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

Liao Meng; Yuan-Yuan Liang; Lei Mei; Jun-Shan Geng; Kong-Qiu Hu; Ji-Pan Yu; Xin-Peng Wang; Toyohisa Fujita; Zhi-Fang Chai; Wei-Qun Shi

doi:10.1021/acs.inorgchem.1c02872

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

Inorg Chem. 2022 Jan 10;61(1):302-316. doi: 10.1021/acs.inorgchem.1c02872. Epub 2021 Dec 15.

Authors

Liao Meng^{1

2}, Yuan-Yuan Liang², Lei Mei², Jun-Shan Geng², Kong-Qiu Hu², Ji-Pan Yu², Xin-Peng Wang¹, Toyohisa Fujita¹, Zhi-Fang Chai^{2

3}, Wei-Qun Shi²

Affiliations

¹ Guangxi Key Laboratory of Processing for Non-ferrous Metals and Featured Materials, College of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, China.
² Laboratory of Nuclear Energy Chemistry, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China.
³ Engineering Laboratory of Advanced Energy Materials, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China.

PMID: 34908402
DOI: 10.1021/acs.inorgchem.1c02872

Abstract

The electron-rich squarate ion (C₄O₄^2-, 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, [(UO₂)(OH)(SA)](Hbpy) (1), [(UO₂)(H₂O)(SA)₂](H₂bpy) (2), (UO₂)(H₂O)(SA)(bpydo)·2H₂O (3), (UO₂)(H₂O)(SA)(phen)·H₂O (4), (UO₂)(OH)(SA)_0.5(phen)·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.