Design and synthesis of a new phosphinite-functionalized clay composite for the stabilization of palladium nanoparticles. Application as a recoverable catalyst for C-C bond formation reactions

Article abstract of DOI:10.1039/c4ra03645j

A new clay composite carrying phosphinite-functionalized ionic liquid moieties (CCPIL) has been designed and synthesized. Palladium nanoparticles were uniformly ligated by CCPIL. After characterization, this composite has been applied as a heterogeneous catalyst for the Suzuki-Miyaura reaction of aryl halides with phenylboronic acid on water in the presence of NaOH as the base. These nanoparticles have also been applied for copper-free Sonogashira-Hagihara reaction in TBAB and Mizoroki-Heck reaction under solvent-free conditions. The catalyst was also found to be stable towards moisture and air and its catalytic activity remained almost unchanged after exposure to air for three months. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra03645j

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Design and synthesis of a new phosphinite-
functionalized clay composite for the stabilization
of palladium nanoparticles. Application as a
recoverable catalyst for C–C bond formation
reactions†
Cite this: RSC Adv., 2014, 4, 27674
a
a
Habib Firouzabadi, Nasser Iranpoor, Arash Ghaderi,ab Mohammad Gholinejad,c
*
*
Sajjad Rahimia and Safura Jokara
A new clay composite carrying phosphinite-functionalized ionic liquid moieties (CCPIL) has been designed
and synthesized. Palladium nanoparticles were uniformly ligated by CCPIL. After characterization, this
composite has been applied as a heterogeneous catalyst for the Suzuki–Miyaura reaction of aryl halides
with phenylboronic acid on water in the presence of NaOH as the base. These nanoparticles have also
been applied for copper-free Sonogashira–Hagihara reaction in TBAB and Mizoroki–Heck reaction
under solvent-free conditions. The catalyst was also found to be stable towards moisture and air and its
catalytic activity remained almost unchanged after exposure to air for three months.
Received 22nd April 2014
Accepted 21st May 2014
DOI: 10.1039/c4ra03645j
www.rsc.org/advances
ILs used. In addition, supporting ionic liquids on inorganic
materials facilitates their purication processes, recycling and
isolation of products from the reaction mixtures.4–6 Incorpora-
tion of ionic liquids may be achieved by two approaches; the
rst approach is their immobilization by covalent binding to
solid support4,5 and the second is the adsorption of ILs on the
surface of a solid support.6
Clay minerals occur widely in nature and their high surface
area and ion exchange properties make them suitable for use as
solid supports.7 A class of clays is the Mg-organosilicates con-
taining pendant organic groups. The composition of these
organo-functionalized clays has been reported as R8Si8M-
g6O16(OH)4, where R is an organic group.8
Transition metals have come to play an important role in
metal-catalyzed reactions in creating C–C bonds.9 One of the
most booming protocols for C–C bond formation between two
sp2 carbons is the Suzuki–Miyaura reaction in which biaryl
compounds are produced.10 Various palladium nanoparticles
have been used as catalysts for the promotion of this reaction11
and numerous reviews are available in the literature in which
the use of palladium nanoparticles in the Suzuki–Miyaura
reaction have been discussed.10a,12 Sonogashira–Hagihara reac-
tion which is one of the most successful methods to form
substituted alkynes, has been widely applied in the synthesis of
natural products and biologically active molecules and different
materials.13 Another palladium-catalyzed reaction which also
proceeds between two sp2 carbons is called the Mizoroki–Heck
reaction. It produces functionalized alkenes that are applied in
the synthesis of many organic compounds.14 Very recently,
palladium nanoparticles supported on amine- and phenyl-
Introduction
In today's economic and industrial environment, the use of less
toxic reaction solvents such as water and ionic liquids are
gaining more importance in view of green chemistry. In addi-
tion, removal of metal wastes from reaction media and the
recovery of expensive metal catalysts are two important issues to
be examined. Along this line, chemists are interested in
researching for using ionic liquids (ILs) as novel and green
solvents, catalysts and reagents.1 ILs have desirable properties,
such as non-volatility, non-ammability and a wide tempera-
ture range over the liquid phase.2 They have polarities similar to
that of light alcohols, in which polar organic compounds, gases
and biocatalysts are dissolved, thus reactions can be performed
under homogeneous conditions.3 By considering these facts,
replacing organic solvents with ionic liquids in organic reac-
tions would be of great interest. However, ionic liquids are
expensive, and their use as a solvent is not desirable economi-
cally. Nevertheless, by immobilization of ILs onto the surface of
a solid support, organic reactions can proceed on the thin IL
layer on the surface of the support and minimize the amount of
aDepartment of Chemistry, College of Sciences, Shiraz University, Shiraz 71454, Iran.
E-mail: rouzabadi@chem.susc.ac.ir; iranpoor@chem.susc.ac.ir; Fax: +98 711228
0926; Tel: +98 711 228 4822
bDepartment of Chemistry, College of Sciences, Hormozgan University, Bandar Abbas,
71961, Iran. Fax: +98 761 766 0032; Tel: +98 761 766 0042
cDepartment of Chemistry, Institute for Advanced Studies in Basic Sciences (IASBS),
Gava zang, Zanjan 45137-6731, Iran. Fax: +98-241-4214949
† Electronic supplementary information (ESI) available: Experimental section and
spectral data for all compounds. See DOI: 10.1039/c4ra03645j
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functionalized clay has been reported to catalyze carbon–carbon
bond formation reactions.15
In recent years, we have reported the synthesis and appli-
cation of functionalized ILs,16a,b and heterogeneous palladium
catalysts for different palladium catalyzed reactions.16c–f In this
study, we combined the advantages of homogeneous IL phase,
phosphorylated ligands and a new heterogeneous clay support
for Pd-catalyzed C–C bond formation in green media.
Results and discussion
Initially, the epoxy-functionalized clay (1) was prepared by the
method reported in the literature.17 In order to build up the
ionic moiety on the surface of the clay, the epoxy-functionalized
clay was dispersed in CH2Cl2 by sonication. 1-Methylimidazole
was added to the reaction vessel followed by diphenyl phos-
phine chloride and the reaction was allowed to stir at 50 ꢀC for
24 h under argon atmosphere. The resulting clay composite
carrying phosphinite-functionalized ionic liquid moieties
(CCPIL, 2) was separated by simple ltration and washed with
CH2Cl2 to give the puried organic-inorganic hybrid material
(Scheme 1).
FT-IR spectrum of 1 showed bands at 3433 cmÀ1 (Si–OH
group) and 565 cmÀ1 (Mg–O absorption) (Fig. 1a). FT-IR spec-
trum of 2 also showed the same bands plus absorption bands
for P–O (1071 cmÀ1), C–O (1138 cmÀ1) and C]C stretching of
aromatic rings (1438 cmÀ1), conrming that phosphinite-
functionalized ionic liquid moieties are covalently bonded to
the clay producing an organoclay composite (Fig. 1b).
Fig. 1 FT-IR spectra of (a) epoxy clay 1 and (b) CCPIL 2.
the %N of 2, which originates from the imidazolium group,
we concluded that 4 mmol of the epoxy group has reacted with
1-methylimmidazole to give CCPIL 2.
According to the %C of 1 in CHN analysis, there are 26 mmol
organic residues per 1 g of the organoclay composite. Based on
Scheme 1 Synthesis of CCPIL and deposition of palladium nanoparticles on it. Reaction conditions: (a) 1-methylimidazole, Ph2PCl, CH2Cl2, 50
ꢀC, under Ar, 24 h; (b) PdCl2 (aqueous solution), rt, 24 h.
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Fig. 5 TEM image of the freshly prepared palladium nanoparticles
supported on CCPIL.
Fig. 6 SEM images of the composite 3 at two magnifications.
Fig. 2 TGA-DTG of (a) epoxy clay 1 and (b) CCPIL 2.
Fig. 3 Solid state UV-vis spectra of PdCl2 (green line); epoxyclay (black
line); CCPIL (red line) and PdNPs supported on CCPIL (blue line).
Fig. 7 SEM-EDX spectrum of palladium nanoparticles supported on
CCPIL.
Thermogravimetric analysis (TGA) of 1 conrms the pres-
ence of organic groups in the material with a decomposition
ꢀ
temperature range of 200–450 C affording 62% residual mass
(Fig. 2a). However, TGA analysis of 2 shows the decomposition
of organic groups at 200–700 ꢀC giving 39% residual mass
(Fig. 2b). In comparison with 1, the decrease of the residual
mass from 62% to 39% in CCPIL 2, veries the increasing
presence of organic groups which is related to the reaction of
epoxide moiety with N-methylimidazole and PPh2Cl.
The complexation of palladium particles with the synthe-
sized organoclay was achieved by stirring the organoclay in an
aqueous solution of PdCl2 at room temperature for 24 h.
Subsequent washing of ltrate with water in a Soxhlet apparatus
(to remove the adsorbed PdCl2) and drying by evacuation at
Fig. 4 XRD spectrum of palladium nanoparticles supported on CCPIL, 3.
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Table 1 Suzuki–Miyaura reaction of various aryl halides with phenylboronic acid in the presence of the catalyst on watera
Entry
1
Substrate
Product
Time (h)
Yield (%)
95 (84)
40 min (12)
2
3
4
91
95
3.5
4
5
9
87
93
10.5
6
7
8
16
2
90
90
96
8
9
18
10
89
85
10
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Table 1 (Contd.)
Entry
11
Substrate
Product
Time (h)
24
Yield (%)
88
12
16
30
85
94
13
a
Reaction conditions: aryl halide (1 mmol), phenylboronic acid (1.2 mmol), Pd nanoparticles supported on CCPIL (0.01 g), NaOH (1.5 mmol) on
water (3 mL) at 80 ꢀC. The data shown in parenthesis refers to the reaction conducted at room temperature.
Table 2 Reusability of the catalyst in reaction between 1-bromo-4-nitrobenzene and phenylboronic acid in the presence of NaOH and 0.01 g of
the catalyst on water at 80 ꢀC
Run
1
2
2
2
3
2
4
5
Time for completion of the reaction (h)
2.25
2.5
50 ꢀC for 24 h resulted in our target material 3 (Scheme 1). The
The X-ray diffraction (XRD) pattern of the PdNPs supported on
amount of loading of palladium on the surface of modied clay CCPIL shows a low-angle reection (d001) corresponding to the
was determined by ICP analysis as 0.072 mmol of palladium per bilayer arrangement of the organic-modied clay.18 The broad in-
1 g of the composite 3, showing the capability of the support to plane reections at higher angles (d020,110, d130,200) and the
absorb about 72% palladium from the solution. It is very likely characteristic reection (060) in smectites conrm the formation
that palladium nanoparticles were mostly ligated by the phos- of 2 : 1 trioctahedral Mg-organosilicate clay with talc-like struc-
phorus group rather than C-2 position of the imidazole.16a
Solid state UV-vis spectrum of palladium chloride shows a (111), (200), (220) and (311) crystallographic planes (Fig. 4).16d
peak at 280 nm corresponding to Pd(II).18 The reduction of Pd(II)
The TEM picture of the supported palladium nanoparticles
ture.17 The characteristic peaks for Pd(0) can also be observed at
to Pd(0) was conrmed by the disappearance of this peak. on the surface of the organoclay composite showed the average
Moreover, increasing the absorption bands in the size of the palladium particles of around 3–5 nm, although
higher regions indicates the formation of Pd(0) nanoparticles19 smaller particles to the size of 1 nm or smaller can also be
(Fig. 3).
observed (Fig. 5).
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Table 3 Optimization of the conditions in the Sonogashira–Hagihara reaction between 1-bromo-4-nitrobenzene and phenylacetylene in the
presence of Pd nanoparticles supported on ionic liquid functionalized clay at 100 ꢀC under argon atmosphere
Entry
Solvent
Base
Time (h)
Conversion (%)
1
TBAB
H2O
H2O/TBAB
DMSO
PEG400
TBAB
TBAB
TBAB
TBAB
TBAB
KOAc
KOAc
KOAc
KOAc
KOAc
NaOAc
None
Cs2CO3
NaOH
Et3N
40 min
6
2.5
3
3
100
92
2
3a
4
100
100
100
100
23
100
Trace
Trace
Trace
5
6
7
8
9
10
11
1
24
12
12
12
12
TBAB
Morpholine
a
TBAB (1 mmol) was added to the reaction mixture.
The morphology of the surface of the composite 3 has been bromo thiophene (4i) and 3-bromo pyridine (4j), led to the
examined by collecting two SEM images at two different corresponding heterobiaryls in high yields (Table 1, entries 8–
magnications (Fig. 6). The SEM-EDX pattern indicates the 10). The catalytic system effect was also manifested in the
presence of palladium nanoparticles in the structure of the clay reaction of aryl chlorides with phenylboronic acid. In these
composite (Fig. 7).
cases, prolonged reaction times were required to give the
The nitrogen adsorption–desorption isotherm for the nal expected products in high yields (Table 1, entries 11–13).
catalyst shows a type-IV curve with H3 hysteresis loop which is
In comparison with our previously reported system using
typical for slit-shaped pores (Fig. S1†).20 BET calculations show a palladium nanoparticles supported on aminoclay,16d the current
specic surface area of 84.8 m2 gÀ1 and a total pore volume of system is more effective for the activation of C–Cl bonds in the
0.09 cm3 gÀ1
.
Suzuki–Miyaura reaction. However, this heterogeneous catalyst
Aer characterization, this composite was used as a catalyst is much less reactive in the Suzuki–Miyaura reaction than the
for the Suzuki–Miyaura reaction. It was found that the use of a reported water-soluble NHC–Pd polymer with triethylene glycol
combination of an aryl halide (1 mmol), phenylboronic acid legs which acts homogeneously.22
(1.2 mmol), palladium nanoparticles deposited on CCPIL (0.01
The reusability of the catalyst was tested upon the reaction of
g containing 7.2 Â 10À4 mmol Pd) and NaOH (1.5 mmol), fol- 1-bromo-4-nitro benzene with phenylboronic acid employing
lowed by heating in water (3 mL) at 80 ꢀC under air resulted the 0.01 g of the catalyst in the presence of NaOH at 80 ꢀC. In the rst
desired biaryls in excellent yields (Table 1). Since the reactants run, the reaction was completed within 2 h to give the product in
are insoluble in water, this reaction is an “on water” reaction.21 90% isolated yield. Similarly, the reactions for the repeated runs
As shown in Table 1, a range of aryl iodides, bromides and were conducted aer separation of the catalyst. Aer completion
chlorides reacted with phenylboronic acid to give the desired of the reaction in the rst run, the aqueous solution was
products in high yields. The reaction of iodobenzene (4a) was removed by a syringe. Ethyl acetate (5 mL) was added to the
faster in comparison with the aryl iodides bearing an electron reaction mixture to extract the organic compounds.
donating group (4b) or having steric hindrance (4c) (Table 1,
The ethyl acetate solution was removed by a syringe and the
entries 1–3). This reaction could be easily conducted at room residual catalyst was dried under a nitrogen ow. Aer complete
temperature although for a longer reaction time (Table 1, entry drying, the catalyst was charged again into the vessel containing
1). In the case of 1-iodo-4-aminobenzene (4b), the Suzuki– the reacting substrates, and the reaction was performed under
Miyaura reaction was performed exclusively upon the carbon similar conditions. This recycling was repeated for ve
halide bond and the N-arylation product was not observed consecutive runs. As shown in Table 2, no signicant changes
(Table 1, entry 2). The coupling reaction of phenylboronic acid on the reaction times were observed during the ve runs con-
with both electron-releasing and electron-withdrawing aryl ducted. This observation indicates that the amount of Pd
bromides afforded products in high yields (Table 1, entries 4– leached into the reaction mixture should be low. This has been
10). Heterocyclic bromides such as 5-bromo pyrimidine (4h), 3- quantitatively approved by ICP analysis, which showed that
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Table 4 Sonogashira–Hagihara reaction of aryl halides with phenylacetylene in the presence of Pd nanoparticles supported on ionic liquid
functionalized clay and KOAc in molten TBAB under argon atmosphere
Isolated Yield
Entry
1
Aryl halide
Product
Time (h)
0.13
(%)
96
2
0.33
91
3
4
1
1
90
88
5
2.25
91
6
7
8
0.67
1
90
82
77
1.5
9
4
68
70
10
3.5
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Table 4 (Contd.)
Isolated Yield
(%)
Entry
11
Aryl halide
Product
Time (h)
4
76
80
12
3.75
aer recycling the catalyst for ve times, only 7% of Pd leached
from the support into the reaction mixture.
We have also shown that the catalyst is stable towards air.
For this aim, we prepared the catalyst and le it in air for three
months. Then, the catalyst was applied for the reaction of
1-bromo-4-nitrobenzene with phenylboronic acid under the
above-mentioned conditions. The desired nal product was
obtained in 88% isolated yield within 2.25 h.
Scheme 2 Mizoroki–Heck reaction of bromobenzene with n-butyl
acrylate in the presence of Pd(0) nanoparticles under neat conditions.
This composite has also been used for Sonogashira–Hagi-
hara reaction. Inspired by the successful experience from our
previous work,16c we initiated the optimization study under the
conditions reported therein, namely, using KOAc as the base
and molten TBAB as the solvent at 100 ꢀC. Employing these
conditions, the reaction of 1-bromo-4-nitrobenzene with phe-
nylacetylene went to completion within 40 min (Table 3, entry
1). Switching the solvent to water gave the desired product in
92% conversion aer 6 h (Table 3, entry 2). Addition of TBAB to
the reaction mixture made a positive impact on the reaction
(Table 3, entry 3). Different solvents as well as bases were tested
for this reaction (Table 3). These studies, like our previous
study, led uꢀs to choose TBAB as the solvent and KOAc as the
base at 100 C as the optimized conditions.
For example the reaction of p-chloroacetophenone with phe-
nylacetylene gave the desired product in 80% isolated yield
within 3.75 h (Table 4, entry 12). The acetyl group remained
intact in this reaction and no condensation reaction was
observed.
The catalyst was also recovered for ve runs without any
signicant decrease in its reactivity (Table 5). ICP analysis
showed only 3% leaching of the catalyst aer the 5th run.
In order to show the wide range of application of the catalyst,
we have also applied this nanocatalyst in the Mizoroki–Heck
reaction. For this purpose, the reaction between bromobenzene
and n-butyl acrylate using nPr3N as the base at 130 ꢀC as the
model reaction was studied. In the result presented in Scheme
2, the reaction proceeded smoothly to completion aer 12 h in
Utilizing the optimized conditions, a wide range of aryl
halides including aryl chlorides reacted with phenylacetylene to
give the desired products in high to excellent yields (Table 4).
ꢀ
the absence of solvent at 130 C to give the desired product in
77% isolated yield.
Table 5 Reusability of the catalyst in reaction between 1-bromo-4-
nitrobenzene and phenylacetylene in the presence of KOAc and 0.01 g
of the catalyst in TBAB at 100 ꢀC
Conclusions
In conclusion, we have designed and synthesized a new class of
ionic liquid supported on the surface of organofunctionalized
clay. To this support, a phosphinite group was anchored to act
as a suitable ligand for transition metals. Palladium nano-
particles were deposited on the surface of this support. This
Run
1
2
3
4
5
Time for completion of the
reaction (min)
40
40
45
45
45
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composite has been applied as a recyclable catalyst for the
Suzuki–Miyaura reaction of aryl halides (I, Br and Cl) with
phenylboronic acid on water under air atmosphere with small
amount of Pd leaching. The composite was stable towards air
and moisture. In order to show the merit of the compound as a
catalyst, we performed the Sonogashira–Hagihara reaction in
molten TBAB and also the Mizoroki–Heck reaction under
solvent-free conditions.
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Acknowledgements
The authors are thankful for the support of this work by
National Elite Foundation of Iran by grant number BN048, Iran
National Science Foundation (INSF-Grant number 92027195),
Shiraz University Research Council and Institute for Advanced
Studies in Basic Sciences (IASBS) Research Council. A.G. and
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