Ligand exchange processes in zirconocene dichloride–trimethylaluminum bimetallic systems and their catalytic properties in reaction with alkenes

Abstract

Ligand exchange processes in the systems L₂ZrCl₂–AlMe₃ (L₂ZrCl₂: Cp₂ZrCl₂, (CpMe)₂ZrCl₂, (C₅Me₅)₂ZrCl₂, Me₂SiCp₂ZrCl₂, Me₂Si(C₅Me₄)₂ZrCl₂, rac-Me₂C(2-Me-4-Bu^t-Cp)₂ZrCl₂, meso-Me₂C(2-Me-4-Bu^t-Cp)₂ZrCl₂, rac-Me₂C(3-Bu^t-Cp)₂ZrCl₂, Ind₂ZrCl₂, rac-H₂C(Ind)₂ZrCl₂, rac-Me₂C(Ind)₂ZrCl₂, rac-Me₂Si(Ind)₂ZrCl₂, rac-C₂H₄(Ind)₂ZrCl₂, rac-C₂H₄(THInd)₂ZrCl₂, rac-Me₂Si(THInd)₂ZrCl₂) and Cp₂ZrMeCl_2−n–AlMe₃ (n = 1, 2) were studied by NMR spectroscopy with the goal to establish the structures and dynamic features of probable intermediates in the zirconocene-catalyzed reactions of alkenes with AlMe₃. The effect of solvent, the organoaluminum compound concentration and the addition of (ClAlMe₂)₂ on the activation parameters of the alkyl exchange in the trimethylaluminum dimer was studied as well. The constants and activation parameters of the methyl group exchange in the monoalkyl-substituted ansa-complexes L₂ZrMeCl (L₂ = rac-Me₂C(2-Me-4-Bu^t-Cp)₂, rac-Me₂C(3-Bu^t-Cp)₂, rac-H₂CInd₂, rac-Me₂CInd₂, rac-Me₂SiInd₂, rac-H₄C₂Ind₂) were established for the first time. The catalytic activity and chemoselectivity of zirconocenes in the reaction of alkenes with AlMe₃ were evaluated and compared with the exchange and equilibrium constants of ligand exchange processes. A mechanism of the reaction was proposed.