Ouali Nadjet, Bocquet Bernard, Rigault Stéphane, Morgantini Pierre-Yves, Weber Jacques, Piguet Claude
Department of Inorganic, Analytical, and Applied Chemistry, University of Geneva, 30 quai E. Ansermet, CH-1211 Geneva 4, Switzerland.
Inorg Chem. 2002 Mar 25;41(6):1436-45. doi: 10.1021/ic010801i.
Variable-temperature (1)H and (13)C NMR measurements of the D(3)-symmetrical triple-helical complexes Ln(L1-2H)(3)(-) (L1 = pyridine-2,6-dicarboxylic acid; Ln = La-Lu) show evidence of dynamic intermolecular ligand-exchange processes whose activation energies depend on the size of the metal ion. At 298 K, the use of diastereotopic probes in Ln(L3-2H)(3)(-) (L3 = 4-ethyl-pyridine-2,6-dicarboxylic acid) shows that fast intramolecular P <==> M interconversion between the helical enantiomers occurs on the NMR time scale. Detailed analyses of the paramagnetic NMR hyperfine shifts according to crystal-field independent techniques demonstrate the existence of two different helical structures, one for large lanthanides (Ln = La-Eu) and one for small lanthanides (Ln = Tb-Lu), in complete contrast with the isostructurality proposed 25 years ago. A careful reconsideration of the original crystal-field-dependent analysis shows that an abrupt variation of the axial crystal-field parameter A(0)2<r (2)> parallels the structural change leading to some accidental compensation effects that prevent the detection of structural variations according to the classical one-nucleus method. Crystal structures in the solid state and density functional theory calculations in the gas phase provide structural models that rationalize the paramagnetic NMR data. A regular triple-helical structure is found for small lanthanides (Ln = Tb-Lu) in which the terdentate chelating ligands are rigidly tricoordinated to the metals. A flexible and distorted structure is evidenced for Ln = La-Eu in which the central pyridine rings interact poorly with the metal ion. The origin of the simultaneous variation of structural parameters and crystal-field and hyperfine constants near the middle of the lanthanide series is discussed together with the use of crystal-field-independent techniques for the interpretation of paramagnetic NMR spectra in axial lanthanide complexes.
对D(3)对称三螺旋配合物Ln(L1 - 2H)(3)(-)（L1 = 吡啶 - 2,6 - 二羧酸；Ln = La - Lu）进行变温(1)H和(13)C NMR测量，结果表明存在动态分子间配体交换过程，其活化能取决于金属离子的大小。在298 K时，在Ln(L3 - 2H)(3)(-)（L3 = 4 - 乙基 - 吡啶 - 2,6 - 二羧酸）中使用非对映异位探针表明，在NMR时间尺度上，螺旋对映体之间发生快速的分子内P <==> M互变。根据晶体场无关技术对顺磁NMR超精细位移进行的详细分析表明，存在两种不同的螺旋结构，一种是大镧系元素（Ln = La - Eu）对应的结构，另一种是小镧系元素（Ln = Tb - Lu）对应的结构，这与25年前提出的同构性形成了完全对比。对原始晶体场依赖分析的仔细重新审视表明，轴向晶体场参数A(0)2<r (2)>的突然变化与导致一些意外补偿效应的结构变化平行，这些补偿效应阻止了根据经典单核方法检测结构变化。固态晶体结构和气态密度泛函理论计算提供了使顺磁NMR数据合理化的结构模型。发现小镧系元素（Ln = Tb - Lu）具有规则的三螺旋结构，其中三齿螯合配体与金属刚性三配位。对于Ln = La - Eu，证明了一种灵活且扭曲的结构，其中中心吡啶环与金属离子的相互作用较弱。讨论了镧系元素系列中间附近结构参数、晶体场和超精细常数同时变化的起源，以及使用晶体场无关技术解释轴向镧系配合物中的顺磁NMR光谱。
