Bulky Cp ligands

Tris(cyclopentadienyl)lanthanide complexes with steri-cally more crowded Cp ligands such as C5Me4R (R = Me, Et, Tr, and SiMe3) are not assessable by simple metathesis between lanthanide trihalides and the respective alkali metal salt of the bulky Cp ligand. For instance, Cp 3Sm, obtainable from Cp 2Sm and cyclooctatetraene, reacts with THF with ring-opening forming Cp 2Sm[0(CH2)4Cp ](THF) (equation 14). 

Turner and co-workers and Bochmann el al. have reported that propylene is polymerized by in situ generated base-free cations Cp2M(R) (49,51). In contrast, Cp 2M(CH3)(THT) (M = Zr, Hf) complexes catalyze propylene oligomerization under mild conditions (PhNMe2, 25°C, 1 atm) by an insertion/)J-CH3 elimination process (Scheme 8) (53). The conventional J3-H elimination chain transfer process is disfavored by severe steric interactions between the C)J-substituents and the bulky Cp ligands in the transition state (Scheme 8). The Hf system produces only Cs and C, products whereas the Zr system produces oligomers up to C24 under these conditions. 

Direct (3-Mo elimination was observed when activating zirconocene methyl neopentyl complexes with B(C6F5)3.555 With sterically bulky Cp ligands, instantaneous isobutylene elimination is observed at — 75 °G however, for the bis-(Cp) compound, the zwitterionic neopentyl complex species is stable at 0 °C but undergoes clean and reversible f3-Me elimination at 25 °C (Scheme 177). This finding is consistent with (3-Me elimination as the major chain-transfer pathway in propylene polymerizations using a sterically encumbered metallocene catalyst. 

Lanthanides. The advantage of synthesizing f element derivatives with only one bulky Cp ligand is to have compounds more reactive, as the other coordination positions are more accessible for substitution. 

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