Tuning of the electrochemical recognition of substrates as a function of the proton concentration in solution using pH-responsive redox-active receptor molecules

Reaction Of ferrocene-1,1'-dicarbaldehyde and ethane-1,2-diamine yielded the Schiff-base derivative 2,5,19,22-tetraaza[6.6](1,1')ferrocenophane-1,5-diene, 1 the molecular structure of which has been determined by single-crystal X-ray analysis. Hydrogenation of 1 with LiAlH4 resulted in the corresponding amine 2,5,19,22-tetraaza-[6.6](1,1') ferrocenophane 2 which was characterised crystallographically. The protonation behaviour of 2 (denoted as L) and its complex formation with copper(II), nickel(II) and zinc(II) has been studied by potentiometric titrations in tetrahydrofuran-water (70:30 v/v) (0.1 mol dm(-3) NBu(4)ClO(4), 25 degrees C). The complexes [M(HL)](3+), [ML](2+), [M(OH)](+) and [ML(OH)(2)] age formed. An electrochemical study of compound 2 has also been performed under the same conditions at which the potentiometry was carried out and the pK(a) values for the mixed valence (FeFeIII)-Fe-II and oxidised (FeFeIII)-Fe-III species determined by fitting the curve of E(1/2) versus pH. From those data the Pourbaix diagram of the redox-active 2 has been calculated. Compound 2 can be considered as a selective electrochemical sensor for copper(II).