A cooperative model for metallocene catalyst activation by methylaluminoxane

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 <i>via</i> DFT. These activators can reversibly form an outer-sphere ion-pair (OSIP) [SBIZrMe₂AlMe₂] [(MeAlO)_<i>n</i>(Me₃Al)_<i>m</i>Me] 3 ([<i>n</i>,<i>m</i>]^- = [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-<i>n</i>,<i>m</i> (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 <i>vs.</i> [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).