Implications of Indenyl Substitution for the Structural Chemistry of Rare-Earth Metal (Half-)Sandwich Complexes and Performance in Living Isoprene Polymerization

Lanthanum indenyl half-sandwich complexes of the composition (Ind^R)­La­(AlMe₄)₂ were synthesized in high crystalline yields by a salt-metathesis protocol applying La­(AlMe₄)₃ and Li­(Ind^R). In the solid state, the parent indenyl (Ind) and 2-ethylindenyl (Ind^Et) complexes exhibit a dimeric structural motif with the methyl groups of the linearly aligned La­(μ-CH₃)Al moieties being cis-positioned to the indenyl ligand. In contrast, 1-trimethylsilyl indenyl (Ind^Si) directs the η¹-coordinated methyl group of the bridging aluminato ligand into a trans-position, while 2-tert-butyl indenyl afforded the monomeric half-sandwich complex (Ind^tBu)­La­(AlMe₄)₂. The reactions of Lu­(AlMe₄)₃ with 1 or 2 equiv of Li­(Ind^R) gave predominantly bis­(indenyl) sandwich complexes (Ind^R)₂­Lu­(AlMe₄). All (half-)­sandwich complexes were characterized by X-ray structure analysis, ¹H/¹³C­{¹H} NMR and FTIR spectroscopy, and microanalysis. The performance of all half-sandwich complexes in isoprene polymerization was assessed upon activation with [Ph₃C]­[B­(C₆F₅)₄], [Ph­NMe₂H]­[B­(C₆F₅)₄], or B­(C₆F₅)₃. The choice of indenyl ligand and cocatalyst had a major impact on the polymerization efficiency and stereospecificity. The highest selectivities could be achieved with the binary catalyst systems (Ind^Et)­La­(AlMe₄)₂/​[Ph₃C]­[B­(C₆F₅)₄] (cis/trans content 10.4/85.9) and (Ind^Si)­La­(AlMe₄)₂/​B­(C₆F₅)₃ (cis/trans content 77.0/13.0).