J. Scott McIndoe, PhD , University of Victoria
Methylaluminoxane (MAO) is a poorly-defined mixture of oligomers made by partially hydrolyzing trimethylaluminum. It is used to activates olefin polymerization catalysts by abstraction of a methyl ligand from the metal center to generate a cationic species with a vacant coordination site, capable of binding olefins and inserting them into the M-C bond. MAO is typically used in large excess.
Our studies of this complex mixture employed electrospray ionization mass spectrometry (ESI-MS) as the analytical tool of choice. However, this technique is not well-adapted to the analysis of extremely air- and moisture-sensitive materials, and the project involved the design and construction of a source housing with improved sealing and an exhaust whose diameter could be throttled back to allow a positive pressure to be maintained inside (thus preventing ingress of air). This new design, in conjunction with an adjacent inert atmosphere glovebox, allowed the convenient handling of MAO solutions and gave us the capability to perform extended analyses.
Analysis of unadulterated MAO provided only very weak signals, in keeping with the idea that these oligomers are neutral and hence invisible to ESI-MS (which requires ions to be pre-formed in solution). However, addition of charged adulterants provides a huge increase in ion current and the appearance of ions whose m/z, isotope pattern, and product ion MS/MS spectrum can all be measured. Particularly effective are the halides, which produce a broad range of species whose identity can be established with confidence, at least in terms of proportion of AlMe3 : Al : O : Me. We have written two programs to allow us to assign these spectra, because the combinatorial possibilities from many small components rapidly become unwieldy. Fortunately, the application of computing power and chemical intuition renders the problem tractable, and we have assigned all prominent species (relative intensity >20%, typically 20-30 ions per spectrum) in the spectra generated from adding trace amounts of halide ion to MAO. How confident are we that the species we see are real and not a function of decomposition during the ESI-MS experiment? Well, we have a nice confirmation of the extent of decomposition in the appearance of signals m/z 2 higher than the signal associated with the ion of interest; these are due to the reaction Al-Me + H2O -> Al-OH + CH4, a reaction confirmed by isotopic labeling of the water (addition of trace D2O results in peaks m/z 3 higher). Compromised samples show high abundance of these decomposition products, and +4, +6 etc species also appear, greatly complicating the appearance of the spectrum and reducing the signal-to-noise ratio considerably.
All species fall in the compositional range [Me1.27-1.59AlO0.73-0.88]n, matching very closely the literature values from 1H NMR studies of [Me1.4-1.5AlO0.75-0.8]n. The most abundant ion we observe is at m/z 1853, and its composition is [Me45Al30O23]-. More specifically, we also know it contains nine AlMe3 molecules, so it may be written [Me18Al21O23(AlMe3)9]-. Accounting for the MeAlO backbone as (MeAlO)18, the remainder is Al3O5, which also accounts for the negative charge (3 x 3 - 5 x 2 = -1). The formula may then be elaborated as [(Al3O5)(MeAlO)18(AlMe3)9]-. Mass spectrometry can take us no further structurally, but there are consistent features in the species we observe: they can be assigned the general formula [(Al2xO3x+1)(MeAlO)y(AlMe3)z]-, except when they contain a halide, in which case the formula is [(Al2O3)x(MeAlO)y(AlMe3)z + X]-. There are further limits on the relative values of x, y and z that give additional insight into the types of ions most likely to acquire a charge. Higher concentrations of halide result in decomposition of the higher molecular weight ions, with the appearance of a suite of species around m/z 600.
We were also interested in the hydrolysis process by which AlMe3 is transformed into MAO. We tracked this process by allowing slow infusion of water into an AlMe3 solution, and observed the slow build-up of higher-nuclearity species. Di-, tri-, tetraaluminum (and higher) methylaluminoxane compounds are observed to form, and their intensity appears to be a factor of both their abundance and their Lewis acidity (i.e. their propensity to associate with an anion to form an -ate complex). We repeated this reaction under a wide variety of conditions to test the generality of the aggregation processes observed.
The postdoctoral fellow hired in Feb 2010 continued until July 2011, when he found full-time employment as the Department of Chemistry's mass spectrometry professional. The MSc student (jointly co-supervised with Dr Irina Paci) hired May 2010 failed to pass his graduate coursework, and left at the end of December 2010. One of my senior PhD students worked on the project in her final year (thesis title "New Methodology for Probing Catalytic Reactions by ESI-MS"); she was responsible for the instrument modifications that led to the pressurized source being used for routine analysis in the project, and designed and implemented this as a permanent improvement on our mass spectrometer, and she graduated August 2011. She is now a postdoctoral fellow in the Department of Chemistry, University of Melbourne. The training received - a combination of mass spectrometric expertise and experience in handling very air-sensitive materials - and experience gained resulted in both of the principal contributors being sought-after in the job market. Collectively, we expect to publish three manuscripts from the work: one on our methodological improvements, one on our insights into MAO speciation, and one on the hydrolysis of AlMe3, and all three of these papers are in preparation. We plan to continue to pursue this area, as the new direction we've established in our research has proved to be successful and the results to date have raised further questions that require further work: notably, the ability to expressly target particular species of interest, in order to prove their significance as active co-catalysts.