Cation-π interactions between potassium ions and aromatic rings. Crystal structures of three potassium complexes of calix[6] arene

Three potassium complexes of calix[6]arenes, [K₂(MeOH)₅] {p-H-calix[6] arene-2H} (1), [K₂(MeOH)₄]{p-tert-butylcalix[6]arene-2H} (2) and [K₂(H₂O)₅] {p-H-calix[6]arene-2H} (3), have been prepared and characterized by X-ray diffraction. In each of 1-3, the doubly deprotonated macrocyclic ligand incorporates two K⁺ ions and adopts the usually observed double partial cone conformation. The structures of 1 and 2 are similar in that one K₊ ion (K1) is positioned near the centre of the cavity of the macrocycle and binds to four phenolic oxygens and two methanol ligands, while the other K⁺ ion (K2) binds to one phenolic oxygen and four methanols in 1, or three methanols in 2, and the two K⁺ ions are connected to each other by one bridging methanol and one phenolic oxygen. A major structural difference between 1 and 2 is a monomeric structure for 1 and a polymeric one for 2. In 1 and 2, the K1 ion also makes close contacts with the aromatic rings whose hydroxyl oxygens are simultaneously bonded to the K1. What is more, in 1, the K2 ion rides on the phenyl ring. In the structure of 3, each of the two K⁺ ions are mirror-related and linked to each other through three bridging waters. They each bind to the phenolic oxygen of one partial cone fragment and make close contacts with the phenyl plane of the other partial cone fragment; each one is also bonded to four water molecules. Close contacts between K⁺ ion and phenyl rings, observed in 1, 2 and 3, are indicative of the presence of the cation-π interaction, an important mechanism of molecular recognition that is assumed for ion selectivity in potassium channels.