Intramolecular Conversion of Pentaaquahydroperoxidochromium(III) Ion to Aqueous Chromium(V): Potential Source of Carcinogenic Forms of Chromium in Aerobic Organisms

The decomposition of the title compound (H2O)(5)CrOOH2+ (hereafter CraqOOH2+) in acidic aqueous solutions is kinetically complex and generates mixtures of products (Cr-aq(3+), HCrO4-, H2O2, and O-2). Relative yields of individual products vary greatly with reaction conditions and initial concentrations of CraqOOH2+. At pH 5.5 in the presence of O-2, the reaction was complete in less than a minute and generated chromate in about 70% yield. These findings, in addition to poor reproducibility of kinetic data, are indicative of the involvement of one or more reactive intermediates that consume additional amounts of CraqOOH2+ in post-rate-determining steps. The kinetics were simplified in the presence of H2O2 or ABTS(2-), both of which are capable of scavenging strongly oxidizing intermediates. The measured rate constant in 0.10 M HClO4 at low O-2 concentrations (<= 0.03 mM) was independent of the concentration of the scavengers and was, within error, the same for ABTS(2-), k = 4.9 (+/-0.2) x 10(-4) s(-1), and H2O2, k = 5.3 (+/-0.7) x 10(-4) s(-1). At a constant ionic strength of 1.0 M, the reaction in the presence of either H2O2 or ABTS(2-) obeyed a two-term rate law, k(obs)/s(-1) = 6.7 (+/-0.7) x 10(-4) + 7.6 (+/-1.1) x 10(-4) [H+]. Both in the presence and absence of ABTS2- as the scavenger, the yields of H2O2 increased with increasing [H+]. These results are discussed in terms of a dual-pathway mechanism for the decay of CraqOOH2+. The H+-catalyzed path leads to the dissociation of H2O2 from Cr(III), while in the H+-independent reaction, CraqOOH2+ is transformed to Cr(V). Both scavengers rapidly remove Cr(V) and simplify both the kinetics and products.