10634
J. Am. Chem. Soc. 1999, 121, 10634-10635
Scheme 18
Low-Temperature Spectroscopic Observation of
Chain Growth and Migratory Insertion Barriers in
(r-Diimine)Ni(II) Olefin Polymerization Catalysts
Steven A. Svejda, Lynda K. Johnson, and Maurice Brookhart*
Department of Chemistry, UniVersity of North Carolina at
Chapel Hill, Chapel Hill, North Carolina 27599-3290
ReceiVed June 10, 1999
Scheme 2
Access to well-defined transition metal olefin polymerization
catalysts has allowed detailed mechanistic studies of this important
class of catalysts. While much attention has centered on early
transition metal d0 and lanthanide d0fn systems,1-7 we have focused
on developing single-site late metal catalysts. Our initial report8
that Ni(II) and Pd(II) aryl-substituted R-diimine complexes bearing
bulky ortho substituents were capable of polymerizing ethylene
and R-olefins has stimulated extensive experimental9-20 and
theoretical21-27 work on these catalysts. The Pd(II) systems yield
highly branched polyethylene under typical polymerization condi-
tions, while the microstructure of polyethylene produced by the
Ni(II) systems is dependent on monomer concentration, reaction
temperature, and ligand structure.
Extensive NMR spectroscopic studies of Pd(II) complexes8,28
(1) Bochmann, M. J. Chem. Soc., Dalton Trans. 1996, 255-270.
(2) Kaminsky, W.; Arndt, M. Polymerization, Oligomerization, and Co-
polymerization of Olefins. In Applied Homogeneous Catalysis with Organo-
metallic Compounds; Cornils, B., Herrmann, W. A., Eds.; VCH: New York,
1996; Vol. 1, pp 220-236.
(3) Brintzinger, H. H.; Fischer, D.; Mulhaupt, R.; Rieger, B.; Waymouth,
R. M. Angew. Chem., Int. Ed. Engl. 1995, 34, 1143-1170.
(4) Coates, G. W.; Waymouth, R. M. Science 1995, 267, 217-219.
(5) Yang, X.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1994, 116,
10015-10031.
(6) Coughlin, E. B.; Bercaw, J. E. J. Am. Chem. Soc. 1992, 114, 7606-
7607.
have established that (1) the catalyst resting state is an alkyl olefin
complex and chain growth is dependent only on the rate of
migratory insertion, (2) the barriers to migratory insertion are in
the range 16.9-17.6 kcal/mol, and (3) the alkyl cation formed
following insertion is a â-agostic complex and Pd can rapidly
migrate along the chain via â-hydride elimination/reinsertion
reactions. This process is responsible for the formation of branches
in the polyethylene produced (see Scheme 1)..
For use in polymerizations, analogous Ni-based cationic alkyl
complexes are generated by the activation of nickel dihalide
complexes with methylaluminoxane (MAO). The difficulty in
preparation and the high reactivity of cationic nickel alkyl
complexes has to date precluded mechanistic studies similar to
those reported for the palladium systems. Here we describe the
(7) Crowther, D. J.; Baenziger, N. C.; Jordan, R. F. J. Am. Chem. Soc.
1991, 113, 1455-1457.
(8) Johnson, L. K.; Killian, C. M.; Brookhart, M. J. Am. Chem. Soc. 1995,
117, 6414-6415.
(9) Galland, G. B.; Souza, R. F. d.; Mauler, R. S.; Nunes, F. F.
Macromolecules 1999, 32, 1620-1625.
(10) Schleis, T.; Spaniol, T. P.; Okuda, J.; Heinemann, J.; Mulhaupt, R. J.
Organomet. Chem. 1998, 569, 159-167.
preparation of (R-diimine)Ni(CH3)(solvent)+BAr′4 (BAr′4
≡
-
-
B(3,5-C6H3(CF3)2)4-) salts and the utilization of these complexes
to study the mechanistic details of the chain growth process which
allows a quantitative comparison of the Ni and Pd systems.
The (R-diimine)NiMe2 complexes 2a,b are the precursors to
the cationic nickel methyl complexes. These dimethyl complexes
are air-sensitive and thermally unstable at 25 °C.29 Isolation of
the clean complexes can be accomplished by rapid filtration
through Florisil at -78 °C.30 Protonation of dimethyl complexes
2a,b with [H+(OEt2)2][BAr′4-] yields the cationic ether adducts
3 (Scheme 2). Rigorous exclusion of water is required to avoid
the facile formation of aquo adducts 4. Addition of stoichiometric
amounts of water to CH2Cl2 solutions of ether adducts 3 at -78
°C results in quantitative conversion to the aquo adducts 4.
The kinetics of chain growth were monitored by generation of
the nickel methyl ethylene complexes, 5a,b, from the solvent
adducts in CDCl2F, and the sequence of migratory insertion
reactions was followed by low-temperature 1H NMR spectroscopy
as the alkyl chain grew (Scheme 3). Clean generation of the
methyl ethylene complex 5a from 3a is not straightforward since
migratory insertion of 5a is competitive with associative displace-
(11) Yang, K.; Lachicotte, R. J.; Eisenberg, R. Organometallics 1998, 17,
5102-5113.
(12) Ganis, P.; Orabona, I.; Ruffo, F.; Vitagliano, A. Organometallics 1998,
17, 2646-2650.
(13) Pellecchia, C.; Zambelli, A.; Mazzeo, M.; Pappalardo, D. J. Mol. Catal.
1998, 128, 229-237.
(14) Zeng, X.; Zetterberg, K. Macromol. Chem. Phys. 1998, 199, 2677-
2681.
(15) Carfagna, C.; Formica, M.; Gatti, G.; Musco, A.; Pierleoni, A. Chem.
Commun. 1998, 1113-1114.
(16) Fusto, M.; Giordano, F.; Orabona, I.; Ruffo, F.; Panunzi, A. Orga-
nometallics 1997, 16, 5981-5987.
(17) Yang, K.; Lachicotte, R. J.; Eisenberg, R. Organometallics 1997, 16,
5234-5243.
(18) Pappalardo, D.; Mazzeo, M.; Pellecchia, C. Macromol. Rapid Commun.
1997, 18, 1017-1023.
(19) Pellecchia, C.; Zambelli, A.; Oliva, L.; Pappalardo, D. Macromolecules
1996, 29, 6990-6993.
(20) Pellecchia, C.; Zambelli, A. Macromol. Rapid Commun. 1996, 17,
333-338.
(21) Froese, R. D. J.; Musaev, D. G.; Morokuma, K. J. Am. Chem. Soc.
1998, 120, 1581-1587.
(22) Musaev, D. G.; Froese, R. D. J.; Morokuma, K. Organometallics 1998,
17, 1850-1860.
(23) Deng, L.; Woo, T. K.; Cavallo, L.; Margl, P. M.; Ziegler, T. J. Am.
Chem. Soc. 1997, 119, 6177-6186.
(24) Musaev, D. G.; Froese, R. D. J.; Svensson, M.; Morokuma, K. J. Am.
Chem. Soc. 1997, 119, 367-374.
(25) Musaev, D. G.; Svensson, M.; Morokuma, K.; Stromberg, S.;
Zetterberg, K.; Siegbahn, P. E. M. Organometallics 1997, 16, 1933-1945.
(26) Deng, L.; Margl, P. M.; Ziegler, T. J. Am. Chem. Soc. 1997, 119,
1094-1100.
(28) Tempel, D. J.; Brookhart, M. Organometallics 1998, 17, 2290-2296.
(29) Solutions of 2 in CD2Cl2 eliminate ethane readily above 0 °C.
(30) The nickel dimethyl complexes were prepared using a procedure
similar to that earlier reported: Svoboda, M.; tom Dieck, H. J. Organomet.
Chem. 1980, 191, 321-328. See Supporting Information for experimental
details.
(27) Musaev, D. G.; Froese, R. D. J.; Morokuma, K. New J. Chem. 1997,
21, 1269-1282.
10.1021/ja991931h CCC: $18.00 © 1999 American Chemical Society
Published on Web 10/27/1999