Palladium-Schiff Base Complex Immobilized Covalently on Magnetic Nanoparticles as an Efficient and Recyclable Catalyst for Heck and Suzuki Cross-Coupling Reactions

Article abstract of DOI:10.1007/s10562-015-1636-y

A new palladium-Schiff base complex immobilized covalently on magnetic nanoparticles (Pd-imino-Py-γ-Fe₂O₃) was synthesized via the reaction of chloro-functionalized γ-Fe₂O₃ with iminopyridine followed by the reaction with palladium acetate. Characterization of Pd-imino-Py-γ-Fe₂O₃ was carried out by various techniques such as XRD, SEM, TEM, FT-IR, TGA, ICP, XPS, VSM and elemental analysis. Pd-imino-Py-γ-Fe₂O₃ was successfully applied as a magnetically recyclable heterogeneous catalyst in Heck and Suzuki cross-coupling reactions of various aryl halides (iodide, bromides and chlorides as challenging substrates) with olefins and phenylboronic acid, respectively. The synthesized catalyst was separated easily by using an external magnet and recycled eight runs without appreciable loss of its catalytic activity and leaching of the significant quantity of Pd. Graphical Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-015-1636-y

Catal Lett
DOI 10.1007/s10562-015-1636-y
Palladium-Schiff Base Complex Immobilized Covalently
on Magnetic Nanoparticles as an Efficient and Recyclable Catalyst
for Heck and Suzuki Cross-Coupling Reactions
1
1
1
2
Sara Sobhani
•
Zahra Mesbah Falatooni
•
Solmaz Asadi
•
Moones Honarmand
Received: 25 August 2015 / Accepted: 27 September 2015
Ó Springer Science+Business Media New York 2015
Abstract
immobilized covalently on magnetic nanoparticles (Pd-
A
new palladium-Schiff base complex
Graphical Abstract
R = OMe, Cl, CN, NO
2
imino-Py-c-Fe O ) was synthesized via the reaction of
2
3
n
R′ = CO
2
Bu , CO
2
Me, CO
2
Et, Ph
chloro-functionalized c-Fe O with iminopyridine fol-
lowed by the reaction with palladium acetate. Charac-
2
3
X
+
R
R'
R
R'
X = I, Br, Cl
terization of Pd-imino-Py-c-Fe O3 was carried out by
2
various techniques such as XRD, SEM, TEM, FT-IR,
TGA, ICP, XPS, VSM and elemental analysis. Pd-imino-
Py-c-Fe O was successfully applied as a magnetically
O
O
O
O
O
O
OAc
O
Si
γ-Fe
O
Si
O
OAc
Pd
2
3
2
3
Pd
N
N
AcO
OAc
recyclable heterogeneous catalyst in Heck and Suzuki
cross-coupling reactions of various aryl halides (iodide,
bromides and chlorides as challenging substrates) with
olefins and phenylboronic acid, respectively. The syn-
thesized catalyst was separated easily by using an
external magnet and recycled eight runs without appre-
ciable loss of its catalytic activity and leaching of the
significant quantity of Pd.
N
N
OH
X
+
B
R
OH
R
X = I, Br, Cl
R = OMe, Cl, CN, NO
2
Keywords Palladium Á Magnetic nanoparticles Á
Heterogeneous catalyst Á Heck and Suzuki reactions
1
Introduction
Heck and Suzuki cross-coupling reactions are the most
straightforward and powerful transformations for the con-
struction of carbon–carbon bonds which can lead to
important substituted olefins and biphenyls. These products
play a significant role in medicinal as well as structural
chemistry and agriculture. They are used as versatile
building blocks for the synthesis of many organic inter-
mediates in material sciences, pharmaceuticals and herbi-
cides [1–4]. Heck and Suzuki cross-coupling reactions
have been commonly carried out by using homogeneous
palladium catalysts in the presence of phosphine [1, 5, 6],
&
Sara Sobhani
sobhanisara@yahoo.com; ssobhani@birjand.ac.ir
1
2
Department of Chemistry, College of Sciences, University of
Birjand, Birjand, Iran
Department of Chemical Engineering, Faculty of Mining,
Chemical and Material Engineering, Birjand University of
Technology, Birjand, Iran
123

S. Sobhani et al.
imidazole [7], pyridine [8], N-heterocyclic carbenes [9],
dibenzylideneacetone (dba) [10], palladacycles [11], thiols
immobilized covalently on c-Fe O (Pd-imino-Py-c-
2
3
Fe O ) and used it as a magnetically recyclable heteroge-
2
3
[
12], porphyrins [13], phthalocyanines [14], bidentate [15]
neous palladium catalyst in Heck and Suzuki cross-cou-
pling reactions.
and pincer [16] ligands. They suffer from some drawbacks
such as requiring large amount of the catalyst, product
contamination, and difficulty in catalyst separation from
the reaction mixture and recycling which is a serious
problem in the pharmaceutical industry. Considering this
intrinsic limitation of homogeneous Pd (expensive metal)
catalysts, in addition to environmental and economical
concerns, many attempts have been directed toward the
development of new strategies to immobilize the homo-
geneous Pd catalysts onto various solid supports in the
form of either metal complexes (by covalent anchoring) or
metal nanoparticles (by adsorption) [17]. In heterogeniza-
tion of Pd catalysts, surface covalent anchoring of Pd
complexes is preferred over simple Pd adsorption as it is
robust enough to withstand the reaction conditions and the
catalyst can be reused several times without significant
leaching during the reaction workup.
2 Experimental
2.1 General Information
Chemicals were purchased from Merck Chemical Com-
pany. NMR spectra were recorded on a Bruker Avance
DPX-400 using deutrated CDCl as solvent and TMS as
3
internal standard. The purity of the products and the pro-
gress of the reactions were accomplished by TLC on silica-
gel polygram SILG/UV254 plates. TEM analysis was
performed using TEM microscope (Philips CM30). FT-IR
spectra were recorded on a Shimadzu Fourier Transform
Infrared Spectrophotometer (FT-IR-8300). Thermo gravi-
metric analysis (TGA) was performed using a Shimadzu
thermo gravimetric analyzer (TG-50). Elemental analysis
was carried out on a Costech 4010 CHN elemental ana-
lyzer. The morphology of the products was determined by
using Hitachi Japan, model s4160 Scanning Electron
Microscopy (SEM) at accelerating voltage of 15 kV.
Power X-ray diffraction (XRD) was performed on a Bruker
D8-advance X-ray diffractometer with Cu Ka
(k = 0.154 nm) radiation. This system was equipped with
a concentric hemispherical (CHA) electron energy analyzer
(Specs model EA10 plus) suitable for X-ray photoelectron
spectroscopy (XPS). The content of Pd in the catalyst was
determined by OPTIMA 7300DV ICP analyzer. Room
temperature magnetization isotherms were obtained using a
vibrating sample magnetometer (VSM, Lake Shore 7400).
Among various solid supports used for the heteroge-
nization of Pd catalysts, magnetic nanoparticles (MNPs)
have emerged as one of the most useful solid materials
[
18–23]. The main advantage of MNPs is their simple
separation from the reaction mixture by using an external
magnet, without any need for filtration or centrifugation.
Also, most magnetic-supported catalysts can be reused
many times while keeping their initial activity. Moreover,
the outstanding dispersion of supported catalysts on MNPs
in different solvents is additional benefit, as this makes the
active sites of the catalyst more accessible to the reactants.
The presence of supported catalysts on the surface of
MNPs solves the diffusion limitation problems, which are
often observed for supported catalysts for example in
microporous or mesoporous matrices [24].
Schiff bases are an important class of organic ligands in
coordination chemistry and have been extensively used as
metal Schiff base complexes to catalyze organic reactions
due to the advantages such as their cost-effective, ease of
synthesis, availability and thermal and chemical stability
2.2 Synthesis of Iminopyridine
Pyridine-2-carbaldehyde (0.5 g) was added to a magneti-
cally stirring mixture of 4-aminophenol (0.5 g) in MeOH
(10 mL). After refluxing for 3 h, the mixture was cooled to
room temperature. The resulting yellow crystals
(iminopyridine ligand) was separated by filtration, washed
with MeOH (3 9 10 mL) and dried under vacuum.
[
25]. However, the use of metal Schiff base complexes as
catalyst in homogeneous solution suffers from deactivation
because of formation of l-oxo and dimeric peroxo- species,
which have been demonstrated to be inactive in various
reactions [26, 27]. Moreover, difficulties in separation and
reusability of homogeneous metal (particularly expensive
metals) Schiff base complexes have limited the applica-
tions of these catalysts. For overcoming the aforemen-
tioned drawbacks, metal Schiff base complexes can be
immobilized on suitable solid supports.
2.3 Synthesis of Schiff Base Immobilized on c-Fe O
2
3
(Imino-Py-c-Fe O )
2
3
A solution of chloro-functionalized c-Fe O [34] (1.7 g)
2
3
in DMF (10 mL) was added dropwise to a stirring
solution of iminopyridine (0.15 g) and NaH (0.005 g) in
DMF (5 mL), under Ar atmosphere at 80 °C within 1 h.
The reaction mixture was stirred at 80 °C for another
On continuation of our research interest in developing
new heterogeneous catalysts [28–41], herein, we have
synthesized
a
new palladium-Schiff base complex
1
23

Palladium-Schiff Base Complex Immobilized Covalently on Magnetic Nanoparticles…
2
4 h. The resulting Schiff base immobilized on c-Fe O
(100 MHz, CDCl ): d 14.1, 52.0, 127.9, 129.6, 135.8,
2
3
3
was separated by an external magnet, washed with ace-
138.9, 169.1.
tone (3 9 10 mL) and dried under vacuum. The loading
-
1
amount of Schiff base was 0.28 mmol g catalyst based
on elemental analysis.
2.5.4 (E)-Ethyl cinnamate (1d)
1
3
H NMR (400 MHz, CDCl ): d 1.43 (t, 3H, J = 8.0 Hz),
3
3
.19 (q, 2H, J = 8.0 Hz), 6.35 (d, 1H, J = 16 Hz),
3
4
2
.4 Synthesis of Palladium-Schiff Base Complex
Immobilized on c-Fe O (Pd-Imino-Py-c-Fe O )
7.27–7.28 (m, 3H), 7.41–7.42 (m, 2H), 7.63 (d, 1H,
3
13
J = 16.0 Hz); C NMR (100 MHz, CDCl ): d 14.2, 60.0,
2
3
2
3
3
118.2, 127.9, 128.7, 130.1, 134.3, 144.3, 166.6.
The synthesized Schiff base immobilized on c-Fe O
2
3
(
1.7 g) was added to a solution of palladium acetate
2.5.5 (E)-1,2-Diphenylethene (1e)
(
0.12 g) in dry acetone (5 mL). The reaction mixture was
1
stirred at room temperature for 24 h. The solid was sepa-
rated by an external magnet, and washed with acetone
H NMR (400 MHz, CDCl ): d 7.18 (s, 2H), 7.33 (t, 2H,
3
3
3
3
J = 6.8 Hz), 7.42 (t, 4H, J = 7.0 Hz), 7.58 (d, 4H,
13
J = 7.2 Hz); C NMR (100 MHz, CDCl ): d 126.7,
(
3 9 10 mL) and dried under vacuum to afford Pd-imino-
3
Py-c-Fe O .
127.8, 128.8, 137.4.
2
3
2
.5 General Procedure for Heck Coupling Reaction
2.5.6 (E)-n-Butyl 3-(4-methoxyphenyl)acrylate (1f)
1
3
A mixture of aryl halide (1 mmol), olefin (1.5 mmol), Et N
3
H NMR (400 MHz, CDCl ): d 0.91 (t, 3H, J = 7.0 Hz),
3
(
2 mmol) and Pd-imino-Py-c-Fe O (0.25 mol%, 0.009 g)
1.35–1.40 (m, 2H), 1.60–1.62 (m, 2H), 3.72 (s, 3H), 4.13
2
3
3
3
(m, 2H, J = 7.0 Hz), 6.24 (d, 1H, J = 16.0 Hz), 6.81 (d,
was stirred at 100 °C in DMF (2 mL) for an appropriate
time (Table 2). The catalyst was separated by an external
magnet, washed with EtOH, dried and reused for a con-
secutive run under the same reaction conditions. Evapo-
ration of the solvent of the filtrate under reduced pressure
gave the crude products. The pure products were isolated
by chromatography on silica gel eluted with n-hex-
ane:EtOAc (50:1).
3
3H, J = 4.8 Hz), 7.39 (d, 3H, J = 4.2 Hz), 7.58 (d, 1H,
3
3
13
J = 16.0 Hz); C NMR (100 MHz, CDCl ): d 13.6, 19.1,
30.7, 55.0, 64.0, 114.1, 115.5, 127.0, 129.5, 144.0, 161.2,
3
167.1.
2.5.7 (E)-1-Methoxy-4-styrylbenzene (1g)
1
H NMR (400 MHz, CDCl ): d 3.89 (s, 3H), 6.92 (d, 2H,
3
3
3
3
J = 8.4 Hz), 6.99 (d, 2H, J = 16.4 Hz), 7.09 (d, 2H,
2
.5.1 (E)-n-Butyl Cinnamate (1a)
3
J = 16.0 Hz), 7.36 (t, 2H, J = 7.6 Hz), 7.46–7.52 (m,
1
3
13
3H); C NMR (100 MHz, CDCl ): d 55.3, 114.1, 126.3,
126.6, 127.2, 127.8, 128.2, 128.7, 130.1, 137.7, 159.3.
H NMR (400 MHz, CDCl ): d 0.88 (t, 3H, J = 7.6 Hz),
3
3
1
3
.34–1.36 (m, 2H), 1.59–1.62 (m, 2H), 4.13 (t, 2H,
3
J = 6.8 Hz), 6.36 (d, 1H, J = 16.0 Hz), 7.29–7.30 (m,
3
H), 7.43–7.45 (m, 2H), 7.60 (d, 1H, J = 16.4 Hz);
13
3
C
2.5.8 (E)-n-Butyl 3-(4-chlorophenyl)acrylate (1h)
NMR (100 MHz, CDCl ): d 13.7, 19.2, 30.8, 64.3, 118.2,
3
1
3
1
28.0, 128.8, 130.1, 134.4, 144.4, 166.8.
H NMR (400 MHz, CDCl ): d 0.90 (t, 3H, J = 7.2 Hz),
3
1
.33–1.39 (m, 2H), 1.57–1.64 (m, 2H), 4.13 (t, 2H,
3
3
J = 6.8 Hz), 6.64 (d, 1H, J = 16.0 Hz), 7.45 (d, 2H,
2
.5.2 (E)-Methyl Cinnamate (1b)
3
3
3
J = 8.0 Hz), 7.63 (d, 1H, J = 16.0 Hz), 7.74 (d, 2H,
13
J = 8.0 Hz); C NMR (100 MHz, CDCl ): d 13.7, 19.2,
1
H NMR (400 MHz, CDCl ): d 3.74 (s, 3H), 6.38 (d, 1H,
J = 16.4 Hz), 7.31–7.33 (m, 3H), 7.45–7.47 (m, 2H), 7.63
3
3
3
3
30.7, 64.4, 118.8, 129.1, 129.4, 132.9, 136.0, 143.0, 166.7.
13
d, 1H, J = 16.4 Hz); C NMR (100 MHz, CDCl ): d
(
3
5
1.6, 117.7, 128.1, 128.9, 130.3, 134.3, 144.8, 167.3.
2.5.9 (E)-1-Chloro-4-styrylbenzene (1i)
1
2
.5.3 (E)-Methyl 2-Methyl-3-phenylacrylate (1c)
H NMR (400 MHz, CDCl ): d 7.07 (s, 2H), 7.28–7.39 (m,
3
1
H), 7.43–7.46 (m, 2H), 7.50–7.52 (m, 2H); C NMR
3
5
1
H NMR (400 MHz, CDCl ): d 2.15 (s, 3H), 3.83 (s, 3H),
(100 MHz, CDCl ): d 127.7, 128.5, 128.8, 129.0, 129.9,
130.0, 130.4, 134.3, 136.9, 138.1.
3
3
3 13
.21–7.41 (m, 5H), 7.92 (d, 1H, J = 16.0 Hz); C NMR
7
123

S. Sobhani et al.
2
.5.10 (E)-n-Butyl 3-(4-nitrophenyl)acrylate (1j)
2.6 General Procedure for Suzuki Coupling
Reaction
1
3
H NMR (400 MHz, CDCl ): d 0.92 (t, 3H, J = 7.2 Hz),
3
1
.35–1.44 (m, 2H), 1.62–1.69 (m, 2H), 4.17–4.24 (m, 2H),
3
.53 (d, 1H, J = 16.0 Hz), 7.63–7.68 (m, 3H), 8.20 (d,
A mixture of aryl halide (1 mmol), phenylboronic acid
6
2
1
1
(1.5 mmol), Et N (2 mmol) and Pd-imino-Py-c-Fe O
3
2 3
3
13
H, J = 8.0 Hz); C NMR (100 MHz, CDCl ): d 13.6,
9.1, 30.6, 64.7, 122.5, 124.0, 128.6, 140.5, 141.5, 148.3,
(0.25 mol%, 0.009 g) was stirred at 100 °C in DMF
(2 mL) for an appropriate time (Table 3). The catalyst was
separated by an external magnet, washed with EtOH, dried
and reused for a consecutive run under the same reaction
conditions. Evaporation of the solvent of the filtrate under
reduced pressure gave the crude products. The pure prod-
ucts were isolated by chromatography on silica gel eluted
with n-hexane:EtOAc (50:1).
3
66.0.
2
.5.11 (E)-n-Butyl 3-(4-cyanophenyl)acrylate (1k)
1
3
H NMR (400 MHz, CDCl ): d 0.77 (t, 3H, J = 7.0 Hz),
3
1
.30–1.38 (m, 2H), 1.57–1.64 (m, 2H), 4.14 (t, 2H,
3
3
J = 6.8 Hz), 6.45 (d, 1H, J = 16.0 Hz), 7.54 (d, 2H,
3
3
J = 8.4 Hz), 7.57 (d, 1H, J = 15.2 Hz), 7.60 (d, 2H,
1
3
3
3
1
J = 8.0 Hz); C NMR (100 MHz, CDCl ): d 13.5, 19.0,
3
0.5, 64.4, 113.0, 118.1, 121.6, 128.3, 132.4, 138.4, 141.8,
65.7.
2
.5.12 (E)-4-Styrylbenzonitrile (1l)
1
H
NMR (400 MHz, CDCl ):
3
d
J = 16.0 Hz), 7.22 (d, 1H, J = 16.0 Hz), 7.31–7.34 (t,
7.09 (d, 1H,
3
3
3
H, J = 4.0 Hz), 7.38–7.41 (t, 2H, J = 4.0 Hz), 7.54 (d,
3
1
3
H, J = 7.2 Hz), 7.58 (d, 2H, J = 8.4 Hz), 7.64 (d, 2H,
3
2
3
13
J = 8.4 Hz); C NMR (100 MHz, CDCl ): d 111.7,
3
1
20.2, 127.8, 128.0, 128.1, 129.8, 130.0, 133.5, 133.6,
1
37.4, 142.9.
2
.5.13 (E)-1-Nitro-4-styrylbenzene (1m)
1
H NMR (400 MHz, CDCl ): d 7.06–7.18 (m, 1H),
3
7
.40–7.45 (m, 4H), 7.49–7.66 (m, 4H), 8.15–8.26 (m, 2H);
1
3
C NMR (100 MHz, CDCl ): d 125.2, 127.4, 128.0, 128.2,
3
2 3
Fig. 1 XRD patterns of Pd-imino-Py-c-Fe O
1
30.0, 134.4, 137.3, 145.0, 147.8.
2 3
Scheme 1 Synthesis of Pd-imino-Py-c-Fe O
1
23

Palladium-Schiff Base Complex Immobilized Covalently on Magnetic Nanoparticles…
Fig. 2 SEM of Pd-imino-Py-c-
2 3
Fe O
Fig. 3 a TEM of Pd-imino-Py-
c-Fe and b particle size
distribution histogram of Pd-
imino-Py-c-Fe
(a)
2 3
O
2 3
O
(b)
50
4
0
3
0
2
0
1
0
0
7
9
1
1
1
3
1
5
1
7
1
9
2
1
2
3
Particle size (nm)
1
00
123 °C
9
7.57 %
9
8
6
4
2
0
8
6
4
9
9
9
9
8
8
8
813 °C
5.41 %
8
0
200
400
600
800
1000
4
000 3600 3200 2800 2400 2000 1600 1200 800
400
Temperature (°C)
-
1
Wave number (cm )
2 3
Fig. 5 TGA of Pd-imino-Py-c-Fe O
Fig. 4 FT-IR spectra of c-Fe
black)
2 3 2 3
O (gray), Pd-imino-Py-c-Fe O
(
123

S. Sobhani et al.
2.6.4 4-Cyanobiphenyl (2d)
1
H NMR (400 MHz, CDCl ): d 7.41–7.51 (m, 3H),
3
1
3
7
.56–7.60 (m, 2H), 7.68–7.74 (m, 4H);
C NMR
(
100 MHz, CDCl ): d 112.0, 120.1, 128.3, 128.8, 129.8,
3
130.2, 133.7, 140.3, 146.8.
3
Results and Discussion
3
.1 Synthesis and Characterization of Pd-Imino-Py-
c-Fe O
2
3
Pd-imino-Py-c-Fe O was synthesized by the simple pro-
2
3
cedure shown in Scheme 1. At first, chloro-functionalized
2 3
Fig. 6 XPS spectrum of Pd-imino-Py-c-Fe O
c-Fe O [34] was reacted with iminopyridine ligand to
2
3
obtain Schiff base immobilized on c-Fe O . Finally, the
2
3
reaction of imino-Py-c-Fe O with Pd(OAc) in dry ace-
2
3
2
tone led to the formation of palladium-Schiff base complex
supported on c-Fe O MNPs (Pd-imino-Py-c-Fe O ).
2
3
2 3
The synthesized catalyst was characterized by XRD (X-
ray diffraction), SEM (scanning electron microscopy),
TEM (high resolution transmission electron microscopy),
FT-IR (Fourier transform infrared spectroscopy), TGA
(thermogravimetric analysis), elemental analysis, ICP (in-
ductively coupled plasma), elemental analysis, XPS (X-ray
photoelectron spectroscopy) and VSM (vibrating sample
magnetometer).
As presented in Fig. 1, the reflection planes of (2 2 0), (3
Fig. 7 Magnetization curves of c-Fe
O
2 3
2 3
and Pd-imino-Py-c-Fe O
1 1), (4 0 0), (4 2 2), (5 1 1) and (4 4 0) at 2h = 30.3°,
5.7°, 43.4°, 53.8°, 57.4° and 63.0° were readily recog-
3
nized from the XRD pattern of Pd-imino-Py-c-Fe O . The
2
3
2
.6.1 Biphenyl (2a)
observed diffraction peaks was indicated that c-Fe O
2
3
magnetic nanoparticles mostly exist in face-centered cubic
structure. The average crystallite size of Pd-imino-Py-c-
Fe O (16 nm) was estimated using the Debye–Scherrer
1
3
H NMR (400 MHz, CDCl ): d 7.46 (t, 2H, J = 7.2 Hz),
3
3
3
.56 (t, 4H, J = 8.0 Hz), 7.72 (d, 4H, J = 6.8 Hz);
13
7
C
2
3
NMR (100 MHz, CDCl ): d 127.2, 127.3, 128.8, 141.3.
formula.
3
SEM images of Pd-imino-Py-c-Fe O were showed
2
3
2
.6.2 4-Methoxybiphenyl (2b)
uniformity and spherical morphology of MNPs (Fig. 2).
As can be seen from the TEM images, imino-Py-c-
Fe O has spherical morphology with relatively good
1
H NMR (400 MHz, CDCl ): d 3.83 (s, 3H), 6.99 (d, 2H,
3
2 3
3
3
J = 8.8 Hz), 7.31 (t, 1H, J = 7.2 Hz), 7.43 (t, 2H,
monodispersity. The particle size distribution of Pd-imino-
3
3
J = 8.0 Hz), 7.55 (t, 4H, J = 8.8 Hz);
13
C NMR
Py-c-Fe O was evaluated using TEM (Fig. 3). The aver-
2
3
(
100 MHz, CDCl ): d 56.5, 115.3, 127.8, 127.9, 129.3,
age diameter of nanoparticles was 15 nm.
3
1
29.9, 134.9, 142.0, 160.3.
FT-IR spectra of c-Fe O and Pd-imino-Py-c-Fe O
were shown in Fig. 4. The band at around 550–650 cm
2
3
2 3
-
1
2
.6.3 4-Nitrobiphenyl (2c)
was assigned to the stretching vibrations of Fe–O bond in
c-Fe O and Pd-imino-Py-c-Fe O . In the spectrum of Pd-
2
3
2 3
1
H NMR (400 MHz, CDCl ): d 7.43–7.52 (m, 3H),
imino-Py-c-Fe O , the C-H stretching mode of methylene
2 3
3
-
can be observed at 2933 cm . Two absorption bands at
1
7
1
.62–7.64 (m, 2H), 7.73–7.76 (m, 2H), 8.29–8.30 (m, 2H);
3
-1
C NMR (100 MHz, CDCl3): d 125.2, 128.5, 128.9,
30.0, 130.3, 139.9, 148.2, 148.7.
1506 and 1442 cm
was attributed to the stretching
1
vibration of C=C bonds. The presence of bands at around
1
23

Palladium-Schiff Base Complex Immobilized Covalently on Magnetic Nanoparticles…
-
1
1
351 and 1265 cm
was allocated to C-N and C-O
The magnetization curves of c-Fe O and Pd-imino-Py-
2
3
stretching vibrations, respectively. Peaks were appeared at
-
c-Fe O were measured at room temperature using a
2
3
1
1
600 and 1373 cm in the spectrum of Pd-imino-Py-c-
Fe O , related to COO (in acetate) and confirmed the
vibrating sample magnetometer (VSM). No reduced
remanence and coercivity were detected, indicating both
unmodified and Pd-imino-Py-c-Fe O are superparamag-
2
3
presence of Pd in the catalyst. These results indicated that
2
3
Schiff base were successfully grafted onto the c-Fe O
MNPs.
netic (Fig. 7). The value of saturation magnetic moments
-
2
3
1
of c-Fe O and Pd-imino-Py-c-Fe O are 68.9 emu g .
2
3
2 3
The thermogravimetric analysis (TGA) of Pd-imino-Py-
c-Fe O was used to determine the thermal stability and
3.2 Catalytic Activity of Pd-Imino-Py-c-Fe O
2 3
2
3
content of organic functional groups on the surface of
in Heck and Suzuki Cross-Coupling Reactions
MNPs (Fig. 5). TG curve of Pd-imino-Py-c-Fe O showed
2
3
the weight loss around 110 °C, which was related to the
adsorbed water molecules on the support. The organic parts
were decomposed completely in the temperature range of
Following the successful synthesis of Pd-imino-Py-c-
Fe O , its effectiveness for Heck and Suzuki cross-cou-
2
3
pling reactions were examined. At first, Heck cross-cou-
pling reaction of iodobenzene with n-butyl acrylate in the
presence of 0.25 mol% of Pd-imino-Py-c-Fe O and Et N
1
23–813 °C. According to the TGA, the amount of com-
ponents supported on c-Fe O is estimated to be
2
3
2
3
3
-
.26 mmol g . These results were in good agreement with
1
0
as a base was chosen as a model reaction (Table 1). This
the elemental analysis data (N = 0.75 % and C = 6.25 %)
and ICP. The ICP analysis showed that 0.26 mmol of
Pd(II) was anchored on 1.0 g of MNPs [Pd(II)
wt% = 2.8 %].
reaction was investigated under solvent-free conditions and
in different solvents such as DMF, CH CN, toluene, EtOH
3
and H O (Table 1, entries 1–6). The best yield of the
2
desired product was obtained in DMF at 100 °C (Table 1,
entry 2). A similar reaction at lower temperatures required
longer reaction time and produced the desired product in
lower yields (Table 1, entries 7 and 8). Increasing the
reaction temperature to 120 °C did not have any significant
In the X-ray photoelectron spectrum (XPS) of Pd-imino-
Py-c-Fe O , binding energies at 338.6 and 343.5 eV cor-
responding to Pd 3d5/2 and Pd 3d3/2, respectively, con-
2
3
firmed the presence of Pd(II) in the catalyst (Fig. 6).
Table 1 Heck coupling
a
Yield (%)
Elntry
Catalyst (mol%)
Solvent
Tem. (°C)
Base
Time (h)
reaction of iodobenzene with n-
butyl acrylate under different
conditions
1
2
3
4
5
6
7
8
9
Pd-imino-Py-c-Fe
Pd-imino-Py-c-Fe
Pd-imino-Py-c-Fe
Pd-imino-Py-c-Fe
Pd-imino-Py-c-Fe
Pd-imino-Py-c-Fe
Pd-imino-Py-c-Fe
Pd-imino-Py-c-Fe
Pd-imino-Py-c-Fe
Pd-imino-Py-c-Fe
Pd-imino-Py-c-Fe
Pd-imino-Py-c-Fe
Pd-imino-Py-c-Fe
Pd-imino-Py-c-Fe
Pd-imino-Py-c-Fe
Pd-imino-Py-c-Fe
Pd-imino-Py-c-Fe
Pd-imino-Py-c-Fe
–
O
2 3
O
2 3
O
2 3
O
2 3
O
2 3
O
2 3
O
2 3
O
2 3
O
2 3
O
2 3
O
2 3
O
2 3
O
2 3
O
2 3
O
2 3
O
2 3
O
2 3
O
2 3
(0.25)
(0.25)
(0.25)
(0.25)
(0.25)
(0.25)
(0.25)
(0.25)
(0.25)
(0.25)
(0.25)
(0.25)
(0.25)
(0.25)
(0.12)
(0.2)
–
100
100
reflux
100
reflux
100
rt
Et
Et
Et
Et
Et
Et
Et
Et
Et
3
N
3
N
3
N
3
N
3
N
3
N
3
N
3
N
3
N
1.5
0.5
1
90
DMF
95
CH
3
CN
46
Toluene
EtOH
0.5
0.5
24
24
1
16
55
2
H O
15
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
21
80
80
120
100
100
100
100
100
100
100
100
100
100
100
0.5
1.5
3
96
10
11
12
13
14
15
16
17
18
19
20
K
2
CO
3
67
Na
2
CO
3
51
NaOAc
KOH
–
1.5
2
63
44
24
0.5
0.5
0.5
0.5
24
24
Trace
78
Et
Et
Et
Et
Et
Et
3
N
3
N
3
N
3
N
3
N
3
N
81
(0.5)
96
(1)
97
Trace
30
2
Pd(OAc) (0.25)
Reaction conditions iodobenzene:n-butyl acrylate:base = 1:1.5:2
a
Isolated yield
123

S. Sobhani et al.
2 3
Table 2 Heck coupling reaction of various aryl halides with olefins catalyzed by Pd-imino-Py-c-Fe O
a
1
23

Palladium-Schiff Base Complex Immobilized Covalently on Magnetic Nanoparticles…
Table 2 continued
a
Reaction conditions molar ratio of aryl halide:olefin:Et
a
3
N:catalyst = 1:1.5:2:0.0025, 100 °C, DMF
3
Isolated yield. Trans isomer was obtained based on JHH value (16.0–16.4 Hz) in the H NMR spectra of the products
1
effect on the progress of the reaction (Table 1, entry 9).
The influence of different bases on the reaction of
iodobenzene with n-butyl acrylate in DMF at 100 °C was
studied (Table 1, entries 10–13). Comparison of the results
(Table 1, entry 2). It is worth to note that the reaction did
not proceed well in the absence of the base and only a trace
amount of the desired product was formed after 24 h
(Table 1, entry 14). Similar reactions were also performed
in the presence of different catalytic amounts of Pd-imino-
Py-c-Fe O (Table 1, entries 15–18) and found that
with those obtained in the presence of Et N showed that
3
Et N was the best choice among the examined bases
3
2 3
123

S. Sobhani et al.
4
000 3600 3200 2800 2400 2000 1600 1200 800
400
-
1
Wave number (cm )
Fig. 11 FT-IR spectra of recovered Pd-imino-Py-c-Fe
line) and fresh Pd-imino-Py-c-Fe (continious line)
O
2 3
(dotted
Fig. 8 Catalyst separation from the reaction mixture using an
external magnet
2 3
O
9
5
9
5
9
5
1
00
94
9
4
9
4
9
4
93
olefins under optimized reaction conditions (Table 1, entry
8
0
2) was investigated (Table 2).
6
0
As shown in Table 2, Pd-imino-Py-c-Fe O was effec-
2
3
4
0
tive for coupling reactions of iodobenzene with various
olefins to give the corresponding products in 86–96 %
2
0
(Table 2, entries 1–5). It was observed that iodobenzene
0
functionalized with electron-withdrawing or electron-do-
nating groups reacted with olefins and generated the cor-
responding products in good to high yields (Table 2,
entries 6–9). This catalytic system was also successfully
applied for the coupling reaction of substituted aryl bro-
mides with various olefins (Table 2, entries 10–15).
Although aryl chlorides are not as reactive and are less
commonly employed in palladium-catalyzed coupling
reactions, Heck reaction of aryl chlorides with olefins was
successfully performed in the presence of Pd-imino-Py-c-
Fe O and the desired products were produced in good to
1
2
3
Run number
4
5
6
7
8
Time = 0.5 h
Fig. 9 Reusability of Pd-imino-Py-c-Fe
butyl cinnamate (1a)
O
2 3
for the synthesis of n-
2
3
high yield (Table 2, entries 16–19).
The possibility of leaching of active species from the
catalyst into the reaction mixture is known as a key factor
of the catalyst stability. To investigate the extent of Pd
leaching from Pd-imino-Py-c-Fe O , the Heck cross-cou-
2
3
pling reaction of iodobenzene with n-butyl acrylate was
carried out in the presence of 0.25 mol % of the catalyst at
100 °C. The supported catalyst was filtered off after 50 %
of the Heck reaction was proceeded and the filtrate was
allowed to react further. The catalyst filtration was per-
formed at the reaction temperature (100 °C) in order to
avoid possible recoordination or precipitation of soluble Pd
upon cooling. The result showed that after this hot filtra-
tion, further reaction was not observed. ICP analysis of the
remaining solution revealed that there was no Pd in the
reaction mixture. These results confirmed that the sup-
ported palladium MNPs was stable and could tolerate the
present reaction conditions without leaching of the signif-
icant quantity of Pd.
Fig. 10 TEM image of Pd-imino-Py-c-Fe
2
O
3
after eight times reuse
0
.25 mol % of the catalyst, which was used in the model
reaction was the best amount (Table 1, entry 2). The model
reaction was examined in the absence of the catalyst and in
the presence of Pd(OAc) (Table 1, entries 19 and 20) and
2
low amounts of the desired product were obtained after
2
4 h.
In order to establish the generality of this method, the
Heck coupling reaction of a variety of aryl halides with
1
23

Palladium-Schiff Base Complex Immobilized Covalently on Magnetic Nanoparticles…
2 3
Table 3 Suzuki coupling reaction of various aryl halides with phenylboronic acid catalyzed by Pd-imino-Py-c-Fe O
a
Reaction conditions molar ratio of aryl halide:phenylboronic acid:Et
a
Isolated yield
3
N:catalyst = 1:1.5:2:0.0025, 100 °C, DMF
123

S. Sobhani et al.
Table 4 Catalytic activity of Pd-imino-Py-c-Fe
coupling reactions
O
2 3
in comparison with other supported Pd catalysts on MNPs used for Heck and Suzuki cross-
a
b
c
TOF (h
-1
Entry
1
Reaction
Heck
Catalyst (mol%) [Ref]
Pd(0)-ZnFe (4.62) [43]
Tem. (°C)
Ar-X
Time (h)
Yield (%)
TON
)
d
O
2 4
78
I
2–14
6–12
10
80–96
17–21
1–10
1–3
2
Br
I
65–90
d
99
14–19
2
3
Heck
Heck
Pd(0)-NH
2
-Fe
O
3 4
(5) [44]
130
120
20
d
Br
I
10–24
1–4
93–99
19–20
1–2
Pd(0)-Fe
3
O
4
@C (0.308) [45]
66–100
65–98
4.3–58
88–98
51–90
87–95
63–90
60–90
73–92
61–84
43–63
79–96
13–70
41
11–16.7
11–16.3
0.7–9.8
88–98
4.2–16.7
3.6–8.1
0.18–0.98
11–12.2
6.4–11.2
43.5–380
12–28
Br
Cl
I
2–4
4–10
8
4
5
Heck
Heck
Pd(II)-phosphine-Fe
3 4
O @SiO
2
(1) [46]
100
100
Br
I
8
51–90
Pd(II)-isatin-c-Fe
O
2 3
(0.5 and 1.5) [38]
0.5–4
2–7
174–190
51–128
40–60
Br
Cl
I
0.5–6
0.5–6
3–8
8–120
e
6
7
8
9
Heck
Heck
Heck
Heck
Suzuki
Pd(II)-DABCO - c-Fe
O
2 3
(1 and 3) [35]
100
24–92
2–184
Br
Cl
I
20–84
2.5–28
0.7–2
10–24
0.5–3
0.5–6
9
14–21
Pd(II)-acetamidine-c-Fe
O
2 3
(0.12) [40]
100
658–800
108–583
342
233–1600
22–1150
38
Br
Cl
I
f
Pd(II)-BIP -SiO
2
@c-Fe
2
O
3
(0.5) [41]
100
0.25–3
2.5–4
5–7
80–98
82–92
75–91
83–96
87–94
85–92
94–100
85–100
11–43
160–196
164–184
150–182
332–384
348–376
340–368
188–200
170–200
22–86
53–784
46–74
Br
Cl
I
21–33
this work
Pd(II)-imino-Py-c-Fe
2
O
3
(0.25)
100
0.5–2.5
0.5–2
4–5
133–768
174–744
69–92
Br
Cl
I
1
0
Pd(II)-salen-SiO @Fe
2
O
3 4
(0.5) [47]
100–130
1
188–200
57–67
Br
Cl
I
3
6
4–14
d
1
1
2
Suzuki
Suzuki
Pd(0)-ZnFe
O
2 4
(4.62) [43]
78
3–8
90–95
19–20
6–7
d
Br
I
3.5–9
1–2
85–91
18–20
2–6
1
Pd(0)-Fe
3
O
4
@C (0.308) [45]
78–153
72.3–100
54–98
6–95
12–16.2
9–16.3
1–15.8
180–198
52–104
166–190
47–194
51–62
8.1–16.7
4.6–16.3
1–4
Br
Cl
I
1–3
1–5
1
3
4
Suzuki
Suzuki
Pd(II)-phosphine-Fe
3 4
O @SiO
2
(0.5) [46]
60
1.5
90–99
26–52
83–95
60–97
76–93
63–96
81–96
41–50
90–98
91–92
82–86
120–132
5.2–10.4
62–380
43–80
1
10
Br
I
10
1
Pd(II)-isatin-c-Fe
O
2 3
(0.5 and 1.5) [38]
100
100
100
0.5–3
3/4–4.5
1–1.5
0.5–2
0.5–2
5–10
1–2
Br
Cl
I
13–62
1
5
6
Suzuki
Suzuki
Pd(II)-acetamidine-c-Fe
O
2 3
(0.12) [40]
(0.5) [41]
525–800
675–800
342–417
180–196
182–184
164–172
262–1600
337–1600
34–75
Br
Cl
I
f
Pd(II)-BIP -SiO
1
2
@c-Fe
2
O
3
90–196
61–74
Br
Cl
2.5–3
4–5
33–43
1
23

Palladium-Schiff Base Complex Immobilized Covalently on Magnetic Nanoparticles…
Table 4 continued
a
b
c -1
TOF (h )
Entry
Reaction
Suzuki
Catalyst (mol%) [Ref]
Pd(II)-imino-Py-c-Fe
Tem. (°C)
Ar-X
Time (h)
Yield (%)
TON
1
7
2
O
3
(0.25) [this work]
100
I
0.5–1.5
2.5–3
86–95
85–94
86–91
344–380
340–376
344–364
229–760
113–150
76–80
Br
Cl
4.5–5
a
b
c
d
e
f
Isolated yield
TON = turnover number (ratio of moles of product formed to moles of catalyst)
TOF = turnover frequency (TON per hour)
Determined by GC
DABCO: 1,4-diazabicyclo[2.2.2]octane
BIP: bis(imino)pyridine
For practical applications of a heterogeneous catalyst,
for the Heck and Suzuki cross-coupling reactions
reusability of the catalyst is a very significant factor [42].
To clarify this issue, catalytic recycling experiments were
carried out using Heck cross-coupling reaction of
iodobenzene with n-butyl acrylate under optimized reac-
tion conditions. After the reaction was completed, it was
cooled to room temperature and the whole amount of Pd-
imino-Py-c-Fe O simply was separated from the product
(Table 4). It is appeared that Pd-imino-Py-c-Fe O showed
2
3
good catalytic activity, but comparing with the others the
advantage is not remarkable, even some catalytic systems
are much better than ours for coupling reaction of aryl
iodides or bromides. More importantly, the procedure fol-
lowed by us provides significant advantages in terms of
TON and TOF compared with the other catalytic systems
toward aryl chlorides as challenging substrates.
2
3
by an external magnet (Fig. 8).
The recovered catalyst was washed with EtOH, dried at
room temperature and reused for a similar reaction. It was
found that Pd-imino-Py-c-Fe O could be recycled and
2
3
4
Conclusions
reused for eight consecutive trials without loss of its cat-
alytic activity (Fig. 9).
In conclusion, a novel covalently immobilized palladium-
Schiff base complex on magnetic nanoparticles (Pd-imino-
Py-c-Fe O ) has been synthesized and characterized by
ICP and elemental analyses (N = 0.75 % and
C = 6.25 %) of Pd-imino-Py-c-Fe O after eight reuses
showed that only a very small amount (\1 %) of Pd was
removed from the catalyst. Comparison of TEM image
2
3
2
3
different techniques. Pd-imino-Py-c-Fe O not only exhi-
2
3
bits an impressive catalytic activity in Heck and Suzuki
cross-coupling reactions, but also possesses high stability.
As magnetic nanoparticles were used as the solid support,
the present catalyst was simply separated from the reaction
mixture by an external magnet, and reused at least for eight
consecutive runs without Pd leaching and significant loss of
its catalytic activity. The merits associated with Pd-imino-
Py-c-Fe O hopefully will render it a valuable catalyst in
(
Fig. 10) and FT-IR spectrum of used catalyst (Fig. 11)
with one of the fresh catalyst (Fig. 3) showed that the
morphology and structure of Pd-imino-Py-c-Fe O
remained intact after eight recoveries.
2
3
Encouraged by the satisfactory results with the Heck
cross-coupling reaction, the Pd-imino-Py-c-Fe O catalyst
2
3
was applied to the Suzuki coupling reaction (Table 3).
As the results of Table 3 indicate, the electron-neutral,
electron-rich and electron-poor iodobenzenes reacted with
phenylboronic acid to generate the Suzuki cross-coupling
products in 86–95 % (Table 3, entries 1–3). This method
was also applicable for the cross-coupling reaction of dif-
ferently substituted bromobenzenes and chlorobenzenes
with phenylboronic acid (Table 3, entries 4–10).
2
3
practical synthesis.
Acknowledgments We are thankful to University of Birjand
Research Council for their support on this work.
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