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Heterogenised Rh(I), Ir(I) metal complexes with chiral triazadonor ligands: a cooperative effect between support and complex

Camino Gonz alez-Arellano a,c , Avelino Corma b , Marta Iglesias a, * , F elix Sanchez c, *a Instituto de Ciencia de Materiales de Madrid, CSIC, Cantoblanco, 28049 Madrid, Spain

b Instituto de Tecnolog a Qu mica, UPV-CSIC, Avda de los Naranjos s/n 46022, Valencia, Spainc Instituto de Qu mica Org a nica, CSIC, Juan de la Cierva, 3. 28006 Madrid, Spain

Received 15 March 2004; accepted 10 April 2004Available online

Abstract

Rhodium and iridium complexes of the chiral triaza ligands, { N ,N 0-bis{[(2 S )-(1-benzylpyrrolidinyl)]methyl}amine ( 2), N ,N 0-bis{[(2S )-(1-benzylpyrrolidinyl)]methyl}-N-propylamine ( 3), N ,N 0-bis{[(2 S )-(1-benzylpyrrolidinyl)]methyl}-N-[3-(triethoxysilyl)pr-o-pyl]amine ( 4)}, are described. All ligands form one to one [ML] species with the above metal ions. The structures of thesecomplexes were elucidated by analytical and spectroscopic data (elemental analysis, mass spectroscopy, IR, 1H and 13 C NMR). Thexationof thepreformedtriethoxysilyl-rhodiumand iridium complexes, onmesoporous solids (MCM-41,SBA-15), andtheir use,underheterogeneousconditions, for thehydrogenation reactionsarereported.The catalyticactivityandselectivity ofheterogenisedcomplexesare higher to that observed under homogeneous conditions, as a consequence of the complex- and/or reagents-to-support interaction.Thestable covalent bond between support andcomplex allows therecoveryand recyclingof theheterogenisedcatalysts fora number of cycles, moreover atomic absorption analysis of the reaction solutions shows that there is not any metal leaching into the solutions. 2004 Elsevier B.V. All rights reserved.

Keywords: Rhodium; Iridium; Hydrogenation; Immobilisation; Mesoporous

1. Introduction

In 1994 Togni and Venanzi [1] reported very prom-ising results with nitrogen donor ligands in asymmetriccatalysis. Six years later, Fache et al. [2] published aprecise review on the current state of the investigationsin this area and recently, one revision [3] shows theprogress accumulated since then in catalytic transfor-mations using tetraaza ligands. Nitrogen-containing li-gands have several distinct advantages. First, they arelargely available in enantiomerically pure form, both inthe chiral pool (quinine, cinchonine, and sparteine) or ascheap industrial chemical intermediates. On the otherhand, chirality on the nitrogen atom is difficult to ob-tain. Contrary to the phosphines, the chiral nitrogenatoms epimerise instantaneously at room temperature;

the formation of a stable chiral centre on a nitrogenatom is, however, possible by using bicyclic structures.

The second advantage of the nitrogen-containing li-gands lies in the chemistry of the nitrogen functionalgroup itself. The chemistry of these is not always easy,but it has received such abundant attention that thereexist, in most cases, numerous synthetic solutions toeach possible transformation of these compounds. As aresult, these synthetic possibilities allow tailor-mademodications for the preparation of ligands with specicphysicochemical properties. In particular, the interac-tions with the transition metals may be widely varied bypreparing X-type ligands (amides, sulfonamides), L-typeligands (amines) or p -type ligands (imines).

Nitrogen-containing ligands are being used more andmore in asymmetric catalysis. They turn out to be suit-able for any type of catalysis and especially for hetero-geneous catalysis [46], which is one of their mainadvantages over phosphines. In addition, nitrogen-containing ligands may be used in asymmetric catalysis

* Corresponding authors. Tel.: +34913349000; fax: +34913720623.E-mail address: [email protected] (M. Iglesias).

0020-1693/$ - see front matter 2004 Elsevier B.V. All rights reserved.doi:10.1016/j.ica.2004.04.024

Inorganica Chimica Acta 357 (2004) 30713078

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with transition metals, which are less expensive thannoble metals [7].

Starting from the readily available LL -proline, ourgroup [812] reported the preparation of the tetraazaC 2-symmetric ligands and their metal-complexes (Rh(I),Ir(I), Cu(I), Mn(II)). These complexes were tested on theasymmetric hydrogenation, cyclopropanation and oxi-dation reactions, showing that ligands gave only poorenantioselectivities ( < 20%) but excellent chemical yields(> 85%). These catalysts have been heterogenised andthe strategy used to preserve as much as possible thecoordination sphere of the metal [13]. The heterogenisedcomplexes are still more stable than their homogeneouscounterpart and they can be used several times withoutdecreasing activity. Nevertheless, the enantioselectivityachieved with the heterogeneous systems remains closeto that obtained with the homogeneous ones.

Herein, we present the synthesis and characterisationof a new family of chiral triaza ligands and their Rh(I)

and Ir(I) complexes and investigate their co-ordinationchemistry and catalytic properties in detail, and howefficient and selective heterogeneous zeolite systems in-volving this type of ligands could be designed (seeFig. 1).

2. Experimental

2.1. General

Preparation of organometallic complexes was carriedout under dinitrogen by conventional schlenk-tubetechniques. The starting complexes [Rh(cod)Cl] 2[1416]and [Ir(cod)Cl] 2 [17] were prepared using reportedmethods. All solvents were carefully degassed beforeuse. The silylating agent iodopropyltriethoxysilane wasobtained by halogen change starting from the commer-cial chloropropyltriethoxysilane. C, H and N analysiswas carried out by the analytical department of the In-stitute of Materials Science (CSIC). Metal contents wereanalysed by atomic absorption using a PerkinElmerAAnalyst 300 atomic absorption and plasma ICP Per-kinElmer 40 spectrometers. Mass spectra were per-formed on a Hewlett-Packard 1100 MSD mass

spectrometer (ESI-MS, APCI-MS) with positive mode.IR spectra were recorded on a Bruker IFS 66v/S spec-trophotometer (range 4000200 cm 1) in KBr pellets;1H, 13C NMR spectra were taken on a Varian XR300and a Bruker 200 spectrometers. 1H NMR chemicalshifts are given in ppm using tetramethylsilane as an

internal standard. High resolution13

C MAS or CP/MAS-NMR spectraof powdered samples, in some casesalso with a Toss sequence, in order to eliminate thespinning side bands, were recorded at 100.63 MHz, 6 l s90 pulse width, 2 ms contact time and 510 recycledelay, using a Bruker MSL 400 spectrometer equippedwith an FT unit. The spinning frequency at the magicangle (54 440) was 4 KHz.Optical rotation values weremeasured at the sodium-D line (589 nm) with a Perkin Elmer 241 MC polarimeter. Gas chromatographyanalysis was performed using a Hewlett-Packard 5890 IIwith a ame ionisation detector in a cross-linkedmethylsilicone column [18]. The inorganic support foranchoring was a purely siliceous MCM-41 [24] andSBA-15 [19].

2.1.1. Synthesis of ligandsLigand N ,N -bis{[( S )-1-benzylpyrrolidin-2-yl]methyl}

amine ( 2) was prepared according to modied publishedmethods using ethyl chloroformate/trimethyl amine asactivating reagent [20].

2.1.2. Synthesis of N,N-bis { [(S)-1-benzylpyrrolidin-2- yl]methyl } propan-1-amine ( 3)

To a solution of amine ( 2) (0.405 g, 1 mmol) in ace-

tonitrile (25 ml), K 2CO 3 (1.42 g, 10 mmol) was addedand the mixture stirred for 10 min, then propyl iodide(0.195 ml, 2 mmol) was added dropwise. The mixturewas reuxed for 8 h and puried by ash chromatog-raphy. (ethyl acetate:ethanol 20:1). The product wasisolated as yellow oil (65%); a 25D 142 (c: 1.49,CHCl 3); 1H NMR (CDCl 3): d (ppm) 7.407.18 (10H,m, H arom ); 4.10 (2H, d, AB, J 16 Hz); 3.30 (2H, d, AB,J 16.0 Hz); 3.022.90 (2H, m, H 5a ;5a 0); 2.672.50 (4H,m, H 2;20, H 7a ;7a 0); 2.462.24 (4H, m, H 7b ;7b 0, H 8); 2.23 2.12 (2H, m, H 5b ;5b 0); 2.071.89 (2H, m, H 3a ;3a 0); 1.82 1.56 (6H, m, H 3b ;3b 0, H 4;40); 1.44 (2H, sex, H 9 , J 8.9Hz); 0.84 (2H, t, H 10 , J 8Hz). 13C NMR (CDCl 3):d (ppm) 139.00 (C arom ); 129.02 (C arom ); 128.18 (C arom );126.85 (C arom ); 62.50 (C 2); 60.25 (C 7); 59.65 (C H 2Ph);57.65 (C 8); 54.74 (C 5); 30.38 (C 3); 22.49 (C 4); 20.24 (C 9);11.88 (C 10). EM, m = z (%): 406.

2.1.3. Synthesis of N,N-bis { [(S)-1-benzylpyrrolidin-2- yl]methyl }-3-(triethoxysilyl)propan-1-amine ( 4)

To a solution of 2 (0.405 g, 1 mmol) in DMF (25 ml),K 2CO 3 (1.42 g, 10 mmol) and NBu 4HSO 3 (0.320 g, 0.1mmol) were added. The solution was stirred for 10 minand iodopropyltriethoxysilane (0.332 g, 1 mmol)) wasadded dropwise. The mixture was reuxed for 8 h and

N

N

NR2: R = H,3: R = (CH 2)2CH 3,4: R = (CH 2)3Si(OEt) 3,

2

3 45

67

7'

2'

3' 4'

5'

6'

Fig. 1. Chiral triaza ligands.

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puried by ash chromatography (ethyl acetate:ethanol1:1). The product was isolated as yellow oil (69%).a 25D 1:94 (c: 0.77, CHCl 3). IR (lm, cm

1): m 1079(SiO). 1H NMR (CDCl 3): d (ppm) 7.407.18 (10H,m, H arom ); 4.08 (2H, d, AB, J 10 Hz); 3.80 (q, J 27Hz, OC H 2CH 3); 3.22 (2H, d, AB, J 10 Hz); 2.982.85

(2H, m, H 5a ;5a 0); 2.652.47 (4H, m, H 2a ;2a 0, H 7a ;7a 0); 2.46 2.28 (4H, m, H 7b ;7b 0, H 8); 2.212.18 (2H, m, H 5b ;5b 0);2.031.90 (2H, m, H 3a ;3a 0); 1.761.59 (6H, m, H 3b ;3b 0,H 4;40); 1.581.47 (2H, m, H 9); 0.770.43 (3H, m, H 10).13C NMR (CDCl 3): (ppm) 140.00 (C arom ); 128.96(C arom ); 128.15 (C arom ); 126.77 (C arom ); 62.36 (C 2); 60.30(C 7); 59.71 (CH 2Ph); 58.65 (OCH 2); 58.30 (C 8); 54.76(C 5); 30.42 (C 3); 22.49 (C 4); 20.36 (C 9); 18.30(OCH 2CH 3); 7.97 (C 10). EM, m = z (%): 567 (M ,15).

2.2. Preparation of rhodium and iridium complexes ( 5 10)

2.2.1. Typical procedureOnly the preparation of [Rh(2)COD]PF 6 CH 2Cl 2 (5)

is described in detail. AgPF 6(116 mg, 0.46 mmol) inTHF (40 ml) was added to [Rh(cod)Cl] 2(114 mg, 0.23mmol) in THF (10 ml) and the mixture was stirredvigorously at room temperature for 30 min. Precipitatedsilver chloride was ltered off and the yellow solutionwas treated with the ligand (166 mg, 0.46 mmol) inTHF. The mixture was stirred for 12 h under reux. Thesolvent was evaporated under reduced pressure to 2 ml.Careful addition of diethyl ether caused the precipita-tion of a solid which was collected by ltration,washed

with diethyl ether and dried under vacuum (10 3

mmHg) to give the light brown cationic complex. Yield:67%. m.p.: 100102 C. a 25D 0:56 (c: 0.60,CHCl 3).Found: C, 48.8; H, 5.0; N, 4.9; Rh, 12.5%. Calc. forC33H 47N 3Cl2F 6PRh (804): C, 49.1; H, 5.8; N, 5.2; Rh,12.7. IR (KBr,cm 1): m 837 (PF); 474 (RhN); 320(NRhN). 1H NMR (CDCl 3, ppm): d 8:057.90 (2H,m, H arom ); 7.567.25 (8H, m, H arom ); 5.705.45 (4H, m,cod); 4.003.40 (8H, m, CH 2Ph, H 5a ;5a 0, H 2;20); 3.002.70(4H, m, H 7 ,70); 2.692.20 (10H, m, H 5b ;5b 0,cod); 2.19 1.23 (8H, m, H 4;40, H 3;30).

13C NMR (CDCl 3 , ppm):d 131 :45 (C arom ); 130.17 (C arom ); 129.10 (C arom );128.64 (C arom ); 61.43 (C 2); 57.10 (CH 2Ph); 54.90 (C 5);54.31 (C 7); 28.43 (C 3); 26.29 (C 4). UVVis k , nm): 386.K M (X1 cm 2 mol 1) 125.3. EM, m = z (%): 574 ([Rh(2)]-[PF 6 + CH 2Cl 2],103Rh).

Complexes 610 were prepared through a proceduresimilar to that given for 5 starting from [MCl(cod)] 2(M Rh, Ir) and using 0.46 equivalents of the appro-priate ligand.

2.2.2. [Rh(3)H 2O]PF 6 ( 6 )Brown. Yield: 79%. m.p.: > 230 C. a 25D 0:65 (c:

0.99, CH 3OH). Found: C, 48.0; H, 6.5; N, 6.0; Rh,15.8%. Calc. for C 27H 39N 3F 6PRh (653): C, 48.2; H, 6.1;

N, 6.2; Rh, 15.3. IR (KBr, cm 1): m 837 (PF); 406(RhN); 318 (NRhN). 1H NMR (CDCl 3 , ppm):d 7:487.45 (5H, m, H arom ); 7.237.16 (5H, m, H arom );4.474.43 (2H, m, C H 2Ph); 4.254.19 (2H, m, C H 2Ph);3.813.71 (4H, m, H 2;20, H 5a ;5a0); 3.553.41 (6H, m, H 8,H 7;70); 1.901.75 (2H, m, H 5b ;5b 0); 1.701.60 (8H, m,

H 4a ;4a 0, H 3a ;3a 0); 1.281.15 (2H, m, H 9); 0.900.70 (3H,m, H 10). UVVis: k max 352 nm. K M(X1 cm 2 mol 1) 118.2. m/z: 508 ([Rh( 3)][H 2O+PF 6],103 Rh).

2.2.3. [Rh(4)H 2O]PF 6 ( 7 )Yield: 56%. m.p.: > 220 C. Found: C, 47.2; H, 6.3; N,

4.9; Rh, 12.1%. Calc. for C 33H 55N 3F 6O4PRhSi (653): C,47.5; H, 6.6; N, 5.0; Rh, 12.3. IR (KBr, cm 1): m 836(PF); 475 (RhN). 1H NMR (CD 3OD, ppm):d 7:507.35 (10H, m, H arom ); 4.404.31 (2H, m,CH 2Ph); 4.124.02 (2H, m, C H 2Ph); 3.523.43 (4H, m,H 2;20, H 5a ;5a 0); 3.123.01 (6H, m, H 8, H 7;70); 2.632.40(2H, m, H 5b ;5b0); 2.332.24 (2H, m, H 3a ;3a0); 2.101.85(4H, m, H 4;40); 1.871.80 (2H, m, H 3b ;3b 0); 1.501.40 (2H,m, H 9); 0.980.85 (2H, m, H 10). EM, m = z (%): 586([Rh( 4)][H 2O+PF 6], 103 Rh, as hydrolyzed species).

2.2.4. [Ir(2)COD] ( 8)Dark brown. Yield: 79%. m.p.: 138140 C.

a 25578 50 (c: 0.53, CHCl 3). Found: C, 57.7; H, 6.9; N,6.3; Ir, 29.0%. Calc. for C 32H 46N 3Ir (663): C, 57.6; H,6.9; N, 5.9; Ir, 10.9. IR (KBr, cm 1): m 496 (IrN). 1HNMR (CDCl 3 , ppm): d 7:697.26 (10H, m, H arom );4.514.43 (2H, m, C H 2Ph); 4.194.08 (2H, m, C H 2Ph);

3.923.73 (2H, m, H 2;20); 3.723.43 (6H, m, H 5a ;5a 0,H 7;70); 2.201.36 (18H, m, H 5b ;5b 0, cod, H 4;40, H 3;30).

13CNMR (CD 3OD, ppm): d 132 :66 (C arom ); 131.86(C arom ); 131.35 ( C arom ); 130.60 ( C arom ); 67.36 ( C 2); 60.02(C H 2Ph); 56.45 ( C 5); 50.25 (C 7); 28.92 (C 3); 23.73 (C 4).UVVis k , nm): 433,358. K M (X1 cm 2 mol 1) 10.2.EM, m = z (%): 663 ([Ir( 2)COD] ,193 Ir); 661 ([Ir( 2)-COD] ,191 Ir).

2.2.5. [Ir(3)H 2O]Cl Et 2O ( 9)Yellow. Yield: 57%. m.p.: 155157 C. a 25578 1:2

(c: 0.69, CH 3OH). Found: C, 51.0; H, 6.8; N, 5.9; Ir,26.8%. Calc. for C 31H 51N 3ClIrO 2 (723.5): C, 51.3; H,7.0; N, 5.7; Ir, 26.6. IR (KBr, cm 1): m 482 (IrN). 1HNMR (CD 3OD, ppm): d 7:607.57 (2H, m, H arom );7.467.40 (2H, m, H arom ); 4.47, 43.4 (2H, AB, J 11.1Hz, d, C H 2Ph); 3.813.68 (2H, m, H 2;20); 3.533.39(2H, m, H 5a ;5a 0); 3.283.17 (2H, m, H 5b ;5b 0); 2.85 2.78 (2H, m, H 7a ;7a 0); 2.662.63 (2H, m, H 7b ;7b 0); 2.38 2.33 (4H, m, H 3a ;3a 0,H 8); 2.251.99 (4H, m, H 4;40);1.971.77 (2H, m, H 3b ;3b 0); 1.471.25 (2H, m, H 9); 0.98 0.86 (3H, m, H 10). 13C NMR (CD 3OD, ppm): d 129 :69(C arom ); 128.59 (C arom ); 126.41 (C arom ); 64.60 (C 2);57.80 ( C H 2Ph); 55.50 (C 8); 55.30 (C 7); 54.30 (C 5); 28.43(C 3); 21.29 (C 4); 18.00 (C 9); 11.05 (C 10). UVVis k ,

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nm) 433,358. K M (X1cm 2mol 1) 129.5. EM, m = z (%): 406 ( 3).

2.2.6. [Ir(4)H 2O]Cl ( 10)Yield: 57%. m.p.: > 220 C. Found: C, 48.2; H, 6.5; N,

4.9; Ir, 23.1%. Calc. for C 33H 55N 3ClIrO 4Si (812.5): C,

48.7; H, 6.8; N, 5.2; Ir, 23.6. IR (KBr,cm1

): m 478(IrN). 1H NMR (CD 3OD, ppm): d 7:627.38 (10H,m, H arom ); 4.694.51 (2H, m, C H 2Ph); 4.28 (2H, AB,J 14.2 Hz, d, C H 2Ph); 3.803.59 (2H, m, H 2;20); 3.49 3.23 (8H, m, H 5a ;5a0, H 7a ;7a 0); 2.251.91 (10H, m, H 4;40,H 3;30,H 8); 1.251.14 (2H, m, H 9); 0.950.79 (2H, m,H 10). 13C NMR (CD 3OD, ppm): d 131 :90, 131.79,131.06, 130.35 (C arom ); 71.63 (C 2); 69.93 ( C H 2Ph); 60.90(C 8); 59.49 (C 7); 55.54 (C 5); 30.58 (C 3); 27.28 (C 4); 24.18(C 9); 23.11 (C 10). K M (X1cm 2mol 1) 93.2.

2.3. Heterogenisation of M-complexes ( Rh-Support , Ir-

Support )

2.3.1. Anchoring on SBA-15 and MCM-41-zeolite 2.3.1.1. Anchoring on MCM-41-zeolite: Route A . Thesupported M-complexes ( Rh4-MCM-41 , Ir4-MCM-41 )were prepared as we have previously described [13].Thus, a solution of metal complex bearing a triethoxy-silyl group (0.5 mmol) in ethanol (2 ml) was added to awell-stirred toluene suspension (40 ml) of the inorganicsupport (1 g) and the mixture was stirred at 80 C for24 h. The solid was then ltered and Soxhlet-extractedfor 724 h to remove the remaining non-supportedcomplex from heterogenised catalyst. The resulting solidwas dried in vacuum and analysed.

Rh4-(MCM-41) : Elemental analysis indicated 0.53mass % Rh. IR (KBr, cm 1): m 3410 (OH); 1237 (vs,support); 1120 (CSi); 1080 (vs, support); 802 (PF); 454(MN). 13C NMR (solid): d (ppm) 73.7 (C 2); 61.1(C H 2CH 3); 59.040.3 ( C H 2CH 2CH 2Si, C 5); 39.521.0(CH 2C H 2NH, C 3, C4, CH 3); 14.7 (C H 2CH 2Si); 7.8(CH 2Si).). UVVis (solid) (nm): 210.

Ir4-(MCM-41): Elemental analysis indicated 1.2 mass% Ir. IR (KBr, cm 1): m 3410 (OH); 1237 (vs, sup-port); 1120 (CSi); 1080 (vs, support); 451 (MN). 13CNMR (solid): d (ppm) 128.9 (C arom ); 70.0 (C 2); 60.1

(C H 2CH 3); 59.040.0 ( C H 2CH 2CH 2Si, C 5); 39.521.0(C 3, C4, CH 3); 15.2 (C H 2CH 2Si); 8.1 (CH 2Si).). UVVis(solid) (nm): 209, 260, 310.

2.3.1.2. Anchoring on SBA: Route B . The supported Rhand Ir-complexes (Rh4-(SBA-15), Ir4-(SBA-15)) wereprepared as follows. A solution of chloropropyltriethoxysilane (0.5 mmol) in ethanol (2 ml) was added to a well-stirred toluene suspension (40 ml) of the support (1 g) andthe mixture was stirred at 80 C for 24 h. The solid wasthen ltered and Soxhlet-extracted for 724 h to removethe remaining non-supported product from heterogenised

ligand. Then, halogen exchange using KI in acetone, atroom temperature for 24 h, leads a white solid that wasdried in vacuum and analysed. Alkylation of the amine 2with this support halide affords the anchored ligand. To asolution of [M(cod)(thf) 2] (M Rh, Ir) (1 mmol) in drythf (20 ml) was added a suspension of the heterogenised

ligand (1 mmol); the mixture was stirred for 6 h at roomtemperature and the pale solid ltered, washed with ace-tonitrile, and dried in vacuum (103 mmHg/8 h) to give theheterogenised complex.

Rh4-(SBA-15) : Elemental analysis indicated 1.31 mass% Rh. IR (KBr, cm 1): m 3400 (OH); 1230 (vs, sup-port); 1118 (CSi); 1080 (vs, support); 853 (PF); 458 (M N). 13C NMR (solid): d (ppm) 129.5 ( C arom ); 70.1 (C 2);59.1 ( C H 2CH 3); 59.040.0( C H 2CH 2CH 2Si,C 5); 4020.0(C 3, C4, CH 3); 13.9 (C H 2CH 2Si); 8.0 (CH 2Si).). UVVis(solid) (nm): 344, 258, 208.

Ir4-(SBA-15) : Elemental analysis indicated 2.63 mass% Ir. IR (KBr, cm 1): m 3550 (OH); 1210 (vs, sup-port); 1120 (CSi); 1068 (vs, support); 458 (MN). 13CNMR (solid) d (ppm) 129.8 (Ph); 67.3 (C 2); 64.4(C H 2CH 3); 57.3 (C H 2CH 2CH 2Si, C 5); 31.2 (C 3 , C4,CH 3); 12.8 (C H 2CH 2Si); 7.6 (CH 2Si). UVVis (solid)(nm): 255, 424.

2.4. Catalytic experiments

The catalytic properties, in hydrogenation reactions,of the above Rh and Ir complexes were examined underconventional conditions for batch reactions in a reactor(Autoclave Engineers) of 100 ml capacity at 313 K

temperature, 4 atm. dihydrogen pressure and ametal:substrate molar ratio of 1/10000. The results weremonitored by gas chromatography using an internalstandard reference. The kinetics results are shown inTable 1 and Fig. 2.

Table 1Asymmetric hydrogenation of diethyl 2-benzylidene succinate a

Catalyst Conv. % b TOF c ;d

5 89.7 13 5786 94.5 23 384Rh4-(MCM-41) 95.9 24 244Rh4-(SBA-15) 81 13 1488 88.8 13 5109 68.9 8334Ir4-(MCM-41) 72.5 15 498Ir4-(SBA-15) 17.8 1701a All reactions were performed in EtOH at 40 C, S/C 10 000 and 4

atm H 2 .b 60 min.c mmol substrate. mmol cat 1 h1 .d Optical yield < 10%.



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2.4.1. Recovery and recycling of catalystsAt the end of the hydrogenation process, the mixture

of reaction was ltered; the residue of zeolite-containingcatalyst was washed with CH 2Cl 2 or acetonitrile tocompletely remove the remains of products and/or re-actants and used again.

3. Results and discussion

3.1. Synthesis of ligands

The ligands were prepared and well characterisedfollowing the procedure showed in Section 2. All reac-

tion steps were ne-tuned for high yield and selectivity.The preparation of diamide N ,N 0-bis[(S )-N-benzylpr-olyl]methyleneamine ( 1) was achieved following amodied published method starting from the easilyavailable LL -proline protected as the N-benzyl derivative,according to Scheme 1. The corresponding amine N ,N -

bis{[( S )-1-benzylpyrrolidin-2-yl]methyl}amine ( 2) wasobtained by reduction of the respective amide withlithium aluminium hydride at reux. All intermediatesand nal products have been obtained with a total yieldof 7080%. Amide 1 is a white solid while amine 2 wasisolated as colourless oils that are stable at low tem-

perature in an inert atmosphere. Amines 3 and 4 wereobtained starting from 2 by alkylation with propyliodideor iodopropyltriethoxysilane. The ligands were charac-terised unequivocally by mass spectrometry, IR and 1H,13C NMR spectroscopy and gave satisfactory elementalanalyses. Mass spectrometry shows the highest ions atm/z 364, 406 and 567 which corresponds to the molec-ular weights of compounds 2, 3, and 4, respectively. The1H and 13C NMR spectra obtained are in agreementwith those expected for these triaza ligands. The ligandshave two optically active centres; both have the S ab-solute conguration.

3.2. Preparation of complexes

The rhodium and iridium complexes ( 510 ) wereobtained by treating the dimeric [M(cod)Cl] 2 (M Rh,Ir) with two equivalents of AgPF 6 in tetrahydrofuran.An equimolar amount of the corresponding multitopicligands was added to the solution containing thecationic mononuclear [M(cod)(THF) x ]PF 6 species(Scheme 2). The mononuclear complexes were preparedin good yield ( > 60%) by ligand exchange of L with[M(cod)(THF) 2]PF 6 in THF. The reaction of N ,N -bis{[( S )-1-benzylpyrrolidin-2-yl]methyl}amine ( 2) with

[Ir(cod)(THF) 2]PF 6 also provided a neutral M-complex,[Ir( N,N -bis{[(S )-1-benzylpyrrolidin-2-yl]methyl}amine-1)(cod)] ( 8) as the main product of the reaction, whichpresents the ligand as a deprotonated anionic species;starting from rhodium compound only traces of theneutral species were detected as a black solid. Thecomplexes were isolated by precipitation from diethylether as microcrystalline air stable solids. Elementalanalysis of C, H, N and metal is consistent with theproposed stoichiometry. Mass spectrometry (ESI-MSand APCI-MS with positive mode) indicated a mono-meric formula in accordance with the results of IR andNMR studies. The complexes have their highest ions atm/z values corresponding to the loss of F , and PF 6 inthe molecular species.

The IR spectra of the free ligands exhibit the char-acteristic bands of amines. The IR spectra of Rh-amine

1

N

NO

N

Bz

BzH4LiAl

3, R = (CH 2)2CH34, R = (CH 2)3Si(OEt) 3

6, 9; R: (CH 2)2CH37, 10 ; R: (CH 2)3Si(OEt) 3

2

I-R

[MCl(cod)]2L-Pro

5, 8

[MCl(cod)]2

N

NN

Bz

BzM X

N

NN

Bz

BzM XR

N

NN

Bz

BzR

H

N

NN

Bz

Bz

H

Scheme 1. Synthesis of ligands and complexes.

1/ 2[MCl(cod)]2 +THF

[M(cod)(THF) x]PF 6AgPF 6 AgCl

[M(cod)(THF) x]PF 6 +

+

Ligand THF [M(ligand)(L')]X

M = Rh, Ir, ligand = 2 , 3, L' = cod, solvent, X = PF 6

Scheme 2.

-- -- -- -- -- -- -- --0

10

20

30

40

50

60

70

80

90

100

Ir4-SBA-15

Ir4-MCM-41

Rh4-SBA-15

C o n v e r s

i o n

( % )

Catalyst

30 min45 min60 min75 min

Rh3Rh2 Rh4-MCM-41

Ir3

Fig. 2. Kinetic prole for the hydrogenation of diethyl 2-benzylidenesuccinate.



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complex 5 show m(NH) band shifted at higher fre-quencies (3400 cm 1). The m(PF) frequency appears at835 cm 1 as medium bands characteristic of the non-coordinated PF 6 anion. The

13C NMR spectra of amine-complexes show only small chemical shift differ-ences compared with the free ligands.

The IR spectrum of the mononuclear Ir-complex 8does not show the expected m(NH) and m(PF) bands.In the Ir-amine complexes 9, 10 , the 13C NMR signalsshow only small chemical shift differences comparedwith the free ligand and band due to m(PF) was notobserved.

Electronic absorption spectra of the rhodium andiridium complexes, measured between 800 and 200 nm,show a clear maximum at ca. 300 nm ( 2 6000 8000)and one shoulder at ca. 400 nm ( e 15002000). Takinginto account their energy position and intensity, thebands at 400 nm could be assigned [21] to dd transi-tions localised on the metal ion. At shorter wavelengths,the bands at 300 nm may be assigned to the M ! ligandcharge transfer transition and intra-ligand transitions.The conductivity data suggest 1:1 electrolytes for allcationic complexes.

3.3. Heterogenisation of complexes

The solids employed in the present study as supportsof the active chiral complexes were purely siliceousMCM-41 and SBA-15. The two solids were selected tocompare topologies and the complex would be pre-sumably located in the interior of the mesoporous

channels. The heterogenisation of homogeneous cata-lysts is a eld of continuing interest: indeed, althoughsome of these organometallic complexes exhibit re-markable catalytic properties (activities and selectivity)they are difficult to separate, intact, from the reactionmedium. Thus, unless the activity of the homogeneouscatalysts is exceptionally high, their heterogenisation isstill currently economical, but also a toxicological andenvironmental challenge. We have considered onestrategy for heterogenisation, which preserves as muchas possible the coordination sphere of the metal. This isachieved by anchoring the homogeneous catalyst to aninorganic support (MCM-41, SBA-15) via covalentbonds between the solid (silanol groups SiOH) andone ligand that have appropriate groups (Si(OEt) 3) at aposition remote to the metal centre (Scheme 3(a)).Preparations of heterogenised materials of complexesbearing a triethoxysilylpropyl group were carried out, bycontrolled hydrolysis of Si-OEt bonds and reaction withthe free silanol (SiOH) on the surface of support. Theresulting catalytic material is very stable and the speciesare covalently bonded to the surface showing only mi-nor frequency shifts from those of the correspondingneat complex, as conrmed by IR and UVVisspectroscopy. The elemental analysis of C, H, N and M

also conrms the M/ligand (1:1) stoichiometry. It isunlikely that the nature of the complex is substantiallyaltered under the relatively mild conditions of the an-choring reaction [22]. Thermogravimetric analysis showsthat the total weight loss is associated with the metalcomplex content corresponding to composition of the

organic ligands. The loading of metal was always ca.$ 12% (0.1%) measured by atomic absorption of metal of the digested samples. These values have beenused for calculating the ratio of catalyst/substrate in thereaction tests. IR and electronic spectra (diffuse reec-tance) of heterogenised complexes are coincident withthat recorded for homogeneous complexes. The IRspectra of the zeolite-supported catalyst show bandsthat correspond to the complexes and support. Peaksdue to the support dominate the spectra. These includethe OH vibration in the range 37003300 cm 1. Someof the bands characteristic of the complexes could be,however, distinguished. Major framework bands appeararound 1140, 1040, 960, 785 and 740 cm 1. Vibrationalbands due to PF, CN appear around 840 and450 cm 1, respectively, broadly similar to those of theneat complex. The UVVis electronic spectra data aregiven in Section 2. The band maximum is not signi-cantly altered on zeolite heterogenised complex. Thepositions and relative intensities of these bands aresimilar to those of the free complexes.

Another alternative route was shown in Scheme 2(b)that consists in anchoring of iodopropyltriethoxysilaneto the support or by introducing a chloropropyltrieth-oxysilane moiety in support preparation and halogen

exchange with iodide. Alkylation of amine 2 with thehalide heterogenised material gives supported amineswhich react with the starting metal complex andM-support complex was obtained with spectroscopicalcharacteristics similar to those obtained by route A.

Also, we have obtained the supported complexes byanchoring the ligand to the support (Scheme 2C) andreaction of this previously heterogenised ligand with thestarting complex to give the respective heterogenisedcomplex. These products show analytical and spectro-scopical properties similar to that obtained via route A.

3.4. Catalytic experiments

3.4.1. Hydrogenation of alkenesThe use of cationic complexes with chiral nitrogen-

containing ligands is an attractive approach for thecatalytic hydrogenation of alkenes. In the rst stage,metal complexes were evaluated as catalysts in the hy-drogenation of diethyl 2-benzylidene succinate to in-vestigate the potential of ligands 2, 3 and 4 for catalysis(Table 1 and Fig. 2). In the presence of 0.1 mol% of catalysts in ethanol at 313 K under a pressure of 4 barH 2 ,the substrate was fully converted to the alkane in60 min. The results of these studies reveal that cationic



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catalysts are very effective for this transformation af-fording the product with higher rate and comparableenantioselectivity to that achieved with the analogoustetraaza-derivatives [23]. The results show that rhodiumcomplexes are more efficient than iridium complexes.The change of substituents on primary amine nitrogenplays a signicative role on activity that increases whenmoving from hydrogen to propyl group.

A study about the inuence of structural charac-teristics of ligand and substrates with these promisingcatalysts in homogeneous media or supported on ze-olites is in progress and will be presented extensivelyin future. When transition metal complexes have beensupported on carriers such as polymers or silica, etc.,it is generally accepted that a moderate to strong re-duction in the reactivity has been observed; however,in our case the turnover numbers for hydrogena-tion increase indicating the cooperative effect of thesupport.

Soluble complexes could be used only once,becausethey deteriorate completely by the end of the rst

catalytic run. Supported-complexes could be recoveredfor recycling and reused retaining most of their cata-lytic activity. After each cycle, the liquid phase thatwas separated from the reaction mixture was exam-ined for catalytic activity in the hydrogenation of alkenes under the same conditions used with the solidcatalyst. The results show that the liquid phases areinactive for hydrogenation. Moreover, atomic emissionanalyses did not detect rhodium in the liquid phasesfrom the rst, second and third cycles. Based on thelower detection limit of the instrument, the amount of rhodium leaching into the liquid phases must be lessthan 0.2% of the Rh on the catalyst.

Nature of the support . Other point that is alsoclear from the results is that the activity for theheterogenised complexes depends on a small butsignicant extent on the support. These results are areection of the differences in the structure of thesolid (topology, surface and silanol nests). A higheractivity is obtained when MCM-41 was used assupport, when compared with mesoporous SBA-15.

A

O

O

OH

Toluene

60C / 24 h

SiO

Support : MCM-41, SBA-15

N

NN

Bz

M BzX

N

NN

Bz

MBz

X

B

O

O

OH

SiO

Toluene

60C / 24 h

Heterogenised complex

N

NN

Bz

BzH

O

O

OH

SiO

N

NN

Bz

Bz[MCl(cod)]2

OH

OH

OH

OH

OH

OH

Toluene

60C / 24 h

[MCl(cod)]2

C

N

NN

BzBz

(EtO) 3Si O

O

OH S u p p o r t

S u p p o r t

S u p p o r t

S u p p o r t

S u p p o r t

S u p p o r t

S u p p o r t

SiO

N

NN

Bz

BzOH

OH

OH S u p p o r t

Si(OEt) 3(CH 2)3Cl(CH2)3Cl

O

O

OH

SiO

(CH2)3I


KI

(EtO) 3Si


Scheme 3. Heterogenisation by covalent bond (tethering).



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Nature of the recovered materials . One of the mainadvantages of the heterogeneous catalysts is the possi-bility for their recovery and reuse. This possibility wasexplored thoroughly with Rh4-(MCM-41) . The resultsobtained indicate that after the rst reaction, the re-covered catalyst does not show a signicant degree of

leaching of rhodium. The spectral data for recoveredmaterial are essentially the same before and after thehydrogenation reaction.

4. Conclusion

A series of new microcrystalline air-stable rhodiumand iridium complexes with chiral triaza ligands havebeen synthesised and fully characterised even in opti-cally pure form. We have shown that these cationicRh(I), Ir(I) complexes are efficient catalysts for the hy-drogenation of olens. Heterogenisation on MCM-41

and SBA-15 increases the activity of the homogeneouscatalysts for different substrates and can be recoveredand reused, retaining most of their catalytic activity.Moreover, the heterogenised complexes are signicantlymore stable than the corresponding homogeneouscomplexes over prolonged reaction times and werehandled without special care in standard conditions.

Acknowledgements

Financial support by the Direcci on General de In-vestigaci on Cient ica y T ecnica of Spain (Project

MAT2003-07945-C02-02) is gratefully acknowledged.

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