二元和三元铀（VI）-L-氟化物体系的电位滴定和多核核磁共振研究，其中L为α-羟基羧酸盐或甘氨酸。

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

作者信息

Szabó Z, Grenthe I

机构信息

Department of Chemistry, Inorganic Chemistry, Royal Institute of Technology (KTH), Stockholm, Sweden.

出版信息

Inorg Chem. 2000 Oct 30;39(22):5036-43. doi: 10.1021/ic000400n.

DOI:10.1021/ic000400n

PMID:11233200

Abstract

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 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 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 UO2LF3(2-) 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 UO2(2+)(aq). The ligand-exchange dynamics of UO2(O-CH2-COO)2F3- and the activation parameters for the fluoride exchange in D2O (k(obs) = 12 s(-1), deltaH(double dagger) = 45.8 +/- 2.2 kJ mo(-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, 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, UO2LF3(2-), and one binary complex, UO2L2 (L = H2N-CH2COO-), with chelate-bonded glycine; log beta(9) = 13.80 +/- 0.05 for the equilibrium UO2(2+) + H2N-CH2COO- + 3F- = UO2(H2N-CH2COO)F3(2-) and log beta(11) = 13.0 +/- 0.05 for the reaction UO2(2+) + 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.

摘要

采用电位滴定法以及¹H、¹⁷O和¹⁹F核磁共振波谱法，研究了二元和三元U(VI)-L-F⁻体系（其中L为乙醇酸、α-羟基异丁酸或甘氨酸）在1.0 M高氯酸钠中的平衡、结构和配体交换动力学。L可能以两种方式键合：要么通过羧酸根基团，要么通过形成螯合物。在乙醇酸体系中，螯合物是在pH约为3时由α-羟基上的质子解离形成的，这表明其解离常数在与铀(VI)配位时急剧增加，至少增加了10¹³倍。羧酸盐配位的UO₂LF₃²⁻中的L交换遵循Eigen-Wilkins机制，正如之前在乙酸盐体系中发现的那样。水交换速率k(aq) = 4.2×10⁵ s⁻¹，与先前测定的UO₂²⁺(aq)的值非常吻合。UO₂(O-CH₂-COO)₂F₃⁻的配体交换动力学以及重水中氟交换的活化参数（k(obs) = 12 s⁻¹，ΔH‡ = 45.8 ± 2.2 kJ mol⁻¹，ΔS‡ = -55.8 ± 3.6 J K⁻¹ mol⁻¹）与相应的草酸盐配合物非常相似，有两条平行途径，一条用于氟，一条用于α-氧代羧酸盐。UO₂(O-CH₂-COO)₂²⁻和UO₂(草酸盐)₂²⁻中的L交换情况也是如此。α-氧代羧酸盐的交换通过质子辅助的螯合环开环然后解离进行。由于我们无法确定是否也存在一条与H⁺无关的平行途径，所以只能给出速率常数的上限，k₁ < 1.2 s⁻¹。这个值比之前研究的三元体系中的值要小。通过¹⁹F核磁共振波谱法研究的三元铀(VI)-甘氨酸-氟体系中的平衡和动力学表明，形成了一种主要的三元配合物UO₂LF₃²⁻和一种二元配合物UO₂L₂（L = H₂N-CH₂COO⁻），其中甘氨酸通过螯合键合；对于平衡UO₂²⁺ + H₂N-CH₂COO⁻ + 3F⁻ = UO₂(H₂N-CH₂COO)F₃²⁻，logβ₉ = 13.80 ± 0.05，对于反应UO₂²⁺ + 2H₂N-CH₂COO⁻ = UO₂(H₂N-CH₂COO)₂，logβ₁₁ = 13.0 ± 0.05。甘氨酸盐交换包括环开环，随后是质子辅助步骤。环开环速率为139 ± 9 s⁻¹，与H⁺浓度以及溶剂H₂O或D₂O均无关。