Alkali-metal ion coordination in uranyl(VI) poly-peroxide complexes in solution. Part 1: the Li⁺, Na⁺ and K⁺--peroxide-hydroxide systems.

Abstract

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-η(2)-η(2), with the second peroxide coordinated "end-on", η(2), to one of the uranyl groups, and M[(UO2)(O2)(OH)]4(3-), with a four-membered ring of uranyl ions linked by μ-η(2)-η(2) peroxide bridges. The stoichiometry and equilibrium constants for the reactions: M(+) + 2UO2(2+) + 2HO2(-) + 2H2O → M[(UO2)(O2)(OH)]2(-) + 4H(+) (1) and M(+) + 4UO2(2+) + 4HO2(-) + 4H2O → M[(UO2)(O2)(OH)]4(3-) + 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.