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)₂ Ln(AlMe₄ )] (Ln=Y, Lu) and [(2,4-dipp)₂ Y(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(C₆ F₅ )₃ led to the isolation and full characterization of the single-component catalyst {{(2,4-dtbp)La[(μ-Me)₂ AlMe(C₆ F₅ )]}[Me₂ Al(C₆ F₅ )₂ ]}₂ . The reaction of the latter complex with 10 equivalents of isoprene could be monitored by ¹ H 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 ].