Combined computational and experimental study of uranyl(VI) 1:2 complexation by aromatic acids

The bis(salicylhydroxamato) and bis(benzohydroxamato) complexes of UO(2)(2+) 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 UO(2)(2+) was investigated computationally for comparison. Experimental data indicate 5-fold UO(2)(2+) coordination with mean equatorial U-O distances of 2.42 and 2.40 A for the salicyl- and benzohydroxamate systems, respectively. DFT calculations on microsolvated model systems [UO(2)L(2)OH(2)] indicate UO(2)(2+) eta(2)-chelation via the hydroxamate oxygen atoms in excellent agreement with experimental data; calculated complex stabilities support that UO(2)(2+) prefers hydroxamate over carboxylate coordination. The 414 nm absorption band of UO(2)(2+) 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 [UO(2)L(2)OH(2)], however, occasionally fail to reproduce accurately experimental UV-vis spectra, which are dominated by UO(2)(2+) <-- L(-) charge-transfer contributions. We additionally show that the U(VI) 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 U(VI).