Aminopiperidine based complexes for lactide polymerisation?

Dalton Transactions Pub Date: 2016-02-18 DOI: 10.1039/C5DT04695E

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)Mex (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 (Pr = 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.

Graphical abstract: Aminopiperidine based complexes for lactide polymerisation
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