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 βx values of [UO2Br(x)](2-x) (x = 1, 2) are 3.45 and 5.42, respectively. The much smaller log βx 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.

Effects of coordinating heteroatoms on molecular structure, thermodynamic stability and redox behavior of uranyl(vi) complexes with pentadentate Schiff-base ligands

“…14,15 Indeed, thermodynamic stability of a UO 2 2+ -halido complex in DMF follows the order of hardness of halide ligands, Cl − > Br − > I − . 16 In contrast, such a trend in complexation between UO 2 2+ and heteroatoms like N, O and S, seems not to be well understood systematically, although it would provide essential information to understand the fundamental nature of UO 2 2+ in more depth and to design molecular structures of ligands exclusively interacting with UO 2 2+ . Indeed, several extracting reagents have been successfully developed for separation of Am( iii ) and Cm( iii ) from Ln( iii ) on the basis of difference in coordinating affinities of these metal ions with soft-donor atoms incorporated in the designed ligands.…”

Exploring the catalytic activity of Lewis-acidic uranyl complexes in the nucleophilic acyl substitution of acid anhydrides

“…Although no isosbestic points were observed in Fig. 4, principal component analysis (PCA) 42,43 has suggested that a total of 2 components are involved in this spectral series. This result corroborates the occurrence of eqn (3) in the current system.…”

Unprecedented Catalytic Behavior of Uranyl(VI) Compounds in Chemical Reactions

“…Indeed, the equatorial coordination environment in the case of uranyl complexes which contains a robust axial structure may offer some selectivity for the chemical reaction. For the nucleophilic acyl substitution of acetic anhydride, Takao et al [110] have experimented with four different uranyl complexes 17-20 [111][112][113][114] as catalyst (Figure 9). For this, substrates such as acetic anhydride and ethanol were chosen for a model reaction system.…”