Mechanism of Action of MAO’s Molecular Cousin

Abstract

The aluminum-alkyl borate (AAB) salt {[iBu₂(DMA)Al]₂(μ-H)}⁺[B(C₆F₅)₄]⁻ (AlHAl_DMA; DMA = N,N-dimethylaniline) is able of fully activating dichloride precatalysts for olefin polymerization and serving as an impurity scavenger, thus deserving to be called a molecular cousin of the well-established methylaluminoxane (MAO). With respect to MAO, it offers the advantage of having a well-defined molecular structure, which was exploited herein to investigate its mechanism of action as a cocatalyst. Particularly, the reaction of the precatalyst (Me₂SiCp₂)ZrCl₂ with AlHAl_DMA and with stable [AliBu₂(L)]⁺, modeling the putative abstracting species [AliBu₂(DMA)]⁺, was studied. The latter reaction led to the isolation of a rare, singly bridged Zr–(μ-Cl)–Al heterodinuclear adduct (2), which is a plausible intermediate of chloride abstraction from the precatalyst. Addition of di-iso-butylaluminum hydride (DIBAL-H) to 2 yielded a mixture of several multinuclear Zr/Al adducts with bridging μ-Cl and μ-H fragments (3–6), which were fully characterized by in-depth 2D NMR spectroscopy. Analogous products were observed in the reaction between (Me₂SiCp₂)ZrCl₂ and AlHAl_DMA, reinforcing the hypothesis that they are intermediates of chloride/hydride exchange, which generates a polymerization-active Zr–H species. The solid-state structure of [(Me₂SiCp₂)Zr]₂(μ-H)(μ-Cl)(μ² -iBu₂AlH₂) (5) was determined by single-crystal X-ray diffraction. The presence of the μ-H fragment in AlHAl_DMA appears to be relevant also for determining the excellent impurity scavenging properties of this cocatalyst, as it was found to react more rapidly than Al–iBu moieties upon exposure of solutions of this cocatalyst to atmospheric oxygen and moisture.