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Organometallics 2009, 28, 4464–4474 DOI: 10.1021/om900300w
Application of Chiral Amine-Imine Ligands in Palladium-Catalyzed Polyketone Synthesis: Effect of Ligand Backbone on the Polymer Stereochemistry M. Rosa Axet,† Francesco Amoroso,† Giovanni Bottari,‡ Angela D’Amora,† Ennio Zangrando,† Felice Faraone,‡ Dario Drommi,‡ Maria Saporita,‡ Carla Carfagna,§ Paolo Natanti,§ Roberta Seraglia,^ and Barbara Milani*,† †
Dipartimento di Scienze Chimiche, Universit a di Trieste, Via Licio Giorgieri 1, 34127 Trieste, Italy, Dipartimento di Chimica Inorganica, Chimica Analitica e Chimica Fisica, Universit a di Messina, Italy, § Istituto di Scienze Chimiche, Universit a di Urbino, Piazza Rinascimento 6, 61029 Urbino, Italy, and ^CNR, Istituto di Scienze e Tecnologie Molecolari Sez. di Padova, Corso Stati Uniti 4, 35127 Padova, Italy ‡
Received April 21, 2009
Two pairs of enantiomerically pure chiral bidentate nitrogen-donor ligands (N*-N) featuring one sp3 and one sp2 nitrogen atom have been applied to the CO/vinyl arene copolymerization reaction catalyzed by the corresponding monocationic palladium(II) complexes [Pd(CH3)(CH3CN)(N*-N)][PF6]. The ligand fragment containing the sp2 nitrogen atom is a 2-pyridinyl or an 8-quinolinyl building block, while the chiral framework of the ligand derives from (S)-(þ)-2,20 -(2azapropane-1,3-diyl)-1,10 -binaphthalene or from trans-2,5-dimethylpyrrolidinyl. Ligands with the binaphthyl moiety generate catalysts showing a moderate activity, while ligands having the pyrrolidinyl fragment result in very low activity species. The stereochemistry of the synthesized polyketones depends on the sp2 nitrogen-containing fragment: ligands with the 2-pyridinyl group lead to atactic copolymers, whereas those with the quinolinyl moiety give an isotactic polyketone. The effect of the nature of the quinone, added to the reaction mixture as oxidant, has been investigated. MALDI-TOF analysis reveals the formation of several polymeric chains, differing in the presence of various end-groups. Introduction Control of polymer architecture is one of the major goals in metal-catalyzed polymerization reactions.1-3 The development of ligand synthesis as well as the coordination and organometallic chemistry allows the fine-tuning of the chemical environment around the metal center, which, together with the possibility to combine more active metals in one single reactor, opens access to new polymeric materials with precisely designed chain microstructures.4 During the last two decades considerable interest has been addressed to the palladium-catalyzed CO/alkene copolymerization yielding perfectly alternating polyketones.5-10 De*Corresponding author. Phone: 0039 040 5583956. Fax: 0039 040 5583903. E-mail:
[email protected]. (1) Coates, G. W.; Hustad, P. D.; Reinartz, S. Angew. Chem., Int. Ed. 2002, 41, 2236–2257. (2) Coates, G. W. J. Chem. Soc., Dalton Trans. 2002, 467–475. (3) Domski, G. J.; Rose, J. M.; Coates, G. W.; Bolig, A. D.; Brookhart, M. Prog. Polym. Sci. 2007, 32, 30–92. (4) Busico, V. Macromol. Chem. Phys. 2007, 208, 26–29. (5) Drent, E.; Budzelaar, P. H. M. Chem. Rev. 1996, 96, 663–681. (6) Bianchini, C.; Meli, A. Coord. Chem. Rev. 2002, 225, 35–66. (7) Nakano, K.; Kosaka, N.; Hiyama, T.; Nozaki, K. Dalton Trans. 2003, 4039–4050. (8) Durand, J.; Milani, B. Coord. Chem. Rev. 2006, 250, 542–560. (9) Su arez, E. J. G.; Godard, C.; Ruiz, A.; Claver, C. Eur. J. Inorg. Chem. 2007, 2582–2593. (10) Anselment, T. M. J.; Vagin, S. I.; Rieger, B. Dalton Trans. 2008, 4537–4548. pubs.acs.org/Organometallics
Published on Web 07/08/2009
spite polyketones being withdrawn from the market, the research in this field is still flourishing at both the academic and industrial level.11-18 In contrast to isotactic polyolefins, which, due to the phenomenon of cryptochirality, are optically inactive macromolecules,19,20 CO/R-olefin copolymers are an example of polymers with main chain chirality;21 thus, when they have an isotactic microstructure obtained via asymmetric catalytic polymerization, implying enantiomerically (11) Scarel, A.; Axet, M. R.; Amoroso, F.; Ragaini, F.; Elsevier, C. J.; Holuigue, A.; Carfagna, C.; Mosca, L.; Milani, B. Organometallics 2008, 27, 1486–1494. (12) Villagra, D.; Lopez, R.; Moya, S. A.; Claver, C.; Bastero, A. Organometallics 2008, 27, 1019–1021. (13) Bianchini, C.; Meli, A.; Oberhauser, W.; Segarra, A. M.; Passaglia, E.; Lamac, M.; Stepnicka, P. Eur. J. Inorg. Chem. 2008, 441–452. (14) Flapper, J.; Reek, J. N. H. Angew. Chem., Int. Ed. 2007, 46, 8590– 8592. (15) Bianchini, C.; Meli, A.; Oberhauser, W.; Claver, C.; Garcia Suarez, E. J. Eur. J. Inorg. Chem. 2007, 2702–2710. (16) Malinova, V.; Rieger, B. Biomacromolecules 2006, 7, 2931–2936. (17) Benito, J. M.; de Jesus, E.; de la Mata, F. J.; Flores, J. C.; Gomez, R. Organometallics 2006, 25, 3045–3055. (18) Durand, J.; Scarel, A.; Milani, B.; Seraglia, R.; Gladiali, S.; Carfagna, C.; Binotti, B. Helv. Chim. Acta 2006, 89, 1752–1771. (19) Beckerle, K.; Manivannan, R.; Lian, B.; Meppelder, G.-Jan M.; Raabe, G.; Spaniol, Thomas, P.; Ebeling, H.; Pelascini, F.; M€ ulhaupt, R.; Okuda, J. Angew. Chem., Int. Ed. 2007, 46, 4790–4793. (20) Carpentier, J.-F. Angew. Chem., Int. Ed. 2007, 46, 6404–6406. (21) Nozaki, K. Synthesis of chiral, optically active copolymers. In Catalytic Synthesis of Alkene-Carbon Monoxide Copolymers and Cooligomers; Sen, A., Ed.; Kluwer Academic Publishers: Dordrecht, 2003; pp 217-235. r 2009 American Chemical Society
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pure, chiral ligands, they lead to optically active materials.22-24 As a consequence, the control of the stereochemistry of the synthesized macromolecules is a main target, and in the case of vinyl arene comonomers a considerable number of nitrogen-donor ligands have been applied to this reaction with this aim.8,25 This collection of data defines a relationship between the symmetry of the ligand and the tacticity of the obtained copolymer: generally, C2v symmetric ligands lead to syndiotactic copolymers, while polyketones with an isotactic microstructure are obtained with ligands of C2 symmetry. Atactic CO/styrene polyketones are obtained with P-N ligands of C1 symmetry26 or with pyridine-imidazoline ligands.27,28 Up to now all the bidentate nitrogen-donor ligands applied to the CO/vinyl arene copolymerization are characterized by sp2 nitrogen atoms only.8 Moreover, while the polyketones with a syndiotactic or an atactic microstructure can be synthesized in high yield and with controlled molecular weight, the synthesis of the isotactic copolymer is still a challenge. Atropisomeric ligands featured by the biphenyl or the binaphthyl backbone as chiral framework have found wide application in asymmetric catalysis in general29,30 and in stereocontrolled CO/alkene copolymerization in particular. Palladium complexes with the BINAPHOS ligand are successful in catalyzing both CO/propene and CO/4-Mestyrene living copolymerization, yielding the corresponding polyketones with an isotactic microstructure.24,31-33 Also a palladium-based in situ catalytic system involving chiral diketimines gave highly isotactic CO/4-tBu-styrene copolymers with low productivities but with molecular weight values in the range of 50 000.34 Finally, atropisomeric P-N ligands were applied in the CO/styrene and in the CO/ethylene copolymerization.35-37 Very recently, we have shown that the electronic unbalance of the two nitrogen-donor atoms has a remarkable positive effect on the productivity of the reaction and on the molecular weight of the synthesized copolymers.11 (22) Brookhart, M.; Wagner, M. I.; Balavoine, G. G. A.; Haddou, H. A. J. Am. Chem. Soc. 1994, 116, 3641–3642. (23) Bartolini, S.; Carfagna, C.; Musco, A. Macromol. Rapid Commun. 1995, 16, 9–14. (24) Nozaki, K.; Sato, N.; Takaya, H. J. Am. Chem. Soc. 1995, 117, 9911–9912. (25) Consiglio, G.; Milani, B., Stereochemical aspects of cooligomerization and copolymerization; In Catalytic Synthesis of Alkene-Carbon Monoxide Copolymers and Cooligomers; Sen, A., Ed.; Kluwer Academic Publishers: Dordrecht, 2003; pp 189-215. (26) Sperrle, M.; Aeby, A.; Consiglio, G.; Pfaltz, A. Helv. Chim. Acta 1996, 79, 1387–1392. (27) Bastero, A.; Ruiz, A.; Claver, C.; Castill on, S. Eur. J. Inorg. Chem. 2001, 3009–3011. (28) Bastero, A.; Claver, C.; Ruiz, A.; Castillon, S.; Daura, E.; Bo, C.; Zangrando, E. Chem. Eur. J. 2004, 10, 3747–3760. (29) Noyori, R.; Takaya, H. Acc. Chem. Res. 1990, 23, 345–350. (30) Chen, Y.; Yekta, S.; Yudin, A. K. Chem. Rev. 2003, 103, 3155– 3212. (31) Nozaki, K.; Sato, N.; Tonomura, Y.; Yasutomi, M.; Takaya, H.; Hijama, T.; Matsubara, T.; Koga, N. J. Am. Chem. Soc. 1997, 119, 12779–12795. (32) Nozaki, K.; Komaki, H.; Kawashima, Y.; Hiyama, T.; Matsubara, T. J. Am. Chem. Soc. 2001, 123, 534–544. (33) Iggo, J. A.; Kawashima, Y.; Liu, J.; Hiyama, T.; Nozaki, K. Organometallics 2003, 22, 5418–5422. (34) Reetz, M. T.; Aderlein, G.; Angermund, K. J. Am. Chem. Soc. 2000, 122, 996–997. (35) Gsponer, A.; Consiglio, G. Helv. Chim. Acta 2003, 86, 2170– 2172. (36) Sirbu, D.; Consiglio, G.; Milani, B.; Kumar, P. G. A.; Pregosin, P. S.; Gischig, S. J. Organomet. Chem. 2005, 690, 2254–2262. (37) Leone, A.; Consiglio, G. Helv. Chim. Acta 2006, 89, 2720–2727.
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Chart 1. The Studied Amine-Imine Ligands and Their Numbering Scheme
We have recently studied the coordination chemistry to palladium(II) of chiral, enantiomerically pure, amine-imine ligands S1-S4 (N*-N; Chart 1) and the catalytic behavior of the related complexes in the asymmetric allylic alkylation of 1,3-diphenylallyl acetate with dimethyl malonate.38-40 These ligands can be visualized as a combination of a fragment having the sp2 nitrogen atom that is a pyridinyl, ligands S1 and S3, or a quinolinyl, ligands S2 and S4, with a chiral part comprising the sp3 nitrogen atom that derives either from the (S)-(þ)-2,20 -(2-azapropane-1,3-diyl)-1,10 -binaphthalene, ligands S1 and S2, or from the trans-2,5dimethylpyrrolidinyl, ligands S3 and S4. The two pairs of ligands differ also in the electronic properties of the sp3 nitrogen atom inside the chiral framework: while the pKa values for pyridine and quinoline are quite close (ca. 5), those for dialkyl-substituted azepine and dialkyl-substituted pyrrolidine are ca. 9 and 10, respectively.38 Finally, in ligands S1 and S3 the pyridinyl moiety is separated from the chiral framework by a methylenic spacer, while in ligands S2 and S4 the quinolinyl fragment is directly bound to the chiral part of the ligand. This difference should impart more rigidity in ligands S2 and S4 with respect to S1 and S3 that should manifest in a higher flexibility. We have now applied these ligands to the Pd-catalyzed synthesis of CO/vinyl arene polyketones, with the aim of investigating the effect of the ligand backbone on the stereocontrol of the polymerization reaction.
Results and Discussion Synthesis and Characterization of Pd Complexes [Pd(CH3)(CH3CN)(N*-N)][X] S1b-d, S2b-S4b (N*-N = S1-S4). The synthesis of the monocationic Pd(II) complexes [Pd(CH3)(CH3CN)(N*-N)][X] S1b-d and S2b-S4b (N*-N= S1, X=PF6 S1b, BF4 S1c, OTf (OTf=triflate) S1d; N*-N= S2, X = PF6 S2b; N*-N = S3, X = PF6 S3b; N*-N = S4, X=PF6 S4b) was performed starting from [Pd(CH3COO)2] following the five-step procedure reported in the literature,41-43 (38) Drommi, D.; Saporita, M.; Bruno, G.; Faraone, F.; Scafato, P.; Rosini, C. Dalton Trans. 2007, 1509–1519. (39) Brancatelli, G.; Saporita, M.; Drommi, D.; Nicol o, F.; Faraone, F. J. Organomet. Chem. 2007, 692, 5598–5604. (40) Saporita, M.; Bottari, G.; Brancatelli, G.; Drommi, D.; Bruno, G.; Faraone, F. Eur. J. Inorg. Chem. 2008, 59–72. (41) R€ ulke, R. E.; Ernsting, J. M.; Spek, A. L.; Elsevier, C. J.; Van Leeuwen, P. W. N. M.; Vrieze, K. Inorg. Chem. 1993, 32, 5769–5778. (42) Groen, J. H.; Delis, J. G. P.; vanLeeuwen, P.; Vrieze, K. Organometallics 1997, 16, 68–77. (43) Milani, B.; Marson, A.; Zangrando, E.; Mestroni, G.; Ernsting, J. M.; Elsevier, C. J. Inorg. Chim. Acta 2002, 327, 188–201.
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Axet et al.
Figure 1. (a) ORTEP drawing (thermal ellipsoids 40% probability level) of complex S2a; (b) side view along the C21-C22 bond. Selected coordination bond lengths [A˚] and angles [deg]: Pd-N(1) 2.076(11), Pd-N(2) 2.224(12), Pd-C(1) 2.052(16), Pd-Cl(1) 2.295(4); N(1)-Pd-N(2) 78.0(5), C(1)-Pd-N(1) 96.1(6), C(1)-Pd--N(2) 174.0(6), C(1)-Pd-Cl(1) 87.0(5), N(1)-Pd-Cl(1) 175.4(3), N(2)-Pd-Cl(1) 99.0(3). Scheme 1. Synthetic Pathway for the Monocationic Palladium Complexes S1b-d and S2b-S4b
which involves a dehalogenation reaction of the neutral derivatives [Pd(CH3)(Cl)(N*-N)] S1a-S4a (N*-N = S1-S4) as last step (Scheme 1). Single crystals suitable for X-ray analysis of the neutral derivative S2a were obtained upon addition of diethyl ether to a chloroform solution of the complex, at the temperature of 4 C. A view of the molecular structure of complex S2a with the atom-numbering scheme is shown in Figure 1. The palladium atom exhibits a distorted square-planar geometry, being coordinated by the nitrogen donors from the bidentate chelating ligand and a chlorido and a methyl group. In the unit cell only one isomer is present, which corresponds to the complex with the methyl group trans to the sp3 N donor, in accordance with the unique species detected in solution (see below). The Pd-N(2) bond length of 2.224(12) A˚ is significantly longer than the Pd-N(1), 2.076(11) A˚, due to the trans influence exerted by the methyl group. The two naphthyl moieties are planar and form a dihedral angle of 62.4(2), a value slightly larger than that measured in metal complexes reported so far ( 2σ(I), max. positive and negative peaks in ΔF map 0.452, -0.739 e A˚-3. Flack parameter indicating the absolute configuration=0.10(7). Reactivity of Complexes [Pd(CH3)(CH3CN)(N*-N)][PF6] S1b and S2b with Carbon Monoxide. The reactivity of the complexes [Pd(CH3)(CH3CN)(N*-N)][PF6], S1b and S2b, with carbon monoxide was studied by 1H and 13C NMR spectroscopy. CD2Cl2 (0.70 mL) was added to an NMR tube (5 mm) charged with the complex, 7.510-3 mmol. CO was bubbled for 5 min through a needle inserted into the rubber cap of the NMR tube. The NMR spectra were recorded after a total time of 10 min. The solution was always kept at room temperature. Copolymerization Reactions. Copolymerization reactions were performed in a thermostatted glass reactor equipped with a magnetic stirrer under CO atmosphere. After introduction of the catalyst precursor, the benzoquinone, the solvent (20 mL), and the aromatic alkene (10 mL), carbon monoxide was bubbled for 10 min into the reaction mixture, heated at 30 C. The system was then closed and connected to a balloon containing CO. After the proper reaction time, the reaction mixture was poored into methanol (100 mL). The polymer precipitated as a white or a gray solid (if palladium black was formed). The suspension was stirred at room temperature for 1 h; then the polymer was filtered off, washed with methanol, and dried under vacuum until constant weight. Separation of Copolymer from Homopolymer. When the solid isolated at the end of the copolymerization test was a mixture of the polyketone and the corresponding polyolefin, a separation of the two macromolecules was required before the characterization. The procedure for the separation was different depending on the relative amount of the macromolecules determined from the 1H NMR spectra. (a) When the amount of homopolymer was higher than 50%, the solid was dissolved in the minimum amount of CH2Cl2. The solution was added dropwise to diethyl ether (diethyl ether volume was 3 times higher than the volume of CH2Cl2). The polyketone readily precipitated. The suspension was stirred at room temperature overnight; then the solid was filtrated under vacuum, washed with diethyl ether, and vacuum-dried. (b) When the amount of homopolymer was lower than 50%, the procedure was the same as above, but the filtration of the solid was made after 2 h from its precipitation. Molecular Weight Measurement. The molecular weights (Mw) of copolymers and molecular weight distributions (Mw/Mn) were determined by gel permeation chromatography versus polystyrene standards. The analyses were recorded on a Kanuer HPLC (K-501 pump, K-2501 UV detector) with a Plgel 5 μm 104 A˚ GPC column and chloroform as solvent (flow rate 0.6 mL min-1). CO/styrene samples were prepared as follows: 2 mg of the copolymer were solubilized with 120 μL of 1,1,1,3,3,3hexafluoro-2-propanol (HFIP), and chloroform was added up to 10 mL; instead, CO/4-Me-styrene copolymers were directly soluble in chloroform. The statistical calculations were performed with the Bruker Chromstar software program. MALDI/MS Measurement. MALDI mass measurements were performed on an Ultraflex II instrument (Bruker Daltonik, (62) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837–838.
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Bremen, Germany), operating in reflectron positive ion mode. The instrumental conditions were as follows: IS1=25 kV; IS2 = 21.65 kV; reflectron potential=26.3 kV; delay time=0 ns. 2-(pHydroxyphenylazo)benzoic acid (HABA) was used as matrix (satured solution in CHCl3). A 10 mg amount of copolymer was dissolved in 1 mL of HFIP, and 5 μL of this solution was added to the same volume of the matrix solution. About 1 μL of the resulting solution was deposited on the stainless steel sample holder and allowed to dry before introduction into the mass spectrometer. Three independent measurements were done for each sample. External mass calibration was done using the Peptide Calibration Standard, based on the monoisotopic values of [M þ H]þ of angiotensin II, angiotensin I, substance P, bombesin, ACTH clip (1-17), ACTH clip (18-39), somatostatin 28 at m/z 1046.5420, 1296.6853, 1347.7361, 1619.8230, 2093.0868, 2465.1990, and 3147.4714, respectively.
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Acknowledgment. This work was supported by MIUR (PRIN No. 2005035123, No. 2007HMTJWP_002), by the European Network “PALLADIUM” (5th Framework Program, contract No. HPRN-CT-2002-00196). Engelhard Italia is gratefully acknowledged for a generous gift of [Pd(AcO)2]. Fondazione CRTrieste is also gratefully acknowledged for the generous donation to the Dipartimento di Scienze Chimiche of a Varian 500 NMR spectrometer. Supporting Information Available: Full crystallographic data for the structure of S2a are provided as a CIF file; a table with selected NMR data of the complexes. The 13C NMR spectra of two polyketones. The MALDI-TOF spectra of two polyketones. This material is available free of charge via the Internet at http://pubs.acs.org.
