Bio-inspired Calix[6]Arene–Zinc Funnel Complexes

A bio-inspired supramolecular system is presented. A calix[6]arene possessing three imidazolyl arms on alternate phenolic positions binds a zinc ion. The resulting complex contains a hydrophobic pocket, which has a flattened conic shape. The system behaves as a selective molecular funnel for neutral guests that bind the metal centre. The exceptional stability of these tetrahedral dicationic complexes is exemplified by the acetaldehyde ternary adduct that was analysed by X-ray crystallography. The ligand is deeply buried in the heart of the calixarene cavity, pointing its methyl group selectively towards the centre of one of the aromatic walls, thereby establishing a stabilizing CH/π interaction. Protic guests undergo hydrogen bonding with the phenolic oxygens of the calixarene structure. The selectivity of the binding in the cavity is based on both the affinity of the donor atom of the guest ligand for the zinc ion and the relative host–guest geometries. The helical shape of the tris-imidazolyl groups binding the metal centre is the base of the chirality of the system. The twisted calix[6]arene structure of the zinc funnel complexes is shown to provide a new example of a cavity suitable for host–guest chiral induction. Formation of helical zinc complexes.

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