Iron–sulfur clusters in ionic polymers on electrodes

Abstract

Iron–sulfur clusters have been incorporated in cationic polymers at electrode surfaces; the methodology for the assembly of such structures, and their characterisation by reflectance Fourier-transform (FT) IR, EPR and electrochemical techniques, is given. The stoichiometries of the cluster-electrode assemblies are self regulated by the necessity to maintain charge neutrality during redox cycling. Thus the assembly produced by the incorporation of [Fe4S4(SPh)4]2– in [C4H4N(CH2)3NMe3]+3[BF4]– polymer (C4H4N = pyrrol-1-yl) adopts a composition close to [C4H4N(CH2)3NMe3]+3[Fe4S4(SPh)4]2–[BF4]–. Electron-transfer and ligand-exchange reactions of polymer-bound clusters have been studied. Electrostatic (ion-pairing) effects are shown to modulate redox potentials and thus E°′ values for [Fe4S4(SPh)4]2–/3– and other couples are generally shifted to potentials positive of those observed in solution. Chronocoulometric measurements show that apparent diffusion coefficients (Dapp) for the one-electron reduction of [Fe4S4(SCH2Ph)4]2– and [Fe4S4(SPh)4]2– are about an order of magnitude greater than that for the reduction of the corresponding trianionic cubanes. This appears to be a consequence of different mechanisms of charge propagation within the polymer films: electron hopping enhances Dapp for the 2-/3- couples whereas for the 3-/4- process, a physical diffusional mechanism probably dominates, with Dapp close to the value for pure diffusion. As estimated by assumption of the simple Dahms-ruff model, the rate constant kex for the electron self exchange reaction between [Fe4S4(SCH2Ph)4]2– and [Fe4S4(SCH2Ph)4]3– within the cationic film, is relatively high at ca. 103 dm3 mol–1 s–1. This is turn suggests that polymer-confirmed clusters operating at the 2-/3- level might well be effective mediators for electron transfer to (co-trapped) substrate reducing sites.