Anion interaction with ferrocene-functionalised cyclic and open-chain polyaza and aza-oxa cycloalkanes

Abstract

A family of ferrocene-functionalised receptors of different topologies have been used as receptors for anions. The compounds have been designed to contain both amine nitrogen and ether oxygen atoms and comprises from monoaza to pentaaza derivatives both open-chain (L¹, L², L³) or cyclic (L⁴, L⁵) and having from one to five ferrocenyl groups. Solution studies directed to determine the protonation constants of L¹, L² and L³ have been carried out in water (0.1 mol dm⁻³ KNO₃, 25 °C) and those of L⁴ and L⁵ in 1,4-dioxane–water (70∶30 v/v, 0.1 mol dm⁻³ KNO₃, 25 °C). The protonation behaviour of the receptors can be explained taking into account electrostatic considerations. Speciation studies in the presence of phosphate have been carried out in water for L¹, L² and L³ and in dioxane–water for L⁴ and L⁵. Speciation studies have also been performed in the presence of ATP with L¹, L² and L³ in water. Selectivity of a mixture of receptors against a certain anion is discussed in terms of ternary diagrams. The shift of the redox potential of the ferrocenyl groups as a function of the pH has been studied. The difference between the oxidation potentials at basic and acidic pH has been determined experimentally and is compared with that theoretically predicted using an electrostatic model previously reported. The electrochemical shift in the presence of ATP and phosphate has been measured in water for L¹, L² and L³ and in the presence of phosphate and sulfate in 1,4-dioxane–water for L⁴ and L⁵ as a function of the pH. The electrochemical response found against those anions is quite poor with maximum cathodic shifts of ca. 30–40 mV. The electrochemical response induced by HSO₄⁻ and H₂PO₄⁻ has also been studied in acetonitrile solutions where a large cathodic shift for H₂PO₄⁻ up to ca. 200 mV was found.