Zirconium-alkyl isomerizations in zirconocene-catalyzed olefin polymerization : a density functional study

Abstract

The migration of a Zr center between adjacent carbon atoms of a Zr-bound alkyl group is investigated by a density functional study of alternative reaction paths available to a (C5H5)2Zr−alkyl cation. This migration is found to occur by the classical reaction route, i.e. by β-H transfer, olefin rotation, and reinsertion into the Zr−H bond without loss of the olefin ligand, rather than by a concerted Zr/H exchange. The activation barrier is determined to decrease from 75 kJ/mol for the degenerate isomerization of a zirconocene ethyl cation to 49 kJ/mol for the isomerization of a primary to a secondary zirconocene propyl cation and 41 kJ/mol for its back-reaction. It is further reduced to 40 kJ/mol for the isomerization of a primary to a tertiary zirconocene isobutyl cation and 31 kJ/mol for its reverse, which model the isomerization process competing with stereoregular chain growth in zirconocene-based catalysts. The facility with which the zirconocene isobutyl cation transfers its β-H atom is connected with a particularly soft stretching vibration of the agostic β-C−H bond. Substantial stabilization of positive charge at the β-carbon atom by two alkyl substituents appears to be the cause. Reaction paths by which an olefin ligand in an intervening isomerization intermediate can change its coordination from one enantioface to the other have also been identified.