Supported Palladium on Magnetic NanoparticlesâStarch Substrate

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Supported palladium on magnetic nanoparticles-starch substrate (Pd-MNPSS): highly efficient magnetic reusable catalyst for C-C coupling reactions in water Maryam Tukhani, Farhad Panahi, and Ali Khalafi-Nezhad ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03923 • Publication Date (Web): 01 Dec 2017 Downloaded from http://pubs.acs.org on December 2, 2017

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ACS Sustainable Chemistry & Engineering

Supported palladium on magnetic nanoparticles-starch substrate (PdMNPSS): highly efficient magnetic reusable catalyst for C-C coupling reactions in water Maryam Tukhani,a Farhad Panahi,a,b,c,* and Ali Khalafi-Nezhad*,a a

Department of Chemistry, College of Sciences, Shiraz University, Shiraz 71454, Iran.

b

Mahshahr Campus, Amirkabir University of Technology, Mahshahr, Iran

c

Department of Polymer Engineering and Color Technology, Amirkabir University of Technology,

Tehran, Iran. Email: [email protected]; [email protected] Abstract A novel heterogeneous Pd catalyst system (Pd-MNPSS) was developed using immobilization of Pd species (PdII and Pd0) on a starch-functionalized magnetic nanoparticles. In order to synthesize PdMNPSS catalyst, first magnetic nanoparticles (MNPs) were prepared and coated with a silica layer (Fe3O4@SiO2) to increase their stability and functionalization capability. The Fe3O4@SiO2 particles were reacted with thionyl chloride (SOCl2) to generate chlorosilyl groups on the surface of MNPs. Reaction of starch with chloro-functionalized MNPs leading to a magnetic reusable poly hydroxy functionalized substrate (MNPSS), which is highly suitable for immobilization of Pd metal on its surface. This catalyst system was designed to be applied in Pd-catalyzed organic coupling reactions in water. Finally, the Pd-MNPSS catalyst was prepared via reaction of MNPSS and Pd(OAc)2. After characterization of the Pd-MNPSS catalyst it was applied in the Heck and Sonogashira coupling reactions in water solvent and excellent results were obtained. The catalyst system was separated from the reaction mixture employing an outside magnetic field. In these processes the catalyst was reusable for 5 times without remarkable decreasing in its activity. The Pd-MNPSS catalyst has many advantages especially in workup process related to our previously reported catalyst systems, Pd supported on silica-starch substrate (PNP-SSS) and Pd supported on silica-cyclodextrin substrate (PNP-SCD). Keywords: Supported palladium catalyst; Magnetic nanoparticles; Starch; Heterogeneous catalyst; Coupling reactions in; Water

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Introduction Development of heterogeneous Pd catalyst systems for application in C-C coupling reactions is of great importance and received considerable attentions in both industrial and academic researches.1-4 Two important and well-known Pd-catalyzed coupling reactions are Heck and Sonogashira, which are essential in synthesis of many advanced materials containing C-C double and triple bonds, respectively.5-7 There are many attempts to find appreciate Pd catalyst systems (cover maximum of the twelve green chemistry principles) to accomplish these valuable reactions under environmentally benign conditions.8 In this regards, different types of palladium catalyst systems (homogeneous and heterogeneous) have been developed. However, many of the reported palladium–catalyst systems suffer from main drawbacks such as tedious and time-consuming workup process, high cost and difficulty in synthesis of these catalyst systems, and use of different additives. Besides, the complexity of separation, recovery, and reusability of the catalyst and more importantly leaching of Pd as an expensive metal, are other significant confronts.9-19 So, the present challenges on this field rely on the development of high performance palladium-catalyzed systems incorporating sustainable and environmentally benign reaction conditions. Naturally occurring substrates (NOSs) such as starch, cyclodextrins, cellulose, chitosan, agarose, pectin etc. are good candidates to be used as supports in order to provide an environmentally benign and practical palladium catalysts systems.20-32 In addition to support role for NOSs, they can act as a reducing agent to form metal nanoparticles and stabilize them.31 These materials have received considerable attentions due to having potential in design of heterogeneous Pd catalyst systems with high reactivity and selectivity in environmentally benign/acceptable media. Other interesting features of NOSs are abundant availability, low price, environmental friendly, and structure diversity. In our research group, silica-starch substrate (SSS) was synthesized using the reaction of starch and chlorine-functionalized silica and it was used as support to immobilize Pd nanoparticles. Thus, a heterogeneous Pd catalyst system (PNP-SSS) (Figure 1a) was synthesized and it was successfully used in C-C coupling reactions including Heck, Sonogashira33 and Suzuki34 in water media, while it was reusable for several times with approximately consistent catalytic activity. In another attempt silica supported cyclodextrins were designed and prepared for efficient size-controlled synthesise of Pd nanoparticle on its surface (PNP-SCD) (Figure 1b). Thus a heterogeneous palladium nano catalyst system was designed for efficient Heck reaction in water solvent.35


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a)

b)

OH O HO

O

HO

O O

O

HO

HO

OH

O

O O

O O

HO HO

O HO

O O

OH O

O HO

OH

O HO

O OH O O OH HO HO O OH OH

O

OH HO

O

HO

O

O O

O

O

OH

HO

OH O OH

OH

O

OH O

O O

O PNP-SSS

HO

Pd nanoparticle

n = 0,1,2

PNP-SCD

Figure 1. A representation to show the chemical structure of PNP-SSS and PNP-SCD catalysts

During our study on the synthesis of magnetic nanoparticles (MNPs) supported catalysts36-42 we found that they have many advantages in comparison with silica supports especially in work up process, thus we decided to synthesize a magnetic reusable Pd catalyst system based on an organicinorganic starch-based substrate. In this situation, the supported palladium on magnetic nanoparticles-starch substrate (Pd-MNPSS) as a new heterogeneous Pd catalyst system was successfully synthesized. Herein, the catalytic activity of Pd-MNPSS was checked in the Heck and Sonogashira reactions as two important C-C coupling processes in water as a green solvent. Results and discussion Catalyst synthesis and its characterization The supported palladium on magnetic nanoparticles-starch substrate (Pd-MNPSS) catalyst was prepared according to the following synthetic route shown in Scheme 1. Silica coated MNPs (Fe3O4@SiO2) were prepared based on a known procedure in the literature with some modifications.43 The SiO2 layer not only prevents from aggregation of Fe3O4 nanoparticles, even ready them for further surface functionalization.44 Because, the surface of silica coated MNPs have more silanol groups, it was treated with thionyl chloride (SOCl2) to produce a chlorine functionalized magnetic nanoparticle substrate (Fe3O4@SiO2@Cl).45 The prepared Fe3O4@SiO2@Cl was reacted with starch to produce a magnetic nanoparticles-starch substrate (MNPSS) via the reaction with Si-Cl groups on the MNP surface. Finally, treatment of MNPSS with Pd(OAc)2 leading 3 ACS Paragon Plus Environment


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to the stabilization of Pd species on its surface in order to produce supported Pd on magnetic nanoparticles-starch substrate (Pd-MNPSS) catalyst.33

Scheme 1. A synthetic pathway toward synthesis of Pd-MNPS

The FT-IR of the catalyst and its comparison with starting materials show the peaks that confirms the successful synthesis of this catalyst (Figure S1). According to the FT-IR spectrum of Fe3O4@SiO2, the absorption peaks observed at 590 cm-1 is associated to the vibration of the Fe-O bond.46 The broad band depicted at 3408 cm-1 is attributed to the stretching vibration of O-H.47 Two peaks observed at 1131 cm-1 and 976 cm-1 are assigned to the symmetrical and asymmetrical vibrations of the Si-O-Si bonds, confirming silica coated layer on MNPs surface.48


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The attendance of vibration band at 594 cm-1 is referring to Si-Cl bond in the structure of Fe3O4@SiO2@Cl material.45 Other peaks related to Fe3O4@SiO2 are observed with little shift due to the change in chemical environment. Considering the FT-IR spectrum of the MNPS substrate, the appeared bands at 2270, 2886, 2836 cm-1 may be because of the stretching vibration for C–H and C–C bonds.49 Furthermore, the observed peak at 1409 cm-1 is related to the stretching vibration of C–O bond.33,50 The elemental analysis results of the MNPSS support shows the existence of the C atom (33.1 %wt), which confirms the presence of starch connected to the MNPs surface. Based on the elemental analysis data, it can be calculated the amount of starch unit (glucose) per gram of material which is equal to 4.4 mmol/g. In order to study the surface morphology of the Pd-MNPSS catalyst it was evaluated by scanning electron microscopy (SEM). The SEM image shows a relatively homogeneous and uniform particles distribution which suitable for catalysis purposes (Figure 2).



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Figure 2. Different magnifications of SEM images of the Pd-MNPSS catalyst According to the amount of 2.43% for Pd based on energy-dispersive X-ray (EDX) spectra, the amount of the immobilized Pd on the surface of MNPSS which is calculated to be 0.23 mmol.g-1 (Figure 3). Also, the Pd content of catalyst was evaluated by ICP analysis after its treatment with concentrated HCl and HNO3. According to the ICP results, the Pd quantity was estimated to be 25.8 ppm (25.8 mg.L-1). This amount is equal to 2.58% w/w (0.24 mmol.g-1 of Pd). The quantification analysis revealed the Pd content is about 2.71% in Pd–MNPSS catalyst, which is in accordance with the ICP and elemental analysis results. 6 ACS Paragon Plus Environment

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Figure 3. The EDX analysis of the Pd-MNPSS catalyst A representative TGA curve of the Pd-MNPSS catalyst shows in Figure 4. It demonstrates two key mass losses. The weight loses observed at ~ 85-185 ◦C was allocated to the adsorbed water (3.2%). The weight loses depicted at ~ 180-430 ◦C is associated to the decomposition of the grafted starch to magnetic nanoparticle support. According to this part of the thermogram it is possible to estimate the amounts of linked starch to MNP support. It seems that about 11% (w/w) of starch grafted to the MNP surface. Considering the high temperature needed for starch removal from MNP surface, confirming elevated thermal stability of the MNP–starch substrate. Also TGA represents the covalently anchoring of starch to MNPs.



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Figure 4. The thermal gravimetric analysis of the Pd-MNPSS catalyst Some of the transmission electron microscopy (TEM) images of the Pd-MNPSS catalyst are depicted in Figure 5.

Figure 5. The TEM images of the Pd-MNPSS catalyst system from different situations (a-c). The TEM image of Pd-MNPSS after reusing from a Heck reaction run (d)


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As shown in Figure 5, the magnetic nanoparticles with near sphere-shaped morphology are produced with moderately monodispersity using our procedure. The crystallinity and phase composition of the Pd-MNPSS catalyst was investigated with X-ray powder diffraction (XRD) and it confirms the nano-crystalline structural of the catalyst. As shown in Figure 6, the XRD pattern of the Pd-MNPSS catalyst shows the characteristic peaks of Fe3O4@SiO2, mainly.

Figure 6. The XRD pattern of the Pd-MNPSS catalyst As presented in Figure 6 the reflection planes of (220), (311), (400), (422), (511) and (440), are in good conformity with the face centered cubic of Fe3O4 (JCPDS Card no. 19-0629).39 In addition to the original peaks, the XRD pattern of catalyst also shows other peak at 2θ = 40.0° which is attributed to the Pd species, representing also immobilization of Pd nanoparticles on the surface of magnetic nanoparticle starch substrate. The Pd-MNPSS was evaluated further by X-ray photoelectron spectroscopy (XPS) (Figure 7).



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Figure 7. The XPS analysis of the Pd-MNPSS catalyst The existence of Fe (2p), Pd (3d), O (1s) and C(1s) signals originated from catalyst further confirms the structure of catalyst. The schematic deconvolution of the Pd (3d) as two main doublet peaks is illustrated in Figure 7b. The doublet with a binding energy at 335.08 (Pd, 3d5/2) and 340.38 eV (Pd, 3d3/2) can be indexed to the Pd (0) state. The other set of peaks at 336.18 (Pd, 3d5/2) and 341.08 eV (Pd, 3d3/2) are ascribed to the Pd (II) oxidation state.48 The calculated integration areas of these two doublets demonstrated that Pd (0) was formed as the major phase on the catalyst surface (68 %). The magnetization measurements of Fe3O4@SiO2@Starch@Pd catalyst were examined by a vibrating sample magnetometer (VSM) in the employed magnetic field from -9000 to +9000 Oe, at room temperature (Figure 8).


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Figure 8. The VSM analysis of the Fe3O4@SiO2@Starch@Pd catalyst at room temperature. The PdMNPSS catalyst is easily separated from the reaction mixture after completion of the reaction. The value of the saturation magnetization is about 15 emu g-1 for Fe3O4@SiO2@Starch@Pd catalyst (Figure 8). The reduce in the saturation magnetization of Fe3O4@SiO2@Starch@Pd catalyst in comparison with Fe3O4@SiO2 may be attributed to the existence of Pd species immobilized on the MNPSS substrate.51 As shown in Figure 8, in spite of the negligible in the saturation magnetization related to naked MNPs, the catalyst can be proficiently and simply remove from the reaction media using an external magnet. After preparation and characterization of the Pd-MNPSS catalyst, its catalytic activity was checked in C-C coupling reactions. Two important Pd-catalyzed coupling including Heck and Sonogashira reactions were investigated with Pd-MNPSS catalyst. Pd-MNPSS-catalyzed Heck and Sonogashira reactions First, Heck reaction was checked using Pd-MNPSS catalyst. The reaction model involving iodobenzene and styrene was selected and different conditions were checked in order to find optimum conditions (Table 1).



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Table 1. Optimization conditions for Pd-MNPSS-catalyzed Heck reaction between Iodobenzene and Styrenea

Entry

Catalyst (mol%)

Solvent

Base

Temp (° C)

Time (h)

Yield (%)

1

none

DMF

K2CO3

120

24

0

2

0.48

DMF

K2CO3

120

24

93

3

0.48

DMF:H2O K2CO3

120

24

85

120

24

90

(5:1) 4

0.48

DMF:H2O K2CO3 (1:5)

5

0.48

Toluene

K2CO3

100

24

60

6

0.48

CH3CN

K2CO3

100

24

75

7

0.48

none

K2CO3

100

24

25

8

0.48

H2O

K2CO3

100

24

89

9

0.41

H2O

K2CO3

100

24

89

10

0.36

H2O

K2CO3

100

24

80

11

0.24

H2O

K2CO3

100

24

65

12

0.17

H2O

K2CO3

100

24

45

13

0.096

H2O

K2CO3

100

24

10

14

0.60

H2O

K2CO3

100

24

89

15

0.36

H2O

K2CO3

80

24

40


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16

0.36

H2O

K2CO3

r.t

24

0

17

0.36

H2O

K2CO3

100

12

89

18

0.36

H2O

K2CO3

100

6

89

19

0.36

H2O

K2CO3

100

4

89

20

0.36

H2O

K2CO3

100

2

70

21

0.36

H2O

Et3N

100

4

30

22

0.36

H2O

DABCO

100

4

45

23

0.36

H2O

NaOH

100

4

15

24

0.36

H2O

K3PO4

100

4

35

a Reaction

conditions: iodobenzene (1.0 mmol), styrene (1.2 mmol), solvent (5.0 mL) and base (2.0

mmol). b Isolated yields. Without use of catalyst no product was detected in DMF solvent (Table 1, entry 1). While, by use of only 0.48 mol% of Pd catalyst approximately 93% of coupling product was isolated (Table 1, entry 2). In order to increase in sustainability of the method we tried to use water as solvent. First, the fraction of water related to DMF solvent was increased and reaction yield in ratio of 5:1 of water:DMF was obtained about 90% which is comparable with DMF solvent (Table 1, entries 3&4). Of course in other solvents the reaction yield was decrease significantly (Table 1, entries 5&6). Under solvent free conditions only 25% of product was detected (Table 1, entry 7). In pure water the reaction yield was 89% which was satisfactory for our purposes (Table 1, entry 8). It seems that the hydrophilic nature of the catalyst is highly affects the reaction yield in aqueous media. Thus we continued our optimization with water solvent. Next, different catalyst loading was checked and 0.36 mol% of catalyst as optimum amount was selected (Table 1, entries 9-14). At lower temperatures the reaction yield was decreased and no coupling product at room rt was observed (Table 1, entries 15&16). Reaction time was controlled and it was observed that reaction is completed after 4h from start of the reaction. About 70% of product is produced with past of only 2h (Table 1, entries 17-20). Also, K2CO3 was the superior base



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tested in this protocol (Table 1, entries 21-24). Thus, the optimized conditions were found to be entry 19 of Table 1 for coupling of iodobenzene and styrene using Pd-MNPSS catalyst in water. Subsequently, the generality and limitation of the methodology was checked using different coupling partners under optimized conditions (Scheme 2). First, styrenes were reacted with aryl halides and some different stilbenes52 were synthesized. Both electron-rich and electron-poor aryl halides gave satisfactory yields. Aryl iodides reacted faster than aryl bromides and chlorides and the reaction yields for them were higher in comparison. Moderate to good yields of coupling product were obtained for aryl bromides in longer times. Average yields for aryl chlorides were obtained after 24h.53 Both styrenes with electron donating and electron withdrawing groups were tested. There is good competent between Br/I, Br/Cl and I/Cl bis-halides and remarkable yields for active halide was produced. Acrylates were also used as alkene coupling partners and some cinnamate54,55 derivatives were synthesized under optimum conditions with acceptable yield. Notably, a moderate to good yields of coupling product was isolated using the reaction of 1-hexene and 4-halophenols using Pd-MNPSS catalyst under optimized conditions.56 Overall, the Pd-MNPSS-catalyzed heck reaction of diverse alkenes and aryl halides is efficiently carried out under optimized conditions.


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X

Y

Pd-MNPSS (0.36 mol%) +

Y

K2CO3

R

R

H2O, reflux

3a-o

CH3

Br

3a X = I, 4h, 90% X = Br, 12h, 85% X = Cl, 24h, 61%

3b X = I, 8h, 88% X = Br, 12h, 78%

3c X = I, 2h, 85%

Cl

Br

Br

3d X = I, 4h, 88%

Cl

3e X = I, 5h, 80%

Cl

O

3h X = I, 2h, 92% X = Br, 8h, 87% O

Cl

O2N

Br 3j X = I, 4h, 91% O

OMe

3i X = I, 4h, 90% O

O

O

3k X = I, 4h, 96% X = Br, 8h, 87% X = Cl, 24h, 70%

OMe Cl

3l X = I, 5h, 91% X = Br, 10h, 85%

O OMe

O2N

3f X = I, 5h, 89% X = Br, 12h, 80%

OMe

3g X = I, 2h, 94% O2N X = Br, 8h, 89% X = Cl, 24h, 72%

O2N

Cl

OMe

3m X = I, 3h, 94% X = Br, 8h, 85% X = Cl, 24h, 68%

OMe 3n X = I, 5h, 91% X = Br, 10h, 82% X = Cl, 24h, 64%

HO

3o X = I, 10h, 80% X = Br, 24h, 64% X = Cl, 48h, 52%

Scheme 2. Pd-MNPSS-catalyzed Heck reaction of different aryl halides and alkenes. Reaction conditions: alkene (1.2 mmol), arylhalide (1.0 mmol), H2O (5mL), K2CO3 (2.0 mmol). All yields are isolated.



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Next the activity of Pd-MNPSS catalyst was evaluated in Sonogashira coupling reaction, in order to further show its performance and utility of in organic synthesis. Thus a model reaction containing iodobenzene and phenylacetylene was selected and different circumstances were investigated. Results of optimization study for Sonogashira reactions using Pd-MNPSS catalyst are summarized in Table 2. First, we checked the cooper-free Sonogashira reaction in DMF solvent at 120 °C and 90% of product was isolated after 24h (Table 2, entry 1). In DMF:H2O (1:5) mixture as solvent the reaction yield was decreased to 80% (Table 2, entry 2). However, in pure water as solvent, 83% of product was produced (Table 2, entry 3). Catalyst loading was investigated and 0.48 mol% of Pd was accepted as optimal for Pd-MNPSS-catalyzed Sonogashira coupling reaction (Table 2, entries 4-8). By decrease of temperature the reaction yield was significantly decreased and at rt no product observed (Table 2, entries 9&10). Reaction time was continuously checked and maximum yield of product was obtained after 10h and after that no progress in reaction yield was observed (Table 2, entries 11-14). Other bases were tested but no superiority was observed, thus K2CO3 was selected as best one (Table 2, entries 15-18). Finally, entry 12 of Table 2 was selected as the optimum conditions for Pd-MNPSS-catalyzed Sonogashira reaction. Table 2. Optimization study for Pd-MNPSS-catalyzed Sonogashira reaction between Iodobenzene and phenyl acetylenea

Time

Entry

Catalyst (mol%)

Solvent

Base

Temp ( °C)

1

0.41

DMF

K2CO3

120

24

90

2

0.41

K2CO3

120

24

80

3

0.41

H2O

K2CO3

100

24

83

4

0.36

H2O

K2CO3

100

24

75

DMF: (1:5)

H2O

(h)

Yield (%)b


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5

0.24

H2O

K2CO3

100

24

70

6

0.17

H2O

K2CO3

100

24

55

7

0.48

H2O

K2CO3

100

24

87

8

0.55

H2O

K2CO3

100

24

87

9

0.48

H2O

K2CO3

80

24

20

10

0.48

H2O

K2CO3

r.t

24

0

11

0.48

H2O

K2CO3

100

12

87

12

0.48

H2O

K2CO3

100

10

87

13

0.48

H2O

K2CO3

100

8

75

14

0.48

H2O

K2CO3

100

6

60

15

0.48

H2O

NaOH

100

10

15

16

0.48

H2O

DABCO

100

10

40

17

0.48

H2O

Et3N

100

10

35

18

0.48

H2O

K3PO4

100

10

25

a Reaction

conditions: iodobenzene (1.0 mmol), phenyl acetylene (1.2 mmol), (5.0 ml), base (2.0

mmol). b Isolated yields. The optimized conditions was used to synthesize diverse 1,2-diarylethynes57,58 using the reaction of phenyl acetylene and aryl halides (Scheme 3). Both electron-rich and electron-poor aryl halides underwent in the coupling reactions well to afford 1,2-diarylethyne products in high yields. Excellent yields for aryl iodides were obtained and aryl bromides gave satisfactory yields in more reaction times. During the reaction progress no homocoupling product of phenylacetylene was detected. Overall, this catalyst system was established to be one of the efficient heterogeneous Pd catalyst systems for Sonogashira coupling reactions in water solvent.



Page 18 of 32

Scheme 3. The Pd-MNPSS-catalyzed Sonogashira reaction of different aryl halides and phenylacetylene. Reaction condition: phenylacetylene (1.2 mmol), arylhalide (1.0 mmol), H2O (5.0 mL), K2CO3 (2.0 mmol). All yields are isolated. Heterogeneity tests of Pd-MNPSS catalyst in Heck and Sonogashira C-C coupling reactions The reusability of Pd-MNPSS catalyst was investigated in both Heck and Sonogashira reactions using the model reactions under optimized conditions. The Pd-MNPSS catalyst showed at least five times of reusability without remarkable decreasing in its catalytic performance in these reactions (Table 3&4).


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Table 3. Reusability of Pd-MNPSS catalyst in the Heck reactiona

Run

1

2

3

4

5

Time (h)

4

4

4

5

6

Yield (%)

89

88

86

85

83

Recovered catalyst (%)

>99

99

98

>97

97

Pd content (ppm)

25.8

ND

ND

ND

24.6

Pd leached during reaction (ppm)

0.2

0.1

0.2

0.3

0.2

Pd leached after hot filtration (ppm)

0.3

ND

ND

ND

0.1

Yield of hot filtration test (%)b

96

95

Pd content (ppm)

25.8

ND

ND

ND

23.9

Pd leached after hot filtration (ppm)

0.45

ND

ND

ND

0.22

Yield of hot filtration test (%)

Supported Palladium on Magnetic NanoparticlesâStarch Substrate

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