EPR SPECTROSCOPIC STUDIES ON THE FORMATION OF CHROMIUM(V) PEROXO COMPLEXES IN THE REACTION OF CHROMIUM(VI) WITH HYDROGEN-PEROXIDE

The Cr(V) products of the reaction of Cr(VI) with H2O2 were studied by EPR spectroscopy. In addition to the well-characterized tetrakis(eta( 2)-peroxo)chromate(V) complex, [Cr(O-2)(4)](3-), with g(iso) = 1.9723 (A(iso) = 18.4 x 10(-4) cm(-1)), three new species were observed with isotropic EPR parameters, g(iso) = 1.9820, g(iso) = 1.9798 (A(iso) = 1 6.3 x 10(-4) cm(-1)), and g(iso) = 1.9764 (A(iso) = 18.1 x 10(-4) cm(- 1)). While [Cr(O-2)(4)](3-) is stable at high concentrations of H2O2 a nd in alkaline solution, the species with a signal at g(iso) = 1.9798 is stabilized at low relative concentrations of H2O2 and in neutral so lution. The signal at g(iso) = 1.9764 is most prominent in weakly acid ic (pH = 4-7) solutions and low relative concentrations of H2O2. Final ly, the signal at 1.9820 is only minor and is apparent at low pH value s and low [H2O2]. From the pH and [H2O2] dependences, and by analogy w ith the V(V) chemistry, the species giving rise to the signals at g(is o) = 1.9820, g(iso) = 1.9798, and g(iso) = 1.9764 are assigned as the oxo(eta(7)-peroxo)chromium(V), [Cr(O)(O-2)(OH2)(n)](+), aquaoxobis(eta (2)-peroxo)chromate(V), [Cr(O)(O-2)(2)(OH2)](-), and the hydroxotris(e ta(7)-peroxo)chromate(V), [Cr(O-2)(3)(OH)](2-), complexes, respectivel y. The implications of these Cr(V) peroxo species for understanding th e in vitro DNA damage caused by Cr(VI) and H2O2 and the genotoxicity o f carcinogenic Cr(VI) complexes are discussed.