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Potentiometric and Multinuclear NMR Study of the Binary and Ternary Uranium(VI)−L−Fluoride Systems, Where L Is α-Hydroxycarboxylate or Glycine

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journal contribution
posted on 06.10.2000, 00:00 by Zoltán Szabó, Ingmar Grenthe
Equilibria, structures, and ligand-exchange dynamics in binary and ternary U(VI)−L−F- systems, where L is glycolate, α-hydroxyisobutyrate, or glycine, have been investigated in 1.0 M NaClO4 by potentiometry and 1H, 17O, and 19F NMR spectroscopy. L may be bonded in two ways:  either through the carboxylate end or by the formation of a chelate. In the glycolate system, the chelate is formed by proton dissociation from the α-hydroxy group at around pH 3, indicating a dramatic increase, a factor of at least 1013, of its dissociation constant on coordination to uranium(VI). The L exchange in carboxylate-coordinated UO2LF32- follows an Eigen−Wilkins mechanism, as previously found for acetate. The water exchange rate, kaq = 4.2 × 105 s-1, is in excellent agreement with the value determined earlier for UO22+(aq). The ligand-exchange dynamics of UO2(O−CH2−COO)2F3- and the activation parameters for the fluoride exchange in D2O (kobs = 12 s-1, ΔH = 45.8 ± 2.2 kJ mol-1, and ΔS = −55.8 ± 3.6 J K-1 mol-1) are very similar to those in the corresponding oxalate complex, with two parallel pathways, one for fluoride and one for the α-oxocarboxylate. The same is true for the L exchange in UO2(O−CH2−COO)22- and UO2(oxalate)22-. The exchange of α-oxocarboxylate takes place by a proton-assisted chelate ring opening followed by dissociation. Because we cannot decide if there is also a parallel H+-independent pathway, only an upper limit for the rate constant, k1 < 1.2 s-1, can be given. This value is smaller than those in previously studied ternary systems. Equilibria and dynamics in the ternary uranium(VI)−glycine−fluoride system, investigated by 19F NMR spectroscopy, indicate the formation of one major ternary complex, UO2LF32-, and one binary complex, UO2L2 (L = H2N−CH2COO-), with chelate-bonded glycine; log β(9) = 13.80 ± 0.05 for the equilibrium UO22+ + H2N−CH2COO- + 3F- = UO2(H2N−CH2COO)F32- and log β(11) = 13.0 ± 0.05 for the reaction UO22+ + 2H2N−CH2COO- = UO2(H2N−CH2COO)2. The glycinate exchange consists of a ring opening followed by proton-assisted steps. The rate of ring opening, 139 ± 9 s-1, is independent of both the concentration of H+ and the solvent, H2O or D2O.