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

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• 作者：Zehuai Mou ; Bo Liu ; Xinli Liu ; Hongyan Xie ; Weifeng Rong ; Lei Li ; Shihui Li ; Dongmei Cui
• 刊名：Macromolecules
• 出版年：2014
• 出版时间：April 8, 2014
• 年：2014
• 卷：47
• 期：7
• 页码：2233-2241
• 全文大小：428K
• 年卷期：v.47,no.7(April 8, 2014)
• ISSN：1520-5835

文摘

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 L^1鈥?Ln(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 L^3鈥?Y(Cl)(CH₂SiMe₃)(THF) (L³ (6a), L⁴ (7a)), L^3鈥?Y(Cl)(N(SiHMe₂)₂)(THF) (L^3聽(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鈥?b>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鈥?.89 vs 0.80鈥?.84) despite the bulkiness of the ligands. Interestingly, the mixed halogen complexes 6a鈥?b>11a were single-site initiators, where the 蟽-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.