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

Abstract

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.