Catalytic activity in olefin polymerization depends not only on the catalyst but also, crucially, on activator/alkylator/scavenger “packages.” Along with binary mixtures containing Lewis or Brønsted acids and Al-alkyl systems, methylaluminoxane (MAO), a still ill-defined oligomeric compound, is the only single-component cocatalyst known to fulfill all three roles effectively and simultaneously. Herein, we report a simple molecular alternative, Al-H-Al⁺[B(C₆F₅)₄]⁻, an unusual borate salt containing a homodinuclear Al-cation (Al-H-Al⁺ = [iBu₂(DMA)Al]₂(μ-H)⁺). Unlike the simpler [AliBu₂]⁺[B(C₆F₅)₄]⁻, this species is easily synthesizable and stable at room temperature. Importantly, Al-H-Al⁺[B(C₆F₅)₄]⁻ can be used as a stand-alone cocatalyst for molecular olefin polymerization catalysis, representing an unprecedented molecular activator able to completely activate dichloride metallocene and prototypical post-metallocene precatalysts. Furthermore, spectroscopic and polymerization studies suggest that Al-H-Al⁺ is the true activating species formed in situ in the binary cocatalyst [PhMe₂NH]⁺[B(C₆F₅)₄]⁻/AliBu₃. As little as 50 equiv of Al-H-Al⁺[B(C₆F₅)₄]⁻ are required for efficient catalyst activation and impurity scavenging, orders of magnitude below the amounts usually required with MAO or AliBu₃. The high, yet “tamed,” Lewis acidity of cationic Al-H-Al⁺ is likely responsible for the increased scavenging ability. Unlike MAO, the well-defined structure of Al-H-Al⁺[B(C₆F₅)₄]⁻ offers easy avenues for further tuning, making it the prototype of a promising cocatalyst family.

The established model cluster (AlOMe)₁₆(AlMe₃)₆ for methylaluminoxane (MAO) cocatalyst has been studied by density functional theory, aiming to rationalize the different behaviors of unmodified MAO and TMA-depleted MAO/BHT (TMA = trimethylaluminum; BHT = 2,6-di-tert-butyl-4-methylphenol), highlighted in previous experimental studies. The tendency of the three model Lewis acidic sites A–C to release neutral Al fragments (i.e., AlMe₂R; R = Me or bht) or transient aluminum cations (i.e., [AlMeR]⁺) has been investigated both in the absence and in the presence of neutral N-donors. Sites C are most likely responsible for the activation capabilities of TMA-rich MAO, but TMA depletion destabilizes them, possibly inducing structural rearrangements. The remaining sites A and B, albeit of lower Lewis acidity, should be still able to release cationic Al fragments when TMA-depleted modified MAOs are treated with N-donors (e.g. [AlMe(bht)]⁺ from MAO/BHT). These findings provide tentative interpretations for earlier observations of donor-dependent ionization tendencies of MAO and MAO/BHT and how TMA depleted MAOs can still be potent activators.