EPR Spectroscopic Studies on the Formation of Chromium(V) Peroxo Complexes in the Reaction of Chromium(VI) with Hydrogen Peroxide

Abstract

The Cr(V) products of the reaction of Cr(VI) with H2O2 were studied by EPR spectroscopy. In addition to the well-characterized tetrakis(η2-peroxo)chromate(V) complex, [Cr(O2)4]3-, with giso = 1.9723 (Aiso = 18.4 × 10-4 cm-1), three new species were observed with isotropic EPR parameters, giso = 1.9820, giso = 1.9798 (Aiso = 16.3 × 10-4 cm-1), and giso = 1.9764 (Aiso = 18.1 × 10-4 cm-1). While [Cr(O2)4]3- is stable at high concentrations of H2O2 and in alkaline solution, the species with a signal at giso = 1.9798 is stabilized at low relative concentrations of H2O2 and in neutral solution. The signal at giso = 1.9764 is most prominent in weakly acidic (pH = 4−7) solutions and low relative concentrations of H2O2. Finally, the signal at 1.9820 is only minor and is apparent at low pH values and low [H2O2]. From the pH and [H2O2] dependences, and by analogy with the V(V) chemistry, the species giving rise to the signals at giso = 1.9820, giso = 1.9798, and giso = 1.9764 are assigned as the oxo(η2-peroxo)chromium(V), [Cr(O)(O2)(OH2)n]+, aquaoxobis(η2-peroxo)chromate(V), [Cr(O)(O2)2(OH2)]-, and the hydroxotris(η2-peroxo)chromate(V), [Cr(O2)3(OH)]2-, complexes, respectively. The implications of these Cr(V) peroxo species for understanding the in vitro DNA damage caused by Cr(VI) and H2O2 and the genotoxicity of carcinogenic Cr(VI) complexes are discussed.