Pyridylamido Hafnium and Zirconium Complexes: Synthesis, Dynamic Behavior, and Ethylene/1-Octene and Propylene Polymerization Reactions

Hafnium and zirconium pyridylamido complexes were prepared by the reaction of deprotonated ligand with MCl₄ followed by alkylation with MeMgBr. ¹H NMR analysis of the isolated products revealed the presence of two complexes in each sample in about a 93:7 ratio. Both complexes were shown by 1D NOESY experiments to be in dynamic equilibrium with each other at ambient temperature. An analysis of the chemical shift differences between major and minor isomers together with low-temperature NOE experiments revealed that the difference between major and minor isomers is due to the rotation of the 2-iPr-phenyl group attached to the chiral center. Magnetization transfer experiments conducted at temperatures between 10 and 40 °C yielded ΔH^‡ and ΔS^‡ of 14.3(6) kcal/mol and −11(2) cal/mol·K, respectively. Density functional theory calculations resulted in very similar activation parameters. Additionally, this fluxional process was calculated to occur in the cationic species with a rate comparable to that of propylene propagation kinetics. Both complexes have been studied as procatalysts for ethylene/1-octene copolymerization and propylene polymerization reactions. ¹³C NMR analysis of polypropylene obtained under high-temperature polymerization conditions allowed for the unequivocal determination of the identity of a second 2,1-regioerror.