Effect of Large Electrolyte Anions on the Sequential Oxidations of Bis(fulvalene)diiron Attached to Glassy Carbon by an Ethynyl Linkage

Two ethynyl-derivatized isomers of bis­(fulvalene)­diiron (BFD, 1,1′-biferrocenylene) were prepared and covalently attached to glassy carbon electrodes through their ethynyl group by three different electrode modification methods. Cyclic voltammetry and square wave (SW) voltammetry were used to characterize surface coverages of 1.4–5.5 × 10^–10 mol cm^–2, the higher of these corresponding to roughly a monolayer, based on computation of an idealized close-packing structure for ethynylbis­(fulvalene)­diiron (E-BFD) on a solid surface. In a dichloromethane solution containing a smaller electrolyte anion such as [PF₆]⁻ or [ClO₄]⁻, the E-BFD-modified electrodes exhibited two quasi-Nernstian one-electron oxidations. In contrast, the current for the second oxidation process, [E-BFD]^+/2+, was diminished in electrolytes containing one of the large fluoroaryl borate anions, [B­(C₆F₅)₄]⁻ or [B­(C₆H₃(CF₃)₂)₄]⁻. The effect was enhanced for electrodes having higher surface coverages being probed at shorter voltammetric time scales. SW voltammetry showed that the diminished currents for [E-BFD]^+/2+ in large-anion electrolytes are not caused by slow electron transfer. Rather, they are attributed to mixed diffusivity of the counter-anions at the electrode/solution interface, as [E-BFD]⁺ and the anion form the optimum (lowest-energy) configuration of a 1:1 ion pair. The interior transport of the anion required to reach this configuration may be sterically encumbered, accounting for the diminished charge transfer observed with electrolytes containing large anions.