Ionic liquid effects on the redox potential of ferrocene

The ionic liquid, IL, dependence of the mid-point potential of the ferrocene|ferrocenium, Fc0/+, couple versus the decamethylferrocene|decamethylferrocenium, DmFc0/+, couple was studied in eleven ILs and in dichloromethane with added IL as the supporting electrolyte. The difference in mid-point potential between Fc and DmFc indicates that Fc0/+ couple is clearly dependent on the IL structure. A variation of about 0.100 V over the range of ILs studied under neat conditions and by ca. 0.050 V when studied in dichloromethane is reported. Meanwhile, a variable potential shift ranging from 0.014 to 0.082 V was observed when data was compared in these two systems (ILs vs. dichloromethane). The effect of water and lithium ion on the Fc0/+ and DmFc0/+ redox potential was also evaluated and found to be minimal.

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► The ferrocene midpoint potential alters by as much as 0.1 V in different ionic liquids.
► The ferrocene midpoint potential is dependent on the anion and less dependent on the cation.
► Decamethylferrocene is considered less sensitive than ferrocene to the solvation effects of ILs.
► The presence of lithium or water does not affect the reference potential.
► Published ferrocene midpoint potentials in ionic liquids can be compared using this approach.