Synthesis, electrochemistry and complexation studies of ferrocene crown ethers

Voltammetric and UV spectrophotometric investigations of alkali and alkaline earth metal cations binding by the new ferrocene macrocyclic ligands 1-4 have been performed in acetonitrile electrolyte. Stability constants were determined from UV measurements. All the metal cations surveyed formed 1:1 stoichiometric complexes with 2 and 3, with the exception of barium which produced a 1:2 intermolecular sandwich complex with 2, No complexation occurred with 1. Electrochemical studies have demonstrated that the binding of Li+, Na+, K+, Mg2+, Ca2+ and Ba2+ with 2 resulted in shifts of the ferrocene oxidation wave to more positive potentials, The same features were observed with 3 in the presence of Li+, Na+, Ki(+) and Mg2+, and with 4 and Ba2+, In contrast, a different type (two-wave behaviour) of positive shift revealed the binding of Ca2+ and Ba2+ with 3, allowing the amperometric titration of these metal cations. The magnitude and the type of potential shift could nor be related to the radius and charge to radius ratio of the cations, or to the strength of the host-guest association. It can be assumed that additional effects such as an effective charge transfer interaction between the cationic guest and the redox centre control the electrochemical recognition behaviour of these redox ionophores. Single crystal X-ray structure of [3-Ba](ClO4)(2) has also been determined to gain better understanding of the barium complexation in 3. (C) 1997 Elsevier Science S.A.