Progressive release of a palladium-pyridyl complex from a layer-by-layer multilayer and illustrative application to catalytic Suzuki coupling

Article abstract of DOI:10.1039/c0cc01986k

Quartz slides bearing layers of a palladium azopyridine complex are seen to liberate catalytic amounts of a soluble active palladium species which can be used for Suzuki coupling.

Full text of DOI:10.1039/c0cc01986k

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Progressive release of a palladium-pyridyl complex from a layer-by-layer
multilayer and illustrative application to catalytic Suzuki couplingw
Shuiying Gao, Zhaoliang Zheng, Jian Lu and Rong Cao*
¨
Received 21st June 2010, Accepted 23rd August 2010
DOI: 10.1039/c0cc01986k
Quartz slides bearing layers of a palladium azopyridine complex
are seen to liberate catalytic amounts of a soluble active
palladium species which can be used for Suzuki coupling.
The layer-by-layer (LbL) technology has become a popular
technique for thin film preparation because of its simplicity,
1
robustness, and versatility. It has been proved that the LbL
method is simple but efficient for constructing metal–ligand
multilayers. For instance, palladium–ligand multilayers were
successfully prepared by alternate adsorption of organic
2
ligand and palladium ion layers onto solid slides, and
functional palladium-pyridyl complexes were loaded within
3
multilayers. Palladium-catalyzed Suzuki cross-coupling
reaction of aryl halides with arylboronic acids is one of
the most powerful and popular methods for C–C bond
4
formation. Palladium-phosphine catalysts have been widely
5
used in homogeneous catalysis. However, most phosphine
ligands are sensitive to air and moisture under homogeneous
conditions. Although heterogeneous Pd catalysts can be
6
recovered easily, their activity and selectivity are not satisfactory
Scheme 1 The self-assembly process of two-step LbL palladium-
and high catalyst concentration is required as compared with
homogeneous catalysts. Therefore, employment of inexpensive
air and moisture stable nitrogen-based ligands for palladium
catalysts has attracted increasing attention. Therefore, to
develop a new strategy for making palladium-pyridyl catalysts
is important. It is generally known that the components of
LbL multilayers are released in polar solvent. A subsequent
release of incorporated functional molecules is found to have
based multilayer.
substrates can absorb palladium metal through metal ion
coordination bonding (see Fig. S1). The absorbance of
PEI/Pd film at 220 nm is due to a charge transfer band
1
0
between Pd ion and amine. UV-vis spectroscopy is used to
characterize the growth process of the multilayer films. As
shown in Fig. 1, the UV absorbance increases linearly with the
7
potential applications such as drug delivery systems. Hence,
number of layers of PdCl /Azpy. The film growth shows a
2
2
good linear correlation with R = 0.9989 between the optical
we tried to utilize a palladium/ligand multilayer as a catalyst
reservoir susceptible to liberate progressively catalytic
amounts of an active Pd species in solution. To our knowledge,
no such strategy has been reported so far.
absorption at lmax = 306 nm and the number of ligand layers,
suggesting that Azpy is incorporated into the films with a
0
Herein, air and moisture resistant ligand 4,4 -azopyridine
(
(
Azpy) is employed to construct palladium multilayers
8
Scheme 1) because Azpy has metal-bridging pyridine sites,
in which N atoms of rigid rod pyridine groups can coordinate
palladium centers well. PEI–(PdCl /Azpy) multilayer films
were prepared by alternately immersing a quartz slide pre-
coated with a PEI layer in PdCl and Azpy aqueous solutions
See ESI). PEI can form the layer on arbitrary substrates.
2
n
2
9
(
After solid substrates are pre-coated with a PEI layer, the
State Key Laboratory of Structural Chemistry,
Fujian Institute of Research on the Structure of Matter,
The Chinese Academy of Sciences, Fuzhou 350002, China.
E-mail: rcao@fjirsm.ac.cn; Fax: +86 59183796710;
Tel: +86 59183796710
2 n
Fig. 1 UV-vis spectra of the (PdCl /Azpy) films. Inset: increase in
w Electronic supplementary information (ESI) available: The experi-
the absorbance at 306 nm as a function of the number of layers of
Pd/Azpy.
2 n
mental details for the preparation of (PdCl /Azpy) , its characterization
and catalytic experiments. See DOI: 10.1039/c0cc01986k
7
584 Chem. Commun., 2010, 46, 7584–7586
This journal is c The Royal Society of Chemistry 2010

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maximum of the K
layers is stronger than that of the PdCl
K PdCl -based and Pd(OAc) -based systems may have a
2
PdCl
4
-based and Pd(OAc)
2
-based multi-
2
-based multilayer. The
2
4
2
5
d
more complicated structure. In addition, other terminal
pyridyl ligands, such as 4-bipyridine (bpy), 1,2-bis(4-pyridyl)-
ethane (bpe) and 1,3-di(4-pyridyl)propane (dpp) have been
demonstrated to form Pd-pyridyl multilayers (see Fig. S5).
Suzuki reactions were carried out by employing the solid
slide coated with (Pd/Azpy)
n
multilayers. First, the effect of the
metal complex precursor on Suzuki couplings was investigated
(
(
(
Table S1). The results indicate that the catalytic activity of the
PdCl /Azpy)10-loaded slide is equivalent to that of the
Pd(OAc) /Azpy)10-loaded slide, whereas the catalytic activity
2
2
of (K PdCl /Azpy) is lower. The Suzuki couplings of various
2
4
10
Fig. 2 XPS spectrum of a (PdCl
2
/Azpy)
5
film deposited on the
aryl halides with arylboronic acids were investigated using the
slide coated with (PdCl /Azpy) films catalyst. As shown in
single-crystal silicon substrate.
2
8
Table 1, the para-substituted aryl bromides bearing electron-
regular growth. The multilayer composition is also confirmed
by X-ray photoelectron spectroscopy (XPS) (see Fig. S2). The
withdrawing (e.g., CN, COMe, CF ) and electron-donating
3
substituents such as OMe gave the corresponding biaryls in
N
1s peak at 399.38 eV comes from the pyridine nitrogen of
high yields. Steric hindrance with the ortho substituents
affected the reaction progress (Table 1, entry 7). Longer
reaction time and higher temperature were required for
complete conversion (Table 1, entry 8). The para-substituted
arylboronic acids bearing electron-donating substituents (e.g.,
Me, MeO) also favored the coupling reactions (Table 1, entries
9 and 10). Even for the ortho-substituted arylboronic acid
(e.g., 2-MeO), it also gave high yield (Table 1, entry 11). For
deactivated phenylboronic acids with e-withdrawing substituents
Azpy, whereas the peak at 400.31 eV is attributed to the azo
group. Thus, the terminal pyridine moieties allow facile
coordination to the palladium centers. Photoelectrons with
energies of 337.7 and 342.9 eV are observed, indicating that the
Pd oxidation state in the (PdCl /Azpy) multilayer is +2 (Fig. 2).
2
n
The AFM images of the films were taken to provide detailed
information about the surface morphology and homogeneity
of the deposited film (Fig. S3). The root-mean-square (RMS)
roughness of (PdCl /Azpy)
2
5
and (PdCl /Azpy)10 surfaces is
2
(e.g., 4-CO H), the couplings gave the desired products in high
2
2.344 nm and 2.752 nm, respectively. The surface exhibits
yields (Table 1, entry 12). Under similar conditions, the
morphology with small islets distributed in the films.
catalytic system is not suitable for aryl chlorides (Table 1,
entry 13).
In order to demonstrate the versatility of this deposition
methodology, the effect of the metal complex precursor such
In order to determine if the catalyst was functioning in a
homogeneous solution phase, we were led to carry out the
following assays: The quartz slide coated with (PdCl /Azpy)
8
as K
2 4 2
PdCl and Pd(OAc) salts was evaluated. UV-vis
measurements of the K PdCl -based and Pd(OAc) -based
4
2
2
2
2
assembly show a good linear correlation with R = 0.9973
multilayers was immersed in a K CO , EtOH and H O mixed
2 3 2
and 0.9980, respectively (see Fig. S4). The UV-vis absorption
solution for 30, 60, 90, 120 and 150 min, respectively, to study
8
Table 1 Suzuki cross-coupling of aryl halide with arylboronic acid using (Pd/Azpy) films-loaded quartz slide
1
R
2
R
a
Entry
X
T/1C
Time/h
Yield (%)
1
2
3
4
5
6
7
8
9
H
H
H
H
H
H
H
H
H
H
4-Me
4-MeO
2-MeO
4-CO
H
I
50
50
50
50
50
50
50
100
50
50
50
100
100
1
2
4
4
4
4
20
24
4
4
4
24
24
98
98
97
95
98
97
38
85
96
95
97
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Cl
4-MeO
4-CN
4-COMe
4-CF
2-MeO
2-MeO
H
H
H
H
H
3
10
11
12
13
b
2
H
90
27
2 3 2
General procedure: 1 mmol of aryl halide, 1.05 mmol of arylboronic acid, 3 mmol of K CO , in H O–EtOH (4 : 3) under ambient atmosphere.
a
b
Isolated yields. Yield after recrystallization from ethanol–water.
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the release behavior of Pd-Azpy complex. UV-vis spectra were
used to monitor the change of films (see Fig. S6). This led us to
observe that the decrease of the absorbance at 306 nm is
obvious during the previous 30 min due to the desorption of
Pd/Azpy multilayers. After 30 min, the absorbance slowly
decreases. After (PdCl /Azpy) multilayers are completely
catalyst-loaded slides, thus reducing the amount of waste.
Therefore, the approach of catalyst-loaded slides helps to
1
2
improve atomic economy in chemical reaction.
In conclusion, we have shown that Pd-complex loaded slides
can be conveniently used as a catalyst reservoir for the Suzuki
reaction. The strategy used in this work may open a new door
for exploring Pd-pyridyl complexes as catalysts for the Suzuki
reaction. Further applications of this approach to other
catalytic systems will be reported in a full paper.
2
8
desorbed into the aqueous solution, the amount of Pd is
determined as 0.59 ppm (ICP-AES) in the solution. As
2 n
expected, the released Pd-azpy complex from (PdCl /Azpy)
multilayer is highly active for catalyzing the Suzuki cross-
coupling reactions under mild reaction conditions. In order to
further demonstrate the released Pd-Azpy complex as the
active species for Suzuki reactions, the following experiments
were performed. During the coupling reaction of aryl bromide
with phenylboronic acid, the quartz slide coated with
We acknowledge the financial support from 973 Program
(2007CB815303, 2006CB932903), NSFC (20731005, 20821061
and 20901078), Fujian Key Laboratory of Nanomaterials
(2006L2005), the Key Project from CAS.
Notes and references
(
PdCl /Azpy)8 multilayer was removed from the reaction
2
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a K
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9
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586 Chem. Commun., 2010, 46, 7584–7586
This journal is c The Royal Society of Chemistry 2010