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Synthesis and catalytic activity of DAB-dendrimer encapsulated Pd nanoparticles for the Suzuki coupling reaction
Inorganica Chimica Acta 359 (2006) 4909–4911 www.elsevier.com/locate/ica
Note
Synthesis and catalytic activity of DAB-dendrimer encapsulated Pd nanoparticles for the Suzuki coupling reaction Julietta Lemo, Karine Heuze´ *, Didier Astruc
*
Groupe Nanosciences Mole´culaires et Catalyse, LCOO, UMR N 5802, Universite´ Bordeaux I, 351 cours de la libe´ration, 33405 Talence Cedex, France Received 28 November 2005; accepted 31 December 2005 Available online 24 February 2006 Dedicated to Professor Dr. Dr. h.c. mult. Wolfgang A. Herrmann.
Abstract Palladium based dendrimer-encapsulated metal nanoparticles (DEMNs) are prepared by reduction of an aqueous solution of palladium (II) and five generations of amino terminated dendrimers (DAB (diaminobutane) dendrimers). The average size of the palladium nanoparticles formed is 1.7–2.8 nm. Catalytic performances of these DEMNs have been studied in the Suzuki cross-coupling reaction. 2006 Elsevier B.V. All rights reserved. Keywords: Dendrimers; Nanoparticles; Palladium; Catalysis
Catalysis by dendrimer-encapsulated metal nanoparticles (DEMNs) is a recent concept that has largely been developed by Crooks and his group using PAMAM dendrimers [1,2]. Many examples of DEMNs are now available in the literature for the formation of stable and well-defined nanomaterials based on reduced metals such as Cu, Au, Ag, Pt, and Pd [3]. Dendrimers are particularly attractive hosts for catalytically active metal nanoparticles because of their well-defined nano-environment in the interior of the dendrimer and their chemical versatility [4]. For instance, Crooks studied catalytic properties of PAMAM dendrimer palladium nanoparticles in organic [5], fluorous [6] and supercritical solvent [7] for allylic hydrogenation. Kaneda et al. [8] investigated the case of derivatized DAB-dendrimer palladium nanoparticles. It was also demonstrated that Pd or Au DENs have a high catalytic reactivity in Heck [9] and Suzuki [10] cross-coupling reactions as well as in the Stille [11] reaction with Pt DENs in aqueous medium and at room temperature. *
Corresponding authors. Tel.: +33 05 40 00 62 74; fax: +33 05 40 00 69 94 (K. Heuze´), Tel.: +33 05 40 00 62 71; fax: +33 05 40 00 66 46 (D. Astruc). E-mail addresses: [email protected] (K. Heuze´), [email protected] (D. Astruc). 0020-1693/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2005.12.070
Since the large majority of catalytic studies have been carried out with PAMAM-DEMNs, we report here, for comparison purposes, the synthesis, the characterization, and the catalytic properties of nearly monodisperse palladium nanoparticles prepared within the interior of five different generations of amino-terminated DAB-dendrimers: DABG1, DAB-G2, DAB-G3, DAB-G4 and DAB-G5, respectively, for dendrimers of generation 1–5. These DEMNs catalysts have been prepared by encapsulation of Pd(II) within the dendrimer followed by a chemical reduction to yield the corresponding Pd(0) nanoparticles [10a,10b]. The comparison of the catalytic efficiency of PAMAM-DEMNs and DAB-DEMNs, an exercise that is unprecedented, should allow to further optimize the nature and molecular engineering of the dendrimer host in catalysis by DEMNs. The size of the nanoparticle usually depends on the number of Pd(II) initially loaded into the dendrimer [1c], so two synthetic methods were investigated depending on the Pd/dendrimer ratio. Following El Sayed’s method A [10a], we have prepared palladium DENs by addition of DAB-dendrimers to an aqueous solution of K2PdCl4 (10 eq. Pd2+). Prior to reduction, the pH of the solution was adjusted to 4 with HCl (1 M) to prevent the complexation of Pd2+ to the peripherical primary amine groups [1b,12].
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J. Lemo et al. / Inorganica Chimica Acta 359 (2006) 4909–4911
Then, once Pd2+ bind to the interior tertiary amines, the reduction with NaBH4 was carried out to form palladium DENs, and the initially yellow solution turned to brown without the formation of Pd black, except in the case of DAB-G1 and DAB-G2 dendrimers. TEM pictures (Fig. 1a) revealed relatively monodisperse nanoparticles with a mean diameter of 1.7–1.9 nm, except for DAB-G1 DEMNs that form aggregates of about 60 nm (resulting from the agglomeration of Pd particles adsorbed to the dendrimer exterior amine groups). Following Christensen’s method B [10b], DAB-dendrimer and K2PdCl4 (60 eq. Pd2+) were mixed in water followed by the addition of NaBH4 without adjusting the pH. The color of the solution changed from yellow to dark brown with the formation of Pd black. TEM pictures showed DEMNs of 1.8–2.8 nm (Fig. 1b). Catalytic performances of the Pd DEMNs (method A synthesis) were examined in the Suzuki–Miyaura reaction between iodobenzene and phenylboronic acid (Eq. (1)). The results are gathered in Table 1. In the case of low generation dendrimers, DAB-G1, DAB-G2, and DAB-G3, the open structure favors the formation of catalytically inactive Pd black. However, they exhibit a higher catalytic activity (entries 1–3) than DEMNs prepared with higher generation dendrimers (entries 4 and 5). In the latter case, the lower accessibility of the catalytic sites for the substrate entering
Fig. 1. TEM of palladium nanoparticles with DAB-G3 dendrimer. Table 1 Suzuki-reaction with DENs palladium prepared by method A Entry
DENs Gn
n (Pd2+) mol
Time
Yielda (%)
Formation of Pd black
1 2 3
G1 G2 G3
4.2 · 106 4.2 · 106 7.5 · 106
4
G4
7.5 · 106
G5
7.5 · 106
1h 1h 1h 4h 1h 4h 24 h 4 days
100 100 60 98 40 66 97 40
yes yes yes yes little little little little
5 a
Reaction conditions. Iodobenzene (1 mmol), phenylboronic acid (3 mmol) in CH3CN/H2O (3/1), NaOAc (6 mmol), 10 ml of catalyst solution at 100 C. GC yields were used after extraction with pentane.
the dendrimer results in a diminishing catalytic activity (increasing reaction time with increasing dendrimer generation). Also, DEMNs prepared with DAB-G4 and DABG5 have a full filled and more compact structure, providing an efficient Pd encapsulation. Therefore, less Pd black was formed during the catalysis reaction than in the case of low generation dendrimers. It is noteworthy that in an analogous experiment of entry 3, the pH was adjusted to 4 before addition of Pd2+ (instead of after the Pd2+ addition), resulting in a 5% yield in 4 h (instead of 98% yield), enlightening the crucial importance of the DEMNs synthetic method in the catalysis properties of such nanoparticles. NaOAc I
+
B(OH)2
100˚C CH3CN/H2O palladium nanoparticle cat.
Suzuki-reaction with DENs palladium
ð1Þ DEMNs prepared with method B and from DAB-G2, DAB-G3 and DAB-G4 dendrimers were used as well in the Suzuki cross-coupling reaction, under the same reaction conditions (Eq. (1)). In all cases, quantitative yields were obtained after 20 h along with the formation of Pd black, indicating an overall lower reactivity than DEMNs prepared with method A. In our case, the size of the particles is decisive, the smaller the particles, the more efficient they would be. The recovery and re-use of these palladium nanoparticles catalysts were investigated in the Suzuki reaction (Eq. (1)). Palladium DENs remain in the aqueous phase and these could be separated after reaction by the addition of pentane and washed with CH2Cl2 (3 · 20 ml). An aqueous solution containing the catalyst was re-injected in a new reaction medium yielding 40% of the product after 24 h, for DENs-G3 (prepared with method A), and yielding 5% in 24 h for DENs-G3 (prepared with method B). TEM analysis of the crude solution after the first reaction cycle revealed the coalescence of nanoparticles. In summary, we have prepared nearly monodisperse DAB-dendrimer encapsulated palladium nanoparticles active in the catalysis of the Suzuki reaction in aqueous media. The reactivity of palladium DENs based on DABdendrimers is similar to the reactivity of palladium DENs based on PAMAM-dendrimers for the Suzuki catalysis reactions [10a,10b]. However, the best way to increase the reactivity as well as the recoverability of such dendrimerencapsulated palladium nanoparticles is probably to derivatize the DAB-dendrimer exterior amines by functionalized alkyl chains as suggested by Yeung and Crooks [9a] or Kaneda et al. [8]. Acknowledgments Financial support from the Institut Universitaire de France (IUF), the Centre National de la Recherche Scientifique (CNRS), the territory of Nouvelle-Cale´donie and the University of Bordeaux 1 is gratefully acknowledged.
J. Lemo et al. / Inorganica Chimica Acta 359 (2006) 4909–4911
References [1] (a) M. Zhao, L. Sun, R.M. Crooks, J. Am. Chem. Soc. 120 (1998) 4877; (b) R.M. Crooks, M. Zhao, L. Sun, V. Chechik, L.K. Yeung, Acc. Chem. Res 3 (24) (2001) 181; (c) R.M. Crooks, B.I. Lemon III, L. Sun, L.K. Yeung, M. Zhao, Top. Curr. Chem. 212 (2001) 81; (d) Y. Niu, R.M. Crooks, C.R. Chimie 6 (2003) 1049. [2] L. Balogh, D.A. Tomalia, J. Am. Chem. Soc 120 (1998) 7355. [3] (a) K. Esumi, A. Suzuki, A. Yamahira, K. Torigoe, Langmuir 16 (2000) 2604; (b) F. Gro¨hn, B.J. Bauer, Y.A. Akpalu, C.L. Jackson, E. Amis, J. Macromol. 33 (2000) 6042; (c) P.F. Floriano, C.O. Noble IV, J.M. Schoonmaker, E.D. Poliakoff, R.L. McCarley, J. Am. Chem. Soc. 123 (2001) 10545; (d) R.W.J. Scott, O.M. Wilson, R.M. Crooks, J. Phys. Chem. B 109 (2005) 692; (e) Y. Niu, R.M. Crooks, Chem. Mater. 15 (2003) 3463; (f) R. Narayanan, M.A. El-Sayed, J. Phys. Chem. B 108 (2004) 8572; (g) D. Astruc, F. Lu, J. Ruiz, Angew. Chem. Int. Ed. 117 (2005) 2; (h) Y. Chung, H.K. Rhee, J. Mol. Catal. A 206 (2003) 291. [4] (a) G.E. Oosterom, J.N.H. Reek, P.C.J. Kamer, P.W.N.M. Van Leeuwen, Angew. Chem. Int. Ed. 40 (10) (2001) 1828;
[5] [6] [7] [8] [9]
[10]
[11] [12]
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(b) D. Astruc, F. Chardac, Chem. Rev. 101 (9) (2001) 2991; (c) G.R. Newkome, C.N. Moorefield, F. Vo¨gtle, Dendrimers and Dendrons: Concepts, Syntheses and Applications, Wiley-VCH, Weinheim Germany, 2002; (d) D. Astruc (Ed.), Dendrimers and Nanosciences, C.R. Chimie, vols. 8–10, Elsevier, New York, 2003. S.K. Oh, Y. Niu, R.M. Crooks, Langmuir 21 (2005) 10209. V. Chechik, R.M. Crooks, J. Am. Chem. Soc. 122 (2000) 1243. L.K. Yeung, C.T. Lee Jr., K.P. Johnston, R.M. Crooks, Chem. Commun. (2001) 2290. M. Ooe, M. Murata, T. Mizugaki, K. Ebitani, K. Kaneda, Nano Lett. 2 (9) (2002) 999. (a) L.K. Yeung, R.M. Crooks, Nano Lett. 1 (1) (2001) 14, See also Ref. [7] in supercritical medium; (b) E.H. Rahim, F.S. Kamounah, J. Frederiksen, J.B. Christensen, Nano Lett. 1 (9) (2001) 499; (c) M. Ooe, M. Murata, T. Mizugaki, K. Ebitani, K. Kaneda, J. Am. Chem. Soc. 126 (2004) 1604. (a) Y. Li, M.A. El Sayed, J. Phys. Chem. B 105 (2001) 8938; (b) M. Pittelkow, K. Moth-Poulsen, U. Boas, J.B. Christensen, Langmuir 19 (2003) 7682; (c) F. Lu, J. Ruiz, D. Astruc, Tetrahedron Lett. 45 (2004) 9943. H. Ye, R.M. Crooks, J. Am. Chem. Soc. 127 (2005) 5097. G.J.M. Koper, M.H.P. van Genderen, C. Elissen-Roman, M.W.P.L. Baars, E.W. Meijer, M. Borkovec, J. Am. Chem. Soc. 119 (1997) 6512.
Note
Synthesis and catalytic activity of DAB-dendrimer encapsulated Pd nanoparticles for the Suzuki coupling reaction Julietta Lemo, Karine Heuze´ *, Didier Astruc
*
Groupe Nanosciences Mole´culaires et Catalyse, LCOO, UMR N 5802, Universite´ Bordeaux I, 351 cours de la libe´ration, 33405 Talence Cedex, France Received 28 November 2005; accepted 31 December 2005 Available online 24 February 2006 Dedicated to Professor Dr. Dr. h.c. mult. Wolfgang A. Herrmann.
Abstract Palladium based dendrimer-encapsulated metal nanoparticles (DEMNs) are prepared by reduction of an aqueous solution of palladium (II) and five generations of amino terminated dendrimers (DAB (diaminobutane) dendrimers). The average size of the palladium nanoparticles formed is 1.7–2.8 nm. Catalytic performances of these DEMNs have been studied in the Suzuki cross-coupling reaction. 2006 Elsevier B.V. All rights reserved. Keywords: Dendrimers; Nanoparticles; Palladium; Catalysis
Catalysis by dendrimer-encapsulated metal nanoparticles (DEMNs) is a recent concept that has largely been developed by Crooks and his group using PAMAM dendrimers [1,2]. Many examples of DEMNs are now available in the literature for the formation of stable and well-defined nanomaterials based on reduced metals such as Cu, Au, Ag, Pt, and Pd [3]. Dendrimers are particularly attractive hosts for catalytically active metal nanoparticles because of their well-defined nano-environment in the interior of the dendrimer and their chemical versatility [4]. For instance, Crooks studied catalytic properties of PAMAM dendrimer palladium nanoparticles in organic [5], fluorous [6] and supercritical solvent [7] for allylic hydrogenation. Kaneda et al. [8] investigated the case of derivatized DAB-dendrimer palladium nanoparticles. It was also demonstrated that Pd or Au DENs have a high catalytic reactivity in Heck [9] and Suzuki [10] cross-coupling reactions as well as in the Stille [11] reaction with Pt DENs in aqueous medium and at room temperature. *
Corresponding authors. Tel.: +33 05 40 00 62 74; fax: +33 05 40 00 69 94 (K. Heuze´), Tel.: +33 05 40 00 62 71; fax: +33 05 40 00 66 46 (D. Astruc). E-mail addresses: [email protected] (K. Heuze´), [email protected] (D. Astruc). 0020-1693/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2005.12.070
Since the large majority of catalytic studies have been carried out with PAMAM-DEMNs, we report here, for comparison purposes, the synthesis, the characterization, and the catalytic properties of nearly monodisperse palladium nanoparticles prepared within the interior of five different generations of amino-terminated DAB-dendrimers: DABG1, DAB-G2, DAB-G3, DAB-G4 and DAB-G5, respectively, for dendrimers of generation 1–5. These DEMNs catalysts have been prepared by encapsulation of Pd(II) within the dendrimer followed by a chemical reduction to yield the corresponding Pd(0) nanoparticles [10a,10b]. The comparison of the catalytic efficiency of PAMAM-DEMNs and DAB-DEMNs, an exercise that is unprecedented, should allow to further optimize the nature and molecular engineering of the dendrimer host in catalysis by DEMNs. The size of the nanoparticle usually depends on the number of Pd(II) initially loaded into the dendrimer [1c], so two synthetic methods were investigated depending on the Pd/dendrimer ratio. Following El Sayed’s method A [10a], we have prepared palladium DENs by addition of DAB-dendrimers to an aqueous solution of K2PdCl4 (10 eq. Pd2+). Prior to reduction, the pH of the solution was adjusted to 4 with HCl (1 M) to prevent the complexation of Pd2+ to the peripherical primary amine groups [1b,12].
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J. Lemo et al. / Inorganica Chimica Acta 359 (2006) 4909–4911
Then, once Pd2+ bind to the interior tertiary amines, the reduction with NaBH4 was carried out to form palladium DENs, and the initially yellow solution turned to brown without the formation of Pd black, except in the case of DAB-G1 and DAB-G2 dendrimers. TEM pictures (Fig. 1a) revealed relatively monodisperse nanoparticles with a mean diameter of 1.7–1.9 nm, except for DAB-G1 DEMNs that form aggregates of about 60 nm (resulting from the agglomeration of Pd particles adsorbed to the dendrimer exterior amine groups). Following Christensen’s method B [10b], DAB-dendrimer and K2PdCl4 (60 eq. Pd2+) were mixed in water followed by the addition of NaBH4 without adjusting the pH. The color of the solution changed from yellow to dark brown with the formation of Pd black. TEM pictures showed DEMNs of 1.8–2.8 nm (Fig. 1b). Catalytic performances of the Pd DEMNs (method A synthesis) were examined in the Suzuki–Miyaura reaction between iodobenzene and phenylboronic acid (Eq. (1)). The results are gathered in Table 1. In the case of low generation dendrimers, DAB-G1, DAB-G2, and DAB-G3, the open structure favors the formation of catalytically inactive Pd black. However, they exhibit a higher catalytic activity (entries 1–3) than DEMNs prepared with higher generation dendrimers (entries 4 and 5). In the latter case, the lower accessibility of the catalytic sites for the substrate entering
Fig. 1. TEM of palladium nanoparticles with DAB-G3 dendrimer. Table 1 Suzuki-reaction with DENs palladium prepared by method A Entry
DENs Gn
n (Pd2+) mol
Time
Yielda (%)
Formation of Pd black
1 2 3
G1 G2 G3
4.2 · 106 4.2 · 106 7.5 · 106
4
G4
7.5 · 106
G5
7.5 · 106
1h 1h 1h 4h 1h 4h 24 h 4 days
100 100 60 98 40 66 97 40
yes yes yes yes little little little little
5 a
Reaction conditions. Iodobenzene (1 mmol), phenylboronic acid (3 mmol) in CH3CN/H2O (3/1), NaOAc (6 mmol), 10 ml of catalyst solution at 100 C. GC yields were used after extraction with pentane.
the dendrimer results in a diminishing catalytic activity (increasing reaction time with increasing dendrimer generation). Also, DEMNs prepared with DAB-G4 and DABG5 have a full filled and more compact structure, providing an efficient Pd encapsulation. Therefore, less Pd black was formed during the catalysis reaction than in the case of low generation dendrimers. It is noteworthy that in an analogous experiment of entry 3, the pH was adjusted to 4 before addition of Pd2+ (instead of after the Pd2+ addition), resulting in a 5% yield in 4 h (instead of 98% yield), enlightening the crucial importance of the DEMNs synthetic method in the catalysis properties of such nanoparticles. NaOAc I
+
B(OH)2
100˚C CH3CN/H2O palladium nanoparticle cat.
Suzuki-reaction with DENs palladium
ð1Þ DEMNs prepared with method B and from DAB-G2, DAB-G3 and DAB-G4 dendrimers were used as well in the Suzuki cross-coupling reaction, under the same reaction conditions (Eq. (1)). In all cases, quantitative yields were obtained after 20 h along with the formation of Pd black, indicating an overall lower reactivity than DEMNs prepared with method A. In our case, the size of the particles is decisive, the smaller the particles, the more efficient they would be. The recovery and re-use of these palladium nanoparticles catalysts were investigated in the Suzuki reaction (Eq. (1)). Palladium DENs remain in the aqueous phase and these could be separated after reaction by the addition of pentane and washed with CH2Cl2 (3 · 20 ml). An aqueous solution containing the catalyst was re-injected in a new reaction medium yielding 40% of the product after 24 h, for DENs-G3 (prepared with method A), and yielding 5% in 24 h for DENs-G3 (prepared with method B). TEM analysis of the crude solution after the first reaction cycle revealed the coalescence of nanoparticles. In summary, we have prepared nearly monodisperse DAB-dendrimer encapsulated palladium nanoparticles active in the catalysis of the Suzuki reaction in aqueous media. The reactivity of palladium DENs based on DABdendrimers is similar to the reactivity of palladium DENs based on PAMAM-dendrimers for the Suzuki catalysis reactions [10a,10b]. However, the best way to increase the reactivity as well as the recoverability of such dendrimerencapsulated palladium nanoparticles is probably to derivatize the DAB-dendrimer exterior amines by functionalized alkyl chains as suggested by Yeung and Crooks [9a] or Kaneda et al. [8]. Acknowledgments Financial support from the Institut Universitaire de France (IUF), the Centre National de la Recherche Scientifique (CNRS), the territory of Nouvelle-Cale´donie and the University of Bordeaux 1 is gratefully acknowledged.
J. Lemo et al. / Inorganica Chimica Acta 359 (2006) 4909–4911
References [1] (a) M. Zhao, L. Sun, R.M. Crooks, J. Am. Chem. Soc. 120 (1998) 4877; (b) R.M. Crooks, M. Zhao, L. Sun, V. Chechik, L.K. Yeung, Acc. Chem. Res 3 (24) (2001) 181; (c) R.M. Crooks, B.I. Lemon III, L. Sun, L.K. Yeung, M. Zhao, Top. Curr. Chem. 212 (2001) 81; (d) Y. Niu, R.M. Crooks, C.R. Chimie 6 (2003) 1049. [2] L. Balogh, D.A. Tomalia, J. Am. Chem. Soc 120 (1998) 7355. [3] (a) K. Esumi, A. Suzuki, A. Yamahira, K. Torigoe, Langmuir 16 (2000) 2604; (b) F. Gro¨hn, B.J. Bauer, Y.A. Akpalu, C.L. Jackson, E. Amis, J. Macromol. 33 (2000) 6042; (c) P.F. Floriano, C.O. Noble IV, J.M. Schoonmaker, E.D. Poliakoff, R.L. McCarley, J. Am. Chem. Soc. 123 (2001) 10545; (d) R.W.J. Scott, O.M. Wilson, R.M. Crooks, J. Phys. Chem. B 109 (2005) 692; (e) Y. Niu, R.M. Crooks, Chem. Mater. 15 (2003) 3463; (f) R. Narayanan, M.A. El-Sayed, J. Phys. Chem. B 108 (2004) 8572; (g) D. Astruc, F. Lu, J. Ruiz, Angew. Chem. Int. Ed. 117 (2005) 2; (h) Y. Chung, H.K. Rhee, J. Mol. Catal. A 206 (2003) 291. [4] (a) G.E. Oosterom, J.N.H. Reek, P.C.J. Kamer, P.W.N.M. Van Leeuwen, Angew. Chem. Int. Ed. 40 (10) (2001) 1828;
[5] [6] [7] [8] [9]
[10]
[11] [12]
4911
(b) D. Astruc, F. Chardac, Chem. Rev. 101 (9) (2001) 2991; (c) G.R. Newkome, C.N. Moorefield, F. Vo¨gtle, Dendrimers and Dendrons: Concepts, Syntheses and Applications, Wiley-VCH, Weinheim Germany, 2002; (d) D. Astruc (Ed.), Dendrimers and Nanosciences, C.R. Chimie, vols. 8–10, Elsevier, New York, 2003. S.K. Oh, Y. Niu, R.M. Crooks, Langmuir 21 (2005) 10209. V. Chechik, R.M. Crooks, J. Am. Chem. Soc. 122 (2000) 1243. L.K. Yeung, C.T. Lee Jr., K.P. Johnston, R.M. Crooks, Chem. Commun. (2001) 2290. M. Ooe, M. Murata, T. Mizugaki, K. Ebitani, K. Kaneda, Nano Lett. 2 (9) (2002) 999. (a) L.K. Yeung, R.M. Crooks, Nano Lett. 1 (1) (2001) 14, See also Ref. [7] in supercritical medium; (b) E.H. Rahim, F.S. Kamounah, J. Frederiksen, J.B. Christensen, Nano Lett. 1 (9) (2001) 499; (c) M. Ooe, M. Murata, T. Mizugaki, K. Ebitani, K. Kaneda, J. Am. Chem. Soc. 126 (2004) 1604. (a) Y. Li, M.A. El Sayed, J. Phys. Chem. B 105 (2001) 8938; (b) M. Pittelkow, K. Moth-Poulsen, U. Boas, J.B. Christensen, Langmuir 19 (2003) 7682; (c) F. Lu, J. Ruiz, D. Astruc, Tetrahedron Lett. 45 (2004) 9943. H. Ye, R.M. Crooks, J. Am. Chem. Soc. 127 (2005) 5097. G.J.M. Koper, M.H.P. van Genderen, C. Elissen-Roman, M.W.P.L. Baars, E.W. Meijer, M. Borkovec, J. Am. Chem. Soc. 119 (1997) 6512.