Mixed Allyl–Borohydride Lanthanide Complexes: Synthesis of Ln(BH4)2(C3H5)(THF)3 (Ln = Nd, Sm), Characterization, and Reactivity toward Polymerization

New mixed allyl–borohydrido lanthanide complexes Ln­(BH4)2(C3H5)­(THF)3 (Ln = Nd (1), Sm (2)) could be prepared in good yield by reacting Ln­(BH4)3(THF)3 (Ln = Sm, Nd) with 1/2 equiv of Mg­(C3H5)2(THF)x. X-ray structure analysis revealed monomeric structures with two terminal BH4 ligands, one π-allyl ligand, and three THF molecules. In an assessment of isoprene polymerization, 1 afforded trans-1,4-polyisoprene in good yield, as a single component or in combination with Mg cocatalyst. Both 1 and 2 were found to be extremely active toward ε-caprolactone polymerization.