A combined experimental-theoretical approach was taken to elucidate the reduction mechanisms of five representative aromatic N-oxides (ANOs) by FeII-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 FeL26- (1:2 FeII-tiron complex), to some extent by type II complexation with free FeII, but not by weak type III complexation.
Bibliographical noteFunding Information:
This material is based upon work supported by the National Science Foundation under Grants CBET-1125713 and CHE-1507981. We are thankful to Mr. Guangfeng Zhou at Temple University and Dr. Christopher Cramer at the University of Minnesota for assistance in computational calculations, to Dr. Chris Schafmeister and Mr. Yanfeng Fan at Temple University for assistance in synthesizing P15NO, and to Dr. Wenqing Xu at Villanova University for assistance in setting-up cell experiments.
© 2015 American Chemical Society.