Biological electron transport and catalysis is often mediated by metal clusters, on which the impacts of ligand coordination and redox can be softened by distribution over multiple metal centres. There are ways to measure relative redox levels within a cluster, but it is harder to assign each level to a particular metal site. A team led by Ted Betley has now addressed this problem by studying a series of well-defined triiron complexes with multi-wavelength anomalous diffraction (MAD). Writing in the Proceedings of the National Academy of Sciences, the team describe how symmetry, electronic delocalization and metal–metal bonding affect MAD data. Their work will enable us to better interpret MAD data on more complicated systems such as metalloenzymes.
Taking [Fe3L(pyridine)] as a model compound, in which L6− is a hexaamido ligand derived from cis,cis-1,3,5-triaminocyclohexane, Betley and colleagues also prepared [Fe3L(μ3-Cl)]− (shown in the image), [Fe3L(μ3-NPh)]2− and [Fe3L(μ3-NPh)] to get a series of complexes that differ in terms of symmetry, redox, Fe–Fe bonding and ligation. After collecting full diffraction data from each complex using 30.5 keV X-rays, the team performed MAD by collecting several partial datasets in the range of 7.085 keV to 7.150 keV — the region around the Fe K-edge (the energy required to eject a 1s electron). Using the atomic coordinates obtained from the full dataset, they refined each partial dataset only in terms of Fe scattering factors and arrived at a plot of real scattering factor (f′) versus energy for each Fe in each complex. The minimum of such an f′ plot corresponds to a XANES edge, with the benefit that one knows which Fe gives rise to the spectral feature. For the almost C3-symmetric complex [Fe3L(μ3-Cl)]−, the three f′ plots overlap, indicating that each Feii site is equivalent. By contrast, the asymmetric complex [Fe3L(pyridine)] also has three formally Feii sites but gives rise to f′ minima at energies spanning a 10 eV range — almost as wide as the range previously observed for Fe0 all the way to Feiii. “What is really cool about the technique is that we can see how different the sites really are even though they are formally at the same oxidation level,” notes Betley. “The analysis revealed how simple perturbations can have a substantial impact on relative oxidation levels within clusters.”
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