Adenine nucleobase directed supramolecular architectures based on ferrimagnetic heptanuclear copper(II) entities and benzenecarboxylate anions

Abstract

Two planar organic anions, benzoate and benzene-1,4-dicarboxylate (terephthalate), have been selected as potential π-stacking intercalators among ferrimagnetic [Cu7(μ-adeninato)6(μ3-OH)6(μ-H2O)6]2+ heptameric discrete entities. The resulting supramolecular architecture is highly dependent on the negative charge density distribution, mainly located in the carboxylate groups of the organic anions. In this sense, the benzoate anion, with just one carboxylate group, does not allow its intercalation between the adeninato ligands as it would imply a high steric hindrance among the heptameric entities. As a consequence, these benzoate anions are located inside the voids of the crystal structure reducing the accessible volume of compound [Cu7(μ-adeninato)6(μ3-OH)6(μ-H2O)6](benzoate)2·~17H2O (1). On the contrary, the terephthalate anion, containing two carboxylate groups at opposite sites, adopts a π-stacking sandwich arrangement between two adeninato ligands that affords the porous open structure of formula [Cu7(μ-adeninato)6(μ3-OH)6(μ-H2O)6](terephthalate)·nH2O (2a, 2b; n: 12 and 24, respectively). In addition to that, the less directional nature of the π-stacking interactions in comparison to the complementary hydrogen bonding based supramolecular metal-organic frameworks (SMOFs), suits them with a flexible architecture able to reversibly adsorb/desorb water (up to a 25–30% at 20 °C) altogether with the expansion/shrinkage of the crystal structure. The bridging adeninato and hydroxido ligands are effective magnetic exchange mediators to provide a ST = 5/2 ferrimagnetic state for the heptanuclear entity.