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-defined, and the molecular structures of 1a, 2a, 3b, 4b, 5a, and 7a were further characterized by single crystal X-ray diffraction analysis. Complexes 1–5 showed similar high activity toward the ROP of rac-LA at room temperature, and both the alkyl species participated in initiation, of which the lutetium complexes exhibited slightly higher selectivity than their yttrium analogues (Pr = 0.85–0.89 vs 0.80–0.84) despite the bulkiness of the ligands. Interestingly, the mixed halogen complexes 6a–11a were single-site initiators, where the σ-halogen moiety remaining on the central metal showed, for the first time, facilitating the heteroselectivity up to Pr = 0.98. This result sheds new light on designing specifically selective catalytic precursors.