VIETNAM JOURNAL OF CHEMISTRY VOL. 50(5) 542-549 OCTOBER 2012
THE SUZUKI REACTION USING PALLADIUM COMPLEX IMMOBILIZED ON MAGNETIC NANOPARTICLES (CoFe204) AS A CATALYST
Phan Thanh Son Nam, Nguyen Tan Tal Ho Chi Minh City University of Technology, Vietnam
Received 14 January 2012
Abstract
Cobalt superparamagnetic spinel femte {CoFei04) nanoparticlcs were synthesized following a microemulsion method, and functionalized with Schiff-base groups on the surface to form immobilized bidentate ligands. The fiinctionalized nanoparticlcs were complcxed with palladium acetate, affording the immobilized palladium con5)Iex catalysts with a palladium loading of 0.21 mmol/g. The catalyst was characterized by X-ray powder diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), thermogravimctric analysis (TGA), Fourier transform infrared (FT-IR), and elemental analysis (EA). The immobilized palladium complex was used as an efficient catalyst for the Suzuki reaction of iodobenzene and phenylboronic acid. Recovery of catalyst was facilely achieved by simple magnetic decantation The immobilized palladium complex catalyst could be reused several times without significant degradation m catalytic activity.
Keywords: Superparamagnetic, palladium, Suzuki reaction, immobilized.
1. INTRODUCTION
The immobilization of homogeneous catalysts to facilitate easy catalyst recovery and recycling, as well as product separation is a longstanding pursuit of catalysis science [1]. When the size of the support material is decreased to nanometer scale, the activity of nanoparticle-supported catalysts could be significantly improved, compared to homogeneous catalysts immobilized on conventional support matnces [2, 3]. However, m this case, facile separation and recycling of nanoparticle materials from reaction media still remains a challenge [3].
This issue can be addressed by using magnetic supports, allowing the catalyst to be easily separated from the liquid reaction media with application of an external magnetic field [3]. Recently, superparamagnetic spinel ferrite nanoparticlcs have been intensively investigated because of their remarkable electrical and magnetic properties, and wide practical applications, especially m biomedical diagnostics and in catalysis [4, 5].
Suzuki reactions have attracted significant interests in organic synthesis, in particular as convenient techniques for the formation of biaryl derivatives [6]. These biaryl units have exhibited practical applications in the production of pharmaceuticals, herbicides, as well as engineering materials such as conducting polymers and liquid
crystals [6, 7]. Catalysts used in the standard Sumki processes are generally based on homogeneous palladium phosphine complexes, which are rarely recoverable without elaborate and wastefiil procedures, and therefore commercially undesirable [6]. In this context, heterogeneous palladium catalysts have recently emerged as a greener alternative to homogeneous processes so that catalysts can be recovered and reused [7]. In this paper, we wish to report for the first time in Vietnam, to our best knowledge, the Suzuki reaction of iodobenzene with phenylboronic acid using palladium catalyst immobihzed on superparamagnetic nanoparticlcs. The magnetic catalyst could be facilely isolated from the reaction mixture by simple magnetic decantation, and reused without significant degradation in activity.
2. EXPERIMENTAL
2.1. Materials and instrumentation
Chemicals were purchased from Sigma-Aldrich, Fisher, and Merck and used as received without further purification imless otherwise noted. Distilled water was purged witfi nifrogen for 2 h prior to use.
A Fischer Scientific FS60H was used to sonicate samples. Fourier transform infrared (FT-IR) spectra were obtained on a Bruker TENSOR37 instnmient
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with samples being dispersed on potassium bromide pallets. Scanning electron microscope (SEM) studies were conducted on a JFM 5500. Transmission electron microscope (TEM) studies were performed using a JEOL JEM 1400, in which samples were dispersed on holey carbon grids for TEM observation. A Netzsch Thermoanalyzer STA 409 was used for simultaneous thermal analysis combining thermogravimctric analysis (TGA) and differential scanning calonmetry (DSC) with a heating rate of 10°C/min under a nitrogen atmosphere. X-ray powder diffraction (XRD) patterns were recorded using Cu Ka radiation source on a XPERT-PRO powder diffractometer.
GC-MS analyses were performed using an Agilent GC-MS 6890. GC analyses were performed using a Shimadzu GC-17A equipped with a FID detector and a 30 m x 0.25 mm x 0.25 pm DB-5 column. The temperature program for GC analyses heated samples from eO^C to 200°Cat 10°C/min, held at 200"C for 2 min, from 200°C to 300°C at 50°C/min, and held at 300°C for 2 min.
2.2. Synthesis of amino-functionalized magnetic nanoparticles
Cobalt spinel ferrite (CoFe204) nanoparticles were synthesized following a microemulsion method [8]. CoFe204 nanoparticles (2.2 g) were dispersed in a mixture of ethanol and water (300 ml, 1:1 by volume). Ammonium hydroxide (35 ml, 25% v/v aqueous solution) was added, and the mixture was stirred vigorously at 60 °C for 24 h. The nanoparticles were washed with copious amounts of deionized water, ethanol, and n-hexane via magnetic decantation. The resulting product was redispersed in a mixture of ethanol and water (300 ml, 1:1 by volume), and sonicated for 30 min at room temperature. 3-(Trimethoxysilyl)propylamine (2 g) was then added, and the solution was heated at 60°C with vigorous stirring for 36 h. The final product was washed with copious amoimts of deionized water, ethanol, and «-hexane by magnetic decantation, and dried imder vacuum at room temperature overnight to yield amino-functionalized magnetic nanoparticles (1.60 g).
2.3. Synthesis of palladium catalyst immobilized on magnetic nanoparticles
The amino-functionalized magnetic nanoparticles (1.60 g) were added to a round-bottom flask containing ethanol (99.5%, 240 ml) and 2- acetyl pyridine (16 ml, 77 mmol). The resulting mixture was sonicated for 30 min, and then heated at
Phan Thanh Son Nam, et al reflux with rapid stirring for 36 h. After that, the reaction mixture was cooled to room temperature, washed with copious aipounts of ethanol and n- hexane by magnetic decantation, and dried under vacuum at room temperature to yield the immobilized Schiff base (1.38 g). The immobilized Schiff base (1.38 g) was then added to the round- bottom flask containing the solution of palladium acetate (0.1452 g, 0.64 mmol) in acetone (240 ml).
The mixture was then stirred vigorously at room temperature for 36 h. The solid was then separated by magnetic decantation, washed with copious amounts of acetone and dried under vacuum at room temperature to yield the immobilized palladium catalyst (1.27 g).
2.4. Catalysis studies
Unless otherwise stated, a mixture of iodobenzene (0.12 ml, 1.08 mmol), phenylboronic acid (0.2046 g, 1.68 mmol), K3P04 (0.8628 g, 3.24 mmol), and hexadecane (0.12 ml) as the intemal standard in dimethylformalmide (5 ml) were added to a round-bottom flask containing the required amount of the immobilized palladium catalyst. The flask was heated at the required temperature with magnetic stirring. Reaction conversions were monitored by withdrawing aliquots (0.1 ml) from the reaction mixture at different time intervals, and quenching with aqueous 5% Na2C03 solution. The organic components were extracted into diethylether, dried over Na2S04 and analyzed by gas chromatography (GC) with reference to hexadecane.
Product identity was also further confirmed by gas chromatography - mass spectroscopy (GC-MS).
3. RESULTS AND DISCUSSION
Cobah spinel ferrite (CoFe204) nanoparticles were synthesized foUowmg a microemulsion method [8]. It was previously reported that magnetic nanoparticles synthesized in basic aqueous media are covered with a number of hydroxyl (-0H) groups, due to the adsorption of hydroxyl groups and protons (H*) on the bare atoms of the metal and oxygen, respectively [9]. The hydroxyl groups on the surface of the magnetic nanoparticles were then enriched with an aqueous solution of ammonia, facilitating the surface modification step. The resulting nanoparticles were fiinctionalized with 3- (trimeliioxysilyl)- propylamine to create surface amino groups, according to a slightly modified literature procedure [8, 9]. The amino-fimctionalized magnetic nanoparticles were allowed to react with 2-acetyl pyridine to form immobilized bidentate ligands.
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which were complexed with palladium acetate using a literature procedure previously reported by Clark and
The Suzuki reaction using palladium complex...
co-workers, affording the immobilized palladium complex catalyst (scheme 1) [10].
1 SDS, MeNH:-
Scheme i. Synthesis of the immobilized palladium catalyst on magnetic nanoparticles The immobilized palladium complex catalyst
was then characterized using a vanety of different techniques. SEM and TEM studies showed diameters of approximately 30-40 nm for the superparamagnetic nanoparticles. However, particle agglomeration was clearly observed on the TEM image, and the pnmary particle size was likely closer to 5-10 nm in diameter (figure 1). It should be noted that most oxide particles, regardless of
composition, aggregate on TEM grids and the TEM images do not imply that the nanoparticles aggregate similariy in solution. XRD studies showed that the superparamagnetic nanoparticles are CoFe204 spinel ferrites, with XRD patterns being consistent with literature [3, 8, 9] (figure 2). No impurity peak was observed in the XRD difiractogram of the modified superparamagnetic nanoparticles.
Figure 1: SEM (left) and TEM (right) micrographs of the functionaHzed superparamagnetic nanoparticles TGA analysis of the amino-functionalized
magnetic nanoparticles indicated that 0.71 mmol/g of the amine was immobilized on the particles. This amine loading was also supported by the elemental analysis result of the nitrogen content
on the particles. TGA analysis of the immobilized Schiff base ligand exhibited a Schiff base loading of approximately 0.43 mmol/g (figure 3). As expected, AAS analysis of the immobilized palladium complex exhibited a palladium loading of 0.21
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mmol/g. It should be noted that the metal loading of several immobilized palladium complex catalysts for cross-coupling reactions was reported to be in the range of 0.1 - 0.5 mmol/g [U]. It was previously found that higher palladium loading was unnecessary as increasing the catalyst loading on the solid support to over 0.5 mmol/g could make a number of active sites inaccessible to the reactants [12]. As the catalyst was designed for Suzuki reaction where a base was required, it was unnecessary to block the free amino groups on the surface of the catalyst. Indeed, it was previously reported that the presence of an amine could increase the stability of the palladium catalyst in the Suzuki and the Heck reactions [11, 12], However, the effect of free amino groups on the activity of the catalyst still needs further investigation.
Phan Thanh Son Nam, et al.
Counts 1500 i
1000
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V/^/^.'•H,••U'^-V^..v•^••^•^^
20 30 40 50 90 70 80 90 Position (*'2ThetaI
Figure 2: X-ray powder diffractogram of the functionalized superparamagnetic nanoparticles
DTA/(uV/mg) texo
100 200 300 400 500 600 700 800 900 Temperature / °C
Figure 3: TGA graph of the immobilized bidentate iminopyridine ligand FT-IR spectra of both the unfunctionalized and
functionalized magnetic nanoparticles showed the presence of an Fe-O sfretching vibration at approximately 590 cm"', an 0-H stretching vibration due to physisorbed water and potentially surface hydroxyls near 3400 cm'', arid an 0-H deformation vibration near 1630 cm'', respectively [8]. The significant features observed for the amine- fimctionalized nanoparticles, the immobilized Schiff base, and the immobilized palladium catalyst is the appearance of the peaks near 1015 cm'' for Si-0 stretching. For the immobihzed ligand and the catalyst, there were also the appearance of the peaks near 2950 cm'due to the -CH2 and aromatic C-H stretching vibrations, and the presence of the imine C=N sfretching vibration near 1600 cm'' which was overlapped with the 0-H deformation vibration (Figure 4) [8]. However, the FT-IR spectra exhibited little meaningfiil data due to the low
loading of the ligand and the palladium complex on the magnetic nanoparticles.
The immobilized palladium complex catalyst was assessed for its activity initially in the Suzuki reaction between iodobenzene and phenylboronic acid to form biphenyl as the principal product (scheme 2). As DMF is normally the solvent of choice for cross-coupling reactions [11], it was decided to carry out the Suzuki reaction in DMF at 120''C, using 0.5 mol% of the catalyst. It is generally accepted that a base is obviously necessary to accelerate the transmetallation step in the catalytic cycle of the Suzuki reaction [13]. The most commonly used base in the Suzuki reaction is Na2C03 or K2C03, but sfronger bases such as NaOH, K3PO4 and Ba(0H)2 were previously reported to give better results in some cases [14]. In this research, however, the reaction using K2CO3 afforded the coupling product in a lower conversion
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compared to the reaction using K)P04 as a base (Figure 5). After 7 h, a conversion of 86% was observed for the case of K2CO3, while the reaction using K3PO4 proceeded with up to 99% conversion being achieved under the same condition. Organic bases such as tnethyl amine and piperidme exhibited
The Suzuki reaction using palladium complex...
less reactivity compared to K3PO4. Indeed, Styring and co-workers also reported similar effects of bases in the Suzuki reaction, where the combination of DMF as the solvent and K3PO4 as the base exhibited dramatically better conversion than other bases [!5J.
3SX wx 3S0 21KD 2^ :ir. SOI iso] MOS UOO 130a 130C 900 BOD TOO too an «o WQvenumbercm-1
Figure 4: FT-IR spectra of the immobilized palladium catalyst B(0H)2
6
K3PO4 [Pd]a-^«
" X: I. Br, CI R: H, CH3, COCH3
Scheme 2: The Suzuki reaction of aryl halides and phenylboronic acid using the palladium complex catalyst immobilized on the magnetic nanoparticles
Figure 5: Effect of different bases on reaction conversions
100
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0 1 2 3 4 5 6 7 Time (h) Figure 6: Effect of temperature on reaction
conversions
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It was then decided to use K3PO4 as the base for the Suzuki reaction in further studies. The effect of the reaction temperature was then investigated varying from 80°C to 120°C, using 0.5 mol%
catalyst in DMF, and in the presence of K3PO4 as the base. Experimental results showed that the reaction rate increased slightly when increasing reaction temperature from 1 OO^C to 120°C. Increasing reaction temperature to above lOO^C was therefore unnecessary. As expected, decreasing the reaction
Phan Thanh Son Nam, et al.
temperature from lOO^C to SO^C resulted in a significant drop in the conversion of iodobenzene, from 99% to only 79% after 7 h (figure 6). Indeed, the temperature range of 90''C to 120°C has been the most commonly used for Suzuki transformation using different types of palladium catalysts [12]. The most effective reaction temperature for the Suzuki reaction using the immobilized palladium catalyst in this research was therefore in good agreement with the literature.
lUO 80 60 40 20 ni
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Figure 7: Effect of catalyst concentration on reaction conversions
1 2 3 4 5 Time (h)
Figure 8: Effect of substituent on reaction conversions
With these results in mind, we therefore studied the effect of catalyst concentration on reaction conversions, using DMF as the solvent and K3PO4 as the base at 100°C. As with previous reports, the higher the catalyst concentration was used, the higher the reaction rate was observed. Almost quantitative conversion of iodobenzene to biphenyl was achieved within 2 h at the palladium concentration of 0.5 mol% relative to iodobenzene.
Decreasing the catalyst concentration resulted in a drop in reaction rate, with 99% conversion being obtained after 4 h at palladium concenfrations of 0.3 mol% (figure 7). The reaction using 0.1 mol%
catalyst proceeded with slower rate, with a conversion of 80% being achieved after 7 h. The catalyst concenfrations used in this study were comparable to those of several previous reports covering different aspects of the Suzuki reaction, where the palladium concenfrations varied from less than 0.1 mol% to more than 1 mol%, depending on the nature of the catalysts as well as the subsfrates [11-13].
In order to investigate the effect of different substituents on reaction conversions, the study was then extended to the reaction of substituted iodobenzenes containing election-donating {i.e.
4-iodotoluene) and election-withdrawing (i.e.
4-iodoacetophenone) groups. It was observed that the reaction of 4-iodotoluene with phenylboronic acid proceeded with slower rate than the Suzuki reaction of iodobenzene. As expected, the reaction rate of the Suzuki between 4-iodoacetophenone and phenylboronic acid was higher than the case of iodobenzene (figure 8). This result indicated that the Suzuki reaction using the superparamagnetic catalyst was favored by elecfron-withdrawing groups on benzene nng, while election-donating groups slowed down the cross-coupling processes. It was also previously reported that the use of election- withdrawing ring substituents normally lead to enhanced reactivity in palladium-catalyzed cross- coupling reactions [15]. The effect of substituents on reaction conversions of iodobenzene derivatives observed in this research was therefore in good agreement with the literature.
Although the Suzuki reaction of iodoarene derivatives with phenylboronic acid is successful in most cases, several efforts have been devoted to the investigation on the cross-coupling of bromoarene and chloroarene [11, 12]. The reason for this frendis that iodoarene derivatives are normally significantly more expensive than bromoarenes, while chloroarenes require lowest cost and therefore they are the most desirable starting inaterials. However,
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chloroarenes are unreactive in most cases, though the Suzuki reactions of activated chloroarene (i.e.
containing strong electron-withdrawing groups) are usually successful by using special catalyst systems [11]. We therefore decided to investigate the Suzuki reaction of bromobenzene and chlorobenzene with phenylbomic acid, respectively, using the superparamagnetic catalyst. As expected, it was
The Suzuki reaction using palladium complex...
observed that the Suzuki iwction of bromobenzene proceeded significantly slower compared with the case of iodobenzene, with a conversion of 88%
being observed after 7 h. The Suzuki reaction of chlorobenzene proceeded with difficulty, though the reaction still afforded a conversion of over 50% after 7 h (figure 9).
100
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_ 1 0 0
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Run Figure 9: Effect of halide on reaction
conversions
An important point concemmg the use of a heterogeneous catalyst is its Kfetime. In the best case the catalyst can be recovered and reused before it eventually deactivates completdy. At the same time, the catalyst recovery can also reduce the environmental pollution caused by heavy metals used in the catalyst system [16]. The immobilized palladium catalyst was therefore investigated for recoverability and reusability in the Suzuki reaction of iodobenzene and phenylboronic acid. After the reaction, an external magnetic field was applied on the outer surface of the glass reaction vessel containing the modified magnetic nanoparticles using a small permanent magnet. The reaction solution was then easily removed from the reaction vessel by decantation while the external magnet held the magnetic nanoparticles stationary inside the vessel [3]. The magnetic catalyst was washed with acetone and «-hexane to remove any physisorbed reagents, dried under vacuum at room temperature overnight, and reused in further reactions imder identical conditions to the first run. It was observed that the immobilized palladium catalyst could be reused several times without significant degradation in activity (figure 10).
4. CONCLUSIONS
In summary, palladium complex immobilized on cobalt superparamagnetic spinel ferrite (CoFe204)
Figure 10: Catalyst recycling studies
nanoparticles was synthesized, and characterized by X-ray powder diffraction (XRD), ti^smission elecfron microscope (TEM), thermogravimetnc analysis (TGA), Fourier ti^nsform infi^red (FT-IR), and elemental analysis (EA). The immobilized palladium complex was used as an efficient catalyst for the Suzuki reaction of iodobenzene and phenylboronic acid to form biphenyl as the principal product. Recovery of catalyst was facilely achieved by simple magnetic decantation. The immobilized palladium complex catalyst could be reused several times without significant degradation in catalytic activity. Our results here demonstrate the feasibiliV of applying magnetic nanoparticles as catalyst supports for immobilizing homogeneous catalysts.
The unique properties of the particles such as nanometer-sized, magnetic, and facilely fimctionalized via silane chemistry, offer potential advantages over conventional catalyst support materials, and would be interested to the chemical industry. Current research in our laboratory has been directed to the design and immobilization of several catalysts on magnetic nanoparticles for a wide rang(
of organic transformations.
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Corresponding author. Phan Thanh Son Nam
Ho Chi Minh City University of Technology, Viemam 268 Ly Thuong Kiel, 10 District, Ho Chi Minh City Email: [email protected] / [email protected]
