Molecular clips based on propanediurea. Exceptionally high binding affinities for resorcinol guests

A series of new receptor molecules derived from 2,4,6,8-tetraazabicyclo [3. 3.1]nonane-3,7-dione (propanediurea) is described. These molecules possess a cavity which is defined by two nearly parallel aromatic side walls positi oned on top of a bis-urea framework. The resulting "U-shaped" clip molecule s are ideal hosts for the complexation of flat aromatic guest molecules. Th e affinity of these new propanediurea based molecular clips for dihydroxybe nzene derivatives is exceptionally high, with association constants up to K -a = 2 400 000 L mol(-1). Comparison of the binding mechanism of a variety of clip and half clip hosts, in conjunction with NMR, IR, and X-ray studies , has enabled the reason for this high binding to be elucidated. It is show n that subtle sub-angstrom changes in the geometry of the clip molecules ha ve a great impact on their binding properties.