Chiral Lanthanocene Derivatives Containing Two Linked Amido−Cyclopentadienyl Ligands:  Heterobimetallic Structure and Lactone Polymerization Activity

Reaction of 2 equiv of dilithium amido−cyclopentadienide Li 2 (C 5 R 4 SiMe 2 NCH 2 CH 2 X) (C 5 R 4 = C 5 Me 4, C 5 H 3 t Bu; X = OMe, NMe 2 ) with anhydrous LnCl 3 (Ln = Y, Lu) gave C 2 -symmetric complexes of the type Li[Ln(η 5:η 1 -C 5 R 4 SiMe 2 NCH 2 CH 2 X) 2 ] containing a heterobimetallic core. The molecular structure of Li[Y(η 5:η 1 -C 5 Me 4 SiMe 2 NCH 2 CH 2 OMe) 2 ] was determined by single-crystal X-ray structural analysis, which showed it to exhibit both a pseudotetrahedral yttrium as well as a tetrahedral lithium center. In accordance with a formal 20-electron configuration at the rare earth metal center Ln, the amido-nitrogen atoms are shown to be pyramidalized. In the case of Li[Y(η 5:η 1 -C 5 H 3 t BuSiMe 2 NCH 2 CH 2 X) 2 ], mixtures of the two diastereomers in relative ratios depending on the reaction time are formed. The ( R, S )-diastereomer is the kinetic product that rearranges to the thermodynamically preferred C 2 -symmetric ( R, R )-diastereomer in donor solvents such as THF. The molecular structures of diastereomerically pure ( R, S )-Li[Y(η 5:η 1 -C 5 H 3 t BuSiMe 2 NCH 2 CH 2 OMe) 2 ] and ( R, R )-Li[Y(η 5:η 1 -C 5 H 3 t BuSiMe 2 NCH 2 CH 2 NMe 2 ) 2 ], as determined by single-crystal X-ray structural analyses, confirm the significantly different steric congestion. All heterobimetallic complexes were shown to be active in the ring-opening polymerization of ε-caprolactone. The poly(ε-caprolactone)s obtained are of high molecular weight ( M n > 30 000) and moderate polydispersity ( M w / M n < 2.0) and are produced by the action of the nucleophilic amido-nitrogen atom.