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)(y)Me]. 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.