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

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-N-oxide antibiotics, viz carbadox. The reaction occurred primarily at the quinoxaline-di-N-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-N-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-center dot 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-N-oxide.