Mass spectrometric characterization of methylaluminoxane-activated metallocene complexes

Sheet Models for Methylaluminoxane (MAO) Activators? A Theoretical Case Study involving rac-Me2Si(η5-C9H6)2Zr (SBIZr) Complexes

Activation of SBIZrMe2 or SBIZrMeCl and a sheet model for an active component of hydrolytic MAO, (MeAlO)16(Me3Al)6, (16,6) has been studied by DFT. Contact ion-pair formation occurs through the intermediacy of SBIZrMe(Cl) or SBIZrMe2 reacting with sheet 16,6 to furnish SBIZrMe-μ-X(MeAlO)16(Me3Al)6 (2, X=Me, Cl). Contact ion-pairs 2 would be in equilibrium with heterodinuclear catalyst precursors [SBIZrMe2AlMe2][(MeAlO)16(Me3Al)6X] (3 (X=Me, Cl) through reversible binding of Me3Al at higher Al : Zr ratios. Calculations show that formation of ion-pairs 3 from contact ion-pairs 2 is more favourable for the SBIZr compared with the parent Cp2Zr complexes. TD-DFT calculations were conducted on relevant SBIZr complexes to relate the results to earlier spectroscopic studies of catalyst activation using UV-Vis spectroscopy. Finally, propene insertion into ion-pairs 2, SBIZrMe-μ-MeB(C6F5)3 (6) and [SBIZrMe][B(C6F5)4] (7) was studied at M06-2X/TZVP level of theory. These studies suggest that contact ion-pairs 2 are significantly less reactive towards insertion than 6 or 7, in disagreement with experiment.

Cages versus Sheets: A Critical Comparison in the Size Range Expected for Methylaluminoxane (MAO)

New cage models (MeAlO)n (Me3 Al)m (n=16, m=6 or 7) isomeric with previously reported sheet models for the principle activator found in hydrolytic MAO (h-MAO) are compared at M06-2X and MN15 levels of theory using density functional theory with respect to their thermodynamic stability. Reactivity of the neutrals or corresponding anions with formula [(MeAlO)16 (Me3 Al)6 Me]- towards chlorination, and loss of Me3 Al is explored while reactivity of the neutrals towards formation of contact- and outer-sphere ion pairs from Cp2 ZrMe2 and Cp2 ZrMeCl is examined. The results suggest on balance that a cage model for this activator is less consistent with experiment than an isomeric sheet model, although the latter are more stable based on free energy.

Ionization of Cp2 ZrMe2 and Lewis Bases by Methylaluminoxane: Computational Insights

The interactions of the Lewis bases CO, octamethyltrisiloxane (OMTS) and 2,2'-bipyridine (bipy) with a sheet model for the principal activator (MeAlO)₁₆ (Me₃ Al)₆ (16,6) in hydrolytic methylaluminoxane (MAO) were investigated by DFT. These studies reveal that OMTS and bipy form adducts with Me₃ Al prior to methide abstraction by 16,6 to form the ion-pairs [Me₂ Al(κ² -L)][16,6] (5: L=OMTS, 6: L=bipy, [16,6]^- =[(MeAlO)₁₆ (Me₃ Al)₆ Me]^- ) while CO simply binds to a reactive edge site without ionization. The binding and activation of Cp₂ ZrMe₂ with 16,6 to form both neutral adducts 1 Cp₂ ZrMe₂ ⋅16,6 and contact ion-pairs 4 and 7, both with formula [Cp₂ ZrMe][μ-Me(MeAlO)₁₆ (Me₃ Al)₆ ], featuring terminal and chelated MAO-anions, respectively was studied by DFT. The displacement of the anion with either excess Cp₂ ZrMe₂ or Me₃ Al was also studied, forming outer-sphere ion-pairs [(Cp₂ ZrMe)₂ μ-Me][16,6] (2) and [Cp₂ Zr(μ-Me)₂ AlMe₂ ][16,6] (3). The theoretical NMR spectra of these species were compared to experimental spectra of MAO and Cp₂ ZrMe₂ and found to be in good agreement with the reported data and assignments. These studies confirm that 16,6 is a very suitable model for the activators present in MAO but highlight the difficulty in accurately calculating thermodynamic quantities for molecules in this size regime.

In-Situ Analysis of Anionic Coordination Polymerizations by Electrospray-Ionization Mass Spectrometry

Anionic coordination polymerizations proceed via highly reactive intermediates, whose in situ analysis has remained difficult. Here, we show that electrospray-ionization mass spectrometry is a promising method to obtain detailed information on the polymerization process. Focusing on polymerization reactions of 1,3-dienes initiated by CoCl₂ /RLi (R=Me, nBu, tBu, Ph), we directly observe the growing polymer chains and characterize the active anionic cobalt centers by gas-phase fragmentation experiments. On the basis of these results, we suggest a plausible mechanism for the polymerization reaction. Moreover, the ESI mass spectra permit the determination of molecular weight distributions, which are in good agreement with those derived from NMR-spectroscopic as well as MALDI mass-spectrometric measurements, and afford a wealth of kinetic data.

Transition Metal-(μ-Cl)-Aluminum Bonding in α-Olefin and Diene Chemistry

Olefin and diene transformations, catalyzed by organoaluminum-activated metal complexes, are widely used in synthetic organic chemistry and form the basis of major petrochemical processes. However, the role of M−(μ-Cl)−Al bonding, being proven for certain >C=C< functionalization reactions, remains unclear and debated for essentially more important industrial processes such as oligomerization and polymerization of α-olefins and conjugated dienes. Numerous publications indirectly point at the significance of M−(μ-Cl)−Al bonding in Ziegler−Natta and related transformations, but only a few studies contain experimental or at least theoretical evidence of the involvement of M−(μ-Cl)−Al species into catalytic cycles. In the present review, we have compiled data on the formation of M−(μ-Cl)−Al complexes (M = Ti, Zr, V, Cr, Ni), their molecular structure, and reactivity towards olefins and dienes. The possible role of similar complexes in the functionalization, oligomerization and polymerization of α-olefins and dienes is discussed in the present review through the prism of the further development of Ziegler−Natta processes and beyond.

Thermodynamics of metallocene catalyst activation: alignment of theory and experiment

Three equilibria involved in metallocene catalyst activation, including dissociation of R₆Al₂ (R = Me, Et or i-Bu) and related species such as [L₂ZrMe₂AlMe₂][B(C₆F₅)₄] (L₂ = Cp₂, 1,2-ethylenebis(η⁵-indenyl), Me₂C(η⁵-C₅H₄)₂) or [(L₂ZrMe)₂μ-Me][MePBB] (L₂ = (h⁵-1,2-Me₂C₅H₃)₂, [MePBB]^- = [MeB(Ar_F)₃]^- with Ar_F = o-C₆F₅-C₆F₄) are studied by DFT using various approaches to account for the enthalpy and entropy changes in gas and condensed phases. These studies reveal that both low energy vibrations and translational entropy conspire to cause significant deviations between theory and experiment when it comes to the free energy change in condensed or even gas phase. Alignment of theory with experiment requires in addition, consideration of specific solvation of reactants and products.

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.

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.

Modular Ion Mobility Calibrants for Organometallic Anions Based on Tetraorganylborate Salts

Organometallics are widely used in catalysis and synthesis. Their analysis relies heavily on mass spectrometric methods, among which traveling-wave ion mobility spectrometry (TWIMS) has gained increasing importance. Collision cross sections (CCS) obtainable by TWIMS significantly aid the structural characterization of ions in the gas phase, but for organometallics, their accuracy has been limited by the lack of appropriate calibrants. Here, we propose tetraorganylborates and their alkali-metal bound oligomers [M_n-1(BR₄)_n]^- (M = Li, Na, K, Rb, Cs; R = aryl, Et; n = 1-6) as calibrants for electrospray ionization (ESI) TWIMS. These species chemically resemble typical organometallics and readily form upon negative-ion mode ESI of solutions of alkali-metal tetraorganylborates. By combining different tetraorganylborate salts, we have generated a large number of anions in a modular manner and determined their CCS values by drift-tube ion mobility spectrometry (DTIMS) (^DTCCS_He = 81-585, ^DTCCS_N2 = 130-704 Ų). In proof-of-concept experiments, we then applied these ^DTCCS values to the calibration of a TWIMS instrument and analyzed phenylcuprate and argentate anions, [Li_n-1M_nPh_2n]^- and [M_nPh_n+1]^- (M = Cu, Ag), as prototypical reactive organometallics. The ^TWCCS_N2 values derived from TWIMS measurements are in excellent agreement with those determined by DTIMS (<2% relative difference), demonstrating the effectiveness of the proposed calibration scheme. Moreover, we used theoretical methods to predict the structures and CCS values of the anions considered. These predictions are in good agreement with the experimental results and give further insight into the trends governing the assembly of tetraorganylborate, cuprate, and argentate oligomers.

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 (Me₃Al) in the presence of donors such as octamethyltrisiloxane (OMTS). Ionization by both methide ([Me]⁻) and [Me₂Al]⁺ 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 [Me₂Al]⁺ abstraction, while [Me]⁻ abstraction from Me₃Al‐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(Me₃Al)₄Me]⁻=[7,4]⁻ are especially stable compared to higher homologues, even though their neutral precursors are unstable.

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.

Synthesis of zirconocene complexes and their use in slurry-phase polymerisation of ethylene

A new family of zirconocene complexes of the type (^3-RInd^#)₂ZrX₂ (where Ind^# = C₆Me₅H and R = Me, Et and Ph) have been synthesised and fully characterised. Six new crystal structures have been reported (meso-(^3-EtInd^#)₂ZrBr₂, rac-(^3-EtInd^#)₂ZrCl₂, rac-(^3-EtInd^#)₂Zr(CH₂Ph)₂, meso-(^3-EtInd^#)₂Zr(CH₂Ph)₂, meso-(^3-MeInd^#)₂ZrBr₂ and meso-(^3-MeInd^#)₂Zr(CH₂Ph)₂). The complexes were studied for slurry-phase ethylene polymerisation when immobilised on solid polymethylaluminoxane (sMAO). Variation in the initiation group was found to have greater influence over polymerisation activity for meso-catalysts than rac-catalysts, with meso-alkyl catalysts showing higher polymerisation activities than meso-halide. Below 70 °C, polymerisation activity follows the order sMAO-meso-(^3-EtInd^#)₂Zr(CH₂Ph)_2, sMAO-meso-(^3-EtInd^#)₂ZrCl₂ and sMAO-meso-(^3-EtInd^#)₂ZrBr₂ (activities of 657, 561, and 452 kg_PE mol_M⁻¹ h⁻¹ bar⁻¹, respectively). sMAO-meso-(^3-EtInd^#)₂ZrBr₂ produces HDPE with the highest molecular weight, followed by sMAO-meso-(^3-EtInd^#)₂ZrCl₂ and sMAO-meso-(^3-EtInd^#)₂Zr(CH₂Ph)₂ (M_w of 503, 406, and 345 kg mol⁻¹, respectively, at 50 °C). sMAO-meso-(^3-MeInd^#)₂ZrBr₂ produced HDPE with almost identical molecular weights to sMAO-meso-(^3-EtInd^#)₂ZrCl₂ (395 kg mol⁻¹ at 50 °C).

A new family of zirconocene complexes of the type (^3-RInd^#)₂ZrX₂ (where Ind^# = C₆Me₅H and R = Me, Et and Ph) have been synthesised and fully characterised.

Polymethylaluminoxane organic frameworks (sMAOF) – highly active supports for slurry phase ethylene polymerisation

A series of modified solid polymethylaluminoxane (sMAO) catalyst supports have been developed for slurry phase ethylene polymerisation, using aryl di-ol modifier groups.

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.

The activator methylaluminoxane is made by hydrolysis of trimethylaluminum. Analysis using ESI-MS reveals rapid formation of small oligomers is followed by slower aggregation to the larger precursors most capable of releasing [Me₂Al]⁺.

Fair Look at Coordination Oligomerization of Higher α-Olefins

Coordination catalysis is a highly efficient alternative to more traditional acid catalysis in the oligomerization of α-olefins. The distinct advantage of transition metal-based catalysts is the structural homogeneity of the oligomers. Given the great diversity of the catalysts and option of varying the reaction conditions, a wide spectrum of processes can be implemented. In recent years, both methylenealkanes (vinylidene dimers of α-olefins) and structurally uniform oligomers with the desired degrees of polymerization have become available for later use in the synthesis of amphiphilic organic compounds and polymers, high-quality oils or lubricants, and other prospective materials. In the present review, we discussed the selective dimerization and oligomerization of α-olefins, catalyzed by metallocene and post-metallocene complexes, and explored the prospects for the further applications of the coordination α-olefin dimers and oligomers.

The mechanism of the reaction between MAO and TMA: DFT study of the electronic structure and characterization of transition states for [AlOMe]6, [AlOMe]9 and [AlOMe]16 cages

Methylaluminoxane (MAO) and trimethylaluminium (TMA) are relevant compounds in organometallic catalysis. Despite many published studies, aspects of their interaction persist an unsolved puzzle. Hence, in this work, we used quantum mechanic approaches based on density functional theory to study this topic. Our calculations revealed that interaction between MAO and TMA occurs initially by the formation of an intermediary Lewis adduct. In agreement with the latent acidity concept, the activation energy for the tensioned Al-O bond break is small, and changes with the local environment of the MAO cages. Breakage of bond belonging to two square faces requires between 4.20 and 5.80 kcal/mol, whereas square-hexagonal faces demand 0.61-9.43 kcal/mol. The products of this reaction present a terminal, acidic 3-coordinate aluminum atom, that can be capped by another TMA molecules. However, our computations suggest that entropic effects may prevent this reaction from occurring at all these sites in the MAO models studied. Additionally, we also characterize the inter/intramolecular methane elimination mechanism. These reactions are not feasible at room temperature but may occur at high temperatures.

Reactivity Trends of Lewis Acidic Sites in Methylaluminoxane and Some of Its Modifications

The established model cluster (AlOMe)16(AlMe3)6 for methylaluminoxane (MAO) cocatalyst has been studied by density functional theory, aiming to rationalize the different behaviors of unmodified MAO and TMA-depleted MAO/BHT (TMA = trimethylaluminum; BHT = 2,6-di-tert-butyl-4-methylphenol), highlighted in previous experimental studies. The tendency of the three model Lewis acidic sites A-C to release neutral Al fragments (i.e., AlMe2R; R = Me or bht) or transient aluminum cations (i.e., [AlMeR]+) has been investigated both in the absence and in the presence of neutral N-donors. Sites C are most likely responsible for the activation capabilities of TMA-rich MAO, but TMA depletion destabilizes them, possibly inducing structural rearrangements. The remaining sites A and B, albeit of lower Lewis acidity, should be still able to release cationic Al fragments when TMA-depleted modified MAOs are treated with N-donors (e.g. [AlMe(bht)]+ from MAO/BHT). These findings provide tentative interpretations for earlier observations of donor-dependent ionization tendencies of MAO and MAO/BHT and how TMA depleted MAOs can still be potent activators.

Paraffin-Inert Atmospheric Solid Analysis Probe: A Fast and Easy Approach To Characterize Extremely Air-Sensitive Organometallic Complexes by Mass Spectrometry

Rational characterization of most organometallic compounds is hampered by their high reactivity, in particular, toward oxygen and water. Mass spectrometry experiments require physical introduction of the sample in the ionization source. So, the main challenge is to transfer air-sensitive organometallic compounds from inert atmosphere to the ionization source. In this aim, we have developed an easy technique that allows the analysis of air-sensitive compounds using the atmospheric solid analysis probe (ASAP). This method consists of a glass capillary filled with the sample (solid or liquid) and sealed by a paraffin plug to maintain the inert sample until the ionization process. It is illustrated through the structural characterization of a new highly air-sensitive dinuclear zirconium complex supported by an original switchable stilbene platform.

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

Gas-phase oxidation of air-sensitive organometallic compounds does not proceed to a significant extent in mass spectrometric analysis unless a vacant coordination site is generated, making nitrogen generators a suitable source of desolvation gas.

Zirconocene-Catalyzed Dimerization of α-Olefins: DFT Modeling of the Zr-Al Binuclear Reaction Mechanism

Zirconocene-mediated selective dimerization of α-olefins usually occurs when precatalyst (η⁵-C₅H₅)₂ZrCl₂ is activated by minimal excess of methylalumoxane (MAO). In this paper, we present the results of density functional theory (DFT) simulation of the initiation, propagation, and termination stages of dimerization and oligomerization of propylene within the framework of Zr-Al binuclear mechanism at M-06x/DGDZVP level of theory. The results of the analysis of the reaction profiles allow to explain experimental facts such as oligomerization of α-olefins at high MAO/(η⁵-C₅H₅)₂ZrCl₂ ratios and increase of the selectivity of dimerization in the presence of R₂AlCl. The results of DFT simulations confirm the crucial role of the presence of chloride in the selectivity of dimerization. The molecular hydrogen was found in silico and proven experimentally as an effective agent that increases the rate and selectivity of dimerization.

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 (t_½∼2 s, up to 25 exchanges of Me for Et) than iBu₃Al (t_½∼40 s, up to 11 exchanges) or Oct₃Al (t_½∼200 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.

Oxidation States, Stability, and Reactivity of Organoferrate Complexes

We have applied a combination of electrospray-ionization mass spectrometry, electrical conductivity measurements, and Mössbauer spectroscopy to identify and characterize the organoferrate species R _nFe _m^- formed upon the transmetalation of iron precursors (Fe(acac)₃, FeCl₃, FeCl₂, Fe(OAc)₂) with Grignard reagents RMgX (R = Me, Et, Bu, Hex, Oct, Dec, Me₃SiCH₂, Bn, Ph, Mes, 3,5-(CF₃)₂-C₆H₃; X = Cl, Br) in tetrahydrofuran. The observed organoferrates show a large variety in their aggregation (1 ≤ m ≤ 8) and oxidation states (I to IV), which are chiefly determined by the nature of their organyl groups R. In numerous cases, the addition of a bidentate amine or phosphine changes the distributions of organoferrates and affects their stability. Besides undergoing efficient intermolecular exchange processes, several of the probed organoferrates react with organyl (pseudo)halides R'X (R' = Et, ⁱPr, Bu, Ph, p-Tol; X = Cl, Br, I, OTf) to afford heteroleptic complexes of the type R₃FeR'^-. Gas-phase fragmentation of most of these complexes results in reductive eliminations of the coupling products RR' (or, alternatively, of R₂). This finding indicates that iron-catalyzed cross-coupling reactions may proceed via such heteroleptic organoferrates R₃FeR'^- as intermediates. Gas-phase fragmentation of other organoferrate complexes leads to β-hydrogen eliminations, the loss of arenes, and the expulsion of organyl radicals. The operation of both one- and two-electron processes is consistent with previous observations and contributes to the formidable complexity of organoiron chemistry.

Oxidation of Methylalumoxane Oligomers

The anions formed from methylalumoxane (MAO) and suitable donors (e.g. octamethyltrisiloxane) are amenable to mass spectrometric (MS) analysis. Their composition as deduced from this data allows direct insight into the chemical transformations of their neutral precursors. One such process is oxidation, which is well-known to be facile for MAO without any clear idea of what actually occurs at a molecular level. Addition of O₂ to MAO results in immediate gelation, but MS analysis reveals no corresponding change to the composition of the principal oligomeric anions. A slow (hours) reaction does occur that involves net incorporation of Me₂ AlOMe into the oligomeric anions, and the identities of the OMe-containing anions were confirmed by ¹ H NMR spectroscopy, MS/MS analysis, and addition of an authentic sample of Me₂ AlOMe to MAO. The result tallies with the fact that addition of O₂ to MAO produces Me₂ AlOMe from free Me₃ Al which eventually leads to formation of oxidized MAO oligomers and changes in ion abundance. Aging of the oxygenated MAO results in further growth of the oligomers similar to that of the non-oxidized species. Mass spectrometric analysis therefore reveals useful insights into the environmental history of a given MAO batch.

The Multifaceted Role of Methylaluminoxane in Metallocene-Based Olefin Polymerization Catalysis

In single-site olefin polymerization catalysis, a large excess of cocatalyst is often required for the generation of highly active catalysts, but the reason for this is unclear. In this work, fundamental insight into the multifaceted role of cocatalyst methylaluminoxane (MAO) in the activation, deactivation, and stabilization of group 4 metallocenes in the immobilized single-site olefin polymerization catalyst was gained. Employing probe molecule FT-IR spectroscopy, it was found that weak Lewis acid sites, inherent to the silica-supported MAO cocatalyst, are the main responsible species for the genesis of active metallocenes for olefin polymerization. These weak Lewis acid sites are the origin of AlMe₂⁺ groups. Deactivation of metallocenes is caused by the presence of silanol groups on the silica support. Interaction of the catalyst precursor with these silanol groups leads to the irreversible formation of inactive metallocenes. Importantly, a high concentration of MAO (14 wt% Al) on the silica support is necessary to keep the metallocenes immobilized, hence preventing metallocene leaching and consequent reactor fouling. Increasing the loading of the MAO cocatalyst leads to larger amounts of AlMe₂⁺, fewer silanol groups, and less metallocene leaching, which all result in higher olefin polymerization activity.

Group 4 permethylindenyl constrained geometry complexes for ethylene polymerisation catalysis

The synthesis and characterisation of six permethylindenyl constrained geometry complexes is reported. Slurry phase ethylene polymerisation studies demonstrate that activities increase with increasing electron donating character of the amido fragment.

Formation, Structure, and Composition of Methylaluminoxane

The structurally ill-characterized methylaluminoxane (MAO) is the activator of choice in olefin polymerization catalysis. We have carried out large scale and systematic quantum chemical calculations to simulate the thermodynamics of its formation by controlled hydrolysis of trimethylaluminum (TMA), extending the studies up to 25 Al atoms, and thus, to the real size domain of MAO. In agreement with previous postulates on its structure, MAO is shown to favor cage-like structures, which commonly contain associated TMA, regardless of size or shape. The sites containing associated TMA are reactive, and explain the function of MAO as a catalyst activator. The compositions of MAOs show overall agreement with experiments, and exhibit structural transitions from chains to rings to sheets to eventually cages as a function of size. The most stable cage structure is obtained for a composition of (MeAlO)₁₆ (Me₃ Al)₆ , which is in precise agreement with mass spectrometric studies of corresponding anions, and adapts a tubular molecular structure with a molecular weight of 1360 g mol^-1 . Our mass spectrometric measurements enable detection of both major and minor anion species.

Alkene Metalates as Hydrogenation Catalysts

First-row transition-metal complexes hold great potential as catalysts for hydrogenations and related reductive reactions. Homo- and heteroleptic arene/alkene metalates(1-) (M=Co, Fe) are a structurally distinct catalyst class with good activities in hydrogenations of alkenes and alkynes. The first syntheses of the heteroleptic cobaltates [K([18]crown-6)][Co(η4 -cod)(η2 -styrene)2 ] (5) and [K([18]crown-6)][Co(η4 -dct)(η4 -cod)] (6), and the homoleptic complex [K(thf)2 ][Co(η4 -dct)2 ] (7; dct=dibenzo[a,e]cyclooctatetraene, cod=1,5-cyclooctadiene), are reported. For comparison, two cyclopentadienylferrates(1-) were synthesized according to literature procedures. The isolated and fully characterized monoanionic complexes were competent precatalysts in alkene hydrogenations under mild conditions (2 bar H2 , r.t., THF). Mechanistic studies by NMR spectroscopy, ESI mass spectrometry, and poisoning experiments documented the operation of a homogeneous mechanism, which was initiated by facile redox-neutral π-ligand exchange with the substrates followed by H2 activation. The substrate scope of the investigated precatalysts was also extended to polar substrates (ketones and imines).

Organometallic chemistry using partially fluorinated benzenes

Fluorobenzenes, in particular fluorobenzene (FB) and 1,2-difluorobenzene (1,2-DiFB), are versatile solvents for conducting organometallic chemistry and transition-metal-based catalysis.

Stability and Unimolecular Reactivity of Palladate(II) Complexes [Ln PdR3 ]- (L=Phosphine, R=Organyl, n=0 and 1)

The reduction of Pd^II precatalysts to catalytically active Pd⁰ species is a key step in many palladium-mediated cross-coupling reactions. Besides phosphines, the stoichiometrically used organometallic reagents can afford this reduction, but do so in a poorly understood way. To elucidate the mechanism of this reaction, we have treated solutions of Pd(OAc)₂ and a phosphine ligand L in tetrahydrofuran with RMgCl (R=Ph, Bn, Bu) as well as other organometallic reagents. Analysis of these model systems by electrospray- ionization mass spectrometry found palladate(II) complexes [L_n PdR₃ ]^- (n=0 and 1), thus pointing to the occurrence of transmetallation reactions. Upon gas-phase fragmentation, the [L_n PdR₃ ]^- anions preferentially underwent a reductive elimination to yield Pd⁰ species. The sequence of the transmetallation and reductive elimination, thus, constitutes a feasible mechanism for the reduction of the Pd(OAc)₂ precatalyst. Other species of interest observed include the Pd^IV complex [PdBn₅ ]^- , which did not fragment via a reductive elimination but lost BnH instead.

Exposing elusive cationic magnesium–chloro aggregates in aluminate complexes through donor control

Judicious choice of Lewis donor provides control over the aggregation state of the [Mg_xCl_2x−1]⁺ cation in a series of Mg battery relevant magnesium aluminates.

Electron ionization mass spectrometric analysis of air- and moisture-sensitive organometallic compounds

Electron ionization (EI) is a reliable mass spectrometric method for the analysis of the vast majority of thermally stable and volatile compounds.