Lactide polymerization with iron alkoxide complexes

Abstract

This report presents the results of a study on the preparation of iron alkoxide complexes chelated by diiminopyridine ligands and their role in the room temperature polymerization of rac-lactide. Reaction of N,N′-(p-R-C6H4CH2)2-diiminopyridines (R = H (1), F (2)) with FeX2 (X = Cl, Br) yielded the homoleptic complexes [(1)2Fe][FeX4] or [(2)2Fe][FeX4], respectively. Treating the latter with Na[BPh4] afforded the anion exchange product [(2)2Fe][BPh4]2, which was characterized by 1H NMR and absorption spectroscopy, combustion analysis, and single crystal X-ray diffraction. Various attempts to grow crystals of [(1)2Fe][FeX4] and [(2)2Fe][FeX4] culminated in the isolation of single crystals of [(2)2Fe][Cl6Fe2O] that was characterized by X-ray diffraction. Attempted synthesis of well-defined, mononuclear alkoxide derivatives from [(1)2Fe]2+ or [(2)2Fe]2+ gave mostly intractable products, but in one case we obtained the crystallographically characterized sodium iron cluster Na4Fe2(OC6H4F)8(THF)2. An aryloxide derivative proved accessible by reaction of NaOC6H4F with the mono-ligand precursor LFeCl2 (L = N,N′-dimesityl-diiminopyridine), but characterization of LFe(OC6H4F)2 was limited to a single crystal X-ray diffraction analysis, owing to unsuccessful attempts at isolating pure samples. The difficulties encountered in the isolation of pure alkoxide derivatives prompted us to use in-situ generated LFe(OEt)2 for studying the polymerization of rac-lactide. This system was found to be moderately active at room temperature and with a slight preference for the formation of a heterotactic polymer (Pr = 0.54–0.65). Large polydispersities of 1.5–2.0 indicated the presence of transesterification side-reactions, which were confirmed by the presence of peaks with m/z = n 144 + M(EtOH) + M(Na+) and m/z = (n + 0.5) 144 + M(EtOH) + M(Na+) in MALDI-MS.