Abiotic reduction of nitroaromatic compounds by aqueous iron(II)-catechol complexes

Complexation of iron(II) by catechol and thiol ligands leads to the formation of aqueous species that are capable of reducing substituted nitroaromatic compounds (NACs) to the corresponding anilines. No reactions of NACs are observed in Fe^II-only or ligand-only solutions. In solutions containing Fe^II and tiron, a model catechol, rates of NAC reduction are heavily dependenton pH, ligand concentration, and ionic strength. Observed pseudo-first-order rate constants (k_obs) for 4-chloronitrobenzene reduction vary by more than 6 orders of magnitude, and the variability is well described by the expression k_obs = k_FeL2^6- [FeL₂^6-], where [FeL₂^6-] is the concentration of the 1:2 Fe^II-tiron complex and k_FeL2 ^6- is the bimolecular rate constant for 4-chloronitrobenzene reaction with this species. The high reactivity of this Fe^II species is attributed to the low standard one-electron reduction potential of the corresponding Fe^III/Fe_II redox couple (E_H ⁰ = -0.509 V vs NHE). The relative reactivity of different NACs can be described by a linear free-energy relationship (LFER) with the one-electron reduction potentials of the NACs, EH^1′(ArNO₂). The experimentally derived slope of the LFER indicates that electron transfer is rate determining. These findings suggest that Fe^II-organic complexes may play an important, previously unrecognized, role in the reductive transformation of persistent organic contaminants.