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, [Fe^II(TBDAP)(NO₂)(CH₃CN)]⁺ (<b>1</b>) (TBDAP = <i>N</i>,<i>N</i>'-di-<i>tert</i>-butyl-2,11-diaza[3.3](2,6)-pyridinophane). Treatment of <b>1</b> with 2.5 equiv of triflic acid (HOTf) affords the {FeNO}⁶ (<b>2</b>) intermediate, which was characterized using a combination of various physicochemical techniques and DFT calculations. Isotopic labeling using Na¹⁵NO₂ confirmed the formation of <b>2</b> 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 <b>2</b>, suggesting that the rate-determining step involves an associative reaction between Fe(II) and NO⁺. Electrochemical analysis showed a reversible redox couple for <b>2</b>, 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.