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Review

Recent Novel Hybrid Pd–Fe3O4 Nanoparticles as Catalysts for Various C–C Coupling Reactions

1
Department of Chemistry, Pusan National University, Busandaehak-ro 63beon-gil, Geumjeong-gu, Busan 609-735, Korea
2
Department of Physics, Pusan National University, Busandaehak-ro 63beon-gil, Geumjeong-gu, Busan 609-735, Korea
3
Clean Fuel Laboratory, Korea Institute of Energy Research, Dajeon 305-343, Korea
*
Author to whom correspondence should be addressed.
These authors contributed equally to this work.
Submission received: 31 May 2019 / Revised: 17 June 2019 / Accepted: 25 June 2019 / Published: 3 July 2019
(This article belongs to the Special Issue Synthesis and Application of Novel Nanocatalysts)

Abstract

:
The use of nanostructure materials as heterogeneous catalysts in the synthesis of organic compounds have been receiving more attention in the rapid developing area of nanotechnology. In this review, we mainly focused on our own work on the synthesis of hybrid palladium–iron oxide nanoparticles. We discuss the synthesis of Pd–Fe3O4—both morphology-controlled synthesis of Pd–Fe3O4 and transition metal-loaded Pd–Fe3O4—as well as its application in various C–C coupling reactions. In the case of rose-like Pd–Fe3O4 hybrid nanoparticles, thermal decomposition can be used instead of oxidants or reductants, and morphology can be easily controlled. We have developed a method for the synthesis of nanoparticles that is facile and eco-friendly. The catalyst was recyclable for up to five continual cycles without significant loss of catalytic activity and may provide a great platform as a catalyst for other organic reactions in the near future.

1. Introduction

In recent years, fusion multimetallic nanoparticles (NPs) have generally been synthesized for use as catalysts, due to properties such as high selectivity for target material, catalytic activity, and physical/chemical stability when compared with equivalent catalysts based on a single metal [1,2,3,4,5,6,7,8,9,10]. In addition, their preparation has been optimized towards the design and synthesis via capping agent for controlled shape, size, and crystal structure [11,12,13,14,15]. In addition, various solvents—such as organic and inorganic solvents dispersed in hydrophilic or hydrophobic capping agents—are important for biological applications involving the effective dispersion of NPs [16,17].
In particular, homogeneous palladium catalysts have exhibited good performance with respect to reaction activities and turnover numbers (TONs). On the other hand, homogeneous catalysts have some decisive foibles, such as problems of recyclable and recovery, which lead to significant losses of costly metal [7]. Many studies have reported increased functionality using the incorporation of two or more clear nanomaterials [18,19,20,21,22]. Among various hybrid multimetallic NPs, palladium–iron oxide (Pd–Fe3O4) has attracted much attention owing to the high catalytic performance (Pd) and magnetically recoverable (Fe3O4) properties of each of the components of the nanocatalyst.
In a recent report, Hyeon et al. 2011 studied the facile synthesis of Pd–Fe3O4 NPs which were used to enable a catalytic effect for cross-coupling reactions. In addition, Wang and coworkers have reported Pd NPs embedded on carbon-coated Fe3O4 microspheres with magnetic properties. Chen et al. 2012 reported on magnetically divisible hybrid Pd/Fe3O4@charcoal catalysts which are made up of active metal of 10 nm-sized Pd NPs and loaded in a 120 nm-sized iron oxide/carbon matrix [23,24,25].
As is well known, the Suzuki–Miyaura coupling, Mizoroki–Heck, and Sonogashira reactions using Pd catalyst—called C–C coupling reactions—are important in chemical, pharmaceutical, and agricultural industries [26,27,28]. Numerous previous works have reported the use of heterodimer Pd–Fe3O4 NPs applied in C–C coupling reactions [23], Pd/Fe3O4@C [29], and hyperbranched polyglycerol functionalized Pd/Fe3O4@SiO2 catalyst [30].
In this review, we concentrate on latter exploitations in the synthesis of hybrid Pd–Fe3O4 nanocatalysts and various strategies for (1) urchin-like FePd–Fe3O4 for magnetic properties [31], (2) magnetically recoverable Pd–Fe3O4 hybrid nanocatalysts [32], (3) effectiveness of high metal-loaded NPs [7], (4) morphology impact of an organic capping agent of hybrid Fe3O4/Pd NPs [33], (5) rose-like Pd–Fe3O4 hybrid nanocomposites of morphology control via thermal decomposition, and [34,35] (6) various transition metal-loaded Pd–Fe3O4 heterobimetallic nanoparticles (Scheme 1) [36,37,38,39].

2. Urchin-like FePd–Fe3O4: Nanocomposite Magnets

High saturation magnetization (MS) and large magnetic coercivity (HC) of magnetic materials are necessary for high-density power and data storage applications [40,41,42]. However, most magnetic materials contain only one of these two properties. For example, Fe, Co, and FeCo exhibit high MS and low HC (i.e., soft magnetic materials). By contrast, NdFeB and CoPt show low MS and high HC (i.e., hard magnetic materials). Thus, the exchange coupling between hard and soft materials, with high Ms and Hc, has attracted much attention [43,44,45,46]. This concept was first proposed by F. Kneller and R. Hawing in 1991 [47], and there was a requirement that the size of the hard magnetic phase should be more than an almost half-domain wall width of the soft magnetic phase to maximize exchange coupling between the soft and hard magnet. However, experimentally, it is difficult to precisely fabricate the nanostructure strongly coupled with different magnetic properties into desirable nanosized materials [48,49]. FePt nanoparticles were firstly synthesized by Sun et al. [50] and have been demonstrated to be a suitable material for numerous magnetic nanocomposites due to huge magnetocrystalline anisotropy (Ku ~ 6.6 × 107 erg cm−3). After chemical reaction processing, FePt-based nanocomposites, such as FePt–Fe3O4 or FePt/Fe3O4 core–shell NPs, were converted to L10-FePt-based nanocomplexes that show high magnetic property [45,51]. However, sometimes, L10-FePt-Fe3Pt complexes were formed that showed soft magnetic behavior due to thermodynamic instability. Therefore, it is difficult to synthesize strongly exchange-coupled nanocomposite magnets. Interestingly, FePd exhibits huge magnetocrystalline anisotropy (Ku ~ 1.0 × 107 cm−3). It also shows eutectoid reaction at exact temperatures depending on the Fe/Pd ratio. Therefore, it is possible to synthesize thermodynamically stable α-Fe and L10-FePd through eutectoid reaction [52].

2.1. Synthesis of Urchin-Like Pd–Fe3O4 and L10-FePd–Fe Nanocomposite Magnets

Hayashi et al. reported the one-pot synthesis of urchin like FePd−Fe3O4 composites and their change into L10-FePd−Fe nanocomplex magnets [53]. Urchin-like nanocomposite with various Fe/Pd ratios (45:55, 49:51, 67:33, and 74:26) were synthesized in the following order. Figure 1 shows the synthetic process, illustration, and high-resolution TEM (HRTEM) image of urchin-like FePd–Fe3O4 composites (Scheme 2).
These urchin-like FePd–Fe3O4 nanocomplexes were the precursors in the synthesis of Ll0-FePd–Fe nanocomposite. Urchin-like FePd–Fe3O4 composites with various Fe/Pd ratios were annealed at various temperatures (350, 400, 450, 500, and 550 °C) under mixed gas conditions (Ar and H2). These composites differed in phase according to annealing temperature and Fe/Pd ratio. Figure 1a shows a HRTEM image of L10-FePd–Fe nanocomposite magnet (Fe/Pd = 67:33) heat-treated at 500 °C that has two different domains. One of them is the (111) planes of L10-FePd with 0.27 nm lattice spacing. The other is (110) planes of α-Fe with 0.20 nm lattice spacing. In addition, it is possible to distinguish L10-FePd from α-Fe using energy dispersive spectroscopy (EDS) with elemental mappings of Pd (red) and Fe (blue). Pd (red) collaborating with Fe (blue) indicates L10-FePd, and separated Fe (blue) indicates α-Fe (b–d).

2.2. Magnetic Properties of L10-FePd–Fe Nanocomposites Magnets

Figure 2 summarizes the annealing temperature and Fe/Pd ratio dependent magnetic properties measured by vibrating sample magnetometer at room temperature (RT). Figure 2a shows magnetic hysteresis curves of Fe67Pd33–Fe3O4 nanocomposites annealed at 350 and 450 °C, respectively. Both hysteresis rings show a single-phase-like performance (no double hysteresis loop), indicating L10-FePd and Fe have exchange interaction. Figure 2b indicates heat-treated temperature reliant on MS and HC of Fe67Pd33–Fe3O4 nanocomposites. Both MS and HC increase with increasing annealing temperature over 500 °C, owing to the increases in L10-FePd phase and grain size of α-Fe. On the other hand, HC abruptly decreases at 550 °C due to materialization of the fcc FePd phase. Therefore, 500 °C is an optimum annealing temperature. Figure 2c shows the magnetic hysteresis loop of Fe45Pd55 and Fe67Pd33–Fe3O4 nanocomposite annealed at 500 °C. From the single phase-like hysteresis exchange coupling of L10-FePd and α-Fe is inferred. Figure 2d indicates the magnetic properties of L10-FePd–Fe nanocomplex magnets on the basis of Fe concentration, and it can be possible to tune exchange coupling among L10-FePd and α-Fe by controlling the ratio of Fe phase.

3. Magnetically Recyclable Pd–Fe3O4 Hybrid Nanocatalyst: Application in Mizoroki–Heck Reaction

Pd is one of the most beneficial metal catalysts in organic synthesis for numerous C–C bond coupling reactions. On the other hand, Fe3O4 is one of the most used catalyst supports owing to its low price, easy separation, high magnetic properties, and easy reusability [54]. In organic synthesis, the Heck reaction is an important C–C coupling reaction which plays a significant role in medicinal, agrochemical, and fine chemical industries [55]. Li et al. [56] worked on one-step synthesis of Pd/Fe3O4 nanocomposites in 4-(2-hydroxyethyl)-1-piperazineethansulfonic acid (HEPES) buffer solution as an active catalyst for Suzuki coupling reaction. In another work, Li et al. [57] focused on the one-pot solvothermal synthesis of Pd/Fe3O4 nanocomposites as an environmental catalyst for Suzuki–Miyaura coupling reactions. Chung et al. [58] worked on Heck and Sonogashira coupling reactions using eco-friendly Pd–Fe3O4. Prasad et al. [59] worked on magnetically recyclable Pd–/Fe3O4-catalyzed Stille coupling reactions of organostannanes with aryl halides. Byun et al. [60] reported on systematic works of magnetically environmental Pd–Fe3O4 heterodimeric nanocrystal-catalyzed organic C–C coupling reactions. Elazab et al. [3] reported on highly efficient Pd–Fe3O4 on graphene support as a catalyst for Suzuki and Heck coupling reaction. Sreedhar et al. fabricated magnetically recyclable catalysts of Pd/Fe3O4 Hiyama coupling of aryl halides with aryl siloxanes [61]. The purpose of this work was to synthesis of Pd–Fe3O4 hybrid nanostructures and to evaluate their catalytic activity in Heck reaction.

3.1. Synthesis and Characterization of Pd–Fe3O4 Hybrid Nanocatalyst

The Pd–Fe3O4 was synthesized via a procedure modified from the literature (Scheme 3) [62,63].
X-Ray Diffraction (XRD) was an effective tool to identify the existence of Pd nanoparticles on Fe3O4 (Figure 3). The diffraction peaks at 18.2, 30.1, 35.4, 43.1, 53.4, 56.9, and 62.9° (2 θ) correspond to (111), (220), (311), (400), (422), (511), and (440) planes of Fe3O4, and illustrate the fcc nature of Fe3O4 (JCPDS no. 19-0629). Similarly, the existence of peaks at 40.1, 46.5, and 68.0° (2 θ) are attributed to the (111), (200), and (220) plane of fcc Pd (JCPDS no. 46-1043).
The TEM images (Figure 4a,b) show spherically shaped nanoparticles with numerous cracks on the surface of the spheres, which suggest the porous structure of the sphere. The high-angle annular dark-field scanning TEM (HAADF-STEM) image (Figure 4c) and elemental mapping (Figure 4d,e) show that O and Fe are spread all over the composite structures, from the inside to the outside, illustrating a hybrid Pd–Fe3O4 arrangement. Figure 4f shows that most of the Pd NPs are scattered on the exterior of the hybrid nanocomplexes.
The value of saturation magnetization for the Pd–Fe3O4 was 58.8 emu g−1. The supermagnetic characteristics of the Pd–Fe3O4 nanocomposites spheres are balanced with the fact that the spheres are compiled with major magnetic nanocrystals, which permits the nanoparticles to quickly gather together in the occurrence of an exterior magnetic field and effortlessly disperse in solution when the exterior magnetic field is eliminated, as shown in Figure 5b. The magnetization hysteresis curve of the synthesized Pd–Fe3O4 nanoparticles at 300 K is depicted in Figure 5c.

3.2. Pd–Fe3O4 Hybrid Nanocatalyst: Application in Mizoroki–Heck Reaction

Heck reaction of iodobenzene and styrene was selected as a model reaction to investigate the catalytic efficiency and stability of the hybrid nanostructures (Table 1). Firstly, the impact of catalyst loading was observed by using various amounts of catalysts in the range of 0.054 to 0.162 mol % (entries 1–3, Table 1). The best performance was obtained with 0.108 mol % (entry 2, Table 1). Then, the effect of temperature was also analyzed. Increasing the reaction temperature from 60 to 110 °C had a crucial effect on the advancement of the reaction (entries 2, 4–7, 9, and 10, Table 1). It can be noted that even at low temperature (80 °C), the reaction still gave 51.3% yield in 3 h (entry 6, Table 1). With increases in reaction time, the yield also increased (entry 8, Table 1).
Durability is an essential factor to analyze the practical applicability of a catalyst. The synthesized catalyst was utilized for the Heck coupling reaction of iodobenzene with styrene at 110 °C for 3 h. After finishing the reaction, it was possible to separate the catalyst using an exterior magnet. It was washed with EtOH and H2O and dried under the vacuum. The catalyst was then used for the next catalytic cycle. After five runs, the yield of the targeted product decreased from 99% to 85% (Figure 6).
The TEM image of reused Pd–Fe3O4 nanoparticles showed that they kept their sphere-shaped structure after being used for five catalytic cycles (Figure 7a). To recognize the actual catalytic active sites through the reaction, a hot-filtration experiment was necessary (Figure 7b). A Heck reaction was carried out by taking a similar substrate was that the catalytic activity was filtered off after reacting for 0.5 h (55% yield). Using the same reaction conditions, the resulting solution was heated, and the reaction was immobile after 1 h (75% yield). This indicated that the leached Pd species can also result in activation of the catalyst for Heck reaction. On the other hand, owing to the lack of Pd (supported Pd nanoparticles), the reaction could not continue. As a result, for Pd–Fe3O4 hybrid nanocatalysts, both Pd leaching into the reaction complex and the supported Pd nanoparticles are the vigorous catalytic species, and the Heck coupling reaction begins partially homogenously in the complex, even if the initial catalyst is heterogeneous.
Put simply, a highly active Pd–Fe3O4 has been synthesized using a facile method. The Pd–Fe3O4 showed good magnetic properties and exhibited good dispersion in solvent. At low temperature (80 °C), the catalyst exhibited excellent catalytic activity. This suggests a number of benefits, including easy synthesis, excellent reactivity, and perfect robustness. The magnetic characteristics of the catalyst permit it to be segregated and recoverable.

4. Bifunctional Catalyst of Pd/Fe3O4/C: High content of Nanoparticles

High metal loading and high dispersion greatly influences the catalytic activity of nanoparticles due to the use of material with a high pore volume [7,64]. Some materials, such as silica or carbon, have been used as supporting materials due to their high pore volume, so that many metals can be loaded in the pore [65,66]. High pore volume materials have been used to avoid agglomeration of nanoparticles [67]. Carbon materials have been developed as supporting materials for various metals with good thermal stability and mechanical stability [21]. Activated charcoal can be used as a supporting material because charcoal has a low price, large pore volume, and large surface area [7].
Due to some advantages, the use of charcoal can result in the high metal loading of nanoparticles, such as Pd and Fe3O4 [7]. Many researchers have used Pd and Fe3O4 nanoparticles as catalysts in various synthesis organic chemistry reactions, such as Suzuki–Miyaura coupling reaction [7,64]. Besides using supporting material, the stability of Pd nanoparticles can be increased by immobilizing Pd nanoparticles on magnetic nanoparticles [23,68,69]. Magnetic nanoparticles, such as Fe3O4, have unique physical properties and possess several advantages, such as easy preparation, easy separation, low toxicity, and low cost [70,71,72]. Fe3O4 nanoparticles also have good catalytic activity because of the presence of two metals in the supporting material that increase catalytic activity in organic reactions [25].

4.1. Melt Infiltration Method

4.1.1. Synthesis of Pd/Fe3O4/Charcoal Catalyst and Suzuki–Miyaura Coupling Reaction

High metal loading and uniformity of particle size in supported material are very important to increase the catalytic activity of the material. Magnetic Pd/Fe3O4/charcoal catalyst was successfully synthesized using various methods. One of them involved using mixed metal hydrate salt via the solid-state grinding method and without the addition of surfactant [7] (Scheme 4).
Pd/Fe3O4/charcoal catalyst was successfully synthesized using two steps. The first was co-solid milling and the second was thermal decomposition under N2 gas flow (Scheme 4). Pd(NO3)3·2H2O and Fe(NO3)3·9H2O were used as hydrate salts for the synthesis of Pd/Fe3O4 nanoparticles. In the co-solid grinding method, both hydrate salts were melt-infiltrated and entered into charcoal pores. Furthermore, thermal decomposition was carried out in 400 °C under N2 conditions to form Pd and Fe3O4 nanoparticles.
Pd and Fe3O4 were successfully located in the charcoal using TEM imaging (Figure 8a). Based on HRTEM analysis in Figure 8b, Pd has been loaded with an average diameter size of 5 nm with a lattice distance of 0.255 nm ((111) planes of Pd). In addition, Fe3O4 with a greater size than Pd was also observed with the diameter around 9 nm and lattice distance of 0.253 nm ((311) planes). XRD analysis result shows that the Pd on Pd/Fe3O4/charcoal has a fcc structure. In this study, Pd hydrate salt was used at 20 wt %. Based on the ICP–AES result, the amount of Pd in the composition was 19.2 wt %. This shows that this method produces high metal loading on porous charcoal.
In Table 2, Pd/Fe3O4/charcoal was applied as heterogeneous catalyst in the Suzuki–Miyaura coupling reaction. 4-Bromoanisole and phenylboronic acid were reacted with in the presence of potassium carbonate. DMF/water (4:1) is the best solvent for this reaction with a turnover frequency of 25. The best result was obtained by adding Pd (20 wt %)/Fe3O4(10 wt %)/charcoal in the reaction with %conversion of >99%. This result is better compared to commercial Pd/charcoal catalyst. The high conversion of reactant into product using Pd (20 wt %)/Fe3O4(10 wt %)/charcoal catalyst is due to the uniform and small size of particles.
Table 3 summarized of scope of substrate in Suzuki–Miyaura coupling reaction using Pd/Fe3O4/charcoal catalyst. This catalyst can catalyze the reactions with good to excellent yield. This showed that Pd/Fe3O4/charcoal catalyst has good activity in this reaction. High metal loading, particle size, and uniformity of particles were the factors affected this reaction and producing a high conversion.

4.1.2. Recycling and Pd Leaching Test

Pd(20 wt %)/Fe3O4(10 wt %)/charcoal can be separated using an external magnet upon completion of the reaction, due to the superparamagnetic character of Fe3O4 which makes it possible to reuse several times. After being used three times (Table 4), it produced high catalysis activity with a conversion of >99%. The HRTEM images of the recovered Pd/Fe3O4/charcoal has been shown in Figure 9.
After Suzuki–Miyaura coupling reaction, the filtrate solution was characterized using ICP–AES. The result showed that the Pd level was 0.48 ppm, which is negligible, and that there was almost no Pd leaching at the catalyst during the Suzuki–Miyaura coupling reaction.

4.2. Stöber Method

4.2.1. Synthesis of Fe3O4@C–Pd Catalyst and Suzuki–Miyaura Coupling Reaction

Traditionally, the synthesis of silica spheres uses the Stöber method. According to a recent report, some studies successfully applied an improved Stöber method in the synthesis of resin spheres composed of resorcinol–formaldehyde (RF) that were transformed to carbon spheres (Scheme 5) [73,74,75,76].
Pd was effectively embedded in the silica based on TEM imaging (Figure 10a). In the HRTEM analysis in Figure 10b, Pd has been loaded with an average diameter size of 10 nm with an interplanar spacing of 0.223 nm ((111) planes of Pd). In this study, the Pd content in the catalysts were measured by ICP–AES and the Pd loading amount reached 8.73 wt %.

4.2.2. Catalytic Efficiency of Fe3O4@C–Pd-550 Nanocomposite

Fe3O4@C–Pd-550 nanocomposite was used as a catalyst for producing structurally diverse aryl halides (Table 5). The reaction gave good yield in the case of aryl bromide compared with aryl chloride (entries 1 and 2). On the other hand, the aryl bromide containing electron-withdrawing group gave better yield than electron-donating group (entries 3 and 4).

5. Hybrid Fe3O4/Pd Catalysts: Impact of Organic Capping Agents

Fe3O4 nanoparticle is a heterogeneous catalyst that is easy to use and can be used repeatedly, which makes it an environmentally friendly catalyst [77]. The synthesis of Fe3O4 nanoparticles can be carried out in various ways, such as using co-precipitation [78]; electrochemical [79], sonochemical [80], and microemulsion techniques [81]; and hydrothermal processes [82]. Fe3O4 nanoparticles can be supported by adding other materials to the surface of Fe3O4 to provide support as a heterogeneous catalyst system, such as by adding Pd nanoparticles, and this is desired due to the ability of Pd as a catalyst with high reaction speed and high turnover rate (TON) [33].
Pd/Fe3O4 microsphere nanoparticles have attracted the attention of researchers because of their good catalytic activity [7]. This is because of the stability of the Pd/Fe3O4 microsphere when dispersed in organic and inorganic solvents. The dispersion stability can be controlled using hydrophilic or hydrophobic capping agents and, also, surface modification [16,17]. Pd/Fe3O4 microspheres were widely developed due to their advantage of being easily separated from the product using external magnets [7]. Several techniques have been advanced for the synthesis of Pd/Fe3O4 microspheres using a variety of capping agents to enhance the dispersion stability of Pd/Fe3O4 nanoparticles in solvents. Some capping agents have been used to improve dispersion stability, such as chitosan [83,84,85], metal [86], SiO2 [87], and carbon [88], among others.

5.1. Immobilization of Pd NPs onto Each Fe3O4 Microsphere

The high dispersion stability of Pd/Fe3O4 nanoparticles using various capping agents can increase catalytic activity in organic reactions. Functional groups, such as poly(vinylpyrrolidone) (PVP), sodium citrate (Na3Cit), and poly(ethylene glycol) (PEG) can be used as capping agents for the synthesis of Fe3O4 using a solvothermal method, and these have different levels of dispersion in water [33] (Scheme 6). Fe3O4 has microsphere shapes with different sizes. Fe3O4 microspheres with Na3Cit as capping agent have a smaller average size than Fe3O4 with PEG and Fe3O4 with PVP. Fe3O4 synthesized with capping agents have smaller average size compared to Fe3O4 synthesized without capping agent.
Pd nanoparticles were immobilized to the Fe3O4 microsphere. Based on TEM images, Fe3O4 microsphere particles have average size of 3 nm. Meanwhile, the average size of Pd nanoparticles immobilized onto Fe3O4 microsphere with various capping agents in an aqueous solution was 4 nm, and they showed good dispersion with the exception of PVP–Fe3O4 microspheres (Figure 11, Table 6).
Based on XRD in Figure 12, Fe3O4 microsphere exhibited fcc structure with (111), (200), and (220) reflection (JCPDS No. 46-1043). In addition, there was a cubic spinel structure with (220), (311), (400), (422), and (511) reflection (JCPDS No. 19-0629).

5.2. Suzuki Coupling Reaction Using Pd/Fe3O4 Nanoparticles with Various Capping Agents

Pd/Fe3O4 microsphere was used in Suzuki coupling reaction as a catalyst. Table 7 showed the effects of catalysts, temperatures, reaction times, solvents, and bases on the Suzuki coupling reaction using phenylboronic acid and bromobenzene. The optimum conditions were obtained when reacting phenylboronic acid and bromobenzene using Na3Cit–Fe3O4/Pd (0.05 mol %), H2O, 50 °C, and 7 h reaction time with %yield of 98% (entry 5). In addition, raising the temperature to 100 °C and reducing the reaction time to 1.5 h resulted in a high %yield (98%, entry 11). When comparing between Pd/Fe3O4 with Na3Cit used as a capping agent and Pd/Fe3O4 using other capping agents, Na3Cit–Fe3O4/Pd had better effectiveness in producing high yield
Based on the results of 5-times recyclability test, the yield did not change significantly. This showed that Na3Cit–Fe3O4/Pd has good effectiveness for the Suzuki coupling reaction (Figure 13).

6. Flower-Like Pd–Fe3O4 and Pd–Fe3O4 Hybrid Nanocatalyst-Embedded Au Nanoparticles

Several attempts have been taken to design new hybrid nanocomplexes with well-defined multicomponents by controlling the size and shapes of these materials through solution growth structure [89,90,91]. Nasrollahjadeh et al. [4] reported on the eco-friendly synthesis of Pd/Fe3O4 NPs using Euphorbia condylocarapa M. bieb root extract and applied as a magnetically recyclable catalyst for Suzuki and Sonogashira coupling reaction. Hoseini et al. [92] worked on the magnetic Pd/Fe3O4/r–GO nanocomposite as an effective and environmental catalyst for the Suzuki–Miyaura coupling reaction in water. Jang et al. [23] reported on facile synthesize Pd–Fe3O4 heterodimer as a magnetically recoverable catalyst for C–C coupling reaction. The use of hybrid Pd–Fe3O4 catalyst for C–C coupling reaction has also been vigorously researched through hyperbranched polyglycerol-inserted Pd–Fe3O4@SiO2 [30]. Yeo et al. [93] developed a Pd–Fe3O4 core–satellite heterostructure as an effective candidate for the decarboxylative coupling reaction in aqueous solution. We focused on the synthesis of Pd–Fe3O4 by controlling shape and then the immobilization of Au NPs onto this Pd–Fe3O4 support (Scheme 7). The synthesized Pd–Fe3O4 catalyst showed good catalytic performance for Sonogashira coupling reactions. On the other hand, the Au/Pd–Fe3O4 hybrid nanocomposites exhibited excellent catalytic performance for the tandem synthesis of 2-phenylindoles with great magnetic recoverability.

6.1. Synthesis of Pd–Fe3O4 and Au/Pd–Fe3O4 Nanocomposites

Figure 14a,b depicts the scanning electron microscopy (SEM) images of the flower-like Pd–Fe3O4 and Au/Pd–Fe3O4 hybrid nanocomplexes. Au NPs were regularly distributed onto the Pd–Fe3O4 supports. Figure 13d and Figure 14c exhibits the TEM images of Pd–Fe3O4 and Au/Pd–Fe3O4. The TEM image shows Pd–Fe3O4 nanocomplexes with a general Fe/Pd ratio of 64:36 (Figure 14c). The immobilization of Au NPs can be visualized from Figure 14d. Figure 14e displays the XRD patterns of the nanocomposites with total element ratios of 7:77:16 (Au/Fe/Pd) and 64:36 (Fe/Pd). The XRD pattern of Pd–Fe3O4 matched well with the Pd crystal structure and lattice planes of the cubic spinel structure of Fe3O4 (JCPDS no. 19-0629). On the other hand, the immobilized Au NPs are match well with fcc Au crystallizations (JCPDS no. 04-0784).

6.2. Flower-Like Pd–Fe3O4: Application in Sonogashira Coupling Reactions

To assess the catalytic performance of the as-prepared Pd–Fe3O4 catalyst, the Sonogashira reactions of iodobenzene and phenylacetylenes were chosen as a model reaction under numerous conditions (Table 8). While optimizing the consequence of solvent, it was observed that the use of more polar solvent gave more yields, which is due to the great solubility of reactant and nanocatalyst in the reaction mixture (entries 1–3, Table 8). The influences of different bases were also studied (entries 3–6, Table 8). The catalyst also showed good activity although when the temperature was reduced to 90 °C (entry 7, Table 8).
Next, we tried to find the optimized conditions to check Turn over frequency (TOF). We then analyzed the impact of the catalyst amount and the reaction time. A decrease in the product conversion was observed when utilizing less catalyst and shorter reaction times (entries 9 and 10, Table 8). The Pd–Fe3O4 (TOF: 66.7) exhibited good catalytic efficiency than when Pd–Fe3O4 (TOF: 18) was synthesized by microbes and heterodimeric Pd/Fe3O4 (TOF: 4.2) [4,58].
The scope of substrate in Sonogashira coupling reaction catalyzed by Pd–Fe3O4 has been displayed in Table 9. Under our catalytic conditions, both electron donating and withdrawing substituents were smoothly coupled with arylacetylene with good conversion rates (entries 2–7, Table 9). Electron withdrawing substituents can normally produce advantageous consequences in Pd-catalyzed reactions by facilitating the rate-limiting oxidation step [94]. When the m-CH3 and –CF3 groups were utilized, product conversion was slightly improved but, comparatively, the conversion was good in the case of the m-CF3 group (entries 8 and 9, Table 9).

6.3. Pd–Fe3O4 Supported Au Nanocatalyst: Applications for Tandem Synthesis of 2-Phenylindoles

To appraise the catalytic efficiency of the Au/Pd–Fe3O4 nanocatalyst, the tandem reaction of 2-phenylindoles with phenylacetylenes were exhibited as a model reaction under dissimilar environments (Table 10).
Compared to piperidine and LiOAc, CsOAc gave good conversion (48%). This can be explained from the hard–soft acid and base (HSCB) theory that e Cs+ is the best Pearson acid to eliminate iodide from the intermediate Pd NPs by maximizing the soft–soft interface [95]. The dual catalytic system exhibits higher catalytic performance than that of single catalytic system because of the electron transfer across the interface. Hence, the Au/Pd–Fe3O4 catalyst exhibited good catalytic performance compared to Pd–Fe3O4 since Au NPs are very efficient in activating phenylacetylene (entries 5 and 6, Table 10) [96]. At high temperature (150 °C), the expected conversion (97%) was obtained (entry 7, Table 10). The effect of catalyst, as well as the reaction time, was also analyzed. A decrease of product conversion (59% and 38%) was observed while using 6 h and a lower amount of catalyst (entries 8–10, Table 10). The optimal reaction conditions were as follows: Au/Pd–Fe3O4 (Au base: 0.18 mol %; Pd base: 0.5 mol %); DMSO (2.5 mL), 150 °C, and 9 h (entry 8, Table 10). Table 11 exhibits the comparison of different Pd based catalyst in organic reaction.

7. Transition Metal Loading Pd–Fe3O4 Heterobimetallic Nanoparticles

Hybrid Pd/Fe3O4 nanoparticles (NPs) are the key factor in many catalytic reactions for organic transformation, due to the superior catalytic performance and magnetic recoverability [97]. Control of the composition, morphology, and architecture has attracted increasing attention in tailoring the resulting properties [98,99,100]. One of the developed methods is doped transition metal, and metal oxide on Pd–Fe3O4 for the synthesis of heterobimetallic NPs. Transition metal and metal oxide nanoparticles often provide an alternative to noble metals, with easy availability and low cost [101,102]. In addition, the composition of bimetallic and trimetallic NPs hybrid NPs compared to monometallic NPs is very promising and synchronously benefits from increasing selectivity, efficiency, and stability, owing to synergistic substrate activation [39,86,101,103,104,105]. NPs from transition metal loaded on hybrid Pd/Fe3O4 have been developed by our group and include Cu/Pd–Fe3O4 [38], Cu2O/Pd–Fe3O4 [37], MnO/Pd–Fe3O4 [36], CoO/Pd–Fe3O4 [36], and Ni/Pd–Fe2O3 [39].

7.1. Synthesis of Hybrid Cu-Doped Pd–Fe3O4 Nanocatalyst

The synthesis of Cu-doped Pd–Fe3O4 nanocomposites were carried out by decomposition of Fe(CO)5 and continued by the reduction of Pd(OAc)2 and Cu(acac)2 in presence of oleylamine (OAm) and 1-octadecene (ODE), and is outlined in Scheme 8a. The molar ratios of Pd/Fe/Cu were varied in the synthesis along with increasing amounts of NaOL. A high load of NaOL enlarged the BET surface areas of the Cu-doped Pd–Fe3O4 nanocomposites, demonstrating the result of NaOL on the surface area and morphology. Pd–Fe3O4 seed particles formed and aggregated, and nanosheets grew from the aggregate surface of Pd–Fe3O4. The morphology obtained without NaOL (Cu-doped Pd–Fe3O4-0) was spherical structures. In the presence of NaOL, Cu-doped Pd–Fe3O4 hybrid obtained a sheet-assembled formation, and not sphere-shaped (Scheme 8a). Cu-doped Pd–Fe3O4-n nanocomposites consist of crystal structures of fcc Pd crystal structure, and the cubic spinel structure of Fe3O4. The magnetic properties of the Cu-doped Pd–Fe3O4-0.3 nanocomposite have saturation magnetization value 9.2 emu g−1. In addition, the remanence and coercivity of the hybrid nanocomplexes were close to zero, representative of superparamagnetism.

7.2. Synthesize Cu2O/Pd–Fe3O4 Nanocatalyst

The controlled thermal decomposition of iron pentacarbonyl and reduction of Pd and Cu (OAc)2 were used in the synthesis of Cu2O/Pd–Fe3O4 nanocomposites as shown in Scheme 8a. The Pd precursor and quantity of reducing agent affected the morphology of the nanocomposites. The flower-like morphology with a regular diameter of 173 nm was obtained using this method. Cu2O/Pd–Fe3O4 nanocomposites confirm the fcc Pd crystal structure and cubic spinel structure of Fe3O4. Uniform distribution of Pd, Fe, and Cu substances over the whole nanocomposite verified the Cu2O/Pd–Fe3O4 hybrid structure. The saturation magnetization analysis confirmed superparamagnetism. The specific surface areas of Cu2O/Pd–Fe3O4 nanocomposites were higher compared to Pd–Fe3O4.

7.3. Hybrid MnO and CoO/Pd–Fe3O4 Nanocomplexes

The fabrication of MnO/Pd–Fe3O4 and CoO/Pd–Fe3O4 nanocomplexes were similar to previous work with the modification of metal precursors such as Mn(acac)2 or Co(acac)2 in OAm and ODE in Scheme 8b,c. The morphology of hybrid MnO/Pd–Fe3O4 and CoO/Pd–Fe3O4 obtained using this method corresponded to uniform hierarchical, and the nanosheets emitted small seed particles in the center (Scheme 8b,c).
The crystal structure of the nanocomposite established the fcc structure of Pd and the cubic spinel structure of Fe3O4 (Figure 15). The addition of metal source decreased the intensity of the Fe3O4 peak, attributing the crystallization of Fe3O4 to disorder caused by Mn and Co ions. The crystalline MnO and CoO did not appear, suggesting that the overhead oxides had an amorphous structure.

7.4. Synthesis of Hybrid Ni–Pd–Fe3O4 Nanocomposites

Pd–Fe3O4 hybrids loaded with transition metal Ni were also fabricated using thermal decomposition and reduction methods. In this method, the amount of triphenylphosphine (TPP) had an effect on controlling the morphology of the nanocomposites. The amount of TPP used was 0.5 and 1 mmol. To synthesize Ni-doped Pd–Fe3O4 hybrid nanoparticles (NPFNPs), Pd(OAc)2, Fe(CO)5, and Ni(acac)2 were used as a salt (Scheme 8d). Spherical morphology with a rough surface was obtained when 0.5 mmol TPP was used, and the average particle diameter around 244 ± 38 nm. Increasing TPP to 1 mmol resulted in the morphology changing to a more impressive nanosheet at the corners, which an average particle diameter of 215 ± 17 nm. On the other hand, the crumpled ball morphology regularly collapsed when applying an excess amount of TPP, confirming the amount of TPP plays an essential role in the morphology-controlled synthesis of nanocomposites.

7.5. Applications of Transition Metal-Loaded Pd–Fe3O4 Heterobimetallic Nanoparticles in Organic Reactions

Pd catalysis in organic transformations such as the Suzuki–Miyaura, Heck, Sonogashira, tandem reactions, hydroboration, etc., are essential in different organic synthesis procedures with huge interest and in many fields of application. As previously mentioned, the advantages of the combination of transition metal and metal oxide loading on Pd–Fe3O4 heterobimetallic nanoparticles, as see in Scheme 9, will be discussed.

7.5.1. Tandem Synthesis of 2-Phenylbenzofurans

The catalytic performance of Cu-doped Pd–Fe3O4 was studied for tandem synthesis of 2-phenylbenzofurans from 2-iodophenols with phenylpropiolic acids as a model reaction in Scheme 8a. Regarding the Cu-doped Pd–Fe3O4-0.3 nanocomposite, the Cu-doped Pd provides impressive catalytic performance, and superior stability was achieved using recovery and leaching experiments. The comparison of the catalytic activity and conversion with heterogeneous catalysts used in earlier work by our group is shown in Figure 16.

7.5.2. C–H Arylation of 1-Butyl-4-Nitro-1H-Imidazoles

The catalytic performance of Cu2O/Pd–Fe3O4 was investigated for C–H arylation, where the benchmark substrates are 1-butyl-4-nitro-1H-imidazoles with iodobenzenes (Scheme 9b). Good catalytic performance was observed for the hybrid Cu2O/Pd–Fe3O4 catalyst compared with Pd–Fe3O4 or other catalysts, as shown in Table 12. The remarkable result is due to the electron transfer across the metal–oxide interface and the synergetic effect of Cu and Pd–Fe3O4.
The Cu2O/Pd–Fe3O4 catalyst also showed good conversion with numerous substituted aryl iodides, as shown in Table 13. The effect of electron-donating substituents and electron-deficient progressed with high reactivity, except the COMe group.

7.5.3. Synthesis of Alkylboronates from Styrene

The hydroboration of styrene with B2Pin2 was used in the catalytic test of MnO/Pd–Fe3O4 and CoO/Pd–Fe3O4 (Table 14). THF is efficiently catalyzed this reaction. MeOH acts as a hydrogen donor. The high yield was obtained while using as an additive (entry 2) and not as a solvent (entry 1). The best base was Cs2CO3 with the highest yield of 67%. Furthermore, the hybrid CoO/Pd–Fe3O4 was observed to have better catalytic performance than the other nanocomposites MnO/Pd–Fe3O4, Pd–Fe3O4, and Pd/charcoal, due to electron transfer beyond the metal and oxide interface. The recyclability and leaching tests suggest the superior catalytic performance and impressive stability of the nanocatalyst.

7.5.4. Suzuki–Miyaura Coupling Reaction

The catalytic properties of Ni/Pd–Fe3O4 applied in Suzuki–Miyaura C–C coupling reaction were studied using bromobenzene and phenylboronic acid. A combination of water and water (1:1) at 50 °C (Scheme 9d) was used. The yields of the products and TOFs for the reactions containing the NPFNP-1 Ni-doped Pd–Fe3O4 hybrid nanoparticles (NPFNPs), NPFNP-2, and Pd–Fe3O4 as the catalyst was obtained under giving conditions. NPFNP-2 presented relatively high catalytic performance compared to NPFNP-1 and Pd–Fe3O4 with a similar mol % of Pd as shown in Figure 16b and Figure 17a. The high catalytic activity of NPFNP-2 can be indicative of a higher number of surface deficiencies caused by the changes in morphology and the synergistic properties of individual components. The defect surface and synergistic effects facilitated the oxidative addition reaction of aryl halide resulting in an increase in yield.

8. Conclusions

The synthesis of hybrid Pd–Fe3O4 nanoparticles was reviewed with a focus on urchin-like FePd–Fe3O4, Pd/Fe3O4, Pd/Fe3O4/charcoal, flower-like Pd–Fe3O4, and transition metal-loaded Pd–Fe3O4 nanocomposites which act as successful catalysts for various C–C coupling reactions. We reviewed all of the hybrid Pd–Fe3O4 nanocomposites that showed better catalytic performance and reusability than many previously reported catalysts, because of magnetic properties. Hybrid Pd–Fe3O4 NPs exhibited high performance, stability, and recyclability with respect to morphology and magnetic properties. We anticipate that our methodology can be developed for other catalytic systems in the near future.

Author Contributions

K.H.P., J.C.P. and S.P. provided academic direction and worked hardly to get the fundings. S.J. and S.A. contributed equally on that work. Specially, S.J. collected materials and wrote the introduction and conclusion part. S.A. contributed in results and discussion part and checked English style as well as grammatical errors throughout the manuscript. D.A., S.S. and M.Y. contributed to the materials and methods and results and discussion part.

Funding

This research was financially supported by Basic Science Research Program as well as National Research Foundation of Korea (NRF). It was also supported by the Ministry of Science, ICT & Future Planning (NRF-2017R1A4A1015533 and NRF- 2017R1D1A1B03036303 and NRF-2018R1D1A1B07045663). This study was supported in part by NRF Korea (NRF-2018R1D1A1B07045663) and Korea Basic Science Institute (KBSI) grant (C38529) to Sungkyun Park.

Conflicts of Interest

The authors declare no conflict of interest.

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Scheme 1. C–C coupling reactions catalyzed by novel hybrid Pd–Fe3O4 nanoparticles.
Scheme 1. C–C coupling reactions catalyzed by novel hybrid Pd–Fe3O4 nanoparticles.
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Scheme 2. Synthetic process, illustration, and TEM image of the urchin-like FePd–Fe3O4 composite nanoparticles.
Scheme 2. Synthetic process, illustration, and TEM image of the urchin-like FePd–Fe3O4 composite nanoparticles.
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Figure 1. (a) HRTEM image of the L10-FePd–Fe nanocomposite grain with L10-FePd or Fe and (b) indicate EDS elemental mappings of Pd (red) and Fe (green) combined signals (c), (d) indicate single element Pd (red) (c) and Fe (green), respectively. Reproduced with permission from Sun, Nano Letters, published by American Chemical Society, 2013.
Figure 1. (a) HRTEM image of the L10-FePd–Fe nanocomposite grain with L10-FePd or Fe and (b) indicate EDS elemental mappings of Pd (red) and Fe (green) combined signals (c), (d) indicate single element Pd (red) (c) and Fe (green), respectively. Reproduced with permission from Sun, Nano Letters, published by American Chemical Society, 2013.
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Figure 2. (a) Room temperature magnetic hysteresis loop of Fe67Pd33 heat-treated at 350 °C (black) and 450 °C (red), (b) indicates that annealing temperature is reliant on Ms and Hc of Fe67/Pd33–Fe3O4 nanocomposites. (c) Magnetic hysteresis loop of Fe45Pd55 (black) and Fe67Pd33–Fe3O4 (red) nanocomposite annealed at 500 °C, (d) Fe concentration reliant on Ms and Hc for the complexes heat-treated at 500 °C. Reproduced with permission from Sun, Nano Letters, published by American Chemical Society, 2013.
Figure 2. (a) Room temperature magnetic hysteresis loop of Fe67Pd33 heat-treated at 350 °C (black) and 450 °C (red), (b) indicates that annealing temperature is reliant on Ms and Hc of Fe67/Pd33–Fe3O4 nanocomposites. (c) Magnetic hysteresis loop of Fe45Pd55 (black) and Fe67Pd33–Fe3O4 (red) nanocomposite annealed at 500 °C, (d) Fe concentration reliant on Ms and Hc for the complexes heat-treated at 500 °C. Reproduced with permission from Sun, Nano Letters, published by American Chemical Society, 2013.
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Scheme 3. Synthetic scheme of Pd–Fe3O4 hybrid nanocatalyst.
Scheme 3. Synthetic scheme of Pd–Fe3O4 hybrid nanocatalyst.
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Figure 3. The XRD spectrum of Pd–Fe3O4.
Figure 3. The XRD spectrum of Pd–Fe3O4.
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Figure 4. (a,b) TEM images of Pd–Fe3O4 at different magnifications, (c) HAADF-STEM image of Pd–Fe3O4, and EDX mapping of (d) O, (e) Fe, and (f) Pd.
Figure 4. (a,b) TEM images of Pd–Fe3O4 at different magnifications, (c) HAADF-STEM image of Pd–Fe3O4, and EDX mapping of (d) O, (e) Fe, and (f) Pd.
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Figure 5. (a) The amplified magnetization hysteresis curves of Pd–Fe3O4; (b) The suspensions before and after magnetic separation by an external magnet; (c) The magnetization hysteresis loop of Pd–Fe3O4.
Figure 5. (a) The amplified magnetization hysteresis curves of Pd–Fe3O4; (b) The suspensions before and after magnetic separation by an external magnet; (c) The magnetization hysteresis loop of Pd–Fe3O4.
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Figure 6. The recyclability test of Pd–Fe3O4 for Heck reaction over five consecutive cycles.
Figure 6. The recyclability test of Pd–Fe3O4 for Heck reaction over five consecutive cycles.
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Figure 7. (a) TEM image of recycled Pd–Fe3O4 after five cycles and (b) hot-filtration experiment in the Heck reaction of iodobenzene and styrene.
Figure 7. (a) TEM image of recycled Pd–Fe3O4 after five cycles and (b) hot-filtration experiment in the Heck reaction of iodobenzene and styrene.
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Scheme 4. The synthetic process of Pd/Fe3O4/charcoal nanoparticles.
Scheme 4. The synthetic process of Pd/Fe3O4/charcoal nanoparticles.
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Figure 8. (a) TEM image with bar scale of 50 nm, (b) HRTEM image with bar scale of 5 nm, and (c) XRD spectra Pd/Fe3O4/charcoal, respectively. Reproduced with permission from Park, New J. Chem., published by The Royal Society of Chemistry, 2014.
Figure 8. (a) TEM image with bar scale of 50 nm, (b) HRTEM image with bar scale of 5 nm, and (c) XRD spectra Pd/Fe3O4/charcoal, respectively. Reproduced with permission from Park, New J. Chem., published by The Royal Society of Chemistry, 2014.
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Figure 9. HRTEM images of the recovered Pd (20 wt %)/Fe3O4(10 wt %)/charcoal catalysts after recycling (a) two times and (b) three times. All bars represent 20 nm. Reproduced with permission from Park, New J. Chem.; published by The Royal Society of Chemistry, 2014.
Figure 9. HRTEM images of the recovered Pd (20 wt %)/Fe3O4(10 wt %)/charcoal catalysts after recycling (a) two times and (b) three times. All bars represent 20 nm. Reproduced with permission from Park, New J. Chem.; published by The Royal Society of Chemistry, 2014.
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Scheme 5. The synthetic process of Fe3O4@C–Pd nanoparticles.
Scheme 5. The synthetic process of Fe3O4@C–Pd nanoparticles.
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Figure 10. TEM image of (a) Fe3O4@C–Pd-350 and (b) HRTEM image of Fe3O4@C–Pd-550, respectively.
Figure 10. TEM image of (a) Fe3O4@C–Pd-350 and (b) HRTEM image of Fe3O4@C–Pd-550, respectively.
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Scheme 6. Dispersion stability of Pd/Fe3O4 nanoparticles using capping agent in water.
Scheme 6. Dispersion stability of Pd/Fe3O4 nanoparticles using capping agent in water.
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Figure 11. TEM images of immobilized of Pd onto Fe3O4 using various capping agents.
Figure 11. TEM images of immobilized of Pd onto Fe3O4 using various capping agents.
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Figure 12. XRD spectrum of each Fe3O4/Pd catalysts. Reproduced with permission from Park, ChemCatChem; published by Wiley, 2014.
Figure 12. XRD spectrum of each Fe3O4/Pd catalysts. Reproduced with permission from Park, ChemCatChem; published by Wiley, 2014.
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Figure 13. Recycling test of Na3Cit–Fe3O4/Pd catalyst.
Figure 13. Recycling test of Na3Cit–Fe3O4/Pd catalyst.
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Scheme 7. Synthetic scheme of Pd–Fe3O4 and Au/Pd–Fe3O4 nanocatalyst.
Scheme 7. Synthetic scheme of Pd–Fe3O4 and Au/Pd–Fe3O4 nanocatalyst.
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Figure 14. SEM images of flower-like Pd–Fe3O4 (a), Au/Pd–Fe3O4 (b) nanocomposites, TEM images of flower-like Pd–Fe3O4 (c), Au/Pd–Fe3O4 (d). Reproduced with permission from Park, Solid State Sciences, published by Elsevier, 2016.
Figure 14. SEM images of flower-like Pd–Fe3O4 (a), Au/Pd–Fe3O4 (b) nanocomposites, TEM images of flower-like Pd–Fe3O4 (c), Au/Pd–Fe3O4 (d). Reproduced with permission from Park, Solid State Sciences, published by Elsevier, 2016.
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Scheme 8. Synthesis of transition metal-loaded Pd–Fe3O4 heterobimetallic nanoparticles.
Scheme 8. Synthesis of transition metal-loaded Pd–Fe3O4 heterobimetallic nanoparticles.
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Figure 15. XRD spectrum of (a) Cu-doped Pd–Fe3O4, (b) Cu2O-doped Pd–Fe3O4, and MnO-, CoO-doped Pd–Fe3O4, respectively. Reproduced with permission from Park, published by (a) Journal of Materials Chemistry A, The Royal Society of Chemistry, 2015; (b) RSC Advances, The Royal Society of Chemistry, 2016; and (c) Catalysis Communications, Elsevier, 2017.
Figure 15. XRD spectrum of (a) Cu-doped Pd–Fe3O4, (b) Cu2O-doped Pd–Fe3O4, and MnO-, CoO-doped Pd–Fe3O4, respectively. Reproduced with permission from Park, published by (a) Journal of Materials Chemistry A, The Royal Society of Chemistry, 2015; (b) RSC Advances, The Royal Society of Chemistry, 2016; and (c) Catalysis Communications, Elsevier, 2017.
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Scheme 9. Schematic applications of transition metal loading Pd–Fe3O4 heterometallic nanoparticles in organic reactions. Reproduced with permission from Park, published by (a) Journal of Materials Chemistry A, The Royal Society of Chemistry, 2015; (b) RSC Advances, The Royal Society of Chemistry, 2016; (c) Catalysis Communications, Elsevier, 2017; and (d) Catalysts, MDPI, 2017.
Scheme 9. Schematic applications of transition metal loading Pd–Fe3O4 heterometallic nanoparticles in organic reactions. Reproduced with permission from Park, published by (a) Journal of Materials Chemistry A, The Royal Society of Chemistry, 2015; (b) RSC Advances, The Royal Society of Chemistry, 2016; (c) Catalysis Communications, Elsevier, 2017; and (d) Catalysts, MDPI, 2017.
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Figure 16. Comparison of Cu-doped Pd–Fe3O4 catalytic activity with heterogeneous catalysts as previously reported by our group. Reproduced with permission from Park, Journal of Chemistry A, The Royal Society of Chemistry, 2015.
Figure 16. Comparison of Cu-doped Pd–Fe3O4 catalytic activity with heterogeneous catalysts as previously reported by our group. Reproduced with permission from Park, Journal of Chemistry A, The Royal Society of Chemistry, 2015.
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Figure 17. Comparison of (a) yield and (b) TOF for the products in the Suzuki–Miyaura coupling reaction. Reproduced with permission from Park, Catalysts, published by MDPI, 2017.
Figure 17. Comparison of (a) yield and (b) TOF for the products in the Suzuki–Miyaura coupling reaction. Reproduced with permission from Park, Catalysts, published by MDPI, 2017.
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Table 1. The reaction of iodobenzene and styrene catalyzed by Pd–Fe3O4 a.
Table 1. The reaction of iodobenzene and styrene catalyzed by Pd–Fe3O4 a.
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EntryPd added bTemp (°C)Time (h)Yield c (%)
10.054110393
20.108110399
30.162110399
40.10860319.8
50.10870334.5
60.10880351.3
70.10880657.1
80.108802491.2
90.10890369.2
100.108100385.4
a Reaction conditions: iodobenzene (5 mmol), styrene (7.5 mmol), NEt3 (7.5 mmol), DMF (10 mL) and nitrogen atmosphere. b Relative to the amount of iodobenzene. c The products were investigated by GC using an internal standard (decane).
Table 2. Screening of optimum conditions in Suzuki–Miyaura coupling reactions. Reproduced with permission from Park, New J. Chem., published by The Royal Society of Chemistry, 2014.
Table 2. Screening of optimum conditions in Suzuki–Miyaura coupling reactions. Reproduced with permission from Park, New J. Chem., published by The Royal Society of Chemistry, 2014.
EntryCatalystsTemp (°C)Time (h)SolventConv. a (%)Product Time Yield (gproduct gPd−1 h−1)
1Pd/Fe3O4/charcoal15030Toluene/H2O (4:1)593.62
2Pd/Fe3O4/charcoal1004Toluene/H2O (4:1)41.84
3Pd/Fe3O4/charcoal1004DMSO/H2O (4:1)2512.0
4Pd/Fe3O4/charcoal1004THF/H2O (4:1)3515.1
5Pd/Fe3O4/charcoal
(Pd 0.5 mol %)
1004DMF/H2O (4:1)6055.2
6Pd/Fe3O4/charcoal1002DMF/H2O (4:1)6055.2
7Pd/charcoal b1004DMF/H2O (4:1)7635.0
8Pd/Fe3O4/charcoal1004DMF/H2O (4:1)>9936.0
9Fe3O4/charcoal1004DMF/H2O (4:1)No reaction
10Commercial Pd/charcoal1004DMF/H2O (4:1)3114.3
The reactions were conducted using 20 wt % of Pd/Fe3O4/charcoal catalyst, 1 mmol of 4-bromoanisole, 1.2 mmol of phenylboronic acid, 2 mmol of K2CO3, 10 mL of DMF, and 2.5 mL of H2O. a Analyzed using 1H NMR and b 20 wt% Pd NPs were used.
Table 3. Suzuki–Miyaura coupling reactions of aryl halides with arylboronic acid. Reproduced with permission from Park, New J. Chem.; published by The Royal Society of Chemistry, 2014.
Table 3. Suzuki–Miyaura coupling reactions of aryl halides with arylboronic acid. Reproduced with permission from Park, New J. Chem.; published by The Royal Society of Chemistry, 2014.
EntryAryl HalideArylboronic AcidProductYield a (%)Product Time Yield (gproduct gPd−1 h−1)
1 Processes 07 00422 i002 Processes 07 00422 i003 Processes 07 00422 i004>9936.2
2 Processes 07 00422 i005 Processes 07 00422 i003 Processes 07 00422 i004>9936.3
3 Processes 07 00422 i006 Processes 07 00422 i003 Processes 07 00422 i004>9936.2
4 Processes 07 00422 i007 Processes 07 00422 i003 Processes 07 00422 i008>9943.1
5 Processes 07 00422 i009 Processes 07 00422 i010 Processes 07 00422 i0114520.9
6 Processes 07 00422 i009 Processes 07 00422 i010 Processes 07 00422 i0116437.9
7 Processes 07 00422 i012 Processes 07 00422 i003 Processes 07 00422 i0134818.9
8 Processes 07 00422 i014 Processes 07 00422 i003 Processes 07 00422 i0159948.6
9 Processes 07 00422 i016 Processes 07 00422 i003 Processes 07 00422 i0178335.5
10 Processes 07 00422 i018 Processes 07 00422 i003 Processes 07 00422 i0199038.5
The reactions were conducted using 20 wt % of Pd/Fe3O4/charcoal catalyst, 1 mmol of 4-bromoanisole, 1.2 mmol of phenylboronic acid, 2 mmol of K2CO3, 10 mL of DMF, and 2.5 mL of H2O. a Analyzed using 1H NMR.
Table 4. Recycling test of Pd/Fe3O4/charcoal catalyst. Reproduced with permission from Park, New J. Chem.; published by The Royal Society of Chemistry, 2014.
Table 4. Recycling test of Pd/Fe3O4/charcoal catalyst. Reproduced with permission from Park, New J. Chem.; published by The Royal Society of Chemistry, 2014.
Recycle RunTemp (°C)Time (h)SolventConv. a (%)Product Yield Time (gproduct gPd−1 h−1)
11004DMF/H2O (4:1)>9946.0
21004DMF/H2O (4:1)>9946.0
31004DMF/H2O (4:1)>9946.0
The reactions were conducted using Pd/Fe3O4/charcoal catalyst with optimum conditions in Table 2 (entry 8). a Analyzed using 1H NMR.
Table 5. The Suzuki coupling reactions of structurally different aryl halides.
Table 5. The Suzuki coupling reactions of structurally different aryl halides.
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EntryAHalogenTime (h)Yield (%)
1HBr692.22
2HCl1263.74
3NO2Br694.51
4OCH3Br687.12
Table 6. Size distribution of Pd/Fe3O4 microspheres using various capping agents.
Table 6. Size distribution of Pd/Fe3O4 microspheres using various capping agents.
Recycle RunCapping AgentAverage Size (nm)
1Na3Cit3.3 ± 0.24
2PEG3.4 ± 0.21
3PVP4.1 ± 0.43
4No capping agent4.7 ± 0.35
Table 7. a Screening of optimum conditions in Suzuki coupling reactions. Reproduced with permission from Park, ChemCatChem., published by Wiley, 2014.
Table 7. a Screening of optimum conditions in Suzuki coupling reactions. Reproduced with permission from Park, ChemCatChem., published by Wiley, 2014.
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EntryCat. (mol %)Temp (°C)Time (h)BaseSolventYield b (%)
11 (Na3Cit)805K2CO3DMF/H2O (4:1)91
21 (Na3Cit)505K2CO3H2O97
30.1 (Na3Cit)505K2CO3H2O65
40.05 (Na3Cit)505Cs2CO3H2O89
50.05 (Na3Cit)507Cs2CO3H2O98
60.05 (Na3Cit)505CsOHH2O66
70.05 (Na3Cit)4012Cs2CO3H2O48
80.05 (Na3Cit)4024Cs2CO3H2O94
90.1 (Na3Cit)4012Cs2CO3H2O76
100.05 (Na3Cit)1001Cs2CO3H2O80
110.05 (Na3Cit)1001.5Cs2CO3H2O98
120.05 (Na3Cit)2524Cs2CO3H2O13
130.05 (PEG)507Cs2CO3H2O89
140.05 (No)507Cs2CO3H2O57
150.05 (PVP)507Cs2CO3H2O37
a The reactions were conducted using 0.5 mmol of bromobenzene, 0.6 mmol of phenylboronic acid, and 3 mL of H2O. b Analyzed using GC–MS.
Table 8. Optimized reaction condition. Reproduced with permission from Park, Solid State Sciences, published by Elsevier, 2016.
Table 8. Optimized reaction condition. Reproduced with permission from Park, Solid State Sciences, published by Elsevier, 2016.
Processes 07 00422 i022
EntryCat. (mol %)Temp (°C)Time (h)BaseSolventConv. (%) a
1112018PiperidineDMF72
2112018PiperidineNMP76
3112018PiperidineDMSO99
4112018Cs2CO3DMSO73
5112018NaOAcDMSO98
6112018K2CO3DMSO78
719018PiperidineDMSO92
80.51203PiperidineDMSO99
90.251203PiperidineDMSO94
100.51201PiperidineDMSO93
Reaction conditions: iodobenzene (1mmol), phenylacetylene (1.1 mmol), base (2 mmol), solvent (5 mL). a Determined using gas chromatography–mass spectrometry (GC–MS).
Table 9. Sonogashira reaction of numerous aryl halides with arylacetylenes catalyzed by Pd–Fe3O4 nanocomposite. Reproduced with permission from Park, Solid State Sciences; published by Elsevier, 2016.
Table 9. Sonogashira reaction of numerous aryl halides with arylacetylenes catalyzed by Pd–Fe3O4 nanocomposite. Reproduced with permission from Park, Solid State Sciences; published by Elsevier, 2016.
EntryAryl HalideArylacetyleneProductConversion (%) a
1 Processes 07 00422 i023 Processes 07 00422 i024 Processes 07 00422 i02572
2 Processes 07 00422 i026 Processes 07 00422 i024 Processes 07 00422 i02776
3 Processes 07 00422 i028 Processes 07 00422 i024 Processes 07 00422 i02999
4 Processes 07 00422 i030 Processes 07 00422 i024 Processes 07 00422 i03173
5 Processes 07 00422 i032 Processes 07 00422 i024 Processes 07 00422 i03398
6 Processes 07 00422 i034 Processes 07 00422 i024 Processes 07 00422 i03578
7 Processes 07 00422 i032 Processes 07 00422 i036 Processes 07 00422 i03392
8 Processes 07 00422 i037 Processes 07 00422 i024 Processes 07 00422 i03899
9 Processes 07 00422 i037 Processes 07 00422 i024 Processes 07 00422 i05294
Reaction conditions: aryl halides (1.0 mmol), arylacetylene (1.1 mmol), piperidine (2.0 mmol) DMSO (5 mL), 120 °C, and 3 h. a Determined using gas chromatography–mass spectrometry (GC–MS).
Table 10. Tandem synthesis of 2-phenylindoles. Reproduced with permission from Park, Nanoscale, published by The Royal Society of Chemistry, 2015.
Table 10. Tandem synthesis of 2-phenylindoles. Reproduced with permission from Park, Nanoscale, published by The Royal Society of Chemistry, 2015.
Processes 07 00422 i039
EntryCatalystTemp (°C)Time (h)BaseConversion (%) a
1Pd–Fe3O412018PiperidineTrace b
2Pd–Fe3O412018Piperidine3 c
3Pd–Fe3O412018Piperidine41
4Pd–Fe3O412018LiOAc45
5Pd–Fe3O412018CsOAc48
6Au/Pd–Fe3O412018CsOAc57
7Au/Pd–Fe3O415018CsOAc97
8Au/Pd–Fe3O41509CsOAc97
9Au/Pd–Fe3O41506CsOAc59
10Au/Pd–Fe3O41509CsOAc38 d
Reaction conditions: Au/Pd–Fe3O4 catalyst (Au base: 0.18 mol %, Pd base: 0.5 mol %), 2-iodoaniline (0.5 mmol), phenylacetylene (0.6 mmol), base (1.0 mmol), DMSO (2.5 mL). a Determined using GC–MS spectroscopy based on 2-iodoaniline. b DMF was used as a solvent. c DMA was used as a solvent. d 0.09 mol % (Au base) of catalyst was used.
Table 11. Comparison of different catalyst. Reproduced with permission from Park, Nanoscale, published by The Royal Society of Chemistry, 2015.
Table 11. Comparison of different catalyst. Reproduced with permission from Park, Nanoscale, published by The Royal Society of Chemistry, 2015.
SI NoCatalystSize (nm)MorphologyApplication
1Pd–Fe3O48.7SphericalMizoroki–Heck reaction
2Pd–Fe3O4213Flower-likeSonogashira coupling reaction
3Au/Pd–Fe3O45.8Flower-likeTandem synthesis reaction
Table 12. Oxidation of benzyl alcohol using various catalysts based on Cu3(BTC)2. Reproduced with permission from Park, RSC Advances, published by The Royal Society of Chemistry, 2016.
Table 12. Oxidation of benzyl alcohol using various catalysts based on Cu3(BTC)2. Reproduced with permission from Park, RSC Advances, published by The Royal Society of Chemistry, 2016.
Processes 07 00422 i040
EntryCatalystsTime (h)Temp (°C)Conv a (%)
1Pd–Fe3O41813062
2Cu2O/Pd–Fe3O41813073
3Cu2O/Pd–Fe3O41814085 b
4Cu2O/Pd–Fe3O4914084 b
5Cu2O/Pd–Fe3O44.514076 b
6Cu2O/Pd–Fe3O4914076 b, c
7Pd/charcoal4.514069 b
8Fe3O4/charcoal4.51400 b
9Cu2O4.51400 b
Reaction conditions: catalyst (Pd base: 5.0 mol %), 1-butyl-4-nitro-1H-imidazole (0.5 mmol), iodobenzene (0.55 mmol), base NaOAc (1.0 mmol), solvent DMSO (3.0 mL). a Determined by 1H NMR. b Iodobenzene (1.0 mmol), NaOAc (2.0 mmol) and DMSO (5 mL) were used. c 2.5 mol % of catalyst was used.
Table 13. Substrate scope. Reproduced with permission from Park, RSC Advances, published by The Royal Society of Chemistry, 2016.
Table 13. Substrate scope. Reproduced with permission from Park, RSC Advances, published by The Royal Society of Chemistry, 2016.
EntrySubstrateConv. (%)EntrySubstrateConv. (%)
1 Processes 07 00422 i041846 Processes 07 00422 i04270
2 Processes 07 00422 i043797 Processes 07 00422 i04439
3 Processes 07 00422 i045848 Processes 07 00422 i04671
4 Processes 07 00422 i047859 Processes 07 00422 i04880
5 Processes 07 00422 i0497510 Processes 07 00422 i05069
Reaction conditions: catalyst (Pd base: 5.0 mol %), 1-butyl-4-nitro-1H-imidazole (0.5 mmol), iodobenzene (1.0 mmol), NaOAc (2.0 mmol), DMSO (5.0 mL).
Table 14. Synthesis of alkylboronates from styrene. Reproduced with permission from Park, Catalysis Communications, published by Elsevier, 2017.
Table 14. Synthesis of alkylboronates from styrene. Reproduced with permission from Park, Catalysis Communications, published by Elsevier, 2017.
Processes 07 00422 i051
EntryCatalystsSolventBaseTime (h)Yield (%) a
1CoO/Pd–Fe3O4MeOHKOtBu12Trace
2CoO/Pd–Fe3O4THFKOtBu1233
3CoO/Pd–Fe3O4THFNaOMe1236
4CoO/Pd–Fe3O4THFCs2CO31267
5MnO/Pd–Fe3O4THFCs2CO31229
6Pd–Fe3O4THFCs2CO31235
7Pd/charcoalTHFCs2CO312Trace
8CoO/Pd–Fe3O4THFCs2CO32463
Reaction conditions: 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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Jang, S.; Hira, S.A.; Annas, D.; Song, S.; Yusuf, M.; Park, J.C.; Park, S.; Park, K.H. Recent Novel Hybrid Pd–Fe3O4 Nanoparticles as Catalysts for Various C–C Coupling Reactions. Processes 2019, 7, 422. https://0-doi-org.brum.beds.ac.uk/10.3390/pr7070422

AMA Style

Jang S, Hira SA, Annas D, Song S, Yusuf M, Park JC, Park S, Park KH. Recent Novel Hybrid Pd–Fe3O4 Nanoparticles as Catalysts for Various C–C Coupling Reactions. Processes. 2019; 7(7):422. https://0-doi-org.brum.beds.ac.uk/10.3390/pr7070422

Chicago/Turabian Style

Jang, Sanha, Shamim Ahmed Hira, Dicky Annas, Sehwan Song, Mohammad Yusuf, Ji Chan Park, Sungkyun Park, and Kang Hyun Park. 2019. "Recent Novel Hybrid Pd–Fe3O4 Nanoparticles as Catalysts for Various C–C Coupling Reactions" Processes 7, no. 7: 422. https://0-doi-org.brum.beds.ac.uk/10.3390/pr7070422

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