Synthesis and structure of aluminium amine-phenolate complexes

Reaction of Al((OPr)-Pr-t)(3) with the tris-phenol amine ligand (LH3)-H-1 in toluene at ambient temperature results in the formation of the iso-propanol adduct [(HOPr)-Pr-t center dot Al(L-1)]. Single crystal X-ray diffraction analysis reveals the structure to be a hydrogen bonded dimer. Reaction of Al((OPr)-Pr-i)(3) (or AlMe3) with (LH3)-H-1 in THF affords the related, and structurally characterised THF adduct, [THF center dot Al(L-1)]. Similar reaction of Al((OPr)-Pr-i)(3) (or AlMe3) with the bis-phenol amine and tetra-phenol diamine ligands, (LH3)-H-2 and (LH4)-H-3, results in the formation and isolation of the complexes [Al(L-2)](2) and [Al(L-3) H] respectively, both of which have been structurally characterised via single crystal X-ray diffraction studies. Reaction of the alkoxide bridged dimer [Al(L-2)](2) with the strong Lewis base HMPA results in the formation of the monomeric HMPA adduct [HMPA center dot Al(L-2)] which was also structurally characterised. The adduct [(HOPr)-Pr-i center dot Al(L-1)] and the dimer [Al(L-2)](2) were tested for their activity in the ring-opening polymerisation (ROP) of rac-lactide under solvent-free conditions (130 degrees C). Under the conditions employed [Al(L-2)](2) failed to produce polymer after 48 h, in stark contrast to [(HOPr)-Pr-i center dot Al(L-1)] 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).