Much or our research over the past decade has dealt with pseudorotaxanes and rotaxanes, which are supramolecular species; see Scheme. The number in the bracket indicates the number of molecular species involved in the mechanically linked compound.

Supramolecular Chemistry with Small Molecules

An example of host design is provided by complexes with 4,4’-bipyridinium salts, aka ‘paraquats’ or ‘viologens’. As shown 5,5’-bis(hydroxymethyl)bis(1,3-phenylene)-32-crown-10 complexes with dimethyl paraquat in acetone with an association constant of ~600 M^-1. Crystals grown from the solution proved by x-ray diffraction not to consist of a pseudorotaxane, but rather what we term a ‘taco complex’, in which the crown ether folds around the guest. The complexation process is primarily driven by hyrdrogen bonding; the 2-protons of the paraquat, being somewhat acidic, H-bond to the ether oxygen atoms of the crown and the 3-protons H-bond to the fluorine atom of the PF₆ counterion.

Based on this structure we conjectured that addition of another loop to the host species would pre-organize it to more readily accept the guest without the entropic penalty associated with folding. The cryptand shown was thus synthesized and indeed its binding constant was 100-fold higher than the crown ether described above. By measurements as a function of temperature the enthalpic and entropic terms in the free energy expression were evaluated; it was found that the enthalpy of complexation was the same for both systems, but the entropy term was significantly more favorable for the cryptand. Note the similarity of the structures of the two complexes; the same types of H-bonding are involved.

For details see Organic Letters 1999, 1, 1001-1004.

Supramolecular Dendrimers

Our group is also interested in the self-assembly of dendritic structures as a means of facititating synthesis and bringing new functiionality to these unique systems. An example of these efforts is the self-assembly of dendritic [2]-, [3]- and [4]-pseudorotaxanes from a triammonium core and dibenzo-24-crown-8 (DB24C8) and its derivatives, i. e., the first, second and third generation benzyl ether denrons (the latter is shown below). We have measured the association constants for these systems in acetone and chloroform. The binding sites of the tritopic guest behave independently with DB24C8. However, the binding of the dendrons occurs in cooperative fashion. In chloroform, in fact, only the completely formed [4]pseudorotaxanes can be detected in solution.

For details see Angew. Chem. Int. Ed. 1998, 37, 3275-3279; Chem. Commun. 1999, 789-790; J. Am. Chem. Soc. 2002, 124, in press.
 

We have also utilized C60 as a core for self-assembled dendrimers as shown below. Model studies with DB24C8 have been successful.