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 NBu₄ClO₄, 25°C). The complexes [M(HL)]³⁺, [ML]²⁺, [M(OH)]⁺ and [ML(OH)₂] are 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 Fe^IIFe^III and oxidised Fe^IIIFe^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).