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 (D_app) 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 D_app for the 2-/3- couples whereas for the 3-/4- process, a physical diffusional mechanism probably dominates, with D_app close to the value for pure diffusion. As estimated by assumption of the simple Dahms-ruff model, the rate constant k_ex 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.