Direct Synthesis of Ion-pair Lanthanide Borohydrides and their High Activity for Polymerization ofL-Lactide and ϵ-Caprolactone

The reactions of LnCl3 (Ln = Nd, Pr, La) with NaBH4 in a THF solution at 75 °C afforded ion-pair lanthanide borohydrides [Ln(BH4)2(THF)5]+[Ln(BH4)4(THF)2]– [Ln = Nd (1), Pr (2), La (3)] in high yields whilst similar reactions at 40 °C yielded the neutral borohydrides [Ln(BH4)3(THF)3] [Ln = Nd (4), Pr (5), La (6)]. The influence of the size of the lanthanide metals on the synthesis of ion-pair lanthanide borohydrides was observed. The same procedure with the middle and latter lanthanide metal chlorides as that for the synthesis of 1–3 did not afford the analogous complexes, but the neutral lanthanide borohydrides [Ln(BH4)3(THF)3] [Ln = Sm (7), Eu (8), Yb (9)] were isolated. Complexes 1–3 showed high activity for the ring-opening polymerizations of ϵ-caprolactone (ϵ-CL) and L-Lactide (L-LA). The comparable study on the reactivity between 1 and 4 revealed that 1 showed somewhat higher activity than 4 in all the polymerizations studied, but no quite difference in chemistry behavior between the two complexes.