Hydrogen-bonded dimers of tetra-urea calix[4]arenes stable in THF.

[structure: see text]. Whereas tetra-urea derivatives of tetra-alkoxy calix[4]arenes 1 exist as single molecules in THF, dimeric hydrogen-bonded capsules are exclusively found for the corresponding calix[4]arene derivatives 3 and 2 with two or four free hydroxyl groups. Comparison with the rigidified tetra-urea 5 suggests that this increased stability of the dimers is due to the stabilization of their four-fold symmetry by intramolecular hydrogen bonds between the phenolic hydroxyl groups.