DOI: 10.1002/cplu.201300361

Cross-Linked Imidazolium Salts as Scavengers forPalladiumRoberto Buscemi, Francesco Giacalone, Santino Orecchio, and Michelangelo Gruttadauria*[a]

Introduction

Platinum group metal (PGM)-catalysed reactions are commonlyused in the manufacture of active pharmaceutical ingredients(APIs) and fine chemicals. The tolerated limits for the metalcontent are becoming increasingly more challenging. Concen-tration levels for many metal ions must be less than 5 ppm.[1]

In recent years, many palladium compounds have been de-veloped as catalysts for new synthetic transformations, such ascarbon–carbon and carbon–heteroatom coupling reactions(e.g. , by Buchwald–Hartwig, Heck, Kumada, Negishi, Nozaki–Hiyama, Sonogashira, Stille, Suzuki–Miyaura and Tsuji–Trost).[2]

Such wide applicability of palladium-based catalysts has al-lowed rapid access to a diverse range of compounds. On theother hand, they also present a problem in that palladium canoften be retained in the isolated product. The use of a support-ed palladium catalyst may avoid the presence of metal in thefinal product, but palladium may leach from the solid catalyst.Indeed, it has been ascertained that, in many cases, supported-palladium-catalysed reactions, such as Suzuki and Heck C�C,coupling are catalysed by leached palladium species.[3] Some-times, such leached palladium species can be recaptured bythe support to give a low-palladium-contaminated product.[4]

On the other hand, such a process is not always operativewhen a supported palladium catalyst is used and it is not pos-sible when a homogeneous palladium catalyst is employed.

To date, several methods are known for palladium removal.The majority of such methods are based on the adsorption of

palladium on a solid scaffold. Thiol groups have been exten-sively used for such a purpose. A material with mercaptopropylmoieties on mesoporous molecular sieves was used for palladi-um(0) and palladium(II) removal.[5] Magnetite nanoparticleswith thiol groups on the surfaces have been reported toremove palladium(II) ions from aqueous and non-aqueous sol-utions.[6] In addition to the thiol moiety, other functionalities,such as amino, thiourea, dimercaptotriazine, triaminotetraace-tic acid or its sodium salt, have been used for the samescope.[1, 7]

This topic is of great interest from an industrial point ofview and many products for palladium removal are commer-cially available, such as QuadraPure, QuadraSil, Smopex, ZEO-prep, ISOLUTE S-Thiol, Thiol-SAMMS, PhosphonicS, Deloxan, Sil-iaBond and SiliaMetS.

Ionic liquids have been extensively used to immobilise palla-dium species and the resulting materials have been successful-ly employed for several C�C coupling reactions.[8] In this con-text, our research group has described the synthesis of palladi-um immobilised on silica gel modified with highly cross-linkedimidazolium networks and the use of these materials in theSuzuki reaction under batch and flow conditions.[9] However,even if ionic-liquid-based materials are widely used to stabilisepalladium species, to give a highly active catalyst for C�C cou-pling reactions, less attention has been devoted to the use ofcovalently supported, ionic-liquid-phase palladium scavengers.Recent examples include guanidinium ionic-liquid-grafted rigidpoly(p-phenylene) microspheres, which have been developedfor [PdCl4]2� scavenging,[10] and supported task-specific ionicliquids, which have been developed through the ionic paircoupling of the imidazolium cation of the modified polystyr-ene support with l-proline. The latter materials have shown anefficient metal-scavenging ability towards Pd(OAc)2.[11]

Herein, we examined the scavenging ability of highly cross-linked imidazolium-based materials towards palladium(0) and

[a] R. Buscemi, Dr. F. Giacalone, Prof. S. Orecchio, Prof. M. GruttadauriaDipartimento di Scienze e Tecnologie BiologicheChimiche e Farmaceutiche (STEBICEF)Sezione di Chimica, Universit� di PalermoViale delle Scienze s/n, Ed. 17, 90128 Palermo (Italy)Fax: (+ 39) 091-596825E-mail : michelangelo.gruttadauria@unipa.it

Supporting information for this article is available on the WWW underhttp://dx.doi.org/10.1002/cplu.201300361.

Five imidazolium-based materials have been synthesised andused for the first time as palladium scavengers. Radical reac-tions of suitable bis-vinylimidazolium salts led to a series of in-soluble materials through homo-polymerisation, immobilisa-tion with a 3-mercaptopropyl-modified silica gel or co-poly-merisation with ethylene glycol dimethylacrylate. These materi-als were screened as palladium scavengers with a set of palla-dium(0) and palladium(II) compounds in different solvents and

at different starting amounts of palladium. In many cases, re-sidual amounts of palladium were lower than 5 ppm, as re-quested for the manufacture of active pharmaceutical ingredi-ents and fine chemicals. The application of one of these mate-rials as a palladium scavenger in a Suzuki coupling reaction re-sulted in a 29-fold abatement of the palladium content in thefinal product with respect to the control reaction.

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palladium(II) compounds largely employed as catalysts in C�Ccoupling reactions.

Results and Discussion

Synthesis and characterisation of scavenging materials

We focused our attention on three different bis-vinylimidazoli-um salts, 1 a–c. In the first step, we used these salts as startingmonomers for their radical polymerisation to give polymers2 a–c (Scheme 1). Then, in a second step, we focused our at-

tention on the use of such a highly cross-linked imidazoliumnetwork covalently supported on silica or co-polymerised inthe presence of ethylene glycol dimethylacrylate (EGDMA;Scheme 2). Bis-vinylimidazolium salts 1 a–c were easily ob-tained through the reaction of 1-vinylimidazole and 1,2-dibro-moethtane, or 1,4-bis(bromomethyl)benzene and 1,3-bis(bro-momethyl)benzene. Radical polymerisation of bis-vinylimidazo-lium salts with AIBN as a radical initiator in ethanol at reflux

gave the polymeric materials. Material 4 was prepared as al-ready described through the radical reaction of salt 1 b in thepresence of a 3-mercaptopropyl-modified silica gel.[12] Highlycross-linked co-polymer 6 was prepared by mixing EGDMA andsalt 1 b and carrying out the polymerisation with AIBN as theradical initiator in ethanol at reflux.

The materials obtained were characterised by 13C{H} CP-MASNMR spectroscopy. Figure 1 a gives the 13C NMR spectrum ofpolymer 2 a and clearly shows signals for the carbon atoms ofthe imidazolium ring at around d= 124 and 137 ppm and thealiphatic carbon atoms in the range of d= 25–60 ppm. Thepattern of the 13C NMR spectrum of material 2 b (Figure 1 b)appears to be different because of the presence of the addi-tional aromatic ring and the deshielded benzylic carbon atoms.With respect to the spectrum of 2 a, the spectrum of material2 b shows fewer signals in the aliphatic region. The spectrumof material 2 c appears to be similar to that of material 2 b (Fig-ure S1 in the Supporting Information).

The NMR spectrum of material 6 indicates the successful co-polymerisation of EGDMA and salt 1 b (Figure S2 in the Sup-porting Information), although nothing can be said about thereal composition of the co-polymer. The signal at d= 176 ppmcan be attributed to the carbonyl groups, whereas signals atd= 123–135 ppm resemble those of material 2 b. Finally, sig-nals at d�50–62 ppm can be attributed to the CH2O carbonatoms and carbon atoms linked to the imidazolium ring. Calcu-lations based on elemental analysis indicated a molar ratio ofEGDMA/1 b of approximately 1.75/1.00.

The surface area of material 4 was 140 m2 g�1 (Figure 2 a).The absence of a silica support gave materials with a muchlower surface area. The surface areas of materials 2 a–c werelow (ca. 20 m2 g�1; Figure 2 b) and that for material 6 was evenlower (ca. 5 m2 g�1).

Palladium scavenging

The next step was to explore thescavenging ability of the pre-pared materials. Materials 2 a–c were tested in the scavengingof aqueous solutions of Na2PdCl4

(Table 1). In this case, the scav-enging property is mainly due toanion exchange between thebromide ions of the resin with[PdCl4]2� ions. For each material,different amounts of palladiumwere examined from 200 to4000 ppm. Material 2 a showedalmost the same percentage ofresidual palladium, regardless ofthe initial amount of palladiumfrom 200 to 3000 ppm (Table 1,entries 1–4). On the other hand,when the amount of Pd was ashigh as 4000 ppm, a greateramount of unretained palladium

Scheme 1. Synthesis of polymers 2 a–c. AIBN = azobis(isobutyronitrile).

Scheme 2. Synthesis of 4 and 6.

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was found (Table 1, entry 5). Materials 2 b and 2 c gave betterresults, especially at 2000 ppm. The latter result could be dueto the higher palladium concentration, which allows better in-teraction with the low-surface-area material. On the otherhand, if a larger amount of palladium was used, leaching wasagain higher, probably owing to saturation of the scavengingsites and/or to their restricted accessibility, especially for theinner sites. The trend in residual palladium as a function of theinitial palladium content is shown in Figure S3 in the Support-ing Information. The lower scavenging ability of material 2 acould be ascribed to its more compact structure, owing to theshort linker. The scavenging ability of material 2 b from

a 2000 ppm solution of palladium was also investigated at dif-ferent mixing times (3, 5 and 24 h). After 3 h the amount of re-sidual palladium was high (18 %), after 5 h it was very low(0.2 %), and the same amount was observed after 24 h.

Because of the interesting results obtained with material 2 b,it was further employed for Pd(OAc)2 removal. Several parame-ters were examined, such as the initial amount of palladium,mixing time t, solvent and temperature (Table 2). Scavengingtests performed at room temperature (Table 2, entries 1–8)showed a residual amount of palladium approximately of 2–3 %, except for entry 1. The best value was observed uponusing an initial amount of palladium of 200 ppm in DMF for

Figure 1. Solid-state 13C NMR spectra of a) 2 a and b) 2 b.

Figure 2. BET curves for 4 and 2 b.

Table 1. Scavenging of Na2PdCl4 with materials 2 a–c in water.[a]

Entry Initial Pd [ppm] Residual Pd [%]2 a 2 b 2 c

1 200 2.0 1.3 1.52 1000 1.8 0.5 1.83 2000 1.5 0.2 0.54 3000 1.7 1.1 3.55 4000 13.3 4.3 7.5

[a] 100 mg of 2 in H2O (5 mL); mixing time of 24 h; results for duplicateexperiments.

Table 2. Scavenging of Pd(OAc)2 with 2 b.[a]

Entry t [h] Initial Pd [ppm] Starting volumeDMF/H2O [mL]

Residual Pd [%]

1 24 139 5:2.5 4.62 24 379 5:2.5 3.33 24 2123 5:0 2.04[b] 24 200 5:0 3.45 24 2000 5:0 2.76 24 4000 5:0 2.17 5 200 5:0 1.88 5 2000 5:0 3.49[c] 24 200 5:0 5.510[c] 5 200 5:0 4.9

[a] 100 mg of 2 b ; results of duplicate experiments. [b] Mean value ofquadruplicate experiments. [c] Temperature, 80 8C.

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5 h (Table 2, entry 7). At a higher temperature (80 8C), theamount of unretained palladium was higher (Table 2, entries 9and 10).

Material 2 b was further tested in the removal of Pd(OAc)2

under other conditions by employing a mixing time of 5 h(Table 3). Three different solvents (DMF, iPrOH/H2O 4/1, ace-

tone/H2O 4/1) were employed with a starting amount of palla-dium of 200 ppm. Leaching was low, especially if iPrOH/H2Owas used (Table 3, entry 2). The same solvent mixture gave anexcellent result when a much higher amount of palladium wasused (2000 ppm, Table 3, entry 5). Under the same conditions,the use of material 2 c gave greater leaching (Table 3, entry 7).Good results were still obtained with 4000 ppm of palladium(Table 3, entries 8 and 9).

In addition to scavenging of palladium(II), the scavenging ofpalladium(0) is also important because palladium is often inthis state during and after coupling reactions. Thus, in additionto another palladium(II) compound, namely, [Pd(PPh3)2Cl2] , sol-utions of two common palladium(0) sources, [Pd(dba)2] (dba:dibenzylideneacetone) and [Pd(PPh3)4] , were examined(Table 4). Two materials, 2 b and 2 c, were tested. [Pd(PPh3)2Cl2]was efficiently scavenged by material 2 b (Table 4, entry 1),whereas when material 2 c was used a large amount of residu-al palladium was found (Table 4, entry 2). A change in the start-

ing solvent mixture did not improve the result (Table 4,entry 3). Similar behaviour was observed in the removal of[Pd(dba)2] , so material 2 b was more efficient than material 2 c(Table 4, entries 4–6). Removal of [Pd(PPh3)4] was not efficient,especially with material 2 c (Table 4, entries 7–10) probably be-cause its bulky size prevented diffusion in the inner scavengingsites of the cross-linked polymeric network.

Next, we investigated the scavenging ability of materials 4and 6. Both materials were tested in selected conditions,based on the results obtained with material 2 b. Table 5 pro-

vides data regarding the removal of Na2PdCl4, Pd(OAc)2 and[Pd(PPh3)2Cl2] with materials 4 and 6. Removal of Na2PdCl4 andPd(OAc)2, with material 4, gave excellent results (Table 5, en-tries 1–3). Even upon starting from a high amount of palladi-um, residual palladium was detected in a low amount. On theother hand, the removal of [Pd(PPh3)2Cl2] gave a poor result(Table 5, entry 4). Results with material 6 were slightly different(Table 5). Although an excellent result was observed in thescavenging of Na2PdCl4 (Table 5, entry 5), scavenging ofPd(OAc)2 and [Pd(PPh3)4] showed high levels of residual Pd(Table 5, entries 6 and 7). On the other hand, a good result wasobserved with [Pd(PPh3)2Cl2] (Table 5, entry 8).

A comparison with literature data highlights the good per-formances obtained with our materials. Guanidinium ionic-liquid-grafted rigid poly(p-phenylene) microspheres efficientlyremoved [PdCl4]2� (530 ppm of Pd) and Pd(OAc)2 (911 ppm ofPd).[10, 11] In our case, we reached almost complete removal ofthe above species upon starting from a much higher amountof palladium (see Table 5). Our materials also show good per-formances compared with commercially available palladiumscavengers. SiliaBond, Thiourea or Triamine removed Pd(OAc)2

from a solution in DMF (1000 ppm), but working at 80 8C.[13]

QuadraPure TU removed Pd(OAc)2 from 100 mL of a 1000 ppmsolution (CH2Cl2 or DMF), but a larger amount of resin was em-ployed (5 g).[14] The scavenging conditions used herein are notfully optimised. Scavenging efficiency is affected by several ex-perimental factors, including the amount of scavenger, contacttime, temperature and concentration, as well as the nature ofthe solvent. Nevertheless, the results reported herein are prom-ising.

Table 3. Scavenging of Pd(OAc)2 with 2 b.[a]

Entry Initial Pd [ppm] Solvent(starting volume [mL])

Residual Pd [%]

1 200 DMF (5) 1.82 200 iPrOH/H2O (4:1) 0.93 200 acetone/H2O (4:1) 1.54 2000 DMF (5) 3.45 2000 iPrOH/H2O (2:0.5) 0.26 2000 DMF/H2O (2:0.5) 0.87[b] 2000 iPrOH/H2O (2:0.5) 3.18 4000 iPrOH/H2O (2:0.5) 1.09 4000 DMF/H2O (2:0.5) 0.9

[a] 100 mg of 2 b ; mixing time 5 h; duplicated experiments. [b] 50 mg of2 c ; duplicated experiment.

Table 4. Scavenging of [Pd(PPh3)2Cl2] , [Pd(dba)2] and [Pd(PPh3)4] with 2 band 2 c.[a]

Entry Pd source Material Solvent ([mL]) Initial Pd[ppm]

ResidualPd [%]

1 [Pd(PPh3)2Cl2] 2 b DMF/H2O (5:2.5) 133.3 0.92 [Pd(PPh3)2Cl2] 2 c DMF/H2O (5:2.5) 133.3 7.03 [Pd(PPh3)2Cl2] 2 c iPrOH/H2O (6:1.5) 133.3 6.04 [Pd(dba)2] 2 b DMF/H2O (5:2.5) 133.3 1.05 [Pd(dba)2] 2 c DMF/H2O (5:2.5) 133.3 7.56 [Pd(dba)2] 2 c iPrOH/H2O (6:1.5) 133.3 4.77 [Pd(PPh3)4] 2 b DMF/H2O (5:2.5) 133.3 5.68 [Pd(PPh3)4] 2 c DMF/H2O (5:2.5) 133.3 169 [Pd(PPh3)4] 2 b DMF (5) 200 6.010 [Pd(PPh3)4] 2 c iPrOH/H2O (6:1.5) 133.3 35

[a] 100 mg of material ; mixing time 5 h; duplicated experiments.

Table 5. Scavenging of different palladium species with 4 and 6.[a]

Entry Material Pd compound Solvent ([mL]) Initial Pd[ppm]

ResidualPd [%]

1 4 Na2PdCl4 H2O (5) 1518 0.0052 4 Pd(OAc)2 iPrOH/H2O (4:1) 685.4 0.203 4 Pd(OAc)2 iPrOH/H2O (4:1) 1393 0.234 4 [Pd(PPh3)2Cl2] DMF/H2O (5:2.5) 93.3 16.55 6 Na2PdCl4 H2O (5) 1518 0.176 6 Pd(OAc)2 iPrOH/H2O (4:1) 1400 12.07 6 [Pd(PPh3)4] DMF/H2O (5:2.5) 93.3 19.28[b] 6 [Pd(PPh3)2Cl2] DMF/H2O (5:2.5) 137.9 1.5

[a] 70 mg of material ; mixing time 20 h. [b] 100 mg of material.

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Finally, we tested one of our materials as a scavenger of pal-ladium in a Suzuki reaction. As a model reaction, we used thereaction between 4-bromobenzaldehyde and phenylboronicacid in the presence of Pd(OAc)2 as the catalyst. To examinethe scavenging ability, we used a large amount of palladium(2 mol %) in the reaction with and without scavenger. Becauseof the good results obtained, we chose material 4 as a scaveng-er. Firstly, we performed the reaction in iPrOH/H2O 4/1 and,after the completion of the reaction, scavenger 4 was added.The material was filtered and the final biphenyl compoundwas analysed to determine the amount of residual palladium.The same procedure was performed for a sample reaction inwhich scavenger 4 was not added (Scheme 3). The amount of

palladium detected was still high after the scavenging proce-dure. Then, we modified the procedure. After completion ofthe reaction, the mixture was filtered through a short pad ofsilica and the solvent was evaporated. Then, one of the sol-vents examined herein (iPrOH/H2O or DMF) and material 4were added. After removal of the scavenging material, theamount of residual palladium in the final compound was ex-amined and it was found to be much lower.

Conclusion

We have reported, for the first time, the use of materials basedon polymerised imidazolium bromide salts as palladium scav-engers from solutions of palladium(0) and palladium(II). Thesematerials were prepared from a suitable bis-vinylimidazoliumdibromide salt and its transformation through a radical reac-tion into an insoluble material.

The materials were screened as palladium scavengers witha set of palladium(0) and palladium(II) compounds in differentsolvents and at different starting amounts of palladium.Among the homo-polymerised scavenging materials (2 a–c),material 2 b, with a para-xylyl linker, gave the best results. Itwas successfully employed for palladium removal from solu-tions of Na2PdCl4, Pd(OAc)2, [Pd(PPh3)2Cl2] and [Pd(dba)2] ,whereas poorer results were obtained in the case of solutionsof [Pd(PPh3)4] . Excellent results were obtained in the removalof palladium from a solution of Na2PdCl4 with materials 4 and6. In addition, material 4 efficiently scavenged Pd(OAc)2. Finally,if 4 was applied as the palladium scavenger in a Suzuki cou-pling reaction, a 29-fold abatement in the palladium contentof the final product with respect to the control reaction wasachieved. This result makes these materials useful scavengers

for palladium with comparable properties to those of commer-cially available scavengers.

Experimental Section

Synthesis of bis-vinylimidazolium salts 1 a–c

Bis-vinylimidazolium salts 1 a–c were obtained through the reac-tion of 1-vinylimidazole and 1,2-dibromoethane, or 1,4-bis(bromo-methyl)benzene and 1,3-bis(bromomethyl)benzene. The procedurewas similar to that reported previously.[9b] A solution of dihalidecompound (0.01 mol) and 1-vinylimidazole (0.021 mol) in toluene(10 mL, in the case of 1,2-dibromoethane) and in chloroform(20 mL, in the case of benzylic compounds) was heated for 24 h inan oil bath at 90 and 50 8C (for solutions in toluene and chloro-form, respectively) with magnetic stirring. After cooling to roomtemperature, the mixture was filtered and washed several timeswith diethyl ether. The solid product was dissolved in methanoland stirred overnight in the presence of activated carbon then fil-tered and dried at 40 8C under reduced pressure.

Synthesis of 2 a–c

Bis-vinylimidazolium salt 1 (3 mmol) was dissolved in ethanol(22 mL) and AIBN was added (10 mol %) under argon. The solutionwas heated at 78 8C overnight. After cooling to room temperature,the white solid formed was filtered and washed with methanoland diethyl ether, then dried in an oven at 40 8C.

Synthesis of 4

Material 4 was prepared as previously reported.[9b] The 3-mercapto-propyl-modified silica gel (500 mg, SH loading 1.2 mmol g�1,0.6 mmol), the bis-vinylimidazolium salt 1 b (2.76 equiv, 1.66 mmol,750 mg), ethanol (10.8 mL) and AIBN (16.3 mg) were placed ina round-bottomed flask and the suspension was degassed by bub-bling with argon for 15 min. The mixture was heated at 78 8Cunder argon with stirring. After 20 h, the reaction mixture wascooled to room temperature, filtered under reduced pressure andwashed with hot methanol then with diethyl ether. The obtainedmaterial was dried in an oven at 40 8C overnight.

Synthesis of 6

Bis-vinylimidazolium salt 1 b (2 mmol) and EGDMA (2 mmol) weredissolved in methanol (50 mL) and AIBN was added (10 mol %)under argon. The solution was heated at 65 8C overnight. Aftercooling to room temperature, the white solid formed was filtered,washed with methanol and diethyl ether, then dried in an oven at40 8C. Elemental analysis found: C 46.97, N 7.00.

Characterisation

Solid-state 13C{H} CP-MAS NMR spectra were recorded on an Agi-lent, 400 MHz spectrometer with samples packed in zirconia rotorsspinning at 15 kHz. Elemental analysis was performed witha Thermo Finnigan Flash Elemental 1112 series instrument. Thespecific surface areas and pore volumes were determined from N2

adsorption/desorption isotherms measured at �196 8C on a Sorpto-matic 1900 (Carlo Erba) instrument. The specific surface areas weredetermined by applying the BET method[15] to the nitrogen adsorp-tion isotherm.

Scheme 3. Procedure 1: Pd(OAc)2 (2 mol %, Pd: 2218 mg), iPrOH/H2O, S-Phos(2 mol %), K3PO4, 70 8C, 3 h, then 4 ; procedure 2: Pd(OAc)2 (2 mol %, Pd:2123 mg); iPrOH/H2O; S-Phos (2 mol %); K3PO4; 70 8C; 3 h; then filtration,evaporation and solvent (iPrOH/H2O or DMF) and 4 were added.

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Scavenging procedure and palladium analysis

The palladium compound was dissolved in the appropriate solventto give the initial amount of palladium reported in Tables 1–5 andthe scavenging material (100 mg) was added. The mixture wasstirred at room temperature for the times reported in Tables 1–5.All data reported are the results of duplicate experiments. Thescavenging material was removed by filtration and the solvent wasevaporated. The residue was mineralised with hot aqua regia(10 mL) and then the solution was concentrated and brought toa known volume with nitric acid (2 %). In the case of incompletemineralisation, the residue was heated in a muffle at 550 8C for20 h and then solubilised with hot aqua regia (10 mL).

Palladium analysis was performed with a PerkinElmer Optima 2100series ICP optical emission spectrometer. Before analysis, a calibra-tion curve in the range 1–10 ppm was made. Solutions at higherconcentration were diluted with nitric acid (2 %). For palladiumconcentrations in the range 50–500 ppb, a PerkinElmer 3100atomic absorption spectrometer with a graphite furnace was em-ployed. To confirm the results, in selected cases, the amount of pal-ladium was also quantified in the final material.

Acknowledgements

This study was supported by the University of Palermo. Experi-mental NMR data were provided by Centro Grandi Apparecchia-ture—UniNetLab—Universit� di Palermo funded by P.O.R. Sicilia2000-2006, Misura 3.15 Quota Regionale.

Keywords: cross-coupling · imidazolium salts · ionic liquids ·palladium · supported catalysts

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Received: October 29, 2013

Revised: December 19, 2013

Published online on January 23, 2014

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