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 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(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]. Equimolar treatment of open half‐sandwich complex [(2,4‐dtbp)Ln(AlMe4)2] with B(C6F5)3 gives isolable compound {{(2,4‐dtbp)La[(μ‐Me)2AlMe(C6F5)]}[Me2Al(C6F5)2]}2 with less tightly bonded contact ion pairs compared to the C5Me5 congener, which markedly affects the polymerization performance.