Tantalum Complexes Incorporating Tris(pyrazolyl)Borate Ligands

Oct 20, 2009 - Four new Ta complexes bearing tris(pyrazolyl)borate ligands, Tp*TaCl(η2-3-hexyne)(benzyl) [1;. Tp* = HB(3,5-dimethylpyrazolyl)3...
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6450

Organometallics 2009, 28, 6450–6457 DOI: 10.1021/om9004719

Tantalum Complexes Incorporating Tris(pyrazolyl)Borate Ligands: Syntheses, Structures, and Ethylene Polymerization Behavior Kenji Michiue,† Toshiyuki Oshiki,‡ Kazuhiko Takai,‡ Makoto Mitani,† and Terunori Fujita*,† †

Research Center, Mitsui Chemicals, Inc., 580-32 Nagaura, Sodegaura, Chiba 299-0265, Japan, and Division of Chemistry and Biochemistry, Graduate School of Natural Science and Technology, Okayama University, Tsushima, Kita-ku, Okayama 700-8530, Japan

‡

Received June 4, 2009

Four new Ta complexes bearing tris(pyrazolyl)borate ligands, Tp*TaCl(η2-3-hexyne)(benzyl) [1; Tp* = HB(3,5-dimethylpyrazolyl)3-], TpMs*TaCl2(η2-3-hexyne) [2; TpMs* = HB(3-mesitylpyrazolyl)2(5-mesitylpyrazolyl)-], TpMs*TaCl2(dN-2,6-iPr2C6H3) (3), and TpMsTaCl2(dN-tBu) [4; TpMs= HB(3-mesitylpyrazolyl)3-], were prepared in moderate to high yields from appropriate TaCl3(R) (R; η2-3-hexyne, dN-2,6-iPr2C6H3, dN-tBu) species and the potassium or thallium salt of the corresponding tris(pyrazolyl)borate ligand. The molecular structures of complexes 3 and 4 established by X-ray analyses showed that these complexes adopted a distorted octahedral geometry in which the Ta metal is coordinated by three N atoms of the tris(pyrazolyl)borate ligand (facial coordination), two cis-located Cl atoms, and the imido group. In combination with dried methylaluminoxane or iBu3Al/Ph3CB(C6F5)4, complexes 1-4 were all active for ethylene polymerization and produced low to very high molecular weight, highly linear polyethylenes. In particular, upon activation with iBu3Al/Ph3CB(C6F5)4, high activities were observed for complexes 3 and 4, which possess the sterically encumbered mesityl-substituted tris(pyrazolyl)borate ligand and the imido group as an auxiliary ligand. The activities, 25.7 kg-PE/mmol-Ta 3 h (complex 3) and 17.1 kg-PE/ mmol-Ta 3 h (complex 4), represent some of the highest values reported to date for Ta-based catalysts. These highly active Ta-based catalysts were long-lived and maintained a practically steady ethylene uptake for 30 min at 80 °C.

Introduction The huge success of highly active molecular group 4 metallocene catalysts and associated catalysts such as the half-sandwich amide catalysts for the (co)polymerization of olefinic monomers has demonstrated that the development of high-activity molecular catalysts is the key to creating differentiated polymers with new or improved performance qualities.1 Therefore, significant effort has been made toward the development of new, highly active, molecular olefin polymerization catalysts by both academic and industrial *Corresponding author. E-mail: [email protected]. jp. (1) (a) Kaminsky, W. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 3911. (b) McKnight, A. L.; Waymouth, R. M. Chem. Rev. 1998, 98, 2587. (c) Bochmann, M. J. Chem. Soc., Dalton Trans. 1996, 255. (d) Brintzinger, H. H.; Fischer, D.; M€ ulhaupt, R.; Rieger, B.; Waymouth, R. Angew. Chem., Int. Ed. Engl. 1995, 34, 1143. (2) (a) Chan, M. C. W. Chem.-Asian J. 2008, 3, 18. (b) Gibson, V. C.; Redshaw, C.; Solan, G. A. Chem. Rev. 2007, 107, 1745. (c) Nomura, K.; Liu, J.; Padmanabhan, S.; Kitiyanan, B. J. Mol. Catal. A: Chem. 2007, 267, 1. (d) Zeimentz, P. M.; Arndt, S.; Elvidge, B. R.; Okuda, J. Chem. Rev. 2006, 106, 2404. (e) Bolton, P. D.; Mountford, P. Adv. Synth. Catal. 2005, 347, 355. (f) Stephan, D. W. Organometallics 2005, 24, 2548. (g) Gibson, V. C.; Spitzmesser, S. K. Chem. Rev. 2003, 103, 283. (h) Suzuki, Y.; Terao, H.; Fujita, T. Bull. Chem. Soc. Jpn. 2003, 76, 1493. (i) Ittel, S. D.; Johnson, L. K.; Brookhart, M. Chem. Rev. 2000, 100, 1169. (j) Britovsek, G. J. P.; Gibson, V. C.; Wass, D. F. Angew. Chem., Int. Ed. 1999, 38, 428. pubs.acs.org/Organometallics

Published on Web 10/20/2009

research groups. As a result, a large number of high-activity molecular catalysts based on a wide array of early and late transition metals have been found.2 Some of these catalysts show (3) (a) Gambarotta, S. Coord. Chem. Rev. 2003, 237, 229. (b) Hagen, H.; Boersma, J.; van Koten, G. Chem. Soc. Rev. 2002, 31, 357. (4) (a) Homden, D. M.; Redshaw, C.; Hughes, D. L. Inorg. Chem. 2007, 46, 10827. (b) Redshaw, C.; Rowan, M. A.; Homden, D. M.; Dale, S. H.; Elsegood, M. R. J.; Matsui, S.; Matsuura, S. Chem. Commun. 2006, 3329. (c) Mountford, P.; Bigmore, H. R.; Zuideveld, M. A.; Kowalczyk, R. M.; Cowley, A. R.; Kranenburg, M.; McInnes, E. J. L. Inorg. Chem. 2006, 45, 6411. (d) Redshaw, C.; Homden, D. M.; Rowan, M. A.; Elsegood, M. R. J. Inorg. Chim. Acta 2005, 358, 4067. (e) Yamada, J.; Nomura, K. Organometallics 2005, 24, 3621. (f) Nomura, K.; Wang, W. Macromolecules 2005, 38, 5905. (g) Lorber, C.; Wolff, F.; Choukroun, R.; Vendier, L. Eur. J. Inorg. Chem. 2005, 2850. (h) Redshaw, C.; Warford, L.; Dale, S. H.; Elsegood, M. R. J. Chem. Commun. 2004, 1954. (i) Gibson, V. C.; Tomov, A. K.; Zaher, D.; Elsegood, M. R. J.; Dale, S. H. Chem. Commun. 2004, 1956. (j) Schmidt, R.; Welch, M. B.; Knudsen, R. D.; Gottfried, S.; Alt, H. G. J. Mol. Catal. A: Chem. 2004, 222, 17. (k) Nakayama, Y.; Bando, H.; Sonobe, Y.; Suzuki, Y.; Fujita, T. Chem. Lett. 2003, 32, 766. (l) Reardon, D.; Guan, J.; Gambarotta, S.; Yap, G. P. A.; Wilson, D. R. Organometallics 2002, 21, 4390. (m) Gibson, V. C.; Redshaw, C.; Elsegood, M. R. J. J. Chem. Soc., Dalton Trans. 2001, 767. (n) Lorber, C.; Donnadieu, B.; Choukroun, R. Organometallics 2000, 19, 1963. (o) Lorber, C.; Donnadieu, B.; Choukroun, R. J. Chem. Soc., Dalton Trans. 2000, 4497. (p) Coles, M. P.; Dalby, C. I.; Gibson, V. C.; Little, I. R.; Marshall, E. L.; da Costa, M. H. R.; Mastroianni, S. J. Organomet. Chem. 1999, 591, 78. (q) Chan, M. C. W.; Chew, K. C.; Dalby, C. I.; Gibson, V. C.; Kohlmann, A.; Little, I. R.; Reed, W. Chem. Commun. 1998, 1673. (r) Chan, M. C. W.; Cole, J. M.; Gibson, V. C.; Howard, J. A. K. Chem. Commun. 1997, 2345. r 2009 American Chemical Society

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Scheme 1

high catalytic activities comparable to those seen for group 4 metallocene catalysts and form distinctive polymers that were previously unobtainable through other means of polymerization. For group 5 metal-based catalysts, a variety of highactivity V-3,4 and Nb5-based catalysts incorporating various ligand sets have been developed, some of which display high catalytic activities even at elevated temperatures. Regarding Ta-based catalysts (the congener metal-based catalysts), Hakala and co-workers discovered amide-pyridine-ligated Ta catalysts that exhibit high ethylene polymerization activities that are similar to those for group 4 metallocene catalysts.6 This discovery has led to an upsurge of interest in the development of Ta-based olefin polymerization catalysts. There are, however, still only limited examples of highly active Ta-based catalysts to date.5a,f,7,8 Previously, we described a strategy to develop highly active olefin polymerization catalysts on the basis of electronically (5) (a) Redshaw, C.; Rowan, M.; Homden, D. M.; Elsegood, M. R. J.; Yamato, T.; Perez-Casas, C. Chem.;Eur. J. 2007, 13, 10129. (b) Pritchard, H. M.; Etienne, M.; Vendier, L.; McGrady, G. S. Organometallics 2004, 23, 1203. (c) Chen, C. T.; Doerrer, L. H.; Williams, V. C.; Green, M. L. H. J. Chem. Soc., Dalton Trans. 2000, 967. (d) Spannenberg, A.; Fuhrmann, H.; Arndt, P.; Baumann, W.; Kempe, R. Angew. Chem., Int. Ed. 1998, 37, 3363. (e) Mashima, K.; Nakayama, Y.; Ikushima, N.; Kaidzu, M.; Nakamura, A. J. Organomet. Chem. 1998, 566, 111. (f) Mashima, K.; Fujikawa, S.; Tanaka, Y.; Urata, H.; Oshiki, T.; Tanaka, E.; Nakamura, A. Organometallics 1995, 14, 2633. (6) (a) Hakala, K.; L€ ofgren, B.; Polamo, M.; Leskel€a, M. Macromol. Rapid Commun. 1997, 18, 635. (b) Polamo, M.; Leskel€a, M.; Hakala, K.; L€ ofgren, B. WO 97/45434. (7) (a) Feng, S.; Roof, G.; Chen, E. Y.-X. Organometallics 2002, 21, 832. (b) Murtuza, S.; Harkins, S. B.; Long, G. S.; Sen, A. J. Am. Chem. Soc. 2000, 122, 1867. (8) (a) S anchez-Nieves, J.; Royo, P; Mosquera, M. E. G. Organometallics 2006, 25, 2331. (b) Mashima, K. Macromol. Symp. 2000, 159, 69. (c) Coles, M. P.; Dalby, C. I.; Gibson, V. C.; Little, I. R.; Marshall, E. L.; da Costa, M. H. R.; Mastroianni, S. J. Organomet. Chem. 1999, 591, 78. (d) Decker, J. M.; Geib, S. J.; Meyer, T. Y. Organometallics 1999, 18, 4417. (e) Antonelli, D. M.; Leins, A.; Stryker, J. M. Organometallics 1997, 16, 2500. (f) Rodriquez, G.; Bazan, G. J. Am. Chem. Soc. 1995, 117, 10155. (g) Mashima, K.; Fujikawa, S.; Nakamura, A. J. Am. Chem. Soc. 1993, 115, 10990. (9) (a) Matsugi, T.; Fujita, T. Chem. Soc. Rev. 2008, 37, 1264. (b) Yoshida, Y.; Matsui, S.; Fujita, T. J. Organomet. Chem. 2005, 690, 4382. (c) Makio, H.; Kashiwa, N.; Fujita, T. Adv. Synth. Catal. 2002, 344, 477. (10) (a) Sakuma, A.; Weiser, M.-S.; Fujita, T. Polym. J. 2007, 39, 193. (b) Nakayama, Y.; Saito, J.; Bando, H.; Fujita, T. Chem.;Eur. J. 2006, 12, 7546. (c) Makio, H.; Fujita, T. Bull. Chem. Soc. Jpn. 2005, 78, 52. (d) Mitani, M.; Saito, J.; Ishii, S.; Nakayama, Y.; Makio, H.; Matsukawa, N.; Matsui, S.; Mohri, J.; Furuyama, R.; Terao, H.; Bando, H.; Tanaka, H.; Fujita, T. Chem. Rec. 2004, 4, 137. (e) Makio, H.; Fujita, T. Acc. Chem. Res. 2009, 42, 1532. (11) (a) Furuyama, R.; Saito, J.; Ishii, S.; Mitani, M.; Matsui, S.; Tohi, Y.; Makio, H.; Matsukawa, N.; Tanaka, H.; Fujita, T. J. Mol. Catal. A 2003, 200, 31. (b) Mitani, M.; Mohri, J.; Yoshida, Y.; Saito, J.; Ishii, S.; Tsuru, K.; Matsui, S.; Furuyama, R.; Nakano, T.; Tanaka, H.; Kojoh, S.; Matsugi, T.; Kashiwa, N.; Fujita, T. J. Am. Chem. Soc. 2002, 124, 3327. (c) Mitani, M.; Furuyama, R.; Mohri, J.; Saito, J.; Ishii, S.; Terao, H.; Nakano, T.; Tanaka, H.; Fujita, T. J. Am. Chem. Soc. 2003, 125, 4293. (d) Matsui, S.; Mitani, M.; Saito, J.; Tohi, Y.; Makio, H.; Matsukawa, N.; Takagi, Y.; Tsuru, K.; Nitabaru, M.; Nakano, T.; Tanaka, H.; Kashiwa, N.; Fujita, T. J. Am. Chem. Soc. 2001, 123, 6847.

flexible ligands combined with transition metals (the ligandoriented catalyst design concept).9 This approach has resulted in the discovery of a number of high-activity catalysts, represented by phenoxy-imine- including phenoxy-ketimine-ligated early transition metal catalysts (FI catalysts),10-12pyrrolide-imine-ligated group 4 metal catalysts (PI catalysts),13 and indolide-imine-ligated Ti catalysts (II catalysts).14 Until recently, group 4 metallocene catalysts were the most active catalysts for ethylene polymerization, but now many of the FI catalysts exhibit substantially higher activities than group 4 metallocene catalysts.11d Notably, Jordan and co-workers have developed group 4 metal-based catalysts supported by tris(pyrazolyl)borate ligands (Tp0 =generic substituted tris(pyrazolyl)borate), which show high ethylene polymerization activities that are similar to those activities for group 4 metallocene catalysts.15,16 We are of the belief that a Tp0 ligand probably possesses electronically flexible properties due to its highly conjugated nature, and thus we are interested in the catalytic properties of Tp0 -ligated transition metal catalysts, other than group 4 metals. As for group 5 metal-based complexes bearing Tp0 ligands for olefin polymerization, although some research has been made with V17,18 and Nb19 complexes, there is no report on Ta complexes for olefin polymerization. We thus investigated Ta complexes containing (12) For recent research on FI catalysts and related complexes, see: (a) Edson, J. B.; Wang, Z.; Kramer, E. J.; Coates, G. W. J. Am. Chem. Soc. 2008, 130, 4968. (b) Terao, H.; Ishii, S.; Mitani, M.; Tanaka, H.; Fujita, T. J. Am. Chem. Soc. 2008, 130, 17636. (c) Theaker, G. W.; Morton, C.; Scott, P. Dalton Trans. 2008, 6883. (d) Yu, S.; Mecking, S. J. Am. Chem. Soc. 2008, 130, 13204. (e) Michiue, K.; Onda, M.; Tanaka, H.; Makio, H.; Mitani, M.; Fujita, T. Macromolecules 2008, 41, 6289. (f) Salata, M. R.; Marks, T. J. J. Am. Chem. Soc. 2008, 130, 12. (g) Parssinen, A.; Luhtanen, T.; Klinga, M.; Pakkanen, T.; Leskela, M.; Repo, T. Organometallics 2007, 26, 3690. (h) Arriola, D. J.; Carnahan, E. M.; Hustad, P. D.; Kuhlman, R. L.; Wenzel, T. T. Science 2006, 312, 714. (i) Mazzeo, M.; Strianese, M.; Lamberti, M.; Santoriello, I.; Pellecchia, C. Macromolecules 2006, 39, 7812. (13) (a) Yoshida, Y.; Mohri, J.; Ishii, S.; Mitani, M.; Saito, J.; Matsui, S.; Makio, H.; Nakano, T.; Tanaka, H.; Onda, M.; Yamamoto, Y.; Mizuno, A.; Fujita, T. J. Am. Chem. Soc. 2004, 126, 12023. (b) Yoshida, Y.; Matsui, S.; Takagi, Y.; Mitani, M.; Nakano, T.; Tanaka, H.; Kashiwa, N.; Fujita, T. Organometallics 2001, 20, 4793. (c) Matsuo, Y.; Mashima, K.; Tani, K. Chem. Lett. 2000, 1114. (14) (a) Matsugi, T.; Matsui, S.; Kojoh, S.; Takagi, Y.; Inoue, Y.; Nakano, T.; Fujita, T.; Kashiwa, N. Macromolecules 2002, 35, 4880. (b) Matsugi, T.; Matsui, S.; Kojoh, S.; Takagi, Y.; Inoue, Y.; Fujita, T.; Kashiwa, N. Chem. Lett. 2001, 566. (15) (a) Lee, H.; Nienkemper, K.; Jordan, R. F. Organometallics 2008, 27, 5075. (b) Michiue, K.; Jordan, R. F. J. Mol. Catal. A 2008, 283, 107. (c) Michiue, K.; Jordan, R. F. Organometallics 2004, 23, 460. (d) Michiue, K.; Jordan, R. F. Macromolecules 2003, 36, 9707. (e) Murtuza, S.; Casagrande, O. L., Jr.; Jordan, R. F. Organometallics 2002, 21, 1882. (16) (a) Gil, M. P.; dos Santos, J. H. Z.; Casagrande, O. L., Jr. Macromol. Rapid Commun. 2001, 202, 319. (b) Furlan, L. G.; Gil, M. P.; Casagrande, O. L., Jr. Macromol. Rapid Commun. 2000, 21, 1054. (17) Scheuer, S.; Fischer, J.; Kress, J. Organometallics 1995, 14, 2627. (18) (a) Casagrande, A. C. A.; Gil, M. P.; Casagrande, O. L., Jr. J. Braz. Chem. Soc. 2005, 16, 1283. (b) Braganc-a, A. L. D.; Zacca, J. J.; Casagrande, O. L., Jr.; Casagrande, A. C. A.; Gil, M. P.; Jordan, R. F. BR Patent PI9904045, 1999. (19) (a) Pritchard, H. M.; Etienne, M.; Vendier, L.; McGrady, G. S. Organometallics 2004, 23, 1203. (b) Jaffart, J.; Nayral, C.; Choukroun, R.; Mathieu, R.; Etienne, M. Eur. J. Inorg. Chem. 1998, 425.

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Organometallics, Vol. 28, No. 22, 2009 Scheme 2 a

Scheme 3 a

Tp0 ligands, leading to the discovery of new highly active Tabased catalysts. Therefore, we herein describe the syntheses, structures, and ethylene polymerization behavior of Ta complexes incorporating Tp0 chelating ligands.

Results and Discussion Synthesis of Tris(pyrazolyl)borate Ta Complexes. Four Ta complexes (1-4) were prepared, using transmetalation, as precatalysts that contain substituted tris(pyrazolyl)borate (Tp0 ) ligands and an imido or hexyne auxiliary ligand (Schemes 1-4). We postulated that the introduction of the imido or hexyne auxiliary ligand presumably stabilizes the high oxidation state of the Ta metal, which may result in the formation of a thermally robust, long-lived olefin polymerization catalyst. Additionally, we believe that the attachment of the sterically hindered imido or hexyne group to the Ta metal may induce effective ion separation between the cationic active species and an anionic cocatalyst after activation, thus leading to high catalytic activity.11d,13b,20 The treatment of TaCl3(η2-3-hexyne)(dimethoxyethane) with KTp* [Tp* = HB(3,5-dimethylpyrazolyl)3-] gave the corresponding Tp*-ligated Ta complex, which reacted with 2 equiv of PhCH2MgCl2 to afford Tp*TaCl(η2-3-hexyne)(benzyl) (complex 1) in 93% yield (Scheme 1). Conversely, the reaction of TaCl3(η2-3-hexyne)(dimethoxyethane) with TlTpMs [TpMs =HB(3-mesitylpyrazolyl)3-] yielded a mixture of TpMsTaCl2(η2-3-hexyne) and TpMs*TaCl2(η2-3-hexyne) [TpMs*=HB(3-mesitylpyrazolyl)2(5-mesitylpyrazolyl)-] (complex 2), as was revealed by the 1H NMR analysis of the reaction mixture. Isolation of TpMsTaCl2(η2-3-hexyne) and/or complex 2 was unsuccessful due to their similar solubility in organic solvents. Complex 2 was successfully obtained in (20) Zuccaccia, C.; Stahl, N. G.; Macchioni, A.; Chen, M.-C.; Roberts, J. A.; Marks, T. J. J. Am. Chem. Soc. 2004, 126, 1448, and references therein.

Michiue et al. Scheme 4 a

60% yield by the reaction of TaCl3(η2-3-hexyne)(dimethoxyethane) with TlTpMs* (Scheme 2). Likewise, the treatment of TaCl3(dN-2,6-iPr2-C6H3)(dimethoxyethane) with TlTpMs* gave TpMs*TaCl2(dN-2,6-iPr2C6H3) (complex 3) in 27% yield (Scheme 3). Alternatively, the reaction of TaCl3(dN-tBu)(dimethoxyethane) with TlTpMs provided a mixture of TpMsTaCl2(dN-tBu) (complex 4) and TpMs*TaCl2(dN-tBu), this being indicated by the 1H NMR analysis of the reaction mixture. Complex 4 was obtained by crystallization in 32% yield (Scheme 4). Isolation of TpMs*TaCl2(dN-tBu) was not successful because of its high solubility in organic solvents. The attempted synthesis of TpMs*TaCl2(dN-tBu) by the reaction of TaCl3(dN-tBu)(dimethoxyethane) with TlTpMs* was unsuccessful. The formation of complex 2 by the reaction of TaCl3(η2-3hexyne)(dimethoxyethane) with TlTpMs and the formation of TpMs*TaCl2(dN-tBu) by the treatment of TaCl3(dN-tBu)(dimethoxyethane) with TlTpMs show that the B-N bonds in these Ta species are labile.21 This fact might suggest that these Ta complexes potentially generate multiple active species under polymerization conditions. Molecular Structures of Complexes 3 and 4. The molecular structures of Ta complexes 3 and 4 were established by X-ray diffraction. ORTEP views and space-filling diagrams are illustrated in Figure 1, and selected bond distances and angles are summarized in Table 1. The core structures of complexes 3 and 4 are similar, and the Ta metal is coordinated by three N atoms (with a facial coordination) of the tris(pyrazolyl)borate ligands, two cislocated Cl atoms, and the imido auxiliary ligand in a distorted octahedral geometry. The structure of complex 3 is less symmetrical than that of complex 4 due to the presence of the 5-mesitylpyrazolyl group. For both complexes, the mesityl groups and the imide group form a deep pocket that shields the Ta metal and the Cl atoms (potential polymerization sites), as illustrated in Figure 1. The Ta-N distances for the pyrazolyl group trans to the imido auxiliary ligand (3: 2.43 A˚; 4: 2.45 A˚) are longer than the Ta-N distances for the pyrazolyl groups trans to the Cl atoms (3: average 2.23 A˚; 4: average 2.24 A˚), probably because of the difference in the trans influence of the imido auxiliary ligand and the Cl atom. The elongation of the Ta-N distance may be attributed to a more electron-donating nature of the imido group than that of the Cl atom. The extent of the trans influence of the imido auxiliary ligand is greater than that of the alkyne auxiliary ligands, Tp*TaCl2(η2-phenylpropyne)22 and (21) For the isomerization of a Tp0 ligand, see: Rheingold, A. L.; White, C. B.; Trofimenko, S. Inorg. Chem. 1993, 32, 3471, and ref 15e. (22) Etienne, M.; Hierso, J.-C.; Daff, P. J.; Donnadieu, B.; Dahan, F. Polyhedron 2004, 23, 379. (23) Hierso, J.-C.; Etienne, M. Eur. J. Inorg. Chem. 2000, 2000, 839.

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Figure 1. Molecular structures and space-filling diagrams of 3 (left column) and 4 (right column). Thermal ellipsoids are shown at 50% probability level. Hydrogen atoms and the solvent molecules are omitted from the ORTEP views.

Tp*TaEtCl(η2-phenylpropyne).23 The average Ta-N distances for the pyrazolyl groups (3: 2.29 A˚; 4: 2.31 A˚) are somewhat longer than those of the reported Tp*TaCl2(η2-phenylpropyne) (average 2.25 A˚),22 Tp*TaEtCl(η2-phenylpropyne) (average 2.28 A˚),23 TpTaMe3Cl (average 2.27 A˚),24 Tp*TaCl(η2CH2dNPh)(CH2tBu) (average 2.27 A˚),25 and [Tp*TaCl3][TaCl6] (average 2.16 A˚).26 As suggested by the space-filling diagrams in Figure 1, the bulky N-2,6-iPr2C6H3 auxiliary ligand in complex 3 is located syn to the 5-mesitylpyrazolyl group, which minimizes the steric interaction between the N-2,6-iPr2C6H3 auxiliary ligand (24) Reger, D. L.; Swift, C. A.; Lebioda, L. Inorg. Chem. 1984, 23, 349. (25) Boncella, J. M.; Cajigal, M. L.; Abboud, K. A. Organometallics 1996, 15, 1905. (26) Mashima, K.; Oshiki, T.; Tani, K. Organometallics 1997, 16, 2760.

and the TpMs* ligand. Because of the steric pressure of the N-2,6-iPr2C6H3 auxiliary ligand, the N(2)-Ta(1)-N(3) angle in complex 3 (85.43°) is larger than the N(2)-Ta(1)-N(7) angle (77.48°) and the N(3)-Ta(1)-N(7) angle (77.32°). The average N-Ta-N angles formed by the tridentate Tp0 ligand (3: 80.08°; 4: 81.76°) are similar for the two cases and are within the range of those for the reported Tp0 Ta complexes (78-83°).22-26 Although the C(37)-N(10)-Ta(1) angle in 3 (178.0°) is almost linear, the C(37)-N(7)-Ta(1) angle in complex 4 (159.7°) considerably deviates from 180°. A similar deviation was observed for Ta(dNR)Cl(diamidopyridine)(pyridine) complexes (R= tBu, 2,6-iPr2C6H3).27 The reasons for these deviations are unclear at the present time. The TadN distances for the imido auxiliary ligands (27) Pugh, S. M.; Blake, A. J.; Gade, L. H.; Mountford, P. Inorg. Chem. 2001, 40, 3992.

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Table 1. Selected Bond Lengths (A˚) and Angles (deg) for Tp TaCl2(dN-2,6-iPr2-C6H3) (3) and TpMsTaCl2(dN-tBu) (4)

Table 2. Ethylene Polymerization Results for Complexes 1-4a

Ms*

Ms*

Tp TaCl2 (dN-2,6-iPr2-C6H3) (3)

Ms

Tp TaCl2 (dN-tBu) (4)

Ta(1)-N(2) Ta(1)-N(3) Ta(1)-N(7) Ta(1)-N(10) Ta(1)-Cl(1) Ta(1)-Cl(2)

2.229(4) 2.224(3) 2.428(4) 1.786(3) 2.3473(9) 2.3588(12)

Ta(1)-N(6) Ta(1)-N(2) Ta(1)-N(4) Ta(1)-N(7) Ta(1)-Cl(1) Ta(1)-Cl(2)

2.234(3) 2.249(3) 2.451(3) 1.770(3) 2.3664(12) 2.3622(11)

N(2)-Ta(1)-N(3) N(2)-Ta(1)-N(7) N(3)-Ta(1)-N(7) Cl(1)-Ta(1)-Cl(2) C(37)-N(10)-Ta(1)

85.43(13) 77.48(13) 77.32(11) 93.88(4) 178.0(3)

N(2)-Ta(1)-N(6) N(6)-Ta(1)-N(4) N(2)-Ta(1)-N(4) Cl(1)-Ta(1)-Cl(2) C(37)-N(7)-Ta(1)

80.71(13) 84.13(11) 80.45(12) 97.30(4) 159.7(3)

M wf tempera- yield e entry complex (μmol) ture (°C) (g) activity (103) 1b 2c 3b 4b 5c 6c 7b 8c 9b 10b 11c 12c 13d 14d 15d 16d

1 1 2 2 2 2 3 3 3 3 3 3 3 3 4 4

2 2 2 2 2 2 2 2 0.2 0.2 0.2 0.2 0.05 0.05 0.05 0.05

60 60 60 80 60 80 80 80 60 80 60 80 40 60 40 60

0.000 0.027 0.173 0.280 0.214 0.348 0.682 1.066 0.393 0.235 1.472 0.846 0.579 0.643 0.426 0.299

0.027 0.173 0.280 0.214 0.348 0.682 1.07 3.93 2.35 14.7 8.46 23.2 25.7 17.1 12.0

143 257 206 273 206 956 830g/4h 1700 24 2536g/3h 1614g/4h 2017g/3h 2313g/3h 642 264

Tm Mw/Mn (°C) 11.0 6.7 28.0 7.4 40.0 3.7 4.6g/2.5h 2.8 2.2 61.9g/2.0h 7.6g/1.9h 15.5g/2.8h 10.3g/2.0h 7.8 5.2

133 133 132 132 132 136 136 135 135 128 132 132 128 136 135

(3: 1.79 A˚; 4: 1.77 A˚) are almost identical for the two cases and are typical for Ta complexes bearing imido auxiliary ligands (1.77-1.79 A˚).28 The Cl-Ta-Cl angles (3: 93.88°; 4: 97.30°) are similar to the angles in [Tp*TaCl3][TaCl6] (95.8-96.1°).26 Ethylene Polymerization Behavior of Complexes 1-4. Ta complexes 1-4 combined with dried methylaluminoxane (DMAO) or iBu3Al/Ph3CB(C6F5)4 as a cocatalyst were evaluated as catalysts for the polymerization of ethylene in toluene solvent for 30 min under 0.97 MPa ethylene pressure. The results are summarized in Table 2. Complex 1, having the Tp* ligand, with DMAO at 60 °C, showed no reactivity toward ethylene under the conditions examined, producing neither polymeric nor oligomeric materials (entry 1). A very low activity (0.027 kg-PE/mmolTa 3 h) was observed when combined with iBu3Al/Ph3CB(C6F5)4 at 60 °C (entry 2). Conversely, complex 2, bearing the sterically more encumbered TpMs* ligand, in association with DMAO or iBu3Al/Ph3CB(C6F5)4 at 60 °C displayed an enhanced activity (DMAO: 0.173 kg-PE/mmol-Ta 3 h; i Bu3Al/Ph3CB(C6F5)4: 0.214 kg-PE/mmol-Ta 3 h, entries 3 and 5). These results are consistent with the general trend that sterically crowded environments around the metal centers normally result in high polymerization activity. One possible explanation for these observations is that the sterically more encumbered TpMs* ligand induces more effective ion separation between the cationic active species and an anionic cocatalyst, and additionally, it gives better steric protection to the coordination sites from electrophilic attack by Lewis acidic compounds in a polymerization medium.11d,13b,20 Raising the polymerization temperature from 60 to 80 °C gave rise to increased catalytic activity (entries 4 and 6). Complex 3, which possesses the imido (dN-2,6-iPr2-C6H3) group in place of the hexyne group (complex 2) as an auxiliary ligand, exhibited 2 to 3 times higher activity on activation with DMAO or iBu3Al/Ph3CB(C6F5)4 at 80 °C (entries 7 and 8). The high polymer yields of entries 7 and 8 may suggest that mass transport effects had an influence on the polymerization activities. Therefore, ethylene polymerizations with complex 3 were conducted at dilute catalyst concentrations (entries 9-14). As anticipated, much higher catalytic activities were obtained, and the maximum activity

of 25.7 kg-PE/mmol-Ta 3 h (entry 14) was achieved with i Bu3Al/Ph3CB(C6F5)4 activation (complex 3: 0.05 μmol, 60 °C, 30 min). The activity, 25.7 kg-PE/mmol-Ta 3 h (2.7 kg-PE/mmol-Ta 3 atm 3 h), is 100 times higher than that of complex 2 and one of the highest for ethylene polymerization ever recorded among Ta-based catalysts. Additionally, complex 4, which has the imido (dN-tBu) group, with iBu3Al/Ph3CB(C6F5)4 also formed a highly active ethylene polymerization catalyst, and the activity of 17.1 kg-PE/mmol-Ta 3 h (1.8 kg-PE/mmol-Ta 3 atm 3 h) was obtained at 40 °C (entry 15).29 These results demonstrate the beneficial effect of the imido auxiliary ligand on catalyst efficiency. In order to examine the capability of the imido auxiliary ligand to induce ethylene insertion, ethylene polymerization with a Ta complex missing a Tp0 ligand, TaCl3(dN-2,6-iPr2C6H3)(dimethoxyethane)/iBu3Al/Ph3CB(C6F5)4, was carried out. As a result, this catalyst system was found to be a poor catalyst for the polymerization (activity 0.148 kg-PE/ mmol-Ta 3 h, 60 °C, 30 min), suggesting that the imido group has a pronounced effect on catalytic activity when combined with the Tp0 ligand. Since complexes 3 and 4 possess the electron-donating imido auxiliary ligands, the catalytically active species stemming from these complexes may have high stability during the course of the polymerization.30 Kinetic profiles of ethylene polymerizations with complexes 3 and 4 combined with i Bu3Al/Ph3CB(C6F5)4 are shown in Figure 2. Significantly, for both cases, the catalyst deactivation is negligible even at 80 °C for 30 min, showing that complexes 3 and 4 generate thermally robust, long-lived catalysts.

(28) (a) Anti~ nolo, A.; Dorado, I.; Fajardo, M.; Garces, A.; Kubicki, M. M.; L opez-Mardomingo, C.; Otero, A.; Prashar, S. J. Organomet. Chem. 2006, 691, 1361. (b) Mashima, K.; Yonekura, H.; Yamagata, T.; Tani, K. Organometallics 2003, 22, 3766. (c) Heinselman, K. S.; Miskowski, V. M.; Geib, S. J.; Wang, L. C.; Hopkins, M. D. Inorg. Chem. 1997, 36, 5530.

(29) Complex 2 displayed practically no reactivity toward ethylene under the conditions identical to entries 13-16. (30) For beneficial effects of electron-donating substituents in ligands on the enhancement of the thermal stability of catalytically active species, see: Matsukawa, N.; Matsui, S.; Mitani, M.; Saito, J.; Tsuru, K.; Kashiwa, N.; Fujita, T. J. Mol. Catal. A 2001, 169, 99.

a Conditions: Argonaut parallel pressure reactor, toluene (5 mL), ethylene (0.97 MPa), polymerization time, 30 min. b DMAO (Al/Ta molar ratio=500). c iBu3Al (Al/Ta molar ratio=50)/Ph3CB(C6F5)4 (B/ Ta molar ratio=1.0). d iBu3Al (Al/Ta molar ratio=100)/Ph3CB(C6F5)4 (B/Ta molar ratio=1.0). eActivity based on polymer yield; units: kg-PE/ mmol-Ta 3 h. f Weight average molecular weight determined by GPC using polyethylene calibration. g Data for the high molecular weight portion of the bimodal GPC peak after deconvolution. h Data for the low molecular weight portion of the bimodal GPC peak after deconvolution.

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Organometallics, Vol. 28, No. 22, 2009

Figure 2. Kinetic profiles of ethylene polymerization with (a) 3/iBu3Al/Ph3CB(C6F5)4 and (b) 4/iBu3Al/Ph3CB(C6F5)4 at 80 °C.38

The above results indicate the high potential of Ta complexes having the TpMs* or TpMs ligands and the imido auxiliary ligands for olefin polymerization. We believe that the electronically flexible properties together with the sterically encumbered nature of these Tp0 ligands are probably responsible for the achievement of high catalytic activities. Although the origin of the beneficial effects of the imido auxiliary ligands on polymerization characteristics is unclear at the present time, we believe that these effects are attributed to the bulky and electron-donating properties of the imido auxiliary ligands. All the Ta complexes yielded highly linear polyethylenes (PEs) (branching less than 1 per 1000 carbon atoms, based on 13 C NMR analyses), which is consistent with the observed Tm values (132-136 °C: entries 2-10, 12, 13, 15, and 16). The relatively low Tm value for entries 11 and 14 (128 °C) is ascribed to the presence of a considerable amount of low molecular weight PEs. GPC analyses show that complexes 1-4 formed low to very high molecular weight PEs of relatively narrow to broad molecular weight distributions. For example, complex 3 with DMAO provided very high or low molecular weight PEs with relatively narrow molecular weight distributions [Mw 1 700 000, Mw/Mn 2.8 (entry 9); Mw 24 000, Mw/Mn 2.2 (entry 10)], suggesting that the polymer may be produced by a single-site catalyst. Conversely, the same complex in association with iBu3Al/Ph3CB(C6F5)4 afforded broad molecular weight distribution PEs (entries 11-14), indicating that multiple active species are in operation in the polymerization. While the reason for the multisite behavior of the Ta complexes is not clear at the current time, we postulate that the behavior originates from isomerization,15c,e,31-33 dechelation or loss of the Tp0 ligands, variable chain transfer to Al, mass transport-limited polymerizations, and/or the heterogeneity of the system.34

Conclusions To explore the potentiality of Ta-based catalysts supported by electronically flexible ligands for olefin polymerization, we (31) Trofimenko, S., Eds. Scorpionates: The Coordination Chemistry of Polypyrazolylborate Ligands; Imperial College Press: London, 1999. (32) (a) Chisholm, M. H.; Eilerts, N. W.; Huffman, J. C. Inorg. Chem. 1996, 35, 445. (b) Darensbourg, D. J.; Maynard, E. L.; Holtcamp, M. W.; Klausmeyer, K. K.; Reibenspies, J. H. Inorg. Chem. 1996, 35, 2682. (33) Kime-Hunt, E.; Spartalian, K.; DeRusha, M.; Nunn, C. M.; Carrano, C. J. Inorg. Chem. 1989, 28, 4392. (34) Higher [Al] conditions resulted in an increase in the high molecular weight portion of the bimodal GPC curve, indicative of the catalyst modification by aluminum species. See: Supporting Information for more detail.

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have designed and synthesized tris(pyrazolyl)borate-ligated Ta complexes (Tp0 Ta complexes) 1-4 bearing alkyne or imido auxiliary ligands. All Ta complexes, when activated with DMAO or iBu3Al/Ph3CB(C6F5)4, were active ethylene polymerization catalysts and produced low to very high molecular weight PEs with relatively narrow to broad molecular weight distributions. In particular, Ta complexes 3 and 4 possessing sterically encumbered mesityl groups on the Tp ligands in association with iBu3Al/Ph3CB(C6F5)4 exhibited very high activities of 25.7 kg-PE/mmol-Ta 3 h (complex 3) and 17.1 kgPE/mmol-Ta 3 h (complex 4). These activities are some of the highest values encountered in Ta-based catalysts. Moreover, complexes 3 and 4 after activation showed a steady rate of ethylene uptake over the 30 min duration at 80 °C, indicative of a thermally robust, long-lived nature. Therefore, Ta complexes 3 and 4 represent a notable addition to the limited list of highperformance Ta-based catalysts for olefin polymerization. The results described herein further indicate the high potential of transition metal complexes that incorporate electronically flexible ligands for olefin polymerization.

Experimental Section General Comments. All manipulations of complex syntheses were performed with the exclusion of oxygen and moisture under nitrogen using standard Schlenk techniques using ovendried glassware. Materials. Dried solvents [1,2-dimethoxyethane, tetrahydrofuran (THF), diethyl ether, and n-hexane] used for complex syntheses were purchased from Wako Pure Chemical Industries, Ltd., and stored on molecular sieves 4A under argon. Dried toluene used for complex syntheses was purified by passage through columns containing activated alumina and an Engelhard Q5 oxygen scavenger. Dried toluene employed as a polymerization solvent was obtained from Wako Pure Chemical Industries, Ltd., and used without further purification. CD2Cl2 and C6D6 were distilled from P4O10 and Na/K, respectively, and were stored under N2. Tantalum(V) chloride was purchased from Nacalai Tesque, Inc. Dried methylaluminoxane (DMAO) was prepared by concentrating MAO (Albemarle, 1.2 M in toluene) in vacuo to remove the remaining trimethylaluminum, providing a solid white powder; the white powder was dissolved in dried toluene prior to use. Ethylene was obtained from Sumitomo Seika Co. Triisobutylaluminum (iBu3Al) was purchased from Tosoh Finechem Corp. TlTpMs,21 TlTpMs*,21 TaCl3(η2-3-hexyne)(dimethoxyethane),35 TaCl3(dN-2,6-iPr2-C6H3)(dimethoxyethane),36 and TaCl3(dN-tBu)(dimethoxyethane)36 were prepared according to literature procedures. All other chemicals were purchased from Wako Pure Chemical Industries, Ltd. and used without further purification. Ligand and Complex Analysis. NMR spectra of Ta complexes were recorded on a JEOL JNM-LA400 (400 MHz) or a JEOL Excalibur 270 spectrometer (270 MHz) from Japan Electron Optics Laboratory Co. Ltd. at ambient probe temperature. 1H and 13C chemical shifts were determined by reference to the residual 1H and 13C solvent signals. The assignment of signals in 13 C NMR spectra for complexes 2, 3, and 4 was made together with the results by distortionless enhancement by polarization transfer (DEPT) NMR experiments recorded on a JEOL JNMECA500 (125 MHz) spectrometer at ambient probe temperature. Coupling constants are reported in Hz. Field desorption (35) Oshiki, T.; Tanaka, K.; Yamada, J.; Ishiyama, T.; Kataoka, Y.; Mashima, K.; Tani, K.; Takai, K. Organometallics 2003, 22, 464. (36) Korolev, A. V.; Rheingold, A. L.; Williams, D. S. Inorg. Chem. 1997, 36, 2647. (37) Soares, J. B. P. Macromol. Symp. 2007, 257, 1.

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mass spectra (FD-MS) were recorded on a JEOL SX-102A instrument. The observed isotope patterns closely matched calculated isotope patterns. The listed m/z value corresponds to the most intense peak in the isotope pattern. Elemental analyses were performed using a PerkinElmer 2400 II Elemental CHN analyzer from PerkinElmer Corp. IR spectra were  E  460-T. recorded using Nicolet PROTEG 2 Tp*TaCl(η -3-hexyne)(benzyl) (1). A Schlenk flask was charged with TaCl3(η2-3-hexyne)(dimethoxyethane) (0.453 g, 0.99 mmol), KTp* (0.332 g, 0.99 mmol), and toluene (16 mL) at 20 °C, and the resulting mixture was stirred for 24 h at this temperature. Insoluble materials were removed from the reaction mixture by centrifugal separation. The supernatant was collected and concentrated in vacuo to give a red solid. The solid was dissolved in THF (8 mL), and then a 0.48 M PhCH2MgCl diethyl ether solution (4.1 mL, 1.97 mmol) was added to this solution with stirring at -78 °C. The resulting solution was allowed to warm to 20 °C and stirred for 24 h. All volatiles were removed in vacuo, the residual solid was extracted with toluene (15 mL), and the extract was concentrated in vacuo to give a solid. The solid was dissolved in n-hexane (25 mL) and stirred for 14 h at room temperature. The precipitate was removed by centrifugal separation, and the supernatant was collected and concentrated in vacuo to give complex 1, Tp*TaCl(η2-3-hexyne)(benzyl) (639 mg, 0.93 mmol), as an orange powder in 93% yield. 1 H NMR (C6D6): δ 0.72 (t; 3H, 3JHH=7.6 Hz, 3-hexyne Me), 1.59 (t; 3H, 3JHH =7.6 Hz, 3-hexyne Me), 1.81 (s; 3H, pz Me), 2.01 (s; 3H, pz Me), 2.04 (s; 3H, pz Me), 2.11 (s; 3H, pz Me), 2.17 (s; 3H, pz Me), 2.39 (d; 1H, 2JHH =13.4 Hz, benzyl H), 2.75 (s; 3H, pz Me), 2.81-2.93 (m; 2H, 3-hexyne CH2), 3.18 (d; 1H, 2JHH= 13.4 Hz, benzyl H), 3.70, 3.85 (both sep; both 1H, 3JHH = 13.4 Hz, 3-hexyne CH2), 5.45 (s; 1H, pz H), 5.59 (s; 1H, pz H), 5.61 (s; 1H, pz H), 6.81-6.86 (m; 3H, Ph), 7.16 (d; 2H, 3JHH = 7.6 Hz, Ph). 13C{1H} NMR (C6D6): δ 12.4, 12.6, 12.8, 13.6, 14.3, 15.6, 15.7, 16.7, 34.4, 35.5, 85.8, 107.4, 107.8, 108.8, 122.3, 127.3, 128.5, 128.7, 143.4, 143.9, 144.3, 151.7, 152.4, 153.2, 247.0, 270.4. FD-MS: 686 (Mþ). Anal. Calcd for C28H39BClN6Ta 3 0.19C6H14: C, 49.78; H, 5.97; N, 11.95. Found: C, 49.91; H, 5.94; N, 11.27. TpMs*TaCl2(η2-3-hexyne) (2). A Schlenk flask was charged with TaCl3(η2-3-hexyne)(dimethoxyethane) (0.282 g, 0.61 mmol), TlTpMs* (0.473 g, 0.61 mmol), and toluene (15 mL) at 20 °C, and the resulting mixture was stirred for 15 h at this temperature. Insoluble materials were removed from the reaction mixture by centrifugal separation, and the supernatant was collected and concentrated in vacuo to give a solid. Toluene (15 mL) was added to the solid, and the mixture was stirred for 14 h at room temperature. The precipitate was removed by centrifugal separation. The supernatant was collected and concentrated in vacuo to 10 mL, and n-hexane (10 mL) was layered. The mixture was stored at -30 °C. Upon standing at this temperature, a red powder was precipitated. The precipitate was collected and washed with n-hexane (5 mL  3) and dried in vacuo to give complex 2, TpMs*TaCl2(η2-3-hexyne) (332 mg, 0.37 mmol), as a red powder in 60% yield. 1 H NMR (C6D6): δ 0.95 (t; 3H, 3JHH=7.6 Hz, 3-hexyne Me), 1.82 (s; 3H, mesityl Me), 1.93 (s; 3H, mesityl Me), 1.94 (s; 3H, mesityl Me), 2.05 (t; 3H, 3JHH = 7.6 Hz, 3-hexyne Me), 2.07 (s; 3H, mesityl Me), 2.09 (s; 3H, mesityl Me), 2.15 (s; 3H, mesityl Me), 2.27 (s; 3H, mesityl Me), 2.28 (s; 3H, mesityl Me), 2.60 (s; 3H, mesityl Me), 3.1-3.17 (m; 2H, 3-hexyne CH2), 3.50-3.59 (m; 2H, 3-hexyne CH2), 5.58 (d; 1H, 3JHH=2.2 Hz, pz H), 5.91 (d; 1H, 3JHH=2.2 Hz, pz H), 5.96 (d; 1H, 3JHH=2.2 Hz, pz H), 6.70 (s; 1H, mesityl H), 6.82 (s; 1H, mesityl H), 6.83 (s; 1H, mesityl H), 6.94 (s; 1H, mesityl H), 6.95 (s; 1H, mesityl H), 6.97 (38) Polymerization conditions: complex 3 or 4 (0.05 mmol), iBu3Al (Al/Ta molar ratio = 400), Ph3CB(C6F5)4 (B/Ta molar ratio = 1.0), toluene (5 mL), ethylene (0.97 MPa), 30 min, 80 °C. See Table S1 in the Supporting Information for more details.

Michiue et al. (s; 1H, mesityl H), 7.21 (d; 1H, 3JHH=2.2 Hz, pz H), 7.35 (d; 1H, JHH=2.2 Hz, pz H), 7.98 (d; 1H, 3JHH=2.2 Hz, pz H). 13C{1H} NMR (C6D6): δ 13.7 (3-hexyne Me), 19.9, 20.3, 20.8, 21.1, 21.18 (mesityl Me); 21.23 (3-hexyne Me), 21.4, 21.8, 21.9, 22.5 (mesityl Me); 30.16, 33.3 (3-hexyne CH2); 108.0, 108.2, 108.7 (pyrazole C-4); 125.6, 127.3, 127.8, 128.4, 128.5, 128.6, 129.3, 129.5, 131.1, 135.8 (unsaturated C); 136.2, 136.9 (mesityl meta); 137.4, 137.7, 138.2, 138.3, 138.4, 138.8, 138.9, 139.7 (unsaturated C); 146.3, 146.5 (pyrazole tertiary carbon); 157.7, 157.8 (pyrazole quaternary carbon). FD-MS: 900 (Mþ). IR (Nujol/CsI): 2522 (ν B-H), 1613 (ν CtC), 303 (ν Ta-Cl). Anal. Calcd for C42H50BCl2N6Ta: C, 55.97; H, 5.65; N, 9.23. Found: C, 55.95; H, 5.59; N, 9.32. TpMs*TaCl2(dN-2,6-iPr2-C6H3) (3). A Schlenk flask was charged with TaCl3(dN-2,6-iPr2-C6H3)(dimethoxyethane) (0.169 g, 0.31 mmol), TlTpMs* (0.236 g, 0.31 mmol), and toluene (7 mL) at 20 °C, and the resulting mixture was stirred for 15 h at 60 °C. Insoluble materials were removed from the reaction mixture by centrifugal separation, and the supernatant was collected and concentrated in vacuo to give a solid. Toluene (15 mL) was added to the solid, and the mixture was stirred for 14 h at room temperature. The precipitate was removed by centrifugal separation, and the supernatant was concentrated in vacuo to give a solid. The solid was washed with n-hexane (5 mL  3) and dried in vacuo to give complex 3, TpMs*TaCl2(dN-2,6-iPr2-C6H3) (82 mg, 0.082 mmol), as a pink powder in 27% yield. 1 H NMR (C6D6): δ 0.91 (d; 3H, 3JHH=6.8 Hz, iPr Me), 1.06 (d; 3H, 3JHH =6.8 Hz, iPr Me), 1.10 (d; 3H, 3JHH =6.8 Hz, iPr Me), 1.40 (d; 3H, 3JHH =6.8 Hz, iPr Me), 1.56 (s; 3H, mesityl Me), 1.87 (s; 3H, mesityl Me), 1.91 (s; 3H, mesityl Me), 2.04 (s; 3H, mesityl Me), 2.08 (s; 3H, mesityl Me), 2.17 (s; 3H, mesityl Me), 2.26 (s; 3H, mesityl Me), 2.27 (s; 3H, mesityl Me), 2.42 (s; 3H, mesityl Me), 3.55 (sep; 1H, 3JHH=6.8 Hz, iPr CH), 4.07 (sep; 1H, 3JHH=6.8 Hz, iPr CH), 5.68 (d; 1H, 3JHH=2.2 Hz, pz H), 5.77 (d; 1H, 3JHH=2.2 Hz, pz H), 5.84 (d; 1H, 3JHH=2.2 Hz, pz H), 6.44 (s; 1H, Ar), 6.61-6.68 (m; 2H, Ar), 6.80-6.84 (m; 4H, Ar), 6.90 - 6.96 (m; 2H, Ar), 7.25 (d; 1H, 3JHH=2.2 Hz, pz H), 7.26 (d; 1H, 3JHH=2.2 Hz, pz H), 8.46 (d; 1H, 3JHH=2.2 Hz, pz H). 13C{1H} NMR (C6D6): δ 19.8, 20.1, 21.26, 21.28, 21.3, 21.6, 21.7, 22.1 (mesityl Me); 22.8, 23.0, 26.7, 27.4, 27.8, 27.9 (iPr Me and CH); 107.0 107.8, 109.5 (pyrazole C-4); 121.6, 122.0, 126.1, 127.7-128.4 (overlaps with C6D6 peaks), 128.5, 128.6, 129.3, 130.7, 136.1, 137.1, 137.4, 137.5, 137.8, 138.0, 138.1, 138.2, 138.4, 138.9, 139.0, 139.2, 147.9, 148.2 (unsaturated C); 150.0 (N-C), 153.2 (5-mesitylpyrazole C-5), 158.1, 159.2 (3mesitylpyrazole C-3). 13C{1H} NMR (CD2Cl2): δ 19.7, 20.2, 20.9, 21.17, 21.24, 21.29, 21.32, 21.4, 21.6 (mesityl Me); 22.5, 22.9, 26.6, 26.9, 27.3, 27.7 (iPr Me and CH); 107.5, 107.9, 109.9 (pyrazole C-4); 121.6, 121.7 (2,6-iPr2-Ph meta); 126.0 (2,6-iPr2Ph para), 127.4, 127.6 (mesityl meta); 127.8 (mesityl ipso), 128.0, 128.1, 128.4, 128.5 (mesityl meta); 129.1, 131.0 (mesityl ipso); 136.7 (5-mesitylpyrazole C-3), 137.3, 137.7, 137.89, 137.93, 138.4, 138.6 (mesityl ortho); 139.0 (3-mesitylpyrazole C-5), 139.1, 139.3, 139.5 (mesityl para); 147.8 (3-mesitylpyrazole C5), 148.3, 148.5 (2,6-iPr2-Ph ortho); 149.6 (N-C), 152.4 (5mesitylpyrazole C-5), 157.4, 159.0 (3-mesitylpyrazole C-3). FD-MS: 993 (Mþ). Anal. Calcd for C48H57BCl2N7Ta: C, 57.96; H, 5.78; N, 9.86. Found: C, 58.49; H, 5.93; N, 9.83. TpMsTaCl2(dN-tBu) (4). A Schlenk flask was charged with TaCl3(dN-tBu)(dimethoxyethane) (0.111 g, 0.25 mmol), TlTpMs (0.191 g, 0.25 mmol), and toluene (7 mL) at 20 °C, and the resulting mixture was stirred for 15 h at 60 °C. Insoluble materials were removed from the reaction mixture by centrifugal separation, and the supernatant was collected and concentrated in vacuo to give a solid. Toluene (15 mL) was added to the solid, and the mixture was stirred for 14 h at room temperature. The precipitate was removed by centrifugal separation, and the supernatant was concentrated in vacuo to give a solid. The solid was washed with petroleum ether (5 mL  3), and the residual 3

Article solid was extracted with toluene (6 mL). The toluene extract was concentrated in vacuo to 3 mL, and n-hexane (10 mL) was layered, then stored at -30 °C. Upon standing at this temperature, a lime green powder was precipitated. The precipitate was collected and washed with petroleum ether (5 mL  3) and then dried in vacuo to give complex 4, TpMsTaCl2(dN-tBu) (71 mg, 0.080 mmol), as a lime green powder in 32% yield. 1 H NMR (C6D6): δ 0.83 (s; 9H, tBu), 2.03 (s; 3H, mesityl Me), 2.08 (s; 6H, mesityl Me), 2.18 (s; 6H, mesityl Me), 2.25 (s; 6H, mesityl Me), 2.41 (s; 6H, mesityl Me), 5.71 (d; 2H, 3JHH=2.2 Hz, pz H), 6.01 (d; 1H, 3JHH=2.2 Hz, pz H), 6.83 (s; 2H, mesityl H), 6.86 (s; 2H, mesityl H), 6.89 (s; 2H, mesityl H), 7.39 (d; 2H, 3JHH= 2.2 Hz, pz H), 7.56 (d; 1H, 3JHH=2.2 Hz, pz H). 13C{1H} NMR (C6D6): δ 21.1, 21.8, 22.4 (mesityl Me); 29.9 (tBu Me), 67.3 (N-C), 108.47, 108.53 (pz C-4); 127.7-128.4 (overlaps with C6D6 peaks), 128.6, 129.0, 130.6, 134.9, 137.0, 137.7, 138.2, 138.6, 139.3 (unsaturated C); 157.7 (5-mesitylpyrazole C-5), 158.4 (3-mesitylpyrazole C-3). 13C{1H} NMR (CD2Cl2): δ 21.1, 21.3, 21.4, 21.7, 22.2 (mesityl Me); 29.6 (tBu Me), 67.2 (N-C), 108.5, 108.9 (pyrazole C-4); 127.4, 128.4, 128.5 (mesityl meta); 130.4, 131.8 (mesityl ipso); 135.4 (5-mesitylpyrazole C-3), 137.3, 137.4 (mesityl ortho); 138.80 (3-mesitylpyrazole C-5), 138.84, 138.9, 139.2 (mesityl para); 156.9 (5-mesitylpyrazole C-5), 158.3 (3-mesitylpyrazole C-3). FD-MS: 889 (Mþ). Anal. Calcd for C40H49BCl2N7Ta 3 0.18C6H14: C, 54.46; H, 5.74; N, 10.82. Found: C, 54.51; H, 5.31; N, 10.16. X-ray Structure Determinations. Single crystals of complexes 3 and 4 3 1.5 toluene suitable for an X-ray analysis were grown by slow evaporation of a CD2Cl2 solution (complex 3) or a toluene/ n-hexane solution (complex 4 3 1.5 toluene). Crystal data, data collection details, and solution and refinement procedures are provided in the Supporting Information. The ORTEP diagrams were drawn with 50% probability ellipsoids. All non-hydrogen atoms were refined with anisotropic displacement coefficients unless otherwise indicated. All hydrogen atoms were included in the structure factor calculation at idealized positions and were allowed to ride on the neighboring atoms with relative anisotropic displacement coefficients. The toluene solvent molecules of 4 3 1.5 toluene were disordered; one was disordered in a plane defined by the six-membered ring of toluene, and the other one was disordered at an inversion center. Each of the toluene molecules was modeled as such. Ethylene Polymerizations (Table 2). Ethylene polymerizations were performed in a parallel pressure reactor (Argonaut Endeavor catalyst screening system) containing eight reaction vessels (15 mL) each equipped with a mechanical stirrer and monomer feed lines. A toluene solution of DMAO or iBu3Al was loaded in each vessel, and a stainless steel manifold was attached. The nitrogen atmosphere was replaced with ethylene, and the solution was saturated with ethylene at the polymerization pressure (0.97 MPa) and thermally equilibrated at the reaction temperature. For runs in which the DMAO was

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employed as a cocatalyst, Ta complex/toluene (0.20 mL of toluene solution of complex followed by a 0.25 mL of toluene wash) was injected sequentially into a DMAO/toluene solution in the reactor to start the reaction. For runs in which iBu3Al/ Ph3CB(C6F5)4 was employed as a cocatalyst, Ta complex/ toluene (0.20 mL of solution), Ph3CB(C6F5)4/toluene (0.50 mL of solution), and toluene wash (0.25 mL) were sequentially injected into a iBu3Al/toluene solution in the reactor to start the reaction. The total volume of the reaction mixture was 5 mL for all polymerizations. The pressure was kept constant by feeding ethylene on demand. After the reaction, the reaction was quenched by injection of excess isobutyl alcohol. The reactor was vented, and the resulting mixture was added to acidified methanol (45 mL containing 0.5 mL of concentrated HCl). The polyethylene was collected by filtration, washed with methanol (10 mL  2), and dried in a vacuum oven at 80 °C for 10 h. Polymer Characterization. Peak melting temperatures (Tm) of PEs were determined by differential scanning calorimetry (DSC) with a Shimadzu DSC-60 instrument. The polymer samples were heated at 50 °C/min from 20 to 200 °C, held at 200 °C for 5 min, and cooled to 0 °C at 20 °C/min. The samples were held at this temperature for 5 min and then reheated to 200 °C at 10 °C/min. The reported Tm values were determined from the second heating scan. Molecular weights (Mw and Mn) and molecular weight distributions (MWDs) of polyethylenes were determined using a Waters GPC2000 gel permeation chromatograph equipped with four TSKgel columns (two sets of TSKgelGMH6-HT and two sets of TSKgelGMH6-HTL) at 140 °C using polyethylene calibration. o-Dichlorobenzene (ODCB) was used as the solvent. Molecular weights and MWDs of the low and high molecular weight portion of the bimodal GPC peak were determined by deconvolution of the GPC trace.37 13C NMR data for the PEs were obtained using o-dichlorobenzene with 25 vol % benzene-d6 as a solvent at 120 °C using an ECX500 spectrometer from Japan Electron Optics Laboratory Co., Ltd., at 125 MHz. The spectra were referenced versus main chain methylene carbon peak, δ13C=29.7 ppm.

Acknowledgment. We thank Mitsui Chemical Analysis & Consulting Service Inc., for elemental analysis, GPC, DSC, and 13C NMR analysis. X-ray diffraction analysis was performed by Dr. Akihito Yamano at PharmAxess, Inc. We thank Mr. H. Makio and Mr. A. Valentine for fruitful discussions and suggestions. Supporting Information Available: Detailed ethylene polymerization results, GPC results for polyethylenes, and summary of crystallographic data for complexes 3 and 4. Crystallographic data for 3 and 4 are available in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org.