Potentiometric and multinuclear NMR study of the binary and ternary uranium(VI)-L-fluoride systems, where L is α-hydroxycarboxylate or glycine

Equilibria, structures, and ligand-exchange dynamics in binary and ternary U(VI)-L-F- systems, where L is glycolate, alpha -hydroxyisobutyrate, or glycine, have been investigated in 1.0 M NaClO4 by potentiometry and H-1, O-17, and F-19 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 -alpha hydroxy group at around pH 3, indicating a dramatic increase, a factor of at least 10(13), 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, k(aq) = 4.2 x 10(5) s(-1), is in excellent agreement with the value determined earlier for UO22+(aq). The ligand-exchange dynamics of UO2(O-CH2-COO)(2)F-3 and the activation parameters for the fluoride exchange in D2O (k(obs) = 12 s(-1), DeltaH(double dagger) = 45.8 +/- 2.2 kJ mol(-1), and DeltaS(double dagger) = -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 alpha -oxocarboxylate. The same is true for the L exchange in UO2(O-CH2-COO)(2)(2-) and UO2(oxalate)(2)(2-), The exchange of alpha -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, k(1) < 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 F-19 NMR spectroscopy, indicate the formation of one major ternary complex, UO2LF32- and one binary complex, UO2L2 (L = H2N-CH2COO-), with chelate-bonded glycine; log beta>(*) over bar * (9) = 13.80 +/- 0.05 for the equilibrium UO22+ + H2N-CH2COO- + 3F(-) = UO2(H2N-CH2COO)F-3(2-) and log beta>(*) over bar * (11) = 13.0 +/- 0.05 for the reaction UO22+ + 2H(2)N-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.