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(μ-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}(AliBu4)(μ-Cl)]2 (Ln = La, Nd) form when [Ln{N(SiMe3)2}2(μ-Cl)(THF)]2 is treated with triisobutylaluminum. By contrast, the [La{N(SiMe3)2}2(μ-Cl)(THF)]2/AlMe3 reaction afforded the isolable heteroaluminate complex [La{N(SiMe3)2}{(μ-CH3)(μ-N(SiMe3)2)AlMe2}(μ-Cl)]2, highlighting the distinct reactivity of aluminum alkyls. Complex [Nd{N(SiMe3)2}(AliBu4)(μ-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(μ-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(iBu3Al-μ–H-AliBu3)]n, formed upon reaction with triisobutylaluminum.