Electrochemical and Spectral Properties of Ferrocene (Fc) in Ionic Liquid: 1-Butyl-3-methylimidazolium Triflimide, [BMIM][NTf₂]. Concentration Effects

Abstract

Several earlier studies of the electrochemical oxidation of ferrocene (Fc) in room-temperature ionic liquids revealed an essentially nonlinear dependence of the oxidation current on the Fc concentration in its relatively dilute solutions, with its formally calculated diffusion coefficient strongly increasing with the concentration. Since no plausible mechanism leading to this very unusual finding had been proposed, our study of Fc solutions in 1-butyl-3-methylimidazolium triflimide, [BMIM][NTf(2)], was performed to verify whether the above observation originated from an incorrect determination of the dissolved Fc concentration. Our observations have demonstrated that reliable control of the Fc concentration in solution is complicated by factors such as the low amount of Fc used to prepare small-volume solutions or the great difficulty to dissolve completely a solid powder in a solvent with an extremely high viscosity. An unexpected additional complication is related to a sufficiently high volatility of Fc which manifests itself even at room temperature and especially at elevated temperatures or/and in the course of vacuum treatment of its solutions or its solid powder. Parallel measurements of electrochemical responses and UV-visible spectra for several series of Fc solutions of various concentrations (prepared with the use of different procedures) have shown a perfect parallelism between the peak current and the intensity of the absorption band in the range of 360-550 nm, leading us to the conclusion of a linear relationship between the oxidation current and the molecularly dissolved Fc concentration. The relations of these measured characteristics with the estimated Fc concentration in these solutions have demonstrated a much greater dispersion (attributed to the difficulty of a precise measurement of the latter) but without a significant deviation from the linearity in general. This finding has allowed us to estimate the diffusion coefficient of this species: D = (1.7 +/- 0.2) x 10(-7) cm(2)/s. The extinction coefficients for the maximum of the absorption band (at 440 nm) of Fc have been compared for a series of solvents: [BMIM][NTf(2)], acetonitrile, THF, heptane, CH(2)Cl(2), ethanol, and toluene. A simple method to estimate reliably the concentration of solute Fc in ionic liquids based on spectroscopic measurements has been proposed, owing to the proximity of Fc absorption properties for a great variety of solvents.

Diffusion regimes at nanoelectrode ensembles in different ionic liquids

“…For the case of Fc in [BMIm][N(Tf) 2 ] the value determined by us at 20 • C, namely 2.9 × 10 −7 cm 2 s −1 , is in reasonable agreement with the literature values of 3.77 × 10 −7 cm 2 s −1 [45], measured at 26 • C. Higher temperature means lower viscosity and higher D values; it was reported that each two-degree shift results in 10% change of the IL viscosity [46], so that, taking into account this factor, the two values are almost the same. Note that recently, for the same probe and IL, Vorotyntsev measured at 20 • C a D value of 1.7 × 10 −7 cm 2 s −1 [46]. On the other hand, the D value determined by us for Fc in [BmPy][N(Tf) 2 ], namely 0.7 × 10 −7 cm 2 s −1 , is about three times smaller than the value 2.31 × 10 −7 cm 2 s −1 , measured by Compton's group [45].…”

The Kinetics of Ferrocene Volatilisation from an Ionic Liquid

“…Fc sublimes relatively quickly at atmospheric temperature and pressure, [6,7] as well as in vacuo. [4,8] Ionic liquids (ILs) have also found widespread applications, particularly with respect to typically possessing extremely low vapour pressures, [9] wide electrochemical windows and inherent conductivity. [10] Their low vapour pressure has led to their application in in situ electrochemistry-XPS investigations, [11] as well as non-volatile electrolytes for gas sensors.…”

“…Therefore, there is currently a heavy reliance on in situ reference couples, such as cobaltocenium j cobaltocene, [1] TMPD j TMPD + · and Ag( 0 ) j Ag I , [14] as well as a significant number of iron-based compounds, [1,15,16] although the use of ferrocene j ferrocenium [1,4,5,7,8,[15][16][17][18][19][20] appears to be especially prevalent. It has previously been observed that placing ILs in vacuo or under a flow of inert gas can result in the volatilisation of dissolved Fc, [4,7,8] and this fact has been partially attributed [8] to discrepancies in the literature regarding diffusion coefficient values, D, and the erroneous assumption that D is strongly related to Fc concentration. [1,8] The non-volatility of ILs encourages their application as thin films on the surfaces of electrodes [21] which can be problematic to characterise, and change with temperature, water content, etc.…”

Hydrodynamic sono-voltammetry of ferrocene in [Tf2N]− based ionic liquid media

“…Some recent works relate to the electrochemical and hydrodynamic behavior of organic and organometallic redox couples, such as tris(4-bromophenyl)amine (TBPA/TBPA + ), cobaltocene (Cc/Cc + ) and ferrocene (Fc/Fc + ) in RTILs [11][12][13][14][15][16][17][18]. In all cases, the diffusion coefficient of the studied electro-active species appears, irrespectively of ionic liquid, two orders of magnitude lower than that measured in a more conventional organic solvent.…”