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, 1H NMR, IR, UV‐Vis and EPR spectroscopies one can propose the compositions: [Cu2(L1)(H2O)4]Cl2 (1), [Cu3(L2)2(H2O)6]Cl2 (2), [Cu(L3)(H2O)2]Cl (3), [Fe2(L1)(H2O)8]Cl2 (4), [Fe3(L2)2(H2O)12]Cl2 (5) and [Fe(L3)(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.