A molecular receptor consisting of a spacer bearing two cofacially disposed terpyridyl-palladium-ligand (terpy-Pd-L) units rigidly separated by about 7 Ă
has been investigated for molecular recognition of planar aromatic molecules. It is found that although the receptor forms stable 1:2 host-guest association complexes with 9-methylanthracene (9-MA), the guest undergoes very rapid site exchange within the receptor and with external free 9-MA. A crystal structure of the 2:1 adduct shows one 9-MA in the molecular cleft defined by the two terpy-Pd-L units and the other resides on an outside face of one terpy-Pd-L unit. To establish the site residency time of the guests, a number of tethered molecules were prepared. These involve an anthracene molecule tethered to a pyridine ligand bound to the palladium atoms to form intramolecular host-guest adducts. Rotating-frame Overhauser effects were used to infer the site residency of the anthracene guests in the receptor. Variable-temperature 1 H NMR spectroscopy of the intramolecular host-guest complexes has revealed that the site residency time of the anthracene guests is 1.6 Ű 10 Ű5 sec at 20°C and 1.3 sec at Ű90°C in acetone solution. Whereas the guests are thermodynamically stable, they are kinetically very labile. A crystal structure of one of the tethered host-guest adducts reveals the expected structure which is the same as that determined in solution by 1 H rotating-frame Overhauser enhancement spectroscopy experiments.T he current intense interest in supramolecular chemistry is the result of several parallel trends in chemistry. Synthetic chemistry has evolved to a stage where traditional methods can produce just about any molecule of modest size and complexity. These methods usually involve the formation of kinetically stable bonds in a predetermined sequence. The construction of each of these bonds allows for the deployment of a variety of synthetic methods that serve as alternatives in the synthetic orchestration. Although powerful, these methods have limitations when confronted with the task of producing the very large complex molecular structures that exist in biology and that are required for the development of material science. It is clear that a new, nontraditional, approach is required for the construction of structurally defined molecules that are in the nanoscale domain. To achieve this aim, inspiration is drawn from biology.The overall structures of biology are usually the result of thermodynamically controlled self-assembly. The process requires first the construction of kinetically stable bonds between a sequence of molecular units. The relative geometries of these bonds and the substituents on the units possess all of the information required for the molecule to self-assemble into a defined structure, the properties of which are unique to the self-assembled molecule. Molecular recognition, a fundamental property of biological molecules, is manifested in the self-assembled structure. Synthetic supramolecular structures are generally formed by the pri...