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

A series of novel neutral nickel complexes <b>3a</b>–<b>g</b> and <b>4a</b>–<b>d</b> bearing the β-ketiminato ligands [(2,6-ⁱPr₂C₆H₃)­NCHCHC­(R)­O]­Ni­(R′)­(L) (for <b>3a</b>–<b>g</b>, R′ = Me, L = Py, and R = ^tBu (<b>3a</b>), Ph (<b>3b</b>), 1-naphthyl (<b>3c</b>), 9-anthryl (<b>3d</b>), PhNMe₂(<i>p</i>) (<b>3e</b>), PhOMe­(<i>p</i>) (<b>3f</b>), PhCF₃(<i>p</i>) (<b>3g</b>); for <b>4a</b>–<b>d</b>, R′ = Ph, L = PPh₃, and ^tBu (<b>4a</b>), Ph (<b>4b</b>), 1-naphthyl (<b>4c</b>), 9-anthryl (<b>4d</b>)) have been synthesized and characterized. The molecular structures of <b>3b</b>–<b>d</b>,<b>f</b>,<b>g</b> and <b>4a</b>,<b>c</b> 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 Ni^II–Ph complexes <b>4a</b>–<b>d</b> proved to be more active than the corresponding pyridine Ni^II–Me complexes <b>3a</b>–<b>d</b>, probably due to the easier dissociation of PPh₃ relative to a pyridine from a nickel center