Reduction of Carbadox Mediated by Reaction of Mn(III) with Oxalic Acid

Abstract

Manganese­(III) geocomponents are commonly found in the soil environment, yet their roles in many biogeochemical processes remain unknown. In this study, we demonstrated that Mn^III generated from the reaction of MnO₂ and oxalic acid caused rapid and extensive decompositions of a quinoxaline-di-<i>N</i>-oxide antibiotics, viz carbadox. The reaction occurred primarily at the quinoxaline-di-<i>N</i>-oxide moiety resulting in the removal of one O from N1-oxide and formation of desoxycarbadox. The reaction rate was accelerated by increasing amounts of Mn^III, carbadox and oxalate. The critical step in the overall reaction was the formation of a quinoxaline-di-<i>N</i>-oxide/Mn^III/oxalate ternary complex in which Mn^III functioned as the central complexing cation and electron conduit in which the arrangement of ligands facilitated electron transfer from oxalate to carbadox. In the complex, the CC bond in oxalate was cleaved to create CO₂^–• radicals, followed by electron transfer to carbadox through the Mn^III center. This proposed reaction mechanism is supported by the reaction products formed, reaction kinetics, and quantum mechanical calculations. The results obtained from this study suggest that naturally occurring Mn^III–oxalic acid complexes could reductively decompose certain organic compounds in the environment such as the antibiotic quinoxaline-di-<i>N</i>-oxide