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) Å. In the presence of ethylene and Ni(COD) ₂ or B(C ₆F 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). Ni-ce cats: Two neutrally charged bimetallic phenoxyiminato Ni ^II ethylene polymerization catalysts are reported. Significant Ni⋯Ni cooperative effects are shown by increased product polyethylene branching in ethylene homopolymerizations and by enhanced norbornene co-monomer incorporation selectivity in ethylene-norbornene copolymerization compared with monometallic controls.