Solid-state supramolecular chemistry of porphyrins. Stacked and layered heterogeneous aggregation modes of tetraarylporphyrins with crown ethers

Abstract

New crystalline materials of mixed composition based on the interaction between tetraarylporphyrin and 18-crown-6 derivatives have been prepared and characterized by X-ray diffraction analysis. Free crown ether macrocycles (18-crown-6 and dibenzo-18-crown-6) associate to manganese- or zinc-tetraphenylporphyrin in aqueous solution through a bridging molecule of water which simultaneously coordinates to the axial site of the porphyrin metal core and hydrogen bonds to the oxygens of the crown ether. This ternary mode of self-assembly can lead to the formation of monomeric, oligomeric and stacked polymeric entities, depending on the symmetry of the crown structure and the preferred coordination geometry of the metal ion. Sodium or potassium 18-crown-6 chlorides were found to be excellent templates for the construction of non-interpenetrating β-molecular networks from zinc-tetra(4-carboxyphenyl)porphyrin building blocks. The resulting layered motifs incorporate the crown ether moieties within the interporphyrin cavities. These arrays are stabilized by strong hydrogen bonds between the self-complementary carboxylic groups as well as by ion pairing, as their formation is associated with proton transfer from one of the carboxylic groups to the chloride anion and expulsion of hydrochloric acid. Molecules of the methanol solvent, which coordinate axially to the central metal ions of the porphyrin and crown ether moieties in one layer while hydrogen bonding to the carboxylic groups of another layer, contribute to the tight packing of the molecular layers along the third dimension. The experimentally established geometries and packing modes of these aggregates provide useful information for further crystal engineering efforts of networked multi-porphyrin domains.