The effect of donor groups and geometry on the redox potential of copper Schiff base complexes

We report a study which correlates the metrical parameters of the unsubstituted tetradentate copper Schiff base complexes containing N2O2, N2N2 and N2S2 donors with their respective redox potentials. To achieve this aim we were required to structurally characterise many of the seminal species including, [CuAmbpr-H2], [CuH4Amben][ClO4]2, [CuH4Ambpr][ClO4]2, [CuH4Ambbu][ClO4]2, CuH4Salpr and [Cu(SSalen)2][ClO4]2 which were absent from the crystallographic catalogue. The oxidative dehydrogenation of CuH4Salen is revisited through the isolation and structural characterisation of (N-salicyl-N′-salicylidene-1,2-ethylenediamine)copper(II) (CuH3Salen). The redox potentials of the three series of compounds are measured, clearly identifying the operating ranges of each donor set. The modulating effect of coordination geometry on redox potential is evident in the series of N2O2 complexes. This study forms the basis of the rational synthesis of tuneable copper redox sensors by demonstrating the regions in which the various donor sets operate.