Host-Guest Chemistry: Receptor for Caffeine
In cooperation with the group of Prof. Siegfried Waldvogel at the University of Bonn, we have studied a receptor for caffeine based on a triphenylene skeleton. The hexahydroxy triphenylene ketals are functionalized with three urea units pointing to the same side of the aromatic plane. The distance between the three urea arms is ca. 10 A which makes the space between the arms large enough to accomodate guests with a threefold (pseudo)symmetry such as triacetylbenzene, trinitrotoluene, or caffeine, which instead of a third carbonyl group bears an imine functionality.
By NMR titration, the binding constants for several substrates as well as for control receptors have been determined (caffeine in methylene chloride: 35000 M-1) and provide insight into the binding mode in solution. The major energetic contribution comes from hydrogen bonding to the distal urea protons (those more distant from the aromatic plane). The other urea hydrogens point towards the aromatic plane rather than towards the substrate and hardly contribute to caffeine binding. In addition, a minor effect comes from stacking of the two pi-surfaces.
In the solid state, this mode of binding is realized as well. The X-ray structure of the receptor-substrate complex indeed shows the shorter, distal NH-O hydrogen bonds and the parallel orientation of the two planes.