Ancillary Ligand-Free Mixed Chlorido/Isobutylaluminato Lanthanide Complexes in Isoprene Polymerization

Ternary catalyst mixtures are employed for industrial-scale rare-earth-metal-promoted diene polymerization. Thereby, the role of organoaluminum compounds in activating the rare-earth-metal component is still poorly understood. Mixed chlorido/amido complexes [Ln{N(SiMe3)(2)}(2)(mu-Cl)(THF)](2) are selected as well-defined precursors according to the chlorination/alkylation activation sequence. The heteroleptic lanthanide tetraisobutylaluminate complexes [Ln{N(SiMe3)(2)}(AliBu(4))(mu-Cl)](2) (Ln = La, Nd) form when [Ln{N(SiMe3)(2)}(2)(mu-Cl)(THF)](2) is treated with triisobutylaluminum. By contrast, the [La{N(SiMe3)(2)}(2)(mu-Cl)(THF)](2)/AlMe3 reaction afforded the isolable heteroaluminate complex [La{N(SiMe3)(2)}{(mu-CH3)(mu-N(SiMe3)(2))AlMe2}(mu-Cl)](2), highlighting the distinct reactivity of aluminum alkyls. Complex [Nd{N(SiMe3)(2)}(AliBu(4))(mu-Cl)](2) features catalytic activity in isoprene polymerization for the fabrication of high cis-selective polyisoprene and displays single-component polymerization activity. Compounds [Ln{N(SiMe3)(2)}(2)(mu-Cl)(THF)](2) are best accessed from La[N(SiMe3)(2)](3)/LaCl3(THF) mixtures since starting out from LaCl3(THF)/K[N(SiMe3)(2)] leads to alkali-metal contaminations as revealed by the isolation of the mixed hydrido/isobutylaluminato complex [K(iBu(3)Al-mu-H-AliBu(3))](n), formed upon reaction with triisobutylaluminum.