Imino-Amido Hf and Zr Complexes: Synthesis, Isomerization, and Olefin Polymerization

New hafnium and zirconium imino-amido complexes containing trimethylethylidene
bridging groups are reported. Investigation into the thermal stability
of these complexes revealed that they undergo facile rearrangement
of the ligand backbone at 80 °C to produce isomeric imino-amido
complexes. Mechanistic experiments and DFT calculations indicate that
this isomerization reaction proceeds via a direct 1,2-methyl shift
rather than a multistep mechanism involving the metal center. The
initial imino-amido complexes (pre isomerization) exhibit high ethylene/1-octene
copolymerization activities and generate polymers with very high molecular
weights and relatively low comonomer contents. On the other hand,
the isomerized complexes display significantly lower polymerization
activities and give polymers with lower molecular weights and broad
molecular weight distributions, indicative of multisite behavior.