Nicolle Gaëlle M, Yerly Fabrice, Imbert Daniel, Böttger Ulrike, Bünzli Jean-Claude, Merbach André E
Laboratory of Inorganic and Bioinorganic Chemistry Institute of Molecular and Biological Chemistry, Swiss Federal Institute of Technology, EPFL-BCH, 1015 Lausanne, Switzerland.
Chemistry. 2003 Nov 21;9(22):5453-67. doi: 10.1002/chem.200305049.
We report the study of binuclear Ln(III) chelates of OHEC (OHEC=octaazacyclohexacosane-1,4,7,10,14,17,20,23-octaacetate). The interconversion between two isomeric forms, which occurs in aqueous solution, has been studied by NMR, UV/Vis, EPR, and luminescence spectroscopy, as well as by classical molecular dynamics (MD) simulations. For the first time we have characterized an isomerization equilibrium for a Ln(III) polyaminocarboxylate complex (Ln(III)=Y, Eu, Gd and Tb) in which the metal centre changes its coordination number from nine to eight, such that: Ln(2)(ohec)(H(2)O)(2) r<==>Ln(2)(ohec)+2 H(2)O. The variable temperature and pressure NMR measurements conducted on this isomerization reaction give the following thermodynamic parameters for Eu(III): K(298)=0.42+/-0.01, DeltaH(0)=+4.0+/-0.2 kJ mol(-1), DeltaS(0)=+6.1+/-0.5 J K(-1) mol(-1) and DeltaV(0)=+3.2+/-0.2 cm(3) mol(-1). The isomerization is slow and the corresponding kinetic parameters obtained by NMR spectroscopy are: k(298)(is)=73.0+/-0.5 s(-1), DeltaH++(is)=75.3+/-1.9 kJ mol(-1), DeltaS++(is)= +43.1+/-5.8 J K(-1) mol(-1) and DeltaV++(is)=+7.9+/-0.7 cm(3) mol(-1). Variable temperature and pressure (17)O NMR studies have shown that water exchange in Gd(2)(ohec)(H(2)O)(2) is slow, k(298)(ex)=(0.40+/-0.02)x10(6) s(-1), and that it proceeds through a dissociative interchange I(d) mechanism, DeltaV( not equal )=+7.3+/-0.3 cm(3) mol(-1). The anisotropy of this oblong binuclear complex has been highlighted by MD simulation calculations of different rotational correlation times. The rotational correlation time directed on the Gd-Gd axis is 24 % longer than those based on the axes orthogonal to the Gd-Gd axis. The relaxivity of this binuclear complex has been found to be low, since 1) only Gd(2)(ohec)(H(2)O)(2), which constitutes 70 % of the binuclear complex, contributes to the inner-sphere relaxivity and 2) the anisotropy of the complex prevents water molecules from having complete access to both Gd(III) cages; this decreases the outer-sphere relaxivity. Moreover, EPR measurements for the Gd(III) and for the mixed Gd(III)/Y(III) binuclear complexes have clearly shown that the two Gd(III) centres interact intramolecularly; this enhances the electronic relaxation of the Gd(III) electron spins.
我们报道了对八氮杂环二十六烷 -1,4,7,10,14,17,20,23 - 八乙酸酯（OHEC）的双核Ln(III)螯合物的研究。通过核磁共振（NMR）、紫外/可见光谱（UV/Vis）、电子顺磁共振（EPR）和发光光谱，以及经典分子动力学（MD）模拟，研究了在水溶液中两种异构体形式之间的相互转化。我们首次表征了一种Ln(III)聚氨基羧酸盐配合物（Ln(III)=Y、Eu、Gd和Tb）的异构化平衡，其中金属中心的配位数从九变为八，即：[Ln₂(ohec)(H₂O)₂]²⁻ ⇌ [Ln₂(ohec)]²⁻ + 2H₂O。对该异构化反应进行的变温和变压NMR测量给出了Eu(III)的以下热力学参数：K(298)=0.42±0.01，ΔH⁰ = +4.0±0.2 kJ mol⁻¹，ΔS⁰ = +6.1±0.5 J K⁻¹ mol⁻¹，ΔV⁰ = +3.2±0.2 cm³ mol⁻¹。异构化过程缓慢，通过NMR光谱获得的相应动力学参数为：k(298)(is)=73.0±0.5 s⁻¹，ΔH⁺⁺(is)=75.3±1.9 kJ mol⁻¹，ΔS⁺⁺(is)= +43.1±5.8 J K⁻¹ mol⁻¹，ΔV⁺⁺(is)=+7.9±0.7 cm³ mol⁻¹。变温和变压¹⁷O NMR研究表明，[Gd₂(ohec)(H₂O)₂]²⁻中的水交换缓慢，k(298)(ex)=(0.40±0.02)×10⁶ s⁻¹，并且它通过解离交换I(d)机制进行，ΔV≠ = +7.3±0.3 cm³ mol⁻¹。通过对不同旋转相关时间的MD模拟计算，突出了这种长方形双核配合物的各向异性。沿Gd - Gd轴的旋转相关时间比基于与Gd - Gd轴正交的轴的旋转相关时间长24%。已发现这种双核配合物的弛豫率较低，原因如下：1）仅占双核配合物70%的[Gd₂(ohec)(H₂O)₂]²⁻对球内弛豫率有贡献；2）配合物的各向异性阻止水分子完全进入两个Gd(III)笼；这降低了球外弛豫率。此外，对Gd(III)以及混合Gd(III)/Y(III)双核配合物的EPR测量清楚地表明，两个Gd(III)中心发生分子内相互作用；这增强了Gd(III)电子自旋的电子弛豫。
