Aluminum, Calcium and Zinc Complexes Supported by Potentially Tridentate Imino-Phenolate Ligands: Synthesis and Use in the Ring-Opening Polymerization of Lactide
Résumé
The coordination chemistry of the potentially tridentate phenoxyethyl- and benzylaminoethyl-iminophenol pro-ligands {ONO}H and {ONN}H on to calcium, zinc and aluminum centers has been studied. {ONO}Ca(N(SiMe3)2)(THF) (1) was prepared by a one-pot salt metathesis procedure but the analogous reaction with {ONN}H led to intractable mixtures. Reaction of {ONO}H and {ONN}H with ZnEt2 (0.5 or 1 equiv.) systematically led to isolation of the corresponding homoleptic complexes {ONO}2Zn (2) and {ONN}2Zn (3). The dimethylaluminum complexes {ONO}AlMe2 (4) and {ONN}AlMe2 (5) were readily prepared by treatment of AlMe3 with 1 equiv. of the corresponding pro-ligands. Compounds 2 and 4 both feature monomeric structures in the solid state, with chelating iminophenolate ligands and free-hanging phenoxyethyl arms. The amido complex 1 was shown to be a moderately active initiator for the controlled ring-opening polymerization (ROP) of racemic lactide at room temperature, yielding polylactides with high initiation efficiencies, relatively narrow polydispersities and a slight heterotactic bias. Immortal polymerizations were achieved by combining excess isopropanol to 1, offering up to 50 macromolecules per metal center, with well-controlled molecular features. The dimethylaluminum compounds 4 and 5 initiated the controlled ROP of lactide in the presence of 1 equiv. of benzyl alcohol as a co-initiator but required higher temperatures. Copyright © 2012 John Wiley & Sons, Ltd.