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 Mw /Mn =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 ].