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) [(CH₃)FI²-Ni₂] and {2,7-di-[2,6-(3,5-di-trifluoromethyl-phenylimino)methyl]-1,8-naphthalenediolato} -bis-Ni^II(methyl)(trimethyl-phosphine) [(CF₃)FI ²-Ni₂)], are reported. The diffraction-derived molecular structure of (CF₃)FI²-Ni₂ reveals a Ni⋯Ni distance of 5.8024(5) Å. In the presence of ethylene and Ni(COD)₂ or B(C₆F₅)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) [(CH₃)FI-Ni] and [2-tert-butyl-6-((2,6-(3, 5-ditrifluoromethyl-phenyl)phenylimino)methyl)phenolato]-Ni^II-methyl- (trimethylphosphine) [(CF₃)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 (CF₃)FI²-Ni ₂ vs. monometallic (CF₃)FI-Ni), as well as by enhanced norbornene co-monomer incorporation selectivity, with bimetallic (CH ₃)FI²-Ni₂ and (CF₃)FI ²-Ni₂ enchaining approximately three- and six-times more norbornene, respectively, than monometallic (CH₃)FI-Ni and (CF ₃)FI-Ni. Additionally, (CH₃)FI²-Ni₂ and (CF₃)FI²-Ni₂ 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.