Olefin insertion using ansa-zirconocenes and methylaluminoxane (MAO) involves Janus-like, sheet anions

Area of Science:

• Catalysis
• Polymer Chemistry
• Organometallic Chemistry

Background:

• Zirconocene complexes activated by methylaluminoxane (MAO) are crucial for industrial olefin polymerization.
• MAO, a vital activator, forms complex sheet-like anions with metallocene catalysts.
• Understanding the precise structure of these active species is essential for optimizing polymerization.

Purpose of the Study:

• To investigate the structural differences between two isomeric contact ion-pairs, 1a and 1b.
• To theoretically model the abundant sheet anion [(MeAlO)16(Me3Al)6Me]- found in MAO/metallocene mixtures.
• To correlate theoretical findings with experimental observations in olefin polymerization.

Main Methods:

• Computational theoretical studies were employed to model sheet anions.
• Propene insertion barriers were calculated for the isomeric ion-pairs 1a and 1b.
• Comparison of theoretical insertion barriers with experimental data.

Main Results:

• Two isomeric contact ion-pairs, 1a and 1b, were identified as active species.
• Significant differences in propene insertion barriers were found between 1a and 1b.
• Theoretical results for the more reactive isomer 1a closely matched experimental data.

Conclusions:

• The detailed structure of the sheet counter-anion significantly influences catalyst reactivity.
• Isomeric ion-pairs exhibit reactivity variations exceeding two orders of magnitude.
• These findings have critical implications for designing efficient polymerization catalysts.