Non-Heme Diiron Model Complexes Can Mediate Direct NO Reduction: Mechanistic Insight into Flavodiiron NO Reductases

Flavodiiron nitric oxide reductases (FNORs), a common enzyme family found in various types of pathogenic bacteria, are capable of reducing nitric oxide (NO) to nitrous oxide (N₂O) as a protective detoxification mechanism. Utilization of FNORs in pathogenic bacteria helps them survive and proliferate in the human body, thus causing chronic infections. In this paper, we present a new diiron model complex, [Fe₂((Py₂PhO₂)­MP)­(OPr)₂]­(OTf), with bridging propionate ligands (OPr⁻) that is capable of directly reducing NO to N₂O in quantitative yield without the need to (super)­reduce the complex. We first prepared the diferric precursor and characterized it by UV–vis, IR, NMR and Mössbauer spectroscopies, cyclic voltammetry, and mass spectrometry. This complex can then conveniently be reduced to the diferrous complex using CoCp₂. Even though this diferrous complex is highly reactive, we have successfully isolated and characterized this species using X-ray crystallography and various spectroscopic techniques. Most importantly, upon reacting this diferrous complex with NO gas, we observe quantitative formation of N₂O via IR gas headspace analysis, the first demonstration of direct NO reduction by a non-heme diiron model complex. This finding directly supports recent mechanistic proposals for FNORs.