PCCP Blog

Ionic liquids are the new black; a nonsense statement, but very true. Recyclable ionic liquids are the new solvents of choice as they allow for extensive recycling, novel processes and treatments and higher durability of devices. Ionic liquids are also just new, with unexplored physical and chemical properties. In a recent PCCP paper Torriero and co-workers target the issue of referencing electrochemical measurements in ionic liquids.
Four classical metallocene redox couples are investigated in this paper. Ferrocene/ferrocenium is the reference redox couple of choice, but has been shown to be sensitive so specific solvation and ion pairing. This is also the case for ionic liquids, just much more extreme. In the four studied ionic liquids the variation is a full 100 mV. A similar large variation is found for cobaltocenium/cobaltocene. This variation presents a major issue if either of these redox couples are used as an internal reference in electrochemical experiments.
The per-methylated derivatives sandwich complexes, derived from 1,2,3,4,5-pentamethyl-cyclopentadienyl or Cp* (Cp-star), vary, but vary less. The relative change, measured as the difference in reduction potential of between Cp*₂Fe and Cp*₂Co⁺, is only a few millivolts. The result presented in this paper shows that the Cp* sandwich complexes may show promise as the new standard in electrochemical experiments in ionic liquids.

The limitation of the suggested internal references are discussed in full in:

Assessment of permethylated transition-metal sandwich complexes as internal reference redox systems in ionic liquids
Angel A. J. Torriero, Jaka Sunarso, Maria Forsyth and Cristina Pozo-Gonzalo
Phys. Chem. Chem. Phys., 2013, 15, 2547-2553
DOI: 10.1039/C2CP43177G

by Dr Thomas Just Sørensen