Visible-Light-Activated Suzuki-Miyaura Coupling Reactions of Aryl Chlorides over the Multifunctional Pd/Au/Porous Nanorods of CeO2 Catalysts

Article abstract of DOI:10.1021/acscatal.5b01173

Activation of aryl chlorides for Suzuki-Miyaura coupling (SMC) reactions is particularly challenging for heterogeneous catalysts due to the chemically inert nature of the C-Cl bond. Herein, the multifunctional Pd/Au/porous nanorods of CeO₂ (PN-CeO₂) catalysts with a well-defined spatial configuration deliver the first example of heterogeneous catalysts to activate the strong C-Cl bond under the irradiation of visible light (>400 nm) at room temperature. PN-CeO₂ with strong basicity not only provides the photogenerated electrons to enrich the electron density of metal nanoparticles but also generates holes for activation of arylboronic acids. Meanwhile, due to the strong local surface plasma resonance, the hot electrons from Au nanoparticles excited by visible light can be injected into Pd nanocatalysts that are spatially contacted with Au nanoparticles. Thus, Pd nanocatalysts with significantly enriched electron density efficiently activate the aryl chlorides under the visible light irradiation at room temperature. The high catalytic activity and reusability of multifunctional photocatalysts associated with full use of the photogenerated electrons and holes inspire the future exploitation for the activation of unreactive chemical bonds under mild conditions.

Full text of DOI:10.1021/acscatal.5b01173

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Article
Visible-Light-Activated Suzuki-Miyaura Coupling Reactions of Aryl Chlorides
over the Multifunctional Pd/Au/Porous Nanorods of CeO2 Catalysts
Sai Zhang, Chun-Ran Chang, Zheng-Qing Huang, Yuanyuan Ma, Wei Gao, Jing Li, and Yongquan Qu
ACS Catal., Just Accepted Manuscript • Publication Date (Web): 28 Sep 2015
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Visible-Light-Activated Suzuki-Miyaura Coupling Reactions of Aryl
Chlorides over the Multifunctional Pd/Au/Porous Nanorods of CeO Catalysts
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†
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Sai Zhang, Chunran Chang, Zhengqing Huang, Yuanyuan Ma, Wei Gao, Jing Li, Yongquan
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, ‡
Qu*
†
Center for Applied Chemical Research, Frontier Institute of Science and Technology and State
Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an, China,
710049
§
School of Chemical Engineering and Technology, Xi’an Jiaotong University, Xi’an, China,
710049
‡
MOE Key Laboratory for Nonequilibrium Synthesis and Modulation of Condensed Matter,
Xi’an Jiaotong University, Xi’an, China, 710049
ABSTRACT. Activation of aryl chlorides for SuzukiꢀMiyaura coupling (SMC) reactions is
particularly challenging for heterogeneous catalysts due to the chemically inert nature of CꢀCl
bond. Herein, the multifunctional Pd/Au/porous nanorods of CeO (PNꢀCeO ) catalysts with a
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wellꢀdefined spatial configuration deliver the first example of heterogeneous catalysts to activate
the strong CꢀCl bond under the irradiation of visible light (> 400 nm) at room temperature. PNꢀ
CeO with strong basicity not only provides the photoꢀgenerated electrons to enrich the electron
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density of metal nanoparticles, but also generates holes for activation of arylboronic acids.
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Meanwhile, due to the strong local surface plasma resonance, the hot electrons from Au
nanoparticles excited by visible light can be injected into Pd nanocatalysts spatially contacted
with Au nanoparticles. Thus, Pd nanocatalysts with significantly enriched electron density
efficiently activate the aryl chlorides under the visible light irradiation at room temperature. The
high catalytic activity and reusability of multifunctional photoꢀcatalysts associated with full use
of the photoꢀgenerated electrons and holes inspire the future exploitation for the activation of
unreactive chemical bonds under mild conditions.
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KEYWORDS: photoꢀcatalysis, porous CeO , spatial configuration, aryl chlorides, Suzuki
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coupling reaction
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1
. INTRODUCTION
Development of friendly photoꢀcatalytic materials for organic reactions with a high activity and
stability under ambient conditions is of great significance for both the fundamental studies and
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practical applications. One particular focus has been placed on utilizations of the photoꢀ
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generated electrons or holes for reduction or oxidation of organic molecules.
Despite the
simultaneous utilization of photoꢀgenerated electrons and holes for an organic chemical reaction
presents a novel and promising catalytic processes for the economical and green synthesis, it's
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seldom reported and still a challenge to scientific community.
SuzukiꢀMiyaura crossꢀcoupling (SMC) reactions have become arguably one of the most
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powerful and convenient methods for the formation of CꢀC bond.
Generally, aryl iodides and
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bromides are employed as substrates for the SMC reactions due to their high activity.
However, activation of aryl chlorides is particularly challenging for SMC reactions due to the
strong CꢀCl bond, despite their benefits of easy availability and low cost. Especially for
heterogeneous catalysts, successful examples using aryl chlorides are quite rare. Stoichiometric
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amount of strong bases and high temperature (>100 ºC) are required.
The key advantages of
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SMC reactions with mild reaction conditions cannot be highlighted. Therefore, to seek for
novel heterogeneous catalysts that can activate aryl chlorides and facilitate the SMC reactions at
mild conditions is highly significant and desirable.
Inspired from the recent research of homogeneous SMC reactions, great efforts have been
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focused on designing electronꢀrich catalytic sites by introducing electron donators (ligands).
The electron donator can provide the requisite electron density on palladium (Pd) to afford the
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oxidative addition of Pd with halogenoꢀbenzene and facilitate the coupling reactions. Hence,
activation of aryl chlorides by heterogeneous Pd may be realized at mild conditions by increasing
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the electron density of Pd through a dedicated design of heterogeneous catalysts. A convenient
and sustainable approach for enriching electron density of Pd is a photoꢀassisted process, in
which the photoꢀgenerated electrons are injected into Pd nanocatalysts under the light irradiation.
Photoꢀcatalytic activation or assistance of aryl iodides for the SMC reactions has been studied
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under ambient conditions,
while activation of aryl chlorides by heterogeneous catalysts stays
essentially untouched at room temperature. To realize the activation of aryl chlorides for SMC
reactions by photoꢀgenerated electrons and holes at mild conditions is important for chemical
synthesis.
Ceria, as an important semiconductor, has been investigated as photoꢀcatalysts for various
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chemical reactions.
Since the visible light occupies ~ 50% of the whole solar spectrum, it is
significant to utilize visible light for catalytic reactions. Despite the large band gap of ceria (3.2
eV), with the help of the strong localized surface plasma resonance (LSPR) of Au nanoparticles,
Au/CeO systems can efficiently utilize the photoꢀgenerated electrons/holes under the irradiation
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of visible light.
In this study, we report a novel Pd/Au/porous nanorods of CeO (PNꢀCeO ) photoꢀcatalysts
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with wellꢀdefined spatial configuration to successfully activate aryl chlorides for SMC reactions
under the visible light irradiation. Au nanoparticles are first loaded on PNꢀCeO by depositionꢀ
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precipitation method. Pd nanocatalysts are selectively photoꢀdeposited on the surface of Au
nanoparticles. PNꢀCeO as the supports can donate electrons to metal catalysts because of its
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strong basicity. Meanwhile, PNꢀCeO exhibits a response to visible light, in which the photoꢀ
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generated electrons and holes can be used for the desired photoꢀcatalytic reactions. Au
nanoparticles not only absorb visible light but also serve as charge mediator to transfer electrons
to Pd nanocatalysts. Under the synergistic action of PNꢀCeO and Au nanoparticles to enrich the
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electron density of Pd nanocatalysts under visible light (> 400 nm), the multifunctional
Pd/Au/PNꢀCeO catalysts realize the activation of aryl chlorides with a wide scope for SMC
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reaction at room temperature. Photoꢀgenerated electrons and holes were used to efficiently
activate the aryl chlorides and aryl boronic acids, respectively. The decreased conversion of aryl
chlorides and the poor selectivity of the cross coupling product in the presence of electron or
hole scavengers further demonstrate the activation of aryl chlorides and aryl boronic acids by
photoꢀgenerated electrons and holes, respectively.
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2. EXPERIMENTAL SECTION
All chemicals (AR grade) were used as received. Water with a resistivity of 18.2 Mꢁ · cm was
used for all experiments. All glassware were thoroughly washed by aqua regia (a volume ratio
of 1: 3 of concentrated nitric acid and hydrochloric acid) to avoid any possible contamination.
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.1 Synthesis of porous nanorods of ceria (PN-CeO ). PNꢀCeO was synthesized by our
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previously reported method, which involved a twoꢀstep hydrothermal method. Briefly, a
mixture of 1.367 g of Ce(NO ) · 6H O and 19.2 g of NaOH in 70 mL of water was treated with
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hydrothermal process at 100 °C for 24 h in a Pyrex bottle to obtain nonporous Ce(OH) /CeO2
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nanorods. Subsequently, the PNꢀCeO was synthesized by hydrothermal treatment on nonporous
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Ce(OH) /CeO nanorods in a stainless autoclave at 160 °C temperatures for 12 hours.
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.2 Synthesis of Au/PN-CeO catalysts. Au/PNꢀCeO catalysts with a 3 wt% metal loading
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was prepared by a modified depositionꢀprecipitation method.
Typically, 50 mg of PNꢀCeO2
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was suspended in 5 mL of HAuCl solution (7.6×10 mM) at room temperature for 1 h. After
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adding 200 mg of urea, the solution was heated to 80 C and maintained at this temperature for 2
hours under vigorous stirring. After the reaction solution was cooled to room temperature, 5 mL
of freshly prepared NaBH solution (0.56 mg/mL) was added. The reaction was continued for
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another 1 hour at room temperature. Finally, the Au/PNꢀCeO catalysts were washed for four
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times with distilled water and dried overnight at 60 C.
The Pd/PNꢀCeO catalyst with a 0.5% metal loading was prepared by the same process,
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except Na PdCl was used for the precursor for metal deposition.
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2.3 Synthesis of Pd/Au/PN-CeO catalysts. Photoꢀdeposition of Pd nanoparticles on Au/PNꢀ
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CeO was conducted using a deꢀaerated aqueous methanol solution (5 vol %) of Na PdCl under
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light irradiation. The Pd precursor was reduced by photoꢀgenerated electrons. Metallic Pd
species were deposited on the surface of Au nanoparticles. The resultant powder was washed
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repeatedly with distilled water and then dried at 60 C overnight under air.
2.4 Photo-catalytic SMC reaction. In a typical experiment for SMC reaction, aryl chlorides
(0.2 mmol), aryl boronic acid (0.24 mmol), K CO (0.6 mmol) and catalysts (15 mg) were mixed
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in 2 mL of DMF and H O solvent with a volume ratio of 1:1 in a 10 mL quartz Schlenk flask.
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The reaction was stirred continuously under the irritation of a Xe lamp (150 W) equipped with an
optical filter (cut off < 400 nm) for desired time. The reaction system was cooled by a
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mechanical fan and the room temperature was 25 C. The products were extracted with 6 mL
ethyl acetate twice. After purification on a microꢀcolumn filled with silica gel, the products were
analyzed by GCꢀMS and GC (Agilent 7890A GC and 597C MS, the column is HPꢀ5MS). mꢀ
Xylene was used as the internal standard.
2.5 Characterization of Catalysts. The phase evolution of asꢀsynthesized nanostructures was
monitored by powder Xꢀray diffraction (XRD). The XRD patterns with diffraction intensity
versus 2
θ were recorded in a Shimadzu Xꢀray diffractometer (Model 6000) using Cu Kα
radiation. Transmission electron microscopy (TEM) studies were conducted on a Hitachi HTꢀ
7700 transmission electron microscope with an accelerating voltage of 120 kV. High resolution
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TEM was performed on JEMꢀ2100F (JEOL, Japan) operating at 200 kV. Xꢀray photoelectric
spectra (XPS) were acquired using a Thermo Electron Model KꢀAlpha with Al Kα as the
excitation source. The contents of Pd and Au were determined by inductively coupled plasma
optical emission spectroscopy (ICPꢀOES) analysis (Agilent 7500ce)
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3. RESULTS AND DISCUSSIONS
The synthetic method and characterization of PNꢀCeO have been reported previously.
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Briefly, the CeO /Ce(OH) nanorods as precursors were obtained in the first hydrothermal
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treatment at 100 °C, and the formation of PNꢀCeO were occurred at the second hydrothermal
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dehydration and oxidation at 160 °C. TEM image of PNꢀCeO (Figure S1a) shows a rodꢀlike
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morphology with a clear porous structure. XRD pattern of PNꢀCeO exhibits a well defined
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cubic fluorite structure (Figure S1b). Au nanoparticles with a size of 4.28 ± 1.05 nm (Figure 1a)
were deposited on the surface of PNꢀCeO by a depositionꢀprecipitation method. High resolution
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TEM (HRTEM) image (Figure 1b) revealed one continuous lattice fringe with a planar spacing
of 0.24 nm, in good agreement with the (111) crystalline plane of Au. After photoꢀdeposition of
Pd, the average size of metal nanoparticles was 5.14 ± 1.01 nm (Figure 1c), which was larger
than that of Au nanoparticles. The structural characterizations indicated that Pd was selectively
deposited on the surface of Au nanoparticles. After careful HRTEM studies, two different
continuous lattice fringes in the noble metal nanoparticles were observed (Figure 1d). The
measured planar spacing of 0.24 nm and 0.22 nm are in good agreement with the (111)
crystalline of Au and Pd, respectively. Energyꢀdispersive Xꢀray spectroscopy (EDS) spectra of
the noble metal nanoparticles (Figure S2) confirmed the coꢀexistence of Au and Pd in one
particle. The results indicate that Pd nanoparticles synthesized by photoꢀdeposition method are
spatially deposited on the surface of Au nanoparticles. It can be explained by the preferential
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reduction of palladium ion at Au nanoparticles with rich of electrons under the light irradiation.
Determined by the ICPꢀOES, the contents of Au and Pd in the catalysts were 2.95 wt% and 0.41
wt%, respectively.
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As a comparison, Au/PNꢀCeO (3%) and Pd/PNꢀCeO (0.5%, Figure S3) were also prepared
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by depositionꢀprecipitation method. In Figure 1e, PNꢀCeO exhibits weak visible light
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absorption at wavelengths above 400 nm. After deposition of noble nanoparticles on PNꢀCeO2
supports, the catalysts display enhanced visible light absorption, compared with pure PNꢀCeO2.
The absorption peak at 550 nm in the spectrum of Au/PNꢀCeO corresponds to the LSPR of Au
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nanoparticles. Although the characteristic absorption peak of Au nanoparticles in Pd/Au/PNꢀ
CeO is not clear, it delivers the highest visible light adsorption.
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The SMC reactions between chlorobenzene and phenylboronic acid could not be occurred in
dark by Au/PNꢀCeO , Pd/PNꢀCeO and Pd/Au/PNꢀCeO catalysts (Table S1). Meanwhile, no
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catalytic activity was observed under visible light irradiation for 4 hours in the absence of
catalysts or in the presence of pure PNꢀCeO catalysts (Table S1). In contrast, Pd/Au/PNꢀCeO2
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catalysts give a yield of 90.1% for the cross coupling product at the same reaction conditions.
Figure 2a shows time course of biphenyl yield catalyzed by Pd/Au/PNꢀCeO under visible light
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irradiation (> 400 nm). With the increase of photoꢀirradiation time, the yield of biphenyl
increases continuously and reaches 98.8% at 5 h. A conversion of chlorobenzene of 99.6% is
also observed. Therefore, the SMC reactions of aryl chlorides can be effectively activated by
visible light at ambient conditions, thus making them very easy to handle. To be mentioned, the
reaction temperature was also raised to 30.9 ºC after the visible light irradiation for 5 hours,
despite the reaction system was cooled by a mechanical fan. This can be attributed to the
photothermal heating originated from Au nanoparticles due to the strong LSPR effect. At this
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temperature, no catalytic activity of Pd/Au/PNꢀCeO was observed for the SMC reaction
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between chlorobenzene and phenylboronic acid in the dark. The results further confirm the
activation of chlorobenzene is realized by Pd/Au/PNꢀCeO under the visible light irradiation.
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Longꢀterm stability and reusability are the most important features of heterogeneous catalysts.
After photoꢀcatalytic SMC reaction of chlorobenzene and phenylboronic acid, the Pd/Au/PNꢀ
CeO catalysts were recovered from the reaction solution via centrifugation and reused for the
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same reaction. As shown in Figure 2b, Pd/Au/PNꢀCeO catalysts delivered high catalytic
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activity at least for 6 cycles. The unaltered morphology of used Pd/Au/PNꢀCeO catalysts
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illustrates their structural stability (Figure S4a). Two different continuous lattice fringes in the
noble metal nanoparticles were also observed from the HRTEM image of the used catalysts
(Figure S4b), suggesting the spatial configuration of Au and Pd components in the catalysts is
well preserved.
To study the synergistic effect of Pd/Au/PNꢀCeO catalysts, Pd/PNꢀCeO (Pd loading 0.5
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wt%) and Au/PNꢀCeO (Au loading 3 wt%) were also employed to catalyze the SMC reactions
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under the same conditions. For Pd/PNꢀCeO , the yield increased very slowly and only reached
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9.15% after 5 h under the visble light irradaition. This can be attributed to the poor
responsiveness of visible light for Pd/PNꢀCeO catalysts and the SMC reaction of chlorobenzene
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cannot be efficiently activated under visible light irradiation. For Au/PNꢀCeO catalysts,
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although the yield of biphenyl was higher than that catalyzed by Pd/PNꢀCeO , it tended to be
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saturated after 3 h reaction and reached at 37.6% after 5 h reaction. The higher catalytic activity
of Au/PNꢀCeO catalysts over Pd/PNꢀCeO may be attributed to a better responsiveness towards
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visible light due to the strong LSPR of Au nanoparticles. Hence, more photoꢀelectrons can be
generated on the surface of Au nanoparticles, which can enhance the yield of biphenyl. However,
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Au is not a common and effective catalysts for SMC reactions, and the final yield cannot reach
a satisfactory value. Compared with the biphenyl yields catalyzed by Pd/PNꢀCeO (9.15%) and
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Au/PNꢀCeO (37.6%), the Pd/Au/PNꢀCeO shows the highest catalytic activity with a product
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yield of 98.8% at the same conditions for 5 h. This value was even 2.1 times higher than the
total yield of Au/PNꢀCeO and Pd/PNꢀCeO , indicating the high catalytic activity of Pd/Au/PNꢀ
2
2
CeO for activation of aryl chlorides under mild conditions.
2
The results ambiguously demonstrate the synergistic effect in Pd/Au/PNꢀCeO catalysts,
2
which presents a surprising and highꢀefficient catalytic activity for SMC reaction of
chlorobenzene at room temperature under the visible light irradiation. The direct evidence of Au
nanoparticles for visible light activation of chlorobenzene was illustrated by studying the
dependence of wavelength of incident light and catalytic activity. The yields of biphenyl
catalyzed by Pd/Au/PNꢀCeO were recorded under visible light irradiation at each wavelength
2
for 6.5 h. In Figure 2c, a correspondence was observed clearly between the yields of SMC
reactions and the characteristic LSPR absorption of Au nanoparticles. The yields of SMC
reactions did not follow the absorption of Au nanoparticles completely, which can be explained
2
1,36
by the light scattering of the catalysts.
Thus, Au nanoparticles in Pd/Au/PNꢀCeO catalysts
2
function as the visible light absorber and transfer the excited electrons to Pd for chemical
reactions. If the excited photoꢀelectrons stay on the surface of Au nanoparticles, the yield of
SMC reactions was unsatisfied (Figure 2a) due to the low catalytic activity of Au nanoparticles
for CꢀC coupling. When Pd is spatially deposited on the surface of Au nanoparticles, the excited
electrons can be efficiently transferred from Au nanoparticles to Pd nanocatalysts, resulting in a
significantly enriched electron density on Pd nanocatalysts and a subsequent enhanced catalytic
activity for the SMC reactions.
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The catalytic activity of Pd/Au/PNꢀCeO under the irradiation of 420 nm incident light is
2
comparable to that under the irradiation of 550 nm light (Figure 2c). It can be attributed to the
relative high light absorption of PNꢀCeO near ultraviolet (UV) region as shown in Figure 1e.
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The SMC reaction between chlorobenzene and phenylboronic acid was also carried out under the
light irradiation without the optical filter. As shown in the Figure S5, a biphenyl yield of 99.2%
was observed after 4 h of the SMC reaction under both UV and visible light irradiation. The
enhanced catalytic activity can be attributed to the high light absorption of PNꢀCeO at UV
2
region, in which more photoꢀgenerated electrons and holes can be used for the activation of
reactants.
To exam the spatial configuration of Pd and Au for synergistic catalytic effect, SMC reactions
were also catalyzed by the mixed catalysts containing Pd/PNꢀCeO and Au/PNꢀCeO under the
2
2
same conditions. The contents of Pd in Pd/PNꢀCeO and Au in Au/PNꢀCeO catalysts are
2
2
controlled as same as those in Pd/Au/PNꢀCeO catalysts. The mixed catalysts exhibited zero
2
activity for SMC reaction under dark (Figure 2d). Under the visible light irradiation for 5 h, the
yield of biphenyl only reached 43.7%, which almost coincided with the total yields catalyzed by
Pd/PNꢀCeO and Au/PNꢀCeO , respectively. No increase in catalytic activity for the mixed
2
2
catalysts indicates the blocked charge transfer between Pd and Au in the mixed catalysts due to
the separately spatial configuration of two metals.
Besides, supports also can significantly affect the electron density of the loaded metal
37,38
catalysts.
In our previous report, PNꢀCeO with a strong basicity can donate electrons to Pd
2
3
2
catalysts and enhance their activity for CꢀC coupling reactions. Herein, XPS technique was
used to evaluate the contribution of PNꢀCeO towards the electron density of noble metals. The
2
zero valent state of Pd(3d ) and Au(4f ) binding energy is close to 335.1 eV and 84.0 eV,
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respectively.
With higher electric density, the binding energy for Pd(3d) and Au(4f) core
level could be much close to their zero valent state. As shown in Figure S6a and S6b, the
binding energies of Au(4f ) and Pd(3d ) are 335.5 eV and 84.3 eV, respectively, which
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demonstrates the high electron density of noble metals anchored on PNꢀCeO2.
Due to the structural defects of oxygen vacancy in PNꢀCeO support, the band gap of 2.63 eV
2
for PNꢀCeO is derived from UVꢀvis adsorption spectra (Figure S7), which is lower than the
2
value of ideal CeO crystal (3.2 eV). As evidenced from absorption spectrum (Figure S7a), PNꢀ
2
CeO can absorb part of visible light and generate electron/hole pairs. The photoꢀgenerated
2
electrons from PNꢀCeO under visible light irradiation can be injected to Au nanoparticles,
2
mediate the electron transfer and enrich the electron density of Pd nanocatalysts. In order to
exam the role of PNꢀCeO support, Au/Pd nanocatalysts with a mass ratio of 1:6 (Figure S8)
2
23
were synthesized according to a previous report. The SMC reaction between chlorobenzene
and 4ꢀmethyphenylboronic acid was tested by Au/Pd nanocatalysts at the similar conditions. In
the absence of PNꢀCeO support, 1.98% conversion of chlorobenzene was obtained after 5 h
2
reaction under the visible light irradiation (Table S2). In contrast, the selfꢀcoupling product of 4ꢀ
methyphenylboronic acid was obtained with a high selectivity of 99.4%. Therefore, the presence
of PNꢀCeO is critical and beneficial to increase the electron density of Pd nanocatalysts and
2
activate chlorobenzene for SMC reactions.
The activation of chlorobenzene by Pd nanocatalysts with an enriched electron density under
the visible light irradiation can be also confirmed by investigating on the SMC reactions between
chlorobenzene and 4ꢀmethylphenylboronic acid in the presence of electron scavenger KBrO at
3
various concentrations. As shown in Table 1, the conversion of chlorobenzene was reduced from
8
9.3% to 42.5% and 10.5% under the visible light irradiation after adding 0.2 mmol and 2 mmol
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of KBrO , respectively (Entry 1, 3 and 4). In contrast, the conversions of chlorobenzene were
3
almost same for the SMC reactions performed at high temperature in the absence (Entry 2) or
presence of various amount of KBrO (Entry 5, 6). The results clearly reveal that the photoꢀ
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generated electrons indeed activate chlorobenzene at room temperature. Moreover, the barely
changed catalytic activity of Pd/Au/PNꢀCeO in the absence or presence of KBrO for high
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temperature SMC reaction indicates the unchanged oxidation states of metal during the reaction,
despite the oxidant nature of KBrO3.
The activation of chlorobenzene by the photoꢀgenerated electrons at room temperature can be
further revealed from the selectivity of cross coupling products under the visible light irradiation.
In the absence of KBrO , the visible light irradiation resulted in a high selectivity of 95.5%
3
towards the cross coupling product between chlorobenzene and 4ꢀmethylphenylboronic acid and
a low selectivity of 4.5% for the selfꢀcoupling product of 4ꢀmethyphenylboronic acid (Entry 1).
Increasing the amount of added KBrO , the selectivity of the cross coupling product decreased
3
from 95.5% (Entry 1) to 50.8% (Entry 3) and 16.7% (Entry 4), respectively. In contrast, the
selectivity of the selfꢀcoupling product of 4ꢀmethyphenylboronic acid increased obviously from
4.5% (Entry 1) to 49.2% (Entry 3) and 83.3 % (Entry 4) with increasing amount of KBrO added,
3
respectively. The observed selectivity can be attributed to the blocked pathway of the activation
of chlorobenzene by the photoꢀgenerated electrons in the presence of electron scavenger KBrO3.
Hence, the selfꢀcoupling reaction of 4ꢀmethyphenylboronic acid dominated the products.
Control experiments of the SMC reaction activated by high temperature in the absence (Entry 2)
and presence of KBrO (Entry 5, 6) delivered the similar selectivity for the cross coupling and
3
selfꢀcoupling products, indicating KBrO did not interfere the selectivity of the SMC reactions.
3
Therefore, the decreased conversion of chlorobenzene, the decreased selectivity of cross
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coupling product and the increased selectivity of the selfꢀcoupling products in the presence of
electron scavenger KBrO confirm that the photoꢀgenerated electrons is necessary for activation
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of chlorobenzene during SMC reaction.
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The above control experiments indicate chlorobenzene becomes active on the surface of Pd
nanocatalysts with a high electron density. To prove this assumption, chlorobenzene in the gas
−
phase, on electrically neutral Pd cluster or on Pd cluster with one extra electron, Pd , were
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calculated to determine the optimized structures, as shown in the supporting information (model
of calculations) and Table S3. Compared with the C−Cl bond length of 1.760 Å in gasꢀphase,
the C−Cl bond of chlorobenzene adsorbed on Pd cluster slightly elongates to 1.779 Å. In
6
−
contrast, the C−Cl bond of chlorobenzene anchored on Pd6 cluster significantly elongates to
−
1
.826 Å. Meanwhile, the C−Cl bond order of chlorobenzene on Pd cluster decreases form 1.08
6
(CꢀCl bond order in gas phase) to 0.95. Both the greatly increased bond length and decreased
bond order of C−Cl in chlorobenzene manifest the activation of C−Cl bond by negatively
charged Pd clusters. To further understand the interaction between Pd clusters and
chlorobenzene molecule, Mulliken population analysis was performed (Table S3). When
−
chlorobenzene molecule adsorbs on Pd6 cluster, −0.208 e of the Mulliken charge of
−
chlorobenzene indicates the electron transfer from Pd6 cluster to chlorobenzene. However,
there is almost no charge transfer when chlorobenzene molecule adsorbs on Pd cluster.
6
Therefore, the negatively charged Pd cluster can benefit the activation of chlorobenzene.
Next, the role of photoꢀgenerated holes from PNꢀCeO was also studied. Arylboronic acid can
2
ꢀ
combine with OH in basic solution and in principle adsorb to basic sites of PNꢀCeO via
2
electrostatic interaction. When the photoꢀgenerated holes are diffused to the adsorbed sites of
arylboronic acid, the adsorbed molecules could be oxidized accompanied with the cleavage of Cꢀ
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B bond. Therefore, arylboronic acid can be activated by the photoꢀgenerated holes under the
visble light irradiation. To demonstrate this, a common hole scavenger, methanol, was
introduced into the SMC reactions of chlorobenzene and 4ꢀmethyphenylboronic acid. For the
visible light induced SMC reactions, the conversions of chlorobenzene decreased from 89.3% to
72.8% and 35.9% after adding 0.5 mL and 2 mL methanol (Table 1, Entry 1, 7 and 8),
respectively. The decreased selectivity of the cross coupling product from 95.5% to 48.4% and
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2.4% and the appearance of selfꢀcoupling product of chlorobenzene also indicated that the
activation of 4ꢀmethyphenylboronic acid was curbed in the presence of methanol. The influence
of methanol for the SMC reactions were also excluded by performed the SMC reactions in
o
presence of various amounts of methanol at 100 C under dark. The similar conversions of
chlorobenzene and selectivity of cross coupling product for all reactions (Table 1, Entry 2, 9 and
1
0) indicated methanol did not affect the SMC reactions in both activity and selectivity. The
above observations suggest that the photoꢀgenerated holes from PNꢀCeO are essential for
2
activation of aryl boronic acid under visible light irradiation. Similar results have been
previously reported for both homogeneous and heterogeneous activation of benzeneboronic acid
2
0,41,42
under light irradiation.
Based on above analysis, the electron transfer process is proposed in Figure 3. Under the
visible light irradiation, the hot electrons generated from Au nanoparticles (strong LSPR effect)
can be efficiently injected in spatially attached Pd. In bulk Au and Pd, Au gains s,p electrons
43
and loses d electrons whereas Pd loses s,p electrons but gains d electrons. For lateꢀtransition
metals like Pd, the dꢀcharacter is much more important than s, pꢀcharacter in defining their
4
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chemisorption and catalytic properties.
The Pd nanocatalysts in our synthesis are tightly
deposited on the surface of Au nanoparticles through the photoꢀdeposition. Therefore, the
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chemical potentials of the electrons in the Au and Pd nanoparticles are in equilibrium,
indicating the easy electron transfer between Au and Pd. When the Pd/Au/PNꢀCeO catalysts is
2
under the illumination of the visble light, the hots electrons generated from Au with an excited
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hot state for up to 0.5ꢀ1 ps will flow across the Au and Pd interface immediately, which
enriches the electron density of Pd and enables SMC reactions. This possible electron transfer
2
3,24
has also been proved both in previous experiments and theoretical calculation results.
The hot electrons generated from Au nanoparticles also has the possibility to be injected into
2
,47
conduction band of ceria.
However, the hot electrons are easier to be injected into Pd due to
their equilibrium chemical potentials of electrons in our catalysts. With the continual
consumption of electrons on Pd for SMC reactions, more and more hot electrons should be
+
supplied for chemical reaction. Afterwards, the Au state needs to be supplemented with the
0
electrons for recovery into the Au state. Therefore, the photoꢀgenerated electrons from PNꢀ
CeO have to be injected into Au nanoparticles to mediate the electron transfer. The spatially
2
selective deposition of Pd nanocatalysts on the surface of Au nanoparticles further confirms the
possibility of electron transfer from PNꢀCeO to Au nanoparticles.
2
Figure 3 also illustrate a proposed plausible reaction mechanism. Under the irradiation of the
visible light, the incident photons are absorbed by Au nanoparticles through their LSPR
excitation, in which the generated hot electrons can be efficiently injected into Pd nanocatalysts
and enrich the electron density of Pd nanocatalysts. The Pd nanocatalysts with high electron
density can activate aryl chlorides and facilitate the first step of oxidative addition reaction of
II
SMC reactions by accelerating the formation of active radical ligand ArPd Cl. Meanwhile, the
electron/hole pairs are generated in PNꢀCeO upon absorption of the incident visible light. The
2
generatedꢀholes are used to activate various arylboronic acids by cleaving the CꢀB bonds. The
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generatedꢀelectrons are employed to mediate the electron transfer process. When the oxidized
arylboronic acids are transferred to adjacent Pd nanocatalysts and meet with the activated aryl
chlorides, the cross coupling reaction happens and gives the final products.
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The reaction protocol was further extended to various aryl chlorides and substituted
arylboronic acids to verify the scope of catalytic systems by Pd/Au/PNꢀCeO under the visible
2
light illumination (Table 2). By substituting chlorides to bromides and iodides, the yields for
iodobenzene and bromobenzene reached 98.1% (0.5 h, Entry 1) and 98.3% (1 h, Entry 2),
respectively, which can be explained by the weaker CꢀBr and CꢀI bonds. With the strong
electronꢀwithdrawing group of –NO on aryl chloride, the conversion of aryl chloride and the
2
yield of the cross coupling product can reach 91.6% and 89.6% for 6 h reaction (Entry 3),
respectively. With the substituent of –OCH (Entry 4) and –CH OH (Entry 5) groups for
3
2
chlorobenzene, the yield of the cross coupling products were reduced to 80.6% and 56.8% at the
same reaction conditions, respectively. When the strong electron donating substituent of the
conjugated alkynyl was introduced to phenyl chloride, the conversion of aryl chloride was only
2
0.1% and a low yield of 15.3% for the cross coupling product was obtained for 6 h visible light
irradiation (Entry 6). For the substituted aryl boronic acids, high catalytic activities were
observed for aryl boronic acids with the electronꢀdonating groups. The yields of the cross
coupling products reached 99.0%, 96.8% and 76.8% for arylboronic acids with the substituents
of –CH (Entry 7), aryl (Entry 8) and tertiary butyl (Entry 9), respectively. With the electronꢀ
3
withdrawing group of –CHO in arylboronic acids, the conversion of aryl chloride and the yield
of the cross coupling product only reached 50.5% and 48.7%, respectively, under the same
reaction conditions (Entry 10), respectively.
4
. CONCLUSION
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In conclusion, we have designed Pd/Au/PNꢀCeO catalysts and experimentally demonstrated
2
the first example to activate aryl chlorides efficiently for SMC reactions at room temperature
under the visible light irradiation (> 400 nm). The synergistic effect of each component
contributes to the activation of reactants. Due to the strong LSPR effect, the hot electrons
excited from Au nanoparticles are efficiently transferred to Pd nanocatalysts spatially contacted
with Au and enrich the electron density of Pd nanocatalysts, which enables the activation of the
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CꢀCl bond of aryl chlorides. Photoꢀgenerated electrons from PNꢀCeO can be used to mediate
2
the electron transfer process for SMC reaction. Meanwhile, the arylboronic acids can be
activated by photoꢀgenerated holes from PNꢀCeO and transferred to adjacent Pd nanocatalysts.
2
When the activated aryl chlorides and oxidized arylboronic acids meet on the surface of Pd, the
CꢀC coupling reactions are realized. These results can serve as inspiration for the further
exploitation of the supported metal nanocatalysts at mild conditions for the chemical reactions
that happen under the restrict conditions previously.
ASSOCIATED CONTENT
Supporting Information. This file provides more detailed information regarding structural
characterization of PNꢀCeO , EDS spectrum of Pd/Au/PNꢀCeO , TEM image of Pd/PNꢀCeO ,
2
2
2
TEM and HRTEM images of Pd/Au/PNꢀCeO catalysts undergoing 6 cycles of SMC reaction,
2
0.5
XPS of Pd/Au/PNꢀCeO catalysts, UVꢀvis absorption spectra of PNꢀCeO in water, plot of (αhv)
2
2
versus hv for PNꢀCeO2, TEM images of Au and Pd/Au nanoparticles, UVꢀvis absorption spectra of
Au and Pd/Au nanoparticles in water, theoretical calculation on the activation of aryl chlorides
on Pd clusters with different electronic states, and control experiment of SMC reactions. This
material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Eꢀmail: yongquan@mail.xjtu.edu.cn.
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Notes
The authors declare no competing financial interest
ACKNOWLEDGMENT
We acknowledge the financial support from a NSFC Grant 21201138. This work was also
partially funded by the Ministry of Science and Technology of China through a 973ꢀprogram
under Grant 2012CB619401 and by the Fundamental Research Funds for the Central
Universities under Grant xjj2013102 and xjj2013043. Authors thank the State Key Laboratory
for Mechanical Behavior of Materials and Dr. Yuanbin Qin for TEM technical support.
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Figure 1. TEM images and HRTEM images of (a, b) Au/PNꢀCeO and (c, d) Pd/Au/PNꢀCeO2
2
catalysts. (e) UVꢀvis absorption spectra of various catalysts in water (0.2 mg/mL).
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Figure 2. (a) The time course of biphenyl yield by various catalysts; (b) the cycling stability of
Pd/Au/PNꢀCeO catalysts for SMC reactions of chlorobenzene and phenylboronic acid; (c) the
2
wavelength dependent activity of biphenyl yield by Pd/Au/PNꢀCeO catalysts; (d) the time
2
course of biphenyl yield by mixed catalysts and the total of biphenyl yield by pure Au/PNꢀCeO2
and Pd/PNꢀCeO catalysts. Reaction conditions: 1 mL of water, 1 mL of DMF, 0.6 mmol
2
K CO , 0.2 mmol of chlorobenzene, 0.24 mmol of phenylboronic and 15 mg of catalysts. Yields
2
3
were determined by GCꢀMS and GC with mꢀxylene as the interior label.
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Figure 3. Schematic view of the proposed photoꢀcatalytic reaction mechanism over the
Pd/Au/PNꢀCeO catalysts under visible light irradiation.
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Table 1. The SMC reaction of chlorobenzene and 4ꢀmethylphenylboronic acid with different
addition *
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Cl
+
B(OH)2
+
+
Selectivity (%)
Reaction
model
KBrO3
(mmol)
MeOH Conversion
Entry
(mL)
(%)
1
2
3
4
5
6
7
8
9
hv
ꢀ
ꢀ
0.2
2
0.2
2
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
0.5
2
0.5
2
89.3
93.5
42.5
10.5
94.7
95.2
72.8
35.9
85.6
86.0
Trace
8.5
Trace
Trace
10.3
9.6
47.71
52.3
9.6
95.5
78.3
50.8
16.7
73.5
72.8
48.39
42.4
74.7
70.5
4.5
13.2
49.24
83.3
16.2
17.6
3.9
5.3
15.7
18.6
o
100 C
hv
hv
o
100 C
o
100 C
hv
hv
o
100 C
o
1
0
100 C
10.9
*
Reaction condition: 1 mL of water, 1 mL of DMF, 0.6 mmol K CO , 0.2 mmol of
2 3
chlorobenzene, 0.24 mmol of 4ꢀmethylphenylboronic acid and 15 mg of Pd/Au/PNꢀCeO2
catalysts.
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Table 2. The scope of Pd/Au/PNꢀCeO catalysts for the SMC reaction *
2
Pd/Au/PN-CeO2
0
1
2
3
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8
9
0
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3
4
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9
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0
R1
X
R
2
B(OH)
2
R
1
R
2
+
Time Conversion
Entry
1
Substrate
Product
Yield (%)
98.1
(h)
(%)
I
0.5
99.1
2
3
4
Br
1
6
6
6
6
5
5
5
98.6
91.6
88.6
58.1
20.1
99.1
96.8
78.9
98.3
89.6
80.6
56.8
15.3
99.0
96.8
76.9
O N
2
Cl
O N
2
O
Cl
O
H2
HO C
H2
HO C
a
5
Cl
a
6
Cl
7
8
9
B(OH)2
B(OH)2
B(OH)2
B(OH)2
1
0
OHC
CHO
5
50.5
48.7
*
Reaction conditions: 1 mL of water, 1 mL of DMF, 0.6 mmol K CO , 0.2 mmol of aryl
2 3
chlorides, 0.24 mmol of arylboronic acids and 15 mg of catalysts. Yields were determined by
GCꢀMS and GC with mꢀxylene as the interior label.
a
With 0.3 mmol of arylboronic acids.
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