Stability of Metal-Carbon Bond versus Metal Reduction during Ethylene Polymerization Promoted by a Vanadium Complex: The Role of the Aluminum Cocatalyst Khalil Feghali, David J. Harding, Damien Reardon, Sandro Gambarotta,* and Glenn Yap Department of Chemistry, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada Qinyan Wang NOVA Chemicals, 2928 16th Street, N.E. Calgary, Alberta T2E 7K7, Canada Received December 6, 2001 The dinuclear and trivalent complex {[(Me 3 Si)NCH 2 CH 2 ] 2 N(Me 3 Si)} 2 V 2 (µ-Cl) 2 (1) is the precursor to mono- and dinuclear alkyl derivatives that are thermally stable. For example, treatment with MeLi gives a stable methyl derivative, probably isostructural with 1, which upon further treatment with pyridine affords the mononuclear complex {[(Me 3 Si)NCH 2 - CH 2 ] 2 N(Me 3 Si)}V(CH 3 )(pyridine) (2). However, reaction of 1 with Me 2 AlCl, AlMe 3 , or PMAO- IP yields the tetrametallic species {[(Me 3 Si)NCH 2 CH 2 ] 2 N(Me 3 Si)} 2 V 2 (µ-Cl) 2 (AlMe 2 ) 2 (3), where the central core of 1 was preserved except for the vanadium centers, which were reduced to the divalent state. The two Me 2 Al residues remained coordinated to the amido ligand. The reduction of vanadium to the divalent state relates to the relatively short life of 1 as an ethylene polymerization catalyst. A similar reaction of 1 with AlCl 3 resulted in dispropor- tionation forming the tetravalent complex {[(Me 3 Si)NCH 2 CH 2 ] 2 N(Me 3 Si)}VCl 2 AlCl 3 (4) and the pentanuclear mixed-valent V(II)/V(III) species [(AlCl 2 ){[(Me 3 Si)NCH 2 CH 2 ] 2 N(Me 3 Si)}V] 2 - [(µ-Cl) 6 V]‚(toluene) 2 (5). The fact that complex 5 contains a divalent vanadium atom stripped of its ligand system indicates that two different reaction mechanisms are operating to reduce the vanadium center and that the differing Lewis acidity of the two aluminum species is the determining factor. Introduction Research into the area of Ziegler-Natta olefin po- lymerization has traditionally focused on the group 4 metals since these catalysts tend to be both efficient and selective. 1 In addition, most of the industrially success- ful catalysts are based on Cp and its derivatives, as these ligands are both robust and highly tunable. Nevertheless, some nonmetallocene systems have found industrial applications. Particularly relevant to this work is the V(acac) 3 catalyst used for the commercial production of ethylene-propylene-diene elastomers (EPDM). 2 A recent study of this system in our group highlighted a number of problems with the process. 3 The investiga- tion confirmed that the Al cocatalyst plays a pivotal role in the catalyst activation by generating, in addition to the V-R function, empty coordination sites on vana- (1) See for example: (a) Mohring, P. C.; Coville, N. J. J. Organomet. Chem. 1994, 479, 1. (b) Erker, G.; Nolte, R.; Tsay, Y. H.; Kruger, C. Angew. Chem. 1989, 28, 628. (c) Erker, G.; Nolte, R.; Aul, R.; Wilker, S.; Kruger, C.; Noe, R. J. Am. Chem. Soc. 1991, 113, 7594. (d) Kaminsky, W.; Kulper, K.; Brintzinger, H. H.; Wild, R. W. P. Angew. Chem., Int. Ed. Engl. 1985, 6, 507. (e) Coates, G. W.; Weymouth, R. M. J. Am. Chem. Soc. 1991, 113, 6270. (f) Ewen, J. A. J. Am. Chem. Soc. 1984, 106, 6355. (g) Guerra, G.; Cavallo, L.; Moscardi, G.; Vacatello, M.; Corradini, P. J. Am. Chem. Soc. 1994, 116, 2988. (h) Jordan, R. F. Adv. Organomet. Chem. 1991, 32, 325. (i) Brintzinger, H. H.; Fisher, D.; Mulhaupt, R.; Rieger, B.; Waymouth, R. M. Angew. Chem., Int. Ed. Engl. 1995, 34, 1143. (j) Bochmann, M. J. Chem. Soc., Dalton Trans. 1996, 255. (2) See for example: (a) Doi, Y.; Suzuki, S.; Soga, K. Macromolecules 1986, 19, 2896. (b) Ouumi, T.; Soga, K. Makromol. Chem. 1992, 193, 823. (c) Gumbolt, A.; Helberg, J.; Schleitzer, G. Makromol. Chem. 1967, 101, 229. (d) Adisson, E. J. Polym. Sci., Part A: Polym. Chem. 1994, 32, 1033. (e) Davis, S. C.; von Hellens, W.; Zahalka, H. In Polymer Material Encyclopedia Vol. 3; Salamone, J. C., Ed.; CRC Press Inc.; Boca Raton, FL, 1996. (f) Sinn, H.; Kaminski, W. In Advances in Organometallic Chemistry; Stone, F. G. A., West, R., Eds.; Academic Press: New York, 1980. (g) Doi, Y.; Tokuhiro, N.; Nunomura, M.; Miyake, H.; Suuki, S.; Soga, K. Transition Metals and Organometallics as Catalysts for Olefin Polymerization; Kaminsky, W., Sinn H., Eds.; Springer-Verlag: Berlin, 1988. (h) Carrick, W. L. J. Am. Chem. Soc. 1958, 80, 6455. (i) Christman, D. L. J. Polym. Sci. 1972, A-1, 471. (j) Pasquon, I. G.; Giannini, U. In Catalysis; Anderson, J. R., Boudart, M., Eds.; Springer-Verlag: Berlin, 1984. (k) Carrick, W. L.; Kluiber, R. W.; Bonner, E. F.; Wartman, L. H.; Rugg, F. M.; Smith J. J. J. Am. Chem. Soc. 1960, 82, 3883. (l) Lehr, M. H. Macromolecules 1968, 1, 178. (m) Christman, D. L.; Keim, G. I. Macromolecules 1968, 1, 358. (n) Lehr, M. H.; Carmen, C. J. Macromolecules 1969, 2, 217. (o) Duck, E. W.; Grant, D.; Horder, J. R.; Jenkins, D. K.; Marlow, A. E.; Wallis, S. R.; Doughty, A. G.; Maradon, J. M.; Skinner, G. A. Eur. Polym. J. 1974, 10, 481. (p) Schuere, S.; Fisher, J.; Kress, J. Organometallics 1995, 14, 2627. (q) Feher, F. J.; Blanski, R. L. J. Am. Chem. Soc. 1992, 114, 5886. (r) Feher, F. J.; Walzer, J. F.; Blanski, R. L. J. Am. Chem. Soc. 1991, 113, 3618. (s) Feher, F. J.; Blanski, R. L. Organometallics 1993, 12, 958. (t) Cucinella, S.; Mazzei A. U.S. Patent 3,711,455, CI. 260-85.3, 1973. (u) Boor, J., Jr.; Youngman, E. A. J. Polym. Sci. A 1966, 4, 1861. (v) Zambelli, A.; Proto, A.; Longo, P. In Ziegler Natta Catalysis; Fink, G., Mulhaupt, R., Brintzinger, H. H., Eds.; Springer- Verlag: Berlin, 1995. (3) Ma, Y.; Reardon, D. F.; Gambarotta, S.; Yap, G. P. A. Organo- metallics 1999, 18, 2773. 968 Organometallics 2002, 21, 968-976 10.1021/om011040u CCC: $22.00 © 2002 American Chemical Society Publication on Web 02/05/2002