Toward a Better Understanding of the Regioselectivity of the Al(III)−Protocatechuic Acid Complexation Reaction

Protocatechuic acid presents two complexing sites in competition to fix metal: the carboxylic and catechol functions. Even in acidic aqueous medium, where the free ligand is fully protonated, Al(III) forms a chelate with the doubly deprotonated catechol group. To gain a better understanding of the complexation mechanism and to explain the regioselectivity of the reaction, reaction pathways involving either the catechol group or the carboxylic one have been calculated at the B3LYP/6-31G(d,p) level of theory. All the intermediate species have been identified, and both processes present the following different steps: metal attack with the coordination of Al(III) to an oxygen atom; deprotonation of hydroxyl groups; ring closure to form a chelate. Whatever the complexing site, a bidentate complex is more stable than a monodentate one. From an energetic point of view, the reaction pathway corresponding to a chelate formation with the catechol function is favored; notably the energy barrier necessary to close the ring involving the metal ion is calculated to be lower than that of carboxyl function.