ARTICLE IN PRESS
Catalysis Communications 9 (2008) 273–275 www.elsevier.com/locate/catcom
Palladium nanoparticles immobilized in ionic liquid: An outstanding catalyst for the Suzuki C–C coupling Je´roˆme Durand a, Emmanuelle Teuma a, Franc¸ois Malbosc b, Yolande Kihn c, Montserrat Go´mez a,* a
Laboratoire He´te´rochimie Fondamentale et Applique´e, UMR CNRS 5069, Universite´ Paul Sabatier, 118 route de Narbonne, 31062 Toulouse Cedex 9, France b SOLVIONIC, Parc technologique Delta Sud, 09340 Verniolle, France c CEMES, UPR 8011 CNRS, 29 rue Jeanne Marvig, 31055 Toulouse Cedex 4, France Received 11 May 2007; received in revised form 13 June 2007; accepted 13 June 2007
Abstract Palladium nanoparticles, stabilised by [BMI][PF6], have been generated from [PdCl2(cod)] under hydrogen atmosphere, leading to a high catalytic activity in the Suzuki cross-coupling reaction without palladium leaching up to ten cycles. 2007 Elsevier B.V. All rights reserved. Keywords: Palladium; Nanoparticles; Ionic liquids; C–C coupling; Catalyst reusing
1. Introduction Palladium nanoparticles stabilised by polymers, [1,2] dendrimers [3–7] or ligands [8–11] have been used as efficient catalysts for C–C coupling reactions [12,13]. Palladium nanoparticles only stabilised by electrostatic effect, prepared by chemical reduction of palladium acetate by tetraalkylammonium salts, showed a high activity in Suzuki cross-coupling reactions under biphasic conditions (ammonium salt/water) [14–16]. Recently, Dupont and co-workers have described the synthesis of free-ligand palladium nanoparticles prepared by decomposition of a starting molecular palladacycle in presence of an allene in dichloromethane. Further dispersion of these nanoparticles in the ionic liquid (IL) [BMI][PF6] ([BMI][PF6] = 1-nbutyl-3-methylimidazolium hexafluorophosphate) gave excellent activities in Heck coupling reactions [17]. From a mechanistic point of view, Hu et al. proposed that for the Suzuki coupling catalyzed by starting Pd nanoparticles, discrete Pd(II) species are involved, regenerating Pd(0) *
Corresponding author. Tel.: +33 561557738; fax: +33 561558204. E-mail address:
[email protected] (M. Go´mez).
1566-7367/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2007.06.015
nanoparticles after the reductive elimination, [2] while Dupont et al. suggested that Pd nanoparticles precursors behave as reservoir of molecular palladium species, which are the true catalytic species in Heck reactions [17]. With the aim to put more light in the nature of active catalytically species involved in the Pd-catalyzed Suzuki C–C coupling, we have interested in nanoparticles only stabilised by IL without ‘‘passivating’’ agents on the metallic surface. 2. Experimental 2.1. Synthesis of Pd nanoparticles 0.025 mmol of palladium precursor (7.1 mg for [PdCl2(cod)]; 4.4 mg for PdCl2; 12.9 mg for [Pd2(dba)3 Æ CHCl3]) in 5 mL of [BMI][PF6] was stirred at room temperature under argon in a Fischer–Porter bottle until complete dissolution. The system was then pressurised with three bar of dihydrogen and stirred at room temperature for 2 h, leading to a black solution. In the case of [Pd2(dba)3 Æ CHCl3], heating at 60 C was necessary to obtain decomposition. The residual gas was then released and the volatiles removed under reduced pressure.
ARTICLE IN PRESS 274
J. Durand et al. / Catalysis Communications 9 (2008) 273–275
2.2. General procedure for Suzuki coupling reaction To 1 mL of a 5 mM solution palladium nanoparticles in [BMI][PF6] prepared as described above, bromobenzene (0.21 mL, 2 mmol), phenyl boronic acid (268 mg, 2.2 mmol, 1.1 eq.) and Na2CO3 (445 mg, 4.2 mmol, 2.1 eq.) dissolved in 1.5 mL of water were added under argon. The mixture was then stirred at 100 C. After 1 h, the solution was allowed to come to room temperature and biphenyl was extracted with hexane (5 · 5 mL). The combined extracts were then dried over Na2SO4, filtered up and volatiles removed under reduced pressure affording a white crystalline solid. For the recycling experiments, after hexane extraction and removal of the aqueous phase, the ionic liquid solution containing the catalyst was consecutively washed with diethyl ether (3 · 5 mL) and water (3 · 5 mL), and dried under vacuum at 60 C for 4 h. The successive catalytic experience was then run, adding the reactants as described previously. The images of particles in the IL were obtained in a transmission electron microscope running at 120 kV. A drop of IL was deposited on a holey carbon grid and the excess of IL was removed in order to obtain a film as thin as possible. Images were recorded on the film of IL lying on the holes of the grid. The elemental analysis of palladium was determined by ICP-MS and carried out by Antellis. 3. Results and discussion Palladium nanoparticles were prepared in [BMI][PF6] by decomposition of [PdCl2(cod)] under hydrogen atmosphere, based on the methodology described by Chaudret in organic solvents [18]. TEM analyses of these particles dispersed in the IL (Fig. 1), shows star-like shaped interparticles organizations. The ‘‘branches’’ are constituted by small nanoclusters, showing a diameter between 6– 8 nm. The solid isolated by centrifugation revealed a high crystalline material, which shows fcc packing like that exhibited by the bulk metal (see below).
In contrast, no super-structures were observed for nanoparticles formed from PdCl2 salt (Fig. 2a) or from [Pd2(dba)3 Æ CHCl3] (Fig. 2b), giving in both cases quite well-dispersed material formed by nanoparticles with a mean size ca. 7 nm. As a result, the electrostatic stabilisation of palladium nanoparticles by the ionic liquid depends on the palladium precursor. Preformed Pd nanoparticles in [BMI][PF6] were used as catalytic precursor for Suzuki C–C coupling between bromobenzene and phenyl boronic acid (Scheme 1) [19,20]. For material formed from [PdCl2(cod)], after 1 h at 100 C using 0.25 mol% of palladium (based on the starting precursor), total conversion of bromobenzene was reached, isolating 92% of biphenyl. The palladium content in biphenyl (determined by ICP-MS) is in the range of 3–5 ppm, values that are within the allowed levels of palladium for active pharmaceutical constituents [21]. Under these conditions, iodobenzene was also activated, but not chlorobenzene. After an appropriated work-up, IL phase containing the catalyst was reused up to 10 times, observing a slight diminution of yield after the eighth run (Fig. 3). The palladium content in the biphenyl isolated after each recycling keeps on low, reaching 200 ppm in the last two runs. It is noteworthy that both [PdCl2(cod)] as molecular catalytic precursor and palladium powder as heterogeneous catalyst, were not active, even at high concentration of catalyst (2.5 mol%). Nanoparticles derived from PdCl2 were less active, isolating 66% of biphenyl under the same conditions described above (palladium content in biphenyl: 7.4 ppm); its catalyst reuse leading to an isolated yield of 53% after the second run, shows that the reusability of this system is not as good as for particles obtained from [PdCl2(cod)]. However, nanoparticles formed from [Pd2(dba)3 Æ CHCl3] were not active, probably due to the presence of organic stabilizers
Fig. 2. TEM images of palladium nanoparticles from: (a) PdCl2 and (b) [Pd2(dba)3 Æ CHCl3], both dispersed in [BMI][PF6].
Br
B(OH)2 Na2CO3 (aq), [Pd]coll +
Fig. 1. TEM images of palladium nanoparticles from [PdCl2(cod)] dispersed in [BMI][PF6]: (a) representative micrograph; (b) isolated ‘‘star’’; (c) a ‘‘branch’’ view.
[BMI][PF6], 1 h
Scheme 1. C–C coupling catalyzed by Pd nanoparticles, [Pd]coll, in [BMI][PF6].
ARTICLE IN PRESS J. Durand et al. / Catalysis Communications 9 (2008) 273–275
the yield remained the same), pointing to a heterogeneous nature of the catalyst [23].
100 Biphenyl yield (%)
275
80 60
4. Summary
40
In conclusion, we have demonstrated that Pd nanoparticles (generated from [PdCl2(cod)] under hydrogen atmosphere) stabilised by [BMI][PF6] lead to a highly active catalyst (0.25 mol% of palladium) for the Suzuki C–C coupling, which could be reused up to seven times without observing neither activity loss nor metal leaching (less than 5 ppm of Pd in biphenyl). The inactivity detected for both molecular precursor [PdCl2(cod)] and palladium powder agrees with a catalyst colloidal nature for our preformed palladium nanoparticles. The investigation of our new efficient catalytic system is going on in our laboratory, basically related to other C–C couplings of pharmaceutical interest.
20 0 1
2
3
4
5
6
7
8
9
10
Runs
Fig. 3. Histogram representing biphenyl yield after each catalyst reused (relative incertitude: 1%).
Acknowledgements The authors thank CNRS, Solvionic and Universite´ Paul Sabatier for financial support. References
Fig. 4. TEM images of palladium nanoparticles from [PdCl2(cod)]: (a) nanoparticles dispersed in [BMI][PF6] (after centrifugation); (b) solid separated by centrifugation; (c) solid separated by centrifugation showing its crystalline structure.
hindering the substrate-metal contact. Note that the detected acid gas after Pd nanoparticles synthesis from [PdCl2(cod)] (during the Fischer–Porter bottle depressurisation) points to HCl formation, in agreement with a heterolytic hydrogen activation promoted by palladium chloride species [22]. Therefore a low content of chloride ions at the surface of the active nanoparticles is expected. The catalytic behaviour of both solution and solid phase separated by centrifugation of Pd nanoparticles obtained from [PdCl2(cod)], was examined. Under our catalytic conditions, the gray solution (constituted by 4.7 · 10 4 mol% of palladium) gave 60% yield of biphenyl, while the black solid was not active. TEM analysis of solution shows the presence of organised particles (Fig. 4a) but only agglomerates are observed for the solid phase (Figs. 4b and c). Consequently, the organization of nanoparticles in IL is related to the catalytic activity. Addition of mercury, after 30 min of catalytic reaction (60% yield), stopped the evolution of the reaction (2 h later
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
Y. Li, E. Boone, M.A. El-Sayed, Langmuir 18 (2002) 4921. J. Hu, Y. Liu, Langmuir 21 (2005) 2121. L.K. Yeung, R.M. Crooks, Nano. Lett. 1 (2001) 14. M. Onoe, M. Murata, T. Mizugaki, K. Ebitani, K. Kaneda, J. Am. Chem. Soc. 126 (2004) 1604. Y. Li, M.A. El-Sayed, J. Phys. Chem. B 105 (2001) 8938. M. Pittelkow, K. Moth-Poulsen, U. Boas, J.B. Christensen, Langmuir 19 (2003) 7682. K.R. Gopidas, J.K. Whitesell, M.A. Fox, Nano. Lett. 3 (2003) 1757. D. Astruc, F. Lu, J.R. Aranzaes, Angew. Chem. Int. Edit. 44 (2005) 7852. L. Strimbu, J. Liu, E. Roman, A.E. Kaifer, Langmuir 19 (2003) 483. M. Moreno-Manas, R. Pleixats, S. Villarroya, Organometallics 20 (2001) 4524. V. Kogan, Z. Aizenshtat, R. Popovitz-Biro, R. Neumann, Org. Lett. 4 (2002) 3529. M. Moreno-Man˜as, R. Pleixats, Acc. Chem. Res. 36 (2003) 638. P. Migowski, J. Dupont, Chem. Eur. J. 13 (2007) 32. V. Calo`, A. Nacci, A. Monopoli, F. Montingelli, J. Org. Chem. 70 (2005) 6040. M.T. Reetz, J.G. de Vries, Chem. Commun. (2004) 1559. J.G. de Vries, Dalton Trans. (2006) 421. C.C. Cassol, A.P. Umpierre, G. Machado, S.I. Wolke, J. Dupont, J. Am. Chem. Soc. 127 (2005) 3298. K. Philippot, B. Chaudret, C.R. Chimie 6 (2003) 1019. P.J. Dyson, T.J. Geldbach, in: Metal Catalysed Reactions in Ionic Liquids, Springer, 2005. D. Zhao, M. Wu, Y. Kou, E. Min, Catal. Today 74 (2002) 157. C.E. Garrett, K. Prasad, Adv. Synth. Catal. 346 (2004) 889. A.S. Berenblyum, A.P. Aseeva, L.I. Lakhman, I.I. Moiseev, J. Organomet. Chem. 234 (1982) 237. J.A. Widegren, R.G. Finke, J. Mol. Catal. A: Chem. 198 (2003) 317.
