Studies on speciation of aluminium complexes with malic acid by high field ²⁷Al NMR spectroscopy

Abstract

High field variable <em>p</em>H ²⁷Al NMR spectral studies of AlCl₃-malic acid system show the formation of as many as four complex species in the <em>p</em>H range 2-8. The binding of malate to aluminium takes place through the hydroxyl and carboxyl groups resulting in five- and six-membered chelate rings with deprotonation of the hydroxyl group. Malate functions either as a bidentate or a tridentate ligand. The 1:1 AI-malate chelate has been isolated and characterised by elemental analysis, potentiometric titration and IR spectroscopy. The results indicate deprotonation of hydroxyl group and bidentate nature of carboxylate groups. A bridged polymeric structure has been proposed for the chelate.