Synthesis and Characterization of Multi-Alkali-Metal Tetraphenolates and Application in Ring-Opening Polymerization of Lactide

A series of alkali metal complexes supported by two bulky
tetraphenols were synthesized and characterized. The reactions of
α,α,α′,α′-tetra­(3,5-di-tert-butyl-2-hydroxyphenyl)-p-xylene­(para-tetraphenol) with nBuLi,
sodium, and KSi­(NMe2)3 gave bimetallic complexes 1, 2, and 3, respectively. Treatments
of the α,α,α′,α′-tetra­(3,5-di-tert-butyl-2-hydroxyphenyl)-m-xylene­(meta-tetraphenol) with 2 or 4 equiv of nBuLi afforded complexes 4 or 5, while
the reactions of meta-tetraphenol with sodium and
KSi­(NMe2)3 gave only trimetallic complexes 6 and 7 for the additional p−π interaction.
Complexes 1–7 were all characterized
by single-crystal X-ray diffraction techniques. In the presence of
benzyl alcohol, all complexes are active catalysts for the ring-opening
polymerization of l-lactide. Comparatively, bimetallic complexes 1, 2, and 3 are more efficient catalysts
because of their symmetric structures, in which complex 3 presents as a rare highly active potassium catalyst for the ring-opening
polymerization of lactide, leading to polymers with good molecular
weight control and narrow molecular weight distributions.