Methylaluminoxane’s Molecular Cousin: A Well-defined and “Complete” Al-Activator for Molecular Olefin Polymerization Catalysts

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.