Bis(tetramethylaluminate) Complexes of Yttrium and Lanthanum

May 11, 2010 - Bis(tetramethylaluminate) Complexes of Yttrium and Lanthanum. Supported by a Quinolyl-Substituted Cyclopentadienyl Ligand: Synthesis an...
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Organometallics 2010, 29, 2588–2595 DOI: 10.1021/om100283b

Bis(tetramethylaluminate) Complexes of Yttrium and Lanthanum Supported by a Quinolyl-Substituted Cyclopentadienyl Ligand: Synthesis and Performance in Isoprene Polymerization Rannveig Litlabø,† Markus Enders,‡ Karl W. T€ ornroos,† and Reiner Anwander*,†,§ †

Department of Chemistry, University of Bergen, All egaten 41, 5007 Bergen, Norway, Anorganisch-Chemisches Institut, Universit€ at Heidelberg, Heidelberg, Germany, and § Institut f€ ur Anorganische Chemie, Universit€ at T€ ubingen, Auf der Morgenstelle 18, 72076 T€ ubingen, Germany

‡

Received April 8, 2010

Quinolyl-substituted half-sandwich complexes (CpQ)Ln(AlMe4)2 (CpQ = 2,3,4,5-tetramethyl-1-(8quinolyl)cyclopentadienyl; Ln = Y, La) were obtained in quantitative yield via protonolysis reactions utilizing HCpQ and homoleptic tetramethylaluminates Ln(AlMe4)3. X-ray structure analyses revealed that the quinolyl-substituted cyclopentadienyl ligand coordinates to the rare-earth metal centers in an η5:η1 fashion through the Cp ring carbon atoms and the N atom of the quinolyl substituent. The complexes (CpQ)Ln(AlMe4)2 show good activity and high trans-1,4-stereoselectivity (maximum 93%) in the polymerization of isoprene upon activation with the organoborates [Ph3C][B(C6F5)4] and [PhNMe2H][B(C6F5)4]. The effects of metal size, cocatalyst, temperature, and solvent were assessed in these polymerization reactions, and the performance of such N-donor-functionalized complexes was compared to that of (CpR)Ln(AlMe4)2 containing different types of nondonor-functionalized cyclopentadienyl ligands.

Introduction In the early 1990s Bercaw et al.1 described scandium alkyl and hydride complexes supported by a linked tert-butylamido tetramethylcyclopentadienyl ligand (Chart 1, A) and their capability to act as living R-olefin polymerization catalysts. Since then, such divalent linked amido-cyclopentadienyl ligands have gained considerable importance in rare-earth metal chemistry.2 Due to easier to access and more electron deficient (rare-earth) metal centers in comparison to their ansametallocene analogues, these types of complexes revealed not only higher activity in polymerization reactions1a,b,3 (so-called *To whom correspondence should be addressed: Fax: þ49(0)7071-292436. E-mail: [email protected]. (1) (a) Piers, W. E.; Shapiro, P. J.; Bunel, E. E.; Bercaw, J. E. Synlett 1990, 74. (b) Shapiro, P. J.; Bunel, E.; Schaefer, W. P.; Bercaw, J. E. Organometallics 1990, 9, 867. (c) Shapiro, P. J.; Schaefer, W. P.; Labinger, J. A.; Bercaw, J. E.; Cotter, W. D. J. Am. Chem. Soc. 1994, 116, 4623. (2) (a) Arndt, S.; Okuda, J. Chem. Rev. 2002, 102, 1953. (b) Okuda, J. Dalton Trans. 2003, 2367. (c) Hou, Z. Bull. Chem. Soc. Jpn. 2003, 76, 2253. (d) Braunschweig, H.; Breitling, F. M. Coord. Chem. Rev. 2006, 250, 2691. (3) (a) Hultzsch, K. C.; Voth, P.; Beckerle, K.; Spaniol, T. P.; Okuda, J. Organometallics 2000, 19, 228. (b) Hou, Z.; Koizumi, T.; Nishiura, M.; Wakatsuki, Y. Organometallics 2001, 20, 3323. (c) Arndt, S.; Beckerle, K.; Hultzsch, K. C.; Sinnema, P.-J.; Voth, P.; Spaniol, T. P.; Okuda, J. J. Mol. Catal. A 2002, 190, 215. (4) (a) Tian, S.; Arredondo, V. M.; Stern, C. L.; Marks, T. J. Organometallics 1999, 18, 2568. (b) Arredondo, V. M.; Tian, S.; McDonald, F. E.; Marks, T. J. J. Am. Chem. Soc. 1999, 121, 3633. (c) Seyam, A. M.; Stubbert, B. D.; Jensen, T. R.; O'Donnell, J. J.; Stern, C. L.; Marks, T. J. Inorg. Chim. Acta 2004, 357, 4029. (d) Hong, S.; Marks, T. J. Acc. Chem. Res. 2004, 37, 673. (5) (a) Trifonov, A. A.; Spaniol, T. P.; Okuda, J. Organometallics 2001, 20, 4869. (b) Trifonov, A. A.; Spaniol, T. P.; Okuda, J. Dalton Trans. 2004, 2245. (c) Robert, D.; Trifonov, A. A.; Voth, P.; Okuda, J. J. Organomet. Chem. 2006, 691, 4393. pubs.acs.org/Organometallics

Published on Web 05/11/2010

“constrained geometry catalysts”) but also distinct selectivity in carbon-heteroatom bond formation reactions: e.g., hydroamination4 and hydrosilylation.5 In contrast to the numerous examples of Ln(III) complexes containing such dianionic linked amido-cyclopentadienyl ligands, there have not been many reports on rare-earth metal(III) half-sandwich complexes with side chains carrying neutral N donors.6-11 This is surprising, since the envisaged bis(alkyl) complexes can display enhanced activity upon cationization. A patent from Christopher et al. reports on the scandium complex [C5Me4(CH2)2NMe2]Sc(CH2C6H5)2, which polymerizes ethylene upon cationization by B(C6F5)3, forming {[C5Me4(CH2)2NMe2]Sc(CH2C6H5)}þ[(C6H5CH2)B(C6F5)3]-.6 In 2003 the groups of Schumann and Hessen independently utilized the monoanionic ligand [C5H4(CH2)2NMe2] for the synthesis of the bis(trimethylsilylmethyl) complexes [C5H4(CH2)2NMe2]Ln(CH2SiMe3)2 (Ln = Sc, Y)7 and a 1,3-butadiene complex of scandium, [C5H4(CH2)2NMe2]Sc(2,3-dimethyl-1,3-butadiene)8 (Chart 1, B), respectively. These studies also included the chiral phenyl-substituted derivatives [(S)-C5H4CHPhCH2NMe2]Ln(CH2SiMe3)2 (Ln = Sc, Lu).7 (6) Christopher, J. N.; Squire, K. R.; Canich, J. A. M.; Shaffer, T. D. (Exxon) World Pat. WO 2000018808, 2000. (7) Schumann, H.; Herrmann, K.; Erbstein, F. Z. Naturforsch., B: Chem. Sci. 2003, 58, 832. (8) Beetstra, D. J.; Meetsma, A.; Hessen, B.; Teuben, J. H. Organometallics 2003, 22, 4372. (9) Li, X. F.; Nishiura, M.; Hu, L. H.; Mori, K.; Hou, Z. J. Am. Chem. Soc. 2009, 131, 13870. (10) Panda, T. K.; Hrib, C. G.; Jones, P. G.; Jenter, J.; Roesky, P. W.; Tamm, M. Eur. J. Inorg. Chem. 2008, 4270. (11) Fedushkin, I. L.; Dechert, S.; Schumann, H. Organometallics 2000, 19, 4066. r 2010 American Chemical Society

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Chart 1. Examples of X-ray Structurally Authenticated Rare-Earth Metal Complexes with N-Functionalized Side Chains and Alkyl Actor Ligands (A-D) and a Chromium(III) Derivative (E)

Recently Hou et al. attempted the synthesis of the complex [C5Me4(o-C6H4NMe2)]Sc(CH2SiMe3)2 supported by an N,Ndimethylanilinyl-functionalized cyclopentadienyl ligand introduced by Enders12 (vide infra); however, they could only isolate the methylene-bridged binuclear complex {[C5Me4(o-C6H4N(Me)CH2-μ]Sc(CH2SiMe3)}2 (75% yield; Chart 1, C).9 Additionally, imidazolin-2-imino-functionalized half-sandwich complexes (Tamm et al.;10 Chart 1, D) and bis(amino)-functionalized derivatives such as {1,2-[(CH2)2NMe2]2C5H3}LnI2(thf) (Schumann et al.)11 have been described. For comparison, there have been a considerable number of examples of corresponding metallocene derivatives carrying amino-functionalized cyclopentadienyl ligands.13 In 2001 Enders et al. reported that N,N-dimethylanilinyland quinolyl-functionalized cyclopentadienyl ligands, with the neutral nitrogen functionality linked to the cyclopentadienyl ring via a rigid C2 spacer (Chart 1, C and E), are favorable for the formation of chelate Cr(III) metal complexes in comparison to similar ligands containing more flexible spacers.14 For example, the reduced flexibility of these ligands prevents intermolecular donor-metal interactions and the formation of insoluble coordination polymers. Recently our group introduced bis(tetramethylaluminate) rare-earth metal complexes of the type (L)Ln(AlMe4)2 (12) Enders, M.; Ludwig, G.; Pritzkow, H. Organometallics 2001, 20, 827. (13) For examples, see: (a) Herrmann, W. A.; Anwander, R.; Munck, F. C.; Scherer, W. Chem. Ber. 1993, 126, 331. (b) Vandenhende, J. R.; Hitchcock, P. B.; Lappert, M. F.; Nile, T. A. J. Organomet. Chem. 1994, 472, 79. (c) Molander, G. A.; Schumann, H.; Rosenthal, E. C. E.; Demtschuk, J. Organometallics 1996, 15, 3817. (d) Schumann, H.; Rosenthal, E. C. E.; Demtschuk, J. Organometallics 1998, 17, 5324. (14) (a) Enders, M.; Fernandez, P.; Ludwig, G.; Pritzkow, H. Organometallics 2001, 20, 5005. (b) Fernandez, P.; Pritzkow, H.; Carbo, J. J.; Hofmann, P.; Enders, M. Organometallics 2007, 26, 4402. (c) Mark, S.; Gaidzik, N.; Doye, S.; Enders, M. Dalton Trans. 2009, 4875. (15) (a) Anwander, R.; Klimpel, M. G.; Dietrich, H. M.; Shorokhov, D. J.; Scherer, W. Chem. Commun. 2003, 1008. (b) Dietrich, H. M.; Zapilko, C.; Herdtweck, E.; Anwander, R. Organometallics 2005, 24, 5767. (c) Dietrich, H. M.; T€ ornroos, K. W.; Herdtweck, E.; Anwander, R. Organometallics 2009, 28, 6739. (d) Fischbach, A.; Perdih, F.; Herdtweck, E.; Anwander, R. Organometallics 2006, 25, 1626. (e) Le Roux, E.; Nief, F.; Jaroschik, F.; T€ ornroos, K. W.; Anwander, R. Dalton Trans. 2007, 4866. (f) Zimmermann, M.; T€ornroos, K. W.; Anwander, R. Angew. Chem., Int. Ed. 2007, 46, 3126. (g) Litlabø, R.; Lee, H. S.; Niemeyer, M.; T€ornroos, K. W.; Anwander, R. Dalton Trans. 2010, DOI: 10.1039/B925837J.

(L = monoanionic ancillary ligand) as alternative bis(hydrocarbyl) derivatives.15,16 With the aim of gaining a fundamental understanding of ancillary ligand effects, and hence to elucidate any structure-reactivity relationship in polymerization catalysis, we created a bis(tetraalkylaluminate)based postmetallocene library.16 As part of these studies a comprehensive account of half-sandwich complexes (CpR)Ln(AlMe4)2 containing various substituted cyclopentadienyl ancillary ligands was given,16-18 where special emphasis was put on the implications of Ln(III) metal cation size, substitution pattern of the cyclopentadienyl ligand, and type of cocatalyst for the polymerization of isoprene. Herein, we extend these studies to the rigid quinolylsubstituted cyclopentadienyl ligand CpQ (Chart 1, E).19 We were especially interested in how the amino donor would cope with the highly Lewis acidic and nitrophilic Al(III) centers, both during the synthesis of the envisaged bis(tetramethylaluminate) complexes (CpQ)Ln(AlMe4)2 and in group 13 (B(III), Al(III)) cocatalyzed isoprene polymerization.

Results and Discussion Synthesis and Characterization of (CpQ)Ln(AlMe4)2. Protonolysis of homoleptic complexes Ln(AlMe4)3 (Ln = Y (1a), La (1b))20 with 1 equiv of quinolyl-substituted cyclopentadiene HCpQ (CpQ = 2,3,4,5-tetramethyl-1-(8-quinolyl)cyclopentadienyl) in hexane at ambient temperature yielded the corresponding bis(tetramethylaluminate) complexes (CpQ)Ln(AlMe4)2 (2: Ln = Y (a), La (b)) in quantitative yields (Scheme 1). Instant gas evolution followed by formation of an orange-brown suspension evidenced the acid-base reaction (16) Zimmermann, M.; T€ ornroos, K. W.; Sitzmann, H.; Anwander, R. Chem. Eur. J. 2008, 14, 7266. (17) Zimmermann, M.; T€ ornroos, K. W.; Anwander, R. Angew. Chem., Int. Ed. 2008, 47, 775. (18) Zimmermann, M.; Volbeda, J.; T€ ornroos, K. W.; Anwander, R. C. R. Chim. 2010, DOI: 10.1016/j.crci.2010.03.019. (19) Enders, M.; Rudolph, R.; Pritzkow, H. Chem. Ber. 1996, 129, 459. (20) (a) Evans, W. J.; Anwander, R.; Ziller, J. W. Organometallics 1995, 14, 1107. (b) Zimmermann, M.; Frøystein, N. Å.; Fischbach, A.; Sirsch, P.; Dietrich, H. M.; T€ornroos, K. W.; Herdtweck, E.; Anwander, R. Chem. Eur. J. 2007, 13, 8784.

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Scheme 1. Synthesis of (CpQ)Ln(AlMe4)2 (2) via Protonolysis

between Ln(AlMe4)3 and the substituted cyclopentadiene. Remarkably, the hard N-donor functionality did not engage in any donor cleavage reactions of the tetramethylaluminate moieties. Such donor-induced Al(alkyl)3 separations display prominent reaction pathways in organo-rare-earth metal chemistry and very often lead to (multiple) hydrogen abstraction at Ln(III)-bonded alkyl ligands.21 The light yellow powdery products 2 were sparingly soluble in aliphatic hydrocarbon solvents but soluble in aromatic solvents. The 1H NMR spectra of complexes 2 show the expected set of signals for the coordinated CpQ ligand. The four methyl groups of the Cp ring appear as two singlets at 1.71 and 1.93 ppm and at 1.80 and 2.08 ppm, in complexes 2a,b, respectively. The signals of the quinolyl protons are found in the range from 6.54 to 8.76 ppm. All signals (except the 2-quinolyl protons in 2a) are shifted 0.2-0.3 ppm upfield compared to those of the HCpQ proligand, which suggests an additional coordination of the N donor to the rare-earth metal center. When the temperature was lowered from þ25 to -80 C, the signals of complex 2a were gradually shifted further upfield by 0.3-0.4 ppm. Only one signal is observed for the aluminum-bonded methyl groups at 25 C (2a, -0.15 ppm; 2b, -0.19 ppm), which implies a rapid exchange between bridging and terminal methyl groups. Variable-temperature 1H NMR studies on complex 2a did not reveal any signal splitting at temperatures from þ25 to -80 C; however, at -80 C significant broadening of the [AlMe4] signal was observed (Δν1/2 ≈ 100 Hz at -80 C). Single crystals of 2a,b suitable for X-ray structure analysis were grown from C6D6/hexane and toluene/hexane mixtures, respectively, at -30 C. Complex 2a crystallizes in the monoclinic space group C2/c along with half a molecule of benzene. Complex 2b is isostructural with 2a, but is metrically triclinic, in space group P1 with one molecule of toluene per two molecules of the product (cf. the Experimental Section). Both complexes obtain the same coordination geometry with two η2 coordinated [AlMe4] moieties and the CpQ ligand coordinated both through a normal η5 connection to the Cp ring and via the nitrogen atom of the quinolyl. A molecular drawing of complex 2a is representatively shown in Figure 1, and selected bond distances and angles for complexes 2a,b are given in Table 1. The most striking feature of the solid-state structures of complexes 2a,b is the presence of two planar η2 coordinated [AlMe4] moieties ( — C1-Ln-C2-Al1 = 0.42 (2a), 3.03 (2b); ( — C5-Ln-C6-Al2 = 1.53 (2a), -0.59 (2b)). This differs from the structural motifs found earlier for the (21) (a) Holton, J.; Lappert, M. F.; Ballard, D. G. H.; Pearce, R.; Atwood, J. L.; Hunter, W. E. J. Chem. Soc., Dalton Trans. 1979, 54. (b) Klimpel, M. G.; Eppinger, J.; Sirsch, P.; Scherer, W.; Anwander, R. Organometallics 2002, 21, 4021. (c) Dietrich, H. M.; Grove, H.; T€ornroos, K. W.; Anwander, R. J. Am. Chem. Soc. 2006, 128, 1458. (d) Fischbach, A.; Anwander, R. Adv. Polym. Sci. 2006, 204, 155.

Figure 1. Molecular structure of 2a (ORTEP drawing). Atoms are represented by atomic displacement ellipsoids set at the 50% level. Hydrogen atoms and solvent molecules are omitted for clarity.

donor-free half-sandwich bis(tetramethylaluminate) complexes (CpR)Ln(AlMe4)2,15a-c,16 where one of the ligands adopts a bent η2 coordination. In complexes 2, the hard quinolyl competes for this “vacant” coordination site, thus accomplishing steric saturation of the Ln(III) metal center and rendering almost unperturbed η 2 -bonded AlMe4 ligands. A similar coordination was found in [(η5PC4Me4)Ln(AlMe4)2]2, where intermolecular La- - -P donor interactions counteract any additional weak La- - -CH3(aluminate) bonding.15e The Ln-C bond distances (LnC(av) = 2.645 A˚ (2a), 2.76 A˚ (2b)) are more comparable to those found in [AlMe4] moieties with bent η2 coordination than to planar distances (Ln-C(av) planar vs bent for complexes (C5Me4SiMe3)Y(AlMe4)2, [1,3-(Me3Si)2C5H3]Y(AlMe4)2, and [1,2,4-(Me3C)3C5H2]La(AlMe4)2: 2.525 A˚ vs 2.675 A˚, 2.560 A˚ vs 2.624 A˚, and 2.705 A˚ vs 2.794 A˚, respectively),16 reflecting the increased coordination number in complexes 2. The mean metal-ring-carbon distances, Ln-C(CpQ)(av) (2.664 A˚ (2a), 2.789 A˚ (2b)), are slightly elongated in comparison to those found in the less sterically crowded (C5Me5)Y(AlMe4)215c and (C5Me5)La(AlMe4)215b (Ln-C(C5Me5)(av) 2.631 and 2.777 A˚, respectively). The Ln-N bond distances (2.567 A˚ (2a), 2.701 A˚ (2b)) are comparable to those found in [C5H4(CH(Ph)CH2NMe2)]Y(C5Me5)Cl13d (Y-N = 2.501 A˚), {1,2-[(CH2)2NMe2]2C5H3}LaI2(thf)11 (La-N = 2.772 and 2.820 A˚), and {1,3-[(CH2)2NMe 2]2C 5 H3 }LaI 2(thf)11 (La-N = 2.712 and 2.728 A˚). The imidazolin-2-imino-functionalized half-metallocenes reported by Tamm et al. did, however, show considerably shorter Ln-N distances (Y-N(av) 2.353 A˚), in accordance with increased electron density at the N atom of the imidazolin2-imino functionality.10 The closest Al- - -N distances in complexes 2a,b are 3.983 and 4.248 A˚, respectively. The interplanar angle between the quinolyl moiety and the Cp ring of about 61 ( — planeCp-planeQ = 60.9 (2a), 60.7 (2b)) is considerably more acute than those found in chromium(III) and rhodium(III) complexes, exhibiting an almost orthogonal orientation,14,22 but similar to the angle found in the cobalt(III) complex (CpQ)CoI2 ( — planeCp-planeQ = 63.0).12 (22) (a) Enders, M.; Fernandez, P.; Mihan, S.; Pritzkow, H. J. Organomet. Chem. 2003, 687, 125. (b) Enders, M.; Kohl, G.; Pritzkow, H. J. Organomet. Chem. 2004, 689, 3024.

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Table 1. Selected Interatomic Distances and Angles for (CpQ)Ln(AlMe4)2 (2) 2b (Ln = La) 2a (Ln = Y)

molecule 1

molecule 2

2.754(10) 2.848(9) 2.699(10) 2.730(9) 2.707(8)-2.870(9) 2.51 2.701(7) 2.043(10) 2.053(11) 1.983(11) 1.985(11) 1.991(11) 2.067(11) 1.980(12) 1.978(11) 3.366(3) 3.277(3)

2.737(10) 2.856(9) 2.715(9) 2.737(9) 2.702(8)-2.865(8) 2.52 2.683(7) 2.056(10) 2.041(11) 1.985(11) 1.988(10) 2.061(9) 2.082(11) 1.977(11) 1.981(11) 3.361(3) 3.278(3)

74.7(3) 76.3(3) 87.8(3) 85.1(3) 87.3(4) 85.0(3) 112.2(4) 111.4(4) 106.59(8) 117.6(5) 123.2(6) 117.9(8)

74.8(3) 78.0(3) 87.8(4) 85.0(3) 85.5(3) 84.6(3) 112.2(4) 111.9(4) 106.01(8) 118.2(5) 122.5(6) 117.9(8)

Bond Distances (A˚) Ln-C1 Ln-C2 Ln-C5 Ln-C6 Ln-C(CpQ) Ln-Cg Ln-N Al1-C1 Al1-C2 Al1-C3 Al1-C4 Al2-C5 Al2-C6 Al2-C7 Al2-C8 Ln 3 3 3 Al1 Ln 3 3 3 Al2

2.599(2) 2.762(2) 2.635(2) 2.583(2) 2.568(2)-2.756(2) 2.38 2.567(2) 2.045(2) 2.041(3) 1.981(2) 1.988(2) 2.086(2) 2.068(3) 1.978(3) 1.980(2) 3.2277(7) 3.1290(7) Bond Angles (deg)

C1-Ln-C2 C5-Ln-C6 Ln-C1-Al1 Ln-C2-Al1 Ln-C5-Al2 Ln-C6-Al2 C1-Al1-C2 C5-Al2-C6 Al1 3 3 3 Ln 3 3 3 Al2 Ln-N-C17 Ln-N-C9 C9-N-C17

78.14(7) 82.39(8) 87.20(9) 82.98(8) 82.13(8) 83.77(8) 111.68(10) 111.66(10) 103.124(19) 117.23(13) 124.87(14) 117.19(18)

Polymerization of Isoprene. We have recently reported that the metal cation size, the substituents on the cyclopentadienyl ligand, and the type of cocatalyst (borate vs borane) crucially affect the performance of half-sandwich complexes (CpR)Ln(AlMe4)2 in isoprene polymerization.16-18 These investigations revealed good to excellent catalytic activities for precatalysts activated by B(C6F5)3 and [Ph3C][B(C6F5)4]/ [PhNMe2H][B(C6F5)4], respectively. Furthermore, increased trans-1,4-selectivity with increasing size of the rare-earth metal cation was observed. The highest stereoselectivities were obtained for the precatalyst/cocatalyst systems (C5Me4SiMe3)La(AlMe4)2/B(C6F5)3 (trans-1,4-content 95.6%, Mw/ Mn = 1.26) and (C5Me5)La(AlMe4)2/B(C6F5)3 (trans-1,4content 99.5%, Mw/Mn = 1.18). In order to extend these investigations and to examine the implications of hemilabile N-donor functionalities, we carried out preliminary tests on 2a,b as precatalysts in isoprene polymerization (Table 2). In addition to different Ln size and cocatalysts, we also tried to assess the influence of the solvent and temperature. Effect of the Boron Cocatalyst. As in our previous studies, the type of boron cocatalyst had a tremendous influence on the catalyst activity. The anilinium borate [PhNMe2H][B(C6F5)4] (B), gave high activity in all polymerizations performed at 40 C, while the trityl borate [Ph3C][B(C6F5)4] (A) showed high activity in most of the corresponding reactions. Using the same preformation time, borane B(C6F5)3 (C) did, however, show extremely low activity and gave only minimal yields even for reactions performed in toluene at 40 C for 24 h (Table 2, runs 4 and 15). We have shown previously that the reaction of (C5Me5)La(AlMe4)2 with B(C6F5)3 (C) occurs instantly via a very fast sequential CH3/C6F5 exchange

process with formation of the ion pair {{(C5Me5)La[(μ-Me)2AlMe(C6F5)]}þ[Me2Al(C6F5)2]-}2 (authenticated by the X-ray structure).17 Moreover, the analogous reaction of dimeric [(C5Me4H)La(AlMe4)2]2 with C was only completed after 3 h.18 This latter observation points toward the initial cleavage of the dimeric structure as the rate-limiting step. The increased formal coordination number of 8 of the lanthanum center in [(C5Me4H)La(AlMe4)2]2 versus 7 in (C5Me5)La(AlMe4)2 might also slow down this CH3/C6F5 exchange process. Therefore, the extremely low activity of eight-coordinate complexes 2 upon activation with borane C might be explained by sterically oversaturated rare-earth metal centers. Due to this low activity and general shock sensitivity of such alkylaluminate/borane mixtures, we did not further investigate into this. The stereoselectivity was less affected by the choice of the cocatalyst, with all systems showing high trans-1,4-selectivity. The borate [Ph3C][B(C6F5)4] (A) displayed slightly higher selectivity than the borate [PhNMe2H][B(C6F5)4] (B) throughout the studies, except for the system CpQLa(AlMe4)2 (2b)/[PhNMe2H][B(C6F5)4] (B)/hexane/40 C, showing a trans-1,4-selectivity of 93.1% (Table 2, run 19). When the polymerization was performed in toluene at 40 C, complex 2b gave appreciably higher trans-1,4-selectivity upon activation with borane C compared to borates A and B (Table 2, run 15). As previously reported for the substituted half-sandwich complexes (CpR)Ln(AlMe4)2,16-18 complex 2a yielded tight ion pairs [CpQY(AlMe4)][B(C6F5)4] (3a) upon reaction with 1 equiv of [Ph3C][B(C6F5)4] (A) or [PhNMe2H][B(C6F5)4] (B) (Scheme 2). 1H NMR spectroscopic studies of the two reactions showed instant disappearance of the signals of complex 2a and formation of the corresponding cationic fragment.

Cp Y(AlMe4)2 (2a) CpQY(AlMe4)2 (2a) CpQY(AlMe4)2 (2a) CpQY(AlMe4)2 (2a) CpQY(AlMe4)2 (2a) CpQY(AlMe4)2 (2a) CpQY(AlMe4)2 (2a) CpQY(AlMe4)2 (2a) CpQY(AlMe4)2 (2a) CpQY(AlMe4)2 (2a) CpQY(AlMe4)2 (2a) CpQY(AlMe4)2 (2a) CpQLa(AlMe4)2 (2b) CpQLa(AlMe4)2 (2b) CpQLa(AlMe4)2 (2b) CpQLa(AlMe4)2 (2b) CpQLa(AlMe4)2 (2b) CpQLa(AlMe4)2 (2b) CpQLa(AlMe4)2 (2b) CpQLa(AlMe4)2 (2b) CpQLa(AlMe4)2 (2b) CpQLa(AlMe4)2 (2b) CpQLa(AlMe4)2 (2b)

Q

precatalyst A A B C A B A B A B A B A B C A B A B A B A B

cocatalystb toluene toluene toluene toluene toluene toluene hexane hexane hexane hexane hexane hexane toluene toluene toluene toluene toluene hexane hexane hexane hexane hexane hexane

solvent 24 2 2 24 8 8 24 24 2 2 8 8 2 2 24 8 8 24 24 2 2 8 8

time (h) 40 40 40 40 -40 -40 40 40 40 40 -40 -40 40 40 40 -40 -40 40 40 40 40 -40 -40

temp (C) 85.9 85.9 74.7 81.5 38.9 89.4 88.4 90.1 88.4 85.0 88.7 70.9 89.0 44.7 90.4 93.1 89.0

15 >99 >99 19 >99 25 1 >99 3 15 5 >99 >99

trans-1,4

>99 >99 >99 1

yield (%)

5.2

42.7 4.8 2.1

1.3 4.6 19.6 5.8

0.6

14.9 1.3

1.3 1.3 3.8 6.0

cis-1,4

5.8

12.6 4.8 4.9

13.8 6.6 9.5 5.2

46.2 9.3 11.6 9.9 11.0

12.8 12.8 21.4 12.5

3,4

1.11

9.3

2.3 3.3 15.9

3.9 0.8

1.21 1.58 1.13 1.39 1.28

3.6

2.7 15.5 10.6 6.1 9.2

7.4 7.4 3.5 0.3

Mnd(x 105)

1.06

1.03 1.37 1.13 1.91 1.07

1.25 1.14 1.12 1.32

Mw/Mnd

7

4