Cationic Allyl Complexes of the Rare-Earth Metals: Synthesis, Structural Characterization, and 1,3-Butadiene Polymerization Catalysis

Monocationic bis-allyl complexes [Ln(eta(3)-C3H5)(2)(thf)(3)](+) [B(C6X5)(4)](-) (Ln = Y, La, Nd; X = H, F) and dicationic mono-allyl complexes of yttrium and the early lanthanides [Ln(eta(3)-C3H5)-(thf)(6)](2+)[BPh4](2)(-) (Ln = La, Nd) were prepared by protonolysis of the tris-allyl complexes [Ln(eta(3)-C3H5)(3)(diox)] (Ln = Y, La, Cc, Pr, Nd, Sm; diox = 1,4-dioxane) isolated as a 1,4-dioxane-bridged dimer (Ln=Ce) or THF adducts [Ln(eta(3)-C3H5)(3)(thf)(2)] (Ln=Ce, Pr). Allyl abstraction from the neutral tris-allyl complex by a Lewis acid, ER3 (Al(CH2SiMe3)(3), BPh3) gave the ion pair [Ln(eta(3)-C3H5)(2)(thf)(3)](+)[ER3(eta(1)-CH2CH=CH2)](-) (Ln = Y, La; ER3 = Al(CH2SiMe3)(3), BPh3). Benzophenone inserts into the La-C-allyl bond of [La (eta(3)-C3H5)(2)(thf)(3)](+)[BPh4](-) to form the alkoxy complex [La{OCPh2(CH2CH=CH2)}(2)(thf)(3)](+)[BPh4](-). The monocationic half-sandwich complexes [Ln(eta(5)-C5Me4SiMe3)(eta(3)-C3H5)(thf)(2)](+)[B(C6X5)(4)](-) (Ln=Y, La; X=H, F) were synthesized from the neutral precursors [Ln(eta(5)-C5Me4SiMe3)(eta(3)-CH5)(2)(thf)] by protonolysis. For 1,3-butadiene polymerization catalysis, the yttrium-based systems were more active than the corresponding lanthanum or neodymium homologues, giving polybutadiene with approximately 90% 1,4-cis stereoselectivity.