Pillaring of CdCl₂-like layers in lanthanide metal-organic frameworks: synthesis, structure, and photophysical properties

Abstract

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 [M₂(H₂O)₄] [{C₅H₃N(COO)₂}₂{C₆H₄ (COO)₂}], M=La (I), Pr (II), and Nd (III). The structure consists of M₂O₁₄N₂ dimeric units connected by pyridine-2,3-dicarboxylate moieties to form two-dimensional 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. The connectivity within the layers closely resembles that of the CdCl₂ layered structure with 3⁶ topology. To the best of our knowledge, this is the first observation of CdCl₂ topology in lanthanide metal-organic framework compounds. Partial substitution of La³⁺ in I by Eu³⁺ and Tb³⁺ (2 and 4%) gives rise to characteristic red/pink or green emission, which suggests a ligand-sensitized metal-centered emission. The Nd compound III shows interesting UV and blue emission through an up-conversion process.