Half‐Sandwich Bis(tetramethylaluminate) Complexes of the Rare‐Earth Metals: Synthesis, Structural Chemistry, and Performance in Isoprene Polymerization

Abstract

The protonolysis reaction of [Ln(AlMe₄)₃] with various substituted cyclopentadienyl derivatives HCp^R gives access to a series of half‐sandwich complexes [Ln(AlMe₄)₂(Cp^R)]. Whereas bis(tetramethylaluminate) complexes with [1,3‐(Me₃Si)₂C₅H₃] and [C₅Me₄SiMe₃] ancillary ligands form easily at ambient temperature for the entire Ln^III cation size range (Ln=Lu, Y, Sm, Nd, La), exchange with the less reactive [1,2,4‐(Me₃C)₃C₅H₃] was only obtained at elevated temperatures and for the larger metal centers Sm, Nd, and La. X‐ray structure analyses of seven representative complexes of the type [Ln(AlMe₄)₂(Cp^R)] reveal a similar distinct [AlMe₄] coordination (one η², one bent η²). Treatment with Me₂AlCl leads to [AlMe₄] → [Cl] exchange and, depending on the Al/Ln ratio and the Cp^R ligand, varying amounts of partially and fully exchanged products [{Ln(AlMe₄)(μ‐Cl)(Cp^R)}₂] and [{Ln(μ‐Cl)₂(Cp^R)}_n], respectively, have been identified. Complexes [{Y(AlMe₄)(μ‐Cl)(C₅Me₄SiMe₃)}₂] and [{Nd(AlMe₄)(μ‐Cl){1,2,4‐(Me₃C)₃C₅H₂}}₂] have been characterized by X‐ray structure analysis. All of the chlorinated half‐sandwich complexes are inactive in isoprene polymerization. However, activation of the complexes [Ln(AlMe₄)₂(Cp^R)] with boron‐containing cocatalysts, such as [Ph₃C][B(C₆F₅)₄], [PhNMe₂H][B(C₆F₅)₄], or B(C₆F₅)₃, produces initiators for the fabrication of trans‐1,4‐polyisoprene. The choice of rare‐earth metal cation size, Cp^R ancillary ligand, and type of boron cocatalyst crucially affects the polymerization performance, including activity, catalyst efficiency, living character, and polymer stereoregularity. The highest stereoselectivities were observed for the precatalyst/cocatalyst systems [La(AlMe₄)₂(C₅Me₄SiMe₃)]/B(C₆F₅)₃ (trans‐1,4 content: 95.6 %, M_w/M_n=1.26) and [La(AlMe₄)₂(C₅Me₅)]/B(C₆F₅)₃ (trans‐1,4 content: 99.5 %, M_w/M_n=1.18).