Hafnium Bis(phenoxyimino) Dibenzyl Complexes and Their Activation toward Olefin Polymerization

Hf bis(phenoxyimino) dibenzyl complexes bearing benzylimino and perfluorophenylimino groups were
prepared by a direct reaction between the desired ligand precursors and Hf(CH2Ph)4. In the solid state,
the Hf complexes adopt an octahedral configuration where imino nitrogens are in positions cis to each
other and oxygens occupy apical vertices of the coordination polyhedron. According to 1H NMR and
2D-NOESY studies, this cis-N,N configuration was also maintained in solution. In addition, the structures
of Hf bis(phenoxyimino) dibenzyl complexes were studied with DFT calculations. Activation of the Hf
dibenzyl complexes with B(C6F5)3 led to the highly air-, moisture-, solvent-, and temperature-sensitive
cationic bis(phenoxyimino) Hf monobenzyl species, which were further investigated by 1H and 19F NMR,
1H−13C heteronuclear correlation, 2D-NOESY, and ESI-MS methods. It was observed that the cationic,
benzylimino-substituted Hf complexes have a propensity toward CH activation. The correlation between
the stability of the cationic species toward CH activation and the catalytic behavior of the MAO-activated
dibenzyl complexes was established. Accordingly, the MAO-activated Hf bis(phenoxyimino) dibenzyl
complexes and their dichloro analogues, depending on the ligand substitution, have from low to very
high catalytic activities (up to 14 000 kg of PE/((mol of cat.) h (bar of ethylene))) in ethylene
polymerizations. In general, the produced polyethylene had a monomodal molar mass distribution with
relatively narrow polydispersity. Polypropylene produced with perfluorophenyl-substituted Hf complexes
had a large amount of 2,1-misinsertions in the polymer chains. To further shed light on the catalytic
reaction, polymerizations of ethylene and propylene with the cationic species generated from Hf bis(phenoxyimino) dibenzyl complexes with B(C6F5)3 were studied with 1H NMR.