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

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Abstract

Reaction of 2 equiv of dilithium amido−cyclopentadienide Li2(C5R4SiMe2NCH2CH2X) (C5R4 = C5Me4, C5H3tBu; X = OMe, NMe2) with anhydrous LnCl3 (Ln = Y, Lu) gave C2-symmetric complexes of the type Li[Ln(η5:η1-C5R4SiMe2NCH2CH2X)2] containing a heterobimetallic core. The molecular structure of Li[Y(η5:η1-C5Me4SiMe2NCH2CH2OMe)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(η5:η1-C5H3tBuSiMe2NCH2CH2X)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 C2-symmetric (R,R)-diastereomer in donor solvents such as THF. The molecular structures of diastereomerically pure (R,S)-Li[Y(η5:η1-C5H3tBuSiMe2NCH2CH2OMe)2] and (R,R)-Li[Y(η5:η1-C5H3tBuSiMe2NCH2CH2NMe2)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 (Mn > 30 000) and moderate polydispersity (Mw/Mn < 2.0) and are produced by the action of the nucleophilic amido-nitrogen atom.

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