Highly branched ferrocene-functionalised polyazacycloalkanes as electroactive receptors for transition-metal ions

Abstract

Reaction of N,N′,N″,N‴-tetrakis (2-aminoethyl)-1,4,8,11-tetraazacyclotetradecane (taec) with ferrocenecarbaldehyde and further reduction with LiAlH₄ resulted in the synthesis of the redox-active receptor N,N′,N″,N‴-tetrakis(4-ferrocenyl-3-azabutyl)-1,4,8, 11-tetraazacyclotetradecane (tfabc). The protonation behaviour of tfabc and its complex formation with Cu^II, Zn^II and Cd^II have been studied in tetrahydrofuran–water (70 : 30 v/v)(0.1 mol dm^–3 NBu^t₄ClO₄, 25 °C). The potentiometric and electrochemical data indicate the formation of 1 : 1 and 1 : 2 (tfabc: M²⁺) stoichiometries. The E_½ values for tfabc shift as a function of the pH from 389 (pH 10) to 482 mV (pH 2.5) and E_½vs. pH curves for the tfabc–H⁺–M²⁺ systems (M = Cu, Zn or Cd) show that tfabc electrochemically recognises the presence of copper(II) selectively at pH < 5. The shift of E_½ appears to be caused by electrostatic forces and the E_½vs. pH curves can be predicted in terms of the distribution diagrams of the tfabc–H⁺–M²⁺ systems.