Construction of Uranyl Organic Hybrids by Phosphonate and in Situ Generated Carboxyphosphonate Ligands

The hydrothermal reaction of uranyl ions with (5-methyl-1,3-phenylene)diphosphonic acid (H₄MPDP) in the presence of additives such as nitric acid, N-bearing species, and heterometal ions yielded five new uranyl organic hybrids: (H₃O)[(UO₂)₅(H₂O)₄(H₃DPB)₂(H₂DPB)(HDPB)]·2H₂O (1), (Hphen)(phen)[(UO₂)₃(H₂DPB)(HDPB)] (2), (H₂dipy)[(UO₂)₃(MPDP)₂] (3), Zn(bipy)(UO₂)(MPDP) (4), and Co(bipy)(UO₂)(MPDP)·H₂O (5) (H₅DPB = 3,5-diphosphonobenzoic acid; phen = 1,10-phenanthroline; dipy = 4,4'-bipyridine; bipy = 2,2'-bipyridine). Single-crystal X-ray diffraction (XRD) demonstrates that 1 and 2 are 3D frameworks constructed of uranyl centers and carboxyphosphonate DPB ligands; the latter were formed via the in situ oxidation of H₄MPDP. In the homometallic uranyl diphosphonate 3, less common UO₆ square bipyramids connected by MPDP ligands were incorporated to form the 2D assembly. A further introduction of heterometal ions produced two heterobimetallic uranyl phosphonates 4 and 5. Both of them show layered structures, formed by UO₆ square bipyramids linked by MPDP ligands with heterometal-centered polyhedra decorated on the sides of the layers. It is found that the pH and heterometal ions have significant effects on the structures of the complexes. In addition to the syntheses and XRD characterization, the spectroscopic properties of these uranyl complexes were also addressed. To complement the experimental results, density functional theory calculations were carried out on several model complexes that feature a homo- or heterobimetallic molecular skeleton. Geometrical/electronic structures, IR spectra, and electronic absorptions were discussed.