Mesoporous Ethane−Silicas Functionalized with a Bulky N

Dec 2, 2010 - The Journal of Physical Chemistry C ..... Yu Horiuchi , Dang Do Van , Yusuke Yonezawa , Masakazu Saito , Satoru Dohshi , Tae-Ho Kim , Ma...
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J. Phys. Chem. C 2010, 114, 22221–22229

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Mesoporous Ethane-Silicas Functionalized with a Bulky N-Heterocyclic Carbene for Suzuki-Miyaura Coupling of Aryl Chlorides and Benzyl Chlorides Hengquan Yang,* Xiaojing Han, Guang Li, Zhancheng Ma, and Yajuan Hao School of Chemistry and Chemical Engineering, Shanxi UniVersity, Taiyuan 030006, People’s Republic of China ReceiVed: September 7, 2010; ReVised Manuscript ReceiVed: NoVember 17, 2010

By co-condensation of IMes [IMes ) N,N′-bis(2,6-dimethylphenyl)imidazol-2-ylidene]-bridged organosilane and bis(triethoxysilyl)ethane in the presence of template, a new mesoporous ethane-silica with a built-in bulky N-heterocyclic carbene (NHC) precursor in the framework was synthesized. N2 sorption, XRD, and TEM characterizations revealed that the synthesized material had an ordered mesostructure. FT-IR and solid state NMR investigations confirmed that the IMes moiety was covalently integrated with the solid materials. Such a functionalized material was able to coordinate Pd(OAc)2, leading to an active solid catalyst for Suzuki-Miyaura couplings of challenging aryl chlorides and benzyl chlorides under the relatively mild conditions. By using isopropyl alcohol as solvent and KOtBu as base, a 78% yield for biphenyl was achieved in the presence of 0.5 mol % Pd at 80 °C within 24 h. This solid catalyst could be reused eight times without a significant decrease in activity. The high recyclability may be attributed to the functionalized, stable nanopores that efficiently prevent the in situ formed Pd nanoparticles from the aggregation into the less active large particles in the catalytic reaction. This study not only supplies a novel functionalized periodic mesoporous organosilica (PMO) but also provides an efficient solid catalyst for Suzuki-Miyaura couplings of challenging substrates. 1. Introduction The discovery of periodic mesoporous organosilicas (PMOs) that were synthesized by condensation of bridged organosilanes [(OR′)3Si-R-Si(OR′)3, R ) -CH2CH2- or -C6H4-] in the presence of templates in 1999 represented one of the most important advances in the field of mesoporous materials.1-12 Similar to pure silica mesoporous materials discovered in 1992, PMOs have high surface areas and ordered nanopores, but fundamentally distinguish themselves from pure silica mesoporous materials because of organic groups as an integral part of the framework. Such unique compositions make PMOs own an improved hydrophobicity, and enhanced hydrothermal and mechanical stability. With these properties, PMOs usually exhibit unique performances in the fields of catalysis, adsorption, dielectric materials, and drug delivery.13-17 For example, Yang’s group found that PMO-type catalysts exhibited much higher catalytic activity than the pure silica counterparts.15 It was also found that PMOs materials showed a significantly enhanced stability against the structural collapse in a strong base medium.16,17 More interestingly, the flexible synthesis protocols of PMOs allow the functional PMOs to be fabricated by the co-condensation of a purposely designed group-bridged trialkoxysilane and other small bridged organosilanes such as bis(trialkoxysilyl)ethane in the presence of templates.17 Such functionalized materials, which are often called bifunctional PMOs, enable functional groups to be homogeneously distributed in the framework at the molecular level and their loadings to be tuned in a controlled fashion. This novel concept is inspiring researchers to create solid catalysts with ligand built in the mesoporous framework. For example, diaminocyclohexane-, tartardiamide-, and binol-functionalized PMOs were * To whom correspondence should be addressed. E-mail:hqyang@ sxu.edu.cn. Fax: +86-351-7011688. Phone: +86-351-7010588.

successfully synthesized and showed promising performances in catalysis and separation.18-20 However, in contrast to the everexpanding applications in catalysis, the catalytically active PMOs are still limited. Our current interest concerns the Pd-catalyzed Suzuki-Miyaura coupling reaction because it is currently a fundamental reaction for the synthesis of polymers and agrochemical and pharmaceutical compounds.21,22 Particular attention has been paid to the development of efficient solid catalysts for this coupling of aryl chlorides that has proven difficult but is highly desirable to be used as substrates in view of the wide diversity, ready availability, and low cost in comparison with aryl bromides and iodides.23 Among the active catalysts for this coupling reaction, palladium N-heterocyclic carbene (Pd-NHC) type complexes have been recently found to be more appealing because they are air-stable and have lower toxicity, compared to the wellknown Pd-phosphine systems.24 However, like most of the metal complex catalysts, Pd-NHC complexes suffer from practical problems such as catalyst separation and catalyst recycling. To address these problems, Pd-NHC complexes were immobilized on silica and polymers for easy recovery and reuse.25-31 However, these reported heterogeneous catalysts usually exhibit low activity toward challenging aryl chlorides except for only a few examples.22 The low activity is mainly attributed to the fact that these immobilized NHCs are small ligands that lack bulky electron-donating groups on the imidazole ring.22,32 Furthermore, most of the reported heterogeneous catalysts exhibited low recyclability. The low recyclability is partially related to mesostructure collapse of the catalyst in the basic reaction medium and the growth of Pd nanoparticles into less active large particles during the catalytic reaction (even for these systems using molecular catalysts, the growth of Pd nanoparticles occurs because Pd(0) nanoclusters are in situ formed as the reaction proceeds).33,34

10.1021/jp108519f  2010 American Chemical Society Published on Web 12/02/2010

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IMes [IMes ) N,N′-bis(2,6-dimethylphenyl)imidazol-2ylidene] is one of the bulky, stable, and versatile NHC ligands. Various metal complexes of IMes such as Pd, Cu, Au, and Ru have been synthesized and recently found to be active in many catalytic reactions.35 For example, the hydrogenated IMes is a key ligand of the well-known Grubbs catalyst for the olefin metathesis reaction.36 Pd-IMes complex was found to efficiently catalyze Suzuki-Miyaura couplings of less reactive aryl chlorides.37-40 Recently, Hesemann and co-workers have reported the successful synthesis of an IMes-bridged oragnosilane and IMes-functionalized mesoporous silica.41 However, to our knowledge, there is no report on the synthesis of IMesfunctionalized PMO, its metal loading, and catalytic performances in Suzuki-Miyaura couplings of aryl chlorides. Driven by the unique properties of PMOs and the potential applications of IMes in catalysis, we herein report the synthesis of new channel-like PMO materials (IMes-functionalized mesoporous ethane-silica, denoted as MES-IMes) by co-condensation of IMes-bridged organosilane and bis(triethoxysilyl)ethane in the presence of P123. Furthermore, we developed an active and recyclable solid catalyst for Suzuki-Miyaura coupling of challenging aryl chlorides as well as benzyl chlorides by deriving MES-IMes with Pd(OAc)2. The highly stable, functionalized nanopores were found to efficiently prevent the growth of the in situ formed Pd nanoparticles, thereby improving the recyclability. 2. Experimental Section 2.1. Reagents and Materials. Pluronic P123 (EO20PO70EO20) was purchased from Sigma Company. Pd(OAc)2 was obtained from Kaida Metal Catalyst & Compounds Co. Ltd. (China, purified before use). Bis(triethoxysilyl)ethane was obtained from Gelest Co. (Germany). Pd/C (with a Pd content of 1 wt %) was purchased from Alfa Aesar. Arylboronic acids were obtained from Beijing Pure Chemical Co. Ltd. Most of the aryl chlorides were purchased from Aladdin Company (China). Other reagents were obtained from Shanghai Chemical Reagent Company of Chinese Medicine Group. All solvents were of analytical quality. 2.2. Synthesis of Bis(4-bromo-2,6-dimethylphenyl)diazabutadiene (1). A 9.4 g sample of glyoxal (40 wt % in water) was added dropwise to 100 mL of methanol containing 25.1 g of 4-bromo-2,6-dimethylaniline.41 A few drops of formic acid was also added into this system as a catalyst. The reaction system was stirred at room temperature for 16 h and at 60 °C for another 4 h. The yellow solid was achieved after filtration and then washed with methanol. A 10.9 g sample of the title compound was obtained (yield ca. 70.0%). 1H NMR (CDCl3) δ 8.04 (s, 2 H), 7.10-7.44 (m, 4 H), 2.25 (s, 12 H). 2.3. Synthesis of 1,3-Bis(4-bromo-2,6-dimethylphenyl)-1Himidazol-3-ium Trifluoromethane Sulfonate (2). A 3.0 g sample of compound 1 was dissolved in 37 mL of dry dichloromethane, leading to a yellow solution. A 1.19 g sample of chloromethyl pivalate and 1.6 g of silver trifluoromethanesulfonate were added to the above solution. The reaction mixture was heated to reflux for 24 h. After the solution was cooled to room temperature, the formed precipitate (silver chloride) was removed by filtration. The homogeneous solution was concentrated, giving a crude product. The pure title product was obtained through a recrystallization in a mixture of methanol and diethyl ether (yield 65.0%). 1H NMR (CDCl3) δ 9.87 (s, 1 H), 8.31 (s, 2 H), 7.84 (s, 4 H), 2.19 (s, 12 H). 2.4. Synthesis of 1,3-Bis(4-triethoxysilyl-2,6-methylphenyl)-1H-imidazol-3-iumtrifluoromethane Sulfonate (3). A 2.0 g sample of compound 2, 0.17 g of Rh(cod)(CH3CN)2BF4,

Yang et al. 5.5 g of triethoxysilane, 4.95 g of triethylamine, and 1.64 g of tetrabutylammonium iodide were dissolved in 15 mL of dry DMF. This resulting solution was stirred at 125 °C for 24 h and then at 135 °C for 16 h under N2 atmosphere. After cooling, the solvents were removed by distillation and the resulting brown residue was dissolved with dichloromethane. After removal of the precipitation by filtration, the DCM solution was washed with cold water (5 × 50 mL). The organic phase was dried over sodium sulfate and filtered. After concentration and washing the organic solvents, compound 3 was achieved (yield 50-60%). 1H NMR (CDCl3) δ 9.61-9.81 (m, 1 H), 7.65-7.85 (s, 2 H), 7.20-7.41 (m, 4 H), 3.80-3.82 (m, 12 H), 2.13 (s, 12 H), 1.25-1.32 (m, 18 H). 2.5. Synthesis of PMOs. A 0.66 g sample of P123, 4.19 g of KCl, and 4.5 g of distilled water were added into 20 mL of HCl solution (2 mol/L). After this solution was stirred at 35 °C for 5 h, a total 5.2 mmol of 3 and bis(triethoxysilyl)ethane dissolved in 2.9 g of ethanol were added dropwise to the above solution. The molar fraction of 3 in the total siliceous precursors was tuned from 5% to 10%, 15%, and 20%. After being stirred for 24 h at this temperature, the resulting suspension was transferred into autoclaves. The temperature of the autoclaves was raised to 100 °C and then kept at this temperature for 20 h. After the hydrothermal process, the precipitated solid was isolated by filtration, washed with water and ethanol, and then dried at room temperature. The template was extracted twice with a diluted ethanolic HCl solution (0.5 g of 36 wt % HCl solution/100 mL of ethanol) under the refluxing conditions. According to the molar fraction of compound 3 in the total silicon precursors, four materialssMES-IMes(5%), MESIMes(10%), MES-IMes(15%), and MES-IMes(20%)swere obtained (the number in parentheses means the molar percent of compound 3 in the total silicon precursors). 2.6. Coordination of MES-IMes(χ) Materials with Pd(OAc)2. A 0.6 g sample of MES-IMes(χ) (dried at 100 °C for 4 h) and 0.24 g of potassium tert-butoxide (KOtBu) were dispersed in 4.8 mL of dry THF. After the mixed system was stirred at 40 °C for 6 h under N2 atmosphere, THF was removed through a filtration. The solid was repeatedly washed with THF and dried under vacuum. The resulting solid material was dispersed into 4.5 mL of dry toluene containing 0.012 g of Pd(OAc)2. After the solution was stirred at 50 °C for 5 h, the solid material was isolated by centrifugation, and then washed with dry toluene. After drying under vacuum, the solid catalyst MES-IMes(χ)-Pd was eventually obtained. 2.7. General Procedure for Suzuki-Miyaura Couplings of Aryl Chlorides and Arylboronic Acids. A mixture of aryl chlorides (2 mmol), phenylboronic acid (2.2 mmol), potassium tert-butoxide (3 mmol), isopropyl alcohol (6 mL), and the catalyst MES-IMes(χ)-Pd was stirred at 80 °C under N2 atmosphere for a given time. At the end of the reaction, the mixture was cooled to room temperature and the product was collected by repeated extractions with diethyl ether. The combined organic layers were concentrated and the resulting product was purified by column chromatography on silica gel. The product was confirmed by1H NMR. Its purity was confirmed by GC. The procedures for the coupling reactions of benzyl chloride and arylboronic acids were the same as those of aryl chlorides. The recycling test for the Suzuki-Miyaura coupling was conducted as follows. For the first run, the mixture of 12 mmol of 4′-chloroacetophenone, 13.2 mmol of phenylboronic acid, 18 mmol of KOtBu, MES-IMes(20%)-Pd (0.5 mol % Pd with respect to aryl chloride), and 36 mL of isopropyl alcohol was

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SCHEME 1: Synthesis Routes for IMes Precursor-Bridged Triethoxysilane [IMes, 1,3-Bis(2,6-dimethylphenyl)imidazol2-ylidene]

stirred at 80 °C. At the end of the reaction the system was cooled to room temperature. The product was collected by repeated extractions with diethyl ether. The isolated catalyst was washed with diethyl ether, methanol, and acetone in sequence and dried under vacuum. The recovered catalyst was weighed again. The fresh solvent and substrates were added, but the molar ratios of substrates and solvent to Pd remained the same as those in the first run. 2.8. Characterization and Analysis. Small-angle powder X-ray powder diffraction was performed on a Rigaku D/Max 3400 powder diffraction system (Cu Ka, 40 kV, 30 mA). N2 adsorption analysis was carried out on a micrometritics ASAP2020 volumetric adsorption analyzer (before the measurements, samples were outgassed at 393 K for 6 h). The Brunauer-Emmett-Teller (BET) surface area was evaluated from data in the relative pressure range of 0.05-0.15. The total pore volume was estimated from the amount adsorbed at the relative pressure of ca. 0.99. Pore diameters were determined from the adsorption branch by using the Barrett-Joyner-Halenda (BJH) method. FT-IR spectra were collected on a PE-1730 infrared spectrometer. Pd contents were analyzed by inductively coupled plasma-atomic emission spectrometry (ICP-AES, AtomScan16, TJA Co.). X-ray photoelectron spectra (XPS) were recorded on a Kratos Axis Ultra DLD, and the C1S line at 284.8 eV was used as a reference. TEM micrographs were taken with a JEM-2000EX transmission electron microscope at 120 kV. Solid state NMR spectra were recorded on an Infinityplus 300 MHz spectrometer: for 13C CP-MAS NMR experiments, 75.4 MHz resonant frequency, 4 kHz spin rate, 4 s pulse delay, 1.0 ms contact time, hexamethyl benzene as a reference compound; for 13Si MAS NMR experiments, 79.6 MHz resonant frequency, 4 kHz spin rate, 4.0 s pulse delay, TMS as a reference compound. GC analysis was conducted on a SP-GC6800A. 3. Results and Discussion 3.1. Synthesis of IMes Precursor-Bridged Organosilane and Mesoporous Ethane-Silica. To incorporate IMes precursor into a material framework through a hydrolysis-condensation process, we need to derive the IMes precursor with hydrolyzable alkoxysilyl groups. We followed a modified route for introduction of two trialkoxysilyl groups onto the terminals of IMes precursor (Scheme 1).41 At first, commercially available 4-bromo2,6-dimethylaniline as a starting material was reacted with glyoxal, giving a bis-Shiff’s base compound 1. Compound 1 was then subjected to a ring-closing step in the presence of a silver salt, leading to a bromo-substituted IMes precursor (compound 2). This compound was then silylated with HSi(OEt)3 in the presences of a Rh complex and tetrabutylammo-

SCHEME 2: Synthesis Procedure for Mesoporous Ethane-Silica MES-IMes(χ)

nium iodide as catalysts, eventually affording an IMes precursorbridged organosilane (compound 3). Tetrabutylammonium iodide was found to be very important to obtain compound 3 in a high yield. The whole process included 3 steps and the yields for every step were over 50%. The mesoporous ethane-silicas were fabricated via a modified procedure of a typical synthesis of SBA-15 (Scheme 2).42 A mixture of compound 3 and bis(triethoxysilyl)ethane (BTEE) was used as the siliceous precursor. Methanol was found to be a good solvent that can assist the dissolution of 3 in BTEE. Previous studies revealed that the presence of bulky groupbridged organosilane usually influenced the mesostructural orderings, and even induced phase changes.43 With these findings in consideration, the molar fraction of 3 in the initial total siliceous precursors [M3/(M3 +MBTEE), wherein M is the molar number] was tuned from 5% to 10%, 15%, and 20%. After removal of templates, four MES-IMes(χ) materials were accordingly obtained, wherein χ equaled 5%, 10%, 15%, and 20% according to the molar fraction of 3 in the initial gel mixture. 3.2. Structural Characterization. N2 sorption isotherms of MES-IMes(χ) are presented in Figure 1. The corresponding pore size distribution curves are displayed in Figure S1 (Supporting Information). Their textural parameters measured with N2 sorption are summarized in Table 1. The synthesized samples all exhibit type IV sorption isotherms with sharp capillary

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Figure 1. N2 adsorption/desorption isotherms of MES-IMes(χ): (a) MES-IMes(5%), offset vertically by 300; (b) MES-IMes(10%), offset vertically by 200; (c) MES-IMes(15%), offset vertically by 100; and (d) MES-IMes(20%).

TABLE 1: Physical Parameters of MES-IMes(χ) Materials samples

Sa (m2/g)

Pb (cm3/g)

Dc (nm)

MES-IMes(5%) MES-IMes(10%) MES-IMes(15%) MES-IMes(20%)

1040 783 733 681

0.96 0.74 0.72 0.67

5.5 5.5 5.4 5.3

Nd (wt %) 0.81 1.45 1.86 1.96

(0.92) (1.63) (2.21) (2.65)

a BET surface area. b Single point pore volume calculated at a relative pressure P/P0 of 0.99. c BJH method from adsorption branch. d Determined by combustion chemical analysis; the data in parentheses are the theoretical values.

condensation steps, characteristic of the SBA-15 type of mesoporous materials with uniform pore size distributions. It is worthwhile to note that the capillary condensation step slightly shifts toward the lower pressure with the increase in the molar fraction of 3, indicating a gradual decrease in pore size. Meanwhile, the specific surface area and pore volume also decrease with the increase in the molar fraction of 3 (Table 1). These physical property changes of the synthesized materials with the introduced amount of 3 perhaps reflect the changes of the interactions between the silicon species and template molecules. For the MES-IMes(20%) sample, the BET specific surface area, pore volume, and pore size are 681 m2/g, 0.67 cm3/g, and 5.3 nm, respectively (in Table 1). Such values fall in the ranges of a typical mesoporous material like SBA-15. Figure 2 displays the XRD patterns of MES-IMes(χ) samples. The XRD pattern of MES-IMes(5%) exhibits an intense d100 diffraction peak at 2θ ) 0.91°, as well as two weak diffraction peaks at 2θ ) 1.68° and 1.85°, which can be indexed to the (100), (110), and (200) diffraction peaks of a typical 2D hexagonal phase (like a mesoporous material SBA-15), respectively. As the amount of 3 in the initial gel mixture increases, the d100 diffraction peak slightly shifts toward lower diffraction angles. These results mean a little expansion of the unit cell size. At the same time, (110) and (200) diffraction peaks become weak, indicating that ordering of the synthesized material decreases. TEM images of MES-IMes(20%) samples are shown in Figure 3. The incidence directions parallel and perpendicular to the pore axis are clearly observed in Figure 3a,b, further confirming that the hexagonal arrangement of the highly ordered mesopores is present throughout the sample. These findings are consistent with the results of XRD and N2 sorption isotherms.

Figure 2. XRD patterns of MES-IMes(χ): (a) MES-IMes(5%); (b) MES-IMes(10%); (c) MES-IMes(15%); and (d) MES-IMes(20%).

3.3. Compositional Characterization. To clarify the compositions of the synthesized materials, we employed FT-IR and solid state NMR to further characterize the synthesized MESIMes(χ) samples. For comparison, mesoporous pure ethanesilica (denoted as MES, which was not incorporated with IMes precursor) was also synthesized. Their FT-IR spectra are shown in Figure 4. The FT-IR spectra of samples clearly exhibit peaks at 2980, 2920, and 1414 cm-1. The two former peaks correspond to the stretching vibrations of C-H bonds and the last one is assigned to the bending vibrations of the C-H bonds. Compared with MES, a new peak around 1545 cm-1 is also observed on the MES-IMes(χ) samples, which is related to the vibrations of the imidazole ring. It is worthy to note that its intensity increases as the molar fractions of 3 increase. The FT-IR results also indicate that IMes precursor was incorporated in the materials. The quantitative determinations of 3 incorporated into the obtained materials were made with elemental analysis. The results are also included in Table 1. Elemental analysis showed that the N content on the MES-IMes(χ) samples gradually increased as the fraction of 3 in the initial gel mixture increased, suggesting that the amount of the incorporated IMes moiety increased. However, the determined N contents are all lower than the theoretical values calculated from the initial molar compositions. For example, the N content on MES-IMes(20%) determined by elemental analysis is 1.96 wt %, which is lower than the theoretical value (2.65 wt %). Namely ca. 74% of IMes precursor in the initial gel mixture was incorporated into MESIMes(χ). These results may be due to the partial decomposition of IMes precursor in the acidic conditions. On the basis of the determined N content, the IMes precursor present on MESIMes(20%) is estimated to be ca. 0.7 mmol/g, which is sufficiently high to perform catalytic reactions. Figures S2 and S3 (Supporting Information) show the 29Si MAS NMR and 13C CP-MAS NMR spectra of MES-IMes(20%), respectively. In Figure S2, the 29Si MAS NMR spectrum shows both “T” and “Q” silicon series. The “T” bands include three signals centered at -79, -64, and -56 ppm. The signal at -79 ppm is attributed to T3 [SiC(OSi)3] for Si species bonded with the phenyl groups of the IMes precursor, indicating that IMes precursor was incorporated in the framework through a Si-C linkage. The signals at -64 and -56 ppm can be assigned to T3 [SiC(OSi)3] and T2 [SiC(OH)(OSi)2] for Si species bridged by the ethane moiety, respectively. The signals at -110 and

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Figure 3. TEM images of MES-IMes(20%): (a) in the direction perpendicular to the pore axis and (b) along the direction of the pore axis. The bar is 50 nm.

Figure 4. FT-IR spectra for MES-IMes(χ): (a) MES; (b) MESIMes(5%); (c) MES-IMes(10%); (d) MES-IMes(15%); and (e) MESIMes(20%).

-101 ppm correspond to Q4 [Si(OSi)4] and Q3 [Si(OH)(OSi)3] silicon sites, respectively. The presences of Q bands indicates that a portion of Si-C bonds in bridged organosilanes decomposed. Compared with Hesemann’s work,41 we believe that the high temperature (100 °C) under the acidic conditions is the major reason for Si-C bond cleavage. This presumption is in agreement with Yang’s report.17 These findings are supported by the elemental analysis results that the N contents present on the solid material were lower than the theoretical values. The 13C CP-MAS NMR spectrum of MES-IMes(20%) clearly shows the signals for saturated C at 8-20 ppm, and for the two aryls as well as the central imidazole rings in the range of 125-145 ppm (Figure S3, Supporting Information). The resonance at 59 ppm is attributed to (EtO)3Si- groups that did not completely hydrolyze. It should be noted that the characteristic signals of the template [at 71 ppm, -(CH2CH2O)n-] were not observed. This is an indication that the content of the residual template was below the detection limit of solid state NMR. 3.4. Coordination of the Hybrid Materials with Pd(OAc)2. To probe the coordination capacity of IMes in the material framework, we treated the synthesized materials with a strong base KOtBu to yield carbenes that are reported to own a high coordination capacity toward Pd(OAc)2.36,37 A 2 wt % loading

Figure 5. XPS spectra: (a) the elemental survey scan of MESIMes(20%)-Pd and (b) Pd XPS spectra of Pd(OAc)2, MES-Pd, and MES-IMes(20%)-Pd.

of Pd(OAc)2 with respect to MES-IMes(χ) was used, leading to MES-IMes(χ)-Pd. ICP-AES analysis revealed that Pd(OAc)2 in solution was completely loaded onto MES-IMes(χ) under the investigated conditions. XPS spectroscopy was employed to investigate Pd species on the solid surface. In Figure 5a, an XPS elemental survey scan of surface elements of MESIMes(20%)-Pd reveals that silicon, oxygen, carbon, nitrogen, palladium, fluorine, sulfur, and chlorine element are present on the materials. The presence of chlorine element on the solid catalyst can be explained by the fact that trifluoromethane sulfonate (CF3SO3-) groups of IMes precursor were partially exchanged by Cl- due to the presence of excessive HCl during the course of material synthesis and template removal. The

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Figure 6. TEM images of fresh MES-IMes(20%)-Pd and reused MES-IMes(20%)-Pd: (a) fresh MES-IMes(20%)-Pd; (b) MES-IMes(20%)-Pd used once; (c) MES-IMes(20%)-Pd used four times; and (d) MES-IMes(20%)-Pd used eight times. The bar is 50 nm.

existence of Cl- and CF3SO3- is probably indicative of the fact that a portion of IMes precursor exists in the framework in the form of imidazolium salt. Figure 5b presents the XPS spectra of pure Pd(OAc)2, MES-Pd, and MES-IMes(20%)-Pd. Pd(OAc)2 exhibits two major peaks centered at 344.0 and 338.6 eV, which are assigned to Pd(II) 3d3/2 and Pd(II) 3d5/2 signals, respectively. Compared with pure Pd(OAc)2, the peaks for MES-IMes(20%)Pd underwent apparent changes in view of the shapes and locations. Four peaks at 343.0, 340.6, 337.9, and 335.7 eV seemingly appear. These results indicate that Pd(II) and Pd(0) species are simultaneously formed on the surface of MESIMes(20%)-Pd. To clarify these two types of Pd species, we synthesized mesoporous pure ethane-silica (MES, without IMes moiety) and derived this material with Pd(OAc)2 under the same conditions as MES-IMes(20%). The Pd XPS spectrum of MESPd shows two peaks at 343.0 and 337.9 eV, which are attributable to Pd(II) 3d siganls. In comparison with Pd(OAc)2, these two peaks have apparent shifts. We deduce that such changes may be caused by the interactions of Pd(OAc)2 with surface Si-OH. On the basis of these results, we speculate that Pd(0) and Pd(II) species for MES-IMes(20%)-Pd are related to the interactions with IMes ligand and surface Si-OH groups,

respectively (as proposed in Figure S4, Supporting Information). It should be noted that the Pd(0) clusters were not observed by TEM on the fresh MES-IMes(20%)-Pd (Figure 6a). 3.5. Suzuki-Miyaura Coupling Reactions. To investigate the catalytic activities of the synthesized materials with different amounts of IMes, MES-IMes(χ) materials were coordinated with 2 wt % Pd(OAc)2, resulting in the solid catalysts MES-IMes(χ)Pd. The MES (mesoporous pure ethane-silica) was also treated with Pd(OAc)2 for comparison. Their catalytic activities were examined with Suzuki-Miyarura coupling of chlorobenzene and phenylboronic acid at 80 °C under N2 atmosphere. The molar ratios of Pd to chlorobenzene were kept at 0.5 mol % for all cases. The reaction results are summarized in Table 2. Only a trace amount of product was detected over MES treated with Pd(OAc)2 (denoted as MES-Pd). However, MES-IMes(5%)-Pd afforded a conversion of 21% under the same conditions. It is noteworthy that the increase in the IMes amount resulted in an increase in conversion. A conversion of 82% was obtained on MES-IMes(20%)-Pd within 24 h. The significantly improved conversions verified the role of IMes ligand in the framework in promoting the coupling reaction. On this occasion, the ratio of IMes/Pd is estimated to be ca. 10. In our experiments, it was

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TABLE 2: The Suzuki-Miyaura Couplings of Chlorobenzene and Phenylboronic Acids over MES-IMes(χ)-Pd Catalysts with Different Amounts of IMes

samples

MES

MES-IMes-(5%)

MES-IMes-(10%)

MES-IMes-(15%)

MES-IMes-(20%)

MS-IMes(20%)b

Pd/Cc

% conva

trace

21

56

66

82

68

28

a The conversions were determined with GC. b Mesoporous silica functionalized with IMes (namely, the framework does not contain ethane moiety). c The commercial Pd/C catalyst with a Pd content of 1 wt %.

found that lowering the ratio of IMes/Pd led to a slight reduction in conversion. In view of the stability of the heterogeneous catalysts, the excessive IMes precursors also may be desirable because the uncoordinated IMes precursors in the framework as ionic liquids help to stabilize the in situ formed Pd(0) nanoparticles against growth.44,27 To further evaluate the catalytic performances of PMO-type catalyst, we also synthesized a mesoporous silica functionalized with IMes [MS-IMes(20%), without ethane moieties bridged in the framework] and derived it with Pd(OAc)2 [denoted as MS-IMes(20%)-Pd]. At the same reactions conditions, MS-IMes(20%)-Pd gave a 68% conversion (Table 2). Clearly, MES-IMes(20%)-Pd is more active than MSIMes(20%)-Pd. The improved activity may be due to the organic microenvironment of Pd sites.15 This comparison further highlights the outstanding performances of PMO-type catalysts. Under the same reaction conditions, the commercial Pd/C (1 wt % of Pd) gave a 28% conversion, which was much lower than that for MES-IMes(20%)-Pd (82%). Recently, several types of heterogeneous catalysts for Suzuki-Miyaura couplings were reported such as Pd/C, mesoporous silica-supported palladium, polymer-incarcerated palladium, polyurea-encapsulated palladium, and dendrimer-supported palladium catalysts (in ref 31). These systems successfully catalyzed Suzuki-Miyaura couplings of aryl bromides but failed in couplings of aryl chlorides (especially deactivated aryl chlorides) except for only a few examples.22,31 Although our catalyst MES-IMes(20%)-Pd took 24 h to achieve a good conversion, it could still represent one of the most efficient heterogeneous catalysts for SuzukiMiyaura coupling of aryl chlorides in view of the relatively mild conditions along with a low loading of Pd (0.5 mol %). To explore the substrate scope for this solid catalyst, Suzuki-Miyaura couplings of other aryl halides and phenylboronic acid over MES-IMes(20%)-Pd were further examined. The corresponding results are included in Table 3. The solid catalyst gave biphenyl in an isolated yield of 78% at 80 °C within 24 h (Table 3, entry1). For aryl chloride bearing an electron-withdrawing group, a yield of as high as 98% was obtained in a shorter time (Table 3, entry 2). To our delight, for deactivated aryl chlorides (with an electron-donating group), 70-72% yields could also be achieved (Table 3, entries 3 and 4). The high activity may be contributed by the bulky IMes ligands incorporated in the framework and the high surface area of the solid catalyst. Bromobenzene as well as deactivated aryl bromides were easy to convert to the corresponding products in excellent yields (Table 3, entries 5-7). We further tested the catalytic performances for the coupling of various arylboronic acids with aryl chlorides. The results are summarized in Table 4. For electron-rich arylboronic acids in spite of ortho- and para-substitutions, MES-IMes(20%)-Pd gave excellent yields (Table 4, entries 1-4). As high as 90% yield was obtained for electron-deficient arylboronic acids (Table 4, entry 5). For 1-naphthalene boronic acid, an 85% yield was achieved. Obviously, MES-IMes(20%)-Pd also has a good tolerance toward the substituents on arylboronic acids.

TABLE 3: Suzuki-Miyaura Couplings of Various Aryl Halides and Phenylboronic Acid over MES-IMes(20%)-Pd

a

The isolated yield.

TABLE 4: Suzuki-Miyaura Couplings of Various Aryl Boronic Acids and 4′-Chloroacetophenone over MES-IMes(20%)-Pd

a

Isolated yields.

The recyclability of MES-IMes(20%)-Pd was investigated with the consecutive Suzuki-Miyaura couplings of 4′-chloroacetophenone with phenylboronic acid. Due to the unavoidable loss of solid catalyst during the course of recovery and washing, the reaction scale was amplified 6 times to ensure that the catalyst of good quality was available for us to perform the consecutive recycling reactions. The recycling results of MESIMes(20%)-Pd are summarized in Table 5. Under the scale-up

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TABLE 5: Recyclability Test of MES-IMes(20%)-Pd with Suzuki-Miyaura Couplings

cycles

1

2

3

4

5

6

7

8

time (h) yielda (%)

18 99

18 99

18 99

18 99

18 93

18 90

23 93

23 90

a

TABLE 6: The Coupling Reactions of Various Arylboronic Acids and Benzyl Chlorides over MES-IMes(20%)-Pd

Isolated yield.

conditions, the fresh solid catalyst gave the product in a 99% yield within 18 h. After the first reaction cycle, the catalyst could be recovered after a centrifugation followed by a filtration. After washing and drying, the recovered catalyst was directly used for the next reaction cycle. From the second to the fourth reaction cycle, acetylbiphenyl in 99% yields were achieved without a prolonged reaction time. The yield was observed to decrease from the fifth reaction cycle, and a 90% yield was obtained within 18 h for the sixth reaction cycle. For the eighth reaction cycles, a yield of 90% could still be achieved by slightly prolonging the reaction time to 23 h. Although the activity of the solid catalyst decreases after the consecutive recycling reactions, it represents one of the most robust heterogeneous catalysts for Suzuki-Miyaura coupling of the challenging aryl chlorides in view of the relatively low Pd loading.25,31 FT-IR investigations of the MES-IMes(20%)-Pd used once preliminarily prove that the NHC structure survived the catalytic reactions. To determine the heterogeneous nature of the catalytic reaction, we conducted a hot filtration experiment. After the coupling reaction of chlorobenzene with phenylboronic acid proceeded for 8 h (the conversion at 55%) the reaction was stopped and the filtrate was immediately collected under the hot conditions. A further 5% increase in conversion was detected after heating this filtrate at 80 °C for another 16 h (40% of the initial KOt-Bu amount was added to the filtrate). These results may indicate that the conversion was at least partially contributed by the leached Pd species in the solution. However, N element in the filtrate was not detected by elemental analysis, excluding the possibility that the leached Pd were Pd-NHC species. Probably as many references reported, the solid catalyst acts as a Pd reservoir where the leached Pd in the solution redeposits at the end of the catalytic reaction. To confirm this possibility in our case, we further determined the Pd residues in filtrates that were collected after the coupling reactions over MES-IMes(χ)-Pd proceeded for 24 h and then cooling. The results are listed in Table S1 (Supporting Information). Interestingly, Pd residue in the filtrates gradually decreases as the IMes amount on the solid materials increases. For MES-IMes(20%)Pd, the Pd concentration in the filtrate is less than 0.5 ppm. These results demonstrate the role of this solid material in capturing Pd from the solution at the end of catalytic reaction although the reaction nature probably involves homogeneous pathways. To further understand the reason for the high recyclability for Suzuki-Miyaura coupling of aryl chlorides, TEM was employed to characterize the fresh and reused MES-IMes(20%)Pd. The TEM images are presented in Figure 6. The fresh catalyst MES-IMes(20%)-Pd showed well-defined nanochannels (Figure 6a). Metal particles were not observed throughout the solid catalyst. However, for MES-IMes(20%)-Pd used once, it was found that the Pd nanoparticles appeared and most of them were dispersed in the nanochannels (Figure 6b). More interestingly, for MES-IMes(20%)-Pd used four times, the fine Pd

a

Isolated yield.

nanoparticles still were observed to be located inside the nanochannels (Figure 6c). MES-IMes(20%)-Pd used eight times shows similar observations (Figure 6d). The ordered mesostructures of the synthesized material remained almost intact even after undergoing eight reaction cycles in a strong base medium (Figure 6d). Our previous investigation and other groups’ work revealed that even for these systems with molecular catalysts, once the catalytic reaction started, Pd(0) in situ formed and then evolved into Pd nanoclusters.6 Due to the high surface energy, the unstable nanoclusters are prone to aggregation and agglomeration into larger particles, leading to the decease in activity of the reused catalysts.27,45,46 However, for our catalysts, the nanochannels probably plus ionic liquids-layer (imidazolium salt) on the surfaces of MES-IMes efficiently prevented the aggregation or agglomeration of Pd nanoparticles through the spatial restrictions and electrostatic interactions, repectively.44,47,48 The limited growth of Pd nanoparticles and the highly stable ethane frameworks ensured that the reused catalyst had so high activity for aryl chlorides. 3.6. The Coupling Reactions of Benzyl Chloride and Arylboronic Acids. Encouraged by the impressive results of our catalyst in Suzuki-Miyaura couplings of aryl chlorides and boronic acids, we wanted to test the catalytic activity toward the couplings of benzyl chlorides and arylboronic acids. This coupling reaction provides an important alternative to Friedel-Crafts reaction for the synthesis of diarylmethane.49-51 However, to our knowledge, few heterogeneous catalysts have been reported. The couplings of benzylic chlorides over MES-IMes(20%)Pd were performed at 80 °C also using isopropyl alcohol as solvent. The results are summarized in Table 6. With use of arylboronic acids such as phenylboronic acid and 1-naphthalene boronic acid, the solid catalyst MES-IMes(20%)-Pd afforded the corresponding products in yields of 75% and 79%, respectively. For electron-rich arylboronic acids, various diarylmethanes in 75-95% yields were obtained (Table 6, entries

Synthesis of a New Mesoporous Ethane-Silica 3-7). For electron-deficient arylboronic acid, the solid catalyst also gave a yield of 85% (Table 6, entry 8). These results demonstrate that MES-IMes(20%)-Pd has high activity for the couplings of benzyl chlorides and arylboronic acids. 4. Conclusions We have successfully synthesized a new mesoporous ethanesilica with a well-known NHC ligand (IMes) precursor in the framework by co-condensation of IMes precursor-bridged triethoxysilane and bis(triethoxysilyl)ethane in the presence of template P123. The synthesized material had a large surface area, high pore volume, and ordered mesostructure. The IMes ligand with a concentration of ca. 0.7 mmol/g can be incorporated on the PMO materials. Such a material was able to coordinate Pd(OAc)2, leading to an active solid catalyst for Suzuki-Miyaura couplings of challenging aryl chlorides. This PMO-type of catalyst exhibits higher activity than mesoporous silica-based catalyst and commercial Pd/C catalyst. It could be reused eight times in a strong base medium without a significant decrease in activity, representing a robust solid catalyst for Suzuki-Miyaura reaction of aryl chlorides. The high recyclability may be attributed to the highly stable, functionalized nanopores that efficiently prevent the growth of the in situ formed Pd nanoparticles. Furthermore, this solid catalyst was active for the couplings of benzyl chloride and arylboronic acids, giving satisfactory yields. This study not only supplies a novel functionalized PMO material but also provides an efficient solid catalyst for Suzuki-Miyaura couplings of challenging substrates. Acknowledgment. We acknowledge the New Teacher Foundation from Education Ministry of China (200801081035), Shanxi Natural Science Foundation for Youths (2009021009), the Natural Science Foundation of China (NSF20903064), Shanxi University Innovative Experimental Project for Undergraduates, and Jiangsu Key Lab. of Fine Petrochemistry for financial support (KF0802). Supporting Information Available: Additional results include pore size distribution curves, 29Si MAS NMR, 13C CPMAS NMR, the proposed Pd species, Pd concentrations in the filtrates and the 1H NMR data for the coupling products. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Inagaki, S.; Guan, S.; Fukushima, Y.; Ohsuna, T.; Terasaki, O. J. Am. Chem. Soc. 1999, 121, 9611. (2) Melde, B. J.; Holland, B. T.; Blanford, C. F.; Stein, A. Chem. Mater. 1999, 11, 3302. (3) Asefa, T.; MacLachlan, M. J.; Coombs, N.; Ozin, G. A. Nature 1999, 402, 867. (4) Hoffmann, F.; Cornelius, M.; Morell, J.; Froba, M. J. Nanosci. Nanotechnol. 2006, 6, 265. (5) Jiang, D. M.; Yang, Q. H.; Wang, H.; Zhu, G. R.; Yang, J.; Li, C. J. Catal. 2006, 239, 65. (6) Loyt, D. A.; Shea, K. J. Chem. ReV. 1995, 95, 1430. (7) Wan, Y.; Zhang, D. Q.; Zhai, Y. P.; Feng, C. M.; Chen, J.; Li, H. X. Chem. Asian J. 2007, 2, 875. (8) Qiao, S. Z.; Lin, C. X.; Jin, Y. G.; Li, Z.; Yan, Z. M.; Hao, Z. P.; Huang, Y. N.; Lu, G. Q. J. Phys. Chem. C 2009, 113, 8673. (9) Cho, E. B.; Kim, D.; Jaroniec, M. J. Phys. Chem. C 2008, 112, 4897. (10) Bao, X. Y.; Zhao, X. S. J. Phys. Chem. B 2005, 109, 10727.

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