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

High Resolution Image Download MS PowerPoint Slide The reduction of nitrite (NO 2 – ) 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)(NO 2 )(CH 3 CN)] + ( 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 Na 15 NO 2 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.