV. The qualitative analysis of LDH/Tris/Pd was
performed using energy-dispersive X-ray (EDX) spectros-
copy. EDX analysis of the prepared catalyst was performed
on an FESEM instrument (SIGMA). TGAwas performed on a
TA instruments (model 951 DUPONT) apparatus. Ultra-
sonication was done in a 2200 ETH-SONICA ultrasound
cleaner with a frequency of 45 KHz.
2.2. Preparation of LDHs
CaAl LDH material was provided by the coprecipitation
method in a one-pot reaction [15]. An aqueous solution
containing a molar ratio of 2:1 frommixed metal solutions,
calcium nitrate tetrahydrate (Ca(NO3)2$4H2O) and
aluminum nitrate nonahydrate (Al(NO3)3$9H2O) dissolved
in deionized (DI) water, was prepared. Then, under
vigorous stirring, at 65 C, the mixed metal solution was
slowly added to a 2 M sodium hydroxide solution to
maintain a constant pH of approximately 10 or 11. After
completion of the addition, the LDH was aged for 18 h at
65 C. In the next step, the obtained white powder was
separated by centrifugation, washed three times with DI
water, and dried overnight at 100 C.
2.3. Preparation of LDH coated with (3-chloropropyl)
triethoxysilane (LDH/(CH2)3-Cl)
LDH/(CH2)3-Cl was synthesized via surface reaction of
LDH by using 3-chloropropyltrimethoxysilane. LDH (1.0 g)
and toluene (50.0 mL) were mixed into a 250 mL round-
bottom flask. Then 2.0 mL of 3-chloropropyltrimethoxy-
silane was added and the mixture was refluxed at 110 C
with continuous stirring for 12 h. The resulting LDH/(CH2)3-
Cl was collected and washedwith toluene and EtOH several
times and dried at 70 C until complete dryness.
2.4. Anchoring of Tris onto LDH/(CH2)3-Cl surface (LDH/Tris)
A mixture of 3.0 g of Tris and 50 mL of EtOH was stirred
at 80 C. Then,1.0 g of LDH/(CH2)3-Cl was added to the flask.
The reaction was conducted for 48 h. The crude product
was filtered off and washed with ethanol three times to
remove all unreacted Tris. The final product (white solid)
was dried under vacuum overnight.
2.5. Synthesis of LDH/Trisesupported palladium NPs (LDH/
Tris/Pd(0))
LDH/Tris (0.5 g) was added to a flask containing a so-
lution of PdCl2 (2 mmol, 0.34 g) in CH3CN (10 mL) and
stirred for 24 h at 50 C. After cooling, the resulting product
was collected on a filter, washed four times with acetone
(8 mL), and dried in an oven at 70 C. Moreover, with the
use of NaBH4, as a reducing agent, Pd(II) reduced to Pd(0):
the obtained catalyst was dispersed in 20 mL CH3CN,
aqueous solution of NaBH4 (0.2 g in 20 mL) was added to a
round-bottom flask, and the mixture was stirred at room
temperature for 5 h. Then, with the use of centrifuge, blacksolid catalyst was collected, washed with DI water
(3 4 mL) and CH3CN (4 mL), and finally dried under
vacuum for 18 h. The concentration of palladium in LDH/
Tris/Pd(0), which was 13.52 wt %, was determined by
inductively coupled plasma optical emission spectrometry
( ICP-OES).
2.6. General procedure for the SuzukieMiyaura coupling
reaction using LDH/Tris/Pd
A mixture of ArX (1 mmol), PhB(OH)2 (1.1 mmol), K2CO3
(2mmol), catalyst (0.05mol % Pd), and H2O/EtOH (1:1, 5mL)
were placed in a 25 mL Schlenk tube and the mixture was
stirred at room temperature. After the completion of the re-
action monitored by thin-layer chromatography (n-hexane/
acetone 10:4), the catalyst was separated by centrifugation,
and then the reaction mixture was washed three times with
ethyl acetate. The combined organic layer was dried over
magnesium sulfate and evaporated in a rotary evaporator
under reduced pressure to obtain the biaryl product.
3. Results and discussion
The catalyst was prepared using a simple and efficient
method and characterized by FT-IR, X-ray diffraction (XRD),
SEM, EDX, TEM, and X-ray photoelectron spectroscopy
(XPS).
In this study, we used NaBH4 as a reducing agent and
reduced Pd(II) to Pd(0) to prepare LDHs/Tris/Pd. After the
addition of NaBH4, the yellow color of the Pd(II) solution
immediately changed into black, which indicated the for-
mation of Pd NPs (Scheme 1).
Conversion of Pd(II) to Pd(0) in the resulted Pd-
supported NPs was confirmed by the UVevis spectrum.
The disappearance of the peak at 420 nm, which belongs to
Pd(II) species, ascertains the formation of the Pd(0) cata-
lysts (Fig. 1).
3.1. Characterization of the prepared LDH/Tris/Pd
The FT-IR spectra of the LDH, LDH/(CH2)3-Cl, LDH/Tris,
and LDH/Tris/Pd are shown in Fig. 2, peak assignments
appeared as “X value”. The broad observed peaks, in all
samples, shown at 3400e3650 and 535e580 cm1 were
attributed to the Al-OH/H2O stretching and metaleoxygen
vibrations, respectively. The appearance of the strong peak
at 1385 cm1 imputed to the interlayer NO3 anion is the
other notable finding for these figures. In Fig. 2b observed
peaks at 1057 and 1138 cm1 attributed to SieOeM and Si
eOeSi confirm that the (3-chloropropyl)triethoxysilane
have functionalized the surface of the LDH. The successful
reaction of Tris with LDH/(CH2)3-Cl was confirmed with
observed peaks at 1353, 1385, and 3423 cm1 attributed to
stretching vibration of CeN, CeO, and NeH groups (Fig. 2c).
The reduction in the intensity of OH groups at 3641 cm1 in
Fig. 2d demonstrated the coordination of Pd NPs on the
surface of the functionalized LDH.
To check the thermal degradation property of LDH/Tris/
Pd, the TGA of the catalyst was performed. Catalyst started
to decompose at less than 150 C attributed to the loss of
the trapped H2O and interlayer anions. The weight loss at a
Scheme 1. Synthetic route of LDH/Tris/Pd.
Fig. 1. UVevis spectrum of the (a) PdCl2 and (b) LDH/Tris/Pd(0).
Fig. 2. FT-IR spectra of (a) LDH, (b) LDH/(CH2)3-Cl, (c) LDH/Tris, and (d) LDH/Tris/Pd.
R. Ghorbani-Vaghei et al. / C. R. Chimie 21 (2018) 644e651646
R. Ghorbani-Vaghei et al. / C. R. Chimie 21 (2018) 644e651 647higher temperature (150e300 C) could be mainly imputed
to the evaporation and subsequent decomposition of (3-
chloropropyl)triethoxysilane groups. The final degrada-
tion temperature occurred at 400e700 C, ascribed to the
release of Tris. The maximum mass loss is 39.10%, which
was attributed to the strong interaction among LDH, Tris,
and Pd. As a result, the catalyst was stable up to 600 C and
could be safely used in organic reactions at high tempera-
tures (Fig. 3).
Fig. 4 shows the XRD patterns of LDH and LDH/Tris/Pd.
The successful syntheses of CaAl-LDHs are established by
comparison of the XRD pattern of synthesis LDHwith those
previously reported [15,16]. The observed tetragonal phase
of LDH/Tris/Pd indicated that the coordinating Tris did not
change the crystallinity of the LDH. The appearance of the
peaks related to palladium in 2q ¼ 39.8, 47.5, 68.4, and
81.7 showed the formation of Pd(0) NPs in LDH/Tris/Pd.
The surface morphology of the LDH/Tris/Pd was char-
acterized by SEM and TEM. SEM results show separate
hexagonal structure attributed to LDH/Tris/Pd. On the other
hand, TEM images of LDH/Tris/Pd demonstrated that
palladium NPs are bonded to the organic matrix in the
catalyst (Fig. 5).
The LDH/Tris/Pd was analyzed by EDX and the outcome
was acceptable with expected elements such as Ca, Al, Si, C,
O, N, and Pd. EDX analysis of the catalyst is shown in Fig. 6.
The surface turnover of the LDH/Tris/Pd was investi-
gated by XPS analysis. The elemental peaks are dominated
by O, Ca, Al, and C of LDH/Tris/Pd (Fig. 7a). The presence of
C, N, and Si in the LDH/Tris/Pd samplewas confirmed by the
characteristic peaks for C 1s at binding energy (BE) of about
284.8 eV, N at BE of about 398 eV, and Si 2p at BE of about
102.5 eV (Fig. 7bed). In Pd 3d XPS spectra of LDH/Tris/Pd,
more intense peaks (335 and 340 eV) were assigned to
metallic palladium, Pd(0) (Fig. 7e). In addition, the char-
acteristic peaks of C 1s included three peaks at 284.5 eV
(CeC bonding), 286.3 eV (CeN bonding), and 289.1 eV
(CeNþ bonding) indicating that the Tris was successfullyFig. 3. Thermogravimetric diagintroduced onto the LDH surface by covalent linkages
(Fig. 7b).
The concentration of palladium in LDH/Tris/Pd, which
was 13.52 wt %, was determined by ICP-OES.
3.2. Evaluation of the catalytic activity of LDH/Tris/Pd
through the SuzukieMiyaura coupling reaction
To check the catalytic activity of LDH/Tris/Pd, we used
the as-prepared supported catalyst for the SuzukieMiyaura
coupling reaction. In the first step, we determined the best
reaction conditions, studying the effect of media upon the
reaction. As a model reaction, the coupling reaction of
iodobenzene with phenylboronic acid in H2O/EtOH (1:1) in
the presence of LDH/Tris/Pd was chosen. A variety of pa-
rameters such as solvent, base, temperature, and catalyst
loading impressed the coupling rate.
First, we have studied themodel reaction using K2CO3 in
different solvents and observed that H2O/EtOH (1:1) was
the best solvent. The use of other organic solvents (Table 1,
entries 1e5) was not useful to the process. In the following,
we tried to find the effect of different organic and inorganic
bases on this reaction (Table 1, entries 5e11). Results
indicated that K2CO3 was themost efficient and other bases
were less effective. In addition, the effect of temperature on
this model reactionwas studied and it was found that room
temperature was the best condition (Table 1, entries 5 and
12e14). To investigate the effect of the catalyst amount, the
catalyst studies were carried out in the presence of
different amounts of the catalyst and results indicated that
0.05 mol % was the best choice (Table 1, entries 5 and 15
e17). Thus, the optimum conditions selected are iodo-
benzene (1 mmol), phenylboronic acid (1.1 mmol), LDH/
Tris/Pd (0.05mol %), H2O/EtOH (5mL,1:1), K2CO3 (2mmol),
and rt.
Using theoptimized reaction conditions (Table 1, entry5),
avarietyof aryl iodides, bromides, andchlorideswithvarious
substituent groups were examined in the SuzukieMiyauraram of the LDH/Tris/Pd.
Fig. 6. EDX spectrum of LDH/Tris/Pd.
Fig. 5. FESEM image of (a) LDH/Tris/Pd and (b) TEM image of LDH/Tris/Pd.
Fig. 4. XRD pattern of (a) LDH and (b) LDH/Tris/Pd.
R. Ghorbani-Vaghei et al. / C. R. Chimie 21 (2018) 644e651648
Fig. 7. XPS spectra of (a) LDH/Tris/Pd, and high-resolution XP spectrum of (b) C 1s, (c) N 1s, (d) Si 2p, and (e) Pd 3d.
R. Ghorbani-Vaghei et al. / C. R. Chimie 21 (2018) 644e651 649coupling reaction (Table 2). According toTable 2, aryl halides
containingbothelectrondonatingandelectronwithdrawing
groups reacted with phenylboronic acid and afforded the
corresponding biaryl products in high yields. It was found
when substituted located at para- ormeta-position, the yield
was better than ortho-position due to steric effects.
In view of economical purposes and green chemistry,
recyclability and reusability of the catalyst were investi-
gated in the reaction of phenylboronic acid with 1-iodo-3-Table 1
Optimization studies of the reaction of 4-iodotoluene with phenylboronic acid in
Entry Solvent Pd (mol %) Base
1 Toluene 0.05 K2CO3
2 CH3CN 0.05 K2CO3
3 EtOH 0.05 K2CO3
4 H2O 0.05 K2CO3
5 H2O/EtOH 0.05 K2CO3
6 H2O/EtOH 0.05 DABCO
7 H2O/EtOH 0.05 HMTA
8 H2O/EtOH 0.05 Et3N
9 H2O/EtOH 0.05 Na2SO4
10 H2O/EtOH 0.05 Na3PO4
11 H2O/EtOH 0.05 Na2CO3
12 H2O/EtOH 0.05 K2CO3
13 H2O/EtOH 0.05 K2CO3
14 H2O/EtOH 0.05 K2CO3
15 H2O/EtOH 0.04 K2CO3
16 H2O/EtOH 0.03 K2CO3
17 H2O/EtOH 0.06 K2CO3
a Reactions were carried out using 1mmol of iodobenzene, 1.1 mmol of phenylb
of Pd).
b Yields are given for isolated products.nitrobenzene. At the end of the reaction, catalyst was
separated by centrifugation, washed three times with H2O
and hot EtOH, dried in an oven, and used for another batch
of the reaction. The results indicated that this catalyst can
be successfully reused six times without any significant loss
in activity (Fig. 8).
Another important parameter of a catalyst, which was
studied, is heterogeneity of the catalyst. For this purpose,
the phenomenon of leaching was studied by ICP-OESthe presence of LDH/Tris/Pd.a
Temperature Time (min) Yieldb (%)
rt 180 50
rt 180 60
rt 150 65
rt 180 50
rt 40 98
rt 120 65
rt 180 50
rt 120 70
rt 240 40
r.t 210 40
rt 180 45
40 40 85
60 40 85
80 40 80
rt 60 90
rt 90 87
rt 40 98
oronic acid, and 2mmol of base in the presence of LDH/Tris/Pd (0.05mol %
Fig. 8. Recyclability tes
Table 2
SuzukieMiyaura cross-coupling reaction of aryl halides with phenyl-
boronic acid.a
Entry Aryl halide Product Time (min)/yieldb
1 20/95
2 40/98
3 35/98
4 10/92
5 60/75
6 30/90
7 40/99
8 75/87
9 120/66
10 40/90
11 25/70
12 20/77
13 150/60
14 120/88
a Reaction conditions: ArX (1 mmol), phenylboronic acid (1.1 mmol),
K2CO3 (2 mmol), H2O/EtOH (5 mL, 1:1), and catalyst (0.05 mol % Pd).
b Isolated yields.
Fig. 9. TEM image of LDH/Tris/Pd after six uses.
R. Ghorbani-Vaghei et al. / C. R. Chimie 21 (2018) 644e651650analysis of the resulting reaction solution mixture. It was
shown that palladium content of the used catalyst was the
same as that of the fresh one (13.48%). In addition, the TEM
image of the catalyst after recycling did not show a
considerable change in the morphology, which clearly
indicated that LDH/Tris/Pd is recyclable and stable under
the reaction conditions (Fig. 9).
The filtration test for the SuzukieMiyaura reaction be-
tween bromobenzene and phenylboronic acid using LDH/
Tris/Pd as a catalyst was performed to check the leaching of
Pd during the reaction. A catalytic run was started as for a
standard reaction, and after reaction for 30 min (the reac-
tion was completed in 60 min), corresponding to 50% con-
version, the reaction mixture was stopped and centrifuged
to afford a clear filtrate. Then the mixture without the solid
catalyst was treated as a standard catalytic run for another
30 min, and the conversion did not proceed significantly.
4. Conclusions
LDH/Tris/Pd was successfully synthesized and used in
the SuzukieMiyaura coupling reaction. The structure of the
catalyst was exactly characterized by various techniques.t of LDH/Tris/Pd.
R. Ghorbani-Vaghei et al. / C. R. Chimie 21 (2018) 644e651 651The results indicated that the stability of the catalyst was
much improved and LDH/Tris/Pd was an effective and
recyclable catalyst for coupling reactions in aqueous media.
Low cost, good stability, excellent workup, and reusability
are some of the most notable advantages of catalysts.
Acknowledgments
The authors wish to thank the Bu-Ali Sina University
and the Center of Excellence in Development of Environ-
mentally Friendly Methods for Chemical Synthesis
(CEDEFMCS) for financial support to this research.
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