Comparison of the chelating power of hydroxyflavones

A series of hydroxyflavones, widely occurring in nature, have been used as model compounds for the study of the behaviours of humic substances towards aluminium ion complexation. The chelating power of 3-hydroxy-4-keto, 5-hydroxy-4-keto and catechol functional groups have been compared in methanol solution, first when these binding sites are isolated, and then when they are in competition on the a same ligand. Electronic and vibrational spectroscopy investigations were performed to characterize the Al(III) complexes: stoichiometry, stability constants and involved groups in multi-site ligands. Quantum chemical calculations have been carried out in order to know the structural modifications generated to the ligand by complexation. The influence of the medium, acidic, neutral and alkaline, has been studied. The 3-hydroxy-4-keto group has the strongest chelating power in acidic and neutral media whereas catechol presents the greatest ability to chelate Al(III) in alkaline solution. This investigation has shown that capacity of different studied functions to chelate Al(III) is the same when they are in competition and when they are individually taken. (C) 2003 Elsevier Science B.V. All rights reserved.