Unusual robust luminescent porous frameworks self-assembled from lanthanide ions and 2,2′-bipyridine-4,4′-dicarboxylate

Three lanthanide(III) coordination polymers, [{Ln(2))(bpdc)(3)(H2O)(3)}center dot H2O](n), (Ln = Sm, 1: Eu, 2: Tb, 3), were self-assembled from 2,2'-bipyridine-4,4'-dicarboxylic acid (H(2)bpdc) and corresponding lanthanide(III) salts under hydrothermal conditions. The compounds were found to be isomorphous and isostructural. Single-crystal X-ray diffraction studies show that compounds 1-3 present a three-dimensional network with one-dimensional hydrophilic microchannels that are occupied by guest water molecules. An edge-sharing dinuclear polyhedral [Ln(2)O(14)] was found to be the building unit of the network, in which two eight-coordinated LnO(8) cores exhibit distinct coordination environments, that is, one is in a dicapped trigonal prism and the other is in a dodecahedron. Surprisingly, the bpdc ligand exhibits four different types of bonding characteristics in a framework, including the bis(monodentate), bis(syn,syn-bridging bidentate), bis(syn,anti-bridging bidentate), and bis(chelating-bridging bidentate) modes. The pyridyl nitrogen atoms of the 2,2'-bipyridyl unit in the bpdc ligand are uncoordinated in an anti conformation along the central C-C bond of the ligand, resulting from the high oxophilic nature of lanthanide(III) ions. A thermogravimetric analysis of I showed a high thermal stability (decomposing under N-2 at T > 470 degrees C, indicating that the coordination habit of the metal ions with the bpdc ligand has a profound effect on the overall rigidity of the framework and the thermal stability of the compound. Photoluminescence measurements indicate that europium compound 2 and terbium compound 3 are strong red and green emitters, respectively.