Uranyl-Halide Complexation in N,N-Dimethylformamide: Halide Coordination Trend Manifests Hardness of [UO2]2+

Complexation of [UO2]2+ with Cl−, Br−, and I− in N,N-dimethylformamide (DMF) was studied by UV-vis absorption spectroscopy and extended X-ray absorption fine structure (EXAFS) to clearly differentiate halide coordination strengths to [UO2]2+. In the Cl− system, it was clarified that the Cl− coordination to [UO2]2+ in DMF proceeds almost quantitatively. The coordination number of Cl− increases up to 4 with increasing its total concentration, i.e., the limiting complex is [UO2Cl4]2−. Logarithmic gross stability constants of [UO2Clx]2−x (x = 1-4) were evaluated as log 1 = 9.67, log 2 = 15.49, log 3 = 19.89, and log 4 = 24.63 from UV-vis titration experiment. The EXAFS results well demonstrated not only the Cl− coordination, but also the DMF solvation in the equatorial plane of [UO2]2+. The similar experiments were also performed for the Br− and I− systems. As a result, the Br− coordination to [UO2]2+ stops at the second step, i.e., only [UO2Br]+ and UO2Br2 were observed. The molecular structure of each occurring species was confirmed by EXAFS. The evaluated log x of [UO2Brx]2−x (x = 1, 2) are 3.45 and 5.42, respectively. The much smaller log x than those of [UO2Clx]2−x indicates that Br− is much weaker ligand to [UO2]2+ than Cl−. The EXAFS experiments revealed that the presence of I− in the test solution does not modify any coordination structure around [UO2]2+. Thus, I− does not form any stable [UO2]2+ complexes in DMF. Consequently, the stability of the halido complexes of [UO2]2+ in DMF is exactly in line with the hardness order of halides. This trend clearly manifests the hardness of [UO2]2+.