Efficient and Heteroselective Heteroscorpionate Rare-Earth-Metal Zwitterionic Initiators for ROP of rac-Lactide: Role of σ-Ligand
作者：Zehuai Mou, Bo Liu, Xinli Liu, Hongyan Xie, Weifeng Rong, Lei Li, Shihui Li and Dongmei Cui
關鍵字：oxophosphine; iminophosphine; metathesis reaction; rac-LA; heteroselectivity
論文來源：期刊
具體來源：http://pubs.acs.org/doi/pdf/10.1021/ma500209t
發表時間：2014年
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.
