Hybrid donor extractants are a promising class of compounds for the separation of trivalent actinides and lanthanides. Here, we investigated a series of sterically loaded diphosphonate ligands based on bipyridine (BiPy-PO-iPr and BiPy-PO-cHex) and phenanthroline (Phen-PO-iPr and Phen-PO-cHex). We studied their complex formation with nitrates of trivalent f-elements in solvent extraction systems (Am and Eu) and homogeneous acetonitrile solutions (Nd, Eu, and Lu). Phenanthroline extractants demonstrated the highest efficiency and selectivity [SF(Am/Eu) up to 14] toward Am(III) extraction from nitric acid solutions among all of the studied diphosphonates of N-heterocycles. The binding constants established by UV–vis titration also indicated stronger binding of sterically impaired diphosphonates compared to the primary substituted diphosphonates. NMR titration and slope analysis during solvent extraction showed the formation of 2:1 complexes at high concentrations (>10^–3 mol/L) for phenanthroline-based ligands. According to UV–vis titrations at low concentrations (10^–5–10^–6 mol/L), the phenanthroline-based ligands formed 1:1 complexes. Bipyridine-based ligands formed 1:1 complexes regardless of the ligand concentration. Luminescence titrations revealed that the quantum yields of the complexes with Eu(III) were 81 ± 8% (BiPy-PO-iPr) and 93 ± 9% (Phen-PO-iPr). Single crystals of the structures [Lu(μ²,κ⁴-(iPrO)₂P(O)Phen(O)₂(OiPr))(NO₃)₂]₂ and Eu(Phen-PO-iPr)(NO₃)₃ were obtained by chemical synthesis with the Phen-PO-iPr ligand. X-ray diffraction studies revealed a closer contact of the f-element with the aromatic N atoms in the case of sterically loaded P═O ligands compared with sterically deficient ligands. Density functional theory calculations allowed us to rationalize the observed selectivity trends in terms of the bond length, Mayer bond order, and preorganization energy.

中文翻译：

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通过空间位阻增强选择性的方法：通过负载二膦酸酯改进 Am(III)/Eu(III) 溶剂萃取

杂化供体萃取剂是一类很有前途的化合物，可用于分离三价锕系元素和镧系元素。在这里，我们研究了一系列基于联吡啶（BiPy-PO-iPr 和 BiPy-PO-cHex）和菲咯啉（Phen-PO-iPr 和 Phen-PO-cHex）的空间负载二膦酸酯配体。我们在溶剂萃取系统（Am 和 Eu）和均相乙腈溶液（Nd、Eu 和 Lu）中研究了它们与三价 f 元素硝酸盐的络合物形成。菲咯啉萃取剂表现出最高的效率和选择性 [ SF(Am/Eu) 高达 14] 在所有研究的 N-杂环的二膦酸盐中从硝酸溶液中提取 Am(III)。UV-vis 滴定确定的结合常数也表明与主要取代的二膦酸盐相比，空间受损的二膦酸盐的结合更强。溶剂萃取过程中的 NMR 滴定和斜率分析表明，对于基于菲咯啉的配体，在高浓度 (>10 ^–3 mol/L) 下形成了 2:1 的配合物。根据低浓度（10 ^–5 –10 ^–6mol/L)，基于菲咯啉的配体形成 1:1 的配合物。无论配体浓度如何，基于联吡啶的配体都会形成 1:1 的复合物。发光滴定显示与 Eu(III) 配合物的量子产率为 81±8% (BiPy-PO-iPr) 和 93±9% (Phen-PO-iPr)。[Lu(μ ² ,κ ⁴ -(iPrO) ₂ P(O)Phen(O) ₂ (OiPr))(NO ₃ ) ₂ ] ₂和Eu(Phen-PO-iPr)(NO ₃ )结构的单晶) ₃是通过化学合成与 Phen-PO-iPr 配体获得的。X 射线衍射研究揭示了f的更密切接触与空间不足的配体相比，在空间负载的 P=O 配体的情况下，具有芳香族 N 原子的 - 元素。密度泛函理论计算使我们能够根据键长、Mayer 键序和预组织能对观察到的选择性趋势进行合理化。