Cationic Alkyl Complexes of the Rare‐Earth Metals: Synthesis, Structure, and Reactivity

Abstract

Cationic alkyl complexes of the rare‐earth metals [LnR m(L)n](3–m)+ (R=alkyl; m=1, 2; L=Lewis base) were virtually unknown species until recently. Because of their increased 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 or Brønsted acids in the presence of weakly coordinating anions. Not only monocationic but also dicationic alkyl derivatives have been shown to be accessible. In the context of modeling homogeneous ethylene polymerization using a mixture consisting of LnR 3/AlR 3/[NMe 2 HPh][B(C 6 F 5)4], such dications were discovered. Some thermally robust examples of mono‐ and dicationic alkyl complexes have been structurally characterized as solvent‐separated ion pairs. Neutral and monoanionic macrocycles such as crown ethers or aza‐crown ethers as well as amidinato, β‐diketiminato, and substituted cyclopentadienyl ligands are suitable ancillary ligands to stabilize the cationic alkyl fragments.