Homoleptic magnesium and calcium complexes supported by constrained reduced Schiff base ligand for lactide polymerisation: DFT analysis of lactide/ligand interactions

Abstract

A homoleptic calcium complex supported by a constrained reduced Schiff base ligand was synthesised and found to be highly active for the ring-opening polymerisation (ROP) of 100 equiv. lactide within 2 min at room temperature. However, the magnesium analogue has significantly lower reactivity. Single-crystal X-ray diffraction revealed a 6-coordinate metal environment for both complexes. Density functional theory (DFT) calculations showed that the calcium complex can asccommodate up to 7-coordination, thereby facilitating polymerisation without detachment of the dimethylamino sidearm, unlike in the magnesium complex. The participation of the phenoxy group and secondary amine (N–H) were observed to facilitate polymerisation via hydrogen bonding.