Electrochemical synthesis and characterization of palladium nanoparticles on nafion–graphene support and its application for Suzuki coupling reaction

Tetrahedron Letters 54 (2013) 3457–3461

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Electrochemical synthesis and characterization of palladium nanoparticles on nafion–graphene support and its application for Suzuki coupling reaction Suresh S. Shendage, Umakant B. Patil, Jayashree M. Nagarkar ⇑ Department of Chemistry, Institute of Chemical Technology (Deemed University), Nathalal Parekh Marg, Matunga (E), Mumbai 400 019, India

a r t i c l e

i n f o

Article history: Received 4 April 2013 Revised 18 April 2013 Accepted 20 April 2013 Available online 27 April 2013 Keywords: Electrochemical Palladium nanoparticles Nafion–graphene Heterogeneous catalyst

a b s t r a c t The electrochemically deposited palladium nanoparticles on nafion–graphene support showed an excellent catalytic activity for Suzuki coupling reactions. The as obtained catalyst was characterized by SEM, TEM, EDAX, XRD, and TGA. The particle size of palladium nanoparticles (Pd NPs) determined from TEM was in the range of 4–12 nm. The mean diameter of Pd NPs was found to be 8.1 ± 1.9 nm. The recyclability of the catalyst was examined and it did not show any significant loss of catalytic activity for five consecutive cycles. Ó 2013 Elsevier Ltd. All rights reserved.

Palladium-catalyzed Suzuki cross-coupling reaction of aryl halides with aryl boronic derivatives is a very suitable and extensively studied approach to C–C bond formation. This reaction is important because biaryl moiety is present in natural products, pharmaceuticals, and herbicides.1–4 Metallic palladium and palladium complexes are widely used as catalysts for coupling reactions because of high activity and selectivity.5–8 However, separation of the catalyst from the product is always a challenging task. A heterogeneous catalyst in such reactions finds an attractive solution to these problems as the Pd catalyst supported on microporous polymers,9 carbon materials,10 amorphous or mesoporous silica,11 inorganic material,12 molecular sieves,13and zinc ferrite14 can be easily separated from the product and successfully reused. It is well known that support material plays a very important role in catalytic activity. Recently graphene has attracted much attention because of its unique properties such as, large specific surface area,15 and thermal and electrical conductivity.16 Graphene, has exhibited huge application potential in different fields, such as electronic devices17 nanocomposite materials,18 and fuel cells.19 However, growing world population with increasing industrial demands has led to the situation where the protection of the environment has become a major concern and crucial factor for the future growth of industrial processes. Electrochemical methods are suitable for the preparation of metal nanoparticles because size and morphology of nanoparticles can be controlled by changing electrochemical conditions.20 These methods are generally chosen

because they are not aggressive to the environment. They demonstrate high reproducibility, simple methodology and can be used at ambient temperature. Besides these advantages, electrochemical processes are of heterogeneous nature which allows easy recovery of products. The other advantages are short reaction time, simple operation, no side reaction, and purity of product. The catalytic activity of metal nanoparticles (NPs) very much depends on their sizes, shapes, and method of preparation.21 Most of the previous Letters demonstrate the electrochemical preparation of Pd NPs and their application as catalysts for Suzuki coupling reaction.22,23 Moreover, in spite of the progress offered by the electrochemical synthesis of NPs, the use of electrochemically prepared Pd NPs as catalysts in coupling reactions is scanty.24 Similarly, little

⇑ Corresponding author. Tel.: +91 22 3361 1111/2222; fax: +91 22 3361 1020. E-mail address: [email protected] (J.M. Nagarkar). 0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tetlet.2013.04.092

Figure 1a. SEM image of Pd/Nf–G composite on platinum plate.

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solution at constant potential and their application as heterogeneous catalysts for Suzuki coupling reaction. Nafion is thermally and chemically stable in the operating temperature of the reaction. Here nafion is used to disperse graphene and stabilize it on the electrode surface. To the best of our knowledge electrochemically deposited palladium NPs on Nf–G support has not been reported earlier for Suzuki coupling reaction. The as-prepared catalyst shows excellent activity and good recyclability for Suzuki coupling reaction. Graphene was prepared by chemical reduction (hydrazine hydrate as a reducing agent) of graphene oxide (GO). GO was prepared by modified Hummers method (Supplementary data). The catalyst Pd/Nf–G composite was prepared by electrochemical

Figure 1b. TEM image of Pd/Nf–G composite.

attention has been given toward electrochemical deposition of Pd NPs on different supporting material and its applications for Suzuki coupling reaction. We herein, report the electrochemical deposition of Pd NPs on Nafion–graphene (Pd/Nf–G) from aqueous

Figure 2c. TGA curve of Pd/Nf–G composite.

Figure 2a. XRD pattern of Pd/Nf–G composite.

Figure 2b. EDAX of Pd/Nf–G composite.

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X R1

+

B(OH)2

Pd/Nf -G, K2CO3 Ethanol/H2 O, 80o C

R2

R1

R2

R 1=CH3 , OCH 3, NO2 , COCH3 , OH, H R 2=CH3 , OCH 3, Cl, H X=I, Br Scheme 1. Suzuki coupling reaction of aryl halides with phenylboronic acid.

Table 1 Optimization of reaction parameters for Suzuki reaction catalyzed by Pd/Nf–Ga

a

b

Entry

Solvent

Base

Pd loading (mol %)

Time (h)

Yieldb (%)

1 2 3 4 5 6 7c 9 10 11 12

Ethanol/H2O Ethanol/H2O Ethanol/H2O Ethanol/H2O Water Ethanol Ethanol/H2O Ethanol/H2O Ethanol/H2O Ethanol/H2O Ethanol/H2O

K2CO3 Na2CO3 K3PO4 Cs2CO3 K2CO3 K2CO3 K2CO3 K2CO3 K2CO3 K2CO3 K2CO3

0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.5 0.4 0.3 0.2

1 1 1 1 1 1 24 1 1 1 1

94 93 80 55 37 90 52 96 94 94 80

Reaction condition: iodobenzene (1 mmol), phenyl boronic acid (1.5 mmol), base (3 mmol), solvent (5 mL), catalyst Pd/Nf–G. Temp 80 °C. Isolated yield based on column chromatography.

deposition of Pd NPs on Nf–G film from aqueous solution containing palladium salt at room temperature at 0.2 V constant potential. The conventional three electrode cyclic voltammetry PGSTAT302N (potentiostat/galvanostat) equipped with GPES software was employed for electrochemical deposition of Pd NPs. Sat-

urated calomel electrode, platinum rod, and Nf–G modified platinum plate were the reference, counter, and working electrodes respectively (Supplementary data). The morphology and formation of Pd NPs on Nf–G support was confirmed by analytical techniques such as scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDAX, Quanta-200 at 20 kv as operating voltage), XRD (Rigaku Miniflex model by using CuKa = 1.54 Å with scanning range 0–50°), transmission electron microscopy (TEM, PHILIPS Model: CM200, Operating voltages: 20–200 kv Resolution: 2.4 Å), thermogravimetric analysis (TGA). Figure 1a displays the SEM image of Pd/Nf–G composite deposited on platinum plate. Figure 1a shows well dispersed spherical Pd NPs. The TEM image shows the presence of uniform well-dispersed Pd nanoparticles on Nf–G support. The particle size distribution histogram of Pd NPs is shown in Fig. S4 Supplementary data. Particle size of Pd NPs ranges from 4 nm to 12 nm (Fig. 1b). The average particle size was found to be 8.1 ± 1.9 nm. EDAX analysis (Fig. 2b) evidently shows the presence of Pd in the sample and that the sample consists mostly of carbon with oxygen most likely due to the presence of a few unreduced oxygen functional groups. It also shows the presence of fluorine and sulfur indicating the exis-

Table 2 Pd/Nf–G catalyzed Suzuki reaction of various aryl halides with phenylboronic acida Entry

Aryl halide

I

Boronic acid

Product(s)

B(OH)2

3

88

1

89

1

92

1.5

93

1.5

93

Cl

4

I

94

B(OH) 2

3

I

1

B(OH) 2

2

I

Yieldb (%)

B(OH) 2

1

Br

Time (h)

Cl B(OH)2

5

H3 CO

OCH3

I 6

B(OH) 2

(continued on next page)

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Table 2 (continued) Entry

Aryl halide

I

Boronic acid

B(OH)2

Product(s)

Time (h)

Yieldb (%)

1.5

94

1.5

93

1.5

95

1.5

92

1

92

3

89

3

90

4

90

H 3CO

7

OCH 3 I

B(OH)2

HO

8

OH

B(OH)2

I

O2 N

9

NO 2 I

B(OH)2

H3 COC

10

COCH 3

I

B(OH) 2

11

OCH 3

Br

B(OH)2

12

Br

B(OH)2

13

O2 N

NO 2 B(OH)2

B(OH) 2

14

a b

Reaction condition: aryl halide (1 mmol), phenyl boronic acid (1.5 mmol), base (3 mmol), solvent (5 mL), catalyst Pd/Nf–G (1 mg). Temp 80 °C. Isolated yield based on column chromatography.

tence of nafion in Pd/Nf–G composite. Figure 2a displays the XRD pattern of Pd/Nf–G composite which show characteristic peak of nafion at 16.5°. A broad peak between 24° and 28° corresponds to graphene.25 The other two peaks at 40.1° and 46.8° are the characteristic peaks of palladium. We have also examined the thermal stability of the prepared Pd/Nf–G composite under nitrogen atmosphere using thermal gravimetric analysis (TGA). As shown in Figure 2c, the Pd/Nf–G exhibits initial weight loss below 200 °C which may be due to moisture loss. Next peak from 300 to 600 °C is attributed to the decomposition of the –SO3H groups and degradation of the polymer main chain. Pd–graphene shows much higher thermal stability with much less mass loss up to 800 °C. The mass loss of Pd graphene is attributed to the existence of a few oxygen functional groups since it is well known that chemical reduction of GO results in thermal degradation pattern similar to that of GO. The amount of Pd in the catalyst (Pd/Nf–G) measured by inductively coupled plasma atomic emission spectrometry (ICPAES) was 32 wt.%. Initially we have carried out the Suzuki reaction of iodobenzene with phenylboronic acid in the presence of electrochemically pre-

pared Pd/Nf–G composite as heterogeneous catalyst (Scheme 1). The reaction was optimized for parameters such as, solvent, catalyst loading, and base. Solvent: ethanol/water (1:1), catalyst loading (1 mg), and base: K2CO3 at 80 °C temperature were the optimized reaction conditions26 (See Table 1). The applicability of the prepared Pd/Nf–G catalyst for Suzuki coupling reaction of the different aryl halides and boronic acids under optimized conditions (Table 2, entries 1–14) was studied. The electron donating and electron withdrawing aryl iodides and phenyl boronic acids gave excellent yields of corresponding products (Table 2, entries 6–10). However, longer reaction time was required for substituted aryl bromide but afforded excellent yield of the products (Table 2, entries 2, 12–14). The reusability of the Pd/Nf–G catalyst was tested for the reaction of iodobenzene and phenylboronic acid. The catalyst was separated from the reaction mixture by decantation, washed with ethyl acetate and dried at room temperature. It was then re-used in a subsequent run. It was found that the Pd/Nf–G catalyst could be recycled for five times without losing the catalytic activity (Table 3). The activity of the catalyst only dropped slightly in 5th

S. S. Shendage et al. / Tetrahedron Letters 54 (2013) 3457–3461 Table 3 Recyclability studya

a

b

Entry

1

2

3

4

5

Run Yieldb (%)

1st 94

2nd 94

3rd 93

4th 93

5th 91

Reaction condition: iodobenzene (1 mmol), phenyl boronic acid (1.5 mmol), base (3 mmol), solvent (5 mL), catalyst Pd/Nf–G (1 mg). Temp 80 °C. Isolated yield based on column chromatography.

run. The amount of Pd lost after 5th cycle was found to be 0.4 wt.% as determined by ICP-AES. This clearly indicates the high catalytic activity and stability of P/Nf–G. In conclusion we have fabricated a Pd/Nf–G catalyst by electrochemical deposition of palladium on Nf–G support from aqueous solution containing palladium ions. The catalyst was characterized by various techniques. This was further used as an excellent catalyst for the Suzuki–Miyaura coupling reactions. The high catalytic activity was attributed to the small size and uniform dispersion of Pd NPs in ethanol/water (1:1 v/v) solvent. In addition it can be recycled for five cycles without much loss in catalytic activity. Acknowledgement The authors are thankful to UGC New Delhi, India for the award of Faculty improvement programme (FIP). Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.tetlet.2013. 04.092. References and notes 1. Suzuki, A. In Modern Arene Chemistry; Astruc, D., Ed.; Wiley-VCH: Weinheim, 2003; pp 53–106. 2. Fihri, A.; Bouhrara, M.; Nekoueishahraki, B.; Basset, J. M.; Polshettiwar, V. Chem. Soc. Rev. 2011, 40, 5181–5203. 3. Zhou, Z.-Z.; Liu, F.-S.; Shen, D.-S.; Tan, Ch.; Luo, L.-Y. Inorg. Chem. Commun. 2011, 14, 659–662. 4. Hajduk, P. J.; Bures, M.; Prastgaard, J.; Fesik, S. W. J. Med. Chem. 2000, 43, 3443– 3447. 5. Mondal, J.; Modak, A.; Bhaumik, A. J. Mol. Catal. A: Chem. 2011, 350, 40–48.

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