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

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journal contribution
posted on 2019-07-18, 16:19 authored by Dominic Diether, Cäcilia Maichle-Mössmer, Reiner Anwander
Lanthanum indenyl half-sandwich complexes of the composition (IndR)­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 (IndEt) 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 (IndSi) 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 (IndtBu)­La­(AlMe4)2. The reactions of Lu­(AlMe4)3 with 1 or 2 equiv of Li­(IndR) gave predominantly bis­(indenyl) sandwich complexes (IndR)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 (IndEt)­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).