Anion Influence on the Structures of a Series of Copper(II) Metal−Organic Frameworks

The main aim of the work herein presented is to investigate the influence of different anions on the overall structure of a series of metal−organic frameworks. The reactions between CuCl₂, Cu(OAc)₂, and CuSO₄ and the two bipyridylurea ligands L¹ and L² [L¹ = 1,3-bis(pyridin-4-ylmethyl)urea; L² = 1,3-bis(pyridin-3-ylmethyl)urea; see Scheme in paper] have been carried out and the crystal structure of five of the resulting metal−organic assemblies determined. These crystal structures have shown that the geometry and size of the corresponding anions together with their coordinating and hydrogen-bonding properties are essential in determining the final structures of the assemblies. Particularly interesting, because of their potential as nanoporous materials, are the assemblies obtained from the reaction between each of the two ligands (L¹ and L²) and CuCl₂, which yield noninterpenetrating 2D metal−organic layers made of squares of ca. 15 × 15 Å. These layers stack on top of each other, producing infinite 3D channels filled with solvent molecules. The thermal stabilities of the five metal−organic frameworks prepared have been studied by means of thermogravimetric analysis. Preliminary X-ray powder diffraction studies of one of these metal−organic frameworks indicate that upon heating the assembly changes to a different crystalline structure. Interestingly, the original structure reforms upon exposure of this sample to traces of water.