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

Inorganic Chemistry, ISSN: 0020-1669, Vol: 53, Issue: 1, Page: 105-115
2014
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Two divalent ytterbium (Yb) complexes, 1 and 2, supported by new [ONOO]-type amine bis(phenolate) ligands L were synthesized in good yield by an amine elimination reaction of Yb(N(SiMe ))(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 ion. Two new trivalent ytterbium (Yb ) aryloxide complexes, 4 and 5, bearing [ONOO]-type amine bis(phenolate) ligands L and L were prepared by double-protonation reaction of Yb(CH)THF with one equivalent of the ligand precursor, then one equivalent of phenol. Complex 4 has a symmetric dimeric structure with a YbO core bridging through the oxygen atoms of the OCH -4-CH groups. Complex 5 is a THF-solvated monomer. Each six-coordinated Yb ion in both complexes adopts a distorted octahedron. All the complexes synthesized together with the known Yb 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 (M /M: 1.07-1.16 for complex 1 and 1.07-1.10 for complex 4) and experimental molar mass M values in good agreement with theoretic M 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 P (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 (P = 0.82) and 3 (P = 0.67), indicating the framework of L in the unit of [YbL] is crucial for determining activity and stereoselectivity of either Yb or Yb complexes. © 2013 American Chemical Society.

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