Fluorenyl Half-Sandwich Bis(tetramethylaluminate) Complexes of the Rare-Earth Metals: Synthesis, Structure, and Isoprene Polymerization

Aiming at half-sandwich complexes of the type (Flu^R)­Ln­(AlMe₄)₂, homoleptic tetramethylaluminates Ln­(AlMe₄)₃ (Ln = Y, La, Nd, and Lu) were treated with equimolar amounts of R-substituted potassium fluorenyls. The salt metathesis reaction of La­(AlMe₄)₃ with K­(Flu) (Flu = fluorenyl = C₁₃H₉) and K­(Flu^<i>t</i>Bu) (Flu^<i>t</i>Bu = di­(<i>tert</i>-butyl) fluorenyl) selectively gave the desired half-sandwich complexes (Flu^R)­La­(AlMe₄)₂. The corresponding reactions of Y­(AlMe₄)₃ with K­(Flu)/K­(Flu^<i>t</i>Bu) and Lu­(AlMe₄)₃ with K­(Flu^<i>t</i>Bu) gave mixtures of half-sandwich and sandwich complexes, while treatment of Lu­(AlMe₄)₃ with K­(Flu) produced only the lutetocene complex (Flu)₂Lu­(AlMe₄). Sterically more demanding 1-trimethylsilyl fluorenyl (Flu^Si) allowed for the isolation of half-sandwich complexes for the entire Ln­(III) size range (Ln = La, Nd, Y, and Lu), in crystalline yields up to 94%. Upon activation with routinely employed borate or borane activators [Ph₃C]­[B­(C₆F₅)₄], [PhNMe₂H]­[B­(C₆F₅)₄], and B­(C₆F₅)₃, highly active initiators for isoprene polymerization were obtained. The catalyst activity as well as molecular weight (distribution) and stereoregularity of the obtained polyisoprenes are governed by the rare-earth metal size, fluorenyl ligand, and cocatalyst: highest activity for La/Flu^Si/[Ph₃C]­[B­(C₆F₅)₄], lowest <i>M</i>_w/<i>M</i>_n = 1.11 for La/Flu/[PhNMe₂H]­[B­(C₆F₅)₄], maximum <i>trans</i>-1,4 selectivity = 85% for La/Flu^<i>t</i>Bu/[PhNMe₂H]­[B­(C₆F₅)₄], maximum <i>cis</i>-1,4 selectivity = 78% for Lu/Flu^Si/[Ph₃C]­[B­(C₆F₅)₄]. The formations of the active species were investigated by NMR spectroscopy revealing not only established cationization pathways but also fluorenyl abstraction in lanthanum complexes (Flu)­La­(AlMe₄)₂ and (Flu^<i>t</i>Bu)­La­(AlMe₄)₂ by trityl borate [Ph₃C]­[B­(C₆F₅)₄]. The reaction of half-sandwich complexes (Flu^R)­Ln­(AlMe₄)₂ with equimolar amounts of Me₂AlCl did not give access to catalytically active species. Crystallization of binary mixtures (Flu^Si)­Y­(AlMe₄)₂/Me₂AlCl in distinct molar ratios of 1:1 and 1:1.7 yielded complexes [(Flu^Si)­Y­(AlMe₄)­(μ-Cl)]₂ and (Flu^Si)₆Y₆Cl₁₂, respectively