title: Mixed-Ligand Uranyl Polyrotaxanes Incorporating a Sulfate/Oxalate Coligand: Achieving Structural Diversity via pH-Dependent Competitive Effect.
abstract: 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.
authors.affiliation: Laboratory of Nuclear Energy Chemistry, Institute of High Energy Physics, Chinese Academy of Sciences , Beijing 100049, China.|School of Nuclear Science and Technology, University of South China , Hengyang 421001, Hunan Province, China.
authors.forename: Zhen-Ni
authors.initials: ZN
authors.lastname: Xie
authors.affiliation: Laboratory of Nuclear Energy Chemistry, Institute of High Energy Physics, Chinese Academy of Sciences , Beijing 100049, China.
authors.forename: Lei
authors.initials: L
authors.lastname: Mei
authors.affiliation: Laboratory of Nuclear Energy Chemistry, Institute of High Energy Physics, Chinese Academy of Sciences , Beijing 100049, China.
authors.forename: Kong-Qiu
authors.initials: KQ
authors.lastname: Hu
authors.affiliation: School of Nuclear Science and Technology, University of South China , Hengyang 421001, Hunan Province, China.
authors.forename: Liang-Shu
authors.initials: LS
authors.lastname: Xia
authors.affiliation: Laboratory of Nuclear Energy Chemistry, Institute of High Energy Physics, Chinese Academy of Sciences , Beijing 100049, China.|School of Radiological and Interdisciplinary Sciences and Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education Institutions, Soochow University , Suzhou 215123, China.
authors.forename: Zhi-Fang
authors.initials: ZF
authors.lastname: Chai
authors.affiliation: Laboratory of Nuclear Energy Chemistry, Institute of High Energy Physics, Chinese Academy of Sciences , Beijing 100049, China.
authors.forename: Wei-Qun
authors.identifier: http://orcid.org/0000-0001-9929-9732
authors.initials: WQ
authors.lastname: Shi