Since its discovery, methylalumoxane (MAO) has become of great importance as a cocatalyst in homogeneous olefin polymerization. The working principles of single-site polymerization catalysts are well-understood, but those of the cocatalyst MAO itself are not. Thus far structural and functional investigations have yielded limited insights and often give contradicting results. MAO's complex nature is due to multiple equilibria between undefined oligomers and “free” trimethylaluminum. Fundamental studies do not clearly portray the molecular structure, and the exact functioning of MAO remains a topic of debate. This comprehensive overview starts with the historical background of MAO and then focuses on its synthesis and large-scale production, structural characterization, properties, and different roles in the activation of olefin polymerization catalysts. Also given is an overview of potential modifications, immobilization on surfaces, and other alternative applications that have been reported for MAO.

Invited for the cover of this issue is the group of Sjoerd Harder at the University of Erlangen-Nürnberg, Germany. The cover image shows the birth of methylalumoxane MeAlO (MAO), or at least one of the possible MAO species, by reaction of trimethylaluminum with water.

A bimetallic aluminum/zirconium complex Cp*₂Zr(Me)OAl(DIPH) [DIPH-H₂ = 3,3′-bis(2-methylallyl)-(1,1′-biphenyl)-2,2′-diol; Cp* = C₅Me₅] was prepared in good yield by the reaction of (DIPH)AlMe with Cp*₂Zr(Me)OH. In contrast to Roesky's catalyst, Cp₂Zr(Me)O(Me)Al(DIPP-nacnac) {DIPP-nacnac = CH[(CMe)(2,6-iPr₂C₆H₃N)]₂}, it contains no Al–Me functionality and has increased Lewis acidity at Al. Crystal structures and the NMR spectra of the aluminum complex (DIPH)AlMe and the bimetallic Al/Zr species reveal in both cases dimeric complexes. The crystal structure of [Cp*₂Zr(Me)OAl(DIPH)]₂ shows a planar Al₂O₂ core and a terminal Cp*₂Zr(Me)O group. Ethylene polymerization in toluene was studied for the bimetallic complex with a variety of scavengers. Under comparable polymerization conditions, the methylalumoxane-activated species show high activities that are similar to those obtained with Cp*₂ZrCl₂ and Roesky's bimetallic complex Cp₂Zr(Me)OAl(Me)(DIPP-nacnac), with a relatively narrow polydispersity index (2.47).

Organocalcium compounds have been reported as efficient catalysts for various alkene transformations. In contrast to transition metal catalysis, the alkenes are not activated by metal–alkene orbital interactions. Instead it is proposed that alkene activation proceeds through an electrostatic interaction with a Lewis acidic Ca²⁺. The role of the metal was evaluated by a study using the metal-free catalysts: [Ph₂N⁻][Me₄N⁺] and [Ph₃C⁻][Me₄N⁺]. These “naked” amides and carbanions can act as catalysts in the conversion of activated double bonds (CO and CN) in the hydroamination of ArNCO and RNCNR (R=alkyl) by Ph₂NH. For the intramolecular hydroamination of unactivated CC bonds in H₂CCHCH₂CPh₂CH₂NH₂ the presence of a metal cation is crucial. A new type of hybrid catalyst consisting of a strong organic Schwesinger base and a simple metal salt can act as catalyst for the intramolecular alkene hydroamination. The influence of the cation in catalysis is further evaluated by a DFT study.

Organocalcium compounds have been reported as efficient catalysts for various alkene transformations. In contrast to transition metal catalysis, the alkenes are not activated by metal–alkene orbital interactions. Instead it is proposed that alkene activation proceeds through an electrostatic interaction with a Lewis acidic Ca²⁺. The role of the metal was evaluated by a study using the metal-free catalysts: [Ph₂N⁻][Me₄N⁺] and [Ph₃C⁻][Me₄N⁺]. These “naked” amides and carbanions can act as catalysts in the conversion of activated double bonds (CO and CN) in the hydroamination of ArNCO and RNCNR (R=alkyl) by Ph₂NH. For the intramolecular hydroamination of unactivated CC bonds in H₂CCHCH₂CPh₂CH₂NH₂ the presence of a metal cation is crucial. A new type of hybrid catalyst consisting of a strong organic Schwesinger base and a simple metal salt can act as catalyst for the intramolecular alkene hydroamination. The influence of the cation in catalysis is further evaluated by a DFT study.

In analogy to the previously reported tetranuclear magnesium hydride cluster with a bridged dianionic bis-β-diketiminate ligand, a related zinc hydride cluster has been prepared. The crystal structures of these magnesium and zinc hydride complexes are similar: the metal atoms are situated at the corners of a tetrahedron in which the vertices are bridged either by dianionic bis-β-diketiminate ligands or hydride ions. Both structures are retained in solution and show examples of H⁻⋅⋅⋅H⁻ NMR coupling (Mg: 8.5 Hz; Zn: 16.0 Hz). The zinc hydride cluster [NN-(ZnH)₂]₂ thermally decomposes at 90 °C and releases 1.8 equivalents of H₂. In contrast to magnesium hydride clusters, there is no apparent relationship between cluster size and thermal decomposition temperature for the zinc hydrides. DFT calculations reproduced the structure of the zinc hydride cluster reasonably well and charge density analysis showed no bond paths between the hydride ions. This contrasts with calculations on the analogous magnesium hydride cluster in which a counter-intuitive H⁻⋅⋅⋅H⁻ bond path was observed. Forcing a reduced H⁻⋅⋅⋅H⁻ distance in the zinc hydride cluster, however, gave rise to a H⁻⋅⋅⋅H⁻ bond path. Such weak interactions could play a role in H₂ desorption. The presumed molecular product after H₂ release, a Zn(I) cluster, could not be characterized experimentally but DFT calculations predicted a cluster with two localized ZnZn bonds.

The superbulky deca-aryleuropocene [Eu(Cp^BIG)₂], Cp^BIG=(4-nBu-C₆H₄)₅-cyclopentadienyl, was prepared by reaction of [Eu(dmat)₂(thf)₂], DMAT=2-Me₂N-α-Me₃Si-benzyl, with two equivalents of Cp^BIGH. Recrystallizyation from cold hexane gave the product with a surprisingly bright and efficient orange emission (45 % quantum yield). The crystal structure is isomorphic to those of [M(Cp^BIG)₂] (M=Sm, Yb, Ca, Ba) and shows the typical distortions that arise from Cp^BIG⋅⋅⋅Cp^BIG attraction as well as excessively large displacement parameter for the heavy Eu atom (U_eq=0.075). In order to gain information on the true oxidation state of the central metal in superbulky metallocenes [M(Cp^BIG)₂] (M=Sm, Eu, Yb), several physical analyses have been applied. Temperature-dependent magnetic susceptibility data of [Yb(Cp^BIG)₂] show diamagnetism, indicating stable divalent ytterbium. Temperature-dependent ¹⁵¹Eu Mössbauer effect spectroscopic examination of [Eu(Cp^BIG)₂] was examined over the temperature range 93–215 K and the hyperfine and dynamical properties of the Eu^II species are discussed in detail. The mean square amplitude of vibration of the Eu atom as a function of temperature was determined and compared to the value extracted from the single-crystal X-ray data at 203 K. The large difference in these two values was ascribed to the presence of static disorder and/or the presence of low-frequency torsional and librational modes in [Eu(Cp^BIG)₂]. X-ray absorbance near edge spectroscopy (XANES) showed that all three [Ln(Cp^BIG)₂] (Ln=Sm, Eu, Yb) compounds are divalent. The XANES white-line spectra are at 8.3, 7.3, and 7.8 eV, for Sm, Eu, and Yb, respectively, lower than the Ln₂O₃ standards. No XANES temperature dependence was found from room temperature to 100 K. XANES also showed that the [Ln(Cp^BIG)₂] complexes had less trivalent impurity than a [EuI₂(thf)_x] standard. The complex [Eu(Cp^BIG)₂] shows already at room temperature strong orange photoluminescence (quantum yield: 45 %): excitation at 412 nm (24270 cm⁻¹) gives a symmetrical single band in the emission spectrum at 606 nm (ν_max=16495 cm⁻¹, FWHM: 2090 cm⁻¹, Stokes-shift: 2140 cm⁻¹), which is assigned to a 4f⁶5d¹4f⁷ transition of Eu^II. These remarkable values compare well to those for Eu^II-doped ionic host lattices and are likely caused by the rigidity of the [Eu(Cp^BIG)₂] complex. Sharp emission signals, typical for Eu^III, are not visible.