Organostannoxane-Supported Palladium Nanoparticles. Highly

Organometallics 2009, 28, 5883–5888 DOI: 10.1021/om900487h

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Organostannoxane-Supported Palladium Nanoparticles. Highly Efficient Catalysts for Suzuki-Coupling Reactions Vadapalli Chandrasekhar,* Ramakirushnan Suriya Narayanan, and Pakkirisamy Thilagar Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208016, India Received June 9, 2009

A phosphine-rich hexameric organostannoxanne, [n-BuSn(O)COOL]6 (PDrum; L = p-PPh2-C6H4-), has been synthesized by a 6:6 reaction between [n-BuSn(O)OH]n and p-Ph2PC6H4COOH. 119Sn{1H} NMR of PDrum revealed a single resonance at -481.0 ppm, indicating that it possesses the characteristic drum structure that is well-known among organostannoxanes. Reduction of PdCl2 with NaBH4 in the presence of PDrum afforded a palladium nanocomposite, NP-1. HRTEM of NP-1 indicates that the average nanoparticle size is about 3 nm. The latter has been shown to be very effective in mediating Suzuki-coupling reactions involving a number of different substrates. NP-1 can be recycled, and HRTEM analysis of the used catalyst reveals some agglomeration. Treatment of NP-1 with n-hexane and cooling to -25 °C afforded another palladium nanocomposite, NP-2. The latter also is very effective in catalyzing the Suzuki-coupling reaction. HRTEM of NP-2 shows an average nanoparticle size of about 4.3 nm, which remains nearly unchanged even after catalysis. NP-2 is a composite also containing some amounts of SnO2.

Introduction Organostannoxanes have been attracting interest for several reasons.1-3 The remarkable structural variation of these compounds, often induced by a subtle modification of the reaction conditions or the reagents employed, has been a major driving force for the sustained activity in this area.4-6 Another reason of interest is the utility of organostannoxane frameworks as scaffolds for supporting an interesting functional periphery.7 Since the current knowledge on the synthesis of organostannoxane assemblies is reasonably advanced, it is possible to prepare specific designer stannoxanes exclusively.1 By an appropriate choice of ligands dendrimer-like molecules can be constructed containing a stannoxane core and a functional periphery. Utilizing this approach, we have decorated the stannoxane cores with electro- and photoactive ligands.8,9 We have also utilized the stannoxane synthesis methodology for the preparation of multisite *To whom correspondence should be addressed. E-mail: vc@iitk. ac.in. Tel: (þ91) 512-259-7259. Fax: (þ91) 512-2597436. (1) Chandrasekhar, V.; Gopal, K.; Thilagar, P. Acc. Chem. Res. 2007, 40, 420. (2) Chandrasekhar, V.; Nagendran, S.; Baskar, V. Coord. Chem. Rev. 2002, 235, 1. (3) Holmes, R. R. Acc. Chem. Res. 1989, 22, 190. (4) Garcı´ a-Zarracino, R.; H€ opfl, H. J. Am. Chem. Soc. 2005, 127, 3120. (5) Beckmann, J.; Dakternieks, D.; Duthie, A.; Lewcenko, N. A.; Mitchell, C. Angew. Chem., Int. Ed. 2004, 43, 6683. (6) Ma, C.; Sun, J. Dalton Trans. 2004, 1785. (7) Delavaux-Nicot, B.; Kaeser, A.; Hahn, U.; Gegout, A.; Brandli, P. -E.; Duhayon, C.; Coppel, Y.; Saquet, A.; Nierengarten, J.-F. J. Mater. Chem. 2008, 18, 1547. (8) Chandrasekhar, V.; Nagendran, S.; Bansal, S.; Kozee, M. A.; Powell, D. R. Angew. Chem., Int. Ed. 2000, 39, 1833. (9) Chandrasekhar, V.; Thilagar, P.; Steiner, A.; Bickley, J. F. Chem. Eur. J. 2006, 12, 8847. r 2009 American Chemical Society

coordination ligands.10 In view of this success, we turned our attention to the use of organostannoxanes for stabilizing palladium nanoparticles. Accordingly, in this paper we report the preparation of a phosphine-rich monoorganostannoxane cage, [n-BuSn(O)O2CC6H4-p-PPh2]6 (PDrum), and the nanocomposites NP-1 and NP-2. These nanocomposites have been shown to be extremely efficient, reusable catalysts for the Suzuki-coupling reaction.

Results and Discussion The reaction of the organotin oxide hydroxide [n-BuSn(O)OH]n with any carboxylic acid, RCOOH, in a 6:6 ratio is known to afford the hexameric cage [n-BuSn(O)O2CR]6, which possesses a drumlike structure.1 The core of this cage is made up of a Sn6O6 framework which is supported by bidentate carboxylate ligands. We reasoned that the reaction of p-(diphenylphosphino)benzoic acid (LCOOH) with [n-BuSn(O)OH]n would afford a phosphine-rich organostannoxane. Accordingly, the reaction of LCOOH (L = p-PPh2C6H4-) with [n-BuSn(O)OH]n in a 6:6 stoichiometry afforded the hexameric cage [n-BuSn(O)O2CL]6 (PDrum) (Scheme 1). The 119Sn{1H} NMR spectrum of PDrum shows the presence of a single peak at -481.0 ppm, which is the signature of organostannoxanes with a drum structure.6-10 The 31P{1H} NMR spectrum of PDrum shows a singlet a t -4.6 ppm which is comparable to that of LCOOH (-4.2 ppm), indicating that the phosphine units in the organostannoxane drum are present in a chemical environment similar to that in the parent ligand.11 (10) Chandrasekhar, V.; Thilagar, P.; Sasikumar, P. J. Organomet. Chem. 2006, 691, 1681. (11) Phadnis, P. P.; Dey, S.; Jain, V. K.; Nethaji, M.; Butcher, R. J. Polyhedron 2006, 25, 87. Published on Web 09/25/2009

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Organometallics, Vol. 28, No. 20, 2009 Scheme 1. Synthesis of PDrum

Scheme 2. General Synthetic Scheme for the Suzuki-Coupling Reaction

Phosphine ligands have been found to be the best candidates for the stabilization of palladium catalysts in homogeneous systems.12 Phosphine ligands have been used as additives and have also been found to enhance the catalytic activity/selectivity in heterogeneous systems.13 In recent years the use of phosphine-containing ligands in stabilizing Pd nanoparticles is also being studied.14 In view of this, we have reasoned that the phosphine-rich stannoxane PDrum would be able to stabilize Pd nanoparticles. Accordingly, PDrum was treated with PdCl2 and subsequently with NaBH4. After the workup a nanocomposite containing palladium nanoparticles and organostannoxane cages was isolated as a shiny black material, NP-1. The 119Sn{1H} NMR spectrum of NP-1 revealed a resonance at around -480.0 ppm. The 31P{1H} NMR spectrum of NP-1 showed two sharp signals at þ29.5 and -4.6 ppm. Since the proximity of nanoparticles is expected to reduce the isotropy of the (12) (a) Martin, R.; Buchwald, S. L. Acc. Chem. Res. 2008, 41, 1461. (b) Fu, G. C. Acc. Chem. Res. 2008, 41, 1555. (c) Hartwig, J. F. Nature 2008, 455, 314. (13) (a) Nishio, R.; Suguira, M.; Kobayashi, S. Org. Lett. 2005, 7, 4831. (b) Leadbeater, N. E.; Marco, M. Chem. Rev. 2002, 102, 3217. (c) Dahan, A.; Portnoy, M. J. Am. Chem. Soc. 2007, 129, 5860. (14) (a) Taruma, M.; Fujihara, H. J. Am. Chem. Soc. 2003, 125, 15742. (b) Penno, D.; Lillo, V.; Koshevoy, I. O.; Sanau, M.; Ubeda, M. A.; Lahuerta, P.; Fernandez, E. Chem. Eur. J. 2008, 14, 10648. (c) Schmidt, F. K.; Belykh, L. B.; Goremyka, T. V. Kinet. Catal. 2003, 44, 623.

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surrounding atoms and hence broaden the NMR signals emanating from the latter,15 we investigated the origin of the NMR signals in NP-1. Suspecting that these could be due to excess free ligand and its fully oxidized form, we deliberately oxidized PDrum and prepared PODrum, which contains phosphine oxides in the periphery. The latter showed a singlet at ∼29.6 ppm in its 31P{1H} NMR spectrum, similar to one of the resonances found in NP-1 (Supporting Information). Addition of an excess of PDrum and PODrum to NP-1 did not result in any additional signals in the 31P{1H} NMR spectrum (Supporting Information), strongly suggesting that the signals observed in NP-1 are the result of the presence of these species. Oxidation of phosphine ligands in presence of Pd and also Rh nanoparticles has been documented earlier.16 In order to test if any molecular complexes are present as a contaminant with NP-1, we reacted bis(benzonitrile)palladium dichloride with PDrum and attempted to isolate molecular complexes. However, we were unable to isolate pure compounds. The 31P{1H} NMR spectrum of the reaction mixture showed a cluster of peaks at ∼22 ppm in addition to the peak due to phosphine oxide at 29.5 ppm (Supporting Information). Since NP-1 does not show any peaks at ∼22 ppm, it is reasonable to conclude that it is not contaminated with any molecular complex(es). In order to remove the excess PDrum and PODrum from NP-1, it was dissolved in chloroform/methanol (1:1) and the solution was stirred for 6 h, treated with n-hexane, and cooled to -25 °C for 24 h (Supporting Information). The dark brown particles that deposited were separated from the solution via a commercially available PTFE membrane filter (pore size of 0.22 μm).17 This process was repeated three times to collect Pd nanoparticles, NP-2. The 31P{1H} NMR spectrum of the latter was flat, consistent with expectation.18 A TGA analysis of NP-1 and NP-2 reveals an interesting difference (Supporting Information). While NP-1 has a char residue of 19.5% (∼900 °C), NP-2 has a much higher char residue of 35.5% (∼900 °C), suggesting that the latter might contain some SnO219 along with the palladium nanoparticles. Powder XRD of NP-2 also seems to indicate the presence of SnO2 (Supporting Information). HRTEM of NP-1 indicates that the size of the palladium nanoparticles ranges between 1.5 and 4.0 nm, with the bulk of the particles showing a size of about 3.0 nm (Figure 1). HRTEM of NP-2 reveals that it possesses an average size of 4.3 nm (Figure 2). NP-1 and NP-2 are both dispersible in a number of organic solvents, including CHCl3, CH2Cl2, DMF, DMSO, MeOH, EtOAc, toluene, and THF. These nanocomposites can be reconstituted several times by drying and redissolving without any noticeable formation of palladium black, thus indicating the crucial stabilizing role of the organostannoxane drum. (15) (a) Zelakiewicz, B. S.; De Dios, A. C.; Tong, Y. J. Am. Chem. Soc. 2003, 125, 18. (b) Badia, A.; Gao, W.; Sing, S.; Demers, L.; Cuccia, L.; Reven, L. Langmuir 1996, 12, 1262. (16) (a) Wu, L.; Li, Z.-W.; Zhang, F.; He, Y.-M.; Fan, Q.-H. Adv. Synth. Catal. 2008, 350, 846. (b) Gl€ockler, J.; Kl€utzke, W.; Meyer-Zaika, W.; Reller, A.; García-García, F. J.; Strehblow, H.-H.; Keller, P.; Rentschler, E.; Kl€aui, W. Angew. Chem., Int. Ed. 2007, 46, 1164. (17) Horinouchi, S.; Yamanoi, Y.; Yonezawa, T.; Mouri, T.; Nishihara, H. Langmuir 2006, 22, 1880. (18) Badetti, E.; Caminade, A.-M.; Majoral, J.-P.; Moreno- Ma~ nas, M.; Sebastian, R. M. Langmuir 2008, 24, 2090. (19) de Monredon, S.; Cellt, A.; Ribot, F.; Sanchez, C.; Armelao, L.; Gueneau, L.; Delattre, L. J. Mater. Chem. 2002, 12, 2396.

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Figure 1. HRTEM image of NP-1 along with the particle size distribution of the nanoparticles.

Figure 2. HRTEM image of NP-2 along with the particle size distribution of the nanoparticles.

The catalytic activity of NP-1 and NP-2 was examined for the Suzuki-coupling reaction (Scheme 2), in view of the versatility of the latter for generating biaryls.20 In the case of NP-1, the catalytic reactions were performed in refluxing toluene, whereas in case of NP-2 the reaction conditions were further optimized and the reaction was carried out in lower boiling methanol. In all the reactions K2CO3 was used as the base. The results of the study carried out are summarized in Tables 1 and 2. The reactivities of NP-1 and NP-2 appear to be similar. Although aryl bromides and aryl iodides are the preferred substrates, the reaction works well even for some aryl chlorides (Table 1, entries 3 and 14; Table 2, entry 3). A variety of aryl bromides have been used, including those that contain heteroatoms (Table 1, entries 12 and 13; Table 2, entry 10). Also, both electron-withdrawing and electronreleasing substituents have been used with equal success (Table 1, entries 4-8; Table 2, entries 4-7). These are also very effective for higher aromatics (Table 1, entries 9-11; Table 2, entries 8 and 9) and for functional group containing substrates (Table1, entries 6-8; Table 2, entries 6 and 7). The (20) (a) Miyaura, N.; Suzuki, A. Chem. Rev. 1995, 95, 2457. (b) Littke, A. F.; Fu, G. C. Angew. Chem., Int. Ed. 2002, 41, 4176.

catalytic efficiency of these nanocomposites is perhaps due to the geometry of the Sn6O6 cage. The angular separation of the phosphine groups imposed by the organostannoxane structure and the steric bulk of the organostannoxane cage, presumably, are responsible for stabilizing palladium nanoparticle domains while at the same time allowing access to the reactants and products. In order to assess if the catalytic reactions with NP-1 and NP-2 are homogeneous, we performed a Hg(0) poisoning test. Accordingly, to the reaction mixture we added mercury (∼2000 times in excess by weight vis- a-vis the catalyst). No change in reactivity was observed. Earlier studies suggest that in most cases Hg(0) does not effect a homogeneous reaction while it forms an amalgam with a heterogeneous catalyst and poisons it.21,22 In order to assess if NP-1 and NP-2 could be reused, we performed recycling reactions using the example cited in Table 1, entry 1 as the test case. In the case of NP-1 we used an in situ recycling methodology.23 After completion of the (21) Weddle, K. S.; Aiken, J. D., III; Finke, R. G. J. Am. Chem. Soc. 1998, 120, 5653. (22) Eberhard, M. R. Org. Lett. 2004, 6, 2125. (23) Narayanan, R.; El-Sayed, M. A. J. Am. Chem. Soc. 2003, 125, 8340.

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Table 1. Suzuki-Coupling reactions Catalyzed by NP-1a

Table 2. Suzuki-Coupling Reactions Catalyzed by NP-2a

a

Reaction conditions: ArX:PhB(OH)2:K2CO3 stoichiometry 1:1.2:2; 5 mg of NP-2 in 5 mL of methanol. The reaction is allowed to complete as monitored by TLC. The yields reported are isolated yields; the purity of product was checked by 1H NMR and melting points. In the body of the table, the “dollar sign” label indicates that recycling experiments were carried out for this reaction using used NP-2 via 0.22 μm PTFE membrane filter.

a

Reaction conditions: ArX:PhB(OH)2:K2CO3 stoichiometry 1:1.5:2; 5 mg of Pd nanoparticles in 10 mL of toluene. The reaction was monitored by TLC. Yields represent isolated values; the purity of products was checked by 1H NMR as well as by melting points. In the body of the table, the “dollar sign” label indicates that in situ recycling experiments were carried out for this reaction 10 times using aryl halide as the limiting reagent.

first cycle fresh amounts of reagents were added to the same reaction flask and the reaction was continued. In this manner the reaction was carried out for 10 cycles without any loss of activity. In the case of NP-2 the catalyst was recovered, by simple filtering (using a 0.22 μm PTFE membrane filter)17,24 and was reused as such for the next cycle. Fresh iodobenzene, phenylboronic acid, and K2CO3 (1:1.2:2) were charged into the flask and the reaction was continued. We have found that at the end of the next cycle the isolated yield of the products was ∼95%, indicating that the catalyst efficiency is not diminished during the reaction. Thus, both NP-1 and NP-2 were found to be quite effective, even in recycling experiments. In order to understand the fate of the palladium nanoparticles after the reaction, we examined the nature of the catalyst after the reaction (first cycle) was completed. We isolated the catalyst (NP-1), and its HRTEM was recorded. This experiment indicated that some agglomeration of the nanoparticles occurred (average size ∼45 nm), while particles with an average size of ∼3 nm were also present (Figure 3). As indicated above, this observation does not seem to affect the reactivity of the nanoparticles. In the case of NP-2 the particle size remains nearly invariant even after catalysis (Supporting Information). Finally, in order to (24) Niembro, S.; Shafir, A.; Vallribera, A.; Alibes, R. Org. Lett. 2008, 10, 3215.

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Figure 3. HRTEM images of NP-1 after the first cycle of catalysis. The reaction was between iodobenzene and phenylboronic acid.

assess if the stannoxane cage is really required to stabilize the nanoparticles, we attempted to prepare the palladium nanoparticles by using LCOOH alone. Although formation of the palladium nanoparticles was observed, these agglomerate into palladium black even after a single catalytic cycle, underscoring the need for the dendrimer-like stannoxane cage for stabilization of palladium nanoparticles. It must be mentioned, that in recent years, other inorganic cages/rings such as silsequioxanes25 and cyclophosphazenes18 have also been used for generating dendrimer-like ligands for similar applications.

Conclusion In conclusion, we have designed and assembled a new functional organostannoxane cage, PDrum, that contains phosphine ligands in its periphery. We have successfully utilized this dendrimer-like molecule to stabilize the palladium nanoparticles NP-1 and NP-2, which have been shown to be excellent reusable catalysts for the Suzuki-coupling reaction.

Experimental Section General Remarks. [n-BuSn(O)OH]n and 4-(diphenylphosphino)benzoic acid were purchased (Aldrich) and were used as such without further purification. Solvents were stored over appropriate reagents and distilled under nitrogen prior to use. Melting points were measured using the JSGW apparatus and are uncorrected. Elemental analyses were carried out by using a Thermo Quest CE Model EA/110 CHNS-O elemental analyzer. 1H, 13P, and 119Sn NMR were recorded on a JEOL-JNM LAMBDA Model 400 NMR spectrometer in CDCl3 solutions. The chemical shifts are referenced with respect to tetramethylsilane (1H), 85% H3PO4 (31P), and tetramethyltin (119Sn), respectively. Powder XRD patterns have been recorded with a Rich Seifert Debye 2002 powder diffractometer using Cu KR radiation (λ = 1.5418 A˚). TGA measurements were carried out using a Perkin-Elmer Pyris6 thermogravimetric analyzer at a heating rate of 10 °C/min under an argon atmosphere. High Resolution Transmission Electron Microscopy (HRTEM). HRTEM images were recorded on a FEI Technai 20 U Twin transmission electron microscope, at an operating voltage of 200 kV. A drop of a methanol/dichloromethane solution of the nanoparticle was spotted on a commercially available 400 mesh Cu grid coated with a thin carbon film. (25) Naka, K.; Itoh, H.; Chujo, Y. Nano Lett. 2002, 2, 1183.

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Scanning Electron Microscopy. SEM images were recorded on a FEI QUANTA 200 HV instrument at an operating voltage of 20 kV. The samples are gold-coated prior to imaging. Synthesis. [n-BuSn(O)O2C-C6H4-p-PPh2]6 (PDrum). [n-BuSn(O)OH]n (0.30 g, 1.44 mmol) and p-(diphenylphosphino)benzoic acid (0.44 g, 1.44 mmol) were taken up in toluene (80 mL) and heated under reflux for 6 h. The water formed in the reaction was removed by using a Dean-Stark apparatus. The reaction mixture was filtered and evaporated to afford a solid identified as the title product, which was purified subsequently by recrystallization. Yield: 0.70 g (95.0%). Mp: >290 °C dec. 1H NMR (400.0 MHz, ppm): 0.81 (t, J = 7.35 Hz, 18H), 1.32 (m, J = 5.86 Hz, 12H), 1.50 (m, 12H), 1.73 (m, 12H), 7.23 (m, 60H), 7.91 (m, 24H). 31P{1H} NMR (161.0 MHz, ppm): -4.6 (s). 119Sn{1H} NMR (150.0 MHz, ppm): -481.0 (s). Anal. Calcd for C152H154O18P6Sn6 (including two molecules of toluene): C, 57.65; H, 4.90. Found: C, 57.41; H, 4.87. [nBuSn(O)O2C-C6H4-p-P(O)Ph2]6 (PODrum). A stoichiometric amount of PDrum (0.30 g, 0.1 mmol) and hydrogen peroxide (0.5 mL) were taken up in chloroform (50 mL) and stirred for 3 h. The reaction mixture was filtered and evaporated to afford a solid identified as the title product which was purified subsequently by recrystallization. Yield: 0.29 g (95.0%). Mp: >300 °C. 1H NMR (400.0 MHz, ppm): 0.81 (t, J = 7.35 Hz, 18H), 1.32 (m, J = 5.86 Hz, 12H), 1.50 (m, 12H), 1.73 (m, 12H), 7.23 (m, 60H), 7.91 (m, 24H). 31 P{1H} NMR (161.0 MHz, ppm): 29.5 (s), 119Sn{1H} NMR (150.0 MHz, ppm): -481.0 (s). Anal. Calcd for C138H138O24P6Sn6: C, 53.69; H, 4.5. Found: C, 54.16; H, 4.83. Palladium Nanoparticles. Synthesis of NP-1. A solution containing 1 mmol of PdCl2 and 1 mmol of PDrum in 75 mL of CHCl3/MeOH (3:1) was stirred for 30 min. To this mixture was slowly added a methanolic solution of NaBH4 (10 mmol) for 10 min. The color of the solution changed from yellow to dark brown and then finally to black. The reaction mixture was stirred further for 6 h, resulting in a dark brown colloidal solution. All the volatiles were evaporated in vacuo, and the resulting solid was dissolved in water (20 mL) (to remove excess NaBH4 and other inorganic salts) and extracted with chloroform. The combined chloroform solution was evaporated under reduced pressure, affording the shiny black material NP-1. The specific details of the reaction are given below. PDrum (2.98 g, 1.00 mmol), PdCl2 (0.18 g, 1.00 mmol), and NaBH4 (0.38 g, 10.0 mmol). Yield: 90%. 1H NMR (400 MHz, ppm): 0.81 (t, J = 7.56 Hz), 1.18 (m), 1.62 (m), 7.39 (m), 7.47 (m), 7.57 (m), 7.59 (m). 31P NMR (161 MHz, ppm): -5.2 (s), 31.1(s). 119Sn NMR (150 MHz, ppm): -480.0 (s). Synthesis of NP-2. To the NP-1 particles was added a chloroform/methanol (1:1) mixture, and the reaction mixture was vigorously stirred for 6 h. To this was added n-hexane, and the reaction mixture was kept at -25 °C for 1 day. Precipitated brown particles were collected by filtering via a commercially available 0.22 μm PTFE membrane filter, and this process was repeated three times. A brown powder was obtained whose 31 P{1H} NMR revealed the absence of peaks. The specific details of the reaction are given below. PDrum (2.98 g, 1.00 mmol), PdCl2 (0.177 g, 1.00 mmol), and NaBH4 (0.38 g, 10.0 mmol). Yield: 65%. 1H NMR (400 MHz, ppm): no signal. 31P NMR (161 MHz, ppm): no signal. General Procedure for the Suzuki-Coupling Reactions and Catalyst Recycling. Catalysis by NP-1. In a 25 mL flask was added the aryl halide, phenylboronic acid, and K2CO3 in a mole ratio of 1:1.5:2, respectively. To this was added 5.00 mg of NP-1 and 10 mL of distilled toluene. The reaction mixture was heated to reflux and stirred. The reaction was monitored by TLC. At the end of the reaction all the volatiles were removed in vacuo and the residue was extracted with a hexane-ethyl acetate solvent mixture (15 mL  3). Evaporation of the combined organic extracts afforded pure Suzuki-coupled products.

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The purity of the products was checked by proton NMR as well as the melting point measurements. Recycling experiments were carried out for the Suzukicoupling reaction between iodobenzene and phenylboronic acid. For these experiments we used the same reaction mixture, assuming aryl halide as the limiting reagent. The catalytic activity remained unchanged even after 10 consecutive cycles. Catalysis by NP-2. Even though the Suzuki-coupling reaction proceeds well for NP-2 using toluene as solvent, we optimized the reaction conditions to the more environmentally friendly (lower boiling) solvent methanol. In a 25 mL flask was added aryl halide, phenylboronic acid, and K2CO3 in a mole ratio of 1:1.2:2, respectively, 5 mg of NP-2, and 5 mL of distilled methanol. The reaction mixture was heated to reflux and stirred until the complete formation of products as detected by TLC. After all the volatiles were removed in vacuo, the residue was extracted with a hexane-ethyl acetate solvent mixture (15 mL  3). The precipitated catalyst was recovered by simply filtering via a 0.22 μm PTFE membrane filter, and the solid mixture was washed three times with 10 mL of H2O, dried under reduced pressure, and then used directly for the next cycle. The coupling product in the combined organic solution was evaporated to get the pure Suzuki-coupled products. The purity of the products was checked by the proton NMR as well as the melting point measurements.

Chandrasekhar et al. Mercury Poisoning Experiment. To a typical reaction mixture described above (with NP-1 or NP-2) approximately 10 g of mercury was added and the reaction continued. No effect on the reaction was observed.

Acknowledgment. We are grateful to the Department of Science and Technology (DST), New Delhi, India, for financial support, including support for a CCD X-ray diffractometer facility at IIT-Kanpur. R.S.N. thanks the Council of Scientific and Industrial Research, India, for Senior Research Fellowship. V.C. is grateful to the DST for a J. C. Bose National fellowship. V.C. is a Lalit Kapoor chair professor. We thank the electron microscopy facility of the Department of Metallurgical Engineering and Materials Science, IIT Kanpur, for HRTEM measurements. We also thank the Advanced Centre for Materials Science for SEM and powder XRD measurements. We are extremely thankful to an anonymous reviewer for his/her suggestions. Supporting Information Available: Figures giving experimental details, powder XRD data, TGA curves, and NMR spectra of Suzuki-coupled products. This material is available free of charge via the Internet at http://pubs.acs.org.