Synthesis of palladium nanocatalysts with cucurbit[n]uril as both a protecting agent and a support for Suzuki and Heck reactions

Abstract

Pd nanocatalysts based on cucurbit[n]uril were successfully synthesized by an operationally simple one-pot liquid-phase method using different reducing agents. Through changing the reducing agents and the ratio of feedstock, well-defined Pd nanostructures with various shapes were obtained. The results of morphological analysis demonstrate that cucurbit[6]uril plays a key role in guiding the formation of Pd nanostructures, and is responsible for the Suzuki and Heck coupling reactions with high efficiency.

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1. Introduction

Since palladium-catalyzed C–C bond formation reactions have been developed in the early 1970s, they have acquired an essential role in modern organic chemistry.¹ Among these reactions, the cross-coupling reactions based on palladium catalysts (e.g. Suzuki and Heck reactions) have become one of the most powerful tools in the pharmaceutical, agrochemical, and fine-chemical industries for the C–C bond formation.^2–4 Homogeneous palladium catalysis possesses many merits, such as high reaction rate, high turnover number, and efficient selectivity;^5–8 however, one of the great drawbacks of such catalysis is that the products might be contaminated by metal leaching, which is particularly unacceptable for the production of pharmaceuticals and fine chemicals. Additionally, the lack of reuse of the catalyst leads to high expenses for large-scale reactions.⁹ It is well-known that nano-sized metal particles have a high surface-to-volume ratio that can dramatically enhance the interaction between reactants and catalysts.¹⁰ Additionally, nanocatalysis can make the products easily removable from the reaction mixtures and make the catalysts recyclable. Thus nanocatalysis is a promising alternative to the homogeneous catalysis to afford the same products with high reaction rates and high yields.¹¹

The catalytic activity of metal nanoparticles (NPs) highly depends on their sizes and shapes, thus the development of an efficient, controlled, and cost-effective design for metal NPs is a goal of great importance in nanocatalysis fields.^12,13 Special emphasis has been given to the achievement of a high degree of control over the size and shape of metal NPs using various synthetic strategies, such as varying reducing agents, or solvents.^14–16 With the development of nanotechnology, there are also many works on the improvement of the nanocatalyst supports.^17,18 For instance, Glaspell et al. prepared nanocatalysts on tailored shape supports by using MgO nanocubes and ZnO nanobelts as supports for Au and Pd NPs, respectively.¹⁹ Firouzabadi et al. used aminopropyl-functionalized clay as a support to prepare a Pd nanocatalyst which was an efficient catalyst for Heck and Suzuki reactions.²⁰ Although some efficient Pd nanocatalysts have been reported, preparation of nanocatalysts practically using inexpensive, environmentally benign, and readily available feedstocks under mild conditions is still a challenging goal.²¹ Meanwhile, there are few studies focusing on the development of new types of materials that act as both supports and capping agents for fabrication of metal NPs.²²

Cucurbit[n]uril (CB[n], n = 4–10) is a family of macrocyclic molecules, which is often used as a host molecule and a ligand in supramolecular and coordination chemistry, respectively.^23–25 In the past two years, CB[n] has been used to prepare nanomaterials owing to the special electrostatic potential distribution at the CB[n] molecule surface.^26–30 For instance, Scherman and co-workers systematically studied the distinct surface-enhanced Raman spectra of gold nanoparticles capped by CB[5] or CB[7].³¹ In addition, CB[n] has been used as a ligand to prepare the precursor for nanomaterial fabrication. For example, Rica and Velders utilized a supramolecular aggregate constructed by CB[7] binding to Ag ions as a precursor to prepare Ag₂S nanocrystals, which is a promising approach for the fabrication of nanostructures of functional materials.³²

The defining features of CB[n] (n = 5–8) are their two identical portals lined by carbonyl groups that provide two negative fringes capable of binding to the surface of metals or other nanostructures (Fig. 1). The electrostatic interaction between CB[n] and surface atoms of metal NPs may stabilize the nanostructure from agglomeration. Importantly, such weak interactions do not impose restrictions on the special chemical properties of metal NPs.^33,34 In spite of the polar fringed portals, the CB[n] are non-polar and thus nearly insoluble in all common organic solvents and water. Another impressive feature of CB[n] is the rigid cyclic structure that has high thermal and chemical stability against many oxidizing agents.³⁵

Fig. 1 Illustration of CB[n] with two identical negatively charged portals.

Based on these unique properties of CB[n] and the merits of both protecting agents and supports for nanocatalysts, we have prepared Pd NPs using NaBH₄ as a reducing agent and CB[6] as a protecting agent in our previous work.²⁸ The excellent catalytic performance of as-prepared Pd NPs based on CB[6] for the Suzuki reaction inspired us to prepare Pd NPs based on CB[n] (n = 5–8) with controlled shapes and sizes and to further investigate other types of reactions using Pd NPs as nanocatalysts. In this paper, we prepared a series of Pd NPs based on CB[n], CB[n]–Pd NPs (n = 5–8), under different conditions by using a facile one-pot liquid-phase method. Furthermore, we investigated the morphological features responsible for the high catalytic activity in Suzuki and Heck reactions.

2. Experimental

Materials and analytical procedures

CB[n] (n = 5–8) were prepared according to the previous literature,^36,37 from glycoluril and excess amounts of formaldehyde. Other starting materials and solvents were obtained from commercial sources and used without further purification. All solutions were prepared with deionized water (Milli-Q, 18.2 M Ω cm).

Transmission electron microscopy (TEM) images were recorded with a JEOL-2010 electron microscope working at 200 kV. The samples were prepared by the addition of a drop of the product in ethanol onto a continuous carbon-coated copper TEM grid and drying in air. Powder X-ray diffraction (XRD) patterns were recorded on a Rigaku-Dmax2500 diffractometer using Cu Kα radiation (λ = 0.154 nm). Analysis of the Pd content was carried out on an Ultima2 inductively coupled plasma optical emission spectrometer (ICP-OES). Fourier-transform infrared (FT-IR) spectra were taken on a Magna 750 FTIR spectrometer with samples as KBr pellets in the range of 450–4000 cm⁻¹. The gas chromatography–mass spectrometry (GC-MS) measurements were performed on a Varian 450-GC/240-MS. ¹H NMR (400 MHz), and ¹³C NMR (100 MHz) spectra were recorded in CDCl₃ solutions using a Bruker AVANCE 400 spectrometer.

Preparation of Pd NPs capped by CB[n] with different reducing agents (CB[n]–Pd NPs, n = 5–8)

(1) CB[n]–Pd NPs reduced by sodium borohydride: 0.2 mmol PdCl₂ and 0.2 mmol CB[n] were mixed in 20 mL of water at room temperature (25 °C). The mixture was stirred for half an hour to form a brown uniform suspension, and then a freshly prepared NaBH₄–alcohol solution (2 mmol in 20 mL) was added rapidly. The reduction occurred instantaneously and was characterized by a color change from brown to black. Stirring was continued for another 3 h and the final products were separated by centrifugation, washed three times with aqueous ethanol (v/v, 1/1) to remove excess salt, and dried at 70 °C for 10 h in air. The CB[6]–Pd NPs with different molar ratios between CB[6] and PdCl₂ ranging from 6 : 1 to 1 : 2 were prepared by using the same procedure described above. The Pd NPs prepared with the molar ratio of CB[6] to PdCl₂ as 1 : 1 are denoted as CB[6]–Pd NPs 1.

(2) Pd NPs based on CB[6] reduced by the polyol process: 0.2 mmol PdCl₂ and 0.2 mmol CB[6] were mixed in 40 mL ethylene glycol (EG), in the presence of 0.2 mmol tetramethylammonium bromide (TMAB) as a phase transfer agent, and stirred for 2 h at 160 °C. After the mixture cooled down to room temperature naturally, the brown products were separated by centrifugation, washed three times with aqueous ethanol (v/v, 1/1), and dried at 70 °C for 10 h in air. Using ethanol (EtOH) as both a solvent and a reducing agent, Pd NPs in the presence of CB[6] were also obtained. Substrates were mixed in 40 mL EtOH at 80 °C and kept to react for 6 h. The final products reduced by EG and EtOH are denoted as CB[6]–Pd NPs 2 and CB[6]–Pd NPs 3, respectively.

(3) Pd NPs based on CB[6] reduced by ascorbic acid (AA): 0.2 mmol PdCl₂, 0.2 mmol CB[6] and 2.52 mmol (0.300 g) potassium bromide were mixed in 20 mL water. The mixture was stirred for half an hour to form a brown uniform suspension, and then 0.34 mmol (0.060 g) AA in 20 mL water was injected into the suspension slowly. The reaction mixture was kept for another 6 h with stirring at 80 °C in air. After cooling down to room temperature, the product was separated by centrifugation, washed three times with aqueous ethanol (v/v, 1/1), and dried at 70 °C for 10 h in air. The final product is denoted as CB[6]–Pd NPs 4.

Coupling reactions catalyzed by CB[n]–Pd NPs

General procedure for the Suzuki coupling reaction: the Suzuki coupling reaction between arylboronic acids and aryl halides was performed with CB[n]–Pd NPs (n = 6, 7) and CB[6]–Pd NPs 3 that were reduced by NaBH₄ and ethanol, respectively. Typically, the catalytic reactions were carried out in a 10 mL round bottom flask under ambient atmosphere. Aryl halides (1 mmol, 1 equiv.), arylboronic acids (1.2 mmol, 1.2 equiv.), sodium carbonate (3 mmol, 3 equiv.), and CB[n]–Pd NPs (5 mg, 0.5 mol%) were mixed in solvent (3 mL of water and 3 mL of ethanol). Then the reaction was run at the indicated temperature. After completion of the reaction, the mixture was extracted with ethyl acetate (3 × 10 mL) and the combined organic phase was dried over anhydrous Na₂SO₄. After filtration, the solvent was removed under reduced pressure to yield the crude product. The crude product was directly analyzed by GC-MS with an external standard method. The desired product was purified by chromatography on silica gel (ethyl acetate/hexane gradient).

General procedure for the Heck coupling reaction: a mixture of aryl halide (1 mmol, 1 equiv.), alkene (1.5 mmol, 1.5 equiv.), sodium carbonate (2 mmol, 2 equiv.), and CB[6]–Pd NPs 1 (5 mg, 0.5 mol%) in 3 mL DMF was stirred in a 15 mL pressure tube at 140 °C for 24 h. After the reaction was complete, the solid catalyst was separated by centrifugation, and the filtrate was diluted with water followed by extraction with ethyl acetate (3 × 10 mL), and dried over anhydrous Na₂SO₄. The solvent was removed and the crude product was directly analyzed by GC-MS. The product was further purified by column chromatography (silica gel, ethyl acetate/hexane gradient).

Recycling catalytic activity investigation: five recycles of the activity were examined for CB[6]–Pd NPs. After the 1st run, the catalyst was separated and washed with aqueous ethanol solution to remove salt and adsorbed organic substrate, followed by drying overnight at 70 °C prior to being reused. The catalyst was used for the 2nd run without further activation and the same process was repeated for the next run.

Applications of hot filtration (Suzuki coupling of iodobenzene with phenylboronic acid and Heck coupling of iodobenzene with n-butyl acrylate): hot filtration was carried out for 5 min for the Suzuki coupling reaction and for 6 h for the Heck coupling reaction, respectively. The filtrates were further reacted under the same conditions for another 20 min for the Suzuki coupling reaction and 6 h for the Heck coupling reaction, respectively. After the reaction, the samples of the reaction mixture were analyzed by GC and ICP.

3. Results and discussion

Preparation and characterization of CB[n]–Pd NPs

A series of CB[n]–Pd NPs were prepared with corresponding CB[n] (n = 5–8), which were stable for more than half a year both in the solid state and in solution.

As shown in Fig. 2A–D, Pd NPs reduced by NaBH₄ in the presence of CB[5], CB[8], CB[6], and CB[7] were obtained, respectively. Previous studies reveal that the two carbonyl rims of CB[n] have a known affinity for metal ions, which can be used as a switch through varying the chemical environment.^38–40 Before reducing agents are added, the Pd ions can bind to carbonyl groups at the portals of CB[5] forming a Pd–CB[5] complex as the metal precursor. In the meantime, CB[5] molecules themselves are capable of forming a number of hydrogen bonds between hydrogen atoms on the molecule outside wall and the solvent molecules.⁴¹ As shown in Fig. 2A, the Pd–CB[5] complexes were reduced into Pd NPs with irregular shapes and large sizes. CB[8] has the largest molecular volume in common CB[n] (n = 5–8)²⁵ and it can stabilize Pd NPs through the electrostatic interaction. However, the large macrocyclic molecule cannot hamper the insertion of Pd ions through the gap between CB[8] molecules which results in the Pd NPs growing into bigger ones (Fig. 2B).

Fig. 2 TEM of CB[n]–Pd NPs (n = 5–8). (A) CB[5]–Pd NPs, (B) CB[8]–Pd NPs, (C) CB[6]–Pd NPs, (D) CB[7]–Pd NPs. (Insets are the corresponding HRTEM images.)

Fig. 2C and D show the TEM images of CB[6]–Pd NPs and CB[7]–Pd NPs, respectively. Both CB[6] and CB[7] have moderate sizes of the portals and they can bind to the surface atoms of Pd through electrostatic interaction and effectively block the Pd NPs from agglomeration. As these images exhibit, near spherical Pd NPs are obtained with surfaces being covered by {111} and {100} facets. The HRTEM images also clearly confirm the uniform distribution of the near spherical Pd NPs in the presence of CB[6] or CB[7] (insets of Fig. 2C and D, respectively). Determined by statistical analysis of HRTEM images of more than 150 particles, the average size of CB[6]–Pd NPs is 3.3 ± 0.2 nm, and that of CB[7]–Pd NPs is 2.5 ± 0.2 nm (Fig. S1, ESI†). CB[7] solubility in water is about 2 × 10⁻²–3 × 10⁻² M at room temperature, which is larger than that of CB[6].²⁵ Therefore, CB[7] molecules may be adsorbed onto the surface of Pd NPs more effectively than CB[6] to restrain the growth of Pd NPs, leading to a smaller average size.

The crystallinity and crystal structure of these Pd NPs prepared in the presence of four different CB[n] were investigated by means of XRD. As shown in Fig. 3A, the reflections for these Pd NPs appear at around 40°, 46°, 68°, and 82°. These peaks correspond to (111), (200), (220), and (311) crystalline planes of the face-centered cubic (fcc) lattice, respectively (JCPDS no. 46-1043). The XRD patterns also demonstrate that Pd NPs prepared in the presence of CB[6] have the best crystallization without any impurity phase. So we investigated Pd NPs prepared with different molar ratios of PdCl₂ to CB[6]. Pd black appears immediately if the amount of CB[6] is too little and well-crystallized Pd NPs are obtained when the molar ratio of CB[6] to the metal precursor is in an appropriate range, as confirmed by their XRD (Fig. S2, ESI†). This phenomenon also demonstrates the key role of CB[6] in protecting Pd NPs. As shown in the TEM images (Fig. 4), CB[6]–Pd NPs with various shapes were prepared by varying the molar ratio of feedstock. When the molar ratio of PdCl₂ to CB[6] is 1 : 2, nearly spherical Pd NPs with an average size of 3.3 nm are obtained with a wider size distribution than that of CB[6]–Pd NPs 1 (Fig. S2, ESI†), which arises from more insoluble CB[6] in the reaction system providing more nucleation sites. The shapes of Pd NPs are changed with the variation of capping agents. When the molar ratio of PdCl₂ to CB[6] is 2 : 1, CB[6] molecules tend to be adsorbed onto Pd NPs forming cubic crystals with surfaces being covered by {100} facets, as seen in the HRTEM image (Fig. 4D). It is found that if the capping agents are further reduced to make the molar ratio of PdCl₂ to CB[6] 3 : 1, Pd NPs with irregular shapes appear. Obviously, the number of molecules adsorbed onto the surface of Pd NPs is decreased as the amount of capping agents is reduced. Given the impact of CB[n] on the synthesis system, using CB[n] as capping agents to prepare metal NPs is more complicated than using other common capping agents, such as PVP or OL. Aforementioned results indicate that if we exquisitely control the synthesis conditions, the growth of the Pd NPs can be regulated.

Fig. 3 XRD patterns of the (A) CB[n]–Pd NPs prepared with the corresponding CB[n] or (B) CB[6]–Pd NPs reduced by different reducing agents (a) NaBH₄, (b) EtOH, (c) EG, (d) AA.

Fig. 4 TEM and HRTEM images of CB[6]–Pd NPs prepared with different ratios of CB[6] to PdCl₂, 1 : 2 (A) and (B); 2 : 1 (C) and (D); 3 : 1 (E) and (F). (Insets are the corresponding FFT patterns.)

Then we investigated the influence of different reducing agents on the synthesis of CB[6]–Pd NPs. The polyol process involves the reduction of a precursor by ethylene glycol or other hydroxy compounds at elevated temperatures in the presence of protecting agents.^42–49 By using EG as both solvent and reducing agent, Pd NPs with various shapes were obtained in the presence of CB[6] at 160 °C. It is observed from the HRTEM images (Fig. 5a–d) that the anisotropic growth of Pd has started to appear. Many of these Pd NPs display nonspherical shapes, that is, decahedron (Fig. 5a), truncated bipyramid (Fig. 5b and c), and nanobar (Fig. 5d). As shown in the TEM image (Fig. 5A), however, most of these as-prepared Pd NPs are nanobars. EG is a weak reducing agent compared to NaBH₄, and thus a slower reduction procedure induces the slightly anisotropic growth of the Pd seeds which are capped by CB[6] molecules into nonspherical shapes. Generally, obtaining twinned Pd nanostructures requires precise manipulation to effectively eliminate oxidative etching.^50–52 It is noticed that decahedron Pd NPs which have a five-fold twin structure are obtained in the presence of CB[6] using EG as the reducing agent in air even at an elevated temperature. To verify the key role of CB[6] in the formation of twinned nanostructures, we use ethanol as the reducing agent which has weaker reducing ability than EG.⁵³ As expected, the HRTEM images reveal that Pd NPs with twinned structures are easily obtained by using CB[6] as a protective agent without complicated manipulation (Fig. 5e–h). The well-defined Pd NPs have abundant twin planes and the particle size is mainly less than 10 nm. These studies indicate a special effect of CB[6] on guiding and stabilizing twin formation for Pd NPs.

Fig. 5 TEM and HRTEM images of CB[6]–Pd NPs reduced by EG (A), the corresponding HRTEM (a–d) show the anisotropic growth, and by EtOH (B), the corresponding HRTEM (e–h) show the abundant twin planes.

Based on above experimental data, we choose ascorbic acid as a mild reducing agent, which could sustain a slow reduction to ensure a uniform growth of Pd nanostructures. Fig. 6 shows the TEM and HRTEM images of Pd nanostructures prepared with ascorbic acid as a reducing agent in water and in the presence of CB[6] and bromide ions at 80 °C. Bromide ions have been proven to chemisorb onto the surface of Pd NPs and alter the order of surface free energies for different facets. As a result, bromide ions can greatly promote the formation of a {100} surface.^54–57 Unexpectedly, Pd NPs with a pyramidal shape enclosed by {111} facets were obtained. The spacing of 0.23 nm corresponds to the {111} interplanar distance of fcc Pd. And the corresponding fast Fourier transform (FFT) diffraction pattern of the HRTEM lattices also indicates that the pyramid Pd NPs are enclosed by {111} facets (inset of Fig. 6B). Such result may be ascribed to the coordination of CB[6] and bromide ions to Pd(II) ions, which forms the Pd@Br@CB[6] complex as a metal precursor.⁵⁸

Fig. 6 TEM and HRTEM images of CB[6]–Pd NPs reduced by AA in the presence of bromide (inset is the corresponding FFT pattern).

We further characterized the resultant phases of these CB[6]–Pd NPs reduced with different reducing agents by means of XRD (Fig. 3B). The XRD investigation reveals the well-crystallized structures of these Pd NPs crystallized in fcc (JCPDS no. 46-1043). The XRD patterns also show that the peaks for CB[6]–Pd NPs 1 are fairly broad, indicating a decrease in the crystallite size, and the shift of the (111) XRD peak position of CB[6]–Pd NPs 1 to the smaller angles also proves that.^59,60

The morphological analysis results indicate that shape-controlled CB[6]–Pd NPs can be obtained by adjusting the synthesis conditions such as varying the ratio of feedstock and employing different reducing agents. Importantly, the successful preparation of twinned structures of Pd NPs under mild reaction conditions demonstrates unambiguously the ability of the CB[6] in mediating and stabilizing twin planes in Pd nanocrystals. It also demonstrates the power of CB[6] in guiding the formation of Pd NPs that are unconventional.

Catalytic performance in the coupling reactions

Based on the aforementioned morphological analysis, CB[n]–Pd NPs with near spherical shapes or with abundant twin planes were chosen as nanocatalysts to investigate Suzuki–Miyaura and Mizoroki–Heck coupling reactions. The three well-defined nanocatalysts are CB[7]–Pd NPs, CB[6]–Pd NPs 1 and CB[6]–Pd NPs 3. Recently, Pd NPs of different origins have been successfully utilized in the Suzuki and Heck coupling reactions.^21,61–64 However, most of these catalysts require a very large amount of catalyst loading and run under relatively harsh reaction conditions, typically, refluxing in organic solvent for a long time or running in a nitrogen atmosphere.^65,66 And such Pd NPs that were prepared by complicated synthetic methods cannot meet the requirements of large-scale application. Moreover, the final product tends to agglomerate into a larger one than the as-prepared NPs after the post-treatment process, which often results in the loss of catalytic activity.^67,68 As described above, various CB[n]–Pd NPs were prepared through a very simple one-pot process without any further post-treatment, and the shapes as well as the catalytic activity of the as-prepared Pd NPs were maintained.

The Pd NPs chosen as catalysts exhibit excellent catalytic activity towards Suzuki and Heck reactions and the reactions tolerated a wide range of functional groups. When iodobenzene and arylboronic acid are used, yield up to 99% is obtained in the presence of three CB[n]–Pd NPs at 40 °C in 20 min (entries 1–6, Table 1). The excellent yields under such extremely mild conditions inspired our further exploration of the catalytic effect of CB[n]–Pd NPs on C–C coupling reactions between bromobenzene and arylboronic acid. Entries 7 to 33 in Table 1 convincingly show the outstanding catalytic activities of CB[n]–Pd NPs towards both bromobenzene and its derivatives. Under such mild conditions, aryl bromides with both electron-withdrawing and electron-donating groups at the para-position can couple with arylboronic acids to afford the corresponding products in high yields. Even sterically-hindered o-MeOC₆H₄Br is able to react with phenylboronic acid, affording the product in 94.5% yield (entry 32, Table 1).

Table 1 Suzuki cross-coupling reactions of aryl halides with arylboronic acids catalyzed by the CB[n]–Pd NPs.^a (CB[6]–Pd NPs 1, CB[7]–Pd NPs and CB[6]–Pd NPs 3, denoted as catalysts 1, 2 and 3, respectively)

Entry	Ar^1–X	Ar^2	T/°C, Time /min	Cata.	Yield^b (%)
a Reaction conditions: aryl halides (1 mmol), arylboronic acid (1.2 mmol), Na2CO3 (3 mmol), solvent: H2O/EtOH (3 mL/3 mL), CB[n]–Pd NPs (0.5 mol%). b Yields of biphenyl are determined by GC using an external standard method based on aryl halides (entries 1–3, 7–9 and 40–42); other products are isolated in yields based on aryl halides. c Solvent: H2O/DMF (1.5 mL/1.5 mL).
1	C6H5I	C6H5	40, 20	Cata. 1	99
2				Cata. 2	95
3				Cata. 3	99
4	C6H5I	4-MeC6H5	40, 20	Cata. 1	99
5				Cata. 2	98
6				Cata. 3	99
7	C6H5Br	C6H5	60, 20	Cata. 1	99
8				Cata. 2	98.6
9				Cata. 3	99
10	C6H5Br	4-MeC6H5	60, 20	Cata. 1	98
11				Cata. 2	98
12				Cata. 3	99
13	C6H5Br	4-MeOC6H5	60, 20	Cata. 1	98
14				Cata. 2	98
15				Cata. 3	99
16	4-MeC6H4Br	C6H5	60, 20	Cata. 1	98
17				Cata. 2	96.5
18				Cata. 3	99
19	4-MeC6H4Br	4-MeC6H5	60, 30	Cata. 1	99
20				Cata. 2	99
21				Cata. 3	99
22	4-MeOC6H4Br	C6H5	80, 30	Cata. 1	88
23				Cata. 2	94.6
24				Cata. 3	95
25	4-MeOC6H4Br	4-MeC6H5	80, 30	Cata. 1	93
26				Cata. 2	98.5
27				Cata. 3	99
28	4-MeCOC6H4Br	C6H5	80, 30	Cata. 1	99
29				Cata. 2	99
30				Cata. 3	99
31	2-MeOC6H4Br	C6H5	80, 180	Cata. 1	92
32				Cata. 2	94.5
33				Cata. 3	92
34	1-Bromonaphthalene	C6H5	80, 180	Cata. 1	95
35				Cata. 2	95
36				Cata. 3	98
37	4-MeCOC6H4Cl	C6H5	120, 180	Cata. 1	89.3
38				Cata. 2	88
39				Cata. 3	92
40	C6H5Cl	C6H5	120, 180	Cata. 1	87^c
41				Cata. 2	83^c
42				Cata. 3	92.9^c
43	4-MeOC6H4Br	C6H5	80, 30	CB[5]–Pd NPs	78.3
44				CB[7]–Pd NPs	94.6
45				CB[8]–Pd NPs	99

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There are rare examples for the Suzuki reaction catalyzed by Pd NPs with high yields under such mild conditions.^69–71CB[6]–Pd NPs also show good catalytic activity towards deactivated aryl chlorides, as shown in entries 37–42 of Table 1. Specifically, up to 92.9% yield (entry 42 in Table 1) of biphenyl is obtained when the reaction is catalyzed by CB[6]–Pd NPs 3 in a mixed solvent composed of DMF and H₂O, implying that the twinned nanostructures with {111} twin planes are highly active for deactivated aryl chloride. Most of the CB[6]–Pd NPs 2 are nanobars with {100} enclosed, as mentioned above, and only gave 83% yield. As shown in Table 1 entries 43–45, the catalytic performances of Pd NPs prepared in the presence of different CB[n] were tested. CB[5]–Pd NPs only gave 78.3% yield when deactivated aryl bromides were employed (entry 43, Table 1). CB[7]–Pd NPs gave improved outcomes compared with CB[6]–Pd NPs 1, reaching 94.6% yield. While CB[8]–Pd NPs showed the best catalytic activity with 99% yield, even better than CB[6]–Pd NPs 3 (entry 45, Table 1). The good catalytic performance of CB[7]–Pd NPs can be attributed to the small and uniform size distribution of Pd NPs prepared in the presence of CB[7]. As previous studies have indicated CB[8] is capable of encapsulating aromatic molecules in its cavity, and thus increases the contact chances of substrates with Pd NPs in this catalytic system.^23,24,35 This feature resulted in the best catalytic performance of CB[8]–Pd NPs for Suzuki coupling reactions, compared with other CB[n]–Pd NPs.

We further studied the morphology of CB[6]–Pd NPs after five cycles of the reactions, as shown in Fig. 7; the near spherical shape and average size are not changed obviously, indicating the good protection by CB[6] for the NPs in catalysis. The nanocatalysts can be recovered easily without the loss of activity.⁷² High yield is obtained even after five cycles using iodobenzene and phenylboronic acid as substrates (Fig. 8), proving that the Pd NPs based on CB[n] possess high catalytic activities and good stabilities.

Fig. 7 TEM (A) and HRTEM (B) images of CB[6]–Pd NPs 1 after catalyzing the Suzuki coupling reaction.

Fig. 8 Recycling of CB[6]–Pd NPs 1 and Pd/C (5 wt%) in Suzuki and Heck reactions between iodobenzene and phenylboronic acid and n-butyl acrylate, respectively.

The Heck reaction has emerged as one of the most powerful tools in organic synthesis.^73–75 Herein we studied the catalytic activity of CB[6]–Pd NPs 1 in the Heck reaction. Initially, the reaction of iodobenzene with styrene was chosen as a model reaction in the presence of CB[6]–Pd NPs 1. Bases, solvents, temperature, and catalyst loading were screened to optimize the reaction (Table 2). K₃PO₄ and NaOAc give moderate yields (entries 7 and 8 in Table 2), organic base, Et₃N, gives only 20% yield (entry 9 in Table 2). Inorganic base, Na₂CO₃, improves the yield up to 85% under the same reaction conditions (entry 1 in Table 2). DMF is found to be more effective than other solvents. Reducing or enhancing the catalyst loading leads to decrease in yields (entries 2–4 in Table 2). We studied the morphology of CB[6]–Pd NPs 1 after the reaction (entry 3, Table 2), as shown in Fig. S3 (ESI†), the as-prepared near spherical CB[6]–Pd NPs 1 have been aggregated into large particles, ca. 20 nm, indicating that the high catalyst loading induced Pd NPs aggregation for the Heck coupling reaction in this catalytic system. With the optimized conditions in hand, the scope of the Heck reaction was explored with 0.5 mol% catalyst loading. A range of aryl iodides and aryl bromides can undergo the Heck reaction to produce trans products in good yields (Table 3). The yields of aryl iodides and bromides with butyl acrylate catalyzed by CB[6]–Pd NPs 1 are higher than those with styrene (entries 1–5 in Table 3), implying that the substrates with electron-withdrawing groups are preferred in the catalytic system.

Table 2 Optimization of the Heck reaction of iodobenzene and styrene catalyzed by CB[6]–Pd NPs 1^a

Entry	Cata./mol%	T/°C	Base	Solvent	Yield^b (%)
a Reaction conditions: aryl halides (1 mmol), olefin (1.5 mmol), base (2 mmol), solvent (3 mL), CB[6]–Pd NPs 1 (5 mg, 0.5 mol%). b Isolated yields.
1	0.5	140	Na2CO3	DMF	85
2	1	140	Na2CO3	DMF	60
3	3	140	Na2CO3	DMF	42
4	0.2	140	Na2CO3	DMF	68
5	0.5	120	Na2CO3	DMF	52
6	0.5	140	K2CO3	DMF	78
7	0.5	140	K3PO4	DMF	66
8	0.5	140	NaOAc	DMF	45
9	0.5	140	Et3N	DMF	20
10	0.5	70	K2CO3	H2O	52
11	0.5	70	Na2CO3	H2O	55
12	0.5	140	Na2CO3	DMF/H2O (2/1)	80

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Table 3 Heck coupling reactions catalyzed by CB[6]–Pd NPs 1^a

Entry	Ar–X	R	Yield^b (%)
a Reaction conditions: aryl halides (1 mmol), olefin (1.5 mmol), base (2 mmol), solvent (3 mL), CB[6]–Pd NPs 1 (5 mg, 0.5 mol%). b Isolated yields.
1	C6H5I	C6H5	85
2	C6H5I	CO2Bu^n	99
3	C6H5I	CO2Bu^t	97.5
4	4-MeOC6H4I	C6H5	85
5	4-MeOC6H4I	CO2Bu^n	99
6	4-MeOC6H4I	CO2Bu^t	80.3
7	C6H5Br	C6H5	82
8	C6H5Br	CO2Bu^n	88
9	C6H5Br	CO2Bu^t	83.5
10	4-MeOC6H4Br	C6H5	76
11	4-MeOC6H4Br	CO2Bu^n	85
12	4-MeOC6H4Br	CO2Bu^t	71.5
13	4-MeCOC6H4Br	C6H5	92
14	4-MeCOC6H4Br	CO2Bu^n	90
15	4-MeCOC6H4Br	CO2Bu^t	85.5

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We also carried out recycling reactions with respect to the Heck reaction, employing 4-iodobenzene and n-butyl acrylate as coupling partners in the presence of CB[6]–Pd NPs 1. A 99% yield was obtained in the first cycle, and the catalyst was recovered easily through centrifugation for the next run. The recycled catalyst was used in the model reaction under the same conditions, giving 97%, 96%, 98%, and 95% isolated yields for the second, third, fourth and fifth runs, respectively (Fig. 8). The results demonstrate that the Pd NPs based on CB[6] are reusable nanocatalysts, suitable for both water and organic solvents, and can be separated for reuse owing to the solubility and polarity of CB[6].

For the measurement of the Pd leaching during the Suzuki coupling reaction, a hot-filtration experiment was run to investigate if the reaction proceeded in a heterogeneous or homogeneous fashion. The catalysts were removed after a certain time (5 min for the Suzuki coupling reaction with 35.5% conversion, 6 h for the Heck coupling reaction with 40% conversion) and the resulting filtrates were monitored after additional reaction time under identical reaction conditions (20 min for the Suzuki reaction and 6 h for the Heck reaction). The ICP result of the filtrate shows that the amount of Pd leaching into the reaction system is negligible and the concentration of Pd in the filtrate is very low (0.12 ppm for the Suzuki reaction, 0.3 ppm for the Heck reaction). The filtrates give no coupling products after the removal of catalysts from the reaction systems, as analyzed by GC. These results suggest that the catalytic process may occur on the surface of Pd NPs, i.e. the Suzuki and Heck coupling reactions catalyzed by CB[n]–Pd NPs are heterogeneous reaction pathways.^76,77

4. Conclusions

We reported a new type of Pd nanostrucutres based on CB[n], which exhibits excellent catalytic efficiency towards Suzuki and Heck reactions. It is demonstrated that the CB[6] serves as stabilizer for keeping the Pd nanostructures from agglomeration via electrostatic interaction between CB[6] and metal nanostructures. More importantly, such relatively weak electrostatic interactions can maintain the active sites exposed on the surface of the nanocatalyst and thus it shows high catalytic activity. The work also demonstrates the unconventional effect of CB[n] on guiding and stabilizing Pd nanostructure formation.

Acknowledgements

We thank 973 Program (2011CB932504, 2012CB821705), NSFC (20731005, 20873151, and 91022007), Fujian Key Laboratory of Nanomaterials (2006L2005) and Key Projects from CAS and FJIRSM (SZD 07002) for the support of this work.

Notes and references

1. C. Barnard, Platinum Met. Rev., 2008, 52, 38.
2. X.-F. Wu, P. Anbarasan, H. Neumann and M. Beller, Angew. Chem., Int. Ed., 2010, 49, 9047.
3. K. C. Nicolaou, P. G. Bulger and D. Sarlah, Angew. Chem., Int. Ed., 2005, 44, 4442.
4. C. Torborg and M. Beller, Adv. Synth. Catal., 2009, 351, 3027.
5. M. Beller, Chem. Ing. Tech., 2006, 78, 1061.
6. N. Selander and K. J. Szabö, Chem. Rev., 2011, 111, 2048.
7. C. E. Tucker and J. G. De Vries, Top. Catal., 2002, 19, 111.
8. B. M. Bhanage and M. Arai, Catal. Rev. Sci. Eng., 2001, 43, 315.
9. C. E. Garrett and K. Prasad, Adv. Synth. Catal., 2004, 346, 889.
10. D. Astruc, F. Lu and J. R. Aranzaes, Angew. Chem., Int. Ed., 2005, 44, 7852.
11. V. Polshettiwar and R. S. Varma, Green Chem., 2010, 12, 743.
12. B. R. Cuenya, Thin Solid Films, 2010, 518, 3127.
13. Z.-Y. Zhou, N. Tian, J.-T. Li, I. Broadwell and S.-G. Sun, Chem. Soc. Rev., 2011, 40, 4167.
14. J. Chen, B. Lim, E. P. Lee and Y. Xia, Nano Today, 2009, 4, 81.
15. Y. Xia, Y. Xiong, B. Lim and S. E. Skrabalak, Angew. Chem., Int. Ed., 2008, 48, 60.
16. A. R. Tao, S. Habas and P. Yang, Small, 2008, 4, 310.
17. L. Feng, D. T. Hoang, C.-K. Tsung, W. Huang, S. H.-Y. Lo, J. B. Wood, H. Wang, J. Tang and P. Yang, Nano Res., 2011, 4, 61.
18. Y. Yamada, C.-K. Tsung, W. Huang, Z. Huo, S. E. Habas, T. Soejima, C. E. Aliaga, G. A. Somorjai and P. Yang, Nat. Chem., 2011, 3, 372.
19. G. Glaspell, H. M. A. Hassan, A. Elzatahry, L. Fuoco, N. R. E. Radwan and M. S. El-Shall, J. Phys. Chem. B, 2006, 110, 21387.
20. H. Firouzabadi, N. Iranpoor, A. Ghaderi, M. Ghavami and S. J. Hoseini, Bull. Chem. Soc. Jpn., 2011, 84, 100.
21. R. Narayanan, Molecules, 2010, 15, 2124.
22. A. Z. Moshfegh, J. Phys. D: Appl. Phys., 2009, 42, 233001.
23. J. Lagona, P. Mukhopadhyay, S. Chakrabarti and L. Isaacs, Angew. Chem., Int. Ed., 2005, 44, 4844.
24. J. W. Lee, S. Samal, N. Selvapalam, H.-J. Kim and K. Kim, Acc. Chem. Res., 2003, 36, 621.
25. K. Kim, N. Selvapalam and D. H. Oh, J. Inclusion Phenom. Macrocyclic Chem., 2004, 50, 31.
26. A. Corma, H. Garcia, P. Montes-Navajas, A. Primo, J. J. Calvino and S. Trasobares, Chem.–Eur. J., 2007, 13, 6359.
27. T.-C. Lee and O. A. Scherman, Chem. Commun., 2010, 46, 2438.
28. M. Cao, J. Lin, H. Yang and R. Cao, Chem. Commun., 2010, 46, 5088.
29. T. Premkumar, Y. Lee and K. E. Geckeler, Chem.–Eur. J., 2010, 16, 11563.
30. X. Lu and E. Masson, Langmuir, 2011, 27, 3051.
31. R. W. Taylor, T.-C. Lee, O. A. Scherman, R. Esteban, J. Aizpurua, F. M. Huang, J. J. Baumberg and S. Mahajan, ACS Nano, 2011, 5, 3878.
32. R. Rica and A. H. Velders, J. Am. Chem. Soc., 2011, 133, 2875.
33. P. D. Jadzinsky, G. Calero, C. J. Ackerson, D. A. Bushnell and R. D. Konberg, Science, 2007, 318, 430.
34. N. R. Jana and X. Peng, J. Am. Chem. Soc., 2003, 125, 14280.
35. K. Kim, N. Selvapalam, Y. H. Ko, K. M. Park, D. Kim and J. Kim, Chem. Soc. Rev., 2007, 36, 267.
36. A. Day, A. P. Arnol, R. J. Blanch and B. Snushall, J. Org. Chem., 2001, 66, 8094.
37. J. Kim, I.-S. Jung, S.-Y. Kim, E. Lee, J.-K. Kang, S. Sakamot, K. Yamaguchi and K. Kim, J. Am. Chem. Soc., 2000, 122, 540.
38. H.-J. Buschmann, E. Cleve, L. Mutihac and E. Schollmeyer, J. Inclusion Phenom. Macrocyclic Chem., 2009, 65, 293.
39. J. Lin, J. Lü, M. Cao and R. Cao, Cryst. Growth Des., 2011, 11, 778.
40. A. L. Koner, C. Márquez, M. H. Dickman and W. M. Nau, Angew. Chem., Int. Ed., 2011, 50, 545.
41. D. Bardelang, K. A. Udachin, R. Anedda, I. Moudrakovski, D. M. Leek, J. A. Ripmeester and C. I. Ratcliffe, Chem. Commun., 2008, 4927.
42. Y. Sun and Y. Xia, Science, 2002, 298, 2176.
43. B. Wiley, T. Herricks, Y. Sun and Y. Xia, Nano Lett., 2004, 4, 1733.
44. F. Kim, S. Connor, H. Song, T. Kuykendall and P. Yang, Angew. Chem., Int. Ed., 2004, 43, 3673.
45. S. H. Im, Y. T. Lee, B. Wiley and Y. Xia, Angew. Chem., Int. Ed., 2005, 44, 2154.
46. B. Wiley, Y. Xiong, Z.-Y. Li, Y. Yin and Y. Xia, Nano Lett., 2006, 6, 765.
47. S. E. Skrabalak, L. Au, X. Li and Y. Xia, Nat. Protoc., 2007, 2, 2182.
48. J. Chen, B. Wiley and Y. Xia, Langmuir, 2007, 23, 4120.
49. S. E. Skrabalak, B. J. Wiley, M. Kim, E. V. Formo and Y. Xia, Nano Lett., 2008, 8, 2077.
50. Y. Xiong, J. Chen, B. Wiley, Y. Xia, S. Aloni and Y. Yin, J. Am. Chem. Soc., 2005, 127, 7332.
51. B. Wiley, Y. Sun and Y. Xia, Acc. Chem. Res., 2007, 40, 1067.
52. Y. Xiong, H. Cai, Y. Yin and Y. Xia, Chem. Phys. Lett., 2007, 440, 273.
53. D. Li and S. Komarneni, J. Nanosci. Nanotechnol., 2008, 8, 3930; A. Pal, S. Shah and S. Devi, Mater. Chem. Phys., 2009, 114, 530.
54. J. A. Schimpf, J. B. Abreu and M. P. Soriaga, J. Electroanal. Chem., 1994, 364, 247.
55. C. A. Lucas, N. M. Markovic and P. N. Ross, Phys. Rev. B: Condens. Matter, 1997, 55, 7964.
56. Y. Xiong and Y. Xia, Adv. Mater., 2007, 19, 3385.
57. Y. Xiong, H. Cai, B. Wiley, J. Wang, M. J. Kim and Y. Xia, J. Am. Chem. Soc., 2007, 129, 3665.
58. X. Huang, H. Zhang, C. Guo, Z. Zhou and N. Zheng, Angew. Chem., Int. Ed., 2009, 48, 4808.
59. K. Ebitani, K.-M. Choi, T. Mizugaki and K. Kaneda, Langmuir, 2002, 18, 1849.
60. J.-Y. Wang, Y.-Y. Kang, H. Yang and W.-B. Cai, J. Phys. Chem. C, 2009, 113, 8366.
61. L. Yin and J. Liebscher, Chem. Rev., 2007, 107, 133.
62. C. C. Cassol, A. P. Umpierre, G. Machado, S. I. Wolke and J. Dupont, J. Am. Chem. Soc., 2005, 127, 3298.
63. J. D. Senra, L. F. B. Malta, A. L. F. Souza, L. C. S. Aguiar and O. A. C. Antunes, Adv. Synth. Catal., 2008, 350, 2551.
64. Z. Li, J. Chen, W. Su and M. Hong, J. Mol. Catal. A: Chem., 2010, 328, 93.
65. L. Wu, Z.-W. Li, F. Zhang, Y.-M. He and Q.-H. Fan, Adv. Synth. Catal., 2008, 350, 846.
66. Y. Kitamura, S. Sako, A. Tsutsui, Y. Monguchi, T. Maegawa, Y. Kitade and H. Sajiki, Adv. Synth. Catal., 2010, 352, 718.
67. R. G. Heidenreich, J. G. E. Krauter, J. Pietsch and K. Köhler, J. Mol. Catal. A: Chem., 2001, 182, 499.
68. Y. Li, X. M. Hong, D. M. Collard and M. A. El-Sayed, Org. Lett., 2000, 2, 2385.
69. R. Narayanan and M. A. El-Sayed, J. Phys. Chem. B, 2004, 108, 8572.
70. Y.-R. Liu, L. He, J. Zhang, X. Wang and C.-Y. Su, Chem. Mater., 2009, 21, 557.
71. G. M. Scheuermann, L. Rumi, P. Steurer, W. Bannwarth and R. Mülhaupt, J. Am. Chem. Soc., 2009, 131, 8262.
72. S. Mostafa, F. Behafarid, J. R. Croy, L. K. Ono, L. Li, J. C. Yang, A. I. Frenkel and B. R. Cuenya, J. Am. Chem. Soc., 2010, 132, 15714.
73. J. G. de Vries, Dalton Trans., 2006, 421; W. Cabri and I. Candiani, Acc. Chem. Res., 1995, 28, 2.
74. I. P. Beleskaya and A. V. Cheprakov, Chem. Rev., 2000, 100, 3009.
75. A. B. Dounay and L. E. Overman, Chem. Rev., 2003, 103, 2945.
76. J.-S. Chen, A. N. Vasiliev, A. P. Panarello and J. G. Khinast, Appl. Catal., A, 2007, 325, 76.
77. Y. Ji, S. Jain and R. J. Davis, J. Phys. Chem. B, 2005, 109, 17232.

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Footnote

† Electronic supplementary information (ESI) available: Analytical data and spectra. See DOI: 10.1039/c1cy00324k

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