Functional Models for the Mono- and Dinitrosyl Intermediates of FNORs: Semireduction versus Superreduction of NO

The reduction of NO to N₂O by flavodiiron nitric oxide reductases (FNORs) is related to the disruption of the defense mechanism in mammals against invading pathogens. The proposed mechanism for this catalytic reaction involves both nonheme mono- and dinitrosyl diiron(II) species as the key intermediates. Recently, we reported an initial account for NO reduction activity of an unprecedented mononitrosyl diiron(II) complex, [Fe₂(N-Et-HPTB)(NO)(DMF)₃](BF₄)₃ (1) (N-Et-HPTB is the anion of N,N,N',N'-tetrakis(2-(l-ethylbenzimidazolyl))-2-hydroxy-1,3-diaminopropane; DMF = dimethylformamide) with [Fe^II{FeNO}⁷] formulation [Jana et al. J. Am. Chem. Soc. 2017, 139, 14380]. Here we report the full account for the selective synthesis, characterization, and reactivity of FNOR model complexes, which include a dinitrosyl diiron(II) complex, [Fe₂(N-Et-HPTB)(NO)₂(DMF)₂](BF₄)₃ (2) with [{FeNO}⁷]₂ formulation and a related, mixed-valent diiron(II, III) complex, [Fe₂(N-Et-HPTB)(OH)(DMF)₃](BF₄)₃ (3). Importantly, whereas complex 2 is able to produce 89% of N₂O via a semireduced mechanism (1 equiv of CoCp₂ per dimer = 50% of NO reduced), complex 1, under the same conditions (0.5 equiv of CoCp₂ per dimer = 50% of NO reduced), generates only ∼50% of N₂O. The mononitrosyl complex therefore requires superreduction for quantitative N₂O generation, which constitutes an interesting dichotomy between 1 and 2. Reaction products obtained after N₂O generation by 2 using 1 and 2 equiv of reductant were characterized by molecular structure determination and electron paramagnetic resonance spectroscopy. Despite several available literature reports on N₂O generation by diiron complexes, this is the first case where the end products from these reactions could be characterized unambiguously, which clarifies a number of tantalizing observations about the nature of these products in the literature.