Synthesis and structure of aluminium amine-phenolate complexes

Reaction of Al(Oⁱ Pr)₃with the tris-phenol amine ligand L¹ H₃in toluene at ambient temperature results in the formation of the iso-propanol adduct [HOⁱ Pr·Al(L¹)]. Single crystal X-ray diffraction analysis reveals the structure to be a hydrogen bonded dimer. Reaction of Al(Oⁱ Pr)₃(or AlMe₃) with L¹ H₃in THF affords the related, and structurally characterised THF adduct, [THF·Al(L¹)]. Similar reaction of Al(Oⁱ Pr)₃(or AlMe₃) with the bis-phenol amine and tetra-phenol diamine ligands, L² H₃and L³ H₄, results in the formation and isolation of the complexes [Al(L²)]₂and [Al(L³)H] respectively, both of which have been structurally characterised via single crystal X-ray diffraction studies. Reaction of the alkoxide bridged dimer [Al(L²)]₂with the strong Lewis base HMPA results in the formation of the monomeric HMPA adduct [HMPA·Al(L²)] which was also structurally characterised. The adduct [HOⁱ Pr·Al(L¹)] and the dimer [Al(L²)]₂were 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)]₂failed to produce polymer after 48 h, in stark contrast to [HOⁱ Pr·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).