Rare‐earth metal derivatives supported by aminophenoxy ligand: Synthesis, characterization and catalytic performance in lactide polymerization

Abstract

A library of rare‐earth metal derivatives supported by an aminophenoxy ligand was prepared and their catalytic performance in lactide polymerization was investigated. It was found that the synthetic strategy had a profound effect on the formation of aminophenoxy rare‐earth metal complexes. Amine elimination between Ln[N(SiMe3)2]3(μ‐Cl)Li(THF)3 (Ln = Yb, Y) and 1 equiv. of the aminophenol [HONH] ([HONH] = ο‐OCH3‐C6H4NHCH2(3,5‐tBu2‐C6H2‐2‐OH)) in toluene gave the unexpected heterobimetallic bis(aminophenoxy) rare‐earth metal complexes [ON]2LnLi(THF)2 (Ln = Yb (1), Y (2)). When the reactions were carried out in THF and TMEDA, amine elimination produced the aminophenoxy rare‐earth metal amide complexes {[ON]LnN(SiMe3)2}2 (Ln = Yb (5), Y (6)) in ca 85% isolated yields. Complexes 5 and 6 could also be obtained from salt metathesis reaction of {[ON]LnCl(THF)}2 (Ln = Yb (3), Y (4)) with NaN(SiMe3)2 in a 1:2 molar ratio. In addition, treatment of complexes 3 and 4 with NaOAr (Ar = &bond;C6H4‐4‐tBu) and (SiMe3)2NC(NPri)2Na in 1:4 and 1:2 molar ratios provided the corresponding aminophenoxy rare‐earth metal derivatives {[ON](μ‐OAr)Ln(μ‐OAr)Na(THF)2}2 (Ln = Yb (7), Y (8)) and {[ON]Ln[(iPrN)2CN(SiMe3)2]}2 (Ln = Yb (9), Y (10)), respectively. These complexes were fully characterized, and their molecular structures were determined using single‐crystal X‐ray diffraction. Polymerization experiments showed that complexes 1, 2, 5, 6, 9 and 10 were highly active for the ring‐opening polymerization of l‐lactide in toluene, and complex 1 promoted l‐lactide polymerization in a controlled fashion. The polymerization of rac‐lactide initiated by the neutral aminophenoxy rare‐earth metal complexes 5, 6, 9 and 10 in THF afforded heterotactic polymers.