Conformational and coordination equilibria on DOTA complexes of lanthanide metal ions in aqueous solution studied by H-1-NMR spectroscopy
Conformational and coordination equilibria on DOTA complexes of lanthanide metal ions in aqueous solution studied by H-1-NMR spectroscopy
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DOI：
10.1021/ic961364o
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发表时间：
1997-05-07
影响因子：
4.6
通讯作者：
Merbach, AE
中科院分区：
文献类型：
--
作者：
Aime, S;Botta, M;Merbach, AE
不同三价阳离子（Sc，Y，La，Ce --> Lu）（DOTA = 1，4，7，10-tetraaza-1，4，7，10-tetrakis（羧甲基）环十二烷）的DOTA络合物的可变温度，压力和离子强度H-1 NMR研究产生的数据与迄今为止使用的模型相矛盾，只有两个对映体对的非对映异构体，不同的配体构象。两种异构体的平衡不能解释最近观察到的异构体比例在系列结束时的明显逆转。由于两种构象异构体都可能失去它们的内层水分子，配位平衡可能叠加在该构象平衡上，如[Ln（DOTA）（H2O）（x）]（-）（Ln = Yb，Lu; x = 1，0）的异构化的大的正反应体积所示。[Nd（DOTA）（H2O）]（-）和[Eu（DOTA）（H2O）]（-）的异构化是纯构象的，如接近零的反应体积所示。测得的异构化熵和异构化熵与该模型一致。各种非配位盐的异构化平衡的移动取决于配体构象，而不是存在或不存在的内球水分子。这是由于弱离子结合和一种配体构象的水溶剂稳定性，而不是溶液中本体水的活性的降低，如通过Eu（III）的配位数敏感跃迁F-5（0）-> D-7（0）的UV-vis测量作为离子强度的函数所示。F-19-NMR位移和[Eu（DOTA）（H2O）]（-）的H-1-NMR光谱中出现对应于[Eu（DOTA）F]（2-）的第三组共振证明，氟离子优先取代内配位层中的水分子，对于构象异构体之一。
A variable-temperature, -pressure, and -ionic strength H-1 NMR study of the DOTA complexes of different trivalent cations (Sc, Y, La, Ce --> Lu) (DOTA = 1,4,7,10-tetraaza-1,4,7,10-tetrakis(carboxymethyl)cyclododecane) yielded data that are in contradiction with the hitherto used model of only two enantiomeric pairs of diastereoisomers that differ in the ligand conformations. A two-isomer equilibrium cannot explain the newly observed apparent reversal of the isomer ratio at the end of the series. As both conformers may lose their inner sphere water molecule, a coordination equilibrium may be superimposed on this conformational equilibrium, as shown by large positive reaction volumes for the isomerization of [Ln(DOTA)(H2O)(x)](-) (Ln = Yb, Lu; x = 1, 0). The isomerization of [Nd(DOTA)(H2O)](-) and [Eu(DOTA)(H2O)](-) is purely conformational, as shown by near-zero reaction volumes. The measured isomerization enthalpies and entropies agree with this model. The shift of the isomerization equilibria by a variety of non-coordinative salts depends on the ligand conformation rather than the presence or absence of the inner sphere water molecule. This results from weak ion binding and water solvent stabilization of one ligand conformation, rather than the decrease of the activity of the bulk water in the solution, as shown by UV-vis measurements of the coordination number sensitive transition F-5(0) --> D-7(0) of Eu(III) as a function of ionic strength. Fluoride ions replace a water molecule in the inner coordination sphere, preferentially for one of the conformational isomers, as proven by F-19-NMR shifts and the appearance of a third set of resonances corresponding to [Eu(DOTA)F](2-) in the H-1-NMR spectrum of [Eu(DOTA)(H2O)](-).