Visible-Light-Activated SuzukiâMiyaura Coupling Reactions of Aryl

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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 Downloaded from http://pubs.acs.org on September 29, 2015

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Visible-Light-Activated Suzuki-Miyaura Coupling Reactions of Aryl Chlorides over the Multifunctional Pd/Au/Porous Nanorods of CeO2 Catalysts Sai Zhang, † Chunran Chang, §Zhengqing Huang, §Yuanyuan Ma,† Wei Gao,† Jing Li,† Yongquan 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 Suzuki-Miyaura coupling (SMC) reactions is particularly challenging for heterogeneous catalysts due to the chemically inert nature of 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. PNCeO2 with strong basicity not only provides the photo-generated electrons to enrich the electron 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 photo-catalysts associated with full use of the photo-generated electrons and holes inspire the future exploitation for the activation of unreactive chemical bonds under mild conditions. KEYWORDS: photo-catalysis, porous CeO2, spatial configuration, aryl chlorides, Suzuki coupling reaction


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1. INTRODUCTION Development of friendly photo-catalytic materials for organic reactions with a high activity and stability under ambient conditions is of great significance for both the fundamental studies and practical applications.1

One particular focus has been placed on utilizations of the photo-

generated electrons or holes for reduction or oxidation of organic molecules.2-5 Despite the simultaneous utilization of photo-generated electrons and holes for an organic chemical reaction presents a novel and promising catalytic processes for the economical and green synthesis, it's seldom reported and still a challenge to scientific community.6 Suzuki-Miyaura cross-coupling (SMC) reactions have become arguably one of the most powerful and convenient methods for the formation of C-C bond.7-10 Generally, aryl iodides and bromides are employed as substrates for the SMC reactions due to their high activity.11-13 However, activation of aryl chlorides is particularly challenging for SMC reactions due to the strong C-Cl bond, despite their benefits of easy availability and low cost.

Especially for

heterogeneous catalysts, successful examples using aryl chlorides are quite rare. Stoichiometric amount of strong bases and high temperature (>100 ºC) are required.14,15 The key advantages of SMC reactions with mild reaction conditions cannot be highlighted.16 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 focused on designing electron-rich catalytic sites by introducing electron donators (ligands).17,18 The electron donator can provide the requisite electron density on palladium (Pd) to afford the oxidative addition of Pd0 with halogeno-benzene and facilitate the coupling reactions.19 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 photo-assisted process, in which the photo-generated electrons are injected into Pd nanocatalysts under the light irradiation. Photo-catalytic activation or assistance of aryl iodides for the SMC reactions has been studied under ambient conditions,20-24 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 photo-generated electrons and holes at mild conditions is important for chemical synthesis. Ceria, as an important semiconductor, has been investigated as photo-catalysts for various chemical reactions.25-27 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/CeO2 systems can efficiently utilize the photo-generated electrons/holes under the irradiation of visible light.28-31 In this study, we report a novel Pd/Au/porous nanorods of CeO2 (PN-CeO2) photo-catalysts with well-defined spatial configuration to successfully activate aryl chlorides for SMC reactions under the visible light irradiation. Au nanoparticles are first loaded on PN-CeO2 by depositionprecipitation method. Pd nanocatalysts are selectively photo-deposited on the surface of Au nanoparticles. PN-CeO2 as the supports can donate electrons to metal catalysts because of its strong basicity.32 Meanwhile, PN-CeO2 exhibits a response to visible light, in which the photogenerated electrons and holes can be used for the desired photo-catalytic 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 PN-CeO2 and Au nanoparticles to enrich the


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electron density of Pd nanocatalysts under visible light (> 400 nm), the multifunctional Pd/Au/PN-CeO2 catalysts realize the activation of aryl chlorides with a wide scope for SMC reaction at room temperature. Photo-generated 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 photo-generated electrons and holes, respectively. 2. EXPERIMENTAL SECTION All chemicals (AR grade) were used as received. Water with a resistivity of 18.2 MΩ · 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. 2.1 Synthesis of porous nanorods of ceria (PN-CeO2). PN-CeO2 was synthesized by our previously reported method,33 which involved a two-step hydrothermal method. Briefly, a mixture of 1.367 g of Ce(NO3)3 · 6H2O and 19.2 g of NaOH in 70 mL of water was treated with hydrothermal process at 100 °C for 24 h in a Pyrex bottle to obtain nonporous Ce(OH)3/CeO2 nanorods. Subsequently, the PN-CeO2 was synthesized by hydrothermal treatment on nonporous Ce(OH)3/CeO2 nanorods in a stainless autoclave at 160 °C temperatures for 12 hours. 2.2 Synthesis of Au/PN-CeO2 catalysts.

Au/PN-CeO2 catalysts with a 3 wt% metal loading

was prepared by a modified deposition-precipitation method.4,34 Typically, 50 mg of PN-CeO2 was suspended in 5 mL of HAuCl4 solution (7.6×10-3 mM) at room temperature for 1 h. After adding 200 mg of urea, the solution was heated to 80 oC 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 NaBH4 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/PN-CeO2 catalysts were washed for four times with distilled water and dried overnight at 60 oC. The Pd/PN-CeO2 catalyst with a 0.5% metal loading was prepared by the same process, except Na2PdCl4 was used for the precursor for metal deposition. 2.3 Synthesis of Pd/Au/PN-CeO2 catalysts. Photo-deposition of Pd nanoparticles on Au/PNCeO2 was conducted using a de-aerated aqueous methanol solution (5 vol %) of Na2PdCl4 under light irradiation. The Pd precursor was reduced by photo-generated electrons. Metallic Pd species were deposited on the surface of Au nanoparticles. The resultant powder was washed repeatedly with distilled water and then dried at 60 oC 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), K2CO3 (0.6 mmol) and catalysts (15 mg) were mixed in 2 mL of DMF and H2O solvent with a volume ratio of 1:1 in a 10 mL quartz Schlenk flask. 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

mechanical fan and the room temperature was 25 oC. The products were extracted with 6 mL ethyl acetate twice. After purification on a micro-column filled with silica gel, the products were analyzed by GC-MS and GC (Agilent 7890A GC and 597C MS, the column is HP-5MS). mXylene was used as the internal standard. 2.5 Characterization of Catalysts. The phase evolution of as-synthesized nanostructures was monitored by powder X-ray diffraction (XRD). The XRD patterns with diffraction intensity versus 2θ were recorded in a Shimadzu X-ray diffractometer (Model 6000) using Cu Kα radiation. Transmission electron microscopy (TEM) studies were conducted on a Hitachi HT7700 transmission electron microscope with an accelerating voltage of 120 kV. High resolution


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TEM was performed on JEM-2100F (JEOL, Japan) operating at 200 kV. X-ray photoelectric spectra (XPS) were acquired using a Thermo Electron Model K-Alpha with Al Kα as the excitation source. The contents of Pd and Au were determined by inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis (Agilent 7500ce) 3. RESULTS AND DISCUSSIONS The synthetic method and characterization of PN-CeO2 have been reported previously. Briefly, the CeO2/Ce(OH)3 nanorods as precursors were obtained in the first hydrothermal treatment at 100 °C, and the formation of PN-CeO2 were occurred at the second hydrothermal dehydration and oxidation at 160 °C. TEM image of PN-CeO2 (Figure S1a) shows a rod-like morphology with a clear porous structure. XRD pattern of PN-CeO2 exhibits a well defined cubic fluorite structure (Figure S1b). Au nanoparticles with a size of 4.28 ± 1.05 nm (Figure 1a) were deposited on the surface of PN-CeO2 by a deposition-precipitation method. High resolution 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 photo-deposition 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. Energy-dispersive X-ray spectroscopy (EDS) spectra of the noble metal nanoparticles (Figure S2) confirmed the co-existence of Au and Pd in one particle. The results indicate that Pd nanoparticles synthesized by photo-deposition 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 ICP-OES, the contents of Au and Pd in the catalysts were 2.95 wt% and 0.41 wt%, respectively. As a comparison, Au/PN-CeO2 (3%) and Pd/PN-CeO2 (0.5%, Figure S3) were also prepared by deposition-precipitation method.

In Figure 1e, PN-CeO2 exhibits weak visible light

absorption at wavelengths above 400 nm. After deposition of noble nanoparticles on PN-CeO2 supports, the catalysts display enhanced visible light absorption, compared with pure PN-CeO2. The absorption peak at 550 nm in the spectrum of Au/PN-CeO2 corresponds to the LSPR of Au nanoparticles. Although the characteristic absorption peak of Au nanoparticles in Pd/Au/PNCeO2 is not clear, it delivers the highest visible light adsorption. The SMC reactions between chlorobenzene and phenylboronic acid could not be occurred in dark by Au/PN-CeO2, Pd/PN-CeO2 and Pd/Au/PN-CeO2 catalysts (Table S1). Meanwhile, no catalytic activity was observed under visible light irradiation for 4 hours in the absence of catalysts or in the presence of pure PN-CeO2 catalysts (Table S1). In contrast, Pd/Au/PN-CeO2 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/PN-CeO2 under visible light irradiation (> 400 nm). With the increase of photo-irradiation 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/PN-CeO2 was observed for the SMC reaction between chlorobenzene and phenylboronic acid in the dark. The results further confirm the activation of chlorobenzene is realized by Pd/Au/PN-CeO2 under the visible light irradiation. Long-term stability and reusability are the most important features of heterogeneous catalysts. After photo-catalytic SMC reaction of chlorobenzene and phenylboronic acid, the Pd/Au/PNCeO2 catalysts were recovered from the reaction solution via centrifugation and reused for the same reaction.

As shown in Figure 2b, Pd/Au/PN-CeO2 catalysts delivered high catalytic

activity at least for 6 cycles. The unaltered morphology of used Pd/Au/PN-CeO2 catalysts 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/PN-CeO2 catalysts, Pd/PN-CeO2 (Pd loading 0.5 wt%) and Au/PN-CeO2 (Au loading 3 wt%) were also employed to catalyze the SMC reactions under the same conditions. For Pd/PN-CeO2, the yield increased very slowly and only reached 9.15% after 5 h under the visble light irradaition.

This can be attributed to the poor

responsiveness of visible light for Pd/PN-CeO2 catalysts and the SMC reaction of chlorobenzene cannot be efficiently activated under visible light irradiation.

For Au/PN-CeO2 catalysts,

although the yield of biphenyl was higher than that catalyzed by Pd/PN-CeO2, it tended to be saturated after 3 h reaction and reached at 37.6% after 5 h reaction. The higher catalytic activity of Au/PN-CeO2 catalysts over Pd/PN-CeO2 may be attributed to a better responsiveness towards visible light due to the strong LSPR of Au nanoparticles.35 Hence, more photo-electrons 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,18 and the final yield cannot reach a satisfactory value. Compared with the biphenyl yields catalyzed by Pd/PN-CeO2 (9.15%) and Au/PN-CeO2 (37.6%), the Pd/Au/PN-CeO2 shows the highest catalytic activity with a product 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/PN-CeO2 and Pd/PN-CeO2, indicating the high catalytic activity of Pd/Au/PNCeO2 for activation of aryl chlorides under mild conditions. The results ambiguously demonstrate the synergistic effect in Pd/Au/PN-CeO2 catalysts, which presents a surprising and high-efficient 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/PN-CeO2 were recorded under visible light irradiation at each wavelength 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 by the light scattering of the catalysts.21,36 Thus, Au nanoparticles in Pd/Au/PN-CeO2 catalysts function as the visible light absorber and transfer the excited electrons to Pd for chemical reactions. If the excited photo-electrons 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 C-C 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/PN-CeO2 under the irradiation of 420 nm incident light is comparable to that under the irradiation of 550 nm light (Figure 2c). It can be attributed to the relative high light absorption of PN-CeO2 near ultraviolet (UV) region as shown in Figure 1e. 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 PN-CeO2 at UV region, in which more photo-generated 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/PN-CeO2 and Au/PN-CeO2 under the same conditions. The contents of Pd in Pd/PN-CeO2 and Au in Au/PN-CeO2 catalysts are controlled as same as those in Pd/Au/PN-CeO2 catalysts. The mixed catalysts exhibited zero 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/PN-CeO2 and Au/PN-CeO2, respectively. No increase in catalytic activity for the mixed 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 catalysts.37,38 In our previous report, PN-CeO2 with a strong basicity can donate electrons to Pd catalysts and enhance their activity for C-C coupling reactions.32 Herein, XPS technique was used to evaluate the contribution of PN-CeO2 towards the electron density of noble metals. The zero valent state of Pd(3d5/2) and Au(4f7/2) binding energy is close to 335.1 eV and 84.0 eV,


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respectively.39,40 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(4f7/2) and Pd(3d5/2) are 335.5 eV and 84.3 eV, respectively, which demonstrates the high electron density of noble metals anchored on PN-CeO2. Due to the structural defects of oxygen vacancy in PN-CeO2 support, the band gap of 2.63 eV for PN-CeO2 is derived from UV-vis adsorption spectra (Figure S7), which is lower than the value of ideal CeO2 crystal (3.2 eV). As evidenced from absorption spectrum (Figure S7a), PNCeO2 can absorb part of visible light and generate electron/hole pairs. The photo-generated electrons from PN-CeO2 under visible light irradiation can be injected to Au nanoparticles, mediate the electron transfer and enrich the electron density of Pd nanocatalysts. In order to exam the role of PN-CeO2 support, Au/Pd nanocatalysts with a mass ratio of 1:6 (Figure S8) were synthesized according to a previous report.23 The SMC reaction between chlorobenzene and 4-methyphenylboronic acid was tested by Au/Pd nanocatalysts at the similar conditions. In the absence of PN-CeO2 support, 1.98% conversion of chlorobenzene was obtained after 5 h reaction under the visible light irradiation (Table S2). In contrast, the self-coupling product of 4methyphenylboronic acid was obtained with a high selectivity of 99.4%. Therefore, the presence of PN-CeO2 is critical and beneficial to increase the electron density of Pd nanocatalysts and 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 4-methylphenylboronic acid in the presence of electron scavenger KBrO3 at various concentrations. As shown in Table 1, the conversion of chlorobenzene was reduced from 89.3% to 42.5% and 10.5% under the visible light irradiation after adding 0.2 mmol and 2 mmol


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of KBrO3, respectively (Entry 1, 3 and 4). In contrast, the conversions of chlorobenzene were almost same for the SMC reactions performed at high temperature in the absence (Entry 2) or presence of various amount of KBrO3 (Entry 5, 6). The results clearly reveal that the photogenerated electrons indeed activate chlorobenzene at room temperature. Moreover, the barely changed catalytic activity of Pd/Au/PN-CeO2 in the absence or presence of KBrO3 for high 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 photo-generated electrons at room temperature can be further revealed from the selectivity of cross coupling products under the visible light irradiation. In the absence of KBrO3, the visible light irradiation resulted in a high selectivity of 95.5% towards the cross coupling product between chlorobenzene and 4-methylphenylboronic acid and a low selectivity of 4.5% for the self-coupling product of 4-methyphenylboronic acid (Entry 1). Increasing the amount of added KBrO3, the selectivity of the cross coupling product decreased from 95.5% (Entry 1) to 50.8% (Entry 3) and 16.7% (Entry 4), respectively. In contrast, the selectivity of the self-coupling product of 4-methyphenylboronic acid increased obviously from 4.5% (Entry 1) to 49.2% (Entry 3) and 83.3 % (Entry 4) with increasing amount of KBrO3 added, respectively. The observed selectivity can be attributed to the blocked pathway of the activation of chlorobenzene by the photo-generated electrons in the presence of electron scavenger KBrO3. Hence, the self-coupling reaction of 4-methyphenylboronic acid dominated the products. Control experiments of the SMC reaction activated by high temperature in the absence (Entry 2) and presence of KBrO3 (Entry 5, 6) delivered the similar selectivity for the cross coupling and self-coupling products, indicating KBrO3 did not interfere the selectivity of the SMC reactions. Therefore, the decreased conversion of chlorobenzene, the decreased selectivity of cross


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coupling product and the increased selectivity of the self-coupling products in the presence of electron scavenger KBrO3 confirm that the photo-generated electrons is necessary for activation of chlorobenzene during SMC reaction. 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 Pd6 cluster or on Pd6 cluster with one extra electron, Pd6 , were 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 gas-phase, the C−Cl bond of chlorobenzene adsorbed on Pd6 cluster slightly elongates to 1.779 Å. In −

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 Pd6 cluster decreases form 1.08 (C-Cl 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). chlorobenzene molecule adsorbs on Pd6

−

When

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 Pd6 cluster. Therefore, the negatively charged Pd cluster can benefit the activation of chlorobenzene. Next, the role of photo-generated holes from PN-CeO2 was also studied. Arylboronic acid can combine with OH- in basic solution and in principle adsorb to basic sites of PN-CeO2 via electrostatic interaction. When the photo-generated holes are diffused to the adsorbed sites of arylboronic acid, the adsorbed molecules could be oxidized accompanied with the cleavage of C-


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B bond.20 Therefore, arylboronic acid can be activated by the photo-generated holes under the visble light irradiation.

To demonstrate this, a common hole scavenger, methanol, was

introduced into the SMC reactions of chlorobenzene and 4-methyphenylboronic 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 42.4% and the appearance of self-coupling product of chlorobenzene also indicated that the activation of 4-methyphenylboronic 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 presence of various amounts of methanol at 100 oC under dark. The similar conversions of chlorobenzene and selectivity of cross coupling product for all reactions (Table 1, Entry 2, 9 and 10) indicated methanol did not affect the SMC reactions in both activity and selectivity. The above observations suggest that the photo-generated holes from PN-CeO2 are essential for activation of aryl boronic acid under visible light irradiation.

Similar results have been

previously reported for both homogeneous and heterogeneous activation of benzeneboronic acid under light irradiation.20,41,42 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 and loses d electrons whereas Pd loses s,p electrons but gains d electrons.43 For late-transition metals like Pd, the d-character is much more important than s, p-character in defining their chemisorption and catalytic properties.44 The Pd nanocatalysts in our synthesis are tightly deposited on the surface of Au nanoparticles through the photo-deposition. Therefore, the


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chemical potentials of the electrons in the Au and Pd nanoparticles are in equilibrium,23,45 indicating the easy electron transfer between Au and Pd. When the Pd/Au/PN-CeO2 catalysts is under the illumination of the visble light, the hots electrons generated from Au with an excited hot state for up to 0.5-1 ps46 will flow across the Au and Pd interface immediately, which enriches the electron density of Pd and enables SMC reactions. This possible electron transfer has also been proved both in previous experiments and theoretical calculation results.23,24 The hot electrons generated from Au nanoparticles also has the possibility to be injected into conduction band of ceria.2,47 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 electrons for recovery into the Au0 state. Therefore, the photo-generated electrons from PNCeO2 have to be injected into Au nanoparticles to mediate the electron transfer. The spatially selective deposition of Pd nanocatalysts on the surface of Au nanoparticles further confirms the possibility of electron transfer from PN-CeO2 to Au nanoparticles. 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 SMC reactions by accelerating the formation of active radical ligand ArPdIICl. Meanwhile, the electron/hole pairs are generated in PN-CeO2 upon absorption of the incident visible light. The generated-holes are used to activate various arylboronic acids by cleaving the C-B bonds. The


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generated-electrons 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. The reaction protocol was further extended to various aryl chlorides and substituted arylboronic acids to verify the scope of catalytic systems by Pd/Au/PN-CeO2 under the visible 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 C-Br and C-I bonds. With the strong electron-withdrawing group of –NO2 on aryl chloride, the conversion of aryl chloride and the yield of the cross coupling product can reach 91.6% and 89.6% for 6 h reaction (Entry 3), respectively.

With the substituent of –OCH3 (Entry 4) and –CH2OH (Entry 5) groups for

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 20.1% 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 electron-donating groups. The yields of the cross coupling products reached 99.0%, 96.8% and 76.8% for arylboronic acids with the substituents of –CH3 (Entry 7), aryl (Entry 8) and tertiary butyl (Entry 9), respectively. With the electronwithdrawing 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/PN-CeO2 catalysts and experimentally demonstrated 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 C-Cl bond of aryl chlorides. Photo-generated electrons from PN-CeO2 can be used to mediate the electron transfer process for SMC reaction.

Meanwhile, the arylboronic acids can be

activated by photo-generated holes from PN-CeO2 and transferred to adjacent Pd nanocatalysts. When the activated aryl chlorides and oxidized arylboronic acids meet on the surface of Pd, the C-C 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 PN-CeO2, EDS spectrum of Pd/Au/PN-CeO2, TEM image of Pd/PN-CeO2, TEM and HRTEM images of Pd/Au/PN-CeO2 catalysts undergoing 6 cycles of SMC reaction, XPS of Pd/Au/PN-CeO2 catalysts, UV-vis absorption spectra of PN-CeO2 in water, plot of (αhv)0.5 versus hv for PN-CeO2, TEM images of Au and Pd/Au nanoparticles, UV-vis 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 *E-mail: [email protected]. 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 973-program 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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(18) Choudary, B. M.; Madhi, S.; Chowdari, N. S.; Kantam, M. L.; Sreedhar, B. J. Am. Chem. Soc. 2002, 124, 14127-14136. (19) Martin, R.; Buchwald, S. L. Acc. Chem. Res. 2008, 41, 1461-1473. (20) Li, X. H.; Baar, M.; Blechert, S.; Antonietti, M. Sci. Rep. 2013, 3. 1743 (21) Xiao, Q.; Sarina, S.; Bo, A.; Jia, J.; Liu, H.; Arnold, D. P.; Huang, Y.; Wu, H.; Zhu, H. ACS Catal. 2014, 3, 1220-1230. (22) Fang, P. P.; Jutand, A.; Tian, Z. Q.; Amatore, C. Angew. Chem. Int. Ed. 2011, 50, 1218412188. (23) Wang, F.; Li, C.; Chen, H.; Jiang, R.; Sun, L. D.; Li, Q.; Wang, J.; Yu, J. C.; Yan, C. H. J. Am. Chem. Soc. 2013, 135, 5588-5601. (24) Sarina, S.; Zhu, H.; Jaatinen, E.; Xiao, Q.; Liu, H.; Jia, J.; Chen, C.; Zhao, J. J. Am. Chem. Soc. 2013, 135, 5793-5801. (25) Vivier, L.; Duprez, D. ChemSusChem 2010, 3, 654-678. (26) Sun, C.; Li, H.; Chen, L. Energy Environ. Sci. 2012, 5, 8475-8505. (27) Zhang, Y.; Zhang, N.; Tang, Z. R.; Xu, Y. J. ACS Sustainable Chem. Eng. 2013, 1, 12581266. (28) Tanaka, A.; Sakaguchi, S.; Hashimoto, K.; Kominami, H. ACS Catal. 2012, 3, 79-85. (29) Kominami, H.; Tanaka, A.; Hashimoto, K. Appl. Catal., A 2011, 397, 121-126. (30) Li, B.; Gu, T.; Ming, T.; Wang, J.; Wang, P.; Wang, J.; Yu, J. C. ACS Nano 2014, 8, 8152-8162. (31) Xiao, M.; Jiang, R.; Wang, F.; Fang, C.; Wang, J.; Yu, J. C. J. Mater. Chem. A 2013, 1, 5790-5805. (32) Zhang, S.; Li, J.; Gao, W.; Qu, Y. Nanoscale 2015, 7, 3016-3020.


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(33) Li, J.; Zhang, Z.; Tian, Z.; Zhou, X.; Zheng, Z.; Ma, Y.; Qu, Y. J. Mater. Chem. A 2014, 2, 16459-16466. (34) Hugon, A.; Delannoy, L.; Louis, C. Gold Bull. 2008, 41, 127-138. (35) Talley, C. E.; Jackson, J. B.; Oubre, C.; Grady, N. K.; Hollars, C. W.; Lane, S. M.; Huser, T. R.; Nordlander, P.; Halas, N. J. Nano Lett. 2005, 5, 1569-1574. (36) Wustholz, K. L.; Henry, A. I.; McMahon, J. M.; Freeman, R. G.; Valley, N.; Piotti, M. E.; Natan, M. J.; Schatz, G. C.; Duyne, R. P. V. J. Am. Chem. Soc. 2010, 132, 10903-10910. (37) Shi, J. Chem. Rev. 2012, 113, 2139-2188. (38) Chng, L. L.; Erathodiyil, N.; Ying, J. Y. Acc. Chem. Res. 2013, 46, 1825-1837. (39) Murdoch, M.; Waterhouse, G.; Nadeem, M.; Metson, J.; Keane, M.; Howe, R.; Llorca, J.; Idriss, H. Nat. Chem. 2011, 3, 489-492. (40) Chen, L.; Chen, N.; Hou, Y.; Wang, Z.; Lv, S.; Fujita, T.; Jiang, J.; Hirata, A.; Chen, M. ACS Catal. 2013, 3, 1220-1230. (41) Zou, Y. Q.; Chen, J. R.; Liu, X. P.; Lu, L. Q.; Davis, R. L.; Jorgensen, K. A.; Xiao, W. J. Angew. Chem. Int. Ed. 2012, 51, 784-788. (42) Pitre, S. P.; McTiernan, C. D.; Ismaili, H.; Scaiano, J. C. J. Am. Chem. Soc. 2013, 135, 13286-13289. (43) Gao, F.; Goodman, D. W. Chem. Soc. Rev. 2012, 41, 8009-8020. (44) Liu, P.; Nørskov, J. K. Phys. Chem. Chem. Phys. 2001, 3, 3814-3818. (45) Hu, J. W.; Li, J. F.; Ren, B.; Wu, D. Y.; Sun, S. G.; Tian, Z. Q. J. Phys. Chem. C 2007, 111, 1105-1112. (46) Yamada, K.; Miyajima, K.; Mafuné, F. J. Phys. Chem. C 2007, 111, 11246-11251.


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(47) Primo, A.; Marino, T.; Corma, A.; Molinari, R.; Garcia, H. J. Am. Chem. Soc. 2011, 133, 6930-6933.


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Figure 1. TEM images and HRTEM images of (a, b) Au/PN-CeO2 and (c, d) Pd/Au/PN-CeO2 catalysts. (e) UV-vis 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/PN-CeO2 catalysts for SMC reactions of chlorobenzene and phenylboronic acid; (c) the wavelength dependent activity of biphenyl yield by Pd/Au/PN-CeO2 catalysts; (d) the time course of biphenyl yield by mixed catalysts and the total of biphenyl yield by pure Au/PN-CeO2 and Pd/PN-CeO2 catalysts. Reaction conditions: 1 mL of water, 1 mL of DMF, 0.6 mmol K2CO3, 0.2 mmol of chlorobenzene, 0.24 mmol of phenylboronic and 15 mg of catalysts. Yields were determined by GC-MS and GC with m-xylene as the interior label.


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Figure 3. Schematic view of the proposed photo-catalytic reaction mechanism over the Pd/Au/PN-CeO2 catalysts under visible light irradiation.


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Table 1. The SMC reaction of chlorobenzene and 4-methylphenylboronic acid with different addition * Cl

+

B(OH)2

+

+

Selectivity (%) Reaction KBrO3 MeOH Conversion model (mmol) (mL) (%) 1 hv 89.3 Trace 95.5 4.5 2 100 oC 93.5 8.5 78.3 13.2 3 hv 0.2 42.5 Trace 50.8 49.24 4 hv 2 10.5 Trace 16.7 83.3 o 5 100 C 0.2 94.7 10.3 73.5 16.2 2 95.2 9.6 72.8 17.6 6 100 oC 7 hv 0.5 72.8 47.71 48.39 3.9 8 hv 2 35.9 52.3 42.4 5.3 o 0.5 85.6 9.6 74.7 15.7 9 100 C 10 100 oC 2 86.0 10.9 70.5 18.6 * Reaction condition: 1 mL of water, 1 mL of DMF, 0.6 mmol K2CO3, 0.2 mmol of chlorobenzene, 0.24 mmol of 4-methylphenylboronic acid and 15 mg of Pd/Au/PN-CeO2 catalysts. Entry


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Table 2. The scope of Pd/Au/PN-CeO2 catalysts for the SMC reaction * R1

X

Pd/Au/PN-CeO2

+

R2

Substrate

Entry

R1

B(OH)2

R2

Time Conversion (h) (%)

Product

Yield (%)

1

I

0.5

99.1

98.1

2

Br

1

98.6

98.3

3

O2N

Cl

O2N

6

91.6

89.6

4

O

Cl

O

6

88.6

80.6

5a

H2 HO C

H2 HO C

6

58.1

56.8

6

20.1

15.3

5

99.1

99.0

5

96.8

96.8

5

78.9

76.9

5

50.5

48.7

6a

Cl

7

B(OH)2

8

B(OH)2

9 10

Cl

B(OH)2

OHC

B(OH)2

CHO

* Reaction conditions: 1 mL of water, 1 mL of DMF, 0.6 mmol K2CO3, 0.2 mmol of aryl chlorides, 0.24 mmol of arylboronic acids and 15 mg of catalysts. Yields were determined by GC-MS and GC with m-xylene as the interior label. a With 0.3 mmol of arylboronic acids.


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TOC Graphic


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