Redox behaviour and stable anion radicals of cyclic polycalicenes

Abstract

Cyclic polycalicenes (cyclic bicalicene 1, cyclic tercalicene 2, and cyclic quatercalicene 3) have been characterized by cyclic voltammetry and EPR spectroscopic measurements. The redox waves are completely reversible at –70°C in the reduction process but remain irreversible in the positive voltage region. The cyclic polycalicenes with larger π conjugations are more easily reduced, irrespective of the 16π, 22π and 28π conjugations at the periphery. In particular, 1 shows large reduction potentials, indicating an ‘aromatic’ system with a 16π-electron periphery. The spin density distribution of the anion radicals (1˙^–, 2˙^– 3˙^–) indicates a localization of the unpaired electron, to some extent, on the five-membered rings, implying a polar structural contribution. It has also been shown that the relatively long chemical bonds at specific positions in the cyclic polycalicenes influence the spin density distribution. The EPR results obtained from the cyclic polycalicenes are quite different from those of the anion radical of calicene itself.