Pillaring of $CdCl_2$-Like Layers in Lanthanide Metal-Organic Frameworks:Synthesis, Structure, and Photophysical Properties

A hydrothermal reaction of lanthanide salts, pyridine-2,3-dicarboxylic acid, benzene-1,4-dicarboxylic acid, and water gave rise to a new series of three-dimensional mixed carboxylates (homocyclic and heterocyclic) of lanthanides with the general formula $[M_{2^-}(H_2O_4)][\{C_5H_3N(COO)_2\}_2\{C_6H_4-(COO)_2\}]$, M=La (I), Pr (II), and Nd (III).T he structure consists of M2O14N2 dimeric units connected by pyridine-2,3-dicarboxylate moieties to form twodimensional layers that are pillared by terephthalate units.The structures also possess two coordinated water molecules, which are arranged to form one dimensional helical chains and can be reversibly adsorbed.T he connectivity within the layers closely resembles that of the $CdCl_2$ layered structure with $3^6$ topology.To the best of our knowledge,this is the first observation of $CdCl_2$ topology in lanthanide metal–organic framework compounds.Partial substitution of $La^{3+}$ in I by $Eu^{3+}$ and $Tb^{3+}$ (2 and 4%) gives rise to characteristic red/pink or green emission, which suggests a ligand-sensitized metal-centered emission.T he Nd compound III shows interesting UV and blue emission through an up-conversion process