Organometallics 2010, 29, 703–706 DOI: 10.1021/om900923t
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Synthesis of a New C3-Symmetric Tripodal P4-Tetradentate Ligand and Its Application to the Formation of Chiral Metal Complexes Dirk Penno,† Igor O. Koshevoy,† Francisco Estevan,† Mercedes Sana u,† † ,‡ Maria Angeles Ubeda, and Julia Perez-Prieto* †
Departamento de Quı´mica Inorg anica, Facultad de Quı´mica, Universidad de Valencia, Dr. Moliner 50, 46100 Burjassot, Valencia, Spain and ‡Instituto de Ciencia Molecular (ICMOL), Polı´gono La Coma s/n, 46980 Paterna, Valencia, Spain Received October 21, 2009 Summary: A novel C3-symmetric tetradentate tripodal ligand with phosphorus as coordinating atoms has been synthesized in good yields. Its coordination ability through the four phosphorus atoms, three at the arms and one at the bridging position, is shown by formation of rigid Pd(II) and Rh(I) complexes. These C3-symmetric complexes are intrinsically chiral; experimental evidence for their configurational stability is included.
The synthesis of tripodal ligands and their facial coordination to metals to generate C3-symmetric complexes are of interest in the research area of selective catalysis. In particular, C3-symmetric chiral complexes are of great significance since, in principle, orders of rotational symmetry higher than 2-fold would lead to an increased control of stereochemistry.1,2 This type of complex is rather unusual, and most of them are based on tripodal nitrogen,3 although examples of coordination through phosphorus, oxygen, selenium, and sulfur have also been reported.4 Gade et al. emphasized the interest in ligands that can provide a relatively rigid and well-defined geometry, such as 1,1,1-tris(oxazolinyl)alkane compounds, which facially coordinate as a tridentate ligand to transition metals.5 *Corresponding author. Tel: þ34-963543050. Fax: þ34-963543274. E-mail:
[email protected]. (1) Nugent, W. A.; RajanBabu, T. V.; Burk, M. J. Science 1993, 259, 479. (2) Moberg, C. Angew. Chem., Int. Ed. 1998, 37, 248. (3) (a) Forcato, M.; Lake, F.; Blazquez, M. M.; Renner, P.; Crisma, M.; Gade, L. H.; Licini, G.; Moberg, C. Eur. J. Inorg. Chem. 2006, 1032. (b) Hammes, B. S.; Ramos-Maldonado, D.; Yap, G. P. A.; Liable-Sands, L.; Rheingold, A. L.; Young, V. G.; Borovik, A. S. Inorg. Chem. 1997, 36, 3210. (c) Memmler, H.; Kauper, U.; Gade, L. H.; Stalke, D. Organometallics 1996, 15, 3637. (4) (a) Parkin, G. New J. Chem. 2007, 31, 1996. (b) Brule, E.; Pei, Y.; Lake, F.; Rahm, F.; Moberg, C. Mendeleev Commun. 2004, 14, 276. (c) Di Furia, F.; Licini, G.; Modena, G.; Motterle, R.; Nugent, W. A. J. Org. Chem. 1996, 61, 5175. (d) Nugent, W. A.; Harlow, R. L. J. Am. Chem. Soc. 1994, 116, 6142. (e) Ward, T. R.; Venanzi, L. M.; Albinati, A.; Lianza, F.; Gerfin, T.; Gramlich, V.; Ramos Tombo, G. M.; Gerardo, M. Helv. Chim. Acta 1991, 74, 983. (f) Burk, M. J.; Harlow, R. L. Angew. Chem., Int. Ed. Engl. 1990, 29, 1462. (5) Gade, L. H.; Marconi, G.; Dro, C.; Ward, B. D.; Poyatos, M.; Bellemin-Laponnaz, S.; Wadepohl, H.; Sorace, L.; Poneti, G. Chem.; Eur. J. 2007, 13, 3058. (6) Bringmann, G.; Breuning, M.; Pfeifer, R.-M.; Schreiber, P. Tetrahedron: Asymmetry 2003, 14, 2225. (7) (a) Zheng, J.; Yang, N.; Liu, W.; Yu, K. J. Mol. Struct. 2008, 873, 89. (b) Bailey, P. J.; Dawson, A.; McCormack, C.; Moggach, S. A.; Oswald, I. D. H.; Parsons, S.; Rankin, D. W. H.; Turner, A. Inorg. Chem. 2005, 44, 8884. r 2010 American Chemical Society
Chart 1. C,P3-Tetradentate Ligand
The most common method for synthesizing C3-symmetric chiral metal complexes is the coordination of chiral ligands to the metal ion,1-6 although there are examples of chiral complexes generated from achiral ligands.7 We have recently reported the synthesis of a chiral Pd complex with a C3-symmetric propeller-shaped ligand without any chiral center; the enantiomers are configurationally stable and can be isolated by chiral HPLC. This Pd complex possesses a rigid phosphorus-tripodal ligand (HTIMP3, Chart 1),8 whose coordination to other metals, such as Mo, Ag, Ir, and Rh, via the three phosphorus atoms has been also demonstrated.8,9 In the cases of the Pd, Ir, and Rh complexes, there is an additional coordination to the metal through the central carbon bridge of the P-tripodal ligand; that is, the ligand actually acts as a C,P3-tetradentante ligand. Aimed at synthesis of new C3-symmetric ligands with phosphorus coordinating atoms and with a view to obtaining chiral C3-symmetric complexes of potential interest in catalysis, we focused on the synthesis of new rigid tripodal phosphanes using 3-methylindole as the starting material. Herein we report the efficient synthesis of a new tripodal P4-tetradentate ligand, namely, tris(2-diphenylphosphino)3-methyl-1H-indol-1-yl)phosphine (TIPP3), and an investigation of its coordination to transition metals, in particular Pd(II) and Rh(I). The structure of the ligand and that of both metal complexes was confirmed by NMR spectroscopy (1H, 13C, and 31P) and X-ray analysis of their crystal structure. NMR experiments demonstrated the axial chirality and (8) Ciclosi, M.; Lloret, J.; Estevan, F.; Lahuerta, P.; Sana u, M.; Perez-Prieto, J. Angew. Chem., Int. Ed. 2006, 45, 6741. (9) (a) Ciclosi, M.; Lloret, J.; Estevan, F.; Sana u, M.; Perez-Prieto, J. Dalton Trans. 2009, 5077. (b) Ciclosi, M.; Estevan, F.; Lahuerta, P.; Passarelli, V.; Perez-Prieto, J.; Sanau, M. Dalton Trans. 2009, 2290. (c) Ciclosi, M.; Estevan, F.; Lahuerta, P.; Passarelli, V.; Perez-Prieto, J.; Sanau, M. Adv. Synth. Catal. 2008, 350, 234. Published on Web 01/12/2010
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Scheme 1. Preparation of Tris(2-diphenylphosphino)-3-methyl1H-indol-1-yl)phosphine, as Well as the Rh(I) and Pd(II) Complexes
Figure 1. ORTEP view of TIPP3 (1). Hydrogen atoms are omitted for clarity. Only the ipso C atoms of the phenyl rings are shown for clarity. Important bond distances (A˚) and angles (deg): P-N(1), 1.729(7); P-N(2), 1.732(7); P-N(3), 1.739(8); N(1)-P-N(2), 101.0(4); N(1)-P-N(3), 100.9(4); N(3)-P-N(2), 101.0(3).
configurational stability of the complexes at temperatures up to 85 °C. The ligand TIPP3 (1) was synthesized with a satisfactory yield by treatment of 2-(diphenylphosphino)-3-methylindole10 with n-butyllithium, followed by the addition of phosphorus trichloride (Scheme 1). The structure of this compound was established by means of NMR spectroscopy (see Figures S1 and S2 in the Supporting Information) and finally confirmed by X-ray analysis of a single crystal grown in CH2Cl2/hexane (Figure 1). The perspective view showed the three-blade propeller shape of the molecule.11 Subsequently, the coordination chemistry of this new tripodal tetradentate ligand to Pd and Rh was explored, as these metals are the most promising for applications in catalysis. Thus, TIPP3 easily reacted with bis(benzonitrile)palladium(II) dichloride at room temperature, yielding the corresponding red, cationic metal complex [Pd(η4-PP3TIPP3)Cl]þ (Scheme 1), isolated as its chloride salt (2). This compound shows a molar conductivity (ΛM) of 84 S cm2 mol-1 in dichloromethane at room temperature, similar to a 1:1 electrolyte.12 Single-crystal X-ray diffraction revealed that the geometry of the metal complex is trigonal bipyramidal and the ligand is tetracoordinated to the metal. The view along the 3-fold molecular axis of the palladium complex revealed its axial chirality (Figure 2). Table 1 summarizes selected bond distances and angles. (10) Yu, J. O.; Lam, E.; Sereda, J. L.; Rampersad, N. C.; Lough, A. J.; Browning, C. S.; Farrar, D. H. Organometallics 2005, 24, 37. (11) For other examples of chiral phosphanes with similar symmetry see: Benincori, T.; Marchesi, A.; Mussini, P. R.; Pilati, T.; Ponti, A.; Rizzo, S.; Sannicolo, F. Chem.;Eur. J. 2009, 15, 86. (12) Svorstol, H.; Hoiland, J.; Songstad Acta Chem. Scand. 1984, B38, 885.
Figure 2. ORTEP view of the Pd(II) complex 2. Hydrogen atoms are omitted for clarity. Only the ipso C atoms of the phenyl rings are shown for clarity. Important bond distances (A˚) and angles (deg): Pd(1)-P, 2.153(2); Pd(1)-Cl(1), 2.361(2); Pd(1)-P(1), 2.491(2); Pd(1)-P(2), 2.373(2); Pd(1)-P(3), 2.492(2); P-Pd(1)-Cl(1), 176.38(9); Cl(1)-Pd(1)-P(1), 101.64(9).
The NMR spectra of 2 are in agreement with the symmetry found in the X-ray structure. The 31P NMR spectrum (Figure S3) shows a quadruplet at lower field for the central phosphorus bridge and a doublet at higher field for the pendant phosphorus atoms. In the 1H NMR spectrum (Figure S4) the singlet at 1.67 ppm indicates the three methyl substituents attached to the methine are equivalent. In addition, TIPP3 was reacted with (1,5-cyclooctadiene)rhodium(I) chloride dimer at room temperature, giving rise to the red, neutral [Rh(η4-PP3-TIPP3)Cl] complex 3 (Scheme 1 and Figure S5). The solid-state structure of 3 (Figure 3) was analyzed by X-ray diffraction of a single crystal (grown in CH2Cl2/hexane), which demonstrated a similar geometry to that of 2 (Table 1 compares selected bond distances and angles of 2 and 3). In addition, complex 2 was reacted with AgBF4 followed by treatment with (R)-phenylethylamine. The 19F NMR spectrum shows one signal at -154 ppm, which is indicative
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Table 1. Selected Crystal Structure Data for the Tripodal Ligand 1 and Metal Complexes 2 and 3a ligand 1
Pd complex 2
Rh complex 3
Distances (A˚) (Pc-N) (Pc-M) (Ps-M) (M-Cl)
1.729, 1.732, 1.739
1.660, 1.673, 1.680 2.153 2.373, 2.491, 2.492 2.361
1.681, 1.699, 1.700 2.106 2.314, 2.322, 2.378 2.430
Angles (deg) (M-Pc-N) (Pc-M-Ps) a
109.8, 110.6, 111.2 82.71, 81.73, 79.90
111.4, 112.2, 112.9 81.00, 83.58, 82.93
c = central, s = side arm, M = metal.
Figure 3. ORTEP view of the Rh(I) complex 3. Hydrogen atoms are omitted for clarity. Only the ipso C atoms of the phenyl rings are shown for clarity. Important bond distances (A˚) and angles (deg): Rh(1)-P, 2.106(2); Rh(1)-Cl(1), 2.430(2); Rh(1)-P(1), 2.378(2); Rh(1)-P(2), 2.314(2); Rh(1)-P(3), 2.322(2); PRh(1)-Cl(1), 173.65(8); Cl(1)-Rh(1)-P(1), 103.40(8).
of uncoordinated BF4. The formation of diastereoisomers would lead to a doubling of the 31P NMR signals and demonstrate the existence of the complex as a racemic mixture. In fact, the 31P NMR spectrum of the complex shows two signals of the same intensity, at -2.8 and -3.2 ppm, for the pendant phosphines (Figure S6), which proves that the axial chirality of the complex is retained in solution. Inversion between P and M enantiomers of C3-symmetric chiral metal complexes with ligands devoid of a chiral center has been studied by NMR spectroscopy performed at different temperatures.13,7b The configurational stability of complexes 2 and 3 was assessed by registering their NMR spectra at temperatures ranging from -50 to 85 °C, using either dichloromethane or toluene as the solvent. Complexes 2 and 3 possess a considerable intramolecular crowding between the three methyl groups. 1H NMR spectra of these complexes show the three methyl groups are equivalent. Consequently, the indolyl groups are twisted in the same sense. A broadening of the signals should have been expected while the temperature was varied in the range -50 to 85 °C if racemization occurred. In conclusion, a novel rigid C3-symmetric ligand with phosphorus as coordinating atoms was synthesized in good (13) Chmura, A. J.; Chuck, C. J.; Davidson, M. G.; Jones, M. D.; Lunn, M. D.; Bull, S. D.; Mahon, M. F. Angew. Chem., Int. Ed. 2007, 46, 2280.
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yields. A key feature of this ligand is the orientation and steric demand of its pendant ligands, whose coordination to Pd and Rh leads to new examples of rare inherently chiral C3-symmetric metal complexes. These new complexes show a unique configurational stability. Also of considerable importance is the stability of compounds 2 and 3 in solution, as five-coordinated d8 complexes are usually unstable and are sometimes proposed as intermediates in catalytic systems.14
Experimental Section General Procedures. All manipulations were performed under purified argon in standard (Schlenk) glassware. 2-Diphenylphosphino-3-methylindole was obtained according to the literature.10 All the other reagents were commercially available. Solvents were of analytical grade and were dried according to standard procedures. The 1H, 13C, 31P, and 19F NMR spectra were recorded on a Bruker DRX-400 spectrometer (at 400, 100.5, 161.8, and 376.2 MHz, respectively), using tetramethylsilane (1H and 13C NMR) or H3PO4 85% (31P NMR) as reference. Chemical shifts are reported in ppm. The coupling constants (J) are in hertz (Hz). Elemental analyses were provided by Centro de Microanalisis Elemental, Universidad Complutense de Madrid. Column chromatography was performed on silica gel (35-70 mesh). Solvent mixtures are volume/volume mixtures, unless otherwise specified. Conductivity measurements were made with a Crison GLP 31 conductimeter at 25 °C. Synthesis of TIPP3 (1). n-Butyllithium (2.05 mL, 1.6 M in hexane, 3.28 mmol) was slowly added to a solution of 2-diphenylphosphino-3-methylindole (1 g, 3.17 mmol) in THF (25 mL) at -78 °C. After stirring for 15 min at the same temperature, the solution was warmed to room temperature and stirred for another 15 min. After cooling again at -78 °C, PCl3 (92.5 mL, 1.06 mmol) was added. The solution was allowed to warm to room temperature and subsequently refluxed for 5 h. The solution was evaporated to dryness, leaving a light brown solid, which was extracted with CH2Cl2 (50 mL) and filtered over silica. The obtained colorless solution was concentrated under reduced pressure, and upon addition of diethyl ether, the product precipitated as a white solid (328 mg). Further precipitation from the mother solution yielded more compound (148 mg). Yield: 476 mg (46%). Crystals suitable for singlecrystal X-ray diffraction were grown from a dichloromethane solution of the product carefully layered with hexane. 1 H NMR (CDCl3): δ 1.76 (s, 9H), 6.49 (t, J=7.2, 6H), 6.64 (d, J = 8.4, 3H), 6.93-7.19 (m, 27H), 7.24-7.29 (m, 3H), 7.53 (d, J = 7.8, 3H). 13C NMR (CDCl3): δ 10.7 (s, CH3), 114.0 (s, CH, indolyl), 119.4 (s, CH, indolyl), 120.8 (s, CH, indolyl), 124.9 (s, CH, indolyl), 125.8 (s, Cquart, indolyl), 127.2 (s), 127.9 (d, J = 5.5 Hz), 128.3 (d, J = 7.9 Hz), 128.5 (s), 131.4 (d, J = 18.1 Hz), 133.4 (d, J=42.4 Hz), 133.8 (d, J = 31.0 Hz), 134.2 (d, J = 20.7 Hz), 136.8 (d, J = 11 Hz, Cquart, indolyl), 136.9 (d, J = 9.7 Hz, Cquart, indolyl), 142.0 (m, Cquart, indolyl). 31P NMR (CDCl3): δ -30.0 (d, J = 197 Hz, 3P), 73.4 (q, J = 197 Hz, 1P). Anal. Calcd for C63H51N3P4: C 77.24, H 5.43, N 4.43. Found: C 77.69, H 5.28, N 4.31. Synthesis of [Pd(η4-PP3-TIPP3)Cl]Cl (2). Tris(2-(diphenylphosphino)-3-methyl-1-indolyl)phosphine (TIPP3) (50 mg, 0.051 mmol) was added to a solution of bis(benzonitrile)palladium(II) dichloride (20 mg, 0.051 mmol) in dichloromethane (20 mL). The color of the solution immediately changed from yellow to red. After stirring for 1 h, the solution was evaporated to dryness, and the solid red residue was purified by column chromatography (silica, 2 8 cm, chloroform to (14) (a) Crabtree, R. H. The Organometallic Chemistry of Transition Metals, 2nd ed.; John Wiley and Sons: New York, 1994. (b) Koshevoy, I. O.; Sizova, O. V.; Tunik, S. P.; Lough, A.; Po€e, A. J. Eur. J. Inorg. Chem. 2005, 4516.
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chloroform/methanol, 10:1). The red band was collected and evaporated to dryness. Recrystallization from dichloromethane/ diethyl ether gave a red microcrystalline solid. Yield: 47 mg, 82%. Crystals suitable for single-crystal X-ray diffraction were grown from a dichloromethane solution of the product carefully layered with hexane. 1 H NMR (CDCl3): δ 1.67 (s, 9H), 6.40 (d, J = 8.4, 3H), 6.92-6.96 (m, 6H), 7.02-7.07 (m, 6H), 7.18-7.24 (m, 6H), 7.38-7.42 (m, 15H), 7.48-7.51 (m, 3H), 7.58 (d, J = 8.0, 3H). 13 C NMR (CDCl3): δ 10.1 (s), 112.6 (s, CH, indolyl), 121.9 (s, CH, indolyl), 124.8 (s, CH, indolyl), 125.3 (m), 128.2 (m), 128.3 (s, CH, indolyl), 128.9 (m), 129.0 (m), 129.8 (m), 130.8 (s), 131.0 (m), 131.8 (m), 131.9 (s), 134.2 (m), 135.1 (m), 139.6 (m). 31P NMR (CDCl3): δ -3.4 (d, J=8.4, 3P), 95.6 (q, J=8.4, 1P). MS (FAB): m/z (%) 1114 (100), [M - Cl]þ; 1079 (5), [M - 2Cl]þ. Synthesis of [Rh(η4-PP3-TIPP3)Cl] (3). Tris(2-(diphenylphosphino)-3-methyl-1-indolyl)phosphine (TIPP3) (30 mg, 0.031 mmol) was added to a solution of (1,5-cyclooctadiene)rhodium(I) chloride dimer (7.9 mg, 0.016 mmol) in dichloromethane (10 mL). The color of the solution immediately changed from yellow to red. After stirring for 1 h, the solution was evaporated to dryness and the solid red residue was purified by recrystallization from tetrahydrofuran/hexane, giving a red microcrystalline solid. Yield: 33 mg, 97%. Crystals suitable for single-crystal X-ray diffraction were grown from a dichloromethane solution of the product carefully layered with hexane. 1 H NMR (toluene-d8): δ 1.60 (s, 9H), 6.69 (d, J = 8.4, 6H), 6.79 (m, 3H), 6.84 (m, 3H), 6.97-7.10 (m, 15H), 7.20 (m, 3H), 7.34 (m, 6H), 7.50 (m, 6H). 13C NMR (CDCl3): δ 10.1 (s, CH3), 113.6 (s, CH, indoly1), 120.3 (s, CH, indolyl), 122.0 (s, CH, indolyl), 125.1 (s, CH, indolyl), 126-140. 31P NMR (CDCl3): δ 5.0 (dd, JP-Rh =138.4 Hz, JP-P0 =30.4 Hz), 131.3 (dq, JP-Rh = 168.2 Hz, JP-P0 = 30.4P). MS (FABþ): m/z (%) 1112 (100), [M]þ; 1076 (16), [M - Cl]þ. Anal. Found: C 67.27, H 4.59, N 3.93. Calcd for C63H51ClN3P4Rh: C 68.02, H 4.62, N 3.78. Crystallographic data were collected on a Kappa CCD diffractometer for compounds 1 and 2 and on a Kappa 2000 for compound 3. The structures were solved by direct methods and refined using the SHELXTL15 program. Non-hydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atoms were added in calculated positions. The H atoms of the water molecule on compound 2 could not be located. In compound 1 the solvent molecules could not be modeled properly, due to the low crystal quality. The program SQUEEZE, part of the PLATON package of crystallographic software, was used to calculate the solvent disorder area and
remove its contribution to the overall intensity data.16 An improvement was observed in all refinement parameters and residuals. Proof of the Chirality of Complex 2. A methanol solution of AgBF4 was added to a dichloromethane solution of complex 2. The solution became a brighter red and a precipitate was formed, which was filtered and dried. The 31P NMR spectrum of the product (red solid) in CDCl3 showed two signals, one at 80 ppm (broad band, 1P) and the other at -1 ppm (doublet, 3P). A drop of (R)-phenylethylamine was added to the NMR tube, and the 31P NMR spectrum of the complex showed two signals of the same intensity, at -2.8 and -3.2 ppm, for the pendant phosphines (Figure S6). X-ray crystal structure data for 1: C63H51N3P4. rhombohedral, space group R3, a =b =35.013(5) A˚, c =28.018(6) A˚, V= 29746(8) A˚3, Z = 18, Fcalcd = 0.979 g cm-3, crystal dimensions 0.24 0.28 0.29 mm3; Mo KR radiation, 298(2) K; 4417 reflections, 4471 independent (μ=0.149 mm-1); refinement (on F2), 635 parameters, 0 restraints, R1= 0.1071 (I > 2σ) and wR2 (all data) = 0.2786, GOF = 1.122, max./min. residual electron density 0.476/-0.433 e A˚-3. X-ray crystal structure data for 2 3 Cl- 3 2CH2Cl2 3 (H2O): C65H55PdCl6N3OP4. triclinic, space group P1, a = 12.5610(3) A˚, b = 15.5230(4) A˚, c = 16.3090(5) A˚, R = 94.3130(14)°, β = 91.6340(14)°, γ = 99.015(2)°, V = 3129.20(16) A˚3, Z = 2, Fcalcd = 1.419 g cm-3, crystal dimensions 0.22 0.26 0.28 mm3; Mo KR radiation, 298(2) K; 27 53 reflections, 9845 independent ( μ = 0.699 mm-1); refinement (on F2), 724 parameters, 0 restraints, R1= 0.0827 (I > 2σ) and wR2 (all data)=0.2629, GOF=1.101, max./min. residual electron density 2.177/-0.745 e A˚-3. X-ray Crystal Structure Data for 3 3 CH2Cl2. C64H53RhCl3N3P4. monoclinic, space group P2(1)/c, a = 12.2170(2) A˚, b = 18.7130(3) A˚, c = 25.2030(5) A˚, β = 97.0640(7)°, V = 5718.09(17) A˚3, Z=4, Fcalcd =1.391 g cm-3, crystal dimensions 0.25 0.29 0.32 mm3; Cu KR radiation, 298(2) K; 15 747 reflections, 6853 independent (μ=5.096 mm-1); refinement (on F2), 680 parameters, 0 restraints, R1 = 0.0736 (I > 2σ) and wR2 (all data) = 0.2141, GOF = 1.037, max./min. residual electron density 0.888/-1.310 e A˚-3.
(15) Sheldrick, G. M. SHELXTL 6.1; Bruker AXS, Inc.: Madison, WI, 2001.
(16) Spek, A. L. PLATON; Utrecht University: Utrecht, The Netherlands, 2005.
Acknowledgment. This work was supported by the Spanish Ministry of Science and Technology [CTQ200506909-C02-01/BQU and CTQ2008-06466]. Supporting Information Available: NMR spectra of compounds 1-3. This material is available free of charge via the Internet at http://pubs.acs.org.
