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Boletín de la Sociedad Chilena de Química

versión impresa ISSN 0366-1644

Bol. Soc. Chil. Quím. v.47 n.3 Concepción sep. 2002

http://dx.doi.org/10.4067/S0366-16442002000300005 

Bol. Soc. Chil. Quím., 47, 227-233 (2002) ISSN 0366-1644

RHODIUM(III) AND RUTHENIUM(II) COMPLEXES WITH THE
CHIRAL PHOSPHINE-ALCOHOL Ph2PCH2CHMeCH2OH.
SYNTHESIS AND CHARACTERISATION

JULIO SANHUEZA, RAUL CONTRERAS AND MAURICIO VALDERRAMA*

Departamento de Química Inorgánica. Facultad de Química.
Pontificia Universidad Católica de Chile. Casilla 306. Santiago. Chile.

(Received: December 4, 2001 - Aceptado: Abril 23, 2002)

ABSTRACT

Reaction of the dinuclear complex [{(h5-C5Me5)RhCl} 2(µ-Cl)2] with the chiral phosphine (R)-Ph2PCH2CHMeCH2 OH leads to the complex [{(h5-C5Me5)RhCl 2(h1-Ph2PCH2 CHMeCH2OH-P)](1). This reaction, in the presence of AgBF4, yields the cationic compound [{(h5-C5Me5)RhCl(h 2- Ph2PCH2CHMeCH2OH-P,O)]BF 4(2). Variable-temperature 1H NMR and circular dichroism experiments support the stereoselective h2-chelate coordination of the ligand and the proposed configuration for the metal centre. In a similar way, the reaction of the dimer [{(h6-C6Me6)RuCl} 2(µ-Cl)2] with the ligands (R)- and (S)-Ph2PCH2CHMeCH2 OH afford the neutral complexes [(h6-C6Me6)RuCl 2{h1-PPh2CH2 CHMeCH2OH-P}] [R-ligand (3), S-ligand (4)], which in turn react with AgBF4 to give the cationic compounds [(h6-C6Me6)RuCl{h 2-PPh2CH2CHMeCH2 OH-P,O}]BF4 [R-ligand (5), S-ligand (6)]. All complexes have been characterised by elemental analysis, IR and multinuclear NMR spectroscopies.

KEY WORDS: Rhodium, ruthenium, chiral bidentate ligand, chiral phosphine, chiral-at-metal complexes.

RESUMEN

La reacción del complejo dinuclear [{(h5-C5Me5)RhCl} 2(µ-Cl)2] con la fosfina quiral (R)-Ph2PCH2CHMeCH2 OH conduce a la formación del complejo neutro [{(h5-C5Me5)RhCl 2(h1-Ph2PCH2 CHMeCH2OH-P)](1). Si la reacción se realiza en presencia de AgBF4 se obtiene el complejo catiónico [{(h5-C5Me5)RhCl(h 2-Ph2PCH2CHMeCH2 OH-P,O)]BF4(2). Experimentos de 1H NMR a temperatura variable permiten demostrar que la coordinación de tipo quelato de la fosfina es altamente estereoselectiva y medidas de dicroismo circular avalan la configuración propuesta para el centro metálico. De forma similar, la reacción del dímero [{(h6-C6Me6)RuCl} 2(µ-Cl)2] con los ligandos (R)- y (S)-Ph2PCH2CHMeCH2 OH genera los complejos neutros [(h6-C6Me6)RuCl 2{h1-PPh2CH2 CHMeCH2OH-P}] [(R)- (3), (S)- (4)], los cuales mediante una posterior reacción con AgBF4 forman los compuestos catiónicos [(h6-C6Me6)RuCl{h 2-PPh2CH2CHMeCH2 OH-P,O}]BF4 [(R)- (5), (S)- (6)]. Todos los complejos han sido caracterizados mediante análisis elemental y espectrocopía IR y de RMN.

PALABRAS CLAVES: Rodio, rutenio, ligandos bidentados quirales, fosfinas quirales, complejos con metales quirales.

INTRODUCTION

Recently, much attention has been focussed on the synthesis of transition metal complexes containing chiral ligands due to their potential applications in stoichiometric or catalytic asymmetric reactions.1-4) In particular, half-sandwich compounds possessing stereogenic metal centres and chiral ligands have been used in the elucidation of the stereochemical course of the reactions.5) These types of complexes, showing an h6-MeC6H4Pri or h5-C5Me5 groups and optically active chelate ligands, have been recently reported as catalysts in enantioselective Diels-Alder reactions.6-10)

On the other hand, the design and use of so-called hemilabile chelate ligands are the subject of considerable interest. They contain a soft donor (e.g., phosphorus) strongly bonded to the metal centre with a hard donor (e.g., oxygen) forming only a weak contact to the metal centre. This combination of donor atoms can provide free co-ordination sites by decoordination of the most labile bonded atom.11-17)

Recently, we have described the reaction of the chiral ligand (S)-PPh2CH2CHMeCH2 OH with the dinuclear complex [{(h5-C5Me5)RhCl} 2(µ-Cl)2] showing that the phosphine-alcohol can acts as a monodentate P-donor or a bidentate chelate P,O-donor ligand. A stereoselective h2-coordination of the phosphine ligand was found in the formation of the cationic complex (SRh,SC)-[(h5-C 5Me5)RhCl{h2-(S)-PPh 2CH2CHMeCH2OH-P,O}]BF 4, whose crystal structure has been determined by X-ray diffraction.18)

Continuing our research in the synthesis and characterisation of organometallic compounds containing chiral ligands, we describe in this work the synthesis of the isomer (R)-PPh2CH2CHMeCH2 OH and its cationic complex with the fragment {(h5-C5Me5)RhCl}. The presence of quiral ancillary ligand of known absolute configuration, permit us to assign the absolute configuration at the metal by X-ray diffraction and circular dichroism. Also, we describe the reactions of the S- and R-ligands with the dinuclear complex [{(h6-C6Me6)RuCl} 2(µ-Cl)2] to form neutral and cationic complexes.

EXPERIMENTAL

All reactions were routinely performed under a purified nitrogen atmosphere, by using standard Schlenk-tube techniques. Solvents were dried, distilled, and stored under a nitrogen atmosphere. The starting complexes [{(h5-C5Me5)RhCl} 2(µ-Cl)2],19) [{(h6-C6Me6)RuCl} 2(µ-Cl)2]20) and the ligand (S)-PPh2CH2CHMeCH2 OH21) were prepared by published procedures. Carbon and hydrogen analyses were performed using a Fisons EA 1108 microanalyzer. FTIR spectra were recorded on a Bruker Vector-22 spectrophotometer using KBr pellets. 1H- (200 MHz), 13C- (50 MHz) and 31P- (81 MHz) NMR spectra were recorded on a Bruker AC-200P spectrometer. Chemical shifts are reported in ppm relative to SiMe4 (1H) and 85% H3PO4 (31P, positive shifts downfield) as internal and external standards, respectively. Circular dichroism (CD) and optical rotatory dispersion (ORD) spectra were determined in CHCl3 in a 1.0 cm path length cell by using a Jovin Yvon CD 6 instrument.

Synthesis of (R)-PPh2CH2CHMeCH2 OH

To a mixture of potassium phosphide (13.0 mL, 0.5 mol L-1 in THF; 6.53 mmol) and lithium diisopropylamide (LDA, 4.8 mL, 1.5 mol L-1 in THF; 7.18 mmol) at 0º C, a solution of (R)-(-)-3-bromo-2-methyl-1-propanol (1 g; 6.53 mmol) in tetrahydrofuran (10 mL) was added dropwise. After stirring the reaction mixture for 30 min at room temperature (r.t.), the solvent was removed by vacuum and the residue was treated with a mixture of free-oxygen water (20 mL) and benzene (30 mL). The organic layer was separated and dried with MgSO4. The solvent was evaporated to give a colourless oil. Traces of Ph2PH [31P{1H} NMR (CDCl3, 295 K) d - 41.2 (s)] were eliminated by heating the mixture at 100 ºC in high vacuum. Yield 1.41 g (84%). 1H NMR (CDCl3, 295 K): d 1.1 [d, 3J(HH) = 6.6 Hz, 3H, Me], 1.7-1.9 (m, br, 1H, CHMe), 1.9-2.0 (m, 1H, PCH), 2.2-2.3 (m, 1H, PCH), 3.6 [dd, 3J(HH) = 6 Hz, 4J(PH) = 1.5 Hz, 2H, CH2O], 7.3-7.6 (m, 10H, Ph). 31P{1H} NMR (CDCl3, 295 K) d - 21.9 (s).

[(h5-C5 Me5)RhCl2{h1 -(R)-PPh2CH2CHMeCH2 OH-P}] (1)

[{(h5-C5Me 5)RhCl}2(µ-Cl)2] (138 mg; 0.22 mmol) was dissolved in tetrahydrofuran (20 mL) under nitrogen. (S)-PPh2CH2CHMeCH2 OH (117 mg; 0.45 mmol), dissolved in tetrahydrofuran (10 mL), was added dropwise and the reaction mixture was stirred at r.t. for 2 h. The reaction mixture was evaporated to dryness, the solid residue formed was extracted with dichloromethane and chromatographed on Kieselgel. Careful addition of n-hexane caused the precipitation of a red solid, which was filtered off, washed with cold n-hexane (3 x 5 mL), and dried under vacuum. Red crystals were obtained from dichloromethane-n-hexane. Yield 117 mg, 46%. Anal. Found: C, 55.6; H, 6.3. C26H34Cl2OPRh requires C, 55.0; H, 6.0%). IR spectrum (KBr): n(OH) 3442. 1H NMR (CDCl3, 295 K): 0.45 [d, 3H, 2J(HH) = 6.4 Hz, Me], 1.30 [d, 15H, 3J(PH) = 3.5 Hz, C5Me5], 2.2-2.4 (m, 2H, CHMe, PCH), 3.0-3.2 (m, 3H, PCH, CH2O), 7.5-7.9 (m, 10H, Ph). 31P{1H} NMR (CDCl3, 295 K): 25.5 [d, 1J(RhP) = 141 Hz].

[(h5-C5 Me5)RhCl{h2-(R)-PPh 2CH2CHMeCH2OH-P,O}]BF 4 (2)

A mixture of the rhodium complex [{(h5-C5Me5)RhCl} 2(µ-Cl)2] (204 mg; 0.33 mmol) and AgBF4 (128 mg; 0.66 mmol) in acetone (15 mL), was stirred for 1 h at r.t. The precipitated silver chloride was removed by filtration through Kieselguhr. To the resulting solution, (R)-PPh2CH2CHMeCH2 OH (170 mg; 0.66 mmol) dissolved in tetrahydrofuran (10 mL) was added dropwise. After stirring for 2 h, the solution was evaporated to a small volume and the complex precipitated by addition of diethyl ether. The solid was filtered off, washed with diethyl ether and dried under vacuum. Yield, 80%. Anal. Found: C, 50.9; H, 5.1. C26H34BClF4OPRh requires C, 50.5; H, 5.5%. IR spectrum (KBr): n(O-H) 3361, n(BF4) 1084 and 521 cm-1. 1H NMR (CDCl3, 295 K): d 0.8 [dd, 3J(HH) = 6.6 Hz, 4J(PH) = 2.4 Hz, 3H, Me], 1.5 [d, 3J(PH) = 3.6 Hz, 15H, C5Me5], 1.7-1.9 (m, 1H, CH), 2.0-2.2 (m, H, PCH), 2.7 [dt, 2J(HH) = 12,8 Hz, 2J(PH) = 5.9 Hz, 1H, PCH], 3.85 (m, 2H, CH2O), 5.4 (s, br, 1H, OH), 7.4-7.9 (m, 10H, Ph). 31P{1H} NMR (CDCl3, 295 K): d 25.3 [d, 1J(RhP) = 140.5 Hz].

[(h6-C6 Me6)RuCl2{h1 -PPh2CH2CHMeCH2OH-P}] [3, 4]

[{(h6-C6Me 6)RuCl}2(µ-Cl)2] (250 mg; 0.37 mmol) was dissolved in dichloromethane (15 mL) under nitrogen. R or S-PPh2CH2CHMeCH2OH (mg; 0.75 mmol), dissolved in tetrahydrofuran (15 mL) was added dropwise and the reaction mixture was stirred at r.t. for 2 h. The reaction mixture was evaporated to dryness and the solid residue obtained was extracted with the minimum amount of dichloromethane. Careful addition of n-hexane caused the precipitation of red solids, which were filtered, washed with n-hexane and dried under vacuum.

(3) [(h6-C6Me6)RuCl 2{h1-(R)-PPh2CH2 CHMeCH2OH-P}]. Yield 153 mg, 72%. Anal. Found: C, 56.2; H, 6.2. C28H37Cl2OPRu requires: C, 55.7; H, 6.4%. IR spectrum (KBr): n (O-H) 3443 cm-1. 1H NMR (CDCl3, 295 K): d 0.33 [d, 3J(HH) = 6.4 Hz, 3H, Me] 1.7 (s, 18H, C6Me6), 1.8-2.0 (m, 1H, CHMe), 2.2-2.4 (m, 1H, PCH), 2.8-3.2 (m, 3H, CH2O, CHP), 7.4-7.9 (m, 10H, Ph). 31P{1H} NMR (CDCl3, 295 K): d 22.5 (s).

(4) [(h6-C6Me6)RuCl 2{h1-(S)-PPh2CH2 CHMeCH2OH-P}]. Yield 178 mg, 84%. Anal. Found: C, 56.0; H, 6.2. C28H37Cl2OPRu requires: C, 55.7; H, 6.4%. IR spectrum (KBr): n (O-H) 3482 cm-1. 1H NMR (CDCl3, 295 K): d 0.35 [d, 3J(HH) = 6.4 Hz, 3H, Me] 1.7 (s, 18 H, C6Me6), 1.8-2.0 (m, 1H, CHMe), 2.2-2.4 (m, 1H, PCH), 2.8-3.2 (m, 3H, CH2O, PCH), 7.5-7.9 (m, 10H, Ph). 31P{1H} NMR (CDCl3, 295 K): d 22.5 (s).

[(h6-C6Me6)RuCl{ h2-PPh2CH2CHMeCH 2OH-P,O}]BF4 [5, 6]

A mixture of complex 3 or 4 (100 mg; 0.17 mmol) and AgBF4 (33 mg; 0.17 mmol) in chloroform (15 mL) was stirred for 1 h at r.t. The precipitated silver chloride was removed by filtration through Kieselguhr. The resulting solution was evaporated to a small volume (3-5 mL) and the complex precipitated by addition of n-hexane. The red solid obtained was filtered off, washed with n-hexane and dried under vacuum.

(5) [(h6-C6Me6)RuCl{h 2-(R)-PPh2CH2CHMeCH 2OH-P,O}]BF4. Yield 158 mg, 74%. Anal. Found: C, 51.6; H, 5.6. C28H37BClF4OPRu requires: C, 52.1; H, 5.7%. IR spectrum (KBr): n (OH) 3381, n (BF4) 1098, 507 cm-1. 1H NMR (CDCl3, 295 K): d 0.65 [dd, 3J(HH) = 6.6 Hz, 4J(PH) = 2.4 Hz, 3H, Me], 1.8 (s, 18H, C6Me6), 1.8-2.0 (m, 2H, CHMe, PCH), 2.30 ([dt, 2J(HH) = 13 Hz, 2J(PH) = 5.8 Hz, 1H, PCH], 3.6-3.8 (m, 2H, CH2O), 4.9 (m, 1H, OH), 7.4-7.8 (m, 10H, Ph). 31P{1H} NMR (CDCl3, 295 K): d 22.7 (s).

(6) [(h6-C6Me6)RuCl{h 2-(S)-PPh2CH2CHMeCH 2OH-P,O}]BF4. Yield 162 mg, 79%. Anal. Found: C, 51.8; H, 5.5. C28H37BClF4OPRu requires: C, 52.1; H, 5.7%. IR spectrum (KBr): n (OH) 3382, n (BF4) 1098, 507 cm-1. 1H NMR (CDCl3, 295 K): d 0.65 [dd, 3J(HH) = 6.7 Hz, 4J(PH) = 2.5 Hz, 3H, Me] 1.8 (s, 18H, C6Me6), 1.8-2.0 (m, 2H, CHMe, PCH), 2.33 [dt, 2J(HH) = 13 Hz, 2J(PH) = 5.8 Hz, 1H, PCH], 3.6-3.8 (m, 2H, CH2O), 4.9 (m, 1H, OH), 7.4-7.8 (m, 10H, Ph).31P{1H} NMR (CDCl3, 295 K): d 22.7 (s).

RESULTS AND DISCUSSION

The phosphine-alcohol (R)-Ph2PCH2CHMeCH2 OH was prepared by reaction of (R)-(-)-BrCH2CHMeCH2OH with Ph2PK in tetrahydrofuran solution in the presence of lithium diisopropylamide and characterised by 1H and 31P{1H} NMR spectroscopy (see Experimental Section). The treatment of the phosphine-alcohol with the dinuclear complex [{(h5-C5Me5)RhCl} 2(µ-Cl)2] in tetrahydrofuran solution, in a 2:1 molar ratio, afforded the neutral compound [(h5-C5Me5)RhCl 2(h1-PPh2CH2 CHMeCH2OH-P)](1). Complex 1 reacts with thallium tetrafluoroborate in refluxing acetone to give the caà onic complex [(h5-C5Me5)RhCl(h 2-PPh2CH2CHMeCH2 OH-P,O)]BF4(2), that contains the phosphine-alcohol ligand bound in its bidentate form. This compound can be alternatively prepared by reaction of complex [{(h5-C5Me5)RhCl} 2(µ-Cl)2] with the phosphine-alcohol in the presence of silver tetrafluoroborate.

These complexes were isolated as stable microcrystalline solids. The IR spectrum of complex 1 shows a strong absorption band at 3442 cm-1 corresponding to the OH group. As expected, complex 2 exhibits the n(OH) stretching, shifted to lower frequencies (3361 cm-1) relative to complex 1, together with the characteristic bands of the uncoordinated anion (BF4-: ca. 1100, 520 cm-1). Their 1H NMR spectra in CDCl3 exhibited the expected doublet resonances assigned to methyl groups of C5Me5 ring together with multiplet signals corresponding to the CH2 and CH protons of the carbon chain. In complex 1 the methyl group bound to the asymmetric carbon appears as a doublet signal at d 0.45 ppm due to H-H coupling [2J(HH) = 6.4 Hz]. However, for complex 2 the methyl group appears as a doublet of doublets at d 0.8 ppm due to H-H and P-H couplings [3J(HH) = 6.6, 4J(PH) = 2.4 Hz]. The 31P{1H} NMR spectra show in all cases a doublet resonance at d 25-26 ppm with a 1J(RhP) coupling in the range 140-141 Hz.

Variable-temperature experiments (213-333 K) of complex 2 in CDCl3 show no evidence of the presence of two possible diastereomers in solution. This result indicates that the h2-chelate co-ordination of the R-ligand is highly diastereoselective. On the other hand, the circular dichroism spectrum of complex 2 shows positive Cotton effect centred at about 404 nm, and it is approximately a mirror image of the circular dichroism spectrum of the recently described chiral at metal complex (SRh,SC)-[(h5-C 5Me5)RhCl{h2-(S)-PPh 2CH2CHMeCH2OH-P,O}]BF 4 18) (Figure 1). Considering that the major contribution of the spectrum corresponds to the metal chromophore, the circular dichroism spectra data and the reported diffractometric results6) allows us to propose the RRh and RC configurations for complex 2.


Figure 1. (a) CD spectrum (CDCl3, 1.58 x 10-4 mol L-1) of complex (SRh,SC)-[(h5-C 5Me5)RhCl{h2-(S)-PPh 2CH2CHMeCH2OH-P,O}]BF 4. (b) CD spectrum (CHCl3, 1.88 x 10-4 mol L-1) of complex 2 (RRh,RC).

In order to gain some insight into the stereoselective process of h2-coordination of the chiral ligands, we carried out the reactions of R- and S-phosphine-alcohols with the dinuclear ruthenium complex [{(h 6-C6Me6)RuCl} 2(µ-Cl)2]. Thus, the reaction in dichloromethane solution, in 2:1 molar ratio, yielded the neutral complex [(h6-C6Me6)RuCl 2{h1-PPh2CH2 CHMeCH2OH-P}]( R-ligand 3, S-ligand 4) which, in turn, reacted in dichloromethane solution with silver tetrafuoroborate rendering the corresponding cationic complex [(h6-C6Me6)RuCl{h 2-PPh2CH2CHMeCH2 OH-P,O}]BF4 (R-ligand 5, S-ligand 6). These compounds can be prepared in high yields by direct reaction of the dinuclear complex [{(h6-C6Me6)RuCl} 2(µ-Cl)2] with the ligands in tetrahydrofuran, in the presence of AgBF4. The reactions are summarised in Scheme 1.


All ruthenium complexes were isolated as stable red solids. The IR spectra of complexes 3 and 4 showed broad n(OH) absorption bands centred at 3443 and 3482 cm-1, respectively. The IR spectra of complexes 5 and 6 showed the n(OH) stretching shifted to lower frequencies, 3381 and 3382 cm-1, respectively, together with the absorptions of the uncoordinated anion (BF4-: 1098 and 507 cm-1).

As expected, the diastereomer neutral complexes (3, 4) and the cationic chiral at metal complexes (5, 6) showed similar NMR spectra. The 1H NMR spectra of complexes 3 and 4 exhibited a singlet resonance at d 1.7 ppm assigned to the methyl groups of the arene ring, a doublet resonance at d 0.35 ppm [3J(HH) = 6.4 Hz] assigned to the methyl bound to asymmetric carbon, and multiplets resonances corresponding to CH and CH2 protons of the carbon chain. These resonances were assigned with the aid of COSY experiments and reported data.6,9) The proton of the free OH group is not observed in the 1H NMR spectra probably due to its rapid exchange with the deuterium of the solvent. The 31P{1H} NMR spectra showed a singlet resonance at d 22.5 ppm.

On the other hand, the 1H NMR spectra of complexes 5 and 6 showed the presence of the arene and the phosphine-alcohol resonances at the required proportions. In particular, the signals centred at d 0.65 (dd) and 2.30 (dt) ppm, due to the resonances of the methyl and PCH2 groups of the carbon chain, respectively, were similar to those appearing in the proton spectra of the cationic rhodium (III) complex described above,6) and are indicative of the chelate form of the ligand. The 31P{1H} NMR spectra showed a singlet resonance at d 22.7 ppm.

Similarly to rhodium complex 2, variable-temperature 1H NMR experiments (213-233 K) of complexes 5 and 6 in CDCl3 solution, show the existence of only one diastereomer. These results show the high stereoselectivity of the isomer ligands in the chelate complexation to the metal centre. Unfortunately, all attempts to prepare suitable crystals of ruthenium cationic complexes to determine the absolute configuration of the metal centre by difractometric means, have so far been unsuccessfull. The circular dichroism spectra of complexes 5 and 6 in CHCl3 solution show the expected pattern associated to the formation of the corresponding diastereomers (Figure 2).


Fig. 2. (a) CD spectrum (CHCl3,10-3 mol L-1) of complex 5. (b) CD spectrum (CHCl3,10-3 mol L-1) of complex 6.

CONCLUSIONS

The results reported in this paper show that the chiral phosphines (R)- and (S)-PPh2CH2CHMeCH2 OH behave either as monodentate P-donor or chelate bidentate P,O-donor ligands toward the organometallic fragments "(C5Me5)Rh" and "(C6Me6)Ru". Variable temperature 1H NMR experiments demonstrate that the h2-chelate coordination is higly stereoselective. For the cationic rhodium complex 2, the configuration of the metal centre has been determined using circular dichroism spectra and the previously reported diffractometric results of the corresponding diastereomer.

ACKNOWLEDGEMENTS

We thank the "Fondo de Desarrollo Científico y Tecnológico, Chile" (Grant Nº 8980007) for financial support

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Author to whom correspondence should be addressed. jmvalder@puc.cl