Synthesis, Conformations and Redox Properties of Diametrical Calix[4]arene diquinones

Several calix[4]arenediquinones 3a-e having quinone moieties in opposite positions (diametrical) have been synthesized by oxidation of 25,27-dialkoxycalix[4]arenes 2a-e with Tl(NO3)3 . 3H2O in organic solvents. The conformational properties of these compounds have been established in the solid state by X-ray crystallography and in solution by variable-temperature dynamic NMR. We found that the calix[4]arenediquinones 3a-e are more flexible than the parent 25,27-dialkoxycalix[4]arenes 2a-e in solution and tend to assume two different partial cone structures in the solid state, depending on the steric bulkiness of the alkoxy groups. Cyclic voltammetry reveals that the redox properties of calix[4]arenediquinones 3a-e depend on their conformational properties.