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.