Mixed-Ligand Uranyl Polyrotaxanes Incorporating a Sulfate/Oxalate Coligand: Achieving Structural Diversity via...

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.