Modifying Methylalumoxane via Alkyl Exchange

Methylalumoxane (MAO) ionizes highly selectively in the presence of octamethyltrisiloxane (OMTS) to generate [Me2Al∙OMTS]+ [(MeAlO)16(Me3Al)6Me]. We can take advantage of this transformation to examine the reactivity of a key component of MAO using electrospray ionization mass spectrometry (ESI-MS), and here we describe the reactivity of this pair of ions with other trialkyl aluminum (R3Al) components. This alkyl exchange reaction bears relevance to the various modified methylalumoxanes (MMAOs) available, which differ from regular MAO in being adulterated with different alkyl groups. We found Et3Al to exchange much faster and extensively (t½ ~ 2 sec, up to 25 exchanges of Me for Et) than iBu3Al (t½ ~ 40 sec, up to 11 exchanges) or Oct3Al (t½ ~ 200 sec, up to 7 exchanges). The exchanges are reversible and the methyl groups on the cation are also observed to exchange with the added R3Al species. These studies are the first to offer concrete insights into the solution transformations of MAO.