Structural Variation within Heterometallic Uranyl Hybrids Based on Flexible Alkyldiphosphonate Ligands

Five novel zinc uranyl diphosphonates have been hydrothermally synthesized by using a series of flexible diphosphonate ligands, including ethane-1,2-diyldiphosphonic acid (H4EDP), propane-1,3-diyldiphosphonic acid (H4PDP), and butane-1,4-diyldiphosphonic acid (H4BDP). Compound Zn­(H2tib)­(UO2)2(EDP)­(HEDP)­(H2EDP)0.5·3H2O (EDP-ZnU1, tib = 1,3,5-tri­(1H-imidazol-1-yl)­benzene) comprises dimeric U2O12 unit condensed by two UO7 pentagonal bipyramids, which are further connected by Zn-centered polyhedra and EDP ligands resulting in a 3-dimensional framework. Compound [Zn­(bipy)­(H2O)]­(UO2)­(PDP) (PDP-ZnU1, bipy = 2,2′-bipyridine) also features U2O12 dimers and Zn-centered polyhedra, but a layered arrangement is formed. Different from that in PDP-ZnU1, the uranium exists in the form of UO6 tetragonal bipyramid and is surrounded by four PDP ligands to generate the layered structure of Zn­(bipy)­(UO2)­(PDP) (PDP-ZnU2). ZnO2N2 tetrahedra are connected on both sides of the layers. Both Zn2(phen)4(UO2)3(BDP)­(HBDP)2·4H2O (BDP-ZnU1, phen = 1,10-phenanthroline) and Zn2(bipy)2(UO2)3(HBDP)2(H2BDP)2 (BDP-ZnU2) contain U2O12 dimers and UO6 tetragonal bipyramids. In BDP-ZnU1, uranyl centers are bridged by BDP to form a 2-dimensional structure, on which Zn­(phen)2 are decorated. Whereas in BDP-ZnU2, uranyl phosphonate layers are connected by bridging ZnO3N2 to produce framework structure. All of these compounds have been investigated by IR and photoluminescent spectroscopy. Their characteristic green light emissions have been attributed to transition properties of uranyl dications.