Effects of the solvent and temperature on the 2:1 catechol–Al(III)-complex

The influence of temperature and solvent effects on the stability of the complex formed by two molecules of 1,2-dihydroxybenzene and one molecule of AlCl3 were experimentally and theoretically studied, by means of UV spectroscopic methods and Density Functional Theory methods. The changes of the stability constant with the temperature were analyzed using the van’t Hoff equation, while the variations with the permittivity of the reaction medium were explained with an equation proposed by us. The experimental and theoretical data obtained allowed proving that the increase in the hydrogen-bond donor ability of the solvents favors a higher thermodynamic stability of the reactants with respect to the complex and, therefore a decrease in the corresponding stability constant. The non-planar structure proposed for the 2:1 ligand–metal complex is coherent with the small batochromic shift experimentally observed. In the complex molecule, the planes containing the phenyl rings are tilted by approximately 89° with each other. It was concluded that the complexation reaction is an endothermic process in which the solvent–solute interactions play an essential role.