Dimeric capsules by the self-assembly of triureidocalix[6]arenes through hydrogen bonds

A number of calix[6]arenes bearing ureas at the upper rim positions of alte rnate rings 1, 3 and 5 were prepared and studied in detail by NMR spectrosc opy and gel permeation chromatography, N-Unsubstituted ureas were shown to dimerize through a cyclic array of hydrogen bonds to give cylindrical cavit ies capable of encapsulating small molecules such as dichloromethane, benze ne and fluorobenzene, Slow equilibria between dimer and monomer were observ ed in [D-6]DMSO-CDCl3 mixtures. By contrast, N-substituted ureas are monome ric, All urea monomers with bulky O-substituents display a solvent-dependen t, slow equilibrium between C-3v and C-s cone conformations.