Biphasic oxidation of diaquadichloro(1,10‐phenanthroline)chromium(III) by N‐bromosuccinimide and the formation of chromium(IV) and chromium(V)

Abstract

The kinetics of oxidation of diaquadichloro(1,10-phenanthroline)chromium(III) complex, [Cr III (phen)(H 2 O) 2 Cl 2 ] + , by N-bromosuccinimide (NBS) is biphasic.The first faster step involves the oxidation of Cr(III) to Cr(IV). The second slower step is due to the oxidation of Cr(IV) to Cr(V). The reaction product is isolated and characterized by electron spin resonance (ESR), IR, and elemental analysis. The chromium(V) product is consistent with the formula [Cr V (phen)Cl 2 (O)]Br. The rate constants k f and k s , for the faster and the slower steps respectively, were obtained using an Origin 9.0 software program. Values of both k f and k s , varied linearly with [NBS] at constant reaction conditions. The effect of pH on the reaction rate is investigated over the pH (4.11-6.01) range at 25.0 • C. The rate constants k f and k s increased with increasing pH. This is consistent with hydroxo forms of the chromium species being more reactive than the aqua forms. Chromium(III) complexes, more often than not, are inert. The oxidation of the Cr(III) complex to Cr(IV), most likely, proceeds by an outer sphere mechanism. Since chromium(IV) is labile the mechanism of its oxidation to chromium(V) is not certain.