Mechanistic Investigation Towards Catalytic NO2– Reduction by Nonheme Fe(II)-Nitro Complexes

Nitrite (NO₂^–) reduction to gaseous nitric oxide (NO) is a 2H⁺/1e^– transfer process that can be catalyzed by heme enzyme nitrite reductase (NiR). It is critical in maintaining the balance of the global nitrogen cycle because it is the first committed step in the denitrification process. In addition, despite the lack of an NiR, the high artery and veinous NO₂^– concentration (~ 500 nM) in mammals is proposed to generate NO as a gaseous vasodilator under hypoxic conditions. Therefore, NO₂^– also has therapeutic values to treat human diseases, such as hypertension. The active site of the native cytochrome cd₁ NiR reveals three key features to achieve catalytic NO₂^– reduction to NO with high efficiency: (i) a low spin (LS) Fe(II)-nitro adduct that NO₂^– binds to Fe(II) via N atom. (ii) a labile Fe(II)-NO^• adduct with weak π-backbonding interaction. (iii) secondary-sphere (or H-bonding) interaction. Inspired by cytochrome cd₁NiR, three nonheme Fe(II)-nitro complexes have been synthesized and characterized by variable techniques including X-ray crystallography, X-ray absorption spectroscopy (XAS), infrared spectroscopy (IR), and density functional theory (DFT). Reactivity studies have shown that these Fe(II)-nitro complexes show catalytic NO₂^– reduction to NO via the NiR-like pathway. The turnover frequency (TOF) and NO₂^–-to-NO conversion percentage have been determined, which is reflective of the efficiency and selectivity of catalysis, respectively. In addition, control reactions using redox-inert Zn(II) analogs underscores the role of Fe and turn-on other ligand assisted chemistry. The results of this work will be presented.

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