"Crowned" Fe4S4 clusters as electrochemical metal ion sensors

A series of Fe4S4 cluster compounds, I, III, and V, in which the cuboidal c luster core is appended with four crown ether thiolate ligands. and II and IV, bearing thiolate ligands without crown ether parts, has been synthesize d and characterized. The spectroscopic and electrochemical properties of th ese compounds are determined by the electronic nature of the thiolate ligan ds. Only in the case of III, where a very short alpha-thioacetyl linker was used to connect the crown ether ligands to the cluster core, was a restric ted conformational freedom of the ligand observed. A detailed electrochemic al study of the influence of alkali and earth alkali metal ions (Li+, Na+, K+, Mg2+, and Ba2+) on the reversible 2(-)/3(-) reduction of the cluster co mpounds was performed. In the case of the crown ether appended clusters I, III, and V, the addition of these metal ions resulted in an anodic shift, i .e. in positive direction, of the reduction potential (modulation effect) a nd to larger current responses (promotion effect). The magnitude of the mod ulation effects is determined by the binding affinity of the metal ions in the crown ether ligands, and by the distance between bound metal ions and t he redox active cluster core. Variation of the linker between the cluster c ore and the metal ion binding site resulted in cluster compounds with almos t inverse selectivities for e.g. K+ and Ba2+ in the case of I and III. For the large effects found for compound I a lariat binding mode is proposed.