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

The electron-rich squarate ion (C 4 O 4 2- , 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, [(UO 2 )(OH)( SA )](H bpy ) ( 1 ), [(UO 2 )(H 2 O)( SA ) 2 ](H 2 bpy ) ( 2 ), (UO 2 )(H 2 O)( SA )( bpydo )·2H 2 O ( 3 ), (UO 2 )(H 2 O)( SA )( phen )·H 2 O ( 4 ), (UO 2 )(OH)( SA ) 0.5 ( phen )·H 2 O ( 5 ), [(UO 2 )( SA )( OA ) 0.5 ](H phen ) ( 6 ), and [(UO 2 )( SA )( OA ) 0.5 ](H vidpy ) ( 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 H vidpy 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.