Combined Computational and Experimental Study of Uranyl(VI) 1:2 Complexation by Aromatic Acids

The bis(salicylhydroxamato) and bis(benzohydroxamato) complexes of UO22+ in aqueous solution have been investigated in a combined experimental and computational effort using extended X-ray absorption fine structure and UV−vis spectroscopy and density functional theory (DFT) techniques, respectively. The experimentally unknown bis(benzoate) complex of UO22+ was investigated computationally for comparison. Experimental data indicate 5-fold UO22+ coordination with mean equatorial U−O distances of 2.42 and 2.40 Å for the salicyl- and benzohydroxamate systems, respectively. DFT calculations on microsolvated model systems [UO2L2OH2] indicate UO22+ η2-chelation via the hydroxamate oxygen atoms in excellent agreement with experimental data; calculated complex stabilities support that UO22+ prefers hydroxamate over carboxylate coordination. The 414 nm absorption band of UO22+ in aqueous solution is blue-shifted to 390 and 386 nm upon complexation by salicyl- and benzohydroxamate, respectively. Calculated time-dependent DFT excitation energies of [UO2L2OH2], however, occasionally fail to reproduce accurately experimental UV−vis spectra, which are dominated by UO22+ ← L− charge-transfer contributions. We additionally show that the UVI large-core pseudopotential approximation recently developed by some of the authors can routinely be applied for electronic structure calculations not involving uranium 5f occupations significantly different from UVI.