Electrochemistry in Media of Exceptionally Low Polarity: Voltammetry with a Fluorous Solvent

Abstract

This work demonstrates the first cyclic voltammetry in a perfluorocarbon solvent without use of a cosolvent. The novel electrolyte tetrabutylammonium tetrakis[3,5-bis(perfluorohexyl)phenyl]borate (NBu(4)BArF(104); 80 mM) allows for voltammetry of ferrocene in perfluoro(methylcyclohexane) by lowering the specific resistance to Ω268 k cm at 20.8 °C. Despite significant solution resistance, the resulting voltammograms can be fitted quantitatively without difficulty. The thus determined standard electron transfer rate constant, k°, for the oxidation of ferrocene in perfluoro(methylcyclohexane) is somewhat smaller than for many solvents commonly used in electrochemistry, but can be explained readily as the result of the viscosity and size of the solvent using Marcus theory. Dielectric dispersion spectroscopy verifies that addition of NBu(4)BArF(104) does not significantly raise the overall polarity of the solution over that of neat perfluoro(methylcyclohexane).

Figure 1

Structures of perfluoro(methylcyclohexane) (1) and NBu₄BArF₁₀₄ (2).

Figure 2

Cyclic voltammogram (CV) of perfluoro(methylcyclohexane) containing 80 mM NBu₄BArF₁₀₄, scan rate = 100 mV/s, showing the electrochemical background. Data is corrected for solution resistance.

Figure 3

CVs of a 0.1 M NBu₄ClO₄/THF solution containing 0, 25, 50, or 75 mM perfluoro(methylcyclohexane): scan rate = 10 mV/s, T = 21°C.

Figure 4

CVs of 5.43 mM ferrocene and 80 mM NBu₄BArF₁₀₄ in perfluoro(methylcyclohexane) with various scan rates. Data is corrected for solution resistance.

Figure 5

CVs of varied concentrations of ferrocene in 80 mM NBu₄BArF₁₀₄/perfluoro(methylcyclohexane): scan rate = 10 mV/s, T = 21°C. The inset shows the linear relationship between the limiting current and the ferrocene concentration.

Figure 6

CV of 1.36 mM ferrocene normalized to the diffusion-limited current (solid) along with a fit based on α = 0.61, k° = 7.13 × 10⁻⁴ cm/s, D(Fc) = 2.05 × 10⁻⁶ cm² s⁻¹ and D(Fc⁺) = 2.85 × 10⁻⁷ cm² s⁻¹(dots).

Figure 7

Log-log plot of k° versus τ_L along with a linear fit for the literature data (open circles, [32,40,41]) only. The fit is extrapolated to the τ_L for perfluoro(methylcyclohexane) (filled circle).

Figure 8

Dielectric dispersion spectrum of perfluoro(methylcyclohexane) with and without 80 mM NBu₄BArF₁₀₄.