Cationic rare-earth metal bis(tetrahydridoborato) complexes: direct synthesis, structure and ring-opening polymerisation activity toward cyclic esters

Rare-earth metal tris(borohydrides) [Ln(BH(4))(3)(thf)(3)] (Ln = Y, La, Nd, Sm) are converted with one equivalent of the Bronsted acid [NEt(3)H](+)[BPh(4)](-) in thf into the monocationic bis(borohydride) complexes [Ln(BH(4))(2)(thf)(5)](+)[BPh(4)](-). They efficiently initiate the ring-opening polymerisation of epsilon-caprolactone.