Molecular meccano.: 47.: [C-H-O] interactions as a control element in supramolecular complexes:: Experimental and theoretical evaluation of receptor affinities for the binding of bipyridinium-based guests by catenated hosts

Macrocyclic receptors incorporating two facing pi-electron-rich aromatic surfaces, held at a distance of approximately 7 Angstrom by polyether spacers, bind bipyridinium-based guests. This recognition motif, which is dictated by pi-pi stacking and [C-H ... O] hydrogen-bonding interactions, has led to the development of efficient template-directed syntheses of mechanically interlocked molecules, such as catenanes and rotaxanes. By employing a supramolecularly assisted synthetic methodology based on these interactions, we have self-assembled two novel [3]catenanes, each incorporating two 1,5-dioxynaphtho-38-crown-10 components and one bipyridinium-based tetracationic cyclophane component. Single-crystal X-ray analyses of these [3]catenanes revealed that they possess internal cavities bounded on two opposite sites by pi-electron-rich 15-dioxynaphthalene units separated by a distance of approximately 7-8 Angstrom. Despite the presence of apparently ideal "binding pockets", these mechanically interlocked compounds steadfastly refuse to bind bipyridinium-based guests in solution, as demonstrated by both H-1 NMR and UV-vis spectroscopy. AMBER* and HF/3-21G calculations on appropriate models show that the absence of [C-H ... O] hydrogen-bonding interactions is responsible for the instability of these geometrically ideal complexes. The [C-H ... O] bond appears to be quantitatively much more important than pi-pi stacking interactions in these particular systems.