Theoretical and spectroscopic investigations of a complex of Al(III) with caffeic acid

We investigate theoretically, by quantum DFT calculations, and by vibrational and electronic spectroscopies the complexation of Al(III) by caffeic acid. This compound presents two potential chelating sites in competition (catechol and carboxylic groups). In methanol solution, four different complexed species have been determined by spectrophotometry. The predominant species observed at low concentration of Al(III) has a 1: 1 stoichiometry. The quantum chemical calculations show that the ligand involved in this complex undergoes small changes in electronic delocalization, compared with free caffeic acid. The structures of free and complexed ligands are slightly different. Calculations of vibrational transitions of caffeic acid and the 1: 1 complex have permitted complete assignment of the experimental spectra. Modeling of electronic spectra in a vacuum and in methanol shows the necessity to take into account solvent effects in order to reproduce the experimental features. The assignments of UV-visible spectra of free and complexed ligands are very comparable, due to the similarity between the involved MOs in electronic transitions. The good accordance between theoretical and experimental data confirms that chelation of aluminum ion preferentially occurs at the deprotonated catechol site. This study illustrates that, in humic substances that include numerous polyphenolic and carboxylic functions in competition, the chelation does not involve the favored carboxylate group.