Neutral and Cationic Rare-Earth Metal Alkyl Complexes: Synthesis, Characterization and Polymerization Catalysis

Cationic alkyl complexes of the rare-earth metals were virtually unknown species until recently. Because of their high Lewis-acidity/electrophilicity they should have considerable potential as homogeneous catalysts in olefin polymerization and in organic transformations. They can be generated by treating the neutral rare-earth metal precursors containing at least two alkyl groups R with suitable Lewis acids or Brensted acids in the presence of weakly coordinating anions. In this work, not only monocationic but also dicationic alkyl derivatives have been shown to be accessible. Such dications were discovered in the context of modelling homogeneous ethylene polymerization using a mixture consisting of LnR3 / A1R3 / [NMe2HPh][B(C6F5)4]. Some thermally robust examples of mono- and dicationic alkyl complexes have been structurally characterized as solvent-separated ion pairs. Neutral and monoanionic ligands such as crown ethers and substituted cyclopentadienyls are suitable ancillary groups to stabilize the cationic alkyl fragments.