Mass Spectrometric Characterization of Methylaluminoxane‐Activated Metallocene Complexes

Abstract

Electrospray-ionization mass spectrometric studies of poly(methylaluminoxane) (MAO) in the presence of [Cp2 ZrMe2 ], [Cp2 ZrMe(Cl)], and [Cp2 ZrCl2 ] 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 (Me3 Al)y-z (Me2 AlCl)z Me](-) (z=1, 2, 3…︁) are observed using metal chloride complexes and under some conditions may predominate over their non-halogenated precursors [(MeAlO)x (Me3 Al)y Me](-) . 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 [Cp2 ZrCl2 ] when compared with [Cp2 ZrMe2 ]. 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 [Cp2 ZrCl2 ], 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.

Droplet Spray Ionization from a Glass Microscope Slide: Real-Time Monitoring of Ethylene Polymerization

“…Because of poor ion intensity by direct analysis of the catalytically active species in toluene, the mixture was diluted fourfold with acetonitrile to enable the acquisition of stable mass spectra 16 containing four cationic species: ion 2 (m/z 235), ion 3 (m/z 255), 2·MeCN (m/z 276) and 3·MeCN (m/z 296) (see Figure 2a). These or similar species have been observed by ESI-MS previously, [14][15][16][17][18] and were easily identified by CID (see Figure S3 in the Supporting Information) and by their isotopic patterns. Cp 2 ZrCl 2 /MAO mixtures are known to be air-sensitive, and Cp 2 ZrOH, m/z 237, overlaps with 2 (m/z 235) and distorts its isotope profile.…”

“…The signal for the [M+2] isotopomer of 2 was consistently enhanced by this artifact, which has been observed before. 18 It is not clear whether this compound was formed by hydrolysis in solution or in the gas-phase, and it is somewhat surprising that these sensitive ions can be observed at all by this open air technique. The clear and steady mass spectra demonstrated that a signal from the sample could be consistently acquired via the droplet spray.…”

“…Although this reaction system is quite complicated and still under debate, this catalytic mixture has been the subject of much inquiry, 10-13 including a few in-depth mass spectrometry studies. [14][15][16][17][18] The generally accepted mechanism is shown in Scheme 1. The ligand-exchange reaction of 1 with MAO, in excess, rapidly leads to the catalytically active species 2.…”

“…Furthermore, in very nice recent studies of zirconocene activation with MAO, 17,18 PhF was used as the solvent to monitor the reaction of Cp 2 ZrCl 2 with MAO by ESI-MS and study the chemistry of these intermediates in solution. Therefore, the utility of droplet spray for in situ analysis and real-time monitoring was demonstrated in PhF (see Figure 1b).…”

Stability and Unimolecular Reactivity of Palladate(II) Complexes [LnPdR3]− (L=Phosphine, R=Organyl, n=0 and 1)

“…Usóna nd co-workers prepared af ew examples of these compounds by the transmetallationo fP d II precursors with organometallic reagents in 1980. [11] As we [12] and others [13] have previously shown, in situ formed anionic organometallate complexes can be selectively probedb ye lectrospray-ionization mass spectrometry (ESI-MS). [10] Analogous reactions are well known for many other metals.…”