Hydrogen-Bonding Clusters Leading to Formation of Supramolecular Dimers of Metalloporphyrin Receptors: Modulation of Lewis Acidity by π-π Interactions

Two remarkable crystal structures are reported of a cyclic receptor 1, containing two metalloporphyrin units. The overall crystal structure of 1 provides the first direct evidence that pi-stacking between two metalloporphyrins reduces the Lewis acidity of the metal ion and thereby dramatically reduces the affinity of zinc for external ligands; this effect was previously suggested indirectly by solution state binding studies. In addition, crystallising 1 from a different combination of solvents and the ability of 1 to distort its structure leads to the remarkable observation of a supramolecular dimer of inter-penetrating macrocycles, 4, held together by clusters of hydrogen-bonded methanol molecules.