Note pubs.acs.org/Organometallics
Multiple Carbon−Carbon and Carbon−Metal Bond Formation from an Iridium-pybox Complex and Electron-Poor Terminal Alkynes: Synthesis of Iridium Complexes with a Novel κ4N,N,N,C Tetradentate Ligand Paloma Paredes, Esmeralda Vega, Josefina Díez, and M. Pilar Gamasa* Departamento de Quı ́mica Orgánica e Inorgánica, Instituto Universitario de Quı ́mica Organometálica “Enrique Moles” (Unidad Asociada al CSIC), Universidad de Oviedo, 33006 Oviedo, Principado de Asturias, Spain S Supporting Information *
ABSTRACT: The reaction of the complex [Ir(η2-C2H4){κ3N,N,N-(S,S)-iPr-pybox)}][PF6] (1) with methyl propynoate in acetonitrile yields the new complex 2 with complete selectivity. In the presence of NaCl, the reaction of 1 with propynoate esters in acetonitrile affords complexes 3a,b. The addition of methanol to complex 3a affords complex 4, whose structure has been determined by X-ray diffraction analysis.
A
Now we report an unprecedented coupling reaction between the Ir(I)-pybox system and propynoate esters. The whole process comprises the formal carbometalation of the alkyne with participation of the iridium center and C-2 of the oxazoline ring.
symmetric synthesis catalyzed by transition-metal complexes bearing enantiopure pyridine-2,6-bis(oxazoline) ligands has received considerable attention in recent years.1 Despite the fact that the catalytic species are usually generated in situ by mixing the metallic precursors and pybox ligands, we and others have reported that active catalysts can be preformed and thereafter used.2 On the other hand, the metal ligands rarely behave as reactive components in transition-metalpromoted processes. In relation with the pyridine-2,6-bis(oxazoline) system, it has been reported that the nucleophilic ring-opening reaction of an oxazoline ring in the presence of Pd(II) and Au(III) species forms a new complex containing a metal−nitrogen σ-bond (Scheme 1).3,4
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RESULTS AND DISCUSSION The reaction of a solution of the iridium complex 1 and methyl propynoate in acetonitrile at room temperature (1/2 molar ratio) resulted in the regio- and stereoselective formation of the new complex 2, which was isolated as an orange solid in 74% yield. In addition, the reaction of 1 with HCCE (E = CO 2 Me, CO2 Et) and NaCl (1/3/1.9 molar ratio) in acetonitrile at room temperature directly produced the complexes 3a,b in good yields (66−88% yield) (Scheme 2). The 1H and 13C{1H} NMR spectra of complexes 2 and 3a,b are in agreement with the presence of a tetradentate ligand.5 1 1 H H COSY and 1H13C HSQC correlation experiments have been performed on complex 3a for the assignation of the 1H and 13C NMR signals. The spectra exhibit resonances assignable to the oxazoline, pyridine, and oxazolidine rings. Thus, the most significant resonances in the 1H NMR spectrum (δ, ppm) are as follows: (i) 9.87 (Ir−CH, singlet), (ii) 3.61 (CO2CH3, singlet), (iii) 3.08 (CH3CN, singlet), (iv) oxazoline ring 5.27 (OCH2, multiplet), 5.14 (OCH2, triplet, JHH = 9.8 Hz), and 4.59 (CHiPr, m), (v) oxazolidine ring 4.46 (OCH2,
Scheme 1. Transformation of the pybox Ligand upon Reaction with Na[AuCl4] or Pd(OAc)2
Received: February 10, 2012 Published: May 1, 2012 © 2012 American Chemical Society
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Scheme 2. Synthesis of Complexes 2 and 3a,b
multiplet), 3.08 (OCH2, multiplet), and 6.83 (NH, broad doublet). The most significant resonances in the 13C{1H} NMR spectrum are found at 171.7 (oxazoline C-2), 155.9 (IrCH), 160.9, 154.5, 137.2 (quaternary carbons, CCO2Me, oxazolidine C-2 and CO2Me), 113.1 (CH3CN), 50.6 (CO2Me) and 2.9 ppm (CH3CN). The NMR spectra of complexes 2 and 3b confirm also the presence of a single isomer and are in accordance with the pattern shown for 3a. Moreover, the additional signals due to the σ-alkynyl ligand of complex 2 are observed in the 13C{1H} NMR spectrum at 113.9 (Ir−CCCO2Me) and 98.4 ppm (Ir−CCCO2Me). Regarding the orientation of ligands L and MeCN, the NMR spectroscopic data definitely do not allow us to ascertain the stereochemistry of complexes 2 (L = CCCO2Me) and 3a,b (L = Cl). The isomer with NCMe cis to the alkenyl group is tentatively proposed on the basis of the crystal structure of the derivative 4 (see below). It was interesting to note that three new σ bonds of different natures were formed in the synthesis of complex 2: Ir−C(sp), Ir−C(sp2), and C(sp2)−C(sp3). On the other hand, the process implies a rare alkyne carbometalation as well as an oxidative hydroalkynylation of the iridium center. Overall, the original κ3N,N,N pybox ligand has been transformed into a new tetradentate κ4N,N,N,C ligand and the Ir(I) is oxidized into Ir(III). A tentative reaction course for synthesis of complex 2 is outlined in Scheme 3. First, we can assume the exchange of the labile ethylene ligands in complex 1 by the methyl propynoate
ligands to give the complex [Ir(η2-HCCCO2Me)2{κ3N,N,N(S,S)-iPr-pybox}][PF6] (intermediate A).6 The oxidative addition of the C(sp)−H bond would generate the intermediate complex B, which would suffer the regioselective cis insertion of the Ir−H bond into the alkyne to form the Ir(alkynyl, alkenyl) complex C. The formation of the C−C bond (intermediate metal carbene D) would occur by intramolecular nucleophilic attack of the terminal carbon of the Ir-alkenyl group at the CN bond of the oxazoline group. Finally, intramolecular deprotonation by the basic nitrogen center would lead to the observed metal complex 2.7 Since crystals of compounds 2 and 3a suitable for X-ray diffraction could not be obtained, the complex 3a was transformed into its derivative 4 (78% yield) by stirring in methanol at room temperature (Scheme 4).8 The complex 4 was isolated as an orange solid and its structure determined by a single-crystal X-ray diffraction study (see Figure 1 and the Supporting Information).
Scheme 3. Transformation of the pybox Ligand
In the crystal of complex 4 one dichloromethane solvation molecule per formula unit of the complex is found. Complex 4 shows a pseudo-octahedral geometry with the iridium atom bonded to the chloride atom, to the imidate nitrogen, and to the nitrogen and carbon atoms of the new tetradentate ligand (Figure 1). The bond angles around the iridium atom are in the range of 174.4(2)° (N(2)−Ir(1)−N(4)) and 78.0(2)° (N(2)− Ir(1)−N(1)). The structure confirms the coupling between the CN bond of one oxazoline unit and the alkyne, leading to a nonplanar oxazolidine ring. Accordingly, the carbon−nitrogen bond length N(3)−C(12) (1.523(8) Å) (oxazolidine ring) is longer than that of N(1)−C(6) (1.288(9) Å) (oxazoline ring), and the angles around the C(12) atom are in the range of 102.6(5)−119.3(5)°, typical of an sp3-carbon atom. The distance Ir−C(18) (2.002(6) Å) is slightly shorter than that of an Ir−C single bond,9 while the distance C(18)−C(19) (1.335(9) Å) is slightly longer than that expected for a CC double bond. In relation with the imidate ligand, the structure reveals a cis relationship between the methoxy group and the iridium center. According to the resonance forms of the imidate function, the distance C(23)−O(6) (1.324(9) Å) is slightly shorter than that of a C(sp2)−O single bond (1.34 Å) and longer than that expected for a C(sp2)−O double bond (1.21 Å).10 In the same way, the angle C(23)−O(6)−C(24)
Scheme 4. Synthesis of Complex 4
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(m, 1H, CHiPr), 4.42 (m, 2H; OCH2), 3.67 (s, 3H; CO2Me), 3.56 (s, 3H; CO2Me), 3.00 (m, 1H, CHiPr), 2.90 (m, 1H, CHMe2), 2.55 (s, 3H, MeCN), 2.12 (m, 1H, CHMe2), 1.21 (d, JH,H = 6.7 Hz, 3H; CHMe2), 1.06 (d, JH,H = 7.0 Hz, 3H; CHMe2), 1.00 (d, JH,H = 6.7 Hz, 3H; CHMe2), 0.77 (d, JH,H = 7.0 Hz, 3H; CHMe2) ppm. 13C{1H} NMR (100.62 MHz, acetone-d6, 298 K): δ 173.0 (s, OCN), 160.7, 154.0, 153.8, 135.8 (4s, OC−N, 2CO2Me, CHCCO2Me), 156.9 (s, IrCH), 143.5 (s, C2,6 C5H3N), 141.3 (s, C4 C5H3N), 124.8, 124.7 (2s, C3,5 C5H3N), 121.1 (s, MeCN), 113.9 (s, CCCO2Me), 98.4 (s, C CCO2Me), 73.5, 73.0 (2s, OCH2), 69.5, 63.0 (2s, CHiPr), 50.8, 50.4 (2s, CO2Me), 31.0, 29.1 (2s, CHMe2), 19.9, 19.0, 17.7, 14.1 (4s, CHMe2), 2.1 (s, MeCN) ppm. Synthesis of Complex 3a. To a solution of methyl propynoate (0.033 mL, 0.45 mmol) in 7 mL of acetonitrile were added [Ir(η2C2H4)2(iPr-pybox)][PF6] (1; 104 mg, 0.15 mmol) and NaCl (16 mg, 0.285 mmol) under a nitrogen atmosphere. The reaction mixture was stirred for 2 h at room temperature, and then the volatiles were removed in vacuo. The residue was dissolved in CH2Cl2 and the solution filtered through a cannula transfer. The addition of a mixture of diethyl ether and hexane (1/3) afforded an orange solid that was washed with hexane (3 × 5 mL). Yield: 79 mg, 66%. IR (KBr, ν(CO2Me), ν(PF6)): 1700 (m), 844 (vs) cm−1. Molar conductivity (acetone, S cm 2 mol − 1 , 293 K): 132. Anal. Calcd for C23H31ClF6IrN4O4P (800.15 g/mol): C, 34.52; H, 3.91; N, 7.00. Found: C, 34.12; H, 3.88; N, 6.98. MS-ESI: m/z 655.2 ([M − PF6]+, 100%); 614.2 ([M − PF6 − MeCN]+, 85%). 1H NMR (300.13 MHz, acetone-d6, 298 K): δ 9.87 (s, 1H, IrCH), 8.60 (t, JH,H = 8.0 Hz, 1H, H4 C5H3N), 8.22 (d, JH,H = 8.0 Hz, 1H, H3,5 C5H3N), 8.11 (d, JH,H = 8.0 Hz, 1H, H3,5 C5H3N), 6.83 (br, 1H, NH), 5.27 (t, JH,H = 9.8 Hz, 1H, OCH2), 5.14 (t, JH,H = 9.8 Hz, 1H, OCH2), 4.59 (m, 1H, CHiPr), 4.46 (m, 2H, OCH2), 3.61 (s, 3H, CO2Me), 3.08 (m, 4H, MeCN, CHiPr), 2.53 (m, 1H, CHMe2), 2.11 (m, 1H, CHMe2), 1.20 (d, JH,H = 6.6 Hz, 3H, CHMe2), 1.09 (d, JH,H = 7.0 Hz, 3H, CHMe2), 1.00 (d, JH,H = 6.6 Hz, 3H, CHMe2), 0.91 ppm (d, JH,H = 7.0 Hz, 3H, CHMe2). 13 C{1H} NMR (100.62 MHz, acetone-d6, 298 K): δ 171.7 (s, OC N), 160.9, 154.5, 137.2 (3s, OC−N, CO2Me, CHCCO2Me), 155.9 (s, IrCH), 143.7 (s, C2,6 C5H3N), 140.8 (s, C4 C5H3N), 125.0, 124.1 (2s, C3,5 C5H3N), 113.1 (s, MeCN), 74.3, 73.4 (2s, OCH2), 68.7, 63.5 (2s, CHiPr), 50.6 (s, CO2Me), 31.2, 30.0 (2s, CHMe2), 20.4, 20.0, 17.6, 15.1 (4s, CHMe2), 2.9 (s, MeCN) ppm. Synthesis of Complex 4. A solution of complex 3a (0.24 g, 0.3 mmol) in methanol (6 mL) was stirred at room temperature for 12 h, under a nitrogen atmosphere. The solvent was concentrated to ca. 1 mL, and diethyl ether was added (30 mL). The resulting orange solid was washed with diethyl ether (3 × 5 mL) and vacuum-dried. Yield: 195 mg, 78%. IR (KBr, ν(CO2Me), ν(CO2Me), ν(CN), ν(PF6) cm−1): 1696 (m), 1653 (m), 1635 (m), 845 (vs) cm−1. Molar conductivity (acetone, S cm2 mol−1, 293 K): 126. Anal. Calcd for C24H35ClF6IrN4O5P (832.19 g/mol): C, 34.64; H, 4.24; N, 6.73. Found: C, 34.78; H, 4.26; N, 6.63. FAB-MS: m/z 614.14 ([M − PF6 − HNC(OMe)Me]+, 100%). 1H NMR (400.13 MHz, acetone-d6, 298 K): δ 10.39 (s, 1H; IrCH), 8.53 (t, JHH = 7.9 Hz, 1H; H4 C5H3N), 8.18 (d, JHH = 7.9 Hz, 1H; H3,5 C5H3N), 8.07 (d, JHH = 7.9 Hz, 1H; H3,5 C5H3N), 6.45 (br, 1H; NH), 5.20 (t, JHH = 9.8 Hz, 1H; OCH2), 5.06 (m, 1H; OCH2), 4.56 (m, 1H; CHiPr), 4.43 (m, 2H; OCH2), 4.17 (s, 3H; OMe), 3.59 (s, 3H; CO2Me), 3.09 (m, 1H; CHiPr), 2.84 (s, 3H; HNC(OMe)Me), 2.35 (m, 1H; CHMe2), 2.02 (m, 1H; CHMe2), 1.03 (d, JHH = 7.3 Hz, 3H; CHMe2), 0.96 (m, 6H; CHMe2), 0.84 (d, JHH = 6.9 Hz, 3H; CHMe2) ppm. 13C{1H} NMR (100.62 MHz, acetone-d6, 298 K): δ 176.5 (s, HNC), 172.0 (s, OCN), 160.6, 154.7, 137.0 (3s, OC−N, CO2Me, CHCCO2Me), 156.0 (s, IrCH), 143.5 (s, C2,6 C5H3N), 139.5 (s, C4 C5H3N), 124.2, 123.8 (2s, C3,5 C5H3N), 73.8, 73.0 (2s, OCH2), 68.2, 64.0 (2s, CHiPr), 57.1 (s, OMe), 50.3 (s, CO2Me), 31.0, 30.0 (4s, CHMe2), 19.7, 19.2, 17.5, 14.9 (4s, CHMe2), 18.4 (s, HNC(OMe)Me) ppm.
Figure 1. ORTEP-type view of molecular structure of complex 4 showing the atom-labeling scheme. Thermal ellipsoids are shown at the 20% probability level. Hydrogen atoms, except H(3N), and the PF6 anion have been omitted for clarity. Selected bond lengths (Ǻ ) and angles (deg): N(1)−C(6), 1.288(9); N(3)−C(12), 1.523(8); Ir− C(18), 2.002(6); Ir−N(4), 2.076(5); N(4)−C(23), 1.265(9); C(23)− O(6), 1.324(9); Ir−N(1), 2.071(5); Ir−N(3), 2.109(5); Ir−N(2), 1.975(6); C(23)−O(6)−C(24), 118.7(6); Cl(2)−Ir−C(18), 171.31(17); N(1)−Ir−N(3), 156.4(2); N(2)−Ir−N(4), 174.4(2).
(118.7(6)°) is close to 120° and the N(4)−C(23) distance is 1.265(9) Å. These values are in accordance with those reported for the complex [Ir(η5-C5Me5)(η3CH2CHCHPh){NHC(OMe)Me}][OTf].11
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SUMMARY In conclusion, a novel reactivity of Ir(I)-pybox complexes toward alkyl propynoates is reported. Interestingly, various single processesC(sp)−H oxidative addition, Ir−H insertion into alkyne, and cyclizationare involved. They have finally resulted in the formation of various C−Ir, C−C, and C−H bonds. The key Ir−H/alkyne insertion occurs with complete stereo- and regioselectivity, affording ultimately a single isomer. From a synthetic point of view, new enantiopure Ir(III) complexes with a novel κ4N,N,N,C ligand have been easily prepared from readily available Ir(I)-pybox and propynoate esters.
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EXPERIMENTAL SECTION
Synthesis of Complex 2. To a solution of methyl propiolate (0.027 mL, 0.38 mmol) in 5 mL of acetonitrile was added [Ir(η2C2H4)2(iPr-pybox)][PF6] (1; 0.104 g, 0.15 mmol) under a nitrogen atmosphere. The reaction mixture was stirred for 10 min at room temperature, and then the volatiles were removed in vacuo. The residue was dissolved in CH2Cl2 and the solution filtered through a cannula transfer. The addition of a mixture of diethyl ether and hexane (1/3) afforded an orange solid, which was washed with hexane (3 × 5 mL). Purification by silica gel chromatography (4/1 ethyl acetate/ hexane) afforded complex 2 as an orange solid. Yield: 95 mg, 74%. IR (KBr, ν(CC), ν(CO2Me), ν(PF6) in cm−1): 2106 (s), 1685 (br s), 843 (vs). Molar conductivity (acetone, S cm2 mol−1, 293 K): 108. Anal. Calcd for C27H34F6IrN4O6P (847.76 g/mol): C, 38.25; H, 4.04; N, 6.61. Found: C, 38.59; H, 4.35; N, 6.97. MS-ESI: m/z 662.1 ([M − PF6 − MeCN]+, 100%). 1H NMR (400.13 MHz, acetone-d6, 298 K): δ 9.69 (s, 1H, IrCH), 8.60 (t, JH,H = 7.9 Hz, 1H; H4 C5H3N), 8.28 (d, JH,H = 7.9 Hz, 1H; H3,5 C5H3N), 8.12 (d, JH,H = 7.9 Hz, 1H; H3,5 C5H3N), 7.11 (br, 1H; NH), 5.23 (dd, JH,H = 10.4 Hz, JH,H = 9.4 Hz, 1H; OCH2), 5.09 (dd, JH,H = 9.4 Hz, JH,H = 8.0 Hz, 1H; OCH2), 4.62 3800
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(9) (a) Paredes, P.; Dı ́ez, J.; Gamasa, M. P. J. Organomet. Chem. 2008, 693, 3681−3687. (b) Cuervo, D.; Dı ́ez, J.; Gamasa, M. P.; Gimeno, J.; Paredes, P. Eur. J. Inorg. Chem. 2006, 599−608. (10) March, J. In Advanced Organic Chemistry, 4th ed.; WileyInterscience: New York, 1992; p 21. (11) Chin, C. S.; Chong, D.; Lee, B.; Jeong, H.; Won, G.; Do, Y.; Park, Y. J. Organometallics 2000, 19, 638−648.
ASSOCIATED CONTENT
S Supporting Information *
Text, a table, and a CIF file giving details of the synthetic experimental procedures and preparation and characterization of complex 3b, details of the X-ray analysis, and crystallographic data for complex 4. This material is available free of charge via the Internet at http://pubs.acs.org. CCDC 863787 also contains supplementary crystallographic data for complex 4.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Spanish Government (MEC and MICINN) (Projects CTQ2006-08485 and CTQ201017005) and Consolider Ingenio 2010 (CSD2007-00006)). P.P. and E.V. thank the Spanish Ministerio de Educación y Ciencia for Ph.D. fellowships.
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REFERENCES
(1) Desimoni, G.; Faita, G.; Quadrelli, P. Chem. Rev. 2003, 103, 3119−3154. (2) (a) Cuervo, D.; Gamasa, M. P.; Gimeno, J. Chem. Eur. J. 2004, 10, 425−432. (b) Paredes, P.; Dı ́ez, J.; Gamasa, M. P. Organometallics 2008, 27, 2597−2607. (c) Panera, M.; Dı ́ez, J.; Merino, I.; Rubio, E.; Gamasa, M. P. Inorg. Chem. 2009, 48, 11147−11160. Recent examples: (d) Tse, M. K.; Bhor, S.; Klawonn, M.; Anilkumar, G.; Jiao, H.; Spannenberg, A.; Dçbler, C.; Mägerlein, W.; Hugl, H.; Beller, M. Chem. Eur. J. 2006, 12, 1875−1888. (e) Evans, D. A.; Fandrick, K. R.; Song, H.-J.; Scheidt, K. A.; Xu, R. J. Am. Chem. Soc. 2007, 129, 10029− 10041. (f) Milczek, E.; Boudet, N.; Blakey, S. Angew. Chem., Int. Ed. 2008, 47, 6825−6828. (g) Tondreau, A. M.; Darmon, J. M.; Wile, B. M.; Floyd, S. K.; Lobkovsky, E.; Chirik, P. J. Organometallics 2009, 28, 3928−3940. (3) The following examples are given for the ring opening of the bound oxazoline ring of a pybox ligand. (a) Palladium(II) complex: Kazi, A. B.; Jones, G. D.; Vicic, D. A. Organometallics 2005, 24, 6051− 6054. (b) Gold(III) complex: Corma, A.; Domı ́nguez, I.; Doménech, A.; Fornés, V.; Gómez-Garcı ́a, C. J.; Ródenas, T.; Sabater, M. J. J. Catal. 2009, 265, 238−44. (4) The ring opening of the bound oxazoline ring of 2-(2-pyridynyl) oxazoline (pyox) coordinated to gold(III), under alkaline conditions, to give the amido alkoxo neutral adducts [Au(N,N′,O)Cl] has been also reported: Cinellu, M. A.; Maiore, L.; Minghetti, G.; Cocco, F.; Stoccoro, S.; Zucca, A.; Manassero, M.; Manassero, C. Organometallics 2009, 28, 7015−7024. (5) The new complexes have been characterized by 1H and 13C NMR spectra. DEPT experiments were carried out for all of the compounds. (6) We have previously observed that the single ethylene/alkyne exchange reaction takes place in the case of complex 1 and dimethyl acetylendicarboxylate, under the same reaction conditions, to give the complex [Ir(η2-MeO2CCCCO2Me)2{κ3N,N,N-(S,S)-iPr-pybox}] [PF6]; see: Díez, J.; Gamasa, M. P.; Gimeno, J.; Paredes, P. Organometallics 2005, 24, 1799−1802. (7) Attempts directed at characterizing intermediates by monitoring the reaction (NMR; MeCN-d3, −20 °C) have been unsuccessful. (8) Other reactions of coordinated nitriles with protic nucleophiles, such as amines, alcohols, and water, have been reported; see: (a) Michelin, R. A.; Mozzon, M.; Bertani, R. Coord. Chem. Rev. 1996, 147, 299−338. (b) Chin, C. S.; Chong, D.; Lee, B.; Jeong, H.; Won, G.; Do, Y.; Park, Y. J. Organometallics 2000, 19, 638−648. 3801
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