Angew. Chem. Int. Ed. http://doi.org/f25t8s (2015)

Spatial control in the formation of covalent bonds can be exerted by using template molecules. Template-directed approaches have, for example, been used to form large porphyrin-based macrocycles starting with porphyrin dimer subunits. In these techniques, the initial subunits polymerize until the recognition sites of the template are saturated. Now, Harry Anderson and colleagues at the University of Oxford have prepared two supramolecular systems each consisting of two 6-arm star-shaped molecules that can template the formation of either an 8-porphyrin or a 10-porphyrin macrocycle. The template arms end with a 4-pyridyl group that binds to the zinc centre of the porphyrin, guiding the polymerization of the porphyrin dimer all around the two templates. One of the arms of each template is not engaged with the porphyrin and sits in the centre of the resulting 2:1 complex.

The researchers then looked at the dynamics of the complexes using exchange NMR spectroscopy, which probes specific hydrogen atoms as they exchange position by conformational dynamics. They found that both the template rings and the porphyrin macrocycle rotate by 60° in a stepwise fashion. They confirm this mechanism through a series of control experiments in which either a palladium compound that binds to the loose arms of the two templates is used, or the 6-arm templates are replaced by 5-arm templates. In both cases the resulting complexes remain static.

The intramolecular stepwise rotation described by Anderson and colleagues mimics the working principle of a caterpillar track.