Solution-dispersible, colloidal, conjugated porous polymer networks with entrapped palladium nanocrystals for heterogeneous catalysis of the Suzuki-Miyaura coupling reaction

Article abstract of DOI:10.1021/cm202283z

Conjugated nanoporous polymer colloids (CNPCs) consisting of covalently cross-linked poly(p-phenyleneethynylene) networks were synthesized by using the Sonogashira coupling reaction in a toluene-in-water miniemulsion. The synthesized CNPCs having a uniform particle size distribution exhibit high porosity with a specific surface area of 421 m²/g and a dual distribution of pore size in the micropore and mesopore ranges. They are amenable to postfunctionalization and enhancement of their dispersibility in solvents, and retain their native photoluminescence. The modified CNPCs allow for in situ incorporation of palladium nanocrystals to form the Pd@CNPC composite materials. The Pd@CNPCs are validated to have excellent catalytic activity, outstanding reusability, and exceptionally high TOF (44100 h ^-1) for the Suzuki-Miyaura coupling reaction.

Full text of DOI:10.1021/cm202283z

Article
pubs.acs.org/cm
Solution-Dispersible, Colloidal, Conjugated Porous Polymer
Networks with Entrapped Palladium Nanocrystals for Heterogeneous
Catalysis of the Suzuki−Miyaura Coupling Reaction
Peng Zhang,†,‡ Zhihuan Weng,‡,§,⊥ Jia Guo,*,† and Changchun Wang†
†State Key Laboratory of Molecular Engineering of Polymers, Department of Macromolecular Science, Fudan University,
Shanghai 200433, P. R. China
§Institute for Molecular Science, National Institute for Material Science, Okazaki 444-8787, Japan
S
* Supporting Information
ABSTRACT: Conjugated nanoporous polymer colloids (CNPCs) consisting of covalently cross-linked poly(p-phenyleneethynylene)
networks were synthesized by using the Sonogashira coupling reaction in a toluene-in-water miniemulsion. The synthesized CNPCs
having a uniform particle size distribution exhibit high porosity with a specific surface area of 421 m2/g and a dual distribution of pore
size in the micropore and mesopore ranges. They are amenable to postfunctionalization and enhancement of their dispersibility in
solvents, and retain their native photoluminescence. The modified CNPCs allow for in situ incorporation of palladium nanocrystals to
form the Pd@CNPC composite materials. The Pd@CNPCs are validated to have excellent catalytic activity, outstanding reusability,
and exceptionally high TOF (44100 h−1) for the Suzuki−Miyaura coupling reaction.
KEYWORDS: conjugated polymer networks, composite colloids, heterogeneous catalysis, nanoporosity, Pd nanocrystals
reported,9 the micropore sizes obtained (<1 nm), as estimated by
capture of CO2, were unfortunately too small to allow nitrogen gas
to freely permeate. Also, the conjugation of polyurea networks was
broken by the urea moieties (−NHCONH−); therefore, the first
unequivocal example of CMPs with suitable solution properties
has yet to be reported.
CMPs as catalytic support are of distinct advantage over
other known porous organic materials since they can serve as
an ideal platform for chemically entrapping catalytic modules
into highly stable, covalently cross-linked, conjugated polymer
networks by taking advantage of their permanent porosity and
the ability to tailor their compositions and properties at the
molecular level. The introduction of catalytic moieties into
CMPs has two main strategies. The first is the bottom-up
approach by which the building blocks with catalytic function
groups are applied directly to construct the polymer networks.
INTRODUCTION
Conjugated microporous polymers (CMPs), which continue to
■
attract increasing attention as a class of microporous organic
polymers, have been proving to be invaluable tools in the fields
of gas storage,1 catalysis,2 light harvesting,3 and supercapacitive
energy storage.4 Because of their chemical diversity, CMPs have
been thoroughly explored compared to other known micro-
porous organic polymers, e.g., hyper-cross-linked polymers
(HCPs),5 polymers of intrinsic microporosity (PIMs),6 and
covalent organic frameworks (COFs).7 To date, extensive
research has led to rapid development in the synthesis of
various CMPs.8 These systems are greatly promising for
advanced applications in organic electronics and luminescent
sensors. To achieve this end, it is necessary to endow CMPs
with solution properties to enable the fabrication of archi-
tectures such as coatings, films, or colloids. However, the cross-
linked rigid skeletons of CMPs are insoluble and infusible, and
retain monolithic morphologies during postmodification. Although
studies on the development of processable microporous polyurea
networks by an organic sol−gel strategy have recently been
Received: August 4, 2011
Revised: November 1, 2011
Published: November 3, 2011
© 2011 American Chemical Society
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ultrosonicated for 10 min (SONICS-750 with a tip operated at
75 W). The miniemulsion was heated to 70 °C, and stirred for 24 h
under N2. After reaction, the mixture was cooled to room temperature
and the product was obtained by precipitation in methanol and
thoroughly washed with methanol, acetone, and chloroform to remove
the surfactants and any unreacted monomer or catalyst residues.
The further purification was carried out by Soxhlet extraction from
methanol for 24 h. The product was dried in vacuum for 12 h at 40 °C.
(Yield, 92%). N-[2-(4-iodophenoxy)ethyl]-N,N-dimethyl amine syn-
thesized as previously reported (see the Supporting Information) was
used as end-capper to react with the residual alkyne bonds on the
surface of CNPCs. The dried powder of CNPCs (200 mg) was re-
dispersed in a mixture of toluene (25 mL) and triethylamine (1.5 mL),
and mixed with tetrakis-(triphenylphosphine)-palladium(0) (50 mg),
copper iodide (15 mg) and end-capper (291 mg, 1 mmol). The reac-
tion was allowed to proceed for 24 h at 70 °C under N2. After being
cooled, the precipitates were filtered off, washed with methanol,
acetone and chloroform, and dried in vacuum.
Synthesis of Pd@CNPCs. The end-capped CNPCs (230 mg) was
dispersed in water (20 mL) and mixed with the acetone solution
(5 mL) of Pd(OAc)2 (70 mg). After stirring at 90 °C for 1 h, the
reaction mixture was filtered off, washed with hot water and acetone,
and dried in vacuum. To determine the loading amount of Pd nano-
crystals, the obtained Pd@CNPCs (3 mg) was dispersed in 13 M nitric
acid (5 mL) and the mixture was heated at 80 °C to dissolve
completely. After being cooled to room temperature, the solution was
diluted for measuring the amount of Pd by ICP.
General Procedure for the Suzuki Coupling Reaction by
Using Pd@CNPCs. 1-Iodo-4-methylbenzene (1 mmol), phenyl-
boronic acid (1.1 mmol), K2CO3 (1.5 mmol) and Pd@CNPCs (0.1
mol %) were added to 3 mL of DMF/H2O mixture with a volume
ratio of 1:1, and then the mixture was heated to 50 °C with stirring for
the time indicated under N2. After the completion of the reaction, the
mixture was filtered to collect the catalyst for the next cycle. The
organic phase in the filtrate was extracted with dichloromethane by
twice, and dried over MgSO4. Finally, quantitative and qualitative
analysis of the product was conducted with GC and GC/MS. The
leaching of Pd in each cycle also was monitored by ICP.
This allows the catalytically active sites to be embedded onto
the pore wall and thus facilitates their homogeneous
distribution through CMPs. Lin et al. utilized the functionalized
monomers with Ru and Ir complexes to synthesize the photo-
active microporous polyphenylene for the heterogeneous photo-
catalysis.2b Cooper et al. developed the bottom-up method-
ology to synthesize a series of metal-anchored microporous
poly(aryleneethynylene) networks for reductive amination.2c
Unlike the above-mentioned, the second strategy is to in-
corporate metal nanocrystals into CMPs via post treatment
method. Thomas et al. immobilized the PVA-stabilized Pd
nanocrystals onto the surface of covalent triazine frameworks,10
or reduced the poly(thienylene arylene) network-loaded palladium
salts for in situ incorporation of Pd nanocrystals into organic
microporous polymer.8e For enhancing the metal−support
interaction, Cooper et al. used supercritical CO2 to encapsulate
the Pd nanocrystals by decomposition of a CO2-soluble Pd
complex in CMP networks.11 Despite the advent of pioneer
studies, it is still rarely investigated because inferior dispersibility
of metal nanocrystal/CMP composite materials largely restrict
their diverse applications. Additionally, pore sizes imposed by
CMPs are predominated in the micropore regime and thus in
situ incorporation of metal nanocrystals into CMP networks
appears to be rather challenging.
Herein, we report a template synthesis of conjugated nano-
porous polymer colloids (CNPCs) based on a heterogeneous
coupling reaction in an oil-in-water miniemulsion system. The
colloidal conjugated networks thus obtained were regularly
spherical, consisted of fully fused organic skeletons, and had
inherent, locked-in, open nanopores. Of more significance, they
were unique in that they were amenable to size-controllable
synthesis and enhancement of their dispersibility in solvents,
and retained their native photoluminescence; all of the fea-
tures are not accessible with solid-state CMPs. Therefore,
these systems may offer new possibilities in terms of
enhanced performances in multiple disciplines, even for
the usual applications known for high-surface-area materials.
Of particular appeal to us is the possibility of complexing metal
nanocrystals within the porous networks, in view of the fact that
unsaturated bonds such as ethynylene or vinylene in conjugated
polymers can be directly employed to form metal-complexed
polymers.12 Thus, we have embarked on an exploration of the
possibility of synthesizing solution-dispersible CNPCs with
embedded metal nanoparticles, and present here our initial
study on the in situ incorporation of Pd nanocrystals within
CNPCs with a view to obtaining highly efficient heterogeneous
catalysts.
Characterization. Solid-state 13C cross-polarization magic-angle
spinning (CP/MAS) NMR spectrum was measured on a Varian
Infinity plus 300WB spectrometer at a MAS rate of 5 kHz and a CP
contact time of 1.4 ms. HR TEM was carried on a JEM-2010
transmission electron microscope at an acceleration voltage of 200 kV
(JEOL, Japan). The sample was prepared by drop-casting an aqueous
dispersion onto a copper grid. EDX spectrum was measured on the
TEM instrument with an EDXS spectrometer. Nitrogen sorption
measurements were collected at 77 K by an ASAP2020 volumetric
adsorption analyzer (Micromeritics, USA). The samples were degassed
at 200 °C for 12 h before measurement. The fluorescence emission
spectra of the end-capped CNPCs dispersed in various solvents were
recorded with a RF-5301PC spectrometer under the excitation
wavelength of 365 nm (Shimadz, Japan). The hydrodynamic diameter
of the CNPCs in water was achieved using a Zetasizer Nano at 25 °C
(Malvern, UK).
EXPERIMENTAL SECTION
■
Materials. 1,3,5-Triethynylbenzene, 1,4-dibromobenzene, and tetrakis-
(triphenylphosphine)-palladium(0) were purchased from Alfa Aesar
(US). Copper iodide, triethylamine, cetyltrimethylammonium bro-
mide, and Pd(OAc)2 were purchased from Aladdin (China). All
reagents for the catalysis of the Suzuki coupling reaction were purchased
from TCI (Japan).
RESULTS AND DISCUSSION
■
Conjugated polymer colloids are emerging as viable alternatives
to conventional conjugated polymers in that they have well-
defined colloidal nanostructures that can be conveniently
incorporated within devices by either spin-coating or inkjet
printing.13 Meanwhile, the intrinsic conductivity and lumines-
cence of the conjugated polymers are retained, even though the
polymeric chains become interpenetrated within the sterically
confined micro- or nanoparticles.14 The notoriously difficult
processing of CMPs seems to be overcome to some extent by
adopting the form of polymer colloids. However, we are aware
that the reported conjugated polymer colloids have been
nonporous and engineered with linear or ladder polymers
Synthesis of CNPCs End-Capped with N-[2-(4-iodophenoxy)-
ethyl]-N,N-dimethylamine. 1,3,5-Triethynyl benzene (150 mg,
1 mmol), 1,4-dibromobenzene (236 mg, 1 mmol), tetrakis-(triphenyl-
phosphine)-palladium(0) (58 mg), and copper iodide (19 mg)
were degassed using three evacuation-N2-backfill cycles. A mixture
of toluene (1.5 mL) and triethylamine (1.5 mL) was added and
stirred until the solid dissolved completely. The yellow solution
obtained was mixed with 30 mL of a 1% aqueous cetyltrimethyl-
ammonium bromide (CTAB) solution with a syringe. The mixture was
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substituted with alkyl side chains. Such one-dimensional
conjugated polymers tend to preferentially intertwine or
stack, and so do not provide sufficient spatial isolation upon
evacuation of the solvent molecules. This leads to nonporous,
compacted microstructures. Although the conventional ap-
proaches for the synthesis of polymer colloids, such as emulsion
or dispersion polymerization, are equally suited for the syn-
thesis of CMP colloids, the generation of permanent pores in
heterogeneous systems, as well as definite control over the
colloidal morphology of rigid networks, apparently remains
unexplored.
In our work, the conjugated nanoporous polymer colloids
(CNPCs) were synthesized by the polymerization of poly(p-
phenyleneethynylene) (PPE) networks by a classical Sonogashira−
Hagihara coupling reaction of terminal alkynes with aryl
halides, as depicted in Figure 1a. To achieve confinement of
bonding. Dynamic light scattering (DLS) was employed to
ascertain the aqueous stabilization in the presence of
surfactants. Figure 1c shows the narrowly distributed histogram
of the hydrodynamic diameters of the CNPCs in water, which
gave an average particle size of 120 nm and a polydispersity
index (PDI) of 0.076. This implies that the resultant product is,
as expected, highly dispersed individually in water and is
invariant in size when compared with its form in the dry state.
The molecular composition of the CNPCs synthesized in the
miniemulsion was verified by solid-state 13C cross-polarization
magic-angle spinning (CP/MAS) NMR. In Figure S1a
(Supporting Information), signals are seen at δ = 131.6,
124.0, and 90.8 ppm, which can be reasonably assigned to the
aromatic and acetylene carbons of the designed PPE networks,
respectively.8a
Porosity was measured by means of nitrogen sorption
analysis at 77 K. In Figure 2, it can be seen that the CNPCs
Figure 1. (a) Schematic illustration of network-inclusive CNPCs
synthesized by the Sonogashira coupling reaction in a toluene-in-water
miniemulsion. (b) TEM image of CNPCs. (c) Histogram of the
hydrodynamic diameters of the CNPCs in aqueous solution.
morphology in this synthesis, the evolution of the PPE
networks had to be controlled by an emulsion technique. A
thermodynamically stable heterophase miniemulsion system
consisting of surfactant-stabilized nanodroplets in a continuous
phase was obtained by power ultrasound emulsification.15 Each
droplet can be regarded as an individual batch reactor suitable
for producing nano- or submicrometer-scaled materials through
a variety of polymerization reactions. Here, 1,3,5-triethynyl-
benzene and 1,4-dibromobenzene monomers together with the
catalyst were dissolved in toluene, and an aqueous solution
containing CTAB as surfactant was added to prepare a toluene-
in-water pre-emulsion. Upon ultrasound emulsification, the
obtained stable and homogeneous miniemulsion with a light-
yellow color underwent polymerization to form a PPE network
in the toluene nanodroplets. After the reaction, the as-
synthesized products were readily dispersed in water, showing
a latex-like appearance. The TEM image displayed in Figure 1b
demonstrated the formation of uniform spherical CNPCs with
a diameter of approximately 100 nm. It is surprising that the
cross-linked networks had a totally rigid conformation, yet were
capable of forming regular CNPCs aided by covalent chemical
Figure 2. (a) N2 sorption isotherm of CNPCs and (b) their pore size
distribution curve calculated by the NLDFT model.
displayed a typical type IV gas sorption isotherm, in accordance
with IUPAC classification, indicative of mesoporous character.
The Brunauer−Emmett−Teller (BET) model was applied to
the adsorption branch, resulting in a specific surface area of
421 m2/g and a pore volume of 0.27 cm3/g. From a de Boer
statistical thickness (t-plot) analysis, the CNPCs displayed a
micropore contribution of 213 m2/g (50.6%) and a mesopore
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contribution of 208 m2/g (49.4%). As compared with the
similar CMP networks synthesized by Cooper et al.,8a the
specific surface area of CNPCs was apparently lower. Also,
their shapes of N2 sorption isotherms were somewhat different,
showing a significant hysteresis that was still present at low
pressures. It is probably owing to a swelling of the PPE
networks upon gas adsorption, which could frequently be
observed in a soft network as increasing numbers of adsorbate
molecules condense in the pores.8e,16 It is assumed that the
CNPC networks might have more structural defects retaining
the residual nonreactive groups, and thus decrease the given
surface areas. Elementary analysis in Table S1 (see the
Supporting Information), showed a lower carbon content of
the CNPCs than that of the similar-structured CMPs, which
proves the incomplete polymerization occurring in the
heterophase system. The reason may lie in that the diffusion
of monomers is limited in the surfactant-confined toluene
nanodroplets, and they are probably entrapped within the
formed network colloids, resulting in obstruction of further
evolution of the PPE networks.
the end-capping components might be overlapped by the broad
peak or spinning sidebands, thereby without showing obvious
lines. Elemental analysis (see Table S1 in the Supporting
Information) on the modified structure showed nitrogen
content of 1.46%, which was applied to determine 20.7% of
the end-capping groups. Although it is possible to under-
estimate the nitrogen content because of the observed
incomplete combustion of the sample, it still allows
approximate quantification of the end-capping components.
As is evident from the fluorescence emission spectra
compiled in Figure 3a, when the end-capped CNPCs were
dispersed in various polar solvents, a single narrow emissive
The pore-size distribution was calculated by the nonlocal
density functional theory (NLDFT) model and the population
was found to be centered at 0.6 nm, 1.3 and 3.1 nm,
respectively, as shown in Figure 2b. To the best of our
knowledge, the most known CMPs have rather small pore sizes
of less than 2 nm, and are thus utilized extensively to explore
the capacity of gas uptake. However, mesoporosity imposed by
conjugated polymer networks has rarely been considered,
which is largely due to the difficulty of inhibiting the occurrence
of rotation or bending imposed by the nonplanar conjugated
moieties. Therefore, it is of considerable interest that the PPE
network colloids display a dual distribution of nanopore sizes,
which are equally populated in the mesopore and micropore
ranges, respectively. Compared with the typical microporsity of
the similar CMPs reported, the larger pore sizes of CNPCs
might be attributed to the intrinsically defective, locked-in
network that elicits a flawed pore size distribution. Besides the
structural factor, it is also likely that because structural changes
in measurement are not considered in evaluation of pore sizes,
the typical methods just like NLDFT model might be not
fully fit to determine the reliable or comparable pore size
distribution.
Fabrication of CNPCs provides an important practical
advantage in that the prominent optical properties of
conjugated polymers are translated into a high-surface-area,
dispersed, aqueous form that makes them superbly useful as
sensors as a result of the possibility of enhanced three-
dimensional energy migration. Before measurement, the
CNPCs were subjected to an end-capping reaction with N-
[2-(4-iodophenoxy)ethyl]-N,N-dimethylamine to remove the
surfactants that might induce variations in luminescence. Solid-
state 13C CP/MAS NMR was used to access the molecular
structure of the end-capped CNPCs. The spectrum in
Figure S1b (Supporting Information) showed the expected
peaks of PPE networks, corresponding to Car−C sites at 131.6
ppm, Car−H sites at 123.0 ppm, and sp-hybridized −Car−C
C− sites at 90.2 ppm. They are in agreement with those
assigned in Figure S1a in the Supporting Information. The
others could be well-attributed to the end-capping groups on
the modified CNPCs. The peaks at 44.6 and 51.5 ppm
corresponded to the −N−CH3 and −CH2−N− groups. The
peaks at 171.1 and 156.0 ppm could be ascribed to the Car−O
and Car−H sites, respectively. The other types of carbon sites in
Figure 3. (a) Fluorescence emission spectrum of the CNPCs in THF,
dioxane, ethanol, chloroform and water, respectively, excited with the
same wavelength of 365 nm. (b) Photographs of dispersions of the
CNPCs in different solvents without (up) and with (bottom) UV light
irradiation (365 nm).
peak was observed in each case at a similar position of 450 nm,
but with remarkably differential intensities. Figure 3b displays
photographs of dispersions of these materials in different
solvents with and without UV light irradiation (365 nm). A
stable colloidal solution emitting blue fluorescence was
obtained in each case, which nicely corresponded to the
spectral data compiled in Figure 3a. Of more interest, the
unique optical properties are unprecedented when compared
with those of conventional conjugated polymers. As far as we
are aware, upon transition of a PPE or its derivatives from their
solution to nonsolvent conditions, the solvatochromic behavior
is substantially dependent on the occurrence of aggregate-
induced planarization, which elicits increased conjugation and
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thus a lower band gap.17 In sharp contrast to these previous
studies, regardless of the solvent polarity, the present PPE
network colloids possess single, nearly symmetrical emissive
profiles, all of which resemble those of well-dissolved linear
PPE chains in a good solvent. The results imply that the PPE-
based CNPCs are most likely to have a chemically condensed
singular microstructure, rather than being a tight agglomeration
of polymeric fragments. Simultaneously, the accessible high-
surface-area CNPCs allow for pronounced solvation of the
conjugated moieties, leading to increased structural rigidity and
hence reduced molecular rotation. As a result, the fluorescence
intensity of the CNPCs was approximately doubled in THF
compared to that in aqueous solution. Overall, a relatively high
degree of native, solution-state, nonsolvatochromic fluores-
cence is obtainable, which promises attractive applicability of
the CNPCs in optical sensors.
Table 1. Suzuki Coupling Reaction of Various Arylhalides
with Phenylboronic Acid in the Presence of Pd@CNPCs.
a
Taking into account the mesoporosity and solution proper-
ties of CNPCs, we tried to synthesize Pd nanocrystals in situ
throughout the colloidal network. Pd(OAc)2 in acetone was
mixed with end-capped CNPCs in aqueous solution in order to
complex the Pd with the ethynylene components. Thereafter,
the reaction was allowed to proceed at 90 °C for 1 h for
thermal decomposition of the loaded Pd(OAc)2.18 The solution
turned black as the reaction progressed, whereas the acetone
was removed by bubbling nitrogen through the mixture. The
TEM image shown in Figure 4a confirmed that the metal
a
Reaction conditions: arylhalide (1 mmol), phenylboronic acid (1.1 mmol),
K2CO3 (1.5 mmol), Pd@CNPCs (0.1 mol %), DMF/H2O (1.5 mL/
1.5 mL), 50 °C. GC yields [%]. DMF/H2O (2.5 mL/0.5 mL).
b
c
solvents. The porosity of the Pd@CNPCs was measured to
ensure the preservation of open pores derived from the organic
moieties. The N2 sorption isotherm is presented in Figure S3
(see the Supporting Information), from which it is clear that
the micropores were characterized by a typical type I sorption
profile. The apparent surface area and the pore volume were
estimated to be 276 m2/g and 0.15 cm3/g, respectively, both of
which are lower than previously obtained values owing to
internally immobilized Pd nanocrystals. The pore-size distribu-
tion shown in the inset of Figure S3 (see the Supporting
Information) was determined by employing the NLDFT model
to account for the less pronounced pores with a width of 1.2 nm,
which is comparable to that calculated from the microstructure of
the bulk CMPs. We assume that the mesoporous components in
CNPCs are filled or blocked with Pd nanocrystals so as to attain
open micropores for accessibility of guest molecules.
The resultant Pd@CNPCs were tested as catalysts in the
Suzuki−Miyaura coupling reaction of arylboronic acids with
various types of aryl halides in the presence of different bases
and solvents. The as-synthesized catalyst powder was readily
redispersed in polar solvents, even without any stirring, and was
amenable to facile centrifugal separation from the reaction
solutions. To optimize the overall process, the reaction of
1-iodo-4-methylbenzene with phenylboronic acid was selected
as a model reaction that could be conveniently carried out at
50 °C. As listed in Table S2 (see the Supporting Information),
the reaction did not proceed at all upon addition of CNPCs
alone. Conversely, high yields of products were obtained in the
presence of Pd@CNPCs. The efficiency of Pd@CNPCs
containing 0.1 mol % of Pd was thoroughly investigated by
varying the types of solvents and bases. Dioxane and
acetonitrile proved to be poor solvents, whereas water and
toluene afforded moderate yields for the same process. In sharp
contrast, high activity was found in DMF, EtOH, and mixtures
of either EtOH/H2O or DMF/H2O at a 1:1 volume ratio,
indicative of good suitability for the present system. Among the
various bases screened in DMF/H2O, K2CO3 provided the
highest coupling yield of 97.2% in 3 h. As a result, the optimum
Figure 4. (a) TEM image of Pd@CNPCs and (b) HR TEM image
revealing crystalline Pd nanoparticles incorporated within and attached
to the CNPCs.
nanocrystals had been embedded into the colloids. The HR
TEM image in Figure 4b revealed the incorporated nano-
particles with a highly crystalline area. Together with
observation of the EDX pattern in Figure S2 (see the
Supporting Information), we determine that the CNPCs
serve as platforms to enable the isolated dispersion of Pd
nanocrystals within the network, thereby preventing the
aggregation of Pd nanocrystals and their precipitation from
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likelihood of a further enhancement of TOF upon lowering the
Pd concentration because the activity of the catalyst is quite
remarkable at low concentrations.
Commercially available Pd on charcoal (Pd@C) was used as
a reference to establish an approximate activity ranking. The
reaction of entry 1 in Table 1 as a test model was monitored as
a function of reaction time. Using Pd@C and Pd@CNPCs with
equivalent concentrations of Pd, it can be seen from Figure 5a that
higher activity was achieved with Pd@CNPCs, allowing for
complete conversion (97.2%) in 3 h, whereas with Pd@C, a
reaction time of more than 8 h was needed to afford just 91.2%
of the product. Subsequently, the reusability and recovery of Pd@
CNPCs were evaluated, with particular consideration of the solid
catalyst used in the reaction. For the same coupling reaction as
studied above, it is evident from Figure 5b that high coupling
yields were attained in all five reaction runs, accompanied by very
low leaching of the Pd catalyst as determined by ICP. The results
again indicate that the recoverable catalytic activity of Pd@CNPCs
makes it an attractive alternative to commercially available Pd
catalysts such as Pd on charcoal.
CONCLUDING REMARKS
■
To summarize, PPE-based CNPCs have been synthesized by
Sonogashira coupling reaction of 1,3,5-triethynylbenzene and
1,4-dibromobenzene in a toluene-in-water miniemulsion.
Polymerization was carried out with the assistance of a
surfactant in the toluene nanodroplets, leading to solution-
dispersible spherical nanoparticles with a relatively narrow size
distribution. The resultant CNPCs retained a well-defined
conjugated framework architecture, which afforded them both
micro- and mesoporosity simultaneously. It is reasoned that the
origin of this mesoporosity may be a surfactant-confined
evolution of the PPE network in the toluene phase. The end-
capped CNPCs could be dispersed in a number of solvents and
emitted intense blue fluorescence in each case with the single,
equally positioned, symmetrical emission profile. No solvato-
chromic behavior depending on forced planarization of the
conjugated moieties was observed because the CNPCs were
constructed entirely from three-dimensional scaffolds and did
not comprise agglomerates of polymeric fragments that would
easily give rise to a solvatochromic effect. Moreover, the highest
intensity of fluorescence emission was found upon dispersion
of the CNPCs in THF, largely due to the solvation-induced
stiffness of backbones against free rotation or bending of the
conjugated units. In this context, Pd nanocrystals were
synthesized in situ on the end-capped CNPCs as a support,
and unequivocally showed an isolated distribution of as-
synthesized nanocrystals. The obtained Pd@CNPCs have been
applied as an effective and reusable catalyst for the Suzuki−
Miyaura coupling of a variety of haloarenes with arylboronic
acids. Extraordinarily high activities with TOF up to 44 100 h−1
and very low leaching of the catalyst during a recycling
procedure demonstrate that Pd@CNPCs are attractive as a
promising alternative to commercially available Pd catalysts. We
therefore speculate that this innovative system may have great
potential, and opens the way for designing a variety of metal-
hybrid porous materials based on the CNPCs with nano-
porosity, photoluminescence properties, and conjugated
scaffolds as supports.
Figure 5. (a) Comparison of catalytic activity of the Pd@C (□) and
Pd@CNPCs (●) catalysts in the Suzuki coupling reaction (entry 1 in
Table 1). (b) Reusability of the Pd@CNPCs catalysts confirmed by
both the conversions (histogram) and Pd leaching amount (−*−) in
five reaction runs (entry 1 in Table 1).
conditions were attained with the Pd@CNPCs in the presence
of K2CO3 in DMF/H2O. We also examined the scope and
limitation of the Pd@CNPCs-catalyzed cross-coupling reaction
between phenylboronic acid and various iodo and bromo
derivatives. All of the reactions indicated in Table 1 were
completed very efficiently, giving excellent yields of the
corresponding bis(aryl) products. Aryl iodides, as expected,
were rapidly converted and gave high yields. Similarly, high
catalytic activity was observed for the aryl bromide derivatives,
regardless of the effects of electron-donating or electron-
withdrawing functions. We also note that all of the reactions
proceeded smoothly in relatively short times, one of which was
even less than 15 min (entry 6). When the concentration of Pd
was fixed at 0.1 mol %, the reaction of entry 6 was scaled-up by
using 30 mmol of substrate, 33 mmol of phenylboronic acid,
45 mmol of K2CO3, and 0.001 mmol of catalyst, and carried out
at 80 °C. The reaction proceeded rapidly and 24.5% of the aryl
halide had been converted after 10 min. This resulted in a
turnover number (TON) of 7350 and a turnover frequency
(TOF) of 44100 h−1. To the best of our knowledge, this is the
highest TOF hitherto obtained for a Suzuki−Miyaura reaction
with a heterogeneous catalyst. Moreover, we highlight the
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Chemistry of Materials
Article
2009, 21, 1291. (g) Lu, W.; Yuan, D.; Zhao, D.; Schilling, C. I.; Plietzsch,
ASSOCIATED CONTENT
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O.; Muller, T.; Brase, S.; Guenther, J.; Blumel, J.; Krishna, R.; Li, Z.; Zhou,
̈
̈
S
* Supporting Information
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2010, 49, 9504.
Synthetic procedure, solid-state 13C NMR spectra, nitrogen
adsorption isotherm, elementary analysis, and catalytic activity
(PDF). This material is available free of charge via the Internet
at http://pubs.acs.org/.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: guojia@fudan.edu.cn.
■
Present Address
⊥Department of Chemistry, University of California, Riverside,
CA 92521.
Author Contributions
‡These authors contributed equally.
ACKNOWLEDGMENTS
■
We acknowledge the support by the National Natural Science
Foundation of China (Grant 21004012), the Research Fund for
the Doctoral Program of Higher Education of China (Grant
20100071120007), and the Fundamental Research Funds for
the Central Universities.
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