MS32-P04 Porous supramolecular architectures based on π-stacking interactions between discrete metal-adenine entities and theobromine/caffeine methylxanthinesTaking into account the great potential of MOFs, this work is focused on a relatively recent new class of analogous porous materials in which coordination bonds are released from guiding the crystal structure and supramolecular interactions play this role.1 Herein we present the results obtained for π-π stacking interactions as the driving force to develop porous supramolecular metal-organic frameworks (SMOFs) as an alternative to more directional hydrogen bonding interactions. In particular, the use of adenine and methylxanthines has given rise to several compounds presented in this communication. The blue compound [Cu7(μ-ade)6(μ3-OH)6(μ-H2O)6](theo)2·28H2O (1) (ade: adeninato, theo: theobrominato) is formed by wheel-shaped cationic units where the Cu(II) atoms are bridged by hydroxide anions, water molecules, and adeninato ligands with a m-κN3:κN9 coordination mode. The supramolecular assembly (Figure 1) takes place mainly through π-π stacking interactions involving the adeninato ligands of the rigid heptameric entities and theobrominate moieties. Compound 1 exhibits an open-framework with voids representing 37% of the unit cell volume, but the plasticity of the π-π interactions causes a reversible shrinkage of the porous system upon activation that precludes the adsorption of gas molecules.
Dark purple compounds [Cu2(μ-ade)4(H2O)2]·3Htheo·7H2O (2), [Cu2(μ-ade)4(H2O)2]·2Htheo·18H2O (3) and [Cu2(μ-ade)4(H2O)2]·(caf)·~6H2O (4) (Htheo: theobromine, caf: caffeine) contain neutral windmill units in which two copper atoms are bridged by four m-κN3:κN9 adeninato ligands. Their crystal structures highly depend on the supramolecular interactions of the theobromine and caffeine bases. In compound 2, two theobromine molecules are hydrogen bonded to the Hoogsteen face of two trans-arranged adeninato ligands, whereas a third theobromine molecule is joined to the Watson-Crick face of one of the previous adeninato ligand. In compound 3, with a lower amount of theobromine, the Watson-Crick interaction is not present. In both compounds, the 3D crystal structure requires the additional presence of π-π stacks between the theobromine molecules.
In compound 4, as the methyl groups of the caffeine molecule do not allow hydrogen bond interactions, the adeninato ligands are hydrogen bonded among them to generate, together with π-stacking interactions, supramolecular sheets containing rectangular windows in which the caffeine molecules are located. Only compound 4 showed permanent porosity, adsorbing a significant amount of CO2 (0.88 mmol of CO2/g at 5 bar and 273 K). The magnetic characterization of the compounds indicate a ferromagnetic behaviour for 1 and strong intradimeric antiferromagnetic interactions in compounds 2 and 4.
 (a) Thomas-Gipson, J.; Pérez-Aguirre, R.; Beobide, G.; Castillo, O.; Luque, A.; Pérez-Yáñez, S.; Román, P.; (2015) Cryst. Growth Des., 15, 975-983. (b) Beobide, G.; Castillo, O.; Luque, A.; Pérez-Yáñez, S. (2015) CrystEngComm, 17, 3051-3059. Keywords: SMOFs, supramolecular interactions, π-π stacking