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.