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

A series of magnesium and zinc complexes, [(L1-3)(2)M-2(-OBn)(2)] (M = Mg (1-3), Zn (4-6)), have been synthesized by the reaction of NNO-tridentate ketiminate ligands (L1-3-H) and a stoichiometric amount of (MgBu2)-Bu-n (or ZnEt2) and BnOH. In addition, the reaction of these ketiminate ligands (L1-4-H) with a half equivalent of (MgBu2)-Bu-n (or ZnEt2) in toluene provides [M(L1-4)(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 [(L-1)(2)Mg-2(-OBn)(2)] (1), [(L-3)(2)Mg-2(-OBn)(2)] (3), [(L-1)(2)Zn-2(-OBn)(2)] (4), [Mg(L-1)(2)] (7), [Zn(L-1)(2)] (11) and [Zn(L-4)(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 [(L-3)(2)Zn-2(-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.