Chapter 3. The Use of Organometallic Building Blocks in the Synthesis. of Shape-Persistent Multi(NCN-Metal) Complexes

Transcription

1 Chapter 3 The Use of rganometallic Building Blocks in the Synthesis of Shape-Persistent ulti(c-etal) Complexes Abstract: Palladated and platinated octakis- and dodecakis(c-pincer) complexes based on shape-persistent aromatic backbones were prepared. For the synthesis of these complexes, monometallated C-building blocks were coupled to rigid octakisand dodecakis(benzylic bromide) cores in a one-pot reaction. Following this modular approach, eight or twelve metal centers were selectively introduced under very mild conditions, thereby minimalizing the risk of destroying the individual organometallic moieties, a crucial aspect in the synthesis of multimetallic materials. The octakis(c-pincer) complexes have a silicon atom as the central branching point, affording tetrahedral, spherical structures. The dodecakis analogs are based on a persubstituted benzene core, giving the multimetallic complexes more flattened spherical geometries. H. P. Dijkstra, C. A. Kruithof,. Ronde, R. van de Coevering, D. J. Ramón, D. Vogt, G. P.. van Klink, G. van Koten, J. rg. Chem., in press. 61

2 rganometallic Building Blocks in the Synthesis of Shape-Persistent ulti(c-etal) Complexes 3.1. Introduction In the field of homogeneous catalysis there is currently wide interest in the application of tailored/engineered organic molecules as soluble support materials for anchored catalytically-active metal complexes. 1,2 Such organic materials often have a periphery bearing multidentate ligands or ligand precursors and are usually designed in such a way that the resulting multimetallic catalysts can easily be removed after catalysis from the product-containing solution for reuse. 3 A crucial aspect in the preparation of multimetallic materials is the sequence in which the metals and appropriate organic functionalities are introduced into the system. ften the organic core bearing the multidentate ligand-sites is prepared as far as possible, prior to the metalation procedures, i.e. the metal centers are introduced in the last step to give the multimetallic species (Route A, Scheme 1). 4 This approach has the advantage that a minimal number of steps is performed with the organometallic species itself. A disadvantage of this method is that incomplete metalation can occur, especially when increasing numbers of ligand-sites have to be metalated. An alternative strategy to multimetallic materials is to anchor an appropriately functionalized, monometalated ligand to a support material (Route B, Scheme 1). 5 This approach has the advantage that all ligand-sites in the final multimetallic complex are metalated since the metal is already present in the step in n + n A n B Y n + n X = metal center = support = ligand Scheme 1. Two routes to prepare multimetallic materials. 62

3 Chapter three which the multimetallic material is constructed. A prerequisite of this approach is that the organometallic moiety has to be resistent to the reaction conditions used for the coupling of the monometallic building blocks to the support materials. In this chapter, the use of organometallic mono(c-pincer) building blocks in the synthesis of palladated and platinated octakis- and dodecakis(c-pincer) complexes is discussed Results and Discussion Synthesis of rganometallic Building Blocks. For the synthesis of multi(cmetal) complexes 7 and 9, organometallic building blocks 3a and 3b were prepared (Scheme 2). The synthesis began with the iodination of 1 using a lithiation/iodination reaction sequence, resulting in ligand precursor 2 in 66% yield. Treatment of 2 with Pd(dba) 2 resulted in the formation of palladium(ii) pincer 3a (72%), 6 while the corresponding platinum(ii) compound 3b was prepared in 83% yield by reaction of 2 with [Pt(p-tol) 2 SEt 2 ] 2. 7 (i) I (ii) ( = Pd) (iii) ( = Pt) I 1 2 3a: = Pd 3b: = Pt Scheme 2. (i) n-bui, hexanes, 70 rt, 4 h, followed by I 2, THF, rt, 3 h; (ii) Pd(dba) 2, C 6 H 6, rt, 6 h; (iii) [Pt(p-tol) 2 SEt 2 ] 2, C 6 H 6, reflux, 3 h. Synthesis of ctakis(c-pincer-metal) Complexes. The synthesis of the central organic core for complexes 7a c started with the lithiation of iodoarene 4 (Scheme 3). The in situ prepared aryllithium compound was reacted with tetrachlorosilane, thereby obtaining tetraarylsilane 5 in 90% yield. Subsequently, the methoxy groups of 5 were converted into bromides by treatment with acetyl bromide and trifluoroborane etherate, resulting in the formation of octakis(benzylic bromide) 6 in 52% yield. Reaction of this octakis(benzylic bromide) with eight equivalents of palladium(ii) building block 3a in acetone at room temperature in the presence of tetrabutylammonium fluoride, gave the corresponding octakis(c-pd II ) complex 7a (67% yield). Platinum complex 7b was analogously obtained by reaction of 6 with eight equivalents of platinum(ii) building block 3b in 44% isolated yield. eutral 63

4 rganometallic Building Blocks in the Synthesis of Shape-Persistent ulti(c-etal) Complexes octakis(c-pd II ) complex 7a was readily converted into the corresponding ionic complex 7c by treatment with eight equivalents of silver tetrafluoroborate in wet acetone. e e I e (i) e e (ii) e e e 4 e e 5 6 e 2 e 2 e 2 e 2 e 2 6 e 2 + (iii) 8 I 3a: = Pd 3b: = Pt e 2 e 2 e 2 e 2 e 2 e 2 7a: = Pd, = I 7b: = Pt, = I 7c: = Pd, = H 2 (BF 4 ) (iv) Scheme 3. (i) 2 eq. t-bui, Et 2, 90 C, 15 min followed by 0.25 eq. Cl 4, 90 C rt, 20 h; (ii) Ac, BF 3 Et 2, CH 2 Cl 2, reflux, 20 h; (iii) Bu 4 F, K 2 C 3, 18-crown- 6, acetone, rt, 20 h; (iv) AgBF 4, wet acetone, rt, 1 h. Synthesis of Dodecakis(pincer-metal) Complexes. The synthesis of dodecakis(c-pincer) complexes 9a c is outlined in Scheme 4. Reaction of twelve equivalents of organometallic building block 3a with dodecakis(benzylic bromide) 8 in the presence of tetrabutylammonium fluoride, resulted in the formation of 64

5 Chapter three dodecakis(c-pd II ) complex 9a. The corresponding platinum(ii) complex 9b was obtained analogously by reaction of twelve equivalents of 3b with 8. The neutral complex 9a could easily be converted into the corresponding ionic complex 9c with silver tetrafluoroborate in wet acetone (Scheme 4) I 3a: = Pd 3b: = Pt (i) e 2 e 2 e 2 e 2 e 2 e 2 e 2 e 2 e 2 e 2 e e 2 2 e 2 e 2 e 2 e 2 9a: = Pd, = I 9b: = Pt, = 9c: = Pd, = H 2 (BF 4 ) (ii) Scheme 4. (i) Bu 4 F, K 2 C 3, 18-crown-6, acetone, rt, 20 h (9b was prepared in the presence of i); (ii) AgBF 4, wet acetone, rt, 1 h. Important to note is that in the synthesis of 7 and 9 eight or twelve metal centers, respectively, are introduced into the multimetallic material in a one-pot reaction under very mild reaction conditions. With the metal already present in the pincer moiety in the final construction step of the multimetallic material, this route results in the formation of fully metalated materials. The alternative route - preparation of the octakis- or dodecakis(c-pincer) ligands followed by permetalation - is a method which can lead to incomplete metalation (see hexakis(c-pd II ) complex 11, Chapter 2) and enhanced decomposition because more severe reaction conditions are required. Furthermore, in contrast to the shapepersistent macromolecular complexes discussed in Chapter 2, which are rather flat, two-dimensional materials, complexes 7 and 9 possess more three-dimensional 65

6 rganometallic Building Blocks in the Synthesis of Shape-Persistent ulti(c-etal) Complexes geometries. Complexes 7a c are based on a tetraaryl substituted silicon center, giving the molecules their spherical, three-dimensional geometries. Complexes 9a c are based on a hexaaryl substituted benzene core surrounded by a more flexible organometallic core, affording a flattened-spherical geometry. These materials will be further investigated as homogeneous catalysts in organic transformations and will also be used to study the influence of shape-persistence and geometry on the retention rates of macromolecular catalysts by nanofiltration membranes Concluding Remarks A new modular approach to prepare palladated and platinated octakis- and dodecakis(c- II ) complexes 7 and 9, respectively, was developed. In this approach, functionalized monometallic C-pincer building blocks were selectively coupled to multifunctionalized organic supports. These reactions were performed under very mild reaction conditions, e.g. in acetone at room temperature, minimalizing the risk of destroying the individual organometallic moieties. Clearly, complexes 7 and 9 possess shape-persistent cores, surrounded by a more flexible organometallic core, affording these structures more spherical geometries. A high degree of shape-persistence together with a three-dimensional geometry is expected to be important to obtain optimal retentions of these molecules by nanofiltration membranes. This can lead to continuous catalytic processes in a nanofiltration membrane reactor in which the homogeneous catalysts are retained very efficiently Experimental Section General: All reactions were carried out using standard Schlenk techniques under an inert nitrogen atmosphere unless stated otherwise. Et 2, THF and hexanes were carefully dried and distilled from a/benzophenone prior to use. CH 2 Cl 2 was distilled from CaH 2. All standard reagents were purchased. Compounds 1, 8 4, 9 8, 9 10 Pd(dba) 2 7 and [Pt(p-tol) 2 SEt 2 ] 2 were prepared according to literature procedures. 1 H (200 or 300 Hz) and 13 C (50 or 75 Hz) R spectra were recorded on a Varian AC200 or Varian 300 spectrometer at 25 C, chemical shifts are in ppm referenced to residual solvent resonances. ADI-TF-S spectra were acquired using a Voyager-DE Bio-Spectrometry Workstation mass spectrometer equipped with a nitrogen laser emitting at 337 nm. The matrix (3,5-dihydroxybenzoic acid) and the sample were dissolved in THF or CH 2 Cl 2 (~30 mg/ml) and 0.2 µl of both solutions were mixed and 66

7 Chapter three placed on a gold ADI target and analyzed after evaporation of the solvent. Elemental microanalyses were performed by Dornis und Kolbe, ikroanalytisches aboratorium, üllheim a.d. Ruhr, Germany. Synthesis of 4-tert-butyldimethylsiloxyl-2,6-bis[(dimethylamino)methyl]-1- iodobenzene (2): n-bui (11.3 m, 18.0 mmol) was added to a solution of 1 (5.0 g, 15.5 mmol) in hexanes (50 m) at 70 C. After addition was completed, the temperature was allowed to rise to room temperature and stirring was continued for 4 h. Subsequently, iodine (5.1 g, 20.0 mmol) in THF (15 m) was added and stirring was continued for 3 h at room temperature. The reaction mixture was poured into ahs 3 (aq) (2, 100 m) and the organic layer was collected. The aqueous layer was extracted with Et 2 (3 50 m) and the combined organic layer was washed with brine (50 m), dried over gs 4 and filtered. After evaporation of all volatiles a brown oil remained, which was flame-distilled to yield a yellow oil. Yield: 4.6 g (66%). This product (2) was used without further purification. 1 H R (CDCl 3, 200 Hz): δ 6.86 (s, 2H, ArH), 3.47 (s, 4H, CH 2 ), 2.30 (s, 12H, CH 3 ), 0.97 (s, 9H, CCH 3 ), 0.19 (s, 6H, CH 3 ). Synthesis of 4-tert-butyldimethylsiloxyl-2,6-bis[(dimethylamino)methyl]-1- (iodopalladio)benzene (3a): A solution of 2 (2.35 g, 5.23 mmol) and Pd(dba) 2 (2.85 g, 5.23 mmol) in benzene (30 m) was stirred at room temperature for 6 h. Subsequently, this mixture was filtered over Celite and all volatiles were evaporated. The residue was extracted with CH 2 Cl 2 (10 m) and pentane was added dropwise to this solution resulting in the formation of a yellowish solid. The procedure was repeated until a white solid was obtained. This solid was collected and dried in vacuo. Yield: 2.06 g (72%). 1 H R (CDCl 3, 300 Hz): δ 6.39 (s, 2H, ArH), 3.94 (s, 4H, CH 2 ), 3.01 (s, 12H, CH 3 ), 0.96 (s, 9H, CCH 3 ), 0.16 (s, 6H, CH 3 ). 13 C R (CDCl 3, 75 Hz): δ , , , , 74.21, 55.20, 25.86, 18.30, Anal. Calcd for C 18 H 33 I 2 Pd: C, 38.96; H, 6.00;, Found: C, 38.88; H, 5.93;, Synthesis of 4-tert-butyldimethylsiloxyl-2,6-bis[(dimethylamino)methyl]-1- (iodoplatino)benzene (3b): A mixture of 2 (1.03 g, 2.30 mmol) and [Pt(p-tol) 2 SEt 2 ] 2 (1.07 g, 1.15 mmol) in benzene (20 m) was heated at reflux for 3 h. After cooling the solution to room temperature, all volatiles were evaporated. The light-yellow solid was washed with Et 2 (2 15 m) and the remaining white solid was dissolved in 67

8 rganometallic Building Blocks in the Synthesis of Shape-Persistent ulti(c-etal) Complexes CH 2 Cl 2 (5 m). Upon dropwise addition of hexanes (20 m) the product precipitated as a white solid. This solid was collected and dried in vacuo. Yield: 1.23 g (83%). 1 H R (C 6 D 6, 300 Hz): δ 6.34 (s, 2H, ArH), 3.19 (s, 3 J Pt,H = 21.9 Hz, CH 2 ), 2.75 (s, 3 J Pt,H = 19.2 Hz, CH 3 ), 1.06 (s, 9H, CCH 3 ), 0.19 (s, 6H, CH 3 ). 13 C R (CDCl 3, 75 Hz): δ , , , , 76.59, 55.93, 25.79, 18.27, Anal. Calcd for C 18 H 33 I 2 Pt: C, 33.59; H, 5.17;, Found: C, 33.68; H, 5.08;, Synthesis of tetra[3,5-bis(methoxymethyl)phenyl]silane (5): To a solution of 4 (4.91 g, 16.8 mmol) in Et 2 (60 m) was added t-bui (20 m, 30 mmol) at 90 C. After stirring the white suspension for ca. 15 min, Cl 4 (0.4 m, 3.49 mmol) was added, and the resulting red-brown suspension was allowed to reach room temperature and was stirred for an additional 20 h. An extra amount of t-bui (5 m, 7.50 mmol)) was added, followed by H 2 (100 m) to hydrolyze the excess of t- Bui. The organic layer was separated and the aqueous layer was extracted with Et 2 (2 100 m). The combined organic layers were dried over a 2 S 4, filtered, and concentrated in vacuo. The free 3,5-bis(methoxymethyl)benzene was removed by bulb-to-bulb distillation ( C, mm Hg) to yield a yellow, viscous oil. Yield: 8.7 g (90 %). 1 H R (C 6 D 6, 200 Hz): δ 7.87 (s, 8H, ArH), 7.55 (s, 4H, ArH), 4.18 (s, 12H, CH 2 ), 3.07 (s, 48H, CH 3 ). 13 C R (C 6 D 6, 75 Hz): δ 139.0, 135.1, 134.7, 128.9, 74.5, R (C 6 D 6, 59 Hz): δ Synthesis of tetra[3,5-bis(bromomethyl)phenyl]silane (6): To a solution of 5 (2.16 g, 3.14 mmol) in CH 2 Cl 2 (500 m) was added dropwise a solution of BF 3 Et 2 (13 m, 0.10 mol) and acetyl bromide (8 m, 0.10 mol) in CH 2 Cl 2 (60 m) at 0 C. The slightly brown solution was heated to reflux for 20 h, whereupon the reaction mixture was allowed to cool to RT. Aqueous a 2 C 3 (5%, 450 m) was added slowly to hydrolyze the excess of BF 3 Et 2. ext, the organic layer was separated, washed with water (2 200 m), dried over a 2 S 4, filtered, and concentrated in vacuo. The product was purified by crystallization from a CH 2 Cl 2 /pentane mixture (v/v 5:1) at 25 C, to yield a white powder (52%). 1 H R (CDCl 3, 300 Hz): δ 7.56 (s, 4H, ArH), 7.45 (s, 8H, ArH), 4.46 (s, 16H, CH 2 ). 13 C R (CDCl 3, 75 Hz): δ , , , , Anal. Calcd for C 32 H 28 8 : C, 35.59; H, 2.61;, Found: C, 35.74; H, 2.65;,

9 Chapter three Synthesis of 7a: Bu 4 F (1 in THF, 1.05 m, 1.05 mmol) was added to a mixture of 6 (0.14 g, 0.13 mmol), 3a (0.58 g, 1.05 mmol), K 2 C 3 (0.73 g, 5.25 mmol) and 18- crown-6 (20 mg, 76 µmol) in acetone (15 m) and this mixture was stirred at room temperature for 20 h. Subsequently, all volatiles were evaporated and the residue was extracted with CH 2 Cl 2 (10 m). Upon dropwise addition of Et 2 (20 m) an offwhite solid precipitated, which was collected. This procedure was repeated twice and the product was dried in vacuo. Yield: 0.34 g (67%). 1 H R (CDCl 3, 300 Hz): δ 7.65 (s, 4H, ArH), 7.50 (s, 8H, ArH), 6.44 (s, 16H, ArH), 4.92 (s, 16H, CH 2 ), 3.92 (s, 32H, CH 2 ), 2.95 (s, 96H, CH 3 ). 13 C R (CD 2 Cl 2, 75 Hz): δ , , , , , , , , 77.44, 70.83, Anal. Calcd for C 128 H 172 I 8 16 Pd 8 8 : C, 38.85; H, 4.38;, Found: C, 39.03; H, 4.46;, Synthesis of 7b: Bu 4 F (1 in THF, 0.75 m, 0.75 mmol) was added to a mixture of 6 (0.10 g, 93 µmol), 3b (0.48 g, 0.75 mmol), K 2 C 3 (0.52 g, 3.76 mmol) and 18- crown-6 (20 mg, 76 µmol) in acetone (15 m) and this mixture was stirred at room temperature for 20 h. Subsequently, all volatiles were evaporated and the residue was extracted with CH 2 Cl 2 (10 m). Upon dropwise addition of Et 2 (20 m) a white solid precipitated, which was collected. This procedure was repeated twice and the product was dried in vacuo. Yield: 0.19 g (44%). 1 H R (CD 2 Cl 2, 300 Hz): δ 7.66 (s, 4H, ArH), 7.61 (s, 8H, ArH), 6.49 (s, 16H, ArH), 4.98 (s, 16H, CH 2 ), 3.94 (s, 3 J Pt, H not resolved, 32H, CH 2 ), 3.05 (s, 3 J Pt, H not resolved, 96H, CH 3 ). 13 C R (CD 2 Cl 2, 75 Hz): δ , , , , , , , , 77.93, 70.72, ADI-TF-S: m/z ([ I] +, calcd ). Anal. Calcd for C 128 H 172 I 8 16 Pt 8 8 : C, 32.94; H, 3.72;, Found: C, 33.16; H, 3.81;, Synthesis of 7c: A solution of AgBF 4 (19.2 mg, 98.8 µmol) in water (0.5 m) was added to a suspension of 7a (48.9 mg, 12.4 µmol) in acetone (10 m) and the resulting mixture was stirred at room temperature in the absence of light for 1 h. The reaction mixture was filtered through Celite and the Celite was washed with acetone (10 m). The filtrate was concentrated to 3 m and dropwise addition of Et 2 (10 m) resulted in an off-white precipitate, which was collected and dried in vacuo. Yield: 39.4 mg (84%). 1 H R (acetone-d 6, 200 Hz): δ 7.73 (m, 12H, ArH), 6.58 (s, 16H, ArH), 5.07 (s, 16H, CH 2 ), 4.00 (s, 24H, CH 2 ), 2.76 (s, 96H, CH 3 ). 13 C R (acetone-d 6, 75 Hz): δ , , , , , , , , 73.49, 70.14,

10 rganometallic Building Blocks in the Synthesis of Shape-Persistent ulti(c-etal) Complexes Synthesis of 9a: Bu 4 F (1 in THF, 0.97 m, 0.97 mmol) was added to a mixture of 8 (0.12 g, 79 µmol), 3a (0.53 g, 0.95 mmol), K 2 C 3 (0.66 g, 4.80 mmol) and 18- crown-6 (25 mg, 95 µmol) in acetone (20 m) and the resulting mixture was stirred at room temperature for 20 h. Subsequently, all volatiles were evaporated and the residue was extracted with CH 2 Cl 2 (10 m). Upon dropwise addition of acetone (20 m) a white solid precipitated, which was collected. This procedure was repeated twice and the product was dried in vacuo. Yield: 0.25 g (53%). 1 H R (CD 2 Cl 2, 300 Hz): δ 6.96 (br s, 18H, ArH), 6.26 (br s, 24H, ArH), 4.69 (br s, 24H, CH 2 ), 3.83 (br s, 48H, CH 2 ), 2.96 (br s, 144H, CH 3 ). 13 C R (CD 2 Cl 2, 75 Hz): δ 157,30, , , , , , , , , 74.22, 70.20, Anal. Calcd for C 198 H 258 I Pd : C, 39.86; H, 4.36;, Found: C, 40.08; H, 4.28;, Synthesis of 9b: Bu 4 F (1 in THF, 0.97 m, 0.97 mmol) was added to a mixture of 8 (0.12 g, 79 µmol), 3b (0.61 g, 0.95 mmol), i (0.83 g, 9.6 mmol), K 2 C 3 (0.66 g, 4.80 mmol) and 18-crown-6 (25 mg, 95 µmol) in acetone (20 m) and the resulting mixture was stirred at room temperature for 20 h. Subsequently, all volatiles were evaporated and the residue was extracted with CH 2 Cl 2 (10 m) and filtered. Upon dropwise addition of ethyl acetate (20 m), a white solid precipitated, which was collected. This procedure was repeated twice and the product was dried in vacuo. Yield: 0.34 g (67%). 1 H R (CD 2 Cl 2, 300 Hz): δ 7.00 (br s, 18H, ArH), 6.29 (br s, 24H, ArH), 4.69 (br s, 24H, CH 2 ), 3.84 (br s, 3 J Pt,H not resolved, 48H, CH 2 ), 3.03 (br s, 3 J Pt,H not resolved, 144H, CH 3 ). 13 C R (CD 2 Cl 2, 75 Hz): most relevant peaks δ , , , 76.10, 70.60, ADI-TF-S: m/z ([ ] +, calcd ). Anal. Calcd for C 198 H Pt : C, 36.78; H, 4.02;, Found: C, 36.94; H, 4.20;, Synthesis of 9c: A solution of AgBF 4 (75 mg, 0.38 mmol) in water (1 m) was added to a suspension of 9a (0.19 g, 32.0 µmol) in acetone (10 m) and the resulting mixture was stirred at room temperature in the absence of light for 1 h. The reaction mixture was filtered through Celite and the Celite was washed with acetone (10 m). The filtrate was concentrated to 3 m and dropwise addition of Et 2 (10 m) resulted in an off-white precipitate, which was collected and dried in vacuo. Yield: 0.17 g (91%). 1 H R (acetone-d 6, 200 Hz): δ 7.07 (s, 12H, ArH), 7.05 (s, 6H, ArH), 6.45 (s, 24H, ArH), 4.75 (s, 24H, CH 2 ), 4.00 (s, 48H, CH 2 ), 2.80 (s, 144H, CH 3 ). 70

11 Chapter three 13 C R (acetone-d 6, 75 Hz): δ , , , , , , , , 73.51, 69.90, References and otes 1 a) Knapen, J. W. J.; van der ade, A. W.; de Wilde, J. C.; van eeuwen, P. W...; Wijkens, P.; Grove, D..; van Koten, G. ature 1994, 372, 659. b) Kragl, U.; Dreisbach, C. Angew. Chem. Int. Ed. 1996, 35, 642. c) Reetz,. T.; ohmer, G.; Schwickardi, R. Angew. Chem. Int. Ed. 1997, 36, d) Giffels, G.; Beliczey, J.; Felder,.; Kragl, U. Tetrahedron: Asymm. 1998, 9, 691. e) inkmann,.; Giebel, D.; ohmer, G.; Reetz,. T.; Kragl, U. J. Cat. 1999, 183, 163. f) Hovestad,. J.; Eggeling, E. B.; Heidbüchel, H. J.; Jastrzebski, J. T. B. H.; Kragl, U.; Keim, W.; Vogt, D.; van Koten, G. Angew. Chem. Int. Ed. 1999, 38, g) de Groot, D.; Eggeling, E. B.; de Wilde, J. C.; Kooijman, H.; van Haaren, R. J.; van der ade, A. W.; Spek, A..; Vogt, D.; Reek, J.. H.; Kamer, P. C. J.; van eeuwen, P. W... Chem. Commun. 1999, h) Kleij, A. W.; Gossage, R. A.; Klein Gebbink, R. J..; Reyerse, E. J.; inkmann,.; Kragl, U.; utz,.; Spek, A..; van Koten, G. J. Am. Chem. Soc. 2000, 122, i) Albrecht,.; Hovestad,. J.; Boersma, J.; van Koten, G. Chem. Eur. J. 2001, 7, j) Rissom, S.; Beliczey, J.; Giffels, G.; Kragl, U.; Wandrey, C. Tetrahedron: Asymm. 1999, 10, 923. k) Dwars, T.; Haberland, J.; Grassert, I.; Kragl, U. J. ol. Cat. A: Chem. 2001, 168, For reviews on dendritic catalysts see: a) Kreiter, R.; Kleij, A. W.; Klein Gebbink, R. J..; van Koten, G. Topics in Current Chemistry, Dendrimers IV; Vögtle, F., Ed.; Springer-Verlag Berlin Heidelberg, 2001, 217, 163. b) Kleij, A. W.; Klein Gebbink, R. J..; van Koten, G. Dendrimers and other Dendritic Polymers; Frechet, J., Tomalia, D., Eds.; Wiley, ew York, c) osterom, G. E.; Reek, J.. H.; Kamer, P. C. J.; van eeuwen, P. W... Angew. Chem. Int. Ed. 2001, 40, For an overview on homogeneous catalyst recycling using ultra- or nanofiltration techniques, see: Dijkstra, H. P.; van klink, G. P..; van Koten, G. Acc. Chem. Res., in press, manuscript available on the world wide web: 4 a) Constable, E. C.; Thompson, A.. W. J. Chem. Soc., Dalton Trans. 1992, b) Armspach, D.; Cattalini, C.; Constable, E. C.; Housecroft, C. E.; Phillips, D. Chem. Commun. 1996, c) Huck, W. T. S.; Snellink-Ruël, B.; van Veggel, F. C. J..; Reinhoudt, D.. rganometallics 1997, 16, d) Huck, W. T. S.; Prins,. J.; Fokkens, R. H.; ibbering,...; van Veggel, F. C. J..; Reinhoudt, D.. J. Am. Chem. Soc. 1998, 120, e) Hoare, J..; orenz, K.; Hovestad,. J.; Smeets, W. J. J.; Spek, A..; Canty, A. J.; Frey, H.; van Koten, G. rganometallics 1997, 16, f) Dijkstra, H. P.; eijer,. D.; Patel, J.; Kreiter, R.; van Klink, G. P..; utz,.; Spek, A..; Canty, A. J.; van Koten, G. rganometallics, 2001, 20, g) Dijkstra, H. P.; Albrecht,.; van Koten, G. Chem. Commun. 2002, a) Albrecht,.; Gossage, R. A.; Spek, A..; van Koten, G. Chem. Commun. 1998, b) Tzalis, D.; Tor, Y. Chem. Commun. 1996, c) Albrecht,.; Schlupp,.; Bargon,.; van Koten, G. 71

12 rganometallic Building Blocks in the Synthesis of Shape-Persistent ulti(c-etal) Complexes Chem. Commun. 2001, Alsters, P..; Baesjou, P. J.; Janssen,. D.; Kooijman, H.; cherer-roetman, A.; Spek, A..; van Koten, G. rganometallics 1992, 11, Canty, A. J.; Patel, J.; Skelton, B. W.; White, A. H. J. rganomet. Chem. 2000, 599, Davies, P. J.; Veldman,.; Grove, D..; Spek, A..; utz, B. T. G.; van Koten, G. Angew. Chem. Int. Ed. 1996, 35, See ref. 4f and: Duchêne, K. -H.; Vögtle, F. Synthesis 1986, Rettig,. F.; aitlis, P..; Cotton, F. A.; Webbs, T. R. Inorg. Synth. 1971,