Acid–base and coordination properties of 2-phenyl-3-hydroxy-4-quinolones in aqueous media

Abstract

The acid–base and coordination properties of 2-phenyl-3-hydroxy-4(1H)-quinolone (1) and 1-methyl-2-phenyl-3-hydroxy-4(1H)-quinolone (2) were characterized by potentiometric, UV-Visible and fluorescence titrations in water containing 5 or 30% vol MeCN and in a micellar solution of a cationic surfactant. The first dissociation constants (pK_a1) corresponding to OH deprotonation of 1 and 2 are about 10 and ligand 1 undergoes a second NH deprotonation with a pK_a2 about 12, which is reduced to 10.4 in the presence of a cationic surfactant. More detailed complexation studies were performed with more soluble ligand 1, which forms stable complexes of 1 : 1 and 1 : 2 compositions with Fe(III), Cu(II), Zn(II), Pb(II) and Me₂Sn(IV) cations in neutral solutions. The most unusual behavior is observed with Zn(II), which strongly promotes NH deprotonation of ligand 1 with formation of the Zn(L)₂²⁻ complex at a pH about 8. The formation of this complex is confirmed by the results of ¹H NMR titrations in DMSO-d₆. Binding of all cations is accompanied by the appearance of a new absorption band in the range 385–405 nm with concomitant disappearance of the band at 350–360 nm in the free ligand. Interactions of 1 and 2 with Zn(II) and Me₂Sn(IV) are accompanied by strong and selective fluorescence enhancements with the blue shift of the emission bands allowing ratiometric detection of these cations. Complexation with transition and heavy metal ions as well as with lanthanides induces fluorescence quenching. Ligand 2 is characterized by X-ray crystallography.