Cyclic triureas -: synthesis, crystal structures and properties

The synthesis of 24-membered macrocycles is described, in which rigid xanthene units (X) and/or diphenyl ether units (D) as flexible analogues are linked via urea groups. All four possible combinations ( XXX, XXD, XDD, DDD) have been obtained with yields of 40 - 72% for the cyclisation step. In two cases, the respective cyclic hexamers (XXDXXD, XXXXXX) were also isolated. Two compounds have been characterised by a single crystal X-ray analysis of the free triurea ( XXD, XDD) and one example ( DDD) by its complex with tetrabutylammonium chloride. It shows the chloride anion in the centre of the macrocycle, held by six NH center dot center dot center dot Cl- hydrogen bonds. The interaction with various other anions has been studied by H-1 NMR. Complexation constants for chloride, bromide and acetate have been measured for all trimers by UV spectrophotometry. Molecular dynamics simulations have been carried out to determine the conformation of the free receptors in chloroform and acetonitrile. They show that in chloroform, intramolecular hydrogen bonding occasionally facilitated by trans -> cis isomerisation of an amide bond dominates the conformation of the macrocycles while in acetonitrile ( the solvent used for complexation measurements), the ligating urea NH protons are properly arranged for the complexation of anions, however, their strong solvation is counteractive to the complexation.