posted on 2014-04-10, 00:00authored byAnnaïg Le Person, Aurélien Moncomble, Jean-Paul Cornard
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 AlIII only, a 2:1 complex is observed,
and the involvement of an aluminum dimer was evidenced.