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Pd-Fe3O4 nanocomposites as efficient catalysts for hydroboration of styrene
Catalysis Communications 100 (2017) 52–56
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Short communication
Hierarchical hybrid MnO/Pd-Fe3O4 and CoO/Pd-Fe3O4 nanocomposites as efficient catalysts for hydroboration of styrene
MARK
Hyunje Wooa, Junha Parka, Jiwoong Kimb, Sungkyun Parkb, Kang Hyun Parka,⁎ a b
Department of Chemistry and Chemistry Institute for Functional Materials, Pusan National University, Busan 609-735, Republic of Korea Department of Physics, Pusan National University, Busan 609-735, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Keywords: Hierarchical Hybrid Nanocomposites Catalyst Hydroboration
We report a one-pot synthesis of hierarchical MnO/Pd-Fe3O4 and CoO/Pd-Fe3O4 nanocomposites containing scrolled multilayered nanosheets by controlled thermal decomposition of Fe(CO)5 and reduction of metal precursors [Pd(OAc)2]. During the synthetic process, particles of amorphous MnO and CoO are immobilized on the Pd-Fe3O4 support. These hierarchical nanocomposites follow a stepwise growth mechanism, initially forming PdFe3O4 seeds aggregated from nuclei, followed by growth to produce multilayered MnO/Pd-Fe3O4 or CoO/PdFe3O4 structures. To the best of our knowledge, this is the first report on the hydroboration of styrene with bis (pinacolato)diboron using heterogeneous nanocatalysts. Among these catalysts, CoO/Pd-Fe3O4 nanocomposites showed excellent performance due to the presence of a dual Pd and CoO catalytic system.
1. Introduction Hybrid Pd-Fe3O4 nanostructures have recently received much attention due to the high catalytic activity (Pd) and magnetic recyclability (Fe3O4) of their components. Wang and coworkers [1] developed a carbon-coated Fe3O4 magnetic support for the immobilization of palladium nanoparticles (NPs). Hyeon et al. [2] synthesized Pd-Fe3O4 heterodimer nanocrystals towards cross-coupling reactions. Our group also previously reported flower-like Pd-Fe3O4 nanocomposites for tandem reactions [3,4]. The fabrication of hierarchical micro-/nanoarchitectures with controlled morphology, orientation and dimensionality, is a significant challenge for nanoscience [5–7]. Currently, three-dimensional (3D) hierarchical architectures, which are ordered assemblies using nanoparticles [8,9], nanorods [10,11] and nanoplates [12,13] as building blocks, have received much attention because of their potential applications in adsorption, catalysis, sensors, etc. [14–16]. Although such architectures, including inorganic [17] and organic nanostructures [18] have already been extensively studied, it is crucial to develop simple and reliable synthetic methods utilizing designed chemical components. So far, important progress has been made in the synthesis of flower-like nanostructures, including those based on Ni, CuO, α-Fe2O3 and ZnO. The growth mechanism and peculiar structure of the above compounds have been successfully investigated [19–22]. These nanostructures can be used in sensors, lithium-ion batteries, and photocatalysis because of their large specific surface areas. However,
⁎
Corresponding author. E-mail address: [email protected] (K.H. Park).
http://dx.doi.org/10.1016/j.catcom.2017.06.033 Received 21 March 2017; Received in revised form 18 June 2017; Accepted 20 June 2017 Available online 21 June 2017 1566-7367/ © 2017 Elsevier B.V. All rights reserved.
determining the growth mechanism of Pd-Fe3O4 flower-like nanocomposites still remains a great challenge. Our ongoing interest in hybrid metal-metal oxides combined with various transition metal NPs motivated us to investigate MnO- and CoO-supported Pd-Fe3O4 nanocomposites. Alkylboronates are versatile intermediates for a wide range of applications in organic synthesis [23–25]. In recent years, transition metal-catalyzed hydroboration reactions have been actively developed as a powerful synthetic method for the preparation of alkylboranes, due to the associated high-atom economy and broad functional group tolerance [26,27]. To the best of our knowledge, the hydroboration of styrene with bis(pinacolato)diboron (B2Pin2) catalyzed by heterogeneous nanocomposites has not yet been reported. In this work, we developed a facile one-pot synthesis of hybrid MnO/Pd-Fe3O4 and CoO/Pd-Fe3O4 nanocomposites. The associated stepwise mechanism involves the initial formation of Pd-Fe3O4 seeds aggregated from nuclei and their subsequent growth to form the multilayered MnO/Pd-Fe3O4 or CoO/Pd-Fe3O4 structure. It is the first report of styrene hydroboration with B2Pin2 using heterogeneous nanocatalysts. 2. Experimental 2.1. Synthesis of MnO/Pd-Fe3O4 and CoO/Pd-Fe3O4 nanocomposites [3,4] A three-necked flask containing a mixture of sodium oleate (0.3 g)
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and 1-octadecene (ODE) (10 mL) was heated to 200 °C under Ar flow and magnetic stirring until the sodium oleate dissolved completely. The solution was cooled to room temperature and a mixture of Pd(OAc)2 (0.112 g) and oleylamine (OAm) (10 mL) was added. The flask was heated to 60 °C and further to 120 °C at a rate of 6 °C/min. The reaction mixture was kept at 120 °C for 30 min, followed by addition of Fe(CO)5 (0.15 mL) under a blanket of Ar gas. The solution was further heated to 160 °C at a rate of 4 °C/min and kept at this temperature for 30 min. A solution of Mn(acac)2 or Co(acac)2 in OAm (31 mg/5 mL) was injected and the reaction mixture kept at 160 °C for 30 min. Subsequently, the heating source was removed, and the solution was cooled to room temperature. The black product obtained was precipitated by addition of ethanol and hexane and separated by centrifugation.
Table 1 ICP molar ratio and BET surface area of hybrid nanocomposites. Nanocomposite
Pd
Fe
Mn
Co
Pd-Fe3O4 seed Pd-Fe3O4 MnO/Pd-Fe3O4 CoO/Pd-Fe3O4
42 38 37 24
58 62 55 72
– – 8 –
– – – 4
(300 MHz) spectrometer. Chemical shift values were recorded as parts per million (ppm) relative to the tetramethylsilane internal standard unless otherwise indicated, and coupling constants are given in Hz.
2.2. Synthesis of Pd-Fe3O4 nanocomposites
3. Results and discussion
A three-necked flask containing a mixture of sodium oleate (0.3 g) and ODE (10 mL) was heated to 200 °C under Ar flow and magnetic stirring until the sodium oleate dissolved completely. The solution was cooled to room temperature and a mixture of Pd(OAc)2 (0.112 g) and OAm (10 mL) was added. The flask was heated to 60 °C and further to 120 °C at a rate of 6 °C/min. This mixture was kept at this temperature for 30 min, followed by addition of Fe(CO)5 (0.15 mL) under a blanket of Ar gas. The solution was further heated to 160 °C at a rate of 4 °C/ min and kept at this temperature for 30 min. The heating source was removed and the solution was cooled to room temperature. The black product was precipitated by adding ethanol and hexane, and separated by centrifugation. The Pd-Fe3O4 nanocomposites were used as a control. To obtain the Pd-Fe3O4 seeds, the reaction mixture was immediately transferred to an ice-bath to quench the synthesis when the temperature reached 160 °C.
3.1. Synthesis and characterization of the hybrid MnO/Pd-Fe3O4 and CoO/ Pd-Fe3O4 The MnO/Pd-Fe3O4 and CoO/Pd-Fe3O4 nanocomposites were synthesized by facile decomposition of Fe(CO)5 and reduction of Pd(OAc)2 and metal precursors [Mn(acac)2 or Co(acac)2] in OAm and ODE. The metal content of the synthesized nanocomposites was estimated by ICPAES (Table 1). The morphology and structure of MnO/Pd-Fe3O4 and CoO/PdFe3O4 were characterized by TEM (Fig. 1). Fig. 1a and b shows a clear view of the uniform hierarchical MnO/Pd-Fe3O4 and CoO/Pd-Fe3O4 nanocomposites, with an average particle diameter of 167 and 168 nm, respectively. Both nanocomposites consist of nanosheets growing out of the small seed particles in the center through (Fig. S1a–d). As a control experiment, Pd-Fe3O4 nanocomposites were also synthesized without the addition of Mn and Co precursors (Fig. S2), showing almost the same morphology as MnO/Pd-Fe3O4 and CoO/Pd-Fe3O4. The crystal structure of the samples was characterized by powder XRD as shown in Fig. 2a. It can be seen that all diffraction peaks are characteristic of the face-centered cubic (fcc) structure of Pd (JCPDS No. 46-1043) and the cubic spinel structure of Fe3O4 (JCPDS No. 190629). Interestingly, the intensity of the Fe3O4 peak was drastically decreased by addition of Mn(acac)2 or Co(acac)2, suggesting that Mn and Co ions disrupt the crystallization of Fe3O4 [28]. Additionally, these XRD patterns do not show the presence of any crystalline MnO and CoO, indicating that the above oxides form an amorphous phase. This is explained by the high solution temperatures (> 200 °C) and hydrothermal conditions generally required for crystalline MnO and CoO NPs syntheses [29]. The high-resolution TEM (HR-TEM) images shown in Fig. 2b demonstrate the formation of highly crystalline Pd particles with MnO/Pd-Fe3O4 and CoO/Pd-Fe3O4 lattice fringes. The line spacing of 0.225 nm could be indexed to Pd (111) reflections of the fcc structure. The high-angle annular dark-field scanning TEM (HAADFSTEM) and the corresponding elemental mapping images show uniform metal mixing in the entire nanocomposite, confirming the hybrid MnO/ Pd-Fe3O4 and CoO/Pd-Fe3O4 structure (Fig. S3). XPS elemental analysis was carried out to probe the chemical states of MnO and CoO (Fig. S4). In Fig. S4a, the Mn 2p peak of MnO/PdFe3O4 can be deconvoluted into two components. The most intense peak at 641.4 eV is assigned to Mn 2p3/2, and the one at 653.3 eV corresponds to Mn 2p1/2. Both peaks indicate the presence of Mn (II) on the surface of MnO/Pd-Fe3O4 [30]. In the case of Co, the two major peaks shown in Fig. S4b are positioned at 780.9 and 796.9 eV and are assigned to Co 2p3/2 and Co 2p1/2 transitions of the CoO phase, respectively [31]. The Co 2p3/2 peak overlaps strongly with Fe LMM Auger peaks. Furthermore, two satellites located approximately 6 eV above the primary binding energy peaks were detected at 786.9 and 803.4 eV, and these were used as a fingerprint for the recognition of high-spin Co (II) species in CoO [32]. Fig. S4c and d show the Fe 2p regions of MnO/Pd-Fe3O4 and CoO/Pd-Fe3O4, indicating that both
2.3. General procedure for hydroboration of styrene The catalyst (Pd base: 1.0 mol%), styrene (0.104 g, 1.0 mmol), B2Pin2 (0.279 g, 1.1 mmol), cesium carbonate (0.652 g, 2.0 mmol), methanol (0.202 mL, 5 mmol) and THF (3.0 mL) were mixed in a Schlenk tube and vigorously stirred at 60 °C. Subsequently, the crude reaction mixture was purified by silica gel column chromatography (hexane:ethyl acetate = 20:1) to afford the desired product. 2.4. Recyclability and Pd leaching tests After the reaction, the catalyst particles were separated by external magnet. The recovered particles were reused as a catalyst for a subsequent reaction. For checking Pd leaching degree, poly(4-vinylpyridine) (PVPy) (300 equiv. with respect to the total Pd content) was employed prior to initiation of the reaction. 2.5. Catalyst characterization The morphology of each catalyst sample was characterized by transmission electron microscopy (TEM) (FEI, Tecnai F30 Super-Twin, National Nanofab Center, South Korea); samples were prepared by placing a few drops of the corresponding colloidal solution on carboncoated copper grids (200 mesh, F/C coated, Ted Pella Inc., Redding, CA, USA). FE-scanning electron microscopy (SEM) (HITACHI_S_4800, National Nanofab Center, South Korea) was also used for characterization. The metal content of each catalyst sample was determined by inductively coupled plasma atomic emission spectrometry (ICP-AES, iCAP 6300 instrument). Powder X-ray diffraction (XRD) patterns were recorded on a Rigaku D/MAX-RB (12 kW) diffractometer. X-ray photoelectron spectroscopy (XPS) (Axis Supra, Shimadzu) was employed to characterize the structural and chemical properties of the nanocomposites. The reaction products were analyzed by 1H nuclear magnetic resonance (NMR) spectroscopy using a Varian Mercury Plus 53
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Fig. 1. TEM images of MnO/Pd-Fe3O4 (a) and CoO/PdFe3O4 (b) hybrid nanocomposites.
nanocomposites grow layer by layer, similarly to what is described as the terrace-step-kink model [34]. Since rolling of the layered structure into nanotubes has been reported [35], the multilayered MnO/PdFe3O4 and CoO/Pd-Fe3O4 structure could also roll up and scroll into flower-like nanostructures during the growth process. Furthermore, uniform sheet-assembled MnO/Pd-Fe3O4 nanocomposites were obtained after the injection of the Mn(acac)2/OAm mixture, possibly due to Ostwald ripening. Although the formation mechanism of the PdFe3O4 nanocomposites needs to be further elucidated, it is thought to involve homogeneous nucleation and solution-phase growth. 3.2. Catalytic hydroboration of arylalkens with B2Pin2 The catalytic activity of MnO/Pd-Fe3O4 and CoO/Pd-Fe3O4 was investigated for the hydroboration of styrene with B2Pin2 (Table 2). Preliminary screening to determine the most suitable solvent system revealed that THF could efficiently catalyze this reaction (Table 2, entries 1–2). MeOH was used as a hydrogen donor in the previously reported mechanism, and the yield dramatically increased when used as an additive (entry 2) and not as a solvent (entry 1). The effects of bases (KOtBu, NaOMe, and Cs2CO3) in the standard reaction were also investigated, with the highest yield obtained for Cs2CO3 (67%). Moreover, the hybrid CoO/Pd-Fe3O4 composite exhibited better catalytic activity than MnO/Pd-Fe3O4, Pd-Fe3O4, and Pd/charcoal [36], owing to electron transfer across the metal-oxide interface [37,38] (Table 2, entries 4–7). The yield was almost unchanged by increased reaction times and catalyst quantity (Table 2, entries 8–9). The following conditions proved to be optimal: CoO/Pd-Fe3O4 (Pd base: 1 mol%), B2pin2 (1.1 eq.), Cs2CO3 (2.0 eq.), MeOH (5 eq.) in THF at 60 °C under Ar for 12 h. (Table 2, entry 4). The CoO/Pd-Fe3O4 catalyst was recycled three times, maintaining its initial activity (> 65%) during recycling. In metal leaching tests, the hot filtered solution after the catalytic reaction was analyzed by ICP-AES, but the content of Co, Pd, and Fe was too small to confirm Pd leaching extent, measured to be 0.10 ppm, 0.13 ppm and 0.68 ppm, respectively. Also, the hydroboration in the presence of PVPy, which behaves as a poison to trap homogeneous Pd species through chelation in the solution phase, exhibited no obvious change in catalytic activity, demonstrating a heterogeneous manner [39]. We can conclude that the leaching and recyclability tests confirmed the high catalytic activity and stability of the nanocomposites.
Fig. 2. (a) XRD patterns and HR-TEM images of (b) MnO/Pd-Fe3O4 and CoO/Pd-Fe3O4.
Fe2 + and Fe3 + are present in Fe3O4. An additional satellite peak (720 eV) appears between the Fe 2p1/2 (725.1 eV) and Fe 2p3/2 (711.9 eV) components, being characteristic of Fe3 + in γ-Fe2O3 and suggesting that the Fe3O4 NPs are partly oxidized [33]. For Pd, the most intense peaks at binding energies of around 335.7 eV (3d5/2) indicate that both MnO/Pd-Fe3O4 and CoO/Pd-Fe3O4 nanocomposites mostly contain metallic Pd (Fig. S4e and f). To illustrate the morphology evolution of the MnO/Pd-Fe3O4 nanocomposites, we used TEM to characterize the products obtained at various stages of a typical synthesis (Scheme 1). Initially, spherical Pd nanoparticles with a diameter of 4.7 nm were obtained at 120 °C. Interestingly, when the reaction temperature was increased to 160 °C after the injection of Fe(CO)5, Pd-Fe3O4 seeds were formed. It seems that the synthesis involves the formation of Fe NPs from Fe(CO)5 and their subsequent mild oxidation to Fe3O4 NPs, which produces PdFe3O4 seeds. Fig. S5 shows the elemental mapping of Pd-Fe3O4 seeds, indicating that Pd NPs are mainly present in the core region (red) and Fe3O4 is located in the outer shell region (green). Broad Pd and Fe3O4 peaks (Fig. 2a) in the corresponding XRD patterns also confirm the seed crystal structure. Once the Pd-Fe3O4 seed particles are formed, the
4. Conclusions A one-pot synthesis of CoO/Pd-Fe3O4 and MnO/Pd-Fe3O4 hybrid nanocomposites through controlled thermal decomposition of Fe(CO)5 and reduction of metal precursors [Pd(OAc)2] was successfully developed. The associated morphology evolution was shown to involve the initial formation of Pd-Fe3O4 seeds followed by subsequent growth to 54
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Scheme 1. Schematic illustration of the morphological evolution process of MnO/Pd-Fe3O4 nanocomposites.
Table 2 Synthesis of alkylboronates from styrene.
O
+
O
O
O B
B
B O
O
Entry
Catalyst (mol%)
Time (h)
Base
Solvent
Yield (%)a
1 2 3 4 5 6 7 8 9
CoO/Pd-Fe3O4 (1) CoO/Pd-Fe3O4 (1) CoO/Pd-Fe3O4 (1) CoO/Pd-Fe3O4 (1) MnO/Pd-Fe3O4 (1) Pd-Fe3O4 (1) Pd/charcoal (1) CoO/Pd-Fe3O4 (1) CoO/Pd-Fe3O4 (2)
12 12 12 12 12 12 12 24 12
KOtBu KOtBu NaOMe Cs2CO3 Cs2CO3 Cs2CO3 Cs2CO3 Cs2CO3 Cs2CO3
MeOH THF THF THF THF THF THF THF THF
Trace 33 36 67 29 35 Trace 63 63
Reaction condition: styrene (1.0 mmol), B2Pin2 (1.1 mmol), catalyst (Pd base: 1.0 mol%), base (2.0 mmol), MeOH (5 mmol), solvent (3.0 mL) 60 °C, 12 h. a Isolated yields.
References
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Acknowledgments This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF2015R1D1A1A02060684 and NRF-2017R1D1A1B03036303). One of us (S. P.) thanks for the support from Korea Atomic Energy Research Institute and NRF-2015R1D1A1A01058672.
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Short communication
Hierarchical hybrid MnO/Pd-Fe3O4 and CoO/Pd-Fe3O4 nanocomposites as efficient catalysts for hydroboration of styrene
MARK
Hyunje Wooa, Junha Parka, Jiwoong Kimb, Sungkyun Parkb, Kang Hyun Parka,⁎ a b
Department of Chemistry and Chemistry Institute for Functional Materials, Pusan National University, Busan 609-735, Republic of Korea Department of Physics, Pusan National University, Busan 609-735, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Keywords: Hierarchical Hybrid Nanocomposites Catalyst Hydroboration
We report a one-pot synthesis of hierarchical MnO/Pd-Fe3O4 and CoO/Pd-Fe3O4 nanocomposites containing scrolled multilayered nanosheets by controlled thermal decomposition of Fe(CO)5 and reduction of metal precursors [Pd(OAc)2]. During the synthetic process, particles of amorphous MnO and CoO are immobilized on the Pd-Fe3O4 support. These hierarchical nanocomposites follow a stepwise growth mechanism, initially forming PdFe3O4 seeds aggregated from nuclei, followed by growth to produce multilayered MnO/Pd-Fe3O4 or CoO/PdFe3O4 structures. To the best of our knowledge, this is the first report on the hydroboration of styrene with bis (pinacolato)diboron using heterogeneous nanocatalysts. Among these catalysts, CoO/Pd-Fe3O4 nanocomposites showed excellent performance due to the presence of a dual Pd and CoO catalytic system.
1. Introduction Hybrid Pd-Fe3O4 nanostructures have recently received much attention due to the high catalytic activity (Pd) and magnetic recyclability (Fe3O4) of their components. Wang and coworkers [1] developed a carbon-coated Fe3O4 magnetic support for the immobilization of palladium nanoparticles (NPs). Hyeon et al. [2] synthesized Pd-Fe3O4 heterodimer nanocrystals towards cross-coupling reactions. Our group also previously reported flower-like Pd-Fe3O4 nanocomposites for tandem reactions [3,4]. The fabrication of hierarchical micro-/nanoarchitectures with controlled morphology, orientation and dimensionality, is a significant challenge for nanoscience [5–7]. Currently, three-dimensional (3D) hierarchical architectures, which are ordered assemblies using nanoparticles [8,9], nanorods [10,11] and nanoplates [12,13] as building blocks, have received much attention because of their potential applications in adsorption, catalysis, sensors, etc. [14–16]. Although such architectures, including inorganic [17] and organic nanostructures [18] have already been extensively studied, it is crucial to develop simple and reliable synthetic methods utilizing designed chemical components. So far, important progress has been made in the synthesis of flower-like nanostructures, including those based on Ni, CuO, α-Fe2O3 and ZnO. The growth mechanism and peculiar structure of the above compounds have been successfully investigated [19–22]. These nanostructures can be used in sensors, lithium-ion batteries, and photocatalysis because of their large specific surface areas. However,
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Corresponding author. E-mail address: [email protected] (K.H. Park).
http://dx.doi.org/10.1016/j.catcom.2017.06.033 Received 21 March 2017; Received in revised form 18 June 2017; Accepted 20 June 2017 Available online 21 June 2017 1566-7367/ © 2017 Elsevier B.V. All rights reserved.
determining the growth mechanism of Pd-Fe3O4 flower-like nanocomposites still remains a great challenge. Our ongoing interest in hybrid metal-metal oxides combined with various transition metal NPs motivated us to investigate MnO- and CoO-supported Pd-Fe3O4 nanocomposites. Alkylboronates are versatile intermediates for a wide range of applications in organic synthesis [23–25]. In recent years, transition metal-catalyzed hydroboration reactions have been actively developed as a powerful synthetic method for the preparation of alkylboranes, due to the associated high-atom economy and broad functional group tolerance [26,27]. To the best of our knowledge, the hydroboration of styrene with bis(pinacolato)diboron (B2Pin2) catalyzed by heterogeneous nanocomposites has not yet been reported. In this work, we developed a facile one-pot synthesis of hybrid MnO/Pd-Fe3O4 and CoO/Pd-Fe3O4 nanocomposites. The associated stepwise mechanism involves the initial formation of Pd-Fe3O4 seeds aggregated from nuclei and their subsequent growth to form the multilayered MnO/Pd-Fe3O4 or CoO/Pd-Fe3O4 structure. It is the first report of styrene hydroboration with B2Pin2 using heterogeneous nanocatalysts. 2. Experimental 2.1. Synthesis of MnO/Pd-Fe3O4 and CoO/Pd-Fe3O4 nanocomposites [3,4] A three-necked flask containing a mixture of sodium oleate (0.3 g)
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and 1-octadecene (ODE) (10 mL) was heated to 200 °C under Ar flow and magnetic stirring until the sodium oleate dissolved completely. The solution was cooled to room temperature and a mixture of Pd(OAc)2 (0.112 g) and oleylamine (OAm) (10 mL) was added. The flask was heated to 60 °C and further to 120 °C at a rate of 6 °C/min. The reaction mixture was kept at 120 °C for 30 min, followed by addition of Fe(CO)5 (0.15 mL) under a blanket of Ar gas. The solution was further heated to 160 °C at a rate of 4 °C/min and kept at this temperature for 30 min. A solution of Mn(acac)2 or Co(acac)2 in OAm (31 mg/5 mL) was injected and the reaction mixture kept at 160 °C for 30 min. Subsequently, the heating source was removed, and the solution was cooled to room temperature. The black product obtained was precipitated by addition of ethanol and hexane and separated by centrifugation.
Table 1 ICP molar ratio and BET surface area of hybrid nanocomposites. Nanocomposite
Pd
Fe
Mn
Co
Pd-Fe3O4 seed Pd-Fe3O4 MnO/Pd-Fe3O4 CoO/Pd-Fe3O4
42 38 37 24
58 62 55 72
– – 8 –
– – – 4
(300 MHz) spectrometer. Chemical shift values were recorded as parts per million (ppm) relative to the tetramethylsilane internal standard unless otherwise indicated, and coupling constants are given in Hz.
2.2. Synthesis of Pd-Fe3O4 nanocomposites
3. Results and discussion
A three-necked flask containing a mixture of sodium oleate (0.3 g) and ODE (10 mL) was heated to 200 °C under Ar flow and magnetic stirring until the sodium oleate dissolved completely. The solution was cooled to room temperature and a mixture of Pd(OAc)2 (0.112 g) and OAm (10 mL) was added. The flask was heated to 60 °C and further to 120 °C at a rate of 6 °C/min. This mixture was kept at this temperature for 30 min, followed by addition of Fe(CO)5 (0.15 mL) under a blanket of Ar gas. The solution was further heated to 160 °C at a rate of 4 °C/ min and kept at this temperature for 30 min. The heating source was removed and the solution was cooled to room temperature. The black product was precipitated by adding ethanol and hexane, and separated by centrifugation. The Pd-Fe3O4 nanocomposites were used as a control. To obtain the Pd-Fe3O4 seeds, the reaction mixture was immediately transferred to an ice-bath to quench the synthesis when the temperature reached 160 °C.
3.1. Synthesis and characterization of the hybrid MnO/Pd-Fe3O4 and CoO/ Pd-Fe3O4 The MnO/Pd-Fe3O4 and CoO/Pd-Fe3O4 nanocomposites were synthesized by facile decomposition of Fe(CO)5 and reduction of Pd(OAc)2 and metal precursors [Mn(acac)2 or Co(acac)2] in OAm and ODE. The metal content of the synthesized nanocomposites was estimated by ICPAES (Table 1). The morphology and structure of MnO/Pd-Fe3O4 and CoO/PdFe3O4 were characterized by TEM (Fig. 1). Fig. 1a and b shows a clear view of the uniform hierarchical MnO/Pd-Fe3O4 and CoO/Pd-Fe3O4 nanocomposites, with an average particle diameter of 167 and 168 nm, respectively. Both nanocomposites consist of nanosheets growing out of the small seed particles in the center through (Fig. S1a–d). As a control experiment, Pd-Fe3O4 nanocomposites were also synthesized without the addition of Mn and Co precursors (Fig. S2), showing almost the same morphology as MnO/Pd-Fe3O4 and CoO/Pd-Fe3O4. The crystal structure of the samples was characterized by powder XRD as shown in Fig. 2a. It can be seen that all diffraction peaks are characteristic of the face-centered cubic (fcc) structure of Pd (JCPDS No. 46-1043) and the cubic spinel structure of Fe3O4 (JCPDS No. 190629). Interestingly, the intensity of the Fe3O4 peak was drastically decreased by addition of Mn(acac)2 or Co(acac)2, suggesting that Mn and Co ions disrupt the crystallization of Fe3O4 [28]. Additionally, these XRD patterns do not show the presence of any crystalline MnO and CoO, indicating that the above oxides form an amorphous phase. This is explained by the high solution temperatures (> 200 °C) and hydrothermal conditions generally required for crystalline MnO and CoO NPs syntheses [29]. The high-resolution TEM (HR-TEM) images shown in Fig. 2b demonstrate the formation of highly crystalline Pd particles with MnO/Pd-Fe3O4 and CoO/Pd-Fe3O4 lattice fringes. The line spacing of 0.225 nm could be indexed to Pd (111) reflections of the fcc structure. The high-angle annular dark-field scanning TEM (HAADFSTEM) and the corresponding elemental mapping images show uniform metal mixing in the entire nanocomposite, confirming the hybrid MnO/ Pd-Fe3O4 and CoO/Pd-Fe3O4 structure (Fig. S3). XPS elemental analysis was carried out to probe the chemical states of MnO and CoO (Fig. S4). In Fig. S4a, the Mn 2p peak of MnO/PdFe3O4 can be deconvoluted into two components. The most intense peak at 641.4 eV is assigned to Mn 2p3/2, and the one at 653.3 eV corresponds to Mn 2p1/2. Both peaks indicate the presence of Mn (II) on the surface of MnO/Pd-Fe3O4 [30]. In the case of Co, the two major peaks shown in Fig. S4b are positioned at 780.9 and 796.9 eV and are assigned to Co 2p3/2 and Co 2p1/2 transitions of the CoO phase, respectively [31]. The Co 2p3/2 peak overlaps strongly with Fe LMM Auger peaks. Furthermore, two satellites located approximately 6 eV above the primary binding energy peaks were detected at 786.9 and 803.4 eV, and these were used as a fingerprint for the recognition of high-spin Co (II) species in CoO [32]. Fig. S4c and d show the Fe 2p regions of MnO/Pd-Fe3O4 and CoO/Pd-Fe3O4, indicating that both
2.3. General procedure for hydroboration of styrene The catalyst (Pd base: 1.0 mol%), styrene (0.104 g, 1.0 mmol), B2Pin2 (0.279 g, 1.1 mmol), cesium carbonate (0.652 g, 2.0 mmol), methanol (0.202 mL, 5 mmol) and THF (3.0 mL) were mixed in a Schlenk tube and vigorously stirred at 60 °C. Subsequently, the crude reaction mixture was purified by silica gel column chromatography (hexane:ethyl acetate = 20:1) to afford the desired product. 2.4. Recyclability and Pd leaching tests After the reaction, the catalyst particles were separated by external magnet. The recovered particles were reused as a catalyst for a subsequent reaction. For checking Pd leaching degree, poly(4-vinylpyridine) (PVPy) (300 equiv. with respect to the total Pd content) was employed prior to initiation of the reaction. 2.5. Catalyst characterization The morphology of each catalyst sample was characterized by transmission electron microscopy (TEM) (FEI, Tecnai F30 Super-Twin, National Nanofab Center, South Korea); samples were prepared by placing a few drops of the corresponding colloidal solution on carboncoated copper grids (200 mesh, F/C coated, Ted Pella Inc., Redding, CA, USA). FE-scanning electron microscopy (SEM) (HITACHI_S_4800, National Nanofab Center, South Korea) was also used for characterization. The metal content of each catalyst sample was determined by inductively coupled plasma atomic emission spectrometry (ICP-AES, iCAP 6300 instrument). Powder X-ray diffraction (XRD) patterns were recorded on a Rigaku D/MAX-RB (12 kW) diffractometer. X-ray photoelectron spectroscopy (XPS) (Axis Supra, Shimadzu) was employed to characterize the structural and chemical properties of the nanocomposites. The reaction products were analyzed by 1H nuclear magnetic resonance (NMR) spectroscopy using a Varian Mercury Plus 53
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Fig. 1. TEM images of MnO/Pd-Fe3O4 (a) and CoO/PdFe3O4 (b) hybrid nanocomposites.
nanocomposites grow layer by layer, similarly to what is described as the terrace-step-kink model [34]. Since rolling of the layered structure into nanotubes has been reported [35], the multilayered MnO/PdFe3O4 and CoO/Pd-Fe3O4 structure could also roll up and scroll into flower-like nanostructures during the growth process. Furthermore, uniform sheet-assembled MnO/Pd-Fe3O4 nanocomposites were obtained after the injection of the Mn(acac)2/OAm mixture, possibly due to Ostwald ripening. Although the formation mechanism of the PdFe3O4 nanocomposites needs to be further elucidated, it is thought to involve homogeneous nucleation and solution-phase growth. 3.2. Catalytic hydroboration of arylalkens with B2Pin2 The catalytic activity of MnO/Pd-Fe3O4 and CoO/Pd-Fe3O4 was investigated for the hydroboration of styrene with B2Pin2 (Table 2). Preliminary screening to determine the most suitable solvent system revealed that THF could efficiently catalyze this reaction (Table 2, entries 1–2). MeOH was used as a hydrogen donor in the previously reported mechanism, and the yield dramatically increased when used as an additive (entry 2) and not as a solvent (entry 1). The effects of bases (KOtBu, NaOMe, and Cs2CO3) in the standard reaction were also investigated, with the highest yield obtained for Cs2CO3 (67%). Moreover, the hybrid CoO/Pd-Fe3O4 composite exhibited better catalytic activity than MnO/Pd-Fe3O4, Pd-Fe3O4, and Pd/charcoal [36], owing to electron transfer across the metal-oxide interface [37,38] (Table 2, entries 4–7). The yield was almost unchanged by increased reaction times and catalyst quantity (Table 2, entries 8–9). The following conditions proved to be optimal: CoO/Pd-Fe3O4 (Pd base: 1 mol%), B2pin2 (1.1 eq.), Cs2CO3 (2.0 eq.), MeOH (5 eq.) in THF at 60 °C under Ar for 12 h. (Table 2, entry 4). The CoO/Pd-Fe3O4 catalyst was recycled three times, maintaining its initial activity (> 65%) during recycling. In metal leaching tests, the hot filtered solution after the catalytic reaction was analyzed by ICP-AES, but the content of Co, Pd, and Fe was too small to confirm Pd leaching extent, measured to be 0.10 ppm, 0.13 ppm and 0.68 ppm, respectively. Also, the hydroboration in the presence of PVPy, which behaves as a poison to trap homogeneous Pd species through chelation in the solution phase, exhibited no obvious change in catalytic activity, demonstrating a heterogeneous manner [39]. We can conclude that the leaching and recyclability tests confirmed the high catalytic activity and stability of the nanocomposites.
Fig. 2. (a) XRD patterns and HR-TEM images of (b) MnO/Pd-Fe3O4 and CoO/Pd-Fe3O4.
Fe2 + and Fe3 + are present in Fe3O4. An additional satellite peak (720 eV) appears between the Fe 2p1/2 (725.1 eV) and Fe 2p3/2 (711.9 eV) components, being characteristic of Fe3 + in γ-Fe2O3 and suggesting that the Fe3O4 NPs are partly oxidized [33]. For Pd, the most intense peaks at binding energies of around 335.7 eV (3d5/2) indicate that both MnO/Pd-Fe3O4 and CoO/Pd-Fe3O4 nanocomposites mostly contain metallic Pd (Fig. S4e and f). To illustrate the morphology evolution of the MnO/Pd-Fe3O4 nanocomposites, we used TEM to characterize the products obtained at various stages of a typical synthesis (Scheme 1). Initially, spherical Pd nanoparticles with a diameter of 4.7 nm were obtained at 120 °C. Interestingly, when the reaction temperature was increased to 160 °C after the injection of Fe(CO)5, Pd-Fe3O4 seeds were formed. It seems that the synthesis involves the formation of Fe NPs from Fe(CO)5 and their subsequent mild oxidation to Fe3O4 NPs, which produces PdFe3O4 seeds. Fig. S5 shows the elemental mapping of Pd-Fe3O4 seeds, indicating that Pd NPs are mainly present in the core region (red) and Fe3O4 is located in the outer shell region (green). Broad Pd and Fe3O4 peaks (Fig. 2a) in the corresponding XRD patterns also confirm the seed crystal structure. Once the Pd-Fe3O4 seed particles are formed, the
4. Conclusions A one-pot synthesis of CoO/Pd-Fe3O4 and MnO/Pd-Fe3O4 hybrid nanocomposites through controlled thermal decomposition of Fe(CO)5 and reduction of metal precursors [Pd(OAc)2] was successfully developed. The associated morphology evolution was shown to involve the initial formation of Pd-Fe3O4 seeds followed by subsequent growth to 54
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Scheme 1. Schematic illustration of the morphological evolution process of MnO/Pd-Fe3O4 nanocomposites.
Table 2 Synthesis of alkylboronates from styrene.
O
+
O
O
O B
B
B O
O
Entry
Catalyst (mol%)
Time (h)
Base
Solvent
Yield (%)a
1 2 3 4 5 6 7 8 9
CoO/Pd-Fe3O4 (1) CoO/Pd-Fe3O4 (1) CoO/Pd-Fe3O4 (1) CoO/Pd-Fe3O4 (1) MnO/Pd-Fe3O4 (1) Pd-Fe3O4 (1) Pd/charcoal (1) CoO/Pd-Fe3O4 (1) CoO/Pd-Fe3O4 (2)
12 12 12 12 12 12 12 24 12
KOtBu KOtBu NaOMe Cs2CO3 Cs2CO3 Cs2CO3 Cs2CO3 Cs2CO3 Cs2CO3
MeOH THF THF THF THF THF THF THF THF
Trace 33 36 67 29 35 Trace 63 63
Reaction condition: styrene (1.0 mmol), B2Pin2 (1.1 mmol), catalyst (Pd base: 1.0 mol%), base (2.0 mmol), MeOH (5 mmol), solvent (3.0 mL) 60 °C, 12 h. a Isolated yields.
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Acknowledgments This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF2015R1D1A1A02060684 and NRF-2017R1D1A1B03036303). One of us (S. P.) thanks for the support from Korea Atomic Energy Research Institute and NRF-2015R1D1A1A01058672.
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