Ligand Steric and Electronic Effects on β-Ketiminato Neutral Nickel(II) Olefin Polymerization Catalysts

A series of novel neutral nickel complexes 3a–g and 4a–d bearing
the β-ketiminato
ligands [(2,6-iPr2C6H3)­NCHCHC­(R)­O]­Ni­(R′)­(L) (for 3a–g, R′ = Me, L = Py, and R = tBu (3a), Ph (3b), 1-naphthyl (3c), 9-anthryl
(3d), PhNMe2(p) (3e), PhOMe­(p) (3f), PhCF3(p) (3g); for 4a–d, R′ = Ph, L = PPh3, and tBu (4a), Ph (4b), 1-naphthyl (4c), 9-anthryl
(4d)) have been synthesized and characterized. The molecular
structures of 3b–d,f,g and 4a,c were further confirmed
by X-ray crystallographic analysis. These complexes were employed
in ethylene polymerization to systematically investigate ligand steric
and electronic effects on the catalytic properties, including activity,
molecular weight (MW), and branching number of the polyethylene obtained.
The complexes bearing more bulky ligands showed higher activities
and produced more branched polyethylene. Electron-deficient ligands
were found to increase the catalytic activity, decrease the MW, and
enhance the branching content of the polyethylene. In addition, phosphine
NiII–Ph complexes 4a–d proved to be more active than the corresponding pyridine NiII–Me complexes 3a–d, probably due to the easier dissociation of PPh3 relative
to a pyridine from a nickel center.