Unusual Robust Luminescent Porous Frameworks Self-Assembled from Lanthanide Ions and 2,2‘-Bipyridine-4,4‘-dicarboxylate

Three lanthanide(III) coordination polymers, [{Ln2(bpdc)3(H2O)3}·H2O]n (Ln = Sm, 1; Eu, 2; Tb, 3), were self-assembled from 2,2‘-bipyridine-4,4‘-dicarboxylic acid (H2bpdc) 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 [Ln2O14] was found to be the building unit of the network, in which two eight-coordinated
LnO8 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 1 showed a high thermal
stability (decomposing under N2 at T > 470 °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.