Landweber-EXAFS structural analysis of aqueous polynuclear U(VI) complexes with tartaric acid

In coordination chemistry, an open question is if the structure of an aqueous metal complex is equal to the structure of its solid form. While the structure of the solid can usually be determined with great reliability and precision by XRD, determination of the structure in solution by EXAFS may be much more biased. An intrinsic problem of EXAFS shell fitting is that the radial pair distribution function (RPDF) is approximated by Gaussians functions imitating the coordination shells. Different combinations of shells can yield different structures with similar fit quality, thereby making the structural solution non-unique. Even the so-called F-test often does not allow obtaining a unique solution.
Therefore, we developed two methods which enable the direct calculation of the RPDF and the spatial structure of metal complexes in solution. Solely based on the FEFF scattering theory, the Landweber inverse method [1] yields the RPDF for the aqueous bi- and tri-nuclear U(VI)-tartaric acid complexes without predefined assumptions about the form of the RPDF. With this information and density functional theory (DFT) calculations, the spatial structures of the complexes are refined by Monte Carlo Target Transformation Factor Analysis [2], which also include the calculation of higher order scattering events. Using this combinatorial structural analysis, we were able to show that in the (UO2)3(H-1Tar)3(OH)25- complex a central equatorial oxygen atom at a radial U-O distance of 2.22 Å connects the three uranium atoms symmetrically. Consequently, the formula of the aqueous complex corresponds to its stoichiometric equivalent (UO2)3(mue3-O)(H-1Tar)35-.

[1] Rossberg & Funke (2010), J. Synchr. Rad. 17, 280-288.
[2] Rossberg et al. (2005), Anal. Bioanal. Chem. 383, 56-66.