Redox Control of the Binding Modes of an Organic Receptor

The modulation of noncovalent bonding
interactions by redox processes
is a central theme in the fundamental understanding of biological
systems as well as being ripe for exploitation in supramolecular science.
In the context of host–guest systems, we demonstrate in this
article how the formation of inclusion complexes can be controlled
by manipulating the redox potential of a cyclophane. The four-electron
reduction of cyclobis­(paraquat-p-phenylene) to its
neutral form results in altering its binding properties while heralding
a significant change in its stereoelectronic behavior. Quantum mechanics
calculations provide the energetics for the formation of the inclusion
complexes between the cyclophane in its various redox states with
a variety of guest molecules, ranging from electron-poor to electron-rich.
The electron-donating properties displayed by the cyclophane were
investigated by probing the interaction of this host with electron-poor
guests, and the formation of inclusion complexes was confirmed by
single-crystal X-ray diffraction analysis. The dramatic change in
the binding mode depending on the redox state of the cyclophane leads
to (i) aromatic donor–acceptor interactions in its fully oxidized
form and (ii) van der Waals interactions when the cyclophane is fully
reduced. These findings lay the foundation for the potential use of
this class of cyclophane in various arenas, all the way from molecular
electronics to catalysis, by virtue of its electronic properties.
The extension of the concept presented herein into the realm of mechanically
interlocked molecules will lead to the investigation of novel structures
with redox control being expressed over the relative geometries of
their components.