Positively Charged Lanthanide Complexes with Cyclen-Based Ligands: Synthesis, Solid-State and Solution Structure, and Fluoride Interaction

The syntheses of a new cyclen-based ligand L2 containing four N-[2-(2-hydroxyethoxy)­ethyl]­acetamide pendant arms and of its lanthanide­(III) complexes [LnL2(H2O)]­Cl3 (Ln = La, Eu, Tb, Yb, or Lu) are reported, together with a comparison with some LnIII complexes of a previously reported analogue L1 in which two opposite amide arms have been replaced by coordinating pyridyl units. The structure and dynamics of the LaIII, LuIII, and YbIII complexes in solution were studied by using multinuclear NMR investigations and density functional theory calculations. Luminescence lifetime measurements in H2O and D2O solutions of the [Ln­(L2)­(H2O)]3+ complexes (Ln = Eu or Tb) were used to investigate the number of H2O molecules coordinated to the metal ion, pointing to the presence of an inner-sphere H2O molecule in a buffered aqueous solution. Fluoride binding to the latter complexes was investigated using a combination of absorption spectroscopy and steady-state and time-resolved luminescence spectroscopy, pointing to a surprisingly weak interaction in the case of L2 (log K = 1.4 ± 0.1). In contrast to the results in solution, the X-ray crystal structure of the lanthanide complex showed the ninth coordination position occupied by a chloride anion. In the case of L1, the X-ray structure of the [(EuL1)2F] complex features a bridging fluoride donor with an uncommon linear Eu–F–Eu entity connecting two almost identical [Eu­(L1)]3+ units. Encapsulation of the F– anion within the two complexes is assisted by π–π stacking between the pyridyl rings of two complexes and C–H···F hydrogen-bonding interactions involving the anion and the pyridyl units.本文报道了一种基于环四氮杂环十二烷（cyclen）的新型配体L2及其三价镧系配合物[LnL2(H2O)]Cl3（Ln=La、Eu、Tb、Yb或Lu）的合成，其中配体L2带有四个N-[2-(2-羟基乙氧基)乙基]乙酰胺侧臂；同时还将其与此前报道的类似配体L1的部分三价镧系配合物进行了对比，配体L1中两个相对的酰胺侧臂被具有配位能力的吡啶基取代。通过多核核磁共振（NMR）波谱研究与密度泛函理论（density functional theory）计算，对溶液中LaIII、LuIII及YbIII配合物的结构与动态行为开展了研究。针对[Ln(L2)(H2O)]³+配合物（Ln=Eu或Tb）在H₂O与D₂O溶液中的发光寿命进行测试，以此探究与金属离子配位的水分子数目，结果表明缓冲水溶液中该配合物存在一个内界配位水分子。结合吸收光谱、稳态与时间分辨发光光谱，对氟离子与上述配合物的结合行为展开研究，结果显示配体L2体系的结合作用意外微弱（log K=1.4±0.1）。与溶液中的实验结果不同，该镧系配合物的X射线单晶衍射结构显示其第九配位位点被氯离子占据。对于配体L1而言，[(EuL1)₂F]配合物的X射线晶体结构呈现出一个桥联氟给体，其以罕见的直线型Eu–F–Eu结构连接两个几乎完全一致的[Eu(L1)]³+单元。两个配合物中吡啶环之间的π-π堆积作用，以及氟阴离子与吡啶基之间的C–H···F氢键相互作用，共同辅助了氟阴离子在两个配合物间的包合过程。