Complexation facilitated reduction of aromatic N -oxides by aqueous Fe II-tiron complex: Reaction kinetics and mechanisms

Yiling Chen, Huichun Zhang

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8 Scopus citations

Abstract

Rapid reduction of carbadox (CDX), olaquindox and several other aromatic N-oxides were investigated in aqueous solution containing FeII and tiron. Consistent with previous work, the 1:2 FeII-tiron complex, FeL26-, is the dominant reactive species as its concentration linearly correlates with the observed rate constant k obs under various conditions. The N-oxides without any side chains were much less reactive, suggesting direct reduction of the N-oxides is slow. UV-vis spectra suggest FeL26- likely forms 5-or 7-membered rings with CDX and olaquindox through the N and O atoms on the side chain. The formed inner-sphere complexes significantly facilitated electron transfer from FeL26- to the N-oxides. Reduction products of the N-oxides were identified by HPLC/QToF-MS to be the deoxygenated analogs. QSAR analysis indicated neither the first electron transfer nor N-O bond cleavage is the rate-limiting step. Calculations of the atomic spin densities of the anionic N-oxides confirmed the extensive delocalization between the aromatic ring and the side chain, suggesting complex formation can significantly affect the reduction kinetics. Our results suggest the complexation facilitated N-oxide reduction by FeII-tiron involves a free radical mechanism, and the subsequent deoxygenation might also benefit from the weak complexation of FeII with the N-oxide O atom.

Original languageEnglish (US)
Pages (from-to)11023-11031
Number of pages9
JournalEnvironmental Science and Technology
Volume47
Issue number19
DOIs
StatePublished - Oct 1 2013

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