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(AlMe 4 ) 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(AlMe 4 ) 2 ], [(2,4‐dipp)La(AlMe 4 ) 2 ], and [(2,4‐dtbp)Ln(AlMe 4 ) 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(AlMe 4 )] (Ln=Y, Lu) and [(2,4‐dipp) 2 Y(AlMe 4 )]. 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(AlMe 4 ) 2 ] with B(C 6 F 5 ) 3 led to the isolation and full characterization of the single‐component catalyst {{(2,4‐dtbp)La[(μ‐Me) 2 AlMe(C 6 F 5 )]}[Me 2 Al(C 6 F 5 ) 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(AlMe 4 ) 2 ] and GaMe 3 (OEt 2 ) leading to [(2,4‐dtbp)La(GaMe 4 ) 2 ].