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

The electron-rich squarate ion (C4O42-, SA(2-)) possesses electronic delocalization over the entire molecule and good redox activity, and the functionalization of metal-organic complexes with the SA(2-) group is desirable. In this work, a mixed-ligand method is used to construct novel uranyl squarate coordination polymers utilizing 4,4'-bipyridine (bpy), 4,4'-bipyridine-N,N'-dioxide (bpydo), 1,10-phenanthroline (phen), 4,4'-vinylenedipyridine (vidpy), and in situ formed oxalate (OA(2-)) as ancillary ligands. Seven mixed-ligand uranyl compounds, [(UO2)(OH)(SA)](Hbpy) (1), [(UO2)(H2O)(SA)(2)](H(2)bpy) (2), (UO2)(H2O)-(SA)(bpydo)center dot 2H(2)O (3), (UO2)(H2O)(SA)(phen)center dot H2O (4), (UO2)(OH)-(SA)(0.5)(phen)center dot H2O (5), [(UO2)(SA)(OA)(0.5)](Hphen) (6), and [(UO2)(SA)-(OA)(0.5)](Hvidpy) (7), with varying crystal structures were synthesized under hydrothermal conditions. Compound 1, together with bpy molecules filling in the interlayer space as template agents, has a two-dimensional (2D) network structure, while 2 gives a one-dimensional (1D) chain based on mononuclear uranium units. Compound 3 shows a neutral 2D network through the combined linkage of SA(2-) and bpydo. Both 4 and 5 have a similar chain-like structure due to the capping effect of phen motifs, while phen molecules in 6 act as templating agents after protonation. Similar to 6, compound 7 has a sandwich-like structure in which the Hvidpy motifs locate in the voids of layers of 2D uranyl-squarate networks. The redox properties of typical mixed-ligand uranyl-squarate compounds, 1, 4, and 5 with high phase purity, are characterized using cyclic voltammetry. All three of these uranyl coordination compounds show anode peaks (E-a) at 0.777, 0.804, and 0.760 V, respectively, which correspond to the oxidation process of SA(2-) -> SA. Meanwhile, cathodic peaks (E-c) 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 SA(2-) 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.

Automated summary

In this study, seven mixed-ligand uranyl squarate coordination polymers with different crystal structures were synthesized, some of which exhibited good redox activity.