Biphasic Oxidation of cis-Diaquabis(1,10-phenanthroline)chromium(III) by N-Bromosuccinimide and the Formation of Chromium(IV) and Chromium(V)

The oxidation of cis-diaquabis(1,10-phenanthroline)chromium(III) [cis-Cr-III(phen)(2)(H2O)(2)](3+) by N-bromosuccinimide (NBS) to yield cis-dioxobis(1,10-phenanthroline)chromium(V) has been studied spectrophotometrically in the pH 1.57-3.56 and 5.68-6.68 ranges at 25.0 degrees C. The reaction displayed biphasic kinetics at pH < 4.0 and a simple first order at the pH > 5.0. In the low pH range, the reaction proceeds by two successive steps; the first faster step corresponds to the oxidation of Cr(III) to Cr(IV), and the second slower one corresponds to the oxidation of Cr(IV) to Cr(V), the final product of the reaction. The formation of both Cr(IV) and Cr(V) has been detected by electron spin resonance (ESR). The ESR clearly showed the formation and decay of Cr(IV) as well as the formation of Cr(V). Each oxidation process exhibited a first-order dependence on the initial [Cr(III)]. The pseudo-first-order rate constants k(34) and k(45), for the faster and slower steps, respectively, were obtained by a computer program using Origin7.0. Both rate constants showed first-order dependence on [NBS] and increased with increasing pH.