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.

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 HO and TMA and tracking the first half hour of reactivity.

Spectroscopic Studies of Synthetic Methylaluminoxane: Structure of Methylaluminoxane Activators.

Hydrolysis of trimethylaluminum (Me Al) in polar solvents can be monitored by electrospray ionization mass spectrometry (ESI-MS) using the donor additive octamethyltrisiloxane [(Me SiO) SiMe , 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.

Step-by-step real time monitoring of a catalytic amination reaction.

The multiple reaction monitoring mode of a triple quadrupole mass spectrometer is used to examine the Buchwald-Hartwig amination reaction at 0.1% catalyst loading in real-time using sequential addition of reagents to probe the individual steps in the cycle. This is a powerful new method for probing reactions under realistic conditions.

Modifying methylalumoxane via alkyl exchange.

Methylalumoxane (MAO) ionizes highly selectively in the presence of octamethyltrisiloxane (OMTS) to generate [MeAl·OMTS] [(MeAlO)(MeAl)Me]. 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 (RAl) components. Using continuous injection methods, we found EtAl to exchange much faster and extensively at room temperature in fluorobenzene (t∼2 s, up to 25 exchanges of Me for Et) than iBuAl (t∼40 s, up to 11 exchanges) or OctAl (t∼200 s, up to 7 exchanges).

Synthesis, characterization and mass-spectrometric analysis of [LSn(IV)F] salts [L = tris ((1-ethyl-benzoimidazol-2-yl)methyl)amine, x = 1-4].

A series of complexes with the formulae [(BIMEt3)SnF4-x][OTf]x with x = 1-4 has been synthesized by successive fluoride abstraction from SnF4 with TMSOTf in the presence of the tetradentate nitrogen donor BIMEt3 (tris ((1-ethyl-benzoimidazol-2-yl)methyl)amine). Single crystal X-ray diffraction and heteronuclear NMR spectroscopic analysis provided insight into these new main group cations. Electrospray ionization mass spectrometric analysis on solutions containing the different salts allowed for successful detection of the cations [(BIMEt3)SnF]3+, [(BIMEt3)SnF2]2+ and [(BIMEt3)SnF3]+.

Oxidation of Methylalumoxane Oligomers.

The anions formed from methylalumoxane (MAO) and suitable donors (e.g. octamethyltrisiloxane) are amenable to mass spectrometric (MS) analysis.

Oxidation of Titanocene(III): The Deceptive Simplicity of a Color Change.

Reduction of red CpTiCl (Cp = cyclopentadienyl) with zinc dust in acetonitrile produces a blue solution of [CpTi(NCMe)], which when exposed to air rapidly discolors to bright yellow. This behavior makes the blue solution a handy visual indicator for the presence of oxygen, but the chemistry is considerably more complicated than the primary colors suggest at first glance. Real-time mass spectrometric and colorimetric analysis reveals that oxidation from Ti(III) to Ti(IV) produces a host of oxygen-containing complexes, whose appearance parallels the observed color changes.

"Masked" Lewis-acidity of an aluminum α-phosphinoamide complex.

The reaction of PhP(DIPP)NH with AlMe cleanly gives an aluminum amide complex that crystallizes as a centrosymmetric dimer with a six-membered Al-N-P-Al-N-P ring. In aromatic solvents the dimer remains intact but the Al-P bond is readily broken upon addition of THF to form PhP(DIPP)NAlMe·THF. Efforts to use [PhP(DIPP)NAlMe] as a "masked" Lewis acidic activator for olefin polymerization catalysts were unsuccessful but the complex showed a Frustrated Lewis pair reactivity instead.

Real-time analysis of Pd(dba) activation by phosphine ligands.

A combination of UV-Vis spectroscopy and electrospray ionization mass spectrometry is used for real-time monitoring of Pd(dba) activation with sulfonated versions of PPh and Buchwald-type ligands. This provides insight into the effect of ligand and preparation conditions on activation and allows for establishment of rational activation protocols.

Insight into Oxide-Bridged Heterobimetallic Al/Zr Olefin Polymerization Catalysts.

Reaction of (TBBP)AlMe⋅THF with [Cp* Zr(Me)OH] gave [(TBBP)Al(THF)-O-Zr(Me)Cp* ] (TBBP=3,3',5,5'-tetra-tBu-2,2'-biphenolato). Reaction of [DIPPnacnacAl(Me)-O-Zr(Me)Cp ] with [PhMe NH] [B(C F ) ] gave a cationic Al/Zr complex that could be structurally characterized as its THF adduct [(DIPPnacnac)Al(Me)-O-Zr(THF)Cp ] [B(C F ) ] (DIPPnacnac=HC[(Me)C=N(2,6-iPr -C H )] ). The first complex polymerizes ethene in the presence of an alkylaluminum scavenger but in the absence of methylalumoxane (MAO).