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

Abstract

The potential of the phenoxy-thioether moieties as ancillary ligands in the coordination chemistry 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 (L₁1-H), CH₃ (L₂2-H), Br (L₃3-H), CF₃ (L₄4-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 L₁1-H and L₄4-H. In contrast, homoleptic yttrium complexes bearing three phenoxy-thioether ligands were favored with L₂2-H and L₃3-H. The activities of all the synthesized complexes toward the ring-opening polymerization of ε-caprolactone and L- and rac-lactide were investigated, also in combination with an alcohol as an external chain transfer agent. Polyesters with controlled molecular parameters (M_n, end groups) and low polydispersities were obtained. The monoinsertion adduct, produced by the reaction of {(L₁1)₂ScN(SiHMe₂)₂} and 1 equiv. of ε-caprolactone, was isolated proving that a coordination–insertion mechanism of ring-opening polymerization was operative. In the polymerization of rac-lactide, yttrium complexes exerted a high degree of stereocontrol producing heterotactic polylactides (P_r up to 0.91).