title: Experimental and Computational Evidence for the Reduction Mechanisms of Aromatic N-oxides by Aqueous Fe(II)-Tiron Complex.
abstract: 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.
authors.affiliation: Department of Civil and Environmental Engineering, Temple University , Philadelphia, Pennsylvania 19122, United States.
authors.forename: Yiling
authors.initials: Y
authors.lastname: Chen
authors.affiliation: Kuang Yaming Honors School, Nanjing University , Nanjing 210093, People's Republic of China.|Institute for Computational Molecular Science, Temple University , Philadelphia, Pennsylvania 19122, United States.
authors.forename: Hao
authors.initials: H
authors.lastname: Dong
authors.affiliation: Department of Civil and Environmental Engineering, Temple University , Philadelphia, Pennsylvania 19122, United States.
authors.forename: Huichun
authors.initials: H
authors.lastname: Zhang
mesh_terms: D014013:1,2-Dihydroxybenzene-3,5-Disulfonic Acid Disodium Salt / Q000737:chemistry*; D004579:Electron Transport; D005296:Ferrous Compounds / Q000737:chemistry*; D010084:Oxidation-Reduction; D010087:Oxides / Q000737:chemistry*; D011725:Pyridines / Q000737:chemistry
chemical_list: D005296:Ferrous Compounds; D010087:Oxides; D011725:Pyridines; D014013:1,2-Dihydroxybenzene-3,5-Disulfonic Acid Disodium Salt; C013229:pyridine N-oxide