Synthesis, electrochemical, spectral, and antioxidant properties of complexes of flavonoids with metal ions

Complexes of metal ions with the flavonoids quercetin (H2L1), rutin (H2L2) and 3-hydroxyflavone (HL3) have been synthesized. From the data obtained by elemental analyses, molar conductance, atomic absorption, thermal analyses, H-1 NMR, IR, UV-Vis and EPR spectroscopies one can propose the compositions: [Cu-2(L-1)(H2O)(4)]Cl-2 (1), [Cu-3(L-2)(2)(H2O)(6)]Cl-2 (2), [Cu(L-3)(H2O)(2)]Cl (3), [Fe-2(L-1)(H2O)(8)]Cl-2 (4), [Fe-3(L-2)(2)(H2O)(12)]Cl-2 (5) and [Fe(L-3)(H2O)(4)]Cl (6). We have used the assumption that the electrochemical oxidation can be used as a model for antioxidant activities for the flavonoids. The cyclic voltammograms show irreversible oxidation processes at +0.58 V and +0.91 V for (1), + 0.82 V for (2), + 0.89 V for (3), + 0.58 V and + 0.90 V for (4), + 0.78 V for (5) and + 0.86 V for (6) versus Ag/AgCl. These values are lower than those for the free flavonoids indicating a more favorable antioxidant activity. Furthermore, these complexes were also more effective than the uncomplexed flavonoids scavenging superoxide radicals generated by reduction of the oxygen-saturated solution. The superoxide radical scavenging activities of flavonoid complexes seemed to correlate with the redox potentials of the flavonoid.