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 accommodate 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.