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

The reduction of NO to NO 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, Fe(-Et-HPTB)(NO)(DMF) () (-Et-HPTB is the anion of N,N,N',N'-tetrakis(2-(l-ethylbenzimidazolyl))-2-hydroxy-1,3-diaminopropane; DMF = dimethylformamide) with [Fe{FeNO}] formulation [Jana et al. 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, Fe(-Et-HPTB)(NO)(DMF) () with [{FeNO}] formulation and a related, mixed-valent diiron(II, III) complex, Fe(-Et-HPTB)(OH)(DMF) (). Importantly, whereas complex is able to produce 89% of NO via a semireduced mechanism (1 equiv of CoCp per dimer = 50% of NO reduced), complex , under the same conditions (0.5 equiv of CoCp per dimer = 50% of NO reduced), generates only ∼50% of NO. The mononitrosyl complex therefore requires superreduction for quantitative NO generation, which constitutes an interesting dichotomy between and . Reaction products obtained after NO generation by using 1 and 2 equiv of reductant were characterized by molecular structure determination and electron paramagnetic resonance spectroscopy. Despite several available literature reports on NO 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.