Synthesis and structural determination of zinc complexes based on an anilido-aldimine ligand containing an O-donor pendant arm: zinc alkoxide derivative as an efficient initiator for ring-opening polymerization of cyclic esters
Zinc complexes bearing the anilido-aldiminate AAOMeOMeOMeOMeligand (AAOMeOMeOMeOMe-H = (E)-2,6-diisopropyl-N-(2-(((2-methoxyethyl)imino)methyl)phenyl)aniline) were synthesized in a stepwise method and were structurally characterized. The reaction of AAOMeOMeOMeOMe-H (1) with one equivalent of diethyl zinc (ZnEt2) furnishes a three-coordinated and mononuclear zinc complex [(AAOMeOMeOMeOMe)ZnEt] (2). Further reaction of 2 with a stoichiometric amount of benzyl alcohol (BnOH) affords a four-coordinated and dinuclear zinc benzylalkoxide complex [(AAOMeOMeOMeOMe)Zn(μ-OBn)]2 (3). In the presence of two equivalents of AAOMeOMeOMeOMe-H with ZnEt2, a homoleptic and four-coordinated zinc complex [(AAOMeOMeOMeOMe)2Zn] (4) is formed. The geometry around the zinc centres of 3 and 4 are both distorted tetrahedrals, while 2 adopts a different coordination mode with a slightly distorted trigonal planar geometry. The variable-temperature 1H NMR studies of 3 illustrate that 3 exhibits a dinuclear structure in solution at low temperature as well as in the solid state. While raising the temperature, it drifts towards dissociation to form a mononuclear zinc benzylalkoxide species, which coexists 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 alkoxide 3 were demonstrated to have living characteristics and to proceed in a controlled manner with narrow polydispersity indices (PDIs < 1.12). An efficient catalytic performance for the β-BL polymerization with a high monomer-to-initiator ratio (1200/1) initiated by 3 has also been reported.
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