Complexation Facilitated Reduction of AromaticN-Oxides by Aqueous Fe^II–Tiron Complex: Reaction Kinetics and Mechanisms

Abstract

Rapid reduction of carbadox (CDX), olaquindox and several other aromatic N-oxides were investigated in aqueous solution containing Fe(II) and tiron. Consistent with previous work, the 1:2 Fe(II)-tiron complex, FeL2(6-), is the dominant reactive species as its concentration linearly correlates with the observed rate constant kobs 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 FeL2(6-) 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 FeL2(6-) 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 Fe(II)-tiron involves a free radical mechanism, and the subsequent deoxygenation might also benefit from the weak complexation of Fe(II) with the N-oxide O atom.

Interactions and Reductive Reactivity in Ternary Mixtures of Fe(II), Goethite, and Phthalic Acid Based on a Combined Experimental and Modeling Approach

“…For pCNB, the same mobile phase was used but gradient elution was run to separate pCNB and its products. Aqueous Fe­(II) and total Fe were analyzed by a modified ferrozine method. ,, Briefly, 100 μL of reagent A (10 mM ferrozine with 0.1 M ammonia acetate) was added to 1 mL samples. The absorbance was measured by a UV–visible spectrophotometer (Agilent 8670) at 562 nm for Fe­(II) concentrations.…”

“…In anoxic environments, the Fe­(II)/Fe­(III) redox reaction plays an important role in biogeochemical redox processes, for example, the transformation of pollutants in the environment. − Over the past 3 decades, Fe­(II) complexed with iron oxides (or surface-bound Fe­(II)), an environmentally important reductant, has been extensively investigated for its ability to reduce a large number of contaminants. − Fe­(II) complexation with iron oxides can significantly enhance the reductive reactivity by lowering the redox potential of Fe­(II). , Numerous factors can affect the reductive reactivity of the surface-bound Fe­(II), including pH, ,,− Fe­(II) concentration, , Fe­(II) surface speciation, − ,, second metal oxides, and properties of the mineral oxides. ,− A common observation is that the reduction kinetics strongly depended on the extent of Fe­(II) adsorption, which was largely influenced by both solution pH and the soluble Fe­(II) concentration . Despite the above significant body of work, little is known about how ligands affect the reductive reactivity of Fe­(II)/iron oxides.…”

Effects of Second Metal Oxides on Surface-Mediated Reduction of Contaminants by Fe(II) with Iron Oxide

“…Gradient elution was run to separate p CNB and its reaction products. Aqueous Fe­(II) and total Fe were analyzed by a modified ferrozine method. , A total of 100 μL of reagent A (10 mM ferrozine with 0.1 M ammonia acetate) was added to 1 mL samples. The absorbance was measured by an ultraviolet–visible (UV–vis) spectrophotometer (Agilent 8670) at 562 nm for Fe­(II) concentrations.…”

Facile one-pot synthesis of water-soluble fcc FePt3 alloy nanostructures

“…The UV-Vis absorption spectrum of tiron-coated FePt 3 NPs displays a broad absorption band around 650 nm (Fig. 8) which is due to electron transfer from Pt to the antibonding π-molecular orbitals of tiron [3,10,31,38]. Valence tautomerization of tiron (OH to =O) of tiron may also contribute to the emergence of these broad absorption band (Fig.…”

“…Valence tautomerization of tiron (OH to =O) of tiron may also contribute to the emergence of these broad absorption band (Fig. 9) [1,3,7,10,12,24,31,34,38].…”