Palladium nanoparticles supported on polymer: An efficient and reusable heterogeneous catalyst for the Suzuki cross-coupling reactions and aerobic oxidation of alcohols

    Palladium nanoparticles supported on Polymer: An efficient and reusable heterogeneous catalyst for the Suzuki cross-coupling reactions and aerobic oxidation of alcohols Kazem Karami, Mahdiyeh Ghasemi, Nasrin Haghighat Naeini PII: DOI: Reference:

S1566-7367(13)00134-9 doi: 10.1016/j.catcom.2013.04.003 CATCOM 3468

To appear in:

Catalysis Communications

Received date: Revised date: Accepted date:

9 February 2013 4 April 2013 6 April 2013

Please cite this article as: Kazem Karami, Mahdiyeh Ghasemi, Nasrin Haghighat Naeini, Palladium nanoparticles supported on Polymer: An efficient and reusable heterogeneous catalyst for the Suzuki cross-coupling reactions and aerobic oxidation of alcohols, Catalysis Communications (2013), doi: 10.1016/j.catcom.2013.04.003

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ACCEPTED MANUSCRIPT Palladium nanoparticles supported on Polymer: An efficient and

reactions and aerobic oxidation of alcohols

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Kazem Karami, Mahdiyeh Ghasemi, Nasrin Haghighat Naeini

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reusable heterogeneous catalyst for the Suzuki cross-coupling

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ABSTRACT Polymer supported palladium nanoparticles catalyst was synthesized and characterized by Scanning electron microscopy (SEM), Transmission electron microscopy (TEM) and X-ray diffraction (XRD) techniques. This catalyst exhibits good activity and stability in Suzuki cross-coupling reaction of arylboronic

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acid with arylhalids and oxidation reaction of alcohols to corresponding aldehydes or ketones without over

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oxidation. After reaction, the catalyst can be separated by simple method and used many times in repeating

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cycles without considerable loss in its activity.

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1.

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Department of Chemistry, Isfahan University of Technology, Isfahan 84156/83111, Iran

Keywords: Pd nanoparticles on polymer, Suzuki coupling, Oxidation of alcohols, Heterogeneous catalyst

1. Introduction Many transition metals have been recognized as powerful and versatile catalysts for cross-coupling reactions and oxidation of alcohols, which these reactions play an important role in pharmaceutical, industry, natural products, and advanced materials [1,2]. Among different transition metals, palladium catalyst precursors,

 Corresponding author. Tel.: +98 3113912351; fax: +98 3113912350; E-mail address: [email protected]

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ACCEPTED MANUSCRIPT particularly nanoscale palladium particles have been developed, and widespread applications in organic synthesis occurred only during the last decade [3,4]. The large surface-to-volume ratio of metallic nanoparticles

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makes them very attractive as catalysts in chemical reactions over the other bulk catalytic materials [5].

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Catalytic activity of palladium complexes can be tuned by ligands such as phosphines, amines, carbenes, dibenzylideneacetone (dba), etc.

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In recent years, palladium catalyzed C-C bond formation has been evolved into a general technique for coupling

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different substrate under various conditions [6,7]. The cross-coupling reaction of arylboronic acid with arylhalids (Suzuki-miyaura reaction) and aerobic oxidation of alcohols to corresponding aldehydes without over

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oxidation have significant importance, and are well-establish methodology in multiple organic transformations [8-10]. For these reactions, many heterogeneous and homogeneous palladium catalysts have been suggested [11-

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13]. Homogeneous palladium catalysts have higher reaction rate and turnover number (TON) and higher

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selectivity than heterogeneous, but suffer from the problems concerning need of sensitive and high price ligands,

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separation from reaction mixture and reuse of expensive metal catalysts . To overcome these problems,

homogeneous palladium catalysts have been immobilized on solid supports, such as zeolites and molecular sieves [14], activated carbon [15], metal oxides [16], porous glass [17], mesoporous supports [18,19] etc. In

this area, polymers play a significant role offering different ways of metal attachment to the polymer matrix via covalent or non-covalent bonding [20].These supported catalysts are very important from economical and environmental point of view. In this paper we reported an efficient and easy method for preparation of stable and active heterogeneous palladium nanoparticles catalyst based on styrene-divinylbenzene (PdNPs/PS). Similar cases of this compound

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ACCEPTED MANUSCRIPT have already been synthesized [21-25]. After well characterization of the PdNPs/PS, it was used as catalyst in the Suzuki cross-coupling reaction and aerobic oxidation of alcohols without any further treatment. Results

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showed that the catalyst is highly active and stable. After completion of the reactions, PdNPs/PS is easily

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recyclable and can be used several times without significant loss of activity.

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2. Experimental

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2.1. Reagents and Equipments

All reactants were purchased from Aldrich Chemical Company or Merck and used as received. Solvents were

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used without further purification or drying. Fourier transform infrared (FT-IR) spectra were obtained in KBr pallets with a Jasco FT/IR 680 plus instrument. Scanning electron microscopy (SEM) studies were conducted

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on a Philips (Model XL-30) instrument. Transmission electron micrographs (TEM) were recorded in a Philips

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(Model CM 120) at 120 kV. X-ray diffraction (XRD) patterns were measured using a Scintag X-ray

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diffractometer with X-ray wavelength of 1.54 Å(Cu Kα) radiation source. Gas chromatographic (GC) analyses were performed using a Agilent Technologies 6890N chromatograph equipped with a flame ionization detector (FID) and an HB-50+ column (length = 30 m, inner diameter 320 μm, and film thickness = 0.25 μm). The temperature program for the GC analysis was from 70 to 200 oC at 20 oC/min, held at 200 oC for 0 min, heated from 200 to 280 oC at 10 oC/min and held at 280 oC for 1 min. The inlet and detector temperatures were set at 260 oC and 280 oC respectively. Products were identified by comparison with authentic samples.

2.2. Catalyst preparation

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ACCEPTED MANUSCRIPT After preparation of dibenzylideneacetone ligand (dba) [26], tris(dibenzylideneacetone)dipalladium(0) complex was prepared by treating the system PdCl2/dba/NaOAc to give a dark purple precipitate [27]. PdNPs/PS were

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synthesized in two steps [28]. At first, Pd2(dba)3 (78.6 mg, 0.086 mmol), butan-1-ol (0.080 mL, 0.864 mmol),

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styrene (0.447 mL, 7.77 mmol), divinylbenzene (0.056 mL, 0.432 mmol) and THF (4 mL) were added to a 50 mL round bottom flask equipped with condenser. The mixture was stirred at 90 °C for 6 h. After cooling to r.t.,

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AIBN initiator (0.01 g, 0.06 mmol) was added. The mixture was further stirred at 90 °C for 4 h. The catalyst

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was collected by filtration, washed with THF (10 mL), filtered and dried at r.t. in air. The catalyst was crushed

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and obtained as a gray powder (0.5 mol% of Pd).

2.3. Typical procedure for Suzuki-Miyaura cross-coupling reaction

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A round-bottomed flask equipped with a condenser and stir bar was charged with aryl halide (0.25 mmol),

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phenylboronic acid (0.375 mmol), Na2CO3 (1.25 mmol) and Pd catalyst (18.5 mg, 0.5 mol% of Pd) in

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DMF:H2O ratio of 1:1 (4 mL). The flask was placed in an oil bath, and the mixture was stirred and heated at 110 °C over 12 hours. After completion of the reaction, the catalyst was easily recovered by filtration and the conversion of the reaction was determined by GC.

2.4. Typical procedure for oxidation reaction of alcohols A round-bottomed flask equipped with a condenser and stir bar was charged with primary or secondary alcohols (0.5 mmol), K2CO3 (0.58 mmol) and Pd catalyst (18.5 mg, 0.5 mol% of Pd) in toluene (4 ml). The flask was placed in an oil bath, and the mixture was stirred and heated at 85 °C over 15 hours under air. After completion

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ACCEPTED MANUSCRIPT of the reaction, the catalyst was easily recovered by filtration and the conversion of the reaction was determined

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by GC.

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2.5. General procedure for recycling reactions

When the corresponding Suzuki or oxidation reaction according to the procedure described in previous sections

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was finished, the catalyst was washed with acetone and water. It was dried under air and reused without any

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pretreatment for repeating cycles.

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3. Results and Discussions

3.1. Synthesis and characterization of catalyst

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The Palladium catalyst was designed by the sequence of reactions given in Scheme 1. Palladium nanoparticles

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were generated from Pd2(dba)3 in a mixture of butan-1-ol and THF before the polymerization. Butan-1-ol would

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inhibit the aggregation of palladium nanoparticles. Crosslinked polystyrene was prepared by free radical polymerization between styrene and divinylbenzene monomer in the present of AIBN as initiator. The color of the catalyst was black, which indicates that the Pd in the catalyst was in Pd(0) form. In addition to, the filtrate was clear and colorless. Thus, the Pd content in the catalyst was estimated by assuming that all the Pd in the Pd2(dba)3 was entrapped in the catalyst. Scheme 1. Herein, characterization the crystalline structure of the PdNPs/PS by X-ray powder diffraction (XRD) is given and summarized in table 1. The XRD pattern of the fresh Pd catalyst exhibits a broad reflection corresponding to

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ACCEPTED MANUSCRIPT the polymer and also shows the (111), (200), (220), (311) and (222) crystallographic planes of the Pd(0) nanoparticles. This is in good agreement with the results obtained from literature [29]. The same result was also

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obtained for the XRD pattern of the catalyst after its use in the Suzuki reaction for fifth run. The Pd nanoparticle

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size was estimated from the XRD pattern using the Scherrer equation and was found close to the size observed in TEM even after fifth run. This reveals showes the excellent stability and recovery of the catalyst.

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To obtain the morphology and particle size of the catalyst, scanning electron microscopy (SEM) and

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transmission electron microscopy (TEM) images of the catalyst were carried out. The SEM images show the uniform distribution of palladium onto polymer, and particles with diameter in the range of nanometers (Fig. 1).

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According to the TEM images, the palladium particles were formed and are well dispersed through the support

Table 1

Fig. 1.

Fig. 2.

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surface in nanometer size about 50 (Fig. 2).

Previous researches have been shown that commercially available samples of Pd2(dba)3 may readily contain various amount of PdNPs in a wide range of sizes (10−200 nm) and Pd2(dba)3 with high purity that prepared by common procedures can be decomposed to PdNPs in the solvent or by preliminary heating [30]. However, characterization of a heterogeneous Pd catalyst on a molecular level is still a problem, although TEM, X-ray diffractometry, and IR spectroscopy allow important insights into the structure [31]. The information obtained from the papers show that probably Pd2(dba)3 complex during the first step of the reaction was converted to

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ACCEPTED MANUSCRIPT Pd(dba) [32] and deposited on the polymer support by π-π interactions between the aromatic rings [33]. (schem 2.)

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Scheme. 2.

3.2. Catalyst testing for the Suzuki reaction

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To test the activity of the Pd catalyst, it was first used in Suzuki coupling reactions. In this reason, iodobenzene

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and phenylboronic acid were initially selected as the test substrate. The reaction conditions were optimized, and the results are presented in Table 2. It was found that the best system for this reaction was DMF:H 2O (1:1) in

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combination with Na2CO3, which yielded a 100% conversion of iodobenzene at 100 °C within 12 h (entry 8). Table 2

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To demonstrate the versatility of the catalytic system, we investigated the reaction using a variety of aryl halides

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with phenylboronic acid under the optimized condition (Table 3). As shown in Table 3, aryl halides with

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electron-withdrawing substituents in para positions reacted smoothly but the efficiencies were lower for the substrates with electron-donating groups (entries 3, 10, 13, 14). Also aryl halides with ortho substitution were poor substrates, which is because of sterically hindered (entry 11). In our catalytic system, the more easily accessible and cheaper aryl chlorides also participated in these reactions (entry 12). Few heterogeneous Palladium catalysts were found to convert activated aryl chlorides at high temperature [34, 35]. Table 3

3.3. Catalyst testing for the oxidation of alcohols

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ACCEPTED MANUSCRIPT The activity of the Pd catalyst was also tested for the selective oxidation of alcohols using air as the source of molecular oxygen. Benzyl alcohol was selected as the test substrate. The oxidation of benzyl alcohol was carried

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out using PdNPs/PS, toluen and atmospheric air as the source of molecular oxygen at 80 °C for 9 h.

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Unfortunately, very low conversion was observed. Changing the solvent wasn't very effective on the conversion of alcohols. To get the better conversion, the reaction conditions were changed and K2CO3 was used as base in

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the reaction and to our surprise, complete conversion finally took place at 85°C in 15 h giving 98% of

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benzaldehyde (Table 4). In fact the presence of the base strongly increased the catalytic activity. To demonstrate the versatility of the catalytic system for the oxidation, different alcohols were chosen, and excellent results

Table 4

Table 5

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were obtained for alcohols that have at least a benzylic ring (Table 5).

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3.4. Heterogeneity and recyclability The reusability of supported catalysts is very important theme and makes them useful for commercial applications. Therefore, the recovery and reusability of our supported catalyst was initially investigated in the Suzuki-Miyaura reaction of iodobenzene with phenylboronic acid under the same condition. Before running another cycle, the solid was separated by simple filtration and washed with acetone, water and then dried. The catalyst is highly reusable. It was used for 5 runs and retained its catalytic activity in these repeating cycles. The results are shown in Fig. 3. Fig. 3.

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ACCEPTED MANUSCRIPT The recovery and recyclability becomes more important in the case of oxidation reactions. To test this, a series of 4 consecutive runs of the oxidation of benzyl alcohol using our supported catalyst under air atmosphere was

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carried out. The catalyst can be reused without considerable loss in its activity. The results are shown in Fig. 4.

Fig. 4.

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It is noteworthy that in each run, reaction was done under the same condition and time was constant too.

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We did not observe the palladium black formation in the reaction mixture even after the 5th run. This

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observation confirmed negligible palladium leaching from our polymeric supported system, which improve the

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reusability of nanocatalyst.

4. Conclusion

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In conclusion, we have reported the simple preparation of palladium nanoparticles dispersed on the surface of

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polymer and its application for the Suzuki coupling in DMF:H2O (1:1); and aerobic oxidation of alcohols in

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toluene using air as the source of molecular oxygen. High temperatures stability, being insensitive to oxygen, total separation from the reaction products and recyclability of the catalyst can also be regarded as advantages of this method.

Acknowledgment We gratefully acknowledge the funding support received for this project from the Iranian Nano Technology Initiative Council. We also thank to the partial support of this study by Department of Chemistry, Isfahan University of Technology.

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[7] S.G. Newman, M. Lautens, J. Am. Chem. Soc. 133 (2011) 1778–1780.

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[11] Y. He, C. Cai, Catal. Commun. 12 (2011) 678-683. [12] K. Karami, N. Rahimi, M. Bahrami Shehni, Tetrahedron Lett. 53 (2012) 2428-2431. [13] K. Karami, M. Bahrami Shehni, N. Rahimi, Appl. Organomet. Chem. (2012) doi:10.1002/aoc.2966. [14] L. Djakovitch, K. Koehler, J. Am. Chem. Soc. 123 (2001) 5990–5999. [15] F. Zhao, B.M. Bhanage, M. Shirai, M. Arai, Chem. Eur. J. 6 (2000) 843-848. [16] A. Biffis, M. Zecca, M. Basato, Eur. J. Inorg. Chem. (2001) 1131–1133. [17] J. Li, A.W.H. Mau, C.R. Strauss, Chem. Commun. (1997) 1275–1276. [18] N. Batail, M. Genelot, V. Dufaud, L. Joucla, L. Djakovitch, Catal. Today. 173 (2011) 2-14.

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ACCEPTED MANUSCRIPT [19] V. Polshettiwar, A. Molnar. Tetrahedron 63 (2007) 6949-6976. [20] V. Andrushko, D. Schwinn, C.C. Tzschucke, F. Michalek, J. Horn, C. Mossner, W. Bannwarth, Helv.

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Chim. Acta 88 (2005) 936–949.

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[21] R. Akiyama, S. Kobayashi, Angew. Chem., Int. Ed. Engl. 40 (2001) 3469-3471. [22] Y. Uozumi, Top. Cur. Chem. 242 (2004) 77-112.

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[23] Y. Liu, C. Khemtong, J. Hu, Chem. Commun. 4 (2004) 398-399.

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[24] Y. Liu, X. Feng, D. Bao, K. Li, M. Bao, J. Mol Catal. A: Chem. 323 (2010) 16-22.

[25] C.A. McNamara, M.J. Dixon, M. Bradley, Chem. Rev. 102 (2002) 3275-3300.

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[26] A.I. Vogel, Practical organic chemistry, fourth ed., Longman, London;New York, 1978.

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[27] T. Ukai, H. Kawazura, Y. Ishii, J.J. Bonnet, J.A. Ibers, J. Organomet. Chem. 65 (1974) 253-266.

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[28] C.M. Park, M.S. Kwon, J. Park, Synthesis. 22 (2006) 3790-3794.

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[29] S. Martinez, A. Vallribera, C.L. Cotet, M. Popovici, L. Martin, A. Roig, M. Moreno- Manas, E. Molins, New. J. Chem. 29 (2005) 1342-1345. [30] S.S. Zalesskiy, V.P. Ananikov, Organomet. 31 (2012) 2302−2309. [31] L. Yin, J. Liebscher, Chem. Rev. 107 (2007) 133-173. [32] C. Amatore, A. Jutand, Coord. Chem. Rev. 178–180 (1998) 511–528. [33] A. Biffis, M. Zecca, M. Basato, J. Mol. Catal. A: Chem, 173 (2001) 249-274. [34] S. Prockl, W. Kleist, M.A. Gruber, K. Kohler. Angew. Chem., Int. Ed. 43 (2004) 1881-1882. [35] K. Kohler, W. Kleist, S.S. Prockl, Inorg. Chem. 46 (2007) 1876-1883.

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ACCEPTED MANUSCRIPT Table 1 Results obtained from powder XRD patterns of Pd catalyst Sample

Crystallographic planes of PdNPsa

Crystal system

Space group

Cubic Cubic

Fm3m Fm3m

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Fresh Pd catalyst 111, 200, 220, 311, 222 Recovered Pd catalystb 111, 200, 220, 311, 222 a according to Miller indices b after its use in the Suzuki reaction for fifth run

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50 50 50 50 50 70 90 100 100

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Time (h)

12 12 12 12 12 12 12 12 10

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Temperature (°C)

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Base

K3PO4.3H2O NaOAc Me3N KOH Na2CO3 Na2CO3 Na2CO3 Na2CO3 Na2CO3

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1 2 3 4 5 6 7 8 9

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Entry

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Table 2 Optimizing of the reaction condition for the Suzuki reaction of iodobenzene with phenylboronic acid

Yield (%)

72 30 75 61 80 87 93 100 73

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Entry

Substrate

Product

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1

I

100

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I

3

CH3

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CH3

94

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CH3

NO2

87

100

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Yield (%)

CH3

2

I

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Table 3 Suzuki-Miyaura cross-coupling reaction of various aryl halides with phenylbronic acid

NO2

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5

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Br

80

Br

CH3

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69 CH3 N

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Br S

8 S

56

Br

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72

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Br

H

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10

O

Br

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H

O

H

H

O

Cl

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12

Cl

43

48

NO2

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13

O

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90

75

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NO2

Cl

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14

H O

O

70

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Method: aryl halides (0.25 mmol), phenylboronic acid (0.375 mmol), Na2CO3 (1.25 mmol), DMF:H2O (1:1) (4 mL) and 18.5 mg catalyst (0.5 mol% Pd) at 100 °C for 12 h.

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Temperature (°C) 80 80 80 80 85 85

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Solvent Toluene CH3CN CH2Cl2 Toluene Toluene Toluene

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Base K2CO3 K2CO3

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Entry 1 2 3 4 5 6

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Optimizing of the reaction condition for oxidation reaction of benzyl alcohol

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Time (h) 9 9 9 14 14 15

Yield (%) Trace Trace Trace 27 85 98

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Entry

Substrate

Product

1

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Table 5 Aerobic oxidation of alcohols

CHO

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OH

2

OH

4

5

100

O

100 O

CHO

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OH

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OH

3

98

CHO

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OH

Yield (%)

Trace

Trace

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Method: 0.5 mmol alcohol, 0.58 mmol K2CO3, 0.185 mg (0.5 mol% Pd) and 4 mL toluene at 85 °C for 15 h.

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Scheme 1. Preparation of palladium catalyst in crosslinked polystyrene

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Fig. 1. SEM images corresponding to Pd catalyst

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Fig. 2. TEM image corresponding to Pd catalyst

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Fig. 3. A recyclability test for catalyst in Suzuki–Miyaura cross-coupling reaction of iodobenzene with phenylboronic acid (reaction conditions are the same as given in Table 3)

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Fig. 4. A recyclability test for catalyst in oxidation reaction of benzyl alcohol (reaction conditions are the same as given in Table 5)

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Scheme 2. How dispersion of nanoparticles on polymer support

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Graphical Abstract

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ACCEPTED MANUSCRIPT Highlights Synthesis and caractrization of Polymer supported Pd nanoparticles catalyst (PdNPs/PS).

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Cross-coupling reaction of arylboronic acid with arylhalids by PdNPs/PS catalyst.

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Oxidation reaction of alcohols to aldehydes or ketones by PdNPs/PS catalyst.

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This heterogeneous catalyst is recyclable and reusable, and can be used several times.

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

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