Electrochemical sensing of anions by redox-active receptors built on the ferrocenyl cyclam framework

A range of ferrocenyl derivatives, L1-L3 and L1Cu, has been prepared and their electrochemical recognition properties in organic solvents have been studied. These compounds are built on the same cyclam framework (ferrocenyl trans di-substitued cyclam), but they differ by the nature of their anions binding sites. While L1 and its corresponding N-methylated L2 derivative can interact with anionic species via H-bonding, strong electrostatic and hard acid-hard base interactions with anionic species are mainly involved in the ammonium derivative L3 and the copper(II) complex L1Cu, respectively. Significant CV perturbations in the characteristic potential of the Fc/Fc+ subunit in the presence of anions were found with L1 and L3. Selectivity of the electrochemical process is likely to be based on the formation of multiple H-bonds between the secondary amines of L1 and the phosphate anions (both H-donor and acceptor) when the topology of the ammonium derivatives, L3, governs mainly the anion complexation. Clear cyclic voltammetric features, such as a well-defined two-wave behavior allowing an accurate amperometric titration, require that the redox receptor contain an appropriate binding site and the ferrocene group in close proximity. Lewis acid-base interactions were shown to influence more deeply the electroactivity of Lewis acid center, namely the copper center in L1Cu, than that of the ferrocenyl redox probe that is not directly connected to the associated anion. However, the precipitation of the host structure (e.g., L3 or L1Cu) in the presence of oxo-anions shows that a compromise has to be found between the establishment of strong interactions and the formation of soluble ion-pairs based complexes.