Dinickel Bisphenoxyiminato Complexes for the Polymerization of Ethylene and α-Olefins

Dinuclear nickel phenoxyiminato olefin polymerization
catalysts
based on rigid p-terphenyl frameworks are reported.
Permethylation of the central arene of the terphenyl unit and oxygen
substitution of the peripheral rings ortho to the aryl–aryl
linkages blocks rotation around these linkages, allowing atropisomers
of the ligand to be isolated. The corresponding syn and anti dinickel
complexes (25-s and 25-a) were synthesized
and characterized by single-crystal X-ray diffraction. These frameworks
limit the relative movement of the metal centers, restricting the
metal–metal distance. Kinetics studies of isomerization of
a ligand precursor (7-a) allowed the calculation of the
activation parameters for the isomerization process (ΔH⧧ = 28.0 ± 0.4 kcal ×
mol–1 and ΔS⧧ = −12.3 ± 0.4 cal mol–1 K–1). The reported nickel complexes are active
for ethylene polymerization (TOF up to 3700 (mol C2H4) (mol Ni)−1 h–1) and
ethylene/α-olefin copolymerization. Only methyl branches are
observed in the polymerization of ethylene, while α-olefins
are incorporated without apparent chain walking. These catalysts are
active in the presence of polar additives and in neat tetrahydrofuran.
The syn and anti isomers differ in polymerization activity, polymer
branching, and polymer molecular weight. For comparison, a series
of mononuclear nickel complexes (26, 27-s, 27-a, 28, 30) was prepared
and studied. The effects of structure and catalyst nuclearity on reactivity
are discussed.