Mechanism of Action of MAO's Molecular Cousin

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.