Structural characterization of aluminium(III) and iron(III) complexes of coumarinic acid in aqueous solutions from combined experimental and theoretical investigations

Here we have studied the complexation of coumarin-3-carboxylic acid with Al(III) and Fe(III) at 37 degrees C and in 0.16 mol L-1 NaCl. The protonation constant of coumarin-3-carboxylic acid was determined in order to evaluate the competition of the ligand for the metal cations and H+. Speciation profiles obtained by potentiometric titrations and supported by UV and H-1- and C-13-NMR data show that a complexation occurs at a ligand-to-Al(III) ratio of 1:1 and at a ligand-to-Fe(III) ratio of 1:2. These data were supported by a computational evaluation, which highlighted the different structural behaviour of the two metals in the complexes. Indeed, the Al(III) cation forms bidentate complexes involving a 6-membered ring by binding both the carboxyl oxygens of the ligand. In contrast, the Fe(III) ion preferentially forms monodentate tetrahedral complexes on the carboxylate moiety of the HCCA ligand.