Uranyl–halide complexation in N,N-dimethylformamide: halide coordination trend manifests hardness of [UO2]2+

Abstract

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⁻ almost quantitatively increases up to 4, i.e., the limiting complex is [UO₂Cl₄]²⁻. Logarithmic gross stability constants of [UO₂Cl_x]^2−x (x = 1–4) were evaluated as log β₁ = 9.67, log β₂ = 15.49, log β₃ = 19.89, and log β₄ = 24.63 from UV-vis titration experiments. The EXAFS results well demonstrated not only the Cl⁻ coordination, but also the DMF solvation in the equatorial plane of [UO₂]²⁺. The interaction of Br⁻ and I⁻ with [UO₂]²⁺ in DMF was also investigated. 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 β_x values of [UO₂Br_x]^2−x (x = 1, 2) are 3.45 and 5.42, respectively. The much smaller log β_x than those of [UO₂Cl_x]^2−x indicates that Br⁻ is a 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.