[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 Hosts1

• Published in: Journal of the American Chemical Society 1999-02, Vol.121 (7), p.1479-1487
• Main Authors: Houk, K. N, Menzer, Stephan, Newton, Simon P, Raymo, Françisco M, Stoddart, J. Fraser, Williams, David J
• Format: Article
• Language: English
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja982748b

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/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 π−π stacking interactions in these particular systems.