New [ONOO]-Type Amine Bis(phenolate) Ytterbium(II) and -(III) Complexes: Synthesis, Structure, and Catalysis for Highly Heteroselective Polymerization of rac-Lactide
journal contributionposted on 06.01.2014, 00:00 authored by Sheng Yang, Kun Nie, Yong Zhang, Mingqiang Xue, Yingming Yao, Qi Shen
Two divalent ytterbium (YbII) complexes, 1 and 2, supported by new [ONOO]-type amine bis(phenolate) ligands La,b were synthesized in good yield by an amine elimination reaction of YbII(N(SiMe3)2)2(TMEDA) (TMEDA = tetramethylethylenediamine) with one equivalent of the ligand precursor. X-ray structural determination showed complexes 1 and 2 both have a monomeric structure. Each adopts distorted octahedral coordination geometry around the six-coordinate YbII ion. Two new trivalent ytterbium (YbIII) aryloxide complexes, 4 and 5, bearing [ONOO]-type amine bis(phenolate) ligands Lc and Ld were prepared by double-protonation reaction of Yb(C5H5)3THF with one equivalent of the ligand precursor, then one equivalent of phenol. Complex 4 has a symmetric dimeric structure with a Yb2O2 core bridging through the oxygen atoms of the OC6H4-4-CH3 groups. Complex 5 is a THF-solvated monomer. Each six-coordinated YbIII ion in both complexes adopts a distorted octahedron. All the complexes synthesized together with the known YbII complex 3 were evaluated in the ring-opening polymerization (ROP) of rac-lactide (rac-LA). Complexes 1 and 4 were found to be extremely active for controlled ROP of rac-LA, as judged by narrow molar mass distributions (Mw/Mn: 1.07–1.16 for complex 1 and 1.07–1.10 for complex 4) and experimental molar mass Mn,exp values in good agreement with theoretic Mn,calcd values calculated on a single PLA chain produced per metal center of initiator. Complex 5 is less controlled. Complexes 1, 4, and 5 exhibited the same high stereoselectivity to give heterotactic polylactide with a Pr (probability of racemic enchainment of monomer units) ranging from 0.97 to 0.99. Dramatic decreases in activity and stereoselectivity were found for complexes 2 (Pr = 0.82) and 3 (Pr = 0.67), indicating the framework of L in the unit of [YbL] is crucial for determining activity and stereoselectivity of either YbII or YbIII complexes.