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

Monocationic bis‐allyl complexes [Ln(η 3 ‐C 3 H 5 ) 2 (thf) 3 ] + [B(C 6 X 5 ) 4 ] − (Ln=Y, La, Nd; X=H, F) and dicationic mono‐allyl complexes of yttrium and the early lanthanides [Ln(η 3 ‐C 3 H 5 )(thf) 6 ] 2+ [BPh 4 ] 2 − (Ln=La, Nd) were prepared by protonolysis of the tris‐allyl complexes [Ln(η 3 ‐C 3 H 5 ) 3 (diox)] (Ln=Y, La, Ce, Pr, Nd, Sm; diox=1,4‐dioxane) isolated as a 1,4‐dioxane‐bridged dimer (Ln=Ce) or THF adducts [Ln(η 3 ‐C 3 H 5 ) 3 (thf) 2 ] (Ln=Ce, Pr). Allyl abstraction from the neutral tris‐allyl complex by a Lewis acid, ER 3 (Al(CH 2 SiMe 3 ) 3 , BPh 3 ) gave the ion pair [Ln(η 3 ‐C 3 H 5 ) 2 (thf) 3 ] + [ER 3 (η 1 ‐CH 2 CHCH 2 )] − (Ln=Y, La; ER 3 =Al(CH 2 SiMe 3 ) 3 , BPh 3 ). Benzophenone inserts into the LaC allyl bond of [La(η 3 ‐C 3 H 5 ) 2 (thf) 3 ] + [BPh 4 ] − to form the alkoxy complex [La{OCPh 2 (CH 2 CHCH 2 )} 2 (thf) 3 ] + [BPh 4 ] − . The monocationic half‐sandwich complexes [Ln(η 5 ‐C 5 Me 4 SiMe 3 )(η 3 ‐C 3 H 5 )(thf) 2 ] + [B(C 6 X 5 ) 4 ] − (Ln=Y, La; X=H, F) were synthesized from the neutral precursors [Ln(η 5 ‐C 5 Me 4 SiMe 3 )(η 3 ‐C 3 H 5 ) 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.