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 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).