Rare earth complexes of phenoxy-thioether ligands: synthesis and reactivity in the ring opening polymerization of cyclic esters

The potential of the phenoxy-thioether moieties as ancillary ligands in the coordination chem. of group 3 metals was explored. The amine elimination reactions between yttrium and scandium amido precursors {M [N(SiHMe2)2]3 (THF)n} with the proligands {4,6-tBu2-OC6H2-2-CH2S(2-R-C6H4)}H [R = H (L1-H), CH3 (L2-H), Br (L3-H), CF3 (L4-H)], bearing different substituents at the ortho position of the thiophenol aryl ring, were studied. The outcome of aminolysis reactions gave different results depending on the structure of the proligand and the reaction conditions. Heteroleptic scandium and yttrium amido complexes with general formula {(L)2M [N(SiHMe2)2] (THF)n} were prevailingly obtained from phenoxy-thioether proligands L1-H and L4-H. In contrast, homoleptic yttrium complexes bearing three phenoxy-thioether ligands were favored with L2-H and L3-H. The activities of all the synthesized complexes toward the ring-opening polymn. of ε-caprolactone and L- and rac-lactide were investigated, also in combination with an alc. as an external chain transfer agent. Polyesters with controlled mol. parameters (Mn, end groups) and low polydispersities were obtained. The monoinsertion adduct, produced by the reaction of {(L1)2ScN(SiHMe2)2} and 1 equiv. of ε-caprolactone, was isolated proving that a coordination-insertion mechanism of ring-opening polymn. was operative. In the polymn. of rac-lactide, yttrium complexes exerted a high degree of stereocontrol producing heterotactic polylactides (Pr up to 0.91).