Hetero-bimetallic Complexes of Titanatranes with Aluminum Alkyls

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Organometallics 2010, 29, 3500–3506 DOI: 10.1021/om100119g

Hetero-bimetallic Complexes of Titanatranes with Aluminum Alkyls: Synthesis, Structural Analysis, and Their Use in Catalysis for Ethylene Polymerization Prabhuodeyara M. Gurubasavaraj and Kotohiro Nomura*,† Graduate School of Materials Science, Nara Institute of Science and Technology (NAIST), 8916-5 Takayama, Ikoma, Nara 630-0101, Japan. †Present address: Department of Chemistry, Tokyo Metropolitan University, 1-1 Minami Osawa, Hachioji, Tokyo 192-0397, Japan, Received February 13, 2010

Hetero-bimetallic Ti-Al complexes containing tris(aryloxo)amine ligands of type [TiR0 {(μ2-O-2,4R2C6H2-6-CH2)(O-2,4-R2C6H2-6-CH2)2N}][R0 2Al(μ2-OiPr)] [3a,b and 4a; R = Me (3), tBu (4); R0 = Me (a), Et (b)] have been prepared by reacting Ti(OiPr)[(O-2,4-R2C6H2-6-CH2)3N] [R = Me (1), tBu (2)] with 1.0 equiv of AlR0 3 in toluene. Crystallographic analyses of 3a,b and 4a reveal that these complexes have a distorted octahedral geometry around Ti and a distorted tetrahedral geometry around Al. Complexes 3a,b and 4a exhibited moderate catalytic activities for ethylene polymerization in toluene in the presence of methylaluminoxane (MAO) at 80-120 °C, and 4a showed higher activity than 3a,b. The tert-butyl analogue (4a) itself polymerizes ethylene without cocatalysts to afford high molecular weight polymer with a uniform distribution, clearly suggesting a hypothesis that cleavage of Ti-O bonds would generate the catalytically active cationic species in this catalysis.

Introduction Olefin polymerization by transition metal complex catalysts has been one of the most attractive research fields since the discovery of Ziegler-Natta catalysts.1 Even a few decades after its first discovery, this field continues to attract considerable attention in both academic and industrial communities. In the typical polymerization process, the transition metal complexes are activated by different types of cocatalysts to form cationic species, which are proposed to catalyze the polymerization process.2 Aluminum reagents such as alkyl aluminum and methylaluminoxane (MAO) play an important role as cocatalysts,3 and these Al complexes are also involved in the catalysis by *Corresponding author. Tel and fax: þ81-42-677-2547. E-mail: [email protected]. (1) For selected reviews/accounts, see: (a) Britovsek, G. J. P.; Gibson, V. C.; Wass, D. F. Angew. Chem., Int. Ed. 1999, 38, 428. (b) McKnight, A. L.; Waymouth, R. M. Chem. Rev. 1998, 98, 2587. (c) Kaminsky, W.; Arndt, M. Adv. Polym. Sci. 1997, 127, 144. (d) Bochmann, M. J. Chem. Soc., Dalton Trans. 1996, 255. (e) Marks, T. J. Acc. Chem. Res. 1992, 25, 57. (f) Jordan, R. F. Adv. Organomet. Chem. 1991, 32, 325. (g) Kaminsky, W., Ed. Metalorganic Catalysts for Synthesis and Polymerization: Recent Results by Ziegler-Natta and Metallocene Investigations; Springer-Verlag: Berlin, 1999. (2) Chen, E. Y-X.; Marks, T. J. Chem. Rev. 2000, 100, 1391. (3) (a) Boor, J., Jr. Ziegler-Natta Catalysts and Polymerizations; Academic Press: New York, 1979. (b) Chien, J. C. W., Ed. Coordination Polymerization; Academic Press: New York, 1975. (c) Andresen, A.; Cordes, H. G.; Herwig, H.; Kaminsky, W.; Merk, A.; Mottweiler, R.; Pein, J.; Sinn, H.; Vollmer, H. J. Angew. Chem., Int. Ed. Engl. 1976, 15, 630. (d) Sinn, H.; Kaminsky, W.; Vollmer, H.-J.; Woldt, R. Angew. Chem., Int. Ed. Engl. 1980, 19, 390. (e) Sinn, H.; Kaminsky, W. Adv. Organomet. Chem. 1980, 18, 99. (4) Recent review article: The Chemistry of Catalyst Activation: The Case of Group 4 Polymerization Catalyst. Bochmann, M. Organometallics 2010, in press (web released on June 29, DOI: 10.1021/om1004447). Related references are cited therein. pubs.acs.org/Organometallics

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forming heterobimetallic complexes or counteranions with titanium complexes.4-6 These bimetallic complexes exhibit high activity in the catalysis and produce polymers with different microstructures. The activation of metal complexes is usually associated with a decrease in complex stability due to the electronic deficiency of the metal center,2,4 but this can be prevented by modification of ligands with several donor atoms that can coordinate when necessary.7 In this way, oxygen donor ligands such as alkoxides,8a,b aryloxides,8c,d and carboxylates8e are extremely versatile, because appropriate substitution patterns allow substantial modification of both the steric and electronic properties of the metal center. (5) For example (heterobinuclear AlMe3 adducts),4 see: (a) Bochmann, M.; Lancaster, J. J. Angew. Chem., Int. Ed. Engl. 1994, 33, 1634. (b) Petros, R. A.; Norton, J. R. Organometallics 2004, 23, 5105. (c) Schr€oder, L.; Brintzinger, H. H.; Babushkin, D. E.; Fischer, D.; M€ulhaupt, R. Organometallics 2005, 24, 867. (d) Bryliakov, K. P.; Babushkin, D. E.; Talsi, E. P.; Voskoboynikov, A. Z.; Gritzo, H.; Schr€oder, L.; Damrau, H. R. H.; Wieser, U.; Schaper, F.; Brintzinger, H. H. Organometallics 2005, 24, 894. (e) Bolton, P. D.; Clot, E.; Cowley, A. R.; Mountford, P. Chem. Commun. 2005, 3313. (f) Bolton, P. D.; Clot, E.; Cowley, A. R.; Mountford, P. J. Am. Chem. Soc. 2006, 128, 15005. (6) (a) Janas, Z.; Jerzykiewicz, L. B.; Sobota, P.; Szczegot, K.; Wiceniewska, D. Organometallics 2005, 24, 3987. (b) Fandos, R.; Gallego, B.; Otero, A.; Rodriguez, A.; Ruiz, M. J.; Terreros, P.; Pastor, C. Organometallics 2007, 26, 2896. (c) Gurubasavaraj, P. M.; Mandal, S. K.; Roesky, H. W.; Oswald, R. B.; Pal, A.; Noltemeyer, M. Inorg. Chem. 2007, 46, 1056. (d) Gurubasavaraj, P. M.; Roesky, H. W.; Mandal, S. K.; Oswald, R. B.; Pal, A.; Herbst-Irmer, R. Inorg. Chem. 2008, 47, 5324. (7) (a) Verkade, J. G. Acc. Chem. Res. 1993, 26, 483. (b) Verkade, J. G. Cood. Chem. Rev. 1994, 137, 233. (8) (a) Gueta-Neyroud, T.; Tumanskii, B.; Kapon, M.; Eisen, M. S. Macromolecules 2007, 40, 5261. (b) Allcock, H. R.; Patterson, D. B.; Evans, T. L. Macromolecules 1979, 12, 172. (c) Phomphrai, K.; Fenwick, A. E.; Sharma, S.; Fanwick, P. E.; Caruthers, J. M.; Delgass, W. N.; Abu-Omar, M. M.; Rothwell, I. P. Organometallics 2006, 25, 241. (d) Michalczyk, L.; Gala, S. de.; Bruno, J. W. Organometallics 2001, 20, 5547. (e) Dang, Y. Coord. Chem. Rev. 1994, 135, 93. r 2010 American Chemical Society

Article

Atrane ligands contain a neutral nitrogen atom that facilitates coordination in a chelate fashion when necessary by providing the metal with additional electronic density. There have been many reports on the synthesis of complexes containing atrane ligands,9 including main group metals to transition metal complexes. Titanatranes among group 4 metals have received considerable interest because of their applications in organic synthesis10a-f and syndiospecific styrene polymerization.10g-k Previous work from our group demonstrated that these titanatranes can be applied as catalyst precursors for olefin polymerization.11,12 We first reported that the titanium aryloxide/ isopropoxide complexes containing a tris(aryloxo)amine ligand, Ti(OiPr)[(O-2,4-R2C6H2-6-CH2)3N] [R=Me (1), tBu (2)], were effective as catalyst precursors for ethylene polymerization in the presence of MAO, and the activities remarkably increased at higher temperature, even at 100-120 °C. Moreover, the activity increased upon addition of a small amount of AlMe3.11a Titanatranes containing a bis(aryloxo)(alkoxo)amine ligand, TiX[{(O2,4-Me2C6H2-6-CH2)2(OCH2CH2)}N] (X = OiPr, OtBu), also exhibit moderate catalytic activities, and the activity increased upon addition of a small amount of AlMe3.11b Later, we reported that the activity was also improved when the monomeric titanatrane containing a terminal aryloxo ligand, Ti(O-2,6-iPr2C6H3)[{(O-2,4-Me2C6H2-6-CH2)2(OCH2CH2)}N], was used in place of the above titanatranes.11c,d Inspired by the fact that the activity increases upon addition of a small amount AlMe3, we successfully isolated the Ti-Al hetero-bimetallic complex [TiMe{(O-2,4-Me2C6H2-6-CH2)2(μ2OCH2CH2)N}][Me2Al(μ2-OtBu)], which showed moderate catalytic activity for ethylene polymerization without cocatalysts at 120 °C, affording high molecular weight polymer with a uniform distribution.11b The result clearly suggests that the cationic species formed by cleavage of the Ti-O bonds play an important role as the active species for polymerization (Scheme 1). The above observation prompted us to isolate the heterobimetallic complexes of tris(aryloxo)amine-based titanatranes by reacting with aluminum alkyls and to explore their reaction chemistry. This is also because well-characterized and catalytically active bimetallic complexes with aluminum alkyl compounds are still limited.4,11b In this paper, we thus present the synthesis and structural analysis of hetero-bimetallic Ti-Al (9) Some examples on atrane ligand based complexes: (a) Voronkov, M. G.; Dyakov, V. M.; Kirpichenko, S. V. J. Organomet. Chem. 1982, 233, l. (b) Schrock, R. R. Acc. Chem. Res. 1997, 30, 9–16. (c) Gade, L. H. Chem. Commun. 2000, 173. (10) (a) Nugent, W. A. J. Am. Chem. Soc. 1992, 114, 2768. (b) Nugent, W. A.; Harlow, R. L. J. Am. Chem. Soc. 1994, 116, 6142. (c) Furia, F. D.; Licini, G.; Modena, G.; Motterle, R.; Nugent, W. A. J. Org. Chem. 1996, 61, 5175. (d) Bonchio, M.; Calloni, S.; Furia, F. D.; Licini, G.; Modena, G.; Moro, S.; Nugent, W. A. J. Am. Chem. Soc. 1997, 119, 6935. (e) Nugent, W. A. J. Am. Chem. Soc. 1998, 120, 7139. (f) Bonchio, M.; Licini, G.; Modena, G.; Bortolini, O.; Moro, S.; Nugent, W. A. J. Am. Chem. Soc. 1999, 121, 6258. (g) Kim, Y.; Hong, E.; Lee, M. H.; Kim, J.; Han, Y.; Do, Y. Organometallics 1999, 18, 36. (h) Kim, Y.; Han, Y.; Hwang, J.-W.; Kim, M. W.; Do, Y. Organometallics 2002, 21, 1127. (i) Kim, Y.; Do, Y. J. Organomet. Chem. 2002, 655, 186. (j) Lee, K.-S.; Kim, Y.; Ihm, S.-K.; Do, Y.; Lee, S. J. Organomet. Chem. 2006, 691, 1121. (k) Lee, J.; Hong, Y.; Kim, J. H.; Kim, S. H.; Do, Y.; Shin, Y. K.; Kim, Y. J. Organomet. Chem. 2008, 693, 3715. (11) (a) Wang, W.; Fujiki, M.; Nomura, K. Macromol. Rapid Commun. 2004, 25, 504. (b) Padmanabhan, S.; Katao, S.; Nomura, K. Organometallics 2007, 26, 1616. (c) Padmanabhan, S.; Wnag, W; Katao, S.; Nomura, K. Macromol. Symp. 2007, 260, 133. (d) Gurubasavaraj, P. M.; Nomura, K. Inorg. Chem. 2009, 48, 9491. (12) Report concerning ethylene/1-hexene polymerization using titanium complexes containing tris(alkoxo)amine ligands: (a) Sudhakar, P.; Amburose, C. V.; Sundararajan, G.; Nethaji, M. Organometallics 2004, 23, 4462. (b) Sudhakar, P.; Sundararajan, G. Macromol. Rapid Commun. 2005, 26, 1854.

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Scheme 1

Scheme 2

complexes of the type [TiR0 {(μ2-O-2,4-R2C6H2-6-CH2)(O-2,4R2C6H2-6-CH2)2N}][R0 2Al(μ2-OiPr)] (R0 = Me, Et) and their use as catalysts for ethylene polymerization. Through these results, we wish to introduce our proposed hypothesis that cleavage of Ti-O bonds generates cationic species that polymerize ethylene without any additional cocatalysts.

Results and Discussion 1. Reactions of Ti(OiPr)[(O-2,4-R2C6H2-6-CH2)3N] [R = Me, tBu] with Al Alkyls. Syntheses and Structural Analyses of Bimetallic Ti-Al Complexes [TiR0 {(μ2-O-2,4-R2C6H2-6-CH2)(O-2,4-R2C6H2-6-CH2)2N}][R0 2Al(μ2-OiPr)] (R0 =Me, Et). On the basis of the assumption that a selective cleavage of one of three Ti-O σ bonds in the titanatranes would afford the catalytically active cationic alkyl-Ti species, we thus explored the possibility of cleavage of the Ti-O bond by reacting Ti(OiPr)[(O-2,4R2C6H2-6-CH2)3N] [R = Me (1), tBu (2)] with 1.0 equiv of AlMe3 in toluene (Scheme 2). The reaction yielded the expected Ti-Al hetero-bimetallic complexes [TiMe{(μ2-O-2,4-R2C6H26-CH2)(O-2,4-R2C6H2-6-CH2)2N}][Me2Al(μ2-OiPr)] [R = Me (3a), tBu (4a)] as orange microcrystals [yield: 62% (3a), 72% (4a)] after recrystallization from a chilled toluene solution (-20 °C). A similar reaction of 1 with AlEt3 afforded [TiEt{(μ2-O-2,4-Me2C6H2-6-CH2)(O-2,4-Me2C6H2-6-CH2)2N}][Et2Al(μ2-OiPr)] (3b) in 76% yield. The resultant complexes were identified by NMR spectra and elemental analyses, and their structures were determined by X-ray crystallography, as described below. These complexes are soluble in toluene, THF, hexane, and chloroform and were very stable as microcrystals, which can be stored for long periods in the drybox without partial decomposition. Moreover, these complexes are rather stable even toward air and moisture for short periods; no significant decomposition was observed even after exposure to air atmosphere; this is contrary to many Ti-methyl and Al-methyl compounds. The 1H NMR spectra of 3a,b and 4a in toluene-d8/C6D6 were analogous to each other and exhibit different sets of resonances for the aryloxo arms due to the oxygen bridge formed between titanium and aluminum, which alters the symmetry of the molecule.13 The resonances are sharp, indicating that the molecules preserve a monomeric form even in solution, as exemplified in Figure 1 (3b). Two singlet resonances ascribed to AlMe2 were (13) 1H NMR spectra measured at various temperatures are shown in the Supporting Information.

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Figure 1. 1H NMR spectra in toluene-d8 for [TiEt{(μ2-O-2,4Me2C6H2-6-CH2)(O-2,4-Me2C6H2-6-CH2)2N}][Et2Al(μ2-OiPr)] (3b) measured at (a) 25 °C and (b) 80 °C.13

observed in the spectra for 3a and 4a, indicating their nonequivalent nature of the methyl groups on Al. Protons ascribed to the ethyl group attached to Al in 3b showed a multiplet (Figure 1a measured at 25 °C), probably because the two protons attached to Al were nonequivalent in addition to coupling to methyl protons. Resonances ascribed to the Ti-Me in these complexes (3a and 4a) were observed around 1-2 ppm; a different set of resonances ascribed to protons in the aryloxo arms was observed due to the steric bulk (bridged with Al alkyls, aryloxo susbstituent). Note that the resultant Ti-Al bimetallic complexes showed high stability in solution; no significant changes in the 1H NMR spectra were observed in toluene-d8/benzene-d6 even at high temperature (110 °C), as shown in Figure 1 (selected data for 3b).13 2. Structural Analysis for the Hetero-bimetallic Complexes 3a,b and 4a. Suitable microcrystals of 3a,b and 4a for their crystallographic analyses were grown from a chilled toluene solution (-20 °C), and their structures are shown in Figures 2 and 3. Selected bond distances and angles are summarized in Table 1.14 These complexes have a distorted octahedral geometry around titanium consisting of a C-Ti-N axis [174.04(7)° for 3a; 173.1(2)° for 3b; 176.15(10)° for 4a] and a distorted plane of one alkoxo and three aryloxo ligands [total bond angles of O(1)-Ti(1)-O(2), O(2)-Ti(1)-O(4), O(3)Ti(1)-O(4), and O(1)-Ti(1)-O(3) were 357.06-366.38°]. The coordination sphere of titanium consists of one methyl (ethyl in the case of 3b), four oxygen atoms including three anionic oxygens from the atrane ligand, and the titanium atom in 3a, (14) Structural analysis results including a table summarizing crystallographic data and collection parameters of complexes 3a, 3b, and 4a, structure reports, and CIF files are shown in the Supporting Information. Complex 4a showed a mixture of two complexes that possessed similar bond distances and angles. Toluene molecules that could not be defined were also observed as disordered molecules (reason for the level A errors in the check CIF file). Details are shown in the Supporting Information.

Gurubasavaraj and Nomura

3b, and 4a, ligated via a transannular interaction stemming from the bridgehead amino nitrogen. The Ti(1)-O(1) (in OiPr) bond distances [2.067(19)-2.075(3) A˚] were longer than the Al(1)-O(1) distances [1.825(2)1.827(13) A˚], and the Ti(1)-C(1) bond distances [2.092(7)2.113(3) A˚] were similar to those in [TiMe{(μ2-O-CH2CH2)(O-2,4-Me2C6H2-6-CH2)2N}][Me2Al(μ2-OR00 )] (R00 = iPr, tBu) [2.103(2), 2.117(2) A˚, respectively] reported previously.11b These results clearly indicate that the resultant complexes possess TiMe bonds by replacement of the OiPr group with AlMe3 or AlEt3. The Ti-OiPr bond distances in 3a [2.071(13) A˚] and 3b [2.075(3) A˚] are much longer than the Ti-OiPr bond length in their precursor complex 1 [1.745(5) A˚],15 and the Ti-OiPr bond length in 4a [2.067(19) A˚] is significantly longer than in its precursor complex of 2 [Ti-OiPr, 1.778(4) A˚].15 Moreover, the Ti(1)-O(2) distances [2.056(4)-2.095(2) A˚] are relatively close to those of the Ti(1)-OiPr bond [Ti(1)-O(1), 2.067(19)2.075(3) A˚] but longer than those of both the Ti(1)-O(3) and the Ti(1)-O(4) bonds [1.819(12)-1.841(4) A˚]. These may suggest that cleavages of both the Ti-OiPr bond and Ti(1)-O(2) bond that would generate a precursor of the catalytically active species. These complexes also have a distorted tetrahedral geometry around Al, and the C(2)-Al(1)-C(3) bond angles in these complexes [C(3)-Al(1)-C(5) in 3b] were 114.3(2)-115.50(9)°, although the O(1)-Al(1)-O(2) bond angles in both 3b and 4a [80.92(18)° and 82.38(9)°, respectively] were smaller than that in 3a [113.14(8)°] probably due to an increased steric bulk. The Al(1)-O(2) bond distances [1.835(4)-1.845(2) A˚, oxygen in the aryloxo arm] were longer than the Al(1)-O(1) distances [1.825(2)-1.827(4) A˚, oxygen in the OiPr group]. Since no significant differences in the bond distances [Al(1)-O(1) and Al(1)-O(2)] were seen in [TiMe{(μ2-O-CH2CH2)(O-2,4-Me2C6H2-6-CH2)2N}][Me2Al(μ2-OR00 )] (R00 = iPr, tBu) [1.825(2)1.8366(14) A˚],11b the observed difference would be due to an increased steric bulk in the aryloxo arm. 3. Ethylene Polymerization by [TiR0 {(μ2-O-2,4-R2C6H2-6CH2)(O-2,4-R2C6H2-6-CH2)2N}][R0 2Al(μ2-OiPr)] (R = Me, t Bu; R0 =Me, Et)-MAO Catalyst Systems. Ethylene polymerizations using 3a,b and 4a were conducted in toluene in the presence of MAO, and the results are summarized in Table 2.16 The white solid MAO, prepared by removing AlMe3 and toluene from the commercially available MAO (PMAO-S, Tosoh Finechem Co.), was used, because all the complexes were sensitive to the amount of AlMe3 contained in MAO. The results from 1,2-MAO catalyst systems11a are also included for comparison. These complexes showed moderate catalytic activities, and the activity at 100 °C (under optimized Al/Ti molar ratios, Al/Ti = 3000) increased in the order 4a (2640 kg PE/mol Ti 3 h) > 3a (233) > 3b (78). The facts suggest that the activity was influenced by the ortho-substituent in the aryloxo arm (tBu vs Me) as well as the alkyl group on both Ti and Al (Me vs Et). One reason for observed higher catalytic activities by 4a than 3a would be due to an increase in the stability of the catalytically active species by introduction of the bulky tBu group. The catalytic activity was also affected by the Al/Ti molar ratios and the polymerization temperature; the activity increased from 80 to 100 °C but decreased at 120 °C. The observed facts, (15) Kol, M.; Shamis, M.; Goldberg, I.; Goldschmidt, Z.; Alfi, S.; Hayut-Salant, E. Inorg. Chem. Commun. 2001, 4, 177. (16) We could not conduct the ethylene polymerization in n-octane because these complexes (3a,b and 4a) showed low solubility in n-octane, although the activities of 1 and 2 in n-octane were higher than those in toluene.11a

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Figure 2. ORTEP drawings of [TiMe{(O-2,4-Me2C6H2-6-CH2)2(μ2-O-2,4-Me2C6H2-6-CH2)N}][Me2Al(μ2-OiPr)] (3a, left) and [TiEt{(O-2,4-Me2C6H2-6-CH2)2(μ2-O-2,4-Me2C6H2-6-CH2)N}][Et2Al(μ2-OiPr)] (3b, right). Thermal ellipsoids are drawn at the 50% probability level, and H atoms are omitted for clarity.14 Table 1. Selected Bond Distances and Bond Angles for [TiMe{(μ2O-2,4-R2C6H2-6-CH2)(O-2,4-R2C6H2-6-CH2)2N}][Me2Al(μ2OiPr)] [R = Me (3a), tBu (4a)] and [TiEt{(μ2-O-2,4-Me2C6H2-6CH2)(O-2,4-Me2C6H2-6-CH2)2N}][Et2Al(μ2-OiPr)] (3b)a 3a

3b

4a

Selected Bond Distances (A˚) Ti(1)-O(1) Ti(1)-O(2) Ti(1)-O(3) Ti(1)-O(4) Ti(1)-N(1) Ti(1)-C(1) Al(1)-O(1) Al(1)-O(2) Al(1)-C(2)

2.071(13) 2.072(11) 1.819(12) 1.841(14) 2.439(15) 2.106(19) 1.827(13) 1.844(15) 1.956(2)

2.075(3) 2.056(4) 1.830(4) 1.836(4) 2.424(5) 2.092(7) 1.827(4) 1.835(4) 1.950(6) Al(1)-C(3)

2.0673(19) 2.095(2) 1.8283(19) 1.8237(19) 2.381(2) 2.113(3) 1.825(2) 1.845(2) 1.954(3)

Selected Bond Angles (deg)

Figure 3. ORTEP drawings of [TiMe{(O-2,4-tBu2C6H2-6-CH2)2(μ2-O-2,4-tBu2C6H2-6-CH2)N}][Me2Al(μ2-OiPr)] (4a) (one of two molecules in the crystal). Thermal ellipsoids are drawn at the 50% probability level, and H atoms and solvent molecules are omitted for clarity. Detailed results are shown in the Supporting Information.14

especially temperature dependence, were somewhat different from those in the ethylene polymerization using 1,2-MAO catalyst systems.11a,17 The activities of 3a and 4 conducted under the same conditions were similar to (or higher than) those of 1 and 2, although the activities of 3b were somewhat lower than those of 1.18 These results, especially with 4, clearly

O(1)-Ti(1)-O(2) 70.48(5) 70.22(15) O(1)-Ti(1)-O(3) 90.57(5) 91.63(17) O(1)-Ti(1)-O(4) 160.13(5) 161.15(18) O(1)-Ti(1)-N(1) 89.11(5) 91.36(17) O(2)-Ti(1)-O(3) 156.71(5) 155.9(2) O(2)-Ti(1)-O(4) 91.61(5) 91.75(17) O(2)-Ti(1)-N(1) 83.20(4) 82.83(17) O(3)-Ti(1)-O(4) 104.40(6) 103.96(18) O(3)-Ti(1)-N(1) 83.09(5) 81.92(18) O(1)-Ti(1)-C(1) 96.01(7) 94.6(2) O(2)-Ti(1)-C(1) 95.62(6) 95.9(2) O(3)-Ti(1)-C(1) 99.86(6) 101.3(2) N(1)-Ti(1)-C(1) 174.04(7) 173.1(2) Ti(1)-O(1)-Al(1) 102.93(6) 103.4(2) Ti(1)-O(2)-Al(1) 102.32(6) 103.9(2) O(1)-Al(1)-O(2) 113.14(8) 80.92(18) C(2)-Al(1)-C(3) 115.50(9) 114.3(2) C(3)-Al(1)-C(5) a The details are shown in the Supporting Information.14

70.99(7) 89.71(8) 163.60(9) 88.79(7) 155.50(9) 94.73(10) 83.80(7) 101.95(8) 80.78(8) 94.73(10) 95.88(10) 100.74(10) 176.15(10) 103.66(9) 101.90(9) 82.38(9) 114.90(14)

suggest that the bimetallic complexes, first formed in the reaction mixture containing the precursors of 1 and 2 in the

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Table 2. Ethylene Polymerization by Ti(OiPr)[(O-2,4-R2C6H2-6-CH2)3N] [R = Me (1), tBu (2)] and [TiR0 {(μ2-O-2,4-R2C6H2-6-CH2)(O-2,4-R2C6H2-6-CH2)2N}][R0 2Al(μ2-OiPr)] [R = Me (3), tBu (4); R0 = Me (a), Et (b)] in the Presence of MAO Cocatalysta run

complex (μmol)

MAO/mmol

temp/°C

yield/mg

activityb

Mwc  10-4

Mw/Mnc

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

3a (1.0) 3a (1.0) 3a (1.0) 3a (1.0) 3a (1.0) 3b (1.0) 3b (1.0) 3b (1.0) 3b (1.0) 3b (1.0) 1 (1.0)d 1 (1.0)d 1 (0.1)f 4a (1.0) 4a (1.0) 4a (0.1) 4a (0.1) 4a (0.1) 4a (0.1) 4a (0.1) 2 (0.1)f

3.0 3.0 3.0 2.0 1.0 3.0 3.0 3.0 2.0 1.0 3.0 3.0 4.0

80 100 120 100 100 80 100 120 100 100 80 100 100 100 100 80 100 120 100 100 100

195 233 60 164 103 56 78 46 34 11 48.5 76.8 76.1 120 114 182 264 118 193 167 185

195 233 60 164 103 56 78 46 34 11 146 230 761 120 114 1820 2640 1180 1930 1670 1850

15.6 2.66 16.2 101 101 112 97.1 72.2 154 242

4.2 1.5 2.0 2.0 2.8 5.7 2.7 4.3 3.8 4.8

52.3 28.4 115

3.7e 2.2e 5.1

3.0 3.0 3.0 2.0 1.0 4.0

37.6 68.0 72.1 52.8 69.3 0.91

12 4.9 6.5 3.5 2.3 1.5

a Conditions: toluene 30 mL, ethylene 8 atm, MAO (prepared by removing AlMe3 and toluene from commercially available MAO), 60 min. b Activity = kg PE/mol Ti 3 h. c GPC data in o-dichlorobenzene vs polystyrene standards. d Time 20 min. e Small amounts of low molecular weight polymers were also observed in the GPC trace (run 12: Mw = 2100, Mw/Mn = 2.8. run 13: Mw = 7255, Mw/Mn = 1.2; Mw = 535, Mw/Mn = 1.4). f Cited from ref 11a.

presence of MAO and a small amount of AlMe3, play a role in this catalysis.18 The resultant polymers were linear polyethylene with high molecular weights and rather broad but monomodal molecular weight distributions.19 The Mw values decreased upon increasing the Al/Ti molar ratios and the polymerization temperature, probably due to an increased degree of chain transfer to Al. The Mw values in the resultant polymers prepared with 3a,b and 4a were higher than those with 1 (runs 12, 13) as well as with 2 (run 21). In particular, 3b afforded polymer with a Mw value of 2.42  106 with unimodal molecular weight distribution (run 10).19 Note that complex 4a showed moderate catalytic activity (120 kg PE/mol Ti 3 h) even in the absence of MAO, affording ultrahigh molecular weight polyethylene with a rather broad but unimodal molecular weight distribution (Mw = 1.15  106, run 14).19 The result was reproducible, although the observed catalytic activity was lower than those in the presence of MAO. Similar attempts using 3a,b, however, afforded negligible amounts of polymers in the absence of MAO. These results clearly support our hypothesis that ethylene polymerization took place accompanied by cleavage of Ti-O bonds and would suggest a plausible mechanism where the cationic species plays a key role as the catalytically active species (17) One probable reason we may take into consideration is that these polymerizations were conducted not in octane, as in the previous report,11a but in toluene, which may coordinate to Ti. Coordination of toluene in the proposed catalytically active species, for examples: Scollard, J. D.; McConville, D. H.; Payne, N. C.; Vittal, J. J. Macromolecules 1996, 29, 5241. (18) A reviewer commented that we should consider the reason that the activities of 3b were low. The activities of 3a,b with high Al/Ti molar ratio (Al/Ti = 3000) at 120 °C were close, probably due to alkyl exchange under these conditions. A reviewer suggested fast reduction to Ti(III) by 3b, but we do not have firm results for explaining the experimental observations. We recently conducted alkene/alkyne insertion chemistry of these alkyl complexes: Saeed, I. Nomura, K. Unpublished results, manuscript in preparation. (19) The resultant polymers possessed rather broad but unimodal molecular weight distributions probably because the resultant polymers were insoluble owing to their ultrahigh molecular weights (heterogeneous nature in the reaction mixture).

Scheme 3

for ethylene polymerization (Scheme 3), as assumed in the previous report.11b,20 It may also be assumed that the equilibrium is sensitive to the steric bulk on the alkoxide ligand on Al, as also described in the previous report.11b The assumed scheme shown here provides a unique concept for designing catalytically active species that are formed by cleavage of Ti-O bonds in the titanatranes without any additional cocatalysts. Although the observed activity in the present system is still not high enough, the present results introduce a new promising approach, and we will explore the possibility in more detail.20

Experimental Section General Procedures. All experimental manipulations were carried out under an atmosphere of dry nitrogen using standard Schlenk techniques or using a Vacuum Atmospheres drybox unless otherwise specified. All chemicals used were of reagent grades and were purified by standard purification procedures. (20) A reviewer commented that we speculate a tetracoordinate active species formed from the octahedral complex (Scheme 3), but partial detachment of only one of the O-donors to form a pentacoordinate active species is also possible as the catalytically active species. As shown by the temperature dependence in the 1H NMR spectra, we have never observed the actual catalytically active species or zwitterionic species (by NMR spectra). The role of MAO to improve the activity also seems obvious (the activity of 4a in the presence of MAO was much higher than that without MAO), and we need to explain the role in this catalysis (maybe effective for dissociation of the Ti-O arm to generate the active species or shift the equilibrium between dormant and active species). The related chemistry is under investigation.

Article Toluene (anhydrous grade, Kanto Kagaku Co., Ltd.) for polymerization was stored in a bottle in the drybox in the presence of molecular sieves (a mixture of 3A 1/16, 4A 1/8, and 13X 1/16). Polymerization-grade ethylene (purity >99.9%, Sumitomo Seika Co. Ltd.) was used as received. Toluene and AlMe3 from commercially available methylaluminoxane [PMAO-S, 9.5 wt % (Al) toluene solution, Tosoh Finechem Co.] were removed under reduced pressure (at ca. 50 °C for removing toluene and AlMe3 and then heated at >100 °C for 1 h for completion) in the drybox to give white solids. Tris(2-hydroxy-3,5-dimethylbenzyl)amine and tris(2-hydroxy-3,5-di-tert-butylbenzyl)amine were prepared according to a published procedure.15 The titanatranes containing phenoxy terminal ligands Ti(OiPr)[(O-2,4-R2C6H2-6CH2)3N] [R = Me (1), tBu (2)] were prepared according to the previous report.15 Molecular weights and molecular weight distributions for polyethylene were measured by gel permeation chromatography (Tosoh HLC- 8121GPC/HT) with a polystyrene gel column (TSK gel GMHHR-H HT  2, 30 cm 7.8 mmj, ranging from