Mass Spectrometric Characterization of Methylaluminoxane

Electrospray ionization mass spectrometric studies of poly­(methylaluminoxane) (MAO) in the presence of Cp₂ZrMe₂, octamethyltrisiloxane (OMTS), or [Bu₄N]Cl in fluorobenzene solution are reported. The results concur with the hypothesis that MAO is partially ionized in sufficiently polar media, where the contact ion pairs are of the general formula [Me₂Al]­[(MeAlO)_x(AlMe₃)_yMe]. A limited number of compositions of this type are detected; the most abundant has x = 23 and y = 7, where the cation [Me₂Al­(OMTS)]⁺ (with m/z 293) and the anion (with m/z 1853) were detected in the presence of OMTS, which stabilizes the Me₂Al cation. The results demonstrate that MAO ionizes Cp₂ZrMe₂ by acting as a source of this electrophilic cation, giving rise to an ion pair featuring a large and weakly coordinating aluminoxanate counterion.