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 [FeII(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 (N2O22–) ligand, and (iii) N–O bond
cleavage. A reaction pathway is proposed for each of the isomers (cis and trans) 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 cis form in
the septet state.