Supramolecular recognition: on the kinetic lability of thermodynamically stable host-guest association complexes

Abstract

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.

Figure 1

An illustration of the extended structure of [9 (L = Cl)]²⁺⋅2(9-MA). The box is the unit cell, 9-MA molecules of the stack are shown as space-filling models, and the terpy-Pd-Cl units are shown as stick models, as are all other molecules not involved in this stack. Palladium atoms in the stack are shown as blue spheres.

Figure 2

A ball-and-stick representation of a “side” view for one of the molecules in the unit cell of 17. Counter ions and solvent molecules have been removed for clarity.

Figure 3

A ball-and-stick representation of a “front” view for one of the molecules in the unit cell of 17. Counter ions and solvent molecules have been removed for clarity.

Figure 4

An illustration of a “side” view of three molecules of 17 that belong to an infinite stack of molecules in the crystal of [17]⁴⁺. Counter ions and solvent molecules have been removed for clarity.