Abstract

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.

Researchers from UNIST and Jeonbuk National University have, for the first time, captured and analyzed a short-lived iron (Fe)-based intermediate involved in converting nitrite (NO₂^–) to nitric oxide (NO)—a key process in the nitrogen cycle and biological signaling. This discovery, made at ultra-low temperatures, provides new insights into how vital molecules are produced in nature and in biological systems.

Using a specialized Fe(ll)-nitrite complex and reaction conditions at -40°C, Professor Jaeheung Cho from the Department of Chemistry at UNIST, in collaboration with Professor Kyung-Bin Cho at Jeonbuk National University isolated the elusive {FeNO}⁶ intermediate, a critical step preceding NO release. Spectroscopic and computational analyses confirmed that this species forms after NO₂^– accepts a proton and undergoes bond cleavage, with the nitrogen-oxygen ion binding to Fe. Further electron transfer then liberates NO. 

The study also revealed that the reaction pathway varies depending on whether proton and electron transfers occur sequentially or simultaneously, providing nuanced insight into reaction mechanisms.

Professor Cho remarked, “This is the first direct observation of the intermediate in NO₂^– reduction to NO. Understanding this step could inform targeted therapies for vascular diseases and inspire the design of new catalysts with improved efficiency.”

According to the research team, this discovery advances fundamental knowledge of nitrogen cycle chemistry and biological NO production, with potential applications in medicine and sustainable catalysis. By elucidating the reaction pathway, the research opens avenues for developing innovative treatments and catalytic systems.

These findings were published in the Journal of the American Chemical Society (JACS) on March 20, 2026. The study has been supported by the Ministry of Science and ICT (MSIT), the National Research Foundation of Korea (NRF), and the Ministry of Health and Welfare (MOHW).

Journal Reference

Seungwon Sun, Youngjin Jeon, Youngseob Lee, et al., “Unveiling an {FeNO}6 Intermediate: A Sequential Mechanistic Investigation of Nitrite Reduction in a Mononuclear Iron(II) Complex,” JACS,  (2026).