title: Rare earth complexes of phenoxy-thioether ligands: 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 (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 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 (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 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).
authors.affiliation: Department of Chemistry and Biology, University of Salerno. I-84084, via Ponte don Melillo, Fisciano, Salerno, Italy. mmazzeo@unisa.it
authors.forename: Mina
authors.initials: M
authors.lastname: Mazzeo
authors.forename: Roberta
authors.initials: R
authors.lastname: Tramontano
authors.forename: Marina
authors.initials: M
authors.lastname: Lamberti
authors.forename: Alessia
authors.initials: A
authors.lastname: Pilone
authors.forename: Stefano
authors.initials: S
authors.lastname: Milione
authors.forename: Claudio
authors.initials: C
authors.lastname: Pellecchia
mesh_terms: D004952:Esters / Q000737:chemistry*; D008024:Ligands; D008674:Metals, Rare Earth / Q000737:chemistry*; D015394:Molecular Structure; D009942:Organometallic Compounds / Q000138:chemical synthesis* / Q000737:chemistry*; D010636:Phenols / Q000737:chemistry*; D058105:Polymerization; D013440:Sulfides / Q000737:chemistry*
chemical_list: D004952:Esters; D008024:Ligands; D008674:Metals, Rare Earth; D009942:Organometallic Compounds; D010636:Phenols; D013440:Sulfides; C042329:phenoxy radical