Synthesis, Structure, and Polymerization Activity of Neutral Halide, Alkyl, and Hydrido Yttrium Complexes of Isopropylidene-Bridged Cyclopentadienyl-Fluorenyl Ligands

Reactions of the anionic complex [(Cp-CMe2-Flu)YCl2]-[Li(ether)4]+ (1) (Cp = C5H4, Flu =
9-C13H8), prepared in situ from YCl3(THF)3.5 and 1 molar equiv of the dilithium salt [Cp-CMe2-Flu]Li2, with equimolar amounts of RLi give alkyl mono-THF complexes [(Cp-CMe2-Flu)]Y(R)(THF) (R = CH(SiMe3)2, 3; CH2SiMe3, 4) in high yields. The solid-state structure
of 3 was established by X-ray diffraction, showing the fluorenyl moiety symmetrically
coordinated to yttrium in an intermediary η3-η5 mode. Hydrogenolysis of 3 and 4 with H2 or
PhSiH3 gives the hydride {[(μ:η5,η5-Cp-CMe2-Flu)]Y(μ-H)(THF)}2 (5). The solid-state structure
of 5 was determined by X-ray diffraction, revealing a dimeric structure with both bridging
Cp-CMe2-Flu and hydride ligands (Y−H = 1.99(4)−2.01(4) Å). Complex 5 is the first
structurally characterized example of a group 3 metal hydride stabilized by a fluorenyl ligand.
Reaction of 1 with PhCH2MgBr gives, instead of a benzyl derivative, the neutral base-free
bromo complex {[(η5,η5-Cp-CMe2-Flu)]Y(μ-Br)}2 (6), which shows a dimeric structure in the
solid state with chelating Cp-CMe2-Flu and bridging bromide ligands. Introduction of the
bulky tert-butyl substituent on the Cp ring of the ligand system enabled the preparation of
the neutral chloro complex [(3-tBuCp)-CMe2-Flu]YCl(THF) (7), using a salt elimination
between the dilithium salt of the ligand and YCl3(THF)3.5. Reaction of 7 with LiCH(SiMe3)2
gives the alkyl complex {[(3-tBuCp)-CMe2-Flu)]Y(CH(SiMe3)2) (8), which contains no THF
molecule in its coordination sphere in contrast to unsubstituted analogues 3 and 4.
Preliminary studies of the catalytic activity of these new complexes for ethylene and MMA
polymerization are reported.