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

Abstract

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 FeIV‐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.