Lanthanide bis(borohydride) complexes coordinated by tetradentate phenoxide ligand: Synthesis, structure, and catalytic activity in ring-opening polymerization of rac-lactide and ε-caprolactone

Abstract

New lanthanide bis(borohydride) complexes coordinated by monoanionic tetradentate phenoxide ligand [(C5H4NCH2)2NCH2(3,5-tBu2C6H2O)]Ln(BH4)2 [Ln = Y (1), Nd (2), Sm (3)] were synthesized by the reactions of Ln(BH4)3(THF)3 with an equimolar amount of the lithium phenoxide [(C5H4NCH2)2NCH2(3,5-tBu2C6H2O)Li] in THF at 60 °C. The complexes were isolated after recrystallization from toluene in 75, 64, and 69% yields, respectively. X-ray analysis revealed that in 1 the phenoxide ligand is tetradentate, while two borohydride groups are coordinated to the Y3+ ion in a κ3-fashion. Compounds 1–3 initiate ring-opening polymerization (ROP) of ɛ-caprolactone (ɛ-CL) and racemic lactide (rac-LA) in toluene at 25 °C. The obtained polylactides are characterized by atactic structures Pr = 0.54–0.58, and rather narrow polydispersities (Mw/Mn = 1.1–1.5). For ɛ-caprolactone polymerization, the resulting polymers have moderate polydispersities (Mw/Mn = 1.4–2.4) and molecular weights (6.2–54.1·103).