Electrochemical and quantum chemical investigation of tetranitrocalix[4]arenes: molecules with multiple redox centers

The calix[4]arene skeleton is electrochemically inactive, but it is a useful stable frame for building "smart" molecules and supramolecular assemblies. Suitable substitution on the upper (and/or lower) rim leads to unusual and surprising properties in this system. Polynitrocalix[4]arenes with reducible nitro groups located at the upper rim represent molecules with multiple redox centers where the potential for interactions between them is the focus of interest. The title compounds are promising precursors, e.g., for design and synthesis of sensors. In this work, the stepwise reduction of two tetranitrocalixarenes was investigated electrochemically, and the results were correlated with quantum chemical calculations. The order of individual electron transfers was described as a consequence of molecular geometry. Two independent pairs of equivalent nitro groups were identified whose reduction potential depends upon their respective locations in the molecule. All nitro groups are electronically isolated and thus are reduced independently yielding poly radical ions. The increasing charge has negligible impact on the geometry of the calixarene, which maintains its pinched shape even when carrying an overall molecular charge of -4.