Kinetics and structural study on complexes of Al(III) with o-hydroxy-benzophenones

The structure, stability and formation rate of complexes formed between AlCl3 and 2-hydroxy-benzophenone or 2-hydroxy4-methoxy-benzophenone in methanol were investigated using UV-visible spectroscopy and the AM1 method. Applying the molar ratio method it was determined that in both cases a complex is formed between a ligand molecule and a molecule of the metallic reagent. The 1: 1 stoichiometric composition of these two complexes was confirmed by kinetic experiments. These permitted to determine that the formation rate and stability of the complex formed between 2-hydroxy-4-methoxy-benzophenone and AlCl3, are greater than those of the other complex. In the theoretical study, it was proposed that the formation of the complexes involves one simple covalent bond between the aluminium atom and the oxygen atom of o-hydroxyl group of the o-hydroxy-benzophenones and a stronger coulombic attraction between the central atom and the carbonyl oxygen atom of the ligands. Using the calculated thermodynamic magnitudes, molecular structural parameters and energy and reactivity indices it was predicted that the complex formed by AlCl3 and 2-hydroxy-4-methoxy-benzophenone exhibits higher thermodynamic stability than the AlCl3-2-hydroxy-benzophenone complex. It was also observed that the planarity of the molecules in both complexes is greater than that of the respective free molecules of the ligands. The greater planarity of the complexed molecules accounts for the calculated values of dipolar moment and batochromic shifts. The formulated theoretical conclusions satisfactorily match the experimental determinations performed. (C) 2002 Elsevier Science B.V. All rights reserved.