Mixed-Ligand Uranyl Polyrotaxanes Incorporating a Sulfate/Oxalate Coligand: Achieving Structural Diversity via pH-Dependent Competitive Effect

A mixed-ligand
system provides an alternative route to tune the structures and properties
of metal–organic compounds by introducing functional organic
or inorganic coligands. In this work, five new uranyl-based polyrotaxane
compounds incorporating a sulfate or oxalate coligand have been hydrothermally
synthesized via a mixed-ligand method. Based on C6BPCA@CB6 (C6BPCA = 1,1′-(hexane-1,6-diyl)­bis­(4-(carbonyl)­pyridin-1-ium),
CB6 = cucurbit[6]­uril) ligand, UPS1 (UO2(L)0.5(SO4)­(H2O)·2H2O, L = C6BPCA@CB6) is formed by
the alteration of initial aqueous solution pH to a higher acidity.
The resulting 2D uranyl polyrotaxane sheet structure of UPS1 is based on uranyl-sulfate ribbons connected by the C6BPCA@CB6 pseudorotaxane linkers. By using oxalate ligand instead of sulfate,
four oxalate-containing uranyl polyrotaxane compounds, UPO1–UPO4, have been acquired by tuning reaction
pH and ligand concentration: UPO1 (UO2(L)0.5(C2O4)0.5(NO3)·3H2O) in one-dimensional chain was
obtained at a low pH value range (1.47–1.89) and UPO2 (UO2(L)­(C2O4)­(H2O)·7H2O)­obtained at a higher pH value range
(4.31–7.21). By lowering the amount of oxalate, another two
uranyl polyrotaxane network UPO3 ((UO2)2(L)0.5(C2O4)2(H2O)) and UPO4 ((UO2)2O­(OH)­(L)0.5(C2O4)0.5(H2O)) could be acquired at a low pH value
of 1.98 and a higher pH value over 6, respectively. The UPO1–UPO4 compounds, which display structural diversity
via pH-dependent competitive effect of oxalate, represent the first
series of mixed-ligand uranyl polyrotaxanes with organic ligand as
the coligand. Moreover, the self-assembly process and its internal
mechanism concerning pH-dependent competitive effect and other related
factors such as concentration of the reagents and coordination behaviors
of the coligands were discussed in detail.

混合配体体系通过引入功能性有机或无机共配体，为调控金属有机化合物的结构与性能提供了新的可行途径。本研究采用混合配体水热合成法，成功制备了五种结合硫酸根或草酸根共配体的新型铀酰基聚轮烷（polyrotaxane）化合物。以C6BPCA@CB6配体（其中C6BPCA为1,1'-(己烷-1,6-二基)双(4-(羰基)吡啶-1-鎓)，CB6为葫芦[6]脲（cucurbit[6]uril））为基础，将初始水溶液的pH调高至更强酸性条件下，得到了UPS1（化学式为UO₂(L)₀.₅(SO₄)(H₂O)·2H₂O，其中L = C6BPCA@CB6）。UPS1所形成的二维铀酰聚轮烷片层结构，以铀酰-硫酸根链为骨架，通过C6BPCA@CB6假轮烷连接单元实现互联。将硫酸根共配体替换为草酸根配体后，通过调控反应pH与配体浓度，成功获得四种含草酸根的铀酰聚轮烷化合物UPO1~UPO4：在低pH范围（1.47~1.89）下得到一维链状结构的UPO1（化学式为UO₂(L)₀.₅(C₂O₄)₀.₅(NO₃)·3H₂O）；在较高pH范围（4.31~7.21）下得到UPO2（化学式为UO₂(L)(C₂O₄)(H₂O)·7H₂O）。通过降低草酸根的投料量，分别在pH=1.98的低酸度条件与pH>6的高酸度条件下，得到了另外两种铀酰聚轮烷网络结构化合物UPO3（化学式为(UO₂)₂(L)₀.₅(C₂O₄)₂(H₂O)）与UPO4（化学式为(UO₂)₂O(OH)(L)₀.₅(C₂O₄)₀.₅(H₂O)）。UPO1~UPO4系列化合物通过pH依赖的草酸根竞争效应展现出丰富的结构多样性，是首例以有机配体作为共配体的混合配体铀酰聚轮烷体系。此外，本研究还详细探讨了该自组装过程及其内在机制，包括pH依赖的竞争效应、试剂浓度以及共配体配位行为等相关影响因素。