Macrocyclic effect on anion binding. A potentiometric and electrochemical study of the interaction of 21- and 24- membered polyazaalkanes with [Fe(CN)6]4– and [Co(CN)6]3–

Abstract

The interactions of the cyclic polyazaalkanes 1,4,7,10,13,16,19-heptaazacyclohenicosane, 1,4,7,10,13,16,19,22-octaazacyclotetracosane and of the open-chain counterparts 1,17-bis(methylamino)-3,6,9,12,15-pentaazaheptadecane and 1,20-bis(methylamino)-3,6,9,12,15,18-hexaazaicosane with the complex anions hexacyanoferrate(II) and hexacyanocobaltate(III) have been studied at 298.15 K in 0.15 mol dm^–3 NaClO₄ by using potentiometric and electrochemical techniques. Both types of polyamines, cyclic and linear, form stable complexes with 1 : 1 anion : receptor stoichiometry. This result has been obtained either by treatment of electromotive force data with the computer program SUPERQUAD or by analysing the variations of [Fe(CN)₆]^3––[Fe(CN)₆]^4– voltammograms with the ratio [Fe(CN)₆]^4– : polyamine. A macrocyclic effect for anion binding is observed, namely the stability of the ‘super-complexes’ of the cyclic ligands is higher than those formed by their open-chain counterparts. Both types of ligands interact more strongly with [Fe(CN)₆]^4– than with [Co(CN)₆]^3–. From the electrochemical measurements it has been estimated that the affinity of [Fe(CN)₆]^3– for these polyammonium receptors is similar to that presented by the equally charged complex anion [Co(CN)₆]^3–.