Theoretical study of the uranyl complexation by hydroxamic and carboxylic acid groups

A theoretical study on the complexation of uranyl cation (UO2(2+)) by three different functional groups of a calix[6]arene cage, that is, two hydroxamic and a carboxylic acid function, has been carried out using density functional theory calculations. In particular, interaction energies between the uranyl and the functional groups have been used to determine their affinity toward uranyl, whereas pKa calculations give some information on the availability of the functional groups in the extraction conditions. On the one hand, calculations of the interaction energies have pointed out clearly a better affinity with the hydroxamic groups. The stabilization of this complex was rationalized in terms of a stronger electrostatic interaction between the uranyl cation and the hydroxamic groups. The presence of a water molecule in the first coordination sphere of uranyl does not destabilize the complex, and the most stable complex is obtained with two functional groups and two water molecules, leading to a coordination number of 8 for the central uranium atom. On the other hand, pKa theoretical evaluation shows that both hydroxamic (deprotonated on the oxygen site) and carboxylic groups are potential extractants in aqueous medium with a preference for carboxylic functions at low pH. Moreover, these data allowed to unambiguously identify the oxygen of the alcohol function as the favored deprotonation site on the hydroxamic function.