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(IndR). 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(mu-CH3)Al moieties being cis-positioned to the indenyl ligand. In contrast, 1-trimethylsilyl indenyl (Ind(Si)) directs the eta(1)-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))(2)Lu(AlMe4). All (half-)sandwich complexes were characterized by X-ray structure analysis, H-1/C-13{H-1} 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 (IndSi)La(AlMe4)(2)/B(C6F5)(3) (cis/trans content 77.0/13.0).