Porous Supramolecular Architectures Based on π-Stacking Interactions between Discrete Metal-Adenine Entities and the Non-DNA Theobromine/Caffeine Nucleobases

This work is aimed at the analysis of the pi-pi stacking interactions as the driving force to develop porous supramolecular metal-organic frameworks (SMOFs) as an alternative to more directional hydrogen bonding interactions. Four compounds based on the interaction between rigid copper/adenine entities and theobromine/caffeine molecules have been synthesized: [Cu-7(mu-ade)(6)(mu(3)-OH)(6)(mu-H2O)(6)](theo)(2)center dot similar to 28H(2)O (1), [Cu-2(mu-ade)(4)(H2O)(2)]center dot 3Htheo center dot similar to 7H(2)O (2), [Cu-2(mu-ade)(4)- (H2O)(2)]center dot 2Htheo center dot similar to 18H(2)O (3), and [Cu-2(mu-ade)(4)(H2O)(2)]center dot(caf)-center dot similar to 6H(2)O (4). The blue compound 1 is formed by wheel-shaped cationic heptameric units where the copper atoms are bridged by hydroxide anions, water molecules, and adeninato ligands with a mu-kappa N3:kappa N9 coordination mode. The assembly of the heptameric entities and the theobrominate anions takes place mainly through pi-pi stacking interactions involving the adeninato ligands and theobrominate moieties. Although compound 1 exhibits an open-framework with voids representing 37% of the unit cell, the plasticity of the pi-pi interactions causes a reversible shrinkage of the porous system upon activation that precludes the adsorption of gas molecules. Dark purple compounds 2-4 contain neutral windmill units in which two copper atoms are bridged by four mu-kappa N3:kappa N9 adeninato ligands. Their final crystal structure highly depends on the supramolecular interactions of the theobromine and caffeine molecules. 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 ligands. In compound 3, with a lower amount of theobromine, the Watson-Crick interaction is not present. In both compounds, the three-dimensional (3D) crystal structure requires the additional presence of pi-pi 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 pi-stacking interactions, two-dimensional 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 these compounds indicates a ferrimagnetic behavior for 1 and strong intradimeric antiferromagnetic interactions in compounds 2 and 4.