The Complexation of Al^III, Pb^II, and Cu^II Metal Ions by Esculetin: A Spectroscopic and Theoretical Approach

UV–visible absorption spectroscopy combined with quantum chemical calculations and, notably, Time-Dependent Density Functional Theory were used to probe the structure of metal complexes with esculetin in dilute aqueous solution, at pH = 5. For the 1:1 complex formation, the studied metal ions can be classified according to their complexing power: aluminum­(III) > copper­(II) > lead­(II). For the three complexes, a chelate is formed with the fully deprotonated catechol moiety and an absorption band is observed at the same wavelength. In all cases, a pronounced ionic character is calculated for metal–ligand bonds. However, the complexes differ in their coordination sphere. Copper and lead are bound to two water molecules leading to a square plane geometry and a hemidirected complex, respectively, whereas aluminum atom has an octahedral environment involving three water molecules and a hydroxide ion. For Al^III only, a 2:1 complex is observed, and the involvement of an aluminum dimer was evidenced.