Inner and Outer Beauty - Beauty in Chemistry - page 12

Fig. 16 Fujita's highly charged octahedra. Although these assemblies have precisely the same
elements of symmetry as the M 4 L 6 assemblies of Raymond and coworkers they differ in that the
ligands are the threefold symmetrical unit, while the metals provide the twofold axis. Reprinted
with permission
4 How the Electronic Structure Affects Guest Chemistry
At the outset of this chapter, we noted that the beauty of symmetry and pattern is
ultimately the beauty of simplicity. The elegance of the chemistry of these supra-
molecular capsules, too, lies in the profound chemical consequences of simple
changes wrought by the defined microenvironments within these assemblies. The
earliest examples of altered chemical activity within supramolecular coordination
compounds come from Fujita and coworkers, in which they employed their palla-
dium-vertexed octahedra (Fig. 16 ) in the Diels-Alder cycloaddition of isoprene
with naphthoquinone [ 26 ], accelerating this bimolecular addition 113-fold.
The basis of the rate acceleration by this host is an increased effective molarity
within the assembly cavity. This principle has been demonstrated with other
supramolecular compounds that possess a defined inner space [ 27 , 28 ]. This is a
powerful but narrow capability of these assemblies, employing size- and shape-
complementarity to bring molecules together in the promotion of bimolecular
reactions. Importantly, this phenomenon does not depend on perturbation of the
potential energy surface to effect the rate accelerations.
This is not to say, however, that supramolecular assemblies can only affect
reactivity kinetically (i.e., by bringing reagents into proximity or providing a spatial
barrier between them, as in the P 4 example earlier). An important example of the
perturbation of a thermodynamic equilibrium by a supramolecular coordination
cage is the increased acidity of encapsulated amines within the M 4 L 6 assembly
shown in Fig. 17 [ 29 ]. A wide variety of amines can be encapsulated within the
supramolecular framework, bound as the protonated ammonium species.
In basic aqueous solution, the equilibrium between free amines and their conju-
gate ammonium ions strongly favors the free amine. In the presence of the M 4 L 6