Discrete Ionic Complexes of Highly Isoselective Zirconocenes. Solution Dynamics, Trimethylaluminum Adducts, and Implications in Propylene Polymerization

Discrete ionic complexes belonging to two main families of highly isoselective polymerization zirconocene systems, C1-symmetric {Cp/Flu} (a,b) and C2-symmetric {SBI} (c) systems, have been scrutinized. The ion pair reorganization processes for inner-sphere ion pairs (ISIPs) 3a,b-MeB(C6F5)3, 3c-MeB(C6F5)3, and 3c-MeAl(C6F5)3 and for the outer-sphere ion pair (OSIP) AlMe3 adducts [6b,c]+[B(C6F5)4]–, quantified by dynamic NMR analysis, were found to feature lower activation barriers for the {SBI}-based systems in comparison to those for the {Cp/Flu}-based congeners. The higher electrophilic character of the {Cp/Flu}-based cationic systems was corroborated by UV/vis spectroscopy studies coupled with TD-DFT calculations. These fundamental differences between the ionic systems of the two metallocene families are discussed in light of their respective propylene polymerization performances, and reasons for the higher productivity of {SBI} systems in comparison to {Cp-Flu} systems are proposed.

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