Polymerization of ɛ-caprolactone by lanthanide trisborohydrides and crystal structure of [Ce(BH4)2(THF)5][Ce(BH4)4(THF)2]

Complexes of lanthanide trisborohydrides, formally Ln(BH4)3(THF)n (Ln=La, Ce, Sm, Yb), were synthesized and their catalytic activity for polymerization of ɛ-caprolactone was studied. All the complexes can catalyze this polymerization. It was found that the catalytic activities decreased in the order of La>Ce>Sm>Yb. The crystal structure of cerium trisborohydride was determined for the first time by single crystal X-ray diffraction analysis. It is an ionic pair complex of [Ce(BH4)2(THF)5][Ce(BH4)4(THF)2]. The cationic part involves two η3-BH4 ligands, while the anionic part involves two η3-BH4 groups and two η2-BH4 groups.