Ligand Steric and Fluoroalkyl Substituent Effects on Enchainment Cooperativity and Stability in Bimetallic Nickel(II) Polymerization Catalysts

The synthesis and characterization of two neutrally charged bimetallic Ni(II) ethylene polymerization catalysts, {2,7-di-2,6-(3,5-di-methylphenylimino)methyl]1,8-naphthalenediolato}-bis-Ni(II) (methyl)(trimethylphosphine) (CH(3) )FI(2) -Ni(2) ] and {2,7-di-2,6-(3,5-di-trifluoromethyl-phenylimino)methyl]-1,8-naphthalenediolato}-bis-Ni(II) (methyl)(trimethyl-phosphine) (CF(3) )FI(2) -Ni(2) )], are reported. The diffraction-derived molecular structure of (CF(3) )FI(2) -Ni(2) reveals a Ni???Ni distance of 5.8024(5)??. In the presence of ethylene and Ni(COD)(2) or B(C(6) F(5) )(3) co-catalysts, these complexes along with their monometallic analogues 2-tert-butyl-6-((2,6-(3,5-dimethylphenyl)phenylimino)methyl)-phenolate]-Ni(II) -methyl(trimethylphosphine) (CH(3) )FI-Ni] and 2-tert-butyl-6-((2,6-(3,5-ditrifluoromethyl-phenyl)phenylimino)methyl)phenolato]-Ni(II) -methyl-(trimethylphosphine) (CF(3) )FI-Ni], produce polyethylenes ranging from highly branched M(w) =1400 oligomers (91?methyl branches per 1000?C) to low branch density M(w) =92?000 polyethylenes (7?methyl branches per 1000?C). In the bimetallic catalysts, Ni???Ni cooperative effects are evidenced by increased product polyethylene branching in ethylene homopolymerizations (～3× for (CF(3) )FI(2) -Ni(2) vs. monometallic (CF(3) )FI-Ni), as well as by enhanced norbornene co-monomer incorporation selectivity, with bimetallic (CH(3) )FI(2) -Ni(2) and (CF(3) )FI(2) -Ni(2) enchaining approximately three- and six-times more norbornene, respectively, than monometallic (CH(3) )FI-Ni and (CF(3) )FI-Ni. Additionally, (CH(3) )FI(2) -Ni(2) and (CF(3) )FI(2) -Ni(2) exhibit significantly enhanced thermal stability versus the less sterically encumbered dinickel catalyst {2,7-di-(2,6-diisopropylphenyl)imino]-1,8-naphthalenediolato}-bis-Ni(II) (methyl)(trimethylphosphine). The pathway for bimetallic catalyst thermal deactivation is shown to involve an unexpected polymerization-active intermediate, {2,7-di-2,6-(3,5-di-trifluoromethyl-phenylimino)methyl]-1-hydroxy,8-naphthalenediolato-Ni(II) (methyl)-(trimethylphosphine).