Heteroditopic ferrocene-based ureas as receptors for anions and cations

Abstract

The synthesis of structurally new types of ferrocene-based ureas, in which the ferrocene moiety is simultaneously attached to two urea groups, have been prepared directly from 1,1-bis(isocyanato)ferrocene 1. Receptors 2a, 2b and 2d show spectral and electrochemical anion-sensing capability for hydrogenphosphate and fluoride anions, in addition 2d also shows a sensing capability for acetate anion. Heteroditopic receptor 2a, bearing a pyridine binding site, does not complex Cu²⁺ cations but instead an intramolecular redox reaction takes place to give the oxidized form 2a⁺. The results obtained by using the heteroditopic receptor 2b, containing two crown ethers units, demonstrate that K⁺ cations can only be electrochemically detected in the presence of hydrogenphosphate anion. Whereas the new structural motif 2d, in which the two urea groups are part of the macrocycle framework, is a selective electrochemical sensor for Li⁺ cation in the presence of alkali metal ions. The preferred binding modes and their extent are proposed for the most representative complexes by means of DFT-based theoretical calculations.