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.