Synthesis and structural determination of zinccomplexes based on an anilido-aldimine ligandcontaining an O-donor pendant arm: zinc alkoxidederivative as an efficient initiator for ring-openingpolymerization of cyclic esters

Abstract

Zinc complexes bearing the anilido-aldiminate AAOMe ligand (AAOMe-H = (E)-2,6-diisopropyl-N-(2-(((2-methoxyethyl)imino)methyl)phenyl)aniline) were synthesized in a stepwise method and were structurallycharacterized. The reaction of AAOMe-H (1) with one equivalent of diethyl zinc (ZnEt2) furnishes a threecoordinatedand mononuclear zinc complex [(AAOMe)ZnEt] (2). Further reaction of 2 with a stoichiometricamount of benzyl alcohol (BnOH) affords a four-coordinated and dinuclear zinc benzylalkoxidecomplex [(AAOMe)Zn(μ-OBn)]2 (3). In the presence of two equivalents of AAOMe-H with ZnEt2, a homolepticand four-coordinated zinc complex [(AAOMe)2Zn] (4) is formed. The geometry around the zinc centresof 3 and 4 are both distorted tetrahedrals, while 2 adopts a different coordination mode with a slightlydistorted trigonal planar geometry. The variable-temperature 1H NMR studies of 3 illustrate that 3 exhibitsa dinuclear structure in solution at low temperature as well as in the solid state. While raising thetemperature, it drifts towards dissociation to form a mononuclear zinc benzylalkoxide species, whichcoexists in solution. The ring-opening polymerizations of ε-caprolactone (ε-CL) and β-butyrolactone (β-BL)catalyzed by complexes 3 and 4 are investigated. The ε-CL and β-BL polymerizations initiated by zinc alkoxide3 were demonstrated to have living characteristics and to proceed in a controlled manner withnarrow polydispersity indices (PDIs < 1.12). An efficient catalytic performance for the β-BL polymerizationwith a high monomer-to-initiator ratio (1200/1) initiated by 3 has also been reported