Ferrocene-based small molecules for dual-channel sensing of heavy- and transition-metal cations

The synthesis, electrochemical, optical, and cation-sensing properties of ferrocene-pentakis(phenylthio)benzene dyads, linked through a putative cation-binding 2-azadiene bridge, are presented. Dyad 5 behaves as a highly selective dual-redox and chromogenic chemosensor molecule for Pb2+ cations; the oxidation redox peak is anodically shifted (Delta E-1/2 = 125 mV), and the low energy band of the absorption spectrum is red-shifted (Delta lambda = 119 nm) upon complexation with this metal cation. Linear sweep voltammetry and spectroelectrochemical studies revealed that Cu2+ and Hg2+ metal cations induced oxidation of the ferrocene unit. The isomeric dyad 7, in which the nitrogen atom and the ferrocene unit are in closer proximity, has shown its ability for sensing both Pb2+ and Hg2+ ions; the oxidation redox peak is anodically higher shifted (Delta E-1/2 = 340 mV), and the low energy band of the absorption spectrum is lower red-shifted (Delta lambda = 61 nm) that those found for dyad 5. The changes in the absorption spectra are accompanied by dramatic color changes which allow the potential for "naked eye" detection. A further exciting property of dyad 7 is that it behaves as an electrochemically induced switchable chemosensor for Pb2+ and Hg2+ because of the low metal-ion affinity of the oxidized 7(+) species for these metal cations. The experimental data and conclusions about the ion-sensing properties are supported by DFT calculations.