Electrochemical Behavior of Cobaltocene in Ionic Liquids

The electrochemical behavior of cobaltocenium has been studied in a number of room temperature aprotic ionic liquids. Well defined, diffusion controlled, anodic and cathodic peaks were found for the Cc+/Cc (cobaltocenium/cobaltocene) reduction/oxidation on gold, platinum and glassy carbon electrodes. Values of the peak separation parameters suggest quasireversibility or even irreversibility for the redox process. The difference between the ferrocene/ferrocenium and cobaltocenium/cobaltocene couples has been evaluated as equal to (1.350 ± 0.020) V. Values of the cobaltocenium (Cc+) diffusion coefficients D have been calculated on the basis of the Randles–Sevcik equation.

Finally, electrode potentials may be expressed versus an inner reference organometallic redox system, which consists of a large cation and its reduced form. The oxidized and reduced forms, both of large radius, may be assumed to be solvated similarly in different solvents. This leads directly to the assumption that redox potentials of such couples should be comparable in different solvents. Consequently, a given redox couple may be regarded as a universal potential reference. Organometallic redox couples such as bis(biphenyl)chromium(0)/(I) (BCr|BCr ? ), ferrocene|ferrocenium (Fc|Fc ? ), and cobaltocene|cobaltocenium (Cc|Cc ? ) have been investigated in various RTILs as described in the literature [10,[19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34], including a study on the applicability of cobaltocenium reduction as a reference for ionic liquids [30] and a detailed study on the simultaneous presence of both (Cc|Cc ? ) and (Fc|Fc ? ) couples in RTILs [32]. The general aim of the present study was to conduct a systematic investigation of the Cc/Cc ? redox reference system in various aprotic ionic liquids at different electrodes.

Water Content and Purity
The water content in aprotic ionic liquids, analyzed with a standard Karl-Fisher titrant (HYDRANAL Ò Composite 1, 1 mL/10 mg H 2 O), was below the detection limit. All of the RTILs were colorless. The purities of ionic liquids for electrochemical purpose were analyzed with cyclic voltammetry on platinum, gold and glassy carbon working electrodes. No reduction or oxidation peaks were detected between the anodic and cathodic decomposition potentials.
Before measurements, the electrodes were polished with aluminum oxide paste in water (Al 2 O 3 , 150 mesh, Merck) and then washed with acetone. The counter electrode was a platinum sheet (0.5 9 1.0 cm). The reference electrode consisted of a silver wire immersed in a solution of AgClO 4 (0.01 molÁdm -3 ) and cryptand 222 (0.1 molÁdm -3 ) in acetonitrile [17]. The reference electrode compartment was separated by a glass frit from the cell containing the ionic liquid. Preparation of the solutions, weighing of the samples, and cell assembly were performed in a glove-box under a dry argon atmosphere. Tested electrolytes were deaerated with argon for 30 min prior to measurements. Voltammetric curves were obtained with the lAutoLab Electrochemical System (Eco Chemie, The Netherlands) at (25 ± 0.1)°C. The initial scan was carried out to more negative potentials (reduction of cobaltocene: Cc ? ? e -? Cc 0 ) followed by the reverse anodic scan (oxidation of Cc 0 ). Two reduction/oxidation scans were recorded in each case. The baseline of each neat ionic liquids was measured before experiments with Cc/Cc ? solutions. The ohmic resistance R between electrodes was determined from impedance spectra (using an ac impedance analyzer Atlas-Sollich, Poland), in the frequency range of 100 kHz to 1 Hz with 10 mV amplitude.

Electrolyte Conductivity and IR Ohmic Drop
The conductivity of the electrolyte influences the resistance between electrodes of the cell. In the case of ionic liquid electrolytes, the specific conductivity is typically between ca. 10 and 0.01 mSÁcm -1 , according to Ref. [8], which may lead to resistances differing by three orders of magnitude. The resistance was obtained by deconvolution of impedance spectra according to an equivalent circuit, consisting of resistance R in series with the Warburg impedance and charge transfer resistance, in parallel to the double layer capacity. The resistance R, determined from impedance spectra, was between ca. 200 X (Et 3 SNTf 2 ) and 2,400 X (MePrPipNTf 2 ). At a typical current level of 10 lA, the IR distortion of the potential was in the range of 0.2-24 mV. Figure 1 presents typical cv curves, after background current and IR drop corrections, for cobaltocenium/cobaltocene (Cc ? /Cc) reduction/oxidation in BuMeImOTf ([CcPF À 6 ] = 9.18 mmolÁL -1 ) at potential sweep rates from (2 to 200) mVÁs -1 , versus the Ag|Ag ? 222, AN reference electrode. Potentials of the peak maximum, E pa and E pc , for Cc ? cathodic reduction and Cc anodic oxidation were -854 and -762 mV, respectively. Similar CV curves were obtained for a number of ionic liquids as solvents recorded at the three different electrodes (Pt, Au, GC). Measurements of DE 1/2 (Cc ? /Cc 0 ) over a period of 24 h gave stable values within ca. 1-2 mV, indicating no significant changes in the liquid junction potential between RTILs and the reference electrode electrolyte. Differences between the cathodic and anodic peak potentials, E pa -E pc , (E pa ? E pc ), and E p -E p/2 values (E p/2 is the half-peak potential), and peak current densities j pa and j pc , in the studied ionic liquids, are collected in Table 1. The difference between the cathodic and anodic peak potentials, E pa -E pc , is C81 mV, while the value predicted by the theory for reversible processes, according to Ref. [38], is (57-60) mV depending on the switching potential. The results indicate a quasi-reversible redox process for the cobaltocenium/cobaltocene couple. A similar behavior was found for the ferrocene|ferrocenium couple in a number of ionic liquids in our previous paper [34]. On the other hand, the E p -E p/2 values are close to 56 mV, typical of reversible processes [38]. In some protic RTILs, the E p -E p/2 values are close to the theoretical value of 56 mV, but in some cases they are higher (even as much as 70 mV). The formal potential (E pa ? E pc ) for the Cc ? /Cc couple in aprotic ionic liquids may be approximated by the value-(831 ± 13) mV (versus the Ag|Ag ? 222 in AN reference). Formal potentials obtained in this study for cobaltocene may be referred to potentials for ferrocene measured in a number of protic and aprotic ionic liquids [34]. Table 2 presents differences between formal potentials of ferrocene/ferrocenium and cobaltocenium/cobaltocene couples in ionic liquids as well as molecular liquids; these results were calculated from the (E pa ? E pc ) values measured versus the cryptate electrode Ref. [34], or versus reference systems mentioned in references [15,[28][29][30][31][32][33][34][35][36][37][38][39][40][41]. Inspection of Table 2 shows that the E 1/2 (Fc/Fc ? )-E 1/2 (Cc ? /Cc) difference may be approximated by the value (1.350 ± 0.020) V and is in agreement with values obtained by other authors [15,32,40,41]. Such good agreement of the potential difference (±0.02 V) suggests that the solvation of both reference couples is nearly independent of the electrolyte.
According to the Stokes-Einstein equation, Eq. 2, the main factor influencing the diffusion coefficient is the medium's viscosity, g, where k B is the Boltzmann constant and r is the cobaltocenium radius. Table 3 shows the Dg values calculated on the basis of literature data on ionic liquid viscosities published in references [5,[42][43][44][45][46][47][48], which correspond to the Walden product. Inspection of Table 3 suggests that the Dg product may be approximated by the value (0.80 ± 0.4) 9 10 -7 cmÁgÁs -2 . This result may also suggest that the cobaltocenium radius is constant and independent of the medium. An interesting aspect is a comparison (ratio) of diffusion coefficients of both popular metallocecenes, ferrocene and cobaltocenium, used as electrode potential references, D(Fc)/D(Cc ? ). Here, the solvation of both forms may be different due to the fact that ferrocene is a neutral molecule, while cobaltocenium is a cation. A comparison of D(Cc ? ) values (Table 3) with the corresponding D(Fc) literature   [49] and D(Cc ? ) = 1.30 9 10 -5 cm 2 Ás -1 [39]}. All of this information suggests that the Fc molecule and Cc ? cation, although of similar shape and radius, are probably solvated differently, as suggested in Ref. [50]. The cobaltocenium cation may interact with anions present in the electrolyte and therefore have a higher effective radius and, hence, a somewhat lower diffusion coefficient. In general, diffusion coefficients of large organic compounds determined in RTILs are on the order of 10 -7 cm 2 Ás -1 which is two orders of magnitude lower than in conventional molecular solvents [51].  Values of E p -E p/2 are close to 56 mV, typical of reversible processes. On the other hand, the difference between cathodic and anodic peak potentials, E pa -E pc , is C81 mV, while the value predicted by theory for reversible processes is 57-60 mV, which indicates a quasi-reversible redox process 3. The E 1/2 (Fc/Fc ? )-E 1/2 (Cc ? /Cc) difference may be approximated by the value (1.350 ± 0.020) V 4. Values of the cobaltocenium (Cc ? ) diffusion coefficients D are in the range of 0.5 9 10 -7 cm 2 Ás -1 -5.2 9 10 -7 cm 2 Ás -1 , depending on the medium's viscosity g.