Aminoclay decorated with nano-Pd(0) picolinic acid complex as a novel efficient, heterogeneous, and phosphine ligand-free catalyst in Heck reaction under solvent-free conditions

Article abstract of DOI:10.1007/s11164-017-2921-8

A new heterogeneous phosphine ligand-free nano-Pd(0) picolinic acid complex was synthesized under mild and ecofriendly conditions with a short and simple process in green solvent. In this work, as a novel functionalization method, the surface of aminoclay was decorated via ionic bond formation between the picolinic acid complex of nano-Pd(0) and the amino group on its surface. Then, the prepared heterogeneous catalyst was characterized by TEM, FE-SEM, ICP, IR, XRD, TGA, EDX, XPS and UV–visible techniques and was subsequently employed as an efficient and reusable heterogeneous catalyst for selective Heck coupling of aryl halide and n-butyl acrylate under solvent-free conditions. The catalyst was recoverable simply by centrifuging and was reusable for at least seven runs with a minimum amount of palladium leaching. Also, the mercury poisoning test confirmed the heterogenous nature of the catalyst.

Full text of DOI:10.1007/s11164-017-2921-8

Res Chem Intermed
DOI 10.1007/s11164-017-2921-8
Aminoclay decorated with nano-Pd(0) picolinic acid
complex as a novel efficient, heterogeneous,
and phosphine ligand-free catalyst in Heck reaction
under solvent-free conditions
1
1
Nafiseh Fahimi
•
Ali Reza Sardarian
Received: 22 November 2016 / Accepted: 24 February 2017
Ó Springer Science+Business Media Dordrecht 2017
Abstract A new heterogeneous phosphine ligand-free nano-Pd(0) picolinic acid
complex was synthesized under mild and ecofriendly conditions with a short and
simple process in green solvent. In this work, as a novel functionalization method,
the surface of aminoclay was decorated via ionic bond formation between the
picolinic acid complex of nano-Pd(0) and the amino group on its surface. Then, the
prepared heterogeneous catalyst was characterized by TEM, FE-SEM, ICP, IR,
XRD, TGA, EDX, XPS and UV–visible techniques and was subsequently employed
as an efficient and reusable heterogeneous catalyst for selective Heck coupling of
aryl halide and n-butyl acrylate under solvent-free conditions. The catalyst was
recoverable simply by centrifuging and was reusable for at least seven runs with a
minimum amount of palladium leaching. Also, the mercury poisoning test con-
firmed the heterogenous nature of the catalyst.
Keywords Nano-palladium (0) Á Solvent-free Á Aminoclay Á Heterogeneous
catalyst Á Phosphine ligand-free Á Heck reaction
Electronic supplementary material The online version of this article (doi:10.1007/s11164-017-2921-8)
contains supplementary material, which is available to authorized users.
&
Ali Reza Sardarian
sardarian@shirazu.ac.ir
Nafiseh Fahimi
fahimi.nafiseh@shirazu.ac.ir
1
Department of Chemistry, College of Sciences, Shiraz University, Shiraz 71946- 84795, Iran
123

N. Fahimi, A. R. Sardarian
Introduction
One of the most powerful and widely used methods in the formation of C–C bonds
is the Heck reaction [1], which is a Pd-catalyzed versatile and efficient tool for the
generation of C–C bonds [2–4] and a key step in the synthesis of organic, important
biologically active and pharmaceutical compounds such as jiadifenin (1) [2],
methyl(E)-3-(1-benzyl-3-isopropyl-1H-pyrazol-5-yl)acrylate (2) [3], tryptanthrin
(
3) and phaitanthrin (4) [4] (Fig. 1).
Over the past decades, there has been a tendency to look for the efficient catalytic
systems, such as palladium–phosphine complexes [5–8], palladacycle complexes
[
[
7–10], palladium nanoclusters immobilized on supports [9–13], palladium salts
12–16] and palladium complexes immobilized on supports [17–19].
Phosphorus ligands, such as phosphines and phosphinites, are among the
important ligands in organometalic chemistry and their metal complexes have been
used in many catalytic reactions [20]. However, phosphorus ligands, which are very
often moisture- and air-sensitive, create problems in catalytic systems [21] and have
a negative impact on the natural environment [20]. On the other hand, these ligands
are comparatively difficult to make, are expensive [22] and often lead to competitive
degradation of the Pd catalyst through the P–C bond cleavage of a coordinated
phosphine ligand [23]. Therefore, there is a strong endeavor to avoid their
application [20] and thus the introduction of effective phosphine ligand-free Pd(0)
complex catalysts has been the subject of much research [6–10].
Homogeneous catalysts offer several advantages concerning high efficiency,
better tuning of chemoselectivity, regioselectivity and stereoselectivity [24].
However, the majority of homogeneous catalysts have not been commercialized
because of their difficult separation from the reaction systems [25]. In order to
overcome this problem, great efforts have been made to develop heterogeneous
catalysts via immobilization and stabilization of homogeneous catalysts on various
support materials [11, 12, 26–28]. Different types of palladium catalysts based on
clay support have been introduced and applied for coupling reactions, including
palladium-containing silicoaluminophosphate-31 (Pd-SAPO-31) [23], Mg–Al-lay-
ered double hydroxide (LDH) [24], montmorillonite-supported Pd complex [29, 30],
hydroxyapatite-supported palladium (PdHAP-1) [31], PdCl2 on sepiolites [32],
nanoparticles of palladium on montmorillonite modified with quaternary ammo-
nium surfactant [33] or immobilized in [bmim][PF ] supported on laponite [34], and
6
palladium nanoparticles supported on aminopropyl-functionalized clay [22].
Fig. 1 Some biological active and pharmaceutical compounds synthesized through C–C bond formation
in a key step
1
23

Aminoclay decorated with nano-Pd(0) picolinic acid…
Heck reactions have been mainly performed in various organic solvents such
DMF, DMSO, toluene, EtOH, and CH CN [1, 15] which are toxic, so that their
3
disposal need special care and cost. For resolving these drawbacks, elimination of
toxic organic solvents and performing the reactions under solvent-free conditions,
which is one aspect of green chemistry [16], has been the subject of much research
in Mizoroki–Heck reactions [20–22].
Regarding the ability of picolinic acid derivatives for chelation with palladium
35] and in continuation of our efforts to find an efficient, heterogeneous, cost-
[
effective, and phosphine ligand-free palladium complex catalyst for C–C bond
formation via cross-coupling reaction under mild conditions [36, 37], we decided to
design a novel heterogeneous phosphine ligand-free aminoclay@picolinic acid
nano-Pd(0) complex via surface ionic functionalization of aminoclay with a
picolinic acid nano-Pd(0) complex.
Experimental
All NMR spectra were taken on a Bruker Avance DPX-250 MHz instrument using
CDCl3 as solvent and TMS as an internal standard (1HNMR 250 MHz and
1
3CNMR 62.9 MHz). All reagents were purchased from Aldrich, Fluka, and Merck,
and were used without any further purification. FT-IR spectra were recorded on a
Shimadzu FT-IR 8300 spectrophotometer, and UV–Vis spectra were recorded on
Gold, Spectrumlab 53, BEL photonics. The size and morphology of the
nanoparticles were observed using a Zeiss EM10C-80 kV transmission electron
microscope (TEM) with an acceleration voltage 80 kV and Holey carbon-coated
grid Cu Mesh 300. Field emission scanning electron microscopy (FE-SEM) images
were obtained on a Mira 3-XMU. The Pd loading and leaching test was carried out
with an inductively coupled plasma (ICP) analyser (Varian, Vista-pro). X-ray
powder diffraction (XRD) spectra were taken on a Bruker AXS D8-advance X-ray
diffractometer with Cu Ka radiation (m = 1.5418). Thermal gravimetric analyses
were recorded on Perkin-Elmer Pyris Diamond model. X-ray photoelectron
spectroscopy (XPS) measurements were conducted with a Thermo Scientifi,
ESCALAB 250Xi with Mg X-ray resource. Products were characterized with
1
13
spectroscopic analysis ( H and CNMR) as shown in the supporting information.
Preparation of the immobilized nano-Pd(0) complex of picolinic acid
on amino clay
The aminoclay was prepared by the method reported in the literature [38].
Conventionally, an aminopropyl-functionalized magnesium (organo) phyllosilicate
clay is synthesized at room temperature by drop-wise addition of 3-aminopropy-
ltriethoxysilane (1.3 mL, 5.85 mmol) to a solution (25 mL) of magnesium chloride
(
0.84 g, 3.62 mmol) in ethanol (25 mL). The white slurry obtained after 5 min was
isolated by centrifugation, washed with ethanol (50 mL) and dried at 40 °C. Then,
the prepared aminoclay was added to a solution of picolinic acid (5.85 mmol,
0
.719 g) in ethanol (25 mL) and stirred for 2 h at room temperature. Then, a black
123

N. Fahimi, A. R. Sardarian
NH2
NH2 NH2 NH2NH2
EtOH
r.t
MgCl2
OH
N
Si
O
12h
O
O
O
NH2
NH NH NH
2
2
2
EtOH
r.t
2
h
pd(0)
N
N
O
O
O
O
NH NH NH NH
3
3
3
3
NH NH NH NH
3
3
3
3
EtOH
r.t
2
h
NH NH NH NH
3
3
3
3
NH NH NH NH
3
3
3
3
Et
H
O
EtOH
r.t
Pd(OAc)2
Pd(0)
Et
2
h
O
H
Scheme 1 General illustration for the preparation of aminoclay@picolinic acid nano-Pd(0) complex
ethanolic solution of palladium (0), which was prepared from stirring an ethanol
solution of palladium acetate (5.85 mmol) at room temperature, was added and the
resultant mixture was stirred for 2 h. The catalyst was centrifuged and washed
thoroughly with ethanol (3 9 5 mL) until the washing became colorless. The
catalyst was obtained as a uniform black powder after drying overnight in air and
then in an oven at 120 °C (see general illustration for preparation of aminoclay
immobilized nano-Pd(0) complex in Scheme 1) [39, 40].
Preparation of n-propan-1-amminium picolinate nano-Pd(0) complex
catalyst
n-Propylamine (5.85 mmol, 0.345 g) was added to a solution of picolinic acid
(
5.85 mmol, 0.719 g) in ethanol (25 mL) and stirred for 2 h at room temperature.
Then, a black ethanolic solution of palladium (0), which was prepared from stirring
an ethanol solution of palladium acetate (5.85 mmol) at room temperature, was
added and the resultant mixture was stirred for 2 h. The catalyst was centrifuged and
washed thoroughly with ethanol (3 9 5 mL) until the washing became colorless.
The catalyst was obtained as a uniform black powder after drying overnight in air.
1
23

Aminoclay decorated with nano-Pd(0) picolinic acid…
General procedure for Mizoroki–Heck reaction using the nano-Pd(0)
complex supported on aminoclay
Aryl halide (1 mmol) and n-butyl acrylate (1.5 mmol) were added to a flask
containing the aminoclay@picolinic acid nano-Pd(0) complex catalyst (0.5 mg of
-
the catalyst, containing 1.2 9 10 mmol of palladium) and n-Pr N (1.5 mmol,
3
3
0
.29 mL) in the absence of solvent. The mixture was stirred at 120 °C in the air.
After completion of the reaction (monitored by TLC), the reaction mixture was
diluted with diethyl ether (5 mL) and the catalyst was separated by centrifuging.
The diluted reaction mixture was extracted with water (3 9 15 mL). The ethereal
layer was dried over anhydrous MgSO and condensed under the reduced pressure
4
to provide the crude product. The crude product was purified by column
chromatography using ethyl acetate and hexane mixtures as eluent to obtain the
pure Heck product.
Results and discussion
The aminoclay was synthesized according to the literature procedure [40] using
magnesium chloride and 3-aminopropyltriethoxysilane (APTES) in an ethanol
solution. Then, picolinic acid was anchored on the surface of the aminoclay. The
addition of aminoclay@picolinic acid prepared in the previous step to the solution
of Pd(OAc) in ethanol resulted in an aminoclay@picolinic acid complex of nano-
2
Pd(0). After separation of the nano-Pd(0) complex from the reaction mixture by
centrifugation and washing with ethanol, it was dried under vacuum and
characterized by TEM, FE-SEM, ICP, IR, XRD, TGA, EDX, XPS and UV–visible
techniques as follows.
The presence of stretching and bending vibrations of primary amine at 3440 and
-
620 cm , respectively, and the IR peaks of C–Si, Si–O–Si and Mg–O at 1141.8,
1
1
1
-
1
033 and 563 cm in the FT-IR absorption spectrum of aminoclay in Fig. 2 clearly
demonstrate the structure of the aminoclay. The shift of two unsymmetrical and
-
symmetrical carbonyl bands at 1720 and 1519 cm in the FT-IR spectrum of
1
-
picolinic acid to lower frequencies of 1650 and 1473 cm , and also the presence of
1
?
-1
a broad stretching band of the -NH3 group at 3217 cm in the FT-IR spectrum
of aminoclay@picolinic acid, confirm the formation of ammonium picolinate on the
surface of the aminoclay. These two carbonyl peaks shift to higher frequencies at
-
1
1
685 and 1477 cm in the spectrum ofthe supported nano-Pd(0) complex on
aminoclay in comparison to the spectrum of aminoclay@picolinic acid, which
obviously can be attributed to the involvement of the carboxylate anion group in
chelation with palladium (Fig. 2).
The appearance of the characteristic bands of Pd(0) at the (111), (200), (220), and
311) crystallographic planes in the XRD spectrum of the aminoclay@picolinic acid
(
nano-Pd(0) complex (Fig. 3a) clearly exhibit the presence of palladium (0)
nanoparticles while these bands were not observed in the XRD spectrum of
Pd(OAc) (Fig. 3c). The broad band in-plane reflections at angles (d020, 110 = 0.42
2
and d130, 200 = 0.24 nm) confirm the formation of aminoclay (Fig. 3b). The average
123

N. Fahimi, A. R. Sardarian
Fig. 2 FT-IR of aminoclay, aminoclay@picolinic acid, picolinic acid, and aminoclay@picolinic acid
nano-Pd(0) complex
Fig. 3 Powder XRD patterns of the aminoclay@picolinic acid nano-Pd(0) complex (a),
aminoclay@picolinic acid (b) and Pd(OAc) (c)
2
particle size of the palladium (0) nanoparticles was calculated by the Scherrer
equation [41] and was estimated to be 10–15 nm.
The UV–Vis spectrum of the resulting catalyst (Fig. 4b) showed the complete
conversion of Pd(II) to Pd(0) by the absence of the peak at *400 nm, which is
attributed to Pd(II) in Pd(OAc) (Fig. 4c).
2
1
23

Aminoclay decorated with nano-Pd(0) picolinic acid…
Fig. 4 UV-visible spectra of aminoclay@picolinic acid (a), aminoclay@picolinic acid nano-Pd(0)
complex (b) and Pd(OAc) (c)
2
The ICP technique was employed for the calculation of palladium loading on the
surface of the aminoclay. According to this analysis, there are 2.5 mmol (0.27 g) of
palladium in 1 g of the catalyst, which shows the high affinity of palladium for the
formation of a complex with picolinate anion.
TEM and FE-SEM images are exhibited in Fig. 5. They show Pd(0) particles
with diameters in the range of nanometers on the layered structure of the clay.
The EDS pattern spectrum of the aminoclay@picolinic acid nano-Pd(0) complex
illustrates a peak at 2.86 keV, which confirms the presence of 26.16 wt% palladium
nanoparticles in the structure of the catalyst (Fig. 6). This finding is in good
agreement with the results obtained from the ICP analysis.
The TGA curves of aminoclay@picolinic acid (a) and the supported Pd complex
b) under N2 atmosphere are presented in Fig. 7. The pink TGA curve (TG%) of
(
aminoclay@picolinic acid (a) show a small mass loss (*4%, w/w) around 100 °C,
which is apparently associated with adsorbed water on the surfaces of the clay. The
second weight loss (*26%, w/w) began at 220 °C due to thermodegradation of the
pyridine ring, and a further degradation started at 400 °C (*15%, w/w) which
belongs to the release of CO . Finally, the reduction in weight percentage of the
2
catalyst at temperatures up to *510 °C is related to the decomposition of the clay
support (Fig. 7).
The TGA curve of the supported Pd complex (b) showed a large decomposition
profile from 250 to 350 °C which is related to the picolinic acid Pd complex
(
*50%, w/w) and is evidence for the thermal stabilization of picolinate ion after
making a complex via the chelation of Pd, which has a great effect on its catalytic
activity and recyclability in a Heck reaction that usually requires heating.
To prove the presence of an ionic bond between the amino group of aÞminoclay
and picolinic acid and the absence of an amide bond, using n-propylamine and
picolinic acid as the model substrates, two reactions were designed and performed
(Scheme 2). In the first reaction, n-propylamine and picolinic acid were mixed in
ethanol as solvent at room temperature to form the corresponding salt, and in the
second experiment, n-propylamine and picolinic acid reacted in the presence of
123

N. Fahimi, A. R. Sardarian
Fig. 5 TEM (a, b) and FE-SEM (e, f) images of the aminoclay@picolinic acid nano-Pd(0) complex
Fig. 6 EDS spectrum of the aminoclay@picolinic acid nano-Pd(0) complex
1
23

Aminoclay decorated with nano-Pd(0) picolinic acid…
Fig. 7 TGA curves of aminoclay@picolinic acid (a) and the aminoclay@picolinic acid nano-Pd(0)
complex (b)
Scheme 2 Model reactions of N-propylamine and picolinic acid under two different conditions (a) and
(
b)
dicyclohexylcarbodiimide (DCC) and triethylamine in dichloromethane at 37 °C to
provide the desired amide [42]. After purification of the product of both
experiments, their IR spectra were taken (Fig. 8). The IR spectrum of n-propan-
1
broad peak in 3417 cm , which is related to -NH3 , and a strong absorption at
-amminium picolinate showed the same pattern as aminoclay@picolinic acid: a
1
-
?
-
650 cm , which is attributed to the carbonyl group of picolinate ion.
1
1
The X-ray photoelectron spectrum of the aminoclay@picolinic acid nano-Pd(0)
complex exhibits peaks at 335.0 and 340.3 eV which are assigned to 3d5/2 and 3d3/2
of palladium in the zero oxidation state, respectively (Fig. 9).
For studying the efficiency of the aminoclay catalyst prepared in the Heck
reaction, the reaction of n-butyl acrylate with iodobenzene was chosen as a model
reaction and then the effects of solvent, base, temperature, and the amount of the
catalyst were studied to optimize these reaction parameters. According to the results
obtained, which are summarized in Table 1, the greatest efficiency was achieved
when 0.5 mg (equivalent to 0.12 mol% of Pd) of catalyst was used at 120 °C in the
presence of tri(n-propyl)amine as the base under solvent-free conditions (Table 1,
entry 5).
With the optimized reaction parameters in hand, the scope and versatility of the
Heck reaction between different aryl halides and n-butyl acrylates were explored in
123

N. Fahimi, A. R. Sardarian
Fig. 8 FT-IR of n-propan-1-amminium picolinate, aminoclay@picolinic acid and N- propylpicolinamide
Fig. 9 XPS spectrum of the aminoclay@picolinic acid nano-Pd(0) complex
the presence of the aminoclay@picolinic acid nano-Pd(0) complex catalyst
(
Scheme 3).
The reactivity of iodo-, bromo- and chlorobenzene were examined by performing
the cross-coupling reactions separately. The results clearly exhibited more reactivity
of iodobenzene than its brombenzene counterpart and also bromobenzene relative to
chlorobenzene (Table 2, entries 1–3). This trend was also observed when the other
aryl iodides and aryl bromides were applied [Table 2, entries (1, 2), (5, 6), (7, 8), (9,
1
0), (11, 12)]. Thus. in this catalytic media, aryl iodides, in general, reacted more
efficiently to produce the corresponding products in higher yields and shorter
reaction times with significant turnover numbers (TONs) and turnover frequencies
1
23

Aminoclay decorated with nano-Pd(0) picolinic acid…
Table 1 Optimization of the reaction parameters of the cross-coupling reaction between iodobenzene
and n-butyl acrylate in the presence of the aminoclay@picolinic acid nano-Pd(0) catalyst
Entry
Solvent
Base
Cat. (mg)
Pd, mol %)
Temp.
(°C)
Time
(h)
Yield (%)
(
1
2
3
4
5
6
7
8
9
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
n-Pr
n-Pr
n-Pr
n-Pr
n-Pr
n-Pr
n-Pr
n-Pr
3
N
N
N
N
N
N
N
N
0.5
r.t
–
–
(
0.12)
3
3
3
3
3
3
3
0.5
50
–
–
(
0.12)
0.5
80
24
24
0.25
24
24
24
24
24
24
24
24
24
24
24
24
10
90
94
88
85
79
60
50
70
60
43
60
55
45
54
(
0.12)
0.5
100
120
100
100
100
100
100
100
100
100
100
100
100
100
(
0.12)
0.5
(
0.12)
0.5
(
0.17)
1.0
(
0.24)
1.0
(
0.24)
K
2
CO
3
1.0
(
0.24)
10
11
12
13
14
15
16
17
CsCO
3
1.0
(
0.24)
Et
3
N
1.0
(
0.24)
(NH4) CO
2
3
1.0
(
0.24)
H
2
O
n-Pr
n-Pr
n-Pr
n-Pr
n-Pr
3
3
3
3
3
N
1.0
(
0.24)
DMF
N
N
N
N
1.0
(
0.24)
Toluene
EtOH
1.0
(
0.24)
1.0
(
0.24)
DMSO
1.0
(
0.24)
(
TOFs = TONs per unit time). These significant differences in the behavior of aryl
halides in their reaction with n-butyl acrylate in the presence of the aminoclay@pi-
colinic acid nano-Pd(0) complex provided a suitable condition for selective cross-
coupling of aryl iodides against aryl bromides and chlorides. The reaction of
1
-bromo-4-iodobenzene and 1-bromo-4-chlorobenzene with n-butyl acrylate
123

N. Fahimi, A. R. Sardarian
Scheme 3 Heck coupling reaction of n-butyl acrylate with aryl halide in the presence of the
aminoclay@picolinic acid nano-Pd(0) complex catalyst
confirmed this selectivity. In these reactions, n-butyl 4-bromocinnamate and n-butyl
4
-chlorocinnamate were obtained as the only products, respectively (Table 2,
entries 16 and 17). Electron-withdrawing groups on the aromatic ring increased the
reaction time (Table 2, entries 7, 8, 13, 18, 19), while the electron-donating groups
decreased it (Table 2, entries 5, 6, 9, 10, 11, 12, 20). 2-Lodo-1-methyl-4-
nitrobenzene, as a substrate, which bears electron-withdrawing and electron-
donating groups simultaneously, carried out the coupling reaction in short reaction
times to give the corresponding a, b- unsaturated ester in high yield (Table 2, entry
2
1). In addition, sterically hindered 1-iodonaphthalene was expeditiously coupled
with n-butyl acrylate in high yields (Table 2, entry 4). 5-Bromopyrimidine, as a
heterocyclic compound, gave the related products in quantitative yields (Table 2,
entry 14). The other alkenes like styrene, methyl acrylate and ethyl acrylate also
reacted with iodobenzene but with lower efficiency and longer reaction times than
n-butyl acrylate (Table 2, entries 22–24). The reaction between iodobenzene and n-
butyl acrylate was examined in the presence of the aminoclay@picolinic acid nano-
Pd(0) catalyst which had remained for more than 1 year in the sample tube in the
laboratory, and the result was exactly the same as the result in Table 2, entry 1
(
Table 2, entry 25).
The efficiency of the present catalytic system was compared with the other
literature-reported systems in the Heck coupling reaction between iodobenzene and
n-butyl acrylate. According to the data gathered in Table 3, highlights of our
catalytic systems are: (1) avoiding employment of toxic solvents, (2) using low
amounts of Pd, (3) shorter reaction times, (4) producing excellent yields, (5)
working under solvent-free and (6) relative high TONs and TOFs.
The reaction between iodobenzene and ethyl acrylate was also run in the
presence of the aminoclay itself and the n-propan-1-amminium picolinate nano-
Pd(0) complex (Table 3, entries 10 and 11). As can be seen, the aminoclay did not
show any activity but the n-propan-1-amminium picolinate nano-Pd(0) complex
catalyzed the reaction to some extent although not as well as the aminoclay@pi-
colinic acid nano-Pd(0) complex (Table 3, entry 9).
Proposed mechanism
According to the literature information [48, 49] and the results obtained in this
study, the following mechanism can be suggested. In the first step, Pd(0) is
converted to Pd(II) through oxidative addition of aryl halide, then syn addition of
the palladium intermediate I on alkene results in a r-complex II. After b-hydride
1
23

Aminoclay decorated with nano-Pd(0) picolinic acid…
Table 2 Heck cross-coupling reactions of aryl halides with n-butyl acrylate in the presence of the
aminoclay@picolinic nano-Pd(0) complex
a
b
c
Entry
1
Substrate
Product
Time (h) Yield (%) TON TOF
I
0
.25
94
783
3132
40
O
O
Br
Cl
2
3
18
30
87
725
"
76
89
633
741
21
"
I
O
O
4
0.58
1277
I
5
6
O
0.5
24
92
86
767
717
1534
30
MeO
MeO
O2N
O
Br
I
MeO
7
8
9
0.33
10
95
90
92
792
750
767
2400
75
O
O
Br
O2N
O2N
I
0.5
1534
O
O
Br
1
0
1
15
88
89
733
742
49
I
1
O
O
0.67
1107
Br
1
2
20
85
708
35
123

N. Fahimi, A. R. Sardarian
Table 2 continued
Entry Substrate
a
b
c
Product
Time (h) Yield (%) TON TOF
O
I
O
13
0.25
93
775
3100
NC
" NC
O
I
N
1
4
5
N
O
34
2
88
85
733
708
21
N
N
"
I
O
O
1
O
354
O
" O
" O
O
Br
O
1
1
6
7
1
5
87
88
725
733
48
Cl
Cl
O
Br
O
0.33
2221
I
"Br
O
O2N
Br
O2N
O
1
1
8
9
5
7
89
84
742
700
148
100
"
O2N
O
O2N
Br
O
O
OH
O
O
2
2
0
1
0.5
0.33
35
94
82
77
783
683
642
1566
2070
18
I
HO
O2N
I
O2N
O
I
d
2
2
1
23

Aminoclay decorated with nano-Pd(0) picolinic acid…
Table 2 continued
a
b
c
Entry
Substrate
Product
Time (h) Yield (%) TON TOF
O
I
e
2
3
O
30
2
60
80
94
500
17
O
I
f
24
O
666
333
O
I
g
O
25
0.25
783
3132
Reaction condition: aryl halide (1 mmol) and n-butyl acrylate (1.5 mmol), 0.5 mg of the catalyst and n-
Pr
N (1.5 mmol, 0.29 mL) under solvent-free condition at 120 °C
Yields refer to the isolated pure products
3
a
b
c
d
e
f
N
product/Npalladium, N denotes molar numbers
N
product/(Npalladium 9 t), N denotes molar numbers, and t denotes reaction time (h)
Using styrene instead of n-butyl acrylate
Using methyl acrylate instead of n-butyl acrylate
Using ethyl acrylate instead of n-butyl acrylate
Using catalyst 1 year after preparation
g
elimination, in a third step, the desired product is formed, and finally reductive
elimination by (n-pr) N on the Pd(II) intermediate III provides Pd(0) species
3
(
Scheme 4).
Recyclability of catalyst
After the completion of the model reaction between iodobenzene with n-butyl
acrylate, the reaction mixture was diluted with diethyl ether, centrifuged and
decanted to recover the catalyst. This recovered catalyst was used in the next run
after washing with diethyl ether and drying. The results showed that the catalyst
keeps its catalytic activity and did not lose its efficiency after seven cycles. But in
the eighth cycle, the reactivity of catalyst broke down and the yield of product
decreased from 94% in the first run to 81% and the yield of the recovered catalyst
declined from 98 to 84% (Fig. 10). The amount of leached Pd in the reaction
solution was measured by the ICP technique. This study revealed that no palladium
leaching was observed after the first run and the amount of palladium leaching after
8
successive runs was determined to be 10%.
In addition to measuring the Pd leaching, which is actually an indication for the
heterogeneity of the clay catalyst, a mercury poisoning test was also performed,
according to the literature report [50], on the Heck cross-coupling reaction of
bromobenzene with n-butyl acrylate as a model. Therefore, on the basis of this
123

N. Fahimi, A. R. Sardarian
1
23

Aminoclay decorated with nano-Pd(0) picolinic acid…
Scheme 4 Plausible mechanism for the Mizoroki–Heck coupling reaction of aryl halide with n-butyl
acrylate
Fig. 10 Reusability of the aminoclay@picolinic acid nano-Pd(0) complex catalyst
report, the model reaction was initially run in the absence of mercury under the
reaction condition applied in Table 2. Analysis of the reaction mixture, after 9 h
stirring at 120 °C by the GC technique, showed the production of only 58% of the
desired product. Then, an excess amount of mercury, 300 molar equivalents relative
to the clay nano-Pd(0) catalyst, was added to the reaction mixture which was stirred
for a further 9 h under the same conditions when the desired product was obtained
in 57% yield.
123

N. Fahimi, A. R. Sardarian
Fig. 11 TEM image of the
aminoclay@picolinic acid nano-
Pd(0) complex catalyst after the
eighth run
The TEM image of the recovered aminoclay@picolinic acid nano-Pd(0) complex
after the eighth run showed some extent of aggregation which can explain the other
responsible reason for the decrease in the catalytic activity of the catalyst (Fig. 11).
Conclusions
Aminoclay surface was simply functionalized by a nano-Pd(0) picolinic acid
complex via a short green procedure at room temperature. Full characterization of
the catalyst was performed with TEM, FE-SEM, EDS, TGA, FT-IR, XRD, ICP,
XPS and UV–visible techniques. This novel heterogeneous phosphine ligand-free
catalyst was employed in the cross-coupling reaction of a wide spectrum of aryl
iodides, bromides and chlorides with n-butyl acrylate under solvent-free conditions.
The results clarified the selective coupling of aryl iodides in comparison with
bromides and chlorides in short reaction times with high to excellent yields. TONs
and TOFs in the present work are significantly higher than many recent reports. The
catalyst was easily separated from the reaction media by centrifuging and reused
efficiently for eight runs.
Acknowledgement We gratefully acknowledge the support of this work by Shiraz University Research
Council.
Compliance with ethical standards
Conflict of interest The authors declare that they have no conflict of interest.
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