“yl”-Oxygen Exchange in Uranyl(VI) Ion: A Mechanism Involving (UO₂)₂(μ-OH)₂²⁺ via U-O_yl-U Bridge Formation

Szabó and Grenthe (Inorg. Chem. 2007, 46, 9372-9378) suggested from NMR spectroscopy that the “yl”-oxygen exchange in dioxo uranium(VI) ion in acidic solution occurs via an OH-bridged binuclear complex (UO₂)₂(μ-OH)₂ ²⁺. Here, an “yl”-oxygen exchange pathway involving the (UO₂)₂(μ-OH)₂ ²⁺ is studied by B3LYP density functional theory calculations. The oxygen exchange takes place via an intramolecular proton shuttle between the oxygen atoms in (UO₂)₂(μ-OH)₂(H₂O)₆ ²⁺. The direct proton transfer from the hydroxo bridge or from the coordinating water to the “yl”-oxygen in (UO₂)₂(μ-OH)₂(H₂O)₆ ²⁺ appears to be negligible because of an exceedingly high activation barrier (~ 170 kJ mol^-1). The exchange mechanism in (UO₂)₂(μ-OH)₂(H₂O)₆ ²⁺ can be described by a multi-step pathway that leads to the formation of an oxo bridge between two uranyl(VI) centers (U-O_yl-U bridge). The activation enthalpy Δ^‡ H of the reaction obtained at the B3LYP level is 94.7 kJ mol^-1 and is somewhat larger than the experimental value of 80 ± 14 kJ mol^-1. However, the discrepancy between theory and experiment is at the acceptable level. The formation of an oxo bridge between the two uranyl(VI) centers was found to be the key step in proton shuttling, indicating that uranyl(VI) complexes with a stable oxo bridge (such as trinuclear (UO₂)₃(μ₃-O)(OH)₃ ⁺) may have even faster “yl”-oxygen exchange rates than (UO₂)₂(μ-OH)₂ ²⁺.