Paddle-Wheel Shaped Copper(II)-Adenine Discrete Entities As Supramolecular Building Blocks To Afford Porous Supramolecular Metal-Organic Frameworks (SMOFs)

The present work assesses the ability of [Cu-2(mu-adenine)(4)(X)(2)](2+) and [Cu-2(mu-adenine)(2)(mu-X)(2)(X)(2)] (X: Cl or Br) metal-nucleobase dinuclear entities to build up supramolecular metal organic frameworks (SupraMOFs) based on the complementary hydrogen bonding interactions established by the Watson Crick and Hoogsteen faces of adjacent adenine moieties. The noncoplanar disposition of these synthons in the [Cu-2(mu-adenine)(4)(X)(2)](2+) building unit leads to an open framework with one-dimensional (1D) channels of ca. 6 angstrom in compounds [Cu-2(mu-adenine)(4)(Cl)(2)]Cl-2 center dot similar to 2MeOH (1, SMOF-1) and [Cu-2(mu-adenine)(4)(Br)(2)]Br-2 center dot similar to 2 MeOH (2, SMOF-2) sustained through the hydrogen bonding base pairing interactions among the Watson Crick faces. In the case of the second building unit, [Cu-2(mu-adenine)(2)(mu-X)(2)(X)(2)], the coplanar arrangement of the two adenines in the dimeric unit does not allow a three-dimensional (3D) supramolecular architecture based only on the complementary hydrogen bonding interactions between the nucleobases. Therefore, other supramolecular interactions involving the halide ions and solvent molecules are crucial for determining the features of the crystal packing. In compound [Cu-2(mu-adenine)(2)(mu-Cl)(2)(Cl)(2)]center dot 2MeOH (3, SMOF-3), base pairing interactions between adjacent adenines produce ID supramolecular ribbons of dinuclear entities. These ribbons establish additional hydrogen bonds between the Hoogsteen face and the chloride anions of adjacent ribbons that are also reinforced by the presence of pi-pi stacking interactions among the adenines leading to a rigid synthon that gives rise to a robust 3D skeleton with the presence of micropores occupied by solvent molecules. In the case of the bromide analogue, the weaker hydrogen acceptor capacity of the bromide allows the solvent molecules to disrupt the self-assembly process of the dinuclear entities and prevents the formation of an open-framework supramolecular structure leading to the nonporous [Cu2(mu-adenine)(2)(mu-Br)(2)(Br)(2)]center dot 2PrOH (4) compound. According to gas adsorption studies, SMOF-1, SMOF-2, and SMOF-3 present a surface instability that creates a diffusion barrier that can be permeated only by strong interacting adsorbate molecules with high kinetic energy such as CO2 but not N-2, H-2, and CH4. This feature makes them attractive for selective gas adsorption and separation technologies.