Synthesis and structure of aluminium amine-phenolate complexes

Abstract

Reaction of Al(OⁱPr)₃ with the tris-phenol amine ligand L¹H₃ in toluene at ambient temperature results in the formation of the iso-propanol adduct [HOⁱPr·Al(L¹)]. Single crystal X-ray diffraction analysis reveals the structure to be a hydrogen bonded dimer. Reaction of Al(OⁱPr)₃ (or AlMe₃) with L¹H₃ in THF affords the related, and structurally characterised THF adduct, [THF·Al(L¹)]. Similar reaction of Al(OⁱPr)₃ (or AlMe₃) with the bis-phenol amine and tetra-phenol diamine ligands, L²H₃ and L³H₄, results in the formation and isolation of the complexes [Al(L²)]₂ and [Al(L³)H] respectively, both of which have been structurally characterised viasingle crystal X-ray diffraction studies. Reaction of the alkoxide bridged dimer [Al(L²)]₂ with the strong Lewis base HMPA results in the formation of the monomeric HMPA adduct [HMPA·Al(L²)] which was also structurally characterised. The adduct [HOⁱPr·Al(L¹)] and the dimer [Al(L²)]₂ were tested for their activity in the ring-opening polymerisation (ROP) of rac-lactide under solvent-free conditions (130 °C). Under the conditions employed [Al(L²)]₂ failed to produce polymer after 48 h, in stark contrast to [HOⁱPr·Al(L¹)] which after 24 h and 48 h produced narrow molecular weight polymer (24 h: yield = 25%, M_n = 14500 and PDI = 1.05; 48 h: yield = 65%, M_n = 47700 and PDI = 1.06).