Binding of hydroxychromones with Al in methanol

The chelation of 3-hydroxy-, 5-hydroxy- and 5,7-dihydroxychromone with Al³⁺ was studied spectrophotometrically. Both the molar ratio method and method of continuous variation (Job’s method) were used to determine the composition of the complexes. In all cases, complex formation involved 1:1 aluminum ion to hydroxychromone ratios. For the 5-hydroxy and 5,7-dihydroxy derivatives, this composition agrees with that for the corresponding hydroxyflavone. However, 3-hydroxyflavone (flavonol) forms a 2:1 complex with Al³⁺ in methanol in contrast to the results for 3-hydroxychromone in this study. The importance of the cinnamoyl mesomeric form for stabilizing the complex between flavonol and Al³⁺ is invoked to explain this discrepancy.