Supramolecular recognition: On the kinetic lability of thermodynamically stable host–guest association complexes

A molecular receptor consisting of a spacer bearing two cofacially disposed terpyridyl–palladium–ligand (terpy-Pd-L) units rigidly separated by about 7 Å 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 × 10(−5) sec at 20°C and 1.3 sec at −90°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.