English

• 0.   Introduction

  C-C couplings of aryl halides containing Sonogas-hira, Suzuki and Heck coupling reactions have great significance for the development of organic chem- istry^[1-5]. To our knowledge, C(sp²)-C(sp) bonds are usually intermediates or precursors for synthesizing natural products, bioactive drugs, and other mater- ials^[6-8]. Remarkably, one of the most common methods to form C(sp²)-C(sp) bonds is Sonogashira coupling reaction^[9-10]. The classic Pd(Ⅱ) complexes with Cu(Ⅰ) salt as co-catalysts have been employed to catalyze the Sonogashira reaction through homogeneous process such as Pd(PPh₃)₂Cl₂/CuI/Et₃N (Et₃N=triethylamine) combination^[11]. Despite homogeneous catalysts exhibit superior catalytic activity, it remains challenging for the recovery and reuse of catalysts in which metal ions (e.g. Pd²⁺, Cu⁺) may contaminate the target products^[9]. Subsequently, a series of palladium nanoparticles as heterogeneous catalysts have been designed and synthesized, which have potential to take the place of palladium complexes to solve these problems^[12-14]. The use of supported catalysts shows growing importance for product purification and catalyst reuse. However, preparation of some supported catalysts is inefficient and the unclearly active sites of metal nanoparticles can complicate their catalytic mechanisms^[15]. Therefore, simple and efficient methods are promising for prepar-ing heterogeneous catalysts with definite structures. Fortunately, the atomically precise metal nanoclusters with well-defined structures can be chosen as model catalysts to provide a new perspective, which is helpful to understand the relationship between struc-tures and properties at atomic level^[16-18]. For example, Jin et al. reported that the Au3 facet on spherical Au₂₅(SR)₁₈ and the waist sites of rod-shaped Au₂₅(PPh₃)₁₀(C≡CPh)₅X₂ were the active sites in catalytic the semihydrogenation of terminal alkynes^[19]. Zhu′s group confirmed one-core-atom loss could boost hydroamina-tion reaction of alkynes by comparing catalytic perfor-mance between Au₂₅ and Au₂₄ nanoclusters^[20].

  In addition, the harsher conditions are sometimes required in order to efficiently conduct Sonogashira cross-coupling reactions, such as high temperature^[21-22], harmful solvents^[23-25] and larger amount of catalysts^[26]. Hence, the development of eco-friendly and moderate systems is also highly desirable.

  In this work, atomically precise Pd₃Cl nano-clusters with positive charges and rod-shaped SBA-15 with negative charges are selected to fabricate well-defined Pd₃Cl/SBA-15 catalysts by electrostatic attrac-tion strategy. The Pd₃Cl nanoclusters are successfully and readily supported on SBA-15 rods with high dispersity. The as-obtained Pd₃Cl/SBA-15 catalysts showed good catalytic activity by using water as green solvent at 55 ℃. The structure of Pd₃Cl nanoclusters consist of Pd₃ kernel protected by phosphine ligands and one chlorine atom. We demonstrated that the internal Pd atoms with positive charge due to bridging ligands can synergistically promote catalytic perfor-mance.

  1.   Experimental

  1.1   Synthesis

  1.1.1   Preparation of Pd₃Cl nanocluster

  The Pd₃Cl clusters were achieved by modifying our previous strategy^[27]. PdCl₂ (0.710 g, 4 mmol) was dissolved with hydrochloric acid (665 μL, 12 mol·L^-1), and the above solution was diluted to 5 mL with deionized water. 0.6 mL of as-prepared solution (0.6 mL, 0.48 mmol) was added into 10 mL of tetrahydro-furan (THF) containing triphenylphosphine (0.310 g, 1.2 mmol). After 10 minutes for vigorous stirring, NaBH₄ (80 mg, 2.1 mmol) dissolved in ethanol (5 mL) was added to the above solution. The reaction was maintained for 2 hours under vigorous stirring. The reaction solution was then centrifuged to remove the precipitate and evaporated the THF solvent with rotary evaporator. After that, the product was washed with ethanol for several times.

  1.1.2   Preparation of SBA-15

  4.0 g of copolymer P123 (EO₂₀PO₇₀EO₂₀, M_a=5 800) was dissolved with 30 mL water and HCl (120 mL, 2.0 mol·L^-1). After that, the above solution was stirred for 4 h. Subsequently, 8.5 g of TEOS (tetra ethyl oxysilane) was added and the solution was stirred at 1 100 r·min^-1 for 5 min. Then, the reaction system was kept for 24 h at 313 K. After that, the above solution was transferred to the Teflon-lined autoclave, and further treated at 403 K for 24 h. The final product was washed with water for several times, and calcined at 773 K for 6 h.

  1.1.3   Preparation of the Pd₃Cl/SBA-15 catalysts

  100 mg of SAB-15 was dispersed in CH₂Cl₂ (10 mL) under ultrasonication for a few minutes. 5 mg of Pd₃Cl nanoclusters in CH₂Cl₂ was added into to the above solution under magnetic stirring for 45 minutes. Pd₃Cl/SBA-15 catalysts were then obtained by centrifugation at 10 000 r·min^-1.

  1.2   Characterization methods

  Silica gel 60 F254 and Merck Kieselgel 200-300 were chosen for analytical thin-layer chromatography (TLC) and prepared column chromatography. The yield was determined on Shimadzu GC 2010 plus by using flame ionization detector. UV-Vis spectra were measured on Techcomp UV1000 spectrophotometer from 300 to 700 nm. ¹H NMR and ¹³C NMR spectra were recorded on a Bruker AM 400 spectrometer which were operated at 400 and 100 MHz respectively (CDCl3 as solvent). The Fourier transform infrared (FT-IR) spectra were conducted on Bruker Tensor 27 instrument in the range of 4 000~200 cm^-1. Mass spectrometric analysis was performed by matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS). The X-ray diffraction (XRD) patterns were recorded on SmartLab 9 KW with Cu Kα radiation (λ=0.154 nm), and the 2θ range (40 kV and 100 mA) was from 10° to 90° in the step of 10°·min^-1. Transmission electron microscopy (TEM) and EDS (energy dispersive X-ray spectroscopy) mapping were performed in a JEM-2010 microscope with an accelerating voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) measurements were acquired on ESCALAB 250Xi with Al Kα radiation. Pd₃Cl nanoclusters loaded on the SBA-15 catalysts were measured by Inductively Coupled Plasma Mass Spectrometry (ICP-MS). ζ-potential analysis was determined by Malvern analyzer under an aqueous solution. Electrospray ionization mass spectrometry (ESI-MS) was recorded on MicroTOF-QIII high-resolution mass spectrometer, and the samples were infused into the chamber at 5 μL·min^-1 directly. The N₂ adsorption/desorption isotherms were performed at liquid nitrogen temperature (-196 ℃) on a Micro-meritics ASAP 2020M+C instrument, and the surface area of samples were calculated by Brunauer-Emmett-Teller (BET) theory.

  1.3   Typical procedure for Sonogashira reaction

  0.5 mmol phenylacetylene (55 μL), 0.5 mmol iodobenzene (56 μL), Pd₃Cl/SBA-15 catalysts (30 mg), and water (1 mL) were added into 10 mL shrek tube at 55 ℃ for 12 h in Ar. The final reaction product was obtained by centrifugation, and the yield was tested by GC (gas chromatography) equipment. The recovered catalyst was washed with n-hexane for next run under the same reaction conditions.

  2.   Results and discussion

  2.1   Design and characterization of Pd₃Cl/SBA-15 catalyst

  The Pd₃Cl/SBA-15 catalysts can be fabricated through electrostatic attraction between positively charged Pd₃Cl clusters and negatively charged SBA-15 rods (Scheme 1).

  Scheme 1

  

  Scheme 1.  Schematic illustration for the formation of Pd₃Cl/SBA-15 by electrostatic attraction strategy

  下载: 全尺寸图片 幻灯片

  To demonstrate the feasibility of the proposed strategy, atomically precise Pd₃Cl nanoclusters were initially synthesized. UV-Vis spectrum of as-prepared Pd₃Cl nanoclusters (Fig. 1a, the red line) exhibited three distinctive absorption peaks at about 340, 418, and 485 nm. As depicted in the matrix-assisted laser desorption ionization mass spectrum (MALDI-MS), the as-obtained Pd₃Cl nanoclusters present a molecular ion peak at 1 511 (m/z) (theoretical M_w=1 511), and two fragments were captured for losing of the PPh₃ units (Fig. 1b, the blue line). TEM shows that the average size of as-prepared Pd₃Cl clusters is around 1.4 nm (Fig. 2a) and the size distribution pattern was shown in Fig.S1. In addition, Pd₃Cl were maintained at 150 ℃ in Ar to test the thermal stability. Notably, the UV-Vis spectrum of the Pd₃Cl nanocluster after thermal treatment is same to that of fresh Pd₃Cl nanocluster (Fig. 1a, the blue line). The MALDI-MS analysis also showed a sharp peak at 1 511 (m/z) after thermal treatment (Fig. 1b, the red line). Therefore, Pd₃Cl possessed good thermal stability and the structure of Pd₃Cl remained intact in the process of anneal treatment.

  Figure 1

  

  Figure 1.  (a) UV-Vis absorption spectra of the as-prepared Pd₃Cl nanocluster (in dichloromethane, red line) and the redissolved nanoclusters after thermal treatment at 150 ℃ in Ar for 1 h (blue line); (b) Positive mode MALDI mass spectrum of as-prepared Pd₃Cl nanocluster and after thermal treatment, the peak except the position of 1511 is the fragment of Pd₃Cl

  下载: 全尺寸图片 幻灯片

  Figure 2

  

  Figure 2.  (a) TEM image of as-prepared Pd₃Cl nanoclusters; (b) SEM image of the as-obtained SBA-15 rods; (c) EDS elemental mapping images of the as-prepared Pd₃Cl/SBA-15 catalysts

  下载: 全尺寸图片 幻灯片

  Furthermore, the synthesis of SBA-15 materials followed the previously reported procedure^[28]. The SEM (Fig. 2b) and TEM (Fig.S2) images show that the as-prepared SBA-15 present rode-like morphologies with well-ordered mesoporous structures. As displayed in Fig.S3, the broad diffraction peak of SBA-15 was assigned to the characteristic peak of amorphous silica. The surface ζ potential of SBA-15 showed -20.1 mV that evidenced SBA-15 with negative charge surface (Fig.S4).

  To obtain further insight into the catalysts, various characterization techniques were employed to determine the as-prepared Pd₃Cl/SBA-15 samples. The elemental maps of Si, O and Pd of Pd₃Cl/SBA-15 catalyst were shown in Fig. 2c. From the elements mapping, Pd₃Cl nanoclusters were uniformly dispersed on a SBA-15 support by electrostatic attraction strategy. The loading of catalyst is 0.8%(w/w) palladium by Inductively Coupled Plasma Mass Spectrometry (ICP-MS). No obvious change was observed from the XRD pattern after Pd₃Cl loading, which suggests that the SBA-15 rods are chemically stable (Fig.S3). The XPS directly illustrates the inexistence of any interaction between Pd₃Cl nanoclusters and SBA-15 supports during loading, for the reason that the Pd₃d bond energy (Pd₃Cl/SBA-15) agreed well with that of fresh Pd₃Cl (Fig.S5). The Fourier transform infrared spectroscopy (FT-IR) confirmed the successful fusion of Pd₃Cl and SBA-15 (Fig.S6). N₂ adsorption and desorption Isotherms of SBA-15 and Pd₃Cl/SBA-15 catalyst were shown in Fig.S7. The BET surface area of SBA-15 rod was determined to be 588 m²·g^-1, which decreased to 468 m²·g^-1 after the loading of Pd₃Cl nanoclusters. In general, the structural integrity of Pd₃Cl nanoclusters supported on SBA-15 was maintained.

  2.2   Catalytic properties of Pd₃Cl/SBA-15 for sonogashira C-C coupling

  We selected Sonogashira C-C coupling as a model reaction to explore the catalytic performance of the as-synthesized Pd₃Cl/SBA-15 catalysts. To increase sustainability of the reaction system, water was used as environment-friendly solvent. Next, the impact of diversified bases was initially evaluated. We found that the reaction activity reached only 20%/25% using K₂CO₃ or Cs₂CO₃ as base at 65 ℃ (Table 1, entries 1~2), and without base, products were not detected (Table 1, entry 3). Interestingly, the yield was markedly improved to 90% in the presence of Et₃N as a base (Table 1, entry 4). The highest yield (92.8%) was achieved when the temperature decreased to 55 ℃ (Table 1, entry 5). However, with a further decrease in temperature, the activity was obviously decreased to 72% (Table 1, entry 7). Under optimal temperature (55 ℃), 82.4% of product was produced with pyrrolidine as base (Table 1, entry 6). Then, 10 or 20 mg of Pd₃Cl/SiO₂ were used, and the lower yield was produced (Table 1, entries 8~9). Eventually, entry 5 of Table 1 was chose as the optimum conditions for Pd₃Cl/SBA-15-catalyzed Sonogashira reaction. For comparison, we also investigated the catalytic activity of Pd₃Cl/CeO₂ catalyst, which have a lower yield (88%, Table 1, entry 10) than that of Pd₃Cl/SBA-15 catalyst (92.8%, Table 1, entry 5). We also directly grew ZIF-8 outside the heterogeneous Pd₃Cl/CeO₂ catalyst (denoted as Pd₃Cl/CeO₂@ZIF-8). The yield (77%, Table 1, entry 11) was lower than that of Pd₃Cl/CeO₂ catalysts. Because the small ZIF-8 cores (< 2 nm) limited the access of reactants to Pd₃Cl nanoclusters and the diffusion of reactants inside the ZIF-8 pores^[29]. The TEM image of Pd₃Cl/SBA-15@ZIF-8 was shown in Fig.S8. It was worth noting that no product was observed in blank experiments, in which Pd₃Cl nanoclusters were absent and SBA-15 or CeO₂ oxide were present (Table 1, entry 12~13). This demonstrated that catalytic activity arose out of the Pd₃Cl nanoclusters.

  Table 1

  

  Table 1.  Sonogashira cross-coupling reaction with Pd₃Cl/SAB-15 and related catalysts^a

  下载: 导出CSV

  Entry	Catalyst	Temperature / ℃	Base	Yield / %b
  1	Pd3Cl/SBA-15	65	K2CO3	20
  2	Pd3Cl/SBA-15	65	Cs2CO3	25
  3	Pd3Cl/SBA-15	65	none	n.r.
  4	Pd3Cl/SBA-15	65	Et3N	90
  5	Pd3Cl/SBA-15	55	Et3N	92.8
  6	Pd3Cl/SBA-15	55	C4H9N	82.4
  7	Pd3Cl/SBA-15	40	Et3N	72
  8	Pd3Cl/SBA-15	55	Et3N	87c
  9	Pd3Cl/SBA-15	55	Et3N	85d
  10	Pd3Cl/CeO2	55	Et3N	88
  11	Pd3Cl/CeO2@ZIF-8	55	Et3N	77
  12	SBA-15	55	Et3N	n.r.e
  13	CeO2	55	Et3N	n.r.f
  14	Pd3Cl/SBA-15-400	55	Et3N	67
  15	Pd3Cl/CeO2-400	55	Et3N	71
  a Reaction conditions: iodobenzene (0.5 mmol), phenylacetylene (0.5 mmol), base (1 mmol), H2O (1 mL), 12 h, 30 mg catalyst, 0.8%(w/w) of palladium, 0.753 μmol of Pd3Cl, 1.4×10-5(n/n) of substrate; b Yield was detected by GC; c 20 mg of Pd3Cl/SiO2; d10 mg of Pd3Cl/SiO2; e 30 mg SBA-15 was used, n.r.=no reaction; f 30 mg CeO2 was used

  2.3   Scope of the catalytic reaction: derivatives of aryl iodide and phenylacetylene

  In order to further evaluate Pd₃Cl/SBA-15 catalysts, we extended the study to different combina-tions of aryl iodides and alkynes under the optimized conditions. Whatever electron-rich or electron-poor phenylacetylene derivatives showed satisfactory yield (Table 2, entries 1~3). Both iodobenzene with electron-withdrawing and that with electron-donating groups were examined. Notice that, aryl iodide with withdraw-ing group showed excellent yields (93.4%, Table 2, entry 4). In contrast, when iodobenzene was attached with the electron-donating group, the yield varied from 93.4% to 81.3% (Table 2, entry 5). Lower activity (67.1%, Table 2, entry 6) was observed when iodo-benzene with stronger electron-donating group for Sonogashira coupling reaction.

  Table 2

  

  Table 2.  Catalytic performance of difference substrates with Pd₃Cl/SBA-15^a

  下载: 导出CSV

  Entry	Aryl halide	Terminal alkyne	Product	Yieldb / %
  1				90.3
  2				91.2
  3				88.5
  4				93.4
  5				81.3
  6				67.1
  a Reaction conditions: 30 mg Pd3Cl/SBA-15 catalyst, 0.5 mmol aryl halide, 0.5 mmol terminal alkyne, 1 mL H2O, 55 ℃ reaction temperature, 12 h; b Yield was detected by GC

  The recyclability of the Pd₃Cl/SBA-15 catalyst was examined (Fig. 3). In this test, the Pd₃Cl/SBA-15 catalyst was recovered by centrifugation after Sono-gashira reaction for next run. We found that the activity of the catalyst decreased only 8% even after eight cycles, which suggested the high stability of the designed catalyst based on electrostatic attraction strategy.

  Figure 3

  

  Figure 3.  Recyclability of Pd₃Cl/SBA-15 catalyst in Sonogashira C-C coupling reaction

  下载: 全尺寸图片 幻灯片

  2.4   Evidence for the importance of structural integrity of Pd₃Cl nanocluster

  To understand the nature of Pd₃Cl/SBA-15 catalysts after reaction, several experiments were performed. Any shift of Pd3d energy was not observed from XPS spectra (Fig.S5). The XRD patterns (Fig.S3) and FT-IR spectra (Fig.S6) also remained almost unchanged before and after reaction, indicating the integrity of Pd₃Cl nanocluster was preserved during reaction process.

  To further reveal the relationship between catalytic property and structure of nanocluster, Pd₃Cl/SBA-15 catalysts were calcined at 400 ℃ in Ar atmosphere. It was worth mentioned that the activity decreased to 67% after calcination at 400 ℃ (Table 1, entry 14). Not surprisingly, similar results were observed when calcined Pd₃Cl/CeO₂ at 400 ℃ (71%, Table 1, entry 15).

  Furthermore, the binding energy of Pd3d_5/2 (as-prepared Pd₃Cl/SBA-15) was 337.1 eV from XPS result and agree well with the fresh Pd₃Cl nanocluster, indicating that Pd was positively charged due to the interaction between Pd and the ligands (Fig. 4(a, b)). The bond energy of Pd3d_5/2 decreased to 335.9 eV after calcination Pd₃Cl/SBA-15 at 400 ℃ from XPS spectrum (Fig. 4c). According to previous report, we classified 335.9 eV as the binding energy of Pd(0), which indicated that the ligands on the surface of Pd₃Cl nanocluster were removed and Pd^δ+ were reduced to a metallic state, while the Pd3d_5/2 peak at 337.5 eV was assigned to Pd^{2+ [30]}. Meanwhile, we also found that Pd(0) was produced from the XRD pattern (Fig. 5), in which the position of 40° was the (111) plane of Pd(0) (PDF No.88-2335)^[31]. Both of these suggested the Pd(0) atoms were exposed on the cluster surface through heat treatment, which were responsible for the decrease in activity. However, this was contrary to the conven-tional opinion that the higher catalytic performance could be obtained when more surface atoms were bare^[32]. The above results indicated the importance of intact structure of Pd₃Cl nanocluster for catalytic performance. The synergistic effect between Pd^δ+ (0 < δ < 2) and the ligand is the key to the catalytic reaction.

  Figure 4

  

  Figure 4.  XPS spectra in Pd₃d: (a) Fresh Pd₃Cl/SBA-15 catalyst; (b) As-prepared Pd₃Cl/SBA-15; (c) Pd₃Cl/SBA-15 calcined by 400 ℃

  下载: 全尺寸图片 幻灯片

  Figure 5

  

  Figure 5.  XRD patterns of Pd₃Cl/SBA-15 and Pd₃Cl/SBA-15 calcined by 400 ℃

  下载: 全尺寸图片 幻灯片

  We tried to investigate the catalytic process of Sonogashira cross-coupling reaction through homo-geneous Pd₃Cl nanocluster. The FT-IR spectra of phenylacetylene+Pd₃Cl+Et₃N were investigated (Fig.S9), the C≡C stretching increased from 2 111 to 2 118 cm^-1 and the position of C≡C-H stretching disapp-eared. This suggests that the H atom from C≡C-H group is removed and then may interact with the nanocluster to form an intermediate^[33]. Furthermore, the Electrospray mass spectrometry (ESI-MS) was adopted for insight into the intermediate. In the presence of Et₃N, Pd₃Cl can interact with phenylacetylene, and new peak appear at m/z=1 577.1 referred to Pd₃…C≡C-Ph, and the simulation result agree well with exper-imental result (Fig.S10). Based on above experimental results and previous reports^[27], the terminal alkyne may be activated with a base for forming Pd₃…C≡C-Ph, which further interacted with iodobenzene to generate products.

  3.   Conclusions

  Well-defined Pd₃Cl/SBA-15 catalysts have been designed and readily synthesized by electrostatic attraction between positively charged atomically precise Pd₃Cl clusters and negatively charged mesoporous SBA-15 rods. It was found that the recyclable hetero-geneous Pd₃Cl/SBA-15 catalysts were robust, stable, and presented efficient catalytic activities for the water-mediated Sonogashira C-C coupling reaction. We demonstrated the structure integrity of Pd₃Cl and the synergistic effect between the ligand and Pd^δ+ (0 < δ < 2) are key for the excellent stability and high activity in Sonogashira reaction. This synthetic strategy may also be applicable to design other atomically precise metal cluster-based heterogeneous catalysts.

  Conflict of interest: The authors declare that they have no conflict of interest.

  Supporting information is available at http://www.wjhxxb.cn

  
  
• 1. [1]

     Chinchilla R, Najera C. Chem. Rev., 2007, 107(3):874-922 doi: 10.1021/cr050992x

  2. [2]

     Biffis A, Centomo P, Zotto A D, et al. Chem. Rev., 2018, 118(4):2249-2295 doi: 10.1021/acs.chemrev.7b00443

  3. [3]

     Chinchilla R, Najera C. Chem. Rev., 2014, 114(3):1783-1826 doi: 10.1021/cr400133p

  4. [4]

     Yuan B Z, Pan Y Y, Li Y W, et al. Angew. Chem. Int. Ed., 2010, 49(24):4054-4058 doi: 10.1002/anie.201000576

  5. [5]

     Lin J Z, Abroshan H, Liu C, et al. J. Catal., 2015, 330:354-361 doi: 10.1016/j.jcat.2015.07.020

  6. [6]

     Paterson I, Davies R D M, Marquez R. Angew. Chem. Int. Ed., 2001, 40(3):603-607 doi: 10.1002/1521-3773(20010202)40:3<603::AID-ANIE603>3.0.CO;2-O

  7. [7]

     Bagley M C, Dale J W, Merritt E A, et al. Chem. Rev., 2005, 105(2):685-714 doi: 10.1021/cr0300441

  8. [8]

     Brunsveld L, Meijer E W, Prince R B, et al. J. Am. Chem. Soc., 2001, 123(33):7978-7984 doi: 10.1021/ja010751g

  9. [9]

     Karak M, Barbosa L C A, Hargaden G C. RSC Adv., 2014, 4(96):53442-53466 doi: 10.1039/C4RA09105A

  10. [10]

      Nicolaou K C, Bulger P G, Sarlah D. Angew. Chem. Int. Ed., 2005, 44(29):4442-4489 doi: 10.1002/anie.200500368

  11. [11]

      Dissanayake K C, Ebukuyo P O, Dhahir Y J, et al. Chem. Commun., 2019, 55(34):4973-4976 doi: 10.1039/C9CC01365B

  12. [12]

      Kozell V, McLaughlin M, Strappaveccia G, et al. ACS Sustainable Chem. Eng., 2016, 4(12):7209-7216 doi: 10.1021/acssuschemeng.6b02170

  13. [13]

      Seth J, Kona C N, Das S, et al. Nanoscale, 2015, 7(3):872-876 doi: 10.1039/C4NR04239E

  14. [14]

      Elhage A, Lanterna A E, Scaiano J C. ACS Sustainable Chem. Eng., 2018, 6(2):1717-1722 doi: 10.1021/acssuschemeng.7b02992

  15. [15]

      Zhang X Y, Sun Z C, Wang B, et al. J. Am. Chem. Soc., 2018, 140(3):954-962 doi: 10.1021/jacs.7b09314

  16. [16]

      Zhou Y, Li G. Acta Phys. Chim. Sin., 2017, 33(7):1297-1309

  17. [17]

      Chakraborty I, Pradeep T. Chem. Rev., 2017, 117(12):8208-8271 doi: 10.1021/acs.chemrev.6b00769

  18. [18]

      Jin R C, Zeng C J, Zhou M, et al. Chem. Rev., 2016, 116(18):10346-10413 doi: 10.1021/acs.chemrev.5b00703

  19. [19]

      Li G, Jin R C. J. Am. Chem. Soc., 2014, 136(32):11347-11354 doi: 10.1021/ja503724j

  20. [20]

      Wang L Q, Shen K Q, Chen M Y, et al. Nanoscale, 2019, 11(29):13767-13772 doi: 10.1039/C9NR04219A

  21. [21]

      Yin L X, Liebscher J. Chem. Rev., 2007, 107(1):133-173 doi: 10.1021/cr0505674

  22. [22]

      Tukhani M, Panahi F, Khalafi-Nezhad A. ACS Sustainable Chem. Eng., 2018, 6(1):1456-1467 doi: 10.1021/acssuschemeng.7b03923

  23. [23]

      Li G, Jiang D E, Liu C, et al. J. Catal., 2013, 306:177-183 doi: 10.1016/j.jcat.2013.06.017

  24. [24]

      Gholinejad M, Bahrami M, Nájera C, et al. J. Catal., 2018, 363:81-91 doi: 10.1016/j.jcat.2018.02.028

  25. [25]

      Moon J, Jang M, Lee S. J. Org. Chem., 2009, 74(3):1403-1406 doi: 10.1021/jo802290r

  26. [26]

      Li Z M, Yang X J, Liu C, et al. Prog. Nat. Sci.:Mater. Int., 2016, 26(5):477-482 doi: 10.1016/j.pnsc.2016.09.007

  27. [27]

      Fu F Y, Xiang J, Cheng H, et al. ACS Catal., 2017, 7(3):1860-1867 doi: 10.1021/acscatal.6b02527

  28. [28]

      Wang Q, Wang Z C, Zheng T H, et al. Nano Res., 2016, 9(8):2294-2302 doi: 10.1007/s12274-016-1116-8

  29. [29]

      Teng J, Chen M Q, Xie Y Y, et al. Chem. Mater., 2018, 30(18):6458-6468 doi: 10.1021/acs.chemmater.8b02884

  30. [30]

      Li Z P, Yang X C, Tsumori N, et al. ACS Catal., 2017, 7(4):2720-2724 doi: 10.1021/acscatal.7b00053

  31. [31]

      Wang Z C, Chen W, Han Z L, et al. Nano. Res., 2014, 7(9):1254-1262 doi: 10.1007/s12274-014-0488-x

  32. [32]

      Li Q, Das A, Wang S X, et al. Chem. Commun., 2016, 52(99):14298-14301 doi: 10.1039/C6CC07825G

  33. [33]

      Liu Y Y, Chai X Q, Cai X, et al. Angew. Chem. Int. Ed., 2018, 57(31):9775-9779 doi: 10.1002/anie.201805319

• 

  Scheme 1  Schematic illustration for the formation of Pd₃Cl/SBA-15 by electrostatic attraction strategy

  下载: 全尺寸图片 幻灯片
  

  Figure 1  (a) UV-Vis absorption spectra of the as-prepared Pd₃Cl nanocluster (in dichloromethane, red line) and the redissolved nanoclusters after thermal treatment at 150 ℃ in Ar for 1 h (blue line); (b) Positive mode MALDI mass spectrum of as-prepared Pd₃Cl nanocluster and after thermal treatment, the peak except the position of 1511 is the fragment of Pd₃Cl

  下载: 全尺寸图片 幻灯片
  

  Figure 2  (a) TEM image of as-prepared Pd₃Cl nanoclusters; (b) SEM image of the as-obtained SBA-15 rods; (c) EDS elemental mapping images of the as-prepared Pd₃Cl/SBA-15 catalysts

  下载: 全尺寸图片 幻灯片
  

  Figure 3  Recyclability of Pd₃Cl/SBA-15 catalyst in Sonogashira C-C coupling reaction

  下载: 全尺寸图片 幻灯片
  

  Figure 4  XPS spectra in Pd₃d: (a) Fresh Pd₃Cl/SBA-15 catalyst; (b) As-prepared Pd₃Cl/SBA-15; (c) Pd₃Cl/SBA-15 calcined by 400 ℃

  下载: 全尺寸图片 幻灯片
  

  Figure 5  XRD patterns of Pd₃Cl/SBA-15 and Pd₃Cl/SBA-15 calcined by 400 ℃

  下载: 全尺寸图片 幻灯片

  Table 1.  Sonogashira cross-coupling reaction with Pd₃Cl/SAB-15 and related catalysts^a

  Entry	Catalyst	Temperature / ℃	Base	Yield / %b
  1	Pd3Cl/SBA-15	65	K2CO3	20
  2	Pd3Cl/SBA-15	65	Cs2CO3	25
  3	Pd3Cl/SBA-15	65	none	n.r.
  4	Pd3Cl/SBA-15	65	Et3N	90
  5	Pd3Cl/SBA-15	55	Et3N	92.8
  6	Pd3Cl/SBA-15	55	C4H9N	82.4
  7	Pd3Cl/SBA-15	40	Et3N	72
  8	Pd3Cl/SBA-15	55	Et3N	87c
  9	Pd3Cl/SBA-15	55	Et3N	85d
  10	Pd3Cl/CeO2	55	Et3N	88
  11	Pd3Cl/CeO2@ZIF-8	55	Et3N	77
  12	SBA-15	55	Et3N	n.r.e
  13	CeO2	55	Et3N	n.r.f
  14	Pd3Cl/SBA-15-400	55	Et3N	67
  15	Pd3Cl/CeO2-400	55	Et3N	71
  a Reaction conditions: iodobenzene (0.5 mmol), phenylacetylene (0.5 mmol), base (1 mmol), H2O (1 mL), 12 h, 30 mg catalyst, 0.8%(w/w) of palladium, 0.753 μmol of Pd3Cl, 1.4×10-5(n/n) of substrate; b Yield was detected by GC; c 20 mg of Pd3Cl/SiO2; d10 mg of Pd3Cl/SiO2; e 30 mg SBA-15 was used, n.r.=no reaction; f 30 mg CeO2 was used

  下载: 导出CSV

  Table 2.  Catalytic performance of difference substrates with Pd₃Cl/SBA-15^a

  Entry	Aryl halide	Terminal alkyne	Product	Yieldb / %
  1				90.3
  2				91.2
  3				88.5
  4				93.4
  5				81.3
  6				67.1
  a Reaction conditions: 30 mg Pd3Cl/SBA-15 catalyst, 0.5 mmol aryl halide, 0.5 mmol terminal alkyne, 1 mL H2O, 55 ℃ reaction temperature, 12 h; b Yield was detected by GC

  下载: 导出CSV
•