Carbon-Bridged Bis(phenolato)lanthanide Alkoxides: Syntheses, Structures, and Their Application in the Controlled Polymerization of ε-Caprolactone

A convenient method for the synthesis of lanthanide alkoxo complexes supported by a carbon-bridged bis(phenolate)
ligand 2,2‘-methylenebis(6-tert-butyl-4-methylphenoxo) (MBMP2-) is described. The reaction of (C5H5)3Nd with
MBMPH2 in a 1:1 molar ratio in THF gave the bis(phenolato)lanthanide complex (C5H5)Nd(MBMP)(THF)2 (1) in a
nearly quantitative yield. Complex 1 further reacted with 1 equiv of 2-propanol in THF to yield the bis(phenolato)lanthanide isopropoxide [(MBMP)2Nd(μ-OPri)(THF)2]2 (2) in high yield. Complex 2 can also be synthesized by the
direct reaction of (C5H5)3Nd with MBMPH2 in a 1:1 molar ratio and then with 1 equiv of 2-propanol in situ in THF.
Thus, the analogue bis(phenolato)lanthanide alkoxides [(MBMP)2Ln(μ-OR)(THF)2]2 [R = Pri, Ln = Yb (3); R =
Me, Ln = Nd (4), Yb (5); R = CH2Ph, Ln = Nd (6), Yb (7)] were obtained by the reactions of (C5H5)3Ln (Ln =
Nd, Yb) with MBMPH2 and then with 2-propanol, methanol, or benzyl alcohol, respectively. The ytterbium complex
{[(MBMP)2Yb(THF)2]2(μ-OCH2Ph)(μ-OH)} (8) was also isolated as a byproduct. The single-crystal structural analyses
of complexes 1−3 and 8 revealed that the coordination geometry around lanthanide metal can be best described
as a distorted tetrahedron in complex 1 and as a distorted octahedron in complexes 2, 3, and 8. A O−H···Yb
agostic interaction was observed in complex 8. Complexes 2−7 were shown to be efficient catalysts for the controlled
polymerization of ε-caprolactone.