Electrochemical properties of n-sulfonatothiacalyx[4]arene complexes with Fe3+ and [Co(dipy)3]3+ ions

The electrochemical properties of n-sulfonatothiacalyx[4]arene (H4XNa4) complexes with [Co(dipy)(3)](3+) and Fe3+ ions were studied by means of cyclic voltammetry in aqueous solution at pH 2.5. The observed single-electron reduction of [Co(dipy)(3)](3+) bound extraspherically to the upper rim and Fe3+ ion bound intraspherically to the lower rim of n-sulfonatothiacalyx[4]arene in binary [Co(dipy)(3)](3+) center dot H3X5-, H3X5- center dot Fe3+, and ternary [Co(dipy)(3)](3+) center dot H3X5- center dot Fe3+ heterometal complexes was more difficult than in the free state. The reversible single-electron transfer to the metal ion results in lower binding energy ([Co(dipy)(3)](3+), Delta Delta G (0) = 3.9 kJ/mol) or in full fast dissociation of the complex (Fe3+). The ternary complex in the solution forms the aggregates, in which inner encapsulated Fe(III) and Co(III) ions are not reduced on the electrode. Their quantitative reduction takes place by the relay mechanism of intra- and intermolecular electron transfer through electrochemically generated [Co(dipy)(3)](2+) outer ions.