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 [FeII{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.