Probing the Design of a Novel Ditopic Anion Receptor

In an attempt to increase host-guest selectivity an open chain ditopic receptor 5 composed of two tetrahedral anion binding subunits which are connected by a p-xylene bridge was synthesized. The preparation produced the target host 5 in four steps in 23% yield starting from the macrotricyclic modules 4 and 6. The selectivity advantage of the ditopic receptor design was investigated by using the dimensional probes 10–14. Association constants of the probes with the ditopic 5 or monotopic 2 were calculated from a Benesi-Hildebrand treatment of the optical effects occurring on host-guest complexation. The comparison of selectivity ratios derived therefrom reveals a sharp increase with 13 and 14 demonstrating the participation of both receptor subunits of 5 in binding these guests. The superiority of the ditopic vs. a monotopic receptor design amounts to a factor of 3. The p-xylene connecting unit is sufficiently rigid to define a minimum distance of approach of the two subunits of the ditopic host 5.