Synthesis of samarium(II) borohydrides and their behaviour as initiators in styrene and ε-caprolactone polymerisation

摘要

The bisborohydrido samarium compound Sm(BH_4)_2(THF) _2 (1) was prepared in high yield by comproportionation between Sm and 2 equivalents of Sm(BH_4)_3(THF)_3. Reaction of 1 with KCp* (Cp* = C_5Me_5) afforded the half-sandwich Cp*Sm(BH_4)(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-(BH_4) bond can be proposed.