Articles Tridentate Cobalt Catalysts for Linear Dimerization and Isomerization of r-Olefins Brooke L. Small* Chevron Phillips Chemical Company LP, 1862 Kingwood Drive, Kingwood, Texas 77339 Received March 19, 2003 Upon activation with modified methylalumoxane (MMAO), tridentate pyridine bisimine cobalt catalysts dimerize R-olefins with lower productivity than analagous iron systems, as indicated by comparing the batch dimerization of 1-butene (TON ∼42 000 for Co, ∼147 000 for Fe). The cobalt-produced dimers are extremely linear (>97%) and contain only traces of trimeric species. The cobalt catalysts also have a tendency to isomerize R-olefins, as evidenced by the approximately equal levels of dimerization and isomerization achieved when 1-butene is dimerized. When the cocatalyst is changed to diethylaluminum chloride, isomerization occurs exclusively to give cis- and trans-2-olefins selectively. To mitigate the effects of feed isomerization, dimerization of propylene was also studied. GC analysis of the products reveals a stepwise oligomerization process that makes linear hexenes, nonenes, and dodecenes, with the hexenes comprising up to 70% of the product mix. The hexenes are over 99% linear and may contain over 50% 1-hexene. Catalyst productivity is high, with turnover numbers exceeding 200 000 mol propylene/mol Co (17 000 g oligomers/g Co complex). Introduction The dimerization of olefins by transition metal com- plexes represents an important class of industrially relevant chemistry. 1 For example, ethylene dimerization to 1-butene can provide a source of comonomer in the production of polyethylene, 2 and olefins such as propyl- ene and butene are dimerized to give C 6 -C 8 materials that serve as feedstocks for gasoline blending or alcohol production. 3 While most dimerization catalysts produce branched dimers from propylene and higher olefins, we recently reported a family of iron-based catalysts that make predominantly linear dimers (up to 80% linear- ity). 4 Due to a unique mechanism of dimerization in which the regiochemistry of olefin insertion changes from 1,2 to 2,1 between the first and second steps, 5 the major product is the head-to-head dimer (Scheme 1). The major byproduct in the reaction is the methyl- branched dimer, which results from two successive 2,1 insertions followed by chain termination. In addition, approximately 15% of the product is a lightly branched olefin trimer, which possesses an average of g1.2 branches per molecule. The final product in the reaction is the undimerized olefin substrate, which often contains several percent of isomerized material due to chain transfer following an initial 2,1 insertion (Scheme 2). In an attempt to expand this dimerization chemistry to other transition metals, several tridentate cobalt complexes were synthesized and tested for their dimer- ization ability. 6 Remarkably, some of these cobalt catalysts are much more selective for producing linear dimers than their iron analogues, while other systems are highly selective for isomerizing the starting mater- ial. Herein are reported the details of this cobalt catalyst study. Results and Discussion Cobalt complexes 1-4, shown in Figure 1, were synthesized by reported methods and tested for their ability to dimerize R-olefins. 7-9 In efforts to compare * Corresponding author. 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J. Organometallics 2001, 20, 5738. (b) Small, B. L.; Baralt, E. J.; Marcucci, A. J. (Chevron Phillips) U.S. Patent 6291733, 2001. (5) (a) Small, B. L.; Brookhart, M. Macromolecules 1999, 32, 2120. (b) Pellecchia, C.; Mazzeo, M.; Pappalardo, D. Macromol. Rapid Commun. 1998, 19, 651. (6) Cobalt complexes containing pyrrole ligands have previously been reported for the dimerization of ethylene to high purity 1-butene: Wu, A. U.S. Pat. 5414178 (Phillips Petroleum), 1995. (7) For specific ligand syntheses, see the following references: (a) Small, B. L.; Brookhart, M. J. Am. Chem. Soc. 1998, 120, 7143. (b) Alyea, E. C.; Merrell, P. H. Synth. React. Inorg. Metal-Org. Chem. 1974, 4 (6), 535. 3178 Organometallics 2003, 22, 3178-3183 10.1021/om030210v CCC: $25.00 © 2003 American Chemical Society Publication on Web 07/04/2003 Downloaded via HEBREW UNIV OF JERUSALEM on January 13, 2020 at 15:10:12 (UTC). 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