Published on Web 02/14/2003
Mechanistic Studies of Nickel(II) Alkyl Agostic Cations and Alkyl Ethylene Complexes: Investigations of Chain Propagation and Isomerization in (r-diimine)Ni(II)-Catalyzed Ethylene Polymerization Mark D. Leatherman, Steven A. Svejda, Lynda K. Johnson, and Maurice Brookhart* Contribution from the Department of Chemistry, UniVersity of North Carolina at Chapel Hill, CB# 3290 Venable Hall, Chapel Hill, North Carolina 27599-3290 Received August 12, 2002 ; E-mail:
[email protected] Abstract: The synthesis of a series of (R-diimine)NiR2 (R ) Et, nPr) complexes via Grignard alkylation of the corresponding (R-diimine)NiBr2 precursors is presented. Protonation of these species by the oxonium acid [H(OEt2)2]+[BAr′4]- at low temperatures yields cationic Ni(II) β-agostic alkyl complexes which model relevant intermediates present in nickel-catalyzed olefin polymerization reactions. The highly dynamic nature of these agostic alkyl cations is quantitatively addressed using NMR line broadening techniques. Trapping of these complexes with ethylene provides cationic Ni alkyl ethylene species, which are used to determine rates of ethylene insertion into primary and secondary carbon centers. The Ni agostic alkyl cations are also trapped by CH3CN and Me2S to yield Ni(R)(L)+ (L ) CH3CN, Me2S) complexes, and the dynamic behavior of these species in the presence of varied [L] is discussed. The kinetic data obtained from these experiments are used to present an overall picture of the ethylene polymerization mechanism for (R-diimine)Ni catalysts, including effects of reaction temperature and ethylene pressure on catalyst activity, polyethylene branching, and polymer architecture. Detailed comparisons of these systems to the previously presented analogous palladium catalysts are made.
Introduction
Significant recent research has been directed toward the design and application of catalyst systems based on late transition metals capable of effecting the polymerization of olefins to high molar mass homo- and copolymers.1-12 Reports from these laboratories13,14 and DuPont15-19 have explored Ni(1) Keim, W.; Appel, R.; Storeck, A.; Kruger, C.; Goddard, R. Angew. Chem., Int. Ed. Engl. 1981, 20, 116-117. (2) Mo¨hring, V. M.; Fink, G. Angew. Chem., Int. Ed. Engl. 1985, 24, 10011003. (3) Ostoja Starzewski, K. A.; Witte, J. Angew. Chem. 1985, 97, 610-612. (4) Klabunde, U.; Ittel, S. D. J. Mol. Catal. 1987, 41, 123-134. (5) Brookhart, M.; DeSimone, J. M.; Grant, B. E.; Tanner, M. J. Macromolecules 1995, 28, 5378-5380. (6) Small, B. L.; Brookhart, M.; Bennett, A. M. A. J. Am. Chem. Soc. 1998, 120, 4049-4050. (7) Small, B. L.; Brookhart, M. Macromolecules 1999, 32, 2120-2130. (8) Britovsek, G. J. P.; Gibson, V. C.; Kimberly, B. S.; Maddox, P. J.; McTravish, S. J.; Solan, G. A.; White, A. J. P.; Williams, D. J. Chem. Commun. 1998, 849-850. (9) Britovsek, G. J. P.; Gibson, V. C.; Wass, D. F. Angew. Chem., Int. Ed. 1999, 38, 429-447. (10) Younkin, T. R.; Connor, E. F.; Henderson, J. I.; Friedrich, S. K.; Grubbs, R. H.; Bansleben, D. A. Science 2000, 287, 460-462. (11) Hicks, F. A.; Brookhart, M. Organometallics 2001, 20, 3217-3219. (12) Ittel, S. D.; Johnson, L. K.; Brookhart, M. Chem. ReV. 2000, 100, 11691203. (13) Johnson, L. K.; Killian, C. M.; Brookhart, M. J. Am. Chem. Soc. 1995, 117, 6414-6415. (14) Gates, D. P.; Svejda, S. A.; On˜ate, E.; Killian, C. M.; Johnson, L. K.; White, P. S.; Brookhart, M. Macromolecules 2000, 33, 2320-2334. (15) Brookhart, M. S.; Johnson, L. K.; Killian, C. M.; Arthur, S. D.; Feldman, J.; McCord, E. F.; McLain, S. J.; Kreutzer, K. A.; Bennett, A. M. A.; Coughlin, E. B.; Ittel, S. D.; Parthasarathy, A.; Tempel, D. J. WO Patent Application 9623010 to DuPont, April 3, 1995. 3068
9
J. AM. CHEM. SOC. 2003, 125, 3068-3081
(II)- and Pd(II)-R-diimine systems that exhibit several unique features, including the copolymerization of ethylene and functionalized olefins,20-23 the oligomerization of ethylene and R-olefins,24-26 the living polymerization of ethylene and R-olefins,27,28 and the homopolymerization of cyclic16 and internal acyclic olefins.29 The microstructure of polyethylene produced by nickel(II)and palladium(II)-diimine catalysts varies from strictly linear (16) McLain, S. J.; Feldman, J.; McCord, E. F.; Gardner, K. H.; Teasley, M. F.; Coughlin, E. B.; Sweetman, K. J.; Johnson, L. K.; Brookhart, M. Macromolecules 1998, 31, 6705-6707. (17) Guan, Z.; Cotts, P. M.; McCord, E. F.; McLain, S. J. Science 1999, 283, 2059-2062. (18) Cotts, P. M.; Guan, Z.; McCord, E. F.; McLain, S. J. Macromolecules 2000, 33, 6945-6952. (19) McCord, E. F.; McLain, S. J.; Nelson, L. T. J.; Arthur, S. D.; Coughlin, E. B.; Ittel, S. D.; Johnson, L. K.; Tempel, D. J.; Killian, C. M.; Brookhart, M. Macromolecules 2001, 34, 362-371. (20) Johnson, L. K.; Mecking, S.; Brookhart, M. J. Am. Chem. Soc. 1996, 118, 267-268. (21) Mecking, S.; Johnson, L. K.; Wang, L.; Brookhart, M. J. Am. Chem. Soc. 1998, 120, 888-899. (22) Johnson, L.; Bennett, A.; Dobbs, K.; Hauptman, E.; Ionkin, A.; Ittel, S.; McCord, E.; McLain, S.; Radzewich, C.; Yin, Z.; Wang, L.; Wang, Y.; Brookhart, M. Polym. Mater. Sci. Eng. 2002, 86, 319. (23) McLain, S. J.; Sweetman, K. J.; Johnson, L. K.; McCord, E. F. Polym. Mater. Sci. Eng. 2002, 86, 320-321. (24) Killian, C. M.; Johnson, L. K.; Brookhart, M. Organometallics 1997, 16, 2005-2007. (25) Tempel, D. J. Ph.D. Dissertation, University of North Carolina at Chapel Hill, 1998. (26) Svejda, S. A.; Brookhart, M. Organometallics 1999, 18, 65-74. (27) Killian, C. M.; Tempel, D. J.; Johnson, L. K.; Brookhart, M. J. Am. Chem. Soc. 1996, 118, 11664-11665. (28) Gottfried, A. C.; Brookhart, M. Macromolecules 2001, 34, 1140-1142. (29) Leatherman, M. D.; Brookhart, M. Macromolecules 2001, 34, 2748-2750. 10.1021/ja021071w CCC: $25.00 © 2003 American Chemical Society
Nickel(II) Alkyl Agostic Cations
ARTICLES
Scheme 1. Proposed Mechanism of Ethylene Polymerization
to highly branched, while the microstructures of poly(R-olefins) produced by these same catalyst systems are characterized by fewer branches than are expected from 1,2-monomer enchainment (chain-straightening).13,14 The degree of branching in the polyethylene and the polymer architecture can depend on a number of factors: the metal, the steric bulk of the diimine ligand, and the reaction conditions. Polyethylenes produced by palladium(II)-diimine catalysts are generally highly branched (e.g., >70 methyls/1000 carbons) regardless of ligand structure or reaction conditions, although the precise polymer architecture varies from linear polyethylene with short chain branches, obtained at high ethylene pressures, to a more hyperbranched structure at lower ethylene concentrations.17 In contrast, polyethylenes prepared from nickel(II)-diimine catalysts display overall branching numbers that are quite variable. Polymerizations run at low temperatures and high ethylene pressure using nickel complexes bearing relatively nonbulky diimine ligands yield nearly linear polyethylene, while higher temperatures and/ or lower ethylene concentrations in combination with nickel complexes bearing sterically demanding diimine ligands result in more highly branched polyethylene.12-14,30 In the case of nickel-catalyzed polymerization of R-olefins, the degree of chain-straightening observed in the resulting poly(R-olefin)s is strongly affected by diimine ligand symmetry and reaction conditions.25 Palladium catalysts generally produce more chainstraightened poly(R-olefin) products than their nickel analogues, but unlike the nickel systems, the degree of branching seen in poly(R-olefin)s produced by the palladium complexes is relatively independent of ligand structure and reaction conditions. Mechanistic studies of ethylene polymerizations catalyzed by both nickel(II)- and palladium(II)-diimine complexes have led to the proposed polymerization mechanism shown in Scheme 1.13,25,31-35 Following initiation, the mechanism is characterized by chain propagation, chain isomerization, and chain transfer steps. To date, the chain propagation step is the best-studied portion of the overall mechanism. Low-temperature NMR studies of ethylene polymerization have established the catalyst (30) Killian, C. M. Ph.D. Dissertation, University of North Carolina at Chapel Hill, 1996. (31) Tempel, D. J.; Brookhart, M. Organometallics 1998, 17, 2290-2296. (32) Svejda, S. A.; Johnson, L. K.; Brookhart, M. J. Am. Chem. Soc. 1999, 121, 10634-10635. (33) Tempel, D. J.; Johnson, L. K.; Huff, R. L.; White, P. S.; Brookhart, M. J. Am. Chem. Soc. 2000, 122, 6686-6700. (34) Shultz, L. H.; Tempel, D. J.; Brookhart, M. J. Am. Chem. Soc. 2001, 123, 11539-11555. (35) Shultz, L. H.; Brookhart, M. Organometallics 2001, 20, 3975-3982.
Scheme 2. Isomerization Equilibria in (R-diimine)Pd Complexes
resting state for both metals under these conditions as the alkyl ethylene complexes, implying turnover-limiting migratory insertion of ethylene. Activation barriers to ethylene insertion have been measured for both palladium and nickel complexes. As previously reported, the activation barriers to ethylene insertion in the chain propagation step are 4-5 kcal/mol lower for the nickel complexes as compared to their palladium congeners.32 Mechanistic studies of the chain isomerization process have focused on (R-diimine)Pd(II) complexes. As established by these studies, alkyl chain isomerization occurs in the β-agostic species, which can be formed either after olefin insertion from the metal(alkyl)olefin resting state or by ethylene loss from the resting state. The barrier to interconversion of the resulting β-agostic species (via β-H elimination, olefin rotation, and reinsertion) is quite low (
