Semirigid Tripodal Ligand Based Uranyl Coordination Polymer Isomers Featuring 2D Honeycomb Nets.

We report five novel uranyl coordination polymers, [(CH)NH]UO(BTPCA) (1), [(CH)NH]UO(BTPCA) (2), [(CH)NH][UO(BTPCA)][UO(BTPCA)]·(HO) (3), (CH)NH(BTPCA)·(HO) (4), and [(CH)NH]UO(BTPCA) (5), by the utilization of semirigid ligand 1,1',1″-(benzene-1,3,5-triyl)tripiperidine-4-carboxylic acid (HBTPCA) and uranyl nitrate through solvothermal reactions. Single-crystal X-ray diffraction analysis reveals that the five compounds share a similar structure composition and local coordination mode to the exclusion of disordered water or DMF molecules. Each UO(COO) motif is connected to six neighboring units through three BTPCA ligands, generating an infinite uranyl honeycomb (6, 3) net. The structures of all the five compounds consist of 2D honeycomb nets of various degrees of distortions, which are induced by the flexibility of piperidine rings. The dimethylamine cations and solvent molecules fill in the space between layers. Therefore, these five compounds are isomers in a broad sense. Notably, both compounds 3 and 4 possess 2-fold interpenetrated structures. For compound 5, the distance between the neighboring 2D honeycomb nets is 7.253 Å. This is the largest distance between the 2D honeycomb nets in uranyl-based coordination polymers, to the best of our knowledge. In addition, compounds 1, 2, and 4 are also characterized by infrared spectroscopy (IR), thermogravimetric analysis (TG), powder X-ray diffraction (PXRD), and luminescence properties.