Polarographic and spectrophotometric investigation of the reduction and deprotonation of (1,3,6,8,10,13,16,19-octa-azabicyclo-[6,6,6]-icosane)cobalt(III) ion, Co(sep)3+, in the presence of different anions

Abstract

The reduction of the title compound has been studied polarographically in different aqueous media. Remarkable specific anion effects have been observed. In the presence of OH^– ions there is a strong displacement of E_1/2 towards negative values. This has been interpreted as due to a deprotonation of the Co(sep)³⁺ ion, which can be treated as an association with OH^–. Using the DeFord–Hume equation, the association constants have been determined at a constant ionic strength (I) of 0.5 mol dm^–3(NaCl). Values of 50 mol dm^–3 and 2 × 10³ mol³ dm^–9 have been obtained for β₁ and β₃, whereas β₂ appears to be too small to be measured. These results are supported by visible spectrophotometric and circular dichroism data. Analogous polarographic measurements were performed at I= 0.4, 1.0 and 2.0 mol dm^–3. The differences between the results in the various media, as well as the changes in E_1/2 in neutral solutions with different supporting electrolytes, are correlated with the effects of the different salts on the solubility in water of benzoic acid and of naphthalene (Setchenov coefficients). Hence, the hydrophobic character of the anions seems to be more relevant than the electrostatic effects.