Efficient and Heteroselective Heteroscorpionate Rare-Earth-Metal Zwitterionic Initiators for ROP of rac-Lactide: Role of σ-Ligand

A series of oxophosphine (3,5-Me2Pz)2CHP(R)2O (Pz = pyrazole; R = tBu (HL1), Cy (HL2)) and iminophosphine (3,5-Me2Pz)2CHP(R)2NAr (R = Cy, Ar = Ph (HL3); R = Ph, Ar = Ph (HL4), Ar = 2,6-Me2-phenyl (HL5)) heteroscorpionate ligands were synthesized. Abstraction of the methide proton of these ligands by rare-earth-metal tris(alkyl)s, Ln(CH2SiMe3)3(THF)2, afforded the corresponding zwitterionic bis(alkyl) complexes L1–5Ln(CH2SiMe3)2(THF) (L1, Ln = Y (1a), Lu (1b); L2, Ln = Y (2a), Lu (2b); L3, Ln = Y (3a), Lu (3b); L4, Ln = Y (4a), Lu (4b); L5, Ln = Y (5a), Lu (5b), while metathesis reaction of the lithium salts of LiL3 and LiL4 with YCl3(THF)2 or YBr3(THF)2 followed by treatment with LiCH2SiMe3 and KN(SiHMe2)2, respectively, afforded the first heteroscorpionate yttrium mixed halogen/alkyl or amido complexes L3–4Y(Cl)(CH2SiMe3)(THF) (L3 (6a), L4 (7a)), L3–4Y(Cl)(N(SiHMe2)2)(THF) (L3 (8a), L4 (9a)), L4Y(Br)(CH2SiMe3)(THF) (10a), and L4Y(Br)(N(SiHMe2)2)(THF) (11a). The structures of these complexes were well-def...