The fabrication of palladium-pyridyl complex multilayers and their application as a catalyst for the Heck reaction

Article abstract of DOI:10.1039/c1jm11759a

Through layer-by-layer (LbL) technology, (PdCl₂/bpy)_n multilayer films were prepared by alternately immersing quartz slides pre-coated with a PEI layer in PdCl₂ and bpy aqueous solutions (PEI = poly(ethylenimine), bpy = 4,

Full text of DOI:10.1039/c1jm11759a

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PAPER
The fabrication of palladium–pyridyl complex multilayers and their
application as a catalyst for the Heck reaction†
*
Shuiying Gao, Yuanbiao Huang, Minna Cao, Tian-fu Liu and Rong Cao
Received 22nd April 2011, Accepted 4th August 2011
DOI: 10.1039/c1jm11759a
Through layer-by-layer (LbL) technology, (PdCl2/bpy)n multilayer films were prepared by alternately
immersing quartz slides pre-coated with a PEI layer in PdCl2 and bpy aqueous solutions (PEI ¼ poly
(ethylenimine), bpy ¼ 4,40-bipyridyl). The (PdCl2/bpy)n multilayers were characterized by UV-vis
spectroscopy, atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). UV-vis
spectra show that the successful fabrication of (PdCl2/bpy)n multilayers depends on the coordination
interaction between palladium ions and pyridine moieties. AFM images show that the nanostructure
aggregates are of Pd form on the surface. With an increase in the number of layers, more
particle aggregates are distributed on the surface. When released from the substrate, Pd nanostructure
aggregates are responsible for the activity of the catalyst. The XPS analysis demonstrates that the Pd
oxidation state is +2. The application of (PdCl2/bpy)n multilayers as a catalyst for the Heck reaction
was also investigated. Palladium–pyridyl complex loaded slides can be used as a catalyst reservoir.
During the Heck reaction process, the desorbed Pd–bpy complexes from the solid slides are soluble
molecular Pd species and are the catalytically active species, which show high activity for the Heck
reaction of aryl bromides. Catalyst loading is as low as 3.4 ꢀ 10ꢁ6 mol% and high turnover numbers
(TON) of up to 2.0 ꢀ 107 are achieved.
applications of a subsequent release of functional supermolecular
1. Introduction
complexes from solid surfaces will be an interesting research
topic.
Transition-metal coordination chemistry provides new oppor-
tunities for preparing self-assembled multilayers. The coordina-
tion chemistry of metals in solution may be utilized to construct
thin films at the molecule level. Layer-by-layer (LbL) technology
has become a popular technique for thin film preparation
because of its simplicity, robustness, and versatility.1–4 Metal ion
coordination-based multilayers are prepared by alternate
adsorption of organic ligand and metal ion layers onto solid
slides. LbL self-assembly based on metal ion–ligand interactions
can yield functional organic superlattices.5–12 For example, self-
assembled multilayers containing metal ions and bis(phospho-
nate) moieties were prepared by metal–ligand coordination
chemistry.5 Recently, functional palladium–pyridyl complexes
were successfully loaded within LbL multilayers by alternate
adsorption of palladium ions and pyridyl ligand layers onto solid
slides.6–12 It is known that a subsequent release of incorporated
LbL hydrogen-bonded functional molecules can have potential
applications such as in drug delivery systems.13 Exploring the
Palladium-catalyzed coupling of olefins with aryl and vinyl
halides, known as the Heck reaction, is a powerful and conve-
nient one-step tool for the construction of carbon–carbon
bonds.14–17 The traditional Heck reaction is typically performed
with phosphine ligands in the presence of a suitable base under
an inert atmosphere.18–21 However, phosphine ligands are toxic,
air-sensitive, and expensive.22 Therefore, large-scale industrial
application is limited, and the development of phosphine-free
catalysts for C–C bond-forming reactions has become a current
focus. In recent years, numerous efforts have been made to
develop more efficient catalyst systems. For example, ligand-free
Heck reaction Pd(OAc)2 catalyst systems remain interesting.23–30
In the absence of a ligand, Pd(OAc)2 between 0.01 and 0.1 mol%
can catalyze the Heck reaction of aryl bromides. However, both
the base and the solvent were found to have a fundamental
influence on the efficiency of the reaction. Much attention has
been devoted to phosphine-free nitrogen-based catalysts. Func-
tional N,O-ligand Pd(II) complexes have proven to be highly
active and stable phosphine-free catalysts for the Heck
reaction.31–33 Another interesting class of ligands is N,N-ligands
such as pyridyl ligands.34,35 Palladium(II) complexes derived
from bidentate nitrogen ligands such as 2,20-bipyridyl and 1,2-bis
(2-pyridylethynyl)benzenes, are effective catalysts for the Heck
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) 591
83796710; Tel: (+86) 591 83796710
† Electronic supplementary information (ESI) available: Analytical data
and spectra. See DOI: 10.1039/c1jm11759a
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reaction of aryl iodides under phosphine-free conditions.24
However, palladium/2,2-bidentate pyridyl cationic homoge-
neous catalysts were not effective for the Heck olefination of less
reactive aryl bromides. To date, palladium(II) complexes con-
structed from 4,4-bidentate nitrogen ligands, such as 4,4-bipyr-
idyl, have rarely been explored as catalysts. Compared to
palladium(II) complexes constructed from 2,2-bidentate nitrogen
ligands, 4,4-bidentate nitrogen ligands easily form infinite
frameworks with palladium ions in solution and such palladium
complexes are not dissolved in the most polar solvent. Such
insoluble palladium complexes as heterogeneous catalysts show
much lower activity than their homogeneous counterparts.
Although heterogeneous catalysts are recovered and reused, the
selectivity and activity of homogeneous catalysts under mild
reaction conditions are unbeaten by their heterogeneous
counterparts.36–39
a takeoff angle of 45ꢂ using a PHI Quantum 2000 scanning
ESCA microprobe (Physical Electronics, USA) with an Ala
X-ray line (1486.6 eV). All AFM images were taken on a single-
crystal silicon slide using a Veeco Multimode NS3A–02Nano-
scopeIII atomic force microscope with silicon tips. Height images
of the films were recorded using tapping-mode AFM. Gas
chromatography–mass spectrometry (GC–MS) was performed
on a 430 GC (Varian, USA). 1H NMR (400 MHz) and 13C NMR
(100 MHz) spectra were obtained on a Bruker AVANCE 400
spectrometer. Analysis of Pd content was measured by an
inductively coupled plasma OES spectrometer (ICP) (Ultima2,
France).
2.2. Layer-by-layer assembly PEI–(PdCl2/bpy)n multilayer
films
In this report, we will explore a strategy which makes
heterogeneous Pd catalysts with low activity in the conventional
catalytic reaction, become highly active species. The strategy is
carried out using layer-by-layer (LbL) technology. Recently, we
have reported azopyridine/palladium complex films as highly
active catalysts for the Suzuki reaction.40 In this paper, we will
report the fabrication of palladium–pyridyl multilayer films from
4,4-bipyridyl (bpy) and palladium dichloride using the LbL
technique (Scheme 1), and the application of the films as a
catalyst reservoir for the Heck reaction.
The quartz slides (size 25 mm ꢀ 12 mm ꢀ 1 mm) (or single-
crystal silicon slides) were cleaned with a ‘‘piranha solution’’ at
80 ꢂC for 40 min, and thoroughly rinsed with distilled water.
Further purification was carried out by immersion in ꢂa H2O/
H2O2/NH3OH (5 : 1 : 1) (V/V/V) bath for 30 min at 70 C. The
clean slides were first immersed in PEI solution for 20 min. The
pre-coated PEI slides were then alternately immersed in PdCl2
aqueous solution (5 mM, 10 mL) and bpy (2 mM, 10 mL) ethanol
solution for 30 min. The substrates were washed with water and
dried with a nitrogen stream after each immersion. By repeating
the above procedure, PEI–(PdCl2/bpy)n multilayer films were
prepared.
2. Experimental
2.3. Analysis of Pd content in (PdCl2/bpy)10 multilayers
2.1. Materials and equipment
The quartz slide coated with (PdCl2/bpy)10 was immersed into an
NaOH aqueous solution (7 mL). UV-vis spectra were used to
monitor the absorbance change of the coated quartz slide. When
the coated quartz slide exhibited no absorbance, the aqueous
solution (7 mL) was used for analysis of Pd content.
Poly(ethylenimine) (PEI, MW ¼ 60 000, 50 wt% aqueous solu-
tion) and 4,40-bipyridyl (bpy) were purchased from Aldrich
Chemical Co. All reagents were used as received, and solutions
were prepared with deionized water (Milli-Q, 18.2 MU cm). UV-
vis absorption spectra were recorded on a quartz slide with
a Lambda35 spectrophotometer (Perkin–Elmer, USA). High-
resolution X-ray photoelectron spectra (XPS) were collected at
2.4. Preparation of Pd–bpy precipitates
An aqueous solution of PdCl2 was mixed with an ethanol solu-
tion of bpy and the mixture was stirred for 3 h. Pd–bpy precip-
itates were obtained and collected by centrifugal separation,
washed with H2O and EtOH, and dried at 50 ꢂC for 6 h.
2.5. Application as a catalyst for the Heck reaction
In a typical reaction, the reaction vessel was charged with
bromobenzene (1 mmol), styrene (1.5 mmol), Na2CO3
(2.0 mmol), and the catalyst (the quartz slide with a (PdCl2/
bpy)10 film) in N,N-dimethylformamide (DMF, 6 mL). The
ꢂ
reaction mixture was heated to 140 C for 24 h. At the end of
the reaction, the reaction mixture was cooled to room temper-
ature, diluted with EtOAc, and washed with 1 N aq HCl and
water. The combined organic phase was dried over anhydrous
Na2SO4. After removal of the solvent, the residue was subjected
to column chromatography on silica gel using ethyl acetate and
hexane mixture to afford the product with high purity. 1H
NMR (400 MHz) and 13C NMR (100 MHz) spectra were
recorded in CDCl3 solution.
Scheme 1 The self-assembly process of palladium–pyridyl complex
multilayers on a solid substrate pre-coated with a PEI layer.
16468 | J. Mater. Chem., 2011, 21, 16467–16472
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substrates can absorb metal ions through metal ion coordination
bonding to the primary, secondary, and tertiary amine groups of
PEI. PEI–(PdCl2/bpy)n multilayer films were prepared by alter-
nately immersing a quartz slide pre-coated with PEI layer in
PdCl2 and bpy aqueous solutions (Scheme 1). (PdCl2/bpy)n
multilayers were characterized by UV-vis spectra, atomic force
microscopy (AFM) and X-ray photoelectron spectroscopy
(XPS). UV-vis spectroscopy is often used to follow the growth of
films built by LbL self-assembly. The self-assembly of the PEI–
(PdCl2/bpy)n film is based on the coordination bond between
palladium ions and the two terminal pyridyl groups of bpy. From
UV-vis spectra and from plotting absorbance vs. number of Pd/
bpy bilayers, the deposition mechanism of (PdCl2/bpy)n multi-
layers is self-assembly and their growth is lateral.41 Therefore, the
film exhibits nearly linear growth (276 nm) for the first ten layers
(Fig. 1), suggesting that bpy molecules were incorporated into
the films. Thus, metal–organic coordination interactions are
responsible for the LbL deposition between palladium ions and
bpy.
3. Results and discussion
3.1. Preparation and characterization
We used PEI to pre-coat the substrate slides. PEI can form layers
on arbitrary substrates. The technique is simple but efficient.
After solid substrates are pre-coated with a PEI layer, the
The atomic force microscopy (AFM) images provide valuable
information about surface topography at the molecular level.
The surface morphology was characterized by the AFM images
(Fig. 2). As seen in the AFM images, the films are relatively
Fig. 1 UV-vis spectra of PEI–(PdCl2/bpy)n films.
Fig. 2 Typical height images of the films on the single-crystal silicon substrate (scan area is 1.0 ꢀ 1.0 mm2). a: (PdCl2/bpy)2 (Rrms ¼ 1.03 nm), b:
(PdCl2/bpy)4 (Rrms ¼ 1.42 nm), c: (PdCl2/bpy)6 (Rrms ¼ 1.14 nm), d: (PdCl2/bpy)8 (Rrms ¼ 1.55 nm), e: (PdCl2/bpy)10 (Rrms ¼ 1.70 nm), f: (PdCl2/bpy)12
(Rrms ¼ 1.25 nm).
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uniform and smooth. Clearly, the silicon surface with (PdCl2/
bpy)2 appears to be very smooth and shows no existence of
aggregate particles. For (PdCl2/bpy)4, some aggregates are
observed, but aggregate particles are not obvious. Compared
with the (PdCl2/bpy)4 surface, the (PdCl2/bpy)6 film surface
presents more aggregates. After 8 bilayers, the aggregates on the
modified surface protrude more and the size of the aggregates
increases. The distribution of the aggregates is uniform and
compact. With the increase in the number of layers, more particle
aggregates are distributed on the surface and the island-shaped
nanostructures are observed in the surface of the films. For the
first four bilayers, the surface coverage is loose. After the number
of bilayer reaches to six, the surface coverage is denser. This is
attributed to the deposition mechanism of self-assembly and
lateral growth,41 consistent with the results of UV-vis spectra. In
other words, during the subsequent deposition process, as well as
surface coverage on top of the preceding layer, there is also
a lateral coverage to the side of the layers. The change of the
root-mean-square (RMS) roughness can explain the deposition
mechanism. For example, the RMS roughness of (PdCl2/bpy)2
and (PdCl2/bpy)4 films is 1.03 nm and 1.42 nm, respectively,
whereas the RMS roughness of (PdCl2/bpy)6 and PdCl2/bpy)8
films is 1.14 nm and 1.55 nm, respectively. The RMS roughness
of (PdCl2/bpy)10 and (PdCl2/bpy)12 films is 1.70 nm and 1.25 nm,
respectively. Obviously, RMS roughness alternately increases or
decreases with the number of bilayers.
The multilayer elemental composition is confirmed by XPS.
XPS measurements were performed to identify the elemental
composition of the multilayer films deposited on the single-
crystal silicon substrate (Fig. 3a). The spectra show all the
expected elements in the multilayers. As expected, the survey
spectrum of the film shows the presence of C1s (287.4 eV), N1s
(402.8 eV), Cl2p (200.4 eV), and Pd3d (340.3 eV), because the
peaks are consistent with characteristic peaks of the corre-
sponding elements and these peaks exhibit a strong and sharp
shape. The O1s peak is from carbon dioxide in air. Besides the
sharp XPS peaks, some satellite peaks are observed in the XPS
spectra. The satellite peaks have weak intensity and exhibit
a broad shape, which are not ascribed to XPS peaks. This is
because weak spectrum lines are not found in characteristic
peaks of the corresponding elements. The shoulder of the Pd3d
peaks on the low binding energy side is ascribed to satellite peaks.
Photoelectrons with energies of 337.7 and 343.0 eV are observed,
indicating the Pd oxidation state in the (PdCl2/bpy)n multilayer is
+2 (Fig. 3b).42
Fig. 3 XPS spectrum of a (PdCl2/bpy)5 film deposited on a single-crystal
silicon substrate (a) survey, (b) Pd3d
.
in (PdCl2/bpy)10 multilayers is 3.6 mg (3.4 ꢀ 10ꢁ6 mol%). In order
to study the release behavior of Pd/bpy multilayers, the quartz
slide coated with (PdCl2/bpy)10 multilayers was immersed into
a DMF and Na2CO3 mixed solution at 140 ꢂC for 90 min. Then,
the UV-vis spectrum showed no absorbance of the coated quartz
slide films, suggesting that all the Pd–bpy complexes of the
multilayer were desorbed into the mixture. During the 90 min,
the amount of Pd catalyst changed with time and reached
a maximum. Once the catalyst is desorbed into the solution, the
Pd(II) catalyst can not be recovered and re-deposited onto
the solid slide. The UV-vis spectra confirm the results. After the
Heck reaction was finished, the UV-vis spectrum does not exhibit
absorbance of immersed quartz slide. Therefore, the amount of
Pd catalyst in (PdCl2/bpy)10 multilayers is 3.4 ꢀ 10ꢁ6 mol%.
3.2.2. Effect of base and solvent. Heck reactions were carried
out employing the solid slide coated with (PdCl2/bpy)10 multi-
layers as a catalyst. The coated slide was immersed into the
reaction mixture. After the reaction finished, the solid slide was
removed from the reaction mixture, and then the desired product
was obtained by extraction and purification. During the coupling
reaction process, the desorbed Pd–bpy complexes from the
multilayer are soluble molecular Pd species and are the catalyt-
ically active species. The active species shows high activity for
Heck reactions.
3.2. Catalytic application of the (PdCl2/bpy)10 film loaded
quartz slide for the Heck reaction
3.2.1. Analysis of Pd content in (PdCl2/bpy)10 multilayers.
Before the application of (PdCl2/bpy)n multilayers as a catalyst
for the Heck reaction, there is a need to determine the amount of
Pd in the (PdCl2/bpy)10 multilayers. However, during the cata-
lytic reaction process, we can not perform on-line detection of the
Pd concentration. Therefore, we found an alternative method of
determining the amount of Pd. The method is as follows: after
(PdCl2/bpy)10 multilayers are completely desorbed into the
aqueous solution (7 mL), the amount of Pd was determined as
0.52 ppm (ICP) in the solution. In other words, the amount of Pd
In order to optimize the efficiency of the catalytic system, we
examined the Heck reaction between bromobenzene and styrene
under a variety of reaction conditions, and the results are
16470 | J. Mater. Chem., 2011, 21, 16467–16472
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Table 2 Heck reactions of aryl halides with olefins using a (Pd/bpy)10
film loaded solid slidea
presented in Table 1. Among the bases employed, Na2CO3 and
K2CO3 give high yields using the present protocol (Table 1,
entries 1 and 2). K3PO4 and NaOAc also give moderate yields
(Table 1, entries 3 and 4). The effect of the solvent is also
investigated. It is found that N,N-dimethylformamide (DMF),
N-methyl-2-pyrrolidone (NMP), and N,N-dimethylacetamide
(DMA) perform well under similar conditions but several other
solvents including dimethyl sulfoxide (DMSO) and 1,4-dioxane
give inferior results (Table 1, entries 9 and 10).
Yield
(%)b
Entry
R1, X
R2
TON
1
2
3
4
5
6
7
8
H, Br
Ph
Ph
Ph
Ph
Ph
Ph
94
95
90
80
78
65
70
0
2.76 ꢀ 107
2.79 ꢀ 107
2.64 ꢀ 107
2.35 ꢀ 107
2.29 ꢀ 107
1.91 ꢀ 107
2.06 ꢀ 107
0
It is found that in the absence of ligand, PdCl2 can catalyze the
Heck reaction, but the maximum yield is 20% under our reaction
4-CN, Br
4-COMe, Br
4-CF3, Br
4-MeO, Br
4-NO2, Br
H, Br
H, Br
H, Br
H, I
H, I
ꢂ
conditions at 140 C for 24 h with 0.001% mol PdCl2 (Table 1,
entry 11). With 0.001 mol % Pd(OAc)2, isolated yield is not
obtained (Table 1, entry 12). It is clear that our catalyst is more
effective than classical palladium(II) salts. Although functional
N,O-polydentate Pd(II) complexes are active for the Heck
reaction in DMF with high turnover numbers up to ca. 104,32,33 it
needs 0.01% mol catalyst loading. By the LbL approach, our
catalyst loading is as low as 3.4 ꢀ 10ꢁ6 mol% and high turnover
numbers (TON) of up to 2.0 ꢀ 107 were achieved. Therefore,
through the use of LbL palladium–pyridyl complex loaded slides,
the amount of palladium catalyst loading may be controlled to
a very low level.
n
CO2 Bu
t
CO2 Bu
9
CO2Me
Ph
0
0
10
11
12
13
94
92
84
60
2.76 ꢀ 107
2.70 ꢀ 107
2.47 ꢀ 107
1.76 ꢀ 107
n
CO2 Bu
t
CO2 Bu
CO2Me
H, I
H, I
a
General procedure: 1 mmol of aryl bromide, 1.5 mmol of styrene, 2
mmol of Na2CO3, DMF (6 mL) at 140 ꢂC for 24 h under ambient
atmosphere. b Isolated yield. TON ¼ mol product/mol Pd.
activity. However, for n-butyl acrylate, the reaction gives
a moderate yield (Table 2, entry 7), because n-butyl acrylate is
more active than methyl acrylate and t-butyl acrylate. Besides
aryl bromides, the reaction between aryl iodides and olefins (e.g.
3.2.3. Effect of the substrates. To explore the scope of the
catalytic reactions, we examined the cross-coupling of aryl
halides with terminal olefins with Na2CO3 as base in DMF at
140 ꢂC. The results are presented in Table 2. In the Heck reaction
of aryl bromide derivatives and styrene, it is clear from Table 2
that aryl bromides containing electron-withdrawing groups (e.g.
CN, COMe, CF3) give high yields (Table 2, entries 2–4), and
electron-donating groups (e.g. MeO) also give moderate yields
(Table 2, entry 5). In the Heck reaction of bromobenzene and
terminal olefins such as methyl acrylate and t-butyl acrylate, the
reaction shows no activity (Table 2, entries 8–9). Methyl acrylate
and t-butyl acrylate are less reactive than styrene, resulting in no
n
t
Ph, CO2 Bu, CO2 Bu, CO2Me) shows high activity under the
same reaction conditions (Table 2, entries 10–13), because
iodobenzene is more active than bromobenzene. Kawano et al.
reported that dichloro-(2,20-bipyridine) palladium(II) complexes
were not effective for the Heck reaction of less reactive aryl
bromides.24 In our study, a catalyst with very low loading is
shown to be an extremely active catalyst for the Heck reaction of
aryl bromides and styrene.
Table 1 Effect of the base and solvent on the Heck reaction using a (Pd/
bpy)10 film loaded solid slidea
3.3. Comparison of catalytic performances with the
heterogeneous counterpart
It should be noted, if PdCl2 and bpy were put together in solu-
tion, precipitate was formed immediately and the product
showed very low activity in Heck reactions (Table 3, entry 1).
This was because the Pd–bpy precipitate acted as a heteroge-
neous catalyst and was not recovered and reused due to catalyst
Yield
(%)b
Entry
Base
Solvent
TON
1
2
3
4
5
6
7
8
K2CO3
Na2CO3
K3PO4
NaOAc
Et3N
Cs2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
DMF
DMF
DMF
DMF
DMF
DMF
DMA
NMP
DMSO
Dioxane
DMF
DMF
88
94
72
75
10
5
88
92
5
2.58 ꢀ 107
2.76 ꢀ 107
2.12 ꢀ 107
2.20 ꢀ 107
2.94 ꢀ 107
1.47 ꢀ 106
2.58 ꢀ 107
2.70 ꢀ 107
1.47 ꢀ 106
0
Table 3 Heck reactions of aryl halides with olefins using Pd–bpy
precipitate as a catalysta
Time
(h)
Yield
(%)b
9
Entry
cycles
10
11c
12d
0
20
0
2.00 ꢀ 104
1
2
1st
2nd
24
24
50
trace
0
a
General procedure: 1 mmol of aryl bromide, 1.5 mmol of styrene, 2
mmol of base, solvent (6 mL) at 140 ꢂC for 24 h under ambient
a
General procedure: 1 mmol of aryl bromide, 1.5 mmol of styrene, 2
mmol of Na2CO3, in DMF at 140 ꢂC under ambient atmosphere,
catalyst: Pd–bpy precipitate (20 mg). b Isolated yield.
b
c
atmosphere. Isolated yield. TON ¼ mol product/mol Pd. with
0.001% mol PdCl2. d with 0.001% mol Pd(OAc)2.
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poisoning (Table 3, entry 2). Therefore, we did not directly use
the palladium–bpy complex as the catalytically active species.
The LbL approach which uses the same precursors is more
successful than those traditional methods of directly using
palladium–bpy complexes. In our study, the catalytically actives
species are those released from the palladium–bpy complex
loaded LbL multilayer slides. The released Pd–bpy complexes of
loaded LbL multilayer slides are soluble molecular Pd species
and the reaction is a homogeneous process. During the Heck
reaction process, the catalytically active Pd(0) complex is formed
in situ from the Pd(II) complex. The Pd(0) species is subject to
two competing processes: one is to aggregate to form palladium
clusters which will turn into insoluble palladium–black and the
other process is to form catalytic palladium species. By using the
LbL approach, catalyst loading can be controlled to a very low
level. Such low Pd concentration suppresses the formation of Pd-
black and keeps all the metal available for catalysis.26 Thus, the
catalytic palladium species outrun the aggregation process. The
bpy ligand plays two important roles: one is to control the Pd
concentration at a low level (3.4 ꢀ 10ꢁ6 mol%) through LbL
assembly of palladium ions and pyridine moieties; the other is to
stabilize Pd colloids released from the substrate to avoid Pd-
black formation during the catalytic reaction process. However,
soluble molecular Pd–bpy complexes cannot be obtained
through the traditional solution method and the palladium–bpy
precipitates can only act as a heterogeneous catalyst with low
activity. The other important advantage is the low amount of the
catalyst needed in the reaction. For example, using conventional
palladium/nitrogen complexes such as N,N,O-terdentate amido/
pyridyl carboxylate Pd(II) complexes, generally requires 0.01%
mol catalyst loading and the palladium has the potential to
remain in the product after isolation. In our LbL multilayer
slides, catalyst loading is controlled to a very low level (10ꢁ6 mol
%) and high turnover numbers (TON) of 107 are achieved. Such
low catalyst loading is significant for synthesizing pharmaceuti-
cally active ingredients, because there are typically strict guide-
lines limiting the levels of palladium metal in drugs.43
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~
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We are thankful for financial support from the 973 Program
(2007CB815303, 2011CB932504), NSFC (20731005, 20821061,
20901078, and 21173222), Fujian Key Laboratory of Nano-
materials (2006L2005), the Key Project from CAS and FJIRSM
(SZD07002).
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