Rare-Earth-Metal Pentadienyl Half-Sandwich and Sandwich Tetramethylaluminates–Synthesis,Structure, Reactivity,and Performance in Isoprene Polymerization

Targeting the synthesis of rare-earth-metal pentadienyl half-sandwich tetramethylaluminate complexes, homoleptic Ln(AlMe₄)₃] (Ln=Y, La, Ce, Pr, Nd, Lu) were treated with equimolar amounts of the potassium salts K(2,4-dmp) (2,4-dmp=2,4-dimethylpentadienyl), K(2,4-dipp) (2,4-dipp=2,4-diisopropylpentadienyl), and K(2,4-dtbp) (2,4-dtbp=2,4-di-tert-butylpentadienyl). The reactions involving the larger rare-earth-metal centers lanthanum, cerium, praseodymium, and neodymium gave selectively the desired half-sandwich complexes (2,4-dmp)La(AlMe₄)₂], (2,4-dipp)La(AlMe₄)₂], and (2,4-dtbp)Ln(AlMe₄)₂] (Ln=La, Ce, Pr, Nd) in high crystalline yields. Smaller rare-earth-metal centers yielded preferentially the sandwich complexes (2,4-dmp)₂Ln(AlMe₄)] (Ln=Y, Lu) and (2,4-dipp)₂Y(AlMe₄)]. Activation with fluorinated borate/borane co-catalysts gave highly active catalyst systems for the fabrication of polyisoprene, displaying molecular weight distributions as low as M_w/M_n=1.09 and a maximum cis-1,4 selectivity of 90.4 %. The equimolar reaction of half-sandwich complex (2,4-dtbp)La(AlMe₄)₂] with B(C₆F₅)₃ led to the isolation and full characterization of the single-component catalyst {{(2,4-dtbp)La(μ-Me)₂AlMe(C₆F₅)]}Me₂Al(C₆F₅)₂]}₂. The reaction of the latter complex with 10 equivalents of isoprene could be monitored by ¹H NMR spectroscopy. Also, a donor-induced aluminato/gallato exchange was achieved with (2,4-dtbp)La(AlMe₄)₂] and GaMe₃(OEt₂) leading to (2,4-dtbp)La(GaMe₄)₂].