Are Methylaluminoxane Activators Sheets?

Area of Science:

• Organometallic Chemistry
• Catalysis
• Computational Chemistry

Background:

• Methylaluminoxane (MAO) is a crucial co-catalyst in olefin polymerization.
• The precise structure and ionization mechanisms of MAO precursors remain incompletely understood.
• Understanding MAO's role is vital for optimizing Ziegler-Natta and metallocene catalysis.

Purpose of the Study:

• To investigate the structural properties of neutral methylaluminoxane (MAO) sheet models.
• To elucidate the ionization mechanisms leading to ion-pair formation during trimethylaluminum (Me3Al) hydrolysis in the presence of octamethyltrisiloxane (OMTS).
• To compare the stability of resulting anions and their neutral precursors.

Main Methods:

• Density functional theory (DFT) calculations were employed to model neutral MAO sheet structures.
• Polarizable continuum model (PCM) calculations in fluorobenzene (PhF) and o-difluorobenzene (DFB) were used to study ionization processes.
• Analysis of ion-pair formation via methide ([Me]-) and [Me2Al]+ abstraction was performed.

Main Results:

• DFT calculations identified MAO structures with 5-coordinate and 4-coordinate aluminum (Al) as likely precursors to ion-pairs.
• Ionization mechanisms were found to be dependent on MAO molecular weight and coordination number.
• Low molecular weight, 5-coordinate sheets favor [Me2Al]+ abstraction, while higher molecular weight, 4-coordinate sheets favor [Me]- abstraction from Me3Al-OMTS.

Conclusions:

• The study provides insights into the structural precursors and ionization pathways of MAO.
• Anion stability analysis suggests specific MAO-derived anions, like [7,4]-, exhibit significant stability.
• These findings contribute to a deeper understanding of MAO's role in catalytic systems and the stability of potential catalytic intermediates.