Aluminum, calcium and zinc complexes supported by potentially tridentate iminophenolate ligands: synthesis and use in the ring‐opening polymerization of lactide

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.