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–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.