Experimental and Computational Evidence for the Reduction Mechanisms of Aromatic N-oxides by Aqueous FeII–Tiron Complex

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• 作者：Yiling Chen ; Hao Dong ; Huichun Zhang
• 刊名：Environmental Science & Technology
• 出版年：2016
• 出版时间：January 5, 2016
• 年：2016
• 卷：50
• 期：1
• 页码：249-258
• 全文大小：445K
• ISSN：1520-5851

文摘

A combined experimental-theoretical approach was taken to elucidate the reduction mechanisms of five representative aromatic N-oxides (ANOs) by Fe^II–tiron complex and to identify the rate-limiting step. Based on the possible types of complexes formed with the reductant, three groups of ANOs were studied: type I refers to those forming 5-membered ring complexes through the N and O atoms on the side chain; type II refers to those forming 6-membered ring complexes through the N-oxide O atom and the O atom on the side chain; and type III refers to complexation through the N-oxide O atom only. Density functional theory calculations suggested that the elementary reactions, including protonation, N–O bond cleavage, and the second electron transfer processes, are barrierless, indicating that the first electron transfer is rate-limiting. Consistent with the theoretical results, the experimental solvent isotope effect, KIE_H, for the reduction of quinoline N-oxide (a type III ANO) was obtained to be 1.072 ± 0.025, suggesting protonation was not involved in the rate-limiting step. The measured nitrogen kinetic isotope effect, KIE_N, for the reduction of pyridine N-oxide (a type III ANO) (1.022 ± 0.006) is in good agreement with the calculated KIE_N for its first electron transfer (1.011–1.028), confirming that the first electron transfer is rate-limiting. Electrochemical cell experiments demonstrated that the electron transfer process can be facilitated significantly by type I complexation with FeL₂^6– (1:2 Fe^II–tiron complex), to some extent by type II complexation with free Fe^II, but not by weak type III complexation.