Modifying methylalumoxane via alkyl exchange

Abstract

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. Using continuous injection methods, we found Et₃Al to exchange much faster and extensively at room temperature in fluorobenzene (t_½ ∼2 s, up to 25 exchanges of Me for Et) than iBu₃Al (t_½ ∼40 s, up to 11 exchanges) or Oct₃Al (t_½ ∼200 s, 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 results point to the reactive components of MAO having a structure that deviates significantly from the cage-like motifs studied to date.