Electrochemical properties of outer-sphere associates of bipyridyl and sepulchrate metal complexes with (thia)calix[4]arenes

Abstract

The electrochemical one-electron reduction (oxidation) of bipyridyl metal complexes ([Co(bipy)3]3+, [Cr(bipy)3]3+, [Fe(bipy)3]2+, [Ru(bipy)3]2+ (as well as Co(III) sepulcrate)) with water-soluble (thia)calix[4]arenes has been studied by means of cyclic voltammetry. It has been shown that [M(bipy)3]3+/2+ bind to (thia)calix[4]arenes via sulfonate groups of the upper rim. Oxidized forms bind stronger than reduced ones leading to reduction (oxidation) of half-wave cathodic shift. The effect of predominant stabilization of oxidized forms of metal complexes for carboxylated calix[4]arene is stronger than for thiacalix[4]arene (ΔΔG0 = − 7.8 ÷ − 12.5 and − 3.7 kJ/mol, respectively). The redox-switchable outer-sphere binding of Co(III) sepulchrate via lower rim of carboxylated calix[4]arene has been revealed using cyclic voltammetry. The binding constants of outer-sphere associates based on calix[4]arenes and unstable metal complexes ([Co(sep)]2+, [Ru(bipy)3]3+, [Co(bipy)3]2+) have been calculated for the first time using 1H NMR titration and cyclic voltammetry data.