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C−H Bond Activation by σ‑Bond Metathesis as a Versatile Route toward Highly Efficient Initiators for the Catalytic Precision Polymerization of Polar Monomers Benedikt S. Soller, Stephan Salzinger, Christian Jandl, Alexander Pöthig, and Bernhard Rieger* WACKER-Lehrstuhl für Makromolekulare Chemie, Technische Universität München, Lichtenbergstraße 4, 85748 Garching bei München, Germany S Supporting Information *
ABSTRACT: Rare earth metals show high activities toward C− H bond activation of heteroaromatic substrates and even methane. In this work, we demonstrate the suitability of this synthetic approach to rare earth metallocenes and show the applicability of the resulting complexes as highly efficient initiators for rare earth metal-mediated group transfer polymerization. Bis(cyclopentadienyl)(4,6-dimethylpyridin-2-yl)methyl lanthanide complexes exhibit unprecedented initiation rates for rare earth metal-mediated dialkyl vinylphosphonate polymerization and facilitate an efficient initiation for a broad scope of Michael acceptor-type monomers. ince the first reports on living polymerizations of acrylic monomers using early transition metal initiators by Collins, Ward,1 and Yasuda et al.2 in 1992, researchers have devoted their efforts in the optimization of reaction conditions and initiator efficiency and the extension of this method to a variety of (meth)acrylates and (meth)acrylamides.3 With respect to the propagation mechanism, this type of polymerization is recognized as coordinative-anionic or coordination−addition polymerization, and due to its similarity to silyl ketene acetalinitiated group transfer polymerization, it is also referred to as transition metal-mediated GTP.3d,4 Rare earth metal-mediated group transfer polymerization (REM-GTP) is of particular interest, as recent publications have shown that its applicability is not limited to common acrylic monomers, but also facilitates the polymerization of several other monomer classes, i.e., dialkyl vinylphosphonates (DAVP), 2-isopropylene-2-oxazoline (IPOx), and 2-vinylpyridine (2VP).4b,5 Moreover, our group reported on the development of a surface-initiated group transfer polymerization (SI-GTP) mediated by rare earth metal catalysts allowing the perfect decoration of substrates with polymer brushes of specific functionality.6 Recently, the modification of silicon nanoparticles to form thermoresponsive and photoluminescent hybrid materials using SI-GTP was published.7 The applicability of REM-GTP to new monomers enables the precise synthesis of tailor-made functional materials, as this polymerization method combines the advantages of both living ionic and coordinative polymerizations. According to its highly living character, REM-GTP leads to strictly linear polymers with very narrow molecular weight distribution (PDI < 1.1), exhibits a linear increase of the average molar mass upon monomer conversion, and allows the synthesis of block copolymers as well as the introduction of chain end functionalities.3 The coordination of the growing chain end at
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© XXXX American Chemical Society
the catalyst suppresses side reactions and allows stereospecific polymerization as well as activity optimization by variation of both the metal center and the catalyst ligand sphere.3,4b REM-GTP initiation usually proceeds via nucleophilic transfer of a strongly basic ligand, e.g., hydride, methyl, or CH2TMS, to a coordinated monomer (Scheme 1a; this is not the case for divalent rare earth metal centers, for which redox initiation occurs).3d,8 Accordingly, for zirconocene systems, a variety of strategies for the synthesis of enolate initiators, which follow a faster initiation mechanism over an eight-electron process (Scheme 1b), has been presented.3d,9 Surprisingly, only little effort was devoted to the development of new initiating species for rare Scheme 1. Possible Initiation Reactions for REM-GTP of DAVP: Nucleophilic Transfer via a (a) 6e− or (b) 8e− Process and (c) Deprotonation of the Acidic α-CH
Special Issue: Mike Lappert Memorial Issue Received: November 20, 2014
A
DOI: 10.1021/om501173r Organometallics XXXX, XXX, XXX−XXX
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Organometallics earth metal-based catalysts, which would in turn allow the introduction of novel chain end functionalities. In previous work, we have shown that late lanthanide metallocenes are highly active catalysts for DAVP polymerization.5c However, in detailed mechanistic studies we found that the traditionally used strongly basic methyl and CH2TMS initiators lead to an inefficient, slow initiation by deprotonation of the acidic α-CH (Scheme 1c).10 Other initiating groups, such as sterically crowded Cp3Ln complexes and thiolato complexes [Cp2Ln(StBu)]2, were found to efficiently initiate DAVP polymerization; however, further development of Cp3Ln complexes is limited and thiolate end groups were found to be prone to elimination.10 Moreover, these complexes are not suitable initiators for sterically less demanding or weaker coordinating monomers such as IPOx or 2VP.5f,10 Accordingly, the development of new initiators for REMGTP, which facilitate an efficient initiation for a broad scope of
Figure 2. Linear increase of the number-averaged molecular weight during DEVP polymerization using Cp2Y(CH2(C5H2Me2N)) and corresponding polydispersity (7.4 mg catalyst, 10 vol % DEVP in 20 mL of toluene, 30 °C).
(thf) (bdsa = bis(dimethylsilylamide, N(SiMe2H)2). We first evaluated the reaction between Cp2Ln(bdsa)(thf) and acetone resulting in the quantitative formation of Cp 2 Ln(N(SiMe2OiPr)(SiMe2H)) by hydrosilylation of the carbonyl moiety. Surprisingly, the more reactive Cp2Ln(CH2TMS)(thf) precursor leads to no reaction with a large variety of substrates up to elevated reaction temperatures, at which decomposition was observed, revealing a pronounced kinetic limitation for the protonolysis reaction. Only with isobutyrophenone the formation of the corresponding alkoxide from the nucleophilic attack of the alkyl ligand was observed. Despite numerous attempts and the use of different precursor complexes and substrates, the formation of enolate rare earth complexes by protonolysis of classical α-CH-acidic substrates could not be facilitated. This is in agreement with literature results, where full conversions for alkyl ligands were observed in NMR-scale reaction, but only low yields of the corresponding enolate were achieved, due to the formation of side products.11,12 A similar approach to functionalize rare earth metal alkyl complexes is to use the high activity of group 3 elements for σ-bond metathesis.13 The C−H activation of pyridines14 and other heteronuclear15 and also internal alkynes16 was well studied by Teuben et al. Recently, Mashima et al. reported on the introduction of chain end functionality for 2VP polymerization by initial C−H bond activation of nonclassical CH-acidic substrates via alkylyttrium-mediated σbond metathesis.5d,17 In order to evaluate the applicability of this approach to rare earth metallocenes, we reacted Cp2Y(CH2TMS)(thf) with 2,4,6-trimethylpyridine in toluene solution, yielding the desired Cp2Y(CH2(C5H2Me2N)) after stirring at room temperature for 30 min (Scheme 3). The synthesis of Cp 2 Y(CH 2 (C 5 H 2 Me 2 N)) from Cp2YCH2TMS(thf) and sym-collidine is quantitative within 30 min, and no difference in activity or molecular weight for the polymerization of DEVP was observed for the isolated complex or an in situ generated catalyst (see Figure S1). Interestingly, σbond metathesis and synthesis of the smaller lutetium cation was not nearly as active as for the larger yttrium analogon. Cp2Lu(CH2(C5H2Me2N)) had to be stirred overnight for complete conversion and was recrystallized for purification. Therefore, an in situ activation is possible only for Cp2Y(CH2(C5H2Me2N)). The (4,6-dimethylpyridin-2-yl)methyl ligand can coordinate to the metal center in the form of both a carbanion and an
Scheme 2. Attempted Synthesis of Enolate and Enamide Rare Earth Metallocene Initiators via Deprotonation of Classical α-CH-Acidic Substrates
monomers and which lead to a stable end group functionalization via a C−C bond, is still of current interest. Inspired by the use of enolate-type initiators in zirconium-mediated GTP, our group focused on the development of enolate or enamide initiators (Scheme 2) in order to facilitate an initiation over an eight-electron process. Such initiators simulate the active propagating species and bypass the ineffective initiation step starting from the alkyl initiator.
Figure 1. Conversion-reaction time plot for the polymerization of DEVP using Cp2Y(CH2(C5H2Me2N)) (7.4 mg catalyst, 10 vol % DEVP in 20 mL of toluene, 30 °C).
As synthetic routes via salt metathesis from lithium enolates and rare earth metal chlorides and via thermolysis of alkyl complexes in the presence of tetrahydrofuran are restricted to selected systems only,11 we decided to investigate the accessibility of rare earth enolates via α-CH-deprotonation of the respective carbonyls (or oxazoline/phosphonate) by amide and alkyl precursors Cp2Ln(bdsa)(thf) and Cp2Ln(CH2TMS)B
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Organometallics
Table 1. Comparison of Cp2Ln(CH2(C5H2Me2N)) Initiators for the REM-GTP of DEVP and IPOx (Toluene, 30 °C)5f,10 catalyst
monomer
[Mon]0/ [Cat]0
init. perioda
Mnb [kDa]
PDIb
I*tb [%]
I*b [%]
TOFc [h−1]
TOF/I*t [h−1]
Cp2Y(CH2(C5H2Me2N)) [Cp2Y(StBu)]2 Cp2Lu(CH2(C5H2Me2N)) [Cp2Lu(StBu)]2 [Cp2YbMe]2 Cp2Y(CH2(C5H2Me2N)) Cp2Lu(CH2(C5H2Me2N)) [Cp2YbMe]2
DEVP DEVP DEVP DEVP DEVP IPOx IPOx IPOx
600 600 600 600 600 200 200 200
− 5s − 15 s 80 s −d −d −
140 150 480 210 910 20 38 21
1.02 1.18 1.13 1.27 1.52 1.26 1.39 1.04
73 46 16 35 54 −d −d −e
68 65 21 47 11 89 59 95
59 400 44 000 46 000 103 000 4300 −d −d 380
81 000 96 000 300 000 290 000 8000 −d −d −e
Initiation period, reaction time until 3% conversion is reached. bDetermined by GPC-MALS, I *t = Mth/Mn, Mth = [Mon]0/[Cat]0 × MMon × conversion (I *t at the maximum rate, I * at the end of the reaction). cDetermined by 31P (DEVP) or 1H (IPOx) NMR spectroscopic measurement. d Not determined due to incomplete conversion (Yield = 80% (Y), 75% (Lu)). eNot determined. a
Scheme 3. Synthesis of Cp2Ln(CH2(C5H2Me2N)) via C−H Bond Activation by σ-Bond Metathesis
enamide (Figure 4). Accordingly, initiation of DAVP polymerization could occur via all three routes presented in Scheme 1. ESI-MS analysis of produced DEVP oligomers shows a chain end functionalization by (4,6-dimethylpyridin-2-yl)methyl and does not provide any evidence for initial deprotonation (Scheme 1, Figure S5), which is in accordance with the observed high initiation rates. The crystal structure of Cp2Y(CH2(C5H2Me2N)) (Figure 3) shows the formation of
Figure 4. Coordination of the heteroaromatic initiator to the metal as carbanion or enamide and proposed eight-membered-ring transition state for the initiation of DEVP.
In order to verify the suitability of these complexes as initiators for REM-GTP, we carried out polymerization experiments with diethyl vinylphosphonate (DEVP). Hereby, for the first time using a Cp2LnX initiator, polymerization of DEVP could be facilitated without observation of an initiation period (Figure 1, Table 1). Kinetic measurements revealed a linear increase of the number-averaged molecular weight upon conversion, narrow polydispersity throughout the whole reaction, and activities comparable to those observed for the corresponding thiolato complexes (Figure 2, Table 1).10 In contrast to the previously applied thiolato complexes, the initiator efficiency of the yttrium complex remains constant throughout the whole polymerization (Figure 1), indicating the initiator efficiency of 68% to be mainly a result of an initial deactivation by impurities (e.g., water). Whereas thiolates are not stable and eliminate during vinylphosphonate polymerization, (4,6-dimethylpyridin-2-yl)methyl initiators form a stable C−C bond. The stable end group is important to prevent unwanted side reactions from olefinic chain ends and opens up new approaches for polymeric surface modifications. The formed dimer of Cp2Y(CH2(C5H2Me2N)) does not hamper the initiation and facilitates the propagation with a high initiator efficiency. In the case of the lutetium compound a significant drop of I* is observed. This is attributed to a more stable dimer and the general high polymerization activity. In such cases, the propagation rate surpasses the dissolving of the dimer by coordination and fewer complexes initiate. For thiolato compounds dimers prevent the coordination of IPOx or 2VP completely and no polymerization occurs. Moreover, (4,6-dimethylpyridin-2-yl)methyl complexes initiate the polymerization of not only vinyl phosphonates but also IPOx, even though only materials with rather broad polydispersity could be obtained, indicating a slow and nonuniform initiation (Table 1). Nevertheless, the described complexes are the first systems
Figure 3. ORTEP drawing of Cp2Y(CH2(C5H2Me2N)) with 50% ellipsoids. All H atoms have been omitted for clarity. Selected bond distances (Å) and angles (deg): C1C2, 1.456(5); C6C8, 1.504(5); Y1C1, 2.641(4); Y1C1C2, 87.2(2); N1C2C1, 115.1(3).
dimers and indicates a partial double bond character of the C1−C2 bond (1.456(5) Å) compared to the nonactivated C6− C8 methyl group (1.504(5) Å). This is in agreement with crystal structure data of similar ortho-alkylpyridines.18 From experimental data, initiation by nucleophilic transfer is evident; whether it proceeds via a six- or eight-electron process may be revealed only by theoretical calculations. C
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(8) (a) Boffa, L. S.; Novak, B. M. Macromolecules 1994, 27, 6993− 6995. (b) Boffa, L. S.; Novak, B. M. Tetrahedron 1997, 53, 15367− 15396. (c) Soller, B. S.; Zhang, N.; Rieger, B. Macromol. Chem. Phys. 2014, 215, 1946−1962. (9) (a) Mariott, W. R.; Chen, E. Y. X. Macromolecules 2004, 37, 4741−4743. (b) Mariott, W. R.; Chen, E. Y. X. Macromolecules 2005, 38, 6822−6832. (c) Rodriguez-Delgado, A.; Chen, E. Y. X. Macromolecules 2005, 38, 2587−2594. (d) Li, Y.; Ward, D. G.; Reddy, S. S.; Collins, S. Macromolecules 1997, 30, 1875−1883. (e) Nguyen, H.; Jarvis, A. P.; Lesley, M. J. G.; Kelly, W. M.; Reddy, S. S.; Taylor, N. J.; Collins, S. Macromolecules 2000, 33, 1508−1510. (10) Salzinger, S.; Soller, B. S.; Plikhta, A.; Seemann, U. B.; Herdtweck, E.; Rieger, B. J. Am. Chem. Soc. 2013, 135, 13030−13040. (11) Evans, W. J.; Dominguez, R.; Hanusa, T. P. Organometallics 1986, 5, 1291−1296. (12) (a) Nakajima, Y.; Okuda, J. Organometallics 2007, 26, 1270− 1278. (b) Heeres, H. J.; Maters, M.; Teuben, J. H.; Helgesson, G.; Jagner, S. Organometallics 1992, 11, 350−356. (13) (a) Watson, P. L. J. Am. Chem. Soc. 1983, 105, 6491−6493. (b) Watson, P. L. J. Chem. Soc., Chem. Commun. 1983, 276−277. (c) Waterman, R. Organometallics 2013, 32, 7249−7263. (14) (a) Duchateau, R.; van Wee, C. T.; Teuben, J. H. Organometallics 1996, 15, 2291−2302. (b) Duchateau, R.; Brussee, E. A. C.; Meetsma, A.; Teuben, J. H. Organometallics 1997, 16, 5506− 5516. (15) (a) Deelman, B.-J.; Booij, M.; Meetsma, A.; Teuben, J. H.; Kooijman, H.; Spek, A. L. Organometallics 1995, 14, 2306−2317. (b) Ringelberg, S. N.; Meetsma, A.; Troyanov, S. I.; Hessen, B.; Teuben, J. H. Organometallics 2002, 21, 1759−1765. (16) Quiroga Norambuena, V. F.; Heeres, A.; Heeres, H. J.; Meetsma, A.; Teuben, J. H.; Hessen, B. Organometallics 2008, 27, 5672−5683. (17) Thompson, M. E.; Baxter, S. M.; Bulls, A. R.; Burger, B. J.; Nolan, M. C.; Santarsiero, B. D.; Schaefer, W. P.; Bercaw, J. E. J. Am. Chem. Soc. 1987, 109, 203−219. (18) Guan, B.-T.; Wang, B.; Nishiura, M.; Hou, Z. Angew. Chem., Int. Ed. 2013, 52, 4418−4421.
exhibiting high initiator efficiencies for both DAVP and IPOx polymerization. In conclusion, we have shown that σ-bond metathesis gives highly efficient initiators for REM-GTP via C−H bond activation of 2,4,6-trimethylpyridine. The rate of the σ-bond metathesis depends heavily on the size of the used metal, and in the case of yttrium also the in situ preparation of (4,6dimethylpyridin-2-yl)methyl initiators is possible. The obtained catalysts show high activities and no initiation period. Accordingly, the molecular weight of PDEVP increases linearly with monomer conversion and the polydispersity remains remarkably narrow. We attribute the living character to a mechanistic match between initiation and propagation, both following an eight-electron process. Further studies to apply initiators from C−H activation to new catalysts for a stereospecific polymerization of polar monomers are currently under way.
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ASSOCIATED CONTENT
S Supporting Information *
Detailed procedures for complex synthesis, polymerizations, and oligomerization reactions and detailed information for single-crystal X-ray structure determination (CCDC No. 1031228). This material is available free of charge via the Internet at http://pubs.acs.org.
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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 The authors thank Peter T. Altenbuchner and Alexander Kronast for valuable discussions. S.S. is grateful for a generous scholarship from the Fonds der Chemischen Industrie.
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REFERENCES
(1) Collins, S.; Ward, D. G. J. Am. Chem. Soc. 1992, 114, 5460−5462. (2) Yasuda, H.; Yamamoto, H.; Yokota, K.; Miyake, S.; Nakamura, A. J. Am. Chem. Soc. 1992, 114, 4908−4910. (3) (a) Yasuda, H.; Ihara, E. Macromol. Chem. Phys. 1995, 196, 2417−2441. (b) Yasuda, H.; Ihara, E. Adv. Polym. Sci. 1997, 133, 53− 101. (c) Yasuda, H. Prog. Polym. Sci. 2000, 25, 573−626. (d) Chen, E. Y. X. Chem. Rev. 2009, 109, 5157−5214. (4) (a) Webster, O. Adv. Polym. Sci. 2004, 167, 1−34. (b) Salzinger, S.; Rieger, B. Macromol. Rapid Commun. 2012, 33, 1327−1345. (5) (a) Seemann, U. B.; Dengler, J. E.; Rieger, B. Angew. Chem. 2010, 122, 3567−3569. (b) Rabe, G. W.; Komber, H.; Häussler, L.; Kreger, K.; Lattermann, G. Macromolecules 2010, 43, 1178−1181. (c) Salzinger, S.; Seemann, U. B.; Plikhta, A.; Rieger, B. Macromolecules 2011, 44, 5920−5927. (d) Kaneko, H.; Nagae, H.; Tsurugi, H.; Mashima, K. J. Am. Chem. Soc. 2011, 133, 19626−19629. (e) Zhang, N.; Salzinger, S.; Rieger, B. Macromolecules 2012, 45, 9751−9758. (f) Zhang, N.; Salzinger, S.; Soller, B. S.; Rieger, B. J. Am. Chem. Soc. 2013, 135, 8810−8813. (g) Altenbuchner, P. T.; Soller, B. S.; Kissling, S.; Bachmann, T.; Kronast, A.; Vagin, S. I.; Rieger, B. Macromolecules 2014, 47, 7742−7749. (6) (a) Zhang, N.; Salzinger, S.; Deubel, F.; Jordan, R.; Rieger, B. J. Am. Chem. Soc. 2012, 134, 7333−7336. (b) Yang, J.; Liang, Y.; Salzinger, S.; Zhang, N.; Dong, D.; Rieger, B. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 2919−2925. (7) Kehrle, J.; Höhlein, I. M. D.; Yang, Z.; Jochem, A.-R.; Helbich, T.; Kraus, T.; Veinot, J. G. C.; Rieger, B. Angew. Chem. 2014, 126, 12702− 12705. D
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