Magnesium and zinc complexes bearing NNO-tridentate ketiminate ligands: synthesis, structures and catalysis in the ring-opening polymerization of lactides

Abstract

A series of magnesium and zinc complexes, [(L^1–3)₂M₂(μ-OBn)₂] (M = Mg (1–3), Zn (4–6)), have been synthesized by the reaction of NNO-tridentate ketiminate ligands (L^1–3-H) and a stoichiometric amount of MgⁿBu₂ (or ZnEt₂) and BnOH. In addition, the reaction of these ketiminate ligands (L^1–4-H) with a half equivalent of MgⁿBu₂ (or ZnEt₂) in toluene provides [M(L^1–4)₂] (M = Mg (7–10), Zn (11–14)) in good yields. All of these complexes have been fully characterized by NMR spectroscopy and elemental analysis, and the molecular structures of [(L¹)₂Mg₂(μ-OBn)₂] (1), [(L³)₂Mg₂(μ-OBn)₂] (3), [(L¹)₂Zn₂(μ-OBn)₂] (4), [Mg(L¹)₂] (7), [Zn(L¹)₂] (11) and [Zn(L⁴)₂] (14) have been further confirmed by X-ray crystallographic studies. X-ray diffraction studies revealed that complexes 1, 3 and 4 were dimeric in the solid state, bridging through the benzyloxy groups, while the solid-state structures of 7, 11 and 14 revealed a mononuclear species, six-coordinated by N, N, and O atoms of two ketiminate ligands, forming a distorted octahedron around the metal centre. Complexes 1–6 acted as efficient initiators for the ring-opening polymerization of lactides, producing PLAs with controlled molecular weights and narrow molecular weight distributions. Complex 6 [(L³)₂Zn₂(μ-OBn)₂] exhibited the highest activity towards the ROP of lactides. And complexes 1–6 initiated rac-lactide polymerization to afford heterotactic enriched polymers (P_r up to 0.82). In addition, complexes 7–14 were also shown to efficiently catalyze the ring-opening polymerization of lactides in the presence of BnOH.