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 A 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 x 10(-5) sec at 20 degrees C and 1.3 sec at -90 degrees 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.
Goshe
Andrew J
AJ
Department of Chemistry, University of Chicago, 5735 South Ellis Avenue, Chicago, IL 60637, USA.
Steele
Ian M
IM
Ceccarelli
Christopher
C
Rheingold
Arnold L
AL
Bosnich
B
B
D018360:Crystallography, X-Ray; D006860:Hydrogen Bonding; D007700:Kinetics; D046911:Macromolecular Substances; D009682:Magnetic Resonance Spectroscopy; D008956:Models, Chemical; D008958:Models, Molecular; D011485:Protein Binding; D013696:Temperature; D013816:Thermodynamics
D046911:Macromolecular Substances