Alkali-metal ion coordination in uranyl(VI) poly- peroxide complexes in solution. Part 1: the Li+, Na+ and K+ – peroxide–hydroxide systems
Résumé
The alkali metal ions Li+, Na+ and K+ have a profound influence on the stoichiometry of the complexes formed in uranyl(VI)–peroxide–hydroxide systems, presumably as a result of a templating effect, resulting in the formation of two complexes, M[(UO2)(O2)(OH)]2− where the uranyl units are linked by one peroxide bridge, μ-η2-η2, with the second peroxide coordinated “end-on”, η2, to one of the uranyl groups, and M[(UO2)(O2)(OH)]43-, with a four-membered ring of uranyl ions linked by μ-η2-η2 peroxide bridges. The stoichiometry and equilibrium constants for the reactions: M+ + 2UO22+ + 2HO2− + 2H2O → M[(UO2)(O2)(OH)]2− + 4H+ (1) and M+ + 4UO22+ + 4HO2− + 4H2O → M[(UO2)(O2)(OH)]43− + 8H+ (2) have been measured at 25 °C in 0.10 M (tetramethyl ammonium/M+)NO3 ionic media using reaction calorimetry. Both reactions are strongly enthalpy driven with large negative entropies of reaction; the observation that ΔH(2) ≈ 2ΔH(1) suggests that the enthalpy of reaction is approximately the same when peroxide is added in bridging and “end-on” positions. The thermodynamic driving force in the reactions is the formation of strong peroxide bridges and the role of M+ cations is to provide a pathway with a low activation barrier between the reactants and in this way “guide” them to form peroxide bridged complexes; they play a similar role as in the synthesis of crown-ethers. Quantum chemical (QC) methods were used to determine the structure of the complexes, and to demonstrate how the size of the M+-ions affects their coordination geometry. There are several isomers of Na[(UO2)(O2)(OH)]2- and QC energy calculations show that the ones with a peroxide bridge are substantially more stable than the ones with hydroxide bridges. There are isomers with different coordination sites for Na+ and the one with coordination to the peroxide bridge and two uranyl oxygen atoms is the most stable one.