Boron Nitride Nanosheet-Anchored PdâFe CoreâShell Nanoparticles

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Boron Nitride Nanosheet-Anchored Pd-Fe Core-Shell Nanoparticles as Highly Efficient Catalysts for Suzuki–Miyaura Coupling Reactions Qinrui Fu, Yuan Meng, Zilin Fang, Quanqin Hu, Liang Xu, Wenhua Gao, Xiao-Chun Huang, Qiao Xue, Ya-Ping Sun, and Fushen Lu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13570 • Publication Date (Web): 04 Jan 2017 Downloaded from http://pubs.acs.org on January 4, 2017

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ACS Applied Materials & Interfaces

Boron Nitride Nanosheet-Anchored Pd-Fe Core-Shell Nanoparticles as Highly Efficient Catalysts for Suzuki–Miyaura Coupling Reactions

Qinrui Fu,† Yuan Meng,† Zilin Fang,† Quanqin Hu,† Liang Xu,† Wenhua Gao,† Xiaochun Huang,† Qiao Xue,† Ya-Ping Sun,‡ and Fushen Lu†,*

†

Department of Chemistry and Key Laboratory for Preparation and Application of Ordered

Structural Materials of Guangdong Province, Shantou University, Guangdong 515063, China ‡

Department of Chemistry and Laboratory for Emerging Materials and Technology, Clemson

University, Clemson, South Carolina 29634-0973, USA

ABSTRACT Boron nitride nanosheets (BNNS) were used to anchor bimetallic Pd-Fe nanoparticles for Suzuki–Miyaura coupling catalysts. The bimetallic nanoparticles were found to be core-shell in structure, and their formation was likely facilitated by their interactions with the BNNS. The PdFe/BNNS catalysts were highly effective in representative Suzuki–Miyaura reactions, with performances matching or exceeding those of the state-of-the-art methods. Specifically, the superior catalytic activities were characterized by generally shortened reaction times, minimal Pd usage, excellent reusability of the catalysts and high or nearly quantitative conversion yields in a benign solvent system without the need for any special conditions, such as ligands and surfactants or inert gas protection. The obvious advantages of the Pd-Fe/BNNS over similar

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catalysts based on other supports, such as reduced graphene oxide (rGO), suggest that BNNS may be developed into a versatile platform for many other important catalytic reactions. KEYWORDS: boron nitride sheets, Pd-Fe nanoparticles, core-shell, catalysts, Suzuki–Miyaura reactions.

INTRODUCTION Suzuki–Miyaura coupling represents a reaction that is highly versatile, applicable to a variety of biaryl derivatives, and characterized by many advantageous features including mild reaction conditions and the easy handling of reagents and byproducts.1-3 While homogeneous catalysts for this coupling have afforded relatively high yields and fast reaction rates, their shortcomings, such as generally requiring ligands that may be air and/or moisture sensitive and issues with the recovery of catalysts from reaction mixtures, have prompted the development of many heterogeneous catalysts for Suzuki–Miyaura coupling.4-7 These catalysts are mostly based on the immobilization of palladium (Pd) nanoparticles on a solid support; the selection of supports has included silica,8 alumina,9,10 and zeolites.11,12 Building upon the popular use of carbon as a support for Pd/C catalysts, recent studies have employed carbon nanomaterials in the design and fabrication of catalysts.13-15 In fact, the heterogeneous catalyst consisting of Pd on graphite oxide (GO) exhibited activities nearly matching those of high-performance homogeneous catalysts.16,17 The successful use of GO, which is a two-dimensional (2D) carbon nanomaterial formed via the exhaustive oxidation of graphite (and thus more vulnerable under reductive conditions), has presented excellent opportunities for other 2D nanomaterials, especially boron nitride nanosheets (BNNS), which not only support the same catalytic activities but are also physicochemically


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highly stable and structurally robust. BNNS from the exfoliation of hexagonal boron nitride (h-BN) are analogous to few-layer graphene nanosheets (Scheme 1), except for stronger interlayer interactions.18-23 Among their uniquely advantageous properties are high stability, excellent chemical and oxidation resistance under extreme conditions, and the electronegative characteristics according to results from theoretical calculations.24-26 The use of BNNS to load metal nanoparticles has been explored for various catalytic reactions27-32 in which beneficial effects due to the possible mixing of the dz2 metal orbitals with the N-pz and B-pz orbitals in boron nitride were discussed.32-34 In the work reported here, we found that our specifically prepared BNNS could anchor Pd-based catalytic nanoparticles for use in Suzuki–Miyaura coupling reactions with performances matching or exceeding the state of the art.10,13,16,35-51

Scheme 1

In a catalyst designed to minimize Pd usage,35-37 Lipshutz and co-workers demonstrated that Pd-Fe bimetallic nanoparticles coupled with phosphine ligands and a special surfactant are ideally suited for pseudo-homogeneous catalysis in Suzuki–Miyaura coupling reactions. They reduced the Pd content in the catalysts to a few hundred ppm (parts-per-million).35 As also found in the work reported here, the BNNS-anchored Pd-Fe bimetallic nanoparticles in a Pd core-Fe shell configuration were highly effective in the heterogeneous catalysis of Suzuki–Miyaura coupling in a benign ethanol-water mixture with yields up to 99% and Pd usage down to 200 ppm. There was no need for any ligands, surfactants, or inert gas protection, and the cumulative



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turnover number was more than 13,000 after six cycles. Hypothetically, the core-shell structure may be advantageous for preventing the active Pd-cores from aggregating and thus better preserving their catalytic activity in the coupling reactions.

RESULTS AND DISCUSSION The BNNS were prepared using a method already reported in the literature52 in which pristine hexagonal boron nitride (h-BN) was exfoliated in an ethanol-water mixture with the aid of vigorous sonication. The BNNS harvested from the exfoliation were re-dispersed in the ethanolwater mixture, and Pd and Fe salts were added to the dispersion, followed by vigorous stirring to ensure that the solution was well-mixed. Then, the metal salts were slowly reduced by sodium borohydride at a low temperature for the formation of Pd-Fe bimetallic nanoparticles on the BNNS supports. The Pd loading on the catalysts (including the metals and BNNS) was purposely kept low, approximately 0.2 wt% (versus the Fe loading of 11.0 wt%), based on the inductively coupled plasma atomic emission spectrometry (ICP-AES) results. The BNNS-anchored Pd-Fe nanoparticles were characterized using electron microscopy techniques. As shown in the TEM images (Figure 1), the BNNS had a lateral size on the order of a micron and showed clear lattice fringes in high-resolution mode (Figure S1), on which the PdFe nanoparticles were fairly distributed. The core-shell structure of the bimetallic nanoparticles was revealed under high-resolution TEM (Figure 1), with the core Pd lattice spacing of 0.228 nm (111) and the shell Fe lattice spacing of 0.279 nm (102). In multiple TEM images, 200 Pd-Fe core-shell nanoparticles were randomly selected for size measurements, from which the average diameter of the Pd core was estimated to be 1.6 nm.


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Figure 1 Further confirmation of the Pd-Fe core-shell structure was achieved via STEM imaging in the Z-contrast mode and three-dimensional mapping (Figure 2, see also Supporting Information). The results also suggested a relatively thick Fe shell and that the bimetallic core-shell nanoparticles were indeed anchored on the BNNS. Most of the core-shell nanoparticles were on or close to the defective edges of the BNNS, as revealed by the TEM and EDS mapping images (Figure 2 and Figure S2). In addition, the Pd nanoparticles were embedded in the relatively thick Fe shells, which could prevent Pd nanoparticles from aggregating. The BNNS are electronegative according to the results from theoretical studies24-26 and favorable for the adhesion of metal nanoparticles. The formation of Pd-Fe core-shell nanostructures may be rationalized as a result of Fe being stronger than Pd in binding to the BNNS surfaces.53

Figure 2

The surface composition of the Pd-Fe/BNNS was analyzed by X-ray photoelectron spectroscopy (XPS, Figure 3). The binding energies were calibrated using the C 1s signal at 284.8 eV (Figure S3). The peaks centered at 711.0 and 724.4 eV may be assigned to Fe3+ 2p3/2 and Fe3+ 2p1/2, respectively (Figure 3).54 The significant O 1s peak at 531.1 eV might be an indication that the outer shell of the Pd-Fe nanoparticle was mainly Fe2O3, probably due to the air exposure during the preparation and storage of the nanoparticles. The relatively much weaker



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signals of Pd 3d (Figure 3) were consistent with the Pd core being small and embedded in a thick Fe shell (capped by Fe2O3). Nevertheless, the Pd peaks were split roughly into two doublets corresponding to different Pd valences. The doublet centered at 335.7 eV and 341.0 eV was likely due to Pd0, while the other doublet at 343.5 eV and 337.8 eV was probably due to Pd2+,55 which again might be a result of the air exposure during the preparation and storage of the nanoparticles.

Figure 3

The Pd-Fe/BNNS catalysts were evaluated in a series of representative Suzuki–Miyaura coupling reactions,10,13,16,35-51 including the bromobenzene-phenylboronic acid pair and those between substituted aryl halides and phenyl- or 1-naphthylboronic acids (Table 1). The reaction media significantly influenced the Pd-Fe/BNNS-catalyzed coupling (Table S1). The ethanolwater mixture apparently surpassed other commonly used solvents because the poor solubility of biphenyl compounds in the aqueous-ethanol mixture probably drove the equilibrium forward. Therefore, the coupling reactions were carried out in a benign aqueous-based solvent mixture, i.e., water-ethanol in a one-to-one volume ratio. For the catalytic coupling of bromobenzene and phenylboronic acid, as an example, a mixture of bromobenzene, the Pd-Fe/BNNS catalyst, and potassium tert-butoxide was dispersed in the solvent with mild sonication, and phenylboronic acid was added to the dispersion with stirring. The reaction was rather efficient, achieving greater than 90% yields in 30 min, while the results from control experiments using BNNSsupported catalysts without either Pd or Fe suggested no or rather poor reaction yields (Table 1).


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However, a lower Pd loading on the Pd-Fe/BNNS catalysts had no or even a positive effect on the reaction yield, reaching 99% with a Pd:Fe ratio down to 1:100 and a total Pd usage of only approximately 0.02 mg for 1 mmol of reactants. Therefore, Pd-Fe/BNNS catalysts with a low Pd:Fe ratio (1:100) and a minimal amount of Pd (approximately 0.02 – 0.09 mg per 1 mmol of reactants) were used in all of the following reactions, yielding excellent coupling results (Table 1). With these catalysts, derivatized bromobenzenes of either an electron-donating or electronwithdrawing substituent could be coupled with phenylboronic acid in high yields in 3.5 h or less (Table 1). For the more active reactant iodobenzene, the coupling was quantitative within only 10 min. When potassium tert-butoxide (pH ~14.0) was replaced by a slightly weaker base (KOH, pH ~13.4), the changes in the yields of biphenyl and its derivatives were marginal (Table 1). However, when potassium phosphate (pH ~12.9) was used as the base, the yields were significantly improved for the Pd-Fe/BNNS-catalyzed coupling reactions involving heteroatomcontaining reactants. For example, the coupling of 4-bromoaniline and 4-bromophenol with phenylboronic acid in the presence of potassium phosphate showed reaction yields of 84% and 92%, respectively. In contrast, the same reactions in the presence of potassium tert-butoxide resulted in much lower yields (< 20%) and several byproducts. The obviously lower efficiency of potassium tert-butoxide was likely because the strong base might deprotonate hydroxyl and amine groups and cause nucleophilic substitution between aryl halides themselves.56,57 PdFe/BNNS generated somewhat lower yields in the coupling reactions involving heterocyclic reactants, as demonstrated by coupling 3-bromopyridine with phenylboronic acid pinacol ester. It is worth noting that Pd-Fe/BNNS was still better than its counterpart (GO-supported Pd2+



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catalysts).16 The decreased yield might be attributed to the inhibition of active catalytic sites caused by the complexation between supported metals and heterocyclic compounds in ligandfree coupling reactions.58

Table 1

The performance of the Pd-Fe/BNNS catalysts is clearly better than the state-of-the-art ligand-free anchored-Pd nanoparticle catalysts for Suzuki–Miyaura coupling reactions, especially considering the minimal Pd usage and shorter reaction time (Table S2). In addition, the much higher turnover frequency (TOF, calculated from the nearly complete reactions) is important, it is one to two orders of magnitude higher than the previously reported values for the same reactants with other catalysts (Table S2). In the coupling of 2-bromonaphthalene with phenylboronic acid, for example, the Pd-Fe/BNNS catalyst achieved a TOF of 3,233 versus the much lower TOF value of only 94 for the same reaction catalyzed by graphite oxide (GO)anchored Pd2+ (denoted as Pd2+-GO), for which the Pd nanoparticles were generated in situ during the coupling reaction.16 For the coupling reactions of aryl halides with phenylboronic or 1-naphthyboronic acid, the results obtained with the Pd-Fe/BNNS catalysts were also comparable to those based on pseudohomogeneous Pd-Fe catalysts, which required complexation with a phosphine ligand and the use of a special surfactant.35 For the catalytic coupling of 4-bromoanisole with 1-naphthylboronic acid as a specific example, comparable TOFs were achieved using the Pd-Fe/BNNS catalysts, which did not require any ligands, special surfactants, or inert gas protection. However, as


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reported for the pseudo-homogeneous Pd-Fe catalysts, no coupling product was detected in the absence of the phosphine ligand, and the reaction yield decreased to 39% when the special surfactant was not used. The obviously high performance of the Pd-Fe/BNNS catalysts is probably a result of synergetic effects due to the Pd-Fe nanoparticles in a core-shell configuration (Figures 1 and 2) and their interactions with the BNNS support. For the former, other bimetallic nanoparticles including Pd-Co and Pd-Ni anchored on the BNNS were prepared using the same method and experimental procedure, and their catalytic activities were similarly evaluated. They were both relatively inefficient for the coupling of bromobenzene with phenylboronic acid as a benchmark reaction, with observed conversion yields of 9% and 42% for the Pd-Co/BNNS and Pd-Ni/BNNS, respectively. Similarly, BNNS-anchored Pd or Fe nanoparticles were much less effective or inactive for the coupling reaction (Table 1). For the latter interactions, a possible mode is the predicted overlap between the dz2 orbitals in the metals and the pz orbitals in the boron and nitrogen in the BNNS based on results from theoretical calculations,32-34 which is conceptually and mechanistically similar to the chelation of the metals by heteroatom-containing ligands.59 Considering the possible synergetic effects that are responsible for the observed high catalytic activities of Pd-Fe/BNNS, one may argue that the interactions between the BNNS and their anchored metal nanoparticles could even be more significant than those in the homogeneous metal-ligand catalysts.35 As a control, rGO was used in the place of BNNS for Pd-Fe/rGO catalysts. These catalysts were found to be significantly less effective than Pd-Fe/BNNS under the same reaction conditions. Using the bromobenzene-phenylboronic acid pair as a benchmark, the yield from the use of the Pd-Fe/rGO was only 64%, compared to the nearly quantitative



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coupling with Pd-Fe/BNNS (Table 1). According to the XPS spectra (Figure S3) in the Supporting Information, Pd-Fe/rGO was not remarkably different from Pd-Fe/BNNS. However, the Pd nanoparticles were generally located on top of the Fe2O3 surfaces, and no apparent coreshell structure was found in the TEM and EDS mapping images of the Pd-Fe/rGO samples (Figure S4 and S5). Hypothetically, the significantly improved catalytic activity of Pd-Fe/BNNS compared with Pd-Fe/rGO was probably due to the synergistic effects and electronic interactions between the bimetallic core-shell nanoparticles and the BNNS supports. Further theoretical and experimental investigations are needed for an understanding of the mechanistic details of the bimetallic core-shell nanoparticle-BNNS interactions and the associated effects on the catalytic performance. Nevertheless, Pd-Fe/BNNS is obviously an excellent catalyst for Suzuki–Miyaura coupling, regardless of the mechanistic origins. The Pd-Fe/BNNS catalysts could be recycled and reused, and their high catalytic activities were preserved (Figure 4). Specifically, for the coupling of bromobenzene with phenylboronic acid, the average yield was still greater than 90% for six cycles. After six cycles, the Pd and Fe contents in the recovered catalysts were determined by ICP analysis, and the results suggested only minimal losses of the metals, less than 10% for Pd and approximately 5% for Fe. For the coupling of 4-bromoanisole and 1-naphthylboronic acid, the average yield was still up to 90% for five cycles. This is in contrast to the result of pseudo-homogeneous catalysis with phosphine ligand-complexed Pd-Fe and a special surfactant,35 where the addition of more Pd was required to maintain the catalytic activity after every other cycle.

Figure 4


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CONCLUSION In summary, due to their unique and/or advantageous characteristics, BNNS are ideally suited for supporting metal nanoparticles in the design and fabrication of high-performance heterogeneous catalysts. In the immobilization of Pd-Fe bimetallic nanoparticles for catalytic Suzuki–Miyaura coupling, BNNS apparently played roles beyond that of a simple support, with more specific interactions not only contributing to the formation of Pd-Fe core-shell nanostructures in the catalyst fabrication but also enabling the Pd-Fe/BNNS catalysts to achieve excellent performance that matches or exceeds the state of the art in representative Suzuki– Miyaura reactions. The superior catalytic activities were reflected by the high or nearly quantitative conversion yields in a benign solvent system and without the need for any special conditions (such as specific ligands, special surfactants, or inert gas protection), the generally shortened reaction time, the minimal Pd usage, and the excellent reusability of the catalysts (the high catalytic performance preserved after multiple cycles). The obvious advantages of the PdFe/BNNS over similar catalysts based on other supports, such as rGO, suggest that BNNS may be developed into a versatile platform for heterogeneous catalysts for other important reactions, which will be pursued in further investigations. EXPERIMENTAL SECTION Materials. Hexagonal boron nitride flakes (5-10 μm in diameter), 4-methoxybiphenyl (98%), and 1-bromo-4-nitrobenzene (98%) were obtained from Alfa Aesar, potassium tert-butoxide from Xiya Reagent, phenylboronic acid pinacol ester (97%) from Energy Chemical. Sodium tetrachloropalladate (98%), bromobenzene (99.5%), iodobenzene (99%), sodium borohydride



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(98%), phenylboronic acid (97%), 1-naphthylboronic acid (97%), 4-bromotoluene (99%), 4bromoanisole (99%), 4-bromoacetophenone (98%), 4-bromophenol (98%), 4-bromoaniline (99%), 3-bromopyridine (98%), biphenyl (99.5%), 4-acetyl-biphenyl (98%), 4-phenyltoluene (98%),

4-bromobenzaldehyde

(99%),

2-bromonaphthalene

(98%),

nickel(II)

chloride

hexahydrate (98%), cobalt(II) chloride hexhydrate (98%), iron(III) chloride (98%), potassium phosphate, and graphite (99.95%) were purchased from Aladdin, Shanghai. Biphenyl-4carboxaldehyde (>95%) and 4-nitrobiphenyl (98%) were supplied by Tokyo Chemical Industry and J&K Scientific Ltd., respectively. Other reagents and solvents were obtained from Xilong Chemical. Measurements. Transmission electron microscopy (TEM) images were taken on a Tecnai G2 F20 with an accelerating voltage of 200 kV. Scanning transmission electron microscopy (STEM) imaging and energy-dispersive X-ray spectroscopy (EDS) mapping were carried out on a Talos F200X electron microscope equipped with high-angle annular dark field (HAADF) detectors. The specimens for the electron microscopy imaging experiments were prepared by depositing a drop of a dilute dispersion of the material being analyzed onto a holey carbon-coated copper grid. Surface composition analysis was performed by using a Kratos Axis UltraDLD X-ray photoelectron spectroscopy (XPS) instrument (Al Kα radiation, hν = 1486.6 eV). The metal contents in catalysts were determined on a Shimadzu ICPE-9000 inductively coupled plasma atomic emission spectrometry (ICP-AES) instrument. Pd-Fe/BNNS. For the preparation of the BNNS, pristine h-BN (500 mg) was suspended in an ethanol-water mixture (volume ratio 1:1, 300 mL) and sonicated in a bath sonicator (JeKen PS40 bath sonicator, 240 W) for 15 h. The mixture was allowed to stand undisturbed for 8 h for the


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precipitation of large h-BN pellets. The supernatant was collected and filtered through a polytetrafluoroethylene (PTFE) membrane (pore size 0.22 μm), and the BNNS sample obtained as a white solid was dried under ambient conditions. The BNNS (60 mg) was dispersed in an ethanol-water mixture (volume ratio 1:1, 80 mL) with the aid of mild sonication. A Na2PdCl4 solution (30 mmol/L, 60 μL) and FeCl3 powder (30 mg) were added to the solution and followed by vigorous stirring for 5 h at room temperature. The reaction mixture was cooled in an ice-water bath, and its pH was adjusted to ~8 via the slow addition of a dilute NaOH solution. After stirring for 1 h, a NaBH4 solution (0.1 mol/L, 40 mL) was added dropwise, and the reaction mixture was stirred for another hour in the ice-water bath. The resulting mixture was filtered through a PTFE membrane filter (pore size 0.22 μm), and the solid was repeatedly washed with anhydrous ethanol and then dried to obtain Pd-Fe/BNNS. The Pd-to-Fe ratio in the Pd-Fe/BNNS was varied to achieve catalysts with different metal contents and compositions, for which the same experimental procedures and conditions were used except for the different amounts of Pd and Fe precursors. Their metal loadings were measured to be 10.9 wt% Fe for Fe/BNNS, 5.5 wt% Pd for Pd/BNNS, and 5.5 wt% Pd and 11.0 wt% Fe for the comparative Pd-Fe/BNNS. Other Catalysts. Pd-Co/BNNS and Pd-Ni/BNNS were prepared using the same experimental procedures and conditions except for CoCl2 and NiCl2 as precursors for the second metal, respectively. The metal loadings based on the ICP-AES results were 0.2 wt% Pd in both catalysts and 11.8 wt% Co or Ni in Pd-Co/BNNS or Pd-Ni/BNNS, respectively. Graphite oxide (GO) was prepared via the modified Hummers' method.60-62 Briefly, graphite (1.0 g) was pre-oxidized by (NH4)2S2O8 (0.9 g) and P2O5 (0.9 g) in concentrated H2SO4 (10 mL)



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at 80 °C. After work-up, the resulting solid was subjected to further oxidation by KMnO4 (5.0 g) in concentrated H2SO4 (40 mL).62 The reaction mixture was diluted, washed with aqueous H2O2 and HCl solutions, and dialyzed against fresh water, followed by sonication for complete exfoliation (GO dispersion). The Pd-Fe/rGO catalysts were similarly prepared by replacing BNNS with GO with the metal contents of 0.2 wt% Pd and 11.0 wt% Fe. Suzuki–Miyaura Coupling Reactions. In a typical reaction for biphenyl products, phenyl halides (1.0 mmol, 1 equivalent), base (KOH, (CH3)3COK or K3PO4, 2.0 mmol, 2 equivalent), and the catalyst (10 mg) were dissolved or dispersed in an ethanol-water mixture (volume ratio 1:1, 10 mL) with the aid of mild sonication. Phenylboronic acid (1.2 mmol, 1.2 equivalent) was added to the dispersion with stirring. At the end of the reaction, the reaction mixture was filtered through a PTFE membrane filter (pore size 0.22 μm), and the solid was washed with anhydrous ethanol. The filtrate was collected and dried over anhydrous MgSO4. The products were quantified by gas chromatography (Agilent Technologies 6890N) analysis with the commercial biphenyl compounds as standards for the calculation of the reaction yields. The coupling reaction of 4-bromoanisole and 1-naphthylboronic acid was carried out using reactant ratios and experimental parameters mimicking those in the recent report by Lipshutz and co-workers.35 Briefly, 4-bromoanisole (0.5 mmol), 1-naphthylboronic acid (0.75 mmol), (CH3)3COK (1.0 mmol), and the Pd-Fe/BNNS catalyst with 0.2 wt% Pd and 11.0 wt% Fe (20 mg) were mixed in an ethanol-water mixture, followed by refluxing for 12 h. The crude product was purified on a silica column with hexane as the eluent. 3-Bromopyridine and phenylboronic acid pinacol ester were coupled using the same procedure except for potassium phosphate as the base. The other workup procedures were the same as those for the biphenyl products described above.


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The Pd-Fe/BNNS catalysts were collected post-reaction in the filtration step and washed repeatedly with distilled water to remove the adsorbed bases and any other residual species. They were dispersed in the ethanol-water mixture and reduced with NaBH4. The detailed procedure and reaction conditions for the recycling of Pd-Fe/BNNS were described in the Supporting Information.

ASSOCIATED CONTENT Supporting Information Supplemental Video showing the 3-dimensional mapping of multiple core-shell Pd-Fe nanoparticles; the supplementary TEM, merged elemental mapping images and EDS spectrum of Pd-Fe/BNNS; XPS, TEM and EDS mapping images of Pd-Fe/rGO; reaction conditions for the recycling of Pd-Fe/BNNS; 1H NMR spectra of biphenyl derivatives.

AUTHOR INFORMATION Corresponding Author *

E-mail: [email protected].

ACKNOWLEDGMENTS This work was supported by the NSFC (51272152 and 21671127) and Guangdong Natural Science Foundation (S2013010014171). Additional supports from Guangdong Province (2014KCXTD012) and SRF for ROCS (SEM) were also acknowledged.



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Scheme and Figure Captions Scheme 1. Schematic structures of graphene, graphene oxide, and boron nitride nanosheet. Figure 1. TEM images of Pd-Fe/BNNS (0.2 wt% Pd and 11.0 wt% Fe) at low magnification (a,b) and high magnification showing the core-shell structures (c) and lattice spacings (d). Figure 2. STEM images of a core-shell Pd-Fe nanoparticle in the bright field mode (a) and dark field (Z-contrast) mode (b), a STEM image of multiple core-shell Pd-Fe nanoparticles in the bright field mode (c) and EDS mapping of the nanoparticles shown in panel c (d–i), and representative three-dimensional mapping of multiple core-shell Pd-Fe nanoparticles (j). Figure 3. XPS survey spectrum of Pd-Fe/BNNS (a) and enlarged spectra in Fe 2p (b) and Pd 3d (c) regions. Figure 4. Recyclability of Pd-Fe/BNNS for Suzuki–Miyaura coupling reactions.


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Table 1. Results of Suzuki–Miyaura Coupling Reactions with Pd-Fe/BNNS as Catalysts.

Entry

Ar-X

Ar′-R

Pd (μmol)

Fe (μmol)

Time

Base

1

Bromobenzene

phenylboronic acid

0

20

30 min

(CH3)3COK KOH

Yield (%) undetectable undetectable

2

Bromobenzene

phenylboronic acid

5

0

30 min

(CH3)3COK KOH

25 28

3

Bromobenzene

phenylboronic acid

5

20

30 min

(CH3)3COK KOH

92 90

4

Bromobenzene

phenylboronic acid

0.2

20

30 min

(CH3)3COK KOH K3PO4

99 93 95

5

Iodobenzene

phenylboronic acid

0.2

20

10 min

(CH3)3COK KOH

100 96

6

4-Bromotoluene

phenylboronic acid

0.2

20

45 min

(CH3)3COK KOH

96 90

7

4-Bromoacetophenone

phenylboronic acid

0.2

20

45 min

(CH3)3COK KOH

95 98

8

4-Bromobenzaldehyde

phenylboronic acid

0.2

20

45 min

(CH3)3COK KOH

93 92

9

1-Bromo-4-nitrobenzene

phenylboronic acid

0.2

20

45 min

(CH3)3COK KOH

98 95 99 91 98

10

4-Bromoanisole

phenylboronic acid

0.2

20

1h

(CH3)3COK KOH K3PO4

11

4-Bromoaniline

phenylboronic acid

0.2

20

3.5 h

KOH K3PO4

20 84

12

4-Bromophenol

phenylboronic acid

0.2

20

1.5 h

KOH K3PO4

85 92

13

2-Bromonaphthalene

phenylboronic acid

0.2

20

1.5 h

(CH3)3COK KOH

97 98

14

4-Bromoanisole

1-naphthylboronic acid

0.4

40

12 h

(CH3)3COK KOH

92 89

15

3-Bromopyridine

phenylboronic acid pinacol ester

0.4

40

4h

KOH K3PO4

29 66



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Page 24 of 29

Scheme 1

C

B

N Graphene

Graphene Oxide

Boron Nitride Nanosheet

ACS Paragon Plus Environment

Page 25 of 29


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 1



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 29

Figure 2 (b)(a)

(a)

5 nm

5 nm

(d)

(c)

(e)

20 nm

Fe

20 nm

(f)

20 nm

20 nm

Pd

(g)

(h)

(i)

20 nm

20 nm

20 nm

(j)

(g)

Fe

10 nm

Pd


Page 27 of 29

Figure 3

Intensity (a.u.)

(a)

N 1S

O 1S Fe 2p B 1S C 1S Pd 3d

600

400

200

0

Binding Energy (eV) (b)

Fe 2p3/2

Intensity (a.u.)

Fe 2p1/2

740

730

720

710

700

Binding Energy (eV) (c)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Pd0 3d5/2

Pd0

Pd2+ 3d5/2 3d3/2

Pd2+ 3d3/2

345

342

339

336

333

Binding Energy (eV)


330


Figure 4

Biphenyl 1-(4-Methoxyphenyl) naphthalene

100 80

Yield (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 29

60 40 20 0 1

2

3

4

Number of Cycles


5

6

Page 29 of 29

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Graphical Table of Content


Boron Nitride Nanosheet-Anchored PdâFe CoreâShell Nanoparticles

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