Mass Spectrometric Characterization of Methylaluminoxane-Activated Metallocene Complexes

• Published in: Chemistry : a European journal 2015-02, Vol.21 (7), p.2980-2991
• Main Authors: Trefz, Tyler K., Henderson, Matthew A., Linnolahti, Mikko, Collins, Scott, McIndoe, J. Scott
• Format: Article
• Language: English
• Subjects: Alkylation; Aluminum; Anions; Chemistry; Dichlorides; electrospray ionization; Ionization; Ions; mass spectrometry; Metallocenes; methylaluminoxane; polymerization; Precursors; Spectroscopy
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201405319

Electrospray‐ionization mass spectrometric studies of poly(methylaluminoxane) (MAO) in the presence of [Cp2ZrMe2], [Cp2ZrMe(Cl)], and [Cp2ZrCl2] in fluorobenzene (PhF) solution are reported. The results demonstrate that alkylation and ionization are separate events that occur at competitive rates in a polar solvent. Furthermore, there are significant differences in ion‐pair speciation that result from the use of metallocene dichloride complexes in comparison to alkylated precursors at otherwise identical Al/Zr ratios. Finally, the counter anions that form are dependent on the choice of precursor and Al/Zr ratio; halogenated aluminoxane anions [(MeAlO)x(Me3Al)y−z(Me2AlCl)zMe]− (z=1, 2, 3…︁) are observed using metal chloride complexes and under some conditions may predominate over their non‐halogenated precursors [(MeAlO)x(Me3Al)yMe]−. Specifically, this halogenation process appears selective for the anions that form in comparison to the neutral components of MAO. Only at very high Al/Zr ratios is the same “native” anion distribution observed when using [Cp2ZrCl2] when compared with [Cp2ZrMe2]. Together, the results suggest that the need for a large excess of MAO when using metallocene dichloride complexes is a reflection of competitive alkylation vs. ionization, the persistence of unreactive, homodinuclear ion pairs in the case of [Cp2ZrCl2], as well as a change in ion pairing resulting from modification of the anions formed at lower Al/Zr ratios. Models for neutral precursors and anions are examined computationally. Activation of [Cp2ZrCl2] by excess methylaluminoxane (MAO) results in extensive anion chlorination at low Al/Zr ratios, as was revealed by electrospray ionization mass spectrometry. Al/Zr ratios of ≥500:1 are required to observe the same anion distribution using [Cp2ZrMe2] and MAO at lower Al/Zr ratios. Furthermore, less MAO is needed to fully activate an alkylated metallocene catalyst (vs. unalkylated) under otherwise identical conditions.