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PAPER

Cite this: DOI: 10.1039/c3gc42311e

Received 9th November 2013,Accepted 29th January 2014

DOI: 10.1039/c3gc42311e

www.rsc.org/greenchem

A highly water-dispersible/magnetically separablepalladium catalyst based on a Fe3O4@SiO2

anchored TEG-imidazolium ionic liquid for theSuzuki–Miyaura coupling reaction in water†

Babak Karimi,*a Fariborz Mansouria and Hojatollah Valib

A novel ionic liquid functionalized magnetic nanoparticle was prepared by anchoring an imidazolium

ionic liquid bearing triethylene glycol moieties on the surface of silica-coated iron oxide nanoparticles.

The material proved to be an effective host for the immobilization of a Pd catalyst through a subsequent

simple ion-exchange process giving a highly water dispersible, active and yet magnetically recoverable Pd

catalyst (Mag-IL-Pd) in the Suzuki–Miyaura coupling reaction in water. The as-prepared catalyst displayed

remarkable activity toward challenging substrates such as heteroaryl halides and ortho-substituted aryl

halides as well as aryl chlorides using very low Pd loading in excellent yields and demonstrating high TONs.

Since the catalyst exhibited extremely low solubility in organic solvent, the recovered aqueous phase con-

taining the catalyst can be simply and efficiently used in ten consecutive runs without significant decrease

in activity and at the end of the process can be easily separated from the aqueous phase by applying an

external magnetic field. This novel double-separation strategy with negligible leaching makes Mag-IL-Pd

an eco-friendly and economical catalyst to perform this transformation.

Introduction

Palladium-catalyzed cross-coupling of aryl halides and triflateswith arylboronic acids, universally called the “Suzuki–Miyaurareaction”, is one of the most important and versatile trans-formations in biaryl synthesis from both academic and indus-trial points of view.1 This is because the resulting biaryls aremultipurpose building blocks in the field of liquid crystals,pharmaceuticals, conducting polymers, herbicides, naturalproducts, functional materials and ligands for catalysis.2 Thegreat significance of the palladium-catalyzed Suzuki–Miyaurareaction has been documented by awarding the 2010 NobelPrize in Chemistry to Professor Akira Suzuki.3 Whereas theSuzuki reaction has been conventionally performed usinghomogeneous palladium catalysts in the presence of phos-phine ligands4 and N-heterocyclic carbenes5 or palladacyclecomplexes,6 they suffer from some drawbacks such as catalystseparation from the reaction mixture, catalyst recycling, high

catalyst loading, deactivation of catalyst through the agglo-meration of Pd particles, and product contamination which is aserious problem in the pharmaceutical industry. Consideringthis intrinsic limitation of homogeneous catalysts, in additionto environmental and economical (Pd is an expensive metal)concerns, many attempts have been directed toward the devel-opment of new strategies to immobilize the homogeneous Pdcatalysts onto various solid supports in the form of eithermetal complex or metal nanoparticles in order to improvetheir recovery and recycling properties.7 In this context,numerous solid supports such as mesoporous and amorphoussilica,8 polymers,9 zeolites,10 metal oxides11 and carbonmaterial12 have been adopted for heterogeneous Pd-catalyzedSuzuki reaction. The key factor of these protocols is decreasingthe energy and time used for achieving chemical transform-ations and separations.

In recent decades, magnetic nanoparticles that have beenstudied extensively for various biological and medical appli-cations13 have emerged as smart and promising supports forimmobilization of catalysts because magnetic-supported cata-lysts can be easily separated from the reaction medium usingan external permanent magnet, which provides a simple separ-ation of the catalyst without the need for filtration, centrifu-gation or other tedious workup processes.14 Notably, thisseparation technique has great significance for nano-sizedcatalyst supports in which the conventional filtration method

†Electronic supplementary information (ESI) available: Characterization data forthe catalyst and Suzuki products. See DOI: 10.1039/c3gc42311e

aDepartment of Chemistry, Institute for Advanced Studies in Basic Sciences (IASBS),

P.O. Box 45137-66731, Gava Zang, Zanjan, Iran. E-mail: karimi@iasbs.ac.ir;

Fax: (+98)-241-4214949bAnatomy and Cell Biology and Facility for Electron Microscopy Research,

McGill University, 3450 University St, Montreal, Quebec H3A 2A7, Canada

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results in the loss of catalyst particles and product contami-nations. Besides the easy separation, an interesting feature ofmagnetic nanoparticles is their convenient surface modifi-cation that provides a wide range of magnetic-functionalizedcatalysts showing identical and sometimes even higher activitythan their homogeneous counterparts in organic transform-ations. In recent years, magnetic nanoparticles as catalysts orcatalyst supports have been extensively used in a variety ofimportant organic reactions including C–C couplings,15 C–Xcouplings,16 reductions,17 oxidations18 and multicomponentreactions19 with a high level of activity. Despite the consider-able success achieved in the field of magnetically separablenanocatalysts, the application of these systems in water as agreen solvent has still remained a great challenge and has notbeen addressed properly, especially for Suzuki reaction inwhich water-soluble boronic acids fall off to the homocouplingreaction or hydrolytic deboronation as side reactions. Thanksto their nano-metric sizes, although magnetic nanoparticlesare well-dispersed in many solvents, they cannot provide a suit-able exposure of their catalytically active centers in the proxi-mity of organic substrates when water is used as the reactionmedium. To the best of our knowledge, there are only a fewreports on the application of magnetically separable Pd-cata-lyzed Suzuki reaction in neat water;20 however, many of themwere necessarily conducted in the presence of large amountsof ionic liquids or n-Bu4NBr (TBAB) as a phase transferreagent. Therefore, it seems that there is still much room tomake novel magnetically recyclable catalysts displayingimproved catalytic performance in pure water. Very recently,we have introduced a novel water-soluble N-heterocyclic palla-dium polymer with triethylene glycol (TEG) legs as a highlyrecyclable catalyst in the Suzuki–Miyaura coupling of arylhalides in neat water.21 Furthermore, it was found that thiswater-soluble catalyst exhibited much better catalytic activityand yet superior reusability in comparison with its previouslydeveloped water-insoluble analog.22 However, despite theexcellent reusability, recycling of this catalyst system requires atime consuming dialysis technique. Therefore, we were verymuch interested in designing and preparing a novel catalystsystem by emphasizing controlling its high solubility (dis-persion) in water while avoiding time consuming recyclingstages. We hypothesized that incorporating TEG functional-ities into a magnetically-based supported catalyst should notonly result in superior water-dispersible character but also itmay lead to higher catalytic performance and faster recycling.Taking cues from these criteria, herein we wish to report onthe preparation of a highly water-dispersible/magneticallyseparable Pd catalyst for Suzuki reaction based on a TEG sub-stituted imidazolium ionic liquid. To do this, we first preparedrelatively monodispersed Fe3O4@SiO2 core–shell magneticnanoparticles using a reverse-micelle template protocol accord-ing to a reported procedure with slight modifications.23 Theresulting silica-coated iron oxide magnetic nanoparticles werethen modified with TEG-functionalized IL and subsequentsimple ion exchange with PdCl4

2− provides the correspondingmagnetic catalyst which is denoted Mag-IL-Pd. In this system,

TEG tags of functionalized magnetic nanoparticles result in ahydrophilic character for the catalyst and provide superior dis-persion of catalyst particles in water. In addition, supported ILnot only stabilizes the Pd nanoparticles during the reactionbut also creates an ionic liquid based nano-environmenton the surface of magnetic nanoparticles to enhance theirinteraction with aryl halides, thus improving the reactionefficiency.

ExperimentalGeneral information

All chemicals and solvents were purchased from commercialsuppliers. Liquid NMR was obtained on a DMX-250 andDMX-400 MHz Bruker Avance instrument using CDCl3 as asolvent and TMS as an internal standard. Thermogravimetricanalysis was conducted from room temperature to 800 °C inan oxygen flow using a NETZSCH STA 409 PC/PG instrument.The structure of the prepared materials were observed bytransmission electron microscopy (Philips CM-200 and TitanKrios TEM) and were verified further by the nitrogen sorptionanalysis (Belsorp, BELMAX, Japan). The content of palladiumin the catalyst was determined using atomic absorption spec-trometry (Varian). FT-IR spectra were recorded using a BrukerVector 22 instrument after mixing the samples with KBr. Themagnetic properties of the prepared materials were measuredusing a homemade vibrating sample magnetometer (Meghna-tis Daghigh Kavir Company, Iran) at room temperature from−10 000 to +10 000 Oe. Gas chromatography analyses were per-formed on a Varian CP-3800 using a flame ionization detector(FID).

Synthesis of silica-coated Fe3O4 magnetic nanoparticles

Magnetite/silica core/shell nanoparticles were prepared byusing a micro-emulsion method in the presence of sodiumdodecylbenzenesulfonate as a surfactant. 1.75 g of sodiumdodecylbenzenesulfonate was dissolved in 15 mL of xylene bysonication for 30 min to prepare a microemulsion. On theother hand, an aqueous solution of iron salts was prepared bydissolving 0.75 mmol of FeCl2·4H2O and 1.5 mmol of Fe-(NO3)3·9H2O in 0.75 mL deionized water (DW). Under vigorousstirring the iron salts solution was added to the microemulsionand the mixture was stirred at room temperature for about12 h. After 12 h, the reverse-micelle solution was graduallyheated to 90 °C under a continuous flow of argon gas for 1 h.When the temperature reached to 90 °C, 1 mL of hydrazine(34 wt% aqueous solution) was injected dropwise into the solu-tion (formation of a black precipitate was observed) and themixture was stirred at 90 °C for 3 h under a flow of argon gas(5 mL min−1). After 3 h, the temperature was cooled down to40 °C within an hour and at this temperature 2 mL of TEOSwas added dropwise into the mixture under vigorous stirring.Then the solution was stirred for another 15 h to achieveformation of silica shells on the surface of the magnetitenanoparticles. Finally, the prepared Fe3O4@SiO2 magnetic

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nanoparticles were collected with the aid of a magnetic barand repeatedly washed with DW and EtOH to completeremoval of the surfactant and then dried overnight in an ovenat 80 °C. At the end, a reddish brown powder was obtained.

Synthesis of 1-(2-(2-methoxyethoxy)ethoxy)-2-bromoethane(TEG-Br)

TEG-Br was prepared according to the Apple reaction.24 In atypical experiment, triethylene glycol monomethyl ether(1.64 g, 10 mmol) was added to 50 mL dry CH2Cl2 in a welldried flask under an argon atmosphere. Then, 4 g (12 mmol)carbon tetrabromide was added to this solution under stirringand the reaction mixture was cooled to 0 °C. At 0 °C, a solutionof 3.9 g (15 mmol) triphenyl phosphine in 10 mL dichloro-methane was added dropwise to the reaction mixture. Afterstirring for 5 h the solvent from the reaction mixture was eva-porated out. 25 mL diethyl ether was added to the reactionmixture, kept for 5 min and filtered. The same process(addition of ether and filtration) was repeated thrice. Theresidue was subjected to flash column chromatography toobtain the pure TEG-Br in 84% yield. The spectral data forTEG-Br are as follows: 1H-NMR (250 MHz, CDCl3, 25 °C, TMS):δ = 3.34 (s, 3H), 3.43 (t, 2H, J = 6.4 Hz), 3.50–3.52 (m, 2H),3.60–3.66 (m, 6H), 3.77 (t, 2H, J = 6.4 Hz); 13C-NMR (250 MHz,CDCl3, 25 °C, TMS): δ = 30.32, 59.00, 70.48, 70.56, 70.57, 71.15,71.89 (C7H15BrO3).

Synthesis of 1-(2-(2-(2-methoxyethoxy)ethoxy)ethyl)-3-(3-(trimethoxysilyl)propyl)-1H-imidazol-3-ium chloride (TEG-IL)

For the preparation of TEG-IL all reactions were performedunder a dry argon atmosphere and all solvents were dried inthe presence of suitable reagents and freshly distilled prior touse. In a typical experiment, a suspension of sodium imidazo-lide in dry THF was prepared from the reaction between driedimidazole (20 mmol) and NaH 95% (0.77 g) in a dried twoneck flask containing dry THF (40 mL) under an argon atmos-phere. Then, 20 mmol TEG-Br was added to the solution ofsodium imidazolide and the mixture was heated to reflux for24 h and then cooled to room temperature. After removal ofTHF under reduced pressure, a solution containing 20 mmol(4.4 g) 3-chloropropyltrimethoxysilane (98%, Merck) in 40 mLabsolute toluene was added to the mixture. Again, the reactionmixture was heated to refluxing temperature of toluene for48 h, and after cooling to room temperature, maintained over-night for phase separation. Then, the upper phase of themixture (toluene) was removed with a pipette. Subsequently,the residual oil was washed with absolute toluene (3 times)and CH2Cl2 to remove unreacted substrates and precipitatedNaCl from the reaction mixture. Finally, after removing thesolvent under vacuum at room temperature, a highly viscoseyellow IL was obtained (TEG-IL) in 89% yield. 1H NMR(250 MHz, CDCl3, 25 °C, TMS): δ = 10.10 (s, 1H), 7.64–7.77(m, 2H), 4.53–4.59 (m, 2H), 4.26–4.35 (m, 2H), 3.87 (m, 2H),3.35–3.65 (m, 17H), 3.34 (s, 3H), 1.96–2.13 (m, 2H),0.60–0.68 (m, 2H).

Immobilization of TEG-IL onto silica-coated Fe3O4 magneticnanoparticles

1 g of Fe3O4@SiO2 magnetic nanoparticles was dispersed in50 mL toluene by sonication for 30 min, and then a solutioncontaining 1.65 g TEG-IL in 20 mL toluene was added to it.The mixture was stirred in refluxing temperature of toluene for24 h under an argon atmosphere. After 24 h, the nanoparticleswere separated using an external magnet and washed repeat-edly with MeOH, EtOH and CH2Cl2. Finally, drying under anair oven for 24 h at 80 °C afforded the Fe3O4@SiO2 supportedIL (Mag-IL). The loading of IL was determined by thermalanalysis.

Preparation of Mag-IL-Pd catalyst

Synthesis of Mag-IL-Pd catalyst was carried out via a simpleion-exchange of chloride with Pd salt. In a typical procedure,500 mg of Mag-IL nanoparticles was sonicated in 20 mLacetone (Merck) for 30 min. 15.1 mg of Na2PdCl4·3H2O wasdissolved in 3 mL acetone and added to Mag-IL nanoparticlesunder stirring under an Ar atmosphere (dropwise and for10 min). The mixture was stirred for 12 h at room temperature,and then nanoparticles were collected with a magnet andwashed with acetone and EtOH and finally dried at 80 °C for5 h. The content of Pd was estimated to be 0.084 mmol g−1

based on AAS.

General procedure for the Suzuki coupling reaction usingMag-IL-Pd catalyst

The Suzuki reaction was performed in a 10 mL round-bottomed flask, aryl halide (0.5 mmol), arylboronic acid(0.6 mmol), K2CO3 (0.75 mmol), Mag-IL-Pd (0.025–0.5 mol%with respect to aryl halide) and water (3 mL) were charged andstirred at r.t. to 120 °C under an argon atmosphere. The reac-tion progress was monitored by GC. After completion of thereaction, the mixture was allowed to cool to room temperature.The product was extracted with addition of 3 mL n-hexanewhile the Mag-IL-Pd catalyst was remaining in aqueous phase.The organic layer was dried with MgSO4 and the solvent wasthen removed under reduced pressure to get the crudeproduct. Pure products were obtained after recrystallization orby isolation of the residue by column chromatography onsilica using the n-hexane and ethyl acetate mixture asan eluent. All the products were characterized by 1H NMR and13C NMR.

Procedure for the reusability test of Mag-IL-Pd catalyst

4-Bromoanisole (1 mmol), phenylboronic acid (1.2 mmol),K2CO3 (1.5 mmol) and the Mag-IL-Pd catalyst (0.025 mol% Pd)were added to 3 mL distilled water and heated under an argonatmosphere in an oil bath at 80 °C. After completion of thereaction, the product was extracted with hexane and theaqueous phase containing the Mag-IL-Pd catalyst was loadedwith the reactants and base for the next run. The catalyst inwater could be reused for 10 subsequent runs without any lossof activity and no need for washing and purification of the

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catalyst after each run. After 10 runs reusing the catalyst bythis technique, the catalyst was separated from water by usingan external magnet.

Results and discussion

To prepare the catalyst, initially magnetite/silica core/shellnanoparticles were prepared using a Fe+2/Fe+3 mixture via amicro-emulsion method in the presence of sodium dodecyl-benzenesulfonate (SDBS) as a surfactant (Scheme 1). In thismethod, SDBS provides an emulsion in xylene/H2O solventwhich is a platform for nucleation and limited growth andsilica-coating of iron oxide magnetic nanoparticles providingprecise control of the size, shape and core–shell structure ofnanoparticles.

Then, the imidazolium based ionic liquid (TEG-IL) that hasTEG functionality on one side and a trimethoxysilyl moiety onthe other side was synthesized and subsequently grafted to thesurface of silica-coated iron oxide magnetic nanoparticles inrefluxing toluene for 24 h. Finally, treatment of IL-function-alized magnetic nanoparticles with Na2PdCl4·3H2O in acetoneat room temperature for 12 h provides the Mag-IL-Pd catalystthrough a simple ion-exchange of Cl− with PdCl4

2−

(Scheme 1).The structure of prepared materials was characterized by

various techniques including TEM, TGA, FT-IR, VSM, AAS andN2 adsorption–desorption analysis. FT-IR spectra ofFe3O4@SiO2 magnetic nanoparticles showed characterizationpeaks for Fe–O–Fe, Si–O–Si and O–H groups (Fig. 1a), whichindicates the presence of iron oxide and silica phases. For IL-functionalized Fe3O4@SiO2 (Mag-IL) and Mag-IL-Pd catalysts,additional peaks were observed for CvC, CvN, aromatic C–Hand aliphatic C–H, which confirms successful immobilization

of TEG-IL on the surface of magnetic nanoparticles (Fig. 1band 1c). Also, the presence of TEG-IL on the surface of magneticnanoparticles was proved by TG analysis. While the TG curveof Fe3O4@SiO2 has no weight loss after 300 °C, for Mag-IL andMag-IL-Pd catalysts a sharp decrease in weight was observed at300 °C–700 °C (Fig. 2).

In addition, TG analysis indicated that the prepared Mag-IL-Pd catalyst has high thermal stability and negligible ILleaching up to about 300 °C. Based on TG analysis the amountof loaded IL on the surface of magnetic nanoparticles was1.3 mmol IL g−1. The prepared material showed typically typeIV N2 adsorption–desorption isotherm with H3 hysteresisloops according to the IUPAC classification (Fig. S1–S3†). Also,the Brunauer–Emmett–Teller (BET) surface areas forFe3O4@SiO2, Mag-IL and Mag-IL-Pd catalysts were 312, 47.5and 35 m2 g−1, respectively. The large surface area ofFe3O4@SiO2 magnetic nanoparticles was related to thesmall particle size of this material as could be observedin TEM images. After immobilization of IL the surface area

Scheme 1 General route for the synthesis of Fe3O4@SiO2 magneticnanoparticles and Mag-IL-Pd catalyst.

Fig. 1 FT-IR spectra for Fe3O4@SiO2 (a), Mag-IL (b) and Mag-IL-Pdcatalysts (c).

Fig. 2 TG analysis of Fe3O4@SiO2 (a), Mag-IL (b), Mag-IL-Pd catalyst(c) and recycled Mag-IL-Pd catalyst after 10 cycles (d).

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dramatically decreased to 47.5 m2 g−1 because the IL occupiedthe surface of nanoparticles and filled completely the inter-particles spaces. Furthermore, a small decrease in the surfacearea from Mag-IL to Mag-IL-Pd catalyst (47.5 to 35 m2 g−1) con-firmed that the ion-exchange of Cl− with PdCl4

2− was carriedout effectively. In addition, AAS analysis indicates the presenceof Pd in the catalyst and the content of Pd was estimated to be0.084 mmol Pd g−1. The structure of the prepared materialswas further verified using transmission electron microscopy(TEM) images. As can be clearly seen, Fe3O4@SiO2 nanoparti-cles have a well-defined core–shell structure with relativelynarrow size distribution (Fig. 3a). A magnified TEM image of asingle Fe3O4@SiO2 nanoparticle indicated that the diameter ofthe iron oxide core and the thickness of silica shell are about10 and 5–7 nm, respectively (Fig. 3b). After functionalizationwith TEG-IL and the subsequent ion-exchange process thenanoparticles kept their core–shell structure as shown in theedge of their TEM images (Fig. 3c and 3d).

All of these observations confirmed the successful immobil-ization of TEG-IL onto the Fe3O4@SiO2 nanoparticles. It is alsoworth mentioning that although the TEM image of the Mag-IL-Pd catalyst showed a little agglomeration of magnetic nano-particles, no aggregation of Pd nanoclusters had occurred orwas detected in the sample either before or after catalysis (videinfra). As shown in Fig. 4a the nanoparticles provide a homo-geneous and stable suspension after dispersing in water dueto the presence of a hydrophilic ionic liquid in their surfaces.Interestingly, this suspension is even stable for several days atroom temperature without any precipitation. To further shedlight on this issue, we prepared a sample of our catalyst inwater according to its actual concentration in the describedprotocol. We then performed a dynamic light scattering (DLS)

analysis in order to measure the zeta potential and possiblyquantify the stability of colloidal dispersion of our catalyst inwater. It is generally accepted that zeta potentials greater than30 mV (or more negative than −30 mV for negatively chargedparticles) are generally considered as a value to prove the stabi-lity of colloidal dispersion.25 We have obtained an initial valueof 45.9 mV at 25 °C after mixing of the colloidal solution for5 min before analysis (ESI, Fig. S8†). This value certainly high-lights the notion that our catalyst affords a stable colloidalmixture in water. The formation of stable colloidal dispersionof the present catalyst system was further confirmed by per-forming zeta potential analysis of the above mixture after twodays which gave a value of 41.1 mV (ESI, Fig. S9†). This clearlydemonstrates that our catalyst system provided a consistentlystable dispersion in pure water.

The magnetic properties of the Fe3O4@SiO2, Mag-IL andMag-IL-Pd catalysts were investigated using a vibrating samplemagnetometer at room temperature. As illustrated in Fig. 5,the magnetization curves of the prepared materials exhibit nohysteresis loop which demonstrates its superparamagneticcharacteristics. Moreover, the strong magnetic properties ofthe prepared catalyst were revealed by complete and easy

Fig. 3 TEM images of Fe3O4@SiO2 (a, b), Mag-IL (c) and Mag-IL-Pdcatalyst (d).

Fig. 4 Good dispersion and solubility of Mag-IL-Pd catalyst in water (a),distribution of Mag-IL-Pd catalyst in a biphasic water/n-hexanesystem (b), and easy separation of Mag-IL-Pd catalyst using an externalmagnet (c).

Fig. 5 The VSM curves of Fe3O4@SiO2 (A), Mag-IL (B), Mag-IL-Pd cata-lysts (C) using a KAVIR magnetometer.

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attraction using an external magnet (Fig. 4c). The saturationmagnetization of the Fe3O4@SiO2 magnetic nanoparticles wasfound to be 8.64 emu g−1 as measured using a homemadevibrating sample magnetometer (Meghnatis Daghigh KavirCompany, Iran). As expected, after functionalization withTEG-IL and subsequent ion-exchange with PdCl4

2− it wasreduced to 5.74 emu g−1 and 5.58 emu g−1, respectively.

It is believed that the immobilization of TEG-IL on thesurface of magnetic nanoparticles could provide a good distri-bution of catalytically active Pd species on the support andstabilizes the Pd nanoparticles during the chemical reaction,and also protects the Pd nanoparticles from aggregation. Inaddition, the hydrophilic character of supported IL provides ameans of good dispersion of magnetic nanoparticles inaqueous medium for developing this system as a semi-homo-geneous Pd catalyst for organic reactions in neat water as asafe and green solvent.

The catalytic activity of the prepared Mag-IL-Pd catalyst wasevaluated in Suzuki cross-coupling reaction in water. Althoughthe boronic acids are water-soluble, as mentioned above,usually the efficiency of Suzuki reaction in neat water is ham-pered by side reactions of boronic acids themselves such ashomo-coupling reaction or hydrolytic deboronation. Inaddition, the major drawback associated with magnetic sup-ported Pd catalysts when water is the reaction solvent is thepoor interaction of active Pd with organic substrates. To over-come this drawback, typically a mixture of organic solventsand water is used as the reaction medium for magnetic sup-ported Pd-catalyzed Suzuki reaction.26 It is supposed that thismagnetic Pd catalyst with its functionalized hydrophilic ionicliquid provides a good interaction between the water-solubleboronic acids and hydrophobic aryl halides and improves theefficiency of Suzuki reaction in neat water.

The reaction of 4-bromoanisole and phenyl boronic acidwas used for initial studies and after optimization of time andtemperature (Table S1†) the best result was obtained after 12 hat 60 °C using 0.1 mol% of Mag-IL-Pd catalyst and K2CO3 as abase. We next investigated the impact of a number of organicas well as inorganic bases under similar reaction conditions ina more systematic manner (Table 1). Under the described con-ditions at 60 °C in neat water the best catalytic performancewas obtained using K2CO3 (Table 1, entry 2).

Having optimized the reaction conditions, we next pro-ceeded to examine the scope and limitation of this Mag-IL-Pdcatalyst in the Suzuki cross-coupling reaction of various typesof iodo-, bromo-, and chloroaryl derivatives and arylboronicacids (Table 2). In general, all of the coupling products wereobtained in good to excellent yields with high TONs using aslow as 0.025–0.5 mol% of Mag-IL-Pd (Pd mol%) and K2CO3 asa base during the 4–24 h in neat water. As expected, aryliodides were rapidly converted to the respective products usingonly 0.025 mol% of catalyst for 4–6 h at 60 °C (Table 2, entries1–6). Moreover, bromobenzene and aryl bromides bearing theelectron-withdrawing group at the para position were reactedwith boronic acids in high yields for 6–7.5 h again using0.025 mol% of the catalyst (Table 2, entries 8–13). It is worth

mentioning that the Suzuki reactions of iodo and bromoarenes with phenyl boronic acid effectively proceeded evenat room temperature with good yields but longer reactiontimes were required to allow satisfactory conversion (Table 2,entries 7 and 11). Also, electron-rich aryl bromides such asthose bearing OMe, OH or NH2 groups at the para or metaposition, which are classified as highly challenging couplingpartners, selectively furnished the corresponding productsafter 12 h at 80 °C using 0.025 or 0.05 mol% of the catalyst(Table 2, entries 14–18). Remarkably, no product arising fromthe homocoupling of aryl boronic acids or cross-coupling withOH and NH2 functional groups were detected in this sub-strates. Then, we focused our investigation on the Suzuki reac-tion of sterically hindered aryl bromides and boronic acidsand interestingly it was found that the system was effective forthe synthesis of mono or di-ortho substituted biaryl com-pounds using as low as 0.025–0.5 mol% of the catalyst at60–80 °C as exemplified by entries 19–24 in Table 2.

Since heteroaromatic compounds are very important frag-ments in most of the drugs, the cross-coupling reactions ofthese compounds are in high demand, but deactivation ofthe transition metal catalysts with the heteroatoms such asnitrogen or sulfur usually hampers the efficiency of these typesof coupling reactions.27 Thus, we examined the efficiency ofour system for Suzuki reaction of heteroaryl bromides oriodides with boronic acids. As can be seen in entries 25–30 ofTable 2, using 0.05 or 0.1 mol% of the Mag-IL-Pd catalyst at60 °C or 80 °C was quite effective for cross-coupling reaction ofheteroaryl compounds with boronic acids in excellent yields.However, when heteroatom-containing boronic acid was usedas a coupling partner, moderate yields were obtained (Table 2,entry 31). Encouraged by these results, we next turned ourattention to the use of more challenging chloroarenes for thisreaction. Our preliminary investigations showed that appli-cation of the optimized coupling protocol for the Suzuki reac-tion of aryl chlorides in water at 80 °C was inefficient and gave

Table 1 Optimization of the Suzuki coupling reaction of 4-bromoani-sole and phenyl boronic acid in the presence of Mag-IL-Pd

Entry T (°C) t (h) Base Yielda (%)

1 60 6 K2CO3 522 60 12 K2CO3 963 60 12 Cs2CO3 634 60 12 tBuOK 525 60 12 Na2CO3 636 60 12 NaOAc·3H2O 417 60 12 Et3N 518 60 12 K3PO4 39

a Reaction conditions: 4-bromoanisole (0.5 mmol), phenyl boronic acid(0.6 mmol), base (0.75 mmol), and H2O (3 mL), under an argonatmosphere, GC yield.

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a considerable amount of the products arising from the homo-coupling of the employed boronic acid. However, we foundthat by changing the solvent to t-BuOH–H2O (1 : 1), andincreasing the catalyst loading to 0.2 mol% at 80 °C, the coup-ling reaction of aryl chlorides bearing electron-withdrawinggroups went to completion within 24 h, affording the corres-ponding cross-coupled products in moderated yields (Table 2,entry 32).We found that by changing the reaction solvent toH2O–DMF (1 : 1) and increasing the reaction temperature to120 °C resulted in excellent yields of the Suzuki coupling pro-ducts (Table 2, entries 33 and 34), but due to the environ-mental impact of DMF we decided not to continue thisprotocol for the rest of the aryl chlorides in the present form.

The recycling and recovery of the supported catalysts are avery important issue from both the practical and environmentpoints of view. Since the primary target of this study was todevelop a highly dispersible as well as easily recoverable cata-lyst, we further explored the reusability of the catalyst in themodel reaction between 4-bromoanisole and phenylboronicacid in the presence of 0.025 mol% of catalyst at 80 °C for

12 h. But our separation technique was different from otherconventional magnetic supported catalysts. The presence of ahydrophilic ionic liquid in the surface of magnetic nanoparti-cles provides a means of complete dispersion of the catalystinto the aqueous phase, and as far as we examined, the catalysthad no affinity to the organic phase. Considering this property,after the first use of the catalyst in the above mentionedSuzuki reaction, the product was simply extracted withn-hexane while the Mag-IL-Pd catalyst remained in the aqueousphase (Fig. 4b). In the next stage, the aqueous phase contain-ing the Mag-IL-Pd catalyst was recharged with fresh couplingpartners and K2CO3

28 for the next run without any washingand purification of the catalyst. The results indicated that thissimple separation method could be repeated for 10 consecu-tive runs and the recovered aqueous phase containing theMag-IL-Pd catalyst showed remarkably constant catalyticactivity in all the 10 cycles (Fig. 6). Finally, the magnetic sup-ported catalyst was easily and completely separated from theaqueous phase using an external magnetic field (Fig. 4c).However, it is worth mentioning that the complete magnetic

Table 2 The Suzuki reaction of various aryl halides and boronic acids in the presence of Mag-IL-Pd catalyst

Entry Ar1 Ar2 X T (°C) t (h) x (mol%) Yielda (%) TON/TOF (h−1)

1 Ph Ph I 60 6 0.025 95 3800/633.32 Ph 4-Me–C6H4 I 60 6 0.025 >99 4000/666.73 4-NO2–C6H4 Ph I 60 4 0.025 99 3960/9904 4-NO2–C6H4 4-Me–C6H4 I 60 4 0.025 93 3720/9305 4-OMe–C6H4 Ph I 60 6 0.025 100 4000/666.76 4-OMe–C6H4 4-Me–C6H4 I 60 6 0.025 95 3800/633.37 Ph Ph I r.t. 24 0.025 72 2880/1208 Ph Ph Br 60 7.5 0.025 82 3280/437.39 Ph 4-Me–C6H4 Br 60 7.5 0.025 97 3880/517.310 4-CHO–C6H4 Ph Br 60 6 0.025 100 4000/666.711 4-CHO–C6H4 Ph Br r.t. 24 0.025 100 4000/166.712 4-CHO–C6H4 4-Me–C6H4 Br 60 6 0.025 100 4000/666.713 4-CO2H–C6H4 Ph Br 60 6 0.025 100 4000/666.714 4-OMe–C6H4 Ph Br 80 12 0.025 100 4000/333.315 4-OMe–C6H4 4-Me–C6H4 Br 80 12 0.025 90 3600/30016 4-OH–C6H4 Ph Br 80 12 0.05 100 2000/166.717 4-NH2–C6H4 Ph Br 80 12 0.05 97 1940/161.718 3-OH–C6H4 Ph Br 80 12 0.05 100 2000/166.719 2-CHO–C6H4 Ph Br 80 12 0.025 100 4000/333.320 4-CHO–C6H4 2-Me–C6H4 Br 60 8 0.025 100 4000/50021 2-CHO–C6H4 2-Me–C6H4 Br 80 12 0.1 100 1000/83.322 2-Me–C6H4 2-Me–C6H4 Br 80 15 0.1 95b 950/63.323 2-Et–C6H4 2-Me–C6H4 Br 80 15 0.1 89b 890/59.324 2,6-Me–C6H3 Ph Br 80 20 0.5 87b 174/8.725 2-Thiophenyl Ph I 60 12 0.05 >99 2000/166.726 5-Pyrimidinyl Ph Br 80 12 0.1 100 1000/83.327 2-Thiophenyl Ph Br 80 12 0.1 100 1000/83.328 3-Pyridyl Ph Br 80 15 0.05 97 1940/129.329 3-Thiophenyl Ph Br 80 15 0.1 100b 1000/66.730 2-Pyridyl Ph Br 80 15 0.1 100b 1000/66.731 Ph 3-Pyridyl Br 80 24 0.05 51 1020/42.532 4-NO2–C6H4 Ph Cl 80 24 0.2 37b,c 185/7.833 4-NO2–C6H4 Ph Cl 120 24 0.2 96b,d 480/2034 4-NO2–C6H4 4-Me–C6H4 Cl 120 24 0.2 85b,d 425/17.7

a Reaction conditions: aryl halide (0.5 mmol), boronic acid (0.6 mmol), K2CO3 (0.75 mmol), and 3 mL H2O, under an argon atmosphere, GCyields. b 0.5 eq. TBAB was added. cH2O–t-BuOH (1 : 1) was used as a solvent. dH2O–DMF (1 : 1) was used as a solvent.

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separation of this catalyst from the water consumes more time(after five minutes) in comparison to other magnetic materialsdue to its high water-dispersity. Moreover, owing to the highlywater-dispersible feature of the designed catalyst we developeda novel double-separation technique (strategy) for complete andeasy recycling of the catalyst for the first time. Since we alsorecycled n-hexane in each reaction cycle, we were able toperform all of the 10 consecutive runs by employing as low as50 mL of n-hexane for the extraction of coupling products,giving high TON of 39 760. It is also worth mentioning that theamount of leached Pd species into the aqueous phase in theselected runs of recycling test (runs 2, 10) and also into thefinal extracted n-hexane phase was negligible (less than detec-tion limit) as measured by ICP-AES analysis. The recycled cata-lyst after the 10th run was analyzed with ICP-AES, TEM andTGA. ICP-AES indicated no decrease in Pd content of the cata-lyst after 10 cycles. The Pd content of the catalyst after 10 runswas the same as the fresh catalyst (0.084 mmol Pd g−1). TEMimage of the recovered catalyst after the 10th reaction run alsoindicated that the Mag-IL-Pd catalyst maintained its core–shellstructure after using in 10 consecutive Suzuki reactions(Fig. 7). However, as can be seen it is very difficult (actuallyimpossible in this system) to discriminate between the mag-netic nanoparticles and the Pd nanoparticles possibly gener-ated in the catalyst matrix. Therefore, in order to assesswhether the Mag-IL-Pd catalyst system could be a source ofin situ generation of Pd nanoparticles in the course of reaction,the kinetics of the Suzuki reaction between bromobenzeneand phenylboronic acid in the presence of 0.025 mol% of Mag-IL-Pd have been investigated in the presence of an excess ofHg(0) (Pd–Hg; 1 : 400) following the condition demonstrated inTable 2, entry 8.22 This new control experiment led to aremarkable decrease in the catalytic activity upon addition ofHg(0), giving the respective biaryl in 18% conversion after 8 hat 60 °C, which is a clear indication that Mag-IL-Pd mightindeed be a source of generation of Pd nanoparticles (ESIFig. S10 and S11†). It is also worth noting that the existence ofan induction period of about 15 min can be also considered asa strong evidence for the generation of Pd nanocluster during

the reaction pathway (ESI Fig. S12†). Therefore, although wewere not capable to see any Pd nanoparticles in the TEMimages of the recovered Mag-IL-Pd, based on both theobserved induction period and also positive Hg poisoningexperiments, it would be impossible to exclude the generationof Pd nanoparticles.22 Therefore, we believe that the Pd nano-particles are most likely in sub-nanometer sizes and are highlydispersed on the magnetic support thus estimating their sizedistribution is technically impossible at this stage.

On the other hand, TG analysis of recovered catalystshowed that the amount of supported IL was slightly less thanthe fresh catalyst (Fig. 2d). The fact that Pd content of the finalmagnetically recycled catalyst has not reduced and that thecatalyst has operated in a homogeneous pathway (as expectedfrom its high dispersible behaviour in water), provides at leasta noticeable feature concerning the present catalyst system.We believe that in the present catalytic system while Mag-IL-Pdis acting as a semi-homogeneous or even as a reservoir forhighly active and soluble Pd species in water, the imidazoliumfunctionalities on the surface of magnetic nanoparticles mayalso create a nano-ionic liquid environment to continuouslyre-capture the palladium nanoparticles, thus resulting in sig-nificant Pd preservation at the end of the process.

Conclusions

We demonstrated the preparation of a novel ionic liquid func-tionalized magnetic nanoparticle by anchoring an imidazo-lium ionic liquid bearing triethylene glycol moieties on thesurface of silica-coated iron oxide nanoparticles as a newhighly water-dispersible/magnetically separable support for cata-lysis of organic reaction in aqueous medium. A simple ion

Fig. 6 Recycling of the Mag-IL-Pd catalyst in the Suzuki reaction of4-bromoanisole and phenylboronic acid.

Fig. 7 TEM image of the recycled catalyst after 10th reaction runs.

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exchange reaction of Cl− by PdCl42− provides a high perform-

ance magnetically separable Pd catalyst that has remarkableefficiency and reusability for the Suzuki coupling reaction of awide range of activated and deactivated aryl halides withboronic acids in water. Interestingly, the prepared catalystshowed improved activity in the Suzuki coupling reaction ofhighly challenging substrates such as heteroaryl- and ortho-substituted aryl halides using very low Pd loading, affordingthe corresponding cross-coupled products in excellent yieldsand TONs. It is believed that the presence of a hydrophilicionic liquid on the structure of the catalyst not only provides agood distribution of Pd nanoparticles on the surface of thesupport and its stability during the reaction but also generatesa stable dispersion (solubility) of the catalyst in water as thereaction medium and exposes the active Pd sites to substrateslike homogeneous systems. Since the catalyst exhibited extre-mely low solubility in organic solvent, the recovered aqueousphase containing the catalyst can be simply and efficientlyused in ten consecutive runs without a significant decrease inactivity and at the end of the process can be easily separatedfrom the aqueous phase by applying an external magneticfield. This novel double-separation strategy with negligibleleaching makes the Mag-IL-Pd an eco-friendly and economiccatalyst to perform this transformation. Altogether, the possi-bility to perform the reaction in water, and at the same timethe easy and rapid recycling protocol via the described double-separation technique, makes this protocol a valid candidatetowards the goal of green chemistry. We speculate that thiscatalyst system would open a new challenging area in thedesign of new types of water dispersible/magnetically separ-able catalyst systems in other important industrial as well assynthetic transformations.

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28 The amount of base needed in each run was estimated bypH analysis of the recovered aqueous reaction phase.

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