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

• Published in: Macromolecules Vol. 47; no. 7; pp. 2233 - 2241
• Main Authors: Mou, Zehuai, Liu, Bo, Liu, Xinli, Xie, Hongyan, Rong, Weifeng, Li, Lei, Li, Shihui, Cui, Dongmei
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 08.04.2014
• Subjects: Applied sciences; Exact sciences and technology; Organic polymers; Physicochemistry of polymers; Polymerization; Preparation, kinetics, thermodynamics, mechanism and catalysts; Ring opening polymerization; Ligand effect; Complex catalyst polymerization; Complex catalyst; Experimental study; Tertiary phosphine imine; Solution polymerization; Tertiary phosphine oxide; Lactic acid polymer; Stereospecificity; Lanthanide Complexes; Stereospecific polymerization; Chemical synthesis; Catalyst activity; Room temperature; Crystalline structure
• ISSN: 0024-9297; 1520-5835
• DOI: 10.1021/ma500209t

A series of oxophosphine (3,5-Me2Pz)2CHP(R)2O (Pz = pyrazole; R = t Bu (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 (P r = 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 P r = 0.98. This result sheds new light on designing specifically selective catalytic precursors.