The Lanthanide Contraction Revisited

A complete, isostructural series of complexes with La−Lu (except Pm) with the ligand TREN-1,2-HOIQO has been synthesized and structurally characterized by means of single-crystal X-ray analysis. All complexes are 1D-polymeric species in the solid state, with the lanthanide being in an eight-coordinate, distorted trigonal-dodecahedral environment with a donor set of eight unique oxygen atoms. This series constitutes the first complete set of isostructural complexes from La−Lu (without Pm) with a ligand of denticity greater than two. The geometric arrangement of the chelating moieties slightly deviates across the lanthanide series, as analyzed by a shape parameter metric based on the comparison of the dihedral angles along all edges of the coordination polyhedron. The apparent lanthanide contraction in the individual Ln−O bond lengths deviates considerably from the expected quadratic decrease that was found previously in a number of complexes with ligands of low denticity. The sum of all bond lengths around the trivalent metal cation, however, is more regular, showing an almost ideal quadratic behavior across the entire series. The quadratic nature of the lanthanide contraction is derived theoretically from Slater's model for the calculation of ionic radii. In addition, the sum of all distances along the edges of the coordination polyhedron show exactly the same quadratic dependence as the Ln−X bond lengths. The universal validity of this coordination sphere contraction, concomitant with the quadratic decrease in Ln−X bond lengths, was confirmed by reexamination of four other, previously published series of lanthanide complexes. Owing to the importance of multidentate ligands for the chelation of rare-earth metals, this result provides a significant advance for the prediction and rationalization of the geometric features of the corresponding lanthanide complexes, with great potential impact for all aspects of lanthanide coordination.本研究通过单晶X射线衍射分析，完成了以TREN-1,2-HOIQO为配体、覆盖镧至镥（钷除外）的完整同构配合物系列的合成与结构表征。 固态下所有配合物均为一维聚合物物种，中心镧系离子处于八配位畸变三角十二面体配位环境中，其给体位点由8个互不相同的氧原子构成。 该系列是首例以配齿数大于2的配体构建的、覆盖La至Lu（不含钷）的完整同构配合物体系。 通过基于配位多面体所有棱边二面角对比的形状参数度量分析，发现该系列螯合基团的几何排布在整个镧系中存在微小偏移。 单根Ln-O键长所呈现的表观镧系收缩，与此前在诸多低配齿配体配合物中观测到的预期二次递减规律存在显著偏差。 但三价金属阳离子周围所有键长的总和则更具规律性，在整个系列中呈现近乎完美的二次变化趋势。 镧系收缩的二次变化特性可通过斯莱特离子半径计算模型从理论上推导得出。 此外，配位多面体所有棱边的总长度与Ln-X键长呈现完全一致的二次依赖关系。 通过重新检视另外4组已发表的镧系配合物系列，证实了这种伴随Ln-X键长二次递减的配位球收缩现象具有普适性。 鉴于多齿配体在稀土金属螯合领域的重要性，本研究成果为相关镧系配合物几何特征的预测与合理化阐释提供了重要进展，对镧系配位化学的所有研究方向均具有潜在的深远影响。