Structure, equilibrium and ligand exchange dynamics in the binary and ternary dioxouranium(vi)-ethylenediamine-N,N′-diacetic acid-fluoride system: a potentiometric, NMR and X-ray crystallographic study†
The structure, thermodynamics and kinetics of the binary and ternary uranium(VI)-ethylenediamine-N,N′-diacetate (in the following denoted EDDA) fluoride systems have been studied using potentiometry, 1H, 19F NMR spectroscopy and X-ray diffraction. The UO22+–EDDA system could be studied up to −log[H3O+] = 3.4 where the formation of two binary complexes UO2(EDDA)(aq) and UO2(H3EDDA)3+ were identified, with equilibrium constants logβ(UO2EDDA) = 11.63 ± 0.02 and logβ(UO2H3EDDA3+) = 1.77 ± 0.04, respectively. In the ternary system the complexes UO2(EDDA)F−, UO2(EDDA)(OH)− and (UO2)2(µ-OH)2(HEDDA)2F2(aq) were identified; the latter through 19F NMR. 1H NMR spectra indicate that the EDDA ligand is chelate bonded in UO2(EDDA)(aq), UO2(EDDA)F− and UO2(EDDA)(OH)− while only one carboxylate group is coordinated in UO2(H3EDDA)3+. The rate and mechanism of the fluoride exchange between UO2(EDDA)F− and free fluoride was studied by 19F NMR spectroscopy. Three reactions contribute to the exchange; (i) site exchange between UO2(EDDA)F− and free fluoride without any net chemical exchange, (ii) replacement of the coordinated fluoride with OH− and (iii) the self dissociation of the coordinated fluoride forming UO2(EDDA)(aq); these reactions seem to follow associative mechanisms. 1H NMR spectra show that the exchange between the free and chelate bonded EDDA is slow and consists of several steps, protonation/deprotonation and chelate ring opening/ring closure, the mechanism cannot be elucidated from the available data. The structure (UO2)2(EDDA)2(µ-H2EDDA) was determined by single crystal X-ray diffraction and contains two UO2(EDDA) units with tetracoordinated EDDA linked by H2EDDA in the “zwitterion” form, coordinated through a single carboxylate oxygen from each end to the two uranium atoms. The geometry of the complexes indicates that there is no geometric constraint for an associative ligand substitution mechanism.