A highly active multi-usable palladium pyridylfluorene film-based catalyst for C-C cross-coupling reactions

Article abstract of DOI:10.1002/aoc.3391

The two terminal pyridyl nitrogen atoms of 2,7-bis(4-pyridyl)fluorene (1) were coordinated to Pd(II) ions to give self-assembled, multilayer films using the layer-by-layer (LbL) method. The films were prepared by alternately dipping the substrate, pre-coated with a polyethyleneimine layer, in aqueous solutions of PdCl₂ and ethanol solutions of 1. The resulting films were characterized using UV-visible absorption spectroscopy, atomic force microscopy (AFM), X-ray photoelectron spectroscopy, scanning electron microscopy (SEM) and inductively coupled plasma atomic emission spectroscopy (ICP-AES). UV-visible spectra and SEM images show almost uniform growth of the film in a near ideal LbL manner. AFM images show that nanostructured aggregates of Pd(II) complexes form on the surface. With an increase in the number of Pd(II)/1 bilayers, more particulate aggregates are distributed on the surface. When released from the substrate, the Pd(II) complex nanostructure shows high catalytic activity for Suzuki-Miyaura and Mizoroki-Heck cross-coupling reactions. The catalyst loading is as low as 9.1 × 10^-3 mol% Pd, as measured using ICP-AES, and high turnover numbers of up to 1.08 × 10⁴ are obtained.

Full text of DOI:10.1002/aoc.3391

Full paper
Received: 18 August 2015
Revised: 5 September 2015
Accepted: 6 September 2015
Published online in Wiley Online Library: 14 October 2015
(wileyonlinelibrary.com) DOI 10.1002/aoc.3391
A highly active multi-usable palladium
pyridylfluorene film-based catalyst for C–C
cross-coupling reactions
Xiuhua Zhaoa, Yayun Zhaob, Jie Zhangb and Xing Lia,b*
The two terminal pyridyl nitrogen atoms of 2,7-bis(4-pyridyl)fluorene (1) were coordinated to Pd(II) ions to give self-assembled,
multilayer films using the layer-by-layer (LbL) method. The films were prepared by alternately dipping the substrate, pre-
coated with a polyethyleneimine layer, in aqueous solutions of PdCl2 and ethanol solutions of 1. The resulting films were charac-
terized using UV–visible absorption spectroscopy, atomic force microscopy (AFM), X-ray photoelectron spectroscopy, scanning
electron microscopy (SEM) and inductively coupled plasma atomic emission spectroscopy (ICP-AES). UV–visible spectra and
SEM images show almost uniform growth of the film in a near ideal LbL manner. AFM images show that nanostructured aggre-
gates of Pd(II) complexes form on the surface. With an increase in the number of Pd(II)/1 bilayers, more particulate aggregates
are distributed on the surface. When released from the substrate, the Pd(II) complex nanostructure shows high catalytic activity
for Suzuki–Miyaura and Mizoroki–Heck cross-coupling reactions. The catalyst loading is as low as 9.1× 10ꢀ3 mol% Pd, as measured
using ICP-AES, and high turnover numbers of up to 1.08 × 104 are obtained. Copyright © 2015 John Wiley & Sons, Ltd.
Additional supporting information may be found in the online version of this article at the publisher’s web site.
Keywords: Pd(II) complex; fluorene pyridine; multilayer film; Suzuki–Miyaura reaction; Mizoroki–Heck reaction
multilayer Pd films. However, these two ligands include the noble
Introduction
metal platinum, and organic analogues would be preferred.
Two spirobifluorene ligands, namely 2-Br-2′,7,7′-tri(4-pyridyl)-9,9′-
Cross-coupling reactions, such as Suzuki–Miyaura[1–3] and
spirobifluorene[41] and 2,2′,7,7′-tetra(4-pyridyl)-9,9′-spirobifluorene,[42]
Mizoroki–Heck reactions,[4–7] are very powerful methods for the
were applied to assemble Pd films and Cu films, respectively, but the
application of spirobifluorene ligands is greatly restricted in organic
formation of carbon–carbon chemical bonds. Palladium-based
catalysts have been most frequently investigated, with many
synthesis because of their low yields.
examples giving high catalytic activity and selectivity which have
In the work reported herein, 2,7-bis(4-pyridyl)fluorene (1),[43]
been achieved through marked advances in the design of
which is easy to synthesize and offers pyridyl nitrogen atoms to
supporting ligands.[8–10] Despite these notable advantages, homo-
readily bond to metal centers, was used to assemble polyethyleneimine
geneous palladium catalysts suffer from high catalyst cost,[11] and
(PEI)–(PdCl2/1)n multilayer films using layer-by-layer (LbL) self-
assembly methods. The resulting films have the advantages of
low Pd loading of the catalyst and involving a simple co-ligand,
and may find further applications in catalysis. The PEI–(PdCl2/1)n
multilayer films were prepared using the LbL technique, and their
use as catalysts in Suzuki–Miyaura and Mizoroki–Heck reactions
was explored.
often poor stability to air and moisture.[8]
In order to improve the stability and reusability of these
catalysts,[12] heterogeneous Pd catalysts have been successfully
designed with the active Pd species immobilized onto organic
and inorganic supports, based on carbon,[13–16] zeolite,[17–20] oxide
and natural mineral,[21–23] polymer[24–27] and silicon[28] matrices.
Although most of these heterogeneous catalysts show excellent
catalytic activity, the conditions required for cross-coupling
typically involve high temperatures (above 100 °C)[29] and relatively
Experimental
high catalyst loadings (3–10 mol%).[30] In order to solve these
Materials and methods
problems, film-based catalysts have been prepared by alternating
adsorption of an organic ligand and metal ion layers onto solid
substrates,[31,32] resulting in metal–ligand complexes loaded within
multilayer films.[33–36] The key step in the formation of such a
film-based catalyst is to select an appropriate ligand which
supports the growing film and imparts good catalytic properties.
Gao et al. reported (PdCl2/Azpy)n[37] and (PdCl2/bpy)n[38] multilayer
films, which were used to efficiently catalyze Suzuki–Miyaura and
Mizoroki–Heck reactions, respectively. Subsequently, two metal
complex ligands, trans-[Pt(PPh3)2(C≡ CPy)2][39] and [Pt2(dppm)2
(C≡ CPy)2],[40] with conjugated pyridyl groups, were used to form
PEI (molecular weight = 6000 g molꢀ1, 10 wt% aqueous solution)
was purchased from Aldrich Chemical Company. All aqueous
*
Correspondence to: Xing Li, Faculty of Science, Ningbo University, Zhejiang,
315211, China. E-mail: lix905@126.com; lixing@nbu.edu.cn
a Faculty of Science, Ningbo University, Zhejiang, 315211, China
b Faculty of Materials Science & Chemical Engineering, Ningbo University, Zhejiang,
315211, China
Appl. Organometal. Chem. 2015, 29, 840–845
Copyright © 2015 John Wiley & Sons, Ltd.

Palladium pyridylfluorene film-based catalyst for C-C reactions
solutions were prepared with distilled water. All solvents were of
best-available commercial quality and used as received. The
reagents were of analytical grade and used as received. All catalytic
reactions were carried out in air. All other synthetic reactions were
performed using a three-necked flask utilizing a double-manifold
vacuum system with high-purity nitrogen flow.
Analysis of Pd content in PEI–(PdCl2/1)10 multilayer film
A quartz slide coated with PEI–(PdCl2/1)10 film was immersed into
an aqueous solution of NaOH (1M, 7 mL), and the change in absor-
bance of the coated quartz slide was monitored using UV–visible
spectra. When the coated quartz slide exhibited no absorbance,
the aqueous solution was used for analysis of Pd content.
Physical measurements
Release property of PEI–(PdCl2/1)n multilayer films
Infrared spectra (KBr pellets) were recorded with an FTIR-8900 spec-
trometer (Shimadzu, Japan) in the range 400–4000 cmꢀ1. NMR
spectra were obtained from solutions in CDCl3 using Bruker DRX-
400 spectrometers. UV–visible absorption spectra were recorded
with a Lambda 35 spectrophotometer (PerkinElmer, USA). Emission
spectra were measured with an F-4600 fluorescence spectropho-
tometer. Thermogravimetric measurements were carried out in air
with a heating rate of 10 °Cminꢀ1 using an SII TG/DTA 7300 integra-
tion thermal analyzer. Atomic force microscopy (AFM) images were
obtained with a single-crystal silicon slide using a Veeco Multimode
NS3A-02NanoscopeIII atomic force microscope with silicon tips.
Height images of the films were recorded using tapping-mode
AFM. High-resolution X-ray photoelectron spectra (XPS) were col-
lected at a takeoff angle of 45° using a PHI Quantum 2000 scanning
ESCA microprobe (Physical Electronics, USA) with an Al Kα X-ray line
(1486.6 eV). Analysis of Pd content was carried out with inductively
coupled plasma atomic emission spectroscopy (ICP-AES) using an
Ultima 2 instrument (France). Scanning electron microscopy (SEM)
images were obtained with a Hitachi SU-70 field-emission scanning
electron microscope and energy-dispersive X-ray analyses (EDS)
were carried out using the SEM instrument equipped with an
energy-dispersive spectrometer. Transmission electron microscopy
images were recorded with a JEM 2010 transmission electron
microscope. Powder X-ray diffraction data were collected with a
Bruker D8 Focus X-ray diffractometer using Cu Kα radiation.
A quartz slide coated with PEI–(PdCl2/1)n film was immersed into a
mixed solution of K2CO3 (3mmol) in EtOH (3mL) and H2O (4mL).
The coated quartz slide was monitored using UV–visible spectros-
copy at 20, 40, 60, 80, 100, 120, 140 and 160 min.
General procedure for suzuki–miyaura and mizoroki–heck
cross-coupling reactions
For Suzuki–Miyaura cross-coupling reactions, the reaction vessels
were charged with aryl halide (1.0 mmol), arylboronic acid
(1.2mmol), K2CO3 (3.0 mmol) and the catalyst (PEI–(PdCl2/1)10
films loaded on quartz slides) in solutions of H2O–EtOH (7mL,
4:3 v/v). The reaction mixture was heated to 50 or 100 °C for dif-
ferent reactions in air. After the end of the reaction, the reaction
system was cooled to room temperature, and extracted with
ethyl acetate (3× 15 mL). Then the combined organic phase
was dried over anhydrous Na2SO4. After removal of the solvent,
the crude product was purified with silica gel column chroma-
tography using petroleum ether to afford the product with high
purity.
For Mizoroki–Heck cross-coupling reactions, samples of aryl
halide (1.0mmol), styrene (or t-butyl acrylate) (1.5mmol), Na2CO3
(2.0mmol) and PEI–(PdCl2/1)10 films loaded on quartz slide as
catalysts were added to a flask containing N,N-dimethylformamide
(DMF, 6 mL). Then the resulting mixture was stirred at 140 °C for
24h under ambient atmosphere, cooled to room temperature
and extracted with ethyl acetate (3× 15 mL). The combined
organic phase was dried over anhydrous Na2SO4, concentrated
and purified using silica gel column chromatography eluting with
ethyl acetate.
LbL assembly of PEI–(PdCl2/1)n multilayer films
Quartz slides (25 mm × 12 mm × 1 mm) (or single-crystal silicon
slides) were cleaned with piranha solution (7:3 (v/v) 98% H2SO4–
30% H2O2) (Caution! Piranha solution is extremely corrosive) at
80 °C for 40 min, and thoroughly washed with distilled water.
Further purification was carried out by immersion in a H2O–H2O2–
NH4OH (5:2:1v/v/v) bath at 70 °C for 30min. The clean slides were
first immersed in PEI solution for 20 min and washed with distilled
water. Then the pre-coated PEI slides were alternately immersed
in aqueous solutions of PdCl2 (5mM) and ethanol solutions of 1
(5mM) each for 30 min. The substrates were washed with water
and dried after each immersion at room temperature. By repeating
the two above-mentioned steps, the PEI–(PdCl2/1)n multilayer films
were prepared.
Multi-usable experiments of PEI–(PdCl2/1)n multilayer films
In the first catalytic cycle, a quartz slide coated with PEI–(PdCl2/1)10
multilayer film was immersed into a solution of 4-ethoxyiodobenzene
(1 mmol), 4-methylphenylboronic acid (1.2 mmol), K2CO3 (3 mmol)
in EtOH (3 mL) and H2O (4 mL). After the mixture was stirred for
20 min at 50 °C, the quartz slide was removed and the resulting
solution was heated at reflux for a further 20 h at 50 °C.
Subsequently, the mixture was extracted with ethyl acetate
(3 × 15 mL). The water phase was collected and recharged with
4-ethoxyiodobenzene (1 mmol), 4-methylphenylboronic acid
(1.2 mmol), K2CO3 (3 mmol). EtOH (3 mL) and sufficient H2O were
added to make the solution volume to 7 mL. The coated quartz
slide was re-immersed into the new reaction mixture. After the
mixture was stirred for 20 min, the quartz slide was again
removed from the reaction mixture and the released PdCl2/1
complex was used to catalyze a second Suzuki–Miyaura reaction.
By repeating these steps, the catalytic behavior of the film was
investigated over several cycles.
Preparation of PdCl2/1 composite
An aqueous solution of PdCl2 (5mM, 2 mL) was quickly added to an
ethanol solution of 1 (5mM, 2 mL), and the resulting mixture was
stirred at 50 °C for 20 h. Then the suspension was centrifuged and
the supernatant removed. The precipitate was re-suspended and
centrifuged sequentially with H2O (2 mL) and EtOH (2mL). The
resulting PdCl2/1 composite was used directly following removal
of the ethanol supernatant.
Appl. Organometal. Chem. 2015, 29, 840–845
Copyright © 2015 John Wiley & Sons, Ltd.
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X. Zhao et al.
Results and discussion
UV–visible spectroscopy of PEI–(PdCl2/1)n multilayer films
PEI was used to pre-coat the substrate slide and form a base layer
offering primary, secondary and tertiary amine groups which
can bind metal ions through coordination interactions. The
PEI–(PdCl2/1)n films were prepared through a LbL self-assembly
method based on coordination bonding between Pd centers and
pyridyl N atoms from 1, alternating adsorption of metal ions and
1 onto the substrate slide. The growth of the film was monitored
using UV–visible spectroscopy. The resulting spectra are shown in
Fig. 1. The maximum absorption peak of the PEI–(PdCl2/1)n films
centered at 355 nm is similar to that of 1 with a red shift of
30 nm, consistent with coordination of metal ions.[44] The absor-
bance of the film increases with an increase in the number of depo-
sition layers, with a close-to-linear relationship (R2 = 0.9761)
between the optical absorption at λmax = 355 nm and the number
of bilayers. This indicates that 1 is incorporated uniformly in the
multilayer PEI–(PdCl2/1)n films for at least the first 10 layers.
Figure 2. XPS spectra of PEI–(PdCl2/1)10 film deposited on a single-crystal
silicon substrate. Inset: Pd3d
.
Analysis of Pd content in PEI–(PdCl2/1)10 film
AFM images of PEI–(PdCl2/1)n multilayer films
The Pd loading in the PEI–(PdCl2/1)n films was determined using
ICP-AES following complete desorption of the multilayer film in
aqueous NaOH. Under these conditions, Pd ions cannot be
reabsorbed onto the PEI base layer as confirmed from UV–visible
spectra. When the UV–visible spectra of the quartz slide show no
absorbance at 355 nm, all the PdCl2/1 of the original multilayer is
desorbed into the solution and available for analysis. Therefore,
the amount of Pd catalyst in PEI–(PdCl2/1)10 multilayers is deter-
mined to be 0.39 ppm (9.7μg or 9.1× 10ꢀ8 mol; see supporting in-
formation for the calculation).
XPS measurements were performed to identify the elemental
composition of the multilayer films deposited on single-crystal sili-
con substrates. As expected, the survey spectrum (Fig. 2) of the
PEI–(PdCl2/1)10 film shows the existence of C1s (285.0eV), Cl2p
(199.0 eV), N1s (399.2 eV), O1s (533.0eV), OKLL (981.1eV) and Pd3d
(337.8 eV), consistent with characteristic peaks of the corresponding
elements. Photoelectrons with energies of 338.1 and 343.3 eV are
observed in the inset of Fig. 2, indicating the Pd(II) oxidation state
in the PEI–(PdCl2/1)10 film.
AFM images provide valuable information about the surface
morphology at a molecular level. For the PEI–(PdCl2/1)5 film, some
aggregates are observed on the surface of the film with a
root-mean-square (RMS) roughness of 11.7 nm (Fig. 3(a)). The
stereoscopic image shows that five layers of film grow vertically
on the substrate (Fig. 3(b)). With an increase in the number of layers,
more particulate aggregates are formed and island-shaped
structures are observed on the surface of the film. As shown in
Figs. 3(c) and (d), the surface of the PEI–(PdCl2/1)10 film is rougher
than that of the five-layer film, and the boundaries between par-
ticulates are less distinct. The RMS roughness of PEI–(PdCl2/1)10 is
16.1 nm, larger than that of PEI–(PdCl2/1)5, indicating that the
roughness of the film increases with the number of layers. Further-
more, the mean diameter of the aggregates increases with the
number of layers and for (PdCl2/1)5 and (PdCl2/1)10 the mean
diameters are 50.397 and 55.541 nm, respectively.
Figure 3. AFM images of films on single-crystal silicon substrate (scan area
1.0 × 1.0 μm2): (a) height image of PEI–(PdCl2/1)5 (Rrms = 11.7 nm); (b)
stereoscopic image of PEI–(PdCl2/1)5; (c) height image of PEI–(PdCl2/1)10
Figure 1. UV–visible spectra of PEI–(PdCl2/1)n (n = 1–10) films. Inset: plot of
absorbance at 355 nm as a function of number of bilayers of PEI–(PdCl2/1)n.
(Rrms = 16.1 nm); (d) stereoscopic image of PEI–(PdCl2/1)10
.
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Appl. Organometal. Chem. 2015, 29, 840–845

Palladium pyridylfluorene film-based catalyst for C-C reactions
Desorption of PEI–(PdCl2/1)10 multilayer film
10% of the original value, i.e. 0.77 absorption units, every 20 min. This
indicates that desorption of the film is almost in a LbL manner in
accord with the assembly behavior discussed above with each layer
desorbing over an average of 20min.
The PEI–(PdCl2/1)n multilayer films serve as reservoirs of homoge-
neous catalytically active palladium species. The release of PdCl2/1
from the PEI–(PdCl2/1)10 film was monitored using UV–visible
spectroscopy (Fig. 4(c)). The decrease of the absorbance at
355 nm is close to linear over 160 min, decreasing by approximately
Two parallel PEI–(PdCl2/1)10 films were prepared to explore the
morphology before and after desorption of several layers. As
shown in Fig. 4(a), the SEM image reveals that colloid-like partic-
ulates are distributed over the PEI–(PdCl2/1)10 film, with holes of
different size and depth apparent on the film surface. This is
consistent with the surface roughness observed using AFM and
discussed above. Following SEM observations, the composition
of the same PEI–(PdCl2/1)10 film was determined using EDS
(Fig. 4(d)). The expected elements, Pd, Cl, C and N, are present
in the EDS spectrum consistent with PEI, PdCl2 and 1. The peaks
of Si, Pt, O and Na are derived from the single-crystal silicon sub-
strate, from platinum spray of the SEM experiment and from
glassware.
The second PEI–(PdCl2/1)10 film was treated with K2CO3, EtOH
and H2O in four 20 min cycles to remove four layers of PdCl2/1 (as
monitored using UV–visible spectroscopy). The surface of the film
after this desorption shows fewer particulates and holes (Fig. 4(b))
and a generally smoother surface consistent with LbL desorption
and the UV–visible spectra shown in Fig. 4(c). However, as discussed
later, this difference in film morphology has an effect on the cata-
lytic properties of the desorbed materials.
Identifying the materials released from the film during the
desorption process is extremely challenging given the very low
concentration in solution. However, fluorescence emission spectra
can give some evidence concerning the nature of the released
materials. The fluorescence spectrum of 1 exhibits two peaks at
397 and 414 nm (Fig. 5(a)). A test composite was formed from
PdCl2 and 1 (1:1) in EtOH–H2O (4:3). The fluorescence spectrum
of the composite shows a peak at 458 nm (Fig. 5(b)), which is
attributed to the vibronic transitions from excitation state S1 to
ground state S0.[45–47] The fluorescence spectrum of the desorbed
materials from PEI–(PdCl2/1)n film (a broad emission band
between 366 and 550nm centered at 430 nm) is likely to corre-
spond to a mix of free 1 and PdCl2/1 composite or aggregated
materials (Fig. 5(c)).
Figure 4. (a) SEM image of PEI–(PdCl2/1)10 film. (b) SEM image of PEI–
(PdCl2/1)10 film after desorption of 4 × 20 min. (c) UV–visible spectra of
PEI–(PdCl2/1)10 multilayer film for various desorption times (inset: decrease
in absorbance at 355 nm as a function of desorption time). (d) EDS
spectrum of PEI–(PdCl2/1)10 film.
Figure 5. Fluorescence emission spectra: (a) compound 1 in a solution of
EtOH–H2O (4:3 v/v) upon excitation at 325 nm; (b) PdCl2/1 composite
suspended in a solution of EtOH–H2O (4:3) upon excitation at 376 nm; (c)
fractions released from the film in a solution of EtOH–H2O (4:3) upon
excitation at 346 nm.
Appl. Organometal. Chem. 2015, 29, 840–845
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X. Zhao et al.
Catalytic application of PEI–(PdCl2/1)10 film on quartz slides for
suzuki–miyaura cross-coupling reactions
The Suzuki–Miyaura cross-coupling reactions were carried out
employing PEI–(PdCl2/1)10 multilayers on solid slides as a source
of active catalyst. As evident from Table 1, para-substituted aryl bro-
mides with electron-withdrawing groups (e.g. COMe, CF3) and
electron-donating groups (e.g. 4-OMe) give the corresponding
biaryls in high yields. However, in the case of an ortho substituent
(e.g. 2-OMe), a lower yield is observed, presumably due to steric hin-
drance of the bromide restricting access by the catalytically active
species (Table 1, entry 5). A longer reaction time (24 h) and a higher
temperature (100 °C) are required to improve the yield (Table 1,
entry 6). However, the analogous ortho-substituted arylboronic acid
(e.g. 2-MeO) couples well with bromobenzene (Table 1, entry 9). Un-
surprisingly, a para-substituted aryl iodide also cross-couples with
an arylboronic acid in high yield (Table 1, entry 10). Gao et al. re-
ported that (PdCl2/Azpy)n films are not suitable for aryl chlorides.[38]
In our study, under similar conditions, even an aryl chloride is effec-
tively cross-coupled in moderate yield and relatively mild conditions
in the aqueous reaction medium (Table 1, entry 11).
Figure 6. Yields obtained with multi-usable film-based catalyst in
Suzuki–Miyaura coupling reactions. General conditions: 1.0 mmol
of 4-ethoxyiodobenzene, 1.2 mmol of 4-methylphenylboronic acid,
3.0 mmol of K2CO3, in H2O–EtOH (4:3 v/v) at 50 °C under ambient
atmosphere for 20 h. Catalyst loading of 9.1 × 10ꢀ3 mol% for
PEI–(PdCl2/1)10 film; 3.0 mol% for 1, PdCl2 and PdCl2/1 composite.
In order to further investigate the catalytic activity of the
PEI–(PdCl2/1)10 film, a series of control experiments were performed
under similar conditions (Fig. 6). It is observed that 1 is inactive as a
catalyst in the absence of a palladium source (Table S1, entry 1);
PdCl2 exhibits moderate catalytic activity in the reaction of 4-
ethoxyiodobenzene and 4-methylphenylboronic acid (Table S1,
entry 2). The PdCl2/1 composite materials also display high catalytic
activity in the Suzuki–Miyaura reaction in the first cycle (Table S1,
entry 3). After reaction, the walls of the flask were coated with palla-
dium black. The mixture was extracted with CH2Cl2 (3 × 15 mL) and
the organic phase worked up to give the cross-coupled product.
The aqueous phase was recharged in the same reaction vessel with
ethanol (to give a total volume of 7 mL), arylboronic acid, aryl
bromide and K2CO3, and complexes were collected and re-used in
a second reaction sequence. However, the residual Pd complex in
the reaction displays low activity due to catalyst decomposition
(Table S1, entry 4). In contrast, the PEI–(PdCl2/1)10 multilayer films can
be successfully applied as a source of catalytically active materials in
multiple, successive cross-coupling reactions under similar condi-
tions. In each cycle, desorption of PdCl2/1 from the multilayer film
into the reaction mixture takes place over an initial period of
20min. The slide is removed and the reaction mixture heated to
50 °C (Table S1; Fig. 6). The desorbed materials show high catalytic
activity for Suzuki–Miyaura cross-coupling reaction with the biphenyl
products being isolated after reaction in good yield (Table S1, entries
5–9; Fig. 6). However, after five catalytic runs, the yield decreases
rapidly (Table S1, entries 10 and 11), indicating that the catalytic ac-
tive species in the solution are gradually used up in the reactions.
Table 1. Suzuki–Miyaura cross-coupling reactions of aryl halides with
arylboronic acid using PEI–(PdCl2/1)10 films on quartz slidesa
Catalytic application of PEI–(PdCl2/1)10 film on quartz slides for
mizoroki–heck reaction
To further explore the scope of the catalytic reactions, Mizoroki–
Heck cross-coupling reactions between aryl halides and terminal
olefins were explored with PEI–(PdCl2/1)10 film as the source of
the catalyst. It is evident from Table 2 that the reaction is only mod-
erately sensitive to the electronic nature of the aryl bromides with
both electron-withdrawing groups (e.g. CN, COMe, CF3) (Table 2,
entries 2–4) and electron-donating groups (e.g. MeO) (Table 2, entry
5) giving high yields of coupled products. However, in the
Mizoroki–Heck reaction of bromobenzene and the terminal olefin
H2C¼CHCOOtBu, the reaction proceeds only in moderate yield
(Table 2, entry 6), indicating that H2C¼CHCOOtBu is a less active
coupling partner than styrene. The Mizoroki–Heck reactions be-
tween aryl iodides and terminal olefins were also carried out,
resulting in high conversion under the same conditions (Table 2,
entries 7 and 8). In 2002, Kawano et al. reported that dichloro(2,2′-
bipyridine) palladium(II) complex is not a suitable catalyst for the
Mizoroki–Heck reaction of less reactive aryl bromides.[48] However,
in this work, the catalyst derived from the multilayer film proves to
be an extraordinarily active catalyst for the Mizoroki–Heck reaction
of aryl bromides and terminal olefins, even with the very low
Entry
R1
R2
X
T (°C),
time (h)
Yield (%)b TON (× 104)c
1
H
H
H
H
H
H
H
Br
Br
Br
Br
Br
50, 4
50, 4
50, 4
50, 4
50, 4
98
96
95
95
57
85
96
95
95
98
55
1.08
1.06
1.04
1.04
0.63
0.93
1.06
1.04
1.04
1.08
0.60
2
4-CF3
4-COMe
4-OMe
2-OMe
2-OMe
H
3
4
5
6
Br 100, 24
7
4-Me
Br
50, 4
50, 4
8
H
4-OMe Br
2-OMe Br
9
H
50, 4
10
11
4-OEt
H
4-Me
H
I
50, 4
Cl
100, 24
aGeneral conditions: 1.0 mmol of aryl halide, 1.2 mmol of arylboronic
acid, 3 mmol of K2CO3, in H2O–EtOH (4:3 v/v) under ambient
atmosphere.
bIsolated yield.
cTurnover number = mol product/mol Pd.
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Appl. Organometal. Chem. 2015, 29, 840–845

Palladium pyridylfluorene film-based catalyst for C-C reactions
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olefins using PEI–(PdCl2/1)10 films loaded on quartz slidesa
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Entry
R1
R2
X
T (°C), Yield (%)b TON (× 104)c
time (h)
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1
2
3
4
5
6
7
8
H
Ph
Ph
Br 140, 24
Br 140, 24
Br 140, 24
Br 140, 24
Br 140, 24
94
93
90
85
81
65
90
94
1.03
1.02
0.99
0.93
0.89
0.71
0.99
1.03
4-CN
4-COMe Ph
4-CF3
Ph
Ph
4-OMe
H
H
H
COOtBu Br 140, 24
COOtBu
Ph
I
I
140, 24
140, 24
aGeneral conditions: 1.0 mmol of aryl halide, 1.5 mmol of olefin, 2 mmol
of Na2CO3, in DMF (6 mL) under ambient atmosphere.
bIsolated yield.
cTurnover number = mol product/mol Pd.
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In this work, the supported PEI–(PdCl2/1)n films are used as a
catalyst reservoir, showing some distinctive and advantageous
characteristics: (1) very low Pd loading of 9.1 × 10ꢀ3 mol%, reducing
the cost; (2) high catalytic yield up to 98%; (3) reusability of five
catalytic cycles without significant decrease in yield; (4) mild reac-
tion conditions in air and environmentally ‘green’ reaction solvent
(EtOH–H2O); (5) the experimental operation is simple and easy
and the supported films are, once prepared, ready to use in syn-
thetic chemistry; and (6) the amount of Pd(II) can be controlled by
altering the release time to reduce waste.
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Conclusions
In our work, PEI–(PdCl2/1)n multilayer films were fabricated from
treatment of a solid support carrying a PEI base layer alternately
with PdCl2 and 1 using the LbL self-assembly technique. The multi-
layer films were used as reservoirs of an active catalyst for Suzuki–
Miyaura and Mizoroki–Heck cross-coupling reactions. Although
the novel catalyst system has extremely low Pd loading of parts
per million, it exhibits high catalytic activity and the multilayer films
could be re-used five times with a yield greater than 82%. The films
as catalyst will be appealing for industrial applications in terms of
high catalytic efficiency and low cost.
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Acknowledgments
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The authors acknowledge financial support of the NSFC (21571110),
the Ningbo Foundation (2014C50037 and 2014A610106), the Ningbo
University Foundation (py2013001 and G14034), and sponsored
K. C. Wong Magna Fund in Ningbo University.
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Additional supporting information may be found in the online ver-
sion of this article at the publisher’s web site.
Appl. Organometal. Chem. 2015, 29, 840–845
Copyright © 2015 John Wiley & Sons, Ltd.
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