Synthesis of samarium(ii) borohydrides and their behaviour as initiators in styrene and ε-caprolactonepolymerisation

Abstract

The bisborohydrido samarium compound Sm(BH₄)₂(THF)₂ (1) was prepared in high yield by comproportionation between Sm and 2 equivalents of Sm(BH₄)₃(THF)₃. Reaction of 1 with KCp* (Cp* = C₅Me₅) afforded the half-sandwich Cp*Sm(BH₄)(THF)₂ (2). The ¹H NMR borohydride resonances of both complexes, observed at very high field, are typical of divalent Sm compounds. X-Ray single crystal analysis revealed polymeric and dimeric molecular arrangements for 1 and 2, respectively. Complex 1 was found moderately active towards styrene polymerisation when activated with Al(ⁱBu)₃, or with a borate/Al combination, whereas 2 showed no activity. Ring-opening polymerisation of ε-caprolactone could be performed rapidly at room temperature with both initiators. Complex 2 leads to narrow polydispersities and higher activity. Two different mechanisms, by monoelectronic transfer or by insertion into the Sm–(BH₄) bond can be proposed.