Department of Civil and Environmental Engineering, Temple University , Philadelphia, Pennsylvania 19122, United States.
Kuang Yaming Honors School, Nanjing University , Nanjing 210093, People's Republic of China.
Environ Sci Technol. 2016 Jan 5;50(1):249-58. doi: 10.1021/acs.est.5b04900. Epub 2015 Dec 18.
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, KIEH, 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, KIEN, for the reduction of pyridine N-oxide (a type III ANO) (1.022 ± 0.006) is in good agreement with the calculated KIEN 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.
采用实验-理论相结合的方法阐明了 Fe(II)-tiron 配合物还原五种代表性芳香族 N-氧化物（ANOs）的还原机制，并确定了速率限制步骤。基于与还原剂形成的可能类型的配合物，研究了三组 ANOs：I 型是指通过侧链上的 N 和 O 原子形成 5 元环配合物的那些；II 型是指通过 N-氧化物 O 原子和侧链上的 O 原子形成 6 元环配合物的那些；III 型是指仅通过 N-氧化物 O 原子形成配合物的那些。密度泛函理论计算表明，包括质子化、N-O 键断裂和第二次电子转移过程在内的基本反应是无势垒的，这表明第一次电子转移是限速步骤。与理论结果一致，实验溶剂同位素效应，KIEN，对于喹啉 N-氧化物（III 型 ANO）的还原，得到 1.072 ± 0.025，表明质子化未参与限速步骤。吡啶 N-氧化物（III 型 ANO）还原的实测氮动力学同位素效应，KIEN（1.022 ± 0.006）与第一次电子转移的计算 KIEN（1.011-1.028）非常吻合，证实第一次电子转移是限速步骤。电化学池实验表明，电子转移过程可以通过 FeL2(6-)（1:2Fe(II)-tiron 配合物）与 I 型配合物显著促进，在一定程度上可以通过游离 Fe(II)与 II 型配合物促进，但不能通过弱 III 型配合物促进。
