Aminopiperidine based complexes for lactide polymerisation

Abstract

Herein we report the synthesis and characterisation of a series of salalen and salan ligands derived from 2-(aminomethyl)piperidine. Depending on the choice of starting salicylaldehyde, a bicyclic salan type ligand (1–3H₂) or imino salalen type ligand (4–6H, 7–9H₂) were prepared. The ligands were successfully complexed to group 4 metals and aluminium; with hafnium and zirconium octahedral complexes, M(1–3)₂, were realised; whilst with aluminium tetrahedral and trigonal bipyramidal complexes, Al(1–9)Me_x (x = 1,2), were isolated. The complexes have been characterised in solution via¹H and ¹³C{¹H} NMR spectroscopy and in the solid state by X-ray crystallography. The group 4 complexes were observed to have a fac–fac arrangement of ligands and there were two isomers present when 3H₂ was ligated. The imino aluminium complexes Al(7–9)Me were isolated as a mixture of diastereoisomers. The resultant complexes were trialed in the ring opening polymerisation of rac-lactide with both heterotactic and isotactic PLA being demonstrated. Tacticity was found to be dependent on the nature of the ligand and metal used; the M(1–3)₂ complexes were generally found to have a heterotactic preference (P_r = 0.67–0.76) and the aluminium polymerisation outcome was dictated more by the steric influence of the ligand, particularly for Al(4/6)Me₂/Al(7/9)Me.