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Alkali-metal ion coordination in uranyl(VI) poly-peroxide complexes in solution. Part 1: the Li+, Na+ and K+ - peroxide-hydroxide systems
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Organic Chemistry.ORCID iD: 0000-0002-7552-1076
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2015 (English)In: Dalton Transactions, ISSN 1477-9226, E-ISSN 1477-9234, Vol. 44, no 4, 1549-1556 p.Article in journal (Refereed) Published
Abstract [en]

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)(O-2)(OH)](2)(-) where the uranyl units are linked by one peroxide bridge, mu-eta(2)-eta(2), with the second peroxide coordinated "end-on", eta(2), to one of the uranyl groups, and M[(UO2)(O-2)(OH)](4)(3-), with a four-membered ring of uranyl ions linked by mu-eta(2)-eta(2) peroxide bridges. The stoichiometry and equilibrium constants for the reactions: M+ + 2UO(2)(2+) + 2HO(2)(-) + 2H(2)O -> M[(UO2)(O-2)(OH)] 2 - + 4H(+) (1) and M+ + 4UO(2)(2+) + 4HO(2)(-) + 4H(2)O -> M[(UO2)(O-2)(OH)](4)(3-) + 8H(+) (2) have been measured at 25 degrees 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 Delta H(2) approximate to 2 Delta H(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)(O-2)(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.

Place, publisher, year, edition, pages
2015. Vol. 44, no 4, 1549-1556 p.
Keyword [en]
Water Exchange-Reaction, Basis-Sets, Chemical-Equilibria, Free-Energy, Solvation, Pseudopotentials, Hydrogen, Atoms
National Category
Chemical Sciences
URN: urn:nbn:se:kth:diva-159355DOI: 10.1039/c4dt02104eISI: 000346907800008ScopusID: 2-s2.0-84919933637OAI: diva2:785159

QC 20150202

Available from: 2015-02-02 Created: 2015-01-29 Last updated: 2015-02-02Bibliographically approved

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