Open-chain polyazaalkane ferrocene-functionalised receptors for the electrochemical recognition of anionic guests and metal ions in aqueous solution

Abstract

New open-chain polyazaalkane N-terminal ferrocene-functionalised receptors, L¹, L² and L³, have been synthesized and characterised and their potential use as selective anion and cation sensing receptors in aqueous solution investigated. Solution studies by potentiometric methods have been carried out in the presence of H⁺ and Cu²⁺ in water (25 °C, 0.1 mol dm^–3 KNO₃). The results have been compared with those of the analogous non-functionalised receptors triethylenetetramine (L⁴), tetramethylenepentamine (L⁵) and N,N″‴-dimethylpentaethylenehexamine (L⁶). Compounds L¹–L³ form both mono- and di-nuclear complexes with Cu²⁺. The electrochemical responses as a function of the pH in the presence of the Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺ or Pb²⁺ cations in water and the ATP, phosphate, sulfate and nitrate anions in thf–water (7∶3 v/v, 25 °C, 0.1 mol dm^–3 [Buⁿ₄N][ClO₄]) for L¹, L², L³ and for 1,4,7,10,13,16-hexaferrocenyl-1,4,7,10,13,16-hexaazacyclooctadecane (L⁷) in 1,4-dioxane–water (7∶3 v/v, 25 °C, 0.1 mol dm^–3 KNO₃) have also been determined. The studies have been directed to determine the pH ranges of selectivity in the electrochemical recognition of cations and anionic guests, pointing out the ability of molecules L¹–L³ and L⁷ to act as selective anion-sensing or cation-sensing receptors. The anion ATP can be selectively detected using the L² receptor in the presence of the competing anions phosphate and nitrate.