Rare-Earth-Metal-Hydrocarbyl Complexes Bearing Linked Cyclopentadienyl or Fluorenyl Ligands: Synthesis, Catalyzed Styrene Polymerization, and Structure-Reactivity Relationship

A series of rare-earth-metalhydrocarbyl complexes bearing N-type functionalized cyclopentadienyl (Cp) and fluorenyl (Flu) ligands were facilely synthesized. Treatment of [Y(CH2SiMe3)3(thf)2] with equimolar amount of the electron-donating aminophenyl-Cp ligand C5Me4H-C6H4-o-NMe2 afforded the corresponding binuclear monoalkyl complex [({C5Me4-C6H4-o-NMe(mu-CH2)}Y{CH2SiMe3})2] (1?a) via alkyl abstraction and C?H activation of the NMe2 group. The lutetium bis(allyl) complex [(C5Me4-C6H4-o-NMe2)Lu(?3-C3H5)2] (2?b), which contained an electron-donating aminophenyl-Cp ligand, was isolated from the sequential metathesis reactions of LuCl3 with (C5Me4-C6H4-o-NMe2)Li (1 equiv) and C3H5MgCl (2 equiv). Following a similar procedure, the yttrium- and scandiumbis(allyl) complexes, [(C5Me4-C5H4N)Ln(?3-C3H5)2] (Ln=Y (3?a), Sc (3?b)), which also contained electron-withdrawing pyridyl-Cp ligands, were also obtained selectively. Deprotonation of the bulky pyridyl-Flu ligand (C13H9-C5H4N) by [Ln(CH2SiMe3)3(thf)2] generated the rare-earth-metaldialkyl complexes, [(?3-C13H8-C5H4N)Ln(CH2SiMe3)2(thf)] (Ln=Y (4?a), Sc (4?b), Lu (4?c)), in which an unusual asymmetric ?3-allyl bonding mode of Flu moiety was observed. Switching to the bidentate yttriumtrisalkyl complex [Y(CH2C6H4-o-NMe2)3], the same reaction conditions afforded the corresponding yttrium bis(aminobenzyl) complex [(?3-C13H8-C5H4N)Y(CH2C6H4-o-NMe2)2] (5). Complexes 15 were fully characterized by 1H and 13C NMR and X-ray spectroscopy, and by elemental analysis. In the presence of both [Ph3C][B(C6F5)4] and AliBu3, the electron-donating aminophenyl-Cp-based complexes 1 and 2 did not show any activity towards styrene polymerization. In striking contrast, upon activation with [Ph3C][B(C6F5)4] only, the electron-withdrawing pyridyl-Cp-based complexes 3, in particular scandium complex 3?b, exhibited outstanding activitiy to give perfectly syndiotactic (rrrr >99?%) polystyrene, whereas their bulky pyridyl-Flu analogues (4 and 5) in combination with [Ph3C][B(C6F5)4] and AliBu3 displayed much-lower activity to afford syndiotactic-enriched polystyrene.