Visible light-driven cross-coupling reactions at lower temperatures using a photocatalyst of palladium and gold alloy nanoparticles

Article abstract of DOI:10.1021/cs5000284

Palladium (Pd)-catalyzed cross-coupling reactions are among the most important methods in organic synthesis. We report the discovery of highly efficient and green photocatalytic processes by which cross-coupling reactions, including Sonogashira, Stille, Hiyama, Ullmann, and Buchwald-Hartwig reactions, can be driven with visible light at temperatures slightly above room temperature using alloy nanoparticles of gold and Pd on zirconium oxide, thus achieving high yields. The alloy nanoparticles absorb visible light, and their conduction electrons gain energy, which is available at the surface Pd sites. Results of the density functional theory calculations indicate that transfer of the light excited electrons from the nanoparticle surface to the reactant molecules adsorbed on the nanoparticle surface activates the reactants. When the light intensity was increased, a higher reaction rate was observed, because of the increased population of photoexcited electrons. The irradiation wavelength also has an important impact on the reaction rates. Ultraviolet irradiation can drive some reactions with the chlorobenzene substrate, while visible light irradiation failed to, and substantially improve the yields of the reactions with the bromobenzene substrate. The discovery reveals the possibility of using low-energy and -density sources such as sunlight to drive chemical transformations.

Full text of DOI:10.1021/cs5000284

Research Article
pubs.acs.org/acscatalysis
Visible Light-Driven Cross-Coupling Reactions at Lower
Temperatures Using a Photocatalyst of Palladium and Gold Alloy
Nanoparticles
Qi Xiao,† Sarina Sarina,† Arixin Bo,† Jianfeng Jia,‡ Hongwei Liu,† Dennis P. Arnold,† Yiming Huang,†
Haishun Wu,‡ and Huaiyong Zhu*,†
†School of Chemistry, Physics and Mechanical Engineering, Faculty of Science and Technology, Queensland University of
Technology, Brisbane, QLD 4001, Australia
‡School of Chemical and Material Science, Shanxi Normal University, Linfen 041004, China
ABSTRACT: Palladium (Pd)-catalyzed cross-coupling reac-
tions are among the most important methods in organic
synthesis. We report the discovery of highly efficient and green
photocatalytic processes by which cross-coupling reactions,
including Sonogashira, Stille, Hiyama, Ullmann, and Buch-
wald−Hartwig reactions, can be driven with visible light at
temperatures slightly above room temperature using alloy
nanoparticles of gold and Pd on zirconium oxide, thus
achieving high yields. The alloy nanoparticles absorb visible
light, and their conduction electrons gain energy, which is
available at the surface Pd sites. Results of the density
functional theory calculations indicate that transfer of the
light excited electrons from the nanoparticle surface to the
reactant molecules adsorbed on the nanoparticle surface activates the reactants. When the light intensity was increased, a higher
reaction rate was observed, because of the increased population of photoexcited electrons. The irradiation wavelength also has an
important impact on the reaction rates. Ultraviolet irradiation can drive some reactions with the chlorobenzene substrate, while
visible light irradiation failed to, and substantially improve the yields of the reactions with the bromobenzene substrate. The
discovery reveals the possibility of using low-energy and -density sources such as sunlight to drive chemical transformations.
KEYWORDS: alloy nanoparticles, cross-coupling, photocatalysis, surface plasmon resonance, visible light
catalysts’ stability and reusability. Thus, new catalytic systems
based on efficient, recyclable catalysts and green energy sources
for effective chemical transformations are highly desirable but
remain a significant challenge.
1. INTRODUCTION
Palladium-based catalysts are widely used in cross-coupling
reactions for the formation of carbon−carbon bonds. A large
variety of homogeneous catalytic systems based on Pd(II) or
Pd(0) have become powerful and versatile tools in modern
organic synthesis,1−3 being employed from the total synthesis
of natural products to the preparation of new materials to bio-
organic chemistry.4,5 In addition, recent years have seen the
imperative to develop “green” cross-coupling reactions using
more environmentally friendly catalysts and methods.4 For
example, Pd nanoparticles (PdNPs) have emerged as promising
catalysts that can function under moderate conditions and be
recycled more readily,6,7 although the catalytic efficiency of the
NPs is often not as good as that of the homogeneous Pd
catalysts. Many of the catalytic reactions (with homogeneous
Pd catalysts or PdNP catalysts) are thermally driven to achieve
viable efficiency, but the heating also has negative side effects.
For example, it makes the synthesis reactions energy-intensive
as we have to heat the entire reaction system, including the
reactor and solvent. High reaction temperatures may also
increase the extent of formation of unwanted side products in
some reactions.8−11 Furthermore, heating may compromise
Light is one potentially sustainable energy source with which
to drive chemical reactions.12−19 Very recently, we discovered
that Au−Pd alloy NPs can strongly absorb visible light and
efficiently enhance the extents of conversion of several
reactions, including the Suzuki−Miyaura cross-coupling reac-
tions, at temperatures slightly above room temperature.19 Two
other research groups also found almost at the same time that it
is an effective approach to use irradiation to accelerate Suzuki
cross-coupling reactions by using Au−Pd bimetallic nanostruc-
tures.20,21 We believe that the conduction electrons of the NPs
gain the energy of the incident light, generating electrons at
high energy levels (light-excited electrons). These light-excited
electrons are available at the surface Pd sites of the alloy NPs.
The surface Pd sites have good affinity for the reactant
Received: January 9, 2014
Revised: April 1, 2014
Published: April 11, 2014
© 2014 American Chemical Society
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molecules, and the electrons at these sites enhance their
intrinsic ability to activate the reactant molecules. The charge
heterogeneity of the alloy NP surface, because of the different
electronegativities of gold and palladium, also plays a key role in
the catalytic reactions. The proposed mechanism of light-
excited electrons of alloy NPs and the interaction of the
adsorbed reactant molecules with the excited electrons and the
NP surface is not specific to the Suzuki−Miyaura cross-
coupling reaction. It is rational to hypothesize that the Au−Pd
alloy structure is likely to be efficient for driving a variety of Pd-
catalyzed cross-coupling reactions with visible light. A challenge
is that the activation of various reactant molecules for different
reactions may not be the same, but the study of several cross-
coupling reactions can provide insight into the mechanism of
the photocatalytic cross-coupling processes in general.
In this study, five different cross-coupling reactions were
investigated to confirm the general applicability of Au−Pd alloy
NP photocatalysts under visible light irradiation, namely, the
Sonogashira, Stille, Hiyama, and Ullmann C−C couplings and
the Buchwald−Hartwig amination (C−N cross-coupling). We
found that visible light can drive these cross-coupling reactions
with the alloy NP catalyst at temperatures slightly above room
temperature, and the performance of the alloy catalyst depends
on the intensity and wavelength of the light irradiation. A
mechanism is proposed on the basis of the results of the density
functional theory (DFT) calculations and experimental
observation: light absorption of the alloy NPs generates excited
electrons, and the light-excited electrons with sufficient energy
are able to transfer into the lowest unoccupied molecular
orbital (LUMO) of the reactant molecules adsorbed on the
NPs, weakening the chemical bonds of the molecules and
facilitating the reactions.
2. RESULTS AND DISCUSSION
2.1. Catalyst Synthesis and Characterization. In this
study, Au−Pd NPs were supported on zirconia (ZrO2) powder
as photocatalysts (for the detailed method, see the
Experimental Section). The NPs of pure gold and pure
palladium on the ZrO2 support (AuNPs@ZrO2 and PdNPs@
ZrO2) were also prepared under synthetic conditions similar to
those used for the synthesis of the alloy NPs. Figure 1 shows
the transmission electron microscopy (TEM) analysis of the
alloy NPs; the Au−Pd alloy NPs are distributed evenly on the
ZrO2 particle surface, and the mean diameters of the Au−Pd
alloy NPs are <7 nm (Figure 1b). The elemental composition
of the as-prepared Au−Pd alloy NPs was studied using energy
dispersion X-ray spectroscopy (EDX). As shown in Figure 1e,
line scan analysis for a typical Au−Pd alloy NP shows that Au
and Pd are distributed fairly uniformly in an alloy NP.
Figure 1. (a) TEM image of the Au−Pd alloy NPs. (b) Particle size
distribution of the Au−Pd alloy NPs. (c and d) High-resolution TEM
(HR-TEM) images of the Au−Pd alloy NPs. (e) Line profile analysis
of a typical Au−Pd NP providing information about the elemental
composition and Au/Pd distribution of the NP.
The important feature of the Au−Pd alloy NPs, which makes
them useful in photocatalysis, is that they strongly absorb
visible light mainly through the localized surface plasmon
resonance (LSPR) effect of AuNPs.19−21 Figure 2 shows the
diffuse reflectance ultraviolet−visible extinction (DR UV−vis)
spectra of the samples. Here, the spectrum of the Au−Pd alloy
NP sample is clearly different from the spectra of the pure
metal NPs. The ZrO2 support exhibited little absorption of light
with wavelengths longer than 400 nm. Hence, the light
absorption in the spectra of the Au−Pd alloy NPs@ZrO2
samples, which is the difference between light absorption of
the alloy NPs@ZrO2 and that of the ZrO2 support alone, is due
to the absorption of the alloy NPs.22,23 The difference between
the two spectra of supported and colloid metal NPs can be
Figure 2. DR UV−vis spectra of the Au−Pd alloy NPs@ZrO2 catalyst
and their comparison with pure AuNPs@ZrO2, PdNPs@ZrO2, and
Au−Pd alloy NPs without the ZrO2 support in an aqueous suspension.
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Table 1. Performance of Au−Pd Alloy NPs, AuNPs, and PdNPs for Cross-Coupling Reactions under Visible Light Irradiation
(red numbers) and in the Dark (black numbers)
a
The yields were calculated from the product content and the aryl iodide conversions measured by gas chromatography (GC). A Nelson halogen
lamp (wavelengths of 400−750 nm) was used as the visible light source, and the light intensity was measured to be 0.45 W/cm2. The reaction was
b
conducted at 45 2 °C for 24 h. For the detailed reaction conditions, see the Experimental Section. TOF (turnover frequency) values were
calculated on the basis of the amount of Pd metal.
attributed to scattering. Because the distances between the alloy
NPs on the ZrO2 support are much smaller than those between
the alloy NPs in their colloid suspension, the light scattering is
much stronger for the supported alloy NPs than that for the
unsupported NPs.24 The light scattering is generally more
significant at longer wavelengths (<600 nm).
shows that visible light irradiation can drive the same reactions
on the Au−Pd alloy NPs under much milder reaction
conditions (45 °C), achieving very good yields, and no
additional additives such as cocatalysts or phosphine ligands
are required. The photocatalytic process at low reaction
temperatures proposed in this study is thermodynamically
preferred. Thus, utilizing light energy to promote the efficiency
of this process is a green approach due to its lower energy
input.
As shown in Table 1, for all the reactions PdNP catalysts
exhibited better activity under irradiation than in the dark. The
absorption of visible light and UV light by PdNPs can excite the
electron interband transition,31 and these excited electrons at
the surface of the PdNPs enhance the catalytic ability of the
PdNPs. AuNPs exhibit no catalytic activity for these reactions,
whether under light irradiation or in the dark, except for the
Buchwald−Hartwig reaction (17% under light, 15% in the
dark). The Au−Pd alloy NPs exhibited superior catalytic
activity when irradiated; the calculated TOF values for Au−Pd
alloy NPs are much higher than those of pure AuNPs or PdNPs
(approximately 1.7−4.2-fold) under light irradiation. This can
be attributed to the fact that the charge heterogeneity of the
alloy NP surface is greater than those of AuNP and PdNP
surfaces,19 which leads to a stronger interaction between the
alloy NPs and reactant molecules.32,33 The performance of the
Au−Pd alloy NPs strongly depends on the Au/Pd ratio for the
reactions in the presence and absence of light. It has been
shown that when the Au/Pd molar ratio is 1/1.86, the surface
charge heterogeneity is the greatest and results in optimal
catalytic activity.19
2.2. Photocatalytic Reactions. 3-Iodotoluene was used as
the aryl halide substrate to react with various coupling partners
using Au−Pd alloy NPs under visible light irradiation (Table
1). The reactions were also conducted in the dark but with
other conditions identical. For example, the temperature of the
reaction mixture in the dark was kept the same as that of the
reaction mixture under light (45 °C) by a water bath. The data
in Table 1 show the results of visible light-enhanced cross-
coupling reactions using the Au−Pd alloy NP catalyst.
It is evident that irradiation increased the extents of
conversion of all the cross-coupling reactions (compared with
the same reactions conducted in the dark). Control experi-
ments using the support ZrO2 (without Au−Pd alloy NPs) as
the catalyst were performed. No conversion was observed for
the reaction when the system was illuminated with light or
when the reaction was conducted in the dark because ZrO2 has
a large band gap (5 eV) and exhibits negligible light absorption
in the visible range.25 Undoubtedly, the catalytic activity is due
to the Au−Pd alloy NPs, and the catalytic enhancement
observed when the system was irradiated is due to the alloy
NPs.
There have been many reports of cross-coupling reactions
catalyzed via homogeneous or heterogeneous processes, but
most of them need to be conducted at elevated temperatures
(≥100 °C), even under reflux conditions.26−30 This study
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Table 2. Au−Pd Alloy NPs Catalyzed Cross-Coupling Reactions with Different Aryl Halides under Visible Light Irradiation (red
a
b
numbers) and in the Dark (black numbers) [values in parentheses are the TOF values (h−1)]
a
The yields were calculated from the product content and the aryl halide conversions measured by GC. The products were analyzed by GC and mass
b
spectrometry. For detailed reaction conditions, see the Experimental Section. TOF values were calculated on the basis of the amount of Pd metal.
c
Reaction time of 14 h.
It is known that AuNPs exhibit strong visible light absorption
due to the LSPR effect.34,35 LSPR is the resonant light-induced
coherent oscillation of charges at the metal−dielectric interface,
established when the frequency of the incident light matches
the frequency of metal surface electrons oscillating against the
restoring force of their positive nuclei. Therefore, in addition to
creating a NP surface with greater surface charge heterogeneity,
gold in the Au−Pd alloy NPs enhances the ability of the NP to
harvest the light energy [compared with that of PdNPs (see
Figure 2)]. When the alloy NPs are irradiated with light, the
conduction electrons gain the energy of the incident light,
yielding Pd sites with light-excited electrons at the alloy NP
surface. Hence, the intrinsic catalytic activity of the Pd sites is
significantly enhanced at low reaction temperatures.
The general applicability of the Au−Pd alloy NP-photo-
catalyzed cross-coupling reaction was investigated with a series
of differently substituted aryl halides. As shown in Table 2, the
light irradiation remarkably promoted the reaction in each case.
High selectivities for the desired cross-coupling products were
achieved regardless of whether the substituents were electron
donors or acceptors. Thus, the visible light photocatalytic
process using Au−Pd alloy NPs can drive various cross-
coupling reactions with a broad range of substrates.
It is well-known that activation of C−Br and C−Cl bonds is
much more difficult than activation of the C−I bond and in
general requires harsher reaction conditions in the heteroge-
neous catalysis system.36 In this study, we examined the
catalytic activity of Au−Pd alloy NPs for reactions using
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Table 3. Examples of Bromobenzene and Chlorobenzene as Substrates for Cross-Coupling Reactions Using Au−Pd Alloy NPs
under Visible Light and UV Light Irradiation
a
The yields were calculated from the product content and the aryl halide conversions measured by GC. The values in parentheses are the data for
b
reactions controlled under the same conditions in the dark. Reaction temperature of 65 °C. The UV light reaction was conducted under UV lamp
(UVP Blak-Ray B100AP High Intensity UV Lamp, 100 W, 365 nm UV) irradiation with a light intensity of 0.9 W/cm2, and the other reaction
c
conditions were kept the same. TOF values were calculated on the basis of the amount of Pd metal.
bromobenzene and chlorobenzene as substrates with light
irradiation. Visible light can activate bromobenzene effectively,
but the yields are much lower than those using iodobenzene
(Table 3, entries 1−3). Surprisingly, we found that ultraviolet
(UV) irradiation with higher intensity and energy could not
only substantially improve the yields of the reactions (Table 2,
entries 1−3) but also activate the reactions with chloroben-
zenes that can hardly be activated under visible light irradiation
(Table 3, entries 4 and 5). These results indicate that one can
improve the catalytic activities by increasing light intensity and
using shorter wavelength light.
2.3. Impact of Light Intensity and Wavelength. We
investigated the dependence of catalytic activity on light
intensity, and the results of the representative examples of
Sonogashira, Hiyama, and Stille reactions are depicted in Figure
3. When the irradiation intensity was increased from 0.1 to 0.2,
0.3, 0.4, and 0.5 W/cm2 with other reaction conditions
unchanged, the extent of conversion of the reactions on the
Au−Pd alloy NPs increased. There is a positive relationship
between the intensity and reaction rate.
As shown in Figure 3, the results clearly exhibit an almost
linear dependence. The contributions of light irradiation to the
conversion efficiency were calculated by subtracting the extent
of conversion of the reaction in the dark from the overall extent
of conversion observed when the system was irradiated, with
both reactions occurring at an identical reaction temperature.
Here the conversion of the reaction in the dark is regarded as
the contribution of the thermal effect. The relative contribu-
tions of light and thermal processes to the conversion
efficiencies are shown in Figure 3. We can see that the higher
the light intensity, the greater the contribution of irradiation to
the overall conversion rate. When the light intensity is 0.1 W/
cm2, the light contributions for these reactions were only 47%
(Sonogashira), 20% (Hiyama), and 14% (Stille), and when the
light intensity increased to 0.5 W/cm2, 90, 89, and 63%,
respectively, of the conversion are due to irradiation. A stronger
light intensity will induce a larger population of electrons at
higher energy levels and create a stronger electromagnetic field
around the NPs (surface enhancement effect), as reported for
AuNPs.37 The transfer of the light-excited electrons of a metal
nanoparticle to molecules adsorbed on the nanoparticles is
well-known.38,39 Such transfer induces the reaction of the
molecules. The surface enhancement effect also contributes to a
stronger interaction between the NPs and reactant molecules,
and thus enhanced catalytic activity of the coupling reactions.
A useful tool for determining whether an observed reaction
occurs via a photoinduced process or a thermocatalytic process
is the action spectrum, which should show one-to-one mapping
between the wavelength-dependent photocatalytic rate and the
light extinction spectrum.40,41 In this study, the reaction rates of
the photocatalyic cross-coupling reactions using Au−Pd alloy
NPs at 40 2 °C under irradiation with different wavelengths
were determined. Five LED lamps with wavelengths of 400 5,
470 5, 530 5, 590 5, and 620 5 nm were used, and the
rates were converted to the apparent quantum efficiencies
(AQEs). The AQE was calculated using the relationship AQE
(%) = [(Ylight − Ydark)/(the number of incident photons)] ×
100, where Ylight and Ydark are the amounts of products formed
under light irradiation and dark conditions, respectively. The
plot of AQE versus the respective wavelengths is the action
spectrum of the reaction. The action spectra of Sonogashira and
Stille reactions are shown in Figure 4 as representative
examples. Each action spectrum in the figure is compared
with the light absorption spectrum of the Au−Pd alloy NPs and
the spectrum of AuNPs.
The action spectra exhibit a few interesting features. First,
action spectra of the cross-coupling reactions do not follow the
absorption spectrum of the Au−Pd alloy NPs@ZrO2 catalyst,
which includes a substantial contribution from scattering. It
means that the scattering has little impact on the catalytic
performance. Second, a correspondence is observed between
the AQE of the Sonogashira reaction and the light absorption
of AuNPs (Figure 4a), which show the characteristic LSPR
absorption peak in the range between 500 and 550 nm.34,35
The PdNPs exhibited a high catalytic activity for the
Sonogashira reaction in the dark (Table 1), indicating that
the PdNPs can readily activate reactants of this reaction. Given
that alloying with Au results in a loss of most of the activity in
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Figure 4. Action spectra for (a) Sonogashira and (b) Stille cross-
coupling reactions. The light absorption spectra (left axis) are DR-
UV−vis spectra of AuNPs (purple) and the Au−Pd alloy NPs (blue).
3-Iodotoluene was used as the aryl halide substrate to react with
corresponding coupling partners under visible light irradiation. The
AQE values were calculated on the basis of the average of three
experiments. For details, see the Experimental Section.
NPs, these light-excited electrons can migrate to the surface Pd
sites where they function like the light-excited electrons of the
PdNPs, resulting in a significant enhancement of the catalytic
performance of the alloy NPs. The action spectrum suggests
that the enhancement of the catalytic performance is mainly
due to the LSPR absorption of gold in the alloy NPs. The
situation of the Stille reaction is similar to that of the
Sonogashira reaction (Table 1); the action spectrum of the
Stille reaction likewise follows the absorption spectrum of
AuNPs (Figure 4b). Here, we can confirm that it is the Au that
acts as an antenna that harvests visible light enhancing the
reaction yield in alloy NP-catalyzed reactions.
The dependence of photocatalytic activity on light intensity
and wavelength indicates that electrons excited by light
absorption are responsible for the observed photocatalytic
activity.42 Because the rate of the catalyzed reactions is expected
to depend on the population of electrons with sufficient energy
to initiate the reaction of the reactant molecules, one can
increase the number of light-excited electrons by applying a
high light intensity. The electron energy sufficient to initiate the
reaction of the molecules on the metal NPs is dependent on the
actual reaction in question. Tuning the irradiation wavelength
can increase the number of light-excited electrons with
sufficient energy to induce the reaction and may also assist us
in understanding the mechanism of the reactions.
Figure 3. Dependence of the catalytic activity of Au−Pd alloy NPs for
(a) Sonogashira, (b) Hiyama, and (c) Stille cross-coupling reactions
on the intensity of light irradiation. The numbers with percentages
show the contribution of the light irradiation effect. 3-Iodotoluene was
used as the aryl halide substrate to react with the corresponding
coupling partner under visible light irradiation. The reaction
conversions were based on the average of two experiments. In the
reactions for determining the light intensity dependence, a photometer
was used to measure the light intensity; the other experimental
conditions were kept the same. For the reaction conditions, see the
Experimental Section.
the dark and AuNPs exhibited no activity for this reaction
(Table 1), the reaction apparently takes place only at the Pd
sites and is not affected by the higher surface charge
heterogeneity of the alloy NPs. Thus, the function of gold in
the alloy NP is apparently only to absorb the light energy. The
light absorption of Au−Pd alloy NPs is obviously stronger than
that of the pure PdNPs at all wavelengths (Figure 2). Gold
nanostructures exhibit strong LSPR absorption of visible light,
which can excite electrons to high energy levels. In the alloy
2.4. Impact of Temperature. Another important feature
of the photocatalytic process on metal NP catalysts is that the
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photocatalytic activity of the NPs can be increased by elevating
the reaction temperature.43,44 This feature is also observed from
the alloy NP photocatalysts in this study. For example, the
extent of conversion of the Sonogashira reaction at 30 °C was
only 11%, whereas the extent of conversion reached 100%
when the temperature was increased to 60 °C in the same time
interval (Figure 5a, blue dots). At a higher temperature, more
Because the photocatalytic activity of the alloy NP
photocatalysts varies with temperature, the apparent activation
energies of Sonogashira and Hiyama reactions were estimated
by using the Arrhenius equation and kinetic data for
photocatalytic reactions conducted over the range of 30−60
°C. As shown in Figure 6, the difference between the activation
Figure 6. Apparent activation energies of (a) Sonogashira and (b)
Hiyama reactions calculated for the photoreaction and the reaction in
the dark. The contribution of light irradiation is calculated from the
difference in the extents of conversion of two processes (with and
without light) and presented as a percentage (Y axis). 3-Iodotoluene
was used as the aryl halide substrate to react with corresponding
coupling partners under light irradiation and in the dark. The light
intensity was 0.45 W/cm2.
energies of the light-enhanced process and the process in the
dark (ΔEa) indicates the reduction in the measured activation
energy due to irradiation. For example, the estimated activation
energy is ∼217 kJ/mol for the Sonogashira reaction in the dark
(Figure 6a), while it is ∼120 kJ/mol for the photocatalytic
reaction under visible light irradiation. Thus, light irradiation
can reduce the activation energy of the Sonogashira reaction by
97 kJ/mol, which represents 44% of the “uncatalyzed”
activation energy. Similarly, the activation energy of the
Hiyama reaction was reduced by 66% (Figure 6b). The fact
that irradiation substantially reduces the activation energy
demands that the photocatalytic process have a mechanism
different from that of the process in the dark.
2.5. Proposed Mechanism. The cross-coupling reactions
involve two essential steps: breaking the carbon−iodine bond
in aryl iodide and activation of the coupling partner molecules
that then facilitates transmetalation.45,46 It has been postulated
that the transfer of an electron from the Pd atoms to the
halogen atoms is involved in facilitating carbon−halogen bond
cleavage in the Pd heterocatalysis.20 This is effectively the
heteroanalogue of the familiar homogeneous oxidative addition
step involving ligated Pd(0). In this study, when the PdNPs
were irradiated with light, enhanced catalytic performance was
observed. Because the absorption of light by PdNPs excites the
electrons of the PdNPs to the high-energy band, it is deduced
that the light-excited electrons at the surface of the PdNPs can
enhance the catalytic ability. Given that AuNPs exhibit no
catalytic activity for most of the reactions, it is reasonable to
believe that the light-excited electrons at the surface Pd sites of
the alloy NPs facilitate the reactions. The linear dependence of
the photoinduced reaction rate on the light intensity, observed
in Figure 3, usually suggests an electron-driven chemical
process on the metal surface.42 Also, transient electron transfer
from a light-excited metal NP to a chemically adsorbed
molecule is well-known.38,39 Electrons of alloy NPs are excited
Figure 5. Dependence of catalytic activity on different reaction
temperatures for (a) Sonogashira and (b) Hiyama reaction under a
thermal heating process in the dark (triangles) and the light irradiation
process (circles). 3-Iodotoluene was used as the aryl halide substrate to
react with corresponding coupling partners under light irradiation and
in the dark. The light intensity was 0.45 W/cm2.
electrons of the metal NPs populate higher energy levels, but
these electrons can still gain energy from the incident light.
Thus, upon irradiation, the number of electrons with sufficient
energy to initiate the reaction of the molecules adsorbed on the
metal NPs is greater. At a low reaction temperature, the
photoexcitation contributes dominantly to the photocatalytic
activity and the photothermal effect would contribute much
less.20 For many catalyzed reactions involving the interaction
between light-excited electrons of a catalyst with reactant
molecules, high reaction temperatures are not a prerequisite to
drive them. Light irradiation can yield excited electrons with
sufficient energy at low reaction temperatures and facilitate the
reactions of the reactants. The metal NPs have the capacity to
couple the stimuli of light irradiation and heat to drive the
catalytic reaction.43 This property not only distinguishes them
from semiconductor photocatalysts but also reveals the
potential of the NPs to utilize the infrared radiation in sunlight,
which accounts for a large fraction of the solar spectrum and
could be used to heat the NPs, further facilitating the reaction.
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Figure 7. Proposed mechanism for the photocatalytic cross-coupling reactions. (a) Light irradiation excites electrons of an alloy NP to high energy
levels, and the transfer of the excited electrons with sufficient energy from the alloy NP to the LUMO of molecules adsorbed on the NP can take
place, activating the reaction. The electrons finally return to the metal NP at a lower energy.38 (b) At higher reaction temperatures, more excited
electrons populate higher energy levels of the metal NPs, which can be readily transferred to the LUMO of the absorbed molecule. (c) Under a
higher irradiation intensity, more electrons of the metal NPs populate higher energy levels, resulting in more electron transfers to the LUMO of the
absorbed molecule and higher reaction rates. (d) DFT calculations on the reactant iodobenzene before (left) and after (right) the transfer of one
electron from the NP surface to the reactant molecule. The C−I bond is elongated because of the electron transfer, which facilitates the activation of
reactions. (e) Schematic diagram of the pathway for the photocatalytic cross-coupling reactions.
by light irradiation; the excited electrons transfer from the alloy
NP surface to the LUMO of a molecule adsorbed on the NP,
activating the substrate. The electron can finally return to the
metal at a lower energy (Figure 7a−c). We therefore performed
DFT calculations on the transfer of one electron from the NP
surface to the reactant iodobenzene molecule. The simulation
suggests that when one electron enters an unoccupied orbital,
the C−I bond will become longer (from 0.214 to 0.300 nm), so
the cleavage of the C−I bond will be much easier (Figure 7d).
For the reactions in which the rate-determining step is C−I
bond cleavage, irradiation will facilitate the transfer of an
electron from the NP to the adsorbed aryl iodide molecule,
yielding a transient negative ion species. Loss of iodide ion will
afford either an adsorbed phenyl radical or a true organo-
metallic aryl−palladium iodide complex on the surface and then
activate the reactions (Figure 7e).
Given that the PdNPs exhibited catalytic activity for most of
the reactions in the dark, it is believed that activating
iodobenzene molecules on the surface Pd sites is not difficult.
The fact that the Buchwald−Hartwig coupling reaction on
PdNPs did not occur in the dark suggests that the surface Pd
sites cannot activate aniline in the dark. The significant catalytic
activity of PdNPs for this reaction under light should be due to
the activation of aniline on the PdNP surface under irradiation.
Aniline has a strong interaction with AuNP surfaces and is often
used for the surface-enhanced Raman spectral study,34 and this
explains the observed catalytic activity of the AuNPs in the
dark. The fact that the Au−Pd alloy NPs exhibited catalytic
activity superior to that of both the AuNPs and the PdNPs in
the dark indicates that the charge heterogeneity of the alloy NP
surface also enhances the catalytic activity.
The heterogeneous catalytic processes of the coupling
reactions examined in this study have multiple steps and are
by no means well-understood.47,48 In this regard, we focused
only on possible roles for the light-excited electrons afforded at
the NP surfaces by irradiation. The delineation of subsequent
steps is a profound challenge but is beyond the scope of this
preliminary survey.
3. CONCLUSIONS
The findings in this study demonstrate that irradiation of Au−
Pd alloy NPs can significantly enhance the intrinsic catalytic
activity of Pd at lower temperatures for a number of Pd-
catalyzed cross-coupling reactions. An outstanding feature of
the Au−Pd alloy NPs is their ability to efficiently concentrate
the energy of a photon flux to a very small volume and to
transfer this energy to the adsorbed molecules to induce their
reaction on the surface. These catalytic cross-coupling
processes are due to the interaction of light-excited electrons
of the catalyst with the reactant molecules, while high
temperatures are not a prerequisite for driving them. The
reaction rate depends on the number of light-excited electrons
and the number of reactant molecules on the catalyst surface.
The number of reactant molecules on the surface depends
mainly on the affinity of the surface for the reactants. Pd sites
have a strong affinity for many organic molecules. The number
of light-excited electrons can be increased by applying a high
light intensity. Although we focused on Au−Pd nanostructures,
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the discussed mechanisms may be universal, and similar
principles could be used in the design of various photocatalysts
comprising any of a range of plasmonic metals and a
catalytically active transition metal.37 Thus, we have been able
to utilize a low-energy and -density source to drive a wide range
of useful chemical transformations. In these reaction systems,
the energy efficiency is high, because of the specific absorption
of the light only by the NP catalysts, and not by the solvent, the
oxide support, the atmosphere, or the reaction vessel. The
findings reported here reveal the possibility of green cross-
coupling reactions driven by visible light at temperatures
slightly above room temperature.
intensity was measured to be 0.45 W/cm2. The temperature of
the reaction system was carefully controlled with an air
conditioner attached to the reaction chamber. The reaction
system under light illumination was maintained at the same
temperature as the corresponding reaction system in the dark
to ensure that the comparison is meaningful. All the reactions
in the dark were conducted using a water bath placed above a
magnetic stirrer to control the reaction temperature; the
reaction flask was wrapped with aluminum foil to avoid
exposure of the reaction mixture to light. At given irradiation
time intervals, 2 mL aliquots were collected and then filtered
through a Millipore filter (pore size of 0.45 μm) to remove the
catalyst particulates. The liquid-phase products were analyzed
with an Agilent 6890 gas chromatography (GC) HP-5 column
to measure the change in the concentrations of reactants and
products. An Agilent HP5973 mass spectrometer was used to
identify the product. For the reactions using H2O as the
solvent, the product was extracted with dichloromethane
(CH2Cl2) before GC analysis. The GC conversions and
selectivities were calculated from the product content and the
aryl halide conversions.
4.4.1. Sonogashira Cross-Coupling Reactions. Aryl iodide
(1 mmol), alkyl alkyne (1.2 mmol), photocatalysts (50 mg),
cetyltrimethylammonium bromide (CTAB) (1 mmol), and
K3PO4 (2 mmol) were added to 10 mL of H2O. The reaction
temperature was 45 2 °C, under a 1 atm argon atmosphere,
with a reaction time of 24 h.
4.4.2. Hiyama Cross-Coupling Reactions. Aryl iodide (1
mmol), trimethoxyphenylsilane (1.5 mmol), photocatalysts (50
mg), and tetrabutylammonium fluoride (TBAF) (1.2 mmol)
were added to 5 mL of toluene. The reaction temperature was
45 2 °C, with a reaction time of 24 h.
4. EXPERIMENTAL SECTION
4.1. Chemicals. Zirconium(IV) oxide (ZrO2, <100 nm
particle size, TEM), gold(III) chloride trihydrate (HAuCl4·
3H2O, ≥99.9% trace metal basis), palladium(II) chloride
(PdCl2, ReagentPlus, 99%), sodium borohydride, powder
(NaBH4, ≥98.0%), hydrochloric acid [HCl, 32% (w/w),
analytical reagent, Chem-Supply], and N,N-dimethylformamide
(DMF, anhydrous, 99.8%, total impurities, <0.005% water)
were purchased from Sigma-Aldrich (unless otherwise noted)
and used as received without further purification. The water
used in all experiments was prepared by being passed through
an ultrapurification system.
4.2. Preparation of Catalysts. Au−Pd alloy NPs (1.5 wt %
Au−1.5 wt % Pd supported on ZrO2, Au/Pd molar ratio of 1/
1.86) were prepared by the impregnation−reduction method.
ZrO2 powder (2.0 g) was dispersed into a HAuCl4 (15.2 mL,
0.01 M) and PdCl2 (28.3 mL, 0.01 M) aqueous solution under
magnetic stirring at room temperature. A lysine (16 mL, 0.53
M) aqueous solution was then added to the mixture while it
was vigorously stirred for 30 min, and the pH value was
determined to be 8−9. To this suspension was added dropwise
a freshly prepared aqueous NaBH4 (3 mL, 0.35 M) solution in
20 min, followed by an addition of an HCl (3 mL, 0.3 M)
solution. The mixture was aged for 24 h, and then the solid was
separated by centrifugation, washed with water (three times)
and ethanol (once), and dried at 60 °C in a vacuum oven for 24
h. The dried powder was used directly as a catalyst. Pure Au
NPs (3 wt %) and Pd NPs (3 wt %) were prepared via a similar
method but using different quantities of HAuCl4 and PdCl2
aqueous solutions.
4.4.3. Stille Cross-Coupling Reactions. Aryl iodide (1
mmol), tributylphenylstannane (1.2 mmol), photocatalysts
(50 mg), cetyltrimethylammonium bromide (CTAB) (1
mmol), and NaOH (3 mmol) were added to 10 mL of H2O.
The reaction temperature was 45 2 °C, under a 1 atm argon
atmosphere, with a reaction time of 24 h.
4.4.4. Ullmann Cross-Coupling Reactions. Aryl iodide (1
mmol), photocatalysts (50 mg), and NaOH (3 mmol) were
added to 10 mL of an EtOH/H2O mixture [1/1 (v/v)]. The
reaction temperature was 45 2 °C, with a reaction time of 24
h.
4.3. Characterization of Catalysts. A transmission
electron microscope (TEM) study and line profile analysis by
an energy dispersion X-ray (EDX) spectrum technique of the
photocatalysts were conducted on a Philips CM200 TEM with
an accelerating voltage of 200 kV. Element line scanning was
conducted on a Bruker energy dispersion X-ray (EDX) scanner
attached to a JEOL-2200FS TEM with a scanning beam
diameter of ≥1.0 nm. The Au and Pd contents of the prepared
catalysts were determined by energy dispersion X-ray spectrum
(EDS) technology using the attachment to a FEI Quanta 200
environmental scanning electron microscope (SEM). Diffuse
reflectance UV−visible (DR-UV−vis) spectra of the sample
powders were examined with a Varian Cary 5000 spectrometer
with BaSO4 as a reference.
4.4. Photocatalytic Reactions. A 25 mL Pyrex round-
bottom flask was used as the reaction container, and after the
reactants and catalyst had been added, the flask was sealed with
a rubber septum cap. The flask was irradiated with magnetic
stirring using a halogen lamp (from Nelson, wavelength in the
range of 400−750 nm) as the visible light source, and the light
4.4.5. Buchwald−Hartwig Cross-Coupling Reactions. Aryl
iodide (1 mmol), aniline (1.2 mmol), photocatalysts (50 mg),
and potassium tert-butoxide (t-BuOK) (3 mmol) were added to
10 mL of N,N-dimethylformamide (DMF). The reaction
temperature was 45 2 °C, with a reaction time of 24 h.
4.4.6. Action Spectrum Experiments. LED lamps (Tongyi-
fang, Shenzhen, China) with wavelengths of 400 5 nm (TYF-
H030 G45), 470 5 nm (TYF-H030 G35), 530 5 nm (TYF-
H030 G35), 590 5 nm (TYF-H030 G38), and 620 5 nm
(TYF-H030 G32) were used as the light source. The light
intensity was measured to be 0.50 W/cm2, and the other
reaction conditions were identical to those of typical reaction
procedures.
4.5. DFT Calculation. The geometries of all the species
were optimized at the level of DFT with Becke’s49 three-
parameter exchange and Lee−Yang−Parr correlation func-
tional50 implemented in Orca.51 Ahlrichs’ triple-ζ valence basis
set52 TZVP was employed to describe the orbitals of all atoms
involved. The iodobenzene molecule and its corresponding
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Research Article
(27) For Stille reactions, see: (a) Espinet, P.; Echavarren, A. M.
negative ion were fully optimized as defined by the B3LYP/
TZVP method.
Angew. Chem., Int. Ed. 2004, 43, 4704−4734. (b) DelPozo, J.;
́
́
Carrasco, D.; Perez-Temprano, M. H.; García-Melchor, M.; Alvarez,
R.; Casares, J. A.; Espinet, P. Angew. Chem., Int. Ed. 2013, 52, 2189−
2193.
AUTHOR INFORMATION
Corresponding Author
*E-mail: hy.zhu@qut.edu.au.
■
(28) For Hiyama reactions, see: (a) Diebold, C.; Derible, A.; Becht, J.
M.; Le Drian, C. Tetrahedron 2012, 69, 264−267. (b) Zhang, L.; Li, P.;
Li, H.; Wang, L. Catal. Sci. Technol. 2012, 2, 1859−1864.
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P.; Bhattacharya, S. J. Organomet. Chem. 2013, 724, 281−288. (b) Liu,
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Punniyamurthy, T. Org. Lett. 2007, 9, 3397−3399. (d) Pan, K.; Ming,
H.; Yu, H.; Huang, H.; Liu, Y.; Kang, Z. Dalton Trans. 2012, 41,
2564−2566. (e) Kidwai, M.; Mishra, N. K.; Bhardwaj, S.; Jahan, A.;
Kumar, A.; Mozumdar, S. ChemCatChem 2010, 2, 1312−1317.
(f) Jammi, S.; Sakthivel, S.; Rout, L.; Mukherjee, T.; Mandal, S.;
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
The authors gratefully acknowledge financial support from the
Australia Research Council (ARC DP110104990).
■
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