Tridentate anilido-aldimine magnesium and zinc complexes as efficient catalysts for ring-opening polymerization of 8-caprolactone and L-Lactide

A novel tridentate anilido-aldimine ligand, [o-C₆H ₄(NHAr)-HC= NCH₂CH₂NMe₂] (Ar = 2,6-ⁱPr₂C₆H₃, L-H, 1), has been prepared by the condensation of N, N-dimethylethylenediamine with one molar equivalent of 2-fluoro-benzaldehyde in hexane, followed by the addition of the lithium salt of diisopropylaniline in THF. Magnesium (Mg) and zinc (Zn) complexes supported by the tridentate anilido-aldimine ligand have been synthesized and structurally characterized. Reaction of L-H (1) with an equivalent amount of MgⁿBu₂ or ZnEt₂ produces the monomeric complex [LMgⁿBu] (2) or [LZnEt] (3), respectively. Experimental results show that complexes 2 and 3 are efficient catalysts for ring-opening polymerization of e-caprolactone (CL) and L-lactide (LA) in the presence of benzyl alcohol and catalyze the polymerization of ε-CL and L-LA in a controlled fashion yielding polymers with a narrow polydispersity index. In both polymerizations, the activity of Mg complex 2 is higher than that of Zn complex 3, which is probably due to the higher Lewis acidity and better oxophilic nature of Mg²⁺ metal.