Photocatalytic Suzuki coupling reaction using conjugated microporous polymer with immobilized palladium nanoparticles under visible light

Article abstract of DOI:10.1021/acs.chemmater.5b00516

Extensive research has been done to develop heterogeneous Pd catalysts for Suzuki coupling reactions, such as the immobilization or stabilization of Pd nanoparticles (NP) on silica, carbon nanotubes, polymers, metal oxides, dendrimers, and graphene layers. Upon photoirradiation, both the average energy and the effective flow of free electrons toward the Pd NPs would be amplified, which can be exploited and harvested to catalyze reactions. Suzuki coupling reactions are performed under homogeneous conditions using a variety of phosphine ligand or palladium catalytic complexes. Despite the relatively high efficiency of these homogeneous systems, the recovery of the costly and potentially toxic palladium catalysts remains as a pressing issue in the development toward a greener, more sustainable chemical industry.

Full text of DOI:10.1021/acs.chemmater.5b00516

Communication
pubs.acs.org/cm
Photocatalytic Suzuki Coupling Reaction Using Conjugated
Microporous Polymer with Immobilized Palladium Nanoparticles
under Visible Light
Zi Jun Wang, Saman Ghasimi, Katharina Landfester, and Kai A. I. Zhang*
Max Planck Institute for Polymer Research, D-55128 Mainz, Germany
S
* Supporting Information
irst discovered in the late 1970s, palladium (Pd)-catalyzed
Suzuki cross-coupling reaction is one of the most powerful
pores, have been applied as organic, heterogeneous visible light
photocatalysts for a number of photoredox reactions.41−45 With
their broad light absorption and tunable semiconductor-range
band gaps, we envision the formation of a hybrid hetero-
junction, i.e., a metal−semiconductor contact between Pd NPs
and CMP, would result in a positively charged region on the
CMP and a negatively charged site on the Pd NPs at the
interface due to the Schottky effect. Upon photoirradiation,
both the average energy and the effective flow of free electrons
toward the Pd NPs would be amplified, which can be exploited
and harvested to catalyze reactions.
In this work, we aim to utilize the photoamplified
heterojunction effect at the interface between a semiconductive
conjugated microporous polymer and Pd NPs. We demonstrate
the use of conjugated microporous polymer supported Pd NPs
(Pd@CMP) as an efficient heterogeneous, visible light
photocatalyst for Suzuki coupling at room temperature.
A conjugated microporous poly(benzoxadiazole) network
(B-BO3) was chosen as the semiconductive polymer support
(Figure 1a), on which Pd NPs with sizes between 5 and 10 nm
F
one-step pathways for the formation of carbon−carbon bonds
in organic synthesis.1,2 Typically, Suzuki coupling reactions are
performed under homogeneous conditions using a variety of
phosphine ligand/palladium catalytic complexes.3−5 Despite
the relatively high efficiency of these homogeneous systems, the
recovery of the costly and potentially toxic palladium catalysts
remains as a pressing issue in the development toward a
greener, more sustainable chemical industry. Heterogeneous
catalysts are favored over homogeneous catalysts because of
their facile recyclability and easy compatibility with continuous
flow systems.6−11 Extensive research has been done to develop
heterogeneous Pd catalysts for Suzuki coupling reactions,12
such as the immobilization or stabilization of Pd nanoparticles
(NPs) on silica,13−15 carbon nanotubes,16 polymers,17−19 metal
oxides,20,21 dendrimers,22,23 and graphene layers.24 However,
due to the high activation barrier of substrates associated with
the rate-limiting steps, the majority of these systems with
heterogeneous catalysts require elevated temperature (≥100 °C
or under reflux conditions), intensive heating, and prolonged
reaction time, which may lead to undesired side reactions and
compromise the stability of the catalysts.12,25
Sunlight is a clean, abundant, and sustainable source of
energy, and the visible light range of the spectrum accounts for
44% of the total solar energy as opposed to only 3% from the
ultraviolet (UV) light.26 Current research in the field of
heterogeneous visible light photocatalysis is predominantly
focused on the photoredox reactions27 such as the oxidation of
alcohols and amines or the hydroxylation of aromatic
compounds,28,29 C−H activation reaction,30 hydrogen gen-
eration,31 and oxygen activation,32 whereas there have been
very few reports on classic cross-coupling reactions, such as
Suzuki coupling reactions. Recently, Wang et al. reported the
harvesting of visible light with plasmonic Au−Pd nanostruc-
tures and their use as photocatalysts for Suzuki coupling
reactions.33 Zhu et al. demonstrated also that Au−Pd alloy NPs
can strongly absorb visible light and efficiently catalyze several
cross-coupling reactions at ambient temperature.34−36 Apart
from plasmonic metals, Li et al. reported that mesoporous g-
C3N4, a well-established visible light nonmetal photocatalyst,
can be used as photoactive anchoring support for Pd
nanoparticles to form a Mott−Schottky heterojunction and
increase the electron density surrounding Pd NPs via light-
amplified effect.37−40
Figure 1. (a) Polymer backbone structure of B-BO3. (b) TEM image
of B-BO3 immobilized with Pd NPs with sizes between 5 and 10 nm.
(c) UV/vis DR spectra of Pd@B-BO3 and B-BO3. (d) VB (HOMO)
and CB (LUMO) band positions of Pd@B-BO3 and B-BO3 (vs SCE)
measured by cyclic voltammetry.
Received: January 28, 2015
Revised: February 28, 2015
Conjugated microporous polymers (CMPs), combining
semiconductive π-conjugated skeletons with permanent nano-
© XXXX American Chemical Society
A
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Chemistry of Materials
Communication
were subsequently immobilized, resulting in the hybrid material
Pd@B-BO3 (Figure 1b). To note, the functional groups inside
B-BO3 were end-capped in order to avoid undesired side
reactions during the light-induced Suzuki coupling later. About
3 wt % of Pd content in Pd@B-BO3 was determined via
inductively coupled plasma (ICP). The synthetic and character-
istic details of the materials are described in the Supporting
Information.
trace conversion, and by extending the reaction time to 5 h, a
conversion of 13% was observed (entry 2). This is similar to
previous report on reactivity of Pd NPs at room temperature in
dark.46 No conversion was also observed without base (entry
3), which is essential to activate boronic acid and facilitate
transmetalation.4 Bare B-BO3 without immobilization of Pd
NPs as a control experiment only led to trace yield of the
product, which is likely obtained due to the residual amount of
Pd (∼50 ppm) present in B-BO3 after its preparation. Using
Pd@C as photocatalyst led to a conversion of 16%, indicating a
minor contribution of the Pd plasmonic effect compared to the
photocatalytic activity afforded by the hybrid heterojunction
between Pd and B-BO3.
To understand the specific role of the photogenerated
electron−hole pairs during the photocatalytic Suzuki coupling
reaction, a hole (diisopropylethylamine (DIPEA)) and a radical
scavenger (benzoquinone) were added to the reaction mixture,
respectively. In the same time, the occurrence of a possible
homocoupling of both aryl reactants was also investigated. 4-
Fluoroiodobenzene was taken as the model aryl halogenide in
order to differentiate the homocoupled products. Under the
same reaction condition as entry 1, 4-fluorobiphenyl; as
product was obtained in a yield of 94% (entry 6). By adding
DIPEA, i.e., when the hole was blocked, 4-fluorobiphenyl was
obtained with a yield of 21%, as well as yielding the
homocoupled side product 4,4′-difluorobiphenyl with a
conversion of 17% (entry 8). Via addition of benzoquinone,
where the generated electron was blocked, 4-fluorobiphenyl
was still obtained in a yield of 30% without any observation of
homocoupled side product (entry 9). By blocking both hole
and electron upon addition of both scavengers, only traces of 4-
fluorobiphenyl were obtained, along with 8% of the
homocoupled 4,4′-difluorobiphenyl (entry 10). It can be
observed that both generated hole and electron pair were
essential for the light-induced Suzuki coupling reaction. And by
only blocking one of the photogenerated electron−hole pairs,
Pd@B-BO3 was still active enough to precede the coupling
reaction with one activated aryl species under lowered reaction
rate.
The Brunauer−Emmett−Teller (BET) surface area of B-BO3
was 612 m2/g with a total pore volume of 0.37 cm3/g. The BET
surface area of Pd@B-BO3 was 176 m2/g with a pore volume of
0.11 cm3/g. This is likely due to the weight increase after
immobilization with Pd NPs. The pore size of ca. 1.4 nm
remained. The UV/vis DR spectrum of Pd@B-BO3 showed a
broader absorption than that of B-BO3 (Figure 1c). This could
be attributed to the immobilization of Pd NPs, which also
absorb in the visible range due to the interband transition and
plasmonic effects. Cyclic voltammetry showed that Pd@B-BO3
exhibited lower valence band (VB) (+1.43 eV vs SCE) and
conduction band (CB) (−0.95 eV vs SCE) than B-BO3 with a
VB of +1.35 eV (vs SCE) and CB of −1.08 eV (vs SCE),
respectively. The electron paramagnetic resonance (EPR)
showed a large increase of the intensity of Pd@B-BO3 in
dark and under light irradiation compared to B-BO3 (Figure S5
in Supporting Information). This could also be caused by the
plasmonic effects on the Pd NPs. On the other hand, the
intensive signal also indicates a longer living electron−hole pair
inside Pd@B-BO3 than in B-BO3.
The coupling of iodobenzene with phenylboronic acid was
chosen as a model reaction to study the catalytic activity of
Pd@B-BO3 under visible light irradiation. To avoid the
photoheating effect, the reaction vial was submerged in a
water bath (Figure S12 in Supporting Information). The
screening and control experiments are listed in Table 1. The
reaction gave the desired product in an almost quantitative
yield at room temperature in 2 h (entry 1). An identical
reaction system, which was conducted in dark, resulted only in
Table 1. Screening and Mechanism Study of the
Photocatalytic Suzuki Coupling Reaction Using Pd@B-BO3
a
On the basis of the observations during the screening and
control experiments, we propose a possible reaction mechanism
as displayed in Figure 2. Upon light irradiation, charge
separations in the form of electron−hole pairs can be generated
within the semiconductive polymer B-BO3. The photoexcited
energetic electrons further amplify the Schottky effect by
migrating toward the noble metal center (i.e., Pd NPs in this
case). With the Pd NPs functioning as active center, the C−I
bond of iodobenzene is attacked by the photogenerated
b
entry
X
reaction condition variations
light
yield (%)
1
2
H
H
H
H
H
H
F
-
+
-
98
c
-
13
3
no K2CO3
+
+
+
+
+
+
+
+
-
4
no Pd@B-BO3
B-BO3 as photocatalyst
Pd@C as photocatalyst
-
-
5
trace
d
6
16
7
94
e
8
F
hole scavenger
radical scavenger
radical and hole scavengers
21
9
F
30
f
10
F
trace
a
Reaction conditions: iodobenzene (0.5 mmol), phenylboronic acid
(0.55 mmol), K2CO3 (0.75 mmol), Pd@B-BO3 (10 mg, 3 wt % Pd
content), DMF/H2O (1.5 mL/1.5 mL), white LED lamp (1.2 W/
b
cm2), water bath, room temperature, 2 h. Isolated yields after
c
d
chromatography. In dark, 5 h. 6 mg of Pd@C (5 wt % Pd content).
e
21% yield of 4-fluorobiphenyl and 17% yield of 4,4′-difluorobiphenyl.
Trace amount of 4-fluorobiphenyl and 8% yield of 4,4′-difluor-
Figure 2. Suggested reaction mechanism of the photocatalytic Suzuki
coupling via the light-induced heterojunction at the interface of B-BO3
and Pd NPs.
f
obiphenyl.
B
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Communication
electrons. Xiao et al. showed via the density functional theory
(DFT) calculation, when an electron enters an unoccupied
orbital of the iodobenzene molecule, the C−I bond elongates
from 0.214 to 0.300 nm,35 facilitating the cleavage of the C−I
bond and yielding a transient radical anion, which initially
forms an aryl complex with Pd. At the same time, the hole
inside the semiconductive polymer B-BO3 activates the C−B
bond of phenylboronic acid, which is transferred into an
BO3 as photocatalyst (Figure S11 in Supporting Information).
TEM images of the used Pd@B-BO3 showed a similar structure
as the unused one, while a slight weight loss of Pd was
determined via ICP (Figure S9 in Supporting Information).
In conclusion, this work demonstrated that Pd@B-BO3, a
type of metal-immobilized microporous polymer semiconduc-
tor photocatalyst, can be used as efficient, heterogeneous
photocatalysts for the Suzuki coupling reactions via the
photoamplified Schottky effect at the metal−semiconductor
interface. The coupling reactions of aryl halides with
arylboronic acid yielded excellent conversions in best cases
under visible light irradiation at room temperature. Given the
heterogeneous nature, the photocatalyst could be easily
separated and recycled for further coupling reactions without
significant loss in activity. We also envision that this novel genre
of photocatalysts could be applied to other classic cross-
coupling reactions, such as Sonogashira, Stille, Hiyama, and
other catalytic reactions.
−
electronically negative B(OH)3 species by catching an OH−
from the basic reaction medium, forming another aryl Pd
complex. The remaining steps, the reductive elimination and
transmetalation, should not be affected by the photoactivation
and are followed by the formation of the final product.47,48
We then further tested the feasibility of Pd@B-BO3 as
photocatalysts for a series of Suzuki coupling reactions of
phenylboronic acids with various aryl halides. As listed in Table
2, aryl iodides (for example entries 6 and 10), as expected, were
Table 2. Scope of the Photocatalytic Suzuki Coupling
Reaction with Various Substituents Using Pd@B-BO3 as
Photocatalyst
ASSOCIATED CONTENT
■
a
S
* Supporting Information
Polymer synthesis, general procedure of the photocatalytic
1
Suzuki coupling reaction, FT-IR, H, 13C, solid state 13C CP/
MAS NMR and EPR spectra, N2 sorption isotherms, and pore
size distribution. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: kai.zhang@mpip-mainz.mpg.de.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The Max Planck Society is acknowledged for financial support.
Z.J.W. is a recipient of a fellowship through funding of the
Excellence Initiative (DFG/GSC 266) of the graduate school of
excellence “MAINZ” (Materials Science in Mainz).
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