Exchange of oxygen in uranyl(VI) and water: two binuclear scenarios in acid and in base

The mechanism of exchange between oxygen in UO₂ ²⁺ and that in solvent water has been disputed over last 50 years. It is well–known that the rate of “yl”–oxygen exchange depends heavily on pH, and that there is virtually no exchange at low pH. With increase of pH (pH > 2) the exchange becomes appreciable, and under highly alkaline solution there is a rapid oxygen exchange. These observations led to an idea that there are at least two different exchange mechanisms; one dominating under weakly acidic to neutral pH and another mechanism at very high pH. Szabó and Grenthe [1, 2] used NMR spectroscopy to identify the species involved in “yl”–oxygen exchange and they suggested two binuclear complexes as key species; (UO₂)₂(OH)₂ ²⁺ at low pH and [(UO₂(OH)₄ ^2-)(UO₂(OH)₅ ^3-)] at high pH. How the oxygen exchange takes place in these complexes in atomic scale, however, remains unidentified because the lifetimes of the intermediate species and the transition states of the oxygen exchange are too short to be detected spectroscopically. That is to say, we know they (dimer complexes) did it but we do not know how they managed to do it. Our attempt here is to identify the “yl”–oxygen exchange pathways in these complexes using quantum chemical method thereby proving that oxygen exchange through these complexes are indeed possible – thereby bringing end to the long–disputed arguments over the “yl”–oxygen exchange mechanisms.
First, we studied the “yl”–oxygen exchange pathway via (UO₂)₂(OH)₂ ²⁺ [3]. We used hybrid density functional theory (DFT) with Becke’s three–parameter hybrid functional and Lee–Yang–Parr’s gradient–corrected correlation functional (B3LYP) employing conductor–like polarizable continuum model (CPCM) using Gaussian 09 program (Gaussian Inc.). The small core effective core potential and corresponding basis set was used on uranium and oxygen. Direct proton transfer from the hydroxo bridge or from the coordinating water to the “yl”–oxygen in (UO₂)₂(OH)₂ ²⁺ can be ruled out because we found exceedingly high activation barrier (~170 kJ mol^–1) through these mechanisms. The exchange mechanism in (UO₂)₂(OH)₂ ²⁺ can be described by a multi–step proton transfer pathway that involves the formation of an oxo bridge between the two uranyl(VI) centres (U–Oyl–U bridge). The activation enthalpy 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 is at the acceptable level.
Second, we tried to identify the oxygen exchange pathway through [(UO₂(OH)₄ ^2-)(UO₂(OH)₅ ^3-)]. For this attempt, we first studied the speciation of uranyl(VI) in highly alkaline solution by quantum chemical calculations as well as X–ray absorption spectroscopy (XAS). Although various previous studies assumed that hydrolysis of UO₂(OH)₄ ^2– produces UO₂(OH)₅ ^3–, our B3LYP calculations together with previous theory work by others [4,5] suggest that hydrolysis of UO₂(OH)₄ ^2– yields UO₃(OH)₃ ^3–. We studied this point further using XAS at the Rossendorf Beamline (ROBL) in ESRF, Grenoble, France, and we found evidence of the existence of new species UO₃(OH)₃ ^3– in XANES spectra. The sample which contained further hydrolyzed species showed clear shift of the uranium L_III absorption edge compared to the sample containing only UO₂(OH)₄ ^2–.Similar energy shift was observed in Pa(V) when speciation changed from spherical Pa⁵⁺ to mono–oxo PaO³⁺ [6]. Therefore the species beyond UO₂(OH)₄ ^2– is better assigned to UO₃(OH)₃ ^3– rather than UO₂(OH)₅ ^3–. Likewise, the complex described as [(UO₂(OH)₄ ^2-)(UO₂(OH)₅ ^3-)] by Szabó and Grenthe should better be written as [(UO₂(OH)₄ ^2-)(UO₃(OH)₃ ^3-)]. We then studied the “yl”–oxygen pathway within [(UO₂(OH)₄ ^2-)(UO₃(OH)₃ ^3-)] by DFT, and found a realistic pathway which has the activation Gibbs energy of 56.3 kJ mol^–1 at the B3LYP level, which is again in good agreement with the experimental value of 60.8 ± 2.4 kJ mol^–1 obtained by Szabó and Grenthe [2].
Our calculations confirm the “yl”–oxygen exchange mechanisms through (UO₂)₂(OH)₂ ²⁺ and [(UO₂(OH)₄ ^2-)(UO₃(OH)₃ ^3-)], and underscores the role of binuclear species. The formation of U–O_yl–U bridge seems to play a key role in facilitating intramolecular proton shuttling among the oxygen atoms thereby contributing to faster “yl”–oxygen exchange.

[1] Szabó, Z. and Grenthe, I., Inorg. Chem. 2007, 46, 9372–9378.
[2] Szabó, Z. and Grenthe, I., Inorg. Chem. 2010, 49, 4928–4933.
[3] Tsushima, S., Inorg. Chem. 2012, 51, 1434–1439.
[4] Shamov, G. A. and Schreckenbach, G., J. Am. Chem. Soc. 2008, 130, 13735–13744.
[5] Bühl, M. and Schreckenbach, G., Inorg. Chem. 2010, 49, 3821–3827.
[6] Le Naour, C. et al., Inorg. Chem. 2005, 44, 9542–9546.