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 (sucha st hose that could be expected to form at typical non-heme iron sites in biology) are extremely rare.U sing the TMG 3 tren co-ligand, we have prepared ah igh-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,M çssbauer and NMR spectroscopy, X-ray crystallography,a nd DFT calculations.T he data show that its electronic structure is best described as ah igh-spin iron(IV) center bound to atriplet NO À ligand with avery covalent iron À NO bond. This finding demonstrates that this high-spin iron nitrosyl compound undergoes iron-centered redoxc hemistry, leading to fundamentally different properties than corresponding low-spin compounds,w hichu ndergo NO-centered redox transformations.

Hemilabile Proton Relays and Redox Activity Lead to {FeNO}x and Significant Rate Enhancements in NO2– Reduction

“…This significant gap indicates that the BS(2,2) solution is likely a closer single-determinant approximation to the electronic structure of this MNIC than the closed-shell solution. The Mulliken spin population on NO (−1.07) in the BS(2,2) solution (Figure 5) is consistent with that observed for triplet NO anion in other FeNO complexes, 71 where the deviation from a formal spin population of −2 arises in part from the metal−ligand covalency, which also reduces the spin population assigned to Fe (1.76). Together with the Mulliken spin populations on PDI (−0.65), this calculation suggests {FeNO} 7 character with intermediate-spin Fe, a triplet NO − ligand, and a singly reduced PDI ligand, Fe(III)(↑↑↑)NO − (↓ ↓)PDI •− (↓).…”

Tetranuclear Fe Clusters with a Varied Interstitial Ligand: Effects on the Structure, Redox Properties, and Nitric Oxide Activation

“…A similarly large shift was observed upon reduction for a structurally related mononuclear trigonal bipyramidal Fe-NO complex, 18 and more generally for non-heme {FeNO} 7 /{FeNO} 8 complexes. 19 An analogous species is not observable for the μ 4 -O clusters.…”

A Combined Probe-Molecule, Mössbauer, Nuclear Resonance Vibrational Spectroscopy, and Density Functional Theory Approach for Evaluation of Potential Iron Active Sites in an Oxygen Reduction Reaction Catalyst

“…Specifically, only the species corresponding to Doubl 4 red in Figure 1B was perturbed upon NO (g) treatment, generating a new doublet species Doubl 4 NO with Mossbauer parameters δ = 0.60 mm/s and ΔE Q = 0.56 mm/s (see Table 1). The significant reduction in the isomer shift and quadrupole splitting of Doubl 4 red upon NO (g) treatment combined with the lack of consistency with previously reported Mossbauer parameters for nonheme iron-NO adducts 44,45 is most consistent with a reaction of this Fe site with NO (g) to generate oxidized iron(III) species through the oxidative cleavage of NO. The inability to resolve additional doublets in Figure 1C suggests that such sites, likely binuclear and differing only in terminal N and O ligation (see below), are quite similar in geometric and electronic structure, giving rise to very similar Mossbauer parameters.…”