New [ONOO]-type amine bis(phenolate) ytterbium(II) and -(III) complexes: synthesis, structure, and catalysis for highly heteroselective polymerization of rac-lactide.

Two divalent ytterbium (Yb(II)) complexes, 1 and 2, supported by new [ONOO]-type amine bis(phenolate) ligands L(a,b) were synthesized in good yield by an amine elimination reaction of Yb(II)(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 Yb(II) ion. Two new trivalent ytterbium (Yb(III)) aryloxide complexes, 4 and 5, bearing [ONOO]-type amine bis(phenolate) ligands L(c) and L(d) 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 Yb(III) ion in both complexes adopts a distorted octahedron. All the complexes synthesized together with the known Yb(II) 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 Yb(II) or Yb(III) complexes.