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 FeNO6 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-diaza3. 3 (2, 6) -pyridinophane). Treatment of 1 with 2. 5 equiv of triflic acid (HOTf) affords the FeNO6 (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 FeNO6 as an isolable and reactive intermediate, offering new insight into the mechanistic landscape of nitrite reduction.