Mixed-Ligand Uranyl Squarate Coordination Polymers: Structure Regulation and Redox Activity.

The electron-rich squarate ion (CO, ) possesses electronic delocalization over the entire molecule and good redox activity, and the functionalization of metal-organic complexes with the group is desirable. In this work, a mixed-ligand method is used to construct novel uranyl squarate coordination polymers utilizing 4,4'-bipyridine (), 4,4'-bipyridine-,'-dioxide (), 1,10-phenanthroline (), 4,4'-vinylenedipyridine (), and in situ formed oxalate () as ancillary ligands. Seven mixed-ligand uranyl compounds, (UO)(OH)() (), (UO)(HO)() (), (UO)(HO)()()·2HO (), (UO)(HO)()()·HO (), (UO)(OH)()()·HO (), (UO)()() (), and (UO)()() (), with varying crystal structures were synthesized under hydrothermal conditions. Compound , together with molecules filling in the interlayer space as template agents, has a two-dimensional (2D) network structure, while gives a one-dimensional (1D) chain based on mononuclear uranium units. Compound shows a neutral 2D network through the combined linkage of and . Both and have a similar chain-like structure due to the capping effect of motifs, while molecules in act as templating agents after protonation. Similar to , compound has a "sandwich-like" structure in which the H motifs locate in the voids of layers of 2D uranyl-squarate networks. The redox properties of typical mixed-ligand uranyl-squarate compounds, , , and with high phase purity, are characterized using cyclic voltammetry. All three of these uranyl coordination compounds show anode peaks () at 0.777, 0.804, and 0.760 V, respectively, which correspond to the oxidation process of → . Meanwhile, cathodic peaks () at -0.328, -0.315, and -0.323 V corresponding to the reduction process of U(VI) → U(V) are also observed. The results reveal that all three of these uranyl coordination compounds show good redox activity and, most importantly, the interplay between two different redox-active motifs of organic linker and uranyl node. This work enriches the library of redox-active uranyl compounds and provides a feasible mixed-ligand method for regulating the synthesis of functional actinide compounds.