Theoretical Study of Nitric Oxide Reduction Catalyzed by a Mononuclear Nonheme Iron Complex

In contrast with the extensive studies on the mechanism of nitric oxide (NO) reduction to nitrous oxide (N₂O) catalyzed by heme-type complexes, the analogous reaction catalyzed by mononuclear nonheme-type complexes remains scarcely explored. In this study, density functional theory (DFT) calculations were performed on the nonheme mononuclear iron complex [Fe^II(Me₃TACN)(S₂SiMe₂)] (TACN = 1,4,7-triazacyclononane) to unveil the mechanism of the NO reduction to N₂O. The results reveal that the NO reduction can be divided into three steps: (i) N-N bond formation between NO and an iron nitrosyl complex, (ii) change in the coordination mode of the resulting hyponitrite (N₂O₂^2-) ligand, and (iii) N-O bond cleavage. A reaction pathway is proposed for each of the isomers (<i>cis</i> and <i>trans</i>) of the intermediate iron hyponitrite complex. A detailed electronic structure analysis confirmed the complex interplay between the spin states of iron and the radical species of the two NO molecules. By tracing the most stable potential energy surface, the possible pathways for the reduction of NO to N₂O were determined. The potential energy surfaces show that the N-O bond cleavage of the N₂O₂ moiety during N₂O formation proceeds only when the N₂O₂ intermediate is in the <i>cis</i> form in the septet state.