Tetramethylcyclopentadienyl-supported rare-earth metal bis(tetramethyl)aluminate complexes: Synthesis, structural chemistry, cation formation, and isoprene polymerization

The protonolysis reaction of [Ln(AlMe4)3] with H(Cp′) (Cp′ = C5Me4H) gives access to half-sandwich complexes [(Cp′)Ln(AlMe4)2]. X-ray structure analyses of the samarium, neodymium, and lanthanum derivatives reveal a distinct [AlMe4] coordination (one η2, one bent η2) for the two smaller rare-earth metals. The lanthanum complex displays an unprecedented dimeric structure with two μ2-η1:η2 coordinating [AlMe4] ligands in the solid state. Treatment of complexes [(Cp′)Ln(AlMe4)2] with perfluorinated organoborates and -boranes produces discrete contact ion-pairs, which are characterized by 1H, 13C, 27Al, 19F, and 11B NMR spectroscopy and act as efficient initiators for the fabrication of trans-1,4 polyisoprene. The polymerization performance is hereby affected by the rare-earth metal cation size, the type of boron cocatalyst, and the polymerization temperature.