Formation and Structure of Hydrolytic Methylaluminoxane Activators.

Methylaluminoxane (MAO) activators have sheet structures which form ion-pairs on reaction of neutral donors such as octamethyltrisiloxane (OMTS). The ion-pairs can be detected by electrospray ionization mass spectrometry (ESI-MS) in polar media. The growth of these reactive precursors during hydrolysis of Me3 Al can be monitored using ESI-MS. Density functional theory, combined with numerical simulation of growth, indicates that this process involves rapid formation of low MW oligomers, followed by assembly of these species into low MW sheets. These can grow through further addition of low MW oligomers or by fusion into larger sheets. The mechanism of these growth processes leads to the prediction that even-numbered sheets should be favored, and this surprising result is confirmed by ESI-MS monitoring experiments of both activator growth and MAO aging.

Spectroscopic Studies of Synthetic Methylaluminoxane: Structure of Methylaluminoxane Activators.

Hydrolysis of trimethylaluminum (Me3 Al) in polar solvents can be monitored by electrospray ionization mass spectrometry (ESI-MS) using the donor additive octamethyltrisiloxane [(Me3 SiO)2 SiMe2 , OMTS]. Using hydrated salts, hydrolytic methylaluminoxane (h-MAO) features different anion distributions, depending on the conditions of synthesis, and different activator contents as measured by NMR spectroscopy. Non-hydrolytic MAO was prepared using trimethylboroxine. The properties of this material, which contains incorporated boron, differ significantly from h-MAO. In the case of MAO prepared by direct hydrolysis, oligomeric anions are observed to rapidly form, and then more slowly evolve into a mixture dominated by an anion with m/z 1375 with formula [(MeAlO)16 (Me3 Al)6 Me]- . Theoretical calculations predict that sheet structures with composition (MeAlO)n (Me3 Al)m are favoured over other motifs for MAO in the size range suggested by the ESI-MS experiments. A possible precursor to the m/z 1375 anion is a local minimum based on the free energy released upon hydrolysis of Me3 Al.

Are Methylaluminoxane Activators Sheets?

Density functional theory calculations on neutral sheet models for methylaluminoxane (MAO) indicate that these structures, containing 5-coordinate and 4-coordinate Al, are likely precursors to ion-pairs seen during the hydrolysis of trimethylaluminum (Me3 Al) in the presence of donors such as octamethyltrisiloxane (OMTS). Ionization by both methide ([Me]- ) and [Me2 Al]+ abstraction, involving this donor, were studied by polarizable continuum model calculations in fluorobenzene (PhF) and o-difluorobenzene (DFB) media. These studies suggest that low MW, 5-coordinate sheets ionize by [Me2 Al]+ abstraction, while [Me]- abstraction from Me3 Al-OMTS is the likely process for higher MW 4-coordinate sheets. Further, comparison of anion stabilities per mole of aluminoxane repeat unit (MeAlO)n , suggest that anions such as [(MeAlO)7 (Me3 Al)4 Me]- =[7,4]- are especially stable compared to higher homologues, even though their neutral precursors are unstable.

Competitive Ligand Exchange and Dissociation in Ru Indenyl Complexes.

Kinetic profiles obtained from monitoring the solution-phase substitution chemistry of [Ru(η5-indenyl)(NCPh)(PPh3)2]+ (1) by both electrospray ionization mass spectrometry and 31P{1H} NMR are essentially identical, despite an enormous difference in sample concentrations for these complementary techniques. These studies demonstrate dissociative substitution of the NCPh ligand in 1. Competition experiments using different secondary phosphine reagents provide a ranking of phosphine donor abilities at this relatively crowded half-sandwich complex: PEt2H > PPh2H ≫ PCy2H. The impact of steric congestion at Ru is evident also in reactions of 1 with tertiary phosphines; initial substitution products [Ru(η5-indenyl)(PR3)(PPh3)2]+ rapidly lose PPh3, enabling competitive re-coordination of NCPh. Further solution experiments, relevant to the use of 1 in catalytic hydrophosphination, show that PPh2H out-competes PPh2CH2CH2CO2Bu t (the product of hydrophosphination of tert-butyl acrylate by PPh2H) for coordination to Ru, even in the presence of a 10-fold excess of the tertiary phosphine. Additional information on relative phosphine binding strengths was obtained from gas-phase MS/MS experiments, including collision-induced dissociation experiments on the mixed phosphine complexes [Ru(η5-indenyl)PP'P″]+, which ultimately appear in solution during the secondary phosphine competition experiments. Unexpectedly, unsaturated complexes [Ru(η5-indenyl)(PR2H)(PPh3)]+, generated in the gas-phase, undergo preferential loss of PR2H. We propose that competing orthometallation of PPh3 is responsible for the surprising stability of the [Ru(η5-indenyl)(PPh3)]+ fragment under these conditions.

Handling considerations for the mass spectrometry of reactive organometallic compounds.

Mass spectrometry is a powerful tool in disparate areas of chemistry, but its characteristic strength of sensitivity can be an Achilles heel when studying highly reactive organometallic compounds. A quantity of material suitable for mass spectrometric analysis often represents a tiny grain or a very dilute solution, and both are highly susceptible to decomposition due to ambient oxygen or moisture. This complexity can be frustrating to chemists and analysts alike: the former being unable to get spectra free of decomposition products and the latter often being poorly equipped to handle reactive samples. Fortunately, many creative solutions to such problems have been developed. This review summarizes some key methods for handling reactive samples in conjunction with the various ionization methods most frequently employed for their analysis.

Real-time analysis of methylalumoxane formation.

Methylalumoxane (MAO), a perennially useful activator for olefin polymerization precatalysts, is famously intractable to structural elucidation, consisting as it does of a complex mixture of oligomers generated from hydrolysis of pyrophoric trimethylaluminum (TMA). Electrospray ionization mass spectrometry (ESI-MS) is capable of studying those oligomers that become charged during the activation process. We have exploited that ability to probe the synthesis of MAO in real time, starting less than a minute after the mixing of H2O and TMA and tracking the first half hour of reactivity. We find that the process does not involve an incremental build-up of oligomers; instead, oligomerization to species containing 12-15 aluminum atoms happens within a minute, with slower aggregation to higher molecular weight ions. The principal activated product of the benchtop synthesis is the same as that observed in industrial samples, namely [(MeAlO)16(Me3Al)6Me]-, and we have computationally located a new sheet structure for this ion 94 kJ mol-1 lower in Gibbs free energy than any previously calculated.

Reactive metallocene cations as sensitive indicators of gas-phase oxygen and water.

Analysis of highly reactive compounds at very low concentration in solution using electrospray ionization mass spectrometry requires the use of exhaustively purified solvents. It has generally been assumed that desolvation gas purity needs to be similarly high, and so most chemists working in this space have relied upon high purity gas. However, the increasing competitiveness of nitrogen generators, which provide gas purity levels that vary inversely with flow rate, prompted an investigation of the effect of gas-phase oxygen on the speciation of ions. The most reactive species studied, the reduced titanium complex [Cp2Ti(NCMe)2]+[ZnCl3]- and the olefin polymerization pre-catalyst [Cp2Zr(µ-Me)2AlMe2]+[B(C6F5)4]-, only exhibited detectable oxidation when they were rendered coordinatively unsaturated through in-source fragmentation. Computational chemistry allowed us to find the most plausible pathways for the observed chemistry in the absence of observed intermediates. The results provide insight into the gas-phase oxidation or hydrolysis of these reactive species.

Modifying methylalumoxane via alkyl exchange.

Methylalumoxane (MAO) ionizes highly selectively in the presence of octamethyltrisiloxane (OMTS) to generate [Me2Al·OMTS]+ [(MeAlO)16(Me3Al)6Me]-. We can take advantage of this transformation to examine the reactivity of a key component of MAO using electrospray ionization mass spectrometry (ESI-MS), and here we describe the reactivity of this pair of ions with other trialkyl aluminum (R3Al) components. Using continuous injection methods, we found Et3Al to exchange much faster and extensively at room temperature in fluorobenzene (t½âˆ¼2 s, up to 25 exchanges of Me for Et) than iBu3Al (t½âˆ¼40 s, up to 11 exchanges) or Oct3Al (t½âˆ¼200 s, up to 7 exchanges). The exchanges are reversible and the methyl groups on the cation are also observed to exchange with the added R3Al species. These results point to the reactive components of MAO having a structure that deviates significantly from the cage-like motifs studied to date.