Nitric Oxide Reduction at a Single Iron Site Facilitated by Second Coordination Sphere Hydrogen Bonding via a Putative Fe(IV)-Oxo Intermediate

Metalloenzymes use second coordination sphere (SCS) hydrogen bond (H-bond) donors and acceptors to facilitate small molecule activation by lowering the activation energies for chemical transformations or by stabilizing reactive intermediates. Borovik and co-workers studied the role of SCS groups in dioxygen and chalcogen activation in well-defined mononuclear model complexes. Here, we use the same complex, K[Fe^II(H₃L)], to study the role of H-bonding for nitric oxide (NO) activation. Remarkably, in solution, K[Fe^II(H₃L)] is capable of direct NO reduction to nitrous oxide (N₂O) at a single Fe center. Initial formation of a hs-{FeNO}⁷ intermediate is followed by fast attack by a second NO molecule to form N₂O and a putative Fe^IV═O intermediate, which then undergoes multiple decomposition pathways, forming an antiferromagnetically coupled <i>S</i>_t = 1/2 Fe^III/Fe^IV dimer and a diamagnetic diferric species as confirmed by Mössbauer and EPR spectroscopy. Notably, in the absence of the SCS H-bond donors, only the formation of a stable hs-{FeNO}⁷ species is observed. Intermediates of NO reduction were observed by reacting K[Fe^II(H₃L)] in the solid state with NO, which leads to the formation of hs-{FeNO}⁷ species, N₂O, and the Fe^IV═O intermediate, as confirmed by IR spectroscopy. To the best of our knowledge, direct reduction of NO at a single Fe center has not been reported. Furthermore, the ability to form a high-valent metal-oxo species directly from NO is unprecedented for nonheme iron chemistry. This novel reactivity has significant implications in biology and for catalytic systems for NO_<i>x</i> reduction.