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, 116 (32), 15836-15841

Exposing the Inadequacy of Redox Formalisms by Resolving Redox Inequivalence Within Isovalent Clusters

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Exposing the Inadequacy of Redox Formalisms by Resolving Redox Inequivalence Within Isovalent Clusters

Amymarie K Bartholomew et al. Proc Natl Acad Sci U S A.

Abstract

In this report we examine a family of trinuclear iron complexes by multiple-wavelength, anomalous diffraction (MAD) to explore the redox load distribution within cluster materials by the free refinement of atomic scattering factors. Several effects were explored that can impact atomic scattering factors within clusters, including 1) metal atom primary coordination sphere, 2) M-M bonding, and 3) redox delocalization in formally mixed-valent species. Complexes were investigated which vary from highly symmetric to fully asymmetric by 57Fe Mössbauer and X-ray diffraction to explore the relationship between MAD-derived data and the data available from these widely used characterization techniques. The compounds examined include the all-ferrous clusters [ n Bu4N][(tbsL)Fe33-Cl)] (1) ([tbsL]6- = [1,3,5-C6H9(NC6H4-o-NSi t BuMe2)3]6-]), (tbsL)Fe3(py) (2), [K(C222)]2[(tbsL)Fe33-NPh)] (4) (C222 = 2,2,2-cryptand), and the mixed-valent (tbsL)Fe33-NPh) (3). Redox delocalization in mixed-valent 3 was explored with cyclic voltammetry (CV), zero-field 57Fe Mössbauer, near-infrared (NIR) spectroscopy, and X-ray crystallography techniques. We find that the MAD results show an excellent correspondence to 57Fe Mössbauer data; yet also can distinguish between subtle changes in local coordination geometries where Mössbauer cannot. Differences within aggregate oxidation levels are evident by systematic shifts of scattering factor envelopes to increasingly higher energies. However, distinguishing local oxidation levels in iso- or mixed-valent materials can be dramatically obscured by the degree of covalent intracore bonding. MAD-derived atomic scattering factor data emphasize in-edge features that are often difficult to analyze by X-ray absorption near edge spectroscopy (XANES). Thus, relative oxidation levels within the cluster were most reliably ascertained from comparing the entire envelope of the atomic scattering factor data.

Keywords: bioinorganic; clusters; redox distribution.

Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.
Fig. 1.
Synthesis of 14.
Fig. 2.
Fig. 2.
X-ray single-crystal structures obtained at 30.5 keV (100 K) for (A) (tbsL)Fe3(py) (1), (B) [nBu4N][(tbsL)Fe33–Cl)] (2), (C) (tbsL)Fe33–NPh) (3), and (D) [K(C222)]2[(tbsL)Fe33–NPh)] (4). Disorder ratio of the phenyl group on the μ3–NPh fragment in 3 is 48:52. Hydrogen atoms, cations, and cocrystallized solvent molecules have been omitted for clarity. Thermal ellipsoids set at 50% probability level. Fe, orange; C, gray; Cl, green; Si, pink; and N, blue.
Fig. 3.
Fig. 3.
Crystal-structure cores with bond metrics indicated and zero-field 57Fe Mössbauer spectra of the all-ferrous materials (A and D) [(tbsL)Fe33–Cl)] (2), (B and E) (tbsL)Fe3(py) (1), and (C and F) [(tbsL)Fe33–NPh)]2− of (4). (GI) Single-crystal X-ray fluorescence scan collected around the Fe K edge in steps of 1 eV at 100 K; anomalous scattering factor f (filled circles) and interpolation (solid line) for each iron center of (G) [(tbsL)Fe33–Cl)] (2), (H) (tbsL)Fe3(py) (1), and (I) [(tbsL)Fe33–NPh)]2− (4).
Fig. 4.
Fig. 4.
Crystal-structure cores for (tbsL)Fe33–NPh) (3) (A) and [(tbsL)Fe33–NPh)]2− of 4 (B) with relevant bond metrics indicated. (C) Single-crystal X-ray fluorescence scan collected around the Fe K edge in steps of 1 eV at 100 K and (D) anomalous scattering factor f (filled circles) and interpolation (solid line) for each iron center of (tbsL)Fe33–NPh) (3). (E) X-ray fluorescence scan overlay for (tbsL)Fe33–NPh) (3, red) and [(tbsL)Fe33–NPh)]2– (4, blue). (F) Overlay of anomalous scattering factor plots for each iron center of (tbsL)Fe33–NPh) (3, red) and [(tbsL)Fe33–NPh)]2– (4, blue). Expanded view of the right-hand side of (G) the X-ray fluorescence and (H) f of 3 and 4 highlighting the observed shift.

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