Unveiling an {FeNO} ⁶ Intermediate: A Sequential Mechanistic Investigation of Nitrite Reduction in a Mononuclear Iron(II) Complex

The reduction of nitrite (NO2–) to nitric oxide (NO•) is a fundamental transformation within both the global nitrogen cycle and enzymatic signaling pathways. Although extensively investigated, the elusive {FeNO}6 intermediate implicated in the 2H+/1e– reduction pathway has rarely been observed or isolated due to the inherent instability. Here, we present a comprehensive mechanistic investigation of nitrite reduction by a mononuclear iron(II)-nitrite complex, [FeII(TBDAP)(NO2)(CH3CN)]+ (1) (TBDAP = N,N′-di-tert-butyl-2,11-diaza[3.3](2,6)-pyridinophane). Treatment of 1 with 2.5 equiv of triflic acid (HOTf) affords the {FeNO}6 (2) intermediate, which was characterized using a combination of various physicochemical techniques and DFT calculations. Isotopic labeling using Na15NO2 confirmed the formation of 2 via heterolytic N–O bond cleavage. Kinetic studies revealed a HOTf-independent rate constant and a markedly negative value of activation entropy for the formation of 2, suggesting that the rate-determining step involves an associative reaction between Fe(II) and NO+. Electrochemical analysis showed a reversible redox couple for 2, and subsequent one-electron reduction by ferrocene released NO•. The generation of NO• was confirmed through trapping experiments using [Co(TPP)], resulting in the formation of [Co(TPP)(NO)]. The experimental findings establish {FeNO}6 as an isolable and reactive intermediate, offering new insight into the mechanistic landscape of nitrite reduction.