Lanthanide–organic assemblies with pyrazinetetracarboxylic and benzophenone-3,3′,4,4′-tetracarboxylic acids

Abstract

Two ligands of potentially high denticity have been used to synthesize novel lanthanide–organic assemblies under hydrothermal conditions. Pyrazinetetracarboxylic acid (H₄PZTC) reacts with terbium nitrate to give the complex [Tb₂(H₂PZTC)₃]·3.5H₂O (1). Three bis(ONO-chelating) ligands are arranged around each metal ion and a two-dimensional assembly with a hexagonal arrangement of the cations is formed. Reaction with europium nitrate in the presence of additional potassium ions yields the complex [EuK(PZTC)(H₂O)₄] (2), which is an intricate three-dimensional framework with the ligand bound to eight cations in the ONO-chelating, OO-chelating or monodentate mode. No significant channel is formed in either compound. The three isomorphous complexes [Ln(HBPTC)(H₂O)₂]·2H₂O with Ln = Eu (3), Gd (4) and Tb (5), where H₄BPTC is benzophenone-3,3′,4,4′-tetracarboxylic acid, have been obtained from the reaction of the lanthanide nitrates with the corresponding dianhydride. The ligand is bound to five metal atoms through the three carboxylate groups, with chelating, bridging or monodentate coordination modes. A “brick wall” subunit is formed and further bridging gives rise to a two-dimensional assembly of the “double floor” type, with both metal ion and ligand defining nodes of similar topology. These are the first examples of use of these two ligands in the domain of lanthanide–organic polymers and frameworks.