A cooperative model for metallocene catalyst activation by methylaluminoxane

Abstract

Activation of rac-Me₂Si(η⁵-Ind)₂ZrMe₂ (SBIZrMe₂) and sheet models for MAO, (MeAlO)₆(Me₃Al)₄ (6,4), (MeAlO)₇(Me₃Al)₅ (7,5), and (MeAlO)₂₆(Me₃Al)₉ (26,9) was studied via DFT. These activators can reversibly form an outer-sphere ion-pair (OSIP) [SBIZrMe₂AlMe₂] [(MeAlO)_n(Me₃Al)_mMe] 3 ([n,m]⁻ = [7,4]⁻and [26,8]⁻) or a contact ion-pair (CIP) SBIZrMe-μ-Me-6,4 (2b) from SBIZrMe₂. Dissociation of Me₃Al from 3 to form CIP SBIZrMe-μ-Me-n,m (2) is generally unfavourable but reversible in toluene continuum. Propene insertion involving CIP 2 features uniformly high barriers of 90–100 kJ mol⁻¹, which are much higher than those experimentally observed for MAO-activated catalysts, though the calculated barriers do track with the coordinating ability of the MAO-based anion, as also suggested by the position of the Me₃Al-binding equilibria. The binding of the neutral sheet 6,4 to anion [7,4]⁻ leads to a hybrid anion [13,8]⁻. The barrier to propene insertion involving CIP SBIZrMe-μ-Me-13,8 (2e) is lower than 60 kJ mol⁻¹. Formation of [SBIZrMe₂AlMe₂][13,8] (3e) from SBIZrMe₂, 7,5 and 6,4 is favorable, though dissociation into 2e and ½ Al₂Me₆ is not. Simulations of catalyst speciation vs. [Al] at constant [Zr] indicate that the formation of species such as 2e or 3e from two components of MAO explains the high activity observed for MAO-activated metallocene complexes at sufficiently high Al : Zr ratios. Dedicated to Walter Kaminsky (1941–2024).