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) has been studied by 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 is much higher than experiment for MAO-activated catalysts, though the calculated barriers do track with the coordinating ability of the MAO-based anion, as also suggested from the position of the Me3Al-binding equilibria. Binding of 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 formation of species like 2e or 3e from two components of MAO may explain the high activity seen for MAO-activated metallocene complexes at sufficiently high Al:Zr ratios.