Heptazine-Based Porous Framework Supported Palladium

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Heptazine-based Porous Framework Supported Palladium Nanoparticles for Green Suzuki-Miyaura Reaction Zhi-Li Du, Qin-Qin Dang, and Xian-Ming Zhang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b05039 • Publication Date (Web): 28 Mar 2017 Downloaded from http://pubs.acs.org on April 1, 2017

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Industrial & Engineering Chemistry Research

Heptazine-based Porous Framework Supported Palladium Nanoparticles for Green Suzuki-Miyaura Reaction Zhi-Li Du, Qin-Qin Dang* and Xian-Ming Zhang*

School of Chemistry& Material Science, Shanxi Normal University, Linfen, Shanxi 041004, China KEYWORDS: heptazine, Suzuki-Miyaura coupling reaction, heterogeneous catalysis.

Abstract A newly heterogeneous palladium catalyst has been synthesized by immobilizing Pd(OAc)2 onto a nitrogen rich heptazine-based porous framework (Cy-pip). Highly dispersed Pd particles and abundant distributed N atoms in the framework make Pd/Cy-pip very efficient towards the Suzuki-Miyaura reaction in aqueous medium at 40 oC without inert atmosphere and phase transfer agents. Pd/Cy-pip catalytic system is superior to many POPs supported Pd catalysts such as Pd/COF-LZU1, Pd@CNPCs, Pd-CIN-1, and MsMOP-1 that required toxic organic solvents or high temperature. Furthermore, the strong interaction between N donor sites and Pd nanoparticles in the framework extends excellent recyclability without significant deactivation of the catalyst after five cycles. Introduction Palladium (Pd)-catalyzed Suzuki-Miyaura reactions are of strategic importance in recent years because of its applications in the synthesis of pharmaceuticals, herbicides, natural products, intermediate and conducting polymers1-4. Homogeneous palladium catalysts5-7 often suffer from high cost of ligands, separation difficulty, nonreusability and product contamination, which limits their industrial application. To overcome these problems, several remarkable heterogeneous catalyst systems have been explored by immobilization Pd into various supports8-11. A number of porous solid materials such as activated carbon 12, mesoporous silicates13, zeolites14 and metal-organic frameworks15-17 have been employed as supports for Pd catalysts. However, the limitations and drawbacks also existed in these catalytic systems. For pure inorganic material supports, the weak affinity between supports and substrate could cause severe diffusion problem of reactants and product18. In addition, grafting ligands to inorganic supports inevitably leads to pore blockage and low ligand loading. Metal-organic frameworks (MOFs) composed of metal ions and multidentate organic ligands also could be used as solid catalyst for Suzuki reaction. Nevertheless, MOFs suffer from some drawbacks such as low stability to moisture, thermal treatment and inaccessibility of substrate to active sites surrounded by bulky ligands. Porous organic polymers (POPs) with excellent stability, high surface area and designable porosity have broad prospects to be used as supports for metal species. Among most POPs, N-rich polymer frameworks offer a large number of metal binding sites for immobilization

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metal species. Recently palladium immobilized POPs catalyst has been reported as a candidate heterogeneous catalyst for Suzuki–Miyaura coupling reaction. In 2011, Wang et al19 synthesized Pd/COF-LZU1 (PdII-containing imine-linked covalent organic framework) which can catalyze Suzuki reaction. However, harsh conditions (150 oC in p-xylene solvent) were employed. Wang et al20 successfully synthesized heterogeneous catalysts Pd@CNPCs by immobilization palladium on cross-linked poly(p-phenyleneethynylene) networks. The Pd@CNPCs showed effective catalytic performance for the reaction of a variety of aryl bromides with arylboronic acids in DMF/H2O at 80 oC. Bhaumik et al21 immobilized Pd nanoparticles onto the surface of a porous covalent imine network (CIN-1) and further applied in the cross-coupling reaction between aryl bromides and phenylboronic acid in DMF at 80 oC. Sirilet al22 successfully synthesized a series of aryl compounds catalyzed by palladium–polyaniline (Pd–PANI) nanocomposite in toluene at 90 oC. Jiang et al23 immobilized Pd at the metal-free porphyrin-based covalent organic framework (H2P-Bph-COF) to heterogeneously catalyze the Suzuki-coupling reaction in toluene at 110 oC. Esteves et al24 synthesized Pd(OAc)2@COF-300 (an imine-linked covalent organic framework COF-300 modified by of Pd(OAc)2) which can efficiently catalyze Suzuki coupling reaction in MeOH/H2O at 70 oC. Although these Pd immobilized POPs catalysts have excellent catalytic activity, they required high temperature or toxic solvents such as p-xylene, N,N-dimethylformamide and toluene, which definitely consumes more energy or causes serious environment problems. Therefore, design and development of novel and green heterogeneous catalysts to be performed under mild condition are of great significance. Heptazine-based porous frameworks constructed from heptazine units and piperazine linkers have plenty of anchor sites to stabilize and disperse Pd active sites for heterogeneous catalysis. Recently we have developed a new heptazine-based polymer network (Cy-pip)25 with a number of nitrogen sites as Lewis bases, which could be used as efficient heterogeneous catalyst for Knoevenagel reaction. Based on our previous work, herein we immobilize Pd on the heptazine-based porous frameworks (Cy-pip) and subsequently explore catalytic activity of the system for green Suzuki-Miyaura coupling reaction in ethanol/water. The catalytic reactions can be efficiently performed at 40 oC. In addition, the heterogeneous catalyst exhibits excellent recycling capability and good stability.

Results and discussion The polymer network Cy-pip was synthesized as previous method25. Presence of heptazine and piperazine functional group in the framework provided binding sites for immobilization palladium species. The catalyst Pd/Cy-pip can be easily prepared by immobilizing Pd(OAc)2 onto Cy-pip polymer network in acetone by refluxing for 48 h. Inductively coupled plasma mass spectrometry (ICP), powdered X-ray diffraction (PXRD), N2 adsorption, X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM) were employed to characterize the Pd/Cy-pip catalyst. The Pd content was 2.14 mol% in Pd/Cy-pip catalyst as determined by ICP. Scheme 1 presents the structural representation of the Pd/Cy-pip heterogeneous catalyst.


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Scheme 1. Structural representation of the Pd/Cy-pip heterogeneous catalyst (most Pd species existed as metallic Pd(0) states with small fraction of Pd(II) in Pd/Cy-pip catalyst as seen from XPS spectra). PXRD pattern of Pd/Cy-pip is shown in Figure S1. Some distinct peaks are observed at 2θ values of 40.1°, 45.9°, and 66.7° degrees for Pd/Cy-pip, which correspond to (111), (200), (220) crystal planes of face-centered cubic Pd nanoparticles, respectively.26, 27 The distinct PXRD pattern revealed that Pd nanoparticles were supported on the Cy-pip framework. 28-30 A broad peak around 20.3 degrees confirmed the amorphous structure of the polymer network. The porosity in the Cy-pip and Pd/Cy-pip materials was determined by nitrogen adsorption-desorption isotherm at 77K (Figure S2). The BET surface areas for the Cy-pip and Pd/Cy-pip samples were 105.8 m2 g-1and 72.6 m2 g-1, with corresponding pore volumes of 0.43 cm3 g-1 and 0.33 cm3 g-1, respectively. The decrease in surface area and pore volume reflected that Pd species are successfully anchored on the Cy-pip polymer network. Pore size distributions (Figure S3) showed no remarkable change of the pore size distribution for the Pd/Cy-pip material compared with Cy-pip. To investigate the oxidation state information of palladium, XPS analysis of Pd 3d binding energies was performed. As illustrated in Figure 1, both Pd(0) (Pd-3d5/2 335.5 and Pd-3d3/2 341.0 eV)31 and Pd(II) (Pd-3d 5/2 337.4 and Pd-3d 3/2 342.7 eV)15b, 17 are present in the Pd/Cy-pip material. Based on the peak areas of Pd 3d 5/2 at 335.5 and 337.4 eV, the ratio of Pd(0) to Pd(II) in Pd/Cy-pip is calculated to be 1.3. Interestingly some Pd(II) ions were reduced to Pd(0) in the preparation of the Pd/Cy-pip catalyst. We hypothesized that heptazine and piperazine moieties in Pd/Cy-pip could assist reduction of Pd(II) into Pd(0). Vaidhyanathan R et al has reported that nitrogen-containing groups such as imine and triazine functionalities can act as reducing agents and stabilizer in the formation of palladium nanoparticles21, 32. TEM was further used to detect the degree of palladium dispersion in the Pd/Cy-pip catalyst. As shown in Figure 2, TEM images showed the high electron density spherical spots of palladium particles uniformly dispersed on Cy-pip networks. The average size is about 3-5 nm in diameter. TEM results suggested that Cy-pip could serve as effective platform for good dispersion of palladium.



0

Pd 3d5/2

0

Pd 3d3/2 2+

Intensity (a.u.)

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346

Pd 3d5/2

2+

Pd 3d3/2

344

342

340

338

336

334

332

Binding Energy (E)

Figure 1. XPS spectra of Pd/Cy-pip.

Figure 2. TEM image of Pd/Cy-pip. The catalytic performance of Pd/Cy-pip was evaluated by Suzuki-Miyaura coupling of 4-bromobenzaldehyde with phenylboronic acid as model reaction (Scheme 2). In order to optimize the reaction conditions, initial catalytic reactions were tested with different parameters including solvent, temperature, the catalyst loading and base. The results are summarized in Table 1. Among the different solvents, the water was proved to be the poor solvent (31.5%, Table 1, entry 1) while dimethylformamide (DMF) could afford moderate conversion 77.3% (Table 1, entry 2). In contrast, high conversion of 97.7% was obtained by using ethanol as solvent (Table 1, entry 3). This may be attributed to the good solubility of reactant, inorganic base and biaryl product in the solvent. Guan et al has reported that addition of suitable amount of water could accelerate conversion of the substrates33. Therefore the catalytic activity was also examined in mixed solvents with different combination of ethanol and water (Table 1, entries 4-6). High conversion of 99.3% and 99.8% could be achieved for 3:1 (v:v) and 1:1 (v:v) ethanol/water mixture. However, further increase of water amount in the ethanol/water mixture (v:v=1:3) would lead to lower conversion (54.1% Table 1, entry 4) which may be due to the poor solubility of reactants and biaryl product in the mixed solvent. Because water is much cheaper and greener than ethanol, the Suzuki-Miyaura coupling reaction in ethanol/H2O (v:v = 1:1) was an excellent choice. The catalyst loading also affects the catalytic activities. The conversion decreased to 73.5% upon palladium loading halved to 0.5 mol% (Table 1, entry 10). When the amount of K2CO3 base was decreased from 2 mmol to 1.2 mmol, the conversion decreased to 60% (Table 1, entry 11). No conversion of 4-bromobenzaldehyde was observed at 80 oC even for 16 h in the absence of a catalyst


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(Table 1, entry12). The effect of temperature on the catalytic performance was also investigated (Table 1, entries 6-9). Only 78% conversion could be detected at room temperature for 3 h (entry 9). The satisfactory conversion (97%) can be obtained at 40 oC for 1 h (Table 1, entry 8). When the temperature rose to 60 oC and 80 oC, the conversion reached more than 99% for 1 h (Table 1, entries 6-7). We also noted that the reaction can proceed smoothly in relatively short time. The conversion of entry 8 in Table 1 as a test model was monitored vs reaction time. Table S1 showed the conversion of 4-bromobenzaldehyde vs time at 40 oC. The reaction conducted rapidly with 30.5% conversion of the aryl halide at 40 oC for 1 min when the Pd catalyst was fixed at 1 mol%. This resulted in a turnover frequency (TOF) of 1830 h-1, which is comparable to Pd(OAc)2@COF300 (an imine-linked covalent organic framework COF-300 modified by of Pd(OAc)2)24 and the Pd-CIN-1 catalyst (Pd nanoparticles immobilized on the surface of a porous covalent imine network CIN-1)21.

Scheme 2. Pd/Cy-pip catalyzed aryl bromide with phenylboronic acid. Table 1. Suzuki–Miyaura coupling reaction of 4-bromobenzaldehyde with phenylboronic acid catalyzed by Pd/Cy-pip.

Entry

Solvent

Tem/℃

Time (h)

Conv. (%)

1

H2O

80

5

31.5

2

DMF

80

5

77.3

3

Ethanol

80

5

97.7

4

Ethanol/H2O=1:3

80

5

54.1

5

Ethanol/H2O=3:1

80

1

99.3

6

Ethanol/H2O=1:1

80

1

99.8

7

Ethanol/H2O=1:1

60

1

99.2

8

Ethanol/H2O=1:1

40

1

97

9

Ethanol/H2O=1:1

25

3

78

10 b

Ethanol/H2O=1:1

80

1

73.5

11 c

Ethanol/H2O=1:1

80

1

60

Ethanol/H2O=1:1

80

16

/

12d a

Conditions: 4-bromobenzaldehyde (1 mmol), phenylboronic acid (1.2 mmol), K2CO3 (2 mmol), Pd/Cy-pip (1 mol%, Based on Pd), EtOH (1 mL), H2O (1 mL).b Cy-pip-Pd (0.5 mol%). c K2CO3 (1.2 mmol). d Without catalyst Pd/Cy-pip.



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Finally, the optimized condition for Suzuki-Miyaura reaction was defined as: 1 mol% Pd/Cy-pip catalyst, ethanol/H2O (EtOH/H 2 O v:v = 1:1) solvent, 2 mmol K 2 CO 3 base, and at 40 oC for 1 h. Under optimized condition, we further explored the substrate scope of 4-substituted aryl bromides with different electronic and steric characters. The results are summarized in Table 2. In general, aryl bromides containing electron-withdrawing functional groups such as –CHO、–COCH3 and –NO2 exhibited excellent conversion (>97%, Table 2, entries 2-4), whereas electron-donating groups gave rise to relative low conversion (63.6% for –OCH3 and 79.1% for –CH3) (Table 2, entries 5-6). Normally, electron-withdrawing substitutes promoted Suzuki coupling reaction and exhibited a positive effect due to the activation of C-Br bond34. The catalytic trend is consistent with the reported ones. Pd/Cy-pip catalytic system is superior in comparison with many other POPs supported Pd catalysts for Suzuki reaction such as Pd/COF-LZU1 (PdII-containing imine-linked covalent organic framework)19, Pd@CNPCs (Pd immobilized on cross-linked poly(p-phenyleneethynylene) networks)20, Pd-CIN-1 (Pd nanoparticles immobilized on the surface of a porous covalent imine network CIN-1)21, and MsMOP-1 (metallosalen-based microporous organic polymer)35 which required toxic organic solvents or high temperature. Pd/Cy-pip herein could achieve a green and efficient catalysis under mild reaction conditions without inert atmosphere and phase transfer agents. The superior catalytic performance may be attributed to uniform dispersion of Pd nanoparticles and unique porosity that allows the reactant substrates to access the active sites. The catalyst also displays a steric selectivity for substrates with different molecular sizes. Under the standard condition, catalytic reaction of 2-bromonaphthalene with phenylboronic acid only afforded 35.5% conversion (Table 2, entry 7). This is probably due to the large steric hindrance of 2-bromonaphthalene which makes reaction substrate difficult to gain access to Pd nanoparticles in the polymer network. Furthermore, the catalytic properties for 4-substituted boronic acid with different electronic character were also tested (Table 2, entries 8 and 9). 4-methylphenylboronic acid bearing electron-rich group resulted in conversion of 87.5%, which exhibits better activity than Pd-MOF (m-Terphenyl Anchored Palladium Diphosphinite PCP-Pincer Complexes)36. However, strong electron -withdrawing groups lead to low conversion (24.5% for –NO2), which may be due to the low solubility of 4-nitrophenylboronic in the solvents. Table 2. Catalytic Suzuki coupling of various aryl bromide with phenylboronic acid by Pd/Cy-pip in Ethanol/H2O (EtOH/H2O v:v = 1:1).

Entry

R1

R2

Product

1

Ph

H

88.1

2

4-CHOC6H4

H

97

3

4-COCH3C6H4

H

98.8


Conv. (%)

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4

4-NO2C6H4

H

99.6

5

4-OCH3C6H4

H

63.6

6

4-CH3C6H4

H

79.1

H

35.5

7

8

Ph

-CH3

87.5

9

Ph

-NO2

24.5

Reaction conditions: R-Br (1 mmol), phenylboronic acid (1.2 mmol), K2CO3 (2 mmol), Pd/Cy-pip (1 mol%), EtOH (1 mL), H2O (1 mL), 40 oC for 1 h. For practical application of a heterogeneous catalytic system, the stability and reusability are very important aspects. Therefore, we have studied the recyclability of Pd/Cy-pip by employing 4-bromobenzaldehyde and phenylboronic acid as model substrates. As shown in Figure S4, no significant loss of activity was observed after five runs. The recovered Pd/Cy-pip catalyst after five runs was characterized by TEM (Figure S5). TEM analysis of recovered Pd/Cy-pip catalyst showed that most palladium nanoparticles are distributed uniformly in the Cy-pip polymer network with only a little palladium aggregations. The Pd/Cy-pip catalyst after five runs has also been characterized by XRD as seen in Figure S6. Compared with fresh prepared Pd/Cy-pip, the diffraction patterns were well maintained after five runs, which indicated the stability of the Pd/Cy-pip catalysts. ICP revealed 1.96 mol% Pd content in the recovered catalyst, almost same to the fresh prepared ones (2.14 mol%), indicating very low metal leaching. Hot-filtration test was further carried out by using 4-bromobenzaldehyde as standard substrate. The Pd/Cy-pip catalyst was removed from the reaction mixture after the substrates reacted at 40 oC for 0.2 h (79.5%). The mother solution didn’t show further conversion for additional 7 h (Figure 3). These results demonstrated the superior catalytic activity of Pd/Cy-pip in Suzuki-Miyaura coupling reactions. The heptazine and piperazine functionalities in the network can anchor Pd atoms and further help to disperse and stabilize Pd nanaoparticles. The π-π interaction between Cy-pip and aryl substrate molecules favoured reactant substrate to access Pd nanoparticles. The proposed mechanism of Pd/Cy-pip catalyzed Suzuki-Miyaura reaction of aryl bromide with phenylboronic acid is illustrated in Scheme 3. Firstly, oxidation addition of arylbromide and well-dispersed Pd nanoparticle gave organopalladium species. With the assistance of base, boronic acid converted to boronate complex, followed by transmetallation with aforementioned organopalladium species afforded the diarylpalladium intermediate. Finally, the desired biphenyl product was obtained through reductive elimination, accompanied by restoration of original palladium catalyst.



100

a

80

Conversion [%]

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b

60 40 20 0 0

1

2

3

4

5

6

7

t (h)

Figure 3. Hot filtration experiments (a) with Pd/Cy-pip catalyst, (b) removing Pd/Cy-pip catalyst after 0.2 h.

Scheme 3. Mechanistic cycle of the Suzuki-Miyaura reaction catalyzed by Pd/Cy-pip. Conclusions In this work we developed a newly heterogeneous palladium catalyst by immobilizing Pd(II) onto a nitrogen rich heptazine-based porous framework (Cy-pip). Owing to high dispersion of Pd catalytic sites, the materials exhibited highly efficient activity in Suzuki reactions in aqueous medium at mild conditions without inert atmosphere and phase transfer agents. The catalyst also demonstrated excellent recyclability without significant loss of the deactivation. Author information Corresponding Author E-mail: [email protected], [email protected] Acknowledgements This work was supported by 973 Program 2012CB821701, NSFC 21101102, Plan for 10000 Talentsin China, Shanxi Province Science Foundation for Youths (2012021008-2) and Innovative Experimental Program for Undergraduates of Shanxi Normal University (SD2015CXXM-82). Supporting Information


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Materials and Measurements.Preparation of the catalyst. Heterogeneous catalytic Suzuki-Miyaura coupling reaction. This information is available free of charge via the Internet at http: //pubs.acs.org.

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Suzuki-Miyaura cross-coupling reaction in aqueous media. Green Chem. 2012, 14 (7), 1964-1970. (34) Grushin, V. V.; Alper, H. Transformations of Chloroarenes, Catalyzed by Transition-Metal Complexes. Chem. Rev. 1994, 94 (4), 1047-1062. (35) Li, H.; Xu, B.; Liu, X.; A, S.; He, C.; Xia, H.; Mu, Y. A metallosalen-based microporous organic polymer as a heterogeneous carbon-carbon coupling catalyst. J. Mater. Chem. A 2013, 1 (45), 14108-14114. (36) Lipke, M. C.; Woloszynek, R. A.; Ma, L.; Protasiewicz, J. D. m-Terphenyl Anchored Palladium Diphosphinite PCP-Pincer Complexes That Promote the Suzuki-Miyaura Reaction Under Mild Conditions. Organometallics 2009, 28 (1), 188-196.



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Table of Contents (TOC)


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Heptazine-Based Porous Framework Supported Palladium

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