Structural and Spectroscopic Characterization of a High-Spin {FeNO}⁶ Complex with an Iron(IV)-NO^- Electronic Structure

Although the interaction of low-spin ferric complexes with nitric oxide has been well studied, examples of stable high-spin ferric nitrosyls (such as those that could be expected to form at typical non-heme iron sites in biology) are extremely rare. Using the TMG₃tren co-ligand, we have prepared a high-spin ferric NO adduct ({FeNO}⁶ complex) via electrochemical or chemical oxidation of the corresponding high-spin ferrous NO {FeNO}⁷ complex. The {FeNO}⁶ compound is characterized by UV/Visible and IR spectroelectrochemistry, Mössbauer and NMR spectroscopy, X-ray crystallography, and DFT calculations. The data show that its electronic structure is best described as a high-spin iron(IV) center bound to a triplet NO^- ligand with a very covalent iron-NO bond. This finding demonstrates that this high-spin iron nitrosyl compound undergoes iron-centered redox chemistry, leading to fundamentally different properties than corresponding low-spin compounds, which undergo NO-centered redox transformations. One-electron oxidation of the high-spin ferrous nitrosyl complex [Fe(TMG₃tren)(NO)]²⁺ yields a rare high-spin (S=1) ferric NO adduct ({FeNO}⁶). Spectroscopic investigations and DFT calculations indicate that this species has an Fe^IV-NO^- electronic structure. This finding demonstrates that high-spin non-heme iron nitrosyl complexes have fundamentally different redox behavior compared to corresponding low-spin heme systems.