[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 π-electron-rich aromatic surfaces, held at a distance of approximately 7 Å by polyether spacers, bind bipyridinium-based guests. This recognition motif, which is dictated by π- π 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 π-electron-rich 1,5-dioxynaphthalene units separated by a distance of approximately 7-8 Å. 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 ¹H NMR and UV-vis spectroscopy. AMBER* and HF/321G 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 π- π stacking interactions in these particular systems.