Template-Directed Synthesis of Molecular Nanorings and Cages

This Account is about templates as construction tools: molecules for making molecules. A template organizes the reactants and provides information to promote formation of a specific product, but it is not part of the final product. We have developed many different strategies for using oligopyridines as templates for the synthesis of alkyne-linked π-conjugated metalloporphyrin oligomers. These compounds include some of the largest macrocycles ever synthesized, such as a 50-porphyrin ring with a diameter of 21 nm containing a ring of 750 C-C bonds. Metalloporphyrins are excellent models for exploring template directed synthesis, as they can be functionalized in many different positions and the central metal (typically Zn or Mg) provides a handle for coordination to templates. Classical template-directed macrocyclization reactions have a 1:1 complementarity between the template and the product. This strategy works well for preparing nanorings of 5-7 porphyrin units, but larger templates are laborious to synthesize. Rings of 8 or more porphyrin units are most easily prepared using "nonclassical" strategies, in which several small templates work together to direct the formation of a large ring. In the Vernier approach, a mismatch between the number of binding sites on the template and the building block leads to a mathematical amplification of the length scale: the number of binding sites in the product is the lowest common multiple of those in the template and the building block. For example, a 40-porphyrin ring can be prepared by coupling a linear decamer in the presence of an octadentate template. Linear Vernier templating opens up intriguing possibilities for self-replication. When several small radial oligopyridine templates bind inside a large nanoring they can form complexes with some vacant coordination sites that display correlated motion like the caterpillar tracks of a bulldozer. These caterpillar track complexes can be used in template-directed synthesis and they provide the most convenient route to 8- and 10-porphyrin rings. Russian doll complexes provide another strategy for template-directed synthesis: a number of specifically designed ligands bind to a central nanoring to form a template for constructing a larger concentric nanoring. The same oligopyridine templates that are used to prepare nanorings can also be used to synthesize three-dimensional nanotubes and nanoballs. Again, nonclassical approaches, in which several small templates work together cooperatively, are much simpler than creating a single large template with sufficient binding sites to define the whole geometry of the product. Oligopyridine ligands can also be used as shadow mask templates to control the demetalation of magnesium porphyrin nanorings, because metal centers that are not coordinated by the template can be selectively demetalated with acid. Thus, the template forms a permanent shadow on the porphyrin nanostructure that remains after the template has been removed. Shadow mask templates provide a simple route to heterometalated molecular architectures. The insights emerging from these studies are widely applicable, and there are many opportunities for inventing new ways of using templates to control reactions.